The Thermo Scientific™ ARL™ QUANT’X Energy-Dispersive X-ray Fluorescence (EDXRF) Spectrometer is employed in quality control of limestone used in coal-fired power plants to perform flue gas desulfurization (FGD). The article first gives a short introduction on FGD and then concentrates on sample preparation, measurement conditions, as well as precision and accuracy of EDXRF for the quantification of major and minor constituents of limestone.
Industry Background
What is Flue Gas Desulfurization?
A flue gas desulfurization (FGD) processing unit, generally known as a scrubber, eliminates sulfur dioxide (SO2) from the exhaust flue gases in power plants in which coal is burnt. Airborne SO2 forms sulfuric acid (H2SO4) by oxidizing within airborne water droplets, resulting in “acid rain”. Due to the stringent environmental emissions regulations for SO2 in countries across the globe, technologies for SO2 removal and related analysis continue to develop.
How Does a Scrubber Work?
During the combustion process, SO2 is formed due to the reaction of sulfur in the coal with oxygen in the air. In order to eliminate the SO2, the flue exhaust gases from a coal-fired power plant are usually passed through a spray of limestone slurry in the scrubber. The resultant reaction typically traps over 90% of the SO2. Nearly 80%–85% of the global FGD units set up in power plants use wet limestone scrubbing compared to other technologies.
What Happens to the SO2 Captured in a Scrubber?
The SO2 trapped in the scrubber mixes with the limestone slurry to produce CaSO3 (and CO2), which is further oxidized to the key byproduct — calcium sulfate (CaSO4) usually known as gypsum. Gypsum is recyclable or profitable for several industrial applications; the gypsum produced by a power plant and left unused is disposed of in permitted landfills together with byproducts of scrubber that are not reusable.
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Figure 1. Wet scrubber using a limestone slurry to remove sulfur dioxide from flue gas.
Table 1. Concentration ranges and required reproducibility.
| Element/Oxide |
Concentration |
Required Std. Dev |
| CaCO3 |
> 95.0 % |
0.10 |
| SiO2 |
< 3.0 % |
0.05 |
| MgCO3 |
< 2.0 % |
0.05 |
| Al2O3 |
< 1.5 % |
0.05 |
| Fe2O3 |
< 3.0 % |
0.03 |
X-Ray Fluorescence (XRF) Application
The quality of the limestone used for scrubbing can be established by means of element/oxide analysis — an ideal application for XRF. Table 1 shows typical ranges and reproducibilities of interest for limestone analysis.
Instrumentation
An ARL QUANT’X XRF Spectrometer has been employed to obtain limits of detection and precision for the analysis of limestone. The ARL QUANT’X Spectrometer is an EDXRF system that offers a cost-effective and quick analytical potential. It is equipped with an air-cooled Rh end-window tube with a slender Be window (0.05 mm) and has a maximum power of 50 W. The ARL QUANT’X Spectrometer is fitted with an electrically cooled silicon drift detector (SDD) which has an area of 30 mm2. The instrument includes a total of nine primary beam filters, guaranteeing that an optimal excitation condition is found at all times. An optional 10-position sample changer enables unattended analysis.
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Sample Preparation and Analytical Conditions
In sample preparation, the powder is ground to obtain a particle size of ~325 mesh, and then, the sample is pressed into a pellet with a diameter of 40 mm at a pressure of 200 kN.
The two excitation conditions used are represented in Table 2. The first condition excites the lightest elements Mg, Al, and Si, whereas the second condition excites Ca and Fe. In order to guarantee a maximum count rate with no saturation of the detector, the current is adjusted for each sample. A total live measurement time of 150 s was taken. All measurements were conducted in vacuum.
Table 2. Excitation conditions used for limestone analysis
| Condition |
Voltage (kV) |
Filter |
Atmosphere |
Live Time (s) |
Elements |
| Low Za |
4 |
None |
Vacuum |
120 |
Mg, Al, Si |
| Mid Za |
18 |
Thin Pd |
Vacuum |
30 |
Ca, Fe |
Calibration and Repeatability
With the help of certified reference materials, calibration curves have been created for the five elements of interest (Ca, Mg, Si, Al, Fe) in limestone. Figures 1, 2, and 3 illustrate the resulting calibrations for magnesium, calcium, and iron, respectively.
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Figure 2. Calibration curve for magnesium in limestone.
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Figure 3. Calibration curve for calcium in limestone.
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Figure 4. Calibration curve for iron in limestone.
An instrument repeatability test was performed by running a sample for 10 repeat analyses without replacing the sample. Table 3 summarizes the results.
Table 3. Repeatability for element oxides in pressed limestone powder
| Run # |
MgO |
Al2O3 |
SiO2 |
CaO |
Fe2O3 |
| % w/w |
% w/w |
% w/w |
% w/w |
% w/w |
| Run 1 |
0.789 |
0.2014 |
0.977 |
53.89 |
0.1045 |
| Run 2 |
0.795 |
0.2015 |
0.978 |
53.96 |
0.1048 |
| Run 3 |
0.798 |
0.2020 |
0.985 |
54.00 |
0.1034 |
| Run 4 |
0.799 |
0.2016 |
0.986 |
54.01 |
0.1044 |
| Run 5 |
0.798 |
0.2008 |
0.982 |
54.04 |
0.1050 |
| Run 6 |
0.793 |
0.1990 |
0.983 |
54.07 |
0.1052 |
| Run 7 |
0.801 |
0.2005 |
0.982 |
54.07 |
0.1037 |
| Run 8 |
0.795 |
0.2024 |
0.981 |
54.12 |
0.1050 |
| Run 9 |
0.807 |
0.2010 |
0.985 |
54.15 |
0.1048 |
| Run 10 |
0.795 |
0.2004 |
0.984 |
54.09 |
0.1037 |
| Average |
0.797 |
0.2010 |
0.982 |
54.04 |
0.1045 |
| SD |
0.005 |
0.0009 |
0.003 |
0.08 |
0.0006 |
| RSD (%) |
0.62 |
0.47 |
0.30 |
0.15 |
0.61 |
Conclusion
With the ARL QUANT’X Spectrometer, exceptional calibration curves and repeatabilities can be realized to determine the suitability of limestone minerals that are used in power plant scrubbers. The method is fast as well as ideal for multiple oxides of interest at both low (below 0.25%) and high (nearing 100%) concentration levels.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.
For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.