Experimental study on sulfur-containing products in pressurised oxy-fuel pyrolysis of pulverised coal

Abstract

Although the contribution of the pressurised oxy-fuel combustion technology for improving energy conversion efficiency and reducing SOx emissions is well recognised, the mechanism of sulfur transformation in the initial pyrolysis step has not yet been elucidated because of the complexity of the pyrolysis process. Clarifying such scientific fundamentals and overcoming technological challenges can effectively advance the implementation of pressurised oxy-fuel combustion technology and realise cleaner production. Herein, the transformation mechanisms of sulfur among solid, liquid, and gaseous states during the pressurised oxy-fuel pyrolysis of pulverised coal were experimentally investigated. The analysis of the gaseous products indicated that the concentration of H2S, the most abundant gaseous product, decreased from 840.3 to 32.1 ppm as the operating pressure increased from 0.1 to 3.1 MPa. Similar trends were observed for CH3SH and SO2. Conversely, COS concentration first increased from 61.4 to 143.4 ppm, after which it decreased to 30.4 ppm, a level similar to H2S concentration. The decreasing sulfur concentration in the volatiles is attributable to the oxidation of low-valence sulfur to form high-valence sulfur and the subsequent absorption by alkali metals in the char residue. The sulfur concentration in liquid tar, which was determined by the competitive reactions of the cracking of heavy hydrocarbons and the polymerisation of light volatiles, first decreased from 6.25 to 3.17 mg/L, followed by an increase to 5.03 mg/L as the operating pressure increased. To further investigate the sulfur transformation, the physicochemical, morphological, and structural properties of pyrolysed char were characterised. The crystal phase, microstructure, average particle size, specific surface area, and surface elemental content of the char affected the distribution and conversion of organic and inorganic sulfur in the char residue. The total sulfur content in the char residue typically increased with operating pressure. Moreover, the effect of operating temperature on the sulfur-containing products was investigated. Based on the experimental results, a binary correlation fitting was applied to predict the various trends in different sulfur-containing gaseous products of pressurised oxy-fuel pyrolysis.