Biochar carbon capture

Biochar is created by the pyrolysis of wood and other biological material. Heating the biological material to very high temperatures in the absence of oxygen causes the substances within them to break down into gases, leaving just carbon.

This is how charcoal is created from wood.

It’s interesting from a climate change point of view as it converts wood - a natural accumulation of atmospheric carbon - into a solid, stable form of carbon which can be buried or stored.

By controlling the process, you can extract either more heat energy or more carbon from the wood. In this way, it provides an interesting way of paying for itself - by extracting a portion of the energy contained in the wood, and burying the rest, you reduce the overall amount of carbon in the ecosystem.

I wanted to understand this method of carbon capture. Can it capture all our emissions or just a fraction? How much land would be needed? Can it pay for itself with generated energy? Does the carbon even stay in the ground long term?

Why don’t we just plant trees?

There’s been a lot of talk in the media recently about tree planting, proposing it as a way to offset carbon dioxide emissions.

This just isn’t an answer. The carbon which is absorbed by trees while growing is released again when they die - either through rotting or burning. Simply planting trees will not have a long term effect on atmospheric carbon dioxide.

We’ve spent the last 200 years digging up carbon from deep underground and releasing it. The only way we can reduce this again is by returning that carbon to a stable, non-atmospheric form.

Carbon captured by trees

In the UK, growing poplar accumulates about 7.3 tonnes of carbon per hectare per year. Other species would accumulate significantly less - 3.6 tonnes for Sitka spruce, 2.4 tonnes for beech. In the tropics this could be as much as 10-15 tonnes per hectare per year.. The carbon makes up about half the weight of the wood (the rest being other elements - including lots of water).

The time the trees would grow depends on their species. Once the system was established you’d only need to harvest and pyrolyse a fraction of the trees each year - 26 years (harvesting 1/26th per year) for poplar, 55 years for sitka spruce, 92 years for beech.

A good estimate might be 6 tonnes of carbon, or 12 tonnes of wood (which is 30% water - so 8.4 tonnes of dry wood), per hectare per year in the UK. Carbon dioxide weighs 3.67x as much as pure carbon - so about 22 tonnes of CO2 per hectare per year in the UK.

What does pyrolysis give you?

This study from China looks at the production of briquette charcoal from sawdust. Sawdust was pyrolysed at various temperatures and the liquid and gas products produced were measured.

They found that pyrolysis left somewhere between 60% (at 250 ℃) and 20% (at 950 ℃) carbon by weight. The rest of the wood turned into liquids and gases. The liquid was a complex mix of oils, while the gas was primarily methane. At higher temperatures, gases dominated - eventually up to 60% by weight.

Wood is a complex mixture of cellulose, hemicellulose, lignin and other things. Pyrolysing causes those to break down, and the hydrogen atoms to escape in the form of hydrocarbons. What’s left is the biochar - made up of a lot of organic carbon and some hydrogen. A value often used to assess biochar is the H/Corg ratio - the ratio of hydrogen to organic carbon. A lower number means more carbon, and is generally achieved through higher pyrolysis temperatures.

A good estimate is 25% of your incoming wood turning into pure biochar, containing around 40% of the carbon in the tree.

The rest of the carbon would be in the gases and liquids you extracted. Some would be used to power the pyrolysis process, and the rest could be used to generate electricity or power vehicles. This would release CO2, returning it to the carbon cycle. It would be a carbon neutral fuel - that carbon was only captured from the air as the tree grew, so isn’t adding to the amount of carbon in the ecosystem.

So 1kg of dry wood turns into 250g of biochar and 750g of hydrocarbon fuel. Let’s say 10% of that would be needed to power the pyrolysis process, that leaves 675g of burnable fuel.

Using the biochar

Then we need to get rid of our biochar. The most straightforward method is to mix it in with soil - which also has benefits for soil quality. How long it stays stable in soil depends how much hydrogen is left. With an H/Corg ratio under 0.4 the biochar is thought to stay in the soil for more than 1000 years. With a ratio of 0.2 that number goes above 2000 years.

Adding biochar to soil also boosts fertility (PDF).

How much can be sequestered

If we take that biochar - which is in a relatively stable form of carbon - and either bury it or turn it into the soil, we’re moving that carbon out of the ecosystem. If our trees absorbed 22 tonnes a year, 40% of that is 8.8 tonnes CO2 per hectare per year.

Global CO2 emissions in 2019 were around 33 gigatonnes (that’s 33 billion tonnes).

So to absorb the entire CO2 output of humanity, we’d need to be growing, harvesting, pyrolysing and burying trees over about 3 billion hectares, which is 30 million square km.

That’s a big area. About 20% of all the land in the world, or 66% of all world forests. Even assuming a 66 year growth cycle, you’d still need to be cutting down, pyrolysing and replanting ~1% of the world’s forests each year.

But our process is also yielding biofuels at around 5.67 tonnes per hectare per year. If every kg of our biofuel means one less kg of fossil fuels being extracted, we need to sequester less carbon. If we turned our 3 billion hectares into fuels, we’d have about 17 billion tonnes of biofuels each year. The world used about 13,975 million tonnes of oil equivalent (MTOE) in 2019 - 3 billion tonnes less than our theoretical pyrolysis yield. In theory if we pyrolysed 1% of the worlds forest each year, we’d be carbon negative by the same amount that we’re carbon positive today.

Realistic amounts of carbon

Of course it’s unrealistic to actively cultivate 66% of the world’s forests. What if we just pyrolyse the wood products which are already being burned or discarded.

Just in the EU there’s already over 1 billion tonnes of wood and plant matter being burned. If we pyrolysed that instead, we’d be taking 942 million tonnes of CO2 out of the ecosystem each year. That’s about 3% of global emissions.

And we have all the technology and knowledge to do this today.

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