flue gas temperature
TRANSCRIPT
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56 National Thermal Power Corporation
Opmizaon of Flue Gas Exit Temperature at
Air Pre Heater outlet in a 500 MW Unit BHEL Boiler
AVS Rao, Sheik Zaheer Ahamad, M Prasad & ISS Reddy
KEYWORDS: Flue Gas Exit Temperature, Air Pre Heater (APH), DMAIC, CTQ,
Multiple Regression, Partial Least Square Regression (PLSR), Design
of Experiment (DOE), Fractional Factorial, Variance Influence Factor
(VIF), Multi Collinearity.
1.0 Introduction
One of the units in NTPC, Ramagundam had been experiencing a high FlueGas Exit Temperatures at Air Pre Heater (APH) outlet. It was observed that
the Average Flue Gas Temperature in the last six months was more than 145
deg C, while the Design Flue Gas Exit Temperature at APH outlet is 125 deg
C at rated parameters.
DMAIC (Define, Measure, Analyse, Improve and Control) methodology of
Six Sigma Quality Management was adopted as a Performance Improvement
Strategy to determine the root cause of the problem and implement the
solution thereof. The purpose of this report is to present the results of the
teams problem-solving efforts and explain the solutions adopted.
In order to improve / reduce Exit Flue Gas Temperature at the APHsoutlet, eighteen parameters were considered and Regression and Partial
Least Square Regression Analysis was done and then a Fractional Factorial
Design of Experiments (DOE) was conducted.
Following the DMAIC Performance Improvement Problem-Solving
strategy of Six Sigma Quality Management, it was observed that four factors,
viz. Average Mill Outlet Temperature, Total Primary Air Flow, Total Secondary
Air Flow and Burner Tilt contributed significantly to the high Flue Gas Exit
Temperature at APHs outlet. In addition to this, it was also observed through
the Statistical Analysis that variation in Fuel Air Dampers, SAPH-A and B
Outlet Dampers and Wind Box DP were having a Statistically significantinteraction effect on the Exit Flue Gas Temperature.
This report presents a detailed explanation of the steps we followed using
the Performance Improvement Problem-Solving strategy Define, Measure,
Analyze, Improve, and Control (DMAIC) of Six Sigma to determine how we
can achieve minimum Flue Gas Exit temperature by just changing operational
practices without any modification and/or without any investment.
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Optimization of Flue Gas Exit Temperature at Air Pre Heater outlet in a 500 MW Unit BHEL Boiler 57
2.0 Improvement Opportunity
It was observed that an average gap in the Exit Flue Gas Temperature at
APHs outlet from the Design Temperature was around 20.0 deg C during
January, 2011. So it was thought that if this gap could be reduced even by
half, i.e. say reduced to 11 deg C, then an improvement in Heat Rate to the
tune of roughly 11Kcal was possible.
3.0 Current Performance
Current Performance Level: The average APHs outlet Flue Gas
Temperature during the month of January, 2011 was observed as 147.5
deg C. The maximum average reached (in one of the 15 minute blocks)
during the month was 172 deg C; please see Drawing (1) below.
Data Collection: Every fifteen minute cumulative data was collected on
the following 18 Key Performance Input Variables (KPIVs) and 8 Key
Performance Output Variables (KPOVs) during the month of January,
2011 for analysis purpose.
CAPABILITY ANALYSIS UNIT-7
Process Data
LB 120
Target *
USL 135
Sample Mean 147.759
Sample N 2761
StDev (within) 0.631459StDev (overall) 9.69413
Observed performance
PPM < LB 0.00
PPM > USL 886997.46
PPM Total 886997.46
Exp. within performance
PPM < LB *
PPM > USL 1000000.00
PPM Total 1000000.00
Exp. within performance
PPM < LB *
PPM > USL 905944.62
PPM Total 905944.62
Potential (within) capability
Cp *
CPL *
CPU 6.74Cpk 6.74
Overall capability
Pp *
PPL *
PPU 0.44
Ppk 0.44
Cpm *
l l l l l l l l
120.0 127.5 135.0 142.5 150.0 157.5 165.0 172.5
Within
Overall
LB USL
Unit-7 Capability Analysis-Jan 2011
Drawing (1)
The Key Performance Input Variables (KPIVs) considered were:
Coal Flow1.
Primary Air Flow2.
Secondary Air Flow3.
Burner Tilt Position Cornerwise (Give Corner Numbers)4.
Each Mill Outlet Temp (which are in service)5.
Wind Box DP6.
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APH Soot Blowing Timings7.
Air Pre Heater Air Inlet Temp (APH-wise)8.
Air Pre Heater Outlet Damper Position (APH-wise)9.
Super Heater Spray (tph)10.
Steam Flow11.
Re-Heater Spray (tph)12.FAD Position13.
OFD Position14.
Wall Blowing Status (timings and no. of Blowers Operated)15.
LRSB Operation Details (timings, date, number of Blowers16.
(Group-wise))
Mill Combination Details (which mill, feeding rate each mill (tph))17.
Load in MW18.
The following parameters were not considered for selection as Input
Variables as they, were beyond the control of Normal Operators.Mill Combination Depending upon the availability
Wear Out of Combustion Components
APH Internal Condition
The Key Performance Output Variables (KPOVs) considered were:
1. Flue Gas Outlet Temperature at PAPH-A
2. Flue Gas Outlet Temperature at PAPH-B
3. Flue Gas Outlet Temperature at SAPH-A
4. Flue Gas Outlet Temperature at SAPH-B
Measurement System Analysis: As the data tags used were sourced from
PI Server (Process Information Server), they were considered most
reliable and stable and as such Measurement System Analysis (MSA) was
not required.
Target Performance Level: The required target APH outlet Flue Gas
Temperature was 132 deg C with allowable 5 deg C deviation from upper
side.
Normality Test results showed that APH Exit Flue Gas Temperature was
responsive to some Input Variables and therefore Statistical Tools can be
used to analyze the data.
4.0 Data Analysis and Interpretation
The 15 minute average data collected around 18 input variables and
8 output variables during January, 2011 was analyzed using Minitab-
Statistical Analysis Software for understanding the Process Capability,
Correlation of Input Variables with Output Variables etc. During Regression
Analysis it was found that there was Multi-Collinearity among the Input
Variables.
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Optimization of Flue Gas Exit Temperature at Air Pre Heater outlet in a 500 MW Unit BHEL Boiler 59
Regression Analysis
Predictor Coefcient Se Coefcient T P Vif
Constant -188.5 203.1 -0.93 0.354
Fw Temp At Eco Inlet -0.02803 0.01308 -2.14 0.032 9.913
Fw Temp At Eco Ol Lhs 1.28405 0.08315 15.44 0.000 23.671
Fw Temp At Eco Ol Rhs 0.32995 0.08727 3.78 0.000 32.563
Ms Temp Lhs -0.00972 0.01823 -0.53 0.594 1.427
Ms Temp Rhs -0.08661 0.01433 -6.05 0.000 3.258
Pah-A Fg Ol Dmaper -0.28817 0.09190 -3.14 0.002 40.607
Sah-A Fg Ol Damper -1.3339 0.9318 -1.43 0.153 19.199
Sah-A Ol Damper2 0.5145 0.2261 2.28 0.023 39.740
Pah-B Fg Ol Damper 0.05151 0.08334 0.62 0.537 38.787
Sah-B Fg Ol Damper 1.540 2.610 0.59 0.555 23.743
Bli 0.033857 0.003968 8.53 0.000 70.241
Fw Flow -0.003565 0.002560 -1.39 0.164 36.367
Total Air -0.033538 0.003330 -10.07 0.000 34.286
Total Fuel -0.04104 0.01088 -3.77 0.000 26.289
Bt Corner-1 Fdbk 0.04183 0.01610 2.60 0.009 2.135
Bt Corner-2 Fdbk 0.23895 0.02453 9.74 0.000 13.552
Bt Corner-3 Fdbk -0.23302 0.04414 -5.28 0.000 31.201
Bt Corner-4 Fdbk 0.08246 0.05716 1.44 0.149 31.829
Furn Wb Dp-1 -0.04787 0.06216 -0.77 0.441 18.490
Furn Wb Dp Lhs 0.21911 0.06130 3.57 0.000 19.710
Aux Air Damper Control 0.06351 0.02296 2.77 0.006 7.403
U Ofa Position 0.0733 0.1084 0.68 0.499 276.472
NORMALITY TEST
Unit-7 APHs Outlet FGTNormal
Mean 147.8StDev 9.693
N 2761
AD 24.097
P-Value
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Lower Ofa Position 0.0020 0.1090 0.02 0.986 276.872
Fad -A 0.13286 0.02731 4.87 0.000 4.784
Fad -B -0.05300 0.01928 -2.75 0.006 5.356
Fad -C 0.1934 0.1110 1.74 0.082 2.583
Fad -D 0.07092 0.02895 2.45 0.014 4.707
Fad -E 0.07978 0.02862 2.79 0.005 3.544Fad -F -0.03438 0.01569 -2.19 0.029 3.773
Fad -G 0.00635 0.04108 0.15 0.877 9.274
Fad H -0.020400 0.009823 -2.08 0.038 2.924
Fad -J 0.00542 0.01177 0.46 0.645 5.914
Fad -K 0.014036 0.009818 1.43 0.153 5.378
Mill A (Tph) 0.02146 0.02064 1.04 0.299 6.946
Mill B(Tph) 0.04156 0.01401 2.97 0.003 9.494
Mill C (Tph) -0.04593 0.02313 -1.99 0.047 1.816
Mill D (Tph) -0.03589 0.01568 -2.29 0.022 10.143
Mill E (Tph) 0.06877 0.02553 2.69 0.007 1.832
Mill F (Tph) -0.03352 0.01438 -2.33 0.020 9.412
Mill G (Tph) 0.04066 0.03197 1.27 0.204 10.529
Mill H (Tph) 0.01426 0.01179 1.21 0.227 9.675
Mill J (Tph) 0.02730 0.01337 2.04 0.041 18.297
Mill K (Tph) 0.00993 0.01341 0.74 0.459 24.144
Sec Air Flow (Tph) 0.012923 0.003175 4.07 0.000 11.781
Sh Spray L(Tph) -0.04677 0.01461 -3.20 0.001 7.407
Sh Spray R(Tph) -0.19350 0.01907 -10.15 0.000 4.608
Rh Spray L(Tph) 0.06820 0.02577 2.65 0.008 2.923
Rh Spray R(Tph) -0.02590 0.01838 -1.41 0.159 4.001
S = 2.18163, R-Sq = 93.5%, R-Sq (adj) = 93.2%.
This means 93.2 % of the variation is explained by the Predictors.
Analysis of Variance (ANOVA)
Source DF SS MS F P
Regression 48 79204.2 1650.1 346.69 0.000
Residual Error 1159 5516.3 4.8
Total 1207 84720.5
As the variance factor of most of the parameters was exceeding 5, it
indicates the existence of Multi Collinearity. That means the parameters are
inter-related and that limits the application of Multiple Linear Regression
for our purpose.
The objective was to identify the Independent Factors which could
explain the variation in the Dependent Variable. Hence, Partial Least
Square (PLS) Regression was used to filter out the Least Impacting Input
Variables. After three trials of PLS Regression, it was found that the following
eight variables were impacting the most to the extent of 94.5%.
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Optimization of Flue Gas Exit Temperature at Air Pre Heater outlet in a 500 MW Unit BHEL Boiler 61
The larger the bar represents the larger the effect and the + & - signs
show their impact being positive or negative on the required end result (i.e.,
Exit Flue Gas Temperature in this case).
The variables found to be affecting the Flue Gas Temperatures are:
Average Mill Outlet Temperature1.
Total Primary Air2.
Total Secondary air3.
Burner Tilt4.
Wind Box DP5.
SAPH-A Gas Damper Position6.
SAPH-B Gas Damper Position7.
FAD-D, E, F Position8.
A Design of Experiment (DOE) Plan was prepared for a Two LevelFractional Randomized Plan for Unit-7 (with Resolution-IV) and was
conducted on 10th and 11th August, 2011. Average data was collected at
5 minutes interval for 15 minutes after each setting and after waiting for
10 minutes for stabilization for 36 Hrs. and collected data for 16 Hrs. with
required settings. In the Process, minimum average temperature of APHs
outlet Flue Gas Temperature achieved was 133.5 deg C and the maximum
average temperature achieved was 170 deg C.
Predictors
PLS Coefcient Plot(response is AV. Fg Temp BEF PAH-A (DEG C))
10 components
1.5
1.0
0.5
0.0
0.5
1.0
Coefcients
l l l l l l l l l l
1 5 10 15 20 25 30 35 40 45
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Coefcients Term Coefcients SE Coefcients T P VIF
Constant 367.991 33.1322 11.1068 0.000
Coal Flow -0.646 0.1002 -6.4480 0.000 2.83463
P. Air Flow 0.101 0.0145 6.9696 0.000 2.88370
S. Air Flow 0.058 0.0072 8.0254 0.000 1.42337
F. WB DP 0.003 0.0293 0.0981 0.922 1.09665
Burner Tilt % (calculated) -0.379 0.0690 -5.4849 0.000 1.18030
Av. Mill O/L temp -1.307 0.0557 -23.4625 0.000 1.07640
SAPH_A GD POS 0.000 0.0240 0.0180 0.986 1.05573
SAPH_B GD POS 0.020 0.0235 0.8333 0.410 1.01454
FAD-D 0.016 0.0283 0.5749 0.569 1.07696
Summary of Model:
S = 2.83424, R-Sq = 95.54%, R-Sq (adj) = 94.49%
PRESS = 476.167, R-Sq (Pred) = 93.05%.
Using Statistical tools, the Main Effects and Interaction Effects of the
Input Variables on the average Flue Gas Exit Temperature at the APHs outlet
were studied and it was found that the following four parameters are having
the maximum influence.
Average Mill Outlet Temperature1.
Total Secondary Air Flow2.
Total Primary Air Flow3.
Burner Tilt4.
DOE-Randomized two level Plan for Unit-7-Rdm (Resolution-IV)
Run Order
Total
Primary
Air
Total
Secondary
Air
Mills
Outlet
Temp-Av
Burner Tilt
Position
Wind Box
DP L&R
APH-Sec-
A-GD-
Position
APH-Sec-
B-GD-
Position
FAD-
D,E&F
Position1 700 1225 90 20 70 65 90 15
2 700 1150 90 40 70 90 65 15
3 610 1150 90 20 100 90 90 15
4 610 1225 75 40 70 90 90 15
5 610 1225 90 40 100 65 65 15
6 610 1150 75 20 70 65 65 15
7 700 1150 75 40 100 65 90 15
8 610 1225 90 20 70 90 65 30
9 700 1225 75 40 70 65 65 30
10 700 1225 90 40 100 90 90 30
11 610 1150 75 40 100 90 65 30
12 700 1150 75 20 70 90 90 30
13 610 1225 75 20 100 65 90 30
14 610 1150 90 40 70 65 90 30
15 700 1225 75 20 100 90 65 15
16 700 1150 90 20 100 65 65 30
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Optimization of Flue Gas Exit Temperature at Air Pre Heater outlet in a 500 MW Unit BHEL Boiler 63
The Main Effects of Wind Box DP, SAPH A & B Gas Damper Positions
and FAD-D Position on FGT are not very significant but their interactions
are Statistically significant.
Since contribution
to sum of squares
is negligible,
factors 3,6,7,8 are
removed from the
ANOVA table
Analysis: Result of DOE
Sl No. Model term Sum of Squares % Contribution
1 PA Flow 66.898 2.94
2 SA fow 105.164 4.62
3 WB DP 0.07123 0.01
4 Burner tilt 34.0668 1.50
5 Mill outlet temp 1844.91 80.95
6 SAPH-A 12.9722 0.57
7 SAPH-B 1.32397 0.06
8 FAD-D 0.42901 0.02
1x5 52.6667 2.32
1x8 89.1388 3.92
Error 71.67929 3.15
SS total 2279.32 100.06
Variables influencing most:
Average Mill Outlet Temperature
Total Secondary Air Flow
Total Primary Air Flow
Coal Flow
Variables Showing Interaction Effects:
Wind Box Dp
Saph-A Gas Damper Position
Saph-B Gas Damper Position
Fad-D,E,F Position
Burner Tilt
5.0 Recommendation(s)
Based on the analysis of DOE data, it was recommended to run Unit - 7 with
the following settings to achieve minimum FGT at APHs outlet:
a. Maintain the Average Mill Outlet Temperature at around 90 deg C
b. Maintain the Total Secondary Air at around 1100 TPH
c. Maintain the Total Primary Air at around 660 TPH
d. Maintain Burner Tilt at 45 % (slightly above horizontal)
Since the effect of Wind Box DP, SAPH A & B Gas Damper Positions and
FAD-D Position are not very significant but their interactions are Statistically
significant, the following was recommended:
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a. Keep Wind Box DP at practical minimum between 70 and 100
mmwc
b. Keep SAPH A & B Gas Dampers at practical maximum between 65%
to 100%
c. Keep FAD-D at practical maximum position between 15 and 40 %.
6.0 Verification of Results
The unit was run with the recommended settings from 2nd November, 2011
and continuing till date. For a period of one month, i.e. in November 2011,
it was observed that the Flue Gas Temp at APHs outlet came down from
an average of 141.8 to 133.3. However, if only the data collected with the
desired setting (285 Hrs.) was considered in the month of November 2011,
the average APH Exit Flue Gas Temperature was found to be 129.2 Deg Cent.
Hence for all practical purposes a reduction of 8.5 deg C has been achieved
resulting in Heat Rate reduction of 9.7 Kcal/KWH.
RAMAGUNDAM
Unit-7 APHs FGT Capability after Implementation Nov-11(using 95.0% condence)
Process Data
LSL 120
Target 130
USL 135Sample Mean 128.742
Sample N 1529
StDev (within) 0.466578
StDev (overall) 6.53012
Observed performance
PPM < LSL 102681.49PPM > USL 162851.54
PPM Total 265533.03
Exp. within performance
PPM < LSL 0.00PPM > USL 0.00
PPM Total 0.00
Exp. overall performance
PPM < LSL 90336.40PPM > USL 168939.85
PPM Total 259276.26
l l l l l l l l l l l l l
115 120 125 130 135 140 145
LSL Target USL
Potential (within) capability
Z.Bench *
Z.LSL 18.74
Z.USL 13.41
Cpk 4.47
Upper CL 4.60
Overall capability
Z.Bench 0.65
Z.LSL 1.34
Z.USL 0.96
Ppk 0.32
Upper CL 0.34
Cpm 0.25
Within
Overall
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Optimization of Flue Gas Exit Temperature at Air Pre Heater outlet in a 500 MW Unit BHEL Boiler 65
RAMAGUNDAM
Capability analysis Unit 7(Comparison before and after)
l l l l l l l l l l l l l l l l
120.0 127.5 135.0 142.5 150.0 157.5 165.0 172.5
LB USL
Single Sample Verification: One time sample was taken again on 4th
December, 2011 for final verification of the results where an improvement
of 12.8 Deg C was observed as follows:
Ad-Hoc Trend
12/4/2011 6:45 PM 4:00 hours 12/4/2011 10:45:00 PM
l 7HHL100F903 Sec Air
Flow 1095 .96851 Tonnes/hr
O Avg Exit Flue Gas Temp Value
137.462
u 7 Total PA Out 663.60 Primary Air
Flow lph
O G7 MW 518.67651 Generation
MW
n AVG Mill Outlet Temp Value
89.1245
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7.0 Limitations
Burner Tilt could not be maintained at 3 Deg Up instead of 20 Deg Up as
it was equally important to maintain rated Steam Parameters. Therefore,
the effect of Burner Tilt was not considered.
It was observed that sometimes Fire in Mill Rejects increased a little; may
be because of high Mill Outlet Temperatures.
Maintaining Primary Air Flow and Secondary Air Flows as defined,
becomes difficult at times because of various reasons, like Coal Quality,
Mills and other combustion parameters.
APH washing was carried out in the month of October and effect
of washing was also found significant as was deduced from overall
calculations.
In the process of observing APH Exit Flue Gas Temperatures in the past
years, the following variables were also found to have reasonable impact
on Exit Gas Temperature:
Mills Healthiness (Fineness, PF Velocity in Coal Pipes,PF Distribution depending upon Orifice Wear Velocities,
Velocities)
Coal Quality Better the quality, higher the temperatures.
8.0 Conclusion
Based on the analysis of DOE data, it can be concluded that by just changing
settings of four parameters, namely Mill Average Outlet Temperature,
Total Secondary & Primary Air and Burner Tilt, it was possible to obtain a
minimum FGT of 129.2 deg C at APHs outlet without any modification and/or investment. That is just by changing operating practices; it is possible to
achieve a reduction of 10.8 deg C in FGT at APHs outlet.