Steam boilers: how to reduce nitrogen oxides (NOx) emissions?

In recent years, one of the concerns that industries are dealing with the most is the environmental issue and how we can help to reduce pollution. Reducing nitrogen oxide (NOx) emissions in steam boilers and industrial burners can be one of our greatest allies in combating environmental impact.

Boiler NOx, which is composed almost entirely of nitric oxide (NO) leaving the stack and usually 1% or 2% of nitrogen dioxide (NO2), has been extensively studied and analyzed with the intention of developing control measures to help reduce NOx emissions while maintaining the thermal efficiency of industrial boiler systems.

Thanks to this research, new special design techniques have been developed for new industrial burners and boilers for thermal power plants to reduce their pollutant emissions.

One such technique is Forced Internal Recirculation (FIR), which combines premixed combustion with partial internal recirculation of combustion gases and the state of the air. In this way it is possible to control the combustion-air mixture of industrial boiler systems achieving:

• To achieve a stable and uniform combustion.
• Minimize maximum flame temperatures.
• Avoid the formation of high oxygen pockets.
• Improve heat transfer to reduce combustion temperatures.

It also reduces NOx formation while maintaining good burning characteristics of the fuel. An example of this is the conventional burners that require a 35% FIR to achieve NOx levels below 9 ppm.

FIR or FGR for NOx reduction in steam boilers?

Forced Internal Recirculation (FIR) is more energy efficient than competitive burners that use high levels of FGR, low excess air or water/steam injection. In addition, energy penalties of competitive methods include higher stack losses and fan power requirements.

However, for existing plants, the FGR (Flue Gas Recirculation) method offers the most favorable economic conditions and potentially the highest emission reduction. When using FGR, NOx formation is reduced by diluting the gas-air mixture and thus lowering the flame temperature.

The flue gases must be mixed with the combustion air passing through the industrial burners, so flue gas recirculation through the bottom of the boiler has little effect on NOx formation. In fact, studies have shown reductions of over 50% in emissions with flue gas recirculation of between 20 and 40%.

Flue gas recirculation through the bottom of the combustion chamber was quite ineffective in reducing NO formation * the reduction was only about 15% to 20% with O2 levels below about 0.8%, flue gas recirculation had very little effect on NOx formation.

These results represent the theory that the flue gas must be mixed in the flame to be effective and, when the mixture is good, the flue gases will serve as a diluent, thus reducing the flame temperature peak and NOx formation.

In addition, experience suggests that reducing NOx emissions from modern industrial burners to 30 ppm or less, corrected to 3% O2 as currently required by some countries in their air quality, requires reducing the average oxygen content of the burner air to 18% (fresh air contains 20.9%O2). Knowing that and the oxygen content of the process flue gas, you can determine the percentage of FGR needed.

If you want more information about our industrial burners and NOx reduction in steam boilers, get in touch with us Please contact us and we will be happy to advise you.