Chemical Looping Combustion (CLC) is a promising carbon capture technology to contribute to the development of BioEnergy with Carbon Capture and Storage (BECCS) technologies. In CLC, the oxygen needed for combustion is supplied by a solid oxygen carrier. During the last years, intensive research has been conducted to identify potential oxygen carriers. Iron and manganese-based minerals allowed obtaining high CO2 capture efficiencies, but lower values for the combustion efficiency, that is, high oxygen demand values, due to the high content of volatiles in biomass. Manganese-iron mixed oxides doped with titanium possess the capability of releasing molecular oxygen under specific conditions which would enhance volatile conversion. The objective of the present work was to evaluate the potential of one of these materials ((Mn66Fe)-Ti7) to improve the combustion efficiency of CLC with biomass in a 0.5 kWth continuous CLC unit. Almost 100% of CO2 capture efficiency was obtained in all the experiments. Once the operational conditions for the synthetic material (Mn66Fe)-Ti7 were optimized, it was possible to reach oxygen demand values about 10% with the advantage that the (Mn66Fe)-Ti7 oxygen carrier presented magnetic properties that would facilitate its separation from biomass ashes in the fuel reactor.

Chemical Looping Combustion (CLC) using renewable solid fuels appears as an important option to reach negative carbon emissions. In this work, three types of forest and agricultural residues (pine sawdust, olive stone and almond shell) were tested between 900-980 degrees C in a 0.5 kW(th) unit with an iron ore as oxygen carrier (Tierga ore) working under In situ Gasification-Chemical Looping Combustion (iG-CLC) mode. Specific solids inventories lower than 1000 kg/MWth were tested as they were consider more representative of what can be used in a larger CLC unit. CO2 represented about 70% in the fuel reactor outlet gas stream, followed by unburnt compounds: H-2, CO and CH4. CO2 capture efficiencies increased with the fuel reactor temperature achieving almost 100% of capture with the three biomasses at temperatures above 950 degrees C. In contrast, no clear trend with the fuel reactor temperature was observed for the total oxygen demand, achieving values about 25%. The major contribution to this value comes from the unburned volatiles with a small contribution coming from tar (approximate to 1%). Regarding tar, naphthalene was the major compound found at the different operating conditions. The present results support the consideration of the CLC process with biomass (bio-CLC) as a promising Bio-Energy with Carbon Capture (BECCS) technology.

In this work, SO2 retention via calcium-based sorbents added in a continuous bubbling fluidized bed combustor (similar to 3 kW(th)) operating in oxy-fuel combustion mode is analyzed. Tests were performed at different operating temperatures with three sorbents, two limestones and one dolomite, and with three coals, ranging from lignite to anthracite, to analyze the influence of coal rank, type of sorbent, sorbent particle size, and O-2/CO2 feeding ratio on the sulfation process. It was found that the combustor temperature had a strong influence on the limestones sulfur retention with a maximum at 900-925 degrees C. The behavior of the limestones was qualitatively similar with the three coals, attaining the highest sulfur retention values working with the lignite and the lowest working with the bituminous coal. On the contrary, with the dolomite the sulfur retention was hardly affected by the combustion temperature and the sulfur retentions attained were higher than with the limestones. The sulfur retention increased with diminishing the Ca-based sorbent particle size, and it was hardly affected by the O-2/CO2 ratio fed into the combustor. (C) 2014 Elsevier Ltd. All rights reserved.

Among the different pollutant gases released from oxy-coal combustion, mercury is the responsible for important operational issues in the CO2 processing unit, being able to cause material corrosion. Some studies concerning the fate of mercury during oxy-coal combustion are referred to pulverized coal (PC) boilers. However, the fate of mercury emissions in fluidized bed (FB) combustors has yet to be elucidated. In this work, mercury emissions from a 3 kWth bubbling FB combustor operating under oxy-fuel combustion conditions have been evaluated. For this purpose, two Spanish Ca-based sorbents and two Spanish coals were used. The effects of type of Ca-based sorbent, the Ca/S molar ratio, temperature and recycled gases (NO, SO2 and H2O) typical in oxy-fuel combustion process on the distribution of mercury species have been analyzed. It was observed that the presence of Ca-based sorbents in the combustor favored mercury fixation as particle-bound mercury which exhibited a maximum at a temperature about 925 degrees C corresponding to the highest degree of limestone sulfation. SO2 recirculation inhibited the Hg-0 oxidation and thus the mercury fixation as particle-bound mercury decreased. However, neither NO nor steam recirculation affected mercury speciation. (C) 2015 Elsevier Ltd. All rights reserved.

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.