Exhaust Gas Economizer in Boiler is a critical heat-recovery component used in coal-fired thermal power plants to reduce flue-gas heat losses. In these plants, a major portion of heat energy is carried away by hot gases leaving the boiler. An exhaust gas economizer, commonly called a Flue Gas Economizer, is installed in the boiler’s flue gas outlet path and before the air preheater to capture waste heat and use it to preheat the feedwater. This recovered heat directly reduces the boiler’s firing rate, lowers fuel consumption, minimizes emissions and improves overall plant efficiency.

In thermal power plants, installing or optimizing an economizer typically improves boiler efficiency by 3–8%, depending on factors such as:

• Flue gas inlet temperature
• Feedwater temperature
• Tube design
• Ash characteristics
• Soot blowing effectiveness

This makes the economizer one of the most important heat-recovery devices in any boiler system.

Construction of Exhaust Gas Economizer in Boiler

A Flue Gas Economizer is a convective-type heat exchanger made of multiple rows of tubes arranged in the path of hot flue gases. The flue gas flows outside the tubes, while boiler feedwater flows inside the tubes, allowing heat to transfer through the tube metal.

Key Components of Economizer:

1. Tube Banks: Rows of steel tubes where feedwater flows inside and flue gas flows outside. These tubes transfer heat from the gas to the water.

2. Inlet Header: A pipe-like chamber that receives cold feedwater and distributes it evenly into all the economizer tubes.

3. Outlet Header:  Collects the heated feedwater from the tubes and sends it toward the boiler drum.

4. Casing: A metal enclosure around the economizer that directs the flue gas flow over the tubes.

5. Support Structure:  Hangers, beams, and frames that hold the tube bundles securely and allow for thermal expansion.

6. Soot Blowers: Steam or air nozzles used to remove ash and soot deposits from the tube surfaces during operation.

7. Tube Shields (if required):  Stainless steel plates installed on the front of tubes in high-erosion or high-wear areas.

8. Insulation: Installed around the casing to prevent heat loss and protect surrounding equipment.

Working Principle of Exhaust Gas Economizer in Boiler

The Flue Gas Economizer works on the principle of convective heat transfer. Hot flue gases leaving the boiler pass over the outside of the economizer tubes, while boiler feedwater flows inside them. As the gases move across the tube surface, heat is transferred through the tube metal into the feedwater.

How It Works (Step-by-Step)

1. Hot flue gases enter the economizer section after leaving the boiler’s main heating surfaces.
2. These gases flow across the outside of multiple tube banks arranged in rows.
3. Feedwater enters the economizer inlet header and is distributed into the tubes.
4. As flue gas moves over the tube surfaces, heat passes through the tube walls and warms the water inside.
5. The feedwater temperature increases significantly before it reaches the boiler drum or next stage.
6. Cooled flue gases then move to the air preheater or directly toward the stack.

By raising feedwater temperature:

• The boiler requires less heat input, reducing coal consumption.
• Stack losses decrease, improving thermal efficiency.
• Boiler operation becomes more stable and efficient.

Heat Recovery & Efficiency Improvement

Efficiency Gain

For conventional coal-fired power plants: 3–8% improvement in boiler efficiency

Rule of Thumb

For every 22°C drop in flue gas temperature → approx. 1% fuel saving

Basic Heat Recovery Formula

Advantages of Economizer in Thermal Power Plants

1. Fuel Saving – Reduced furnace heat duty lowers coal consumption.

2. Increased Boiler Efficiency – Typically 3–8%, depending on design.

3. Reduced Emissions – Lower coal firing → less CO₂, NOₓ, and SO₂.

4. Reduced Thermal Stress – Higher feedwater temperature means stable drum condition.

5. Improved Heat Transfer in Other Sections – Lower flue gas temp entering APH improves air preheating efficiency.

Problems, Causes and Solutions of Exhaust Gas Economizer in Boiler

🔷 Problem:  Soot Fouling / Ash Deposition

Cause: High ash coal, Poor combustion & high unburnt carbon, Inadequate soot blowing, High α-quartz content in ash (abrasive)

Solution:

• Use long retractable soot blowers
• Maintain coal fineness
• Optimize secondary/tertiary air distribution
• Regular water-wash during outages

🔷 Problem: Acid Dew Point Corrosion (H₂SO₄ Attack)

Cause:

• Sulfur in coal → SO₂ → SO₃ → H₂SO₄
• Metal temperature < acid dew point (~140–160°C)
• Moisture during shutdowns

Solution:

• Ensure feedwater inlet temperature is > 120°C
• Avoid cold-end condensation
• Dry the economizer immediately after water-wash
• Improve combustion to reduce SO₃ formation

🔷 Problem: External Erosion

Cause:

• High-velocity flue gas laden with ash
• CFBC boilers with cyclone issues
• High fines (<1 mm) in coal

Solution:

• Install SS 304H erosion shields
• Correct cyclone performance
• Control coal size & ash flow patterns
• Reduce localized gas velocities

🔷 Problem: Oxygen Pitting / Water-Side Corrosion

Cause: Poor deaeration, high DO, low pH
Solution:

• Maintain DO < 7 ppb
• Add oxygen scavengers (e.g., hydrazine/sulfite)
• Maintain feedwater pH 8.8–9.2

🔷Problem: Flow Accelerated Corrosion (FAC)

Cause: High velocity inside tubes, Low pH, inadequate chemistry

Solution: Maintain pH in the recommended range, Control feedwater flow velocity

🔷Problem: Tube Rupture (Localized Thinning)

Cause:

• Erosion + corrosion combined
• Overheating due to deposits
• Poor welding quality

Solution:

• Quarterly UT thickness survey
• Use proper welding procedures (ASME/IBR)
• Replace degraded tubes promptly

🔷 Problem: Thermal Shock

Cause:

• Rapid startup/shutdown
• Sudden feedwater temperature changes

Solution:

• Controlled boiler warm-up schedule
• Avoid cold feedwater during startup

Cleaning & Maintenance Practices of Exhaust Gas Economizer in Boiler

A. Online Cleaning (During Operation)

• Regular sootblowing with dry, superheated steam
• Monitor stack temperature for fouling indication

B. Offline Water-Wash 

Procedure:

• Use warm alkaline solution + trisodium phosphate
• Maintain drain pH 5–11
• Wash top rows first, then lower rows
• Remove ash lumps from hopper
• Dry the tubes by firing immediately after wash

C. Routine Maintenance

• Daily: monitor ΔP, stack temp, soot blowers
• Monthly: inspect headers, casing, seepage
• Quarterly: UT thickness measurement
• Annually: deep cleaning + tube replacement as required

Example of Fuel Saving with an Economizer (Simple Calculation)

Assume a 600 MW thermal power plant has:

• Coal Consumption: 420 TPH
• Flue gas Economizer inlet temperature: 300°C
• Flue gas Economizer outlet temperature: 200°C
• Temperature drop of flue gas due to economizer: 100°C

Note: Rule of thumb: Every 22°C drop in flue gas temperature = 1% fuel saving

Step 1: Calculate percentage fuel saving

Fuel Saving %= 100 / 22 = 4.5 %

So the economizer gives 4.5% fuel saving

Step 2: Convert % fuel saving to coal saving

Coal saved per hour: coal consumption per hour x fuel saving % = 420 x 0.045 = 18.9 ton per hour

Coal saved per day: 18.9 x 24 = 453.6 ton per day

Coal saved per year: 453.6 x 365 = 165564 ton per year

Step 3: Cost saving

Assume the Coal cost: ₹5,000 per ton (example)

Cost Saving per year = Coal saved per year x Coal cost= 165564 x 5000 = Rs 827820000 = 82.78 crore