Numerical Investigation Of Riser Of The Pressurized Dense Transport Bed Gasifier

To understand the hydrodynamics characteristics and gasification process in the riser of dense transport bed gasifier, we have developed the numerical methods of the gas-solid multiphase flow and gasification process with Geldart B particles under the framework of two-fluid model in this work. Based on the developed methods, the effects of operating pressure and temperature on the flow behaviors have been investigated. And we have also studied some important operating parameters on the gasification performances. Besides the study on the dense transport bed, a novel riser with an enlarged bottom has been designed and the flow behaviors in the novel riser have been investigated with CFD method.It is important to predict the gas-solid behavior in the riser, as it is foundation for the numerical simulation of the gasification process. The Eulerian-Eulerian two-fluid model coupled with the kinetic theory of granular flow has been employed to predict the gas-solid flow hydrodynamics in the riser of dense transport bed. The detailed predicted results of two test cases are presented and compared with experimental data. The predicted results using the EMMS/Matrix and the Gidaspow drag models are compared in details. Both drag models can predict the overall axial profiles of solid volume fraction and Gidaspow model can get a better distribution especially at the bottom of the riser. The EMMS/Matrix drag model can predict well the radial distributions of solids volume fraction and axial velocity. The Gidaspow drag model coupled with partial differential equation of granular temperature can greatly improve the radial distributions. The comparisons of 2D and 3D simulation results reveal that the two-dimensional model can be used as a simplified and fast method for the CFD simulation of gas-solid flow in the riser under the DSU regime.To further understand the hydrodynamics in the riser under high pressure and temperature, the effects of operating pressure and temperature on the gas-solid flow behaviors have been studied. The operating pressure varies between 0.1 MPa and 1.0 MPa, and the solids circulating flux increases from 200 to 1400 kg/m2s. When operating pressure is below 0.3 MPa, the gas-solid flow will successively experience Fluid Dominated (FD) zone, Particle-fluid Compromising (PFC)/FD zone and PFC zone with the increase of solids circulating flux. Under the same superficial gas velocity and solids circulating flux, the solids volume fraction decreases while the solids axial velocity increases as operating pressure increases, especially at the core of the riser. In this work, the gas density is kept constant by proper combination of pressure and temperature, thus the effect of temperature on gas-solid flow behavior is mainly reflected through the variation of gas viscosity. With the same gas density, the increase of gas viscosity resulting from higher temperature has minor effects on the axial profiles of solids volume fraction. And it makes the solids axial velocity increases at the core and decreases slightly near the wall.The two-fluid model coupled with related chemical reaction model has been employed to predict the gasification process in the riser of dense transport bed and circulating fluidized bed gasifier. The predicted syngas composition out of the riser agrees well with the experimental data. And the gas-solid flow characteristics, temperature distribution, species distribution, reaction characteristics in the gasifier are obtained. It reveals that this method is suitable to model the gasification process. The effects of some important operating parameters such as C2/Coal mass ratio, Steam/Coal mass ratio, solids circulating flux and operating pressure on the performance of the dense transport gasifier have been studied. Under high solids circulating flux, the increase of O2/Coal mass ratio will increase the temperature by promoting char combustion reaction, which is helpful to improve char gasification in the upper region. As a result, both the carbon conversion ratio and the syngas heating value increase. Under high solids circulating flux, the temperature in the riser decreases obviously as the steam/coal mass ratio increases from 0.05, resulting in the decrease of CO, the increase of CO2 and H2 mole fraction, and slightly decrease of syngas heating value. As the solids circulating flux increases, the axial and radial profiles of solids volume fraction are more uniform, leading to more uniform temperature profiles, which can effectively avoid the ash agglomerating phenomena. Higher operating pressure can improve the coal processing capacity of the dense transport bed gasifier, and promote char combustion and gasification reaction rate. As a result, the increase of operating pressure improves the carbon conversion and the syngas quality.To investigate the effect of riser geometry, a two-stage riser with an enlarged bottom has been proposed. The enlarged bottom is defined as the first reaction zone and the upper section is the second reaction zone. As the pressure drop increases from 1.4 kPa to 57.1 kPa, the solids circulating flux experiences an overall increase. Between 6.9 kPa and 16.6 kPa, the solids circulating flux almost keeps constant. In the second zone, the flow regime experiences successively dilute pneumatic transport, fast fluidization and dense suspension upflow as the pressure drop increases. Detailed comparisons of flow behaviors between the two-stage riser and typical riser reveal that the turbulent fluidization in the first zone can efficiently enhance the back-mixing of particles and the gas-solid contact. In the second zone, the dense suspension upflow can also be achieved in the two-stage riser under the same gas velocity, but it needs much more solids inventory, which will increase significantly the average solids residence time. The parametric studies of the diameter and height of the enlarged bottom section (D1 and H1) have been performed. As D1 decreases, the flow regime in the first zone changes from turbulent fluidization to fast fluidization and the flow behaviors in the second zone change little. The increase of H1 extends the PFC/FD zone and the corresponding solids circulating flux keeps constant. These results provide theoretical bases for the optimization of riser structure.