Effect of hematite addition to CaSO4 oxygen carrier in chemical looping combustion of coal char

Abstract

Chemical looping combustion (CLC) is a very promising technology, which combines the potential to reduce costs and the energy penalty significantly for CO₂ capture. For CaSO₄ oxygen carriers, their low reactivity and the evolution of sulfur species limit their practical application in CLC. In this study, CaSO₄ oxygen carriers decorated with hematite were prepared by a mechanical blending method with natural anhydrite as active support and hematite as additive. Experiments on the gasification and CLC of coal char in a steam medium were conducted in a laboratory-scale fluidized-bed reactor at atmospheric pressure. The effects of the reaction temperature, iron–sulfur ratio and cycle numbers on the performance of CaSO₄/Fe₂O₃ oxygen carriers were investigated in terms of carbon conversion and CO₂ yield as well as the rate of evolution of SO₂. Temperature and hematite favored an enhancement of the carbon conversion and CO₂ yield. The evolution of SO₂ increased with a rise in reaction temperature, but the peak time was delayed, which was ascribed to the sulfur-suppressing effect of hematite. The iron–sulfur ratio had slight influence on the product gas concentrations and there was no obvious decrease in the rate of evolution of SO₂ when the iron–sulfur ratio reached 0.13. Redox cycling tests showed that the cumulative rate of evolution of SO₂ in reduction/oxidation decreased first and increased subsequently with the increase of cycle number. Related chemical equations involving an oxygen transfer mechanism of iron-catalyzed reduction were proposed.