PERFORMANCE AND MODELING OF FLUIDIZED-BED COAL GASIFIERS

Near future commercialization plans are being considered for pressurized fluidized-bed coal gasifiers. This work explores the constraints of these reactors and their commercial potential. A unique predictive kinetic model that accounts for coal/char oxidation and gasification, water-gas shift, heat losses, and recycle and purge streams was developed. The model predicts carbon conversion, reactor temperature, bed holdup, and 30 other state variables. The simulation results are in good agreement with pilot-plant data. The model has been used to understand the kinetic, process, and design constraints, and their thermodynamic consequences.; Pilot-plant simulation indicates that they have been operated in the wrong regime for commercial scaleup. The inability to operate in a commercially attractive regime can be attributed to excessive fines recycle and the complex ash-separation bottom zone which requires large amounts of excess gas to operate. The usefulness of this classifying bottom zone is in doubt.; Commercially-sized reactor simulation indicates that both of the currently developed designs are of no commercial value for fuelgas generation from high-grade bituminous coal. One of the developed designs (U-Gas) may be marginally attractive with a catalyst.; Simulation with Western coal indicates that fluidized-bed gasifiers are suitable for partly dried Western coals and lignites. This advantage is due to faster reaction rates and a more favorable stoichiometry.; Under common gasification conditions with Eastern coal the operable range was determined to be kinetically constrained and far from thermodynamic equilibrium. For Western coal the operable range is closer to equilibrium. Equilibrium can be approached by lowering the steam-to-oxygen ratio.; A unique stoichiometric invariance technique has been developed for coal gasification. The relation permits the calculation of steam-free mass balances; provides a data consistency cross-check; substitutes for invalid measurements; and enables on-line metering of critical process parameters such as carbon conversion and cold-gas efficiency. The stoichiometric invariance has been used to uncover gross measurement errors in the pilot-plants.