What is the Pyrolysis?

Pyrolysis is an alternative to incineration for volume reduction and partial disposal of solid waste. The word pyrolysis comes from two Greek words meaning ‘fire’ and ‘breakdown’. Therefore pyrolysis is defined as breakdown by heat.

Pyrolysis is an irreversible chemical change brought about by the action of heat in an atmosphere devoid of oxygen. Because most organic substances are thermally unstable, they can, upon heating in an oxygen-free atmosphere, be split through a combination of thermal cracking and condensation reactions into gaseous, liquid and solid fractions.

In contrast to the combustion process, which is highly exothermic, the pyrolytic process is highly endothermic. For this reason the term destructive distillation is often used as an alternative term for pyrolysis. Destructive distillation of wood to produce methanol and coal gasification is other common pyrolytic processes.

Normal combustion, as in conventional incineration requires the presence of sufficient amount of oxygen, which will ensure complete oxidation of organic matter. Using cellulose (C₆H₁₀O₅) to represent organic matter, the reaction is:

C₆H₁₀O₅ + 60₂ ———- > 6CO_z + 5H₂0 + Heat

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 pyrolysis, the reaction would be:

3 (C₆H₁₀O₅) ______ > 8H₂0 + C₆H₈0 + 2CO + 2C0₂ + CH₄ + H₂ + 7C

CO + H₂ ______ > HCHO (formaldehyde)

CO + 2H₂ ______ > CH₃OH (methanol)

When the solid waste is predominantly cellulose, under slow heating at a moderate temperature, the destruction of bond is selective (i.e. the weakest breaking first) and the products are primarily a non-combustible gas and a non-reactive char. On the other hand, when the solid 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. Normally, pyrolysis is carried out at temperature between 500-1000°C to produce three component streams.

(а) Gas: It is a mixture of gas stream containing hydrogen, carbon monoxide, carbon dioxide, methane and some hydrocarbons.

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

(c) Char. Consisting of almost elemental carbon along with inert material that might have entered the process

The char, liquid and gas have a large calorific value. This calorific value should be utilised by combustion. Part of this heat obtained by combustion of either char or gas is often used as process heat for the endothermic reaction. It has been observed that even after supplying the heat necessary for pyrolysis, certain amount of excess heat still remains which can be commercially exploited.

Incineration/Combustion

Much of solid waste is combustible, and the destruction of this fraction, coupled with energy recovery, is an option in solid waste management. Combustion is a chemical reaction where the elements in the fuel are oxidised. In waste-to-energy plants, the fuel is, of course, the solid waste. The major oxidisable elements in the solid wastes are carbon and hydrogen, lb a lesser extent, sulphur and nitrogen are present. With complete oxidation, carbon is oxidised to carbon dioxide, hydrogen to water and sulphur to sulphur dioxide. Some fraction of the nitrogen may be oxidised to nitrogen oxides.

For proper incinerator operation, sufficient air must be supplied to meet the requirements of:

1. Primary and secondary combustion

2. Turbulence for mixing the air and the solid waste.

The combustion reaction is a function of oxygen, temperature, time and turbulence. There must be a sufficient excess of oxygen to drive the reaction to completion in a short period of time. The oxygen is most frequently supplied by forcing air into the combustion chamber with the aid of an air blower. Sufficient time must be provided for the combustion reactions to proceed.

Conventional incineration

The basic arrangement of the conventional incinerator is shown in operation begins with the unloading of solid wastes from collection trucks into a storage bin. Storage capacity usually averages about the volume of one day. Overhead crane is used to batch load wastes into the combustion chamber.

Although the solid waste may have some heat value, it is normally quite wet and is not autogenous (self-sustaining in combustion) until it is dried. Conventionally, auxilliary fuel is provided for the initial drying stages. In addition to fuel, air may be supplied by means of an air blower.

Combustion chamber consists of bottom grates on which the combustion occurs. As the solid waste enters the combustion chamber and its temperature increases, volatile materials are driven off as gases. Rising temperatures cause the organic component to thermally crack and form gases. When the volatile compounds are driven off, fixed carbon remains. When the temperature reaches the ignition temperature of carbon, it is ignited. Most combustors operate in the range of 980 to 1090°C, which ensures good combustion and elimination of odours.

The heat liberated during combustion can be recovered by providing a system of pipes in which water is turned to steam to generate electricity.

Some fly ash and other particulate matter may be carried through the chamber. To meet local air pollution control legislations, some form of air pollution control devices (scrubber and baghouse) are provided. The cleaned gas is discharged through stack. Bulk volume reduction in incinerators is about 90%. Thus, about 10% of the material must still be carried to a landfill.