Image: Flue gas damper / guillotine damper (site archive)

Under ultra-low emission requirements, SCR denitrification systems in coal-fired power plants must operate reliably across the full load range. At low loads, however, the economizer absorbs proportionally more heat, and the flue gas temperature at the SCR inlet falls below the catalyst's minimum ammonia-injection temperature (about 300 °C), forcing the denitrification system offline and pushing NOx emissions above limits. Lifting the SCR inlet temperature back above 300 °C at low loads is the core of "full-load denitrification" retrofit — and the flue gas guillotine damper (bypass duct damper) is the key enabling component in the most widely used solution.

1. Main Technical Routes for Full-Load Denitrification

Routes for raising the low-load SCR inlet temperature include: economizer gas bypass, economizer sectioning, feedwater bypass (water-side substitution), No. 0 high-pressure heater, and duct supplementary firing. Among them, the economizer gas bypass requires relatively modest retrofit work and offers direct control, making it one of the most common engineering choices: part of the flue gas bypasses the economizer, reducing its heat absorption and raising the gas temperature at the economizer outlet — i.e., the SCR inlet.

2. Two Typical Bypass Arrangements

· Tap at economizer inlet: a bypass duct is taken from the economizer inlet and merges downstream at the economizer outlet — hot gas skips the economizer and mixes with the main flue stream;
· High-temperature gas injection: gas is drawn from the furnace or another high-temperature zone into the duct ahead of the SCR.

The operating logic is the same: at low loads the bypass guillotine damper opens, increasing the hot bypass flow and raising the SCR inlet temperature; at high loads it closes so all gas passes through the economizer, preserving boiler efficiency. Field results show that, combined with hot-water recirculation and similar measures, this scheme can raise the economizer outlet temperature from about 250 °C to above 300 °C (a typical retrofit case: 249.9 °C → 301.8 °C), meeting full-load ammonia-injection requirements. Designs generally require boiler-efficiency loss of no more than 1 percentage point with the bypass open.

3. Key Technical Requirements for the Guillotine Damper

· High-temperature resistance and deformation control: bypass gas is hot — the body and blade must withstand high temperatures with controlled thermal deformation, remaining free-moving without jamming;
· Tight closure: at high loads the bypass must seal tightly — any leakage creates a hot-gas short circuit that directly affects boiler efficiency and temperature-field uniformity;
· Reliable, remotely interlocked operation: the damper cycles with load, requiring a dependable electric actuator with position feedback, interlocked with the unit load control for automatic operation;
· Maintainable sealing: elastic seal strips should be inspectable and replaceable, with link mechanisms adjustable for stroke.

4. Industry Trend

According to GEP Research's 2026 report, about 15% of new or retrofitted coal-fired plant projects adopt integrated desulfurization–denitrification–dust-removal units, with multi-pollutant synergistic removal becoming the trend. Full-load denitrification retrofits combined with FGD upgrades mean dampers and guillotine doors in flue ducts must handle wider load ranges and more complex gas conditions — demanding higher sealing performance and reliability.

We manufacture flue gas dampers, guillotine dampers and isolation gates with electric actuators for remote interlocked control, customizable for full-load denitrification and FGD retrofit duties — contact us for selection advice.

References: "Research and Application of Full-Load Denitrification Technology for Coal-Fired Boilers" (Zhejiang Electric Power), "Retrofit Design of Gas-Temperature-Adjusting Bypass for SCR Operation at Low Loads" (China Electric Power), GEP Research "Global and China Coal Power FGD Industry Development Research Report (2026)" and other public sources, compiled by this site.