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

Innovative solutions to the valorisation of troublesome wastes, especially those with a high moisture content, are currently particularly sought. An abundant amount of various organic waste generated by various industry sectors could be used to obtain perspective groups of products like hydrocarbons or hydrogen, which are widely demanded in multiple production processes. Herein, we proposed a two-stage processing route in which hydrothermal processing of apple pomace was integrated with steam gasification. Firstly, apple pomace, as an exemplary high moisture content raw material, was converted into hydrochar in a single solvent system (water) and a binary solvent system (water mixed with ethanol). This stage was designed to analyze the effect of the medium type on the change in the yield and quality of resultant groups of bioproducts, particularly solid residues called hydrochar. The quality of the hydrochars was examined by infrared spectroscopy, proximate, and ultimate analysis. We found a noticeable effect of the application of a mixed solvent medium on the conversion level of the pristine structure. The upgrading of the resultant hydrochars through steam gasification to ensure the sustainability of conversion was the subsequent step that we aimed at. The steam gasification was studied at a temperature 900°C. The kinetics and composition of the syngas were analysed. It was found that hydrochars produced under different conditions exhibit variable properties as a result of various levels of degradation of the original components of the raw material. The main gas products of the steam gasification process of all hydrochars were hydrogen, methane, carbon dioxide, and carbon monoxide. Whereas the solid bioproduct obtained in the aqueous system is characterised by the highest reactivity, conversion rate, and yield of the most important products, i.e. hydrogen, methane, and carbon monoxide, the one obtained in the ethanol system exhibited the lowest values of these parameters. The hydrochar obtained in the binary system was characterized by intermediate properties.