Computational Fluid Dynamics Simulations Of Catalytic Coal Gasification In Jetting Fluidized Beds

Coal is still the main energy resource in China, but its inefficient utilization has brought severe environmental pollution problems for the sustainable development of China’s economy and society. As an alternative of coal, the utilization of natural gas can reduce the air pollution caused by the direct coal combustion. Given the fact that China is rich in coal but short of oil and natural gas, it is a realistic choice to produce the synthetic natural gas from coal to guarantee the energy safety and to improve the ecological environment. The coal-to-gas technology represented by the catalytic coal gasification is paid great attention.Catalytic coal gasification is a process that comines the endothermic steam gasification and the exothermic mathanation in a single fluidized bed, and produces methane-rich raw gas with the help of alkali metal catalysts. To support the research and development of industrial scale-up of jetting fluidized bed catalytic coal gasifiers, a two fluid model (TFM) was incorporated with the kinetic theory of granular flow (KTGF) to provide the rheological properties of solids, and a comprehensive CFD model was established to describe the gas-solid hydrdnamics and the catalytic coal gasification. Validations of the models were perfomed with a small-scale jetting fluidized bed coal gasifier and a pilot-scale catalytic coal gasifier, respectively. The main research results of this paper were as follows.Firstly, a well documented, literature reported bench-scale atmospheric jetting fluidized bed coal gasifier was chosen as the benchmark case. The numerical simulation results were carefully processed and several key gas-solid hydrodynamic properties, such as the jet height, the bubble size, the bubble velocity and the amount of solids entrained from the annulus to the jet were validated against literature correlations. The distributions of the jet temperatures and the gasification reaction rates were predicted, and the simulated gas species compositions (CO, CO2, H2) were validated against the experimental data and compared with those from the CVM modeling. It was found that the KTGF simulation results agreed with the experiment data better than the CVM, confirming that the KTGF model could improve the descriptions of gas-solid hydrodynamics of the jetting fluidized bed.Secondly, the TFM-KTGF model was applied in a pilot-scale pressurized jetting fluidized bed catalytic coal gasifier with embedded air jets. The bed expansion was validated against the empirical correlation, and some key jetting characteristics, including the jet height, the axial distributions of particle content, gas and solid velocities were presented. The flow pattern of double jets belonged to the isolated jets category, indicating that two embedded jets developed separately without coalescing with each other. It was found that embedded jets changed the original flow pattern, and could split the nearby rising bubbles by providing sufficient momentum to the particles in the wake, which subsequently could reach the bubble roof. The simualion results showed that embedded jets reassigned the oxygen distribution and avoided the hot spot at the bed bottom. A high temperature zone was predicted along the jet pathway due to a rapid combustion of char particles. The maximum temperature was, however, controlled below the ash softening temperature due to the improved gas-solid mass and heat transfers benefited from the reduction of bubble sizes, and large formation of clinkers was avoided in the jet zone. The distributions of gas species compositions were predicted, and the computed carbon and steam conversions agreed with the experimental results.Thirdly, the effects of oxygen/steam ratio and steam/coal ratio on the maximum temperature and methane yield were investigated. It was found that even with a low oxygen/steam ratio, the maximum temperature in the embedded jet region was still above 1500K, which reinforced the importance of defining and monitoring the jet temperature for an industrial jetting fluidized bed gasifier. The steam/coal ratio was found critical in the CH4 production. A high steam/coal ratio resulted in a low steam conversion, which made a large amount of unreacted steam. CO was thus consumed quickly by the water-gas shift reaction, which limited the CH4 production. The accuracy of the TFM-KTGF model was verified at different operation conditions, and the CFD model was able to provide technical supports for the industrial scale-up of the catalytic coal gasfication.Finally, the two-fluid model was further applied in the modeling of the combustion process of wastes and primary air on the moving grate. A coupling simulation procedure between the moving-grate combustion and the furnace combustion was established. The effects of geometric structure of the moving grate and the heating value of wastes on the grate combustion were investigated. It was found that the high moisture and low heating value of the wastes resulted in the non-uniform velocity and temperature distributions. The results provided solid foundations for the optimal design and operation of moving-grate waste incinerators.