Contrasting Effects of Sulfur Dioxide on Cupric Oxide and Chloride during Thermochemical Formation of Chlorinated Aromatics

Abstract

Sulfur dioxide (SO2) gas has been reported to be an inhibitor of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) formation in fly ash. However, other research has suggested little or no inhibitory effect of SO2 gas. Although these studies focused on reactions between SO2 gas and gas-phase chlorine (Cl) species, no attention was paid to thermochemical gas-solid reactions. In this study, we found contrasting effects of SO2 gas depending on the chemical form of copper (CuO vs CuCl2) with a solid-phase inorganic Cl source (KCl). Chlorinated aromatics (PCDD/Fs, polychlorinated biphenyls, and chlorobenzenes) increased and decreased in model fly ash containing CuO + KCl and CuCl2 + KCl, respectively, with increased SO2 injection. According to in situ Cu K-edge and S K-edge X-ray absorption spectroscopy, Cl gas and CuCl2 were generated and then promoted the formation of highly chlorinated aromatics after thermochemical reactions of SO2 gas with the solid-phase CuO + KCl system. In contrast, the decrease in aromatic-Cls in a CuCl2 + KCl system with SO2 gas was caused mainly by the partial sulfation of the Cu. The chemical form of Cu (especially the oxide/chloride ratio) may be a critical factor in controlling the formation of chlorinated aromatics using SO2 gas.

Amino Compounds as Inhibitors of De Novo Synthesis of Chlorobenzenes

“…A study by Fujimori et al . 28 also showed similar distribution ratios, but the absolute yield of each CBz in the present study was higher because of the larger amount of CuCl 2 ·2H 2 O added, which is necessary for determining the mechanism. As shown in Fig.…”

“…The degree of chlorination did not decrease, but the total chlorine from CBzs declined by 36%. In Yan’s 29 study, (NH 4 ) 2 SO 4 reduced the yield of PCDD/Fs in the gas phase by about 93% (about 60% PCDDs and 98% PCDFs); when gaseous SO 2 was used 28 , the yields of CBzs, PCDDs, and PCDFs were reduced by about 50%, 30%, and 50%, respectively. Thus, (NH 4 ) 2 SO 4 and SO 2 are effective inhibitors of dioxin and CBzs formation.…”

“… 40 and Fujimori et al . 28 proposed that SO 2 and H 2 O can convert CuCl or CuCl 2 to CuSO 4 or CuO by detecting the residues, which was also proved by the thermodynamic equilibrium calculation 41 . However, the fundamental information that enables an understanding of these mechanisms is still insufficient.…”

“…On the basis of the known mechanism of dioxin formation, diverse inhibitors have been used in the source control or end-of-pipe removal for dioxin, including nitrogen-containing compounds (NH 3 , urea and (NH 4 ) 2 SO 4 ) 24 25 26 27 , sulfur-containing compounds (elemental sulfur, SO 2 , (NH 4 ) 2 SO 4 and coal) 25 28 29 30 31 , hydroxy-functional groups 23 and selective catalysts for reduction (SCR) 32 33 34 35 . Among them, the most well-known dioxin inhibitors are highly efficient SCR, such as VO x /TiO 2 32 , V 2 O 5 /WO 3 33 and TiO 2 /V 2 O 5 /WO 3 34 35 catalysts, which usually require a complex preparation process and have high cost.…”

“… 27 indicated that SO 2 was more effective than NH 3 in inhibiting the formation of PCDD/Fs, while the contrast 29 between (NH 4 ) 2 SO 4 and CO(NH 2 ) 2 was contrary. In recent years, the suppression mechanisms of the formation of chlorinated aromatic pollutants by sulfur-containing and nitrogen-containing compounds have been discussed 20 28 37 38 39 , including (1) transformation of Cl 2 to HCl, (2) sulfonation of dioxins or precursors and (3) conversion of metal chlorides (CuCl 2 or FeCl 3 ) with high catalytic activity to inert compounds. Yan et al .…”