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Polycyclic Aromatic Hydrocarbons (PAHs)
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10 Environmental impact associated with pulverized fuel ash: Polycyclic Aromatic Hydrocarbons (PAHs)
Incomplete combustion of fossil fuels and waste results in the formation of soot and tar. These materials contain all sorts of organic compounds (hydrocarbons), including polycyclic aromatic hydrocarbons (PAHs). Some PAHs are known to have carcinogenic properties, while others do not. When considering the health issues associated with pulverized fuel ash and other airborne particulate materials, it is therefore important to distinguish between carcinogenic and non-carcinogenic PAHs. PAHs consist of two or more benzene rings. At room temperatures, those with two or three rings possess a high vapor pressure and are therefore known as more volatile PAHs. PAHs with more than three rings are much less non-volatile. The carcinogenic PAHs all have four to six benzene rings, while most mutagenic PAHs have four or more rings. Hence, it is only non-volatile PAHs with four or more rings that represent a major health hazard. These PAHs are normally bounded to very fine particulate materials known as aerosols. PAHs are products of incomplete combustion. In principle, therefore, coal combustion can lead to their formation. At modern pulverized coal-fired power stations, however, combustion is virtually complete. First, the coal is pulverized to form a fine powder. Furthermore, during firing, the temperatures, boiler residence times and fuel-air ratios are such that almost all the carbon is converted into carbon dioxide. Consequently, almost no hydrocarbons are formed. The low carbon content of pulverized fuel ash – in practice about 5 per cent - is evidence of the near-complete conversion. The introduction of low-NOx burners has not affected this situation. As their name suggests, low-NOx burners are designed to minimize the formation of NOx. This is achieved by keeping the combustion temperature and oxygen surplus as low as possible. Such conditions might be expected to lead to the formation of PAHs, but an optimum balance has been struck so as to minimize NOx formation while achieving high carbon conversion rates. Consequently, PAH formation remains minimal.
KEMA-report 50030086-KPS/MEC 00-6050.
External reviewer:
Dr P. de Voogt, senior lecturer at the Universityof Amsterdam, faculty of Science, department of Environment and Toxicological Chemistry
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