Three-Dimensional Numerical Simulation Of Combustion/Pyrolysis/Gasificatio N Processes Of Combustible Solid Waste/Coal

The discharge of solid waste has caused great harm to the ecology and environment in our country. How to effectively treat and utilize combustible solid waste, improve the coal conversion efficiency and reduce gas pollutant emissions have became hot issues in fields of energy and power. A typical combustion, pyrolysis, or gasification power system of combustible solid waste or coal is a dense gas-solid reaction system. Currently, studies on hydrodynamics of dense gas-solid system have progressed to reach a high level. However, the full knowledge on gas-particle flow characteristics, heat and mass transfer and chemical reaction mechanism is still lacking, especially in industrial application, which leads to some difficulties in scaling design and configuration optimization. The present work is devoted to revealing the flow and reaction characteristics of combustion/pyrolysis,/gasification processes by numerical simulation approach.The Eulerian multiphase model for dense gas-solid flow coupled with chemical reaction has been improved. The overall submodel of gaseous pollutant emissions was introduced. A three-dimensional (3D) comprehensive model for combustion/pyrolysis/gasification processes of combustible solid waste/coal was established. The model can simultaneously predict complex gas-solid flow, product gas, tar content, and gaseous pollutant emissions.On the basis of multiphase particle-in-cell (MP-PIC) method, a 3D Eulerian-Lagrangian model for combustion/pyrolysis/gasification processes of combustible solid waste/coal has been established, which uses an LES approach for fluid phase and a discrete particle method for particle phase. In MP-PIC, a computational particle represents a number of physical particles with identical properties, and the particle collisions and heterogeneous chemical reactions are solved on the Eulerian grid. By this way, the efficient solution of dense gas-solid reaction system is achieved.Simulations were carried out in a bubble fluidized bed and in a spouted bed for coal and combustible solid waste gasification using the Eulerian-Lagrangian model. The flow patterns conform to the physical characteristic of real system, and the predicted product gas compositions compare well with experimental data under different operating conditions. Then we used Eulerian-Eulerian model and Eulerian-Lagrangian model to investigate the combustion of solid waste in a circulating fluidized bed (CFB), respectively. The gas pollutant emissions such as S2 NO and N2O were highly focused on. The results show that both of two models can effectively predict gas-solid flow and combustion characteristics. Compared with the Eulerian-Eulerian model, the results of Eulerian-Lagrangian model can capture the particle clusters and have smaller average relative errors of product gas compositions.Based on the Eulerian-Lagrangian model for dense gas-solid reaction system, the gasification characteristics in fluidized beds were studied for five kinds of typical components in municipal solid waste (MSW). The flow patterns, product gas, gaseous pollutant emissions, tar contents, heating value of syngas, gas yield, carbon conversion efficiency, and cold gas efficiency were discussed under different air equivalence ratios. The relationship between gasification characteristic and flow regime was analyzed. In addition, we selected various MSW components and proportions to mix and simulated the gasification of the mixed MSW. The gasification characteristics are combined with mixture temperature, particle composition, and linear superposition of single-component results to reveal the synergy mechanism of multi-components in the gasification process.The 3D Eulerian-Lagrangian model is applied to simulate the co-combustion of MSW and coal in a 75 t/h industrial-scale CFB boiler, which includes chamber, cyclone, loop-seal, external heat exchanger, etc. The simulation predicted profiles of local high temperature, areas prone to erosion, and gaseous pollutant emissions. The gas-solid flow mechanisms and combustion reaction characteristics in the CFB boiler are analyzed from the full-loop perspective.