Inert pyrolysis of stoker-coal fines

Abstract

This study provided information on the relative stability of nitrogen-, sulphur-, carbon- and hydrogen-containing species retained in the char during pyrolysis of the stoker fines. The presence of these principal constituents was determined by elemental analyses on the remaining char; this allowed a simple (but effective) means of correlating the loss of sulphur and nitrogen species against a variety of pyrolysis conditions. Inert pyrolysis experiments were performed in a CF-IR Lindberg tube furnace. Furnace temperatures studied ranged from 700 °C to 1300 °C while the residence time ranged from 1 to 40 minutes. The coal fines from typical stoker-sized coal included three size ranges (i.e. d < 0.16, 0.16 < d < 0.28 and 0.28 < d < 0.40 cm). The experimental results indicated the relative stability of the carbon species. Carbon evolution was least affected by an increase in temperature and relatively insensitive to changes in coal rank, although the highest-rank coal evolved the least carbon and the lignite coal evolved the most. Hydrogen evolution was also relatively insensitive to coal rank; however, the extent of evolution was large under all conditions tested and increased significantly with increasing temperature. Carbon evolution clearly approached a fixed, asymptotic value with increasing time and temperature, but hydrogen evolution was almost complete at 1100 °C. Nitrogen and sulphur both exhibited behaviour generally intermediate to that observed for carbon and hydrogen. Sulphur evolution consistently exceeded that of carbon but the dependence on temperature was essentially identical for both species. In contrast, nitrogen evolution at 700 °C was almost identical to that of carbon and relatively insensitive to coal rank. However, nitrogen evolution exhibited a stronger temperature dependence and at 1100 °C it far exceeded the evolution of the carbon species. Particle size was shown to have little influence on the evolution of the various species over the range tested in this study; therefore, the conclusions are relevant to stoker-coal fines in general.