Types of Technologies for Converting LFG into Energy
A number of factors, including the availability of an energy market, project costs, potential revenue sources, and other technical considerations, can determine which technologies are most appropriate for a particular LFGE project. Technologies for converting LFG into energy include the following.
Approximately 70 percent of the LFGE projects currently in operation in the U.S. are used to generate electricity, either for onsite use or to sell to the grid (U.S. EPA 2008c). Electricity from LFG can be generated using a variety of technologies, including internal combustion engines, gas turbines and microturbines, with two-thirds of LFGE electricity generation projects using internal combustion engines or turbines. One million tons of landfilled MSW can produce an electricity generation capacity of 0.8 MW (U.S. EPA 2007a).
Direct use of LFG
Direct use of LFG, which involves transmitting the medium-Btu gas via pipeline to be combusted by an end user, accounts for approximately 30 percent of all LFGE projects in the U.S. (U.S. EPA 2008c). LFG can be combusted by end users to fuel boilers, dryers, kilns, greenhouses and other thermal applications. Current industries using LFG include automobile manufacturing, chemical production, food processing, pharmaceutical, cement and brick manufacturing, wastewater treatment, consumer electronics and products, and prisons and hospitals (U.S. EPA 2007a). One million metric tons of landfilled MSW can produce between 8,000 and 10,000 pounds of steam per hour when LFG is used to fuel a boiler (U.S. EPA 1996). The economics of an LFG project improve the closer the landfill is to the end user. The piping distance from an LFG project to its end user is typically less than 10 miles, although piping LFG up to 20 miles can be economically feasible, depending on the gas recovery at the landfill and the energy load at the end-use equipment (U.S. DOE Undated).
Combined Heat and Power
One specific type of direct use of LFG is as a fuel source for combined heat and power (CHP) or cogeneration systems that generate both electricity and thermal energy. CHP systems can achieve substantially higher efficiencies than separate heat and power systems that do not use the waste heat produced in electricity generation. Thermal energy cogenerated by LFGE projects can be used for on-site heating, cooling, and/or process needs, or piped to nearby industrial or commercial users to provide a second revenue stream for the project (U.S. EPA 2008c). CHP is often a better economic option for end users located near the landfill or for projects where the end user generates both electricity and waste heat.
Production of alternate fuels from LFG can involve several technologies, including the following.
Municipalities can deliver LFG to the natural gas pipeline system as both a high- and medium-Btu fuel. Upgrading LFG to produce high-Btu gas involves separating CH4 from the CO2 components of LFG. The separated CH4 can be sold to natural gas suppliers or used in applications requiring high-Btu fuel. Although expensive, newly developing technologies are reducing the cost of these types of projects, which are ideally suited for larger landfills located near natural gas pipelines.
LFG can also be converted to vehicle fuel. Vehicle fuel applications involve using LFG to produce compressed natural gas (CNG), liquefied natural gas (LNG), or methanol. This process involves removing CO2 and other trace impurities from LFG to produce a high-grade fuel that is at least 90 percent CH4. Currently, CNG and LNG vehicles comprise a very small portion of automobiles in the U.S., so there is not a significant demand for these vehicle fuels. However, with growing interest in alternative fuels, demand is expected to increase.
—From the Draft Clean Energy Strategies Guide: Landfill Gas to Energy at www.epa.gov.