What's special about biochar?

“Biochar is a form of biomass that has been thermally decomposed in an oxygen-limited environment. Its potential to enhance crop yields, bolster plant disease resistance, detoxify soil, and sequester carbon is well documented.”(1) And that’s just for farmers...

I started researching biochar in 2009 at Byron Bay in NSW, Australia while studying a Diploma in Permaculture which I completed in 2010. What fascinated me the most about it was it’s almost ‘magical’ properties – even at that early stage of world biochar research, there seemed to be unlimited potential with an exponentially growing list of applications eg. water filtration, growing systems, hard infrastructure, Carbon fibre, PV cells, solid state batteries, thermal storage etc. The list is still growing. But – having trained in medical and environmental science in a former life, over the following years I tried to science out these ‘magical’ properties and demystify this Carbon-based and ‘Carbon negative’ material of the future which can help combat climate change.

What makes biochar special? As it turns out, there are many chemical and physical properties in variable combinations of importance depending on the biochar. But one specific property caught my attention – surface area. There seemed to be an improvement of biochar efficacy eg.adsorption, electron transfer etc. in a number of biochar materials chemistry research papers I have read over the years when the surface area of a specific biochar is higher than it’s competitors in the study - but not always.

Firstly, to be clear, the surface area of a piece of biochar is irregular and forms a ‘3D hierachical biochar matrix’ down to the nanoscale level. There are micropores (less than 2nm), mesopores (2nm to 50nm) and macropores (greater than 50nm) across the surface which greatly contribute to the overall surface area. This allows for great adsorptive capacities and also allows small dimension molecules, such as gases and solvents, to be absorbed(2). Although numbers greatly vary, the overall surface area of 1 gram of biochar could be anything from 300m2 upwards. In a kelp battery study(3), an activated Carbon electrode (which could be fabricated from biochar feedstock with subsequent activation) was built with 4425 m2 per gram surface area. Incredible (though I couldn’t find the specific species of ‘brown macroalgae’ in the paper). Fast growing and Carbon sequestering hemp and bamboo are also promising candidates. The advantage for soil applications is the enormous variety of microbes and fungi that can find shelter and mine resources in the pores. The surface is also hydrophilic or ‘water loving’. In fact, some biochar can adsorb more than 10 times it’s weight in water – but there is a a fuzzy logic around ‘Water Holding Capacity’ (WHC), which is affected by different soil types/hydraulics and surface area. The general increase of WHC with biochar is great for growing systems as this water is conveniently slow released – essentially, a primitive form of irrigation, though there is also the issue of ‘plant available water’ which is too complex to explain in this article. Then there’s dye removal, heavy metal removal, pesticide and herbicide removal, toxin removal and more – many chemicals love to bond on to the surface chemistry of the biochar. There are many different binding/bonding sites on the surface. The nature of these binding sites also varies between biochars. It partly depends on the chemical characteristics of the soil that biomass is grown which sucks up minerals during the plant’s life and after pyrolysis (breakdown and volatilisation of the biomass under heat, which can take place in limited or no oxygen), those minerals are locked into the ‘3D hierarchical biochar matrix’ of the surface and many provide unique binding sites. The number of these binding sites will also vary between pieces of biochar. So, it seems there are a few variables in play here: surface area, type of binding sites and the number of binding sites. Every piece of biochar is unique!

So, if surface area is so beneficial, how can it be increased? The most simple way is to burn the biomass with a low moisture content (MC) <15% in a stove or kiln that is energy efficient so the biomass burns at a high ‘Highest Temperature Treatment’ (HTT) eg. 750 degrees or even hotter (in the case of controllable pyrolysis temperatures in some continuous biochar kilns) though there are other variables too. But, there is a trade-off here – the higher the temperature of the stove or kiln, the lower the ‘mass yield’ (the yield of biochar mass after a burn). So, as a biochar maker, also known as a ‘Charista’ you need to make a choice – higher surface area V higher mass yield? How can you choose, well, it depends on the application! For eg., air and water filtration work more effectively with higher surface areas (indeed, there are many ‘activated Carbons’ that exploit this property). Most growing systems that I’ve researched and invented or adapted operate well with a trade-off between surface area and mass yield using mainly Kon-Tiki biochar kilns (using ‘flame cap’ operating software) and Top-Lit UpDraft (TLUD) stoves for biochar production(4). I’m now developing a new kiln called the ‘Flame Cap ‘Algorithm’ Panel Kiln’ with expandable volume, minimal feedstock processing and easy logistics. If abundance of biomass is not an issue, then go for a higher surface area by drying out your biomass as much as possible below 15% MC but the mass yield will be lower. This should cover many applications – I don’t remember ever reading a statement or conclusion that listed high surface area of biochar as a disadvantage for a given application.

In conclusion, although there may be many new terms introduced in this article, I hope I have enticed you to do more research about biochar – especially it’s high surface area property. Biochar has proven durability in the field in the past with ‘Terra Preta’ (Dark Earth) and a massive Carbon removal potential with a 100 million years ‘half life’ if it meets the 'Inertinite Benchmark of Random Reflectance' >2% (5) . Crystallisation of phytoliths/plant stones/plant opals/PhytOCs could be a major factor here for permanence too. Biochar applications are growing exponentially year by year. I think it’s difficult to screw up Civilisation with biochar if it's feedstock is using biomass waste that would have otherwise released Carbon emissions. Biochar Civilisation could advance with ‘sustainable adaptation’ to climate change using 'Carbon removal' as it's key. I say, research, design, build, test, develop and commercialise as many biochar making technologies and applications as possible and help save ‘Planet A’!

REFERENCES

1. Joseph, S. and Taylor, P., ‘A farmer’s guide to the production, use, and application of biochar’, 2024, ANZBIG, p.12

2. Lehmann, J & Joseph, S, ‘Biochar for Environmental Management: Science, Technology and Implementation’(2nd Edition), 2015, p.95 (note: 3rd edition is now available)

3. Zeng, J, Wei, L and Guo, X, ‘Bio-inspired high-performance solid-state supercapacitors with the electrolyte, separator, binder and electrodes entirely from kelp’, 2017, Journal of Materials Chemistry A, p.1

4. www.permachar.net

5. Sanei, H, Rudra, A, et al, 'Assessing biochar’s permanence: An inertinite benchmark', 2024, International Journal of Coal Geology, p.1