Boilers, Gas, Water Treatment

Qualities of Limestone That Influence Wet FGD Performance

Issue 6 and Volume 115.

By Brad Buecker, Contributing Editor

With the projected rapid growth in flue gas scrubber construction, plant personnel at facilities where wet flue gas desulfurization (WFGD) is selected will be tasked with choosing the proper reagent for the system. In some cases, this may involve a choice between selecting a reagent of high quality, with potentially high transportation costs versus a lower quality material located closer to the plant.

Removing Sulfur Dioxide from Flue Gas

In wet scrubbers, sulfur dioxide (SO2) must be transferred from the gas phase to the liquid phase. This, of course, is the function of water in slurry sprays or baths. The reaction of sulfur dioxide with water is an equilibrium reaction and without a compound to bind with SO2 it will just come back out of solution.

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Some theoretical chemists argue that true H2SO3 does not exist and that SO2 retains its molecular character and is surrounded by water molecules. However, when SO2 is added to water the pH drops, which suggests that Equation 1 is correct and that the following dissociation reactions are reasonable.

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Regardless of the subtleties of this argument, it is the acidic nature of this solution that drives the scrubbing reaction. The principal component of all limestones by definition is calcium carbonate (CaCO3). While CaCO3 is only very slightly soluble in water (as evidenced by the use of limestone as construction materials over thousands of years), it will dissolve almost completely in well-designed scrubbing systems.

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When we combine these three equations, the fundamental limestone scrubbing process emerges.

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Without going into additional detail, most wet-limestone scrubbers are equipped with forced-oxidation systems to convert the reaction products in Eq. 4 into a final byproduct of gypsum (CaSO4•2H2O). What we wish to examine is how limestone impurities affect the efficiency of the scrubbing process.

Limestone Impurities

Limestone deposits are numerous throughout the United States, with large seams in Appalachia, the Ohio Valley, the Southeast (particularly in or near Florida), the Mid- and Upper-Mississippi Valley and in the Central- and Southern-Plains. High-quality limestone, whose calcium carbonate content is at or above 95 percent, may be found in many seams. These stones typically are quite reactive and will work well in scrubbers when ground to the proper size (90 percent passing through a 325-mesh screen is a common rule-of-thumb). However, many formations contain fine grains of silica-bearing compounds commonly known as chert. Silica (SiO2) is the primary component of sand, and although the grains within limestone deposits are typically very fine they still represent inert material that reduces the quality of the limestone. Thus, for any given amount of limestone delivered to the plant, higher silica content equates to a reduction in available active content.

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Inert materials can also cause other difficulties, most notably regarding product separation. Hydrocyclones (or hydroclones, for short) are commonly utilized in scrubbing systems to separate heavier, fully-reacted gypsum solids from lighter intermediate compounds and partially-reacted limestone (see Figure 1).

Orifices at the overflow and underflow discharge ports are set to provide maximum efficiency. If the inert content of the limestone changes substantially, the inerts, and the density change they impart, can influence hydroclone operation and reduce efficiency.

Inert issues may also arise from chemistry that is seemingly deceptive. In some FGD systems, soluble magnesium products are added for process improvement, as magnesium ions (Mg+2) will assist in holding onto bisulfite (HSO3) and sulfite (SO3-2) ions until they can react with calcium. Some limestone formations contain a high concentration of dolomite (CaCO3•MgCO3), where a substantial portion of the calcium and magnesium carbonate exists in a 1:1 molecular ratio. These deposits may have well over 90 percent combined carbonate content. A first thought might be that this material would be great in a scrubber, but dolomite is quite un-reactive. Thus, un-reacted dolomite can behave like inerts in the scrubber and also reduce the value of the raw product.

Two other impurities that can cause difficulties are iron (Fe) and manganese (Mn). Iron is usually found in higher concentrations than its cousin, Mn. Both of these compounds will catalyze the oxidation of intermediate reaction products to gypsum and can cause severe scaling in scrubbers that are not forced oxidized. Iron and manganese can also negatively influence filtration systems for byproduct dewatering. During testing, stones that had a noticeable iron content were found to influence iron particulate blinding of vacuum filter cloth. Also, some stones caused gypsum crystals to change shape from flat platelet configuration to needle-like structures. This crystal geometry change further hurt dewatering capabilities.

The upshot is that owners, operators and chemists of planned wet-limestone FGD systems should carefully evaluate the quality of limestone reagent to be used and discuss these issues with the equipment supplier. Good upfront work can prevent operating nightmares later. In addition to basic chemistry analyses, relatively straightforward tests are available to evaluate limestone reactivity before any material is accepted for use.

Author: Brad Buecker is a contributing editor for Power Engineering and also serves as a process specialist with Kiewit Power Engineers, in Lenexa, Kan. He has nearly 30 years of experience in, or affiliated with, the power industry, much of it in chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power in Springfield, Ill., and Kansas City Power & Light Co.’s La Cygne, Kan., station. He has an A.A. in pre-engineering from Springfield College in Illinois and a B.S. in chemistry from Iowa State University. He has written many articles and three books for PennWell on steam generation topics. He is a member of the ACS, AIChE, ASME, and NACE. He is also a member of the ASME Research Committee on Power Plant & Environmental Chemistry, the program planning committee for the Electric Utility Chemistry Workshop and the program planning committee for Coal-Gen.

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