A fuel cell is an electrochemical device similar to a battery. Although both batteries and fuel cells generate power through an internal chemical reaction, a fuel cell differs from a battery in that it uses an external supply that continuously replenishes the reactants in the fuel cell. A battery, on the other hand, has a fixed internal supply of reactants. A fuel cell can supply power continuously as long as the reactants are replenished, whereas a battery can generate only limited power before it must be recharged or replaced. Most types of fuel cells can operate on a wide variety of fuels including hydrogen, digester gas, natural gas, propane, landfill gas, diesel, or other combustible gas. In some cases, such as in a wastewater treatment plant (WWTP), methane (biosolids gas) from anaerobic digesters can be reused in the fuel cell instead of flaring off the excess gas. Other advantages of fuel cells include few moving parts, modular design, and negligible emission of pollutants.