Comparison of natural ilmenites as oxygen carriers in chemical-looping combustion and influence of water gas shift reaction on gas composition

Chemical-looping combustion (CLC) is a promising technology for CO2-capture for storage or reuse as a method to mitigate CO2 emissions from the use of fossil fuels. In a CLC system the oxygen carrier is of great importance. Environmentally sound and low cost materials seem to be preferable especially for CLC of solid fuels. The natural occurring ore ilmenite has already been the target of different studies in order to work out its feasibility as oxygen carrier for different fuels. The initial part of this work is a screening of five commercial available ilmenite minerals as oxygen carrier, crushed and sieved to 125-180 mu m. The screening includes an examination of the sulfur released during the first heat up and the activation of the oxygen carrier, indicated by the fuel conversion using alternating reduction (syngas 50 vol.% CO in H-2) and oxidation conditions (10 vol.% O-2 in N-2). The five first cycles were carried out at 850 degrees C to avoid initial agglomeration whereas the main activation cycles have been performed at 950 degrees C in a tubular quartz reactor under fluidized bed conditions. From these experiments it is concluded that rock ilmenites are preferable as oxygen carriers since they revealed an improved fuel conversion, although offering a higher sulfur content, which is released during the initial heat up. One ilmenite mineral was used to investigate the influence of the catalyzed and non-catalyzed water gas shift reaction (WGS) on the fuel gas conversion at 950 degrees C. It was shown, that the WGS has an influence on the final gas composition. Experiments with only H-2 or CO separately as fuel diluted with N-2 have been performed to eliminate the effect of the WGS. Experiments with syngas and therefore activated WGS allow determining its influence on gas composition. To quantify the reaction progress, the law of mass action for the gas consumption is compared to the equilibrium constant at the corresponding temperature. The results from these experiments agree with the results from catalyzed WGS experiments. Therefore, CaO in the size fraction 180-250 mu m was added to the oxygen carrier since it is a well-known WGS catalyst. As a result the overall conversion of hydrogen and carbon monoxide were improved. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.