“There is one way we could save ourselves, and that is through the massive burial of charcoal” James Lovelock

Converting biomass into charcoal type char which can be used to improve soil fertility, while also trapping carbon dioxide, certainly has major attractions. Some energy is generated too. But a key issue is whether, in net climate terms, the loss of (some) biomass for direct conversion to energy is balanced by the gain from CO2 entrapment and extra CO2 absorption by more fertile soils- especially if the combustion route also used geo-sequestration i.e. CCS?

A parametric study of bio-sequestration by Malcolm Fowles at the Open University, suggested that from a global warming perspective we should displace coal with biomass if the latter’s conversion efficiency is much over 30%. Otherwise we should sequester carbon from biomass rather than generate energy.

However, this was only a preliminary study and he felt that a more comprehensive analysis might shift the balance more towards bio- sequestration. He did not include carbon savings from hydrogen and other pyrolysis products, or crucially from reduced soil emissions- that’s hard to assess after all. And costs were not included in his model, although qualitatively and intuitively he felt bio-sequestration should be cheaper than geo-sequestration by CO2 capture and storage. (Fowles, M. (2007), “Black carbon sequestration as an alternative to bio-energy’, Biomass and Bioenergy 31: 426-432, doi:10.1016/j.biombioe.2007.01.012)

Clearly though there are lot of unknowns- for example as to the permanence of bio-sequestration – how long will the carbon stay trapped in the soil? Some say thousand of years, based on historical examples of charcoal use. But then that was in traditional ‘no til’ agricultural contexts: farming methods would now have to change if we wanted to avoid releasing the stored carbon.

There are also strong views about the likely impact if biochar production was adopted on a wide scale. While some see it as a major way to deal with climate problems, the fear of vast agri-business plantations worries some people, Guardian correspondent George Monbiot especially, although even he accepts that there could be niche uses. http://www.guardian.co.uk/environment/2009/mar/24/george-monbiot-climate-change-biochar

Biochar can be produced by pyrolysis at around 500 degrees C, either slowly (over days, the traditional approach e.g. in kilns), which results in about equal amounts of biochar (about 35% of the original biomass), liquid and gaseous fuels; or rapidly (e.g. flash pyrolysis, in seconds), which gives less biochar (about 15% converted) less gaseous products, but more liquid ‘bio-oil’ products (about 75%). In addition there is high temperature (800C) gasification, which typically, over hours, yields a low proportion of solids (only about 10% biochar), but a high proportion of gaseous products (about 85%).

Clearly with fast pyrolysis or gassifiation the processing throughputs can be larger, but slow pyrolysis gives you more biochar in the mix. For example, BEST Energy in Australia, have developed a slow pyrolysis approach called Argichar, in which between 25 and 70 % by weight of the dry feed material is converted to a high-carbon char material, while also generating syngas: see www.ecovoice.com.au/enews/enews-47/Images%2047/Brief%20BEST%20pyrolysis%20and%20Agrichar%202007.pdf

Potential

How much carbon sequestration might be achieved? Globally, according to Professor Tim Lenton, from UEA, “Biochar has the potential to sequester almost 400 billion tonnes of carbon by 2100 and to lower atmospheric carbon dioxide concentrations by 37 parts per million.” How does that compare to other approaches, like Carbon Capture and Storage? Biochar production removes CO2 from the air, while CCS aims to remove it from the exhaust gases of power plants- in large quantities. According to Bruce Tofield, from the Low Carbon Innovation Centre, UEA ‘In the UK biochar might yield a few million tonnes CO2 saving with current biomass sources – CCS needs to aim for over 100 m tonnes’.

However, that doesn’t mean turning biomass into biochar is a bad idea, and some environmenalists are quite enthusiastic. In ‘The Renewable World’, a new book from the World Future Council, Herbie Girardet and Miguel Mendonca (Green Books) are very keen on techniques for improving soil fertility and biological carbon dioxide absorption, and talk of ‘carbon farming’. They note that ‘by pyrolysing one tonne of organic material which contains about half a tonne of carbon, about half a tonne of CO2 can be removed from the atmosphere and stored in the soil, while the other half can be used as carbon neutral fuel.’ However they add that ‘a major question that needs an urgent answer is how enough organic matter can be made available to produce significant amounts of biochar. Opponents argue that farming communities in developing countries may be forced to produce fast-growing tree monocultures on precious agricultural land to produce biochar to counter climate change for which they are not even responsible’. But they point to sewage as an example of a less contentious feedstock.

There are no doubt many other niche sources of biomass like this, as well as novel sources like algae, although there may also be competing uses -e.g. sewage gas is one of the cheapest renewable energy sources for electricity generation. But then we are back with the question of which is most effective at reducing carbon dioxide?

The Royal Society’s recent review of Geoengineering commented ‘It remains questionable whether pyrolysing the biomass and burying the char has a greater impact on atmospheric greenhouse gas levels than simply burning the biomass in a power plant and displacing carbon-intensive coal plants’. It concludes ‘biomass for sequestration could be a signi?cant small-scale contributor to a geoengineering approach to enhancing the global terrestrial carbon sink, and it could, under the right circumstances, also be a benign agricultural practice. However, unless the sustainable sequestration rate exceeds around 1 GtC/yr, it is unlikely that it could make a large contribution. As is the case with biofuels, there is also the signi?cant risk that inappropriately applied incentives to encourage biochar might increase the cost and reduce the availability of food crops, if growing biomass feedstocks becomes more pro?table than growing food’.

That is a point picked up by James Bruges in the new Schumacher society report ‘The Biochar Debate’ (Green Books). He argues for a global Carbon Maintenance Fund, rather than just awarding carbon credits. But that is rather going ahead of ourselves. First we have to see if the biochar option makes sense. The Royal Society pointed out that so far there was not enough research on the topic. Defra has commissioned the UK Biochar Research Centre (UKBRC) to review the impacts of biochar. Hopefully that will provide some answers.

More at http://www.biochar-international.org/