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Lime Kiln Combustion, ANSYS Fluent CFD Simulation Training

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The present problem simulates the combustion process of methane gas in a vertical lime kiln using ANSYS Fluent software.

This product includes a Mesh file and a comprehensive Training Movie.

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Description

Project Description

The present problem simulates the combustion process of methane gas in a vertical lime kiln using ANSYS Fluent software. The construction of a vertical lime kiln consists of two main parts, including the combustion zone and the preheating zone. The fuel and air enter the building from the middle of the lime kiln and form a combustion reaction. As a result of the chemical reaction of combustion, a significant amount of heat is produced, increasing the temperature inside the lime kiln. On the other hand, some calcium carbonate enters the lime kiln from the upper part. This calcium carbonate or limestone reacts separately by receiving heat from the combustion of the desired fuel and carbon dioxide production.

As a result, it can produce calcium oxide or the same as quicklime and separate it from carbon dioxide. In this project, only the occurrence of the chemical reaction of burning inside a lime kiln is investigated. The combustion reaction defined in the model consists of performing a chemical reaction between air and methane. In the middle part of the lime kiln and from four side inputs, the flow, including 0.9 methane (CH4) and 0.1 nitrogen (N2), enters the lime kiln with a speed equal to 1.357 m.s-1 and a temperature equal to 300 K. Simultaneously, from the central area in the middle of the lime kiln, an initial airflow and a fuel flow enter the furnace.

So that the airflow contains 0.23 oxygen (O2) and 0.77 nitrogen (N2) with speed equal to 15.1 ms-1 and a temperature equal to 400 K and a fuel flow containing 0.9 methane (CH4) and 0.1 nitrogen (N2), with a speed equal to With 8.413 ms-1 and a temperature of 300 K. Simultaneously, the flow of calcium carbonate (CaCO3) or limestone enters from the upper part of the furnace with a speed of 0.0031 m.s-1 and a temperature of 300 K and moves downwards. Simultaneously, secondary airflows containing 0.23 oxygen (O2) and 0.77 nitrogen (N2) enter from the bottom of the furnace at a speed of 14.74 m.s-1 and a temperature of 300 K and move upwards.

There are also two outputs for the model; Thus, the reaction products are discharged from the outlet at the bottom of the vertical lime kiln at atmospheric pressure, and the excess gases produced are sucked into the outside space by a suction fan. In addition, porous materials have been used inside the lime kiln. The porous area is made of aluminum with a porosity coefficient of 0.3 and has an inertial resistance equal to 907.4 1.m-1 and a viscous resistance equal to 1100000 1.m-2.

Lime Kiln Geometry & Mesh

The present model is designed in three dimensions using Design Modeler software. The model is a semi-designed vertical furnace due to its symmetrical geometric structure and to reduce the computational cost. The model has four fuel inlets in the middle of the side surface of the lime kiln and one fuel inlet in the center of the furnace and has a primary air inlet in the center of the lime kiln and a secondary air inlet in the lower part of the furnace, and an outlet for products in the lower part of the furnace. And a particular outlet for discharging gases in the upper part of the furnace.

lime kiln

We carry out the model’s meshing using ANSYS Meshing software, and the mesh type is unstructured. The element number is 2219550. The following figure shows the mesh.

lime kiln

Lime Kiln CFD Simulation

We consider several assumptions to simulate the present model:

  • We perform a pressure-based solver.
  • The simulation is steady.
  • The gravity effect on the fluid is equal to -9.81 m.s-2 along the vertical axis.

The following table represents a summary of the defining steps of the problem and its solution:

Models (lime kiln)
 
Viscous   k-epsilon
  k-epsilon model standard
  near wall treatment standard wall functions
Species   Species Transport
  reactions volumetric
  number of volumetric species 6
Energy   On
Boundary conditions (lime kiln)
 
4 Inlet – Gas   Velocity Inlet
  velocity magnitude 1.357 m.s-1
  temperature 300 K
  CH4 mass fraction 0.9
Inlet – Gas Original   Velocity Inlet
  velocity magnitude 8.143 m.s-1
  temperature 300 K
  CH4 mass fraction 0.9
Inlet – Lime Kiln   Velocity Inlet
  velocity magnitude 0.0031 m.s-1
  temperature 300 K
  CaCO3 mass fraction 1
Inlet – Primary Air   Velocity Inlet
  velocity magnitude 15.1 m.s-1
  temperature 400 K
  O2 mass fraction 0.23
Inlet – Secondary Air   Velocity Inlet
  velocity magnitude 14.74 m.s-1
  temperature 300 K
  O2 mass fraction 0.23
Product Output   Pressure Outlet
  gauge pressure 0 pascal
Exhaust Gases   Exhaust Fan
  gauge pressure 0 pascal
  pressure jump 100000 pascal
Outer Walls   Wall
  wall motion stationary wall
  heat flux 0 W.m-2
Symmetry Walls   Symmetry
Methods (lime kiln)
 
Pressure-Velocity Coupling   SIMPLE
  pressure second order
  momentum first order upwind
  turbulent kinetic energy first order upwind
  turbulent dissipation rate first order upwind
  energy first order upwind
  all species second order upwind
Initialization methods   Hybrid

Results & Discussions

At the end of the solution process, two-dimensional and three-dimensional contours related to pressure, temperature, velocity, and mass fraction O2, CH4, H2O, CO2, N2, and CaCO3 were obtained. The resulting images show that the reaction products, including carbon dioxide and water vapor, are produced due to the combustion reaction between the reactants (methane fuel and airflow). Significant heat is also generated when a combustion reaction occurs. The heat generated by the combustion reaction can contribute to the dissociation of calcium carbonate or limestone and the production of quicklime.

There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.

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