What is biochar and how is it made?

Biomass heated in an oxygen-deficient environment produces biochar, a solid, carbon-rich material. In the near or full absence of oxygen, that process is called pyrolysis. Biomass used to make biochar can include various materials such as agricultural or forest residues, construction debris, organic waste, seaweed, and more. Biochar is also gaining attention for its potential as an ingredient within engineered materials, such as cement.

Biochar achieves carbon removal by storing CO₂ that was removed from the atmosphere by photosynthesis and plant growth. Without further intervention, the carbon from that CO₂ will stay stored for as long as the plant remains alive, after which the plant biomass will decompose, cycling most of the carbon back into the atmosphere. Technological interventions in this carbon cycle, like pyrolysis, can store the carbon that nature captured. Pyrolysis converts some of the biomass carbon into chemical compounds with higher resistance to environmental degradation. How much of the carbon converts to these more recalcitrant forms depends on the type of biomass and the heating conditions, including temperature and time. After biochar production, the highly recalcitrant fraction of carbon can be considered stored for long periods of time, as long as the biochar does not combust.

Common methods of biochar production include:

• Industrial pyrolysis systems: Modern pyrolysis systems heat biomass in a low-or-no oxygen environment, creating biochar and gaseous compounds known as synthesis gas. Synthesis gas is typically combusted in a controlled system to provide heat for the system. Low air emissions can be achieved for these systems, which is important to maximizing environmental benefits of biochar production.

• Kon-Tiki kilns: This is a less complex and therefore more widely deployable method of biochar production, but air pollution is likely to be higher than for industrial and can be significant enough to undermine carbon benefits of biochar production. Biomass is lit on fire in an open-top, conical kiln. Biomass is continually added to the process and finally quenched with water. 

• Trench mounds: Trench production is one of the simplest forms of biochar production and is used often by smallholder farmers. Biomass is buried in a trench, covered, and combusted to produce biochar. Air pollution is likely to be significant and even higher than Kon-Tiki kilns.  

How biochar stores carbon over the long term

Durability, or permanence, is a key metric when quantifying carbon removal from biochar. A common metric is the amount of carbon that remains stored in biochar 100 years or more after it is applied to soil. 

Quantification of durability relies on experiments and models to project permanence. There are a number of established and emerging approaches to evaluating biochar recalcitrance to chemical decomposition and permanence in storage. Most of these are based on short-term incubation experiments in laboratories, where scientists seal biochar in a container with other organic matter and measure carbon changes over a period of weeks to months. These approaches typically treat biochar as having multiple “pools” of carbon that differ by their resistance to decomposition. 

While these experiments offer insights into decomposition dynamics, they have two significant limitations. First, these approaches do not measure biochar decomposition and durability in the real world (e.g., after biochar is applied to agricultural soils), meaning these approaches do not capture the effects of actual soil and other environmental factors. Research has shown that different soils can strongly influence decomposition rates. Second, the short duration of these experiments means there is significant potential for error in extrapolation to centuries and beyond.

Why accurate durability is critical in crediting

CDR certification methodologies use a range of approaches to quantify permanence. Most common is the use of biochar chemical composition, namely the hydrogen-to-organic-carbon ratio (H/Corg), and models that correlate H/Corg with projected permanence from decay models applied to biochar decomposition experiments, in order to calculate a permanence factor. Other methods include measurement of the random reflectance of biochar to estimate the amount of highly recalcitrant carbon in biochar.

The risks of inaccurate durability estimates for carbon removal buyers are significant. Overestimating durability can lead to ineffective spending on carbon removal, reputational risk from overstated climate impact, and delayed progress on net-zero and compliance goals. Buyers need to understand and engage with durability estimation methods to ensure they are procuring high-quality biochar credits.

Why underground coal isn’t a valid durability comparison

Recent research proposes a new approach to quantifying biochar durability. The authors assert that random reflectance—a parameter developed in the field of organic petrology (the study of organic matter in rocks)—can be used to characterize durability. Using a component of coal (known as “inertinite”) as an analogue for biochar, they propose a benchmark random reflectance (Ro) of 2% as an “inertinite benchmark” for biochar. They argue that the entire fraction of biochar that meets that benchmark will degrade over approximately 100 million years. This is a large departure from other estimates of biochar durability and Carbon Direct believes this to be at odds with more conservative models.

There are several reasons to doubt this method. The coal-biochar analogy is inadequate because coal buried underground is not exposed to the same degradation conditions as biochar in soil. Underground coal has lower exposure to oxygen, water, and ultraviolet (UV) light, all of which are plentiful in surface soils. Each of these is associated with well-understood decay mechanisms. Furthermore, the process of geologic burial dramatically changes the physical structure of organic matter. Additionally, the permanence factor that the reflectance method allows (readily up to 1.0, or 100% carbon retention over extremely long time scales) contradicts experimental evidence that some carbon loss will occur over years or decades. 

Where biochar science is headed next

Biochar stability science will keep advancing. Research into biochar degradation pathways in soil will continue to refine permanence models and inform the relevance of coal analogues. Long-term field measurement studies with a high density of in-soil sampling to quantify persistence over time should be a near-term priority, and biochar CDR projects can contribute. Additional studies should investigate pathways to increase durability and rigorously develop modern isotopic, spectroscopic, and chemical techniques for robust assessment of biochar's durability. 

Conclusion: Crediting biochar means getting durability right

Biochar is a promising solution for both soil health and climate impact. Nevertheless, carbon removal certification protocols will need updating as the science of biochar durability evolves. Carbon Direct cautions against excessively generous claims of biochar CDR durability, in both permanence factor and timeframe, given the important uncertainties outlined above.

Protocols should not outpace science in terms of permanence assessment, and crediting should stay rooted in a science-backed understanding of what drives biochar permanence, including production conditions (e.g., high temperatures) and biochar composition (namely, low H/Corg). At a time when investment in biochar is booming and the science of biochar is evolving, the voluntary carbon market must keep pace.

Read our full scientific white paper, Biochar’s Long Game, for a deeper look at biochar durability and protocol best practices.