Effect of hydrothermal carbonization and torrefaction on spent coffee grounds

E. Sermyagina¹*, C. Mendoza¹² and I. Deviatkin³

¹LUT University, Department of Energy Technology, PL 20, 53851 Lappeenranta, Finland
²Federal University of Minas Gerais, 31270-901 Belo Horizonte, MG Brazil
³LUT University, Department of Sustainability Science, PL 20, 53851 Lappeenranta, Finland
*Correspondence: ekaterina.sermyagina@lut.fi

Abstract:

Coffee is one of the most tradable commodities worldwide with the current global consumption of over 10 billion kilograms of coffee beans annually. At the same time, a significant amount of solid residues, which are known as spent coffee grounds (SCG), is generated during instant coffee manufacturing and coffee brewing. Those residues have a high potential in various applications, yet they remain mostly unutilized. The current work presents the experimental comparison of two pretreatment technologies – hydrothermal carbonization (HTC) and torrefaction – for converting SCG into a valuable char. The results showed that low-temperature torrefaction (< 250 °C) has a negligible effect on feedstock properties due to initial pre-processing of coffee beans. However, the energy conversion efficiency of torrefaction at higher temperatures is comparable with that of HTC. The average energy yields for high-temperature torrefaction (> 250 °C) and HTC were on the level of 88%. Devolatilization and depolymerization reactions reduce oxygen and increase carbon contents during both processes: chars after torrefaction at 300 °C and HTC at 240 °C had 23–28% more carbon and 43–46% less oxygen than the feedstock. Both pretreatment methods led to a comparable increase in energy density: the highest HHV of 31.03 MJ kg^-1 for torrefaction at 300 °C and 32.33 MJ kg^-1 for HTC at 240 °C, which is similar to HHV of anthracite. The results showed that both processes can be effectively used to convert SCG into energy-dense char, even though HTC led to slightly higher energy densification rates.

Key words:

biomass pretreatment, hydrothermal carbonization, spent coffee grounds, torrefaction