Thermal gravimetric analysis (TGA) with crucibles of different shapes and sizes are used to evaluate reactions of volatiles generated from pyrolysis of two coals based on the understandings obtained from decomposition of CaCO3 and Polyvinylpyrrolidone. The TGA experiments were carried out in a flow of argon at a heating rate of 10 degrees C min(-1). The decomposition of CaCO3 shows an increase in derivative thermal gravimetric (DTG) peak temperature with an increase in the sample height or in the free space volume in the crucibles, which are attributed to the reverse reaction between the volatile product CO2 and the solid product CaO. The depolymerization of Polyvinylpyrrolidone supported on a coal-derived coke shows a decrease in DTG peak depth with an increase in the sample height or in the free space volume of the crucibles, indicating that the volatile intermediates generated from the deploymerization are reactive to form solid. The DTG curves of the two coals are very similar. They show a main pyrolysis peak which varies little with changes in the sample height and in the free space volume in the crucibles, suggesting that the extent of volatiles' reaction to form solid in the crucibles is very limited. (C) 2014 Elsevier B.V. All rights reserved.