A Model for Nitrogen Chemistry in Oxy-Fuel Combustion of Pulverized Coal

Abstract

In this work, a model for the nitrogen chemistry in the oxy-fuel combustion of pulverized coal has been developed. The model is a chemical reaction engineering type of model with a detailed reaction mechanism for the gas-phase chemistry, together with a simplified description of the mixing of flows, heating and devolatilization of particles, and gasÀsolid reactions. The model is validated by comparison with entrained flow reactor results from the present work and from the literature on pulverized coal combustion in O 2 /CO 2 and air, covering the effects of fuel, mixing conditions, temperature, stoichiometry, and inlet NO level. In general, the model provides a satisfactory description of NO formation in air and oxy-fuel combustion of coal, but under some conditions, it underestimates the impact on NO of replacing N 2 with CO 2 . According to the model, differences in the NO yield between the oxy-fuel combustion and the conventional combustion of pulverized coal can mostly be attributed to the recycling of NO (reburning effect) and to changes in the mixing patterns between fuel and oxygen. For pulverized-fuel combustion at high temperatures, we think that NO is mainly reduced by heterogeneous reactions involving both char and soot. Here, the tar yield of the volatiles is mainly converted to soot and H 2 , limiting the concentration of hydrocarbons and thereby the importance of gas-phase removal of NO. Our work emphasizes the need for accurate descriptions of mixing, volatile composition (fate of tar), and heterogeneous reactions. Furthermore, more work is desirable on the reduction of NO by CO on char at higher temperatures.

Homogeneous and heterogeneous contributions of CO 2 and recycled NO to NO emission difference between air and oxy-coal combustion

“…Homogeneous reductions of recycled NO (accounting for nearly 60%) were more important than the reactions between NO and char during the lignite and bituminous coal combustion, whereas the reactions between recycled NO and char contributed more to NO reduction (approximately 55%) than homogeneous NO removal in the anthracite combustion. This result differs from both Okazaki's result [5] and Hashemi's result [15]. Due to the abundant NO from the recycled flue gas in O 2 /RFG atmosphere, the dominant factor for NO removal is the amount of reducing agents in the gas or solid sample.…”

“…Okazaki and Ando [5] found homogeneous NO removal reductions are more important than heterogeneous reactions for reducing NO exhaust during their experiment. In contrast, Hashemi et al proposed a conclusion that heterogeneous NO removal reactions are more important [15]. Thus, the major recycled NO reduction route is still under debate.…”

Performance of Pulverized Coal Combustion under High Temperature Air Diluted by Steam

“…On the other hand, the peak points in curves of Figure 5 present the trapped soot at the bottom of combustion chamber. Besides, reliable combustion evaluates by soot amounts the formation which shows the impure carbon particles due to the incomplete combustion process [27].…”

Nitrogen Oxide Evolution in Oxy-Coal Combustion

“…Then to assess the conversion of HCN to NO a published kinetic model for HCN oxidation was used. This model was developed by Dagaut and Glarborg [4] and enhanced with reactions pertinent to CO 2 by Glarborg and co-workers; see Alzueta et al [5] and Hashemi et al [6].…”