Charcoal is, of course, nothing new. People have been making it for millennia, chiefly for fuel. The process is simple: take wood, or straw or the waste from crops, and heat it in the absence of oxygen. Traditionally, this was done by heaping earth on top of the lit biomass so that it smouldered for a long time. Modern kilns can make the process more efficient, but the principle remains the same.
There is much about biochar that remains a puzzle, however. Take the soil-fertility effects. What is it about biochar that improves soil so much? "The simple answer is that we don't know exactly," says Simon Shackley of the University of Edinburgh. "It's probably a combination of several factors. Charcoal is very porous, so it acts like a sponge in retaining water, and the nutrients dissolved in water, which is something poor soils aren't very good at. And [its porous nature] also means it provides a good material for the growth of lots of important bacteria."
Another factor in its favour is that using biochar as a fertiliser can displace artificial nitrogen fertilisers, which give off nitrous oxide, a greenhouse gas 300 times more powerful than carbon dioxide. And biochar is not toxic, adds Tim Lenton of the University of East Anglia. "No one has yet said there is some great hidden danger associated with it."
But Saran Sohi, a lecturer in soil science, warns that anyone hoping that biochar alone will solve fertility problems is probably deluded; biochar is not enough by itself to make the difference that terra preta -- "dark earth" --does to thin Brazilian soils. "Terra preta soils also contain other nutrients, from the other substances they contain -- things like bones, which are rich in phosphorus" [essential for healthy plant growth], he says. The biochar undoubtedly plays a role in holding these nutrients together, ensuring they remain available to plant roots, but the nutrients must be provided by other means. "No one has yet succeeded in recreating terra preta," Shackley adds.
To produce biochar on an industrial scale, traditional methods of charcoal production would be impractical. Instead, researchers are looking to pyrolysis -- a form of controlled thermal decomposition of organic material in the absence of oxygen, at heat that can reach 500° to 600° degrees Celsius.
Using pyrolysis also allows the capture of the syngas and the tarry liquid byproducts, both of which can be used as fuel to generate electricity or for the heating process.
The amount of biochar to be produced depends on accelerating or slowing down the pyrolysis process: fast methods produce 20% biochar and 20% syngas, with 60% bio-oil, while slow methods produce about 50% char and far smaller quantities of oil. "It's much easier to do slow pyrolysis as well," notes Adrian Higson of the United Kingdom's National Non-Food Crops Centre (NNFCC). "And cheaper." As modern pyrolysis plants can be run entirely from the syngas, the output is between three and nine times the energy input required, according to the Institute for Governance and Sustainable Development (IGSD).
What to use to make the char? Tearing down forests to turn into charcoal would be insane in climate-change terms. But there is plenty of other material. Agriculture produces large amounts of plant and animal waste -- straw, husks, dung. Even human waste -- sewage sludge, or some forms of household rubbish -- could be used.
And using waste products creates a double carbon saving: if left to rot, they produce methane, a greenhouse gas 20 times more powerful than carbon dioxide. But the difficulty is in gathering the waste -- and making it economic to do so. Farmers will require some persuasion that the trouble of conserving and cooking their waste to a charcoal makes financial sense, and they may need new machinery to do so. At a municipal waste level, the problem will be sorting the organic waste, which can be turned to char, from the rest of the rubbish -- and proving that this is cheaper and more beneficial than merely burying it.
The IGSD suggests a way of marrying small-scale and industrial methods for producing the char, that if refined could enable the economically viable production of biochar in urban, rural and even poor regions. It suggests three possible systems. The first is a centralised scheme, whereby all waste biomass in a given region would be brought to a central plant for processing; the second is a decentralised system in which each farmer or a small group of farmers would have their own fairly low-tech pyrolysis kiln.
The third system proposes a mobile alternative, in which a vehicle equipped with a pyrolyser -- powered using syngas -- would visit small farms, returning the biochar to the farmers to use, while collecting the bio-oil to be transported to a refinery and turned into liquid biofuel for vehicles. As an example, the IGSD cites Brazil's sugar cane industry, in which the tops of the canes, normally burned in the field, and the bagasse -- the residue from sugar production -- could be turned efficiently into biochar. It estimates that of the 460 megatonne annual sugar cane harvest, as much as 230 megatonnes could be available for pyrolysis.
A clutch of companies is now working on these problems, and seeking to commercialise biochar as a medicine for both climate and soil, and as an energy source.
As Mike Mason, founder of the carbon-offsetting company Climate Care, bought by JP Morgan, somewhat ruefully notes, he had been planning by now to spend most of his time charging round Africa looking at elephants. But instead he decided that climate change was too great a problem to leave alone, and with his new company, Biojoule, has been investigating ways to turn biochar into a viable business. In Ontario, Canada, Dynamotive is making biochar and up to 130 tonnes a day of bio-oil at a wood-products mill. Crucible Carbon, based in Australia, predicts that its technology will allow carbon sequestration from biochar at the cost of about US$13 (20 Australian dollars) a tonne.
Yet even without the logistical problems, others are less sure of the absolute benefits of the product. Robert Trezona, head of research and development at the Carbon Trust, a UK government-funded body that helps businesses cut their greenhouse-gas emissions, worries that seeing biochar as the main output from cooking biomass might be to miss the point.
The Carbon Trust is running a competition to develop pyrolysis plants, but with the aim of manufacturing liquid transport fuels from biomass, using fast pyrolysis techniques, to which biochar is merely a byproduct of questionable usefulness. "Producing liquid biofuels for transport is going to be very important in cutting emissions. We don't know the same about biochar," Trezona says. In fact, encouraging small farmers to produce biochar by traditional, low-tech methods may actually result in more greenhouse-gas emissions than simply burning the plants for fuel or discarding them, he says.
"This is very much unproven," Trezona asserts. "You want to be able to show that it stays in the soil for hundreds of years, and to prove that is difficult."
The Carbon Trust is not allowing companies applying to it for funding to count the biochar byproduct of pyrolysis as part of the carbon savings they produce. "We are a long way from having enough technical evidence to create a proper case for biochar," says Trezona. "Even the soil-improvement benefit is a new unexpected finding."
Flannery disagrees. "At least half of the carbon in charcoal is still sequestered 500 years later. This has been known for a long time, from radiocarbon dating from charcoal by paleontologists," he says.
Even if biochar does not fulfil all of the potential claimed for it, it could still make an important contribution. Al Gore, the former US vice-president and environmental campaigner, likes to point out that the search for a "silver bullet" to solve the problem of climate change has been a distraction. Instead, he argues, though there may be no silver bullet, "there is silver buckshot". Only by bringing many different methods of cutting emissions or absorbing carbon to bear can we reduce atmospheric levels of carbon to within the limits of safety. And of those possible methods, few are as simple and cheap as biochar.
Johannes Lehmann of Cornell University makes the point that "biochar sequestration does not require a fundamental scientific advance and the underlying production technology is robust and simple, making it appropriate for many regions of the world".
But no one should doubt that rolling out this technology will be a mammoth task. The problem is twofold: developing a model for biochar production that reliably reduces greenhouse gases but is easily replicable in small farms in poor countries; and in the developed world, changing the business model of large farms so that collecting and cooking their waste is a better option than not.
The huge US agribusinesses may be easy to reach, and good candidates to start using their waste for char, but they are likely to need financial incentives before they begin to see the point. The poor farmers of the developing world might be glad of the husbandry advice and techniques that would help them revitalise their own soils with biochar -- but how to reach them all? That may prove impossible.
These problems of economics and communication will be the real hurdles at which biochar may fall, just as they have been the reasons why we have failed to capitalise on other ways of cutting carbon, from the very simple -- small alterations to wood-fired cooking stoves in Africa and India can reduce the indoor air pollution from cooking fires that kills millions, yet hardly any homes have them -- to the complex challenges, such as adopting renewable energy. A massive effort will be required to overcome the inertia that has been the downfall of other great climate ideas.
Homepage photo by Biochar.org
Copyright The Financial Times Limited 2009