PCBs, thermal combustion

VAC TRAX is an ex situ thermal desorption process that separates contaminants such as volatile organic compounds (VOCs), semivolatile organic compounds (SVOCs), polychlorinated biphenyls (PCBs), and radioactive materials from soils, sludges, and solid trash. This process can be applied to mixed and unmixed waste streams. Because the nitrogen atmosphere in which the process occurs is inert, no combustion of organic material takes place. 

The electric infrared incineration technology is a mobile thermal processing system that is suitable for soils or sediments contaminated with organic compounds, polychlorinated biphenyls (PCBs), and metals. Liquid organic wastes can be treated after mixing with sand or soil. Electrically powered silicon carbide rods heat organic wastes to combustion temperature while any remaining combustibles are incinerated in an afterburner. 

Tetrachlorodibenzo-p-d1oxin (TCDD). At the detection limit of ten parts per trillion, no TCDD was found in the effluents from the combustion of coal in three different boilers at the Ames power plant (see summary table in reference 16 for description of boilers). This observation was confirmed at a second smaller coal-fired power plant located at Iowa State University. Even when the coal fuel was supplemented with RDF, which should contain the precursor compounds, no dioxins were observed in the vapor and particle samples taken from the effluents. Thus no de novo synthesis occurred during the combustion of coal alone and if dioxins were formed from precursor compounds in the co-combustion of coal and RDF, they were destroyed in the efficient combustion as explained above for the thermal destruction of the PCBs present in the RDF. 

Chlorinated Benzenes. Ten chlorinated benzenes were targeted for analysis in the stack effluents. The analytical results when coal alone was combusted are shown in Table IV. When the coal fuel was supplemented with RDF up to 20%, no consistent increase in the amounts of the chlorinated benzenes occurred although barely detectable amounts of the tetra-, penta- and hexa- isomers were observed during some of eleven different combustions of coal with 20% RDF. Based on these results it appears as if the dichlorobenzenes, reported to be present in the RDF at the 8000 pg/kg level (15,16), were thermally destroyed with high efficiency in much the same manner as that documented above for the PCBs present in the RDF. This suggestion has not been supported so far by experimental results. 

Thermal breakdown of PCDEs is possible at high temperatures, but PCDEs can also be converted into PCDFs during combustion. The pyrolytic behavior of PCDEs was investigated at different temperatures in the 1970s, but no PCDFs were detected then [72]. Later studies have verified that PCDEs can form PCDFs at elevated temperatures. Thermal conversion of PCDEs into PCDFs is at a similar rate to that of PCBs into PCDFs [15]. 

HALOGENATED WAXES (1336-36-3) see PCBs or polychlorinated biphenyls. HALOWAX 1014 (1335-87-1) C,oH,CI Combustible solid (flash point unknown). Violent reaction with strong oxidizers, strong acids, powdered aluminum, potassium, sodium. On small fires, use dry chemical powder (such as Purple-K-Powder), alcohol-resistant foam, water spray, or COj extinguishers. Thermal decomposition releases toxic hydrogen chloride and phosgene gases. 

Chemical analyses of combustion and pyrolysis products from various PCB replacements would be of little practical value without a discussion of the toxicology of these products and implications for human health risks. However, basing a human health risk assessment upon toxicities of individual thermal decomposition products is in itself a risky venture. 

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