Kinetic Study On No-Char Reaction At High Temperature

Nowadays, the pollution caused by NO_x emitting from coal-fired boiler power plant becomes more and more serious. It is very important for NO_x control to predict accurately the formation and reduction of NO_x during pulverized coal combustion. Since NO-char reaction plays a very significant role in NO_x formation and there has not yet been a common understanding on the NO-char reaction mechanism all over the world, further more, the kinetic parameters obtained differs from each other largely, a detailed study on the kinetics of NO-char reaction at high temperature has been conducted in this paper.Firstly, the characteristics and development of char pore structure under high-temperature reducing condition has been studied. Five coal-chars were prepared in a simulator of high-temperature entrained flow reactor (SHEFR) which can simulate the temperature and gas composition of a real pulverized coal combustion environment. The pore structure of chars was measured by mercury porosimetry and nitrogen adsorption. Via the study on the pore structure parameters and pore diameter distribution of different coal-chars in the expariment, the following conclusions were obtained. Low-rank coal-char has larger specific surface area, pore volume, porosity and higher reactivity than. In addition, the development of char pore structure under the high temperature reducing conditions was also studied in this experiment. Under the high temperature reducing conditions, the chars collected at various temperatures and residence times have the similar adsorption isotherms which belong to type II isotherm, gasification of chars has significant impact on the structure and morphology of the char particles, and the surface area development of chars could be attributed to micro-pores.Then, the kinetic characterization of NO-char reaction was performed by isothermal thermogravimetry in the temperature range of 973~1573K, and the discrete random pore model (DRPM) was applied to describe the NO-char reactions. Acid treatment on the YB and SH chars was applied to obtain demineralized chars. According to the experimental results, the following conclusions could be obtained. Experimental data for all the chars can be unified into a single master curve. The presence of catalytic metal matter increases the reactivity and decreases the activation energies of chars with NO. The DRPM can be used to well describe the reaction of all chars. However, due to the accumulation of metal catalyst on char surface, it underestimates the reaction rates at high carbon conversions.Next, high-temperature drop tube furnace (DTF) was adopted to study high-temperature NO-char reaction and a one-dimention model of NO-char reaction was built to analyze the experimental data, and the following conclusions can be obtained. The mass transfer limitations of NO have a great influence on the reactivity of chars with NO and different surface area bases result in great differences in values of effectiveness factors. Presenting results on Hg surface-area-normalized basis leads to better reduction of data scatter, compared with those on BET surface-area-normalized basis, and the values of Hg surface area (measured by mercury porosimetry) are more stable during the process of reaction than those of BET surface area (measured by nitrogen adsorption). The deactivation of chars during the high temperature NO-reaction was observed. The extent of char deactivation is closely associated with parent coal rank. As the coal rank increases, the Hg2/Hg1 ratios (ratio of the Hg surface area after reaction to the Hg surface area before reaction) increase monotonously, which implies the Hg surface area basis is more appropriate for high-rank coal chars. The BET2/BET1 (ratio of the BET surface area after reaction to the BET surface area before reaction) ratios for almost all chars are lower than the corresponding Hg2/Hg1 ratios, especially for the high-rank coal chars. This also indicates that Hg surface area is a better basis for normalizing the reactivity of different coal chars than BET surface area. In addition, the comparison of TGA and DTF experimental data for kinetic analysis suggests that thermogravimetric analysis as a rapid and simple method could be an effective method used in studying the reactivity of coal char towards NO at high temperatures. At high temperature, O2 prevented the NO-char reaction, whereas CO and SO2 had an enhanced effect.In this study, a normalized correlation factor X is proposed to analyze the effect of metal concentration on char reactivity, and it is observed that the catalytic activity in the decreasing sequence is Mg, K, Na, Ca and Fe at temperatures of 1273~1573 K. It should be noticed that Mg has the highest catalytic activity although its content in chars is very low, which is less studied in other literatures. The scattering of activation energies obtained from different chars is due to the existence of metal catalysts. The effect of metal catalysts could significantly increase the char reactivity by reducing the activation energy. Addintionally, a normalized parameter mc is proposed to analyze the effect of main metal oxide content on the char reactivity. The increase of the main metal oxide content in a certain range appreciably accelerates the reduction of NO by char particles. The reactivity of NO-char reaction increases with mc value linearly.Finally, a new kinetic model of high-temperature NO–char reaction was proposed, which took into account both the pore diffusion and thermal annealing of char. The NO–char reaction at high temperature was predicted by a combination of the model and Fluent 6.2. For all chars, the predicted results agree very well with experimental data at 1273, 1373 and 1473 K, but the model based on previous reaction mechanism underestimates the experimental data at 1573 K. In this connection, a new mechanism was proposed, in which more NO is consumed per amount of carbon in comparison to previous mechanism. And this can be used to explain the failure of the model at 1573 K. The model in which the Hg surface area is used as the area basis successfully normalizes the reactivity of chars with different ranks, which is very important in predicting the NO–char reaction at high temperature.