Bio-Crude Production Improvement during Hydrothermal Liquefaction of Biopulp by Simultaneous Application of Alkali Catalysts and Aqueous Phase Recirculation

Abstract

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.

Hydrotreating of bio-crude obtained from hydrothermal liquefaction of biopulp: effects of aqueous phase recirculation on the hydrotreated oil

“…20 The difference can be explained by the presence of alkali catalysts in that study, which promotes the production of light polar organics and elevates the TOC and TN saturation levels. 22 Furthermore, the pH values of each sample before and after surplus water removal were monitored. As it can be seen, the pH slightly decreased along with recirculation from 7.2 to 6.3.…”

“…† Based on the optimized HTL condition in our previous studies, 234 g of homogeneous biopulp slurry containing around 38 g of the dry matter (ash-free) was injected into the reactor ($20% of dry matter) in the reference run (C0). 20,22 The reactor was then fully purged using N 2 gas to minimize the oxidization and subsequently pressurized up to 2 MPa. A 1.75 kW electrical ceramic heater with a 4 K min À1 heating rate was employed to maintain the desired operational temperature (350 C).…”

“…Moreover, the inorganic content of the biopulp and the utilized analysis techniques are represented in our previous study. 22 The pre-sulfided commercial Ni–Mo/Al 2 O 3 catalyst was supplied by Shell A/S, Fredericia, Denmark. Dichloromethane (DCM) (99.9% purity) and acetone solvents used in product separation were purchased from VWR.…”

“…Nevertheless, due to the lower heat transfer capacity of the utilized autoclave reactor, the cooling rate was relatively slow (5 K min −1 ) in comparison to its micro-batch counterparts employed in our previous study. 22 As further discussed in the following sections, the lengthy heating/cooling rates expose the products to the hot medium, which might render unexpected variations in the structure of the HTL products.…”

“…The separation procedure is extensively elaborated elsewhere. 22 The extra water of the recovered aqueous phase was partially evaporated employing a lab-scale vacuum rotary evaporator (Büchi R210), where the temperature was isothermally kept at 65 °C while pressure was gradually decreased to 60 mbar. To tackle the foaming risk during operation, ten droplets of n -octanol were added to the round bottle flask to suppress foaming during the process.…”