PCB in municipal waste incineration

NIPSECT has made a literature study for the Danish Environmental Protection Agency (DEPA) about what happens when PCB-contaminated waste is incinerated at municipal solid waste incinerators.

PCB

 PCB

This study reviews the available scientific and technical literature about the fate and mass balance of  PCB in non-hazardous waste (< 50 ppm) burned in a conventional municipal solid waste incinerator with a flue gas temperature of 850oC.

The destruction and formation of PCB in waste incineration has not been studied as much as for dioxins and furans (PCDD/F) for which an official air emission limit value has been established. It is generally agreed that the mechanisms of formation in the incinerator plant of these related chemicals are quite similar but not identical.

What is then the behavior PCB in municipal waste, when it is burned in a conventional waste incinerator? PCB is non-flammable but most PCB in such waste is destroyed after a few seconds residence time at the high temperatures (>850 oC) in the oven, and very little PCB survived the combustion process and ended up in the flue gasses. In MSWIs the destruction/removal of low-level PCB in waste was > 90 % of the content in the waste. A minor part of the PCB in waste will end up in the bottom ash, the more the higher level in the waste. The PCB pattern of the bottom ashes corresponds to the PCB pattern of the waste showing that PCB was unchanged.

At the high temperatures present the PCB surviving the combustion process (or formed) will be vaporized, as a part of the raw flue gas. Gradually during the cooling of the flue gas PCB will condense and adsorb on boiler ash or fly ash. In the same time most of the PCB in the waste (mainly ndl-PCB) is destroyed by the combustion, new dioxin-like PCB (dl-PCB) is formed in the boiler and/or in the flue gas. Calculated as TEQs the PCB concentration in the flue gas typically increased 10 times compared with the PCB-TEQ content in the waste. The faster the raw flue gas was cooled to less than 250 oC, the less dl-PCB was formed. Among the formed dl-PCB congeners PCB126, PCB77 and PCB169 dominated. That is also the most toxic PCB congeners. The changed PCB pattern of the raw flue gas is very much different from the original PCB pattern of the burned waste.

During the following flue gas cleaning the PCB pattern changes very little, but the modern and efficient flue gas equipment, for instance ”dioxin filters” with bag filters and addition of active coal is able to remove > 99 % of the dl-PCB from the flue gas. In a modern MSWI only about 1 % of the PCB in the raw flue gas is emitted to ambient air, and 99 % ends up the various ash fractions, mainly fly ashes.

This literature survey shows that globally there are many different technologies and operation conditions applied for waste incineration and that have improved by time. Furthermore, the methods of PCB determination and calculation in waste differ in the published studies making comparisons difficult and uncertain.

The conclusion is that in order to reduce the emission of PCB from MSWIs as much as possible, the burned waste should contain as little PCB as possible. Addition of shredder waste will most likely increase the PCB concentration of the waste and increase the PCB concentration in the raw flue gas and bottom ash 2-5-fold.

Most PCB in the waste will be destroyed, if the combustion process is as optimal as possible regards high temperature, long residence time, and appropriate oxygen pressure. A regular and constant operation of the incineration plant is important, because start-up and close-down of operations diminish the destruction of PCB and increases new-formation and emission of PCB (and dioxin) from MSWIs considerably caused by memory effects.

A rapid and efficient cooling of the raw gas to less than 250 oC reduces the new-formation of dl-PCB, and a dioxin filter is able to catch most of the PCB-containing particles/fly ash, which means that the emission of PCB to the environment will be minimal, if the collected ash fractions also are handled with care.

If the MSWI operates optimal and steady-state regards the combustion and the flue gas cleaning is completely efficient, it will – with the present knowledge – not be problematic at a MSWI to incinerate waste containing up to 10 mg PCB/kg. This concentration level is also used as an accept limit value at German MSWIs. It is also evaluated that co-incineration of 10 % shredder waste in a modern MSWI will not change the air emissions significantly but the ash fractions will be more contaminated with PCB and eventually change properties relevant for the deposition.

Studies of incineration in MSWIs of waste with higher PCB concentrations are lacking.
It has not been possible in this study to make a complete assessment of, which literature data can directly be associated with the conditions at the Danish MSWIs, and therefore it was impossible to make a detailed comparison and evaluation. Initiation of such a study should be considered.

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