Sulfur transformation during internals-regulated pyrolysis of coal and waste tires: Characteristics and mechanisms

Internals-regulated pyrolysis, characterized by co-current heat and volatile flow, shows great potential for industrial application, but its sulfur transformation behavior is still unclear. This study systematically compares sulfur transformation in internals-regulated and traditional pyrolysis using waste tires and Shenmu coal as feedstocks. For both feedstocks, internals-regulated pyrolysis retains more sulfur in char and tar while reducing sulfur retention in pyrolytic gas across the entire temperature range, compared with traditional pyrolysis. The cracking of sulfur-containing oil into gaseous sulfur species is significantly suppressed in internals-regulated pyrolysis, resulting in lower gaseous sulfur yield and higher liquid-phase sulfur yield. In addition, a two-stage reactor was used to investigate the influence of temperature profiles (high-to-low in internals-regulated versus low-to-high in traditional pyrolysis) on sulfur transformation during volatile-char interactions. When volatiles from the first stage pass through a high-temperature char bed, H 2 S release increases and char sulfur content decreases, due to temperature-enhanced desulfurization by reducing gases (H 2 and CO). In contrast, when volatiles pass through a low-temperature char bed, H 2 S release decreases and char sulfur content increases, primarily due to the adsorption of H 2 S by the char matrix. Based on these findings, a mechanistic framework for sulfur transformation in both pyrolysis modes is proposed.