OCR Text |
Show -3- full understanding of NOx formation is still not possible. Recognizing these limitations, the CEMFLAM consortium members are commencing CEMFLAM II to undertake further work. This paper makes use of the information obtained in the CEMFLAM I trials, together with in-house modelling programs developed by FCT to make a combined theoretical and empirical assessment of the combustion conditions in two rotary kilns and to estimate the combustion related NOx reductions which could be achieved by optimizing burner design and fuel usage. 2. HOx FORMATION IN KILN FLAMES NOx formation in kiln flames is generally by both thermal and fuel routes (for coal, oil and petroleum coke). OWing to the very high temperatures which occur ie., above 2000 0 C (3600 0 F), thermal NOx is generally the dominant mechanism. In gas fired kilns fuel NOx is absent so all the NOx is thermal NOx. However, it should be noted that the absence of fuel NOx in gas fired kilns does not necessarily lead to a reduction in NOx emissions since flame temperatures are often higher. Thermal NOx is formed by the combination of atmospheric nitrogen and oxygen at very high temperatures. The high temperatures are required because of the high activation energy of the reaction, it is therefore highly temperature dependent. The reaction takes place between oxygen radicals, nitrogen radicals and molecular nitrogen in the Zeldovich reaction couple. Apart from temperature, the in-flame oxygen concentration and the residence time in the high temperature zones influence the final thermal NOx emissions. Most fuels, other than gas contain nitrogen bound as an organic compound in the structure. When the fuel is burnt this organic nitrogen becomes converted into a range of cyanide and amine species some of which are subsequently oxidised to NOx, depending on the local oxygen availability, but this mechanism is less dependent on temperature. A third mechanism of NOx formation has been identified by some workers which involves the fixation of nitrogen by hydrocarbon compounds in fuel rich areas of the flame. This mechanism, known as prompt NOx, probably only accounts for 5% of NOx created in practical kiln flames. Prompt NOx will therefore not be be specifically considered in this paper, but the mechanism of both thermal NOx and fuel NOx formation are described in more detail. |