Hydrothermal Conversion of Food Waste to Carbonaceous Solid Fuel—A Review of Recent Developments

Abstract

This review critically discussed recent developments in hydrothermal carbonization (HTC) of food waste and its valorization to solid fuel. Food waste properties and fundamentals of the HTC reactor were also covered. The review further discussed the effect of temperature, contact time, pressure, water–biomass ratio, and heating rate on the HTC of food waste on the physiochemical properties of hydrochar. Literature review of the properties of the hydrochar produced from food waste in different studies shows that it possesses elemental, proximate, and energy properties that are comparable to sub-bituminous coal and may be used directly as fuel or co-combusted with coal. This work conclusively identified the existing research gaps and provided recommendation for future investigations.

Continuous Hydrothermal Carbonization of Olive Pomace and Orange Peels for the Production of Pellets as an Intermediate Energy Carrier

“…Hydrothermal carbonization (HTC) is a thermochemical process that is applied at mild reaction temperatures, typically ranging from 180 to 250 • C, with water as the reaction medium [9,10]. The major advantage of this process is the direct applicability of wet (biomass) streams, which excludes the use of an energy-intensive drying step [11].…”

“…The major advantage of this process is the direct applicability of wet (biomass) streams, which excludes the use of an energy-intensive drying step [11]. During the HTC process, multiple reactions take place simultaneously, including hydrolysis, dehydration, decarboxylation, condensation and repolymerization [10,[12][13][14]. The effect of these reactions is a decrease in both the atomic H/C and atomic O/C ratio, moving towards values that are typical for lignite [15,16].…”

“…The most important process parameter is the reaction temperature. Higher temperatures result in more degradation reactions, lowering the solid yield [9,10,13]. The solid hydrochar product obtained at higher temperatures typically has better fuel properties, but the liquid stream becomes less suitable for digestion due to the formation of phytotoxic components [17][18][19][20].…”

“…The solid yield decreases with extended reaction time, although an initial increase in yield can be observed due to secondary char formation, i.e., the repolymerization of the dissolved components in the aqueous phase to hydrochar [11,22,23]. The biomass to water ratio influences the product yield, as a higher water concentration results in more extensive hydrolysis reactions, resulting in a lower solid yield [10]. The type of applied feedstock has a high influence on both the solid yield and final properties.…”

Investigation of the Effects of Cohydrothermal Carbonization on the Physicochemical Characteristics and Combustion Behavior of Straw Hydrochars

“…[4] As a new technology, hydrothermal carbonization (HTC) has been shown to be able to improve the fuel quality of biomass while consuming less energy. [5,6] Studies have shown that the reaction temperature is the main factor in HTC [7] ; hemicellulose (decomposed at 180 °C) and cellulose (decomposed at 220 °C) are decomposed into monomers, and the lignin component is more stable at 260 °C. [8,9] Specifically, hemicellulose and cellulose usually decompose completely at treatment temperatures above 230 °C and 270 °C, respectively.…”