The effects of temperature, mixing and volatile release on no reduction mechanisms by coal reburning

Abstract

The effects of temperature, mixing and devolatilization rates on NO reduction by coal reburning were studied experimentally using a 0.2-MW pilot-scale furnace. A mathematical model was used to investigate the sensitivity of the reburning process to the devolatilization behavior and mixing rates of the reburn coal. The model considers the major NO reduction pathways relevant to reburn using a global chemistry approach, namely, hydrocarbon fragments, volatile fuel-nitrogen species, and heterogeneous NO reduction by char particles. The predicted and experimental results both show that increased volatile yields and faster devolatilization rates can lead to improved NO reduction in the reburn zone provided that the reburn zone is operated fuel rich so as to limit the consumption of volatile gases by oxygen. As reburn zone conditions become increasingly leaner, rapid coal pyrolysis results in the earlier consumption of volatile gases by oxygen as mixing takes place, leading to less NO reduction. Increasing the temperature increases the rate of coal devolatilization near the reburn coal injectors, leading to less efficient NO reduction. The magnitude of this effect was found to vary with the coal type. Consequently, while it is known that rank plays an important part in determining the effectiveness of a coal as a reburn fuel, the results presented also show that simple rank indicators such as proximate volatile yield can only provide an initial indication of the suitability of a particular coal as a reburn fuel. Of further significance to the reburning process is the specific coal pyrolysis behavior, which is controlled by the process temperature and heating rate and can vary between coals of similar rank but different morphology. For coal reburn, the model predictions suggest that the NO reduction by CH fragments is the dominant NO-reduction mechanism in the reburn zone, accounting for nearly 70% of the total reduction observed.