Abstract
Hydrothermal carbonization (HTC) represents an efficient and valuable pre-treatment technology to convert waste biomass into highly dense carbonaceous materials that could be used in a wide range of applications between energy, environment, soil improvement and nutrients recovery fields. HTC converts residual organic materials into a solid high energy dense material (hydrochar) and a liquid residue where the most volatile and oxygenated compounds (mainly furans and organic acids) concentrate during reaction. Pristine hydrochar is mainly used for direct combustion, to generate heat or electricity, but highly porous carbonaceous media for energy storage or for adsorption of pollutants applications can be also obtained through a further activation stage. HTC process can be used to enhance recovery of nutrients as nitrogen and phosphorous in particular and can be used as soil conditioner, to favor plant growth and mitigate desertification of soils. The present review proposes an outlook of the several possible applications of hydrochar produced from any sort of waste biomass sources. For each of the applications proposed, the main operative parameters that mostly affect the hydrochar properties and characteristics are highlighted, in order to match the needs for the specific application.
Highlights
The increasing need of finding new renewable energy alternatives to fossil fuels together with the need to safely dispose of organic waste has pushed, in the last few years, the investigation for more efficient and reliable technologies for waste biomass energy exploitation and conversion towards valuable materials
Volpe et al [19], when performing thermos gravimetric thermal stability for the tea waste, due to the presence of hemicellulose and lignin which start to analysis (TGA) on olive mill waste hydrochar, found that while cellulose peak is significantly reduced degrade above 210 °C, while polysaccharide and protein present in the penicillin mycelial waste were for Hydrothermal carbonization (HTC) temperature of 250 ◦ C, the peak associated to lignin increases [44]
It has been demonstrated that during HTC, the phosphorous element segregates into the solid phase and treatment of the recovered hydrochar by an acidic leaching leads to the removal of P which moves to the aqueous phase
Summary
The increasing need of finding new renewable energy alternatives to fossil fuels together with the need to safely dispose of organic waste has pushed, in the last few years, the investigation for more efficient and reliable technologies for waste biomass energy exploitation and conversion towards valuable materials. At high temperature and pressure, even at subcritical conditions, water undergoes a dramatic properties change acting more as an organic solvent and its increased ion product favors reactions that are typically catalyzed by acids or bases, promoting biomass decomposition through hydrolysis, dehydration and decarboxylation reactions [17,18] By means of these reactions, it is possible to increase the carbon content of the initial feedstock, by removing most of the more volatile oxygenated compounds (furans and low molecular fatty acids) that are typically moved to the aqueous phase [19].
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