Thermodynamics on sulfur migration in CaSO₄ oxygen carrier reduction by CO

Abstract

CaSO₄ is an attractive oxygen carrier for chemical looping combustion(CLC) because of its high oxygen capacity and low price. The utilization of a CaSO₄ oxygen carrier suffers the problems of sulfur release, and deactivation caused by sulfur loss. With respect to the fact that partial sulfur release could be recaptured and then recycled to CaSO₄ by CaO sorbent, the mixture of CaSO₄-CaO can be treated as an oxygen carrier. Thermodynamics of CaSO₄ and CaSO₄-CaO reduction by CO have been investigated in this study. The sulfur migrations, including the sulfur migration from CaSO₄ to gas phase, mutual transformation of sulfur-derived gases and sulfur migration from gas phase to solid phase, were focused and elucidated. The results show that the releases of S₂, S₈, COS and CS₂ from CaSO₄ oxygen carrier are spontaneous, while SO₂ can be released at high reaction temperatures above 884 °C. SO₂ is the major emission source of sulfur at low CO/CaSO₄ molar ratios, and COS is the major part of the byproducts as soon as the ratio exceeds 4 at 900 °C. Under CO atmosphere, all the sulfur-derived gases, SO₂, S₂, S₈ and CS₂, involved are thermodynamically favored to be converted into COS substance, and are spontaneously absorbed and solidified by CaO additive just into CaS species, which may be recycled to CaSO₄ as oxygen carrier in the air reactor. But high reaction temperatures and high CO₂ concentrations are adverse to sulfur capture.

Keywords

Chemical looping combustion / CO₂ separation / CaSO₄ oxygen carrier / Sulfur migration / Sulfur capture

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