What is FGR technology for boilers? – Industrial Boiler

Summary

Flue Gas Recirculation (FGR) is a straightforward yet effective technique in boiler combustion control that redirects a controlled fraction of exhaust gases back into the burner air stream to temper flame temperatures, thereby curbing nitrogen oxide (NOₓ) formation and fine-tuning combustion performance.

Principle of FGR

In an FGR system, typically 5–25% of the boiler’s flue gases—composed mainly of CO₂, H₂O, and N₂—are extracted downstream of the economizer and reintroduced at the burner inlet through insulated ductwork equipped with a modulating damper. This inert gas mix dilutes the incoming air-fuel charge, reducing the oxygen concentration and overall heat capacity, which lowers the peak flame temperature and slows the combustion rate. As NOₓ formation increases exponentially with temperature, even a modest 7% drop in flame temperature achieved by 10% FGR can cut NOₓ emissions by up to 50%.

Key Benefits

Significant NOₓ Reduction

By tempering combustion temperatures, FGR routinely slashes thermal NOₓ production by 30–50%, helping boilers meet stringent emissions standards without exotic catalysts.

Enhanced Combustion Stability and Efficiency

Recycled flue gases preheat the combustion air, recovering waste heat and promoting more uniform flame propagation; while slight efficiency penalties can occur at high recirculation rates, proper tuning maintains or even marginally improves overall boiler efficiency.

Flexible Retrofit and Control

Both external and internal FGR configurations exist: external systems use dedicated ducting and fans, whereas internal FGR leverages burner head geometry to draw back gases automatically. Either approach integrates with existing forced-draught burners and combustion management systems, enabling simple installation and precise modulation of recirculation rates.

With its low capital cost, reliable performance, and compliance support for evolving air-quality regulations worldwide, FGR remains a cornerstone technology for NOₓ control and boiler optimization.