Flameless Combustion in Gas Turbines

In high-temperature systems, particularly gas turbines and industrial furnaces, improving energy efficiency and reducing emissions is of great importance importance. Conventional combustion methods can lead to both thermal stresses and high NOₓ emissions due to the formation of high-temperature pockets and flame kernels. At this point, flameless combustion emerges as an innovative approach that combines the advantages of high efficiency and low emissions.

Flameless Combustion

Flameless combustion is a combustion method in which fuel (methane) and oxidizer (air) are mixed at high temperature and low oxygen concentration, resulting in combustion without the formation of a distinct flame kernel. It is called “flameless” because the combustion reaction is distributed over a large volume rather than concentrated at a single point, eliminating the visible bright flame. This results in lower peak temperatures.

Comparison with Conventional (Flame) Combustion

Conventional Combustion:

• High temperatures lead to increased formation of pollutants such as NOₓ.
• The flame kernel can cause significant thermal stresses within the combustion chamber.

Flameless Combustion:

• Low NOₓ and CO Emissions: The absence of high-temperature pockets reduces NOₓ production, and partial combustion products such as CO are minimized.
• More Uniform Temperature Distribution: Without a concentrated flame kernel, thermal loads and temperature gradients are significantly reduced.
• Higher Efficiency and Lower Fuel Consumption: Energy utilization is improved through enhanced heat recovery and increased internal and external flue gas recirculation.

Flame and Flameless Combustion

As shown in the figure above, the left image displays a distinct flame kernel, while the right image shows no such visible kernel. Combustion occurs homogeneously throughout the chamber.

Process of Achieving Flameless Combustion

To achieve flameless combustion, the following steps are typically followed:

1. Preheating the fuel and air above the autoignition temperature.

2. Reducing the O₂ concentration in the air—for example, by diluting air with exhaust gases to lower the oxygen level.

3. Increasing the recirculation of hot combustion products within the combustion chamber.

4. Using burner and combustion chamber designs that enhance mixing and turbulence.

Example Analysis and Results

In a comparative study, two analyses were performed using the ANSYS Fluent software to simulate both flame and flameless combustion in a gas turbine combustor. To enable flameless combustion, the methane fuel and air were preheated above the autoignition temperature of approximately 600°C. Additionally, the oxygen concentration in the air was reduced from 23% to 10%.

Analysis results revealed that in conventional flame combustion, high-temperature peaks and a distinct flame front were observed. In flameless combustion, the temperature distribution was significantly more uniform, with markedly lower peak temperatures. The following images present the simulation results obtained using ANSYS Fluent.

Flame and flameless combustion analysis results