Evaluation of the Effect of Sulfur on Iron-Ore Oxygen Carrier in Chemical-Looping Combustion

Chemical-looping combustion (CLC) is an emerging combustion technology that can be used to meet the demand of energy production with inherent separation of CO2. Because of the presence of sulfur contaminants in fossil fuels, the gaseous products of sulfur species and the interactions between these sulfur contaminants and the oxygen carrier are significant concerns in chemical-looping combustion. Experiments on chemical-looping combustion of a sulfur-containing gaseous fuel with iron ore as the oxygen carrier were performed by thermogravimetric analysis and Fourier transform infrared (TGA-FTIR) spectroscopy. The effects of reaction atmosphere (N-2 and CO2), H2S concentration, and pressure on the reactivity of iron ore in the presence of H2S were investigated. The evolution of gaseous sulfur species was also investigated in both N-2 and CO2 atmospheres. With a higher concentration of H2S in the gaseous fuel, the weight loss was slower, and a weight gain was even observed due to the sulfidation of iron ore. Sulfidation of iron ore was observed in both N-2 and CO2 atmospheres, and elevated pressure contributed to a higher sulfidation rate. Compared with N-2 atmosphere, CO2 atmosphere gave higher concentrations of COS and an initial SO2 peak but a lower concentration of CS2. Furthermore, the effect of sulfidation on the structure of the iron ore was investigated in a fluidized bed, and scanning electron microscopy with energy-dispersive analysis by X-rays (SEM-EDAX) and X-ray diffraction (XRD) were utilized to characterize the iron ore. The sulfidation of iron ore caused a decrease in both surface area and pore volume, and the porous surface of the oxygen carrier became smoother and almost imperforate. FeS was the only iron sulfide observed during the sulfidation process. The vulcanized iron ore could be regenerated with air calcination treatment, and the addition of H2O in the gaseous fuel could prevent the sulfidation of iron ore. The addition of CaO in the oxygen carrier was effective in mitigating the sulfidation and reducing the emission of gaseous sulfur species.