Airborne pollutants formed in industrial processes

Dear Control Engineering: In your posting about mercury from power plants, you mentioned that there are other pollutants that are created in industrial processes, notably dioxin. So what is that story?

There are two key groups of similar toxic gasses that are a particular concern: dioxins and furans (D/F). These include tetra-, penta-, hexa-, hepta-, and octa- chlorinated dibenzo dioxins and furans. There are more than 200 variations of these in total, with some regarded as the most toxic substances known. They form in combustion processes, particularly where there are sufficient levels of precursors and when the combustion gas has gotten down to the 450 to 650 °F range. Allow me to quote a concise explanation from a Dow Website:

“Dioxins and furans are unwanted by-products of incineration, uncontrolled burning, and certain industrial processes. Industrial sources of dioxin to the environment include incinerators, metal smelters, cement kilns, the manufacture of chlorinated organics, and coal burning power plants. Dioxin is also produced by non-industrial sources (now considered by the U.S. EPA to be the greatest source in the U.S.), like residential wood burning, backyard burning of household trash, oil heating, and emissions from diesel vehicles. According to the EPA, cigarette smoke also contains a small amount of dioxin. Of the 210 dioxin and furan compounds, 17 are generally the focus of regulatory action.

“Just as combustion provides a means for dioxin formation, so too does it allow for its destruction, through careful controls. A high combustion temperature, adequate combustion time, and turbulence to distribute heat all contribute to maximize dioxin destruction. Dioxin formation following combustion is prevented by quickly cooling combustion gases, and minimizing the presence of certain metals known to promote dioxin formation. Dow has worked to prevent dioxin formation in its processes by shutting down older, less efficient units; adding new equipment to existing units; and employing advanced controls. Dow has also implemented new technology to recycle wastes as useful raw materials, thereby minimizing the need for incineration.”

Dow makes the point that well-regulated burning combined with rapid cooling of combustion gasses to minimize the time spent in the prime formation temperature range goes a long way to preventing formation. Counter that with some guy burning trash in a barrel. Throw a few plastic bottles into the pile and the D/F formation could be more than a full-scale cement plant with an effective control system.

Peter Welander, pwelander@cfemedia.com

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