COAL CHAR GASIFICATION KINETICS IN A JET-FLUIDIZED BED (COMBUSTION, REACTION, RATE PHENOMENA)

MING-YUAN KAO, Purdue University

Abstract

The kinetics of gasification of -10 + 65 mesh coke and coal char with O(,2) and O(,2)-CO(,2) mixtures was studied in a continuous, bench scale, jet-fluidized reactor over a wide range of temperatures (900(DEGREES)C to > 1400(DEGREES)C) and bed depths under atmospheric pressure. The measured specific reaction rates of carbon ranged from 0.01 to 0.1 gram carbon reacting per gram carbon in the reactor per minute. From experimental data and observations, an isothermal two-zone model was developed which states that the jet-fluidized bed is composed of a lower CO(,2)-producing, exothermic zone and an upper CO-producing, endothermic zone. Excellent heat transfer inside the jet-fluidized bed provides nearly isothermal operating conditions. The CO(,2)-C reaction, which occurs on the high internal surface areas of the porous coke or coal char particles, determines the overall gasification rate of carbon. Oxygen in the input gas is consumed by the fast O(,2)-CO gas phase reaction just outside the particles. The experimental data are well correlated by a rate equation based on the isothermal two-zone model with the CO(,2)-C reaction as the rate-controlling step. The rate constant k, termed the "overall reactivity coefficient," is defined as the specific reaction rate of carbon per atmosphere CO(,2). The overall reactivity coefficients of carbon at various temperatures obtained in this study agree well with those measured independently in a fixed bed reactor, giving an estimated activation energy of 45,600 cal/mole for both Inland Steel Production coke and Universal Prepared coal char. Thus, the experimental data provide strong evidence that coke or coal char gasification in the jet-fluidized reactor is controlled by the chemical kinetics of the CO(,2)-C reaction inside the porous particles and that the effects of mass transfer from bulk gas to solids on gasification rate are minimal. This investigation demonstrated that the jet-fluidized reactor system is capable of sustained steady state operation. The reactor had been operated at much higher temperatures than permissible in the conventional fluidized reactor and with a fluidized bed maintained above liquid slag. Substantially higher specific reaction rates were obtained compared to those of the conventional fluidized reactor. Jet-fluidization is a promising new technique with potential new applications in coal gasification and metallurgical reduction.

Degree

Ph.D.

Subject Area

Materials science

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