Analysis of the parameters affecting energy consumption of a rotary kiln in cement industry Adem Atmaca * , Recep Yumrutas ¸ University of Gaziantep, Department of Mechanical Engineering, 27310 Gaziantep, Turkey highlights We analyzed a rotary kiln and investigated the first law and second law efficiency values. Performance assessment of a kiln indicates that the burning process involves energy and exergy losses. The anzast layer affect the efficiency and production capacity of the kiln. The specific energy consumption for clinker production is determined. article info Article history: Received 22 August 2013 Accepted 15 February 2014 Available online 25 February 2014 Keywords: Cement Rotary kiln Specific energy consumption Energy Exergy abstract In this study, the effects of refractory bricks and formation of anzast layer on the specific energy con- sumption of a rotary kiln are investigated. Thermodynamic analysis of the kiln is performed to achieve effective and efficient energy management scheme. Actual data, which are taken from a cement plant located in Gaziantep, Turkey, are used in numerical calculations to obtain energy balance for the system. It is calculated that 12.5 MWof energy is lost from the surface of the kiln which accounts for the 11.3% of the total energy input to the unit. The specific energy consumption for clinker production is determined to be 3735.45 kJ/kg clinker. The formation of anzast layer and the use of high quality magnesia spinel and high alumina refractory bricks provide 7.27% reduction in energy consumption corresponding to a saving of 271.78 MJ per ton of clinker production. It is recognized that the anzast layer has an important role for durability of the refractory bricks and heat transfer out of the kiln. The applications prevent the emission of 1614.48 tons of CO 2 per year to the atmosphere. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Cement industry is one of the most energy intensive industries in the world. It is essential to investigate the feasibility of reducing coal consumption and greenhouse gas emissions of the rotary kilns in the industry. In comparison to the other industrial sectors, cement industry has been consuming the highest proportion of energy. A typical well-equipped plant consumes about 4 GJ energy to produce one ton of cement. At the same time, this sector is one of the worst pollutant sector [1], which emits an increasing amount of greenhouse gases such as carbon dioxide, nitrogen oxide, chloro- fluorocarbons and methane. For each ton of clinker produced, an equivalent amount of greenhouse gases are emitted [2,3]. Cement production in the world is about 3.6 billion ton per year [4]. About 2% of the electricity produced in the whole world is used during the grinding process of raw materials [5]. Total electrical energy con- sumption for cement production is about 110 kWh/t of cement, roughly two thirds of this energy is used for particle size reduction [6]. Because of high energy consumption rates and high environ- mental impact of the process, the manufacturing process has been considered by the investigators for many years. Schuer et al. [7] studied energy consumption data and focused on the energy saving methods for German cement industry considering electrical and thermal energy saving methods. Saxena et al. [8] investigated energy efficiency of a cement plant in India. Worell et al. [9] dealt with energy analysis in the U.S. cement industry for the years 1970 and 1997. Engin and Ari [10] analyzed a dry type rotary kiln system with a kiln capacity of 600 t clinker per day. They found that about 40% of the total input energy was lost through hot flue gas, cooler stack and kiln shell. The study indicates that for a dry type cement production process, the carbon dioxide emission intensity for kiln * Corresponding author. Tel.: þ90 342 317 1734; fax: þ90 342 360 1170. E-mail addresses: [email protected], [email protected](A. Atmaca), [email protected](R. Yumrutas ¸ ). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng http://dx.doi.org/10.1016/j.applthermaleng.2014.02.038 1359-4311/Ó 2014 Elsevier Ltd. All rights reserved. Applied Thermal Engineering 66 (2014) 435e444
Analysis of the parameters affecting energy consumption of a rotarykiln in cement industry
Adem Atmaca*, Recep YumrutasUniversity of Gaziantep, Department of Mechanical Engineering, 27310 Gaziantep, Turkey
h i g h l i g h t s
� We analyzed a rotary kiln and investigated the first law and second law efficiency values.� Performance assessment of a kiln indicates that the burning process involves energy and exergy losses.� The anzast layer affect the efficiency and production capacity of the kiln.� The specific energy consumption for clinker production is determined.
a r t i c l e i n f o
Article history:Received 22 August 2013Accepted 15 February 2014Available online 25 February 2014
Keywords:CementRotary kilnSpecific energy consumptionEnergyExergy
http://dx.doi.org/10.1016/j.applthermaleng.2014.02.031359-4311/� 2014 Elsevier Ltd. All rights reserved.
a b s t r a c t
In this study, the effects of refractory bricks and formation of anzast layer on the specific energy con-sumption of a rotary kiln are investigated. Thermodynamic analysis of the kiln is performed to achieveeffective and efficient energy management scheme. Actual data, which are taken from a cement plantlocated in Gaziantep, Turkey, are used in numerical calculations to obtain energy balance for the system.It is calculated that 12.5 MW of energy is lost from the surface of the kiln which accounts for the 11.3% ofthe total energy input to the unit. The specific energy consumption for clinker production is determinedto be 3735.45 kJ/kg clinker. The formation of anzast layer and the use of high quality magnesia spinel andhigh alumina refractory bricks provide 7.27% reduction in energy consumption corresponding to a savingof 271.78 MJ per ton of clinker production. It is recognized that the anzast layer has an important role fordurability of the refractory bricks and heat transfer out of the kiln. The applications prevent the emissionof 1614.48 tons of CO2 per year to the atmosphere.
� 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Cement industry is one of the most energy intensive industriesin the world. It is essential to investigate the feasibility of reducingcoal consumption and greenhouse gas emissions of the rotary kilnsin the industry. In comparison to the other industrial sectors,cement industry has been consuming the highest proportion ofenergy. A typical well-equipped plant consumes about 4 GJ energyto produce one ton of cement. At the same time, this sector is one ofthe worst pollutant sector [1], which emits an increasing amount ofgreenhouse gases such as carbon dioxide, nitrogen oxide, chloro-fluorocarbons and methane. For each ton of clinker produced, anequivalent amount of greenhouse gases are emitted [2,3]. Cement
production in the world is about 3.6 billion ton per year [4]. About2% of the electricity produced in the whole world is used during thegrinding process of raw materials [5]. Total electrical energy con-sumption for cement production is about 110 kWh/t of cement,roughly two thirds of this energy is used for particle size reduction[6]. Because of high energy consumption rates and high environ-mental impact of the process, the manufacturing process has beenconsidered by the investigators for many years. Schuer et al. [7]studied energy consumption data and focused on the energysaving methods for German cement industry considering electricaland thermal energy saving methods. Saxena et al. [8] investigatedenergy efficiency of a cement plant in India. Worell et al. [9] dealtwith energy analysis in the U.S. cement industry for the years 1970and 1997. Engin and Ari [10] analyzed a dry type rotary kiln systemwith a kiln capacity of 600 t clinker per day. They found that about40% of the total input energy was lost through hot flue gas, coolerstack and kiln shell. The study indicates that for a dry type cementproduction process, the carbon dioxide emission intensity for kiln
A. Atmaca, R. Yumrutas / Applied Thermal Engineering 66 (2014) 435e444436
feed preparation process is about 5.4 kg CO2 per ton cement pro-duced. Camdali et al. [11] have calculated the enthalpies going intoand leaving the rotary kiln in cement industry and the heat lossesfrom the system by conduction, convection and radiation accordingto the first law of thermodynamics. Furthermore, exergy analysis ofthe system is made based on the second law of thermodynamics.Kabir et al. [12] analyzed a pyroprocessing unit of a typical dryprocess cement plant. In order to enhance the energy performanceof the unit, they considered conservation of heat losses from thesystem. Application of waste heat recovery steam generator andsecondary kiln shell were suggested. They showed that power andthermal energy savings of 42.88 MWh/y and 5.30 MW can beachieved respectively. Atmaca et al. [13e15] have employed energyand exergy analysis on a pyroprocessing unit in Turkey, the rate ofheat loss is reduced from 22.7 MW to 17.3 MWby the application ofinsulation to the system. They determined that 1056.7 kW ofelectricity can be generated by using the waste heat, and annualemission rates have been reduced by 8.2%.
In this study, thermal performance of the rotary kiln presentedin a cement plant is investigated using energy analysis based thefirst and second laws of thermodynamics. The data collected from acement plant located in Gaziantep, Turkey, are used in numericalcalculations to obtain realistic performance parameters. The effectsof the anzast layer and thickness, type and composition of re-fractory bricks on the performance parameters of the kiln areexamined. The literature survey indicates that studies on rotary kilnis limited in number and scope, and this paper can contribute to abetter understanding of rotary kiln operation and parametersaffecting its performance.
2. System description
Cement production is a long process which consumes largeamounts of fossil fuels and electricity. The process includes fivemain stages: (a) mining and grinding of raw materials into finepowder, (b) blending the farine in homogenization silos prior topreheating in four staged cyclone preheaters, (c) increasing thetemperature of farine (pre-calcination) in preheating tower withflue gases from the kiln, (d) burning the prepared mixture of farinein a rotary kiln (calcination) after the preheating tower and (e)grinding the clinker in a cement mill.
Clinker production in rotary kiln system is the most energyintensive stage in cement production, accounting for about 90% oftotal thermal energy use [8].
In the present study, Gaziantep cement plant located in theSouth-east of Turkey is considered as a case study for the thermalenergy analysis. Annual cement production capacity of the plantis 1.5 million tons. The cement plant operates on a dry cementprocess line. The rotary burner is a refractory lined tube type kilnwith a diameter of 4.2 m and 59 m length. It is inclined at anangle of 3.5�, and its rotational speed is 1e2 rpm. The averageclinker production capacity of the rotary kiln is 65 t/h. Four stagecyclone type pre-heater is used to pre-calcinate the raw materialbefore it enters the kiln. In a typical dry rotary kiln system, pre-calcination gets started in the pre-heaters, and approximatelyone third of the raw material would be pre-calcined at the end ofpre-heating. The temperature of the pre-heated material is about1000 �C. The raw material passes through the rotary kiln towardsthe flame. In the calcination zone, calcination process, combina-tion of alumina, silica and ferric oxide with lime take place atabout 1500 �C. Pulverized coal is burnt in the rotary kiln to reachthe required reaction temperature. After the combustion and thereactions inside the kiln, clinker, the semi product of cement isproduced. Clinker is rapidly cooled in cooling unit after the rotarykiln. Fast cooling of the clinker enables heat recovery from
clinker, and improves the product quality [9]. The clinker isground together with gypsum and other pozzolans materials andfinally cement is produced. The flow diagram of the rotary kiln isshown in Fig. 1.
3. Thermodynamic analysis of the rotary kiln
The rotary kiln is heart and the most energy consuming part of aconventional cement plant. Thermodynamic analysis of the kilnsystem is performed in this section to achieve effective and energyefficient management scheme. Energy and exergy analyses for thekiln unit of the cement factory are performed by using the first andsecond laws of thermodynamics. Specific heat capacity, input andoutput mass of each item, temperature, pressure values and con-stant specific heat of the input and output materials are determinedfor the operating rotary kiln. Cement production is a continuousprocess. Stopping the production process in order to change therefractories is a long, costly and undesirable process. The refractorybricks of the rotary kiln are changed when they lose their thermalproperties. In order to enter into the rotary kiln and measure thethickness of the anzast layer, we waited for the appropriate time.Manymeasurements have been taken for about 3 years and averagevalues are used. During the analysis, the following assumptions aremade: (1) the system is assumed to be steady state, steady flowprocess, (2) kinetic and potential energy chances of input andoutput materials are negligible, (3) the gases inside the kiln areassumed to be ideal gases, (4) electrical energy produces the shaftwork in the system, (5) the ambient and kiln average surfacetemperatures are constant throughout the period of the study.
In order to find heat and work interactions, energy and exergyefficiencies, and the rate of irreversibility in a steady state flowprocess, the following balance equations are applied. The massbalance for an open system operating under steady state conditionsis expressed as:
X_min ¼
X_mout (1)
where _m is the mass flow rate of the kiln, subscripts “in” and “out”in all expressions stand for input and output values of eachparameter.
The general energy balance can be expressed as:
X_Ein ¼
X_Eout (2)
_Qnet;in � _Wnet;out ¼X
_mouthout �X
_minhin (3)
where _Q is the rate of heat transfer, _W is the rate of work, _m is massflow rate, and h is enthalpy. The first law (energy or energetic)
Table 1Clinker composition.
Chemical name Chemical structure Chemical form Percentage (%)
Calcium ferrite 4CaO. Al2O3.Fe2O3 C4AF 10.4Di-calcium silicate 2CaO.SiO2 C2S 13.2Calcium aluminate 3CaO. Al2O3 C3A 9.1Tri-calcium silicate 3CaO.SiO2 C3S 60.2Potassium oxide K2O e 2.5Sulfur trioxide SO3 e 2.1Magnesium oxide MgO e 1.2Sodium oxide Na2O e 1.3Total e e 100
Table 2Kiln zones and refractory materials.
Kiln zones Refractory materials
1 Chain zone: The “front end” of the kiln,it is typically lined with coarseaggregate monolithic.
Coarse aggregate
2 Preheating: Usually the longest zone asthe name suggests this section is thepre-heating section of the kiln. It isgenerally lined with alkali resistantrefractories such as 40e50% aluminabricks.
Mid/high alumina
3 Calcining: Higher in temperature thanthe chain and pre-heating zones it iscommonly lined with higher aluminarefractory materials.
High alumina
4 Burning zone: The eutectic temperaturebetween the “free” lime in the calcinedfeed and alumina-silica materials is inthe region of 1100e1300 �C, that is whythe burning section of the kiln is linedwith basic refractories such asmagnesia-chrome or magnesia-spinel asthese materials form no eutectics with
Magnesia-alumina-spinel
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efficiency is defined as the ratio of energy output to the amount ofenergy input, which is:
hI ¼P _EoutP _Ein
(4)
The general exergy balance is expressed as:
X_Exin �
X_Exout ¼
X_Exdest (5)
X�1�T0
Tp
�_Qp� _Wnet;outþ
X_minjin�
X_moutjout ¼
X_Exdest
(6)
where _Qp is the heat transfer rate through the boundary at tem-perature Tp at location p. The subscript zero indicates properties atthe dead state of P0 and T0. The subscript dest indicates destruction.The second-law (exergy or exergetic) efficiency may generally bedefined as the rate of exergy output divided by the rate of exergyinput:
hII ¼P _ExoutP _Exin
(7)
In this study, we use Eq. (9) and use exergies of outgoing andinput materials to the unit. Maximum improvement in the exergyefficiency for a process is obviously achieved when the exergy loss
Fig. 2. The thermal resistance network for heat transfer through the mantle of therotary kiln.
or irreversibility is minimized. Higher exergy efficiency permits abetter matching of energy sources and uses [16].
Internal energy change and enthalpy change values are:
Du ¼Z2
1
cðTÞdT ¼ cavgðT2 � T1Þ (8)
Dh ¼ Duþ y DP (9)
where cavg is average specific heat, y is specific volume and DP ispressure change. Due to negligible pressure change the enthalpychange is equal to the internal energy change. The enthalpy valuesof the input and output materials can be expressed with referenceto ambient conditions:
Dhin ¼ cavgðT1 � T0Þ (10)
Dhout ¼ cavgðT2 � T0Þ (11)
lime at the temperatures encountered inthe “hot” zones. The three sections arecommonly determined by differentclinker coating conditions, a stablecoating is essential for extendedrefractory life.
4.1 Upper transition: In the upper transitionzone the coating is usually thin ornonexistent; temperatures are generallyin the region of 1250 �C. About 30% of thelength of the burning zone is called as theupper transition zone.
Magnesia-alumina-spinel
4.2 Sintering: Coatings in sintering zone areusually thick and stable. This zone is aboutthe 50% of the burning zone length.
Magnesia-alumina-spinel
4.3 Lower transition: The lower transition zoneencounters the most severe conditions inthe kiln. Temperatures here are at thehighest and the coating is often unstableand thin, clinker fluids are also present.This zone is about 20% of the burning zonelength.
Magnesia-alumina-spinel
5 Discharge: There is a chamber between therotary kiln and the clinker cooler. The clinkerdischarges into firing hood. High wear oftenoccurs at the ring end of the discharge zone.The discharge zone is commonly lined withbasic or high alumina refractories [18].
Magnesia-alumina-spineland/or mid/high alumina
Fig. 4. Presentation of refractory arrangement and anzast layer in rotary kiln.
A. Atmaca, R. Yumrutas / Applied Thermal Engineering 66 (2014) 435e444438
where T1 and T2 are the input and output temperatures of thematerials and T0 is the ambient air temperature.
For incompressible substances the entropy change is:
s2 � s1 ¼ cavg lnT2T0
(12)
For ideal gases the entropy change is:
s2 � s1 ¼ cp;avg lnT2T0
� R lnP2P0
(13)
Since the pressures of the input and output materials are equal,their Ds values are expressed as:
Dsin ¼ cp;avg lnT1T0
(14)
Dsout ¼ cp;avg lnT2T0
(15)
After obtaining the entropy and enthalpy values of the input andoutput materials, the exergy values of input and output materials inthe rotary kiln are calculated from the equations.
Djin ¼ Dhin � T0Dsin (16)
Djout ¼ Dhout � T0Dsout (17)
4. Heat loss calculation of the rotary kiln
Due to temperature difference between inner surface andambient air temperature, therewill be heat transfer from the kiln toatmosphere. The heat transfer from the rotary kiln takes place dueto conduction, convection and radiation. Substantial quantity ofheat is transferred to the atmosphere from the surface. This heattransfer is considered waste heat. Conservation of this heat willimprove the thermal efficiency of the rotary kiln.
To obtain the general energy balance of the system, the energyconsumed during the formation of clinker is calculated. The clinkercomposition which is taken from the facility laboratory is shown in
Fig. 3. Rotary kiln zones.
Rconv;1 ¼ 12pr4L1h1
Rcond;1 ¼ 12pL1k1
lnr3r4
Rcond;2 ¼ 12pL1k2
l
Rcond;3 ¼ 12pL1k3
lnr1r2
Rconv;2 ¼ 12pr1L1h2
Rrad ¼ 12pr1L1hrad
Table 1. Al2O3, MgO, CaO, SiO2 and Fe2O3 percentages in the man-ufactured cement has been analyzed to be 3, 1.76, 51.2, 26.5 and4.5%, respectively. Formation energy of the clinker is calculated byusing the Zur Strassen equation [17].
Formation energy ðkcal=kgÞ ¼ 4:11½Al2O3�þ6:48½MgO�þ7:646½CaO��5:116½SiO2��0:59½Fe2O3� (18)
Energy is transferred by mass, heat and work within the rotarykiln which we choose as the control volume. The simplifications ofthe one dimensional heat conduction, convection and radiationequations in a cylindrical structure for the case of constant con-ductivity for steady conduction with no heat generation is applied(Fig. 2). The rate of heat transfer between the control volume and itssurroundings is calculated from the following equations:
_Q total ¼ Tin � ToutRtotal
(19)
where Rtotal is the total thermal resistance of the system andcalculated from
Ash 2241.99 1.3 290 710 546.00 1.16 340.04 129.81Total e 12,885 e e e e e 1581 603.56TOTAL e 211,577 e e e e e 62,102.68 34,649.86
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Table 4Mass and energy balance of the unit.
Input materials ṁ (kg/h) S _Ein (kW) Percentage (%)
Farine 105,000 20,089.47 18.1Pulvarized coal 7200 112.6 0.1Primary air 9866 87.48 0.1Secondary air 89,511 23,240.13 20.9Electrical work e 4341.5 3.9Combustion of coal e 63,311.65 56.9Total 211,577 111,182.83 100
Output materials ṁ (kg/h) S _Eout (kW) Percentage (%)
Formation of clinker e 36,537.63 32.9Clinker 65,200 24,293.94 21.8Hot gas 133,492 36,227.74 32.6Dust and ash 12,885 1581.00 1.4Heat transfer from the kiln e 12,542.51 11.3Total 211,577 111,182.83 100
A. Atmaca, R. Yumrutas / Applied Thermal Engineering 66 (2014) 435e444440
where h is the convection coefficient, k is the thermal conductivity,and hrad is the radiation heat transfer coefficient and its value isdetermined from,
hrad ¼ 3s�T2out;surf þ T2out
��Tout;surf þ Tout
�(22)
where 3is the emissivity of the surface, and s is StefaneBoltzmanconstant as 5.67 � 10�8 W/m2 K4.
5. Results and discussion
The effects of the refractory bricks and formation of anzast layeron specific energy consumption (SEC) of the kiln are investigated inthis study. For that reason, thermodynamic analysis was performedto find performance parameters of the kiln such as heat losses,efficiency and SEC. The energy and exergy calculations are doneusing MS Excel Professional Plus 2013 which is a commercialsoftware. The software makes it possible to analyze the wholesystem by considering their interactions with each other. Actualdata are used in numerical calculations, and the performance pa-rameters are obtained. The results obtained are presented asTables and figures, and they are discussed in this chapter.
In dry process, the raw ingredients are prepared and storedwithout addition of water, and the kiln is commonly divided intofive zones (Table 2 and Fig. 3). The refractory brick arrangement and
Fig. 6. Energy bala
anzast layer in rotary kiln are presented in Fig. 4. Tin is the innertemperature of the rotary kiln and Tsurf is the surface temperatureof the mantle of the kiln. r1, r2, r3 and r4 are the inner radiuses of therotary kiln. r1 � r2 gives the thickness of the steel mantle, r2 � r3 isthe thickness of the refractory bricks and r4 is the average radius ofthe anzast layer.
The surface of the kiln is divided into 4 sections with differentsurface temperature values. The type and length of refractory ma-terials used are presented in Fig. 5.
5.1. Energy and exergy analysis of the kiln
Energy balance for the kiln is defined that energy input is equalto energy output for the steady state operation. Total energy inputto the kiln consists of energy entering by raw materials, electricityconsumed and the combustion of pulverized lignite coal. Totalenergy output consists of the energy absorbed by raw materials,heat loss and hot gas leaving from the kiln. Results obtained from acase study can be given to explain energy analysis for the kiln.
The results of the energy and exergy analysis for the rotary kilnunit are presented in Table 3. The relevant data and constants areobtained from on site measurements. The work transfer due toelectricity and heat lost values are calculated. The given andcalculated values including mass flow rates ð _mÞ, input (Tin) andoutput (Tout) temperatures, constant pressure specific heats (cp),enthalpy (Dh) and entropy (Ds) changes, energy and exergy valuesof the raw materials, first law (hI) and second law (hII) efficiencies,and average ambient air temperature (T0) are given in Table 3. Thematerial and energy balance for the unit is presented in Table 4. Thespecific heat capacity of the each input and output material hasbeen calculated using the empiric correlation below which prac-tices upon the Kirchhoff law [19].
Cp ¼ aþ bT þ cT2 þ dT3 (23)
Here, a, b, c and d are the constants for raw material, and Trepresents temperature of each material. The constants of eachcomponent of the input and output materials are taken fromRefs. [20,21].
Total energy input to the kiln is calculated to be 111.18 MW. Heatlost from the surface of the kiln is equal to 12.5 MWwhich accountsfor the 11.3% of the total energy input to the unit. Based on thecalculated values given in Table 3, the first law efficiency of the kiln
nce of the RK.
Fig. 7. The energy band diagram (Sankey) of the rotary kiln.
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is calculated from Eq. (4) to be 62.10 MW/111.18 MW ¼ 0.558 or55.8%. The energy lost account to 44.2% of the inlet energy. Thesecond law efficiency of the rotary kiln is calculated from Eq. (7) tobe 34.649 MW/89.49 kW ¼ 0.387 or 38.7%. This corresponds to anexergy loss of 61.3% in the unit.
Fig. 7 shows a Sankey diagram indicating magnitudes andpercentages of energy flows and losses while Fig. 8 shows aGrassmann diagram with the corresponding data for exergy.Operation of the system involves thermal energy inputs in theform of hot gas, electricity, and thermal energies of burning coaland raw materials. The output includes thermal energies con-tained in clinker and hot gas as well as energy losses with heatlosses, leaking dust and ash and the energy consumed during theformation of clinker which are the unavoidable waste of burningprocess. The input energy is dominated by the combustion ofpulverized lignite coal with a 56.9% contribution while inlet airaccounts for 20.9% and precalcined farine accounts for 18.1% of thetotal energy input (Fig. 8). 32.9% of the input energy is lost duringthe formation of clinker. Heat loss from the surface of the kilnaccounts for 11.3% and hot gas leaving the system accounts for
Fig. 8. The exergy band diagram (Grassmann) of the rotary kiln.
32.6% of the output energies (Table 4). The exergy flow diagram inFig. 8 shows that most of the exergy input to the system is due tocombustion of fuel (69.9%) followed by the exergy of primary andsecondary air streams (13.5%) and the precalcined farine (11.8%).As a result, combustion process has the greatest contribution ofinput energy and input exergy. An examination of output exergiesshows that exergy loss is responsible for 61.3% of the all outputexergies (Fig. 6).
5.2. Specific energy consumption (SEC) of the kiln
The specific energy consumption (SEC) of the system is calcu-lated by using the data taken from the factory area for one year(Table 5). For the production of 65,200 kg/h clinker, the factoryconsumes 4341.5 kW of electricity and 63,311.65 kW of energy bythe combustion of pulverized lignite coal. Thus, the average SEC is(63,311.65 þ 4341.5)*3600/65,200 ¼ 3735.45 kJ/kg clinker.
5.2.1. Effect of anzast layer on the efficiency of the kilnThe formation of an anzast layer inside the kiln has important
effects on energy and exergy efficiency of the unit. Some advan-tages of the anzast layer formation are summarized below:
- An anzast layer protects the refractory bricks against hightemperature values,
- Reduces the deformations on the bricks due to hot clinker flow,- Supports bricks during continuous rotation of the kiln,- Reduces the heat transfer rate and coal consumption.
In order to maintain appropriate anzast layer inside the kiln, thesilicate module of the farine has kept as low as possible to provideeasy sintering. Silica has an abrasive effect on the bricks. Therefore,the amount of free silica has been decreased by using iron oxideminerals instead of sand during production of farine. In this way,thematerials containing higher silicawere able tomelt easily underlower temperature values. The experienced staff have an importantrole to maintain the desired conditions. The thickness of the anzastlayer is measured in each section of the kiln, and the averagethickness of the layer is found to be 450 mm. After obtaining thethickness of the anzast layer, the first law efficiency of the kiln iscalculated to be 69419.89 kW/116614.2 kW ¼ 0.595 or 59.5%. Thesecond law efficiency of the rotary kiln is calculated to be40215.96 kW/93151.44 kW ¼ 0.431 or 43.1%. The SEC of the unit isfound to be 3441.26 kJ/kg clinker. SEC value is calculated from,65760.99 kJ/s � 3600/66450 kg/h ¼ 3562.67 kJ/kg clinker.
Table 5Monthly SEC of the rotary kiln under standard conditions.
a The specific combustion energy of the coal is calculated to be 31,100 kJ/kg coal.
Table 6Properties of new refractory bricks.
Content Section 1magnesiachromite
Section 2magnesiaspinel
Section 3highalumina
Section 4alumina
Mg0 (%) 65e70 80e84 10e20 12e16Al2O3 (%) 2e5 10e14 75e80 65e70Cr2O3 (%) 2e4 e e e
CaO (%) 10e16 2e5 e e
Fe2O3 (%) e e 4e7 3e6SiO2 (%) 3e5 1e4 2e8 4e10Apparent porosity (%) 18 17 20 22Bulk density (g/cm3) 3.05e3.1 2.9e3.05 2.7e2.96 2.65e3.1Thermal conductivity
at 1000 �C (W/mK)3.5 2.4 1.8 2.2
Cold crushing strength(MPa)
55 60 63 61
Thickness (mm) 250 300 350 250
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5.2.2. Effect of the type and quality of the refractory bricks on theefficiency of the kiln
Refractory materials play a critical role in the rotary kiln lin-ing. The main contents of these bricks are mainly based onalumina, magnesia and chrome (Table 6). While replacing thebricks, it is recognized that the thickness of the old bricks arereduced by half in some regions of the kiln. It is seen that, there isonly magnesia chromite and alumina bricks inside the kiln. Thesecond quality old bricks with poor thermal properties arereplaced with bricks which have higher Mg and Al content. Thenew chrome ore free bricks have resistance against high thermo-mechanical and thermochemical loads with redox conditions,alkali and sulfate attack. Thus, the service life of the bricks areincreased considerably.
The heat transfer losses from the surface of the unit and the coalconsumption of the unit decreased considerably after replacinghigh quality refractory bricks inside the kiln. The first and secondlaw efficiency and SEC of the unit have been evaluated again. Basedon the calculated values in Table 7, the first law efficiency of the kilnis calculated to be 72838.4 kW/118973.65 kW ¼ 0.612 or 61.2%. Thesecond law efficiency of the rotary kiln is calculated to be42874.65 kW/95089.68 kW¼ 0.451 or 45.1%. SEC value is calculatedfrom, 64222.21 kJ/s� 3600/66750 kg/h¼ 3463.67 kJ/kg clinker. Theenergy consumption and clinker production of the unit on a monthbasis is given in Table 8.
5.2.3. Effect of ambient air temperature on the efficiency of the kilnThe highest and lowest ambient air temperatures are obtained
from Turkish State Meteorological Service [22]. The monthly
Table 7Mass and energy balance of the unit after installing refractory bricks with betterthermal properties.
Input materials ṁ (kg/h) S _Ein (kW) Percentage (%)
Farine 105,000 22,784.4 19.59Pulvarized coal 6810 106.5 0.09Primary air 10,106 59.74 0.05Secondary air 89,511 29,105.84 25.03Electrical work e 4341.5 3.73Combustion of coal e 59,880.71 51.5Total 211,427 116278.7 100
Output materials ṁ (kg/h) S _Eout (kW) Percentage (%)
Formation of clinker e 35,411.75 30.45Clinker 66,750 25,999.44 22.36Hot gas 133,242 43,566.14 37.47Dust and ash 11,435 1703.75 1.47Heat transfer from the kiln e 9597.62 8.25Total 211,427 116,278.7 100
changes in SEC of the rotary kiln with respect to ambient air tem-perature are shown in Fig. 9. The data indicates that at higherambient temperatures (during summer months), both the first andsecond law efficiencies increase. This corresponds to higher rates ofclinker production. The average air temperatures for winter andsummer can be taken as 5 �C and 30 �C, respectively. As a result, theefficiencies are higher in summer than in winter. The temperaturedifference between themantle of the kiln and the surrounding air islower in summer, and less heat is lost. As a result, coal consumptionof the unit is lower in summer days.
5.3. Emissions reduction
After obtaining a suitable anzast layer and using better qualityrefractory bricks inside the kiln, the heat transfer from the surfaceand coal consumption of the kiln have been decreased consider-ably. Clinker production has been increased at the same time. At theend of 2nd year, the average coal consumption of the unit hasdecreased from 63,668.46 t/y to 61,931.66 t/y. The amount of coalsaved per year is 1736.8 tons.
The amount of carbon dioxide emission per kg of coal burned is0.93 kg [23,24]. Thus, 1,614,480 kg of CO2 emission has been pre-vented by saving 1736.8 tons of coal in a year.
Nitrogen oxides are formed during fuel combustion in rotarykilns. The NOx emissions result from the oxidation of nitrogen in the
3350
3400
3450
3500
3550
3600
3650
3700
3750
3800
Jan
Feb
Marc
hApr
ilM
ayJu
ne July
Aug Sept
OctNov Dec
Months
SEC
(kJ
/kg
clin
ker)
Standart conditions
After efficiency enhancement studies
Fig. 9. SEC of the rotary kiln with respect to months.
A. Atmaca, R. Yumrutas / Applied Thermal Engineering 66 (2014) 435e444 443
fuel as well as in incoming combustion air. The quantity of NOx
formed depends on the type of fuels, its nitrogen content, com-bustion temperature, etc. The emission factor for NOx in cementprocess is 1.4 kg/t coal burned for both dry and wet process kilns.Thus, 2431.5 kg of NOx/y emission has been prevented.
The emission of SO2 into the atmosphere is known to cause theformation of acid rain and smog. Sulfur dioxide may come from thesulfur content in ores and in combusted fuel which will vary fromplant to plant. The emission factors of dry kilns suggested by US EPA(US Environmental Protection Agency) is 3.5 S kg SO2/ton of coalburned, where S is the sulfur content in the fuel in percent [25,26].The properties of the coal are presented in Table 3. The emissionfactor for SO2 is calculated to be 0.0455 kg SO2/t coal burned. About79 kg of SO2 emission is prevented yearly.
6. Conclusions
The analysis and performance assessment of the rotary kilnindicate that the clinker formation process involves energy andexergy losses, and the process is affected by certain parameters. Themain results of the study can be summarized as follows:
- The first law efficiency of the rotary kiln is determined to be55.8% while the second law efficiency is 38.7%. The energy lostfrom the system is calculated to be 12.5 MW. The specific energyconsumption for clinker production is determined to be3735.45 kJ/kg clinker.
- It is calculated that 32.9% of the energy is lost during the for-mation of clinker and 32.6% of the total energy exits with hot gasstreams.
- The quality and type of the refractory used inside the kiln affectthe performance of the rotary kiln significantly. After theapplication of anzast layer and new refractory bricks inside thekiln, the first and second law efficiency and SEC values of thesystem are calculated as 61.2%, 45.1% and 3463.67 kJ/kg clinker,respectively.
- With the help of efficiency enhancement studies, annual clinkerproduction of the kiln has been increased from 529,894.5 tons to555,928.5 tons. There is 4.68% increase in production capacity ofthe unit.
- The rotary kiln operates for about 7750 h in a year. The annualtotal coal consumption of the facility has been decreased from63,668.46 tons to 61,931.66 tons. Coal consumption of the kilndecreased by 2.72%, that’s 1736.8 tons of coal per year has beensaved after the application of anzast layer and new refractorybricks. With decrease in coal consumption, annual CO2, NOx andSO2 emissions rates of the facility are decreased by 1,614,480 kg,2431.5 kg and 79 kg respectively.
- The ambient air conditions affect efficiency and production ca-pacity of the kiln. The SEC of the kiln increase in winter monthsdue to lower ambient temperatures. It appears that the losses(particularly heat losses) increase in winter months.
- Reduction in fuel consumption in a rotary kiln operation can beachieved byminimizing various losses occurring in the unit [27].According to the results, increase in combustion efficiency willbe the main parameter on the system efficiency. Minimizingheat losses by effective insulation, reducing the temperature ofgases at the outlet by more effective heat transfer in the unit,and minimizing air and steam leak by effective sealing are somemeasures that can help reduce energy consumption. Furtherstudies on the topic may involve the investigation of the pa-rameters effecting the system performance and optimization ofthem for best operation. A thermoeconomic analysis of thesystem can also provide significant information indicating costallocation in the system.
Special thanks
I would first like to thank my mother, Elif and my father KemalAtmaca who have died in a terrible traffic accident in 27th ofNovember 2013, without their continuous support and encour-agement I never would have been able to achieve my goals. I wouldlike to express my deepest appreciation to my parents.
Acknowledgements
The authors acknowledge the support provided by the ScientificResearch Unit (GUBAP) of the University of Gaziantep, Dr. NihatAtmaca from the University of Gaziantep, and Huseyin Sencan,Mehmet Marasli, Deniz Ozdil and Erkan Demirel from LimakCement Group.
NomenclatureA cross-sectional area (m2)c specific heat (kJ/kg K)_E energy rate (kW)_Ex exergy rate (kW)h specific enthalpy (kJ/kg) or heat convection
coefficient(W/m2 K)k thermal conductivity (W/mK)L length (m)m mass (kg)ṁ mass flow rate (kg/s)P pressure (Pa)r radius (m)R thermal resistance (K/W)Q heat transfer (kJ)_Q heat transfer rate (kW)s specific entropy (kJ/kg K)Ṡ entropy rate (kW)T temperature (K)t tonT0 ambient temperature, �C or Kv specific volume (m3/kg)W work (kJ)Ẇ work rate or power (kW)y year
Greek lettersh1 first law (energy) efficiency (%)h2 second law (exergy) efficiency (%)3 emissivityj flow exergy (kJ/kg)s StefaneBoltzman constant as 5.67 � 10�8 W/m2 K4
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