Soot formation during rapid pyrolysis of bio-oil and its fractions in a drop-tube furnace at high temperatures

This paper studies the soot formation during rapid pyrolysis of bio-oil and its fractions in a drop-tube furnace at 1300 degrees C. Formulated samples are prepared to represent the water-soluble faction of bio-oil. Micromorphology analysis results show that particles with aerodynamic diameter less than 1 mu m are soot, while those with aerodynamic diameter between 1 and 10 mu m are spherical chars. The soot yield from the pyrolysis of the waterinsoluble fraction (i.e., pyrolytic lignin) is similar to 77 mg/g, much higher than those for bio-oil (similar to 31 mg/g) and the formulated water-soluble fraction (similar to 23 mg/g). To understand the synergies between the water-insoluble and the water-soluble fractions of bio-oil, the soot yield is also estimated based on the soot yields of these individual fractions and their respective contents in bio-oil, and then compared with the soot yield of bio-oil. Such interactions are found to have an insignificant impact on soot formation during pyrolysis. To understand the roles of organics and water in soot formation, water is removed from the water-soluble fraction for pyrolysis. Organics in the water-soluble fraction show a positive effect on soot formation when pyrolysing with pyrolytic lignin, leading to a similar to 50 % increase in the soot yield. In contrast, water greatly suppress the soot formation during pyrolysis, resulting in a similar to 47 % reduction in the soot yield when water is added to the mixture of organics and pyrolytic lignin. Analysis of average soot cluster size shows different degrees of soot coagulation and agglomeration during the pyrolysis of bio-oil and its fractions. Soot coagulation and agglomeration are heavily dependent on the soot concentration in the gas phase, since soot concentration reflects the collision chance among soot particles. Water in bio-oil can significantly decrease the average size of soot clusters through decreasing the soot yield and suppressing soot coagulation.