The Combustion Characteristics Of Chars Derived From Coal Topping Process

China is rich in coal and the resources of natural gas and petroleum are limited. A lack of transportation liquid fuel hampers the fast development of China. A so called"Coal-topping process"applies abundant low rank coal resources to extract liquid products by flash pryolysis under mild conditions for further obtaining light fuel oil and valuable chemicals, through which the problem of liquid fuel shortage may be partly relieved. Up to now, coal-topping process has been applied in circulating fluidized bed system. By considering the fact that pulverized-coal combustion is the main way for power generation in China, the idea of combining coal-topping process with a pulverized coal boiler was proposed to expand its application, i.e. the pulverized coal is pyrolized firstly to obtain liquid and gaseous products, and then the remaining char will be burned in the pulverized coal furnace. The combustion behavior of the char determines the performance of the pulverized burner and furnace, and the pulverized coal combustion is more sensitive to the type of coal. Herein, the combustion characteristics of the char obtained under different coal-topping conditions were studied to test the feasibility of the char as the fuel for pulverized coal boiler. By evaluation of the relation between the pyrolytic conditions and the char's physical properties, as well as the influence of the char with different physical properties on its combustion characteristics, the effect and the mechanism of the coal-topping conditions on the char's combustion characteristics were studied.Datong Bituminous coal with different particle size group were pyrolyzed for obtaining chars at different pyrolytic temperatures in a spout entrained reactor. The chemical compositions of the chars were characterized by proximate and ultimate analysis. The change of functional group and carbon structure of the parent coal and the obtained chars were analyzed by a flourier transform infrared spectrometer and an X-ray diffraction. Pyrolysis degree of coal (Dv) was defined to reveal quantatively the effects of the pyrolytic conditions on the char composition and carbon structure.Pore structures of the parent coal and the chars were characterized by nitrogen adsorption analysis. The morphology of the coal and chars was observed under scanning electron microscopy. The NLDFT method was applied to calculate the pore size and the pore surface area distributions, instead of the commonly used BJH method, based on the adsorption data. The BET area was obtained similarly. Pore structure evolution and its reasons during the pyrolysis process were analyzed.The combustion characteristics of Datong bituminous coal, chars and Yangquan anthracite were studied via simultaneous thermal analysis (STA) under experimental conditions of low heating rates. The ignition temperature and the weight average apparent activation energy were obtained and used to evaluate the reactivity of the chars, the parent coal, and the Yangquan anthracite. The ignition type of the chars, the parent coal, and the Yangquan anthracite was also analyzed. Based on the thermogravimetric analysis, the ignition behavior and combustion reactivity under low heating rate of the chars and the parent coal, as well as those of Yangquan anthracite were compared. The major influencing factors to combustion characteristics of the chars, such as material compositions, pore structures, and carbon structure, were analyzed.A drop tube furnace (DTF) was designed, constructed and commissioninged for testing the char combustion characteristics under simulated conditions relavent to pulverized coal boilers. The combustion tests on Datong bituminous coal, chars and Yangquan anthracite were carried out in the drop tube furnace. The particle resident time in the DTF was estimated by the Computational Fluid Dynamics (CFD) software FLUENT. The conversion of combustibles and carbon, as well as the specific reaction rate of combustibles as a function of reaction time were calculated based on the experimental data of different fuels applied. The combustion reactivity under high heating rate of the parent coal, derived char sand Yangquan anthracite is compared. The pore structure evolution during combustion process of chars pyrolyzed were analyzed. The influence of volatile matter content and pore specific surface area on the specific reaction rate of combustibles was discussed. Results obtained from DTF were also compared with those obtained by thermogravimetric analysis.Results from this work show that, reactivities of all derived chars are higher than that of Yangquan anthracite, indicating that it may be feasible to use coal-topping char as fuel of pulverized coal boiler. Thermal deactivation was observed under the pyrolysis conditions in this work. The reactivity of all chars is lower than that of the parent coal. Changes in material composition and pore structure affect the reactivity for chars derived at temperatures applied. Carbon structure ordering contributes to thermal deactivation only when coal pyrolysis degree reaches to a certain level. Combustion reactivity of the chars is determined by their composition, pore structure and carbon structure. The reactivity of chars shows a trend of decreasing as the volatile matter decreases and the fuel ratio increases. Results obtained by thermogravimetric analysis are basically consistent with those obtained from DTF .