Chemical-Looping Combustion with Natural Gas Using NiO-Based Oxygen Carriers
CO2 capture and storage has been proposed as an alternative to mitigate global warming. Chemical-looping combustion 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 presented in this thesis has been conducted in two chemical-looping combustors, and focuses on the experimental evaluation of NiO-based oxygen carriers prepared by commercially available production methods. Investigations in a 10-kW reactor system using natural gas as fuel evaluate the long-term performance of different oxygen carriers. The most important results in this thesis concern particles prepared by spray-drying, which were subjected to > 1000 h of operation with fuel. Fuel conversion was good, 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. No decrease in reactivity was seen during these long-term tests. Experiments in a 300-W chemical-looping combustor 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.