Water - Gas Shift Modeling of Coal Gasification in an Entrained - Flow Gasifier

Most of the reaction rates for the water-gas shift (WGS) reaction were obtained from experiments under simplified laboratory conditions with specific catalysts. A few of the reaction rates without using catalysts were obtained under supercritical (water) conditions, with the pressure much higher than those in a typical gasifier. In either case, it is not clear how the published reaction rates can be trustfully used to predict the actual WGS reaction rate in a gasifier without the presence of a catalyst and under different temperature and pressure conditions than those in the laboratory. This study focuses first on reviewing the published WGS reaction rates with and without the presence of catalysts, followed by calibrating the WGS reaction rate to match the experimental data taken from the Japanese CRIEPI research gasifier. The 3-D Navier-Stokes equations and nine species transport equations are solved with seven global gasification reactions (three heterogeneous and four homogeneous,) and a two-step thermal cracking model for volatiles. The Chemical Percolation Devolatilization (CPD) model is used for the devolatilization process. Three different cases with three different finite rates for the WGS reaction (Jones’s rate under catalytic conditions and Wade’s and Sato’s rates under noncatalytic conditions) are conducted. The result shows that the three originally published rates are all too fast and overpredict the experimental WGS reaction rate. Adding a backward WGS reaction rate doesn't slow down the reaction rate, resulting in the same gas composition and temperature at the gasifier exit as that calculated without adding the backward WGS reaction. The pre-exponential rate constant value (A) of each reaction rate is therefore adjusted to match the experimental data. The results show that all three WGS reaction rates can match the experimental data reasonably well. The exit temperature can be matched within 2% (20K). The mole fractions of CO and H2O can be matched fairly well within 4 percentage points (or 10%); however, the simulated H2 mole fractions are always 7-9 percentage points (or about 40%) higher than the experimental data. The suggested calibrated reaction rates are documented.