Biofuels hold great promise for the future of transportation energy, but how near we are to that future is, at best, an educated guess with many variables at play. A study led by Scott Paap (Systems Research and Analysis Dept.), “Biochemical production of ethanol and fatty acid ethyl esters from switchgrass: A comparative analysis of environmental and economic performance” (published in Biomass and Bioenergy), takes a close look at the biochemical production of fatty acid ethyl esters (FAEE), one of several candidate biofuel molecules, and how it measures up against cellulosic ethanol production.
The study found that biochemical production of cellulosic ethanol outperforms FAEE biochemical production under the current technology development state (using both economic and environmental metrics), primarily due to FAEE’s low fermentation yields. The study also identified pathways to improve the FAEE process and evaluated the prospects for it matching or exceeding ethanol process performance in the long term.
“This is a great example of techno-economic modeling helping to inform and guide research,” said Blake Simmons (Biofuels & BioMaterial Science and Technology Group), vice president of the Deconstruction Division at the Joint BioEnergy Institute (JBEI) and a Sandia senior manager. “This is really powerful insight into where we are relative to existing technology and where we need to be in order to fulfill our mission of replacing petroleum as a transportation fuel.”
“The potential process energy savings from removing distillation steps has been touted as an advantage of producing drop-in biofuels such as FAEE,” explains Paap. “However, when I searched the literature, I found that no one had performed a direct comparison of the efficiency of the fuel production processes. Our analysis began as an attempt to quantify the potential cost and energy savings of producing water-immiscible biofuels.”
The comparison between the processes to produce ethanol and FAEE is imperfect, says Scott, because ethanol is a gasoline additive/replacement and FAEE is intended for diesel engines. However, the results are broadly applicable to other water-immiscible biofuels, most of which suffer from similarly low maximum theoretical fermentation yields when compared to ethanol.
This point also underscores the need to extend the scope of such comparisons beyond the biorefinery to distributing fuel to end users, and ultimately to combustion in engines. “The process model allows you to take a step back and analyze how a specific technology fits into the bigger picture. When a process is in the early stages of development, any estimates of energy use or greenhouse gas emissions, for example, will have a large degree of uncertainty. So, the model is useful for comparing different options and their relative performance rather than arriving at absolute numbers,” says Scott.
The study was part of an Early Career Laboratory-Directed R&D project.