Mathematical Modeling of a Single-Stage, Downward-Firing, Entrained-Flow Gasifier

Gasifiers are the centerpieces of coal-fired integrated gasification combined cycle (IGCC) plants. Mathematical models of gasifiers have been developed in recent literature to describe the physical and chemical processes taking place inside the reactor vessels. These models range from simple one-dimensional (1D) steady-state equilibrium models to higher-order, sophisticated, dynamic 2D and 3D computational fluid dynamics (CFD) models that describe coupled gas–solid hydrodynamics, heat and mass transfer, and reaction kinetics over the complex gasifier geometry. In the current work, a 1D steady-state model of a single-stage, downward-firing, oxygen-blown, slurry-fed, entrained-flow gasifier has been developed for use in the context of IGCC process simulation. In this mathematical model, mass, momentum, and energy balance equations for solid and gas phases are considered. The model includes a number of heterogeneous and homogeneous chemical reactions along with devolatilization and drying of the slurry feed. The solid–gas heterogeneous reaction rates are calculated using the unreacted shrinking-core model. A detailed model of the radiative heat transfer has been developed considering interactions between the solids and all internal gasifier surfaces (side wall, top, and bottom surfaces), as well as interactions between the surfaces themselves. No a priori wall temperature profile is assumed in this model. The heat loss from the gasifier wall to the environment is also considered in the energy balance equations. In slurry-fed gasifiers, recirculation near the inlet of the gasifier is promoted by rapid mixing of the slurry feed with a portion of the hot reaction products. This violent mixing results in a significant rise in temperature that helps in evaporating the water and devolatilizing the coal. The recirculation is achieved by appropriately designing the feed burner and feeding the oxygen through a swirling annular injector. In the current gasifier model, a heuristic recirculation model has been developed and the conservation equations have been appropriately modified. The equations describing the gasifier are formulated as a set of ordinary differential equations (ODEs) in Aspen Custom Modeler (ACM). The ODEs are discretized using finite differences, and the resulting highly nonlinear system of algebraic equations is solved using a Newton-type method. The gasifier model is then validated using pilot plant and industrial data. This paper presents a number of parametric studies that have been performed using the 1D steady-state gasifier model to provide insight into the gasifier performance as the inlet and operating conditions change. Results are presented as profiles for species concentration and gas, solid, and wall temperatures. The effect of coal feed types on composition are also presented. In addition, a radiant syngas cooler (RSC) model has been developed in Aspen Plus and coupled with the gasifier model, thereby enabling the RSC exit stream composition to be compared to available industrial data.