Combustion Modification Control of Nitrogen Oxides, USEPA, Office of

Research and Development, National Risk Management Research Laboratory,

Research Triangle Park, NC, EPA Report EPA/600/F-95-012, August 1995

Combustion Modification Control of

Nitrogen Oxides

Taking Research from Concept to Implementation 

An analysis of the impacts of EPA's NOx control technology research on the environment 

Introduction 

EPA's efforts in research and development of nitrogen oxide (NOx) control technologies by 

means of modifying the combustion process have played a major role in reducing stationary 

source NOx emissions by over 3 million tons (2.73 x 10^6 tonnes) annually, and have led to at 

least three low NOx burner technologies now commercially offered by major equipment vendors. 

These accomplishments have been made with an average total EPA investment of less than $4 

million per year over the past 20 years. 

NOx formed during the combustion process has been seen as a major air pollution problem since 

environmental issues first rose to the national forefront less than 30 years ago. Since its 

inception, EPA has played a major role in the development of NOx control technologies through 

research conducted and directed by the Agency. EPA has been involved in the full scope of 

technology development, from investigations of the fundamental science of NOx formation to the 

full-scale field demonstration of new control technologies. These successful efforts have 

contributed to a reduction of NOx emissions through the New Source Performance Standards 

(NSPS) and the 1990 Clean Air Act Amendments (CAAAs), both of which are based on 

combustion modification control of NOx.[1] Research at EPA's Air and Energy Engineering 

Research Laboratory (AEERL) continues to develop more effective and efficient control 

technologies and to transfer those technologies to markets in both the U.S. and abroad. 
Background 

NOx, which includes nitrogen oxide (NO) and nitrogen dioxide (NO2), is an important pollutant 

for several reasons. NOx is a primary contributor to ozone non-attainment, and it contributes to 

acid deposition, forest damage, and visibility problems in addition to direct adverse health 

impacts from NO2. NOx from stationary combustion sources is a major contribution to total 

emissions, and control of these emissions can result in significant improvements to the 

environment. However, control of NOx is a complex process affected by the nitrogen content of 

the fuel, the amount and distribution of air in the combustion process, temperature, unit load, and 

burner design, among other factors. Therefore, NOx emissions vary significantly with changes in 

temperature and air/fuel mixing, and are controlled primarily by modifying the basic combustion 

process, with the result that combustion modification NOx controls directly affect not only 

emissions, but often the efficiency and operability of the unit as well. The need to minimize 

emissions and maximize operating efficiency then makes NOx control a technically challenging 

endeavor that requires understanding of complex issues from combustion chemistry to plant 

operations, as well as an understanding of the economic issues related to plant fuel consumption 

and maintenance. 

In the late 1960's and early 1970's, when environmental issues became more visible, little was 

known regarding the mechanisms of NOx formation, particularly from the combustion of coal. 

The chemical and physical processes of pulverized coal combustion were not well understood, 

and the need to control the formation of NOx added an additional layer of complexity to the 

problem. During the same period, energy efficiency also became increasingly important, leading 

to a large effort aimed at developing a better understanding of how pulverized coal burns. As a 

part of this effort, EPA sought to determine the mechanisms that govern the formation of NOx 

during coal combustion as a basis for reducing NOx emissions from utility boilers. EPA's early 

efforts focused on the prevention of NOx through modification of the combustion process, since 

this approach held the promise of higher emissions reductions and greater economic efficiency 

than the use of flue gas treatment for NOx control. 

Research Accomplishments 

Fundamental Research 

One of the major accomplishments of EPA's NOx program came out of the Fundamental 

Combustion Research (FCR) program. Work conducted by EPA personnel at EPA's Air and 

Energy Engineering Research Laboratory (AEERL) led to the discovery of the role of "fuel 

nitrogen" (i.e., nitrogen bound in the fuel) during the combustion of oil, synfuels, and pulverized 

coal.[2] This work not only determined the mechanisms for conversion of fuel nitrogen to NOx, 

but it allowed for the development of low NOx technology based on the principle of preferential 

conversion to molecular nitrogen (N2) under fuel-rich combustion conditions. Based on the study 

of a wide range of U.S. and non-U.S. coals, it identified the need to maximize the volatilization 

of fuel nitrogen within the rich zone to enhance NOx control. 

This fundamental information made possible many of the subsequent combustion modification 

controls for NOx from coal-fired boilers. Additional fundamental information developed under 
this program advanced the state of the art of technology development; EPA fundamental research 

on oxidative pyrolysis of nitrogen compounds also identified nitrous oxide (N2O) as a major 

byproduct and hypothesized the N-C-O intermediate mechanism.[2] The importance of these 

developments was highlighted by a recent NOx program review, during which experts in the field 

of NOx control technology development noted that nearly all the advances in combustion 

modification NOx control technology during the late 1980's and early 1990's were made possible 

by EPA-sponsored FCR during the 1970's. Thus the investments made 20 years ago continue to 

pay dividends toward a cleaner atmosphere. 

Applied Research 

Applied research sponsored by EPA on combustion modification NOx control technology played 

a major role in the development of state of the art low NOx burner technology designs. These 

designs, and other combustion modification NOx controls, are based on creation of a fuel-rich 

primary zone to convert fuel nitrogen to N2 rather than NOx, followed by controlled air addition 

to burn out the balance of the fuel. 

The rich fireball concept for tangentially fired boilers is the basis for ABB-CE's Low NOx 

Concentric Firing System, the most commonly used low NOx combustion system offered today 

for tangentially fired boilers. The cover shows the conceptual design of the rich fireball and its 

implementation in a full-scale boiler.[2] In this concept, part of the combustion air is diverted 

toward the furnace walls to create an internal fuelrich central core. Two EPA/CE retrofit 

demonstrations at 400 and 180 MW have produced NOx levels of 0.41 and 0.35 lb/10^6 Btu 

(0.20 and 0.17 kg/kJ), respectively.[3,4] The Distributed Mixing Burner (DMB) for wall-fired 

boilers is shown in an artist's cutaway in Figure 1. In this concept, part of the combustion air is 

introduced through tertiary ports to create a fuel-rich flame zone. The concept can be adapted to 

a variety of burner designs; however, the burner shown is a generic design with a divided 

secondary air throat. The DMB is the result of an EPA development program that resulted in the 

design of a burner now commercially offered by the German firm L & C Steinmuller GmbH, 

which has achieved NOx levels of about 0.4 lb/10^6 Btu on a 700 MW retrofit boiler. The 

development program also provided a wealth of data on the design and operational behavior of 

low NOx burners for wall-fired boilers, forming much of the technical foundation for the 

evolution of today's advanced burners. EPA was also a major sponsor of the developmental 

efforts that led to the XCL burner (shown in Figure 2) now offered by Babcock & Wilcox as the 

heart of their most advanced low NOx combustion system.[5] This burner is based on the 

Babcock & Wilcox Dual Register Burner (DRB) with air velocities and fuel injector designs 

optimized as part of the Limestone Injection Multistage Burner (LIMB) demonstration at Ohio 

Edison Edgewater's 108 MW boiler. The designs were optimized to accommodate short firing 

depths and limited fan pressures. During the demonstration, the burner achieved an emission rate 

of 0.48 lb/10^6 Btu (0.23 kg/kJ).[5] 
Figure 1. Artist's cutaway view of the Distributed Mixing Burner (DMB). 

Figure 2. Schematic drawing of the Babcock & Wilcox XCL Burner.[5] A coal impeller may 

also be added in the primary burner barrel to accommodate short firing depths. 

EPA also played a significant role in the development of another NOx control technology, natural 

gas reburning. Reburning injects fuel, usually natural gas, downstream of the primary 

combustion zone both to provide a portion of the total heat input (usually less than 15%) and to 

destroy NO formed within the primary zone. The foremost advantage of reburning (or "fuel 

staging") is its ability to be used in applications where air staging or low NOx burners are not 

possible, such as in wet bottom boilers that cannot operate at the lower furnace temperatures 

often caused by staging without "freezing" the liquid slag formed during the combustion process. 

Reburning has recently been demonstrated in two full-scale utility boilers under EPA 

cosponsorship. The demonstration project at Ohio Edison's Niles plant provided long term 

operational data on the use of natural gas reburning on a 108 MW cyclone boiler. A 300 MW 

wet bottom wall-fired boiler in Ukraine (see Figure 3) was also the site of an AEERL 

demonstration of natural gas reburning. AEERL, with ABB-CE, provided the conceptual design 

for the system. The final engineering drawings were produced by the All Russian Heat 
Engineering Institute in Moscow, and the system was installed on a boiler at the Ladyzhinskaya 

power station south of Kiev in Ukraine. On both units, NOx reductions of up to 50% were 

common at full load conditions.[6,7] 

Figure 3. Conceptual design of the reburn system installed in Ukraine as part of a joint EPARussia-Ukraine project. 

Technology Transfer 

As an outgrowth of the NOx program, EPA has held, and continues to hold, a series of technical 

meetings to relay information to industry and other researchers. The first meeting that was 

national in scope was the Stationary Source Combustion Symposium, held in 1975. From this 

meeting, a biannual series of symposia have been sponsored by EPA. Starting with the 1980 

symposium, sponsorship has been joint with the Electric Power Research Institute (EPRI), and 

the NOx symposia have become the major NOx control technology forums worldwide. The ninth 

Joint Symposium on Stationary Combustion NOx Control was held in May 1993, and attracted 

over 500 attendees from government, industry, and the research community from the U.S. and 

six other countries. In addition to information on utility boiler NOx control, the symposia have 

presented information on fundamental NOx formation and destruction mechanisms, control of 

industrial sources, and regulatory impacts on cost and performance. In addition to the symposia, 

transfer of technical information has taken place through the publication of symposia 

proceedings and over 200 EPA technical reports and conference papers on topics including 

descriptions of fundamental NOx formation and destruction mechanisms, emissions from low 
NOx burners for residential furnaces, and combustion modification techniques for full-scale 

utility boilers. 

Program Impact 

Much of EPA's research and development effort on combustion modification NOx control 

focused on fundamental and pre-commercial science and technology development; the end result 

of EPA sponsored work is therefore indirect and difficult to quantify precisely. However, these 

efforts provided, at a minimum, critical developmental seeds for four commercially available low 

NOx burner systems: the Babcock & Wilcox XCL burner, with over 11,000 MW of burner 

capacity under contract; ABB-CE's Low NOx Concentric Firing System and the tangential low 

NOx combustion system offered by NEI, with 25,000 MW of planned or installed capacity; and 

Steinmuller's staged mixing burner, with over 25,000 MW of installed capacity overseas. At an 

average estimated cost of $20 per installed kilowatt, the value of these systems is over $1.2 

billion. 

The AEERL has conducted an active combustion modification control technology research and 

development program since 1975. The NSPS for NOx from utility boilers resulting in part from 

these efforts have yielded a reduction of NOx emissions of approximately 1 million tons per year 

from utility units alone since 1985. Over 250,000 MW of utility generating capacity will use 

some form of combustion modification NOx control to meet the Title IV NOx reduction 

provisions of the CAAAs. Prior to the passage of the CAAAs, the NSPS were the primary form 

of national NOx emissions regulations for both utility and industrial sources. The impacts of the 

NSPS are clearly seen in Figure 4 by the drop in the rate of increase of national NOx emissions 

following promulgation of the NSPS requirements in 1971 and 1977. The average national NOx 

emission factor from coal-fired utility boilers has been reduced from 0.97 lb/million Btu (0.46 

kg/kJ) in 1970 to approximately 0.83 lb/10^6 Btu (0.40 kg/kJ) in 1992, and will be lowered to 

below 0.50 lb/10^6 Btu (0.24 kg/kJ) after the year 2000 when the NOx provisions of the CAAAs 

take effect.[8-10] While some of the emissions reductions can be ascribed to increased efficiency 

and lower electricity consumption, these reductions would not have been possible without the 

regulations on new sources; these regulations, in turn, were made possible to a large degree 

through the efforts of EPA in the area of NOx control technology research and development. 
Figure 4. National emissions of NOx from utility and industrial sources, 1940-2010 (actual and 

projected).[8-10] 

Not only have the designs of new units incorporated the results of EPA's research and 

development efforts, but the emissions levels mandated by the CAAAs are a direct result of 

EPA's work to develop burner technologies which can be retrofitted to boilers built prior to the 

NSPS. Title IV of the CAAAs set a Phase I goal of an annual reduction in NOx emissions from 

utility boilers of 2 million tons for the purpose of minimizing acid deposition. The emission rates 

of 0.45 lb/10^6 Btu (0.22 kg/kJ) for tangentially fired boilers and 0.50 lb/10^6 Btu (0.24 kg/kJ) 

for wall-fired boilers were based to a large degree on the performance of EPA-sponsored retrofit 

control technology. Additional reductions are expected to be attained by the mandated controls 

associated with Phase II of Section 407 of the CAAAs. Annual costs associated with the Phase I 

controls have been estimated at less than $3 per electric customer, less than the cost of one meal 

at a typical fast-food restaurant. 

In addition to these reductions, Title I of the CAAAs requires control of NOx for the purpose of 

reducing ambient ozone levels in certain areas of the country. Emission standards for Title I are 

typically lower than those for Title IV; both of these programs are possible in large part because 

of the advances in fundamental combustion science and technology development sponsored or 

conducted by EPA. 

Current combustion modification NOx control research is focused on methods to enhance reburn 

technology. Existing reburn technology is capable of achieving 50% NOx reduction with 15 to 

18% of the total heat input in the natural gas reburn fuel. EPA is exploring ways to obtain higher 

levels of NOx control or to minimize the amount of reburn fuel needed to obtain current levels of 

NOx reduction. Potential methods being considered include pulsed combustion, controlled 

mixing of the reburn fuel with the furnace gases, water injection, ammonia injection, or 
alternative reburn fuels. Promising techniques may be tested on full-scale boilers in the U.S., 

Russia, or Ukraine. 

Future research will likely focus on application of combustion modification (CM) technology to 

industrial boilers, which account currently for 28% of stationary source NOx emissions. In 

addition, CM combined with Selective Catalytic Reduction (SCR) and Selective Non-Catalytic 

Reduction (SNCR) will be studied. Some flue gas treatment processes which reduce NOx can 

create N2O, a greenhouse and ozone-depleting gas. EPA's combustion NOx control research 

program is well positioned to help mitigate this problem. 

In conclusion, advances in fundamental science, application development, and full-scale 

technology application associated with NOx control have led to major decreases in the total 

emissions of NOx from stationary combustion sources, and EPA has played a major role in each 

of these areas. The NOx reduction emissions due to these advances continue to benefit the health 

and environment of the people of the U.S. and many other parts of the world. 

References 

1. Clean Air Act Amendments of 1990, Public Law 101-549, November 15, 1990. 

2. Pulverized Coal Combustion: Pollutant Formation and Control, 1970-1980. U.S. 

Environmental Protection Agency, Air and Energy Engineering Research Laboratory, 

Research Triangle Park, NC. Report EPA-600/8-90-049 (NTIS PB90-229253), May 

1990. 

3. "Field Evaluation of a Low-NOx Firing System for Tangentially Coal-Fired Utility 

Boilers," U.S. Environmental Protection Agency, Air and Energy Engineering Research 

Laboratory, Research Triangle Park, NC. Report EPA-600/7-85-018 (NTIS PB85-

201093), May 1985. 

4. "Performance Results from the Tangentially Fired LIMB Demonstration Program at 

Yorktown Unit No. 2," J.P. Clark, M.R. Gogineni, R.W. Koucky, A.F. Kwasnik, C.H. 

Francis, and D.G. Lachapelle, presented at the 1993 SO2 Control Symposium, Boston, 

MA, August 23-27, 1993. 

5. "Demonstration of Sorbent Injection Technology on a Wall-Fired Utility Boiler 

(Edgewater LIMB Demonstraton)," U.S. Environmental Protection Agency, Air and 

Energy Engineering Research Laboratory, Research Triangle Park, NC. Report EPA600/R-92-115 (NTIS PB92-201136), June 1992. 

6. "Long-Term NOx Emissions Results with Natural Gas Reburning on a Coal-Fired 

Cyclone Boiler," R. Borio, R. Lewis, D. Steen, and A. Lookman, Proceedings: 1993 Joint 

Symposium on Stationary Source Combustion NOx Controls, EPRI TR-103265, 1993. 

7. "Three-Stage Combustion (Reburning) Test Results from a 300-MWe Boiler in the 

Ukraine," R.C. LaFlesh, R.D. Lewis, R.E. Hall, V.R. Kotler, and Y.M. Mospan, 

Proceedings: 1993 Joint Symposium on Stationary Source Combustion NOx Controls, 

EPRI TR-103265, 1993. 

8. State Energy Data Report: Consumption Estimates, 1960-1990. U.S. Department of 

Energy, Energy Information Administration, Washington, DC. Report DOE/EIA0214(90), May 1992. 
9. Electric Power Annual: 1992. U.S. Department of Energy, Energy Information 

Administration, Washington, DC. Report DOE/EIA-0348(92), January 1994. 

10. National Air Pollutant Emission Trends: 1900-1992. U.S. Environmental Protection 

Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC. 

Report EPA-454/R-93-032 (NTIS PB94-152097), October 1993.