CO2 Capture using Chemical-Looping Combustion – Operational Experience with Gaseous and Solid Fuels

Doctoral thesis, 2011

CO2 capture and storage has been proposed as an alternative to mitigate global warming. Chemical-looping combustion (CLC) is an unmixed combustion concept where fuel and combustion air are kept separate by means of an oxygen carrier, and the CO2 capture is inherently achieved. The work in this thesis focuses on the experimental evaluation of different oxygen carriers and fuels in three CLC reactors. These continuous units are based on interconnected fluidized-bed technology and feature a fuel reactor (FR) and an air reactor (AR) as the principal reaction chambers. Fuel is oxidized to CO2 and H2O in the FR with oxygen supplied by the oxygen carrier, which is based on metal oxide. Oxygen-carrier particles are then re-oxidized in the AR. A 10 kW chemical-looping combustor for gaseous fuels was used to evaluate the long-term performance of different Ni-based oxygen carriers. The most important findings concern particles prepared by spray-drying, which were subjected to more than 1000 h of operation with natural gas as fuel. Fuel conversion was high, and increased with (a) decreased circulation, and (b) increased fuel-reactor temperature. Combustion efficiency close to 99% was accomplished using these spray-dried particles. At the end of the test series, the continuous loss of fine material was 0.003%/h, which corresponds to a particle life time of 33000 h. Experiments in a 300 W chemical-looping combustor for gaseous fuels investigate the possibility to optimize the methane conversion – while retaining the oxygen-transport capacity – by mixing different NiO-based oxygen carriers. The study confirmed that such optimization is indeed feasible. A 10 kW CLC unit designed for solid fuels was used for both continuous and batch tests. In the continuous tests, three oxygen carriers were used; (a) ilmenite, an iron-titanium mineral, (b) ilmenite in combination with limestone, and (c) manganese ore. Generally, longer residence time of the fuel and increased temperature in the FR had a beneficial effect on gasification. Compared to ilmenite, the use of ilmenite+limestone as oxygen carrier improved gas conversion as well as the rate of char gasification. It was shown that this result was due to the effect of limestone on the water-gas shift equilibrium. In another study, in-bed fuel feed was found to significantly improve gas conversion, mainly caused by increased contact between the oxygen carrier and volatile gases produced in the fuel chute. The use of a manganese ore as oxygen carrier greatly enhanced the rate of gasification. Furthermore, gas conversion also improved using the manganese ore. A concern with the manganese ore is the large production of fines. The batch tests in the 10 kW unit involved the feeding of five fuels to the FR in batches of 20-25 g at four temperatures, using ilmenite as oxygen carrier. By using devolatilized fuel, it was possible to determine (a) oxygen demand associated with syngas from char gasification as well as (b) kinetics of gasification. The minimum oxygen demand for char was found to be around 5%.