At that time, German researchers decided to start the H2Wood project, with the aim of producing hydrogen fuel from wood – burning hydrogen only produces water as a by-product, and the gas can also be used in fuel cells, generating electricity directly.
The idea is that wood waste and old wood can be used to produce biohydrogen through biotechnological processes that are in line with the requirements of a circular economy.
The chosen route uses wood to produce sugar, and then bacteria genetically modified for this purpose feed on the sugar and produce hydrogen. This process also generates carbon dioxide (CO2), but this unwanted byproduct is immediately used to grow microalgae, which also produces hydrogen.
The proposal was successful at the laboratory scale, and now the team is working on building a pilot plant to produce hydrogen, a key step between laboratory and commercial scale. The plant is expected to start operating in 2025.
Biohydrogen
The biohydrogen production process begins with pre-treatment of old wood and waste.
First, wood waste, such as pallets or old garden fencing, is ground up and broken down into its basic components. To do this, the wood is boiled under pressure at a temperature of 200°C in a mixture of ethanol and water. Lignin, as well as adhesives, solvents and paints from wood waste, dissolves in ethanol, separating chemical contaminants from the wood fibers.
In the next step, the fraction of wood fibers remaining after boiling, cellulose and part of hemicellulose, are broken down into individual sugar molecules (glucose and xylose), which serve as food or substrate for hydrogen-producing microorganisms.
“Separating wood into its parts is a process that requires expertise. This is where we benefit from the many years of experience we have gained in building our lignocellulose biorefinery,” said Professor Ursula Schlesmann from the Fraunhofer Institute for Interfacial Engineering and Biotechnology.
To convert the produced sugar into hydrogen, the researchers created two linked fermentation processes using hydrogen-producing bacteria and microalgae.
Bacteria produce hydrogen and carbon dioxide. The carbon dioxide is separated from the gas mixture and transferred to the algae reactor, which is a photobioreactor. Microalgae use the gas as a carbon source and reproduce. Unlike bacteria, they do not need sugar. In the second step, the microalgae are transferred to a specially designed reactor, where they release hydrogen through direct photosynthesis.
Modular factory
The team expects a high yield in biohydrogen production: initially, about 0.2 kilograms of glucose can be produced from one kilogram of old wood. “We can then produce 50 liters of hydrogen using anaerobic microorganisms,” Schlesman said.
During fermentation with anaerobic bacteria, carbon dioxide is produced in equal proportions, i.e. 50%. Once the hydrogen is separated from the gas mixture, about two kilograms of carbon dioxide in the photobioreactor can produce one kilogram of microalgae biomass. This biomass contains up to 50% starch, in addition to the pigment lutein. For example, leftover algal biomass can be used in plastic components, again with the help of bacteria.
The pilot plant, which includes three bioreactors, is currently under construction, which is being implemented in modular form – meaning the plant will be expandable, simply by adding more reactors. Moreover, it will be possible to combine different process steps in a standardized way, which is an ideal prerequisite for testing new technologies and ways to increase the productivity of the process as a whole. Technological innovation
