Corn ethanol and biodiesel currently rank as major alternative transportation fuels largely due to the government subsidy and Renewable Fuels Standard (RFS) which sets the goal of producing 7.5 billion gallons/year of biofuels by 2012. However, they are seriously constrained by large land use requirements (1), food-fuel conflict, and high eutrophication potentials (2). Consequently, cellulosic ethanol and hydrogen are considered as viable alternatives due to their potential for minimizing land use requirements and avoiding the food-fuel conflict. Candidates for cellulosic ethanol include crop residues, grasses, and solid wastes. Hydrogen used in fuel cells is predominantly produced from natural gas through reforming, although it can be produced from other sources such as water and biomass. Life cycle analyses (LCAs) of cellulosic ethanol (3, 4) and hydrogen (5) outline several advantages such as higher net energy and lower GHG emission. A recent study reveals a prospect of integrating hydrogen in biofuel production to reduce the land requirements and recycle CO2 (6). However, existing LCA studies rely primarily on energy consumption and emissions of a fewer pollutants. Production and utilization of alternative fuels require the support of ecological goods and services; ignoring them in LCA may skew the conclusions and decision making since not all resources are renewable and plentiful.

We are conducting a comprehensive "ecologically-based LCA" of cellulosic ethanol and hydrogen by accounting for contributions of ecosystem goods and services such as water, land, pollination, soil fertility and erosion, atmospheric gases and natural cycles. The study results in input-output hybrid models of the selected fuels (7). The raw data are available in diverse units of mass and energy. Thermodynamic approaches based on energy, exergy and cumulative exergy are used to obtain "mid-point" and "end-point" indicators. These results are based mainly on "input-side" information and can complement results from conventional LCA, which is mainly based on emissions and their impact.

The results indicate the benefits of biomass based fuels in terms of their lower impact on climate change and lower reliance on fossil fuels. However, their high reliance on ecosystem goods and services such as water, soil, pollination, and land identify potential vulnerabilities. Such information could be used for smart decisions about the fuel mix and the fuel value chain.

We also employ metrics based on quantities such as energy, mass, industrial ecological cumulative exergy consumption (ICEC), and ecological cumulative exergy consumption (ECEC) to possibly derive additional insight not available hitherto. The advantage of using exergy analysis is that it allows integration of materials and energy in a scientifically rigorous manner and facilitates a comparison across disparate units. Input-output hybrid models expand the boundary of analysis minimizing truncation errors. In addition, inclusion of ecological goods and services ensures analysis at the ecosystem level and assists in holistic decision making. We also explore the relationship between energy/exergy consumption and human health impacts over the life cycle of cellulosic ethanol and hydrogen. It is possible that such a relationship can be used as proxy for predicting impacts of emerging technologies for which little information is available. We will present disaggregated information in physical units such as mass, energy, and exergy for each resource considered in the study as well as aggregated metrics such as efficiency, industrial cumulative degree of perfection (ICDP), and sustainability index, etc. to offer a multi-faceted analysis.

References:

(1) Hill, J.; Nelson, E.; Tilman, D.; Polasky, S.; Tiffany, D., 2006. Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels. Proceedings of the National Academy of Sciences 103 (30), 11206-11210.

(2) Baral, A.; Bakshi, B. R., 2006. Comparative study of biofuels vs. petroleum fuels using input-output hybrid life-cycle assessment. AIChE 2006 Annual Meeting, San Francisco, CA.

(3) Spatari, S.; Zhang, Y.; MacLean, H. L., 2005. Life cycle assessment of switchgrass- and corn stover-derived ethanol-fueled automobiles. Environmental Science and Technology 39 (24), 9750-9758.

(4) Farrel, A.E.; Plevin, R.J.; Turner, B.T.; Jones, A.D.; O'Hare, M.; Kammen, D.M., 2006. Ethanol can contribute to energy and environmental goals. Science, 311, 506-508.

(5) MacLean, H. L.; Lave, L. B, 2003. Life cycle assessment of automobile/fuel options. Environmental Science and Technology 37 (23), 5445-5452.

(6) Agrawal, R.; Singh, N. R.; Ribeiro, F. H.; Delgass, W. N., 2007. Sustainable fuel for the transportation sector. Proceedings of the National Academy of Sciences 104 (12) 4828-4833.

(7) Ukidwe, N. U.; Bakshi, B. R., 2004. Thermodynamic accounting of ecosystem contribution to economic sectors with application to 1992 US economy. Environmental Science and Technology 38 (18), 4810-4827.