Sustainable H2-rich bio-syngas production from high moisture wastes: Steam gasification of hydrochars obtained from apple pomace in single and binary solvent systems

Innovative solutions dedicated to valorization of high moisture content wastes into perspective groups of products like hydrocarbons or hydrogen are sought. Herein, we propose a two-stage processing route involving hydrothermal processing of apple pomace (AP) with the subsequent steam gasification. AP was converted into hydrochar using either a single solvent (water or ethanol) or a binary solvent (water mixed with ethanol). The effect of the medium type on the change in the yield and quality of the resultant hydrochar was studied, including comprehensive characterization. The subsequent step was to upgrade the resultant hydrochars through steam gasification at 900 °C. A noticeable influence of the application of a type of solvent medium on the conversion level of the pristine structure, as well as on the effectiveness of steam gasification, was noted. Syngas included hydrogen, methane, carbon dioxide, and carbon monoxide, with hydrogen being the dominant component (56–60 %). The high share of methane in bio-syngas translates into a relatively high energy potential of the residual gas after possible hydrogen separation. The aqueous system produced the most reactive and high-yielding hydrochar, while the ethanol system exhibited the lowest values due to differences in porous structure, elemental composition, and structure of surface functional groups.