Effect of the temperature on the spent coffee grounds torrefaction process in a continuous pilot-scale reactor

Abstract

Several studies in Literature have analyzed the torrefaction treatment process of biomass in batch reactors. Nevertheless, in order to check the industrial applicability of a process, it is more interesting to carry out torrefaction tests in continuous pilot plant reactors. Thus, the present study reports the results of continuous semi-industrial scale (500 kg/h) torrefaction experiments employing spent coffee grounds (SCG) as a feedstock in a horizontal rotary reactor. Torrefaction tests were carried out at three different temperatures (210 °C, 235 °C, and 260 °C) for 90 min, in order to evaluate the yields of solid, liquid (water and tar) and gaseous products and their composition. In particular, the effectiveness of the decarbonization and deoxygenation processes was investigated, analyzing the carbon and oxygen distribution. Within the temperature range, torrefaction yielded solid of 55.2–85.8 wt% with an increase of the higher heating value (HHV) of 5.1–15.4% compared to the raw material. Liquid fraction yield was in the range of 11.3–34.3 wt%, with a moisture content of 73.7–91.9% and HHV between 18.8 and 23.4 MJ/kg on dry basis. Gas fraction represented 2.9–10.5 wt% of the raw biomass withHHVbetween 1.95 and 3.55 MJ/Nm3. Acids (mainly acetic acid) were the major compounds in the tar fraction, accounted up to 68 wt%, while CO2 was the main product of the deoxygenation reactions in the gaseous fraction (76.6–86.9 vol%). From the perspective of mass yield and energy density enhancement, 260 °C was considered to be the optimal temperature for torrefaction of SCG.