Air Pollution Control Equipment Services, Emissions

Properly Monitor Your Scrubber Chemistry

Issue 7 and Volume 112.

By Brad Buecker, Contributing Editor

Within the last several years the flue gas desulfurization (FGD) market has mushroomed and many utilities have purchased and are erecting wet FGD systems. Critical to proper operation of these systems will be accurate liquids and solids monitoring of the process streams. This article examines many of the most important analyses for which new scrubber personnel must prepare and equip laboratories.

Corrosion Prevention

Chlorine in coal converts to hydrogen chloride (HCl) during combustion. HCl, of course, is an acid and it reacts with limestone to produce calcium and magnesium chloride (CaCl2 and MgCl2). Both are very soluble salts. Chloride concentrations may reach several thousand mg/L in scrubber quenchers. High alloy metals to resist chlorides are expensive, so often materials are selected to handle the calculated maximum chloride concentration, but no more. So, if the process is allowed to operate at a higher chloride concentration, severe corrosion can result.

More than one technique is available to monitor chlorides. Most sophisticated is ion chromatography (IC), which can also analyze many other anions from FGD or other process streams. For those with limited budgets, chloride monitoring by ion specific electrode is an alternative. This technique operates similarly to other ion specific methods, in which the electrode senses the ion of particular importance. Yet another possibility is titration analysis, such as the method offered by the Hach Co. Very simple in nature, it gives good results for the chloride concentrations typically encountered in wet scrubbers.

Scrubber Slurry Alkalinity

Limestone (CaCO3) or hydrated lime [Ca(OH)2] are the principal reagents in wet scrubbers. Consider the simplified reaction outlined below, which illustrates the initial reaction of limestone with the acidic solution produced by sulfur dioxide absorption (SO2 + H2O ⇔ 2H++ + SO3-2) into the scrubber slurry sprays.

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Extremely important is constant measurement of the alkalinity in the scrubber module or modules, as too much alkalinity will waste reagent while lean alkalinity will impair SO2 removal. The technique universally employed in wet scrubbers is pH monitoring. These measurements must be continuous, with control of reagent feed rates based upon the readings. For the lab staff, grab-sample pH analyses are very important to make sure that the in-line probes/monitors are accurate.

Scrubber Slurry Density

Most systems utilize slurry sprays to contact the flue gas. As sulfur dioxide is absorbed and water evaporates, the slurry density increases. The slurry circulating pumps can only handle so much mass before electrical requirements are exceeded. Like pH, scrubbers are equipped with continuous density monitors, typically utilizing radioactive detectors. Again, the lab staff needs to monitor density on a grab sample basis to ensure the accuracy of the continuous instruments. A common method is use of a pycnometer, where the vessel is first filled with water and weighed, and then is rinsed and filled with slurry sample and weighed. The specific gravity ratio of the slurry vs. water is then compared to a chart or table, which gives the slurry density.

Solids Analyses

Equation 1 outlined the primary scrubbing reaction. Of course, as sulfur dioxide is removed from the flue gas, it becomes a solid material that must be handled and removed from the scrubber. In the absence of any other reactants, 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 greatly influences chemistry. 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 sulfate ions co-precipitates 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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Control of solids chemistry offers interesting challenges and is extremely critical to operation. Experience has shown that operation in either a completely oxidized state (no calcium sulfite-sulfate hemihydrate in the scrubbing slurry) or a completely un-oxidized state (no gypsum in the slurry) minimizes scaling in the scrubber. Scale buildups can be extremely problematic, as deposit formation on scrubber internals and subsequent gas flow restrictions may cause unit de-rates and even forced outages if gas flow is severely restricted.

A typical deciding factor on the choice of oxidized or non-oxidized byproduct involves the handling characteristics and commercial value of the solid. Calcium sulfite and calcium sulfite-sulfate hemihydrates are soft materials that tend to retain water. They have little value as a chemical commodity. For this reason, many scrubbers are equipped with forced-air oxidation systems to introduce additional oxygen to the scrubber slurry. A properly designed oxidation system will convert all of the liquid sulfite ions to sulfate ions. Sulfate of course precipitates with calcium as gypsum, which forms a cake-like material when subjected to vacuum filtration. In many cases, 85 to 90 percent of the free moisture in gypsum can be extracted by this relatively simple mechanical process. Low moisture is a common requirement of wallboard manufacturers. Removal of free moisture also reduces the chloride concentration of the byproduct, which is a critical aspect for wallboard production.

The technique that has proven itself very well for scrubber solids analysis is thermogravimetry. A thermogravimetric analyzer (TGA) is a quantitative not a qualitative instrument, so the operator needs to have a good idea of the primary constituents in the sample before analysis. If the sample compounds decompose at distinct and separate temperatures, it becomes easy to calculate the concentration of the original materials. Wet-limestone scrubber byproducts lend themselves well to this technique. The following equations illustrate the decomposition chemistry of wet-limestone FGD solids. The typical decomposition temperature ranges are also shown.

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Figure 1 illustrates a TGA analysis of a pre-dried scrubber solids sample containing all three of the major constituents. For the moment we will ignore the decomposition shown at 600 C. This will be addressed shortly.

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The calculations to determine original constituent concentrations are straightforward. The molecular weight of gypsum is 172 and that of the water forced out is 36, so the initial gypsum content is determined by multiplying the weight loss (5.772 percent) times a factor of 172 ÷ 36 (4.78). For calcium sulfite-sulfate hemihydrate, the factor is 131.9 ÷ 9 (14.6), where the mole ratio of calcium sulfite to calcium sulfate is assumed to be 85:15. For the calcium carbonate decomposition, the factor is 100.1 ÷ 44 (2.27). Thus, for the analysis shown in Figure 1, the gypsum content is 27.6 percent, the calcium sulfite-sulfate hemihydrate content is 12.0 percent and the calcium carbonate content is 22.3 percent.

This sample comes from a wet-limestone scrubber that serves the dual purpose of removing SO2 and flyash from the gas stream. The analytical procedure includes an isothermal step at 600 C, where the furnace atmosphere is temporarily switched from nitrogen to air. This allows any unburned carbon to combust, which otherwise would decompose during the final weight loss step. The effect is clearly illustrated by the vertical slope at 600 C in Figure 1. Following the isothermal step, the initial heating rate resumes and the furnace ramps up to 1000 C. The calcium carbonate decomposition then proceeds unmasked.

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Another valuable aspect of this technique is to determine the efficiency of the scrubbing process, commonly described as reagent utilization. Figure 2 shows an analysis of the byproduct from another FGD system with forced oxidation. As can be seen, the vast majority of the byproduct, nearly 96 percent, consists of gypsum, with just under 2 percent un-reacted calcium carbonate in the byproduct. This represents very good limestone utilization. Depending upon the size of the steam generator, and, of course, the back-end scrubber, each percentage increase in unused limestone can cost the utility thousands of dollars per year.

Author: Brad Buecker is a contributing editor for Power Engineering. He recently joined Nalco Co. as an Industry Technical Consultant having previously served as an Air Quality Control Specialist and Plant Chemist for Kansas City Power & Light Co. He has previous experience as a chemical cleaning services engineer, a water and wastewater system supervisor and a consulting chemist for Burns & McDonnell Engineers. He also served as a results engineer, flue gas desulfurization (FGD) engineer and analytical chemist for City Water, Light & Power, Springfield, Ill. Buecker has written more than 70 articles and columns on steam generation, water treatment and FGD chemistry, and he is the author of three books on steam generation topics published by PennWell Publishing, Tulsa, Okla. Buecker has an AA in pre-engineering from Springfield College in Illinois and a BS in chemistry from Iowa State University. He is a member of ACS, AIChE, ASME, and NACE.