Thermal (or thermochemical) MSW-to-energy conversion systems consist of primary technologies which convert the municipal waste into heat or gaseous and liquid products, together with secondary conversion technologies which transform these products into more useful forms of energy, such as heat and electricity.
A wide array of thermal waste-to-energy technologies are available to convert the energy stored in solid wastes into more useful forms of energy. These technologies can be classified according to the principal energy carrier produced in the conversion process. Energy carriers are in the form of heat, gas, liquid and/or solid products, depending on the extent to which oxygen is admitted to the conversion process (usually as air).
The three predominant methods for thermal MSW-to-energy conversion are combustion in excess air, gasification in limited air, and pyrolysis in the absence of air.
Combustion
Conventional MSW combustion technologies (also known as incineration) raise steam through the combustion of municipal wastes. This steam may then be expanded through a conventional turbo-alternator to produce electricity. Fluidized bed combustors (FBC), which use a bed of hot inert material such as sand, are a more recent development. Bubbling FBCs are generally used at 10-30 MWth capacity, while Circulating FBCs are more applicable at larger scales.
Gasification
Gasification of municipal wastes takes place in a limited supply of oxygen and occurs through initial devolatilization of the waste, combustion of the volatile material and char, and further reduction to produce an energy-rich gas known as synthetic gas (a mixture of carbon monoxide and hydrogen). Synthetic gas (or syngas) has a lower calorific value than natural gas but can be efficiently used as fuel for boilers, engines, and combustion turbines after cleaning the syngas of impurities like tars and particulates.
Pyrolysis
Pyrolysis enables municipal solid wastes to be converted to a combination of solid char, syngas and bio-oil. Pyrolysis technologies are generally categorized as “fast” or “slow” according to the time taken for processing the feed into pyrolysis products. Using fast pyrolysis, bio-oil yield can be as high as 80 percent of the product on a dry fuel basis. Bio-oil can act as a liquid biofuel or as a feedstock for chemical production.
