RELEASE OF VOLATILE SULFUR-COMPOUNDS DURING LOW-TEMPERATURE PYROLYSIS OF COAL

The behaviour of sulfur structures in coal during low temperature pyrolysis has been studied. Nine low rank coals with high organic and pyritic sulfur contents were heated in a swept fixed bed reactor (N2) up to 850-degrees-C, and the evolved sulfur compounds were determined by sulfide electrode (H2S) and Fourier transform infrared (FT-i.r.) spectroscopy (SO2, COS). Evolution of H2S as a function of temperature passes through two peaks between 500-560-degrees-C and 630-700-degrees-C, related to the decomposition of organic and pyritic sulfur, respectively. Assignment of organic sulfur structures and pyritic sulfur to the two observed peaks of H2S evolution were tested using float-sink fractions with different mineral matter and pyrite contents (0.3-33 wt%). An important role of coal organic matter in pyrite decomposition to H2S was found. The evolution of COS with pyrolysis temperature, followed a trend similar to that for H2S. The evolved SO2 was related to the decomposition of iron sulfate from weathering of pyrite, but also with the presence of oxidation reactions during pyrolysis. The amount of sulfur removed from coal by pyrolysis varied from 35 to 64% of the total sulfur content for the assayed samples. Pyrolysis temperatures below 700-degrees-C were sufficient to obtain desulfurization levels close to 90% of the total amount of sulfur which can be removed by pyrolysis.