Characterisation and fluidised bed gasification of selected high-ash South African coals
Engelbrecht, André Daniël
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South Africa has abundant coal reserves and produces approximately 75 % of its primary energy from coal. Based on scientific analysis, however, it is generally accepted that a link exists between climate change and the use of fossil fuels such as coal. The development of clean coal technologies (CCT) has therefore received increased attention worldwide. Integrated gasification combined cycle (IGCC) technology, utilising fluidised bed gasification, has been identified as a potential CCT that can be applied in South Africa. A suite of four South African coals was identified as being possible fuels for IGCC power stations which would operate for three to four decades, towards the middle of this century. The selected coals are from New Vaal, Matla, Grootegeluk and Duvha collieries. These coals were subjected to detailed characterisation, thernogravimetric analysis and fluidised bed gasification tests to access their suitability for use in IGCC power stations. The characterisation performed consisted of standard coal analytical methods, petrographic techniques and physical analysis. The results of the analysis showed that the coals are low in grade and rich in inertinite. The vitrinite random reflectance (Rr), which is regarded as a reliable rank parameter, showed that the coals selected are representative of the rank variation within South African bituminous coals, with the coals of lower rank having larger surface areas and porosities. Gasification reactivity experiments were carried out in a thermogravimetric analyser (TGA) at 87.5 kPa, between temperatures of 875 °C and 950 °C, with 100 vol. % CO2 as the reacting gas. The results of the tests show that the reactivity of coal char increases with a decrease in the rank of the coal. The reactivity of the New Vaal coal, which has the lowest rank, is comparable to the reported reactivity of some overseas lignite coals. The results also show that the grain model can be used to describe char conversion and that the Arrhenius equation describes the effect of temperature on the reaction rate constant. Pilot-scale fluidised bed gasification tests were carried out on the four selected coals at 925 °C and 950 °C. The results show that the fixed carbon conversion achieved in the gasifier correlates well with the rank parameter (Rr) of the coal. Although the order of ranking of reactivities of the different coals in the fluidised bed gasifier and the TGA are the same, the variation in the reactivity index in the fluidised bed gasifier is significantly lower than in the TGA. This was attributed to the large amount of fixed carbon that is converted in the FBG by means of the partial combustion reaction which is less sensitive to the reactivity of the char. The low volatile matter content and the high ash content of the coals tested, together with high gasifier heat losses and nitrogen dilution, contributed to the low calorific value of the gas produced. No agglomeration and clinkering of the coal was observed during the gasification tests and it was concluded that this can be attributed to the low Free Swelling Index (FSI) and Roga Index (RI) of the coals tested. It was concluded that fluidised bed gasifiers are able to utilise high-ash South African coals and are therefore a candidate technology for IGCC power stations. Due to the relatively low reactivity of most South African bituminous coals, a secondary combustion stage may be required after the fluidised bed gasifier in order to achieve acceptable overall carbon conversions.
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