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What Is Biochar?

When we talk about climate science, much of today's public discussion is focused on lowering carbon emissions, i.e. reducing the amount of carbon dioxide (CO2) that is produced globally each year. Because greenhouse gases like CO2, nitrous oxide, and methane absorb thermal energy, as levels of these atmospheric greenhouse gases rise, so, unfortunately, does the global temperature. In fact, 66% of that rise in temperature can be attributed to carbon dioxide alone.

To avert the extreme consequences of global warming, climate stakeholders must work to not only limit carbon dioxide emissions, but also to extract some of the existing CO2 from the air. When it comes to carbon removal, biochar projects are emerging at a rapid pace all over the world. The methodology could hold the key to removing large amounts of CO2 from the atmosphere while also offering a variety of other benefits: it reduces nitrous oxide emissions in soil, improves soil health, and is a vector for clean energy production. So let’s talk about this promising charcoal-like substance!

What is biochar?

Biochar is black, porous and looks similar to charcoal but its properties and uses are far different: by definition biochar is obtained from a pyrolysis process and is used for environmental purposes such as soil amendment or remediation.

What are its properties?

  • Extremely porous - it has a deceptively high surface area, making it a powerful adsorbing matrix with many potential applications.
  • Very high adsorption capacity - capturing contaminants and volatile compounds brought into contact with it.
  • Resistant to microbial degradation - hence, it remains in soils for centuries or even millenia.

Different types of biochar courtesy of UC Davis Biochar database

How is it produced?

Biochar is the solid product of pyrolysis of biomass. Pyrolysis happens when a biological material is brought to high temperatures (500-700°C) without oxygen. Unlike in a ‘standard‘ burning process, i.e. combustion, a portion of the carbon of the parent material remains as biochar after pyrolysis instead of being emitted as CO2. Biochar can have very different properties depending on the feedstock material and the processing conditions used to make it. Biochar made from hardwoods for instance tends to have a higher carbon content, higher surface area and is more resistant to degradation in soil whereas biochar made from chicken manure will have higher proportions of phosphorus and ash.

The production of biochar can be very low-tech, such as KonTiki kilns, or rather high-tech with more advanced systems that when producing biochar, also take advantage of the heat generated in the process and use it for instance to dry the feedstock or generate electricity.

Low-tech Kon Tiki kiln

High-tech pyrolysis plant

Why is biochar an essential part of the carbon removal conversation?  

The international scientific community identified biochar as a promising net negative emissions technology (NET) for the first time in the 2018 IPCC report. In recent years, Biochar has become more well known because of its climate change mitigation potential: since about half of the carbon in the original material remains when it is turned into biochar and resists degradation for centuries or even millennia, putting biochar into soil means locking carbon away and removing CO2 from the atmosphere long term. Today, biochar is one of the only shovel-ready and scalable NETs available and it is estimated that biochar can contribute to removing between a 0.5 to 2 gigatonnes of CO2 per year globally (Fuss et al., 2018).

Exemplary pyrolysis process courtesy of SYNCRAFT

Is biochar new? 

Even though all this might make biochar sound like a new technological approach, biochar has actually been around for millenia. The term itself however, was first coined in 2005. Biochar is a naturally occurring material, an intrinsic component of soil organic matter created in forest fires and exists in natural ecosystems as Pyrogenic Carbon (PyC). It has been used for millennia as part of ancestral practices. The discovery of fertile black soils in the Amazon forest (terra preta) is what spurred the research into biochar and its soil improving properties. In fact, in terra preta soils, biochar has been found to make up as much as 30% of organic matter. Adding biochar to agricultural soil can improve soil water retention, nutrient retention and uptake, boost microbial life and hence improve yields.

Terra Preta: Fertile black soils found in the Amazon forest

What are common biochar uses?

Given its porosity, molecular strength and adsorption properties, biochar has many different uses and applications:

  • In agriculture as a soil amendment, to improve compost, and as an animal feed
  • For soil remediation: its adsorption properties can be used to immobilize certain contaminants such as heavy metals
  • As a material composite
  • In construction materials

Biochar actually has over 50 possible uses! With all of these benefits and uses, biochar has the potential to reduce carbon dioxide and nitrous oxide emissions at scale, with many co-benefits depending on its end use.

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