Formation of dioxin

Mere destruction of the original hazardous material is not, however, an adequate measure of the performance of an incinerator. Products of incomplete combustion can be as toxic as, or even more toxic than, the materials from which they evolve. Indeed, highly mutagenic PAHs are readily generated along with soot in fuel-rich regions of most hydrocarbon flames. Formation of dioxins in the combustion of chlorinated hydrocarbons has also been reported. We need to understand the entire sequence of reactions involved in incineration in order to assess the effectiveness and risks of hazardous waste incineration. 

Concern has been expressed about the possible formation of dioxins and furans. However, measurements during experiments indicated that the emissions of dioxins and furans were not significantly elevated. Dioxin emissions with or without plastic input appeared to be about a factor of 100 below the standard of 0.1 ng/Nm TEQ TCCD (toxicity equivalent in relation to the toxic dioxin TCCD) (a.7). This might be due to the benefit of the strongly reducing atmosphere and the high temperature of 2100 °C. In total, until now the conclusion has been that at current PVC levels in MSW, pretreatment for chlorine removal is unnecessary. 

In the post-combustion chamber temperatures of 900 °C to 1200 °C are reached. The kiln can - like any rotary kihi - handle solid, fluid and gaseous waste streams. Based on the heat capacity of the waste, halogen content, and potential slag formation, an optimal mixture of wastes is determined. By choosing the feed carefully, production of high-quality HCl can be assured. Furthermore, in this way a minimum formation of dioxins and furans can be ensured. 

Heat is transferred between the two reactors by circulating sand between the gasifier and combustor. Both reactors are of the riser type with a very short residence time. They allow for a high PVC waste throughput. The gasifiers have a reducing atmosphere, which prevents the formation of dioxins. 

FIGURE 7.1 Formation of dioxin and dibenzofuran (from Crosby 1998). 

The selectivity of the catalyst is of major importance in the case of chlorinated VOCs the oxidation products should not contain even more harmful compounds than the parent-molecule, for example, formation of dioxins should be avoided. In addition, the minimization of CI2 and maximization of HCl in a product gas should be achieved [61]. These are just a few examples of why researchers are continuing the search for VOC oxidation catalysts as well as new reactor concepts. The new possibilities include, for example, utilization of nanosized gold catalysts in the oxidation of sulfur-containing VOCs and microwave-assisted processes where combination of adsorption and oxidation is used in low-concentration VOC oxidation [62, 63]. 

Secondary metal production is advantageous not only with respect to energy consumption but also to environmental impact as compared to primary metal production. The important environmental problems pertain to the formation of dioxins during thermal processing and the generation of dusts of the oxides of zinc, lead, and other metals in secondary steel, copper, and zinc production. 

Sydox [Sydney oxidation] A process for destroying polychlorinated biphenyls by oxidation. A catalyst containing ruthenium is used, and the temperature is kept below 100°C to prevent the formation of dioxins. Developed by J. Beattie at the University of Sydney in the 1980s by 1991 it had not been piloted. 

Furthermore, if the syngas is combusted in a gas turbine where excess oxygen is present, the high combustion temperature does not favor formation of free chlorine. In addition, post-combustion formation of dioxin or furan is not expected to occur because very little of the particulates that are required for post-combustion formation of these compounds are present in the flue gas. 

The environmental impact of PVC is discussed with reference to the toxicity of vinyl chloride, plasticisers and heavy metal stabilisers, the formation of dioxins and acid rain resulting from the incineration of PVC waste, and plasticiser migration from PVC products. A brief examination is also made of the recyclability of PVC. 18 refs. 

Alzeta states that their TPUs can achieve 99.99% destruction with emissions of nitrogen oxides and carbon monoxide of <10 parts per million (ppm) corrected for 3% oxygen emissions. They also state the Alzeta systems prevent the formation of dioxins and furans, and are available with several different operating systems and capacities for different site requirements. 

The Alka/Sorb air pollution control system is designed to remove dioxin, furans, toxic metals, acid gases, and particulates from industrial and medical incinerator off-gas. The Alka/Sorb system consists of a dry treatment/wet scrubbing process during which incinerator off-gas is cooled, contacted with an alkaline powder, injected with a sorbent, filtered by a baghouse and then wet-scrubbed for final removal of trace acid gases. Two central parts of the Alka/Sorb system include a wet-acid scrubber and a patented sorbent called Diox-Blok, which prevents the formation of dioxins and furans in air emissions. 

The atmosphere within the treatment chamber is essentially anaerobic. In the relative absence of oxygen and low operating temperature, the potential for formation of dioxins and furans during thermal desorption is essentially eliminated. 

Careful control of the conditions of synthesis of chlorinated phenols will reduce the formation of dioxins, and this is now the rule for avoiding their formation. Likewise, macro-contaminants, such as inactive isomers, are now recognized as unwanted components of chemicals that may cause harm in the environment or to humans. However, a good understanding of the mechanism of toxic action and how the chemical interacts with the receptor is needed to be able to recognize that toxicity is isomer-specific and to then use this in the intelligent design of low-risk products. 

As discussed in the earlier survey (1), a biogenic source of polychlorinated dibenzo-p-dioxins and dibenzofurans is peroxidase-catalyzed transformation of chlorophenols as first reported by Oberg and Rappe (2041-2044). More recent studies confirm these observations (2045-2048). In addition to lactoperoxidase and horseradish peroxidase, human leukocyte myeloperoxidase catalyzes in vitro formation of dioxins and dibenzofurans from chlorophenols (2046, 2047). Formation rates are in the pmol/mol range (Scheme 3.6) demonstrating that a human biosynthesis of dioxins and furans is not only possible but also likely. These observations are reinforced by the reported in vivo (rats) conversion of the pre-dioxin nona-chloro-2-phenoxyphenol to octachlorodibenzo-p-dioxin (OCDD) (2049), and the production of hepta- and octachlorodibenzo-p-dioxin in the feces of cows fed pentachlorophenol-treated wood (Scheme 3.7) (2050, 2051). 

Ferrario J, Byrne C, Cleverly D (2000) Summary of Evidence for the Possible Natural Formation of Dioxins in Mined Clay Products. Organohalogen Compds 46 23... 

Horii Y, van Bavel B, Kannan K, Petrick G, Nachtigall K, Yamashita N (2008) Novel Evidence for Natural Formation of Dioxins in Ball Clay. Chemosphere 70 1280... 

Although formation of dioxins is local, environmental distribution is global. Dioxins are found throughout the world in practically all ecosystems. The highest levels of these compounds are found in some soils, sediments, and food, especially dairy products, meat, fish, and shellfish. Very low levels are found in plants, water, and air [116]. 

Formation of dioxins in laboratory irradiation experiments on pen-tachlorophenol adsorbed on soils may be prevented by adding fulvic acids [79]. The photodegradation of propachlor (2-chloro-N-isopropylace-tanilide) and propanil showed faster rates in soils than in water samples [80]. The presence of HSs reduces the photodegradation rate of the latter compounds in water but leads to an enhancement in soil. Hydroxy and dechlo-rinated derivatives were the major photoproducts observed. 

Hutzinger O, Fiedler H (1988), NATO / CCMS B2, Report Number 173, Formation of Dioxins and Related Compounds in Industrial Processes . North Atlantic Treaty Organization, Committee on Challenges of Modem Society... 

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