Combustion and Sootblowing Optimization Help Reduce Emissions and Improve Performance and...

Combustion and Sootblowing

Optimization Help Reduce

Emissions and Improve

Performance and Availability of

Boilers Firing Western Fuels

Nenad Sarunac, Carlos E. Romero,

Harun Bilirgen and Edward K. Levy Energy Research Center

Lehigh University

610-758-4090

EPGA's 2005 Power Generation Conference, Hershey, PA September 7-8, 2005 2

Presentation Outline

Introduction

Fuel Properties

Combustion Optimization

Case Studies

Sootblowing Optimization

Case Study

Predicting Catastrophic Slagging/Fouling

Events

Early Warning Advisor


Introduction

Fuel switching has become a popular technique

for complying with the SO2 emission

requirements.

Blends of Eastern bituminous (EB) and low-

sulfur sub-bituminous (PRB) coals are often

used.

Fuel blending also affects unit operability, NOx

emissions, and overall unit performance.

Important for units equipped with low-NOx firing

systems and for furnaces with high volumetric

heat release.


Potential Constraints PRB coals have a significantly higher moisture content and lower HHV compared to EB coals.

Larger coal flow rate is needed to maintain power output.

Mill capacity can become a constraint; unit derate might be required.

Mill drying capacity might be insufficient, resulting in a low mill exit temperature.

Coal pipe plugging problems, especially during the startup.

Gas/Oil burners are needed in PA ducts.

PRB coals are highly reactive; special care must be taken to prevent auto ignition in bunkers.

Parameter Units Medium-S Eastern

Coal

PRB Coal 60/40 PRB/EB

Blend

Moisture % 6.4 28 19.4

Ash % 6.7 4.5 5.4

Nitrogen % 1.4 0.7 1.0

Sulfur % 1.6 0.2 0.8

HHV BTU/lb 13,387 8,751 10,605

Ash Softening

Temperature

oF 2,714 2,160 2,380


Effect of Coal Moisture on Mill Temp.

130

135

140

145

150

155

160

165

25 30 35 40

Coal Moisture [%]

Mil

l T

em

pe

ratu

re [

De

g.

F]


Potential Impact on NOx Formation PRB coals are highly reactive: low LOI.

PRB coals have lower nitrogen content, compared to EB coals.

PRB coals contain greater portion of fuel nitrogen in a volatile form.

Larger portion of fuel nitrogen is released in the oxygen-deficient combustion zone.

EB coals contain a larger portion of fuel nitrogen in char.

Larger portion of fuel nitrogen is released in the oxygen-rich combustion zone.

Parameter Units Medium-S Eastern

Coal

PRB Coal 60/40 PRB/EB

Blend

Moisture % 6.4 28 19.4

Ash % 6.7 4.5 5.4

Nitrogen % 1.4 0.7 1.0

Sulfur % 1.6 0.2 0.8

HHV BTU/lb 13,387 8,751 10,605

Ash Softening

Temperature

oF 2,714 2,160 2,380


Slagging and Fouling Potential PRB coals have significantly lower ash fusion

temperature compared to EB coals.

Low melting compounds formed in furnace when firing

PRB coal blends.

Furnaces designed for EB coals will be more prone to

slagging when firing PRB/EB coal blends.

PRB ash is more reflective, reducing radiation heat flux

to walls.

Higher FEGT

Increased risk of furnace slagging

Increased risk of convective pass fouling

Sootblowing strategy needs to be changed or optimized,

or additional sootblowers have to be installed.



Pioneered by the Energy Research Center

(ERC).

Early on used as an alternative to hardware

modifications for emissions reduction or

performance improvement.

Now is used in conjunction with hardware

modifications to maximize their effectiveness.

Based on modifications to the boiler control

settings.


Combustion Optimization: ERC Approach

Based on more than 15-year experience with

performance and combustion optimization of utility

boilers, ERC has developed practical approach for

combustion optimization.

Requires detailed understanding of the underlying

physics and boiler operation.

Data-based approach: field testing is needed

Database is created by performing a series of

parametric and factorial tests: Determine effect of individual parameters

Determine inter-dependencies between parameters.

Comprised of seven steps.

Continuous, proactive process.


Optimization Goals

Combustion

Optimization

NOx Control

and Performance

Improvement

Reduce

Furnace

Slagging

Mercury

Reduction


Applications and Achieved Results ERC combustion optimization approach and Boiler OP was used to optimize

30 + 4 power plants:

Firing Arrangement: Tangentially- and wall-fired boilers

Firing Systems: Conventional, Low-NOx, LNCFS I, II, III, TFS2000

Size Range: 80 to 750 MW

Fuels: Eastern and western fuels, PRB, fuel blends, co-firing with fuel oil,

natural gas and coke oven gas.

Optimization Goals:

Target NOx emission level with minimum performance impact

Best unit performance

Reduced slagging

Minimize mercury emissions and activated carbon usage

Achieved Results:

NOx Emissions Reduction: 5 to 35 %

Performance Improvement: 50 to 120 BTU/kWh

Mercury Emission Reduction: 30 to 80 %


Case Studies:

Unit A - 100% PRB Coal

Unit B - PRB/EB Blend

Unit C - PRB/EB Blend


Case Studies: Unit A Unit Description:

650 MW

Opposed Wall-Fired Subcritical Boiler

DRB-XCL Burners

Four Burner Elevations

Fuel: 100 % Western (PRB) Coal

Optimization Objectives:

Determine optimal boiler control settings over a range of target NOx levels

Optimize heat rate at target NOx level, while maintaining acceptable CO emissions.

Results:

37% Max. NOx Reduction

30% Sustainable NOx Reduction


0.15

0.20

0.25

0.30

5 10 15 20 25

Top Row SA Flow as Percentage of Total SA Flow

NO

x E

mis

sio

n R

ate

[lb

/MB

tu]

2.8 % O2, 6/2/99

3.1 % O2, 6/2/99

3.5 % O2, 6/2/99

3.5 % O2 6/3/99

Baseline

3

4

5

6

7

8

10

11

12

13

14

1 BL

2 & 9 BL

Unit A: Wall-Fired, DRB-XCL Burners


Unit A: Wall-Fired, DRB-XCL Burners

9,600

9,650

9,700

9,750

0.15 0.2 0.25 0.3 0.35

NOx Emission Rate [lb/MBtu]

Ne

t U

nit

He

at

Ra

te [

Btu

/kW

h]

Parametric Test

Baseline

Split Fire

Optimal (Session 65)

37% Max NOx Reduction

30% Sustainable

NOx Reduction


PRB/EB Fuel Blending

Firing of PRB/EB fuel blends represents a

challenge to low-NOx operation and combustion

optimization.

Fuel blend becomes additional variable. Affects NOx emissions.

Affects unit performance.

Affects operating constraints.

Affects FEGT and slagging.

Effect of fuel blend on low-NOx and optimal

settings needs to be accounted for.

CEM SO2 emission rate can be used as an on-

line indication of changes in fuel blend.


NOx vs. PRB Content in Fuel Blend

0.19

0.20

0.21

0.22

0.23

0.24

0.25

56 58 60 62 64 66 68

PRB in Blend [%]

NO

x E

mis

sio

n R

ate

[lb

/MB

tu]

NOx decreases as PRB content is increased.


Unit Heat Rate vs. PRB Content in

Fuel Blend

0

20

40

60

80

100

56 58 60 62 64 66 68

PRB Coal in Blend [%]

Ch

an

ge

in

ne

t U

nit

He

at

Ra

te [

Btu

/kW

h]

Unit heat rate increases as PRB content increases.


FEGT vs. PRB Content in Fuel Blend

1,850

1,900

1,950

2,000

56 58 60 62 64 66 68

PRB in Blend [%]

FE

GT

[F

]

FEGT increases as PRB content increases.


Effect of Fuel Blend on NOx Emissions

0.00

0.10

0.20

0.30

0.40

0.5 1.0 1.5 2.0 2.5

SO2 Emission Rate [lb/MBTU]

NO

x E

mis

sio

n R

ate

[lb

/MB

TU

]

More PRB More EB

85/1

5 P

RB

/EB

Ble

nd


Case Studies: Unit B Unit Description:

285 MWnet

Tangentially-Fired Subcritical CE boiler

Twin Furnaces (Separate SHT and RHT Furnace)

Four Mills per Furnace (8 Total)

TFS 2000R Ultra Low NOx System, 2 Levels of SOFAs

Fuel: Blend of EB and PRB Coals

Furnace slagging and convective pass fouling are major operational

constraints.

Optimization Objectives:

Determine optimal control settings resulting in low NOx operation, best

unit performance and FEGT below the slagging/fouling limit.

Results:

Optimal settings, resulting in acceptable slagging, were determined

over a range of target NOx levels.


Unit B: T-Fired, TFS 2000, Divided Furnace

2,020

2,040

2,060

2,080

2,100

2,120

2,140

2,160

0.10 0.12 0.14 0.16 0.18 0.20

Target NOx [lb/MBtu]

SH

T F

urn

ace F

EG

T [

F]

Test Data

Optimal Settings

FEGT Constrained Settings

Optimal settings constrained to maintain FEGT at 2,100 oF.


Unit B: T-Fired, TFS 2000, Divided Furnace

0

10

20

30

40

50

60

0.10 0.12 0.14 0.16 0.18 0.20

Target NOx [lb/MBtu]

Ne

t U

nit

He

at

Ra

te D

evia

tio

n [

Btu

/kW

h]

Optimal

FEGT Constrained

Constrained optimal settings result in higher heat rate.


Recommendations for Units

Firing PRB Coals or Blends Install instrumentation for on-line FEGT measurement.

Determine furnace operating conditions that mitigate slagging.

Field testing needed

Determine the effect of sootblowing on FEGT and slagging.

Field testing needed

Sootblowing optimization

Perform laboratory analysis of mineral constituents in the coal ash.

Chemical fractionation and CCSEM to characterize fuel and identify problem species and melting point.

Drop tube furnace tests to evaluate effects of O2 level, temperature and residence time on slagging.

Advanced on-line fuel characterization to determine slagging indices in real time (under development).


Furnace

Wa llb lowers

R etractab le

Sootb lowers

FEGT

Install Instrumentation for

FEGT Measurement

• Uncertainty in calculated

value of FEGT is of the

same order of magnitude

as the difference in FEGT

corresponding to the clean

and dirty furnace

conditions.

• Calculated values of FEGT

do not always follow the

same trend as

measurements.

Why is FEGT

Measurement Needed?


CONCLUSIONS: PRB Blend, Combustion

Optimization

PRB content in fuel:

Has a direct impact on emissions, performance,

and unit operability.

Affects FEGT.

Can be directly related to furnace slagging and

convection pass fouling.

Needs to be incorporated into a combustion

optimization program as an additional independent

parameter.

Sootblowing Optimization


Slagging and Fouling

Furnace and convective pass slagging and fouling have a

negative effect on boiler performance, emissions, and unit

availability.

Furnace Slagging: Reduces heat transfer to waterwalls.

Increases amount of heat available to convection pass. Higher FEGT.

Higher steam temperature.

Higher desuperheating spray flows. Reduced performance.

Higher NOx emissions.

Convective Pass Slagging and Fouling: Reduces heat transfer in convection pass.

Lower steam temperature. Reduced performance.

Lower desuperheating spray flows.

Increased flue gas temperature at boiler exit. Reduced

performance.


0.38

0.39

0.40

0.41

0.42

0.43

0.44

2,740 2,750 2,760 2,770 2,780 2,790 2,800 2,810 2,820

Furnace Exit Temperature [oF]

NO

x E

mis

sio

n R

ate

[lb

/MB

tu]

Effect of Slagging on NOx Emissions


Sootblowing Over-cleaning of furnace walls can result in:

Low steam temperatures

Increased moisture levels and erosion damage in last stages of LP turbine.

Lower turbine and unit power output (due to reduced reheat steam temperature).

Heat rate penalty

Sootblowing of boiler convective pass increases heat transfer in that region:

Increases steam temperatures and desuperheating sprays.

Reduces flue gas temperature at boiler exit.

For best performance it is important to maintain an optimal balance between furnace and convective pass heat transfer. Sootblowing Optimization


Optimization Challenge

Determine which portions of boiler to clean and on

what schedule.

Consider trade-offs: NOx

Opacity

Steam Temperatures

Heat Rate

Other Factors: Tube life, Sootblower steam or air

consumption, and Maintenance cost.

For utilities affected by the Ozone Transport Season

(OTS) regulations, the trade-offs will be different during

the OTS than for the rest of the year.


Sootblower Characterization

The unique feature of the ERC approach.

Sootblowers are grouped according to their

location and HT surface that is being impacted.

Characterization tests are performed for each

sootblower group. One sootblower group is activated at a time.

Plant response to sootblower activation is

recorded, i.e., effect of sootblowing on: Surface cleanliness of boiler HT sections,

Steam temperature and attemperating spray,

NOx emission rate, opacity, etc.


Case Study: Unit D - 435 MW T-Fired Unit

Project Objectives:

Control slagging of platen SHT and pendant RHT.

Improve main steam temperature control.

Eliminate variations in sootblowing practices among unit

operators.

Technical Approach:

Perform sootblowing characterization.

Regroup sootblowers (based on test results).

Develop, implement and test optimal sootblowing

schedule.


Unit D: Sootblower Locations

NORTH SOUTH9

13 6

2

7

8 4

3

1

5

14

11

12

151819

2021

22

2425

10

9

136

2 10

7 14

113

4 8 15 18 19

5

1

12

2120

22

25 24

A 12 A 13 A 14 A 15 A 1 A 2 A 3 A 4 A 5 A 6 A 7 A 8 A 9 A 10 A 11

B 15 B 18 B 19 B 20 B 22 B 1 B 4 B 7 B 8 B 9 B 10 B 11 B 12

W 5 W 6 W 1 W 2 W 3 W 4

C 14 C 17 C 19 C 20 C 1 C 3 C 4 C 4 C 7 C 8 C 9 C 11

W 15

W 12

W 20

W 16 W 13 W 14

W 7 W 8 W 9

W 19 W 10

W 17 W 18

D 15 D 16 D 19 D 21 D 22 D 1 D 2 D 3 D 4 D 5 D 6 D 7 D 8 D 9 D 11 D 14

Pla

ten S

HT

Fro

nt

Pendant

SH

T

Rear

Pendant

SH

T

Fro

nt

Pendant

RH

T

Rear

Pendant

RH

T

Horizontal

SHT

Economizer

Economizer

Pla

ten S

HT

Fro

nt

Pendant

RH

T

Rear

Pendant

RH

T

Fro

nt

Pendant

SH

T

Rear

Pendant

SH

T

Horizontal

SHT

Economizer

Economizer

Retractables

Water Lance

Wall Blowers

W 11

WEST FRONTNORTH SIDE SOUTH SIDE EAST

REAR

EAST

REAR


Unit D: Optimal Sootblowing Schedule

Developed based on sootblower characterization

test data, such as: Effect of individual sootblower group on:

FEGT

Steam Temperature

Stack opacity and NOx

Visual inspections of critical boiler areas

Discussions with boiler operators.

Optimal sootblowing schedule implemented in

January 2003.

Results: Positive feedback from plant engineers.

No slagging incidents since.

Improved steam temperature control.


CONCLUSIONS: Sootblowing

Furnace and convective pass slagging and fouling have a negative effect on boiler performance and emissions.

Increased maintenance cost.

Operators are typically given insufficient guidance concerning sootblower operation.

Considering its importance, sootblower operation needs more attention.

Optimization challenge: Which boiler sections to clean and when.

ERC has developed practical, knowledge-based sootblowing optimization approach.

On-line implementation accomplished through IntelliCLEAN, an intelligent adaptable sootblower optimizer.

Implemented on 500 and 400 MW tangentially-fired boilers.

Evaluation of benefits in progress.

Predicting Catastrophic

Slagging/Fouling Events:

Early Warning Advisor


Catastrophic Slagging/Fouling

Occurs when FEGT exceeds ash softening temperature.

Normal Prevention Approach: Maintain FEGT below a predetermined value.

Slagging occurs in the furnace or on the pendant superheater.

Fouling with subsequent plugging typically occurs in the pendant RHT.

Point Of No Return: When bridging occurs between the tubes.


Early Warning Advisor: EWA

EWA detects the onset of a slagging-fouling-

plugging event while there is still time to take

corrective action.

EWA uses information on:

Coal quality,

Boiler operating conditions,

Sootblowing practice,

Boiler performance parameters

to determine when an episode of accelerated

rate of convective pass plugging has begun.


EWA Methodology

Multi-variable approach. Process data are used.

Based on statistical method:

Principal Component Analysis (PCA)

Determines departure from “normal

conditions” earlier than single-parameter

(uni-variant) methods. Important to detect upset as early as possible.


Case Study: Unit E - RHT Plugging

600 MW wall-fired boiler

Fuel: 100% PRB

Pendant RHT plugging event is analyzed.

PCA was used to obtain early warning of the

upcoming catastrophic fouling/plugging event.


Pendant

RHT

Side Elevation of Unit E Boiler

Pendant

RHT


Unit E: Early Detection of Abnormal

Condition


CONCLUSIONS: Early Warning Advisor

Convection pass of PRB-fired boilers is susceptible to flyash fouling/plugging.

Pendant RHT

Principal Component Analysis (PCA) was successfully applied for early detection of catastrophic fouling/plugging event.

Provided 5-day advanced warning of catastrophic plugging event.

EWA can be used to provide early warning of other abnormal events.

Questions …

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