Biochar explained

What is it, how do you make it...everything you need to know about biochar

What is biochar? Our definition/meaning

There is no consensus on a single definition of biochar, so here is ours:

Biochar is the carbon rich remains of organic material that has been heated to decompose and remove most of the hydrogen and oxygen containing molecules. The process is called pyrolysis and is carried out in the absence of air/oxygen to prevent the material burning. The resulting residue is black and contains the “skeletal” remains of the starting material e.g. wood. The finer the starting material the finer the end product.

It is carbon, containing the natural minerals found in the starting materials. The carbon can be present as amorphous or graphitic phases depending on pyrolysis temperature. Under all formation conditions the surface of the carbon is covered in oxygen containing groups. These groups facilitate both ion exchange and electron exchange, leading to its ability to hold nutrients and to encourage plant growth.

It’s an ancient technology believed to be over 7000 years old, used by people in the Amazon basin in the form of Terra Preta soil.

How does pyrolysis work and what equipment is needed to produce it?

Biochar is made by heating organic materials such as wood in the absence of air (oxygen), a process called pyrolysis or sometimes carbonisation. Temperatures of 200-700C can be used to obtain different types of biochar or other products including wood acid and wood oil/bio-oil. Once completed it must be cooled before introduction of oxygen or it will ignite and burn, like charcoal on a barbecue.

The equipment to produce it ranges from the simplest, heating a container with small holes to enable the off gases to escape but not let the air in, to modern chemical manufacturing systems with heat recovery and process control. The latter have a much better environmental performance than the simplest systems. The approaches can be divided into whether the objective of the process is to make biochar or bio-oil. Bio-oil is typically made by heating the material quickly and often to higher temperatures. This maximises bio-oil production: biochar is a byproduct and is often burnt to provide heat for the process. Maximising biochar production requires slower rates of heating, where a typical burn could take over 8 hours.

Properties & Characterisation

Biochar is largely made up of carbon in the form of the skeletal remains of the starting ingredients, wood, straw, crop wastes etc. It also contains residual mineral matter (ash) and organic matter. At the time of manufacture it will contain essentially no water. The higher the mineral content of the feedstock the even greater proportion in the final product since the volatile components are lost through the process. As an example, Chicken litter is a feedstock with high mineral content.

The physical form of biochar will reflect the feedstock. Wood in sections will produce the charcoal used in barbecues for example. Smaller sized feedstocks such as wood chips, straw or compost will produce correspondingly smaller particles. The physical form is important for handling at the production and use stage. Biochar can be quite soft and friable leading to dust production on handling or grinding and suitable precautions are needed.

The structure and surface properties are important in its interaction with the soil and the plant rhizosphere. Woody materials typically retain their pore structures. Materials with high mineral content or biochar made at lower temperatures may have melted components eg minerals or tars that can block the pores. This reduces porosity.

The pore structure provides some of the most important properties of biochar.

Pore sizes will vary from relatively large 100μm to very fine <1μm. These pores give it a large surface area, allowing absorption of liquids including water, but also provide a home for the surrounding microbial community within the soil. They also allow fine roots and hyphae to access water and minerals held within the pores.

The carbonaceous surface of biochar is decorated with oxygen groups these endow the surface with ion exchange properties, both cationic (eg K+ and Ca2+) and anionic eg NO3 and PO43-). This valuable property allows it to retain nutrients and minerals from its environment. In the soil this can help it hold onto Nitrogen (N), Phosphorus (P) and Potassium (K), reducing run off and the need for constant fertiliser application. It can also hold minerals such as Magnesium (Mg) which is beneficial to plant growth and health.
biochar under an electron microscope

Image courtesy of Dr Jocelyn of Biochar Industries / Biochar Projects and Friends of the Char

Differences between materials

Charcoal / BBQ charcoal vs biochar

Charcoal or BBQ Charcoal is made from wood and is used as a fuel. It’s typically much larger in size and can be soaked in additional liquid fuel. Some regard charcoal as having too much residual organic matter which in turn can be phytotoxic i.e. not useful for soil amendment.

IBI and the EBC (European Biochar Certificate) set 0.7 as the maximum H/Corg ratio for a carbonised product to be considered as biochar.

Biochar is used as soil amendment and is much smaller in size, typically <8mm.

Horticultural charcoal vs biochar

Horticultural charcoal and biochar are essentially the same thing with a different name. Horticultural charcoal is charcoal that has been crushed into smaller particle sizes, it may contain more volatiles, depending on the processing conditions.

Biochar vs activated carbon

Biochar can be activated using one of two physical processes to increase its surface area.

Steam activation

Steam is introduced to remove some carbon from it at higher temperatures, essentially opening up the pores, increasing its surface area/porosity.

Chemical activation

The feedstock is inoculated with a chemical such as phosphoric acid, dried and pyrolysed, preventing material shrinkage, again increasing its surface area/porosity.

Ash vs biochar

Ash is produced when organic material is burnt. It contains calcium compounds, potassium salts, phosphates and trace elements including iron manganese, zinc, copper and some heavy metals.

Biochar combines carbon and the ash components described above.

Typical sources of feedstock

There is no recipe for the best feedstock to use, as every situation is different. The following feedstocks have been documented as having favourable properties for soil amendment.

wood feedstock

Pros and benefits

Cons and risks