CSBE-SCGAB
Thursday, 18 September 2014 11:15

From the forest to the highway: Can wood pellets be a feedstock for future biorefinery?

We face significant challenges with the continued use of oil, from concerns about carbon emissions contributing to climate change to on-going depletion of finite oil reserves affecting the lifestyles of future generations. It is inevitable that we will have to evolve from a finite, hydrocarbon driven global industry to a more sustainable, “carbohydrate based” society. Being a forest rich province, transformation to bioeconomy provides enormous opportunities to British Columbia for the effective utilisation of its abundantly available forest biomass.

One of the consequences of climate change we had to face in BC was the mountain pine beettle infestation of the trees. This beetle generally plays an important role in the life of the forest, attacking old and weakened trees and promoting the growth of young forests. However, warmer summers and mild winters in BC resulted in the spread of the epidemic to more than half a million hectares in 1990. Growing imbalance on the climate change has caused a much vigorous spread by the end of the decade reaching approximately 15 million hectares of infested trees. This has created an abundance of wood beyond its traditional applications such as lumber or pulp and paper and created potential opportunities to use these biomass resources as a future feedstock for biorefinery.

Despite the abundant availability, one of the biggest challenges for the large scale processing of biomass is their low bulk density and high moisture content which limit their long distance transportation over long distances. It has been shown that in order meet IEA target of reducing the carbon emissions by 2050, we have to trade and process approximately 100 billion cubic meters of biomass, which is an order of magnitude higher than today’s global energy commodity infrastructure. This is where densification technologies can make a difference. Densification processes such as pelletisation can enhance the bulk density of biomass 4-6 times higher thus improving the tradability of biomass over long distances.

Twenty years ago, it would have been difficult to imagine the export of wood pellets from Canada to Europe or Asia. However, a combination of higher oil prices, relief from oil import dependency and carbon reduction strategies of several nations have made the co-firing of biomass for electricity production more economically and socially desirable. Together with these factors, densification of biomass substantially changed the economics of moving biomass around the globe. In the last 10 years, global wood pellets production has increased from zero to 16 million tonnes. Canada, together with the USA, is the world’s largest pellet exporters exceeding two million tonnes in 2013 with British Columbia being the dominant exporter of pellets as was triggered by the mountain pine beetle epidemic. Most of the Canadian wood pellets are exported to Europe over a distance of 16,000 km.

biomass-travel

 

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.

Dr. Linoj Kumar can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..

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).

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Linoj Kumar

BC regional director

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