## Reliant Energy Tangential Low NOx System at Limestone Unit 2 Cuts Texas Lignite, PRB and Pet Coke NOx

Ron Pearce Reliant Energy Incorporated Lim e stone Station Jewett, TX

John Grusha Foster W h eeler Energy Corporation Perryville C o rporate Park Clinton, NJ 08809-4000



### ABSTRACT

Reliant Energy owns and operates two large 820 M W lignite-fired units at the Lim e stone Electric Generating Station located approxim atel y 125 m iles northwest of Houston, Texas. The C-E boilers are controlled circulation, tangentially fi red, eight-corner units with 10 fuel elevations.

In respon se to rec e nt Te xas air quality regu lati o n s, Relian t Energy re tro f itted Lim e stone #2 with Foster W h eeler Energy Corporatio n’s Tangen t ial Low N O x (TLN3) system . It was chosen because of delivery schedules and es tablished lo w NOx operating perform a nce on si m ilar lignite

f i red units r e trof itted by FW EC . A com p lete TLN3 syst em was engineered, fabricated an d delivered to the plant in only sixteen weeks. I t was in sta l led and m a de opera tio nal in seve n weeks. The system was fully operational in May of 2000 and was t uned by Septem ber to consistently achieve a NOx perform a nce guara ntee below 0.20 lb/MMBtu, with very low CO and LOI levels, while firing a range of Texas lignite.

1Texas lignite and Powder River Basin coal have b een fired separately and as blends. In addition, pet coke has also been co-fired with both coal s. This paper presents the FW EC Tangential Low NOx com b ustion system and provides results on e m issions, boiler perform a nce and operational dif f

e rences between the various f u e l com b inatio ns. Based o n over 14 months of positiv e resu lts and successf ul operation

, Reliant En ergy Incorp or ated is proceeding with m odifying Lim e stone #1 with an enhanced FWEC TLN3 system in the Fall of 2001.



### INTRODUCTION

The sta tion is in an ozo n e attainm e nt reg i on of east Texas. I t is the T e xas Natu ra l Resour ce Conservatio n Comm ission’s (TNRCC) positio n that the em issions from plants located in the attainm e nt regions affect the com p liance of non-a ttainm ent areas such as Dallas an d Houston. The TNRCC established NOx e m is sion requirem e nt s of 0.165 lb/MMBtu for this region to be effective in 2003. In addition, the Lim e stone un its are under strict CO lim i t ations of 0.11 lb/MMBtu, which equates to approxim a tely 60 ppm ( c orr.3%O2) over an 8 hour rolling

average.

The Reliant requirem ents for th e retrofit were to reduce N O x levels from a baseline of 0.42 lb/MMBtu to under 0.20 lb/MMBtu , over a normal unit 30 day load cycling period w h ile firing a range of Texas lignite. In addition, the system ha d to have the capability of firing PRB and pet coke. Boiler perform a nc e and operation, furnace sl agging, L O I, com ponent longevity, etc., were also not to be negatively effected. The m o st cr itical p e rf ormance requ ire m ent however was tha t carbon m o noxide levels not exceed 60 ppm (corr .3%O2) over any 8 hour period, regardless of fuel. Controlling CO levels to such low leve ls, under ultra low NOx air staging con d itions and load cycling operation, is one of the challeng es facing low NOx burner designers. CO is not easily predictable like NOx, unbur ned carbon or boiler perform a nc e. CO for m ation is related prim arily to the degree of fuel and air m i xing. FW EC TLN design requirem ents stress fuel / air balance and m i xing for successful ultra low tangen tial coal firing. Industry experience verifies that im proper air or fuel distri bution, jet penetration and m i xing can result in very high CO levels, particularly with reactive coals lik e lign ite and PRB.

FW EC designed and supplied a TLN3 system in tended to m eet all of Relia nt Energy’s Lim e stone unit ob jec t ives while n o t exc eedin g the estab l ished m a ximum CO level. FW EC applied their proprietary com puter windbox si m u lation and design program to optim ize all nozzle tips, dam p ers and dam p er operation prio r to fabrication. TLN 3 system s are designed specifically to operate w ith fu el and air com p artm ent da m p e r s in a controllable range throughout a wide unit load range. E xperience shows that low NOx tangential system s operating with higher

num b ers of idle com p artm ents and excessive le akage through closed da mpers under deep staged low NOx conditions tend to exhi bit higher CO levels due to poor jet penetration and m i xing.

Beside the m a in windbox m odificati ons, the secondary air ducts we re also m odified to induce separated overfire airflow while provid ing good corner to corner distribution.

A unique aspect of the Lim e stone #2 retrofit was the ability to ev alu a te and co mpare TLN3 system perform a nce firing a range of Texas lign ite, PRB, lignite & PR B blends as well as pet coke co-firing. Results were also compar ed to the unm odifie d Lim e stone #1 unit.

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### Limestone #2 TLN3 System

• A “single” level of Sepa rated Over Fi re Ai r (SOF A) was installed a s th e prim ary m e ans of incorporating verti cal air staging with in the com bustion zone. The system was designed and positioned for high NOx reduction efficiency and included airflow mon itoring, nozzle direction and velocity

contro l featu r es for exten s ive CO control flex ibility.

• A network of secondary air ducting was supplied to re-route and induce secondary air to the new SOFA windboxes. It included all as sociated hangers, expansion joints, and other boiler hous e ste e l modif i cation s to accom m odate this addition.

• All the original m a in windbox nozzle tips were replaced with FW EC’s inwindbox (close coupled) over fire air nozzle tips, boundary air nozzle tips for fireball shape control and “Double Shr oud” air and coal nozzle tips. This

arrangem ent provided a high degree of fuel -air mixing, required for ultra low NOx staged firing conditions. FW EC does not use or require any “special“co al nozzle tip s with TLN system s.

FIGURE 1.

3• Coal and air nozzle tips include the FW EC Double Shroud design (patent applied) that provides added therm a l distortion resistance over convention tangential n o zzle tips.

• Coal nozzle tips also include a sp ecial design feature that allows en tire installation and replacem ent from the furnace side without removing stationary coal nozzle assem b lies, great ly redu cing insta lla tion tim e and cost.

• Main windbox dam p ers were m odified to im prove secondary airflow co ntrol, biasing and vertical stag ing capab ility.

• “Lower Furnace Stoichiom e try Control“ (LFSC) feature was provided. T h is is an integral part of all TLN system s to m i nim i ze and control the f u el-rich lower furnace hopper conditions comm only found on tangential lo w NOx retrofits.

• One (1) elevation of n e w “coal nozzle” and nozzle tip assem b lies was supplied for enhanced pet coke firing. The assemblies con t ain special features that increase recircula tion of pet coke partic les for earlier ignition.

• Special air biasing logic was inco rpor ated in to the units dam p er control system f

o r addition a l f i r e ball shapin g, sl agging a nd gas outlet prof ile con t rol.

• As part of the TLN retrofit, Relian t Energy upgraded all windbox damper drives with a m o re reliable dam p er dr ive d e sign that inc l ude d positive p o sitio n feedback.



### Lo w NOx Lignite Firing

Since these are m i ne-mouth units, a range of Te xas lignite is m o st comm on fired fuel. Fuel characteristics, higher heating v a lues and ash slagging properties can change often. Consequently, furnace conditions an d em issions also fluctu ate as fuel ch aracteristics change. In general, however when com p aring the pre-retr ofit to post TLN retrofit conditions or even

com p aring the uncontrolled unit #1 to unit #2, there are s e veral differences th at were read il y noted.

1. Furnace cleanliness im proved following the TLN3 retrofit 2. Heat rate im proved by 1 to 2%. 3. SH and RH tem p eratures were m a intained at 996 F and 1002 F. 4. Unit was no longer sensitive to sla gging at lower operating oxygen levels. 5. Unburned carbon has rem a ined very low (~ 0.6%) 6. FEGT tem p eratu r es d i d NOT noticeably change. 7. Lower furnace hopper conditions rem a ined good with dull orange color, good visibility and no vi sible slag buildup

48. Average NOx e m issions were reduced by 55 % from pre-retrofit levels to approxim a tely 0.19 lb/MMBtu at full lo ad over the range of lignite, while

m a intaining an averag e CO level of 15 ppm over each 8 hour period. (F IGURE 2)

FIGURE 2.

Post Retrofit TLN3 NOx and CO Emissions – Texas Lignite

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### Lo w NOx PRB Firi ng

Firing PRB (Caballo Ro jo) coal rev ealed an entire new set of resultant changes with regards to

em ission and boiler operation. Specifically, eval uations m a de during extended periods of PRB coal firing with the TLN3 controlled unit #2 were com p ared to sim ilar load con d itions on the uncontrolled unit #1.

1. SH and RH tem p eratures were m e t and d i d n o t appear to change no ticeab ly between lignite and P RB. 2. Based on infrared pyrometer readings, th e furnace tem p eratures increased ~ 100 F o

. 3. Unit 2 furnace slagg i ng was less co mpared to th e uncontro lled unit #1. 4. There was a noticeable difference in flyash consistency. Unit #1 had a f i ner ash versus a more granular consistency on unit #2. 5. Lower furnace hopper conditions rem a ined good with ov erall dull o r ange colo r, g ood visibility and no vi sible slag buildup. 6. LOI increas e d as expected when compared to lignite bu t re m a ined less than 1%. 7. PRB required “aggressive” soot blower ope ration in the econom izer / convective heat section in order to keep the furnace outlet tem p e r atures down. Even so the econom izer outlet tem p eratures were too high to tolerate extended opera tion at full load. (FIGURE 3)

FIGURE 3

Range of Econom i zer Outlet Ga s Temperatures Firing PRB

68. NOx emiss i ons w ere significan tly low er w h en firing PRB as com p ared to lignite, for the

sam e level of staging. NOx averaged 0.137 lb /MMBtu over an extended norm a l unit load cycling p e riod while m a inta ining C O levels below 23 ppm

. ( FIGURE 4)

FIGURE 4

Post Retrofit TLN3 NOx and CO Emissions – PR B Coal

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### Lo w NOx PRB and Lignite Bl end Firin g

A series of tests were conducted to evaluate PRB and lignite co-f iring. This included docum enting the effects on e m issions as well bo iler perform a nce and ope ration. Two m e thods of

blending were evalua ted inc l udin g yard blen ding and de dicated m ills. Overall th e f o llowing general observations were seen.

1. NOx e m issions and LOI levels depe nded on the ratio of PRB to lignite. 2. Interm ediate blends of PRB / Lignite exhibi ted som e undesirable s l agg i ng condition s when the ratio was between 30% to 70% PRB by hea ting va lu e. It appea r s th at the eu tec t ic of the slag is a result of the intera ctive chem istry of the two fuel s in this range of operation. 3. Blending of the fuel with a dedicated pulveri zer provides better furnace control. However, this is m o re cum b ersom e from a fuel-handling viewpoint.



### Pet Coke Co-Firing w i th Lignite and PRB

A dedicated level of pet coke nozzle tips and stationary nozzle assemblies were designed by FW EC and provided as part of the F W EC TLN3 retrofit on unit #2. Co-f iring up to 23% (BTU basis) pet coke was evaluated to docum ent the effects on boiler perform a nce and em issions. Tabulated results ar e listed on TABLE A.

Pet Coke and Lignite:

1. Initially NOx em ission initially increased to 0.25 lb/MMBtu slightly as the percentage of

pet coke incr eased. Selec tive s t agin g of the com bustion air yielded reduced average value of 0.21 lb/MMBtu while firing a 20% Btu blend of pet coke with the lignite. 2. Pet coke does not m i x well in pulverizers with othe r f u els. T h is is m o re of a f unction of hardness and type of pet coke. If the pe t co ke is in “sh o t” typ e or signif i can tly harder th an the prim ary fuel, the Ray m ond bow l m ills will prefer th e prim ary fuel and reject m u ch of the pet coke 3. Co-firing pet coke tends to reduce steam temperatures slightly because furnace clean lines s improves sig n ificantly. However, due to the high m a ss flow of the lignite,

tem p erature s can still be m a intained.

Pet Coke and PRB:

1. NOx e m ission increased as the percentage of pet coke increased. At approxim a tely 20% pet cok e on a Btu b a sis, the NOx em issions reached 0.1 67 lb/M MBtu. CO was very low

. 2. Steam tem p eratu r es cou l d not be m a intained co-f iring pe t cok e with PRB. As a result the evaluation period test was a relative l y sho r t. It is be lie ved tha t th is is due to th e reduced m a ss associated with the pet coke and the PRB. In addition the lower ash loading an d cleaner furnace surfaces asso ciated with the p e t coke co-firing contributed to lower firing zone temperatur es and the res u ltan t decrease in steam

tem p eratures.

8NOx versus Load Com p arison of Lignite & P R B

1. The Lim e stone units are norm a lly base loaded. So, there is not a gr eat d eal of data at reduced load. The data that is available indicates that PRB NOx e m ission rates are relatively constant between 50 % to 100 % lo ad. Lignite NOx e m ission rates increase significan tly at redu ced loads. Th is is depicted in Figure 5 below.

FIGURE 5

NOx Versus Load for PRB & Lignite After Retrofit

9TABLE A - TLN3 FUE L PERF OR MANCE S U MMARY DATA TAB LE

S H R H

TEST NOx CO Out Te m p Out Te m p FURNACE P ANE L(S )

FUE L D U R A T I O N l bs/M M B tu pp m d e g F d e g F M W g SL A G * SL A G *

L I GNI T E 3 0 DA. 0. 1 9 3 15. 4 9 9 6 1 0 0 2 8 1 0 1 3

PRB (100%) ~ 9 DAYS 0.137 22 1003 1009 808 2 2

L I GNI T E / 20% Pet Coke ( B T U ) 2 W E E K S 0. 21 21 1000 997 797 1 2

33% PRB / 67 % L I GNI T E ( B T U ) ~24 HRS. 0. 165 49 998 1004 813 2 3

45% PRB / 55 % L I GNI T E ( B T U ) ~24 HRS. 0. 18 25 991 999 803 3 5

71% PRB / 29 % L I GNI T E ( B T U ) ~24 HRS. 0. 159 22 990 998 806 3 4

PRB / 20% PE T C OKE ( B T U ) ~ 8. 5 HRS 0. 167 3 954. 4 968. 6 807 1 2

* H i gh er nu mb er s d e no te wor s e slag co nd itio n s

.



### Conclusions

1. The FW EC TLN3 system has dem o nstrated the following:

a. Fuel f i ring f l exibility ov e r a rang e of Texas lign i te, as well as PRB and pet coke. b. Reduced furnace slaggin g com p ared to the p r e-retrofit sys t em

. c. Im proved boiler efficiency (heat ra te reduction and relatively low LOI). d. Good reduction of NOx em issions without a si gnificant increase in CO while firing a range of fuels.

e. Durable erosion resistant com ponents, solid engineering and construction. f. Special coal nozzle tips de signed to be replaced without rem oving stationary coal nozzles result in installation cost savings.

g. Co-firing lignite and PRB produced NOx emissions that wer e rela tive l y proportional to the b l end ratios o f

the fuels. h. Em ission results with the TLN3 system co nfir m e d m o st predicted design perform a nce values. In addition actual m a in windbox and overfire d a m p er positions clo s ely

m a tched damper positions initially predic ted with the F W EC TLN design program. This design tool allows TLN systems to be optim ally sized for em issions and optim al fuel-air m i xing and air flow control. 2. Based on these results, PRB clearly and repeatedly showed lower NOx levels com p ared to the Texas lignites. However, boiler m odi fi cations to m a intain the econom izer gas outle t tem p eratur e with in equipm en t design lim its would be required. 3. Pet coke can provide a good fu el supplem ent in som e cases, but it does tend to increase NOx em issions. Special air s t aging consider ations can provide som e additional NOx reduction. 4. Blending fuels can sometim es allow the operato r to “enjoy” the worst of both fuels if care is not taken to consider how all the fuel firing equipm ent is reacting to it. 5. Reliant Energy Inc. has ordered a second TLN 3 from Foster Wheeler Energy Corporation to be installed on Unit #1. This de cision is based on the perform a nce of Unit #2 TLN3 and the working relationship establ ished between the two organizations.

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### Acknow ledgements

The following individuals need sp ecial acknowledgem e nt for their s upport of this successful low NOx retrofit at Reliant E n ergy’s Lim e stone Plant.

Brad Moulton -FW E C Bruce McCrea - FW EC Stefan Laux - FW EC Sara W o ldehanna - FWEC Dennis O’Gallagher – L C I Engineering Ken McCarthy- FW EC Wayne S. Jones – REI Fred Lem o i n e - REI Yiu Lam Lau – REI Neal Irv in – REI



### Referen ces

1. Lon g Te rm Re sults fr om the First US L o w NOx C o nversi o n o f

a Ta ng entia l Lig n ite F i red Unit –

J. Gr us ha, S. Wol d eha n na , K

. M c C a rt hy

, an d G. Hei n z – P r ese n t e d at 24 th In ternation a l Tech n i cal Confere n ce on Coal Utilizatio n & Fu el System s, Clearwater, FL., March 8-11 , 19 99

2. A ppl i c at i o n of Si mul a t

i o n T o ol s and E

x

peri ment al

M e t

h o dol ogi es i n t h e Desi gn an d P e rfor m a nce Op timiza tio n of Lo w NOx Retro f

its to a Lignite Ta ng ential Fired Bo iler – J. G r usha , S

. Wol d eha n na, Presen ted at EPRI-DOE EPA Co m b in ed Utility Air Po llu tan t Con t ro l Sy m p o s iu m , Atla n t a, GA., Au gust 16 -2 0, 1 999

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