Design and Operation of a 10 kWth Chemical-Looping Combustor for Solid Fuels

Licentiatavhandling, 2007

The use of a 10 kWth chemical-looping combustor for solid fuels was investigated. The unit was designed, dimensioned and built and two types of fuel were tested: a South African coal and a petroleum coke from Mexico. Chemical-looping combustion is an alternative to normal combustion for CO2 capture. With this process, the fuel and the combustion air are never mixed and the oxygen is transferred to the fuel via an oxygen carrier in the form of a metal oxide circulating between two reactors, e.g. interconnected fluidized beds. The result is that the CO2 produced is not diluted by the nitrogen in the air and can be recovered by condensing and removing the water resulting from the combustion. The oxygen carrier tested was ilmenite, an iron titanium oxide. The aim of the work was to find out if chemical-looping combustion was suitable for solid fuels, i.e. if the process itself worked and if the testing would reveal any potential killer for the scaling-up from the 10 kW unit to larger pilot plants and industrial facilities. In particular, key issues such as the risk for metal oxide particles agglomeration, the solid fuel conversion, the actual CO2 capture of the process as well as the gas conversion of the carbon-containing gases resulting from fuel devolatilization and gasification were considered and evaluated. Investigations of the reactivity of the particles as well as their integrity in terms of attrition and fragmentation were also performed. The results gave proof of the concept. No agglomeration was found during the testing itself and ilmenite appeared to be a suitable material. For the South African coal, a total of more than 22 h operation gave CO2 capture efficiencies ranging from 82 to 96% and solid fuel conversions up to nearly 80% while the oxygen demand, i.e. the part oxygen lacking to convert the carbon containing gases to CO2, was in the range 16-22%, based on CO and CH4 measurements. For the petroleum coke, 11 h operation gave CO2 captures between 60 and 75% principally due to the lower reactivity of this fuel. The solid fuel conversion ranged between 66 and 78% while the oxygen demand based on CO, CH4 and H2 measurements varied between 23 and 30%.