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 utilisation. There are some important characteristics of this liquid that are summarised in the table below.
Typical properties and characteristics of wood derived crude bio-oil
|Physical property||Typical value|
|HHV as produced||16-19 MJ/kg|
|Viscosity (40C and 25% water)||40-100 cp|
|Vacuum distillation residue||up to 50%|
|Ready substitution for conventional fuels in many stationary applications|
|Heating value of 17 MJ/kg at 25% wt. water, is about 40% that of fuel oil/diesel|
|Does not mix with hydrocarbon fuels|
|Not as stable as fossil fuels|
|Quality needs definition for each application|
The liquid has a distinctive odour – 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 miscible with polar solvents such as methanol, acetone, etc. but totally immiscible with petroleum-derived fuels.
The density of the liquid is very high at around 1.2 kg/litre compared to light fuel oil at around 0.85 kg/litre. 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 atomisers 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 40oC) 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 vaporised once they have been recovered from the vapour phase. If the liquid is heated to 100oC 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 bio-oil has 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.