Reliant Energy Tangential Low NOx System at 

 Limestone Unit 2 

Cuts Texas Lignite, PRB and Pet Coke NOx 

Ron Pearce 

Reliant Energy Incorporated 

Limestone Station 

Jewett, TX 

John Grusha 

Foster Wheeler Energy Corporation 

Perryville Corporate Park 

Clinton, NJ 08809-4000 

ABSTRACT 

Reliant Energy owns and operates two large 820 MW lignite-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-corner units with 10 fuel elevations. 

In response 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 lb/MMBtu, with very low CO 

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

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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 successful operation, Reliant Energy Incorporated is proceeding with modifying Limestone 

#1 with an enhanced FWEC TLN3 system in the Fall of 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. 

The TNRCC established NOx emission requirements of 0.165 lb/MMBtu for this region to be 

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

0.11 lb/MMBtu, which equates to approximately 60 ppm (corr.3%O2) over an 8 hour rolling 

average. 

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

lb/MMBtu to under 0.20 lb/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 effected. The most critical performance requirement however was that

carbon monoxide levels not exceed 60 ppm (corr.3%O2) 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 airflow 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. 

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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. 

FIGURE 1.

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• 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 

Since these are 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. 

1. Furnace cleanliness improved following the TLN3 retrofit 

2. Heat rate improved by 1 to 2%. 

3. SH 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 

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

approximately 0.19 lb/MMBtu at full load over the range of lignite, while 

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

FIGURE 2.

Post Retrofit TLN3 NOx and CO Emissions – Texas Lignite 

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Low NOx PRB Firing 

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

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

coal firing with the TLN3 controlled unit #2 were compared to similar load conditions on the 

uncontrolled unit #1. 

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) 

FIGURE 3

Range of Economizer Outlet Gas Temperatures Firing PRB 

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8. NOx emissions were significantly lower when firing PRB as compared to lignite, for the 

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

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

FIGURE 4

Post Retrofit TLN3 NOx and CO Emissions – PRB Coal 

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Low NOx PRB and Lignite Blend Firing

A series 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. 

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

2. 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. 

3. 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 

A dedicated 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 lb/MMBtu slightly as the percentage 

of pet coke increased. Selective staging of the combustion 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 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 of the 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 lb/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. 

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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 % to 100 % load. Lignite NOx emission rates increase 

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

FIGURE 5

NOx Versus Load for PRB & Lignite After Retrofit 

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TABLE A - TLN3 FUEL PERFORMANCE SUMMARY DATA TABLE 

SH RH

TEST NOx CO Out Temp Out Temp FURNACE PANEL(S)

FUEL DURATION lbs/MMBtu ppm degF degF MW g SLAG* SLAG*

LIGNITE 30 DA. 0.193 15.4 996 1002 810 1 3

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

LIGNITE / 20% Pet Coke (BTU) 2 WEEKS 0.21 21 1000 997 797 1 2 

33% PRB / 67 % LIGNITE (BTU) ~24 HRS. 0.165 49 998 1004 813 2 3 

45% PRB / 55 % LIGNITE (BTU) ~24 HRS. 0.18 25 991 999 803 3 5 

71% PRB / 29 % LIGNITE (BTU) ~24 HRS. 0.159 22 990 998 806 3 4 

PRB / 20% PET COKE (BTU) ~ 8.5 HRS 0.167 3 954.4 968.6 807 1 2 

* Higher numbers denote worse slag conditions . 

Conclusions 

1. The FWEC TLN3 system has demonstrated the following: 

a. Fuel 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 blend ratios of the fuels.

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. Pet 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 firing 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. 

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Acknowledgements 

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 

Stefan Laux - FWEC 

Sara Woldehanna - FWEC 

Dennis O’Gallagher – LCI Engineering 

Ken McCarthy- FWEC 

Wayne S. Jones – REI 

Fred Lemoine - REI 

Yiu Lam Lau – REI 

Neal Irvin – REI 

References 

1. Long Term Results from the First US Low NOx Conversion of a Tangential Lignite Fired Unit – J. Grusha, 

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

Utilization & Fuel Systems, Clearwater, FL., March 8-11, 1999

2. Application of Simulation Tools and Experimental Methodologies in the Design and Performance 

Optimization of Low 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

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