Translating analytical pyrolysis fingerprints to Thermal Stability Indices (TSI) to improve biochar characterization by pyrolysis-GC-MS

Biochar thermosequences produced from the charring of poultry manure and freshwater macroalgae feedstocks between 300 and 700 °C were analyzed by pyrolysis-GC-MS to assess the nature of the thermochemical conversion of N-rich feedstocks. With increasing charring temperature (TCHAR), the products of intact lignin, protein and polysaccharides decreased whereas those of charred aromatic domains (monocyclic and polycyclic aromatic hydrocarbons, benzonitriles) increased. These results are in agreement with thermosequences obtained from lignocellulosic feedstocks under the same analytical conditions. Therefore, we aimed to create a universal proxy of the degree of thermochemical alteration of biochar thermosequences from diverse series of feedstocks (gorse wood, chestnut wood, rice straw, poultry manure, freshwater algae and tannin) using Principal Components Analysis (PCA). From the PCA the relation of pyrolysis products with charring intensity was established and translated to Thermal Stability Indices (TSIp), which then gave rise to calculations of TSI of the biochars (TSIb). The TSIp varied only slightly between the different feedstocks suggesting that they could be used in future research to interpret pyrolysis fingerprints for a wide range of different biochars. In addition, TSIb can be used as a thermostability proxy for biochars and were found to be more reliable than other pyrolytic proxies such as the benzene/toluene or naphthalene/C1-naphthalenes ratios. This study marks the first attempt to develop a single proxy of stability for biochars at TCHAR from pyrolysis fingerprints using 157 common pyrolysis products. These proxies will provide a simple measure for the usefulness of a biochar for C sequestration and/or soil amelioration.