Where does airborne mercury come from, and how do we get rid of it?

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Dear Control Engineering: When I read about new EPA regulations on mercury emissions from coal-fired power plants, I want to know where the mercury comes from, and how to get rid of it.

Power plants do not create mercury, lead, arsenic, or other heavy metals any more than they would be able to create gold. Those pollutants are elemental metals and present in coal. The problem is that when you burn coal, you vaporize those metals and send them out the stack. There is some controversy as to exactly what happens in the atmosphere, but eventually those metals condense, either in their elemental form or after reacting with something else in the process (e.g., chlorine), and fall to earth in rain drops. Rain goes into rivers and lakes, and fish accumulate the metals.

(There are other types of pollutants, such as dioxin, that can be formed in a combustion process, but that’s a different issue.)

Mercury emissions have gone down, at least as a function of total electricity produced, but many plants still have little in the way of abatement equipment. Conventional FGD scrubbers can help with mercury to some extent, but if you really want to cut it down, activated charcoal injection is usually best. If the fluegas is at the right temperature, mercury will condense and stick to finely-ground activated carbon particles blown into the stream. The particles are then removed in a baghouse.

This process works, but the result is contaminated charcoal. The usual disposal method is landfilling or burial. If you put it in an old coal mine, you’re returning it whence it came.

Peter Welander, pwelander(at)cfemedia.com