High-tech charcoal fights climate change An updated version of a technique used by Amazonian Indians hundreds of years ago offers a way to store carbon for hundreds or thousands of years while producing nonfossil fuelsa double whammy for researchers seeking tools for fighting climate change.
forest debrissdrives off volatile substances and unstable C, producing gas and oils that can be used for energy and leaving behind stable, C-rich charcoal. The concept is not new. Amazonian Indians mixed a combination of charcoal and organic
efficient cook stoves for poor families in Kenya to industrialscale units for processing suburban yard waste. Despite the apparent benefits, not everyone is enthusiastic about biochar. Almuth Ernsting, codirector of the nonprofit organization Biofuel Watch, worries that large-scale production could lead to the razing of tropi-
Extracting and storing energy with the biochar process.
Biocharscharcoal produced by heating organic material in the absence of oxygen (O2)snot only contains stable carbon (C), but may also help boost soil fertility. But the benefits depend on a complex combination of factors that must be controlled to make it economically attractive while ensuring that it is a net greenhouse gas (GHG) sink, rather than a source of emissions, according to an ES&T research article (Environ. Sci. Technol. 2009, DOI 10.1021/ es902266r) by Kelli Roberts and colleagues from Cornell University and the University of New South Wales. While biochar “can play an important role in reducing our greenhouse gas emissions,” Roberts says, the added advantage is that “it’s actually sequestering carbon and not just offsetting emissions.” The principle behind biochar is straightforward. Pyrolysis of biomasssfrom grass clippings to cornstalks to pine-beetle-infested
matter into the soil to make it more fertile. Scientists believe this terra preta, or “dark earth”, allowed large civilizations to thrive in places where the soil would otherwise have been too poor to produce large harvests. The stability of that C led researchers to investigate biochar for carbon capture and storage (CCS). Besides charcoal, “terra preta also [contains] animal waste, animal bones, and pottery shards,” Roberts says. “Those soils are found to be very fertile, and that’s what [biochar researchers] are trying to replicate.” Biochar’s effect on soil fertility appears to depend on the type of soil to which it is added, she says. Biochar “acts like a sponge,” holding nutrients and water, Roberts says, but farmers may still need to add nitrogen to the soil. One advantage of biochar, Roberts says, is its flexibility. Cornell University researchers are exploring possibilities ranging from fuel-
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cal forests for biochar feedstock plantations. “Human pressures on ecosystems and biomass are already unsustainable, from high demand for pulp and paper, agrofuels, animal feed, etc.,” Ernsting says. “Adding another huge demand will exacerbate existing problems and lead to more land conversion and more small farmers and indigenous people losing their land.” Roberts and her colleagues considered that hazard in calculating biochar’s profitability and its value for climate change mitigation. Of the various organic materials, or feedstocks, they considered, yard waste was most profitable. Agricultural residue, such as cornstalks, could be moderately profitable, but energy crops, such as switchgrass, carried the risk of a net increase in emissions because of land-use change. “We don’t want that to happen,” Roberts says. “If you put a switchgrass plantation where a
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tropical rain forest was, or if you [displace] food crops, it doesn’t make sense, and we’ll be in a worse position than when we started.” Biochar’s greatest promise lies in its C-storage capacity, which could make it attractive for emissions trading programs. Unlike forests, which could be burned or cut down, biochar “can’t be easily reversed,” and the amount of C is known and traceable, Roberts says. There are still several obstacles to including biochar in climate change mitigation programs, according to Simon Shackley of the University of Edinburgh School of Geosciences, who specializes in biochar policy issues. One is lack of data about the stability of biochar CCS under different environmental conditions.
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Field trial results have varied, indicating a need for better understanding of how biochar behaves in different types of soil, Shackley says. Ernsting noted that most field studies have been short, leaving biochar’s long-term fate unknown. Biochar’s profitability as part of a trading scheme will also depend on C prices, Shackley says. If prices remain low, it might be more profitable simply to produce energy from biomass instead of making biochar, he says. Nevertheless, biochar’s versatilitysfor energy production, CCS, and as a soil additivescould give it an advantage over other CCS methods. Unlike underground sequestration, in which C is gradually mineralized for long-term storage, “biochar gives carbon a use,”
Shackley says. “That’s a more philosophical point, but it makes biochar different from other options.” Roberts also sees biochar as one of many strategies that scientists and policy makers can use to mitigate climate change. She notes, however, that its carbon storage potential does not eliminate the need to decrease GHG emissions. “It might be able to provide a certain percentage of [carbon] offsets, but we certainly need many, many approaches, and the most important is reducing emissions,” Roberts says. “We hope people don’t [perceive] it as a silver bullet, but we want to put it out there as part of a multi-benefit approach to mitigating climate change.” —BARBARA FRASER
January 15, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9
