United States
Office of
Research and
Washington DC 20460
National Risk
Research Laboratory
Cincinnati OH 45268
Air Pollution
Prevention and Control
Research Triangle Park NC 27711
                    August 1995
ŁEPA   Combustion Modification
         Control of Nitrogen Oxides

         Taking  Research from
         Concept to Implementation


                        Improved Technology for Environmental Protection
 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 106 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 environmen-
 tal  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 Amend-
 ments (CAAAs), both of which are based on combustion
 modification control of NOx.1 Research at EPA's Air
 Pollution Prevention and Control Division (APPCD)
 [formerly the Air and Energy Engineering Research
 Laboratory (AEERL)] continues to develop more effec-
 tive and efficient control technologies and to transfer
 those technologies to markets in both the U.S. and

 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 operabil-
 ity 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 understand-
 ing of the economic issues related to plant fuel con-
 sumption 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 pro-
 cesses 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 reduc-
 tions 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 NO
 program came out of the Fundamental Combustion
 Research (FCR) program. Work conducted by EPA
 personnel at the APPCD 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 funda-
 mental research on oxidative pyrolysis of nitrogen
compounds also identified nitrous oxide (N2O) as a

                              Printed on Recycled Paper

                       Improved Technology for Environmental Protection
major byproduct and hypothesized the N-C-O intermedi-
ate 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 divi-
dends 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 combus-
tion 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 fuel-rich central
core. Two  EPA/CE retrofit demonstrations at 400 and
180 MW have produced NOx levels of 0.41 and 0.35 Ib/
106 Btu (0.20 and 0.17  kg/kJ), respectively.34 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 second-
ary air throat. The DMB is the result of an  EPA develop-
ment 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/106 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 burn-
ers. EPA was also a major sponsor of the developmen-
tal 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
           Air Port
Figure 1.  Artist's cutaway view of the Distributed Mixing
         Burner (DMB).
optimized as part of the Limestone Injection Multistage
Burner (LIMB) demonstration at Ohio Edison
Edgewater's 108 MW boiler. The designs were opti-
mized to accommodate short firing depths and limited
fan pressures. During the demonstration, the burner
achieved an emission rate of 0.48 lb/106 Btu (0.23 kg/

EPA also played a significant role in the development of
another NOx control technology, natural gas reburning.
Reburning injects fuel, usually natural gas,  downstream
                               Outer Secondary Air
Primary Air/Coal
Inner Secondary Air
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

                       Improved Technology for Environmental Protection
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
abiliy 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 tem-
peratures 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 APPCD demonstration
of natural gas reburning. APPCD, with ABB-CE, pro-
vided 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

Figure 3.
Conceptual design of the reburn system installed in
Ukraine as part of a joint EPA-Russia-Ukraine
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 mecha-
nisms, 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

Program Impact

Much of EPA's research and development effort on
combustion modification NOx control focused on funda-
mental 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 capac-
ity;  and Steinmuller's staged mixing burner, with over
25,000 MW of installed capacity overseas. At an aver-
age estimated cost of $20 per installed kilowatt, the
value of these systems is over $1.2 billion.

The APPCD has conducted an active combustion
modification control technology research and develop-
ment 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 promulga-
tion of the NSPS requirements in 1971 and 1977. The

                       Improved Technology for Environmental Protection
           1950   1960
1970  1980
1990  2000 2010
Figure 4.  National emissions of NOx from utility and industrial
         sources, 1940-2010 (actual and projected).8'10
average national NOx emission factor from coal-fired
utility boilers has been reduced from 0.97 Ib/million Btu
(0.46 kg/kJ) in 1970 to approximately 0.83 lb/106 Btu
(0.40 kg/kJ) in 1992, and will be lowered to below 0.50
lb/106 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.

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 technolo-
gies 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 utiliy boilers of 2
million tons for the purpose of minimizing acid deposi-
tion. The emission rates of 0.45 lb/106 Btu (0.22 kg/kJ)
for tangentially fired boilers and 0.50 lb/106 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 esti-
mated 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.


 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

                      Improved Technology for Environmental Protection
      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
      Demonstration)," U.S. Environmental Protection
      Agency, Air and Energy Engineering Research
      Laboratory, Research Triangle Park, NC. Report
      EPA-600/R-92-115 (NTIS PB92-201136),June
 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 Station-
      ary Source Combustion NOx Controls, EPRI TR-
 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 Esti-
      mates, 1960-1990. U.S. Department of Energy,
      Energy Information Administration, Washington,
      DC. Report DOE/EIA-0214(90), May 1992.
 9.    Electric Power Annual:  1992. U.S. Department of
      Energy, Energy Information Administration, Wash-
      ington, DC. Report DOE/EIA-0348(92) January
 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.


C. A. Miller
R.E. Hall
U.S. Environmental Protection Agency
Combustion Research Branch (MD-65)
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
(919) 541-0554 (FAX)
   Internet:  amiller®inferno.rtpnc.epa.gov
           bob_hall@qmip. rtpnc.epa.gov