Please note it's not a waste incineration but a postcombustion treatment of vapours of a ceramic kiln
Dear Dr. Squassabia,
Temperature is not the only parameter that would guarantee dioxin-free incineration; very important is the concentration of oxygen in the post-incineration air, but also the residence time of HCl and gaseous organics at high temperature. Although dioxins are mainly formed at much lower temperatures (300-400 deg. C), it is very possible to have dioxins generated even at temperatures higher than 950 C. This may occur when: 1. post-incineration is incomplete due to insufficient oxygen to be supplied continuously; 2. residence time at temperature higher than 950 C is insufficient; 3. post-incineration gases are slowly cooled. However, to avoid dioxins generation, the process temperature in the post-incineration chamber is recommended to be at least 1100-1200 C. Also in case of waste incineration, the post-incineration treatment is addressed to gases exclusively, since the solid or liquid waste is burnt in the primary chamber at temp of 850 C or lower.
Thanks Carmen for your answer. I confirm to you that the engineer who are designing this postcombustor are not planning to reach this Temperatures. so , as I understand there's a risk of dioxin formation.
They also say that there's no risk beacause there's no chlorine but just chloride. Chloride can form dioxin just in the presence of Copper . I 've many doubts because Chloride at high temperature can generate chlorine. Oxygen will pe a focal point but they will be burn in presence of oxygen.
The mechanism by which dioxins are formed is not very well elucidated. Chlorine atoms (as elemental chlorine or included in chlorinated compounds) and organic compounds (either simple or complex molecules) are required to come in contact at temperatures above 250 ° C and combine to make various dioxins and furans. Your explanation that chlorides can form dioxins only in the presence of copper is a bit doubtful to me. This is maybe not for the case of high temperatures. I worked for some projects on the incineration of PCBs-contaminated oils, also another one aimed on incineration of medical waste (such waste contains a lot of PVC). Documentary studies has shown me that the risk of dioxin formation is very high in the presence of chlorine or chlorinated compounds, regardless the material to be incinerated (solid or liquid). Also, many studies have reported the presence of dioxins in post-incineration gases above permitted limits (0.1 ng/m3) in incinerators operating even at temperatures above 1200 C. My opinion is that the risk of dioxin formation exists in the case you referred to, but more advanced documentary study would be required, also some more opinions from specialists. However, some sampling and GC/MS analysis of the combustion gases would answer this question.
Best regards.
I agree with you Carmen and your explanation. Please just note that the company that is going to install this postcombustor ( not me ) is underlining the fact that with the HCL gas ( they are burning not liquid or solid , but emission of a kiln so it's gas phase ) Dioxin cannot generate: this was something very strange to me. Of course we will require more analysis after the installation.
Yes, when speak about post-combustion we refer to gaseous compounds, also in case of gases from ceramic kiln or gases from waste incineration. More documentary analysis is required for this issue. For me it is much unclear why this risk exists also during slow cooling since it is recommended to have a very rapid gases cooling below 200 C to avoid dioxins generation, it means that gases (chlorine and organics) may be combined to form dioxins even after post-incineration phase. Therefore the mechanism is complex, dioxins family compounds are multiple but this rather insufficient understanding of the chemical mechanisms makes the process still at risk of dioxin formation. Good luck in your research and development!
What puzzles me is: where does this HCl come from?
And: how comes that it cannot be neutralized and scrubbed away as calcium chloride, for instance?
Looking at the future: VOC-laden off-gases are bound to be treated without any combustion, making resort to low-temperature thermal plasma.
In the meanwhile, today VOC-laden off-gases can be pyrolyzed (not burnt: in a reducing environment then) in a dual-chamber reactor operating alternately, warmed with methane in a shift mode..
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