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1 April 2008 Limitations of Char from Biomass
Vince Gutschick
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Day and Hawkins (BioScience 57: 814–815 [doi:10.1641/B571002]) claim that they have a technology to convert biomass to a fertilizer that is part char and part ammonium carbonate. Various claims for the product are not credible. Foremost, the purported nitro gen (N) content is too high. One hundred kilograms (kg) of (assumed dry) biomass, as plant material, should contain about 2% N or 2 kg N, even less if it is high-cellulose material such as grain stover. The product is said to be 56.4% (56.4 kg) ammonium bicarbonate, which in turn is 17.7% N. (I note also that the quotation to 3 significant figures is inappropriate, given the variability in feedstocks.) Thus, the total N would be 0.177 • 56.4 = 10.0 kg N, far too high. The authors also claim that additional N is sequestered in the char as ammonium nitrate, making the N content even less believable. In a digital publication (Energy [doi:10.1016/j.energy.2004.07.016]), Day and colleagues disclose that the ammonia could have been (but was not) made with a hydrogen byproduct of pyrolysis.

The ability of biochar to increase the availability of several nutrients is also conditional, not absolute or necessarily positive. Nutrient retention by strong adsorption is useful in soils with very low ion-exchange capacity (sands, e.g.) but can be competitive with plant uptake in richer soils. One of the academic research groups (see  www.css.cornell.edu/faculty/lehmann/biochar/Biochar_home.htm) shows biomass gains only at low applications, in soil initial conditions that are not well specified. The value of biochar for sequestering carbon (about half of the original input to the pyrolyzer) is perhaps more credible for the moderate term (century scale). The sequestration should be debited for the fuel use in pyrolysis, which is not specified.

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Vince Gutschick "Limitations of Char from Biomass," BioScience 58(4), 285, (1 April 2008). https://doi.org/10.1641/B580420
Published: 1 April 2008
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