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Biochar properties affected more by intensity of pyrolysis process than feedstock type
Given the variability in pyrolysis processes and their accompanying process conditions in combination with a wide range of available biomass feedstocks for biochar production (including wood, energy crops, agricultural waste residues, etc.), biochar physicochemical properties are expected to vary.
Ronsse and coauthors address this topic by characterizing biochar produced from various feedstock biomasses and under a range of slow pyrolysis process conditions. Feedstocks used were pine wood, wheat straw, green waste and dried algae. Process conditions varied were the highest treatment temperature (HTT) and duration at the HTT (residence time). Four different HTT (300, 450, 600, and 750 °C) and two different residence times (10 and 60 min) were tested.
The biochars produced were then characterized by yield, proximate analysis (i.e. moisture, volatile matter and ash content), elemental analysis, heating value, and surface area. Higher volatile matter is known to decrease plant growth. The biochar was then incubated with soil to determine biological degradation.
In proximate analysis, it was found that the yield of biochar decreased under the highest HTT and longest residence time. The pH was affected by feedstock type and increased with HTT and residence time. The higher heating values increased with higher HTT but were not affected by feedstock type. Surface area was highest for wood and at high HTT, and decreased with longer residence time.
In the biological degradation experiments, soil incubation tests showed that the addition of biochar to the soil initially reduced the carbon mineralization rate (i.e. less carbon captured) compared against the control soil samples. This effect was more pronounced when adding chars with high fixed carbon content (resulting from more severe thermal treatment), as chars with low fixed carbon content (produced through mild thermal treatment) had a larger amount of volatile, more easily biodegradable, carbon compounds.
Once the relationships between feedstock, production process, and biochar properties are known, this knowledge will permit the production of tailor-made biochar. More specifically, an appropriate selection of biomass feedstock and pyrolysis parameters could be made that yields a biochar optimized for a specific behavior within the soil both in terms of soil fertility improvement and enlarging the pool of stable carbon within the soil.
Ronsse F, van Hecke S, Dickinson D, Prins W (2013) Production and characterization of slow pyrolysis biochar: influence of feedstock type and pyrolysis conditions. GCB Bioenergy, 5, 104–115.. doi: 10.1111/gcbb.12018 Read this paper
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