Seven CEOs—from Exelon, Great Plains Energy, National Grid, NRG Energy, Duke Energy, FPL Group, Great River Energy—explain how global warming is affecting their customers, shareholders, and...
Coal After MATS
A strategy for completely removing mercury from environmental emissions.
Mercury is a trace element in coal which currently is emitted from coal fired electric generating power plants. The ultimate presence of mercury in the environment results in a cumulative poisoning of people as illustrated in Figure 1.
Many industries produce contaminants such as mercury as byproducts in their production processes which are expelled in the air and water as pollutants into our environment. These contaminants have been deemed detrimental to our ecosystems in general and human health specifically and are thus heavily regulated. Ironically, the EPA cites recent increases in the contamination of waterways as attributable to industry's increased use of clean air technology. Such is also going to be the case when powered activated carbon (PAC), a sorbent which can capture most of the gaseous mercury from coal-fired power plants, is used. As illustrated in Figure 2, mercury will then be spread throughout the power plant processes in an effort to clean the air and much of it will end up in the retention ponds. There, anaerobic bacteria has a high probability to transform the toxic mercury into the highly toxic methyl mercury.
What is needed is technology that removes the mercury and concentrates it in a single small location instead of contaminating our air and waterways. We have such a process, discussed below.
Since there seem to be no two power plants in the US which are configured the same, a common solution is desirable. An assumption built into our solution is that nitrous oxides (NOx) and sulfur oxide (SOx) are removed before the mercury treatment system. The mercury removal equipment will be installed prior to the final emission stack. There are two treatment options for mercury removal: one is chemical and the other is biological. Both require a catalytic oxidation of the exit flue gas stream followed by a high-efficiency, low-pressure-drop water-based scrubber.
Let's first consider a chemical treatment option. In this case, the function of the low-temperature oxidizing catalyst is to convert all mercury to the water-soluble oxidized mercury (charge +2), identified by the symbol Hg(+2). Next, a circulating water-based scrubber will capture the Hg(+2) and pump a draw-off stream to a proprietary concentration and treatment facility to remove and concentrate the mercury (Figure 3). Mercury Hg(+2) is reacted with sodium sulfide, precipitated, filtered, and removed as a small amount of solid waste. Several methods of treatment are available.
Now let's consider a biological