Model establishment for the kinetic evaluation of synergistic effect on the coal char gasification with H2O and CO2 mixtures

A kinetic model derived from the classical Å esták-Berggren equation was established to simulate the isothermal gasification process of coal char with different partial pressures of H2O, CO2 and their mixtures. The effects of H2O and CO2 on the reaction could be expressed by the kinetic parameters η and φ which represent the conversion corresponding to maximum reaction rate (xmax) influenced by reactant gas and the gas distribution at the active sites, respectively. The results indicate that the relationship between η and xmax can be expressed as xmax = 0.509-0.429η. The parameter φ of co-gasification is superior to that of H2O or CO2 gasification, indicating that the distribution of the gasifying agents during co-gasification is more uniform than that during H2O or CO2 gasification. The synergistic behavior observed during co-gasification was evaluated by the model and it indicates that the interaction between H2O and CO2 results in the improvement of coal physical structure and accelerates the co-gasification reaction rate. Kinetic compensation effect at different partial pressures indicates that the dependence of intrinsic reaction rate on the partial pressures of gasifying agent is gradually weakened as the temperature approaches the isokinetic temperature Tiso for in-situ gasification. It is also confirmed by the nth order reaction rate equation that the reaction order n tends to approach zero when the temperature approaches Tiso. The reaction rate is only affected by the temperature when the gasification temperature exceeds Tiso and the partial pressure of reactant gas is higher than 0.03 MPa which is the minimum pressure required for full coverage of all the active sites.