The effect of inorganic and organic chlorine on formation of highly chlorinated organic compounds during incineration: Laboratory pilot study

Aliphatic liquid as a basic fuel was incinerated in a laboratory scale pilot plant. Inorganic chlorine and organic chlorine mixed with basic fuel were used as additive chemicals. Sodium chloride (NaCl) and tetrachloroethylene (C₂Cl₄) were used as the sources of inorganic and organic chlorine. Combustion parameters were adjusted for optimum combustion and, consequently, the amount of particles in flue gases was low. The concentrations of chlorine in flue gases were high enough for possible formation reactions of organic chlorinated compounds in all of the chlorine input tests. An increase in chlorine input did not significantly increase the amounts of highly chlorinated organic compounds, like PCDD/Fs. The main result was that chlorophenol concentrations increased in parallel with organic chlorine input. Comparing organic chlorine to inorganic chlorine tests showed that more highly substituted PCDD/F congeners were formed when organic chlorine was the additive chlorine source. The formation of highly chlorinated organic compounds such as PCDD/Fs requires not only chlorine and aliphatic fuel to be formed, but some catalysts are also needed.     

Chlorinated naphthalene formation from the oxidation of dichlorophenols

Polychlorinated naphthalenes (PCNs) formed along with dibenzo-p-dioxin and dibenzofuran products in the slow combustion of dichlorophenols (DCPs) at 600 degrees C were identified. Each DCP reactant produced a unique set of PCN products. Major PCN congeners observed in the experiments were consistent with products predicted from a mechanism involving an intermediate formed by ortho-ortho carbon coupling of phenoxy radicals; polychlorinated dibenzofurans (PCDFs) are formed from the same intermediate. Tautomerization of the intermediate and H2O elimination produces PCDFs; alternatively, CO elimination to form dihydrofulvalene and fusion produces naphthalenes. Only trace amounts of tetrachloronaphthalene congeners were formed, suggesting that the preferred PCN formation pathways from chlorinated phenols involve loss of chlorine. 3,4-DCP produced the largest yields of PCDF and PCN products with two or more chlorine substituents. 2,6-DCP did not produce tri- or tetra-chlorinated PCDF or PCN congeners. It did produce 1,8-DCN, however, which could not be explained.     

Radical/radical vs radical/molecule reactions in the formation of PCDD/Fs from (chloro)phenols in incinerators

Pathways from chlorinated phenols as precursors to PCDD/Fs are discussed with focus on the effect of (poly)chlorination on thermochemistry and rate in the displacement of chlorine from a chlorophenol molecule by a (chloro)phenoxy radical (reaction (A) as a key example). Through measurements on the respective methylethers (anisoles) the O-H bond of 2,4,6-TCP turns out to be 5 kcal/ mol, and that in PCP 4 kcal/mol, less strong than O-H in phenol itself. On this basis it is concluded that-in contrast with earlier proposals--displacements such as in reaction (A) are at least as slow as reaction (B) of phenoxy radical with chlorobenzene. PhO. + PhCl -->PhOPh + Cl. reaction B Compared with condensation of two (chloro)phenoxy radicals, such radical/molecule reactions are therefore an insignificant pathway to dioxins in incinerators.     

Thermodynamic analysis and kinetic modelling of dioxin formation and emissions from power boilers firing salt-laden hog fuel

Both organic chlorine (e.g. PVC) and inorganic chlorides (e.g. NaCl) can be significant chlorine sources for dioxin and furan (PCDD/F) formation in combustion processes. This paper presents a thermodynamic analysis of high temperature salt chemistry. Its influence on PCDD/F formation in power boilers burning salt-laden wood waste is examined through the relationships between Cl2, HCl, NaCl(g) and NaCl(c). These analyses show that while HCl is a product of combustion of PVC-laden municipal solid waste, NaCl can be converted to HCl in hog fuel boilers by reactions with SO2 or alumino-silicate materials. Cl2 is a strong chlorinating agent for PCDD/F formation. HCl can be oxidized to Cl2 by O2, and Cl2 can be reduced back to HCl by SO2. The presence of sulphur at low concentrations thus enhances PCDD/F formation by increasing HCl concentrations. At high concentrations, sulphur inhibits de novo formation of PCDD/Fs through Cl2 reduction by excess SO2. The effect of NH3, CO and NOx on PCDD/F formation is also discussed. A semi-empirical kinetic model is proposed. This model considers both precursor and de novo formation mechanisms. A simplified version is used as a stack emission model. The kinetic model indicates that stack dioxin emissions will increase linearly with decreasing electrostatic precipitator (ESP) efficiency and exponentially with increasing ESP temperature.     

Mechanism of the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans from chlorophenols in gas phase reactions

The pyrolysis of chlorinated phenates at a temperature of about 280 degrees C results in the formation of definite chlorinated dibenzodioxin (PCDD) congeners [1-3]. It is shown that in gas phase reactions chlorophenols react in the presence of oxygen above 340 degrees C not only to PCDD but also to chlorinated dibenzofurans (PCDF). The mechanism of this reaction of chlorophenols to PCDD and PCDF was elucidated. In a first step phenoxyradicals are formed which are capable of forming PCDDs and PCDFs. This is confirmed by the oxygen dependency of the reaction. In an argon atmosphere no dimerization of chlorophenols could be observed at 420 degrees C. By the identification of intermediates and by analyzing the PCDF isomers formed from individual chlorophenols the reaction pathway is elucidated. As intermediates in the formation of PCDFs polychlorinated dihydroxybiphenyls (DOHB) were identified. These are most likely formed by the dimerization of two phenoxy radicals at the hydrogen substituted carbons in ortho-positions under simultaneous movement of the hydrogen atoms to the phenolic oxygen PCDDs are formed in the gas phase via ortho-phenoxyphenols (POP) analogous to the pyrolysis of phenates, but due to the radical mechanism in the first condensation step to POPs not only a chlorine atom is capable for substitution but also the hydrogen atoms. The formation of the DOHBs and their condensation to PCDFs and hydroxylated PCDFs as well as the ratio of PCDD to PCDF formed show a strong dependency on the reaction temperature, the substitution pattern of the chlorophenols and the oxygen concentration.     

A modeling evaluation of the effect of chlorine on the formation of particulate matter in combustion

The effect of chlorine on the fuel-rich oxidation of hydrocarbons and on the molecular weight growth of aromatics is analyzed by simulating experiments featuring a model chlorinated additive CH3Cl in a jet-stirred/plug-flow reactor and premixed flames. The kinetic model used in this work emphasizes the role of resonantly stabilized radicals in the formation and growth of aromatics, and considers soot inception as the net effect of molecular weight growth and graphitization of aromatic structures. Chlorinated hydrocarbons decompose at temperatures significantly lower than hydrocarbons, producing reactive Cl-atoms, which have a strong tendency to go to HCl. The HCI, tying up the H-atoms, inhibits hydrocarbon oxidation. The model is able to predict not only the levels but the shape of the experiments quite well and also the surprising finding of an increased soot formation associated with lower PAH levels found in rich flames with significant levels of chlorine. Based on reaction kinetic analysis, chlorine addition to the fuel enhances soot formation by promoting the formation of aromatic-ring compounds and accelerating the abstraction of aromatic H-atoms from stable PAH molecules. This process activates the transformation of aromatics to soot.     

Formation of aromatic chlorinated compounds catalyzed by copper and iron

This study shows the catalyzing effects of iron and copper on the formation of chlorinated compounds such as chlorobenzenes (ClBzs), chlorophenols (CIPhs), polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). Both total concentrations and congener distributions have been studied. The parameters and conditions varied during the combustion tests were the complete and incomplete combustion and the metal and chlorine addition. The incomplete combustion promoted the formation of organic chlorinated compounds in flue gas particles. Highly chlorinated congeners of PCDD/F were dominant in the flue gas particles, whereas the importance of lower chlorinated congener were increased in the gas phase. In the complete combustion conditions the concentrations of PCDD/Fs increased when the degree of chlorination were high, nevertheless the concentrations of tetra and penta PCDD/Fs were higher in the gas phase than the concentrations in the fly ash particles. Organic chlorine promoted the formation of chlorinated compounds more effectively than inorganic chlorine, which instead promoted the formation of PCDD/Fs in the gas phase, especially with copper catalyst. Different concentration levels of chlorinated compounds were observed in the gas phase and in particles when the chlorine source and combustion conditions were varied from incomplete to optimum conditions. Both copper and iron seem to have a catalytic effect on PCDD/F formation.     

Metal catalyzed formation of chlorinated aromatic compounds: a study of the correlation pattern in incinerator fly ash

Chlorinated aromatics are unintentionally formed and released from combustion and other thermal processes involving organic matter and chlorine. The catalytic activity of incinerator fly ash in the low-temperature formation of chlorinated aromatics has been demonstrated in both laboratory experiments and full-scale trials. Copper has been shown to be an effective catalyst, but several other transition metals possess a similar activity. Here results are reported from a series of full-scale combustion trials with different fractions of household and industrial wastes, with waste from forestry as a reference fuel. The composition of elements and chlorinated aromatics in the fly ash was evaluated with principal component analysis and partial least squares regression. The observed correlation pattern indicates that metals other than copper are of equal importance for the catalytic activity. Chromium and nickel are two of these metals, which may contribute to the de novo formation of chlorinated benzenes, phenols, PCDD and PCDF.     

Formation of PCDD/Fs and PCBs in the process of production of 1,4-dichlorobenzene

The occurrence and distribution of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) in a typical process of production of 1,4-dichlorobenzene (p-DCB) were investigated systematically in this study. P5CDFs and H6CDFs were the main congeners of PCDD/Fs, while only low levels of other PCDD/Fs were detected in the samples. The concentrations of PCBs were up to 4614 ng/g in the semi-manufactured products, and 1797 ng/g in the product p-DCB. It was also found that high chlorinated PCB congeners increased with the further chlorination. The levels of PCDD/Fs decreased obviously in the purification process, while PCBs could not be removed effectively. Polychlorophenols and other important dioxin precursors (1,4-dichloro-2-phenoxybenzene and 2-phenyl-2',5,5'-trichlorophenol) were also analyzed. Polychlorophenols which ortho- and para-positions of the hydroxyl were substituted by chlorine were the main congeners. In addition, a possible pathway of the formation of PCDD/Fs and PCBs in the production of p-DCB was presented. The results of this article suggest that re-estimation on the risk of using p-DCB products, especially the products for daily use such as mothballs, is expected.     

Effect of matrix on heterogeneous phase chlorine substitution reactions for dibenzo-p-dioxin and HCl in air

Five materials were used in gas-solid phase reactions between dibenzo-p- dioxin(DD) and HCl in order to determine the role of organic and inorganic components in fly ash on chlorine substitution reactions of chlorinated dioxins. The five solids were: granular activated carbon, silica gel, diatomaceous earth, Tenax-GC, and fly ash. Conditions for reactions were 10 min at 150°C with 5% HCl in air. Extent of chlorination was measured using GC/MS analyses of extracts of fly ash after treatment processes and was expressed as the ratio monochloroDD/original unreacted DD. These ratios were: fly ash, 1.082; silica gel, 0.059; activated carbon, 0.024; and Tenax-GC, 0.001. Measurement of similar behavior on diatomaceous earth was impossible since starting material and possible products were irreversibly adsorbed completely. The major chlorinated dioxin produced under these conditions was 2-chlorodibenzo-p- dioxin which is the isomer favored through an electrophilic substitution mechanism.     

Partitioning of dioxins and dibenzofurans: Whole blood, blood plasma and adipose tissue

It has become relatively common to estimate human dioxin body burden and to document dioxin exposures by measuring dioxin and dibenzofuran congeners in adipose tissue, whole blood or blood plasma, and reporting these values on a lipid basis. It has not been determined whether these three types of specimens contain identical dioxin and dibenzofuran levels. This paper compares paired plasma and adipose tissue and paired whole blood and adipose tissue in analyses from two groups of patients. The first group consists of twenty U.S. veterans with paired plasma and adipose specimens. The second group consists of four German adults with whole blood compared to adipose tissue. Forty-eight analyses were performed. The results suggest that for some higher chlorinated compounds, such as OCDD, plasma lipid values may be higher than adipose lipid values, but whole blood lipid values for the higher as well as lower chlorinated PCDD/Fs may be relatively similar. On the other hand, the values for the lower chlorinated PCDD/Fs, such as TCDD, are similar in blood plasma, adipose tissue and whole blood. Total PCDD/F dioxin “toxic equivalents” are similar within each of the two series reported here, using the current “International Dioxin Toxic Equivalent” system.     

Dioxin formation in stretched flames

Numerical simulations of stretched laminar twin premixed flames are carried out in order to understand the gasphase formation of dioxins in situations analogous to the post combustor and following regions of a typical incinerator. A previously developed chemical kinetic mechanism, that describes dioxin formation in terms of some generic species, is used in the calculations. The results indicate a temperature region favorable to the gasphase production of dioxins that lies between 1100 and 1500 K, as well as a weak dependence of dioxin formation on the oxygen concentration in the flames. The effect of oxygen is better described by observing the consumption of some of the generic species. Though no measurements of dioxin concentration in idealized flows, such as those simulated, are available, the results are in qualitative accord with total measurements of dioxin concentration (due to both gas- and solid-phase processes) obtained by other investigators. The numerical predictions identify some flames that are ignited, in which dioxin consumption takes place, and others which are unignited, in which significant dioxin production occurs as the result of largely isothermal mixing. The calculations indicate that unignited flames containing no initial fuel favor dioxin formation significantly over those that contain some initial quantity of fuel. Finally, some implications regarding incinerator practice are discussed.     

Breakpoint chlorination curves of greywater.

A study on chlorination of raw greywater with hypochlorite is reported in this paper. Samples were chlorinated in a variety of conditions, and residual chlorine (Cl2) was measured spectrophotometrically. For each sample, the chlorination curve (chlorine residuals versus chlorine dose) was obtained. Curves showed the typical hump-and-dip profile attributable to the formation and destruction of chloramines. It was observed that, after reactions with strong reductants and chloramines-forming compounds, the remaining organic matter exerted a certain demand of chlorine. The evolution of chlorination curves with addition of ammonia and dodecylbencene sulfonate sodium salt and with dilution of the greywater sample were studied. In addition, chlorination curves at several contact times have been obtained, resulting in slower chlorine decay in the hump zone than in the dip zone. In addition, the decay of coliforms in chlorinated samples was also investigated. It was found that, for a chlorination dosage corresponding to the maximum of the hump zone (average 8.9 mg Cl2/ L), samples were negative in coliforms after 10 to 30 minutes of contact time. After-growth was not observed within 3 days after chlorination. Implications in chlorination treatments of raw greywater can be derived from these results.     

Microbial degradation of chlorinated dioxins

Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) were introduced into the biosphere on a large scale as by-products from the manufacture of chlorinated phenols and the incineration of wastes. Due to their high toxicity they have been the subject of great public and scientific scrutiny. The evidence in the literature suggests that PCDD/F compounds are subject to biodegradation in the environment as part of the natural chlorine cycle. Lower chlorinated dioxins can be degraded by aerobic bacteria from the genera of Sphingomonas, Pseudomonas and Burkholderia. Most studies have evaluated the cometabolism of monochlorinated dioxins with unsubstituted dioxin as the primary substrate. The degradation is usually initiated by unique angular dioxygenases that attack the ring adjacent to the ether oxygen. Chlorinated dioxins can also be attacked cometabolically under aerobic conditions by white-rot fungi that utilize extracellular lignin degrading peroxidases. Recently, bacteria that can grow on monochlorinated dibenzo-p-dioxins as a sole source of carbon and energy have also been characterized (Pseudomonas veronii). Higher chlorinated dioxins are known to be reductively dechlorinated in anaerobic sediments. Similar to PCB and chlorinated benzenes, halorespiring bacteria from the genus Dehalococcoides are implicated in the dechlorination reactions. Anaerobic sediments have been shown to convert tetrachloro- to octachlorodibenzo-p-dioxins to lower chlorinated dioxins including monochlorinated congeners. Taken as a whole, these findings indicate that biodegradation is likely to contribute to the natural attenuation processes affecting PCDD/F compounds.     

Formation of PCDD/PCDF

One option of recycling used contaminated packaging is to recover its high energy content. This can be performed in a normal multi-fuel power plant by co-combustion of packaging-derived fuel (PDF) or refuse-derived fuel (RDF) with fossil fuels, such as coal or peat. This work includes the results of 17 co-combustion tests and an evaluation of the results by the Principal Component Analysis (PCA) and the Partial Least Squares Projections to Latent Structures (PLS). PCA and PLS calculations showed that especially Pb, but also Cr, and Cu correlated with lower chlorinated furans (PCDFs) in the fly ash. Correlation between Sn and lower chlorinated dioxins (PCDDs) in the fly ash was also noticed. CO and PAH emission in the flue gas correlated with total PCDD/Fs in the flue gas. In a real full-scale combustion process, a single parameter in fuel, flue gas or a combustion parameter did not provide a guide to PCDD/F formation or to a level of the total PCDD/F emission, but correlations between different parameters and PCDD/Fs could be found. Although PDFs and RDF had catalytic heavy metals and chlorine, the co-combustion results showed that they can be co-combusted with peat and coal in a fluidized-bed boiler at least up to 26 % with very low total PCDD and PCDF emissions.     

Review of concentrations and chemistry of trichloroacetate in the environment

This paper reviews the concentrations of trichloroacetate (TCA) in the atmosphere-plant-soil system. Data originate mainly from Europe. The median TCA concentration in rainwater and canopy drip decreased until 1995. From then the median TCA concentration in rainwater remains rather constant while for canopy drip later data are not available. The same seems to hold for concentrations in air although a very limited data set is available. The median concentrations in coniferous needles and groundwater are constant for the period observed. The median TCA concentrations in soil decreased until 1992 and then remained constant.The TCA formation from chlorinated solvents in the atmosphere may explain a substantial percentage of the TCA amount in the atmosphere. The TCA concentrations in rainwater and canopy drip indicate that there will be other sources contributing to 10-50%. Waste incineration, biomass burning and natural formation in the marine boundary layer are potential candidate sources of TCA, but nothing can be said as yet on their TCA emission rates. Anthropogenic emissions of chlorine could also be a source.TCA can be formed from chlorinated solvents by biota. However, for coniferous trees the uptake of TCA from soil may be the predominant route. Biotic and abiotic reactions can cause to formation of TCA in soil, but also formation of TCA from chlorinated solvents by biota that excrete TCA, may contribute. Mass balance calculations of the bioactive soil top layer show that the production rate of TCA in certain soil types could be substantial. The mass balance calculations could not distinguish between natural and anthropogenic sources in soil.     

Development of expanded and core kinetic models for the gas phase formation of dioxins from chlorinated phenols

Expanded, 45 reaction, and core, 12 reaction, kinetic models have been developed that account for the major features in the homogeneous formation of polychlorinated dibenzo-p-dioxins (PCDD) from the oxidation of 2,4,6-trichlorophenol (P). The expanded and core schemes provide good agreement between experimental and calculated yields of PCDDs using the CHEMKIN combustion package or the React kinetic program, respectively. Steady-state approximations of the reaction kinetic models including radical-molecule and radical-radical formation pathways of PCDD, as well as oxidative destruction pathways of chlorinated phenoxyl radicals, reveal a competition between reactions of chlorinated phenoxyl radicals with chlorinated phenols, recombination reactions of chlorinated phenoxyl mesomers, and destruction/decomposition of phenoxyl radicals.     

Formation and decomposition of chloroaromatic compounds in chlorine-containing benzene/oxygen flames

Premixed chlorine-containing, fuel-rich, low-pressure benzene/oxygen flames were analysed for the formation of (oxygenated) chloroaromatic compounds and their radicals by means of the condensation/radical-scavenging method (Hausmann, M., Homann, K.-H., 1995. Ber. Busenges. Phys. Chem. 99, 853-862). Several chlorinated organic compounds (methyl chloride, t-butyl chloride, chlorobenzene, chloroform) were used as additives within a maximum concentration of 10% of total fuel. Product identification and quantification were performed by GC/MS. The extent of formation of chloroaromatic compounds in these flames was largest in the cases of chlorobenzene and chloroform as additives. For chlorobenzene, 12 different chloroaromatics could be analysed in between C7H7Cl and C12H9Cl. Their formation is mainly due to conversion of initial chlorobenzene into substituted or oxidised derivatives, or growth products. Additional chlorination of aromatics is shown to be of minor importance in chlorobenzene-containing flames. Three isomeric (o/m/p) scavenging products could be identified for the chlorophenyl radical. In the chloroform case, 15 chloroaromatics could be analysed in between C6H5Cl and C14H9Cl. The weak C-Cl bond in chloroform is responsible for the high extent of chloroaromatics formation, either by Cl abstraction from the additive or by chlorination reactions via Cl radicals. Additionally, specific pathways to (di)chloroaromatics and chlorinated fulvene-type structures are outlined via CHCl2 and CCl2 radicals.     

The effects of temperature and oxygen content on the PCDD/PCDFs formation in MSW fly ash

In this study, the effects of the temperature, oxygen content in the gas stream and carbon content in ash particles on PCDD/Fs formation on the fly ash surface were investigated. The optimum temperatures for dioxin formation were found at 350 degrees C for boiler ash, 300 degrees C for cyclone ash and 250 degrees C for ESP ash, respectively. Preliminary results indicate that the optimum temperature will decrease as the particle size decreases. When the O2 concentration is varied between 0% and 100%, the optimum oxygen content for PCDD/Fs formation is found to be at 7.5% for cyclone ash, and the PCDD/PCDF ratio increases with the increase of oxygen content. Dioxin formation is observed even for the gas containing no oxygen passed through the fly ash. Hence, other reacted routes that do not need O2 for dioxin formation take place on fly ash. The carbon content in fly ash is varied between 0% and 20% in this study, and the results have indicated that the maximum dioxin formation is to be found at 5%. The precursors are not injected into the fly ash or gas stream in all formation experiments, however, dioxin is still formed in fly ash. Consequently, other chlorinated routes besides Deacon reactions may take place on the fly ash surface.     

酪氨酸氯化过程中光谱变化特征及消毒副产物的生成规律

考察酪氨酸在不同投氯量条件下氯化后的余氯,紫外吸光度值和荧光光谱,以及消毒副产物对羟基苯乙腈(4-HBC)的生成特性。结果表明,随着投氯量的增加,余氯呈现先增加再减小再增加的趋势。在投氯量为0～0.5 mmolCl2/L时,增加投氯量可提高氯化后溶液的UV254、UV274和UV280值以及4-HBC的生成量,表明低投氯量时氯化可提高溶液中不饱和键的含量;而投氯量为0.5～1 mmol Cl2/L时,增加投氯量降低UV254、UV274和UV280值以及4-HBC的生成量,表明过量的氯亦可破坏溶液中的不饱和键。荧光光谱测试实验亦发现:在投氯量为0.05 mmol Cl2/L时,酪氨酸溶液氯化后的荧光峰强度明显增加,表明氯化可生成荧光强度较高的产物。过量的氯(0.5～1 mmol Cl2/L)则可破坏溶液中的荧光结构,降低荧光峰强度直至未检出。