Natural fibres, such as hemp and flax, have recently regained attention as sustainable alternatives to synthetic textile fibres. During processing, a lignocellulosic byproduct, the protective shell known as hurds (hemp) and shives (flax), is generated, presenting opportunities for valorisation into high-value materials such as biochar. However, the potential of these byproducts as precursors for biochar suitable for energy storage applications remains largely unexplored.

This study addressed two main research questions: Are hemp hurds and flax shives suitable alternatives to hard carbon anode materials? How does the composition change when hemp hurds and flax shives are co-pyrolysed? To answer these questions, biochars derived from hemp hurds, flax shives, and their 50 wt% (COHF) mixture were produced and evaluated for their carbon, hydrogen, nitrogen, inorganic, and heavy metal compositions.

The samples were pyrolysed in a muffle furnace under an inert atmosphere at 400, 650, and 900 °C. The resulting biochars were analysed for thermal degradation using thermogravimetric analysis (TGA), elemental composition using an elemental analyser, and heavy metal content using microwave plasma atomic emission spectroscopy (MP-AES).

All three feedstocks produced biochars with high carbon contents. Hemp hurds biochar contained 74.1 ± 3.9 % and 77.1 ± 2.7 % carbon at 650 °C and 900 °C, respectively, whereas flax shives biochar reached 64.1 ± 7.5 % at 650 °C. The COHF mixture yielded 70.4 ± 4.0 % and 69.3 ± 1.1 % carbon at 650 °C and 900 °C, respectively. Thermogravimetric analysis revealed similar thermal degradation behaviours across all feedstocks. Co-pyrolysis produced, to some extent, a synergistic effect on elemental composition, resulting in reduced concentrations of arsenic, nickel and copper, and the COHF biochar contained notably lower heavy metal concentrations than the individual biochars at all temperatures. However, the concentrations of iron, aluminium, and potassium increased, whereas calcium, sodium, magnesium, manganese, and phosphorus were present at similar or slightly elevated concentrations.

These findings suggest that hemp hurds and flax shives are promising feedstocks for high-carbon biochar and may serve as precursors for hard carbon anode materials. The co-pyrolysis of both materials produced biochar with reduced metal impurities, indicating a possible synergistic effect between the two materials. Overall, the valorisation of hemp and flax byproducts into biochar represents a sustainable and socially responsible pathway for circular material use in the textile and energy industries