More Methane, Less Acid Gas

In last month's issue, we shared some thoughts about anaerobic digestion and introduced an Energy & Environmental Research Center (EERC) project, funded by Xcel Energy, which will test and demonstrate a novel technology to enhance anaerobic digestion of biomass to produce a biogas containing increased methane content and significantly reduced hydrogen sulfide.

Hydrogen sulfide is a toxic gas produced by sulfate-reducing bacteria under anaerobic conditions. In anaerobic digestion processes, hydrogen sulfide can become problematic because it 1) contributes to foul odors, 2) contributes to sulfur dioxide emissions when combusted, 3) creates a corrosive environment when present with moisture, and 4) can poison, or reduce the effectiveness, of fuel cells.

Current technologies to control hydrogen sulfide from anaerobic digestion processes rely on removing the hydrogen sulfide after it has formed, typically using expensive scrubbers.

The EERC technology produces a biogas with significantly reduced hydrogen sulfide through the selective death of sulfate-reducing bacteria, the root cause of sulfide production.

Since the sulfate-reducing bacteria also compete with methane-producing bacteria for available biomass carbon, reducing the sulfate-reducing bacteria population allows more carbon to be converted to methane. Biogas quality is also improved through a reduction in carbon dioxide production, another metabolic end product of sulfate-reducing bacteria.

The overall effect of the EERC technology is a higher British thermal unit-containing biogas-one with more methane, and less carbon dioxide and hydrogen sulfide-from the same quantity of biomass.

The EERC has filed a patent application on the process and has successfully demonstrated process capabilities in controlling hydrogen sulfide formation in agriculture processing wastewaters. The Xcel Energy project will demonstrate the EERC technology on dairy manure, an abundant biomass resource. The EERC has partnered with Haubenschild Farm Dairy Inc., a 1,000-acre, 1,000-cow dairy near Princeton, Minn. The farm has an operational anaerobic digester that processes 20,000 gallons of manure waste into nearly 23,000 cubic feet of biogas per day. The EERC will conduct bench-scale testing at its laboratories in Grand Forks, N.D., using dairy manure samples collected from the farm and pilot-scale testing on-site at the farm in Princeton. The pilot system will consist of a skid-mounted anaerobic digester to simulate operation of the full-scale digester at the farm. The project is scheduled to begin in July, and valuable information will be developed regarding engineering design and realistic cost estimates for enhancing traditional digestion processes to produce greater quantities of gas at a lower cost.

Dan Stepan is a senior research manager at the EERC in Grand Forks, N.D. Reach him at or (701) 777-5247.