Techno-economic Analysis of Farm Scale Plug-flow Anaerobic Digestion

Treating animal wastes through anaerobic digestion (AD) yields methane-rich biogas that can be used for power generation or heating, and a nutrient-rich digestate that can be land applied as fertilizer. Anaerobic digestion also reduces odors from stored and land applied manures. Despite these benefits, AD deployment rates in the United States (US) are only 5% for dairy farms identified as being suitable for AD by the US Environmental Protection Agency. The objective of this study was to analyze the economic and technical limitations of farm-scale plug-flow anaerobic digesters using a simple model permitting insight into the fundamental constraints on the technology. A model was developed to determine the cost of methane produced via AD based on operation size. For context, the cost of ADmethane was then compared to commercial methane costs (i.e., natural gas). The analysis shows how critical farm size is to making AD-methane cost-competitive with natural gas. At low herd sizes (below 400 animals), carbon credits and odor reductions alone appear insufficient to overcome the relatively low commercial energy rates in the US. However, moderate reductions in digester cost and interest rate, coupled with moderate increases in amortization period, and/or natural gas prices appear could make AD more competitive with commercial energy in the US even at relatively small herd sizes (ca. 200 animals)