A relationship between biochar reflectance and carbonization temperature

This study presents a correlation between mean random reflectance (R o ) measurements of biochar and carbonization temperature (CT). It uses 21 biochar samples produced from wheat straw, rice straw, and rice husk biomasses at laboratory-controlled conditions at CTs from 350 to 650 °C at 50 °C steps, a heating rate of 10 °C/min, and a residence time of 60 min. Mean R o values, ranging from 0.52 % to 3.87 % across all samples, were calculated from 500 individual measurements in each sample and plotted against the CT. The rice husk biochar yields bimodal R o distributions due to reduced carbonization of the SiO 2 -rich surface of the hulls, resulting in retarded mean R o values. Thus, two strong polynomial CT-R o correlations (r 2 = 0.98, p < 0.0001, MAE = ±12.89 °C; r 2 = 0.99, p < 0.0001, MAE = ±12.91 °C) were established using only the wheat and rice straw biochar samples. The correlations were used to calculate CTs from a test set of wheat straw and wood biochar samples with R o values ranging from 1.65 % to 4.30 %. The results were compared with the CTs derived from a correlation based on charcoal. The new biochar correlations yield CTs that are from 13 °C to 67 °C higher than the charcoal correlation, but up to 3.5 %R o , the difference is 50 °C or less. The charcoal-based correlation thus provides a minimum estimate of the degree of carbonization. Another correlation, based on the numerical fitting of published heating temperature and R o data yields much higher CT at R o values above ∼ 3.5 %.