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108 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 8 Supplying Energy from Waste by Combustion In most European countries, by far the most common method of extracting energy from solid waste either in untreated form or as a prepared fuel is combustion (or incineration) with energy recovery. Virtually all wastes can be treated by combustion and the technology is usually regarded as the default against which other options are measured. Combustion is a well established technology and the main technical challenges have been addressed. There remains however significant technical uncertainty surrounding the impact of the composition of the waste on the performance of the heat exchange surfaces, particularly fouling and corrosion. Other barriers remain, such as the largely negative public perception of such sites in some countries, including the UK. 8.1 Process Description Untreated Waste Waste derived fuel Combustion Heat and/or Electricity Process Description A combustion system in its simplest form comprises two basic elements: a furnace where the fuel is burned and a boiler that recovers the heat from the combustion as steam or hot water. When the fuel enters the high temperature environment in a furnace it will first dry and then decompose, or pyrolyse, into volatile tars and gases and char components. These components then react with air to release the energy contained in the heating value of the fuel. This energy is released as radiation to the walls of the furnace and into a flow of hot flue gases to be captured by heat exchange surface in the boiler that generates steam for use either in the process, or by a turbine to produce electricity, or both (CHP). A furnace essentially consists of a box, lined either with refractory or water tubes, and a burner where the air and fuel mix and burn. There are two main categories of burner for solid waste and biomass applications, grate and fluidised bed. The first has evolved from designs that have been used widely throughout the 19 th and 20 th centuries whilst the second is a fairly recent innovation from 1970’s onwards. Grate burners In grate-firing the fuel burns in a layer on a grid. Air for combustion is blown both through the grid and over the top of the fuel layer. The processes of drying, pyrolysis and combustion of the volatiles and char take place sequentially as the material proceeds through the boiler on the grate. Various types of grids or grates have evolved to move the fuel through the boiler and eventually remove the ash. Some grates vibrate, some move slowly forward on chains whilst others have a reciprocating action. Grates are reliable, but are considered somewhat inflexible and are designed to cope with a limited range of fuels. Experience in the UK has shown that high, and variable, moisture content fuels such as poultry litter can lead to uneven combustion and high dust emissions.
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Small industrial-commercial and domestic boilers are small grate burners that burn almost exclusively clean wood in the form of logs, chips and pellets. Logs are used mostly for domestic applications, pellets for domestic and commercial and chips for commercial to large commercial and industrial. Fluidised bed burners In a fluidised bed furnace, the fuel burns in a bed of sand or other mineral that is violently agitated by the combustion air. The fuel is fed at a controlled rate to keep the temperature of the sand bed at 800 to 900°C. With moderate air velocities the bed has the appearance of a boiling liquid, hence the name bubbling fluidised bed (BFB). If a higher velocity is used the sand will be carried out of the furnace and must be recycled to the base via a cyclone – this is known as a circulating fluidised bed (CFB). Heat is removed and steam is raised by tubes in the bed of sand (not needed for biomass fuels), the walls of the furnace and in the exhaust flue. This type of boiler is proving very popular for medium to large industrial boilers for coal and other solid fuels and is taking an increasing share of this market. A great advantage of the fluidised bed to the power plant operator is its fuel flexibility. This feature has been used to great advantage by CHP plants in the Nordic Countries, where it is common practice to fire wood chip, coal, peat, oil and wastes both together and separately. This flexibility is bought at the cost of greater complexity however and they are unlikely to be cost effective lower output alternatives. Good modern examples of fluidised bed burners from the Bioenergy capital grants scheme are at Lockerbie (43MWe) and Wilton (31MWe). Both are fuelled by a mix of forestry wood chip and graded waste wood from wood recycling operations. Steam Boilers There are two types of boiler in common usage, fire tube and water tube. Fire tube boilers, as the name suggests, transfer the heat from the fire to the water by passing the hot flue gasses through tubes inserted in a pressure vessel containing the boiling water. Typically the gas will pass backwards and forwards three times through the boiler – three tube passes. They are inexpensive and manufactured in volume. Typically the furnace supplier will purchase the boiler and adapt it to his system. Fire tube boilers are manufactured in sizes up to 50 tonnes steam/h and a typical steam pressure of 20bar, although 40bar is possible. They produce only saturated steam but a variant is available that allows for superheating by installing a heat exchanger after the second tube pass. This is a cost effective solution for industrial plants that require smaller turbines, typically 1 - 2MWe. Water tube boilers generate steam inside of tubes that are installed around the furnace and in banks across the flue gas. The temperatures and pressure of the steam can be much higher than in water tube boilers. Utility boilers can reach supercritical conditions. For large biomass boilers 520°C and 120 bar is not uncommon. Smaller units that might be appropriate for a CHP installation would have temperatures of 450°C and 50 bar. They can be manufactured in much larger sizes than fire tube boilers and tend to overlap the top end of the fire tube range. Grate burners can be installed with either fire tube or water tube boilers. Fluidised beds are normally installed with water tube boilers. Control of emissions The combustion gases are cleaned by a sequence of process stages that remove particulates, acid gases and trace organic compounds such as dioxins and furans. The number of stages their type and complexity depends upon the composition of the waste, its legal classification under the Waste Incineration Directive and the size of the installation. The main species to be removed and abatement technologies are summarised below in Table 55. 109
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South East 327 - - 55% of MSW by 20
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Appendix 3 MBT Methods Specific in
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Figure 27 Orchid Environmental proc
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