Primary Measures for Reduction of PCDD/F in Co-Combustion of Lignite Coal and Waste: Effect of Various Inhibitors

Abstract

Co-combustion of coal and waste in power plants poses both environmental and economic challenges, especially because of the high polychlorinated dibenzo-p-dioxin and furan (PCDD/F) emissions from solid waste. In this study, we performed a series of experiments focusing on the prevention of PCDD/F formation by the use of various inhibitors added to the fuel before combustion. A mixture of lignite coal, solid waste, and poly(vinyl chloride) (PVC) was thermally treated in a laboratory-scale furnace at 400 degrees C. Twenty different additives were investigated at a level of 10 wt% of the total fuel during the experiments. We have divided them into four general groups according to their chemical nature: metal oxides, N-containing compounds, S-containing compounds, and N- and S-containing compounds. The resulting values showed a significant reduction of PCDD/F levels when N- and S-containing compounds were used as additives to the fuel. Principle component analysis (PCA) was used to illustrate the effect of the 20 different inhibitors on the congener patterns emitted. As a result, the most effective inhibitors for PCDD/F formation in flue gases were determined to be (NH4)2SO4 and (NH4)2S2O3; they are inexpensive and nontoxic materials. Both compounds can suppress the formation of toxic compounds such as PCDD/Fs by more than 98-99%, and the most toxic PCDD/F congeners were not detectable in most of the samples. Thus, these compounds were also studied as a lower percentage of the fuel. (NH4)2SO4 resulted in a greater than 90% reduction of PCDD/F even when composing only 3% of the fuel combusted. However, less than 5% (NH4)2S2O3 resulted in far weaker inhibition. The PCDD/F homologue distribution ratio for samples with varying percentages of (NH4)2SO4 and (NH4)2S2O3 was also investigated. Higher percentages of the inhibitors produced a lower percentage of lower chlorinated PCDDs. The opposite effect was found for PCDFs.