The Research In Supercritical Water Gasification Of Sewage Sludge Under Different Catalytic Conditions And The Migration Of Transformation Rules Of Heavy Metals

Supercritical water gasification can not only make organic matters in the sludge be released and reutilized, but also have a good immobilization effect on the heavy metals in the sludge. Therefore, it has become one of the most ideal ways for the resource utilization of and hazard-free disposal of sludge. In this paper, the excess sludge with the characteristics of high ash purchased from the wastewater treatment plant in Shenzhen City was used as the research object, the dried excess sludge as raw material was treated in the intermittent supercritical water reaction facility in the experiment to explore the gas, liquid and solid products' distribution laws and the migration and transformation laws of heavy metals after the supercritical water gasification of sludge.The analysis results of supercritical water gasification products showed that the four factors, i.e., the water-material ratio, pressure, temperature and time, exhibited different effects on the gasification rates of H₂ and CH4, the research showed that optimum parameters of supercritical gasification of sludge were the water-material ratio of 10: 1 with the reaction pressure of 25 MPa at 400?for 20 min. Ca O, Al Cl₃, Pd catalysts exhibited significant catalytic effects on the organic matter in sludge, the order of the catalytic ability was Pd>Ca O>Al Cl₃, energy gas gasification rate reached 43.73% with the output of H₂ and CH4 reaching 133.99L/Kg and 35.26L/Kg, respectively.As many as 30 kinds of organic compounds mainly including 6 categories that were alkanes, olefins, phenols, alcohols, carboxylic acids and organic matter containing N, were detected in oil products by GC-MS under different catalytic conditions. The hexadecane acid had the best directional catalytic effect with the catalytic rate of 26.72% among all the detected organic matters. By combining the analysis results with FTIR results, it was found that the catalytic treatment of supercritical water gasification can split the –OH,-CH₃,-CH₂,-C=O and other functional groups with poor stability in the organic matters in sludge. The results of elemental analysis and particle size analysis showed that the sludge's residue quality significantly reduced, physical particle size decreased and specific surface area increased accordingly.The heavy metals were detected by ICP-AES in the water solution, oil and solid products after supercritical water gasification. The contents of heavy metals in water solution and oil products were relatively low, and the Pb, Cr, Cd contents in the corresponding water solution and oil products were lower than the detection limit. According to the heavy metal migration and transformation laws, a variety of heavy metals were mainly enriched in the solid residue after supercritical water gasification treatment and the enrichment rate reached more than 99%.The study on heavy metals speciation indicated that the root reason that the heavy metals mainly existed in the form of the residue was that the supercritical water gasification can promote their transformation from the unstable state to the stable state. XRD analysis found that the metals like Pb, Ni, etc. were easy to form the crystalline compound in the residue, and the immobilization on the heavy metals by the lattice was the important mechanism for the existence of heavy metals in the form of residue. The assessment analysis of biological environmental risk factors showed that the risk coefficient of sludge residue after supercritical water gasification was relatively low, and the heavy metals were effectively immobilized.