Chemical transformations of solid waste typically involve a change of phase. To reduce the volume and to recover conversion products, the principal chemical processes used to transform municipal solid waste include:

1) Combustion

2) Pyrolysis

3) Gasification

All three of these processes are often classified as thermal processes.

1. Combustion

Combustion is defined as the chemical reaction of oxygen with organic materials, to produce oxidized compounds accompanied by the emission of light and rapid generation of heat. In the presence of excess air and under ideal conditions, the combustion of the organic fraction of municipal solid waste can be represented by the following equation:

$\text{Organic matter} + \text{excess air} \rightarrow N_2 + CO_2+ H2_O + O_2 + \text{ash} + \text{heat}$

Excess air is used to ensure complete combustion.

2. Pyrolysis

Pyrolysis is the term used to describe the process. In contrast with the combustion process, which is highly exothermic, the pyrolytic process is highly endothermic. For this reason, destructive distillation is often used as an alternative term for pyrolysis.

The characteristics of the three major component fractions resulting from the pyrolysis of the organic portion of municipal solid waste are:

• A gas stream containing primarily hydrogen, methane, carbon monoxide, carbon dioxide and various other gases, depending on the organic characteristics of the waste material being pyrolyzed.

• A tar and oil stream that is liquid at room temperature and contains chemicals such as acetic acid, acetone and methanol

• A char consisting of almost pure carbon plus any inert material that may have entered the process.

The reaction is as follows:

$3(C_6H_{10}O_5) \rightarrow 8H_2O + C_6H_8O +2CO + 2CO_2 + CH_4 + H_2 + 7C$

3. Gasification

The gasification process involves partial combustion of a carbonaceous fuel so as to generate a combustible fuel gas rich in carbon monoxide, hydrogen and some saturated hydrocarbons’, principally methane. The combustible fuel gas can then be combusted in an internal combustion engine or boiler.