Combustion processes, chlorinated dioxins

This structure looks nothing like the structures of chlorine-containing compounds used in industrial processes. In fact, no dioxin is deliberately manufactured anywhere in the diverse chlorine industry. Nevertheless, dioxins are of concern for two reasons First, dioxins appear to be inevitable trace by-products of some reactions involving chlorine, particularly combustion and second, dioxins accumulate in the biosphere, where they have highly deleterious effects. 

Dioxin or furan refers to molecules or compounds composed of carbon and oxygen. These compounds when reacted with halogens such as chlorine or bromine acquire toxic properties. Most research on halogenated dioxin and furan has been concerned with chlorinated species. It is generally accepted that dioxin and furan are by-products of combustion processes including domestic and medical waste combustion or incineration processes.7 In combustion processes, hydrocar-... 

Polychlorinated Dibenzo-(p)-Dioxins and Dibenzo-Furans. Another group of compounds that we need to specifically address are the polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzo-furans (PCDFs) (Fig. 2.15). The PCDDs and PCDFs are not intentionally produced but are released into the environment from various combustion processes and as a result of their occurrence as unwanted byproducts in various chlorinated chemical formulations (e.g., chlorinated phenols, chlorinated phenoxy herbicides see Alcock and Jones, 1996). Because some of the PCDD and PCDF congeners are very toxic (e.g., 2,3,7,8-tetrachloro dibenzo-p-dioxin, see margin), there have been and still are considerable efforts to assess their sources, distribution, and fate in the environment. Similarly to the PCBs or DDT (see above), the PCDDs and PCDFs are highly hydrophobic and very persistent in the environment. It is therefore not surprising that they have also been detected everywhere on earth (Brzuzy and Hites, 1996 Lohmann and Jones, 1998 Vallack et al., 1998). Finally, we should note that polybrominated diphenylethers (PBDEs, see margin) that, like the PBBs (see above), are used as flame retardants, are of increasing environmental concern (de Boer et al., 2000). 

Minute amounts of dioxin are created in certain combustion processes and by a few chemical manufacturing processes, including the use of chlorine in paper bleaching. 

A study sponsored by the American Society of Mechanical Engineers (ASME), involving the analysis of over 1700 test results from 155 large-scale, commercial incinerator facilities throughout the world, found no relationship between the chlorine content of waste and dioxin emissions from combustion processes. Instead, the study stated, the scientific literature is clear that the operating conditions of combustors are the critical factor in dioxin generation. 

Recent studies have indicated that humans may be exposed to 2,3,7,8-tetrachlorodibenzo-p- dioxin(2,3,7,8-TCDD), a toxic and teratogenic substance in laboratory animals (1). A fraction of beef fat samples from cattle known to have grazed on pasture treated with herbicide 2,4,5-T (which contains trace quantities of 2,3,7,8-TCDD) have been reported to contain low part per trillion (ppt) levels of 2,3,7,8-TCDD (2,3) Two studies of bovine milk reported no detectable chlorodioxins however (4,5). Combustion processes have been reported to produce chlorinated dioxins which enter the air as fly ash and soot (6.7.8). Three different studies of human milk have been carried out by various workers to determine if humans contain detectable concentrations of 2,3,7,8-TCDD (9,20,22). 

Polychlorinated biphenyls (PCBs) are mixtures of biphenyls with 1 to 10 chlorine atoms per molecule. They are oily fluids with high boiling points, great chemical resistance, and low electrical conductivity, and have been used as plasticizers in polyvinyl polymers, insulators and coolants in transformers and heat exchange fluids. Dioxins and furans (see Section 9.1) are not produced intentionally but are derived from combustion processes of chlorinated aromatic hydrocarbons, pesticides and chlorinated solvents. 

Emission of hydrogen chloride is the third most important contribution to the global acidification from human activities. The two first are SO2 and NO, The HCI is a local pollutant, contrary to the other two. It is soluble in water and easily dissolved in rain droplets and, therefore, usually falls down near the emission source. The hydrogen chlorine emissions from combustion and gasification processes has been calculated to 3,5 Mt./year, The major part of the estimated global contribution of HCI to the atmosphere is evaporation from the oceans. Even with a redeponation of 90-% HCI to the oceans, the estimated emission will reach approximately 120 Mt./year, The majority of the emitted chlorine from a combustion process will leave as HCI in the gas phase which may cause problems like corrosion and formation of dioxins. 

Several countries have introduced stringent emission limits (0.1 ng-TE/Nm ) for chlorinated dioxins and furans emitted from combustion sources, in particular solid waste incinerators, because of concerns over their adverse health effects. Technologies for reducing their formation and emission in incineration processes have been studied extensively and can be applied in modern incineration plants. Activated carbon injection and fabric filtration are currently practiced in many installations. However, to minimize capital cost, a more fundamental approach is needed to control and limit formation of these pollutants in incineration processes, e.g., involving the postcombustion zone, the combustion chamber, and waste feeding. ... 

In the last twenty years, PCDD and PCDF were identified as by-products in many industrial processes which involve chlorine or chlorinated compounds. Additionally both groups of compounds were found to be formed in a broad range of combustion processes, including accidental fires. Municipal waste incineration is particularly considered to be a very important, if not the most important, of the identified source of environmental dibenzo-p-dioxin and dibenzofuran contamination. As a consequence, the evaluation and close control of new and existing installations for their dioxin releases has become a major concern. Based on this relative importance of municipal waste incinerators, and taking into account the relative toxicity data actually available for PCDD and PCDF, it was decided to prepare and certify a crude fly ash extract (CRM 429) for the twelve more toxic PCDD and PCDF [18,19]. 

Sources From Combustion Processes. In 1977, PCDD and PCDF were detected in particulate emissions from municipal incinerators (5). Similar data were soon reported from around the world. In 1980, workers at Dow Chemical Company, using the most sensitive and specific analytical techniques of that time, detected a range of PCDD in residues from many combustion processes (6). These workers postulated that all combustion processes that contain chlorine sources produce PCDD. Although this postulate is not supported in all situations, dioxin emissions from combustion sources are major contributors to PCDD in the environment (7). 

More recently, environmental contamination by chlorinated dioxins and furans released during combustion processes has been recognized. These substances are formed not only during incineration of chlorinated phenols, but also during combustion of materials in home fireplaces and municipal incinerators (10). Chlorinated dioxins and dibenzofurans also are formed during fires involving polychlorinated biphenyls (15). 

Polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), often termed dioxins , consist of 210 different compounds which have similar chemical properties. This class of compounds is persistent, toxic, and bioaccumulative. They are generated as byproducts during incomplete combustion of chlorine containing wastes like municipal solid waste, sewage sludge, and hospital and hazardous wastes. Industrial processes such as bleaching of wood pulp in the manufacture of paper products, can also produce PCDDs and PCDFs [39]. 

The notable human tolerance toward dioxin probably lias its roots in histoiy. Dioxin was found in 8000-year old sediments in Japan and dioxin levels of 30 ppt were detected in 170-year old soil samples from Rothamsted, England (90 ppt was the measured level in 1995). Industrial development obviously increased the amounts of dioxin, but its origin in the early or pre-historical samples is a much more interesting question. In 1977, evidence was found that dioxins are formed regularly in combustion processes. Wood contains about 0.2% of chlorine in the form of chloride salts, some of which is transformed into dioxins during burning. About 5 billion t of wood are burnt annually on Earth, which may form about 551 of dioxin. The human body has learnt to tolerate dioxins in small amoimts. 

There has been considerable concern throughout Europe about the incineration of wastes, yet in Japan about 70% of all MSW is incinerated, and plans are that this should increase to 90% by the year 2000 [33]. Incinerators that are poorly operated may run at temperatures too low to burn potentially hazardous intermediates of the combustion process such as the products from pyrolysis which are believed to be the precursors in the combustion processes [34]. Of particular concern has been the discovery of extremely toxic materials such as dioxins (chlorinated dibenzo-/ -dioxins and benzofuran dioxins), in the flue gases of some incinerators. Such is the level of concern that many European countries have increased the legislative and environmental controls on incinerator operators, and some are moving to ban the incineration of plastics [35], and particularly PVC. In incinerators where the temperature is below about 1400 K, dehydrochlorination of PVC occurs, accompanied by the formation of polyenes. The polyenes can then cyclise and be oxidised, and then be attacked by chlorine-containing species to produce dioxins, the most toxic of which is 2,3,7,8-tetra-chlorodibenzo-/7-dioxin (TCDD), the material at the centre of the disaster at Sveso, Northern Italy, in 1979. More than 70 dioxins are known to exist (Figure 13.11). 

Combustion and Incineration Sources Dioxin-like compounds can be generated and released to the environment from various combustion processes when chlorine donor compounds are... 

Dioxins and furans form in virtually every combustion process of solid materials if chlorine, oxygen and organic matter are present within an appropriate temperature range. In waste combustion conditions, two possible ways for dioxins/fiirans to... 

Chlorinated organic compounds (dioxins, other halocarbons) Combustion of municipal wastes, paper processing, cleaning solvents Toxic effects including birth defects, reproductive failure, cancer, and systemic poisoning. 

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