Gasification Characteristics And Kinetics Study Of Coal Char With H₂O And CO₂

The development of coal char gasification technology in mixtures of H2O and CO2 has greatly improved the efficiency of coal utilization and can meet requirements of energy conservation and emission reduction. However, in order to guide the recycling of CO2 to the gasifier and replace partial steam for the co-gasification reasonably, specific co-gasification reaction mechanisms and related basic theories are urgently needed. At the present stage, it is void of kinetic model for co-gasification kinetic study, so the correlation study is significant.In this paper, a thermal gravimetric analyzer NETZSCH STA 449 F3 was used to study the gasification characteristics and kinetics of Yining coal char with H2O and CO2. The maximum reaction rate occurred at the initial stage was investigated and it was found to depend on the gas adsorption behavior to a great extent. A modified adsorption kinetic model (Modified-VM) was established to simulate the reaction rate of C-H2O and C-CO2 over the entire range of carbon conversion. In addition, the isothermal gasification reactivities of coal char at different partial pressure of reactant gas were examined and the existence of kinetic compensation effect in terms of different partial pressures was illustrated. It was demonstrated that the correlation between intrinsic reaction rate and the species as well as partial pressure of gasifying agents was gradually weakened with the temperature increasing. When the gasification temperature was higher than the isokinetic temperature, the reaction rate was only affected by the temperature in the minimum limits of reactant gas concentration was guaranteed. This result was also confirmed by the n-th order reaction rate equation, in which the order n tended to zero when the temperature approached to the isokinetic value. Furthermore, the extended integrated core model (eICM) was put forward to discriminate the different behaviors of coal char gasification with pure agent and with mixture agents. From the analysis of the kinetic parameters, the mechanism of the action of different gasifying agents on the gasification process were illustrated. Moreover, the difference of non-isothermal and isothermal gasification characteristic was studied to disclose the essence of gasification in two ways, and to offer the objective evidence which can explain the synergistic effect during the co-gasification. The main conclusions are shown as follows:(1) The maximum reaction rate of this tested char occurred at the initial stage of gasification was related to the gas adsorption behavior, thus the modified kinetic model which coupled the adsorption kinetics with volumetric kinetic model was proposed to simulate the gasification reaction rate over the entire range of carbon conversion and it showed an excellent fitting result. The modified model can be expressed as:(2) The extended integrated core model (or co-gasification kinetic model) can be used to simulate the gasification behavior with an excellent fitting result no matter what the gasification agent was. It (eICM) can be described as follows: dx/dt=kx(1-x)n[-ln(1-x_]-The value of isokinetic temperature Tiso was 1109℃ which was calculated by the eICM at different partial pressures. Reaction rate hardly relied on the partial pressure and species of gasifying agents and the order n approached to zero when the temperature closed to Tiso. The reaction rate only depended on the temperature once the partial pressure of reactant gas was higher than 0.3 which was the minimum limit to guarantee the full coverage of total active sites.(3) The study of co-gasification kinetics indicated that the roles of H2O and CO2 on the co-gasification were so complicated that they were not simply involved in the reaction consumption, but also participated in the improvement of coal physical structure and further to facilitate the co-gasification rate. The kinetic parameters proved that the synergistic effect existed in the co-gasification of non-isothermal process contributed to the improvement of frequently factor. It can be speculated that the interaction between pore production and pore expansion in the coexistence of H2O and CO2 could increase the chemical dissociative adsorption rate of agents on the surface of CaO to produce more oxygen free radical, which can accelerate the collision frequency of the oxygen free radical to coal matrix thus improve the co-gasification reaction rate.