Catalytic role of conditioner CaO in nitrogen transformation during sewage sludge pyrolysis

Thermal disposal of sewage sludge is likely to cause serious nitrogen related environmental pollution since it contains considerable amounts of nitrogen, the species of which are quite different from those in coal. Considering that lime (CaO) is a widely applied chemical conditioner for sewage sludge dewatering, this study investigated the catalytic role of conditioner CaO in nitrogen transformation during sewage sludge pyrolysis in a drop-tube/fixed-bed furnace at 873 K, 1073 K and 1273 K, respectively. Model compounds were also used to further clarify the mechanisms involved. According to the results, conditioner CaO increased the fraction of more stable protein-N as well as amine-N. The solid phase reactions produced CaCxNy, thus enhancing the nitrogen retention in char. Correspondingly, decreased relative ratio of nitrates-N/nitrites-N and oxygenated organics in sludge conditioning contributed to less NO emission. Meanwhile, conditioner CaO promoted the conversion of HCN to NH3, as well as the deamination of proteins, amine, and other N-containing compounds in tar and char, leading to increased NH3 generation. Subsequently, CaCx, the decomposition product of CaCxNy, captured NH3, driving down the final production of NH3. In addition, Ca(OH)2 hindered the transformation of nitrile-N in char to HCN, decreasing HCN generation. CaO reacted with HCN, further reducing its releasing amount. CaCxNy derived from different sources decomposed to produce a very large amount of N2. These indicate that reusing conditioner CaO is a promising strategy for reducing the productions of NOx precursors efficiently and increasing the formation of non-polluting N2 dramatically.