Incineration

Spittelau incineration plant in Vienna.
SYSAV incineration plant in Malmö, Sweden capable of handling 25 metric tons (28 short tons) per hour household waste. To the left of the stack, a new identical oven line is under construction (March 2007).

Incineration is a waste treatment technology that involves the combustion of organic materials and/or substances.[1] Incineration and other high temperature waste treatment systems are described as "thermal treatment". Incineration of waste materials converts the waste into incinerator bottom ash, flue gases, particulates, and heat, which can in turn be used to generate electric power. The flue gases are cleaned of pollutants before they are dispersed in the atmosphere.

Incineration with energy recovery is one of several waste-to-energy (WtE) technologies such as gasification, Plasma arc gasification, pyrolysis and anaerobic digestion. Incineration may also be implemented without energy and materials recovery.

In several countries there are still expert and local community concerns about the environmental impact of incinerators (see The argument against incineration).

In some countries, incinerators built just a few decades ago often did not include a materials separation to remove hazardous, bulky or recyclable materials before combustion. These facilities tended to risk the health of the plant workers and the local environment due to inadequate levels of gas cleaning and combustion process control. Most of these facilities did not generate electricity.

Incinerators reduce the volume of the original waste by 95-96 %, depending upon composition and degree of recovery of materials such as metals from the ash for recycling.[2] This means that while incineration does not completely replace landfilling, it reduces the necessary volume for disposal significantly.

Incineration has particularly strong benefits for the treatment of certain waste types in niche areas such as clinical wastes and certain hazardous wastes where pathogens and toxins can be destroyed by high temperatures. Examples include chemical multi-product plants with diverse toxic or very toxic wastewater streams, which cannot be routed to a conventional wastewater treatment plant.

Waste combustion is particularly popular in countries such as Japan where land is a scarce resource. Denmark and Sweden have been leaders in using the energy generated from incineration for more than a century, in localised combined heat and power facilities supporting district heating schemes.[3] In 2005, waste incineration produced 4.8 % of the electricity consumption and 13.7 % of the total domestic heat consumption in Denmark.[4] A number of other European Countries rely heavily on incineration for handling municipal waste, in particular Luxemburg, The Netherlands, Germany and France. [2]

Contents

Technology

Types of incinerators

An incinerator is a furnace for burning waste. Modern incinerators include pollution mitigation equipment such as flue gas cleaning. There are various types of incinerator plant design: moving grate, fixed grate, rotary-kiln, fluidised bed.

Moving grate

Control room of a typical moving grate incinerator overseeing two boiler lines

The typical incineration plant for municipal solid waste is a moving grate incinerator. The moving grate enables the movement of waste through the combustion chamber to be optimised to allow a more efficient and complete combustion. A single moving grate boiler can handle up to 35 metric tons (39 short tons) of waste per hour, and can operate 8,000 hours per year with only one scheduled stop for inspection and maintenance of about one month's duration[5]. Moving grate incinerators are sometimes referred to as Municipal Solid Waste Incinerators (MSWIs).

The waste is introduced by a waste crane through the "throat" at one end of the grate, from where it moves down over the descending grate to the ash pit in the other end. Here the ash is removed through a water lock.

Municipal solid waste in the furnace of a moving grate incinerator capable of handling 15 short tons (14 metric tons) of waste per hour. The holes in the grate elements supplying the primary combustion air are visible.

Part of the combustion air (primary combustion air) is supplied through the grate from below. This air flow also has the purpose of cooling the grate itself. Cooling is important for the mechanical strength of the grate, and many moving grates are also water cooled internally.

Secondary combustion air is supplied into the boiler at high speed through nozzles over the grate. It facilitates complete combustion of the flue gases by introducing turbulence for better mixing and by ensuring a surplus of oxygen. In multiple/stepped hearth incinerators, the secondary combustion air is introduced in a separate chamber downstream the primary combustion chamber.

According to the European Waste Incineration Directive, incineration plants must be designed to ensure that the flue gases reach a temperature of at least 850 °C (1,560 °F) for 2 seconds in order to ensure proper breakdown of organic toxins. In order to comply with this at all times, it is required to install backup auxiliary burners (often fueled by oil), which are fired into the boiler in case the heating value of the waste becomes too low to reach this temperature alone.

The flue gases are then cooled in the superheaters, where the heat is transferred to steam, heating the steam to typically 400 °C (752 °F) at a pressure of 40 bars (580 psi) for the electricity generation in the turbine. At this point, the flue gas has a temperature of around 200 °C (392 °F), and is passed to the flue gas cleaning system.

At least in Scandinavia scheduled maintenance is always performed during summer, where the demand for district heating is low. Often incineration plants consist of several separate 'boiler lines' (boilers and flue gas treatment plants), so that waste receival can continue at one boiler line while the others are subject to revision.

Fixed grate

The older and simpler kind of incinerator was a brick-lined cell with a fixed 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

The rotary-kiln incinerator[6] used by municipalities and by large industrial plants. This design of incinerators have 2 chambers a primary chamber and secondary chamber. The primary chamber in a rotary kiln incinerator consist of an inclined refractory lined cylindrical tube. Movement of the cylinder on its axis facilitates movement of waste. In the primary chamber, there is conversion of solid fraction to gases, through volatilization, destructive distillation and partial combustion reactions. The secondary chamber is necessary to complete gas phase combustion reactions

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".[7] A diagram of a rotary-kiln incinerator can be found here.

Fluidized bed

A strong airflow is forced through a sandbed. The air seeps through the sand until a point is reached where the sand particles separate to let the air through and mixing and churning occurs, thus a fluidised bed is created and fuel and waste can now be introduced.

The sand with the pre-treated waste and/or fuel is kept suspended on pumped air currents and takes on a fluid-like character. The bed is thereby violently mixed and agitated keeping small inert particles and air in a fluid-like state. This allows all of the mass of waste, fuel and sand to be fully circulated through the furnace.

Specialized incineration

Furniture factory sawdust incinerators need much attention as these have to handle resin powder and many flammable substances. Controlled combustion, burn back prevention systems are very essential as dust when suspended resembles the fire catch phenomenon of any liquid petroleum gas.

Use of heat

The heat produced by an incinerator can be used to generate steam which may then be used to drive a turbine in order to produce electricity. The typical amount of net energy that can be produced per ton municipal waste is about 2/3 MWh of electricity and 2 MWh of district heating[2]. Thus, incinerating about 600 metric tons (660 short tons) per day of waste will produce about 17 MW of electrical power and 1200 MWh district heating each day.

Pollution

Incineration has a number of outputs such as the ash and the emission to the atmosphere of flue gas. Before the flue gas cleaning system, the flue gases may contain significant amounts of particulate matter, heavy metals, dioxins, furans, sulfur dioxide, and hydrochloric acid.

In a study[8] from 1994, Delaware Solid Waste Authority found that, for same amount of produced energy, incineration plants emitted fewer particles, hydrocarbons and less SO2, HCl, CO and NOx than coal-fired power plants, but more than natural gas fired power plants. According to Germany's Ministry of the Environment, waste incinerators reduce the amount of some atmospheric pollutants by substituting power produced by coal-fired plants with power from waste-fired plants[9].

Gaseous emissions

Dioxin and furans

The most publicized concerns from environmentalists about the incineration of municipal solid wastes (MSW) involve the fear that it produces significant amounts of dioxin and furan emissions.[10] Dioxins and furans are considered by many to be serious health hazards. Older generation incinerators that were not equipped with adequate gas cleaning technologies were indeed significant sources of dioxin emissions. Today, however, due to advances in emission control designs and stringent new governmental regulations, incinerators emit virtually no dioxins. In 2005, The Ministry of the Environment of Germany, where there were 66 incinerators at that time, estimated that "...whereas in 1990 one third of all dioxin emissions in Germany came from incineration plants, for the year 2000 the figure was less than 1 %. Chimneys and tiled stoves in private households alone discharge approximately twenty times more dioxin into the environment than incineration plants."[9]. According to the United States Environmental Protection Agency, incineration plants are no longer significant sources of dioxins and furans. In 1987, before the governmental regulations required the use of emission controls, there was a total of 10,000 grams (350 oz) of dioxin emissions from U.S. incinerators. Today, the total emissions from the 87 plants are only 10 grams (0.35 oz) yearly, a reduction of 99.9 %. Backyard barrel burning of household and garden wastes, still allowed in some rural areas, generates 580 grams (20 oz) of dioxins yearly. Studies conducted by EPA[11] demonstrate that the emissions from just one family using a burn barrel produces more emissions than an incineration plant disposing of 200 metric tons (220 short tons) of waste per day.

Generally the breakdown of dioxin requires exposure of the molecule to a sufficiently high temperature so as to trigger thermal breakdown of the molecular bonds holding it together. When burning of plastics outdoors in a burn barrel or garbage pit such temperatures are not reached, causing high dioxin emissions as mentioned above. While the plastic does burn in an open-air fire, the dioxins remain after combustion and float off into the atmosphere.

Modern municipal incinerator designs include a high temperature zone, where the flue gas is ensured to sustain a temperature above 850 oC for at least 2 seconds befores it is cooled down. They are equipped with auxiliary heaters to ensure this at all times. These are often fueled by oil, and normally only active for a very small fraction of the time. A side effect controlling dioxin is the potential for generation of reactive oxides (NOx) in the flue gas, which must be removed with SCR or SNCR (see below).

For very small municipal incinerators, the required temperature for thermal breakdown of dioxin may be reached using a high-temperature electrical heating element, plus an SCR stage.

CO2

As for other complete combustion processes, nearly all of the carbon content in the waste is emitted as CO2 to the atmosphere. MSW contain approximately the same mass fraction of carbon as CO2 itself (27%), so incineration of 1 metric ton (1.1 short tons) of MSW produce approximately 1 metric ton (1.1 short tons) of CO2.

In the event that the waste was landfilled, 1 metric ton (1.1 short tons) of MSW would produce approximately 62 cubic metres (2,200 cu ft) methane via the anaerobic decomposition of the biodegradable part of the waste. This amount of methane has more than twice the global warming potential than the 1 metric ton (1.1 short tons) of CO2, which would have been produced by incineration. In some countries, large amounts of landfill gas are collected, but still the global warming potential of the landfill gas emitted to atmosphere in the US in 1999 was approximately 32 % higher than the amount of CO2 that would have been emitted by incineration.[12]

In addition, nearly all biodegradable waste has biological origin. This material has been formed by plants using atmospheric CO2 typically within the last growing season. If these plants are regrown the CO2 emitted from their combustion will be taken out from the atmosphere once more.

Such considerations are the main reason why several countries administrate incineration of the biodegradable part of waste as renewable energy[13]. The rest - mainly plastics and other oil and gas derived products - is generally treated as non-renewables.

Different results for the CO2 footprint of incineration can be reached with different assumptions. Local conditions (such as limited local district heating demand, no fossil fuel generated electricity to replace or high levels of aluminum in the waste stream) can decrease the CO2 benefits of incineration. The methology and other assumptions may also influence the results significantly. For example the methane emissions from landfills occurring at a later date may be neglected or given less weight, or biodegradable waste may not be considered CO2 neutral. A recent study by Eunomia Research and Consulting on potential waste treatment technologies in London demonstrated that by applying several of these (according to the authors) unusual assumptions the average existing incineration plants performed poorly for CO2 balance compared to the theoretical potential of other emerging waste treatment technologies.[14].

Other emissions

Other gaseous toxins in the flue gas from incinerator furnaces include sulfur dioxide, hydrochloric acid, heavy metals and fine particles.

The steam content in the flue may produce visible fume from the stack, which can be perceived as a visual pollution. It may be avoided by decreasing the steam content by flue gas condensation, or by increasing the flue gas exit temperature well above its dew point. Flue gas condensation allows the latent heat of vaporization of the water to be recovered, subsequently increasing the thermal efficiency of the plant.

Flue gas cleaning

The quantity of pollutants in the flue gas from incineration plants is reduced by several processes.

Particulate is collected by particle filtration, most often electrostatic precipitators (ESP) and/or baghouse filters. The latter are generally very efficient for collecting fine particles. In an investigation by the Ministry of the Environment of Denmark in 2006, the average particulate emissions per energy content of incinerated waste from 16 Danish incinerators were below 2.02 g/GJ (grams per energy content of the incinerated waste). Detailed measurements of fine particles with sizes below 2.5 micrometres (PM2.5) were performed on three of the incinerators: One incinerator equipped with an ESP for particle filtration emitted 5.3 g/GJ fine particles, while two incinerators equipped with baghouse filters emitted 0.002 and 0.013 g/GJ PM2.5. For ultra fine particles (PM1.0), the numbers were 4.889 g/GJ PM1.0 from the ESP plant, while emissions of 0.000 and 0.008 g/GJ PM1.0 were measured from the plants equipped with baghouse filters[15][16].

Acid gas scrubbers are used to remove hydrochloric acid, nitric acid, hydrofluoric acid, mercury, lead and other heavy metals. Basic scrubbers remove sulfur dioxide, forming gypsum by reaction with lime.[17]

Waste water from scrubbers must subsequently pass through a waste water treatment plant.

Sulfur dioxide may also be removed by dry desulfurisation by injection limestone slurry into the flue gas before the particle filtration.

NOx is either reduced by catalytic reduction with ammonia in a catalytic converter (selective catalytic reduction, SCR) or by a high temperature reaction with ammonia in the furnace (selective non-catalytic reduction, SNCR).

Heavy metals are often adsorbed on injected active carbon powder, which is collected by the particle filtration.

Solid outputs

Operation of an incinerator aboard an aircraft carrier

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 4-10 % by volume and 15-20 % by weight of the original quantity of waste[2][18], and the fly ash amounts to about 10-20 % 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 often contain high concentrations of heavy metals such as lead, cadmium, copper and zinc as well as small amounts of dioxins and furans[19]. The bottom ash seldom contain significant levels of heavy metals. While fly ash is always regarded as hazardous waste, bottom ash is generally considered safe for regular landfill after a certain level of testing defined by the local legislation. Ash, which is considered hazardous, may generally only be disposed of in landfills which are carefully designed to prevent pollutants in the ash from leaching into underground aquifers - or after chemical treatment to reduce its leaching characteristics[20]. In testing over the past decade, no ash from an incineration plant in the USA has ever been determined to be a hazardous waste. At present although some historic samples tested by the incinerator operators' group would meet the being ecotoxic criteria at present the EA say "we have agreed" to regard incinerator bottom ash as "non-hazardous" until the testing programme is complete.

Other pollution issues

Odor pollution can be a problem with old-style incinerators, but odors and dust are extremely well controlled in newer incineration plants. They receive and store the waste in an enclosed area with a negative pressure with the airflow being routed through the boiler which prevents unpleasant odors from escaping into the atmosphere. However, not all plants are implemented this way, resulting in inconveniences in the locality.

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, environmental activists 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 and a great many local and national air quality regulatory agencies worldwide.

The argument for incineration

The argument against incineration

Decomissioned Kwai Chung Incineration Plant from 1978. It is currently being demolished.

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 in North America

The first full-scale, municipally operated, waste-to-energy facility in the U.S. was the Arnold O. Chantland Resource Recovery Plant, built in 1975 located in Ames, Iowa. This plant is still in operation and produces refuse-derived fuel that is sent to local power plants for fuel.[34] The first commercially-successful incineration plant in the U.S. was built in Saugus, Massachusetts in October 1975 by Wheelabrator Technologies, and is still in operation today.[18]

Several older generation incinerators have been closed; of the 186 MSW incinerators in 1990, only 89 remained by 2007, and of the 6200 medical waste incinerators in 1988, only 115 remained in 2003.[35] Between 1996 and 2007, no new incinerators were built. The main reasons for lack of activity have been:

Despite these problems, there has been renewed interest in waste-to-energy in the U.S., Canada, and the UK. Projects to add capacity to existing plants are underway, and municipalities are once again evaluating the option of building incinerators rather than continue landfilling municipal wastes.

Incineration in Europe

In Europe, with the ban on landfilling untreated waste, scores of incinerators have been built in the last decade, with more under construction. Recently, a number of municipal governments have begun the process of contracting for the construction and operation of incinerators. In Europe, some of the electricity generated from waste is deemed to be from a 'Renewable Energy Source (RES)' and is thus eligible for tax credits if privately operated.

Incineration in the United Kingdom

The technology employed in the UK waste management industry has been greatly lagging behind that of Europe due to the wide availability 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 through the use of alternative methods of waste treatment. It is the UK Government's position that incineration will play an increasingly large role in the treatment of municipal waste and supply of energy in the UK.

In the UK in 2008, plans for potential incinerator locations exists for approximately 100 sites. These have been interactively mapped by UK NGO's. [36][37][38][39]

Small incinerator units

An example of a low capacity, mobile incinerator.

Small scale incinerators exist for special purposes. For example, the small scale [40] incinerators are aimed for hygienically safe destruction of medical waste in developing countries. Simple, mobile incinerators are becoming more widely used in developing countries where the threat of avian influenza is high. Small incinerators can be quickly deployed to remote areas where an outbreak has occurred to dispose of infected animals quickly and without the risk of cross contamination.

Incinerators

See also

References

  1. Knox, A. (2005) Overview of incineration, An Overview of Incineration and EFW Technology as Applied to the Management of Municipal Solid Waste (MSW), University of Western Ontario, Canada
  2. 2.0 2.1 2.2 2.3 Waste to Energy in Denmark, publication by Ramboll (2006)
  3. Kleis, Heron and Dalager, Søren (2004) 100 Years of Waste Incineration in Denmark, A historical review of incineration in Denmark
  4. Danish Energy Statistics 2005 by the Danish Ministry of Energy.
  5. Vestforbrænding anlæg 6 - Danmarks største forbrændingsovn, Brochure (in Danish) on the largest incineration line in Europe, 2004
  6. Rotary-kiln incinerators An excellent detailed description of rotary-kiln incinerators
  7. Photos of rotary-kiln incinerators with afterburners.
  8. Waste-to-Energy Compared to Fossil Fuels for Equal Amounts of Energy (Delaware Solid Waste Authority)
  9. 9.0 9.1 9.2 Waste incineration a potential danger: bidding farewell to dioxin spouting, Report by Germany's Ministry of the Environment
  10. Beychok, Milton R. (January 1987). "A data base for dioxin and furan emissions from refuse incinerators". Atmospheric Environment 21 (1): 29–36. doi:10.1016/0004-6981(87)90267-8. ISSN 0004-6981. 
  11. Evaluation of Emissions from the Burning of Household Waste in Barrels, EPA November 1997
  12. 12.0 12.1 Themelis, Nickolas J. An overview of the global waste-to-energy industry, Waste Management World 2003
  13. Energy From Waste, from the homepage of the UK Renewable Energy Association
  14. 14.0 14.1 D. Hogg, et al (2008) Greenhouse Gas Balances of Waste Management Scenarios, Report for the Greater London Authority, Eunomia
  15. 15.0 15.1 PM Emission from CHP Plants <25MWe, poster by Malene Nielsen et. al, National Environmental Research Institute of Denmark
  16. 16.0 16.1 Emissionsfaktorer og emissionsopgørelse for decentral kraftvarme, Kortlægning af emissioner fra decentrale kraftvarmeværker, Ministry of the Environment of Denmark 2006 (in Danish)
  17. Kraftvärmeverket, avfall blir el och värme, brochure on the SYSAV incinerator in Malmö, Sweden (in Swedish)
  18. 18.0 18.1 Waste-to-Energy: Less Environmental Impact than Almost Any Other Source of Electricity, Integrated Waste Services Association homepage
  19. Behaviour of metals in MSW fly ash during roasting with chlorinating agents, PhD Thesis by Chan, C.C., Chemical Engineering Department at University of Toronto, 1997.
  20. 20.0 20.1 Environmental and Health Risks Associated with the Use of Processed Incinerator Bottom Ash in Road Construction, Report AEAT/ENV/R/0716 by BREWEB October 2003
  21. http://www.elc.org.uk/papers/2005vansteenis.doc
  22. http://www.grundon.com/PDFs/leaflets/technical/Haz_Landfill_Flyer_Mar_2004.pdf
  23. [1]
  24. [2]
  25. [3]
  26. Mole Valley - Capel Action Group
  27. Suffolk Together - We say no to incinerators
  28. Incinerators - Weapons of Mass Destruction, Country Doctor Magazine January 31. 2005, by Dick van Steenis
  29. http://www.noharm.org/details.cfm?ID=1963&type=document
  30. Economic Analysis and Land Use Policy, Session of Workshop sponsored by United States Environmental Protection Agency and NCERQA, Washington 1999
  31. Zero Waste: A Global Perspective Presentation by Dr Paul Connett, St Lawrence University
  32. Energy from Waste: The Myths Debunked, Press Conference by opposers to incineration in Ontario, USA
  33. On the Main EU Directives on Waste, Briefing document by Friends of the Earth
  34. Arnold O. Chantland Resource Recovery Plant, Ames City Government Homepage
  35. Waste Incineration: A Dying Technology
  36. Household Waste Incinerators · UK Without Incineration Network
  37. Map launched of all planned UK incinerators - letsrecycle.com
  38. Friends of the Earth: New map shows over 100 communities threatened by rubbish-burners
  39. Channel 4 - News - 30 new rubbish incinerator plants planned for the UK
  40. Waste Disposal Unit from the homepage of the Indian Centre for Renewable Energy, Appropriate Technology and Environment

External links

Anti-incineration groups

UK Without Incineration Network

Burn barrels

Burn Barrel Organization
EPA Fact Sheet
Emissions Information

EU information

EU Directive on waste incineration
BREF Drafts & Papers

International Solid Waste Association position

position papers

Overviews

Incineration article
FAQ's on incineration

Tutorial

Flash presentation of SYSAV, a large incineration plant in Malmö, Sweden.
Incineration Tutorial from Rensaleer Polytechnic Institute