Chemical Looping Combustion in a 10 kWth Prototype Using a CuO/Al2O3 Oxygen Carrier: Effect of Operating Conditions on Methane Combustion

Chemical looping combustion (CLC) currently is an attractive option to decrease the greenhouse gas emissions that affect global warming, because it is a combustion process with inherent CO2 separation and with low energy losses. The CLC concept is based on the transfer of oxygen from the combustion air to fuel by means of an oxygen carrier in the form of a metal oxide. The system consists of two separate but interconnected reactors, normally fluidized-bed type. In the fuel reactor, the oxygen carrier particles react with fuel and generate a gas stream mainly composed of CO2 and H2O. The reduced metal oxide is later transported to the air reactor, where oxygen from the air is transferred to the particles; in this way, one can obtain the original metal oxide ready to be returned to the fuel reactor for a new cycle. In this work, a 10 kWth pilot plant that is composed of two interconnected bubbling fluidized-bed reactors has been designed and built to demonstrate the CLC technology. The prototype was operated for 200 h, 120 h of which involved the burning of methane. The effect of the operating conditions (oxygen carrier-to-fuel ratio, fuel gas velocity, oxygen carrier particle size, and fuel reactor temperature) on fuel conversion was analyzed working with a CuO-Al2O3 oxygen carrier prepared by dry impregnation. In addition, the behavior with respect to attrition, agglomeration, and reactivity of the oxygen carrier was analyzed. It was found that the most important parameter that was affecting the CH4 conversion was the oxygen carrier-to-fuel ratio. Complete methane conversion, without CO or H2 emissions, was obtained with this oxygen carrier working at 800 °C and oxygen carrier-to-fuel ratios of >1.4.