Abstract:
The moving bed coal gasifier produces 89 percent of the world's gas from coal. In South Africa the largest sector of the chemical and petrochemical industries is dependant on products from these gasifiers. Little is known about the transfer processes in the gasifier because it is not possible to get accurate information on the transfer process by means of experimentation.
In this study the transfer processes in the gasifier were simulated by computational fluid mechanics. Firstly a one-dimensional numerical model was developed to simulate the mass and energy transfer of the height dimension of the gasifier. The model consists of reaction rates of seven reactions that occur in the gasification process. The results obtained compare well with those of similar published models as well as with observations on gasifiers in operation.
Secondly, a three-dimensional chemical reacting flow model that incorporated the mass and momentum transfer of the fluid in a moving packed bed, was developed. The model was compared with the one-dimensional model, as well as with results achieved from gasifier excavations. These comparisons proved that the three-dimensional model was a more reliable simulation of the gasification process. Finally, the model was applied to study some of the transfer mechanisms in the gasifier. It became clear from the simulation that the fundamental cause of temperature instabilities in the gasifier was the non-homogeneous spread of coal particle size inside the gasifier. It was finally demonstrated that the model developed could be applied to a wide range of studies on gasification.
