2.            STATE OF THE ART TECHNOLOGY

The basic principles of incineration are as follows: the heat produced from burning waste is transferred to water boilers to produce steam that in turn drives steam turbines-generators to produce electricity. Newer methods of waste incineration include gasification and pyrolysis, which heat the waste under controlled conditions to produce low-to-medium-heating fuel gases, together with tars, char and ash. These products are ultimately dependent on the type of reactor and the type of waste, but most systems produce a raw gas suitable for direct firing in kilns or boilers.

Mass burning

Mass burning is the oldest, simplest and most popular method of recovering energy from municipal solid waste (MSW). In mass burn systems untreated waste is simply incinerated. The heat given off is converted into steam, which can then be passed through a turbine to generate electricity, used directly to supply heat to nearby industries or buildings, or to produce both electricity and low temperature heat suitable for space heating. Producing electricity from high temperature steam, and usable heat as a by-product, is called co-generation or "combined heat and power (CHP)." This results in more efficient use of fuel.

There are several types of MSW mass burn combustion systems for recovering energy from waste, including refractory and modular furnaces; but "water-wall" furnaces are the most popular at the present time. Water-wall technology is similar to the furnaces used at coal burning power plants. Exhaust gases are generally passed through a "scrubber", an "electrostatic precipitator" or a "fabric filter baghouse" in order to remove particulates such as "fly ash", and acid gases, before they are released through the stack.

The most common incineration technologies

Water-wall technology

MSW is received into a pit where an overhead crane mixes the waste to evenly distribute combustible materials and moisture, and removes oversized materials. The crane feeds waste into a charging hopper from which it is then fed onto the grate, usually by means of a hydraulic ram. The grate agitates and transports the waste across the combustion chamber. The waste is transformed into a sterile (biologically inactive) ash, which can be used in construction or disposed of to landfill.

Air for combustion is introduced from under the grate (underfire air) and from nozzles located in the furnace above the grate (overfire air). Underfire air initiates combustion and keeps the grate cool. Overfire air helps to mix the combustion gases and ensure more complete combustion of volatiles. Non-combustible material and ash are discharged from the end of the grate into a water quench pit from where it is removed for further treatment and recovery.

Energy is recovered from the hot flue gases in a water-wall boiler. These boilers normally comprise superheaters and economisers to increase energy recovery. Energy is transferred from the hot flue gases to water in the boiler tubes, generating hot water and steam and cooling the flue gases. The steam is used to turn a turbine and generate electricity. It is also possible to use the steam as a heat source for space heating or industrial processes. The cooled flue gases pass through pollution abatement plant consisting of semi-dry scrubbers (acid gas removal) and fabric filters (fine particulate removal), before exhausting to air via a stack.