Variations in microscopic properties of biomass char: Implications for biochar characterization

Biochar, a carbon-rich material produced during the thermochemical conversion of biomass, is considered to be an important material for environmental applications and sustainable agriculture. With interest in its use growing, it is essential to understand how pyrolysis parameters control biochar properties and, subsequently, which analytical methods can be applied effectively in characterizing biochar. While various methods are already in use, the potential of reflected light microscopy analysis has not yet been widely explored. This paper focuses on examining the microscopic properties of various biochars by implementing reflected light microscopy and assessing whether this technique has the potential to deliver quick, reliable, and essential information for biochar characterization. To support microscopic observations, selected physical (density and surface area) and chemical parameters (carbon and sulfur content, functional group distribution) of biochar samples were also determined. Megascopically and microscopically there is a considerable difference among biochars of different feeds including their size and shape, porosity, ratio of the abundance of cellular to non-cellular pores, inorganic matter content, the ratio of thick-walled to thin-walled inertinite, or the ratio between high-reflectance inertinite to low-reflectance inertinite and their reactivity. The biochars studied also showed variations in the oxygenated groups contribution, a wide range of carbon content (14.7 to 98.8%), density (1.45 to 2.47 g/cm3), and surface areas (from <1 to 363.6 m2/g). Our results demonstrate that microscopic characteristics and reflectance of biochar can provide crucial diagnostic value for predicting biochar properties and, in combination with other physical and chemical properties, help to enhance information about the biomass conversion process and potential practical use of biochar.