Biomass – an underestimated source of green hydrogen

Researchers want to produce hydrogen from regional wood waste in the future. Green waste and sewage sludge can help produce green hydrogen for the energy and transport transition. If chipboard or MDF panels are used, they must first be freed from adhesives. Then, however, the regenerative energy carrier can be used by local businesses and energy suppliers. Biogenic hydrogen would have the potential to cover the energy needs of the industrial and heavy transport sectors – a real wild card for the energy transition.

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A climate-neutral circular economy based on wood would have many advantages, in the Black Forest region (Schwarzwald) for example, where wood is the most important economic good. During its processing into furniture and building materials or during the demolition of buildings, considerable amounts of wood scrap accumulates. The disposal usually costs even more money. Up to now, waste wood and scraps have generally been utilized in wood-burning systems for energy.

As early as the summer of 2021, the region in southern Germany has been pursuing a new path: Out of the wood waste is to emerge green hydrogen. “Following a bioeconomic approach, with the aid of biotechnological processes, we want to produce climate-neutral biohydrogen as well as additionally usable substances, such as carotenoids or proteins, from waste wood and wood scraps,” says Ursula Schließmann. She works at the Fraunhofer Institute for Interfacial Engineering and Biotechnology (Fraunhofer IGB) and coordinates the joint project H2Wood – BlackForest.

By using scrap wood, CO₂ can be saved in two ways. On the one hand, renewable biohydrogen can substitute existing fossil fuels. On the other hand, scrap and waste wood will provide not only hydrogen. Through the new biotechnological approach, the energy recovery of the wood waste is combined with a material utilization. “The CO₂ released from the wood is bound in the form of carbon-based co-products,” explains Schließmann, “This way, it is fed back into the natural carbon cycle.”

So far, however, no plant yet exists that produces biohydrogen on a large scale. At Fraunhofer IGB, the processes required for this are now being prepared and investigated, before they are implemented in the pilot plant at the digital engineering center Campus Schwarzwald in Freudenstadt.

The German education ministry (BMBF) is funding the project in the Schwarzwald until mid-2024 with around 12 million euros. Partners of the project are, in addition to Fraunhofer IGB, also the Fraunhofer Institute for Manufacturing Engineering and Automation (Fraunhofer IPA), the research center IFF of Universität Stuttgart (Institut für industrielle Fertigung und Fabrikbetrieb) and Campus Schwarzwald.

Remove adhesives and varnishes

The first step and prerequisite for biotechnological conversion is a pretreatment. Because wood waste like particleboard or MDF contains adhesives such as resins and phenols or even varnishes. These chemical components would have to be removed, because only then could bacteria and microalgae do their work, the researcher explained. In addition, the wood must be broken down into its building blocks so that the obtained cellulose can be split into individual sugar molecules, which in turn serve as food for the H₂-producing microorganisms.

For the biotechnological conversion of the wood sugar, Fraunhofer IGB is relying on a fermentation process with bacteria, which metabolize the various sugars into CO₂, organic acids and ethanol. The metabolic products of the bacteria serve as nutrients for the microalgae. These synthesize carotenoids or proteins as co-products and also release hydrogen in the process.

The idea that green hydrogen has the potential to meet the energy needs of the industrial and heavy transport sector of a region is supported by the current study by Fraunhofer IPA “Industrielle Wasserstoff-Hubs in Baden-Württemberg” (industrial hydrogen hubs in the German state of Baden-Württemberg). Its conclusion: Decentralized hydrogen production and use pays off if distribution centers, known as hubs in neo-German, are strategically placed and connected in the right way. Electrolyzers in these hubs would then be operated with green electricity. To keep transport costs low, the centers must be close to the consumers. Another criterion: The industry at the site must have a need for process heat, high-temperature processes and hydrogen gas, for nitrogen fertilizer production for example.”

“Ideal locations are near busy roads with truck depots where H₂ refueling stations can be set up” says Jürgen Henke from Fraunhofer IPA. With the help of the location criteria, the research team was able to identify suitable sites in Baden-Württemberg. Particularly in the metropolitan regions Rhein-Neckar Karlsruhe. Computer simulations made at Fraunhofer IPA show that 30 percent of fossil energy can be replaced with regionally produced green hydrogen within ten years – and that’s if only building on free state-owned spaces.

Project: Hydrogen from plant waste

Along with wood, green waste is a largely untapped resource. Around 4.6 million tonnes was collected in the brown garbage bins of German residents alone in the previous year, according to the national environmental agency (Umweltbundesamt). Added to the waste from public parks, gardens, agriculture, food production, sewage sludge and cafeteria leftovers – all in all, a good 15 million tonnes.

The majority ends up in composting facilities or is incinerated to generate heat and electricity. “But the green waste is much too good for that,” stressed Johannes Full, head of the sustainable development of biointelligent technologies group at Fraunhofer IPA, “it would be more pragmatic to generate hydrogen from it and to capture the resulting CO₂, store it or use it in the long term.”

How that works Fraunhofer IPA is demonstrating at a business from the metal industry. There, waste from fruit and wine growers in the surrounding area, cardboard and waste wood as well as cafeteria waste can be converted into hydrogen. This is then used directly in the metal processing. To do this, the fruit scraps and cafeteria waste are first fermented with the help of bacteria in dark containers, thus generating H₂ and CO₂. Then, the fermented mass is brewed in a biogas plant to make methane.

Light flashes decompose banana peels

At the technical university TH Lausanne in Switzerland as well, a team of researchers, led by Hubert Girault, is converting biomass into hydrogen – by means of photopyrolysis. In a reactor is a xenon flash lamp that emits high-energy light. The team has tried it with banana peels, gnawed corn cobs, orange peels, coffee bean skins and coconut shells. These were first dried at 105 °C for 24 hours and then milled.

The researchers put the powder into the reactor at ambient pressure. Then, the xenon lamp sends flashes into the biomass, which turns into hydrogen and biochar. The process is complete after only a few milliseconds. From every kilogram of biomass are obtained about 100 liters of hydrogen and 330 grams of biochar. That corresponds to about one third of the mass of the dried banana peels started with.

The young Swiss company H2Valais now wants to employ this process on a large scale. The photopyrolysis, however, is competing with the hydrothermal gasification of biomass that startups like SCW Systems in the Netherlands and TreaTech in Switzerland are using. In this, wet biomass is subjected to a pressure of 250 to 350 bar and a temperature of 400 to 700 °C. Under these conditions, methane and hydrogen are formed within a few hours. This shows once again: There are a diversity of approaches for obtaining H₂ from biomass. This potential for the energy transition should be unlocked sooner than later.

Containerized unit turns pellets into pure H₂

The joint project BiDroGen is likewise realizing the goal of converting wood into hydrogen. The companies BtX Energy and A.H.T. Syngas Technology is getting 630,800 euros of funding for it from the German economy ministry. The project aims to develop to market maturity a compact container solution for the decentralized production of hydrogen from pelletized wood waste.

The basis is the already existing gasifier technology from BtX to separate pure hydrogen from mixed gases. The goal is to accordingly maximize the hydrogen content of the wood gas produced from the pellets through innovative catalysts, to guarantee the gas purity for downstream processes and to enable separation of H₂ from the product gas stream. This is how very pure hydrogen is to be obtained from pelletized scrap wood. One kilogram of pure hydrogen can be obtained from 12 to 15 kg of wood, depending on the gas quality. That corresponds to an efficiency of over 50 percent.

The mobile container solution would then provide decentralized green hydrogen. For application, the company sees great potential particularly in rural areas. For the clean transport transition, it could be a very useful card in the deck, as municipalities could procure hydrogen-powered vehicles right away, even if there is no hydrogen refueling station in the region yet.

Author:

Niels Hendrik Petersen