INFORM November/December 2025

16 • inform November/December 2025, Vol. 36 (10)

Usually, a single-phase water emulsion in oil is produced but that emulsion can be easily broken if the water content is high enough. The organic phase contains less than 10 percent moisture whereas the aqueous phase is mostly water with dis solved light organic oxygenates. The most abundant perma nent gases produced during biomass pyrolysis are CO and CO 2 , with lesser amounts of methane, hydrogen and light hydrocar bons (ethane, ethylene, propane, propylene, and butanes). PYROLYSIS PARAMETERS Biomass pyrolysis can be considered slow or fast depending on the temperature, pressure and residence time to maximize products. Process temperatures are usually between 450 °C and 650 °C. For maximum biochar the recommendation is a slow pyrolysis process at lower temperature, higher pressure, and a long residence time (hours) performed in kilns or retorts. Maximizing liquid bio-oil yields requires a fast pyrolysis process using short vapor residence times (seconds) at temperatures up to 500 °C and atmospheric or sub-atmospheric pressure. Fast pyrolysis processes involve high heat transfer rates, on the order of 1000 °C per second, that result from injecting ambient temperature biomass into a high temperature reac tor for less than one second. Increasing the temperature up to 700 °C or higher in a fast pyrolysis process results in greater vapor cracking and higher pyrolysis gas production. BIO-MASS COMPOSITION In general, if it is in the feedstock it will end up in the bio-oil. There are many databases available that contain large librar ies of biomass elemental and chemical compositions. The average elemental composition of biomass is 51 weight per cent carbon, 41 weight percent oxygen, 6 weight percent hydrogen with a balance of nitrogen and sulfur (https://doi. org/10.1016/j.fuel.2009.10.022). The largest variation in the biomass elemental composition is in the sulfur and nitrogen contents, but they are present in low concentrations that can be significant depending on the bio-oil application. Most biomass is 25 percent lignin and 75 percent carbo hydrate, a combination of cellulose and hemicellulose. Woody biomass tends to have higher lignin content than herbaceous biomass (grasses, stover, and straws) and certain biomass, like softwood, can have a high extractives content. Extractives include materials like waxes, fatty acids, resins, gums, and tannins. Another highly variable component of biomass is the ash content. For most debarked woody biomass, the ash content is less than 1 weight percent while feedstocks like rice straw or rice hulls can contain up to 20 weight percent ash. While ash has little impact on the pyrolysis process, some ash com ponents act as catalysts promoting char formation or vapor cracking. Green wood can be up to 50 weight percent water but thermochemical biomass conversion processes include a dry ing step to control the moisture content of the feedstock. Feedstocks and process conditions do impact bio-oil characteristics and compositions, but in a non-selective way.

Technology developers use catalysts and reagents in the pro cess strategically to control a bio-oil’s physical properties and chemical composition. CATALYTIC FAST PYROLYSIS For catalytic fast pyrolysis, a technology developer uses cat alysts like zeolites, metal oxides, or solid acids to control the chemistry of the vapors and produce a higher quality bio-oil. Quality in this case is broadly defined as less acidic and more thermally stable. The catalysts promote deoxygenation of pyrolysis vapors by hydrodeoxygenation (removing oxygen as H 2 O), decarboxylation (removing oxygen as CO 2 ), and decar bonylation (removing oxygen as CO) to destroy acidic compo nents and increase the hydrocarbon content of the bio-oil. The goal is to improve the thermal stability of bio-oils for direct use or upgrade them to transportation fuels using conven tional petroleum refining technology like hydrotreating and hydrocracking. The various fast pyrolysis pathways are summarized below. Catalytic biomass pyrolysis processes can be performed in-situ if the biomass and catalyst are mixed directly in the primary conversion reactor or ex-situ if the pyrolysis vapors interact with the catalyst in a secondary downstream reactor. The in-situ process has the advantage of fewer reactors and a lower capital cost. The ex-situ process avoids potential catalyst poisoning by separating the biomass ash from the catalyst and provides increased flexibility with independent temperature control in each reactor. In addition to catalysts, reagents or gases act as another variable in the biomass pyrolysis process. Hydrogen is added most often. In combination with an appropriate catalyst, it enhances the hydrodeoxygenation pathway for pyrolysis vapor deoxygenation. Increasing the pressure of the catalytic fast pyrolysis process with hydrogen achieves almost complete deoxygenation. Each fast pyrolysis process has strengths and weak nesses, so selecting the appropriate pathway depends mainly on the downstream use of the bio-oil. Deoxygenation of bio mass pyrolysis vapors is useful for upgrading bio-oil to fuels. However, complete deoxygenation may be counterproductive if the intention is to isolate specific bio-based chemicals from

Fast Pyrolysis

Vapor Phase Upgrading

Fast Pyrolysis

Catalyst Added

Catalytic Fast Pyrolysis

Biomass + Heat

Bio-oil

H ₂ Added

Reactive Catalytic Fast Pyrolysis

High Pressure

Hydropyrolysis

Summary of pyrolysis pathways.

Other

Nitrogen Compounds

Phenols

PAH

Monoaromatic

Furan

Carbonyl

Aliphatic

Alcohol

Acid

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