Almatis-TU DelftSeminar on NumericalModelingof Rotary ...ta.twi.tudelft.nl/nw/...kiln/Fluid_and_Combustion.pdf · and an aluminium-bearing material. A typical kiln arrangement : Reverberatory

Almatis-TU Delft Seminar on

Numerical Modeling of Rotary KilnsJune 9, 2011

Room Vassiliadis, 16th floor, EWI Building, Mekelweg 4, Delft

Organizers: M. Pisaroni (TU Delft), R. Sadi (Almatis) and D. Lahaye (TU Delft)

Calcium Aluminate Cements

Calcium Aluminate Cements are very white, high purity hydraulic bonding agents providing controlled setting times and strength development for today's high performance refractory products.

Almatis' calcium aluminate cements are for use in refractories designed for service up to 1800°C :• Vibration and self-flowing castable mixes,• Quick-setting mortar, joining mixtures, self-leveling floors, and compensation compounds• Refractory castables for steel, nonferrous metals and petrochemicals industries.

Calcium Aluminate Cements Production

The cement is made by fusing together a mixture of a calcium-bearing material (limestone) and an aluminium-bearing material.

A typical kiln arrangement :Reverberatory furnace in which the hot exhaust gases pass upward as the lump raw material mix passes downward.

The calcined material drops into the "cool end" of the kiln. The melt overflows the hot end of the furnace into a cooler in which it cools and solidifies. The cooled material are crushed and grounded.

In the case of high-alumina refractory cements, where the mix only sinters, a rotary kiln must be used.

Rotary Kiln Philosophy

Fundamentally, rotary kilns are heat excangers in which energy from hot gas phase isextracted by the bed material.

During the passage, various heat exchange processes: drying, heating, and chemicalreactions that cover a broad range of temperatures.

Cylindrical vessel, inclined slightly to the horizontal, which is rotated slowly about its axis.

The material is fed into the upper end of the cylinder. As the kiln rotates, material gradually moves down towards the lower end, and may undergo a certain amount of stirring and mixing.

Hot gases: opposite direction as the process material.

The hot gases is be generated by a flame projected from a burner-pipe inside the kiln.

MODELING APPROACH

• STEP1: Heat transfer in the lining (2D)

Combustion Model (1D) Setup and Test

• STEP2: Combustion and heat transfer in the gas and in the lining (3D) Nox Temperature

• STEP3: Granular material thermal and motionanalysis Chemical

TURBULENT NON-PREMIXED COMBUSTION IN A ROTARY KILN

BURNER

AIR INLET• CONVECTION/CONDUCTION/RADIATION

• NON PREMIXED COMBUSTION

• POLLUTANT EMISSION

•TURBULENCE

•MIXING AND TRANSPORT OF CHEMICALSPECIES

TURBULENT NON-PREMIXED COMBUSTION IN A ROTARY KILN

BURNER

• CONVECTION/CONDUCTION/RADIATION

• NON PREMIXED COMBUSTION

• POLLUTANT EMISSION

•TURBULENCE

•MIXING AND TRANSPORT OF CHEMICALSPECIES

TURBULENT NON-PREMIXED COMBUSTION IN A ROTARY KILN

ROTATING WALL(REFRACTORY)

• CONVECTION/CONDUCTION/RADIATION

• NON PREMIXED COMBUSTION

• POLLUTANT EMISSION

•TURBULENCE

•MIXING AND TRANSPORT OF CHEMICALSPECIES

THE COMBUSTION

Combustion Air Natural Gas Cooling Air A/G ratio : 9

3800 [m3/h] 425 [m3/h] 300 [m3/h]

TEMP 500 - 600 °C 50 - 100 °C 50 - 100 °C

COMPOSITION 23 % O2 90 - 88.7 % CH4 23 % O2

5.6 % C2H6

1.6 - 1.5 % C3H8

0.9 % C4H10

1.4 % CO2

0.4 - 1.9 % N2

COMBUSTION MECHANISMS:

FAST : 6 SPECIES, 1 REACTION CH4 +2O2 CO2 + 2H2OH2 CO2 H2O O2 N2 CH4

DETAILED MECHANISM WITH INTERMEDIATE REACTIONS:

MODEL 1: 12 STEP 16 SPECIESMODEL 2: 15 STEP 19 SPECIES

DETAILED MECHANISM WITH MORE SPECIES

MODEL 3: 463 REACTIONS 70 SPECIES

MODEL 4: N-ALKANES (VERY DETAILED)

ARRHENIUS COEFFICIENTS: dependence of the rate constant k of Chemical reactions on the temperature T and activation energy Ea.

COMBUSTION MODEL:

Eddy Break-Up (EBU) Models

• Track individual mean species concentrations on the grid through transport equations.

• The reaction rates are calculated as functions of :o mean species concentrations, o turbulence characteristics o temperature.

• A mean enthalpy equation is solved.

The mean temperature, density and viscosity are then calculated

Standard EBU Model : reaction rate is dictated by the turbulent mixing time scale.

• Individual species are transported at different rates according to their own governing equations.

• The reaction rate model: expression that takes the turbulent micromixing process into account.

Hybrid Kinetics/EBU Model assumes that the reaction rate is also affected by finite-rate chemical kinetics.

Mean enthalpy Species concentrations

TURBULENCE MODELS:

K-Epsilon turbulence model: transport equations are solved for the turbulent kinetic energy and its dissipation rate.

Realizable Two-Layer K-Epsilon model combines Realizable K-Epsilon model: different transport equation for epsilon.Critical coefficient of the model, is expressed as a function of mean flow and turbulence properties.

Two-layer approach: alternative to the low-Reynolds number approach. The computation is divided into two layers. (Shear driven Wolfstein model)

• Layer adjacent to the wall: turbulent dissipation rate and turbulent viscosity are functions of wall distance. • Entire flow: equation for the turbulent kinetic energy.

FLOW MODELS:

The Ideal Gas (with combustion) model allows thermodynamic polynomial for calculation of specific heat of individual components of the mixture.

Multi-Component Gas model used to simulate a miscible mixture of two or more pure gases (MECHANISM).

Segregated Flow model solves the flow equations (one for each component of velocity, and one for pressure) in a segregated (uncoupled) manner. Linkage between momentum and continuity equations is achieved with a predictor-corrector approach (Rhie-and-Chow-type pressure-velocity coupling combined with SIMPLE algorithm).

Segregated Fluid Enthalpy model solves the total energy equation with chemical thermal enthalpy as the independent variable. Temperature is then computed from enthalpy according to the equation of state.

Segregated Species model solves the species continuity equations for a multi-component fluid mixture. Together with global mass continuity, these equations provide a means for updating the field of N mass fractions defining the mixture composition.

OTHER MODELS:

Participating Media Radiation Model

Nox Emission model provides a framework for modeling the NOx transport equation. Nox Zeldovich Model

NOx is considered to be a passive scalar, not influencing density calculations. Therefore, in steady-state simulations, the NoxEmission model should be used as a post-processing tool (it should be activated after the flow field has converged).

SOLVERS

MESH

MESH

TEMPERATURE PROFILE

TEMPERATURE PROFILE

TEMPERATURE PROFILE

TEMPERATURE PROFILE

CH4 PROFILE

O2 PROFILE

CO2 PROFILE

GAS-SOLID INTERFACE TEMPERATURE

INCIDENT RADIATION

INCIDENT RADIATION

INCIDENT RADIATION

RADIATIVE ABSORPTION

RADIATIVE ABSORPTION

RADIATIVE ABSORPTION

TURBULENT LENGTH SCALE

VELOCITY VECTORS

VELOCITY VECTORS

END OF PART 1

The power of Fire, or Flame, for instance, which we designate by some trivial chemical name, thereby hiding from ourselves the essential character of wonder that dwells in it as in all things, is with these old Northmen, Loke, a most swift subtle Demon of the brood of the Jötuns... From

us too no Chemistry, if it had not Stupidity to help it, would hide that Flame is a wonder. What is Flame?

Carlyle on Heroes Odin and Scandinavian Mythology

Realizable K-Epsilon model

Standard K-Epsilon model

BACK

Standard EBU

Individual species transported at different rates according to their own governing equations derived from the instantaneous governing equations for species i:

Time average of the instantaneous governing equation:

Reaction rate is modeled is modeled through an expression that takes the turbulent micromixing process into account.

BACK

Hybrid EBU

This model accounts for finite rate effects by assuming that the actual reaction rate is the minimum of the reaction rates from (STANDARD)

and

Hence

BACK

SOLVERS:

Wall distance

SIMPLE [Semi-Implicit Method for Pressure-Linked Equations]

Segregated species

Segregated energy

K-Epsilon Turbulence Solver

K-Epsilon Turbulence Viscosity Solver

Nox solver

BACK