Characterization of Size-Segregated Soot from Pine Wood Pyrolysis in a Drop Tube Furnace at 1300 °C

This study deploys a Dekati low-pressure impactor (DLPI) to separate soot from pyrolysis of pine wood in a drop tube furnace at 1300 degrees C according to its aerodynamic diameter for subsequent characterization. Under the experimental conditions, it was determined that the total amount of pine wood fed into the reactor must be 150 mg or less for effective soot separation without overloading the DLPI. The result shows that the carbonaceous materials with aerodynamic diameters <= 10 mu m (CM10) from pine wood pyrolysis have a unimodal distribution with a fine mode of 0.077 mu m. The carbonaceous materials with aerodynamic diameters between 1 and 10 mu m (CM1-10) are primarily char fragments but only account for <2% of the CM10. The CM with aerodynamic diameters <= 1 mu m (i.e., CM1) accounts for >98% of the CM10 and is soot. Further analysis shows that the key elements present in soot (i.e., CM1) are Na, K, and Cl because of the condensation of these inorganic species during the cooling of flue gas or as the result from the reactions between soot and alkali metals to form organic bonds, although these inorganic species contribute only to similar to 0.2% of the soot yield. The (Na + K)/Cl molar ratios of >3 in the segregated soot samples indicate that the alkali metals in soot mainly are not in the form of NaCl and KCl. The results also show similar to 85% of Na in soot is water-soluble while only similar to 12% of K in soot is water-soluble, indicating that soot preferentially reacts with K over Na to form an organic bond. Moreover, the contents of water-insoluble Na and K in incipient soot are higher than those in mature soot and soot clusters. Characterization using transmission electron microscopy shows that the soot samples have a disordered inner core(s) and clear fringe structure. There are slight increases in soot fringe and tortuosity during soot growth.