Quantification of the persistent aromatic carbon content in biochar

Quantification of the time-dependent carbon sequestration by biochar remains a challenge. Recently, hydrogen pyrolysis was suggested to identify the content of stable polycyclic aromatic carbon (SPAC) or persistent aromatic carbon (PAC), which would not degrade to a relevant extent for centennial timescales due to their high degree of aromaticity and condensation (>7 rings). However, hydrogen pyrolysis is too expensive and laborious for broad application or even use in routine analysis.

Here, we test a suite of analytical methods in order to identify alternatives to hydrogen pyrolysis for the quantification of SPAC/PAC in routine analysis of biochar or other pyrogenic carbons. We use 34 experimental biochars out of two precursors, wood and straw, produced at different highest treatment temperatures (HTT = 400-800 °C) and eight industrial biochars obtained from different feedstocks and different pyrolysis technology with HTT of 550-1200 °C. The methods include elemental analysis to obtain molar ratios of H/C and O/C, electrical conductivity of the solid as a proxy for the degree of condensation, thermogravimetric analysis coupled to differential scanning calorimetry (TG-DSC) to assess thermal stability (R50) and Raman spectroscopy. Raman spectra of the D- & G band provide information on the nanostructural development and should allow relative quantification of the semi-persistent and persistent carbon fractions. An incubation experiment (biochar + sand + compost microbial consortium) under laboratory conditions will provide direct data on biochar mineralization to quantify the semi-persistent fraction (SPC).

Using a broad range of HTT and two different precursors as well as industrial biochars out of existing commercial pyrolysis reactors, we aim to cover a relevant parameter space to identify the possible range of SPAC/PAC and SPC content in industrial biochars. In an ideal case, this work will enable the use of low-cost technology such as Raman and/or electrical conductivity of the solid to quantify or estimate SPAC/PAC with a sufficient accuracy and precision.