Dual role of conditioner CaO in product distributions and sulfur transformation during sewage sludge pyrolysis

Product distributions and sulfur transformation during sewage sludge pyrolysis are essential to subsequent thermal disposal process, both of which are strongly affected by calcium-based compounds. Considering that lime (CaO) is widely applied in sludge conditioning pre-treatment, this study investigated the dual role of conditioner CaO in sludge pyrolysis behavior and sulfur evolution at the temperature range of 873-1273 K. Specific compositions in three phases, such as solid residual, tar and gas, were also determined to further clarify the mechanisms involved. According to the results, conditioner CaO increased the fraction of ash and more stable organics in sludge particles, which contributed to higher yield of char comprising lots of carbon and hydrogen. The ring-opening of aromatic hydrocarbons followed by addition reaction of H2O in solid phase facilitated H2 and CO generation, leaving a large number of COH and CO in char. Meanwhile, conditioner CaO catalyzed tar cracking, further increasing the productions of H2 and CO. More interestingly, residual calcium was still capable of fixing sulfur in solid product. Raw sludge matrix prevented sulfur releasing mainly through cyclization reactions, and the relative ratio of char-S only accounted for less than 50%. Correspondingly, most of SO2, CS2, C2H6S2, H2S, COS, and CH4S were emitted. In contrast, char-S yields reached up to 92.8% mainly through forming thermal stable sulfide and sulfate promoted by conditioner CaO, which drove down the final production of sulfurous gases. These indicate that reusing conditioner CaO is a promising strategy for increasing combustible gases and char yields efficiently and reducing the formation of polluting S-containing gases dramatically.