A1 Journal article (refereed), original research

Effect of hydrothermal carbonization and torrefaction on spent coffee grounds

Open Access publication

Publication Details
Authors: Sermyagina Ekaterina, Mendoza Clara, Deviatkin Ivan
Publisher: Estonian Agricultural University
Publication year: 2021
Language: English
Related journal or series: Agronomy Research
Volume number: 19
ISSN: 1406-894X
JUFO level of this publication: 1
Open Access: Open Access publication


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.

Last updated on 2021-15-04 at 12:55

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