NOx REDUCTION FROM A 44 MW WALL-FIRED 

BOILER UTILIZING OXYGEN ENHANCED 

COMBUSTION

Lawrence E. Bool III 

Praxair Inc. 

175 East Park Drive, Tonawanda, NY 14150 

Jurron Bradley 

Praxair Inc. 

175 East Park Drive, Tonawanda, NY 14150 

Copyright © 2003, Praxair Technology, Inc. All rights reserved. 

ABSTRACT 

Praxair has developed a novel method of using oxygen to bring about additional NOx 

reduction from low NOx combustion systems burning pulverized coal or other nitrogen 

containing fuels. This paper presents results of field tests in a commercial wall-fired 

boiler burning pulverized bituminous coal. 

In pilot- and full-scale burner testing NOx emissions were reduced by as much as 60%, 

to below 0.15 lb/MMBtu, when a small portion of low-NOx burner combustion air was 

replaced with locally injected oxygen. Oxygen injection allowed burner operation at 

lowered stoichiometric ratio, and raised the temperature of the primary combustion zone 

to favor NOx reduction kinetics. Oxygen addition also reduced carbon in ash. These 

promising results led to beta testing in a commercial 44 MW wall-fired boiler beginning 

fourth quarter of 2002. Testing as part of the demonstration project showed greater than 

40% reduction in NOx emissions, reduction in LOI, enhanced flame stability, and 

unanticipated opacity reductions. 

INTRODUCTION 

Although oxygen enhanced combustion has been adopted for many high temperature 

industrial furnaces for productivity improvement, energy reduction and NOx reduction, 

economic considerations have limited the application of oxygen in the utility industry. 

However, as NOx emissions limits continue to tighten, the ability of oxygen enhanced 

combustion to reduce NOx without the need for expensive equipment, such as SCR 

systems, makes its use more attractive. Praxair has developed oxygen enhanced NOx 

reduction technologies for both combustion zone and post combustion NOx control1,2 . 

This paper describes the application of oxygen to prevent the formation of NOx in the 

combustion zone. 
Using oxygen in the combustion zone of the boiler can significantly reduce NOx 

formation by enhancing the effectiveness of currently available control techniques such 

as low NOx burners with overfire air (OFA). For example, oxygen can support deep 

staging of the furnace while avoiding many of the problems inherent with staging. Even 

small amounts of oxygen can enhance the effectiveness of staging beyond that available 

with air alone3,4 – leading to significant reductions in NOx formation. Finally since 

oxygen is fed to the burner zone, it can be preferentially fed to specific burners in order to 

minimize burner to burner variations3, 4

, which can significantly reduce NOx and 

unburned carbon in ash (UBC). 

Oxygen enhanced staged combustion can mitigate both of these problems. Since less 

nitrogen is present in the flame when some of the combustion air in the first stage is 

replaced with oxygen, the flame temperature is higher for a given stoichiometric ratio. 

This increase in temperature has two effects. First, increasing the temperature under fuel 

rich conditions drives the reactions involving fuel nitrogen species towards the formation 

of N2 rather than NOx. Second, the increase in temperature, coupled with higher local 

oxygen inlet concentrations leads to increased devolatilization rates and yields. The 

higher volatile yield means that the combustibles in the gas phase increase as compared 

to the baseline – leading to a more fuel rich gas phase which inhibits NOx formation from 

the volatile nitrogen species. Therefore oxygen enhanced combustion both creates more 

fuel rich conditions and increases the flame temperature in the fuel rich zone, both of 

which reduce NOx formation. The enhanced devolatilization also yields less residual 

carbon that must be burned in the second stage, leading to lower NOx formation from 

char and lower UBC. 

Experimental work performed at two different facilities was used to demonstrate the 

concept of oxygen enhanced combustion for NOx control. Detailed experiments were 

performed using the 5 MMBtu/hr pilot-scale facility at the University of Utah. Full-scale 

experiments were performed in ALSTOM Power’s Industrial Scale Burner Facility 

(ISBF) in Windsor, CT using a 25 MMBtu/h commercially available Radially Stratified 

Flame Core (RSFCTM) burner. These tests indicated NOx formation is significantly 

reduced as the burner zone stoichiometric ratio decreases and that the oxygen enriched 

combustion provides additional NOx reduction beyond that available from simple airbased staging3

. Similar data demonstrated that oxygen can enhance staging for NOx 

control over a wide range of conditions, and that the method for oxygen introduction is 

critical to the NOx reduction achieved4. 

Based on these successful tests a full-scale demonstration of the technology was 

performed during the last quarter of 2002. This demonstration used the existing burners 

and overfire air system at the James River Power Station, Unit 3 owned by City Utilities 

of Springfield, MO to demonstrate the effectiveness of oxygen enhanced combustion for 

NOx control. 
DESCRIPTION OF JAMES RIVER UNIT 3 

The James River Power Station, Unit 3 is a Riley Stoker, pulverized coal, sub-critical 

steam generator with a capacity of 44 MW. The unit is equipped with 3 Attrita 

pulverizers. Six D B Riley CCV Low NOx burners are arranged in two elevations on the 

front wall of the boiler. An overfire air system consisting of 5 ports had previously been 

installed on a single elevation above the top row of burners. The OFA ducts were 

designed such that each duct is divided into two parts in a 1/3-2/3 arrangement. City 

Utilities normally fires a blend of PRB and bituminous coal in Unit 3 and typically does 

not utilize the existing OFA system. Excess oxygen is measured with in-situ probes 

before the air heater. NOx, SOx, and opacity are measured in the stack. Plant operating 

data are automatically recorded in a data acquisition system. For this test an independent 

testing contractor, GE Mostardi Platt, installed a 12-point gas-sampling grid immediately 

below the in-situ oxygen probes. This grid was used to measure gas composition in 

various locations in the duct and to derive an average over the entire duct. Ash samples 

were collected under specific conditions for loss on ignition (LOI) analysis. 

 

The demonstration program at James River consisted of two phases. In the first phase the 

plant fired only the bituminous coal it typically uses in its blend. This allowed the test 

team to determine the effectiveness of the technology with a coal similar to those used for 

the single burner testing. For the second phase of testing the plant switched back to its 

normal PRB-bituminous blend for this Unit. 

In each phase plant operating data and gas composition data was taken at full load under 

a wide range of burner stoichiometric ratios and oxygen replacement rates, including 

baseline data with no oxygen added. The overall and burner stoichiometric ratios were 

calculated using measured plant data and these flue gas compositions. 

O2 IMPLEMENTATION AT JAMES RIVER UNIT 3 

For the demonstration at James River an 

oxygen delivery system was designed and 

constructed to allow a wide range of oxygen 

replacement rates to be tested. Liquid oxygen 

was delivered to the plant using tanker trucks 

and then stored in two cryogenic storage tanks 

(Figure 1). Atmospheric vaporizers were used 

to convert the oxygen from liquid to gas. The 

gaseous oxygen was then piped to a flow skid 

located near the burner front. This flow skid 

consisted of a total flow controller followed by 

metered flow to each burner. A remote switch 

was installed to allow the operator to shut the 

oxygen flow off at any time. This remote 

switch was also tied into the plant control 

Figure 1. Oxygen tank installation at 

James River 
system such that any condition leading to loss of fuel flow would shut the system down. 

The design of the oxygen storage and delivery system was such that it could be 

constructed and installed with little or no impact on the plant operation. In fact, the 

minor burner modifications required for oxygen enhanced combustion were performed 

while the unit was operating at full load. For a commercial operation the only major 

modification to this system would be use of an on-site oxygen generator in place of the 

liquid oxygen delivery and storage. 

FULL LOAD RESULTS 

During the first phase of the test program the unit was held at full load to explore the 

effect of air-alone staging on NOx emissions. Plant operating data, as well as emission 

information and ash samples were collected over a wide range of operating conditions. 

Various oxygen injection methods and replacement rates were evaluated to confirm the 

effectiveness of oxygen for NOx reduction and the importance of the injection strategy 

on the technology performance. Periodically the burner would be returned to the ‘as 

found’ condition and data collected without oxygen to evaluate the repeatability of the 

air-only conditions. These air-only results provided insight on the minimum NOx the 

plant could consistently achieve while keeping other operational and emissions 

parameters, such as LOI and opacity, within acceptable ranges. 

Impact of staging and oxygen injection on LOI 

Representative data on the effect of both staging and oxygen injection on LOI are shown 

in Figure 2. These data show that as the burner stoichiometric ratio is reduced from the 

unstaged condition to approximately 0.92 the LOI increased significantly. However, 

even when the burner stoichiometric ratio was further reduced to approximately 0.85 and 

a small amount of oxygen was added the LOI was comparable to, or only slightly higher 

than, the unstaged air case. This is true even for oxygen replacements less than 5%. 

These data suggest that oxygen allows deeper staging with little or no impact on LOI. 

Impact of staging and oxygen injection on NOx

The effect of staging with oxygen on NOx emissions is shown in Figure 3. Based on 

opacity and other limitations the plant is limited to burner zone stochiometric ratios 

between 0.97 and 1.0 with the air staging system. The NOx reductions shown in Figure 3 

were calculated from the staged air baseline case. These data show that even very small 

oxygen replacements yielded NOx reductions of approximately 40%. This result is 

particularly encouraging as the burners at this facility are first generation low NOx 

burners. Even higher reductions are expected from more advanced low NOx burner 

designs, such as those used in the single-burner pilot and full-scale tests. The data also 

demonstrates that low oxygen replacement rates help to maintain flame stability and LOI 

– allowing operation at lower burner stoichiometric ratios than could be achieved 

otherwise. The staging curve for the bituminous coal, not shown, indicate that even low 
levels of oxygen addition enhance the staging process, leading to NOx levels lower than 

could be achieved with air alone at a given SR 

Figure 2. Effect of staging on LOI – Full Scale 

James River Unit 3

Bituminous Coal

0.0

5.0

10.0

15.0

20.0

25.0

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20

Burner

LOI (%)

Air

Oxygen enhanced

Figure 3. NOx reduction with small amounts of oxygen replacement

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

0.75 0.80 0.85 0.90 0.95 1.00

Burner Zone SR

NOx Reduction from Baseline

Blend

Bituminous
One interesting aspect of these results is that, unlike previous single burner data, these 

low levels of oxygen replacement yielded significant improvements in NOx emissions 

under relatively lean conditions (SR>0.85). The observed NOx reductions at these burner 

stoichiometric ratios are probably due in large part to stabilization of the flame and 

reducing the flame standoff distance. Limited data at higher oxygen replacements 

(Figure 4) suggest that even greater reductions may be achievable under richer conditions 

and higher oxygen use (<10% replacement). These conditions more closely match the 

conditions in the pilot-scale work. 

Impact on CO emissions 

Another critical factor is the impact of oxygen enhanced combustion on CO emissions. 

Although reducing the burner zone stoichiometric ratio causes an increase in CO being 

generated in the burner zone, it is generally understood that CO emissions are typically 

controlled through the overfire air design and overall boiler operation. Good mixing of 

the overfire air and adequate excess oxygen are critical to minimize CO emissions. Data 

from the demonstration program are shown in Figure 5. As can be seen in the figure CO 

emissions increased to various degrees as the burner zone stoichiometric ratio was 

reduced. As part of the testing the system was operated on air under staged conditions 

where burner zone stoichiometric ratio and excess air levels were lower than typical of 

normal boiler operation. This led to higher CO emissions. The data also suggest that 

oxygen addition had little or no impact CO emissions and may, in fact, have resulted in 

Figure 4. NOx reductions from PRB-bituminous blend 

90% PRB-10% Bowie Blend 

Full Load

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

0.75 0.80 0.85 0.90 0.95 1.00

Burner Stoichiometric Ratio

NOx Reduction from Baseline

Nozzle 2

Nozzle 2 - 2x O2

Nozzle 3
lower emissions than air alone. This suggests that oxygen enhanced combustion, when 

coupled with a well designed staged combustion system operated with adequate excess 

air, should not significantly impact CO emissions. 

Impact of staging and oxygen injection on flame stability 

Comparing the NOx reductions and the LOI demonstrate that deeper staging and oxygen 

addition yields significant reductions in NOx emissions with little or no impact on LOI. 

Observations of the flames with and without oxygen addition indicated that flame 

stability and burner standoff distances were significantly improved when oxygen was 

used. Figures 6 and 7 show the typical trend observed during the testing. As the burner 

zone was staged deeper with air alone the flame became less stable and typically had a 

fairly significant standoff distance. Flame impingement on the rear wall was noticed 

when the overfire was used with air alone. However, when even small amounts of 

oxygen were used the flame stability and flame attachment were significantly improved. 

These small amounts of oxygen replacements also shortened the flames enough to 

eliminate this problem under almost all stoichiometric ratio conditions. 

Figure 5. CO emissions 

0.85 0.90 0.95 1.00 1.05 1.10 1.15

Burner Zone SR

Corrected CO concentration (ppm)

Air only

x replacement

2x replacement

3x replacement

Average of all data
Impact of staging and oxygen injection on opacity

One factor that often limits the use of staged combustion systems is the stack opacity, 

which can increase to unacceptable levels when overfire air systems are used. Although 

the mechanisms leading to changes in opacity are complex, one potential explanation is 

that the increased carbon in the ash and residual soot can lead to degradation in ESP 

performance. Figure 8 illustrates how staging impacted stack opacity during testing with 

the bituminous coal. As noted on the plot these data do not include the opacity spikes 

typical during the sootblowing cycle. These data were removed in order to better focus 

on the effect of burner stoichiometric ratio and oxygen use on the stack opacity. As can 

be seen from the data going from an unstaged condition to a staged condition with air 

alone resulted in a slight increase in stack opacity. However, when small amounts of 

oxygen were used the stack opacity values were actually lower than the unstaged air case. 

Figure 9, where all the stoichiometric ratio data for a given oxygen replacement are 

combined, confirms that on average adding even small amounts of oxygen can 

significantly reduce stack opacity. 

Figure 6. Typical staged air flame, note 

standoff distance and lack of 

definition 

Figure 7. Typical staged flame with 

small O2 addition, note clear and well 

attached 

Burner Face 

Burner Face 
CONCLUSIONS 

Figure 9. Effect of oxygen replacement on opacity 

O2 Replacement

Opacity [%]

Figure 8. Effect of staging on opacity – bituminous coal 

0.70 0.80 0.90 1.00 1.10 1.20

Burner Zone SR

Opacity [%]

Air

Oxygen enhanced
The results of the demonstration project at James River Unit 3, and the single burner 

work that preceded it, demonstrated that oxygen enhanced combustion can substantially 

reduce NOx emissions from coal-fired power plants. The burner observations indicate 

that flame stability was dramatically improved with oxygen addition. The use of oxygen 

also reduced LOI and opacity as compared to the air- alone staging, with measured LOI 

being comparable or only slightly higher than the unstaged air-only condition. The 

measured opacity was actually lower in many cases with oxygen-enhanced staging than 

the original unstaged condition for the bituminous coal. Observations of the plant 

operation and the available data suggest that deep staging with oxygen didn’t lead to 

significant operational problems, such as slagging. Efficiency estimates suggest that 

staging without oxygen addition can lead to lower boiler efficiencies, while staging with 

oxygen results in little or no degradation in boiler efficiency as compared to the unstaged 

case. Finally, CFD models coupled with correlations based on the current understanding 

of waterwall wastage suggests that deep staging with air alone can result in significant 

increases in waterwall wastage. However, using even small amounts of oxygen under 

deeply staged conditions leads to corrosion rates comparable to those associated with 

mildly staged systems. Based on these conclusions, this project successfully 

demonstrated that oxygen enhanced combustion leads to significant reductions in NOx 

emissions without many of the problems typically associated with staged combustion 

systems. 

AKNOWLEDGEMENTS 

The authors wish to thank City Utilities and the personnel at the James River Power 

Station for their support and hospitality this demonstration project. The authors also 

wish to thank U.S. Department of Energy NETL and Praxair for providing funding for 

this development program. Finally, the authors would like to acknowledge the 

contributions of the following team members: 

E. Eddings, R. Okerlund – University of Utah 

M. Cremer, D. Wang, - Reaction Engineering International 

N. Nsakala, C. Maney, G. Richards, R. MacWhinnie - Alstom Power 

S. Johnson, Quinapoxet Solutions 

S. Plasynski, B. Lani, DoE NETL 

"This paper was prepared with the support of the U.S. Department of Energy, 

under Award No. DE-FC26-00NT40756. However, any opinions, findings, 

conclusions, or recommendations expressed herein are those of the author(s) 

and do not necessarily reflect the views of the DOE". 

REFERENCES 
1. Kobayashi, H., Bool, L.E., Riley, M, “NOx Reduction Using Coal Based Reburning”, 

U.S. Patent No. 6,202,949, March 2001 

2. Bool, L, and Kobayashi, H., “Reagent Delivery System”, U.S. Patent No. 6,254,379, 

July 3, 2001 

3. Bool, L., Kobayashi, H., Thompson, D., Eddings, E., Okerlund, R., Cremer, M, and 

Wang, D., “Oxygen for NOx Control – A Step Change Technology?”, 19th Annual 

International Pittsburgh Coal Conference, Pittsburgh, PA Sept 2002. 

4. Patents pending 

KEY WORDS 

NOx, oxygen, staging, low NOx burner