Coal conversion submodels for design applications at elevated pressures. Part II. Char gasification

Abstract

This paper surveys the database on char gasification at elevated pressures, first, to identify the tendencies that are essential to rational design of coal utilization technology, and second, to validate a gasification mechanism for quantitative design calculations. Four hundred and fifty-three independent tests with 28 different coals characterized pressures from 0.02 to 3.0 MPa, CO 2 and steam mole percentages from 0 to 100%, CO and H 2 levels to 50%, gas temperatures from 800 to 1500 °C, and most of coal rank spectrum. Only a handful of cases characterized inhibition by CO and H 2, and only a single dataset represented the complex mixtures of H 2O, CO 2, CO, and H 2 that arise in practical applications. With uniform gas composition, gasification rates increase for progressively higher pressures, especially at lower pressures. Whereas the pressure effect saturates at the higher pressures with bituminous chars, no saturation is evident with low-rank chars. With fixed partial pressures of the gasification agents, the pressure effect is much weaker. Gasification rates increase for progressively higher gas temperatures. In general, gasification rates diminish for coals of progressively higher rank, but the data exhibit this tendency only for ranks of hv bituminous and higher. These tendencies are interpreted with CBK/G, a comprehensive gasification mechanism based on the Carbon Burnout Kinetics Model. CBK/G incorporates three surface reactions for char oxidation plus four reactions for gasification by CO 2, H 2O, CO and H 2. Based on a one-point calibration of rate parameters for each coal in the database, CBK/G predicted extents of char conversion within ±11.4 daf wt% and gasification rates within ±22.7%. The predicted pressure, temperature, and concentration dependencies and the predicted inhibiting effects of CO and H 2 were generally confirmed in the data evaluations. The combination of the annealing mechanism and the random pore model imparts the correct form to the predicted rate reductions with conversion. CBK/G in conjunction with equilibrated gas compositions accurately described the lone dataset on complex mixtures with all the most important gasification agents, but many more such datasets are needed for stringent model evaluations. Practical implications are illustrated with single-particle simulations of various coals, and a 1D gasifier simulation for realistic O 2 and steam stoichiometries. The rank dependence of gasification rates is the determining factor for predicted extents of char conversion at the gasifier outlet. But soot gasification kinetics will determine the unburned carbon emissions for all but the highest rank fuels. Only gasification kinetics for gas mixtures with widely variable levels of H 2O, H 2, and CO are directly relevant to gasifier performance evaluations.