# FOSTER WHEELER

SUCCESSFUL NOx REDUCTION AND

CONVERSION TO POWDER RIVER BASIN

FUEL ON WALL FIRED BOILERS

Sara Woldehanna, Firing systems Engineer

Tom Steitz, P.E., Firing systems Manager

Brian Exner, Performance Engineer

Foster Wheeler Energy Corporation Perryville Corporate Park, Clinton, NJ 08809

Brian Skinner, Manager or Performance Resources

Jim Smoeyink, Operations Team Leader

Northern Indiana Public Service Company
5265 Hohman Avenue, Hammond, IN, U.S.A

Presented at:
ASME International Joint Power Generation Conference Houston, Texas, U.S.A. October 14-16, 1996

Power Engineering PowerGen Conference

Orlando, Florida, U.S.A. December 4-6, 1996

# SUCCESSFUL NOx REDUCTION AND CONVERSION TO POWDER

RIVER BASIN FUEL ON WALL FIRED BOILERS

Sara Woldehanna, Firing Systems Engineer Tom Steitz, Firing Systems Manager Brian Exner, Performance Engineer

Foster Wheeler Energy Corporation, Perryville Corp. Park, Clinton, NJ, 08809, USA; Ph: (908) 713-3005 Fax: (908) 713-2405

Brian Skinner, Manager of Performance Resources, Jim Smoeyink, Operations Team Leader,

Northern Indiana Public Service Company, 5265 Hohman Aye, Hammond, IN, USA; Ph: (219) 853-5200 Fax: (219) 956-2398

ABSTRACT This paper presents the project goals and evaluations along with the operating results of the Low NOx/Powder River Basin (PRB) fuel conversion at Northern Indiana Public Service Company’s (NIPSCO), Schahfer unit 15, 500 MW boiler. In adherence to the New Source Performance Standards (NSPS), the original firing equipment produced NOx emissions slightly below 0.70 lb/ MMBtu burning Bituminous coal with overt fire air. The overall objectives were to burn cost effective PRB coal at NOx emissions below 0.45 lb/ MMBtu while maximizing boiler performance. Working closely with NIPSCO, Foster Wheeler Energy Corporation (FWEC) designed and supplied the necessary Low NOx burners and pressure parts modification to enable PRB coal firing on this unit. Post-retrofit NOx emissions, without overfire air, are consistently below 0.35 lb/MMBtu with repeatable loss of ignition performance below 1%. The pressure part modifications met NIPSCO’s boiler performance requirements, with successful unit operation above 90% MCR. Experiences with PRB coal at other Foster Wheeler units are also briefly described.

Northern Indiana Public Service Company: Schahfer
unit 15

INTRODUCTION TO BOILER AND ORIGINAL EQUIPMENT The unit, a 500 MW Foster Wheeler boiler, is owned by Northern Indiana Public Service Company (NIPSCO) and was commissioned into service in 1979. The original equipment was designed to perform under the New Source Performance Standards (NSPS) of the 1970’s. The boiler is a natural circulation, reheat type steam generator rated for a maximum capacity of 3,800,000 lbs/hr of superheated steam at 2620 psig and 1005 F design outlet (Fig. 1). At 100% MOR, reheat steam was designed at 3,366,000 lbs/hr of flow at 562 psig with an outlet temperature of 1005 F. Superheat steam temperature control is achieved by sprays. Originally, six MB-23 vertical spindle mills were used for coal pulverizing. These mills were eventually replaced by six MPS 89 N mills by NIPSCO. At MOR, the design coal flow for the specified fuel was 480 klb/hr resulting in approximately 80 Klb/hr coal flow per mill. Coal is fed to a balanced draft opposed fired furnace with 24 burners. The furnace is 59 1⁄2 feet wide by 52 feet deep. Each wall has 4(columns) by 3 (rows) of burners with one row of overfire air ports located above the center line of each burner column. The original design fuel is a Bituminous coal with higher heating value of 11,000 Btu/lb (refer to Table 1 for proximate and ultimate analysis of design coal). At MCR the total heat input from coal was designed to be approximately 5600 MMBtu/hr, resulting in a relatively cool furnace. The efficiency of the unit at MCR was predicted to be 87%. The original burners were Foster Wheeler’s turbulent type with scroll inlets designed to achieve NOx slightly less than 0.70 lbs/ MMBtu with overfire air. The twenty-four turbulent burners had 48’ throat diameters with an inner barrel housing oil ignition equipment.

OBJECTIVE OF PROJECT To meet 1990 Clean Air Act, NIPSCO decided to explore different options to bring NOx down from the nominal current values of 0.70 lbs/ MMBtu to below the regulatory limits of 0.45 lbs/ MMBtu. In conjunction with attaining this objective, NIPSCO wanted the option of burning Powder River Basin (PRB) coal, which is significantly different from the Eastern Bituminous coal for which this unit was designed Requirements of the 1990 Clean Air Act have increased utility interest in burning PRB coal. This coal is not only inexpensive to burn, but is low in sulfur content. Thus, PRB can be an economical alternative to various methods of flue gas desulfrization. However, burning PRB is not without its disadvantages.FWEC was contracted by NIPSCO, with a two fold purpose in mind:

• conduct a study on the effect of burning PRB fuel on boiler performance, recommend modifications, and design and install required modifications;

• modify or supply any combustion equipment to reduce NOx reduction below regulatory limits. Although the project was broken down into two smaller projects related to these two objectives, award of both to FWEC allowed for the natural mating of the two designs. FWEC undertook both these projects which included design and supply of pressure part modifications and low NOx IFS burners.

IMPACT OF BURNING PRB WITH ORIGINAL PRESSURE PARTS PRB is not considered to be a high slagging coal in the traditional sense of the word. Unlike high slagging eastern coals, the ash accumulation caused by burning PRB is thin. However, the impact on surface effectiveness is generally the same as that of high slagging coals. The ash in PRB coal contains high amounts of calcium oxide ( CaO). Because the calcium oxide is white, it causes the ash to be light colored and to have a reflective quality. When fired, a thin layer of light-colored ash quickly builds on the furnace water walls and any other radiant heat transfer surface. The reduction in surface effectiveness causes less heat to be absorbed in the water walls, roof and radiant superheater. The suppressed absorption in the furnace results in a high furnace exit gas temperature. PRB also has a relatively high fouling potential which must be considered. Due to the high gas temperatures, the hemispherical softening temperature of the ash can be exceeded at the furnace exit and into the convection surface, causing the ash to be “sticky”. As a result, a layer of ash forms on the convection surfaces, and reacts chemically with the tubes and the hot flue gas to form a tenacious deposit which is difficult
to remove. Fouling in the convection surfaces (HRA) generally begins as the ash accumulates in the back spacing between the tubes. This is known as “profiling”. Profiling with PRB coals is not uncommon in the primary superheater and the reheater. Over time, the backspace can become completely closed. This not only insulates a significant area of the heat transfer surface, but it changes the gas flow around the tubes and thus reduces the effectiveness of the surface. Experience has indicated that fouling in the HRA is more significant in the primary superheater and the upper banks of the reheater. The severity of the fouling appears to be reduced in the lower HRA. Consequently, from a design standpoint, it is desirable to have ample clear space between elements in the upper HRA. If ample space between coils is not maintained, the ash accumulation may bridge the space between tubes. The higher gas temperature also helps drive the transfer of heat in spite of the decreased surface effectiveness. This is due to the higher log mean temperature differential (LMTD) between the gas and the steam. Even with the higher LMTD, heat transfer to the fouled tubes can not be maintained at the rate achieved with the clean tubes The elevated flue gas temperature is then carried through the entire boiler. The result is a higher gas temperature leavingthe economizer. If significant, this can potentially weaken the structural supports and the flue plate in the area above and below the lower economizer. All structural equipment must be carefully examined for the gas temperatures experienced when burning PRB. The high gas temperatures combined with the high moisture content of PRB together results in lower boiler efficiency. In some cases, as much as a 5% reduction in boiler efficiency has been noted when switching to PRB coal firing.

PROPOSED BOILER AND BURNER MODIFICATIONS To achieve the above expressed objectives, FWEC performed a study, and proposed a retrofit of the turbulent burners and OFA system with FWEC’s low NOx IFS burner, as well as changes to the original pressure parts to accommodate firing PRB coal.

Recommendations for Pressure Part Modifications As an initial step, an engineering study was performed to predict the effects of burning PRB coal with the original pressure parts. In order to establish a baseline for the study, the performance of the boiler was predicted when firing the current Eastern Bituminous coal, utilizing the FWEC Boiler Performance computer program. The boiler load for this prediction wasapproximately 90% of the original MCR due to a unit derating necessitated by high gas temperatures entering the air heater experienced while firing Eastern Bituminous coal. The performance model was made without the hot precipitator as it was to be converted to a cold precipitator during the modification outage. A summary of this performance appears in Table 2, Column I. Performance predictions were then calculatedutilizing the original pressure parts when firing PRB coal. The predictions were made utilizing computer analysis with heat transfer surface effectiveness correlations adjusted to reflect typical operating characteristics of PRB coals. As seen in Table 2, Column II, the furnace exit gas temperature (FEGT) was predicted to increase when switching to PRB coals. This is due mainly to the highly reflective ash associated with this fuel. In order to minimize potential fouling of the convection surface, it was desirable to maintain the FEGT below 2300F, the ash softening temperature in a reducing atmosphere of the design PRB coal (see Table 1). At 90% MCR, it was predicted that the FEGT could be maintained below 2300F when switching to PRB coal. Additionally, the gas temperature entering the horizontal primary superheater was predicted to be 1884F. This was found to exceed the acceptable gas temperature for the original arrangement of this equipment and could lead to excessive fouling and possibly plugging of the convection surface. In order to optimize the conversion to PRB coal, the following design modification were recomended:

• Increase the clear side spacing in the primary superheater to minimize the potential for fouling and plugging;

• Add a vertical loop or ‘dogleg to the top of the primary superheater in order to maintain superheater capability that may be lost due to the above arrangement modification. This would also reduce flue gas temperatures entering the horizontal primary superheater to further reduce the propensity for fouling and plugging;

• Increase the heating surface in the primary superheater by increasing the tube diameter to help maintain final steam temperature and to reduce gas temperatures in the HRA;

• Add a loop to the top of the upper economizer to further reduce the flue gas temperature;

• Add furnace wall blowers to maintain the highest degree of furnace cleanliness possible. A summary of the performance predictions for the modified pressure parts can be seen in Table 2, Column Ill. It should be noted that the addition of the primary superheater dogleg was predicted to reduce the gas temperature entering the horizontal primary superheater by 86F. The gas temperature entering the air heater was predicted to drop by 10F because of the recommended modifications which results in a slight increase in boiler efficiency. Additional boiler modifications were to be considered if a greater reduction in flue gas temperature entering the air heater was required based on operational experience.

Low NOx Burner Modification To reduce NOx from the baseline values of 0.70 lbs/MMBtu to 0.45 lbs/ MMBtu, FWEC recommended replacing the original turbulent burners with the current state-of -the-art FWEC patented IFS burners (Fig. 2). Since the prediction of NOx using only these burners was comfortably below 0.45 lbs/ MMBtu, FWEC proposed that the current overfire air ports be blanked off to maximize combustion efficiency. In addition, FWEC believed their closure would allow for increased reheat steam temperature capability at low loads following the retrofit. The proposed Low NOx coal wall fired burner was FWEC’s third generation Internal Fuel StagedTM (IFS) design. This Low NOx burner is based on a concept patented by FWEC in the 1970’s. The IFS burner became available for commercial operation in 1990 and has since become a proven design with operational experience on a number of boilers. The IFS burner two-stages the secondary air flow and internally stages the fuel and primary air flows such that co-axial flames are developed within the burner’s throat for NOx reduction, while maintaining classical turbulent burner flame patterns and low windbox to furnace pressure drop. A fundamental design philosophy was strictly adhered to during the development of this burner:

• Flame length and envelope should be equivalent to that of the turbulent burners that have been in historical use;

• Combustion air flow and swirl to each burner should be independently controllable;

• Primary air/coal velocity should be adjustable to insure optimum relation between primary and secondary air streams.

• Primary and secondary air pressure drop should be no greater than for the turbulent burners, to ensure the use of existing fans.

The following design features of the IFS burner are integral in the reduction of NOx and stable combustion:

• Dual Series register provides flame shape control and NOx reduction by using a two-stage secondary air adjustment;

• Adjustable coal nozzle allows primary air/coal velocity to be optimized without changing primary air flow, thus ensuring optimized NOx reduction with good combustion;

• Internal Fuel Staged Nozzle segregates the coal intomultiple concentrated stream and develops a coaxial inner flame zone . This results in the volatiles of the coal being driven out and burned under more reducing conditions than otherwise would occur. The volatiles contain a high percentage of the fuel bound nitrogen, and combustion under reducing conditions converts bound nitrogen into free N2, substantially reducing NOx formation.

RETROFIT AND MODIFICATION The proposed modifications were accepted by NIPSCO and implemented during a unit outage in May, 1994. FWEC designed and supplied 24 IFS burners and all necessary auxiliary equipment. Existing burner throatopenings were utilized with some modifications. Twenty four existing gas ignitors were modified to fit new burners. In order to equalize conduit to conduit coal distribution, eighteen coal conduit ceramic lined resistors were designed and supplied. The design and fabrication of these resistors were based on a coal flow study. Existing overfire air ports had been blanked off prior to this outage. During the outage, the recomended heat transfer surface modifications were also installed. The primary superheater was redesigned and a vertical superheater “dogleg” was added. An additional upper economizer loop was added and extra sootblower openings to adequately clean the new vertical primary superheater were installed. FWEC offered NIPSCO the following primary guarantees with this conversion:

• The pressure part modifications will not prevent the boiler from achieving 90% of original MCR;

• When operating at 90% of original MCR (3,420,000 lbs/hr main steam flow), the superheater spray flow will not exceed 275,000 lbs/hr (+10,000 lbs/hr tolerance);

• NOx emissions, measured at the economizer outlet, without the use of overfire air, shall not exceed 0.45 lbs/MBtu over the load range;

• Unit carbon monoxide (00) shall not exceed 100 parts per million ( ppm) with the unit operating at guaranteed NOx emissions levels at full load;

• Unburned carbon in ash will be less than or equal to 3%.

• Windbox to furnace differential will not exceed 3.5 in WO over the load range with all twenty four burners in service.

POST RETROFIT PERFORMANCE Following the modifications, FWEC conducted tests to optimize the burners and evaluate post retrofit performance. The post-retrofit NOx emissions as measured by the CEMS during all the optimization tests were consistently below 0.35 lb/ MMBtu (Fig. 3). CO emissions at the end of the optimization testing were always less than 75 ppm (Fig. 4). LOI values were less than 1.2 % for all tested samples (Fig. 5 and 6), 50% the contract requirements.

At the conclusion of the optimization tests, NOx guarantee tests were performed by an independent third party contractor, Mostardi Platt. The tests were performed at nominal full (90% of original MCR) and intermediate loads. Low excess air operation (10%) during these tests produced less than 0.25 lb/MMBtu NOx, again with less than 1% LOI values (Fig. 7). Prorating the NOx results to 100% MCR at 20% equates the NOx emissions to below 0.4 lb/MMBtu, without over fire air. Prorated emissions at 100% MCR and 10% excess air fall well below 0.35 lb/MMBtu. Furnace to windbox pressure drop was only 2.1” WO (Fig. 8).The pressure part modifications allowed for successful operation even above the predicted 90% MCR (Fig. 9). Further increased capacity would be available should additional pressure part modifications recommended by FWEC be implemented. The superheat spray flow was consistently below 225,000 lbs/hr, well below guarantee levels of 275,000 lbs/hr (Fig. 10).To compare the effectiveness of the pressure part modifications, refer back to Table 2. Column IV is a baseline test taken before the modification of the unit using the Bituminous coal fired in 1993. It can be seen that the gas temperatures were higher than those originally predicted (column I, Table 1). Even with these higher than predicted baseline temperatures, post retrofit gas temperatures were reduced to target levels (see columns Ill, V and VI of Table 2).

PROBLEMS ENCOUNTERED The only adverse operational experience encountered has has been burner throat eyebrows. Eyebrows are formations of slag on the burner throat refractory, and usually form on the upper hemisphere of the throat. They are commonly seen with high slagging coals where the ash suspended in the hot combustion gas comes in contact with the refractory walls that are relatively hotter than tube surfaces. Even though the ash has little tendency to accumulate on clean surfaces to begin with, after a while, a thin layer of ash starts to accumulate on the refractory walls. This thin layer will have a higher surface temperature than the retractory, and ash particles now suspended in the combustion gas will have more of a propensity to attach to this thinly layered surface. Thus, after the initial deposition, the ash accumulation tends to grow very fast. Even though eye brows are mainly a result of fuel properties, different flow patterns are suspected to accelerate or delay their formation of eyebrows. NIPSCO and FWEC have decided to jointly tackle the formation of these eyebrows over long term operation through the exploration of different flow patterns through testing and flow modeling.

PRB Experiences at other Units Portland General Electric’s Bordman Station, Unit 1 is a 500 MW Foster Wheeler Unit commissioned into service in 1980. The boiler was designed to burn AMAX coal from the Eagle Butte mine in the Powder River Basin of Wyoming (Table 3) to meet the NSPS NOx emissions standard of 0.7lbs/MMBtu. The boiler is opposed fired with four levels of burners with four burners in each deck. Eight FW MB-23 mills supply the burners. The Bordman Station Unit 1 was equipped with what was at the time the most advanced FW combustion system: the Controlled Flow! Split Flame Burner (CF/SF) and overfire air ports (The CF/SF burner is an earlier generation FW low NOx burner and varies in some aspects from later generation burners; one key difference is that the CF/SF has a four port coal tip instead of the six port tip of the IFS burner.) Four overfire air ports were located on each wall. The following performance guarantees were offered at the MCR steam flow of 3,825 klb/hr: boiler efficiency will not be less than 84.86 + 0 33 % superheat and reheat steam temperatures will be 1005 + 10 F; and reheat spray flows under specifically defined inlet steam temperatures will be 0 klb/hr. Testing conducted in 1981 resulted in NOx emissions of 0.384 lbs/ MMBtu without overfire air and 0.248 lbs/ MMBtu with overf ire air. UBC in fly ash was approximately 0.33%. While turbine operation restriction forced steam temperature set points to 950 F, there was no reheat spray, and boiler efficiency was 84.7%, well within guarantee levels. Central and South West Service’s Oklaunion Unit 1 is a 720 MW Foster Wheeler unit commissioned into service in 1986 and was designed to burn sub bituminous coal with PRB characteristics (Table 3) . The boiler is opposed fired with three decks of CF/SF low NOx burners with five burners in each deck. Six FW MBF mills supply the burners. The following performance guarantees were offered at the MCR steam flow of 4,913 klb/hr: NOx emissions will be less than or equal to 0.5 lbs/ MMBtu; boiler efficiency will not be less than 84.99 ± 0.33 %; and superheat and reheat steam temperatures will be 1005 + 10 F. Testing conducted by an independent party in 1988 resulted in NOx emissions of 0.37 lbs/ MMBtu at MCR conditions with superheat and reheat temperatures at 996 F and 994 F respectively. The boiler efficiency at guarantee conditions was 84.84%, well within guarantee levels. Grand River Dam’s Unit 2 is a 570 MW Foster Wheeler unit commissioned into service in 1986. The unit was designed to burn either a Wyoming sub bituminous coal (Rawhide) or a 50/50 blend of the Rawhide with an Oklahoma strip mine bituminous coal (Table 3) The boiler is opposed fired with three decks of CF/SF low NOx burners with four burners in each deck. Six FW MBF mills supply the burners. Four overfire air ports were located on each wall. The following performance guarantees were offered at the MCR steam flow of 3,900 klb/hr: NOx emmissions will be than or equal to 0.5 lbs/ MMBtu; superheat and reheat steam temperatures will be 1005 + 10 F; and boiler efficiency will not be less than 86.23 + 0.33 % for PRB firing, and 88.38 + 0.33% for the blend firing. Testing conducted on the pure Rawhide Wyoming coal resulted in superheat temperatures of 1004 F and reheat temperatures of 993 F. NOx emissions for these tests are not available. NOx compliance tests conducted a few months later with a 50/50 Wyoming/Oklahoma blend resulted in 0.42 lbs/ MMBtu with over fire air. Boiler efficiency calculations are not available.

CONCLUSION Based on the operating results at NIPSCO and the other units described above, when firing PRB coal NOx levels in the range of 0.35 to 0.45 with low NOx burners only, and 0.3 to 0.4 with low NOx burners coupled with overf ire air are easily achieved. Low excess air operation is a viable option due to the volatility of the fuel, with NOx levels below 0.3 lbs/ MMBtu with burners alone and LOI less than 1%. Boiler and HRA pressure parts can be studied and modified to permit operation with the inexpensive PRB coal. Most performance predictions for the units described have been achieved or exceeded.

Acknowledgments FWEC would like to acknowledge the major contributions of NIPSCO to this project. The authors also appreciate the important assistance of Timothy Sweeney and Al Edwards of Foster Wheeler Energy Corporation to the creation of this paper.

### Table 1: Coal Analyses for the Various Coals

Constituent (on as received basis) Bituminous coal used in predictions Design PRB coal used in predictions Pre-retrofit test coal (Bituminous) Typical PRB coal during post retrofit optimization

Volatile Matter % Appx 38 34.87 32.53

Fixed Carbon % Appx 43 44.7 32.18

H2O % 11.03 29.5 13.27 29.63

Ash % 8.01 5.01 7.16 5.66

S2 % 0.41 0.34 0.40 0.33

H2 % 4.2 3.45 4.51 3.47

Carbon % 63 49.1 63.64 48.79

N2 % 1.4 0.72 1.34 0.79

O2 % 11.03 11.8 9.68 11.35

HHV 11.000 8.500 11.205 8.407

CaO % of Ash 4.35 24.5 5.35 20.77

MgO % of Ash 1.3 4.5 1.27 3.73

Ash Softening Temp in Reducing atm, F 2460 2300 2345 2144

Ash Softening Temp in Oxidizing atm, F 2550 2350 2520 NA



### Table 2: Gas Temperatures at various points in the gas pass

Case I: Baseline prediction II: PRB / No

change III: PRB w/

modification IV: Baseline

Test V: NOx Test VI: Optimiz. Test

Main steam flow, klbh 3420 3420 3420 3431 3400 3138

Superheat spray flow, klbh 16 146 242 64 225 120

Furnace Exit Gas Temperature, F 2121 2290 2289 2254 2246* 2241*

Gas entering horizontal superheater, F 1745 1884 1798 1883 NA NA

Gas leaving lower economizer, F 768 822 812 809 742 755

Unit efficiency % 88.51 84.61 84.75 85.89 NA NA

Case I: Predictions with original boiler pressure parts with Eastern Bituminous coal fired prior to pressure part modifications Case II: P redictions with original boiler pressure parts with PRB fuel Case III: Predictions with pressure part modifications and PRB fuel Case IV: Baseline testing with original pressure parts and Eastern Bituminous test coal Case V: NOx Guarantee post retrofit test: after phase I modifications, firing PRB coal Case VI: Test conducted during final phase of Low NOx Burner Optimization period, firing PRB coal

• Denotes measured FEGT

### Table 3: Design Coal Analyses

Constituent (on as received basis) AMAX coal of Portland Electric Sub Bit. Coal of Central & South West Grand River Dam’s sub bituminous coal Grand River Dam’s bituminous coal

Volatile Matter % 31.2 30.05 30.05 35.5

Fixed Carbon % 32 32.94 32.94 48.5

H 2 O % 30.4 31 31 4.62

Ash % 6.4 6.01 6.01 7.7

S 2% 0.3 0.4 0.4 2.69

H 2% 3.4 3.25 3.25 5

Carbon % 47.85 46.81 46.81 69.5

N 2% 0.62 0.66 0.66 1.4

O 2% 10.83 11.87 11.86 5.27

HHV 8020 8055 8055 12000

CaO % of Ash 22.85 20.53 20.68 4.6

MgO % of Ash 4.74 4.59 4.62 1

Ash Softening Temp in Reducing atm, F 2193 2178 2180 NA

Ash Softening Temp in Oxidizing atm, F 2255 2234 2234 NA