NOx formation during combustion is mainly caused by high-temperature reactions between nitrogen and oxygen, together with the nitrogen contained in certain fuels.
In industrial combustion systems, NOx formation is influenced by several factors, including:
Flame temperature
Oxygen concentration
Residence time
Fuel composition
Fuel-air mixing
Combustion staging
Furnace atmosphere
Burner design
For natural-gas-fired industrial burners, thermal NOx is often the dominant formation mechanism, particularly when combustion produces localized high-temperature zones.
A simplified relationship is:
High Temperature + Oxygen + Nitrogen + Sufficient Residence Time
↓
NOx Formation
Understanding this mechanism is essential when designing or selecting Low NOx burners.
NOx is a general term for nitrogen oxides generated during combustion.
The two most important compounds are:
Nitric oxide (NO)
Nitrogen dioxide (NO₂)
In many high-temperature combustion processes, NO is formed first and can subsequently be oxidized to NO₂.
NOx is therefore not a single substance produced by one reaction. It is the result of several chemical pathways that can occur under different combustion conditions.
Combustion air contains a large amount of nitrogen.
Although nitrogen is relatively stable under normal conditions, very high temperatures provide sufficient energy for nitrogen and oxygen to participate in reactions that would otherwise occur very slowly.
The basic concept is:
N₂ + O₂
↓
High-Temperature Reaction
↓
NO
↓
Further Oxidation
↓
NO₂
The higher the temperature and the more favorable the local combustion conditions, the greater the potential for NOx formation.
NOx formation during combustion is generally divided into three major mechanisms:
Thermal NOx
Prompt NOx
Fuel NOx
For industrial natural-gas combustion, thermal NOx is often the most important mechanism.
However, the relative contribution of each mechanism depends on the fuel and combustion conditions.
Thermal NOx is formed primarily from the nitrogen and oxygen present in combustion air at high temperatures.
It is strongly associated with:
High flame temperature
Oxygen availability
Residence time
Local combustion conditions
Thermal NOx becomes particularly important when combustion creates extremely hot regions.
This is why a burner can produce significant NOx even when the overall furnace temperature appears relatively moderate.
The key factor is often the local peak flame temperature, not simply the average furnace temperature.
Temperature has a strong influence on the rate of thermal NOx formation.
When the flame temperature becomes very high, nitrogen and oxygen reactions become much more active.
For example:
Moderate Flame Temperature
→ Relatively Lower Thermal NOx Formation
Very High Local Flame Temperature
→ Rapidly Increased Thermal NOx Formation
This is one of the fundamental reasons why Low NOx burners are designed to control peak flame temperature.
Importantly, reducing NOx does not necessarily mean lowering the required process temperature.
The objective is often to maintain the required furnace temperature while avoiding excessively hot local flame zones.
Prompt NOx forms through reactions involving hydrocarbon fragments and atmospheric nitrogen in the early region of a flame.
It is associated particularly with the initial flame chemistry near the fuel-rich reaction zone.
Prompt NOx is generally more significant in certain combustion conditions than in others, and its contribution can depend on:
Fuel composition
Flame structure
Mixing
Temperature
Local equivalence ratio
For many industrial natural-gas burners operating at high temperatures, thermal NOx is usually the more important design consideration.
Fuel NOx originates from nitrogen compounds contained in the fuel itself.
When fuel contains chemically bound nitrogen, some of that nitrogen can be converted into nitrogen oxides during combustion.
This mechanism is particularly relevant for fuels with higher nitrogen content.
For relatively clean natural gas, fuel NOx is generally much less significant than thermal NOx.
The fuel composition therefore matters when evaluating NOx formation.
Oxygen availability influences NOx formation.
When sufficient oxygen is present in a high-temperature region, nitrogen-oxygen reactions can proceed more readily.
However, this does not mean that simply reducing combustion air is always a good NOx-reduction strategy.
Too little air can cause:
Incomplete combustion
Higher CO
Flame instability
Unburned fuel
Therefore, burner design needs to balance:
NOx
CO
Flame Stability
Combustion Efficiency
Residence time refers to how long combustion gases remain under conditions favorable for NOx formation.
If gases remain at very high temperatures for longer periods, there is more opportunity for nitrogen and oxygen reactions to occur.
Therefore:
High Temperature + Long Residence Time
can increase NOx formation.
Low NOx burner designs can influence this by controlling flame structure, gas recirculation, mixing, and heat-release distribution.
Fuel-air mixing determines the local combustion environment.
If fuel and air mix rapidly and create a concentrated high-temperature flame, localized NOx formation can increase.
Controlled mixing can distribute the combustion process more gradually.
This is why burner geometry is important.
Factors such as:
Fuel velocity
Air velocity
Injection angle
Swirl
Mixing location
Recirculation
can all influence the resulting flame structure.
The burner determines how fuel and air enter the furnace and how combustion develops.
A burner can influence:
Flame temperature
Oxygen distribution
Mixing rate
Flame length
Flame shape
Recirculation
Heat-release distribution
A conventional burner may produce a concentrated high-temperature flame.
A Low NOx burner may instead use:
Staged combustion
Internal FGR
External FGR
Controlled mixing
Premixed combustion
Distributed combustion
to modify the combustion environment.
Staged combustion separates the combustion process into different stages.
Instead of immediately supplying all of the oxygen required for complete combustion, the burner controls when and where fuel and air react.
A simplified concept is:
Primary Combustion
↓
Controlled-Oxygen Zone
↓
Secondary Air
↓
Combustion Completion
This can reduce the formation of extremely hot oxygen-rich regions.
The objective is to maintain complete combustion while reducing peak temperature.
Flue Gas Recirculation, or FGR, returns part of the combustion exhaust to the combustion process.
The recirculated gas dilutes the fresh combustion mixture and increases its effective heat capacity.
This can:
Reduce oxygen concentration
Reduce peak flame temperature
Modify flame structure
Reduce thermal NOx formation
FGR can be implemented internally within the burner or through an external recirculation system.
A common misconception is:
"If the furnace temperature is high, NOx must be high."
This is not necessarily true.
Two furnaces may operate at the same process temperature but have very different NOx emissions.
Why?
Because the burner flame may have different:
Peak temperatures
Oxygen concentrations
Mixing patterns
Residence times
Recirculation levels
For example:
Furnace A
High local flame temperature → Higher NOx potential
Furnace B
Controlled flame temperature → Lower NOx potential
Both may maintain the same required process temperature.
Fuel type is only one part of the combustion equation.
Two natural-gas burners can have different NOx emissions because of differences in:
Burner geometry
Fuel-air mixing
Flame temperature
Air staging
Fuel staging
FGR
Flame shape
Operating conditions
This is why Low NOx performance is fundamentally related to combustion-system design, not simply fuel selection.
Furnace geometry can influence the way the flame develops.
Important parameters include:
Furnace volume
Chamber dimensions
Burner position
Burner angle
Exhaust position
Product location
Furnace pressure
A burner designed for a large furnace may behave differently when installed in a compact chamber.
Poor matching between burner and furnace geometry can create:
Local hot spots
Excessive wall temperatures
Uneven combustion
Unexpected NOx performance
Therefore, burner selection should consider the entire furnace.
When multiple burners are used, their flames can interact.
If burners are positioned too close together or their flame directions are poorly coordinated, they can create overlapping high-temperature regions.
This may affect:
Flame temperature
Heat distribution
Furnace-wall temperature
NOx formation
Burner number, spacing, angle, and firing strategy should therefore be considered together.
Not necessarily.
Excess air affects oxygen availability, flame temperature, heat transfer, and exhaust losses simultaneously.
Increasing excess air can sometimes reduce flame temperature because additional air absorbs heat. However, it can also increase oxygen availability and exhaust losses.
The actual NOx response depends on the combustion system.
Therefore, excess air should not be treated as a simple one-variable NOx control method.
Not automatically.
A burner producing extremely low NOx under one condition may not necessarily provide the best overall combustion performance.
A practical evaluation should consider:
NOx
CO
Combustion stability
Heat output
Turndown
Fuel consumption
Temperature uniformity
The goal is to achieve low-emission combustion without compromising the industrial process.
Low NOx burners generally attack the problem at the combustion stage.
The main approaches include:
Reduce localized temperature peaks without reducing required furnace temperature.
Prevent excessive oxygen from being present in the hottest combustion regions.
Control where and when fuel and air react.
Dilute the combustion mixture and increase heat capacity.
Create a more suitable flame structure.
Avoid concentrated high-temperature zones.
For natural-gas-fired industrial burners, the most important NOx mechanism is often thermal NOx.
The main factors are:
High Local Flame Temperature
Oxygen Availability
Residence Time
Combustion Conditions
Fuel composition can also affect emissions, but natural gas generally contains very little fuel-bound nitrogen compared with many other fuels.
This is why burner design plays such an important role in controlling NOx from natural-gas combustion.
DYDTEC Combustion develops industrial burners, Low NOx burners, linear burners, thermal air furnaces, and combustion-system integration solutions.
NOx control can involve coordination between:
Burner-head design
Fuel injection
Combustion-air distribution
Flame structure
Staged combustion
Gas recirculation
Furnace geometry
Heat load
Temperature control
DYDTEC Combustion was established in 2012 and has R&D and manufacturing bases in Shanghai and Yangzhou.
Its product portfolio includes 100+ burner models covering 200+ application scenarios, allowing combustion configurations to be matched to different industrial heating requirements.
For an OEM project, NOx performance is best evaluated together with the furnace, fuel, combustion air, exhaust system, heat load, and process temperature rather than treating the burner as an isolated component.
Before selecting a Low NOx burner, it is useful to establish:
Furnace dimensions
Operating temperature
Burner position
Exhaust position
Furnace pressure
Heat load
Heating rate
Temperature profile
Operating range
Product requirements
Fuel type
Fuel pressure
Fuel composition
Air pressure
Air temperature
Excess-air level
Available airflow
NOx limit
Measurement basis
CO requirement
Applicable local emissions requirements
These parameters determine which NOx formation mechanisms are most relevant and which reduction strategy is appropriate.
For many high-temperature natural-gas combustion systems, the main cause is thermal NOx formation caused by high local flame temperatures in the presence of oxygen.
The three commonly discussed mechanisms are thermal NOx, prompt NOx, and fuel NOx.
High temperatures accelerate reactions between nitrogen and oxygen, making thermal NOx formation much more significant.
Oxygen is an important reactant in NOx formation, but NOx depends on the combination of oxygen availability, temperature, residence time, and combustion chemistry.
Yes. Natural-gas combustion can produce NOx, primarily through thermal NOx formation.
It can, but industrial processes often cannot simply reduce their operating temperature. Low NOx burner technology instead seeks to control local flame conditions while maintaining the required process temperature.
FGR introduces combustion products into the combustion process, diluting the mixture and reducing peak flame temperature.
It controls the availability of fuel and oxygen during different stages of combustion, reducing intense high-temperature zones.
No. NOx must be evaluated together with CO, flame stability, heat output, turndown, fuel consumption, and process performance.
NOx formation during combustion is primarily a result of what happens inside the high-temperature combustion zone.
The most important factors are:
Temperature
Oxygen Availability
Residence Time
Fuel Composition
Fuel-Air Mixing
Flame Structure
These factors determine how much nitrogen is converted into nitrogen oxides.
For industrial burners, the most effective NOx strategy is therefore not simply to reduce fuel or lower the furnace temperature. It is to control the combustion environment so that excessive local flame temperatures and unfavorable oxygen conditions are avoided while stable and complete combustion is maintained.
NOx is largely a combustion-condition problem. By controlling peak flame temperature, oxygen availability, mixing, residence time, staging, and recirculation, a properly designed burner can significantly reduce NOx while continuing to provide the heat required by the industrial process.
