Energy Sources, Part A, 31:1573–1582, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1556-7036 print/1556-7230 online
DOI: 10.1080/15567030802094011
Biofuels from Agricultural Biomass
A. DEMIRBAS1
1
Sila Science, Trabzon, Turkey
Abstract Biofuels are liquid fuels which can be produced from agricultural biomass.
Agriculture-based biofuels include bioethanol, biodiesel, biomethanol, methane, and
bio-oil components. Various agricultural residues, such as grain dust, crop residues,
and fruit tree residues, are available as the sources of agricultural energy. Bio-energy
from biomass, both residues and energy crops, can be converted into modern energy
carriers. Bioethanol is derived from renewable sources feedstock, which are typically
plants such as wheat, sugar beet, corn, straw, and wood. Biodiesel is a non-fossil
fuel alternative to petrodiesel which can be obtained from vegetable oil and animal
fats by transesterification. Bio-oils are liquid or gaseous fuels made from biomass
materials, such as agricultural crops, municipal wastes, and agricultural and forestry
by-products via biochemical or thermochemical processes.
Keywords biodiesel, bioethanol, biogas, biomethanol, bio-oil
Introduction
Biomass includes wood and logging residues, agricultural crops and their waste byproducts, the organic portion of municipal solid waste, animal wastes, municipal biosolids
(sewage), waste from food processing, and aquatic plants and algae. Biomass provides
a clean, renewable energy source that could dramatically improve our environment,
economy, and energy security. Energy can be obtained from direct combustion of biomass
by burning dry organic matter, such as woody scraps, grasses, and agricultural residues.
The importance of biomass varies significantly across regions as shown in Table 1.
Bioenergy, the energy from biomass, has been used for thousands of years, ever
since people started burning wood to cook food or to keep warm, and today, wood is
still our largest biomass resource for bioenergy. Many countries in the developing world
still use wood as their primary fuel (Demirbas and Demirbas, 2007). Wood is one of the
carbonaceous fuels. The carbonaceous fuels are responsible for over 80% of the world’s
energy production. Figure 1 shows carbon, hydrogen, and oxygen contents of fuels.
In the future, biomass has the potential to provide a cost-effective and sustainable
supply of energy, while at the same time aiding countries in meeting their greenhouse
gas reduction targets. By the year 2050, it is estimated that 90% of the world population
will live in developing countries. In developed countries there is a growing trend towards
employing modern technologies and efficient bio-energy conversion using a range of biofuels, which are becoming cost-wise competitive with fossil fuels (Puhan et al., 2005).
The electricity is produced by direct combustion of biomass, advanced liquefaction,
gasification, and pyrolysis technologies, which are almost ready for commercial-scale use
Address correspondence to Professor Ayhan Demirbaş, P. K. 216, TR-61035 Trabzon, Turkey.
E-mail: [email protected]
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A. Demirbas
Table 1
The importance of biomass in different world regions
Region
Share of biomass in
final energy consumption
Africa (average)
Burundi
Ethiopia
Kenya
Somalia
Sudan
Uganda
South Asia (average)
East Asia (average)
China
Latin America (average)
Europe (average)
North America (average)
Middle East (average)
62.0
93.8
85.6
69.6
86.5
83.7
94.6
56.3
25.1
23.5
18.2
3.5
2.7
0.3
(Demirbas, 1998). Residues remaining after the harvest of crop and forestry products are
being proposed as a substantial energy source for generation of electrical power. Also a
number of crops and crop residues may fit modern bioenergy chains (Haberl and Geissler,
2000; Hoogwijk et al., 2003; Reijnders, 2004; Pimentel et al., 2005).
The pyrolysis products are divided into a volatile fraction consisting of gases, vapors,
and tar components and a carbon-rich solid residue. The pyrolysis process consists of
a very complex set of reactions involving the formation of radicals. The gasification of
biomass is a thermal treatment, which results in a high production of gaseous products
and small quantities of char and ash (Demirbas, 2002a). Hydrogen is produced from solid
wastes by pyrolysis (Demirbas et al., 1996).
Figure 1. Carbon, hydrogen, and oxygen contents of fuels.
Biofuels from Agricultural Biomass
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Figure 2. Worldwide electricity generation by fuels 1980–2020 (billion kilowatt hours).
Direct combustion is the old way of using biomass. Biomass thermo-chemical conversion technologies such as liquefaction, pyrolysis, and gasification are certainly not
the most important options at present (Demirbas, 1994); combustion is responsible for
over 97% of the world’s bio-energy production (Demirbas, 2004). Faaij (2006) suggested
that gasification is commercially available with high overall efficiency of about 40–50%
when used with a combined cycle or 15–30% when used with combined heat and power
gas engine. Pyrolysis is less well developed than gasification. Faaij (2006) suggested
that pyrolysis is not commercially available. It produces 60–70% of the heat content of
bio-oil/feed stock, smaller capacities compared to gasification and anaerobic digestion.
Figure 2 shows the worldwide electricity generation by fuels for 1980–2020. Natural
gas and renewables are the fastest growing primary energy sources in the world (Demirbas, 2006a). As can be seen from Figure 2, renewable energy is a promising alternative
solution because it is clean and environmentally safe. Due to economic, environmental,
and technological changes, natural gas and renewables have become the fuels of choice
for new power plants.
Agriculture and Environment
Agricultural residues such as straws, nut shells, fruit shells, fruit seeds, plant stalks and
stovers, green leaves, and molasses are potential renewable energy resources. Current
disposal methods for these agricultural residues have caused widespread environmental
concerns. For example, disposal of rice and wheat straw by open-field burning causes air
pollution (Zhang and Zhang, 1999).
Agricultural energy or green energy production is the principal contributor in economic development of a developing country. Its economy development is based on
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A. Demirbas
agricultural production and most people live in the rural areas. Implementation of integrated community development programs is therefore very necessary. It is believed that
integrated community development contributes to push up socio-economic development
of the country.
Carbon dioxide (CO2 ) and carbon monoxide (CO) are the main greenhouse gases
associated with global warning (Demirbas, 2003b). Biomass energy generates far less air
emissions than fossil fuels, reduces the amount of waste sent to landfills, and decreases
our reliance on foreign oil. Heavy metal contents in crops depend on their bioavailability
in the soil. However, the uptake of heavy metals from soil is not a simple function of
total soil heavy metal content (Tuzen et al., 1998). Contamination has been related to
the composition of gasoline, motor oil, and car tires, and to roadside deposition of the
residues of crops materials.
Municipal solid waste (MSW) compost is increasingly used in agriculture as a soil
conditioner and also as a fertilizer. MSW would be landfilled and critics are concerned
with its often elevated metal concentrations. MSW compost has also been reported to
have high salt concentrations, which can inhibit plant growth and negatively affect soil
structure (Hargreaves et al., 2008). The potential for excessive amounts of trace metals
to contaminate the food chain through MSW compost additions is thought to depend on
the source material used in the compost and the final concentration of the metals in the
compost (He et al., 1995). MSW compost tends to have higher concentrations of metals
when sewage sludge is added with the feedstock, with the lowest metal concentrations
found in MSW compost, which has been made from source-separated waste (Richard
and Woodbury, 1992). Sewage sludge should not be added to the compost at any point
since it will raise the metal content of the compost (Richard and Woodbury, 1992).
Bioethanol
Ethanol, or ethyl alcohol, is an alcohol made by fermenting and distilling simple sugars.
Bioethanol is a fuel derived from renewable sources of feedstock; typically plants, such
as wheat, sugar beet, corn, straw, and wood. The cellulose, or carbohydrates, contained in
biomass, can be turned into bioethanol through hydrolysis, fermentation, and distillation
(Demirbas, 2006b). First, the hydrolysis process converts the plant material into sugars.
The efficiency of converting cellulose to glucose, or sugar, may depend on pretreatments
to structurally and chemically alter the material. Next, the anaerobic biological process of
fermentation converts the sugars into alcohol, usually by the action of yeast (Demirbas,
2007).
Bioethanol can be produced from a wide array of carbohydrates having a general
chemical formula of (CH2 O)n . Commercial yeasts, such as Saccharomyces ceveresiae, are
used in the fermentation of sucrose. The chemical reaction involved in forming ethanol
is composed of the hydrolysis of sucrose followed by the fermentation of simple sugars
(Baltz et al., 1982). First, as shown in Eq. (1), glucose and sucrose are formed by the
invertase enzyme in the yeast catalyzing the sucrose.
C12 H22 O11 ! C6 H12 O6 C C6 H12 O6
Sucrose
Glucose
(1)
Fructose
Second, another enzyme present in the yeast called zymase converts the glucose and the
fructose into ethanol, as seen in Eq. (2).
C6 H12 O6 ! 2C2 H5 OH C 2CO2
(2)
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Figure 3. Flow chart for the production bioethanol from cereal grain or straw.
This hydrolysis is followed by fermentation, distillation, and dehydration to yield
waterless ethanol. Figure 3 depicts the progression of biomass from basic plant matter
into ethanol and animal feed.
Bioethanol is a petrol additive/substitute. It is possible that wood, straw, and even
household wastes may be economically converted to bioethanol.
Bioethanol can be used as a 5% blend with petrol under the EU quality standard EN
228. This blend requires no engine modification and is covered by vehicle warranties.
With engine modification, bioethanol can be used at higher levels, for example, E85 (85%
bioethanol).
Gluco-amylase enzyme converts the starch into D-glucose. The enzymatic hydrolysis is then followed by fermentation, distillation, and dehydration to yield anhydrous
bioethanol. Corn (60–70% starch) is the dominant feedstock in starch-to-bioethanol
industry worldwide.
Biodiesel
Biodiesel is a non-fossil fuel alternative to petrodiesel which can be obtained from
vegetable oil and animal fats by transesterification (Demirbas, 2002b). Chemically, most
biodiesel consists of alkyl (usually methyl) esters instead of the alkanes and aromatic
hydrocarbons of petroleum-derived diesel (Ma and Hanna, 1999; Demirbas, 2003).
The biodiesel esters were characterized for their physical and fuel properties including density, viscosity, iodine value, acid value, cloud point, pure point, gross heat of
combustion, and volatility. The biodiesel fuels produced slightly lower power and torque,
and higher fuel consumption than No. 2 diesel fuel. Biodiesel is better than diesel fuel
in terms of sulfur content, flash point, aromatic content, lubricity, and biodegradability
(Bala, 2005).
The cost of biodiesels varies depending on the base stock, geographic area, variability
in crop production from season to season, the price of the crude petroleum, and other
factors. Biodiesel has over double the price of petroleum diesel. The high price of
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A. Demirbas
biodiesel is, in large part, due to the high price of the feedstock. However, biodiesel
can be made from other feedstocks, including cooking oil, palm oil, inedible tallow, pork
lard, and yellow grease (Demirbas, 2005).
Biodiesel from virgin vegetable oil reduces carbon dioxide emissions and petroleum
consumption when used in place of petroleum diesel (Carraretto et al., 2004). Biodiesel
is technically competitive with or offer technical advantages compared to conventional
petroleum diesel fuel. The presence of oxygen in biodiesel improves combustion and,
therefore, reduces hydrocarbon, carbon monoxide, and particulate emissions; oxygenated
fuels also tend to increase nitrogen oxide emissions (Demirbas, 2005; Demirbas, 2009a).
A small percentage of biodiesel can be used as an additive in low-sulfur formulations
of diesel to increase the lubricity lost when the sulfur is removed (Dunn, 2001; Dorado
et al., 2003).
Biomethanol
Methanol, is also known as “wood alcohol.” Generally, methanol is easier to find than
ethanol. Sustainable methods of methanol production are currently are not economically
viable. Methanol is produced from synthetic gas or biogas and evaluated as a fuel for
internal combustion engines. The production of methanol is a cost-intensive chemical
process. Therefore, in current conditions, only waste biomass such as old wood or biowaste is used to produce methanol (Vasudevan et al., 2005).
Biomass resources can be used to produce methanol. The pyroligneous acid obtained
from wood pyrolysis consists of about 50% methanol, acetone, phenols, and water
(Demirbas and Gullu, 1998; Gullu and Demirbas, 2001). As a renewable resource,
biomass represents a potentially inexhaustible supply of feedstock for methanol production. The composition of bio-syngas from biomass for producing methanol is presented
in Table 2.
The energy value of residues generated worldwide in agriculture and the forestproducts industry amounts to more than one-third of the total commercial primary energy
use at present (Hall et al., 1993). Bio-energy supplies can be divided into two broad
categories: (a) organic municipal waste and residues from the food and materials sectors;
and (b) dedicated energy crops plantations. Bio-energy from biomass, both residues and
energy crops, can be converted into modern energy carriers such as hydrogen, methanol,
ethanol, or electricity (Azar et al., 2003). Figure 4 shows production of biomethanol from
carbohydrates by gasification and partial oxidation with O2 and H2 O.
Table 2
Composition of bio-syngas from
biomass gasification
Constituents
% by volume
(dry and nitrogen free)
Carbon monoxide (CO)
Hydrogen (H2 )
Carbon dioxide (CO2 )
Methane (CH4 )
Ethene (C2 H4 )
28–36
22–32
21–30
8–11
2–4
Biofuels from Agricultural Biomass
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Figure 4. Biomethanol from carbohydrates by gasification and partial oxidation with oxygen gas
and water vapor.
Bio-oil and Biogas
The term bio-oil is used mainly to refer to liquid fuels. There are several reasons for
bio-oils to be considered as relevant technologies by both developing and industrialized countries. They include energy security reasons, environmental concerns, foreign
exchange savings, and socioeconomic issues related to the rural sector.
Bio-oils are liquid or gaseous fuels made from biomass materials, such as agricultural
crops, municipal wastes, and agricultural and forestry by-products via biochemical or
thermochemical processes. They can substitute conventional fuels in vehicle engines—
either totally or partially in a blend (Demirbas, 2009b). The organic fraction of almost any
form of biomass, including sewage sludge, animal wastes, and industrial effluents, can
be broken down through anaerobic digestion into methane and carbon dioxide mixture
called “biogas.” Biogas is an environmentally friendly, clean, cheap, and versatile fuel.
There is an increasing world-wide demand for energy crops and animal manures for
biogas production (Amon et al., 2007). Methane production was measured for 60 days in
1 L eudiometer batch digesters at 311 K. Maximum methane yield per hectare from late
ripening maize varieties ranged between 7,100 and 9,000 Nm3 CH4 ha 1 . The highest
methane yield per hectare was achieved from digestion of whole maize crops. Digestion
of corns only or of corn cob mix resulted in a reduction in methane yield per hectare of
70 and 43%, respectively (Amon et al., 2007).
Pyrolysis/cracking is defined as the cleavage to smaller molecules by thermal energy.
Hydrogen can be produced economically from agricultural biomass (Encinar et al.,
1998). Biomass can be thermally processed through gasification or pyrolysis to produce
hydrogen. The main gaseous products from biomass are the following (Wang et al.,
1997):
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A. Demirbas
Pyrolysis of biomass ! H2 C CO2 C CO C Gaseous and liquid hydrocarbons
Catalytic steam reforming of biomass ! H2 C CO2 C CO
Fisher-Tropsch synthesis of (H2 C CO) ! Gaseous and liquid hydrocarbons
(3)
(4)
(5)
The conventional pyrolysis of biomass is associated with the product of interest, that
is the high charcoal content, but the fast pyrolysis is associated with the products of
interest, which are tar at low temperature (675–775 K) (Bridgwater, 2003) and/or gas at
high temperature (Encinar et al., 1998).
Conclusion
Biomass is a general term for material derived from growing plants or from animal wastes.
The modernization of biomass technologies, leading to more efficient biomass production
and conversion, is one possible direction for biomass use in developing countries. It is
critical, therefore, that the biomass processes used in developing countries are sustainable.
In industrialized countries, the main biomass processes utilized in the future are
expected to be the direct combustion of residues and wastes for electricity generation,
bioethanol and biodiesel as liquid fuels, and combined heat and power production from
energy crops. The future of biomass electricity generation lies in biomass integrated
gasification/gas turbine technology, which offers high-energy conversion efficiencies.
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