Post on 31-Jul-2020
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.