Characterization of Biocrude Produced by Hydrothermal Liquefaction of Municipal Sewage Sludge in a 500 mL Batch Reactor

Abstract

This paper investigates the production of biocrude via hydrothermal liquefaction (HTL) of sewage sludge. Results on the effect of the separation protocol of the oily/aqueous phases highlighted the superior performance of inertial separation over liquid−liquid extraction, with a biocrude yield of 43% attained at a HTL temperature of 350 °C. Experimental results revealed that processing time exerts a nonmonotonic effect on the biocrude yield with maximum yields of 35 and 43% after 30 and 10 min processing times at 300 and 350 °C, respectively. The best results in terms of energy recovery (about 59%) and reduced content of S in the biocrude (about 1%) are obtained at 350 °C. A minimum yield of water-soluble compounds (29%) is achieved at 10 min of processing time at 350 °C, whereas the maximum yield (49%) is attained after 30 min of processing time at the same temperature. From both NMR and LC-MS analyses, the biocrude obtained proved to be a complex mixture mainly constituted by aliphatic compounds, with small amounts of aromatic compounds (including PAHs) and olefins. Finally, inorganic elements present in the sludge report entirely to the solid phase.

Unravelling the Role of Biochemical Compounds within the Hydrothermal Liquefaction Process of Real Sludge Mixtures

“…No appreciable differences emerged from the bidimensional spectra recorded for the three mixed sludges; therefore, for simplicity, in Figures 6 and 7 we have reported only the spectra for MS:TS 75:25 (for the bio-crude and aqueous phases, respectively). In good agreement with the data reported in Figure 5, the spectra showed a broader collection of signals due to the presence of amines and alcohols, compared to those observed in the case of pure MS reported in [47]. A similar enrichment was also evident when comparing the spectra of the mixed sludges and those of the pure TS and MS.…”

“…A detailed description of the experimental apparatus and protocols adopted for both HTL tests and products separation was reported in previous works by this research group [46,47], and it is briefly summarised below.…”

“…The chemical analysis of both the bio-oil and the aqueous phase was carried out by mono-and bi-dimensional nuclear magnetic resonance (1D/2D NMR) spectroscopy, together with liquid chromatography-mass spectrometry (LC-MS) analysis. More details on the experimental conditions are reported in [47]. Briefly, 1 H-NMR, 1 H, 1 H-COSY and 1 H, 13 C-HSQC DEPT spectra were recorded at 400 MHz on a Bruker (Milano, Italy) 400 MHz spectrometer.…”

“…With reference to the content of this section, it is highlighted that the data for pure TS will be illustrated in detail in a parallel forthcoming paper, and the data for pure MS were from [47]; when necessary, they will be referred to here for comparison purposes.…”