Received: February 1st, 2021 ; Accepted: March 28th, 2021 ; Published: April 6th, 2021 ; Correspondence: [email protected] 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
