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110 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Table 55 Pollution and the abatement methods for combustion of waste Pollutant Method of removal Particulates or aerosols Fabric or electrostatic filters, for smaller boilers and cleaner fuel cyclones or multiclone. Acid gasses mainly from sulphur and Dry or liquid carbonate wash or dry chlorine absorber. Dioxin and other organic compounds Active carbon absorber, can be combined with dry carbonate scrub Oxides of nitrogen. Injection of ammonia or urea into combustion zone. Catalytic removal from the flue gas. The main concerns on pollution from incinerators relate to the health effects of dioxins and other chemicals such as PCBs (poly chlorinated biphenyls) and PAHs (poly aromatic hydrocarbons). These compounds accumulate in body fat and are persistent. Dioxins are not present in nature. The term is loosely used to describe the whole family of toxic organic micro-pollutant emissions that may be formed during and after combustion. They have been associated with poorly run incineration in the past when chlorine containing wastes such as PVC, paper and card and treated wood were burnt, and this association persists. Understanding of how dioxins are formed has led to advances in process design and control that have resulted in the emissions from the incineration sector falling dramatically in the UK. Scientific studies have shown that the impact of emissions from an individual plant is very low. Dioxin emissions have reduced by more than 90% in the UK in the past 20 years. Nevertheless there remains acute public concern over the issue, despite the fact that local authorities have access to this information. PCBs are classified as persistent in the environment and as being probably carcinogenic to humans. They have also been linked to sub-chronic effects such as reduced male fertility and long-term behavioural and learning impairment. They may originate from flexible plastic waste, paint waste, as well as transformers and capacitors. PAHs are a large group of chemical compounds and are carcinogenic. They may originate from waste wood, tar or fat waste for example. Sampling and analysis costs for all of these compounds are high due to the specialised nature of the equipment and trained laboratory personnel required. This restricts the frequency of data collection. For this reason the public express concern regarding the reliability of data. A simpler, on-line test would be more satisfactory but due to the nature of dioxins this is a long way off. Smaller installations burning wood that do not have particulate removal equipment have also given rise to concern recently. This is due to the nature of emissions from wood firing. The ash composition contains alkali components that vaporise at the furnace temperature to later condense in the cold atmosphere as aerosols with a size that is respirable. There is a similar problem with condensable organics in batch fed wood combustion devices where poor combustion control allows the formation of aerosols of organic compounds and soot. These are particularly worrying as they contain carcinogenic materials such as furans and aromatics. This is only likely to be a problem in urban areas where the aggregate effect of several installations in a small area coupled with other sources such as diesel traffic could give unacceptable impacts. Local Authorities are being issued with revised guidelines on this issue and this should resolve the situation. The ash leaves the process as two distinct streams – bottom ash that falls out of the combustion grate or bed, and fly ash that is separated from the flue gases. Bottom ash is considered to be inert and, after the separation of metals, is either disposed of to general landfill or used as aggregate. Fly ash, however, can contain heavy metal contamination and, with certain chlorine containing wastes, dioxin so must be
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 disposed of in a controlled landfill site. Recent European practice involves melting fly ash to form a glass that reduces the mobility of the metal ions and destroys dioxin. Following this treatment the ash can be used as aggregate or filler. Impact of waste composition on boiler performance There are two main impacts that must be managed by the designers and operators of waste combustion plant; acid gas and ash fouling. Chlorine is always present in mixed waste streams and derives largely from plastics but also from paper and card. The chorine is converted into hydrochloric acid in the combustion chamber, and this can cause excessive corrosion in the boiler, particularly at high temperatures. Restricting the temperature and pressure of the steam produced by the incinerator limits the corrosion risk but also restricts the electrical efficiency that can be achieved. Circulating fluidised bed combustors have an advantage in treating wastes with a high chlorine content in that the high temperature boiler surface can be located in the circulating bed material where the chlorine concentration is lower. The oxides and salts of silicon and alkali metals are present in biomass derived wastes to a much larger extent than fossil fuels. The content increases with the proportion of annual growth material in the fuel. Thus, straw and other annual crop residues contain large quantities as does the bark and outer layers of trees. These components have low melting points that can lie below the temperature of the furnace, they also have a propensity to form low melting point eutectic mixtures with other ash components. The liquid ash can adhere to heat transfer surfaces causing a reduction in output, sites for corrosion, and occasionally physical damage. Fluidised beds are also susceptible to agglomeration and eventual sintering and fusing due to the sticky nature of soft and liquid ash. Problems caused by ash chemistry are some of the most intractable in the industry and require the application of substantial knowledge and experience in sourcing and blending feedstocks and instituting regular boiler cleaning procedures. Suppliers of fluidised bed boilers have learned to manage the chemistry of their bed materials to avoid low melting point eutectics by the addition of materials such as dolomite. A comprehensive review and a discussion of best practice for waste combustion is given in the supporting documentation for the Waste Incineration Directive 196 Co-firing biofuels with fossil fuels The concept of co-firing biofuels with coal is one means of increasing the combustion of biomass in an existing power plant without incurring excessive capital costs. This approach has received increased attention recently, particularly in Denmark, the Netherlands and the US. The details of over 300 co-firing installations can be found on the IEA Bioenergy Agreement Task 32 database of co-firing installations. 197 The way in which biomass is fired depends on the quantities involved. Where biomass quantities are small (2- 5%), the biomass is mixed with the coal at the mill inlet. Where biomass constitutes 5-25% of the total fuel, shredded biomass is fired through dedicated burners. Beyond the 25% level, new concepts will be needed because combustion of that amount of biomass will have a substantial impact on both furnace and ash behaviour. Current opinion within the industry is that this will probably mean converting the fuel to gas. Current practice in the UK is to receive the fuel on site in the form of pellets that are stable, easy to handle and break easily in the coal mills. 196 European Integrated Pollution and Prevention and Control Bureau Waste Incineration Bref (08.06) http://eippcb.jrc.es/pages/FActivities.htm 197 http://www.ieabcc.nl/database/cofiring.php 111
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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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