Bio-oil may be seen as a two-phase microemulsion. In the continuous phase
are the decomposition products of hollocellulose; in the discontinuous phase
are the pyrolytic lignin macromolecules. Hollocellulose is the fibrous residue
that remains after the extractives, lignin, and ash-forming elements have been
removed from the biomass. The same as crude petroleum oil, which is extracted
from the ground, pyrolysis liquid and biocrude contain tar as their heaviest
component.
Bio-oil is a class-3 substance falling under the flammable liquid designation
in the UN regulations for transport of dangerous goods (Peacocke and
Bridgwater et al., 2001, p. 1485).