Effect of Different Hydrothermal Parameters on Calorific Value and Pyrolysis Characteristics of Hydrochar of Kitchen Waste

Abstract

Kitchen waste (KW) has high water content and organic matter, which has great potential for energy application, but how to treat it effectively has always been a difficult problem. In this study, the feasibility of transforming kitchen waste into hydrochar as a solid fuel to replace the primary energy by hydrothermal carbonization (HTC) is put forward. An experimental study of proportioned KW under HTC conditions (reaction temperature, residence time, liquid–solid ratio) is carried out by elemental analysis, industrial analysis, energy density calculation, and calorific value measurement. The results show that a proper extension of the residence time to 3.5 h at a liquid–solid ratio of 10 and a reaction temperature of 265 °C for the kitchen waste after HTC would result in a maximum calorific value of 30.933 MJ/Kg for the prepared hydrochar. The maximum ash content of hydrochar at a 265 °C reaction temperature is 3.94% < 10%, which is much lower than the standard of extra-low-ash coal, and the study shows that the hydrochar prepared from kitchen waste by HTC for combustion will greatly improve the combustion efficiency. The three types of kitchen waste under HTC conditions are compared with the three typical types of coal in China by the van Krevelen diagram. The results show that the combustion properties of hydrochar from kitchen waste prepared by the HTC method are better than those of lignite close to bituminous coal and can provide a theoretical basis for replacing primary energy in the future. Finally, by combining thermogravimetric analysis with mathematical modeling, it is calculated that the hydrochar of kitchen waste has a lower activation energy than those of other biomass, which can better facilitate the reaction. The experimental results further determine the feasibility of hydrochar as an alternative primary energy source and provide a theoretical basis for the future conversion of kitchen waste into hydrochar as a solid fuel through HTC.