Methane Bio Gas April 2308

Methane/biogas Dwight Adams Biogas refers to the gas produced from biological decomposition of organic matter such as food waste, grass clippings, waste water sludge, or animal manures in the absence of oxygen [1]. Depending on how the decomposition occurs (e.g. in a landfill or in an anaerobic digester where the process can be controlled), the composition is typically 50-75% methane (CH4) and 25-50% carbon dioxide (CO2), with smaller amounts of other gases such as nitrogen, hydrogen, and hydrogen sulfide (that imparts the smell of rotting eggs). Biogas can be used as a replacement for natural gas (methane) for electricity production, space heating, water heating, and process heating. It can be compressed to replace natural gas for use in vehicles, internal combustion engines, or fuel cells [2-6].

Illustration of anaerobic digestion and uses of the products, from Ref. 2. Wet-stream municipal solid waste (MSW) is a widely-used source of material suitable for anaerobic digestion for methane production. There have been many studies of anaerobic digestion of MSW, including one by J.F.K. Earle, D.P. Chynoweth, and R.A. Nordstedt [3] from which many of the data quoted here are taken. The typical 65% methane, 35% carbon dioxide mixture of the biogas depends on a number of parameters including the composition of the waste stream, the length of retention time in the digester, and prevailing physico-chemical conditions in the digester. The heating value for biogas straight from the digester is 400 to 600 Btu/SCF

(units are British thermal units per standard cubic foot). A ton of biodegradable MSW produces up to 1100 standard cubic feet of methane [4]. Using combined heat and power with 80% efficiency, this quantity of methane would produce 1.2 MW from the 150 t/d Alachua County wet-stream waste, enough to supply power for the plant plus extra for sale to the utility [4-6]. Raw biogas may be burned as fuel in either internal combustion engines or in gas turbines. The latter are preferred since impurities in raw biogas may corrode internal combustion engines. The gas can be cleaned of CO2, H2S and other impurities to produce “synthetic natural gas” for injecting into the gas mains, as a vehicle fuel or for producing electricity. Many cities in Europe, including Berne Switzerland run their buses on synthetic methane produced from sewage sludge. In addition to wet-stream MSW, sewage sludge might be co-digested, which would bring the total power produced (combined heat and power) to 3-5 MW when the savings in processing energy for the present aerobic digestion of sludge is taken into account. Co-digestion of MSW with sludge or animal waste is common in European countries that have a long-history of anaerobic digestion [4]. An anaerobic digester that would handle all the wet-stream waste plus the sewage sludge for Alachua County would cost in the range of $10,000,000 [4]. If the output is 5 MW, this is a cost per kW of $2,000. This can be compared with the price per kW of $3,000 for the biomass power plant presently under consideration by the City of Gainesville [7]. Recently, fuel cells have been developed that can operate directly from raw (digester) biogas, ADG. In 2001, a first-of-its kind 200 kW fuel cell power plant was operated on digester gas at the Yonkers, NY wastewater treatment facility [8]. Based on the successes at the Yonkers demonstration project, several commercial fuel cells have been installed at sewage wastewater treatment plants in Boston, MA, Portland, OR, Calabassas, CA, and Cologne, Germany. Recently, the New York Power Authority announced that eight AGD-fueled fuel cells will be sited at various wastewater treatment plants serving New York City [8]. Commercial versions of fuel cell power plants designed to operate on ADG are now available. An ADG fuel cell that produces 1 MW began operation at King County WA waste water treatment plant at Renton in 2004 [9]. Fuel cells, unlike combustion technologies, have very low emissions of carbon monoxide, sulfur oxides, nitrogen oxides, and non-methane organic carbon. The fuel cell at Yonkers would reduce greenhouse emissions by 8,900 MT/yr. Electrical efficiency is up to 50%, while overall efficiency is as high as 70-80% with heat recovery. In the US there is the potential to provide 2 GW from sewage treatment plants. The quantity of MSW exceeds that of sewage sludge by at least a factor of 10, which translates to over 20 GW of power from anaerobic digestion of various wastes without including any from animal manures [4,8,9]. A Whole Foods grocery store in Connecticut has installed a fuel cell to provide all its power [10]. This has great advantage over central power production because of the higher efficiency of the fuel cell compared to conventional power plants and there is no loss in transmission lines. Using a compact modular anaerobic digester [3,11] located in a shopping center, the food waste from a grocery store and surrounding restaurants could provide ADG for the fuel cell, while eliminating hauling of the waste to a disposal site and the transmission of power back to the shopping center.

In addition to the fuel value of the methane produced by anaerobic digestion of wastes, a “liquor” and a solid digestate consisting of about 60% by weight of the solids in the incoming waste remains after digestion [4,8]. Thus, for Alachua County with about 150 t/d of wet-stream waste, which is roughly 50% solids and 50% water, these two materials would amount to about 45 t/d. These are valuable fertilizers, especially the solid fraction that is a humus-like material high in organic carbon. As discussed more fully in the section on Biosolids, this material is quite useful as a soil amendment, potting material, or if dried and pelletized, is a marketable organic fertilizer [12]. Its use in this way contributes to reducing greenhouse gases through the carbon sequestered in the soil. References 1. http://en.wikipedia.org/wiki/Biogas 2. “Turning Waste into Gold in the Future Cityscape,” Stephen Salter, Water Sentinel, SeptOct 2007, p 16. 3. “Anaerobic Composting of Municipal Solid Waste: Biogas Utilization,” J.F.K. Earle, D.P. Chynoweth, and R.A. Nordstedt, Florida Cooperative Extension Service Bulletin 271, June 1991. 4. “Producing Energy and Fertilizer from Organic Municipal Solid Waste,” U. Zaher, D-Y. Cheong, B. Wu, and S. Chen, Washington State University, Ecology Publication No. 0707-024, June 26, 2007. 5. Biogas Resource Center, www.energynetwork.net/resource_center/launch_documents/biogas.php 6. Basic Data on Biogas-Sweden at www.energynetwork.net/resource_center/launch_documents/documents/Basic_data_on_biogas_in_S weden_2006_(11p).pdf 7. City of Gainesville Biomass Proposal, http://www.gru.com/Pdf/futurePower/BindingProposals/Binding%20Proposal%20Recom mendation%20CC%20Presentation%204-28-08.pdf. 8. “Technical Assessment of Fuel Cell Operation on Anaerobic Digester Gas at the Yonkers, NY Waste Water Treatment Plant, R.J. Spiegel and J.L. Preston, Waste Management 23, 709 (2003). 9. “Lessons Learned from the World’s Largest Digester Gas Fuel Cell,” E. Allen, J. Hennessy, G. Bush, C. Nelson, Presentation for WEFTEC Conference, Nov. 2, 2005. 10. “Whole Foods to be Powered by Fuel Cell,” www.hydrogencarsnow.com/blog2/index.php/hydrogen-economy/whole-foods-marketto-be-powered-by-fuel-cell/. 11. Dr. Jose Sifontes, Sigarca, Inc., Gainesville, FL. 12. Dr. Amir Varshovi, GreenTechnologies, Gainesville, FL.