Density Functional Theory Study On The Formation Mechanism Of Dioxins In Lncineration

Dioxins are a series of chlorinated compounds which cantains an oxygen heterocyclic ring. Dioxins are commonly regarded as highly toxic compounds that are environmental pollutants and persistent organic pollutants (POPs). Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) constitute the main body of dioxins. As by-products of various high-temperature processes, such as waste incineration, fossil fuel combustion and metal smelting, dioxins come into focus for the high toxicity. In order to reduce pollutants emission, to gain insight into the formation mechanism of PCDD/Fs in waste incineration is of great necessity. The online monitoring of dioxins turned out to be elusive and costly. Affected by this, the formation mechanism of dioxins could still be patchy and unreliable. Formation routes from chlorophenols (CPs) are the most important ones of PCDD/Fs congeners. Considerable studies have been conducted on the PCDD/Fs formations under various experimental conditions. However the products distribution of CPs condensation differs from field tests of waste incinerators:the ratio of PCDDs and PCDFs is more than one, and the distribution in waste incinerators occurs the other way round. The catalytic reaction of amorphous 12C- and 13C-labeled carbon revealed that approximately half of PCDDs are formed via the condensation of monocyclic aromatic precursors; however, PCDFs are primarily released from the oxidation of the precursors with two or more aromatic rings, such as PAHs and elemental carbon. Though organic substances could be fully decomposed in incinerators, carbon compounds are unfortunately found to be regenerated in the cooling down flue gas. The transformation of carbon compounds in flue gas is proved to be a continuous growth process: short chain hydrocarbons → benzene series → polycyclic aromatic hydrocarbons (PAHs) → graphite or amorphous carbon. Previous works show that ortho-benzyne (o-C6H4) is an initiator of PAHs when aromatic fuels are burned. Considering the structural similarity between DF and PAHs, the formation route of PAHs could be applied the same to DF, and therefore benzyne and PAHs are potential sources for dioxins. The chlorination of dibenzofuran (DF) could play an important role in forming PCDFs. On the one hand, the complete distributions for PCDF congeners shows broadly consistent between the chlorination model and the data from incinerators; In addition, the chlorination of DF vapor by copper (II) chloride is proved to be a highly efficient process. On the other, the superior concentration of DF supports the hypothesis. The concentration of DF in flue gas is 50-700 times that of total PCDFs, and 2600 times during combustion disturbances.This study focuses on DF and dibenzo-p-dioxin (DD) formation processes initiated by benzyne and PAHs. Phenyl radical, phenoxy radical, phenylperoxy radical, phenol and phenylhydroperoxide are chosen for addition reagents for predicting the dioxin formation initated by benzyne. The PAHs oxidation initated by·OH and ·OC1 is proved to be a feasible formation route of DF in this study. All the reaction pathways are predicted using the M06-2X-GD3 method, which is based on the dispersion-corrected density functional theory (DFT-D3). The main conclusions of the research are as follows:1. Dioxins formation from the bimolecular reactions of benzyne with phenyl, phenoxy, phenylperoxy radicalsBenzyne is a highly reactive intermediate, which has a cyclic structure with a carbon-carbon triple bond. The bond angle of this triple bond is bended under the influence of tensile force, i.e.,127.4°. Phenyl radical, phenoxy radical, phenylperoxy radical, phenol, and phenyl hydroperoxide can react with benzyne. Once one of carbon in the triple bond of benzyne is bounded, the other carbon will be a reactive site which gives chance to form DF or DD.a. The bimolecular reactions of benzyne with phenyl radical, phenoxy radical, phenylperoxy radical, phenol, or phenyl hydroperoxide could produce DF or DD; The standard reaction Gibbs energies remain below zero within temperature range 298-1500 K.b. The activation enthalpies for all the elementary reactions are lower than 31.27 kcal/mol, except the dehydration-cyclization reaction of 2,2’-biphenol (67.65 kcal/mol), and thus these reactions are easy to occur.c. Biphenyl-2-yl, the addcuct of benzyne and phenoxy radical, could produce DF via cyclization and H-leaving reactions. The reactions are spontaneous processes and no extra radical or molecule is needed for assisting this reaction route. Once one of the 2’-H is replaced by OH, Cl, CH3, or OCH3, the standard reaction Gibbs energy will decrease still further.2. Dioxins formation from the bimolecular reactions of phenylperoxy radical with aromatic hydrocarbonsPhenylperoxy radical is an important combustion intermediate, and derived from the oxidation of phenyl radical. In addition to the decomposition reaction, i.e., C6H5OO·→ C6H5O·+ O(3P), phenylperoxy radical may produce dioxins via other reation routes. Detailed conclusions are as follows:a. The outer oxygen atom in a phenylperoxy radical can add to the aromatic ring of benzene, phenol, toluene, anisole, biphenyl, diphenyl ether, and chlorobenzene, and the subsequent elimination reactions will generate diphenyl peroxide.b. The displacement reaction, in which the chlorine atom of chlorobenzene is replaced by phenylperoxy radical, creates diphenyl peroxide. The chloric substituent in phenylperoxy radical can accelerate the formation of diphenyl peroxide, and the ortho-chloric substituent in chlorobenzene could play the same role.c. Diphenyl peroxide is a potential precursor of dioxins, for it will spontaneously form two molecular phenoxy radicals.3. Sources of aryneThe exploration for the sources of aryne comes to the conclusions like these:a. The H-abstraction reaction of phenyl radical is a possible source of benzyne, for the standard reaction Gibbs energies are always below zero within the studied temperature range from 298 K to 1500 K.b. The Diels-Alder reaction between butadiyne and ethyne is a reversible reaction. The standard reaction Gibbs energies will increase to positive value, if the reaction temperature rises up to 1300 K.c. The bimolecular reaction of O2 (1△g) with naphthalene or anthracene has a high energy barrier, up to 71.07 kcal/mol, thus it will occur only in high-temperature.4. DF formation mechanism from PAHsPAHs are usually the products of the imperfect combustion of fuel. In flame, aromatic compounds might be attacked by active radicals, and then be disintegrated. Hydroxyl radical (·OH) turns out to be one of the dominant chain carriers, and plays an important role in various aspects of combustion chemistry. In addition to·OH, chlorine monoxide radical (CIO·) becomes important in the flame of chlorinated organic compounds. The predicted mechanisms are summarised as follows:a. Dibenzofuran will be formed if·OH or·OC1 adds to C8a, and the order of reactivity follows as 9H-fluoren-9-one> 9-methylfluorene> fluorene> phenanthrene.b. The oxidations initiated by CIO·are more favorable processes, considering that the standard reaction Gibbs energies and the standard reaction enthalpies are at least 20.73 kcal/mol lower than those of the equivalent reactions initiated by·OH.c.·OH, HOO·,·H,·CH3, and O2 can eliminate the hydrogen atom bonding with C9 of phenanthrene, fluorene, and 9-methylflurene.9-methylflurene is more likely to lose the hydrogen atom than fluorene, and phenanthrene is the hardest one.d. The reaction channel from fluorene and O2 to 9H-fluoren-9-one and H2O seems very important, not only because it contains only three elementary reactions, but because the standard reaction Gibbs energies are lower than -80.07 kcal/mol.e. DF may also stem from the fact that ·OH or CIO·adds to the C9 of 9H-fluoren-9-one, and it is worth mentioning that an additional oxidant-molecular or atomic oxygen-is needed and more heat is released in this reaction route.f. The partial oxidation of anthracene might generate 9-anthrone, 9H-xanthene, 9H-xanthen-9-one, anthraquinone, 9H-fluoren-9-one, DD, and DF. Comparing with 9H-fluoren-9-one, 9-methylfluorene, fluorene, and phenanthrene, anthracene is more difficult to produce DF, for more elementary reactions are included in the formation channels.