Carbon removal and the biochar route

Blog Post

What carbon removal means, the main methods and where FENIX fits

Misplaced carbon

This time, our starting line is a little different. We normally pick up from biomass and talk about residues that might end up in landfills or incinerated, and we follow that waste all the way through its transformation into biochar. Today we take a detour and focus on the carbon side of it.

The first idea to establish is that carbon exists on our planet in a fixed quantity, distributed among different spaces: atmosphere, oceans, living beings, soils, rocks and fossil deposits. The issue is that this carbon has been distributed differently recently, and it is gaining territory in the atmospheric reservoir.

Where is carbon and what is going on?

Carbon has many roles on the Earth. It helps regulate the planet’s temperature, forms the chemical backbone of every living thing, makes up the food we eat, influences the acidity of the oceans, underpins the structure and fertility of our soils, and much else besides. [1]

Most of it is stored, but it does not stay put. It moves between those reservoirs continuously. A clear example is photosynthesis, the process in which plants use sunlight to combine carbon dioxide with water and build sugar molecules. When animals eat plants, this carbon is passed on to them, and it eventually returns to the soil or the atmosphere. That’s how carbon circulates.

Human activity in the last centuries has altered that circulation. The carbon in coal and oil was captured by living organisms tens to hundreds of millions of years ago and has been locked underground ever since. Burning it returns that carbon to the atmosphere in form of greenhouse gases such as CO₂ among others, in a very short span, at a pace faster than natural sinks can remove it. The result is a strengthened greenhouse effect, bringing rising temperatures, changes in rainfall, variations in sea level and disruption to ecosystems. [2]

Two sides of the carbon balance

There are two broad ways climate mitigation can change the amount of greenhouse gases accumulating in the atmosphere: by reducing the emissions going in, or by increasing the amount being taken out.

Reducing emissions means making a source of greenhouse gases smaller. This can mean burning less fossil fuel, preventing methane (CH₄) from being released during waste treatment, or replacing a high-emission fuel or process with a lower-emission alternative.[3] [4]

Carbon dioxide removal works on the other side of the balance. A human activity causes CO₂ from the atmosphere to be taken up and durably stored, strengthening a carbon sink.[5]

The two can happen within the same system. A process can remove and store atmospheric carbon while also avoiding emissions elsewhere. What matters is keeping those effects separate when assessing its climate benefit.

Carbon removal

Taking CO₂ out of the atmosphere is only the first step. For it to count as carbon dioxide removal, that carbon also needs to be durably stored.

The Intergovernmental Panel on Climate Change (IPCC), the United Nations (UN) body that assesses the scientific evidence on climate change, defines carbon dioxide removal as “anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products”.[6] It covers deliberate strengthening of biological, geochemical and chemical sinks, and excludes uptake not directly caused by human activities. Capture applied to CO₂ from fossil fuel use is not removal. But the same technology can form part of a removal pathway when what it captures is biogenic or drawn directly from ambient air, and is then durably stored.

The methods that meet this definition differ enormously in how they work and in how long they hold on to the carbon, and that is what the rest of this section is about.

The routes

There is no single way to classify carbon-removal methods. For its Sixth Assessment Report, the IPCC organises them according to how the CO₂ is removed and, separately, how long the resulting carbon storage can last.

Land-based biological methods use biological carbon uptake on land. They include afforestation and reforestation, soil carbon sequestration, biochar and bioenergy with carbon capture and storage.
Ocean-based biological methods enhance biological uptake and storage associated with marine and coastal systems, including ocean fertilisation and blue carbon management.
Geochemical methods accelerate reactions that remove CO₂ through minerals and ocean chemistry, such as enhanced weathering and ocean alkalinity enhancement.
Chemical methods use engineered processes to extract CO₂ directly from ambient air. Direct air carbon capture and storage, using approaches such as solid sorbents or liquid solvents, is the main example.

The same taxonomy separately considers storage timescales:

Figure 1. IPCC taxonomy on carbon dioxide removal [7]

How removal is judged

There is no official four-point test for whether one removal method is “good”, but several questions help determine how effective a route is.[8]

The first is how long the carbon stays there. Different methods store carbon over very different timescales, ranging from decades to thousands of years or longer. Shorter-term storage can still have a climate benefit, but the possibility that the carbon eventually returns to the atmosphere has to be taken into account.

The second is what can undo the storage. Carbon stored in vegetation or soils can be released through fire, land-use change, decomposition or changes in management practices. Other routes, such as geological storage, depend on suitable geology and careful operation to prevent leakage. What matters is therefore both the risk of reversal and how much continued management is needed to keep the carbon stored.

The third is the net climate benefit. Removing and storing carbon can itself generate emissions through energy use, transport, processing or other activities. These have to be accounted for when determining how much CO₂ the overall process removes on balance.

The fourth is cost and scalability. A route may store carbon for a very long time, but it also needs to be economically viable and capable of removing meaningful quantities of carbon.

Inside the FENIX system

FENIX works on one of the land-based biological carbon-removal methods described above: biochar. Plants first take CO₂ from the atmosphere and incorporate its carbon into biomass. In FENIX, green waste from the Bourges area, including branches and leaves that would otherwise be composted, is used to produce biochar through pyrolysis.

Part of that biogenic carbon is retained in the biochar in a much more persistent form. Once incorporated into soil, that carbon can remain there far longer than it would in the original biomass. It is estimated that biochar can persist in soils from decades to thousands of years, depending on the feedstock and how it is produced.[9]

FENIX then combines this biochar with digestate to produce a soil improver, testing different mixtures and doses under different soil and climate conditions. The project measures carbon storage and soil greenhouse-gas emissions at different stages of this work. Because some changes in soil carbon can take years to become detectable, it also uses models to simulate the effects of different application scenarios over more than ten years, including how carbon storage and soil emissions develop. In this way, FENIX can assess the carbon-removal route beyond the immediate effects observed after application and under different agricultural conditions.