Pyrolytic destruction of polychlorinated biphenyls in a reductive atmosphere

Abstract

This paper discusses the development of an innovative technology for the pyrolytic destruction of halogenated hydrocarbons in a reductive environment. The technology developed demonstrated the potential for resource recovery, as reaction products could be commercially used. The process was specifically directed at the dehalogenation of polychlorinated biphenyls (PCBs), a class of chlorinated compounds most difficult to thermally decompose due to the high strength of their carbon-chlorine bonds. PCBs have extensive industrial applications, and are regarded as pollutants of serious environmental concern due to their persistence and insidious health effects. Pyrolysis experiments were conducted under controlled laboratory conditions in batch reactor systems with Aroclor 1254 (a mixture of PCB congeners) to evaluate the process feasibility, and to determine the destruction and removal efficiencies (DREs). The pyrolysis was conducted under a variety of operating conditions to investigate the effect on DREs due to changes in process variables such as reactor residence time, reaction temperature, and reactant (methane) concentration. Under optimal conditions — a residence time of 7 minutes, reaction temperature of 1000°C, and stoichiometric excess methane — a DRE of over 99.999% was achieved. The reaction products were dehalogenated hydrocarbons, hydrogen chloride, and soot. The dehalogenated compounds formed were identified as aliphatic and aromatic hydrocarbons by gas chromatography/mass spectrometry (GC /MS). The study also included a scientific analysis of free-radical reactions and associated mechanisms involved in the decomposition of the halogenated hydrocarbons. Achievement of the highest DREs suggests that the process needs further investigation as a method for dehalogenation of chlorinated hydrocarbons with the potential of product recovery for commercial use.