Study on Correlation Characteristics of PCDD/Fs Isomers Generation during the Coal-Combustion Process with Sludge Mixing

The 17 toxic polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) isomers as a persistent and refractory organic pollutant are one of the pollutants produced in solid fuel (including pulverized coal and mixed fuel) combustion, and regulating their distribution characteristics is an important aspect of effectively reducing their toxic equivalent. In this study, a correlation model of PCDD/Fs toxic equivalent was proposed to quantitatively analyze the correlation between HCl and SO2 concentrations in the reaction atmosphere, meanwhile, a pathway-diagnosis method of PCDD/Fs isomers transformation was adopted to analyze the PCDD/Fs distribution characteristics of coal combustion and that mixed with 10% sludge. The results show that the inflection point for the derivative of the relationship between HCl concentration and SO2 concentration moves towards the direction of lower SO2 concentration and higher HCl concentration with the increase of PCDD/Fs toxic equivalent. Besides, the functional pathways of mixing sludge on PCDD/Fs generation in coal combustion were obtained by calculating chlorine substitution probability. It has been deduced that five transformation pathways were improved by mixing sludge. Next, the critical pathways of the influence from mixing sludge on the distribution characteristics of PCDD/Fs isomers in coal combustion were analyzed by drawing the percentage content migration diagram of PCDD/Fs isomers and the average toxic equivalent migration diagram that combined with the toxic equivalent of PCDD/Fs isomers. The results showed that the main reasons for mixing sludge to further reduce the average toxic equivalent of PCDD/Fs isomers in coal combustion were the increased chlorine substitution probability at the 4-position of 12378 Pentachlorodibenzo-p-dioxin (12378-PeCDD), the decreased chlorine substitution probability at the 4-position of 2378 Tetrachlorodibenzo-p-furan (2378-TeCDF), and the increased total chlorine substitution probability of 23478 Pentachlorodibenzo-p-furan (23478-PeCDF), 123478-Hexachlorodibenzofuran (123478-HxCDF) and 123678-HxCDF respectively. Finally, the functional pathway diagram of PCDD/Fs isomers generated from coal combustion with sludge mixing was drawn.