Bio-crude is a black, thickened liquid that can be solid at room temperature and readily separates from its aqueous product following production. It has an elemental composition related to its original biomass feed, although the bio-crude can be somewhat depleted in oxygen and other hetero-atoms due to condensation reactions occuring during the hydrolytic process. For this reason, the molecular weight profile is typically higher than for bio-oils, as the light fragments and pyrolytic sugars readily polymerize during thermal processing.
The liquid is formed by thermolysis of the solid biopolymeric structure of the biomass when subjected to between 250C to 370C in a pressurized, liquid solvent environment. The solvent can be water, recycled bio-crude, or other ionic solvents that are functionally stable at the reaction conditions. It can provide an ionic environment to aid in the thermolytic chemistry and serve as a medium in which to dissolve homogenous catalysts, such as alkali that are frequently used to favor base-catalyzed condensation reactions at lead to more aromaticity and lower oxygen content in the product.
Although it is produced in the presence of water (either as solvent, in the original biomass, or from condensation reactions), the bio-crude readily separates from a water phase and typically contains only 5 to 10% water.
However, while bio-crude is closer in properties to petroleum than are bio-oils, it is not a directly analogous to crude petroleum. The remaining oxygen from the biomass contributes chemical functionality to the bio-crude that is not present in petroleum. This results in the presence of functional groups such as acids, alcohols, ketones, phenolics, methoxy-phenolics, and larger molecular structures derived from the condensation polymerization such as naphthols and benzofurans. However, the remaining chemical functionality is not as pronounce as it is in bio-oil, as the bio-crude is relatively stable over time and when exposed to moderate heat, as it is more of an equilibrium product than bio-oils.
This stability provides a specific advantage, in that it can be heated to processing, filtration, or catalytic conditions with relatively little impact to the functionality or the molecular weight of the bio-crude, in contrast to the thermal instability of bio-oils. This allows for subsequent hydroprocessing to be performed without significant polymer fouling of catalysts.
Characteristics of example bio-crudes are presented in the table below.
Typical properties and characteristics of bio-crude compared to fuel oil or bio-oils. (Table adapted from both Billing in PyNe 39 and Oasmaa et. al, Energy Fuels, 2012, 26(4), 2454-2460.)
|Analysis||Bio-Crude, algal||Bio-oil||Fuel oil|
|C, dry (wt%)||79||56||85|
|H, dry (wt%)||10||6||11.1|
|O, dry (wt%)||3.8 (by dif.)||38||1|
|Water (wt%)||7||20 to 30||0.025|
|Nitrogen (wt%)||5||0 to 0.4||0|
|Sulfur (wt%)||1.4||0 to 0.05||0.2|
|Viscosity @40C||245||13 to 35||3.0 to 7.5|
|Density @15C||0.99||1.10 to 1.30||0.89|
|LHV (MJ/kg)||39 (HHV)||13 to 18||40.3|
|Ready substitution for conventional fuels in many stationary applications|
|HHV of 39 MJ/kg which is higher than for bio-oil|
|Does not mix with hydrocarbon fuels|
|Quite viscous, but nearly as stable as petroleum oils|
|Contains more sulfur and nitrogen, commensurate with biomass origin|