contribution 176

8/13/2019 Contribution 176

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Virtual-Lab of a Cement Clinker Cooler for

Operator Training

Oscar Acuna Carla Martin-Villalba Alfonso Urquia

Universidad Tecnologica de Bolvar, Cartagena, Colombia(e-mail: [email protected]).

Departamento de Informatica y Automatica, UNED, Madrid, Spain(e-mail: [email protected], [email protected])

Abstract: Plant operator training plays a fundamental role in improving the energy efficiencyof the the cement manufacturing process and reducing CO2 emissions. A virtual-lab of aclinker grate cooler, intended for training of cement plant operators, has been developed. Thegrate cooler model has been derived from first principles, and has been validated consultingcement industry experts, and comparing the simulated results with published data and available

information from cement industries. The model has been described in the Modelica language.The Interactive Modelica library has been used to develop the interactive user-to-modelinterface, and the communication between this interface and the model. The virtual-lab,which is completely described in Modelica, has been simulated using the Dymola modelingenvironment. The Interactive Modelica library can be freely downloaded from the websitehttp://www.euclides.dia.uned.es/

Keywords: Computer-aided instruction, Computer simulation, Control education, Dynamicmodelling, Process models, Process simulators, Training, Modelica

1. INTRODUCTION

Cement production is one of the most energy intensive

of all industrial manufacturing processes, due to the veryhigh temperatures required to produce the cement clinker.Depending on the energy efficiency of the cement manufac-turing plant, about 3GJ to 5.5GJ are used for producingone ton of cement. Additionally, cement manufacturinghas a significant environmental impact. Making one tonof cement results in the emission of about one ton of CO2.

The cement production process consists of two main steps(Labahn and Kohlhaas, 1983; Duda, 1984): the productionof clinker from raw materials and the production of cementfrom clinker. Two different processes, wet and dry, areused to produce the cement clinker. A typical dry processfor clinker manufacturing is described below (see Fig. 1).

The raw materials, consisting of a predetermined blendof CaCO3, SiO2, Al2O3 and Fe2O3, are crushed andhomogenized. The grinded raw materials are heated tocalcination temperature in the preheater and passed tothe calciner, where the calcination reactions take place.The meal from calciner is fed to the kiln, where remainingcalcination and other clinkerization reactions occur. Hotclinkers are discharged from the kiln to the cooler, wherethe clinker is cooled by heat exchange with ambient air.

Various types of clinker coolers are used in industry(Deolalkar, 2009). The grate cooler is one of them. It

This work has been supported by Vicerrectora Academica de la

Universidad Tecnologica de Bolvar and by Universidad Nacionalde Educacion a Distancia (UNED), under the grant Proyectos deInvestigacion propia de la UNED 2010 Ref. 2010V-PUNED/0001.

consists of air-permeable grills through which the coolingair flows. The hot clinker from the kiln, fed directly onthe grill at one end of the cooler, moves along the cooler

length. Air current passes through the grills and the clinkerbed lying on them. Hot clinkers are typically fed to thecooler at 12500C14000C and they are cooled down to atemperature of about 1000C.

The biggest opportunities for improving energy efficiencyand reducing CO2 emissions certainly comes from impro-ving the cement manufacturing process. However, energyconsumption and CO2 emissions can also be reduced byimproving the operation of existing cement plants. Plantoperator training plays a fundamental role in this respect(Ziya et al., 2010; Akimasa et al., 2001).

Virtual training simulators, frequently called virtual-labs,are effective educational tools. They provide a flexible

and user-friendly method to define the experiments tobe performed on the mathematical model. Virtual-labsare composed of: (1) the simulation of the mathematicalmodeldescribing the relevant properties of the system; (2)the interactive user-to-model interface, named virtual-labview; and (3) a narrativethat provides information aboutthe system and the use of the virtual-lab. The virtual-lab view is intended to provide a visual representation ofthe model dynamic behavior and to facilitate the usersinteractive actions on the model. The graphical propertiesof the view elements are linked to the model variables,producing a bidirectional flow of information between theview and the model. Any change of a model variable value

is automatically displayed by the view. Reciprocally, anyuser interaction with the view automatically modifies thevalue of the corresponding model variable.


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Raw meal

Exhaust air

Cyclone

Preheated raw

meal CoalTertiary air

CoalKiln

exhaust

Part ially calcined

raw meal

Hot

clinker

Secondary air

Primary

air

Preheater

Rotary kiln

Cooler

Calciner

Hot gases

Clinker

Fig. 1. Dry process for cement clinker manufacture.

The development of a grate cooler virtual-lab, intendedfor training of cement plant operators, is discussed in thispaper. Cooler operation is crucial in a cement industry. Onone hand, the characteristics of the cooling process (e.g.,residence time and temperature gradient of the clinker)strongly affect the clinker quality and, consequently, theproperties of the produced cement. On the other hand,the cooler secondary and tertiary air are used for fuelcombustion in the kiln and the calciner, respectively (seeFig. 1). For this reason, the energy efficiency of theclinker manufacturing process, and the kiln and calcinerperformance, depend on how the cooler is operated.

The paper content is as follows. The modeling hypothesesand the model validation are discussed in Section 2. Thevirtual-lab capabilities, implementation and use are brieflyexplained in Section 3.

2. MODELING OF THE GRATE COOLER

The modeled grate cooler has five fans, whose air flow canbe controlled individually. The secondary air is injected bythe first two fans and the tertiary air by the last three fans(see Fig. 2).

Tertiary air

Hot

clinker

Secondary air

Clinker

Fig. 2. Modeled grate cooler.

The model objective is estimating the

2D temperature distributions of air and clinker, the outlet temperature of the secondary and tertiary

airs, and

the clinker outlet temperatureas a function of

the cooler operating conditions (i.e., inlet temperatu-res and flows of clinker and primary air, grate speedand clinker bed height),

the clinker properties (i.e., clinker particle diameterand sphericity, clinker heat capacity, and density andporosity of the clinker bed),

the air properties (i.e., density, viscosity and heatcapacity of air), the cooler length, and

the heat transfer parameters (i.e., convective heattransfer coefficient between clinker and air, thermal

conductivities of air and clinker, and emissivity ofclinker).

2.1 Modeling hypotheses

The cooler model is primarily based on the model descri-bed by Mujumdar and Ranade (2009). The main modelinghypotheses are described below.

The solid bed is rectangular, with length L and heightH,and contains air and clinker. The clinker moves horizonta-lly with spatially-uniform velocity. The air moves upwardin a vertical direction, i.e. in perpendicular direction to the

grate and clinker movement. The diameter and sphericityof the clinker particles, and the clinker bed density andporosity, are independent of the spatial coordinates.


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The clinker enters into the cooler at uniform temperature.The inlet air is at ambient temperature. Heat conductionin clinker is modeled, while heat conduction in air isassumed negligible. The dominating mode of heat transferin the cooler is convection between the clinker particles andthe flowing air. The convection heat transfer coefficient(hc) can be calculated from the following expression

hc = Nu kg

Ds (1)

wherekg is the thermal conductivity of air and Ds is theclinker particle diameter. The Nusselt number is estimatedfrom the empirical correlation (Perry and Green, 1984)

Nu = 0.0295 Re4/5 Pr1/3 (2)

The Reynolds and Prandtl numbers are

Pr =g Cgpkg

, Re = Ds g ug

(1 ) s g (3)

where g , g , Cgp and ug are the density, viscosity, heatcapacity and velocity of air, is the bed porosity and s theclinker particle sphericity. The thermal conductivity andviscosity of air are calculated at the average temperatureof clinker and air, (Ts +Tg)/2. The heat capacity of air(Cgp ) is assumed a constant.

1 21

M

N

x

L

H

Free board region

Clinker bed

Tertiary

air

Hot

clinker

Secondary

air

Clinker

Fig. 3. Spatial discretization in control volumes.

The solid bed is divided into NM rectangular controlvolumes (see Fig. 3). Each side of the control volume (CV)is a control plane (CP), through which mass and energycan be exchanged with the environment. The stirred tankapproximation is made for the clinker contained in eachCV: 1) the clinker temperature is spatially uniform withinthe CV; and 2) the clinker going out from a CV is atthe same temperature as the clinker contained in the CV.

The stirred tank approximation is also made for the aircontained in each CV. Mass and energy balances for airand clinker are written for each CV of the solid bed asdescribed below.

Steady-state mass balance for air:{air mass flow goinginto the CV through the bottom CP}={air mass flowgoing out through the top CP}. There is no air flowthrough the left and right CP.

Steady-state energy balance for air: {enthalpy inputflow through the bottom CP due to air flow} +{solid-to-gas convective heat} = {enthalpy outputflow through the top CP due to air flow}.

Steady-state mass balance for clinker: {clinker mass

flow going into the CV through the left CP} ={clinker mass flow going out through the right CP}.Clinker doesnt flow through the bottom and top CP.

Steady-state energy balance for clinker: {enthalpy in-put flow through the left CP due to clinker move-ment} + {conductive heat flow for clinker going intothe CV through the four CP} = {enthalpy outputflow through the right CP due to clinker movement}+ {solid-to-gas convective heat} + {thermal energyemitted by radiation through the top CP}. Radiationis only considered for CV of the top layer of the solidbed.

The free board region is divided into N rectangular CVwith length L/N, as shown in Fig. 3. This region onlycontains air. The stirred tank approximation is made forthe air contained in each CV. For the CV placed at thefirst two-fifths of the cooler length, the air goes out fromthe CV through the left CP. The air going out from thefirst CV is the secondary air. The balances in these CVare as follows.

Steady-state mass balance for air:{air mass flow goinginto the CV through the bottom CP} +{air mass flow

going into the CV through the right CP}= {air massflow going out through the left CP}. There is no airflow through the top CP.

Steady-state energy balance for air: {enthalpy inputflow through the bottom CP due to air flow} +{enthalpy input flow through the right CP due to airflow} + {energy received by radiation through thebottom CP} = {enthalpy output flow through theleft CP due to air flow}.

Mass and energy balances for the CV placed in the freeboard region at the last three-fifths of the cooler lengthare derived analogously. In this case, the air goes into theCV through the bottom and left CP, and goes out from

the CV through the right CP. The air going out from thelast CV is the tertiary air.

The cooler efficiency is calculated from the followingexpression:

eff = 100enthalpy flow of secondary air

entalpy flow of output clinker (4)

2.2 Model validation

The model validation has been performed consulting ce-ment industry experts, and comparing the simulated re-sults with published data (Touil et al., 2005; Rasul et al.,2005; Mujumdar et al., 2007) and available informationfrom cement industries.

The cooler model has been described in the Modelica lan-guage (Modelica Association, 2012) and simulated usingDymola (Dynasim AB, 2008). The solid bed, H = 0.8 mhigh andL = 11 m long, has been discretized into 30 20control volumes and the free board region into 30 controlvolumes. The physical properties of clinker and air arespecified in Table 1.

The model has been validated under different experimentalconditions. Some simulation results are shown in Fig. 4.The simulated experimental conditions are: clinker ve-locity 0.1m/s, air inlet temperature 300K, clinker inlet

temperature 1673K and air mass flow 10kg/s per fan. Thesimulated efficiency of the cooler operating under theseconditions is 51%.


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Fig. 5. Modelica description of the virtual-lab main window shown in Fig. 6.

Fig. 6. Main window of the grate cooler virtual-lab.


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The root component (see Fig. 5) hosts a component ofthe MainFrame class, named mW, that generates a mainwindow inside which other components can be placed.Three containers are placed inside mW: pNand pN1of thePanel class, and cV of the Canvas class. The cV componentcontains the 3D animated diagram of the grate cooler. Thisdiagram is composed of drawable elements.

The 2D plots of temperature distribution in clinker andair are described by means of two components of thePlanarGrid class named clinker and air (see Fig. 5). Thisclass components are divided into cells that are coloredaccording to the value of an attribute. In this case, theattribute is the temperature in the associated CV.

The arrows representing the secondary and tertiary airflows, and the clinker output flow are obtained usingcomponents of the Arrowclass. The text above these arrowsis provided by components of the Textclass. Elements of thePolygonclass are used to build the fans and other polygonalfigures. The diagram legend showing the mapping between

temperature and color is provided by a component of theScalarBar class.

The pN container is placed above cV. It contains elementsthat allow performing changes on the model. It hostsinteractive controls of the NumberFieldand CheckBoxclasses.The numeric boxes allow the user to change the inputtemperature and mass flow of clinker, and the air inputtemperature. Check-boxes show and hide secondary win-dows that contain controls to change other air and clinkerproperties, and the solid bed size (length and height).

The pN1 container, placed below cV, hosts interactivecontrols of the CheckBox class. Check-boxes show and hide

windows that contain different temperature plots and thevirtual-lab documentation (a set of HTML pages).

3.3 Virtual-lab setup

A Modelica model has been written to describe the com-plete virtual-lab. This model inherits the VirtualLab Mode-lica class, which is distributed in the Interactive library.Values have been given to the following parameters ofVirtualLab: 1) the model-to-view communication interval,Tcom; 2) the name of the Modelica class describing thevirtual-lab model; and 3) the name of the Modelica classdescribing the virtual-lab view. In addition, the relations-

hip among the model variables and the view variables hasbeen specified. This has been accomplished by writing therequired equations inside the Modelica class that definethe complete virtual-lab.

4. CONCLUSION

The Modelica language, a free Modelica library namedInteractive and the Dymola modeling environment havebeen successfully applied to the development of a virtual-lab for training of cement plant operators. The use ofModelica greatly reduces the modeling effort. The virtual-lab view description and the complete virtual-lab setup

are trivial tasks using Interactive. The developed virtual-lab can be easily distributed to the users, who dont needto install any additional software to run the virtual-lab.

ACKNOWLEDGEMENTS

The authors are grateful to Ing. Marlon Gonzalez for hiscomments and suggestions, which have greatly helped toimprove the virtual-lab of the grate cooler.

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