8/10/2019 2004 Int Ansys Conf 161

1/15

Model of Mass and Energy Transfer in a Clinker RotaryKiln

J.A. Guirao

S. IglesiasNumerical Analysis TEChnologies S.L.

J. Pistono

Universidad de Oviedo

R. Vzquez

IMASA Div. Internacional

Abstract

The purpose of this report is the development of an application allowing the precise simulation of theprocesses which are involved in the operation of a rotary cement kiln, either wet or dry. This will be in use

during cement production in existing kilns and in the design stage of new kilns, including other processes,such as preheating and precalcining. The length of the furnace is divided into as many sections as desired,the last face of the first section being equal to the first face of the second section, and so on. In each section

it is considered: (1) heat transfer by conduction, convection and radiation (2) chemical reactions, including

vaporization of the water content in the wet process (3) entrance and exit of flows of each chemicalspecimen. Every physical property and transfer depends on local (point) conditions. The results are

complete maps of temperature and composition together with their variations during stages of kiln feeding

and fuelling.

Introduction

The development of an application allowing the strict simulation of the processes taking place in a rotarykiln for clinker production will be an important advance in the design procedures for this equipment.

The project initial objectives are summarized as follows:

To build a design tool: development of an application capable of predicting all the variables,chemical as well as thermo-physical in the complete kiln length. Traditionally, the design of rotary

kilns is mainly supported on the knowledge derived from previous constructive experience. A toolsuch as that hereby proposed will constitute the technical basis for decision taking concerning

design, such as length to be assigned to each stretch, chain mass, etc.

To build a supporting tool for other design stages as mechanical analysis.

To build a simulation tool for prediction of the behavior of the equipment in its workingconditions, which will allow making a sensitivity analysis, predicting its behavior in the case of

foreseen changes.

The construction of such a tool requires a detailed analysis of the heat and mass transfer mechanismsoccurring inside the kiln. For that, a sequential analysis methodology has been developed, consisting in the

division of the equipment in parts of a given length. The finite elements method applied to each part of thekiln allows establishing the starting conditions for the following element, obtaining therefore a sequential

resolution.

In each analyzed part the effects derived from the three heat transmission mechanisms (convection,

conduction and radiation), are considered in a coupled way. The convergence conditions in each element

are therefore based on the accomplishment of the energy balance in the element.

8/10/2019 2004 Int Ansys Conf 161

2/15

Parallel to the heat transmission model, development of a detailed model of the chemical and physical

transformations (the evaporation is treated as an equivalent reaction kinetics) inside the kiln, and evolution

of the different species in function of the thermo-dynamical parameters. This model interacts with the

preceding one, since the chemical reactions are either exo- or endo-thermic, and these contributions must

be taken into account in the energy balance of each element.

Finally, a model capable of predicting the product evolution is also needed. Since the kiln is rotating, and

the thermo-physical characteristics of the product vary along the kiln, the product longitudinal speed alsovaries, resulting in variations of the product resident time in each element, of filling index1, etc. All thesecircumstances are again coupled with the thermal, chemical and particle dynamics models.

At present, the three models have been developed, but some parameters of the different models are pendingof adjustment, from experimental data.

The three models described are implemented within the environment of the ANSYS application, and they

interact with a fully parameterized and realistic generation of the kiln geometry itself.

Transfer phenomena

In the simulation the following energy transfer mechanisms are included:

Conduction

In order to analyze the conduction in a cement rotary kiln, its cross section has two parts, depending on the

presence of product, as shown in Figure 1, and the heat flows also through the product, when present; this

is a particular type of conduction in porous solid.

Figure 1. Considered conduction phenomena

1Area filled with product related to total cross section

8/10/2019 2004 Int Ansys Conf 161

3/15

All the conduction phenomena are characterized by the conductivities proper to each material present as a,

function of its temperature.

Convection

The heat transfer associated to convective phenomena has been included by use of transfer correlations for

the evaluation of the film coefficients, function of the non-dimensional numbers that control the transferprocesses. These APDL TABLE functions depends of two variables (local surface temperature and average

flluid bulk temperature). In that way the film transfer coefficients have been taken as depending of the localtemperature of the exchange surfaces applying boundary conditions in table form, i.e. non-linear analysis.

Internal convection is forced convection in circular cross section ducts (figure 2) and external is taken asnatural convection outside of rotating cylinders (figure 3).

Figure 2. Inside convection phenomena considered

Figure 3. Outside convection phenomena considered

8/10/2019 2004 Int Ansys Conf 161

4/15

Radiation

Radiation from the kiln outside shell to the atmosphere, an equivalent simple transfer coefficient is

considered, according to:

( )sgrsgs TThAQ =

( )( )sgsgsr TTTTh ++=22

In the preceding equations, a view factor of 1 has been considered, because the shell surface is fully

surrounded by the atmosphere.

Concerning the inside radiation, we have a far more complex situation, since there exists radiation between

a gas and a surface, the gas being highly participative. It is also necessary to estimate the radiation between

surfaces, and therefore to calculate the shape factors between the several radiant surfaces.

Figure 4. Inside radiation phenomena

As a first approach, and in the aim of not making it too complex, it has been assumed that, as the kiln is

continuously rotating, the inside surface temperatures in the same section are not very different; so, in thisstage of the study, we neglect the value of the radiant energy transmitted between inside surfaces.

Nevertheless, we are at present working in the development of an algorithm for the determination of the

shape factors for each couple of model elements (finite elements), and the solution of the heat transmissionby radiation trough participating media, considering the gas as a mixture of CO2, H2O, N2y O2.

For the modeling of the transfer equipment by inside radiation, the AUX12 Radiation Matrix Method has

been used.

Chemical model

The cement production is obtained by operating a kiln at high temperature, where different chemical

reactions take place until a dry solid called clinker is obtained. Not all these reactions are fully known,

therefore it cannot be ascertained exactly what happens inside the kiln. Given the kiln feed, an approximate

reaction model can be established, with some accuracy, resulting in a quite complex behavior. To simplify,it has been assumed that six main reactions take place:

8/10/2019 2004 Int Ansys Conf 161

5/15

H2O (L) H2O (G)

CaCO3(S) CaO (S) + CO2(G)

2 CaO (S) + SiO2(S) SiO2. 2 CaO (S)

SiO2. 2 CaO (S) + CaO (S) SiO2. 3 CaO (S)

Fe2O3(S) + 4 CaO (S) + Al2O3(S) Fe2O3. 4 CaO . Al2O3(S)

3 CaO (S) + Al2O3(S) Al2O3. 3 CaO (S)

Some of the reactions occur simultaneously, what adds difficulties to their study.

The evolution of the several chemical species is adjusted to the reaction kinetics of the equations present inthe chemical model, each one having a prevalence interval controlled by the temperature of the gas

mixture. At the same time, the thermodynamic characterization of the preceding equations is included in

the kiln energy balances analyzed. This is affected by APDL programming, integrating it within thesolution iterative process of each kiln element.

Figure 5. Chemical Model

The following stages are considered from the chemical point of view:

Evaporation or drying.

Calcining.

Sintering.

Figure 5 shows the evolution of the typical species according to the adopted model.

8/10/2019 2004 Int Ansys Conf 161

6/15

Figure 6. Zones of the kiln chemical model

Model of solid material dynamics

The mass flow speed of the material along a rotary kiln is not constant. However, in an approximate way,

the time of permanence of the material within the kiln can be obtained by the following equation, U.S.Bureau of Mines.

Fndp

lt

=

77.1

t: stay time (min)

l: kiln length (m)

: natural slope of the dry material ()

p: kiln slope ()

d: kiln diameter (m)

n: rotating speed (r.p.m.)

F: factor considered in section reductions ( =1, for constant diameter)

Control of stay time by variations in kiln diameter has shown to give irregular shifting and material

retentions in the transitions; therefore rotary kilns are built with a constant diameter in the whole length. Asit can be observed in the preceding equation, by modifying the slope and the rotation speed of the kiln the

stay time may be controlled.

As already said, the passage speed inside a kiln is not constant and is conditioned by the thermo-chemical

processes and material transport occurring inside the kiln, with strong influence by the raw material

physical characteristics.

Driving the kiln at constant speed, the material shifts at different speed in the different areas, namelydraying, transition, calcining and sintering. Essays have confirmed this with tracers, in which radioactive

isotopes have been used (Na24 and Mn26). The following image show the estimated average speeds of thematerial in the direction of the kiln longitudinal axis.

The speed indicated values are average values obtained by the relation between the length of the considered

area and the stay time of the product in the same. That is to say, that speed is taken as a vector in axialdirection, when in fact, since the kiln is turning, the product is continuously recirculated. On the other hand,

a part of the product that has already passed by a point of the kiln can pass again because there may be a

small return of product, due to the dragging effect of the gas current.

The complexity of the study of the possible flow recirculations inside the kiln, especially in the drying area,where there are chains mixing the product, advised not to consider these recirculations in this first model.

8/10/2019 2004 Int Ansys Conf 161

7/15

The simplified alternative, is a constant speed supposed in each zone, together with the net material balance

of a stationary process. In future studies, it is foreseen to develop models describing with greater

approximation the dynamic behavior of the raw material inside the kiln.

Figure 7. Stages of the solid dynamics in the kiln

Resolution algorithms

Calculation hypothesis

The general hypotheses considered in the calculation are the following:

Stationary process: it is supposed that in any given kiln point the same processes occur, independently

of the time. In general, a cement kiln operates in a continuous way, so that the transitory periods arerestricted to situations where the operation conditions are varied (until reaching another nearly stationary

state). So this hypothesis is quite close to reality. Nevertheless, in later studies is intended to include the

simulation of transitory processes with sensitivity analysis.

The analysis of the radiation inside the kiln is complex. Therefore, it is considered, as a starting point,

that the gas inside the kiln is not fully participative and that the inside surfaces do not interchange net

radiation. This simplification, as previously explained, cannot give big errors because the radianttemperatures involved are very similar. The gas has been taken as a focus of emission, due to its highertemperature, but not absorbing any energy coming from the inside surfaces.

The chemical processes, very complex and partially unknown, has been modeled in a simpler way,

which will have to be revised in later analysis.

A study of the flame is not carried out and therefore it is supposed that the gas average temperature in

each point characterizes it.

No product recirculations or returns caused by the gas current are considered.

General resolution algorithm

The simulation of the heat transfer processes that occur in a cement rotary kiln is undertaken by means ofthe division of the same in a number of finite elements, on which the heat transferred and chemical

processes are studied in detail. After solving an element, the next is solved by applying, as initial contourconditions, those obtained at the end of the preceding element, and continuing this procedure along the

whole length of the kiln, until the last element. As a final checking, it is necessary to do a global energy

balance and compare it with actual data. If there are substantial differences with the measured data, thestarting hypotheses have to be corrected.

8/10/2019 2004 Int Ansys Conf 161

8/15

In the next figure gives the schema of the general calculation algorithm. The main program module

generates a complete model of the kiln. It includes a full set of algorithms to carry out analysis and

calculation.

Initial data

For the development of the main algorithm, it is necessary to know, as starting actual data, those definedbelow; however, for each kiln division, it is necessary to adopt more additional data, which are enumerated

in the following paragraphs:

Kiln geometrical data

Product data.

Initial concentrations. Product massic flow

Gas data.

Mass flow.

Temperature data

Product temperature at kiln inlet.

Outlet gas temperature

Ambient temperature.

Flame average temperature.

8/10/2019 2004 Int Ansys Conf 161

9/15

Figure 8. General resolution algorithm

Kiln division

For the complete kiln calculation, it is divided in N parts, not necessarily equal, solving each of them

independently.

The solution of each division is made starting from a detailed finite element model, reflecting the kiln

geometry as well as the different materials and their position. After establishing the adequate contour

conditions to solve of the elements, this is done using the ANSYS calculation package by finite elements of

general purpose.

The choice of the number of divisions is based on two criteria: calculation accuracy and computing time.

The greater the number of divisions is, the shorter is the length of the element to solve, and therefore, the

calculation accuracy is greater. It must be considered that in each element heat transfer phenomena, all

parameters depending on the temperature, are solved. Initially, its variation along the element must be

estimated or decided, so with smaller element less iterations are needed to obtain a valid solution.

8/10/2019 2004 Int Ansys Conf 161

10/15

On the other hand, greater the number of divisions means, longer the calculation time, exponentially with

the size of the problem. A decision must be taken considering both aspects.

Given that the kiln is divided in N parts, it is possible to define a different geometry for each of them;

obviously this must be consistent with reality, that is to say there cannot be brusque changes in diameter,

thickness, etc.

This possibility of establishing different values for the generic parameters (model parameterization) of each

element is imposed by the product circulation, since it has different speeds depending on each process, as

already stated. That applies to the loading angle, product stay angle, as well as the diameters, and even the

mesh each element.

Figure 9. Finite element model of a kiln element

Transition algorithm

As previously described, the resolution procedure of a kiln is sequential. The continuity between elements

is ensured with a transition algorithm which stores the output parameters of one element, and writes themas starting values of the following element to be calculated.

The reason for this is that to solve an element it is necessary to know initial inlet data, which generally are,

as said, the final data obtained solving the preceding element.

Gas initial temperature.

Local temperature in the border parts of the element feed side.

Gas mass flow.

Product mass flow.

Temperature gradient in the longitudinal direction.

Concentration of the several substances (gas and product).

8/10/2019 2004 Int Ansys Conf 161

11/15

Figure 10. Transition algorithm

This procedure obtains these outlet data of the element "i" and takes them as initial calculation data for theelement "i+1", by means of the automatic construction of a file in which are included all the necessary data

for the solution of the i+1 element.

Post process

The data obtained in the solution of each element gives a description of the complete kiln. The parameters

obtained directly and indirectly from the solution of the finite element model are here listed:

Evolution of the gas average2temperature in the whole kiln length.

Map of local temperature in any point of the product.

Map of local temperature in any point of the refractory.

Map of local temperature in any point of the ring.

Evolution of the mass flow of gas mixture, as well as the concentrations of its different chemicalspecies.

Evolution of the product mass flow, as well as the concentrations of its different chemical species.

Estimated value of physical properties of fluids and solids in any point of the kiln.

2Averaged in each cross section

8/10/2019 2004 Int Ansys Conf 161

12/15

Energy transferred by any of the transfer modes in any point of the kiln. Enthalpy differences in

the gas mixture between any chosen two points of the kiln. Heat flow by longitudinal convectionin any cross section direction of the kiln. Heat implied in the evolution of every chemical reaction.

Because of the architecture of the calculation program ANSYS, the temperature results derivedfrom the complete solution of the kiln can be directly used as solicitation conditions on a finite

element model of the kiln for mechanical analysis.

Figure 11. Energy transferred from the outside to each element

Figure 12. Mass fraction distribution of CIO

8/10/2019 2004 Int Ansys Conf 161

13/15

Figure 13. Temperature distribution in product and gas

8/10/2019 2004 Int Ansys Conf 161

14/15

Figure 14. Temperature distribution in the 50 first meters of the kiln

8/10/2019 2004 Int Ansys Conf 161

15/15

Figure 15. Detailed temperature contour plot of three elements with a refractory failure

Conclusions

A methodology is proposed to undertake the simulation of the thermal and mass transfer behavior of a

rotary cement kiln. The combination of a programming environment with a tool for analysis by the of finiteelements method allows offering very detailed results together with its extension to the whole geometry of

the kiln. The thermal analysis involves the development of chemical models.

Although there is still a lot of work to be done to improve the accuracy of the results, and substituting thesimplified assumption by approaches more closed to actual processes, it can be stated that a fairly complete

model has been built. Properly developed, it will be an efficient tool to help designing and running rotary

cement kilns.

References

[1] ANSYS SAS IP, Inc. (2003), APDL Programmer Guide, Revision 7.0, Canonsburg

[2] TCL-TK Manual Page, http://www.tcl.tk

[3] ANSYS SAS IP, Inc. (2001), UIDL Programmer Guide, Revision 5.5, Canonsburg

[4] Incropera, F.P. y Witt, D.P. de (1990) Fundamentals of Heat and Mass Transfer, John Wiley & Sons,

New York