Transformation of the Sulfur Element during Pyrolysis of Sewage Sludge at Low Temperatures

In this study, the transformation mechanism for the sulfur element during pyrolysis of sewage sludge (SS) was investigated. The amount of H2S, which was mainly produced from methionine pyrolysis at low temperatures, increased stepwise with increasing temperature in the range of 400-600 degrees C. The pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) results showed that a large amount of sulfur-containing heterocyclic compounds was found in the tar from sulfur-containing amino acids. The electrospray ionization Fourier transform ion cyclotron resonance (ESI FT-ICR) MS results further confirmed that the sulfur element in sulfur-containing amino acids was mainly transferred into the tar phase. Polymerization was the main reaction during the low-temperature pyrolysis of sulfur-containing amino acids. In addition, methylation, deamination, desulphurization, and decarboxylation also took place during the pyrolysis. X-ray photoelectron spectroscopy (XPS) results indicated that the content of sulfur-containing inorganic compounds was very low in char. With an increase of pyrolysis temperatures, the heterocyclic compounds further cracked to form H2S. The above results indicated that H2S mainly came from methionine during the pyrolysis of SS at low temperatures. A potential mechanism for the transformation of the sulfur element during the pyrolysis of SS was proposed.