Pyrolysis is an irreversible chemical change brought about by the action of heat in an atmosphere devoid of oxygen. Synonymous terms are thermal decomposition, destructive distillation, and carbonization. In partial combustion, oxygen is present in insufficient quantities to cause complete combustion (i.e., less than SOR). Normal combustion, as in conventional incineration requires the presence of sufficient amount of oxygen which will ensure complete oxidation of organic matter. Using cellulose $\left(\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}_{5}\right)$ to represent organic matter, the reaction is

$\begin{array}{ll}{\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}_{5}+6 \mathrm{O}_{2}} & {\rightarrow 6 \mathrm{CO}_{2}+5 \mathrm{H}_{2} \mathrm{O}+\text { heat }} \\ {\left(\mathrm{CH}_{4}+2 \mathrm{O}_{2}\right.} & {\left.\rightarrow \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O}+\text { heat }\right)}\end{array}$

In order to ensure complete combustion and to remove the heat produced during the reaction, excess air is supplied which leads to air pollution problems.

In the case of partial combustion, the reaction would be

$\begin{array}{ll}{2 \mathrm{CH}_{4}+2 \mathrm{O}_{2}} & \rightarrow {2 \mathrm{CO}+4 \mathrm{H}_{2}} \\ {\mathrm{CO}+\mathrm{H}_{2}} & \rightarrow {\text { HCHO (formaldehyde) }} \\ {\mathrm{CO}+2 \mathrm{H}_{2}} & {\rightarrow \mathrm{CH}_{3} \mathrm{O}_{4}(\text { methanol) }}\end{array}$

Thus even the simplest of hydrocarbons will yield a variety of products under conditions of partial combustion. As the complexity of fuel increase the variety of possible products also increases. Pyrolysis, unlike incineration is an endothermic reaction and heat must be applied to the waste to distil off volatile components.

When the waste is predominantly cellulose under slow heating at a moderate temperature, the destruction of bonds is selective (the weakest breaking first) and the products are primarily a non-combustible gas and anon-reactive char. On the other hand, when the waste is rapidly heated to a high temperature, complete destruction of the molecule is likely to take place. Under intermediate conditions, the system would yield more liquid of complex chemical composition. Normally these two processes are referred to as low temperature and high temperature pyrolysis respectively. Pyrolysis is carried out at temperatures between 500 and $1000^oC$ to produce three component streams.

• i) Gas: It is a mixture of combustible gases such as hydrogen, carbon monoxide, methane, carbon dioxide and some hydrocarbons.

• ii) Liquid: It contains tar, pitch, light oil and low boiling organic chemicals like acetic acid, acetone, methanol, etc.

• iii) Char: It consists of elemental carbon along with the inert materials in the waste feed.

The char, liquids, and gas have a large calorific value. This calorific value should be utilized by combustion. Part of this heat obtained by combustion of either char or gas is often used as process heat for the endothermic pyrolysis reaction. It has been observed that even after supplying the heat necessary for pyrolysis, a certain amount of excess heat still remains which can be commercially exploited. Though a number of investigations have been made, only a few have led to full-scale plants.