(35b) Carbon Footprint Analysis of Biomethane from Anaerobic Digestion of a Food Waste / Manure Mixture in Colorado

This study determined the life cycle greenhouse gas (GHG) emissions of bio-methane produced from anaerobic digestion of a mixture of food waste and cow manure in Colorado. Included in the system boundary for this analysis are long distance semi-truck transport of collected food waste from the Denver metropolitan area, avoid landfilling (and composting in a separate scenario) emissions for the food waste, short haul transport of manure waste from local dairy farms, anaerobic digestion process and biogas purification, distribution of the bio-methane into the natural gas pipeline grid, and combustion of bio-methane. The carbon accounting included both fossil and biogenic carbon. The effects of co-products solid compost sold to retail gardening market and nutrient rich liquid digestate distributed to local farm fields were accounted for in the analysis.

After including emission credits for avoiding landfill emissions of CO₂ and CH₄, bio-methane emits over its life cycle -3.5 kg CO₂ equivalents / kg bio-methane, thus reducing global warming effect with each unit of production of bio-methane. This favorable emission intensity is compared to that of fossil methane of between 3.3 to 5.1 kg CO₂ equivalents / kg fossil methane. Effects of life cycle system parameters in the model on GHG results are investigated in scenario analyses and also in time dependent analysis of landfill emissions.