Hydrothermal Carbonization of Spent Coffee Grounds

Hyeok-Jin Kim,Sea-Cheon Oh

doi:10.3390/app11146542

Abstract

With increasing coffee production and consumption, the amount of coffee by-product is also increasing. Therefore, there is growing worldwide interest in using these by-products as a renewable energy source. In this study, hydrothermal carbonization was conducted with subcritical water to improve the fuel characteristics of spent coffee grounds. The water content was varied, with the mass ratio between the dry sample and water set to 1:1.5 and 1:4. The reaction temperature was increased by 10 °C from 180 to 250 °C. The fuel and thermal characteristics of the reaction products were investigated through mass and energy yields, elemental, proximate, and heating value analysis. In analysis results, as the reaction temperature increased, carbon and fixed carbon content increased, and oxygen and volatile matter content decreased, resulting in an increase in calorific value. Thermogravimetric analysis, derivative thermogravimetry, and Fourier transform infrared spectroscopy were also conducted on the reaction products. To investigate their storage characteristics, chemical oxygen demand analysis was conducted. The results showed that with increasing reaction temperature, the fixed carbon content and heating value increased; also, the fuel characteristics became similar to those of coal. In addition, the reaction products became more hydrophobic as the reaction temperature increased.

Highlights

With increasing coffee production and consumption, the amount of coffee by-product is increasing
These values were calculated using Equations (1) and (2) and the heating value analysis results. For both water content conditions, the mass yield decreased as the reaction temperature increased, while the higher heating value (HHV)
Changes in mass yield, energy yield, and HHV at a reaction temperature of 250 ◦ C were larger than those at 220–240 ◦ C. This appears to be because the hydrothermal carbonization reaction occurred more rapidly near 250 ◦ C

Summary

Introduction

With increasing coffee production and consumption, the amount of coffee by-product is increasing. Hydrothermal carbonization was conducted with subcritical water to improve the fuel characteristics of spent coffee grounds. The fuel and thermal characteristics of the reaction products were investigated through mass and energy yields, elemental, proximate, and heating value analysis. The biodiesel extraction process requires various pretreatment steps, such as sample homogenization, drying, and extraction, and the typical carbonization method has a low yield of high-efficiency fuel because of the time and energy required for the drying process To overcome these shortcomings, there is growing interest in wet processes, that do not require a separate pre-drying process, and hydrothermal carbonization, which is performed at lower temperatures than the typical carbonization process [7,8,9]. The reaction temperature determines the main reaction atmosphere, and hydrothermal reactions are known to be the most active at reaction temperatures between 180 and 250 ◦ C [8,13]

Methods

Results

Conclusion