Pyrolysis is a major process for waste disposal in which waste is thermally degraded in an inert environment. Pyrolysis is usually carried out at a lower temperature (250-900°C) and at low pressure. As shown in Table 6.4 [27-29], there are various modes of pyrolysis depending on the method of pyrolysis and the operating conditions. The heating value of pyrolysis gas typically lies between 5 and 15 MJ/m3 based on MSW and between 15 and 30 MJ/m3 based on RDF [27].

Conventional pyrolysis reactors are: fixed bed, fluidized bed, entrained flow, moving bed, rotary kiln, ablative reactor, and so on. Generally waste pyrolysis generates a large number of compounds, and these need to be tracked for their effective uses. Generally, the pyrolysis process involves three stages: (a) A feed preparation and pretreatment step that includes grinding and drying. The grinding improves the feed quality and subse­quent heat transfer. The drying improves gas-solid contact, heat and mass transfer, and reactions in the reactor. (b) A pyrolysis reactor that generates

Source: Modified from Bridgwater and Bridge, 1991. A review of biomas pyrolysis and pyroly­sis technologies. In A. V. Bridgwater and G. Grassi, (Eds.) Biomass Pyrolysis Liquids Upgrading and Utilisation, London: Elsevier Applied Science, pp.11-92; Bridgwater, 1995 / Thermal biomass conversion technologies, Biomass and Renewable Energy Seminar, Loughborough University, IK, March; and Huber, Iborra, and Corma, 2006. Synthesis of transportation fuels from biomass: chemistry, catalysis, and engineering, Chem. Rev., ACS.

gas, solids containing mineral and metallic compounds, and liquids. (c) An upgrading of pyrolysis gas, liquids, and solids to generate more useful fuel, chemicals, and materials. The impurities in the products such as the pres­ence of arsenic and other materials can make the product very difficult to use and the process useless. These impurities are removed using appropriate purification technologies [30].

The pyrolysis process can also (a) recover organic fractions such as methanol as a material/fuel, (b) recover char for external use, (c) generate more efficient electricity using gas engines or turbines, and (d) reduce flue gas volume after combustion which may reduce flue gas treatment capital costs. The pyrolysis process is used for MSW and sewage sludge, decontamination of soil, synthetic waste, and used tires, cable tails, and metal and plastic materials for substance recovery. In general, pyrolysis is a very versatile process and has been extensively used for waste con­version. When the pyrolysis process is used to generate fuels or chemi­cals, a subsequent upgrading of pyrolysis oil is often carried out. Also, as shown in Table 6.4 [27-29], pyrolysis is often carried out in the presence of hydrogen (i. e., hydropyrolysis) to improve quality and quantity of the oil production.

A novel Chartherm process is a pyrolytic distillation process designed to maximize the useful recovery of both materials and energy from waste [7]. The process was developed by Thermya Co. in France with an industrial capacity of 1,500 kg/hr wood waste. The advantages of this process are (a) low operating temperature (300-400°C) compared to conventional pyrolysis and gasification temperature of 500-1500°C, (b) avoidance of tar and dioxin emissions and thereby reducing gas cleaning equipment, and (c) possibilities of recovering both energy and materials from the waste feed [31, 32].

The process of pyrolysis in its different formats has been extensively used to treat different types of waste. Some of the typical recently reported litera­ture studies on this subject are depicted in Table 6.5. This table by no means gives a complete account of all reported studies.