Energy Balances for the Production of Cellulosic Ethanol

Cellulosic ethanol has the potential to meet a substantial fraction of the US needs for liquid transportation fuels.  To remove the perceived major barriers to the large-scale commercialization of this technology, a lot of attention has been focused lately on overcoming the recalcitrance of plant cell walls to deconstruction, on the development of more efficient fermentation methods and on genetically engineering innovative energy crops that maximize fuel production. However, the commercial success of cellulosic ethanol will depend to a large extent on our ability to design energy efficient plants. 

We report here some recent results from a systematic analysis of the energy requirements of cellulosic ethanol production. A comprehensive process model for the production of ethanol from cellulosic biomass (corn stover, switchgrass etc.) was developed and implemented using the SuperPro simulator package.  We paid particular attention to the operation of the utilities section of the plant, a combined heat and power (CHP) unit that generated the steam and electricity needed for the purification of ethanol and the other plant units. 

A detailed parametric analysis was carried out to study how the energy balances of each plant section and the overall energy efficiency of the plant were influenced by changes in (a) the composition of the biomass feedstock, and (b) the conversion levels of the pretreatment, hydrolysis and fermentation stages. We carefully analyzed the requirements of the utilities section of the ethanol plant where byproduct streams of the production process were utilized for energy generation. One of the key questions asked was whether these streams were a sufficient fuel source for meeting the energy requirements of the plant or whether we needed to provide extra fuel in the form of additional biomass or natural gas.  

Our calculations indicate a significant trade-off between ethanol production and external energy inputs.  Increases in the cellulose and hemicelulose contents of the feedstock or increases in the  conversion of the fermentation stage would result (as expected) to higher ethanol production rates.  This increase, however, would be offset by an increase in the requirement for extra energy inputs in the form on natural gas or biomass fed directly to the utilities section.  Therefore, any changes designed to increase ethanol production may be accompanied by lower overall efficiencies and net energy ratios for the biorefinery. These results cast some doubt on the ultimate effectiveness of efforts aimed at engineering innovative energy crops with high cellulose/hemicellulose content in order to maximize fuel production.