Computational Fluid Dynamics And Second Law Of Thermodynamics Analysis For Coal Gasification

The coal gasification technology converts the solid coal into the clean gaseous fuel for power generation and provides the raw material to the chemical engineering industry. It is useful for the CO₂ capture and clean coal utilization to use coal gasification. Recently, the major project about coal gasification is to be more efficient, lower energy cosumption and less environmental pollution. Numerical simulation has became the main method to provide theoretic direction and technical support to the application of coal gasification based on the accumulated experimental data. As the coal gasification is essentially related to the gas-solid two-phase flow, the Eulerian-Eulerian and the Eulerian-Lagrangian are two methods to simulate the coal gasification. Within the Eulerian-Eulerian the particle are assumed to be pseudo-fluid, which decreases the computational cost by comparing the Eulerian-Lagrangian method. The aim of this thesis was to simulate the coal gasification in the bubbling fluidized bed and the entrained flow gasifier accurately by using Eulerian-Eulerian method. The model were developed to simulate the change of particle diameter and effect of turbulence during the coal gasification. Moreover, it was shown that the two-fluid model was applicable to simulate the non-dense planar gas-solid impinging streams. The research work was done using the open-source software MFIX, and the models involving heat and mass transfer in MFIX were perfected. Meanwhile, the Second Law of Thermodynamics was coupled to conduct the thermal performance analysis on the coal gasification. The main works are listed below.?1?The coal gasification in the bubble fluidized bed was simulated, and the effects of turbulent flow on the gas diffusion and homogeneous reaction were incorporated in simulation. A new model was developed to simulate the change of particle diameter during the coal gasification, so the heterogeneous reaction was modified. The simulation syngas output were was in good agreement with the experimental measurements, and the bed temperature was more uniform than that simulated without modified reaction rate. The separation was showed, i.e., the heavy particles collected at the bottom of the reactor while the light particles existed at the middle and top of the reactor.?2?The planar gas-solid imping streams was simulated using the two fluid model applied to nondense gas-solid two-phase flow. The simulation results from different cases indicated that the planar gas-solid imping streams oscillated periodically that was like to the planar gas impinging streams. The results also showed the detailed information about the oscillation and the influence factor to the oscillation period. The residence time of the particle were calculated and analyzed in the simulation. Comparing the results from k-?-? model and k-?-?-kgs, model, the vortex resulted from oscillation was beneficial to particles returning the impinging zone. Meantime, more kinetic energy of jets would be exhausted during the oscillation.?3?The lab-scale entrained flow gasifier was simulated using the two-fluid model. In the simulation, the two-fluid model was developed by coupling the particle change model, radiation model and particle diffusion model. The results indicated that the turbulence had the great influence on particles diffusion. The gasifier could be divided into combustion zone and gasification zone, and the temperature decreased along the axis and approached to the experimental measurement. In addition, the gas concentration had the good agreements with the experimental results both at the axial line and along the radial direction, which validated the developed two-fluid model.?4?The thermal performance analysis based on the Second Law of Thermodynamics was applied to the impinging streams and coal gasification in the fluidized bed and the entrained flow gasifier. The entropy production rate was used to analyze the oscillation of planar gas-solid impinging streams and the different flow types of gas impinging streams. It was proved that the pressure release at the impinging zone and continuous jets made the gas-solid impinging streams oscillate periodically. The entropy production rate and the exergy efficiency of the gasification in the fluidized bed and the entrained flow gasifier were calculated based on the numerical simulation. All the factors resulting in the entropy production during the gasification process were included in the calculation. The dynamic change and the most influential factor were showed, which was helpful to direct optimization design of the experiment condition.