Lime Kiln Combustion

Lime Kiln Combustion CFD Simulation by ANSYS Fluent, Tutorial

Description

This project simulates the combustion process of methane gas within a vertical lime kiln using ANSYS Fluent. The 3D geometry was designed in Design Modeler as a semi-modeled vertical furnace, leveraging the structure's symmetry to reduce computational cost. The model includes four fuel inlets positioned around the middle of the kiln's side surface, a fifth fuel inlet at the furnace center, a primary air inlet at the center of the kiln, a secondary air inlet in the furnace's lower section, an outlet for reaction products in the lower section, and a dedicated outlet for gas discharge at the top of the furnace. The domain was meshed in ANSYS Meshing, totaling 2,219,550 elements.

Methodology

A vertical lime kiln consists of two main functional zones: the combustion zone and the preheating zone. Fuel and air enter through the middle of the kiln and undergo a combustion reaction, releasing substantial heat and raising the internal temperature. Separately, calcium carbonate (limestone) is introduced from the upper section of the kiln. As it absorbs heat generated by the combustion process, it undergoes a distinct thermal decomposition reaction, releasing carbon dioxide and producing calcium oxide (quicklime). This project focuses specifically on modeling the combustion reaction itself within the kiln, rather than the limestone decomposition process.

The combustion reaction models a chemical interaction between air and methane, represented using the Species Transport model with its volumetric reaction sub-model. Flow entering through the four side inlets and the central inlet consists of 0.9 methane (CH₄) and 0.1 nitrogen (N₂), entering at 1.357 m/s and 300 K, alongside a simultaneous airflow entering from the central region.

The model includes two outlets: reaction products exit through the bottom outlet at atmospheric pressure, while excess gases are drawn out through a separate suction-fan-driven outlet. Porous media was incorporated within the kiln, modeled as aluminum with a porosity coefficient of 0.3, an inertial resistance of 907.4 1/m, and a viscous resistance of 1,100,000 1/m². The standard k-epsilon turbulence model and the energy equation were used to solve the turbulent flow field and capture temperature variation throughout the domain.

Conclusion

Results include 2D and 3D contours of pressure, temperature, velocity, and mass fractions for O₂, CH₄, H₂O, CO₂, N₂, and CaCO₃. The results confirm that reaction products — including carbon dioxide and water vapor — form as a result of the combustion reaction between methane fuel and air, releasing significant heat in the process. This generated heat is precisely what drives the subsequent dissociation of calcium carbonate into quicklime, linking the combustion process directly to the kiln's primary industrial function.