(165f) Bio-Oil Microexplosions in the Thermochemical Refining of Biomass for Biofuels

Richard Hermann¹, Andrew Teixeira², Jake Kruger¹, Wieslaw Suszynski¹, Paul J. Dauenhauer², Lanny Schmidt¹

¹University of Minnesota, Department of Chemical Engineering and Materials Science, 421 Washington Ave. SE, Minneapolis, MN 55455

²University of Massachusetts, Department of Chemical Engineering, 686 North Pleasant Street, Amherst, MA, 01003

Fast pyrolysis is a promising technology for the conversion of lignocellulosic biomass to renewable liquid fuels and chemical products.  The technique involves rapid heating of biomass to 375-525 °C in the absence of oxygen for residence times of 1-2 seconds in order to convert the feedstock’s energy rich carbon backbone into a condensed liquid referred to as “bio-oil” [1]. It has been reported previously that convectively heating bio-oil droplets in a hot, gaseous environment may produce a violent boiling event termed a “microexplosion” in which the droplet is violently blown apart into many smaller secondary droplets [2-3].  Literature has observed both vaporization and microexplosion phenomena in bio-oil systems operating with temperatures from 500-900 °C in environments containing up to 24 mol % oxygen.  It has been suggested that microexplosions occur in multi-component bio-oil fuels due to the formation of a non-volatile shell and superheated volatile core [4], but this mechanism has not been confirmed.  We investigate the microexplosion mechanism by allowing a 5 µL droplet of pine-derived bio-oil to vaporize on an isothermal γ-alumina surface.  During vaporization, the disc was maintained at 500 °C under a constant nitrogen purge.  For these conditions, the bio-oil droplets were observed to evaporate for 1.5-2.5 seconds at which point they either dissipate into smaller drops and form surface char, or undergo a microexplosion event.  A combination of low and high frame rate photography (1000 fps) was used to determine microexplosion event times and to quantitatively assess the droplet vaporization rate. This analysis allowed for the determination of bio-oil vaporization adherence to the classical “D²-law” [5].  Further mechanistic insight was gathered by assessing the role of additives to the raw bio-oil. Specifically, microexplosion frequency and droplet lifetime was assessed for bio-oil/methanol dilutions, addition of solid particulate, and small pore size filtering (0.45 µm). The results generated from these treatment methods provide critical insight into the mechanism of microexplosions in bio-oils and provide potential solutions for either suppressing or enhancing the microexplosion phenomenon.

References

[1] G. W. Huber, S. Iborra, A. Corma, “Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering”, Chem. Review 2006, 106, 4044-4098.

[2] C. R. Shaddix, D. R. Hardesty, “Combustion Properties of Biomass Flash Pyrolysis Oils: Final Project Report” SAND99-8238; (1999).

[3] J. D'Alessio, M. Lazzaro, P. Massoli, V. Moccia, Symposium (International) on Combustion 1998, 27, 1915.

[4] V. Stamatov, D. Honnery, J. Soria, Analysis 2005, 17, 20.

[5] Godsave, G.A.E. Studies of the combustion of drops in a fuel spray: the burning of single drops of fuel. In: Proceedings of the Fourth Symposium (International) on Combusion (1953). Combustion Institute, Baltimore, MD, 818-830.