Wastes represent an increasingly important fuel source. Using wastes as fuel can have important environmental benefits. It can provide a safe and cost-effective disposal options for wastes that could otherwise present significant disposal problems. It can help reduce CO2 emissions, through displacement of fossil fuels. Methane is 23 times more damaging than CO2 for global warming. If biodegradable waste is diverted from landfill methane emissions can be avoided.

Any energy that is recovered from biological wastes can be regarded as renewable energy. It comes from plant material (either directly, or in the case of animal wastes, paper or card, indirectly). As plants grow they absorb carbon dioxide from the atmosphere. When this biomass material is used as a fuel, the carbon dioxide is returned to the atmosphere in a "carbon neutral" cycle. If biomass is used to displace fossil fuels instead of being left to decompose naturally, it will actually help to limit the emission of carbon dioxide and methane into the air.

There are many ways of combining waste disposal with energy recovery. A number of well established technologies are available for generating heat or power from wastes. There are also new technological developments, especially in power generation, which have the potential to increase the efficiency of energy recovery.

Recovering energy from wastes from municipal or industrial sources can turn the problem of waste disposal into an opportunity for generating income from heat or power sales. The safe and cost-effective disposal of these wastes is becoming increasingly important worldwide, especially with the demand for higher environmental standards of waste disposal and the pressure on municipalities to minimise the quantities of waste generated and disposed to land.

The technology

A very wide range of municipal or industrial wastes may be used as fuel. The nature of the waste and the waste disposal method will determine the way that energy can be recovered. Dry household, commercial or industrial wastes can either be burned (combusted) as raw waste, or they may first undergo some sorting or processing to remove waste components that can be recycled separately.

Combustion with energy recovery

Waste combustion with energy recovery is an established way to dispose of wastes. It decreases the volume of the waste and allows for recovery of metals and other potentially recyclable fractions. After further basic treatment, most of the remaining residue can be combined with other materials and used as an aggregate material. Any residue that is landfilled is biologically inactive and does not generate potentially harmful emissions.

The heat recovered from these plants can be used to generate electricity, or can be used for industrial heat applications. The size of energy from waste plant is designed to meet the waste disposal needs of the community, taking into account the potential for waste minimisation and recycling. Plants that generate electricity can typically process between 20,000 and 600,000 tonnes per year, and from this they can generate from 1 to 40 MW of electricity. Power is produced from these wastes by using the steam raised in the combustion process to drive a steam turbine to generate electricity, in a similar manner to a conventional coal fired power station.

Combined heat and power (CHP) is an attractive option where there is a market for the heat . This could be a factory or district heating system for a small community.

Advanced thermal technologies

Where the waste stream is of a uniform nature, for example if it has been processed into a homogenous fuel, it is better suited to the more "advanced technologies", such as gasification or pyrolysis. Wastes that are not uniform in composition, for example municipal wastes, are less suited to treatment by advanced technology, although the technology is rapidly developing to handle more challenging wastes.

Gasification

Gasification is one of the newer technologies that is increasingly being used for waste disposal. It is a thermo-chemical process in which biomass is heated, in an oxygen deficient atmosphere to produce a low-energy gas containing hydrogen, carbon monoxide and methane. The gas can then be used as a fuel in a turbine or combustion engine to generate electricity. Gasifiers fuelled by fossil sources such as coal have been operating successfully for many years, but they are now increasingly being developed to accept more mixed fuels, including wastes. New gas clean-up technology ensures that the resulting gas is suitable to be burnt in a variety of gas engines, with a very favourable emissions profile. Gasifiers operate at a smaller scale than incineration plant, and can also be provided in modular form to suit a range of different scales of operation. A number of British companies are leading in this emerging technology.

Pyrolysis

Pyrolysis is another emerging technology, sharing many of the characteristics of gasification. With gasification partial oxidation of the waste occurs, whilst with pyrolysis the objective is to heat the waste in the complete absence of oxygen. Gas, olefin liquid and char are produced in various quantities. The gas and oil can be processed, stored and transported, if necessary and combusted in an engine, gas turbine or boiler. Char can be recovered from the residue and used as a fuel, or the residue passed to a gasifier and the char gasifed.

Strict environmental standards now apply in all European countries governing the emissions from energy from waste plant, particularly of heavy metals, furans and dioxins. All energy from waste plant must now meet these standards, which can be achieved through the installation of extensive state-of-the-art gas cleaning systems.

• Biomass (Energy from Waste)