The comparison study of multiple biochar stability assessment methods

Abstract

Biochar produced from thermochemical processing biomass is being developed as an effective way to mitigate climate change. The stability of biochar means the potential capability of fixing biochar carbon in the soil environment. The method for biochar stability assessment is significant to its climate change mitigation potential. However, there is no standard method available to use, and the stability of biochar assessed by the diverse methods has a significant difference. Nevertheless, the comparative study on the diversified stability assessment methods is limited. In the present study, the biochars were pyrolyzed from soybean straw, wood sawdust, and Chlorella Vulgaris at temperatures ranging from 300 to 800 °C. Then, the evaluation methods, such as ultimate analysis, proximate analysis, the Edinburgh stability tool, X-ray photoelectron spectroscopy (XPS), solid state 13C nuclear magnetic resonance (NMR), and thermogravimetric analysis, were compared to assess the stability of biochars. The stability indicators obtained by most methods, excluding aromatic C from NMR and C–C/C = C/ C–H from XPS, were closely related to the pyrolysis temperature (ANOVA analysis, p < 0.05). The volatile matter/(fixed carbon + volatile matter) had high correlations with other indicators (Pearson coefficient, |r|>0.36, p < 0.05) except for C–C/C = C/C–H from XPS. The cluster analysis indicates the studied stability methods could be divided into three categories, according to which the proximate analysis may be developed as an alternative to O/C and H/C. These research findings help provide the reference for the classification and selection of stability assessment method for the accurate and efficient evaluation of biochar stability.