An Investigation of NO Reduction by Coal Reburning

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Xa. -Z An Investigation of NO Reduction by Coal Reburning Y.B.Yang, B.M. Gibbs and E. Hampartsoumian School of Process, Environmental and Materials Engineering Department of Fuel and Energy The University of Leeds, Leeds, L S2 9JT, U.K. Abstract In this paper coal reburning for the reduction of NO emission was investigated in a 0.2MWt pilot-scale furnace. Eight bituminous coals were selected for study over a wide range of operational parameters including reburn zone residence time, reburn-fuel fraction, type of coal, furnace temperature level and primary N O concentration. Three-dimensional numerical calculations were carried out in an effort to fully understand the reburning process, the related NO-chemistry and to investigate in detail the sensitivity of the reburning process to the devolatilisation behaviour of the reburn coal and mixing rates. A global N O mechanism model was employed which features the three major NO-reducing agents, i.e. hydrocarbon (CH) fragments, volatile fuel-nitrogen (HCN) species and char particles. The model predictions show that reactions between primary N O reduction and C H fragments are the dominant N O - reduction mechanism in the reburn-zone, accounting for nearly 7 0 % of the total reduction observed. The predictions also serve to validate the experimentally observed effects of increasing the furnace temperature level and number of coal injectors. Parametric analysis shows that increased volatile yields and/or faster devolatilisation rates can lead to improved N O reduction in the reburn zone provided that the rebum 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 N O reduction. The experimental and predicted results highlight the differences in coal pyrolysis behaviour, even between coals of similar rank which would account for the variability observed in the extent on N O reduction for the coals investigated. 1. Introduction The principle of reburning (or fuel-staging) is to destroy NOx already formed in the primary combustion process by introducing a secondary fuel into the furnace [1-6]. Typically three zones are involved (Fig. 1). First, the primary zone, in which the combustion of the main fuel takes place forming N O depending on fuel type, operating and mixing conditions. Next the reburning zone , where additional fuel (not always the same as the main fuel) is introduced to produce hydrocarbon radical fragments and other reducing agents which react with the N O from the primary zone. Finally, in the bum-out zone, over-fire air is added to complete the combustion processes. All the N O reduction routes shown also play a part in the in the burnout zone to a greater or lesser extent. The primary zone may be operated fuel-rich to limit flame temperatures and promote low-NOx production mechanisms, although this is not always possible in practice.