Tannery Sludge Gasification in a Fluidized Bed for Its Energetic Valorization

Abstract

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.