Oxy-fuel combustion is one of the leading options for power generation with CO2 capture. The process consists of burning the fuel with a mixture of nearly pure oxygen and a CO2-rich recycled flue gas, resulting in a product flue gas from the boiler containing mainly CO2 and H2O. Among the possible boiler types, fluidised bed combustors are very appropriate for the oxy-fuel process because they allow the in situ desulphurisation by feeding Ca-based sorbents into the combustor. In this work, the effect of the temperature of the combustor on the retention of the SO2 generated in the combustion of two coals with very different sulphur content (a lignite and an anthracite) has been studied. The experimental facility used was a bubbling fluidised bed (BFB) combustor of similar to 3 kWth. Tests were conducted under oxy-fuel combustion mode and also under enriched-air combustion mode for comparison reasons. A Spanish limestone Granicarb'' was used as Ca-based sorbent for sulphur retention. The temperatures tested were between 800 and 970 degrees C using Ca/S molar ratios between 0 and 3. It was found that in BFB combustors operating under oxy-fuel combustion conditions the optimum temperature to achieve the highest sulphur retention was 900-925 degrees C, whereas operating with enriched air the optimum combustion temperature was 850-870 degrees C. Working at the optimum temperature, the SO2 retentions were lower in oxy-fuel combustion than in enriched air combustion conditions. It was also observed that working with lignite there was 10-15% of sulphur retention by coal ashes, however, working with anthracite the sulphur retention by coal ashes was negligible. This finding was independent of the combustion mode used, oxy-fuel or enriched air. (C) 2012 Elsevier Ltd. All rights reserved.

The interest in the use of solid fuels such as coal in Chemical Looping Combustion (CLC) is growing because of the benefits of the direct use of coal in this technology on the reduction of the costs linked to carbon dioxide capture. In CLC, the oxygen needed for the combustion is supplied by a solid oxygen carrier therefore avoiding the direct contact between fuel and air. Focusing on the use of solid fuels in the in situ Gasification Chemical Looping Combustion (iG-CLC), the oxygen carrier is mixed with the coal in the fuel reactor, where gasification of the coal and reaction of the gasification products with the oxygen carrier particles take place. In this process, the possible loss of oxygen carrier together with the ashes makes it interesting to find inexpensive materials to be used as carriers, such as natural minerals or industrial residues. In this work, an Fe-based bauxite waste is used as oxygen carrier in the combustion of different types of coal. Experiments were performed in a TGA and a batch fluidized bed (FB) using both coal and char from the corresponding coal. The influence of temperature as well as the gasifying agent composition on the performance of coal conversion in an iG-CLC process was evaluated. Tests in a thermogravimetric analyzer (TGA) revealed direct char combustion by oxygen in the bauxite waste material, but no evidences of such direct combustion were found in the experiments in the batch fluidized bed. In this case, gasification of char by H2O or CO2 was found as a necessary step in char conversion. Using steam as gasifying agent, higher char gasification rates were observed than using CO2 and in all conditions lignite presented the highest char gasification rate. At 980 degrees C, the lignite char gasification rate in CO2 doubled the value obtained with anthracite using steam, indicating that recirculated CO2 can be fed to the fuel reactor of a CLC system if lignite is used as fuel. For the rest of the fuels, it was possible to use a mixture of H2O and CO2 as gasifying agent without decreasing the feasibility of the iG-CLC process. The bauxite waste was able to burn the gaseous products generated during the gasification of the different types of coal with high combustion efficiencies. (c) 2012 Elsevier Ltd. All rights reserved.