Solid waste in landfills produces methane, the major constituent in natural gas. When landfills reach capacity, they are typically sealed and covered with layers of dirt. The biological process of decomposition produces a low of grade methane gas. It is possible to "mine" this gas, drill a well and collect it for use as a fuel to generate electricity using an internal combustion engine modified for this purpose or burned in a boiler to make steam. One drawback to this type of system is the quality of gas produced. It is typically very low grade methane gas and contains many compounds and pollutants which must be "scrubbed" or removed from the gas before being burned in an engine. These compounds will cause significant degradation or damage to the engine in a short amount of time if they are not removed.