Chemical Reactions: Gas Flare, 2-step Air-Methane Mechanism Combustion

Gas Flare, Two-Step Air–Methane Mechanism Combustion, ANSYS Fluent CFD Simulation Tutorial

Description

This project simulates combustion in a gas flare, using a two-step methane–air mechanism, in the presence of a crosswind, with ANSYS Fluent.

This case is a clear example of a reacting flow, where the fluid motion and the chemistry are solved together: the flow carries fuel and air into the flame, the combustion reactions release heat and change the gas composition, and the resulting temperature and density fields feed back into the flow. Modeling this coupling is exactly what the reacting-flow (species transport) approach is built for.

A gas flare is a combustion device used in industrial facilities such as oil and gas refineries and at production wells, particularly on offshore platforms, to safely burn off natural gas.

The 3-D geometry was built in Design Modeler. Because the flare is symmetric, only half of it is modeled to cut the computational cost, with a symmetry boundary condition applied. The flare has a cylindrical body with four outlet ducts and sits inside a computational domain that carries the wind flow; this domain is likewise halved along the symmetry plane. The model was meshed in ANSYS Meshing with 1,546,925 elements.

Simulation Methodology

Gas flares burn the natural gas released during oil extraction. During extraction, natural gas accumulates above the oil in the reservoir. Collecting and storing this gas is preferable, but where that is not possible it is flared. Burning the gas in a flare avoids uncontrolled, hazardous release, and converting methane to carbon dioxide before it reaches the atmosphere is less harmful than releasing the methane directly.

To capture the chemistry, the species transport model is used with volumetric reactions enabled, and the eddy-dissipation model estimates the reaction rate. A methane–air mixture burns through a two-step mechanism: first methane and oxygen react to form carbon monoxide (and water), then the carbon monoxide combines with oxygen to form carbon dioxide. Air enters the domain at 0.2 m/s and 300 K, and the fuel enters at 0.1 m/s and 300 K. The realizable k-ε model and the energy equation are enabled to solve the turbulent flow and compute the temperature distribution.

Results & Conclusion

After solving, two- and three-dimensional contours of pressure, temperature, velocity, and the mass fraction of each modeled species were obtained, with the two-dimensional contours shown on the geometry's symmetry plane.

The species mass-fraction contours confirm that the reaction takes place: the carbon dioxide and carbon monoxide contours show these products being generated, while the methane contour shows the hydrocarbon being consumed as the reactant. The contours also show that the crosswind carries the combustion products, such as carbon dioxide and carbon monoxide, away from the flare and disperses them into the surrounding environment.