Kikuyu grass clippings as a bioeconomy feedstock: influence of pyrolysis conditions on biochar properties and its effect on the growth of Swiss chard

Abstract

The integration of urban organic waste streams into the bioeconomy is critical for advancing the water–food–energy nexus. Grass clippings, an abundant and underutilized municipal residue, represent a promising feedstock for producing value‐added materials. This study provides a comprehensive evaluation of biochar produced from Kikuyu grass ( Cenchrus clandestinus ) and its potential for soil amendment and carbon sequestration. Biochar was produced by slow pyrolysis under nitrogen, examining the effects of two feedstock particle sizes and three peak temperatures (400, 500, and 600 °C). Higher pyrolysis temperatures increased biochar pH and the abundance of aromatic carbon structures, but decreased biochar yields correspondingly. The maximum specific surface area was achieved at 500 °C using the larger feedstock size fraction. Expanding upon preliminary findings, this study established a direct, positive correlation between biochar surface area and both water retention capacity and CO 2 adsorption capacity, demonstrating the dual benefits of climate mitigation and soil improvement. The performance of selected biochars as soil conditioners was assessed in a greenhouse trial with Swiss chard ( Beta vulgaris ). Biochar application improved soil water retention and enhanced plant growth significantly compared with untreated controls. Preliminary cost considerations, based on published fertilizer prices, suggest that biochar may offer long‐term economic advantages through soil benefits extending over multiple growing seasons. However, field‐scale validation is required to confirm profitability because the present greenhouse study was not designed as a formal techno‐economic analysis. These findings highlight the potential for valorizing urban grass clippings into biochar as a viable pathway within the circular bioeconomy, contributing to waste reduction, soil improvement, and long‐term carbon sequestration.