Fundamental limits of NO formation in premixed methane-air flames: determination of the mechanism of upstream heat loss and flue-gas recirculation as NOx control strategies by low-pressure flame studies (GPC.5480)

The progressively more stringent regulations for pollutant emissions from domestic, commercial and industrial equipment fuelled by natural gas are driving the designers of combustion systems to search for efficient means of limiting NOX, formation. Although much is known about the chemical mechanisms by which NOX is formed, it is presently not known to what extent the various formation mechanisms are operative in most practical devices, nor is the current understanding of combustion processes capable of predicting the effects of NOX control strategies in these devices. As a result, the intrinsic advantages and limitations of the various control strategies are often unknown, and combustion equipment is still developed by empirical, trial-and-error, methods. Thus, this lack of knowledge results in an inefficient and ineffective development process: not knowing the fundamental controlling factors for NOX formation leads to unnecessary duplication of effort, while potentially fruitful avenues of development are left unexplored. The development process would be greatly facilitated if the designer knew, a priori, the effectiveness of a given control strategy in lowering NO, emissions. This knowledge can only be derived from microscopically correct insight into NO formation chemistry and its response to variations in process conditions, such as dilution with flue gases and heat transfer. Such insight is seriously lacking at present.

The research proposed here is designed to provide this insight for two NOX control strategies: flue-gas recirculation and upstream heat loss (the operating principle of radiant burners). Towards this end, the concentrations of key chemical species and temperature will be measured using non-invasive optical techniques in flat flames subjected to various degrees of flue-gas recirculation and upstream heat loss. Comparison of the experimental results with those of numerical calculations using the best available detailed chemical mechanisms will expose the mechanistic shortcomings of the models, and the mechanisms will be altered to repair the deficiencies.

The results of this project will be utilised for the benefit of the design of low-NOX combustion systems on two levels. On a practical level, the results will be used to show the most promising conditions for minimising NO, formation (for example, combinations of fuel-air ratio and upstream heat loss). This is of particular importance for new high-efficiency, low-emission combustion techniques for the energy--intensive industry being developed by Gasunie and others. On a more fundamental level, the improved chemical mechanism will be used in advanced numerical models, currently being developed at various Dutch universities, to aid the design of combustion systems. The results of this project will significantly improve the accuracy of the NOX predictions of these models, and thus enhance their added value for the design of practical systems.

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Bij het onderzoek zijn vier bedrijven en vier universiteiten betrokken.