Combustion of a German lignite using chemical-looping with oxygen uncoupling (CLOU)
Conference contribution, 2008
Chemical-looping with oxygen uncoupling (CLOU) is a novel method to burn solid fuels in gas-phase oxygen without the need for an energy intensive air separation unit. The carbon dioxide from the combustion is inherently obtained separated from the rest of the flue gases. The technique is based on chemical-looping combustion but involves a completely different reaction mechanism for the fuel oxidation. The process uses three steps in two reactors, one air reactor where a metal oxide captures oxygen from the combustion air (step 1), and a fuel reactor where the metal oxide releases oxygen (step 2) and where this oxygen reacts with a fuel (step 3). This means that the char reacts directly with gaseous O2, which is a very fast reaction. In other proposed schemes for using chemical-looping combustion of solid fuels there is a need for an intermediate gasification step of the char with steam or carbon dioxide to form reactive gaseous compounds which then react with the oxygen carrier particles. The gasification reactions are inherently slow, resulting in slow overall rates of reaction. This is solved in the proposed process, since there is no intermediate gasification step needed and the char reacts directly with gas-phase oxygen. Of course this demands another type of oxygen carrier than those normally used in chemical-looping combustion, and a thermal analysis has identified CuO/Cu2O, Mn2O3/Mn3O4 and Co3O4/CoO as potential systems. Thermodynamic calculations indicate that metal sulphates should not be formed in the fuel reactor during normal operation, although they could be formed locally for Co and Mn-based oxygen carriers at lower temperatures. This work presents results from an investigation of the reaction between a Cu-based oxygen carrier with a German lignite in a batch fluidized bed reactor. A ratio of oxygen carriers to fuel of 6 kg/MJ was employed during the combustion and between 30-45 seconds was needed for 95% conversion of the coal in the temperature interval 850-985C. The oxidation was possible at all temperatures, and a substantial part of the oxidation occurred near the equilibrium O2 concentration. No signs of agglomerations of the oxygen carrier particles were found.