Testing of minerals and industrial by-products as oxygen carriers for chemical-looping combustion in a circulating fluidized-bed 300 W laboratory reactor

Abstract

Chemical-looping combustion (CLC) is a promising technology for future energy production with inherent CO2 separation. One approach is to use minerals or industrial by-products as oxygen carriers to reduce the costs of the process. This study focuses on the investigation of two iron-based oxygen carriers, which were examined under continuous operation in a 300W laboratory reactor. Ilmenite is an iron–titanium oxide mineral, whereas iron oxide scale (IOS) is obtained as a by-product from the rolling of sheet steel. Syngas was used as a fuel – pure and with steam addition to suppress the formation of solid carbon.During the experiments the variables reactor temperature, fuel flow and air flow were changed. Furthermore the effect of steam addition to the fuel was investigated. Particle properties were compared over the span of 85h of continuous operation for ilmenite and 37h for IOS. The analysis is based on gas measurements from the actual CLC operation, but also on scanning electron microscopy, X-ray powder diffractometry and measurements of BET surface area and density.With ilmenite oxygen carrier it was possible to achieve full conversion of syngas up to about 190Wth fuel equivalent at 900°C. With design fuel flow of about 300Wth at 900°C the combustion efficiency was above 98%. There was almost no visible difference in reactivity of fresh activated particles and those used for 85h. Combustion efficiency up to 99% was achieved with IOS oxygen carrier at 900°C and about 100Wth fuel equivalent. At 300Wth fuel equivalent and 900°C a combustion efficiency of only 90% could be reached.Both oxygen carriers were operated for tens of hours, which allowed for a better understanding of lifetime behavior and other basic characteristics. Whereas ilmenite oxygen-carrier particles were mostly stable over the course of 85h of experiments, a large fraction of IOS oxygen-carrier particles had disintegrated to fines after only 37h of experiments. The gathered data indicates that both oxygen carriers could be an alternative to synthesized particles, though with more drawbacks for IOS than for ilmenite.