Sulfur Transformation during Microwave and Conventional Pyrolysis of Sewage Sludge

The sulfur distributions and evolution of sulfur-containing compounds in the char, tar and gas fractions were investigated during the microwave and conventional pyrolysis of sewage sludge. Increased accumulation of sulfur in the char and less production of H2S were obtained from microwave pyrolysis at higher temperatures (500–800 °C). Three similar conversion pathways were identified for the formation of H2S during microwave and conventional pyrolysis. The cracking of unstable mercaptan structure in the sludge contributed to the release of H2S below 300 °C. The decomposition of aliphatic-S compounds in the tars led to the formation of H2S (300–500 °C). The thermal decomposition of aromatic-S compounds in the tars generated H2S from 500 to 800 °C. However, the secondary decomposition of thiophene-S compounds took place only in conventional pyrolysis above 700 °C. Comparing the H2S contributions from microwave and conventional pyrolysis, the significant increase of H2S yields in conventional pyrolysis was mainly attributed to the decomposition of aromatic-S (increasing by 10.4%) and thiophene-S compounds (11.3%). Further investigation on the inhibition mechanism of H2S formation during microwave pyrolysis confirmed that, with the special heating characteristics and relative shorter residence time, microwave pyrolysis promoted the retention of H2S on CaO and inhibited the secondary cracking of thiophene-S compounds at higher temperatures.