Cogen_cementplant

Energy balance and cogeneration for a cement plant

Shaleen Khurana, Rangan Banerjee *, Uday Gaitonde

Indian Institute of Technology, Energy Systems Engineering, IIT Bombay, Powai, Mumbai 400076, India

Received 25 May 2001; received in revised form 3 November 2001; accepted 19 November 2001

Abstract

The cement industry is an energy intensive industry consuming about 4 GJ per tonne of cement pro-duced. A thermodynamic analysis for cogeneration using the waste heat streams is not easily available.Data from a working 1 Mt per annum plant in India is used to obtain an energy balance for the system anda Sankey diagram is drawn. It is found that about 35% of the input energy is being lost with the waste heatstreams. A steam cycle is selected to recover the heat from the streams using a waste heat recovery steamgenerator and it is estimated that about 4.4 MW of electricity can be generated. This represents about 30%of the electricity requirement of the plant and a 10% improvement in the primary energy efficiency of theplant. The payback period for the system is found to be within two years. � 2002 Elsevier Science Ltd. Allrights reserved.

Keywords: Cement; Energy balance; Waste heat recovery; Cogeneration

1. Introduction

The cement industry is an energy intensive industry. In India the industry accounts for 10.3% oftotal fuel consumption in the manufacturing sector [1]. The energy costs account for about 26% ofthe manufacturing cost of cement [2]. In terms of the primary energy usage about 25% of the inputenergy is electricity while 75% is thermal energy [1]. The specific energy consumption varies fromabout 3.40 GJ/t for the dry process to about 5.29 GJ/t for the wet process. The best practicespecific energy consumption in India is 3.06 GJ/t while in some countries of the world it is lowerthan 2.95 GJ/t [1,3]. The higher specific energy consumption in India is partly due to the harderraw material and the poor quality of the fuel. Waste heat recovery from the hot gases in thesystem has been recognized as a potential option to improve energy efficiency [4]. However there

Applied Thermal Engineering 22 (2002) 485–494www.elsevier.com/locate/apthermeng

*Corresponding author. Tel.: +91-22-576-7883; fax: +91-22-572-6875.

E-mail addresses: [email protected], [email protected] (R. Banerjee).

1359-4311/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.PII: S1359-4311(01)00128-4

are few detailed thermodynamic analyses of operating cement plants that evaluate the option ofwaste heat recovery. This paper builds up an energy balance for an operating plant and estimatesthe power that can be generated from the waste heat streams.The process of manufacture of cement can be divided into three basic steps, preparation of raw

materials, pyroprocessing to produce clinker, and grinding and blending clinker with other prod-ucts to make cement. The raw materials obtained from the quarry are crushed, ground and mixedas a slurry in the wet process and a powder in the dry process. This mixture is then fed into acalciner and preheater before being fed into the kiln, for pyroprocessing (clinker formation). Thekiln reaches temperatures greater than 1450 �C [1]. The clinker nodules produced and any ad-ditives are then ground to the desired fineness in the cement grinder. Pyroprocessing consumes99% of the fuel energy while electricity is mainly used to operate both raw material (33%) andclinker (38%) crushing and grinding equipment. Pyroprocessing requires another 22% of theelectricity hence it is the most energy intensive step of the production process [1].

2. System definition and data source

The cement plant considered is Maihar Cement––Unit 2, Madhya Pradesh, India. A schematicof the plant (Fig. 1) shows the flow of various streams and the components of the plant. The plantruns on dry process with a five stage suspension preheater and an inline calciner. The productioncapacity is 3800 tonne per day. The specific energy consumption for the plant is 3.7 GJ per tonne

Fig. 1. Schematic of the Maihar cement plant.

486 S. Khurana et al. / Applied Thermal Engineering 22 (2002) 485–494

of clinker and 87 kWh (0.31 GJ) of electricity per tonne of cement. Since it is one of the moreefficient plants in the country [5] it is suitable as a reference case for study.The system under consideration for the energy balance is enclosed in the rectangular box in Fig.

1. It is the pyroprocessing unit that includes the preheater, the calciner, the kiln and the clinkercooler. The streams into the system are the raw material, the air into the cooler and the coal firedinto the kiln and the calciner. The streams leaving the system are clinker out from the cooler, theexhaust gases from the preheater and the hot air out from the cooler. The system along with allthe data available is summarized in Fig. 2. The composition of coal into the system and clinkerout of the system is represented in Figs. 3 and 4 while the composition of the preheater exhaust isgiven in Table 1.

Fig. 2. Data available for the streams entering the system.

Fig. 3. Composition of coal.

S. Khurana et al. / Applied Thermal Engineering 22 (2002) 485–494 487

3. Mass balance

The stream data obtained from the plant is used to perform a mass balance over the system.The following reactions are known to occur in the system:Calcination reactions

CaCO3 ! CaOþ CO2

MgCO3 !MgOþ CO2Assuming complete combustion of coal

CþO2 ! CO2

4HþO2 ! 2H2O

SþO2 ! SO2Stoichiometric calculations are used to arrive at the flow rate of the remaining streams. Thecomposition of preheater exhaust is known and a species balance on nitrogen, oxygen and carbondioxide gives the flow rate of the exhaust gases. The composition and the flow rate of the raw feed

Fig. 4. Composition of clinker.

Table 1

Composition of preheater exhaust

Species %

CO2 38

N2 57

O2 5


are estimated from the clinker composition and the reactions. The final flow rates of the differentstreams are summarized in Fig. 5.

4. Energy balance

An enthalpy balance for the system is drawn, taking the reference enthalpy to be 0 kJ/kg at 0 �C,1 atm. The specific enthalpy of various components is obtained from Peray [6]. The temperaturesof the streams are measured and the calorific value of coal is obtained from the plant data (Fig. 2).The energy required for the reaction has been estimated using the correlations given in thehandbook [6]. The input energy with various streams is calculated per kg clinker produced. Theoverall energy balance is summarized in Table 2.A component wise energy balance is similarly drawn using the information about the degree of

calcination. The material entering the calciner is 30% calcined and the material leaving the cal-ciner is 96% calcined [5]. It is assumed that the calcination energy is uniformly distributed over thetemperature range to calculate the calcinations energy in each component. It is also assumed that

Fig. 5. Mass flow rates of different streams into the system.

Table 2

Summary of the enthalpy

Stream Flow rate

(kg/kg clinker)

Specific heat

(kJ/kgK)

Temperature

(�C)Enthalpy

(kJ/kg clinker)

Entering the system

Raw feed 1.56 0.9 50 66

Ambient air 2.98 1.0 30 89

Coal 0.15 0.9 50 7

Combustion of coal Net calorific value ¼ 23800 kJ/kg coal 3611

Total 3773

Leaving the system

Clinker 1.00 0.8 100 82

Preheater exhaust 2.27 1.0 280 636

Hot air from cooler 1.42 1.0 400 568

Reaction energy 1850

Total 3136


the coal entering the calciner is fully combusted in the calciner and that entering the kiln iscombusted in the kiln.The energy balance for the entire system is summarized as a Sankey diagram (Fig. 6). The

values are indicated as a percent of the total energy released from the combustion of coal in thecalciner as well as the kiln. The energy released on combustion of coal is about 3600 kJ/kg clinker.It is observed in the enthalpy flow diagram that there is a good agreement between the overall

energy input to the system and out of the system with an inconsistency of about 600 kJ/kg clinkerthat amounts to about 15% of the input energy. Considering the nature of the data sources andthe simplifications made, the energy balance can be said to be in good agreement. Some of thesources of error that have not been considered are the radiation losses predominantly from thekiln shell, the energy lost with the dust leaving with the different streams.A parameter that is used to evaluate the performance of the system is the primary energy

efficiency, defined as

gprimary ¼Qu þ W =gp

Q

where Qu is the energy used up for the reaction,W is the power generated, gp is the efficiency of aconventional power plant and is assumed to be 35% while Q is the thermal energy input.With the current methodology of manufacture, the primary efficiency of the process is about

50% and the remaining 35% of the energy is lost with the flue gases and the hot air, and energyrecovery from these streams would improve the overall efficiency of the system. The energyleaving the system with the two streams can be calculated as the ratio of the enthalpy carried withthe exhaust stream HExhaust and the enthalpy leaving with the hot air HAir to the enthalpy enteringthe system from the combustion of coal Q.

Energy carried with the preheater exhaust stream ¼ HExhaustQ

¼ 18%

Fig. 6. Sankey diagram for the system.


Energy carried with the hot air from the cooler ¼ HAirQ

¼ 16%

The temperature to which the exhaust gas gets cooled in the preheater is limited by the numberof cyclones. Most modern plants have five cyclones due to structural limitations. It is seen that thepreheater system has a high energy efficiency and there are no significant losses. The temperatureof flue gas into the preheater is 900 �C and the material temperature upon heat exchange is about800 �C, which indicates that the process is thermodynamically efficient. The only loss in from thepreheater is in form of exhaust gases (18%). It has been suggested that the process be modified forenhanced waste heat recovery by replacing the preheater system with the waste heat recoverysystem [8], however considering the process specifications and the high efficiency, it is desirable tolook at the option of recovering heat from the existing streams rather than modify the system.Exergy analyses on the preheater cyclones have indicated that the second law efficiency of thepreheater is high [9], hence it is not proposed to make modifications on the preheater instead aretrofit to the existing components is suggested.

5. Power generation

The two waste heat streams are available for power generation and it is proposed that a wasteheat recovery steam generator (WHRSG) be used to generate steam that is passed through asteam turbine to generate power. A schematic is shown in Fig. 7. The nature of the two streams is

Fig. 7. Schematic for the power generation system.


different, the preheater exhaust is heavily dust laden (70 g/Nm3) hence separate exchangers shouldbe designed for the two streams.Based on the temperature of the streams a steam cycle is selected. A pinch point (minimum

approach temperature) of 20 �C is taken. The steam parameters are taken as 10 bar saturated atthe steam turbine inlet. The condenser temperature is selected at 50 �C as the ambient temperaturereaches higher than 40 �C in the summer months. The streams in the WHRSG are represented ona temperature enthalpy diagram in Fig. 8. From the stream temperature it is seen that exchangersin parallel should be used for higher recovery as compared to a series configuration for the twostreams.The preheater exhaust stream gets cooled to 178 �C while the hot air stream gets cooled to

140 �C. Both these stream temperatures are above the acid dew point of the streams. The powergenerated for the system is about 100 kJ/kg clinker which amounts to 4.4 MW with a productionrate of 3800 tonne per day. This amounts to about 30% of the total power requirement of theplant.The design of a WHRSG for the gas streams needs to consider that the flue gases are heavily

dust laden. Prediction of the performance of the heat exchanger would require an estimation ofthe fouling characteristics of the streams. The dust is also likely to cause abrasion particularly atthe bends and needs to be considered while design. Prediction of the fouling characteristics of thedust laden gases requires experimentation. Since the power recoverable is significant, a project topredict the fouling behavior is suggested. Theoretical models for prediction of deposition of dustfrom streams can be used for a first estimate of the fouling characteristics.The implementation of the system would require special consideration of the layout of the

system, and might pose a problem in some of the older plants that have undergone a number ofstructural changes. The primary energy efficiency of the cement plant with the cogenerationsystem is 60% which indicates an improvement of 10%. The cost of electricity supplied by theelectricity board is about Rs. 4.5 (US$ 0.096) per kWh [7] hence for the plant the savings from therecovered energy amount to around Rs. 16 crores (US$ 3.4 million) per annum for a working of330 days in an year, assuming a shutdown of about 5 days for system maintenance due to ex-cessive fouling caused by the dust, every three months. The first cost estimate for the system isabout Rs. 5 crores per MW and taking into account the operating costs, the payback period forthe system is estimated within 2 years.

Fig. 8. Temperature–enthalpy diagram for the HRSG.


The production of cement in India is about 110 Mt per annum, extrapolating the results about450 MW of power can be generated from the various plants in India. A large number of plants inthe country are relatively inefficient when compared with the plant considered with a highertemperature of the exhaust gases, hence it is expected that the power generated will be higher thanestimated.One of the prime considerations in the design of the system has been to make it a retrofit and

using the gas streams only down stream of the process and the operation of the kiln, calciner andpreheater stays unaffected. This will ensure easier acceptability of the option and if required a shutdown of the cogeneration system can be taken without affecting the cement plant output.

6. Conclusion

The data collected from a 1 Mt per annum working cement plant was used to arrive at anenergy balance for the pyroprocessing unit. The Sankey diagram revealed that the efficiency of thepreheater and calciner units is high. The overall thermal efficiency of the plant was found to be50% and is close to the best practice with the current technological limitations. The waste heat wasestimated at 35% of the energy input. A retrofit steam cycle was selected and for the consideredplant about 4.4 MW of power can be generated from the waste heat streams. This represented animprovement of about 10% in terms of primary energy efficiency of the plant. Around 30% of theenergy requirement of the plant can thus be met from the cogeneration system. Extrapolating tothe cement production in India this offers a potential of about 450 MW and is an economicallyviable option for cement plants.

Acknowledgements

The authors would like to acknowledge the support of Maihar Cement, and Mr. R.M. Shah,Technical Advisor, Maihar Cement.

References

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