Understanding Biochar’s Role in Achieving Permanent Carbon Removal: A benchmark for Biochar Carbon Removal (BCR)
Removing carbon from the atmosphere is key to achieving net zero and the urgency of addressing climate change is growing. Sequestering carbon in a geologically stable, solid form is the most durable way of actively reducing the amount of carbon in the atmosphere. Currently, there three main approaches are being explored: Carbon Capture and Storage (CCS), either from biogenic sources (BECCS) or the ambient air (DACCS), Enhanced Rock Weathering (ERW), and Biochar Carbon Removal (BCR). BCR stands out due to its rapid conversion of biomass into inertinite, a highly stable form of carbon.
Understanding Carbon pathways
In nature, atmospheric CO2 is sequestered and transferred into permanent carbon storage through geological processes, encompassing both the inorganic and the organic carbon pathway. CCS and ERW both rely on the inorganic carbon pathway, leading to long-term sequestration by converting CO2 into mineral carbon (carbonates) within sedimentary rocks.
Fig. 1. A simplified schematic representing permanent carbon storage through natural inorganic and organic carbon pathways. In the inorganic pathway, CO2 is mineralized into carbonate minerals, enabling the permanent storage of carbon. Simultaneously, the organic pathway involves the carbonization (maceralization) of biomass into inertinite maceral for permanent carbon storage.(1)
Both the inorganic and the organic carbon pathway ultimately lead to permanent forms of carbon. BCR can be viewed as a shortcut of the organic carbon pathway, wherein the thermochemical conversion of carbon from the original biomass feedstock into inertinite maceral occurs within the reactor in a matter of minutes. This process yields the most stable form of organic carbon.
Thus, carbonates are regarded as the terminus of the inorganic carbon cycle, contributing to the accumulation of stable inorganic carbon. Biochar, or inertinite, represents the most advanced and enduring form within the global organic carbon pool.
New benchmark for organic carbon pathway
Not all biochars are equal(ly stable). How can a mature stage of carbon be defined for both pathways to assure a permanent and quantifiable removal of carbon from the atmosphere? Sanei et al. from Aarhus University and the Geological Survey of Denmark & Greenland have proposed a new benchmark, defining the permanent carbon fraction in any biochar sample (full paper linked below).
In their study, they utilize Random Reflectance (Ro), an optical tool, to quantify the permanent carbon fraction in biochar by the reflection of incident light from its surface. With increased carbonization, the Ro increases until a geologically stable form of carbon is produced, reaching an Ro of 2% (inertinite benchmark, IBRo2%).
Their findings: With a half-life in the range of millions of years, fully carbonized biochar defines the endpoint of the organic carbon cycle. The oxidation kinetic reaction model shows that inertinite biochar (consisting of pure inertinite) has a half-life of approximately 100 million years in a harsh, oxidizing environment. Based on these results, the authors define BCR as a highly durable carbon removal approach.
Fig. 6. Identification of liptinite, semi-inertinite, and inertinite organic pools in a rice straw biochar (production pyrolysis temperature of 600°C) using a combination of the fluorescence and white incident light microscopy (see Table 2). The Ro histogram displays the correlation between organic pool type and reflectance range. (1)
Inertinite constitutes a large fraction of carbon in biochars, but biochar may contain up to three less permanent carbon pools. Apart from inertinite, there are semi-inertinite, liptinite and free hydrocarbons, in descending order of stability. The estimated mean residence time (MRT), derived from modeling biochar’s bacterial decay, only predicts the turnover of the unstable carbon fraction in the biosphere, neglecting the inertinite fraction in biochar and its turnover in the geosphere. Therefore, it is crucial to comprehend, characterize, and quantify the various carbon fractions in biochar.
Why is this research ground-breaking?
Sanei et al. scientifically prove a crucial point that supports the consideration of BCR as a permanent CDR. They also introduce a set of analytical tools that can be utilized to obtain essential information about the carbonization process and improve it, ensuring the maximum yield of inertinite biochar and energy form a system.
Read the full paper here: https://www.sciencedirect.com/science/article/pii/S0166516223002276
Please note: This is the pre-proof version. The final version of the manuscript including the supplementary material will be published soon.
(1) Hamed Sanei, Arka Rudra, Zia Møller Moltesen Przyswitt, Sofie Kousted, Marco Benkhettab Sindlev, Xiaowei Zheng, Søren Bom Nielsen, Henrik Ingermann Petersen, Assessing biochar’s permanence: An inertinite benchmark, International Journal of Coal Geology, 2023, 104409, ISSN 0166-5162, doi.org/10.1016/j.coal.2023.104409.