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Optimization of the Waste Lignocellulosic Biomass Hydrothermal Carbonization Process by Response Surface Methodology

Abstract:

In recent years, various thermochemical processes have been studied to obtain bioenergy. Hydrothermal carbonization (HTC) has proven to be a convenient alternative since it allows biomass with high moisture and ash content to be processed without prior drying processes, obtaining a biofuel with up to 20 to 30 % more calorific value compared to biomass. However, the main challenge is determining optimum points of increase in calorific power and energy performance to transform it into economically feasible. This study proposes the study and optimization of the HTC process with two Chilean biomasses: sawdust (Pinus radiata) and rapeseed (Brassica napus). Mass yield (MY), higher heating value (HHV), and energy yield (EY) were analyzed by applying a Design of Experiments (DoE) with three factors and two levels (23). The variables used and their levels were temperature (T): 190 - 250 °C; time (t): 60-120 min; and biomass/water ratio (B/W): 8 – 14 % for rapeseed and 10 – 16 % for sawdust. For sawdust, increasing the temperature from 190 to 250 °C raised HHV by 23 % and lowered MY (by 21 %) and EY (by 18 %). Rapeseed HHV showed an increase of 23 %, while MY and EY decreased by 11 and 14 %. In the statistical analysis, the most influential variables for both biomasses were temperature and the B/W ratio, similar to what was found in previous studies. New operating points were determined to maximize the HHV of the hydrochar using the Response Surface Methodology and the equations obtained from the DoE (R2 above 0.98). The results achieved were higher than the average indicated in the literature. For sawdust, an EY of 77 % and an HHV of 28.6 MJ/kg (+48 %) were obtained at 280 °C, 100 min, and a B/W of 13 %. On the other hand, for rapeseed, EY=49.2 % and HHV=29.1 MJ/kg (+37 %) were achieved at 280 °C, 90 min, and a B/W ratio of 10 %.