CONTROL TECHNIQUES
FOR
NITROGEN OXIDE EMISSIONS
1 FROM STATIONARY SOURCES
•Jo
U. S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Environmental Health Service
-------
CONTROL TECHNIQUES FOR NITROGEN OXIDES
FROM STATIONARY SOURCES
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Environmental Health Service
National Air Pollution Control Administration
Washington, D.C.
March 1970
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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
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PREFACE
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-
sponsibilities.
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-
mogeneous.
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-
lutants.
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
matter.
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
111
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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.
IV
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NATIONAL AIR POLLUTION CONTROL TECHNIQUES ADVISORY
COMMITTEE
Chairman
Mr. Robert L. Harris, Jr., Director
Bureau of Abatement and Control
National Air Pollution Control Administration
MEMBER
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
CONSULTANT
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
Conservation
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
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FEDERAL AGENCY LIAISON REPRESENTATIVES
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
Development
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
Research
Office of Noise Abatement
Department of the Treasury
Mr. Gerard M. Brannon
Director
Office of Tax Analysis
Atomic Energy Commission
Dr. Martin B. Biles
Director
Division of Operational Safety
Federal Power Commission
Mr. F. Stewart Brown
Chief
Bureau of Power
General Services Administration
Mr. Thomas E. Crocker
Director
Repair and Improvement Division
Public Buildings Service
National Aeronautics and Space
Administration
Major General R. H. Curtin, USAF
(Ret.)
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
Engineering
Office of Construction
VI
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CONTRIBUTORS
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.
Vll
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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
Systems
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
vm
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TABLE OF CONTENTS
Section Page
LIST OF TABLES xi
LIST OF FIGURES xii
SUMMARY xiv
1. INTRODUCTION 1-1
2. DEFINITIONS 2-1
2.1 REFERENCES FOR SECTION 2 2-2
3. SOURCES OF NITROGEN OXIDES 3-1
3.1 REFERENCES FOR SECTION 3 3-5
4. COMBUSTION CONTROL TECHNIQUES 4-1
4.1 COMBUSTION MODIFICATIONS 4-1
4.1.1 Factors Affecting NOX Emissions 4.3
4.1.1.1 Fuel Type and Composition 4.4
4.1.1.2 Heat Release and Transfer Rates 4.4
4.1.2 Modifications of Operating Conditions 4.5
4.1.2.1 Low Excess Air Combustion 4.5
4.1.2.2 Flue Gas Recirculation 4.5
4.1.2.3 Steam and Water Injection 4.7
4.1.3 Design Modifications 4-8
4.1.3.1 Burner Configuration 4-8
4.1.3.2 Burner Location and Spacing 4_8
4.1.3.3 Fluidized Bed Combustion 4.9
4.1.3.4 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
4.2.2.1 Reinluft Char Process 4-13
4.2.2.2 Control by Tyco Laboratories' Modified Lead Chamber
Process 4-13
4.3 REFERENCES FOR SECTION 4 4-14
5. LARGE FOSSIL FUEL COMBUSTION PROCESSES 5-1
5.1 ELECTRIC POWER PLANT BOILERS 5-1
5.1.1 Emissions 5-1
5.1.2 Control Techniques 5-1
5.1.2.1 Conversion to Lower NOX Producing Fuel 5-1
5.1.2.2 Fuel Additives 5-2
5.2.2.3 Combustion Control 5-2
5.1.3 Costs 5.3
5.1.3.1 Limitations with Large Coal-Fired Boilers 5.4
5.1.3.2 Packaged Boilers 5.4
5.2 STATIONARY ENGINES 5-5
ix
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0 . Page
Section
5.2.1 Piston Engines 5-5
5.2.1.1 Emissions 5-6
5.2.1.2 Control Techniques 5-6
5.2.2 Turbine Engines 5-9
5.2.2.1 Emissions 5-9
5.2.2.2 Control Techniques 5-9
5.2.3 Costs 5'10
5.2.3.1 Diesel Engines 5-10
5.2.3.2 Gas Engines 5-11
5.3 REFERENCES FOR SECTION 5 5-11
6. OTHER COMBUSTION PROCESSES 6-1
6.1 DOMESTIC AND COMMERCIAL HEATING 6-1
6.1.1 Emissions 6-2
6.1.2 Control Techniques 6-2
6.2 INCINERATION AND OTHER BURNING 6-3
6.2.1 Emissions 6-3
6.2.2 Control Techniques 6-4
6.2.2.1 Waste Disposal 6-4
6.2.2.2 Forest Wildfires 6-5
6.2.2.3 Controlled Vegetation Burning 6-5
6.2.2.4 Coal Refuse Fires 6-6
6.2.2.5 Structural Fires 6-6
6.2.3 Costs of Control 6-6
6.3 REFERENCES FOR SECTION 6 6-6
7. INDUSTRIAL AND CHEMICAL PROCESSES 7-1
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
7.2.1.1 Emissions 7-12
7.2.2 Organic Oxidations 7-12
7.2.2.1 Emissions 7-13
7.2.2.2 Control Techniques 7-13
7.2.2.3 Costs of Control 7-14
7.2.3 Organic Nitrations 7-14
7.2.3.1 Emissions from Nitration 7-20
7.2.3.2 Control Techniques 7-20
7.2.3.3 Costs of Controls 7-21
7.2.4 Explosives: Manufacture and Use 7-21
7.2.4.1 Emissions 7-23
7.2.5 Other Uses of Nitric Acid 7-24
7.2.5.1 Fertilizer 7_24
7.2.5.2 Metals Pickling 7_25
7.3 PETROLEUM AND NATURAL GAS INDUSTRIES 7.25
7.3.1 Emissions 7-26
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Section Page
7.3.2 Control Techniques 7-31
7.3.3 Costs of Control 7-32
7.4 METALLURGICAL PROCESSES 7-32
7.4.1 Furnaces Related to Hot Metal or Pig Iron Production 7-32
7.4.1.1 Blast Furnace 7-32
7.4.1.2 Blast Furnace Stove 7-33
7.4.2 Furnaces Related to Steel Production 7-34
7.4.2.1 Open Hearth Furnace 7-34
7.4.2.2 Coking Ovens 7-34
7.4.2.3 Miscellaneous Furnaces 7-35
7.4.3 Emissions 7-36
7.4.4 Costs 7-37
7.5 KILNS - CEMENT, LIMESTONE, AND CERAMICS 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 MISCELLANEOUS NOX EMISSION SOURCES 7-42
7.7.1 Process Description 7-42
7.7.1.1 Refractory Fibers 7-42
7.7.1.2 Perlite Expanding Furnaces 7-42
7.7.1.3 Baking and Drying Ovens 7-42
7.7.1.4 Spray Dryers 7-42
7.7.1.5 Welding 7-42
7.7.1.6 Electrostatic Precipitators 7-43
7.7.1.7 High-Level Exposure in Agriculture 7-43
7.7.2 Control Techniques 7-43
7.8 REFERENCES FOR SECTION 7 7-44
8. NITROGEN OXIDES EMISSION FACTORS 8-1
8.1 EMISSION FACTOR ACCURACY 8-1
8.2 REFERENCES FOR SECTION 8 8-4
9. POSSIBLE NEW TECHNOLOGY 9-1
9.1 POTENTIAL COMBUSTION MODIFICATIONS 9-1
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
SUBJECT INDEX 1-1
LIST OF TABLES
Table
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
xi
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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)
Engines
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
xii
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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
Xlll
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SUMMARY
DEFINITION
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.
FACTORS INFLUENCING NOX
FORMATION
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
3.
Nitric oxide tends to react with oxygen as
follows:
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
xiv
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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.
SOURCES
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
gas.
CONTROL TECHNIQUES - COMMER-
CIALLY DEMONSTRATED
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
Source
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.
xv
-------
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-
CONTROL TECHNIQUES
VARIED COMMERCIAL SUCCESS
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
xvi
-------
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-
tions.
Urea Inhibition of Nitrogen
Oxides
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-
trations.
CONTROL TECHNIQUES
(SPECULATIVE)
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
buoyancy.
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
Sieves
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.
xvn
-------
CONTROL TECHNIQUES FOR NITROGEN OXIDE
EMISSIONS FROM STATIONARY SOURCES
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
simultaneously.
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
sources.
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.
1-1
-------
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
combustion.
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).
2-1
-------
2.1 REFERENCES FOR SECTION 2
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.
2-2
-------
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
formed.
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
Table 3-1. TIME FOR NO FORMATION IN A GAS CONTAINING 75 PERCENT
NITROGEN AND 3 PERCENT OXYGEN1
Temperature, F
2,400
2,800
3,200
3,600
Time to form 500-ppm
NO, sec
1,370
16.2
1.10
0.117
NO concentrations at
equilibrium, ppm
550
1,380
2,600
4,150
3-1
-------
Table 3-2. ESTIMATES OF NITROGEN OXIDE EMISSIONS
IN THE UNITED STATES, BY SOURCE, 19682
(tons/year)
Source
Emissions
Mobile fuel combustion
Motor vehicles
Gasoline
Diesel
Aircraft
Railroad
Vessels
Non-highway users
Stationary fuel combustion
Coal
Fuel oil
Natural gas
Wood
Solid waste
Open burning
Conical incinerators
Municipal incinerators
On-site incinerators
Coal waste banks
Forest burning
Agricultural burning
Structural fires
Industrial processed
Total
6,600,000
600,000
40,000
400,000
300,000
300,000
4,000,000
1,110,000
4,640,000
230,000
450,000
18,000
19,000
69,000
190,000
1,200,000
280,000
23,000
200,000
20,669,000
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
control.
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.
3-2
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.
-------
•WASTE
COAL
WASTE
NON-
COMBUSTION
:PETROLEUM=
NOX ESTIMATED TONS, 1968
4,640,000
9,350,000
6,600,000
4,000,000
2,970,000
746,000
190,000
200,000
1,733,000
FUEL TYPE
20,669,000
GAS AND LPG
PETROLEUM
AUTO/GASOLINE
COAL
COAL UTILITIES
WASTE BURNING
COAL WASTES
INDUSTRIAL NON-COMBUSTION
AGRICULTURAL, WOOD, AND
FOREST FIRES
U. S. TOTAL
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.
3-3
-------
NON-
COMBUSTION
PIPELINES AND
GAS PLANTS
NOX ESTIMATED TONS, 1968
4,000,000
2,485,000
2,280,000
825,000
200,000
9,790,000
TYPE OF INSTALLATION
ELECTRICAL UTILITY
INDUSTRIAL COMBUSTION
PIPELINES AND GAS PLANTS
DOMESTIC AND COMMERCIAL
NON-COMBUSTION
U.S. TOTAL
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
Fuel
Natural gas
(l,046Btu/ft3)7
Fuel oil
(149,966Btu/gal)7
Coal
(ll,867Btu/lb)7
Average NOX emissions in boilers
and power plants, Ib NOX/109 Btu
Household and
commercial
111
80-480
337
Industry
205
480
842
Electric
generation
373
680
842
3-4
-------
3.1 REFERENCES FOR SECTION 3
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.
3-5
-------
4. COMBUSTION CONTROL TECHNIQUES
4.1 COMBUSTION MODIFICATIONS
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
combustion.
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-
sidered.
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
4-1
-------
Table 4-1. CALCULATED EQUILIBRIUM CONCENTRATION
OF NITROGEN OXIDES3 (ppm)
Temperature,
°F
80
980
2,060
2,912
Air
NO
3.4 x lO'10
2.3
800
6,100
NO2
2.1 x 1C'4
0.71
5.6
12
Flue gasb
NO
1.1 x 10'10
0.77
250
2,000
NO2
3.3x lO'5
0.11
0.87
1.8
aFor the reactions: N2 + O2
2NO + O2
b3.3% O2, 76% N2 in Hue gas.
2 NO
2N02
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
Where:
(NO), (N2), and O2 - Concentrations, gram-
mols/cm3
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
4-2
difficult to remove it from a gas in trace
amounts.
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
E
a.
Z*
0
h-
o:
H
Z
LU
U
o
u
111
X
0
u
DC.
h-
Z
900
800
700
600
500
400
300
200
100
O
1 1 /
j-
1
— / —
OIL / /
f f
/f —
*
L / / _
/ /
/ / -
/ f
//GAS
S fS —
/ »**
^ ''.**'
2,800 3,000 3,200
TEMPERATURE, ° F
3,400
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
reaction.
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
4-3
-------
even be widely different in identical equip-
ment.
4.1.1.1 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.
4.1.1.2 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
4-4
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.
CL
O-
in
Z
O
600
500
400
O 300
O
o:
h-
z
U- 200
X
o
100
FUEt
' — GAS
.— OIL
I
I
10
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
Conditions
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.
4.1.2.1 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
»-
Q.
Q.
Z
o
LLJ
Z
LU
O
o
a:
H
z
Li.
O
HI
X
o
/uu
600
500
400
300
200
100
0
I I I *D I
1 Iff
_ / _
f
f HORIZONTAL
— / FIRING —
I
......o
— .•••*x) —
TANGENTIAL
_ FIRING _
1 1 1 1
1 1 1 1
1234
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
4-5
-------
Z
o
UJ
240
200
UJ 160
£E
O a
^°-\20
li.
o
10
UJ
Q
X
O
80
40
i r i i
1 1
01234
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.
4.1.2.2 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.
4-6
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
desired.
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
A,
-------
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
z
QL
m
o
z
o
o
o
o
o
O
Z
16
14
12
10
8
6
4
2
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.
4.1.2.3 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
4-7
-------
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
4.1.3.1 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
andDiehl.19
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
4-8
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
Section 4.1.2.1).
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.
4.1.3.2 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.
-------
OIL
ATOMIZER
HTER
GAS
SPUD
COAL
IMPELLER
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.
4.1.3.3 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.
4-9
-------
IGNITION
TRANSFORMER
LIGHTER
OIL ATOMIZERS
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
4-10
unit, which is the same as the emissions
measured from stoker-fired units of similar
capacity.
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.
-------
4.1.3.4 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
possible.
4.2 FLUE GAS CLEANING
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-
lutants.
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
4-11
-------
Table 4-2. TYPICAL COMPOSITIONS AND SOLUBILITY OF STACK GASES
Component
N2
C02
H2O
SO2
NOX
NO (% in NOX)
NO2 (% in NOX)
Volume percent from
combustor
Coala
76.2
14.2
6.0
3.3
0.2
Oilb
77.0
12.0
8.0
3.0
0.15
Gasc
72.3
9.1
16.8
1.8
—
— 0.07d »-
HI 5-10 —
Solubility in water,
V/V, at 1 atm, 25°C
0.0018
0.1449
0.0039
9.41
0.0056
Decomposes6
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
4-12
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
-------
COAL
SUPPLY
LIMESTONE
SUPPLY
MILL
FURNACE
TO STACK
GAS
STACK
REHEATER
AIR
HEATER
•*—AIR
SCRUBBER
SETTLING
TANK
RECYCLE
AND
MAKE-UP
WATER
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
developed.
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.
4.2.2.2 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
4-13
-------
DESULFURIZED
NOX FREE GAS
FLUE GAS
(S02 AND NO)
AIR
(02)
H20
REACTION
CHAMBER
N02
H2S04 (MIST) __
(NO +N02)
NO AND N02
FLUE GAS
SCRUBBER
NO
OXIDIZER
NO
AND
N02
HN03
ABSORBER
NOX
STRIPPER
CONCENTRATION
H2S04
H2S04 AND
NOHS04
HOT STRIPPING
GAS
H2S04
HOT
EVAPORATING'
GAS
HOT GAS
. AND
H20
H2S04
EVAPORATOR
RECYCLE
PRODUCT
HNO-,
PRODUCT
H2S04
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
reduced.
The flue gas treating process is still in the
early stages of development, and Tyco
Laboratories, Inc., is currently investigating
4-14
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
combined.
and the NOX stripper are
4.3 REFERENCES FOR SECTION 4
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
1957.
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,
1967.
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.
4-15
-------
5. LARGE FOSSIL FUEL COMBUSTION PROCESSES
5.1 ELECTRIC POWER PLANT BOILERS
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
3-2.1
5.1.2 Control techniques
5.1.2.1 Conversion to Lower NOx Producing
Fuel
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 4.1.1.1). 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-
5-1
-------
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.
5.1.2.2 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-
roundings.
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.
5.1.2.3 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
boilers.
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
5-2
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
Table 5-1. ESTIMATED CONTROL COSTS AND REDUCTION OF NOX EMISSIONS,
FOR A 1,000-MEGAWATT BOILER USED 6,120 HOURS/YEAR
BY SELECTED METHODS AND BY TON
Control method
Uncontrolled
(base case)
Low excess
air
Two-stage
combustion
Low excess air
plus two-stage
combustion
Flue gas re-
circulation
Low excess air
plus flue gas
recirculation
Water injection
NOX
reduction,
%
0
0
0
33
33
25
50
40
35
90
73
60
33
33
33
80
70
55
10
10
10
Fuel
used
Gas
Oil
Coal
Gas
Oil
Coal
Gas
Oil
Coal
Gas
Oil
Coal
Gas
Oil
Coal
Gas
Oil
Coal
Gas
Oil
Coal
Control
cost/yr,
$
0
0
0
95,000b
-297,000b
- 79,OOQb
0
0
299
95,OOQb
-297,000b
220,000
202,000
202,000
202,000
107,000
95,000b
123,000
144,000
179,000
143,000
NOX
reduction,
tons/yr
53,0003
30,0003
30,000*
17,500
9,900
7,500
26,500
12,000
10,500
47,700
21,900
18,000
17,500
9,900
9,900
42,400
21,000
16,500
5,300
3,000
3,000
NOX
control
costs,
$/ton
0
0
0
5b
-3Qb
-lib
0
0
29
2b
_14b
12
12
20
20
3
5b
8
27
60
48
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.
5-3
-------
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.
5.1.3.1 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 4.1.2.1). 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.
5.1.3.2 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
5-4
-------
in efficiency may result, and the correspond-
ing fuel savings will help to defray alteration
costs.
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.
5.2 STATIONARY ENGINES
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
Fuel
Liquid fuel oil
Liquid or gas
Gas
Percent of Total3
15
19
66
alt is estimated 85 percent of these engines are supercharged;
about 1 percent are the precombustion-chamber type.9
.10
5-5
-------
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.
5.2.1.1 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
engine.
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.
5.2.1.2 Control Techniques
These observations, together with other
general background knowledge, suggest that
the following techniques to control NOX
Table 5-3. ESTIMATED STATIONARY ENGINE DISTRIBUTION IN UNITED STATES, 1969a
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
Turbojet
Turbojet
Super-
charged
Yes
No
Yes
No
Yes
No
—
Fuel
Oil
Oil
Oil
Oil
Gas
Gas
Oil
Gas
Estimated
total hp,
millions
0.7
0.1
14
2
45
8
5
46
Reference
4
5
4,5,6,7
8
9
Oxides of nitrogen emissions,
Ib NOx/lb fuel
(0.02)
0.007-0.010
0.07
0.021-0.047
(0.16)
0.084
0.008-0.01
(0.004)b
lbNOx/106 Btu
(0.9)
0.38-0.55
3.8
1.2-2.6
(8.0)
4.0
0.11-0.32
(0.2)b
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.
5-6
-------
Table 5-4. SUMMARY OF SOURCE SAMPLING TESTS FOR TWO
DUAL-FUEL (GAS AND DIESEL) ENGINES
Fuel
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
39,291
35.6
8,890
0.074
4.04
1,375
77
3,185
105
360
Natural gas
31,767
31.5
7,350
0.075
3.50
1,544
Engine 2
Diesel No. 2
21,639
33.0
4,910
0.066
3.61
1,251
80
1,850
160
514
Natural gas
19,813
34.6
4,540
0.038
1.77
746
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.
5.2.1.2.1 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
diesels.
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.
5.2.1.2.2 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.
5.2.1.2.3 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
5-7
-------
Table 5-5. EFFECT OF FUEL TYPE ON EMISSIONS OF NOX FROM STATIONARY DIESEL ENGINE8
Fuel characteristics
°API
42.8
32.7
32.4
30.9
Methane
FIA Anal.,
Percent
Aromatics
12
28
26
0
Est. net
heat, value
Btu/lb
18,570
18,300
18,290
18,220
21,500
Boiling Point F
50%
dist.
428
400
519
538
259
rmai
Final
514
609
661
656
Viscosity
.U.b.
@ 100° F
33.0
34.3
36.1
38.0
NOX emissions,
Ib NOx/lb fuel
0.023
0.023
0.03
0.03
0.0056b
lbNOx/106Btu
1.24
1.26
1.64
1.65
0.26b
Reference
4
4
4
4
8
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.
5.2.1.2.4 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
Table 5-6. EFFECT OF AIR/FUEL RATIO2 ON NOX PRODUCTION IN
SPARK-IGNITED STATIONARY GAS ENGINES13
fuel, %
100
95
92
87
84
70
65
Ib air/lb fuel
17.1
18
18.6
19.7
20.4
24.4
26.3
Power, % max. IMEP
88.9
80
78
71
misfire
NOX production
Ib NOx/lb fuel
0.038
0.065
0.095
0.069
0.055
0.028
lbNOx/106Btu
1.8
3.0
4.4
3.2
2.6
1.3
aFull throttle; maximum power spark; 1,000 rpm; compression ratio, 8/1; fuel, methane.
5-8
-------
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.
5.2.1.2.5 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.
5.2.1.2.6 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.
5.2.1.2.7 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.
5.2.2.1 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.
5.2.2.2 Control Techniques
5.2.2.2.1 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
emissions.
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
5-9
-------
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.
5.2.2.2.2 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.
5.2.2.2.3 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.
5.2.2.2.4 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
ratio).
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.
5.2.3.1 Diesel Engines
Currently, the only effective way to reduce
NOX is to use engines with design features
that tend to minimize NOX production.
Table 5-7. EFFECT OF AIR/FUEL RATIO3 ON NOX FORMATION IN
EXPERIMENTAL COMBUSTOR RIG16
Air /fuel ratio,
Ib air/lb fuel
53
73
Relative
temperature
at exit of
combustor, F
T
T minus 370
NO concentration, ppm
As
measured
34
10
Corrected
to 15/1 air /fuel
120
50
Calculated
NOX, Ib/lb fuel
0.0017
0.0007
aDirectly related to temperature of gases at combustor exit.
5-10
-------
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.
5.2.3.2 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.
Table 5-8. NITROGEN OXIDE CONTROL COSTS FOR STATIONARY GAS ENGINES
Engine, hp
80
800
4,200
Cost, $/yr
60.00
800.00
930.00
Cost, $/ton
NOX eliminated
420.00
9.30
1.20
REFERENCES FOR SECTION 5
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
1968.
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.
5-11
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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
areas.
"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.
6.1 DOMESTIC AND COMMERCIAL
HEATING
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,
263,964.
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
content.
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
Table 6-1. FUEL CONSUMPTION FOR SPACE HEATING AND COOKING, 1966
Fuel type
Natural gas, 109ft3
Distillate fuel oil,
42-gal bbl
Residual fuel oil,
42-gal bbl
Quantity
5,761
472,778,000
167,471,000
Equivalent heating value,4
10l2Btu
5,945
2,832
1,052
Table 6-2. GAS CONSUMPTION FOR DOMESTIC HEATING IN UNITED STATES
Number of
customers
28,800,000
33,000,000
38,200,000
Total
Type of use
Heating units
( 1 and 2 family)
Hot water
Ranges
Avg heat use,
105 Btu/unit-yr
1,020
300
100
Total heat use/yr,
1012Btu
2,938
990
420
4,348
6-1
-------
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.
6-2
-------
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
above.
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.
6.2 INCINERATION AND OTHER
BURNING
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,
respectively.13
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,
6-3
-------
Table 6-3. ESTIMATED NATIONAL EMISSIONS FROM INCINERATION
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
Total
Quantity,
lO^ tons/yr
57
16
27
77
275
76
146
8
48
730
NOX emission,
1()3 tons/yr
69
19
18
450
280
400
800
23
190
2,249
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.
6.2.2.1 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.
6-4
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-
cumulations.20
-------
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.
6.2.2.2 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.
6.2.2.3 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
6-5
-------
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
6.2.2.4 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 °
6.2.2.5 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,
6-6
National Fire Protection Association,
National Safety Council, and insurance com-
panies.
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
6.3 REFERENCES FOR SECTION 6
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-
sociation.
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,
1969.
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
1960.
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.
278-286.
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.
6-7
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7. INDUSTRIAL AND CHEMICAL PROCESSES
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
acid.
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
NO2:NO).
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
air.
A large amount of research with varying
degrees of success has been carried out on the
7-1
-------
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
7-2
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, 7.2.2.2, and 7.2.3.2 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.
7.1 NITRIC ACID MANUFACTURE
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-
curs:
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
tetroxide.
2NO + O2 -* 2NO2
(2)
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
achieved.
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-
ery.
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
AEFFLUENT TAIL GAS STREAM
STACK
WASTE HEAT I. J 1.
BOILER PI ['
PLATINUM
FILTER
COMPRESSOR — EXPANDER
Figure 7-1. Flow diagram of a typical 120-ton-per-day nitric acid plant utilizing the
pressure process.
7-3
-------
TAIL GAS TO
ATMOSPHERE (Volume
98% HNOj VAPOF
FEED
93% H2S04 •»
60% HN03->
DEHYDRATING
COLUMN
LIQUIC
STFAM
COIL
»
V
^
N
$
!
— >
?
^
l
^
$
\
k
—— "i
\
COUN 1 tKl_UKKCP
CONDENSER
( *
)
(
\
J
A ICONDENSATE
VAPOR J 1
j, . NON-
CONDENSABLE
GASES
BLEACHER
^-»
I ^ 95-99% HN03
TO COOLER AND STORAGE
/APOR
70% H2S04
" ' TO COOLER
*> >
1
"
--
c
r
<
]_
...
•><*
•74.3% N2
Ofl A % Or.
1 0% NO + NOn
ABSORPTION
COLUMN
AIR
B HNO3
BOILER
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
7-4
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
gases.
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-
tive.-
Table 7-1. ESTIMATES OF STRONG ACID REQUIREMENTS AND
NOX EMISSIONS FROM CONCENTRATION OPERATIONS
Use
Nitrobenzene
Toluene diisocyanate
Commercial explosives
Total
Nitric acid requirements
Total HNO3,
tons
90,000
75,000
38,000
203,000
Strong acid,
%
72
51
100
Strong acid,
tons/yr
65,000
38,000
38,000
141,000
NOX,
tons/yr
162
95
95
352
7-5
-------
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
catalyst:
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
methane.
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
7-6
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
steam.
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
7-7
-------
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.
7-8
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
1:1.
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
Section 7.2.3.2.
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
7-9
-------
Table 7-2. ESTIMATED COSTS OF CATALYTIC REDUCTION
FOR THREE CONTROL SYSTEMS
NOX before reduction, ppm
NOX after reduction, ppm
Fuel
Investment, 103 $
$/scfm
Costs, 103 $/yra
Natural gas
Ammonia
Catalyst
Steam
Investment-based costs"
Total cost, 103 $/yr
Total cost, $/ton HNO3
Total cost, $/ton NOX removed
Decolorizer
3000
3000
Natural gas
60
2.60
14
4
(50)
30
(2)
(0.02)
Selective
abater
3000
300
Ammonia
140
6.10
33
12
(21)
70
94
0.95
34
Two-stage non-
selective abater
3000
300
Natural gas
255
11.00
82
8
(144)
128
74
0.75
27
aParentheses indicate negative costs or savings.
"Includes 20 percent return before taxes.
total temperature rise can be used in the
turboexpander.
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
7-10
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.
7.2 NITRIC ACID USES
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
below.
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
Use
Quantity, tons
Ammonium nitrate
Adipic acid
Terephthalic acid
Nitrobenzene
Toluene diisocyanate *
Commercial explosives (e.g.,
Fertilizers (e.g., NH4NO3)
Other uses
Total
4,540,000
560,000
100,000
90,000
75,000
38,000
150,000
569,000
6,122,00019
7-11
-------
7.2.1 Ammonium Nitrate Manufacture
Ammonium nitrate is produced by the di-
rect neutralization of nitric acid with am-
monia:
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.
7.2.1.1 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
decomposition.
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.
CYCLOHEXANE
02
(AIR)
CYCLOHEXANONE-
CYCLOHEXANiOL
02
(HN03)
ADIPIC
ACID
Figure 7-3. Adipic acid synthesis.
7-12
-------
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
(N20).
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.
7.2.2.1 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.
7.2.2.2 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).
PARAXYLENE
°2
(HN03)
TOLUIC ACID
02
(HN03)
TEREPHTHALIC
ACID
Figure 7-4. Terephthalic acid synthesis.
7-13
-------
Table 7-4. ESTIMATED ACID REQUIREMENTS AND NOX EMISSIONS FROM
NITRIC ACID OXIDATION OF ORGANIC COMPOUNDS, 1967
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
1,080.00
1.04
560,000.00
11.60
3,000.00
Terephthalic acid
300. 00a
0.68
100,000.00
13.00
1,000.00
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.
7.2.2.5 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
7-14
-------
and "drowned" in case of abnormal condi-
tions.
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-
eral.
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
absorbent.
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.
7-15
-------
NITRATING HOUSE
ON
MIXED-ACID
STORAGE MIXED-ACID
____, SCALE TANK
MIXED ACID
GLYCERIN
I CATCHJANK
' -
SPENT-ACID
STORAGE TANK
»• BUGGY
v NEUTRALIZER
SPENT-ACID
STOREHOUSE
WASTE WATER
I I
GLYCERIN HEATER HOUSE
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
NITROGLYCERIN
TO POWDER
^. WASTE WATER
. RECOVERED
"^ NITRIC ACID
WEAK SULFURIC
ACID
-------
AGITATOR
GLYCERIN FEED
NITRATOR
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.)
-------
-J
I—»
oo
GLYCERIN
FEED
•AGITATOR
ACID
SEPARATOR
NITROGLYCERIN
WATER
AGITATOR
MECHANICAL WASHER
SODA ASH
SOLUTION
NITRATOR
SPENT ACID
TO LEVELING DEVICE
(SEE BELOW)
NITRO-
GLYCERIN
ACID
LINE OF
SEPARATION
TO DROWNING TANK
SEPARATOR
LEVELING
DEVICE
SPENT ACID
TO AFTER-SEPARATOR
AND STORAGE
TO DROWNING TANK
SPENT ACID
FROM SEPARATOR
LEVELING
DEVICE
WASH WATER TO SEPARATOR
AGITATOR
MECHANICAL
'NEUTRALIZER
.NITROGLYCERIN
EMULSION
TO ADDITIONAL
WASHERS AND
SEPARATORS
Figure 7-7. Biazzi continuous-nitration plant.
(Courtesy of John Wiley and Sons, Inc.)
-------
AIR
GASES TO ABSORPTION TOWER
<-
SPENT-ACID
FEED TANK
CHEMICAL-WARE
BLOCKCOCK
DENITRATING TOWER
(PAULING TOWER)
NITRIC DISTILLATE
SAMPLER
NITRIC ACID
TO STORAGE
THERMOMETER
SULFURIC ACID
I
WATER
SULFURIC ACID
COOLING TUB
TO STORAGE
Figure 7-8. Recovery of spent acid.
(Courtesy of John Wiley and Sons, Inc.)
7-19
-------
7.2.3.1 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.
Table 7-5. ESTIMATED NOX EMISSIONS FROM
ORGANIC NITRATIONS IN 1967
Product
Nitrobenzene
Dinitrotoluene
Estimated acid
consumption,
tons/yr
90,000
75,000
NOX, tons/yr
Lower
limit
8
7
Upper
limit
630
530
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
7-20
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.
7.2.3.2 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
not.
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
7.2.3.3 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-
tors.34
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
Table 7-6. INGREDIENTS IN U.S. INDUSTRIAL
EXPLOSIVES, 1967
Ingredient
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
ingredients
Nitroglycerin
TNT
Total
Quality, tons
643,000
23,000
53,000
76,000
795,000
120,000b
28,000b
10,000b
953,000
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
7-21
-------
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
02NOH2C
CH2ONO2
CH2ONO2
NO2
l
CH3 N02
O2N-N
CH2
N-NO2
PETN
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
^C
H2
RDX
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
STRONG
MIXED
ACID
TOLUENE
HN03
RECOVERY
I
V.
NOX
* D
MONO-REACTOR
E-NITRATION
-»•
ACID
MNT
HN03
DILUTE
H2S04 "
DI-REACTOR
H2S04
CONCEN-
TRATION
ACID
DNT
SELLITE
*•
TRI
TNT
1
-REACTOR
J
PRODUCT
WASH
TNT
TO REDWATER
INCINERATION
Figure 7-9. Trinitrotoluene (TNT) manufacturing diagram.
7-22
-------
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
7.2.4.1 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
production.
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
REQUIREMENTS AND NOX EMISSIONS FROM MANUFACTURE
OF INDUSTRIAL EXPLOSIVES, 1967
TNT (industrial use)
Reactor
Product,
tons
10,000
HNO3,
tons
10,000
NOX,
Ib/ton HNO3
5.0
NOX,
tons/yr
25
Nox, ton/ton
of product
0.003
7-23
-------
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
explosives.18
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
places.
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.
7.2.5.1 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
produced.45
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
Explosive
TNT, 50/50 Amatol, RDX, tetryl,
straight dynamites, AN=fuel oil
AN/DNT
PETN
Nitroglycerin
Straight gelatins, 40%
Straight gelatins, 75%
NO %
1NUX, '"
0
0
0.3-0.6
1.5
0.1
0.1
NOX, Ib/ton
explosives
0
0
10-18
46
3
3
7-24
-------
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.
7.2.5.2 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.
7.3 PETROLEUM AND NATURAL
GAS INDUSTRIES
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
7-25
-------
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
products.
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-
7-26
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
reaction.
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
OVERHEAD
CRUDE-
OIL
o
z
Q.
0.
O
Q
ID
o:
u
LIGHT NAPHTHA
RAW GASOLINE
RAW KEROSINE
MIDDLE DISTILLATE
HEAVY GAS OIL
REDUCED
CRUDE
VACUUM
DIST.
UNIT
RESIDUUM
WET
GAS
H2
GAS
PLANT
CATALYTIC
REFORMER
HYDROGEN
TREATING
H2S
*-FUEL GAS
*-LPG
ALKY. I—ALKYLATE-
1 GASOLINE
H2S
REFORMATS
STRAIGHT-RUN GASOLINE
SULFUR
PLANT
CATALYTIC
CRACKING
CRACKED GASOLINE
LUBE STOCKS
WET GAS
COKER
TREATMENT
ASPHALT
STILL
TREATERS
CHILLERS
FILTERS
GASOLINE
O
. MOTOR
GASOLINE
-AVIATION
GASOLINE
-*-KEROSINE
LIGHT FUEL OIL
AND DIESEL OILS
->-SULFUR
-FLIGHT FUEL OIL
-»HEAVY FUEL OIL
-»-LUBE STOCKS
-*-GREASES
-»-WAXES
-•-ASPHALT
COKE
to
Figure 7-10. Composite processing plan for a modern refinery.
-------
FLUE GAS
REGENERATOR
REGENERATED
CATALYST
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-
7-28
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
-------
Table 7-9. NOX EMISSIONS FROM PETROLEUM AND NATURAL GAS
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.
Operations
Oil and gas production
Gas
Heaters and boilers
Engines
Oil
Diesel engines
Production subtotal
Gas plants
Heaters and boilers
Engines
Turbines
Gas plant subtotal
Gas pipelines
Engines
Turbines
Gas pipeline subtotal
Refining
Gas
Heaters and boilers
Engines
Turbines
Oil
Heaters and boilers
Other
Refining subtotal
Total NOX emissions
Fuel
consumptiona
495C
124C
7.3f> 8
247C
260C
15C
436C
140C
l,656f
40f
54*
39 .7f
Emission
factorb
200d
770e
220h
190d
4,300e
200*
7,300e
2001
210d
4,350e
2001
67h
NOX emissions
Range,
ppm
25-200
250-2,500
25-200
1,000-3,500
50-200
1,500-3,500
50-200
25-200
1,000-3,500
50-200
50-400
Quantity,
103ton/yr
50
48
34
132
24
561
2
587
1,596
14
1,610
173
87
5
56
9
330
2,659
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
7-29
-------
100
70
50
4O
30
20
UJ
O
O
o:
UJ
Q
X
O
10
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
200
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
7-30
which are not consistent with other data now
available.
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
-------
100
_Q
•z.
o
o
H
Z
X
o
o:
z
o
in
—
i
LU
10
0.1
0.01
0.001
/o
OIL WELL PUMP ENGINE
IN GOOD CONDITION
• DATA FROM FIGURE 8-1
O ADDITIONAL TEST DATA
OIL WELL PUMP ENGINE
IN POOR CONDITION
I
I
10
100 1,000
ENGINE LOAD, hp
10.000
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.
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.
Table 7-10. NITROGEN OXIDE EMISSIONS FROM
PETROLEUM AND NATURAL GAS
OPERATIONS IN UNITED STATES, 1967
Petroleum and natural gas
operations
Oil and gas production
Gas plant
Pipeline
Refining
Total
Percent of total 1967
emissions
0.8
3.7 '
10.3
2.1
16.9
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
7-31
-------
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
7-32
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 METALLURGICAL PROCESSES
»
7.4.1 Furnaces Related To Hot Metal
Or Pig Iron Production
7.4.1.1 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
-------
Table 7-11. INVESTMENT COST ESTIMATES FOR REFINERY FURNACES
IN TWO SIZES, AT A STANDARD LOCATION, WITH NITROGEN OXIDES
CONTROL BY FLUE GAS RECIRCULATION,
APRIL THROUGH JUNE 1969a'51
Investment
No recirculation
Material
Labor
Total
30 percent recirculation
Material
Labor
Total
Increased horsepower required
by 30 percent recirculation
Installed
Operating
Furnace No. 1"
$133,900
27,000
$160,900
$151,100
29,900
$181,000
89 hp
63 hp
Furnace No. 2C
$ 712,500
152,700
$ 865,200
1
$ 867,000
$ 180,000
$1,047,000
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
included.
^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
7.4.7.2 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
7-33
-------
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
7.4.2.1 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.
7-34
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
reactions.69
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
7.4.2.2 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
oven.
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.
7.4.2.3Miscellaneous 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
today.74
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.
7-35
-------
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
7-36
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
fuel.
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-
plications.
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,
however.
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-
trol.
7.5 KILNS - CEMENT, LIMESTONE,
AND CERAMICS
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
temperature.
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
kilns.
Table 7-12. FUEL CONSUMED BY CEMENT INDUSTRY IN U.S., 1967
Fuel
Coal, 1 O3 short tons
Oil, 103bbl(42gaVbbl)
Natural gas, 106ft3
Amount consumed
9,096
4,956
159,717
Equivalent Btu, 1012
236
30
196
7-37
-------
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-
ress.
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
7-38
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
7-13.
Table 7-13. ESTIMATED NOX EMISSIONS FROM
KILNS IN UNITED STATES, 1967
Process unit
Cement kilns
Lime kilns
Ceramic kilns
Total
NOX)
tons/yr
108,000
21,000
21 ,000
150,000
Percent of total
U.S. NOX emissions
0.8
0.15
0.15
1.1
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.
7.6 GLASS MANUFACTURE
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-
trol.
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
7-39
-------
GLASS SURFACE IN REFINER
REFINER SIDE WALL
GLASS SURFACE MELTER SI DE WALL THROAT
•MELTER BOTTOM
IN MELTER
NATURAL DRAFT STACK
BACK WALL
COMBUSTION AIR
BLOWER
RIDER ARCHES
MOVEABLE REFRACTORY BAFFLE
Figure 7-14. Regenerative side port glass-melting furnace.
throat of the bridgewall into the "working
zone."
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-
ture.
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
7-40
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.
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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
situation.
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
7-41
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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 MISCELLANEOUS NOX
EMISSION SOURCES
7.7.1 Process Description
7.7.1.1 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
year.92
No specific data are available on the NOX
emitted during fiber production.
7.7.1.2 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
7.7.1.4 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
7-42
-------
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.
7.7.1.6 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
research.
Until more reliable information is available,
one cannot consider electrostatic precipita-
tion a contributor to NOX.
7.7.1.7 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
(N02).
7.7.2 Control Techniques
The first of the combustion sources
described above (Section 7.7.1.1) 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
7-43
-------
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-
ly, NOX control can be an integral consider-
ation.
7.8 REFERENCES FOR SECTION 7
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Nitrogen Oxide Emissions from Stationary
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Emissions from Nitric Acid Manufacturing
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3. Inorganic Chemicals and Gases. Current In-
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1966.
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7. Sandaresan, B. B. and C. I. Harding. Methods of
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8. Janicyck, W. et al. New Prospects in Nitric Acid
Technology. Przemysl Chem. 46(1):28-31, 1967.
9. Gerstle, R. W. and R. F. Peterson. Control of
Nitrogen Oxide Emissions from Nitric Acid Man-
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1967.
10. Private Communications.
11. Hunter, J. B. Platinum Catalysts and Air Pol-
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23, 1968.
12. Andersen, H. C., P. L. Romeo, and W. J. Green.
New Family of Catalysts for Nitric Acid Tail
Gases. Nitrogen 50:33-36, November-December
1967.
13. Decker, L. Incineration Technique for Control-
ling Nitrogen Oxides Emissions. Presented at the
60th Annual Meeting of the Air Pollution Con-
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14. Anderson, H. C. and W. J. Green. Method of
Purifying Gases Containing Oxygen and Oxides
of Nitrogen (Englehard Industries, Inc., U.S.
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Patent Office. 752(5):969, January 31, 1961.
15. Newman, D. J. and L. A. Klein. Apparatus for
Exothermic Catalytic Reactions (Chemical Con-
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Official Gazette U.S. Patent Office.
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31. Crater, W. deC. Nitration. In: Kirk-Othmer
Encyclopedia of Chemical Technology, Standen,
A. (ed.). Vol. 9. New York, Interscience Publish-
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32. Matsuguma, H. J. Nitrobenzene and Nitro-
toluenes. In: Kirk-Othmer Encyclopedia of
Chemical Technology, Standen, A. (ed.). Vol. 13,
2d ed. New York, Interscience Publishers, 1967.
p. 834-853.
33. Urbanski, T. Chemistry and Technology of
Explosives, Jeczalikowa, I. and S. Laverton,
(Trs.). Vol. I. New York, MacMillan Co., 1964
635 p.
34. Private communication with E. I. Du Pont de
Nemours and Co. March 1969.
35. Hardison, L. C. Techniques for Control of Oxides
of Nitrogen. Universal Oil Production Tech. Rev.
March 1967.
36. Russo, R. S. NAAP Pollution Advisory Com-
mittee Meeting, Newport Army Ammunitions
Plant, Newport, Ind. January 30, 1969. National
Air Pollution Control Administration. Washing-
ton, D.C. February 13, 1969.
37. Felder, T. D., Jr. Apparatus for Converting
Oxides of Nitrogen to Innocuous Gases. (E. I. Du
Pont de Nemours and Co., U.S. Patent No. 3,
232, 713). Official Gazette U.S. Patent Office.
«2J(1):261, February 1, 1966.
38. Ephraim, F Inorganic Chemistry, rev. by
Thome, P. C. L. and E. R. Roberts. 6th ed.
London, Oliver and Boyd Ltd., 1954. 956 p.
39. Apparent Consumption of Industrial Explosives
and Blasting Agents in the United States, 1967.
Mineral Industry Surveys; Explosives, Annual.
Bureau of Mines. Washington, D.C. March 1969.
14 p.
40. Cook, M. A. The Science of High Explosives
(ACS Monograph Series Number 139). New
York, Reinhold Publishing Corp., 1958. 440 p.
41. Fordham, S. High Explosives and Propellants.
Oxford, Pergamon Press, 1966. 224 p.
42. Biazzi, M. Biazzi Process. In: Chemistry and
Technology of Explosives, Ornaf, W. and S.
Laverton, (Trs.). Vol. II. Oxford, Pergamon
Press, 1965. p. 114.
43. Higgins, F. B., Jr. Control of Air Pollution from
TNT Manufacturing. Presented at the 60th An-
nual Meeting of the Air Pollution Control As-
sociation. Cleveland. June 11-16, 1967.
44. Summary Report of Air Pollution Evaluations at
Volunteer Army Ammunition Plant, Tyner,
Tenn., 1967. U.S. Army Environmental Hygiene
Agency. Edgewood Arsenal.
45. Fertilizer Technology and Usage, McVickar, M.
H. et al (eds.). Madison, Wisconsin, Soil Science
Society of America, 1963. 464 p.
46. Consumption of Commercial Fertilizers, U.S.
Dept. of Agriculture. Statistical Reporting
Service.
47. Private communication with Chevron Research
Co. February 1969.
48. McGannon, H. E. The Making, Shaping and
Treating of Steel. 8th ed. Pittsburgh, United
States Steel Co., 1964. 1300 p.
49. Kerns, B. A. Chemical Suppression of Nitrogen
Oxides. Ind. Eng. Chem. Process Design Develop.
4(3):263-265,July 1965.
50. Minerals Yearbook 1967, Vol. I-II. Bureau of
Mines. Washington, D.C. 1968. 1262 p.
51. Bartok, W. et al. Systems Study for Nitrogen
Oxide Control Methods from Stationary Sources.
NAPCA Contract PH-22-68-55. Report Number
GR-2-NOS-69. 1969.
52. Bonamassa, F. Emissions to the Atmosphere
from Eight Miscellaneous Sources in Oil
Refineries. Joint District, Federal and State
Project for the Evaluation of Refinery Emissions.
Los Angeles County Air Pollution Control
District. Los Angeles, Calif. Report Number 8.
June 1958.
53. Crude Petroleum, Petroleum Products, and
Natural-Gas-Liquids. Mineral Industry Surveys;
Petroleum Statement, Monthly. Bureau of Mines.
Washington, D.C.
54. Shipments of Fuel Oil and Kerosene in 1967.
Mineral Industry Surveys; Fuel Oil Shipments,
Annual. Bureau of Mines. Washington, D.C.
August 7, 1968. 12 p.
55. Duprey, R. L. Compilation of Air Pollutant
Emission Factors. National Center for Air Pol-
lution Control. Durham, N.C. PHS Publication
Number 999-AP-42. 1968. 67 p.
56. Private communication with A. R. Cunningham.
April 29, 1969.
57. Kridner, K. Growth of the U.S. Gas Transmission
Operations. Pipe Line Ind. 25:35-40, June 1967.
58. Refining Survey. Oil Gas J. 67(12): 116, 1969.
59. Mills, J. L. et al. Emissions of Oxides of Nitrogen
from Stationary Sources in Los Angeles County;
A Summary of Data on Air Pollution by Oxides
of Nitrogen Vented from Stationary Sources.
Joint District, Federal, State, and Industry
Project. Los Angeles County Air Pollution
Control District. Los Angeles, Calif. Report
Number 4. July 1961. 34 p.
60. 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 Pollu-
tion Control District, Los Angeles, Calif. Report
Number 3. April 1962.51 p.
7-45
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61. Nicholls, J. E., I. A. El-Messiri, and H. K. New-
hall. Inlet Manifold Water Injection for Control
of Nitrogen 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.
62. Lee, R. C. and D. B. Wimmer. Exhaust Emission
Abatement by Fuel Variations to Produce Lean
Combustion (SAE Paper No. 680769). Phillips
Petroleum.
63. McGannon, H. E. The Making, Shaping and
Treating of Steel. 8th ed. Pittsburgh, United
States Steel Co., 1964, 1300 p.
64. McGannon, H. E. The Making, Shaping and
Treating of Steel. 8th ed. Pittsburgh, United
States Steel Co., 1964. p. 91.
65. Annual Statistical Report—American Iron and
Steel Institute. New York, American Iron and
Steel Institute, 1967. p. 79.
66. Minerals Yearbook 1967, Vol. MI. Bureau of
Mines. Washington, D.C. 1968. 1262 p.
67. Minerals Yearbook 1967, Vol. I-II. Bureau of
Mines. Washington, D.C. 1968. p. 603.
68. Crawford, A. R., and W. Bartok. Survey of NOX
Emissions in the U.S. with Projections to the
Year 2000. Presented at 62d Annual Meeting, Air
Pollution Control Association. New York. June
22-26, 1969.
69. Private Communication.
70. Thatcher, J. W. Iron and Steel. In: Minerals Year-
book 1967, Vol. I-II. Bureau of Mines. Washing-
ton, D.C. 1968. p. 595.
71. DeCarlo, J. A. and B. D. Watson. Coke and Coal
Chemicals. In: Minerals Yearbook 1967, Vol.
I-II. Bureau of Mines. Washington, D.C. 1968. p.
387.
72. Russell, C. C. Carbonization. In: Kirk-Othmer
Encyclopedia of Chemical Technology, Standen,
A. (ed.), Vol. 4, 2d ed. New York, Interscience
Publishers, Inc., 1964. p. 400-423.
73. Murray, R. W. Foundry Melting-Cupola vs
Induction vs Arc. Iron Age. 199(19): 61-11, May
11, 1967.
74. McGannon, H. E. The Making, Shaping and
Treating of Steel, 8th ed. Pittsburgh, United
States Steel Co., 1964. p. 436-458.
75. OG Gas-Recovery Equipment Raises Yield of
BOF Plant. Iron Age. 198(1):78-9, August 18,
1966.
76. Hale, R. W. The Future of Scrap in Electric
Furnace Steelmaking. Presented at the 41st An-
nual Convention and Equipment Exposition. In-
stitute of Scrap Iron and Steel, Inc. New York.
1969.
77. Why Do Steelmakers Want Plasma Furnaces?
Iron Age. 203(17):26, April 24, 1969.
1A6
78. Bogue, R. H. Cement. In: Kirk-Othmer Encyclo-
pedia of Chemical Technology, Standen, A. (ed.),
Vol. 4, 2d ed. New York, Interscience Publishers,
Inc., 1964. p. 684-710.
79. Roberts, J. E. Lime Kiln Design-1: UCM's
Vertical Lime Kiln. Chem. Eng. Prog. 5P:88-91,
October 1963.
80. Minerals Yearbook 1967, Vol. I-II. Bureau of
Mines. Washington, D.C. 1968. 1262 p.
81. Boynton, R. S. Lime and Limestone. In: Kirk-
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Standen, A. (ed.), Vol. 12, 2d ed. New York,
Interscience Publishers, Inc., 1966. p. 414-460.
82. Levine, S. Fifth Lime Kiln Boosts Warner
Company's Capacity. Rock Prod. 70:60-62, July
1967.
83. Smith, W. S. and C. W. Gruber. Atmospheric
Emission from Coal Combustion-An Inventory
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Number 999-AP-24, April 1966. 112 p.
84. Air Pollution Engineering Manual, Danielson, J.
A. (ed.). National Center for Air Pollution Con-
trol. Cincinnati, Ohio. PHS Publication Number
999-AP-40. 892 p.
85. Handbook of Glass Manufacture, Tooley, F. V.
(ed.), Vols, I and II. New York, Ogden Publishing
Co., 1953. 506 p. and 1960. 282 p.
86. 1963 Census of Manufacturers. Vols. I, II, and
III. Bureau of Census. Washington, D.C. 1966.
87. Consumer Scientific Technical and Industrial
Glassware. Current Industrial Reports Series
MA32E-1962 and 1967. Bureau of Census.
Washington, D.C. October 25, 1963. 3 p. and
August 5, 1968. 4 p. Fibrous Glass. Current In-
dustrial Reports Series MA32J-1962 and 1967.
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1963. 3 p. and May 14, 1968. 1 p.
88. Private Communication.
89. Peckham, G. W. New Electric Furnace Reduces
Melting Costs. Glass Ind. 43:552-553, 568,
573-574. October 1962.
90. Takeno, W. The Formation of Nitrous Vapors in
the Burning of Natural and City Gas in Glass-
blowers' Torches. Chem. Weekblad.
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91. Mineral Wool Furnaces. In: Air Pollution En-
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93. Perlite-Expanding Furnaces. In: Air Pollution En-
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from Stationary Sources in Los Angeles County;
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747
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8. NITROGEN OXIDES EMISSION FACTORS
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.
8.1 EMISSION FACTOR ACCURACY
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-
lems.
"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
8-1
-------
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
10,000
z
LU
O
§ 1,000
H
Z -c
O
00 1
,100
10
LU Z
H- O
1.0
LU ^
O LU
0.1
0.01
4?
4^
:>
-------
known. NOX emission factors from these and
other sources have been compiled by
Duprey.2
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-
sions.
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
DURING COMBUSTION OF FUELS AND OTHER MATERIALS
Source
Average emission factor
Fuels
Coal
Household and commercial
Industry
Utility
Fuel oil
Household and commercial
Industry
Utility
Natural gas
Household and commercial
Industry
Utility
Wood
Combustion sources
Gas Engines
Oil and gas production
Gas plant
Pipeline
Refinery
Gas Turbines
Gas plant
Pipeline
Refinery
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
8-3
-------
Table 8-1 continued. EMISSION FACTORS FOR NITROGEN OXIDES
DURING COMBUSTION OF FUELS AND OTHER MATERIALS
Source
Average emission factor
Other combustion
Coal refuse banks
Forest burning
Agricultural burning
Structural fires
Chemical industries
Nitric acid manufacture
Adipic acid
Terephthalic acid
Nitrations
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.
7,000x0.65=4,550^-^*-
yr
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
8.2 REFERENCES FOR SECTION 8
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.
8-4
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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 POTENTIAL COMBUSTION MODIFI-
CATIONS
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
4.1.3.3).
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.
9-1
-------
9.2 FLUE GAS REMOVAL
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-
moval.
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
produced.
9.3 REFERENCES FOR SECTION 9
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.
9-2
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SUBJECT INDEX
Agriculture, high level exposure in, 7-43
Air combustion, low excess, 4-5—4-6
Ammonium nitrate manufacture, 7-12
B
Baking and drying ovens, 742
Basic oxygen furnace, 7-35
Blast furnace, 7-32
Boilers
[See: Coal-fired boilers; Electric power
plant boilers; Packaged boilers]
Burner configuration, 4-8
Burner Location and spacing, 4-8—4-9
D
Definitions, 2-1
Design modifications, 4-8
Diesel engines, 5-10
Domestic heating, 6-1—6-3
E
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),
5-1-5-11
Commercial heating, 6-1—6-3
Control processes, potential, 4-13
Costs
[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-
esses]
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
Furnaces
[See: Basic oxygen furnaces; Blast furnace;
Cupola furnace; Glass melting furnace;
Open hearth furnace; Perlite expanding
furnaces]
Gas engines, 5-11
Glass manufacture, 7-39
Glass melting furnace, 7-39
1-1
-------
H
Heat release and transfer rates, 4-4
I
Incineration and other burning, 6-3
Industrial and chemical processes, 7-1
K
Kilns, 7-37
Lead chamber process, modified, 4-13—4-14
Limestone wet-scrubbing process, 4-11
M
Metallurgical processes, 7-32
Metals pickling, 7-25
N
Nitric acid manufacture, 7-2
Nitric acid uses, 7-11-7-25
O
Open hearth furnace, 7-34
Operating conditions, modification, 4-5
Organic nitrations, 7-14-
Ovens
[See: Baking and drying ovens; Coking
ovens]
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
R
Refractory fibers, 7-42
Scrubbers
[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,
4-12
Steam and water injection, 4-7
Steel production, 7-34
Structural fires, 6-6
Turbine engines, 5-9
Vegetation burning, controlled, 6-5
W
Waste disposal, 6-4
Welding, 7-42
West-scrubbing process
[See: Limestone wet-scrubbing process]
1-2
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