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Abstract
Integrated gasification combined cycle is an advanced electricity generation technology, based on coal gasification. Wider deployment requires further research into the components of the process, including coal gasification. The coal gasification reactions are the slowest step in the gasifier and therefore determine size and operating conditions. Understanding the rate of the gasification reactions at high temperatures is crucial for accurate design and optimisation of the gasification process. While the rates have been measured at low temperature, limited data exist at the high temperatures and pressures relevant to entrained flow gasifiers. As the reactions are solid gas reactions, at high temperature they depend on both reaction rate and gas diffusion rate, which complicates the extrapolation of low temperature data. High temperature data measured thus far is only applicable to certain coals, as the gasification rates were not related to char properties.
The aim of this thesis was to measure the high temperature gasification rates of three coal char samples and interpret the results in terms of the char morphology and intrinsic reactivity. The results showed that the gasification rate depended on both intrinsic reaction rate and diffusion rate under the experimental conditions. The majority of the char particles were very swollen, with high porosity and thin walls, whereas the conventional analysis approach assumes porous spherical particles. The effectiveness factor approach was modified to incorporate flat-plate geometry, and used to extract the intrinsic reaction rate from the gasification rate data, which showed that the intrinsic reaction rate could be extrapolated to higher temperatures. The application of the effectiveness factor for estimating the gasification rate was demonstrated, and the modified approach was found to give a better estimation of char gasification rate at high temperature. Accurate measurement of char wall thickness and porosity was identified for potential improvement of the approach.
Char conversion in a pilot-scale entrained flow gasifier was then explained in terms of char reaction rate, char morphology and reaction temperature, which showed the need for the development of more accurate models of char conversion in gasifiers, to which the work in this thesis can be applied.