(199e) Rigourous Modeling of An Entrained Flow Gasifier Considering a Heuristic Recirculation Model and a Detailed Energy Balance
AIChE Annual Meeting
Tuesday, October 18, 2011 - 10:10am to 10:35am
Coal-fired gasifiers are the centerpiece of integrated gasification combined cycle (IGCC) power plants. The gasifier produces synthesis gas that is cleaned and sent for combustion in a gas turbine. Several mathematical models have been developed to study the physical and chemical processes taking place inside the gasifier. Such models range from simple one-dimensional (1D) steady-state models to sophisticated dynamic 3D computational fluid dynamics (CFD) models. In this work, a 1D model of a single-stage, entrained-flow, General Electric Energy (GEE)-type gasifier has been developed.
In our 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 unreacted-core shrinking model is used for calculating the rates of the heterogeneous reactions. A detailed model of the radiative heat transfer has been developed considering interactions between all internal surfaces (side wall, top, and bottom surfaces) of the gasifier and the solids along with the interactions between the surfaces themselves. Unlike models developed in many previous studies, no arbitrary wall temperature profile is assumed but an energy balance across the wall including the heat loss from the gasifier wall to the environment is carried out. In the slurry-fed gasifiers, the recirculation near the inlet of the gasifier is promoted for rapid mixing of the slurry feed with a part of the hot reaction products. This vigorous 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 model, a heuristic recirculation model has been developed and the conservation equations have been appropriately modified.
The above formulation results in a highly nonlinear system of differential algebraic equations (DAE) that is solved using a Newton-type method in Aspen Custom Modeler® (ACM). The model is validated with the data from experimental studies and pilot plants. The paper includes species concentration profiles and temperature profiles of the gas and solid phases. A number of parametric studies are carried out to gain more insight into the gasifier performance as the inlet and operating conditions change. The results include the effects of the recirculation ratio on the initial heat up of the slurry feed as the type of coal and its quality are changed. For various inlet conditions, the model can be utilized to predict the profiles of a number of key variables which are very difficult, if not impossible, to measure in an actual gasifier.