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How is CFD modeling of Combustion carried in a Rotary Cement Kiln using ANSYS FLUENT

Introduction

The cement kiln is one important part in cement production industries and it is used to heat the cement mixture. It rotates slowly at its axis. The cement kiln is exposed to carry high temperature cement mixture. The inner surface kiln has high temperature around 1200 C and outer surface is exposed to ambient temperature. To maintain structural strength, its our surface is continuously cooled with air or water jets.

Application of Rotary Kiln

Rotary cylindrical kiln is widely used for industrial application such as

heating, drying
iron ore reduction
Calcining
Pyrolysis of solid fuels like titanium dioxide production, solid waste materials

Process in Cement Kiln

In the cement plant, the Pre-calcined raw material is supplied to the rotary kiln where a series of physical and chemical reaction (combustion) take place to form the clinker
After that, Hot clinker is sent to the cooler
The energy required for clinker formation is achieved by burning fuel (coal, oil, gas) at the bottom end of the kiln through multi-channel burner
Preheated primary air ( 70 – 80°C) is injected in the burner from the channels of swirl and in the axial direction
Secondary air ( ~ 1000 ) is taken out from the cooler through the annular opening area between the burner and end wall of the kiln
Convective and radiative heat transfer between flue gas of combustion and charge takes place
. Flue gas from the kiln flows out of the upper end of kiln into the pre calciner
Solid material like Clinker coming out of the Rotary cement kiln is at high temperature ( 1300 – 1450 °C )and it is cooled to 100-120 °C with the help of ambient air
A schematic representation of the rotary kiln with Material and flue gas flow is given below

Burner for Cement Kiln

Air or fuel staged burner with a long cylindrical mixture region is used for heating calcinated mixture at the end of the rotary kiln
Primary and secondary air are supplied for mixing of gaseous fuel
Fuel can be coal gas or hydrogen rich fuel gas
High velocity flue gas generated from combustion is blown to the centre of cement kilon

A typical type of burner for cement kiln is shown below,

Flame generated by burner is shown below. High temperature flue gases heat mixture

Reaction Zone in Kiln

In cement kiln, a series of chemical reaction are takes place in the different zones of the kiln as per difference in temperature when the kiln feed material enters the high-temperature zones in the rotary kiln)
A series of chemical reactions take place in the rotary kiln which contains the materials ( alumina, quicklime, ferric oxide, silica gel, and other metal oxides react to form four major compounds of cements:
- Quicklime – CaO.SiO2 (C3S)
- Alumina – 2CaO.SiO2 (C2S)
- Calcium oxide – 3CaO.Al₂O₃
- Ferric oxide – 4CaO$Al2O3.Fe2O3
The compounds have their own temperature formation and specific zone of the kiln.
In general, three main zones are observed in the rotary cement kiln which are given below

Combustion Modelling Using CFD for Cement Rotary Kiln

Combustion converts reactants (fuel) into products of combustion by reacting with oxygen from air
During the combustion process is heat release and it is also called exothermic chemical reaction.
Combustion is generally presented as: Fuel + Oxidizer <=> Products of combustion + Energy

Combustion is one of essential subject in chemical engineering, which involves
- turbulent fluid flow
- heat transfer by convection and radiation
- species transport
- heterogeneous and homogenous chemical reaction,
- Particle phase dynamics
- pollutant formation a

Gas-Phase Conservation Equations

The set of conservation equations that are solved in most CFD modelling of combustion
Every flow model in CFD solver like FLUENT or OpenFOAM which solves the conservation of mass (continuity) , energy and particle of gas phase
Mass Conservation

Where Sp is the source term due to combustion particles

Momentum Equation

Where P, τ, ρg and F are pressure and turbulent shear stresses, gravitational force and gravity force respectively

Turbulence Modelling

Turbulence modeling is carried out using a closure model for the Reynolds stress with k-ε turbulence model. For CFD modeling of Turbulence combustion the article on website.
K-equation

ε- equation

Enthalpy Equation

In the enthalpy (energy) equation, there is the source term, S_hdue to combustion That is heat source and heat transfer within the system that affect the temperature of flue gases. In rotary kilns, heat transfer mode is dominated by thermal radiation.

Particle-Phase Conservation Equations

Coal combustion is modeled using the fast chemistry model which is called as eddy dissipation model (EDM) in ANSYS FLUENT. You can visit the blog post on turbulent multiphase modeling of coal combustion
The particle phase is modelled with discrete phase model (DPM) In this model, particles are tracked using Lagrangian equations
At the inlet, Coal particles follows a Rossin-Rammler size distribution and particles are in the Lagrangian frame of reference (LFR) with stochastic trajectories model considering gravity effect

Combustion processes of coal are treated as de-volatilizing first and then char burning
Combustion of volatile gases is rapid and the combustion can be mixing-controlled, complex. For simplicity, details of chemical kinetic rates can be neglected
The net rate of production of product species due to reaction is given below

Where Yp and, YR is the mass fraction of reactant and product species. A and B are empirical constant.

Case study: Problem of Cement Kiln

Details of Cement Kiln:

- - Diameter of rotary cement kiln = 3.5 m
  - Diameter of rotary cement kiln =50 m
  - Thickness = 0.015 m
  - Angle of inclination of kiln with respect to burner axis = 3.5°
  - Speed of rotary kiln (rpm) = 1.5

Fuel Composition:

Coal fuel gas with major species CO, CO₂ and H₂ used

Species	% of Mole n fuel mixture
CO	40
CO₂	30
N₂	3
H₂	23
CH₄	3
H₂O	1

Steps for CFD Modelling

Geometry or CFD domain for kiln and burner:

- The geometry of cement kiln and burners created using ANSYS space claim
- Sufficient length of cement kiln is taken to model essential region

Meshing

- ANSYS meshing platform used to create finite number of cells over the computational domain
- Hex elements in kiln as polyhedral elements in burners less number of cells compared to tetra-hydral or hexa-hydral element and computational cost is reduced
- Good mesh quality (less skewness) can be obtained for complex geometries
- Faster convergence for variables for complex geometries

Boundary Conditions and Selection of Models

Inlet of air and fuel are specified with mass flow rate and temperature
Fuel composition of coal gas used for simulation
Outer surface is specified with the moving wall boundary of 3 rpm and convective heat transfer to ambient at temperature of 300 K
Inlet of cement kiln is specified with particle flow rate, particle size (5mm -20 mm) and temperature (850 °C)
Outlet of cement kiln is specified with pressure outlet
Multiphase model: Discrete Phase Model (DPM)
Turbulence model: K-ε Realizable
Combustion Model:
- Partially premixed FGM (Flammlet Generated Manifold) model with GRI-Mech 3 reaction mechanism
- The follwoing figure shows generation of PDF table based on mean mixture fraction

Simulation:

- CFD simulation of combustion in cement kiln and burners carried out in ANSYS FLUENT as a commercial FVM solver
- Combustion simulation carried out using the Partially premixed model of Flammlet generated manifold (FGM)
- This combustion model is based on combination of progress variable (c) and PDF mean mixture fraction model

CFD Results

- CFD results like velocity, temperature, flow pattern and temperature profiles are presented based on CFD results
- Velocity flue gases is higher near the outlet of burner and it decreases as it flows inside the kiln

Temperature Contours

Temperature of flue gases is higher near the outlet of burner and it decreases as it flows inside the kiln

Molar Concentration of Fuel

Molar concentration of hydrogen is higher near the inlet fuel inlet
After mixing with air its concentration decreases

Temperature Variation along the axis of cement kiln

Temperature of flue gases generated from combustion is maximum near the outlet of burner and it decreases as the distance from burner increases

Summary

CFD results have been presented for velocity, temperature, and temperature profiles
High temperature is observed near the burner outlet in the cement kiln. As the distance from the burner outlet increases the flue gas temperature decreases
Mixing of fuel and air need to be improved in the burner by fixing their inlet on the opposite side of the burner