(595g) Comparative Study of Biofuels Vs Petroleum Fuels Using Input-Output Hybrid Life-Cycle Assessment

Alternative transportation fuels such as ethanol, biodiesel, and hydrogen have become a subject of intense scientific scrutiny in light of dwindling fossil fuel reserves, increasing price and climate change. Ethanol and biodiesel have already been used as transportation fuels for sometime whereas hydrogen fuel is still in research and development phase. To evaluate the desirability and tradeoffs for using alternative fuels, a comprehensive life-cycle assessment (LCA) is recommended. A number of process LCAs have been conducted for alternative transportation fuels. However, process-scale life analysis has certain limitations such as subjectivity involved in system boundary selection and incompleteness of the system [1] that often lead to conflicting conclusions [2,3]. This limitation is also relevant to other life cycle oriented approaches such as net energy analysis [3]. ISO has defined broad guidelines for cut-off criteria; however, they are difficult to meet due to lack of prior knowledge for inputs and processes. In process life cycle inventory, inputs from third tier and beyond are neglected introducing truncation errors. Another important unresolved challenge in LCA studies is that the underlying data are often thermodynamically inconsistent and may not even satisfy the laws of conservation.

To overcome the drawbacks, hybrid models have been introduced for LCA [1], and data reconciliation methods have been developed to enhance the quality of life cycle inventory data by imposing conservation laws [4]. There are three main hybrid models: tiered hybrid, input-output hybrid, and integrated hybrid. Hybrid models have been used for other products or economic sectors; however, application of an input-output hybrid model in life cycle analysis of transportation fuels is novel. Preliminary results obtained from input-output hybrid life-cycle assessment (IOHLCA) indicate that energy returns on investment or net energy of corn ethanol and biodiesel are lower than those of gasoline and diesel, respectively. It suggests that biofuels are inefficiently extracted and processed.

From a climate change perspective, both corn ethanol-based fuels (E10 and E85) and biodiesel-based fuels (BD20 and BD100) appear attractive since their use results in significant reductions of GHG emissions in comparison to gasoline and diesel. They also have lower well-to-wheel emissions of methane. However, use of corn ethanol and biodiesel as transportation fuels increases emissions of PM10, nitrous oxide, nitrogen oxides (NOx) as well as nutrients such as nitrogen and phosphorous; the latter are the main of agents for eutrophication and creation of hypoxic zones in the Gulf of Mexico. Low yields of soybean and limited availability of agricultural lands severely limit the production of biodiesel. As a result biofuels such as corn ethanol and biodiesel can only meet a small portion of growing demand for transportation fuels.

We are also developing a novel thermodynamic input-output hybrid model by incorporating ecological cumulative exergy [5] that captures contributions from ecology and, hence, better reflects the magnitude of sustainability of alternative fuels. The resulting thermodynamic input-output hybrid model will provide a new approach to address the issue of ecologically conscious product design and manufacturing. Indicators such as environmental loading ratio, yield ratio, impact per value added, and sustainability index obtained from such a model can be used to evaluate the tradeoffs and appropriateness of transportation fuels.

References: [1] Suh S., Lenzen M., Treloar G.J., Hondo H., Horvath A., Huppes G., Jolliet O., Klann U., Krewitt W., Moriguchi Y., Munksgaard J., Norris G., 2004. System boundary selection in life-cycle inventories using hybrid approaches. Environmental Science and Technology 38(3), 657-664. [2] Kim S., Dale B., 2005. Environmental aspects of ethanol derived from no-tilled corn grain: nonrenewable energy consumption and greenhouse gas emissions. Biomass and Bioenergy 28, 475-48. [3] Pimentel D., 2003. Ethanol fuels: energy balance, economics, and environmental impacts are negative. Natural Resources Research 12(2), 127-134. [4] Yi, H.-S., Bakshi, B. R., Enhancing Life Cycle Inventories Via Reconciliation across Multiple Scales, AIChE 2005 Annual Meeting, Cincinnati, OH [5] 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.