Numerical simulation of entrained flow gasification: Reaction kinetics and char structure evolution

Numerical simulations can help to improve the design of entrained flow gasifiers. The presented 3D-CFD model, developed using the software ANSYS Fluent 15.0, is validated against experimental data obtained from a pilot-scale pressurized entrained flow reactor. The comparison of the model with experiments shows a good correlation for the investigated Rhenish lignite. It is established that the modeling approach is suitable for predicting pressure- and temperature-dependent conversion rates in entrained flow gasifiers. This work focuses on the comprehensive modeling of char particle reactions. Pore diffusion and boundary layer diffusion become relevant at high temperatures. An nth order effectiveness factor approach with intrinsic reaction kinetics is applied in order to account for these diffusion limitations. Furthermore, the influence of thermal deactivation on the reactivity of the char surface is included. Several submodels are introduced, with focus on the structural evolution of char particles. A new pore structure model for high temperatures is developed that describes the surface area, the diameter, the density, the porosity and the mean pore diameter of the particles as a function of reaction regime and char conversion. User defined functions (UDF) include all submodels as well as the aforementioned effectiveness factor approach for the reactions of char with O2, CO2 and H2O. The computational effort is comparable to combustion simulations performed with standard models available in the software used.