On the Dynamic Modeling of an Entrained Gasifier Using Aspen Custom Modeler

摘要

The gasifier is the heart of the integrated gasification combined cycle (IGCC), a technology thathas emerged as an attractive alternative to conventional coal-fired power plant technology due toits higher efficiency and cleaner environmental performance especially with the option of CO_2capture and sequestration. Understanding the optimal performance of the gasifier is thereforeparamount for the efficient operation of IGCC power plants.Numerous gasifier models of varying complexity have been developed to study the variousaspects of gasifier performance. These range from simple one-dimensional (1D) process-typemodels to rigorous higher order 2-3D models based on computational fluid dynamics (CFD).Whereas high-fidelity CFD models can accurately predict most key aspects of gasifierperformance, they are computationally expensive and typically take hours to days to execute onhigh-performance computers. Therefore, faster 1D partial differential equation (PDE)-basedmodels are required for use in dynamic simulation studies, control system analysis, and trainingapplications. A number of 1D gasifier models can be found in the literature, but most are steadystatemodels that have limited application in the practical operation of the gasifier. As a result,1D PDE-based dynamic models are needed to further study and predict gasifier performanceunder a wide variety of process conditions and disturbances.In the current study, a 1D transient model of a single-stage downward-fired GE/Texaco-typeentrained-flow gasifier has been developed. The model comprises mass, momentum and energybalances for the gas and solid phases. The model considers the initial gasification processes ofwater evaporation and coal devolatilization. In addition, the key heterogeneous andhomogeneous chemical reactions have been modeled. The resulting time-dependent PDE modelis solved using the well-known method of lines approach in Aspen Custom Modeler?, wherebythe PDEs in the spatial domain are discretized and the resulting differential algebraic equations(DAEs) are then integrated over time using a dynamic integrator. The dynamic response resultsof the gasifier performance parameters to certain disturbances commonly encountered duringpractical operation are presented. These disturbances include ramp and step changes to input variables such as coal flow rate, oxygen-to-coal ratio and water-to-coal ratio among others.Comparison of model predictions to available dynamic data will also be discussed.