Incineration: Difference between revisions

For other forms of waste plant that produce energy see waste-to-energy.

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 ash, flue gases, particulate, and heat, which can in turn be used to generate electricity. The flue gases are cleaned for pollutants before it is dispersed in the atmosphere.

Incineration with energy recovery is one of several waste-to-energy (WtE) technologies such as gasification and anaerobic digestion. Incineration may also be implemented without energy and materials recovery, but this is increasingly being banned in OECD countries.

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

Modern 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 replace landfilling, it reduces the necessary volume 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. For example in 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 over more than a century in localised combined heat and power facilities supporting district heating schemes.^[3] In 2003, waste incineration produced 3 % of the electricity consumption and 11 % of the total domestic heat consumption in Denmark. A number of other European Countries rely heavily on incineration for handling municipal waste, in particular Luxemburg, The Netherlands, Germany and France. ^[2]

Technology

Types of incinerators

An incinerator is a furnace for burning refuse, modern incinerators include pollution mitigation equipment such as flue-gas cleaning. There are various types of incinerator plant design:

• Simple
• Fixed or moving grate
• Rotary-kiln
• Multiple/stepped hearth
• Fluidised bed

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.

Fixed or moving grate

These are large fixed hearth incinerators, with a moving grate. The moving grate enables the movement of waste through the combustion chamber to be optimised to allow a more efficient and complete combustion. These incinerators are typically used for combustion of municipal wastes, and are thus referred to as MSWIs: Municipal Solid Waste Incinerators.

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.

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.

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 ^oC for 2 seconds in order to ensure proper breakdown of organic toxins. This includes 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 ^oC at a pressure of 40 bar for the electricity generation in the turbine. At this point, the flue gas has a temperature of around 200 ^oC, and is passed to the flue gas cleaning system.

A single moving grate boiler can handle up to 35 tonnes of waste per hour, and can operate 8,000 hours per year with only one scheduled 'revision' (inspection and maintenance) of about one months duration^[4]. At least in Scandinavia revisions are 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 oven line while the others are subject to revision.

Rotary-kiln

The rotary-kiln incinerator^[5] 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 klin 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".^[6] A diagram of a rotary-kiln incinerator can be found here.

Multiple/stepped hearth

Waste is transported through the furnace by moving teeth mounted on a central rotating shaft.

Fluidised 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.

Specialised incineration

Modern 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 an turbine in order to produce electricity. The typical amount of net energy that can be produced per ton municipal waste is about 0.67 MWh electricity of electricity and 2 MWh district heating^[2]. Thus, incinerating about 600 tonnes per day of waste will produce about 17 MW of electrical power and 1200 MWh district heating.

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, the flue gases may contain significant amounts of particulate matter, heavy metals, dioxins, furans, sulphur dioxide, and hydrochloric acid.

In a study^[7] from 1994, Delaware Solid Waste Authority found that modern incineration plants emitted fewer particles, hydrocarbons and less SO₂, HCl, CO and NO_x than coal-fired power plants, but more than natural gas fired power plants. According to Germany's Ministry of the Environment, modern waste combustors reduce the amount of some atmospheric pollutants by substituting power produced by coal-fired plants with power from waste-fired plants^[8].

Gaseous emissions

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^[9] to the atmosphere. Dioxins and furans are considered by many to be serious health hazards. Older generation incinerators that were not equipped with modern gas cleaning technologies were indeed significant sources of dioxin emissions. Today, however, due to advances in emission control designs and stringent new governmental regulations, modern 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."^[8]. According to the U.S. EPA, modern 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 of dioxin emissions from U.S. incinerators. Today, the total emissions from the 87 plants are only 10 grams, a reduction of 99.9 %. Backyard barrel burning of household and garden wastes, still allowed in some rural areas, generates 580 grams of dioxins yearly. Studies conducted by EPA^[10] demonstrate that the emissions from just one family using a burn barrel produces more emissions than a modern incineration plant disposing of 200 tonnes of waste per day.

Other gaseous toxins in the flue gas from incinerator furnaces include sulphur dioxide, hydrochloric acid, PAHs.

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 flue gas condensation or increased flue gas temperatures.

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). One incinerator equipped only with an ESP for particle filtration emitted 5.3 g/GJ fine particles with sizes below 2.5 micrometres (PM_2.5), while two incinerators equipped with baghouse filters emitted 0.002 and 0.013 g/GJ PM_2.5.^[11]

Acid gas scrubbers are used to remove hydrochloric acid, nitric acid, hydrofluoric acid, tungsten, and other heavy metals. Basic scrubbers remove sulfur dioxide, forming gypsum by reaction with lime.^[12]

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

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

NO_x 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

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][13], and the fly ash amounts to about 10 % to 20 % of the total ash^[14]. 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^[15]. 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^[16]. In testing over the past decade, no ash from a modern incineration plant in the USA has ever been determined to be a hazardous waste^{[citation needed]}. 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^{[citation needed]}.

Other pollution issues

Odour pollution can be a problem with old-style incinerators, but odours and dust are extremely well controlled in a modern incineration plant. 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 odours 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 (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^[8].
• Modern incineration plants generate electricity and heat that can be sold to the regional electric grid and can sell steam to district heating systems or industrial customers.
• The bottom ash residue remaining after combustion has been shown to be a non-hazardous solid waste that can be safely landfilled or possibly reused^[16].
• In densely populated areas, finding space for additional landfills is becoming increasingly difficult^{[citation needed]}.
• Many European countries such as Sweden, Denmark, The Netherlands, France, Switzerland, Austria and Germany, rely heavily on incineration plants. In Europe, it is being banned to landfill waste which is suitable for other treatment - e.g. combustible waste must be treated with energy recovery^[17], although avoidance, reuse and recycling are given even higher priority.
• Fine particles can be efficiently removed from the flue gases with baghouse filters. Even though approximately 40 % of the incinerated waste in Denmark was incinerated at plants with no baghouse filter, Danish incinerators were only responsible for approximately 0.3 % of the total domestic emissions of particulate smaller than 2.5 micrometres (PM_2.5) to the atmosphere in 2006.^[11]
• Incineration of municipal solid waste avoids the release of carbon dioxide and methane. Every ton of MSW incinerated, prevents about one ton of carbon dioxide equivalents from being released to the atmosphere^{[citation needed]}.
• Incineration of medical waste and sewage sludge produces an end product ash that is sterile and non-hazardous^{[citation needed]}.

The argument against incineration

• The end product ash must still be safely disposed of. SAARC (South Asian Association for Regional Cooperation) countries agree that incineration should not be considered as an option for the treatment of their municipal solid wastes for low calorific value and environmental pollution potential^[18]
• There are still concerns by many about the health effects of dioxin and furan emissions into the atmosphere from old incinerators.
• Incinerators emit varying levels of heavy metals such as vanadium, manganese, chromium, nickel, arsenic,mercury, lead and cadmium, which can be toxic at very minute levels
• Other advanced alternative technologies are available such as Mechanical Biological Treatment combined with Anaerobic Digestion (MBT/AD), Autoclaving or Mechanical Heat Treatment (MHT) using steam or Plasma arc gasification PGP, or combinations.
• The expense of building and operating an incinerator which requiring long contract periods to recover initial investment costs, causing a long term lock-in.
• Incinerators produce fine particles in the furnace. Even with modern particle filtering of the flue gases (e.g. ESPs or baghouse filters), some of these are emitted to the atmosphere. As an example, the baghouse filters in a specific modern incineration plant planned for erection in the UK, will only capture 65-70 % particulate smaller than 2.5 micrometres (PM_2.5) ^[19]. PM_2.5 is not separately regulated in the European Waste Incineration Directive even though they are suspected to be linked to infant mortality in the UK ^{[20] [21]}, and PM_2.5 emissions from local incinerators to be a significant PM_2.5 source here^[22]. Italian nanopathologists are concerned about the build up of nanoparticles in human tissue and the food chain. ^{[23] [24] [25]}
• Although waste combustion can be used to generate power, a portion of that power is consumed by fans, pumps and other electrically powered components.
• Local communities are often unpleased with the idea of locating incinerators in their own vicinity. (The Not In My Back Yard phenomenon). Studies in Andover, Massachsuccett linked 10 % property devaluations with closed incinerator proximity ^[26].
• Prevention, minimisation, reuse and recycling of waste should all be preferred to incineration according to the waste hierarchy. Supporters of zero waste consider incinerators and other waste treatment technologies as barriers to recycling and separation beyond particular levels, and that waste resources are sacrificed for energy producion^{[27] [28][29]}.

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 the United States

The first full-scale waste-to-energy facility in the US 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^[30]. 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^[13].

Several older generation incinerators have been closed; 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^[31]. Between 1996 and 2005, no new waste-to-energy plants were built. The main reasons for lack of activity have been:

• Economics. With the increase in the number of large inexpensive regional landfills and, up until recently, the relatively low price of electricity, incinerators were not able to compete for the 'fuel', i.e., waste. In Europe, with the ban on landfilling untreated waste, scores of modern design waste-to-energy plants 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 waste-to-energy plants. A number of Canadian cities are likewise working toward installation of waste-to-energy plants.
• Tax Policies. Tax credits for plants producing electricity from waste were rescinded in the 1990s. In Europe, some of the electricity generated from waste is deemed to be from a 'Renewable Energy Source (RES)'. A new law granting tax credits for such plants was implemented in the U.S. in 2004.
• Landfill bias. In Europe and Japan, the public understands the hazards of landfills, but this awareness is less developed in the U.S., where furthermore there is a lot of open land.

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

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 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 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.

Small incinerator units

Small scale incinerators exist for special purposes. For example, the small scale ^[32] 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^{[citation needed]}. 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^{[citation needed]}.

Incinerators

• Allington Incinerator
• Isle of Man Incinerator
• Kirklees Incinerator
• List of incinerators in the UK
• SELCHP
• Sheffield Incinerator

See also

• Anaerobic digestion
• Biogas powerplant
• Composting
• Gasification
• Plasma Gasification
• Human incineration
• Landfill
• List of solid waste treatment technologies
• Mechanical biological treatment
• Pyrolysis
• Thermal treatment
• Waste Incineration Directive
• Waste management
• Waste-to-energy

External links

Wikimedia Commons has media related to Incineration.

Anti-incineration groups

Anti-incineration Organization based in Syracuse,NY

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

References