Design, modelling and operation of a 100 kW chemical-looping combustor for solid fuels

Doctoral thesis, 2012

With the increasing threat of global warming, technologies for efficient capture and storage of the greenhouse gas CO2 are sought after. Chemical-looping combustion is a novel CO2 capture technology that can be applied when burning gaseous, liquid or solid fuels. By using two interconnected fluidised beds with a bed material capable of transferring oxygen from air to the fuel, a nitrogen-undiluted stream of CO2 can be obtained with no direct efficiency loss. This thesis is focused on design, modelling and operation of a 100 kW chemical-looping combustor for solid fuels. The goal of the test rig is validation of chemical-looping combustion at a scale giving both high operational flexibility and semi industrial conditions. Two analytical models that can be applied to any chemical-looping combustor for solid fuels are presented. The first model finds the residence-time from batch-experiments. The second model estimates the gas conversion of a general fuel as a function of the oxygen carrier bed inventory. Furthermore, a cold-flow model of the 100 kW unit has been constructed. Details about the cold-flow model design and experimental results regarding fluidisation, slugging, residence-time and circulation are presented. The 100 kW unit has been operated for over 23 hours with an ilmenite oxygen carrier, using three different fuels. During this time, no instabilities in the bed inventories have been detected. Experiments aiming for optimal performance showed that gas conversion above 84% and CO2 capture over 99% are possible. A detailed analysis of the relation between the global solids circulation, the fuel reactor bed inventory and the gas conversion was conducted. The results revealed that the bed inventory in the fuel reactor had a strong impact on gas conversion, whereas little effect of overall circulation could be seen under the present conditions.