Bio-Butanol Vs. Bio-Ethanol: A Technical and Economic Assessment

Fermentation-derived butanol is a possible alternative to ethanol as a biomass-based liquid fuel. We compare the production of butanol vs. ethanol from corn or switchgrass by fermentation based on the liquid fuel yield in terms of the lower heating value (LHV). Industrial scale experience on fermentation to butanol (ABE fermentation) or ethanol (yeast) is used to establish a baseline comparison at this time, and to put recent advances in fermentation to butanol in perspective. A dynamic simulation model demonstrates the technical, economic and policy implications for these technologies.

The LHV yield of butanol was only about 50% of the LHV yield of ethanol per mass of corn. This is a very serious disadvantage for butanol since feedstock costs are a significant portion of the fuel price. While some developments based on metabolic and biochemical engineering show improvements for butanol fermentation, there appears to be currently no industrial scale avenue for an economically competitive butanol production by fermentation to rival bio-ethanol due to the poor yield of butanol LHV. This would increase butanol's life-cycle greenhouse gas emission as more feedstock would be needed for the same amount of energy compared to ethanol. Another problem is that a given fermenter volume can produce only about one quarter of the LHV as butanol per time compared to ethanol. This increases capital costs. The sometimes touted advantage of butanol being more compatible with existing pipelines is, according to our techno-economic simulations insufficient to alter the conclusion because of associated capital costs to connect plants.