Effect of hydrothermal carbonization and torrefaction on spent coffee grounds

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

Conversion of cellulose to activated carbons for high-performance supercapacitors

Biomass-derived activated carbons are promising materials that can be used in various applications. Current work investigates the possibilities of the cellulose-derived activated carbons in substituting the commercial alternatives for the supercapacitors’ electrodes with high efficiency, stable performance and relatively low cost. Hydrothermal carbonization (HTC) followed by chemical activation with KOH is used to convert cellulose into highly porous activated carbons. The effect of HTC parameters on the material porosity development and electrochemical properties of the electrodes is evaluated with several variations of the residence time and the weight ratio between cellulose and water during the pretreatment. The analysis shows that intensification of the HTC process (longer residence time and higher water/cellulose ratio) results in increase of the surface area of both hydrochar samples and subsequent activated carbons: with the highest surface area for the sample produced after 2 h HTC treatment with water/cellulose ratio of 6/1 – 2,645 m2 g -1 . As for the electrochemical analysis, the highest values of the specific capacitance are found for the samples produced from 2 h HTC treatment: 110.3 F g -1 (water/cellulose ratio of 3/1) and 102.5 F g -1 (water/cellulose ratio of 6/1). Additionally, it is noted that electrodes produced from the samples treated during 4 h have higher impedance at low operation frequency. The present study proves the possibility to substitute commercial activated carbons with cellulose-derived materials, the porosity of which can be tuned accordingly already during the pretreatment step

Use of principal component analysis to evaluate thermal properties and combustibility of coffee-pine wood briquettes

Submitted: February 1st, 2021 ; Accepted: March 30th, 2021 ; Published: May 21st, 2021 ; Correspondence: [email protected] coffee production chain is a potential source of residual biomass inherent to the high productivity that can contribute to the generation of value-added products. The residues from the coffee sector are typically disposed to landfill without treatment causing potential environmental inconveniences. Briquetting presents an alternative process to produce a uniform fuel with high energy density. Briquettes facilitates easy transportation, enables better handling and storage of biomass residues. Properties such as low equilibrium moisture content, high energy density and compressive strength were reported for different coffee-pine wood briquettes treatments. Moreover, understanding of the thermal properties of the briquettes during combustion is crucial to evaluate their final application. This research is the first study that investigates the combustibility properties and kinetic parameters of the thermal decomposition of briquettes from coffee-pine wood using differential and integral thermal analysis under non-isothermal conditions. Multivariate analysis of the collected parameters through principal components analysis (PCA), was implemented to reduce the dimensionality of the data. The desired profile in the combustibility is directly related to high temperatures and long burning times, thus, the tested briquettes displayed a significant combustibility potential, reporting peak temperatures and burnout times around 600 °C and 27 minutes, respectively. Activation energy kinetic parameter in the range of 12–42 kJ mol-1 and average reactivity of 0.14–0.22 min-1 , were also found. The results revealed the not thermally hard material to degrade when compared to biomasses typically used for combustion

Effect of hydrothermal carbonization and eutectic salt mixture (KCl/LiCl) on the pyrolysis of Kraft lignin as revealed by thermal analysis coupled to advanced high-resolution mass spectrometry.

The production of graphite requires high temperatures, and fossil petroleum, coal, or nutshells are frequently used as a carbon source. As a replacement, Kraft lignin, a by-product of the pulp and paper industry, is a promising carbon source with a prefigured aromatic network. Biomass-based feedstocks with improved characteristics can be obtained by hydrothermal carbonization, but the chemical nature of this process is not fully understood yet. Moreover, adding a eutectic salt mixture (LiCl/KCl) to the pyrolysis of Kraft lignin and HTC lignin can improve the graphitization at lower temperatures. In this study, thermal analysis with online mass spectrometric detection of the evolved gas mixture was applied to explore the influence of the eutectic salt mixture on the char conversion process. Aside from classical pyrolysis gas chromatography mass spectrometry, thermogravimetry coupled with soft photoionization mass spectrometry allowed to identify phenol, hydrogen sulfide, dimethyl sulfide, and various larger lignin fragments. These larger dimeric/trimeric methoxyphenol derivatives were successfully validated by means of high-resolution mass spectrometry equipped with soft atmospheric pressure chemical ionization. The investigations indicated the catalytic influence of the salt mixture on the production process of the biochars, achieving partially graphitization already at relatively low temperatures (700 °C). On the morphology, Raman spectroscopy and electron microscopy revealed the evolution of the carbon structures and revealed that the materials have typical features for amorphous carbon