# TangentialLow NOx System at Reliant Energy's Limestone Unit2 Cuts Texas Lignite, PRB and Pet Coke NOx Ron Pearce Reliant Energy Incorporated Lnnestonl.' Station jewett, TX .Iol1n (irusha Foster WheelerEnergy Corporation Perryville Corporate Park UlI1ton, ]\,J**O~~OY-4000** 

### ABSTRACT Reliant Energy owns and operates two large 820 MW lignite-firedunits atthe LimestoneElectric Generating Station located approximately 125 miles**~orthwest** of Houston, Texas. The C-E boilers are controlledcirculation, tangentially fired, eight-comerunits with 10 fuel elevations. 

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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 ofdelivery schedules and established low NOx operatingperformance 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 Unit2 Cuts Texas Lignite, PRB and Pet CokeNOx Ron Pearce Reliant Energy Incorporated Limestone Station Jewett, TX John (jrusha FosterWheeler EnergyCorporation Perryville Corporate Park L'llIltol1, NJ**O~~09-4000** 

### ABSTRACT Reliant Energy owns and operates two large 820 MW li&nite-firedunits atthe LimestoneElectric Generating Station located approximately 125 miles northwest of Houston, Texas. The C-E boilers are controlledcirculation, tangentially fired, eight-comerunits 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 ordelivery schedules and established low NOx operating performance on similarlignitetired 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 arange ofTexas lignite. 

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IPHAB000262Texas lignite and PowderRiver 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 ofpositive results and sLiccessful operation, Reliant Energy Incorporated is proceeding with modifying Limestone #1 with an enhancedFWEC TLN3 systeminthe Fall 01'2001. 

## INTRODUCTION The station is in an ozone attainment region ofeast 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 ofnon-attainment areas such as Dallas and Houston. "he T'\lRCC established NOx emission requirements of0.165 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 of0.42 Ib/MMBtu to under 0.20 Ib/MMBtu, over anormal unit 30 day load cyclingperiod while firing a range ofTexas lignite. In addition, the system had to have the capability offiring 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 ofthe 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 offuel 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, particularlywithreactive coals likelignite 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 aircompartment dampers in a controllable range throughout awide unit load range. Experience shows that low NOx tangential systems operating with higher numbers ofidle compartments and excessive leakage through closed dampers under deep staged low NOx conditions tendto exhibithigherCO levels due topoorjetpenetration andmixing. Beside the main windbox modifications, the secondary air ducts were also modified to induce separated overfire airt10w while providinggood cornerto cornerdistribution. A unique aspect of the Limestone #2 retrofit was the ability to evaluate and compare TLN3 system performance firing a range ofTexas lignite, PRB, lignite & PRB blends as well as pet coke co-firing. Results were also compared to the unmodified Limestone#1 unit.

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## 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 airto the new SOFA windboxes. It included all associatedhangers, 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 airnozzle tips for fireball shape control and "Double "Shroud" air and coal nozzle tips. This arrangementprovided a high degree offuel-air mixing, required for ultra low NOx staged firing conditions. FWEC does not use or require any "special"coal nozzle tips withTLN systems. CORI\JEF: \/IEV\I



### FIGURE 1.

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## 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, greatlyreducing installationtime and cost.

• Main windbox dampers were modified to improve secondary airflow control, biasing andvertical 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 ofpetcokeparticles for earlierignition.

• Special air biasing logic was incorporated into the units damper control system for additional fireball shaping, slaggingandgas outletprofile control.

• As part ofthe TLN retrofit, Reliant Energy upgraded all windbox damper drives with a more reliable damperdrive design that includedpositiveposition feedback. 

### Low NOx LigniteFiring SlllCe these arc mine-mouth units, a range ofTexas 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. Heatrate improvedby 1 to 2%. 3 SI-I and RH temperatures were maintained at 996 F and 1002 F. 4. Unitwas no longersensitive to slagging at loweroperatingoxygen levels. 5. Unburned carbonhasremained very low**(~0.6%)** 6. FEGT temperatures did NOT noticeablychange. 7. Lower furnace hopper conditions remained good with dull orange color, good visibilityand no visible slag buildup

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## IPHAB0002658. 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 overeach 8hourperiod. 

### (FIGURE 2) 

### LMS2NOx-**LlG~JITE** 

### AFTER RETROFIT

til**r~** ean=.I!:lll3 

# I I 

# I I 

# I

# ~-~iI §
....d 1 _ ....,.. C'



### IbsimBtu 

### LMS 2 CO -LIGNITE AFTER RETROFIT 500

Q U**20~**

## : t of 100 Mean=15.4



### ppm FIGURE 2. PostRetrofitTLN3 NOx and CO Emissions

- Texas Lignite



## s


## IPHAB000266

### Low NOx PRB Firing Firing PRB (Caballo Raja) coal revealed an entire new set ofresultant 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 pyrometerreadings, the furnace temperatures increased 100 FO. 3. Unit2 furnace slagging was less comparedto the uncontrolledunit#1. 4. There was a noticeable difference in flyash consistency. Unit #1 had a finer ash versus a more granularconsistency on unit#2. 5. Lower furnace hopper conditions remained good with overall dull orange color, good visibility andno visible slag buildup. 6. LOI increased as expectedwhen comparedto lignitebutremainedless 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 extendedoperation atfull 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 @ \ 

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# .... 780 DEG FTYP LIGNITE @ MeR /
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# /
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# n n 780 780 soo 810 :320 DEG F 830 840 850 860



### FIGURE 3 Range ofEconomizerOutletGas Temperatures Firing 

### PRB

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## IPHAB0002670.**~OX** emissions were significantly lower when firing PRB as compared to lignite, for the same level ofstaging. NOx averaged 0.137 Ib/MMBtu over an extended normal unit load cycling period while maintaining CO levels below 23 ppm. (FIGURE 4)**,--~--~~~~-c-~~~~~~~~~~-~-~-~-~** 

### LMS2NOx-

### PRBAFTER RETROFIT

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## s**~lean=.1371**

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

### LMS 2CO- PRBAFTER RETROFIT



## - 201 :: ' o W 15 Mean=22.3]



### ppm FIGURE 4 Post Retrofit TLN3 NOx and CO Emissions

- PRB Coal

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## IPHAB000268

### Low NOx PRB and LigniteBlendFiring 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 ofblending 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 ofPRB to lignite. Intermediate blends ofPRB / Lignite exhibited some undesirable slagging conditions when the ratIo was between 30% to 70% PRB by heating value. It appears that the eutectic ofthe slag is aresult ofthe interactive chemistry ofthe two fuels in this range ofoperation. Blending ofthe fuel with a dedicated pulverizer provides better furnace control. However, this IS more cumbersome from a fuel-handling viewpoint. 

### PetCoke 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 ofthe 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. 

### PetCoke andLignite: 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 of0.21 Ib/MMBtu while firing a 20% Btu blend ofpet coke with the lignite. 2 Pet coke does not mix well in pulverizers with otherfuels. This is more ofafunction ofhardness and type ofpet coke. Ifthe pet coke is in "shot" type or significantly harder than the primary fuel, the Raymond bowl mills willpreferthe primary fuel and reject much ofthe petcoke 3. Co-firing pet coke tends to reduce steam temperatures slightly because furnace cleanliness improves significantly. However, due to the high mass flow ofthe lignite, temperatures can still be maintained. 

### PetCoke and PRB: 1. NOx emission increased as the percentage ofpet 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-firingpet coke with PRB. As aresult 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.



## IPHAB000269NOx versus Load Comparison ofLignite & PRB 1. The Limestone units are normallybase loaded. So, there is not a great deal ofdata 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 significantlyatreduced loads. This is depicted in Figure 5 below. LM:::. 2100% PRB TLN3 Retrofit NOxV5 rvl\/V 0.150

145 140 'S" 0 135 

## ro 0130 E E 0 125 u; @. x 0 Z 0 115 0 110 Ci 105 0 100**Ch~rt** AreaI I i 

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## j I ! I 41"11-1 700 800 MW LI"I:,. 2100% Lignite :erro.**P~trQtlttK)":vs** I:vWV 1--**-~---------------------------------,** I i 1 

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800 760 720 680 640 560 520 480 1**~+--~----~----_+--+_-_+--+_-_+-** 400 440 

### FIGURE 5 NOx Versus Load for PRB & Lignite After Retrofit

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## IPHAB000270TABLE A -TLN3 FUELPERFORMANCE SUMMARY DATATABLE
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 FWEC TLN3 system has demonstrated the following: a Flie! firing flexibility over arange ofTexas lignite, as well as PRB andpetcoke. b. Reduced furnace slagging comparedto the pre-retrofit system. c. Improvedboiler efficiency (heatrate reduction andrelativelylow LOI). d. Good reduction ofNOx emissions without a significant increase in CO while firing a range offuels. e. Durable erosion resistantcomponents, solidengineering 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 Mendratios a/thefuels.
. h. Emission results with the TLN3 system confirmedmostpredicted designperformance 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 airflow 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 outlettemperature within equipmentdesign limits wouldbe 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 ofboth fuels if care is not taken to considerhow all the fuel tiring equipmentis reactingto it. 5. Reliant Energy Inc. has ordered a secondTLN3 from FosterWheelerEnergy Corporation to be installed on Unit #1. This decision is based on the performance ofUnit #2 TLN3 andthe workingrelationship establishedbetween the two organizations. 

### Acknowledgements
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## IPHAB000271The following individuals need special acknowledgement for their support ofthis 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 USLowNOx Conversion of a TangentialLignite 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 of Simulation Tools andExperimentalMethodologies in theDesign andPerformance Optimization of LowNOx Retrofits to aLignite TangentialFiredBoiler

- J. Grusha, S. Woldehanna, Presented at EPRI-DOE EPA Combined Utility Air PollutantControl 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 StoichiometryControl LFSC Lignite PRB Pet Coke ultra Low NOx Combustion Modifications



### 5/30/01

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## IPHAB000272