The wood-processing industry produces large quantities of wood waste, which until now has either been disposed of at great expense or utilised for energy. Waste wood treated with wood preservatives can currently only be incinerated in special large-scale power plants, as the exhaust air has to be cleaned of the harmful substances. In the ‘H2Wood – BlackForest’ project, Fraunhofer researchers and their partners have now specifically sought alternative uses for wood waste in the Black Forest region, which is heavily characterised by wood processing.

Biohydrogen production from wood waste

The focus was on the use of regional residual and waste wood for the biotechnological production of green hydrogen and other bio-based substances such as lignin or carotenoids. Two linked fermentation processes with hydrogen-producing bacteria and microalgae were established specifically for this purpose in the project, which has been running since 2021.

Lignin obtained as a by-product

Wood waste such as old pallets or garden fences were first broken down into their basic components such as lignin, cellulose and hemicellulose in an ethanol-water mixture under high pressure. In addition, impurities from adhesives and varnishes were separated from the wood fibres. ‘When the wood is fractionated, the wood fibres are freed from lignin, which forms 20 % to 30 % of the wood cell wall substance alongside cellulose and hemicellulose. This lignin, as one of the by-products, can be used in a variety of ways – for example in composite materials. One application example is car panelling,’ says Ursula Schließmann, project coordinator and deputy institute director at the Fraunhofer IGB in Stuttgart.

CO₂ as a food source for bacteria and microalgae

According to the researchers, the cellulose contained in the wood fibre fraction was then broken down into individual sugar molecules such as glucose. The glucose was then fed to bacteria in the fermenter, where they produced hydrogen and CO₂. According to the researchers, the carbon dioxide separated from the gas mixture was in turn used to propagate microalgae in the photobioreactor.

‘The metabolic products of the bacteria, i.e. the supposed CO₂ waste stream, are therefore food for the microalgae and are not released into the exhaust air as a harmful greenhouse gas,’ explains the project coordinator. ‘The microalgae synthesise carotenoids or pigments from this under the influence of light as further by-products that can be utilised by various industrial sectors.’ In a further step, the microalgae were transferred to a specially developed reactor in which they release hydrogen by means of direct photolysis.