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.

Short summary

The Earth's carbon dioxide removal and storage occur via inorganic and organic pathways: mineralization and maceralization. Biochar, imitating the organic pathway, undergoes controlled pyrolysis to transform biomass feedstock through a carbonization process into the inertinite maceral, which is a permanently stable form of organic carbon. Kinetic modeling in this study confirms inertinite's carbon stability over geological time scale.

Assessing biochar's permanence hence hinges on achieving complete carbonization and transformation. Inertinite serves as the gold standard for organic carbon permanence, guiding this study to measure biochar's carbonization against inertinite characteristics. Analyzing the random reflectance (R_o) of biochar reveals that 76% of studied samples qualify as pure inertinite. Apart from inertinite, other organic pools in biochar, quantifiable through geochemical pyrolysis and R_o methods, affect stability. Determining the carbonization temperature offers insights into biochar's efficiency and stability concerning production variables.

中文翻译：

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评估生物炭的持久性：惯性基准

地球系统自然去除二氧化碳及其永久储存是通过（i）无机碳和（ii）有机碳途径发生的。前者涉及碳的“矿化”和碳酸盐矿物的形成，而后者则采用生物质的“煤质化”或自然碳化成“惰性煤质”。生物炭的生产是一种模仿地质有机碳途径的二氧化碳去除（CDR）方法，利用受控热解将生物质快速碳化并将其转化为惰性显微物质以进行永久储存。因此，评估生物炭持久性的主要挑战是确保实现完全转化。

惰质岩是地壳中最稳定的显微组分，因此被认为是环境中有机碳持久性的最终基准。因此，本研究旨在根据惰性体已确定的成分和微观特征来测量生物炭的碳化程度。 2% 的随机反射率 (R _o ) 被提议作为“惰性基准”(IBR _o 2%)，并用于使用以下公式来量化生物炭中的永久碳库： R _o 频率分布直方图。结果表明，76% 的研究商业生物炭样品的整个 R _o 分布范围远高于 IBR _o 2%，被认为是纯惰性生物炭。典型惰性生物炭的氧化动力学反应模型表明，生物炭中一半碳的降解和损失需要大约 1 亿年的时间范围。该估计假设暴露在表面温度恒定为 30°C 的高氧化环境中，突显了材料固有的“永久”性质。在不太恶劣的环境中，惰性体的预期持久性通常预计会更长。

除了构成典型商业生物炭最大部分的惰质体之外，不完全碳化的生物炭可能含有最多三个其他有机库，按稳定性降序排列。生物炭中这些池的相对浓度可以通过地球化学热解和随机反射方法的组合来量化。此外，R _o 可用于计算生物炭的碳化温度（CT ^o C），这是生物炭碎片在热解过程中暴露的最高温度。该指标提供了有关碳化过程效率以及随后生物炭稳定性的重要信息，涉及生产温度 (PT ^o C)、加热停留时间和热扩散率。

 简短的摘要

地球二氧化碳的清除和储存是通过无机和有机途径进行的：矿化和岩化。生物炭模仿有机途径，经过受控热解，通过碳化过程将生物质原料转化为惰质煤质，这是一种永久稳定的有机碳形式。本研究中的动力学模型证实了惰质岩在地质时间尺度上的碳稳定性。

因此，评估生物炭的持久性取决于实现完全碳化和转化。惰性石是有机碳持久性的黄金标准，指导这项研究根据惰性石特性测量生物炭的碳化。分析生物炭的随机反射率 (R _o ) 表明，76% 的研究样品符合纯惰质体的条件。除了惰质体之外，生物炭中的其他有机物（可通过地球化学热解和 R _o 方法定量）也会影响稳定性。确定碳化温度可以深入了解生物炭的效率和与生产变量相关的稳定性。