Influence of Lime Addition to Ilmenite in Chemical-Looping Combustion (CLC) with Solid Fuels

The influence of calcined and sulfated limestone addition in an oxygen carrier bed of ilmenite has been investigated for chemical-looping combustion (CLC) with solid fuel. The experiments have been performed in a laboratory-batch fluidized-bed reactor where the sample was exposed to alternative oxidizing and reducing conditions at atmospheric pressure and 970 degrees C. In each reducing period, 0.2 g of petroleum coke was introduced to a bed of 20 g of ilmenite and 5 g of limestone particles (180-250 mu m). The limestone was calcined and sulfated to different levels (0, 25, or 40%) and mixed with the ilmenite particles (125-180 mu m). During the reducing period, the mixture was fluidized with a flow of 600 mL(n)/min of 50% steam in nitrogen, and during the oxidation period, the mixture was fluidized with a flow of 600 mL(n)/min of 5% O-2 in nitrogen. Mixing CaO with ilmenite increased the conversion of the gas as well as the conversion rate of the char. Apart of the sulfur released from the fuel reacted with the lime to form CaS/CaSO4. Concerning the mixture of CaSO4/ilmenite, a further improvement was seen on the char conversion because of the release of SO2 from the sulfated limestone particle. However, this beneficial effect disappeared after 10 cycles. Thus, the tests did not show any lasting positive effect from using a mixture of CaSO4/ilmenite compared to CaO/ilmenite. The reason for the improved gas conversion with the addition of CaO to ilmenite seems to be the catalytic effect of CaO on the water gas shift reaction, converting CO to H-2, with the latter being much more reactive toward ilmenite.