United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-85/047 Nov. 1985
Project Summary
Assessment of NOX Emission
Factors for Direct-Fired Heaters
C. R. Newman
Industrial fuel use in non-boiler appli-
cations amounted to about 60 percent
of total industrial fuel consumption in
1980. Historically, air pollution control
of these sources has focused on emis-
sions of particulates and sulfur com-
pounds with much less emphasis on
emissions of oxides of nitrogen (NO,).
NO., however, currently are felt to play
a role in the formation of acid precipita-
tion.
The increasing use and potential for
use of preheated combustion air for
energy conservation may result in in-
creased emissions of NO, from direct-
fired process heaters. Limited data
show that NO* emission rates rise as
combustion air temperature increases.
Studies indicate that a significant
market for high temperature heat re-
covery equipment for use with many
types of industrial sources will exist as
they become proven and are applied to
both new and existing sources. If these
devices, which can preheat combustion
air to 2000°F* and beyond, are applied
extensively, nationwide emissions of
NOX could increase significantly.
Under this work assignment, available
data on emission factors for major
categories of direct-fired heaters were
reviewed. Systematic studies were ana-
lyzed to develop emission factors for
NO, at various levels of combustion air
preheat used in major energy-consum-
ing industries.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
*Readers more familiar with the metric system may
use the conversion factors at the back of this
Summary.
mentedin a separate report of the same
title (see Project Report ordering in-
formation at back).
Introduction
The objective of this report is to provide
technical information on NO, emissions
attributable to energy-intensive direct-
fired process heaters. Specific tasks
undertaken to quantify current NO. emis-
sions and trends in NO, resulting from
technology changes were:
• Define energy-intensive direct-fired
process heaters.
• Describe the fundamentals of NO,
formation.
• Investigate technology changes lead-
ing to energy conservation.
• Determine the effect of energy con-
servation techniques, primarily com-
bustion air preheat, on NO, emissions.
• Develop NO, emission factors for
direct-fired process heaters employing
various levels of combustion air pre-
heat.
Energy-Intensive Heaters
Several energy-intensive industrial
processes were identified as capable of
producing and using high-temperature
preheated combustion air: steel reheat
furnaces; steel soaking pits; aluminum
reheat furnaces; aluminum melting fur-
naces; steel forging furnaces; annealing
furnaces; titanium melting furnaces;
nickel melting furnaces; cement kilns;
and glass melting furnaces. Although
preheated combustion air presents op-
portunities for energy savings in these
industrial process heaters, it also has the
drawback that increased combustion air
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temperatures lead to increases in thermal
NO, emitted from these stationary
sources. NO, emissions could increase
from an estimated 6.73 million tons in
1980 to an estimated 8.31 million tons in
1990 due in part to combustion air
preheat.
Fundamentals of IMOX Formation
Regardless of the type of combustion
device, NOX are formed in combustion
processes by the thermal fixation of
atmospheric nitrogen and by the con-
version of fuel-bound nitrogen. Total NO,
emissions and the contribution of each
mechanism to the total depend on the
heater design, heater operating condi-
tions, and the type of fuel burned.
Combustion generates high tempera-
tures that result in molecular dissociation
of the oxygen in the combustion air. The
separate oxygen atoms react with many
other atoms in the combustion reactions,
but some react with the otherwise stable
Nz molecule to form NO.
By using thermodynamic and chemical
kinetic analyses, it is possible to model
approximately the controlling influences
of combustion stoichiometry, tempera-
ture, and residence time, assuming one-
dimensional homogeneous air/fuel flow.
Explicitly, the theoretical model shows
that the rate of formation of NO is
sensitive to the mole fractions of oxygen
and nitrogen. Also, the model predicts the
extreme sensitivity of NO formation to
temperature: as the temperature in-
creases, the rate constants increase
exponentially. Since preheat of combus-
tion air can increase the peak flame
temperature in the combustion zone, air
preheat can increase thermal NO forma-
tion.
On practical equipment, combustion
fundamentals of thermal NO formation
include mixing, turbulence, heat transfer,
and composition, along with temperature
in the combustion zone. Thus, a quantita-
tive determination of thermal NO forma-
tion at various levels of combustion air
preheat must be based on specific furnace
designs and operating parameters.
In general, all fuel-bound nitrogen can
be assumed to be converted rapidly to NO.
As with thermal NO formation, the con-
version of fuel nitrogen depends on
combustion conditions. This dependency,
however, has been found to be less
sensitive to design and operating param-
eters. For this analysis, it is assumed that
all fuel-bound nitrogen is converted to NO
with no effect by combustion air temper-
ature.
Process Heater
Design and Operation
The primary function of process heater
systems is to provide controlled efficient
conversion of the chemical energy of fuel
into an energy form that can be trans-
ferred as heat to process fluids. Process
heaters do this by introducing the fuel
and air for combustion, mixing these
reactants, igniting the mixture, and dis-
tributing the flame envelope and the
products of combustion. A process heater
should function so that the fuel and air
input is ignited and burned continuously
and immediately upon its entry into the
furnace. The total fuel burning system
required to do this consists of subsystems
for air handling, fuel handling, ignition,
and combustion product removal, plus
the burners and the furnace. Assuming
the burners are chosen properly in fur-
nace system designs, trends in formation
of NOX can be evaluated for many types of
furnaces by analysis of a few basic burner
types.
As industries seek ways of reducing
operating costs by fuel conversion, oper-
ating parameters may be adjusted for
optimum fuel conservation. Optimization
of fuel use using heat recovery tech-
niques, however, may lead to increases in
pollutant emissions. Currently, one of the
more prevalent heat recovery techniques
involves preheating combustion air to
achieve greater furnace efficiency. This
air preheat also increases NO,formation
by increasing the temperature in the com-
bustion zone. A systematic study of
energy-intensive industries reveals two
broad, distinct classifications of industrial
processes capable of using preheated
combustion air. The steel, aluminum, and
other metal processing industries and
petroleum refining industries use baffle
burners capable of using air preheat in
the range of 800-1200°F. Furnaces
common in the glass manufacturing and
cement manufacturing industries can use
preheated combustion air in the range of
1600-2200°F.
Several major companies are involved
in the manufacture of burners capable of
accepting high temperature combustion
air. To evaluate the effect of combustion
air preheat on NO, emissions from direct-
fired process heaters, burners must be
selected as representative of those that
are used or potentially could be used for
the specific furnaces under consideration.
Steel reheating processes, steel soaking
processes, and aluminum melting pro-
cesses can be represented by the Bloom
hot air baffle burner, based on the extent
of its current use in these processes.
Because of process similarity and furnace
design and operation, the Bloom hot air
baffle burner would also be representa-
tive of the nickel and titanium melting
furnaces. The Bloom burner is also used
in refinery process heaters. The North
American refractory-lined burner is used
in the glass industry and, because of
similar process heat requirements, it may
also be considered as representative of
burners used in cement kilns. Thus, by
analysis of the various operating condi-
tions of two burners, the effect of com-
bustion air preheat on NO, emissions
from energy-intensive processes can be
estimated.
Effect of Heater Operation on
NOX Emissions
NOX emission factors have been devel-
oped at various combustion air preheat
levels for two furnaces representing the
broad classifications of direct-fired heat-
ers used in energy-intensive industries,
for a theoretically based model furnace,
and for prototype low-NO, burners capa-
ble of industrial use. Systematic studies
by the Institute of Gas Technology under
contract to the Gas Research Institute
and Southern California Gas Company
provided the basis for development of
these emission factors. These results
were used to develop NO, emissions
factors for the range of process heaters
used in energy-intensive industries at
various levels of combustion air preheat.
Table 1 gives the results of this analysis.
Conversion Factors
Readers more familiar with the metric
system may use the following factors to
convert to that system:
Nonmetric
Times
Yields
Metric
Btu
°F
Ib
ton
1.055
5/9(°F-32)
0.454
907.2
kJ
°C
kg
kg
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Table 1. Emission Factor Summary"
Furnace
Fuel
/VO, Emission Factors, Ib /V02/706
Preheat Temperature. °F
Btu
Steel Reheat
Steel Soaking Pit
Aluminum Reheat
Aluminum Melting
Steel Forging
Annealing
Titanium Me/ting
Nickel Melting
Cement Kiln
Glass Melting
Refinery Process
Coke Oven Gas
Natural Gas
Coke Oven Gas
Natural Gas
Residual Fuel Oil
Natural Gas
Residual Fuel Oil
Natural Gas
Natural Gas
Natural Gas
Natural Gas
Natural Gas
Natural Gas
Natural Gas
Natural Gas
800
0.34
0,34
0.34
O.34
0.64
0.34
0.64
0.34
0.34
0.34
0.34
0.34
—
—
0.34
1200
0.72
0.72
0.72
0.72
1.02
0.72
1.02
0.72
0.72
0.72
0.72
0.72
—
—
0.72
1600
1.40
1.40
1.40
1.40
1.70
1.40
1.70
1.40
1.40
1.40
1.40
1.40
0.80
0.80
1.40
2000
3.20
3.20
3.20
3.20
3.50
3.20
3. SO
3.20
3.20
3.20
3.20
3.20
1.80
1.80
3.20
'These emission factors are adequate for use in emission inventory preparation. Application of
these factors to specific heaters are subject to the following limitations:
(i) The emission factors are based only on experimental data from two types of industrial
furnaces, and they are suggested for application to other industrial furnaces based on
similarities in furnace design and operating conditions.
(ii) Except for the glass melting furnace and cement kiln, the emission factors for 1,600 and
2,000°F air preheat temperatures are extrapolated values.
(Hi) For residual fuel oil. the listed NO, emission factors are for 0.3 weight percent N content and
19.600 Btu/lb heating value, assuming 100% conversion of fuel N to NO.
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C. R. Newman is with GCA/Technology Division. Chapel Hill. NC27514.
J. David Mobley is the EPA Project Officer (see below).
The complete report, entitled "Assessment of NOX Emission Factors for Direct-
Fired Heaters," (Order No. PB86-119 112/AS; Cost: $11.95. subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-85/047
0000529 PS
U S 5NVIR 0TCTION
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