Experimental Investigation And Modeling Of The Interaction Between Ignition And Denitration During Pulverized Coal Reburning

In this paper, based on the theoretical prediction method investigation of homogeneous ignition temperature of pulverized coal, the interaction between two extremely complicated and highly non-linear critical phenomena during pulverized coal reburning, i.e., that on homogeneous and heterogeneous denitration, and that on pulverized coal ignition, was investigated by combining experiment and numerical simulation. For the first time, the abrupt variations of denitration efficiency with excess air coefficient during coal reburning, were found to be relevant with the typical critical phenomenon during coal combustion, i.e., "ignition", the physical essence of the interaction between pulverized coal denitration and ignition was theoretically elucidated, and a reasonable explanation was made on the critical phenomenon about the variation jump of denitration efficiency with excess air coefficient. The investigation results of this paper have important practical significance for the prediction of pulverized coal ignition and combustion characteristics, and the determination of the operating parameters during industrial reburning. Those results also provide a solid scientific guidance for the industrial application of pulverized coal reburning.The fast pyrolysis characteristics of Shenmu bitiminous coal, Erhao bitiminous coal and Jincheng anthracite coal were investigated with curie point pyrolyzer and gas chromatography, and the following experimental results were found. The mass loss of coal pyrolysis mainly occurs in the temperature rising stage, the mass fraction of tar in volatiles is ranked the first for both bituminous coal and anthracite coal, which exceeds 50% for bituminous coal and is higher than that for anthracite coal. According to the release data of pyrolysis products, the single-equation model was used for calculating the kinetic parameters of pyrolysis reactions. Based on these parameters, the exact whole-coupling transient model on single pulverized coal combustion was developed, with various pyrolysis products including tar, heat transfer, mass transfer, and chemical reactions considered, and the igntion and combustion processes of the three coals mentioned above were simulated exactly. The ignition modes of those were found all to be joint heterohomogeneous ignition, the high temperature caused by CO flame on char surface at the beginning of char ignition was validated by simulation results, which also proved that the primary surface reaction product, i.e., CO was ignited by volatile flame. In addition, the energy distribution coefficient during joint heterohomogeneous ignition was also calculated with the above mentioned model.Considering some disadvantages of the whole-coupling transient model, such as too complicated simulation process and too long simulation time, a simplified unsteady homogeneous ignition model of single pulverized coal was developed, based on the ignition limit theory of combustible gas, whose calculation precision was verified with that whole-coupling model. The simulation results of above three coals showed that the theoretical assumptions of the simplified model were reasonable, and it owned high precision, simple process, and short calculation time, which can meet the industrial demand. Based on the above simplified homogeneous ignition theory, the homogeneous ignition model of pulverized coal stream was developed, which can be used in industrial applications. The above investigation about models solves a basic theoretical problem in coal combustion field, which has never been solved well in the past, the research results are much significant for both academic theory and practical application. Ultimately, combined with heterogeneous ignition model (TET) of coal char, the prediction method on critical ignition condition of pulverized coal stream during reburning was developed.In this paper, the experimental system of pulverized coal reburning was designed and established, the variations of denitration efficiency, main gas concentrations in flue gas, and some residual char with excess air coefficient and residence time were investigated, and the on-line images in furnace were also acquired. The model mentioned above was also used to predict the ignition status of reburning coal. The physical essence of the interaction between denitration and ignition during pulverized coal reburning was elucidated, and the intrinsic relationship of the influences of operating parameters on denitration was discussed.At lower reburning temperature, the curves of denitration efficiency, H2, and CO volume fractions in flue gas all present irregular M-shaped variation with increasing excess air coefficient, the ignition status of pulverized coal has key influences on denitration efficiency. When the coal is not ignited yet, the appropriate amount of oxygen is in favor of homogeneous NO reduction, and the optimal residence time of coal in reburning zone is related to the consumption rates of hydrocarbons in fuel gas. When the excess air coefficient is relatively high to cause homogeneous ignition, the hydrocarbons released from pyrolysis and their relevant intermediates important for homogeneous NO reduction are consumed greatly by the volatile flame; then, the denitration efficiency decreases abruptly, and the optimal residence time coincides with the ignition time of volatiles on the whole. At higher excess air coefficient, the coal char is ignited by the volatile flame, and the ignition mode is joint heterohomogeneous ignition mode. The particle temperature rises greatly, the rates of residual volatile release and surface oxidation reaction also increase greatly, which is favorable for the generation of CO and free active sites on char surface. As a result the heterogeneous reducing reactions begin to strengthen and dominate the whole denitration process, and the denitration efficiency increases again. Here, that efficiency increases continuously with prolonging residence time in this experiment, and the determination of optimal residence time should be considered together with the burnout of pulverized coal. At even higher excess air coefficient, the char combustion begins to be controlled by diffusion, the flame begins to move away from char surface, and it is difficult for the oxygen to reach that surface, which goes against char temperature rising and free active site generation. As a result, the heterogeneous reducing reactions are weakened, and the whole denitration efficiency decreases again. At higher reburning temperature the promotion effects of char ignition on denitration are less obvious than those at lower temperature. As to bigger coal particles, the excess air coefficients, at which the above-mentioned phenomena appear, are slightly lower due to their lower ignition temperatures.Aiming at the above results, it is considered that the homogeneous reducing reactions of volatiles do not give their full play due to homogeneous ignition at most industrial reburning conditions. In the initial stage of pulverized coal reburning, the ignition should be avoided as much as possible so that the volatiles are mainly used for NO reduction, instead of consumed by combustion reactions. In the latter stage of reburning, the char ignition should be just ensured, which is in favor of char temperature rising and heterogeneous NO reduction.According to the above analyses, the ideal scheme of pulverized coal reburning is presented and described below. At first little air or part flue gas from tail duct of boiler enters the upstream region of reburning zone with pulverized coal, so that the excess air coefficient in this region is very low, the ignition is avoided, and the homogeneous NO-reduction reactions will maximize and dominate the whole denitration process. Then some air enters the downstream region of reburning zone, the coal char formed in the upstream region will be just ignited, and the heterogeneous NO-reduction reactions will maximize and dominate the whole denitration process. Based on the above scheme, the homogeneous NO-reduction reactions and the heterogeneous NO-reduction reactions will play their roles in two time-stages respectively, which can avoid not only the unfavorable influences of homogeneous ignition on homogeneous NO reduction, but also the unfavorable influences of no ignition on heterogeneous NO reduction. In a word, the above mentioned investigation results have much guiding significance for the determination of operating parameters and the implement of reburning scheme in industry.