The cost of cellulases remains a key issue in the production of cellulosic ethanol, and the impact of enzymes on greenhouse gas (GHG) emissions of cellulosic ethanol has received little attention. This study evaluates life cycle emissions and cellulase production costs for bioethanol production, considering on-site and off-site production options. A complete enzyme production process was simulated using AspenPlus, generating mass and energy balance information required to calculate GHG emissions and financial metrics.
GHG emissions for cellulase production range from 10.2 to 16.0 g CO2 eq g–1 enzyme protein, depending on on-site or off-site production and the method of transportation. Enzyme GHG emissions are predicted to be 258 g CO2 eq. L–1 of ethanol for on-site production, versus 403 g CO2 eq. L–1 for off-site production, based on a 150 MMLY ethanol plant using 11.5 mg enzyme g–1 substrate and a cellulase fermentation yield of 90%.
Cellulase production costs were estimated for a range of conditions, including ethanol plant size, enzyme dose and protein yield for on-site production, and enzyme plant size, protein yield and return on investment for off-site production. On-site production costs range between $3.80 and $6.75 kg–1 protein, versus $4.00 to $8.80 kg–1 for off-site production. In both scenarios, the lowest cost corresponds to a 90% protein yield, and a high enzyme demand and production capacity. An enzyme production cost of $4.70 USD kg–1 corresponds to an enzyme cost of 0.46 USD gal–1 ($0.12 L–1) of ethanol in a 150 MMLY plant using 11.5 mg enzyme g–1 substrate. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd