Determination of the Carbon Sequestration Potential of Hydrochar and Pyrochar from Faecal Sludge, Water Hyacinth and Oakwood

This study investigates the relationship between thermal recalcitrance (R50) and long-term carbon sequestration of biochars. If further seeks to ascertain the impact of ash content on their carbon permanence (Fperm). Chars derived from faecal sludge (FS), water hyacinth (WH) and Oakwood (OW) were produced via hydrothermal carbonisation (HTC, 150–250 °C) and pyrolysis (200–800 °C), with the aim of establishing an empirical link between thermal stability and real carbon sequestration potential over a 100-year period. Biochars were characterised using proximate/ultimate analysis, FTIR and thermogravimetric analysis to determine R50. Fperm and sequestered carbon per tonne feedstock (BC+100) were then estimated using the Woolf et al., method. A H/C-based model for soils at 15, 20 and 25 °C. A threshold was established for R50 ≤ 0.5 (low-temperature pyrolysis and HTC), below which thermal recalcitrance was not a reliable predictor of Fperm. Above this threshold, strong linear correlations (R2 ≈ 0.85) were obtained for all soil temperatures, confirming R50 as a robust predictor of relative permanence for mid- to high-temperature chars. High-temperature pyrochars (≥600 °C) exhibited near-graphitic permanence but with yield penalties, while HTC yielded lignite-like hydrochars with lower sequestration potential. This indicates that compared to low ash OW, both FS and WH require much higher process temperature and severity to produce feasible BC+100 values, making them more costly to sequester. Overall, carbon sequestration potential depended on both feedstock composition and treatment temperature, as well as the temperature of the receiving soil. The results also show that R50 can provide a useful, thermally grounded indicator of long-term carbon permanence.