Effect Of Different Catalysts On The Distribution Of Products From Microwave Pyrolysissludge

Pyrolysis of sludge has become a hot research topic on the resource utilization of sewage sludge. It can catalytically split the macromolecules of organic matters in the sewage sludge and improve the quality of the pyrolysis products by adding catalysts during the process of pyrolysis. CaO and, Fe₂O₃ have catalytic activity in the biomass pyrolysis process. Therefore, CaO and Fe₂O₃ are selected as catalysts for pyrolysis in this study and, sewage sludge from a wastewater treatment plant in Shenzhen as raw material to research the effects of CaO and Fe₂O₃ on the distribution laws of three products from microwave pyrolysis of sewage sludge under different temperatures. On this basis, technologies such as gas chromatograph, GC-MS, XRD, SEM, EDS were used to analyze the gas, liquid and solid products from catalytic pyrolysis, and methods such as TC LP and Tessier were taken to further analyze the heavy metals in the sewage sludge before and after pyrolysis and the analysis results provided strong evidence for the resource utilization of sewage sludge.As seen from the analysis of the results of gas production through microwave-assisted catalytic pyrolysis with CaO and Fe₂O₃ under the condition of no catalysts added, the process of hydrogen production from pyrolysis can be divided into three stages : initial stage of gas production, middle stage of gas production and late stage of gas production. CaO had the best effect on the catalytic pyrolysis of sewage sludge for hydrogen production. At 900℃, production rate was up to 143 m L / min, but the total amount of hydrogen output was maximum at 800℃, coming to 38.2L/kg. Unlike the trend of changes in the rate of hydrogenproduction, the change of methane production ratein pyrolysis process was divided into two stages: the initial and late stages of gas production. Fe₂O₃ had the best effect on catalytic pyrolysis for methane production, and the methane production rate was up to 89 m L/min at 900℃. The total methane output was also maximum at 900 ℃, coming to 15.9L/kg. Compared with the conditions of no catalysts added, CaO and Fe₂O₃ had significant effects of catalytically splitting the molecules of organic matters in the sludge, the gas output came to the maximum of 630L/kg at pyrolysis temperature of 800℃ with CaO as catalyst, followed by the effect of Fe₂O₃ with peak gas output of 511L/kg at 700℃. The pyrolysis temperature had significant effects on the gas production rates and the total output in the pyrolysis process. The gas production rates increased sharply as the temperature increased and came to the highest rates at pyrolysis temperature of 900℃.As seen from the analysis of bio-oils from microwave-assisted catalytic pyrolysis of sewage sludge with different catalysts, it showed that bio-oil from the pyrolysis without catalysts added contained large amounts of hexadecanoic acid, with the content reaching 42.42% at 500℃, and also toxic organic molecules such as phenol. After catalytic pyrolysis with catalysts, the contents of hexadecanoic acid and oxygen dropped, ramatically, with the lowest content of hexadecanoic acid at only 10.26% under the conditions of Fe₂O₃ catalytic pyrolysis and the te mperate at 900℃. What’s more, the content of aromatic in the bio-oil reduced dramatically, and as a result, the quality of bio-oil significantly improved.There were a large amount of heavy metals in the sewage sludge. The ecological risk index of heavy metals in the raw sewage sludge was up to 169.91, and after catalytic pyrolysis, the inorganic elements such as Si, Mg and, Ca concentrated in the sludge surface with glass beads formed, effectively reducing the leaching of heavy metals in sewage sludge. As a result, the large amount of directly usable and indirectly usable biont in the heavy metal of the sewage sludge was transformed into the non-usable, thus, the ecological risk index of sludge residue dropped significantly after pyrolysis, with the lowest index of 15.73 under the conditions of Fe₂O₃ catalytic pyrolysis and the temperate at 900℃. The results indicated CaO and, Fe₂O₃ can effectively improve the safety performance of the s ludge residue after pyrolysis.