43 Performance Analysis of Supercritical Boiler

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 2, May-August (2012), © IAEME

422

PERFORMANCE ANALYSIS OF SUPERCRITICAL BOILER

Sanjay Kumar Patel1 Dr. A.C. Tiwari2

University institute of technology University institute of technology

Rajiv Gandhi Proudyogiki Vishwavidyalya Rajiv Gandhi Proudyogiki Vishwavidyalya

Bhopal, India Bhopal, India

Email- [email protected] Email- [email protected]

Abstract

Coal fired power generation is switching over to supercritical (SC) and ultra supercritical (USC) plants which

operate with steam on higher temperature and above critical pressure to produce power output at higher thermal

efficiency. Due to involvement of high heat resistant material, manufacturing cost of the components of

supercritical plants are increases, but due to higher efficiency its operating cost is low as compare to subcritical

plants. An analysis has been made in the study to explore the possibilities of operating power plants with steam

at higher temperature and pressure. Due to high efficiency of this plant 15 % lower co2 emission is achieved by

high steam parameters as compare to subcritical plants. Analysis shows that for different operating condition of

boilers and turbine, if there is an increment in the load of boiler and drop in the load of turbine higher efficiency

is obtained. There are two parameters boiler maximum continuous rating (BMCR) and turbine maximum

continuous rating (TMCR) are varied by increasing the value of steam flow rate of superheaters and reheaters.

By increasing or decreasing these values we can find out which condition is best for power generation. A

comparative study between subcritical and supercritical boilers and analysing the performance of boilers, Factor

affecting efficiency of boilers has carried out with identification and analysis for improved working of

supercritical plants.

Keywords:

Supercritical-Boilers, steam-turbine, BMCR, TMCR, rankine cycle, superheaters

Introduction: Supercritical is a thermodynamic phase that describes the state of a substance where there is no clear distinction

between liquid phase and gaseous phase. (i.e. they are a homogeneous fluid). Water reaches this state at a

pressure above 22.1 MPa (221 bar), also known as ‘supercritical pressure’ of water. Beyond this pressure, it is a

homogeneous mixture of water and steam, as shown in Fig-1. Up to an operating pressure of around 19 MPa in

the evaporator part of the boiler, there is a non-homogeneous mixture of water and steam in the evaporator. Up

to an operating pressure of around 19 MPa in the evaporator part of the boiler, there is a non-homogeneous

mixture of water and steam in the evaporator.

Up to an operating pressure of around 19 MPa in the evaporator part of the boiler, there is a non-homogeneous

mixture of water and steam in the evaporator. In this case, a drum-type boiler is used because the steam needs to

be separated from water in the drum of the boiler before it is superheated and led into the turbine. Above an

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online)

Volume 3, Issue 2, May-August (2012), pp. 422-430

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 2, May

operating pressure of 22.1 MPa in the evaporator part of the boiler, the cycle medium is a single

with homogeneous properties and there is no need to separate steam f

Once-through boilers are therefore used in supercritical cycles.

cycle, on which a typical steam turbine power plant operates. At working pressures in exc

pressure, the Rankine cycle becomes supercritical cycle. The region below critical point is the subcritical region

having a non-homogeneous mixture of water and steam. Figure

‘A’ on the T-S diagram represents the critical point.

technologies employed in the modern coal

Subcritical boilers operate below 220 bars, the supercritical pressure of water.

homogeneous mixture of water and steam in the evaporator part of the boiler.

used because the steam needs to be separated from water before it is superheated and led into the turbine. The

remaining water in the drum re-enters the boiler for further conversion to steam. The water circulation system

can be a natural circulation or a forced (assisted) circulation

Steam Conditions

Today’s supercritical coal fired power plants permits efficiencies that exceed 45%, depending on cooling

conditions. Options to increase the efficiency above 50

steam conditions as well as on improved process and component quality.

MPa/600°C/620°C are achieved using steels with 12

achieved using Austenite, which is a proven, but expensive material. Nickel

permit 35 MPa/700°C/720°C, yielding efficiencies up to 48%.

1-2: HP Turbine Expansion

2-3: Reheat

3-4: IP + LP Turbine Expansion

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

6359(Online) Volume 3, Issue 2, May-August (2012), © IAEME

423

operating pressure of 22.1 MPa in the evaporator part of the boiler, the cycle medium is a single

with homogeneous properties and there is no need to separate steam from water in a drum.

Fig-1. Water Phase Diagram

through boilers are therefore used in supercritical cycles. A critical point can be illustrated on a Rankine

cycle, on which a typical steam turbine power plant operates. At working pressures in excess of this critical


homogeneous mixture of water and steam. Figure-2 shows the supercritical Rankine cycle. Point

S diagram represents the critical point. Based on the operating pressures, there are two basic boiler

technologies employed in the modern coal-fired power plants. These are Subcritical and

Subcritical boilers operate below 220 bars, the supercritical pressure of water. This means that there is a non

homogeneous mixture of water and steam in the evaporator part of the boiler. In this case a drum

be separated from water before it is superheated and led into the turbine. The

enters the boiler for further conversion to steam. The water circulation system

can be a natural circulation or a forced (assisted) circulation.


conditions. Options to increase the efficiency above 50 % in ultra-supercritical power plants rely on elevated

l as on improved process and component quality. Steam conditions up to 30

MPa/600°C/620°C are achieved using steels with 12 % chromium content. Up to 31.5 MPa/620°C/620°C is

achieved using Austenite, which is a proven, but expensive material. Nickel-based alloys, e.g. Inconel, would

720°C, yielding efficiencies up to 48%.

4-5: Condenser

5-6: Feedwater heating and pumping

IP + LP Turbine Expansion 6-1: Boiler

Fig-2. Supercritical Rankine Cycle


August (2012), © IAEME

operating pressure of 22.1 MPa in the evaporator part of the boiler, the cycle medium is a single-phase fluid

A critical point can be illustrated on a Rankine

ess of this critical


2 shows the supercritical Rankine cycle. Point

Based on the operating pressures, there are two basic boiler

and Supercritical

This means that there is a non-

In this case a drum-type boiler is

be separated from water before it is superheated and led into the turbine. The

enters the boiler for further conversion to steam. The water circulation system


supercritical power plants rely on elevated

Steam conditions up to 30

MPa/620°C/620°C is

based alloys, e.g. Inconel, would

Feedwater heating and pumping



424

BOILER LOAD CONDITIONS

Boiler Maximum Continuous Rating (BMCR): Boiler Maximum Continuous Rating (BMCR)

is the maximum rating specified for the boiler. This corresponds to 109.94% of Turbine maximum

continuous rating. Turbine Maximum Continuous Rating (TMCR): Turbine Maximum Continuous Rating

(TMCR) is the basis of steam generator output and is equal to the turbine generator maximum

guaranteed rating.

Constant Pressure Operation Above 90% TMCR, the main steam pressure remains constant at the rated value, condition, while the

load is controlled by throttling main steam flow with the designated partial arc control valve. Below

30% TMCR, the main steam pressure remains constant at the minimum. The minimum constant pr. is

92 bar. The start-up and re-circulation system is designed to provide the necessary mass flow for

adequate cooling of the evaporator during start-up and low load operation. A minimum of 30% of

TMCR flow is maintained up to a boiler load of 30% TMCR. In this re-circulation system, the

feedwater flows through the boiler feedwater line to the economizer, to the evaporator and then to the

water separator. From the separator the recirculated water returns through the Boiler Recirculation

Pump to the boiler feedwater line, where it is mixed with feedwater.

Fig- 4 Once through operation of supercritical boiler

Water separator HP BPV

LP BPV

Fig-3 Cycle of supercritical power plant

Division Superheater

Platen Superheater

Final Superheater

HP TBN

Reheater

LP IP t

Condenser

COP

LP HTR

Deaerator

BFP HP HTR

BCP

Economizer

Evaporator

BFP Economizer Water Wall Separator Superheater

BCP



425

Performance of supercritical boiler:

1. Efficiency calculation of boiler:

There are two methods to calculate the efficiency of boiler. That is, Heat loss method and heat input-

output method. To calculate the efficiency of boiler correctly we use heat loss method.

ηb = (1 - L ) X 100 [%] .......................................1

Hf + Ba

Where, H f : Higher heating value of fuel [J/kg]

Ba : Total heat credit [J/kg]

Heat Loss Items of Boiler (a) Dry Gas Loss

(b) Heat Loss of Water Contents Caused by Hydrogen Combustion in Fuel

(c) Heat Loss of Unburned Carbon

(d) Water Loss in Fuel

(e) Water Loss in Combustion Air

(f) Radiation Loss

(g) Unaccounted Losses

Heat Distribution of Boiler (a) Economizer

The heat absorption rate in economizer of subcritical boiler is twice more than that of supercritical boiler.

(b) Furnace (Radiation)

The radiation heat absorption of subcritical boiler in furnace is less than that of supercritical boiler at every load.

0

2000

4000

6000

8000

10000

12000

0 20 40 60 80 100 120Load

Heat Absorption Rate in Economiser

subcritical supercritical

02000400060008000

100001200014000

0 20 40 60 80 100 120Load

Radiation Absorption Rate in Furnace




426

(c) Furnace (Convection)

The convection heat absorption of supercritical boiler in furnace is twice more than that of subcritical boiler.

(d) Primary Superheater

The heat absorption rate in primary superheater of supercritical boiler is twice more than that of subcritical

boiler.

(e) Secondary superheater

The heat absorption rate in secondary superheater of subcritical boiler is 3 times more than that of supercritical

boiler.

0

2000

4000

6000

8000

10000

12000

0 20 40 60 80 100 120Load

Convective Heat Absorption Rate in Furnace


0

5000

10000

15000

20000

25000

30000

0 20 40 60 80 100 120Load

Heat Absorption Rate in primary superheater


020,00040,00060,00080,000

100,000120,000140,000

0 20 40 60 80 100 120Load

Heat Absorption Rate in secondary superheater




427

(f) Final Superheater

The heat absorption rate in final superheater of subcritical boiler is approximately the same that of supercritical

boiler.

(g) Primary Reheater

The heat absorption rate in primary reheater of subcritical boiler is 10 times more than that of supercritical

boiler.

(h) Final Reheater

The heat absorption rate in final reheater of subcritical boiler is much more than that of supercritical boiler.

0

5000

10000

15000

0 20 40 60 80 100 120Load

Heat Absorption Rate in Final superheater


020,00040,00060,00080,000

100,000120,000

0 20 40 60 80 100 120Load

Heat Absorption Rate in Primary Reheater


0

5,000

10,000

15,000

20,000

25,000

30,000

0 20 40 60 80 100 120Load

Heat Absorption Rate in Final Reheater




428

(i) Air Preheater

The heat absorption rate in air preheater of subcritical boiler is much more than that of supercritical boiler.

(j) Heat Absorption Rate of Each Part in Boiler

The heat absorption rate in water wall of supercritical boiler is approximately twice more than that of subcritical

boiler. The heat absorption rate in economizer of subcritical boiler is approximately 4 times more than that of

supercritical boiler.

Efficiency of Boiler The boiler efficiency of supercritical boiler is a little lower than that of subcritical boiler.

CONCLUSIONS

Analysis shows that higher output can be obtained with high temperature steam at supercritical

pressure comparing with the output of subcritical units operating with same steam flow rates. Thermal

efficiency of supercritical plant is high as well as emission is also reduced due to higher efficiency.

Performance of supercritical boiler is calculated by different graphical representation and it is

compared to subcritical boilers curves. The increased pressure also increases cycle efficiency and,

although this effect is a second-order effect compared with the effect of temperature, it can still make

0

500

1,000

1,500

2,000

0 20 40 60 80 100 120Load

Heat Absorption in Air preheater


0

20

40

60

Economiser Waterwall Superheater Reheater

Boiler Heat Absorption Rate (%)


0

20

40

60

80

100

Subcritical Supercritical

Boiler Efficiency (%)

Boiler Efficiency Total Loss



429

an important contribution to increasing overall plant efficiency. However Supercritical boilers operate

in a higher pressure and temperature zone as compared to subcritical boilers leading to increased

thermal efficiencies.

REFERENCES

1. Bejan A., Tsatsaronis, G., and Moran A., 1996, Thermal Design and Optimization, Wiley,

New York.

2. Kotas T.J., 1985, The Exergy method of Thermal Power analysis, Butterworth.

3. Nag P.K., Power plant engineering, 2nd Ed., Tata Mc Graw – Hill, New York, 1995.

4. Dr. gupta A.V.S., second low analysis of super critical cycle.

5. Viswanathan, R., 2001, Boiler materials for ultra supercritical coal power plants, USC

Materials quarterly report, EPRI Inc., Oct-Dec 2001.

6. Kiameh, P. (2002), Power Generation Handbook, McGraw-Hill Handbooks.

7. Rajput, R.K. (2001), Thermal Engineering, Laxmi, New Delhi.

8. Babcock & Wilcox power generation groups technical papers.

Appendix: select data

Table 1

Operating condition

BMCR SH control point 50%

TMCR

Steam flow superheater kg/hr 2225,000 963,760

Steam flow reheaters kg/hr 1741,820 836,410

Steam temp. superheater 0 c 540 540

Steam temp. reheaters 0 c 568 568

Reheat entering temp. 0 c 299 289

Reheat entering pressure bar 45.39 22.15

Feed water temperature 0 c 289.64 244.34

Boiler efficiency % 86.28 86.85

Table 2

performance

load BMCR TMCR 80%TMCR 60%TMCR

Steam flow superheater kg/hr 2225,000 2,023,750 1,572,470 1,158,410

Steam flow reheaters kg/hr 1,741,820 1,678,370 1,328,960 996,950

Superheater outlet temp. 0 c 540 540 540 540

Superheater outlet press. bar 250 248.48 232.35 174.92

Reheat inlet temp. 0 c 299 296 281 286

Reheat otlet temp. 0 c 568 568 568 568

Reheat inlet pressure bar 46.37 44.80 35.49 26.56

Reheat oulet pressure bar 44.71 43.21 34.21 25.56

Reheat pressure drop. bar 1.65 1.58 1.27 1.0

Feed water temperature 0 c 289.64 286.23 270.35 254.09

Fuel fired kg/hr 471800 438100 354900 272400

Efficiency % 86.28 86.29 86.69 86.88



430

Table 3

performance

load 50% TMCR 30% TMCR Both HPH out

Steam flow superheater kg/hr 963,760 596,100 1,839,500

Steam flow reheaters kg/hr 836410 517200 1,784,200

Superheater outlet temp. 0 c 540 540 540

Superheater outlet press. bar 147.30 91.0 246.98

Reheat inlet temp. 0 c 289 294 309

Reheat otlet temp. 0 c 568 540 568

Reheat inlet pressure bar 22.15 13.33 48.52

Reheat oulet pressure bar 21.29 12.77 46.86

Reheat pressure drop. bar .86 .55 1.66

Feed water temperature 0 c 244.34 219 196.15

Fuel fired kg/hr 231100 147300 463100

Efficiency % 86.85 86.24 87.31

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43 Performance Analysis of Supercritical Boiler