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

Torrefaction – the process for biofuels production by using different biomasses

D. Urbancl*, J. Krope and D. Goričanec

University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova street 17, SI2000 Maribor, Slovenia
*Correspondence: danijela.urbancl@um.si

Abstract:

Torrefaction process is a mild pyrolysis, where biomass material is converted into solid fuel with higher heating value. The results of torrefaction at different temperatures in a range from 220 to 400 °C for three varied materials, oak wood, mixed wood and dehydrated, granulated sewage sludge are presented. The torrefaction process started with warm up stage, which took place for 30 minutes, after that sample was torrefied for 2 hours at constant temperature. The process continued with cool down stage. The energy demands were covered by electric power, while the flue gasses were not integrated in the process. The influence of the operating temperatures are analysed in order to determine optimal operation parameters to get the torrefied biomass with highest calorific value. Furthermore, the optimal operation time according to the largest increase in calorific value for each material is evaluated. The results of calorific value, mass drop and chemical compositions such as elemental analyses are also presented. Results show that heating values increase with raising temperature for both wood samples. The heating values for sewage sludge increases to approximately 320 °C, after that temperature are unchangeable. Torrefied oak wood samples were more fragile at higher temperatures in comparison to raw or torrefied oak wood samples at lower temperatures. At torrefied sewage sludge samples the changes in fragility are not detected due to pre-prepared granulates of sludge.

Key words: