Since building the Arne Næss Memorial Troll I’ve been doing some further fun experiments with biochar.
Biochar, basically, is charcoal made from any kind of organic material (biomass), not just wood. Like charcoal, it’s made by heating the biomass in the absence of oxygen, a process called pyrolysis. Biochar is useful for two main reasons: as a soil improver, and as a way of sequestering carbon dioxide from the atmosphere. Also, the gases produced by pyrolysis (mainly hydrogen and methane) can be burnt as fuel.
I have been using an extremely simple method that makes about half a kilo of biochar at a time. Take an old metal paint tin with a lid. Fill with biomass - basically, anything organic which is carbon-rich. Finely divided materials like sawdust and straw make the best biochar, but I’ve also used nutshells, orange peels, sticks… Put the lid on, but bend the rim back in one place to let the gases out. Place on hot embers inside the woodstove. First smoke, then jets of flame will come out of the tin. (The combustion of the gases produced by pyrolysis is now helping to heat the stove.) Once the flame stops, take the tin out, let it cool, and open it to reveal your biochar.
So far I’ve been composting the biochar, but I’m going to start experimenting with using it in a potting mix, to see how the plants respond. For this, I’ve soaked the biochar in diluted urine (as a source of nitrogen), then mixed it up in equal parts with sand and commercial potting compost.
Covering the earth with charcoal?
There was a big debate about biochar in last week’s Guardian, between George Monbiot, who thinks it’s being sold as a “miracle mass fuel cure“, and defenders of biochar, including James Lovelock, who agrees that “it would be wrong to plant anything specifically to make charcoal” but that biochar has net benefits if it’s made from agricultural wastes.
Monbiot says what’s being proposed amounts to “turning the planet’s surface into charcoal”. But is it really? I calculated just how much biochar would really be needed to store all the excess carbon dioxide in the atmosphere…
- The total weight of the atmosphere over each square metre of the Earth’s surface is 10 tonnes. (A handy figure to remember, that.)
- Carbon dioxide is now present in the atmosphere at 385 parts per million by volume (ppmv), equivalent to 582 parts per million by mass (ppmm).
- This is 35% higher than pre-industrial levels (284 ppmv = 429 ppmm).
- So the excess CO2 which needs to be removed is 153 ppmm.
- Conveniently, 1 tonne = 1 million grams. So one part per million of 10 tonnes is 10 grams. The 153 ppmm CO2 we need to remove comes out at 1530 grams per square metre of the Earth’s surface.
- By mass, CO2 is 30% carbon and 70% oxygen. So if all that excess carbon dioxide were fixed as charcoal (which is more-or-less pure carbon), it would come to 459 grams per square metre of the earth’s surface.
- The density of charcoal is 208 kg/m3. So this hypothetical global layer of charcoal would be just 0.459 / 208 = 0.002m, or 2 mm thick.
At first this sounds like a tiny amount. All the extra carbon dioxide that we’ve added to the atmosphere, amounts to what? 2mm of charcoal - the thickness of a pencil lead! But if the excess CO2 was magically taken out of the atmosphere, this layer of charcoal would cover everything - the land, the cities, the ocean, the ice caps…
Realistically, if we are talking about sequestering that CO2 as carbon in the soil, then we aren’t talking about the whole of the earth’s surface, but only about the arable land, because that’s (a) where most of the crop wastes are, (b) where the people are to do the charring, (c) where people are interested in improving the soil, and (d) arable lands are practically man-made ecosystems anyway, so if we need to modify land on a planetary scale, that’s the place to start.
About 2.7% of the Earth’s surface area is arable land - so in the unlikely event that this charcoal were spread out evenly over all the arable land in the world, there would be 17kg of charcoal per square metre, in a layer 8cm thick. This is indeed a large amount; typical carbon content of Terra Preta is only about 5kg per square metre - even though the black soil layer is many centimetres thick. And in the more realistic case that charcoal is only applied to selected soils, over say 1/6 of all the arable land, we’d need to apply 125kg per square metre, which is a 50cm thick layer of charcoal.
Suppose we look at the amount of biochar that would be needed to capture all our global carbon dioxide emissions, which came to 27 billion tons in 2004 - that’s 8 billion tons of carbon, which at 5kg a square metre (a typical rate for applying biochar) would need to be applied over 0.16 gHa of the world’s 1.36 gigahectares of arable land - i.e. 1/9 of the earth’s arable land. That amount would need to be applied every year to stop atmospheric CO2 concentrations from rising further.
I can certainly imagine this as a feasible option - at any rate, more feasible than letting the CO2 continue to build up in the atmosphere, or trying to get rid of it in outlandish and untested ways.
To come down to earth, if we wanted to apply 5 kg/m2 of biochar to our land, which is about 8000 square metres, we’d be making 40 tonnes of it. At the moment I’m still experimenting on a slightly smaller scale…