A modified two-stage quartz reactor was designed to further investigate the mechanism of sulfur transformation based on the interactions between volatiles and char during brown coal pyrolysis at different temperatures. With the two-stage reactor, the active volatiles generated from the coal fast pyrolysis can be in direct contact with the nascent char produced from the coal temperature-programmed pyrolysis. The results show that the interactions between volatiles and char have a pronounced effect on the sulfur transformation in the upgrading process of brown coal by pyrolysis, with the effect being a function of temperature. The interactions between volatiles and char promote more sulfur into the gas phase at lower temperatures of below 600 degrees C, while remain more sulfur in char at higher temperatures of above 600 degrees C. The evolutions of H-2, CH4, CO and CO2 at different reaction stages play important roles for the formation of sulfur species in char, which are the main evidences to reveal the mechanism of sulfur transformation. The XPS data of chars obtained from the different types of experiments indicate that the distribution of sulfur forms on the char surface is closely related to sulfur transformation. A comprehensive mechanism of sulfur transformation due to the interactions between volatiles and char during brown coal pyrolysis was also explored. (C) 2012 Elsevier Ltd. All rights reserved.