Life-Cycle Assessment for the Production of Bioethanol from Willow Biomass Crops via Biochemical Conversion*

We conducted a life-cycle assessment (LCA) of ethanol production via bioconversion of willow biomass crop feedstock. Willow crop data were used to assess feedstock production impacts. The bioconversion process was modeled with an Aspen simulation that predicts an overall conversion yield of 310 liters of ethanol per tonne of feedstock (74 gal per US short ton). Vehicle combustion impacts were assessed using Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) models. We compared the impacts of bioconversion-produced ethanol with those of gasoline on an equivalent energy basis. We found that the life-cycle global warming potential of ethanol was slightly negative. Carbon emissions from ethanol production and use were balanced by carbon absorption in the growing willow feedstock and the displacement of fossil fuel–produced electricity with renewable electricity produced in the bioconversion process. The fossil fuel input required for producing 1 MJ of energy from ethanol was 141 percent less than that from gasoline. More water was needed to produce 1 MJ of ethanol fuel than 1 MJ of gasoline. The life-cycle water use for ethanol was 169 percent greater than for gasoline. The largest contributors to water use were the conversion process itself and the production of chemicals and materials used in the process, such as enzymes and sulfuric acid. The Energy Independence and Security Act (EISA) mandates that at least 16 billion gallons (61 billion liters) per y of cellulosic fuel be in production by the year 2022 (EISA 2007). To meet this ambitious goal, many feedstocks, with appropriate conversion technologies, will be required for fuel production. Woody biomass will play an important role in supplying feedstock for biofuels production. The National Academy of Sciences (NAS 2009) projects that 124 million dry tons (112 million tonnes) per y of woody biomass will be available for use by 2020, without compromising the environment. The Consortium for Research on Renewable Industrial Materials (CORRIM) has comprehensively assessed the life-cycle impacts of solid wood products. The current work by CORRIM expands that research portfolio to investigate production of fuels from woody biomass. In this project, the production of ethanol The authors are, respectively, Masters Student, College of the Environment, School of Environmental and Forest Sci., Univ. of Washington, Seattle ([email protected]); Masters Student, College of the Environment, School of Environmental and Forest Sci., Univ. of Washington, Seattle; Consultant, WoodLife Environmental Consultants, LLC, Corvallis, Oregon ([email protected]); PhD Candidate, PhD Student, and Senior Research Associate, College of Environmental Sci. and Forestry, State Univ. of New York, Syracuse ([email protected], [email protected], [email protected]); Denman Professor of Bioresource Sci. and Engineering, School of Forest Resources, Univ. of Washington, Seattle ([email protected] [corresponding author]); and Professor Emeritus, Univ. of Idaho, Moscow (lrkmjohnson@ frontier.com). This paper was received for publication in February 2012. Article no. 12-00022. * This article is part of a series of nine articles addressing many of the environmental performance and life-cycle issues related to the use of wood as a feedstock for bioenergy. The research reported in these articles was coordinated by the Consortium for Research on Renewable Industrial Materials (CORRIM; http://www.corrim.org). All nine articles are published in this issue of the Forest Products Journal (Vol. 62, No. 4). Forest Products Society 2012. Forest Prod. J. 62(4):305–313. FOREST PRODUCTS JOURNAL Vol. 62, No. 4 305 using bioconversion is investigated with willow biomass as the feedstock. Willow is considered a good bioconversion feedstock because the carbohydrates can be recovered with good yields without extensive pretreatment (Sassner et al. 2005). Companion CORRIM biofuels investigations reported in this issue of the Forest Products Journal use softwood residual feedstocks. Hardwood feedstocks were chosen for this study because they do not exhibit the recalcitrance reported for softwoods (Mansfield et al. 1999). Other benefits of using willow as a feedstock include high biomass production, suitability for cultivation on marginal land, ease of vegetative propagation from dormant hardwood cuttings, broad genetic base and ease of breeding, and ability to resprout after multiple harvests (Keoleian and Volk 2005). Life-cycle assessment (LCA) of ethanol produced by bioconversion of willow has been investigated for Europe using the information of feedstock production available in literature from the United Kingdom (Stephenson et al. 2010). In this study, the impacts associated with conversion were estimated using the Aspen model developed by the National Renewable Energy Laboratory (NREL; Aden et al. 2002), and emissions from vehicle use were estimated using data from the Conservation of Clean Air and Water in Europe and the European Council for Automotive Research and Development (Stephenson et al. 2010). Environmental impacts were calculated using Environment Development of Industrial Products methodology. Stephenson found that ethanol produced using bioconversion reduced life-cycle greenhouse gas (GHG) emissions and fossil energy requirements by 90 and 83 percent, respectively, compared with gasoline. There have also been LCA studies for production of ethanol using bioconversion of poplar feedstocks (González-Garcia et al. 2010). Poplar is similar to willow in terms of growth, harvesting, and biomass composition. Results from these LCAs should be similar to those using willow. In the González-Garcı́a study, feedstock data were obtained from the literature on poplar crops grown and harvested in Spain. They used the Aspen model (Aden et al. 2002) to assess the impacts of the bioconversion process and used available literature to estimate emissions associated with vehicle usage. The Institute of Environmental Studies (CML), a European LCA impact indicator method, was used for impact characterization. GonzálezGarcı́a et al. (2010) found that compared with gasoline, the life-cycle GHG emissions for bioconversion ethanol (E100) were 80 percent lower than those for gasoline, and fossil fuel use was decreased by 78 percent. This life-cycle GHG emission that was smaller than that reported by Stephenson et al. (2010) could be explained by González-Garcı́a et al. (2010) not accounting for the impact of excess electricity production on the life-cycle impacts. In the present work we refine and expand on the previous LCA work by using life-cycle inventory (LCI) databases developed for US data and by using actual operations data for feedstock production and harvesting. Further, we investigate life-cycle water consumption, which may have a significant environmental impact for biorefineries using a bioconversion approach.