In this work, design of experiments–response
surface methodology (RSM) was implemented to predict the importance
of hydrothermal carbonization (HTC) key parameters and their interactions
in the preparation of canola-stalk-derived hydrochar via HTC technique.
According to the RSM results, temperature and reaction time were found
to be the most important control factors. The possible optimum conditions
were found to be 207 °C and 82 min for temperature and time,
respectively, in order to achieve a hydrochar with the maximum mass
yield (solid yield 53.38%), carbon recovery rate (52.66), and O/C
ratio (0.69). Furthermore, the optimized hydrochar was successfully
activated via potassium hydroxide (KOH), under mild activation conditions.
Synthesized microporous activated carbon demonstrated the highly improved
Brunauer–Emmett–Teller (BET) surface area of 474.87
m2 g–1 compared to the low BET surface
area of mesoporous hydrochar (S
BET of
2.69 m2 g–1). Porous activated carbon
was used as an adsorbent for methylene blue removal that showed a
promising dye removal capacity of 93.4 mg g–1. The
morphological and chemical compositions of the solid materials were
analyzed by various techniques, including elemental analysis, field
emission scanning electron microscopy (FESEM), BET analysis, Fourier
transform infrared (FTIR) spectroscopy, and energy-dispersive X-ray
spectroscopy.
This study focuses on the valorization of the organic fraction of municipal solid waste (biopulp) by hydrothermal liquefaction. Thereby, homogeneous alkali catalysts (KOH, NaOH, K2CO3, and Na2CO3) and a residual aqueous phase recirculation methodology were mutually employed to enhance the bio-crude yield and energy efficiency of a sub-critical hydrothermal conversion (350 °C, 15–20 Mpa, 15 min). Interestingly, single recirculation of the concentrated aqueous phase positively increased the bio-crude yield in all cases, while the higher heating value (HHV) of the bio-crudes slightly dropped. Compared to the non-catalytic experiment, K2CO3 and Na2CO3 effectively increased the bio-crude yield by 14 and 7.3%, respectively. However, KOH and NaOH showed a negative variation in the bio-crude yield. The highest bio-crude yield (37.5 wt.%) and energy recovery (ER) (59.4%) were achieved when K2CO3 and concentrated aqueous phase recirculation were simultaneously applied to the process. The inorganics distribution results obtained by ICP reveal the tendency of the alkali elements to settle into the aqueous phase, which, if recovered, can potentially boost the circularity of the HTL process. Therefore, wise selection of the alkali catalyst along with aqueous phase recirculation assists hydrothermal liquefaction in green biofuel production and environmentally friendly valorization of biopulp.
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