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ENERGY AND EXERGY ANALYSIS OF THERMAL POWER PLANTS

BASED ON ADVANCED STEAM PARAMETERS

Suresh M.V.J.J.*, K.S. Reddy and Ajit Kumar Kolar

Heat Transfer and Thermal Power Laboratory, Department of Mechanical Engineering Indian Institute of Technology Madras, Chennai – 600 036

Phone: +91-44-22575664, Fax: +91-44-22570509* E-mail: [email protected]

AbstractThe present study deals with the comparison of energy and exergy analysis of thermal power plants based on

advanced steam parameters in Indian climatic conditions. The study involves coal-based thermal power plantsusing subcritical, supercritical, and ultrasupercritical steam conditions. The design configurations of 500 MWeunit size were considered. The study encompasses the effect of condenser pressure on plant and exergyefficiency. The effect of high grade coal on performance parameters as compared to typical Indian low gradecoal was also studied. The major exergy loss took place in coal combustion followed by the steam generator. Dueto condenser pressure limitation, the maximum possible plant efficiency was found to be about 41% for

supercritical steam power plant and 44.4% for the ultrasupercritical steam power plant.

Keywords: Coal-based thermal power plant, Energy, Exergy, Supercritical and Ultrasupercritical steam

1. IntroductionEnergy in general, and electricity in particular, plays a vital role in improving the standard of life. India has

abundant proven reserves of coal of about 95 billion tonnes (Ministry of Coal, 2006) and thus coal-based thermal power plants dominate source-wise mix with 55% installed capacity of a total of about 1,24,000 MWe(Ministry of Power, 2006). The power plants in India are coal-based operating on sub-critical steam conditions.The indigenous coal used is of low grade with mineral matter content as high as 45%. In order to addressincreasing electricity demand and concern for environmental safety it is imperative to install power plants basedon advanced coal technologies which are (more) energy efficient, environmentally acceptable, and economically

viable. Energy and exergy analysis provides insight into losses in various components of a power generatingsystem. Unlike energy, exergy is not generally conserved but is destroyed. So, the majority of the causes ofirreversibilities like heat transfer through a finite temperature difference, chemical reactions, friction, and mixingare accounted by exergy analysis (Cengel and Boles, 2004).

Rosen (2001) has compared the performance of operating coal-fired and nuclear steam power plant located inCanada of unit size of approximately 500 MWe using a process-simulator, Aspen Plus. Chew (2003) reviewedthe sensitivity of supercritical steam plant cycle performance to operating conditions. Recently,

Bugge et al. (2006) have presented the status and perspectives for the AD700 technology which involves thedevelopment of a coal-fired power plant with steam temperature of 700

oC. The plant efficiencies of old power

plants in India are still around 30% based on lower heating value (LHV) of fuel and modern subcritical cycles(500 MWe unit size) have attained efficiencies of about 35 - 38% (LHV). Further improvement in plantefficiency can be achieved by using supercritical steam conditions. Enhanced efficiency results in reducedemission of CO2 / unit kWh. The first supercritical coal fired power plant in India is being installed at Sipat,Chattisgarh with unit size of capacity 660 MWe (NTPC annual report, 2006). An attempt has been made in this

paper to predict the possible improvement in efficiency obtained with thermal power plants based on advancedsteam conditions in Indian climatic conditions for Indian low grade and an imported high grade coal.

2. System ConfigurationsThe detailed process flow sheets of the considered 500 MWe unit size coal-based power plants involvingsubcritical, supercritical and ultrasupercritical steam conditions are shown in Figures 1-3 respectively. An

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16 Advances in Energy Research (AER – 2006)

operating pulverized coal combustion sub-critical steam power plant unit of National Thermal Power

Corporation, India was chosen as a reference for comparison. It involves steam conditions of 174.5 bar / 540oC /

540oC. The plant uses a single stage reheating with the maximum feed water temperature at the inlet of

economizer being 256oC. The supercritical steam power plant shown in Fig. 2 has steam parameters of 290 bar /

582o

C / 580o

C / 580o

C with 2-stage reheating and feed water temperature of 300o

C at the inlet of economizer(Kjaer, 2006). One of the design configurations of an ultrasupercritical thermal power plant presented by Kjaer(2006) has been considered for the study. This plant involves steam parameters of 375 bar / 700 oC / 720oC /720oC with 2-stage reheating and feed water temperature of 350oC at the inlet of economizer.

H

H

H

H

F F

F F

H

FBFP

HPH2 HPH1LPH3 LPH2 LPH1

Condenser

Cooling Water

DA

Fuel

Combustor

Stack

Air

LPTLPT

IPTIPTHPT

Reheater

Evaporator

Superheater

Economiser

Air Preheater

Figure 1: Flowsheet for the 500 MWe subcritical steam power plant

H

H

H

H

H

F FF FF F F F F

Stack

HPH3 HPH2 HPH1 LPH6 LPH5 LPH4 LPH3 LPH2 LPH1

Cooling Water

Condenser

DA

LPTLPTLPTLPTIPTIPTIPTHPTVHPT

FuelCombustor

Air

Economiser

Reheater 2

Reheater 1

Superheater

Figure 2: Flowsheet for the 500 MWe supercritical steam power plant

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Energy and Exergy Analysis of Thermal Power Plants Based on Advanced Steam Parameters 17

Cooling w ater

HPH2 HPH1 LPH5 LPH4 LPH3 LPH2 LPH1

Condenser

LPTLPTIPT

DA

IPTHPT

FuelCombustor

Superheater

Reheater 2

Reheater 1

Economiser

Stack

F F F

H

H

H

H

F

H

F F F

Figure 3: Flowsheet for the 500 MWe ultrasupercritical steam power plant

3. System SimulationThe performance of the considered state-of-the-art power plant configurations of 500 MWe unit size wasestimated by a detailed component-wise modeling followed by a system simulation. A flow-sheet computer program, “Cycle-Tempo” was used for this study (Cycle-Tempo, 2006). To calculate exergy quantities, thereference-environment model used by Gaggioli and Petit (1977) was considered.

3.1 Assumptions

The following assumptions were made to carry out the simulation: 1).The kinetic and potential exergies are

neglected; 2).The reference state for water / steam is saturated liquid at a temperature of 25oC; 3).The incoming

fuel temperature is 25oC; 4).Isentropic efficiency of pumps / fans: 75%; 5).Generator efficiency: 97%;

6).Excess air: 20%, and 7).Condenser pressure: 10 kPa.

For the overall unit, the plant efficiency is evaluated as:

η = net electricity produced / energy content (Lower Heating Value) of fuel (1)

and the exergy efficiency is evaluated as:

ψ = net electricity produced / exergy content of fuel (2)

3.2 Fuel Characteristics

The characteristics of the considered fuel are presented in Table 1. The composition of the coal used for the present study represents that of the typical Indian coal (Singareni Mines - SM) with lower heating value of

14.5 MJ/kg (VTPS, 2003). With a view to compare performance of the considered systems using high grade

(low mineral matter) coal, a typical South African (SA) coal (Barroso et al., 2006) having low mineral matterwas also considered.

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Table 1: Characteristics of the considered coals

Type India (SM) South Africa (SA)

Proximate analysis (% by weight, dry basis)

Fixed carbon 25.5 60.5

Volatile matter 31.4 22.8

Mineral Matter 43.1 16.7

Elemental analysis (% by weight)

Carbon 38.9 68.1

Hydrogen 2.6 3.5

Oxygen 6.7 7.5

Nitrogen 0.7 1.7

Sulphur 0.6 0.5

Moisture 5.7 2.4

Lower Heating Value (MJ/kg) 14.5 25.7

Fuel Exergy (MJ/kg) 16.5 27.5

It needs to be emphasized here that in spite of high mineral matter, the Indian coal is of high quality with regard

to sulphur content which is in general less than 0.6% (Parivesh, 2006) thus having minimal negativeenvironmental impact.

4. Results and DiscussionThe comparison of plant and exergy efficiencies of the various power plant configurations is presented inTable 2. The results show an increase of 3% points in plant efficiency of supercritical power plant and about6.3% points for ultrasupercritical power plant as compared to subcritical power plant. The enhancement inefficiency can be accounted by the increased steam parameters at the turbine inlet. The comparison of the key

performance parameters with different types of coal is shown in Table 3.

Table 2: Comparison of efficiencies

Sl No. Plant Parameters Plant Efficiency (%) Exergy Efficiency (%)

1 Subcritical 174.5/540/540 38.1 33.3

2 Supercritical 290/582/580/580 41.1 35.9

3 Ultrasupercritical 375/700/720/720 44.4 38.8

Table 3: Comparison of performance parameters for low (SM) and high (SA) grade coal

Coal Consumption

(t/hr)

Plant efficiency

(%)

Exergy efficiency

(%)Sl No. Plant

SM SA SM SA SM SA

1 Subcritical 326 184 38.1 38.2 33.3 35.7

2 Supercritical 303 170 41.1 41.2 35.9 38.4

3 Ultrasupercritical 279 157 44.4 44.5 38.8 41.6

The study highlights the reduction of about 43% in fuel consumption using high grade coal as compared to thetypical Indian coal. There is about 14% reduction in fuel consumption using ultrasupercritical steam conditions

when compared to subcritical steam conditions based power plant which is due to enhanced overall efficiency ofthe plant. The study also shows that there is only a slight increase in the plant efficiency using high grade coalwhich can be accounted to reduction in energy consumption of the auxiliaries. In the case of exergy efficiency, a

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significant increase can be noted when SA coal is used as compared to SM coal. This can be attributed to the

presence of lower mineral matter in the SA coal.

A detailed energy and exergy balance was also carried out for the considered power plants. The comparison ofenergy balance is shown in Table 4 and an exergy balance is represented in Table 5.

Table 4: Comparison of energy balance using low grade (SM) coal

Sl No. PlantPlant efficiency

(%)Heat rejected in

cooling water (%)Heat rejected

through stack (%)Other losses

(%)

1 Subcritical 38.1 53.1 7.8 1.0

2 Supercritical 41.1 49.9 7.8 1.2

3 Ultrasupercritical 44.4 46.1 8.3 1.2

Table 5: Comparison of exergy balance

Subcritical Supercritical UltrasupercriticalSl No.

Components(%) SM SA SM SA SM SA

1 Exergy efficiency 33.3 35.8 35.9 38.7 38.8 41.6

2 Loss due to coal combustion 31.9 25.5 31.9 25.5 31.9 25.5

3 Loss in steam generator 18.0 20.9 15.2 17.9 12.3 14.5

4 Loss in stack 4.7 5.1 4.7 5.1 4.7 5.2

5 Loss in turbine 3.8 3.8 3.3 3.3 3.2 3.2

6 Loss in condenser andcooling water sink 3.3 3.3 3.1 3.1 2.9 2.9

7 Loss in feed water heaters 1.0 1.0 1.1 1.1 1.7 1.7

8 Other losses 4.0 4.6 4.8 5.3 4.5 5.4

It is evident from the above table that the majority of the irreversibilities are accounted by the exergy balance. Itis observed from Table 4, that the energy losses are associated mainly with heat rejection in cooling water and

stack. The cooling water flow rate decreases from about 16,600 kg/s in the case of subcritical power plant to12,500 kg/s in the case of ultrasupercritical power plant for a temperature rise of 10oC across the condenser. This

is attributed to reduction in heat rejection in the condenser as the steam conditions are increased from subcritical

to supercritical. Energy balance leads to misconception because the energy rejected in cooling water sink is oflow grade. The exergy balance discloses that the maximum exergy destruction takes place in coal combustion process followed by the steam generator. The increase in the exergy efficiency results from the reduced exergylosses in the steam generator. The reduced losses in the steam generator can be attributed to the smaller heattransfer temperature difference as the steam parameters are increased from subcritical to ultrasupercriticalconditions. The exergy loss in the case of steam turbine also decreases with the enhancement in steam

parameters. This is due to higher operating steam parameters resulting in lower steam throughput and dryersteam exhaust. The exergy loss associated with the cooling water reduces from 3.3% to about 2.9% as the resultof smaller steam mass flow rate for a constant power output rating of 500 MWe.

Condenser pressure is one of the most important parameters influencing the plant and exergy efficiency, which

depends on the local cooling water conditions. The cooling water temperature representing Indian conditions wasconsidered as 25oC. The lowest possible cooling water temperature was considered as 10oC, which is obtained in

Denmark (seawater). The variation of the plant and exergy efficiency with condenser pressure using low grade(SM) coal was studied and is represented in Figures 4-5. It is observed that the plant efficiency of the consideredsystem configurations increases by about 3 percentage points with the reduction in condenser pressure fromabout 10 kPa (the condenser pressure used in India) to 2.3 kPa [the lowest condenser pressure attained in practice

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(Denmark), Kjaer, 2006]. This gain in efficiency is not possible in Indian cooling water conditions thus limiting

the maximum possible efficiency to about 44.4%.

36

38

40

42

44

46

48

2 4 6 8 10

Condenser Pressure (kPa)

P l a n t E f f i c i e n c y ( %

Ultrasupercritical PlantSupercritical PlantSubcritical Plant

32

34

36

38

40

42

2 4 6 8 10

Condenser Pressure (kPa)

E x e r g y E f f i c i e n c y ( % )

Ultrasupercritical PlantSupercritical PlantSubcritical Plant

Figure 4: Variation of plant efficiency Figure 5: Variation of exergy efficiencywith condenser pressure with condenser pressure

5. Conclusions

A plant efficiency of 38.1% was obtained using subcritical power plant. The study discloses a 3 percentage

points gain in plant efficiency for supercritical power plant over the subcritical plant. Similarly a gain of 6

percentage points in plant efficiency was obtained with ultrasupercritical power plant. A reduction of about43.5% was obtained in fuel consumption using high grade (SA) coal as compared to typical low grade (SM)Indian coal. There was about 14% reduction in fuel consumption with ultrasupercritical power plant as comparedto subcritical power plant. The energy losses were associated mainly with heat rejection in condenser and stack

whereas major exergy losses took place in the combustor and steam generation unit. With low grade (SM) coal,the exergy loss in the combustor was about 32%. In the case of steam generator, the exergy loss reduced to 12%from about 18% as the steam parameters were increased from subcritical to supercritical conditions using low

grade (SM) coal. Due to condenser pressure limitation, the maximum possible overall energy efficiency wasfound to be about 44.4% with the ultrasupercritical power plant. Thus, installing coal-based thermal power plants

based on advanced steam parameters in India will be a prospective option aiding energy self-sufficiency.

AcknowledgementsOne of the authors, Suresh MVJJ thanks Energy Technology Section, Delft University of Technology, The

Netherlands for licensing a copy of Cycle-Tempo.

References

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