Pyrolyzers have been used since ancient times to produce charcoal (Figure 3.2). Early pyrolyzers operated in batch mode using a very slow rate of heating and for long periods of reaction to maximize the production of char. If the objective of pyrolysis was to produce the maximum amount of liquid or gas, then the rate of heating, the peak pyrolysis temperature, and the duration of pyrolysis had to be chosen accordingly. These choices also decided what kind of reactor was to be used. Table 3.5 shows how the yield is maximized for different choices of heating rate, temperature, and gas residence time.

Modern pyrolyzers are more concerned with gas and liquid products, and require a continuous process. A number of different types of pyrolysis reactor have been developed. Based on the gas-solid contacting mode, they can be broadly classified as fixed bed, fluidized bed, and entrained bed, and then further subdivided depending on design configuration. The following are some of the major pyrolyzer designs in use:

• Fixed or moving bed

• Bubbling fluidized bed

• Circulating fluidized bed

• Ultra-rapid

• Rotating cone

• Ablative

• Vacuum

Except for the moving bed other gasifier types are illustrated in Figure 3.9.