Suppression of dioxins in waste incinerator emissions by recirculating SO2

Unravelling the impacts of sulfur dioxide on dioxin catalytic decomposition on V2O5/AC catalysts

Dioxins are high chlorine, toxic, and persistent organic pollutants that exert significant pressure on both human and the environment. From the analysis of current pollutant removal of the whole life cycle, such as integrated removal of NOx, SO2 and dioxins in a system, the dioxins oxidation activity as well as the distribution of oxidation products in the presence of SO2 are still a challenge. In this study, dibenzofuran (DBF) was regarded as a model dioxin compound, and V2O5/AC was used as a catalyst to investigate the impact of SO2 on degradation activity and the degradation path of DBF. Various characterization results showed that SO2 could promote the transformation of DBF to intermediates through a reaction with lattice oxygen and lower the apparent activated energy of DBF catalytic oxidation on V2O5/AC catalysts. The density functional theory (DFT) calculations confirmed that SO2 improved the oxidation ability of lattice oxygen on V2O5/AC. The ethyl hydrogen fumarate intermediate decreased and the small-molecule byproducts increased, providing further evidence that SO2 accelerates the degradation of DBF and its intermediates. However, the formation of VOSO4 would inevitably deteriorate the adsorption and oxidation abilities of V2O5/AC. A model is pioneered to describe the relationship between SO2 promotion and VOSO4 inhibition on DBF catalytic oxidation on a V2O5/AC catalyst. This study is expected to provide theoretical guidance for the collaborative abatement of multi-pollutants in flue gas.

PCDD/Fs, PAHs and HCl emissions from co-combustion of lignite and chicken manure in a circulating fluidized bed boiler with compact refractory casting

Chicken manure (CM) should be used in energy production due to its high production potential for the waste-to-energy approach. Co-combustion of CM with lignites may be a good practice in terms of reducing its environmental impact and the need for fossil fuels. However, the level of organic pollutants originated from CM combustion is not clear. This study investigated the potential of CM to be combusted in a circulating fluidized bed boiler (CFBB) with a local lignite. Combustion and co-combustion tests of CM and Kale Lignite (L) were performed in the CFBB to measure PCDD/Fs, PAHs and HCl emissions. CM burned in the upper parts of the boiler due to its high volatile matter content and low density compared to coal. This caused the bed temperature to decrease with the increase in the amount of CM in the fuel mixture. It was also observed that the combustion efficiency increased as the share of CM in the fuel mixture increased. Total PCDD/F emissions increased with CM share in the fuel mixture. However, all are less than emission limit value (100 pg I-TEQ/m3). Co-combustion of CM with lignite at different ratios did not have a significant effect on HCl emissions. PAH emissions were found to increase with the increase of the CM share when the CM share was more than 50% by weight.

Formation and inhibition of Polychlorinated-ρ-dibenzodioxins and dibenzofurans from mechanical grate municipal solid waste incineration systems

This study is carried out in two full-scale (300 t/d) municipal solid waste incinerators (MSWI), focusing on the inhibition effect on polychlorinated-ρ-dibenzodioxins and dibenzofurans (PCDD/F) formation by the Sulfur-, Phosphorus-, and Nitrogen-containing inhibitors. The inhibition efficiencies of total PCDD/F range from 45.77 % to 58.55 %, meanwhile, from 50.1 % to 57.6 % for toxic PCDD/F. X-ray photoelectron spectroscopy results conduct the inhibition effect on the three key factors of PCDD/F formation: catalytic metal, carbon source and chlorine source. Inhibitors can increase the proportion of inorganic chlorine form at the ash surface. The changes of sulfur and phosphorus forms support the inhibition mechanisms of PCDD/F. De novo synthesis is the stable inhibition pathway in this study, meanwhile, the chlorophenols-route and dibenzodioxin and dibenzofuran chlorination also work in some tests. The results are the basics for further industrial application of PCDD/F inhibitors and benefit in controlling the PCDD/F emission from MSWI.

Development of phosphorus-based inhibitors for PCDD/Fs suppression

Inhibition mechanisms of sulfur-, nitrogen- and phosphorus- based inhibitors on the de novo synthesis of polychlorinated dibenzo-p-dioxins, and dibenzofurans (PCDD/F) were studied by exploring speciation evolution of carbon, chorine and copper in fly ash under laboratory-scale experiments. Significant inhibition of PCDD/Fs by thiourea (TUA) and ammonium dihydrogen phosphate (ADP) was observed as 97.2% and 98.2%, respectively, except for potassium dihydrogen phosphate (PDP). ADP and PDP exhibited better inhibition on PCDFs than on PCDDs, whereas TUA exhibited the opposite effect. After adding inhibitors, the proportion of C-O/C=O/O-C=O bonds at the surface of fly ash increased, and stronger oxidation of carbon occurred, together with the conversion from Cu²⁺ to Cu⁺ and the inhibition of organic chlorine formation. Kinetic model results indicated that TUA might either suppress the carbon gasification or promote the decomposition of PCDD/Fs, resulting in a remarkable inhibition of PCDD/Fs formation. Simulated chemical reaction equilibrium further comfirmed that catalytic metal could be deactivated into CuS and Cu₂S by sulfur, and into Cu₂P₂O₇ by phosphorus. Moreover, NH₃, decomposed from TUA and ADP, was able to convert Cl₂ into HCl, albeit with a weaker chlorination ability. This study of inhibition mechanisms is useful for the exploration and utilization of efficient inhibitors in full-scale incinerators.

Synergistic Elimination of NOx and Chloroaromatics on a Commercial V2O5-WO3/TiO2 Catalyst: Byproduct Analyses and the SO2 Effect

The synergistic control of multipollutants is the frontier of environmental catalysis. This research is in the infancy stage, and many uncertainties still remain. Herein, we investigated the reaction characteristics of synergistic elimination of NO_x and chloroaromatics on a commercial V₂O₅-WO₃/TiO₂ catalyst. The reaction byproducts were qualitatively and quantitatively analyzed, and their origins were clarified. In particular, the origins of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) from the synergistic reaction with or without SO₂ were first explored; this is crucial for assessing the environmental risk by applying such a synergistic system. Experimental results indicate that during the synergistic reaction, the V₂O₅-WO₃/TiO₂ catalyst was deactivated at 200 and 250 °C, whereas the 300 °C was sufficient to durably convert the NO and chlorobenzene at the turnover frequency (TOF) of 7.23 × 10^-4 and 1.32 × 10^-4 s^-1, respectively. A range of aromatics, alkenes, and alkanes, particularly their chlorinated congeners, were observed in the off-gases and on the catalyst surface, where those of 3-chlorobenzonitrile, 4-chloro-2-nitrophenol, and inorganic CS₂ were first discovered. In the time-on-stream test at 250 °C, the PCDD/Fs collected from the off-gases was measured at 0.0514 ng I-TEQ Nm^-3, but the most toxic dioxins congener, 2,3,7,8-TCDD, was not observed. The alkalinity of selective catalytic reduction reaction likely facilitated the chlorophenol formation, which eventually promoted PCDD/F generation. The SO₂ was found to benefit polychlorinated byproduct generation, but the addition of which distinctly inhibited PCDD/F formation.

Influences of P-N-containing inhibitor and memory effect on PCDD/F emissions during the full-scale municipal solid waste incineration

Influence of inhibition and memory effect on PCDD/Fs are carried out in a full-scale (23 t h^-1) municipal solid waste (MSW) incinerator under three conditions. Ammonium dihydrogen phosphate (ADP) is injected into the post combustion zone of incinerator. The results show that the inhibition efficiency of PCDD/Fs in flue gas was 53.34% (71% for TEQ), and ADP possibly also didn't have effect on the chlorination reaction of PCDD/F formation. The memory effect phenomenon, PCDD/F adsorption/desorption, is clearly discovered in the wet scrubber system (WSS), resulting in PCDD/F concentration increased by 6.2-19.9 times. Memory effect also changes the distribution of PCDD/Fs by increasing the proportions of low-chlorinated PCDD/Fs, which is easily to be desorbed because of their higher vapor pressure comparing with the high-chlorinated PCDD/Fs. After renewing the scrubbing solution and cleaning the bottom sludge in WSS, the PCDD/F TEQ concentrations decreases from 0.51 to 0.24 ng I-TEQ Nm^-3, further mitigating but still not enough to eliminate the memory effect because the PCDD/F desorption of wall and scrubber fillings in the WSS. The results are paving the way for further industrial optimized design of WSS and assist in controlling the PCDD/F emissions from MSWI.

Suppression of dioxins by S-N inhibitors in pilot-scale experiments

S-N inhibitors like thiourea and sewage sludge decomposition gases (SDG) are relatively novel dioxins suppressants and their efficiencies are proven in numerous lab-scale experiments. In this study, the suppression effects of both thiourea and SDG on the formation of dioxins are systematically tested in a pilot-scale system, situated at the bypass of a hazardous waste incinerator (HWI). Moreover, a flue gas recirculation system is used to get high dioxin suppression efficiencies. Operating experience shows that this system is capable of stable operation and to keep gaseous suppressant compounds at a high and desirable molar ratio (S + N)/Cl level in the flue gas. The suppression efficiencies of dioxins are investigated in flue gas both without and with addition of S-N inhibitors. A dioxin reduction of more than 80 % is already achieved when the (S + N)/Cl molar ratio is increased to ca. 2.20. When this (S + N)/Cl molar ratio has augmented to 4.18 by applying suppressant recirculation, the residual PCDD/Fs concentration in the flue gas shrank from 1.22 to 0.08 ng I-TEQ/Nm(3). Furthermore, the congener distribution of dioxins is analysed to find some possible explanation or suppression mechanism. In addition, a correlation analysis between (S + N)/Cl molar ratios and PCDD/Fs is also conducted to investigate the chief functional compounds for dioxin suppression.

Municipal solid waste incineration in China and the issue of acidification: A review

In China, incineration is essential for reducing the volume of municipal solid waste arising in its numerous megacities. The evolution of incinerator capacity has been huge, yet it creates strong opposition from a small, but vocal part of the population. The characteristics of Chinese municipal solid waste are analysed and data presented on its calorific value and composition. These are not so favourable for incineration, since the sustained use of auxiliary fuel is necessary for ensuring adequate combustion temperatures. Also, the emission standard for acid gases is more lenient in China than in the European Union, so special attention should be paid to the issue of acidification arising from flue gas. Next, the techniques used in flue gas cleaning in China are reviewed and the acidification potential by cleaned flue gas is estimated. Still, acidification induced by municipal solid waste incinerators remains marginal compared with the effects of coal-fired power plants.

Gaseous emissions from sewage sludge combustion in a moving bed combustor

Substantial increase in sewage sludge generation in recent years requires suitable destination for this residue. This study evaluated the gaseous emissions generated during combustion of an aerobic sewage sludge in a pilot scale moving bed reactor. To utilize the heat generated during combustion, the exhaust gas was applied to the raw sludge drying process. The gaseous emissions were analyzed both after the combustion and drying steps. The results of the sewage sludge characterization showed the energy potential of this residue (LHV equal to 14.5 MJ kg(-1), db) and low concentration of metals, polycyclic aromatic hydrocarbons (PAH), polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF). The concentration of CO, NOx, BTEX (benzene, toluene, ethylbenzene and xylenes) emitted from the sludge combustion process were lower than the legal limits. The overall sludge combustion and drying process showed low emissions of PCDD/PCDF (0.42 ng I-TEQ N m(-3)). BTEX and PAH emissions were not detected. Even with the high nitrogen concentration in the raw feed (5.88% db), the sludge combustion process presented NOx emissions below the legal limit, which results from the combination of appropriate feed rate (A/F ratio), excess air, and mainly the low temperature kept inside the combustion chamber. It was found that the level of CO emissions from the overall sludge process depends on the dryer operating conditions, such as the oxygen content and the drying temperature, which have to be controlled throughout the process in order to achieve low CO levels. The aerobic sewage sludge combustion process generated high SO2 concentration due to the high sulfur content (0.67 wt%, db) and low calcium concentration (22.99 g kg(-1)) found in the sludge. The high concentration of SO2 in the flue gas (4776.77 mg N m(-3)) is the main factor inhibiting PCDD/PCDF formation. Further changes are needed in the pilot plant scheme to reduce SO2 and particulate matter emissions, such as the installation of exhaust gas-cleaning systems. According to previous studies, the efficient operation of such cleaning systems is also effective for metals emission control, which makes the combustion of sewage sludge a feasible treatment method from both energetic and environmental perspectives.