Studies On Physical And Chemical Property And Gasification Reaction Characteristic Of Different Coal Chars At Elevated Temperatures

With the development of modern coal gasification techologies, the large-scaled and high efficient entrained-flow coal gasification technology becomes gradually a key technology and has broad prospects of application. Therefore, it is quite significant to study physical and chemical property and gasification reaction characteristic of coal chars at the conditions closing to entrained-flow gasification. In this study, the main contents and achievments were summarized as following:(1) Different chars of Shenhua coal, Yanzhou coal and liquid phase carbonization coke, were respectively prepared using a muffle and a small-scaled falling bed at the conditions of pyrolysis temperatures ranging from 950℃to 1500℃and slow and rapid heating rate. Firstly, physical and chemical properties of resulting chars were investigated by SEM, XRD and a surface area and porosity analyzer so on, and then some new cognitions were obtained: at high pyrolysis tempeatures, the ashes in chars occur to fuse and agglomerated into spherical granules, and the ash fusion of slow pyrolysis chars is different with that of rapid pyrolysis chars; the ashes of elevated temperature pyrolysis chars contains SiC; carbothermal reactions of CaO and Fe₃O₄ can take place in the high temperature range; BET surface areas of elevated temperature pyrolysis chars from either coal or liquid phase carbonization coke are almost below 10m²/g; the carbon crystallite structures of coal chars prepared at slow heating rate are more ordered than those of coal chars prepared at rapid heating rate; the ordering degree of the carbon crystallite structures of coal chars was higher than that of liquid phase carbonization coke; either Shenfu coal chars or Yanzhou coal chars are difficult to graphitization.(2) The CO₂ gasification reaction characteristic of different chars was investigated by an isothermal thermo-gravimetry in the gasification temperature range of 950～1400℃. The following conclusions were reached. The gasification processes of coal chars were controlled by the chemical reaction in low temperature range (950～1150℃), while by the gas diffusion in high temperature range (1150～1400℃), owing to the ash fusion which retarded seriously the gas to reach the pore surface of coal chars and the chemical reaction was greatly accelerated. The gasification activity of rapid pyrolysis chars was higher, and even several times higher than that of slow pyrolysis chars. The difference in gasification activity of various coal chars diminished in the elevated gasification temperature range. As a whole, the higher pyrolysis temperature resulted in higher gasification activity of liquid phase carbonization coke, which was different from the effect of the pyrolysis temperature on the gasification activity of coal chars. The gasification activity of Shenhua coal chars was comparatively high, while that of Yanzhou coal chars was comparatively low. The gasification activity of carbon from fly ashes was quite high, while that of chars made by liquid phase carbonization was quite low, and even lower than that of graphite. In addition, the steam gasification reactivity and kinetics of Yanzhou coal chars prepared at high temperatures, were also investigated using a fixed-bed gasification experimental set-up at gasification temperatures of 900～1200℃. The results showed that it was quite favorable to simulate the steam gasification process of coal chars using the reaction-diffusion model proposed in this study, and that the increasing partial pressure of steam caused the reactivity of coal chars to increase, but the increasing degree of gasification rate of coal chars with relatively low activity was very small, and even almost unchanged.(3) Rapid heating and pressured pyrolysis chars of Shenhua coal, Yanzhou coal and liquid phase carbonization coke were respectively prepared using a pressured thermo-gravimetric experimental set-up at the conditions of the pyrolysis temperature of 900℃, pyrolysis pressures of 0.1～3MPa and rapid heating rate. Then, BET surface areas of resulting chars were tested by a surface area and porosity analyzer and the CO₂ gasification reaction characteristic of resulting chars was aslo investigated by an isothermal thermo-gravimetry. The main conclusions were that at rapid heating and pressured pyrolysis, the variation of BET surface areas of coal chars was opposite to that of liquid phase carbonization coke during the pyrolysis process, and that the gasification activity of resulting chars prepared at the atmospheric pressure higher than that of resulting chars prepared at high pressures, while effects of pressures on the gasification activity of different carbonaceous materials were distinct at pressured pyrolysis.