Mechanistic insights into effect of feeding rate on soot formation during rapid pyrolysis of biomass model components in a drop-tube furnace at high temperature

Abstract

This paper reports the significant effect of feeding rate on soot formation during rapid pyrolysis of water-washed cellulose and acid-washed lignin (denoted as W-cellulose and A-lignin, respectively) in a drop-tube furnace at 1300 °C and a residence time of ∼0.75 s in argon. Soot produced during W-cellulose pyrolysis has a unimodal distribution, with only a fine mode that its modal diameter increases from 0.043 to 0.246 µm as the feeding rate increases from 40 to 280 mg/min. However, at feeding rates of 12–200 mg/min, soot produced from A-lignin pyrolysis has a bimodal distribution with two fine modes located at 0.077 µm and 0.246 µm, respectively. As the A-lignin feeding rate further increases from 200 to 280 mg/min, the fine mode at 0.077 µm disappears and the particle size distribution of soot becomes unimodal with only a fine mode with diameter of 0.246 µm. Increasing feeding rate increases the yield and particle size of total soot but decreases the yield of non-mature soot for W-cellulose at feeding rates < 280 mg/min and A-lignin at feeding rates < 40 mg/min. The results suggest that a high feeding rate produces high concentration of soot precursors and enhances the collisions for increased formation of incipient soot to large mature soot. As the feeding rate further increases to be more than 40 mg/min, the soot yield from A-lignin pyrolysis levels off, the particle sizes of overall soot decease and the yields of non-mature soot increase. Under such conditions, further results show that soot growth is limited by the availability of soot-forming carbonaceous materials (including small gas molecules e.g. C2H2 and polycyclic aromatic hydrocarbons).