Herein we identified the characteristics of sludge pyrolysis theoretically and experimentally, where the general roles of sludge ash, CaO and Al2O3 were analyzed. Both valuable products and polluting materials were considered. Regarding the latter one, we for the first time proposed the principles of competitive effect and linkage effect in the silicate category, and accordingly, we put forward a general clue for trace element fixations. For syngas, the total yield composed of CO, H-2 and CH4 increased while the char yield decreased in the presence of ash and CaO; for polluting gases, the presence of CaO had a pronounced mitigation effect. Then the distributions of trace elements were identified by analyzing their network roles. For the network modifiers such as Ba and Zn, CaO weakened their fixation due to the competitive effect while Al2O3 and ash had a positive effect due to the linkage effect. For the network formers such as As and the elements with large atomic radius such as Co, CaO improved their fixation due to the linkage effect whilst Al2O3 and ash had an negative effect. Accordingly, we acquire significant ideas for clean and effective utilization of sludge, as well as coal and other solid wastes.