Assessing biochar’s permanence: an inertinite benchmark

The natural removal of carbon dioxide and its permanent storage by the Earth system occurs through (i) inorganic carbon and (ii) organic carbon pathways. The former involves the “mineralization” of carbon and formation of carbonate minerals, whereas the latter employs the “maceralization” or natural carbonization of biomass into the “inertinite maceral”. The production of biochar is a carbon dioxide removal (CDR) method that imitates the geological organic carbon pathway, using controlled pyrolysis to rapidly carbonize and transform biomass into inertinite maceral for permanent storage. Therefore, the main challenge in assessing biochar's permanence is to ensure complete transformation has been achieved.

Inertinite is the most stable maceral in the Earth's crust and is hence considered an ultimate benchmark of organic carbon permanence in the environment. Therefore, this study aims to measure the degree of biochar's carbonization with respect to the well-established compositional and microscopic characteristics of the inertinite. The random reflectance (R_o) of 2% is proposed as the “inertinite benchmark” (IBR_o2%) and applied to quantify the permanent pool of carbon in a biochar using the R_o frequency distribution histogram. The result shows that 76% of the studied commercial biochar samples have their entire R_o distribution range well above IBR_o2% and are considered pure inertinite biochar. The oxidation kinetic reaction model for a typical inertinite biochar indicates a time frame of approximately 100 million years for the degradation and loss of half of the carbon in the biochar. This estimate assumes exposure to a highly oxidizing environment with a constant surface temperature of 30°C, highlighting the inherent “permanent” nature of the material. In a less hostile environment, the expected permanence of inertinite is generally anticipated to be even longer.

In addition to the inertinite that constitutes the largest fraction of the typical commercial biochar, an incompletely carbonized biochar may contain up to three other organic pools in descending order of stability. The relative concentration of these pools in a biochar can be quantified by a combination of geochemical pyrolysis and random reflectance methods. Furthermore, the R_o can be used to calculate the carbonization temperature (CT^oC) of a biochar, which is the maximum temperature to which biochar fragments have been exposed during pyrolysis. This indicator provides important information about the efficiency of the carbonization process and subsequently the biochar's stability, with respect to production temperature (PT^oC), heating residence time, and thermal diffusivity.