COMPARISON OF INSTANTANEOUS, EQUILIBRIUM, AND FINITE-RATE GASIFICATION MODELS IN AN ENTRAINED-FLOW COAL GASIFIER (391-401)

摘要

A coal gasification simulation model involves many submodels and each of the sub-models needs to be investigated and verified. This paper focuses on comparing three different gasification reaction models: instantaneous gasification, global equilibrium, and finite-rate models. The goal is to determine if the simplified instantaneous gasification model can be used to quickly capture acceptable approximations of thermal-flow and reaction behaviors that can be used as a preliminary screening tool of new design ideas for improving gasifiers' performance. The Eulerian-Lagrangian approach is applied to solve the Navier-Stokes equations and eight species transport equations with three heterogeneous global reactions and three homogeneous reactions. The coal particles are tracked with the Lagrangian method. In the instantaneous gasification model, the interphase exchange rates of mass, momentum and energy are assumed to be infinitely fast. Also, the dispersed phase can be simplified as the gas phase, and the complex two-phase flow is then treated as a single-phase flow. Two water shift rates are used. The fast rate is used with the presence of catalyst, while the slow rate is used without catalyst as in a typical entrained-flow gasifier. The results show that reactions in the instantaneous gasification model occur fast and finish quickly; whereas, the reaction in the finite-rate model,which involves gas-solid reactions, occurs slowly. Varying the coal particle sizte of the finite-rate model shows that the syngas heating value of the smaller particle size is closer to the instantaneous gasification model. The water shift rate plays a very important role on affecting the accurate prediction of the syngas composition. The syngas composition of using fast water shift rate is very close to that calculated from the global equilibrium method. The overall result reveals that the instantaneous gasification approach can provide an overall evaluation of relative changes of gasifier performance in terms of temperature, heating value, and gasification efficiency corresponding to parametric variations, but not adequately capture the local gasification process predicted by the finite rate model in most part of the gasifier.