No abstract

Drying coffee grounds involves high energy consumption and represents an important step in using coffee waste materials as green energy. This study analyzes the drying process using thermogravimetric analysis. The kinetics of the drying process of coffee grounds, containing equal proportions of Arabica and Robusta, were evaluated under isothermal conditions at various temperatures: 50, 60, 70, and 80°C, with sample layer thicknesses of 0.6, 1.2, 1.8, and 2.4 mm. The thermogravimetric curves obtained for the coffee grounds samples under conditions of constant temperature allowed the tracing of the drying curves (moisture content—drying time). The influence of the furnace air velocity on the drying and water evaporation process was analyzed. The drying rate has been established to be about 2.6 times slower than the rate of water evaporation under the same conditions; as a result the overall process is controlled by the effective water diffusion in the layer. This aspect is important in industrial practice for sizing tubular dryers. The activation energy was found to be influenced by the thickness of the coffee grounds layer subjected to the drying process and had values between 8.3 kJ/mol and 10.3 kJ/mol.

This paper aims at analysing the thermal behaviour of six types of commercial coffee and the coffee grounds resulting from their use in a professional DeLonghi coffee machine. The influence of composition (Arabica and Robusta) on the thermal behaviour was thus highlighted for the first time. The obtained results indicate that the degradation occurs in three or four stages, based on the composition. The first stage consists in water removal, which can be found in a percentage of 1 up to 5%. The thermal decomposition of the samples starts at temperatures higher than 230 °C, in the case of both, different coffee types and coffee grounds. By comparatively analysing the temperature values at which the degradation starts, it can be noticed that in all the cases there is an increase in the thermal stability of the coffee grounds with approximately 20 degrees, compared to the coffee types they come from. For coffee grounds originating from coffee that contains 35% Arabica and 65% Robusta, the degradation mechanisms change at temperatures higher than 320 °C, as confirmed by the occurrence of an additional degradation stage, compared to the case of other samples. The obtained DSC curves emphasized the presence of two melting peaks and one crystallization peak, which, according to the specialty literature, indicates the presence of coffee oil.