Role of secondary char on the fuel properties and pyrolysis behaviors of hydrochars: Effect of temperature and liquid-solid ratio

Abstract

Hydrothermal carbonization (HTC) is widely recognized as a promising approach for enhancing the fuel properties of agricultural waste. This study investigated the HTC characteristics of corn stalks, focusing on the effect of secondary char (SC) on the fuel properties and pyrolysis behaviors of hydrochars at different temperature and liquid-solid ratio (LSR). Results showed that lower LSR increased the carbon content and higher heating value of hydrochars. Higher temperature strengthened the effect of LSR. At 240 °C, hydrochar yield decreased from 58.56 % to 45.59 % as the LSR increased. The higher LSR enhanced hydrolysis and promoted the transfer of organic components to aqueous phase, thereby facilitating SC formation with a content as high as 12.10 %. The increase in SC was accompanied by larger carbon microspheres and greater deposition coverage, resulting in reduced specific surface area and elevated oxygen-containing functional groups. The covering of SC strengthened the pyrolysis-like reaction of primary char (PC) during HTC (limited by contact with water). These changes influenced subsequent pyrolysis behavior, with PC tending to produce phenols at low LSR but aromatic hydrocarbons at high LSR. This work could enhance the understanding of SC and provide a practical approach combining HTC and extraction to regulate bio-oil preparation by pyrolysis.