PNNL

Abstract

The liquid formed by the fast pyrolysis (FP) of biomass is commonly termed bio-oil. Biomass is converted to bio-oil through FP at 400-550°C typically at 1 atm. During FP, the biomass is held at temperature for a residence time on the order of 1-2 seconds. FP is beneficial from the standpoint that a large portion of the total mass and carbon contained in the biomass is converted and retained in the liquid bio-oil fraction. However, bio-oil is acidic, viscous, and thermally unstable. Bio-oil must be upgraded in order to be considered as a liquid transportation fuel or separated and further treated to produce value-added chemicals. Bio-oil is generally regarded as difficult to upgrade due to its thermal instability and chemical complexity which is derived in part from the incorporation of highly reactive species such as aldehydes and terminal olefins as well as the presence of sugars and anhydrosugar species which undergo secondary polymerization reactions when heated. Upgrading strategies considered in this chapter include methods to stabilize and deoxygenate bio-oil through low temperature hydroprocessing to stabilize bio-oil followed by higher temperature “deep hydrotreating” to remove the majority of oxygen present; converting bio-oil catalytically (often with zeolites such as HZSM-5) prior to condensation with catalysts inside the FP reactor or in a separate fixed or fluidized bed after the FP reactor; catalytic cracking with acid catalysts such as zeolites after condensation; or through physical or chemical separations to improve bio-oil stability, extract valuable chemicals, or produce H¬2. Bio-oil upgrading by co-processing with refinery streams such as vacuum gas oil in a fluid catalytic cracking (FCC) unit in a refinery has also been investigated. Numerous excellent reviews, perspectives and other chapters have been published in the preceding 7 years.[1-10] Reviews specific to individual sections within this chapter are at the beginning of the individual sections. This chapter is intended as an update to the original article authored by Prof. Anthony V. Bridgwater, Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power, First Edition, Edited by Robert C. Brown, © 2011 John Wiley & Sons, Ltd. We gratefully acknowledge the original contribution by Prof. Bridgwater and have kept much of his original discussions intact with updated references. The scope of this updated chapter includes references published since about 2010. In the interest of space many of the original references included in the original version were not included. Thus, Prof. Bridgwater’s original chapter should be viewed as a companion to this updated chapter to provide a holistic set of references to the numerous bio-oil upgrading processes discussed herein.