These gases could cause fire and explosions in some landfills, promoting close monitoring by the California Environmental Protection Agency. Also, under the authority of the Clean Air Act, the U.S. EPA has proposed regulating air pollution emissions from both new and existing municipal solid waste landfills. These regulations require the control of methane and non-methane organic compound emissions. This effectively requires the collection and combustion of landfill gases. So, a good solution to the landfill gas problem is to collect it and use it to produce electricity.
Landfill gas (LFG) is generated by the natural degradation of MSW by anaerobic (without oxygen) micro-organisms. Once the gas is produced, the gas can be collected by a collection system, which typically consists of a series of wells drilled into the landfill and connected by a plastic piping system.
The gas entering the gas collection system is saturated with water, and that water must be removed prior to further processing. The typical dry composition of the low-Btu gas is 57 percent methane (natural gas), 42 percent carbon dioxide, 0.5 percent nitrogen, 0.2 percent hydrogen, and 0.2 percent oxygen. In addition, a significant number of other compounds are found in trace quantities. These include alkanes, aromatics, chlorocarbons, oxygenated compounds, other hydrocarbons and sulfur dioxide.
After dewatering, the LFG can be used directly in reciprocating engines. It can also be further processed into a higher-British thermal unit (Btu) gas (suitable for use in boilers for manufacturing processes, as well as for electricity generation via gas turbines.) The most important part of the scrubbing process is the removal of sulfur dioxide from the gas since it results in corrosion within the combustion equipment.
Further processing into a high-Btu gas requires the removal of carbon dioxide as well as all remaining trace components. The resulting pipeline-quality gas is of high enough quality to be blended with existing natural gas systems; however, since the passage of legislation in 1988 which makes a seller of LFG to a gas utility liable for impacts of toxics in the gas, no LFG has been sold to a gas corporation.
The gas is also suitable for electricity generation applications such as gas turbines and fuel cells. For example, Southern California Edison and Los Angeles Department of Water and Power operate a 40 kilowatt phosphoric acid fuel cell using processed landfill gas at a hotel/convention center complex in the City of Industry.
As of 1995, California has 56 of these “landfill gas recovery facilities;” 14 of them collected only gases and 42 others collected gases to produce electricity. These electricity facilities have an installed capacity of about 246 megawatts.
Current research in the area of landfill gas recovery involves the recirculation of the leachate generated in the landfill by the anaerobic decomposition process. The recirculation of the leachate through the waste in a lined and covered landfill effectively accelerates and enhances the generation of methane gas. This form of landfill design and operation converts the landfill into a bioreactor. Yolo County is currently involved in this research at their Davis landfill.
Since landfill gas recovery facilities are located at existing landfills, there are generally fewer permitting issues associated with them compared to other MSW-to-energy facilities. Some of the issues associated with LFG treatment and power generation equipment include:
- Ability to meet air quality requirements
- Handling and disposal of the condensate from the dewatering process
Landfill gas technology is commercially available in California. Recent proposed and existing environmental regulations and economic conditions (i.e., low avoided cost of electricity pricing, soft energy market brought about by declining oil prices, elimination of various energy and production tax credits and incentives) might slow the pace of the landfill gas project development. On the other hand, the Clean Air Act and concerns about global climate change may encourage development of new landfill recovery projects.