This posting follows my last on biochar and the likelihood of it being used as a long-term form in which to store carbon captured from the atmosphere. In a nutshell (no pun intended), plants absorb CO2 through photosynthesis, are harvested and then pyrolysed to yield this relatively stable form of carbon along with a release of energy and other useful liquid and gaseous products, some of which might also be used to furnish fuels. To be practical, the process must produce more energy overall than it consumes. The biochar is tilled into soil which can improve its fertility, crop yield, fertilizer requirements and water-retention abilities. Thus, many pressing issues are addressed in a single action, in respect to global warming, phosphate and water shortages, and the difficulty in growing enough food to feed the burgeoning world population and alleviating poverty in the developing world. Put in such terms biochar begins to sound little short of a miracle.
Humans emit around 7 billion tonnes of carbon into the atmosphere annually from burning fossil fuels, and so that amount must be absorbed in addition to remediating the levels of CO2 that are already there. In rough numbers, if theories about anthropogenic global warming are correct, it would be a reasonable aim to deplete the amount of CO2 in the atmosphere to pre-industrial levels, or say a drop from 380 to 280 parts per million (ppm), or 100 ppm. The mass of the atmosphere is 5.3 x 10^15 tonnes (less than one millionth the total mass of the Earth), and thus it contains:
(44/30) x (12/44) x 100 x 10^-6 x 5.3 x 10^15 = 2.12 x 10^11 tonnes, or 212 Gt of carbon. In this sum, 30 is asumed to be the average molecular mass of an "air" molecule, 12 is the atomic mass of carbon and 44 the molecular mass of CO2.
Over a 40 year period (so that we have accomplished out feat by 2050, the magic year when all governmental targets are to be met), we thus need to remove 212 + (40 x 7) = 492 Gt of carbon, which works out to 12.3 Gt per year.
If we assume a mean crop-mass of 30 tonnes per hectare per year of which 40% is carbon based on a carbohydrate formula of C6H12O6, this amounts to 0.4 x 30 = 12 tonnes of carbon per hectare per year, and so we would need (12.3/12) x 10^9 ha = 1.02 x 10^7 km^2, i.e. around 10 million square kilometres of land to grow it on. This can be compared with 150 million km^2 for the total land mass of the earth, of which around 15 million km^2 is arable and around another 30 million is pasture land. There are swathes of existing forest (including rainforests) but we don't really want to begin cutting them down, since they are principal carbon-sinks, although growing trees e.g. sycamore etc. as part of a managed sustainable programme (harvesting them at regular intervals) might make a substantial contribution to the total carbon-capture volume.
Not all of the arable crops can be converted to biochar, of course, but manure etc. might be from the animals and humans that eat them. Probably, to achieve the aim of capturing almost 500 Gt of carbon over 40 years would require working close to the limits of the planet's growing capacity, and a concomitantly vast investment in engineering, along with policy, commercial, social and all other aspects in an integrated programme. Like many other postulated sustainable technologies, biochar too may fail the crucial "Scale Test" in the final feasibility analysis.
Finally, what would be the depth of biochar generated by the capture of 492 Gt of biochar (essentially carbon)?
If we assume a density for carbon of 1 tonne/m^3, that gives a volume of 492 x 10^9 m^3 for its biochar. If we use that same land area of 1.02 x 10^7 km^2 = 1.02 x 10^13 m^2, we have a thickness of:
492 x 10^9 m^3/1.02 x 10^13 m^2 = 0.048 m = 4.8 cm,
or a mere sprinkling of around two inches!