Adsorption of Dioxins on the Entering Raw Meal

Cement kilns still emit considerable amounts of dioxins, more precisely polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) whether these kilns co-incinerate waste, or not. The emission level depends on the quality of meals (composition of mixed and ground limestone and shale or clay) on the one hand, the co-firing of waste on the other. Waste can be fired at the lower, discharge end of the rotary kiln, or be introduced to mid-kiln, at the kiln entrance, in the calcination section, or even further downstream along the path of the combustion gases. The temperature/time history of these combustion gases and of the meal are decisive factors in the formation, collection and destruction of dioxins. Still, very little is known on the extent and location of such phenomena along the path of meal, in countercurrent to that of combustion gases. In this study, the adsorption effects of the entering meal on the flows of dioxins were fully investigated in a 4500 ton day–1 clinker production line. The meal was characterised by its Ultimate Analysis, its specific surface and by XRD- and EDS-analysis, indicating that CaO, CaCO3 and SiO2 were the main compositions of it. In laboratory experiments, it was observed that the entering raw meal could indeed adsorb 2,3,7,8-substituted PCDD/Fs from combustion gas thus cleaning the raw gas even before its treatment by dust collection. The results revealed that meal could adsorb 2,3,7,8-PCDD/Fs from RDF incineration flue gas, with an efficiency up to 84.8% in wt. units and 71.0% in I-TEQ units.