and its benefits

What is


Charcoal, biochar and activated carbon from wood stock are all made using the same process: pyrolysis. This is the carbonisation of wood by heating it in an oxygen depleted environment. In simple terms, charcoal for burning is made with very little oxygen present. This prevents any of the tars and oils that inhabit the pores within the structure of the wood from burning away. The charcoal is dense and has a high calorific value. If you introduce some oxygen into the process the volatiles start to burn away, creating a sponge like structure within the wood. The charcoal becomes lighter 

and more friable. This process is known  as ‘activation’. Introducing oxygen is certainly not the best way to activate charcoal (you can’t make high quality activated carbon this way), but it is the easiest . 

So at one end of the scale you have charcoal, in the middle you have biochar (which is partially activated) and at the other end is activated carbon. The ecological value of the charcoal we produce very much depends on how activated it is; the more activated the better! 

How does biochar


The Tawi stove produces biochar; charcoal which has been activated. It achieves this by allowing sufficient oxygen to reach the charcoal to burn out the tiny pores within the wood, forming a sponge like structure. This structure can have a vast surface area, 300 – 2,000 m2/g.

As liquids and gases pass through biochar, some particles are attracted to its surface via a process called ‘adsorption’. These particles remain attached, forming a film. Eventually the structure will become saturated and adsorption will stop. (This short video by ‘Dr. Beckman’ illustrates the principal)

Activated charcoal adsorption has limitations. Although it will adsorb a wide range of impurities and contaminants, such as odours, pigments and chlorine, other substances, such as fluoride and sodium, will pass straight through.


Biochar benefits can roughly be divided into two categories: manufacture and storage.

Manufacture: a piece of Biochar is like a bee hive with no bees; no more than a space with lots of empty compartments, it is of no use by itself. Introduce a swarm of bees to the hive and they will, in time, make you honey. Biochar is much like a hive, but for micro-organisms. It offers them a safe, electron-rich home where they can thrive and produce chemicals beneficial to the plants around them.

Storage: Biochar is very good at retaining moisture and chemical compounds. However, the bonds that hold them within the biochar are weak and can easily be overcome by the plants who need them. This makes biochar an ideal storage structure.

The effectiveness of activated charcoal depends on the degree of activation, and the original source material. The way the Tawi stove makes activated charcoal is not precise, so the degree of activation varies enormously. However, for our intended purpose that doesn’t matter. Commercial water filters use small amounts of highly activated charcoal. We achieve the same results by using a larger quantity of biochar, but we are making it for free!


Although we use the biochar we produce in Kenya for water filtration, please do not do the same with your biochar, as it is possible that the wood you use has picked up chemicals which might survive pyrolysis and still be harmful.

How is biochar


Once you have produced charcoal, how do you know if it’s biochar? There are a number of tests that you can do, from seeing how well it absorbs, to seeing how dense it is. As you will see from the following studies, our charcoal performs extremely well when compared with commercial biochar samples, which is why we can use it so effectively for filtering water or eliminating unwanted odours. 

If you would like to try testing some of the biochar you produce in your Tawi stove, find some simples tests to do on it here

Iodine Test

One of the most common tests done to quantify the filtering capabilities of activated carbon is that of determining the iodine number. This involves stirring a sample of the charcoal in some iodine solution and then doing a titration to determine how much iodine was removed by the sample. 

The pictures opposite give an idea of the filtering capabilities of the samples (the lighter the liquid, the more iodine it absorbed and therefore the better its filtering ability). It also shows how much difference is made by the amount of time the biochar spends in the iodine. 

The full results from our ‘backyard’ iodine number determination can be found here (spoiler – our biochar filtered out more iodine then commercial biochar!)

Samples left in iodine for 30 seconds

Samples left in iodine for 2 days

Key : test-tubes from left to right: commercial activated carbon; commercial biochar; biochar produced in our ‘biocharBBQ’ using apple; oak; pine branches

Filtration Tests

When we first started making these stoves in Kenya, we contacted the Nakuru Defluoridation Company, a company that tests activated carbon for filtering water from the local lakes. 

The results of the tests they ran on our biochar show that, for the first few days, it is extremely effective at removing most dangerous contaminants from the water. The only molecule it failed to remove was fluorine, which is a huge problem in the Lake Bogoria area. This was unsurprising, as the only effective filter they had found for that was bone char! However, their results were hugely encouraging, and their full study can be seen here.

How is biochar


Use your biochar to enrich your garden and deodorize your rubbish and compost bins. You should be able to use all the biochar you produce by mixing it with your compost. This is exactly what the Native Amazonians did and it certainly worked for them!

Improve Soils

Biochar acts like a sponge for nutrients and water. These elements are then available for plants. So if you mix it in with your compost you can ensure that nutrients are retained and don’t just leach away. However biochar’s most important function in soil is to provide a safe home for soil microbes. These in turn produce nutrients which plants feed on. Biochar also improves soil aeration and texture, which benefits other soil dwellers such as worms.

CLICK HERE to learn how to charge your biochar. 

Biochar also plays an important roll in conservation. Poor soils in arid climates stand to benefit most from the addition of biochar. Biochar can be an important ally in reforestation. A small amount of charged biochar, added to the bottom of a planting hole, will act as a vital reservoir for water and nutrients. This will help sustain tree seedlings during periods of drought. Biochar is used in many ‘seed ball’ conservation projects; such as the one recently featured in David Attenborough’s Green Planet series. You can have a look at that project HERE.

Eliminate Odours

Biochar has an incredible ability to absorb odours, especially natural odours such as those produced from food waste. Keep a jar of ground up biochar beside your compost bin, and every time you add food waste just sprinkle some biochar on top before you close the lid. This will eliminate most unpleasant smells, although particularly strong onion may not be completely removed. Biochar is also perfect in a fridge where it can be difficult to remove strong smells from savoury dishes like curry. Just put a small plate of ground biochar in the fridge and the smell will be gone after a few hours. 

Replace Peat

One industry that has only recently come to public attention is that of peat cutting. Peat is used in numerous commercial compost and potting mixes, and has been for a very long time. There are two significant problems with its use. Ancient peat bogs and marshlands store large amounts of carbon. When peat lands are drained and mixed with other soils in potting compost, the carbon dioxide is released into the atmosphere as the peat is broken down by microbes in the mixed soil. The second problem is the loss of vital, diverse and unique habitats caused by the draining of peatlands. Peat plays an important role in horticulture; however, we now have to find sustainable alternatives. The biochar made in Tawi stoves has the potential to be one such alternative.

The following article by Lauren Quinn, from the University of Illinois Urbana Campaign, outlines the damage peat extraction causes and explains how biochar might be able to play a part in mitigating it. We aim to support research like this.    

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Carbon Farmer

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