PCB decomposition and formation in thermal treatment plant equipment
Introduction
Environmental contamination by technical PCB has been intensely focused as the main PCB study since PCB was first detected by Jensen et al. (1969) in a body of a sea eagle. However, by-product PCB is thought to be studied as a further problem, which is unintentionally formed and released from thermal processes involving organic material and chlorine as result of incomplete combustion or chemical reaction. By-product PCB has been verified with the progress of the study of by-product PCDD/DF (Olie et al., 1977, Tiernan et al., 1983). Dioxin-like PCB (dl-PCB) has been predominantly selected as the object compounds for study of by-product PCB, because it has high toxicity as PCDD/DF. In previously studies, dl-PCB was detected from exhaust gas and fly ash in waste incinerators (van Bavel et al., 1992, Sakai et al., 1993). In 2001, annex C of the Stockholm Convention on Persistent Organic Pollutants imposed the duty of measuring by-product PCB which formed and released unintentionally from anthropogenic sources such as waste treatment processes and industrial processes. The aim is for parties to the Stockholm Convention to both reduce and assess PCB emissions (UNEP, 2002). The understanding of PCB formation during incineration and the accumulating of chemical data are important for promoting of “Best available techniques” and “Best environmental practices” (UNEP, 2002, Japan Ministry of the Environment, 2005). Fewer studies investigated other PCB (i.e. bulk-PCB) (Ballschmiter et al., 1987, Sakai et al., 1994, Sakai et al., 2001, Ikonomou et al., 2002, Kim et al., 2004) or made direct observations of incinerator interiors (Fangmark et al., 1993, Fangmark et al., 1994, Sakai et al., 1996, Noma et al., 2006). Sakai et al. (2001) conducted direct observations of an incinerator system in studying bulk-PCB. This is the only study of PCB homologue in flue gas from a boiler outlet and BF outlet of an incinerator. In Japan, waste incinerators are generally comprised of some stages including a primary combustion chamber, secondary combustion chamber, and some flue gas treatment equipments. It is possible that the unintentional formation of PCB and decomposition of PCB in input material occur simultaneously in these various stages. However, to our knowledge, the behavior of PCB has not studied yet.
Therefore, in this study, we conducted experiments on the combustion of waste samples to investigate both the decomposition and unintentional formation of PCB at several stages in thermal treatment plant equipment which was comprised of an electrically-heated rotary kiln primary combustion chamber, an electrically-heated vertical secondary combustion chamber, a gas cooling zone, a bag filter, an activated carbon adsorption tower, and a scrubber for treating flue gas. We used refuse-derived fuel (RDF) or automobile shredder residue (ASR) as input waste samples for the combustion experiments. Recently, RDF and ASR have been increasingly applied to electrical power generation, heat recovery systems, combustion and so on. Chemical information concerning by-product PCB in the combustion of RDF and ASR is essential for the appropriate management of these systems. It is concerned that ASR has greater capacity to produce by-product than general combustion waste, because ASR contains substantial metal such as copper (Cu) and iron (Fe) as well as plastics (Miyakawa et al., 2001, Kida et al., 2004). The ability of Cu and Fe to catalyze the chlorination of organic compounds is well known (Vogg and Stiegilitz, 1986, Addink and Olie, 1995, Ishibashi et al., 2001). Therefore, in this study, we compared the formation of by-product PCB with RDF and ASR.
Section snippets
Thermal treatment plant equipment
Testing was conducted using the thermal treatment plant equipment (Noma et al., 2006). This system comprised an electrically-heated rotary kiln primary combustion chamber, an electrically-heated vertical secondary combustion chamber, and some flue gas treatment equipments which were a gas cooling zone, a bag filter, an activated carbon adsorption tower, and a scrubber. The temperatures of the rotary kiln primary combustion chamber and the secondary combustion chamber were set at 850 °C and 900
Transition of PCB in the thermal experiments
PCB concentrations which were the sum of bulk-PCB concentration and dl-PCB concentration in RDF and ASR were 54 000 ng kg−1 and 90 000 ng kg−1, respectively. Total PCB amounts in several samples were calculated by multiplying sample amount by PCB concentrations. The calculated amounts of RDF, ASR, and gases were 10 kg, 7.6 kg, and 140–236 Nm3, respectively. The ratios of total PCB amount to input at several different stages were calculated to understand the transition of PCB in the thermal experiments (
Conclusions
We investigated both the decomposition and unintentional formation of PCB congeners during combustion experiments of RDF or ASR at several stages in a thermal treatment plant that composed a primary combustion chamber, a secondary combustion chamber, and some flue gas treatment equipments.
PCB concentration and the number of congeners increased at the kiln exit. While this behavior was common to RDF and ASR, the increase in PCB concentration at the kiln exit was greater with the combustion of
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Research fellow of the Japan Society for Promotion of Science.