Numerical analysis of an entrained-flow coal-slurry gasifier with separated oxygen-feeding streams

The characteristic design of a Texaco gasifier, with a coaxial coal-slurry oxygen-jet nozzle and an outer annular oxygen-jet, has a significant influence on the gasification process. For the fuel-oxidant coaxial jet nozzle, the central oxygen-jet flow impinges on the coal-slurry stream and assists in the atomization of the coal slurry. The outer oxygen-jet, which splits the oxygen intake stream into two, makes the flame position away from the nozzle and provides a better protection for the nozzle from being burned down. In the present study, numerical simulations of the coal slurry gasification process inside an entrained-flow gasifier with separated oxygen feeding streams are investigated by using the commercial computational fluid dynamic software, ANSYS/FLUENT. The Eulerian-Lagrangian approach is employed for the multi-phase flow. Coal slurry is supplied for the gasification process with a feeding rate of 47.32kg/s and the coal slurry concentration of 35% of water and 65% of coal. Gasification of coal slurry includes several physical sub-processes, such as moisture vaporization, devolatilization, volatile combustion, char oxidization and water-gas shift reaction. The transport exchanges of mass, momentum, and energy between different phases are taken into account in the present numerical model. The physical models for the gaseous flow include turbulence and radiation models. The coal combustion and gasification processes are considered in five homogeneous reactions and three heterogeneous reactions. The heterogeneous reactions are determined by a finite-rate reaction model and the homogeneous reactions are determined by the finite rate and eddy-dissipation combustion model. Three parameters, namely, positions of the outer annulus feeding tunnel, oxygen/carbon ratio, and oxygen feeding velocity of the outer annulus streams, have been examined to determine the effects of operating conditions on the gasification process. The numerical results show that the flame distribution is affected by the positions of the outer tunnel. As the outer tunnel is moved away from the central axis, the flame surface is extended and leads to a higher temperature in the downstream of the gasifier. In the investigations of the oxygen/carbon ratio, the lower the oxygen/carbon ratio is, the lower outlet temperature and the poorer the coal conversion rate. The distribution of the combustion region and the coal conversion rate is strongly affected by the feeding velocity of the outer annulus stream. The coal conversion rate and the cold gas efficiency are in a descending trend when the oxidant feeding is operated in high velocity. The compositions of the product gases are affected by the WGS. Moreover, the reaction rate of the WGS reaction is sensitive to the temperature of the gasification chamber.