ENERGY AND EXERGY BALANCE OF RAWMILL IN CEMENT PLANT

International Conference on Explorations and Innovations in Engineering & Technology (ICEIET – 2016)

ISSN: 2348 – 8360 http://www.internationaljournalssrg.org Page 26

ENERGY AND EXERGY BALANCE OF RAWMILL IN CEMENT PLANT

S.B.NITHYANANTH1, Senior Assistant Professor, Mechanical Engineering,

H.RAHUL2, PG Research Scholar, Energy Engineering.

Department of Mechanical Engineering, Kumaraguru College of Technology, Coimbatore,

Tamilnadu, India.

ABSTRACT

One of the most energy intensive industries in the world is cement production. In

cement plants in order to produce raw materials preparation, clinker and rotary kilns are widely

used. The objective of this study is to perform energy and exergy analysis of a raw mill (RM)

and raw materials preparation unit in Malabar Cements, Walayar; using the actual operational

data. The RM has a capacity of 120 ton-material hourly. Both energy and exergy efficiencies of

the RM are investigated for the plant performance analysis and improvement, and are determined

to be 77.54% and 11.566%, respectively. In the analysis of energy and exergy utilization the

present technique is used as a useful tool, in developing energy policies and providing energy

conservation measures. Depending upon the material to be ground there are several grinding

methods available in cement industry. About 26% of the total electrical power is used in grinding

the raw materials in cement production process.The energy obtained from the rotary burner is

consumed during grinding process.

1. INTRODUCTION

The basic method of a process investigation is energy balance. It points at the needs to

improve the process and makes the energy analysis possible, it is the key to optimization and is

the basis for developing the exergy balance. Analysis of the energy balance results would

disclose the efficiency of energy utilization in particular parts of the process and allow

comparing the efficiency and the process parameters with the currently achievable values in the

most modern installations. They will also establish the priority of the processes requiring

consideration, either because of their excessive energy consumption or because of their

particularly low efficiency. The modern thermodynamic method used as an advanced tool for

engineering process evaluation is the exergy analysis.The first law of thermodynamics relys on

http://www.internationaljournalssrg.org/



energy analysis, and the first and the second laws of thermodynamics relys on exergy analysis.

The material balance for the considered system also utilises both the analysis. Analysis and

optimization of any physical or chemical process, using the energy and exergy concepts, can

provide the two different views of the considered process.

Discovery of the causes and quantitatively estimate the magnitude of the imperfection of

a thermal or chemical process is the main purpose of exergy analysis. A better understanding of

the influence of thermodynamic phenomena on the process effectiveness, comparison of the

importance of different thermodynamic factors, and the determination of the most effective ways

of improving the process under consideration are the results of exergy analysis. The engineer or

scientist working in the area of energy systems and the environment requires a true

understanding of exergy and the insights it can provide into the efficiency, environmental impact

and sustainability of energy systems.

One of the worst pollutant industries is the cement industries. The primary conditions in

the determination of targets for the studies on energy saving are the collection and evaluation of

periodical data concerning industry and other final energy consuming sectors . The primary

physical energy intensity for cement production has dropped by 30%, from 7.9 GJ/ton to 5.6

GJ/ton since 1970. However the production of cement increased from 570 million tons to 1.6

billion tons per year at the same time. About 5% of global carbon dioxide emissions originate

from cement production as a result of this. About half of this is due to calcinations and the

remaining half is due to combustion processes. Inorder to lower the production costs

considerably in a cement plant energy can be efficiently used during grinding process. It will also

lead to lower the emission rates. Most of the options to reduce CO2 emissions are based on

increasing energy efficiency in various processes in industry. A modern analysis tool which is

used for engineering processes is exergy analysis.It is related to both the first law and second law

of thermodynamics. Identification of the causes of the imperfection of an energy conversion

process is the main purpose of exergy analysis.It leads to a better understanding of the influence

of thermodynamic processes on the process effectiveness, comparison of the importance of

different thermodynamic factors, and the determination of the most effective ways of improving

the process under consideration. The system operation can be improved and a better design and

optimization can be achieved through a better understanding of sites of exergy destructions . A




higher exergetic performance of a system translates into energy savings and environmental

benefits. A modern thermodynamic method used as an advanced tool for engineering process

evaluation is the exergenic analysis. Whereas the energy analysis is based on the first law of

thermodynamics, the exergy analysis is based on both the first and the second laws of

thermodynamics. The material balance for the considered system is also utilised by both the

analyses. Analysis and optimization of any physical or chemical process, using the energy and

exergy concepts, can provide the two different views of the considered process. A better

understanding of the influence of thermodynamic phenomena on the process effectiveness,

comparison of the importance of different thermodynamic factors, and the determination of the

most effective ways of improving the process under consideration can be achieved through

exergy analysis. The insights exergy analysis can provide into the efficiency, environmental

impact and sustainability of energy systems are required for the engineer or scientist working in

the area of energy systems and the environment.

There are many studies about grinding process including those on the effect of farine size

distribution of raw material on pyroprocessing, melting and chemical reaction of clinker mixture.

Some additives during grinding are suggested while preventing to decompose the clinker

composition, and the enhancement in the process was achieved. By varying dead state

temperature s between 18°C and 41°C. Sogut et al. calculated first law(energy) and second-law

(exergy) efficiencies of a raw mill . By considering electrical energy saving methods and thermal

energy saving methods Schueret al . studied energy consumption data and focused on the energy

saving methods for German cement industry . Energy and exergy efficiencies of a raw mill for

analysis and improvement of the plant was studied by Utlu et al. Using energy and exergy

analysis Sogut et al. assessed the performance of a trass mill in a cement plant . Energy

efficiency of a cement plant in India was investigated by Saxena et al. A dry type rotary kiln

system with a kiln capacity of 600 ton clinker per day was analysed by Engin and Arian. They

found that about 40% of the total input energy was lost through hot flue gas, cooler stack and

kiln shell. Energy analysis in the U.S. cement industry for the years 1970and 1997 was dealt with

Worell et al. The study indicates that for a dry type cement production process, the carbon

dioxide emission intensity for kiln feed preparation process is about 5.4 kg CO2 per ton cement

produced.




Iin order to attain energy saving, and financial savings there has been increasing interest

in using energy and exergy analysis modelling techniques for energy-utilization assessments

.Energy and exergy analysis studies conducted on cement factories have done up to now have

been all about rotary kiln. Quite a large quantity of mass and energy flow was observed at the

raw mill as the process was examined according to the distribution of energy . The source of heat

that is needed to obtain raw meal in the raw mill (RM) is the exhausts gas taken from the rotary

kiln. The whole system will be affected by the heat losses in the RM. The problem with the

efficiency of the system will be shown by the Heat losses that come out especially at the

beginning stage of the process. If necessary precautions are taken in the RM heat losses can be

decreased and saving of fuel at the rotary kiln can be achieved.

Most extensive studies in the exergy field was performed by Szargut,Kotas and Wall.

Szargut is the first scientist introducing the cumulative exergy consumption and cumulative

degree of perfection for industrial processes and making the distinction between second law

efficiency (exergetic efficiency or rational efficiency) and cumulative degree of perfection for

industrial processes. However, giving different industrial processes such as sulfuric acid, gas

turbine and refrigeration plants Kotas has followed a similar approach. By establishing the

energy flows in processes and drawing up the exergy losses ‘Wall’ presented the exergy flows

for a pulp and paper mill and a steel plant.An energy balance of a cogeneration system for a

cement plant in Indiana was performed by Khurana et al. They found that about 35% of the input

energy was being lost with the waste heat streams. To recover the heat from the streams using a

waste heat recovery steam generator a steam cycle was selected and it was estimated that about

4.4 MW of electricity could be generated. Energy and exergy analyses for a dry system rotary

burner with pre-calcinations in a cement plant of an important cement producer in Turkey was

carried out by Camdali et al. Heat losses by conduction, convection and radiation from the rotary

burner were obtained to be about 3% of the heat coming into the system.

In this regard, the structure of the paper is organized as follows. The first chapter includes

the concepts of energy and exergy. Chapter 2 includes the previous researches in raw mill

analysis. The description of the cement process and the energy utilization in the Malabar

Cements, Walayar are given in chapter 3. Chapter 4 explains about a raw mill, types of raw mill

and the processes in it. Chapter 5 makes a theoretical analysis using mass, energy and exergy




balance equations. Energy and exergy analysis method is applied to the plant studied and the

results obtained are discussed in Section 6, while Section 7 concludes.

2. Description of cement process and energy utilization in Malabar Cements

Cement production involves the chemical combination of Calcium carbonates

(limestone), silica, alumina, iron ore, and small amounts of other materials, which are chemically

altered through intense heat to form a compound with binding properties as it is highly energy

intensive. Raw materials preparation, clinker production and finish grinding are the main steps in

cement production

Fig. 3.1 Overall Manufacturing Process

2.1 Raw materials preparation

Limestone which is the main raw material is mined at a site close to the cement plant.

The other raw materials like laterite , sweetener limestone are purchased from various parts of

India. These materials are then ground to a fine powder in the proper proportions needed for the

cement. To compensate low amounts of lime in the primary raw material sweetener limestone is

used. As per requirement its concentration is varied. To form slurry these can be ground as a dry

mixture or combined with water . The addition of water at this stage has important implications

for the production process and for the energy demands during production. Production is often

categorized as dry process and wet process. To remove some water from the slurry after




grinding, additionally equipment can be added; the process is then called semi-wet or semi-dry.

In Malabar Cements, they use dry process.

2.2 Clinker production

The mixture of raw materials enters the clinker production (or pyroprocessing) stage.

During this stage, the mixture is passed through a kiln (and a preheated system) and exposed to

increasingly intense heat, up to 1400°C. This process drives off all moisture, dissociates carbon

dioxide from calcium carbonate, and transforms the raw materials into new compounds. The

output from this process, called clinker, must be cooled rapidly to prevent further chemical

changes.

Clinker production is the most energy-intensive step, accounting for about 80% of the

energy used in cement production, Produced by burning a mixture of materials, mainly limestone

(CaCO3), silicon oxides (SiO2), aluminium, and iron oxides, clinker is made by dry process. The

raw materials are ground, mixed, and fed into the kiln in their dry state in the dry process. Once

the materials are ground, they are fed into a kiln for burning. The raw material is preheated (in 4

stages) using the waste heat of the kiln, or it is precalcined in modern kilns. The water is first

evaporated during the burning or pyroprocessing, after which the chemical composition is

changed, and a partial melt is produced. The solid material and the partial melt combine into

small marble-sized pellets called clinker.

Fig. 3.2 Rotary Kiln




2.3 Finish grinding

To produce the cement cooled clinker is ground in tube or roller mills and blended by

simultaneous grinding and mixing with additives (gypsum, fly-ash or blast furnace slags).

Drying of the additives may be needed at this stage.

3. Raw Mill

Raw mill is used to grind the raw materials into the farine which is the semi product of

clinker. It is then fed to the rotary burners which transforms it into clinker which is the semi

product of cement. The production process completes after the grinding process in a cement mill.

The average moisture content of input materials must be as low as possible in order to lower the

cost of grinding and drying process. This may be achieved by using the waste heat from the kiln

effectively. Energy which is transferred from the rotary burner is consumed by grinding process

during drying the materials. Various factors must be considered such as grindability of the input

materials, ambient air conditions, mill size and shape, feed material size, waste heat temperature,

and quantity sucked from the rotary burner as well as moisture content of the input materials

when selecting the most convenient grinding and drying process . The raw mill is a dual chamber

mill with mechanical circulation system which grinds a medium hard material at a capacity of

about 120 tons/h. The mill diameter is 4.2 m.

Fig. 4.1 Central discharge Raw Mill




3.1 Types of Raw mill

There are basically two types of raw mills: Wet raw mills and dry raw mills

3.1.1Wet raw mills

Wet grinding is more efficient than dry grinding because water coats the newly formed

surfaces of broken particles and prevents re-agglomeration. When it is in slurry form the process

of blending and homogenizing the raw material is also much easier. Water in the resultant slurry

has to be removed subsequently it is a disadvantage, and this usually requires a lot of energy.

While energy was cheap, wet grinding was common, but since 1970 the situation has changed

dramatically, and new wet process plant is now rarely installed. Wet grinding is performed by

two distinct means: wash mills and ball mills.

3.1.1.1 Wash mill

Washmill is the earliest raw milling technology, and was used to grind soft materials such

as chalk and clay. It is rather similar to a food processor. It consists of a large bowl (up to 15 m

in diameter) into which the crushed (to less than 250 mm) raw materials are tipped along with a

stream of water. By rotating the sets of harrows the material is stirred. The outside walls of the

bowl consist of gratings or perforated plates through which fine product can pass. Grinding is

largely autogenous (i.e. it takes place by collision between lumps of raw material), and is very

efficient, producing little waste heat, provided that the materials are soft. Typically two or three

wash mills are connected in series, these being provided with successively smaller outlet

perforations. With the expenditure of as little as 5 kWh of electricity per dry tonne The entire

system can produce slurry . Relatively hard minerals (such as flint) in the mix, are more or less

untouched by the grinding process, and settle out in the base of the mill, from where they are

periodically dug out.

3.1.1.2 Ball mills and wash drums

The ball mill allows grinding of the harder limestones that are more common than chalk.

A horizontal cylinder that rotates on its axis is consisted in a ball mill. It holds spherical,

cylindrical or rod-like grinding media of size 15–100 mm that may be steel or a variety of

ceramic materials, and occupy 20–30% of the mill volume. The shell of the mill is lined with




steel or rubber plates. Grinding is effected by impact and attrition between the grinding media.

The various mineral components of the raw meal are fed to the mill at a constant rate along with

water, and the slurry runs from the outlet end. The wash drum has a similar concept, but contains

little or no grinding media, grinding being autogenous, by the cascading action of the larger raw

material pieces. It is suitable for soft materials, and particularly for flinty chalk, where the

unground flint acts as grinding media.

3.1.2 Dry raw mills

The normal technology installed today is dry raw mills, allowing CO2 emissions and

minimization of energy consumption. In general, cement raw materials are mainly quarried, and

thus contain a certain amount of natural moisture. Because of the formation of an intractable

"mud" attempting to grind a wet material is unsuccessful. Since large particles hold moisture

deep in their structure it is much easier to dry a fine material than a coarse one. It is therefore

usual to simultaneously dry and grind the materials in the raw mill. To supply this heat a hot-air

furnace may be used , but usually hot waste gases from the kiln are used. For this reason, the raw

mill is usually placed close to the kiln preheater. Types of dry raw mill include ball mills, roller

mills and hammer mills.

3.1.2.1 Ball mills

These are similar to cement mills, but often with a larger gas flow. To ensure a dry

product without overheating the mill, the gas temperature is controlled by cold-air bleeds . The

product passes into an air separator, which returns oversized particles to the mill inlet.

Occasionally, the mill is preceded by a hot-air-swept hammer mill which does most of the drying

and produces millimetre-sized feed for the mill. Ball mills are rather inefficient to make a tonne

of raw meal typically require 10–20 kWh of electric power. For pre-grinding large wet feeds the

Aerofall mill is sometimes used. It is a short, large-diameter semi-autogenous mill, typically

containing 15% by volume of very large (130 mm) grinding balls. Feed can be up to 250 mm,

and the larger chunks produce much of the grinding action. The mill is air-swept, and the fines

are carried away in the gas stream. Crushing and drying are efficient, but the product is coarse

(around 100 µm), and is usually re-ground in a separate ball mill.




4.1.2.2 Roller mills

Occasionally called vertical spindle mills these are the standard form in modern

installations. In a typical arrangement, the material is fed onto a rotating table, onto which steel

rollers press down. A high velocity of hot gas flow is maintained close to the dish so that fine

particles are swept away as soon as they are produced. The gas flow carries the fines into an

integral air separator, which returns larger particles to the grinding path. The fine material is

swept out in the exhaust gas and is captured by a cyclone before being pumped to storage. The

remaining dusty gas is usually returned to the main kiln dust control equipment for cleaning.

Feed size can be up to 100 mm. Roller mills are efficient, using about half the energy of a ball

mill, and there seems to be no limit to the size available. Roller mills with output in excess of

800 tonnes per hour have been installed. Unlike ball mills, feed to the mill must be regular and

uninterrupted; otherwise damaging resonant vibration sets in.

3.1.2.3 Hammer mills

Hammer mills (or "crusher driers") swept with hot kiln exhaust gases have limited

application where a soft, wet raw material is being ground. Giving it high drying capacity it can

be operated at a higher temperature than other mills, thus its design is simple. However, the

grinding action is poor, and the product is often re-ground in a ball mill.




4. System analysis

Fig. 5.1 Raw Mill Material flow

4.1 Raw Mill specifications

Model

Outside

Diameter

(m)

Length (m) Thickness

(mm)

Mean

Rotating

speed (rpm)

Capacity

(Ton/hr)

Central

discharge 4.2 14.7 80 15.1 120

Table. 5.1 Raw Mill specifications

In order to analyse the raw mill thermodynamically, the following assumptions are made:

1. The system is assumed as a steady state, steady flow process.

2. Kinetic and potential energy chances of input and output materials are negligible.

3. The gases inside the mill are assumed to be ideal gases.

4.Electrical energy produces the shaft work in the system.




The temperature and pressure values, specific heat capacity, the input and the output

mass of each item, and the constant specific heat of the input and output materials are determined

for a currently working raw mill. The following balance equations are applied in order to find

heat and work interactions, energy and exergy efficiencies, and the rate of irreversibility in a

steady state flow process . The mass balance is expressed as

The general energy balance is expressed as

Where, Q is the rate of heat transfer, W is the rate of work (power), m is mass flow rate, and h is

enthalpy. The first law (energy) efficiency is defined as the ratio between the amounts of energy

output and the amount of energy input to a system:

hin = CpT1 and hout = CpT2

The general exergy balance is expressed as:

Ʃ min ψin – Ʃ mout ψout = Ʃ Exdest

Where, ψ is the flow energy and is given by:

ψ = (h-h0)-T0(s-s0)

The second-law (exergy or exergetic) efficiency may generally be defined as the rate of exergy

output divided by the rate of exergy input:

Δhin = Cp(T1-T0) and Δhout = Cp(T2-T0)




Δsin = Cp ln(T1/T0) and Δsout = Cp ln(T2/T0)

5. Results and calculations

Raw materials used in raw mill and their chemical composition are:

Material CaO(%) MgO(%) SiO2(%) Al2O3(%) Fe2O3(%)

MCL 44-52 1 12 1.5 2

Laterite - - 10-12 40-43 32

Sweetener 44-52 3.5 13.5 4 2.4

Table. 6.1 Raw material composition

5.1 Calculation for specific heat capacity

General equation is:

Cp = A+ Bt + Ct2+Dt

3, where t = (temperature in K) /1000 and A, B, C and D are material

constants.

1. Specific heat capacity of CaO

A= 49.95, B=4.88, C= -0.35, D= 0.0461

At t = 313K, Cp = 0.918 kJ/kgK

2. Specific heat capacity of MgO

A= 47.259, B=5.681, C= -0.8726, D= 0.1043

At t = 313K, Cp = 1.224 kJ/kgK

3. Specific heat capacity of SiO2

A= -6.07,B=251.676, C= -324.796, D= 168.56

At t = 313K, Cp =0.767 kJ/kgK

4. Specific heat capacity of Al2O3

A= 102.429, B=38.749, C= -15.9109, D= 2.628

At t = 313K, Cp =1.108 kJ/kgK

5. Specific heat capacity of Fe2O3




A= 93.44, B=108.36, C= -50.86, D= 25.59

At t = 313K, Cp =0.769 kJ/kgK

Specific heat capacity for:

1. MCL (Malabar Cements limestone)

Cp = 0.52 CpCaO + 0.01 CpMgO + 0.12 CpSiO2 + 0.015 CpAl2O3 + 0.02 CpFe2O3

= 0.6136 kJ/kgK

2. Laterite

Cp = 0.12 CpSiO2 + 0.43 CpAl2O3 + 0.32 CpFe2O3

= 0.814 kJ/kgK

3. Sweetener

Cp = 0.52 CpCaO + 0.04 pMgO + 0.13 CpSiO2 + 0.04 CpAl2O3 + 0.024 CpFe2O3

= 0.689 kJ/kgK

4. Raw Meal

At t = 343 K,

Cp = 0.597 kJ/kgK

5. Hot gas from rotary kiln

Mass percentage Cp (600K) Cp (343K)

CO2 57 1.075 0.925

N2 38 1.003 1.043

O2 5 1.076 0.934

Table. 6.2 Hot Gas Composition

At t = 600K, Cp = 1.05 kJ/kgK

At t = 343K, Cp = 0.97 kJ/kgK

6. Dust

At t = 600K, Cp = 0.554 kJ/kgK

7. Leaking Air

At t = 313K, Cp = 0.935 kJ/kgK

At t = 343K, Cp = 0.970 kJ/kgK




5.2 Mass balance

Raw Mill Flow Data (10-05-2014)

Total Farine Production: 2020 Tons

Running hours: 18

Input Material Mass (Ton) Moisture content (%)

MCL 1656 3.04

Laterite 162 10

Sweetener 202 12.93

Table. 6.3 Raw Mill Input Data

INPUT

MATERIAL

MASS FLOW

RATE (kg/hr)

OUTPUT

MATERIAL

MASS FLOW

RATE (kg/hr)

MCL 92000 FARINE 282555.55

LATERITE 9000 HOT GAS 21870.117

LIMESTONE(CG) 11222.22 MOISTURE 4488.8888

HOT GAS 21870.117 STEAM 1066.6612

MOISTURE IN

MCL 2888.88 LEAKING AIR 5467.532

MOISTURE IN

LATERITE 1000

MOISTURE IN

LIMESTONE(CG) 1666.67

RECIRCULATING

LOAD 168333.33

DUST 2000

LEAKING AIR 5467.532

TOTAL 315448.749

315448.749

Table 6.4 Mass balance










5.5 Calculation of Heat Loss

Reynold’s number, Re = VD/ν

Velocity , V = 3.32m/s

Diameter, D = 4.2m

Kinematic viscosity, ν = 16.48 x 10-6

Re = (3.32 x 4.2)/(16.48 x 10-6

)

= 846116.505

Nusselt’s number, Nu = C.Rem

.Prn . (Pr∞/Prw)

0.25

C = 0.076

m = 0.7

T∞ = (35+70)/2

= 52.5

At Pr = 500,

Nu = 0.076 x (846116.505)0.7

x (500)0.36

x (0.698/0.694)0.25

= 10052.36

Nu = h.L/k

10052.36 = h x 4.2/0.02826

h = 67.63W/m2K

Convection heat transfer,

Q = hA(Ts-T∞)




= 67.63 x Π x 4.2 x 14.7 x (70-35) = 459171.906 W

Radiation heat transfer,

Q = έσA(T4-T∞

4) = 0.85 x 5.67 x 10-8 x Π x 4.2 x 14.7 (343

4-308

4) = 45264.009 W

5.6 Monthly Variations in power consumption

The Raw mill is rotated using two electrical motors at an average 15.1 rpm. The variation

of power consumption of these motors is given. The power consumption is higher in monsoon

and winter months, while it is comparatively lower in summer months and it is an interesting

fact. The monsoon season can have a higher amount of moisture in the raw materials. This may

be the reason for increased power consumption. The presence of moisture can affect the

grindability of the raw mill. When the moisture content is more the particle size can’t be easily

reduced to the required limit. Furthermore, the possible reason for high consumption in winter

months can be the effect of ambient temperature as well as the moisture content.

Fig.6.1 Monthly Variations in Power Consumption

5.7 Effect of moisture content

The effect of moisture content on the raw mill efficiencies were studied, and the results

were as expected. The moisture has a negative impact on the raw mill performance. The

variations of first law and second law efficiencies with moisture content are given in Fig.6.2

0

200000

400000

600000

800000

1000000

1200000

ELECTRICITY CONSUMPTION (KW)




Fig. 6.2 Efficiency Vs Moisture content

5.8 Effect of ambient temperature

The changes in efficiency values with respect to ambient temperature are shown in Fig. 6.3. The

data indicates that at higher ambient temperatures, both the first and second law efficiencies

increase.

Fig. 6.3 Efficiency Vs Ambient Temperature

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

6 9 12 15 18 21

EFFI

CIE

NC

Y %

MOISTURE %

1ST LAW EFFICIENCY

2ND LAW EFFICIENCY

0

10

20

30

40

50

60

70

80

90

290 295 300 305 310

EFFI

CIE

NC

Y %

TEMPERATURE (K)

1ST LAW EFFICIENCY

2ND LAW EFFICIENCY




7. Conclusions

The analysis and performance assessment of a raw mill indicate that the grinding process

involves energy and exergy losses and the process is affected by certain parameters. The main

results of the study can be summarized as follows:

The first law efficiency of the raw mill is determined to be 77.54% while the second law

efficiency is 11.556%. The electrical power input from the main driving motors to the

system is average 2459.39 kW per day while the energy lost from the system is calculated

to be 5966.18 kW.

Operation of the system involves both thermal and electrical energy (and exergy) inputs

in the form of hot gas, electricity, and thermal energies (and exergies) with return and

raw materials. The output includes thermal energies (and exergies) contained in farine

and hot gas as well as energy (and exergy) losses with heat losses and leaking air and

steam. Heat loss accounts for 22.462%. An exergy balance shows that 88.434% of all

exergy input are lost in the process. It appears that reduction in fuel and electricity

consumption in a raw mill operation can be achieved by minimizing various losses

occurring in the unit. Minimizing heat losses by effective insulation, reducing the

temperature of gases at the outlet by more effective heat transfer in the unit, and

minimizing air and steam leak by effective sealing are some measures that can help

reduce energy consumption.

Special considerations should be given to large ball mills so that utilization of all waste

heat from the kiln is maximized for drying during grinding process. Each ton of hot gas at

600 K supplied from the kiln increases the farine production capacity of a standard mill.

Efficiency and production capacity of the mill is affected by the ambient air conditions.

Both the first law and second law efficiencies of raw mill and the production rate increase

in summer months due to higher ambient temperatures. It appears that the losses

(particularly heat losses) increase in winter months.

The effect of moisture content of raw materials is also a key factor in deciding the

efficiencies of the raw mill. In monsoon months, the raw materials will have higher

moisture content, and hence effective drying techniques should be employed. A higher

mass flow rate ok hot gas from the rotary kiln will be a better option.




The raw mills are big grinding facilities consisting of dozens of different machinery.

There are many different parameters affecting the grinding behaviour of the unit. Mill

size, ball charge rate, shape, temperature and humidity of the entering raw materials,

circulating load within the system, ambient air conditions, rotational speed of the mill,

temporary stops for the periodical maintenance of the system, chemicals used to speed

the pulverization and to eliminate the sticking problem, efficiency and performance of

each machinery used, the microstructure of the raw materials and vibration characteristics

of the system are some of these parameters. All of these factors affect the specific energy

consumption of the system. Further studies on the topic may involve the investigation of

these parameters on the system performance and optimization of them for best operation.

A thermo-economic analysis of the system can also provide significant information

indicating cost allocation in the system.




8. References

Adem Atmaca, Mehmet Kanoglu. ‘Reducing energy consumption of a raw mill in

cement industry’, 22 April 2012, Energy 42 (2012) 261-289.

Zafer Utlu, Ziya Sogut, Arif Hepbasli, Zuhal Oktay. ‘Energy and exergy analyses

of a raw mill in a cement production’, 12 June 2006, Applied Thermal

engineering 26 (2006) 2479-2489.

‘Cement Manufacturer’s Hand Book’ by Kurt E Peray

‘Hand Book for designing Cement Plants’ by S P Deolalkar

Cetin Hosten, Berkan Fidan. ‘An industrial comparative study of cement clinker

grinding systems regarding the specific energy consumption and cement

properties’, 4 January 2012, Powder Technology 221(2012) 183-188.

Schuer A, Leiman A, Ellerbock HG. Possible ways of saving energy in cement

production, Cement Kalk Gips 7 (1992) 175-182.

Cengel YA, Boles MA. Thermodynamics, an engineering approach. 5th ed.

McGraw Hill; 1998.


Documents

ENERGY AND EXERGY BALANCE OF RAWMILL IN CEMENT PLANT