Comparative study of the inherent combustion reactivity of sawdust chars produced by TGA and in the drop tube furnace

Abstract

Thermogravimetric analysis (TGA) has been widely used to obtain inherent combustion reactivity data for biomass fuels. However, fundamental information relating such data to systems that operate under conditions closer to those encountered in pulverized fuel (PF) combustion is relatively scarce. This study therefore, compares the combustion reactivity of sawdust devolatilized under slow heating conditions by TGA and fast heating conditions in a drop tube furnace (DTF) representative of PF combustion. The implications of devolatilization temperature, heating rate and residence time were considered. It was established that the char burnout profiles of the DTF char prepared at 1300 °C and 600 ms was similar to the 700 °C TGA char although, the DTF char showed a lower maximum rate. Scanning electron microscope (SEM) images revealed that the TGA chars retained a cell matrix structure with visible macropore network. In contrast, the DTF chars displayed swollen surfaces with distributed pore networks, resulting in higher BET surface areas. The activation energy of the DTF chars was smaller than those of TGA char, demonstrating the presence of greater internal diffusion control. Finally, it is demonstrated for the first time that the compensation effect for biomass char is considerably less than that reported previously for coal chars.