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.

In the current paper, a one-dimensional partial differential equation (PDE)-based dynamic model and its simulation results are presented for a single-stage down-fired entrained-flow gasifier. The gasifier model comprises mass, momentum, and energy balances for the gas and solid phases. The initial gasification processes of water evaporation and coal devolatilization and the key heterogeneous and homogeneous chemical reactions have also been modeled. The resulting coupled system of PDEs and algebraic equations is solved using the well-known method of lines in Aspen Custom Modeler. In addition to the dynamic gasifier model, efficient control strategies that can satisfactorily perform both servo and disturbance rejection functions have been developed for the entrained-flow gasifier. The dynamic variations of key gasifier output variables in response to the disturbances commonly encountered in industrial operation are presented. Output variables of interest include gas and solid phase temperatures, synthesis gas compositions, and carbon conversion, while disturbances include ramp and step changes in input variables such as coal flow rate, oxygen-to-coal ratio, and water-to-coal ratio among others. Feedstock switchovers have also been studied by simulating transitions from one coal type to another. The gasifier model results are also compared to the dynamic data available in the literature.

Mathematical Modeling and Simulation of a One-Dimensional Transient Entrained-flow GEE/Texaco Coal Gasifier Job S. Kasule Numerous gasifier models of varying complexity have been developed to study the various aspects of gasifier performance. These range from simple one-dimensional (1D) models to rigorous higher order 3D models based on computational fluid dynamics (CFD). Even though high-fidelity CFD models can accurately predict many key aspects of gasifier performance, they are computationally expensive and typically take hours to days to execute even on highperformance computers. Therefore, faster 1D partial differential equation (PDE)-based models are required for use in dynamic simulation studies, control system analysis, and training applications. In the current study, a 1D transient model of a single-stage downward-firing entrained flow General Electric Energy (GEE)/Texaco-type gasifier has been developed. The model comprises mass, momentum and energy balances for the gas and solid phases. A detailed energy balance across the wall of the gasifier has been incorporated in the model to calculate the wall temperature profile along the gasifier length. This balance considers a detailed radiative transfer model with variable view factors between the various surfaces of the gasifier and with the solid particles. The model considers the initial gasification processes of water evaporation and coal devolatilization. In addition, the key heterogeneous and homogeneous chemical reactions have been modeled. The resulting time-dependent PDE model is solved using the method of lines in Aspen Custom Modeler®, whereby the PDEs are discretized in the spatial domain and the resulting differential algebraic equations (DAEs) are then integrated over time using a variable step integrator. Results from the steady-state model and parametric studies have been presented. These results include the gas, solid, and wall temperature profiles, concentrations profiles of the solid and gas species, effects of the oxygen-to-coal ratio and water-to-coal ratio on temperature, conversion, cold gas efficiency, and species compositions. In addition, the dynamic response of the gasifier to the disturbances commonly encountered in real-life is presented. These disturbances include ramp and step changes in input variables such as coal flow rate, oxygen-tocoal ratio, and water-to-coal ratio among others. The results from the steady-state and dynamic models compare very well with the data from pilot plants, operating plants, and previous studies. iii Dedication This work is dedicated to my beloved mother Rosette Nansubuga, my late father Kizza Godfrey, my late uncle Micheal Ssebunya and the rest of my family. iv Acknowledgements I am highly indebted to my Advisor, Prof. Richard Turton for his guidance, patience, support, and unwavering belief in me during this research experience. Special thanks to Dr. Debangsu Bhattacharyya for his selfless help and insightful suggestions throughout this work. Many thanks to Dr. Stephen Zitney for his help with As...

in Wiley Online Library (wileyonlinelibrary.com) Slagging entrained-flow gasifiers operate above the melting temperature of the ash. As slag is highly nonwetting on the surface of char (carbon) particles, it is likely that it will agglomerate into one or several slag droplets and some of these droplets can detach from the char particles. If the slag exists in the form of droplets on the char surface rather than as a solid shell around the unreacted char particle, a shrinking particle model would be more physically realistic representation in comparison to the widely used shrinking core model (SCM). In the early section of the gasifier, the temperature remains below the ash melting temperature and, therefore, the SCM is more appropriate in this region. With this motivation, a novel hybrid shrinking-core shrinking-particle model has been developed. The model provides spatial profile of a number of important variables that are not available from the traditional SCM.