Public Health Service
                   Environmental Health Service


                   Public Health Service
                Environmental Health Service
           National Air Pollution Control Administration
                    Washington, D.C.
                       March 1970

National Air Pollution Control Administration Publication i\o. AP-67
     For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402 - Price $1

   Throughout the development of Federal air
pollution legislation, the Congress has consist-
ently found that the States and local govern-
ments have the  primary responsibility  for
preventing and controlling air pollution  at its
source. Further, the Congress has consistently
declared  that it  is the responsibility of the
Federal government to provide technical and
financial assistance to State and local govern-
ments so that they  can  undertake these re-
   These  principles were reiterated  in the
1967 amendments to the Clean  Air Act. A
key element of that Act directs the Secretary
of Health, Education, and Welfare to collect
and make available information on all aspects
of air pollution  and its  control. Under the
Act,  the issuance  of control  techniques  in-
formation is a vital step in a program designed
to assist the  States in taking  responsible
technological,  social, and political action to
protect the public from the adverse effects of
air pollution.
   Briefly, the  Act calls  for  the Secretary of
Health, Education, and Welfare to define the
broad atmospheric areas  of the  Nation in
which climate, meteorology, and topography,
all of which influence the capacity of air to
dilute and disperse pollution, are generally ho-
   Further,  the Act requires the Secretary to
define  those  geographical  regions  in  the
country  where air pollution is a  problem—
whether interstate  or  intrastate.  These  air
quality  control regions are designated on the
basis  of meteorological, social, and  political
factors  which suggest that a group  of  com-
munities should be treated as  a unit for set-
ting limitations on concentrations of atmos-
pheric pollutants. Concurrently, the Secretary
is  required  to issue  air  quality criteria for
those pollutants he believes  may be  harmful
 to  health  or welfare, and to publish related
 information on the techniques which can be
 employed  to control the sources of those pol-
   Once  these steps have  been taken for any
 region, and for any pollutant or combination
 of  pollutants,  then  the  State or States re-
 sponsible  for the  designated  region  are on
 notice to develop ambient air quality stand-
 ards applicable  to  the region for the pollu-
 tants involved, and to develop plans of action
 for meeting the standards.
   The Department of Health, Education, and
 Welfare  will review,  evaluate, and  approve
 these standards and plans and, once they are
 approved,  the States  will be  expected to take
 action to  control  pollution  sources  in  the
 manner outlined in  their plans.
   At  the  direction  of the Secretary,  the
 National  Air  Pollution  Control  Admin-
 istration has established appropriate programs
 to  carry  out  the  several Federal  responsi-
 bilities specified in the legislation.
   Control  Techniques for  Nitrogen  Oxides
from Stationary Sources is one of a series of
 documents to be produced under the program
 established to carry out the responsibility for
 developing and  distributing  control technol-
 ogy information. Previously,  on February 11,
 1969,  control  technique  information   was
 published  for  sulfur  oxides and  particulate
   In accordance with the Clean  Air Act, a
 National Air  Pollution Control  Techniques
 Advisory Committee was established, having a
 membership  broadly  representative  of  in-
 dustry, universities, and all levels of govern-
 ment.  The committee, whose members are
 listed  following  this discussion,  provided
 invaluable  advice in identifying the  best  pos-
 sible methods  for  controlling the pollution

sources, assisted in determining the costs in-
volved, and gave major assistance in drafting
this document.
   As further required by the Act, appropriate
Federal departments and agencies, also listed
on the following pages, were consulted prior
to issuance of this document. A Federal con-
sultation  committee,  comprising  members
designated by the heads  of 17 departments
and  agencies, reviewed the document,  and
met  with  staff personnel of the National Air
Pollution  Control Administration to discuss
its contents.
   During  1967, at the initiation of the Secre-
tary  of Health, Education,  and Welfare, sev-
eral  government-industry  task  groups were
formed to explore mutual problems relating
to air  pollution  control. One of these, a task
group  on  control technology  research  and
development, looked into ways that industry
representatives could participate in the review
of the control techniques reports. According-
ly, several industrial representatives, listed on
the following pages,  reviewed this document
and  provided  helpful  comments  and sug-
gestions.  In addition,  certain  consultants to
the National Air Pollution Control Admin-
istration also revised  and assisted in preparing
portions of  this document. These also are
listed on the following pages.
   The   Administration  is  pleased  to  ac-
knowledge efforts of each of the persons spe-
cifically  named, as well as those of the many
not so listed  who contributed to the publica-
tion of this volume. In  the last analysis, how-
ever,  the  National  Air  Pollution Control
Administration is responsible for its content.
   The control of air pollutant emissions is a
complex problem because of the variety of
sources and source characteristics.  Technical
factors frequently make necessary the use of
different  control  procedures  for different
types of sources. Many techniques are still in
the development stage, and prudent control
strategy  may  call for the  use of  interim
methods until these techniques are perfected.
Thus, we can expect that we will continue to
improve, refine,  and periodically revise the
control techniques information so that it will
continue  to  reflect  the  most  up-to-date
knowledge  available.
                         John T. Middleton,
        Commissioner, National Air Pollution
                    Control Administration.


                            Mr. Robert L. Harris, Jr., Director
                            Bureau of Abatement and Control
                      National Air Pollution Control Administration

Mr. Herbert J. Dunsmore
Assistant to Administrative
  Vice President of Engineering
U. S. Steel Corporation
Pittsburgh, Pennsylvania

Mr. John L. Gilliland
Technical Director
Ideal Cement Company
Denver, Colorado

Dr. August T. Rossano
Department of Civil Engineering
Air Resource Program
University of Washington
Seattle, Washington

Mr. Jack A. Simon
Principal Geologist
Illinois State Geological Survey
Urbana, Illinois

Mr. Victor H. Sussman, Director
Division of Air Pollution Control
Pennsylvania Department of Health
Harrisburg, Pennsylvania

Dr. Harry J. White, Head
Department of Applied Science
Portland State College
Portland, Oregon

Mr. Robert L. Chass
Chief Deputy Air Pollution
  Control Officer
Los Angeles County Air Pollution
  Control District
Los Angeles, California

Mr. C. G. Cortelyou
Coordinator of Air & Water
Mobil Oil Corporation
New York, New York

Mr. Charles M. Heinen
Assistant Chief Engineer
Chemical Engineering Division
Chrysler Corporation
Highland Park, Michigan

Mr. William Monroe
Chief, Air Pollution Control
Division of Clean Air & Water
State Department of Health
Trenton, New Jersey

Mr. William W. Moore
Vice President, and Manager of
  Air Pollution Control Division
Research-Cottrell, Inc.
Bound Brook, New Jersey

 Department of Agriculture
 Dr. Theodore C. Byerly
 Assistant Director of Science
   and Education

 Department of Commerce
 Mr. Paul T. O'Day
 Staff Assistant to the Secretary

 Department of Defense
 Mr. Thomas R. Casberg
 Office of the Deputy Assistant Secretary
   (Properties and Installations)

 Department of Housing & Urban Development
 Mr. Samuel C. Jackson
 Assistant Secretary for Metropolitan

 Department of the Interior
 Mr. Harry Perry
 Mineral  Resources Research Advisor

 Department of Justice
 Mr. Walter Kiechel, Jr.
 Assistant Chief
 General  Litigation Section
 Land and Natural Resources Division

 Department of Labor
 Dr. Leonard R. Linsenmayer
 Deputy Director
 Bureau of Labor Standards

Post Office Department
Mr. W. Norman Meyers
Chief, Utilities Division
Bureau of Research & Engineering

Department of Transportation
Mr. William H. Close
Assistant Director for Environmental
Office of Noise Abatement
Department of the Treasury
Mr. Gerard M. Brannon
Office of Tax Analysis

Atomic Energy Commission
Dr. Martin B. Biles
Division of Operational Safety

Federal Power Commission
Mr. F. Stewart Brown
Bureau of Power

General Services Administration
Mr. Thomas E.  Crocker
Repair and Improvement Division
Public Buildings Service

National Aeronautics and Space
Major General R. H. Curtin, USAF
Director of Facilities
National Science Foundation
Dr. Eugene W. Bierly
Program Director for Meteorology
Division of Environmental Sciences

Tennessee Valley Authority
Dr. F. E. Gartrell
Assistant Director of Health

Veterans Administration
Mr. Gerald M. Hollander
Director of Architecture and
Office of Construction

Dr. William G. Agnew, Head
Fuels & Lubricants Department
Research Laboratories
General Motors Corporation
Warren, Michigan

Dr. A. D. Brandt
Manager, Environmental Quality Control
Bethlehem Steel Corporation
Bethlehem, Pennsylvania

Mr. John M. Depp, Director
Central Engineering Department
Monsanto Company
St. Louis, Missouri

Mr. Stewart S. Fritts
Operations Consultant
Lone Star Cement Corporation
New York, New York

Mr. J. C. Hamilton
Vice President for Administration
Director of Engineering
Owens-Illinois, Inc.
Toledo, Ohio

Mr. Richard B. Hampson
Manager, Technical Services
Freeman Coal Mining Corporation
Chicago, Illinois

Mr. C. William Hardell
Coordinator, Eastern Region
Air & Water Conservation
Atlantic Richfield Company
New York, New York

Mr. James F. Jonakin
Manager, Air Pollution Control Systems
Combustion Engineering, Inc.
Windsor, Connecticut
Mr. James R. Jones, Director
Coal Utilization Services
Peabody Coal Company
St. Louis, Missouri

Mr. John F. Knudsen
MMD-ED Industrial Hygiene Engineer
Kennecott Copper Corporation
Salt Lake City, Utah

Mr. Edward Largent
Manager, Environmental & Industrial
  Hygiene, Medical Dept.
Reynolds Metals Company
Richmond, Virginia

Mr. Walter Lloyd
Director, Coal & Ore Services Dept.
Pennsylvania Railroad Company
Philadelphia, Pennsylvania

Mr. Michael Lorenzo
General Manager
Environmental Systems Department
Westinghouse Electric Corporation
Washington, D. C.

Mr. J. F. McLaughlin
Executive Assistant
Union Electric Company
St.  Louis, Missouri

Mr. Robert Morrison
President, Marquette Cement Manufac-
  turing Company
Chicago, Illinois

Dr.  Clarence A. Neilson
Director of Laboratories & Manager
  Of Technical Services
Laboratory Refining Dept.

 Continental Oil Company
 Ponca City, Oklahoma

 Mr. James L. Parsons
 Consultant Manager
 Environmental Control
 E.I. duPont de Nemours & Co., Inc.
 Wilmington, Delaware

 Mr. James H. Rook
 Director of Environmental Control
 American Cyanamid Company
 Wayne, New Jersey

 Mr. K. J. Schatzlein
 Chemical Engineer
 Lehigh Portland Cement Company
 Allentown, Pennsylvania

 Mr. T. W. Schroeder
 Manager of Power Supply
 Illinois Power Company
 Decatur, Illinois

Mr. Robert W. Scott
Coordinator for Conservation Technology
Esso Research & Engineering Company
Linden, New Jersey

Mr. Bruce H. Simpson
Executive Engineer, Emissions Planning
Ford Motor Company
Dearborn, Michigan

Mr. Samuel H. Thomas
Director of Environmental Control
Owens-Corning Fiberglas Corporation
Toledo, Ohio

Mr. A. J.  Von Frank
Director, Air & Water Pollution Control
Allied Chemical Corporation
New York, New York

Mr. Earl Wilson, Jr.
Manager, Industrial Gas Cleaning Dept.
Koppers Company, Inc.
Baltimore, Maryland

Mr. Wayne Wingert
Environmental Improvement Engineer
The Detroit Edison Company
Detroit, Michigan

                            TABLE OF CONTENTS
Section                                                                      Page
SUMMARY	     xiv
1.   INTRODUCTION  	     1-1
2.   DEFINITIONS	     2-1
     2.1   REFERENCES FOR SECTION 2	     2-2
     3.1   REFERENCES FOR SECTION 3	     3-5
     4.1   COMBUSTION MODIFICATIONS  	     4-1
           4.1.1 Factors Affecting NOX Emissions	     4.3
         Fuel Type and Composition  	     4.4
         Heat Release and Transfer Rates  	     4.4
           4.1.2 Modifications of Operating Conditions	     4.5
         Low Excess Air Combustion  	     4.5
         Flue Gas Recirculation	     4.5
         Steam  and Water Injection  	     4.7
           4.1.3 Design Modifications	     4-8
         Burner Configuration	     4-8
         Burner Location and Spacing	     4_8
         Fluidized Bed Combustion	     4.9
         Small Combustion Equipment   	     4-11
     4.2   FLUE GAS CLEANING	     4-11
           4.2.1 Control by Limestone Wet-Scrubbing Process  	     4-11
           4.2.2 Potential Control Processes	     4-13
         Reinluft Char Process	     4-13
         Control by Tyco Laboratories' Modified Lead Chamber
                        Process  	     4-13
     4.3   REFERENCES FOR SECTION 4	     4-14
           5.1.1 Emissions	     5-1
           5.1.2 Control Techniques	     5-1
         Conversion to Lower NOX Producing Fuel	     5-1
         Fuel Additives  	     5-2
         Combustion Control  	     5-2
           5.1.3 Costs  	     5.3
         Limitations with Large Coal-Fired Boilers  	     5.4
         Packaged Boilers  	     5.4
     5.2   STATIONARY ENGINES	     5-5


0   .                                                                             Page

           5.2.1  Piston Engines	     5-5
         Emissions	     5-6
         Control Techniques	     5-6
           5.2.2  Turbine Engines	      5-9
         Emissions	     5-9
         Control Techniques	     5-9
           5.2.3  Costs  	     5'10
         Diesel Engines   	     5-10
         Gas Engines	     5-11
      5.3   REFERENCES FOR SECTION 5	     5-11
           6.1.1  Emissions	     6-2
           6.1.2  Control Techniques	     6-2
           6.2.1  Emissions	     6-3
           6.2.2  Control Techniques	     6-4
         Waste Disposal  	     6-4
         Forest Wildfires	     6-5
         Controlled Vegetation Burning	     6-5
         Coal Refuse Fires	     6-6
         Structural Fires	     6-6
           6.2.3  Costs of Control   	     6-6
      6.3   REFERENCES FOR SECTION 6	     6-6
      7.1   NITRIC ACID MANUFACTURE	     7-2
           7.1.1  Emissions	     7.4
           7.1.2  Control Techniques	     7-5
           7.1.3  Costs for Control	     7.9
      7.2   NITRIC ACID USES   	     7-11
           7.2.1  Ammonium Nitrate Manufacture	     7-12
         Emissions	     7-12
           7.2.2  Organic Oxidations	     7-12
         Emissions	     7-13
       Control Techniques	     7-13
       Costs of Control  	     7-14
           7.2.3 Organic Nitrations   	     7-14
        Emissions from Nitration	     7-20
       Control Techniques	     7-20
       Costs of Controls	     7-21
           7.2.4 Explosives: Manufacture  and Use	     7-21
        Emissions	     7-23
           7.2.5 Other Uses of Nitric Acid	     7-24
        Fertilizer  	     7_24
       Metals Pickling   	     7_25
           7.3.1 Emissions	        7-26

Section                                                                      Page

           7.3.2 Control Techniques	    7-31
           7.3.3 Costs of Control  	    7-32
           7.4.1 Furnaces Related to Hot Metal or Pig Iron Production	    7-32
        Blast Furnace	    7-32
        Blast Furnace Stove	    7-33
           7.4.2 Furnaces Related to Steel Production   	    7-34
        Open Hearth Furnace	    7-34
        Coking Ovens	    7-34
        Miscellaneous Furnaces	    7-35
           7.4.3 Emissions	    7-36
           7.4.4 Costs   	    7-37
           7.5.1 Emissions	    7-38
           7.5.2 Control Techniques	    7-39
     7.6   GLASS MANUFACTURE	    7-39
           7.6.1 Emissions	    7-41
           7.6.2 Control Techniques	    7-41
           7.7.1 Process Description	    7-42
        Refractory Fibers	    7-42
        Perlite Expanding Furnaces	    7-42
        Baking and Drying Ovens	    7-42
        Spray Dryers  	    7-42
        Welding	    7-42
        Electrostatic Precipitators   	    7-43
        High-Level Exposure in Agriculture	    7-43
           7.7.2 Control Techniques	    7-43
     7.8   REFERENCES FOR SECTION 7	    7-44
     8.2   REFERENCES FOR SECTION 8	    8-4
           9.1.1 Oxygen Enrichment	    9-1
           9.1.2 Fluid Bed Combustion  	    9-1
           9.1.3 Centralization of Energy Source	    9-1
     9.2   FLUE GAS REMOVAL	    9-2
           9.2.1 Improved Aqueous Scrubbing   	    9-2
     9.3   REFERENCES FOR SECTION 9	    9-2

                              LIST OF TABLES
3-1.     Time for NO Formation in a Gas Containing 75 Percent Nitrogen and 3 Per-
        cent Oxygen	    3-1
3-2.     Estimates of Nitrogen Oxide Emissions in the United States, by Source, 1968.    3-2

 Table                                                                             Page

 3-3.     Comparison of Coal, Oil, and Gas on Equivalent Btu Basis	     3-4
 4-1.     Calculated Equilibrium Concentration of Nitrogen Oxides	     4-2
 4-2.     Typical Compositions and Solubility of Stack Gases	     4-12
 5-1.     Estimated Control Costs and Reduction  of NOX Emissions, for a 1,000-
          Megawatt Boiler Used 6,120 Hours a Year, by Selected Methods and by Ton.     «
 5-2.      Stationary Reciprocating Piston Engines in United States, 1967	       "
 5-3.      Estimated Stationary Engine Distribution in United States, 1969	
 5-4.      Summary of Source Sampling Tests for Two Dual-Fuel (Gas and Diesel)
 5-5.      Effect of Fuel Type on Emissions of NOX from a Stationary Diesel Engine.  .     5-8
 5-6.      Effect of Air/Fuel Ratio on NOX Production in Spark-Ignited Stationary Gas
          Engines	     5-8
 5-7.      Effect of Air/Fuel Ratio on NOX Formation in an Experimental Combustor
          Rig	     5-10
 5-8.      Oxides of Nitrogen Control Costs for Stationary Gas Engines	     5-11
 6-1.      Fuel Consumption for Space Heating and Cooking, 1966	     6-1
 6-2.      Gas Consumption for Domestic Heating in United States	     6-1
 6-3.      Estimated National Emissions from Incineration and Other Burning, 1968.  .     64
 7-1.      Estimates of Strong Acid Requirements and Oxides of Nitrogen Emissions
          from Concentration Operations	     7-5
 7-2.      Estimated Costs of Catalytic Reduction for  Three Control Systems	     7-10
 7-3.      Estimated Nitric Acid Consumption in United States, 1967	     7-11
 7-4.      Estimated Acid Requirements  and Oxides of Nitrogen Emissions from Nitric
          Acid Oxidation of Organic Compounds, 1967	     7-14
 7-5.      Estimated Oxides of Nitrogen Emissions from Organic Nitrations, 1967.   .  .     7-20
 7-6.      Ingredients in U.S. Industrial Explosives, 1967	     7-21
 7-7.      Total United  States Nitric  Acid Requirements and  Oxides of Nitrogen
          Emissions from the Manufacture of Industrial Explosives, 1967	     7-23
 7-8.      Oxides of Nitrogen in Products  of Explosions	     7-24
 7-9.      NOX Emissions from Petroleum and Natural Gas Operations, 1967	     7'29
 7-10.     Nitrogen  Oxides Emissions from Petroleum and Natural Gas Operations in
          United  States,1967	     7.3!
 7-11.     Investment Cost Estimates for Refinery Furnaces in Two Sizes, at a Standard
          Location, with Oxides  of Nitrogen Control by  Flue Gas  Recirculation,
          April-June 1969   	     7.33
 7-12.     Fuel Consumed by Cement Industry in United States, 1967	     7-37
 7-13.     Estimated Oxides of Nitrogen Emissions from  Kilns in United States, 1967.  .     7-38
 8-1.      Emission  Factors  for Nitrogen Oxides During  Combustion  of Fuels  and
          Other Materials	     3.3

                                 LIST  OF FIGURES
Figure                                                                             Page
 3-1.      Total Estimated  Oxides of Nitrogen Emissions from Mobile and Stationary
          Sources in United States (20,669,000 tons),  by Fuel Type and Tonnage, 1968.    3-3
 3-2.      Total Estimated Oxides of Nitrogen Emissions from Stationary Installations
          in United States (9,600,000 tons), by Type of Use and Tonnage, 1968.  ...     3.4

Figure                                                                               Page

4-1.      Calculated Nitric  Oxide Concentration  from Oil- and  Gas-Firing,  0.5 sec.
         Residence Time Excess Air	      4-3
4-2.      Effect of Heat Release of Oxides of Nitrogen Emissions, by Type of Firing  .      4-4
4-3.      Effect of Excess of Air on Oxides of Nitrogen Emissions  from Oil-Fired
         Boilers	      4-5
4-4.      Oxides of Nitrogen Emissions from Oil-Fired Boilers at Low Excess Air  ...      4-6
4-5.      Combined Effect  of Flue Gas Recirculation and Excess Air on  Oxides  of
         Nitrogen Emissions from an Oil-Fired Domestic Heating Furnace	      4-7
4-6.      Multi-Fuel Burner for Gas (Spud-Type Burner),  Oil (Circular Register), and
         Coal (Circular Register)	      4-9
4-7.      Gas-Ring Type Burner	      4-10
4-8.      Limestone Injection Wet-Scrubbing Process	      4-13
4-9.      Flow Diagram for Tyco Process	      4-14
7-1.      Flow Diagram of  a Typical 120-Ton-per-Day Nitric Acid Plant  Utilizing the
         Pressure Process	      7-3
7-2.      Nitric Acid Concentrating Unit	      7-4
7-3.      Adipic Acid Synthesis	      7-12
7-4.      Terephthalic Acid Synthesis	      7-13
7-5.      Batch Process for  the Manufacture of Nitroglycerin	      7-16
7-6.      Schmid-Meissner Continuous-Nitration Plant   	      7-17
7-7.      Biazzi Continuous Nitration Plant	      7-18
7-8.      Recovery of Spent Acid	      7-19
7-9.      TNT Manufacturing Diagram	      7-22
7-10.    Composite Processing Plan for a Modern Refinery	      7-27
7-11.    Fluidized Bed Catalytic Cracking Unit (FCC)	      7-28
7-12.     Oxides of Nitrogen Emissions from Refinery Furnace	      7-30
7-13.     Variation in  Oxides of Nitrogen  Emission Rates from Stationary  Internal
          Combustion Figures	      7-31
7-14.     Regenerative Side Port Glass-Melting Furnace	      7-40
8-1.       Estimation of  Average  Unit Oxides of Nitrogen  Emissions from  Similar
          Combustion Equipment  	      8-2

   In  this  document,  the term  "nitrogen
 oxides" or "NOX" refers to either  or both of
 two gaseous oxides  of nitrogen, nitric oxide
 (NO) and nitrogen dioxide (NO2).  These sub-
 stances are important in air pollution control
 because they are  involved in  photochemical
 reactions in  the atmosphere and because, by
 themselves, they have undesirable physiologi-
 cal effects.
 Chemical Equilibrium
   Under  proper  conditions,  nitrogen  and
 oxygen tend to combine in accordance  with
 the following equation:

             N2 + O2 t; 2 NO

 The equilibrium concentration of  NO varies
 with  temperature;  it  is  negligible  below
 1,000°  F but quite significant above 3,000°
 F. In addition, it is influenced  by gas compo-
 sition;  at a given  temperature, for example,
 the equilibrium concentration of NO in air
 exceeds that of NO in a flue gas of 3 percent
 oxygen  content by a factor of approximately
  Nitric oxide  tends to react with  oxygen as

           NO+ 1/2 O2 ^ NO2

This equation implies the coexistence of NO
and NO2. Calculated equilibria indicate  that
the stability of NO2 decreases with increasing
temperature.  Nevertheless,  from an equilibri-
um standpoint, the absolute concentration of
NO2 increases  with  temperature  while the
ratio of its  concentration to that  of NO
decreases with increasing temperature.
  Chemical equilibria  depend  only on the
initial  and  final states  and not at all  on
reaction mechanisms or intermediate reaction
steps. Equilibrium concentrations are obtained
after  the  lapse  of sufficient reaction  time;
therefore, they  are not necessarily observed
experimentally. Because of the simplicity of
the molecules involved in the foregoing equa-
tions, their thermodynamic properties are ac-
curately known  and equilibrium calculations
are  made easily.  Such calculations serve  as an
estimate for emission quantities and as a guide
in equipment design.

Rates of Formation
  Rates of formation  can be calculated by
kinetic  equations  that depend heavily on
experimental measurements. The  products
obtained depend  in  large measure on the
relative speeds of the reactions that actually
occur.  The  rate  of oxidation of nitrogen  to
NO is highly temperature-dependent; it is very
slow at 500°  F, but fast at 4,000° F. The
underlying  reason is that a high level of en-
ergy is  needed  to  break the N-N  bond  of
molecular nitrogen so that oxygen can react.
Conversely, a smaller but still relatively great
amount of  energy is needed to break the N-0
bond to permit decomposition of nitric oxide
into its elements. This means that high  tem-
peratures  are  required to  form NO;  once
formed, it  resists  any breakdown  into its
elements.   Breakdown becomes  more  and
more  unlikely as temperature decreases, be-
cause  the energy available for thermal break-
down diminishes rapidly with decreasing tem-
perature. Thus, an initially high temperature
followed by quick cooling, even to a  relatively
high  temperature  level,  produces   large
amounts of NO.
  At  temperatures above 2,000°F,  both NO
and NO2 are formed, but the amount of NO2

is usually well under 0.5 percent of the total
NOX.  The  oxidation of  NO to  NC>2  by
oxygen, however, is peculiar in that its rate of
formation decreases with increasing tempera-
ture.  This is one of the few known reactions
that  exhibit such a decrease. The  resultant
slow  oxidation rate at high temperatures ac-
counts in part  for the negligible amounts of
NO2  frequently found  in hot combustion
gases. Another  characteristic of the oxidation
of NO  to NO2 by oxygen only, is the fact
that the rate varies with the square of the NO
concentration.  The rate  of oxidation of NO
by oxygen in air falls off  rapidly, therefore,
with dilution of the NO. A long period of time
may  be required to oxidize trace quantities of
NO  by this mechanism, but  photochemical
reaction in sunlight  accomplishes  such ox-
idation in a much shorter time.
   As in all chemical  reactions, the rates of
formation and  decomposition for NO and
NO2  can be hastened by means of catalysts.
Attainable theoretical  equilibrium concentra-
tions are  not changed by catalysts; only the
time  of attainment is changed. With or with-
out   a  catalyst, equilibrium  concentrations
may  be approached when either a high or low
NOX  concentration is present.

   Mobile sources, the largest  single source
category, contribute over 40 percent of all the
man-made NOX  emitted in  the United States.
Current knowledge on methods of control is
covered in detail in AP-66, Control Techniques
for   Carbon  Monoxide,  Nitrogen  Oxide,
and   Hydrocarbon Emissions  from Mobile
Sources.  The next largest  source is electric
power generation, which is responsible for
nearly 20 percent of all man-made  NOX.
About 40 percent of NOX  emitted  from
stationary installations is attributed to electric-
generating power plants.
   About  1  percent  of the total man-made
NOX emitted to  the ambient air of the United
States is formed by chemical sources, mainly
related to the  manufacture and use of nitric
acid.  Concentrations  from these sources are,
however,  usually  much  greater than  those
from  noncombustion souices, and, therefore,
often give rise to a highly visible, brown-red

Combustion Modifications
  Two-stage combustion in oil- and gas-fired
boilers  has  reduced  NOX  emissions  from
power plant  boilers by  30 to  50 percent.
Low-excess-air operation has  reduced  NOX
emissions from oil- and gas-fired power plant
boilers by 30 to 60 percent, depending upon
the percentage of excess air, the design of the
boiler, and the type of firing. By changing the
firing of power plant boilers from front-wall,
or opposed, firing to tangential firing, NOX
has been reduced from 30 to 40 percent.
  A  modified  two-stage combustion  tech-
nique, combined with low-excess-air firing has
reduced  the stack-gas  NOX  concentration
emitted  by  two  750-megawatt  gas-fired
power-plant  boilers  from 1,500 to 175 parts
per million (ppm). Nominal costs were report-
ed  by  the  company with  no  decrease in
generating capacity.
  All of the above  approaches are based on
considerations of chemical equilibrium and
reaction rate. They involve reduction of peak
gas temperatures, trends away from oxidizing
and   toward reducing atmospheres,  and
changes in the time-temperature history of
the combustion  gases. These approaches are
all commercially  demonstrated for large oil-
and gas-fired boilers, but are yet to be demon-
strated for large coal-fired boilers.

Changes in Fuel or Energy
  Generation of electricity through the use of
nuclear energy is projected  to grow in the
future.  Essentially,  no  nitrogen  oxides are
emitted since this source of energy  does not
depend on  the combustion  of  fossil fuels. In
1968,  12  billion kilowatt-hours  of electric
power generated from  nuclear  energy was
reported in the United States; reliable sources
project 3,000 billion kilowatt-hours  by 1990.

 Waste Disposal
   Substitution of sanitary landfills for open
 burning has  proved to  be  a commercially
 demonstrated  control  technique  in certain
 areas of the country.

 Chemical Sources
   Chemical  vent gases normally are much
 more  highly  concentrated than  combustion
 gases, and the NO content can be more quick-
 ly oxidized by oxygen in air. If uncontrolled,
 these streams of NOX may create air pollution
 problems  when  relatively high ground-level
 concentrations occur.
   Nitrogen oxides from chemical sources may
 be decolorized by catalytic  reduction  using
 fuels,  such as natural gas or hydrogen. Such
 reactions are exothermic and  much heat is
 generated.  Because  of  practical  consider-
 ations,  such as catalyst life and the tempera-
 ture limitations of structural materials, only
 the process of decolorization by reduction to
 NO has been uniformly successful.
   Catalytic reduction of NO2 to NO is not a
 true control technique; it merely decolorizes
 the  stack  gas. Stack velocities and  normal
 atmospheric  turbulence  contribute to  rapid
 dilution, with increasingly slow rates of ox-
 idation of NO by  air. Photochemical reactions
 in the atmosphere can, however, oxidize these
 small NO concentrations to NO2-

 Energy Substitution
   Industrial,  commercial,  and  household
 thermal  requirements  contribute  to above-
 average  NOX concentrations  in urban areas.
 These thermal  requirements  may  be met in
 part  by  central electric-power  stations with
 adequate NOX emission controls. Emissions of
 NOX are easier to control  from such a large
 centrally located complex. The efficiency of a
heating  system,  however,  is  lower  when
 conversion  of fuel to electricity to heat is  re-
quired than it is when the system is designed
to convert fuel to  heat directly.
 Source Relocation
   Relocation of a pollutant source is a means
 of reducing  the exposure of a densely  pop-
 ulated  area to  NOX  emissions. Such  a plan
 entails  retirement of the old  urban electrici-
 ty-generating stations and the building of new,
 more  efficient, power-generating  stations at
 remote locations. Economic  feasibility  may
 be affected, however, by the cost of long-
 distance power transmission.

 Catalytic Abatement
   Catalytic abatement is similar to  catalytic
 decolorization, but  the  NOX  is  reduced
 completely to elemental  nitrogen. In catalytic
 abatement of NOX, sufficient fuel is used to
 give complete  oxygen burnout, followed by
 reduction of NOX to elemental nitrogen. Dif-
 ficulties in  applying  this  principle in com-
 mercial practice have included short catalyst
 life  and  high-temperature problems.   Low
 initial oxygen content in the gas to be treated
 is recommended.

 Caustic Scrubbing
   Solutions or suspensions of caustic or calci-
 um  hydroxide  react with NO 2  and  with
 equimolar ratios of NO  and NO2- The latter
 mixtures are absorbed to form nitrites, just as
 if they  were  the actual  anhydride of nitrous
 acid, N2O3- It follows that scrubbers can be
 designed for a high degree of removal of NOX
 from chemical  vent streams.  When scrubbers
 are used,  a solution of nitrate and  nitrite is
 formed. If no use is made of the solution on
 site, it  can  create a waste-disposal problem.
Also, the nitrite can decompose to regenerate
NOX if the waste is dispersed into a lower-pH
 environment.  NOX  removal  by  means of
 scrubbers may not, therefore, be desirable.

 Nitrogen Oxide Incineration
   Nitrogen   oxide  incineration  involves
reduction of NOX to elemental nitrogen by
burnout with a gaseous fuel. This differs from
 catalytic abatement  in  that  no catalyst  is
used. At least 10 percent more fuel than re-
quired  for reaction with  oxygen and NO  is
added to the vent gas stream to reduce NO

to nitrogen. NOX reductions of 75 to 90 per-
cent have  been  obtained.  Because  of the
reducing (fuel rich) conditions employed, CO
and  HC may  be  present in  the  exit gas,
and  more  air and  a  second  reactor may  be
required to control these pollutants.
   The NOX incineration method  might  be
useful  for  treatment  of nitric  acid absorber
tail gas following commercial nitration opera-
Urea Inhibition of Nitrogen
   In  some cases where strong nitric acid
solutions are used, urea can  be  added  to
inhibit  or  prevent  release  of  NOX.  This
method  has  been used in metal pickling, in
phosphate  rock acidulation, and in some  ni-

Steam and Water Injection
   A potential control technique, steam and
water  injection,  is  based on  lowering the
boiler  peak flame  temperature and diluting
the combustion gases, as in flue-gas recircula-
tion. This  method requires development and
may involve loss in efficiency, but it has been
found  to reduce NOX emissions from internal
combustion engines.
Flue-Gas Recirculation
 ^ Laboratory results of the flue-gas recircula-
tion technique show sizable reductions in NOX
emissions,  particularly  in combination  with
low-excess-air firing, but tests on boiler emis-
sions are inconclusive.  The  recirculated flue
gases should  be injected directly  into  the
flame zone.
 Stack-Gas Treatment
   Several treatments are under development
 for removal of sulfur oxides from flue gases,
 and some of these techniques also  remove
 a  part of the NOX content simultaneously.
 These treatments  are  described  in  AP-52,
 Control Techniques for Sulfur Oxide Air Pol-
 lutants. Difficulties have been encountered in
 operating most of these processes.
   A  disadvantage  of flue-gas treatment of
 large  gas volumes is that scrubbing with water
 cools the gases to  as low as perhaps 125° F,
 and thereby creates a visible plume with poor

 Selective Catalytic Reduction
 of NOX
   This process uses ammonia in the presence
 of  oxygen  to reduce  NOX.  Limited  com-
 mercial experience  in  three  plants  ranged
 from  unsatisfactory to partially successful.
 Adsorption on Molecular
   Molecular  sieves  are  solid chemical struc-
 tures  that can adsorb NOX from very low con-
 centrations in a  gas stream.  The adsorbed
 NOX  can  be  desorbed with hot  air or steam,
 and the  NOX exit  concentration is  greatly
 raised over that of the entering tail-gas stream.
 Process units involving adsorption and desorp-
 tion of NOX oh molecular sieves are offered
 commercially, and  the degree of removal of
 NOX is guaranteed.  No reports of their use in
 an actual plant are available. Limited evidence
 indicates that adsorption is quite satisfactory
 from  a well-dried gas. Some user, such as a
 nitric acid plant,  must be located nearby to
 accept the concentrated NOX gas stream re-
sulting from the desorption step.

                               1.   INTRODUCTION
   Pursuant  to  authority  delegated  to  the
Commissioner of the  National Air Pollution
Control Administration, Control Techniques
for Nitrogen Oxide Emissions from Stationary
Sources is issued  in accordance with Section
107(c) of the Clean Air Act as amended (42
U.S.C. 1857-18571).
   Nitrogen  oxides  in the atmosphere  are
known  to  be involved in the formation of
photochemical smog of the Los Angeles type
through complex reactions with hydrocarbons
and other pollutant gases under the influence
of sunlight.
   Oxides of nitrogen (NOX) are emitted from
both  stationary and  mobile  sources,  in
roughly  equal proportions on a nationwide
basis. They are emitted in significant amounts
as the result of  the fixation  of nitrogen in
combustion processes, but not directly in
proportion to the nitrogen content of fossil
fuels burned.
   Although control  of  NOX   is a complex
problem, there are  some NOX  sources  for
which control techniques are now available at
relatively low cost;  and,  in a  number of in-
stances, promising laboratory  data have indi-
cated that other control techniques are ready
for further testing and development.
   Nitrogen  oxides are emitted  from many
fossil-fuel combustion sources  and, to a lesser
extent,  other  industrial  stationary sources.
Fuel type,  equipment operation  and  design,
and other variables influence the levels of
NOX  emitted by individual sources  to  the
extent   that  currently  available  emission
factors have only  a broad and  generalized ap-
plicability. A number of potential techniques
being developed for  controlling these emis-
sions  are  based  either on  principles  that
prevent the formation of NOX or on methods
for the  removal of NOX  from stack or tail
gases. Except  for  a  few  chemical reactions
that occur when the only reactive constituent
of the tail gas is NOX,  the feasible methods
appear to be confined to  those that control
nitrogen oxides and  sulfur oxides emissions
  Noncombustion  processes  contribute  only
a small  proportion of total  NOX  emissions,
but can present a serious local hazard. Control
technology in this area has been the subject of
intensive  research,  particularly  in  the
manufacture and uses of nitric acid  wherein
NC>2 is a process raw material. Control tech-
nology via   combustion modification in
oil- and gas-fired processes has been  applied,
and this method is recommended for further
application  and  development. Several other
potential techniques must  await the results of
further research and development in  control-
ling nitrogen oxide emissions  from stationary
  While some  data are presented herein on
quantities of nitrogen oxides emitted to the
atmosphere,  the  effects  upon  health and
welfare  of  nitrogen  oxides  and  their sec-
ondary  atmospheric  reaction products  are
considered  in  two companion  documents,
AP-63, Air Quality Criteria for Photochemical
Oxidants and  Air Quality  Criteria  for Ni-
trogen Oxides. *
*To be issued at a later date.

                               2.   DEFINITIONS
   Two oxides of nitrogen are important in
the  atmosphere:  nitric oxide (NO) and ni-
trogen dioxide (NC>2).  Five  more oxides of
nitrogen  are  known:  NC>3, N2O,
N2O4, and ^65. The oxides NC>3 and
are  too  unstable to  exist  at  atmospheric
conditions;  ^03 and  ^05 are the anhy-
drides of nitrous and nitric acid respectively;
and  N2O, the anesthetic known as "laughing
gas," is a stable compound and can support

   The  term NOX is used generally  to rep-
resent the gaseous pollutants NC>2 and NO,
expressed as NO 2- It is known that NO is ox-
idized to  NO2 by oxidants, including  oxygen
in the air. Special precautions in sampling and
analysis are required, therefore, to get  reliable
measurements of NO. This difficulty is serious
enough to cast doubt on most of the  data in
the older literature since the ratio of NO to
NO2 may not be reliable.

   Nitrous oxide (N2O) is formed largely from
the decomposition of nitrogen compounds by
soil  bacteria. It has a mean concentration of
about 0.25 ppm  in  the atmosphere.1  Nitric
oxide and nitrogen dioxide are formed under
high-temperature conditions such as the burn-
ing of fossil fuels, and in much larger amounts
by various biological reactions in nature but
in low concentrations.  Most of the NOX
formed by man is nitric oxide, which is ox-
idized to the more toxic and irritant nitrogen
dioxide by  the oxygen of the air, or, much
more  easily,  by  photochemically reactive
hydrocarbons in the presence of sunlight.

  Two commonly used  methods are available
for determination of  NOX.  They are the
phenol-disulfonic acid method and the mod-
ified  Saltzman method. Of  these two, the
phenol-disulfonic acid method has been used
to obtain the bulk of the data on NOX emis-
sion factors.
   The phenol-disulfonic acid method for the
determination of  NOX is  described in  the
American  Society  for  Testing Materials
(ASTM)  Standard  Procedure D  1608.2  It is
noted  in the procedure  that samples not
analyzed promptly will give low results; this
is a common failing of older data in the liter-
ature.  An important variable  is the  gradual
chemical reduction of  NO2  to NO and, to a
lesser extent, of NO to nitrogen by the action
of SO2 in the presence of t^O, in a flue gas.
The presence of other combustion effluents
can also create uncertainties.
   The Saltzman method for  determination of
NOX is a modification3 of the Greiss Ilosvay
reaction4 described in ASTM Standard Proce-
dure D 1608. This method depends upon the
development of a red-violet  color, which can
be compared to standards  either visually or
spectrophotometrically.  The  comparison
should be made without  delay if the sample
contains  oxidizing or  reducing gases other
than NOX.
   The accuracy  of older NOX  data in  the
literature is  suspect,  unless  the  user is  sure
that the  analyst allowed for the reducing ef-
fect of any SO2 present.
   Flue-gas analyses may be corrected for the
dilution effect of excess air,  and one method
of  reporting would  be  to  recalculate  the
observed  results  to the  basis of 3 percent
excess  oxygen. The extent of this effect can
be illustrated by  the  example of a flue  gas
containing  555 ppm  of  NOX at 1 percent
excess  oxygen; this would be calculated to be
500 ppm on the basis  of sufficient air to give
3 percent ©2 in the flue gas (2 percent excess
O2 is about 10 percent excess air; calculated
dilution ratio is 111:100).


   1.  Goody, R.M. and C.D. Walshaw. The Origin of
       Atmospheric Nitrous Oxide. Quart. J. Roy. Mete-
       orol. Soc. 7P(342):496-500, October 1953.
   2.  Oxides  of Nitrogen  in Gaseous  Combustion
       Products (Phenol-Disulfonic Acid  Procedure)
       (ASTM  Designation: D 1608-60). In: The 1968
       Book of ASTM Standards, Part 23. Philadelphia,
    American  Society  for  Testing and  Materials,
    October 1968. p. 461-465.
3.   Saltzman, B.E. Colorimetric Microdetermination
    of Nitrogen Dioxide in the  Atmosphere.  Anal.
    Chem. 25:1949-1955, December 1954.
4.   Thomas, M.D.  et al. Automatic Apparatus  for
    Determination  of  Nitric Oxide and  Nitrogen
    Dioxide in  the  Atmosphere. Anal.  Chem.
    2S(12):1810-1816, 1956.

                   3.    SOURCES OF NITROGEN OXIDES
  When fossil fuels (gas, fuel oils, and coal)
are burned with air, some of the oxygen and
nitrogen gas present combine to form NO ac-
cording to the following reaction:

             N2 + O2 ^ 2ND

Given time, this reaction continues to an equi-
librium level, which depends upon variables
such as flame temperature, the concentration
of each gas, and the movement of the gases
through zones of different temperatures, pres-
sures,  and  concentrations.  Once NO  is
formed, however, the rate of decomposition is
very slow, too slow for NO to dissociate into
oxygen and nitrogen under ordinary reaction
conditions. As a result, the NO persists, or is
"frozen" in  the  flame  products  after they
leave the high-temperature zone. The data in
Table  3-1  show  the  significant  effect  of
minute changes in residence time at very high
temperatures  on  the rate at  which NO  is
  The NO thus formed can react with more
oxygen to form NO2. Thus,  the main factors
in NOX formation are: the flame temperature,
the length of time the combustion gases are
maintained at  that temperature,  and the
amount of excess  air present in  the flame.
Small combustion  units  operating  under
relatively low-temperature conditions produce
an exhaust gas containing only small amounts
of NOX. The vent gas from a typical domestic
gas-fired water heater, for example, contains
only  10  ppm  NOX. The concentration rises
rapidly with combustion intensity and may
reach 500 to 1,000 ppm or more in  a power-
plant steam boiler.
  The NOX emissions attributed to  man's
activities other than combustion are relatively
small, about 1 percent of total  NOX. The
emissions in this category are mainly from the
manufacture and use of nitric acid. In general,
the NOX  concentrations  arising from these
sources are much greater than those of com-
bustion  sources, and they  often form highly
visible red-brown emissions.
  Table 3-2 lists  estimates of oxides  of  ni-
trogen emissions developed by  the National
Air  Pollution  Control  Administration   by
source, using emission factors  discussed  in
Section 8.
  There appears to be little doubt that NO2
is removed from  the atmosphere by hydro-
lysis to become nitric acid,  which is then
precipitated  as nitrates in rainfall or in  dusts.
The residence time of NO2 in  the atmosphere
is only a few days.3 NOX is an essential part
of  the  natural  nitrogen  cycle  of organic
growth,  decomposition into the atmosphere
and return to the soil as  a natural fertilizer.
The  natural  cycle is estimated to produce a
worldwide NO2 emission of about 500 x 106
                           NITROGEN AND 3 PERCENT OXYGEN1
Temperature, F

Time to form 500-ppm
NO, sec
NO concentrations at
equilibrium, ppm

                     Table 3-2.  ESTIMATES OF NITROGEN OXIDE EMISSIONS
                           IN THE UNITED STATES, BY SOURCE, 19682
           Mobile fuel combustion
             Motor vehicles
             Non-highway users
           Stationary fuel combustion
             Fuel oil
             Natural gas
           Solid waste
             Open burning
             Conical incinerators
             Municipal incinerators
             On-site incinerators
             Coal waste banks
             Forest burning
             Agricultural burning
             Structural fires
           Industrial processed



 tons per year.4  Air pollution by  NOX emis-
 sions tends to become progressively worse as
 population becomes more dense and demands
 for industrial combustion energy  increase.5
 The concentration of NOX in urban areas of
 the United States averages 40 to 50 parts per
 billion (ppb),6 which is strikingly greater than
 the natural background level of 1  ppb; it is
 this  difference  that creates the  need  for
   Figure 3-1  shows the  total amount of man-
 made NOX generated in the United States in
 1968, according  to  the type  of fuel used.
 Both mobile  and stationary sources are sum-
 marized  for  the  sake of completeness and
 comparison.  Emissions from mobile sources
 are treated at length in another  document,
 AP-66,   Control  Techniques  for  Carbon
Monoxide, Nitrogen Oxide, and Hydrocarbon
Emissions from Mobile Sources.
   Figure  3-2  presents graphically  a  1968
estimate of  NOX emissions from stationary
installations  in the United States by  types of
fuel use, as developed by an  independent
source.  After electric power plants and auto-
mobiles, the  largest NOX source is the natural-
gas-driven  compressor  for  oil and gas pipe-
lines and gas plants. This is largely a nonurban
source,  which is discussed in detail in Section
7.3. A comparison between coal, oil, and gas
for ordinary use  in power plants or boilers is
given in Table  3-3. Average emission factors
on a uniform Btu basis are given.  In individual
units, wide variations from these data are to
be expected, as discussed in Section  8  of this
report.  Coal, with its large emission factors,
accounts for some 39  percent of all NOX from
stationary installations  in the  United States,
or  18  percent of  the  total NOX emissions
shown in Table 3-2.

              NOX ESTIMATED TONS, 1968

         FUEL TYPE
Figure 3-1.  Total estimated oxides of nitrogen emissions from mobile and stationary
sources in United States (20,669,000 tons), by fuel type and tonnage, 1968.

                           PIPELINES AND
                             GAS PLANTS
              NOX ESTIMATED TONS, 1968


    Figure 3-2.  Total estimated oxides of nitrogen emissions from stationary installations
    in United States (9,600,000 tons),  by type of use and tonnage, 1968.
                       Table 3-3. COMPARISON OF COAL, OIL, AND GAS
                                ON EQUIVALENT-Btu BASIS6
Natural gas
Fuel oil
Average NOX emissions in boilers
and power plants, Ib NOX/109 Btu
Household and

1.   Boilers, Heaters, and  Steam Generators.  In: Air
    Pollution Engineering Manual,  Danielson, J.A.
    (ed.). National Center for Air Pollution Control.
    Cincinnati,  Ohio.  PHS  Publication Number
    999-AP-40. 1967. p. 540.
2.   National  Air  Pollution  Control Administration,
    Reference Book of Nationwide Emissions. U.S.
    DHEW, PHS, CPENS, NAPCA. Durham, N.C.
3.   Robinson,  E.  and  R.C.  Robbins.  Source,
    Abundance and Fate of Gaseous Atmospheric
    Pollutants.  Stanford  Research Institute. Menlo
    Park, Calif. Final Report. SRI Project PR-6755,
    February 1968. p. 75.
4.  Robinson, E.  and R.C.  Robbins. Source, Abun-
    dance and  Fate of Gaseous Atmospheric Pol-
    lutants. Stanford Research Institute Menlo Park,
    Calif.  Final   Report.  SRI  Project  PR-6755,
    February 1968. p. 85.
5.  The Effects of Air Pollution. National Center for
    Air Pollution  Control. PHS Publication Number
    1556.1967.18 p.
6.  Duprey, R.L.  Compilation of Air Pollutant Emis-
    sion  Factors.  National Center for Air Pollution
    Control. Durham, N.C. PHS Publication Number
    99-AP-42. 1968. 67 p.
7.  Steam Electric Plant Factors. 18th ed. Washing-
    ton,  D. C. National Coal Association, October
    1968.110 p.

  The combustion  of fossil fuels with air as
the oxidant results in the formation of oxides
of nitrogen. These species are formed by the
direct combination of molecular nitrogen and
oxygen in flames. Thus, in the combustion of
natural gas, which is virtually free  of bound
nitrogen, the quantities of NOX produced (in
excess of 1,000 ppm NOX in flue gases  from
some electric utility boilers) clearly indicate
that high-temperature fixation of atmospheric
nitrogen occurs. In contrast, the formation of
sulfur oxide pollutants is solely dependent on
the sulfur content of the fuel.
  Chemically bound nitrogen in the fuel can
also contribute to the overall NOX emission
from combustion  processes,  since  it reacts
with  oxygen  much more  readily  than the
molecular  nitrogen supplied  with  the  com-
bustion air.1  These differences in  reaction
rates are due to the extremely stable nature of
molecular nitrogen. For molecular nitrogen to
react, enough energy must be furnished to dis-
sociate  the N-N bond. The bond dissociation
energy  is 225,000  calories  per gram-mol of
nitrogen, a very high value.
  By contrast, the dissociation energies of the
carbon-nitrogen  bonds in fuel molecules fall
in the more modest range of about 60,000 to
150,000  calories per  gram-mol.  Cor-
respondingly,  the activation energy required
for  oxygen to react with bound nitrogen in
the  fuel is substantially  lower than that re-
quired for reacting with molecular  nitrogen.
Except  at very high temperatures, therefore,
NOX is formed at a higher rate via the oxida-
tion of fuel nitrogen than through  the  reac-
tion of oxygen with molecular nitrogen. The
formation of NOX from  nitrogen in the fuel
may be relatively  unaffected  by changes in
combustion  conditions, according to  the
limited  data available.1'2'3  This problem re-
quires  more research,  particularly on  coal
  The role  of  the  fuel nitrogen content in
combustion  at  the  usual boiler  temperature
has been  studied in laboratory experiments,
which  indicate  NOX  is formed by the  ox-
idation  of fuel nitrogen.1  Elshout and  Van
Duren2  also correlated data on NOX in  fuel
oil combustion  and found a trend toward an
increase in NOX emissions with increasing fuel
nitrogen content. These correlations may be
treated  as tentative, however,  since other
boiler   operating variables  were  not con-
  Recent  data, obtained by Argonne National
Laboratories, from tests in  which oxygen-
argon replaced air, showed  that oxides of ni-
trogen can  be  formed  in  low  temperature
combustions exclusively via the oxidation of
fuel nitrogen. Combustion of a fluidized bed
of coal was run with 20 percent oxygen in
nitrogen  and  with  20 percent  oxygen in
argon.  The amount of NO  produced was es-
sentially the same in both cases and was about
the same  as that  obtained with a conven-
tionally fired package boiler.3
  In high-temperature combustion processes,
the  elementary reactions  between   atmos-
pheric  nitrogen  and oxygen clearly  play a
dominant  role   in  NOX  formation.  Under
typical combustion conditions, the only oxide
of nitrogen of any  consequence  is nitric  ox-
ide.   Thermodynamic equilibrium  highly
favors the formation of NO over that of NO2
at high temperatures (as shown in Table 4-1).
Once  formed at high temperatures, the  NO
can react  with excess oxygen  to yield NO2-
Although  this oxidation  reaction  has a
negative temperature coefficient, i.e., its rate
decreases  with  increasing  temperatures,  the
residence time usually available in combustion
equipment is too  short for an appreciable

                                  OF NITROGEN OXIDES3 (ppm)
3.4 x lO'10
2.1 x 1C'4
Flue gasb
1.1 x 10'10
3.3x lO'5
        aFor the reactions:  N2 + O2
                        2NO + O2
        b3.3% O2, 76% N2 in Hue gas.
2 NO
 fraction (more than 5-10%) of the NO to be
 oxidized  to NO2- The bulk of  the  NC>2,
 favored  by the thermodynamic equilibrium
 under ambient conditions, is produced in the
 atmosphere, rather than within the confines
 of the combustion source.
   The rate of NO formation is very highly
 temperature  dependent,  and  as  a  result,
 Zeldovich  estimated an activation  energy of
 129 Kcal/mole for this reaction.4 He gives the
 following expression for the rate of formation
 of NO, derived  on the basis  of  the  chain
 mechanism of reactions:

      ' = 3.0xlOl4e-129,000/RT(N2)(o2)l/2


 (NO),  (N2), and O2 - Concentrations, gram-
   t = Time, seconds
   T = Temperature, °Kelvin
   R =  Gas constant, cal/gram-mol/°K.

   More recent data verify this magnitude of
 the activation energy.5
   The  very high  activation energy for NO
 formation accounts for the extreme depend-
 ence of  reaction rate on temperature.  The
 gas-phase decomposition of NO back into N2
and O2  has a lower, but still appreciable,
activation energy. This means that  NO, once
formed, is extremely stable; that is why it is
      difficult to remove it from  a gas  in  trace
        Catalysts may speed up both the formation
      and decomposition.
        A number of investigators have studied the
      NO  decomposition  reaction,  and  various
      estimates have  been  made for  the kinetic
      parameters of this process.5'8  It appears that
      below  1100°K, NO decomposition is cat-
      alyzed by surfaces, while above 1400°K it is
      strictly a gas-phase reaction. From the stand-
      point of practical applications, i.e., using com-
      bustion  modifications to control NOX  emis-
      sions,  the most important factor in the NO
      chemistry of flames is the rate of formation
      of NO.
        Since  NO formation is highly temperature
      dependent, the reduction in NOX emission ef-
      fected by relatively modest peak flame tem-
      perature reductions can  be very significant.
      This is,  of  course, an over-simplification  of
      the  actual situation, in that not only  the peak
      flame  temperature,  but  the entire  time-
      temperature  history  of  the combustion
      process requires control. Nevertheless, relative-
      ly simple techniques aimed at the reduction
      of peak flame temperatures have been success-
      ful in accomplishing reductions in NOX  emis-
      sions of the order of 50 percent, compared
      with "standard" operating practices.
        Movement of the combustion gases through
      different zones of temperature, pressure, and
      concentrations  influence  the   formation  of
      NO.  The  principal factors affecting  NOX

production are flame temperature, the excess
air  present in  the flame, and  the length of
time  that  the combustion  gases  are  main-
tained at flame temperature. Most of the NO
is formed in the flame where the highest tem-
peratures prevail. As  indicated, the  level of
emissions is also influenced by the amount of
excess air present. The  lower the excess air,
the lower the NOX emissions and vice versa.
Excess air beyond a certain level dilutes the
flame and, therefore, decreases the flame tem-
perature and decreases NOX.
  When  residence time of combustion gases
at the high flame temperatures is relatively
short,  the  NO reaction is  prevented from
reaching  equilibrium. Figure  4-1  shows
theoretical  concentrations of NO  calculated
on the basis of typical fuel analyses, excess air
conditions, and a residence  time of  0.5  sec-
ond.9 The  strong dependence  of NO forma-
tion on  temperature  is readily apparent in
Figure 4-1. At high temperatures, such as in
the  flame, appreciable  NO is  formed;  but









1 1 /
— / —
OIL / /

f f
/f —
L / / _
/ /
/ / -
/ f
	 S fS —
/ »**
^ ''.**'

   2,800        3,000       3,200

              TEMPERATURE, ° F
Figure 4-1.  Calculated nitric oxide concen-
tration from oil- and gas-firing, 0.5 second
residence time excess air.
below 2,800°F, formation is  negligible (see
Section 3.1  above).  Furthermore, the NO
reaction is reversible, and  some of the NO
decomposes to nitrogen and oxygen.  If suf-
ficient time were available, the decomposition
reaction could also proceed to  equilibrium. In
a boiler, industrial  furnace, or engine, how-
ever,  the  gases move quickly from the hot
flame zone to cooler regions, and the rapid
decrease in temperature freezes the NO con-
centration  at  a  level  exceeding  that cor-
responding to  the  equilibrium value  at ex-
haust or  flue gas temperature. The key  to
reducing NOX  emissions by modification  of
the   combustion  process,  consequently,
depends on some combinations of lowering
peak  flame temperature,  altering  the time-
temperature  profile  in  the  furnace, and
reducing oxygen and nitrogen  availability for
  Because of the potential simplicity of NOX
control via combustion modification, this area
is regarded as the most  promising method  of
controlling  NOX emissions.  If  successfully
applied, the modification of design features or
operating conditions  could  be  the least
expensive  and  most readily applicable tech-
nique of  NOX control  for various  types  of
combustion equipment. At least partial suc-
cess has been  found for NOX reduction by a
number of combustion  modifications, which
will be discussed further.
4.1.1   Factors Affecting NOX Emissions
  Theory predicts that high or low NOX emis-
sions are the result of design or operating con-
ditions on flame temperature, excess air, and
residence time.  Present theory  is useful  in
explaining certain specific phenomena, but is
not comprehensive enough to explain all the
interactions of the many variables having an
effect on the  process. Experience has shown
that the levels of NOX emission in a particular
combustion unit cannot be predicted based
on  theoretical considerations or generalized
data correlations. In fact, not  only do emis-
sions vary extensively  between units having
similar operating conditions, but  they  may

 even be widely different  in identical equip-
 ment. Fuel Type and Composition
   It has been shown that the type of fuel
 fired can have an  effect on the quantity of
 NOX emissions produced, and that these emis-
 sions vary further with the kind of equipment
 in which the fuel is consumed.  This is illustra-
 ted  in  Table  3-3, wherein NOX emissions
 with coal were shown as highest in all categor-
 ies.  Emissions  from  fuel oil firing  are next
 highest, and gas firing is lowest. Recent expe-
 rience  in the electric generation  industry in
 California has shown,  however, that NOX
 emissions with natural gas can be substantially
 higher  than with fuel oil, particularly for large
 boilers (See Section 8).
   As discussed in Section 4.1,  the chemically
 bound  nitrogen in  the fuel (coal and oil) may
 also be converted into NOX. Other fuel prop-
 erties such  as  carbon-to-hydrogen ratio, ash
 and  metals  content, and  fuel  gravity  and
 viscosity may also be important. The effect of
 most of these  factors remains  to  be defined. Heat Release and Transfer Rates
   The  heat release in a given furnace volume
 has  an  important bearing on the  flame tem-
 perature  produced. The amount of cooling
 surface surrounding the  combustion zone not
 only has an effect  on the flame temperature,
 but  also affects the quench rate in the flame.
 The  interaction  between  these  two factors
 influences the  NOX  emission  rate.  In  other
 words,  if heat  release rate increases but heat
 transfer does not keep pace, furnace tempera-
 ture  increases and  so does NOX  formation.
 The   trend in  the  public utility  industry is
 toward  much larger boiler and  turbine instal-
 lations  than those presently  in existence.
 Firing rates  in  these units, of necessity, have
 increased, with  resultant higher flame tem-
 peratures. The installation of a  larger number
 of burners in these units is required to achieve
 the desired firing rate. The result is that  the
 burners and the flame are spread throughout a
 wider portion of the furnace.
  It  is  also  well known  that NOX emissions
decrease with a decrease  in furnace loading, a
direct result of heat release rate. Tests at full,
one-half,  and one-quarter load resulted in ni-
tric  oxide values of 300, 185, and 145 ppm,
respectively.10  Furthermore,  it  has  been
observed  that  emissions  not only vary with
heat release rate, but that they also vary with
the fuel type in different installations at the
same heat release rates. Figure 4-2, based on
data reported by Sensenbaugh and Jonakin,9
illustrates these points.

O  300
U-  200
          ' — GAS
          .— OIL
               15        20        25       30

           HEAT RELEASE, 103 Btu/ft3-hr
 Figure 4-2.  Effect of heat release on oxides
 of nitrogen emissions, by type of firing.

  A number of investigators have noted the
effects of heat release rate in a given furnace
volume   on  NC>2  concentrations.  There is
little  doubt  that the heat release rate is an
important  parameter having a direct bearing
on  peak flame temperature. Furthermore, it
has been suggested  that for very large boilers,
oil  flames, because of higher emissivity, may
have  a more rapid rate of heat removal by
radiation  than nonluminous  gas flames, so
that the  oil flame may give a lower effective
residence time at peak temperatures.11 The
use of carburetted flames has been considered
as one means to increase the emissivity of the

nearly colorless flames normally  obtained in
the combustion of natural gas.

4.1.2  Modifications of Operating
  Combustion modification techniques aimed
at lowering the level of NOX emissions can be
divided into two broad categories. In the first
category of these techniques,  the  operating
conditions in fossil fuel combustion processes
are modified. In the second  category (to be
discussed  in   Section  4.1.3),  combustion
equipment design  modifications can be used
to lower NOX emissions. Some  of the designs
and operating  conditions that  decrease NOX
emissions may  tend  to increase emissions of
CO  and  hydrocarbons  or decrease fuel  ef-
ficiency. Low Excess Air Combustion
  Low excess  air combustion  is one of the
most  promising and  universally applicable
methods  of reducing NOX  by  combustion
modifications. Special control equipment and
very careful supervision are required to  avoid
explosions, carbon monoxide, smoke, and un-
burned fuel emissions;  these  can  be  mini-
mized, however, by the usual operations at
excess air levels from 15 to 50 percent. This
procedure consists of supplying  as close to
stoichiometric  requirements  of air for
complete  burnout of fuel  as is permitted by
the nature of the combustion  process.
  Low excess air  combustion is  a  technique
originally developed in the United  Kingdom
to overcome undesirable  operating conditions
such  as  low-temperature  corrosion and  air
heater plugging in oil fired boilers.12  Later,
European  boiler operators  made  further
refinements  in  this type of operation, claim-
ing  additional  operating  advantages,
particularly improvements in  furnace slagging
and  heating-surface  fouling.   In  the  early
1960's,   this   operating  technique was
introduced to  public utilities in the United
States and subsequently adopted as a regular
practice  at  some oil-fired power stations.
While  theory  predicts a reduction of NOX
emissions with lower excess air, actual data
are still limited on this type of operation. The
investigations  by  Sensenbaugh and  Jonakin
(I960),9 on oil-fired boilers also showed that
NOX  emissions decrease as  the oxygen con-
tent of the flue  gas is  reduced, even at the
higher levels of excess air. As shown in Figure
4-3, NOX reductions of 36 percent in a hori-
zontal oil-fired  boiler and  28 percent in a








I I I *D I
1 Iff
_ / _
— / FIRING —


— .•••*x) —
1 1 1 1
1 1 1 1

           OXYGEN IN  FLUE GAS, percent
Figure 4-3.  Effect of excess air on oxides of
nitrogen emissions from oil-fired boilers.^
tangentially fired unit were obtained as  flue
gas oxygen content was decreased from 3.5 to
4.0 percent to about 2 percent. Subsequent
investigations12"14 at much lower excess air
(2 to 3 percent excess air, 0.4 to 0.6 percent
62) in oil-fired installations indicate NOX re-
ductions of up to 63 percent can be achieved
by  lowering  excess air from  15 percent to
about  2 to 3 percent.  Figure 4-4, based on a
report by Fernandez, Sensenbaugh, and Peter-
son,1 3  illustrates the magnitude of NOX re-
ductions possible in oil-fired boilers operating
at low excess air. Similar effects have been ob-
served in laboratory  tests of an oil-fired do-
mestic heating installation14 with a level of
excess air much higher than that in the larger
utility boilers.
  Laboratory data reported by Bienstock15
indicate  that with a single small burner  and

  UJ  160
  O a
                         i    r   i   i
                         1    1
             OXYGEN IN FLUE GAS, percent

   Figure 4-4. Oxides of nitrogen emissions
   from oil-fired boilers at low excess airJ3

 correspondingly simple  controls,  both  low
 excess air and  two-stage combustion do  give
 lower values of NOX  with  coal. By reducing
 the excess air fed to the primary combustion
 zone  from  22  to  50 percent,  a  62 percent
 reduction of NOX was  obtained  at carbon
 combustion  efficiencies of 98  percent. This
 combustion  efficiency was  improved to 99.2
 percent by injecting an additional 17 percent
 of air just beyond the flame front, with the
 same reduction in NOX.
   There  are  engineering difficulties  in
 reducing these  laboratory  data to practice,
 but in view of the importance of coal as a
 fossil fuel, the incentive for a successful devel-
 opment is great. Flue Gas Recirculation
   The  use of flue gas recirculation in  com-
 bustion processes is not new. This is a process
 in which a portion  of the combustion flue gas
 (say  10-20%  of feed  to  the furnace) is  re-
 circulated into the combustion zone.
   Gas  recirculation as  practiced  in utility
 plants for boiler tube temperature  control
 does not fulfill the  requirements of "flue  gas
 recirculation  for NOX  control," because  the
 recirculated gases are injected downstream of
 the burners.  The flue gas must enter directly
 into the combustion zone if it is to be  ef-
 fective in lowering the  flame temperature and
 reducing NOX formation.
   Processes using this technique provide some
 means for recycling a portion of the flue gases
 produced back to the combustion chamber.
 The effect of recycling on combustion condi-
 tions  depends on (1) the point of injection
 into   the  combustion system  and  (2) the
 amount of gas recycled. The greatest effect on
 flame conditions is  achieved by mixing the
 gases  directly  with  the  combustion  air.
 Injecting flue  gas directly  into  the primary
 flame zone has  the effect of (1)  decreasing
 flame temperature, and (2) diluting the com-
 bustion air (oxygen) and the resultant flue
 gases. Where  flue gas is introduced down-
 stream of the combustion process, the effect
 is largely one  of dilution. The quantity of gas
 recirculated is  of major importance, having a
 direct effect  on  the  combustion conditions
   Some large steam boilers  are designed for
 recirculation of a portion of the flue gases in
 order  to   control  superheat  temperatures.
 Normally, as boiler load decreases, steam tem-
 peratures  tend  to drop off unless  some
 method of control  is   employed.  By  re-
 circulating  an increasing  portion of the flue
 gas as the boiler load decreases, it is possible
 to maintain steam temperature at a constant
 level over a wider load range. Where this type
 of control is used, the flue gases are injected
 to reduce the effectiveness of the furnace heat
 absorption  surface without  interfering with
 the combustion process.  Tests made  during
 the California joint project NOX investigations
 in 1960-1962 concluded that recirculation for
 steam  temperature control was relatively inef-
 fective  in suppressing   NOX.  Recent un-
 published  data,  however, indicate  that  re-
 circulation of 20 percent of the gases  in a
 gas-fired boiler  equipped with recirculation
 steam  temperature control reduced emissions
 by 20  percent.1' It should be recognized that
 the objectives  of recirculation for NOX con-
 trol may not be compatible  with the objec-
 tives for steam temperature control.
  Exhaust gas  recycling has  also  been found
 to be effective in controlling  NOX in internal
 combustion engines. It has been shown1' -1 6
that  an 80  to 85  percent reduction in NOV

can be  achieved in  the internal combustion
engine,  albeit at the expense of operability.
Laboratory tests of an oil-fired domestic heat-
ing  furnace1 3  have  shown   potential
reductions of 70 percent when recycling  50
percent of the  flue gas, as shown in Figure
4-5 The data presented in Figure 4-5 indicate







      0       10      20      30     40     50
                EXCESS AIR,  percent
 Figure 4-5.  Combined effect of flue gas re-
 circulation and excess air on oxides of nitro-
 gen emissions from an oil-fired domestic
 heating furnace.14

 that reductions in emissions  up to 80 percent
 can be achieved by  the combination  of re-
 ducing  excess air from 50 to 20 percent and
 recirculating 50 percent by volume of the flue
 gas. Combining  low excess  air combustion
 with  flue  gas  recirculation  has  a further
 advantage in that it makes it easier to achieve
 smokeless combustion, in addition to the re-
 duction in NOX. Gas  recirculation has also
 been employed  in low-temperature heating
 furnaces in industrial applications to achieve
 greater and more uniform heat transfer into
 the product. Vertical shaft  lime kilns often
 use recirculation to control  calcining tem-
 peratures more effectively. The effect on NOX
 emissions  in the latter cases, however, is not
 known, but further investigation is warranted.
   The  mechanism  for  the reduction of NOX
 emissions  via flue  gas recirculation  has been
 attributed to the  effect of  decreasing flame
 temperature and to  dilution of the available
 oxygen in the flame zone.14 Investigators at
the Bureau of Mines have concluded that for
gas-fired appliances this reduction in NOX is
due  only to  the  lowering of the flame tem-
perature and not to  reactant dilution.17  If
this is true, the ability to accomplish the same
end by  other thermal means suggests the pos-
sibility of  other solutions to  the  NOX
problem. Gas recirculation, regardless of its
action  in reducing NOX,  offers considerable
potential for application to many existing in-
stallations, with  the possible  exception of
those dependent  on high  flame  temperature,
i.e.,  melting  furnaces. The extent  of  ap-
plicability of this type  of combustion modi-
fication must still be investigated. The quan-
tity  of  recirculated gas necessary  to  achieve
the desired effect in different installations is
important and can influence the feasibility of
the  applications. For  instance,  recirculating
large quantities of gas in utility  boilers poses
problems in the handling of large quantities of
gas,  in  addition to the  problem of increased
investment and operating costs.  Steam and  Water Injection
   Flame temperature, as  discussed above, is
one  of the important parameters affecting the
production of NOX. There  are  a number of
possible ways to decrease flame temperature
via thermal  means. For  instance, steam or
water  injection,  in quantities  sufficient to
lower  flame  temperature  to  the  required
extent,  may  offer  a control solution. Water
injection was found to be  very effective in
suppressing  NOX emissions  from an  internal
combustion engine.18 Water injection may be
preferred over steam in many cases, due not
only to its availability and lower cost, but also
to its  potentially greater thermal  effect. In
gas-  or  coal-fired boilers, equipped for stand-
by oil  firing  with steam  atomization,  the
atomizer offers a simple means for injection.
Other installations  will require special rigging
so that  an investigation program will be neces-
sary to determine the degree of atomization
and  mixing  with  the  flame  required,  the
optimum point of injection, and  the quan-
tities of water or steam necessary to achieve
the desired effect. The use of these techniques

 for NOX  control may carry with them some
 undesirable  operating  conditions,  such  as
 decreased  efficiency and higher  corrosion
 rates; and these must also be evaluated.
 4.1.3   Design Modifications Burner Configuration
   The  specific design and configuration of a
 burner has  an  important bearing  on the
 amount of NOX formed. Certain design types
 have been found to give greater emissions than
 others. For example, the spud-type gas burner
 appears to give a higher emission rate than the
 radial  spud  type, which, in turn, produces
 more  NOX than the ring type. (See Figures
 4-6, 4-7).1J  Similarly, the spray  angle  in oil
 atomizers can affect NOX;  narrower  spray
 angles, producing poorer atomization, have
 been reported to provide lower NOX  emis-
 sions,11  contrary to the  results of Barnhart
   Some  burners, such as the cyclone and
 vortex  types, operate under highly  turbulent,
 high intensity conditions.  In field tests of con-
 ventional   utility  burners,11  adjustments
 made on burners to decrease turbulence to a
 minimum reduced NOX emissions by 40 per-
 cent, but resulted in unsatisfactory flame con-
 ditions. Throttling the burner registers to in-
 crease  windbox pressure  and turbulence in-
 creased NOX emissions by more than 15 per-
 cent. It appears in general that long "lazy"
 flames  seem  to favor lower NOX emissions, in
 contrast with the short, intense flames usually
 considered desirable for combustion.
   In the course of the  California investiga-
 tions in  the late 1950's, a two-stage com-
 bustion type of burner was developed and is
 currently being  used in  a number  of Los
 Angeles County power stations. In this type
 of burner,  about 85 to  95 percent of the
 stoichiometric  air needed for combustion is
 admitted  to the  flame  through  the burner
 throat.  The remainder of  the air required for
 complete  combustion  is injected through
 ports above the burner to complete the burn-
 out  of the initial combustion phase. When
 such operating  conditions  were  used,  re-
 ductions of 30 percent in NOX emissions were
observed,  whereas admission of only 90 per-
cent of the stoichiometric air into the primary
zone resulted in a 47  percent  reduction  of
NOX.19  This  technique has  been  applied
successfully to  oil and gas firing,  but the
engineering problems  of applying it to the
coal-fired  boilers have  not  been solved (see
NOX emissions may be explained by a combi-
nation of  several factors. First, there is a lack
of available oxygen for NOX formation when
the first stage is operated under substoichio-
metric air conditions.  Second, the flame tem-
perature is lower under these conditions than
in normal combustion. Third,  to the degree
that heat  is  removed  between stages, the
maximum  flame  temperature in the second
stage is lower  than that for one-stage  com-
bustion. Fourth, the effective residence time
available  for  NOX formation  at  the  peak
temperatures reached in the  second stage may
be reduced.
   Because of the effects observed  with the
use of two-stage combustion, it is worthwhile
to consider the extension of this technique  to
multistage combustion  for improved  NOX
emission control. As a more remote possibil-
ity, the staged injection of the fuel instead  of
the  air may  result in  a  time-temperature-
concentration profile favoring low NOX emis-
sions. Burner Location and Spacing
   Although data are scarce regarding the ef-
fect of burner spacing and location  on NOX,
potentially these are important variables. The
interaction between closely spaced  burners,
especially  in  the center of a multiple-burner
installation, could be  expected  to  increase
flame temperature at these locations. There is a
tendency  toward greater NOX emissions with
tighter spacing  and a  decreased ability  to
radiate  to cooling surfaces.  This  effect  is
illustrated by the higher NOX emissions from
larger boilers with greater multiples of burners
and tighter spacing.
   Tangential firing has been demonstrated  to
be a useful design, representing a change from
other types of burner location  and spacing.

              Figure 4-6.  Multi-fuel burner for gas (spud-type burner), oil (circular register), and
              coal  (circular register).
              (Courtesy of The Bobcock and Wilcox Company)
    Burners in  a  tangentially  fired  boiler  are
    located in the corners, firing on a tangent to a
    circle at the center of the furnace. As a result,
    individual burner flames have very  little  op-
    portunity  to interact  with  one another and
    the flames radiate widely to the surrounding
    cooling  surfaces. Tangential firing  can pro-
    duce reductions in  NOX of up to 50 percent
    compared  with front or opposed fired furn-
    aces.13  This significant effect  may be  ex-
ploited  further for potential NOX  control in
other types of combustion processes.  Fluidized Bed Combustion
  Combustion of fossil fuels in fluidized beds
may prove to be useful for NOX control. Heat
transfer in fluidized-bed combustion processes
is excellent, so that the average bed tempera-
ture is very  low (1,600° to 1,800° F)  com-
pared with conventional flame temperatures.

                GAS INLET
             Figure 4-7. GaS ring-type burner. (Courtesy of The Babcock and Wileox Company)
  Because of  the  low bed  temperatures, the
  thermodynamic equilibrium under these con-
  ditions is  such that  only  small  amounts of
  NOX should be formed from N2-
    In spite of this favorable outlook, exper-
  imental  data  reported  at  the  First  Inter-
  national Conference  on  Fluidized Bed  Com-
  bustion  (Heuston  Woods,  Ohio, November
  1968) did not show the expected  reduction in
  NOX emissions.  For example, Pope, Evans,
  and  Robbins  measured  about  0.3 pound
  NOX/106 Btu fired in their coal-burning pilot
unit,  which  is  the  same  as the emissions
measured  from  stoker-fired units of similar
  The reasons for the high levels  of NOX
emitted from fluidized bed combustions are
not yet known. Possibly,  high local surface
temperatures in the bed, the oxidation of the
nitrogen in the fuel, or both of these factors
may contribute to the observed high levels of
NOX. Continuing developments of this tech-
nique  may  eventually result in  substantial
reductions of NOX.

------- Small Combustion Equipment
  The largest number of combustion devices
in  existence  are  installed in single-family
residences. Most of these devices are small
gas-fired  equipment. These small sources of
NOX, such as hot water heaters and residen-
tial  space-heating  furnaces, are difficult  to
modify economically.
  Premixed  flames  have  been  shown  to
produce  lower quantities of NOX than  dif-
fusion flames in laboratory experiments.20-2 l
The application  of this  type of burner, elim-
inating or limiting  the use of secondary air,
may have an  undeveloped potential for the
economic modification  of existing  and new
gas-fired   equipment. Thus,  substantial re-
ductions  in  NOX  emissions  from  a large
number  of  small  pollution sources  may be

  Removing NOX  from  flue gas provides an
alternative means of controlling combustion-
produced  emissions.  Such  removal  may
ultimately become the most effective control
technique in those cases where combustion
modifications are not possible or are of only
limited benefit.  In  contrast to SC>2  control,
however, very little effort to  date has gone
into research and development of techniques
for  removing nitrogen oxides  from  flue gas.
For  reasons outlined more fully  below, the
only methods now  considered practical are
those that remove both SC>2 and NOX, and
the  amount of  NOX removed  is limited to
about  20 percent or less of that found in an
ordinary   flue gas.  The  processes  covered
below  are discussed from  the standpoint of
SO2  removal  in  the  document,  AP-52,
Control Techniques for Sulfur  Oxide  Air Pol-
  A  systemic  study indicates  that  NOX
removal  from flue gases will continue  to be
more  difficult  than SC>2  removal.22  Com-
plications arise from the  fact that the nitrogen
oxides present exist primarily  as the stable
and  relatively unreactive  nitric oxide,  NO.
Upon  release to the atmosphere, the NO is
oxidized slowly to the more reactive dioxide,
NO2- A second complicating factor is that the
flue gases contain other reactive species such
as  H2O, CO2,  and SO2 in greater concen-
trations  than the nitrogen oxides, and these
species  interfere  with removal.  Typical
concentrations of these  gases for the com-
bustion of coal, oil, and natural gas are shown
in Table 4-2. The relative reactivity of each
gaseous component is suggested  by the data
on  their solubility in water. It must be kept in
mind that  these  solubility data are for  the
pure gas in equilibrium with water, each at 1
atmosphere  pressure, and that  the  nitrogen
oxides, particularly NO2 are present  in  the
stack gases in very low concentration.  Other
problems arise from the need to  process very
large  volumes of gas for NOX  control. A
modern  1,000-megawatt  power plant,  for
example, emits about 2 million cubic feet per
minute of the flue gas. Processing such a high
volume of hot gas presents formidable equip-
ment and engineering problems.
  The only commercial  flue gas process at
present that has been reported to remove ni-
trogen oxides is  Combustion Engineering's
limewater-SO2 scrubbing process. Other flue
gas treating processes under study for SO2
control that  have shown varying abilities to
remove nitrogen  oxides are the Reinluft Char
Process and the Tyco Laboratories' Modified
Lead Chamber Process.

4.2.1   Control by Limestone
       Wet-Scrubbing Process
  Slaked lime or limestone  aqueous  suspen-
sions  which are capable of removing SO2 or
803 as acid gases are capable of removing a
certain amount of NO2, or NO2  plus NO, for
the same  reasons.  There  are  two  basic
variations of the limestone wet scrubbing proc-
ess.  In the first, dry limestone is injected
directly into the boiler furnace where it reacts
with some of the sulfur oxides (20 to 30 per-
cent of the  total SO2, and essentially  all  the
803) ahead of a  water  scrubber; it is then
calcined  to  the more reactive  quicklime  for
use in the scrubber. In the second, limestone
is added directly to the  scrubber. Although


NO (% in NOX)
NO2 (% in NOX)
Volume percent from
— 	 0.07d 	 »-
HI 5-10 —

Solubility in water,
V/V, at 1 atm, 25°C

              aCalculated for burning with 20% excess air a typical high volatile bituminous
               coal of the following composition: carbon, 70.1%; oxygen, 6.6%; hydrogen,
               4.9%; nitrogen, 1.4%; sulfur, 3.0%; ash, 12.7%; H2O, 1.3%.
              ^Calculated for burning with 20% excess air a typical residual fuel oil of the fol-
               lowing composition: 86.5% C, 10.3% H, 2.5% S, 0.7% N.
              cCalculated for burning natural gas with 10% excess air.
              dThis is an average value. Actual values range from 0.02 to 0.15%.
              ePhase diagram in a sealed tube shows equilibrium with water a 25UC, as about
               54 wt % N2O4 (or 120 g/100 g of H2O).23
 limestone in the scrubber is less efficient than
 lime, the second method avoids such potential
 boiler  problems  as abnormal  slagging,  in-
 creased  erosion, and interference with com-
 bustion  and  temperature  control. In both
 cases, the solids from a reactive slurry, which
 combines with most of the sulfur oxides  and
 some  of the nitrogen oxides. Reacted slurry,
 along with fly ash, is removed from the scrub-
 ber for disposal.
   One disadvantage is that scrubbing with the
 water slurry cools the gases to the  saturation
 temperature of about 125°F. This produces a
 visible  and  less buoyant plume;  decreased
 buoyancy reduces  the  plume rise  and gives
 higher ground-level concentrations.
   The early work with limestone and lime
 scrubbing, done in England in the 1930's, was
 carried  out  with  direct addition  of  the
 reactant  to  the scrubber.  James Howden &
 Co., Ltd., and Imperial Chemical Industries,
 Ltd.,  installed lime and chalk scrubbing proc-
esses  at the Fulham power plant in London
 and the Tir John power plant in Swansea. In
pilot plant studies, Howden and ICI report24
 obtaining  60 to  70 percent removal of ni-
 trogen oxides and 97 to 99 percent removal
 of sulfur oxides. These data are not typical of
 later results, and they may have been compli-
 cated by difficulties in the analysis for NOX.
   In the  United  States, UOP studied  wet
 limestone  and dolomite scrubbing at Wiscon-
 sin Electric's Oak Creek power plant. Their
 data2 5 indicate about 20 percent removal of
 nitrogen  oxides.  Combustion Engineering
 studied dry limestone injection into the Boiler
 with  wet  scrubbing at  Detroit Edison's  St.
 Clair power  plant.2 6 Nitrogen oxide removal
 in this study also averaged about 20 percent.
 The close  agreement in  results obtained by
 UOP and  Combustion  Engineering and  the
 large disparity between these results  and those
 of Howden-ICI make the earlier data suspect.
   Combustion Engineering has been offering
its  process (Figure 4-8)  commercially. This
process has already been installed by Union
Electric  Co.  at  its Meramec plant, and it is
being installed by Kansas Power and Light
(KP&L)  at its Lawrence plant. In both cases,
the boilers being  outfitted are rated  at  125

                                                               TO STACK
                                                                           TO DISPOSAL
                  Figure 4-8. Limestone injection wet-scrubbing process.
megawatts. A number of troubles have been
encountered  with the Meramec plant unit.27
This particular unit has not yet been studied
with respect to NOX removal, however.

4.2.2  Potential Control Processes
   Several processes for flue  gas treatment
now in  the research stage have shown
potential for NOX removal  and are still to be

4,2.2.1  Reinluft Char Process
   The  Reinluft Char  Process uses a slowly
moving bed  of  activated  char  to remove
primarily  the sulfur oxides from flue gases.
Some NOX may simultaneously be removed.
   This process is described in Section 4.5.6 of
NAPCA publication  AP-52,  Control  Tech-
niques for Sulfur Oxide Air Pollutants. Control by Tyco Laboratories'
        Modified Lead Chamber Process
   The flue gas treating process being research-
ed by Tyco Laboratories, Inc., under contract
to NAPCA,  utilizes the  chemistry of  the
chamber sulfuric acid process to remove both
sulfur and nitrogen oxides. The basic features
of the process are shown in  Figure 4-9. Con-
centrated sulfuric acid is used to absorb both
the sulfuric acid mist and the mixture of NO
and NO 2 produced by recycling NO 2 into the
flue gas. The acid is heated and stripped of its
nitrogen oxides, which are partially converted
to nitric acid; the remaining nitrogen oxides
are then oxidized for recycle to  the flue gas.
The   stripped  sulfuric  acid,  diluted by
moisture from the flue gas, is concentrated in
an evaporator for recycle to the absorber with
the excess constituting a second acid product.
  Process consideration  requires  that high
NOX  scrubbing efficiencies  in excess  of 90
percent be obtained for the process to be in
NOX balance, since about ten times as much
NOX  is recycled to the  system as is  in the
original flue  gas. Thus, if a target level of 90
percent removal  of the original flue gas ni-
trogen oxides is set, scrubber efficiencies of
99 percent are  required. A second  problem is

                                                      NOX FREE GAS
        FLUE GAS
      (S02 AND NO)
H2S04 (MIST)   __
 (NO +N02)
                                      NO AND N02
                       H2S04 AND
                                                          HOT STRIPPING
                                HOT GAS
                               .  AND
                       Figure 4-9.  Flow diagram for Tyco process.
the very large  heat  load  required to recon-
centrate  the acid diluted by the water vapor
in the flue gas and to strip the nitrogen ox-
ides. This heat load amounts to as much as 40
percent of the  total  heat  load of the boiler.
This would not be an  easy requirement to
meet; recent developments suggest, however,
that this excessive heat load  can be greatly
  The flue gas treating process is still in the
early  stages   of  development,  and  Tyco
Laboratories,  Inc., is currently  investigating
         process modifications that could reduce proc-
         essing costs. Some of the modifications that
         have shown promise of success entail a change
         in the chemistry of  the process whereby the
         NO  oxidizer
       and  the NOX stripper  are

          1. Shaw, J.T. and A.C. Thomas. Oxides of Nitrogen
            in Relation to the Combustion of Coal. Presented
            at  the  7th  International Conference on Coal
            Science. Prague. June 10-14, 1968.

 2.  Elshout, A.J. and H. van Duuren. De Emissie van
    Stikstofoxiden als  Gevolg  van Verbranding-
    sprocessen  in  Vuurhaarden  van  Thermische
    Centrales.  Electrotechniek  (The  Hague).
    46(12):251-256, June 13, 1968.
 3.  Jonke, A.A. Reduction of Atmospheric Pollution
    by Fluidized Bed Combustion. Argonne National
    Laboratory.  Lemont, 111.  Monthly  Progress
    Report No. 8. March 1969.
 4.  Zeldovich, J. The Oxidation of Nitrogen in Com-
    bustion  and  Explosions. Acta Physicochim.
    U.R.S.S. (Moscow). 27(4):577-628, 1946.
 5.  Click, H.S., JJ. Klein, and  W. Squire.  Single
    Pulse Shock Tube Studies  of the Kinetics  of the
    Reaction N2 +  O2 ~£ NO between 2000  and
    3000°K. J. Chem. Phys. 27(4):850-857, October
 6.  Freedman, E. and J.W. Daiber. Decomposition
    Rate of Nitric Oxide Between 3000 and 4300°K.
    J. Chem. Phys. 34:1271-1278, April 1961.
 7.  Wise, H. and M.F. Freeh. Kinetics of Decomposi-
    tion of Nitric Oxide  at Elevated Temperatures. I.
    Rate Measurements in a Quartz Vessel. J.  Chem.
    Phys. 20:22-28, January 1952.
 8.  Yuan, E.L. et al. Kinetics  of the Decomposition
    of Nitric Oxide in the Range 700-1800°. J. Phys.
    Chem. 63:952-956, June 1959.
 9.  Sensenbaugh, J.D.  and J. Jonakin. Effect of
    Combustion Conditions   on Nitrogen  Oxide
    Formation  in  Boiler  Furnaces (ASME  Paper
    60-WA-334). Presented at  the American Society
    Mechanical  Engineers  Meeting.  November
    27-December 2, 1960. 7 p.
10.  Bamhart, D.H. and  E.K.  Diehl. Control  of Ni-
    trogen Oxides in Boiler Flue Gases by Two-Stage
    Combustion. J.  Air  Pollution Control  Assoc.
    ;0(5):397^06, October 1960.
11.  Bartok, W. et al. Systems Study of Nitrogen Ox-
    ides Control Methods for Stationary  Sources.
    Esso  Research  and  Engineering  Co.  under
    NAPCA  contract PH  22-68-55. In press. U.S.
    Department  of Commerce, National Bureau of
    Standards, Clearinghouse for  Federal  Scientific
    and Technical Information, Springfield, Virginia
    22151. 1970.
12.  Chaikivsky, M.  and C.W. Siegmund, Low-Excess-
    Air Combustion  of  Heavy Fuel-High  Tempera-
    ture  Deposits and  Corrosion. J. Engr. Power.
    S7(4):379-388, October 1965.
13.  Fernandez,  J.M., J.D. Sensenbaugh, and D.G.
    Peterson. Boiler  Emissions and  Their Control.
    Presented at Conference on Air Pollution Con-
    trol. Mexico City. April 28, 1966.
14.  Andrews, R.L., C.W.  Siegmund, and D.G. Levine.
    Effect of Flue Gas  Recirculation on Emissions
    from Heating Oil Combustion. Paper presented at
    61st Annual Meeting of the Air Pollution Control
    Association. St. Paul. Minn. June 1968.
15. Bienstock, D., R.L. Amsler, and  E.R. Bauer.
    Formation of Oxides of Nitrogen in Pulverized
    Coal Combustion. J. Air Pollution Control Assoc.
    76:442-445, August 1966.
16. Kopa, R.D. et al. Exhaust Gas Recirculation as a
    Method  of  NOX  Control  in an  Internal-
    Combustion Engine. (APCA Paper No. 60-72).
17. Grumer, J. et al. Effect of Recycling Combustion
    Products on Production of Oxides of Nitrogen,
    Carbon  Monoxide  and  Hydrocarbons  by Gas
    Burner Flames (Paper No. 37A). Presented at the
    16th Annual Meeting  of American Institute of
    Chemical Engineers. New York. 1967.
18. Nicholls, J.E., LA. El-Messin, and H.K. Newhall.
    Inlet Manifold Water Injection for Control of Ni-
    trogen  Oxides-Theory  and Experiment  (SAE
    Paper  No.  690018).  Wisconsin  University.
    Presented  at Automotive Engineering Congress
    and Exposition, 1969 Annual Meeting.  Detroit,
    January 13-17, 1969. 10 p.
19. Barnhart, D.H. and  E.K. Diehl. Control of Ni-
    trogen Oxides in Boiler Flue Gases by Two-Stage
    Combustion. J. Air Pollution  Control Assoc.
    70(5):397-406, October 1960.
20. Richter, G.N., H.H. Reamer, and B.H. Sage. Ox-
    ides of Nitrogen in Combustion—Effects of Pres-
    sure  Perturbations.  J.  Chem.  Eng.  Data.
    8(2):215-221, April 1963.
21. Richter, G.N., H.C. Wiese, and B.H. Sage. Oxides
    of Nitrogen  in  Combustion:  Premixed Flame.
    Combust. Flame. 6(1): 1-8, March 1962.
22. Manny, E.H. and S. Skopp. Potential Control of
    Nitrogen Oxide Emissions  from   Stationary
    Sources. Presented at 62d Annual Meeting of the
    Air Pollution Control  Association.  New  York.
    June 22-26, 1969.
23. Saunders, E. et  al. The Recovery of Nitric and
    Sulfuric Acids from Flue  Gases. Presented at
    American Institute  Chemical  Engineers
    Symposium. Cleveland. May 5, 1969.
24. Pearson, J.L., G. Nonhebel, and P.H.N. Ulander.
    The  Removal of Smoke and  Acid  Constituents
    from Flue Gases by a Non-Effluent Water Process
    J.  Inst. of Fuel. 8(39): 119-156. February 1935.
25. Pollock, W.A., J.P.  Tomany,  and  G.  Frieling.
    Removal  of  Sulfur Dioxide and Fly Ash  from
    Coal Burning Power Plant Flue Gases (ASME
    Paper No. 66-WA/CD-4). Air  Eng.  9(9):24-28,
26. Plumley, A.L. et al. Removal of SO2 and Dust
    from  Stack  Gases.  Proc.  Amer. Power  Conf.
    29:592-614, 1967.
27. McLaughlin,  J.F.  and J.  Jonakin. Operating
    Experience  with  Wet Dolomite   Scrubbing.
    Presented at 62d Annual Meeting of the Air Pol-
    lution  Control  Association.  New  York.  June
    22-26, 1969.

 5.1.1  Emissions
   The largest single source of nitrogen oxide
 pollutants  from stationary sources is boilers
 fired with fossil fuels. Differences between
 individual units may and frequently do have a
 major effect  on the production of pollutants,
 especially nitrogen  oxides since their emission
 is largely determined by the design and opera-
 tion of the equipment used. For example, the
 type of  firing,  and spacing  and location  of
 burners,  particularly in the large boiler units,
 can  have a marked effect on nitrogen oxide
 emissions, as  discussed in Section 4.1.
   The total amount of NOX  emitted in 1968
 by the electric utility industry for industrial
 and  commercial purposes is given in Figure
 5.1.2  Control techniques Conversion  to Lower NOx Producing
   If NOX emissions are high  with coal, inter-
 mediate with oil, and low with natural gas, on
 an equivalent Btu  basis,  substitution of the
 energy source  by  conversion to one of the
 lower NOX producing fuels offers one means
 of reducing total NOX emissions from a given
 piece of equipment. For instance, converting
 an ordinary boiler  designed for coal- and oil-
 firing to the burning of natural gas could result
 in a reduction in NOX emissions. The extent  of
 the possible  reduction is, of course, depend-
 ent on whether the  boiler can be converted to
 the lower NOX producing fuel, the design of
 the unit, the type of burners, the method of
 firing, and  the differences in emission factors
between  the  two fuels. In general,  the  sub-
stitution of either gas or oil for coal is expect-
ed to  give a significant  reduction  in NOX
(Table 3-3). Emission  results for oil and gas
are more comparable; data available indicate
that  emissions  from  very  large  units  can
actually be greater for gas- than for oil-firing.
   Although fuel type can be important in re-
ducing NOX emissions, the techniques used in
making a  conversion are of equal importance;
other changes made at the  same time may
easily result  in higher emissions.  Conversion
offers  an  opportunity  for  a  complete
revamping of the combustion system to one
of lower  polluting  tendencies. To  minimize
NOX, care should be  taken in selecting the
proper  location,  spacing,  and  design  of
burners, to assure as low a heat release as pos-
sible, to minimize flame temperature, and to
provide for  minimum excess air operation.
Provisions  for recirculating a portion of the
flue gases back to the flame zone should also
be considered. In addition, differences in the
emissivities of the fuel types  should be eval-
uated so  that full  advantage of  lower NOX
producing tendencies may  be realized.
   There   are  indications  that  chemically
bound nitrogen in fuel may play an important
part in NOX emissions (Section If so,
burning fuel with the lowest nitrogen content
could reduce NOX  emission  levels. For in-
stance, it  has been  noted  that switching to a
paraffinic,  "low sulfur"  fuel  oil  of low ni-
trogen content in public utility boilers result-
ed in nitrogen oxide emissions approximately
50 percent less than those  obtained when a
"high sulfur" fuel  oil of relatively high ni-
trogen content was burned.1

   Fuel availability and price structure in the
United States are complex. There are many
factors  involved  in  determining  ultimate
availability and price. In any  given  location,
fuel prices and availability vary widely and are
dependent  upon geographical location, user
category, quantity to be used, etc. While con-
version to a lower NOX producing fuel poten-
tially offers  some  assistance for the NOX
problem, widespread substitutions undoubt-

edly would have  an adverse effect  on the
entire fuel supply structure; ultimately, the
availability, of fuel for this purpose would be
limited. The subject of fuel availability, price
structure, and conversion is discussed in some
detail in Section 4 of AP-52, Control Tech-
niques for Sulfur Oxide Air Pollutants. Fuel Additives
   For purposes of this document, a fuel ad-
ditive is  a substance added to any fuel to in-
hibit  formation of  NOX when  the  fuel  is
burned. The  additive can be liquid, solid, or
gas; for liquid fuels, the additive should pref-
erably be a liquid soluble in all proportions in
the fuel, and it should  be  effective  in very
small concentrations. The additive should not
in itself create an air pollution hazard nor be
otherwise deleterious to equipment and sur-
   As far as is known, no  fuel additive  is
available as yet to inhibit the formation of
NOX in combustion systems. The possibility
exists, however, that such an additive may be
developed in the future, and it is believed that
an additive of  this type needs  only to be
capable of partially reducing NOX formation
to be useful. Combustion Controls
   Combustion modifications as  potential
NOX control techniques for stationary sources
of emissions were discussed in Section 4.1.
These techniques have been investigated and
developed in numerous laboratory test instal-
lations and in a limited number of successful
applications  in commercial  public  utility
   Sufficient  knowledge  now exists to have
reasonable confidence  in  the potential of
these techniques for the  abatement of NOX
emissions in much of the existing combustion
equipment. Most of the field test  work has
been done on boilers, especially some of the
larger sized  units  in  public utility  power
plants. There is good reason to be  confident
of  the success  of  such  modifications to this
type  of equipment.  In  smaller units, un-
foreseen  problems,  resulting from  size or
operating requirements, may limit the amount
of NOX reduction.
   Extensive trials of these control techniques
will  be necessary in  many different instal-
lations in a range of sizes to determine their
effectiveness and identify  limitations. It is
reasonable to assume  that NOX reductions of
at least 20  to 30  percent using coal and
perhaps as high as 60 to 90 percent using oil
and gas can be achieved in much of the exist-
ing combustion equipment.  It is obvious that
not  all of the proposed  control techniques
will be applicable to  every  process, but it is
entirely possible  that in most cases, one or a
combination of methods may be found to be
effective in achieving reasonable control.
     Low-excess-air  combustion, flue  gas
recirculation, two-stage combustion, water or
steam injection, and possible combinations of
these methods  appear to  be the most prom-
ising  potential control techniques (Section
4.1.1).  In  large power-generation-station
boilers, fired by  gas and  oil,2'4  applications
have shown that low-excess-air operation may
reduce NOX emissions by 30 to 60 percent,
the  range  of  reduction  depending  on the
lowest level of excess air achieved, design of
the boiler, and the type of firing.
   Similarly, flue  gas recirculation has proved
to be  effective in suppressing NOX emission
from  these  installations.  In a utility  boiler,
reductions of  about  20  percent have been
reported when the  flue  gas was  reinjected
through ports in  the  bottom  of the furnace.
In a test  installation  using  domestic heating
oil, reduction of  NOX emissions by 50 to 65
percent was achieved by  mixing recirculated
flue gas with the  combustion air before com-
bustion.5 Two-stage combustion  was found to
be successful in  reducing NOX  by 30 to 47
percent in utility boilers in  cooperative in-
vestigations conducted in  southern California
by  the Babcock &  Wilcox  Company and
Southern  California  Edison  Company.6
Tangential firing,  as opposed to  front wall or
opposed firing, was  also found to produce
from 30 to 40 percent less NOX-2 >7 The com-
bination  of  reducing   excess  air  and
eliminating firing in the top eight burners in
two  750-megawatt boilers at  the Pacific Gas

and Electric Company's Moss Landing Power
Station successfully  reduced NOX  by about
90  percent,  from 1,500 ppm down to an
average of 175  ppm.8 Substoichiometric air
was supplied  to the bottom of 40 burners and
the top 8 burners were used as ports to supply
air for complete  burnout.
  Steam or  water injection as a method of
control has not  as yet been demonstrated as
an effective control technique. The quantities
of steam or water required to obtain a large
reduction in  NOX may make this application
unrealistic and uneconomical, but the use of
moderate quantities of steam or water to trim
peak  temperatures,  when  used with  other
combustion control modifications, may prove
reasonable and helpful.

5.1.3   Costs
  Estimated  costs of control  of NOX emis-
sions by a new 1,000-megawatt utility power
plant are shown in Table 5-1. Approximately
the same figures will  apply to the modifica-
tion of an existing plant using low excess air
                   FOR A 1,000-MEGAWATT BOILER USED 6,120 HOURS/YEAR
                            BY SELECTED METHODS AND BY TON
Control method
(base case)

Low excess


Low excess air
plus two-stage
Flue gas re-

Low excess air
plus flue gas
Water injection

- 79,OOQb
       Uncontrolled method, total NOX emitted; no reduction.
       bin instances wherein the installation of control equipment results in a savings, control costs
        are listed as negative numbers.

and/or two-stage combustion,  excluding  the
costs  of downtime and  assuming that  space
and  facilities are available.  NOX  reduction
using low excess air shows a net return, for a
large plant where the necessary control equip-
ment and operators are already available. Limitations with Large
       Coal-Fired Boilers
   Low-excess-air  operation  and  two-stage
combustion have not yet been found feasible
for application in large  pulverized coal-fired
boilers.   There are serious engineering  dif-
ficulties  to be overcome  in this type of opera-
tion,  despite the fact that small-scale labora-
tory  experiments indicate  the  technology
might be useful, if it could be made to work
(Section  Achieving  low-excess-air
operation is dependent on obtaining a reason-
able  uniformity  of air/fuel  ratios  in  all
burners,  to  avoid unburned  fuel, increased
carbon  monoxide and  hydrocarbon in flue
gas,  and reduced safety  of  operation. With
pulverized  coal, this is difficult because of the
imbalances  between  primary  air  and  coal
streams in  individual burners, and the further
imbalance  created by mixing  these  streams
with  secondary air at the burner. Use of low
excess air tends to increase the production of
CO and  hydrocarbons. Decreasing  excess air
increases flame  temperature in the furnace,
which can promote slagging and corrosion of
furnace  tubes; but a small amount of steam
or water injection could possibly be used to
control the problem of flame temperature.
   Two-stage combustion has not been applied
in pulverized coal-firing as yet.  This modi-
fication  delays the combustion process, how-
ever,  and its appplication  to pulverized coal
may increase unburned fuel losses to a point
where efficiency  is impaired and the process
becomes uneconomical.
   The effectiveness and  applicability of NOX
control  through  combustion modifications to
industrial  and commercial  boilers will, in
general,  follow the same pattern  as discussed
above for public utility  power plant boilers,
especially  when  these  units are  large and
comparable in design and configuration to the
electric utility  boilers. Since commercial and
industrial boilers are normally smaller in size,
however,  the cost of installing potential con-
trols on a unit basis could be expected to be
higher. When  these  boilers  are fired  by
pulverized coal, application of the techniques
may also  be limited, as, for example, in the
case of utility units. There is reasonable as-
surance, however, that the suggested  controls
can be  effective in gas- or oil-fired boilers of
this category.   Packaged Boilers
  The  majority  of packaged water-tube or
fire-tube boilers are fired with natural gas or
oil. Those fired with coal are usually stoker-or
grate-fired.  For  physical  reasons, packaged
fire-tube  boilers  have a maximum rating of
about  25,000  pounds of steam  per hour;
while water-tube packaged boilers of a capac-
ity  as great  as 250,000 pounds/hour  are now
being sold.  In general, the larger the installa-
tion, the  easier it is to justify installation of
combustion  modification control  equipment.
Modifications to control NOX can, however,
complicate  the automatic controls required
for reliable  operation.  For the gas-  and oil-
fired packaged boiler, modifications may not
be as difficult as they are for coal-fired instal-
lations. Low-excess-air firing  with oil-  and
gas-fired packaged  boilers might be relatively
easy to achieve and have the potential of re-
ducing NOX by 30 to 50 percent.
  For low-excess-air firing, burners will have
to be redesigned and additional instrumenta-
tion supplied to provide closer control of the
air/fuel ratio in  correspondence with  load
modulation. Burner redesign may be relatively
simple and inexpensive; only minor modifica-
tions to the existing burners may  be required
to achieve the level of excess air desired. The
costs of these modifications, however, will be
dependent on the number of burners, the size
of the equipment, and  the necessary changes
in the  control system. These costs will be par-
tially or completely offset by increased  effi-
ciency and decreased maintenance  and operat-
ing costs. An increase of from 1 to 2 percent

in efficiency may result, and the correspond-
ing fuel savings will help to defray alteration
   Many  packaged  boilers  are  designed  for
pressure  firing,  with  the  forced  draft  fan
mounted as a part  of the burner. When flue
gas recirculation  is  considered for NOX con-
trol, the position of the forced draft fan may
complicate the installation of necessary modi-
fications. Some means must be provided for
taking gas near the  boiler exit and injecting it
into the  air stream  so that it mixes with the
combustion air. Normally, this will  require a
separate fan and  additional duct work. Space
is a limiting factor in packaged boilers, and
should be taken into consideration when flue
gas is to be recirculated. Recirculation can re-
duce NOX emissions in packaged boiler instal-
lations potentially  by 30 to 40  percent; if
combined with low-excess-air firing,  reduc-
tions up to 60 to 70 percent may be  achieved.
Recirculation may  also make it possible  to
achieve lower-excess-air  operation  without
smoke. Specific  costs  for conversion of  a
packaged boiler designed for pressure-firing to
flue gas recirculation will be highly dependent
on the amount of gas to be recirculated (fan),
ductwork required,  the size  of the  unit, and
the controls already available.
   Two-stage combustion  has not been used
with packaged boilers and  may be difficult to
install except on larger units. An exception
might  be  the stoker-  or  grate-fired  units,
where overfire air  may create the necessary
turbulence to improve  combustion. In some
packaged boilers, compact size and small com-
bustion space will make two-stage combustion
impractical. Two-stage combustion delays the
combustion  process  by  accomplishing the
initial  combustion phase  under substoichi-
ometric conditions and completing burnout
downstream with air injected through overfire
air ports.  In  many instances, the combustion
zone is too  small for this  technique so the
potential effectiveness has to be evaluated for
each specific design. When it can be applied,
two-stage  combustion could achieve the same
order of NOX reductions (30 to  40 percent) in
smaller packaged boilers as in the larger units,
but costs increase as sizes decrease.

  As has been pointed out in previous discus-
sions, the combustion of fuels with air forms
oxides of nitrogen by the  direct combination
of molecular nitrogen  and oxygen in the
flames. The thermodynamics and kinetics of
the reaction  indicate that  control of NO for-
mation must be  accomplished by lowering
flame  temperatures,  limiting the availability
of the reactants (oxygen and  nitrogen), or
controlling net reaction rates. These principles
apply to combustion inside engines as well as
in a less confined space such as in a furnace.
  The stationary engine  population of the
United States includes both reciprocating pis-
ton  engines  and  gas  turbines. The United
States  reciprocating piston engine population
is distributed9 as given in Table  5-2.

5.2.1   Piston Engines
  No  information on the power capabilities
of the total United  States installed piston
engine population has been located. From dis-
tillate fuel oil consumption in "industrial and
                        Table 5-2.  STATIONARY RECIPROCATING PISTON
                               ENGINES IN UNITED STATES, 1967
Engine type
Full diesel
Dual fuel
Spark-ignition gas
Liquid fuel oil
Liquid or gas
Percent of Total3
                     alt is estimated 85 percent of these engines are supercharged;
                      about 1 percent are the precombustion-chamber type.9

other" stationary engine applications,11 how-
ever, it is estimated that the total current pis-
ton engine capacity may approximate 70 mil-
lion hp,  assuming that two-thirds engines are
gas-fired, and using service time factors of 0.6
for prime  movers in manufacturing installa-
tions, 0.2  in electric utility uses, and a fuel
consumption  of  0.4 pound  fuel/brake  hp-
hour. This estimation is probably low because
some  large  oil-fueled diesel  engines  un-
doubtedly use heavy fuel  oil, which would
not appear in the distillate fuel figures. Esti-
mates  of total United  States NOX produced
by stationary engines are given in Table 5-3.    Emissions
   The NOX emission characteristics of various
types of piston engines  are listed in Table 5-3.
It should be emphasized that these data are
based on experiments  using relatively small
engines,  such as  200-hp truck engines,  and
there may be some question about extrapolat-
ing the data to larger engines. It is believed,
however,  the data do provide a  valid indica-
tion  of the effect of various engine variables.
Data on  the gas turbine engine are from con-
ventional aircraft turbojets. Some preliminary
and tentative observations can be made:
   1. Turbocharging  a piston engine may in-
     crease NO production.
   2. A precombustion-chamber piston engine
     seems  to produce  about  one-third as
     much NOX  as  a direct-injection, piston
   3. An oil-fired turbojet engine may produce
     about 0.1 as much  NOX as an oil-fired,
     direct-injection diesel engine.
   Table 5-4 summarizes data obtained in tests
on two large dual-fuel (oil and gas) stationary
diesel engines. The larger engine operated at a
lower brake  mean effective  pressure (BMEP)
and engine speed than the smaller engine. The
air/fuel ratio was higher than those normally
found in automotive diesels under similar load
conditions,  and  extremely  low particulate
emissions  were  observed. The  data  suggest
that slower engines produce higher concentra-
tions of NOX; this is to  be  expected for the
longer  residence time of the gases at high tem-
peratures in the engine cylinders.   Control Techniques
  These  observations,  together  with other
general background knowledge, suggest that
the  following techniques  to  control  NOX
Type of stationary engine
Reciprocating engine
Diesel, precomb. chamber'5
Diesel, precomb. chamber
Diesel, direct injection
Diesel, direct injection
Gas engine, spark ignited
Gas engine, spark ignited
Gas turbine


total hp,




Oxides of nitrogen emissions,
Ib NOx/lb fuel

lbNOx/106 Btu

  aEstimated figures in parentheses are based on assumption that supercharging doubles NOX emissions.
  "Estimates are based on assumption that gas will emit the same amount of NOX as turbo fuel. This may be
   open to question, and probably penalize the turbojet engine. If the turbine engine responded to gas fuel
   in the same way as the diesel engine, NOX emissions would be of the order of 0.04 Ib/million Btu, a very
   low air pollutant emission level.

                          DUAL-FUEL (GAS AND DIESEL) ENGINES
Airflow, Ib/hr
Air/fuel ratio
Exhaust flow, scfrn3
NOX, Ib/lb fuel
NOx,lb/106 Btu
NOX, ppm
Percent rated load
Brake horsepower
Brake mean effective
pressure, psi
Engine speed, rpm
Engine 1
Diesel No. 2

Natural gas

Engine 2
Diesel No. 2

Natural gas

       a29.92 in. Hg, 60°F.

emissions from stationary engines may be pos-
sible. Applicability to both reciprocating pis-
ton engines and turbine engines is discussed.  Combustion  Chamber  Design—
The  beneficial effect of  the  precombustion
chamber (Table 5-5) suggests that combustion
chamber design and modification may have a
significant effect on NOX emissions.  The pre-
combustion chamber  provides a type of two-
stage combustion, the  first stage  under ex-
tremely rich fuel conditions  and the second
stage, very lean. It can be applied readily to
oil-fired piston engines. It does not  seem to
be as  easily adapted to  the  gas-fired  type.
Initial  cost  of the engine may be increased
slightly, and there may be an  adverse effect
on thermal efficiency if the engine is operated
under  conditions  differing  from optimum
design. Examples of precombustion  chamber
engine designs are  the Caterpillar, the M.W.M.
(Motoren-Werke Mannheim),  the  Mercedes-
Benz OM-621, and the  larger Daimler-Benz
  Other combustion  chamber modifications
that may influence NOX emissions and require
further  investigation include  the use of tur-
bulence chambers, as used in engines manu-
factured  by   International Harvester,
Waukesha,  John  Deere, and Deutz,, and
"energy cells," as used in Minneapolis-Moline
engines. Energy cells provide controlled com-
bustion to prevent high peak pressures and
rough operation.  Fuel Type—There are some in-
dications that by increasing fuel API gravity
(i.e., decreasing specific gravity) a decrease in
NOX emissions occurs in a diesel engine, as
shown in  Table 5-5. NOX  production  de-
creases about 25 percent when the gravity of
the fuel is increased from 30.9 to 42.8° API.
This trend  is also supported by the low emis-
sions obtained with natural gas. There are, of
course,  accompanying  changes  in  boiling
range, viscosity, and cetane number; the latter
effect is probably significant also.  It  is not
clear  which  is  the  controlling  variable,
although the mid-boiling point, or viscosity,
gives the most consistent trend. Thus it ap-
pears that it may be possible to modify NOX
emissions by  tailoring fuel properties.  It will
be appreciated, however, that the use of spe-
cial fuels will add  to fuel costs  because  of
problems associated with availability, refining,
and marketing.   Exhaust  Recycle-This method
has been shown to be an effective means of
controlling  NOX emissions in gasoline engines,
furnaces, etc.12 It may also reduce NOX from
diesel engines if the fresh  air-to-fuel ratio is
held constant, but testing will be required to
confirm this. It will probably mean a decrease
in the  maximum power  capability  of  the

Fuel characteristics

FIA Anal.,
Est. net
heat, value
Boiling Point F

@ 100° F

NOX emissions,
Ib NOx/lb fuel
aPrecombustion chamber, turbo-charged, rated speed, about 25 to 1 air fuel ratio.
bActual data doubled to correct for turbo-charging; data at full throttle, 72% Max. IMEP and 72% stoichiometric
fuel, i.e., A/F = 24.
engine and will  involve a modest increase in
cost since additional  piping  and a  control
mechanism will be needed.  Optimum Air/Fuel  Ratio-The
maximum  power  setting of most stationary
diesel engines is limited by smoking at about a
20-to-l air/fuel ratio.
   It is possible,  therefore, to vary the air/fuel
ratio only by reducing the  amount of fuel in-
jected, which results  in  a proportional in-
crease  in the air/fuel ratio and a correspond-
ing decrease in power.  The  indications are
that as fuel rate is reduced, NOX emissions in
pounds per hour are a  linear  function  of
power.14  This means that the rate of  NOX
formation per pound  of fuel (or million  Btu)
is constant over the  range  in which  specific
fuel  consumption is constant and no benefit
results from  the  overalll leaner or higher air/
fuel operation.
  Gas engines, on the other hand, are usually
set at an air/fuel  ratio giving maximum econ-
omy  of operation; about 2.5 percent excess
oxygen equivalent to about  19-to-l air/fuel
ratio. As shown in Table 5-6, this is very close
to the conditions at which maximum NOX
production is obtained. Table 5-6 also indi-
cates that NOX production can be reduced by
changing to either richer or leaner operation.
Using  richer  mixtures creates the risk  of in-
creasing  the  emissions of  unburned  hydro-
carbons and  CO. This hazard can be avoided
by using higher air/fuel ratios. For example,
with  methane  it is possible to operate with
only 70 percent of the stoichiometric amount

fuel, %

Ib air/lb fuel

Power, % max. IMEP
NOX production

Ib NOx/lb fuel

aFull throttle; maximum power spark; 1,000 rpm; compression ratio, 8/1; fuel, methane.


of fuel without misfiring the engine. At this
point, NOX emissions are reduced to about 30
percent of their maximum value. The penalty
for this type of operation is a loss of power
(e.g., 30 percent at 70 percent stoichiometric
fuel)  equivalent to a 30%  increase in the
initial cost of an engine.
  The reason that adjustment of air/fuel ratio
controls NOX emissions in gas engines and not
in injected diesels  is because the gas engine
operates with a premixed, homogeneous air/
fuel mixture. Thus, flame temperatures are re-
duced  at  high  air-to-fuel ratios.  This is not
the case with the heterogeneous air-fuel mix-
ture present in a diesel engine in which liquid
fuel is sprayed  into the air in the combustion
chamber and combustion takes place in fuel-
rich places.   Modification of Fuel Injection
Timing—It is possible to vary the pressure and
temperature  conditions  in  a fuel-injection
diesel engine by changing the timing of the
fuel injection,  by changing the point in the
cycle at which  fuel injection is started and/or
the rate of fuel addition. Two-stage fuel injec-
tion is also  a possibility. The effect of these
variables on NOX production as well as power
output  and/or  thermal  efficiency  requires
clarification.   Variable  Compression  Ratio-
Variable  compression ratio-compression igni-
tion engines have been developed for the mili-
tary.1 l The compression ratio is changed by
hydraulically raising or lowering the height of
the piston. The pistons are undoubtedly more
expensive  than the conventional type  and
may require somewhat  more maintenance.
Such a system tends to maintain a preset peak
combustion pressure in the combustion cham-
ber. Presumably this would avoid the exces-
sively severe cycles which occur frequently15
in the conventional diesel engine and cause a
disproportionate part of the NOX emissions.
The NOX  emission characteristics of  the vari-
able- compression engine are thus of consider-
able interest.   Treating  Exhaust—NOX can, of
course, be controlled by removal from the ex-
haust gases. The various possibilities would be
analogous to the methods developed for stack
gas treatment previously described.12  Compli-
cating factors peculiar to  the  diesel engine
may be the larger amounts of air used in the
diesel  cycle  (30  percent  or  more above
stoichiometric requirements)  and a relatively
low  exhaust  temperature.  Adding  control
equipment to an engine for exhaust treatment
will obviously increase both initial costs and
operating costs.
5.2.2  Turbine Engines
   Gas turbine  engines in  stationary  applica-
tions in the U.S. are estimated to have a total
current capacity  of 51 million horsepower
(based on Reference 12 assuming 60 percent
of the worldwide stationary  turbine popula-
tion is in the U.S.). Approximately 90 percent
of these engines are fueled with natural gas.   Emissions
   Emissions of NOX from  a  turbojet engine
are already quite low,  about  one-tenth those
of a diesel engine. Even correcting for a dif-
ference in thermal  efficiency (20  versus  30
percent),  a  gas turbine would presumably
emit only about one-sixth of the NOX emitted
by  a diesel.  Further  improvements can  be
made, however, and may be  worthy of con-
sideration if costs,  in terms  of  not only
money but economy,  performance,  and  the
like, are not too high.   Control Techniques   Combustion  Chamber Design-
Exhaust  recirculation has been previously  re-
ported1 2 as capable of making marked reduc-
tions  in  NOX  emissions in an  oil-fired  do-
mestic heater.  It would seem  possible that an
analogous type of recirculation could be de-
veloped within the combustor of a gas turbine
engine, with similar benficial effects  on NOX
  Two or more stage  combustion  is also  an
effective general  method  for holding down
NOX  formation.  In  this type of operation,
partial combustion is carried out initially in a
fuel-rich  primary zone; combustion is then
completed by  the addition of more  air. The

beneficial effect  is probably due to keeping
combustion temperatures down and restrict-
ing the  availability of  oxygen  during the
critical combustion phases.
   It appears,  therefore,  that studies of com-
bustor design  directed  toward encouraging in-
ternal  exhaust recirculation and controlling
oxygen  availability would  be fruitful ap-
proaches to reducing NOX formation in tur-
bine engines. While such studies have not yet
been  reported, it is believed that  improved
combustors could be developed without any
major increase in cost. The effect of design
changes on turbine performance criteria needs
to be studied to  determine what penalties
would be involved by this approach.   Effect of Fuel Composition-It
is well known that the radiation  character-
istics of a flame in a gas turbine engine in-
crease as the  hydrogen content of the fuel is
decreased. This suggests  that the temperature
of the flame  from a fuel with low hydrogen
content  may  be  lower,  because of increased
radiation to the  combustor walls,  than that
from a fuel containing more hydrogen. NOX
emissions in  turbine engines may  thus vary
with fuel characteristics. Data are required. It
is understood, of course, that the use of spe-
cial fuels usually  results  in an increase in fuel
cost.   Treatment  of  Exhaust-The
considerations in treating exhaust gases from
a turbine  engine are essentially the same  as
the diesel engine, with the added  compli-
cation that the turbine utilizes about twice as
much air.  Reduction  of  Turbine  Inlet
Temperature-Gas turbine efficiency depends
directly on the differences in temperature be-
tween  the inlet and  outlet gas; any change
that lowers the inlet temperature requires a
larger turbine for a given power output.
  It is possible that NOX  emissions can  be
lowered  by reducing turbine inlet tempera-
tures. For example, observations demonstrate
that the  NOX  level is determined  by  the
degree of progress of the formation reaction
at the time of thermal quenching.16 It ap-
pears that at low turbine inlet temperatures
this quenching  occurs earlier in the formation
reaction than at higher  turbine inlet tempera-
tures (Table 5-7). The drop in NOX formation
at the higher air/fuel ratios (i.e., lower turbine
inlet temperature) is probably because of the
larger quench  air ratio (higher  excess air/fuel
  This effect requires clarification. The pen-
alties for such a step, however, are reflected in
installation  and  operating  costs.  It  is  esti-
mated  that a 100° F decrease in turbine inlet
temperature will decrease power by  10 per-
cent and  will  increase  fuel consumption by
about 5 percent.

5.2.3  Costs
  Little information is  available on the costs
of reducing NOX emissions from stationary
engines.   Diesel Engines
  Currently, the only effective way to reduce
NOX is to use engines with  design features
that  tend  to   minimize NOX production.
                             EXPERIMENTAL COMBUSTOR RIG16
Air /fuel ratio,
Ib air/lb fuel
at exit of
combustor, F
T minus 370
NO concentration, ppm
to 15/1 air /fuel
NOX, Ib/lb fuel
aDirectly related to temperature of gases at combustor exit.


Engines employing precombustion chambers,
which  minimize  NOX,  may  have  slightly
higher initial costs and fuel consumption than
engines without  precombustion  chambers.
The magnitude of the cost differential  would
have to  be  established by obtaining price
quotations for specific applications.   Gas Engines
   Reduction in  NOX  by  increasing air/fuel
ratio from  the optimum  power setting  de-
creases power output by as much as 30 per-
cent. This would, in turn, increase original
engine costs by as much as 30 percent.
   Water injection into the intake manifold of
 gas  engines has promising potential for NOX
 removal. The initial cost of a system for water
 injection was roughly estimated at $2,000 per
 engine. With water costs of  20^ per thousand
 gallons, the costs of reducing NOX  emissions
 would be as given in Table 5-8.
   Table  5-8 shows very high costs per unit of
 NOX removed for small engines, and low costs
 for  large engines. The costs are based on 20
 percent capital charges,  an injection  rate  of
 1.25 pounds of water per pound of fuel, and
 75 percent reduction in NOX.
Engine, hp
Cost, $/yr
Cost, $/ton
NOX eliminated

 1.  Bartok, W.  et  al.  Systems Study of Nitrogen
    Oxide Control Methods from Stationary Sources.
    NAPCA  Contract  No.  PH-22-68-55,  Interim
    Status Report. May 1, 1969.
 2.  Sensenbaugh, J. and J. Jonakin. Effect of Com-
    bustion Conditions on Nitrogen Oxide Formation
    in Boiler  Furnaces  (ASME Paper No. 60-WA-
    334). Presented at the American Society Mechan-
    ical Engineers Meeting. November 27- December
    2, 1960. 7 p.
 3.  Novobilski, J. A. Top Steam Plant Performance
    Depends on  Good Test and  Results  Program.
    Combustion. 38(8): 10-15, February 1967.
 4.  Chaikivsky, M. and C. W. Siegmund. Low Excess
    Air Combustion of Heavy Fuel-High Temper-
    ature  Deposits and Corrosion.  J. Engr. Power.
    57(4):379-388, October 1965.
 5.  Andrews,  R. L., C.  W.  Siegmund, and D. G.
    Levine. Effect  of  Flue  Gas  Recirculation on
    Emissions  from Heating Oil Combustion. Pre-
    sented at 61st Annual Meeting of the Air Pollu-
    tion Control Association. St.  Paul, Minn. June
 6.  Barnhart,D. and E. K. Diehl. Control of Nitrogen
    Oxides in  Boiler Flue Gases by Two-Stage Com-
    position.  J.  Air  Pollution  Control  Assoc.
    70(5):397-406, October 1960.
 7.  Fernandez, J. M., J. D. Sensenbaugh, and D. G.
    Peterson. Boiler Emissions and Then- Control.
    Presented at Conference on Air Pollution Con-
    trol. Mexico City. April 28, 1966.
 8. Private Communication  with E. W.  Munson.
    Santa  Cruz County United Air Pollution Control
    District. Salinas, Calif. May 9, 1969.
 9. Elonka, S. Oil and Gas Engines: This Year's Sur-
    vey Includes 873 Engines Located Around the
    Globe. Power. 112:S12-S19, October 1968.
10. Power. 110: October 1966.
11. Market Estimates by Private Communication.
12. Bartok, W.  et al. Systems Study for  Nitrogen
    Oxide  Control Methods from Stationary Sources.
    Esso  Research and  Engineering Co.  under
    NAPCA contract PH 22-68-55. In press. U.S. De-
    partment  of  Commerce, National  Bureau of
    Standards, Clearinghouse  for Federal  Scientific
    and Technical Information, Springfield, Virginia
    22151. 1970.
13. Lee, R. C. and D. B. Wimmer. Exhaust Emission
    Abatement by Fuel Variations to Produce Lean
    Combustion (SAE Paper  No. 680769). Phillips
    Petroleum Co. October 1968.
14. Marshall, W. F. and R. W. Hum. Factors Influ-
    encing  Diesel Emissions   (SAE Paper  No.
    680528). Presented at West  Coast Meeting. San
    Francisco. August 12-15, 1968. 9 p.
15. Patterson, D. J. Cylinder Pressure Variations, A
    Fundamental  Combustion Problem.  S.A.E.
    Transactions, 75:621-631,  1967.
16. Sawyer, R. F. et al. Air Pollution Characteristics
    of Gas Turbine Engines. University of California,
    College of Engineering.


                    6.   OTHER COMBUSTION PROCESSES
  Major  amounts of the total fuels burned
and NOX emissions  released in the United
States  are  associated  with small-scale com-
bustion processes. These include important
nonindustrial uses in domestic and  commer-
cial heating, hot water supply, a wide variety
of incinerators, and burning of solid wastes.
This is a much larger stationary use of fuels
than the electric utilities, but it is a less in-
tense type of combustion. Its contribution to
NOX is significant, particularly in residential
  "Commercial"  in  this discussion denotes
multiple-unit heating and  small  appliances
used in business,  which are similar to  house-
hold equipment and appliances such as space
and water heaters, ranges, and clothes dryers.
  Some  30 percent  of the total  fossil fuels
used in the United States for energy produc-
tion (transportation excluded) is consumed in
domestic and commercial heating. This repre-
sents 12,432 X  1012 Btu  annual consump-
tion, and it substantially exceeds the 9,994 X
1012  Btu required for power generation by
the electric utilities.1
   Limited statistics2 for the breakdown into
fuel types within this sector show the follow-
ing division for the approximately 29 million
domestic, automatic central-heating units in
use  in  1967: gas  burners, 18,184,321; oil
burners,  10,692,825; and  coal  stokers,
   The amount of fuel consumed3 in 1966 for
space heating and  cooking  only  is shown in
Table 6-1 by fuel type and equivalent Btu
   The use of gas in the United States for do-
mestic purposes only was  estimated,5  as of
January 1968, as shown in Table 6-2.
   Table 6-2  statistics show the extremly large
Fuel type
Natural gas, 109ft3
Distillate fuel oil,
42-gal bbl
Residual fuel oil,
42-gal bbl
Equivalent heating value,4
Number of
Type of use
Heating units
( 1 and 2 family)
Hot water

Avg heat use,
105 Btu/unit-yr

Total heat use/yr,

number of NOX emitters involved in the do-
mestic and commercial heating categories, and
the substantial quantities of fuel used.
6.1.1  Emissions
   The rate at which fuel is used for domestic
appliances, such as water heaters, ranges, gas
refrigerators, and clothes driers, remains rea-
sonably constant throughout the year, but a
much  larger amount  is required for space
heating during the  6-month  winter period.
Thus,  the rate  of domestic fuel consumption
increases during the winter over the average
percentage consumed annually; in mid-winter
months, in many areas, it can be well over 50
percent of all fossil fuels consumed.
   The contribution of residential space heat-
ing to NOX is  suggested by data on seasonal
and diurnal effects  on fuel  usage  for space
heating from the Continuous Air Monitoring
Projects (CAMP) and other sources, which in-
dicate that the variations in 862  emissions in
urban areas may be correlated with changes in
residential  heating  throughout the  day and
throughout the year.6-7'8
   Significantly, the  products of  domestic
combustion are vented close to ground level.
Under adverse  conditions  such as stagnation
or  aerodynamic downwash,  these  gases be-
come  an immediate part of the  ambient at-
mosphere. The extent  of this pollution is not
as visible with clean burning gas and oil fuels
as chimney  soot was in the past, when  coal
was the main fuel, but the NOX and other less
visible combustion products  formed are still
present. Data from CAMP and other studies
pointed out that chimney gases are poorly dis-
persed; this means that a given amount of
NOX at ground level will have a greatly aggra-
vated effect on ambient air compared to the
same amount emitted from a tall utility stack.
The air layer into which the combustion gases
are distributed is thinner at ground level, and
under  adverse conditions this may be signifi-
cant.  Commercial  buildings and  apartments
will presumably show an intermediate effect.8
  The effect of unit size and severity of oper-
ation report by Mills9 et al. from Los Angeles
data has a direct bearing on NOX emissions
 from domestic heating units  (see Section 3).
 According to their correlation (Figure 3-1), a
 typical gas-fired domestic furnace of 150,000
 Btu capacity produces NOX at about 12 per-
 cent of that produced in a large utility boiler
 of 1010 Btu, for a given amount of fuel. A
 similar ratio  exists for oil-fired  equipment.
 Although  domestic and commercial heating
 units consume some 125 percent of the fuel
 used in utility boilers, their total contribution
 to NOX in the United States  will be only 12
 percent of this ratio, or about 7 percent of all
 NOX  in the United  States, compared  to 44
 percent for the utilities.

6.1.2   Control Techniques
   The  combustion  modifications discussed
above (see  Sections 4.1 and  5.1) as promising
control techniques are theoretically applicable
to small domestic heating units, but there are
serious  obstacles  to their practical  use. The
trend,  in fact, is in the other direction, since
high  efficiency and  compact  size in  the
design  of modern domestic units lead toward
more  intense,   concentrated,  and  high-
turbulence  flames. The result is a higher flame
temperature, and inherently more NOX  per
unit of heat released.1 ° Many domestic instal-
lations are  improperly installed, out of adjust-
ment, or poorly maintained, particularly in re-
gard to the cleaning of flues  and chimneys.
The problem  has  been serious in  European
cities, as well as in the United States.11 Con-
certed  action by the public  is required if the
problem  of faulty and poorly maintained in-
stallations is to be solved.
   The specific modifications considered most
promising (low excess air, flue gas recircula-
tion, two-stage combustion, and water injec-
tion),  all  require  additional equipment and
operating  controls.  Modifications  for each
unit are expensive; the smaller the unit,  the
more  expensive these controls will be com-
pared to total unit cost. Modifications other
than proper adjustment are less expensive and
are easier to apply in new installations; design
and engineering research is required  to  de-
velop such new equipment.

  The  trend  in small  boiler  operation  is
toward  automatic controls. Although certain
of the above modifications for NOX control,
such as burner design and placement, can be
built  into  automatic boilers, other features
will  increase  the complexity  of operation.
Such  problems could make safety the deter-
minant factor.
  Multiple-unit structures, such as apartment
buildings, schools, and commercial houses, in
the  larger  northern cities  require  heating
plants of up to 500 horsepower rating. Com-
bustion modifications are more practical  in
this size range for NOX reduction. Such  con-
trol means are discussed  in full in Section 5.2
  A partial reduction in NOX can be realized
by converting from coal  to other fuels.  This
conversion is substantially complete in most
areas, so that there is little room for further
improvement in this direction. The NOX emis-
sions factors for oil and  natural gas in house-
hold-size units are not sufficiently different to
justify any  preference for either fuel on this
basis (see Sections 3.1 and 5.2.1).
  Domestic NOX emissions may also be  con-
trolled by conversion to electrical heat, which
transfers the problem to the big utility plant,
where  controls  can be  more  effective.  In
1967, there were  approximately 2,486,000
automatic,  domestic, electric heating instal-
lations  versus 29 million gas- and coal-fired
installations.2  Electricity can  be used for all
space heating, cooking,  hot  water heating,
clothes  drying, etc., and  it has advantages in
overall  safety  and convenience.  On the other
hand,  it  is sometimes  the most  expensive
source  of  heat energy.   Low-pressure waste
steam from large utility  plants is also being
used for heating  and refrigeration in various
urban areas in place of fossil fuels.
  In general, in the domestic and commercial
sectors, the smaller  the heat output, the more
numerous are  the sources and the more dif-
ficult is the control of NOX emissions. Elec-
tric heating has the virture of not contributing
to emissions from individual sources; and its
use is expected  to increase,  particularly  in
densely populated areas.
  Preliminary results of a survey conducted
by  the Public  Health  Service indicate that
household, commercial,  and industrial  solid
waste  production in  the  United  States is
about  10 pounds per capita per day, or 360
million tons per year. About 190 million tons
per year (or 5.3 pounds per day per capita) is
collected  for disposal, and  the remainder is
either  disposed of on  site or handled by the
household  or  establishment itself.12   An
estimated 177 million  tons of this material is
burned in the open  or in incinerators.1 3  An
additional 550  million  tons  of  agricultural
waste, 1.1 billion tons of animal  wastes, and
1.5  billion tons  of  mineral wastes  are gen-
erated each year.12 It is estimated that half of
the agricultural wastes are burned in the open
and  that, except for 48 million tons of coal
refuse  consumed by  fire each year, no animal
or mineral wastes are  burned.13 The quantities
of material consumed by forest burning and
structural fires are estimated to  total about
200 million and 8  million  tons  a  year,
   Incineration and open burning are used to
reduce the weight and volume of solid waste.
High-temperature incineration  with excess air
reduces  emissions  of particulate  matter,
carbon monoxide,  and smog-forming  com-
pounds such  as aldehydes, hydrocarbons, and
organic acids, which  typify open burning, but
it  tends to increase nitrogen oxide emissions.
6.2.1  Emissions
   Estimated NOX emissions from incineration
and other burning are shown in Table 6-3.
   NOX levels in incinerator flue  gases are in
the range of 50 to 80 ppm,14 and 8 to 130
ppm in  gas  from open burning, where the
NOX level varies with cycle time.15  Emission
factors1 and  the volumes of various  solid
wastes consumed in each disposition  are diffi-
cult  to measure, particularly for  open burn-
ing. The factor for open burning appears high
relative to factors for incineration, since it
does not  seem reasonable that open  burning,
in which radiation to the sky cools the flame,


                               AND OTHER BURNING, 196813

On-site incineration
Municipal incineration
Conical burner incineration
Open burning
Agricultural burning
Controlled forest burning
Forest wildfires
Structural wildfires
Coal refuse fires
lO^ tons/yr
NOX emission,
1()3 tons/yr
would produce more NOX than combustion in
a closed system (see Section 8).
6.2.2  Control Techniques
   The use of waste dispositions other than
combustion may be the most likely means for
reducing NOX  emissions,  since the methods
normally used for control of other emissions
from incineration, such as particulate matter,
organics, and  carbon  monoxide,  tend  to in-
crease emissions of NOX. The reduction  of
NOX emissions by some of these methods and
by changing combustion  conditions in incin-
erators is discussed below.  Waste Disposal
   From the standpoint of air pollution, the
least satisfactory method of waste disposal is
burning.  Sanitary  land fills  are  good alter-
natives, to  the extent that land usable for this
purpose is  available. Approximately 1  acre-
foot of land is required per 1,000 persons per
year  of operation when waste production is
4.5 pounds per day per capita.16 In addition,
cover material approximating 20  percent by
volume  of  the compacted waste is required;
the  availability of cover material may limit
the use of sanitary land fill.
   Unusual local  factors  may  lead  to a
solution of the land fill site problem. One ref-
erence1 7 indicates that  in a  pilot project
under way the refuse  is shredded and baled
for loading on rail cars for shipment to aban-
doned strip-mine land-fill sites.
  Other  noncombustion  alternatives  may
have  application  in  some  localities.  Com-
posting is  now being tested  on a practical
scale,18 and hog  feeding has been used for
disposal of garbage. Dumping at sea has been
practiced by some seacoast cities, but some of
the garbage  floats and returns to shore unless
dumped at appreciable distances. Such prac-
tices  are  now  forbidden  by  the  Federal
Government.   Elsewhere,  refuse  has  been
ground and  compressed into bales, which are
then wrapped in chicken wire and coated with
asphalt.1 9 The high-density bales sink to the
bottom in the deeper ocean areas and remain
intact. The  practice  of  grinding garbage in
kitchen units and  flushing it down the sewer
has been  increasing. This in turn increases the
load of sewage disposal plants and the amount
of sewage sludge.
  Incinerators  that reduce  NOX by changing
combustion  conditions  are  not  available
except on a developmental basis. Most incin-
erators  are not   provided  with  means  to
remove the  heat of combustion. Accordingly,
the burning rates must often be  curtailed and
overfire air  provided in order to protect the
incinerator  from  heat damage. Gas tempera-
ture must exceed 1,400° F if carbon particles,
organics, carbon monoxide, and odorous com-
pounds are to be consumed. Temperatures in
excess of  1,800°  F  cause  sintering  of ash
particles,  which  can  cause  undesirable ac-

  Data are rather firm in showing that excess
air  increases NOX emission regardless of the
cooling effect it has on the gas temperature.
An  increase from 50 to 300 percent of excess
air  caused  a 32 percent increase in NOX per
pound of  waste  burned.21  Rose  and  co-
workers5  showed  that an  average value  of
1.96 pounds of NOX emitted per ton of waste
of 50 percent excess air, and 2.19 at 150 per-
cent of excess air, an increase of 12 percent.
Also, an increase in  gas  temperature from
roughly  1,500°  to  2,300°  F  caused  the
NOX concentration  to increase from 50  to
100 ppm.  As  expected,  feed rate also  in-
creased the emission of NOX per unit weight
of waste.

  Auxiliary fuel may be used in certain incin-
erators for such purposes  as waste  drying,
startup, supplemental heat,  and afterburning.
Either oil or gas is used, and the NOX  emitted
is,  in part, from the  auxiliary  fuel.  For
example,  in  one  domestic  garbage incin-
erator22  designed  for low particulate  and
organic emissions, natural gas was burned to
increase heat input to 15,000 Btu per pound
of waste  as compared with a heating value for
garbage on the order of 2,500 Btu per pound.
The NOX emissions from burning 12.5 cubic
feet of gas per pound of garbage are estimated
at 2.9 pounds  per ton of garbage, which is a
very  substantial increase as compared with
estimates of emissions without gas burning on
the order of 1  pound per ton.2 3 Figures for a
flue-fed incinerator2 3 are 10 pounds of NOX
emitted per ton with an afterburner and  0.3
without.  These data suggest  that the NOX
emission  from the incineration of refuse is
substantially increased by the auxiliary firing
of oil and gas fuels.

  Heat may be recovered from incinerators
by  steam boilers, thus furnishing energy that
would otherwise  have to  be generated by
burning fossil fuels. Total NOX emissions are
thus  reduced  according to the amounts  of
fossil fuel  not burned. This means of refuse
disposal has already received considerable at-
tention in  Europe,24'28 and  some attention
in the United States. Forest Wildfires
  An estimated 800,000 tons of nitrogen ox-
ide  is emitted  annually  from  forest  wild-
fires. J 3  These  fires are  caused  by  natural
elements such as lightning or by carelessness.
Considerable activity  has been and  is being
directed toward reducing the frequency of oc-
currence and the severity of these fires. These
activities include publishing and advertising
information on fire prevention and  control,
surveillance of forest  areas where fires are
likely to occur, and various fire-fighting and
control  activities. Information on forest fire
prevention  and control is available from the
U.S. Department of Interior and from state
and local agencies.  Controlled Vegetation Burning
  Forest debris, crop residues, scrub, brush,
weeds, grass, and other vegetation are burned
for one or more of the following purposes:
  1. To  control vegetation, insects,  or or-
    ganisms harmful to plant life.
  2. To reduce the volume of waste.
  3. To minimize fire hazards.
  4. To improve land.
NOX emissions  from this burning are
estimated   to be  slightly less than 700,000
tons per year.1 3
  Because  collection   and  incineration  of
these materials would  tend to increase NOX
emissions,  the only current way  to  control
emissions   is to  avoid combustion.  In the
future it may be  possible to develop incin-
eration  processes that can control NOX and
other emissions, as well as particulate matter,
organics, odorous  compounds,  and  carbon
monoxide; or it may be possible to develop
equipment that  can burn these materials as
substitutes  for fossil fuels.
  Other alternatives to incineration are aban-
donment or burying at the site, transport to
and disposal in remote areas, and utilization.
Abandonment or burning at the  site is prac-
tical in  cases where no other harmful effects
will  ensue.  Abandoned or buried vegetation
can have harmful  effects upon plant life by
hosting  harmful  insects  or  organisms, for
example. Agricultural  agencies such as the


 U.S. Department of Agriculture, or state and
 local  agencies  should  be  consulted  before
 these techniques are employed. Other harmful
 aspects  such  as  odor  or water pollution
 potential,  or  fire hazards,  must  also  be
 considered.  Disposal  of  waste materials in
 areas where harmful effects are avoided is pos-
 sible, but is not commonly practiced. Some of
 these waste  materials  can  be  used.  Larger
 forest scraps are  processed by chipping or
 crushing  and are  used  as raw  materials for
 kraft  pulp mills  or for processes producing
 fiberboard, charcoal briquettes, or synthetic
 firewood.19  Composting or  animal  feeding
 are  other possible alternatives to burning.18 Coal Refuse Fires
   An estimated 190,000 tons  of NOX is emit-
 ted  each  year from 19 billion  cubic feet of
 burning coal refuse.1 3 Extinguishing and pre-
 venting these fires are the techniques used for
 eliminating these emissions.  These techniques
 involve cooling and repiling the refuse; sealing
 refuse  with  impervious  material;  injecting
 slurries of noncombustibles into  the refuse;
 minimizing the  quantity  of combustibles in
 refuse;  and  preventing ignition  of refuse.
 These techniques  and  the  status  of future
 plans and research are described and discussed
 in  the  document, Control  Techniques for
 Sulfur Oxide Air Pollutants.3 ° Structural Fires
   Structural  fires emit an estimated 23,000
 tons of NOX  annually.1 3 Prevention and con-
 trol techniques  are used to reduce these emis-
 sions. Use of fireproof construction; proper
^handling, storage, and packaging of flammable
 materials; and publishing and advertising in-
 formation on fire prevention are some of the
 techniques used  to prevent fires. Fire control
 techniques include the  various  methods for
 promptly extinguishing fires: use of sprinkler,
 foam, and inert  gas systems;  provision  of
 adequate fire-fighting facilities and personnel;
 and  provision of adequate alarm systems. In-
 formation on these and other techniques for
 fire  prevention and control are available from
 agencies  such   as local  fire  departments,
National  Fire  Protection  Association,
National  Safety Council, and insurance com-
6.2.3   Costs of Control
   Where substitution of noncombustion alter-
natives for incineration is the most practical
means of controlling NOX emissions, costs of
controls are a function of the relative costs of
noncombustion disposition  and incineration.
The costs reported vary widely with locality.
   Sanitary landfill  costs, including amortiza
tion, have been reported as  $ 1.05  per ton for
27,000 tons per year of refuse and $1.27 per
ton  for 11,000 tons per year.12 The costs do
not  include costs of collection and transporta-
tion to  the site. Operating costs  for  incin-
eratons are  $4 to  $8 per  ton  of  refuse;31
estimated  capital costs are $6,000 to $13,000
per  ton per day.32  Sanitary landfill is  less
costly  than incineration. A report from Phila-
delphia has estimated that  the disposal of
refuse  by railroad shipment  to available aban-
doned coal mines would cost $2.00 per ton
less than incineration.17
 1.  U.S. Gross Consumption of Energy Resources for
    Years  1947-1965.  In:  Petroleum Facts and
    Figures, 1967 ed.  New York, American Petro-
    leum Institute, 1967. p. 251.
 2.  U.S. Automotive Domestic  Control-Heating
    Equipment in Use, 1959-1967. In: Petroleum
    Facts and Figures, 1967 ed. New York, American
    Petroleum Institute, 1967. p. 181.
 3.  Statistical  Abstract of the United States: 1968.
    89th ed. Bureau of the Census. Washington, D. C.
    June 1968. p. 676.
 4.  Crude  Petroleum and  Petroleum  Products,
    December 1967. Mineral Industry Surveys; Petro-
    leum Statement, Monthly. Bureau of Mines.
    Washington, D. C. March 22, 1968. p. 25.
 5.  Market  Statistics from  American  Gas  As-
 6.  Lynn, D.A. and T.B. McMullen. Air Pollution in
    Six Major  U.S. Cities as Measured by the Contin-
    uous Air Monitoring Program.  J. Air Pollution
    Control Assoc. 16:186-190, April 1966.
 7.  Turner,   D.B.  The  Diurnal  and Day-to-Day
    Variations of Fuel Usage for Space Heating in St.
    Louis,  Missouri.  Atmos.  Environ. 2:339-351,
    July 1968.

 8. Halitsky, J. Some Aspects of Atmospheric Dif-
    fusion in Urban Areas. In: Symposium: Air Over
    Cities. Robert A. Taft  Sanitary  Engineering
    Center.  Cincinnati,  Ohio.  Technical  Report
    Number SEC-TR-A62-5. 1961. p. 217-228.
 9. Mills, J.L. et al. Emissions of Oxides of Nitrogen
    from  Stationary Sources in Los Angeles County;
    Oxides  of  Nitrogen Emitted by  Medium and
    Large Sources. Joint District  Federal, State, and
    Industry  Project.  Los  Angeles County Air Pol-
    lution  Control   District,  Los  Angeles,  Calif.
    Report Number 3, April 1961. 51 pages.
 10. A Home  Furnace  the  Size  of a Two-Pound
    Coffee Can.  Chem. Eng. 76(2):73, January  27,
 11. Fournier,  M.  and P. Jacquinot. Fight Against
    Atmospheric Pollution from Domestic Furnaces.
    Control Measures  in Effect in the Special Protec-
    tion Zones in Paris During the Winter of 1965-66
    [Lutte centre la  Pollution Atmospherique due
    aux foyers Domestiques. Controle exerce" dans les
    zones  de  protection  spgciale  a  Paris  (Hiver
    1965-1966]. Pollution  Atmospherique  (Paris).,
    9(34):91-99, April-June 1967.
 12. Black, RJ.  et al.  The  National  Solid Wastes
    Survey. Presented at Annual Meeting  of the In-
    stitute for  Solid Wastes of the American Public
    Works Association. 1968.
 13. NAPCA  Reference Book of Nationwide  Emis-
    sions.  National Air  Pollution  Control  Admin-
    istration. Raleigh, N.C. (To be published).
 14. Rose, A.H.,  Jr.,  et al. Air Pollution Effects of
    Incinerator Firing Practices and Combustion  Air
    Distribution. J.   Air  Pollution  Control Assoc.
    8(4):297-309, February 1959.
 15. Gerstle, R.W. and D.A.  Kemnitz.  Atmospheric
    Emissions from Open Burning. J. Air Pollution
    Control Assoc. 77:324-327, May 1967.
 16. Kirsh, J.B.  Sanitary  Landfill.  In:  Elements of
    Solid Waste  Management Training  Course
    Manual. Public Health Service. Cincinnati,  Ohio.
    1968. p. 1-4.
 17. Air Pollution Problems  from  Refuse  Disposal
    Operations   in the Delaware  Valley.  Dept: of
    Public  Health,  Air  Management   Services.
    Philadelphia, Pa. February 1969.
 18. Wiley, J.S.  et al.  Composting Developments in
    the U.S. Combust. Sci. 6(2):5-9, 1965.
 19. Kurker, C.  Reducing  Emissions from  Refuse
    Disposal  J. Air Pollution  Control Assoc.
    79:69-72, February 1969.
20. Kaiser, E.R. Incinerators to Meet New Air Pol-
    lution  Standards.   In: Proceedings of the Mid-
    Atlantic  Section,  Air  Pollution Control  As-
    sociation. April 20, 1967.
21. Stenburg, R.L. et al. Effect  of Design and Fuel
    Moisture  on  Incinerator Effluents.  J. Air Pol-
    lution  Control  Assoc.  10(2): 114-120,  April
22. Sterling, M. Air  Pollution Control and the Gas
    Industry.  J.  Air  Pollution  Control  Assoc.
    77:354-361, August 1961.
23. Duprey, R.L.  Compilation of Air Pollutant Emis-
    sion  Factors.  National Center for Air Pollution
    Control. Durham, N.C. PHS Publication Number
    999-AP-42. 1968. 67 p.
24. Stabenow, G. Performance and  Design Data for
    Large European  Refuse  Incinerators with Heat
    Recovery.  In:  Proceedings of 1968 National In-
    cinerator   Conference.  New York, American
    Society  of  Mechanical Engineers,  1968.  p.
25. Eberhardt,  H. European Practice in  Refuse and
    Sewage  Sludge  Disposal by Incineration. In:
    Proceedings of 1966 National Incinerator Con-
    ference. New York, American Society of Mech-
    anical Engineers,  1966. p. 124-143.
26. Rogus, C.A. An Appraisal of Refuse Incineration
    in Western  Europe. In: Proceedings of  1966
    National  Incinerator  Conference.  New  York,
    American  Society  of  Mechanical  Engineers,
    1966. p. 114-123.
27. Stabenow,  G. Survey of European  Experience
    with  High  Pressure Boiler  Operation Burning
    Wastes  and  Fuel.  In:   Proceedings  of   1966
    National  Incinerator  Conference.  New  York,
    American  Society  of  Mechanical  Engineers,
    1966. p. 144-160.
28. Rousseau, H. The  Large Plants  for  Incineration
    of Domestic  Refuse in the Paris Metropolitan
    Area. In:  Proceedings of 1968 National  Incin-
    erator Conference.  New  York, American Society
    of Mechanical Engineers, 1968. p. 225-231.
29. Private communication with E.K. Taylor. South-
    east  Air Pollution  Control  Authority.  State of
    Washington. October 1969.
30. Control Techniques for Sulfur Oxide  Air Pol-
    lutants.  National Air Pollution  Control Admin-
    istration. Washington, D.C.  Publication Number
    AP-52. January 1968.
31. Private  communication with  H.L.  Hickman.
    Technical Services, Bureau of Solid Wastes Man-
    agement. Rockville,  Md.  November 1, 1968.
32. Bogne, M.K.  Municipal Incinerators. U.S.  Public
    Health Service, Office of Solid Wastes. 1965.

  Nitrogen oxides are a minor source of air
pollution, incidental  to combustion heat, for
a number of industrial processes having more
serious problems due to emissions of smoke,
particulates, SC>2, or  organic chemical wastes.
For -example, several large industries such as
nonferrous mineral  producers  or pulp  and
paper mills have problems with SC>2, but no
particular concern with NOX. Some industrial
processes that emit only a small percentage of
total national  emissions could cause danger-
ous local concentrations, if uncontrolled.
  The  problem of NOX  emissions has been
researched in the chemical industry  more in-
tensively than  anywhere else because it  may
represent the loss of a valuable raw material.
The following  sections  of this report discuss
commercial processes developed for NOX  con-
trol in  the  manufacture and  uses of nitric
  The  NOX released in vent  gases from the
manufacture and industrial uses of nitric acid
differs  markedly from  that emitted  from a
combustion flue gas in  concentration, total
amount, and the ratio of NC>2 to NO present.
The NOx-containing chemical  gas is common-
ly a process stream which must be  recycled
with maximum NOX recovery in order to have
an economical process. Vent gas is released
only because it is too  impure to recycle or too
low in  concentration for  economic recovery.
The economic  limit with a pure gas, as in ni-
tric acid manufacture, is about 0.1 to 0.3 per-
cent NOX, or 1,000 to 3,000 ppm. The limit
is higher in organic nitrations, such as the
manufacture of nitroglycerine, where  NOX
content of the vent gas may approach 1  per-
cent NOX, or 10,000 ppm.
  The total amount of NOX emitted from all
chemical manufacturing is about 1 percent of
all NOX from man-made sources in the United
States. These processes represent a local nui-
sance problem only in special local areas. The
problem  has  been most serious in  military
ordnance  works, which manufacture large
volumes  of nitric acid and use  it in organic
nitrations. A  single plant like the Volunteer
Ordnance Works has  produced,  for example,
emissions of  NOX equal to all nonmilitary
uses of nitric acid in the United States.
  A high ratio of NO9/NO at high concentra-
tion  causes  the gases  to be  visible as  a
brownish plume. The visibility limit depends
on the total amount of NO2 present in the gas
volume  or layer observed. A convenient rule
of thumb is that a stack plume or air layer
will have a visible brown color when the NO2
concentration exceeds 2,400 ppm divided  by
the stack diameter in inches.1 This means that
the  threshold  of visibility  for  a  2-inch-
diameter stack  is about 1,200 ppm of NO2
and for a 1-foot-diameter stack, 200 ppm of
NO2 (or 2,000 ppm of NOX at a 1:10 ratio of
  The distinction  between NO2  concentra-
tions  and total amount can be  quite impor-
tant in chemical vent  gases, since a short burst
of NO2 at 10,000 ppm may be visible but less
hazardous than  many times  as much NOX
emitted from  a large stack at  a lower con-
centration. The total  amount in  a short, con-
centrated emission may be  too small to have
a detectable effect on NOX levels in ambient
  A large amount of research  with varying
degrees of success has been carried out on the

 development  of processes for the removal of
 NOX from the off-gas resulting from the man-
 ufacture and uses  of nitric acid. The following
 processes are discussed  in detail  in Sections
 7.1  and 7.2  below,  and summarized  briefly
 here to show their general relationship.
   Catalytic reduction reduces  NC>2  to  NO
 and in some cases NO to N2, using a reducing
 gas  such as  methane,  hydrogen, or  carbon
 monoxide.  The  best catalysts to  date  are
 noble metals, which are so sulfur-sensitive
 that this process  cannot be used  with many
 impure industrial gases.
   Catalytic decolorization and power recov-
 ery  is a form of catalytic reduction in which
 the  fuel oxidized  reduces the NO2 present to
 colorless and  nonirritant NO. This is a highly
 exothermic process,  requiring  careful tem-
 perature controls, in  which the heat released
 provides power. Recovery of the heat  energy
 in a turboexpander enables the manufacturer
 to break even on costs.
   Nonselective catalytic abatement requires
 enough added reducing  fuel  to  burn out  the
 oxygen in the tail  gas and then to reduce both
 the  NO2 and  NO  present to N2- This is much
 more difficult technically than decolorization,
 and  commercial results have been less satisfac-
 tory than  decolorization. It requires  equip-
 ment and provisions for  interstage or product
 heat removal.
   Selective  catalytic  abatement involves  the
 direct  reduction, within a narrow temperature
 range, of NOX to  molecular nitrogen by am-
 monia. This reduction is  specific for NOX and
 occurs  without the  simultaneous  reaction
 with the oxygen in the flue gas. The process
 has  been  only partly successful to date  and
 requires further development.
   Caustic scrubbing,  lime-water,  or a lime-
 stone  slurry  can  remove NO2  or  a  1:1 mol
 ratio of NO2/NO  (equivalent to ^03), but
 this  can create a  serious problem of waste
 water  disposal and  this  method  is in only
 limited use.
   Adsorbents  or  absorbents have  a  limited
 capacity. Molecular sieve adsorbents are  of-
 fered commercially for the removal of NOX
from a moisture-free gas. Available informa-
tion indicates that the development of a pro-
cess of  this type applicable  to combustion
flue gases is unlikely.
   NOX  incineration  is a  noncatalytic re-
duction process in which a fuel is added in at
least 10  percent excess over the amount cor-
responding to the NOX and oxygen present,
and the NOX is reduced to N2-
   Details of these control methods are discus-
sed  in Sections 7.1.2,, and in
connection  with specific processes to which
they may be applied.
   Dilution of NOX tail gases in the stack with
large volumes  of air  makes the effluent less
visible; but, because dilution has no effect on
source strength, it has little or no  effect on
ground-level   concentration.  Addition of
diluent air, however, increases stack velocity,
which increases effective stack height and, to
this extent, contributes to decreased ground-
level NOX concentrations.
   All nitric acid manufactured in the United
States is  made by the catalytic oxidation of
ammonia. Air  and ammonia  are preheated,
mixed, and  passed over  a catalyst, usually
platinum-rhodium. The following reaction oc-

      4NH3 + 5O2 -» 4NO  + 6H2O   (1)

The steam is cooled to 100°F or less, and the
NO  then reacts with more oxygen to form
nitrogen  dioxide and its liquid dimer, nitrogen
2NO + O2  -* 2NO2
     is a reddish-brown gas with a sharp odor.
  The liquid and gas then enter an absorber
tower. Added air is directed to the bottom of
the tower and water to the top. The NO2 (or
N2O4) reacts with  water  to form nitric  acid
and NO, as follows:

      3NO2  + H2O -> 2HNO3 + NO   (3)

The  formation of 1 mole of NO for each 2
moles of HNC>3 makes it necessary  to reox-
idize NO after each absorption stage since the
gas rises up the absorber and limits  the level
of  recovery  that  can  be  economically
  Acid product is withdrawn from  the bot-
tom of the tower in concentrations  of 55 to
65  percent. The air entering the bottom of
the tower serves to strip NO 2 from the prod-
uct and to supply oxygen for reoxidizing the
NO formed in making nitric acid (equation 3).
   Normally, pressures of 80 to 120 pounds
per square inch (psi) are employed in the ox-
idation  and  absorption operations.  Tail gas
from the absorber passes through an entrain-
ment eliminator for  removal of nitric acid
mist. Tail  gas is then reheated by exchange
with the gases from ammonia oxidation and is
directed to a turboexpander for energy recov-
   Before  corrosion-resistant  materials  were
developed, the  ammonia oxidation  and ab-
sorber  operations  were  carried  out at  es-
sentially atmospheric pressure. Because of the
low absorption and NO oxidation rates, much
more absorption  volume was required,  and
several  large  towers were placed in series.
Some of these low-pressure units are still in
operation, but they represent less than 5 per-
cent of the current United States nitric acid
capacity.2 Improved technology has made the
"pressure process" more economical, and it is
the preferred process in the United States, so
that over 90 percent  of our nitric acid plant
capacity is  believed to be of the pressure
type.2  A typical flow plan is shown in Figure
7-1. Except for leaks, the only source of NOX
emissions is the tail gas from the unit.
  Combination pressure plants carry  out the
ammonia oxidation step at low pressure and
absorption   at  high pressure.  Intermediate-
pressure plants carry out both operations at
intermediate pressures.
  For  most uses, the 55 to 65 percent  nitric
acid made by ammonia oxidation is satisfac-
tory. For some uses, however, high-strength
acid is required. These requirements are met
by concentrating the lower strength acids.
   Figure 7-2  illustrates a nitric  acid  con-
centration  unit using  extractive distillation
with  sulfuric  acid.  A mixture of  strong
sulfuric acid and 55 to 65  percent nitric acid
is introduced at the top of a packed column,
and flows down the column countercurrent to
the ascending vapors.  Nitric acid leaves the
      . „„„..„  •«-—•• MAIN INLET GAS STREAM
                                                       WASTE HEAT I. J	1.
                                                        BOILER  PI	['
      Figure 7-1.  Flow diagram of a typical 120-ton-per-day nitric acid plant utilizing the
      pressure process.

                                                      TAIL GAS TO
                                                      ATMOSPHERE (Volume

93% H2S04 •»
60% HN03->






— >




—— "i


( *
j, . NON-
I 	 ^ 95-99% HN03

	 70% H2S04
" 	 ' 	 TO COOLER

*> 	 >








•74.3% N2
Ofl A % Or.
1 0% NO + NOn




                       Figure 7-2.  Nitric acid concentrating unit.
top as a 98 percent nitric acid vapor contain-
ing small amounts of NOX and oxygen, which
result from the dissociation of nitric acid. The
vapors pass to a bleacher and a condenser to
condense nitric acid and separate  NOX and
oxygen,  which pass to an absorber column for
conversion to, and recovery of, nitric acid. Air
is admitted to the bottom of the  absorber.
Dilute sulfuric acid is  withdrawn  from the
bottom of the dehydrating tower and is sent
to be concentrated further  or  be used for
other purposes.
   The system usually operates at  essentially
atmospheric pressure.
7.1.1  Emissions
   Absorber tail gas is the principal  source of
NOX  emissions from  nitric acid  manufac-
turing.  Minor  sources  are  nitric  acid  con-
centration  and the filling of storage  tanks and
shipping containers.
   Nitrogen oxide  emissions from nitric acid
manufacturing are estimated at 145,000 tons
per year for  1967,  about 1  percent of the
NOX from all pollution sources or  2 percent
of the pollution from stationary sources. This
figure  is based on  the production of  6.12
million tons of nitric acid in 19673 and an
average emission factor of 45 pounds NOX per
ton of acid.2 Without  controls, an 800-ton-
per-day plant  (the  largest currently built)
would emit about 1,500 pounds of NOX per
hour.  Typical  tail  gas  contains about 0.3
volume-percent  (300  ppm)  of  NOX  with
about  equal concentrations of NO and NO2-2
Tail gas is typically reddish-brown or yellow.
   In  any nitric acid plant, NOX content of
the tail gas  is affected by several variables.
High  levels  may be caused by insufficient air
supply, high temperatures in  the  absorber
tower,  low  pressure, producing acid at
strengths  above  design, and  internal leaks,
which permit  gas from ammonia oxidation
with high nitrogen oxides content to enter the
tail-gas streams.  Careful  control and good
maintenance are required to hold tail-gas ni-
trogen oxide content to a minimum.
   The  Hoko  process4  differs   from other
nitric acid  processes by making  concentrated

(98 to 99 percent) nitric acid directly. It has
found very limited application in the United
States, where acid concentrations of 55 to 65
percent are satisfactory for the ammonium ni-
trate production, which  consumes about 75
percent of all  nitric  acid produced. In  the
Hoko process, ammonia is combusted with air
or oxygen at  atmospheric pressure. The com-
bustion gases are cooled to  about 30°C, as
rapidly as possible, in order to condense and
remove water as dilute nitric acid (1 to 2 per-
cent HNO3).  The gas is then compressed, and
NO is oxidized  to NOo, which is absorbed in
cold concentrated  nitric acid. The absorber
liquid, an  N2O4-HNC>3  solution, is  mixed
with the required volume of water, and the
mixture is admitted to an autoclave and ox-
idized with pure oxygen to make 98 or 99
percent nitric acid. The gas from  the absorber
is scrubbed with weak acid to remove HNO^.
The  NOX in  the tail  gas is said to be  0.05
percent  by  volume.3  A  problem is  the
disposal  of the weak  acid formed by  con-
densing the ammonia oxidation vapors.
  About 5 pounds of NOX per ton of nitric
acid  is emitted in the absorber gas from nitric
acid  concentration units. Typical  NOX  con-
centration  is about   1  percent,2  reflecting
operation at essentially atmospheric pressure.
Although figures are  not available  on  the
volumes of concentrated nitric acid produced
(see  Section  7.2), rough estimates of  con-
centrated HNC>3 requirements and resulting
NOX emissions for the United States in 1967
are given in Table 7-1.
  Not  included  in Table  7-1  are  acid  re-
quirements and  NOX emissions from nitric
acid  concentration operations for producing
military explosives.

7.1.2  Control Techniques
   The  economic necessity of recovering NC>2
formed in the manufacture  and  uses of nitric
acid  has led  to  a large amount of chemical
research that is applicable to problems in NOX
control. It must  be recognized, however, that
most of this research has been for the recov-
ery of  NOX at around 3,000 ppm; and this is
not the same problem as the removal of much
lower  concentrations  of NOX  from stack
   A large body of technology has been devel-
oped for  treating tail gas  from nitric acid
plants.  Catalytic  reduction  is  particularly
suited to  the   nitric  acid manufacturing
process,  and  various  systems   have  found
wide-spread commercial  use. Caustic scrub-
bing removes NOX, but  there are  few com-
mercial applications of this process because of
problems  in  disposing  of the spent  caustic.
Molecular sieve adsorption shows promise and
is now  offered  for  commercial  use with
moisture-free gases.6-7  Absorption in con-
centrated  sulfuric or  nitric acid  has been
investigated8  but does not  appear competi-
Toluene diisocyanate
Commercial explosives
Nitric acid requirements
Total HNO3,
Strong acid,

Strong acid,

   Catalytic reduction is particularly  appli-
 cable  to  the pressure  process  because the
 reactor can be placed in the  tail gas circuit
 after the preheater. Energy generated by the
 reaction of fuel with oxygen and nitrogen ox-
 ides is recovered  in  the  turboexpander, in-
 creasing power recovery. Thus, the expander
 may provide most of the power required by
 the air compressor.2 >9
   Several types of catalytic reduction units
 have been  used with varying  degrees of suc-
 cess. All employ fuel to  reduce nitrogen ox-
 ides, converting NO2  to  NO and NO to N2.
 Nonselective reduction uses hydrogen, hydro-
 carbons, or carbon monoxide5  as fuel, which
 reacts  with oxygen in the tail gas, as well as
 with  the  nitrogen oxides.  In selective re-
 duction, ammonia  reacts with NOX in prefer-
 ence to molecular  oxygen. In all catalytic re-
 ductions, catalyst activity decreases somewhat
 with use.
   Nonselective reduction units include  those
 designed for decolorization and energy recov-
 ery, and units designed  for NOX abatement.
 Decolorization and  power recovery  units
 reduce NOX to NO and burn out part of the
 oxygen, but their  capacity to  reduce NO to
 elemental nitrogen is limited. The purpose of
 nonselective .abatement units is to reduce the
 NO as well.
   In  nonselective  reduction,  the  following
 reactions take place  when a preheated  mix-
 ture of tail gas and methane pass  over the

 CH4  + 4NO2 -»•  4NO  + CO2 + 2H2O (4)

       CH4 + 2O2 -»• C02 +  2H20    (5)

   CH4 + 4NO -»• 2N2 +  CO2 + 2H2O    (6)

   Similar  equations  can  be  written  sub-
 stituting hydrogen for methane; 4 moles of
 hydrogen  is  needed to  replace 1  mole of
   The  kinetics   are such that  reduction
 reaction (4) is faster than reduction reaction
 (5),  but abatement  reaction   (6)  is  much
slower than reaction (5). Thus, decolorization

can be  accomplished by adding just  enough
fuel for partial oxygen burnout;  but  if NOX
abatement is required, sufficient fuel must be
added for complete oxygen burnout.
  Catalytic reduction units for decolorization
and power  recovery  are  used in about  50
nitric  acid  plants in the United  States.10
Many use  natural gas for  fuel; some use
hydrogen. When natural gas  is used,  tail gas
must be preheated to about  900°F to insure
ignition. Preheat  temperatures  as  low  as
300°F will insure ignition with hydrogen.5
  Catalytic  reduction is highly exothermic.
The  temperature  rise for the reaction with
methane is  about 230°F for  each percent
oxygen  'burnout,  and with  hydrogen it is
about 270°F.2 For decolorization, the outlet
temperature is ordinarily limited  to 1200°F,
the maximum  temperature  limit of  turbo-
expanders  with  current  technology.10 In-
creased  power  recovery  may justify  adding
sufficient methane to reach the temperature
limit of the turbine.
   Supported platinum or  palladium catalysts
are employed in decolorization. Catalyst sup-
ports include ceramic spheres, ceramic pellets,
ceramic  honeycomb,  and  woven  nickel-
chromium ribbon. The honeycomb structure
is reported to give low pressure drop  at the
high space  velocities of 100,000 volumes per
hour per volume.  In new decolorization units,
honeycomb supports are favored over ceramic
spheres   and pellets,  which  require  lower
hourly space velocities of 30,000 volumes per
hour per volume.1 x -1 3
  At  a  given  level  of NO2  removal, the
nickel-chromium-supported  catalyst uses
more fuel, but is  said to convert more NO to
N2 than the ceramic supported catalyst.1 °
  Both catalyst and nitric acid manufacturers
report  satisfactory performance for decolor-
ization  units. The reduction  of total  NOX is
limited, but ground-level NO2  concentration
in critical areas near  the plant is reduced sub-
stantially.1 °
  Nonselective catalytic abatement is more
difficult technically than decolorization, and
commercial  results have been  less satisfactory.
Provisions must be made to control the heat

released  in burning out all  the oxygen from
the tail gas, which must be done before exten-
sive NO reduction proceeds.
   As mentioned above, tail gas must be pre-
heated  to 900°F  to insure ignition when
methane is used as the reducing agent. Outlet
temperatures  would   reach  1,370°  and
 1,590°F for 2 and 3 percent oxygen burnout,
respectively. These temperatures  compare to
the 1,200°F  maximum temperature limit for
current  turboexpanders  and  the  1,500° to
 1,600°F  maximum  limit for catalyst.  For
single-stage operation, the oxygen in the tail
gas  cannot exceed  2.6  percent  to  remain
within the temperature limit  of the catalyst,
and  cooling must be  provided to  meet the
turboexpander limit. Older turbines may have
even lower temperature limitations.
   Two  or three plants are known to have
installed  single-space  nonselective  abaters.
They are believed to have been designed for
natural  gas.10 As noted  above, oxygen con-
centration cannot exceed  about 2.6 percent.
The  reactors must be  designed to withstand
1,500° to 1,550°F at 100 to 120 psig, which
requires costly refractories or alloys. Ceramic
spheres  are  used as  catalyst supports, at
hourly  gas  space velocities up  to  30,000
volumes  per  hour per  volume.10  One
company reports that they have been able to
maintain NOX levels of 500 ppm  or less over
an extended period of time. Operation close
to 300 ppm might be obtained. Catalyst sup-
pliers report values of 100 ppm from small-
scale tests with gas of fixed composition. On a
plant scale, the effluent gas must be cooled by
heat exchange  or quench to  meet the tem-
perature  limitation of the turbine. It would be
logical to use a waste heat boiler  to generate
   A somewhat  cheaper  but  less  successful
alternative is two-stage reduction  for abate-
ment.   Two   or  three   commercial   instal-
lations are known in the  United States. One
system  involves  two reactor stages with
interstage heat removal.13 Another  two-stage
system for abatement involves preheating 70
percent of the feed to 900°F, adding fuel,  and
 passing  the mixture  over  the first-stage
 catalyst. The fuel addition to the first stage is
 adjusted to obtain the desired outlet tempera-
 ture. The remaining 30 percent of the tail gas,
 preheated to only 250°F, is used to quench
 the first-stage effluent. The  two streams plus
 the fuel for complete reduction are mixed and
 passed over the second-stage catalyst; the ef-
 fluent passes directly to the turboexpander.
 This system avoids high temperatures, and the
 use of coolers and waste heat  boilers.5'14'15
  Honeycomb  ceramic  catalysts have been
 employed  in  two-stage abatement,  with
 hourly gas-space velocities of about  100,000
 volumes per hour per volume in each  stage.12
  Commercial  experience  with  single-stage
 catalytic abaters has been modestly  satisfac-
 tory, but two-stage units operating on natural
 gas have  not been as successful.  Two-stage
 units designed for abatement have frequently
 achieved abatement for periods of only a few
 weeks,  at which  point  declining  catalyst
 activity results in increasing NO levels. Recent
 data indicate that successful abatement can be
 maintained  for somewhat  longer  periods.
 Units that no longer abate NO emissions can,
 however, continue to  serve for energy recov-
 ery and decolorization.1 °
  The success  of single-stage abaters  com-
pared to the limited success of two-stage units
may result from the  following  factors:  the
catalyst is in a reducing atmosphere; the tem-
peratures are  higher; and spherical rather than
honeycomb catalyst supports are used. It  has
not been practical to change catalyst type in
two-stage  units because the reactors designed
for  a  space velocity of 100,000 volumes  per
hour per  volume would be  too small to ac-
commodate  a spherical  catalyst,  which  ef-
fectively removes NO at a space velocity of
about 30,000. The failure of the honeycomb
catalyst in NO reduction compared to its suc-
cess  in  decolorization  may  reflect  that
reaction kinetics make it much more  difficult
to reduce NO than NO2-1 °
  Fuel  requirements  are  10  to 20 percent
 over  stoichiometric  for nonselective  abate-
 ment  with   methane.  Some  hydrocarbons


and CO appear in treated tail gas.  Further-
more, not all methane is converted in decolor-
ization.  Less  surplus fuel is required when
hydrogen is used.1 °
   Selective  catalyst abatement  must  be
carried  out within  the  narrow temperature
range of 410°  to 520°F. Within these limits,
ammonia will reduce NO2 and NO to molec-
ular nitrogen, without simultaneously reacting
with oxygen.  The reactions are shown in the
following equations:

     8NH3 + 6N02 -"  7N2 +  12H2O  (7)

        4NH3 + 6NO -*• 5N2 + 6H2O     (8)

Above   520°F,  ammonia may  oxidize  to
form NOX; below  410°F, it may form  am-
monium nitrate.2
   Three commercial  plants have  installed
selective abatement units.  Performance  to
date has ranged from unsatisfactory to partly
successful.  The reaction supplies little energy
for recovery in the turboexpander. * °
   A number  of World War II  vintage nitric
acid plants of 55 tons per day capacity are
equipped with reciprocating compressors for
power  recovery.  If selective  abaters were
installed in front of their expanders, ammon-
ium salts could enter the lubricating oil sys-
tem, and an explosion could result. This is not
a problem with turboexpanders.
   Platinum on a honeycomb support is used
in selective abatement. This  catalyst system
has not been very successful up to the present
time; consequently, nitric acid manufacturers
do not  consider selective abatement  to  be
commercially  developed  yet.10  Platinum  on
pellets may be satisfactory for this service.12
The very high cost of platinum catalysts and
the  necessity  of using them only on a gas
stream that is essentially free of sulfur com-
pounds limit the number of  analogous stack
gases with which they can be used.
   Caustic scrubbing is an alternative method
of removing  NOX from  nitric  acid  tail gas.
Sodium hydroxide, sodium carbonate, ammo-
nium hydroxide, and calcium carbonate react
with NOX  to form  nitrite and  nitrate salts.
Examples of 90 percent removal of NOX by
caustic  scrubbing have been reported in the
literature.16 The high removal levels probably
reflect  a  favorable NO/NO 2  ratio of about
  Although  sodium  hydroxide  or  sodium
carbonate scrubbing removes NOX from tail
gas,  it  has  not found extensive use  in the
industry because of the difficulties encoun-
tered in disposing of the spent solution; the
nitrite and nitrate salts contained in the spent
solution become a serious  water pollutant if
released as a liquid effluent. The  concentra-
tions are too dilute for economic recovery of
the salts.10
  Several potential processes for tail gas treat-
ment are still  to  be  tested  commercially.
Molecular sieves have been reported to be
much more effective adsorbents for NOX in
tail gas than silica gel.6-7 They have recently
become  commercially  available for  this
purpose, and limited evidence indicates that
they work well. The  adsorbed NOX can be
desorbed  as enriched NOX or HNO3  by dis-
placement with hot air or steam, and retur led
to the nitric acid plant. Silica gel is probably
more acid resistant than the zeolites.
   NOX can be adsorbed  in nitric acid or
sulfuric acid. Sulfuric acid readily absorbs a
50/50 mixture of NO/NO2. The tail gas also
contains  water,  which  would  dilute  the
sulfuric acid. Reconcentration makes  sulfuric
acid absorption costly.
  Concentrated nitric acid would be expected
to absorb some  NO as well  as  the  50/50
mixture of NO/NO2. Again, the abundance of
water  in  tail  gas would  dilute  the  acid.
Absorption of NOX from tail gas by 20 to 35
percent HNO3,  followed by regeneration of
the contaminated acid with oxygen, has been
proposed in  the literature.8
  Emissions from storage  tanks can  be col-
lected  through a vapor recovery system and
returned to  the process feed, as can emissions
from nitric  acid concentration.  The  equip-
ment should be constructed of stainless steel.
Frequently,  emissions from this source are
disposed  of  by incineration, which reduces

 the NOX content by 75 to 90 percent.17  Ni-
 trogen  oxide  incineration is  covered  in
 7.1.3   Costs for Control
   By  careful design and operation,  some ni-
 tric acid manufacturers attain NOX  levels in
 untreated tail gas of 0.1 to 0.2 percent versus
 a typical level of 0.3 percent.1 °  The optimum
 level  of NOX  in  tail  gas  depends  on  the
 economic factors prevailing at each location:
 0.1  percent reduction  in  NOX means  ap-
 proximately 1  percent improvement in nitric
 acid recovery.  The difference  between  the
 credits for  improved recovery and the added
 cost for additional  absorber stages, increased
 cooling, etc., determines the optimum level. If
 abatement is required, its costs  enter into the
 economic calculations.
   Costs of  emission controls may be partially
 offset by credits for products or energy recov-
 ery. If NOX can be  removed from tail gas and
 recovered at a high concentration, it  may be
 returned  as feed to  the process. The value of
 the recovered  NOX is  a potential credit  for
 developing  an  adsorption  process.  On  the
 other  hand, the cost of recovering  dilute ni-
 trite and nitrate  salts in spent  caustic may
 outweigh their product value. In catalytic re-
 duction,  all  the products are released to the
 atmosphere, but the energy  released in the
 reaction may be economically recovered in a
 turboexpander or waste heat boiler.
   Costs  for  catalytic  reduction   will  be
 affected by  the type  of  treatment
 (nonselective  decolorization  of abatement,
 selective abatement, etc.), the volume of tail
 gas to  be treated, the cost of fuel, and the
 value of the energy recovered.
   Adding catalytic  reduction capabilities to
 an existing unit is usually more difficult and
 costly  than  incorporating the facilities into a
 new unit as it is being designed. For existing
 plants,  modifications to the heat exchanger
 train may be required to obtain and hold tail-
gas temperatures within the operable range.
Physical space limitations  may  preclude
placing the  reactor ahead of  the  turbo-
expander. Additional preheat facilities would
be required in placing  the reactor after the
turboexpander, and energy generated in the
reduction would not be available for recovery
in the turboexpander. For these reasons, costs
of adding  catalytic reduction equipment to
existing plants will vary with each application
and  will depend  upon  the  modifications re-
quired for the installation.
   Rough costs of three catalytic  reduction
alternatives have been derived during the plan-
ning  of a new  300-ton-per-day nitric  acid
plant. These alternatives  are (1)  decolor-
ization, (2)  single-stage  selective abatement,
and   (3) two-stage nonselective  abatement.
The  comparison  is shown in Table 7-2. The
costs are  strongly  affected  by conditions
specific to the location and  should  not be
used for other locations.
  In  this  comparison,  investments for the
abatement  systems are two to four  times the
investment for decolorization. Selective ab-
atement  requires less investment than two-
stage  abatement.  Overall operating  costs, in-
cluding  a 20 percent return before  income
taxes,  are  approximately breakeven  for
decolorization. For most companies, this cor-
responds to an after-tax return of 10 percent,
which is the current cost of capital. Operating
costs  for two-stage abatement are lower than
for  nonselective  abatement, since  steam
credits more than  offset higher investment
and fuel costs.
  In  this plant, a turboexpander augmented
by  a steam  turbine  drives the  compressor
supplying air for  the  oxidation of ammonia.
Without catalytic reduction, tail gas would be
preheated  by exchange  to a  temperature
140°F below the  limit for the turboexpander.
In the decolorization alternative, fuel addition
rate  is set  so that the heat  release meets the
temperature  limit of the turboexpander. The
additional power  recovery at the  expander is
balanced by reducing  steam  to the turbine. In
selective abatement, steam savings are smaller
because the selective reduction provided only
a 60° F  temperature rise. In two-stage, non-
selective abatement, additional steam savings
are obtained by  placing a waste heat boiler
between stages, since only 140° of the 680°F

                             FOR THREE CONTROL SYSTEMS

NOX before reduction, ppm
NOX after reduction, ppm
Investment, 103 $
Costs, 103 $/yra
Natural gas
Investment-based costs"
Total cost, 103 $/yr
Total cost, $/ton HNO3
Total cost, $/ton NOX removed
Natural gas
Two-stage non-
selective abater
Natural gas
              aParentheses indicate negative costs or savings.
              "Includes 20 percent return before taxes.
total  temperature rise  can be  used in the
   Investment-based costs are assumed at 50
percent per annum, including  a 20 percent
return before income tax. Steam is credited at
750  per  thousand  pounds; natural gas  is
charged at 400 per 1,000 Btu; and ammonia,
at $50 per ton. Catalytic life was assumed to
be 6  years for nonselective operation and 2
years for selective.
   To the extent that abatement is not yet
completely  satisfactory, the  basis  for this
study has not  been completely demonstrated.
   In  the  mid-1950's, a  catalytic decolorizer
was installed  on a high-pressure-type  nitric
acid  plant somewhat smaller  than those re-
ferred to in Table 7-2, at a cost of $50,000. A
recent estimate for a catalytic control device
for a  low-pressure nitric acid plant of some-
what greater capacity than the plants in Table

7-2 was $400,000.  This  was to be a unit for
decolorization or one-step, nonselective abate-
ment, the choice depending upon the amount
of synthesis gas (3N2  +  H2) mixed  with the
tail gas. Operating costs per ton of nitric acid
was estimated at $0.90 for decolorization and
$2.30 for abatement.18
   Investments  for  catalytic reduction must
provide for catalyst, reactor vessel,  controls,
and  heat-exchange  equipment. Honeycomb
catalyst costs about  $2,000 per cubic  foot
whereas spherical catalysts  cost about $700
per cubic foot.10 The costs per  scfm of tail
gas are about equal, however, because honey-
comb systems are designed for 100,000 stand-
ard cubic  feet per hour (scfh) of gas per cubic
foot of catalyst, while spherical catalyst sys-
tems are designed for space velocities of about
30,000 scfh of gas per cubic foot of catalyst.
Thus,  catalyst investments  are about $1.20

per scfm of tail gas or $ 100 per ton per day of
nitric acid. If two  catalyst  stages are em-
ployed, the investment must be doubled.
  Heat-exchanger  investments range widely
from a  low of zero  for equipping some new
plants with decolorizers to about $85,000 for
a waste-heat-boiler system in a 300-ton-per-
day nitric  acid plant in a single-stage abater
and perhaps higher costs for two-staged units.
  A complete design  and equipment  list is
needed  to obtain costs for any specific  plant.
In general,  unit investment per pound per day
of NOX removal varies inversely  with plant
size; the small plant is, therefore, at a cost
disadvantage relative  to the large plant.
  Operating costs must cover the cost of fuel,
repair  labor and materials,  catalyst replace-
ment, and  capital  charges. Fuel costs are de-
pendent on location and vary with the type of
fuel burned. Usually natural gas is less expen-
sive than hydrogen because, though costs per
1,000 scf are about the same, only 25 percent
as much is required. Hydrogen costs may ap-
proach  the cost  of methane  when  waste
hydrogen is available,  but the amount avail-
able  from  ammonia manufacture  would
usually  be  adequate  only  for  decolorization,
and the supply would not be available during
the ammonia plant shutdowns.
   Costs were not developed for caustic scrub-
bing because disposal of spent caustic solution
prohibits  use of this method  at most loca-
tions. An estimate of the cost of adding a
two-stage  abater to an existing plant has been
given by  Decker13  as  ranging from  about
$750 to $1125 per ton  per day of nitric acid
capacity for abatement corresponding to that
shown in  Table 7-2. The abater is installed
after  the turbine; thus,  there is no  credit for
energy recovery. This operation is said to re-
quire fuel  costs  of 50c per ton of HNC>3.

   Important uses of nitric acid and the  esti-
mated quantities consumed in each use are
listed in Table  7-3. The basis for these  esti-
mates is  given  in  the corresponding section

   Approximately  75 percent  of the  nitric
acid produced  in the United States is  con-
sumed in  making  ammonium nitrate;  adipic
acid manufacture consumes about 9 percent.
No other  single outlet  consumes more than
about  4  percent.  Other uses include  metal
pickling and etching, nitrations  and  oxida-
tions of organic compounds, and production
of metallic nitrates.
                      Table 7-3. ESTIMATED NITRIC ACID CONSUMPTION
                                  IN UNITED STATES, 1967
          Quantity, tons
                      Ammonium nitrate

                      Adipic acid
                      Terephthalic acid

                      Toluene diisocyanate  *

                      Commercial explosives (e.g.,

                      Fertilizers (e.g., NH4NO3)
                      Other uses







7.2.1   Ammonium Nitrate Manufacture
  Ammonium nitrate is produced by the di-
rect  neutralization  of nitric acid with  am-

        NH3 +  HNO3 -»> NH4NO3

About 0.81 ton  of  nitric acid (100 percent
equivalent) and 0.210 to 0.225 ton of anhy-
drous ammonia are required to make 1 ton of
ammonium nitrate. In actual practice 100 per-
cent nitric acid is not used, and typical feed
acid contains  55 to 60 percent HNO3-  The
product is an aqueous solution of ammonium
nitrate, which may be used as liquid fertilizer
or  converted  into   a  solid  product.  The
strength  of the  solution varies.  The heat of
reaction  is usually used to evaporate part of
the water, giving  typically a solution of 83 to
86 percent ammonium nitrate. Further evapo-
ration to a solid may be accomplished  in a
falling-film  evaporator,20 in a disk-spraying
plant,21  or by  evaporation to dryness  in a
raked shallow open pan (graining).2 *
   Much  of the solid ammonium nitrate  pro-
duced in  the  United States is  formed by
"prilling," a process in which  molten  am-
monium nitrate  flows in droplets  from  the
top  of a  tower countercurrent  to a rising
stream of air, which cools and solidifies the
melt to produce pellets or prills.22
   In  1967, 5.606 x  106  tons of ammonium
nitrate was produced.19 At  a nitric acid  con-
sumption rate of 0.81 ton  per  ton of am-
monium  nitrate, this  production  required
about 4.54 x  106 tons of nitric  acid. About
0.7 x  106 to  0.8 x 106 tons23  was used in
explosives, and  most  of  the  remainder was
used in direct application or  in mixed ferti-
lizers.  Emissions
  No significant amount of NOX is produced
in this process; the most likely source of nitric
acid emissions would be the neutralizer. The
vapor pressure of ammonia, however, is much
higher  than the vapor pressure of nitric acid,
and the release of nitric acid fumes or NOX is
believed to be negligible,24  especially since a
slight excess of NH3 is used to reduce product
  Because NOX emission is negligible, no con-
trols are required.

7.2.2   Organic Oxidations
  Nitric acid is used  as an oxidizing agent in
the  commercial  preparation  of  adipic acid,
terephthalic  acid, and other organic com-
pounds containing   oxygen.   The  effective
reagent is probably  NC>2, which has very
strong oxidizing power.
  Adipic  acid, COOH • (CH2)4 • COOH, is a
dibasic  acid used in  the manufacture of
synthetic  fibers. In the United States, adipic
acid is made in a two-step operation as shown
in Figure 7-3.
  The  first step  is the catalytic oxidation of
cyclohexane  by  air to a mixture of cyclo-
hexanol and cyclohexanone. In  the second
step, adipic acid is made by the catalytic oxi-
dation  of the  cyclohexanol/cyclohexanone
mixture using 45 to  55  percent  nitric acid.
The product is purified by crystallization.25
The whole operation is continuous.
                            Figure 7-3.  Adipic acid synthesis.

  In oxidizing cyclohexanol/cyclohexanone,
nitric acid  is reduced to unrecoverable N2O
and potentially recoverable NO and NO2- Ac-
cording to  Lindsay,26 the nitric acid require-
ment is 1.46 pounds per pound of chemically
formed adipic acid,  of which 0.43 pound is
potentially  recoverable NOX and 1.03 pounds
is unrecoverable N2O.
  Terephthalic acid  is an intermediate in the
production  of  polyethylene  terephthalate,
which  is  used in polyester, films, and other
miscellaneous  products. Some terephthalic
acid can be produced in various ways, one of
which  is by the oxidation of paraxylene by
nitric acid.  The nitric acid oxidation proceeds
in two steps as shown in Figure 7-4. Both
steps take  place in a single reactor. The first
step yields  primarily N2O, while the second
step yields  mostly NO in the offgas.27 Based
on  this information, the nitric acid require-
ment for terephthalic acid  is estimated to be
1.33 pounds per pound of terephthalic acid,
of which 0.76 pound is potentially recover-
able (NO)  and 0.57 pound is unrecoverable
  In 1967, total estimated terephthalic acid
production  was about 395,000 tons, an esti-
mated 150,000 tons  of which was made  via
nitric acid  oxidation. Estimated adipic acid
production  was about 540,000 tons,2 8 all of
which  is believed to be made from the nitric
acid  oxidation of  cyclohexanol/cyclo-
hexanone.  Nitric acid is used for the oxida-
tion of other organic compounds in addition
to terephthalic acid and adipic acid, but none
is believed  to approach these two in product
volume. Emissions
  No data  are available on NOX emissions re-
leased  to the atmosphere from manufacturing
operations using nitric acid  as  an  agent for
oxidizing organic compounds. The offgases
leaving the reactor after nitric acid  oxidation
of organic materials may  contain as much as
50 percent NOX before processing for acid re-
covery.26-27 N2O, one of the principal com-
pounds of the offgas, is not counted as NOX,
since it is not oxidized to NOX in the atmos-
phere and is considered harmless.
  NOX emissions and nitric acid requirements
were roughly estimated, based on  references
26 and 27, by assuming that NOX in adsorber
offgas would  be reduced to  0.3 percent,  a
level typical or nitric acid plants. The absorp-
tion systems recover an estimated  98  to 99
percent of the NOX as nitric acid and recycle
it back to the  unit.  The rough estimates are
shown in Table 7-4.
  The estimated NOX emissions from these
two  sources amount to  0.05 percent of total
NOX emissions from  all  stationary sources, or
0.03 percent of total NOX emissions from all
sources in the United States. Con trol Techniques
  In commercial operations, economy re-
quires the recovery of NOX as nitric acid. It is
recovered by mixing  the offgas  with air and
sending the stream  to an absorbing tower,
where nitric  acid  is  recovered as the stream
descends and unrecoverable ^O and nitrogen
pass off  overhead. The recovery  process is
similar to the NO  oxidation described in Sec-
tion 5; the system is similar to the absorption
sections of a nitric acid plant, as described in
the same section.
  Should the resulting emission rates be con-
sidered too high, further reduction could be
attempted by standard techniques such as in-
cineration in a  reducing flame or  additional
absorption (Section 4.3).
                        Figure 7-4.  Terephthalic acid synthesis.


Production rate,28 106 Ib/yr
Net HNOg consumption,'5 Ib/lb product
Net HNOg consumption, tons/yr
NOX emissions, Ib/ton product
NOX emissions, tons/yr
Adipic acid
Terephthalic acid
300. 00a
             aTerephthalic acid produced by nitric acid oxidation only.
             "After crediting for HNC>3 recovered from offgases.
  A potential, long-range control for elimi-
nating NOX from organic oxidation processing
is the replacement of nitric acid as an oxidant
by  catalytic  processes using air  or  oxygen.
Most  terephthalic  acid  is now produced via
catalytic processes using air oxidation. The
laboratory  catalytic  oxidation  of  cyclo-
hexanol and  cyclohexanone by air to  adipic
acid has also been reported,29 but no com-
mercial process is known.  Costs of Control
  Economy requires that nitric  acid  be re-
covered  from reactor  offgas in  large-scale
organic  oxidations using  nitric  acid  as  the
oxidizing agent. For example, the incentive
for acid recovery  for a  100-million-pound-
per-year  adipic acid plant  would be  about
$ 1.6 million per year. This figure is based on
recovering 0.3 pound of HNO3 per pound of
adipic acid at a nitric acid cost of $3.90 per
100 pounds.30 The optimum economic re-
covery level depends upon economic factors
at each installation.
7.2.3   Organic Nitrations
  Nitration  is the treating  of organic com-
pounds  with  nitric acid  (or NO2) to  produce
nitro  compounds  or nitrates. The following
equations illustrate the  two  most common
types of reaction:
     RH  + HONO2 -»• RNO2 +  H2O  (1)
    ROH + HONO2 -"  RONO2 + H2O (2)

Examples of products  of the first reaction
(C-nitration) are compounds such as nitroben-
zene, nitrotoluenes, and nitromethane. Nitro-
glycerin  (or glyceryl  trinitrate) and nitrocel-
lulose are examples of compounds produced
by the second reaction (O-nitration).
  Nitrating agents used commercially include
nitric acid,  mixed nitric and sulfuric acids
(mixed  acids),  and  NO2. Mixed nitric and
sulfuric  acid is most frequently used. The
sulfuric acid functions to promote formation
of  NO(2  ions  and to absorb the water pro-
duced in the reaction.
  Nitrations are carried out in either batch or
continuous processes. The  trend is toward
continuous processes, since  control is more
easily maintained, equipment is smaller, sys-
tem holdup  is smaller,  and  hazards  are re-
duced. A multiplicity of specialty  products
such as dyes and drugs, which are produced in
small volumes, will continue, however, to  be
manufactured by small batch nitrations.
  Batch nitration  reactors are usually covered
vessels provided  with stirring facilities and
cooling coils or jackets. The reactor bottom is
sloped,  and product is  withdrawn from the
lowest point. When products are potentially
explosive, a   large   tank  containing  water
(drowning tank) is provided so  that  the  re-
actor contents can  be  discharged promptly

and "drowned" in case of abnormal condi-
  When the reaction is completed, the reactor
contents are transferred to a separator, where
the product is separated from the spent acid.
The product is washed, neutralized, and puri-
fied; spent acids  are processed for recovery.
Figure 7-5 illustrates a batch nitration process
for manufacturing nitroglycerin.3 1
   Continuous nitration for nitroglycerin is
carried out in many types of equipment. Two
widely employed  processes are the Schmid-
Meissner process (illustrated  in  Figure  7-6)
and the Biazzi process (illustrated in Figure
7-7).3 1 Both processes provide for continuous
reaction, separation, water  washing, neutrali-
zation,  and purification.  The Biazzi process
makes greater use of impellers for contacting
than   the  Schmid-Meissner,  which uses
compressed air  to provide agitation  during
washing  and  neutralizing.  Both  types  of
equipment can be used for nitrating in gen-
   When mixed acid is used, the spent acid is
recovered  in a system similar to that shown in
Figure 7-8.3 *  The  mixed acid enters the  top
of the denitrating tower. Superheated steam is
admitted at the bottom to drive off the spent
nitric acid and NOX overhead. The gases are
passed through a condenser to liquefy nitric
acid,  which is withdrawn to storage; the  un-
condensed gases are then sent to an absorp-
tion tower. Weak sulfuric acid is withdrawn
from  the bottom of the denitrator tower and
concentrated or disposed of by some conven-
ient arrangement.
   When nitric acid alone is used for nitration,
the weak spent acid is normally recovered by
sending it  to  an absorption tower,  where it
replaces some of the water normally fed as
   Nitrobenzene  and dinitrotoluenes are pro-
duced in  large  volumes  as chemical inter-
mediates.  Explosives  such as TNT, nitrogly-
cerin, and nitrocellulose are produced in  sig-
nificant but lesser volumes.
   Nitrobenzene is manufactured in both con-
tinuous and  batch nitration plants.  Mixed
acids containing 53 to 60 percent H2SO4, 32
to 39 percent HNC>3, and 8 percent water are
used in batch operations, which may process
1,000 to  1,500 gallons of benzene in 2 to 4
hours.  Continuous plants, as typified by the
Biazzi units (Figure 7-7) also use mixed acids.
A 30-gallon continuous nitrator has the capa-
city of a 1,500-gallon batch nitrator. Yields
are 95 to 99 percent of theoretical.32
   The major use  of nitrobenzene is in the
manufacture of aniline. It is  also used as a
solvent. Nitrobenzene production in 1967 was
an  estimated 170,000  tons. Nitric acid  re-
quirements are approximately 0.53 pound per
pound of nitrobenzene.32 On this basis, nitric
acid used in nitrobenzene synthesis was esti-
mated at 90,000 tons for 1967.
   Dinitrotoluene  is  manufactured  in two
stages  in  both continuous and  batch  units.
The first stage is the nitration  of toluene to
mononitrotoluene, which is nitrated to dini-
trotoluene in the  second stage. For making
mononitrotoluene in the batch process, mixed
acid consisting of 28 to 32 percent HNC«3, 52
to 56 percent H2SO4,  and 12 to  20 percent
water is used in equipment sized to handle up
to  3,000  gallons.  Operating  temperature
ranges  from  25° to  40°C. Mononitrotoluene
yields of 96 percent are typical.33
   The second step, the production of dinitro-
toluene,  is carried out separately because it
requires more severe conditions.

   Dinitrotoluene is  made from  mononitro-
toluene using stronger mixed acid containing
28  to  34 percent HNO3, 60 to 64 percent
^2^04, and  5 to  8  percent water. Tempera-
tures are increased to 90" C after all the acid
has  been added.  Dinitrotoluene  yields are
about 96 percent of theoretical.33
   The  principal use of dinitrotoluene is as an
intermediate  in making toluene diisocyanate
(TDI) for use  in polyurethane plastics. It is
usually  supplied as mixtures of the 2,4  and
2,6  isomers.  In  1967,  about  200 million
pounds of toluene diisocyanate was produced,
which  required 75,000  tons of nitric acid,
estimated on the basis of 0.74 pound of nitric
acid per pound of TDI.

                                                             NITRATING HOUSE
                       STORAGE    MIXED-ACID
                        ____,   SCALE TANK
        MIXED ACID
                                                                                    I CATCHJANK
                                                                                    '    -
                                                  STORAGE TANK
                                                                      	»• BUGGY
                                                                     v NEUTRALIZER
                                                                                                               WASTE WATER
     I	I
                                                                                          NEUTRALIZING HOUSE

                                                                                                       CATCH TANKS
                                                NITRIC ACID
                                             RECOVERY HOUSE
                                           SPENT ACID FEED TANK -CT^^_	IT_J

                                 Figure 7-5.  Batch process for the manufacture of nitroglycerin (NG)?1
                                 (Courtesy of John Wiley and Sons, Inc.)
                                                                                                                WASTE WATER
                                                                                                              TO POWDER
                                                                                                           ^. WASTE WATER
                                                                                           .  RECOVERED
                                                                                           "^ NITRIC ACID
                                                                                              WEAK SULFURIC


                                                                                     TO BAFFLED
                                                                                    SETTLING TANKS
                                                                     (PASSED THROUGH ADDITIONAL
                                                                     WASH COLUMNS IF NECESSARY)
                                                                 WASTE WATER
                                                                 BAFFLE TANK
                                        Figure 7-6.  Schmid-Meissner continuous-nitration plant.
                                        (Courtesy of John Wiley and Sons, Inc.)


                                                 MECHANICAL WASHER
                                                                                                    SODA ASH
        SPENT ACID
                               TO DROWNING TANK
                                SPENT ACID
                             FROM SEPARATOR
                                                        WASH WATER TO SEPARATOR
                                          Figure 7-7.  Biazzi continuous-nitration plant.
                                          (Courtesy of John Wiley and Sons, Inc.)

                                                          GASES TO ABSORPTION TOWER

           FEED TANK
                     DENITRATING TOWER
                       (PAULING TOWER)
                                                                    NITRIC DISTILLATE
                                                                   NITRIC ACID
                                                                   TO STORAGE

                                           TO STORAGE
                                Figure 7-8.  Recovery of spent acid.
                                (Courtesy of John Wiley and Sons, Inc.)

-------   Emissions from Nitration
  Relatively large  NOX emissions may origi-
nate in  the nitration reactor and in the deni-
tration of the spent acid. NOX is also released
in auxiliary equipment such  as  nitric  acid
concentrators, nitric  acid plants,  and nitric
acid storage tanks.
  Nitration  reactions per se do not generate
NOX emissions. NOX is formed in side reac-
tions involving the oxidation of organic mate-
rials. Relatively little oxidation and NOX  for-
mation  occur when easily nitratable com-
pounds, such as toluene, are processed. Much
more severe conditions are required in proc-
essing compounds  that are difficult to nitrate,
such as dinitrotoluene; more oxidation takes
place and, thus, more NOX is formed.
  Very few data are reported in the literature
on  NOX emissions from nitrations. For con-
tinuous nitrations, one company has reported
emissions of 0.12 to 0.24 pound NOX per ton
of nitric acid, with a mean of 0.18 at a single
location.34  At the  same location, emissions
averaging 14 pounds of NOX per ton of  acid
were reported  in manufacturing specialty
products  in  small  batch-type  operations.
Further discussion of these small  batch nitra-
tions appears in the emission controls section.
   Using the factors  0.18 and 14 pounds NOX
per ton HNC>3 as lower and upper limits for
nitrations, the NOX emissions in  1967 would
have the range indicated in Table 7-5.


Estimated acid
NOX, tons/yr
 Even  using the upper limit,  NOX emissions
 from  nitrobenzene and  dinitrotoluene syn-
 theses are  relatively small but may present
 local nuisance problems. Since the upper limit

represents  specialty batch  operations  on a
small scale, the emissions are probably much
higher than would be encountered in large-
volume production of these products in either
batch or continuous equipment.  Control  Techniques
  In  large  batch  or continuous  nitrations,
operations  are carried out in closed reactors.
Fumes are  conducted from the reactor, air is
added, and the mixture enters an  absorption
tower for recovery of nitric acid. If too much
NO 2  remains in the residual gas from the ab-
sorber, it may be further reduced by conven-
tional techniques such as caustic scrubbing or
fume burning.
  NOX burning is  carried out under reducing
conditions.  An  excess  of fuel  is used, and
NOX  acting as an oxidant, is reduced to nitro-
gen. The system should have at least 10 per-
cent  excess fuel. Hardison discusses the fuel/
oxidizer ratio limitations in his article.3 5 NOX
reductions  of 75 percent36 to 90 percent35
have  been  anticipated, but not yet confirmed
in plant operation. Burner  and flare designs
are discussed in references 35 and 37.
  Caustic  scrubbing of NOX  is discussed in
the nitric acid section.
  Noncondensable gas from acid denitration
is treated in the same manner as reactor gas. A
common absorber is sometimes employed.
  Small batch nitrators used  in manufactur-
ing specialities such  as  drugs and  dyes are
small-volume, high-intensity NOX emitters. In
one plant,34 reaction times ranged from 3 to
12  hours,  depending on  the  product made.
From 3 to 850 batches of each product were
made each year. Emissions ranged from 1.4 to
260  pounds of NOX per ton  of  nitric acid,
with  a median of 42 pounds per ton of nitric
acid. The median emission was 14  pounds per
ton when one product was excluded from the
calculations. The emissions, which are vented
to individual stacks, are brown in  color for a
few hours per batch.
   Caustic  scrubbing and NOX incineration
were said to be the most plausible controls for
specialty batch nitrations. Catalytic reduction
is usually  ruled out  because  of organic and

otner impurities in the gas. Neither control is
considered highly  efficient in this application.
  The intermittent  character  of emissions
makes them difficult to control  and contri-
butes to very high pollution abatement costs
per ton  of nitric  acid  consumed. Operating
costs  for such equipment  would render ap-
proximately half of the small batch nitrations
so  uneconomical  that  the manufacture  of
these products would be terminated.34 Large
batches  may  be  suitable  for conversion to
continuous operating, but small  batches are
   A potential technique for control of NOX
emissions, which is suitable for some chemical
systems, is the addition of urea to the react-
ants. Urea reacts with ^03 or nitrous acid to
form  CC>2 and nitrogen. In addition to sup-
pressing  NOX formation, the addition of urea
virtually eliminates the oxidation of hydrocar-
bons.38   Costs of Controls
   In  large-scale operations, recovery  of nitric
acid by  absorbing concentration NOX emis-
sions should  be  economical, as discussed  in
Section  5 on oxidation. Fume incinerator in-
vestments are  quoted at $10,000 to  $20,000
by one  source.13  Another suggests that in-
vestments of  $75,000 to $150,000 are neces-
sary for flame abatement facilities for existing
small  batch  nitrators,  and  $75,000   to
 $250,000 for  existing large nitrators. Annual
operating costs were estimated at $25,000  to
 $85,000 per product for small batch nitrators
and $25,000 to $40,000 for continuous nitra-
 7.2.4  Explosives: Manufacture and Use
   Industrial explosives  in  the United  States
consists  of over 80 percent by weight  of am-
monium nitrate and  some 10 percent of nitro
organic  compounds. The following estimates
of ingredients used in  commercial explosives
(Table 7-6) were derived from statistics of the
U.S. Bureau of Mines.39
   Ammonium nitrate (AN) is widely used be-
cause it  is the least costly of all explosives, it
is very  powerful, and  it  is  low in relative
hazard.  Smaller amounts of other explosives
             EXPLOSIVES, 1967
 Ammonium nitrate

   Processed and unprocessed

   In permissiblesa

   In other high-explosives3

   In other blasting agents3

 Total ammonium nitrate

 Fuel oil, carbonaceous material
   and other nonexplosive



Quality, tons








   aAmmonium nitrate estimated at 65 percent of
 permissible, 35 percent of other high explosives and
 60 percent of water gels and slurries and rigidly cart-
 ridged blasting agents.40'41
   "Estimates provided by Du Pont.18

such as nitroglycerin (NG) and trinitrotoluene
(TNT) were consumed in commercial explo-
sives in 1967. The manufacture of ammonium
nitrate has been discussed above. The manu-
facture of TNT is included below under mili-
tary explosives.
  Nitroglycerin is manufactured  in one stage
in both  batch and continuous  units using
mixed acids. Description of specific batch and
continuous  units  are included in the general
description  of nitration above.  Anhydrous
mixed acid containing 45   to  50  percent
HNO3 and  50 to  55  percent H2SC>4 is the
most common nitrating agent. Nitric acid con-
sumption is  about  1  pound  of  nitrogly-
cerin.4 2  TNT also requires about 1 pound of
nitric acid per pound of product.33 The esti-
mated consumption of nitric acid in  1967 in
commercial  explosives, other than ammonium
nitrate,   was 38,000  tons, based on  10,000

tons of TNT and 28,000 tons of nitroglycerin,
as shown in Table 7-6.
   Production and consumption data for mili-
tary explosives are classified. Some of the
more important ingredients in military explo-
sives are known however: TNT, penterythritol
                                         TNT. RDX is used in admixture with TNT, or
                                         compounded with mineral jelly to form a use-
                                         ful plastic explosive. Tetryl is most often used
                                         as a primer for other less sensitive explosives.
                                         Methods of preparation  of these explosives
                                         are discussed in detail by Urbanski.33
                                                                    CH3    N02

 tetranitrate  (PETN),  cyclotrimethylene-tri-
 nitramine (RDX), and trinitrophenylmethyl-
 nitramine (Tetryl). Nitration is  an  essential
 step in the manufacture of each of these. The
 structures of  PETN,  RDX, and  Tetryl are
 shown above.
   PETN is most  commonly used in  conjunc-
 tion with TNT in the form  of pentolites,
 made  by incorporating PETN into molten

                                           TNT  (symmetrical  trinitrotoluene)  is
                                         produced in batch and continuous processes.
                                         The addition of each successive nitro group to
                                         the toluene molecule becomes more difficult,
                                         so that more servere  conditions are required.
                                         Therefore, it  is normal practice to introduce
                                         strong mixed  acids at one end and toluene at
                                         the other in  a multistage-operation as illus-
                                         trated in Figure 7-9.
                                      60% HNO3
                                                        60% HN03

* D






H2S04 "








                Figure 7-9. Trinitrotoluene (TNT) manufacturing diagram.

  The spent acid leaving one stage is fortified
with fresh nitric acid before entering the next
stage. The  spent  acid from  the final stage is
denitrated in equipment illustrated  in Figure
7-5. Nitric acid is recovered from the denitra-
tor overhead, for recycling to the process, and
the uncondensable gas is sent to an absorption
tower to recover NOX as nitric acid.
  TNT is purified by washing with sodium
sulfite  (Sellite).  The principal impurities are
asymmetrical  trinitrotoluenes,  which  react
rapidly with sodium sulfite to form dinitro-
toluene sulfonates, which are  soluble in the
wash water. The waste wash water is disposed
of by  evaporation, the residue  being  incin-
erated.33  The waste water has  a  deep red
color from  the nitro compounds  and is gen-
erally referred to as "redwater."
  Operating conditions for nitrating dinitro-
toluene to   trinitrotoluene  are  severe-
anhydrous mixed acid,  and temperatures of
100° C and higher. Yields of 82 to 90 percent
of theoretical  are  typical, because of oxida-
tion reactions that generate carbon oxides and
NOX.33 It  is,  therefore,  essential to process
the reactor fumes as well  as the denitrator
uncondensibles in an  absorption tower to
minimize net acid consumption.
  For discussion of the various processes used
in manufacturing TNT, the reader is referred
to Urbanski's book.3 3  Emissions
  NOX emissions from TNT nitration  reac-
tors at the new Ordnance Works  at Newport,
Indiana,  have  been estimated  at  approxi-
mately 2.5  pounds per ton of TNT. The NOX
concentration, diluted with 14 volumes of air,
was estimated at 400 ppm.36
  Much higher NOX emissions were reported
at another ammunition plant where the equip-
ment was old  and overloaded.  This plant has
been  an  intense source of NOX, generating
about 130,000 pounds of NOX per day.43'44
A program is planned to reduce emissions.
  Assuming that  the average  TNT  plant
would emit twice as much NOX as the new
plant mentioned above, and  that no NOX is
emitted in nitroglycerin manufacture,18 NOX
emisisons from commercial explosives (Table
7-6) are roughly estimated in Table 7-7.
  Redwater destruction is not employed in
manufacturing TNT  for  commercial use18
because redwater is  not  made.  In  contrast,
redwater production  accompanies  manufac-
ture of military TNT because of the necessity
for washing out isomers and other impurities
that make TNT sensitive  to  shock. Military
ammunition must be  capable of long storage
and must be insensitive to handling.
  Other sources of NOX related to explosives
manufacturing are denitration of spent mixed
acid, nitric acid concentration, and nitric acid
  At  the Newport  plant,  emission  from
denitration and nitric acid concentration were
estimated at   about  12 pounds  per ton  of
nitric  acid after 75 percent reduction in a
fume  burner.  The  NOX concentration was
estimated at 1.7 percent, probably all NO.
  Oxygen  balance  is the most important
factor affecting the  NOX  content  of the
products of explosions. An  oxygen-balanced
explosive  contains just enough oxygen  to
                       Table 7-7. TOTAL UNITED STATES NITRIC ACID
                            OF INDUSTRIAL EXPLOSIVES, 1967

TNT (industrial use)


Ib/ton HNO3


Nox, ton/ton
of product


oxidize all carbon in the explosives to carbon
dioxide  and all  hydrogen to water. A defi-
ciency of  oxygen  produces  toxic  carbon
monoxide whereas an excess of oxygen tends
to  produce NOX.40>41  Ammonium nitrate
itself contains an excess  of one atom of ox-
ygen  per molecule.  Combustible materials,
e.g., 5 percent fuel oil, are usually added to
ammonium nitrate to provide oxygen balance
and increase power. Nitroglycerin has a slight
oxygen surplus, while TNT is short of oxygen.
   Compositions  of the products of explosions
are difficult  to  determine. For this  reason,
most of the information on these products is
obtained by  detailed computer calculations.
The reader  is referred to Cook's book for
information on  methods of calculations.40
NOX is found in explosion products when the
explosive contains  more  oxygen  than  re-
quired for oxygen balance. Calculated NOX in
products from various explosives are listed in
Table 7-8.
   The volume of NOX generated in commer-
cial explosions in 1967 was probably less than
100 tons since  most  of the  28,000 tons of
nitroglycerin  was  used  in  oxygen-balanced
   Although a very small source of NOX emis-
sion, explosions  could be an intense source in
confined spaces underground, and precautions
should be  taken to avoid exposure in such
7.2.5   Other Uses of Nitric Acid
  Other uses of nitric acid include acidulation
of phosphate  rock  and limestone for use  in
fertilizers, the treating of metals, and rocket
fuel oxidation. Fertilizers
  In  the  United States,  sulfuric and phos-
phoric acids are  the principal acids  used  in
acidulating phosphate rock. A few manufac-
turers produce "nitric phosphate" fertilizers
by acidulating phosphate rock with nitric acid
to form phosphoric acid and calcium  nitrate.
In subsequent steps, ammonia is added with
either carbon dioxide, or sulfuric or phos-
phoric acid,  and  "nitric  phosphates" are
formed.  Dibasic  calcium  phosphate  and
ammonium  nitrate are  the useful compounds
  U.S. Department of Agriculture statistics46
do not 'segregate nitric phosphate fertilizers
made by acidulation of phosphoric rock; but
private sources indicate that nitric phosphate
fertilizer made in this  manner was estimated
at 500,000 tons in  1967,  and nitric acid con-
sumption at 150,000 tons.
  NOX emissions are dependent on the quan-
tity  of carbonaceous  material in  the rock,
since NOX is formed as nitric acid oxidizes the
carbonaceous  matter.  The use  of calcined
rock avoids the production of NOX.
  Air pollution abatement by fertilizer manu-
facturers'  efforts has  centered on reducing
                         Table 7-8. NOX IN PRODUCTS OF EXPLOSIONS
TNT, 50/50 Amatol, RDX, tetryl,
straight dynamites, AN=fuel oil
Straight gelatins, 40%
Straight gelatins, 75%
NO %
1NUX, '"

NOX, Ib/ton


particulates and fluorides emissions, which are
severe problems. The water scrubbing used to
reduce these  pollutants would be expected to
reduce NOX emissions to only a minor degree.
Although no  measurements of NOX emissions
are available, brown plumes are said to occur.
   One company47 has found that the addi-
tion  of  urea  to  the  acidulation  mixture
reduces  NOX  emissions and eliminates the
brown plume.47  Urea, as mentioned in the
discussion on nitration controls, reacts with
NO and NO2  to form N2.  Metals Pickling
   The principal use of nitric acid in metals
pickling is in  treating stainless steel. Mill scales
on stainless steels  are hard and are difficult to
remove.  Pickling  procedures  vary; sometimes
a  10 percent sulfuric  acid  bath at  140° to
160° F  is followed by  a bath at 130°  to 150°
F  with  10 percent nitric acid and 4  percent
hydrofluoric  acid. The first  bath loosens the
scale, and the  second  removes it. A continu-
ous system for stainless steel strip consists of
two tanks containing 15 percent hydrochloric
acid, followed by  a tank containing 4 percent
hydrofluoric  and  10  percent  nitric  acid at
150° to  170° F. One effective  method is the
use  of molten salts of sodium hydroxide to
which is  added some  agent such as  sodium
hydride. This may be  followed by a dilute
nitric acid wash.4 8
   Nitric  acid is preferred to other acids for
treating stainless  steel because  it does  not
attack the steel  itself. Sulfuric  and  hydro-
chloric acids,  which are less costly, are  usually
preferred for  other metals.
   No measurements were found of emission
rates from nitric acid pickling of stainless steel.
Treating  equipment  should   be properly
hooded  and  ventilated  and  the   fumes
scrubbed  to  protect workers.  Urea  would
probably control the NOX emissions.
   Nitric  acid is  also  used  in  the chemical
milling of copper or iron from metals that are
not  chemically attacked by  nitric acid, and
for bright-dipping  copper. In  the latter opera-
tion, a cold solution of nitric and sulfuric acid
has  been customarily  used.  It has been re-
ported that copper can be  bright-dipped in
cold nitric acid alone when  urea is added. A
highly acceptable finish is obtained, and NOX
fumes are eliminated.
  Sulfuric acid should not be used with the
nitric acid-urea  mixture  since nitrourea,  an
explosive, can form. Not more than 8 ounces
of urea per gallon should be added, and satis-
factory operation can be obtained with only 2
ounces per gallon.
  In chemical milling, the addition of 6 to 8
ounces of urea per gallon of 40 percent nitric
acid will  reduce  NO2 emissions from  8,000
ppm  to  levels below  10 ppm,  providing a
bubble disperser is used.4 9
  A small, but intense, source of NOX occurs
in the manufacture of tungsten filaments for
lightbulbs. Tungsten filaments are wound  on
molybdenum  cores,  and after heat-treating,
the   cores  are dissolved  in  nitric  acid.
Decker13  describes air pollution  equipment
for reducing  the dense NO2 fumes given off
periodically when trays of the filaments are
dissolved. The fumes pass  over  a charcoal
adsorber  bed, which adsorbs NOX as fumes
are generated and desorbs when no fumes are
being generated. This smooths out  peaks and
valleys in  NOX content in offgases, which are
then heated and combined with carbon mon-
oxide and hydrogen from a rich combustion
flame. The mixture is then passed through a
bed of noble metal catalyst.  A colorless gas is
released from the equipment.
  Oil and gas production, pipeline transporta-
tion,  gas  plant  operation, and  petroleum
refining  as  stationary sources of NOX  are
discussed in this section. Tanker, barge, truck,
and rail transportation, since they are mobile
sources of NOX, are not covered.
  Major   power  consumers in  oil  and gas
production  operations  include  drilling,  oil
well pumping, gathering, gas and water injec-
tion  to  the  producing  formation,  and  oil
dehydration.  Gas plant  operations include
removal of impurities, such as water and sur-
face  compounds, and recovery of  valuable

natural gas liquids, such as ethane, propane,
butane, and natural gasoline.
   In petroleum  refining,  crude oil is  con-
verted into salable products  in the required
volumes.  A simplified flow sheet for a refin-
ery  appears in  Figure  7-10. Crude oil is
charged  to  an atmospheric  pipestill where
light products  are separated and taken over-
head and light catalytic reforming feed,  raw
gasoline,  kerosene,  middle  distillate,   and
heavy gas oil are taken as sidestream products.
The reduced crude  is charged to  a vacuum
pipestill where heavy gas oil, lube stocks, and
residuum are cut.
   Atmospheric  and vacuum gas oils  are
charged  to  catalytic  cracking  units, which
produce  light  ends, cracked gasoline,  and
fractions  for blending distillate and residual
fuels.  Reduced  crude is  used in  making
asphalt or residual fuels,  and is often fed to
coking  to  increase  the  yield  of distillate
   Catalytic  cracking and  coking produce
propylene  and butylene,  which  are  often
alkylated with  isobutane to  make alkylate.
Sometimes  the olefins are polymerized for
gasoline  or  chemical production.  Catalytic
reforming  increases  the  octane number of
naphtha by converting naphthenes  (saturated
cyclic hydrocarbons)  and  paraffins  to   aro-
matics. Hydrogen treating is  used  to reduce
sulfur content, increase stability, and improve
burning characteristics  of  kerosenes   and
middle distillates.
   The relative  volumes of gasoline, kerosene,
middle distillate, heavy fuel oil,  etc., can be
adjusted by diverting heavy gasoline fractions
from gasoline  to middle  distillate and  cat-
cracking  feed,  by diverting  coker  feed to
heavy fuel, etc.
   A fluid-bed catalytic-cracking unit is often
the heart of a modern refinery. Such a unit is
illustrated in Figure 7-11. Preheated gas  oil is
charged  to a  moving stream of hot regen-
erated catalyst while it is  being transferred
from the regenerator to the reactor. The gas
oil is cracked in the reactor; the products then
pass through cyclone separators  for removal
of entrained catalyst and are cut  into prod-

 ucts  in  a fractionator. Coke forms on  the
 catalyst during the reaction.
   Spent  catalyst  is  withdrawn  from   the
 bottom of the reactor and transferred to  the
 regenerator where coke  is burned off.  The
 regenerator flue  gas  passes through cyclone
 separators for catalyst removal  and is  dis-
 charged  through the  stack.  The  hot,  regen-
 erated catalyst flows  back to the reactor,
 supplying heat and  catalyzing the  cracking
   The regenerator flue gas contains about 10
 percent carbon monoxide. This gas is some-
 times fed to a CO boiler where it is burned in
 the preheated air to generate steam. Auxiliary
 fuel is required to maintain satisfactory com-
 bustion conditions.
   Typical  refinery process heaters  are  the
cabin-type furnace, used  for heat releases
above  150 million Btu per  hour, and  the
vertical cylindrical furnace,  used  for  heat
duties below 80 million Btu per hour. Either
type may be  used in  the 80- to  150-million-
Btu-per-hour  range.  Combustion  boxes  are
lined  with refractory. Fuels may  be liquid,
gas, or a combination of both. Gas burners
operate with  10  to  40  percent  excess  air,
liquid  burners with 20 to 80 percent. Stack
temperatures are 400°  to 900° F.
7.3.1   Emissions
   NOX emissions  in the petroleum and natu-
ral gas industries result from the combustion
of fuel in process heaters and boilers, and
from  internal combustion engines  used  to
drive   compressors and electric  generators.
NOX  is   also   released from  the  catalytic-
cracking regenerator and from CO boilers.
   Oil  and gas production,  gas  plants, and
pipeline stations are usually located in remote
areas  far from population centers. Emissions
do not, therefore, contribute substantially to
NOX levels in populous areas. Petroleum refin-
eries,  however, are often  located in or near
densely populated areas.
   Estimates of the amounts of NOX emitted
from petroleum and natural gas operations in
the year 1967  are shown in Table 7-9.

                                                                           DRY GAS
                                WET GAS
                      LIGHT NAPHTHA
                     MIDDLE DISTILLATE
                     HEAVY GAS OIL
                                                                                                      *-FUEL GAS
                                                                                  ALKY. I—ALKYLATE-
                                                                                        1 GASOLINE
                                                        STRAIGHT-RUN GASOLINE
                                              CRACKED GASOLINE
         LUBE STOCKS
                                                  WET GAS
                                .  MOTOR
                          LIGHT FUEL OIL
                         AND DIESEL OILS
                                                                              -FLIGHT FUEL OIL

                                                                              -»HEAVY FUEL OIL
                       -»-LUBE STOCKS

                                      Figure 7-10.  Composite processing plan for a modern refinery.

                FLUE GAS
                                                                     WET GAS TO POLY.
                                                                   OR ALKYLATION UNITS
                                                                   CRACKED GASOLINE
                                                                   LIGHT FUEL OIL
                                                                   RECYCLE GAS OIL
                     HEAVY FUEL OIL
                    Figure 7-11.  Fluidized bed catalytic cracking unit (FCC).
  Diesel oil consumption given in Table 7-9
was taken as residual plus diesel fuel oil con-
sumed in "oil-company use,"5 4 minus fuel oil
"consumed  at refineries."5 3  All diesel  oil
consumed  in  oil  and  gas  production  was
assumed to have been used in diesel engine
prime movers, the use with the highest emis-
sion factor.  Gas consumed  in  oil  and gas
production was estimated by subtracting gas
used in gas plants and estimated losses,  from
the gas field use.50
  Under "refining" in Table 7-9, the numbers
shown for  gas include natural gas, LPG, and
refinery gas,  calculated as natural gas on the
basis of heat of combustion.53  Oil consump-
tion includes acid sludge.5 3
  Allocation of gas plant fuel among "heaters
and  boilers,"  "gas  engines,"  and "gas  tur-
bines" is based on a survey of sources of NOX
emissions in  the  oil and gas industries con-

ducted  by  Esso Research  and Engineering
Company and  funded  by the National Air
Pollution  Control  Administration  under
Contract No.  PH  22-68-55.51 <56  Allocation
of pipeline  fuel between "gas engines" and
"gas turbines" is based on capacities indicated
by Pipe line Industry.5 7  Allocation of gaseous
fuels to the various users in refineries is based
on   the  industry  survey  conducted  by
Esso.5»>5 6
  Emission factors for gas-fired heaters  are
derived from  Figure  7-12.  This relationship
between NOX production rate and fuel-firing
rate is based on emission data obtained in the
petroleum and natural gas survey.5'>s 6 Emis-
sion factors for  diesel  engines  and oil-fired
furnaces are taken from Duprey55 Emission
factors for  gas engines are based on  Figure
7-13, which is reproduced from the studies of
Mills14 et al. Emission factors for gas-turbine

                                     OPERATIONS, 1967
           aOil consumption as million bbl/yr.
            Gas consumption as billion scf/yr.
           bOil emission factors - Ib NOX/1,000 gal.
            Gas emission factors - Ib/million scf .
           cReference 50.
           dReference 51.
           eReference 52.
           f Reference 53.
           ^Reference 54.
           hReference 55.
               Table 5-3.
Oil and gas production
Heaters and boilers
Diesel engines
Production subtotal
Gas plants
Heaters and boilers
Gas plant subtotal
Gas pipelines
Gas pipeline subtotal
Heaters and boilers
Heaters and boilers
Refining subtotal
Total NOX emissions
7.3f> 8
39 .7f


NOX emissions

engines were  from  Table 7-9.  Furnace  and
engine sizes  in the various  applications are
also based on the survey of the petroleum and
natural gas industry.51'56
   Other emissions  in  refining  include NOX
produced in  catalytic-cracking  regenerators
(3,000 tons per year), and NOX from carbon
monoxide burned in CO boilers (6,000  tons
per  year).  The  emissions   from  catalytic-
cracking regenerators  result  from  cracking
4,190  thousand  barrels  of fresh feed  per
day5 8  at an emission rate of 4.2 pounds NOX
per  thousand  barrels.59  There  are  75  CO
boilers in the United States,8 each emitting an
estimated 21  pounds per hour of NOX.

   Catalytic-plant  regenerator  gas  emission
factors  were  derived from the data reported

02 IN FLUE GAS, percent

   O   2.5-3.4
   •   3.5-4.4
   A   4.5-5.4
   •   5.5 +
            8   10
                            20      30    4O   50    70    100

                                    FIRING RATE, million Btu/hr
                                                                             300   4OO  500
             Figure 7-12. Oxides of nitrogen emissions from refinery furnace.51
by Mills5 9 et al., since the factors reported by
Duprey55 were based  on  data which  have
been  superseded.59  The  4.2  pounds  per
thousand  barrels of  fresh  feed is  a  weight
average emission factor for fluid and moving-
bed  catalytic-cracking  units.  NOX in  cat-
alytic-cracker flue  gas would  be  about  10
times as high, based  on Duprey's  factors,5 5
                                 which are not consistent with other data now

                                   CO boiler factors were based on recent tests
                                 by an oil company.6 The data reported by
                                 Mills59 et al. would give 45 x 103 tons per
                                 year of NOX. This is greater than the amount
                                 generated  in  a  conventional  furnace with


                 OIL WELL PUMP ENGINE
                   IN GOOD CONDITION

                 100         1,000

                ENGINE LOAD, hp
 Figure 7-13.  Variation'in oxides of nitrogen
 emission rates from stationary internal com-
 bustion engines.^

much higher flame temperatures; the emission
factor is therefore thought to be too high.
  Emission  factors  for gas  engines are not
firmly  established.  In  Figure 7-13, the high
values for large engines reported by Mills et al.
are  believed  to   reflect  lean   combustion
mixtures, supercharging, low speed, and other
factors  that favor high NOX emissions. (See
Section 5.3.)  The values  for smaller engines
reported by Mills60 et al.  are probably  for
naturally aspirated engines, operated at higher
speed with rich combustion mixtures—factors
that favor low NOX The position of the points
on the plot probably reflect these factors, not
size per se. Thus,  the correlation shown  in
Figure  7-13 is used  as  a  means of inter-
polation  in  estimating  emissions.  Extra-
polation of the correlation for engines larger
than 2,000  horsepower (hp),  the largest size
tested,  was  avoided  and emission  factors
derived for  the 2,000-hp engine were applied
to larger engines.
   It should be emphasized that only meager
data are  available  for  gas engines, and the
emission  factors may  not  be truly repre-
sentative. The use of  Duprey's gas engine
emission  factor,  based  on tests of  engines
averaging about  350 hp52  would result in
estimated pipeline and gas plant emissions of
only 200,000 and 140,000 tons per  year of
   NOX emissions from the  petroleum  and
natural gas industries as a percentage of total
NOX from all sources in the United States are
given in Table 7-10.

Petroleum and natural gas
Oil and gas production
Gas plant
Percent of total 1967
3.7 '
                                               7.3.2   Control Techniques
                                                 Control  of emissions from  steam  boiler
                                               operations  has been covered in  Section 5.1.
                                               Combustion modifications such as low excess
                                               air, flue gas recycle, two-stage  combustion,
                                               and  steam  or water injection  may be ap-
                                               plicable to both boilers and refinery heaters.
                                                 Flue  gas  recirculation has potential as an
                                               NOX control method for process furnaces. It
                                               could be incorporated into new  furnaces and
                                               installed in  existing furnaces. Several factors
                                               mitigate against  two-stage  combustion and
                                               very low excess  air rates in process furnaces.
                                               Excellent combustion conditions are required
                                               to prevent hot spots and coking inside furnace
                                               tubes as well as corrosion  of tube hangers.
                                               Two-stage combustion produces long smoky


flames which impinge on furnace tubes. Most
furnaces are of natural draft design, and the
air rate is controlled  by  damper setting.
Furthermore, many furnaces burn by-product
gas or liquid fuels of fluctuating composition
and heat content. Thus, inadequate control of
air rate and fluctuating fuel quality limit the
reduction in excess air.
   Potential methods of controlling emissions
from spark-fired gas engines include adjusting
fuel/air ratio, reducing intake manifold pres-
sure,  water injection, and  exhaust gas recir-
culation. Tests on laboratory engines indicate
that water injection into  the intake manifold
reduces NOX with no decrease in  maximum
power or fuel economy.6 1  At 93 percent of
theoretical fuel, NOX was reduced from 3,000
ppm  with  no water injection to about 700
ppm at a water injection  rate of 1.25 pounds
per pound of fuel. Water injection  was even
more effective for rich mixtures.
   Operation with rich rather than lean mix-
tures reduces  NOX,  but increases fuel  con-
sumption and CO and hydrocarbon  emissions.
The NOX can potentially be reduced to N2 in
a  catalytic reactor making use of  the  un-
burned hydrocarbons and  carbon  monoxide
in the exhaust gas from fuel rich combustion.
A second reactor may be needed to oxidize
the CO and hydrocarbons.
   In  engines  burning natural gas,  fuel/air
ratios as low as  70 percent of stoichiometric
may  be employed without misfiring.  The
narrower flammability   limits  for  gasoline
cause  misfiring  at such  lean conditions. A
laboratory engine burning methane at 70 per-
cent  of stoichiometric  fuel  produced only
about 30 percent of the NOX produced at 92
percent  of  stoichiometric  methane.62  The
latter  operation  is typical  of many engines
and produces maximum NOX. Operation with
very  lean  mixtures  results in  a  loss  of
maximum power, but fuel economy apparent-
ly was not reduced in a laboratory engine.62
   Reduction  of  manifold pressure  as limited
by maximum speed tends to reduce NOX at
the expense of fuel economy.
   NOX emissions  can be  cut, over  a long
period,  by using gas  turbine drivers in  new

installations and replacing wornout units with
gas turbines.  Gas  turbines  consume  about
11,000 Btu per horsepower-hour compared to
about 6,600 for piston engines.5 7 The cost of
modified aircraft turbines, however, are about
$110  per horsepower  compared with about
$300 for reciprocating engines.5 7 The current
trend is  toward  turbines  that are  better
adapted to automation.
   NOX  from diesel engines may be reduced
by the use of precombustion instead of direct
injection.  Variable compression  ratio, two-
stage fuel injection, and exhaust gas recycle
may be helpful. (See Section 5.2.)
7.3.3   Costs of Control
   Cost  estimates were made for two recent
refinery furnace designs and are tabulated in
Table 7-11. Input rates of 50 million and 300
million  Btu  per  hour,  respectively, were
selected as being representative of the sizes
presently  found in refineries. Estimates for
other  sizes may be extrapolated from these
data.  Cost estimates  were made  only  for
flue gas recirculation  control,  other control
techniques not being suitable to this equip-
ment, as discussed in Section 7.3.2.
7.4.1   Furnaces Related To Hot Metal
       Or Pig Iron Production Blast Furnace
  The  blast furnace  is the central unit in
which iron ore is reduced, in the presence of
coke and limestone, for the production of pig
iron. The blast  furnace itself is normally  a
closed unit and therefore has no atmospheric
emission. A preheated air blast  is supplied to
the furnace from  the  blast furnace  stove,
through nozzle-like openings called tuyeres.
The subsequent reactions in the blast furnace
are not pertinent to this discussion.  Excellent
descriptions are available,  however, such as
the complete  discussion of the process  of
changing raw ore to finished steel published
by United States Steel Corporation.63
   The  hot blast  reacts  with  the coke  to
produce  heat   and carbon monoxide. The
excess carbon monoxide, beyond that needed

                           CONTROL BY FLUE GAS RECIRCULATION,
                               APRIL THROUGH JUNE 1969a'51
No recirculation
30 percent recirculation
Increased horsepower required
by 30 percent recirculation
Furnace No. 1"
89 hp
63 hp
Furnace No. 2C
$ 712,500
$ 865,200
$ 867,000
$ 180,000
530 hp
386 hp
             aBasis of estimates:  only furnace equipment costs, including blowers, are re-
            flected in costs. Nonequipment costs (i.e., foundations, site preparation, instru-
            mentation, feed piping) and contractor's field labor overhead and fees, are not
            ^50 million Btu/hr, vertical cylindrical, heat absorbed.
             C300 million Btu/hr, vertical tube cabin, heat absorbed.
in reducing the ore, leaves the top of the blast
furnace  with  other  gaseous  products and
particulates and is known as blast furnace gas.
This gas is cleaned to remove-the particulates,
which  could  later cause plugging.  It is then
available for  heating purposes. Blast furnace
gas  contains about 1 percent hydrogen and 27
percent carbon monoxide; it  has  a heating
value of approximately 92 Btu/ft3,64 Blast Furnace Stove
  Between 4  and 5 tons of blast furnace gas is
generated for each ton of  pig iron  produced.
Some  18  to  24 percent of this gas is used as
fuel to heat the three stoves which are usually
associated with each blast furnace. Two are
generally  on  heat while the third is on blast.
  The  blast furnace stove is a structure some
26 to 28 feet in diameter and about 120 feet
high.  A  roughly  cylindrical  combustion
chamber extends to the  top of structure and
the hot combustion gases pass through a brick
checkerwork to the bottom by reverse flow
and  thence to the  stack.  The  checkerwork
usually contains 275,000 square feet of heat-
ing surface and has about 85 percent thermal
efficiency.  Unlike  the  conventional  regen-
erators, which extract  heat  from the waste
combustion gases,  the  blast furnace  stove is
heated by burning  fuel. The stored heat is
then used to  preheat air for the combustion
of fuel in the furnace to be served.
   The low calorific value of blast furnace gas
would indicate that due to dilution and low
flame temperature  the NOX  content of the
combustion  gas  from  blast  furnace stoves
should be  low.  A  steel manufacturer  who
requests annonymity gives data on  stack gas
samples ranging from 1.7 to 6.6 ppm NOX, or
an average of 3.6 ppm. However,  the NOX
emission based on the Btu  output is estimated
at 26 pounds NOX per hour or 114  tons per

year for a typical blast furnace.* On June 1,
1968, there  were 173 furnaces on blast in the
United  States,66 which indicates an annual
atmospheric burden of 20,000 tons of NOX
from blast furnace operations. This is 0.7 per-
cent of the estimated  1965 total  industrial
NOX emissions6 7 in the United States.
7.4.2   Furnaces Related to Steel
       Production  Open Hearth Furnace
  Steel making by the  open  hearth process
has been decreasing since it reached a peak in
1956, when  it represented 90 percent, or 103
million tons, of the total production. In 1967,
production had declined to 55  percent and by
1972,  only  25  million tons may be  so-
produced. Phase-out   is  expected   between
1980 and 1990.68 Regardless of this dramatic
decline in the  use of the open  hearth process,
due to  the  inroads of basic oxygen furnace
steelmaking  processes, its  NOX   emission
potential deserves consideration.
  The  open hearth furnace  is both  reverber-
atory and regenerative, like  the glass melting
furnaces (see Section 7.6). It is reverberatory
in that the charge is  melted in a shallow
hearth  by heat from a  flame passing over the
charge   and  by radiation  from  the heated
dome. It is regenerative in that the remaining
heat in  the partially spent combustion  gases
from  the  reverberatory  chamber is  ac-
cumulated  in  a brick-filled  chamber,  or
"checker", and released  to preheat the  in-
coming  combustion air when the  cycle  is
*This calculation is based on a specific gravity of 1.02
 for blast furnace gas64 (0.08233 pound per cubic
 foot). If an average value of 4.5 tons of blast furnace
 gas produced per ton of pig iron is assumed, then
 109,300 cubic feet of blast  furnace gas is produced
 per ton of pig iron, or 7,150,000 cubic feet for 65.4
 tons,  which is  the average production per  blast
 furnace per hour.65 If  an average value of 21 per-
 cent of the blast furnace gas having a Btu content of
 90 is used to heat the stoves,  then 135,180,000
 cubic feet is used, or 65,590,000 cubic feet for each
 of the two  stoves  on  heat.  This amount to  13
 pounds NOX per hour,  or 26 pounds per hour for
 the two stoves, or 114 tons per year.
reversed. Fuel of low calorific value such as
blast furnace gas as well as the combustion air
may be preheated by the checkers in order to
obtain the high temperatures required.
   Hot metal from the blast furnace, pig iron,
scrap iron,  and lime are the  usual  materials
charged to an open hearth furnace. These are
heated over a period averaging 10 hours, at a
temperature  as  high  as  the refractories will
permit. Fuel oil is the preferred  fuel and is
burned with excess air to provide an oxidizing
influence  on the charge.  These  conditions
favor NOX formation, and  an  analysis of  10
open hearth stack samples shows an emission
range of 22.9 to 671 ppm, or an average of
243 ppm. In another series of tests, an average
of 320 ppm was obtained.  These analyses
were taken while an  oxygen lance was being
used  to  enhance  the  steel  producing
   It is estimated that 490 pounds of NOX is
emitted in the production of a 200-ton batch
of steel. These calculations are based  on 3.2
million Btu  of  fuel per  ton  of steel70 and
0.76 pound  of  NOX per million Btu. If the
United States production of steel in 1967 was
127,213,000 tons70   and  55  percent was
produced  in open  hearth furnaces,  then
roughly 85,000  tons  of  NOX  was emitted  to
the  atmosphere  from   these  sources.  This
represents 3.1 percent of the estimated  1965
industrial  NOX emissions  in the United
States.68 Coking Ovens
   Coke is an essential component in making
pig iron and  steel; coke ovens are generally an
integral part of the steel plant complex.  In
1967,  coke  ovens  in   the  United  States
produced 63,775,000 tons of coke. One-sixth,
or 92  million tons, of the total bituminous
coal produced was charged to coke ovens. On
the average, 1.43 tons of coal was required for
each ton of coke produced.71 •
   Conventional  coking is done in long  rows
of slot-type ovens into which coal is charged
through holes in the top of the  ovens. The
sidewalls or liners are built of silica brick, and
the spaces between the chambers are flues in
which  fuel gas  burns to supply the required

heat. Each pound of coal carbonized requires
1,000 to 1,150 Btu and flue temperatures are
as high as 2,700° F.7 2. The remaining heat in
the partially spent combustion  gases is ac-
cumulated in  a  brick  checkerwork,  which
releases it to preheat the combustion air when
the cycle is reversed.  This is a typical regen-
erative  cycle  to conserve  fuel  and  give a
higher flame temperature.
  The coal in the coking chambers undergoes
destructive distillation during a heating  period
of about 18 hours. The noncondensable gase-
ous product is known as coke oven gas and on
a dry basis has a heating value of about 570
Btu/cu ft. Approximately 35 percent  of the
coke oven gas produced is used in heating the
  The trend in new coke oven construction is
toward  higher ovens,  \6l/i  to  19 feet  as
opposed  to the usual 13 to 14 feet, which will
increase  coal  throughput by  75 percent.73
This increase is due in part to higher tempera-
tures permitted by improved refractories and
more uniform heating designs. This  trend will
tend to increase NOX emission.
   Actual analysis of two  coke oven stack gas
samples gave values of 41 to 50 ppm NOX or
an average of  45  ppm.  In  another test, an
average of 28 ppm was obtained.6 9
   It is estimated roughly that in the carbon-
izatlbh  of 92  million tons of coal  in  the
United States during  1967, more than 6,000
tons of  NOX  was emitted.* This represents
0.23 percent of the estimated 1965  industrial
NOX emissions in the United States.68
* These calculations are based on a slot oven holding
20 tons of coal and provided with 30 separate heating
flue combustion chambers. A heating period  of 18
hours is assumed and a value of 1,100 Btu required to
carbonize 1 pound of coal. This calculates to 81,000
Btu/flue/hr. When gas is burned at this rate,  0.005
pound of NOX is produced per hour.59 For 30 flues
over  18  hours,  2.7  pounds of NOX is produced,
which is the amount produced in the carbonization of
20 tons of coal.  For 92 x 106 tons, the NOX produc-
tion equals 6,210 tons. Furnaces
  The cupola  furnace  is  similar in  many
respects to the blast furnace. Its function is to
melt scrap and pig iron for subsequent proces-
sing  in  steel  production.  Like  the  blast
furnace, its off gas contains usable quantities
of carbon monoxide which can be recovered
and burned to preheat the air blast.7 3
  The reducing conditions that tend to exist
in the cupola furnace could not be expected
to produce the quantity of NOX that the high
temperatures would  suggest.  Available data
indicate a concentration  of 13 ppm NOX in
the vent  gases  or  0.11  pound  per ton  of
metal produced.59 A questionnaire survey of
1,680  United  States  foundaries  by  the
National  Air  Pollution   Control
Administration and  the U.S. Department of
Commerce shows that  14,600,000  tons  of
iron  casting shipments  was produced  by
cupolas in 1966. On  this basis, iron cupolas
emitted 805  tons of NOX to the atmosphere
during the year.

  The basic oxygen furnace (EOF)
  The use of air for bottom-blowing in the
refining process for steel production has been
common  practice since  1870 but in recent
years, it has declined to minor importance.
The use of  commercially  pure oxygen for
top-blowing has increased rapidly  and this is
now   a  major  refining  method. Bottom-
blowing  was characteristic  of the Bessemer
process, which is no longer used, whereas top
blowing is characteristic  of the basic oxygen
furnace  (EOF), the  more  common  type
  The  EOF method produces  substantial
amounts of carbon monoxide; and if this is
permitted to  burn off, some NOX is produced
by  reaction  of  the  very  hot gas with  air
outside the furnace. One steel manufacturer
gives  a range of values of from 30 to 80 ppm
or 0.36 to 1.0 pound NOX per ton  of steel.
This can be avoided by use of a suitable hood
system to collect the gas for other uses.7 5 If
these other  uses  include use as  fuel, some
NOX may still be produced.


   Other furnaces using either regenerative or
recuperative preheat of the combustion air, in
addition to the open hearth, coke ovens and
blast stoves are used for such  lesser ap-
plications as  reheating  and  soaking pit
furnaces. Actual analysis of soaking pit stack
gases has given average NOX  values of 25.8
and 21.5 ppm.6 9 These values are in line with
coke oven  emissions (45.6 and 28.3 ppm) but
considerably lower than open hearth  values
(243 and  320 ppm). NOX emissions  from
soaking  pit  stack   gases  are  estimated at
5,500  tons per year  in the  United States,
based on 0.086 pound NOX per ton  of ingot
steel production,69 and U.S. ingot steel pro-
duction of 127 million tons per year.5 9
  Sintering operations are another NOX emis-
sion source,  differing somewhat  from  the
conventional furnace. In this instance, finely
divided iron-bearing material is mixed with a
fuel such  as coke breeze  to give  a product
suitable for blast furnace use. The mixture is
spread on a traveling grate and ignited by gas
burners.  The burning is  continued by air
drawn  down  through the grate. Analysis of
stack gases  from such sintering produces NOX
values of 47.2 to 36.3 ppm.69 NOX emissions
from sintering  stack  gases are estimated at
27,000 tons per year in the  United States,
based on  1.04 pounds NOX  per  ton of
sinter6 9 and sinter production of 51 million
tons per year in  the United States.5 9
7.4.3  Emissions
  Emissions  in  the  steel  industry  and its
related processing have been concerned with
fumes, smoke, and  dust or particulates. The
gases usually considered abnoxious  have been
SC>2, CO, and odors. The presence of oxides of
nitrogen  has been obscured  by the  heavy
emission of particulates and a resulting lack of
physical  evidence.  The  NOX  emissions
observed can be  traced largely  to the  com-
bustion of fuel oils and gas and, in part, to the
burning  of carbon  monoxide,  which  is a
product of the processing operations.

  Planned  equipment and processing changes
for abatement of air pollution must consider

NOX  emissions, e.g., higher temperatures in-
crease NOX production. On the other hand,
the major shift now in progress from the open
hearth to the  basic oxygen furnace in steel
production promises a  substantial reduction
in NOX emissions. The  NOX produced in the
EOF  process is believed  to result  from the
burning of the  carbon monoxide produced in
the operation  and  to some extent  from the
nitrogen content of the  commercially pure
oxygen used.  This may be  controlled  by
proper hooding and gas collection,75  which
may be a  minor source  of NOX when used as
  Regenerative type furnaces will be used in
one capacity  or another for some time, and
may be modified for at least partial control
of NOX from combustion processes, as out-
lined  in  Section 4.1.  In  brief,  these
modifications emphasize avoiding excess  air
which limits the  presence  of  one  of the
reactants in NOX formation, regardless of the
resultant increase in flame temperature.
  Removal of NOX from the products  of
combustion by scrubbing or adsorption have
not as yet reached a stage of development
which seems practical  for metallurgical ap-
  The use of  electricity for heat in  steel
production  is  growing  rapidly,  and this
transfers the  NOX  emissions to  the utility
plant where  easier control  is  expected.
Projections to  1980 indicate that raw steel
production will reach   180 million tons,  of
which 45 million (up from 13 million tons in
1967) will be produced in electric furnaces.76
  Electric induction heating will reduce NOX
emissions  at  the furnace  to the extent that
fuel  combustion is  displaced.  On  the other
hand, electric arc heating does produce NOX.
No  quantitative data appear to be available,
  The  projected  use  of the  plasma  arc
furnace77  may  produce  a  substantial amount
of NOX, and the related processes may require
some  form of inert blanketing if problems
with air pollution are to  be avoided.

7.4.4  Costs
  No  separate  data are available  for  this
industry  on the costs of NOX pollution con-

  The kiln-type furnace is  used in  the pro-
duction of cement clinker and quicklime and
in the firing of ceramic-type products. The
rotary kiln predominates the cement industry
and  is also extensively  used for limestone
calcining. However,  the latter industry uses,
to a  considerable extent, the  vertical shaft
kiln  both in  its  original  form,  which  is
gradually being retired, and the newer high-
capacity  design. For ceramic firing, the tunnel
kiln is used for the smaller, more fragile items
and the beehive kiln for larger items such as
drainage pipe.
  Cement clinker  is produced by  sintering
intimately ground  mixtures of limestone with
suitable clays, shales, or marls. Such mixtures,
either as  dry feed  or wet slurry, are gradually
heated  to  sintering temperature   (up  to
2,900°F)78 in rotary-type kilns. The product
is a  clinker which, when finely ground and
mixed with a small amount of gypsum to con-
trol set, is the portland cement of commerce.
  A rotary kiln is essentially a steel cylinder,
varying in diameter from  8 to 25 feet and 150
to  700  feet  long,  lined with  a refractory
material.  These kilns are  slightly inclined and
are  rotated at about 1 rpm. Raw feed entering
at the high end works its way gradually down
to the lower  end over a period of 2 to  4
hours. Preheated air, in excess, and fuel are
fired at the lower end, so that the combustion
gases  travel  countercurrent  to  the feed.
During the slow travel of the feed down to
the hot end  of the kiln, a series of reactions
takes  place  in a nonreducing atmosphere,
ending  in  a  partial  melting.  This  sintered
material is  the cement clinker referred to
above. The hot clinker is cooled by incoming
combustion  air  that  improves the  overall
thermal efficiency  and gives a higher  peak
   The capacity of individual kilns has  been
increased in recent years to more than 10,000
barrels per day (376 pounds per barrel), but
2,000  to 6,000 barrels per day is representa-
tive of the industry.
   In  the  United States in  1967,  some  188
plants  produced about 369 x 10^ barrels of
cement. Average  fuel consumption in  1967
was  1.25 x  106 Btu/barrel or 6.66 x  106
Btu/ton  of cement. The fuel consumed is
shown in Table 7-12.
   Limestone  rock  occurs  in two principal
variations.  If it is  almost entirely  calcium
carbonate  (CaCC^),  it  is known  as  a  high
calcium limestone. If a  considerable  portion
of the calcium carbonate content is displaced
with magnesium carbonate (from  30 to 45
percent), it is known as a dolomitic limestone.
Either of these limestones, when heated, loses
carbon dioxide (CO2) to form quicklime.
   At calcination temperatures of 1,700° to
2,450°F,  dissociation  of  the  limestone
proceeds gradually inward from the outer sur-
face of the stone particle. Temperatures in the
higher range  are required  for dissociation to
reach  the  center of  the  particle.  Since  a
gradual temperature increase  is preferred, the
rotary kiln is particularly suited to limestone
calcination.  Product heat is recovered as pre-
heat for  the combustion  air, as  in  cement
               Table 7-12. FUEL CONSUMED BY CEMENT INDUSTRY IN U.S., 1967
Coal, 1 O3 short tons
Oil, 103bbl(42gaVbbl)
Natural gas, 106ft3
Amount consumed
Equivalent Btu, 1012

   Excessive heat is avoided if a high quality
or active quicklime is desired. Longer heating
at  sintering  temperature  is  required  to
produce dead-burned dolomitic lime for re-
fractory furnace linings.
   Vertical kilns were first used for limestone
calcination, but these have been  largely  re-
placed with rotary kilns. However, a few new
installations  of vertical kilns of improved
design have been announced.79 The  capacity
limit,  which was  a  major disadvantage, has
been extended  up to 500 tons per day, which
is the equivalent of modern rotary kilns.
   Vertical kilns are  more thermally  efficient
than rotary kilns. Typical fuel consumptions
are 5 x 10^ Btu per ton7 9 for vertical kilns
and 7 x 10^ for rotary.
   The 1967 United States production of lime
was 17,974,000 short  tons in a total of 209
plants. Of this amount, 33 plants produced 63
percent of the total.80 Since fuel consump-
tion statistics are not available for the  man-
ufacture of lime, a value of 6 x 10^ Btu per
ton was used to estimate the thermal require-
ments  for limestone calcination in the United
States  in  1967  at 108 x 1012 Btu. No reliable
breakdown as to the use  of  coal,  oil, or gas
appears to be available, but the literature sug-
gests that a grandual switch to gas  is in prog-
   The ceramics industry embraces  a multi-
tude  of products which include  the more
general  headings of  refractories,  vitreous
enamels, whitewares, abrasives, structural clay
products,  and  ceramic  minerals. The larger
kilns used by the industry are of two types,
tunnel and beehive.
   The tunnel kiln  varies from 2 to  10 feet
wide and  4 to  8 feet high by 75 to  100 feet
long. These kilns  are gas- or oil-fired,  with
burners placed  near the center of the tunnel.
The material being fired  moves slowly into
the hot zone and is then cooled slowly in the
remaining length of travel. Combustion  air is
introduced at the exit end and is preheated as
it passes over the hotware leaving the kiln.
   Beehive kilns  are  shaped as  the name
indicates  and vary  in  size up to  60 feet in
diameter.  Burners  are  spaced  around  the
periphery;  combustion  gases are  diverted
toward  the dome, then  down through the
ceramic material stacked on the floor. Ducts
under the floor lead the gases to a common
stack, which usually serves four kilns.
7.5.1  Emissions
   Particulates  are  the  pollutants associated
with  cement,  lime, and  ceramic kiln  oper-
ations. Limited data are reported in the liter-
ature on NOX emissions  from  tunnel and
beehive ceramic kilns, but no  data have been
found  on NOX from cement or lime  kilns.
Since large volumes of fuel are consumed and
combustion air is  preheated, significant NOX
formation  would  be anticipated in  cement
and lime kiln operations.
   Estimated  NOX emissions from kiln  oper-
ations in the U.S. during 1967 appear in Table
Process unit
Cement kilns
Lime kilns
Ceramic kilns
21 ,000
Percent of total
U.S. NOX emissions
  Cement  and lime kiln  emission  estimates
are  based on the fuel consumptions discussed
in Section 7.5 and on the correlation between
NOX production  rate and heat input to the
combustion unit81 shown  in Figure 8-1.
  There  are  limited NOX emission data for
tunnel kilns used for firing ceramic items such
as pottery. The  results  of five tests varied
from  3 to 66 ppm with an average of  24
ppm.8 2 Gas firing was used in all instances.
  A large  beehive  kiln  which can  accom-
modate 1000 tons of ceramic ware requires a
heat input of up to 15 x 106 Btu per hour.82
Such a kiln, based on similar emission factors
to those used above, will produce 2.2 pounds
of NOX per hour if gas fired and 4.8 if oil
fired.  If four kilns are served by a single stack,
as is usually the case, the stack emission will

be 8.8 pounds of NOX per hour for gas and 19
for oil.
7.5.2  Control Techniques
  The literature shows no references to com-
mercial  installation of  equipment for  re-
moving  NOX from  kiln waste gas or of mod-
ifications  to kiln operations to reduce NOX
production.  Water scrubbing  is  sometimes
used  for particulate removal from  waste  gas
from lime  kilns. In this operation, the  gas
contacts a slurry of calcium hydroxide, which
should remove a 50/50 mixture of NO and
NG*2 and  reduce NOX up to 20 percent (see
Section 4.2.1). Flue gas recirculation, which is
used  to control temperature in  some lime
kilns,83  should  reduce   NOX  emissions  by
lowering flame temperature.
   Combustion  gas in  lime  kilns  contacts
calcium oxide and calcium carbonate as it
passes through the kiln, consequently, some
NOX probably reacts to form calcium  nitrite
and calcium nitrate.
   The nitrites and nitrates would subsequent-
ly decompose in the high-temperature zone of
the   kiln, releasing any  NOX  which  had
reacted.  Thus,  it  is  unlikely that contact
between the combustion gas and lime or lime-
stone  in  the  kiln  results  in  an  overall
reduction in NOX emissions.
   Several of the possible means  of reducing
NOX emissions in kiln  operation are the same
as in the high-temperature combustion dis-
cussed in  detail in Section 4.1. These include:
   1. Conversion to oil and preferably  to gas
     firing. However,  advantage should  be
     taken  of the  economic aspects  of an
     intermittent gas supply by retaining an
     alternate firing means.
   2. Keeping excess air to a minimum.
   3. Automatic control of firing variables.
   4. Modification   of  firing  design such  as
     burner location.
   5. Avoiding peak flame temperature  to  the
     extent the process permits.
Electric heating eliminates all the pollutants
associated with combustion  sources, but its
use in kiln operations is very limited. Another'
means of  emission control in kiln operation is
the choice of kiln type. Some NOX reduction
in limestone calcining is obtained by using a
vertical instead of a rotary kiln, but only be-
cause  less  fuel  is consumed.51  The higher
thermal efficiency, lower  capital and oper-
ating costs, and increased capacity of recently
designed vertical kilns may result in a long-
term trend away from rotary kilns.
   Scrubbing as a means of NOX reduction is
not  as yet feasible  for  combustion gases.
Section 4.2 describes  the  progress  that  has
been made in this area.

   The glass manufacturing industry is made
up of several basically different types of oper-
ation. They are:
   1. Glass container manufacture.
   2. Fiber glass manufacture.
   3. Flat glass manufacture.
   4. Specialty-glass manufacture.
   The raw materials used  in glass manufac-
ture consist primarily of silica sand, soda ash,
limestone, and cullet (crushed waste glass). In
the production of window and plate glass, for
example, temperatures in the range of 2750°
to 2850°F may be required to melt these raw
materials into a viscous liquid.
   The furnaces used are of the pot type if
only a few tons of a specialty glass are to be
produced, or of the continuous tank type for
larger quantities. By far the larger amount of
glass is melted in furnaces, and only these will
be considered in connection with NOX  con-
   Continuous  tank furnaces have  a holding
capacity of up to 1400 tons and a daily out-
put  of as much as 300 tons. Figure 7-14 is a
cut-away view  of a typical side-port, regen-
erative container glass furnace.84 It is also re-
presentative of some specialty glass furnaces.
The "batch" feed is introduced at the left, or
"melting end," which is maintained at as high
a temperature as the production requires. The
glass is heated by direct radiation from the
flames and  the  refractory melter crown. As
the  mass fuses, it passes  into the "fining
zone"  and  finally  through the submerged


                                                               GLASS SURFACE IN REFINER

                                                REFINER SIDE WALL
                                        •MELTER BOTTOM
              IN MELTER


                          COMBUSTION AIR
                                                                 RIDER ARCHES

                     Figure 7-14.  Regenerative side port glass-melting furnace.
throat of the bridgewall into the "working
  The  combustion  gases,  on  leaving  the
melting zone, retain a considerable amount of
heat.  This is reclaimed in a regenerator or
brick  checker chamber. When the firing cycle
is reversed,  combustion air is preheated by
being passed through the brick work. Preheat-
ing  saves fuel but increases the flame tempera-
  A variation on side-port firing is the end-
port fired furnace, in which a single burner
replaces  multiple burners at  one end of the
tank.  It is possible that the different type of
                                      firing  could influence NOX emission. The
                                      combustion gases follow a U-shaped path and
                                      enter a checker chamber for heat reclamation.
                                      Reversal of the cycle is then similar to that in
                                      side-port firing.  Numerous variations on con-
                                      struction and firing include direct firing, and
                                      regenerative, with end point firing.

                                         Coal is  not  used in glass melting. Since
                                      molten glass is conductive, heating is used as a
                                      booster to supplement fuel firing whenever
                                      technically and economically practical. Gas
                                      and, to a lesser  extent, fuel oil are the prefer-
                                      red fuels.

7.6.1  Emissions
  The flue gas from glass-melting tanks is the
major  source  of NOX emissions in the glass
industry.  The  operation  of these tanks is
similar to that of open-hearth furnaces used in
steelmaking;  regenerative  checkerwork sets
absorb heat from the combustion gases for
subsequent  release  to the incoming  com-
bustion air. This  is  accomplished by  a re-
versing valve which puts each checkerwork set
through its heating and cooling cycle in turn.
The  sequence of intense  high-temperature
combustion and  quenching in the checker-
work sometimes raises NOX emission to levels
perhaps fivefold those experienced in a steam
boiler of equivalent heat release. This observa-
tion  is based on measured emissions of 0.87
pound NOx-per  million Btu fired for glass-
melting "tanks  firing  an average of 38  million
Btu  of gas per hour60 compared to  0.13
pound NOX per million Btu of heat input for
gas firing in normal combustion equipment of
this size (see Figure 8-1).
   Normal  combustion supplies  the  lesser
amounts of heat used in glass annealing and
tempering, so that these  processes are only
minor sources of NOX.
  Annual  NOX  emissions  from the  United
States glass industry are roughly estimated at
110,000 tons for 1967, or about 1.3 percent
of total NOX pollutants.  This  is based on
reported fuel usage in 1962,86 and reported
glass production  for 1962  and  1967.87  A
300-ton-per-day  glass  melter,  currently the
largest size, will consume  about  92 million
Btu  per hour  of fuel and  discharge an
estimated 80 pounds per hour of NOX, based
on an emission  factor of  0.87 pound per
million Btu of fuel fired. This factor is taken
from results of tests  run in Los Angeles on
glass-meter stack gas  containing NOX  con-
centrations ranging from 435 to 1,320 ppm,
with an average of 570 ppm.60
  The type of furnace and firing shown  in
Figure 7-14 is reminiscent of that used  in
producing steel  by the open-hearth furnace.
Actual chemical analyses  of  the  stack gas
from  an open-hearth furnace  fired with 70
percent gas and 30 percent  oil varied from
500 to 800 ppm of NOX, with an average of
700 ppm. Similar results were found for four
glass furnaces. The values ranged from 435 to
1,320  ppm, with an average  of 570 ppm.60
Other  analyses of exhaust gases from a large,
regenerative, gas-fired  furnace showed 137
ppm of NOX when amber glass was melted
and 725 ppm when flint glass was melted.84
7.6.2  Control Techniques
  Controls for NOX emissions have not been
developed for the glass industry. Combustion
modifications  for  reducing. flame tempera-
tures do not appear applicable,  since high
flame temperatures are required to heat the
charge  to the 2,600° to 2,850°F temperatures
required in  glass  melting. Operation  with
excess  air might lower NOX levels while main-
taining or increasing temperature; it would re-
quire improved controls. Automatic controls
and changes in  burner positioning are
potential methods of reducing  NOX.
  Control measures which may be specific to
glass melting are:
   1. Minimize hot spots within the furnace. It
     is known  that  minute  changes  in
     residence time in extremely high tem-
     perature  zones can considerably affect
     NOX emission. Table 3-1 illustrates the
  2. Use an end-fired tank which has a single
     large burner and a long path travel. A
     large,  luminous  flame   is  known   to
     produce  less  NOX than an  intense
     restricted  flame;  a tank with such a
     burner might produce less NOX.
   3. Use electric heating as a booster source
     when  high temperature  or increased
     production is  desired. It is favored for
     colored glasses and tends to extend  re-
     fractory life.  An  all-electric melter  re-
     quires  less  capital investment than con-
     ventional melters.8 8 Of importance from
     the pollution standpoint, electric heating
     reduces NOX emission to the extent, at
     least, that  combustible  fuels have been
     displaced.  However,  the economics
     involved are  not  clearly  in  favor  of

     electric heating  except  as  a  "booster,"
     since for most locations power is a more
     costly  source of energy than fuel, even
     though power is 75 percent efficient and
     fuel only 15 to 20 percent.89  The use of
     power  transfers  NOX emissions to  the
     power  plant.
   High temperature flames, particularly those
 enriched with oxygen, are the source of high
 levels of NOX emissions. Consequently, small
 room-scale  or laboratory operations90  which
 use  high  temperature,  oxygen  fortified
 flames—such as  bench-type glass-blowing and
 quartz-working  and  flame  photometry—are
 potentially sources of high levels of NOX.
 7.7.1   Process Description  R efractory Fibers
   The  production of fibers  such  as mineral
 and glass wool requires the melting  of re-
 fractory  material  (300°F).  The molten
 material is reduced to a stream of globules by
 passage  through  a strong blast of highly  super-
 heated  steam  or air.  The   flying molten
 globules develop and trail long fibrous tails.91
 Although much  more is involved,  this  is the
 way rock wool and similar mineral fibers  are
 produced. Once assembled into batts, etc., the
 material is  widely  used   for  insulation
 purposes. Eight to  10 million pounds  of
 alumina-silica  fibers  are   produced   each
   No specific data are  available on the NOX
 emitted during fiber production.  Perh te Expanding Furnaces
   Perlite  is  a glassy  volcanic  rock  which
 occurs as spherical particles.  When perlite is
 heated rapidly, the water which the spherules
 contain  causes them  to puff up or expand
 into a white cellular low density material.93
 In  1967, some  357,560 tons of expanded
 perlite were  produced,94 of which roughly 90
 percent  was used  as  an aggregate in plaster
 and concrete. Essentially all furnaces used to
 expand  perlite are heated with natural gas.
 The temperature required is  in  the range of
1,450°  to  1,800°F. A  value  of 19 ppm of
NOX has been  found in the exhaust gases of
one furnace.9 s
7. 7.1.3  Baking and Drying Ovens
  Ovens  for  baking and drying  are  used
diversely as bakery ovens, sand core ovens for
foundries;  paint baking ovens, plastic curing
ovens, and rock wool drying ovens.96  These
may be direct-fired, in  which case the  com-
bustion  products  are  in  contact  with the
material being heated; or indirect-fired, in
which case air  is heated and then circulated
through the oven.
  Baking and drying ovens vary  considerably
in  size, design, and construction.  Although
some are oil-fired, gas-firing is usually prefer-
red.  The  combustion  air  need  not be  pre-
heated since operating temperatures are usual-
ly in the range of 180° to 650°F. Combustion
gases  are discharged   directly to  the
atmosphere unless they contain fumes that re-
quire  the  use of an afterburner. Although a
relatively large  number of such ovens may be
in use, their average hourly heat requirement
of perhaps 1.2 million Btu is low. NOX stack
emission has been reported to  be  10  to 40
ppm.9 s  Spray Dryers
  In spray drying, a highly dispersed liquid is
brought in contact with a high  temperature
gas. The hot gases are usually obtained from a
direct-fired gas or oil heater. If  a controlled
atmosphere is necessary, indirect  heating is re-
quired.  A cyclone separator or a baghouse is
frequently  used to separate the dried product
from the hot gases.97
  The detergent industry  is a  large user of
spray dryers. The heat requirement of such
dryers is about 15 million Btu per hour, and
the NOX  content of the  stack  gas has been
reported to be about 7 ppm.95
7. 7.1.5  Welding
  The high temperatures  involved in electric
arc  and oxyacetylene welding (3,500°F) are
expected to favor NOX production. Measure-
ments  made under  conditions  of  restricted
ventilation have shown that amounts of NOY

vary from a trace to nearly 35 ppm, depend-
ing on the particular operation and the con-
ditions  involved.  These samples were  taken
from the visible fume stream at a distance of
6 inches from the arc.98 It is estimated that
welding   operations  in the  United   States
contribute roughly 70  tons of NOX per year
to the atmospheric burden. This estimate is
based on three assumptions: that the mean
gas flow  in all welding is 10 cubic feet per
minute, that the concentration  of NOX is 10
ppm, and that data from Los Angeles are
representative of the United States at large.
  Welding operations  in  which no  visible
emissions exist should  not be assumed to be
free of NOX. Electrostatic Precipitators
  Electrostatic  precipitators  are  used
extensively to remove very small particulate
material from  gases. The equipment essential-
ly consists of a series of electrodes which, be-
cause  of their  high  voltage   (20,000   to
100,000  volts in  single-stage units) create a
corona discharge  that ionizes gas molecules
and  electrically charges the particles passing
through the field. The charged particles then
tend to  migrate toward oppositely  charged
surfaces and are removed from the gas stream.
It is suggested  that the corona discharge  forms
ozone  and atomic oxygen, which can react
with nitrogen to form nitrogen oxides.
  Certain data  from  precipitators  tied  to
coal-fired  steam  boilers indicate that such
formation of NOX does occur; other equally
acceptable data show a decrease in  NOX on
passage through an electrostatic precipitator.
This  marked disagreement  in results has not
been explained, and additional information of
interest  may  be developed  by  further
  Until more reliable information is available,
one  cannot consider electrostatic precipita-
tion a contributor to NOX.  High-level Exposure in Agriculture
  A  noncombustion   source   of  nitrogen
dioxide is the gas formed in silos in the first
few  days after filling." Anaerobic  bacterial
action occurring in the green fodder produces
nitrogen dioxide  in quantities  sufficient to
impart  a readily  visible color.  Such a con-
centration  is expected to exceed 40 ppm, if
an  assumed  path of  5  feet is used in the
formula for minimum visibility: 2,400 divided
by the path length in inches  = ppm NC>2.100
Thus, silage gas constitutes a high level point
source of the highly toxic oxide of nitrogen

7.7.2  Control Techniques
  The  first  of the  combustion sources
described above (Section involves the
use of very high temperatures for the produc-
tion of refractory fibers.  Both cupola- and re-
verberatory-type furnaces are used. These fur-
naces are extensively used in the manufacture
of steel, and control of  these NOX emission
resulting from the combustion of fuel has
been described in Section 7.4.2.  Probably the
most practical control is to use as little excess
air  as possible. In addition to partially con-
trolling  the nitrogen oxides, the low-excess-air
technique will provide the highest flame tem-
perature. Also to be avoided are highly tur-
bulent flames or at least local areas of high
turbulence  which produce  extremely  high
local temperature. (See Section 5.2 for a more
detailed description.) In general the long non-
turbulent flame is to be preferred to minimize
NOX emission.
  The next three heating operations (perlite
expanding furnaces, baking and drying ovens,
and  spray dryers) require only moderate tem-
peratures. Under these circumstances, control
measures can be adopted which  would other-
wise be impossible if very high  temperatures
were required.  Such  control  of  the  com-
bustion process has been  described in detail in
Section 4.1. Control of excess air to the com-
bustion zone is probably the most practical
approach for partial NOX reduction. Other ap-
proaches are the use of two-stage combustion
and  flue gas recirculation and the avoidance
of flame turbulence. The present technology
does not provide for complete removal of all
NOX on a practical basis. The status of such
endeavors is the subject of Section 4. In most


cases, participate and fume control have been
the dominant air pollution problems. All con-
trol  and  process  variables  are  interrelated,
however, and in new  construction, particular-
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90. Takeno, W. The Formation of Nitrous Vapors in
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    blowers'  Torches.  Chem.  Weekblad.
    62(19):239-240,  1966.
91. Mineral Wool  Furnaces. In: Air Pollution  En-
    gineering Manual, Danielson, J. A. (ed.). National
    Center for  Air  Pollution Control.  Cincinnati,
    Ohio. PHS  Publication  Number  999-AP-40.
    1967. p. 342.
92. Gould, T.  R. Refractory Fibers. In: Kirk-Othmer
    Encyclopedia of Chemical Technology, Standen,
    A. (ed.), Vol. 17, 2d ed. New York, Interscience
    Publishers, Inc., 1968. p. 285-295.
93. Perlite-Expanding Furnaces. In: Air Pollution En-
    gineering Manual, Danielson, J. A. (ed.). National

    Center for Air  Pollution Control.  Cincinnati,
    Ohio. PHS  Publication  Number  999-AP-40.
94.  Minerals  Yearbook  1967, Vol. HI. Bureau  of
    Mines. Washington, D. C. 1968. p. 834.
95.  Mills, J. L. et al. Emissions of Oxides of Nitrogen
    from Stationary Sources in Los Angeles County;
    Oxides of Nitrogen emitted by  Small  Sources.
    Joint  District,  Federal,  State,  and   Industry
    Project. Los Angeles Country  Air Pollution Con-
    trol District. Los Angeles,  Calif. Report Number
    2. September 1960. 73 p.
96.  Core  Ovens.  In:  Air  Pollution  Engineering
    Manual, Danielson, J.  A. (ed.). National Center
    for Air Pollution Control.  Cincinnati, Ohio. PHS
    Publication Number 999-AP-40. 1967. p. 309.
 97.  Driers.  In:  Air  Pollution Engineering  Manual,
     Danielson, J. A.  (ed.). National Center for Air
     Pollution  Control.  Cincinnati, Ohio.  PHS
     Publication Number 999-AP-40.  1967. p. 367.
 98.  Fay, H., P. H. Mohr, and P. W. McDaniel. Nitro-
     gen  Dioxide  and  Ozone  Concentrations  in
     Welding Operations. Amer. Ind.  Hyg.  Assoc.
     Quart. 18(1): 19-28, March 1957.
 99.  Lowry,  T.  and  L. M.  Schuman.  Silo-Filler's
     Disease;  A  Syndrome   Caused by  Nitrogen
     Dioxide. J.  Amer. Med. Assoc.  162(3): 153-160,
     September 15, 1956.
100.  Hardison, L. C. Techniques for Control of Oxides
     of Nitrogen. Presented at 62d Annual Meeting of
     the Air  Pollution  Control  Association.  New
     York. June 22-26, 1969.

  For  an  accurate  air pollution  survey,
whether  for a single source or for a metro-
politan  area,  pollutant  emissions must  be
identified by type and quantity.  This  deter-
mination—together  with meteorological,  air
quality  and effects sampling programs, and
strong   enforcement   actions-fulfills  the
requirements for local, state, and Federal  air
pollution control activities.
  An adequate  emission investigation will
provide evidence of source emissions and will
define  the location, magnitude,  frequency,
duration, and  relative contribution of these
emissions. This emission survey of pollutants
will  include the determination of emission
rates from fuel combustion at stationary and
mobile sources, from solid waste disposal, and
from industrial process losses.
  Ideally, the determination of emission rates
should include a stack analysis of all sources
of interest, but this is impractical  when an air
pollution survey must cover a large land area
that could  contain  many  thousands  of
sources.  Emissions must be  estimated  for
sources  that  do not have accurate stack gas
analyses. Estimates are arrived  at by the use
of emission factors, which  are estimates of
pollutant emission rates based on past stack
sampling data, material  balances, and engi-
neering  appraisals of other sources similar to
those in question.
  The  accuracy  of emission  estimates  de-
pends upon the use of an accurate  fuel quality
figure as well as an accurate emission factor.
It is important to recognize, however, that the
development of accurate NOX emission  fac-
tors  is very difficult because of the complex
nature  of nitrogen  oxide formation  from
combustion processes.
  There is no convenient way of anticipating
the approximate amount of NOX pollutants
formed  from a given  amount of fuel, as there
is for the oxides of sulfur. The  formation of
SOX is directly related to the concentration of
sulfur in  the fuel. On  the other hand, NOX
can be  formed in  substantial concentrations
of  several hundred  ppm  or  more with no
chemically bound nitrogen in the fuel; e.g., in
natural gas combustion (see Section 4.1).
  The amount of NOX emitted depends upon
equipment design,  a  complex set of combus-
tion conditions, and  operating variables such
as installation  size, type of burner,  cooling
surface  area,  firing  rate, and air/fuel ratio.
Consequently, the  determination of accurate
emission  factors  that  represent average or
typical  emissions  from each class of equip-
ment involves a complicated sampling  pro-
cedure,  which requires the selection of repre-
sentative equipment  and  testing  under
representative  operating  conditions.  The
quotation below, taken from Report No. 3 of
the Joint Los Angeles County  report1  con-
cerns NOX emissions from  the single  most
important emission  source, steam  power
plants,  and succinctly illustrates these  prob-
   "In the determination  of an NOX emission
factor for power  plants, some  130 tests in-
cluding  554  individual  samples  were  con-
sidered. The Department of Water and Power
of the City of Los Angeles has been making
an  extensive study of the effect of operating
variables  on  NOX emissions.  These studies
have included a testing program in which two

 to three tests per week were performed for a
 period greater than a year.
   "The rates of emission of NOX from units
 as complex as these, with the possibility of a
 number of constantly fluctuating operating
 variables, may be assumed to be constantly
 fluctuating  also.  In these circumstances, the
 rate of emission of NOX at any given instant
 in any plant may be different from the rate of
 emission  at the  next  instant.  Experience
 gained during the carrying out of the project
 has  shown  these assumptions to be true. A
 striking example of this variability is the fact
 that samples taken as nearly at the same time
 as possible  from two  probes in  close prox-
 imity to each other as possible show two dif-
 ferent values for NOX concentrations. It was
 found that actual rates of NOX emission from
 sister units  may  be different for operating
 conditions which are  the same for each unit
 within the limits of ability to determine. This
 phenomenon has been verified repeatedly.
   "Thus, it may be seen that in the develop-
 ment of an  emission factor, calculations must
 be based on  averages of  many  data  taken
 under many different conditions.  As pointed
 out in the discussion of asphalt paving plants,
 the use  of such a factor to determine the rate
 of NOX  emission from a single  unit at any
 given time  may produce data which are far
 from reliable for the  conditions  existing  at
 that time.
   "During the carrying out of the test pro-
 gram on  power plant boilers, a  number of
 phenomena  were brought to light.  Instead of
 clarifying the situation, many of these obser-
 vations  served merely  to point up the com-
 plexity of the problems. It should be borne in
 mind that the examples and curves shown are
in each  case for some particular unit and
 should not be construed to be correction fac-
 tors  for  measured emission rates  from any
 other unit.  The degree and direction of the
effect of operating variables upon NOX pro-
duction  must be determined  individually for
each particular unit to be considered."
   The study cited was a joint Federal, state,
county,  and industry project in which Mills et
al. measured NOX emissions from a wide vari-
 ety of gas- and  oil-fired combustion equip-
 ment, ranging from domestic heaters to large
 steam boilers, and were able to establish the
 general relationship between gross heat input
 and NOX  (as NO2>.  The experimental  data
 they obtained are in many cases the best or
 the only actual measurements reported in the
 literature. Their data are reproduced in Figure
 8-1.! Note that in Figure 8-1, the average heat

§ 1,000
Z -c

00 1
H- O
LU ^

known. NOX emission factors from these and
other  sources  have  been  compiled by
   Emission factors have  been used, for the
most part, to represent average emission rates
for a particular type of fuel for a particular
consumption  sector. If  no  adjustments are
made in these factors for specific changes in
type  and  size of equipment, operating rates,
and control equipment over a certain period,
the factors may not represent current emis-
   In  some instances, emission factors used in
this document do  not  agree  with those pub-
lished by Duprey.2  Such  differences  have
been  noted and explained in the description
of  emission  factors  for catalytic  cracking
regenerators and  CO  boilers in Section 7.3;
the emission factors for open burning appear
questionable,  as discussed in Section 6.2.  In
other instances, emission factors were derived
from published or unpublished data if no
published emission factors were  found. Thus,
emission factors were  derived for cement and
lime kilns in  Section 7.5, and  for  iron ore
sintering, coke ovens,  and steel soaking pits in
Section 7.4.
  Table  8-1 is a compilation of average emis-
sion factors for NOX from various types of
sources. These emission rates represent uncon-
trolled sources unless otherwise noted.  For
an  operation in which control equipment is
                     Table 8-1. EMISSION FACTORS FOR NITROGEN OXIDES
      Average emission factor
                       Household and commercial
                     Fuel oil
                       Household and commercial
                     Natural gas
                       Household and commercial
                  Combustion sources
                     Gas Engines
                       Oil and  gas production
                       Gas plant
                     Gas Turbines
                       Gas plant
                     Waste Disposal
                       Open burning
                       Conical incinerator
                       Municipal incinerator
                       On-site incinerator
                8 Ib/ton
               20 Ib/ton
               20 Ib/ton

            12-721b/103 gal
               721b/103 gal
              1041b/103 gal

              1161b/106 ft3
              2141b/106 ft3
              3901b/106 ft3
               11 Ib/ton
              7701b/106 ft3
            4,300 lb/106 ft3
            7,3001b/106 ft3
            4,400 lb/106 ft3

              200 lb/106 ft3
              200 lb/106 ft3
              200 lb/106 ft3

               11 Ib/ton
             0.65 Ib/ton
                2 Ib/ton
              2.5 Ib/ton

                 Table 8-1 continued. EMISSION FACTORS FOR NITROGEN OXIDES
        Average emission factor
               Other combustion
                  Coal refuse banks
                  Forest burning
                  Agricultural burning
                  Structural fires
               Chemical industries
                  Nitric acid manufacture
                  Adipic acid
                  Terephthalic acid
                    large operations
                    small batches
           8 Ib/ton
          11 Ib/ton
           2 Ib/ton
          11 Ib/ton

          57 Ib/ton HNO3 product
          12 Ib/ton product
          13 Ib/ton product

      0.2-14 Ib/ton HNO3 used
       2-260 Ib/ton HNO3 used
utilized, the emission rate given for an uncon-
trolled source must be multiplied by: 1 minus
the percent efficiency of the equipment, ex-
pressed in  hundredths. Emission factors are
from a Public Health  Service Publication,2  a
study contract,3  and an emissions survey.4
   Examples of  how to use  emission factors
are given below.
   1 . Fuel oil combustion.
     Given:  Power  plant  burns 50,000,000
     gal of fuel oil per year.
   50,000,000 x      = 5,200,000
   2. Solid waste disposal.
     Given:  Conical incinerators burns 7,000
     tons of waste per year.


   3. Process industries.
     Given:  A certain large continuous nitra-
     tion employs 40 tons per day of strong
     HNO3- If the control device is 90% effi-
     cient,  what is the  daily evolution  of
     NOX? (EF = emission factor.)

     If EF = 12 Ib/ton HNC>3 used, then
40 x 12( 1.0 - 0.9) - 4.8 Ib NOx/day
1. Mills, J.L. et al. Emissions of Oxides of Nitrogen
  from  Stationary Sources in Los Angeles County;
  Oxides of Nitrogen Emitted by Medium and Large
  Sources. Joint District, Federal,  State, and Indus-
  try Project.  Los Angeles  County Air Pollution
  Control  District.  Los Angeles,  Calif.  Report
  Number 3. April 1961.51 p.
2. Duprey, R.L. Compilation  of Air Pollutant Emis-
  sion  Factors.  National Center for Air Pollution
  Control. Durham, N. C. PHS Publication Number
  999-AP-42. 1968. 67 p.
3. Bartok, W. et al. Systems Study for Nitrogen Oxide
  Control  Methods from Stationary Sources. NAPCA
  Contract PH-22-68-55. Report Number GR-l-NOS-
  69. May 1, 1969.
4. NAPCA Reference Book of Nationwide Emissions.
  National Air  Pollution Control  Administration.
  Raleigh, N. C. June 1969.

                       9.   POSSIBLE NEW TECHNOLOGY
   Besides  the demonstrated  and  promising
 techniques  for  controlling  nitrogen oxide
 emissions from combustion discussed in Sec-
 tions 4 and 5, several methods appear to have
 potential, but have not yet been proved.1

 9.1.1   Oxygen Enrichment
   Oxygen enrichment is becoming increasingly
 more practical as pure oxygen becomes less
 expensive  because of its large-scale. use in
 industry, particularly in the steel industry. In
 the glass industry (see Section 7.6.2), oxygen
 addition is sometimes cheaper than electrical
 heating to boost  furnace capacity. As a meth-
 od of NOX control, the use of pure oxygen in
 the flame gives a higher flame temperature and
 therefore tends to produce more NOX, not less;
 this effect prevails until the oxygen supply is so
 pure that there is not enough N2 left in the
 flame to  produce NOX. Commercial oxygen
 that is  70 percent ©2 and 30 percent N2 is
 probably not pure enough to accomplish this
 result. If furnace operation with enriched oxy-
 gen is to become practical, it will be necessary
 to redesign the heat recovery system to make
 up for the lack of unburned nitrogen gas as a
 heat carrier. This will require substantial de-
 velopmental research,  and the net  effect on
 NOX emissions is  not now predictable.
 9.1.2   Fluid Bed Combustion
   Fluid bed  combustion has  been amply
 demonstrated  as  an  effective  means to even
 out high spot temperatures in the combustion
 zone  of petroleum  catalytic crackers. The
 application  of this  technique to  the com-
bustion  of pulverized  coal was  expected to
give less NOX, but preliminary results failed to
show  the  expected decrease.  The  large
amounts of chemically bound nitrogen in coal
are a possible contributing factor (see Section
   If this problem can be overcome, develop-
ment  work on a new boiler  design can pro-
ceed,  taking into account  the  fact that the
burning section  must be much larger than at
present to allow a maximum fluidizing gas
velocity  of  5  to 6  feet  per second versus
present furnace  gas velocities of 60 feet per
second.  The  boiler  section  can  be  much
smaller than at present because of the greatly
improved rates of heat transfer in a fluid bed.
9.1.3   Centralization of Energy Source
  Electric  boilers  and central  power  are
becoming  increasingly  more  practical  for
domestic and commercial  space heating  as
urban  concentration  increases. The use  of
direct electric heating and the potential use of
waste heat from utility power  plants have
already been  discussed. A logical extension of
that discussion would  be the consideration of
(1) the applicability  of the use of electric boil-
ers to produce steam for heating, (2) the dis-
tribution of a superheated water supply from
a central power plant,  or  (3) the  use of a
circulating  heat  carrier, such as Dowtherm,
that could supply individual flash boilers in a
large apartment of industrial complex. These
alternatives are all subject to the same param-
eters of power costs and the same parameters
involved in the substitution of a large utility
plant, where  problems of air and water pollu-
tion would be centralized rather than spread
out over many smaller units.

   Selective reduction of  NOX by  ammonia
over supported platinum catalyst has already
been  discussed in Section 4.3 in connection
with the content of nitric acid stack-gas emis-
sions.  Such reduction has been used in three
nitric acid plants with very limited success.
   The use of noble metal catalyst restricts the
application of this technique to gases contain-
ing 1 ppm or less sulfur oxides.
   Only the flue gas from gas-fired equipment
could  possibly  meet this  requirement, since
no gas  desulfurization  process can be ex-
pected  to  provide  such  complete  SC>2 re-
   The use  of metal catalysts other than noble
metals for  the selective ammonia reduction of
NO is  discussed in several patents.2'4  Refer-
ences 3 and 4 indicate that these catalysts are
not  sensitive   to  sulfur poisoning.  If  these
claims are  correct, NH3 reduction could be
used in  coal- and oil-  as well as  gas-fired
plants.  Considerable  research would  be
needed,  however, before  such a speculative
system could be developed.
9.2.1   Improved Aqueous Scrubbing
   In Section  4.2, it  was indicated that lime-
water  scrubbing removes  about  20 percent
nitrogen oxides. Such removal is believed to
be  in  the  form of ^03 produced by the
interaction of NO and  NO2 in equal molar
quantities;  this removal  could  perhaps be
improved by increasing  the NO2/NO ratio
from its present flue-gas value of about 0.1 to
a level of 1.0. Techniques that have been sug-
gested1   for achieving  the  more  optimum
NO2/NO ratios include  catalytic oxidation,
homogeneous oxidation with ozone, and addi-
tion of NO2 to the gases. In the case of NO2
addition, part of the NO2 might be obtained
from the nitrites and nitrates formed in  the
scrubber  by  thermally  decomposing  these
salts  and oxidizing  the  concentrated NOX

1. Bartok, W.  et al. Systems Study of Nitrogen Oxide
  Control Methods for Stationary Sources. NAPCA
  Contract PH-22-68-55, Interim Status Report. May
  1, 1969.
2. Anderson, H.C. and C.D. Keith. Method of Purify-
  ing Gases Containing Oxygen and Oxides of Nitro-
  gen (Englehard Industries,  Inc., U.S. Patent No. 3,
  008, 796).  Official  Gazette U.S. Patent Office.
  772(2): 507, November 14, 1961.
3. Nonnenmacher,  H. and K. Kartte.  Selective  Re-
  moval of Oxides of Nitrogen from Gas  Mixtures
  Containing Oxygen (Badische  Anilin-&  Soda-
  Fabrik, U.S. Patent No.  3, 279, 884). Official
  Gazette U.S. Patent Office. 831(3): 1190, October
  18, 1966.
4. Schmidt, K.-H. and V. Schulze. Removal of Nitro-
  gen Oxides from Gases by Catalytic Reduction to
  Nitrogen [Zerfahren zur Entfernung von Stick-
  oxyden auf Gasen durch Katalytische Reduction
  der Selben  zu  Stickstoff]  (Hamburger  Gaswerke
  G.m.b.H., German Patent No. 1, 259, 298). Patent-
  blatt. 88(4): 45 I.January 1968.

                                   SUBJECT INDEX
Agriculture, high level exposure in, 7-43
Air combustion, low excess, 4-5—4-6
Ammonium nitrate manufacture, 7-12
 Baking and drying ovens, 742
 Basic oxygen furnace, 7-35
 Blast furnace, 7-32
   [See:   Coal-fired boilers;  Electric power
   plant boilers; Packaged boilers]
 Burner configuration, 4-8
 Burner Location and spacing, 4-8—4-9

Definitions, 2-1
Design modifications, 4-8
Diesel engines, 5-10
Domestic heating, 6-1—6-3


Electric power plant boilers, 5-5
Electrostatic precipitators, 743
Emission factor accuracy, 8-1
Emissions, factors affecting, 4-3, 7-1
Emissions in United States, 3-1 -3-2
Engines (stationary), 5-5—5-11
   [See:  Diesel engines; Gas engines; Piston
   engines; Turbine engines]
Explosives manufacture and use, 7-21
Coal-fired boilers, limitations, 5-4
Coal refuse fires, 6-6
Coking ovens, 7-34
Combustion equipment (small), 4-11
Combustion modification, 4-1—4-3, 9-1
Combustion  processes  (large  fossil  fuel),
Commercial heating, 6-1—6-3
Control processes, potential, 4-13
   [See: Electric power plant  boilers' Engines
   (stationary);  Incineration  and other burn-
   ing;  Industrial  and  chemical  processes;
   Nitric acid  manufacture;  Organic oxida-
   tions;  Organic nitrations;  Petroleum  and
   natural gas industries; Metallurgical proc-
Cupola furnace,  7-35
Fertilizer, 7-24
Flue gas cleaning, 4-11,9-1
Flue gas recirculation*4-6—4-7
Fluidized bed combustion, 4-9, 9-1
Forest wildfires, 6-5
Formation of nitrogen oxides, 3-1—3-5
Fuel type and composition, 4-3
   [See: Basic oxygen furnaces; Blast furnace;
   Cupola  furnace;  Glass  melting  furnace;
   Open  hearth  furnace; Perlite  expanding
Gas engines, 5-11
Glass manufacture, 7-39
Glass melting furnace, 7-39

Heat release and transfer rates, 4-4
Incineration and other burning, 6-3
Industrial and chemical processes, 7-1
Kilns, 7-37
Lead chamber process, modified, 4-13—4-14
Limestone wet-scrubbing process, 4-11


Metallurgical processes, 7-32
Metals pickling, 7-25


Nitric acid manufacture, 7-2
Nitric acid uses, 7-11-7-25


Open hearth furnace,  7-34
Operating conditions, modification, 4-5
Organic nitrations, 7-14-
   [See:  Baking  and drying ovens; Coking
Packaged boilers, 5-4—5-5
Perlite expanding furnaces, 7-42
Petroleum and natural gas industries, 7-25
Piston engines, 5-5-5-9
Possible new technology, 9-1
Refractory fibers, 7-42
   [See:  Limestone  wet-scrubbing  process;
   Lead chamber process, modified]
Sintering operations, 7-36
Sources of nitrogen oxides, 3-1—3-5
Spray drying, 7-42
Stack  gases,  composition  and solubility of,
Steam and water injection, 4-7
Steel production, 7-34
Structural fires, 6-6
Turbine engines, 5-9
Vegetation burning, controlled, 6-5
Waste disposal, 6-4
Welding, 7-42
West-scrubbing process
   [See: Limestone wet-scrubbing process]