Enhanced carbon retention in Litchi biochar via in-situ limewater coating and self-limited oxygen pyrolysis regulated by water-fire interaction

Abstract Biochar, a carbon-rich material with a porous structure, holds significant potential for mitigating climate change through carbon sequestration. However, its widespread adoption has been hindered by high production costs, primarily associated with oxygen-restricted systems and energy-intensive production processes. This study introduced a cost-effective, field-adapted strategy to enhance carbon retention in biochar derived from Litchi branches through limewater coating and synergistic water-fire interaction. Litchi branches were pretreated with limewater to create a surface coating, then underwent in-situ carbonization via self-oxygen-limited pyrolysis to form a dark-red char which was then quenched with limewater to produce biochar. Calcium (Ca)-mediated carbon retention during pyrolysis was investigated through Fourier-transform infrared spectroscopy (FTIR) coupled with scanning electron microscopy and energy-dispersive spectroscopy (SEM–EDS). The limewater-treated biochar achieved a significantly improved carbon conversion rate (86%) compared to CK (52%), the untreated biochar sample, with an enhanced specific surface area of 280 m 2 g –1 . FTIR and SEM–EDS analyses revealed that the limewater treatment formed a calcium-enriched protective barrier that effectively suppressed the formation of CO x during combustion. Additionally, mineral Ca-carbon composites formed during pyrolysis further improved carbon stabilization and retention. This study offers a practical and scalable solution for producing biochar under field conditions by addressing challenges related to cost-effectiveness and process efficiency, thereby promoting the application of biochar for carbon sequestration. Graphical Abstract