United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-83-052 Jan. 1984
Project Summary
Flare Efficiency Study
Marc McDaniel
A full-scale experimental study was
performed to determine the efficiencies
of flare burners for disposing of hydro-
carbon emissions from refinery and
petrochemical processes. With primary
objectives of determining the combus-
tion efficiency and hydrocarbon de-
struction efficiency for both air- and
steam-assisted flares over a wide range
of operating conditions, the study
provides a data base for defining the air
quality impact of flaring operations.
Test results indicate that flaring is
generally an efficient hydrocarbon
disposal method for the conditions
evaluated.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory. Research Triangle
Park, NC, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The report summarizes an experimen-
tal study to determine the efficiencies of
flare burners as devices for the routine
disposal of hydrocarbon emissions from
refinery and petrochemical processes.
The primary objectives of this study were
to determine the combustion efficiency
and hydrocarbon destruction efficiency
for both air- and steam-assisted flares
over a wide range of operating conditions
that might be encountered in routine in-
dustrial applications. The study excluded
flaring conditions which might represent
large hydrocarbon releases during process
upsets, start-ups, and shutdowns.
Both government and industry envi-
ronmental officials are concerned with
the effects of flaring hydrocarbons on the
air quality. However, since flares do not
lend themselves to conventional emission
testing techniques, few attempts have
been made to characterize flare emissions.
Flare emission measurement problems
include: the effects of high temperatures
and radiant heat on test equipment, the
meandering and irregular nature of flare
flames due to external winds and intrinsic
turbulence, the undefined dilution of
flare emission plume with ambient air,
and the lack of suitable sampling locations
due to flare and/or flame heights,
especially during process upsets when
safety problems predominate.
Previous flare efficiency studies did not
encompass the range of variables en-
countered in the industrial setting. Limited
test conditions of flare types, relief gas
types, Btu content, relief gas flow rate,
and steam-to-relief gas ratios were
previously explored. This study was
intended to add to the available literature
on the subject by testing the flaring of an
olefin (propylene) in both air- and steam-
assisted flares with test variables of relief
gas flow rate, relief gas Btu content, and
steam-to-relief gas ratio.
Separate elements of this flare efficiency
study were sponsored by the U.S.
Environmental Protection Agency (EPA)
and the Chemical Manufacturers Associ-
ation (CMA). Other project participants
included John Zink Company (provided
the flares, test facility, andflare operation)
and Optimetrics, Inc. (operated EPA's
Remote Optical Sensing of Emissions
(ROSE) system). Engineering-Science,
Inc., operated the extractive flare sampling
and analysis systems and prepared this
report.
Technical Summary
Figure 1 is an overview of the equip-
ment used to operate and test the flares.
The test methodology utilized during the
study employed a special 27-ft sample
probe suspended by a crane over the
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Crude Propylene
Volume Tank\—& Slowdown Flare
©o
Engineering Science Analysis Trailers
Figure 1. Flare efficiency test systems.
flare flame. The sample extracted by the
probe was analyzed by continuous
emission monitors to determine concen-
trations of C02, CO, total hydrocarbons
(THC), S02, NOx, and 02. In addition, the
probe tip temperature, ambient air
temperature, and wind speed and direction
were measured. Integrated samples of
the flare gas were collected for hydrocar-
bon specie analysis by gas chromato-
graph. Particulate matter samples were
collected during the smoking flare tests.
Sulfur was tried as a tracer material in an
effort to determine the dilution of the flare
gas between the flare burner and the
sampling probe location; however, use of
this untried sulfur balance method for
determining dilution ratios was unsuc-
cessful.
The term "combustion efficiency" was
used during the study as the primary
measure of the flares' performance.
Conceptually, this term defines the
percentage of flare emissions that are
completely oxidized to CO2. Mathemati-
cally, combustion efficiency is defined as:
% CE = C02
Where:
CO2 — parts per million by volume of
carbon dioxide.
Steam
Measuring Station
CO = parts per million by volume of
carbon monoxide,
THC = parts per million by volume of total
hydrocarbon as methane, and
Soot = parts per million by volume of soot
as carbon.*
Table 1 summarizes the results of the
flare efficiency tests. The rigorous test
program included flare testing under 34
different operating conditions during 3
weeks in June 1982. Test variables
included Btu content of the flare gas
(propylene diluted with nitrogen), flare
gas flow rates, steam flow rates, and air
flow rates. Five of the 34 tests were
divided into 13 subtests for purposes of
data analysis because the flare operation
did not represent steady-state conditions.
The Btu content of the flare relief gas was
varied from 2,183 to 192 Btu/scf for the
steam-assisted flare, and from 2,183 to
83 Btu/scf for the air-assisted flare. The
relief gas flow rates ranged from 703 to
0.35 scfm (purge flow rate) for the steam-
assisted flare, and from 639 to 0.54 scfm
(purge flow rate) for the air-assisted flare.
Conclusions and Observations
• When flares are operated under
conditions representative of good
industrial operating practices, com-
I"! In most cases, the "Soot" term was zero
bustion efficiencies in the flare
plume are greater than 98%.
Steam- and air-assisted flares are
generally an efficient means of
hydrocarbon disposal over the range
of operating conditions evaluated.
Varying flow rates of relief gas have
no effect on steam-assisted flare
combustion efficiencies below an
exit velocity of 62.5 fps.
Varying Btu content of relief gases
has no obvious effect on steam-
assisted flare combustion efficiencies
for relief gases above 300 Btu/scf. A
slight decline in combustion effi-
ciency was noted for relief gases
below 300 Btu/scf.
Flaring with steam-to-relief-gas
ratios above 3.5 Ib/lb may lower
combustion efficiencies.
Flaring low Btu content gases at
high exit velocities may result in
lower combustion efficiencies for
air-assisted flares.
Smoking flares achieve high gaseous
hydrocarbon destruction efficiencies.
In many cases, where high combus-
tion efficiencies were observed, the
CO and hydrocarbon concentrations
observed in the flare plume were
about equal to those found in
ambient air.
Concentrations of NOX emissions in
the flare plume ranged from 0.5 to
8.16 ppm.
The combustion efficiency data
were insensitive to sampling probe
height within the normal operating
heights of the probe.
Further development of a technique
to use sulfur or another material as a
tracer material to determine the
flare dilution ratios is required.
Steam-assisted flares burning relief
gases with less than 450 Btu/scf
lower heating value were not ob-
served to smoke, even with zero
steam assistance.
1 Higher concentrations of total hy-
drocarbons and CO were not ob-
served during the purge rate flare
tests.
1 The meandering of the flame's
position relative to the sampling
probe with varying wind conditions
affected the observed values but had
no apparent effect on the combustion
efficiency values.
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Table 1. Flare Efficiency Test Results
Relief Gas
Test
Number
Flow,
scfm
Heating
Value,
Btu/scf
Steam - to - Relief -
Gas Ratio,
Ib/lb
Combustion*
Efficiency, %
Comments
Steam-Assisted Flare Tests
r
2
3
4
8
7
5
67
17
50
56
61
55
57
59a
59b
60
51
16a
16b
16c
16d
54
23
52
53
473
464
456
283
157
154
149
148
24.5
24.4
245
25.0
24.7
703
660
599
556
591
496
334
325
320
252
194
159
0.356
0.494
0556
0.356
2183
2183
2183
2183
2183
2183
2183
2183
2183
2183
2183
2183
2183
294
305
342
364
192
232
298
309
339
408
519
634
209
267
268
209
0688
0.508
0.448
0
0
0.757
1.56
0.725
0926
3.07
3.45
5.67
6.86
O.150
0
0
0
O
0
0
0.168
0
0
0
0
0
0
77.5
123
99.96
99.82
99.82
99.80"
98.81C
99.84
99.94
99.84
99.45
99.70
82.18
68.95
99.90
99.79
99.86
99.82
97.95
99.13
98.92
98.66
99.73
99.75
99.74
99.78
99.90
100.01
98.82
99.40
Incipient smoking flare
Smoking flare
Smoking flare
Incipient smoking flare
Sampling probe in flare flame
Steam-quenched flame
Steam-quenched flame
No smoke
No smoke
Incipient smoking flare
Smoking flare
Air-Assisted Flare Tests
26
65
28
31
66
29a
29b
64
62
63
33
32a
32b
481.6
159
157
22.7
639
510
392
249
217
121
0.714
0.556
0.537
2183
2183
2183
2183
158
168
146
282
153
289
83
294
228
Air Flow
Hi
0
Hi
Lo
O
Lo
Lo
Lo
Lo
Lo
Lo
Lo
Lo
99.97
99.57"
99.94
99.17
61.94
54.13
64.03
99.74
94.18
99.37
98.24
98.94
98.82
Smoking flare; no air assistance
Detached flame observed
Detached flame; no air assistance
Detached flame; with air assistance
Flame slightly detached
'Does not account for carbon present as soot.
"When soot is accounted for, C£ = 91.21%.
cWhen soot is accounted for. CE - 92.72%.
"When soot is accounted for, CE = 97.95%
Marc McDaniel is with Engineering Science, Inc., Austin, TX 78722.
Bruce A. Tichenor is the EPA Project Officer (see below).
The complete report, entitled "Flare Efficiency Study, "(Order No. PB83-261 644;
Cost: $14.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
•ftUS GOVERNMENT PRINTING OFFICE 1984-«9-015/7265
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