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

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ABSTRACT 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

1 ABSTRACT 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 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. 1

2 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 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 lb/mmbtu for this region to be effective in 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. 2

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.

3 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. 3

4 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 4

5 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 5

6 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 F o. 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 6

7 8. NOx emissions were significantly lower when firing PRB as compared to lignite, for the same level of staging. NOx averaged 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 7

8 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 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. 8

9 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 9

10 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 PRB (100%) ~ 9 DAYS LIGNITE / 20% Pet Coke (BTU) 2 WEEKS % PRB / 67 % LIGNITE (BTU) ~24 HRS % PRB / 55 % LIGNITE (BTU) ~24 HRS % PRB / 29 % LIGNITE (BTU) ~24 HRS PRB / 20% PET COKE (BTU) ~ 8.5 HRS * 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. 10

11 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 24 th International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, FL., March 8-11, 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,

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