Effect of pyrolysis temperature, heating rate, and residence time on rapeseed stem derived biochar

Uncontrolled open burning of unwanted rapeseed stems is often performed in China, resulting in resource wastage and substantial air pollution. The pyrolysis of rapeseed stem renders biochar, which can improve soil structure and reduce heavy metal(loid) leaching. However, due to a lack of experimental evidence, the relationship between the physicochemical properties of rapeseed stem biochar and its pyrolysis conditions remains unclear. Here we show pyrolysis dependent properties of rapeseed stem biochar pyrolyzed under various temperatures (200-700 °C, in 50 °C intervals), heating rates (1, 5, 10, 15, 20 °C/min), and residence times (10, 20, 40, 60, 80, 100 min). A statistical analysis of other biochar data reported in the literature found that pyrolysis temperature is significantly correlated to biochar yield, pH, fixed C and surface area. Pyrolysis temperature was also found to be the most influential parameter on rapeseed stem biochar; demonstrating a positive relationship with pH, microporous structure, surface area, fixed C, and ash content, whilst having a negative relationship with yield, average pore size, functional groups, volatile matter, O and H mass fractions, and the number and density of functional groups. In comparison to other reported biochars, the rapeseed stem biochar generally had higher pH values and lower yield, ash content, and O/C ratios. N2 adsorption/desorption analysis showed that a microporous biochar was achieved at pyrolysis temperatures of 450 °C or higher. The surface area and morphology were significantly influenced by residence time, which is often overlooked in the literature. The results of this study have elucidated the relationship between rapeseed stem biochar and its pyrolysis conditions. A step towards sustainability can now be achieved by optimized pyrolysis of rapeseed stems, to produce biochar with enhanced properties for environmental management.