Bioethanol from corn stover - Global warming footprint of alternative biotechnologies

Bioethanol from residual corn stover could contribute to lowering CO2 loads within the transport sector, if used as an amendment to gasoline. We modelled by life cycle assessment and Monte Carlo simulation seven different technological configurations for producing bioethanol from corn stover based on consistent mass flows and estimated ethanol production extracted from 141 datasets of reasonable quality. By parametrizing key processes and determining their statistical distribution based on actual data, we were able to estimate the Global Warming Potential (GWPs) for all the alternative technologies on a system level. Most of the individual cases showed a net saving in GWP when the savings obtained from recovering energy from anaerobic digestion of the liquid residues and incineration of the solid residues were included. The net savings could in some cases be as high as 900 similar to 1200 kg CO2-eq/t dry corn stover solids. If the residues were not subject to energy recovery, the production of bioethanol and use in gasoline would be a net load to global warming in more than 50% of the technological configurations. The best-practice, defined as the top 15% cumulative probability with respect to GWP, suggests that technologies based on steam explosion and ammonia-based pretreatment appear statistically the most promising and could contribute, with residue energy recovery, to GWP savings of 850-1050 kg CO2-eq/t dry corn stover solids and produce in the range 178-216 kg of bioethanol. This paper provides insights into the key parameters for bioethanol production from corn stover and suggests areas for further research.