Waste Incineration

Introduction

It is estimated that the UK produces some 177 million tonnes of waste each year from households, commerce, and industry, including construction and demolition¹. Of this, according to the Department for Environment, Food & Rural Affairs (Defra) annual survey of municipal solid waste (MSW) for 1999/00, it is estimated that approximately 29 million tonnes of MSW², 5.6 million tones of hazardous wastes and up to 400,000 tonnes of clinical wastes3 were produced during the year.

Waste incineration generally refers to the incineration of MSW however, it must be remembered that significant and increasing amounts of hazardous, clinical, and sewage sludge wastes are also disposed of by incineration mostly in dedicated facilities. There are currently 13 MSW incinerators operating in the U.K. burning approximately 2.0 million tonnes of MSW (8% of the total) each year.

History

The disposal of waste by incineration goes back nearly 130 years with the first fully functional incinerator being constructed in Nottingham in 1874. This facility operated for some 27 years with the ash from the plant being used as a building material. The world's first waste fired electricity generation plant was opened at Shoreditch, London in 1885, and by 1912 there were some 300 waste incinerators in the U.K., 76 of which were generating electricity.

The 1960s and 1970s saw a new period of construction with about 40 new MSW incinerators being built though only five were equipped for power generation as the main objective was volume reduction (the resulting ash from incineration is approximately 10% by volume and 30% by weight of the original MSW) particularly for metropolitan areas thereby reducing transport and disposal costs to landfill. Most of these incinerators had relatively rudimentary emission control equipment compared to modern installations.

In 1989, two new EC Directives⁴ on the Reduction (for existing plant) and Prevention (for new plant) of Air Pollution from Incinerators focused on emissions to air. The date (1/12/1996) by which existing facilities had to comply resulted in the closure of many old and polluting incinerators, as they did not warrant the expenditure needed to enable them to comply. A new waste incineration directive⁵, which takes effect from the beginning of 2003 for new facilities and 2006 for existing plants, addresses emissions to all media, and covers sewage sludge, clinical waste incinerators and other combustion processes as well as MSW operations.

The Technology

There are four main technologies for the incineration of MSW.

1. Mass Burn. This is the most common and simplest form of incineration where the waste is burnt as received with virtually no pre-processing. The waste is fed via a hopper onto a sloping, moving grate that agitates and moves the waste down through the combustion chamber so that by the time it is discharged into the ash pit at the end of the process, all combustible material has been burnt. The hot gasses are directed to a boiler where the heat is extracted to generate steam that drives a turbine connected to an electricity generator. The flue gases then pass through a gas cleaning process to remove ash and pollutants before being discharged to the atmosphere via the chimneystack. One tonne of waste produces a nominal 550 - 650 kilowatt hours of electricity or expressed another way a 100,000 tonne/year incinerator will produce 7 MW of electricity net of power used to run the plant.

2. Fluidised Bed Combustion. An alternative to mass burn is to pre-process the waste to remove the non-combustible and recyclable materials. The waste is then shredded to produce floc type material called coarse Refuse Derived Fuel (cRDF) that has a higher calorific value than that of untreated waste. The combustion bed of the incinerator consists of a mixture of sand and dolomite mineral through which air is pumped in sufficient amounts to create a rapidly moving or 'fluidised' bed. This process improves the combustion efficiency, which in turn generates more energy and reduces pollution. The main disadvantage of this technology apart from being more complex is that throughputs are up to 35% slower than for a mass burn unit. A new incinerator constructed at Dundee is the first of its kind in the UK to use fluidised bed technology and is capable of generating 8.3 MW of electricity from 120,000 tonnes of waste per year6.

3. Gasification. With Gasification, wastes do not need to be pre-sorted but must be crushed. The gasification process involves the waste being heated in a low oxygen atmosphere to produce a low calorific value gas that may be burnt in an engine or turbine that is coupled to an electricity generator.

4. Pyrolysis. As with Gasification the waste only needs to be crushed before heating, this time at high temperature in the absence of oxygen. The heat breaks down complex molecules to produce gases that are burned in a combustion chamber at temperature in the region of 1200 oC. Both Pyrolysis and Gasification are at present more expensive than existing processes. No full-scale facilities are planned for the present in the UK.

All the above technologies are known as 'energy from waste' processes (EfW). Some plants in addition to generating electricity also produce hot water to supply neighbouring properties. Sheffield and Nottingham have operated in this way for some time and now the newer generation of facilities are incorporating this capability as well.

Issues

The issues relating to waste incineration fall under two main categories, those relating to environmental effects from pollution and those relating to waste management policy.

Environmental Impacts

Environmental impacts arise from both the construction of the facility and its operation.

Impacts from construction are similar to those of any large development and include noise, dust, traffic, visual amenity, cultural heritage, and possible damage to fauna and flora. The majority of developments have taken place on brownfield sites so some of the impacts may be less significant with others being controlled through strict planning requirements.

Operational impacts arise from the solid, liquid, and gaseous emissions from the incinerators. Solid residues arise both from the incinerator bottom ash (IBA) that falls into the ash pit at the end of the combustion process and that arising as fly ash from the air pollution control (APC) process used to clean the waste flue gases before they are discharged to the atmosphere. The IBA is essentially an inert sandy gravel material from which ferrous metals are extracted before being either disposed to landfill or being used as a secondary aggregate in road construction. APC residues are much more problematic as in addition to comprising of fine ash particles, dioxins and heavy metal salts, they may also contain significant amounts of unreacted lime and carbon used in the gas cleaning process. APC residues are currently disposed of to specially engineered landfill sites as hazardous waste though a number of studies are looking at ways of immobilising the pollutants within the ash to render it more inert.

Contaminated water is generally formed when water is used in the flue gas cleaning process. This has to be treated by an onsite treatment plant before the water is permitted to be discharged to the foul sewer. Currently in the UK, no MSW incinerators use the wet gas scrubbing process and it is believed none are planned.

Atmospheric emissions of concern fall into five categories: smells and odours, acid gases, heavy metals, particulates and organic compounds.

• Smells and odours emanating from an operation can be controlled by strict plant housekeeping policy and a comprehensive fume extraction system that collects and funnels odours into the combustion air intakes for the incinerator so that they are destroyed by the combustion process.
• Acid gases mainly comprise of hydrogen chloride and sulphur dioxide. Other gases that are of concern are nitrogen oxides and, with global warming now an issue, carbon dioxide.
• Heavy Metals of most concern are cadmium and mercury, which are vaporised in the combustion process. Most other heavy metals are present as metal salts and are captured by the gas cleaning process and removed in the APC residues.
• Particulate matter is the fine material that exists in the gas stream after the gas cleaning process. The new incineration directive imposes a maximum limit of 10 milligrams of particulate matter per cubic meter of flue gas emitted. Concern over particle matter relates not only to the amount but also now attention is focussing on the ultra fine particles that can get deep into the lungs which are less than 10 microns in size (known as PM10s).
• Organic compounds are either present because of incomplete combustion, or are produced in the combustion process. Of all the organic pollutants, a family of chlorinated organic compounds known as dioxins are of most concern. The reason for the concern is that some dioxins exhibit extremely high toxicity as well as the link between long-term exposure to dioxins and cancer. Dioxins are found throughout the world and are widely distributed through the food chain. They are released into the environment through a number of routes ranging from industrial chemicals to combustion processes such as diesel engines, bonfires, waste incinerators, and forest fires.

Tall chimneys can be used to mitigate against localised pollutant deposition because they create a wider dispersion of the flue gasses, however plans for tall chimneys may provoke strong objections by local residents on the grounds of the visual impact of the chimney. In addition, wider dispersion of emissions may provoke more objections to a project from the greater numbers of the public who could be affected.

Pollution from incineration plants and the possible health effects that may arise, is a major issue with the public and fundamental to addressing these publics concerns is that the plants are strictly controlled and regulated both by planning authorities and by the Environment Agency through the integrated pollution control regime (IPC).

Waste Management Policy

An important part of the current UK waste management policy is known as the 'waste management hierarchy' that presents a hierarchy of preferred options for waste management. The highest priority is to minimise the production of waste in the first place. The next is to reuse waste followed by recycling, recovery, and finally the last and least preferred option is disposal, which usually means landfilling. The hierarchy puts incineration of MSW with energy recovery as a higher preferred option to landfilling though the other options of reuse and recycling should be considered over incineration.

There is considerable debate over the role of waste incineration in waste management over the next ten years. On one hand, the Energy from Waste Association (now joined with the Environmental Services Association) states that by 2010 around 15 new plants of an average capacity of 200,000 t/y will be needed to meet government targets. In contrast, groups such as Greenpeace and Friends of the Earth argue there is no place for incineration as it is a polluting process and a waste of valuable materials that can be recycled or reused.

References

1. Department for Environment, Food and Rural Affairs, Digest of Environmental Statistics - 2000.
2. Department for Environment, Food and Rural Affairs, Municipal Waste Management Survey 1999/2000.
3. Waste Strategy 2000 for England and Wales, Part 1, Command Paper 4693-1, May 2000, The Stationary Office.
4. EC Directive references.
5. 2000/76/EC.
6. EfW in the UK, Energy from Waste. www.efw.org.uk.

Bibliography

Allsopp, M et al. Incineration and Human Health: State of Knowledge of the Impacts of Waste Incinerators on Human Health, Greenpeace, March 2001.

An Introduction to Household Waste Management: Energy-from-waste incineration, ETSU, 1998.

Ban the Burn, Friends of the Earth, www.foe.co.uk .

Energy from Waste, Waste Watch Information Sheet, www.wastewatch.org.uk

Household Waste Management in the UK: some examples of current practise, ETSU, 1999.

Incineration of Household Waste: POST149, The Parliamentary Office of Science and Technology, December 2000.

The Public Acceptability of Incineration: Research undertaken for the National Society for Clean Air and Environmental Protection, NSCA, May 2001.