Electrical conductivity of biochar: method optimization and insights from an industrial data set

Depending on its condensed aromatic structures, biochar varies in electrical conductivity. Measuring this conductivity allows to conclude about carbon speciation and, indirectly, about pyrolysis conditions. Recently, the electrical conductivity of dried and non-suspended biochar, termed as solid-state electrical conductivity (SEC), has been adopted as an analytical parameter in the European Biochar Certificate and is now proposed by several commercial and academic laboratories as routine analysis. Here, we present the corresponding measurement setup and test its limitations. Electrical conductivity was measured for 488 biochars produced in pilot plant and industrial pyrolysis systems. As known from literature, SEC of carbonaceous powders depends strongly on measurement conditions, which must be precisely defined to ensure comparability. For the presented setup, biochars should be milled to a median particle size (d<inf>50</inf>) between 25–150 µm and preferably measured under a compaction pressure of ≥ 62 MPa, which minimizes the influence of particle size relative to the effect of carbon speciation. The reproducibility of SEC measurements on a given sample was < 4–6%, and operator-to-operator variability < 3%. Biochars produced at 400 °C to 900 °C predominantly are semi-conductive carbons with SECs between 10^–6 and 10⁴ mS cm^-1, increasing exponentially with pyrolysis temperature. Consequently, SEC correlated well with the hydrogen to carbon molar ratio, which is a widely used parameter to estimate biochar aromaticity and the mean residence time of biochar in soil. The presented method provides a simple tool to approximate carbon speciation and is highly sensitive in monitoring biochar quality and batch homogeneity.