Conversion of spent coffee grounds into biochar using a sustainable solar assisted biomass torrefaction technique

Abstract

Coffee farmers and producers in Kerala (India) face a serious obstacle in their pursuit of a self-sustaining coffee processing technology and fuel deficiency. Research indicates that coffee, as one of the largest industries, generates a huge amount of waste, namely Spent Coffee Grounds (SCGs). This study aims to evaluate the feasibility and efficiency of converting SCG into biochar using a solar-assisted biomass torrefying technique, while keeping frugal or "jugaad" innovation in mind. To torrefy the feedstock, it was intended to use the available SK14 solar cooker in conjunction with a prototype reactor unit. Based on previous research on torrefying coffee waste, this study asks: Can the biomass torrefied using the SK 14 cooker be utilized as fuel? In this context: Solar biomass torrefaction is an endothermic process of converting the feedstock in an inert environment at low temperatures of 200-300 °C provided by concentrated solar energy in order to produce a high yield of solid biochar.

Based on the literature research, it was important to characterize the SK14 cooker, the feedstock, and to understand the operating principles of the chosen reactor, namely Evacuated Tube Vacuum Collectors (EVCs). Since the design of the reactor and cooker were interdependent, COMSOL was used to simulate the heat distribution profile and temperature profile of the reactor model. Literature and simulation results were used to construct a prototype reactor, and torrefaction tests were conducted in Hyderabad (India). SCG was effectively torrefied to generate biochar at 240 °C and 260 °C, as evidenced by high heating values of 26 MJ/kg (21% increase) and 26.3 MJ/kg (22.50% increase), respectively compared to the raw material. The results show that the current system can be utilized as a small-scale solar biomass torrefier, creating biochar that can be used as a fuel. However, the reactor's non-homogeneous heating rate and poor heat retention severely hampered its applicability. Further study is required to find other features and aspects that might not only improve the design and efficacy of the torrefier, but also facilitate its implementation for coffee producers in Kerala.