Valorization of bark for chemicals and materials: A review

The annual bark yield in Canada is as much as 17 million m3. Currently, more than half of the bark is incinerated or landfilled and the remainder of the bark is mainly used as a cheap source of energy in saw/pulp mills. Both bark incineration and landfilling can lead to environmental problems. Due to the abundance of ash in bark and the lower sintering point of bark ash than that of wood ash, the combustion of bark can lead to fouling which would damage the combustors. Bark contains a large fraction of extractives and lignin (up to 50 wt% on a dry basis), which can be utilized as a renewable source of chemicals, particularly aromatic chemicals. The technical routes and technologies on the valorization of tree barks for chemicals and materials are reviewed in this paper. These include direct utilization of bark for wooden panels, and extraction for extractives (mainly tannin) and their the application in resins and foam materials, as well as conversion of bark via thermochemical technologies, mainly phenolysis, direct liquefaction in alcohols and pyrolysis. Finally, some challenges and perspectives on the production of chemicals and materials from bark are discussed.