Sulfur Fate during the Lignite Pyrolysis Process in a Chemical Looping Combustion Environment

摘要

Chemical-looping combustion (CLC) is a novel technology with the feature of CO2 inherent separation in which the fuel is converted via lattice oxygen (instead of gaseous oxygen) provided by the oxygen carrier that circulates between two structurally interconnected but atmosphere-isolated reactors, i.e., fuel reactor and air reactor. In the fuel reactor of in situ gasification CLC (iG-CLC), the pyrolysis and gasification products of coal are oxidized by lattice oxygen in the O-2-free environment. Therefore, the characteristics of sulfur species evolution and distribution in the coal pyrolysis products are significantly different from those in the conventional combustion, gasification, and pyrolysis processes. In this study, a two-stage fluidized bed reactor was utilized to investigate the reaction between the oxygen carrier and in situ coal pyrolysis products, in which the coal and oxygen carrier particles are separately loaded in two reactors. In this way, the influence of oxygen carrier on the coal pyrolysis process could be eliminated. As obtained from the experiment, the distribution of sulfur species in coal pyrolysis products changed significantly after being oxidized by the oxygen carrier. To be more specific, the sulfur species were 70.1% H2S, 0.2% SO2, 0.8% COS, and 13.1% CS2, respectively, during the coal pyrolysis process in the blank experiment loaded with silica sand, whereas the concentrations of the sulfur species (in the same order) changed to 26.0%, 68.2%, 0%, and 0%, respectively, once the pyrolysis products went through the Fe2O3/Al2O3 oxygen carrier. The result indicates that most of the H2S, COS, and CS2 contents could be oxidized by the oxygen carrier to generate SO2 in the CLC environment. The sulfurous gas conversion rate at the CLC experiment was higher than that at blank experiment due to the fast evolution of sulfur in tar, which was also converted by oxygen carrier to enhance the sulfur conversion at the CLC experiment. Most of the H2S could be oxidized by the oxygen carrier to generate SO2 via the reaction H2S(g) + 9Fe(2)O(3) = 6Fe(3)O(4) + H2O(g) + SO2(g), and this has been confirmed by both experiment and HSC simulation. Moreover, scanning electron microscopy and energy-dispersive X-ray spectroscopy results indicated that no metallic sulfide was formed on the surface of reduced oxygen carrier.