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Reductions In NOx and Smog Pollution

NOx formation during the combustion process occurs mainly through the oxidation of nitrogen in the combustion air (thermal NOx) and nitrogen bound in the fuel matrix (fuel NOx). Thermal NOx formation during the combustion process is easily understood and has been adequately described by the Beldovich mechanism. It is suppressed by reducing flame temperatures and limiting oxygen concentration. Fuel NOx formation is a more complex process involving local concentration of oxygen and nitrogen and is reduced by minimizing the avaiability of oxygen during the early stages of the combustion process.

Separated Overfire Air (SOFA) provides staging of combustion air controlling the mixing process of fue and air and the peak temperatures. The effectieness of the NOx control process is dependent on the coal burned, with lower NOx emissions from coals with lower nitrogen contents and lower ratios of fixed carbon to volatile matter (i.e., higher reactivity). Combustion zone geometry is particulary important, with higher heat release rates and shorter residence times all contriibuting to higher NOx levels.