Types of incineration

Municipal incinerators | Industrial waste incineration | Medical waste | Waste to energy schemes | Refuse-derived fuel (RDF) | Prolysis / thermal gasification | Cement kilns

Municipal incinerators

Municipal waste incineration is still the first cause of dioxin pollution, according to a 1999 UNEP study. Over the past few years, many countries have updated their old incinerators or built new ones. In doing so, they have taken advantage of improved technologies for air pollution control. This has resulted in a substantial reduction in toxic emissions.

Although this is an improvement, toxic waste production is still a serious global issue. In fact, the problem has now shifted, and more dioxins and other toxic substances are appearing in the ashes, therefore creating new disposal and pollution problems.

Studies conducted in Europe have reported that emission measurements from some European incinerators fall within the new proposed EU emissions levels, but others exceed this limit.

Industrial/hazardous waste incineration

Only a few recent studies on emission testing of industrial incinerators are published in scientific literature. A study undertaken in Japan performed point measurements on nine industrial waste incinerators. Dioxin emissions were below the new EC limit for two of the incinerators and above this level for the remaining six.

In the US, one study reported on dioxin emissions of mobile soil burning incinerators. On-site remediation of soils, contaminated by these incinerators, is employed where sites contain compounds that are difficult to destroy. Data collected primarily from the trial burns of 16 incinerators, showed that 10 of the incinerators failed to meet the proposed EPA standard. The authors commented that a significant fraction of soil burning incinerators could have problems meeting the proposed future EPA limit.

Medical waste - useful waste into hazardous waste

Only 10 percent or less of a typical hospital's waste stream is potentially infectious. It is possible to sterilise this waste with heat, microwaves and other non-burn disinfection technologies.

The remaining waste is not infectious and often includes paper, plastic and food waste. This is similar to the waste coming from hotels, offices and restaurants, as hospitals serve all of these functions.

Incineration of medical waste has become a formidable chemical pollution problem, that is costly to manage and difficult to contain. Fortunately, there are alternative technologies available to ensure the appropriate sterilisation of medical waste.

Waste to energy schemes

The generation of energy from waste has increased. It is now used extensively by governments and industry, to "green" incineration, thereby making it more acceptable to the public. However, all the negative impacts of incineration also apply to "waste to energy" facilities.

Municipal Solid Waste (MSW) can be:

· Directly combusted in "waste-to-energy" incinerators;

· Processed as refuse-derived fuel (RDF), before incineration (or combustion in power plants); or

· Gasified using pyrolysis or thermal gasification techniques.

Landfill gas recovery is another "MSW-to-electricity" technology that permits electricity production from existing landfills. This is possible via the natural degradation of MSW by anaerobic fermentation (digestion) into landfill gas. Anaerobic digestion can also be used on municipal sewage sludge.

Moreover, the energy used to produce the product is lost and only a fraction of the intrinsic energy content of the materials can be recovered. Reuse and recycling, even from an energy perspective, are the preferred options.

Refuse-derived fuel (RDF)

Refuse-derived fuel (RDF) typically consists of pelletized or fluff MSW that remains after the removal of non-combustible materials such as ferrous materials, glass, grit, and other non combustible materials. The remaining material is then sold as RDF and used in dedicated RDF boilers or co-incinerated with coal or oil in a multi-fuel boiler. However, the environmental concerns of incineration also apply to RDF combustion facilities.

Pyrolysis/thermal gasification

Pyrolysis and thermal gasification are related technologies. Pyrolysis is the thermal decomposition of organic material at elevated temperatures, in the absence of gases such as air or oxygen. The process, which requires heat, produces a mixture of combustible gases (primarily methane, complex hydrocarbons, hydrogen and carbon monoxide), liquids and solid residues.

Thermal gasification of MSW is different from pyrolysis in that the thermal decomposition takes place in the presence of a limited amount of oxygen or air. The generated gas can then be used in either boilers or cleaned up and used in combustion turbine generators.

Both of these technologies are at the development stage, with a limited number of units in operation. Most of the environmental concerns for incineration also apply to pyrolysis and thermal gasification facilities.

Cement kilns

Throughout the world some 60 cement kilns have been modified so that various wastes can be burned along with conventional fuels.

However, cement kilns are designed to make cement and not to dispose of waste. A study by the US Centre for the Biology of Natural Systems, found that emissions of dioxins are eight times higher from cement kilns that burn hazardous waste than those that do not burn it.