APTD-1516
LOW EMISSION BURNERS
FOR AUTOMOTIVE RANKINE
CYCLE ENGINES
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Water Programs
Mobile Source Pollution Control Program
d Automotive Power Systems Development Division
Ann Arbor, Michigan 48105
-------
APTD-1516
LOW EMISSION BURNERS
FOR AUTOMOTIVE RANKINE
CYCLE ENGINES
Prepared by
Herbert R. Hazard,-Robert D. Fischer, and-Clarence McComis
Battelle
Columbus Laboratories
505 King Avenue
Columbus, Ohio 43201
Contract No, EHS 70-117
EPA Project Officers:
*~F. Peter Hutchins and K. F. Barber
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Water Programs
Mobile Source Pollution Control Program
Advanced Automotive Power Systems Development Division
Ann Arbor, Michigan 48105
April 1973
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The APTD (Air Pollution Technical Data) series of reports is issued by
the Office of Air Quality Planning and Standards, Office of Air and
Water Programs, Environmental Protection Agency, to report .technical
data of interest to a limited number of readers. Copies of APTD reports
are available free of charge to Federal employees, current contractors
and grantees, and non-profit organizations - as supplies permit - from
the Air Pollution Technical Information Center, Environmental Protection
Agency, Research Triangle Park, North Carolina 27711 or may be obtained,
for a nominal cost, from the National Technical Information Service,
5285 Port Royal Road, Springfield, Virginia 22151.
This report was furnished to the U.S. Environmental Protection Agency
by Battelle Columbus Laboratories in fulfillment of Contract No. EHS 70-117
and has been reviewed and approved for publication by the Environmental
Protection Agency. Approval does not signify that the contents necessarily
reflect the views and policies of the agency. The material presented in
this report may be based on an extrapolation of the "State-of-the-art."
Each assumption must be carefully analyzed by the reader to assure that it
is acceptable for his purpose. Results and conclusions should be viewed
correspondingly. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
Publication No. APTD-1516
11
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TABLE OF CONTENTS
Page
INTRODUCTION 1
CONCLUSIONS 2
SUMMARY. .......... ...... 3
DEVELOPMENT GOALS. ........ ..... 5
BURNER CONCEPTS. .......... 7
BURNER TEST CONDITIONS AND INSTRUMENTATION . 8
FUEL NOZZLE DEVELOPMENT. 11
BURNER DEVELOPMENT TO REDUCE NO EMISSION LEVELS ..... 12
x
Configuration A................... 12
Configuration B .......... 14
Configuration C 15
Configuration D ....... ...... 18
Configuration E 19
EXHAUST-GAS RECIRCULATION. . 21
Experimental Techniques . . ..... 21
Emission Data for Configuration B 24
Emission Data for Configuration E-2 28
DISCUSSION OF THERMAL AND CHEMICAL ASPECTS OF EGR 32
EFFECT OF FUEL NITROGEN CONTENT ON NO EMISSION. ..... 34
x
ANALYTICAL PREDICTIONS OF HEAT TRANSFER
and NO GENERATION 37
x
AUXILIARY POWER REQUIREMENTS 39
GAS SAMPLING AND ANALYSIS STUDY 40
REFERENCES 43
BATTELLE — COUUIVIBUS
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LOW-EMISSION BURNERS FOR
AUTOMOTOVE RANKINE-CYCLE ENGINES
by
Herbert R. Hazard, Robert D. Fischer,
and Clarence McComis
INTRODUCTION
The Rankine-cycle engine has been selected by the U.S.
Environmental Protection Agency as one of the most promising candi-
dates for the automotive engine of the future because of its potential
for low emission of air pollutants. However, automotive service imposes
severe demands upon a Rankine-cycle combustion system, including an
extremely wide turn-down range, very small size, and low pressure drop
to minimize auxiliary power. In view of the difficulty anticipated in
meeting such requirements, development of combustion technology was one
of the first tasks to be started in the EPA Advanced Automotive Power
Systems Program for development of automotive Rankine-cycle engines.
This report discusses results of a combustion technology program
carried out at Battelle, Columbus Laboratories under this program.
BATTELLE — COLUMBUS
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CONCLUSIONS
In the course of developing a burner to meet all contract re-
quirements, and the 1976 Federal Emission Standards as well, various
burner design features were explored and considerable insight into the
technology of low-emission combustion was obtained. It proved possible
to meet all contract requirements, including burner volume, auxiliary
power limit, and emission limits, by use of a suitable combination of
design features, and with air atomization of fuel.
Burner Configuration E-2, which met all contract requirements,
was a rich-primary burner having internal cooling of the primary zone with
boiler tubes. It could be operated over the firing range from 110 to 1
Ib fuel per hour and met emission goals at all firing rates. Total volume
3
based on exterior dimensions was 0.95 ft , compared with the contract goal
3
of 1.3 ft . Auxiliary power without EGR was 1.49 hp, compared with the
contract goal of 2.0 hp; with 10 percent EGR, auxiliary power was 1.97 hp.
The use of EGR provided a significant reduction of NO emission.
X
Burner B, a cooled, lean-primary burner, could meet emission re-
quirements for firing rates above 20 Ib/hr. However, when fired with 10
percent EGR, at firing rates of 5 and 10 Ib/hr, NO emissions were about
X
36 percent above the goal and were not improved by EGR.
The factor of air atomization, upon which the program was based,
proved very important. It was found that atomizers of conventional design
did not provide good fuel atomization and dispersion over the required
turndown ratio of 100:1, and a commercial acoustic-type of air-atomizing
nozzle of unique design was improved to meet this requirement. Air atomiza-
tion also had a significant effect on combustor geometry, in that atomizing
air provided sufficient turbulence for combustion at the lowest firing
rates, and variable-geometry design proved unnecessary as a means of pro-
moting turbulence.
The effect of fuel nitrogen content upon NO emission from
X
Burner E-l was explored. It was found that 50 to 90 percent of the fuel
nitrogen was converted to NO in combustion. It was concluded that very
X
low fuel nitrogen content should be specified for Rankine-cycle burners.
The nitrogen content of kerosene and gasoline-type fuels is normally below
10 ppm, which should meet all requirements for low-NOx combustion.
BATTELLE — COLUMBUS
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SUMMARY
In the course of the program burners based on four different
burner concepts were developed and evaluated. Each of these was optimized
by experimental development, and data for the optimum configurations are
reported.
Early in the program it was found that emission goals for CO,
HC, and smoke could be easily met, but that NO emission levels far exceeded
X
goals. Accordingly, development was concentrated on meeting NO emission
X
goals, and only NO data are reported for those cotnbustors that did not
X
meet NO goals.
X
Configuration A, the first burner developed, was a compact, un-
cooled, lean-primary burner. It met all contract requirements except for
emission of NO , which far exceeded goals.
X
Configuration B, a cooled, lean-primary burner, was aerodynamically
similar to Configuration A. The use of water-cooled coils to cool the com-
bustion space reduced NO emission significantly, but it still exceeded
X
emission goals.
Configuration C was an uncooled burner with a rich primary zone
followed by a lean secondary zone. The proportion of the total combustion
air admitted to the primary zone was varied over a wide range, and it was
found that lowest NO emission was obtained with about 60 percent of the
X
total air admitted to the primary zone. NO emission was above goals,
X
however.
Configuration D was a cooled, rich-primary burner, in which the
effect of cooling was explored by use of three different water-cooled coils,
used separately and in combination. NO emission for Configuration D met
X
contract goals.
Configuration E was basically similar to Configuration D, with
the exception that a conical cooling coil around the burner throat was re-
placed by a smooth water-cooled conical surface to improve burner aero-
dynamics. In doing this the air film slot at the upstream end of the burner
cylinder was closed. NO emission characteristics were similar to those
X
BATTEULE — COLUMBUS
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for Burner D, but considerably more excess air was needed to meet CO re-
quirements. Accordingly, a film slot was added in the secondary zone to
provide air to burn CO flowing along the burner wall. This modified design,
Configuration E-2, was the final design evaluated and met all contract re-
quirements.
The effect of EGR (Exhaust Gas Recirculation) on NO emission
X •
was explored using Configuration B and Configuration E-2. It was found
that EGR reduced NO emission significantly for both configurations. The
X
use of 10 percent EGR in Configuration B brought NO emission below goals
X
at firing rates of 20 Ib/hr or more, but did not improve NO emission at
X
firing rates of 5 and 10 Ib/hr. The use of EGR in Configuration E-2 also
reduced NO emission significantly, with no significant change in emission
X
of CO and HC. All emissions were below goals for all firing rates and EGR
ratios.
With total auxiliary power limited to 2.0 hp, Configuration E-2
could be fired with about 10 percent of EGR. This would provide the lowest
emission levels of any burner design and operating mode evaluated during
this program.
The effect of fuel nitrogen was investigated by firing fuel doped
with varying amounts of pyridine in Burner E-l. It was found that a signi-
ficant proportion of the nitrogen in the pyridine was converted to NO , the
X
proportion varying from about 90 percent for 0.01 percent fuel nitrogen,
to about 55 percent with 0.44 percent fuel nitrogen. It was concluded
that a significant proportion of fuel nitrogen occurring in natural or-
ganic compounds would be converted to NO in a similar manner, although
X
the exact percentage might be somewhat different than for pyridine, and
that control of fuel nitrogen content would be needed to assure low NO
X
combustion in burners of this type.
BATTELLE — COLUMBUS
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DEVELOPMENT GOALS
The objective of the program was to develop a full-scale burner
suitable for an automotive Rankine-cycle engine. The contract goals in-
clude a 100:1 turndown ratio with firing rates from 2,000,000 Btu/hr to
3
20,000 Btu/hr, combustion volume of 1.3 ft or less, and total mechanical
power input to fans and compressors of 2 hp or less. The fan power was
specified as that needed to overcome burner pressure drop, and does not
include any additional power needed to overcome boiler pressure drop.
Emission goals for the program were the 1980 AAPS Goals, listed
as contract goals in Table 1. For comparison, the 1976 Federal Emission
Standards for Light Vehicles are also tabulated; these are of greater
current interest than the contract goals, although they had not yet been
promulgated when the program was begun.
Emission goals are specified in units of grams per vehicle mile.
For this burner development program, EPA specified that goals be converted
to units of grams per kilogram fuel, or mol ppm, by assuming vehicle opera-
tion at 10 miles per gallon of fuel for all test conditions.
TABLE 1. CONTRACT EMISSION GOALS COMPARED WITH 1976 FEDERAL STANDARDS
Contract Goals ^ 1976 Federal Standards
Pollutant
CO
NO as N00
x 2
HC
Particulates
Smoke
g/mile
4.7
0.4
0.14
e/kg fuel
16.25
1.38
0.48
0.10
mol ppm E/mile
797 3.4
40 0.4
53 0.4
4.2
p/kg fuel mol ppm
11.8 576
1.38 40
1.85 152
-
(no visible smoke) - ~
Mol ppm calculated for 140 percent of stoichiometric air.
BATTELLE - COLUMBUS
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For convenience in day-to-day data reduction and plotting,
emission data were plotted as mol ppm of pollutant as measured. In
order to accommodate the dilution of a pollutant with increasing excess
air, the emission goals were expressed as curves in which concentration
decreased with increasing excess air. This avoided the necessity of
calculating a "corrected" pollutant concentration for every data point,
and also showed the variation of actual concentration with excess air.
Data for final configurations were also plotted as g/kg fuel, which are
not affected by variations of excess air.
BATTELLE - COLUMBUS
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BURNER CONCEPTS
The selection of the burner design and the fuel atomizer was
influenced heavily by the specified turndown ratio of 100:1 and the need
to meet 1976 emission standards. Experimental burners were based upon the
following concepts:
(1) Air atomization of fuel
(2) Swirl stabilization of flame
(3) Both lean and rich primary zones
(4) Both cooled and uncooled primary zones
(5) Exhaust gas recirculation.
The selection of air atomization was heavily influenced by the
requirement for 100:1 turndown ratio. The swirl-stabilized burner was
designed for excellent mixing of fuel and primary air to minimize gradients
of fuel-air ratio to the extent possible, and to provide good combustion
over a wide range of fuel-air ratios. The use of a one-stage burner with
a lean primary zone and the alternative use of a two-stage burner with
a rich primary zone were both explored, and they had different emission
characteristics. In addition, the use of boiler tubing for cooling of the
primary zone was explored as a device for reducing NO emission. The
X
effects of EGR (exhaust gas recirculation) were investigated for the best
cooled lean-primary and rich-primary burners.
No commercially available fuel nozzles were found that could
meet the requirement of 100:1 turndown with good fuel dispersion and
TJ
atomization over the entire range. A Sonicore 125 H resonant acoustic,
air-atomizing nozzle was improved during the program by changing internal
details to improve fuel dispersion, and to improve burner performance at
firing rates below 3 Ib/hr. It is believed that fuel atomization was
unusually fine, accounting for the clear blue flame observed under most
firing conditions. Atomizing air provided most of the turbulence for
combustion at very low firing rates, where burner pressure drop approached
1/10,000 that at full rating.
BATTEULE — COLUMBUS
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BURNER TEST CONDITIONS AND INSTRUMENTATION
All burner tests were run at atmospheric pressure, with com-
bustion air at about 100 F. Jet A kerosene fuel was fired at rates from
1 to 110 Ib/hr, and air-flow rates were varied to provide 60 percent to
185 percent of stoichiometric air at several firing rates. This permitted
plotting data in terms of fuel-air ratio and firing rate over a wide range
of conditions.
Figure 1 is a drawing showing the arrangement of the burner test
rig and Figure 2 is a photograph of the test rig. Burner air flow was mea-
sured with a single ASME orifice having a hot«film sensor at the center.
The linearized output from the hot-film anemometer permitted accurate read"
ings of flow rate over a 100:1 range, equivalent to a 10,000:1 range in ori-
fice pressure drop. This meter was calibrated against a Flow Prover using
several sizes of critical-flow nozzles, and proved linear within 1 percent
over the range of use.
Fuel flow was measured by a Cox Series 12 variable area meter
containing two calibrated rotameter tubes, each with a logarithmic scale
suitable for readings to an accuracy of 1 percent of reading at any point
on the scale. Fuel temperature was held constant to simplify metering.
Exhaust gas was sampled with water-cooled probes. One long,
straight probe was pivoted at its midpoint and arranged so that, with
the pivot on the burner axis, the burner diameter could be traversed
at various distances from the inlet, covering any desired point on the
cross-section. This probe was lined with a quartz tube to avoid catalytic
destruction of N09 in the presence of CO when traversing within the
(1 1 4c
flame. A second probe, placed acroso the burner outlet, contained
U holes spaced to align with the centers of 6 equal annular areas, to
Provide a sample representative of the average outlet analysis. This
Probe was of water-cooled stainless steel construction and was unlined,
as it Was not intended for use in reducing atmospheres. From the probe
the gas sample was passed through an ice trap for water removal before
Numbers in parentheses designate References at end of report, page 43.
BATTEULE — CQUUMBUS
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-Intake silencer
Air intake
Hot-film anemometer probe
X in orifice
Pressure-! p
regulator 1 (/
Dampers for
flow control
Combustor
Gas sampler
1—Flow control valves
FIGURE 1. SCHEMATIC ARRANGEMENT OF TEST RIG FOR
LOW-EMISSION BURNER DEVELOPMENT
FIGURE 2. PHOTOGRAPH OF TEST RIG FOR LOW-EMISSION BURNER DEVELOPMENT
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10
entering the analytical instruments. The gas sample was drawn through
the analytical system with a water aspirator at a rate monitored with a
rotameter.
NO was measured for Configurations A, B, and C (described
Ji
later) with a Mast NO meter, and NO only was measured for Configuration
D with a Beckman NDIR instrument. The electronic circuit of the Mast
meter was modified to provide a 0-200 ppm scale range and the meter was
calibrated before and after each test. Although the Mast NO- meter
measured only NO , it was used to measure NO by conditioning the gas
™ X
sample to oxidize all NO to N02. This was done in an acidified dichro-
mate scrubber which also removed SO-, an interfering compound.
NO was measured for configuration D with a Beckman Model 315
NDIR (nondispersive infrared) analyzer. The instrument was calibrated
daily with zero gas and span gas. The NDIR instrument was used in pre-
ference to the Mast NO- meter when it became available because it greatly
speeded data taking. However, data obtained with both instruments were
closely comparable and appeared reliable and consistent. It was recognized
that, in condensing water from the gas sample before analysis, some in-
determinate amount of N09 might be dissolved in the condensed water and
lost. The NO quantity was later shown to be about 10 ppm, a value small
enough that it would not alter results or conclusions significantly.
When using the NDIR, which is extremely sensitive to water, the gas
sample from the ice trap was passed through a dry-ice trap packed with
glass wool,and a Dryrite dessicent bed. With this drying train the
calibration was unchanged when span gas was bubbled through water, in-
dicating that water removal was adequate,,
it
Near the end of the program a TECO Chemiluminescence Analyzer,
which measures NO, was used with a 300-F sampler and sampling lines to
provide NO analysis of the wet gas sample. This instrument included a
converter to convert N00 to NO, so that either NO or NO could be
2. X
measured by using or bypassing the converter. With this instrument and
the hot sampling system, the NO values measured were somewhat higher than
NO values for the NDIR system with moisture removal. It proved possible
* Thermoelectron Engineering Company
BATTELLE — COLUMBUS
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11
to obtain the same level of values with the NDIR by using the hot sampling
system, an NO to NO converter followed closely by a water trap, and the
NDIR analyzer. These systems were used in studying the effect of fuel
nitrogen on NO emission. For EGR studies an Aerochem chemiluminescence
X
analyzer was used in combination with the TECO converter, with a water
trap between to avoid plugging of the flow-metering capillary tube within
the analyzer. Careful comparisons of analyses of bagged gas samples showed
that this moisture trap did not influence NO and NO data.
X
CO was measured with a Beckman Model 215A NDIR analyzer having
0-250 ppm and 0-2000 ppm scale ranges.
HC (unburned hydrocarbons) was measured with a Beckman 402 hydro-
carbon analyzer. This instrument utilizes a flame-ionization detector,
and the entire instrument and sampling system operate at 400 F to pre-
vent condensing of hydrocarbons before analyses.
Oxygen was measured with a Beckman Process Oxygen Analyzer
having scale ranges of 0-5 and 0-25 percent.
FUEL NOZZLE DEVELOPMENT
Low-emission combustion requires good fuel atomization and uni-
form fuel dispersion within a combustion space to avoid over-rich and
over-lean local regions. Providing good fuel atomization and dispersion
over a flow range of 100:1 proved difficult, as none of the available
commercial nozzles proved satisfactory for this operating range. Use of
an air-assist nozzle, in which compressed air is used to provide atomiza-
tion energy at the lowest fuel rates, proved unsatisfactory because of
p
the poor fuel dispersion at the lowest rates. However, Sonicore acoustic
air-atomizing nozzles, a form of modified Hartman whistle, proved nearly
R
satisfactory. Performance of the Sonicore nozzles for this application
was greatly improved by two changes: the first was to make a nozzle sized
exactly for the flow requirement, using a 0.089-in. air venturi at the
center. The second change was to use 8 radial fuel-admission holes in-
stead of four, to provide more uniform distribution of fuel. With these
BATTELLE — COLUMBUS
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12
changes the nozzle could be operated over the desired flow range and com-
bustion could be maintained at 1 Ib fuel per hr. (However, this required
a reduction of atomizing-air pressure with fuel flow rates below 10 Ib/hr.)
n
The modified Sonicore nozzle, designated the S-089-8 was used
in all of the combustors described in this report* It requires 24 Ib per
hr of atomizing air at 34 psi, equivalent to 0.5 air horsepower. A modi-
fied nozzle with a 0.125 throat diameter gave slightly better atomization
and required 29 Ib atomizing air per hr at a pressure of 15 psi, equivalent
to 0.33 air horsepower. Although the minimum firing rate with this nozzle
was 1.8 Ib fuel per hr, it would be preferable where this minimum rate is
acceptable.
BURNER DEVELOPMENT TO REDUCE NO EMISSION LEVELS
x
The initial research approach was based upon use of a lean-
primary uncooled burner having very fine fuel atomization and high vortex
intensity to minimize gradients of air-fuel ratio to the extent possible.
This burner met all research goals except that of NO emission, which
X
was excessive. Accordingly, other approaches were explored, and the prin-
cipal concern became that of meeting NO emission goals.
X
Configuration A
Figure 3 shows Configuration A, a simple, swirl-stabilized
burner, with all air admitted through a scroll to a 4-in. diam. burner
throat. The air-atomizing fuel nozzle was placed at the center of the
air inlet, so that fuel and air were intimately mixed very close to the
burner throat. A metal disk at the burner outlet restricted the outlet
area to an annular slot. This had little effect on burner pressure drop,
but made a significant difference in flow patterns. Without this disk,
atmospheric air was aspirated into the open combustor outlet, diluting
the burning mixture and distorting test results. This would not be a
problem when firing into a boiler. Configuration A represents the
result of considerable optimization of dimensions and vortex energy.
BATTELt-E - COLUMBUS
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FIGURE 3. BURNER CONFIGURATION A
FIGURE 5. BURNER CONFIGURATION B
30O
200
Q.
Q.
80 lOO 120
Percent Stoichiometric Air
140
FIGURE 4. VARIATION OF NOX WITH PERCENT OF
STOICHIOMETRIC AIR FOR CONFIGURATION A
Fuel flow rate, 50 Ib/hr
160
100
120 140 160
Perc-ent Stoichiometric Air
180
200
FIGURE 6. VARIATION OF NOX WITH PERCENT OF
STOICHIOMETRIC AIR FOR CONFIGURATIONS A AND B
Fuel flow rate, 50 Ib/hr
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14
Configuration A operated well at firing rates from 1 Ib per 'hr
to 110 Ib per hr of fuel, and with air-fuel mixtures varying from about
twice stoichiometric fuel to about twice stoichiometric air (fuel equiva-
lence ratio from 1.8 to 0.6). At the leanest condition (near 200 percent
of stoichiometric air) emission of odor, CO, and HC was becoming apparent,
but the flame was stable.
Figure 4 shows NO emission data for Configuration A at a con-
X
stant firing rate of 50 Ib fuel/hr, over a range of air-flow rates. Air
flow is expressed as percentage of stoichiometric air to provide a linear
scale. The NO level peaks at 250 ppm at 100 percent of stoichiometric
X
air and falls sharply with richer or leaner mixtures. In other tests with
somewhat different swirl intensity, levels as high as 600 ppm NO were
X
measured. NO level was essentially constant when firing rate was varied
X
from 10 to 110 Ib/hr at a constant value of air-fuel ratio. All NO data
X
are expressed in ppm as measured. The "goal" curve includes dilution
corrections for excess air.
The levels of CO, HC, and smoke were very low at all firing
conditions from 120 to 180 percent of stoichiometric air. HC levels
were about 1 ppm, compared with atmospheric levels of 6 to 8 ppm, and
CO levels were generally below 100 ppm. With the blue-flame operation
typical of this burner, no smoke could be measured by the ASTM smoke-
spot technique with 110 percent or more of stoichiometric air.
Configuration B
Figure 5 shows Configuration B, a burner of the same size and
shape as Configuration A except that considerable water-cooled tubing
was added as a way of reducing flame temperature, and length was ex-
tended 4 in. to provide space to burn out CO and HC which was formed
close to the cold surfaces.
Figure 6 shows NO emission levels for both Configuration A
X
and Configuration B. It will be noted that N0x levels were reduced by
30 ppm by the cooling surface in Configuration B. The lower curve,
marked "goal", is the level of NO emission required to meet 1976
X
Federal emission standards for passenger cars. Rather high levels of
excess air are required to meet these goals with these burners, which
affects fan power.
BATTELLE — COLUMBUS
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15
Configuration C
Figure 7 shows Configuration C-2, which incorporates staged
air admission. A rich-primary zone, in which little NO is formed, is
followed by a lean secondary zone where air is added to complete the
combustion and dilute combustion products to a moderate temperature
within a few milliseconds, thus kinetically limiting the amount of
NO formed in the secondary zone.
The data of Figure 4 suggests that the primary zone should be
quite rich to minimize NO formation there. However, if the primary zone
is too rich, the combustion temperatures reached in the secondary zone
cause formation of excessive NO there. Thus, there is an optimum dis-
tribution of air between primary and secondary zones for minimum overall
NO emission. Several uncooled, staged-air-admission burners having
different secondary-air hole patterns and different proportions of pri-
mary and secondary air were studied to explore this effect.
Figure 8 shows the variation of NO with percent of stoichio-
X
metric air for four different uncooled staged-air admission burners
with rich primary zones. A curve for Configuration A is included for
comparison. Configuration C-l looked similar to Configuration C-2,
Figure 7, but the liner was 10 in. in diameter and 14 in. long. Secon-
dary air was admitted through two rings of 8 holes of 5/8-in. diameter
and one ring of 16 holes of 7/16-in. diameter. With this arrangement,
about 60 percent of the total air entered the primary zone and the
overall pressure drop was only 5.5 in. wg (water gage) at 110 Ib
fuel/hr. Fuel and air mixing were relatively poor, the exit gas
being rich at the edges and lean at the center, and the flame was a
heavy yellow in color. However, NO levels were low.
X
IATTELLE - COLUMBUS
-------
ra
H
m
r
r
m
-30 holes -1/4 diam
2 holes-1/2 diam
2 rings of 16 holes -3/8 diom
120 140 160 180 200
Percent Stoichiometric Air
FIGURE 8. VARIATION OF NOx WITH PERCENT OF
STOICHIOMETRIC AIR FOR CONFIGURATION C
Firing rate, 50 Ib/hr
FIGURE 7. BURNER CONFIGURATION C-2
n
a
r
C
5
D
C
(0
Cooling coils - 3/8 tubing
Coil X - 7 turns on cone
Coil Y -10 turns
Coil Z ~5 turns, plus cooled disk
100
Secondary-Air Holes
32 holes 1/4 diam
16 holes 3/8 diom
16 holes 3/8 diam
o
"lOO
120 140 160 ISO
Percent Stoichiometric Air
200
FIGURE 9. BURNER CONFIGURATION D
FIGURE 10. VARIATION OF NOx WITH PERCENT OF
STOICHIOMETRIC AIR FOR CONFIGURATION D
Firing rate, 50 Ib/hr
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17
Configuration C-2, shown in Figure 7, utilized a smaller liner
and smaller secondary-air holes. Pressure drop was 7.8 in. wg at 110 Ib
fuel/hr and the distribution of fuel and air was excellent. The flame
appeared uniform and outlet profiles of oxygen and temperature were flat.
However, NO emission levels were considerably higher than for Configura-
X
tion C-l. It should be pointed out that the exit temperature was nearly
uniform at 3200 F with 120 percent of stoichiometric air, so that the
90 ppm NO measured for this condition is not high in comparison with
X
data for conventional burners.
Configuration C-3 was like Configuration C-2 except that the
primary-zone air was reduced from 60 percent to 40 percent of the total.
The result was an NO emission curve approaching that for Configuration A,
X
with all primary air. It appears that all of the NO of Configuration
X
C-3 is formed in the secondary zone where most of the combustion takes
place.
Configuration D-l was an uncooled version of Configuration D,
shown in Figure 9, The primary-zone swirl register and the secondary-
air hole arrangement were the same as for Configuration C-2, but the
primary zone was lengthened and the liner diameter increased to permit
addition of cooling coils. About 66 percent of the total air entered
the primary zone. Configuration D-l had the lowest NO emission level
X
of any of the staged-air-admission burners for operation with more than
120 percent of stoichiometric air. Fuel and air were well mixed and
outlet temperature and oxygen profiles were nearly flat.
From Figure 8 it may be seen that the NO curve for a lean-
X
primary burner (A) is much steeper than curves for the rich-primary
burners (C-l, C-2, C-3, and D-l). At the richer end of the operating
range the rich-primary burners emit less NO , but they may emit more
X.
than the lean-primary burner at the lean end of the range. It appears
that the peak emission level for the rich-primary burners occurs with
stoichiometric fuel-and-air proportions in the primary zone, followed
by dilution in the secondary zone. The rich-primary burner appears to
offer the advantage of low NO emission if optimized for the desired
X
overall air-fuel ratio.
BATTELLE — COLUMBUS
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18
Configuration D
Figure 9 shows Configuration D, including three separate water-
cooled coils that were used to study cooling effects. Configuration D-l,
discussed above, was uncooled except for a coil placed outside the liner
near the exit to avoid overheating the liner. Configuration D-2 utilized
Coil X, a conical coil placed at the burner end of the liner. Configura-
tion D-3 utilized only Coil Y, a cylindrical coil on the primary-zone
surface. Configuration D-4 utilized both Coil Y and Coil Z, which in-
cluded a water-cooled disk separating the primary and secondary zones
and a small coil extending into the center of the primary zone. For all
of these configurations the estimated primary-zone flow was 66 percent
of the total air flow.
Figure 10shows NO emission data for the cooled, rich-primary
burners. These data were taken with an NDIR analyzer and do not include
NO-. It was later found that N09 values were about 10 ppm. NO data
fi £n X
for configurations A and B are included for comparison.
The lowest NO emission data in Figure 10are for Configuration
D-4, with the most cooling. However, the smallest coil, Coil X, used in
Configuration D-2 was almost as effective as all of the other coils com-
bined. Its position close to the fuel spray appears to provide maximum
NO reduction with minimum heat removal. The large cylindrical Coil Y
proved least effective in reducing NO, although it removed the most heat
of any single coil, at 3.8 percent of the heat released. It appears
significant that a small coil at the right location has a very large
effect on NO emission.
The curves in Figure 10 show that the combination of staged
air admission and cooling resulted in lower NO emission levels than
either staged air admission or cooling used independently. The NO
emission levels for Configurations D-2, D-3, and D-4 are all lower
than those for the uncooled D-l having similar air admission, and
well below those for Configuration B, the cooled lean-primary burner.
Temperature measurements in Configuration D-4 showed that the primary-
zone exit temperature was 2340 F and the outlet temperature was about
2800 F when fired at 50 Ib/hr. These temperatures are too low for NO
formation at high rates.
BATTELLE — COLUMBUS
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19
Configuration E
Configuration E, shown in Figure 11, was an improved rich-
primary burner. It was similar to Configuration D except that a smooth,
water-cooled cone was installed at the burner end to replace the conical
cooling coil, which interfered with air flow from the burner throat, and
a 1/8-in. film-air slot was installed between rings of dilution-air holes
near the outlet. The air flow to the primary zone was 51 percent of the
total. The cooling surfaces removed 10 percent of the heat released at a
firing rate of 50 Ib/hr, this percentage decreasing at higher firing
rates and increasing at lower firing rates. All cooling surface was cooled
in all tests.
When Configuration E-l, without the film-air slot, was evaluated
it was found that CO concentrations along the outside surface of the burner
were high enough to significantly increase overall CO emission levels. Addi-
tion of the film slot, to make Configuration E-2, flattened the burner-
outlet CO profile and reduced the CO emission level significantly.
Figure 12 shows CO, HC, and NO emission levels for Configuration
E-l without the film slot, with a comparative curve for CO with the film
slot. The improvement in CO level with the film slot is evident. The
data of Figure 12 were obtained at a firing rate of 50 Ib/hr, and show
that CO emission goals can be met when firing with 130 percent of stoich-
iometric air or more. HC and NO emissions, measured without the film slot,
are also below emission goals over a wide operating range.
Data for Configuration E-2 were obtained in EGR studies and
are presented in the section that follows.
BATTELLE - COLUMBUS
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Secondary-Air Holes
30 holes, 1/4 diam;
2 holes, 1/2 diam
16 holes, 3/8 diam
16 holes, 3/8 diam
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Percent Stoichiometric Air
FIGURE 11. BURNER CONFIGURATION E
FIGURE 12. VARIATION OF CO, HC, AND NO WITH PERCENTAGE
OF STOICHIOMETRIC AIR FOR CONFIGURATION E-l
Firing rate, 50 Ib/hr
-------
21
EXHAUST-GAS RECIRCUIATION
Experimental Techniques
Exhaust-gas recirculation (EGR) is a recognized technique for
reducing NO emission from combustion systems. The primary effect of
X
EGR is to dilute the flame, with a resulting reduction in flame tempera-
ture. Secondary effects include the inhibiting effect of the recirculated
moisture, and the reduction of peak flame temperatures for stoichiometric
mixtures, which may occur in local regions. The overall effect is a signi-
ficant reduction in NO emission.
X
EGR studies were carried out using a lean-primary burner (Con-
figuration B) and a rich-primary burner (Configuration E), both cooled.
The effects of EGR on both burner types were explored in 242 tests cover-
ing a range of air-fuel ratios, firing rates, and percentages of recircu-
lation for each burner. All cooling surfaces in both of these burners
were cooled in all tests.
Figure 13 shows schematically the arrangement of apparatus used
for EGR studies, and Figure 14 is a photograph of the principal piping,
with emission-measurement instruments in the background. Figure 15 shows
the burner outlet, with the 3-inch recirculation pipe at the center, and
an air-cooled, 12-point gas-sampling rake installed across the burner outlet.
The EGR studies were carried out using the apparatus shown in
Figures 13, 14, and 15. The recirculated exhaust gas was drawn from the
center of the burner outlet and passed through a stainless-steel, water-
cooled pipe in which gas temperature was reduced to about 400 F. The
time for gas flow through the cooled section at 110 Ib fuel/hr was about
30 millisec, comparable to that for gas flow through a boiler. Both air
flow and recirculated-gas flow were controlled by adjustment of dampers.
Gas temperature was controlled by the number of water-cooled sections used;
for low flow rates, only one or two sections were cooled.
The definition of "Percent EGR" used throughout this report is:
^ (recirculated gas) «,
Percent EGR = (recircuiated gas) + (combustion air) + (fuel)
All quantities are in units of weight flow, Ib per hr.
BATTELLE - COLUMBUS
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22
000
000
. = Air orifice
Gas-and-air mixer
Gas damper
/
58"
Gas onfice-
Gas sampling station
3" ID-/
(••-Thermocouple - gas analysis
- Damper for
flow control
-Fan
Water-cooled
exhaust-gas
return line
-Combustor
\
•Emission measurements
FIGURE 13. SCHEMATIC ARRANGEMENT OF APPARATUS FOR EGR STUDIES
FIGURE 14. PHOTOGRAPH OF APPARATUS FOR EGR STUDIES
-------
FIGURE 15. PHOTOGRAPH OF BURNER OUTLET SHOWING WATER-COOLED
RECIRCULATION PIPE AND 12-POINT AIR-COOLED
SAMPLING RAKE
-------
24
Emission Data for Configuration B
Figure 16 shows the variation of HC, CO, NO, and NO with the
X
percentage of stoichiometric air for Configuration B fired at 50 Ib fuel/hr,
and for various amounts of recirculated exhaust gas. HC emissions,
at the top of the figure, were below 5 ppm over a wide range of air-fuel
ratios and recirculation ratios. Only one point exceeded emission goals,
and this was for an extremely rich mixture. EGR appears to have no signi-
ficant effect on HC emissions.
CO emissions decrease with increasing air-fuel ratios, and are
generally lower with EGR than without it. Almost all of the data points
fall to the left of the curve shown on the data plot. In general, emission
goals are met over the firing range from 130 to 170 percent of stoichio-
metric air, and CO levels are below 200 ppm for the leaner mixtures rang-
ing from 150 to 170 percent of stoichiometric air.
NO and NO emissions, at the bottom of Figure 16, show very large
X
effects of EGR and of excess air. At 150 percent of stoichiometric air,
NO emissions levels exceed the goal without EGR, approach the goal with
X
10 percent EGR, and are well below the goal with 15 and 22 percent EGR.
Although earlier data without EGR showed that Configuration B could not
meet emission goals for NO , this burner is very responsive to EGR and
X
could meet all goals with about 15 percent EGR for all EGR rates.
At the outlet of the cooled section of the water-cooled recirculation
loop the CO concentration was always near zero even when CO concentrations
at the burner outlet were above 2,000 ppm. NO and NOX concentrations were
consistently higher in the recirculation loop than at the burner outlet by
a few ppm. With 700 ppm CO at the burner outlet, concentrations of NO and
NOX in the recirculation loop were 7 ppm and 1 ppm higher than at the burner
outlet. With 2,000 or more ppm of CO at the burner outlet, NO and NOX levels
in the recirculation loop were 13 ppm and 6 ppm higher than at the burner
outlet. Total NOX levels for these tests ranged from 30 to 40 ppm. It is
not known whether NO and NOX are formed in the recirculation loop, or whether
the CO in the sample from the burner outlet influences the NO and NOX measure-
ments made by the chemiluminescent analyzer.
BATTELLE — COLUMBUS
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25
1200
800
I
o"
o
400
?2000
.F-ircenf f GR
• 0
o 10
f 15
A 22
-1976 Fed. Std.
100
120 140 160 180 200
Percent Stoichiometric Air
FIGURE 16. VARIATION OF HC, CO, NO, AND NOjj WITH PERCENTAGE
OF STOICHIOMETRIC AIR FOR CONFIGURATION B
Firing rate, 50 Ib/hr
BATTELLE — COLUMBUS
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26
An interesting aspect of the NO and NO data is that the concen-
X
tration of NO (the width of the shaded band between NO and NO curves) is
*• x
about 10 ppm for any level of NO . Thus, the percentage of total NO as
" X
N02 varies from about 15 percent with 90 ppm of NO to 50 percent with 20
ppm of NO «
x
Figure 17 shows emission data for Configuration B expressed as
Emission Index (g emissions per kg fuel), plotted against firing rate. All
points in Figure 17 are for 145 to 155 percent of stoichiometric air except
where noted by numbers beside points, which show the actual percent air.
HC emissions, at the top of the figure, are very low at the higher
firing rates, but rise at rates of 20 Ib/hr and less. However, all values
are below the emission goal except two points, both with richer than optimum
fuel-air ratios.
CO emissions are generally within goals, but three test points
at low firing rates exceed them,. In order to meet CO emission goals at the
lower firing rates relatively lean mixtures, in the range of 150 to 200
percent of stoichiometric air,are required. No data for firing 110 Ib
fuel/hr with 150 percent of stoichiometric air were taken because of fan
capacity limits. However, extrapolation of data for richer mixtures
(plotted against percent of stoichiometric air) indicates a level of about
2 g/kg CO, falling below the curve shown.
NO emission levels, at the bottom of Figure 17, meet goals for
X
firing rates of 20 to 110 Ib/hr with 10 or 15 percent EGR. However, NO
X
emission levels of about 2 g/kg exceed the goal for firing rates of 5 and
10 Ib/hr.
From Figure 17 it is evident that a lean-primary cooled burner
with 10 to 15 percent EGR, fired at 150 percent of stoichiometric air,
can meet all emission goals except for NO emission at very low firing
X
rates. At these lowest firing rates the HC and CO increase if air-fuel
ratio is increased, so that there is no operating point where CO, HC, and
NO emissions are all below the goals. The quantity of NO emitted at the
x x
low firing rates is probably small, however, as the emission index is only
36 percent above the goal.
IATTELLE — COLUMBUS
-------
27
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-------
28
Emission Data For Configuration E-2
Configuration E-2 is the cooled, rich-primary burner shown in
Figure 11. All of the data below were taken with the film-air slot in-
stalled.
Figure 18 shows the variation of HC, CO, NO, and NO with the
X
percentage of stoichiometric air for Configuration E-2 fired at 50 Ib
fuel/hr, and for various percentages of EGR. HC emissions, at the top of
Figure 18, were extremely low for all tests, and well within emission goals,
CO emission levels were very similar to those for the lean-
primary burner, meeting goals with more than 130 percent of stoichiometric
air, and falling to 200 ppm at 150 percent of stoichiometric air. The
effect of EGR appeared negligible, and was less than that shown in Figure
16 for the lean-primary burner.
NO emissions varied both with EGR and with excess air. With
x
no EGR the NO emission level straddled the goal curve over the range of
X
excess air. On some days it was below the goal curve and on other days
it exceeded the goals as shown, depending upon air humidity and other
variables not clearly defined. However, with 16 percent EGR, the NO
X
emission level was at approximately half of the emission goal, and in-
creasing percentages of EGR resulted in further reductions of NO
X
emission levels.
The difference between NO and NO values was measured at a
x
nearly constant value of 10 ppm, representing 10 ppm of NO , as found
in Figure 16. The NO curves are shown separately in Figure 18 because
they intermingle with N02 curves and make it difficult to see the relation
between NO level and percent of EGR.
x
From the data of Figure 18 it appears that Configuration E-2
meets emission goals when fired with 145 to 155 percent of stoichiometric
air, and without EGR. EGR has no effect on HC or CO emissions, but re-
duces NO emission significantly. With about 15 percent EGR, NO emission
x x
levels would be about half the goal, and other emissions would be even less.
BATTELLE — COLUMBUS
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29
I
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Percent EGR
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Percent Stoichiometric Air
FIGURE 18. VARIATION OF HC, GO, NO, AND NOX WITH PERCENTAGE OF
EXCESS AIR AND PERCENT EGR FOR CONFIGURATION E-2
Firing rate, 50 Ib/hr
BATTELLE — COLUMBUS
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30
Figure 19 shows emission data for Configuration E-2 expressed
as g/kg fuel, plotted against firing rate. All points in Figure 17 are
for 145 to 155 percent of stoichiometric air except where noted by numbers
beside points, which show the actual percent air.
HC emissions are generally very low, at zero, except for four
points at low firing rates. HC emissions should be well below emission
goals for all firing conditions above 5 Ib/hr, but it is necessary to
use the optimum air-fuel ratio to avoid excessive HC emission levels at
lower firing rates.
CO emissions are generally below 2 g/kg at firing rates above
20-lb fuel/hr, far below the contract goal of 16.25 g/kg or the 1976 stan-
dard of 11.8 g/kg. However, CO emissions rise rapidly to the range of
6 to 9 g/kg at 5 Ib/hr, with one point at 16.5 g/kg for firing with 144
percent of stoichiometric air. It appears necessary to keep excess air
above 60 percent at this firing rate to assure meeting CO emission goals,
and there is a possibility of excessive CO emission at firing rates below
5 Ib/hr, where flame shape is controlled by atomizing air and little can
be done to influence emissions. CO emissions appear to be independent of
the percent of EGR.
NO emission levels are all below the emission goal of 1.4 g/kg,
X
with almost all points falling at or below 1 g/kg. The NO level is de-
X
pendent upon the percentage of EGR, decreasing as the percentage of EGR
is increased.
From Figure 19 it appears that Configuration E-2, the cooled,
rich-primary burner, can meet all emission goals without EGR, and that
further reduction in NO emission is obtained by use of EGR. It appears
X
desireable to operate with about 150 percent of stoichiometric air over
the range of 10 to 110 Ib/fuel/hr, with somewhat higher air-fuel ratios at
5 Ib/hr and less in order to meet CO and HC emission goals.
BATTELLE — COLUMBUS
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31
3
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Percent
• *
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A 2
a 2
EGR
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6
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6 - -
Percent EGR
1976 Fed. Std.
145-155% stoichiometric air
E - Cooled, rich-primary burner
145 % stoichiometric air
Percent EGR
40 60 80
Fuel Rote, Ib/hr
100
120
FIGURE 19. VARIATION OF EMISSION INDEX WITH FIRING RATE
AND PERCENT EGR FOR CONFIGURATION E-2
Firing rate, 50 Ib/hr
B AT TEULE — COLUMBUS
-------
32
DISCUSSION OF THERMAL AND CHEMICAL ASPECTS OF EGR
EGR is extremely effective in reducing NO emissions, and has
X
the thermodynamic advantage that it does not increase stack loss. Further-
more, data obtained during this program indicates that, pound for pound,
exhaust gas is a more effective diluent than air in reducing NO emission.
X
This is not true of all data; for example, the data for the Steam Engine
(2)
Systems combustion system indicate that exhaust gas and excess air can
be interchanged with exactly the same results. The differences in data
from the two sources, coupled with the differences in the combustion sys-
tems to which they apply, provide a unique insight into the mechanism
through which EGR and air, as diluents reduce the emission of NO .
X
The data reported by SES applies to a premixed, vaporizing
burner in which a homogenous mixture of gasoline vapor and air is burned
in a ported burner much like a natural-gas burner. If the mixture is
truly nomogeneous, all of the mixture passing through the flame front
experiences the same time and temperature history, and peak temperature
is controlled by the overall, or bulk fuel-air ratio. With this system,
when diluent exhaust gas is premixed with the air and fuel, the effect
on NO formation is approximately the same as that obtained by dilution
X
with air, and good correlation of data can be obtained if NO emission
X
is plotted against mass flow of air-plus-gas. This correlation is indica-
tive that the effects of EGR in this system are almost entirely thermal
in nature, and significant effects of gas composition are not evident.
The emission data for Configurations B and~E are completely
different, showing that exhaust gas is considerably more effective as a
diluent than is air. When NO concentrations are plotted against total
X
weight of air plus gas, distinctly different curves appear for each
different level of exhaust gas recirculated. The percentage reduction
of NO caused by introducing exhaust gas as a percentage of the air
X
supplied is about the same at any level of air-fuel ratio, even though
a considerably greater flow of exhaust gas is needed at high values
BATTELLE — COLUMBUS
-------
33
of excess air than at low values. Thus, the NO emission level correlates
X
better with the oxygen content of the air-and-gas mixture entering the
burner than with the mass flow, so that thermal capacity of the bulk mix-
ture is not the most significant variable.
In contrast with the SES burner, in which fuel vapor and combus-
tion air are completely premixed before burning, Configuration B and E are
such that fuel atomization, droplet evaporation, and mixing with combustion
air all take place within the combustion space, and significant gradients
of fuel-air ratios are possible. For example, as a fuel drop evaporates,
the fuel-air mixture at the drop surface may be richer than stoichiometric
but, as the vapor diffuses away from the drop surface, the mixture becomes
leaner, passing through stoichiometric and eventually reaching the bulk
mixture conditions. Another source of fuel-air ratio gradients is flow
eddies, in which local mixtures may vary from rich to lean, passing through
stoichiometric at some surface in the eddy; these may be fixed eddies
attached to a flameholder, or small eddies of fuel-and-air mixture passing
through the burner. The exact form of the local regions in which mixture
gradients occur is not obvious, but the general nature of the phenomena
involved might be considered under the general concept of "unmixedness".
The responsiveness of a burner to EGR, as against the responsiveness to
excess air in reducing NO emission provides a unique and effective way
X
of measuring the degree of unmixedness in a burner«
The physical explanation of the effect of unmixedness on NO
X
emission is that, as the mixture ratio of fuel and air in a local region
varies from rich to lean, temperature gradients corresponding to local
mixture ratios are possible, with the peak temperature corresponding to
the local mixture most closely approaching a stoichiometric mixture.
NO production in these peak-temperature regions would be considerably
greater than that at the average temperature for the overall mixture.
When unmixedness is considered in comparing flame dilution
with exhaust gas with that for dilution with air, it is evident that dilu-
tion with air still permits local hot spots approaching the adiabatic
flame temperature for a stoichiometric mixture. However, premixing of
BATTELLE — COLUMBUS
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34
combustion air with exhaust gas reduces the oxygen content, increasing
the amount of inert diluent that must be heated in the local stoichio-
metric-mixture zone, thus reducing the highest local peak temperatures.
This is reflected in lowering of NO concentration in the exhaust.
X
Another independent factor influencing the effectiveness of
EGR is the water content of the recirculated gas. It is known that air
humidity affects NO production to the extent that addition of one percent
X
of moisture by weight to combustion air will reduce NO production by about
(3) x
20 percent. Considering that the exhaust gas normally contains about ten
percent of moisture, recirculation of ten percent of the exhaust gas would
add approximately 1 percent to the moisture in the combustion air, and this
should reduce NO emission by about 20 percent.
X
EFFECT OF FUEL NITROGEN CONTENT ON NO EMISSION
—. x
In the course of burner development it was found that the per-
centage of chemically combined nitrogen in different lots of fuel varied
considerably, and an influence of fuel nitrogen content upon NO emissions
X
was suspected. This influence was explored by doping a low-nitrogen Jet-A
Fuel with pyridine, C H N, to provide several levels of nitrogen content.
(Only the chemically combined nitrogen in the fuel influences NO emissions,
X
and dissolved nitrogen has no significant effect.)
In operating with doped fuel, Configuration E-l was fired at a
constant fuel rate of 50 Ib/hr and the air flow was varied over a range
from 120 to 175 percent of stoichiometric air. This provided a range of
air-fuel mixtures in the primary zone varying from 60 to 85 percent of
stoichiometric air; thus, the primary zone was fuel-rich under all firing
conditions.
Figure 20 shows the variation of NO and NO emission levels with
fuel nitrogen content and with the percentage of stoichiomentric air. It
is evident from Figure 20 that N0x emission increases significantly with
increasing fuel nitrogen content. It also varies somewhat with percentage
of stoichiometric air.
BATTELI-E — COLUMBUS
-------
35
NO and NO, data using 300 F
sampling system, TECO Chemilumines
cent analyzer with N02 to NO
converter
120 140 160 ISO 200
Percent Stoichlometric Air
FIGURE 20. VARIATION OF NO AND NOXEMISSION WITH FUEL NITROGEN CONTENT
in
° 80
O)
.1 60
"35
if 40
"o
1 20
V
* 9)
c£>L
>e —
—
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-=d?I
lemilu
*-^—
minescence
f
Anolys
Percent
Theoretical Air
o 120
. + 150
175
0 0.1 0.2 0.3
is
0.4 0.
Percent Chemically-Bound Nitrogen in Fuel
FIGURE 21. PERCENT OF FUEL NITROGEN CONVERTED TO
AS FUNCTION OF FUEL NITROGEN CONTENT
BATTEULE - COLUMBUS
-------
36
The data of Figure 20 were taken using a 300 F air-cooled, 12 point
sampling rake and a heated stainless-steel sample line to a TECO
chemiluminescnece analyzer. The analyzer included a converter to convert
N02 to NO, consisting of a 6-ft length of 1/8-in. stainless steel tubing
heated to 1200 F NO was determined by by-passing the converter and NOV
X
was determined by passing the sample through it.
Approximately equal values for NO and NOX are shown in Figure 20,
indicating that N0£ concentration is negligeable. The NO values measured
previously with the NDIR analyzer were also similar. It is believed that
the low N02 concentration reflects reduction of N02 to NO in the presence
of high oxygen atom concentrations, as discussed later in this report.
If the NO formed by fixation of atmospheric nitrogen is subtracted
from the total NO in the exhaust gas, the percentage of fuel nitrogen
converted to NO can be calculated. Such a calculation is very sensitive to
small errors in NO measurement when fuel nitrogen content is low, but it
is possible to obtain consistent curves by careful selection of an NO value
for zero fuel nitrogen.
Figure 21 shows the percent of fuel nitrogen converted to NOX as a
function of fuel nitrogen content and percentage of theoretical air. The
conversion efficiencies shown range from about 90 percent for very small
values of fuel nitrogen, to about 50 percent at the highest value of fuel
(4 5)
nitrogen. These values are within the ranges reported by other experimenters. '
The organic nitrogen content of the undoped fuel was measured in samples
taken from the fuel nozzle before each group of tests. Values of 0.003 and
0.01 percent were measured, equivalent to values of 30 and 100 ppm. These
values are high for Jet A fuel and may reflect some system contamination.
BATTELLE — COLUMBUS
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37
ANALYTICAL PREDICTIONS OF HEAT
TRANSFER AND NO GENERATION
Early in the development of the combustor a computer program was
developed for study of local wall temperatures and heat-transfer rates,
local gas temperatures, and levels of NO generated. With this analytical
approach, excellent predictions of wall temperatures and heat-transfer
rates were possible, but the predicted levels of NO generation were far
lower than the levels measured experimentally. The difference is believed
to be the result of high gas temperatures and high oxygen-atom concentra-
tions within small local regions not treated adequately in the analysis.
The analytical method was based upon the Hottel and Cohen radia-
tion analysis program J , modified to allow for variable flame emissivity.
This was considered essential in this study to show the effect of luminous
portions of the flame occupying only a portion of the combustor, especially
at the lower firing rates. The basic procedure in the Hottel and Cohen
method involves dividing the combustion chamber into a number of gas zones
of different temperatures surrounded by surface zones. The temperature of
each gas zone is taken as the well-stirred temperature considering the flow
into and out of the zone, the heat release within the zone, and the heat
transfer by radiation and convection into and out of the zone. Radiation
interchange is included from each surface to every other surface, from
each surface to all gas zones, and from each gas zone to every other gas
zone. The complexity of this radiation interchange problem plus the itera-
tive nature of the heat balance solutions for each zone temperature requires
the use of a high-speed computer for even a relatively small number of
zones.
With this analytical approach, temperature distributions within
gas zones, wall temperatures, and heat-transfer rates from gas zones and
to walls, were carried out. In general, it was possible to predict quite
well the metal wall temperatures, for example. However, the local gas
temperatures computed were well-mixed, uniform temperatures for each zone,
and these were based upon an assumed rate of heat release by combustion
within each zone. The program did not provide a means for predicting
peak gas temperatures in very small local regions, or to predict local
combustion intensity.
BATTELLE — COLUMBUS
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38
The thermal program was combined with a kinetics program based
upon Zeldovitch kinetics for NO production. The mean zone gas temperatures
were such that the predicted levels of NO generation were much lower than
the actual levels as measured experimentally, indicating a deficiency in
this analytical approach.
The kinetics model assumed equilibrium oxygen-atom concentration,
which, with zone temperature, determines NO generation. However, there
is now considerable evidence that oxygen-atom concentrations in flame
fronts may exceed equilibrium values by many times, and this may be one
factor in the low values of NO predicted. Another contributing factor is
the probability that much of the NO is formed in small local regions where
reaction rates are high and local temperatures are well above the overall
zone temperature used in the calculation.
It was found that the analytical approach used provides excellent
prediction of local wall temperatures and heat-transfer rates, but that the
local gas temperatures, especially in small local zones, may exceed the
predicted temperatures for well-mixed zones. A different type of analy-
tical approach seems necessary for these regions, which probably account
for most of the NO produced. Recent experimental work has shown that NO
levels similar to those found in the Rankine-cycle burner can be produced
(8)
with reaction periods as short as 0.2 millisec under appropriate con-
ditions. The residence time in the Rankine-cycle burner is about 10
millisec at full firing rate, which is about 50 times the period needed
for NO generation.
BATTELLE — COLUMBUS
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39
AUXILIARY POWER REQUIREMENTS
Contract goals included a limit of 2 hp for auxiliary power
for burner operation. This power level is shaft power into fans and
pumps and does not include electric motor or hydraulic drive inefficiency.
Fan power is that for burner pressure drop only, and does not include an
allowance for boiler pressure drop, which would increase the power required,
Table 2 summarizes auxiliary power requirements for Configuration
E-2. Data were calculated for a fuel flow of 110 Ib/hr (2,000,000 Btu/hr)
and 140 percent of stoichiometric air, and the effect of two levels of EGR
are included. Fan efficiency of 70 percent was assumed. Fuel-pump power
is nominal and was assumed to be 0.1 hp, and atomizing-air compressor
power is based upon compressor efficiency of 66 percent.
TABLE 2. SUMMARY OF AUXILIARY POWER REQUIREMENTS
FOR CONFIGURATION E-2
Atomizing air compressor
Fuel pump
Combust ion- air fan
Total auxiliary power
Shaft
No EGR
0.5
0.1
0.94
1.54
Horsepower
10% EGR
0.5
0.1
1.38
1.98
Input
17% EGR
0.5
0.1
1.56
2.16
From Table 2 it can be seen that auxiliary power input for Con-
figuration E-2 is within the 2-hp goal, and that up to 10 percent EGR can
be used before this goal is exceeded. Configuration E-2 meets emission
goals for CO, HC, and smoke without EGR, but is marginal in emission of
NO . The use of 10 percent EGR would reduce NO to a value safely within
x* x
emission goals without exceeding auxiliary power goals.
BATTELLE — COLUMBUS
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40
GAS SAMPLING AND ANALYSIS STUDY
Early in the program it had been observed that NO concentrations
were lower than expected when CO concentrations were above 2,000 ppm. It
was suspected that the high CO concentrations in the gas might be inter-
fering with accurate NO determinations, or that CO and NO were reacting
in the sampling system to destroy NO.
EGR studies offered a means of comparison of NO and NO data
for gas samples taken simultaneously from two parts of the system,, The
samples taken from the burner outlet were often high in CO concentration,
but the samples taken from the EGR loop contained no CO. The CO was burned
to CO in passing through the water-cooled EGR section, in which the resi-
dence time varied from a minimum of 30 millisec to more than one second,
depending on firing rate and percentage of EGR.
Gas samples were taken from the burner outlet using a 12-point
stainless steel rake, air cooled to 300 F. Samples from the EGR loop
were taken at the downstream end of the water-cooled section, shown in
Figure 13, where the gas temperature was 400 F or less, using an uncooled
stainless steel probe at the centerline of the pipe. Sample lines to the
chemiluminescence analyzer were heated to avoid water condensation.
Figure 22 shows the trends of NO and NO data for 64 tests, for
X
samples from the burner outlet and the EGR loop. For Configuration B,
the lean-primary burner, the NO concentration at the burner outlet was
always lower than the NO concentration by about 10 ppm. However, NO
X
concentration in the EGR loop was higher, approaching the NO value.
X
For Configuration E-2, the rich-primary burner, NO concen-
trations at the burner outlet were lower than NO there by about 5 ppm,
X
and lower than the NO concentration in the EGR loop. The NO concen-
X
tration was the same at the burner outlet and in the EGR loop.
IATTELLE — COLUMBUS
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41
40
a 30
a.
TJ
o 20
O
10
—At burner outlet
•—NO
r
In EGR loop
A
Configuration B
Lean primary
Configuration E-2
Rich primary
FIGURE 22. TRENDS OF NO AND NOx DATA AT BURNER OUTLET AND IN EGR LOOP
Points are average data for 64 tests
BATTELLE — COLUMBUS
-------
The data plotted in Figure 22 suggest that the NO entering
the EGR loop is converted to NO in the loop, since the NO concentration
increases and the total NO remains constant. It is believed that the
x
following reaction takes place:
N02 + 0 - NO + 02.
Support for this reaction is provided by the results of un-
published research now in progress at Battelle-Columbus. The same type
of reaction has been observed in bench-scale diffusion flames. In this
research the oxygen-atom concentration at the end of the flame is
measured and is high enough to promote this reaction.
The oxygen-atom reaction is insensitive to gas temperature and
would take place equally well in a hot probe or a cold probe.
It was concluded that the variations in NO and NO concentra-
2t
tions in various parts of the system are characteristic of the system
and do not represent sampling or measurement errors.
BATTELLE — COLUMBUS
-------
43
REFERENCES
(1) C. J. Halstead and A.J.E. Munro, "The Sampling, Analysis, and
Study of the Nitrogen Oxides Formed in Natural Gas/Air Flames",
Proc. Conf. on Natural Gas Research and Technology, Chicago, Illinois,
February 28, 1971, sponsored by the American Gas Association and the
Institute of Gas Technology.
(2) "Design and Development of an Automobile Propulsion System Utilizing
a Rankine-Cycle Engine (Water Base Fluid)", Fourth Quarterly Progress
Report, March 15, 1972, EPA Contract No. 68-04-0004, Mod 2, Steam
Engine Systems Corporation.
(3) F. W. Lipfert, "Correlation of Gas Turbine Emission Data", ASME
Paper No. 72-GT-60. Presented at ASME Gas Turbine Conference,
March, 1972, San Francisco.
(4) D. W. Turner, R. L. Andrews, and C. W. Siegmund, "Influence of
Combustion Modification and Fuel Nitrogen Content on Nitrogen Oxides
Emissions from Fuel Oil Combustion", paper presented at AIChE Meet-
ing of December 1 and 2, 1971, San Francisco.
(5) G. B. Martin and E. E. Berkau, "An Investigation of the Conversion
of Various Fuel Nitrogen Compounds to Nitrogen Oxides in Oil Com-
bustion", paper presented at AIChE National Meeting, Atlantic City,
N.J., August 30, 1971.
(6) H. C. Hottel and E. S. Cohen, "Radiant Heat Exchange in a Gas-Filled
Enclosure: Allowance for Nonuniformity of Gas Temperature", AIChE
Journal, Vol 4, No. 1, March, 1958, pp 3-14.
(7) H. C. Hottel and A. F. Sarofim, "Radiative Transfer", McGraw-Hill,
New York, 1967.
(8) Craig T. Bowman, "Investigation of Nitric Oxide Formation Kinetics
in Combustion Processes: The Hydrogen-Oxygen-Nitrogen Reaction",
Combustion Science and Technology, Vol 3, 37-45 (1971).
BATTELLE - COLUMBUS
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44
i TECHNICAL REPORT DATA
> (Picatc read Inunctions on the revrm before comftletmg) ^
;• REPORT NO. a.
APTD-1516
TITLE AND SUBTITLE
S Low Emission Burners for Automotive Ranklne
Cycle Engines
7. AUTHORI&)
H.R. Hazard, R.D. Fischer, and C. McComis
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Battel le
Columbus Laboratories
505 King Avenue
Columbus, Ohio 1*3201
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. ENVIRONMENTAL PROTECTION AGENCY
Mobile Source Pollution Control Program
Advanced Automotive Power Systems Development 01 v.
| Ann Arbor, Michigan 48105
3, «iciPteNT"s ACCBSSSOWNO,
~,.i.»»ni».ni..ii.». »^ inmiiM n mum HI .WWH^WV*********^™®*^""?"***™
8. REPORT 0AT« I
_AerULJiZl ' '_J
6. PERFO«M(NG OHQANI2ATION COOi 1
S. PERFORMING ONQANlf ItfON *ip0HT M0~|
10. PROGRAM £i.i'fti~ENT *«6". " " "
rf- fitttt ff4£y/w*N¥ii0. ' " ~*
EHS 7Q-H7
13. TYP6 OF REPORT ANO PERIOD COVfcWEO
14. SPONSORING AOf NCY CODE
16. SUPPLEMENTARY NOTES
, •'STRACT
:'. report discusses the results of a combustion technology program. The objective of
, program was to develop a full-scale burner suitable for an automotive Ranklne-
le engine. The contract goals include a 100:1 turndown ratio with firing rates from
«.,000,000 Btu/hr to 20,000 Btu/hr, combustion volume of 1.3 ft3 or less, and total me-
chanical power input to fans and compressors of 2 hp or less. The fan power was speci-
fied as that needed to overcome burner pressure drop, and does not include any addi-
tional power needed to overcome boiler pressure drop. Emission goals for the program
were the I960 AAPS Goals. In the course of the program burners based on four different!
burner concepts were developed and evaluated. Each of these was optimized by experiment
tal development, and data for the optimum configurations are reported. Burner Configu-
ration E-2, which met all contract requirements, was a rich-primary burner having in-
ternal cooling of the primary zone with boiler tubes. It could be operated over the
firing range from 110 to 1 Ib fuel per hour and met emission goals at all firing rates
The effect of EGR (Exhaust Gas Reelrculation) on NOX emission was explored on two
configurations. It was found that EGR reduced NOx emission significantly for both
configurations.
KEY WORDS ANO DOCUMENT ANALYSIS
DESCRIPTORS
Dilution Oxides of nitrogen
engines Analysis
Rankine cycle
Exhaust emissions
velopment
ties
sampling
Nitric oxide
Hydrocarbons
Carbon monoxide
Combustion chamber
^IDENTIFIERS/OPEN ENDED TERMS
Exhaust-gas reel rcuIaUor
Low emission burners
Jet A kerosene fuel
1980 AAPS goals
V. COSATI I ickl/O[i>U|i
138
20M
218
rEMENI
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