Combustion processes, chlorinated

Flame Retardants. Flame retardants are added to nylon to eliminate burning drips and to obtain short self-extinguishing times. Halogenated organics, together with catalysts such as antimony trioxide, are commonly used to give free-radical suppression in the vapor phase, thus inhibiting the combustion process. Some common additives are decabromodiphenyl oxide, brominated polystyrene, and chlorinated... 

One of the concerns that exists in thermally processing chlorinated organics is that the by-products of combustion that are formed may be more harmful than the initial... 

Although the petrochemical and metals industries were the primai y focus of the toxic air pollutants legislation, approximately forty of these substances have been detected in fossil power plant flue gas. Mercury, which is found in trace amounts in fossil fuels such as coal and oil, is liberated during the combustion process and these emissions may be regulated in the future. EPA issued an Information Collection Request (ICR) that required all coal-fired plants to analyze their feed coal for mercury and chlorine. Since these data will be used in making a regulatory decision on mercury near the end ot the year 2000, it is critical that the power industry provide the most accurate data possible. 

The addition of chlorinated waxes will trap free radicals formed during the basic combustion process, by combination with a hydrogen halide, usually HC1. 

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. 

CDDs are released into the air in emissions from municipal solid waste and industrial incinerators. Exhaust from vehicles powered with leaded and unleaded gasoline and diesel fuel also release CDDs to the air. Other sources of CDDs in air include emissions from oil- or coal-fired power plants, burning of chlorinated compounds such as PCBs, and cigarette smoke. CDDs formed during combustion processes are associated with small particles in the air, such as ash. The larger particles will be deposited close to the emission source, while very small particles may be... 

CDDs have been measured in all environmental media including ambient air, surface water, groundwater, soil, and sediment. While the manufacture and use of chlorinated compounds, such as chlorophenols and chlorinated phenoxy herbicides, were important sources of CDDs to the environment in the past, the restricted manufacture of many of these compounds has substantially reduced their current contribution to environmental releases. It is now believed that incineration/combustion processes are the most important sources of CDDs to the environment (Zook and Rappe 1994). Important incineration/combustion sources include medical waste, municipal solid waste, hazardous waste, and sewage sludge incineration industrial coal, oil, and wood burning secondary metal smelting, cement kilns, diesel fuel combustion, and residential oil and wood burning (Clement et al. 1985 Thoma 1988 Zook and Rappe 1994). 

Recently, Wikstrom et al. (1996) studied formation of CDDs, CDFs, and chlorobenzenes in the combustion process. These authors monitored combustion of an artificial fuel where the chlorine level and source were varied in the artificial waste. Different levels of organic chlorine (PVC) and inorganic chlorine (CaCl2 6H20) were added to the fuel. When the level of chlorine in the fuel was <1%, there was no correlation between the quantities of CDDs, CDFs, and chlorobenzenes present. However, when the chlorine level was >1%, increased formation rates were noted for CDDs, CDFs, and chlorobenzenes. 

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. 

HCB has been detected in the environment in some studies carried out in developing countries. Its major source is likely to be combustion processes, especially open combustion of landhlls containing chlorinated products. 

Noticeable amounts of hydrogen chloride can be found in the emission namely due to incineration and combustion process of chlorine containing materials. HCl is quite stable towards oxidising agents and on account of its solubility can be incorporated into the rain or can be adsorbed by dry deposition. It can undergo an oxidation process similar to that of SO2 with OH radicals to yield chlorine atoms... 

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