Optimizing the Size of Anaerobic Digesters

Abstract

Anaerobic digestion of manure from confined feeding operations (CFOs) to produce biogas and in turn electric power in farm or feedlot based units as well as centralized plants is evaluated for two settings in Alberta, Canada: a mixed farming area, Red Deer County, and an area of concentrated beef cattle feedlots, Lethbridge County. Centralized plants transport manure to the plant and digestate back to the source CFO, an added cost relative to farm or feedlot based plants, but gain from the economy of scale in plant capital and operating cost. A centralized plant drawing manure from 61 sources in the mixed farming area, at a manure yield of 34 dry tonne year-1 ha-1 could produce 6.5 net MW of power at a cost of $218 MWh-1. No individual CFO in the mixed farming area, including a 7,500 head beef cattle feedlot, can produce power at a lower cost with a farm or feedlot based unit. A centralized plant drawing manure from 560,000 beef cattle in Lethbridge County, at a manure yield of 280 dry tonne year-1 ha-1, can produce power at a cost of $138 MWh-1. In Lethbridge County, An individual feedlot larger than 40,000 head of beef cattle could produce power at a lower cost than the centralized plant. Commercial processes to recover concentrated nutrients and a dischargeable water stream from digestate are not available. However, we analyze the theoretical impact of digestate processing based on a capital cost of 2/3 of the AD plant itself. Digestate processing shifts the balance in favor of centralized processing, and a feedlot would need to be larger than 250,000 head to produce power at a lower cost than a centralized plant. Power from biogas has a high cost relative to current power prices and to the cost of power from other large scale renewable sources. Power from biogas would need to be justified by other factors than energy value alone, such as phosphate, pathogen or odor control.