Heterogeneity of biochar properties as a function of feedstock sources and production temperatures.

The aim of this study was to quantify the influence of the two main categories of factors determining the yield and properties of biochar, i.e., feedstock properties and production conditions, here represented by the highest treatment temperature (HTT). To achieve this, a wide range of production temperatures (200-650°C) and an extensive set of diverse feedstock (n=12) were used to calculate the sensitivity. The quantitative evaluation was based on statistical analysis of coefficients of variation, and thus derived indices representing the extent of influence of the two factors, i.e., a feedstock-depended heterogeneity (HF) and a temperature-depended heterogeneity (HT). The results showed that both feedstock properties and production conditions are important for determining the yield and properties of biochar, but their respective influence changes with the property or set of properties of interest. The biochar parameters most affected by feedstock properties were e.g., total organic carbon, fixed carbon, and mineral elements of biochar. On the other hand, biochar surface area and pH was mainly influenced by highest treatment temperature. Biochar recalcitrance was mainly determined by production temperature, while the potential total C sequestration (product of recalcitrance and pyrolysis carbon yield) depended more on feedstock. Overall, the work sheds some light on the relative importance of different biochar production process parameters on the final biochar product, which is an important step towards "designed" biochar.