Sulphur dioxide capture under fluidised bed combustion conditions using dolomite and coal ash as sorbents
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A study was done to investigate sulphur dioxide capture capacities of South African mined dolomites, limestone and coal ash derived from high inertinite coal. The study was carried out under atmospheric and pressurised fluidised bed coal combustion conditions using a thermogravimetric analyser. Temperatures between 750-900°C were used. A typical flue gas mixture composed of 2000ppm S02, 5.3% 02 and C02 concentration of 10 and 20% for atmospheric and pressurised sulphation respectively and balance N2 was used. The performance of the sorbents was examined at atmospheric (0.875 bar) pressure where the active part of the sorbent was CaO and at 10 and 15 bar where the active part of the sorbent was CaC03. Coal ash had a calcium content of 7% by weight, this was much lower compared to dolomite and limestone which had between 20-30% calcium by weight. The sorbents' structural characterisation was done using nitrogen adsorption and mercury porosimetry. Compared to coal ash, the raw dolomite and limestone had a relatively smaller internal surface area, between 0.4-1.0 m2/g, for the uncalcined state, then increasing to between 10-15 m2/g on calcination. Coal ash had a relatively high internal surface area of 11 m2/g, which is comparable to the calcined sorbents. The porosity of raw sorbents increased from about 0.2 to 0.35 on calcination. Sulphation at atmospheric pressure showed a calcium conversion of between 22-44% for dolomites, 37-49% for limestone and 13-28% for coal ash after 180 minutes. At atmospheric pressure, dolomites and coal ash reactivity started off relatively fast followed by a sudden decline after 20% conversion, this is attributed to the change from kinetic control to product layer control; limestone did not show a decline in reactivity. Under pressurised conditions the reaction started off relatively slow and no signs of reaction decline were observed up to 180 minutes reaction period. Sulphation at 15 bar was higher than at 10 bar. At 15 bar dolomite calcium conversion observed ranged between 16-34%, for limestone it was between 18-36% and coal ash conversion was relatively higher, at 28-35% calcium conversion. The conversion results obtained compare well with results obtained by other researchers. A modified unreacted shrinking core model with variable effective diffusivity accounting for change in diffusion with growing product layer was successfully used to model results at both atmospheric and pressurised conditions.
- Engineering