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VTT develops new process for full utilization of softwood bark

In Finland, the mechanical and chemical forest industry generates around three million tonnes of softwood bark annually, most of which is used for energy production. Using a method developed by VTT Technical Research Centre of Finland (VTT), a high yield of pure tannins can be extracted from the bark for use as a raw material for resins used in wood products. The residual fibre fraction can be used to produce sugars for fermentation into biochemicals and biofuels or for material applications.

Bark and woodchips in the fuel yard of a biomass-fired combined heat and power (CHP) plant in southern Finland.

Europe’s position in the production of biochemicals from biomass and by-products is limited to a few compounds, while their demand is among the largest in the world. However, Europe has a lot of world leader chemical companies. On the other hand, lignocellulosic waste constitutes one of the most abundant biomass sources and does not compete with the food chain.

Traditionally, tannins are extracted from the bark by hot water extraction. In addition to tree species, the extraction yield is influenced by the origin and processing history of the raw material. From Scandinavian spruce and pine, the yield remains at most 10 percent of bark by weight.

The utilization of bark residues as a source of sugar has been studied through enzymatic hydrolysis. Even in this process, the yields have not been very high, and consequently, much of the bark tannin and carbohydrates have remained unutilized.

New process developed

A new method for the total utilization of the bark has been developed by VTT Technical Research Centre of Finland (VTT) as part of a European project called “Systemic Approach to Reduce Energy Demand and CO2 Emissions of Processes that Transform Agroforestry Waste into High-Added-Value Products(REHAP).

Co-funded under the EU Horizon 2020 research and innovation programme, the project is being run under the EU’s Sustainable Process Industry through Resource and Energy Efficiency (SPIRE) programme.

The new process uses much higher alkaline conditions and a higher temperature, more closely resembling the wood cooking process. About one-third of the bark by weight can be dissolved and isolated as a tannin fraction with significantly fewer impurities – carbohydrates and ash – than in a fraction obtained by hot water extraction.

According to VTT, this tannin extract is a much more reactive resin raw material than normal kraft lignin. The same technology as used for lignin recovery is suitable for the recovery of the tannin. In addition to tannin, there is some lignin in the fraction.

In Finland, the mechanical and chemical forest industry generates around three million tonnes of softwood bark annually, most of which is used for energy production. Using a method developed by VTT Technical Research Centre of Finland (VTT), a high yield of pure tannins can be extracted from the bark for use as a raw material for resins used in wood products.

The fibre fraction can be hydrolyzed considerably more easily than after hot water extraction. The hydrolyzate is well-suited for fermentation. The purpose is to further test the suitability of a fibre fraction for material applications.

Part of an EU project

The 16 partner REHAP consortium aims to revalorize agricultural- and forestry waste –  wheat straw and bark respectively –  through its recovery, conversion into primary sugars, lignin and tannin components and then into secondary intermediary components such as sugar acids, carboxylic acids, aromatics, and resins. These bio-based building blocks can be used to displace fossil-derived components in materials and products.

BBEPP from Belgium (upscaling), FORESA from Spain (tannin fraction for resin) and BIOSYNCAUCHO from Spain (hydrolyzate fermentation) have participated in the further development and validation of the process and the resulting fractions. The project coordinator is Spanish TECNALIA.

More specifically the project aims to produce to bio-based intermediaries and biochemicals 1,4 and 2,3-Butanediol, ester-polyols, phenolic resins, polyurethane (PU) and modified hydrolysis lignin that will be used in products such as glulam boards, insulation foams, cement, and adhesives. This includes:

  • Isolation of tannins and carbohydrates from forestry waste to turn them into bio-phenolic resins for wooden panels and isocyanate-free polyurethanes (PU) for insulating foams, respectively.
  • Isolation of lignin and carbohydrates from agricultural waste to turn them into bio-phenolic resins for wooden panels and bio-superplasticizers for cement, and ester-polyol PU for adhesives, respectively.
  • Fire retardant lignin and sugar-based additives will be also developed.

Developed processing technologies (chemo/thermo/enzymatic and fermentation) will be optimized at pilot scale – technical readiness level (TRL) 6-7 – for further exploitation and replication of results. All products will be integrated into a prototype to demonstrate industrial applicability into the Green Construction sector.

Throughout the project, Life Cycle and Cost Assessment (LCCA), market analysis, business plan, waste management strategy and measures for future standardization will be implemented using a systemic perspective approach.

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