Industrial & Engineering Chemistry Research, Vol.52, No.11, 4059-4071, 2013
Mechanism Investigation of Enhancing Reaction Performance with CaSO4/Fe2O3 Oxygen Carrier in Chemical-Looping Combustion of Coal
CaSO4 as a promising oxygen carrier has a larger oxygen transport capacity. However, the formation of CaO in the CaSO4 reduction process by side reactions can cause sulfur species evolution and decrease its reactivity. An innovative method was investigated by means of adding hematite together with the CaSO4 to be combined oxygen carriers in CLC of coal. The objective was to decrease sulfur species evolution and increase the coal conversion. Experiments were performed in a batch fluidized-bed reactor at atmospheric pressure. Ten reduction/oxidation cycles confirmed the occurrences of side reactions toward sulfur species evolution, although the CaSO4 oxygen carrier showed an increasing reactivity in the initial cycles. With the addition of hematite, the gaseous sulfur species evolution was remarkably decreased in both reduction and oxidation processes during the cycles, while the coal conversion and CO2 capture were enhanced. The extensive physical and chemical properties of the oxygen carrier were characterized by X-ray diffractometer (XRD) analysis and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (SEM-EDX) analysis to detect the mechanism. It was not the desulfurization capacity of iron oxide that lowered sulfur species emission but the suppression of the side reactions by adding hematite.