Incineration

Incineration is a method of disposing of waste by burning it at very high temperatures.Overview of incineration Knox, Andrew, An Overview of Incineration and EFW Technology as Applied to the Management of Municipal Solid Waste (MSW), University of Western Ontario, Canada, February 2005 Incineration is often described as "thermal treatment". In effect, incineration of waste materials converts the waste into gaseous emissions to the atmosphere and residual ash.

Incineration usually functions as an alternative to landfilling and bioremediation. Incineration reduces the overall volume of the waste stream and, especially for hazardous wastes, is intended to reduce the wastes' toxicity and other hazardous characteristics. It is particularly popular in countries such as Japan where land is a scarce resource.

How incinerators work

An incinerator is a furnace for burning refuse. The older and simpler kind of incinerator was a brick-lined cell with a metal grate over a lower ash pit, with one opening in the top or side for loading and another opening in the side for removing incombustible solids called clinkers. Many small incinerators formerly found in apartment houses have now been replaced by trash compacters.

The rotary-kiln incineratorRotary-kiln incinerators An excellent, detailed description of rotary-kiln incinerators used by municipalities and by large industrial plants has a long, slightly inclined cylindrical tube through which refuse is moved continuously. In the first section, the refuse is dried. In the second section, the dried refuse is moved onto a rocking grate where it is ignited and partially burned. The third and last section is a refractory-lined cylinder where combustion is completed. The clinkers spill out at the end of the cylinder. A tall flue gas stack, fan, or steam jet supplies the needed draft. Ash drops through the grate, but many particles are carried along with the hot gases. The particles and any combustible gases may be combusted in an "afterburner".Photos of rotary-kiln incinerators with afterburners. In order to control air pollution, the combustion product gases are further treated with acid gas scrubbers to remove sulfuric acid and nitric acid emissions, and then routed through bag houses to remove particulates before the gases are released into the atmosphere.

The heat produced by the rotary-kiln incinerator can be used to generate steam which may then be used to drive an electrical generator producing a net electrical power for the local utility of about 500 to 600 kWh of per ton of waste incinerated.The ABC of Integrated Waste Management Thus, for example, the production of 50 MW of electrical power would require the incineration of about 2,200 tons per day of waste.

Large-scale municipal and hazardous waste incinerators

The history of municipal solid waste (MSW) incineration is linked intimately to the history of landfills and other disposal alternatives. The merits of incineration are inevitably judged in relation to those of the alternatives. Since the 1970s, recycling and other prevention measures have changed the context for such judgements.

The history of hazardous waste incineration is more recent. It is marked by major milestones in environmental regulation (e.g., RCRA) and the ebb and flow of local opposition over the siting and management of incinerator facilities.

The use of MSW incinerators and medical waste incinerators has been on the decline in the United States. Of the 186 MSW incinerators in 1990, only 112 remained by 2003, and of the 6200 medical waste incinerators in 1988, only 115 remained in 2003."Waste Incineration: A Dying Technology The primary reasons for the decline are most probably the public's opposition to such plants and the newer, stricter governmental emission regulations. Other reasons might be: that it is quite expensive to safely dispose of the residual ash; that it is sometimes less expensive to dispose of municipal wastes in landfills; and that it is politically difficult to replace aging plants.

Emissions from incineration

Incineration itself generates other forms of waste, such as the residual ash and the emission to the atmosphere of combustion product gases and particulate matter. The health hazards associated with the emissions and the ash are often the subject of controversy.

Gaseous emissions

The combustion product gases exhausted to the atmosphere by incineration are a source of concern. The main pollutants in the exhaust gases include acid gases such as hydrogen chloride, sulphur dioxide, nitrogen oxides (referred to as NOx), and carbon dioxide. The most serious environmental concerns about the incineration of municipal solid wastes (MSW)is that it produces significant amounts of dioxin and furan emissionsBeychok, M.R., A data base of dioxin and furan emissions from municipal refuse incinerators, Atmospheric Environment, Elsevier B.V., January 1987 to the atmosphere. Dioxins and furans are considered by many to be serious health hazards. However, advances in emission control designs and very stringent new governmental regulations have caused large reductions in the amount of dioxins and furans produced by incinerating municipal solid wastes.

The quantity of pollutants in the emissions from large-scale incinerators is reduced by a process known as scrubbing as well as other processes.

Solids

Incineration produces fly ash and bottom ash just as is the case when coal is combusted. The total amount of ash produced by municipal solid waste incineration ranges from 15% to 25% by weight of the original quantity of waste, and the fly ash amounts to about 10% to 20% of the total ash. The fly ash, by far, constitutes more of a potential health hazard than does the bottom ash because the fly ash contains toxic metals such as lead, cadmium, copper and zinc as well as small amounts of dioxins and furans.University of Toronto, PhD Thesis Chan, C.C., Behaviour of metals in MSW fly ash during roasting with chlorinating agents, Chemical Engineering Department, University of Toronto, 1997. The bottom ash may or may not contain significant levels of health hazardous materials. In the United States, and perhaps in other countries as well, the law requires that the ash be tested for toxicity before disposal in landfills. If the ash is found to be hazardous, it can only be disposed of in landfills which are carefully designed to prevent pollutants in the ash from leaching into underground aquifers.

The debate over incineration

Use of incinerators for waste management is controversial. The debate over incinerators typically involves business interests (representing both waste generators and incinerator firms), government regulators, and local citizens who must weigh the economic appeal of local industrial activity with their concerns over health and environmental risk.

People and organizations professionally involved in this issue include the U.S. Environmental Protection Agency (U.S. EPA) and a great many local and national air quality regulatory agencies worldwide.

The argument for incineration

The concerns over the health effects of dioxin and furan emissions have been significantly lessened by advances in emission control designs and very stringent new governmental regulations that have resulted in large reductions in the amount of dioxins and furans emissions.

Incinerators can be used for generating electricity or to provide energy in other forms such as generating steam for heating. Such a use is known as waste-to-energy or energy recovery.

In densely populated areas, finding space for additional landfills is becoming very difficult.

Incineration of medical waste produces an end product ash that is sterile and non-hazardous.

The argument against incineration

There are still concerns by many about the health effects of dioxin and furan emissions into the atmosphere.

The expense of building and operating an incinerator.

Although waste incineration can be used to generate energy, a significant amount of that energy is consumed by the use of scrubbers and other methods to clean up the exhaust gases.

With increased recycling of waste, the fuel quality of the municipal waste is decreased because there is less paper and other combustibles in the waste. In fact, additional fuel (i.e., natural gas) may be needed to burn the waste.

The safe disposal of the end product ash must still be dealt with.

Potential technologies

A process that is hoped to end up supplanting incineration of plastics, if proved to be efficient, is thermal depolymerization. The reality however is, that, unlike the high-temperature incineration, the intermediate-temperature thermal depolymerisation produces an almost equal amount of hundreds of chemical compounds, for which there is no application, and many of which are hazardous and harmful.