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Transforming wood waste into green hydrogen and carbon dioxide


Mountains of wood waste could be broken down into hydrogen and carbon dioxide gas to reduce fossil fuel consumption, University of Canterbury researchers say.

Chemical engineers Shusheng Pang​ and Alex Yip​ are developing a novel way to make climate-friendly hydrogen, which is being touted as a replacement for natural gas. Today, green hydrogen is made using electricity, though about 20 per cent of this power is generated by burning fossil fuels.

The process would also capture lots of carbon dioxide, which could be used in plant nurseries, fertiliser factories and chemical plants instead of fossil fuels.

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Some greenhouses burn fossil fuels such as natural gas to create additional carbon dioxide – which boosts plant growth – and heat their facilities.

All plants breathe in carbon dioxide and store the carbon in their leaves and wood. Forests suck up carbon dioxide directly from the atmosphere.

The forestry sector produces approximately 3 million tonnes of wood leftovers, such as unwanted branches, each year. Often this is left behind in plantation forests – and during floods, the debris can wash downstream, causing damage, Pang said.

The forestry sector produces roughly 3 million tonnes of woody residue. Researchers want to transform it into green gas.

Piers Fuller/Stuff

The forestry sector produces roughly 3 million tonnes of woody residue. Researchers want to transform it into green gas.

The researchers see this material as an opportunity to create two products: climate-friendly hydrogen gas and carbon dioxide.

First, wood is chemically processed in a large machine – developed at Canterbury University over 15 years – to create a mixture of gases, including hydrogen, carbon dioxide, carbon monoxide and methane. Then, steam is added and the process converts, purifies and separates this gas mixture into hydrogen in one chamber and carbon dioxide in another.

To boost the creation and separation of the gases, Yip spent 10 years developing special materials, known as catalysts. These make the process more effective, “unlocking” the potential of the wood matter, he said.

Water molecules have two atoms of hydrogen, so by adding steam to the mix, the team is able to extract more hydrogen than is just contained within the wood. For every kilogram of wood, the process extracts about 70 grams of hydrogen.

But at the moment, this takes a long time, Pang said. “We need to get the process fast.”

The machine is currently as large as a two-storey building, Pang added. “For commercial production, we’ll need to increase the scale by 100 times – not the physical size, but the capacity.”

Hydrogen gas is a promising zero-carbon fuel. It can be burnt or chemically combined with oxygen, to release energy and water. It’s expected to replace fossil fuels in the production of lower-carbon steel as well as powering off-road vehicles.

Industry already manufactures plenty of hydrogen gas to create chemicals including methanol and fertiliser – though this is typically from natural gas. In 2019, domestic chemical production from natural gas released 1.8 million tonnes of greenhouse emissions into the atmosphere.

At the moment, the lowest-carbon way to make hydrogen is by using electricity to split water. The Ballance Agrinutrients factory is running a small pilot to make hydrogen via electrolysis, then fertiliser. But hydrogen facilities must build additional renewable generation, such as wind farms, or risk increasing the amount of coal and gas burned to create power.

In the longer-term, the university team’s work could provide a third pathway, for countries such as Aotearoa with large forestry sectors. Capturing and selling the carbon dioxide – because much more of this gas is made during the process – will help make the production economically competitive.

If all goes well, Pang estimates the system could produce hydrogen and carbon dioxide at scale in about a decade. “The potential for this technology and the product is huge.”

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