Italy is the main supplier of leather in Europe, contributing 65% to European production of bovine leather and 15% to the world production. The tanning industry produces wastewater rich in pollutants, especially Cr, which is the chemical species with the strongest environmental impact. The purification process of this wastewater, produces a tanning sludge (TS) with high concentration of Cr(III), generally stored in authorized landfills with high costs. Due the high production of TS, it is necessary to implement sustainable management strategies that aim both to reduce the volume of TS to be disposed of and to produce energy vectors. Therefore, Sludge-to-energy ("StE") technologies are adopted. However, due to the high content of Cr(III) in TS, special attention must be paid to limit/avoid the oxidation of Cr(III) to Cr(VI), extremely harmful. In this context, gasification is particularly suitable for the treatment of TS because, operating in oxygen-deficient conditions, it reduces the risk of oxidation of Cr. In this work, were carried out gasification tests, in a fluidized bed reactor (ID=0.041m and H=1m), of a dried Italian TS, supplied by SSIP. Tests were conducted at 850°C, at a velocity of 0.3 m/s. The volumetric flow rate of fed gas was 350 NL/h, consisting of 85.7% N2 (300 NL/h) and 14.3% air (50 NL/h). The TS (ID<1mm) was sent to the reactor with mass flow rates of 99g/h and 63g/h to obtain two different air/fuel equivalence ratio (ER) values of 0.15 and 0.24, respectively, in order to observe its effects on hexavalent Cr formation. In the test with ER=0.15 (less oxidative conditions) 22% more syngas was produced (CO+H2=77.07%), compared to that with ER=0.24 (CO+H2=59.74%). Consequently, HHV in the test with ER=0.15 (12.0 MJ/Nm3) is greater than that obtained with ER=0.24 (11.6 MJ/Nm3), while the CO2 content is lower (16.16% for ER=0.15 vs 30.10% for ER=0.24). The concentration of Cr(VI) (absent in the raw TS) determined in bottom-ash, fly-ash and tar is always four orders of magnitude lower than the concentration of Cr(III) in the raw TS (25.38 g/kg), demonstrating the correct choice of gasification operating conditions, aimed at limiting oxidation to Cr(VI).
GASIFICATION OF TANNERY SLUDGES IN FLUIDIZED BED: FATE OF Cr(III)
R Migliaccio;M Urciuolo;G Ruoppolo;
2021
Abstract
Italy is the main supplier of leather in Europe, contributing 65% to European production of bovine leather and 15% to the world production. The tanning industry produces wastewater rich in pollutants, especially Cr, which is the chemical species with the strongest environmental impact. The purification process of this wastewater, produces a tanning sludge (TS) with high concentration of Cr(III), generally stored in authorized landfills with high costs. Due the high production of TS, it is necessary to implement sustainable management strategies that aim both to reduce the volume of TS to be disposed of and to produce energy vectors. Therefore, Sludge-to-energy ("StE") technologies are adopted. However, due to the high content of Cr(III) in TS, special attention must be paid to limit/avoid the oxidation of Cr(III) to Cr(VI), extremely harmful. In this context, gasification is particularly suitable for the treatment of TS because, operating in oxygen-deficient conditions, it reduces the risk of oxidation of Cr. In this work, were carried out gasification tests, in a fluidized bed reactor (ID=0.041m and H=1m), of a dried Italian TS, supplied by SSIP. Tests were conducted at 850°C, at a velocity of 0.3 m/s. The volumetric flow rate of fed gas was 350 NL/h, consisting of 85.7% N2 (300 NL/h) and 14.3% air (50 NL/h). The TS (ID<1mm) was sent to the reactor with mass flow rates of 99g/h and 63g/h to obtain two different air/fuel equivalence ratio (ER) values of 0.15 and 0.24, respectively, in order to observe its effects on hexavalent Cr formation. In the test with ER=0.15 (less oxidative conditions) 22% more syngas was produced (CO+H2=77.07%), compared to that with ER=0.24 (CO+H2=59.74%). Consequently, HHV in the test with ER=0.15 (12.0 MJ/Nm3) is greater than that obtained with ER=0.24 (11.6 MJ/Nm3), while the CO2 content is lower (16.16% for ER=0.15 vs 30.10% for ER=0.24). The concentration of Cr(VI) (absent in the raw TS) determined in bottom-ash, fly-ash and tar is always four orders of magnitude lower than the concentration of Cr(III) in the raw TS (25.38 g/kg), demonstrating the correct choice of gasification operating conditions, aimed at limiting oxidation to Cr(VI).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
