Numerical Study Of Particle Flow Behavior In Entrained Flow Gasifier And Development Of A Gasifier Partition Model

This study is conducted to reveal the particle flow behavior in an Opposed Multi-Burner (OMB) gasifier and to develop a gasifier partition model. In the first part of this article, particle residence time distribution (RTD) and particle motion in a commercial scale OMB gasifier are studied using a3D numerical model. In the second part, a comprehensive1D model and a partition model of the OMB gasifier are developed respectively to predict the gasification performance.1. A3D numerical model of the OMB gasifier is established to study the particle RTD. The realizable k-ε model is used to model turbulence. The stochastic trajectory model is employed to track particle motion. The simulation results show that the particle lag time decreases constantly with increasing of the inlet gas velocity, the particle diameter and density. The particle mean residence time increases firstly and then decreases while the inlet gas velocity is increasing. And the inlet gas velocity has significant effects on trajectories of the medium particles (～100μm). The drag force plays an important role in the motion of fine particles throughout the gasifier, while it can be an order of magnitude lower than gravity for large particles. When the diameter of gasifier outlet increases from600mm to1000mm, the particle mean residence time decreases from8.59s to4.82s. The particle residence time calculated under the industrial operating condition is large than which is derived under the experimental condition.2. The hard-sphere model and direct simulation Monte Carlo (DSMC) method are introduced into the3D numerical model mentioned above to take into consideration the inter-particle collision. Effects of particle collision, particle diameter and mechanical characters on the particle velocity, concentration and collision ratio are investigated. The results show that the inter-particle collision has significant effects on particle flow behavior. After taking particle collision into account, the particle concentration around the axes of burner and gasifier increases sharply, and the dispersion of particles in jet flow is enhanced slightly. The inter-particle collision also leads a decreasing of particle mean residence time and a increasing of particle lag time. With the deceasing of particle diameter, the particle concentration around the axes of burner and gasifier deceases slightly, while the particle velocity and collision ratio increases. Smaller restitution coefficient of particle results higher particle concentration near the center of impinging zone. In the full scale gasifier, particles are concentrated in the regions around axes of burner and gasifier. And a high particle concentration region under the burner plane is found and considered as the result of recirculation flow.3. A comprehensive1D model of gasifier is developed, in which the particle RTD is employed to characterize the particle flow. The1D dimensionless distributions of gas temperature and gas compositions are obtained. The effects of particle diameter on the particle temperature and carbon conversion are studied. The reaction rates of coal devolatilization and combustion are extremely high in gasifier. While using Shenfu coal as feedstock, the carbon conversions of fine particles (<100μm), medium particles (about200μm) and large particles (500μm) are100%,97%and85%at the outlet of gasifier, respectively. Fine particles are heated rapidly and converted completely in gasifier.4. Based on the results of3D numerical modeling of the gasifier, the flow field structure of the commercial scale OMB gasifier is investigated in this study. A Reactor Network Model (RNM) is established. The gasifier is divided into different flow zones:the Jet Zone, the Impinging Zone, the Impinging flow Zone, the Recirculation Zone and the Outlet Zone. The different configurations of the RNM are tested by using a Markov chain stochastic model. The Random Pore Model (RPM) and the "effective factor" method are employed to model char gasification under high pressure and temperature. The simulation results show well agreement with industrial data. While using Shenfu coal as feedstock, the temperature around impinging zone is2101℃. The gradients of gas temperature and compositions are high near the Impinging Zone. Heating, combustion and gasification of fine particles (20μm,40μm and60μm) mainly happens in the Jet Zone and the Impinging Zone. While most of the particles with medium diameter (80μm,100μm) are heated and converted in the Impinging Zone and the beginning of the Impinging Flow Zone. Temperature and carbon conversion of large particles (>200μm) increase slowly in the Impinging Flow Zone. With the increasing of O/C ratio, at the outlet of gasifier, the concentration CH4decreases sharply. As increasing the O/C ratio of0.01. the gas temperature at gasifier outlet increases10～15℃. Keeping the temperature constant, as increasing the slurry concentration of1%, the coal and oxygen consumptions decrease about3kg/1000Nm3(CO+H2) and6Nm3/1000Nm3(CO+H2) respectively.