Power generators, oil refinery operators and the chemical industry have turned in recent years to gasification to unlock the power of coal and other carbon based fuels. Gasifiers in commercial operation, especially for power generation, have incurred operational issues. A thorough understanding of their operation is essential for addressing existing challenges and improving future designs. Many complex processes take place in a coal gasifier and the development of computational models is an essential component of gaining this understanding.
Any coal gasification model must be capable of modelling the fundamental processes occurring in the gasifier. Firstly, volatile components in the coal such as light gases and tar are released by pyrolysis which is also known as devolatilisation. The volatile components released undergo homogeneous reactions. These are more commonly modelled as global reactions rather than detailed reactions involving radicals. Following devolatilisation the char residue gasifies.
Approach to modelling
Having considered the fundamental processes, the overall approach to modelling can incorporate different levels of complexity. First is the number of space dimensions; three levels are possible. Secondly, whether time is included. If it is, the model is dynamic. If not, the model is steady state. In zero-dimensional models, the output variables are evaluated in relation to the input variables without considering the details of processes occurring inside the control volume and these are not suitable for modelling gasifiers. One-dimensional models assume that all variables inside the equipment vary along one-space co-ordinate and these allow profiles of variables to be evaluated throughout the system. These have been successfully used to model all types of gasifiers.
Two-dimensional models allow for changes in properties in the axial and radial directions. These may be adequate if the model has cylindrical symmetry. Three-dimensional modelling entails considerable mathematical and computational complexity but in many situations is necessary for a realistic representation particularly of asymmetric geometries.
CFD models are a powerful tool for investigating many types of plant and in the last few decades such modelling has played an important role in improving the performance of pf plant. Using CFD models to describe coal gasification requires additional layers of complexity. Multiple phases are present as the gasifier contains solids and possibly liquids in addition to the gas phase. The phases in the reactor are complex and contain changing chemical mixtures. Both homogeneous and heterogeneous reactions must be considered. In addition to the continuity equation and the equation of motion, the energy equation and mass transfer equations must be solved as coupled equations.
In a moving bed gasifier, the solid fuel is fed at the top of the reactor and slowly flows to the base where the residual solid is removed. Many processes take place such as drying, devolatilisation, gasification and combustion. Several models have been developed which are able to reproduce the processes taking place and make predictions under industrial-scale operating conditions. Many such models have been based on one-dimensional representations. The solid phase flow can be assumed to be plug-flow and the system can be assumed to be at steady-state. This level of sophistication has been found to be adequate in many cases. Unlike entrained-flow and fluidised bed gasifiers, relatively few modelling studies have been performed on moving bed coal gasifiers. Recent work has tended to focus on biomass gasifiers. It is evident that the relatively few models developed recently to simulate moving bed gasifiers have tended to regard the system as consisting of several zones at steady state in which a particular process takes place. Many of the models utilise the Aspen Plus coding. The comparisons that have been made of model results with plant or rig data have indicated reasonable agreement.
Fluidised bed gasifiers
Fluidised bed gasifiers can be considered to consist of two phases: a bubble and an emulsion phase. Bubbles entering the bed expand as they pass up through the bed, hence the bubble size increases with bed height. Each bubble can be assumed to consist of a bubble volume which is surrounded by a bubble cloud. Transport processes occur between the bubble phase, the cloud and the emulsion phase. These have generally assumed steady state. A detailed CSFMB model which was produced by De Souza-Santos and others could model both moving bed and circulating bed gasifiers. CFD modelling has also been extensively used for modelling fluidised bed gasification. The results of the models have been compared with plant and rig data. The comparisons have generally been satisfactory. There is little indication that modelling results have been utilised to solve plant problems.
Entrained flow gasification technology is the most widely used gasification technology but modelling this process is more complicated than modelling fluidised bed gasifiers due to the need to model ash slagging. Unlike FBG modelling for which the motion of the coal particles is generally described by an Eulerian approach, in entrained flow gasifiers which are more lightly loaded, the Lagrangian approach is more suitable. There have been some 1-D models to model entrained flow gasifiers.
More modelling studies have been undertaken for entrained-flow gasifiers than for moving bed or fluidised bed gasifiers. Both 1-D and 3-D models have been developed but the majority are 3-D. These are frequently based on CFD using the commercial software FLUENT. In most cases the results of the modelling studies have given insight into the fundamental processes occurring in the gasifier. This has helped to improve gasifier designs. In some cases, the results have been compared with plant data and it has been possible to choose model inputs to give reasonable fit with the measured data. These remarks can be extended to apply to gasification modelling in general.
There are many processes taking place in gasifiers and developing computational models of these processes is a complex task. These models are vital in understanding the processes taking place. The model inputs tend to be chosen to fit available data rather than ‘a priori’. Insight gained by modelling has advanced the design of gasifiers and can improve gasifier performance. However, there are fewer examples where modelling has directly solved operational problems.
Written by Dr Rohan Fernando, IEA Clean Coal Centre.
Edited by Katie Woodward
Read the article online at: https://www.worldcoal.com/coal/21042014/developments_in_modelling_and_simulation_of_coal_gasification/