Biochar is a charcoal-like substance that is created by heating up biomass (typically agriculture residues) at very high temperatures in a low-oxygen environment – a process known as pyrolysis. By converting biomass into biochar, it effectively halts much of the decomposition of biomass that would have led to the release of carbon dioxide (CO2) and other greenhouse gases. Biochar is typically applied to soils where it is claimed to enhance soil quality, though this is highly dependent on several conditions that still need to be fully understood through large scale field trials. Since biochar is sold to farmers as soil amendments, it is hard to ascertain the additionality of buying biochar offsets. It is also difficult to quantify the permanence of biochar, as well as its soil benefits, as it can vary based on the biochar composition, the features of the soil it is added to, and other environmental conditions. We hope to recommend biochar offsets when we have identified projects that have overcome some of these uncertainties, as we believe biochar has the potential to be a beneficial tool in addressing climate change.
This report was last updated in November 2021.
Bio-oil, a dense liquid that is also created through pyrolysis, can overcome biochar’s additionality and permanence challenges if it is not sold for commercial purposes but is instead injected underground in geologic formations. We have identified one company – Charm Industrial – that produces bio-oil and injects it underground. We recommend purchasing carbon credits through their website.
What is biochar?
Carbon removed from the atmosphere through the process of photosynthesis is released back into the atmosphere when biomass (organic material) biologically degrades. Biochar is a charcoal-like product that is created by heating biomass to very high temperatures in a low-oxygen environment - a process known as pyrolysis. Converting biomass to biochar slows the decaying process and locks stored carbon in place, in turn slowing down its release back into the atmosphere. This charcoal-like substance can be added to soils, which prolongs carbon storage, enhances soil quality, and potentially increases crop yields in some conditions, enabling the soil itself to sequester more carbon. A process known as “fast pyrolysis” can be used to convert biomass into bio-oil which can be used as an energy feedstock or pumped back underground for permanent storage.
Biochar as a carbon offset
While biochar is a relatively mature product, its use as a carbon offset is quite nascent. In fact, at time of writing this overview, major carbon offset registries had not yet begun listing biochar projects. The American Carbon Registry rejected an effort to approve a methodology for biochar projects after a peer review process found limited evidence of the stability of soil carbon sequestration in fields treated with biochar. Another registry, Verra, only recently proposed a methodology for establishing project baselines and project additionality for biochar projects with public review and final approval of this methodology expected by the end of 2021. Despite the lack of existing certification, marketplaces like Puro.Earth have made it possible to buy biochar carbon removal credits from providers outside of traditional carbon registries, and a few biochar and bio-oil producers make it possible to purchase carbon removal credits directly.
When biomass is converted to biochar, it dramatically slows the release of carbon into the atmosphere by preventing the biological decomposition of biomass - avoiding the release of greenhouse gases. For example, projects that collect biologically degrading agriculture wastes and convert them into biochar are generally seen as emissions avoidance projects. Some biochar advocates claim that by removing crop waste from natural cycles of growth and decay (by converting it to biochar), this is the equivalent of a carbon-negative technology. From our perspective, we admit that this can be seen as somewhat of a grey area between avoided emissions and CO2 removal.
Projects that grow plants for the purposes of converting plant biomass to biochar have a more solid claim to be considered CO2 removal as the project boundary includes the removal of carbon from the atmosphere through photosynthesis. Growing plants for this purpose, however, can have negative environmental and land use implications, and we therefore believe this approach should only be pursued if it does not harm food security, biodiversity, rural livelihoods in the process.
Proponents have also claimed that biochar applied to soils can increase crop yields, suggesting higher carbon dioxide uptake by plants through the process of photosynthesis, and therefore adds to carbon removal from the atmosphere. This effect has not been widely studied in field trials and varies significantly due to soil type, biochar composition, and other environmental conditions (see Causality section). We therefore ignore this in determining whether a biochar project is an avoidance or removal project unless strong evidence can be provided on the added soil carbon sequestration claims.
Produced from organic materials that have high carbon content, biochar is made up of anywhere between 50% to 93% carbon. The product fixes carbon that would have otherwise been released into the atmosphere as carbon dioxide (every ton of carbon fixed in biochar results in 3.61 tons of avoided CO2 emissions as CO2 has a higher molecular weight than carbon).These organic materials, typically crop residues like wheat straw, corn stover, almond shells, rice husk, and others, would have been broken down by soil microorganisms, which release CO2 and other gases in the process and return nutrients from crop residues back into the soil. A recent study found that leaving crop residues to decay on agriculture farms may actually store more carbon in the soils than would have been the case had the residues been cleared (though these benefits may be offset by increased emissions of nitrous oxide, a strong greenhouse gas). Converting forestry and crop residues to biochar and applying them to soils leads to longer, more stable storage of carbon in soils than if residues were burned or simply applied to soils (approximately 50% of the original carbon is stored in biochar compared to 3% retained after burning and less than 20% after 5-10 years of biological decomposition). The production of bio-oil through fast pyrolysis that is subsequently sequestered into injection wells also results in a more stable storage of carbon than would have otherwise decomposed.
Biochar and bio-oil's ability to fix carbon that would have otherwise more quickly decomposed is assessed as high, though the longevity of this benefit varies based on several factors, which is covered in the Permanence section.
Biochar is a product that is commonly sold to farms where it is added to soils. Biochar projects that primarily depend on the sale of carbon offsets as opposed to the sale of physical biochar to farms can make a reasonable claim to additionality. These projects tend to provide biochar to end users for free or sold at a substantial discount. Bio-oil that is sequestered underground and not sold to an end user would also satisfy additionality.
Many of the projects we evaluated, however, depend primarily on the sale of biochar to farms as soil amendment, making it hard to determine whether the biochar would have been produced without the sale of offsets. An estimated 90% of biochar produced in Europe, for example, has been used as feed additive in livestock farming for years, suggesting an already robust existing market for the product. A 2018 producer survey estimated total biochar production at 36,700 to 76,600 tons per year in North America and a separate 2018 analysis estimated global biochar production at over 330,000 tons per year which is expected to grow to 884,000 tons per year by 2027. Determining the additionality of offsets that support an already growing industry will require market analyses that can isolate growth in biochar production based on demand for offsets. The ability to purchase biochar offsets is relatively new, thereby limiting robust analysis of their additionality. According to the National Academies of Sciences, biochar sales prices range from $600/ton to $1,030/ton of biochar, corresponding to a carbon price of $230-400/ton of CO2e (assumes 70% carbon content). Biochar offsets sold for substantially less than this carbon price are likely to be heavily dependent on end user sales, suggesting lower levels of project additionality. However, this is an admittedly crude approach to estimating additionality.
With exception of projects that do not generate profits from the sale of biochar or bio-oil project additionality for biochar is typically assessed as low.
The production of biochar depends on a number of key inputs, including energy for the pyrolysis process, biomass feedstock, and pyrolysis units to conduct pyrolysis. The degree of marginal additionality depends on how constrained biomass producers are in obtaining these key inputs in response to revenue from offset sales. Biochar producers who are able to easily access new biomass feedstocks like agriculture waste residues, or are able to easily deploy new, modular pyrolysis units can make a strong case for marginal additionality. Alternatively, biochar producers that struggle to source new biomass feedstocks or depend on large, industrial scale pyrolysis units have a lower degree of marginal additionality. Overall, biochar projects tend to have migh marginal additionality unless a project is unable to readily acquire new pyrolysis capacity or biomass feedstock because of offset sales.
The conversion of biomass feedstock to biochar fixes carbon that would have otherwise been released into the atmosphere as the original biomass degraded. A number of methodologies are used for estimating biochar stability – that is the durability of carbon in biochar. Hydrogen to organic Carbon molar ratios (H:Corg) and Oxygen to Carbon molar (O:C) ratios measure the proportion of H and O relative to C present in biochar (The process of converting organic matter to biochar results in lower H:C and O:C ratios compared to the original product.) Organic materials with low H:Corg and O:C values (e.g. biochar) are less prone to degradation than materials with high H:Corg and O:C values. These values can be determined in laboratory tests and are used in assessing eligibility of biochar projects under Puro Earth’s methodology. Other methods attempt to directly quantify carbon loss over a period of time under laboratory conditions as well as field studies of biochar in soils. For example, observations from 3-5 year-long incubation experiments in controlled environments with additives to promote decomposition are used to measure carbon decomposition and extrapolated to estimate the long-term stability of biochar. A study by the International Biochar Initiative assessed 28 methods to determine biochar carbon stability found that H:Corg measurements correlated closely with directly observed 3-5 year carbon degradation studies. A biochar sample with an H:Corg value of 0.6 was predicted to have a BC+100 score of 65.6%, meaning that 65.6% of the organic carbon measured in biochar will likely remain stable for at least a century (Puro Earth’s methodology requires H:Corg ratios of less than 0.7).
Real world applications of biochar to soil can yield different results from lab tests and incubation studies. While the carbon content in biochar applied to soils can last for decades to centuries, it depends on factors like soil type, biochar production temperature, and biochar composition. Carbon in wood-based biochar may be stored for hundreds to thousands of years compared to decades for manure-based biochar. Higher temperature pyrolysis is strongly correlated to increased biochar stability. Ultimately, biochar is slowly decomposed in soils, and the ability for biochar to resist decay is largely determined by the feedstock and pyrolysis conditions. More research and field trials are needed, however, to achieve greater precision around biochar’s durability in soils. Nonetheless the carbon in biochar distributed in soils is more durable than carbon in original biomass left to degrade naturally.
Carbon stored in bio-oil that is injected into injection wells is even less likely to decompose than biochar applied to soils. This is primarily because bio-oil sinks to the bottom of injection wells, whereas biochar is distributed near soil surface where it interacts with soil microbes and other elements. Injection wells are regulated by the EPA and are used to inject hazardous or non-hazardous wastes and fluids in deep geologic formations. While carbon stored in bio-oil that is injected this way is less reversible than biochar in soils, this storage method is not without its risks. Lax oversight into over 680,000 underground waste and injection wells in the United States could lead to leaks and structural failures, however the EPA has deemed the probability of well failures to be low. Permanence of bio-oil and biochar are assessed to be high.
Puro Earth, a marketplace listing 13 biochar projects as of this writing, lists prices for biochar-based carbon removal credits ranging from $110 (95 EUR at time of writing) to $681 (586 EUR) per ton of CO2. This is substantially more expensive than traditional avoidance-based carbon offset projects, but are cheaper than some of the technological carbon dioxide removal projects like direct air capture. One reason for the cost disparity between projects is that some biochar providers (like CarboCulture) make their biochar available to farmers at a significant discount and depend more heavily on the sale of offsets and are therefore pricing their offsets higher. While we believe biochar projects that rely primarily on revenues from offset sales to have a stronger case for additionality, we consider them high cost. Projects that produce bio-oil from biomass that is sequestered underground (not sold), are priced even higher than biochar offsets and are also considered high cost. For projects that sell to farmers and use carbon credits to lower the cost marginally, their “true” cost is difficult to ascertain, since one would need to understand the additional amount of biochar demanded due to the cost difference. We at Giving Green are planning to take a closer look at these market dynamics in the future.
When added to soil, biochar can create long-term carbon pools in the soil, stimulate microbial benefits, increase soil’s water holding capacity, improve nutrient availability, and decrease susceptibility to plant disease. The impact of applying biochar to soil on increasing crop yields depends on a number of environmental factors like soil type, soil pH, fertilizer inputs, and soil fertility. A meta-analysis on crop yields found that in tropical climates, where acidic soils are more common, biochar elicited a 25% increase in yield, but had almost no effect in temperate climates. Another analysis found the effects of biochar on crop yields ranging from reduction in crop yields by 28% to increases of 39%. These variations and the effects of other soil and environmental conditions demonstrate the need for more well designed, long-term field studies on the complex interaction of biochar with soils.
Biochar holds significant promise as a valuable tool in addressing climate change. However, there is a great deal of uncertainty about the additionality of biochar offsets given that biochar is sold to farmers. The permanence and co-benefits are also difficult to quantify. We therefore cannot recommend biochar offsets at this time until some of these questions can be clarified. However, we can recommend one provider, Charm Industrial, that produces bio-oil and sequesters it underground, addressing some of the additionality and permanence issues that exist with other biochar providers.