Troubleshooting dioxins stack emissions in an industrial waste gas incinerator

An important source of dioxins and furans at present is waste incineration, utmost formed during combustion processes and emitted to the environment without being fully captured by waste-gas treatment equipment. In this study, monitoring campaign of International Toxic Equivalents for dioxins and furans (I-TEQDF), was carried out at pharmaceutical industrial waste incinerator to find a correlation between combustion parameters and feed composition with potential emission. Principal Component Analysis (PCA) shows that high values of dioxin emission correlate with short residence time of the flue gas in the furnace as well as low oxygen concentration. These operating conditions were further investigated, using COMSOL Computational Fluid Dynamics (CFD) simulation to calculate the temperature profiles along the furnace. The results suggest that the flame temperature profile is anticipated to exhibit cold areas (cold spots), which may be used as a proxy for dioxin formation due to incomplete combustion. Additionally, the calculated congeners furan to dioxin concentration ratio, points to their formation via de novo mechanism. SEM-EDS analysis preformed on the bag filter upstream the feed following its filtration, have shown large amount of iron, which may have served as a metal catalytic source for dioxin formation. The iron origin is most likely from corrosion of the feeding pipe, drifted with the waste gas and trapped on the bag filter. The results of this study provide a better understanding of the parameters controlling dioxin formation and emission from the plant and may assist a planning of process optimization in such a plant.

Atline measurement of 1,2,4-trichlorobenzene for polychlorinated dibenzo-p-dioxin and dibenzofuran International Toxic Equivalent Quantity prediction in the stack gas

A home-made analytical instrument based on thermal desorption gas chromatography coupled to resonance enhanced multiphoton ionization time-of-flight mass spectrometry (TD-GC-REMPI-TOFMS) was applied for atline measurement of 1,2,4-trichlorobenzene for the prediction of polychlorinated dibenzodioxin and dibenzofuran (PCDD/F) concentrations in the stack gas of a municipal solid waste incinerator (400 ton/day). Conventional high resolution gas chromatography/high resolution mass spectroscopy (HRGC/HRMS) measurements for the determination of PCDD/F concentrations were performed to compare with TD-GC-REMPI-TOFMS measurements. 1,2,4-Trichlorobenzene correlated with I-TEQ at r = 0.867, 0.953 and 0.944 in unstable, stable and integrated conditions. The correlation was independent of the facility operating conditions observed in this study. Using a linear model to predict I-TEQ by 1,2,4-trichlorobenzene over the test, the average of the relative difference between predicted and measured I-TEQ was 18.9%. 1,2,4-Trichlorobenzene measured by TD-GC-REMPI-TOFMS can be used as a robust indicator of I-TEQ in stack gas.

Suppression of PCDD/Fs during thermal desorption of PCBs-contaminated soil

Thermal treatment of polychlorinated biphenyls (PCBs) contaminated soil was shown in earlier work to generate new PCBs, as well as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). In this study, this thermal desorption was conducted with addition of three distinct inhibitors, including ammonium sulphate, urea and calcium oxide, to inhibit the formation of PCDDs and PCDFs when remediating PCBs-contaminated soil. Experiments were conducted for 40 min at 400 °C after adding 1 wt.% of inhibitor. Both the total PCDD/Fs and international toxic equivalent quantity (I-TEQ) reduced when inhibitors were introduced. Of the three compounds tested, CaO shows the highest inhibition efficiency, 92.2 % for total PCDD/Fs and 95.6 % for I-TEQ. The amount of CaO added also influences the suppression efficiency of PCDD/Fs. These results suggest that promoting desorption and destruction of precursors is probably the mechanism of suppression.

Sulfur recirculation for increased electricity production in Waste-to-Energy plants

Sulfur recirculation is a new technology for reducing boiler corrosion and dioxin formation. It was demonstrated in full-scale tests at a Waste to Energy plant in Göteborg (Sweden) during nearly two months of operation. Sulfur was recirculated as sulfuric acid from the flue gas cleaning back to the boiler, thus creating a sulfur loop. The new technology was evaluated by extensive measurement campaigns during operation under normal conditions (reference case) and operation with sulfur recirculation. The chlorine content of both fly ash and boiler ash decreased and the sulfur content increased during the sulfur recirculation tests. The deposit growth and the particle concentration decreased with sulfur recirculation and the dioxin concentration (I-TEQ) of the flue gas was reduced by approximately 25%. Sulfuric acid dew point measurements showed that the sulfuric acid dosage did not lead to elevated SO3 concentrations, which may otherwise induce low temperature corrosion. In the sulfur recirculation corrosion probe exposures, the corrosion rate decreased for all tested materials (16Mo3, Sanicro 28 and Inconel 625) and material temperatures (450 °C and 525 °C) compared to the reference exposure. The corrosion rates were reduced by 60-90%. Sulfur recirculation prevented the formation of transition metal chlorides at the metal/oxide interface, formation of chromate and reduced the presence of zinc in the corrosion products. Furthermore, measured corrosion rates at 525 °C with sulfur recirculation in operation were similar or lower compared to those measured at 450 °C material temperature in reference conditions, which corresponds to normal operation at normal steam temperatures. This implies that sulfur recirculation allows for higher steam data and electricity production without increasing corrosion.