- 1 -

APS ANALYSIS OF EFFICACY OF LNB’S AND OFA 

AT FOUR CORNERS 

EPA has requested comment on ...”the potential efficacy of LNB + OFA on all Units at 

FCPP.” EPA assessed that Units 1, 2 and 3 have fireboxes (furnaces) too small to 

effectively utilize modern-day low-NOx burners (LNB) and separated overfire air (OFA) 

combustion-based NOx control technologies. In support, EPA correctly referenced the 

“considerable operational problems” experienced on Unit 2 subsequent to retrofit of 

Foster Wheeler LNB in late-1998. EPA concludes that due to their small size (assumably 

meaning fireboxes or furnaces), Units 1 & 2 with LNB and Unit 3 with LNB + OFA will 

not provide significant NOx control beyond present-day levels. EPA further references 

the cell-burner boiler classification of Units 4 & 5 and the higher presumptive levels of 

NOx control established by the EPA for this group of boilers. EPA concludes that 

because higher NOx standards were set for cell burner boilers that these units have 

“inherent design limitations” on the application and effectiveness of OFA compared to 

conventional dry-bottom, wall-fired boilers. APS offers the following comments on the 

potential effectiveness of LNB + OFA on each of the different boilers at FCPP. 

A. Units 1 & 2 - Units 1 & 2 are Riley Stoker pulverized-coal (p.c.), dry-bottom, front 

wall fired boilers designed circa 1961 that went into commercial service in February and 

May of 1963. Each unit is presently rated at an output of 170 MWNe. The unique 

features of the furnaces (fireboxes) on these two units with regards NOx control 

capability or limitations are: a) the closely-pitched horizontal spacing (only 5’-5”) of 

burners in its 3 row x 6 column burner firing pattern; b) tangent-tube furnace construction 

with estimated 4% to 6% ‘tramp’ air infiltration; c) very small upper furnace residence 

time (only 0.41 sec.) for potential OFA addition; d) a common furnace rear wall and 

convection-backpass front wall design that prevents the possibility of adding OFA ports 

to the rear wall; and e) an unfavorable, upper-furnace Radiant Superheat (RSH) design 

arrangement. By far the most problematic of these issues that constrain more aggressive 

NOx reduction levels are the close horizontal burner spacing and the very small upper 

furnace residence time. These two issues will now be discussed in some detail. 

The furnace and convective back-pass design on Units 1 and 2 are shown in Figure 1-1. 

Each of the (18) p.c. fired burners on Units 1 & 2 are designed for 102 million Btu/hr 

heat input with furnace wall throat openings on Unit 1 of 33” diameter (original design) 

and 38” diameter throat openings on Unit 2 with its LNB. The closely-pitched, 

horizontal burner-to-burner throat openings on Units 1 & 2 of only 5’-5” are much closer 

than industry standards [1], and hence atypical of conventional wall-fired-boiler burner 

array spacing of between 8 ft. to 9 ft. separation vertically and horizontally. Reference [2] 

submitted to “Environmental Organizations” including New Mexico Citizens for Clean 

 

1

 Coal Combustion, J. Tomeczek, Krieger Publishing, 1994. Recommended standard is horizontal pitch of 

3 * throat diameter. Thus, desired Unit 1 & 2 horizontal burner spacing would equal 8.25 ft. (3* 33”) to 

9.50 ft. (3* 38”). 

2

 Staudt, J.E., “Assessment of Potential for Further NOx Reduction by Combustion-Based Control at Four 

Corners Steam Electric Station”; Final Report, April 2004, Andover Technology Partners (ATP) 
- 2 -

Air and Water, and others, in April 2004 discusses in some depth the importance of 

adequate separation between LNB in a multiple burner array and the significant risk of 

upsetting near-burner fuel:air mixing and combustion aerodynamics critical to optimal 

NOx control and combustion efficiency if burners are too closely spaced. A key focus of 

discussion was on recognition of two alternate flame types, Type 1 and Type 3, produced 

by different, commercially-available LNB designs and their applicability to different 

furnace designs and burner spacing arrays. These flame types are redrawn and included 

herein as Figures 1-2 and 1-3. Type 1 LNB flames were discussed as slow-diffusion, low 

secondary air swirl designs with narrow flame angle (of spread) and long, luminous flame 

zones caused by the full centerline penetration of the PA/coal jet (see Fig. 1-2). Flame 

attachment is achieved by bluff body flame stabilization [3

]. The ATP report identified 

B&W and Riley Power LNB as having Type 1 flame characteristics, and noted the 

lengthened flames produced by these LNB were typically not compatible with older 

vintage, single wall fired furnaces having relatively shallow furnace depth and limited 

height such as Units 1 & 2. Type 3 low-NOx flames utilize strong secondary air swirl to 

create an annular, internal recirculation zone (IRZ) in the near-burner region along the 

burner axis. In a properly designed Type 3 LNB, the PA/coal streams (jets) partially 

penetrate into the IRZ before being reversed to flow backwards and along the outer 

boundary of the swirling recirculation zone. Near-burner flow patterns within Type 3 

low NOx flames are illustrated in Figure 1-3. Type 3 flames have a wider flame angle (of 

spread) due to the strong secondary air swirl, shorter flame lengths due to the lack of 

central PA/coal jet penetration, and are swirl stabilized for flame attachment. However, 

this flame type requires precise control of SA swirl levels and PA/coal jet velocity to 

achieve the critical balancing of forward momentum forces from the PA/coal jets 

 

3

 Combustion Aerodynamics, Beer, J.M, Chigier, N.A., Krieger Publishing (1972). 
- 3 -

Figure 1-1. Picture of Four Corners Units 1 & 2 Reheat Boiler (Side View) 
- 4 -

Figure 1-2. Type 1 Low NOx p.c. Burner Flame on wall-fired boiler 

Figure 1-3. Type 3 Low NOx pulverized coal flame on Wall-Fired burner 

against the reverse velocity field of the IRZ. ATP identified Foster Wheeler and 

ALSTOM Power LNB as having Type 3 flame characteristics. 
- 5 -

Based of these flame type discussions, the ATP report recognized the likely adverse 

burner-to-burner flame interference occurring with the FW, high swirl type LNB installed 

on Unit 2. ATP assessed the ALSTOM low-NOx burner as a possible replacement for 

the FW burners, but was not able to rectify the high combustion air swirl design of this 

burner with ALSTOM claims that the flames had a narrow angle of spread which avoided 

burner-to-burner interference in closely-pitched burner arrays. The ATP report 

concluded there was no compelling evidence (including ALSTOM field experience) to 

suggest that the ALSTOM low-NOx burner would perform any better than the FW design 

in the tightly-spaced burner array on Units 1 & 2. It was also stated that ...”it may be 

possible that a different burner design might resolve the current problems on Unit 2. But 

that will only be determined with time as the problems of Unit 2 become better 

understood.” 

In 2008 as part of BART evaluations for the FCPP under Regional Haze rules, APS’ 

engineering staff made an updated, detailed study of the technical feasibility and design 

practicability of retrofitting the latest, most-advanced LNB and LNB + OFA systems 

being supplied by U.S. manufacturers on Units 1, 2 and 3 [4

]. These evaluations were 

made to determine whether Units 1, 2 and 3 could achieve NOx emissions ≤ 0.42 

lb/mmBtu (LNB only) or ≤ 0.32 lb/mmBtu (LNB + OFA) and simultaneously control PM 

emissions and stack opacity to acceptable levels without jeopardizing unit reliability, 

efficiency, or output capacity. Central to these evaluations was identifying a 

commercially-proven LNB capable of achieving optimal NOx performance without 

flame interference issues in a close-spaced burner array, and with acceptable luminous 

flame length. To this end, an extensive study was made of technical papers published in 

the last ten years with an emphasis of evaluation and discussion on near-burner mixing 

patterns and combustion aerodynamics of commercial, p.c. fired U.S. LNB designs [5, 6, 7

, 8 ]. This work revealed that a third low-NO burner flame type existed with near-burner 

flame aerodynamics in between Type 1 and Type 3 LNB flames. The aerodynamic flow 

patterns and ignition characteristics of this flame type (designated as Type 1-A) are 

shown in Figure 1-4. In Type 1-A low-NOx flames the overall swirl number (S) for 

collective secondary air and PA/coal streams is in the low-to-moderate range (0.2 < S < 

0.45) which overlaps the swirl number of Type 1 slow-diffusion flames (0.2 < S < 0.3). 

[The swirl number is simply the normalized ratio of angular to axial momentum of the 

combustion air and PA/coal flow streams exiting the burner throat opening.] The bluffbody flame attachment ring and SA diverter create a local IRZ immediately downstream 

of the burner exit driven by the PA/coal jet, and strengthened by the swirl applied to the 

PA stream. Swirl primarily from the outer SA jet creates a second ERZ radially 

 

4

 Allen, C.D., “Engineering Report on Low-NOx Burner + Overfire Air Application and Systems Design 

Options for Four Corners S.E.S, Units 1 – 5 Wall-Fired and Cell Burner Boilers”; Rev. 0, Sept. 2008,.

APS Privileged and Confidential report. 

5

 Gu, M., Zhang, M., et. al., “The Effect of Mixing Characters of Primary and Secondary Air on NOx 

Formation in Swirling Pulverized Coal Flames“; June 2005. 6

 Dorman, D., Courtemanche, B., Yilmaz, A., “Reducing NOx Emissions and Commissioning Time on 

Southern Company Coal Fired Boilers with CFD Analysis”; March 2003. 7

 Hules, K.R., Yilmaz, A., “Bunker to Stack: Cost Reduction and Problem Solving Benefits of CFD for 

Utility Power Generation”; Dec. 2002 8

 Heap, M.P., Davis, K.A., Sarofim, A.F., et. al., “Optimized Fuel Injector Design for Maximum InFurnace NOx Reduction and Minimum Unburned Carbon”; Final Report 1998 
- 6 -

displaced and slightly downstream from the PA-driven IRZ. As depicted in Figure 1-4, 

part of the fuel-rich PA/coal stream is re-circulated back towards the IRZ at the base of 

the flame while the remainder penetrates along the burner axis analogous to Type 1 nearburner flame aerodynamics shown in Figure 1-2. Type 1-A flames have a narrow flame 

angle of spread and normalized luminous flame lengths (flame length/throat diameter) in 

the range of 7 to 7.5 [9

]. Riley Power Inc. (RPI) is the primary U.S. manufacturer of 

LNB with Type 1-A flame characteristics and a photo showing its actual flames in a 

tight-spaced burner layout is shown in Figure 1-5. 

Figure 1-4. Type 1-A Low NOx Pulverized Coal Flame on wall-fired burner 

 

9

 Hayes, J.M., Beer, J.M., “Adrodynamic Modeling and Scale-Up of a Low NOx Oil Burner”; ASME 

Conf., 1996. 
- 7 -

Figure 1-5. Flame Zones at Branch U4 with 122 mmBtu/hr CCV-DAZ Burners in CloseSpaced Cell Burner Layout; note narrow flame angles. 

Table 1-1 shows a summary comparison of key burner design parameters and flame types 

for commercial LNB developed from APS’ most recent studies. Also shown in the table 

are predicted NOx , CO, LOI, and PM emissions based on a retrofit of Units 1 & 2 with 

Riley Power CCV-DAZ low-NOx burners. Note that the Riley LNB is the only design 

APS now evaluates as being capable of achieving NOx emissions ≤ 0.42 lb/mmBtu (LNB 

only) with acceptable flame length and reliable, full-load boiler operation. Performance 

predictions for NOx, CO, etc. in Table 1-1 are based on engineering analysis and 

regression correlation of field data collected from recent LNB retrofits on p.c. fired 

boilers with comparable (or more severe) furnace design parameters and firing similar or 

higher rank high-volatile bituminous coals as Units 1 & 2 at the FCPP [10]. NOx 

performance information supplied by vendors was checked against the EPA Clean Air 

Markets database. Hence, in summary, APS believes it now has identified a LNB design 

that will provide significant NOx control improvements on Units 1 and 2 with burners 

only ( i.e., 46% additional reduction on Unit 1 and 34% additional reduction on Unit 2) 

with acceptable luminous flame length and combustion efficiency performance. 

 

10 Reference: Louisville Gas & Electric, Cane Run Unit 5; and Georgia Power, Harllee Branch Unit 4. 
- 8 -

Table 1-1. Design Parameters and Flame types of Commercial LNB, and Predicted 

Performance of the RPI design burner on Units 1 and 2 

 Burner/Boiler Manufacturer 

Parameter B&W/ 

DRB-4Z 

Riley / 

CCV-DAZ 

FW / Vortex ALSTOM / 

RSFC 

Siemens 

(ABT) / 

Opti-Flow 

LNB 

SA Swirl 0.2 – 0.3 0.2 – 0.45 0.6 – 0.9 0.6 – 1.1 Insuff. data 

PA Swirl zero 0.2 zero 0.2 zero 

PA Vel., ft/sec 65 – 85 65 – 85 85 – 110 about 100 Insuff. data 

Quarl α, deg 25 25 22 25 / 0 ? 

Flame Type 1 1-A 3 3 ? 

Normalized Flame 

Length 

8 – 9 7 – 7.5 5.5 – 6 ~ 6 ? 

Applicable 4C 

Boilers 

4 & 5 1 – 5 3 – 5 3 – 5 ? 

Predicted Performance on Units 1 and 2 

Predicted NOx, 

lb/mmBtu 

≤ 0.42 

Guarantied NOx by 

RPI, lb/mmBtu 

≤ 0.40 

CO at boiler exit, 

ppm 

 < 50 – 60 

Pred. Avg. LOI, Wt. 

% in ash 

 1.0 

Avg. PM Emissions 

at stack, lb/mmBtu 

 0.033 

 
- 9 -

The second major design constraint which limits NOx control effectiveness or reduction 

potential on Units 1 and 2 is the very small upper furnace residence time. EPA is correct 

in identifying this as a major concern. In fact, upper furnace residence time is the 

foremost factor governing the successful application of separated OFA -- in conjunction 

with LNB. Upper furnace residence time (UFRT) is typically defined by the furnace 

volume between the OFA port elevation to the apex of the furnace nose divided by the 

boiler flue gas flow rate [11]. OFA ports are typically placed on the firing wall and 

installed 8 to 10 ft. above the top burner row. Adequate upper furnace residence time is 

required for effective mixing of OFA with upward flowing furnace gases and unburned 

fuel (primarily CO and char particles), and combustion burnout of this un-reacted fuel. 

EPRI guidelines [12] and preliminary rules of thumb applied by U.S. boiler OEM’s for 

evaluating the feasibility of OFA application are roughly: 

• UFRT > 1.0 sec. (ideal); 

• 0.6 < UFRT < 1.0 sec. (acceptable to good); 

• UFRT < 0.5 sec. (insufficient). 

Upper furnace residence time on Units 1 & 2, calculated at a bulk average temperature of 

2500°F and corrected for elevation above sea level, is only 0.41 seconds. With this very 

short UFRT, if a conventional front-wall OFA system were applied to Units 1 & 2 it 

would be almost assured that significant increases in CO emissions, increased unburned 

carbon in the ash, increased furnace exit gas temperatures, and unacceptable PM 

emissions accompanied by noticeable increases in stack opacity would occur [13]. In fact 

the UFRT on Units 1 and 2 for potential application of conventional front wall OFA 

systems is outside (below) the successful experience range of any U.S. LNB/OFA 

manufacturer. This is the reason ATP concluded in 2004, and APS has confirmed with 

its most recent re-evaluations, that Units 1 and 2 in their present furnace design 

configuration are not technically feasible candidates for OFA addition. 

B. Unit 3 - Unit 3 is a Foster Wheeler p.c., dry-bottom, front wall fired boiler also 

designed around 1961 – 1962 that went into commercial service in April of 1964. The 

rated output of Unit 3 is 220 MWNe. A side elevation view of the Unit 3 furnace and 

convective back-pass is shown in Fig 1- 6. The Unit 3 furnace has several advantages 

over Units 1 & 2 with regards to NOx control features: 1) it is designed with moreconventional, approximate 8 ft. horizontal and vertical burner spacing; 2) furnace walls 

are membrane construction versus tangent tube hence furnace ‘tramp’ air infiltration is 

not an issue; 3) the common furnace rear wall and convection back-pass front wall only 

 

11 Some burner/boiler manufacturers apply an upper furnace volume effectiveness factor in calculating 

UFRT’s; another defines the furnace exit at the bottom elevation of furnace WW platens or RSH platens. 12 Eskinazi, D., “Retrofit NOx Controls for Coal-Fired Utility Boilers”; EPRI Research Project 2916-7. 13 Trego, P., Kleisley, R, Panzera, P., “Cromby Unit # 1: Challenges of Retrofitting Low NOx Combustion 

Technology to a Single Wall Fired Boiler”; ASME Joint Power Conference, Oct. 1995. 
- 10 -

Figure 1-6. Side Elevation Drawing of 4C Unit 3 Furnace and Convection Back-Pass 

OFA 

Ports 
- 11 -

extends 16 ft. below the apex of the furnace arch thereby allowing the potential of OFA 

ports addition in a revised burner firing pattern; and 4) the upper furnace Radiant 

Superheat design layout conforms to present-day utility boiler design standards (was 

redesigned/replaced in 1980). The two primary design issues presently limiting the 

effectiveness of NOx control on Unit 3 are: incorrectly-designed, second-generation 

CF/SF type Foster Wheeler low-NOx burners; and a very small upper furnace residence 

time (only 0.40 sec. as on Units 1 & 2) from where front wall OFA ports would have to 

be located to the furnace exit. 

The existing high swirl FW split-flame LNB installed on Unit 3 in 1990 were expected to 

achieve NOx emissions of ≤ 0.45 lb/mmBtu at the furnace design burner zone heat 

release rate (BZRR) of 250 kBtu/hr-ft2

 [14]. This performance was never achieved, not 

due to any burner-to-burner flame interference issues as on Units 1 & 2 but due to an 

incorrect SA-to-PA design velocity ratio at the burner throat. A typical design value for 

this ratio is around 1.0, based on APS’ detailed design review of ten (10) other front wall 

fired boilers equipped with Foster Wheeler CF/SF or IFS design LNB. Although FW 

added an insert piece into the burner nozzles on Unit 3 around 1992 to increase the 

PA/coal jet velocity, this was only able to lower the SA/PA velocity ratio to 1.35 – 1.40. 

The physical and aerodynamic effects of this mismatch in key burner design velocities 

are: 1) to increase shear-induced turbulence (and mixing) between the two steams at the 

base of the flame; and 2) to cause an earlier than desired breakup of the four, 

concentrated, fuel-rich PA/coal jets leaving the burner nozzle [15, 8

, 16 ]. Both of these 

effects raise NOx emissions. Linear regression analysis performed on Unit 3 data for a 

regression of NOx on the SA/PA velocity ratio showed NOx emissions are increased by ~ 

0.08 lb/mmBtu due to this mismatch of SA and PA axial velocities at the burner exit. 

Since the Unit 3 firebox does not have the tight burner spacing constraint of Units 1 & 2, 

two (2) alternate, modern-day low-NOx burner designs are assessed by APS to be most 

advantageous: either the Riley Power CCV-DAZ burner, or the Foster Wheeler “Vortex” 

burner (see Table 1-1). Based on APS’ review of other front wall fired furnaces with 

comparable design features to Unit 3 (e.g., BZRR, balanced draft, membrane wall, ≥ 8’ x 

8’ burner spacing, BZRT ≥ 0.9 sec.), and firing similar high volative bituminous C or B 

grade coals, predicted NOx emissions with these latest, most-advanced (third-generation) 

low-NOx burners would be ≤ 0.42 lb/mmBtu. This represents an additional 29% 

reduction in NOx levels (i.e., (0.59 – 0.42)/0.59 *100 = 29%) with a LNB only changeout on Unit 3. 

Unit 3 does have an option for a more compact burner firing pattern to give space in the 

upper furnace for front- and/or rear-wall OFA ports and slightly improved upper furnace 

residence time for mixing and burnout. This approach involves deleting the two, top row 

“wing” burners and converting to a 4 x 4 firing pattern (16 burners versus 18). The 

existing three ball-tube mills would be retained with the top mill supplying four burners, 

and the bottom two mills each supplying six burners. Front and rear wall OFA ports, 

 

14 Allen, C.D., “Summary Engineering Report on Application of Foster Wheeler CF/SF Low-NOx Burners, 

Four Corners Unit 3”; Oct. 1987. 15 Cremer, M., Davis, K., Letcavits, J. Vierstra, S., et. al., “CFD-Based Development, Design, and 

Installation of Cost-Effective Control Strategies for Coal-Fired Boilers”; 16 Low NOx System CFD Modeling and Design Study, Nevada Power – Reid Gardner 1 & 2; FWEC 

Confidential Report, 1997. 
- 12 -

preferably in an interlaced (staggered) layout, or rear wall only OFA ports (second 

option), would be located at the current elevation of the top row burners (El 5403’-6’, see 

Figure 1-6). Analytical CFD modeling by Reaction Engineering (REI) and B&W has 

shown that front and rear wall OFA port designs, followed by rear wall only OFA ports, 

provide the best mixing and CO/LOI burnout on front wall fired utility boilers with 

limited upper furnace residence time [17, 13]. The UFRT on Unit 3 would be increased 

from 0.40 seconds to 0.52 seconds by the above described burner firing pattern redesign 

and OFA port layout. 

Predicted Unit 3 full-load (220 MWNe) emissions performance with a 20% OFA system, 

as described above, in conjunction with either Riley CCV-DAZ or Foster Wheeler Vortex 

low-NOx burners is [18]: 

1. NOx emissions ≤ 0.32 lb/mmBtu; 

2. Average CO emissions ≤ 125 to 150 ppm at boiler exit; 

3. Carbon in ash of 1.5% to 2.0%, by weight; (i.e., 50% to 100% increase). 

4. PM emissions at stack with existing VS in range of 0.040 – 0.044 lb/mmBtu. 

In summary, APS believes that significant additional NOx control reduction can be 

achieved on Unit 3 – on the order of 46% from present day levels, with properly 

designed, modern-day low-NOx burners in a revised burner firing pattern and in 

conjunction with a front- and rear-wall designed 20% (of total combustion air) OFA 

system. 

C. Units 4 & 5 - Units 4 & 5 are dry-bottom, opposed-fired, cell burner boilers originally 

equipped with three-nozzle cells in a non-symmetrical firing array. In 1989 (U4) and 

1991 (U5), the units were retrofit with “non-plug controls” for NOx (i.e., Foster Wheeler 

CF/SF low-NOx burners on approximate 9.0 ft. horizontal and vertical burner spacing; 4 

row x 6 column firing array). Both units are rated at a nominal 800 MWG electrical 

output and went into service in July, 1969 (U4) and July, 1970 (U5). The boilers on 

Units 4 & 5 are B&W once-through, supercritical water/steam pressure design with 

membrane-wall furnaces and a positive-pressure, gas-side furnace design (versus 

balanced draft). The furnace design uses B&W’s three-pass Universal Pressure (UP) 

flow circuitry with multiple intermediate waterwall collection/distribution headers and 

interconnecting piping located in the upper regions of the windboxes. EPA asserts that as 

cell burner boilers (EPA Phase II, Group 2) Units 4 and 5 have inherent design features 

that limit the effectiveness of combustion-based NOx control versus conventional wallfired burner boilers (EPA Phase II, Group 1). As APS has commented in the past [19], 

cell burner boilers are in fact no different in their baseline, uncontrolled NOx emissions 

or in the NOx control achieved by LNB only than wall-fired boilers in an opposed-wall 

 

17 Davis, K.A., Eddings, J.R., Heap, M.P., “Evaluating the Effects of Low-NOx Retrofits on Carbon in Ash 

Levels”; EPRI-DOE-EPA Combined Mega Symposium, Atlanta, 1999. 18 Reference: Gulf Power, Crist Unit 6 (RPI); FHI – Units LT3 and CCP2, China (FW). 

19 See APS Comments dated March 19, 1996 on Proposed EPA NOx Acid Rain Program – 40 CFR Part 

76, Docket No. A-95-28. 
- 13 -

firing pattern. This is shown by the data and correlation shown in Figure 1-7 where 

uncontrolled NOx emissions of several wall-fired boilers (both single-wall fired and 

opposed-wall fired) are plotted along with uncontrolled NOx emissions from several cell 

burner boilers. This data was taken from EPA’s own database and extensive databases 

collected by the Edison Electric Institute (EEI) in the mid-1990’s. It can be seen that 

baseline, uncontrolled NOx emissions from wall-fired burner boilers with opposed-wall 

firing are in the same range as for cell burner boilers. The distinguishing feature of cell 

burner boilers as a boiler population is that they are all opposed-wall fired with burner 

zone heat release rates in the range of 300 – 500 kBtu/hr-ft2 (FW definition basis for 

BZRR). On the other hand, the much larger wall-fired burner boiler population includes 

a significant percentage (~ 75%) of single-wall fired units with burner zone heat release 

rates in the range of 150 – 300 kBtu/hr-ft2

. Based on a relatively small sample size (n < 

30) analyzed by APS back in 1996, single-wall fired boilers had uncontrolled mean NOx 

levels of 1.04 lb/mmBtu while the larger-sized, higher capacity opposed-wall fired 

boilers had a mean uncontrolled NOx level of 1.25 lb/mmBtu. Thus the mean 

uncontrolled NOx for the entire population of “wall-fired burner boilers” is 

approximately 0.75 * 1.04 + 0.25 * 1.25 = 1.09 lb/mmBtu, while cell burner boilers as a 

group have an approximate 1.25 lb/mmBtu mean uncontrolled NOx rate. Hence, cell 

burner boilers will always produce higher uncontrolled NOx emissions, 

Uncontrolled NOx vs Burner Zone Release Rate

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

100 200 300 400 500 600

Burner Zone Release Rate, kBtu/hr-ft^2

NOx Emissions, lb/mmBtu

Cell Burner Units

Opposed-Wall Fired

Single-Wall Fired

Fitted, Linear Regression

NOx = 0.0009 (BZRR) + 0.81

R^2 = 0.533

Four

Corners

Units 4 & 5

Four

Corners

Units 1,2 & 3

Figure 1-7. Uncontrolled (Base) NOx as a Function of Burner Zone Release Rate 

for Wall-Fired boilers and Cell Burner Boilers 

and in turn – whether retrofit on a plug-in or non-plug-in basis, higher controlled 

emissions when compared to the conventional “wall-fired burner” boiler population. 

However, cell-burner boilers retrofit with non-plug-in combustion-based NOx control 
- 14 -

and opposed-fired conventional wall-fired boilers are similar or analogous in burner 

firing pattern physical appearance, key furnace key parameters, and in their capability for 

comparable NOx reductions. For comparison to Units 1, 2 & 3 at the FCPP, Units 4 & 5 

have a higher BZRR of 380 kBtu/hr-ft2 (see Figure 1-7), but an upper furnace residence 

time of 0.84 seconds. 

As discussed herein it would be expected that LNB with Type 1 or Type 1-A narrow 

angle of spread luminous flames would be most successful if retrofit on cell burner 

boilers on a plug-in basis. In fact this is the case with Riley Power’s CCV and CCVDAZ low-NOx burners having achieved NOx reductions in the range of 50% - 60% using 

“burners only” in cell-fired configurations [6

]. Riley claims they have retrofit over 7,000 

MW of cell-burner-fired utility boilers with their CCV or CCV-DAZ low-NOx burners 

[

20]. B&W has also had some success in retrofitting their DRB-XCL low-NOx burner on 

a plug-in basis for cell burner boilers [21]. 

APS’ retrofit of Units 4 and 5 with Foster Wheeler second-generation CF/SF LNB in a 

non-plug-in design approach achieved NOx reductions of 50% - 52% with burners only. 

Mean NOx emissions were reduced from 1.15 lb/mmBtu to around 0.53 lb/mmBtu [22, 23]. Units 4 & 5 presently operate at annual-average NOx emission of 0.49 lb/mmBtu 

with normal, full-load operation frequently having one (1) pulverizer out of service but 

with its secondary air registers partially open. This creates a slight burner zone “staging” 

effect which further lowers NOx emissions by around 5%, depending on which of the 

eight (8) pulverizers is out of service. As shown in Table 1-1, the widened 9 ft. x 9 ft. 

horizontal and vertical burner spacing on Units 4 & 5, and the 42 ft. firing depth between 

front and rear walls, allows three (3) different modern-day, improved-performance LNB 

to be considered: B&W’s DRB-4Z, Riley’s CCV-DAZ, and FW’s Vortex burner. All 

three of these burner designs, in conjunction with a conventional front- and rear-wall 

20% to 25% OFA system are considered by APS to be “technically feasible” for 

application on Units 4 & 5. [Technical feasibility is defined as being able to operate 

Units 4 & 5 with improved low-NOx emissions while maintaining good boiler operability 

and reliability.] APS’ review of comparably-designed, opposed-wall fired, drum and 

supercritical pressure boilers with equivalent BZRR’s, burner zone and upper furnace 

residence times, conventional 20% - 25% OFA, and firing high volatile bituminous coals 

of equal or higher rank than Four Corners, showed NOx emissions performance of ≤ 0.35 

lb/mmBtu with any of these three different LNB designs [4, 24]. This predicted NOx level 

represents a 28% reduction in NOx emissions, or over seven (7) million pounds of NOx 

reduction annually from each Unit. 

 

20 See Babcock Borsig Power Technical Bulletin No. B-97/02. 21 LaRue, A.D., Costanzo, M.A., et. al., “Update of B&W’s Low NOx Burner Experience”; ASME 

International Joint Power Conference, July, 2000. 

22 Allen, C.D., “Low-NOx Burner Evaluation at the Four Corners Station – Units 4 & 5, Comparative 

Emissions and Thermal Performance”; EPRI Product RP2916-4, Final Report, July, 1994. 23 Allen, C.D., “Review of NOx Emissions Performance Compared to Post-LNB Retrofit Baseline Data, 

Four Corners Unit 5”; Generation Engineering Dept., March, 1995. 24 Reference: Mississippi Power, Jack Watson Unit 5 (RPI); Allegheny Power, Fort Martin Unit 2 and First 

Energy, East Lake Unit 5 (B&W); Tucson Electric, Springerville U3 and Seminole Electric, Palatka Units 1 

& 2 (FW). 
- 15 -