Pyrolysis of Municipal Sewage Sludge within a circular economy vision: Production of sustainable Biofuels and Economic Biofertilizers

This study aims to produce renewable biofuels (bio-oil, syngas) and nutrient-rich biofertilizers (pyrochar or biochar) from Municipal Sewage Sludge (MSS) via the innovative pyrolyis process. The pre-dried MSS was analyzed using proximate and ultimate analyses, FTIR spectroscopy, and DTA-TGA thermogravimetry to evaluate their suitability as a feedstock for thermochemical conversion and biofuels production. The optimization of the pyrolysis reactions was performed using a laboratory-scale fixed-bed reactor and the investigation of the influence of some key operational parameters (final temperature and heating rate) on the pyrolysis products distribution shows that 600 °C and 10 °C/min are the optimal conditions to obtain a maximum bio-oil yield (44.31 wt.%). The characterization of the obtained pyrolysis products indicates (i) a high aliphatic character of the bio-oil, with the occurrence of several organic compounds (n-alkanes, n-alkenes, carboxylic acids and aromatic compounds); (ii) good fuel properties of the pyrolytic oil comparable to those of fossil fuels, promoting its application as liquid biofuel; (iii) a high calorific value of the obtained syngas (HHV around 16.68 MJ/m3) encouraging its use as a source of renewable energy or for pyrolysis reactor heating (iv) a highly porous biochar with good carbon and nitrogen contents which could be investigated as a biofertilizer.