CFD Simulation of Methane Combustion with Excess Air Considering NOx Formation

The process of burning methane (CH4) in the presence of oxygen (O2) results in the production of carbon dioxide (CO2), water vapor (H2O), and heat. This reaction is extremely exothermic, resulting in the release of a substantial amount of energy as heat. Here is the balanced chemical equation for methane combustion:

CH4 + 2O2 → CO2 + 2H2O

However, the oxygen available exceeds the stoichiometric requirement in practical combustion processes with excess air. Regarding 28% excess air, the balanced chemical equation is changed to the following equation:

CH4 + 2.56O2 + 9.6256N2 ® CO2 + 2H2O + 0.56O2 + 9.6256N2

In the present problem, methane combustion with 28% excess air is simulated regarding the formation of Nitrogen Oxide (NOx) using ANSYS Fluent software.

Simulation Process

A cylindrical combustor is considered in which methane is injected with a high-speed jet, expands with little interference from the outer wall, and mixes with low-speed air. A structured mesh grid is generated over the combustor and results in 34100 elements.

In order to model combustion considering the NOx production, Species Transport model is employed in ANSYS Fluent. Eddy Dissipation turbulence-chemistry interaction model is taken into account in this simulation. Note that both Prompt and Thermal NOx formation are included.

Post-processing

The temperature contour gives helpful insights into the thermal distribution within the combustion chamber. During the start of combustion, areas of high temperature are detected very close to the burner or ignition source, indicating the heat release in specific locations. Examining the mass fraction of various species provides insight into the sequence of chemical reactions that occur during combustion. At first, methane is the dominant species due to its role as the primary fuel. During combustion, methane is used up, resulting in higher concentrations of combustion byproducts like carbon dioxide (CO₂) and water vapor (H₂O). In addition, the generation of nitrogen oxides (NOx), such as nitric oxide (NO) and nitrogen dioxide (NO₂), occurs when nitrogen and oxygen react at elevated temperatures.