Effects of calcium-based sorbents on PCDD/F formation from pentachlorophenol combustion process

A new insight into the CaO-induced inhibition pathways on PCDD/F formation: Metal passivation, dechlorination and hydroxide substitution

Ca-based inhibitors (especially CaO) for PCDD/F (polychlorinated dibenzo-p-dioxin and dibenzofuran) formation are considered as economic inhibitors with low toxicity and strong adsorption of acidic gases (e.g., HCl, Cl₂, and SO_x), whereas the insight understanding of its inhibition mechanisms is scarcely explored. Herein, CaO was used to inhibit the de novo reaction for PCDD/F formation (250-450 °C). The evolution of key elements (C, Cl, Cu, and Ca) combined with theoretical calculations was systematically investigated. The concentrations and distribution of PCDD/Fs demonstrated the significant inhibition effect of CaO on I-TEQ (international toxic equivalency) concentrations of PCDD/Fs (inhibition efficiencies: > 90 %) and hepta~octa chlorinated congeners (inhibition efficiencies: 51.5-99.8 %). And the conditions (5-10 % CaO, 350 °C) were supposed to be the preferred conditions applied in real MSWIs (municipal solid waste incinerators). CaO significantly suppressed the chlorination of carbon matrix (superficial organic Cl (CCl) reduced from 16.5 % to 6.5-11.3 %) and the formation of unsaturated hydrocarbons or aromatic carbon (superficial CC decreased from 6.7 % to 1.3-2.1 %). Also, CaO promoted the dechlorination of Cu-based catalysts and Cl solidification (e.g., conversion of CuCl₂ to CuO, and formation of CaCl₂). The dechlorination phenomenon was validated by the dechlorination of highly chlorinated PCDD/F-congeners (via DD/DF chlorination pathways). Density functional theory calculations revealed that CaO facilitated the substitution of Cl by -OH on the benzene ring to inhibit the polycondensation of the chlorobenzene and chlorophenol (Gibbs free energy reduced from +74.83 to -36.62 and - 148.88 kJ/mol), which also indicates the dechlorination effect of CaO on de novo synthesis.

Emission characteristics of dioxins during iron ore Co-sintering with municipal solid waste incinerator fly ash in a sintering pot

The disposing of municipal solid waste incineration(MSWI) fly ashes containing dioxins is an intractable problems. Co-sintering is one of the most ideal methods to dispose MSWI fly ash, because it not only degrades the dioxins but also makes it possible to re-utilize MSWI fly ashes. In the present study, MSWI fly ash(FA) and water washed MSWI fly ash(WFA) were added into the sinter raw mixture in a lab-scale sintering pot. Different effects of fly ash and water washed fly ash on emission characteristics of dioxins were studied, and possible pathways to form dioxins were discussed in detail. During co-sintering, at least 88.9%, 99.1% of dioxins brought in by FA, WFA was decomposed, respectively, and re-synthesis with a significant distribution characteristic of dioxins originated from sintering process. In the preheating and dry zone, the recombination and condensation reaction of precursors were the main formation pathways of 2,3,7,8-PCDDs in the sintering process and 2,3,7,8-PCDFs were formed by de novo synthesis. In addition, the resynthesis process was affected by chlorine and calcium brought in by fly ash. Sufficient chlorine boosted the chlorination of reactants while calcium increased the residence time, causing the emission concentration of toxic dioxins and the adding ratio were nonlinearly dependent. Therefore, the maximum proportion of water washed MSWI fly ash that can be added into the sintering process was 0.5 wt.%. Besides, the possible pathways to form dioxins were proposed.

Inhibition of chlorobenzenes formation by calcium oxide during solid waste incineration

Solid waste incineration is a major emission source of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). The injection of N- and S-containing compounds is an effective way to suppress the formation of PCDD/Fs, but this approach is still shortcoming because additional pollutants such as NH₃ and SO_x are emitted. To avoid the secondary pollutions, a de novo synthesis inhibition mechanism in the presence of CaO was postulated to transform CuCl₂ to CuO and deplete Cl₂ and HCl. Chlorobenzenes (CBzs), which are indicators and precursors of PCDD/Fs, were adopted to prove the inhibitory effect of CaO at 400 °C, using both simulated synthetic ash and extracted air pollution control residues. As the molar ratio of CaO to CuCl₂ exceeded 3, the residual carbon increased, and the inhibition efficiency of CBzs exceeded 93 %. This performance is superior to the corresponding performance of NH₄H₂PO₄, which has been proved to be a potential inhibitor. Furthermore, with CaO, chlorides remained in the solid phase and had inactive catalytic performance; and they were the major products rather than HCl, Cl₂ and Cu₂OCl₂. The addition of CaO during waste incineration therefore can facilitate the abatement of PCDD/Fs contamination and reduce the emissions of acid gas simultaneously.

Inhibition of polychlorinated dibenzo-p-dioxins and dibenzofurans by phosphorus-containing compounds in model fly ash

Waste incineration is a preferred method in China to dispose the municipal solid waste, but controlling the production of highly toxic polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans effectively during incineration is both challenging and imperative. In this study, the suppression of PCDD/Fs by various phosphorus-containing compounds was explored, and the mechanisms responsible for the inhibition were studied in detail. The experiments took place in a lab-scale vertical tubular reactor at 350 °C under a simulated flue gas (12 vol% O₂ in N₂ flow), and both the off-gases and residues were collected for PCDD/Fs analysis. The scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to characterize the reaction residues. The experimental results revealed that NH₄H₂PO₄ and (NH₄)₂·HPO₄ showed the highest inhibitory effect (57.2% and 57.3%, respectively) on the PCDD/Fs formation, followed by CaHPO₄ with inhibition efficiency of 39.1%. In contrast, KH₂PO₄ and K₂HPO₄ barely inhibited the generation of the PCDD/Fs. The inhibitory effect of NH₄H₂PO₄ and (NH₄)₂·HPO₄ was similar to that of nitrogen-based inhibitors. At the same time, it was proven that the inhibitory activity of CaHPO₄ might be due to the reaction of it with Cu²⁺ forming stable compounds.

Pathways and influential factors study on the formation of PBDD/Fs during co-processing BDE-209 in cement kiln simulation system

The thermal processes of cement kilns are sources of polybrominated dibenzofurans and dioxins (PBDD/Fs); however, when co-processing decabromodiphenyl ether (BDE-209) soil in cement kilns, very few reports have investigated the mechanism of PBDD/Fs formation from BDE-209. Therefore, the pathways and factors that influence the formation of PBDD/Fs were investigated using Box-Behnken design (BBD) of the response surface methodology (RSM) at lab-scale. The PBDEs, HBr/Br₂ and PBDD/Fs emissions in flue gas from the simulated thermal process were analyzed using gas chromatography/mass spectroscopy (GC/MS), high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS), and ion chromatography (IC). Density functional theory (DFT) was also used to further discuss the formation of PBDD/Fs. The major products of BDE-209 thermal decomposition in flue gas were 97.1% HBr/Br₂ (a.v. 26.6%/70.6%) > 2.7% PBDEs >0.2% PBDD/Fs. Formation of precursors were the main pathways for PBDD/Fs, and those precursors were dominated by higher-brominated PBDEs (heptã deca-BDEs); debromination of BDE-209 was also a crucial pathway for the formation of PBDD/Fs throughout the thermal process. Interestingly, it was easier to form HpBDD/Fs from OBDD/Fs than from PBDEs. The O₂ percentage and interaction factors of O₂ percentage, temperature, and CaCO₃ percentage have the largest influence on PBDD/Fs emissions and formation.

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.

Synergetic inhibition of PCDD/F formation from pentachlorophenol by mixtures of urea and calcium oxide

Chlorophenols are structurally similar to PCDD/Fs and have been considered as highly potential precursors for PCDD/Fs formation. The suppressing effects of PCDD/F formation from pentachlorophenol (PCP) were investigated on various mass ratios of CaO and urea. The total concentration of 2,3,7,8-PCDD/Fs, mostly dominated by OCDD, was determined to be 48.58-10186ng/mg in inhibitor-reaction systems, being much lower than that in blank reaction system (75654ng/mg). Interestingly, compared with pure CaO and urea reaction system, the concentration and TEQ of formed 2,3,7,8-PCDD/Fs in mixed urea/CaO reaction system were lower, especially with 5-20% urea reaction systems being respectively at decrease by 96.5-99.4% and 99.2-99.7%. The suppression efficiency of TEQ in 5-20% urea reaction systems could be always approximately 100% under 250-350°C. These results suggested that mixed inhibitors, especially 5-20% urea inhibitors, have a synergetic inhibition effect for PCDD/Fs formation from PCP. Mixed inhibitor generated several intermediates, involving CO2, H2O, NH3, Ca(OH)2, CaCO3, HNCO, biuret and ammelide. The complex between PCP and Ca, N-doped species, lower chlorinated phenols and benzenediol, and organic acids were also determined. Synergetic inhibition mechanism may be attributed to accelerated facilitation of acid-base reaction and N doping. The decomposition of PCP itself also contributes to prevent PCDD/Fs formation.

Simultaneous removal of PCDD/Fs, pentachlorophenol and mercury from contaminated soil

Pentachlorophenol (PCP), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), and mercury were simultaneously removed from heavily contaminated soil using a continuous pilot-scale thermal system (CPTS). Operating the system at 700 °C with 22 min of retention time ensured that the residual contaminants in remediated soil are lower in concentration than the soil standards of Taiwan EPA require. Both PCP and PCDD/Fs are effectively destroyed during the treatment at high temperatures in the CPTS, but significant dechlorination of PCDD/Fs is also found, resulting in lower net destruction efficiencies of TCDD/F and PeCDD/F-congeners, compared with those of highly chlorinated Hx-, Hp- and OCDD/F congeners. Moreover, 2,3,7,8-TetraCDD is significantly formed if the retention time is not long enough for total destruction. Inadequate reaction time (or retention time) even may lead to a rise in TEQ-value due to incomplete dechlorination. Mercury is significantly desorbed from contaminated soil and discharged through the exhaust. For PCP and PCDD/Fs, the exhaust discharge percentages including both the remediated soil and the exhaust are <0.03% and 1.14% of the input, respectively, achieved with 700 °C and 33 min retention time. In contrast, some 97.8% of input mercury rate is desorbed and discharged via the exhaust, so that the latter should be carefully cleaned via efficient air pollution control devices, whereas this contribution focuses on the conditions required for reaching adequate soil cleaning.

Influence of calcium hydroxide on the fate of perfluorooctanesulfonate under thermal conditions

To explore the potential fate and transport of perfluorochemicals in the thermal treatment of sludge, perfluorooctanesulfonate (PFOS), a perfluorochemical species commonly dominant in wastewater sludge, was mixed with hydrated lime (Ca(OH)(2)) to quantitatively observe their interaction under different temperatures. The phase compositions of the mixtures after the reactions were qualitatively identified and quantitatively determined using X-ray diffraction technique. The results of the thermogravimetry and differential scanning calorimetry analyses indicate that PFOS gasified directly during the thermal treatment process when the temperature was increased to around 425 °C. However, the formation of CaF(2) at 350 °C suggests that the presence of Ca(OH)(2) in the mixture can lead to the decomposition of PFOS at 350 °C, which is lower than the decomposition temperature of PFOS alone (425 °C). The increase of temperature promoted a solid state reaction between PFOS and Ca(OH)(2), and also enhanced the interaction between the gaseous products of PFOS and CaO (or Ca(OH)(2)). The preferred Ca/F molar ratio to achieve fluorine stabilization by Ca(OH)(2) was above 1:1 in the experiment involving 400 °C and 600 °C treatment. It also showed that equilibrium efficiency is achieved within 5 min at 400 °C and within 1 min above 600°C.

Deactivation of metal chlorides by alkaline compounds inhibits formation of chlorinated aromatics

The inhibitory mechanisms of alkaline compounds on the formation of chlorinated aromatic (aromatic-Cl) compounds in postcombustion fly ash from thermal processes such as municipal solid waste (MSW) incineration are not fully understood. Here, we report quantitative and X-ray spectroscopic evidence that deactivation of metal chloride promoter activity by alkaline compounds inhibits the formation of aromatic-Cl compounds. The formation of aromatic-Cl compounds such as chlorobenzenes and polychlorinated biphenyls in real MSW fly ash was inhibited by the addition of NaOH, Ca(OH)(2), or NaHCO(3), either dry or in solution, with the fly ash. With optimal conditions, the formation of aromatic-Cl compounds was inhibited by more than 95% in comparison with formation in reheated raw MSW fly ash. We prepared simplified model fly ash samples to estimate the influence of alkaline compounds on trace Cu, Fe, Pb, and Zn chlorides, which strongly promote aromatic-Cl compound formation. More than 99% inhibition was observed in some model samples. Cl K-edge X-ray absorption and X-ray diffraction provided clear evidence of promoter deactivation, as NaOH or NaHCO(3) changed to NaCl, and Ca(OH)(2) changed to CaCl(2) or CaClOH by reaction with the metal chlorides. NaOH was the most reactive and useful of the three alkaline compounds tested. We recommend deactivation of metal chlorides as an environmentally friendly method of inhibiting the formation of aromatic-Cl compounds, with the added benefit of changing the alkaline compounds and metal chlorides into harmless chemicals such as NaCl and metal oxides.