Chemical-Looping Combustion with Solid Fuels

Licentiate thesis, 2007

In order to meet the global demand of CO2 neutral energy production, chemical-looping combustion could play an important role. With this technique it is possible to capture CO2 without an excessive energy penalty. By circulating metal oxide particles between two reactors, the air- and fuel reactor, oxygen is transported from the combustion air to the fuel. This enables combustion of a fuel without mixing the flue gases with the nitrogen from the combustion air. In this thesis the reaction of solid fuels with oxygen-carrier material has been investigated using petroleum coke and six different coals with varying content of volatiles. The reactivity was evaluated in a laboratory fluidized bed reactor operated under cyclically oxidizing and reducing conditions. Two different oxygen carrier materials were used in the testing, manufactured particles with iron oxide supported on magnesium alumina oxide and a natural mineral, ilmenite, which is an oxide containing iron and titania. All investigated fuels reacted with the oxygen carrier particles with little or no defluidization. When the solid fuel is added to the fuel reactor it is gasified to mainly CO and H2 which reacts with the oxygen carrier. It was found that the gasification rate of the solid fuel was the rate limiting reaction, as the reaction of oxygen carrier with CO and H2 was found to be quite rapid. In comparison to pure gasification of coals, the presence of an oxygen carrier seems to significantly enhance the gasification rates, most likely because of removal of H2 and CO, which may inhibit the gasification reactions. At a temperature of 950˚C and with 50% steam in the fluidization gas, 95% conversion of the fuels was reached after 4-15 minutes, depending on the fuel. The conversion rate was strongly dependent on 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 showed that it would be between 500 and 2000 kg/MWth depending on which fuel and which oxygen carrier is used.