In Kisumu, Kenya, start-up company Tera Carbon is collecting waste residues from sugarcane, heating this waste biomass without oxygen in a process called pyrolysis, then distributing the resulting charcoal-like substance to sugarcane growers, who mix this “biochar” into their soils. The outcome is that carbon dioxide – which had been removed from the atmosphere by the sugarcane as it grew in the fields of western Kenya – will be stored in the soil for thousands of years.

“It’s a simple process, and the carbon removal is immediate,” says Rob Palmer, Tera Carbon’s co-founder and CEO. Without the production of biochar, he explains, the sugarcane waste, known as bagasse, would be “either combusted or left to decompose, releasing the carbon that was in the plant back into carbon dioxide or methane”.

The technology is already playing a crucial role in carbon markets around the world. So far, biochar projects account for around 90% of the carbon removal credits that have been used to offset emissions and thus “retired” from the market.

Biochar projects are relatively cheap to operate, compared to technologies such as “direct air capture”, which rely on large-scale industrial processes. At the same time, biochar projects are designed to permanently store carbon in the soils. This “permanence” is a distinct advantage over projects that involve planting trees or other vegetation, given that a forest could be destroyed by fire, and it is hard to guarantee that carbon will be permanently stored in wood products that might be produced.

The other advantage of biochar is that the substance has significant benefits for soil health. Palmer notes that farmers across the continent typically struggle with access to the inputs they need to keep soils healthy and productive. “You get farmers either not applying anything to their soils or only being able to choose one of the inputs required,” he says.

“Generally, they’ve been told that chemical fertiliser is the way to go – and it is, in the short term. You see a short-term boost in yield, but you’re unable to balance the downstream negative effects,” he says.

In the region of Kenya where Tera Carbon has launched its project, Palmer says that a sugarcane plant should last for around 25 seasons – but, with increasingly degraded and acidic soils that struggle to retain water, the plants are now lasting for just four seasons.

Biochar can help address these problems when it is applied to soils. The material helps soils retain nutrients, while also improving water retention and encouraging the beneficial activities of micro-organisms. A trial conducted by the Dubai-based International Center for Biosaline Agriculture found that applying 30 tonnes of biochar to a hectare can improve the soil’s water content by 40%.

“If we can get the sort of yield responses that we have seen in the literature – we have seen from trials of anywhere between 20% and 30% yield increases – and we can increase the longevity of these plants, that’s going to be huge,” says Palmer. “And if we can do that, not just in Kenya, but in the whole of Africa, this could have massive impact.”

Tera Carbon’s project in Kenya is just one of a growing number of schemes cropping up across the continent. Katrin Mikolajewski, team leader for certified projects at Atmosfair, a German non-profit that operates carbon offset projects, says that companies purchasing carbon credits often specifically favour biochar as a means of removing carbon. “There is demand from companies for negative emissions,” she says.

Atmosfair has established a project in Ghana, in partnership with Dutch organisation Carboneers, to supply small-scale pyrolysis units to farming communities so they can convert crop residues into biochar. The aim is to remove 13,000 tons of carbon dioxide annually – equivalent, Atmosfair says, to the emissions from flying across the Atlantic in economy class 12,000 times.

Mikolajewski believes there is scope for many similar projects on the continent. “Africa has a lot of biomass residues readily available,” she says. “There’s a lot of capacity and potential to scale up.”

Atmosfair’s project in Ghana is designed at the community level. Unlike industrial-scale biochar production facilities, its pyrolysis units are based on simple technology and are relatively cheap to install. “The sweet point of these decentralised projects is really that the benefits for the farmers and for the people are more in focus,” says Mikolajewski. Farmers, many of whom are women, gain an added source of income for producing biochar. “Basically everybody can participate.”

At the other end of the spectrum, producing biochar on an industrial scale requires significant capital investment in pyrolysis plants. As well as Tera Carbon, another start-up seeking to pioneer biochar production is B10 Char. Founded by two former commodity traders, the company has acquired land in South Africa and invested around $2m in a pilot pyrolysis unit that will produce up to 7,000 tonnes of biochar a year from over 22,000 tonnes of feedstock from timber waste products. The company says that this will equate to the net removal of around 15,000 tonnes of carbon from the atmosphere.

Philip Edmonds, the company’s co-founder, says B10 Char hopes to build further units on the same site. “If this plant is successful, we can put another four or five plants on the same site of identical size. It’ll be a cut and paste job.” He notes that the “biggest challenge” is in ensuring that farmers make use of the biochar.

“Farmers are, let’s say, a conservative species. Quite rightly, they’re not going to accept some new wonder-drug that goes onto their land without trying it for at least one growing season, maybe two growing seasons, on a small part of their farm, in the corner somewhere with a control to make sure that it really is improving.”

B10 Char plans to generate most of its revenues from carbon credits, at least initially. It will sell its biochar at low prices and may give it away for free in some cases to demonstrate the benefits for farmers. Edmonds also notes there are other potential uses for biochar. It can be used in anaerobic digesters to help increase production of biogas, a renewable energy source. Another possibility in South Africa is to use biochar during mine reclamation work, in which soil that was excavated and then left to degrade when a pit was dug is restored after the mine’s closure. There are also possible uses for biochar in steel and concrete production processes. Biochar can even be added to asphalt to improve the quality of road surfaces.

While Edmonds believes biochar is a “low hanging fruit” for carbon removals, he stresses the need for projects to operate on a much bigger scale. “The world as a whole needs to scale up this business by multiple orders of magnitude,” he points out.

The 15,000 tonnes of carbon that B10 Char aims to remove annually with its pilot plant is, he admits, a tiny drop in the ocean of what is required. Researchers at the University of Oxford warned last year that 7bn to 9bn tonnes of carbon dioxide will need to be removed from the atmosphere each year to keep the world on track to limit warming to 1.5 degrees.