Construction and Operation of the ABEC #4 Covered Lagoon Digester and Electrical Generation System

This project used biogas derived from on-site dairy manure to generate low-carbon, renewable electricity and achieve high overall efficiency by capturing waste heat from the power generation system and using it to drive an absorption chiller. The absorption chiller is designed to convert waste heat into chilling capacity, which will be used to chill milk produced by the dairy cows.
The purpose of this project was to document the construction and operation for one year of the American Biogas Electric Company #4 LLC (ABEC #4) covered lagoon digester, with a volume of 25.6 million gallons, along with a 1-megawatt engine-generator. Construction, start-up, and commissioning of the digester was completed in 2017 using the flushed manure from 5,760 “manure equivalent milkers” as influent (one manure equivalent miler represents 100 percent of the manure from a Holstein cow weighing 1,360 pounds). Full operation commenced in February, 2018.
Through the end of December, 2018, ABEC #4 averaged 430,750 cubic feet per day of biogas containing 60 percent methane, approximately 75 cubic feet of biogas per cow per day. The average monthly gross electrical production during this period was 684,000 kilowatt-hours (kWh) for 701 hours of operation per month (out of a possible 720 hours), averaging 976 kilowatts of generating capacity. The parasitic load was 31 kilowatts or 3.1 percent, and the net energy sold to Pacific Gas and Electric Company averaged more than 662,000 kWh per month.
Average monthly income for the project was $127,000, with a simply payback of 6.8 years based on total installed capital cost of $7.63 million and average monthly operating costs of $34,000. The project provided the additional benefit of producing an average of 108 tons per day of fiber bedding/fertilizer.
Environmental benefits of this project include the reduction of more than 21,000 metric tons of carbon dioxide equivalent greenhouse gas, principally methane, along with holding exhaust emissions at 2.6 parts per million (ppm) of oxides of nitrogen (NOx) and 35 ppm carbon monoxide (CO), well under California Air Resources Board limits of 11 ppm NOx and 210 ppm CO. Progress was made in limiting hydrogen sulfide (H2S) emissions, thus prolonging the life of the engine-generator. H2S levels dropped from a high of 5,000 ppm to less than 100 ppm in June while using only the air injection system. A final iron sponge scrubbing resulted in H2S levels averaging 12 ppm going into the engine-generator.