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Energy 36 (2011) 2820e2827

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Energy

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Exergy-based indicators to evaluate the possibilities to reduce fuel consumptionin lime production

Alexis Sagastume Gutiérrez a,*, Carlo Vandecasteele b

aUniversidad de Cienfuegos, Carretera a Rodas kilómetro 4, cuatro caminos, Cienfuegos, CubabDepartment of Chemical Engineering, University of Leuven, de Croylaan 46, B-3001 Heverlee, Belgium

a r t i c l e i n f o

Article history:Received 14 July 2010Received in revised form12 February 2011Accepted 15 February 2011Available online 25 March 2011

Keywords:Exergy efficiencyQuicklimeCleaner productionExergy effectiveness

* Corresponding author. Tel.: þ53 43511963; fax: þE-mail address: asagastume@ucf.edu.cu (A.S. Guti

0360-5442/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.energy.2011.02.023

a b s t r a c t

A new way to evaluate the energetic performance of lime shaft kilns is proposed. Two new exergy-basedindicators are introduced for the evaluation, one to assess the exergy efficiency of limekilns and the otherindicator to assess the effectiveness of the exergy consumption of the dissociation reaction. Thecombination of both indicators provides a clear picture of the energetic performance of the process,highlighting the main potentialities for fuel saving (fuel consumption represents about 50% of totalproduction costs).

The validity of the proposed assessment is examined using some operating data measured ina commercial lime factory. Results show that introduction of exergy-based indicators in the assessmentimproves the evaluation of the energy consumption of the calcination process. In this way the impact ofthe process losses in the fuel consumption is better addressed.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Lime is one of the oldest andmost importantmaterials producedby industry and has many different applications. The most impor-tant factor in the production cost of lime is fuel consumption, rep-resenting ca. 50% of the total costs [1]. Fuel consumption is alsoassociated to CO2 emission in the calcination process: 785 kg of CO2

are emitted per ton of CaCO3 due to the dissociation reaction and200e400 kg are emitted due to fuel combustion, giving a total of1000e1200 kg of CO2 per ton of lime produced. This situationmakeslime one of the industrial products with the highest specific emis-sion of CO2 associated to its production [2,3]. As the quantity of CO2emitted due to the dissociation of CaCO3 is constant, the finalamount of CO2 emitted depends on the efficiency of the fuelconsumption in thekiln. For all these reasons theefficiencyof energyconsumption has been the subject of previous studies [4e9]. Thesestudies use as an efficiency indicator the thermal efficiency of thelime production process defined as the ratio of energy “consumed”to energy supplied. Such an energy based definition does notconsider the impact of internal irreversibilities of the productionprocess on fuel consumption.Moreover, the energy based definitionprovides no information of the potentiality to reuse in the processthe energy loss through the boundaries of the kiln. For example, thepossibility of using the heat loss with the exit gases to preheat the

53 43522762.érrez).

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limestone feed to the kiln depends on the temperature of the exitgases, aspect not considered with the energy approach. To mitigatethis problem, in this paper exergy rather than energy will be used.

The objective of this paper is to present an evaluation meth-odology for the calcination process allowing to decrease energyconsumption. It introduces two new exergy-based indicators onefor exergy efficiency and one for exergy effectiveness.

Cleaner Production has been defined by UNEP-UNIDO (www.unido.org/) as “the continuous application of an integrated environ-mental strategy to processes, products and services to increase efficiencyand reduce risks to humans and the environment”. Cleaner Productionis concerned with the reduction, and possibly elimination of processemissions (at the source rather than by end of pipe treatment), ofraw material consumption, of fuel consumption, and of wasteproduction. It is also concerned with the recycling of residues as rawmaterial in the same or in another process. Cleaner Production aimsat increasing the efficiency, profitability and sustainability of theprocess and at decreasing its environmental impact. So, CleanerProduction focuses on the optimization of the raw material andenergy use and on the reduction and reutilization of residues [10].

For lime production the primary target of Cleaner Productionimprovements should be to decrease fuel consumption and therelated environmental impact, especially the release of carbondioxide, one of the main actors of climate change, by increasingenergy efficiency. The exergy-based indicators proposed may servein the assessment procedure in view of Cleaner Production as theyallow to identify opportunities to decrease energy consumption.

Nomenclature

A surface area per unit length (m2/m3)Aq fraction of calcium oxide available in the quicklime.CP specific heat (KJ/Kg K)dL limestone diameter (m)f fuel consumption per ton of quicklime (KgF/Kgtime)F fuel consumption (Kg)E exergy (KJ)e specific exergy (KJ/Kg)h coefficient of convective heat transfer (W/m2 K)Δh specific enthalpy (KJ/Kg)H enthalpy (KJ)K thermal conductivity (W/m K)M mass flow (Kg/s)PL specific surface area (1/m)q specific heat (KJ/Kg)Q heat (KJ)R resistanceLHV lower heating value of the fuelS entropy (KJ/K)T temperature (K)v velocity (m/s)Ufuel conversion degree of fuelXlime conversion degree of limestoneNu Nusselt numberPr Randlt numberRe Reynold numberz height of the kiln coordinate (m)

Greek lettershe exergy efficiencye exergy effectivenessj porosity

n kinematic viscosity (m2/s)k transient factoru volumetric gas flow4 relation between chemical exergy and lower heating

value of fuel

Subscriptsa airChem chemicalCO2 carbon dioxided dissociationD destructionDiff diffusionfurnace kilnF fuelg gash convectionin inputk conductionloss lossLs limestoneo ambientout outP productQl quicklimesol solidwall Wallb convective mass transfer

SuperscriptsAV avoidableUN unavoidableo chemical

Fig. 1. Lime shaft kiln.

A.S. Gutiérrez, C. Vandecasteele / Energy 36 (2011) 2820e2827 2821

2. Process description

In order to produce lime it is necessary to calcinate limestone bysupplying heat. This process is usually carried out in kilns, thevertical shaft kiln being used most, because of its high energyefficiency compared with other designs. A vertical shaft kiln (Fig. 1)is basically a moving bed reactor with the upward flow of hotcombustion gases counter-current to the downward flow of lime-stone particles that undergo calcination at elevated temperature,usually exceeding 900 �C. For a better understanding the kiln isdivided in three parts, namely the preheating, calcination andcooling zone. In the preheating zone beginning at the top of the kilnand ending when calcination starts, the limestone feed to the kiln isheated up to the dissociation temperature. At the end of the pre-heating zone begins the calcination zone, in which by means of theproper heat supply the CaCO3 dissociates into CaO and CO2. Therequired heat for calcination is supplied by the combustion of fuel(coal, petroleum, gas, etc.). At the end of the calcination zone,begins the cooling zone, in which the quicklime is cooled downwith a flow of air that heats up and is used in the fuel combustion,thus improving the thermal efficiency of the process.

3. Indicators

An alternative way to evaluate the thermal efficiency in the limeproduction is by means of exergy analysis. Exergy analysisaddresses all the energy loss through the boundaries of a process aswell as the energy degradation because of the entropy generation

inside its boundaries. Granovskii et al. [11] outline “It is generallyaccepted that an increase in the efficiency of fossil fuel utilizationmakes industrial technologies more ecologically benign and oftenmore safe. Therefore, exergy methods can help in rationallymodifying contemporary technologies.” Of course, exergy methodscan improve the thermodynamic aspects of contemporary tech-nologies but no further. Exergy accounting is known to provide

A.S. Gutiérrez, C. Vandecasteele / Energy 36 (2011) 2820e28272822

a wide and clear vision of the use and degradation of energy [12].Exergy analysis is relevant in identifying and quantifying both theconsumption of useful energy (exergy) used to drive a process aswell as the irreversibilities (exergy destructions) and the losses ofexergy. The latter are the true inefficiencies and, therefore, anexergy analysis can highlight the areas of improvement of a system[13]. A reduction in the system’s efficiency directly impacts theplant performance through additional fuel consumption [14].Exergy analysis points to the energy loss through the boundaries ofa process as well as to the energy degradation because of theentropy generation inside its boundaries.

Exergy analysis is generally applied in the evaluation of powergeneration systems. Nevertheless, as thermodynamic laws governenergy conversion processes, this method can be applied to allprocesses where energy conversion occurs and not only to powergeneration systems. According to Hoffman [15] the optimization ofa thermodynamic process can be developed as to minimize thedissipation of energy (exergy destruction) or to maximize thepower generation. In the case of lime production, the aim is toreduce the exergy destruction of the process.

The exergetic evaluation of the limestone calcination in lime-kilns will clearly indicate the locations of heat dissipation in theprocess and its influence in the fuel consumption, highlighting thesaving potentialities.

3.1. Exergy efficiency

The exergy efficiency is defined on the basis of an exergybalance.

An exergy balance of a process [16,17] takes into considerationthat the exergy supplied to the process (EF) is partly irreversiblydestroyed (ED), partly loss (Eloss) by exergy transport to thesurroundings, and partly obtained as a product (EP):

EF ¼ ED þ Eloss þ EP (1)

When the exergy content in the product is considered its valu-able part, as is the case in thermoelectric power plants whereelectricity is valuable because of its exergy content, the exergyefficiency is defined as the ratio between product exergy and fuelexergy [12,17e20]:

he ¼ EPEF

¼ 1� ED þ ElossEF

(2)

This definition does not consider that only a part of the thermo-dynamic inefficiencies can be avoided whereas the remaining partcannot. Improvement efforts should be centered on the inefficienciesthat can be avoided. So that, Tsatsaronis and Park [16] distinguishesbetween the avoidable and unavoidable exergy destruction:

ED ¼ EAVD þ EUND (3)

and modify the definition of exergy efficiency [16]:

he ¼ EPEF � EUND

¼ 1� EAVDEF � EUND

(4)

As a second form of the exergy efficiency Kotas [21] proposedthe efficiency defect to define the fraction of the input loss throughirreversibility:

1� he ¼ EDEF

(5)

In equation (5) the exergy loss to the surroundings is consid-ered as external exergy destruction and is included in the term ED.

Granovskii et al. [11] also consider the ratio between the exergydestruction and the exergy supply and call it the depletionnumber.

In the particular case of the calcination process in a lime shaftkiln, this traditional point of view of the exergy efficiency conceptmight lead to the idea that increasing the exergy content in the limeproduced will make the process more efficient. In reality anincrease of the exergy content of the lime product represents anincrease of its thermal exergy, which represents part of the exergyloss in the process. Increase on the exergy content of the limeproduct increases its temperature, and reduces the lime quality.Indeed, the reactivity of lime is reduced by high temperatures, thereduction being proportional to the temperature and the time thatlime is exposed to this temperature [22]. So, a new definition isneeded to calculate the exergy efficiency of the lime calcinationprocess.

If the exergy content in the product is considered as process loss,as is the case in lime production and therefore included in Eloss andequation (3) is taken into account, the exergy balance of equation(1) becomes:

EF ¼ Eloss þ EAVD þ EUND (6)

From equation (6) it appears that the efficiency of the process ismaximal when the sum of the exergy loss and the avoidable exergydestruction equals zero and the exergy supply thus equals theunavoidable exergy destruction (minimum quantity of exergynecessary for the process to take place). The exergy efficiency can inthis case be defined as the ratio between the unavoidable exergydestruction and the exergy supply:

he ¼ EUNDEF

¼ 1� EAVD þ ElossEF

(7)

It is indeed maximum (equal to 1) when the exergy supplyequals the unavoidable exergy destruction.

Equation (7) appears similar to equation (5) proposed by Kotas[21], in both cases the ratio between the exergy destruction and theexergy supply is considered. Nevertheless, these expressions areconceptually different. Equation (5) gives an indirect measurementof the traditional exergy efficiency, with no distinction madebetween avoidable and unavoidable exergy destruction. In equa-tion (7) the unavoidable exergy destruction defines the efficiency ofthe process and the sum of avoidable exergy destruction and exergyloss characterize its inefficiencies.

To illustrate this fact, let us consider a shaft kiln producing26.3 t/(m2 day) of lime under conditions with specific energyconsumptions of 4.1 MJ/Kglime and 4.3 MJ/Kglime, respectively. Thevalue of irreversibilities for the kiln operation with 4.1 MJ/Kglime,where the kiln operates with the lowest fuel consumption, will beconsidered as the unavoidable exergy destruction for this produc-tion rate. Fig. 2 shows the exergy balance in both cases:

In the first case the calculation of the efficiency defect and theproposed exergy efficiency gives 93%. In this case figures coincidebecause the total exergy destruction of the process equals theunavoidable exergy destruction. In the second case, where the totalexergy destruction differs from the unavoidable exergy destruction,calculations of the efficiency defect results in 91%while the exergyefficiency gives 89%. This means that from the total exergydestruction can be saved the 2%, representing the avoidable exergydestruction, by improving the operational criteria of the kiln. In thiscase the new definition of exergy efficiency proposed in this workbetter address the potentialities of fuel saving based on the limits ofthe energy usage in the kiln highlighting the inefficiencies of theprocess.

Fig. 2. Exergy balance of limekiln: a) lime output 26.3 t/(m2 day), air excess number1.1, energy usage 4.1 MJ/KgCaO, b) lime output 26.3 t/(m2 day), air excess number 1.1,energy usage 4.3 MJ/KgCaO. (Exergy values are in MJ/m2 day).

A.S. Gutiérrez, C. Vandecasteele / Energy 36 (2011) 2820e2827 2823

3.2. Exergy effectiveness

The calcination of limestone has some special requirementsassociated to the dissociation of CaCO3. This reaction needstemperatures above 900 �C in the presence of CO2 in thesurrounding gases [1,4,22]. To achieve a sufficient reaction rate it is,however, necessary to increase the temperature of the surroundinggas to values near 1200 �C to increase the heat transfer rate [6]. Inthe case of a limekiln, it is possible to operate in the range ofunavoidable exergy destruction for temperatures below or justabove the dissociation temperature. Under this conditions theexergy efficiencywould be high, but the effectiveness of the processis zero or very low, because only very small amounts of CaCO3

dissociate. A way to assess this situation is through a new indicatorto evaluate the effectiveness of the dissociation reaction:

e ¼ EdEF

(8)

Equation (8) represents the ratio between the exergy irre-versibly consumed by the dissociation reaction and the exergysupplied to the process. In the theoretical case that exergy effi-ciency remains in an acceptable range during operation and thatexergy effectiveness values remains low, this will indicate that thecalcination process is not developing under the proper tempera-ture conditions.

4. Mathematical modeling

4.1. Governing equations

In order to calculate the proposed indicators it is necessary toestablish the exergy supply to the limekiln and the exergydestruction during the process. To do so, the temperature of the exitgases and of the quicklime must be determined. In this casea mathematical model is used to describe the heat transfer processand the temperature profile in the kiln [1]. This model describes theshaft kiln with two systems of differential equations, one systemdescribing the preheating and the cooling zone and the other thecalcination zone. The mathematical model only describes the meantemperature of the limestone particles.

4.1.1. PreheatingIn this zone the limestone is heated to just below the dissocia-

tion temperature. An energy balance for the solids and gas phasegives:

Gas :dTgdz

$ _Mg$cpg ¼ hkðzÞ$A$�Tsol � Tg

�(9)

Solid :dTsoldz

$ _Msol$cpsol ¼ hkðzÞ$A$�Tsol � Tg

�(10)

The limestone surface per unit length, in a section dz, is calcu-lated as:

A ¼ Afurnace$PL$ð1� jÞ (11)

The specific surface area of the lime particle depends on theshape and the size of the particle. For a sphere:

PL ¼ 6dL

(12)

The boundary conditions are:

Tgðz ¼ 0Þ ¼ Tgout

Tsol(z ¼ 0) ¼ 20 �CTsol(z ¼ zph) ¼ 820 �C

The transient heat transfer coefficient (hk) includes heatconduction from the core of the particle to its surface andconvective heat transfer from the surface of the particle to the gasphase:

hk ¼ 11hþ dL=2k$ksol

(13)

The transient factor for a counter-current flow with a capacityflow ratio of 1equals [1]:

k ¼8<:

3 for a plate4 for a cylinder5 for a sphere

(14)

The coefficient of convective heat transfer in a packed bed iscalculated using the model of the hydraulic diameter. In this casethe Nusselt number definition includes the void fraction:

Nu ¼ h$dLkg

$j

ð1� jÞ (15)

The Nusselt number is approximated by the expression [1,23]:

Nu ¼ 2$j

ð1� jÞ þ 1:12$Re0:5$Pr0:33 þ 0:0056$Re$Pr0:33 (16)

Similar to the Nusselt number, the Reynolds number for thepacked bed is also defined including the void fraction:

Re ¼ vg$dL

n$ð1� jÞ

j(17)

The superficial gas velocity (vg) can be calculated with theknown gas volumetric flow at standard temperature and pressure(STP) (u) and the cross-section area of the kiln. Since the gasvolumetric flow changes with the temperature, the superficial gasvelocity is temperature dependent:

vg ¼ uAfurnace

$T

273(18)

4.1.2. Cooling zoneIn the cooling zone the quicklime is cooled down from the

calcination temperature to the lowest possible temperature. Fromthe energy balance is obtained:

A.S. Gutiérrez, C. Vandecasteele / Energy 36 (2011) 2820e28272824

Gas :dTa

$ _Ma$cpa ¼ hkðzÞ$A$ðTsol � TaÞ (19)

Fig. 3. Flows of exergy entrance and leaving the kiln.

dz

Solid :dTsoldz

$ _Msol$cpsol ¼ hkðzÞ$A$ðTsol � TaÞ (20)

The boundary conditions for the cooling zone are:Ta(z ¼ Z) ¼ 20 �C

Tsolðz ¼ ZÞ ¼ Tsolout

4.1.3. Calcination zoneIn this zone the calcination reaction and the fuel combustion

takes place, producing a temperature above 900 �C. From theenergy balance follows:

Gas :dTgdz

$ _MgðzÞ$cpg ¼ dUfueldz

$LCV$ _MgðzÞ � hkðzÞ$A$�Tsol � Tg

�(21)

Solid :dTsoldz

$ _MsolðzÞ$cpsol

¼ hkðzÞ$A$�Tsol � Tg

� � dXlime

dz$DhCO2

$ _MsolðzÞ (22)

Boundary conditionsTsol(z ¼ zc) ¼ 820 �C

Tgðz ¼ zcÞ ¼ Tgzc

In equation (22) the decomposition behavior of the limestoneparticle during the dissociation is included in the term dXlime/dz.The decomposition process involves five sub-processes, which arein series: heat transfer by convection from the surrounding gases tothe solid surface, thermal conduction from the surface to thereaction front, chemical reaction at the front, diffusion of CO2through the porous lime layer to the surface and diffusion of CO2 tothe surroundings gases. In the model it is assumed that the particlegeometry is ideal and that the shrinking core model describes thechemical reaction [1,23,24]. This results in two coupled differentialequations:

dXlime=dt$½Rh þ Rk$f1ðXÞ� ¼ 1 (23)

dXlime=dt$hRb þ RDiff$f1ðXÞ þ Rchem$f2ðXÞ

i¼ 1 (24)

The equations for the resistances and the shape factor weregiven by Chen et al. [23].

4.2. Numerical solution

The mathematical model described above to determine thetemperature profile for the solid and gas phases in a shaft kilnconsists of a system of ordinary differential equations and isa boundary value problem, where the boundary conditions aredistributed between two points. In order to solve this problem thesolution needs to be calculated iteratively using numericalmethods. Considering the characteristics of the mathematicalmodel the appropriate solution method is the RungeeKuttamethod of 4th order. Unfortunately this method does not providea solution when one of the terms equals zero. In this case the finitedifference method is applied.

Considering that, when the temperature of the solid and the gasphases reach the same value in some point of the calcination zone,

the RungeeKutta method, does not provide a solution, the calci-nation zone was divided into 2 zones:

� The calcination zone, where the limestone calcination and fuelcombustion take place.

� The air preheating zone, where the air emerging from thecooling zone is preheated to a temperature higher than thesolid temperature.

The proper system of equations is used for each zone but allzones are coupled by the temperatures of the solid and gas phases.The equations describing the preheating and the burning zone aresolved with the RungeeKutta method, while the air preheatingzone is modeled with finite difference method. The error tolerancewas taken as 10�4 with a step size of 0.015 m.

5. Exergy balance of the lime shaft kiln

In a vertical shaft kiln, there are three input flows of exergy (seeFig. 3): the exergy of the fuel, the exergy of the limestone and theexergy of the combustion air. Also there are three exit flows, theexergy of the exit gas, the exergy of the quicklime and the exergy ofthe heat loss through the kiln walls. Finally, avoidable andunavoidable exergy destruction takes place in the process. From anexergy balance we obtain:

EF þ Ea þ ELs ¼ Eg þ EQl þ Ewall þ EAVD þ EUND (25)

The exergy of the fuel can be calculated from the ratio of thechemical exergy to the lower heating value [21]:

4 ¼ LHVeOq

(26)

This relation is function of the chemical composition of the fuel(H e hydrogen, C e carbon, O e oxygen and S e sulfur). Propervalues and expressions to calculate 4 are provided by Kotas [21].

For gaseous fuel these values are: 1.04(�0.5%) for natural gas,0.985 for hydrogen and 0.973 for carbon monoxide.

For liquid fuel 4 is calculated as:

4 ¼ 1:0401þ 0:1728$HCþ 0:0432$

OC

þ 0:02169$SC$

�1� 2:0628$

Hc

�(27)

and for solid fuel as:

4 ¼ 1:0437þ 0:1882$HCþ 0:0610$

OCþ 0:0404$

NC

(28)

The exergy of the air, the limestone, the exit gases and thequicklime are calculated from:

dE ¼ dH� To$dS (29)

The exergy content of a system is an extensive variable depend-ing on the mass of the system:

Table 1Data from Bes investigation and value of calculated indicators.

d(m) Q1 (t/m2 day)E*Tð

MJtlime

Þ EUND ð MJtlime

Þ l he e Eg EQl

0.08 26.3 4.1 3471 1.10 0.928 0.794 0.070 0.0020.08 26.3 4.1 3481 1.10 0.931 0.794 0.067 0.0020.08 26.3 4.2 3517 1.10 0.918 0.775 0.080 0.0020.08 26.3 4.3 3575 1.10 0.911 0.757 0.086 0.0020.08 26.3 4.0 3637 1.35 0.939 0.814 0.059 0.0020.08 26.3 4.1 3684 1.35 0.928 0.794 0.070 0.0020.08 26.3 4.2 3772 1.35 0.928 0.775 0.070 0.0020.08 26.3 4.1 3844 1.40 0.933 0.794 0.065 0.0020.08 26.3 4.2 3905 1.40 0.925 0.775 0.072 0.0020.08 26.3 4.3 3971 1.40 0.919 0.757 0.079 0.0020.08 29.0 4.1 3471 1.10 0.928 0.794 0.070 0.0020.06 23.6 4.1 3471 1.10 0.928 0.794 0.070 0.0020.06 41.5 4.1 3471 1.10 0.928 0.794 0.070 0.0020.06 47.2 4.1 3471 1.10 0.928 0.794 0.070 0.0020.06 47.6 4.1 3471 1.10 0.928 0.794 0.070 0.002

Fig. 4. Performance of the indicators with the daily operations.

A.S. Gutiérrez, C. Vandecasteele / Energy 36 (2011) 2820e2827 2825

E ¼ e$m (30)

Once established the reference state (To), the specific exergy isa variable of state.

The exergy loss through the wall equals:

Ewall ¼ Qwall$

�1� To

T

�(31)

And the heat loss through the wall can be estimated as 170 KJ/KgCaO [1].

The total exergy destruction of the process is calculated as:

ED ¼ EF � Eloss (32)

6. Case study

The behavior of the proposed indicators is analyzed in a smallcommercial lime factory with data obtained during two different20 day periods of regular work, without and with the imple-mentation of a change of the operational criteria. During the firstperiod the plant was under very inefficient operation conditions,during the second period these criteria were modified and the fuelconsumption was improved.

This particular lime plant was selected because the cooling zonein the lime shaft kiln is out of service, causing considerable losses inprocess performance. This situation permits to highlight somecharacteristics of the proposed indicators.

Also, in the lime plant no information is available regarding theoperation conditions established by the constructor company.Without this information it is not possible to define the unavoid-able exergy destruction during the kiln operation. The shaft kilnoperating in the lime plant has a total length of 11.3 m of the pre-heating and calcination zone. Bes [1] reports information on theexploitation of lime shaft kilns with a total length of 9.5 m for thepreheating and calcination zone. The proposed indicators arecalculated for these data (Table 1). The average value of unavoidableexergy destruction obtained from these calculations is used asa reference to calculate the exergy efficiency of the shaft kiln in thelime plant.

Calculations are performed based on laboratory measurementsof the available CaO percentage in the different production periodsof the factory [6]. The laboratory measurements are carried outaccording the Cuban Standard NC 54-279 based on the AmericanStandard ASTM C-25. Limestone composition is determinedfollowing the Cuban Standard NC 54-27 based on the AmericanStandard ASTM C-25 and ASTM C-110.

As the cooling zone of the kiln in the lime plant is not operating,all the simulations were developed for the behavior of the pre-heating and the calcination zone.

One aspect to remark is that the exergy efficiency for the shaftkiln in the lime plant was calculated with a value of unavoidableexergy destruction obtained from calculations performed with dataobtained from literature [1]. Of course, this introduces an error inthe obtained results. Nevertheless, this value can be used asa reference of the behavior of the limekiln in the plant consideringthat data from Ref. [1] deal with normal vertical shaft kilns withsimilar values for the total length of the preheating and the calci-nation zone as those for the lime plant.

6.1. Indicator performance at plant level

From the data of the plant it appeared that without the change ofthe operational criteria, the plant operated with an average energyconsumption of 9.6 MJ/KgCaO and emitted 1:42 tonCO2

=tonQl. Withthe change of operational criteria implemented these values werereduced to 7.3 MJ/KgCaO and 1:37 tonCO2

=tonQl, respectively. Thesevalues are considerably high. Ref. [1] gives for a good kiln perfor-mance to produce medium burned quicklime, in a normal verticalshaft kiln, an energy consumption of 4.1e4.3 MJ/KgCaO and toproduce hard burned quicklime 4.5e4.6 MJ/KgCaO.

Fig. 4 shows the behavior of the proposed indicators. Black linesrepresent values without the operational criteria change and graylines represent values with the operational criteria change.Withoutthe change of the operational criteria, the average efficiency of thekiln and the effectiveness were remarkably low, 36.5% and 31.8%,respectively. With the implementation of the operational criteriachange these values were improved and the exergy efficiency andthe exergy effectiveness of the dissociation reaction increased to40% and 34.9%, respectively.

Fig. 5 gives the exergy loss according to equation (25) related to

the exergy of the fuel ðEð%Þ ¼ EiEF$100Þ. The same color code is

applied as in Fig. 4 to indicate the values without and with theimplementation of the operational criteria change. It is clear thatwithout and with the change of operational criteria the mainexergy loss of the process is the avoidable exergy destruction,representing 39%, the gas flow contributes with about 16.2% andthe loss with the quicklime flow with 8.3% summing up to 63.5% ofthe exergy supply. With the implementation of the operationalcriteria change the exergy loss distribution shows that the loss byavoidable exergy destruction is reduced to 35.7% and the loss withthe gas flow to 15.2%, at the same time the exergy loss with thequicklime flow increases to 9.1% and the total loss is reduced to ca.

Fig. 5. Exergy loss without and with the operational criteria change.

A.S. Gutiérrez, C. Vandecasteele / Energy 36 (2011) 2820e28272826

59.9%. In both cases the exergy loss through the wall is negligiblewith values ranging from 0.6 to 0.8%.

Although, the change of operational criteria only causes smallimprovements, it is important to remark the obtained results. Withthe operational criteria change the exergy loss with the gas flow isreduced by about 1.0%, the loss with the quicklime flow increases byabout 0.8% and the avoidable exergy destruction is reduced byabout 3.4%. The efficiency increases by about 3.6%. With the oper-ational criteria change the total exergy loss with the gases is slightlyreduced due to a reduction on the exit gas temperature causinga reduction of its specific exergy. Also a decrease of the mass flow ofthe exit gas contributes to the exergy loss reduction according toequation (30). On the contrary, the exergy loss with the quicklimeincreases slightly, as the exit temperature of the quicklime and thespecific exergy remain almost the same, but the mass flow ofquicklime per unit of fuel consumption increases (as the efficiencyincreases reducing the fuel consumption) growing the total exergyloss according to equation (30).

From Table 1 the kilns reported by Bes [1] give an average exergyefficiency and effectiveness of 92.7% and 78.6%, respectively, withan exergy loss of 7.1% in the gas flow and 0.2% in the quicklime.When these values are comparedwith those obtained for the kiln ofthe lime plant considered, it is clear that in the lime plant consid-ered the kiln operates with a very low efficiency even with theoperational criteria change.

The proposed exergy efficiency allows the assessment of theprocesswhere the product is not valuated for its exergy content. Theresults obtained on the case study show that the analyzed plantoperates under very inefficient criteria. If we compare with theresults obtained in Table 1 we find that in this case the loss repre-sents ca. 7% of the exergy supply with the fuel while in the casestudy the loss represents ca. 60% of the fuel consumption. Themajorcontribution to the fuel loss is the avoidable exergy destructionfollowed by the exergy losswith the gasflowandwith the quicklimeflow. The cooling zone is not operative influencing the efficiency ofthe kiln. This contributes with 8e9% of the fuel loss, increasing itssignificance as the limekiln production is increased. Table 1 showthat the losswith the quicklimeflow represents 0.2% of the total lossin a kiln with the cooling zone operating, stressing the importanceof the cooling zone operation. Also the exergy loss with the gas flowis ca. 7% from Table 1 and for the lime plant of the case studyrepresents ca. 15% emphasizing the importance of an adequatecontrol of the temperature of the exit gas flow.

7. Discussion and conclusions

In this work two new exergy-based indicators to evaluate thelimestone calcination in a lime shaft kiln are proposed. The exergy

efficiency indicator permits to highlight the principal opportunitiesof fuel saving by addressing the main areas of heat dissipation. Italso allows the assessment of the process where the product is notvaluated for its exergy content. The indicator for the exergy effec-tiveness of the dissociation reaction allows to assess the produc-tivity of the limekiln.

Using these indicators the Cleaner Production philosophy can beapplied in lime production. The procedure of application of CleanerProduction includes a stagewhere themain potentialities of savingsof a process are identified [10] and the proposed indicators permitto identify the origin of the exergy loss. From their applicationdifferent actions to reduce fuel consumption and consequently theCO2 emission can be derived, increasing the profitability of the limeplant and reducing the associated environmental impacts.

In general the exergy evaluation represents an improvement inthe evaluation of the lime shaft kiln and identifies the main lossesof the calcination process addressing the avoidable losses.

Acknowledgment

Grateful acknowledgment is made to VLIR-UOS (Flanders,Belgium) for their support through the project “ACenter for CleanerProduction to contribute to the socio-environmental developmentof the province of Cienfuegos, Cuba”. The authors also like toacknowledge the help of Soanlys Santiago and Dayni Mederos withthe language.

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