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Waste-to-energy (WtE) or energy-from-waste (EfW) in its strictest sense refers to any waste treatment that creates energy in the form of electricity or heat from a waste source that would have been disposed of in landfill, also called energy recovery.

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Waste-to-energy and incineration

Waste-to-energy has become synonymous with modern incineration practices in the field of waste management. New incinerators are often termed waste-to-energy plants as it is deemed to be a more publicly acceptable title than "incineration". This term is often associated with antequated systems that produced high levels of emmissions and polluted the air of the surrounding environment.

Incinerators and WtE plants are generally considered not to produce renewable energy as a large fraction of the power which is generated comes from plastics (derived from fossil fuels) and other non-renewable sources. It could be debated that the energy generated from the biodegradable waste fraction is renewable however many countries do not credit this.

Incineration is a solid waste treatment technology involving combustion of waste at high temperatures. Incineration and other high temperature waste treatment systems are described as "thermal treatment". In effect, incineration of waste materials converts the waste into heat (that can be used to generate electricity), gaseous emissions to the atmosphere and residual ash.

A waste-to-energy plant (WtE) is a modern term for an incinerator that combusts wastes to produce electricity, deemed to be more publicly acceptable than incinerator. This type of incineration is sometimes called an energy-from-waste (EfW).

Incineration functions as an alternative to landfilling and biological treatment methods such as composting and anaerobic digestion.

Incineration has particularly strong benefits the treatment of certain waste types in niche areas especially for clinical wastes and certain hazardous wastes where pathogens and toxins must be destroyed by high temperatures.

Modern waste-to-energy incinerators are very different from the incinerators that were commonly used until a few decades ago. Unlike modern ones, those plants usually did not include materials separation to remove hazardous or recyclable materials before burning. These incinerators endangered the health of the plant workers and the nearby residents, and most of them did not generate electricity.

The potential of electricity generation using incineration and other non-thermal methods of waste-to-energy such as anaerobic digestion are being increasingly looked at as a potential energy diversification strategy.

Incineration is particularly popular in countries such as Japan where land is a scarce resource. Sweden has been a leader in using the energy generated from incineration over the past 20 years and Denmark also extensively uses incineration in localised combined heat and power facilities supporting district heating schemes.

How incinerators work

An incinerator is a furnace for burning refuse. There are various types:-

Simple incinerators

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 waste compactors.

Rotary-kiln incinerators

The rotary-kiln incinerator 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". 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.

Uses for heat produced by the incinerator

The heat produced by the rotary-kiln incinerator can be used to generate steam which may then be used to drive an electrical generator. The typical range of net electrical energy that can be produced is about 500 to 600 kWh of per ton of waste incinerated. Thus, incinerating about 2,200 tons per day of waste will produce about 50 MW of electrical power.

Pollution

Incineration has a number of outputs such as the ash and the emission to the atmosphere of combustion product gases and particulate matter. Waste-to-energy plants emit less air pollution than coal plants, but more than natural gas plants.

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 emissions 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.

Solid outputs

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. 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. In the UK, all incinerator ash is classed as hazardous and must be disposed of in a hazardous waste designated landfill.

Other pollution issues

Odour pollution can be a problem when the plant is not located in an isolated area. Some plants store the waste in an enclosed area with a negative pressure with the airflow being routed through a boiler or filter which prevents unpleasant odours from escaping into the atmostphere. However, not all plants are implemented this way, resulting in complaints.

An issue that affects community relationships is the increased road traffic of waste collection vehicles to transport municipal waste to the incinerator. Due to this reason, most incinerators are located in industrial areas.

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.
  • 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.

SAARC (South Asian Association for Regional Cooperation) countries agree that incineration as well as unproven technologies such as Plasma, should not be considered as an option for the treatment of their municipal solid wastes for low calorific value and environmental pollution potential.

Trends in incinerator use

The history of municipal solid waste (MSW) incineration is linked intimately to the history of landfills and other waste treatment technology. The merits of incineration are inevitably judged in relation to the alternatives available. Since the 1970s, recycling and other prevention measures have changed the context for such judgements. Since the 1990s alternative waste treatment technologies have been maturing and becoming viable.

Incineration is a key process in the treatment of hazardous wastes and clinical wastes. It is often imperative that medical waste be subjected to the high temperatures of incineration to destroy pathogens and toxic contamination it contains.

Incineration decline in the United States

The use of 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. 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.

Incineration in the United Kingdom

The technology employed in the UK waste management industry has greatly lagged behind that of Europe due to the wide availablility of landfills. The Landfill Directive set down by the European Union led to the Government of the United Kingdom imposing waste legislation including the landfill tax and Landfill Allowance Trading Scheme. This legislation is designed to reduce the release of greenhouse gases produced by landfills though the use of alternative methods of waste treatment. It is the UK Governments position that incineration will play an increasingly large role in the treatment of municipal waste and supply of energy in the UK.

Energy from waste in other technologies

There are a number of other new and emerging technologies that are able to produce energy from waste without burning the waste directly. These technologies are considered to generate renewable energy and are widely perceived to be more publicly acceptable.

Thermal technologies:

Non-thermal technologies:

Related pages


External articles and referecnes

General
Burn Barrels
Burn Barrel Organisation
EPA Fact Sheet
EPA Report
Emissions Information
EU Information
EU Directive on waste incineration
BREF Drafts & Papers
Tutorial
Incineration Tutorial from Rensaleer Polytechnic Institute
ISWA International
ISWA Working Group on thermal treatment of solid waste
Overviews
Incineration article
Incinerator Manufacturers
Inciner8 International
3Ts International
Directories
  • Waste Energy - A visual directory of waste to energy websites. (EnergyPlanet.info)

See also

  • Waste-to-Energy Index of organizations and processes that use sewage, garbage, and other waste products to generate energy efficiently and cleanly.

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