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Show -2- despite this extensive research and a relatively good understanding of the chemical reactions which lead to NOx formation, the predictability of the performance of low NOx burners is not good or easy. This is because NOx formation is heavily dependent on local temperatures and species concentrations which are in turn determined by near field aerodynamics, fuel injection patterns and fuel characteristics, therefore the permutations and combinations of variables is enormous. Hence, even for power boilers the prediction of NOx emissions is based on limited theoretical understanding and a good deal of empirical interpretation. For rotary kilns, the current level of knowledge is much worse than for power boilers. Attempts to produce low NOx burners for kilns have been able to make only limited use of the research undertaken on power boilers because the two types of burner work on fundamentally different principles. On a power boiler, all the combustion air passes through the burner and is therefore under the control of the burner designer. The fuel/air mixing is largely controlled by the flow patterns and turbulence in the secondary and tertiary air stream with primary air generally being used to establish flame stability. With a rotary kiln burner, the burner designer has no control over the secondary air (and in most cases no interest in its aerodynamic flow patterns), and the fuel/air mixing is controlled by the primary airflow and especially its momentum relative to the secondary air momentum. Most attempts to produce low NOx kiln burners have centred on reducing mixing rates by reducing primary air momentum. Whilst this is generally effective at reducing NOx, it has also resulted in operational problems with many kilns as a result of inadequate heat transfer to the product which has led to quality problems and production constraints. In many cases, flame impingement has been a problem with consequential early refractory failure. Recognizing the need for high performance low NOx kiln burners, the International Flame Research Foundation supported by a number of European cement and burner manufacturers, FCT undertook a two year project to investigate the formation of NOx in cement kiln flames. This project, known as CEMFLAM I identified the major factors influencing NOx emissions from coal and petroleum coke fired rotary kiln burners. The CEMFLAM I project advanced the understanding of NOx formation in rotary kilns enormously, but this work represents probably less than 1% of the effort which has been put into power boilers where a |