Influence of Different Catalytic Metals on the Formation of PCDD/Fs during Co-combustion of Sewage Sludge and Coal

Co-combustion of sewage sludge (SS) and coal was developed rapidly in China, however, less attention was paid to the formation characteristics of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and the influence factors, such as the catalytic metals (CuCl_2, FeCl_3, ZnCl_2, CuO, Fe_2O_3, and ZnO), sources of carbon and chlorine. During the co-combustion of SS (2.5 wt.%) and coal (97.5 wt.%), the formation content of PCDD/Fs was 0.51 ng g^(-1) and 0.01 ng I-TEQ g^(-1). By adding metal chlorides (2.0 wt.% CuCl_2, FeCl_3, and ZnCl_2), the formation contents of PCDD/Fs were increased significantly to 148.99, 67.38, and 53.63 ng g^(-1) (i.e., 2.10, 1.09, and 1.34 ng I-TEQ g^(-1)), respectively, while the metal oxides (2.0 wt.% CuO, Fe_2O_3, and ZnO) only increased the PCDD/F contents to 1.27, 0.76, and 0.57 ng g^(-1) (0.03, 0.01, and 0.02 ng I-TEQ g^(-1)), respectively. That is, the promotion effect of different metal additives on PCDD/F formation followed a sequence of CuCl_2 (292×) ＞ FeCl_3 (132×) ＞ ZnCl_2 (105×) ≫ CuO (2.5×) ＞ Fe_2O_3 (1.5×) ＞ ZnO (1.1×), wherein the metal chlorides showed 2 orders of magnitude higher promotion than that of metal oxides. The proportion of highly chlorinated PCDD/Fs was further increased after adding metal chlorides, while the additional metal oxides decreased it. The de novo synthesis was revealed as the major formation pathway of PCDD/Fs except for adding ZnO. The CuCl_2 and FeCl_3 can also enhance the chlorination routes to generate PCDD/Fs. Only ZnO showed the promotion effect on the CP-route. The chlorine source was an essential limiting factor for generating PCDD/Fs, and the chlorine from metal chlorides was the preference source compared with that contained in SS and coal. The results have important reference value for controlling PCDD/Fs formation in the co-combustion process of SS and coal.