Fluidized bed gasification is a promising process technology to manage the growing amount of sewage sludge (SS) requiring disposal. Two samples of SS, produced in different seasons of the year by a municipal wastewater treatment plant, were subjected to gasification at 850 °C in a bench-scale fluidized bed reactor using, as a gasification agent, a nitrogen/air mixture at different values of oxygen/fuel equivalence ratio (ER = 0.1−0.2). The starting materials and the output streams (syngas, tar, and solid residues) were thoroughly characterized. The fate of specific SS constituents and the characteristics of bottom ashes were addressed, so contributing to the problem of a proper SS management approach in the context of the circular economy. Computeraided simulations were also performed, which allowed us to predict the composition of the syngas from SS gasification under operating conditions different from those experimentally investigated (i.e., reactor temperature and ER).

The present article deals with the valorization of the organic content of tannery sludges to produce energy vectors. In this scenario, gasification is a viable option to obtain a flexible gaseous stream (syngas) of interesting energetic value, under operating conditions that do not favor the oxidation of Cr(III) (typically found in tannery sludges) to the more harmful Cr(VI) state. To this end, an industrial tannery sludge was characterized through proximate/ultimate analyses and determination of the heating value, showing its capability to act as a solid fuel in a gasification process, and metal analyses, showing that its Cr(VI) content was below the detection limit (2 ppm). The material was subjected to gasification tests in a lab-scale fluidized bed (FB) reactor. The reactor, with a 41 mm inside diameter and a 1 m height, was electrically kept at an operating temperature of 850 °C. The fluidization velocity was 0.30 m/s at 850 °C, i.e., 7.5 times the value of the minimum fluidization velocity. The gasifying stream was composed by O2 (3% vol.) diluted in N2. The adopted oxidant equivalence ratio (ER) levels were 0.15 and 0.24, to ensure substoichiometric (i.e., reducing) conditions in the FB atmosphere. Under the most reducing operating conditions, it was possible to produce syngas with a lower heating value of 12.0 MJ/N m3 (dry and N2-free basis). It contained, under these conditions, about 42% H2, 36% CO, and 4% CH4, plus 16% CO2 and other components. The tar produced from the process, fully characterized by gas chromatography–mass spectrometry, showed a favorably low concentration of about 25 g/N m3. FB bottom and fly ashes were analyzed for their carbon and metal contents. In bottom ash, the total Cr concentration resulted in the range of 8–12 g/kg, with a Cr(VI) concentration between 8 and 10 ppm. In the elutriated stream, the total Cr concentration was about 55 g/kg, with a Cr(VI) concentration between 4 and 7 ppm. The Cr(VI) concentration was higher when higher values of the ER were used, but it resulted in 3–4 orders of magnitude lower than the total Cr concentration, showing the appropriateness of the process to produce syngas with very limited oxidation of chromium in the solid residues.

Bench-scale fluidized bed fast pyrolysis of as-received and torrefied residual olive stone (OS) was carried out at 500 °C in the presence or absence of a solid acid catalytic bed. Light to mild torrefaction conditions were investigated, with temperatures of 200, 225, and 250 °C, set in a bench-scale fluidized bed torrefier. Light torrefaction temperatures (200 and 225 °C) did not yield appreciable differences in the thermogravimetric pyrolysis studies, despite notable changes in mass and energy yield resulting from the influence of temperature difference. The mass and energy yields decreased from 91.1 to 74.0% and from 92.0 to 80.9%, respectively, moving from light to mild torrefaction. The higher heating value (HHV) increased linearly with an increase in torrefaction temperature, reaching a maximum of 22.1 MJ/kg at 250 °C, which was 9.7% greater than the as-received OS sample. A comparison was made between the influence of light and mild torrefaction on subsequent bench-scale pyrolysis experiments. In particular, the quality of bio-liquids for torrefaction coupled to fast pyrolysis shifted signifcantly towards the production of higher phenolic and aromatic derivatives that may find applications as potentaila drop-in alternative hydrocarbons.

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