Effect of ammonium sulfate and urea on PCDD/F formation from active carbon and possible mechanism of inhibition

Suppressing the formation of chlorinated aromatics by inhibitor sodium thiocyanate in solid waste incineration process

Suppressing the formation of chlorinated aromatics (Cl-aromatics) by chemical inhibitors is an important measure to reduce dioxin emission from the solid waste incineration plants. In this study, we first investigated the reduction effect of a novel inhibitor sodium thiocyanate (NaSCN) on the emission of dioxins in 2 full-scale solid waste incineration systems. Injection of NaSCN solution into the higher temperature flue gas resulted in about 60% reduction in the concentration of total tetra- to octa-chlorinated dibenzo-p-dioxins and dibenzofurans in stack flue gas. The suppression effect was further verified by a laboratory study on the chlorination of naphthalene over model fly ashes with or without NaSCN addition. By characterizing the reaction products between NaSCN and key catalysts Cu and Fe chlorides, two main suppression mechanisms were proposed: (i) reduction of highly active cupric chloride (CuCl₂) and ferric chloride (FeCl₃) to less active cuprous chloride (CuCl) and ferrous chloride (FeCl₂), (ii) sulfidation of Cu chlorides. The laboratory study indicated that the unreacted NaSCN in the combustion flue gas could be mainly decomposed into Na₂S, C₃N₄, Na₂S₂O₃, NaS₂, Na₂SO₄, CO₂, SO_2, NO₂ and COS. These decomposition products are low toxic or can be effectively removed by the air pollution control devices. CAPSULE: NaSCN suppressed the formation of chlorinated aromatics in combustion flue gas mainly through inducing the reduction of highly active Cu (II) and Fe (III) chlorides.

Effect of urea on chlorinated aromatics formation mediated by copper and iron species in combustion flue gas

Urea ((NH₂)₂CO) is widely applied to the reduction of NO_X in modern full-scale solid waste incineration systems, but there is a lack of knowledge about how urea affects the formation and emission of Cl-aromatics. In this study, we investigated the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polychlorinated polychlorinated naphthalenes (PCNs) via electrophilic chlorination and precursor pathway mediated by model fly ashes containing Cu and Fe species with or without urea addition. The results indicated that the addition of urea promoted the direct chlorination of parent aromatics over Cu (Ⅱ) chlorides and the coupling reaction of chlorophenols over Fe species, while suppressed the catalytic chlorination of parent aromatics over Fe (Ⅲ) chlorides and the coupling reaction of chlorophenols over Cu species. The diverse effects should be mainly attributed to the formation of complex salts containing NH₃ and NH₄⁺. The formation of complex salts of Fe chlorides and NH₄Cl could hinder the oxidization of Fe chlorides, and thus maintain the high activity of Fe species for catalyzing the coupling reaction of chlorophenols. The formation of complex salts of Cu (Ⅱ) chloride and NH₃ could prevent the chemical sorption of phenoxyl groups, and thus suppress the coupling reaction of chlorophenols. NH₃ released from the thermal decomposition of urea could not only react with Cl₂ to suppress the catalytic chlorination of aromatics, but also neutralize HCl to accelerate the direct chlorination of aromatics. In general, urea should act as inhibitor for suppressing the formation of Cl-aromatics in solid waste incineration systems.

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.

Influence factors and mass balance of memory effect on PCDD/F emissions from the full-scale municipal solid waste incineration in China

Studies are carried out in two wet scrubbing systems (WSSs) subordinated to two similar full-scale (30 t h^-1) municipal solid waste (MSW) incinerators to explore the influence factors and mass balance of memory effect on polychlorinated-ρ-dibenzodioxins and dibenzofurans (PCDD/Fs) emissions. The results show that the memory effect on two WSSs notably increases the TEQ concentrations by 13.6 and 3 times, respectively, through increase in the total mass concentration and the proportions of low-chlorinated PCDD/Fs, directly resulting in the PCDD/F emissions of 1# MSW incinerator over the national standard. PCDD/F adsorption/desorption in WSSs is the reasonable acting mechanisms of memory effect. In addition, memory effect mainly influences gaseous PCDD/F emissions by elevating the percentage of PCDFs, while slightly affects PCDD/Fs in residuals. A mass balance of PCDD/Fs is established to further analyze the influence factors of memory effect on WSSs, indicating filling as the largest potential source discharging PCDD/Fs into outlet flue gas. The results pave the way for further industrial optimization of WSSs design, such as the filling materials with less adsorption capacity on PCDD/Fs and more reasonable operation.

Inhibition effect of polyurethane foam waste in dioxin formation

The present work has been focused on studying the polychlorinated polychlorodibenzo-p-dioxin/furan (PCDD/F) inhibition in the combustion process by prior addition of an inhibitor to the fuel. Three different experiments of combustion were carried out at 850 °C in a laboratory-scale horizontal tubular quartz reactor, and several compounds were analyzed from the flue gas obtained, such as PCDD/Fs, dioxin-like polychlorinated biphenyls (dl-PCBs), NH₃, HCN, NO_x, HCl, Cl₂, chlorobenzenes (ClBzs), chlorophenols (ClPhs), polycyclic aromatic hydrocarbons (PAHs), volatile and other semivolatile organic compounds. The fuel used was a synthetic waste which was composed of sawdust and PVC (95 wt% and 5 wt%, respectively) and the inhibitors studied were polyurethane foam (PUF) that can be found in mattress waste and gases from the oxidative pyrolysis of PUF. Due to the high N content of PUF, a significant decrease of PCDD/F and dl-PCB formation has been obtained in the experimental runs carried out with the inhibitors studied, particularly with the gases from the oxidative pyrolysis of PUF. In addition, it must be noted that emissions of incomplete combustion products have also decreased.

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.

Inhibition of element sulfur and calcium oxide on the formation of PCDD/Fs during co-combustion experiment of municipal solid waste

In order to investigate the effect of element sulfur (S) and calcium oxide (CaO) on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) during municipal solid waste (MSW) combustion process, MSW was incinerated with S or CaO in a laboratory-scale incinerator and heated in the flow of N₂-O₂ gas mixture at 800°C. It can be concluded that 25% of oxygen concentration is the best condition for the following inhibitors experiments through a series of oxygen variation experiments. With adding S and CaO in MSW incineration, seventeen kinds of 2, 3, 7, 8-substituted PCDD/Fs congeners were analysed with high-resolution chromatography and mass spectrometry method. The results show that S and CaO obviously suppress PCDD/Fs formation. Comparing inhibition effect of S with that of CaO, the inhibitory effect of S on HpCDD/Fs formation was most remarkable, of around 88.1%; while CaO could inhibit the formation of HxCDD/Fs more evidently than sulfur and the inhibition was 85.1%. PCDFs were the main components of dioxins produced from MSW incineration in the experiments, and S and CaO were unable to change the dominating generation route of PCDD/Fs. S and CaO could mainly consume chlorine sources or weaken the chlorination in the PCDD/Fs formation process to restrain the PCDFs formation, but the inhibition mechanisms were different. In addition, some dioxins or precursors might be decomposed by S and CaO.

Amino Compounds as Inhibitors of De Novo Synthesis of Chlorobenzenes

The inhibitory effects of four amino compounds on the formation of chlorobenzenes (CBzs)--dioxin precursors and indicators, and the inhibitory mechanisms were explored. The results show NH4H2PO4 can decrease the total yields of CBzs (1,2di-CBz, 1,3di-CBz, 1,4di-CBz, penta-CBz and hexa-CBz) by 98.1%±1.6% and 96.1%±0.7% under air and nitrogen flow. The inhibitory effects indicated by the total yields of CBzs follow the order NH4H2PO4 > NH4HF2 > (NH4)2SO4 > NH4Br under air flow and NH4H2PO4 ≈ (NH4)2SO4 ≈ NH4HF2 >NH4Br under nitrogen flow. The inhibition mechanism revealed by thermal analysis that CuCl2 was converted to CuPO3 by reacting with NH4H2PO4 below 200 °C, which can block the transfer of chlorine and formation of C-Cl bonds at 350 °C. The effects of the other three inhibitors were weaker because their reactions with CuCl2, which form other copper compounds, and the reaction of CuCl2 with carbon, which forms C-Cl bonds, were almost simultaneous and competitive. Oxygen influenced the yield of CBzs obviously, and the total yield of five CBzs sharply increased with oxygen. Because of their high efficiency, low environmental impact, low cost, and availability, amino compounds--especially NH4H2PO4--can be utilized as inhibitors of CBzs during incineration.