Thermogravimetric Analysis of Flue Gas Desulfurization (FGD)

Significance of the Topic

Determining the composition of flue gas desulfurization (FGD) gypsum is critical for quality control in cement and building material industries. TGA enables precise quantification of moisture, hydrates, carbonates and other components by monitoring mass changes under controlled heating. This information ensures product consistency and compliance with industrial standards.

Objectives and Study Overview

This study presents a TGA-based method to characterize FGD gypsum. Key goals include quantifying:

• Free moisture
• Calcium sulfate dihydrate
• Calcium sulfite hemihydrate
• Calcium hydroxide
• Carbonate content
• Ash and fly ash fractions

The procedure employs a multi-step temperature program to isolate each component's mass change.

Methodology and Instrumentation

Samples of ~1 g ground to <0.5 mm were analyzed with a LECO TGA701 system under a programmed atmosphere. Key parameters:

• Atmospheres: predominantly nitrogen with an oxidation step in oxygen
• Heating profile: incremental ramps from 25 °C to 950 °C in defined steps
• Step durations: hold times to ensure complete decomposition or transitions
• Balance sensitivity: 0.0005 g readability, stable environment

Used Instrumentation

• LECO TGA701 Thermogravimetric Analyzer
• Ceramic crucibles and covers

Main Results and Discussion

Typical analyses of NIST SRM 2429 and gypsum purity standards yielded:

• Dihydrate (CaSO4·2H2O): ~97.6 %
• Sulfite hemihydrate (CaSO3·0.5H2O): ~2.5 %
• Calcium hydroxide: ~0.08 %
• Calcium carbonate equivalent: ~79.2 %
• Total ash: ~80 %
• Fly ash fraction: <0.01 %

Results demonstrated excellent repeatability (standard deviations <0.1 %). The multi-step TGA approach effectively isolates overlapping decomposition events for accurate quantification.

Benefits and Practical Applications

• Comprehensive compositional profile in a single analysis
• High sensitivity to minor constituents
• Improved quality control in gypsum-based materials
• Support for process optimization in FGD operations

Future Trends and Possibilities

Advances may include coupling TGA with evolved gas analysis (FTIR or MS) for real-time speciation, automated data processing for faster throughput, and development of standardized methods for regulatory compliance.

Conclusion

The described TGA method on the LECO TGA701 provides a robust and reproducible protocol for detailed characterization of FGD gypsum. Its ability to distinguish closely overlapping thermal events makes it a valuable tool for industrial quality assurance.