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

Abstract

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 °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 μm). The limestone was calcined and sulfated to different levels (0, 25, or 40%) and mixed with the ilmenite particles (125–180 μm). 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. A part of the sulfur released from the fuel reacted with the lime to form CaS/CaSO 4 . Concerning the mixture of CaSO 4 /ilmenite, a further improvement was seen on the char conversion because of the release of SO 2 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 CaSO 4 /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.