Co-gasification of sewage sludge derived hydrochar and coal: Implications for syngas production and ash content

Abstract

The co-gasification of hydrothermally carbonized sewage sludge-derived hydrochar (HC) with SH coal under a CO₂ atmosphere presents a promising route for sustainable energy generation and CO₂ mitigation. This study systematically investigates the influence of HC-to-coal blending ratios and gasification temperatures on syngas composition, tar formation, and ash content. A blend of 25 wt % HC with 75 wt % coal at 950 °C was found to be optimal, achieving a 21.32 % ash and elevating the syngas lower heating value to 8.49 MJ/Nm³, thereby indicating enhanced gasification efficiency. The optimized blend was further evaluated at 800–950 °C to elucidate temperature-dependent behavior. Gas analysis revealed that CO₂ remained the dominant component (>60 vol %), while H₂ concentration increased from 40 % to 58 % with rising HC content due to intensified thermal cracking and organic matter depolymerization. Concurrently, CO concentration rose significantly from 16 % at 800 °C to 59 % at 950 °C, attributed to enhanced Boudouard and water–gas shift reactions facilitated by the CO₂ environment. Tar analysis showed an increase in polycyclic aromatic hydrocarbons (PAHs) with higher HC ratios, highlighting a trade-off between syngas quality and tar burden. This work provides a comprehensive mechanistic insight into HC–coal co-gasification, underscoring the critical role of temperature and blending optimization in maximizing syngas yield while minimizing undesirable byproducts. The findings support co-gasification as a viable waste-to-energy pathway for sewage sludge valorization and reduced reliance on fossil fuels.