Scope of Flame less Combustion Technology for NOx Reductions

What is Flameless combustion?

  • Flameless combustion, also known as low-NOx (nitrogen oxide) combustion or colorless combustion, is a combustion process in which fuel is burned in the absence of a visible flame.
  • This technique is primarily developed to reduce the formation of nitrogen oxides, which are a group of air pollutants that contribute to smog and acid rain.
  • In a conventional combustion process, such as in a flame-based burner or furnace, high temperatures and oxygen concentrations can lead to the formation of nitrogen oxides.
  • These pollutants are harmful to the environment and human health.
  • Flameless combustion seeks to minimize the production of nitrogen oxides by controlling the combustion process.
Flameless Combustion Techniques
Flameless Combustion Techniques

Features of Flameless combustion

  • While flameless combustion offers several advantages, its implementation requires careful engineering and consideration of specific furnaces and process requirements.
  • It is often applied in industrial settings where energy efficiency, emissions reduction, and safety are critical considerations.
  • Key characteristics of blameless combustion include:
  1. Uniform Mixing: The fuel and air are thoroughly and uniformly mixed before combustion to ensure a more controlled and evenly distributed combustion process.
  2. Lower Peak Temperatures: Flameless combustion aims to reduce the peak temperatures within the combustion zone, as higher temperatures can lead to increased nitrogen oxide formation.
  3. Extended Combustion Time: The combustion process is extended to allow for more complete and efficient fuel oxidation, minimizing the conditions that favor nitrogen oxide formation.
  4. Staged Combustion: In some flame-less combustion systems, the combustion process is staged, meaning that fuel and air are introduced in separate stages to control the combustion reactions.
  5. Advanced Burner Designs: Specialized burners and combustion chambers are designed to facilitate flame-less combustion. These designs often include features such as porous media burners or modified flame shapes.
CFD Results of flame profiles
CFD Results of Flame Profiles


Benefits of flameless combustion in furnaces include:

  • Increased Efficiency: Flameless combustion can lead to higher thermal efficiency in furnaces by optimizing the combustion process and minimizing heat losses.
  • Reduced Emissions: The controlled combustion conditions in flameless systems often result in lower emissions of pollutants such as nitrogen oxides (NOx) and carbon monoxide (CO).
  • Improved Safety: Flameless combustion can enhance safety by eliminating or minimizing visible flames, reducing the risk of flame impingement and associated safety hazards.

What are the Techniques of Cold Combustion?

  • flameless combustion, also known as cold combustion or cool flame combustion, refers to a combustion process that occurs at temperatures lower than those associated with a visible flame.
  • This phenomenon is of interest in various applications due to its potential for reduced emissions and improved efficiency.
  • Here are some techniques and factors associated with flameless combustion:
  1. High Dilution:
    • Flameless combustion often involves high dilution of the fuel-air mixture with inert gases such as nitrogen or carbon dioxide.
    • This helps in reducing the peak temperatures associated with traditional combustion.

low NOX burner FGR

    low NOX burner FGR
  1. Preheating:
    • Preheating the reactants before combustion can lead to flameless combustion. This reduces the ignition delay and promotes a more uniform combustion process.
  2. Catalysis:
    • The use of catalysts can facilitate flameless combustion by lowering the activation energy required for the reaction. This allows combustion to occur at lower temperatures.
  3. Micro-Mixing:
    • Enhancing the mixing of fuel and oxidizer at a small scale can promote uniform combustion and reduce temperature peaks. This can be achieved through advanced burner designs or by using additives that improve mixing.
  4. Inhibition of Free Radicals:
    • Controlling the formation of free radicals, which contribute to high-temperature combustion reactions, can be achieved through the use of inhibitors. This helps in maintaining a flameless or cool flame regime.
  5. Fuel Design:
    • Modifying the molecular structure of the fuel itself can influence combustion characteristics. Certain fuels are more prone to flameless combustion due to their chemical properties.
  6. Pulsed Combustion:
    • Intermittent or pulsed combustion can be employed to achieve flameless conditions. This involves cycling between combustion and non-combustion phases, allowing for heat dissipation during the non-combustion phase.
  7. Pressure Effects:
    • Controlling the pressure of the combustion environment can influence the combustion characteristics. Lower pressures can sometimes lead to flameless combustion.
  8. Controlled Inlet Air Temperature:
    • Adjusting the temperature of the incoming air can impact the combustion process. Lowering the inlet air temperature can help maintain flameless conditions.
  9. Advanced Burner Designs:
    • Using specially designed burners that promote better mixing and controlled combustion can contribute to flameless combustion.
  • Flameless combustion is an area of ongoing research, and different applications may require tailored approaches based on specific requirements. The goal is often to achieve combustion with reduced emissions, lower peak temperatures, and improved overall efficiency.

Applications of Flameless combustion 

  • Flameless combustion, also known as cool flame combustion or low-temperature combustion, refers to the process of burning fuel without a visible flame
  • These techniques can be applied in various combustion systems such as internal combustion engines, gas turbines, boilers, and industrial furnaces to achieve flameless combustion and improve combustion efficiency while reducing emissions.


Burner tiles for FGR low NOx burners

    Burner tiles for FGR low NOx burners


  • This type of combustion often occurs at lower temperatures compared to traditional combustion processes and can offer certain advantages, such as reduced emissions and improved safety in some applications. Here are some techniques used to achieve flameless combustion:

Homogeneous Charge Compression Ignition (HCCI):

    • HCCI engines operate by compressing a homogeneous mixture of air and fuel to the point of autoignition, similar to how a diesel engine operates.
    • .HCCI engines can achieve combustion with very little or no flame propagation.
    • This technique allows for efficient and clean combustion with lower emissions of pollutants like nitrogen oxides (NOx) and particulate matter.
      HCCI engine combustion
      HCCI engine – Flameless combustion

Internal Exhaust Gas Recirculation (EGR):

    • Internal Exhaust Gas Recirculation involves recirculating a portion of exhaust gases back into the combustion chamber.
    • This technique reduces the oxygen concentration in the combustion chamber, which lowers the combustion temperature and reduces the formation of nitrogen oxides (NOx).
      ECU EGR for Diesel Engine for low NOX
      ECU EGR for Diesel Engine for low NOX
  1. Lean Burn Combustion:
    • Lean burn combustion involves burning fuel with an excess of air compared to stoichiometric conditions.
    • This excess of air helps reduce the peak flame temperature and decreases the formation of nitrogen oxides (NOx).
    • Lean burn combustion is commonly used in gas turbines and some internal combustion engines.
      Combustion stability curves
      Combustion stability curves for Gas Turbine
  2. Catalytic Combustion:
    • Catalytic combustion involves using catalysts to facilitate the combustion process at lower temperatures compared to traditional combustion.
    • Catalysts accelerate the reaction rates of combustion, allowing for efficient combustion at reduced temperatures.
      Working principle of Catalytic Thermal Oxidizer
  3. Microflame Combustion:
    • Flameless combustion involves creating small, controlled flames or plasma discharges to initiate combustion.
    • These microflares can help ignite fuel at lower temperatures and can be used in applications such as waste incineration and fuel cells.
  4. Pulsed Combustion:
    • Pulsed combustion involves periodically igniting fuel-air mixtures in a combustion chamber.
    • This technique can help achieve efficient combustion with lower peak temperatures and reduced emissions.
  5. Oxycel Combustion:
    • Oxy-fuel combustion involves using oxygen instead of air as the oxidizer in combustion processes.
    • By using oxygen, the combustion temperature can be controlled more precisely, leading to more efficient and cleaner combustion.

Flam less burners for Low NOX

CFD Modeling of Flameless combustion

  • Computational Fluid Dynamics (CFD) modeling of flameless combustion plays a significant role in understanding, optimizing, and designing combustion systems with reduced emissions.
  • Combustion is a vast topic. You can refer the post of combustion modeling in detail for CFD modeling of turbulent combustion.
  • Here’s how CFD is used in modeling flameless combustion:
  1. Fluid Dynamics Simulation:
    • CFD software simulates the fluid flow and heat transfer within the combustion chamber.
    • It predicts variables such as velocity, pressure, temperature, and species concentration throughout the combustion process.
  2. Chemical Kinetics Modeling:
    • CFD models incorporate chemical kinetics to simulate the combustion reactions accurately.
    • Chemical kinetics models describe the rates of chemical reactions occurring during combustion, including fuel oxidation and pollutant formation.
  3. Turbulence Modeling:
    • Turbulence modeling is essential for capturing the turbulent flow characteristics within the combustion chamber.
    • Different turbulence models, such as Reynolds-averaged Navier-Stokes (RANS) or Large Eddy Simulation (LES), may be employed based on the complexity of the flow.
  4. Fuel Injection Modeling:
    • CFD simulations model the fuel injection process, including fuel atomization, spray formation, and fuel-air mixing.
    • Understanding the behavior of the fuel spray helps optimize combustion efficiency and reduce emissions.
  5. Species Transport and Reaction:
    • CFD models track the transport of various chemical species, including fuel, oxygen, nitrogen, and combustion intermediates.
    • Species transport and reaction models predict the formation and distribution of pollutants such as nitrogen oxides (NOx) and particulate matter.
  6. Radiation Modeling:
    • Radiation modeling accounts for the transfer of thermal radiation within the combustion chamber.
    • It helps predict temperature distributions and radiative heat transfer, particularly in high-temperature combustion environments.
  7. Validation and Verification:
    • CFD results are validated against experimental data to ensure accuracy and reliability.
    • Validation involves comparing CFD predictions with measurements from laboratory experiments or real-world combustion systems.
  8. Parametric Studies and Optimization:
    • CFD simulations allow for parametric studies to explore the effects of different operating conditions, geometry designs, and fuel compositions on combustion performance and emissions.
    • Optimization techniques can be employed to identify the optimal configuration that maximizes combustion efficiency while minimizing emissions.
  9. Combustion Stability Analysis:
    • CFD models help analyze combustion stability, including the onset of flame extinction, flashback, or combustion instability.
    • Understanding combustion stability is crucial for ensuring safe and reliable operation of combustion systems.
CFD results : Temperature contours during combustion
CFD results: Temperature contours during combustion

Flameless Combustion for Hydrogen Firing

  • Flameless combustion, also known as MILD (Moderate or Intensive Low-oxygen Dilution) combustion or flameless oxidation
  • This is an advanced combustion technique that offers several advantages over traditional combustion methods, particularly when firing hydrogen.
  • Here are some key points about flameless combustion for hydrogen firing:

Characteristics of Flameless Combustion

  1. Uniform Temperature Distribution: Flameless combustion provides a more uniform temperature distribution in the combustion chamber, reducing thermal NOx formation and minimizing hotspots.
  2. Low Emissions: Due to the lower peak temperatures, flameless combustion significantly reduces the formation of nitrogen oxides (NOx) and other pollutants.
  3. Improved Efficiency: The process enhances thermal efficiency by achieving more complete combustion and better heat transfer.
  4. Fuel Flexibility: Flameless combustion can be applied to a wide range of fuels, including hydrogen, natural gas, and biogas.

Hydrogen Firing with Flameless Combustion

Hydrogen is an ideal candidate for flameless combustion due to its clean-burning properties. Here are some benefits and challenges specific to hydrogen:


  1. Zero Carbon Emissions: Hydrogen combustion produces only water vapor, eliminating CO2 emissions.
  2. High Energy Density: Hydrogen has a high energy content per unit mass, making it an efficient fuel.
  3. Reduced NOx Formation: The low peak temperatures in flameless combustion help mitigate NOx emissions, which can be a concern with hydrogen’s high flame temperatures.


  1. Safety Concerns: Hydrogen is highly flammable and requires careful handling and storage to prevent leaks and potential explosions.
  2. Burner Design: Specialized burners are needed to maintain the conditions necessary for flameless combustion, particularly with hydrogen’s unique properties.
  3. Dilution Requirements: Achieving the appropriate dilution of air and fuel to enter the flameless combustion regime can be more complex with hydrogen.

Applications and Implementation

  1. Industrial Furnaces: Flameless combustion is used in industrial applications like metal processing and chemical manufacturing, where precise temperature control and low emissions are critical.
  2. Power Generation: Hydrogen-fired flameless combustion can be applied in gas turbines and combined heat and power (CHP) systems to enhance efficiency and reduce emissions.
  3. Research and Development: Ongoing research focuses on optimizing burner designs, improving fuel-air mixing, and ensuring the safety of hydrogen use in flameless combustion systems.


  • Flameless combustion offers a promising approach to hydrogen firing, combining high efficiency and low emissions.
  • While challenges remain in terms of safety and burner design, advancements in technology and engineering are making this a viable option for sustainable energy production and industrial applications.
Flam less burners for Low NOX
Flam less burners for Low NOX

Essential Conditions for Flameless Combustion 

  • The range for achieving flameless combustion of hydrogen involves specific temperature, dilution, and oxygen concentration conditions.
  • Here are the key parameters and their typical ranges:

Temperature Range

  • Entry Temperature:
    • To initiate flameless combustion, the preheated air and the combustion chamber should typically be at a temperature between 800°C and 1000°C (1472°F to 1832°F).
    • This ensures that the hydrogen and air mixture reaches the auto-ignition temperature without forming a visible flame.
  • Combustion Zone Temperature:
    • Once established, the temperature within the flameless combustion zone generally ranges between 800°C and 1400°C (1472°F to 2552°F).
    • The exact temperature depends on the burner design, the extent of dilution, and the specific application requirements.

Dilution and Oxygen Concentration

  • Dilution with Recirculated Flue Gases: Flameless combustion requires significant dilution of the fuel-air mixture with recirculated flue gases. This helps lower the oxygen concentration and increase the specific heat capacity of the mixture, contributing to a uniform temperature distribution.
  • Oxygen Levels: The oxygen concentration in the combustion air is typically reduced to around 10-15%, significantly lower than the 21% found in ambient air. This low oxygen level helps prevent the formation of hot spots and visible flames.

Flow and Mixing Conditions

  • High Turbulence: Effective mixing of fuel, air, and recirculated flue gases is crucial. High turbulence ensures a homogeneous mixture, promoting uniform combustion without localized high-temperature regions.
  • Preheated Air: The combustion air is often preheated to temperatures between 500°C and 700°C (932°F to 1292°F) to facilitate entry into the flameless combustion regime.

Practical Considerations

  • Burner Design: Specialized burners are designed to create the necessary mixing and flow conditions. These burners often include features to recirculate flue gases and maintain the required dilution levels.
  • Fuel Supply: Hydrogen fuel must be supplied at a controlled rate to ensure stable combustion and avoid flame formation.
  • Safety Measures: Robust safety protocols are essential given hydrogen’s flammability, including leak detection, proper ventilation, and explosion prevention measures.

Summary of ranges for Flame less or MILD conbustion

  • The range for effective flameless combustion of hydrogen includes:
    • Initial Temperature: 800°C to 1000°C
    • Combustion Zone Temperature: 800°C to 1400°C
    • Oxygen Concentration: 10-15%
    • Preheated Air Temperature: 500°C to 700°C
  • These conditions help achieve a stable, efficient, and low-emission combustion process, making flameless combustion an attractive option for utilizing hydrogen as a clean fuel.


  • Flame-less combustion can be implemented in various combustion systems, including industrial boilers, furnaces, and gas turbines.
  • It is especially relevant in industries where NOx emissions are a significant environmental concern, and regulatory standards for emissions reduction are stringent.
  • The benefits of flame-less combustion include reduced nitrogen oxide emissions, improved combustion efficiency, and potentially lower pollutant concentrations in the exhaust.
  • However, implementing flame-less combustion can also pose engineering and design challenges, and the technology may require careful optimization for specific applications.
  • Overall, flame-less combustion is part of ongoing efforts to develop cleaner and more environmentally friendly combustion processes.
  • By leveraging CFD modeling, engineers and researchers can gain insights into the complex phenomena occurring during flameless combustion. This knowledge enables the development of more efficient, cleaner, and environmentally sustainable combustion technologies.


  1.  Low NOx burners with FCT by ESA
  2. Lantec Combustion Technology FC Burners
  3. Review on Biomass Flame less combustion

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