This study determined the life cycle greenhouse gas (GHG) emissions and Net Energy Return on Investment of bio-methane produced from anaerobic digestion of a mixture of food waste and cow manure in a facility near LaSalle, Colorado. The analysis compares a business-as-usual (BAU) case with an anaerobic digestion (AD) case. The BAU case includes the composting of food waste with manure plus the production and use of fossil natural gas. The AD case includes production of bioCH4
through an advanced AD process as well as production and use of peat moss in an amount that represents the difference in compost production between the two cases. Included in the system boundary for this AD analysis are long distance semi-truck transport of collected food waste from the Denver metropolitan area, short haul transport of manure waste from local dairy farms, AD process and biogas purification, distribution of the bio-methane into the natural gas pipeline grid, combustion of bio-methane, and distribution of digestate from the AD process to local dairy farm fields for provide both irrigation water and mineral nutrients. 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.