numerical!modeling!of!rotary!kilns! - tu...
Post on 13-Mar-2018
216 Views
Preview:
TRANSCRIPT
The physical phenomena occurring inside a rotary kiln can be separated in two parts: gas phase (freeboard) phenomena and granular bed phenomena. In the freeboard the main phenomena are: • Turbulent non-‐premixed combus?on • Heat Transfer including Radia?on On the granular bed the main phenomena: • Heat Transfer • Chemical Reac?ons • Phase changes Due to the complexity of the physical phenomena of a rotary kiln, one can divide the model in two:
• Freeboard CFD model • Granular Bed model
Turbulent combus?on results from the two-‐way interac?on of chemistry and turbulence. When a fame interacts with a turbulent flow, turbulence is modified by combus?on because of the strong flow accelera?ons through the flame front induced by heat release, and because of the large changes in kinema?c viscosity associated with temperature changes. • Reynolds stress tensor: Realizable k-‐epsilon model (Turbulence Model) • Turbulent scalar flux: Eddy diffusivity model • Mean source term: Eddy break-‐up model (EBU) • RadiaFon: ParFcipaFng Media RadiaFon Model (DOF) • NOx: Zeldovich Model The grid was done using polyhedral elements: 2.8 Million elements
A rotary kiln is a pyro-‐processing device used to raise materials to high temperatures. It is a long horizontal cylinder with a certain inclina?on with respect to its axis. Material within the kiln is heated to high temperatures so that chemical reac?ons can take place. A rotary kiln is therefore fundamentally a heat exchanger from which energy from a hot gas phase is transferred to the bed material. The energy originates from the combus?on of hydrocarbon fuels via a main burner at the hot end. The rotary kiln in ques?on is a counter-‐ current gas direct fired Calcium Aluminate Cement rotary kiln. • Increasing market demand for high purity cement • Unscheduled shutdown due to ring forma?on • Restric?ve emission regula?ons (i.e.: NOx) have triggered the industrial partner’s management to increase its knowledge base on kiln processes. The model is a plaRorm to understand and op?mize the opera?on of the process.
Numerical Modeling of Rotary Kilns M.A. Romero Valle, M. Pisaroni , D. van Puyvelde and D. J. P. Lahaye, Scien?fic Compu?ng Group, DelX Ins?tute of Applied Mathema?cs, Faculty of Electrical
Engineering, Mathema?cs and Computer Science, DelX University of Technology, The Netherlands
Objec?ves
Physical Phenomena
CFD Freeboard Model Industrial Case Results
Further Work
References CounteracFng Ring FormaFon in Rotary Kilns, Accepted for publica?on in Mathema?cs with Industry. M. Pisaroni, D. J. P. Lahaye and R. Sadi. Numerical Modeling of Rotary Kilns, in prepara?on M.A. Romero Valle, M. Pisaroni, D. van Puyvelde, and D. J. P. Lahaye.
The developed one-‐dimensional granular bed model encompasses two phenomena in the kiln: the axial heat transfer and the sintering reac?ons occurring in the bed. A one-‐dimensional axial heat transfer model was developed and validated with data from the literature. The sintering reac?on kine?cs model was developed taking as basis informa?on found in literature and experimental XRD (X-‐Ray Diffrac?on) data handed by the industrial partner. • Energy balance for Granular Bed: • 3-‐D diffusion controlled sintering reacFon model: • Correla?ons coming from literature were used to determine the Heat Transfer Coefficients for the different
heat transfer paths that exist in the granular bed system. • The model uses results from the CFD freeboard model as input. • ValidaFon was done by modeling an inert bed and comparing the model with exis?ng data from the literature. Valida?on Results:
*Experimental data taken from Barr et al. and XRD Data from industrial partner.
Granular Bed Model
3-2 Radiative Heat Transfer 25
mscp,sdTs = qdA [W ], (3-1)
where the left hand side denotes the heating of the bulk material and the right hand side theheat transferred to the material at position z. It is to be noted that the heat transfer pathsare the same as described in Chapter 1.
By rearranging the terms one gets the following:
mscp,sdTs
dz= qLcrd = QLcrd
A= Q
LK[Js≠1m≠1], (3-2)
where ms is the mass flow rate, cp,s the heat capacity of the solids, Ts the temperature ofthe bulk solids, q the heat flux into the bulk solids, Q the heat transfer rate, Lcrd the chordlength for calculating the area of heat exchange, A the heat exchange area between freeboardand granular bed, and LK the total kiln length.
It is to be noted that implicit in the heat transfer rate Q shown above are the heat transferpaths described in Chapter 1. By noting this, one can observe that Equation 3-2 can also beseen as the governing equation of the axial heat transfer model.
In the following sections expressions for the heat transfer paths described in Chapter 1 willbe shown. It is to be noted the following adjustments in nomenclature with respect to Figure1-6 and the present chapter:
Qgæeb = Qradiation,gæs + Qconvection,gæs,
Qgæew = Qconvection,gæw,
Qewæeb = Qradiation,wæs.
On the left hand side one can observe the nomenclature given in Figure 1-6 and on the righthand side one can observe the nomenclature used in the present chapter.
One should note as well that the nomenclature used in the present chapter will be in accor-dance to mechanical and chemical engineering texts where a heat transfer coe�cient is used.We note that the heat transfer rate can be written as [4]:
Q = hA�T,
where Q is the heat flow [W], h the heat transfer coe�cient [W/m2K], A the heat transfersurface area [m2] and �T the temperature di�erence between two materials [K].
3-2 Radiative Heat Transfer
In the present section models considering radiative heat transfer within the kiln will be pre-sented.
Master of Science Thesis M. A. Romero Valle
3-2 Radiative Heat Transfer 25
mscp,sdTs = qdA [W ], (3-1)
where the left hand side denotes the heating of the bulk material and the right hand side theheat transferred to the material at position z. It is to be noted that the heat transfer pathsare the same as described in Chapter 1.
By rearranging the terms one gets the following:
mscp,sdTs
dz= qLcrd = QLcrd
A= Q
LK[Js≠1m≠1], (3-2)
where ms is the mass flow rate, cp,s the heat capacity of the solids, Ts the temperature ofthe bulk solids, q the heat flux into the bulk solids, Q the heat transfer rate, Lcrd the chordlength for calculating the area of heat exchange, A the heat exchange area between freeboardand granular bed, and LK the total kiln length.
It is to be noted that implicit in the heat transfer rate Q shown above are the heat transferpaths described in Chapter 1. By noting this, one can observe that Equation 3-2 can also beseen as the governing equation of the axial heat transfer model.
In the following sections expressions for the heat transfer paths described in Chapter 1 willbe shown. It is to be noted the following adjustments in nomenclature with respect to Figure1-6 and the present chapter:
Qgæeb = Qradiation,gæs + Qconvection,gæs,
Qgæew = Qconvection,gæw,
Qewæeb = Qradiation,wæs.
On the left hand side one can observe the nomenclature given in Figure 1-6 and on the righthand side one can observe the nomenclature used in the present chapter.
One should note as well that the nomenclature used in the present chapter will be in accor-dance to mechanical and chemical engineering texts where a heat transfer coe�cient is used.We note that the heat transfer rate can be written as [4]:
Q = hA�T,
where Q is the heat flow [W], h the heat transfer coe�cient [W/m2K], A the heat transfersurface area [m2] and �T the temperature di�erence between two materials [K].
3-2 Radiative Heat Transfer
In the present section models considering radiative heat transfer within the kiln will be pre-sented.
Master of Science Thesis M. A. Romero Valle
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1300
400
500
600
700
800
900
1000
1100
1200
Axial Position (m)
Te
mp
era
ture
(K
)
Bulk Temperature
Wall Temperature
Gas Temperature
Gas Temperature (Barr et al., 1989)
Bed Temperature (Barr et al., 1989)
• The freeboard CFD model was used to evaluate how by having an excess air one could reduce the peak temperature of the kiln. The model was used to determine that a 120% air excess would reduce the peak temperature to avoid ring forma?on. The result was validated in situ.
• The granular bed model was used to analyze the plant observa?ons regarding the product quality altera?ons with respect to the opera?ng changes derived with the freeboard model. It is concluded that the proposed opera?ng changes increase product quality due to a slower conversion of Alumina.
100% Base Air (ring blockage) 120% Excess Air (no rings)
0 10 20 30 40 50 600
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (min)
Conve
rsio
n
Conversion of Alumina (Al2O3)
A = 14500, Ea=205 Kj/mol, f(alpha)=D4(alpha)
Experimental Values XRD
Transversal Granular Flow + Heat Transfer Model • Discrete Element Modeling • Con?nuum Approach • Energy Balance
!0.6 !0.4 !0.2 0 0.2 0.4 0.6
!1.25
!1.2
!1.15
!1.1
!1.05
!1
!0.95
!0.9
!0.85
!0.8
1900
1905
1910
1915
1920
1925
1930
1935
1940
1945
!0.6 !0.4 !0.2 0 0.2 0.4 0.6
!1.25
!1.2
!1.15
!1.1
!1.05
!1
!0.95
!0.9
!0.85
!0.8
2000
2050
2100
2150
2200
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.2
0.4
0.6
0.8
1
Normalized Kiln Length (!)
Conve
rsio
n / F
ract
ion
Al2O3 Conversion / Liquid Fraction
Al2O3 Conversion
Liquid Fraction
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.2
0.4
0.6
0.8
1
Normalized Kiln Length (!)
Conve
rsio
n / F
ract
ion
Al2O3 Conversion / Liquid Fraction
Al2O3 ConversionLiquid Fraction
top related