Comparison of the Combustion Reactivity of TGA and Drop Tube Furnace Chars from a Bituminous Coal

Although thermogravimetric analysis (TGA) is a widely accepted technique for assessing coal combustion conflicting trends have often reported when reluding appaient TGA reactivities to pulverized fuel (PF) burnor conditions. Therefore, this paper compares the reactivity of chars generated in a drop tube furnace(DTF) to those from TGA. The implications of devolatilization temperature, heating rate and residence time are considered. For the smaller particle size ranges of tile bituminous coal investigated (ATC.), optimized devolatilization procedures were used to generate corresponding TGA burnout rates between the two char types. However, with) fractions of > 75 pill, the DTF chars showed an increased burnout propensity when moving from combustion regime 11 to combustion regime III. Scanning electron microscope (SEM) images and internal surface areas indicate that this is because of incompatible char morphologies, Thus, while chars produced under the conditions of TGA pyrolysis strongly resemble raw coal and display all undeveloped pore network; the DTF chars are highly porous, extensively swollen and possess considerably larger internal surface areas, Subsequently, char burnout variability was quantified, with the reactivity distribution for the DTF samples found to be up to an order of magnitude more significant than for the TGA chars. This is attributed to a fluctuating devolatilization environment on the DTF. Finally, a TGA study ob,observed a robust particle size based compensation effect for the TGA chars, with the relative reaction rates and activation energies demonstrating the presence of internal diffusion control. However this phenomenon was partly alleviated for the DTF chars, since their higher porosities reduce mass transfer restrictions. Moreover, it should be realized that DTF char fractions of < 35 mu m, including thow required to ensure true intrinsic control under the investigated burnout conditions, cannot be produced directly. This is because of bridging and sloughing in the DTF's screw-feeder. Instead, such samples must be created by grinding larger particles, which destroys the char's existing porosity.