Tangential Low NOx System at

Reliant Energy's Limestone Unit 2

Cuts Texas Lignite, PRB and Pet Coke NOx

Ron Pearce

Reliant Energy Incorporated

Lnnestonl.' Station

jewett, TX

.Iol1n (irusha

Foster Wheeler Energy Corporation

Perryville Corporate Park

UlI1ton, ]\,J O~~OY-4000

ABSTRACT

Reliant Energy owns and operates two large 820 MW lignite-fired units at the Limestone Electric

Generating Station located approximately 125 miles ~orthwest of Houston, Texas. The C-E

boilers are controlled circulation, tangentially fired, eight-comer units with 10 fuel elevations.

>_. .,

in respOllse to recent Texas air quality regulations, Reliant Energy retrofitted Limestone #2 with

Foster Wheeler Energy Corporation's Tangential Low NOx (TLN3) system. It was chosen

because of delivery schedules and established low NOx operating performance on similar lignitefired units retrofitted by FWEC. A complete TLN3 system was engineered, fabricated and

delivered to the plant in only sixteen weeks. It was installed and made operational in seven

weeks. The system was fully operational in May of 2000 and was tuned by September to

consistently achieve a NOx performance guarantee below 0.20 Ib/MMBtu, with very low CO

and LOI levels, while firing a range ofTexas lignite.

IPHAB000261
Tangential Low NOx System at

Reliant Energy's Limestone Unit 2

Cuts Texas Lignite, PRB and Pet Coke NOx

Ron Pearce

Reliant Energy Incorporated

Limestone Station

Jewett, TX

John (jrusha

Foster Wheeler Energy Corporation

Perryville Corporate Park

L'llIltol1, NJ O~~09-4000

ABSTRACT

Reliant Energy owns and operates two large 820 MW li&nite-fired units at the Limestone Electric

Generating Station located approximately 125 miles northwest of Houston, Texas. The C-E

boilers are controlled circulation, tangentially fired, eight-comer units with 10 fuel elevations.

in respollse to recent Texas air quality regulations, Reliant Energy retrofitted Limestone #2 with

Foster Wheeler Energy Corporation's Tangential Low NOx (TLN3) system. It was chosen

because or delivery schedules and established low NOx operating performance on similar lignitetired units retrotitted by FWEC. A complete TLN3 system was engineered, fabricated and

delivered to the plant in only sixteen weeks. It was installed and made operational in seven

weeks. The system was fully operational in May of 2000 and was tuned by September to

cOllslsLcntly achieve a NOx performance guarantee below 0.20 IbIMMBtu, with very low CO

and LOI levels, while firing a range of Texas lignite.

>_. .,

IPHAB000262
Texas lignite and Powder River Basin coal have been fired separately and as blends. In addition,

pet coke has also been co-fired with both coals. This paper presents the FWEC Tangential Low

NOx combustion system and provides results on emissions, boiler performance and operational

differences between the various fuel combinations. Based on over 14 months of positive results

and sLiccessful operation, Reliant Energy Incorporated is proceeding with modifying Limestone

#1 with an enhanced FWEC TLN3 system in the Fall 01'2001.

INTRODUCTION

The station is in an ozone attainment region of east Texas. It is the Texas Natural Resource

Conservation Commission's (TNRCC) position that the emissions from plants located in the

attainment regions affect the compliance of non-attainment areas such as Dallas and Houston.

"he T'\lRCC established NOx emission requirements of 0.165 Ib/MMBtu for this region to be

cfTective in 2003. In addition, the Limestone units are under strict CO limitations of

0.1 I Ib/MMBtu, which equates to approximately 60 ppm (corr.3%02) over an 8 hour rolling

average.

The Reliant requirements for the retrofit were to reduce NOx levels from a baseline of 0.42

Ib/MMBtu to under 0.20 Ib/MMBtu, over a normal unit 30 day load cycling period while firing a

range of Texas lignite. In addition, the system had to have the capability of firing PRB and pet

coke. Boiler performance and operation, furnace slagging, LOI, component longevity, etc., were

also not to be negatively eflected. The most critical performance requirement however was that

carbon monoxide levels not exceed 60 ppm (corr.3%02) over any 8 hour period, regardless of

fuel Controlling CO levels to such low levels, under ultra low NOx air staging conditions and

load cycling operation. is one of the challenges facing low NOx burner designers. CO is not

easily predictable like NOx, unburned carbon or boiler performance. CO formation is related

primarily to the degree of fuel and air mixing. FWEC TLN design requirements stress fuel/air

balance and mixing for successful ultra low tangential coal firing. Industry experience verifies

that improper air or fuel distribution, jet penetration and mixing can result in very high CO

levels, particularly with reactive coals like lignite and PRB.

FWEC designed and supplied a TLN3 system intended to meet all of Reliant Energy's

Limestone unit objectives while not exceeding the established maximum CO level. FWEC

applied their proprietary computer windbox simulation and design program to optimize all

nozzle tips, dampers and damper operation prior to fabrication. TLN3 systems are designed

specifically to operate with fuel and air compartment dampers in a controllable range throughout

a wide unit load range. Experience shows that low NOx tangential systems operating with higher

numbers of idle compartments and excessive leakage through closed dampers under deep staged

low NOx conditions tend to exhibit higher CO levels due to poor jet penetration and mixing.

Beside the main windbox modifications, the secondary air ducts were also modified to induce

separated overfire airt10w while providing good corner to corner distribution.

A unique aspect of the Limestone #2 retrofit was the ability to evaluate and compare TLN3

system performance firing a range of Texas lignite, PRB, lignite & PRB blends as well as pet

coke co-firing. Results were also compared to the unmodified Limestone #1 unit.

2

IPHAB000263
Limestone #2 TLN3 System

• A "single" level of Separated Over Fire Air (SOFA) was installed as the

primary means of incorporating vertical air staging within the combustion

zone. The system was designed and positioned for high NOx reduction

efficiency and included airflow monitoring, nozzle direction and velocity

control features for extensive CO control flexibility.

• A network of secondary air ducting was supplied to re-route and induce

secondary air to the new SOFA windboxes. It included all associated hangers,

expansion joints. and other boiler house steel modifications to accommodate

this addition.

• All the original main windbox nozzle tips were replaced with FWEC's inwindbox (close coupled) over fire air nozzle tips, boundary air nozzle tips for

fireball shape control and "Double "Shroud" air and coal nozzle tips. This

arrangement provided a high degree of fuel-air mixing, required for ultra low

NOx staged firing conditions. FWEC does not use or require any

"special"coal nozzle tips with TLN systems.

CORI\JEF: \/IEV\I

FIGURE 1.

3

IPHAB000264
• Coal and air nozzle tips include the FWEC Double Shroud design (patent

applied) that provides added thermal distortion resistance over convention

tangential nozzle tips.

• Coal nozzle tips also include a special design feature that allows entire

installation and replacement from the furnace side without removing

stationary coal nozzle assemblies, greatly reducing installation time and cost.

• Main windbox dampers were modified to improve secondary airflow control,

biasing and vertical staging capability.

• "Lower Furnace Stoichiometry Control" (LFSC) feature was provided. This is

an integral part of all TLN systems to minimize and control the fuel-rich

lower furnace hopper conditions commonly found on tangential low NOx

retrofits.

• One (1) elevation of new "coal nozzle" and nozzle tip assemblies was

supplied for enhanced pet coke firing. The assemblies contain special features

that increase recirculation of pet coke particles for earlier ignition.

• Special air biasing logic was incorporated into the units damper control

system for additional fireball shaping, slagging and gas outlet profile control.

• As part of the TLN retrofit, Reliant Energy upgraded all windbox damper

drives with a more reliable damper drive design that included positive position

feedback.

Low NOx Lignite Firing

SlllCe these arc mine-mouth units, a range of Texas lignite is most common fired fuel. Fuel

characteristics, higher heating values and ash slagging properties can change often.

Consequently, furnace conditions and emissions also fluctuate as fuel characteristics change. In

general, however when comparing the pre-retrofit to post TLN retrofit conditions or even

comparing the uncontrolled unit #1 to unit #2, there are several differences that were readily

noted.

I. Furnace cleanliness improved following the TLN3 retrofit

2. Heat rate improved by 1 to 2%.

3 SI-I and RH temperatures were maintained at 996 F and 1002 F.

4. Unit was no longer sensitive to slagging at lower operating oxygen levels.

5. Unburned carbon has remained very low (~0.6%)

6. FEGT temperatures did NOT noticeably change.

7. Lower furnace hopper conditions remained good with dull orange color, good

visibility and no visible slag buildup

4

IPHAB000265
8. Average NOx emissions were reduced by 55 % from pre-retrofit levels to

approximately 0.19 Ib/MMBtu at ful1 load over the range of lignite, while

maintaining an average CO level of 15 ppm over each 8 hour period. (FIGURE 2)

LMS 2NOx - LlG~JITE AFTER RETROFIT

til

r~ ea n=.I!:lll3 I

I

I

I

I

~-~

iI

1 _§ ....,..

....d

C'

IbsimBtu

LMS 2 CO -LIGNITE AFTER RETROFIT

500

Q

U

20~:

t

of

100

Mean=15.4

ppm

FIGURE 2.

Post Retrofit TLN3 NOx and CO Emissions - Texas Lignite

s

IPHAB000266
Low NOx PRB Firing

Firing PRB (Caballo Raja) coal revealed an entire new set of resultant changes with regards to

emission and boiler operation. Specifically, evaluations made during extended periods of PRB

cllal (mng with the TLN3 controlled unit #2 were compared to similar load conditions on the

uncontrolled unit #1.

SH and RH temperatures were met and did not appear to change noticeably between

lignite and PRB.

2. Based on infrared pyrometer readings, the furnace temperatures increased 100 FO.

3. Unit 2 furnace slagging was less compared to the uncontrolled unit #1.

4. There was a noticeable difference in flyash consistency. Unit #1 had a finer ash versus a

more granular consistency on unit #2.

5. Lower furnace hopper conditions remained good with overall dull orange color, good

visibility and no visible slag buildup.

6. LOI increased as expected when compared to lignite but remained less than 1%.

7. PRB required "aggressive" soot blower operation in the economizer / convective heat

section in order to keep the furnace outlet temperatures down. Even so the economizer

outlet temperatures were too high to tolerate extended operation at full load. (FIGURE 3)

LM2: 2 ECOr'JOMIZER OUTLET GASITEMP. FIRING PRB

~820 MW +/-5 (Full Load / MeR)

30'::,:.

25%

5%

DESIGN 8'\ 0 DEG F @ \

\

....

780 DEG F TYP LIGNITE @ MeR

/

i

,

/ rI

/ r-

/

I n n

780 780 soo 810 :320

DEG F

830 840 850 860

FIGURE 3

Range of Economizer Outlet Gas Temperatures Firing PRB

6

IPHAB000267
0. ~OX emissions were significantly lower when firing PRB as compared to lignite, for the

same level of staging. NOx averaged 0.137 Ib/MMBtu over an extended normal unit load

cycling period while maintaining CO levels below 23 ppm. (FIGURE 4)

,--~--~~~~-c-~~~~~~~~~~-~-~-~-~

LMS 2 NOx - PRB AFTER RETROFIT

, Z1I

IS

/

I

I s

~lean=.1371

L-- ~_~,_~~~~_~_~~~ ___.J

LMS 2 CO - PRB AFTER RETROFIT

- 201

:: ' o

W 15

Mean=22.3]

ppm

FIGURE 4

Post Retrofit TLN3 NOx and CO Emissions - PRB Coal

7

IPHAB000268
Low NOx PRB and Lignite Blend Firing

A senes of tests were conducted to evaluate PRB and lignite co-firing. This included

documenting the effects on emissions as well boiler performance and operation. Two methods

of blending were evaluated including yard blending and dedicated mills. Overall the following

general observations were seen.

I. NOx emissions and LOI levels depended on the ratio of PRB to lignite.

Intermediate blends of PRB / Lignite exhibited some undesirable slagging conditions when

the ratIo was between 30% to 70% PRB by heating value. It appears that the eutectic of the

slag is a result of the interactive chemistry of the two fuels in this range of operation.

Blending of the fuel with a dedicated pulverizer provides better furnace control. However,

this IS more cumbersome from a fuel-handling viewpoint.

Pet Coke Co-Firing with Lignite and PRB

\ Jc'clicat-:d level of pet coke nozzle tips and stationary nozzle assemblies were designed by

FWEC and provided as part of the FWEC TLN3 retrofit on unit #2. Co-firing up to 23% (BTU

basis) pet coke was evaluated to document the effects on boiler performance and emissions.

Tabulated results are listed on TABLE A.

Pet Coke and Lignite:

1. Initially NOx emission initially increased to 0.25 Ib/MMBtu slightly as the percentage

of pet coke increased. Selective staging of the combustion air yielded reduced

average value of 0.21 Ib/MMBtu while firing a 20% Btu blend of pet coke with the

lignite.

2 Pet coke does not mix well in pulverizers with other fuels. This is more of a function

of hardness and type of pet coke. If the pet coke is in "shot" type or significantly

harder than the primary fuel, the Raymond bowl mills will prefer the primary fuel and

reject much ofthe pet coke

3. Co-firing pet coke tends to reduce steam temperatures slightly because furnace

cleanliness improves significantly. However, due to the high mass flow of the lignite,

temperatures can still be maintained.

Pet Coke and PRB:

1. NOx emission increased as the percentage of pet coke increased. At approximately

20% pet coke on a Btu basis. the NOx emissions reached 0.167 Ib/MMBtu. CO was

very low.

2. Steam temperatures could not be maintained co-firing pet coke with PRB. As a result

the evaluation period test was a relatively short. It is believed that this is due to the

reduced mass associated with the pet coke and the PRB. In addition the lower ash

loading and cleaner furnace surfaces associated with the pet coke co-firing

contributed to lower firing zone temperatures and the resultant decrease in steam

temperatures.

IPHAB000269
NOx versus Load Comparison of Lignite & PRB

1. The Limestone units are normally base loaded. So, there is not a great deal of data at

reduced load. The data that is available indicates that PRB NOx emission rates are

relatively constant between 50 (Yo to 100 % load. Lignite NOx emission rates increase

significantly at reduced loads. This is depicted in Figure 5 below.

LM:::. 2 100% PRB TLN3 Retrofit

NOx V5 rvl\/V

0.150

145

140

'S" 0 135

ro 0130 E

E

u; 0 125

@.

x

0

Z 0 115

0 110

Ci 105

0 100

Ch~rt Area I

I i

I i I I

i I i I i

I i I

---...-

! 1 I

'- i ././'

i"-... / .-/ t- --- _10-'

1-----

,

,

I

j I

!

I

41"11-1 700 800

MW

LI"I:,. 2100% Lignite :erro. P~trQtlttK)":vs I:vWV

1-- -~---------------------------------,

I

i 1

t- ~~ ---+--

j

1 I

1 I : ,,,,", I

f---- I I i -- I

I

I -+- _t-l---_~~--+--'~-T-.....-i~-..J----l~~ ..)

: I 1- I:: : =--

t-----,----,-----+---,.·----f-- - ~-- --.:.. . I .

I

I

1

- , ::l -

C5

E

I

I~I~i

i

480 520 560 640 680 720 760 800

1 ~+--~----~----_+--+_-_+--+_-_+-

400 440

FIGURE 5

NOx Versus Load for PRB & Lignite After Retrofit

9

IPHAB000270
TABLE A - TLN3 FUEL PERFORMANCE SUMMARY DATA TABLE

SH RH

TEST "lOx CO Out Temp Out Temp FURNACE PANEL(S)

FUEL DURATIO:'-J Ibs/MMBtu rpm degF degF MWg SLAG' SLAG"

LIGNITE 30 DA. o 193 15.4 996 1002 810 I 3

PRB (100",,) 9 DAYS o 137 22 1003 1009 808 2 2

LllJ'; Ill: ~iJ"" Pel lUKe Il3TU) WEEKS u.21 ~I 100U 997 797 I 2

~~O{I PRB (i7 % L1GNITF iBTU) -24 HRS 0.11i5 49 998 1004 813 2 3

~5"" PIZIl 55" L1G:'-JITE (BTU) -2411RS 0.18 25 991 999 803 3 5

71 'Yo PRB ! 29 % LIGNITE (BTU) -24 HRS 0159 22 990 998 806 3 4

PRB ! 20% PET COKE (BTU) - 8.5 HRS 0.167 3 954.4 968.6 807 I 2

* Higher numbers denote worse slag conditIons.

Conclusions

I. The FW EC TLN3 system has demonstrated the following:

a Flie! firing flexibility over a range of Texas lignite, as well as PRB and pet coke.

b. Reduced furnace slagging compared to the pre-retrofit system.

c. Improved boiler efficiency (heat rate reduction and relatively low LOI).

d. Good reduction of NOx emissions without a significant increase in CO while firing a

range of fuels.

e. Durable erosion resistant components, solid engineering and construction.

f. Special coal nozzle tips designed to be replaced without removing stationary coal

nozzles result in installation cost savings.

g. Co-firing lignite and PRB produced NOx emissions that were relatively proportional

to the Mend ratios a/thefuels. .

h. Emission results with the TLN3 system confirmed most predicted design performance

values. In addition actual main windbox and overfire damper positions closely

matched damper positions initially predicted with the FWEC TLN design program.

This design tool allows TLN systems to be optimally sized for emissions and optimal

fuel-air mixing and air flow control.

2. Based on these results, PRB clearly and repeatedly showed lower NOx levels compared

to the Texas lignites. However, boiler modifications to maintain the economizer gas

outlet temperature within equipment design limits would be required.

3. PCI coke can provide a good fuel supplement in some cases, but it does tend to increase

NOx emissions. Special air staging considerations can provide some additional NOx

reduction.

4. Blending fuels can sometimes allow the operator to "enjoy" the worst of both fuels if

care is not taken to consider how all the fuel tiring equipment is reacting to it.

5. Reliant Energy Inc. has ordered a second TLN3 from Foster Wheeler Energy Corporation

to be installed on Unit #1. This decision is based on the performance of Unit #2 TLN3

and the working relationship established between the two organizations.

Acknowledgements

10

IPHAB000271
The following individuals need special acknowledgement for their support of this successful low

NOx retrofit at Reliant Energy's Limestone Plant.

Brad Moulton -FWEC

Bruce McCrea - FWEC

Skfan LaLlx - FWEC

Sara Woldehanna - FWEC

Dcnllis O'Gallagher - LCI Engineering

Ken McCarthy- FWEC

WayIlc S. Jones - REI

Fred Lemoine - REI

Yiu Lam Lau - REI

Neal Irvin - REI

References

1. Long Term Resultsfrom the First US Low NOx Conversion ofa Tangential Lignite Fired Unit - J. Grusha,

S Woldehanna. K. McCarthy, and G Heinz - Presented at 24th International Technical Conference on Coal

L'liltzallon & Fud Systems, Clearwater, FL., March 8-11, 1999

2. Application ofSimulation Tools and Experimental Methodologies in the Design and Performance

Optimization ofLow NOx Retrofits to a Lignite Tangential Fired Boiler - J. Grusha, S. Woldehanna,

Presented at EPRI-DOE EPA Combined Utility Air Pollutant Control Symposium, Atlanta, GA., August 16-20,

1999

Key Words

Limestone Electric Generating Station

Tangential Low NOx Firing System

TLN

Separated Overfire Air

Boundary Air

Double Shroud Nozzle tips

Lower Furnace Stoichiometry Control

LFSC

Lignite

PRB

Pet Coke

ultra Low NOx Combustion Modifications

5/30/01

11

IPHAB000272