Release of hydrogen sulfide during microwave pyrolysis of sewage sludge: Effect of operating parameters and mechanism

•Release of H2S under different factors from microwave pyrolysis was studied.•The evolution of sulfur-containing compounds in the chars was conducted.•Temperature and catalyst were the main factors influencing the release of H2S during pyrolysis.•Organic sulfur accounted for 60% of total-S in sludge responsible for the formation of H2S.•The mechanism of H2S formation from mercaptan, aromatic-S and sulfate was proposed. The effects of sludge characteristics, pyrolysis temperature, heating rate and catalysts on the release of H2S and mechanism of H2S formation during sludge pyrolysis were investigated in a microwave heating reactor (MHR). The evolution of sulfur-containing compounds in the pyrolysis chars obtained at temperature range of 400–800°C was characterized by XPS. For a given temperature, the maximum concentration of H2S appeared at moisture content of 80%. Compared to the influence of heating rate on the H2S yields, pyrolysis temperature and catalyst played a more significant role on the release of H2S during microwave pyrolysis process. The H2S concentration increased with increasing temperature from 400°C to 800°C while decreased with increasing heating rate. Both the Nickel-based catalyst and Dolomite displayed significant desulfurization effect and Ni-based catalyst exhibited the larger desulfurization capability than that of Dolomite. The organic sulfur compounds accounted for about 60% of the total sulfur in the sludge which was the main reason for the formation of H2S. The mechanism analysis indicated that the cleavage reactions of mercaptan and aromatic-S compounds at temperatures below 600°C and the cracking reaction of sulfate above 700°C respectively were responsible for the H2S release during sludge pyrolysis.