Production of Hydrochar from Kitchen waste through hydrothermal Carbonization

Abstract

Hydrothermal carbonization (HTC) is emerging as a promising waste to energy technology for conversion of organic wastes with high water contents. There have been relatively few experiments focused on evaluating the HTC of solid waste. These experiments have provided valuable information regarding HTC feasibility and potential environmental benefits. However, the studies lack the data necessary to understand how carbonization product composition and reaction extent change with time. The HTC is a novel thermal conversion process that may be a viable means for managing solid waste streams while minimizing greenhouse gas production and producing residual material with intrinsic value. The main objective of this study was to convert kitchen waste into hydro char through hydrothermal carbonization. The specific objectives of this study are to characterize kitchen waste from a university restaurant, to access the effect of processing parameters on hydro char, and to evaluate hydro char. The methodology for characterization for both the waste and hydro char was based on proximate analysis, ultimate analysis and calorific value. A ratio of dry sample to water of 1:9 was used to make a wet sample and was stirred in a thermal reactor at 180 rpm under controlled HTC conditions. A three-factor, five level CCD with 20 experiments was used. The HTC process parameters which included holding temperature (153-2600C), residence time (13-47 minutes), and pressure (64-736kPa) were optimized by a response surface methodology central composite design (RSM-CCD) to tailor the properties of hydro char. The results from proximate analysis of kitchen waste revealed that moisture content, volatile matter, ash content and fixed carbon content were 9.6, 65.1, 13.6 and 11.9% respectively. The proximate analysis of the hydro char showed that moisture, volatile matter, ash content and fixed carbon content were 2.5, 45.6, 16.5 and 35.4% respectively. The ultimate analysis of the hydro char showed that carbon, hydrogen, nitrogen, Sulphur and oxygen were 48.8, 0.5, 0.4, 0.5 and 45.3% respectively. Mass yield of hydro char decreases for all samples with increasing HTC reaction temperature. ANOVA confirmed the adequacy of the model which yielded R² and adjusted R² values of 0.8630 and 0.7398 respectively with a P value of 0.003 and F-value of 7.00. In conclusion, the high heating value, ultimate analysis and proximate analysis suggested that the hydro char from kitchen waste can potentially be utilized as a solid fuel. It was confirmed that valuable products can be generated from organic waste using hydrothermal carbonization. Keywords: Hydrothermal carbonization, Hydro char, Kitchen waste