Fluidised bed gasification of high-ash South African coals : an experimental and modelling study

Abstract

South Africa has large coal reserves and produces approximately 74% of its primary energy from coal. Coal gasification using moving bed gasifiers is one of the most important coal utilisation technologies, consuming ± 17.5% of locally produced coal. This study was motivated by the need to investigate alternative coal gasification technologies for the utilisation of fine, high-ash and caking coals for future Integrated Gasification Combined Cycle (IGCC) and coal to liquids (CTL) plants. These coals are estimated to form a large percentage of the remaining coal reserves in South Africa and could be difficult to utilise efficiently in moving bed gasifiers. Fluidised bed gasification was identified as a technology that could potentially utilise these coals. Coals from the New Vaal and Grootegeluk collieries were selected as being suitable for this investigation. The coals were subjected to detailed characterisation, bench-scale and pilot-scale fluidised bed gasification tests. The results of the pilot-scale atmospheric bubbling fluidised bed gasification tests show that stable gasification is possible at temperatures between 880 °C and 980 °C. The maximum fixed carbon conversion achievable in the pilot plant is, however, limited to ± 88% due to the low reactivity of the coals tested and to thermal fragmentation and attrition of the coal in the gasifier. It was found that oxygen enrichment of the gasification air from 21% to 36% by means of oxygen addition produces a significant increase in the calorific value of the gas (3.0 MJ/Nm3 to 5.5 MJ/Nm3). This observation has not previously been reported at pilot-plant scale. A mathematical model for a bubbling fluidised bed coal gasifier was developed based on sub-models for fluidised bed hydrodynamics, coal devolatilisation, chemical reactions, transfer processes and fines generation. A coal devolatilisation sub-model to predict the products of coal devolatilisation in a fluidised bed gasifier was developed and incorporated into the model. Parameters associated with the rates of the gasification reactions and the devoltilisation process were obtained by means of bench-scale tests. The heat loss parameter (Q) in the model was estimated by means of a heat loss calculation. The results from the pilot-scale gasification tests were used to evaluate the predictive capability of the model. It was found that for temperature, fixed carbon conversion and calorific value of the gas the difference between measured and predicted values was less than 10%. Recommendations are made for further refinement of the model to improve its predictive capability and range of application. The model was used to study the effect of major operating variables on gasifier performance. It was found that increasing the reactant gas (air, oxygen and steam) temperature from 250 °C to 550 °C increases the calorific value of the gas by ± 9.3% and the gasification efficiency by ± 6.0%. Increasing the fluidised bed height has a positive effect on fixed carbon conversion; however, at higher bed heights the benefit of increasing the bed height is less due to the inhibiting effects of H2 and CO on the rates of char gasification.