Incineration and Diversion of Waste away from Landfill
Incineration is a long established means of waste disposal. It relies upon burning waste in excess air. It is important to divert organic matter away for landfill and new waste technologies are being developed to achieve this. Incineration has been around for along time and achieves organic waste diversion away from landfills. To divert organic waste away from landfills is essential to reduce the environmental impact of landfills, reduce the rate they are filled and used up, and in Europe to comply with the EU Waste Directive.
Modern plants are intensively engineered to comply with very stringent environmental legislation. The incineration process destroys both biological and non-biological organic matter. Modern incinerators invariably include some means of recovering some of the energy within the waste.
Incineration provides a substantial reduction in the total volume of waste requiring disposal in landfill. In addition the remaining waste is rendered biologically clean and stable. This is a very stable process: virtually all wastes can be burned and the burning process can be adequately controlled.
Most modern incinerators generate electricity from the surplus heat; this provides an income stream. Also in some instances units can be integrated into a district-heating scheme. As there are many plants around the world which have demonstrated satisfactory technical and financial operation there is little difficulty in obtaining finance for new plant. There are two distinct types of incinerator: moving bed and fluid bed.
The fluid bed technology involves chopping the waste into small fragments and suspending them in a sand bed through which air is blown. The combustion is undertaken in this violently moving bed. This fluid bed offers the prospect of better combustion control, and hence the emissions are more controlled. In practice the problems and cost of reducing the size of the waste often outweighs these benefits, and the bed is more susceptible to blocking than the moving grate. Both of these technologies compete at the 'large end' of the market, that is units that burn over 125,000 tonnes per annum.
All incinerators rely on a high integrity gas cleaning train in order to meet the required emission limits. This equipment will include a neutralising system to remove acid gases, a carbon injection system to reduce emissions of certain organics and heavy metals, and a filter to reduce particulates.
The energy recovery systems used in the UK invariably centre on a boiler. As a result of the pressures and temperatures at which these boilers can operate the net electrical efficiency from the site seldom exceeds 22 per cent. An option exists to improve this efficiency if the waste heat can be put to beneficial use, such as in a district-heating scheme.
Any carbon-containing materials will be converted to carbon dioxide, some metals will be converted to their oxides, and some metals will remain unchanged. The problems with incinerators often arise from materials that do not burn or certain acidic gases. Large metallic or mineral (rock) items can cause physical damage; items such as aerosols or gas canisters can explode and cause problems in the combustion zone. The pressure limits on the boilers are a consequence of the effects of chemical attack by some volatile metals and acids, which inevitably are present in wastes.
Emissions and Residues
The main output from an incinerator is the flue gas, which contains water vapour and most of the carbon in the feed as carbon dioxide as well as the nitrogen and excess oxygen, from the air used for combustion. Acidic gases and dioxins are controlled by addition of lime and carbon, which are removed from the gas stream by a filter. The filter controls the particulate emission from the plant. The solids from this filter are known as 'Air Pollution Control residues' (APC residues) and constitute a significant output as they are classed as hazardous waste. These can contain some fine ash carried over from the combustion (fly ash). Depending upon detailed plant design there may be a separate output of fly ash from some form of capture equipment. Heat is another output from an incinerator: it can be utilised directly as steam by an adjacent plant, converted to hot water for use in a district heating system, or converted to electricity. A modern incinerator can operate as a local Combined Heat and Power (CHP) plant and can be run to provide a range of demands on an hour-by-hour basis.
Incineration is a well-established process which is best employed when linked to local heat or power users are available. This process guarantees 100 per cent diversion of biodegradable waste from landfill. Waste Managers should consider the use of incineration very carefully when selecting a waste technology for a city, town, or area. It may be a better option than other methods.
* The views are solely those of the Author. As elsewhere on this web site, always engage professional advice before placing reliance on any aspect of this article.