Moisture and Heating Rate Effects on Volatile Matter Yields for Coals Available to Purchase

Currently there are two ASTM standard test methods used around the world for the determination of volatile matter in coal. ASTM D3175, Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke, was first published as ASTM D22-16, Laboratory Sampling and Analysis of Coal, in 1916 and uses a minimum volume vertical furnace. ASTM D7582, Standard Test Methods for Proximate Analysis of Coal and Coke by Macro Thermogravimetric Analysis, uses a macro thermogravimetric analyzer (TGA) system. Macro TGA systems use samples that are one gram or larger, whereas micro TGA systems use only milligram-size samples. In both test methods, the furnaces are heated to a final temperature of 950°C and include a seven-minute hold time before final mass measurements. For nearly four decades, fuel scientists have tried to reconcile the differences between the coal volatile matter yields obtained using either a preheated (950°C) minimum volume vertical furnace (ASTM D3175) or a TGA system. This article describes experiments designed to help explain the differences in volatile matter yields from coals with the major focus being the moisture content in the samples analyzed. Parameters studied include predrying coal samples, using different heating rates, and using different intermediate temperatures in two-step procedures for the measurements. Factors including the moisture content and plastic behavior of coals, both of which contribute to the differences between the volatile matter values determined by different methods, will be discussed.