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.

Figure 1: Temperature distribution in CFD Simulation of Methane Combustion with Excess Air Considering NOx Formation

When NOx concentration contours are studied alongside velocity distributions, they show an important connection between residence time, temperature, and pollutant production. The largest NOx concentrations are seen in areas where high temperatures coincide with enough residence duration, particularly in the post-flame zone. The velocity field exhibits a typical jet expansion pattern, with the high-speed core gradually fading as it goes downstream, resulting in recirculation zones that improve fuel and extra air mixing. These flow patterns have a significant impact on both combustion completeness and NOx formation: higher velocities in the central jet region result in shorter residence times and potentially lower NOx production, whereas slower-moving peripheral regions with elevated temperatures become prime zones for thermal NOx formation via the Zeldovich mechanism.

Figure 2: Nox distribution in CFD Simulation of Methane Combustion with Excess Air Considering NOx Formation

Figure 2: Nox distribution in CFD Simulation of Methane Combustion with Excess Air Considering NOx Formation