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Biochar.... a Miracle to Save the Planet?
14 Oct, 2008 10:55 am
Biochar is a kind of charcoal, produced by the low temperature carbonization of biomass. During the pyrolysis process other materials are produced, both gaseous and liquid organic products which might be used to substitute for petroleum in the manufacture both of fuels and organic feedstocks for industry. The carbonaceous residue, biochar, can be added into soil and is reported to improve the quality of some soils making them more fertile. Last but not least, the process is described as "carbon negative", thus purporting the means to actually extract carbon from the atmosphere to alleviate the putative effects of greenhouse gas induced global warming and climate change. A case too far or not?
Back to biochar: which is a kind of charcoal with several appealing qualities. Number one is that if the hypothesis that global warming is causing the world climate to change is correct, in consequence of humans pushing CO2 into the atmosphere, then by growing trees and other forms of biomass which absorb CO2 via photosynthesis, if this is then pyrolysed (heated to cause chemical decomposition) there remains a solid carbonaceous residue which can improve the growing properties of some soils if it is integrated into the top layers. Put another way, carbon is pulled down from the atmosphere and dumped in solid form into the earth. Improving the fertility of soils might also save on the amount of chemical fertilizers that need to be applied to soil to obtain adequate crop-yields, and result in some curbing of our demand upon a declining world resource of phosphate rock which peaked in production twenty years ago. In truth we will have to redesign the way we live, not only to rely far less on personal transportation, but to recycle nitrogenous and phosphate components from human and animal waste, in order to grow food, let alone a crop for biochar. Ideally those two practices can be integrated, so for example, the proverbial chaff from wheat might be converted into a stable form of carbon-rich material that persists in soil for thousands of years, or at least hundreds, actually drawing-down carbon from the atmosphere and cooling the earth, according to the greenhouse effect hypothesis.
There is scarcely doubt that it is the greenhouse effect that keeps the earth warm; a theory advanced by Professor Svente Arrhenius more than 100 years ago. Arrhenius also has his name attached to the pre-exponential or frequency factor in the rate equations of physical chemistry, known colloquially as Arrhenius Equations, that allow us to quantify chemical reactions and other dynamic processes involving molecules. He also proposed an unconventional though now known to be correct theory about ions in solution. For his transgression of prevailing thinking, the Swedish scientific establishment vigorously opposed his being awarded a professorship, although he went on to win the Nobel Prize for chemistry in 1903. A truly brilliant man. The mean global temperature is reckoned at 15 degrees centigrade, and without its influence, it would be nearer minus 15 degrees. Throughout measured geological history, mostly as may be gauged from ice-core samples, every one hundred thousand years the earth experiences an ice-age and then warms into an interglacial period, such as we have now. I have occasionally ruminated that there may have been equally albeit differently advanced civilizations formed on the earth in previous warm times, which were then wiped-away and all evidence of them so, by the following catalogue of ice and glacial grinding. Who knows?
Also as the earth warms the level of CO2 follows the warming, with a lag of roughly 800 years. The present global-warming movement presumes that the current levels of atmospheric CO2 are causing the Earth to warm, and will do so yet more spectacularly in the forthcoming decades. Since these concentrations are unprecedented over 3 million years, this may be true. It is also the case that the mass-balance (as I have written on here before) corresponds closely between the amount of fossil carbon we have burned into CO2 into the atmosphere and its current quantity. The evidence that the atmospheric CO2 is becoming increasingly richer in the lighter 12C isotope also supports the conclusion that the significant increase in this gas since 1950 is indeed derived from fossil fuels. What, nonetheless is not certain is that the amount of CO2 in the atmosphere is warming the Earth. There are credible theories created in very credible minds, that the warming of the Earth has alternative origins. We simply don't know, and it is debatable whether it is imperative or justified to turn over all of civilization to carbon-capture strategies.
However, all roads lead to Rome. Since we are threatened by the depleting resource of fossil fuels, most immediately oil, cutting back on our rate of burning them is the single option to take the edge off this imperative; to buy us some more time to regroup. If the greenhouse-gas theory is true then we shall save ourselves and our offspring generations much suffering. The actions in either case are the same. The best way to capture solar energy is through photosynthesis and thus we may grow our way to hope in terms of food production, energy provision, soil fertility and the remediation of our carbon excess. Socially and spiritually such cooperative and concerted actions may be our saving grace.
Biochar - Atmospheric CO2 "Mitigation".
Here are a few sums about 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!