Evolution of Char Properties during Rapid Pyrolysis of Woody Biomass Particles under Pulverized Fuel Conditions

Rapid pyrolysis of mallee and pine wood particles (250-355 mu m) was conducted in a drop-tube furnace (DTF) at 1300 degrees C in argon atmosphere for collecting partially pyrolyzed char samples at various particle residence times (0.31-0.68 s). A novel feature of the DTF is its double-tube configuration that realizes direct determination of the char yield via experiments. This further enables investigations into the evolutions in the char yield, transformation of alkali and alkaline earth metallic (AAEM) species in chars, and char particle shape/morphology during rapid pyrolysis. The char yield decreases with the particle residence time but remains unchanged at 3.8 and 2.8% after 0.51 and 0.41 s for mallee and pine chars, respectively, indicating that the pyrolysis reactions are mostly completed. Similarly, the retention of AAEM species decreases with particle residence time, although the retention of AAEM species in pine char continues to decrease after 0.41 s. The continuous reduction in O/C and H/C molar ratios after 0.41 s suggests that the nascent char continues to experience thermochemical condensation reactions that lead to the scission of oxygen-containing groups. Chemical fractionation of mallee char with particle residence times of similar to 0.32 and similar to 0.51 s shows an increase in the amount of ion-exchangeable Na and K retained in partially pyrolyzed mallee wood and then a decrease as pyrolysis continues. These can be attributed to the transformation of water-soluble Na and K into ion-exchangeable Na and K, with partial release as a result of char deoxygenation. However, Mg and Ca, presented mainly in water-soluble or ion-exchangeable forms in the parent biomass, were increasingly transformed into acid-soluble or acid-insoluble forms. As pyrolysis progresses, the char particle length and diameter continue to shrink and char particles become increasingly spherical. Mallee wood shows stronger resistance to deformation because its original elongated shape is maintained, while pine char particles rapidly melt at the early stage of pyrolysis.