Oxides of nitrogen (NOx) are a family of highly reactive gases that can be produced naturally, but largely result from fuel combustion (industrial combustion and automobile engines). In the environment, they are pollutants that react with volatile organic compounds in complex ways to produce ground-level ozone, and they also play a role in producing smog and acid rain. NOx formation occurs via different mechanisms: thermal NOx is based on temperature and makes up most of the NOx formed during combustion; fuel-bound NOx comes from nitrogen atoms contained fuels; and prompt NOx is formed when molecular nitrogen in the air combines with the fuel in fuel-rich conditions. This one-page reference outlines these mechanisms largely responsible for NOx formation in industrial combustion

Thermally formed NOx

Thermal NOx is formed by oxidation of N₂ in air and requires sufficient temperature and time to produce NOx. For fuels that contain no nitrogen in the parent molecules (for example, natural gas), this mechanism, also known as the Zeldovich mechanism, produces most of the NOx. This may be approximated by the following integrated rate expression:

where the brackets indicate the volume concentration of the enclosed species, A and b are constants, t is the total reaction time (with θ serving as the dummy variable in the integration), and T is the absolute temperature. So NOx from this mechanism depends on three primary quantities — temperature, oxygen concentration and reaction time — and minimizing any or all of them will reduce NOx. Since temperature is exponentially weighted, the peak flame temperature has an oversized role in NOx formation.

A rule of thumb is that below approximately 1,700K, the residence time in typical gas turbine combustors is not long enough to produce significant thermal NOx. Where temperatures higher than 1,700K cannot be avoided, it is necessary to limit residence time to control NOx formation, which favors very short combustor designs.

Fuel-bound mechanism

If a significant number of fuel molecules contain nitrogen bound in their structure, the overwhelming share of NOx will be formed from the fuel-bound mechanism, leading to a rate equation as shown in Equation (2):

where κ is a constant. Since the reaction is fast, and the fuel concentration is limited by the required stoichiometry, fuel-bound NOx formation may be reduced only by reducing the excess oxygen or switching to lower-nitrogen fuels.

Prompt NOx

The third NOx-formation mechanism is the Fenimore mechanism, also called prompt NOx. It is similar to the fuel-bound mechanism, except that the nitrogen comes directly from the combustion air. Since nitrogen radicals are exceptionally difficult to pare from molecular nitrogen, prompt NOx from this mechanism is usually negligible.

Reducing NOx in burners

Three burner configuration methods exist for reducing nitrogen oxides in burners: staged air, staged fuel and internal fluegas recirculation combined with staged air or staged fuel.

Staged-air burners. These types of burners work by introducing 100% of the fuel into the burner and only part of the combustion air (primary air), thus creating a sub-stoichiometric flame. This flame has a reduced temperature and therefore inhibits NOx formation. The flame is completed with the addition of the secondary air to complete the combustion process. This process allows for greater control at lower burner loads and also accommodates a wider range of fuels.

Staged-fuel burners. This burner method introduces 100% of the combustion air into the burner and splits the fuel supply into primary and secondary volumes. The primary fuel mixes with the combustion air to create a flame (Figure 1). As with staged-air burners, peak flame temperature is lower, and NOx formation is reduced. Secondary fuel is added to complete the combustion process. Staged-fuel burners provide greater NOx reduction, as the fuel supply has a larger effect on NOx formation. This method is more commonly used when a consistent fuel supply is available.

Internal fluegas-recirculation burners. This method combines either staged air or staged fuel with internal fluegas recirculation to help reduce NOx formation. The best results are obtained where internal fluegas recirculation is used to dilute the fuel gas in a staged-fuel burner, creating a gas with a low calorific value.

References

1. Stoeger, J., Burner Technologies and Concepts: Meeting Emissions-Reduction Goals, Chem. Eng., Sept. 2022, pp. 34–39.

2. Vij, A.D., Enclosed Combustion Equipment and Technology, Chem. Eng., January 2018, pp. 46–49.

3. Al-Hajji, M.H., Burner Inspection and Maintenance, Chem. Eng., November 2014, pp. 40–45.

4. Colannino, J., Low-Cost Techniques for NOx Reduction, Chem. Eng., May 2020, pp. 30–36.

5. Richards, G., Weiland, N. and Strakey, P., Combustion Strategies for Syngas and High-Hydrogen Fuel, in “Gas Turbine Handbook,” National Energy Technology Laboratory (NETL), 2006.