Synthesis of biochar-MWCNT composites from waste wooden furniture via tonne-scale mobile high-temperature downdraft gasification

In this study, we report the in-situ formation of biochar–multi-walled carbon nanotube (MWCNT) composites via a mobile high-temperature downdraft gasification process using discarded wooden furniture as feedstock. Rather than producing isolated CNTs, our process results in a composite material where MWCNTs are sparsely and intimately attached to the surfaces of biochar particles. The presence and morphology of the nanotubes were confirmed by high-resolution transmission electron microscopy (HRTEM) on biochar fragments pre-identified by SEM imaging. The observed MWCNTs exhibit hollow cores and concentric walls with an interlayer spacing of ~0.34 nm, an average diameter of ~21.4 nm, wall thickness of ~8.1 nm, and a hollow core of ~6 nm. The in-situ formation of the biochar–multi-walled carbon nanotube (biochar–MWCNT) composites observed in this study is primarily attributed to the presence of trace metallic catalysts, such as iron, originating from surface coatings on discarded wooden furniture. Under high-temperature gasification conditions, these iron-based catalysts facilitate the cracking, recombination, and deposition of carbon-containing gaseous species in the syngas (e.g., CH₄ and other C–H species), thereby inducing the growth of MWCNTs on the surfaces of biochar particles and resulting in a tightly integrated biochar–MWCNT composite structure. In contrast, biochar produced from other biomass feedstocks, including rice straw (low lignin content) and fallen street trees (high lignin content), which lack iron-containing coatings or catalytically active metal species, exhibited no evidence of carbon nanotube formation on their surfaces. This work highlights a sustainable and decentralized approach for converting bulky wooden waste into functional carbon composites, expanding the potential applications of gasification-derived biochar materials. • Biochar-MWCNT composites synthesized from discarded furniture. • Multi-walled carbon nanotubes exhibit average diameter of ~21.4 nm. • Two plausible carbon sources: CH₄ and other C–H species. • Trace iron on furniture surfaces catalyzes carbon nanomaterial formation. • Carbon nanomaterial formation linked to feedstock type and gasification conditions.