Pyrolysis oil is a synthetic fuel under investigation as substitute for petroleum. It is extracted by biomass to liquid technology of destructive distillation from dried biomass in a reactor at temperature of about 500°C with subsequent cooling. Pyrolytic oil (or bio-oil) is a kind of tar and normally contains too high levels of oxygen to be a hydrocarbon. As such it is distinctly different from similar petroleum products.
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Biomass is split into solid and gaseous components under the influence of heat only (anhydrous pyrolysis). The solid component, charcoal, may be used for heating the process, a soil additive (biochar), or as activated carbon in absorption processes. The non-condensable gaseous component, consisting of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2) and methane (CH4), may be burned. The condensible gases, however, may be rapidly cooled to form condensate droplets, which can then be separated from the non-condensable gases due to the substantial difference in density of each fraction. The condensate may be reignited similar to #2 fossil fuel. The heating value is 15-22 MJ/kg.
The oil produced in a pyrolysis process (bio-oil) is acidic, with a pH of 1.5-3.8 (2.8). The acidity may be lessened by the addition of readily-available base compounds. Little work has been done on the stability of bio oil acidity that has been altered with base compounds. While the exact composition of bio-oil depends on the biomass source and processing conditions, a typical composition is as follows: Water 20-28 %; Suspended solids and pyrolitic lignin 22-36 %; Hydroxyacetaldehyde 8-12%; Levoglucosan 3-8 %; Acetic acid 4-8 %; Acetol 3-6 %; Cellubiosan 1-2 %; Glyoxal 1-2 %; Formaldehyde 3-4 %; Formic Acid 3-6%. [1].
The water molecules are split during pyrolysis and held separately in other compounds within the complex pyrolysis liquid. This distinction is significant, as the "water" in pyrolysis oil does not separate like standard fossil fuels. The density is approximately 1.2-1.3 (1.22) kg/l or 10.01-10.85 (10.18) lbs/gallon, which is much higher than that of diesel. The oxygen content is 40-50% (mostly from the "water" content) and no sulfur may be detected normally. The lower heating value is approximately 16-21 (17.5) MJ/kg. The pour point is -12°C to -33°C, and no cloud point could be observed until -21°C. The carbon residue is 17-23 % wt (0.13% ash). The flash point is 40-100°C, and the pyrolysis oil is not auto-igniting in a diesel engine. The cetane number is only 10. The viscosity increases to a maximum in period of 12 months due to polymerization. The pyrolysis oil is not stable reacting with air and degasing. Pyrolysis oil cannot be blended with diesel [2].
It may be very attractive, from an economic standpoint, to first recover valuable bio-chemicals from the bio-oil. The remainder of the bio-oil can then be combusted to generate electricity or converted to a syngas, from which chemicals and clean fuels can be synthesized [3].