Flue Gas Temperature

7/26/2019 Flue Gas Temperature

1/11

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


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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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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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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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


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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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.

Flue Gas Temperature

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