Submitted to: Environmental Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2013
Publication Date: 5/29/2013
Citation: Pourhashem, G., Spatari, S., Adler, P.R., Mcaloon, A.J. 2013. Cost and greenhouse gas emission tradeoffs of alternative uses of lignin for second generation ethanol. Environmental Research Letters. 8 DOI: 10.1088/1748-9326/8/2/025021. Interpretive Summary: Biorefineries under development for converting biomass to ethanol are designed to use a fraction of biomass for ethanol (fuel) and a fraction for power generation. The portion the biomass converted to power is slow to decompose and also resistant to being converted to fuel; therefore, the most economic use of this portion of the biomass is to burn it in a boiler and convert to steam and electricity. A negative result of using crop residues such as straw as a source of biomass for ethanol production is that soil carbon levels can decrease along with crop yields. We examined application of this stable fraction of the remaining biomass to the land, considering the effect on both the economics and the greenhouse gas emissions for production of ethanol. We found that although using this material to generate electricity for the biorefinery was the most economic use, applying it to the land produced ethanol with the lowest greenhouse gas footprint. This finding could help the industry evaluate the different markets for byproducts produced at the biorefinery, considering both the economic and environmental impacts.
Technical Abstract: Second generation ethanol bioconversion technologies are under demonstration-scale development for producing lignocellulosic fuels to meet the federal Renewable Fuel Standards (RFS2) supply requirements by 2022. Bioconversion technology utilizes the fermentable sugars generated from the cellulosic fraction of the feedstock, and most commonly assumes the lignin fraction may be used as a source of thermal and electrical energy recovery. We examine the life cycle greenhouse gas (GHG) emission and techno-economic cost tradeoffs for alternative uses of the lignin fraction of agricultural residues feedstocks for (corn stover, and wheat and barley straw) used within a 2000 ton per day ethanol biorefinery in three locations in the United States. We compare three scenarios in which the lignin is 1) used as a land amendment to replace soil organic carbon; 2) separated, dried and sold for co-firing with coal to produce electricity; and 3) used to produce electricity on-site at the biorefinery. Results from this analysis indicate that for life cycle GHG intensity, amending the lignin to land is the lowest among the three ethanol production options (-19 to -4 g CO2e MJ-1), coal co-firing the lignin is the second lowest (-11 to 4 g CO2e MJ-1), and on-site power generation is highest (25 to 45 g CO2e MJ-1), though still could meet RFS2 advanced fuel requirements. The lignin-land amendment option has the lowest capital cost among the three options due to requiring a small boiler for the biorefinery’s thermal energy needs and use of biogas generated on-site. The need to purchase electricity and uncertain market value of the lignin-land amendment raise the cost compared to on-site power generation and electricity co-production. However, assuming a market value ($50/dry Mg) for nutrient replacement in agricultural soils, the lignin-land amendment option shows promise for abating GHG emissions relative to gasoline.