Biochar

Biochar

Turning biomass into charcoal

What you need to know about biochar

  1. Biochar is the output of heating biomass to high temperatures in an oxygen-poor environment, a process known as pyrolysis.
  2. The end result is a charcoal that is chemically stable, rich in carbon, and can be used as a soil additive and for other purposes.
  3. The feedstock biomass can be any plant matter, but to be additional, projects need to use waste biomass (without another use) as their feedstock.
  4. Different types of plant feedstock create biochar with different amounts of carbon and durability.
  5. The pyrolysis process also creates combustible gases including methane and CO₂ as byproducts, which can be captured and used for fuel.

Turning biomass into charcoal

When organic matter is burned or left to decay, it releases its carbon content back into the atmosphere as CO₂. However, biochar projects circumvent this natural process by heating organic matter (biomass) in the absence of oxygen so that no carbon is released, thereby becoming a method of carbon dioxide removal (CDR).

Biochar is the charcoal-like substance that results from this process, and it could hold the key to removing vast amounts of CO₂ from the atmosphere while also offering a host of other benefits: it reduces nitrous oxide emissions in soil, improves soil health, and can be a vector for clean energy production.

When biochar is applied as a soil enrichment, it helps soils form better aggregates, which helps them retain more water and nutrients. In turn, this supports plant growth, decreases dependence on chemical fertilizers, and reduces leaching and runoff. Applying biochar can improve crop yields while reducing emissions of the land used by about 12%. Experts estimate that widespread use of biochar could durably sequester 2.5 gigatonnes of CO₂ each year. To put that in context, we’ll need to achieve approximately 10 gigatonnes of durable CO₂ removal annually by 2050 across all project types in order to meet our climate targets. That means biochar could be a huge part of global climate efforts.

The history of biochar

Archaeological records point to biochar use in indigenous Amazonian cultures as far back as 2,500 years ago. Known by the Portuguese term “terra preta” (meaning “black earth”), biochar use in agriculture in South America is also referred to by scholars as Amazonian dark earth (ADE). Amazonian communities would ignite a mixture of organic materials, certain tree trunks, and soil, then bury it for combustion, producing biochar, which they’d use as soil fertilizer. ADE is much more fertile than the naturally occurring Amazonian soil, leading scientists to credit the practice in part for forming the basis for a widespread, thriving pre-Columbian civilization with densely populated centers.

However, the earliest biochar practitioners may have been Australian Aborigines, who practiced “fire-stick farming” — igniting moist plant matter to encourage crop growth — as much as 10,000 years ago. Evidence of the Aboriginal use of oven mounds containing charcoal, minerals, organic matter, and clay has been discovered in southeast Australia.

Biochar has also been used for centuries in Japan and China to maintain soil fertility.

How is biochar produced?

While there are multiple methods of producing biochar, the principle behind production is consistent: plants absorb carbon dioxide through photosynthesis. Even on working farms or thriving forests, there will inevitably be waste plant biomass (i.e. corn husks, thinned trees, animal manure). This waste biomass becomes the feedstock for biochar production. The feedstock undergoes pyrolysis, and then becomes biochar. Gasses including methane and CO₂ are also released as byproducts during pyrolysis, which can be captured for use or storage.

All in all, this process creates three byproducts:

Types of feedstock used to create biochar

Biochar can be versatile as a climate solution, since many types of biomass can be used as feedstock for pyrolysis. Different feedstocks have different underlying chemistry, which impacts the stability and carbon composition of the biochar that’s created. Higher stability in the biochar means carbon is less likely to interact with its environment and be released back into the atmosphere. Broadly speaking, there are two main categories of biomass used as feedstock in most biochar projects today.

Wood: Wood tends to produce the most stable, highest-carbon content biochar, especially when exposed to very high temperatures during pyrolysis.

Agricultural residue: Farming creates a large amount of waste biomass including crop residue like corn stalks or animal manure. Biochar created from this agricultural residue usually has lower carbon content compared to wood feedstock.

Biochar stability and permanence

Feedstock exposed to different temperatures can also have different stability. Higher temperatures during pyrolysis tend to create more stable biochar. For biochar, higher stability equals higher “permanence” — the length of time carbon is sequestered. Scientists estimate that biochar can be durable for up to or exceeding 1,000 years, depending on factors including feedstock materials, heat of pyrolysis, and environmental factors. Many projects are rated with permanence levels of hundreds of years or more, which exceeds the permanence of forestry projects.

Biomass sourcing

All carbon projects need to account for lifecycle emissions, such as those produced by transport. Biochar projects need lots of feedstock, and it’s best when that feedstock is co-located with (or near to) the pyrolysis facility or kilns to reduce transportation emissions and costs. Additionally, biomass used as feedstock for biochar shouldn’t have land-use competition — meaning projects aren’t just growing biomass to turn into biochar when that land could otherwise be used for sustainable agriculture or forestry.

Biochar leakage and additionality

If a biochar project uses feedstock that could be used productively elsewhere, this will cause “leakage.” Leakage refers to the possibility that carbon avoided or removed by a climate project shifts or “leaks” to another location, system, or process. Only farming byproducts and waste materials should be used as feedstock— not the ‘fuel’ or the ‘food’ itself.

For example, if manure is being used as fertilizer and is then pulled for use as biochar feedstock, the farm that was originally using the fertilizer will now need to find new fertilizer.

The word “additionality” is used in carbon credit projects to talk about the climate impact the money generated by the sale of credits is directly responsible for. Carbon credits are financially additional when a project can demonstrate it’s only viable because of revenues from carbon credits.

Financial diligence for biochar projects focuses on the proportions of funding going to each of these separate streams, and must establish the minimum carbon credit revenue required to make the project viable.

The varying scales of biochar projects

Industrial pyrolysis projects: These are industrial-scale projects that rely on large-scale, automated pyrolyzers or gasifiers to mass-produce biochar.

Artisanal kiln systems: Individuals can produce biochar using small kilns or open fires. This process is more labor-intensive and produces smaller quantities of material.

Batch systems: Batch systems are between industrial and artisanal methods.

Capture of methane and other process-related gas

The pyrolysis of biomass generates combustible gasses made from methane, CO₂, and similar compounds. These gasses can represent a huge source of external emissions if not accounted for.

Most projects do at least some of both options. Large-scale pyrolyzers can be very efficient at capturing gasses; however, there’s almost always at least some residual emissions that aren’t captured.

Benefits & Risks

Permanence

Biochar has two carbon sink pools: polycyclic aromatic carbon (PAC) and semi-persistent carbon (SPC). SPC is durable for 50-100 years; PAC can persist for 1000 years in many soil types and climates.

Renewable energy

Bio-oil and syngas are byproducts of producing biochar, and can be used to generate heat and electricity.

Biochar uses

Biochar can be used for water filtration, as a soil amendment, or added to animal feed to reduce emissions from livestock agriculture and improve animal health.

Leakage

Biochar projects need to measure and protect against causing emissions leakage from changes in land management outside the project area, such as sourcing new biomass rather than using existing stock.

Additionality

Accurately estimating what would have happened to biomass feedstock in the absence of the biochar project is key to calculating the additionality of the carbon credits.