Bio-oil

Bio-oil (center), char (left), and pine biomass (right) – Photo PNNL

Key Points

• Bio-oil, or pyrolysis oil, is a liquid biofuel produced by biomass pyrolysis:  rapid heating and rapid quenching of biomass to form a liquid.
• As liquefied biomass it is more easily pumped; stored; fed to useful processes; and more compatible to chemical modification, processing, or extraction.
• Bio-oil, is a liquid emulsion of oxygenated organic compounds, polymers, and water
• Bio-oil has properties that are dissimilar to petroleum oil, including
  • Primarily comprising polar molecules, it is not miscible with polar petroleum oils
  • Contains water up to 20 to 30%
  • The compounds contain oxygen, up to 40% by weight, thus lower heating value
  • Acidic and reactive compounds that can react over time, faster with heat
  • Higher density than water
  • Often contains solid inorganics and carbon char.

Further Reading

• Pyne 41:  Funke on comparing bio-oils; Mullen on isolating phenols from bio-oil
• Pyne 40:  Leijenhorst on inorganics in oils; Sandstrom/Johansson on bio-oil fraction composition;
• Pyne 38:  Oasmaa on oil stability; Feng/Meier on CO2 extractable compounds; Sanna on catalytic bio-oil

Bio-oil – Pyrolysis Liquid

Crude pyrolysis liquid or bio-oil is dark brown and approximates to biomass in elemental composition. It is composed of a very complex mixture of oxygenated hydrocarbons with an appreciable proportion of water from both the original moisture and reaction product. Solid char may also be present.

The liquid is formed by rapidly quenching and thus freezing the intermediate products of flash degradation of hemicellulose, cellulose and lignin. The liquid thus contains many reactive species, which contribute to its unusual attributes. Bio-oil can be considered a micro-emulsion in which the continuous phase is an aqueous solution of holocellulose decomposition products, that stabilizes the discontinuous phase of pyrolytic lignin macro-molecules through mechanisms such as hydrogen bonding. Aging or instability is believed to result from a breakdown in this emulsion. In some ways it is analogous to asphaltenes found in petroleum.  Fast pyrolysis liquid has a higher heating value of about 17 MJ/kg as produced with about 25% wt. water that cannot readily be separated.  While the liquid is referred to as ‘bio-oil’, it will not mix with any hydrocarbon liquids.  It is composed of a complex mixture of oxygenated compounds that provide both the potential and challenge for utilization. There are some important characteristics of this liquid that are summarized in the table below.

Typical properties and characteristics of pyrolysis liquids derived from biomass versus a fuel oil. (Table adapted from Oasmaa et. al, Energy Fuels, 2012, 26(4), 2454-2460.)

* Note: Measurement of flash point of bio-oil is problematic due to water evaporation and difficulty in sustaining flame.

The liquid has a distinctive odor – an acrid smoky smell, which can irritate the eyes if exposed for a prolonged period to the liquids.  The cause of this smell is due to the low molecular weight aldehydes and acids.  The liquid contains several hundred different chemicals in widely varying proportions, ranging from formaldehyde and acetic acid to complex high molecular weight phenols, anhydrosugars and other oligosaccharides.

The liquid contains varying quantities of water, which forms a stable single phase mixture, ranging from about 15 wt% to an upper limit of about 30-50wt% water, depending on how it was produced and subsequently collected.  Pyrolysis liquids can tolerate the addition of some water, but there is a limit to the amount of water, which can be added to the liquid before phase separation occurs, in other words the liquid cannot be dissolved in water.  It is mostly miscible with polar solvents such as methanol, acetone, etc. but immiscible with petroleum-derived fuels.

The density of the liquid is very high at around 1.2 g/ml compared to light fuel oil at around 0.85 g/ml.  This means that the liquid has about 42% of the energy content of fuel oil on a weight basis, but 61% on a volumetric basis.  This has implications on the design and specification of equipment such as pumps and atomizers in boilers and engines.

The viscosity of the bio-oil as produced can vary from as low as 25 cSt to as high as 1000 cSt (measured at 40^oC) or more depending on the feedstock, the water content of the oil, the amount of light ends that have been collected and the extent to which the oil has aged. Viscosity is important in many fuel applications (Diebold et al. 1997).

Pyrolysis liquids cannot be completely re-vaporized once they have been recovered from the exit gas of pyrolysis, some of which can already be aerosol at relatively high temperature.  If the liquid is heated to 100^oC or more to try to remove water or distil off lighter fractions, it rapidly reacts and eventually produces a solid residue of around 50wt% of the original liquid and some distillate containing volatile organic compounds and water.  While some bio-oils have been successfully stored for several years in normal storage conditions in steel and plastic drums without any deterioration that would prevent its use in any of the applications tested to date, it does change slowly with time, most noticeably there is a gradual increase in viscosity.  Recent samples that have been distributed for testing have shown substantial improvements in consistency and stability.