Effect of Soot and Copper Combustor Deposits on Dioxin Emissions

Abstract

An experimental study was conducted to investigate the effects of residual soot and copper combustor deposits on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) during the combustion of a chlorinated waste. In a bench-scale setup, distillate fuel oil doped with copper compounds was fired under sooting conditions in a quartz reactor heated by a Linberg furnace to 900°C. Soot- and copper-containing aerosols were carried by the hot flue gas and deposited on a separate quartz "deposition" tube connected to the exit of the quartz reactor. Selected experiments were also conducted to deposit either soot or copper on separate quartz deposition tubes. In a separate setup, 1,6-dichlorohexane was injected into a natural-gas-fired furnace reactor to produce a flue gas that contained 6.2% oxygen, 8.6% carbon dioxide, 10–50 ppm carbon monoxide, and 500 ppm hydrogen chloride. The deposition tubes were separately attached to the furnace reactor and a flue gas slip stream was passed through the tubes into a PCDD/F sampling train. The deposition tubes were maintained at 320°C by an electrical heating tape. Sulfur dioxide (SO2) was injected near the flame zone of the furnace reactor in selected experiments. High levels of PCDD/Fs were measured at the exit of the deposition tubes containing both soot and copper deposits. No PCDD/Fs were found during experiments using a tube with only soot deposits, and trace amounts of PCDD/Fs were found with a deposition tube containing only copper deposits. Repeated experiments with the same deposition tube containing soot and copper deposits resulted in reductions in the amount of PCDD/Fs formed, indicating decreased formation with time. Significantly lower PCDD/F emissions were measured from the experiments with SO2 injection, indicating that SO2 is effective in suppressing PCDD/F formation reactions. It was also found that a deposition tube previously exposed to flue gases from 1,6-dichlorohexane incineration exhibited residual PCDD/F formation reactions when exposed to flue gases from the combustion of natural gas. The results demonstrate that the combustor deposits containing copper and soot can cause "memory effects" that can play an important role in the emissions of PCDD/Fs from commercial incinerators. Key words: Chlorinated waste incineration; dioxin emissions; soot and copper deposits; wall effects; memory effects