Characteristics and Mechanism of Soot Formation during the Fast Pyrolysis of Biomass in an Entrained Flow Reactor

摘要

To understand the effects of biomass origin and temperature on the characteristics of soot formation from biomass, fast pyrolysis of wheat straw and sawdust was performed in an entrained flow reactor in the temperature range of 900-1300 degrees C. The produced soot, permanent gas, and tar were sampled and characterized by transmission electron microscopy (TEM) and gas chromatography/gas chromatography-mass spectroscopy (GC/GC-MS) with respect to yield, morphology, structure, and composition. Moreover, soot formation was modeled in a plug flow reactor (PFR) with a detailed reaction mechanism. Results indicate that the woody biomass produced a significant higher quantity of soot (0.34%-6.30% (dry biomass, db)) than that from straw (0.28%-2.40% (db)), and the woody soot has a more ordered structure. The reason for this was primarily ascribed to the combined effects of high contents of lignin, cellulose, and low content of ash in woody biomass. The carbonization of soot occurred at about 1100 degrees C when primary spherical particles were formed with concentrically stacked graphitic layers. All of the collected tar species were deoxygenated aromatic components, of which benzene and naphthalene were the characteristic species. The aromatic species in sawdust tar were much more heterogeneous than those of straw. Due to the soot formation reactions occurring above 900 degrees C from the secondary decomposition of light CxHy gases, carbon conversions of the two biomasses declined. When the temperature reached 1100 degrees C, the CO generation reactions were strengthened remarkably, which caused the carbon conversion to increase again. A reasonable agreement between the observed and the predicted soot yield was obtained. In this studied case, the HACA (hydrogen abstraction carbon addition) route is the dominant route for soot formation, while the contribution from CPDyl (cyclopentadienyl) dimerization is small.