Energetic valorisation of tannery sludge by gasification in fluidised bed

The present article deals with the valorisation 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 favour 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 characterised through proximate/ultimate analyses and determination of heating value, witnessing its capability to act as solid fuel in a gasification process, and metal analyses, showing its Cr(VI) content below the detection limit (2 ppm). The material has been submitted to gasification tests in a lab-scale Fluidised Bed (FB) reactor. The reactor, 41 mm ID and 1 m height, was electrically kept at the operating temperature of 850°C. The fluidisation velocity was 0.30 m/s at 850°C, i.e. 7.5 times the value of the minimum fluidisation velocity. The gasifying stream was composed by O2 (3% vol.) diluted in N2. The oxidant Equivalence Ratio (ER) ranged from 0.15 to 0.24, so to ensure substoichiometric (i.e., reducing) conditions in the FB atmosphere. Under the most reducing operating conditions, it was possible to produce a syngas with lower heating value of 12.0 MJ/Nm3 (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 characterised by Gas Chromatography- Mass Spectrometry (GC-MS), showed a favourably low concentration of about 25 g/Nm3. FB bottom and fly ashes were analysed for their carbon and metal contents. In bottom ash, the total Cr concentration resulted in the range 8-12 g/kg, with Cr(VI) concentration between 8 and 10 ppm. In the elutriated stream, the total Cr concentration was about 55 g/kg, with Cr(VI) concentration between 4 and 7 ppm. The Cr(VI) concentration was higher when higher values of ER were used, but it resulted 3-4 orders of magnitude lower than the total Cr concentration, witnessing the appropriateness of the process for the production of a syngas with very limited oxidation of chromium in the solid residues.