Incineration

Incineration is a method of disposing of waste by burning it.

Incineration often functions as an alternative to other disposal methods, especially landfilling. 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 particulary popular in countries such as Japan where land is a scarce resource. Incineration is often described as Thermal Treatment.

Incineration and Waste Management

For the Environmental engineer today Incineration may be used as one of a variety of tools to design and implement a waste management strategy.

The basic aim of pro-environmental waste management policies is generally to reduce the impact on the environment by reducing the sum of energy used and emissions to the environment. In the waste management policies of the EU, Incineration comes in step four and five of the hierarchy of waste strategies:

1: Reduce waste at source

2: Reuse products (no or minimal processing)

3: Recyle

4: Dispose with energy recovery

5: Dispose without energy recovery

Aspects of Incineration

Incineration does not divide neatly into separate categories but rather several factors influence the design of incinerators and how harmful they are to the environment. These aspects are discussed in greater depth below:

Scale

The scale of incineration can vary hugely from burn barrels for individual houses to large municipal waste incinerators catering for whole cities.

Small scale describes people burning household waste in small quanties. It is used most in areas with regular municipal waste collection. The low temperatures, lack of control and lack of montitoring mean that this form of incineration is generally the most polluting per kilo of waste burned.

Medium scale incineration is usually for commercial, industrial or institutional waste. For example large hospitals may use an on-site incinerator to dispose of medical waste. The type of waste such incinerators burn is often highly specific.

Large scale incineration is usually for muncipal waste. Such incinerators deal with 100 to 1000 tons of waste per day. The waste they burn is mixed and it's characteristics (e.g. water content and calorific value) may vary significantly.

Waste Types

Most waste generated by human activity can be burnt in an incinerator.

Municipal Waste

Municipal Waste is the largest source of incinerator fuel. It may be sorted or unsorted waste.

Sorted municpal waste, has as the name suggests, been sorted to remove some forms of waste. In developed countries, particularly the EU and Japan, this usually means that recyclable material is removed by residents or by a company before waste is burnt.

Valuable materials, like steel, aluminum, paper and some types of plastics are removed, materials that are not economically recyclable, such as polystyrene, paper towels and wax-coated paper are sent for incineration.

Garden waste and compostable material is also often removed prior to incineration. The high water content of compostable material means that it reduces the efficiency of the incinerator.

Unsorted municipal waste on the other hand has not had potentially valuable material removed from it.

Medical waste

Medical waste incineration entails the combustion of a waste stream with special biological contamination risks. One of the major problems with medical waste is that there are many relatively-small and under-regulated, under-supervised combustion sites.

Chemical Weapons

Chemical weapons are also incinerated. For example, the U.S. has thousands of tons of sarin stockpiled during the Cold War era, as a countermeasure against even larger stockpiles of the Cold War enemies. Having signed the international ban, U.S. Government has been trying its best to destroy that sarin by incineration, but the destruction is hampered by the environmental concerns about incineration.

Furnace Types

An incinerator may utilise any of a number of furnace types. The selection of furnace type will depend on the quantity and type of waste the incinerator will be dealing with.

To select a furnace type the calorific value of the waste to be burned needs to be ascertained. This will vary with the type of waste to be burned (medical vs municipal)and also with the particular city and the level of sorting carried out.

Cement kilns

A versatile type of incinerator is the cement kiln, whose main product is Portland cement. A cement kiln is a rotating cylinder, almost horizontal but slightly inclined, with the upper end continuously fed with a mixture of clay and lime or limestone, and the lower end fed with burning fuel. The temperature of thousands of degrees causes the lime and clay react chemically, and a continuous stream of the white-hot molten portland cement flows from the lower end of the cylinder.

Cement kilns benefit from this fact in their auxiliary role of incinerators, since by their nature, they combine incineration with scrubbing those inorganic gases. Kilns convert and lock those gases and ashes into mineral products. This is done by the molten cement and lime covering the entire walls of the rotating kiln. The alkaline properties of the hot, molten mixture neutralize those gases. A conventional incinerator would produce sulfur dioxide, or when equipped with a scrubber, harmless but cumbersome by-products. A cement kiln converts that sulfur dioxide into the mineral of gypsum, later locked in the portland concrete. The chlorine becomes less toxic calcium chloride, the phosphorus becomes the mineral of apatite, fluorine becomes the mineral of spat, and other similar reactions occur. The only remaining gaseous contaminants that leave a cement kiln are small amounts of nitrogen oxide, and residues of dioxins.

Energy Recovery

A modern incinerator will nearly always encompass some form of energy recovery. The two principle methods of energy recovery used are electricity generation and municipal heating. One incinerator may combine the two methods depending on the demands of the local area.

Electricty Generation

This is where the furnace of the incinerator is used to power an electricity generation plant.

Municipal Heating

This is where the furnace of the incinerator is used to generate steam and hot water which is pumped directly to businesses/institutions and houses in the vicinity of the incinerator. The hot water is then used to heat the buildings.

Emissions from Incineration

Incineration generates several forms of waste itself, such as the emission of unburned gases and metals in, the hazardous secondary products of combustion, and leftover ash. The health hazards associated with these emissions and incinerator wastes are the subject of intense controversy.

Solids

The volume of solids or ash left after incineration is usually from 30% to 10% of the original quantity of waste. The ash is far more concentrated with pollutants than the original waste. The ash is often regulated as a hazardous waste itself and must be landfilled.

This concention of pollutants can allow otherwise unrecoverable metals to be recycled. The portion of metals that could not be separated prior to combustion are periodically removed from the boilers and sent to foundries for recycling.

Gases

The exhaust gases produced as by products of incineration are a major source of councern. Among the unintended pollutants caused by incineration are dioxins and furans, which are the subject of ongoing study and debate.

The quantity of hazardous substances in Incinerator exhaust gases is reduced by a technique known as scrubbing

History

The history of municipal waste 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 disputes over the siting and management of incinerator facilities.

The use of incinerators has been on the decline in the United States due in part to high disposal costs. There were 98 such plants in 2002 and 89 in 2004. The reasons for this might be that it is often cheaper to take waste to a landfill, that it is politically difficult to replace aging plants or that emissions standards or public opposition is causing incinerators to be shut down. The difficulties and costs of disposing of hazardous wastes in America are high due to the regulation of waste transport, storage, managment and incineration.

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 EPA and AQMD.

The Argument Pro-Incineration

• Much of the debate over incineration has focused on the traces of residual by-products of incineration, some of which are highly toxic in minute amounts. As a result, attention often focuses on non-combustible by-products, such as mercury and dioxin, that are equivalent to far less than 1% of the overall waste stream.

• Incinerators can be used for generating electricity or provide energy in other ways such as generating steam for heat. Such a use is known as waste to energy or energy recovery. However, a significant amount of energy is lost due to "scrubbers", and other methods used to clean up the exhaust.

==The Argument against Incineration

• Concerns over the unknown potential side effects of Dioxins and Furans released into the air.

• The expense of building and running an incinerator.

• Once an incinerator is built, waste is required to run it. This provides a dis-incentive to reducing waste production and reusing waste products.

Potential technologies

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