Cellulosic ethanol is currently touted as a potential future sustainable transportation fuel that exerts lower environmental impacts, avoids food-fuel conflict, and more importantly improves energy security. Considering the nascent nature of cellulosic ethanol technology, it is imperative that its benefits and drawbacks be fully explored before the production begins in a commercial scale. Existing process–based life cycle studies provide favorable metrics for cellulosic ethanol in terms of energy and greenhouse gas emissions in comparison to corn ethanol and gasoline (1-5). However, the reliance of cellulosic ethanol production on ecosystem goods and services or natural capital is one important aspect that has not been rigorously examined. There are several critical resources such as fisheries, soil erosion, water, pollination, land, etc. which are as critical as fossil fuel consumption and need to be incorporated in decision-making. Ecologically-based lifecycle assessment (Eco-LCA) is an approach that accounts for contributions of ecological goods and services and helps track the natural capital footprint over the lifecycle (6). Such insight can also be used to design integrated industrial-ecological networks that reduce their impact.

A comprehensive study of cellulosic ethanol derived from five different feedstocks- switchgrass, corn stover, yellow poplar, recycled newspaper and municipal solid waste was conducted using hybrid Eco-LCA which integrates process-level lifecycle inventories with economic-scale life cycle inventories. A preliminary study shows that cellulosic ethanol E85 reduces greenhouse gas emissions by 38%-60% over gasoline. Energy returns on investment for cellulosic ethanol are higher with a range of 3-7. However, in terms of natural resource consumption cellulosic ethanol consumes more of all resources considered in the study except crude oil indicating its relatively higher resource intensity. Some examples of resources that are consumed more by cellulosic ethanol are natural gas, electricity, soil, crushed stone, water, various metal ores and nonmetallic ores. For corn stover-, yellow poplar- and switchgrass-derived ethanol, land use requirements are very large and it is constrained in the same way as corn ethanol. On the other hand, land use requirements for cellulosic ethanol derived from municipal solid waste and recycled newspaper are relatively small since these feedstocks do not require direct land use.

The Eco-LCA results are presented hierarchically with thermodynamic metrics based on energy and exergy used for aggregation. Overall, cellulosic ethanol consumes less non-renewable cumulative exergy which translates into better sustainability indices. Contribution of materials such as soil and detrital matter is significant implying that minimizing soil erosion, particularly for corn stover, would improve the overall system efficiency not including sunlight. Yield ratios of cellulosic ethanol from various feedstocks are better than that of corn ethanol but still lower than that of gasoline.

Our ongoing research work expands the earlier work by considering how industrial and ecological processes could be engineered to reduce the impact of the life cycle. For example, non-renewable inputs in the production system such as chemical fertilizers and fossil fuels may be replaced by biosolids, and renewable fuels by biodiesel and cellulosic ethanol. Such a model will be helpful in predicting reductions in natural resource/energy consumption and environmental impacts when biofuel production systems move closer to a sustainability ideal. The work will involve sensitivity analysis by varying biomass and ethanol yields. A scenario analysis that assumes the use of all lignocellulosic biomass available in the US for cellulosic ethanol production will be conducted. A theoretical scenario involving recycling of CO2 using a hydrogen fuel (7) will also be investigated. This approach has a potential for significantly minimizing land use requirements, and is appealing considering the constraint posed by limited available land for biomass production.

References:

(1) Sheehan, J.; Aden, A.; Paustian, K.; Kendrick, K.; Brenner, J.; Walsh, M.; Nelson, R. Energy and environmental aspects of using corn stover for fuel ethanol. J. Indust. Ecol. 2004, 7 (3- 4), 177-146.

(2) Spatari, S.; Zhang, Y.; MacLean, H.L. Life Cycle assessment of switchgrass- and corn stover-derived ethanol-fueled automobiles. Environ. Sci. Technol. 2005, 39 (24), 9750-9758.

(3) Kalogo, Y.; Habibi, S.; MacLean, H.L.; Joshi, S. Environmental implications of municipal solid waste-derived ethanol. Environ. Sci. Technol. 2007, 41 (1), 35-41.

(4) Levelton Engineering Ltd; S&T Consultants. Assessment of Net Emissions of Greenhouse Gases from Ethanol-Blended Gasolines in Canada: Lignocellulosic Feedstocks. Prepared for Agriculture and Agrifood Canada, Ottawa, ON, 1999, File 499-0893.

(5) Kemppainen, A. J.; Shonnard, D.R. Comparative life-cycle assessments for biomass-to-ethanol production from different regional feedstocks. Biotechnol. Prog. 2005, 21 (4), 1075-1084.

(6) Baral, A.; Bakshi, B.R. Hybrid LCA of corn ethanol, gasoline, soybean biodiesel, and diesel, part I – accounting for natural capital and net energy analysis. Environ. Sci. Technol., submitted 2008.

(7) Agrawal, R.; Singh, N. R.; Ribeiro, F. H.; Delgass, W. N. Sustainable fuel for the transportation sector. Proc. Natl. Acad. Sci. 2007 104 (12) 4828-4833.