Why biochar as a permanent carbon sink is essential for achieving climate targets

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New research proves a half-life of approximately 100 million years for Biochar Carbon Removal (BCR), thus turning the permanence discussion and the role of biochar. These findings could also revolutionize the classification and resulting prices of carbon removal credits.

Carbon sinks are part of the carbon cycle and have played a major role in the Earth's climate since time immemorial. A carbon sink is a carbon reservoir that absorbs carbon and thus reduces the CO₂ concentration in the atmosphere. Against the backdrop of man-made climate change and the urgent need to reduce CO₂ in the atmosphere, they are of particular importance. 

"Carbon sinks are indispensable for a successful climate protection policy", is how the German Federal Environment Agency, Germany's central environmental authority, classifies the importance of CO₂ sinks.

In addition to natural sinks, such as forests or peatlands, technological carbon dioxide removal (CDR) solutions like Biochar Carbon Removal (BCR) help to create more carbon sinks, which is essential for achieving climate targets. In this context, the CO₂ storage potential, i.e. how safely and long-term the carbon is stored in the sinks, is of central importance, as it determines how effective and impactful the CDR solution is considered.

In the case of Biochar Carbon Removal, we are usually talking about carbon sinks in the soil, where the biochar and the carbon it contains is transferred into a CO₂ sink via soil applications such as in agriculture or landscaping. Biochar can also act as a carbon sink through industrial applications, for example in concrete.

Permanence as an indicator of how effective a CDR method is

An important keyword for the evaluation of the carbon sink as a safe CO₂ reservoir is permanence, which means as much as: How long is the carbon stored and how great is the reversibility, i.e. the risk of stored carbon being released back into the atmosphere? 

So how can the permanence of biochar be determined and proven? This is where the term "terra preta", also known as "black earth", often comes up: Man-made pyrogenic carbons older than 3000 years have been found in the world's soils, suggesting that this carbon does not decompose but remains stored in the soil. Although this is an important indication, it is not reliable scientific proof. But how do you prove that the carbon in biochar remains stably stored for centuries, millennia or longer? 

"Unfortunately, we don't have the luxury of experimenting for 1,000 years," says Hamed Sanei, Professor and Director of the Lithospheric Organic Carbon Laboratory (LOC) at the Department of Geoscience at Aarhus University (Denmark), who investigates biochar’s permanence. Previous scientific studies, mostly by soil scientists, have attempted to predict decomposability based on certain patterns and trends, but due to a time-limited observation period this is only partially reliable.

New approach to measuring the stability of carbon in biochar

Professor Hamed Sanei and Dr. Henrik Ingermann Petersen of the Geological Survey of Denmark and Greenland (GEUS) have teamed up to apply a novel scientific approach: they are comparing biochar with geological carbonaceous rocks that are millions of years old. This enables them to provide information about the stability of carbon in biochar and even more than that: they can make statements about which type of biochar stores carbon particularly safely and thus also how to optimize the storage potential in the production of biochar. 

Sanei, Petersen and their team have already published several studies on the subject, relying on well-established measurable parameters of organic geochemistry and petrology that define the degree of preservation of organic carbon in the Earth's crust. These parameters are utilized, comparatively, to infer the organic carbon stability in biochars relative to that preserved in the geological carbonaceous rocks.

Classification of biochar as an inert maceral

Sanei et al. latest study, "Assessing biochar's permanence: An inertinite benchmark" (published on January 5, 2024 in the International Journal of Coal Geology) takes this research method to a new level. 

Contrary to the widely held belief that mineralization is the only permanent method of storing CO₂, this research highlights the importance of the "organic carbon pathway". The Earth stores carbon not only through mineralization, but also by converting biomass into inertinite-maceral, highly carbonized organic carbon that is non-degradable. Once converted to inertinite, the organic carbon is transferred from the biosphere cycle to the geosphere, where it remains for periods of several million years. For context: this process is responsible for the storage of over 15,000,000 gigatons of organic carbon in sedimentary rock.

According to Sanei et al., the production of biochar imitates the geological organic carbon pathway by rapidly carbonizing biomass through pyrolysis and converting it into inert maceral for permanent storage. Assessing biochar’s permanence therefore depends on whether complete carbonization and conversion is achieved. Inertinite is the most stable maceral in the Earth's crust and is considered an ultimate benchmark of organic carbon permanence in the environment, which is why this study measured the degree of biochar’s carbonization with respect to the well-established characteristics of inertinite.

Biochar samples identified as permanent CO₂ reservoirs

Sanei et al. use Random Reflectance (Ro), an optical tool, to quantify the permanent carbon pool in biochar. With increasing carbonization, the Ro value increases until a geologically stable form of carbon is formed and a Ro value of 2% is reached, which is considered the inertinite benchmark. Of all 64 European samples analyzed, 76% could be classified as pure inertinite and therefore as a permanent carbon sink. One sample with biochar from fruit pits achieved a particularly high value of 4.7% and was described by Sanei et al. as a "prime example of pure inertinite biochar". 

The oxidation kinetic reaction model shows that inertinite biochar (consisting of pure inertinite) has a half-life of approximately 100 million years in a harsh, oxidizing environment. Based on these results, the authors define Biochar Carbon Removal as a highly durable carbon removal process.

In addition, the Ro method can be used to calculate the carbonization temperature of the biochar, i.e. the maximum temperature to which the biochar fragments were exposed during pyrolysis. This parameter provides crucial insights into how the production temperature, the residence time during heating, and the thermal conductivity of the feedstock affect the efficiency of the carbonization process. These are all important factors influencing inertization that can be optimized, making Biochar Carbon Removal an even more effective means of achieving negative emissions.

Biochar is an effective CDR method and permanent carbon sink

"I hope that our research results will help biochar to be recognized as a clearly permanent carbon sink and effective CDR method in the future and that we can focus on how we can safely and permanently store as much CO₂ as possible by optimizing biochar production," Hamed Sanei summarizes their findings. 

The results from Hamed Sanei's study are an important indicator for the entire CDR community, as they not only confirm the permanent CO₂ storage capacity of biochar and high effectiveness of Biochar Carbon Removal as a CDR method, but also provide information on how the pyrolysis process can be optimized.

But there’s much more in it that could be ground-breaking: Taking a closer look at the Ro values of biochar could have a big impact on the carbon removal markets. Being able to quantify the permanent carbon pool in biochar and thereby classifying the biochar in terms of their level of inertinite could lead to different “price classes” of Biochar Carbon Removal Credits. The higher the amount of the permanent carbon pool, the higher the price for the credits, thus leveling the price difference that exists today between credits for mineralized carbon in rocks and Biochar Carbon Removal, as this difference is largely based on a perceived reliability of the permanence.  

This could give a boost to financing and faster scaling of Biochar Carbon Removal solutions and help Biochar Carbon Removal being recognized as what it is already: the only CDR method that is ready to scale today. We hope that politicians will focus more on Biochar Carbon Removal as a permanent CDR method in the future and acknowledge its important role in the fight against climate change.

This article is also published on Novocarbo. illuminem Voices is a democratic space presenting the thoughts and opinions of leading Sustainability & Energy writers, their opinions do not necessarily represent those of illuminem.