A Simple and Energy Efficient Approach to Cleaning Biogas

摘要

One of the leading challenges to the beneficial reuse of natural biogas has always been the contaminants it carries along with it- hydrogen sulfide, carbon dioxide, and siloxanes. Newer air quality regulations and more efficient combustion processes have required that the gas produced from anaerobic digestion be cleaned before it is used in downstream equipment. Traditionally, this has been accomplished using activated carbon or iron oxide in Iron Sponge scrubbers; which rely on adsorption or reaction of the sulfide with iron oxide impregnated wood chips. The biggest difference in our wastewater industry, compared to others that deal with cleaning biogas, is our steady supply and access to water, typically pressurized from the plant water systems. This pressurized water is being used as part of a new and simple technology for cleaning biogas. Using the water, a vacuum is generated using a Venturi that draws the raw biogas into the system, eliminating the need for pressurized gas typically required in conventional water scrubbers. The carbon dioxide and hydrogen sulfide preferentially dissolve into the water leaving behind the methane in the biogas. The gas/water mixture is then separated in a gas separator, and the cleaned gas is collected at the top in the gas separating valve. The biogas scrubber technology' was piloted at Milwaukee Metropolitan Sewerage District's (MMSD) South Shore Water Reclamation Facility (SS WRF) located in Oak Creek, Wisconsin. A pilot unit was set up in the Digester gallery of SS WRF. A small stream of raw biogas, produced in Digester 10, was fed to the pilot unit, and the treated gas leaving the unit was returned to the plant's biogas pipeline. Grab samples of biogas going in and out of the pilot unit were collected and tested for methane and carbon dioxide, total sulphur compounds, and siloxanes. The results showed close to 100% removal of hydrogen sulfide, about 30% increase in methane accompanied by about an 80% decrease in carbon dioxide, and around 65% removal of siloxanes as an added benefit.