Somewhat surprisingly, although SO₃
reacts with atmospheric moisture, the
compound passes through wet scrubbers
relatively untouched. Regardless, the
difficulties with SO have led to a number
3
of techniques to prevent its formation or
discharge from the stack. These include
development of SCR catalysts that minimize
SO to SO oxidation and alkaline chemical
23
injection to neutralize the SO prior to
3
removal in a scrubber.

The speciation of mercury as it escapes from the coal during and after combustion is interesting and is dependent upon the coal’s chlorine content. In the absence of chlorine, mercury tends to emerge in its elemental form, Hg⁰. Chlorine oxidizes mercury to primarily Hg^{+ 2}. This has major effects on how mercury is removed from the gas. Oxidized mercury is very soluble and will come out

in wet scrubbing solutions, but elemental mercury is not. So, for utilities burning Eastern bituminous coals, wet scrubbing is a potential solution for removing most of the mercury from the flue gas. Oxidized mercury will also adsorb onto carbonaceous materials, either unburned carbon in the flue gas or artificially injected activated carbon. A point to note about mercury removal in wet scrubbers is that some may be re-emitted by the scrubbing solution. When sulfur dioxide is transferred from the gas phase to the liquid phase (and as it first reacts with the alkaline reagent, either limestone or lime), the anionic product formed is sulfite (SO^3-2). Sulfite is easily oxidized by air in the flue gas or by forced air injection, but it can also be oxidized by mercury, which in turn reverts to its elemental—and insoluble—state.

For PRB coals, which have much less chlorine than their bituminous counterparts, elemental mercury is usually the majority species. While some elemental mercury will adsorb to carbon-based material, the process is much more effective if the mercury can be oxidized. Thus, much testing is underway to oxidize mercury by injection of chloride or bromide salts either into the flue gas or onto the coal before it is combusted.

A factor that negatively influences mercury adsorption by carbonaceous materials is injection of SO for enhanced electrostatic

3
precipitator (ESP) performance. PRB coal
combustion produces a high resistivity ash,
which a small amount of sulfur trioxide
reduces. However, the SO also adsorbs to
3
carbon, thus reducing its effectiveness in
capturing mercury. This is a difficulty that
is still being investigated.

A Different Speciation

A final issue that has been well known for years but remains of concern at utilities that burn several varieties of coal is sodium. Many of the inorganic materials in coal are complex silicon-based structures in which various metals (including sodium, aluminum, potassium and others) are bound together. But sodium can also exist in a salt form, such as chloride. When the coal is combusted this “free” sodium volatilizes, but then begins to condense within an approximate temperature range of 1,300 F to 1,500 F. These temperatures, of course, exist in the boiler backpass. There, the condensing sodium acts as a glue to collect fly ash, which builds up on superheater and reheater tubes.

A problem can arise at utilities where the fuels manager has authority to buy a wide variety of coals on the spot market depending on the lowest price. Even though the fuel may all come from the same geographical region, the coal quality between mines—and sometimes even within a seam, particularly with regard to impurities—may be quite different. In some cases, rapid and severe fouling of the backpass results, with aggravating (or worse) consequences. In at least some of these cases, increased volatile sodium content in the coal undoubtedly is to blame.

 

Author: Brad Buecker is a contributing editor. He can be reached at beakertoo@ aol.com.

How Clean Air Rules
Deter Equipment Investment
Provisions of the Clean Air Act may actually discourage utilities
from lowering emissions and improving efficiency.
By Steve Blankinship, Associate Editor

The law of unintended consequences describes what happens when a simple system tries to regulate something highly complex. The political system that promulgates regulations often produces unintended consequences. And when it comes to power plant emissions, no better example exists of such dysfunctional regulation than the prevention of significant deterioration portion of the Clean Air Act’s New Source Review provisions (PSD/NSR). Like so

many aspects of environmental law, PSD/ NSR and a host of court rulings and interpretations have produced confusion and uncertainty on a grand scale. The result has been to delay or halt new coal plants as well as projects to improve existing plants that, if completed, would lower net sulfur dioxide, nitrous oxide, mercury and carbon dioxide emissions.

Power Engineering magazine has covered issues related to the adverse effects of PSD/

NSR, most recently in guest columns written by contributing editor Robynn Andracsek, P.E., of Burns & McDonnell. As a member of the Kansas City-based firm’s Environmental Studies and Permitting Group, Andracsek helps clients obtain air and operating permits for power plants and refineries. Before joining Burns & McDonnell more than six years ago, she worked for another industry consulting firm. She holds a degree in mechanical engineering and masters