Impact of pyrolysis temperature and residence time on biochar reflectance: A case study of ground coffee biochar

Biochar is a porous, carbonaceous material derived from biomass pyrolysis with diverse applications. It holds significant potential in the agricultural sector (as a soil conditioner or fertilizer carrier), for environmental purposes (e.g., carbon sequestration), and industrial applications (e.g., plastics, paper and textile industries). However, its effective application depends on a clear understanding of biochar's long-term stability and the application of reliable methods to assess its durability and reactivity.

This study aims to characterize the microscopic properties of ground coffee-derived biochar using incident white light microscopy. Furthermore, it investigates the correlations between reflectance, surface area measurements, and elemental composition. A total of 16 samples were examined, including one sample of raw ground coffee and 15 coffee biochar samples produced at three pyrolysis temperatures (300, 600, and 850 °C) and five residence times (1, 3, 6, 12, and 24 hours) at laboratory conditions. For all samples, proximate and ultimate analyses were conducted. Reflectance and surface area measurements were determined for a comprehensive characterization.

For the examined coffee biochars, the random reflectance increased with rising pyrolysis temperature and residence time. Photomicrographs of coffee biochars pyrolyzed at 300 °C, 600 °C and 850 °C illustrate a color change. At 300 °C for 1, 3, and 6 hours, the biochar appears predominantly grey, indicating partial pyrolysis. With extended pyrolysis at 300 °C for 12 hours, as well as at higher temperatures of 600 °C and 850 °C, the biochar exhibits a bright white coloration. The results demonstrate a strong correlation between coffee biochar reflectance and both pyrolysis temperature and residence time, as reasonably expected. The coffee biochar samples, which formed at temperatures of 600 °C and 850 °C, exhibit R_o ranges (2.58 – 5.10%) well above the inertinite benchmark (IBR 2%). BET analysis reveals that specific surface area values range from 0.07 to 1020 m²/g, micropore area from 3 to 641 m²/g, and total pore volume from 0.0008 to 0.56 mL/g, all positively correlated with increasing pyrolysis temperature and residence time. Furthermore, the carbon and oxygen contents of the samples exhibit expected trends, characterized by an increase in carbon (C) and a decrease in oxygen (O).

Coffee biochars exhibit higher reflectance, surface area, and carbon content with increasing pyrolysis temperature and residence time, as expected, indicating greater stability, microporosity, and overall quality. These characteristics support their potential use in environmental applications. Finally, this study also demonstrates that reflectance measurement is the most qualified quality parameter.