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9 Environmental impact associated with pulverized fuel ash: Dioxins
Incomplete combustion of fossil fuels and waste can lead to the production of hydrocarbons. Under certain circumstances, some of the hydrogen atoms in these hydrocarbons can be replaced by atoms of chlorine, fluorine or bromine to form substances called dioxins. There are 210 different types of dioxin, of which a congeneric group of seventeen, the so-called dirty seventeen, are toxic. In principle, any combustion process that takes place in the presence of chlorine can lead to dioxin formation. (Combustion in the presence of fluorine, bromine or a mixture of these halogens can also result in dioxin formation, but, since fluorine and bromine is much less common, they are much less significant in relation to emissions.) Two mechanisms of formation are known: homogeneous gas phase reaction at 850 to 1100 °C and de-novo synthesis. The latter can take place during cooling at temperatures of between 500 and 200 °C. The level of dioxin formation depends on the fuel composition and the combustion conditions. The large-scale combustion of fossil fuels, as in power stations, results in very low levels of dioxin emission. This is mainly because combustion in modern coal-fired power stations is virtually complete. Complete combustion is critical in this context. If combustion is complete, no soot is formed; this is significant because soot particles are believed to have the right carbon structure for the formation of dioxin. The presence of chlorine is also a determining factor, but the chlorine concentration does not significantly influence the amount of dioxin formed. Furthermore, the presence of SO3, formed from SO2, appears to inhibit dioxin formation. In this context too, coal combustion differs from waste incineration, since little SO2 is present in the waste incineration process, whereas coal combustion is associated with high SO2 concentrations. Together, the almost complete combustion and the presence of SO2 account for the very low levels of dioxin formation associated with coal combustion.
KEMA-report 50030086-KPS/MEC 00-6049.
External reviewer:
Dr K. Olie, senior lecturer at the University of Amsterdam, faculty of Science, department of Environment and Toxicological Chemistry
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