By Joseph A. Scroppo and Ren6 M. Laurens (M-C Power Corporation, USA), Greg Bush (King County, Department of Natural Resources, USA), and David Reynolds (CH2M Hill, USA)
Wastewater treatment plants using anaerobic digestion to stabilise and reduce pathogenic microorganisms produce large quantities of gas. This gas contains about 60% methane and 40% CO 2. The digester gas can be used at the site for heat or power. In some cases the CO 2 is removed and the methane is sold as pipeline quality gas. A molten carbonate furl cell, using the digester gas without CO 2 removal, has the potential to provide significant environmental and economic benefits. Engineering studies have shown the benefits of locating a 1 MW molten carbonate fuel cell at a wide variety of US sites. A demonstration plant is being developed with the participation of the US Environmental Protection Agency, King County Department of Natural Resources, Washington State Energy Office, M-C Power Corporation and Puget Sound Energy. The demonstration plant will verify the merits and applicability of MCFC technology throughout the US.
All wastewater treatment plants (WWTPs) produce some solids as a by-product. Anaerobic digestion is a biological process that is widely used for reducing pathogen content and stabilisation of these solids. This process converts approximately 40% of the solids to digester gas. Some typical characteristics of digester gas are shown in Table 1. This gas typically contains approximately 60% methane and 40% carbon dioxide. Therefore, the digester gas has a significant fuel value. Currently, digester gas is used for heating the anaerobic digester, space heating, generation of electricity using internal combustion engines, converted to natural gas, or is flared. Now a new option is emerging for utilising the digester gas: converting it to electricity using molten carbonate fuel cells (MCFCs). These fuel ceils do not require removal of CO 2 from the digester gas, and produce electricity at high efficiency and with very low levels of pollutant emissions.
M-C Power Corporation is one of the world's leading developers and manufacturers of MCFCs for stationery power generation. Currently a development stage company, M-C Power plans to complete its product development and demonstrations in 2001.