Wet limestone FGD solids analysis by thermogravimetry

Mar 18, 2023

Thermogravimetry (TGA) is an analytical method that can improve scrubber performance and eliminate time-consuming and often inaccurate wet-chemistry test methods.

By Brad Buecker, Buecker & Associates, LLC

In a recent article for Power Engineering [1], I wrote about the special properties of limestone, and especially high-purity stone, that have made it a common flue gas desulfurization reagent for numerous coal-fired power plants over the last five decades. Of course, in many areas of the world coal-fired power is being phased out, which makes flue gas desulfurization (FGD) chemistry moot to some readers. However, coal-fired power plants are still common in some countries. And, as I was reminded from feedback to the previous article, many metal-ore smelting facilities exist around the globe where a principal emission is gaseous sulfur dioxide. Wet-limestone FGD (WFGD) scrubbing is becoming a proven process for these applications. [2]

Accurate analysis of scrubber solids is critical for determining reaction efficiency and limestone utilization. This article outlines a technology that a colleague and I helped pioneer in the power industry way back in the late-1980s [3], but which may be unfamiliar to many plant personnel today. The technique is thermogravimetry (TGA). The analytical method allowed us to eliminate time-consuming and often inaccurate wet-chemistry test methods and improve scrubber performance. I brought the concept with me to a second coal-fired power plant, where TGA again proved its value. [4]

Analysis by thermogravimetry is simple in concept. A TGA functions by weighing samples on a precise analytical balance as they are heated. The main features of some designs are a top mounted balance, vertically supported sample pan, an automatically-operated furnace that raises for sample analysis and lowers when analysis is complete, an automatic sample loader, and a personal computer for instrument operation and data analysis. The furnace compartment typically has a port that allows samples to be analyzed in various atmospheres via bottled gases that are connected to the compartment by a manifold, tubing system, and automated sample-switching device. Nitrogen is common to provide an inert atmosphere that eliminates oxidation reactions that might occur with air. Additional discussion of this topic appears later in this document.

A 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 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 reader can refer to Reference 1 for a more detailed description of scrubber process chemistry.) The following equations illustrate the decomposition temperatures and chemistry of wet-limestone FGD solids.

CaSO4·2H2O –> CaSO4 + 2H2O↑­ (160oC to 200oC) Eq. 1

(CaSO3·CaSO4) ·½H2O –> CaSO3·CaSO4 + ½H2O­↑ (400oC to 430oC) Eq. 2

CaCO3 –> CaO + CO2↑­ (650oC to 800oC) Eq. 3

Figure 2 illustrates a TGA analysis of a pre-dried scrubber solids sample from Reference 4 containing all three of the major constituents listed above. For the moment, we will ignore the decomposition shown at 600oC. This will be addressed shortly.

The calculations to determine 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 2, 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 came from a wet-limestone scrubber that served the dual purpose of removing SO2 and the fly ash from the flue gas of a Cyclone boiler. The fly ash loading was much lower than it would have been for a pulverized coal unit, but nonetheless the unburned carbon effects upon the analyses were important. We first attempted to analyze the scrubber solids in a nitrogen atmosphere from beginning to end of the run, but found that volatile discharge and accompanying weight loss from the unburned carbon blended in with the calcium carbonate decomposition. So, we modified the procedure to introduce air to the furnace at 600oC. (We subsequently lowered the temperature to 500oC). The step was isothermal with a 20-minute hold time that allowed all volatile matter and carbon to burn away. The effect is clearly illustrated by the vertical slope at 600oC in Figure 2. Following the isothermal hold step, the computer automatically resumed sample heating to a final temperature of 1000oC. This step clearly separated the carbon vaporization from the calcium carbonate decomposition.

When this boiler and scrubber were installed, no plans were in place to produce a high-gypsum byproduct for potential sale. In part, this probably was because of the relatively poor-quality limestone in the area (CaCO3 content less than 90%), which would not have produced a high-purity byproduct. Instead, the scrubber solids were discharged as a slurry to large, lined holding ponds. Some readers may ask why byproduct analyses were needed at all with the slurry being discarded. Two answers emerged quickly. Initial TGA data indicated unused CaCO3 concentrations in the byproduct of 15-25%, as is evident in Figure 3. When we informed plant operators of this gross inefficiency, they adjusted limestone grinding mill settings to produce a finer reagent size. Subsequently, the unused limestone concentrations dropped to 5-10%, which translated into a large savings in limestone costs. Also, and again referring to Figure 3, a rise in unburned carbon content in the daily scrubber samples typically indicated a problem with one or more of the boiler's coal crushers. The lab technicians would usually detect such upsets before the boiler operators, so they made it a point to notify the operators immediately after seeing a sample weight loss due to carbon decomposition. This allowed for expedient adjustments of a poorly performing coal mill.

High-purity byproduct analyses

My initial work with TGA, which extended over half a decade, was with a forced-oxidation, wet-limestone scrubber that was designed to produce a byproduct suitable for sale to wallboard manufacturers. The required gypsum concentration was 94% or greater. Figure 3 is an analysis of the typical byproduct. (Like many other units in the US, the boiler and scrubber have been retired.)

TGA proved extremely valuable in helping plant chemists verify that the forced oxidation system was operating properly. For starters, the TGA data showed that the scrubber manufacturer had not installed sufficient oxidation capacity to convert all of the calcium sulfite (CaSO3) to CaSO4. Per language in the original contract, the supplier had to install an additional air compressor to ensure complete oxidation.

Following this correction, we periodically noticed an oxidation loss per TGA data. Investigation revealed that the high temperatures of the oxidation air would cause crystal deposition at the openings in the perforated air-injection laterals. As airflow to the slurry dropped the loss of oxidation efficiency was detectable due to an increase in (CaSO3·CaSO4)·½H2O. The unit manager had a water injection system installed to the lateral headers that lowered the oxidation air temperature and eliminated the scale formation.

In another very prominent example of the TGA importance at this plant, over a two-year period we performed several full-scale tests on limestones in close proximity to the plant to see if we could lower transportation costs of the material. All of these stones were of lower quality than the standard. Performance data confirmed that none were suitable as a replacement; a conclusion which in large part was confirmed by TGA results that showed a significant drop in byproduct gypsum content and a significant rise in unreacted CaCO3.

A Quirky Observation

For those contemplating use of a TGA for FGD solids analyses, a special point should be noted. The author discovered early on that calcium sulfite will break down at high temperatures when analyzed in an atmosphere devoid of oxygen. A colleague found a description of this chemistry in a somewhat obscure reference that I no longer have.

CaSO3 –> CaO + SO2­↑ Eq. 4

This phenomenon reappeared during subsequent work at my second utility. Figure 4 illustrates this effect.

The trained analyst can distinguish the end of the calcium carbonate decomposition and the beginning of the calcium sulfite decomposition. This transition is apparent at the shoulder in the decomposition pattern at 750oC. The calcium sulfite breakdown can be eliminated by analyzing the sample in air, and Figure 5 shows a duplicate sample as analyzed in an air atmosphere.

Rather than decompose, a portion of the calcium sulfite appears to oxidize to calcium sulfate at the high temperature, but I do not have actual confirmation of this chemistry.

Thermogravimetry is an excellent method for tracking wet-limestone FGD chemistry, especially when quick results are required, when checking for process upsets, and when making process changes. While its application in the coal-fired power industry may not be as valuable as in the past, it is a technique that may be valuable for other industries such as metal refining, where SO2 scrubbing may be an important process.

References

Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as Senior Technical Publicist with ChemTreat, Inc. He has over four decades of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company's (now Evergy) La Cygne, Kansas station. Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control. He may be reached at [email protected].