9-02: Contribution of enzymes to life cycle greenhouse gas emission of lignocellulosic ethanol

Recent fuel regulations (EISA and California’s Low Carbon Fuel Standard) in various jurisdictions have included requirements for reductions in greenhouse gas (GHG) emissions compared to gasoline on a life cycle basis. Life cycle assessments (LCA) of lignocellulosic ethanol should take into account the entire process from feedstock production through conversion to ethanol, including chemicals and enzymes used in the process. Our aim is to determine the importance of the contribution of enzymes to life cycle GHG emissions of lignocellulosic ethanol and additionally to examine enzyme production costs. A complete enzyme production process, including downstream purification, was simulated using AspenPlus, leading to mass and energy balance information required to calculate GHG emissions and financial metrics. The cradle-to-gate GHG emissions for cellulase production range from 10.2 to 16.0 g CO₂ equivalent (eq) g^-1 enzyme protein, depending on whether cellulase is produced on-site or off-site, and the method of transportation. For a 150 million liter per year ethanol plant and an enzyme dosage of 11.5 mg g^-1, the corresponding GHG contribution of enzymes to the life cycle of ethanol are 258 and 403 g CO₂ eq L^-1 ethanol produced for on-site and off-site enzyme production, respectively. The cost for on-site enzyme production ranges between 3.8 and 6.75 $/kg, versus 4 and 8.8 $/kg for off-site production. Off-site production is favored if the enzyme plant is large and serves multiple ethanol plants; on-site production is favored for large ethanol plants with a high enzyme demand.