Densification processes such as pelletisation increase the bulk density of biomass four to six times higher. The process involves compressing biomass particles of 2-3 mm size through the holes of a die in the pelletiser to form the cylindrical shaped pellets. During the process, the friction between the biomass particle and the walls of the die generates heat which raises the temperature in-situ above the glass transition temperature of lignin. Lignin is believed to soften and facilitate the binding of the biomass particles during the process. The resulting compacted, dense, drier wood pellets are current being used as a feedstock for thermochemical conversion applications such as heat and electricity.
The global trade in wood pellets continues to grow. Despite being a globally traded biomass commodity, their potential as a feedstock for large scale cellulosic ethanol production has not been evaluated. A typical biochemical based biomass-to-ethanol process involves the three main steps of, pretreatment, to recover most of the hemicellulose sugars in the water soluble fraction while enhancing enzyme accessibility to the water insoluble cellulosic component, enzymatic hydrolysis of the cellulose to glucose, followed by fermentation of both the cellulose and hemicellulose derived sugars to ethanol. Unlike the suitability of wood pellets for thermal applications, it was believed that wood pellets would be a recalcitrant feedstock for biochemical conversion process.
Researchers at the Department of Chemical and Biological Engineering, UBC has looked at the possibility of using wood pellets as a feedstock for an enzyme based biochemical conversion to produce simple sugars. A typical biochemical based biomass-to-ethanol process involves the three main steps of, pretreatment, to recover most of the hemicellulose sugars in the water soluble fraction while enhancing enzyme accessibility to the water insoluble cellulosic component, enzymatic hydrolysis of the cellulose to glucose, followed by fermentation of both the cellulose and hemicellulose derived sugars to ethanol. Forest Products Biotechnology/Bioenergy group has been working on improving and optimising all these process steps over the last 20 years, but largely utilising the low dense biomass substrates such as wood chips, saw dust, straw etc.
Prior to doing the work, the researchers anticipated that conditions to make stable and transportable wood pellets will make it harder for biochemical bioconversion. However, when the softwood pellets and softwood chips were subjected to the same set of steam pretreatment and enzymatic hydrolysis, the results were quite surprising. The team found that wood pellets could be processed as effective as “less dense” wood chips! At the optimised process conditions, greater than 80% of the original carbohydrates could be recovered from the wood pellets as monomeric sugars which can be potentially fermented to fuels and chemicals. Subsequently, they have also looked at using steam pretreatment as a process for conditioning the wood pellets for enhancing their transport properties including the durability and strength. Even more surprisingly, the work has shown that steam pretreatment can be integrated into a single process step to both enhance the wood pellet properties and facilitate a direct bioconversion of the resulting wood pellets to monomeric sugars. Now UBC researchers are applying this strategy on a range of other biomass feedstocks including agricultural residues and hardwoods. With the finding that wood pellets can be used as a biorefinery feedstock, the industry expects new markets for British Columbian wood pellets in the global biomass industry including the high value applications such as the production of sugars and fuel ethanol.
Project team: Dr. Linoj Kumar, Dr. Shahab Sokhansanj, Dr. Jack Saddler, Dr. Fahimeh Yazdanpanah, Mr. Bahman Ghiasi, Dr. Zahra Tooyserkani, Dr. Tony Bi and Dr. Jim C Lim of UBC-Chemical and Biological engineering and Mr. Staffan Melin (Wood Pellet Association of Canada).