1. Introduction

Incineration can be defined as a controlled combustion process for burning solid, liquid and gaseous combustible wastes to gases and residue containing non-combustible material.

During combustion, moisture is vaporized whereas the combustible portion is vaporized and oxidized. Carbon dioxide, water vapour, ash and non-combustibles are the end-products.

However, radioactive waste, halogenated plastics, large amounts of reactive chemical waste, mercury, Cd & ampoules of heavy metals should not be incinerated. All such waste having combustion value above 2000 Kcal/kg can be burnt in single-chamber incinerators while those having combustion value of above 3500 Kcal/kg should be burnt in pyrolytic double chamber incinerators.

2. Air for combustion

Air is required in the combustion process, for which it can be supplied beneath the grates (underfire air) or over the fuel bed (overfire air) to provide turbulance. In order to effect complete combustion and promote turbulance, at least 50% excess air should be provided in incinerators. However, too large an excess air would lower furnace temperature. Refractory furnaces require 150 - 200% excess air; whereas water tube wall furnaces require about 50-100% excess air. The total air required for municipal incinerators is split into overfire (70%) underfire (10%) and secondary (20%) for good performance.

3. Types of Incinerators

3.1. Multiple Channel Incinerators

Combustion in multiple channel incinerators proceeds in two stages, primary (solid fuel combustion) in the ignition chamber followed by secondary (gaseous phase combustion) in secondary combustion chamber.

In the ignition chamber, drying, ignition and combustion of waste occurs. The moisture and volatile components of the waste are vaporised and partially oxidised while passing from the ignition chamber through the flame port connecting the ignition chamber with the mixing chamber. The volatile components of refuse and the products of combustion flow from the flame port to mixing chamber in which secondary air is introduced.

Secondary combustion achieves combustion of unburnt furnace gases and carbon suspended in the gases and elimination of odours. The combination of adequate temperature and additional air, along with secondary burners if necessary, help initiate the secondary combustion process. Turbulant mixing occurring as a result of restricted flow areas and abrupt changes in flow directions both in horizontal and vertical plane furthers the gaseous phase reaction. Due to the abrupt changes in directions and expansions, the particulate matter is removed by wall impingement and simple settling. The gases finally escape through a stack or a combination of gas cooler and induced draft system.

Multiple chamber incinerators are of two types: i) Retort, and ii) Inline. Retort type is preferred when the quantity of waste to be burnt is less than 340 kg/hr (750 Ibs/hr) while Inline type is used for higher capacities.

3.2. Municipal Incinerators

Municipal incinerators are constructed and operated for large capacities. In general, such installations have the following components:

i) Reception and Storage ii) Charging Hopper and Chute iii) Furnace iv) Grates and Stoking

The incinerators require a large amount of water for quenching of the clinker, for the removal of fly ash in the water scrubbers and in the boilers.The amount of water required varies from 1500 to 9000 litres/tonne (350 to 2000 gallons per tonne) of refuse burnt depending on the design. Energy consumption by various units in the incinerator varies from 30-50 KWH/tonne of refuse burnt depending on the type of unit.

4. Auxiliary Fuels

Auxiliary fuels will be required in the following cases:

• i) Furnace starting and warming up.
• ii) Promotion of primary combustion when the solid waste is wet or does not have adequate calorific values.
• iii) Completion of secondary combustion to ensure odour and smoke control.
• iv) Additional heat is required for heat recovery units. When the refuse has a low calorific value with lower content,auxiliary fuel will be required.

5. Recovery of Heat

Recovery of heat has been practised extensively in European installations but to a limited extent in USA. The heat recovered can be used for supplying hot water, generating electricity and to heat the plant during winter. Heat is recovered by adopting suitable systems such as

• i) waste heat boiler system with tubes located beyond conventionally built combustion chambers;
• ii) water tube wall combustion chambers;
• iii) combination of the above; and
• iv) integrally constructed boiler and water tube wall combination.

Excess air needed will depend upon the system adopted. Low excess air increases the amount of heat recovered and reduces the capacity of air pollution equipment. The theoretical efficiency of the recovery process can be as high as 70% depending on the type of equipment used. The amount of steam produced varies from 1 to 3.5 kg per kg of solid waste.

6. Products of Incineration

• Siftings
• Residue
• Clinker and Flyash
• Suspended Particulates
• Waste Gas

7. Incineration of Plastics

Plastics found in wastes may be thermoplastics which soften, deform and melt when heated or thermosettings which are stable. Plastics are based on polymers generally containing C, H & O which at normal incineration temperature of $600^oC$ and above get converted to $CO_2$ and $H_2O$.

At temperatures above $600^oC$, nitrogen oxides may be formed if it contains nitrogen as in the case of nylon polyurathans, polyamides and nitriles. Fumes of PVC, HC1 and HF may be released in some wastes. Specially designed incinerators are available for burning waste PVC and recovery of HCl.