Is biochar production really carbon-negative? Bioenergy is seen as one of the primary possibilities for preventing global warming—one of the most serious environmental problems humans have ever faced. In a recent article, “Rethinking biochar” (Environ. Sci. Technol. 2007, 41, 5932–5933), Rebecca Renner describes how pyrolysis of biomass is envisioned as a method for bioenergy production and carbon sequestration that can also have positive effects on soil fertility. During pyrolysis, syngas and/or biooil are produced along with a char residue (biochar). Biochar is highly resistant to microbial degradation upon land application and has a number of positive effects relating to soil fertility. The approach is referred to as carbon-negative because carbon is sequestered in the soil in
the form of biochar, thus releasing less carbon than do carbon-neutral technologies. The term carbon-negative is very compelling, but before it becomes widely used, we question whether it is appropriate. To do this, we ask readers to consider two theoretical cases. In the first case, a certain amount of biomass is used for bioenergy production. Because the carbon is fixed during plant growth, this case is considered carbon-neutral. In the second case, the same amount of biomass is converted to biochar and applied to soil, where it does not decompose. In the first case, no carbon is sequestered, whereas in the second case, all of the carbon is sequestered. However, in the first case, energy is extracted, but in the second case, no energy is extracted. This means that a corresponding amount of fossil fuels would have to be used to produce the energy, effec-
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tively resulting in comparable carbon emissions. The approach described by Renner can be considered a combination of the two cases above, in which some carbon is used for energy extraction and some is used for biochar production. The above considerations are inaccurate for numerous reasons. For example, the energy content per carbon atom of biomass is different from that of fossil fuels, and biochar will eventually be degraded. However, pyrolysis with associated biochar production is not generally associated with smaller carbon emissions than other forms of bioenergy production. SANDER BRUUN and JESPER LUXHØI Faculty of Life Sciences University of Copenhagen, Denmark
[email protected] © 2008 American Chemical Society
