Jun 10, 2009
Biochar – a burning issue
Comprehensive research into the properties, types and applications of biochar are required for its successful and judicious use, say UK experts. They are also calling for land-use accounting to play a part in policy decisions.
Biochar – a charcoal-type material produced by cooking organic matter in a low-oxygen environment (pyrolysing) – holds great potential for carbon mitigation. The manufacturing process releases volatile gases that can be made into liquid fuels or used to generate power or heat. The resulting char supplies a range of fertility-boosting nutrients when spread on soils. And its molecular structure makes biochar resistant to decomposition by micro-organisms – so it fixes carbon for very long time periods.
However, talk of widespread, industrial-scale deployment of biochar, with some even advocating that slow-growing forests are replaced with fast-growing trees for logging, pyrolysing, burying and repeated replanting, means the technology has attracted considerable controversy.
While the debate continues as to whether biochar offers the ultimate win-win solution or paves the way to biodiversity-threatening monocultures, scientists are getting on with more pragmatic challenges – like discovering exactly how it works.
Simon Shackley, lecturer at the newly formed UK Biochar Research Centre (UKBRC), explained that this is no small task. Biochar is a natural product and as such is hard to characterise. Its behaviours are hard to predict and the benefits are context-specific.
The main challenge, he explained, is the number of variables. The feedstock for the pyrolysis process, which can range from kitchen waste to manure to forestry cuttings, determines many of the mineral and structural characteristics of the resulting char. Heating time and temperature also affect the final properties. There is as yet no dominant engineering design for pyrolysis units, so different equipment also produces different results.
What's more, different biochars react in different ways in different soil types and climates – and offer benefits to different crop types. The carbon-fixing performance of a biochar product depends on whether the fuel by-products are used effectively to replace fossil fuels; and the issue of whether a greater emissions reduction is achieved simply by burning the feedstock or whether it should be converted to biochar and a fuel is also dependent on a number of factors – including which fossil fuel is used for comparison.
Shaping the debate
There is widespread agreement that biochar must be used appropriately if it is to be of measureable benefit – but much work is still needed to define "appropriately".
"It's difficult to provide a generic definition of appropriate as it's very context specific," explained Shackley. "The indirect effects on land-use are of course very important; the controversy about biofuels has already reframed the debate in this area. We can't rule out the possibility of dedicated biofuel crop systems with a biochar element – for example sugar cane plantations where the waste is turned into biochar – but we consider that extreme biochar farming would be deeply problematic – in terms of its effects on biodiversity, water and other ecosystem services.
"The biofuels debate told us that a focus on a single issue entails a high risk. And public perception [of the risk] is important – as important as the reality.
"However the risk of over-commercialisation at this stage is unlikely. The real challenge is convincing farmers and industry to take it up – they need lots of evidence."
No to carbon-negativity
The first step to gaining public and industry trust, Shackley thinks, is to avoid presenting biochar as a "free lunch". He has particular reservations about the term "carbon-negative," often touted in biochar circles. "This is not a very useful concept," he observed. "There are too many assumptions and approximations."
Subsequent steps entail gathering the evidence that will guide cost- and carbon-effective production and use of biochar. The UKBRC research team is currently establishing what makes a good biochar – in terms of its stability, its agronomic benefit to soils, and its ability to control and contain contamination. Pilot biochars will then be tested in situ.
"Over the coming years there will be increasing numbers of small field trials across the country," revealed Shackley. "Of course we won't know the long-term effects – but we can't afford to wait for 10 years to find those out."
There are some applications that he described as "no-brainers" – for example for small-scale use in developing countries, where farm or forest waste products are transformed into biochar together with fuel that replaces diesel for off-grid electricity generation.
On the other hand, Shackley pointed out that building a pyrolysis plant in the US to process 70,000 tonnes of corn stalks per annum would require significant demand for the biochar as a primary, high-value product, since electricity generated from the fuel by-products may not be in enough demand to attract a high price. Biochar prices vary greatly; market conditions may exert the strongest influence on the scale and rate of adoption of biochar.
Shackley also noted that if deployment on a global scale is to be managed responsibly, it will be essential for governments to work together to ensure that land-use changes are included in carbon accounting methods.
The challenge for scientists though, emphasised Shackley, lies in characterising and classifying the biochars. "It's not proper science if you can't control the variables. Ultimately we'll probably want to design different biochars for different properties and applications," he reflected.
About the author
Vanessa Spedding is a contributing editor to environmentalresearchweb.