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.