Abstract Biochar pore space consists of porosity of multiple length scales. In direct water holding applications like water storage for plant water uptake, the main interest is in micrometre-range porosity since these pores are able to store water that is easily available for plants. Gas adsorption measurements which are commonly used to characterize the physical pore structure of biochars are not able to quantify this pore-size range. While pyrogenetic porosity (i.e. pores formed during pyrolysis process) tends to increase with elevated process temperature, it is uncertain whether this change affects the pore space capable to store plant available water. In this study, we characterized biochar porosity with x-ray tomography which provides quantitative information on the micrometer-range porosity. We imaged willow dried at 60 °C and biochar samples pyrolysed in three different temperatures (peak temperatures 308, 384, 489 °C, heating rate 2 °C min−1). Samples were carefully prepared and traced through the experiments, which allowed investigation of porosity development in micrometre size range. Pore space was quantified with image analysis of x-ray tomography images and, in addition, nanoscale porosity was examined with helium ion microscopy. The image analysis results show that initial pore structure of the raw material determines the properties of micrometre-range porosity in the studied temperature range. Thus, considering the pore-size regime relevant to the storage of plant available water, pyrolysis temperature in the studied range does not provide means to optimize the biochar structure. However, these findings do not rule out that process temperature may affect the water retention properties of biochars by modifying the chemical properties of the pore surfaces.

中文翻译：

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热解温度对柳生物炭水文孔隙度的影响

摘要 生物炭孔隙空间由多个长度尺度的孔隙组成。在直接保水应用中，例如用于植物吸水的储水，主要关注微米级孔隙率，因为这些孔隙能够储存植物容易获得的水。通常用于表征生物炭物理孔隙结构的气体吸附测量无法量化该孔径范围。虽然热解孔隙率（即在热解过程中形成的孔隙）往往会随着工艺温度的升高而增加，但不确定这种变化是否会影响能够储存植物可用水的孔隙空间。在这项研究中，我们用 X 射线断层扫描表征了生物炭的孔隙度，它提供了微米级孔隙度的定量信息。我们拍摄了在 60 °C 下干燥的柳树和在三种不同温度下热解的生物炭样品（峰值温度 308、384、489 °C，加热速率 2 °C min-1）。样品经过精心准备并通过实验进行追踪，从而可以研究微米级范围内的孔隙度发展。孔隙空间通过 X 射线断层扫描图像的图像分析进行量化，此外，还使用氦离子显微镜检查纳米级孔隙率。图像分析结果表明，原材料的初始孔隙结构决定了研究温度范围内微米级孔隙率的性质。因此，考虑到与植物可用水储存相关的孔径范围，研究范围内的热解温度并不能提供优化生物炭结构的手段。然而，