Influence of Limestone Addition in a 10 kWth Chemical-Looping Combustion Unit Operated with Petcoke

Ilmenite, a natural mineral composed of FeTiO3, is a low-cost material suitable as an oxygen carrier for chemical-looping combustion (CLC) with solid fuels. One option when using the CLC technology with solid fuels is to introduce the fuel directly into the fuel reactor. Once in there, the fuel is gasified and volatiles and gasification products react with the oxygen carrier. In this study, the influence of limestone addition to ilmenite as an oxygen carrier was tested in a continuous 10 kW(th) CLC pilot for solid fuels. The fuel fed was a petcoke, and the gasifying agent was steam. Tests with an ilmenite-limestone mixture as the bed material were performed, and also tests using only ilmenite as the bed material were carried out for comparison. Global solid circulation was varied because it is an important operational parameter, which determines the solid fuel residence time. The experiments were made at two fuel-reactor temperatures: 950 and 1000 degrees C. Generally, a higher residence time of the fuel and a higher temperature increased both gasification and combustion efficiencies. This was seen for both with and without limestone addition. The addition of limestone gave a significant improvement of gas conversion at 950 degrees C, which could be explained by lime catalyzing the water-gas shift reaction. Moreover, the presence of limestone significantly increased the char conversion at both 950 and 1000 degrees C.