Numerical Simulation And Performance Optimization Of Pulverized Coal Gasifier

The pulverized coal gasification technology is widely applied in the chemical industry. The pulverized coal gasifier is the core equipment of the pulverized coal gasification technology, its performance is critical to the successful application of this technology, thus it is vital to evaluate the flow field and gasification performance of a gasifier. In this work, a typical pulverized coal gasifier with a dual-channel nozzle is investigated using software, Fluent, to model 3D cold flow and reacting flow. The major conclusions are as follows:（1） Cold flow simulation of a three-dimensional gasifier is carried out by using a realizable k-ε turbulence model and a discrete phase model（DPM） for coal particles.The simulation results show that, the coal particle trajectories are significantly influenced by the gas flow field. The average residence time of the coal particles is5.55 s with a standard deviation of 2.81 s, and the minimum residence time is 2.39 s.With an increase in the inner inlet velocity and the outer inlet velocity, the average turbulence intensity and the average turbulent kinetic energy of the gasifier increase,while the average residence time, the minimum residence time and the standard deviation decrease. An increase in the outlet velocity, leads to a decrease in the peak turbulence intensity and turbulent kinetic energy at the central axis of the gasifier, the position of the peak moves from 0.15 m to 0.30 m from the gasifier inlets.（2） Due to the complexity of the coal gasification reaction process, a simplified gasification reaction mechanism is established. Reacting flow simulation of a three-dimensional gasifier is carried out by using the simplified reaction mechanism for coal gasification, and the finite-rate/eddy-dissipation model for the gas phase reactions, heterogeneous reactions are modeled using a multiple-surface-reactions model, and the particle flow is simulated using the DPM model. The modeling results indicate that, with the realizable k-ε model, the two competing rates model and a simplified method for the volatiles, the simulation results are in good agreement with the experimental data. The relative errors between the simulation results and the experimental results of CO, H₂, CO₂, H₂ O mole fractions and the carbon conversionrate are 9.65%、-4.62%、-6.25%、-1.11% and 0.99%, respectively. The temperature at the top and outside of the jet is high in the gasifier, with the maximum temperature of about 2400 K, and the temperature at the center of the jet is lowest. The mole fractions of CO₂ and H₂ O are high in the jet zone and the recirculation zone, while the effective gas（CO and H₂） mole fractions is low. The gas species mole fractions approach a steady sate at a distance of 1.5 m from the nozzle. At the outlet of the gasifier, the mole fractions of CO and H₂ are 0.322 and 0.17, respectively, while the mole fractions of CO₂ and H₂ O are 0.159 and 0.259, respectively. The effective gas（CO+H₂） mole fraction is 0.492. Coal particles concentrate in the jet region and in the recirculation zone. The average residence time of coal particles is 0.79 s with a standard deviation of 0.52 s. The minimum residence time is 0.30 s, and the carbon conversion rate is 82.81%.（3） With the increase in the oxygen-coal mass ratio, the effective gas（CO+H₂）mole fraction at the gasifier outlet reduce, while the carbon conversion increases first and then decreases. When the oxygen-coal mass ratio is 1.0, the carbon conversion is highest at 84.3%, while the effective gas mole fraction is 0.456. With the increase of the steam-coal mass ratio, the load ratio, and the coal particle diameter, the carbon conversion and the effective gas at the gasifier outlet decrease. Replacing N2 with CO₂ as the carrier gas increases the effective gas mole fraction at the gasifier outlet by2%. When the gasifying agent mass ratio of CO₂ is increased, the effective gas mole fraction increases, while the carbon conversion rate increases first and then decreases,the optimal mass ratio of CO₂ is 40%.（4） Based on the modeling results for the effective gas（CO+H₂） mole fraction and the carbon conversion rate, the optimal axial expansion aspect ratio of the gasifier is 8.85. When the radial expansion aspect ratio decreases from 5.85 to 9.85, the coal conversion rate increases from 77.4% to 94.4%, and the effective gas mole fraction increases from 0.461 to 0.567.