C? U A U.S. Environmental Protection Agency Industrial Environmental Research
ti • f^ Office of Research and Development Laboratory
Research Triangle Park. North Carolina 27711
BURNER DESIGN CRITERIA FOR
NOX CONTROL FROM LOW-BTU
GAS COMBUSTION: VOLUME II.
ELEVATED FUEL TEMPERATURE
Interagency
Energy-Environment
Research and Development
Program Report
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
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The nine series are.
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2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
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This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
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tems. The goal of the Program is to assure the rapid development of domestic
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essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects; assessments of, and development of, control technologies for energy
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REVIEW NOTICE
This report has been reviewed by the participating Federal Agencies, and approved
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This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600-7-77-094b
December 1977
BURNER DESIGN CRITERIA FOR NOX
CONTROL FROM LOW-BTU GAS
COMBUSTION: VOLUME II. ELEVATED
FUEL TEMPERATURE
by
Donald R. Shoffstall and Richard T. Waibel
Applied Combustion Research
Institute of Gas Technology
I IT Center, 3424 South State Street
Chicago, Illinois 60616
Contract No. 68-02-1360
Program Element No. EHE624a
EPA Project Officer: David G. Lachapelle
Industrial Environmental Research Laboratory
Office of Energy, Minerals and Industry
Research Triangle Park, N.C. 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, D.C. 20460
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ABSTRACT
This program was initiated to provide quantitative data on combustion emissions
from "high-temperature," low-Btu gas. It was intended to complement a recently
completed EPA project that evaluated emissions resulting from the burning of
"ambient-temperature," low-Btu gas. All the experimental results were gathered
from a pilot-scale furnace fired with a moveable vane boiler burner at a heat
input of 0.66 MW (2,250,000 Btu/hr). The gases tested in this program, Winkler
oxygen (WO), Koppers-Totzek oxygen (KTO),and Wellman-Galusha air (WGA),
ranged from 5.8 MJ/m3 (156 Btu/ft3) to 9.9 MJ/m3 (266 Btu/ft3). Measurements
were made of NO emissions, temperatures within the flame, and flame emissivity.
A mathematical model was used to predict the efficiencies of the furnace with
the various fuels and the model agreed well with the experimental measurements.
The NO emissions of the gases tested were ordered by the adiabatic flame
temperature.
ii
-------
TABLE OF CONTENTS
INTRODUCTION
SUMMARY •*
DESCRIPTION OF TEST FACILITY 5
Gas Generating System 5
Pilot-Scale Furnace 8
Analytical Test Equipment 15
NO and NO^ Instrumentation 1?
CO, CH^, and CO2 Measurements 18
Oxygen Measurements 18
Chromatographic Measurements 18
EXPERIMENTAL PLAN 24
NO Emission Levels 26
Temperature Profiles 41
Flame Emissivity Determination 42
DATA CORRELATION 59
NO Emissions 59
Furnace Performance 59
Mathematical Model of Furnace Performance 66
In-the-Flame Analysis ? 3
CONVERSION TABLE 16
ill
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FIGURES
No. Page
1 Schematic Diagram of IGT Gas-Generating System 6
2 Gas-Generating System 7
3 Rectangular Test Furnace 9
4 Removable Sidewall Furnace Panels for Interior
Flame Probing 10
5 Overall System Schematic Diagram of Rectangular
Test Furnace 11
6 Radiant Tube Preheater for Main Furnace
Combustion Air 13
7 Flue-Gas Cooler 14
8 Control Room Facility and Analytical Instrumentation 16
9 Gas-Sampling Probe Head for Nonparticulate
Flue Gases 20
10 ModifiedlFRF Temperature Probe 21
11 General Probe Holder 22
12 Pyroelectric Radiometer (Used for Emissivity
Measurement) 23
13 Diagram of Movable-Vane Boiler Burner 25
14 Method of Measuring Movable-Vane Angle for
Boiler Burner 25
15 NO Emissions Versus Fuel Gas Preheat With
Winkler Oxygen Gas 27
16 NO Emissions Versus Secondary Air Preheat
With Winkler Oxygen Gas 28
17 NO Emissions Versus Secondary Air Preheat
With Natural Gas 29
18 NO Emissions Versus FGR for "Ambient" Winkler
Oxygen Fuel 3 1
IV
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FIGURES, Cont.
No. Page
19 NO Emissions Versus FOR for Winkler Oxygen
Preheated to 800°F 3 2
20 NO Emissions Versus Fuel Gas Preheat With
Koppers-Totzek Oxygen Gas 33
21 NO Emissions Versus Secondary Air Preheat
With Koppers-Totzek Oxygen Gas 34
22 NO Emissions Versus FGR for "Ambient"
Koppers-Totzek Oxygen Fuel 3 6
23 NO Emissions Versus FGR for Koppers-Totzek
Oxygen Fuel Preheated to 800°F 3 7
24 NO Emissions Versus Fuel Gas Preheat With
Wellman-Galusha Air Gas 38
25 NO Emissions Versus Secondary Air Preheat With
Wellman-Galusha Air Gas 39
26 Arrhenius Plot of In [NOJ Versus Reciprocal of
the Adi aba tic Flame Temperature 40
27 Average Gas Temperature Profile for Natural Gas
With a 45-Degree Vane Rotation 43
28 Average Gas Temperature Profile for "Ambient"
Koppers-Totzek Oxygen Gas With a 45-Degree
Vane Rotation 44
29 Average Gas Temperature Profile for Preheated
Koppers-Totzek Oxygen Gas With a 45-Degree
Vane Rotation 45
30 Average Gas Temperature Profile for Preheated
Koppers-Totzek Oxygen Gas With 15% FGR With a
45-Degree Vane Rotation 46
31 Refractory Emissivity Versus Refractory Temperature 49
32 Gas Emissivity Versus Axial Position for Natural Gas 5 1
33 Gas Emissivity for "Ambient" Koppers-Totzek Oxygen Gas 52
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FIGURES, Cont.
No. Page
34 Gas Emissivity Versus Axial Position for Preheated
Koppers-Totzek Oxgyen Gas 5 •>
35 Gas Emissivity Versus Axial Position for Preheated
Koppers-Totzek Oxygen Gas With 15% FGR 54
36 Flame Flow Pattern Tested 56
37 NO Emissions Versus Firing Density for the
Pilot-Scale Furnace 60
38 Comparison of IGT Experimental and B&W Calculated
Furnace Efficiencies 69
39 Comparison of Experimental and Well Stirred Speckled-
Wall Model Calculated Furnace Efficiencies 7 1
40 Variation of Well Stirred Approximation with
Adiabatic Radiance 7 2
41 Comparison of Experimental and Non-Well Stirred
Speckled-Wall Model Calculated Furnace Efficiencies 74
vi
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TABLES
No. Page
1 Synopsis of Furnace Operating Conditions and
Test Results for a 45-Degree Vane Angle 2
2 Species Concentrations, Adiabatic Flame Temperatures,
and Gross Heating Value (Wet) for Medium- and Low-Btu Gases 3
3 Preheater Tube Lengths Versus Preheat Temperature
for Gases Tested 8
4 Calculated and Measured Emissivities 58
5 Projected NO Emission Levels for a Utility Boiler 6 1
6 Pilot-Scale Furnace Efficiencies for Preheated
Fuel Gases 6 2
7 B&W Calculated Boiler Efficiencies Using IGT Data 63
8 Calculated and Measured Radiant Section Exit Gas
Temperature and Emissivities 65
9 Comparison of Measured and Calculated Furnace
Efficiencies 7 5
10 Comparison of Calculated Emissivities 75
vii
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INTRODUCTION
The objective of this work was to provide and evaluate quantitative data on the
environmental quality of effluent combustion products when retrofitting a utility
boiler burner with "high-temperature," low-Btu gas. This program was intended to
compliment a recently completed EPA project, which evaluated the emissions
resulting from burning "ambient temperature," low-Btu gases.
Work is currently being done by government and industry to develop high-
temperature, post gasification sulfur cleanup systems. This is an attractive
development because it enables utilization of the product gas's sensible heat. It is
not obvious what effect these high temperature gases will have on NO emissions,
since the higher fuel temperature which results from using a high-temperature,
sulfur-removal system does not necessarily result in a higher adiabatic flame
temperature, due to the increased water vapor content of the gas. The steam
produced during gasification is not removed by cooling as with conventional
cleanup systems.
To ensure the immediate relevance of the program's results, only gas compositions
from commercially available coal conversion processes (Lurgi, Koppers-Totzek,
Winkler, and Wellman-Galusha) were considered. The fuel gases were selected for
testing based on the results of emission trials with ambient temperature fuel gas.
A summary of these test results is presented in Table 1. The gases selected for
"high-temperature" testing were Winkler oxygen (WO), Koppers-Totzek oxygen
(KTO), and Wellman-Galusha air (WGA). Koppers-Totzek oxygen and Winkler
oxygen gases had emissions equivalent to or greater than those from natural gas
combustion. Wellman-Galusha air gas had the highest level of NO emissions of the
air gases tested. Table 2 presents species concentrations for "high-temperature"
and "ambient-temperature," low-Btu gases. Emission trials were conducted with
these fuel gases preheated to 400°, 800°, and 1200°F. These fuel gases were
tested to evaluate changes in the level of NO emissions, temperatures within the
flame, flame emissivities, and furnace efficiencies when substituted for natural
gas on a utility boiler. The actual testing was done on a pilot-scale test furnace
modeled to simulate the radiant (furnace) section of a boiler.
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Table 1. SYNOPSIS OF FURNACE OPERATING CONDITIONS AND TEST RESULTS
FOR A 45-DEGREE VANE ANGLE
(Part 1)
Fuel
Natural gas
Lurgi oxygen
Winkler oxygen
Koppers-Totzek oxygen
Wellman-Galusha air
Winkler air
Gross heating
value,
Btu/SCF
1035.0
285.4
270.2
285.0
1 59.4
116.3
Adiabatic
flame
temp, °F
3337
3164
3329
3578
2948
2579
H2/CO
ratio
2.2
1.3
0.7
0.5
0.6
Fuel input
vel, ft/s
100.6
100.0
105.6
97.6
136.4
109.6
Combustion
air inlet
vel. ft/s
53.3
51.2
52.3
50.4
54.6
74.7
Table 1. SYNPOSIS OF FURNACE OPERATING CONDITIONS AND TEST RESULTS
FOR A 45-DEGREE VANE ANGLE
(Part 2)
Fuel
Natural gas
Lurgi oxygen
Winkler oxygen
Koppers-Totzek oxygen
Wellman-Galusha air
Winkler air
Gross heat
input,
Btu/hr
2,371,265
2,356,066
2,350,940
2,348,367
2,358,036
2,358.940
Flue products
N2
C02
H20
02
%
72.5
62.9
63.1
65.0
72.3
74.5
8.3
16.8
18.3
20.2
15.8
14.6
16.7
17.8
16.1
12.2
9.2
8.2
2.5
2.5
2.5
2.6
2.7
2.8
Vol of flue
products,
CFH at 60°F
27,357
27,565
26,169
24,356
32,772
38.339
SCFair
SCF fuel
11.23
2.71
2.43
2.51
1.54
1.13
SCF flue products
SCF fuel
12.26
3.41
3.06
3.07
2.33
1.96
Table 1. SYNPOSIS OF FURNACE OPERATING CONDITIONS AND TEST RESULTS
FOR A 45-DEGREE VANE ANGLE
(Part 3)
Fuel
Natural gas
Lurgi oxygen
Winkler oxygen
Koppers-Totzek oxygen
Wellman-Galusha air
Winkler air
Flue-gas
temp, °F
2553
2442
2523
2554
2434
2335
Rear-wall
temp, "F
2309
2300
2309
2327
2066
2012
Emissivity
Calc
0.159
0.194
0.189
0.174
0.154
0.150
Meas
0.177
0.185
0.218
0.190
0.190
0.170
Flue NO,
ppm
65
32
73
104
15
4
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Table 2. SPECIES CONCENTRATIONS, ADIABATIC FLAME TEMPERATURES,
AND GROSS HEATING VALUE (Wet) FOR MEDIUM- AND LOW-Btu GASES TESTED
Fuel
Koppers-Totzek oxygen
Winkler oxygen
Wellman-Galusha air
Temp, °F
90
1200
90
1200
90
1200
CO
H2
C02
CH4
N2
H2O
vol %
52.9
49.0
32.9
25.7
27.9
26.2
34.5
31.7
41.2
32.2
14.9
14.0
9.2
8.6
20.0
15.8
3.2
3.0
0.5
0.5
3.0
2.4
2.7
2.5
1.0
0.9
1.0
0.8
49.6
46.5
1.9
9.3
1.9
23.1
1.9
8.0
Adiabatic
flame
temp, °F
3578
3677
3327
3371
2948
3119
Heating
value,
Btu/SCF
284
266
269
212
166
156
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SUMMARY
All of the experimented results presented in this report were taken from a pilot-
scale furnace fired with a movable-vane boiler burner (MVBB) at a heat input of
j.
2.25 million Btu/hr. In addition to natural gas, three low-Btu gases were tested:
Winkler oxygen, Koppers-Totzek oxygen, and Wellman-Galusha air. The
combustion air was preheated to 325°F and was varied with test fuel gases to give
3% oxygen (dry) in the flue.
Based on a detailed analysis of the experimental data, the following conclusions
were made:
1. No concentration levels for the ambient and preheated low-Btu gases
tested were ordered by adiabatic flame temperature.
2. Of the fuel gases tested, only 800°F preheat Winkler oxygen gas with 15%
FOR and ambient Wellman-Galusha air had NO emission levels projected to
conform with the New Source Performance Standard of 168 ppm (0.2 lb/10
Btu) for gas-fired boilers.
3. The NO emission data yielded an activation energy of 195.2 kcal/mole.
These data also revealed a rate controlling reaction temperature that
provides an empirical method of predicting NO emission levels for a variety
of operating conditions including fuel preheat, air preheat, and external
flue-gas recirculation.
4. An iterative calculation technique using a non-well stirred reactor model
predicted furnace efficiencies for each test fuel in good agreement with
the experimental results.
* It is EPA policy to use metric units.' However, in this report,
English units are occasionally used for convenience. See conversion
table at the back of this volume.
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DESCRIPTION OF TEST FACILITY
GAS GENERATING SYSTEM
The low-Btu gases tested during this program were produced using the gas
generating and fuel preparation facility shown schematically in Figure 1. The
critical item in the gas supply system is the reformer, pictured in Figure 2. This
is a special gas generator that produces varying ratios of H^/CO. Natural gas and
carbon dioxide are fed to the reformer through a specially designed mixing tee.
Additionally, steam may also be mixed depending on the desired H^/CO ratio. The
resulting mixture is supplied to four reaction retorts contained in a vertical
cylindrical furnace, where the endothermic chemical reaction is completed at a
temperature of 2100°F. Globally, the reaction scheme is the water-gas shift. The
gas generated within the reaction tubes passes through water-jacketed coolers,
where it is quenched to prevent deterioration of the H^/CO ratio.
The reformed gas is compressed to 30 psig and pushed through an MEA (methyl-
ethyl-amineJ-CO?, absorbing tower. This tower is used to remove CO? from the
reformed gas if its concentration is above what is needed to to synthesize the
desired low-Btu gas. The reformed gas then enters a mixing chamber, where it is
blended with nitrogen, carbon dioxide, steam, and natural gas to reach the desired
mixture of gas to be modeled. The synthesized gas is then fed to the fuel preheat
furnace.
This preheat furnace has dimensions 3 feet x 4 feet x 4-1/2 feet. The 9-inch-thick
furnace walls are made of firebrick enclosed by 2-inch insulating board and a 1/4-
inch steel plate shell. This fuel preheater is mounted on 6-inch-diameter wheels,
which ride in a steel channel, allowing the distance between the fuel preheater
and pilot scale furnace to be varied. This variation is accomplished through a
pulley system and a 2-hp reversible electric motor.
Six inspirating burners (each of 200,000 Btu/hr) fire the preheater. A W-shaped
fuel preheater pipe made of 2-inch-diameter, 304 stainless steel pipe is inserted
into the preheater. This preheat pipe is fixed to the fuel injection nozzle. The
amount of fuel preheat pipe contained in the preheater can be varied by changing
the preheat furnace position relative to the pilot-scale test furnace. Given in
Table 3 are the gas types tested and the length of pipe in feet that must be
inserted into the preheater with 2000°F walls to achieve the indicated preheat
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STORAGE
BULK
EXISTING
FURNACE
STACK I
FURNACE
FLUE
LOW-Btu
GAS
NATURAL
GAS
DESULFURIZER 1
txJ-MXJ-
WATER
C02 J
COMPRESSOR
C°2 DEMINERALIZER''
(SURGE TANK)
in nIT
MEA SOLUTION
Figure 1. Schematic diagram of IGT gas-generating system
-------
-J
2b. Reformer controls and synthetic gas mixing panel
Za. Natural gas reforming system
Figure 2. Gas-generating system
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temperatures. After preheating, these synthesized gases are fed to the pilot-scale
furnace for combustion testing.
Table 3. PREHEATER TUBE LENGTHS
VERSUS PERHEAT TEMPERATURE
FOR GASES TESTED
Winkler oxygen
Koppers-Totzek oxygen
Wellman-Galusha air
Preheat temperature, °F
400
800
1200
ft
4
2.8
4.8
8
6.2
10.8
15
11.4
20
PILOT-SCALE FURNACE
The experimental work was conducted on a rectangular furnace with a 25-sq-ft
cross-sectional area and a 13-foot length. This furnace can be end- or sidewall-
fired at a rate of 4 million Btu/hr. The furnace is equipped for in-the-flame
sampling, preheated air, and flue-gas recirculation. (See Figure 3.) This furnace
is capable of operating at temperatures up to 3000°F or as low as 1600°F at a
constant (maximum) gas input of 4 million Btu/hr and up to 40% excess air.
Cooling is achieved with cooling coils embedded in the refractory walls. The
furnace is constructed completely of 9-inch-thick cast refractories, with
removable panels in one sidewall to permit insertion of sampling probes (Figure 4.)
The overall furnace system is shown schematically in Figure 5. The system is
flexible enough that the following operating parameters can be independently
varied:
• Heat input, to 4 million Btu/hr (8.0 million for certain burners)
• Air input, to 40% excess
• Heat losses , to the furnace walls by changing flow in water-cooling
tubes cast into the refractories
• Combustion air temperature, to 1000°F
• Flue-gas recirculation capability, to 35% of combustion air
e Furnace pressure, to +0.05 inch of water.
8
-------
•• '
,! ~.
Figure 3. Rectangular test furnace
-------
Figure 4. Removable sidewall furnace panels for interior flame probing
-------
SAFETY SHUTOFF-
V3J
HHf HEATER COMBUSTION ;
AIR BLOWER-F3
MANUAL SHUT-
FLOW CONTROL °ff -V7.V8
,(T2) VALVES-V9.VIO (PI] \ \
j. *£ ll-^t* A \ \_
PITo"o4|||l =£= METERING ORIFICE-OI
IL—
^ I WALL-COOLING TUBES,
' MAIN GAS
I SHUTOFF-VI
SAFETY
SHUTOFF-VZ
METERING PITOT
r—
ft WALL-COOLING
r-J SWITCHING VALVES
•1 T VIZ.VI3
1 d^METERlNG OR
I IP6I — — (T5)
•- ' IwALL-COOLI
WALL-COOLING
BLOWER-F3
,.. „ ING WATER
I -• 1 HEAT EXCHANGER
|— (T6)
FILTERED
AIR INTAKE
^FLUE-GAS COOLER R|VER-WAT[H
"r----r-T IHEAT EXCHANGER FLOWCONTROL
VOLVES-VI4.VI5
WALL-COOLING
PUUP-P3
'/RIVER WATER
LEGEND
$3 ..KM vu.-v
fl
Figure 5. Overall system schematic design of rectangular test furnace
-------
The combustion air for the main furnace can be preheated up to a temperature of
1000°F with a separately fired radiant tube air preheater. The radiant tube
furnace (Figure 6) consists of an insulated airtight steel chamber 4 feet high, 4
feet wide, and 16 feet long. As the combustion air to be preheated passes through
this chamber, it is heated by convection from three 6-inch-diameter "hairpin" gas-
fired radiant tubes.
The radiant tubes and refractory flow passages inside the preheater are arranged
to provide an S-shaped flow pattern, which maximizes residence time for heating
at the maximum allowable pressure drop (20 ounces) for which the flow pattern
will provide the necessary air flow of 75,000 SCF/hr.
The temperature of the air can be regulated by changing the heat input to the
radiant tubes. Ambient temperature air can be supplied by completely shutting
down the preheater or by directing the air through the preheater bypass pipe. The
bypass pipe was installed to allow working on the preheater without shutting down
the air supply to the main furnace. Air bypass is achieved by selective switching
of valves.
Flue products for recirculation back to the burner and main furnace are obtained
from the furnace itself. Flue products can be withdrawn from the furnace flue
passage just prior to the main furnace flue damper. Up to 14,000 SCF/hr of flue
products can be withdrawn from the flue, which provides a 30% recirculation
factor when the furnace is fired at 3.5 million Btu/hr with 20% excess air.
The main furnace flue products are actually pulled from the flue (Figure 5) by the
suction in the inlet side of the main furnace combustion air fan (F4). The flue
products enter the recirculation withdrawal and treatment system at about
2800°F through a short length of internally insulated steel duct. These hot gases
are cooled to about 1Z5°F in a packed-bed water cooler (Figure 7). Cooled city
water (about 70°F) is sprayed down on a bed of refractory packings as the hot
gases pass up through the packed bed. This cooling system lowers the water
content of the flue gas from about 0.008 to about 0.007 Ib/CF, which is the dew
point of the gases at about 125°F. (The lost water content can be added again
later in the system if experimental conditions require this treatment.) The cooled
gases then pass through a flow-control shutoff valve (V25 in Figure 5). This valve
controls the flue-gas flow rate, which regulates the percentage of recirculated
products. This valve is interlocked to an outlet temperature sensor on the gas
12
-------
AIR
INLET
(8 in.)
-16ft
REFRACTORY FLOW
V" PASSAGES
HOT AIR
OUTLET
(12 in. X 12 in.)
Wss*ssffSSSSSSsJs^siS£JfSsJJf}ssSs7JrssSfJssSfss srfssssssssJJJr
z
INTERNAL INSULATION
STEEL SHELL
RADIANT
BURNER
FLUE
COLLECTOR
FLUE
COLLECTOR
Figure 6. Radiant tube preheater for main furnace combustion air
-------
COOLING WATER SYSTEM
60 gpm AT 150 psig
SPRAY MANIFOLD
AND HEADS
COOLED
FLUE
GASES
WIRE MESH LIQUID
DEMISTTER
PERFORATED
2-in. WATER LEVEL
PACKED REFRACTORY
BED-80% VOIDS .
HOT(2800°F)
FLUE GASES
WATER OVERFLOW
TO DRAIN
Figure 7. Flue-gas cooler
14
-------
cooler. If the outlet temperature of the gases exceeds 150°F, which would
damage the combustion air fan, the control valve (VZ5) shuts down. This stops the
flow of flue gases. Beyond the flue-gas control valve, the flue gases are mixed
with the required amount of air for combustion. A control valve (VZ4) regulates
the amount of air pulled in by the fan. The total amount of air for combustion and
flue products is metered with an orifice plate (O7) at the outlet of the fan. The
flue products and air then pass into the air preheater or preheater bypass pipe.
The water used in the flue-gas cooler is clean city water, which is continuously
recycled. A water flow of 60 gpm is supplied at 150 psig by a turbine pump to a
series of spray heads in the gas cooler. The hot (ZOOop) spent water flows out of
the cooler into an atmospheric holding tank. This tank is equipped with a
constant-level overflow to the building drain. In this way, any condensed water
from the combustion is removed and disposed of in the sewer. The water in the
holding tank is periodically treated with sodium hydroxide to prevent acid buildup
in the water due to condensing flue-gas components. One such component
removed by the flue-gas cooler is NO^. The hot water from the water holding
tank is cycled through an American Standard heat exchanger capable of removing
1.5 million Btu/hr of heat from the water. Cooling in the heat exchanger is
provided by a flow of river water at a rate of about 150 gpm at 80 psig. The river
water is supplied by a river adjacent to the test facility through a service pump
(PI) maintained by IGT.
ANALYTICAL TEST EQUIPMENT
Figure 8 is an overall view of furnace controls and the analytical instrumentation
package. The equipment used for concentration measurements of chemical
species during this progam is listed below; these analyzers included the following
items:
1. Beckman 74Z Polarographic Oxygen (O?)
Z. Beckman Paramagnetic Oxygen (02)
3. Beckman NDIR Methane (CH4)
4. Beckman NDIR Carbon Monoxide (CO)
5. Beckman NDIR Carbon Dioxide (CO2)
6. Varian 1200 Flame lonization Chromatograph (Total HC and C^ to CQ)
15
-------
P-15-Z5
Figure 8. Control room facility and analytical instrumentation
16
-------
7. Beckman NDIR Nitric Oxide (NO)
8. Beckman UV-NO2
9. Hewlett-Packard Thermoconductivity Chromatography, Hydrogen (H2),
Nitrogen (N2), Argon (A2), CO, CO2, Cj to C5, Oxygen (O2)
10. Beckman Chemiluminescent NO-NO2
11. Data Integration System.
This instrumentation package allowed concentration measurements of the
following major components: 1) measurement of hydrocarbon compounds Cj to
Cg; 2) independent check of NO-NO2 Chemiluminescent with NDIR-NO and
NDUV-NO2; 3) independent check of paramagnetic O2, polarographic O2, NDIR-
CH4j NDIR-CO, and NDIR-COz with the respective chromatographic species
concentrations; and 4) measurement of hydrogen (H2), argon (A2), and nitrogen
(N2).
The following sections give a general description of the measurement system used
for this program.
NO AND NO2 INSTRUMENTATION
The Chemiluminescent NOX and NDUV-NO2 system was mounted in a roll-around
cabinet that could be placed out at the furnace. This was important in minimizing
sampling distances, which can affect accuracy. The Chemiluminescent unit was
equipped with a carbon converter. Test work by IGT and others has demonstrated
that in a reducing environment the carbon converter maintains a better conversion
efficiency than converters made of stainless steel, quartz, or molybdenum.
The instrumentation was calibrated by using both a permeation tube with a
controlled known release of NOX and certified prepared cylinders of NO and NO2
gases.
The sample gas was drawn from the furnace through a special alumina probe by a
Dia-Pump Model 08-800-73 all stainless-steel and Teflon® pump delivering
approximately 0.4 CF/min. (This sample delivery rate was dictated by the
requirements of the measuring instruments.) The sample is immediately passed
through a stainless-steel large-particle filter. Both the pump and filter were kept
above 100°C to prevent condensation of the water vapor inherent to combustion
products.
17
-------
CO, CH4, AND CO2 MEASUREMENTS
Nondispersive infrared analyzers were used for carbon monoxide, methane, and
carbon dioxide measurements. These analyzers do not affect the sample gas and
can be operated in series. They were calibrated by using certified gases with
known concentrations of the species being determined. The infrared analyzers
require a completely dry sample. Therefore, the sample was first passed through
a water trap and a 3 A molecular drying sieve. A small in-line filter was placed
immediately after the drying tube to trap particles of sieve that may be carried
over by the gas stream.
OXYGEN MEASUREMENTS
A portion of the "conditioned" sample gas is diverted from the NDIR units to a
Beckman Model 600 paramagnetic analyzer. A second oxygen analyzer, a
Beckman Model 742 polarographic, was used as a cross-check on the oxygen
concentration. The Model 742 analyzer has an advantage over the paramagnetic
in time response.
CHROMATOGRAPHIC MEASUREMENTS
As a detailed gas analysis was required, the sample was fed to a Hewlett-Packard
7620-A thermal conductivity chromatograph, which permitted concentration
evaluations of hydrogen, nitrogen, argon, oxygen, carbon monoxide, carbon
dioxide, and hydrocarbons Cj to Cg. To achieve separation of these species, a
helium carrier gas was used in conjunction with a Porapak Q column. Three
temperature program rates were also required, ranging from —100° to 300°C. A
sample loop volume of 100 ml was used to ensure linearity in the hydrogen
response for concentrations up to 60%.
For total hydrocarbon analysis, a Beckman hydrocarbon analyzer was used. A
detailed hydrocarbon analysis could be made using a Varian 1200 flame-ionization
chromatograph. All chromatographic readings were electronically integrated and
printed out as a function of resolution time.
In addition to flue-gas analysis, a major task of this program was to map profiles
of temperature, chemical species, and flame emissivity. Modified designs of the
International Flame Research Foundation (IFRF) were used to construct probes,
which enabled this type of data collection.
18
-------
Figure 9 shows the assembly drawing of the gas-sampling probe used in both the
flame and the flue. To minimize NO^ reduction in the probe, an alumina tube was
inserted for the first 18 inches and was joined to Teflon tubing to carry the gas
sample to the analyzers.
Temperature data were collected using a suction pyrometer; the design is
illustrated in Figure 10. A Pt-Pt Rh 10% thermocouple was used. The efficiency
of the pyrometer was measured at 96% with a 25-second response time.
The gas-sampling probe, suction pyrometer, and radiation cooling target were all
designed to be installed in the general probe holder shown in Figure 11.
To evaluate radiation intensity, which is needed for a determination of flame
emissivity, a PR 200 Pyroeletric Radiometer, manufactured by Molectron Corp. in
Sunnyvale, California, was used. Figure 12 shows the pyroelectric radiometer plus
radiation shield. This radiometer uses a permanently poled lithium tantalate
detector that is capable of resolving radiant power into the nanowatt range while
maintaining a continuous spectral response from the vacuum UV to 500y m. A
built-in optical calibration system in the form of a highly stable LED (light-
emitting diode) that is calibrated against an NBS traceable standard of total
irradiance permits a direct correlation of experimental data from different trials.
19
-------
l-l/2-in. 304 SS TUBING
•l/2-in. 304 SS TUBING
•I-3/4-in. 304 SS TUBING
-2-in. 304 SS TUBING
-2-1/2-in. 304 SS TUBING
3/4-in. 304 SS
TUBING
ALUMINA TUBING IN\
l/8-in. 304 SS TUBING
3/4-in. 304 SS TUBING
Figure 9. Gas sampling probe head for nonparticulate flue gases
-------
WATER
GAS
EXTRACTION
-GAS
-GAS
COOLING
'JACKET
THERMOCOUPLE
HOT JUNCTION"
ALUMINA y SILLIMANITE S REFRACTORY
SHEATH SHIELDS CEMENT PLUG'
SUCTION TIP FOR MEASUREMENTS IN
NATURAL GAS AND OIL FLAMES
SUCTION TIP FOR MEASUREMENTS IN
PULVERIZED-COAL FLAMES
Figure 10. Modified TFRF temperature probe
-------
r-0.0625 in. THICK
NJ
2.50 in. x II go
2.00 in. x 16 go
1.75 in. x II go
Figure 11. General probe holder
-------
L
Figure 1Z. Pyroelectric radiometer
(used for emissivity measurements)
23
-------
EXPERIMENTAL PLAN
All test data were collected using a single register movable-vane boiler burner
(MVBB) with an axial fuel injector. An assembly drawing of the MVBB tested is
shown in Figure 13. The combustion air enters perpendicular to the axes of the
burner and passes through a register of guide vanes that impart a degree of spin to
the air, dependent on the vane orientation.
Figure 14 illustrates how the angle of the movable vane is measured. The ratio of
the average tangential and radial velocity components at the exit of the movable-
vane register depends only on the geometric dimensions of the vanes in the axis
perpendicular cross section (assuming a negligible Reynolds number influence).
A straight pipe was used as the fuel injector, guaranteeing that the fuel had only
an axial velocity. The diameter of the fuel injector was varied for fuel type to
maintain an injection velocity of 100 ft/s at 70°F.
The burner block used during this program had a 30-degree divergent angle with a
15.2-cm (diameter) entrance and a 48.2-cm (diameter) exit to the furnace. To
maximize flame stability the throat nozzle position was used through this
program. The throat nozzle position is located 2.5 cm from the burner block
entrance within the 15.2-cm (diameter) refractory duct connecting the burner
with the block.
Burner operating conditions used throughout the program had the level of flue
oxygen fixed at 3% (dry) with the combustion air preheated to 325°F (unless
specified differently). These conditions are considered typical for utility boiler
burners.
Based on the overall program objective of evaluating changes in boiler
performance and NO emisson levels when retrofitting from natural gas to "high-
temperature" low-Btu gas, it was necessary to select an operating variable to
model the pilot-scale test furnace against a field service boiler. Several
parameters were considered, including operating wall temperature, heat
absorption profile, and firing density. It was decided to match the temperature of
the combustion products from natural gas at the pilot-scale furnace exit to the
temperature measured between the boiler and the secondary superheater section
24
-------
Figure 13. Diagram of movable-vane boiler burner
AIR FLOW
Figure 14. Method of measuring movable-vane angle for boiler burner
25
-------
of a utility boiler. This decision was based on the complexities of multiburner
interactions within a utility boiler versus the single-burner test furnace where
several flames would not view the boiler water walls. The data of most interest in
evaluating performance changes were gas volume, temperature, and gas emissivity
entering the secondary superheater. Thus, baseline data were collected for
natural gas after the furnace load had been adjusted to give an exit gas
temperature of 2550°F.
The first task to be conducted after the baseline furnace operating conditions had
been fixed was to determine the environmental impact of retrofitting a utility
boiler with "high-temperature" low-Btu gases.
NO EMISSION LEVELS
Figures 15 and 16 show the NO emission levels as a function of fuel preheat
temperature and secondary air preheat temperature for Winkler oxygen gas. A
comparison of the two figures shows that the effect of fuel preheat on NO
production is less dramatic than that of combustion air preheat. With fuel
preheat, the NO emissions increase by about 50% from a 400° to 800°F fuel
temperature, but increase about 100% for a similar increase in air preheat. A
comparison of adiabatic flame temperatures shows the flame temperature
increasing from 3171°F at 400°F fuel preheat temperature to 3371°F at 1200°F
fuel preheat temperature. Correspondingly, the adiabatic flame temperature with
air preheat increases from 3327°F at 325°F to 3562°F at 800°F secondary air
preheat temperature. The level of emissions from preheated Winkler oxygen
intermediate-Btu gas is less than that for ambient temperature gas at 325°F air
preheat temperature because the increased water content of the hot gas (23%
versus 2%) lowers the heating value and reduces the adiabatic flame temperature.
The adiabatic flame temperature for preheated fuel with 325°F secondary air does
not equal that for ambient Winkler oxygen with similar air preheat until a fuel
temperature of nearly 1200°F.
Figure 17 shows the NO emissions versus secondary air preheat for natural gas.
The natural gas adiabatic flame temperature is about 3680°F at 325°F and 3880°F
at 800°F secondary air preheat temperature. The NO emissions follow a
consistent trend, increasing as the adiabatic flame temperature rises.
26
-------
50
40-
£
a.
a
*
O
z
30-
20'
WINKLER OXYGEN
GAS NOZZLE, THROAT POSITION
O2 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
10
200 400 600 800 1000
FUEL PREHEAT TEMPERATURE, ° F
1200
Figure 15. NO emissions versus fuel gas preheat with
Winkler oxygen gas
27
-------
240-1
NO
00
200-
160-
o.
o
Z
120-
80-
40'
WINKLER OXYGEN, 830JSCF/hr
GAS NOZZLE THROAT POSITION
02 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
-0.3
•0.2 c
o
J:
z
-0.1
100
300 500
SECONDARY AIR PREHEAT, °F
700
900
Figure 16. NO emissions versus secondary air preheat with
Winkler oxygen gas
-------
240-
200-
160-
a
a 120-
o"
80H
40-
NATURALCAS, 2174SCF/hr
CAS NOZZLE THROAT POSITION
02 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
T~
100
~I 1 \
300 500
SECONDARY AIR PREHEAT, °F
700
- 0.3
0.2 §
|
5
-q.i
900
Figure 17. NO emissions versus secondary air preheat with natural gas
-------
It was demonstrated during a prior phase of this program (EPA Report No. 600/2-
76-098a) that external flue gas recirculation (FGR) was an effective means of
controlling NOX emissions from natural gas flames. Additionally, it was shown
that combustion aerodynamics could be used to recirculate combustion products to
the base of the flame within the furnace, resulting in reduced NOX levels. Using
this control technique, labeled "internal" FGR, reductions of up to 70% of those
achieved using external FGR were possible. The trial sequence required to
evaluate internal recirculation is extremely involved because of the number of
independent variables involved, e.g., fuel injection, injector position, burner block
angle, movable-vane rotation, and wall temperature. Thus, as a first attempt at
evaluating the effectiveness of using combustion aerodynamics as a control
technique, it was necessary to quantify the maximum reduction possible. This was
done using external FGR.
The percentage flue-gas recirculation is defined by the relationship:
°f Products Recirculated -
FGR =
Vol of Air + Vol of Fuel
Figures 18 and 19 show the NO emissions versus FGR for ambient temperatures
and preheated Winkler oxygen intermediate-Btu gas. For the ambient gas, the
emissions reduction is from 70% to 90%, while for the preheated fuels, the levels
were reduced by 60% to 70% at 15% FGR. Comparing the absolute levels, the
emissions for the preheated fuel are lower than those found for the ambient fuel.
The adiabatic flame temperature varies from 3268°F at 0% FGR to 2933°F at
15% FGR for the preheated fuel at 800°F and from 3327°F at 0% FGR to 3007°F
at 15% FGR for the ambient fuel. The extra water content of the preheated fuel
can be viewed as equivalent to additional FGR. On a volume basis, it is equivalent
to 8% FGR. Since the specific heat of water is about 11% higher than that of
CO^/N^ flue-gas mixture, it is equivalent to 9% FGR on a thermal basis.
Although this argument appears valid with zero external FGR, the effect of the
fuel water content appears to diminish as FGR is increased.
Figures 20 and 21 show the effect of fuel preheat and secondary air preheat on NO
emissions from Koppers-Totzek oxygen (KTO) medium-Btu gas. The NO emissions
increase about 15% for the 45 and 60-degree vane angle rotations, about 50% for
the 15-degree angle, and about 100% for the 30-degree vane angle as the fuel
30
-------
60
I
a
*
O
2
WINKLER OXYGEN
GAS NOZZLE, THROAT POSITION
O2 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
O 60°
V 45°
A 30°
D 15"
5 10 15 20
FLUE GAS RECIRCULATION, %
Figure 18. NO emissions versus FOR for "ambient" Winkler oxygen fuel
31
-------
WINKLER OXYGEN
GAS NOZZLE, THROAT POSITION
O2 IN FLUE, 3%
BURNER BLOCK ANGLE, 30"
VANE ROTATION AS LABELED
FUEL TEMPERATURE, 800° F
5 10 15
FLUE GAS RECIRCULATION, %
20
Figure 19. NO emissions versus FGR for Winkler oxygen
preheated to 800°F
32
-------
200.
I
a
O
2
KOPPERS-TOTZEK OXYGEN
GAS NOZZLE, THROAT POSITION
O2 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
O 60°
V45°
A 30°
D 15°
100'
200
400 600 800 1000
FUEL PREHEAT TEMPERATURE, ' F
1200
Figure 20. NO emissions versus fuel gas preheat with
Koppers-Totzek oxygen gas
33
-------
400-
300-
£
a
a
O
200-
100-
KOPPERS-TOTZEK OXYGEN, 7679 SCF/hr
GAS NOZZLE THROAT POSITION
02 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
—O
_0.6
-0.5
4-0-4 g
OQ
O
4-0.3
-o.z
-0.1
tf
I
100
I
300
I I I
500
SECONDARY AIR PREHEAT, °F
I
700
900
Figure Zl. NO emissions versus secondary air preheat
with Koppers-Totzek oxygen gas
-------
preheat increases from 400° to 1200°F. These increases in emissions are less than
the 200% to 300% increases seen for the air preheat trials. The only exception is
the 50% emissions increase seen for the 15-degree angle during the air preheat
trials. The adiabatic flame temperatures for preheated fuel vary from 3537°F at
400°F to 3667°F at 1200°F. With air preheat, the flame temperature increases
from 3578°F at 325°F to 3745°F at 800°F. The adiabatic flame temperature for
preheated fuel is high for KTO because the water content goes from only 1.9% to
9.3% comparing ambient with hot gas. A comparison of emissions with those
previously seen for natural gas shows that they follow the trend suggested by the
adiabatic flame temperatures, i.e., higher emissions with higher adiabatic flame
temperature.
Figures 22 and 23 show the NO emissions versus FGR for ambient and preheated
KTO intermediate-Btu gas. The emissions are reduced by 50% to 60% at 15%
FGR for the ambient gas, which is comparable to the 60% to 65% reduction found
in previous studies with natural gas. With fuel preheated to 800°F, the reduction
in emissions is again 50% to 60% at 15% FGR except for the 45-degree vane angle
data. In general, the emissions are higher than those seen for ambient KTO. The
adiabatic flame temperature varies from 3578°F at 0% FGR to 3191°F at 15%
FGR for ambient KTO and from 3605°F at 0% FGR to 3335°F at 15% FGR for
KTO preheated to 800°F. These emissions data follow the trend of increased
emissions with increased adiabatic flame temperature.
Figures 24 and 25 show the NO emissions from Wellman-Galusha air (WGA) low-
Btu gas versus fuel preheat and secondary air preheat. Here the NO emissions
double with a fuel preheat temperature increase from 400° to 1000°F. For a
similar increase in secondary air preheat, the NO emissions double for the 45 to
60-degree vane angles and increase by 50% and 20% for the 30 and 15-degree
angles, respectively. The adiabatic flame temperature varies from 2916°F at
400°F to 3119°F at 1200°F fuel gas preheat. With secondary air preheat, the
flame temperatures go from 2948°F at 325°F to 3112°F at 800°F air preheat
temperature. The absolute levels for the WGA low-Btu gas NO emissions are
lower than those seen in Figure 17 for natural gas. Because of the low value of
the WGA low-Btu gas NO emissions, no FGR trials were run with this fuel.
Figure 26 shows a plot of In [NO) versus the reciprocal of the adiabatic flame
temperature, T^p. Included are data for the preheated fuels with and without
35
-------
280
KOPPERS-TOTZEK OXYGEN
GAS NOZZLE, THROAT POSITION
02 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
o-v
FLUE GAS RECIRCULATION, %
Figure 22. NO emissions versus FOR for "ambient"
Koppers-Totzek oxygen fuel
36
-------
KOPPERS-TOTZEK OXYGEN
GAS NOZZLE, THROAT POSITION
O2 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
FUEL TEMPERATURE, 800° F
100 <)
a
a
•>
O
z
10 20
FLUE GAS RECIRCULATION, %
30
Figure 23. NO emissions versus FGR for Koppers-Totzek oxygen
fuel preheated to 800°F
37
-------
25
20
E
a
a
•k
O
2
15
10
WELLMAN-GALUSHA AIR
GAS NOZZLE, THROAT POSITION
O2 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
200 400 600 800 1000
FUEL PREHEAT TEMPERATURE, ° F
Figure 24. NO emissions versus fuel gas preheat with
Wellman-Galusha air gas
38
-------
30-.
25-
20-
g
a
u>
VO
O
10-
WELLMAN-GALUSHA AIR, 13,554SCF/hr
GAS NOZZLE THROAT POSITION
5-| O2 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
-0.050
2
ta
e
o
-0.025
100
300
500
SECONDARY AIR PREHEAT, °F
700
Figure 25. NO emissions versus secondary air preheat with
Wellman-Galusha air gas
900
-------
a
a 4.
O
J5
O WINKLER OXYGEN
A KOPPERS-TOTZEK OXYGEN
Q WELLMAN-GALUSHA AIR
V NATURAL GAS
LURGI OXYGEN
olP
E = 195. 2 KCAL/MOLE
0.24
0.25
0.26
0.27
1000/T
0.28
AF'
0.29
0.30
Figure 26. Arrhenius plot of In [NO] versus reciprocal of
the adiabatic flame temperature
-------
FGR and also data from the ambient fuel tests reported earlier. The good
correlation obtained suggests that the reaction can be described by an Arrhenius
rate expression that uses the adiabatic flame temperature:
in [NO] « -EA/RTAF (1)
Further, if we accept that the true activation energy is 134.7 kcal/mole, as
recommended by Thompson, Brown, and Beer*, we can state the following —
-134.7 = -134.7 (2)
RTRXN RCTAF
so that the rate-controlling reaction temperature is a constant factor of the
adiabatic flame temperature. Then, from our plot -
(3)
and thus —
C = 0.690 (4)
Our plot, therefore, would suggest that the rate-controlling reaction
temperatures, Tp^N* equal 0.69 T^p. Such temperatures are low compared to
those seen in the furnace. This plot, however, appears to be a good empirical
method of predicting NO emissions as a function of operating variables, including
fuel and air preheat and FGR.
TEMPERATURE PROFILES
To determine the difference in flame temperature reached within the furnace,
temperature profiles were taken as a function of fuel type and fuel input
conditions. These data were collected for operating conditions yielding the
highest measured level of NO emissions. The fuels tested included natural gas
(baseline conditions), Koppers-Totzek oxygen, Koppers-Totzek oxygen preheated
to 800°F, and Koppers-Totzek oxygen preheated to 800°F with 15% flue-gas
recirculation.
All gas temperature mesurements reported were gathered using a suction
pyrometer. The head of the pyrometer consisted of a thermocouple (Pt-Pt Rh
Thompson, D., Brown, T. D. and Beer, J. M., "NO- Formation in
Combustion," Combust. Flame 20, 69-79 (1972) September.
41
-------
10%) protected by refractory radiation shields. These shields are fitted to the end
of a water-cooled probe through which gases are sucked and the thermocouple
wires passed. The efficiency of the pyrometer was determined to be 99% using
the technique presented by T. Land (Instruments and Automation. Vol. 29, No. 7,
1956).
Baseline data were collected for natural gas at seven axial positions for a
45-degree burner vane rotation. These data are plotted in Figure 27. The axis of
the flame was located at the 15.2-cm axial position by making several radial scans
at different vertical positions. The flame center line was then confirmed by gas
analysis.
Koppers-Totzek oxygen (KTO) has a 241°F higher adiabatic flame (3578°F)
temperature than natural gas (3337°F). The KTO yielded a 110°F higher
maximum average temperature than natural gas. This difference has been
reduced to 70°F in the flue due to wall-gas temperature equilibrium. These
temperature profiles are shown in Figure 28.
Koppers-Totzek oxygen gas preheated to 800 F (3605°F adiabatic flame
temperature) temperature data are presented in Figure 29. The maximum average
temperature is 250°F higher than natural gas. The flue-gas temperature is 240°F
higher, while the wall temperature is 198°F higher than those temperatures
measured during the combustion of natural gas.
Adding 15% FGR to the Koppers-Totzek oxygen gas preheated to 800°F (KTO
800°F) produced the temperature profile data illustrated in Figure 30. The
adiabatic flame temperature (3335°F) is comparable to that of natural gas
(3337°F). The maximum average temperature was 37°F higher than for natural
gas, while the flue-gas temperatures were similar.
FLAME EMISSIVITY DETERMINATION
Using the pyroelectric radiometer with a 16-inch-diameter cooling target, data
were collected for evaluating gas absorptivity. The Schmidt method was
employed to reduce the measured data into absorptivities. The following
measurements were made for each data point:
R! = radiation intensity of flame backed by a cold black target
= e£Ef
R2 = radiation intensity of a hot black target
= Er
42
-------
3100-,
2900-
2700-
2500-
H
<
H
Ul
<
O
UJ
0
<
25
u
-LECENI
2300
60
3000—1
2800-
AXIAL POSITION, cm
O 15.2 O 146. 1
V 57. 2 < 219.1
A105.2 D310.8
O395.6
20
i
40
.0
0-^0
2600-
2400-
2200
1^
40
20 £
RADIAL POSITION, cm
I
20
40
Figure 27. Average gas temperature profile for natural gas
with a 45-degree vane rotation
43
-------
3300-
3100-
2900-
2700-
u
Ct
D
H
g 2500-
o,
w
O 3200-
W
O
Bi
w
< 3000-
AXIAL POSITION, cm
O 15.2 O 146.1
V 57. 2 < 219. 1
A 105. 2 Q 310.8
<>395.6
2467
2800-
2600-
2400-
I
60
f I I I I I I
40 20 ^ 20
RADIAL POSITION, cm
i
40
Figure 28. Average gas temperature profile for "ambient"
Koppers-Totzek oxygen gas with a 45-degree vane rotation
44
-------
W
K
K
u
W
H
o
W
3
3300^
3100-
Z900-
Z700-
2500-
2300
3100—1
2900-
2700 —
2500-r
AXI^L POSITION, cm
O 15.2 O 146. 1
V 57.2 < 219. 1
A105.2 D 310.8
O 395.6
RADIAL POSITION, cm
Figure Z9> Average gas temperature profile for preheated
Koppers-Totzek oxygen gas with a 45-degree vane rotation
45
-------
3000-1
2600-
2200^
1800-
W
K
K
W
OH
s
H
H
O
W
O 3200-
K
W--
2800-
2400-
60
2000-
1
60
1
40
1
20
i
(£,
i
20
i
40
I
60
LEGEND
AXIAL POSITION, cm
O 15. 2 O 146. 1
V 57.2 < 219. 1
A 105.2 D 310.8
. <>395.6
40
I
I
20
I I I I
20 (£
RADIAL POSITION, cm
I
40
I
60
Figure 30. Average gas temperature profile for preheated
Koppers-Totzek oxygen gas with 15% FGR
with a 45-degree vane rotation
46
-------
^3 = radiation intensity of a flame backed by a hot black target
= 6fEf + (1 - af) Er
where 6f is the flame emissivity, af is the flame absorptivity for radiation
originating at the hot target, and Ef and Er are the black body emissive powers
(a T4) at the flame and hot target temperatures. It follows that -
Ro - Ri
af = l - -^ -
As the target temperature Tr is usually colder than the flame temperature Tf, and
given the properties of nonluminous flame radiation, it is expected that af >er.
The relationship between af and ef can be found in Hottel and Sarofim2 as -
_ T a + b — c
t !•
ef Tf
Values of a+b-c can be estimated from Hottel and Sarofim's plots of CO^ and
H20.
R£ is measured from a hot refractory surface that is not black; thus the measured
radiation intensity contains reflected as well as emitted radiation. When siting on
a refractory of reflectivity P r, one measures a leaving flux density Wr (the
apparent emissive power) equal to the sum of the emitted radiation erEr, and the
reflected radiation prHr is the flux density incident on the refractory in question.
When there is no flame, the value of Hf is simply the flux density Wf leaving the
refractory surfaces viewed from the reference spot. When there is a flame, the
value of Hr is equal to the sum of the flame radiation e fEf and the radiation
received from other refractory surfaces after attenuation by the flame
(1 - £f)Wr. The three measurements for the Schmidt are then —
1. Flame with cold black background: R, = EfEf
2. Hot refractory: R2 = Wf = £rEr + PrHr = £fEr + P rWf = Ep^
3. Flame backed by hot refractory: R^ = e fEf + (1 - ef)W*r
Hottel, H. C. and Sarofim, A. F., Radiative Transfer, 300, New York:
McGraw-Hill, 1967.
47
-------
EfEf + (1 - Ef)
EfE£ + (1 - Ef> {e^ + pr [ EfEf + (1 - Ef)W*r]
= EfEf + (1 - Ef) {EfEr + pr [ EfEf + (1 - Ef)
Applying the Schmidt method as before:
R3 -
af=l - J—
R2
the following expression for absorptivity is derived:
E
F (e_ + P e, p-
f r
a, = 1 — (1 — e,.) {e + P [ e -=— + (1 — e,) ( —r\ —
f f r r f E f 1 — p_ (1 — e.
(5)
To quantify the refractory reflectivity, the radiometer was sited on a hot
refractory furnace sidewall through the general probe holder (Figure 11) and then
with the probe holder removed. All precautions were taken to ensure that the
radiometer view angle was identical both with and without the probe holder and
that the cold probe holder did not change the refractory temperature during the
measurements. Figure 31 presents data collected during these reflectance trials
plotted against the refractory emissivity data proved by Babcock and Wilcox (solid
line).
To evaluate the gas composition emissivity using Equation 5, the following
experimental data must be collected: 1) Rj, R2> and Rj are measured using the
pyroelectric radiometer and water-cooled black target; 2) a£ is then calculated
using the Schmidt technique; and 3) the furnace wall temperature is measured
using an optical pyrometer. These measured data are used in conjuction with
Figure 31 to evaluate £ f and Pf (refractory emissivity and reflectivity). The
final item of information needed to use Equation 5 is the flame temperature (Tj).
48
-------
VO
0. 65
0.40
O
1800
2000 2200
REFRACTORY TEMPERATURE, °F
2400
2600
Figure 31. Refractory emissivity versus refractory temperature
-------
These data were presented in Figures 21 through 30. A review of these
temperature profiles points out the large radial temperature gradients that occur
near the furnace burner wall. Because the analysis technique is built around a
uniform radial temperature, the reliability of the emissivities at the front of the
furnace is very small. However, as the gases approach the furnace back wall
(flue), these gradients decrease until, at the last axial position where data are
collected (395.6 cm from the furnace burner wall), the temperature profile is flat.
The flame temperature used in evaluating emissivity is an average of the radial
temperature measurements weighted by their annular area. These values (average
flame temperature Tf) refractory temperature Tr, refractory emissivity er, and
refractory reflectivity p f) are plugged into Equation 5, leaving two values to be
determined: a.^ and e^. A value for the flame emissivity is assumed, which
allows the flame absorptivity to be estimated. This estimated value for a^ is
compared with the measured value. If the two values do not correspond, an
iterative procedure is started and continued until satisfactory agreement has been
reached.
The emissivity profiles evaluated for natural gas, Koppers-Totzek oxygen,
Koppers-Totzek oxygen preheated to 800°F, and Koppers-Totzek oxygen
preheated to 800°F with 15% EFGR are presented in Figure 32 through 35.
The variation of emissivity along the furnace length was observed for all fuel
gases tested. Because gas emissivity is a function of composition, temperature,
and path length, the variation of emissivity along the furnace axis can best be
explained as an aerodynamic effect.
A general aerodynamic characterization of a flame can be made by determining
the different types of flow patterns that exist within a combustion chamber. A
detailed flow analysis of a confined flame reveals that the front section of a
combustion chamber can be divided into four zones: primary jet, primary
recirculation, secondary jet, and secondary recirculation. The primary and
secondary jets contain only particles with a forward flow direction (away from the
burner), whereas the recirculation zones contain gas particles moving in the
reverse flow direction (back toward the burner). The size, shape, and particle
density of the recirculation zones are determined by the velocity, the ratio of gas
to air, the spin intensity of the secondary jet, the burner-block angle, and, for the
50
-------
0.2—1
15°
w
O
o.i—
01
NATURALGAS. 2174SCF/hr
CAS NOZZLE THROAT POSITION
02 IN FLUE. 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
I
50
100
150 200
AXIAL POSITION, cm
250
300
350
400
Figure 32. Gas emissivity versus axial position for natural gas
-------
O.Z—i
H
HH
w 0.1-
o
Ln
I
50
KOPPERS-TOTZEK OXYGEN, 7679 SCF/hr
GAS NOZZLE THROAT POSITION
O2 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION AS LABELED
I
100
150
I
ZOO
250
I
300
350
I
400
AXIAL POSITION, cm
Figure 33. Gas emissivity versus axial position for "ambient"
Koppers-Totzek oxygen gas
-------
O.Z
01
u>
H
t—I
>
I—I
w
Cfl
t—*
w
CO
O
0. 1
KOPPERS-TOTZEK OXYGEN
GAS NOZZLE, THROAT POSITION
O2 IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION, 45°
FUEL TEMPERATURE, 800° F
100 200
AXIAL POSITION, cm
300
400
Figure 34. Gas emissivity versus axial position for preheated
Koppers-Totzek oxygen gas
-------
0.2 •
H
I—(
>
w
0. 1
KOPPERS-TOTZEK OXYGEN, 14% FGR
GAS NOZZLE, THROAT POSITION
Oz IN FLUE, 3%
BURNER BLOCK ANGLE, 30°
VANE ROTATION, 45°
FUEL TEMPERATURE, 800° F
100 200
AXIAL POSITION, cm
300
400
Figure 35. Gas emissivity versus axial position for preheated
Koppers-Totzek oxygen gas with 15% FGR
-------
secondary recirculation zone, the size and shape of the combustion chamber.
Figure 36 shows the type of flow pattern that was observed during this
investigation. This flow pattern is conventionally labeled Type n.
A Type H flow pattern is generated when the secondary jet has a tangential
velocity component large enough to cause the particles to adhere to and pack
tightly against the burner block. This packing creates a low or negative pressure
region in the center of the burner block. The pressure differential between the
furnace and the central region of the burner block causes gas molecules to be
pulled into this region and back toward the burner, thus creating the primary
recirculation zone. When the velocity of the primary jet is greater than the
velocity of the recirculating gases in the primary recirculation zone, the primary
jet penetrates this reverse flow region, and a recirculation lobe occurs on each
side of the burner axis.
A gas analysis of the secondary recirculation zone reveals species concentrations
similar to those measured at the furnace flue. Thus, an emissivity measured at
the furnace front wall would be an addition of the flame plus secondary
recirculation zones. The emissivity from the recirculation zone is approximately
that measured at the rear of the furnace. As the flame spreads, the optical path
of the secondary recirculation zone is decreased, reducing the concentration of
radiating sources. As the combustion continues, the concentrations of CO2 an^-
H2O along the optical path are increased as products of combustion. At the end
of the furnace, the gas temperatures and species concentrations across the width
of the furnace are nearly uniform, and the emissivity is as if from a static mixture
at a uniform temperature and composition. This is confirmed by the emissivities
measured for natural gas combustion (Figure 3Z), where the measured emissivity
at the front of the furnace is different for each vane angle tested; toward the end
of the combustion chamber, however, all the values converged to approximately
0.177.
The variation of emissivity near the furnace front wall as a function of vane angle
occurs due to the tangential velocity component of the secondary air — that is,
how fast the vortex of combustion air will spread upon leaving the burner block.
The larger the tangential velocity component, the higher the rate of post burner-
block expansion and the smaller the contribution of the secondary recirculation
zone to the measured emissivity.
55
-------
Ul
TYPE II
LOW SWIRL INTENSITY
PRIMARY JET VELOCITY
SECONDARY JET VELOCITY
Figure 36. Flame flow pattern tested
-------
Another possibility is that af (which is the value measured) increases for some
distance into the furnace as a consequence of the presence of either soot or cold
pockets of absorbing gas.
Emissivities were calculated using the measured flue-gas temperatures and
compositions for a 45-degree vane angle rotation and CO2 and H^O vapor
emissivities found in Radiative Transfer (Hottel and Sarofim). These calculated
values are listed in Table 4. They spread from a minimum of 0.159 for natural gas
to a maximum of 0.190 for preheated Koppers-Totzek with 15% FOR. These
calculated values are independent of burner operating conditions and the furnace
geometry is considered only when evaluating the mean beam length. Since the
radiometer viewed a collimated beam, the furnace width corresponds to the beam
length.
A second calculation technique developed by Leckner^ using NASA data was also
employed. This method is based on a statistical model using spectral data to
evaluate total emissivities of carbon dioxide and water vapor in homogenous
gases. For carbon dioxide emissivity there is good agreement with Hottel's
emissivities; however, for water vapor, Leckner's emissivities at temperatures
above 1650°F are consistently higher and show that the partial pressure
connection factor is temperature-dependent. This effect is shown in Table 4 with
Leckner's emissivities larger than Hottel's for all the fuel gases tested. The
experimentally evaluated emissivities were between the theoretically determined
emissivities.
These experimental data do provide a basis for recommending the use of
calculated emissivities for extrapolating to the dimensions of commercial boilers.
2 Hottel, H. C. and Sarofim, A. G., Radiative Transfer, 2Z4-37 ,New York:
McGraw-Hill, 1967.
3 Leckner, B., Combustion and Flame 19:33, 197Z.
57
-------
Table 4. CALCULATED AND MEASURED EMISSIVITIES
€calc
ameas
emeas
Difference
calculated emissivity
measured absorptivity
measured emissivity
I 6calc ~cmeas I
€calc
Fuel
Natural gas
Koppers-Totzek
oxygen
Koppers-Totzek
oxygen, 800°F
Koppers-Totzek
oxygen, 800°F
plus 15% FGR
Pco2
0.083
0.204
0.198
0.198
PH^
0.167
0.121
0.143
0.143
Gas temp,
°R
3013
3014
3256
2996
ameas
0.131
0.145
0.129
0.164
€meas
0.177
0.190
0.192
0.215
Hottel-Sarofim
Emissivity
0.159
0.174
0.161
0.190
%Ae
+ 11
+ 9
+ 19
+ 13
Leckner
Emissivity
0.215
0.211
0.202
0.215
%Ac
-18
-10
-5
0
oo
-------
DATA CORRELATION
NO EMISSIONS
Due to the size of the gas-generating system for synthesizing the medium- and
low-Btu test gases, the firing density of the pilot-scale test furnace could not
exceed 6923 Btu/ft^-hr. To permit a better correlation as to the level of
emissions that could be anticipated from a utility boiler, natural gas was fired
with inputs up to 3.75 million Btu/hr. This corresponds to a firing density of
11,538 Btu/ft3-hr, which produced a NO emission level of 225 ppm compared with
the New Source Perfomance Standard for natural gas at 1000 Btu/SCF of 168 ppm
(0.2 pounds NO2/million Btu heat input). These data are graphed in Figure 37.
A 300-MW utility boiler with a radiant chamber 30 x 60 feet in cross section and
80 feet high is fired with 2.86 X 10^ Btu/hr. The firing density for a boiler of this
size at this firing rate is 19,860 Btu/ft -hr. Extrapolating the data of Figure 37
yields a NO emission level for the 19,860 Btu/ft -hr firing density of 458 ppm. To
quantify the anticipated NO emission levels from this 300-MW boiler for each of
the fuel gases tested, it is assumed that an identical firing density-to-NO emission
level relationship would occur for each medium- and low-Btu gas as is represented
for natural gas in Figure 37. From this assumption, the NO emission levels
presented in Table 5 were projected.
Thus, only two fuel gases tested were projected to comply with the 0.2 Ib NO^ per
million Btu performance standard with the burner system operating normally.
High temperature (800°F) Winkler oxygen combustion with 15% FOR in the
combustion air will meet the existing standard. The compliance results from a
2933°F adiabatic flame temperature which is 335°F lower than high temperature
Winkler oxygen gas with no FGR, and 394°F lower than ambient Winkler oxygen
gas. Thus it appears that all gases with an adiabatic flame temperature above
3250°F will require modifications to the burner/boiler system in order to comply
with the source performance standards.
FURNACE PERFORMANCE
The efficiencies evaluated on the pilot-scale furnace for each of the fuels tested
are listed in Table 6. These efficiencies (defined as the fraction of total input
59
-------
400-.
300 -
e
a.
a
Z*
g
H
w
u
O 200
O
W
O
100
- 0.3
- 0.2
3
CQ
c
O
I
I
.O
- 0.1
FUEL: NATURAL GAS
4.0 8.0 12.0 16.0
FIRING DENSITY, Btu/ft3 x 103
20.0
24.0
Figure 37. NO emissions versus firing density for
the pilot-scale furnace
60
-------
Table 5. PROJECTED NO EMISSION
LEVELS FOR A UTILITY BOILER
Fuel
Natural gas
Winkler oxygen
Winkler oxygen,
800 °F preheat
Winkler oxygen,
800 °F preheat plus 15% FOR
Koppers-Totzek oxygen
Koppers-Totzek oxygen,
800 °F preheat
Koppers-Totzek oxygen,
800 °F preheat plus 15% FOR
Wellman-Galusha air
Wellman-Galusha air,
800 °F preheat
NO, ppm
458
514
183
70
733
972
317
106
113
lbNO2/106Btu
0.714
0.766
0.289
0.129
1.019
1.372
0.524
0.198
0.208
Adiabatic
flame
temp, °F
3337
3327
3268
2933
3578
3605
3335
2948
3026
61
-------
enthalpy given up in the furnace by the combustion products) can be evaluated
using the flue-gas temperatures in in Table 6 and the combustion products in
Table 1. Subtracting the flue-gas enthaply from the total enthalpy input gives the
amount of heat transferred in the furnace, and dividing by the total enthalpy input
gives the efficiency. The pilot-scale furnace efficiencies for natural gas,
Koppers-Totzek oxygen, preheated Koppers-Totzek oxygen, and preheated
Koppers-Totzek oxygen plus 15% FOR are within the range of 28.2% to 34.2%.
In addition to changes in the furnace (radiant section) efficiency when natural gas
is replaced with a low-Btu gas, the temperature and volume of combustion gas
products entering the convective section will change, thus varying the amount of
heat absorbed in the convective section. Comparing the volume of combustion
products for the test fuels reveals that Koppers-Totzek oxygen (293 Btu/ft3) has a
9% smaller volume of combustion products than natural gas, while Winkler air (116
Btu/ft3) produces a 42% greater volume of combustion products than natural gas.
These changes in combustion-product volume will result in changes in gas
velocities and shift the heat absorption patterns within the convective section.
Table 6. PILOT-SCALE FURNACE EFFICIENCIES
FOR PREHEATED FUEL GASES
Fuel
Natural gas
Koppers-Totzek oxygen,
800 °F
Koppers-Totzek oxygen,
800 °F plus 15% FOR
Flue-gas
temp, °F
2553
2592
2681
Furnace
efficiency, %
28.6
34.2
28.2
To evaluate a shift in heat absorption when retrofitting a boiler with low-Btu gas
requires a method of calculation. Babcock and Wilcox Co. (B&W) agreed to take
experimental data presented in EPA Report No. 600/7-77-094a and use them for
the input conditions to the convective section to predict differences in overall
boiler efficiency (radiant plus convective) between natural gas and low-Btu gases.
The method of calculation employed by B&W was to use their design computer
program to forecast changes in boiler performance when retrofit with a low-Btu
fuel gas.
62
-------
The particular boiler selected for their calculations was a standard design with a
maximum rated steam How of 2,430,500 Ib/hr at 26ZO psig/1005°F at the
superheater outlet when fired with natural gas. The unit (B&W Contract RB-455)
is installed at the Teche Station of the Central Louisiana Electric Co. and supplies
steam to a Westinghouse turbine having a maximum capacity of 361 MW. This
same unit was reported on by B&W for EPRI Project 265-2, entitled "Low Btu Gas
Study," a program designed to look at retrofitting boilers using only existing
design information.
To update these results, the flue-gas temperatures and gas compositions measured
on the IGT furnace during earler "ambient" temperature fuel-gas trials were used
as input data. These temperatures and gas compositions were assumed to be
identical to those entering the convective section of the boiler. Using these
experimental results, B&W calculated unit efficiencies for natural gas, Winkler
oxygen, and Winkler air fuel gases. The total required output of Unit RB-455
based on a 2,198,000 Ib/hr main steam flow is 2652.5 million Btu/hr. Based on the
experimental data and fuel analysis presented in Table 1, B&W generated the
information in Table 7. The first line shows natural gas firing with the amount of
excess air that the unit was designed for at this load. The second line shows
natural gas firing with input conditions (325°F air preheat and 3% excess oxygen)
identical to those used for the pilot-scale test furnace. The actual output from
firing natural gas with the increased level of excess air is in excess of 2652.5
million Btu/hr. This is because higher gas weights result from the increased
volume of combusiton air creating an overabsorption in the reheater. Similar
reheat overabsorption occurs in the output for Winkler air, which is also greater
than 2652.5 million Btu.
Table 7. B&W CALCULATED BOILER EFFICIENCIES USING IGT DATA
Fuel
Natural gas
Natural gas
Winkler oxygen
Winkler air
IGT
flue-gas
temp, °F
2553
2523
2335
Actual
unit
output,
10s Btu/hr
2652.5
>2652.5
2652.5
>2652.5
Unit
efficiency,
%
85.1
84.8
84.4
80.4
02 in
flue gas,
% by vol
1.1
3.0
3.0
3.0
B&W
furnace
exit-gas
temp, °F
2702
2633
2583
2397
63
-------
It should be noted that these calculation in no way imply that this, or any other
unit, could economically be retrofitting to burn these test fuels. Several other
factors in addition to unit efficiency must be considered in an actual retrofit
study. Two of these important factors are 1) redistribution of heat absorption
between boiler, primary and secondary superheater, and reheat and economizer
surface (a significant redistribution of which will affect metal temperatures,
spray flows, and circulation); and, 2) changes in combustion-air and flue-gas
weight from the original design quantities, which will affect fan and air heater
performance. Both of these factors can have a significant influence on the
feasibility of retrofitting existing units designed for conventional fossil fuel firing.
To aid in determining the value of the experimental data, B&W reran their design
calculation without using the experimental results as input. The new inputs
required for the computer program were the fuel composition and the desired
temperature of combustion products leaving the air heater. The results of these
calculations for the gas temperature and emissivity at the exit of the radiant
furnace section are listed in Table 8. The combustion products temperature
leaving the radiant section shows excellent agreement with the experimental
values. This provides substantiation on how realistically the pilot-scale test
furnace was able to model the radiant section of a utility boiler.
Comparing the gas emissivities of the combustion products at the radiant section
exit as measured by IGT to those calculated by B&W reveals a large difference.
This difference can be understood by looking at the emissivities dependent
variables. These variables are the partial pressure of the radiating gas, the
temperature of the gas mixture, and the distance across (beam length) the
radiating gas. The temperature and partial pressure of the radiating gases are
similar for the IGT test data and the B&W calculated values. Therefore the
difference between the pilot-scale furance and the 360-MW boiler is due to the
different beam length, 4 feet and 11 feet, respectively.
64
-------
Table 8. CALCULATED AND MEASURED
RADIANT SECTION EXIT
GAS TEMPERATURES AND EMISSIVITIES
Fuel
Natural gas
Winkler oxygen
Winkler air
Koppers-Totzek oxygen
Koppers-Totzek oxygen,
800 °F
Koppers-Totzek oxygen,
800 °F plus 15% FOR
IGTa
Tg
2553
2523
2335
2554
2592
2681
C9
0.18
0.22
0.17
0.19
0.19
0.22
B&Wb
T9
2633
2583
2397
—
—
--
€9
0.29
0.35
0.31
--
• —
—
a Experimental data collected on pilot-scale test furnace.
Calculated values based on normal design technique.
65
-------
MATHEMATICAL MODEL OF FURNACE PERFORMANCE
To allow the experimental data to be accurately extrapolated, a mathematical
model was sought that would provide estimates of temperature and emissivity in
agreement with the experimental results. In addition, the model had to be easy to
use (not requiring a high-speed computer) yet provide reliable estimates. The
method selected to compare the experimental results with those calculated by
B&W is the "speckled-wall" model (SWM) developed by Hottel.4 This model makes
use of the follow assumptions:
1. The combustion products and flame in the combustion chamber are assigned
a single mean temperature; this means that the furnace is treated as a well
stirred reactor.
2. The gas is gray.
3. The heat sink surface area is gray and can be assigned a single
temperature.
4. External losses through the furnace walls are negligible, and internal
convection to the refractory furance walls can be neglected.
5. The arrangement of heat sink and refractory surfaces is such that any point
on the furnace walls has the same view-factor to sink surfaces as any other
point. This occurs only when the sink and refractory surfaces are
intimately mixed. The walls are speckled.
The items of information needed to use the model applying the iterative method
described below are the adiabatic flame temperature, the total enthalpy, the total
surface area of the furnace, the ratio of sink surface area to refractory surface
area, and the emissivity of the sink.
The model has the capability to -
1. Account for convective heat flux to the sink
2. Allow for heat losses through the furnace walls
3. Allow for the difference in flue-gas temperature and effective gas
radiating temperatures, because an industrial furnace does not have perfect
stirring.
The model gives substantially correct predictions of furnace performance for a
wide variety of furnaces and can be expected to give a good comparison between
fuels in a particular furnace.
4 Hottel, H. C., J. Inst. Fuel 34; 220,1961.
66
-------
A flame-to-sink energy exchange factor, (GS)R, for radiation heat transfer can be
defined by —
for the case where the heat sink is well distributed. Here Aj is the total furnace
area, Ag is the heat sink area, and e and e are the sink and flame
o ,
emissivities, respectively. Then a total energy exchange factor can be defined to
include radiative and convective heat transfer by —
(GS)R c = (GS)R + hsAg/4a T3gs (7)
where h is the convective heat-transfer coefficient, a is the Stefan-Boltzman
o
constant, and T is the mean temperature bet
A "reducing firing density" can be defined by —
constant, and T is the mean temperature between the sink and the flame
= Hf/[ 0
-------
where n' = r)(T - T )/!.„
AT o Ar
T " Ts/TAF
and A' = A/T
t\I-
where A accounts for departures of the furnace from the well stirred
approximation. The effective radiating temperature of the gas will differ from
the mean enthalpy temperature of the gases at the flue of the furnace by an
amount A > which depends on the firing density and furnace geometry.
The first attempt at using the well stirred speckled-wall model ( A ' = 0) was to
evaluate how it would agree with IGT experimental data and the values calcuated
by B&W. The measured and calculated efficiencies are plotted in Figure 38 versus
adiabatic radiance. Adiabatic radiance is defined as the total radiant energy that
would be emitted from the combustion products of a test fuel at the test fuel's
adiabatic temperature. Figure 38 again illustrates how accurately the pilot-scale
test furnace was able to model the radiant section of a utility boiler. Thus to get
efficiencies calculated with the well stirred speckled-wall model to agree with the
IGT test data would demonstrate the model's ability to predict performance of a
utility boiler radiant section.
To allow the speckled-wall model to be used from a completely theoretical
position with no experimental data needed, an iterative method was devised. A
combustion-products gas temperture is estimated at the exit of the radiant
section. From this temperature an emissivity is calculated using the Hottel-
Sarofim charts. (Any technique for calculating emissivities can be used. The
Hottel-Sarofim method was chosen based on the ease of calculation and the
universal availability of the gas emissivity charts.) This emissivity is plugged into
Equation 6 to evaluate the flame-to-sink energy exchange factor. This permits
the total energy exchange factor, Equation 7, and the reduced firing density,
Equation 8, to be calculated. A reduced furnace efficiency can then be
determined by Equation 9. This reduced efficiency can be turned into a true
furnace efficiency using Equation 10. Having the true furnace efficiency, the
exit-gas temperature can be determined using the heat content of the combustion-
product gases. If the exit-gas temperature derived from the speckled-wall model
agrees with the temperature used to calculate the emissivity, then the solution is
68
-------
35—
\O
25.
O
z
a
u
w
15-
O
IGT EXPERIMENTAL DATA
B&W DESIGN CALCULATION
Till
5678
ADIABATIC RADIANCE, C. QC7T4 (104)
A D AD
10
I
11
Figure 38. Comparison of IGT experimental and B&W calculated
furnace efficiencies
-------
finalized. If the temperature does not agree, then the estimated temperature is
incremented, a new emissivity is calculated and plugged into the speckled-wall
model, and the iteration continues. The procedure is followed until a satisfactory
agreement between estimated and calculated temperatures is achieved, that is,
until the calculated and estimated temperatures are sufficiently close that the
emissivity is constant over the temperature range between them.
Figure 39 compares the efficiencies calculated with the well stirred speckled-wall
model determined using the iterative method discussed above to the IGT measured
efficiencies as a function of adiabatic radiance. These calculated efficiencies are
proportional to the adiabatic radiance. The slope of this linear relationship of
calculated efficiencies to adiabatic radiance is smaller than the slope of the line
relating measured efficiency to adiabatic radiance, indicating that the pilot-scale
test furnace deviated from a well stirred reactor. If the slopes of the
experimental and theoretical lines were equal, then the deviation from a well
stirred reactor would be independent of adiabatic flame temperature and
emissivity. However, the slopes of the relationships illustrated in Figure 39 are
not equal, meaning that as the adiabatic radiance of the fuel gas becomes larger
the deviation of the furnace being modeled will get farther from a well stirred
reactor. The speckled-wall model can make allowances for this deviation by
assigning A an appropriate value. (Refer to Equation 9.) From reviewing the
extreme left (Winkler air) and right (Koppers-Totzek preheated to 425°C) data
points of the theoretical curve of Figure 39, it can be concluded that no
connection need be made for Winkler air ( A = 0); however, a value for A must be
determined that will raise the 425°C preheated Koppers-Totzek oxygen fuel gas
efficiency from 22.9% to 34.2%. The value of A needed to get the calculated
and measured efficiencies to agree can be determined by evaluating the exit-gas
temperature that would result in a furnace efficiency of 34.2%. Having
determined this temperature, 259 2°F, it is used to find the combustion-product
gas emissivity, 0.359. The speckled-wall model is then solved for A , 605°R. A
linear relationship can now be developed between A and adiabatic radiance. This
relationship is shown in Figure 40. This figure illustrates that the departure of the
furnace from a well stirred reactor is dependent on the adiabatic radiance of the
fuel gas. Using Figure 40, A can be determined for each of the fuel gases tested.
This allows the efficiency for each fuel gas to be reevaluated using this non-well
stirred speckled-wall model. As with the well stirred model, the iterative
70
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35-,
25-
ITERATIVE CALCULATION WITH
WELL STIRRED SPECKLED-WALL MODEL
IGT EXPERIMENTAL DATA
u
2
a
o
t—(
CK
U.
15-
I
4
6 7
ADIABATIC RADIANCE,
10
11
Figure 39. Comparison of experimental and well stirred speckled-wall
model calculated furnace efficiencies
-------
600-1
500-
400-
SLOPE = 92.65
'R
10 ADIABATIC RADIANCE
ro
. 300-
200-
100-
567
ADIABATIC RADIANCE,
I
10
11
Figure 40. Variation of well stirred approximation with
adiabatic radiance
-------
procedure is followed. The results of this calculation are shown in Figure 41, and
are listed in Table 9. The figure illustrates that the iterative speckled-wall
calculation can give results in good agreement with the experiment when properly
modified to allow for deviation of the furnace from perfect stirring. Table 10
lists the iteratively evaluated emissivities as well as those calculated by B&W. To
evaluate the effect emissivity has on efficiency, the emissivity calculated by B&W
for Winkler oxygen fuel gas was substituted in the non-well stirred speckled-wall
model to determine an exit-gas temperature. The non-well stirred speckled-wall
model using the B&W computed emissivity yields an exit-gas temperature of
2463op, or 33°F above that determined using the iterative technique. Thus a 7.9%
change in gas emissivity will produce only a 1.4% change in exit-gas temperature.
The non-well stirred speckled-wall model provides a calculation tool that yields
good agreement with the more sophisticated techniques employed by B&W. Using
the iterative technique, accurate predictions of furnace performance can be made
for nonluminous fuels.
IN-THE-FLAME ANALYSIS
Detailed in-the-flame data were collected for natural gas preheated Koppers-
Totzek and preheated Koppers-Totzek with 15% FOR. These data are to aid in
/
quantitative modeling of large-scale turbulent diffusion flames and to provide a
qualitative guide in understanding and controlling NO emission levels. These
measurements included gas species concentrations and temperature profiles. A
supplement volume containing these detailed in-the-flame data is available upon
request from the EPA project officer,(919) 541-2236.
73
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35
_ IGT EXPERIMENTAL DATA
/\ NON-WELL STIRRED ITERATIVE
^ SPECKLED-WALL MODEL CALCULATION
O'
(J
Z
a
u
H^
b
t*
u
15-
Ox
x
ADIABATIC RADIANCE, e. DaT4n( 104)
10
I
11
Figure 41. Comparison of experimental and non-well stirred
speckled-wall model calculated furnace efficiencies
-------
Table 9. COMPARISON OF MEASURED AND CALCULATED
FURNACE EFFICIENCIES
Fuel
Natural gas
Lurgi oxygen
Winkler oxygen
Koppers-Totzek oxygen
Wellman-Galusha air
Winkler air
Koppers-Totzek oxygen,
425°C
Koppers-Totzek oxygen,
425°C plus 15% FOR
tA^dO4)
8.98
8.72
9.61
10.12
6.31
4.47
11.00
9.10
IGTa
28.6
25.2
26.4
33.1
19.0
10.8
34.2
28.2
B&Wb
26.3
—
25.1
—
—
7.8
—
—
SWMC
19.8
19.8
19.9
22.4
16.3
12.2
22.9
20.4
NSWMd
27.0
26.6
29.6
31.4
18.9
12.2
34.2
27.4
u Measured on IGT pilot-scale test furnace.
b Calculated by B&W for Teche Station Boiler (RB-455) using design calculation technique.
c Calculated using the well stirred iterative speckled-wall model (A = 0°R).
d Calculated using the non-well stirred iterative speckled-wall model (A evaluated using Figure 67).
Table 10. COMPARISON OF CALCULATED EMISSIVITIES
Fuel
Natural gas
Winkler oxygen
Winkler air
B&Wa
0.29
0.35
0.31
SWMb
0.30
0.35
0.34
NSWMC
0.33
0.38
0.34
0 Calculated by B&W for Teche Station Boiler (RB-455) using design calculation
technique.
b Calculated using the well stirred iterative speckled-wall model technique
(A = 0°R).
c Calculated using the non-well stirred iterative speckled-wall model technique
(A evaluated using Figure 67).
75
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CONVERSION TABLE
ENGLISH TO SI METRIC CONVERSION FACTORS
To Convert From
lb/106 Btu
106 Btu/hr
Btu
psi
SCFH
ft/3
Inch
Feet
Feet2
Inches of water (pressure)
lb/ft3
gpm
Inch2
°F
°R
To
ng/J
MWt
J
Pa
m'/s
m/s
m
m
m2
Pa
kg/m3
m3/s
m2
°C
°C
Multiply By
4.299 E + 02
2.928751 E-01
1.055 E E + 03
6.894757 E + 03
7.865790 E -06
3.048000 E-01
2.540000 E -02
3.048000 E-01
9.290304 E-02
2.4884 E +01
1.601846 E + 01
6.309020 E - 05
6.451600 E-04
t°C = (t°F- 32)/1.8
t°C= [(5/9)t°R]-273.15
J = Joule
g = gram
s = second
MWt = megawatts thermal
gpm = gallons (U. S. liquid)/minute
Pa = Pascal
m = metre
k = kilo (103)
n = nano (10"9)
M = mega (106)
C = Celsius
F = Fahrenheit
R = Rankine
psi = pounds per square inch
SCFH - standard cubic feet per hour
76
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-77-094b
2.
3. RECIPIENT'S ACCESSION NO.
j. TITLE AND SUBTITLE Burner Design Criteria for NQx
Control from Low-Btu Gas Combustion; Volume II.
Elevated Fuel Temperature
5. REPORT DATE
December 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Donald R. Shoffstall and Richard T. Waibel
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Applied Combustion Research
Institute of Gas Technology
IIT Center, 3424 South State Street
Chicago, Illinois 60616
10. PROGRAM ELEMENT NO.
EHE624a
11. CONTRACT/GRANT NO.
68-02-1360
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final: 10/76-10/77
14. SPONSORING AGENCY CODE
EPA/600/13
is. SUPPLEMENTARY NOTES fERL-RTP project officer is David G. Lachapelle, Mail Drop 65,
919/514-2236.
B. ABSTRACT
gives results of B. program to provide quantitative data on
combustion emissions from high-temperature low-Btu gas. It complements a
recently completed EPA project that evaluated emissions resulting from the bur-
ning of ambient-temperature low-Btu gas. The experimental results were gathered
from a pilot-scale furnace fired with a movable-vane boiler burner at a heat input
of 0.66 MW (2.25 million Btu/hr). The gases tested in this program --Winkler
oxygen, Koppers-Totzek oxygen, and Wellman-Galusha air—ranged from 5. 8 MJ/
cu m (156 Btu/cu ft) to 9. 9 MJ/cu m (266 Btu/cu ft). Measurements were made of
NO emissions, temperatures within the flame, and flame emissivity. A mathemati-
cal model was used to predict the efficiencies of the furnace with the various fuels;
the model agreed well with the experimental measurements. The NO emissions of
the gases tested were ordered by the adiabatic flame temperature.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATl Field/Croup
Air Pollution
Nitrogen Oxides
Manufactured Gas
Natural Gas
Combustion Control
Burners
Design
High-tempera-
ture Tests
Flames
Emissivity
Air Pollution Control
Stationary Sources
Low-Btu Gases
13B
07B
21D
2 IB
13A
14B
13. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
84
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
77
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