Critical Reviews in Plant Sciences, 24:385–406, 2005
c Taylor & Francis Inc.
Copyright ISSN: 0735-2689 print / 1549-7836 online
DOI: 10.1080/07352680500316334
Renewable Energy from Willow Biomass Crops: Life Cycle
Energy, Environmental and Economic Performance
Gregory A. Keoleian
Center for Sustainable Systems, School of Natural Resources and Environment, University of Michigan,
440 Church St., Ann Arbor, Michigan
Timothy A. Volk
SUNY College of Environment Science and Forestry, 133 Illick Hall, Syracuse, New York
Table of Contents
I.
INTRODUCTION ........................................................................................................................................... 386
A. History of Willow Use ............................................................................................................................... 387
II.
WILLOW BIOLOGY AND PRODUCTION ................................................................................................... 387
A. Rapid Growth of Willow ............................................................................................................................ 387
B. Vegetative Propagation ............................................................................................................................... 388
C. Willow Genetic Diversity and Potential ....................................................................................................... 388
D. Coppicing Ability ..................................................................................................................................... 389
E. Production System for Willow Biomass Crops ............................................................................................. 389
III.
ENERGY AND ENVIRONMENTAL PERFORMANCE OF WILLOW BIOMASS ELECTRICITY .............. 390
A. Life Cycle Assessment and Modeling .......................................................................................................... 390
B. Willow Production Model .......................................................................................................................... 390
C. Willow Biomass Energy Conversion Model ................................................................................................. 393
D. Life Cycle Energy Performance .................................................................................................................. 394
1.
Energy Analysis of Co-Firing ........................................................................................................... 394
2.
Energy Analysis of Direct Firing and Gasification .............................................................................. 395
E. Life Cycle Environmental Performance ....................................................................................................... 395
1.
Global Warming Potential ................................................................................................................ 395
2.
Air Pollutant Emissions .................................................................................................................... 397
F. Other Environmental and Rural Development Benefits .................................................................................. 398
IV.
ECONOMICS ................................................................................................................................................. 399
V.
COMPARISON WITH OTHER RENEWABLE ENERGY SOURCES ............................................................ 400
A. Life Cycle Energy and Environmental Performance ...................................................................................... 400
B. Land Use Intensity of Willow and Other Electricity Generating Systems ........................................................ 400
C. Economics of Willow and Other Electricity Generating Systems .................................................................... 400
VI.
CONCLUSIONS ............................................................................................................................................. 402
ACKNOWLEDGMENTS ........................................................................................................................................... 403
REFERENCES .......................................................................................................................................................... 403
Address correspondence to Gregory A. Keoleian, Center for Sustainable Systems, School of Natural Resources and Environment, University
of Michigan, Dana Bldg., 440 Church St., Ann Arbor, MI 48109-1041. E-mail: [email protected]
385
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Short-rotation woody crops (SRWC) along with other woody
biomass feedstocks will play a significant role in a more secure
and sustainable energy future for the United States and around the
world. In temperate regions, shrub willows are being developed as
a SRWC because of their potential for high biomass production in
short time periods, ease of vegetative propagation, broad genetic
base, and ability to resprout after multiple harvests. Understanding
and working with willow’s biology is important for the agricultural
and economic success of the system.
The energy, environmental, and economic performance of willow biomass production and conversion to electricity is evaluated
using life cycle modeling methods. The net energy ratio (electricity
generated/life cycle fossil fuel consumed) for willow ranges from 10
to 13 for direct firing and gasification processes. Reductions of 70 to
98 percent (compared to U.S. grid generated electricity) in greenhouse gas emissions as well as NOx , SO2 , and particulate emissions
are achieved.
Despite willow’s multiple environmental and rural development
benefits, its high cost of production has limited deployment. Costs
will be lowered by significant improvements in yields and production efficiency and by valuing the system’s environmental and rural
development benefits. Policies like the Conservation Reserve Program (CRP), federal biomass tax credits and renewable portfolio
standards will make willow cost competitive in the near term.
The avoided air pollution from the substitution of willow for
conventional fossil fuel generated electricity has an estimated damage cost of $0.02 to $0.06 kWh−1 . The land intensity of about 4.9
× 10−5 ha-yr/kWh is greater than other renewable energy sources.
This may be considered the most significant limitation of willow,
but unlike other biomass crops such as corn it can be cultivated on
the millions of hectares of marginal agricultural lands, improving
site conditions, soil quality and landscape diversity. A clear advantage of willow biomass compared to other renewables is that it is
a stock resource whereas wind and PV are intermittent. With only
6 percent of the current U.S. energy consumption met by renewable
sources the accelerated development of willow biomass and other
renewable energy sources is critical to address concerns of energy
security and environmental impacts associated with fossil fuels.
Keywords
I.
plant biology, renewable electricity, net energy, greenhouse gas emissions reduction, pollution prevention, life
cycle assessment
INTRODUCTION
The 21st century is envisioned to become the “age of biology” as renewable biomass resources replace petroleum and
other fossil fuels for energy and industrial products. Growing
concerns about national energy security, global increases in CO2
emissions, local and regional air and water pollution associated
with the use of fossil fuels, sustainability of natural resources, an
increasing demand for biodegradable products, and need to revitalize rural economies has made the development of sustainably
produced biomass as a feedstock for bioenergy and bioproducts a critical priority in North America, Europe, Australia and
elsewhere (National Research Council, 2000; European Communities, 2001; Baker et al., 1999).
About 6 percent of the 104 exajoules (EJ) (98 quadrillion
BTU) of energy consumed annually in the United States comes
from renewable sources (2002 dataset in Energy Information
Administration, 2003a). Biomass in all of its forms composes
nearly half of these renewable sources, making it the most utilized source of renewable energy. Globally biomass energy contributes an estimated 14 percent of total primary energy demand
(IEA, 1999), although some of this resource is not being produced and harvested in a sustainable manner. The stated goal in
the United States is to triple the use of biomass for bioenergy
and bioproducts by 2010 (Reicher, 2000). The long-term goal
in the EU is to have biomass supply 20 percent of the region’s
primary energy (IEA, 2001).
Biomass that can be converted into bioenergy and bioproducts can come from a variety of sources including forests, agricultural crops, various residue streams, and dedicated woody or
herbaceous crops. The development and deployment of woody
biomass resources has several advantages over agricultural
sources. Woody biomass is available year-round from multiple
sources, so end users are not dependent on a single source of material. The net energy ratios associated with bioenergy and bioproducts from woody biomass are large and positive, meaning
that considerably more energy is produced from these systems
than is used in the form of fossil fuels to produce the biomass
and generate electricity or other end products. In certain regions
of the U.S. the availability of woody biomass resources that can
be produced sustainably is greater than the potential from other
sources.
Most projections of global energy use predict that biomass
will be an important component of future energy supplies, contributing 10 to 45 percent of the total primary energy in the
coming decades. The majority of these studies show that shortrotation woody crops (SRWC) will be the single most important
source of biomass (Berndes et al., 2003). The size of the worldwide contribution of SRWC in the future will be strongly influenced by factors such as land availability, yields, and socioeconomic conditions that will vary from region to region. SRWC in
combination with other sources of woody biomass will be used
for heat and power generation using combustion and gasification
conversion pathways and for the production of bioproducts—
liquid fuels, chemicals and advanced materials currently made
from petroleum products—through thermochemical and biological pathways.
The United States has the potential to sustainably produce
and utilize large amounts of woody biomass. There are over 40
million ha of idle or surplus agricultural land available for the
deployment of SRWC (Graham, 1994). On over 200 million ha
of timberland (timberland is defined as “Forest land that is producing or is capable of producing crops of industrial wood and
not withdrawn from timber utilization by statute or administrative regulation.” (Smith et al., 2001)) in the U.S., the net annual
incremental growth exceeds removals by almost 50 percent. The
ratio is much higher in certain areas of the country where forest
cover predominates. It is 125 percent in the Northeast and 95
percent in the Northcentral U.S. (Smith et al., 2001). Estimates
indicate that of the 14.1 million tonnes per year of mill residues
RENEWABLE ENERGY FROM WILLOW BIOMASS CROPS
generated annually in the U.S., almost 3.5 million tonnes are
available for production of bioenergy and bioproducts at a price
of up to $10/ton (Fehrs, 1999).
Projections indicate that biomass will provide 7.5 exajoules
(7.1 quads) of energy in the U.S. by 2025 (EIA, 2003b). Woody
biomass will make up 67 percent of that total. Dedicated biomass
crops (both woody and herbaceous) are projected to develop
rapidly in the next two decades, and will make up 32 percent
(1.6 exajoules) of the biomass resource in 2025 (EIA, 2003b).
Their deployment will put 6 to 10 million ha of unirrigated land
into productive use depending on the yield, create thousands
of new jobs in rural areas, provide an annual carbon offset of
up to 0.30 Pg or about 60 percent of the targets prescribed in
Kyoto Protocol (Tuskan et al., 2001), and produce an array of
other environmental benefits. As the demand for biobased products and bioenergy grows and conversion technologies improve
in the near future, biomass crops that are selected and grown
for targeted applications in specific geographic locations will
become a significant part of the feedstock supply.
SRWC systems involve genetically improved plant material
grown on open or fallow agricultural land. They require intensive
site preparation, nutrient inputs, and short rotations (3 to 10
years). In northern temperate areas, woody crop development
has focused on willow shrubs (Salix spp.) and hybrid poplar
(Populus spp.), while eucalyptus (Eucalyptus spp.) has been a
model species in warmer climates.
A.
History of Willow Use
The inherent benefits associated with willow have been recognized and recorded for millennia, with references to willow
for medicinal uses and basket production by both the Egyptians
and Romans (Hubbard, 1904; Stott, 1992). During the Roman
Empire willow was cultivated and used for a wide array of products in addition to baskets including medicine, fences, and as
framing for shields. Willow basket production in England goes
back to at least 100 B.C. A large-scale commercial willow basket industry developed rapidly in the 1820s in response to the
Napoleonic blockade and flourished again during the two world
wars due to shortages of other materials (Stott, 1992). Native
Americans in North America have understood the biology and
benefits associated with willow for a long time and passed this
knowledge from generation to generation. The ability of many
species of willow to coppice vigorously and develop from unrooted cuttings was recognized and used by Native Americans
as part of their management and use of the species for medicinal purposes and as construction material for a wide array of
items including sweat lodges, furniture, baskets, rope, whistles,
arrows and nets (Moerman, 1998). In some regions, willow cuttings were actively used to stabilize streambanks that were prone
to erosion (Shipek, 1993).
European immigrants started to cultivate willow in the United
States in the 1840s in western New York and Pennsylvania. By
the late 1800s cultivation of willows for basketry and furniture
387
had spread from the shores of Maryland to the western borders
of Wisconsin and Illinois. New York State dominated willow
cultivation at this time, with 60 percent of the total reported
cultivated area and about 45 percent of the income generated
from willow products (Hubbard, 1904). However, at the end
of the 1800s, demand for willow was already declining due to
competition from cheaper imported material and baskets. Several decades later only isolated pockets of willow cultivation
remained.
Interest in the use of willows as a feedstock for bioenergy and
bioproducts has developed over the past few decades because
of the multiple environmental and rural development benefits
associated with their production and use (Volk et al., 2004c;
Börjesson, 1999). The cultivation of willow was revitalized in
Sweden in the early 1970s in response to the need for alternative sources of domestically produced energy following the
energy crisis (Christersson et al., 1993). Since that time about
17,000 ha of willow biomass crops have been established in Sweden (Verwijst, 2001). In North America willow production was
started again in upstate New York in the mid 1980s. The focus
in both regions was research on cultivation of willow biomass
crops as a locally produced, renewable feedstock for bioenergy
and bioproducts. As the understanding of willow biology and
production systems has grown, applications for willow have expanded to include various agroforestry practices, phytoremediation, and bioengineering.
This chapter will provide a critical review of willow biomass,
its agricultural production and use for electricity generation in
the United States. The following topics are addressed: (1) biology of willow and the characteristics that shape biomass production systems, (2) energy and environmental performance including willow biomass crop production and electricity generation
processes, (3) the economics of willow biomass crops, and (4) a
comparison of willow biomass crops with alternative renewable
and nonrenewable technologies considering energy and environmental performance, land use intensity and economics.
II.
WILLOW BIOLOGY AND PRODUCTION
Willow shrubs have several characteristics that make them
ideal for SRWC systems including the potential for high biomass
production in short time periods, ease of vegetative propagation from dormant hardwood cuttings, broad genetic base and
ease of breeding, and ability to resprout after multiple harvests
(Christersson et al., 1993; Mitchell, 1995; Volk et al., 1999).
Understanding these inherent characteristics and then developing a production system that makes use of them is important in
optimizing the production of willow biomass crops.
A.
Rapid Growth of Willow
Willow’s ability to attain high biomass production levels in
just a few years is due to its ability to reach its maximum annual
increment (MAI) rapidly, tolerance of high planting densities
and its rapid growth rate following coppicing. Many of these
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traits are characteristics of plants that are adapted to disturbed
environments. The initial planting density and the length of the
rotation are two important factors that strongly influence how
long it takes to reach peak mean annual increment and the viability of the stools over multiple generations. For willow biomass
crops using unimproved genetic material with planting densities
of 14,000 to 18,000 plants ha−1 , the peak mean annual increment
is reached in three to five years (Kopp et al. 1997; Willebrand
and Verwijst, 1993; Willebrand et al., 1993). Three years after coppicing, 32 different willow varieties planted at a density
of 18,518 plants ha−1 produced between 5 and 14 stems per
stool (Tharakan et al., 2005a), so the total number of stems
per hectare can range from 92,000 to more than 250,000. The
combination of a high planting density and longer rotations results in a high self-thinning rate, impacting both the long-term
viability of the stand and the potential for increased risk of
infection in weaker and dead plants (Sennerby-Forsse, 1995;
Verwijst, 1991). Recommendations for planting density and rotation length may need to be revised as improved genetic material that has a more upright growth form is deployed (Bullard
et al., 2002a). Higher planting densities have more efficient resource use earlier in the rotation, but result in higher establishment costs, which may make the economics of this approach
less attractive (Bullard et al., 2002a, b). In contrast, lowering
initial planting densities would reduce establishment costs and
delay reaching the peak MAI at least in the first rotation. However, using proper establishment and weed control practices to
minimize initial/annual mortality in the stand and promote the
development of a uniform structure could allow initial planting density to be lowered to about 10,000 plants ha−1 without
compromising production over multiple rotations (SennerbyForsse, 1995). Recommendations to modify planting densities
and rotation lengths need to be considered in the context of the
entire production system since other management decisions and
costs, such as weed control and harvesting efficiency, will be
affected.
As with many facets of willow biomass production systems, the inherent characteristics of the genetic material used
has an impact on planting density and rotation length decisions
(Willebrand et al., 1993; Bullard et al., 2002a). Recently completed research on 32 different willow clones in central New
York indicates that there are at least two different functional
groups with alternative growth strategies for high biomass production in the first rotation (Tharakan et al., 2005a). The first
group is characterized by plants with a large number of small
stems (about 11 per stool), relatively low leaf area index (LAI)
and specific leaf area (SLA), but high foliar N concentrations
and wood specific gravity. The other group is characterized by
plants with a small number of large-diameter stems (about 6
per stool), high LAI and SLA, but low foliar N concentrations
and wood specific gravity. As new willow varieties are deployed,
recommendations for planting densities and rotation lengths will
have to be developed for the different varieties or functional
groups.
B.
Vegetative Propagation
Vegetative propagation from dormant shoots is another characteristic of willow that is an adaptation to survival in disturbed
environments. This characteristic greatly simplifies and speeds
up the breeding, screening, and deployment process for improved genetic material. Fields of willow biomass crops contain
mixtures of different species and hybrids by planting blocks
of different varieties across a field (Abrahamson et al., 2002)
or random mixtures of varieties within each row (Dawson and
McCracken, 1995). Mixtures enhance structural and functional
diversity, reduce the impact of pests and diseases and lower the
potential for widespread crop failures (McCracken and Dawson,
2001).
Many species of shrub willow develop preformed root primordia as part of normal growth. When planted in the proper
conditions, these root primordia can begin to develop into root
apical meristems in 48 hours (Fjell, 1985). Planting stock for
willow biomass crops makes use of these characteristics and
consists of unrooted hardwood cuttings that are harvested from
one-year-old shoots during the dormant season. After harvesting, sorting and grading, plant material, either as 1 to 2 m long
whips or 18 to 25 cm cuttings, is stored at −2 to −4◦ C in plastic
to avoid desiccation (Cram and Lindquist, 1982, Siren et al.,
1987). Cuttings are removed from cold storage one to two days
prior to planting so the root primordia become active and >90
percent of the cutting is inserted in the ground using mechanical
planters (Abrahamson et al., 2002; Danfors et al., 1998). Willow
cuttings are robust enough that they can be left out of cold storage for up to two weeks before being planted and still achieve
survival and first-year production rates necessary for a successful stand, provided that the cuttings are protected from excessive
heat or desiccation prior to planting (Volk et al., 2004b).
C.
Willow Genetic Diversity and Potential
The wide range of genetic diversity within the genus Salix
and the limited amount of domestication of willow as a crop
means that there is a tremendous potential to increase yields of
shrub willows through traditional breeding and hybridization.
There are about 450 species of willow worldwide (Argus, 1997),
ranging from prostrate, dwarf species to trees that can reach
heights of greater than 40 m. The species used in SRWC systems
are primarily shrubs from the subgenus Caprisalix (Vetrix), of
which there are over 125 species worldwide. While they have
many characteristics in common, growth habits, life history, and
resistance to pests and diseases vary considerably. This diversity
is important in the successful development and deployment of
SRWC.
Major efforts to breed S. viminalis and other shrub willows as
a biomass crop have been progressing for more than 15 years in
Sweden (Gullberg, 1993) and the United Kingdom (Lindegaard
and Barker, 1997). This research has provided a large body of information on the inheritance of traits important for biomass production (Rönnberg-Wästljung et al., 1994; Rönnberg-Wästljung
RENEWABLE ENERGY FROM WILLOW BIOMASS CROPS
and Gullberg, 1996; Rönnberg-Wästljung, 2001) and a genetic
map to support breeding for desired traits (Hanleyet al., 2002).
These long-term breeding programs have produced yield increases of 12 to 67 percent in Sweden (Larsson, 1998; Larsson,
2001) and 8 to 143 percent in the United Kingdom (Lindegaard
et al., 2001). The potential to increase willow biomass crop
yields is substantial and will be a key factor in reducing costs to
produce willow biomass (Tharakan et al., 2005b). Breeding in
the United States started in 1995 with the collection of a broad
range of genetic material from several continents and controlled
pollinations starting in 1998. Initial results from these efforts
indicate that yield increases will be similar to what has been
reported from efforts in Sweden and the UK (Kopp et al., 2002).
As with any crop, moving plant material from one region to
another needs to be done carefully so new pests and diseases
are not introduced, varieties are matched to local soil and climate conditions, and catastrophic failures do not occur. To date,
all the Swedish-bred varieties tested in the U.S., which have S.
viminalis in their background, have been severely damaged by
potato leafhopper (Empoasca fabae Harris), a major pest on alfalfa in the Midwest and Northeast U.S. However, initial results
indicate that resistance to this insect can be bred by crossing S.
viminalis with varieties from Asia (Kopp et al., 2001).
D.
Coppicing Ability
Plants’ ability to resprout after being damaged or harvested
has probably developed as a response to high disturbance environments, so it is a characteristic that is often strongly expressed
in pioneer species. Willow is well known for its vigorous coppicing ability and it has been used for centuries to manage willow
for a wide range of applications (Hubbard, 1904; Stott, 1992;
Moerman, 1998).
Short-rotation woody crops that are harvested on rotations of
five years or less are typically managed using a coppice system.
Willow biomass cropping systems have been built around Salix’s
coppicing ability. Coppicing has been reported to reinvigorate
the growth of plants, and in some cases accelerate growth towards the theoretical maximum (Sennerby-Forsse et al., 1992;
Sennerby-Forsse, 1995). Coppicing removes apical dominance,
allowing the development of secondary buds on the remaining stool into shoots (Paukkonen et al., 1992; Sennerby-Forsse
et al., 1992). Coppicing has been shown to increase ethlyene
production (Taylor et al., 1982), which can also promote the
development of secondary buds (Kauppi et al., 1988). Shrub
willow typically has one main and two lateral buds at each bud
group. The arrangement and development of buds varies among
species, and clones of the same species, so the number of stems
per stool varies (Sennerby-Forsse et al. 1992; Sennerby-Forsse
and Zsuffa, 1995).
The quantity, arrangement and rapid rate of leaf area developing from the large number of shoots following coppicing have
been suggested as important factors influencing the productivity of SRWC (Cannell et al., 1987; Cannell et al., 1988; Raven,
389
1992; Ceulemans et al., 1996). Coppicing causes the canopy to
develop rapidly because of the increased number of shoots on
each stool, changes in leaf size and specific leaf area associated
with juvenile growth, increased net photosynthetic rates, and the
rapid growth of shoots (Kauppi et al., 1988; Tschaplinski and
Blake, 1989; Sennerby-Forsse et al., 1992; Ceulemans et al.,
1996). The cause of these changes are not well known, but the
proximity of a relatively small shoot mass to a well developed
root system following coppicing has often been noted as an important factor (Blake, 1983; Kauppi et al., 1988; Tschaplinski
and Blake, 1989; Sennerby-Forsse and Zsuffa, 1995). The stimulating effect of coppicing on leaf area development may contribute to the attainment of maximum biomass yields in a short
period of time in coppice systems (Ferm and Kauppi, 1990;
Sennerby-Forsse et al., 1992). However, in some situations coppicing willow biomass crops after their first year of growth had
no impact on the rate of leaf area development or peak leaf area
compared to uncoppiced plants (Volk, 2002).
A critical design feature for the long-term sustainability of
the willow coppice management system is harvesting after leaf
fall, when the translocation of leaf, branch and stem nutrients
to the root systems for winter storage has occurred (Bollmark
et al., 1999). The availability of these nutrients is essential for
vigorous sprouting the following spring. Nutrients that are not
translocated from the foliage are returned to the system in litter,
which when mineralized can supply 1/3 to 2/3 of the annual
nutrient demand (Ericsson et al., 1992).
E.
Production System for Willow Biomass Crops
Development of the willow biomass production system is
based on the current understanding of the biology and ecophysiology of shrub willows. Further improvements in the production
system and the development of improved management practices
will be based on a growing understanding of processes and properties of SRWC systems. Failure to couple the development of
the system with the biology of this diverse genus could undermine the biological and ecological sustainability of the system.
The cultivation of willow biomass crops occurs on agricultural or fallow land and combines knowledge from the fields of
forestry and agronomy. The system currently being deployed
is based on years of research and operational experience in
Sweden, United States, Canada, and the United Kingdom. It
employs a double-row configuration, which facilitates the use of
agricultural equipment, with a density of 10,000 to 20,000 plants
ha−1 (Abrahamson et al., 2002; Danfors, 1998). The density selected depends on soil fertility, rotation length, climate conditions, and desired dimensions of the end product. Agriculturaltype site preparation and complete weed control that starts the
fall before planting is recommended for sites that are currently
in perennial herbaceous cover. For sites that are planted in an
annual crop, site preparation can begin in the spring that willow
planting will occur. On sites that are prone to erosion, cover crops
can be successfully incorporated during establishment to provide
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G. A. KEOLEIAN AND T. A. VOLK
soil protection while the willow canopy develops (Volk et al.,
2004a). Planting machinery configurations include four- and sixrow planters that use 100 to 200 cm dormant whips. Eighteento 25-cm-long sections are cut off the whips and pressed flush
with the soil surface. Careful matching of willow varieties and
planting sites is necessary to ensure the long-term production of
the system (Verwijst et al., 1996).
Willow biomass crops are typically harvested on a three- to
four-year cycle using modified agricultural equipment that cuts
and chips the biomass in a single operation. The end product
can then be delivered directly to end users. Prototype harvesters
that cut and collect the material as whole stems have been built,
but are currently not commercially available. The advantage of
this type of system is that natural drying of the stems can occur
and material can be stored for a longer period of time. However,
handling costs associated with the whole stem approach are generally greater because the biomass is handled more often before
delivery to the end user.
Willow plants resprout vigorously after each harvest, so seven
to 10 harvests are possible from a single planting. Nutrients,
particularly nitrogen in organic or inorganic form, are applied
after each harvest with rates and types being determined by site
conditions (Mitchell et al., 1999; Ledin and Willebrand, 1995).
Current recommendations in the northeastern U.S. call for the
applications of 100 kg N ha−1 once every three years (Abrahamson et al., 2002; Adegbidi et al., 2003). Nutrient additions are
designed to replace the nutrients removed during harvest rather
than simply trying to maximize production. The use of organic
residues as a nutrient source can transform a waste stream from
one industry into a useful product for willow biomass crops, increase biomass yields and improve the environmental attributes
of these systems (Adegbidi et al., 2003; Labrecque et al., 1998;
Heller et al., 2003).
Experimental yields of short-rotation willow as high as 24 to
30 oven dry tonnes (odt) ha−1 yr−1 have been measured in
Sweden and North America (Adegbidi et al., 2001; Christersson,
1986; Labrecque et al., 2003). Typical yields are more often in
the range of 10 to 12 odt ha−1 yr−1 with second rotation yields
of the best producing willow clones increasing by 20 to 90 percent depending on the site, variety and management practices
(Labrecque et al., 2003; Volk et al., 2001). Commercial yields
have been considerably lower, about four odt ha−1 yr−1 , across
almost 2,000 ha harvested over a three-year period in Sweden
(Larsson et al., 1998) and about six odt ha−1 yr−1 in the first
large-scale field trials harvested in New York in 2001 (Volk,
unpublished data). Difficulties with site preparation and tending
resulting in poor establishment, ineffective weed control and improper nutrient management, along with the use of unimproved
clones, all contributed to these low yields. Larson et al. (1998)
predicted that future commercial yields in Sweden should be
above 7.5 odt ha−1 yr−1 if these issues are addressed according
to current management recommendations. A similar increase in
yield is anticipated for large-scale operations in the U.S. The
use of improved genetic material alone will result in substantial
yield increases ranging from 8 to 143 percent (Larsson, 2001;
Lindegaard et al., 2001; Kopp et al., 2001).
III.
ENERGY AND ENVIRONMENTAL PERFORMANCE
OF WILLOW BIOMASS ELECTRICITY
A.
Life Cycle Assessment and Modeling
Widespread development of bioenergy in general, and willow
biomass-for-bioenergy in particular, is contingent on the energy,
environmental, and economic benefits of this renewable source
relative to other renewable technologies and conventional energy
sources (Abrahamson et al., 1998). Life cycle assessment (LCA)
methodology provides a comprehensive systems-based analysis
of the energy and environmental performance of a product system (International Organization for Standardization, 1997). In
LCA, the material and energy inputs and outputs are quantified throughout a product’s life, from raw material acquisition
through production, use and disposal. Potential environmental
impacts of the product system are then assessed based on this
life cycle inventory. This section will present a comprehensive
life cycle study of a willow biomass plantation in New York
State (Heller et al., 2003; Heller et al., 2004). Previous life
cycle or energy analysis studies of SRWC systems have considered prospective poplar plantations in the United States (Mann
and Spath, 1997) and both willow and poplar biomass production systems under European conditions (Rafaschieri et al.,
1999; Nonhebel, 2002; Matthews, 2001; Hanegraaf et al., 1998;
Borjesson, 1996; Dubuisson and Sintzoff, 1998).
B.
Willow Production Model
A LCA model was developed for the full willow agriculture to electricity production system (Heller et al., 2003; Heller
et al., 2004). In this section, the willow biomass production system developed in New York State is described and evaluated
in terms of energy performance and net greenhouse gas emissions of the biomass feedstock production system. The willow
cropping system boundary is shown schematically in Figure 1.
The willow agricultural production model uses field data collected during the establishment of 65 ha in western New York
in 2000. Table 1 gives a timeline for the major operations undertaken in willow field management. Willow biomass crops are
grown as a perennial with multiple harvest cycles (or rotations)
occurring between successive plantings. The model base-case
scenario assumes seven three-year rotations and includes one
year of site preparation, coppicing after the first year of growth
and the removal of the willow stools at the end of the final rotation (Abrahamson et al., 2002).
Life cycle assessment methodology follows the ISO 14040
guidelines (International Organization for Standardization,
1997). The model was developed using the software program,
Tools for Environmental Analysis and Management (TEAM),
by Ecobalance, Inc. Modules for generalized practices such as
raw material extraction, large market chemical production (including ammonium sulfate), average grid electricity generation,
RENEWABLE ENERGY FROM WILLOW BIOMASS CROPS
FIG. 1.
Schematic of willow biomass production processes and inputs.
transportation fuel production, and transport emissions were
taken from Ecobalance’s Database for Environmental Analysis
and Management (DEAM).
The net greenhouse effect was calculated using global warming potentials from the Intergovernmental Panel on Climate
Change (IPCC, 1997) (direct effect, 100 year time horizon)
(Houghton et al., 1994). Air acidification and eutrophication
impact assessment methods followed those presented by Leiden
University, Centre for Environmental Science (Guinee et al.,
TABLE 1
Willow biomass crop field operation timeline
Year
Season
Activity
0
Fall
1
Spring
1
2
3
4
5
6
7
(8–22)
Winter
Spring
Mow, contact herbicide,
plow, disk, seed
covercrop, cultipack
Disk, cultipack, plant,
pre-emergent
herbicide, mechanical
and/or herbicide
weed control
1st year coppice
Fertilize
Winter
Spring
1st harvest
Fertilize
23
Winter
391
2nd harvest
(Repeat 3 year cycle for
3rd–7th harvest)
Spring/Summer Elimination of willow
stools
2000). These impact potential methods compile the contributions of releases throughout the system life cycle, quantified
relative to a standard. The air acidification potential calculation,
expressed in kg SO2 equivalents ha−1 , includes air emissions
of ammonia, sulfur oxides, and nitrogen oxides. Eutrophication
potential, expressed in kg PO4 equivalents ha−1 , includes contributions from air and water emissions of ammonia and phosphorous, air emissions of nitrogen oxides, and water emissions
of nitrates and phosphates. Data of nutrient run-off or leaching
from willow biomass crops are unavailable and therefore not included in this assessment. Studies have shown that in established
willow crops NO3 -N and NH4 -N concentrations in ground water
were generally less than 0.5 mg L−1 (Aronsson et al., 2000) and
that total N leaching is 1–2 kg N ha−1 (Mortensen et al., 1998)
and so it is expected that this assumption will have little effect
on the eutrophication potential in perennial willow crops.
Energy is consumed and emissions are released in tractor
operations in each field activity, manufacture of agricultural implements and tractors, manufacture of fertilizer and pesticides
and transport of chemicals to the farm. Equipment manufacturing burdens were allocated to the system on a field-hour basis,
distributed over the estimated life of the tractor or implement
(1,200 to 1,500 hours for implements, 12,000 hours for tractors
(Lazarus, 2001). Operation burdens are broken down into fuel
consumption and oil consumption. Fuel consumption was modeled using engineering estimates from the American Society of
Agricultural Engineers (ASAE, 1999a,b) and parameters such
as tillage depth and field speed.
Based on experience at SUNY College of Environmental Science and Forestry, the assumed willow biomass yield for the first
rotation is 10 oven dry tonnes (odt) ha−1 yr−1 . Thus, the first harvest (after three years of growth) is expected to produce 30 odt
ha−1 . Successive rotations have an additional growth advantage
because the willow’s root system is already established. It is
392
G. A. KEOLEIAN AND T. A. VOLK
TABLE 2
Willow biomass characteristics
Willow biomassa
(dry weight)
Carbon
Sulfur
Oxygen
Hydrogen
Nitrogen
Chlorine
Ash
Moisture (at harvest)
Heating value
49.4%
0.05%
42.9%
6.01%
0.45%
265 ppm
1.24%
∼ 50%
19.8 MJ odkg−1
a
Average of stem samples, including bark, of threeyear-old SV-1 and S-365 (willow clones still in use) from
EPRI/USDOE (2000).
expected that this will increase yields in later rotations by 30–
40% (Volk et al., 2001). For successive rotations, the assumed
yield is 13.6 odt ha−1 yr−1 . The assumed composition and energy
content of harvested willow is presented in Table 2.
The inventory is averaged over the biomass harvested from
seven three-year rotations and includes all field preparation,
planting, maintenance, and harvesting activities. Based on experience in New York, the model uses average yields of 13.6
odt ha−1 yr−1 and 100 kg N ha−1 added once per rotation (every
three years) in the form of ammonium sulfate. Fertilization and
harvesting account for the majority of the 98.3 GJ ha−1 of primary energy consumed over seven harvest rotations of willow
biomass crops as shown in Figure 2. Fertilizer manufacturing
itself makes up 91 percent of the energy consumed in the fertilization step, while the nursery production of willow cuttings
FIG. 2. Breakdown by activity type of primary energy used in producing
willow biomass crops (Heller et al., 2003).
constitute 76 percent of the planting step. Overall establishment
of the crop and management through to the end of the first rotation accounts for 36 percent of the total primary energy use.
Direct energy inputs of diesel fuel represent 46 percent of the
total energy use, while indirect inputs (agricultural chemicals,
machinery, nursery operation) compose the balance.
The net energy ratio (harvested biomass energy at the farm
gate divided by fossil energy consumed in production) for agricultural production of willow biomass after the first rotation is
16.6. This ratio increases to 55.3 when considering output and
consumption over the full seven rotations (Table 3). In other
words, according to our model, 55 units of energy stored in
biomass are produced with one unit of fossil energy. The net
energy ratio for the system is directly proportional to total yield
across the reasonable yield range based on experience in New
York (Table 3). Biomass yield is dependent on a wide array of
factors including crop genetics, soil fertility, weather, site preparation, weed competition, insect and disease damage, and animal
browse.
The net energy ratio presented here is on the high end of
values reported in the literature for the production of woody
biomass (see Table 4 in Matthews, 2001). Much of the range
seen in earlier reports and the discrepancy with our estimates
can be attributed to major differences in growing and processing methods (for example, the inclusion of irrigation or active
drying), fertilizer application rates and biomass yield assumptions. When the contributions from storage and drying, fence
erection and maintenance, and transportation are not included
in Matthews’ energy budget of a SRWC production system in the
UK, the resulting net energy ratio is 65 (Matthews, 2001). Mann
and Spath (1997) conducted a LCA study of a biomass gasification combined-cycle system fed with biomass feedstock from a
hybrid poplar cropping system grown on seven-year rotations.
They report a net energy ratio of 55 (recalculated to represent
energy in biomass divided by energy consumed in feedstock
production).
System greenhouse gas flows, including emissions from direct and indirect fuel use, N2 O emissions from applied fertilizer
and leaf litter, and carbon sequestration in below ground biomass
and soil carbon, total to 3.7 Mg CO2 eq. ha−1 over 23 years of
willow energy crops, or 0.68 g CO2 eq. MJ−1 of biomass energy
produced (Heller et al., 2003).
There is considerable interest in using treated wastewater sewage sludge (biosolids) as a nutrient source in SRWC
(Pimentel, 1980; Labrecque et al., 1997; Adegbidi et al., 2003).
About half of the biosolids produced in the United States are
recycled for beneficial use through land application (Goldstein,
2000). Biomass energy crops are particularly attractive means
for treating and utilizing biosolids: a non-food crop further reduces the risk of causing human disease, extensive perennial
roots effectively filter mineral nutrients, and, with proper consideration, it is possible to control the flow of heavy metals in
the system (Aronsson and Perttu, 2001). In addition, the organically bound fraction of nutrients in biosolids are released slowly,
393
RENEWABLE ENERGY FROM WILLOW BIOMASS CROPS
TABLE 3
Effects of yield assumptions on the net energy ratio at the farm gate for willow biomass crops grown in NY
Yield parameters
Starting yield
Model results
Yield increase
Total yield
(odt ha−1 )
(%)
(GJ ha−1 )
(odt ha−1 )
Energy input
(MJodt−1
bmass )
10 (basecase)
6
15
10
10
36
36
36
5
100
5430
3260
8160
4330
7710
274
165
412
219
390
358
596
238
449
252
making them available longer into the SRWC rotation-cycle
when additional amendment application is prohibitive.
We can consider hypothetical scenarios using biosolids as
an alternative fertilizer source for willow plantations. Representative biosolids data from a small, rural municipality (Little
Valley, NY) and a more industrial municipality (Syracuse, NY)
were used. Application rates were 100 kg plant-available N,
based on calculation methods in the New York State regulatory guidelines (New York State Department of Environmental
Conservation, 1999).
Biosolids introduce organic matter to the soil, resulting in
increased soil carbon sequestration (Adegbidi et al., 2003). On
the other hand, adding readily available carbon along with a nitrogen source can increase N2 O production (Beauchamp, 1997)
(not accounted for here). Higher ammonia volatilization is expected with biosolids application. An additional concern with the
use of sewage sludge biosolids is the introduction of trace heavy
metals into the soil. Numerous studies from Sweden (Borjesson,
1999) and Canada (Labrecque and Teodorescu, 2001) demonstrate that heavy metal leaching and/or accumulation are insignificant in willow biomass crops fertilized with biosolids.
Salix clones show marked specificity in taking up some heavy
metals, and some clones have demonstrated a high capacity to
accumulate heavy metals such as cadmium and zinc (Landberg
and Gerger, 1994). If accumulated concentrations in the biomass
Energy ratio
(MJout MJ−1
in )
% of basecase
55.3
33.2
83
44.1
78.5
60
151
80
142
are high, heavy metals could be removed from the ash through
flue gas cleaning when the biomass is combusted (Obernberger
et al., 1997), thus providing a means for concentrating and removing the metals as an environmental pollutant.
The herbicide free scenario presented in Table 4 involves
increased mechanical weed control in place of chemical herbicides. Two additional cultivation passes prior to planting and
four total “first year weed control” cultivation passes were substituted for post-emergent and pre-emergent herbicide applications. While field trials would need to be conducted to demonstrate the effectiveness of weed control under this scenario (it is
assumed that yield is unaffected), the results suggest that such a
scenario has minimal affect on system energy consumption and
tracked emissions.
C.
Willow Biomass Energy Conversion Model
Willow biomass can be converted into a variety of energy
forms and carriers. It can be combusted in a boiler to produce heat for industrial applications and for district heating.
The model presented here highlights conversion to electricity.
Conversion to hydrogen for use as a transportation fuel represents another possible option in the future. A wide range of technologies and configurations exist for converting willow biomass
to electricity. Willow biomass can be co-fired with coal and/or
wood residue, direct fired, or gasified. The system boundaries for
TABLE 4
Willow production alternative management scenarios
Scenario
Fertilizer
Ammonium sulfate (base case)
Sulfur coated urea
Biosolids (Syracuse)
Biosolids (Little Valley)
Herbicide-free
a
b
Energy ratio Global warming potentiala Air acidification potential Eutrophication potentialb
MJout MJ−1
Mg CO2 eq. ha−1
kg SO2 eq. ha−1
kg PO4 eq. ha−1
in
55
45
73
80
54
10.5
11.1
9
8.5
10.8
127.6
77.7
306.1
115.2
130.2
Does not include biomass carbon flows (harvested or below ground) or N2 O from leaf litter.
Does not include nutrient run-off or leaching from fertilizer additions.
13
13.7
65.6
23.7
13.6
394
G. A. KEOLEIAN AND T. A. VOLK
the LCA shown in Figure 3 include all operations required for
production of willow biomass, coal mining and processing, coal
and biomass transportation, manufacturing of co-firing retrofit
equipment, and the avoided operations of wood residue landfill
disposal. The functional unit is one MWh of generated electricity
delivered to the grid.
The co-firing model is based on anticipated operations at the
Dunkirk Power Plant Unit #1. Originally built in the late 1940s,
this is a tangentially-fired pulverized coal boiler with uncontrolled SO2 emissions and low NOx burners with close-coupled
overfire air (OFA) NOx controls. The anticipated co-firing rate
at the Dunkirk plant is 10 to 15 percent. Parameters for co-firing
operation are estimates based largely on experience at other cofiring power plants (EPRI/DOE, 1997). The addition of biomass
handling equipment is expected to result in a slight increase in
plant parasitic load, although the increase in load from biomass
handling will be largely offset by reduced coal pulverizer activity (Lindsey and Volk, 2000). There is an expected slight
decrease in boiler efficiency due to the higher moisture content
of biomass compared with coal. The energy conversion portion of the life cycle inventory makes use of the model developed by Mann and Spath and described in detail in Spath et al.
(1999) and Mann and Spath (2001), with modifications noted
here.
At co-firing rates above about 2 percent, modest power plant
modifications are necessary to accommodate receiving, handling, and feeding of biomass fuel (EPRI/DOE, 1997). Approximate material requirements for the co-firing retrofit, based on
experience at Dunkirk were included in the inventory (Heller
et al., 2004). Manufacture of the original coal-fired boiler was
not included in this LCA. Approximations based on previous
coal-powered boiler LCAs (Spath et al., 1999) suggest that excluding plant manufacturing affects major system indicators (energy, global warming potential) by less than 1 percent.
Baseline emissions for the coal-only (no co-firing) operation
were established using the average of 1996, 1997, and 1998
emissions for Dunkirk Unit #1 as reported by EPA (2000) and
emission calculations provided by NREL (Spath et al., 1999).
Changes in power plant air emissions for co-firing scenarios
are based primarily on fuel-bound effects; i.e., SO2 emissions
decrease to the extent that biomass fuel has lower sulfur content than coal. It is expected that co-firing will provide an additional reduction in NOx emissions due to the higher volatility and
moisture content of biomass. Correlations presented by Tillman
predict a 16.1 percent and 26.4 percent reduction in NOx emissions at 10 percent and 15 percent co-fire, respectively (Tillman,
2000a). These effects depend largely on boiler configuration and
operation, however.
D. Life Cycle Energy Performance
1. Energy Analysis of Co-Firing
Generating electricity with coal alone consumes 11500
MJ/MWhelectricity across the full life cycle, 93 percent of which
is coal used directly at the plant (Heller et al., 2004). Due to the
large quantity of coal processed, upstream energy consumption
FIG. 3. Schematic of biomass energy system including agricultural production, biomass handling, and power generation via co-firing with coal and/or wood
residue or gasification (Heller et al., 2004).
RENEWABLE ENERGY FROM WILLOW BIOMASS CROPS
FIG. 4.
Distribution of upstream energy consumption (energy not used directly as fuel in the power plant) for biomass co-firing.
(i.e., energy not used directly as fuel at the plant) is dominated
by coal mining, transportation, and losses as shown in Figure 4.
Substituting biomass for coal decreases this energy consumption somewhat, but also introduces additional upstream energy
consumption in producing the biomass. As a result, total upstream energy consumption remains nearly unchanged with cofiring. Note that if coal haul losses are not included, upstream
energy consumption for the no-co-fire case decreases to 273
MJ/MWhelec , while the residue/willow blend and all-willow cofire are 320 and 304 MJ/MWhelec , respectively.
Net energy ratio, defined as the electricity produced by the
system divided by the total fossil fuel energy consumed, is a
useful indicator of system performance. The net energy ratio
for the no-co-fire case is 0.313. This increases to 0.341 with
10 percent co-firing, with little difference in net energy ratio
between the two co-firing scenarios.
2.
395
Energy Analysis of Direct Firing and Gasification
The full benefits of willow biomass energy become more
apparent when examining direct fired or gasified willow to electricity rather then co-firing with coal. The net energy ratio for
direct firing using an EPRI model for combustion indicated a
net energy ratio of 9.9. In the case of gasification, the net energy ratios based on a NREL gasifier model and EPRI gasifier
model were 13.3 and 12.8, respectively. The high net energy ratios demonstrate the tremendous fossil energy leveraging of this
renewable energy source. For example, 13 units of electricity are
generated for every one unit of fossil energy consumed across
the full life cycle of willow gasification.
E. Life Cycle Environmental Performance
1. Global Warming Potential
A predominant environmental benefit of biomass energy is its
apparent carbon neutrality with respect to the atmosphere: that
is, the CO2 emitted in utilizing the biomass energy is balanced
by the CO2 absorbed in growing the biomass crop, resulting
in no net increase in atmospheric CO2 . However, other sources
of CO2 emissions that exist in the system (tractor operation,
fertilizer manufacturing, etc.) must be considered. In addition,
emissions of other greenhouse gases, such as N2 O, will also
contribute to the net global warming potential of the system. On
the other hand, there are potential carbon storage pools in the
willow coppice system that deserve attention. Here we demonstrate the relative magnitudes of the various contributors to the
system net global warming potential in an attempt to highlight
their respective importance. Table 5 presents estimates for these
greenhouse gas flows per hectare of willow plantation, accumulated over seven rotations (23 years).
Greenhouse gas (GHG) emissions resulting from diesel fuel
combustion and the manufacture of agricultural inputs are predominantly CO2 ; together these emissions are equivalent to 1.3
percent of the carbon that is harvested as biomass. Considering
396
G. A. KEOLEIAN AND T. A. VOLK
TABLE 5
Greenhouse gas (GHG) flowsa per hectare over seven rotations
for the base case (ammonium sulfate fertilizer)
CO2
Diesel fuel
Ag. inputsb
N2 O from applied N
N2 O from leaf litter
C sequestration
Below-ground
biomass
Soil carbon
Net total
Harvested biomass
3.12
2.97
Other GHG
Mg CO2 eq. ha−1
0.06
0.4
+3.97 (±3.17)c
+7.28 (±5.83)c
−14.1
0
−8.01
−499.2
+11.7 (±9.0)c
Totals
3.18
3.37
3.97
7.28
−14.1
0
3.7
−499.2
a
Positive values indicate additions (releases) to the atmosphere.
Includes fertilizer and herbicide manufacturing and transport, machinery manufacturing, and nursery operations.
c
Bracketed numbers represent the N2 O emission range presented by
the IPCC (1997) estimate.
b
only the CO2 emissions from diesel combustion and agricultural
inputs, 0.31 g C is emitted per MJ of biomass produced.
Matthews (2001) reports a carbon emissions coefficient of
1.4 g C MJ.−1 However, if Matthews’ value is adjusted
by excluding the contributions of major system differences
(fence, storage/drying, transportation) the result is comparable
(0.37 g C MJ−1 ). Fertilizer manufacturing constitutes 75 percent of the greenhouse gas emissions included in “agricultural
inputs.”
As indicated, estimates of N2 O emissions are based on IPCC
guidelines and do not account for differences that may exist due
to fertilizer type, soil type and/or drainage, or other site specific
parameters. Efforts have been made to determine the effect of
fertilizer type on N2 O emissions (for example, see the review
by Harrison and Webb, 2001) and in general, empirical emission measurements from anhydrous ammonia (2 to 5 percent applied N) are higher than the IPCC estimate while emissions from
ammonium-, urea-, and nitrate-based fertilizers tend to be on the
low end of the IPCC estimate range. However, weather, timing of
fertilizer application, and possibly soil type (drainage) are large
factors. N2 O emissions are generally higher under wet conditions (spring application, poorly drained soil) as denitrification
is an anaerobic process. Review of empirical studies suggests
that emissions from nitrate-based fertilizers can be significantly
greater than ammonium-based fertilizers (e.g., ammonium sulfate) under wet conditions. Specific reports of emissions from
ammonium sulfate would suggest that spring application of ammonium sulfate would result in emissions at or below the low
end of the IPCC range (i.e., <0.25% of applied N) (Harrison
and Webb, 2001). Thus the N2 O emission estimates presented
in Table 5 would seem to be at the upper bounds.
Annual leaf senescence in willow biomass crops is significant
(3.8 ± 0.2 Mg ha−1 yr−1 ). Typically, the nutrients contained in
leaf litter are considered to remain within the system, becoming
available for successive plant growth as the leaves decompose.
However, the microbial processes that cause decomposition can
also result in atmospheric losses as N2 O. While losses are small
(again, likely overestimated by the IPCC correlation), the quantity of leaf litter in SRWC as well as the high global warming
potential of N2 O (296 times that of CO2 ) amplify the effect.
Still, our upper-bound estimates of N2 O emissions from leaf
litter amount to only 1.5 percent (uncertainty range: 0.3 to 2.6
percent) of the harvested biomass on a CO2 -equivalents basis.
Below-ground biomass in the form of coarse roots and stools
presents a short-term (1 to 2 decades), reversible carbon storage
pool. Coarse root biomass increases as the willow stand matures,
but is expected to reach a relatively stable level in mature plantations with minor variation due to above ground harvest cycles.
Thus, unlike above ground biomass that accumulates with successive harvests, below ground biomass maintains a steady state
for much of the crop’s lifetime. At the end of the crop’s life,
roots and stumps will likely be left in the soil to decompose,
releasing much of the accumulated carbon as CO2 . If the site
is re-planted to willow, growth of a new root system will offset
CO2 emissions from decomposing old roots and stools, but no
additional net accumulation as below ground biomass will be
realized. Sequestration on this time scale may be relevant under
future carbon emission trading scenarios. The values presented
here are based on the limited data available, but are intended
to provide an order-of-magnitude estimate of below ground sequestration potential.
Our inventory assumes no net change in soil carbon in the
willow system. The potential to sequester carbon in soil under SRWC systems is site-specific and is dependent on factors
such as former and current management practices, climate, and
soil characteristics. Heavy tillage can result in decreases in soil
organic matter. A site history of conventional tillage without sufficient reintroduction of organic matter through crop residues,
cover cropping or manure can lead to a significant depletion
of soil organic matter (Reicosky et al., 1995). Introduction of
SRWC on such a site would likely result in increases in soil
organic matter due to reduced tillage and inputs of leaf litter
and fine root mass. On the other hand, converting grasslands or,
in the extreme, peat bogs which are high in organic matter, to
SRWC may result in decreases in soil organic carbon (Matthews,
2001; Boman and Turbull, 1997). Furthermore, it is expected
that soils will reach a steady state of carbon content slowly, on
a decade time scale, making measurement of change difficult.
Short-term changes in soil carbon under perennial bioenergy
crops have been reported (Zan et al., 2001; Tolbert et al., 2002),
but the long-term significance of these changes remain uncertain. West and Marland (2002) report a carbon sequestration rate
upon switching from a history of conventional tillage to no-till,
averaged across a variety of crops and over an average experiment duration of 17 years. Their reported value (337 ± 108
RENEWABLE ENERGY FROM WILLOW BIOMASS CROPS
kg C ha−1 yr−1 ) amounts to 24.7 ± 7.9 Mg CO2 ha−1 over 20
years. Achieving such a level of sequestration in SRWC would
dominate the other GHG flows in the system, resulting in a net
decrease in global warming potential. It is important to note,
however, that sequestration of carbon in the soil is neither permanent nor constant.
In our model, generating electricity from coal alone releases
978 kg CO2 eq. MWh−1 , 95 percent of which is released in
the form of CO2 at the power plant. Mining and transportation of the coal compose the balance, contributing 42.8 and
7.6 kg CO2 eq. MWh−1 , respectively. Figure 5 demonstrates
how the greenhouse gas emissions are distributed in the cofiring scenarios. GHG emissions from coal mining, transport,
and combustion are reduced by replacing coal with biomass during co-firing. CO2 absorbed in the growing of willow biomass
is credited to the system in the “willow production” step, but
is directly offset by the willow biomass contribution in “power
plant emissions,” resulting in a neutral contribution. Contributions of CO2 emissions from tractor operation during willow
production, as well as N2 O emissions from fertilized agricultural soils, are relatively small (see Heller et al., 2003, for details). While the system is not credited for CO2 absorption during the growth of woody residues, a credit is taken for landfill
397
methane and CO2 emissions avoided in utilizing the residues in
co-firing.
2.
Air Pollutant Emissions
Co-firing biomass provides a significant reduction in SO2
emissions, due to the low sulfur content of biomass. NOx emission predictions based on fuel-bound N demonstrate an increase
in NOx emissions when co-firing the furniture residue/willow
blend (Table 6). This is due to the high nitrogen content of the
furniture wood residues. A 5 percent reduction in total system
NOx emissions is realized with a 10 percent co-fire of all willow (Table 6). The generic residue scenario provides a similar
reduction in NOx emissions due to low N content of the biomass
(data not shown). The empirical correlation presented by Tillman
(2000a) predicts a 16.1 percent reduction in stack NOx emissions at a 10 percent co-fire. This results in total system emissions of 1440 and 1456 g NOx MWh−1 for the residue/willow
blend and the all-willow scenario, respectively (15 percent and 14 percent system reduction relative to no co-fire,
respectively).
The model predicts an approximate 6 percent reduction in
particulate emissions with 10 percent co-firing, all of which is
realized through reduced coal mining operation (Table 6). The
FIG. 5. Distribution of greenhouse gas emissions by life cycle stages and contributing gas species (Heller et al., 2004).
398
G. A. KEOLEIAN AND T. A. VOLK
TABLE 6
Total system air pollutants, global warming potential, and net energy ratio for biomass co-firing, dedicated willow biomass
electricity generation, and other renewable energy sources
NOx
(g NO2 /
MWhelec )
SO2
(g SO2 /
MWhelec )
Non-methane
hydrocarbons
(g/MWhelec )
Particulates
(unspecified size)
(g/MWhelec )
Global warming
potential
(kg CO2
eq./MWhelec )
10% co-fire, residue &
152.4
1868
willow blenda
(−3.2%)
(9.7%)
10% co-fire, all
160.1
1614
willowa
(1.7%)
(−5.2%)
Average US gridc
416.8
3329.9
Willow production & transport with. . . d
NREL gasifier
69.9
645.0
(−83.2%) (−80.6%)
EPRI gasifier
277.3
816.6
(−33.5%) (−75.5%)
EPRI direct-fired
1769.2
278.9
(324.4%) (−91.6%)
BIPVd, f
43.3
247.6
(−89.6%) (−92.6%)
Windd,h
na
30
(−99.1%)
13670
(−9.6%)
13675
(−9.5%)
3207.5
84.1
(−10.1%)b
86.4
(−7.6%)b
44.1
707.6
(−6.3%)
705.8
(−6.6%)
2136.0
905.7
(−7.4%)
882.7
(−9.9%)
989.1
373.9
(−88.3%)
941.9
(−70.6%)
>161.0
(−95.0%)e
512.2
(−84.0%)
20
(−99.4%)
524.6
(1089%)
105.5
(139.1%)
235.9
(434.6%)
67.5
(53.0%)
na
34.0
(−98.4%)
>31.4
(−98.5%)e
>40.8
(−98.1%)e
574.8
(−73.1%)
na
38.9
(−96.1%)
40.2
(−96.0%)
52.3
(−94.7%)
59.4g
(−94.0%)
9.7g
(−99.0%)
CO
(g CO/
MWhelec )
Net
energy
ratio
0.341
0.342
0.257
13.3
12.9
9.9
4.3
30.3
a
Parentheses are percent change relative to coal-only (no co-fire) operation.
Biomass contribution to stack emissions.
c
TEAM database, version 3.0 (Ecobalance).
d
Parentheses are percent change relative to average US grid.
e
Power plant stack emissions not specified; values shown are from feedstock production and transportation only.
f
BIPV = building integrated photovoltaic. Reference: (Keoleian and Lewis, 2003).
g
Global warming potential contains CO2 contributions only.
h
Schleisner (2000).
b
model assumes that there is no change in power plant particulate
emissions with co-firing of biomass, which was the case during
a week-long co-firing test at the Dunkirk power plant.
Co-firing biomass provides significant reductions in mercury
emissions. Metal analysis of willow biomass indicates an average mercury content of 0.005 mg odkg−1 (EPRI/DOE 2000) (Hg
content not available for residue biomass). Assuming that all the
biomass-derived Hg volatilizes during combustion, a 10 percent
co-fire of all willow would reduce the system air emissions of
Hg by 8.4 percent. Contributions of mercury from willow production are negligible. Note that there is a potential for biomass
to contain other heavy metals of concern. Metal content in the
biomass is a function of metal concentrations in the plantation
soil and plant uptake efficiency (Labrecque and Teodorescu,
2001; Landberg and Gerger, 1994).
Table 6 contains the estimated emissions for dedicated willow biomass to electricity systems, using both direct-fired and
gasification conversion technologies. In these scenarios, it is
assumed that willow biomass supplies all of the feedstock en-
ergy to the power plant. Emissions from the production and
transportation of willow are combined with power plant emission factors. Significant pollution prevention (relative to the current U.S. electricity grid) is realized with biomass-generated
electricity.
F.
Other Environmental and Rural Development Benefits
In addition to the environmental benefits quantified in the
LCA, the production and use of willow biomass crops as a
renewable feedstock for bioenergy and bioproducts produces
a suite of environmental and rural development benefits (Volk
et al., 2004c). The perennial nature and extensive fine-root system of willow crops reduces soil erosion and non-point source
pollution, promotes stable nutrient cycling and enhances soil
carbon storage in roots and the soil (Ranney and Mann, 1994;
Aronsson et al., 2000; Ulzen-Appiah, 2002). Bird diversity in
willow biomass crops is similar to diversity in shrub land, successional habitat, and intact eastern forests (Dhondt and Wrege,
RENEWABLE ENERGY FROM WILLOW BIOMASS CROPS
2003). Below ground, free living mites (Acari) have been used
as an indicator of soil biodiversity because of their high population density, species richness, sensitivity to soil conditions
and available well-developed sampling methodologies. Three
years after planting the density and diversity of these mites was
similar to undisturbed old-fields, indicating that perennial willow biomass crops supported a diverse soil biota (Minor et al.,
2004).
Willow biomass crops have the potential to revitalize rural
economies by diversifying farm crops, creating an alternative
source of income for landowners, and circulating energy dollars
through the local economy. Willow energy crops will be produced in close proximity to the power plants because of their
short supply chain due to transportation costs and short storage
time. This provides an important link and business relationship
between the power plant and local community businesses. Because these fuels are available locally, the money that is paid
to obtain their energy is spent locally, thereby encouraging the
local economy and providing income for local businesses (IEA,
2003).
Biomass as a form of renewable energy has the potential to
create more jobs per unit of energy produced than other renewables or fossil fuels (ECOTEC, 1999). Projections indicate that
the development of woody and herbaceous energy crops will
create almost 80,000 jobs by 2020 in Europe (ECOTEC, 1999).
At the regional level, about 75 direct and indirect jobs will be
created and over $520,000 per year in state and local government
revenue would be generated for every 4,000 ha of willow that
is planted and managed as a dedicated feedstock for bioenergy
(Proakis et al., 1999).
These benefits would be substantially enhanced if willow
biomass was used successfully as a feedstock for biorefineries that produce a portfolio of value-added fuels, chemicals,
and materials. Biorefineries will separate willow and other
woody biomass feedstocks into its constituent components (cellulose, hemicellulose and lignin) and use these components
in different conversion and production pathways. Some of the
products that will be produced include ethanol, biodegradable
plastics and polymers, adhesives and thermosetting polymers.
Process improvements leading to efficient extraction and utilization of the hemicellulose component alone have the potential to increase the value of a ton of willow biomass by about
$20.
IV.
ECONOMICS
Despite the numerous environmental and rural development
benefits associated with willow biomass crops, their use as a
feedstock for bioenergy and bioproducts has not yet been widely
adopted in North America due to the current high cost to produce and deliver SRWC to an end user. Current costs to produce
and deliver SRWC are $43–52 odt−1 ($2.60–3.00 GJ−1 ) (Walsh
et al., 1996; Tharakan et al., 2005b). On an energy unit basis,
399
these prices are greater than commonly used fossil fuels like
coal, which for large-scale power producers in the Northeast
have costs about $1.40–1.90 GJ−1 . A commercial willow enterprise will not be viable unless the biomass price, including
incentives and subsidies, is comparable to currently used fossil
fuels, and parties involved in growing, aggregating and converting the biomass, are able to realize a reasonable rate of return
on their investment.
There are two pathways to make willow biomass crops economically viable. One is to lower the cost of production by
reducing operating costs and increasing yields. The other is to
value the environmental and rural development benefits associated with the system. The development of willow biomass crops
is in its infancy so there is potential for significant improvements
in yields and production efficiency. The two areas with the greatest potential for immediate impact are improving yields through
traditional breeding and reducing costs by improving harvesting
efficiency. Other areas such as reducing establishment costs and
improving overall production efficiency are important as well.
An 18 percent increase in yield will reduce the delivered cost
of willow biomass by 13 percent (Tharakan et al., 2005). Harvesting and transportation accounts for 39 to 60 percent of the
cost of willow biomass (Tharakan et al., 2005; Mitchell et al.,
1999). Improving harvesting efficiency by 25 percent can reduce
the delivered cost of willow by approximately $0.50/MMBtu
($7.50/ton). This is an attainable goal with the ongoing development of a willow harvester based on a New Holland forage
harvester.
At the other end of the spectrum, recent policy developments
in the federal Conservation Reserve Program (CRP) and Conservation Reserve Enhancement Program (CREP), federal renewable energy tax credits, and state Renewable Portfolio Standards
(RPS) are policy mechanisms that are beginning to value some
of the benefits associated with willow biomass crops.
Growing willow on CRP land with the landowners receiving
cost share payments for establishment and annual payments for
each hectare enrolled based on soil type significantly reduces
the cost of producing willow feedstock. The delivered price of
willow drops to $1.90 GJ−1 (Tharakan et al., 2005b), which
begins to make it cost competitive with current coal prices. The
reduction will be greater under the CREP because cost share
and annual payments are greater than under CRP. However, only
targeted watersheds in NY are currently eligible for CREP. The
federal biomass tax credit for closed loop biomass, like willow,
is 1.8 c kwh−1 . Making use of this tax credit can reduce the
delivered cost of willow biomass crops to $1.90 GJ−1 (Tharakan
et al., 2005b).
Another recent policy development is the implementation of
renewable portfolio standards in various states in the United
States. The New York State RPS will require that 25 percent
of the electricity retailed in the state comes from renewable resources by 2013. Currently about 19 percent of New York’s
power comes from renewables, mostly hydro. Biomass from a
400
G. A. KEOLEIAN AND T. A. VOLK
variety of sources, including willow biomass, will play a significant role in attaining that target. The incentives under this
program are currently being developed, but will be in place by
the end of 2005.
V.
A.
COMPARISON WITH OTHER RENEWABLE
ENERGY SOURCES
Life Cycle Energy and Environmental Performance
Comparisons with example LCAs of other renewable energy
technologies reveal that biomass affords relatively similar levels
of avoided pollution (Table 6). Biomass outperforms building
integrated photovoltaics (BIPV) from an energy perspective (as
well as the closely correlated global warming potential), but
does not score as well as wind generation. It should be noted,
however, that these studies do not take into account the reliability
of the different generating systems. Both wind and solar are
intermittent sources while biomass permits continual base load
generation. The net energy ratio for willow biomass electricity
ranges from 10 to 13, which is much higher than the net energy
ratio for biomass crops used for transport fuels. The net energy
ratio of biodiesel from soybeans is 3.2 (NREL, 1998). The net
energy ratio for corn ethanol is a controversial metric with values
ranging from 0.77 (Pimentel, 2003) to 1.3 (USDA, 2002). These
data clearly demonstrate that willow biomass is a significantly
more efficient crop for leveraging fossil energy resources.
Co-firing biomass reduces emissions of SO2 , Hg, and (in
most cases) NOx . The extent to which biomass co-firing will
reduce NOx remains case-specific, as it is dependent on not only
biomass composition but also boiler configuration and operating
conditions. Dedicated biomass generation, which in new power
plants will most likely use gasification technology, provides significant reductions in SO2 , NOx , and Hg emissions relative to
the current coal-dominated electricity mix.
Recent forecasts by the Energy Information Administration
(2001) at the U.S. Department of Energy predict little to no
addition of biomass-powered electricity generation, or indeed
any renewable energy sources, when tightened three-pollutant
(NOx , SO2 , Hg) regulations are adopted. Power generators are
expected to instead choose the less expensive option of installing
emission control equipment while maintaining coal as the primary fuel source. However, cases that consider stringent CO2
reductions or an aggressive RPS predict significant increases in
the use of biomass, both in co-firing operations and dedicated
biomass power plants. When a CO2 cap at 7 percent below the
1990 level is assumed, biomass co-firing is predicted to increase
to 50 billion kWh by 2020, more than 700 percent above the
Annual Energy Outlook reference case. Adopting the goal of
20 percent RPS by 2020 is forecasted to increase total biomassfueled generation to 526 billion kWh by 2020, with 85 percent
of this from dedicated power plants. Under this 20 percent RPS
scenario, biomass composes 10 percent of the total electricity
generation (EIA, 2001).
B.
Land Use Intensity of Willow and Other Electricity
Generating Systems
Establishing energy crops requires arable land, and there is
concern over the availability of this resource. According to our
model, supplying enough biomass to support a 10 percent co-fire
of all willow at the Dunkirk facility would require an estimated
2925 ha of SRWC. This is 2.5 percent of the open land with
suitable soils and slopes for willow biomass production within
an 83-km radius around the Dunkirk plant (note that Dunkirk is
on the shore of Lake Erie, so much of the area around the plant
is unavailable) (Neuhauser et al., 1995). Operating a 100 MW
gasifier at 37 percent efficiency and 80 percent capacity would
require 26,865 ha of willow crops, or only 1.3 percent of the total area within an 80-km radius. In an expanded biomass energy
market, however, willow energy crops will be one of many
biomass sources including urban, agricultural and forestry
residues, low value wood from natural forests, and other energy crops such as switchgrass. A recent EIA report estimates
that 3.9 to 5.8 million ha of energy crops will be needed to meet
the 20 percent RPS by 2020 projection (Haq, 2002). The report points out that it is possible to grow biomass energy crops
on Conservation Reserve Program (CRP) land, and that this
projected energy crop acreage represents 24 to 37 percent of
the current allowable CRP land. In addition, acreage devoted to
farms and ranches has been declining steadily since the 1950s
(USDA, 2005). Thus, land use for biomass energy crops is not
expected to conflict with land requirements for food and feed
crop production.
The land intensity of willow biomass was compared with
other renewable and nonrenewable energy technologies in a
study by Spitzley and Keoleian (2004) and results are shown in
Figure 6. The land intensity of biomass is significantly greater
than other electricity generating systems and is a limiting constraint for this renewable energy source. For example, to meet
the 100 million MWh electricity demand in the state of Michigan, 4.9 million ha of willow would need to be planted assuming
4.9 × 105 ha-yr/kWh (willow direct fire). This figure can be contrasted with total land area of the State, which is 14.8 million
ha.
C.
Economics of Willow and Other Electricity
Generating Systems
Figure 7 presents estimates for the levelized cost of willow
and alternative electricity generating technologies from a comparative assessment by Spitzley and Keoleian (2004). This figure
indicates that willow has significant economic advantages over
many other renewable technologies. While the nonrenewable
electricity generating systems have the lowest levelized costs
they pose the highest environmental damage costs from pollutants emitted particularly during fuel combustion. The external
costs from air pollutant emissions for alternative electricity generating systems is compared in Figure 8 (Spitzley and Keoleian,
RENEWABLE ENERGY FROM WILLOW BIOMASS CROPS
FIG. 6.
FIG. 7.
Total life cycle land area requirements for electricity generating technologies (Source: Spitzley and Keoleian, 2004).
Total levelized cost of electricity for electricity generating technologies, including fuel cost (Source: Spitzley and Keoleian, 2004).
401
402
G. A. KEOLEIAN AND T. A. VOLK
FIG. 8.
Total life cycle cost of pollutant damage for electricity generating technologies (Source: Spitzley and Keoleian, 2004).
2004). The fossil based technologies have the greatest external
costs ranging from $0.02 to $0.06/kWh.
VI. CONCLUSIONS
This chapter highlights the opportunities and challenges for
willow biomass crops for bioenergy. It is recognized that willow
biomass will be one of several sources of woody biomass that
will be blended together as a feedstock for bioenergy and bioproducts. Projections for the U.S. and other parts of the world
indicate that short-rotation woody crops like willow will be an
important part of future energy supplies and will help to provide
energy security and sustainability.
Willow shrubs have several characteristics that make it ideal
for SRWC systems including high yields obtained in a few years,
ease of vegetative propagation, a broad genetic base, a short
breeding cycle, and ability to resprout after multiple harvests.
Many of these characteristics have been know for centuries and
have been used in the management of willows for a wide range
of products ranging from medicines to building material and erosion control. There are about 450 species of willow worldwide,
ranging from prostrate, dwarf species to trees with heights of
greater than 40 m. The willow used in woody crop systems are
primarily from the subgenus Caprisalix (Vetrix), which has over
125 species worldwide. While they have many characteristics
in common, growth habits, life history, and resistance to pests
and diseases vary. This diversity is important in the successful
development of woody crops. Understanding the biology and
ecophysiology of willow biomass crops and developing the pro-
duction system using these principles is important to ensuring
the economic and biological sustainability of this system.
For a renewable energy technology to succeed in overcoming
the inertia of our current fossil based economy, the full benefits
of the technology need to be quantified so they can be valued. In
the case of willow biomass energy there are many compelling
advantages over conventional energy systems as well as other
renewable technologies. The high net energy ratio for willow
biomass electricity by direct fire or gasification ranges from 10
to 13 MJ of electricity/MJ of fossil fuel consumed across the
life cycle system. Consequently, willow provides a tremendous
opportunity to conserve fossil resources. This can be contrasted
with the net energy ratio for the current U.S. grid of 0.26. The
net energy for the willow energy system can even be increased
further by substituting bio-based diesel for petroleum diesel in
the operation of farm equipment, which accounts for half of the
total fossil input during agricultural production and substituting
biosolids or other organic amendments for synthetic fertilizers,
which require large amounts of fossil fuels to produce.
The closed carbon cycle for willow biomass production dramatically limits greenhouse gas emissions in the range of 40–50
g CO2 equivalent/kWhelec . This represents a 95 percent reduction in greenhouse gas emissions relative to the current U.S. grid.
Similarly, NOx , SO2 , and particulate emissions are also reduced
by comparable percentages.
While the energy and environmental case for willow is outstanding, current economics have limited its full market penetration. Much of the price differential can be attributed to
“market failure.” In conventional energy markets, social and
RENEWABLE ENERGY FROM WILLOW BIOMASS CROPS
environmental costs associated with the production and use of
fossil fuel use are mostly ignored, resulting in an overprovision
of fossil fuel-based energy relative to socially efficient levels.
Meanwhile, the positive externalities associated with willow are
not valued, leading to an underprovision of this resource. If the
external costs of fossil based electricity of $0.02–$0.06/kWh
were internalized in the price, then willow biomass would be
competitive.
The other approach to making willow, and other renewables,
cost competitive with fossil fuels is to value their environmental and rural development benefits. Results from LCAs quantify
some of these benefits but only a few, such as reduced emissions from power plants, are valued in the market place. Markets
for other benefits like reductions in greenhouse gas emissions
are rapidly developing as the Kyoto Protocol is implemented,
which should begin to shift the market more favorably toward
sustainable energy technologies. Some benefits, such as rural
development, increased biodiversity and improved soil and water quality are harder to value. However, recent changes in state
and federal policies including renewable portfolio standards, the
Conservation Reserve Program, and federal energy tax credits,
are in essence valuing these benefits. The combination of these
policies and improvements in yields and production system efficiencies will make willow biomass cost competitive with fossil
fuels in the near term.
Willow and other sources of woody biomass must compete
with the other renewables such as wind and photovoltaics. One
clear advantage of willow biomass over these technologies is
that it is a stock resource whereas wind and PV are intermittent. Consequently, large-scale wind and PV pose much greater
challenges with grid integration. Given vast differences in the
regional distribution of resources and conditions such as solar radiation, wind speeds, and moisture that affect the development
of renewable technologies at potential sites across the United
States and globally, it is clear that dramatic growth in each of
the renewable energy systems, including willow biomass, will
be necessary to achieve a sustainable energy future.
ACKNOWLEDGMENTS
We wish to acknowledge the life cycle modeling research
of Dr. Martin Heller and David Spitzley and Edwin White for
comments on a earlier draft of this manuscript. This chapter is
based on research funded by several sponsors including: a postdoctoral fellowship grant from the National Research Initiative
Competitive Grants Program of the United States Department
of Agriculture (USDA Award no. 00-35314-09998), the New
York State Energy and Research Authority (NYSERDA), and
the United States Department of Agriculture Cooperative State
Research Education and Extension Service.
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