Pyrolysis of municipal solid waste

Abstract A state‐of‐the‐art review describing the characteristics of municipal solid waste (MSW) and assessing the chemistry and technology of pyrolysis of municipal solid waste is presented. The economics of the pyrolysis process are outlined. Combustibles constitute on average about 60% of the weight of MSW and result in an average heating value (“as received” basis) of about 3,000 to 6,000 Btu/Ib. This makes MSW attractive for thermal treatment. Municipal solid waste can be converted to gas, liquid and solid products by pyrolysis. Due to the complexity in composition of MSW the exact mechanism of pyrolysis is not known. Both homogeneous and heterogeneous reactions occur at the same time and both heat and mass transfer take place during the process. The relative yields of different products depends on the temperature of pyrolysis and the rate of heating. High pyrolysis temperatures and high heating rates favour the production of gases indicating high energies of activation for gasification reactions. At low temperatures, below 800°C, the pyrolysis process is reaction‐rate controlled, while at high temperatures, above 1,200°C, the process is diffusion‐rate controlled. Conditions of good heat and mass transfer are required for gasification of MSW. The residual char after pyrolysis can be gasified by further treatment with steam, hydrogen or carbon monoxide and water. The heat available from the products of pyrolysis is sufficient to sustain the process and yield some excess energy. Three types of reactor design have been generally used in the investigation of pyrolysis of MSW; fixed bed reactor, fluidized bed reactor and rotary kiln reactor. The advantages and weak points of each of these are briefly discussed. The costs of disposal of MSW by pyrolysis appear to be competitive with incineration.