A team from the University of Maine has developed a new method for the fast pyrolysis of lignin at atmospheric pressure and without catalysts into value-added chemicals and fuels. As reported in their paper in the ACS journal Energy & Fuels, the method provides improvements over conventional fast pyrolysis of lignin through the addition of calcium formate salt to the feed prior to fast pyrolysis.
Lignin, which is a significant component of woody biomass, represents a substantial product stream within pulp mills and forest biorefineries, the team notes. The lignin fraction of woody biomass can range between 15 and 30% by mass and 40% by energy.
Currently, only a small fraction of lignin is used for low-grade products, such as dispersants or binding agents, while the remainder is burned as fuel. Despite the recalcitrance of lignin to chemical transformations, there has been renewed interest in converting lignin to valuable chemical products, including gasification to fuel processes, nanostructured carbon-based materials, and liquid chemicals. Of the methods focused on liquid chemical production, pyrolysis represents a rather straightforward approach to breaking down lignin into smaller fragments through intermediate heating in the absence of oxygen and producing a liquid phase called bio-oil.
Despite some success in pyrolysis of lignins, there remain several significant barriers to advancing this technology, including continuous processing because of feeding/agglomeration issues and poor liquid yields.—Mukkamala et al.
The researchers found that the addition of formic acid in the form of a metal removed oxygen from the lignin during fast pyrolysis by deoxyhydrogenation, facilitated by the hydrogen generated during the co-decomposition of calcium formate in situ.
They achieved liquid yields as high as 33%, containing an oil product with a HHV of 41.7 MJ/kg and an O/C ratio of 0.067 and containing largely aromatics.
Initial data suggest that higher liquid yields could be possible, depending upon the formate concentration and optimizing residence times within the pyrolysis reactor. Additionally, feed/ agglomeration issues with the lignin appeared mitigated. Relative to other methods, formate-assisted fast pyrolysis does require the addition of formic acid to the process (calcium can be recycled). However, it is possible that other processes within a biorefinery can produce formic acid, and advantages include atmospheric pressure operation, continuous operation, and no catalysts. This method could be extended to other biomass feedstocks in upstream processing strategies.—Mukkamala et al.
Saikrishna Mukkamala, M. Clayton Wheeler, Adriaan R. P. van Heiningen, and William J. DeSisto (2012) Formate-Assisted Fast Pyrolysis of Lignin. Energy & Fuels doi: 10.1021/ef201756