Global Agriculture: Industrial Feedstocks for Energy and Materials
BM Jenkins, University of California, Davis, CA, USA
r 2014 Elsevier Inc. All rights reserved.
Glossary
Anaerobic digestion The biochemical conversion or
fermentation of organic materials, such as biomass, by
mixed bacterial communities under anaerobic (without free
oxygen) conditions. The process is typically used to produce
a methane-containing biogas. Anaerobic digestion also
occurs naturally in solid waste landfills.
Biochar Charcoal or black carbon produced by the
pyrolysis or other thermochemical processing of biomass.
Biochemical conversion Conversion by biological and
chemical processing, typically through microbial systems but
also involving the use of enzymes or chemical pretreatment.
Biofuel A fuel product made from a biomass feedstock.
Biomass Living material. In the context of energy and
materials, biomass is nonfossil material of biogenic origin.
Biomaterial A material synthesized or produced from
biological resources or biomass.
Biorefinery A processing facility, similar in function to a
petroleum refinery, used to refine or upgrade biomass
feedstocks to higher value fuels, materials, and other products.
Introduction
Technological innovations and the complex composition of
biomass offer immense opportunities for new materials and
products from agriculture. Many of these opportunities also
come with immense challenges. Biomass can be used to produce a wide variety of biofuels to replace petroleum and natural
gas, for example, but large-scale production is at present a
vigorously contested global issue with many uncertainties as to
net environmental, social, and economic benefits despite the
promise of increased renewable energy supplies and enhanced
national and global energy security. Less heavily debated is the
production of new pharmaceuticals, nutritional products, specialty chemicals, and biomaterials, but these also are subject to
concerns over the lifecycle implications of feedstock production
and product manufacturing. Together with new tools and
techniques for crop improvement, genetic modification, and
biomass conversion, agriculture has the capacity to add significant new economic and development value. Agriculture, as a
primary land use sector, also has the option to transform into
new ways that have little to do with food or biomass production, including land conversion to support wind and solar
energy deployment that may be only partially compatible with
more conventional agricultural production but which may offer
greater profit to the land owner. With the overall sustainability
of the current agricultural system still an open question, the
expansion, intensification, or redirection of agriculture toward
new markets will require careful analysis and management.
Although agriculture has long provided many products in
addition to food, it has the productive capacity to supply much
larger quantities of industrial feedstock for other markets, in
Encyclopedia of Agriculture and Food Systems, Volume 3
Gasification The thermochemical conversion of a biomass
feedstock or other organic material by heating and reaction
at elevated temperatures through partial oxidation using a
controlled amount of oxygen, steam, or other oxidant.
Gasification is used principally to produce fuel or synthesis
gases containing carbon monoxide and hydrogen along
with other species.
Lifecycle analysis An accounting technique used for
evaluating and comparing the environmental and other
impacts of a product, process, or system from the start to
end of its useful life.
Pyrolysis The thermochemical heating of a biomass
feedstock or other organic material in the absence of free
oxygen. The process typically produces liquid, gaseous, and
solid (char) products.
Thermochemical conversion Conversion by thermal and
chemical processing such as by combustion, gasification, or
pyrolysis.
many cases in direct competition with food production. European, US, and other national or regional policies to encourage
biofuel production have been heavily criticized for this reason
and for their potential to increase greenhouse gas emissions
rather than reducing them as intended. Increased demand for
biomass, whether food crop or otherwise but especially for
food crops like corn (maize) and soybeans for industrial
products, is also criticized for increasing food and feed prices
with particular impact on the world’s poor. This food versus
fuel debate is characterized by large uncertainties that lead to
difficulties in the design of effective policies for the broader use
of products from agriculture. As the development of agriculture
has had significant global impact due to land conversion,
transitions in agriculture to take advantage of new technologies
and new markets for energy, chemicals, materials, and other
nontraditional products require an improved understanding of
global consequences and a larger systems perspective beyond
what has been applied to date. The success of agricultural and
industrial research in producing new conversion methods and
new products from biomass has created enormous economic
and environmental potential, as well as a greater imperative for
better information and strategies in managing land resources
and the global agricultural enterprise.
Resources and Feedstocks
Photosynthetic Pathways, Efficiencies, and Global Biomass
Production
Biomass is living material, and in the context of energy and
materials from agriculture and other sources, biomass is
doi:10.1016/B978-0-444-52512-3.00156-X
461
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Global Agriculture: Industrial Feedstocks for Energy and Materials
interpreted to mean nonfossil material of biogenic origin.
Biomass includes purpose-grown organisms and crops as industrial feedstocks, crop, and processing residues from agricultural, industrial, and commercial operations, and biogenic
fractions of municipal solid wastes and wastewaters among
other sources. Agriculture is increasing its production of purpose-grown crops for energy and materials including trees and
shrubs, grasses and other herbaceous materials, algae, and
other aquatic and terrestrial species. Primary agricultural residues include cereal straws and stovers, animal manures,
orchard and vineyard prunings, forest slash from timber operations, forest stand improvement thinnings (such as small
trees and brush removed to reduce wildfire intensity), and
green waste from yard- and landscape maintenance. Secondary
residues arise from food processing, lumber production, and
other industrial operations. Black-liquor is a lignin-containing
secondary residue from pulp and paper production, but is
most commonly burned in recovery boilers at the mill in order
to regenerate pulping chemicals and generate steam and
power. Tertiary residues are associated with end-of-use materials such as wastepaper, food scraps and other biogenic
fractions of municipal solid wastes, biosolids from waste water
treatment, and waste fats, oils, and greases (FOG) although
advances in reuse, recycling, and product recovery are expanding the perception of these as resources rather than wastes
and altering their overall economic value and utility in society.
Large amounts of biomass reside in waste landfills that are
now being considered for materials mining to reclaim energy
and product value. Distinct in arising from different economic
objectives, residues and purpose-grown crops nonetheless
share many similarities in composition, use, and lifecycle
impacts.
The total global resource potential in biomass from agriculture and other activities has been variously estimated. The
principal photosynthetic pathway produces carbohydrate
biomass from carbon dioxide and water (Calvin, 1976). The
process is endothermic and requires energy in the form of light
(photons, ν) as well as water as a source of electrons for the
overall reduction of CO2 to carbohydrate:
nCO2 þ nH2 O þ ν ¼ ðCH2 OÞn þ nO2
½1
where n indicates the mean polymer chain length of the
carbohydrate. The photosynthetic efficiency dictates the number of photons needed per mole of carbohydrate. Anoxygenic
photosynthesis also occurs that does not result in oxygen as a
product and does not use water as an electron donor (see
reaction [2] below) (Sato-Takabe et al., 2012; Buchanan, 1992;
Cohen et al., 1975).
An estimated 0.02% of the 175 PW (PW ¼ Petawatt ¼
1015 W) of incoming solar radiation to the Earth (Hubbert,
1971) is used to produce approximately 70 1012 kg (70 Gt
(Gt ¼ Gigaton¼ 109 metric tons ¼ 1012 kg)) of biomass each
year through oxygenic photosynthesis (based on an average
dry matter heating value of approximately 16 MJ kg1) (Jenkins et al., 1998). More detailed analyses of biomass production by type of ecosystem yield global estimates in the
range of 170–220 Gt year1 (Klass, 1998; Hall et al., 1993).
Total plant biomass currently accumulated in all global ecosystems is estimated at approximately 1015 kg (1000 Gt) dry
matter, or approximately 10 times annual production (Hall
et al., 1993; Salisbury and Ross, 1992).
Biomass is also produced through processes other than
oxygenic photosynthesis. Hyperthermophilic bacteria, such as
those associated with hydrothermal vents at the deep ocean
floor, use chemosynthetic pathways for energy and biomass
production and are utilized in symbiotic relationships by other
organisms (e.g., tube worms) for survival in these extreme
environments (Kato et al., 2010). Overall, sulfide chemosynthesis produces elemental sulfur and water in converting
carbon dioxide and hydrogen sulfide:
nCO2 þ 2nH2 S ¼ ðCH2 OÞn þ nH2 O þ 2nS
½2
Oxygen gas is not released by this mechanism. Sulfur metabolizing bacteria that utilize chemosynthetic pathways are now
being investigated for their use in hydrogen sulfide removal
from biogas produced during anaerobic digestion of manure
and other feedstocks (Ho et al., 2013; Camarillo et al., 2013).
Sulfur removal is required for the successful application of
most postcombustion catalysts for NOx emission reductions
from engines burning biogas for power generation (Camarillo
et al., 2013; Liu and Gao, 2011).
Biomass provides approximately 15% of world energy
needs, but in developing countries constitutes a much higher
fraction of energy supply: 35% overall, and in excess of 80% in
many rural areas (Hall et al., 1993; Bain et al., 1998). Globally,
agriculture produces approximately 5 Gt year1 of crop residues of which perhaps less than half may be sustainably
available for energy and industrial purposes although larger
fractions may be used in developing countries (Jenkins, 1995).
Resource-focused studies of global bioenergy potentials range
widely, differing by at least an order of magnitude depending
on the assumptions made regarding land availability and
crop yields (Berndes et al., 2003). Considerations of lifecycle
environmental effects, particularly greenhouse gas emissions,
also influence global potentials (Searchinger et al., 2008;
Fargione et al., 2008).
Plants utilize three principal pathways in assimilating atmospheric carbon dioxide and synthesizing carbohydrate
structures and other compounds through photosynthesis. The
biological or dry matter yield is dependent in part on the
pathway used. Light energy is absorbed in two pigment systems called photosystem I (PS-I) and photosystem II (PS-II).
In both the systems, absorption of light by chlorophyll and
accessory pigments leads to the emission and transport of
electrons against an adverse voltage gradient. As noted above
(refer to reaction [1]), in oxygenic photosynthesis the electrons
are derived from the photolysis of water mediated by a manganese-containing enzyme in PS-II (Salisbury and Ross, 1992;
Marschner, 1986). Electrons are transferred from PS-II through
what is called the Z-scheme to PS-I, storing energy in the carriers adenosine triphosphate (ATP) and the reduced form of
nicotinamide adenine dinucleotide phosphate (NADPH) for
later use in CO2 reduction and carbohydrate synthesis. Mineral
nutrients are directly involved in electron transport, and in
other processes of the plant. The mineral or ash concentration
and composition are often quite important in the subsequent
use of the biomass, and can influence the design of the production and utilization system.
Global Agriculture: Industrial Feedstocks for Energy and Materials
The light reactions store energy in NADPH and ATP. In the
so-called C3 plants, CO2 and water react with ribulose-1,5diphosphate to produce 3-phosphoglyceric acid as part of the
Calvin–Benson cycle. The glyceric acid is subsequently converted using NADPH and ATP into 3-phosphoglyceraldehyde
and then into hexose phosphate and ribulose-5-phosphate.
The latter reacts with ATP to regenerate ribulose-1,5-diphosphate, whereas hexose is used in the synthesis of the
primary storage products, sucrose and starch. The C3 pathway
takes its name from the 3-carbon intermediates produced
during the cycle. C3 plants include the cereals barley, oats,
rice, and wheat, alfalfa (lucerne), cotton, Eucalyptus, sunflower, soybeans, sugar beets, potatoes, tobacco, Chlorella, and
others. Gymnosperms (with a few possible exceptions),
bryophytes, and algae are C3 organisms, as are most trees and
shrubs.
Along the C4 pathway, CO2 combines with phosphoenolpyruvate (PEP) via a PEP-carboxylase catalyzed reaction to
form oxaloacetate, which is reduced to malic-acid (malate) or
aspartic-acid (aspartate), 4-carbon intermediates giving the
pathway its name. These are translocated from the mesophyll
cells where the primary CO2 fixation occurs to the bundle
sheath cells from which CO2 is released for subsequent fixation through reactions of the Calvin–Benson cycle as in C3
plants. Decarboxylation of the acids regenerates PEP. C4 plants
are usually of tropical origin and have higher photosynthesis
rates and higher biological yields (dry matter production)
compared with C3 plants. C4 plants include sugarcane, sorghum, maize, and Bermuda grass. Euphorbia species considered
for direct hydrocarbon production are mostly C3, but a few
have evolved to use the C4 pathway.
The third primary pathway is that of crassulacean acid
metabolism (CAM) used by many succulents. The CAM
pathway also fixes CO2 via PEP carboxylase, but in the CAM
species the stomata are open at night rather than during the
day in order to reduce water loss. Malate is stored in the
vacuoles during the night and then released during the day
when the stomata are closed. CAM plants have lower growth
rates than the C4 species but have high water use efficiency due
to their adaptation to low-water environments, including
semiarid and saltmarsh regions, and epiphytic sites, such as
those used by orchids.
For photosynthetic organisms, biological yield (total biomass) is a function of the net production by photosynthesis
and consumption by respiration, the latter including photorespiration in C3 plants. Respiration provides energy through
the oxidation of organic compounds in generating substrates
for the synthesis of other plant products in essentially the reverse of reaction [1] above. Maximum theoretical photosynthetic efficiencies can be derived based on a minimum
requirement of eight photons of photosynthetically active radiation (PAR, light of 400–700 nm wavelength) per molecule
of CO2 used to produce glucose, the minimum ν in reaction
[1] (Klass, 1998; Hall et al., 1993; Loomis and Williams,
1963). Approximately 43% of the energy in sunlight at the
ground level is contained within the PAR band, and of this a
maximum of approximately 80% is actively absorbed during
photosynthesis. Only approximately 28% of this energy is
stored in glucose. In C4 plants respiration consumes somewhere between 25% and 40% of the energy in glucose. The
463
maximum net efficiency of photosynthesis based on incident
sunlight is therefore 6–7%. Photorespiration in C3 plants
generally leads to efficiencies of approximately 3%, lower than
the C4 plants. Photosynthetic efficiencies can be translated to
biomass yields using site-specific insolation data and the energy content of the biomass (heating value). At maximum
efficiency, theoretical yields can exceed 400 metric tons of dry
matter per hectare per year. Agricultural yields are generally far
below this due to nonoptimal crop conditions, limited inputs,
and losses to diseases and pests. In practice, yields are also
lower because not all biomass is or can be harvested. Genetic
modifications and other crop improvements are widely investigated to increase productivity and yields.
Efficiencies for agricultural crops typically are of the order
of 1%, although tropical crops, such as sugarcane and highyielding grasses, can produce at 2–3% efficiency with dry
matter yields of 50–100 Mg ha1 year1. Intensive production
of green algae can approach 5% efficiency, similar to the best
efficiencies with C4 crops under research conditions. A maize
(corn) crop grown in Davis, California achieved 5.6% photosynthetic efficiency under optimized conditions where only
light was limiting (Loomis and Williams, 1963). Seasonal efficiency for many C3 crops when given sufficient water and
nutrients is closer to 2% (Monteith, 1977). Natural forest efficiencies trend lower. The production of purpose-grown industrial and energy crops seeks to produce biomass at
relatively high photosynthetic efficiency. Owing to the costs of
inputs, an optimal production system based on maximum
profit may not operate at maximum yield; however, yields are
nevertheless of high relative importance to the overall economic feasibility.
Biomass can be used for remediation of environmental
contamination (phytoremediation), and the use of biomass as
fuel in the substitution of fossil resources can help mitigate
greenhouse gas and global climate change impacts, but its
production and use requires careful lifecycle assessment to
ensure net environmental benefits. As noted earlier in the
Section Introduction, many policies and incentives for the
production of biofuels are highly controversial.
Biomass production has significant utility in serving to
store solar energy. Hybrid renewable energy systems using
biomass can take advantage of this attribute in helping to
stabilize electricity grids supplied with high levels of intermittent solar and wind power generation.
Biomass is a distributed resource, and its use to supply
large quantities of energy and materials requires relatively large
amounts of land. The overall conversion efficiency from solar
energy to final energy product is low due to the inherently low
efficiency of photosynthesis. The overall electrical efficiency,
for example, using biomass produced at 2% photosynthetic
efficiency as feedstock for power plants operating at 25%
average thermal efficiency is 0.5%. Solar photovoltaic systems,
including inversion of DC to AC power for grid interconnection, currently generate at efficiencies of 6–12% (with
peak research efficiencies well above this), but suffer from the
intermittency of sunlight. The higher efficiency of direct solar
energy conversion coupled with declining manufacturing costs
generates competition between the agricultural and energy
sectors for land resources, a subject of additional controversy
and developing policy.
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Global Agriculture: Industrial Feedstocks for Energy and Materials
Improvements in biomass-fueled power systems will increase conversion efficiencies, but the overall solar conversion
efficiencies for biomass are likely to remain below 1% for
power. To meet the current (2013–14) world primary energy
demands of approximately 600 EJ (EIA, 2013a) would require
approximately 600 million hectares of land (1 TJ of energy in
biomass per hectare per year) for crops continuously producing at 2% photosynthetic efficiency (approximately 6 kg
per square meter or 60 Mg ha1 year1 dry matter). This is
approximately 40% of the world’s cultivated land area, and
15% of forest lands. It is only approximately 30% of the area
of degraded tropical lands, and half the area of these lands
considered suitable for reforestation (Hall et al., 1993).
Abandoned agricultural land is estimated to range globally
from 385 to 470 million hectares (Campbell et al., 2008). The
actual land requirement to meet the world energy demand
would be much greater than this because of the differences in
conversion efficiencies between the current energy resources
and biomass to satisfy the same end uses, and because not all
land would be kept in continuous production and the overall
photosynthetic efficiency would not likely reach 2%. This estimate does not include aquatic or marine species such as algae
that might also contribute. More considered estimates of the
contributions from bioenergy range from 50 to 240 EJ year1
and closer to 10% of the global energy demand (Berndes et al.,
2003). Meeting the world energy demand from biomass is
neither necessary nor desirable, but the potential scale for industrial feedstock production is large, even for relatively small
shares of the energy market alone and highly significant in
terms of land use impacts, both direct and indirect.
Types of Biomass
Agricultural residues
Agricultural residues are coproducts of the principal commodity production system. These include primary vegetation
as well as animal manures composed of digested feeds. Corn
stover, for example, which is a residue of grain production, is
now receiving considerable attention as an energy resource in
addition to its potential use in new materials. Increasing economic value of residue biomass can lead to changes in the
overall crop production system, such as attention to improving
yields of residues as well as the primary crop. Increased fertilization may be needed to replace nutrients exported with the
residue biomass when harvested. Alternatively, the farming
system may adapt to the application of recycled ash or other
nutrients returned from the biomass utilization system.
Changes in residue management can also lead to addition or
loss of soil carbon, adjustment of fertilizer composition to
reduce the uptake of chlorides and other constituents that can
be detrimental to downstream conversion, modified tillage
strategies to protect against soil erosion when residue cover is
reduced or to take advantage of decreased amounts of residue
needing to be incorporated into the soil, changes in irrigation
practices to manage soil and crop moisture for biomass harvesting, modified chemical applications due to changes in
weed, pest, and disease pressure from residue removal, and
changes in the harvesting system design and operation to integrate primary crop and residue collection.
Controversy over indirect land-use change effects and food
versus fuel impacts associated with purpose-grown crops for
biofuels shifted focus onto agricultural residues as seemingly
benign sources of feedstock. However, residue use also needs
to be carefully evaluated for more global sustainability effects.
Harvesting of corn stover in the midwestern US for biofuels,
for example, may result in losses of soil carbon that increase
the net greenhouse gas emissions above the levels for petroleum-based fuels (Murphy, 2013). Purpose-grown crops
such as mixtures of native grassland perennials may prove
superior in overall environmental performance (Tilman et al.,
2006; Murphy, 2013). Broad generalizations relating to the
relative impacts of agricultural residues and industrial crops
should be carefully inspected.
Other changes to the production system may occur when
modifications are desired in the properties of the biomass.
One example is the delayed harvesting of cereal straws to take
advantage of natural precipitation in the leaching of alkali
metals and chlorine to improve the combustion or gasification
properties for biomass power generation and to reduce the
export of nutrients from the field when harvesting residues.
Residue harvesting reduces air pollution when substituted for
traditional open burning disposal practices with some crops,
and also removes nonvolatile nutrients such as phosphorous
(P) and potassium (K) (Jenkins et al., 1992). Leaching by
precipitation readily removes soluble potassium and a number
of other constituents and returns them to the field before
harvesting, although in-field decomposition of the residue and
dry matter loss may reduce the overall economic value (Bakker
and Jenkins, 2003).
Residue yields from selected crops are listed in Table 1. The
estimated production rates for animal manures are also
shown. When not measured directly, residue yields are frequently estimated from primary crop yields using the harvest
index, the ratio of crop yield to total above ground biomass,
typically determined from test plots (Huehn, 1993). Harvested
yields are typically lower than listed due to losses in collection
and handling, adding to the uncertainty associated with resource supply.
Forest residues and stand improvement biomass
Forest residues include tree tops and branches from timber
harvesting operations referred to as forest slash (due to the
practice of removing them from the commercially valuable
bole of the tree). Biomass can also be collected in the form of
forest thinnings of two forms: (1) lower quality stock that is
often unsuitable for traditional markets and which contributes
to poor forest health or (2) as increased production from more
intensively managed regrowth forests. In many temperate
forests, the mean annual growth far exceeds the mean annual
harvest, and the overall stand quality can be diminished.
Understory brush and dense stands of trees can contribute to
high fuel loadings in forests with increased danger of catastrophic wildfire, higher incidence of crown fires and greater
damage and mortality among mature trees, and high costs of
fire suppression if practiced. Many forests of the western
United States are now particularly at risk of catastrophic fire
due to high fuel loads resulting from more than a century of
fire suppression (Jenkins, 2005).
Global Agriculture: Industrial Feedstocks for Energy and Materials
Table 1
Agricultural residue yields and manure production rates
Crop residue
Yield (Mg ha 1 year 1)
Almonds
Apples
Apricots
Artichokes
Asparagus
Avocados
Barley
Beans
Cherries
Citrus
Corn (maize)
Cotton
Cucumbers
Dates
Figs
Grapes
Lettuce
Melon/squash
Oats
Peaches
Pears
Plums
Potatoes
Prunes
Rice
Safflower
Sorghum (grain and milo)
Sugar beets
Tomatoes
Walnuts
Wheat
2.2
3.7
3.4
3.8
4.9
2.5
2.5
1.9
0.7
1.7
9.0
2.9
3.8
1.7
3.7
3.4
2.2
2.7
2.3
3.4
3.8
2.5
2.7
1.7
6.7
1.9
5.0
4.6
2.9
1.7
3.7
Livestock manures
Production (kg dry matter per animal per day)
Beef cattle
Dairy cattle
Chickens (layers)
Chickens (broilers)
Turkeys
Swine
4.1
5.9
0.04
0.02
0.1
0.5
Source: Adapted from Knutson, J., Miller, G.E., 1982. Agricultural residues in
California, factors affecting utilization. UC Cooperative Extension Leaflet No. 21303.
Berkeley, CA: University of California.
Industrial and energy crops
Terrestrial (land-based) purpose-grown industrial and energy
crops are typically classified as woody or herbaceous although
not all industrial crop types are readily classified in this way.
Jatropha, for example, includes approximately 170 species of
succulent plants, shrubs, and trees. The more common species
of this plant considered for biofuel purposes is the droughtresistant shrub Jatropha curcas that produces an oil-bearing
seed useful for biodiesel production among other uses. The
plant also contains toxic compounds including phorbol esters
and other terpenoid compounds, but even these also have
some beneficial uses (Devappa et al., 2011; Wang et al.,
2013a). The phorbol esters demonstrate insect deterrent and
cytotoxic antitumor and antimicrobial properties (Devappa
et al., 2011). Woody crops are predominantly plantation trees,
465
frequently grown in short-rotation intervals of from 1 to 20
years between harvests. Cultural practices for these crops have
been well established for roundwood and papermaking, and
have been more recently extended to the production of fuel
wood. Herbaceous crops include annual and perennial grasses
and other nonwood plants. Production practices for these
crops are in most cases similar to other agricultural crops, although in both woody and herbaceous crop production, the
end use for the biomass can influence the management and
cultural inputs and practices employed to optimize the production system. A number of more commonly considered
industrial and energy crops are listed in Table 2 but many
more have been investigated for specific material recovery or
energy purposes.
Just as they are for current agriculture, water availability
and water costs are key constraints in industrial biomass
production. Water requirements vary considerably but are
typically in the range of 300–1000 kg per kg of dry matter
produced. Arid or semiarid regions of the world are not anticipated to produce substantial quantities of biomass for energy markets even where water is commonly imported for
agriculture. The exception may be in the production of algae
due to the high availability of solar radiation and the potential
for the use of brackishwater, waste water, or seawater (Renuka
et al., 2013). Lands in these areas might be used in the production of feedstocks for higher value industrial and consumer
products.
Biomass production can play an integral role in managing
salts and remediating other undesirable impacts of irrigated
agriculture in arid or semiarid regions and in reclaiming lands
degraded by unsustainable agricultural practices. Integrated
farm drainage management (IFDM) systems have been
evaluated in California and other irrigated agricultural regions
to reclaim salt-affected soils and to reduce the environmental
impacts of agriculture in these areas (Lin et al., 2002). IFDM
systems employ sequential reuse of water through a cascade
of increasingly salt-tolerant crops with the objective of continuously reducing water volume and increasing salt concentration for ultimate recovery or removal. Fresh irrigation water
applied to high-value vegetable crops, for example, results in
drainage water that can be applied to more salt-tolerant agricultural crops such as cotton or barley. Biomass crops, such
as Eucalyptus or Jose tall wheat grass (Agropyron elongatum)
are suitable for irrigation with the secondary drainage water
from these, with tertiary drainage from the biomass crops
applied to halophytes, and solar evaporators or other salt
separation and purification systems used for final treatment
and salt removal. The biomass crops transpire large amounts
of water and act as biopumps to lower groundwater tables
and reduce salt accumulation in the root zone, and
also help in removing toxic elements, such as selenium, that
is taken up and accumulated in the plant biomass. Crop
harvesting removes undesirable materials from the field for
downstream recovery, utilization, or disposal. Biomass yields
under these conditions are substantially less than the best
yields obtained for the same species under optimal conditions. Where other agronomic or environmental purposes
are involved, yield is not necessarily the primary consideration for these systems although still an important consideration for economic performance. Biomass crops can in
466
Table 2
Global Agriculture: Industrial Feedstocks for Energy and Materials
Selected industrial and energy feedstock crops
Woody species – biomass/fiber/pulp
Alder (Alnus spp.)
Australian pine (Casuarina)
Birch (Onopordum nervosum)
Black locust (Robinia pseudoacacia)
Eucalyptus (Eucalyptus spp.)
Lucaena (Lucaena leucocephala)
Poplar (Populus spp.)
Willow (Salix spp.)
Herbaceous species – biomass/fiber/energy grain
Alfalfa (Medicago sativa)
Corn/Maize (Zea mays)
Flax (Linum usitatissimum)
Hemp (Cannabis sativa)
Jose tall wheatgrass (Agropyrum elongata)
Kenaf (Hibiscus cannabinus)
Miscanthus (Miscanthus spp.)
Napier grass/Bana grass (Pennisetum purpureum)
Spanish thistle or Cardoon (Cynara cardunculus)
Spring barley (Hordeum vulgare)
Switchgrass (Panicum virgatum L.)
Triticale (Triticosecale)
Winter rye (Secale cereale)
Winter wheat (Triticum aestivum)
Herbaceous species – sugar/starch/biomass
Buffalo gourd (Curcurbita foetidissima)
Cassava (Manihot esculenta)
Jerusalem artichoke (Helianthus tuberosus)
Sugar/energy cane (Saccharum spp.)
Sugar/fodder beet (Beta vulgaris)
Sweet sorghum (Sorghum bicolor)
general be used for the restoration of abandoned or degraded
agricultural lands (Campbell et al., 2008), and economic and
environmental reasons are frequently proposed for planting
in marginal areas.
Short-rotation woody crops grown for energy or other
purposes are typically grown in plantations with stand densities ranging up to 10 000 trees per hectare depending on the
size of the tree desired at harvest, harvesting technique employed, and end use intended for the biomass (Smith et al.,
1997). More than 100 million hectares are in industrial tree
plantations (Hall et al., 1993), most in longer rotations for
roundwood and pulp production. Production site selection
depends on soil properties, water availability, slope, climate,
and distance from market. As with agricultural crops, high
yields are associated with better soil types, although trees can
generally take advantage of higher groundwater tables (Siren
et al., 1987). Soil preparation commonly involves land clearing to remove the existing vegetation and eliminate weeds, soil
amelioration to adjust pH and improve tilth, drainage, and
nutrient concentrations, and leveling and in some cases
mulching. The plant bed is prepared in a manner similar to
agricultural crops, although tillage is typically deeper than for
cereals. Following soil preparation, cuttings, slips, or seedlings
are planted either manually or by machine or by machineassist to manual planting (Golob, 1987). Soil moisture management following planting is critical as deeper tillage
Wetland species – biomass/fiber
Cattail (Typha sp.)
Cordgrass (Spartina spp.)
Giant reed (Arundo spp.)
Giant reed (Phragmites spp.)
Reed canary grass (Phalaris arundinacea)
Seed/oilseed/terpenes
Amaranth (Amaranthus spp.)
Castor (Ricinus communis)
Crambe (Crambe abyssinica)
Euphorbia (Euphorbia spp.)
Jojoba (Simmondsia chinensis)
Linseed (Linum usitatissimum)
Oilseed rape (Brassica spp.)
Safflower (Carthamus tinctorius)
Soybean (Glycine max)
Sunflower (Helianthus annuus L.)
Jatropha (Jatropha curcas)
Aquatic species – biomass/lipids
Brown algae (Sargassum spp.)
Giant kelp (Macrocystis pyrifera)
Microalgae (Botryoccus braunii)
Red algae (Gracilaria tikvahiae)
Unicellular algae (Chlorella and Scenedesmus)
Water hyacinth (Eichhornia crassipes)
increases drying rates, potentially leading to inadequate water
availability without rain or irrigation. Cultivation, chemical
application, or mulching (including plastic sheet mulching) to
control weeds, and in some cases pruning to improve shoot
vigor, are typically required following planting for proper
stand establishment. Some crop species, such as Salix (willow),
are highly susceptible to common herbicides. Biological weed
control is sometimes practiced, usually by planting weedcompetitive crops such as Trifolium, although these can also
compete for nutrients and water with the primary crop until
good canopy cover is achieved. Fencing to control grazing by
animals may also be needed. Irrigation and fertilization can be
accomplished in the same manner as for agricultural crops.
Drip irrigation systems can be combined with chemigation
(injection of nutrients and other chemicals into the irrigation
system for distribution to the plants). Frost protection is not
commonly practiced, but frost damage is a concern in many
locales. Where frosts are frequent, more tolerant species or
varieties are selected. Fire suppression may be needed in dry
areas, and varietal or species selection is important in reducing
damage from insects and diseases. Plantation design can include set-aside areas harboring native predators of pests, and
division of the plantation into blocks of different clones or
species to make the overall plantation less susceptible. Owing
to the lower frequency of planting and other operations in tree
plantations, soil erosion rates are generally lower compared
Global Agriculture: Industrial Feedstocks for Energy and Materials
with agricultural crops. Seedling survival rates in wellmanaged plantations are typically approximately 85%. Coppice (resprouting) crops are harvested in 3–10 year cycles, with
up to six cycles before replanting. Mean annual dry matter
increments in practice are 10–20 Mg ha1 year1, generally
declining at higher latitudes for reasons of climate and insolation. Crop improvements may extend these yields to
15–30 Mg ha1 year1 or higher.
Production of herbaceous crops in many respects bears
greater resemblance to conventional forage and other agricultural crops, although many of the same considerations
apply to the production system design as do for short-rotation
woody crops (Smith et al., 1997; Gosse, 1996). Regional mixed
cultivation of perennial and annual species, as opposed to
large-scale monoculture, is seen in many cases to be of benefit
both in terms of environmental and economic performance
(Tilman et al., 2006). Depending on the location, the regional
mix might include legumes and warm and cool season grasses.
Total dry matter yields for herbaceous species under good
management are typically in the range of 10–30 Mg ha1
year1. Sugarcane, a C4 species, is one of the most productive
crops, with world average yields of approximately 35 Mg
ha1 year1 (Hall et al., 1993) with fresh wet-weight yields
including sugar above 250 Mg ha1 year1 although few
countries exceed 100 Mg ha1 year1 fresh-weight (Duke,
1983). Where proposed as energy or industrial crops, particular attention must be given to the composition of the
plant. Herbaceous species, especially grasses, generally contain
more ash than woody species, and the ash is higher in alkali
metals and silica. The latter, for example, combine at high
temperatures in thermochemical systems to form slags and
deposits that increase maintenance costs. Leaching with water
can remove most of the alkali, and delayed harvest to take
advantage of rain-washing can improve the combustion
properties in the same manner as noted earlier for residues
(Jenkins et al., 1998; Huisman, 1999). Chlorine, primarily
supplied in potassium fertilizer (e.g., as muriate of potash or
KCl) or present as chloride in irrigation water, facilitates alkali
volatilization and fouling at high temperatures, accelerates
corrosion of metals, and contributes to acid gas (principally
HCl) and toxic emissions such as dioxins and furans from
combustion systems (Jenkins et al., 2011). Sulfur, although
typically present in plant biomass at low concentrations, also
contributes to fouling and air pollutant emissions. Restricting
chlorine from fertilizers in energy crop production is generally
advantageous, as is proper management of sulfur. These concerns are reduced for crops grown as feedstock for direct
chemical extraction, fermentation, or manufactured fiber
products, where oil, sugar, starch, or structural carbohydrates
are of more importance, unless ligneous residues of the fermentation are to be burned as fuel as is commonly considered
for cellulosic biorefineries. However, in all cases the composition of the feedstock needs to be considered in terms of
the requirements of the manufacturing or conversion process
and any accompanying waste disposal. Consideration also
needs to be given to the introduction of exotic species that may
grow well initially but lack biological disease and pest controls
in new environments and are, therefore, more susceptible to
later damage or that may prove adaptive and invasive to the
detriment of native species or other crops.
467
Except for phytoplankton, aquatic species tend to demonstrate higher yields than terrestrial crops (Klass, 1998). Aquatic
species include both unicellular (e.g., Chlorella, Scenedesmus)
and macroscopic multicellular (e.g., Macrocystis, Gracilaria,
Sargassum) algae (seaweeds), and a number of water and salt
marsh plants such as cordgrass (Spartina spp.), reed (Arundo,
Phragmites), bulrush (Scirpus), and water hyacinth (Eichhornia).
Dry matter yields range between 5 and 75 Mg ha1 year1.
Water hyacinth is considered a nuisance plant in many inland
waterways and is difficult to control, but is hardy and disease
resistant. Many of the aquatic species are currently produced
under much more intensive conditions, and generally for more
valuable products than energy and fuels, for example, Spirulina
for food protein or for antihistamine peptides used in the
prevention of atherosclerosis (Spolaore et al., 2006; Vo and
Kim, 2013), although benefits of its use as a supplement for
animal feeds are still in question (Holman and Malau-Aduli,
2013).
Algae and cyanobacteria represent a significant fraction
(20–30%) of global photosynthetic productivity and are
widely investigated for their energy and industrial product
potential (Waterbury et al., 1979; Spolaore et al., 2006;
Pisciotta et al., 2010). High lipid concentrations in some
strains of algae (up to 50% in Nannochloropsis and 60% in
Botryococcus braunii (Scott et al., 2010)) have encouraged
research and commercial development into the production of
biodiesel fuel from algal extracts and fermentation of algae for
alcohol, biogas, and other products using algal production in
open natural systems, ponds and raceways, as well as in highly
engineered bioreactor systems (Sheehan et al., 1998; Cheng
et al., 2013). Petroleum resources that supply current motor
fuels and other energy demands predominantly originate from
algal and zooplankton biomass deposited during periods of
global warming approximately 90 and 150 Ma ago (Campbell,
1988). Interest in algae is also driven by perceived high
productivities and the potential for carbon uptake and sequestration in mitigating carbon dioxide emissions from
power plants. Single-day maximum productivities obtained in
research trials are up to 50 g of dry matter per square meter per
day, which if sustained throughout the year would equate to
183 Mg ha1 year1, well above yields from the best producing C4 species. Commercial yields are unlikely to exceed
approximately 80 metric tons dry matter per hectare per year
(Scott et al., 2010) although more optimistic estimates are
frequently made. Sea-based floating membrane bioreactors
have also been proposed to enable large-scale off-shore
farming of algal biomass using municipal wastewaters as nutrient sources (Howell, 2009; Harris et al., 2013).
Structure, Composition, and Properties of Biomass
Biomass is a chemically rich feedstock that can be used in its
native form, converted into intermediate chemicals and materials for further downstream processing to final products, or
converted directly to the final product. Composition varies by
the type of feedstock, location of production, and by a number
of other cultural attributes, but can be generally classified into
organic and inorganic constituents all of which can be important in the conversion to energy, materials, and fuels. These
468
Global Agriculture: Industrial Feedstocks for Energy and Materials
lead, mercury, and other heavy metals). The design of any
industrial system requires a careful analysis of the chemical
and physical properties of the intended feedstock to evaluate
influences on process operations and product quality.
The structure of biomass varies depending on the species
and the tissue type. Microstructures of some higher plants
(woods and grasses) are shown in Figures 2–4. The term
‘wood’ more specifically refers to the secondary xylem tissue
arising from cell division in the vascular cambium (Hon and
Shiraishi, 2001), but bark and other tissues are also generally
included when referring to feedstock materials even though
compositions can be widely different. Ash content, for example, is frequently higher in bark than in wood. Its distribution also varies in herbaceous species such as rice (Summers
et al., 2003). Woods are classified as softwoods and hardwoods. Tracheid and parenchyma cells are the principal types
in softwoods, with tracheids in some comprising 90% of the
wood volume. Hardwoods are more complex. Principal cell
types in hardwoods are fibers, vessels, and parenchyma.
Herbaceous species such as the grasses have tubular or solid
stems (culm) divided into node and internode regions, the
latter typically surrounded by leaf sheaths originating from the
nodes. Algae constitute a highly diverse group of eukaryotic
organisms that can be unicellular or multicellular and differ in
fine structure compared with terrestrial green plants (Bouck,
1965). They range in size from microalgae under 5 mm (e.g.,
Scenedesmus) to the macroscopic giant kelp (Macrocystis
pyrifera) exceeding 50 m in length. They also vary substantially
in lipid, carbohydrate, nucleic acid, and protein composition
(Sheehan et al., 1998). Fresh-weight moisture is more than
65% and more commonly more than 85%; dry weight ash
contents can exceed 30%; protein concentrations can approach
Fixed
carbon
Elemental
composition
(Si, Al Ti, Fe,
Ca, Mg, Na, K,
P, S, Cl, C,H,
O,...)
Elemental composition
C
Extractives
(sugars, starch,
lipids, proteins,
inorganics,...)
H
O
N, S, Cl,...
Figure 1 Biomass composition (dashed lines indicate variable concentrations).
Structural components
Pectin,...
Volatile matter
Volatile solids
Dry matter (total solids)
Wet biomass
Ash
(metals,
minerals,
adventitious
materials)
Moisture
constituents can be further divided into a number of components and properties subject to international standards of
analysis (Figure 1).
Fresh or crude biomass contains water as moisture and dry
matter, the latter comprises organic constituents that are
volatile under heating (volatile solids) and an inorganic fraction represented as ash. Moisture content influences the entire
production chain due to its importance in determining the
harvesting conditions, transportation modes and costs, conversion technology type and performance. For the processes
involving thermal transformations (e.g., combustion), the organic fraction is typically classified into volatile matter (not to
be confused with volatile solids) and fixed carbon, but combined these two fractions comprise most of the structural
components of biomass (principally hemicellulose, cellulose,
and lignin but also pectin and other cell wall polymers) and
extractives (e.g., lipids in algae or oilseeds, sugars, starches, and
terpenes and also many other compounds including some of
the inorganic constituents). The ash consists typically of salts
and other minerals, metals, and adventitious material, that is,
materials not inherent in the biomass but added instead
through handling and processing, such as soil accumulated
during harvesting. Organic materials may also be adventitious,
and alterations in the organic composition and structure can
occur through degradation or decomposition of the feedstock
during handling, storage, and processing. More fundamentally, the biomass can be characterized by its elemental
composition that will contain essentially every natural element
but more practically is limited to a smaller number of major
elements with a large number of minor and trace elements at
concentrations of significance, some of which will be important to industrial processing or energy conversion (e.g.,
Lignin
Hemicellulose
Cellulose
Global Agriculture: Industrial Feedstocks for Energy and Materials
half the dry weight; and lipid concentrations range up to 60%
dry weight (Butler, 1931; Renaud et al., 1999; Sheehan et al.,
1998). Cell wall components include cellulose, hemicelluloses, lignin, pectins, arabinogalactan proteins, extensin, carrageenan, agar, and in diatoms sculpted silica composites
(Blumreisinger et al., 1983; Abo-shady et al., 1993; Domozych,
2011; Domozych et al., 2012). The so-called blue-green algae
(prokaryotic cyanobacteria) are now classified with the bacteria. Chitin (polymeric N-acetylglucosamine, also found in
the exoskeletons of arthropods) rather than cellulose is the
primary structural component of the cell wall among the true
fungi (Blumenthal and Roseman, 1957; Sikkema and Lovett,
1984).
The dominant structural compounds making up the plant
biomass are cellulose (six-carbon or C6 polymers) and hemicellulose (predominantly C5 polymers but including C6 species) produced via condensation polymerization of the
monosaccharides (Chum and Baizer, 1985; Schultz and
Taylor, 1989; Sudo et al., 1989; Lynd, 1990; Wyman and
Hinman, 1990; Hon and Shiraishi, 2001). The other primary
structural components are lignins, aromatic polymers of
variable structure derived in one proposed pathway from
coniferyl, sinapyl, and p-coumaryl alcohols. The alcohols arise
through the shikimic acid pathway, and are polymerized into
lignin via free-radical reactions (Salisbury and Ross, 1992).
Organic compounds in biomass also include proteins, triglycerides (fats and oils), terpenes (including isoprenes), waxes,
cutin, suberin, phenolics, phytoalexins (antimicrobial compounds produced by the plant), flavonoids, betalains, alkaloids, and other secondary compounds as well as sugars and
starch. Plants accumulate inorganic materials (ash), sometimes
in concentrations exceeding those of the hemicellulose or
lignin. The composition of wood can depend on the applied
forces during growth (‘reaction’ wood). Compression wood
typically has highly lignified tracheid walls, in contrast to
tension wood with lower lignin concentrations.
469
Cellulose is a linear crystalline polysaccharide of β-Dglucopyranose units linked with (1–4) glycosidic bonds. Cellulose serves as a framework substance, making up 40–50% of
wood. In the cell wall, cellulose exists in the form of thin
threads or microfibrils, each 2–5 nm wide in wood, but up to
30 nm wide in algae (Valonia). The polymer is formed from
repeating units of cellobiose, a disaccharide of β-linked glucose
moieties (Figure 5). The structure of cellulose renders it highly
insoluble and resistant to biochemical degradation. The αlinked disaccharide of glucose forms starch (Figure 6). Starch
is an amorphous material, and as such is more readily degraded by biochemical means than is cellulose and hence the
relative ease of fermenting starch to ethanol whereas cellulose
requires substantial pretreatment to deconstruct it before
fermentation.
Hemicelluloses are matrix substances laid down between
cellulose microfibrils. They are polysaccharides of variable
Figure 3 Scanning electron micrograph of birch hardwood showing
a more complex structure compared with softwoods. Large holes are
vessel elements. Reproduced from Society of Wood Science and
Technology, 2013. Structure of Wood. Available at: http://www.swst.
org/edu/teach/teach1/structure1.pdf (accessed 11.11.13).
Figure 2 Scanning electron micrographs of softwood showing longitudinal tracheids, horizontal rays, and other structures. Left: spruce wood
(rc, a resin canal; bp, a bordered pit). Right: pine wood. Reproduced from Society of Wood Science and Technology, 2013. Structure of Wood.
Available at: http://www.swst.org/edu/teach/teach1/structure1.pdf (accessed 11.11.13).
470
Global Agriculture: Industrial Feedstocks for Energy and Materials
Sugarcane
70×
Figure 4 Scanning electron micrographs of a wheat straw stem (left) showing hollow tubular structure of vascular bundles in culm, and
sugarcane (right) showing solid stem with distributed vascular bundles. Left: Reproduced from Malhotra, V., 2013. Bio-Composites from
Agricultural Raw Materials. Available at: http://www.physics.siu.edu/malhotra/vivek/biocomposites.htm (accessed 11.11.13). Right: Reproduced from
NTNU, 2013. Monocotyledons. Norwegian University of Science and Technology – Trondheim. Available at: http://www.chemeng.ntnu.no/research/
paper/Online-articles/Nonwoods/Nonwood.html (accessed 11.11.13).
CH2OH
H
O
OH
H
H
O
H
O
O
H
CH2OH
OH
H
H
OH
O
H
n
Figure 5 The β-linked glucose units forming cellobiose, the
repeating unit of cellulose. Reproduced from Salisbury, F.B., Ross, C.
W., 1992. Plant Physiology. Belmont, CA: Wadsworth Publishing Co.
CH2OH
CH2OH
H
O
O
H
OH
H
H
OH
H
H
O
O
H
OH
H
H
OH
H
O
n
Figure 6 The α-linked glucose units making up starch. Reproduced
from Salisbury, F.B., Ross, C.W., 1992. Plant Physiology. Belmont,
CA: Wadsworth Publishing Co.
composition containing both five- (including xylose and arabinose) and six-carbon (including galactose, glucose, and
mannose) monosaccharide units. Like starch, hemicelluloses
are mostly amorphous, making them more readily hydrolysable than cellulose. Hemicelluloses constitute 20–30% of
wood, generally with higher concentrations in hardwoods
than softwoods. Hemicelluloses also constitute 20–30% of dry
matter in most other biomass. Partial structures for the primary forms of hemicellulose in hardwood and softwood are
shown in Figure 7.
Lignin is the encrusting substance (‘glue’) solidifying the cell
wall. Lignin is an irregular polymer of phenylpropane units and
the structure varies among different plants. Lignin is thought to
occur through the enzymatic dehydrogenation of phenylpropanes followed by radical coupling (Hon and Shiraishi, 2001).
Softwood lignin is composed principally of guaiacyl units
stemming from the precursor trans-coniferyl alcohol (Figure 8).
Hardwood lignin is composed mostly of guaiacyl and syringyl
units derived from trans-coniferyl and trans-sinapyl alcohols.
Grass lignin contains p-hydroxyphenyl units deriving from
trans-p-coumaryl alcohol. Almost all plants contain all three
guaiacyl, syringyl, and p-hydroxyphenyl units in lignin. A partial
structure of softwood lignin is shown in Figure 9.
Partial compositions of selected biomass materials are listed in Table 3. The four materials – Monterey pine (Pinus
radiata, a softwood), eastern cottonwood (Populus deltoides, a
hardwood), sugarcane bagasse (from Saccharum spp.), and
wheat straw (from Triticum aestivum var. Thunderbird) – were
analyzed as reference materials for the purposes of providing
reference characterizations of industrial and energy feedstocks
(NIST, 2003).
Plants producing large amounts of free sugars, such as
sugarcane and sweet sorghum, are attractive as feedstocks for
ethanol fermentation, as are starch crops such as maize (corn)
and other grains. Current attention is focused on the fermentation of the cellulosic components because of the perceived
economic and energetic advantages for large-scale liquid fuel
production. Cellulose and hemicellulose are not fermentable
by conventional means, as more aggressive pretreatment and
hydrolysis must precede the fermentation step. Lignins are not
yet generally considered fermentable, and thermochemical
means are usually proposed for their conversion to fuels.
Typically, 60–80% of the biomass feedstock mass is ultimately
fermentable based on cellulose and hemicellulose (holocellulose) concentrations.
Oil crops such as rape and canola, soybean, sunflower,
safflower, algae, and others have been widely investigated for
Global Agriculture: Industrial Feedstocks for Energy and Materials
471
COOH
OH
O
O
CH3O
OAc
O
OH
OH
O
OH
OH
O
O
O
O
O
O
OH
OAc
O
O
OAc
OH
OH
(a)
CH2OH
HO
O
OH
O
OH
OAc
CH2OH
O
OH OH
CH2
O
OH
O
O
O
CH2OH
O
OH OH
CH2OH
O
OAc OH
O
OH
O
O
CH2OH
OH
(b)
Figure 7 Partial structures of the principal hemicelluloses in wood (Hon and Shiraishi, 2001): (a) O-acetyl-4-O-methylglucuronoxylan from
hardwood; (b) O-acetyl-galactoglucomannan from softwood. Ac ¼acetyl group.
OCH3
HC
H
CH
OH
HC
H
OCH3
HOC
HOC
H
H
Coniferyl alcohol
OH
CH
OCH3
HC
H
OH
CH
HOC
H
Sinapyl alcohol
p-Coumaryl alcohol
Figure 8 Phenolic subunits of lignin. Reproduced from Salisbury, F.B., Ross, C.W., 1992. Plant Physiology. Belmont, CA: Wadsworth Publishing Co.
their potential in producing diesel fuel substitutes (commonly
referred to as ‘biodiesel’). Although raw plant oils are not
generally satisfactory as fuels for diesel-type (compression-ignited) engines, methyl and ethyl esters formed by reacting the
oil with an alcohol (methanol or ethanol) have lower viscosity
and improved injection and combustion properties making
them suitable for engine use (Peterson et al., 1992). Other
crops, such as Euphorbia, have been investigated for direct
production of hydrocarbons (Nishimura et al., 1977; Nemethy
et al., 1981).
Roughly half of the plant organic matter is carbon, the rest
being made up of 5–7% hydrogen, 30–50% oxygen, along
with nitrogen, sulfur, chlorine, and other elements (Jenkins
and Ebeling, 1985; Jenkins et al., 1998). The ash or inorganic
fraction of biomass may account for more than 30% of the dry
mass in some cases (e.g., animal manures and other waste
materials, some algae). Temperate region woods typically have
ash contents below 1%, whereas some tropical woods contain
up to 5% ash (Chum and Baizer, 1985). Leaves and young
tissues contain more ash than mature wood. Cereal grain
straws, hulls (husks), and other agricultural biomass have ash
contents ranging up to approximately 25% of the dry mass.
The amount of ash in a biomass material, and the composition of the ash are important for the selection of the
conversion technology. As noted earlier in the Section Types of
Biomass, alkali metals (especially the macronutrient potassium) in the ash of many agricultural residues can cause
slagging and boiler fouling and corrosion in the case of direct
firing as a result of high temperature reactions with silica and
other minerals. Chlorine is important in corrosion and as a
facilitator in the alkali reactions leading to deposition in
boilers. Biochemical methods avoid this problem because of
the lower temperatures involved although corrosion can still
be of concern. Other methods, such as supercritical water
oxidation, in which the oxidation is carried out at a relatively
low temperature but high pressure, may suffer less from ash
reactions although salts can be an issue, but such processes are
not yet commercial. In certain circumstances, fouling is reduced by the extraction of the offending elements before
introducing the fuel to the boiler. Such is the case of sugarcane
472
Global Agriculture: Industrial Feedstocks for Energy and Materials
CH2OH
CH2OH
HC
O
O
HC
HCOH
HCH
OH
CH2OH
O
HC
HC
CH3O
HC
O
CH
OCH3
CH3O
O
O
CH
CH2OH
CH
CH2OH
CH3O
HCOH
CH3O
O
OH
CH
O
HOCH2
HCOH
OCH3
O
HC
CH2OH
O
OCH3
CH2OH
HCOH
CH
CH2
O
O
CH3O
O
HC
OCH3
HCOH
O
CH2OH H2COH
CH
O
HC
OCH3
HOH2C
OCH3
CH
HCOH
HOCH2
CH3O
HC
O
HC
HC
CH
CH
OCH3
CH3O
HC
C
H
HOH2C
H
C
H2C
CH3O
HCOH
C
OH
O
CH3O
OCH3
O
HC
O
OCH3
Figure 9 Partial lignin structure of softwood. Reproduced from Salisbury, F.B., Ross, C.W., 1992. Plant Physiology. Belmont, CA: Wadsworth
Publishing Co.
Table 3
Partial compositions (%) of biomass materials (NIST, 2003)
Biomass constituent
Populus deltoides (rm #8492)
Pinus radiata (rm #8493)
Sugarcane bagasse (rm #8491)
Wheat straw (rm #8494)
Ash
95% ethanol extractives
Acid soluble lignin
Acid insoluble lignin
Total lignin
Glucuronic acid
Arabinan
Xylan
Mannan
Galactan
Glucan
1.0
2.4
2.4
23.9
26.2
3.7
0.7
13.9
2.0
0.6
43.2
0.3
2.7
0.7
25.9
26.6
2.6
1.5
6.2
10.9
2.4
42.9
4.0
4.4
2.0
22.3
24.2
1.3
1.8
21.5
0.4
0.6
40.2
10.3
13.0
2.3
15.9
18.0
2.1
2.5
21.7
0.3
0.8
37.6
Each value is mean from round-robin testing of approximately 20 laboratories.
Global Agriculture: Industrial Feedstocks for Energy and Materials
bagasse, commonly used as a fuel in the sugar industry, but
which has passed through the sugar extraction step and had
most of the alkali leached in the process. Direct firing of cane
trash, currently under investigation for increasing the power
generation from the cane industry, does not extract these
elements, and may suffer from deposition common to straws
and other high ash, high alkali biomass fuels.
The energy content or heating value (heat of combustion)
of biomass can be partially correlated with ash concentration.
Woods with less than 1% ash typically have heating values
near 20 MJ kg1. Each 1% increase in ash translates roughly
into a decrease of 0.2 MJ kg1 (Jenkins, 1989) because ash
does not contribute substantially to the overall heat released
by combustion, although elements in the ash may be catalytic
to thermal decomposition. Heating values can also be correlated to carbon concentration, with each 1% increase in carbon
elevating the heating value by approximately 0.39 MJ kg1, a
result identical to that found by Shafizadeh (1981) for woods
and wood pyrolysis products. The heating value relates to the
amount of oxygen required for complete combustion,
with 14 022 J released for each gram of oxygen consumed
(Shafizadeh, 1981). Cellulose has a smaller heating value
(17.53 MJ kg1) than lignin (26.7 MJ kg1) because of its
higher degree of oxidation. Other compounds, such as
hydrocarbons, with lower degrees of oxidation or containing
no oxygen tend to raise the heating value of the biomass.
Proximate and elemental compositions and heating values
for selected biomass are listed in Table 4. The heating value
shown in the table is the constant volume higher heating value
that is one of four that can be defined based on whether the
test is carried out at constant pressure or constant volume and
whether water produced from the hydrogen in the biomass is
in the vapor or liquid phase at the end of the test (Jenkins,
1989). Wheat straw and sugarcane bagasse listed in Table 4 are
not the same samples listed in Table 3 and hence have different compositions. The proximate analysis yields ash content
and the fraction referred to as volatile matter (Figure 1)
evolved during a short heating at 950 1C in a nonoxidizing
atmosphere representing pyrolysis. The difference between the
total dry matter and the sum of ash and volatile matter is fixed
carbon (essentially charcoal). The ultimate analysis yields
elemental concentrations of carbon, hydrogen, nitrogen, and
sulfur, with oxygen usually determined by difference. Although not part of the standard for ultimate analysis, chlorine
is also sometimes analyzed due to its importance in many
conversion systems. The Jose tall wheat grass listed in the
table was grown as a phytoremediation crop under saline
conditions, and has a high Cl content.
473
Logistical Supply Chains
For crop and forest residues and energy crops, supply operations upstream of the conversion facility will typically include collection or harvesting, packaging and processing such
as baling, pelleting or grinding, loading and unloading onto
transport vehicles, transportation on roads, rail, river or marine routes, and in most cases storage, possibly at one or more
intermediate sites (depots) in addition to the primary site of
conversion. Densification of the bulk biomass may be preferred for the purposes of achieving high transportation payloads on trucks and other vehicles but contributes to the cost
of processing. Whether the cost can be justified depends in part
on transport distance and other factors associated with the
handling and use of the feedstock and the cost of transport
otherwise. Conversion into pyrolysis oils, sugars, and other
intermediate products at depots or other satellite facilities may
also be considered for logistical purposes to reduce the total
costs of feedstock transportation to larger central facilities for
final processing and production.
Drying to reduce moisture content may be needed, especially for feedstocks intended for many types of thermochemical conversion facilities. For some facilities, for example,
biochemical conversion facilities using fermentation systems,
the moisture in feedstock may be an important contribution
toward meeting the overall process water demands and may
enhance conversion but adds to transportation costs and increases the risk of spoilage during handling.
The costs associated with feedstock supply, conversion, and
the distribution of finished products influence the optimal size
of a conversion facility (Jenkins, 1997). Economies of scale
typically exist for capital and operating costs so that as the
production capacity or size increases, these costs typically do
not increase in the same proportion and the cost per unit of
product output decreases. However, as the size of the facility
increases, the feedstock demand increases as well, and for
distributed feedstocks such as crop and forest residues and
purpose-grown crops, this means the supply region will generally increase unless crop productivity and yields can be increased to compensate for the added supply needs.
Transportation costs, therefore, increase, but not generally in
direct proportion to the facility size. These competing effects
lead to a system size at which either total cost of production is
minimized (Figure 11) or profit is maximized depending on
the objective function. For example, economies of scale in the
capital and operating costs of facilities to generate electricity
(power plants) from biomass coupled with increasing costs of
feedstock acquisition yield a minimum in the levelized
(amortized) cost of energy from the plant (Figure 11). The
scaling in capital and operating cost may take a functional
form such as
s
C
M
¼
½3
Co
Mo
The use of biomass for industrial products and energy typically
requires a supply chain of multiple operations to deliver biomass feedstock or biomass-derived intermediates from the site
of production to the conversion facility and then the finished
product into final demand (Figure 10). For food processing
facilities, lumber mills, wastewater treatment plants, dairies,
and other animal operations, the supply chain may be relatively simple due to the generation of the feedstock on-site.
where C is the cost; M is the facility size or capacity; the subscript denotes a reference facility of known cost and size; and s
is the scaling factor that specifies how cost varies with size. The
optimum size is sensitive to the scaling relationship and the
value of s. The two cases of Figure 11 were derived for s is
constant for all sizes and s is variable and increasing with size
to account for increasing risk as the facility size becomes very
Proximate and elemental compositions and heating values of selected biomass materials (Jenkins et al., 1998), Composition of selected biomass materials (Jenkins et al., 1998)
Miscanthus Switch
grass
Jose tall wheat
grass
Hybrid
poplar
Willow Water
hyacinth
Nonrecyclable waste
paper
Sugarcane
bagasse
Municipal digester sludge (class B
biosolids)
Typical harvest moisture (% wet basis)
14
14
10
14
14
10
45
45
85
6
50
75
Proximate composition (% dry matter)
Ash
4.88
18.67
Organic fraction
95.12
81.33
Volatiles
76.48
65.47
Fixed carbon
18.64
15.86
14.48
85.52
69.94
15.58
4.90
95.10
78.20
16.90
6.53
93.47
77.03
16.44
12.34
87.66
72.18
15.48
1.6
98.40
86.14
12.26
0.95
99.05
85.23
13.82
22.40
77.60
8.21
91.79
82.50
9.29
3.61
96.39
84.51
11.88
37.91
62.09
53.68
8.41
16.33
18.05
18.90
17.86
18.93
19.38
16.02
21.52
18.50
15.38
19.09
18.98
20.22
20.38
19.24
19.56
20.64
23.44
19.19
24.77
14.69
15.52
16.26
16.08
10.41
10.66
2.40
20.22
9.25
3.85
40
29
14
45
30
21
41
33
26
49
28
22
16
56
6
Alfalfa
straw
Rice
straw
Higher heating value (MJ kg 1)
Moisture free
18.16
15.09
(dry)
Moisture and ash 19.09
18.55
free
Wet
15.62
12.97
a
Structural composition (% dry matter)
Cellulose
29
34
Hemicellulose
12
28
Lignin
9
Ultimate elemental composition (% moisture and ash free)
Carbon
48.40
47.02
48.08
53.31
Hydrogen
6.15
6.39
6.08
4.63
Oxygen (by
43.05
44.77
43.99
41.59
difference)
Nitrogen
1.19
1.07
1.19
0.21
Sulfur
0.19
0.22
0.28
0.11
Chlorine
1.17
0.71
0.71
0.21
Ash analysis (% ash)
SiO2
7.04
Al2O3
1.12
TiO2
0.05
0.41
Fe2O3
CaO
21.37
MgO
5.83
Na2O
11.2
K2O
22.9
P2O5
6.32
SO3
4.27
Cl
CO2
a
74.67
1.04
0.09
0.85
3.01
1.75
0.96
12.3
1.41
1.24
54.64
5.73
0.23
6.16
5.02
2.45
2.16
14.09
2.43
3.03
70.60
1.10
0.06
1.00
7.50
2.50
0.17
12.80
2.00
1.70
36
30
19
50.69
6.08
42.57
52.86
5.05
39.59
51.65
5.99
41.75
49.56
5.95
44.11
52.96
6.82
37.16
53.66
7.60
38.18
49.99
5.86
43.94
58.29
7.18
24.17
0.60
0.06
0.10
2.00
0.37
2.21
0.60
0.02
0.35
0.03
0.01
2.53
0.53
0.38
0.22
0.04
0.15
0.08
9.08
1.72
0.16
66.53
6.98
0.34
3.56
7.14
3.17
1.03
7.00
2.80
2.00
46.71
3.09
0.09
1.00
4.59
2.03
8.60
15.68
2.70
2.05
13.43
0.05
1.17
0.41
0.21
0.76
59.16
5.76
0.31
26.76
0.20
5.26
8.08
1.39
0.06
0.84
45.62
1.16
2.47
13.2
10.04
1.15
19.44
63.97
3.81
0.42
8.37
1.68
0.83
0.23
0.10
1.14
41.87
22.25
3.87
20.90
3.50
1.45
0.26
2.59
1.13
0.90
47.11
17.9
1.22
5.64
8.65
2.98
1.33
1.32
14.65
1.38
0.01
0.21
Structural data representative only and not necessarily from same sample used for elemental analysis and heating value.
Global Agriculture: Industrial Feedstocks for Energy and Materials
Wheat
straw
Type
474
Table 4
Global Agriculture: Industrial Feedstocks for Energy and Materials
Production
Harvesting and handling
• Residues
• Collection
• Purpose-grown
• Processing
Conversion
•
• Storage
crops
• Transportation
(May be performed
multiple times and in
any order depending on
optimal system design)
•
•
•
•
475
Final demand
Pretreatment
Thermal
Biological
Chemical
Mechanical
Thermochemical
Combustion
Gasification
Pyrolysis
Hydroprocessing
Biochemical
Anaerobic (fermentation)
Anaerobic digestion
Alcohol fermentation
Aerobic
Composting
Activated (oxygenated)
waste treatment
Direct hydrogen
Physicochemical
Extraction
Esterification
Manufacturing (may also
employ physical and chemical
processing)
•
•
•
Bioenergy (heat and power)
Process and space heating
Power generation
Biofuels (fuels)
Biomass solids
Charcoal/Biochar
Synthesis gases (CO + H2)
Hydrogen
Biogas (methane + CO2)
including digester gas and
landfill gas
Methanol
Ethanol
Pyrolysis liquids
Biodiesel
Others
Bioproducts (chemicals and
materials)
Fibers
Structural materials
Composite materials
Citric and other acids
Pesticides
Lubricants
Surfactants
Others
Figure 10 Supply-chain processes for biomass to energy and products.
Mopt=1252 MW
constant s
Mopt=305 MW
variable s
0.20
(US$ kWh−1)
0.15
0.10
Total cost
Total cost
0.05
Capital + O&M cost
Capital + O&M cost
Fuel cost
Fuel cost
0.00
1
10
100
1000
M (MW)
1
10
100
1000
10 000
M (MW)
Figure 11 Economy of scale and capacity optimization in biomass conversion (Jenkins, 1997).
large. The cost–size dependence for regionally distributed
feedstock typically follows a different relationship, contributing an increasing cost per unit of production as the facility
scale increases (Figure 11) (Jenkins, 1997). The estimated cost
of power generation is plotted in Figure 11 against the facility
size on a logarithmic scale, which masks to some degree the
broad range around the optimum in which cost does not vary
substantially, a characteristic of larger size facilities. The optimal sizes in both examples are quite large for biomass power
generators and few facilities approach these sizes in practice.
To some extent, this is due to regulations that also affect facility siting and permitting. In California and the US, for example, new facilities must undergo a siting review when the
size exceeds 50 or 80 MW electrical capacity and this adds to
the cost of facility development (Jenkins, 1997).
Consideration of the feedstock properties and availability,
the regional product markets, and the costs and prices associated with production and sales are important to the size,
476
Global Agriculture: Industrial Feedstocks for Energy and Materials
Corn/grain
Forest
Animal fat
Dry mil
FAHC
Grease
FAME
FT Diesel
LCE
HEC
Wal mill
AG Residue
Seed oils
Wood pulp
MSW
OVW
Figure 12 Optimized potential biorefinery locations in the US identified through geospatial modeling of biomass resources, supply infrastructure,
and fuel demand (Parker, 2011).
type, and location of conversion or production facilities.
Geospatial modeling using resource location data to give the
spatial distribution of feedstock coupled with technoeconomic
models of conversion facilities and distribution models of
finished product have been used, for example, to project policy
outcomes for biorefinery development in the US and to
speculate on the optimized structure of the future industry
(Figure 12) (Parker et al., 2010; Tittmann et al., 2010; Parker,
2011).
Chemicals and Materials from Biomass
Products from biomass originate both from direct use or extraction of materials from the parent tissues and via conversion
of the biomass to alternate forms. In related bioprocessing, the
biomass can serve as a substrate by which the cellular mechanisms are used by another organism to express the desired
product, for example, the transient expression of a protein
through the process of agroinfiltration (Joh and VanderGheynst, 2006).
The potential to produce a much larger range of chemicals,
materials, and other products from biomass has been widely
recognized, including direct replacement of products now
synthesized from petroleum, natural gas, and other nonrenewable resources (Johnson, 2007). Biobased plastics, lubricants, solvents, surfactants, composites, and other products
are increasing in market shares as technology improves, new
markets are developed, and demand increases for more
sustainable or ‘green’ products (BIOCHEM, 2010). Global
production of bioplastics is projected to increase five-fold by
2016 to close to 6 million metric tons compared with the
production in 2011, although in this case demand for biodegradable plastics does not appear to be the major driver of
growth as only 13% of the total is anticipated to be in polymers of this type (European Bioplastics, 2013). Biobased
polyethylene terephthalate (PET) is likely to make up the
largest fraction (greater than 40%) of the total. Other drivers,
such as displacement of petroleum and reduced greenhouse
gas emissions, plus economic and market factors account for
the increasing demand of biologically derived materials. For
example, in the making of riboflavin (vitamin B2), single-step
instead of six-step processing, increased productivity, and reduction in cost are quoted as the main forces in the adoption
of a biobased process (Johnson, 2007). Similar rationale exists
for the production of antibiotics and acrylamide monomers
(Johnson, 2007; Scott et al., 2007). Improved processes and
efficiencies in the manufacturing of biobased intermediates
and final products are the subject of substantial research, such
as the use of biomass-derived furans in the catalytic production of p-xylene that is, in turn, used to make terephthalic
acid, a monomer for PET (polyester) plastics (Williams et al.,
2012), or the conversion of biomass-derived carbohydrates to
chemical feedstocks, such as alkenes and aromatics, that are
now principally produced from petroleum (Arceo et al., 2009;
Arceo et al., 2010).
Agriculture and forestry have long supplied materials other
than food and feed for construction, pulp and paper, textiles,
Global Agriculture: Industrial Feedstocks for Energy and Materials
medicine, cosmetics, soaps, and other uses, and the production of materials such as plastics from biomass is not new,
although many new and novel chemicals are now becoming
available. Crude plastics were historically made from natural
proteins, such as those from blood, egg, and milk. Goodyear’s
vulcanization of natural rubber beginning in 1838 provided a
pathway to thermoset materials (Richardson, 2010). Shellac
was produced by dissolving the natural resin secreted by the
lac bug in ethanol (Patel et al., 2013). The conversion of cellulose to nitrocellulose by the addition of nitric acid was discovered in 1846 and used for the preparation of guncotton,
and nitrocellulose modified by camphor as a plasticizer was
being used by 1889 for photographic film by Eastman Kodak
(Michel, 2006; PHS, 2013a). Nitrocellulose is now applied in
such diverse uses as membranes for protein immobilization
and analysis and solid-rocket motors. The compound (as celluloid) was also famously used in the attempted replacement
of natural ivory for billiard balls but the balls had a tendency
to explode on impact. Cellulose acetate, the acetate ester of
cellulose, was first prepared in 1865 and is still widely used for
plastic films and other applications (PHS, 2013b). Viscose, a
polymer of wood pulp, was patented in 1892 and the process
for making cellophane, a viscose plastic film produced by
dissolving cellulose in alkali and carbon disulfide was established by 1913 (PHS, 2013c). Despite the pollution issues
associated with disulfide in the manufacturing process, cellophane is biodegradable and has remained in continuous
production since the 1930s especially for food packaging
(Rojo et al., 2010; PHS, 2013c). Rayon fiber used in textiles is
made through a similar process.
Comprehensive studies have examined candidate materials
from sugars, synthesis gases (syngas), and lignin, all derived
from biomass (Werpy and Petersen, 2004; Holladay et al.,
2007). The study by Werpy and Petersen (2004) identified an
initial set of 300 possible building-block chemicals from
sugars and syngas in integrated biorefineries. Such biorefineries conceptually involve various processing operations
477
within the same production facility (Figure 13) for the
manufacturing of multiple value-added products, similar to
but potentially more complex than modern petroleum refineries that produce a variety of commodity chemicals and
fuels (Johnson, 2007). Screening criteria for the chemical selection included feedstock cost, estimated processing costs,
market information, and compatibility with current and projected future refining operations. From this initial set, a
foundational suite of chemicals was selected based on more
strategic criteria (direct product replacement, novel products,
and building-block intermediates) and the value chains associated with processing to products for final demand
(Figure 14). The resulting network of product interactions has
been widely referenced in relation to potential biorefinery
design and opportunities for products from biomass. The final
suite includes compounds with carbon numbers (number of
carbon atoms in the molecule) ranging from C1 to C6
(Table 5). These compounds have multiple functionality for
further downstream conversion and could be produced from
both lignocellulose and starch in making sugars or syngas as
intermediate chemical platforms, some of the principal platforms under development for energy and products from biomass. The study by Holladay et al. (2007) later identified other
products from lignin using the criteria based on technical
difficulty in production, market potential, market risk, building-block utility, and whether the product could be made as a
single compound or would be present in a mixture with other
compounds (Table 5).
As illustrated in Figure 14, a wide spectrum of products can
be produced from biomass to serve multiple markets in what
is loosely classified a biobased economy. These products can
range from higher value, generally lower production and sales
volume chemicals such as pharmaceuticals, food additives,
and cosmetics, to lower value, higher volume chemicals and
materials such as biobased plastics and composites, chemical
feedstocks, fuels, and environmental products such as biochar
or black carbon now being investigated for its agronomic
Feedstock
•
•
•
•
•
Food and feed grains
Lignocellulosic biomass
Forest biomass
Municipal solid waste
Algae and others
Processing technologies
•
•
•
•
•
Biological processes
Chemical processes
Thermochemical processes
Thermal processes
Physical processes
Products
Materials and energy
•
•
•
•
•
Fuels
Chemicals
Materials (e.g., polymers)
Specialty chemicals and materials
Commodities and goods
Figure 13 Basic integrated biorefinery concept. Adapted from Johnson, F.X., 2007. Industrial Biotechnology and Biomass Utilization, Prospects
and Challenges for the Developing World. Vienna: Stockholm Environment Institute and United Nations Industrial Development Organization.
478
Starch
Intermediate
platforms
Biobased
Syn Gas
Building
blocks
Secondary
chemicals
SG
Intermediates
Products/uses
Industrial
Corrosion inhibitors, dust control,
boiler water treatment, gas
purification, emission abatement,
specialty lubricants, hoses, seals
Transportation
C2
Fuels, oxygenates, anti-freeze, wiper
fluids molded plastics, car seats, belts
hoses, bumpers, corrosion inhibitors
Textiles
Hemicellulose
Carpets, Fibers, fabrics, fabric
coatings, foam cushions, upholstery,
drapes, lycra, spandex
C3
Sugars
Cellulose
Lignin
Glucose
Fructose
Xylose
Arabinose
Lactose
Sucrose
Starch
Safe food supply
Food packaging, preservatives,
fertilizers, pesticides, beverage
bottles, appliances, beverage can
coatings, vitamins
Environment
C4
Water chemicals, flocculants,
chelators, cleaners and detergents
Communication
C5
Molded plastics, computer casings,
optical fiber coatings, liquid crystal
displays, pens, pencils, inks, dyes,
paper products
Housing
Paints, resins, siding, insulation,
cements, coatings, varnishes, flame
retardents, adhesives, carpeting
Oil
Recreation
C6
Footgear, protective equipment,
camera and film, bicycle parts & tires,
wet suits, tapes-CD’s-DVD’s, golf
equipment, camping gear, boats
Health and hygiene
Protein
Ar
Figure 14 Chemical platforms for biobased products from biomass feedstocks (Werpy and Petersen, 2004).
Plastic eyeglasses, cosmetics,
detergents, pharmaceuticals, suntan
lotion, medical-dental products,
disinfectants, aspirin
Global Agriculture: Industrial Feedstocks for Energy and Materials
Biomass
feedstocks
Global Agriculture: Industrial Feedstocks for Energy and Materials
Table 5
479
Building-block chemicals and products from sugars, syngas, and lignin (Werpy and Petersen, 2004; Holladay et al., 2007)
Source
Chemical
Sugars/
syngas
C1: Carbon monoxide (CO) and hydrogen (H2) (syngas)
C3: Glycerol, 3 hydroxypropionic acid, lactic acid, malonic acid, propionic acid, and serine
C4: Acetoin, aspartic-acid, fumaric acid, 3-hydroxbutyrolactone, malic-acid, succinic acid, and threonine
C5: Arbinitol, furfural, glutamic acid, itaconic acid, levulinic acid, proline, xylitol, and xylonic acid
C6: Aconitic acid, citric acid, 2,5 furan dicarboxylic acid, glucaric acid, lysine, levoglucosan, and sorbitol
Process heat and power, syngas, methanol, dimethyl ether (DME), ethanol, mixed alcohols, hydrocarbons (Fischer–Tropsch liquids), C1 –
C7 hydrocarbon or oxygenates, alkylates (benzene, toluene, xylene, and higher), cyclohexane, styrenes, biphenyls, phenol, substituted
phenols, catchols, cresols resorcinols, eugenol, syringols, coniferols, guaiacols, vanillin, vanilic acid, DMSO, aromatic acids, aliphatic
acids, syringaldhyde and aldehydes, quinones, cyclohexanol/al, beta-keto-adipate, carbon fiber, polyelectrolites, polymer alloys, fillers,
polymer extenders, substituted lignins (carbonylated, ethoxylated, carboxylated, epoxidized, and esterified), thermoset resins,
composites, formaldehyde-free adhesives and binders, wood preservatives, nutraceuticals and drugs, and mixed aromatic polyols
Lignin
utility in soils and as a means to sequester carbon (BIOCHEM,
2010; Keiluweit et al., 2010; Woolf et al., 2010).
Biobased plastics (bioplastics) constitute the class of
manufactured plastics that are either entirely or partially
composed of biologically derived materials, that is, biomass.
Plastics certified as compostable frequently contain a high
proportion of biobased materials but not all biobased polymers are biodegradable (BIOCHEM, 2010). Current bioplastics include the bulk materials polylactic acid (PLA),
polyamides (nylon), polyhydroxyalkanoates, polybutyleneterephthalate and PET and polyethylene from among a larger
number of thermoplastics, and polyurethane from among the
thermosetting materials. These are typically prepared from
sugars and starches or from processing residues (e.g., lignin).
PLA is principally made from plant sugars polymerized
through a process of fermentation. Corn is a major source.
Biobased succinic acid, a precursor to various bioplastics, can
be produced from glucose and sucrose sugars and is a replacement for petroleum-based adipic acid (Chimirri et al.,
2010; Liang et al., 2013).
Composite materials are standard products of the wood
industry (e.g., particleboard) but a wider variety of biomass
feedstocks are now being investigated for their application in
this market, including woody and herbaceous biomass such as
Eucalyptus, Athel (Tamarix aphylla), and Jose tall wheatgrass (A.
elongatum) used also in phytoremediation of salt-affected
soils (Pan et al., 2007; Zheng et al., 2006, 2007a). Particleboards made from saline wood generally had equal or better
mechanical qualities compared with boards made from more
conventional nonsaline wood materials (Zheng et al., 2006,
2007a).
Nanomaterials from biomass are also being investigated for
a wide variety of applications. Low-cost carbon fiber production has been the subject of much interest especially in the
wake of recent legislation to increase vehicle fuel economy
standards in the US that add motivation to finding new lightweight materials with good strength and from sustainable
sources. Toward this purpose, lignin has been investigated for
its use in producing carbon fiber but is required to be of high
purity with a narrow molecular weight distribution, and
greater research is needed in the refining of this potential
feedstock (Baker and Rials, 2013). Carbon nanofibers, nanotubes, graphenes, and other materials have also been investigated for use in energy storage devices, particularly ultra- and
supercapacitors. Carbon nanosheets for use in high energy
density supercapacitors have been made from hemp bast
fibers, yielding a maximum energy density of 12 Wh kg1 and
8–10 Wh kg1 after charge times of less than 6 s, comparable
to or greater than the conventional activated carbons (Wang
et al., 2013b). The multilayered structure of the feedstock
biomass in this case provides a unique starting morphology
for producing the carbon sheets.
Biobased lubricants and solvents are similarly classified as
being comprised entirely or in part from biomass. Biobased
lubricants predate petroleum-based compounds and are either
vegetable oil-based or manufactured from modified oils including synthetic esters made from mineral oil-based products
(BIOCHEM, 2010). Finished products can be biodegradable
with improved environmental properties and lower toxicity
allowing use in more sensitive environments. Canola and a
number of other vegetable oils have been investigated as
feedstocks in the production of more environmentally benign
motor oils for engines (Johnson et al., 2002). Solvents are used
in many different applications, such as additives in paints,
degreasing agents, pharmaceutical extractions, stripping
agents, and cleaners. A key property of some biobased solvents
is their low emission of volatile organic compounds that are
both direct health hazards and contribute to reduced air
quality, for example, in reactions with oxides of nitrogen or
NOx in the formation of tropospheric ozone, a breathing
hazard and regulated air pollutant (U.S. EPA, 2013). The fatty
acid methyl ester of soy oil made from soybeans, in addition
to its use as biodiesel, is also a biosolvent, as are lactate esters,
limonene, and other citrus terpenes. Although limonene (as
D-, L-, and DL-limonene), for example, is listed as a generally
recognized as safe synthetic flavoring substance by the US
Food and Drug Administration (CFR, 2006), and high-purity
products may generally prove of low allergenic properties,
oxidized products (such as obtained by exposing to air for
prolonged periods) have been shown to produce sensitivity in
animals (Karlberg et al., 1991) and safety properties need to be
carefully considered in application.
Plant oils are also used in the production of surfactants –
chemicals that lower the surface tension of liquids to allow
better mixing. By definition, biosurfactants have at least
one of the hydrophobic or hydrophilic groups associated with
the surfactant molecule from a biobased source and so can
also be either entirely or partially derived from biomass.
480
Global Agriculture: Industrial Feedstocks for Energy and Materials
Feedstocks include palm oil, sorbitol, sucrose, glucose, and
animal fats such as tallow. Surfactants are common ingredients in detergents but are used in a wide range of other
products including agricultural chemicals, textiles, paints, and
lubricants (BIOCHEM, 2010), and nonionic surfactants have
been employed in the enzymatic hydrolysis of biomass as
pretreatment for fermentation and biofuel production (Zheng
et al., 2007b).
Biochar or black carbon from biomass has received much
attention in recent years for its perceived utility in maintaining
soil carbon stocks and soil fertility. Although carbon produced
from biomass in various stages of activation has long served
for water treatment and environmental remediation, the discovery of the terra preta do índio soils in the Amazon stimulated
new research related to understanding the properties of carbons for soil applications including chars produced in conjunction with energy conversion, especially biomass pyrolysis
but also thermal gasification. The potential new market in
biocarbons is fortuitous also for thermochemical energy conversion systems where char coproducts have in the past posed
a disposal problem given the limited capacity of the activated
carbon markets. The terra preta soils, which originate from preColumbian times and accumulated charcoal, bone, and manure as part of kitchen middens from food preparation and
other activities, have been shown to have lower greenhouse gas
emissions (as CO2) per unit of carbon in the soil compared
with similar soils with low or no black carbon (Liang et al.,
2010). In a study of black carbon from 12 different biomass
feedstocks including hardwoods, softwoods, algal digestate,
walnut shell, and turkey litter, char surface area, an important
measure of sorptive properties, increased with charring temperature whereas chemical functionality could be separated
into wood and nonwood feedstock groups (Mukome et al.,
2013). The presence of carbonate and chloride salts increased
the basicity of chars from nonwood feedstock. Agronomic
properties and impacts can be related to feedstock composition (Mukome et al., 2013). Biochar applications to soils
may also reduce the overall lifecycle greenhouse gas emissions
for bioenergy systems (Gaunt and Lehmann, 2008).
Energy and materials from biomass frequently compete for
common resources, hence in addition to lifecycle environmental concerns, debate also arises over the highest and the
best use of biomass. Policies favoring one use over another can
introduce economic distortions, as in the food-versus-fuel
debate where both sectors may receive government incentives
or subsidies but without substantial coordination aimed at
improving the overall sustainability. Given the limited supplies of biomass, the potential to extract multiple products
through integrated biorefining (cascading the biomass use)
has been suggested as a way to maximize the value of the
resource (Keegan et al., 2013). Such practice already exists in
both the forestry and agriculture sectors, for example, in pulp
and paper manufacturing and sugar milling in which highvalue materials are extracted before processing residues (e.g.,
lignin from wood or bagasse from sugarcane) are utilized for
energy conversion. As energy values increase, however, economic distinctions tend to disappear in many cases, shifting
focus onto other performance metrics as indicators of social
good. Greenhouse gas emissions may be reduced through the
production of biomaterials, many of which can sequester
renewable carbon over long periods of time in addition to
displacing fossil emissions from petroleum or other nonrenewable resources. Significant opportunities exist to improve
material flows through the society to meet both material and
energy needs. Just as integrated technologies may improve the
economic benefits for energy and materials from biomass,
policies that address system-level resource management effects
may realize a greater social benefit. Uncertainties remain in the
assessment of net benefits for both materials and energy
conversion from agricultural feedstocks and distinct needs
exist for improved information.
Energy from Biomass
The potential for biomass to supply much larger amounts of
useful energy has stimulated substantial research and development into systems to convert biomass into the principal
forms of heat, electricity, and solid, liquid, and gaseous fuels
(Bungay, 1981; Hall and Overend, 1987; Kitani and Hall,
1989; Klass, 1998; Brown, 2003). Although substantial debate
continues over the sustainability of large-scale bioenergy development, so do advances continue across a wide range of
conversion technologies and systems to improve the overall
efficiencies and fuel yields, fuel properties, lifecycle costs, and
environmental performance.
Three principal routes exist for converting biomass to energy: (1) thermochemical, (2) biochemical, and (3) physicochemical. In practice, combinations of two or more of these
routes may be used in the generation of the final product or
products. Chemical or biological catalysts are employed in
many cases. Thermochemical conversion includes combustion, thermal gasification, and pyrolysis along with a
number of variants involving microwave, plasma arc, supercritical fluid, hydrothermal, and other processing techniques
(Brown, 2011). Products include heat, fuel gases, liquids, and
solids. Thermochemical techniques tend to be of high rate and
relatively nonselective for individual biomass components in
that the chemically complex biomass is substantially degraded
into simple compounds (e.g., CO, CO2, H2, and H2O), which
may later be reassembled into more complex compounds
through either chemical or biological processing (Chum and
Baizer, 1985). Biochemical conversion includes fermentation
to produce alcohols (ethanol being perhaps the best known),
fuel gases (such as methane by anaerobic digestion), acids, and
other chemicals (Klass, 1998). Among the physicochemical
methods are alkaline and acid processes, esterification, mechanical milling, steam and ammonia freeze explosion and other
explosive decompression processes, and pressing and extrusion, many times in combination with a biochemical or
thermochemical reaction process. One major process under
this category is vegetable oil extraction and esterification to
manufacture biodiesel as a substitute for engine fuel. Biochemical and physicochemical processes in general are intended to be more selective for biomass components and
higher value products, although thermochemical routes can
also be used, as in the indirect production of methanol via
thermal gasification (Hos and Groeneveld, 1987). There are
other means of producing useful energy from biomass, such as
biophotolysis for the production of hydrogen by plants
Global Agriculture: Industrial Feedstocks for Energy and Materials
(Bungay, 1981), and other direct hydrogen routes (Cortright
et al., 2002), but these are not yet demonstrated on a commercial scale. Ethanol is already produced on a large scale as a
biofuel, mostly for light-duty automotive use including as an
oxygenate blendstock for motor gasoline as well as in higher
concentrations for flexible fuel (FlexFuel) and neat (100%)
fuel vehicles. Hydrocarbon liquids from biomass are of particular interest due to their similarity to current gasoline, diesel, and aviation fuels and their fungibility in existing fuel
transportation and distribution systems such as pipelines.
These and other so-called drop-in biofuels avoid the need for
new infrastructure or the use of more expensive truck transport
for large-scale application.
Thermochemical routes generally involve the heating and
thermal decomposition of feedstock biomass to compounds
that can either be directly used or reacted to produce other
and potentially more valuable chemicals and fuels. Gasification and pyrolysis are thermochemical approaches that involve thermolysis of the feedstock to intermediate gases (e.g.,
CO and H2 as synthesis gas or syngas) and liquids (e.g.,
biocrude) that can be burned directly, chemically synthesized
into drop-in liquid fuels along with a wide variety of other
fuels including methane, hydrogen, methanol, and dimethylether, or fermented biochemically to alcohols. These thermal
processes can also produce biochar (see the discussion on
materials from biomass above). Torrefaction is a light pyrolysis that typically is carried out as a pretreatment at temperatures between approximately 200 and 300 1C to produce
a carbonized feedstock or biochar with improved grindability,
densification, and other handling characteristics. Biochar
production can add coproduct value to gasification, pyrolysis,
and other thermochemical conversion processes although
there remain many uncertainties regarding its value for these
purposes. Intermediates such as biocrude produced through
pyrolysis and lipids extracted from algae or oil seeds
(vegetable oils) can be hydrotreated through the addition of
hydrogen for upgrading to renewable diesels and other
hydrocarbons. Combustion of the feedstock generates heat
which can then be used in the generation of electricity or
process steam and hot water, and is historically the largest
energy use of biomass.
Biochemical routes, predominantly fermentation, rely
principally on microbes (yeasts, bacteria, or fungi) for the
conversion to useful fuels and chemicals. In most instances,
hydrolysis of structural polymers (starch, hemicellulose, and
cellulose, see above) through enzymatic or chemical means,
possibly in combination with thermal or mechanical pretreatment, is necessary to generate the sugars or other simpler
compounds upon which the microbes feed. Starch and sugar
fermentation by yeast is well known and has long been used in
making beverage alcohol (ethanol) and is currently used in
producing most of the world’s fuel alcohol from corn (maize)
and sugarcane. The use of lignocellulosic biomass is less well
developed and the subject of much research directed at improving hydrolysis, particularly enzymatic hydrolysis, of both
the hemicellulose and cellulose fractions and simultaneous
cofermentation of the resulting five and six-carbon sugars for
which recombinant organisms have also been developed to
work in association with or in place of yeast. Lignin is mostly
recalcitrant to biological conversion and is often considered
481
for use as boiler fuel in making process heat or steam (such as
that used in distilling the alcohol) and electricity for the
biorefinery but higher economic value may reside in fuel
products if effective conversion techniques can be developed,
for example, gasification to syngas for subsequent catalytic
conversion to substitute natural gas or liquid fuels. Anaerobic
digestion is another type of fermentation that typically uses
mixed populations of bacteria to digest biomass to make
biogas, a multicomponent gas consisting mostly of methane
(CH4, the major component of natural gas) and CO2 with
other more minor species including hydrogen sulfide (H2S).
Sulfide removal is typically needed before the use of biogas in
engines especially those employing catalytic aftertreatment of
combustion products for control of oxides of nitrogen (NOx)
and other air pollutants. Anaerobic digestion is commonly
used in waste water treatment facilities and is becoming more
common for managing animal wastes in concentrated animal
feeding operations such as dairies and hog and poultry farms.
Biogas is used as engine and boiler fuel, sometimes in combined heat and power (CHP) operations, such as for cheese
making at the dairy. Biogas can be upgraded to pipeline
quality methane to complement natural gas or for use as vehicle fuel as compressed natural gas (CNG) or liquefied natural gas (LNG), and can serve as chemical feedstock. Biogas
has also been used in microturbines and for fuel cells for
distributed power applications. Landfill gas is a biogas that is
produced by the anaerobic decomposition of organics in solid
waste landfills and is recovered for electricity generation and
other energy purposes and to reduce hazardous migration of
the gas into nearby structures.
Physicochemical processes are used in the making of biodiesel, a product of the catalyzed reaction between an oil, such
as the vegetable oil, and an alcohol, such as methanol or
ethanol, all of which can be produced from biomass. In the
conventional biodiesel process, triacylglycerides that make up
vegetable oils and other FOG are converted by transesterification into long-chain esters and coproduct glycerol. The
esters (biodiesel) have lower viscosity than the original
vegetable oil and have better injection and spray properties for
use in diesel engines. Biodiesel can also be produced using
enzymatic techniques. Coproduct markets for glycerol exist in
soaps, cosmetics, and many other products but the potentially
large production of fuel could saturate these markets so that
additional uses are being investigated, including production of
other fuels.
Industrial facilities to produce fuels and chemicals from
biomass are typically classified as biorefineries. Integrated
biorefineries can include multiple processing operations in
making a suite of products from the same production facility,
similar to modern petroleum refineries (Figure 15). Biofuels
are often classified as first generation, second generation, and
beyond, the terms intended to convey the development status
and to some extent the perceived sustainability of the production systems although there are not always clear distinctions. First-generation biofuels include ethanol from starch
and sugar as well as biodiesel, both mostly from food crops,
but may also include biogas, syngas, and pyrolytic biooil.
Second-generation biofuels include ethanol, hydrocarbons,
and other fuels produced from lignocellulosic and algal feedstocks. Later generations are associated with more advanced
482
Global Agriculture: Industrial Feedstocks for Energy and Materials
Organic residues
and others
Grasses
Starch
crops
Lignocellulosic
crops
Sugar
crops
Lignocellulosic
residues
Marine
biomass
Oil crops
Oil-based
residues
Separation
Lignin
Fiber
separation
Organic
juice
Oil
Syngas
Anaerobic
digestion
Pyrolytic
liquid
C5 sugars
C6 sugars
Electricity
and heat
Biogas
H2
Legend
Feedstock
Platform
Material
products
Chemical
process
Thermochemical
process
Mechanical/
Biochemical
physical process processes
Biomethane
Biomaterials
Chemicals and
building blocks
Energy products
Link among biorefinery pathways
Fertilizer
Bio-H2
Synthetic biofuels
(FT, DME...)
Bioethanol
Polymers and
resins
Glycerin
Electricity
and heat
Animal
feed
Food
Biodiesel
Figure 15 Partial classification system for biorefining options (Cherubini et al., 2009).
production and processing and improved lifecycle performance. Third-generation biofuels are based on feedstocks
modified to improve processing, such as reduced lignin or
increased oil content. Fourth-generation technologies involve
both improved feedstocks and improved processing and conversion techniques, including genetically modified microorganisms, that result in net negative carbon impacts as an
outcome of the biofuel production system (Lu et al., 2011).
The conversion strategies are integrally coupled to the
properties of the biomass. Moisture has often been used as a
discriminating parameter in the selection between thermochemical and biochemical processes, although by itself this is
not usually sufficient. In many cases, the properties of the
biomass necessary for engineering design have not been
properly characterized before commercial implementation of a
technology. Operation of combustion-type power-generating
stations intended to burn certain types of agricultural residues
(in particular cereal straws and other herbaceous biomass) has
been hindered by fouling of boiler heat transfer surfaces from
the inorganic compounds in the biomass (Turnbull, 1991;
Wiltsee, 2000; Jenkins et al., 1998, 2011), a phenomenon that
was largely overlooked in the design phase of many facilities,
but is well known for coal (Baxter, 1992). Effective biomass
energy development demands a continuing program in resource and technology development.
Thermochemical Conversion
Combustion
Historically, and still so today, the most widely applied energy
conversion method for biomass is combustion. The chemical
energy of the fuel is converted into heat energy. Heat can be
used directly and also transformed by heat engines of all sorts
into mechanical and electrical energy. Theoretically, biomass
could be directly converted into electricity by magnetohydrodynamic power generation in the same way coal might be, but
the technology is still mostly speculative (Rosa, 1961; Mikheev
et al., 2006). Burning of wood and agricultural materials in
open fires and simple stoves for cooking and space heating is
common around the world and a vital source of heat, although less desirable than advanced conversion techniques
from the perspective of atmospheric pollution and undue
health impacts from incomplete combustion (Jenkins et al.,
2011). Electricity generation using biomass fuels has received
considerable attention in recent years, although sugarcane
bagasse has long been used as fuel for electricity generation at
sugar mills and is now an integral component of biofuel
production from sugarcane with surplus power exported for
distribution. Wood also has been widely used for cogeneration
of heat and electricity in sawmills, with heat and steam generation being the primary function for kiln drying of lumber.
Global Agriculture: Industrial Feedstocks for Energy and Materials
A large independent power-generation industry in the US
developed in response to incentives provided directly and
indirectly by federal legislation, with approximately 10 GWe of
electrical-generating capacity from biomass currently operating
in the United States alone (EIA, 2013b). CHP facilities using
wood, straw, and municipal solid wastes are operating in
Europe and elsewhere for electricity generation and district
heating (COGEN Europe, 2011). Additional incentives exist in
the form of renewable portfolio standards (RPS) such as that
in California that now calls for a third of retail electricity sales
to come from renewable resources by the year 2020. Total
installed capacity in biomass power generation around the
world is approaching 50 000 MWe including large-scale solid
fuel combustion as well as small-scale digester and landfill gas
applications (IEA, 2007). In many regions of the world, Asia
being an exception, biomass utilization is below the sustainable resource capacity and potential exists to increase uses for
fuels, heat, and power (Parikka, 2004).
Viewed simply, the complete combustion of biomass in air
transforms the organic fraction into carbon dioxide and water:
Cx Hy Oz þ n1 H2 O þ n2 ð1 þ eÞðO2 þ 3:76N2 Þ ¼ n3 CO2
þ n4 H2 O þ n5 N2 þ n6 O2
½4
where the stoichiometric coefficients n depend on the concentrations of carbon, hydrogen, and oxygen in the original
feedstock (here expressed on a moisture-free, ash-free basis),
and on the amount of excess oxygen, e, added to the reaction.
Feedstock moisture is accounted for in the reaction by the
coefficient n1, and air is approximated in the normal engineering fashion as consisting of 21% by volume oxygen and
79% equivalent nitrogen.
A more sophisticated global combustion reaction for biomass that better accounts for the range of species present is
represented by the mass balance of eqn [5] in which the mass
coefficients mr,i designate the masses of the reactant species and
mp,j designate the product species. In this case, the biomass is
divided into three fractions: an organic phase, a moisture phase,
and an ash phase. The feedstock elemental mass fractions described by the coefficients yi arise from the analysis of the dry
feedstock for C, H, O, N, S, and Cl. Moisture in biomass is
represented by a separate water fraction (free and bound
moisture). The oxidation medium, air, for example, is considered to consist of O2, CO2, H2O, and N2, all in the gas phase.
The reaction is represented as resulting in eleven main gas phase
products and a residual mass containing ash and unreacted
portions of the other element masses from the biomass. The
residual can consist of solids such as particulate matter, carbon
in ash as charcoal or carbonates, and chlorides and sulfates in
furnace deposits. Equation [5] is reasonably general in that it
can equally be used to describe the combustion of other fuels
such as (bio)methane, biodiesel, Fischer–Tropsch liquids, and
other hydrocarbons, biooils from pyrolysis, and many others:
!
mr,1 ∑yi C,H,O,N,S,Cl,ash þ mr,2 H2 OðlÞ þ :::
i ::: þ mr,3 O2 þ mr,4 CO2 þ mr,5 H2 OðgÞ þ mr,6 N2 ¼ :::
::: ¼ mp,1 CH4 þ mp,2 CO þ mp,3 CO2 þ mp,4 H2 þ mp,5 H2 O
þmp,6 HCl þ ::: ::: þ mp,7 N2 þ mp,8 NO þ mp,9 NO2
þmp,10 O2 þ mp,11 SO2 þ mresidual
½5
483
In addition to the product composition, eqn [5] allows for
the estimation of other properties of the reaction, such as the
flame temperature under different burning conditions, an
important parameter for estimating heat transfer, efficiency,
and thermal performance.
Equation [5] also specifies the oxidant-to-fuel ratio, or in
the case of air as the source of oxygen, the air–fuel ratio, along
with the equivalence ratios often used to specify combustion
conditions. The air–fuel ratio, AF, defines the mass of air
added relative to the mass of feedstock, expressed on either a
wet or dry basis. The stoichiometric value, AFs, defines the
special case in which only the amount of air theoretically
needed to completely burn the feedstock is added to the reaction. The air–fuel ratio, the fuel–air equivalence ratio, ϕ, the
air–fuel equivalence ratio or air-factor, λ, and the excess air, e,
are all related as follows:
ϕ¼
AFs
AF
λ¼
1
ϕ
e¼λ1
½6
The combustion regimes are defined by the value of ϕ with
ϕ¼ 1 (e¼ 0) being the stoichiometric combustion, ϕ41 the
fuel-rich regime (insufficient air), and ϕo1 the fuel-lean regime (excess air). The equivalence ratio is particularly important in characterizing the potential air pollutant emissions
from a combustion system. For fuel-rich conditions, concentrations increase for products of incomplete combustion such
as hydrocarbons, CO, and particulate matter. NOx emissions
may peak under slightly fuel-lean conditions due to high flame
temperatures and reaction of nitrogen and oxygen in air (the
so-called thermal NOx). NOx is also produced from nitrogen
in the feedstock (fuel NOx). Other pollutant species are also
produced in varying amounts under all three combustion regimes but are not specifically included in eqn [5].
Fuel moisture is a limiting factor in biomass combustion.
The autothermal limit for most biomass fuels is approximately
65% moisture content wet basis (mass of water per mass of
moist fuel). Above this point, insufficient heat is liberated by
combustion to satisfy the energy needs for evaporation and
product heating. Practically, most combustors require a supplemental fuel, such as natural gas, when burning biomass in
excess of 55% moisture wet basis. In the US, Federal Energy
Regulatory Commission regulations developed after the enactment of the Public Utilities Regulatory Policies Act in 1978
permit up to 25% of the power plant energy input to come
from natural gas or other fossil fuel (depending on air permits) for qualifying biomass cogenerators. Many independent
power producers and biomass facilities employ cofiring for
startup and flame stabilization.
The most common type of biomass-fueled power plant
today utilizes the conventional Rankine or steam cycle, as illustrated in Figure 16. The fuel is burned in a boiler, which
consists of a furnace and one or more heat exchangers to make
superheated steam. Typical medium efficiency units utilize
steam temperatures and pressures of approximately 500 1C
and 6 MPa. The steam is expanded through one or more turbines that drive an electrical generator. The steam from the
turbine exhaust is condensed, and the water recirculated to the
boiler through the feedwater pumps. Combustion products
exit the combustor, are cleaned, and vented to the atmosphere.
484
Global Agriculture: Industrial Feedstocks for Energy and Materials
Stack
Emission control
Flue gas
Superheated steam
Turbine
Generator
Electricity
Boiler
Air
Fuel
Condenser
Water
Flyash
Pump
Bottom ash
Coolant
Figure 16 Schematic of a Rankine or steam cycle power plant.
Typical cleaning devices include wet or dry scrubbers for sulfur
and chloride control, cyclones or other inertial separation
devices, baghouses (filters) or electrostatic precipitators for
particulate matter removal, and selective catalytic or selective
noncatalytic reduction of NOx. Low CO and hydrocarbon
emissions are maintained primarily by proper control of air–
fuel ratio in the furnace and boiler.
Ash fouling on the superheaters, the heat exchangers used
to increase steam temperature above its saturation temperature
and that are commonly located in the hottest parts of the
furnace, has been a severe problem in many biomass-fueled
boilers. Pretreatment of the feedstock to remove much of the
alkali and chloride before firing to the boiler has proved
beneficial in reducing fouling (Jenkins et al., 1998, 2011).
Coproduct bottom and fly ash can be used for industrial
materials or land applied as fertilizers, potentially from the
same fields that produced the feedstock. Ash from municipal
solid waste incinerators, which commonly employ additional
control of chlorides, mercury, and other hazardous products,
may require hazardous waste disposal.
Individual power plants using biomass fuels in the manner
described here typically range up to approximately 50 MW
electrical capacity, which in the US is sufficient to supply the
needs of from 25 000 to 50 000 people. Larger sizes are possible, and size selection is accomplished through an analysis of
fuel resource availability, plant economy, and local regulations
as described earlier under feedstock logistics. The distributed
nature of biomass fuels and the limited economy of scale associated with plants of this type have kept the size of individual facilities relatively small in comparison with coal,
petroleum, or nuclear-fueled power-generating stations. The
largest biomass-fueled power station is a 240 MWe unit in
Finland that operates at 545 1C and 16.5 MPa and produces
process steam and hot water for district heating, but this unit
can also accept peat and coal as fuel (Alholmens Kraft, 2013).
A larger project in the UK (750 MWe) based on replacing coal
with biomass was recently abandoned for financial and feedstock supply reasons, the latter based on uncertainties about
local supplies from the region (Ross, 2013). Conversion of
coal-fired facilities to biomass continues to be considered,
however, in order to help meet the standards for reduced
greenhouse gas emissions.
The efficiencies of biomass power plants are generally
lower than comparable fossil-fueled units because of higher
fuel moisture content, lower steam temperatures and pressures
(due to the need to control fouling at higher combustion gas
temperatures), and to some extent the smaller sizes, with a
proportionately higher parasitic power demand for pumps,
fans, and other electrical devices associated with the power
plant itself. Large fossil-fueled units normally incur approximately 3% parasitic demand but with biomass plants the demand is more commonly approximately 10%. Currently,
biomass power plant efficiencies are in the range of 17–28%,
compared with good single-cycle fossil-fueled units ranging up
to 40% overall efficiency. Gas and distillate (diesel) fired
combined cycle power plants have efficiencies ranging more
than 50% overall. Biomass integrated combined cycles are
projected to exceed 35% electrical efficiency. Cofiring of biomass at 10–15% of energy input in higher capacity, higher
efficiency fossil (e.g., coal) stations can also lead to a higher
efficiency in biomass conversion. Increasing the power generation efficiency is a major goal for advanced biomass designs
(Jenkins et al., 2011).
Gasification and pyrolysis
On heating, biomass fuels will decompose into a number of
gaseous and condensable species, leaving behind a solid carbonaceous residue known as char. This is an early stage of
combustion, and the luminous flame seen when burning
wood and other biomass is a result of the oxidation of volatile
Global Agriculture: Industrial Feedstocks for Energy and Materials
compounds emitted during pyrolysis and gasification of the
feedstock and thermal radiation from soot particles from the
flame giving a characteristic yellow color.
When the fuel–air equivalence ratio, ϕ, of eqn [6] is substantially greater than unity (fuel-rich), the fuel will be only
partially oxidized due to the insufficient oxygen, and the reaction products will consist not only of carbon dioxide and
water, but of large amounts of carbon monoxide and hydrogen in addition to variable amounts of gaseous hydrocarbons
and condensable compounds (tars and oils), along with char
and ash. Other oxidants, including steam, can also be used
instead of air, in which case the reaction product suite will
differ. Reaction conditions can be varied to maximize the
production of fuel gases, fuel liquids, or char (as for charcoal),
depending on the intended energy market or markets. The
term gasification is applied to processes that are optimized for
fuel gas production (principally CO, H2, and light hydrocarbons). Under heating alone without the addition of an
oxidant the feedstock will pyrolyze. Pyrolysis reactors are
typically designed to maximize the production of liquids
through fast rather than slow heating although increasing
interest in biochar or black carbon is now shifting the preferred
product mix. Catalysts are sometimes employed to promote
various reactions, especially the cracking of high molecular
weight hydrocarbons produced during gasification and also in
chemical catalytic synthesis of liquid hydrocarbons and other
products in making transportation biofuels.
Gasification technology was developed more than 200
years ago (Kaupp and Goss, 1984), and lately has been improved primarily for the purposes of providing solid fuels
(biomass, coal, and coke) access to some of the same commercial markets as natural gas and petroleum. Gasifiers have
long been used to convert solid fuels into fuel gases for operating internal combustion engines, both spark ignited (gasoline) and compression ignited (diesels). They can also be
used for external combustion devices, such as boilers and
Stirling engines. The most common types are the direct gasifiers, where the partial oxidation of the feedstock in the fuel
bed provides the heat for pyrolysis and gasification reactions,
which are mostly endothermic. Indirect gasifiers and pyrolysis
reactors use external heat transfer to provide the heat necessary
to pyrolyze the fuel. The heat may be produced by the combustion of some of the original biomass fuel, or by the combustion of output fuel gases, liquids, or char. Allothermal
reactors have been developed to supply heat by internal but
separate burning of the char phase following gasification of the
feedstock, mostly in dual reactor systems (Wilk and Hofbauer,
2013). Gasifiers may have less difficulty with ash slagging
because of the lower operating temperatures compared with
combustors, although slagging, fouling, and bed agglomeration remain problems with some fuels (e.g., straw).
When direct gasifiers are supplied with air to react the
feedstock, the fuel gases will contain large amounts of nitrogen
and the heating value or energy content of the gas will be low
(3–6 MJ m3) in comparison to natural gas (compare methane at 36.1 MJ m3) and other more conventional fuels.
Nonsupercharged engines operating on such gas will be derated in power output relative to their operation on gasoline or
diesel (Jenkins and Goss, 1988). In the case of diesel engines,
the gas cannot be used alone, and pilot amounts of diesel fuel
485
are injected to provide proper ignition and timing. For sparkignited engines, the engine power output is about half that of
the same engine on gasoline, because the air capacity of the
engine (the amount of air drawn into the engine cylinder
during the intake stroke) is reduced by the large volume occupied by the fuel gas, and so not as much fuel can be burned
during each cycle (Jenkins and Goss, 1988). Supercharging the
engine can overcome this in part. For dual-fuel diesel engines,
the gas can generally supply up to 70% of the total fuel energy
without encountering severe knock, which results from the
long ignition delay associated with the producer gas, the
same property which gives the gas excellent octane rating
(Chancellor, 1980; Ogunlowo et al., 1981). The same properties of producer gas which lead to late ignition and knock in
a diesel engine make it quite knock resistant in a spark-ignited
engine, so compression ratios well above 10 can be used. With
proper design of the cylinder head at increased compression
ratio, the engine efficiency can be improved over gasoline,
offsetting some of the derating due to reduced air capacity.
If the gasification reactor uses enriched or pure oxygen, the
fuel gas, or syngas, produced is of higher quality. The cost of
producing the oxygen is high, however, and such systems are
generally proposed for larger scales or for producing higher
value commodities, such as chemicals and liquid fuels.
Methanol, a liquid alcohol fuel, CH3OH, is produced by the
catalytic reaction
CO þ 2H2 ¼ CH3 OH
½7
This reaction is favored by low temperature (400 1C) but by
high pressure (30–38 MPa). Zinc oxide and chromic oxide are
the common catalysts. With copper as the catalyst, the reaction
temperature and pressure can be reduced (260 1C, 5 MPa), but
copper is sensitive to sulfur poisoning and requires good gas
scrubbing (Probstein and Hicks, 1982). Fischer–Tropsch reactions can be utilized to produce a spectrum of chemicals
including alcohols and aliphatic hydrocarbons. Temperature
and pressure requirements are reduced, and greater selectivity
can be obtained by the choice of catalyst.
Liquids, such as gasolines, can be produced via indirect
routes involving the gasification or pyrolysis of the solid biomass to produce reactive intermediates that can be catalytically
upgraded (Kuester et al., 1985; Prasad and Kuester, 1988;
Kuester, 1991; Brown, 2011). Liquids produced directly by
pyrolysis are usually corrosive, suffer from oxidative instability, and cannot be directly used as engine fuels. Many products
are also carcinogenic. Refining of some sort is generally necessary to produce marketable compounds. Despite this, fast
pyrolysis reactors employing biomass and other fuels are in
commercial startup to produce biooils (Ensyn Corp, 2014).
Liquid fuels can also be produced by direct thermochemical
routes, such as by hydrogenation in a solvent with a catalyst
present (Elliott et al., 1991; Bridgwater and Bridge, 1991).
One of the principal technical hurdles, especially at the
small scale, in applying gasifiers for applications other than
direct burning of the raw gas is gas cleaning and purification. Removal of particulate matter and tars from the gas is
critical in downstream power generation and fuel synthesis.
Tars comprise a class of heavy organic materials that are particularly difficult to remove or treat. Systems are available to
produce acceptable gas quality, but generally rely on some
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Global Agriculture: Industrial Feedstocks for Energy and Materials
combination of wet and dry scrubbing and filtering and add
expense to the conversion system. Small-scale gasifiers used for
remote power generation have often been deployed without
adequate handling procedures for tar separated from the gas.
Gas cleaning and tar handling remain critical engineering
challenges for a wider adoption of the technology at all scales.
Advanced power generation options from biomass include
the use of a biomass gasifier to produce fuel gas for a gas
turbine in an integrated gasification combined cycle system
(Figure 17; Meerman et al., 2013). The efficiency of these
systems could be considerably higher than the conventional
Rankine cycle power generation systems. Major engineering
challenges include hot gas cleaning to provide a gas of
Gas
purification
adequate quality to avoid fouling the turbine, and the development of reliable high pressure reactors or compressors and
fuel feeding systems. The use of a gasifier is thought to be an
advantage over a direct combustor because heat loss in the gas
cleaning system is of less concern, most of the fuel energy
being in the form of chemical energy in the product gas. Other
advantages for gasifiers over combustors include the ability to
operate at lower temperatures, and lower gas volumes per unit
of feedstock converted, which assists in the removal of sulfur
and nitrogen compounds to reduce pollutant emissions. Systems of this type are currently under development and several
large-scale demonstration projects have been completed, but
the technology has not yet been deployed commercially for
Topping
cycle
Producer
gas or
syngas
Crude gas
Steam
injection
Burner
Pressurized
air
Gasifier
Tar/
impurities
Gas
turbine
Compressor
Fuel
Ash/
char
Generator
Electricity
Air
Stack
Emission control
Flue gas
Bottoming
cycle
Heat
Recovery
Steam
Generator
Superheated
steam
Steam
turbine
Exhaust
gas
Generator
Electricity
Condenser
Water
Recovery/disposal
Pump
Coolant
Figure 17 Integrated gasification combined cycle advanced power generation concept. Pressurized air gasification shown. A steam-injected gas
turbine option is also shown (IG/STIG).
Global Agriculture: Industrial Feedstocks for Energy and Materials
biomass although it has for coal at larger scales (Stahl and
Neergaard, 1998). Figure 17 also illustrates the possible use of
steam injection for reducing thermal NOx emissions and enhancing the power output of the gas turbine. The high heat
capacity of the steam in comparison to the combustion
products leads to a power increase, and the addition of steam
reduces the flame temperature, which is beneficial for reducing
thermal NOx formation (Weston, 1992). Many other thermochemical conversion options are under development
(Brown, 2011).
Biochemical Conversion
Biochemical conversion relies primarily on the abilities of
microorganisms to convert biomass components into useful
liquids and gases. Yeast fermentation is the principal means of
producing ethanol. Biogas is a product of bacterial fermentation in anaerobic digestion, although bacteria are also used
in alcohol fermentation. Fermentation is a widely used industrial process and an active area of biotechnology research
and development.
Fermentation
Ethanol (C2H6O) is widely produced by fermentation and is
the predominant liquid fuel derived by biochemical means
from biomass. In the United States, ethanol is currently used
as a oxygenate blending agent in gasoline to help reduce air
pollutant emissions from vehicles as well as a major fuel
product in the form of E85, an ethanol–gasoline blend containing nominally 85% ethanol and 15% gasoline although
actual blends may range as low as 70% ethanol. In Brazil,
ethanol is used both as a pure or ‘neat’ (100% ethanol) fuel,
and as a blending agent with gasoline at 20% concentration
(E20). The primary feedstock for ethanol production in the
United States is corn (maize) grain as a source of readily
fermentable starch but sorghum and other agricultural feedstocks are used as well. In Brazil it is sugarcane as a source of
sugar. The yeast Saccharomyces cerevisiae has been the most
widely used organism for ethanol fermentation (Sen, 1989).
The fermentation process involves the necessary pretreatment
of the feedstock to produce a fermentable substrate (sugar),
fermentation, and product separation.
Corn biorefineries producing ethanol are of two types: wet
mills and dry mills. Wet mills extract multiple products from
the grain including oil, fiber, gluten, and starch, with the starch
fraction used for fermentation. Dry mills (Figure 18) use the
entire corn kernel for fermentation. The grain is not directly
fermentable, but requires milling, hydration and gelatinization, and enzymatic or acid hydrolysis of the starch to
fermentable sugars before the actual fermentation. Theoretical
yields from glucose (C6H12O6) are 51% ethanol and 49%
carbon dioxide (mass basis). The overall reaction for the fermentation of glucose to ethanol is
C6 H12 O6 ¼ 2C2 H6 O þ 2CO2
½8
The theoretical yields from starch and sucrose are somewhat
higher. Actual yields from fermentation are lower due to
production of the microbial cell biomass and incomplete
conversion of the substrate.
487
Ethanol concentrations reach approximately 10% in the
fermentation beer before separation; higher concentrations
inhibit the microbial activity. The ethanol is typically separated by steam distillation to 95% concentration, the azeotropic point at which water and ethanol evaporate to yield the
same concentration in the vapor as in the liquid and no further
separation by distillation is possible. The distilled product is
dehydrated using solvents or other desiccants to produce anhydrous ethanol. A high protein stillage is produced and is
used as animal feed (distillers grains). Beverage grade alcohol
includes removal of fusel oils during distillation. For fuel grade
alcohol, this step is usually eliminated.
Cellulosic feedstocks are more difficult to hydrolyze into
monosaccharides for fermentation, thereby incurring more
costly pretreatment. Current methods under development for
cellulosic biomass hydrolysis and fermentation essentially
fall into four categories: (1) concentrated acid hydrolysis
(Figure 19), (2) dilute acid hydrolysis, (3) enzymatic hydrolysis (Figure 20), and (4) thermochemical conversion
(gasification and pyrolysis) followed by fermentation of synthesis gases, a process that attempts to circumvent problems
associated with hydrolysis (Skidmore et al., 2013). Concentrated acid processes have long been used to produce ethanol
from cellulosic materials, but for economic and environmental
reasons in modern applications require substantial recovery
and recycling of the acid, adding expense. Enzymatic hydrolysis is widely investigated in an effort to improve yields
and reduce costs. Enzymatic processes developed following
research during World War II to control deterioration of
military clothing and equipment. This research identified the
fungal organism Trichoderma viride, later renamed Trichoderma
reesei, important to the development of cellulose enzymes
(cellulase) for decrystallization and hydrolysis of cellulose.
Genetically modified and recombinant organisms have been
developed to enhance both the hydrolysis and fermentation of
cellulosic materials (Luli et al., 2008; Singh, 2010). The cost of
enzyme production remains high, however, and major research investigations continue to explore means of reducing
the cost. Alternative techniques are under development to
produce sugar aldonates as the reactive intermediates rather
than sugars to replace the pretreatment, cellulase production,
and enzymatic hydrolysis processes with a single biological
step (Fan et al., 2012).
Cellulose hydrolysis yields hexoses, primarily glucose,
fructose, mannose, and galactose. Hemicellulose hydrolysis
yields mostly pentoses, principally xylose and arabinose. Lignin is currently unfermentable in any commercial application.
The sugars are fermented to ethanol whereas lignin is mostly
considered as boiler fuel (e.g., in steam production for distillation and power generation) or for conversion to other
products, such as aryl ethers (Salanitro, 1995; Sergeev and
Hartwig, 2011; Parthasarathi et al., 2011). Sequestering of
lignin has also been proposed to reduce net greenhouse gas
emissions from biochemical conversion (Murphy, 2013).
Energy balances associated with fermentation of lignocellulosic materials are thought to be improved relative to
those for corn grain, although commercial data are not yet
substantially available. For lignocellulosic feedstocks, overall
ethanol yields are likely to be in the range of 300 l Mg1 of dry
feedstock compared with 400 l Mg1 for corn grain. The ideal
488
Global Agriculture: Industrial Feedstocks for Energy and Materials
Corn grain
Grind
Water
Enzyme (-amylase)
pH adjust (lime)
Cook
(starch solubilization)
Water
Cool
pH adjust (acid)
Enzyme (glucoamylase)
Hydrolysis
Cool
Yeast
Fermentation
CO2
Beer
Stillage
Steam
Steam
Beer still
Drying
Feed
Rectifying column
Steam
Anhydrous
tower
Solvent
stripper
Make-up solvent
Water
Anhydrous ethanol
Figure 18 Process for the production of anhydrous ethanol from corn.
energy recovery assuming complete conversion into ethanol
for cellulose fermentation is approximately 97% of the feedstock energy. The overall reaction of cellulose to glucose is
ðC6 H10 O5 Þn þ nH2 O ¼ nðC6 H12 O6 Þ
½9
with glucose fermented to ethanol as in reaction [8]. The
fractional energy yield for cellulose to ethanol is shown in
Table 6. Actual energy yields are substantially lower. At
300 l Mg1 the energy yield from feedstock is 40%. This does
not include the supplemental energy involved in feedstock
production, handling, and conversion.
As an improvement over the separate hydrolysis and
fermentation (SHF) of the cellulosic biomass, simultaneous
saccharification and fermentation (SSF) processes are perceived to have certain advantages in avoiding end-point
inhibition in the fermentation stage and the possibility of
reducing the fermentation time. The cost of ethanol from SSF
processes may, therefore, be reduced relative to SHF. In SSF, a
mixed culture of organisms is used, typically Brettanomyces
claussenii and S. cerevisiae, with B. clausenii being active early
and the more robust S. cerevisiae continuing to completion.
More recent developments employ simultaneous saccharification and cofermentation of multiple sugars, or combined enzyme production, saccharification, and fermentation
Global Agriculture: Industrial Feedstocks for Energy and Materials
489
Feedstock
Mill
Hemicellulose
reactor
100 °C/1−6 h
9.5% H2SO4
10% glucose
Solids
Stream
Acid
Press
Water
Steam
5−10% C5
5−10% C6
Cellulose
pretreatment
20−30% H2SO4
1−2 h
Lime
Neutralization
Filter
Solids/gypsum
recovery
Dryer
Yeast
Water
Steam
Recycle acid
+ Sugars
Fermentation
Cellulose
reactor
Beer
Steam
Press
CO2
Solids (Lignin +
unreacted cellulose/acid
and sugar recovery)
Distillation
Stillage
Ethanol
Figure 19 Process for fermenting cellulosic feedstocks using concentrated acid hydrolysis with acid recycle. Adapted from Barrier, J.W., Moore,
M.R., Broder, J.D., 1986. Integrated Production of Ethanol and Coproducts from Agricultural Biomass. Muscle Shoals, AL: Tennessee Valley
Authority.
processing in consolidated bioprocessing (Olson et al.,
2012).
Gasification systems coupled with a bioconversion stage
are proposed to take advantage of the capability of anaerobic
organisms, such as Clostridium ljungdahlii to ferment syngas to
ethanol. These systems potentially would operate at a higher
rate than the hydrolytic fermentation systems (Skidmore et al.,
2013).
Thirty-five percent of the weight of ethanol is due to oxygen. In the United States, ethanol is used as an oxygen source
in gasolines where it assists in controlling CO emissions from
engines. At present, ethanol can be blended up to 10% by
volume. Fuel oxygen promotes CO oxidation to CO2, and the
ethanol in the gasoline tends to create a leaning effect in most
spark-ignited engines, causing them to operate at higher
effective air–fuel ratios with lower CO emission rates (OTA,
1990). Ethanol has a high octane rating, as does methanol,
and engines using neat or blended fuels can be designed for
higher compression ratios and higher efficiencies. Therefore,
even though ethanol has a heating value which is 60% that of
gasoline (Table 7), the effective fuel consumption on a
properly configured engine should only be increased by approximately 25% relative to gasoline. Stable blends of ethanol
with gasoline require anhydrous ethanol to avoid phase separation, an issue in ethanol transportation and distribution.
The use of ethanol as blending stock for gasoline in
California has also been controversial due to the higher vapor
pressures when blended compared with gasoline alone, although the vapor pressure of neat ethanol is less than gasoline.
The higher blend vapor pressure can result in higher evaporative losses of reactive hydrocarbons, although improved
automotive fuel systems have reduced this concern in late
model vehicles and blending ratios have shifted from 5.7% to
10%. The hydrocarbons react with NOx in the atmosphere to
produce ozone, a lung irritant and undesirable tropospheric
pollutant. The level of ethanol blending is also a point of
contention between oil refiners and ethanol producers due to
rules under the US federal renewable fuel standard (RFS), a
part of the Energy Independence and Security Act of 2007
(U.S. Congress, 2007; Podkul, 2013). A 10% blend ratio limits
the total amount of ethanol that can be used in motor fuel
(the so-called blend wall), and changes in gasoline demand
could result in refiner demands that are lower than called for
under the RFS, making financing difficult for new biorefining
capacity additions. Vehicle warranties, other than for E85capable vehicles, mostly cover only up to 10% ethanol, adding
to the blend-wall constraints. Further constraints to the growth
of a corn ethanol industry in the United States and elsewhere
arise in California from estimates of lifecycle carbon intensities
(net greenhouse gas emissions) under the low-carbon fuel
standard (LCFS) that requires fuel suppliers to reduce intensity
by 10% (CARB, 2012). Where coal is used for process energy in
biorefineries producing fuel ethanol, estimated carbon intensities exceed those of the reference gasoline from petroleum.
490
Global Agriculture: Industrial Feedstocks for Energy and Materials
Feedstock
Size
reduction
Water
Acid
Dilute acid
pretreatment
Solids
Liquid
Press
Lime
Neutralization/
conditioning
Cellulase
enzymes
Liquid
Saccharification/
fermentor
Ethanol
recovery
Solids/gypsum
recovery
Filter
Ethanol
Solids (lignin utilization)
Press
Wastewater
treatment
Figure 20 Process for fermenting cellulosic feedstocks using enzymatic hydrolysis. Adapted from McMillan, J.D., 2004. Biotechnology routes to
biomass conversion. DOE/NASULGC Biomass and Solar Energy Workshops, August 3–4. Golden, CO: National Renewable Energy Laboratory.
Table 6
Ideal fractional energy yields in the conversion of cellulose into ethanol (actual yields lower)
Compound
Molecular weight (kg kmol 1)
Higher heating value (MJ kg 1)
Molar energy (MJ)
Fractional energy yield
Cellulose
Glucose
Ethanola
162
180
46
17.53
15.67
29.78
2840
2820
2740
1.000
0.993
0.965
a
2 mol of ethanol produced per mole cellulose reacted.
Although concerns such as these will likely be resolved over the
longer term, the role of policy is evident in influencing system
design and industrial capacity.
Anaerobic digestion
Fermentation by anaerobic bacteria is used to produce biogas,
a gaseous fuel consisting of 50–80% by volume methane, 15–
45% carbon dioxide, and 5% water, with small concentrations
of H2S and other species (Krich et al., 2005). The technology is
extensively employed in municipal waste water treatment.
Often the biogas is burned in engines to generate power, with
engine waste heat used to heat the digester for improved
performance. The same biological processes are active in waste
landfills, where gas recovery has become an integral part of
landfill design, both as a means to control gas migration and
as a means of energy recovery. Other products can also be
produced through anaerobic digestion including carboxylic
acids, ketones, and alcohols (Thanakoses et al., 2003).
Table 7
Properties of ethanol and gasoline
Property
Specific gravity (15 1C)
Lower heating value
(MJ kg 1)
Lower heating value (MJ L 1)
Octane number ((R þ M)/2)a
Stoichiometric air–fuel ratio
Lower heating value of
stoichiometric air–fuel
mixture (MJ kg 1)
Enthalpy of vaporization
(kJ kg 1 at 15 1C)
Reid vapor pressure (kPa)b
a
Ethanol
Unleaded regular
gasoline
0.79
26.9
0.78
44.0
21.2
98
9.0
2.7
34.3
88
14.7
2.8
840
16
335
55–103
Average of research (R) and motor (M) octane test methods (pump value).
Reid vapor pressure of 10% ethanol in gasoline is 3−7 kPa higher than gasoline
alone.
b
Global Agriculture: Industrial Feedstocks for Energy and Materials
The overall reaction for anaerobic digestion of the organic
portion of the feedstock (represented as CxHyOz) to biogas is
x y z y z
þ CO2
Cx Hy Oz þ x H2 O ¼
4 2
2 8 4
x y z
þ CH4
þ
ð10Þ
2 8 4
The actual concentration of CO2 is typically reduced due to
its solubility in water.
The digestion of the feedstock is commonly described as
occurring in three stages although the cooperative mechanisms
among the organisms involved has been emphasized
(McInerney and Bryant, 1981; Hills and Roberts, 1982; Parkin
and Owen, 1986). After loading feedstock to the digester
vessel, much of the organic material is solubilized by bacterial
metabolism or chemical hydrolysis. Acid-forming bacteria
utilize the soluble compounds and produce low molecular
weight organic acids, principally acetic acid, that are used by
methanogenic (methane-forming) bacteria for the final conversion to biogas. In a properly operating digester, these processes take place simultaneously within the digester contents,
although in some recent designs the hydrolysis and methanogenic processes have been largely separated (Zhang and
Zhang, 1999). Acid-forming bacteria are generally more robust
than the methanogens, and can overwhelm the system with
acids, causing a decrease in pH and unfavorable conditions for
the methanogens. A properly functioning digester will have a
pH in the range of 7–8. The health of the digester is controlled
by proper management of nutrient loading, retention time,
temperature, and, in dilute slurry digesters, mixing.
Digesters most often operate with dilute slurries. Bioreactor
designs include covered lagoons commonly used in dairy and
other agricultural operations, batch digesters, plug-flow digesters, completely stirred tank reactors, upflow anaerobic
sludge blanket systems, anaerobic sequencing batch reactor
systems, anaerobic-phased solids systems, and in landfills
enhanced bioreactors employing leachate recycle. Plug-flow
reactors utilizing higher solids concentrations are also used.
High solids or dry-fermentation digesters have been developed
to reduce some of the problems associated with handling dilute slurries in tank reactors (Jewell, 1982). These high solids
digesters are similar in nature to landfills, but implemented at
smaller scales and with greater automation.
Digesters can operate on almost all biomass feedstocks,
although with differing conversion rates and efficiencies. They
are well suited for wet or moist feedstocks that would require
drying for thermal conversion systems. Nitrogen availability is
limiting in most cases, particularly when woods or herbaceous
materials are used alone. Animal manures and mixtures of
manures with agricultural residues, such as straw, give better
performance due to improved carbon-to-nitrogen (C/N)
ratios, which are recommended to be in the range of 25–30
overall (Hills and Roberts, 1981). The biogas produced typically has a heating value of 22–24 MJ m3 with yields of 50–
400 l per kg dry solids depending on feedstock and digester
conditions, including temperature, inoculation levels, and
hydraulic retention time. Design operating conditions include
the psychrophilic (ambient temperature or below), mesophilic
(30–40 1C), and thermophilic temperature regimes (50–
60 1C) with conversion rates generally increasing with
491
temperature although heating requirements also increase.
Hyperthermophiles that can withstand even higher temperatures have not yet been employed in commercial systems.
Energy conversion efficiencies for anaerobic digesters are
relatively low if based on the yield of the gas alone. Typical
energy efficiencies of biogas are 20–50%, and up to approximately 20% for electricity. The stabilized sludge remaining
after digestion can be used as fertilizer, unless contaminated
with heavy metals or other toxic materials from the input feed.
Nutrient management is an important issue in many agricultural systems and digesters can be used to advantage, especially in mineralizing nitrogen before land application or
discharge of effluent.
Biogas is a reasonably versatile fuel and can be burned
directly for heating the digester (to improve performance), to
heat water, or for cooking and lighting purposes. The gas can
also be used as fuel for engines, turbines, and boilers, including cogeneration, and more recently in fuel cells (Krich
et al., 2005; Lo Faro et al., 2013). As noted earlier, NOx
emissions from engines are currently a key issue for small digester-power systems. H2S can be scrubbed from the gas to
reduce corrosion and SO2 emissions, although in small-scale
applications such as individual dairy operations, the cost of
more effective removal systems is a concern. Siloxanes typically present in landfill gas may need scrubbing (usually on
charcoal) for some applications, turbines in particular and also
in fuel cells and reciprocating engines. Biogas has been used as
transportation fuel, including low pressure applications with
large flexible bags on the roof of the vehicle serving as the fuel
tank (Stout, 1990), but is now more widely considered for
CNG or LNG applications. CO2 stripping followed by gas
compression is mostly required for these purposes. CO2
stripping has also been used to produce pipeline quality gas
for blending into natural gas distribution systems and to
produce transportation fuels. Small-scale digesters have been
promoted in many areas to improve sanitation as well as to
supply fuel gas. The process is also considered in the design of
integrated biorefineries (Thomsen et al., 2013).
Physicochemical Conversion
Biodiesel has recently emerged as a commercial renewable
alternative to petroleum-derived diesel. Biodiesel can be produced from virgin plant and algal oils and also from waste oils
such as cooking oils and greases, the latter incurring lower
procurement costs and hence lower cost of production. In the
United States, the RFS mandates increasing the amounts of
biodiesel although the amount required is a fairly small share
of the total biofuel under the standard (U.S. Congress, 2007).
Recent reversals in European Union (EU) policies due to sustainability concerns have reduced mandated biofuel volumes
with impacts on the existing biodiesel producers and suppliers.
Although potentially resource and policy constrained, the
current production of biodiesel represents one of the predominant physicochemical conversion techniques.
Unlike petroleum-based diesel fuels, which are primarily
straight or branched chain hydrocarbons, vegetable oils are
primarily mixtures of triacylglycerols composed of fatty acids
esterified to the three –OH positions on glycerol (Goering
492
Global Agriculture: Industrial Feedstocks for Energy and Materials
et al., 1982; Robbelen et al., 1989). Although crude or refined
vegetable oils can be used as fuel for compression-ignited
(diesel) engines, such use is not recommended except on an
emergency basis for short periods of time (Peterson, 1989).
Engine damage arising from fuel polymerization and ring
sticking, along with lubricating oil contamination, is likely to
occur after prolonged operation. Vegetable oils have heating
values similar to diesel fuel (approximately 90% of ASTM No.
2 diesel), but the viscosity is one to two orders of magnitude
higher. The high viscosity causes irregular spray patterns from
fuel injectors inside the engine cylinders, which in turn leads
to injector coking (carbonization of the fuel on the injector
nozzles) and deposits on cylinder walls and pistons.
Vegetable oils also have higher cloud point and pour point
temperatures, which makes them inferior to diesel fuels for
cold weather operation. However, they have higher flash point
temperatures, and are therefore somewhat safer, and are less
toxic than diesel fuels and mostly biodegradable. Stable blends
of diesel fuel with vegetable oils can be made to improve the
fuel properties, but modification of the oil is preferred for
general engine application.
The reaction of a triglyceride, such as a vegetable oil, with
an alcohol in the presence of a catalyst such as sodium hydroxide, produces upon separation a fatty acid ester and glycerol. The ester is an improved diesel fuel substitute over the
original oil and the basis for current biodiesel manufacturing.
When methanol is used, the result is a fatty acid methyl ester.
Fatty acid ethyl esters, produced from oil and ethanol, have
also emerged as diesel fuel replacements, with some possible
advantages over the methyl esters in reduced smoke opacity
and lower injector coking. The esters have viscosities that are
only 2–3 times that of diesel fuel, which leads to reduced
injector coking and decreased deposit formation compared
with the original vegetable oil. The esters have improved cetane numbers relative to the original oils, and in some cases
relative to diesel fuel. This makes them useful as blending
stocks for diesel. Although esters reduce smoke emissions
compared with diesel fuel, NOx emissions are about the same
or higher, as are acrolein emissions. Aromatic aldehyde emissions depend on the aromaticity of the fuel such that blends of
biodiesel with regular diesel tend to have higher emissions
than pure biodiesel fuel (B100) (Qi et al., 2013; Cahill and
Okamoto, 2012).
Fuel specification standards for biodiesel have been developed in Europe (Mittelbach et al., 1992) and the US including standards (ASTM D6751, 2012) for 100% biodiesel
fuel (B100) as blending stock for distillate fuels. Owing to its
typically low sulfur content, biodiesel can be blended with
petroleum diesel that otherwise does not meet newer lowsulfur fuel standards. B20, a 20% biodiesel blend, can be used
in some unmodified diesel engines. The use of B100 may require modifications to the fuel supply system if the engine has
not been warranted for it.
The Challenge of Sustainability
The production of industrial feedstocks by agriculture involves
land and other resources that provide many different agronomic and ecosystem services. As the scale of production
capacity for fuels and other commodity chemicals has increased, so has the debate around the wisdom of converting
forest lands and prime or marginal agricultural lands to purposes other than traditional food, feed, and fiber production,
especially food for a growing global population. Policies intended to improve sustainability in one sector, such as energy
security, have often been contested for their narrow focus and
lack of analysis and consideration of larger system issues, such
as net global greenhouse gas emissions or net energy benefits
(Searchinger, 2008; Pimentel, 1991). The reversal in EU policies regarding biofuels was motivated largely by reconsideration of sustainability impacts associated with resourcing large
supplies of biodiesel and other fuels from palm oil and other
crops where global environmental, social, and economic impacts had not been systematically evaluated (Turner et al.,
2008).
Challenges to US federal policies relating to biofuels developed following the first oil shock of 1973–74 when the US
initiated policy supporting alternative fuels. Debate initially
centered on net energy yields associated with producing
ethanol from corn with suggestions that the amount of energy
embodied in fossil fuels and other inputs to grow corn and
manufacture ethanol exceeded the energy in the ethanol, thus
negating any intended energy benefits (Pimentel, 1991).
Criticisms were largely addressed through improvements in
conversion technologies (Shapouri et al., 2002; Farrell et al.,
2006), but other questions of sustainability continue to arise.
In addition to a more recent debate over the net greenhouse
gas emissions from industrial feedstock production, the effects
on food prices have been called into question (the food-versus-fuel debate) with concerns over food security, especially
for the poor. The Brazilian program to produce ethanol from
sugarcane, widely viewed as a successful national response to
improve domestic energy security, also generates concerns over
environmental sustainability although the net greenhouse gas
impacts are mostly regarded as lower than for corn (Smeets
et al., 2008).
The global energy markets are large and can easily absorb
large quantities of feedstock biomass. Estimates for the United
States suggest that only approximately a third or less of
transportation fuel could be supplied from biomass (Perlack
and Stokes, 2011; Parker et al., 2011). Increasing biofuel demand potentially leads to direct land-use changes (dLUCs) as
well as indirect land-use changes (iLUCs) through marketmediated effects (Searchinger et al., 2008) unless yield intensification on the same land base is able to compensate, a
goal of crop research but not so far achieved. iLUC can cause
substantial emissions of carbon to the atmosphere from
clearing of standing biomass and depletion of carbon stocks in
the soil in response to crop shifting elsewhere in the world
(Fargione et al., 2008; Searchinger et al., 2008). The amount of
carbon released, like many other issues in agricultural sustainability, is highly uncertain and estimates vary widely. Inclusion of iLUC effects in environmental lifecycle assessments
can result in higher attributed greenhouse gas emissions for
some biofuels in comparison with petroleum-derived gasoline
or diesel fuels. Policies to encourage biofuels based solely on
direct substitution effects therefore may fail to achieve the
intended outcomes for total greenhouse gas emission reductions. In California, the development of a LCFS to limit the
Global Agriculture: Industrial Feedstocks for Energy and Materials
carbon intensity of transport fuels and electricity so as to reduce carbon emissions has been particularly contentious
(CARB, 2012). Other standards, such as the US renewable fuel
standard (U.S. Congress, 2007), also require set levels of lifecycle greenhouse gas emission reductions to qualify for a renewable identification number for the fuel, a key objective for
market access and financial success in meeting fuel blending
obligations (U.S. Congress, 2005; 2007). Sustainability considerations for biofuels also apply to issues of environmental
justice and other social implications in addition to environmental and economic effects (RSB, 2010).
Bioenergy production has been viewed as a way to provide
additional economic opportunities for farms and rural areas,
improve the environment, enhance energy security, and help
to stabilize fuel prices. Substitution of biofuels for gasoline
and other petroleum products can result in direct reductions in
greenhouse gas emissions and other benefits, but can also lead
to changes in the type of air pollutants that are emitted, affect
water demand and quality, alter land use, and increase or
destabilize other commodity prices. Demand for wood and
other biomass feedstocks for traditional uses in cooking,
heating, and charcoal making has contributed to deforestation
in many regions of the world in addition to causing adverse
health effects from exposure to smoke (Jenkins et al., 2011).
Concerns have long been expressed over harvesting of agricultural residues for energy and other industrial uses due to
possible agronomic impacts associated with nutrient depletion, changes to soil organic matter and carbon, and soil
erosion (Jenkins et al., 1981). Similar criticisms arise in the
large-scale production of chemicals and materials from agriculture. In adopting new uses for crops and resources, careful
consideration must be given to the lifecycle impacts and sustainability of the entire production system and feedstock
supply chain.
Economic sustainability is a primary factor in motivating
investment financing for industrial feedstock production,
where environmental and social sustainability factors are satisfied. The costs of producing energy and other products from
biomass are heavily influenced by feedstock acquisition costs
as well as by the costs of conversion. For comparison, costs to
generate electricity in the existing biomass-fueled power plants
are approximately US$0.06–0.08 kWh1 (US$17–22 per GJ),
sometimes higher, at an average fuel cost of US$30 per metric
ton, principally as wood residue (Jenkins, 2005). Cost projections for purpose-grown crops range substantially higher,
typically beyond $50 per metric ton dry weight and often more
than $100 per metric ton (Parker et al., 2011).
Exclusive of harvesting and downstream processing and
conversion operations, production costs for agricultural and
other biomass residues are typically allocated to the primary
crop production system and not always separately accounted.
Byproduct or waste biomass may be available at no cost, or in
some cases disposal fees (tipping fees) can be applied to cover
the costs of handling, an advantage to systems receiving
municipal solid wastes. In contrast, industrial and energy crops
assume full allocation of production costs. These costs are
quite variable depending on the species, production site, level
of management, and resulting yield. Total average delivered
costs, including harvesting and transportation (85 km),
for Eucalyptus plantations in northeast Brazil producing at
493
12.5 Mg ha1 year1 have been estimated at US$2.50–3.40 per
GJ, or approximately US$52–67 per metric ton dry matter (CPI
inflation adjusted 2013 US dollars) (Hall et al., 1993). Of the
total, 40% is associated with stand establishment including
nursery production, land, planting, and administration, and
another 10% is associated with plantation maintenance including management, cultivation, and research. Half the delivered cost is in the production of the biomass. Under
Canadian conditions, the total delivered cost including harvesting, chipping, and transportation for a typical 5 year
rotation, 4 rotation cycle forestry crop with a yield of 12 Mg
ha1 year1, was similarly estimated at US$58 per metric ton
(CPI adjusted 2013 US dollars), of which 40% was allocated
to the production system including land, nursery, planting,
and tending (Golob, 1987).
Other energy and chemical feedstock costs influence decisions to produce industrial feedstocks from agriculture.
Natural gas prices in the US in the period 1999–2001, for
example, fluctuated between US$2 and US$15 per GJ. Owing
to the development of hydraulic fracturing (fracking) techniques and expansion of natural gas reserves, prices are now in
the vicinity of US$3 per GJ and anticipated to remain below
US$4 per GJ through 2020, increasing to US$8 per GJ by
2040 (EIA, 2013c). Although policies such as California’s
RPS require increasing supplies of electricity from renewable
sources, resource neutrality within the policy provides no
specific advantage for biomass over other renewable resources
and principal capacity additions have been in wind and solar
(EIA, 2013d). Policy stability is also important in motivating
financing for biorefineries and other industrial applications.
Reversals such as that of the EU over biofuels (colloquially
referred to as the ‘biofuels disaster’) and possible adjustments
in the renewable fuel obligations under the RFS in the US
create significant uncertainties associated with security of investments in facilities with decades-long economic lifetimes.
Research along with genetic and cultural improvements are
projected to reduce biomass production costs, but biomass
production levels will also be influenced by direct and indirect environmental and socioeconomic consequences external to the direct costs of production (Hanegraaf et al.,
1998).
Sustainable large-scale production of industrial feedstocks
will require a broad systems view and well-designed standards
and best practices. International standards for certifying sustainable production of biofuels are currently in development
with early versions in use (RSB, 2010). The principles on
which these are based – attention to law, stakeholder participation, climate change mitigation, human, labor and land
rights, rural and social development, food security, waste
management, resource conservation and environmental protection – apply equally well to agricultural production systems
for biobased products as they do for bioenergy, a point reinforced by the recent name change of one of the principle
cooperative initiatives in international standardization to
broaden from biofuels to biomaterials and biomass production, more generally (RSB, 2013). Local standards may in
some cases exceed international standards but will need close
coordination to avoid conflicts with international agreements
and rules such as those of the World Trade Organization. The
complex issues now being researched and addressed in
494
Global Agriculture: Industrial Feedstocks for Energy and Materials
sustainable industrial feedstock production should provide
new perspectives on the improved sustainability of agriculture
overall.
See also: Agroforestry: Practices and Systems. Air: Greenhouse
Gases from Agriculture. Climate Change: Agricultural Mitigation.
Computer Modeling: Applications to Environment and Food
Security. Economics of Natural Resources and Environment in
Agriculture. Global Food Supply Chains. International and Regional
Institutions and Instruments for Agricultural Policy, Research, and
Development. International Trade. Land Use, Land Cover, and FoodEnergy-Environment Trade-Off: Key Issues and Insights for
Millennium Development Goals. Markets and Prices. Natural
Capital, Ecological Infrastructure, and Ecosystem Services in
Agroecosystems. Soil: Carbon Sequestration in Agricultural Systems
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Relevant Websites
http://www.astm.org/
American Society for Testing and Materials International.
http://www.bioenergykdf.net/
Bioenergy Knowledge Discovery Framework.
http://www.arb.ca.gov/homepage.htm
California Air Resources Board.
http://www.energy.ca.gov/
California Energy Commission.
http://www.caiso.com/Pages/default.aspx
California Independent System Operator.
http://www.cpuc.ca.gov/puc/
California Public Utilities Commission.
http://en.european-bioplastics.org/
European Bioplastics Organization.
http://www.iea.org/
International Energy Agency.
http://www.iso.org/iso/home.html
International Standards Organization.
http://www.wikipedia.org
On-Line Encyclopedia.
http://www.plastiquarian.com/index.php
Plastics Historical Society.
http://rsb.org/
Roundtable on Sustainable Biomaterials.
498
Global Agriculture: Industrial Feedstocks for Energy and Materials
http://www.usda.gov/wps/portal/usda/usdahome
U.S. Department of Agriculture.
http://energy.gov/
U.S. Department of Energy.
http://www.eia.gov/
U.S. Energy Information Administration.
http://www.epa.gov/
U.S. Environmental Protection Agency.
http://www.nist.gov/index.html
U.S. National Institute for Standards and Technology.
http://www.nrel.gov/
U.S. National Renewable Energy Laboratory.
http://www.nsf.gov/
U.S. National Science Foundation.