Product distribution and sulfur behavior in sewage sludge pyrolysis: Synergistic effect of Fenton peroxidation and CaO conditioning

Iron- and calcium-based compounds play an important role in the devolatilization stage of solid fuels thermal disposal. This study aims to investigate the synergistic effects of Fenton's reagent (Fe2+/H2O2) and conditioner CaO on product distribution and sulfur transformation in this process. Four samples were pyrolyzed by using a special drop-tube/fixed-bed furnace at 873-1273 K with a high heating rate. Specific compositions of solid residue, tar and gas were also identified to further clarify the related mechanisms. According to the results, after Fenton peroxidation, both side chain and main chain of organic compounds were prone to crack, forming more gas and less tar with similar composition. When the oxidized sludge was further treated by CaO, altered carbon species evolution left additional COH, CO, C(O, N), OCO in char. Moreover, synergy between the two conditioners was likely to provide Fe2O3, CaO and H2O, further increasing the H2-rich gas yields. Besides, the observation also demonstrates that Fenton treatment favored the char-S production not only through catalyzing the cyclization reactions but also from producing relatively stable sulfonic acid-S/sulfone-S. When temperature increased, the peroxidation hindered H2S, CS2 and CH3SH releasing, while the decomposition of FeSO4 and Fe2(SO4)3 aggravated SO2 polluting. However, it is exciting that sulfur fixation rate eventually reached over 97.0%, since the residual Fe- and Ca-containing compounds cooperatively captured liberated sulfur. Thus reusing conditioners, which originally targeted for improving sludge dewaterability, is a promising strategy for achieving efficient thermal conversion of sewage sludge with low sulfur pollution.