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Bio-oil – Upgrading

Bio-oil produced by the pyrolysis of lignocellulosic materials is among the most complex and inexpensive raw oils that can be derived from biomass and required upgrading prior to use. Typically, bio-oil consists of five major fractions: (i) water, 15 to 30% w/w, (ii) low-boiling oxygenated compounds, 8 to 26% w/w, (iii) phenols derivatives, 2 to -7% w/w, (iv) water-insoluble oligomers derived from lignin, 15 to 25% w/w, and (v) water- soluble products, 10 to 30% w/w.

Upgrading bio-oil to a conventional transport fuel such as gasoline, diesel, gasoline, methane, or liquefied petroleum gas (LPG) requires water removal, deoxygenation (by hydrocracking and/or hydrotreating), followed by product recovery (typically by distillation), which can be accomplished by integrated conventional refinery processes (Table B-28).

Table B-28 Simplified illustration of bio-oil upgrading.

Feedstock	Process (primary)	Product	Process (secondary)	Products
Biomass	Pyrolysis*	Bio-oil
			Hydrocracking
			(plus hydrotreating)	Methane
				Naphtha (gasoline)
				Kerosene (diesel)
			Gasification
			(synthesis gas)	Alcohols
				Hydrocarbons
*Often referred to as thermal cracking in the refining industry.

There is also interest in partial upgrading to a product that is compatible with refinery streams in order to take advantage of the economy of scale and experience in a conventional refinery. Integration into refineries by upgrading through cracking or hydrotreating is a viable option, but in such cases where the bio-oil is blended with a crude oil product, there may be incompatibility issues that arise.

Upgrading bio-oil to a conventional transport fuel such as diesel, gasoline, kerosene, methane, and liquefied petroleum gas (LPG) requires full deoxygenation and conventional refining, which can be accomplished either by integrated catalytic pyrolysis or by decoupled liquid phase hydrodeoxygenation. There is also growing interest in partial upgrading to a product that is compatible with refinery streams in order to take advantage of the economy of scale and experience in a conventional refinery, and the main methods are: (i) hydrodeoxygenation and (ii) catalytic cracking, in situ or ex situ gasification to synthesis gas followed by synthesis to hydrocarbon derivatives or alcohol derivatives.

Hydrothermal liquefaction under a hot-water environment has been proposed as an alternative process to provide better energy efficiency and unique characteristics of bio-oil and other related products compared to pyrolysis-based processes. The optimization of the operating parameters, including temperature, pressure, time, and catalyst, is crucial for improving the performance of these processes. In addition to bio-oil production technologies, several upgrading technologies based on catalytic approaches (e.g., hydrotreatment and esterification) have also been developed to further improve bio-oil quality for a variety of applications.