Capture of CO2 from solid fuels using Chemical-Looping Combustion and Chemical-Looping Oxygen Uncoupling

Doctoral thesis, 2008

In order to meet the global demand of CO2 neutral energy, chemical-looping combustion could play an important role. Oxygen can be transferred from the combustion air to the fuel by means of oxygen carrying metal oxide particles circulating between two reactors, an air- and a fuel reactor. This enables combustion of a fuel without mixing the products of combustion, CO2 and H2O with the nitrogen from the combustion air. In this thesis two main strategies for using solid fuels with oxygen-carrier material have been investigated. The first is introduction of the coal directly to the fuel reactor where the gasification of the coal and subsequent reactions with the metal oxide will occur simultaneously. The second is to use an oxygen carrier which releases O2 in the gas-phase in the fuel reactor. The solid fuel is oxidized through normal combustion. The last alternative is referred to as chemical-looping with oxygen uncoupling (CLOU). The results give proof of concept for both investigated options. A small lab reactor designed for tests of solid fuels has been developed, and an experimental procedure for testing solid fuels established. The procedure involves the cyclic oxidation of oxygen carrier with oxygen containing gas and reduction with solid fuel. Also a larger screening was made of different ores and industrial materials identifying a number of possible low-cost oxygen carrying materials. The gasification rate of the char was the rate limiting reaction, as the reaction of oxygen carrier with gasification products CO and H2 was found to be quite rapid. The presence of an oxygen carrier enhances the gasification rates significantly, most likely because of removal of H2 and CO. The conversion rate of the fuel was dependent on the fraction of steam and SO2 in the fluidizing gas, as well as temperature and fraction of volatiles in the fuel. A preliminary estimation of the oxygen carrier inventory needed in a real CLC-system with solid fuel suggested that it would be between 200 and 2000 kg/MWth depending on which fuel and which oxygen carrier is used. The rates of fuel conversion are much higher in CLOU compared to CLC and the rate increased significantly with increasing temperature. No CO was measured in the CLOU experiment with petroleum coke and oxidation of oxygen carriers could take place at low oxygen concentrations close to the equilibrium partial pressure.