Researchers
in Sweden and Spain have devised a three-step process for the conversion of
precipitated kraft lignin from black liquor into green diesel.
The
kraft process converts wood into wood pulp for paper production. The process
produces a toxic byproduct referred to as black liquor—a primarily liquid
mixture of pulping residues (such as lignin and hemicellulose) and inorganic
chemicals from the Kraft process (sodium hydroxide and sodium sulfide, for
example). For every ton of pulp produced, the kraft pulping process produces
about 10 tons of weak black liquor or about 1.5 tons of black liquor dry
solids.
The magnitude of the recovery
process is often not fully appreciated. Globally over 1.3 billion tons per year
of weak black liquor are processed; about 200 million tons per year of black
liquor dry solids are burned in recovery boilers to recover 50 million tons of
cooking chemicals as Na2O, and to produce 700 million tons of high pressure
steam. This makes black liquor the fifth most important fuel in the world, next
to coal, oil, natural gas, and gasoline. Since black liquor is derived from
wood, it is the most important renewable bio-fuel, particularly in Sweden and
Finland.
The
new three-step process takes a much different approach.
1)First,
a mild Ni-catalyzed transfer hydrogenation/hydrogenolysis using 2-propanol
generates a lignin residue in which the ethers, carbonyls, and olefins are
reduced.
2)An
organocatalyzed esterification of the lignin residue with an in situ prepared tall oil fatty acid anhydride
gives an esterified lignin residue that was soluble in light gas oil.
3)The
esterified lignin residue is then coprocessed with light gas oil in a continous
hydrotreater to produce a green diesel.
This approach will
enable the development of new techniques to process commercial lignin in
existing oil refinery infrastructures to standardized transportation fuels in
the future.