A Practical Guide to Gas-Gas Heaters in FGD Systems

In modern emission control systems, especially in Flue Gas Desulfurization units, the GGH system (Gas-Gas Heater) plays a critical role in both energy recovery and corrosion control.

It is not just a conventional heat exchanger—it is a key component that directly affects system reliability, environmental compliance, and long-term operating cost.

1. What is a GGH System?

A Gas-Gas Heater (GGH) is a heat recovery device designed to transfer heat between:

• Hot untreated flue gas (before the absorber)
• Cold, saturated clean gas (after wet desulfurization)

Its main function is to reheat the cleaned flue gas before it is discharged through the stack.

2. Why GGH is Necessary in FGD Systems

After wet FGD treatment, flue gas typically has the following characteristics:

• Low temperature (approximately 45–60°C)
• High moisture content (saturated condition)
• High risk of acid condensation

If discharged directly, this can lead to:

• Severe chimney corrosion
• Visible white plume emissions
• Poor flue gas dispersion
• Increased environmental compliance risks

A GGH system solves these issues by:

• Raising flue gas temperature (typically to 80–120°C)
• Preventing acid dew point corrosion
• Improving plume lift and dispersion
• Reducing visible emissions

3. Working Principle of a GGH System

A typical GGH operates through a heat exchange process:

1. Hot raw flue gas enters the GGH and transfers heat to the heating element
2. The cooled gas flows into the absorber (scrubber)
3. Clean but cold gas exits the absorber
4. This cold gas passes through the GGH and absorbs heat
5. Reheated gas is discharged through the stack

4. Main Types of GGH Systems

4.1 Rotary GGH

Structure:

• Rotating heat storage wheel
• Divided into sectors for gas flow

Advantages:

• High heat transfer efficiency
• Compact design

Limitations:

• Leakage between clean and dirty gas
• Seal wear and maintenance issues
• Higher operational complexity

4.2 Tubular GGH

Structure:

• Fixed tube bundle (similar to shell-and-tube heat exchangers)

Advantages:

• No gas leakage
• Better corrosion resistance
• Stable and reliable operation

Limitations:

• Larger footprint
• Slightly lower efficiency compared to rotary designs

In recent years, many projects—especially international ones—prefer tubular GGH due to its reliability.

4.3 Heat Pipe GGH

Structure:

• Uses sealed heat pipes for heat transfer

Advantages:

• Complete isolation between gas streams
• Excellent corrosion resistance
• No cross-contamination

Typical Applications:

• High-corrosion environments
• High SO₃ conditions

5. Key Design Considerations

When preparing a technical proposal or quotation, the following factors are critical:

5.1 Acid Dew Point

• Determines minimum outlet temperature
• Typically ranges from 110–140°C depending on SO₃ content

5.2 Material Selection

Common material options include:

• Carbon steel with enamel coating
• ND steel (acid dew point resistant steel)
• Stainless steel (e.g., 316L, 904L)

Material selection directly impacts service life and maintenance cost.

5.3 Fouling and Cleaning

FGD environments often involve:

• Dust accumulation
• Ammonium bisulfate (ABS) formation
• Scaling

Typical solutions:

• Soot blowers
• Steam or water washing systems

5.4 Pressure Drop

GGH design must control system resistance:

• Typical range: 800–1500 Pa
• Excessive pressure drop increases fan energy consumption

5.5 Leakage Control (for Rotary GGH)

Key factors include:

• Seal design
• Pressure balance
• Sector isolation

Leakage is one of the most common operational issues in rotary systems.

6. Is GGH Always Required?

Some projects consider eliminating the GGH system to reduce initial investment.

Alternative solutions include:

• Wet stack with corrosion-resistant lining
• Bypass reheating systems
• Electric reheaters

However, these options often result in:

• Higher operating costs
• Increased corrosion risks
• More visible emissions

For most large-scale industrial and power plant applications, GGH remains the preferred solution.

7. When Should a GGH Be Upgraded?

A GGH retrofit or upgrade should be considered if:

• There is severe fouling or blockage
• Frequent shutdowns are required for cleaning
• Leakage is detected (especially in rotary systems)
• Chimney corrosion is observed

Typical upgrade approaches include:

• Replacing rotary GGH with tubular GGH
• Upgrading materials (e.g., ND steel or enamel coating)
• Adding online cleaning systems

8. Conclusion

A GGH system is not just an auxiliary component—it is a critical element that ensures:

• Efficient energy utilization
• Long equipment service life
• Stable environmental performance

A well-designed GGH improves overall system reliability.
A poorly designed one can become the most maintenance-intensive part of the FGD system.

From an engineering and commercial perspective, GGH should be treated as a core system component rather than a secondary accessory.