ISSN: 0735-2689 print / 1549-7836 online DOI:...

Critical Reviews in Plant Sciences, 24:385–406, 2005Copyright c© Taylor & Francis Inc.ISSN: 0735-2689 print / 1549-7836 onlineDOI: 10.1080/07352680500316334

Renewable Energy from Willow Biomass Crops: Life CycleEnergy, Environmental and Economic Performance

Gregory A. KeoleianCenter for Sustainable Systems, School of Natural Resources and Environment, University of Michigan,440 Church St., Ann Arbor, Michigan

Timothy A. VolkSUNY College of Environment Science and Forestry, 133 Illick Hall, Syracuse, New York

Table of Contents

I. INTRODUCTION ........................................................................................................................................... 386A. History of Willow Use ............................................................................................................................... 387

II. WILLOW BIOLOGY AND PRODUCTION ................................................................................................... 387A. Rapid Growth of Willow ............................................................................................................................ 387B. Vegetative Propagation ............................................................................................................................... 388C. Willow Genetic Diversity and Potential ....................................................................................................... 388D. Coppicing Ability ..................................................................................................................................... 389E. Production System for Willow Biomass Crops ............................................................................................. 389

III. ENERGY AND ENVIRONMENTAL PERFORMANCE OF WILLOW BIOMASS ELECTRICITY .............. 390A. Life Cycle Assessment and Modeling .......................................................................................................... 390B. Willow Production Model .......................................................................................................................... 390C. Willow Biomass Energy Conversion Model ................................................................................................. 393D. Life Cycle Energy Performance .................................................................................................................. 394

1. Energy Analysis of Co-Firing ........................................................................................................... 3942. Energy Analysis of Direct Firing and Gasification .............................................................................. 395

E. Life Cycle Environmental Performance ....................................................................................................... 3951. Global Warming Potential ................................................................................................................ 3952. Air Pollutant Emissions .................................................................................................................... 397

F. Other Environmental and Rural Development Benefits .................................................................................. 398

IV. ECONOMICS ................................................................................................................................................. 399

V. COMPARISON WITH OTHER RENEWABLE ENERGY SOURCES ............................................................ 400A. Life Cycle Energy and Environmental Performance ...................................................................................... 400B. Land Use Intensity of Willow and Other Electricity Generating Systems ........................................................ 400C. 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, Universityof Michigan, Dana Bldg., 440 Church St., Ann Arbor, MI 48109-1041. E-mail: [email protected]

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386 G. A. KEOLEIAN AND T. A. VOLK

Short-rotation woody crops (SRWC) along with other woodybiomass feedstocks will play a significant role in a more secureand sustainable energy future for the United States and around theworld. In temperate regions, shrub willows are being developed asa SRWC because of their potential for high biomass production inshort time periods, ease of vegetative propagation, broad geneticbase, and ability to resprout after multiple harvests. Understandingand working with willow’s biology is important for the agriculturaland economic success of the system.

The energy, environmental, and economic performance of wil-low biomass production and conversion to electricity is evaluatedusing life cycle modeling methods. The net energy ratio (electricitygenerated/life cycle fossil fuel consumed) for willow ranges from 10to 13 for direct firing and gasification processes. Reductions of 70 to98 percent (compared to U.S. grid generated electricity) in green-house gas emissions as well as NOx, SO2, and particulate emissionsare achieved.

Despite willow’s multiple environmental and rural developmentbenefits, its high cost of production has limited deployment. Costswill be lowered by significant improvements in yields and produc-tion efficiency and by valuing the system’s environmental and ruraldevelopment benefits. Policies like the Conservation Reserve Pro-gram (CRP), federal biomass tax credits and renewable portfoliostandards will make willow cost competitive in the near term.

The avoided air pollution from the substitution of willow forconventional fossil fuel generated electricity has an estimated dam-age 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 onthe millions of hectares of marginal agricultural lands, improvingsite conditions, soil quality and landscape diversity. A clear advan-tage of willow biomass compared to other renewables is that it isa stock resource whereas wind and PV are intermittent. With only6 percent of the current U.S. energy consumption met by renewablesources the accelerated development of willow biomass and otherrenewable energy sources is critical to address concerns of energysecurity and environmental impacts associated with fossil fuels.

Keywords plant biology, renewable electricity, net energy, green-house gas emissions reduction, pollution prevention, lifecycle assessment

I. INTRODUCTIONThe 21st century is envisioned to become the “age of bi-

ology” as renewable biomass resources replace petroleum andother fossil fuels for energy and industrial products. Growingconcerns about national energy security, global increases in CO2

emissions, local and regional air and water pollution associatedwith the use of fossil fuels, sustainability of natural resources, anincreasing demand for biodegradable products, and need to revi-talize rural economies has made the development of sustainablyproduced biomass as a feedstock for bioenergy and bioprod-ucts a critical priority in North America, Europe, Australia andelsewhere (National Research Council, 2000; European Com-munities, 2001; Baker et al., 1999).

About 6 percent of the 104 exajoules (EJ) (98 quadrillionBTU) of energy consumed annually in the United States comes

from renewable sources (2002 dataset in Energy InformationAdministration, 2003a). Biomass in all of its forms composesnearly half of these renewable sources, making it the most uti-lized source of renewable energy. Globally biomass energy con-tributes an estimated 14 percent of total primary energy demand(IEA, 1999), although some of this resource is not being pro-duced and harvested in a sustainable manner. The stated goal inthe United States is to triple the use of biomass for bioenergyand bioproducts by 2010 (Reicher, 2000). The long-term goalin the EU is to have biomass supply 20 percent of the region’sprimary energy (IEA, 2001).

Biomass that can be converted into bioenergy and bioprod-ucts can come from a variety of sources including forests, agri-cultural crops, various residue streams, and dedicated woody orherbaceous crops. The development and deployment of woodybiomass resources has several advantages over agriculturalsources. Woody biomass is available year-round from multiplesources, so end users are not dependent on a single source of ma-terial. The net energy ratios associated with bioenergy and bio-products from woody biomass are large and positive, meaningthat considerably more energy is produced from these systemsthan is used in the form of fossil fuels to produce the biomassand generate electricity or other end products. In certain regionsof the U.S. the availability of woody biomass resources that canbe produced sustainably is greater than the potential from othersources.

Most projections of global energy use predict that biomasswill be an important component of future energy supplies, con-tributing 10 to 45 percent of the total primary energy in thecoming decades. The majority of these studies show that short-rotation woody crops (SRWC) will be the single most importantsource of biomass (Berndes et al., 2003). The size of the world-wide contribution of SRWC in the future will be strongly influ-enced by factors such as land availability, yields, and socioeco-nomic conditions that will vary from region to region. SRWC incombination with other sources of woody biomass will be usedfor heat and power generation using combustion and gasificationconversion pathways and for the production of bioproducts—liquid fuels, chemicals and advanced materials currently madefrom petroleum products—through thermochemical and biolog-ical pathways.

The United States has the potential to sustainably produceand utilize large amounts of woody biomass. There are over 40million ha of idle or surplus agricultural land available for thedeployment of SRWC (Graham, 1994). On over 200 million haof timberland (timberland is defined as “Forest land that is pro-ducing or is capable of producing crops of industrial wood andnot withdrawn from timber utilization by statute or administra-tive regulation.” (Smith et al., 2001)) in the U.S., the net annualincremental growth exceeds removals by almost 50 percent. Theratio is much higher in certain areas of the country where forestcover predominates. It is 125 percent in the Northeast and 95percent in the Northcentral U.S. (Smith et al., 2001). Estimatesindicate that of the 14.1 million tonnes per year of mill residues

RENEWABLE ENERGY FROM WILLOW BIOMASS CROPS 387

generated annually in the U.S., almost 3.5 million tonnes areavailable for production of bioenergy and bioproducts at a priceof 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). Woodybiomass will make up 67 percent of that total. Dedicated biomasscrops (both woody and herbaceous) are projected to developrapidly 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 landinto productive use depending on the yield, create thousandsof new jobs in rural areas, provide an annual carbon offset ofup to 0.30 Pg or about 60 percent of the targets prescribed inKyoto Protocol (Tuskan et al., 2001), and produce an array ofother environmental benefits. As the demand for biobased prod-ucts and bioenergy grows and conversion technologies improvein the near future, biomass crops that are selected and grownfor targeted applications in specific geographic locations willbecome a significant part of the feedstock supply.

SRWC systems involve genetically improved plant materialgrown on open or fallow agricultural land. They require intensivesite preparation, nutrient inputs, and short rotations (3 to 10years). In northern temperate areas, woody crop developmenthas focused on willow shrubs (Salix spp.) and hybrid poplar(Populus spp.), while eucalyptus (Eucalyptus spp.) has been amodel species in warmer climates.

A. History of Willow UseThe inherent benefits associated with willow have been rec-

ognized and recorded for millennia, with references to willowfor medicinal uses and basket production by both the Egyptiansand Romans (Hubbard, 1904; Stott, 1992). During the RomanEmpire willow was cultivated and used for a wide array of prod-ucts in addition to baskets including medicine, fences, and asframing for shields. Willow basket production in England goesback to at least 100 B.C. A large-scale commercial willow bas-ket industry developed rapidly in the 1820s in response to theNapoleonic blockade and flourished again during the two worldwars due to shortages of other materials (Stott, 1992). NativeAmericans in North America have understood the biology andbenefits associated with willow for a long time and passed thisknowledge from generation to generation. The ability of manyspecies of willow to coppice vigorously and develop from un-rooted cuttings was recognized and used by Native Americansas part of their management and use of the species for medic-inal purposes and as construction material for a wide array ofitems including sweat lodges, furniture, baskets, rope, whistles,arrows and nets (Moerman, 1998). In some regions, willow cut-tings were actively used to stabilize streambanks that were proneto erosion (Shipek, 1993).

European immigrants started to cultivate willow in the UnitedStates in the 1840s in western New York and Pennsylvania. Bythe late 1800s cultivation of willows for basketry and furniture

had spread from the shores of Maryland to the western bordersof Wisconsin and Illinois. New York State dominated willowcultivation at this time, with 60 percent of the total reportedcultivated area and about 45 percent of the income generatedfrom willow products (Hubbard, 1904). However, at the endof the 1800s, demand for willow was already declining due tocompetition from cheaper imported material and baskets. Sev-eral decades later only isolated pockets of willow cultivationremained.

Interest in the use of willows as a feedstock for bioenergy andbioproducts has developed over the past few decades becauseof the multiple environmental and rural development benefitsassociated with their production and use (Volk et al., 2004c;Borjesson, 1999). The cultivation of willow was revitalized inSweden in the early 1970s in response to the need for alter-native sources of domestically produced energy following theenergy crisis (Christersson et al., 1993). Since that time about17,000 ha of willow biomass crops have been established in Swe-den (Verwijst, 2001). In North America willow production wasstarted again in upstate New York in the mid 1980s. The focusin both regions was research on cultivation of willow biomasscrops as a locally produced, renewable feedstock for bioenergyand bioproducts. As the understanding of willow biology andproduction systems has grown, applications for willow have ex-panded to include various agroforestry practices, phytoremedi-ation, and bioengineering.

This chapter will provide a critical review of willow biomass,its agricultural production and use for electricity generation inthe United States. The following topics are addressed: (1) biol-ogy of willow and the characteristics that shape biomass produc-tion systems, (2) energy and environmental performance includ-ing willow biomass crop production and electricity generationprocesses, (3) the economics of willow biomass crops, and (4) acomparison of willow biomass crops with alternative renewableand nonrenewable technologies considering energy and environ-mental performance, land use intensity and economics.

II. WILLOW BIOLOGY AND PRODUCTIONWillow shrubs have several characteristics that make them

ideal for SRWC systems including the potential for high biomassproduction in short time periods, ease of vegetative propaga-tion from dormant hardwood cuttings, broad genetic base andease 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 develop-ing a production system that makes use of them is important inoptimizing the production of willow biomass crops.

A. Rapid Growth of WillowWillow’s ability to attain high biomass production levels in

just a few years is due to its ability to reach its maximum annualincrement (MAI) rapidly, tolerance of high planting densitiesand its rapid growth rate following coppicing. Many of these


traits are characteristics of plants that are adapted to disturbedenvironments. The initial planting density and the length of therotation are two important factors that strongly influence howlong it takes to reach peak mean annual increment and the viabil-ity of the stools over multiple generations. For willow biomasscrops using unimproved genetic material with planting densitiesof 14,000 to 18,000 plants ha−1, the peak mean annual incrementis reached in three to five years (Kopp et al. 1997; Willebrandand Verwijst, 1993; Willebrand et al., 1993). Three years af-ter coppicing, 32 different willow varieties planted at a densityof 18,518 plants ha−1 produced between 5 and 14 stems perstool (Tharakan et al., 2005a), so the total number of stemsper hectare can range from 92,000 to more than 250,000. Thecombination of a high planting density and longer rotations re-sults in a high self-thinning rate, impacting both the long-termviability of the stand and the potential for increased risk ofinfection in weaker and dead plants (Sennerby-Forsse, 1995;Verwijst, 1991). Recommendations for planting density and ro-tation length may need to be revised as improved genetic ma-terial that has a more upright growth form is deployed (Bullardet al., 2002a). Higher planting densities have more efficient re-source use earlier in the rotation, but result in higher establish-ment costs, which may make the economics of this approachless attractive (Bullard et al., 2002a, b). In contrast, loweringinitial planting densities would reduce establishment costs anddelay reaching the peak MAI at least in the first rotation. How-ever, using proper establishment and weed control practices tominimize initial/annual mortality in the stand and promote thedevelopment of a uniform structure could allow initial plant-ing density to be lowered to about 10,000 plants ha−1 withoutcompromising production over multiple rotations (Sennerby-Forsse, 1995). Recommendations to modify planting densitiesand rotation lengths need to be considered in the context of theentire production system since other management decisions andcosts, such as weed control and harvesting efficiency, will beaffected.

As with many facets of willow biomass production sys-tems, the inherent characteristics of the genetic material usedhas an impact on planting density and rotation length decisions(Willebrand et al., 1993; Bullard et al., 2002a). Recently com-pleted research on 32 different willow clones in central NewYork indicates that there are at least two different functionalgroups with alternative growth strategies for high biomass pro-duction in the first rotation (Tharakan et al., 2005a). The firstgroup is characterized by plants with a large number of smallstems (about 11 per stool), relatively low leaf area index (LAI)and specific leaf area (SLA), but high foliar N concentrationsand wood specific gravity. The other group is characterized byplants with a small number of large-diameter stems (about 6per stool), high LAI and SLA, but low foliar N concentrationsand wood specific gravity. As new willow varieties are deployed,recommendations for planting densities and rotation lengths willhave to be developed for the different varieties or functionalgroups.

B. Vegetative PropagationVegetative propagation from dormant shoots is another char-

acteristic of willow that is an adaptation to survival in disturbedenvironments. This characteristic greatly simplifies and speedsup the breeding, screening, and deployment process for im-proved genetic material. Fields of willow biomass crops containmixtures of different species and hybrids by planting blocksof different varieties across a field (Abrahamson et al., 2002)or random mixtures of varieties within each row (Dawson andMcCracken, 1995). Mixtures enhance structural and functionaldiversity, reduce the impact of pests and diseases and lower thepotential for widespread crop failures (McCracken and Dawson,2001).

Many species of shrub willow develop preformed root pri-mordia as part of normal growth. When planted in the properconditions, these root primordia can begin to develop into rootapical meristems in 48 hours (Fjell, 1985). Planting stock forwillow biomass crops makes use of these characteristics andconsists of unrooted hardwood cuttings that are harvested fromone-year-old shoots during the dormant season. After harvest-ing, sorting and grading, plant material, either as 1 to 2 m longwhips or 18 to 25 cm cuttings, is stored at −2 to −4◦C in plasticto avoid desiccation (Cram and Lindquist, 1982, Siren et al.,1987). Cuttings are removed from cold storage one to two daysprior to planting so the root primordia become active and >90percent of the cutting is inserted in the ground using mechanicalplanters (Abrahamson et al., 2002; Danfors et al., 1998). Willowcuttings are robust enough that they can be left out of cold stor-age for up to two weeks before being planted and still achievesurvival and first-year production rates necessary for a success-ful stand, provided that the cuttings are protected from excessiveheat or desiccation prior to planting (Volk et al., 2004b).

C. Willow Genetic Diversity and PotentialThe wide range of genetic diversity within the genus Salix

and the limited amount of domestication of willow as a cropmeans that there is a tremendous potential to increase yields ofshrub 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 reachheights of greater than 40 m. The species used in SRWC systemsare primarily shrubs from the subgenus Caprisalix (Vetrix), ofwhich there are over 125 species worldwide. While they havemany characteristics in common, growth habits, life history, andresistance to pests and diseases vary considerably. This diversityis important in the successful development and deployment ofSRWC.

Major efforts to breed S. viminalis and other shrub willows asa biomass crop have been progressing for more than 15 years inSweden (Gullberg, 1993) and the United Kingdom (Lindegaardand Barker, 1997). This research has provided a large body of in-formation on the inheritance of traits important for biomass pro-duction (Ronnberg-Wastljung et al., 1994; Ronnberg-Wastljung


and Gullberg, 1996; Ronnberg-Wastljung, 2001) and a geneticmap to support breeding for desired traits (Hanleyet al., 2002).These long-term breeding programs have produced yield in-creases of 12 to 67 percent in Sweden (Larsson, 1998; Larsson,2001) and 8 to 143 percent in the United Kingdom (Lindegaardet al., 2001). The potential to increase willow biomass cropyields is substantial and will be a key factor in reducing costs toproduce willow biomass (Tharakan et al., 2005b). Breeding inthe United States started in 1995 with the collection of a broadrange of genetic material from several continents and controlledpollinations starting in 1998. Initial results from these effortsindicate that yield increases will be similar to what has beenreported from efforts in Sweden and the UK (Kopp et al., 2002).As with any crop, moving plant material from one region toanother needs to be done carefully so new pests and diseasesare not introduced, varieties are matched to local soil and cli-mate 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 bypotato leafhopper (Empoasca fabae Harris), a major pest on al-falfa in the Midwest and Northeast U.S. However, initial resultsindicate that resistance to this insect can be bred by crossing S.viminalis with varieties from Asia (Kopp et al., 2001).

D. Coppicing AbilityPlants’ ability to resprout after being damaged or harvested

has probably developed as a response to high disturbance envi-ronments, so it is a characteristic that is often strongly expressedin pioneer species. Willow is well known for its vigorous coppic-ing ability and it has been used for centuries to manage willowfor a wide range of applications (Hubbard, 1904; Stott, 1992;Moerman, 1998).

Short-rotation woody crops that are harvested on rotations offive years or less are typically managed using a coppice system.Willow biomass cropping systems have been built around Salix’scoppicing ability. Coppicing has been reported to reinvigoratethe growth of plants, and in some cases accelerate growth to-wards the theoretical maximum (Sennerby-Forsse et al., 1992;Sennerby-Forsse, 1995). Coppicing removes apical dominance,allowing the development of secondary buds on the remain-ing stool into shoots (Paukkonen et al., 1992; Sennerby-Forsseet al., 1992). Coppicing has been shown to increase ethlyeneproduction (Taylor et al., 1982), which can also promote thedevelopment of secondary buds (Kauppi et al., 1988). Shrubwillow typically has one main and two lateral buds at each budgroup. The arrangement and development of buds varies amongspecies, and clones of the same species, so the number of stemsper stool varies (Sennerby-Forsse et al. 1992; Sennerby-Forsseand Zsuffa, 1995).

The quantity, arrangement and rapid rate of leaf area devel-oping from the large number of shoots following coppicing havebeen suggested as important factors influencing the productiv-ity of SRWC (Cannell et al., 1987; Cannell et al., 1988; Raven,

1992; Ceulemans et al., 1996). Coppicing causes the canopy todevelop rapidly because of the increased number of shoots oneach stool, changes in leaf size and specific leaf area associatedwith juvenile growth, increased net photosynthetic rates, and therapid growth of shoots (Kauppi et al., 1988; Tschaplinski andBlake, 1989; Sennerby-Forsse et al., 1992; Ceulemans et al.,1996). The cause of these changes are not well known, but theproximity of a relatively small shoot mass to a well developedroot system following coppicing has often been noted as an im-portant factor (Blake, 1983; Kauppi et al., 1988; Tschaplinskiand Blake, 1989; Sennerby-Forsse and Zsuffa, 1995). The stim-ulating effect of coppicing on leaf area development may con-tribute to the attainment of maximum biomass yields in a shortperiod of time in coppice systems (Ferm and Kauppi, 1990;Sennerby-Forsse et al., 1992). However, in some situations cop-picing willow biomass crops after their first year of growth hadno impact on the rate of leaf area development or peak leaf areacompared to uncoppiced plants (Volk, 2002).

A critical design feature for the long-term sustainability ofthe willow coppice management system is harvesting after leaffall, when the translocation of leaf, branch and stem nutrientsto the root systems for winter storage has occurred (Bollmarket al., 1999). The availability of these nutrients is essential forvigorous sprouting the following spring. Nutrients that are nottranslocated from the foliage are returned to the system in litter,which when mineralized can supply 1/3 to 2/3 of the annualnutrient demand (Ericsson et al., 1992).

E. Production System for Willow Biomass CropsDevelopment of the willow biomass production system is

based on the current understanding of the biology and ecophysi-ology of shrub willows. Further improvements in the productionsystem and the development of improved management practiceswill be based on a growing understanding of processes and prop-erties of SRWC systems. Failure to couple the development ofthe system with the biology of this diverse genus could under-mine the biological and ecological sustainability of the system.

The cultivation of willow biomass crops occurs on agricul-tural or fallow land and combines knowledge from the fields offorestry and agronomy. The system currently being deployedis based on years of research and operational experience inSweden, United States, Canada, and the United Kingdom. Itemploys a double-row configuration, which facilitates the use ofagricultural equipment, with a density of 10,000 to 20,000 plantsha−1 (Abrahamson et al., 2002; Danfors, 1998). The density se-lected depends on soil fertility, rotation length, climate condi-tions, and desired dimensions of the end product. Agricultural-type site preparation and complete weed control that starts thefall before planting is recommended for sites that are currentlyin perennial herbaceous cover. For sites that are planted in anannual crop, site preparation can begin in the spring that willowplanting will occur. On sites that are prone to erosion, cover cropscan be successfully incorporated during establishment to provide


soil protection while the willow canopy develops (Volk et al.,2004a). Planting machinery configurations include four- and six-row planters that use 100 to 200 cm dormant whips. Eighteen-to 25-cm-long sections are cut off the whips and pressed flushwith the soil surface. Careful matching of willow varieties andplanting sites is necessary to ensure the long-term production ofthe system (Verwijst et al., 1996).

Willow biomass crops are typically harvested on a three- tofour-year cycle using modified agricultural equipment that cutsand chips the biomass in a single operation. The end productcan then be delivered directly to end users. Prototype harvestersthat cut and collect the material as whole stems have been built,but are currently not commercially available. The advantage ofthis type of system is that natural drying of the stems can occurand material can be stored for a longer period of time. However,handling costs associated with the whole stem approach are gen-erally greater because the biomass is handled more often beforedelivery to the end user.

Willow plants resprout vigorously after each harvest, so sevento 10 harvests are possible from a single planting. Nutrients,particularly nitrogen in organic or inorganic form, are appliedafter each harvest with rates and types being determined by siteconditions (Mitchell et al., 1999; Ledin and Willebrand, 1995).Current recommendations in the northeastern U.S. call for theapplications of 100 kg N ha−1 once every three years (Abraham-son et al., 2002; Adegbidi et al., 2003). Nutrient additions aredesigned to replace the nutrients removed during harvest ratherthan simply trying to maximize production. The use of organicresidues as a nutrient source can transform a waste stream fromone industry into a useful product for willow biomass crops, in-crease biomass yields and improve the environmental attributesof these systems (Adegbidi et al., 2003; Labrecque et al., 1998;Heller et al., 2003).

Experimental yields of short-rotation willow as high as 24 to30 oven dry tonnes (odt) ha−1 yr−1 have been measured inSweden and North America (Adegbidi et al., 2001; Christersson,1986; Labrecque et al., 2003). Typical yields are more often inthe range of 10 to 12 odt ha−1 yr−1 with second rotation yieldsof the best producing willow clones increasing by 20 to 90 per-cent depending on the site, variety and management practices(Labrecque et al., 2003; Volk et al., 2001). Commercial yieldshave been considerably lower, about four odt ha−1 yr−1, acrossalmost 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 firstlarge-scale field trials harvested in New York in 2001 (Volk,unpublished data). Difficulties with site preparation and tendingresulting in poor establishment, ineffective weed control and im-proper nutrient management, along with the use of unimprovedclones, all contributed to these low yields. Larson et al. (1998)predicted that future commercial yields in Sweden should beabove 7.5 odt ha−1 yr−1 if these issues are addressed accordingto current management recommendations. A similar increase inyield is anticipated for large-scale operations in the U.S. Theuse 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 PERFORMANCEOF WILLOW BIOMASS ELECTRICITY

A. Life Cycle Assessment and ModelingWidespread development of bioenergy in general, and willow

biomass-for-bioenergy in particular, is contingent on the energy,environmental, and economic benefits of this renewable sourcerelative to other renewable technologies and conventional energysources (Abrahamson et al., 1998). Life cycle assessment (LCA)methodology provides a comprehensive systems-based analysisof the energy and environmental performance of a product sys-tem (International Organization for Standardization, 1997). InLCA, the material and energy inputs and outputs are quanti-fied throughout a product’s life, from raw material acquisitionthrough production, use and disposal. Potential environmentalimpacts of the product system are then assessed based on thislife cycle inventory. This section will present a comprehensivelife cycle study of a willow biomass plantation in New YorkState (Heller et al., 2003; Heller et al., 2004). Previous lifecycle or energy analysis studies of SRWC systems have consid-ered prospective poplar plantations in the United States (Mannand Spath, 1997) and both willow and poplar biomass pro-duction 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 ModelA LCA model was developed for the full willow agricul-

ture to electricity production system (Heller et al., 2003; Helleret al., 2004). In this section, the willow biomass production sys-tem developed in New York State is described and evaluatedin terms of energy performance and net greenhouse gas emis-sions of the biomass feedstock production system. The willowcropping system boundary is shown schematically in Figure 1.The willow agricultural production model uses field data col-lected during the establishment of 65 ha in western New Yorkin 2000. Table 1 gives a timeline for the major operations un-dertaken in willow field management. Willow biomass crops aregrown as a perennial with multiple harvest cycles (or rotations)occurring between successive plantings. The model base-casescenario assumes seven three-year rotations and includes oneyear of site preparation, coppicing after the first year of growthand the removal of the willow stools at the end of the final rota-tion (Abrahamson et al., 2002).

Life cycle assessment methodology follows the ISO 14040guidelines (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 asraw material extraction, large market chemical production (in-cluding ammonium sulfate), average grid electricity generation,


FIG. 1. Schematic of willow biomass production processes and inputs.

transportation fuel production, and transport emissions weretaken from Ecobalance’s Database for Environmental Analysisand Management (DEAM).

The net greenhouse effect was calculated using global warm-ing potentials from the Intergovernmental Panel on ClimateChange (IPCC, 1997) (direct effect, 100 year time horizon)(Houghton et al., 1994). Air acidification and eutrophicationimpact assessment methods followed those presented by LeidenUniversity, Centre for Environmental Science (Guinee et al.,

TABLE 1Willow biomass crop field operation timeline

Year Season Activity

0 Fall Mow, contact herbicide,plow, disk, seedcovercrop, cultipack

1 Spring Disk, cultipack, plant,pre-emergentherbicide, mechanicaland/or herbicideweed control

1 Winter 1st year coppice2 Spring Fertilize34 Winter 1st harvest5 Spring Fertilize67 Winter 2nd harvest(8–22) (Repeat 3 year cycle for

3rd–7th harvest)23 Spring/Summer Elimination of willow

stools

2000). These impact potential methods compile the contribu-tions of releases throughout the system life cycle, quantifiedrelative to a standard. The air acidification potential calculation,expressed in kg SO2 equivalents ha−1, includes air emissionsof ammonia, sulfur oxides, and nitrogen oxides. Eutrophicationpotential, expressed in kg PO4 equivalents ha−1, includes con-tributions from air and water emissions of ammonia and phos-phorous, air emissions of nitrogen oxides, and water emissionsof nitrates and phosphates. Data of nutrient run-off or leachingfrom willow biomass crops are unavailable and therefore not in-cluded in this assessment. Studies have shown that in establishedwillow crops NO3-N and NH4-N concentrations in ground waterwere generally less than 0.5 mg L−1 (Aronsson et al., 2000) andthat 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 effecton the eutrophication potential in perennial willow crops.

Energy is consumed and emissions are released in tractoroperations in each field activity, manufacture of agricultural im-plements and tractors, manufacture of fertilizer and pesticidesand transport of chemicals to the farm. Equipment manufactur-ing 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 fuelconsumption and oil consumption. Fuel consumption was mod-eled using engineering estimates from the American Society ofAgricultural Engineers (ASAE, 1999a,b) and parameters suchas tillage depth and field speed.

Based on experience at SUNY College of Environmental Sci-ence and Forestry, the assumed willow biomass yield for the firstrotation is 10 oven dry tonnes (odt) ha−1 yr−1. Thus, the first har-vest (after three years of growth) is expected to produce 30 odtha−1. Successive rotations have an additional growth advantagebecause the willow’s root system is already established. It is


TABLE 2Willow biomass characteristics

Willow biomassa

(dry weight)

Carbon 49.4%Sulfur 0.05%Oxygen 42.9%Hydrogen 6.01%Nitrogen 0.45%Chlorine 265 ppmAsh 1.24%Moisture (at harvest) ∼ 50%Heating value 19.8 MJ odkg−1

aAverage of stem samples, including bark, of three-year-old SV-1 and S-365 (willow clones still in use) fromEPRI/USDOE (2000).

expected that this will increase yields in later rotations by 30–40% (Volk et al., 2001). For successive rotations, the assumedyield is 13.6 odt ha−1 yr−1. The assumed composition and energycontent of harvested willow is presented in Table 2.

The inventory is averaged over the biomass harvested fromseven three-year rotations and includes all field preparation,planting, maintenance, and harvesting activities. Based on ex-perience in New York, the model uses average yields of 13.6odt ha−1 yr−1 and 100 kg N ha−1 added once per rotation (everythree years) in the form of ammonium sulfate. Fertilization andharvesting account for the majority of the 98.3 GJ ha−1 of pri-mary energy consumed over seven harvest rotations of willowbiomass crops as shown in Figure 2. Fertilizer manufacturingitself makes up 91 percent of the energy consumed in the fer-tilization step, while the nursery production of willow cuttings

FIG. 2. Breakdown by activity type of primary energy used in producingwillow biomass crops (Heller et al., 2003).

constitute 76 percent of the planting step. Overall establishmentof the crop and management through to the end of the first ro-tation accounts for 36 percent of the total primary energy use.Direct energy inputs of diesel fuel represent 46 percent of thetotal energy use, while indirect inputs (agricultural chemicals,machinery, nursery operation) compose the balance.

The net energy ratio (harvested biomass energy at the farmgate divided by fossil energy consumed in production) for agri-cultural production of willow biomass after the first rotation is16.6. This ratio increases to 55.3 when considering output andconsumption over the full seven rotations (Table 3). In otherwords, according to our model, 55 units of energy stored inbiomass are produced with one unit of fossil energy. The netenergy ratio for the system is directly proportional to total yieldacross the reasonable yield range based on experience in NewYork (Table 3). Biomass yield is dependent on a wide array offactors including crop genetics, soil fertility, weather, site prepa-ration, weed competition, insect and disease damage, and animalbrowse.

The net energy ratio presented here is on the high end ofvalues reported in the literature for the production of woodybiomass (see Table 4 in Matthews, 2001). Much of the rangeseen in earlier reports and the discrepancy with our estimatescan be attributed to major differences in growing and process-ing methods (for example, the inclusion of irrigation or activedrying), fertilizer application rates and biomass yield assump-tions. When the contributions from storage and drying, fenceerection and maintenance, and transportation are not includedin Matthews’ energy budget of a SRWC production system in theUK, the resulting net energy ratio is 65 (Matthews, 2001). Mannand Spath (1997) conducted a LCA study of a biomass gasifica-tion combined-cycle system fed with biomass feedstock from ahybrid poplar cropping system grown on seven-year rotations.They report a net energy ratio of 55 (recalculated to representenergy in biomass divided by energy consumed in feedstockproduction).

System greenhouse gas flows, including emissions from di-rect and indirect fuel use, N2O emissions from applied fertilizerand leaf litter, and carbon sequestration in below ground biomassand soil carbon, total to 3.7 Mg CO2 eq. ha−1 over 23 years ofwillow energy crops, or 0.68 g CO2 eq. MJ−1 of biomass energyproduced (Heller et al., 2003).

There is considerable interest in using treated wastewa-ter 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 arerecycled for beneficial use through land application (Goldstein,2000). Biomass energy crops are particularly attractive meansfor treating and utilizing biosolids: a non-food crop further re-duces the risk of causing human disease, extensive perennialroots effectively filter mineral nutrients, and, with proper con-sideration, it is possible to control the flow of heavy metals inthe system (Aronsson and Perttu, 2001). In addition, the organi-cally bound fraction of nutrients in biosolids are released slowly,


TABLE 3Effects of yield assumptions on the net energy ratio at the farm gate for willow biomass crops grown in NY

Yield parameters Model results

Starting yield Yield increase Total yield Energy ratio

(odt ha−1) (%) (GJ ha−1) (odt ha−1)Energy input(MJodt−1

bmass) (MJout MJ−1in ) % of basecase

10 (basecase) 36 5430 274 358 55.36 36 3260 165 596 33.2 60

15 36 8160 412 238 83 15110 5 4330 219 449 44.1 8010 100 7710 390 252 78.5 142

making them available longer into the SRWC rotation-cyclewhen additional amendment application is prohibitive.

We can consider hypothetical scenarios using biosolids asan alternative fertilizer source for willow plantations. Repre-sentative biosolids data from a small, rural municipality (LittleValley, 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 regula-tory guidelines (New York State Department of EnvironmentalConservation, 1999).

Biosolids introduce organic matter to the soil, resulting inincreased soil carbon sequestration (Adegbidi et al., 2003). Onthe other hand, adding readily available carbon along with a ni-trogen source can increase N2O production (Beauchamp, 1997)(not accounted for here). Higher ammonia volatilization is ex-pected with biosolids application. An additional concern with theuse of sewage sludge biosolids is the introduction of trace heavymetals into the soil. Numerous studies from Sweden (Borjesson,1999) and Canada (Labrecque and Teodorescu, 2001) demon-strate that heavy metal leaching and/or accumulation are in-significant in willow biomass crops fertilized with biosolids.Salix clones show marked specificity in taking up some heavymetals, and some clones have demonstrated a high capacity toaccumulate heavy metals such as cadmium and zinc (Landbergand Gerger, 1994). If accumulated concentrations in the biomass

TABLE 4Willow production alternative management scenarios

Scenario Energy ratio Global warming potentiala Air acidification potential Eutrophication potentialb

Fertilizer MJout MJ−1in Mg CO2 eq. ha−1 kg SO2 eq. ha−1 kg PO4 eq. ha−1

Ammonium sulfate (base case) 55 10.5 127.6 13Sulfur coated urea 45 11.1 77.7 13.7Biosolids (Syracuse) 73 9 306.1 65.6Biosolids (Little Valley) 80 8.5 115.2 23.7Herbicide-free 54 10.8 130.2 13.6

aDoes not include biomass carbon flows (harvested or below ground) or N2O from leaf litter.bDoes not include nutrient run-off or leaching from fertilizer additions.

are high, heavy metals could be removed from the ash throughflue gas cleaning when the biomass is combusted (Obernbergeret al., 1997), thus providing a means for concentrating and re-moving the metals as an environmental pollutant.

The herbicide free scenario presented in Table 4 involvesincreased mechanical weed control in place of chemical herbi-cides. Two additional cultivation passes prior to planting andfour total “first year weed control” cultivation passes were sub-stituted for post-emergent and pre-emergent herbicide applica-tions. While field trials would need to be conducted to demon-strate the effectiveness of weed control under this scenario (it isassumed that yield is unaffected), the results suggest that such ascenario has minimal affect on system energy consumption andtracked emissions.

C. Willow Biomass Energy Conversion ModelWillow biomass can be converted into a variety of energy

forms and carriers. It can be combusted in a boiler to pro-duce 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 repre-sents another possible option in the future. A wide range of tech-nologies and configurations exist for converting willow biomassto electricity. Willow biomass can be co-fired with coal and/orwood residue, direct fired, or gasified. The system boundaries for


the LCA shown in Figure 3 include all operations required forproduction of willow biomass, coal mining and processing, coaland biomass transportation, manufacturing of co-firing retrofitequipment, and the avoided operations of wood residue landfilldisposal. The functional unit is one MWh of generated electricitydelivered to the grid.

The co-firing model is based on anticipated operations at theDunkirk Power Plant Unit #1. Originally built in the late 1940s,this is a tangentially-fired pulverized coal boiler with uncon-trolled SO2 emissions and low NOx burners with close-coupledoverfire air (OFA) NOx controls. The anticipated co-firing rateat the Dunkirk plant is 10 to 15 percent. Parameters for co-firingoperation are estimates based largely on experience at other co-firing power plants (EPRI/DOE, 1997). The addition of biomasshandling equipment is expected to result in a slight increase inplant parasitic load, although the increase in load from biomasshandling will be largely offset by reduced coal pulverizer ac-tivity (Lindsey and Volk, 2000). There is an expected slightdecrease in boiler efficiency due to the higher moisture contentof biomass compared with coal. The energy conversion por-tion of the life cycle inventory makes use of the model devel-oped by Mann and Spath and described in detail in Spath et al.(1999) and Mann and Spath (2001), with modifications notedhere.

At co-firing rates above about 2 percent, modest power plantmodifications are necessary to accommodate receiving, han-dling, and feeding of biomass fuel (EPRI/DOE, 1997). Approx-imate material requirements for the co-firing retrofit, based on

FIG. 3. Schematic of biomass energy system including agricultural production, biomass handling, and power generation via co-firing with coal and/or woodresidue or gasification (Heller et al., 2004).

experience at Dunkirk were included in the inventory (Helleret al., 2004). Manufacture of the original coal-fired boiler wasnot included in this LCA. Approximations based on previouscoal-powered boiler LCAs (Spath et al., 1999) suggest that ex-cluding plant manufacturing affects major system indicators (en-ergy, global warming potential) by less than 1 percent.

Baseline emissions for the coal-only (no co-firing) operationwere established using the average of 1996, 1997, and 1998emissions for Dunkirk Unit #1 as reported by EPA (2000) andemission calculations provided by NREL (Spath et al., 1999).

Changes in power plant air emissions for co-firing scenariosare based primarily on fuel-bound effects; i.e., SO2 emissionsdecrease to the extent that biomass fuel has lower sulfur con-tent than coal. It is expected that co-firing will provide an addi-tional reduction in NOx emissions due to the higher volatility andmoisture content of biomass. Correlations presented by Tillmanpredict a 16.1 percent and 26.4 percent reduction in NOx emis-sions at 10 percent and 15 percent co-fire, respectively (Tillman,2000a). These effects depend largely on boiler configuration andoperation, however.

D. Life Cycle Energy Performance1. Energy Analysis of Co-Firing

Generating electricity with coal alone consumes 11500MJ/MWhelectricity across the full life cycle, 93 percent of whichis coal used directly at the plant (Heller et al., 2004). Due to thelarge quantity of coal processed, upstream energy consumption


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 dominatedby coal mining, transportation, and losses as shown in Figure 4.Substituting biomass for coal decreases this energy consump-tion somewhat, but also introduces additional upstream energyconsumption in producing the biomass. As a result, total up-stream energy consumption remains nearly unchanged with co-firing. Note that if coal haul losses are not included, upstreamenergy consumption for the no-co-fire case decreases to 273MJ/MWhelec, while the residue/willow blend and all-willow co-fire are 320 and 304 MJ/MWhelec, respectively.

Net energy ratio, defined as the electricity produced by thesystem divided by the total fossil fuel energy consumed, is auseful indicator of system performance. The net energy ratiofor the no-co-fire case is 0.313. This increases to 0.341 with10 percent co-firing, with little difference in net energy ratiobetween the two co-firing scenarios.

2. Energy Analysis of Direct Firing and GasificationThe full benefits of willow biomass energy become more

apparent when examining direct fired or gasified willow to elec-tricity rather then co-firing with coal. The net energy ratio fordirect firing using an EPRI model for combustion indicated anet energy ratio of 9.9. In the case of gasification, the net en-ergy ratios based on a NREL gasifier model and EPRI gasifiermodel were 13.3 and 12.8, respectively. The high net energy ra-tios demonstrate the tremendous fossil energy leveraging of this

renewable energy source. For example, 13 units of electricity aregenerated for every one unit of fossil energy consumed acrossthe full life cycle of willow gasification.

E. Life Cycle Environmental Performance1. Global Warming Potential

A predominant environmental benefit of biomass energy is itsapparent carbon neutrality with respect to the atmosphere: thatis, the CO2 emitted in utilizing the biomass energy is balancedby the CO2 absorbed in growing the biomass crop, resultingin no net increase in atmospheric CO2. However, other sourcesof CO2 emissions that exist in the system (tractor operation,fertilizer manufacturing, etc.) must be considered. In addition,emissions of other greenhouse gases, such as N2O, will alsocontribute to the net global warming potential of the system. Onthe other hand, there are potential carbon storage pools in thewillow coppice system that deserve attention. Here we demon-strate the relative magnitudes of the various contributors to thesystem net global warming potential in an attempt to highlighttheir respective importance. Table 5 presents estimates for thesegreenhouse gas flows per hectare of willow plantation, accumu-lated over seven rotations (23 years).

Greenhouse gas (GHG) emissions resulting from diesel fuelcombustion and the manufacture of agricultural inputs are pre-dominantly CO2; together these emissions are equivalent to 1.3percent of the carbon that is harvested as biomass. Considering


TABLE 5Greenhouse gas (GHG) flowsa per hectare over seven rotations

for the base case (ammonium sulfate fertilizer)

Other GHGCO2 Mg CO2 eq. ha−1 Totals

Diesel fuel 3.12 0.06 3.18Ag. inputsb 2.97 0.4 3.37N2O from applied N +3.97 (±3.17)c 3.97N2O from leaf litter +7.28 (±5.83)c 7.28C sequestration −14.1

Below-ground −14.1biomass

Soil carbon 0 0Net total −8.01 +11.7 (±9.0)c 3.7Harvested biomass −499.2 −499.2

aPositive values indicate additions (releases) to the atmosphere.bIncludes fertilizer and herbicide manufacturing and transport, ma-

chinery manufacturing, and nursery operations.cBracketed numbers represent the N2O emission range presented by

the IPCC (1997) estimate.

only the CO2 emissions from diesel combustion and agriculturalinputs, 0.31 g C is emitted per MJ of biomass produced.Matthews (2001) reports a carbon emissions coefficient of1.4 g C MJ.−1 However, if Matthews’ value is adjustedby 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 per-cent of the greenhouse gas emissions included in “agriculturalinputs.”

As indicated, estimates of N2O emissions are based on IPCCguidelines and do not account for differences that may exist dueto fertilizer type, soil type and/or drainage, or other site specificparameters. Efforts have been made to determine the effect offertilizer type on N2O emissions (for example, see the reviewby Harrison and Webb, 2001) and in general, empirical emis-sion measurements from anhydrous ammonia (2 to 5 percent ap-plied N) are higher than the IPCC estimate while emissions fromammonium-, urea-, and nitrate-based fertilizers tend to be on thelow end of the IPCC estimate range. However, weather, timing offertilizer application, and possibly soil type (drainage) are largefactors. N2O emissions are generally higher under wet condi-tions (spring application, poorly drained soil) as denitrificationis an anaerobic process. Review of empirical studies suggeststhat emissions from nitrate-based fertilizers can be significantlygreater than ammonium-based fertilizers (e.g., ammonium sul-fate) under wet conditions. Specific reports of emissions fromammonium sulfate would suggest that spring application of am-monium sulfate would result in emissions at or below the lowend of the IPCC range (i.e., <0.25% of applied N) (Harrisonand Webb, 2001). Thus the N2O emission estimates presentedin 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−1yr−1). Typically, the nutrients contained inleaf litter are considered to remain within the system, becomingavailable for successive plant growth as the leaves decompose.However, the microbial processes that cause decomposition canalso result in atmospheric losses as N2O. While losses are small(again, likely overestimated by the IPCC correlation), the quan-tity of leaf litter in SRWC as well as the high global warmingpotential of N2O (296 times that of CO2) amplify the effect.Still, our upper-bound estimates of N2O emissions from leaflitter amount to only 1.5 percent (uncertainty range: 0.3 to 2.6percent) of the harvested biomass on a CO2-equivalents basis.

Below-ground biomass in the form of coarse roots and stoolspresents a short-term (1 to 2 decades), reversible carbon storagepool. Coarse root biomass increases as the willow stand matures,but is expected to reach a relatively stable level in mature plan-tations with minor variation due to above ground harvest cycles.Thus, unlike above ground biomass that accumulates with suc-cessive harvests, below ground biomass maintains a steady statefor 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 siteis re-planted to willow, growth of a new root system will offsetCO2 emissions from decomposing old roots and stools, but noadditional net accumulation as below ground biomass will berealized. Sequestration on this time scale may be relevant underfuture carbon emission trading scenarios. The values presentedhere are based on the limited data available, but are intendedto provide an order-of-magnitude estimate of below ground se-questration potential.

Our inventory assumes no net change in soil carbon in thewillow system. The potential to sequester carbon in soil un-der SRWC systems is site-specific and is dependent on factorssuch as former and current management practices, climate, andsoil characteristics. Heavy tillage can result in decreases in soilorganic matter. A site history of conventional tillage without suf-ficient reintroduction of organic matter through crop residues,cover cropping or manure can lead to a significant depletionof soil organic matter (Reicosky et al., 1995). Introduction ofSRWC on such a site would likely result in increases in soilorganic matter due to reduced tillage and inputs of leaf litterand fine root mass. On the other hand, converting grasslands or,in the extreme, peat bogs which are high in organic matter, toSRWC may result in decreases in soil organic carbon (Matthews,2001; Boman and Turbull, 1997). Furthermore, it is expectedthat soils will reach a steady state of carbon content slowly, ona decade time scale, making measurement of change difficult.Short-term changes in soil carbon under perennial bioenergycrops have been reported (Zan et al., 2001; Tolbert et al., 2002),but the long-term significance of these changes remain uncer-tain. West and Marland (2002) report a carbon sequestration rateupon switching from a history of conventional tillage to no-till,averaged across a variety of crops and over an average exper-iment duration of 17 years. Their reported value (337 ± 108


kg C ha−1 yr−1) amounts to 24.7 ± 7.9 Mg CO2 ha−1 over 20years. Achieving such a level of sequestration in SRWC woulddominate the other GHG flows in the system, resulting in a netdecrease in global warming potential. It is important to note,however, that sequestration of carbon in the soil is neither per-manent nor constant.

In our model, generating electricity from coal alone releases978 kg CO2 eq. MWh−1, 95 percent of which is released inthe form of CO2 at the power plant. Mining and transporta-tion of the coal compose the balance, contributing 42.8 and7.6 kg CO2 eq. MWh−1, respectively. Figure 5 demonstrateshow the greenhouse gas emissions are distributed in the co-firing scenarios. GHG emissions from coal mining, transport,and combustion are reduced by replacing coal with biomass dur-ing co-firing. CO2 absorbed in the growing of willow biomassis credited to the system in the “willow production” step, butis directly offset by the willow biomass contribution in “powerplant emissions,” resulting in a neutral contribution. Contribu-tions of CO2 emissions from tractor operation during willowproduction, as well as N2O emissions from fertilized agricul-tural soils, are relatively small (see Heller et al., 2003, for de-tails). While the system is not credited for CO2 absorption dur-ing the growth of woody residues, a credit is taken for landfill

FIG. 5. Distribution of greenhouse gas emissions by life cycle stages and contributing gas species (Heller et al., 2004).

methane and CO2 emissions avoided in utilizing the residues inco-firing.

2. Air Pollutant EmissionsCo-firing biomass provides a significant reduction in SO2

emissions, due to the low sulfur content of biomass. NOx emis-sion predictions based on fuel-bound N demonstrate an increasein NOx emissions when co-firing the furniture residue/willowblend (Table 6). This is due to the high nitrogen content of thefurniture wood residues. A 5 percent reduction in total systemNOx emissions is realized with a 10 percent co-fire of all wil-low (Table 6). The generic residue scenario provides a similarreduction 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 emis-sions at a 10 percent co-fire. This results in total system emis-sions of 1440 and 1456 g NOxMWh−1 for the residue/willowblend and the all-willow scenario, respectively (15 per-cent and 14 percent system reduction relative to no co-fire,respectively).

The model predicts an approximate 6 percent reduction inparticulate emissions with 10 percent co-firing, all of which isrealized through reduced coal mining operation (Table 6). The


TABLE 6Total system air pollutants, global warming potential, and net energy ratio for biomass co-firing, dedicated willow biomass

electricity generation, and other renewable energy sources

CO(g CO/

MWhelec)

NOx

(g NO2/MWhelec)

SO2

(g SO2/MWhelec)

Non-methanehydrocarbons(g/MWhelec)

Particulates(unspecified size)

(g/MWhelec)

Global warmingpotential(kg CO2

eq./MWhelec)

Netenergyratio

10% co-fire, residue &willow blenda

152.4(−3.2%)

1868(9.7%)

13670(−9.6%)

84.1(−10.1%)b

707.6(−6.3%)

905.7(−7.4%)

0.341

10% co-fire, allwillowa

160.1(1.7%)

1614(−5.2%)

13675(−9.5%)

86.4(−7.6%)b

705.8(−6.6%)

882.7(−9.9%)

0.342

Average US gridc 416.8 3329.9 3207.5 44.1 2136.0 989.1 0.257Willow production & transport with. . . d

NREL gasifier 69.9(−83.2%)

645.0(−80.6%)

373.9(−88.3%)

524.6(1089%)

34.0(−98.4%)

38.9(−96.1%)

13.3

EPRI gasifier 277.3(−33.5%)

816.6(−75.5%)

941.9(−70.6%)

105.5(139.1%)

>31.4(−98.5%)e

40.2(−96.0%)

12.9

EPRI direct-fired 1769.2(324.4%)

278.9(−91.6%)

>161.0(−95.0%)e

235.9(434.6%)

>40.8(−98.1%)e

52.3(−94.7%)

9.9

BIPVd, f 43.3(−89.6%)

247.6(−92.6%)

512.2(−84.0%)

67.5(53.0%)

574.8(−73.1%)

59.4g

(−94.0%)4.3

Windd,h na 30(−99.1%)

20(−99.4%)

na na 9.7g

(−99.0%)30.3

aParentheses are percent change relative to coal-only (no co-fire) operation.bBiomass contribution to stack emissions.cTEAM database, version 3.0 (Ecobalance).d Parentheses are percent change relative to average US grid.ePower plant stack emissions not specified; values shown are from feedstock production and transportation only.f BIPV = building integrated photovoltaic. Reference: (Keoleian and Lewis, 2003).gGlobal warming potential contains CO2 contributions only.hSchleisner (2000).

model assumes that there is no change in power plant particulateemissions with co-firing of biomass, which was the case duringa week-long co-firing test at the Dunkirk power plant.

Co-firing biomass provides significant reductions in mercuryemissions. Metal analysis of willow biomass indicates an aver-age mercury content of 0.005 mg odkg−1 (EPRI/DOE 2000) (Hgcontent not available for residue biomass). Assuming that all thebiomass-derived Hg volatilizes during combustion, a 10 percentco-fire of all willow would reduce the system air emissions ofHg by 8.4 percent. Contributions of mercury from willow pro-duction are negligible. Note that there is a potential for biomassto contain other heavy metals of concern. Metal content in thebiomass is a function of metal concentrations in the plantationsoil and plant uptake efficiency (Labrecque and Teodorescu,2001; Landberg and Gerger, 1994).

Table 6 contains the estimated emissions for dedicated wil-low biomass to electricity systems, using both direct-fired andgasification conversion technologies. In these scenarios, it isassumed that willow biomass supplies all of the feedstock en-

ergy to the power plant. Emissions from the production andtransportation of willow are combined with power plant emis-sion factors. Significant pollution prevention (relative to the cur-rent U.S. electricity grid) is realized with biomass-generatedelectricity.

F. Other Environmental and Rural Development BenefitsIn addition to the environmental benefits quantified in the

LCA, the production and use of willow biomass crops as arenewable feedstock for bioenergy and bioproducts producesa suite of environmental and rural development benefits (Volket al., 2004c). The perennial nature and extensive fine-root sys-tem of willow crops reduces soil erosion and non-point sourcepollution, promotes stable nutrient cycling and enhances soilcarbon storage in roots and the soil (Ranney and Mann, 1994;Aronsson et al., 2000; Ulzen-Appiah, 2002). Bird diversity inwillow biomass crops is similar to diversity in shrub land, suc-cessional habitat, and intact eastern forests (Dhondt and Wrege,


2003). Below ground, free living mites (Acari) have been usedas an indicator of soil biodiversity because of their high pop-ulation density, species richness, sensitivity to soil conditionsand available well-developed sampling methodologies. Threeyears after planting the density and diversity of these mites wassimilar to undisturbed old-fields, indicating that perennial wil-low biomass crops supported a diverse soil biota (Minor et al.,2004).

Willow biomass crops have the potential to revitalize ruraleconomies by diversifying farm crops, creating an alternativesource of income for landowners, and circulating energy dollarsthrough the local economy. Willow energy crops will be pro-duced in close proximity to the power plants because of theirshort supply chain due to transportation costs and short storagetime. This provides an important link and business relationshipbetween the power plant and local community businesses. Be-cause these fuels are available locally, the money that is paidto obtain their energy is spent locally, thereby encouraging thelocal economy and providing income for local businesses (IEA,2003).

Biomass as a form of renewable energy has the potential tocreate more jobs per unit of energy produced than other renew-ables or fossil fuels (ECOTEC, 1999). Projections indicate thatthe development of woody and herbaceous energy crops willcreate almost 80,000 jobs by 2020 in Europe (ECOTEC, 1999).At the regional level, about 75 direct and indirect jobs will becreated and over $520,000 per year in state and local governmentrevenue would be generated for every 4,000 ha of willow thatis planted and managed as a dedicated feedstock for bioenergy(Proakis et al., 1999).

These benefits would be substantially enhanced if willowbiomass was used successfully as a feedstock for biorefiner-ies that produce a portfolio of value-added fuels, chemicals,and materials. Biorefineries will separate willow and otherwoody biomass feedstocks into its constituent components (cel-lulose, hemicellulose and lignin) and use these componentsin different conversion and production pathways. Some of theproducts that will be produced include ethanol, biodegradableplastics and polymers, adhesives and thermosetting polymers.Process improvements leading to efficient extraction and uti-lization of the hemicellulose component alone have the poten-tial to increase the value of a ton of willow biomass by about$20.

IV. ECONOMICSDespite the numerous environmental and rural development

benefits associated with willow biomass crops, their use as afeedstock for bioenergy and bioproducts has not yet been widelyadopted in North America due to the current high cost to pro-duce and deliver SRWC to an end user. Current costs to produceand deliver SRWC are $43–52 odt−1 ($2.60–3.00 GJ−1) (Walshet al., 1996; Tharakan et al., 2005b). On an energy unit basis,

these prices are greater than commonly used fossil fuels likecoal, which for large-scale power producers in the Northeasthave costs about $1.40–1.90 GJ−1. A commercial willow en-terprise will not be viable unless the biomass price, includingincentives and subsidies, is comparable to currently used fossilfuels, and parties involved in growing, aggregating and convert-ing the biomass, are able to realize a reasonable rate of returnon their investment.

There are two pathways to make willow biomass crops eco-nomically viable. One is to lower the cost of production byreducing operating costs and increasing yields. The other is tovalue the environmental and rural development benefits associ-ated with the system. The development of willow biomass cropsis in its infancy so there is potential for significant improvementsin yields and production efficiency. The two areas with the great-est potential for immediate impact are improving yields throughtraditional breeding and reducing costs by improving harvestingefficiency. Other areas such as reducing establishment costs andimproving overall production efficiency are important as well.An 18 percent increase in yield will reduce the delivered costof willow biomass by 13 percent (Tharakan et al., 2005). Har-vesting and transportation accounts for 39 to 60 percent of thecost of willow biomass (Tharakan et al., 2005; Mitchell et al.,1999). Improving harvesting efficiency by 25 percent can reducethe delivered cost of willow by approximately $0.50/MMBtu($7.50/ton). This is an attainable goal with the ongoing devel-opment of a willow harvester based on a New Holland forageharvester.

At the other end of the spectrum, recent policy developmentsin the federal Conservation Reserve Program (CRP) and Conser-vation Reserve Enhancement Program (CREP), federal renew-able energy tax credits, and state Renewable Portfolio Standards(RPS) are policy mechanisms that are beginning to value someof the benefits associated with willow biomass crops.

Growing willow on CRP land with the landowners receivingcost share payments for establishment and annual payments foreach hectare enrolled based on soil type significantly reducesthe cost of producing willow feedstock. The delivered price ofwillow drops to $1.90 GJ−1 (Tharakan et al., 2005b), whichbegins to make it cost competitive with current coal prices. Thereduction will be greater under the CREP because cost shareand annual payments are greater than under CRP. However, onlytargeted watersheds in NY are currently eligible for CREP. Thefederal biomass tax credit for closed loop biomass, like willow,is 1.8 � c kwh−1. Making use of this tax credit can reduce thedelivered cost of willow biomass crops to $1.90 GJ−1 (Tharakanet al., 2005b).

Another recent policy development is the implementation ofrenewable portfolio standards in various states in the UnitedStates. The New York State RPS will require that 25 percentof the electricity retailed in the state comes from renewable re-sources by 2013. Currently about 19 percent of New York’spower comes from renewables, mostly hydro. Biomass from a


variety of sources, including willow biomass, will play a sig-nificant role in attaining that target. The incentives under thisprogram are currently being developed, but will be in place bythe end of 2005.

V. COMPARISON WITH OTHER RENEWABLEENERGY SOURCES

A. Life Cycle Energy and Environmental PerformanceComparisons with example LCAs of other renewable energy

technologies reveal that biomass affords relatively similar levelsof avoided pollution (Table 6). Biomass outperforms buildingintegrated photovoltaics (BIPV) from an energy perspective (aswell as the closely correlated global warming potential), butdoes not score as well as wind generation. It should be noted,however, that these studies do not take into account the reliabilityof the different generating systems. Both wind and solar areintermittent sources while biomass permits continual base loadgeneration. The net energy ratio for willow biomass electricityranges from 10 to 13, which is much higher than the net energyratio for biomass crops used for transport fuels. The net energyratio of biodiesel from soybeans is 3.2 (NREL, 1998). The netenergy ratio for corn ethanol is a controversial metric with valuesranging from 0.77 (Pimentel, 2003) to 1.3 (USDA, 2002). Thesedata clearly demonstrate that willow biomass is a significantlymore efficient crop for leveraging fossil energy resources.

Co-firing biomass reduces emissions of SO2, Hg, and (inmost cases) NOx. The extent to which biomass co-firing willreduce NOx remains case-specific, as it is dependent on not onlybiomass composition but also boiler configuration and operatingconditions. Dedicated biomass generation, which in new powerplants will most likely use gasification technology, provides sig-nificant reductions in SO2, NOx, and Hg emissions relative tothe current coal-dominated electricity mix.

Recent forecasts by the Energy Information Administration(2001) at the U.S. Department of Energy predict little to noaddition of biomass-powered electricity generation, or indeedany renewable energy sources, when tightened three-pollutant(NOx, SO2, Hg) regulations are adopted. Power generators areexpected to instead choose the less expensive option of installingemission control equipment while maintaining coal as the pri-mary fuel source. However, cases that consider stringent CO2

reductions or an aggressive RPS predict significant increases inthe use of biomass, both in co-firing operations and dedicatedbiomass power plants. When a CO2 cap at 7 percent below the1990 level is assumed, biomass co-firing is predicted to increaseto 50 billion kWh by 2020, more than 700 percent above theAnnual Energy Outlook reference case. Adopting the goal of20 percent RPS by 2020 is forecasted to increase total biomass-fueled generation to 526 billion kWh by 2020, with 85 percentof this from dedicated power plants. Under this 20 percent RPSscenario, biomass composes 10 percent of the total electricitygeneration (EIA, 2001).

B. Land Use Intensity of Willow and Other ElectricityGenerating Systems

Establishing energy crops requires arable land, and there isconcern over the availability of this resource. According to ourmodel, supplying enough biomass to support a 10 percent co-fireof all willow at the Dunkirk facility would require an estimated2925 ha of SRWC. This is 2.5 percent of the open land withsuitable soils and slopes for willow biomass production withinan 83-km radius around the Dunkirk plant (note that Dunkirk ison the shore of Lake Erie, so much of the area around the plantis unavailable) (Neuhauser et al., 1995). Operating a 100 MWgasifier at 37 percent efficiency and 80 percent capacity wouldrequire 26,865 ha of willow crops, or only 1.3 percent of the to-tal area within an 80-km radius. In an expanded biomass energymarket, however, willow energy crops will be one of manybiomass sources including urban, agricultural and forestryresidues, low value wood from natural forests, and other en-ergy crops such as switchgrass. A recent EIA report estimatesthat 3.9 to 5.8 million ha of energy crops will be needed to meetthe 20 percent RPS by 2020 projection (Haq, 2002). The re-port points out that it is possible to grow biomass energy cropson Conservation Reserve Program (CRP) land, and that thisprojected energy crop acreage represents 24 to 37 percent ofthe current allowable CRP land. In addition, acreage devoted tofarms and ranches has been declining steadily since the 1950s(USDA, 2005). Thus, land use for biomass energy crops is notexpected to conflict with land requirements for food and feedcrop production.

The land intensity of willow biomass was compared withother renewable and nonrenewable energy technologies in astudy by Spitzley and Keoleian (2004) and results are shown inFigure 6. The land intensity of biomass is significantly greaterthan other electricity generating systems and is a limiting con-straint for this renewable energy source. For example, to meetthe 100 million MWh electricity demand in the state of Michi-gan, 4.9 million ha of willow would need to be planted assuming4.9 × 105 ha-yr/kWh (willow direct fire). This figure can be con-trasted with total land area of the State, which is 14.8 millionha.

C. Economics of Willow and Other ElectricityGenerating Systems

Figure 7 presents estimates for the levelized cost of willowand alternative electricity generating technologies from a com-parative assessment by Spitzley and Keoleian (2004). This figureindicates that willow has significant economic advantages overmany other renewable technologies. While the nonrenewableelectricity generating systems have the lowest levelized coststhey pose the highest environmental damage costs from pollu-tants emitted particularly during fuel combustion. The externalcosts from air pollutant emissions for alternative electricity gen-erating systems is compared in Figure 8 (Spitzley and Keoleian,


FIG. 6. Total life cycle land area requirements for electricity generating technologies (Source: Spitzley and Keoleian, 2004).

FIG. 7. Total levelized cost of electricity for electricity generating technologies, including fuel cost (Source: Spitzley and Keoleian, 2004).


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 externalcosts ranging from $0.02 to $0.06/kWh.

VI. CONCLUSIONSThis chapter highlights the opportunities and challenges for

willow biomass crops for bioenergy. It is recognized that willowbiomass will be one of several sources of woody biomass thatwill be blended together as a feedstock for bioenergy and bio-products. Projections for the U.S. and other parts of the worldindicate that short-rotation woody crops like willow will be animportant part of future energy supplies and will help to provideenergy security and sustainability.

Willow shrubs have several characteristics that make it idealfor SRWC systems including high yields obtained in a few years,ease of vegetative propagation, a broad genetic base, a shortbreeding cycle, and ability to resprout after multiple harvests.Many of these characteristics have been know for centuries andhave been used in the management of willows for a wide rangeof products ranging from medicines to building material and ero-sion control. There are about 450 species of willow worldwide,ranging from prostrate, dwarf species to trees with heights ofgreater than 40 m. The willow used in woody crop systems areprimarily from the subgenus Caprisalix (Vetrix), which has over125 species worldwide. While they have many characteristicsin common, growth habits, life history, and resistance to pestsand diseases vary. This diversity is important in the successfuldevelopment of woody crops. Understanding the biology andecophysiology of willow biomass crops and developing the pro-

duction system using these principles is important to ensuringthe economic and biological sustainability of this system.

For a renewable energy technology to succeed in overcomingthe inertia of our current fossil based economy, the full benefitsof the technology need to be quantified so they can be valued. Inthe case of willow biomass energy there are many compellingadvantages over conventional energy systems as well as otherrenewable technologies. The high net energy ratio for willowbiomass electricity by direct fire or gasification ranges from 10to 13 MJ of electricity/MJ of fossil fuel consumed across thelife cycle system. Consequently, willow provides a tremendousopportunity to conserve fossil resources. This can be contrastedwith the net energy ratio for the current U.S. grid of 0.26. Thenet energy for the willow energy system can even be increasedfurther by substituting bio-based diesel for petroleum diesel inthe operation of farm equipment, which accounts for half of thetotal fossil input during agricultural production and substitutingbiosolids or other organic amendments for synthetic fertilizers,which require large amounts of fossil fuels to produce.

The closed carbon cycle for willow biomass production dra-matically limits greenhouse gas emissions in the range of 40–50g CO2 equivalent/kWhelec. This represents a 95 percent reduc-tion in greenhouse gas emissions relative to the current U.S. grid.Similarly, NOx, SO2, and particulate emissions are also reducedby comparable percentages.

While the energy and environmental case for willow is out-standing, current economics have limited its full market pen-etration. Much of the price differential can be attributed to“market failure.” In conventional energy markets, social and


environmental costs associated with the production and use offossil fuel use are mostly ignored, resulting in an overprovisionof fossil fuel-based energy relative to socially efficient levels.Meanwhile, the positive externalities associated with willow arenot valued, leading to an underprovision of this resource. If theexternal costs of fossil based electricity of $0.02–$0.06/kWhwere internalized in the price, then willow biomass would becompetitive.

The other approach to making willow, and other renewables,cost competitive with fossil fuels is to value their environmen-tal and rural development benefits. Results from LCAs quantifysome of these benefits but only a few, such as reduced emis-sions from power plants, are valued in the market place. Marketsfor other benefits like reductions in greenhouse gas emissionsare rapidly developing as the Kyoto Protocol is implemented,which should begin to shift the market more favorably towardsustainable energy technologies. Some benefits, such as ruraldevelopment, increased biodiversity and improved soil and wa-ter quality are harder to value. However, recent changes in stateand federal policies including renewable portfolio standards, theConservation Reserve Program, and federal energy tax credits,are in essence valuing these benefits. The combination of thesepolicies and improvements in yields and production system effi-ciencies will make willow biomass cost competitive with fossilfuels in the near term.

Willow and other sources of woody biomass must competewith the other renewables such as wind and photovoltaics. Oneclear advantage of willow biomass over these technologies isthat it is a stock resource whereas wind and PV are intermit-tent. Consequently, large-scale wind and PV pose much greaterchallenges with grid integration. Given vast differences in theregional distribution of resources and conditions such as solar ra-diation, wind speeds, and moisture that affect the developmentof renewable technologies at potential sites across the UnitedStates and globally, it is clear that dramatic growth in each ofthe renewable energy systems, including willow biomass, willbe necessary to achieve a sustainable energy future.

ACKNOWLEDGMENTSWe wish to acknowledge the life cycle modeling research

of Dr. Martin Heller and David Spitzley and Edwin White forcomments on a earlier draft of this manuscript. This chapter isbased on research funded by several sponsors including: a post-doctoral fellowship grant from the National Research InitiativeCompetitive Grants Program of the United States Departmentof Agriculture (USDA Award no. 00-35314-09998), the NewYork State Energy and Research Authority (NYSERDA), andthe United States Department of Agriculture Cooperative StateResearch Education and Extension Service.

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