Coal

O & M Feature: FGD Gypsum Issues

Issue 11 and Volume 111.

Gypsum Analytical Methods

By Brad Buecker, Contributing Editor

Numerous flue gas desulfurization (FGD) systems are being designed and/or installed at utilities throughout the country. In many cases, these will be forced-air oxidation systems that will produce a gypsum byproduct. Disposing of the byproduct, perhaps by sale to wallboard or cement manufacturers or for agricultural purposes, depends upon several factors. Even if the byproduct can only be land-filled, converting the material to gypsum makes it easier to dewater and handle.

We begin by reviewing material covered in a previous article for Power Engineering,1 but which is important in understanding how gypsum is produced.

When sulfur dioxide (SO2) in the flue gas contacts scrubber slurries, the pollutant transfers from the gas to the liquid phase, where the following equilibrium reactions are fundamentally representative of the transfer process.

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Limestone, whose primary components are calcium carbonate (CaCO3) with lesser amounts of magnesium carbonate (MgCO3), when introduced to water will raise the pH according to the following mechanism.

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However, CaCO3 is only slightly soluble in water, so this reaction is minor in and of itself. In the presence of acid, calcium carbonate reacts much more vigorously and it is the acid generated by absorption of SO2 into the liquid that drives the limestone dissolution process.

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Equations 1, 2 and 3 when combined illustrate the primary scrubbing mechanism.

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In the absence of any other factors, (for example, oxygen in flue gas) calcium and sulfite ions will precipitate as a hemihydrate, where water is actually included in the crystal lattice of the scrubber byproduct.

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However, oxygen in the flue gas has a major effect on chemistry, and in particular on byproduct formation. Aqueous bisulfite and sulfite ions react with oxygen to produce sulfate ions (SO4-2).

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Approximately the first 15 mole percent of the sulfate ions co-precipitate with sulfite to form calcium sulfite-sulfate hemihydrate [(CaSO3·CaSO4)·½H2O]. Any sulfate above the 15 percent mole ratio precipitates with calcium as gypsum.

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Calcium sulfite-sulfate hemihydrate is a soft, difficult-to-dewater material that previously has had little practical value as a chemical commodity. Gypsum, on the other hand, is much easier to handle and has practical value. These factors are driving utilities to install forced oxidation systems for gypsum production.

Gypsum Uses

Three major markets exist for gypsum: wallboard manufacture, cement production and soil stabilization. Wallboard manufacture currently leads the way by far. According to the United States Geological Survey (USGS) in 2006, almost 91 percent of domestic consumption, which totaled 41.6 million tons, was accounted for by manufacturers of wallboard and plaster products. Some 3.0 million tons was used for cement production, 1.1 million tons for agricultural applications and small amounts for a range of industrial processes such as smelting and glassmaking. At the beginning of 2006, the capacity of operating wallboard plants in the United States was about 37.6 billion square feet per year.2 At present, synthetic gypsum from FGD systems supplies about one quarter of the wallboard industry’s needs.

Let’s look at issues regarding supply, gypsum quality and applications in these markets.

Gypsum supplied to the wallboard and Plaster of Paris industries must first be calcined (heated) to prepare the material for use.

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The reaction product is simply Plaster of Paris. Wallboard manufacturers rehydrate Plaster of Paris in the manufacturing process so that the re-formed gypsum bonds to the cardboard sheets attached on either side. For this process to be successful, the original gypsum must be rather pure, with a CaSO4·2H2O content of 92 percent or better. Precursors in FGD systems to meet these requirements include the use of high-purity limestone (93 percent or greater calcium carbonate content) and sufficient air compressor capacity to convert all sulfite in the scrubber to sulfate (see equations 5, 6, and 7 above). Obviously, the compressors must be able to introduce at least the stoichiometric amount (1:1 ratio) of oxygen required for the SO2 being removed, but typically the ratio must be at least 1.5:1 and often higher to ensure complete conversion of sulfite to sulfate. Under-designed air compressor capacity results in byproduct quality that cannot meet required standards.3

Moisture content and an interrelated issue of dissolved salt concentration (chlorides in particular) are also very important. Typically, calcining plants require the moisture content of raw material to be at or below 10 percent. Gypsum as it is first produced in the scrubber is a slurry. The common method of moisture reduction, and in part slurry density control in the scrubber, is to route the slurry through one or more banks of hydrocyclones to remove moisture by centripetal action, followed by drying either via vacuum drum or, for consistent solids of 90 percent dryness, belt filters.

Even at 10 percent moisture, solids can contain significant chloride concentrations, as often the chloride levels in the scrubber process slurry can reach or exceed as much as 30,000 parts-per-million (ppm). This is particularly true at plants burning eastern bituminous coal. So, even with moisture reduced to 10 percent, the byproduct chloride concentration can easily exceed the common 100 ppm maximum guideline for wallboard production. This can easily be overcome by installing a freshwater wash at the beginning of either the vacuum drum or the belt filter process. Calcium chloride, and the much smaller concentrations of magnesium chloride, are soluble and quickly wash out of the filter cake.

As mentioned earlier, synthetic gypsum from FGD systems supplies only a portion of what is received by wallboard plants. However, with additional FGD units coming on line, the market may become saturated within five years or so. The housing market downturn in mid-2007 shows that demand can be variable, so utility personnel must give this idea careful investigation before assuming that their gypsum byproduct will find a market in the construction industry.

Calcium sulfate is used in the cement industry as a conditioning agent. Small amounts of the material improve cement’s setting characteristics. I have no exact data regarding gypsum quality requirements for this market, but have learned that they are not as stringent as for the wallboard industry. This may be due to the fact that impurities in synthetic gypsum, such as un-reacted limestone, would not adversely affect cement properties.

Demand for gypsum in the agricultural industry is not nearly as great as for wallboard manufacture. However, reports indicate that some expanded use may be in the offing. Calcium and sulfur are two minor, but essential, elements for crop growth. Gypsum represents a ready and inexpensive source of each. Perhaps the emerging biofuels industry will generate an increased need for additional benign fertilizers. Impurities are much more tolerable in gypsum used for agricultural purposes. Even so, chlorides still must be washed from the solids as the high salinity would otherwise be detrimental.

An important issue for utilities planning to produce commercial-grade gypsum is to speak with potential vendors early in the process to learn where and to whom the byproduct may be offered. Regardless of the opportunity, and even if the byproduct cannot be sold, my personal choice (based on practical experiences) would be to include forced-air oxidation on any wet-limestone scrubber. When dewatered and compacted in a landfill, the gypsum is generally quite stable. Un-oxidized or partially oxidized byproduct containing calcium sulfite does not dewater very readily and often must be blended with fly ash to produce a stable, solid material. For plants equipped with large holding ponds, FGD slurry may be pumped as is. However, the slurry (and particularly any material containing calcium sulfite) becomes anaerobic below the surface where microbes can cause decomposition of the sulfite to sulfides. This produces the familiar “rotten egg” smell, especially if the material is disturbed.

Gypsum Analytical Methods

A quick and efficient method for determining the byproduct gypsum content (and one I have used for over 20 years) is thermogravimetric analysis (TGA). Indeed, EPRI has included this method in its updated FGD analytical procedures.4 Figure 1 illustrates an analysis of a high-purity (gypsum-wise), byproduct sample.

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The first weight loss, beginning at about 160 C, comes from complete dehydration of the gypsum.

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Simple molecular weight calculations show that this sample, taken from material supplied to a wallboard manufacturer, contains 96 percent gypsum. The decomposition at 650 C is from un-reacted calcium carbonate, which calculations show to be at a 1.9 percent concentration.

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This material easily meets wallboard guidelines from a gypsum-purity standpoint.

Regarding quality control measurements of the byproduct’s chloride concentration, a simple method is to wash a measured byproduct sample with a known quantity of deionized water and then analyze the chloride content of the rinse solution with a standard technique such as ion chromatography. From there, it is quite easy to convert the dissolved chloride concentration in the wash water to that of the original sample, and then to determine if the wash system is functioning correctly.

References

1. B. Buecker, “Wet-Limestone Scrubbing Fundamentals”; Power Engineering, pp. 32 – 37, Vol. 110, No. 8, August 2006.

2. http://minerals.usgs.gov/pubs/commodity/gypsum/

3. B.Buecker, “Gypsum Seed Recycle in Limestone Scrubbers”; unpublished paper presented at the Utility Representatives FGD Conference, June 10-12, 1986. Farmington, N.M.

4. C. Dene, Project Manager, “FGD Chemistry and Analytical Methods Handbook, Volume 2 – FGD Chemistry (Revision 2)”; Report 1013347, March 2007, the Electric Power Research Institute, Palo Alto, Calif.