EVALUATION OF PERFORMANCE OF BOILERS IN THERMAL POW ER

STATIONS (BTPS & RTPS)

K.V SREENIVAS RAO 1 AND CHANNAKESHWA PRASAD S.M 2

1Professor, 2M.Tech. Student, Department of Mechanical Engineering, Siddaganga Institute of

Technology, Tumkur- 572103, Karnataka, India, 1Email:-kvsraoiit@yahoo.com

ABSTRACT

The role of efficiency and performance lies in maximizing generation from the thermal power plants.

It enhances energy efficiency of the plant. Presently about 65% of the total power is generated by thermal

power plants. There are thermal units of various capacities in India. It is envisaged that at the end of 2015

thermal generation of 50,000MW will be added to the total power generation capacity of India. The cost

required for obtaining 1 MW installed in the grid is about Rs 6 crores and 2 crores more are required for

the associated transmission system. Considering the massive investment for thermal generation it is

important to give a thought on the returns being obtained from these stations. In order to keep maximum

output from the given input, the units must be run at a maximum possible efficiency. Power plant

performance has various steps in improving the power generation capacity. The areas which are mainly

considered for the station performance are plant maintenance loss, thermal efficiency factors, plant load

factors, forced outages and plant availability factor. In the present study boiler efficiency of the thermal

power plants is assessed by direct and indirect method and compared with MCR value. From the

analysis, it was observed that the boiler efficiency is in the range of 83.66% & 83.5% against the MCR

value of 86.97% (BTPS Unit 1) & (RTPS 1 to V units) respectively. The reduction in boiler efficiency

could be due to wet stack loss, low quality of fuel, improper operating conditions and plant age factor.

Further, the boiler efficiency can be improved by reducing the air-in leakage from the air side to flue gas

side by replacing axial and radial seals.

KEY WORDS: Boiler efficiency, Dry stack loss, Wet stack loss, Combustible in ash loss.

1. INTRODUCTION

Efficiency of the boiler depends on several factors like design, type of fuel, fuel consumption, gas

temperature leaving the boiler, coal fineness in the case of pulverized coal etc. The efficiency of coal-

fired boiler generally is in the range of eighty five to eighty nine percent. The efficiency of boiler can be

calculated by making use of the direct (Input/Output) method or indirect (Losses) method. In the present

work, an attempt has been made to evaluate the efficiency of coal-fired boiler by indirect method, which

is being used in BTPS & RTPS. The main advantages of indirect method over direct method include

higher accuracy and also it takes into account individual losses in calculating efficiency.

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN:2249 – 6890 Vol.2, Issue 1 (2012) 24-35 © TJPRC Pvt. Ltd.,

Evaluation of Performance of Boilers in Thermal Power Stations (BTPS & RTPS) 25

Dry stack loss, wet stack loss and combustibles in ash loss are the three major losses, which accounts

nearly 90% of the boiler losses and the balance of the losses accounts for 10% of the total boiler losses.

From the analysis, it was observed that the boiler efficiency is in the range of 84.856% against the MCR

(maximum continuous rating) value of 86.97%. for RTPS and for BTPS processed value is 83.66%

against the MCR value of 88.97%.In BTPS wet stock loss and combustible in ash loss is more .The

reduction in boiler efficiency could be due to low quality of fuel, improper operating conditions and plant

age factor. Further, the boiler efficiency can be improved by reducing the air-in leakage from the airside

to flue gas side by replacing axial and radial seals. The efficiency can also be improved by periodic

monitoring of coal fineness and classifier setting, such that the presence of unburnt carbon in bottom and

fly ash is reduced.

The efficiency of boiler can be calculated by making of any of the following methods

1. Direct methods or input/output method

2. In direct method or losses method

The direct method is the established way of determining boiler efficiency and consists of measuring

the quantity of heat given to the boiler and to the steam in a period of say one hour. In view of the

inaccuracies of measurement of coal input to the boiler being more. This method has been replaced with

losses method.

NOMENCLATURE

A Ash content in fuel (%)

C Carbon in fuel (%)

c Carbon in dry ash (%)

CAL Combustibles is ash loss (KJ/kg)

C in A Carbon in rough ash and dust (kg/kg of fuel)

Cb Combustibles in bottom ash (%)

Cf Combustibles in fly ash (%)

CO Carbon monoxide at APH inlet (%)

CO2 Carbon dioxide at APH inlet (%)

CV Calorific value of carbon (KJ/kg)

DSL Dry stack loss (KJ/kg)

Db Distribution of bottom ash (%)

Df Distribution of fly ash (%)

GCV Gross calorific value of coal (kcal/kg)

N2 Nitrogen in flue gas (%)

NCV Net calorific value of coal (k cal/kg)

O Oxygen in flue gas (%)

RL Radiation loss ((KJ/kg)

S Sulphur in fuel (%)

SHLBA Sensible heat loss of bottom ash ((KJ/kg)

SHLFA Sensible heat loss of fly ash ((KJ/kg)

SHLWV Sensible heat loss of water vapour (KJ/kg)

T Flue gas temperature at APH outlet (0C)

t Air temperature at F.D fan inlet or

ambient temperature (0C)

Tb Temperature at the bottom of the furnace (0C)

U Total weight of carbon in ash (kg/kg of fuel)

Ub Weight of carbon in bottom ash (kg/kg of fuel)

K.V Sreenivas Rao & Channakeshwa Prasad S.M 26

H Hydrogen in fuel (%)

h Kg of moisture per kg of dry air

M Total moisture in fuel (%)

M1 Main steam flow (Tons/ hour)

MCA Moisture in combustion air (KJ/kg)

Uf Weight of carbon in fly ash (kg/kg of fuel)

WSL Wet stack loss (KJ/kg)

FG FLUE GAS

ηb Boiler efficiency

2. VARIOUS LOSSES IN BOILERS

Dry stack loss

The only components of a fuel that burn to form dry products of combustion are the carbon and

sulphur. Of the two, carbon has the greater significance and hence sulphur, which is very much low

(0.42%) in Indian coal is ignored. Whenever the flue gas leaves the air heaters, it doses no further useful

work. However, it is at a considerably higher temperature then ambient. So it carries valuable heat to the

chimney to be dispersed to the atmosphere. The dry flue gas is the heat carried away by the flue gas that

consists of carbon dioxide, nitrogen, oxygen and carbon monoxide. The major parameter of exit gas

temperature at air preheater (APH) outlet is the proper index for quantifying the dry stack loss.

( ) ( )tTAinCSC

COCODSL −

−++

= 6.3010010012

100

2

The dry stack losses is calculated as a function of quantity of dry flue gases, the temperature

difference between air temperature of forced draft (F.D) fan inlet and flue gas at boiler exit and mean

specific heat of the gas at constant pressure.

%DSL = (DSL / GCV) 100

Wet stack loss

The wet products of combustion are deriving from the moisture and the hydrogen in the fuel. The

losses are calculated taking into account on exit gas temperature. This loss is not controllable and mainly

depends on the fuel characteristics like hydrogen content and inherent moisture in the fuel. The primarily

controllable factor to control this loss is by reduction in exit gas temperature.

( ) ( )[ ]tT

HMWSL −++−

+= 252.424422588.1100

9

%WSL = (WSL / GCV) 100

Combustibles in ash loss

This appears to be a major loss, where carbon as free carbon escapes through the bottom and fly ash.

Unburnt gas is mainly due to incomplete burning of carbon to carbon monoxide instead of carbon

Evaluation of Performance of Boilers in Thermal Power Stations (BTPS & RTPS) 27

dioxide. The amount of combustible materials left over in ash depends on the efficiency of combustion

and also on the coal fineness available from pulverizes.

( )( )CVUCAL =

% CAL = (CAL/ GCV) 100

U = (Ub + Uf)

( )

−

=b

bbb C

CADU

100100100 ( )

−

=

b

fff C

CADU

100100100

Moisture in combustion air

The air used combustion has small amount of moisture, which gives rise to some heat loss. This is

usually quite small and is calculated as:

( )( ) ( )htTAinC

SC

COCO

NMCA −

−++

= 88.1267100

034.3

2

2

% MCA = (MCA / GCV) 100

Sensible heat loss of bottom and fly ash

In modern pulverized fuel boilers, hot ash and slag is collected in the bottom ash hopper where it is

quenched in the water. Also bottom ash hopper water absorbs some portion of radiant heat from the

furnace and adds to the moisture loss in the flue gasses. The fly ash is collected in electrostatic

precipitator (ESP) hoppers at a particular temperature and so there is a loss of sensible heat due to that

also.

( )tT

DASHLBA b

b −

= 15.0100100

( )tTDA

SHLFA b

f −

= 15.0100100

% SHLBA = (SHLBA / GCV) 100

% SHLFA = (SHLFA / GCV) 100

Sensible heat loss of water vapour

This loss refers to the loss of sensible heat in water vapour. The difference between grass calorific

value and net calorific value is the latent heat of vapour; therefore, the sensible heat of water vapour

accounted in this loss can be calculated as follows:

( ) ( )NCVGCVWSLSHLWV −−=

K.V Sreenivas Rao & Channakeshwa Prasad S.M 28

% SHLWV = (SHLWV / GCV) 100

Radiation loss

It is the heat loss from the boiler enclosure through the insulation. Heat is radiated to surrounding

and it is expelled into ambient air in contact with boiler surfaces. Providing least conductive insulation

can minimize the loss. The equivalent loss is approximately 0.2%, which is almost forming 80% of total

radiation.

=

−

5.05.110100

6.3log4238.08167.0 1M

XGCVRL

% RL =(RL / GCV) 100

In the present work, an attempt has been made to evaluate the efficiency of coal-fired boiler (Unit

number 1 to 5) by indirect method, which is being used in RTPS. The main advantages of indirect

method over direct method include higher accuracy and also it takes into account individual losses in

calculating efficiency. Various individual losses that are taken into account while evaluating the

efficiency of the boiler are dry stack loss, wet stack loss, combustibles in ash loss, moisture in

combustion air, sensible heat loss of water vapour, sensible heat loss of bottom ash, sensible heat loss of

fly ash and radiation heat loss.

3. TEST CONDITIONS

During the test period the following conditions would be ensured.

- As for as possible there should not be any variation in the unit load as well as in the operating

conditions

- Preferably no oil burner in service

- Continues blow down in closed position

- H.P. Heaters should be in service.

4. SAMPLE OF ANALYSIS

Following sample taken for analysis

- Raw coal sample at inlet to the coal mills for proximate analysis and ultimate analysis of coal.

- Pulverized coal sample at mill outlet.

- Bottom ash sample.

- Fly ash sample.

- Flue gas sample.

- Mill rejects sample.

Evaluation of Performance of Boilers in Thermal Power Stations (BTPS & RTPS) 29

5. OBSERVATIONS AND ANALYSIS

BOILER EFFICIENCY

Figure 5. 1: Comparison of boiler efficiency of BTPS Unit 1 with MCR value (With 8 Trials)

Figure 5.1 shows the comparison of boiler efficiency with MCR value for BTPS unit 1, boiler

efficiency is in the range of 83.66% against the designed value of 88.97% this due to the increased

losses of wet stock, combustible in ash, & radiation Loss

Fig 5.2 shows boiler efficiency of RTPS 5 units, Boiler efficiency is in the range of (83.% to 84%)

against the design efficiency of 86.56%,this value is satisfactory considering the plant age factors

Fig 5.3 shows the comparison between direct & indirect method of calculation of boiler efficiency.

Boiler efficiency almost remains constant(± 1%) for BTPS unit 1 irrespective of load & coal flow where

as the direct way of calculation changes(± 13%) according coal flow and load and it’s a instantaneous

one

Figure 5.2 : Comparison of boiler efficiency of RTPS Unit 1 to Unit V with MCR value

K.V Sreenivas Rao & Channakeshwa Prasad S.M 30

Figure 5.3 : Comparison of direct and indirect way of calculation of boiler efficiency of BTPS

Unit1 (8 Trails)

Figure 5.4 : Comparison of direct and indirect way of calculation of boiler efficiency of RTPS

Units 1 To Unit V

FIG 5.4 shows the comparison between direct & indirect method of calculation of boiler efficiency

.The direct method is less accurate one & instantaneous values obtained can’t be granted, we see that for

unit 3 & unit 4 boiler efficiency is 117.43% & 116.92% which is over 100%,can’t be used to calculate

the boiler efficiency.

Seeing the fig 5.3 & 5.4 we conclude that losses method is more accurate and it has the following

advantages over the output method

1. No coal weighers are required

2. Accuracy of values is less critical then with the direct method

3. Individual Losses Are Also Measured In Addition To Calculation Of Efficiency

4. The calorific value of the fuel is still required. But any error In its determination has a smaller

effect on the final answer

Evaluation of Performance of Boilers in Thermal Power Stations (BTPS & RTPS) 31

6. REASONS FOR DETERIORATION IN THE BOILER EFFICIEN CY

Dry stack loss

BTPS unit 1(8 trials)

Fig 6.1 shows the percentage of deviation of dry stock losses from MCR value .Flue Gas

Temperature At The APH Outlet Is 1350C ,So Dry Stock Loss Is Matching With The Designed Value

.Losses Are In Designed Range.

Figure 6.1 : Percentage of deviation from MCR value for dry stack losses for BTPS Unit 1(8 trials)

RTPS unit 1 to V

Fig. 6.2 shows the percentage of deviation of dry stack losses from MCR value. It is clear from

figure that the dry stack losses in Unit 1 and 2 are more when compared to the dry stack losses in other

units. This could be due to poor heat transfer in air-preheater. This poor heat transfer results in higher

flue gas temperature at the boiler exit (155 0C for Unit 1 and 2) as against MCR value of 137 0C, the loss

may occur to this following reasons

� Lack of soot blowing.

� Worn out condition of APH baskets

� deposit on boiler heat transfer surface

� High excess air

� Low final feed temperature

� Higher burner tilt at low loads

� Incorrect secondary air to primary air ratio.

Fig 6.3 shows the Dry stack loss for APH outlet temperature values , as the temperature is higher

the dry stack loss is higher, The optimum value for the exit flue gas temperature is 1300c, The value less

than that will result in Dew point sulphuric acid which corrodes APH baskets

K.V Sreenivas Rao & Channakeshwa Prasad S.M 32

Figure 6.2 : Percentage of deviation from MCR value for dry stack losses for RTPS unit 1 to V

0

1

2

3

4

5

6

7

145

147

151

151

152

153

154

157

158

FG TEMP AT APH O/L deg C

DR

Y S

TA

CK

LO

SS

%

Figure 6.3 : FG temperature at APH O/L Vs dry stack loss

Wet stack loss

BTPS UNIT 1 (Trials 8)

Fig. 6.4 shows the percentage of deviation of wet stack losses from MCR value .The deviation is

higher then the designed value . Maximum Wet Stock Loss Is 6.651% against The Designed Value Of

5.19%.

Figure 6.4 : Percentage of deviation from MCR value for wet stack losses for BTPS unit 1 (8 trials)

RTPS unit 1 to V

Fig. 6.5 shows the percentage of deviation of wet stack losses from MCR value. The deviation is

higher in Unit 1 and 2 when compared to other units. Such higher deviation is because of moisture (both

inherent and surface) and hydrogen in fuel. The total moisture in coal is 12.4% (Unit 1), which is quite

Evaluation of Performance of Boilers in Thermal Power Stations (BTPS & RTPS) 33

variation of calorific value. Maximum wet stack loss is 6.150% (Unit 1) as against 4.64% MCR value.

The measures to be taken are

1. Make proper stacking

2. Make good drain arrangements

3. The Water Sprayed On Coal For Not Getting Fired In Coal Yard Must Be Minimized

4. Coal With Less Hydrogen Content Must Be Used

Figure 6.5 : Percentage of deviation from MCR value for wet stack losses for RTPS unit 1 to V

0

2

4

6

8

10

12

6.1

8.3

9.5

9.8

10.5

11.1

12.5

12.7

15.2

15.9

17.7

MOISTURE IN COAL %

WE

T S

TA

CK

LO

SS

%

Figure 6.6 : Moisture in coal V/S wet stack for various value

Fig 6.6 shows as the moisture in coal increases the wet stack loss also increases .the inherent

moisture in the coal can’t be removed but we can avoid wet coal by installing heating stations in coal

handling system, and giving hot air (PA) to mill so that mill outlet temperature is maintained above 80 0C

Combustible in ash

BTPS UNIT 1 (8Trials)

Fig. 6.7 shows the percentage of deviation of combustibles in ash loss from MCR value. It is clear

from figure that the combustible in ash loss in BTPS UNIT 1 is more than the designed value ,this due to

in proper air and coal mixture ,tilt, operation secondary damper opening ,volatile particles in coal

,improper grinding of coal in mills designed value is (0.71%) against the actual average value is

(3.406%)

K.V Sreenivas Rao & Channakeshwa Prasad S.M 34

Figure 6.7 : Percentage of deviation from MCR value for combustibles in ash loss for

BTPS unit 1(8 trials)

RTPS UNIT 1 TO V

Fig. 6.8 shows the percentage of deviation of combustibles in ash loss from MCR value. It is clear

from figure that the combustible in ash loss in Unit 5 is more when compared to the combustibles in ash

loss in other units. This could be due to poor mill performance and improper secondary air for

combustion. This poor mill performance and improper secondary air for combustion results more

combustibles in bottom ash (19% for Unit 5) as against MCR value of 5%.

Measures to be taken are

1. Check and Overhaul Mills

2. Air Flow Optimization

3. Burner Tilt Operation Check

4. Secondary Air Damper Operation Check

5. Poor Quality of Coal, Volatile Materials

6. Crushers house must be used before mills

Figure 6.8 : Percentage of deviation from MCR value for combustibles in ash loss for

RTPS unit 1 to V

Evaluation of Performance of Boilers in Thermal Power Stations (BTPS & RTPS) 35

0

1

2

3

4

5

77.04

80.98

81.88

82.11

82.62

83.1

83.83

84.37

84.66

85.19

86.02

BOILER EFFICIENCY

CO

MB

US

TIB

LE

S IN

ASH

LO

SS

Figure 6.9 : Boiler efficiency v/s combustibles in ash loss for various values

Fig 6.9 shows the decreasing of the boiler efficiency as the combustibles in ash loss increases for

BTPS unit1. This is due to the coal we get is of lesser calorific value and crushers house & magnetic

separator are by passed resulting lesser mill performance and it decreases the combustion efficiency

7. CONCLUSIONS

From the analysis, it was observed that the boiler efficiency is in the range of 83.66% & 83.5%

against the MCR value of 86.97% (BTPS Unit 1) & (RTPS 1 to V units) respectively. The reduction in

boiler efficiency could be due to more wet stack loss, low quality of fuel, improper operating conditions

and plant age factor. Further, the boiler efficiency can be improved by reducing the air-in leakage from

the air side to flue gas side by replacing axial and radial seals. The efficiency can also be improved by

periodic monitoring of coal fineness and classifier setting, such that the presence of unburnt carbon in

bottom and fly ash is reduced.

For control of reheater (RH) spray, burner tilting and coal flow biasing is better, because RH spray

leads to work losses in the high pressure (HP) turbine. Present practice is keeping all mills equal loading.

Biasing of coal flow in elevations with a maximum flow in the lowest elevation and minimum flow in

the upper most elevation will help minimize spray and offer better temperature control.

Optimization of air flow is required, as oxygen at inlet is more; this increases the dry flue gas loss. It

can be reduced to optimum level. Inlet oxygen can be checked in both passes.

REFERENCES

1. Modern Power Station Practice, BHEL, Manual .

2. A.B.Gill, Power Plant Performance, Butterworths: London, 1984.

3. Power Plant Technology- El-Wakil

4. Power Plant Engineering – S.C Arora & Domkundwar.

5. Boiler Performance & Life Extension –BHEL(Tiruchirpalli)

6. Power plant engineering- P.K Nag.