WASTE INCINERATORS

Description

In general waste is incinerated at a sufficiently high temperature (800 °C) to turn some of the substances into gases and release others as an aerosol; and then this mixture of gases and particles is further incinerated at a higher temperature (1000 °C).

There are several types of incinerator. The simplest are refractory-lined chambers into which oil or gas fired burners are directed. Mass burn incinerators have grates on the floor through which air can flow. Rotary kilns rotate to mix hazardous or clinical wastes. Multiple hearth incinerators, which can be used for high throughput of sewage sludge, consist of a many layered column. Sludge is introduced at the top to dry it out before it is moved to sequentially lower chambers. A more flexible design is the >fluidised bed incinerator . In this, a bed of hot sand is fluidised from below by jets of hot air. The waste is dropped into this and primary incineration takes place. Mobile incinerators can be used for the on-site treatment of heavily contaminated soil.

Many substances require little auxiliary fuel for incineration, but this depends upon the calorific value of the waste. For example, the incineration of contaminated soil would require fuel, whereas municipal waste may not, and indeed may even be a source of heat and electrical power. Sewage sludge would usually require to be dried to 25 % dry solids before it could be incinerated autothermally.

Waste incinerators generally have in-built systems to prevent unwanted gaseous emissions. These usually consist of cyclones, bag filters and lime dosing for flue gas desulphurisation.

Noncombustible material forms a loose ash which has to be collected. The ash may contain metal that can be recycled. Fine particles are carried out by the exhaust gases and collected as fly ash. Some systems mix this with cement and sealant to make building aggregate.

Application

Waste incinerators are able to destroy a wide range of contamination. They can have high combustion efficiencies (up to 99 %) for many contaminants. However, they tend to have a high capital cost. Waste incinerators have applications in the incineration of municipal waste, sewage sludge, clinical waste, special (chemical) waste and agricultural waste including animal carcasses. Waste incineration is an integral part of a combined heat and power plant.

Water jet eductors will remove sticky or gummy particulate material from a gas stream. Self-induced spray scrubbers have a lower operating pressure drop and can be slightly less efficient at particle separation; they are generally not used for trace gas removal.

Sizing

A typical waste incinerator dealing with about 20 tonnes/day of waste would have an electrical consumption of about 400 kW. Electrical power is required for compressors, extraction fans, screw conveyors, hydraulics, washing and lighting. Multiple hearth incinerators can deal with up to 300 tonnes/day whereas fluidised bed incinerators tend to take smaller loadings.

In some venturis the cross sectional area of the throat can be varied to keep the operating pressure drop constant. In this way, a constant efficiency can be maintained in spite of variations in throughput.