United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S2-86/080 Dec. 1986
&EPA Project Summary
Evaluation of the Efficiency
of Industrial Flares:
H2S Gas Mixtures and
Pilot Assisted Flares
J. H. Pohl and N. R. Soelberg
The U.S. Environmental Protection
Agency has contracted with Energy and
Environmental Research Corporation to
conduct a research program which will re-
sult in quantification of emissions from,
and efficiencies of, industrial flares. The
program is divided into four phases. Phase
I (Experimental Design) and Phase II (De-
sign of Test Facilities) have been reported
in EPA-600/2-83-070. Phase III (Develop-
ment of Test Facilities) and the initial work
in Phase IV (Data Collection) have been
reported in EPA-600/2-84-095. Further
data collection has been reported in
EPA-600/2-85-106.
Initial results (EPA-600/2-84-095) were
limited to tests conducted burning pro-
pane/IM2 mixtures on pipe flares without
pilot flare stabilization. Further results
(EPA-600/2-85-1O6) reported the influ-
ence of the flared gas and flare head
design on destruction and combustion ef-
ficiency without stabilization by pilot
flares. The current report is the fourth in
the series and presents test data on the
combustion efficiency and destruction ef-
ficiency of (1) gas mixtures containing
H2S, and (2) flare flames with pilot flare
stabilization. The tests were conducted on
3- and 6-in* open pipe flares without
aerodynamic flame stabilization devices.
The following results were obtained from
this work:
• Gas mixtures of HZS/N2 can be
stably flared at much lower volu-
* Readers more familiar with the metric system may
use the conversion factors at the back of this
summary.
metric gas heating values than can
propane/N2 mixtures.
• Destruction and combustion efficien-
cies greater than 98% are obtained
for gas mixtures of H2S/N2 and
H2S/propane/Nz when the gas heat-
ing value is at least 1.2 times the level
required to produce a stable flame.
• For mixtures containing both H2S
and propane, H2S destruction effi-
ciency was consistently higher than
propane combustion efficiency.
• The gas heating value required to
maintain a stable flame, including the
heating value contribution of the pilot
gas, is 3 times lower with pilot assist
than without pilot assist on 3- and
6-in. open pipe flares without aero-
dynamic flame stabilization devices.
• Combustion efficiencies greater than
98% for pilot assisted flares are a-
chieved when the heating value is
greater than 1.2 times that required
to stabilize the flame.
• Increasing the pilot flow from 2 to 5
scfm, or the number of pilot flames
from one to three (on 3- and 6-in.
open flares without other flame stabi-
lization) could decrease the heating
value of the gas required for stability
by about 10-20%.
This Project Summary was developed
by EPA's Air and Energy Engineering Re-
search 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).
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introduction
The test program "Evaluation of the Ef-
ficiency of Industrial Flares" has been
funded by the U.S. EPA and conducted at
the Energy and Environmental Research
Corporation (EER) El Toro Test Site. This
program has been conducted in phases.
Phase I involved construction of a pilot-
scale flare test facility. During Phase II
combustion efficiency tests were con-
ducted on eight commercial and EER pro-
totype flare heads ranging in size between
3 and 12 in. in diameter. During Phase III,
effects of flare head design and gas com-
position on flare combustion and destruc-
tive efficiencies were studied. Commercial
Coanda steam assisted heads, pressure
heads, and an air-assisted flare head were
tested. Also, different gas mixtures con-
taining ammonia, 1,3-butadiene, ethylene
oxide, and hydrogen sulfide (H2S) were
tested.
Objectives
This phase of the work has two objec-
tives: (1) evaluation of H2S destruction ef-
ficiency for H2S-containing flare gases,
and (2) evaluation of the effects of pilot
assist on flare combustion efficiency.
In order to determine the limits of stable
flare operation for these gas mixtures and
pilot assisted flares, and key operating
conditions that affect flame stability and
efficiency, some conditions with poor sta-
bility and low combustion efficiencies
were measured. Such results merely indi-
cated flare operating performance at or
beyond the edge of the operating envel-
ope, and are not indicative of normal com-
mercial flare operation.
Destruction Efficiency of H2s
Before H2S destruction efficiency could
be evaluated, it was necessary to develop
techniques to accurately and reliably
measure H2S at plume concentration
levels of 0-1000 ppm, without interference
from SO2, present in levels between zero
and 10,000 ppm. Methods successfully
adapted for this measurement were
methylene blue and Draeger tubes. For
higher H2S gas concentrations (25 ppm
or greater), gas chromatography was also
used.
Destruction efficiency tests of H2S
were conducted using a 3-in. diameter
open pipe flara Flame stability limit curves
for these tests are shown in Figure 1.
There is good agreement between the cur-
rent 1985, ~5% H2S gas mixture tests
and the 1984, ~5% gas mixture tests. The
stability limit curve for the H2S/N2 gas
mixture tests is much lower than that for
7700
7000
\] 1984 ~ 5% HiS in Propane/N2 Mixtures
V 1985 ~ 5% HiS in Propane/Nz Mixtures
t> HzS/Nz Mixtures
100
0.1
1.0
100.0
10.0
Exit Velocity, ft/s
Figure 1. Flame stability curves for HzS gas mixtures flared using a 3-in. diameter open
pipe flare.
1000.0
the tests for the ~5% H2S in propane/N2
mixture. This shows that gas heating value
is not the only contributing factor to flame
stability. Other factors may be (1) higher
mixture strength in an H2S/N2 mixture
which has equivalent heating value to a
propane/N2 mixture, (2) wider flammable
range in air for H2S than for propane, (3)
lower adiabatic flame temperature of H2S
burned in a stoichiometric air mixture, and
(4) lower ignition temperature of H2S.
The combination of these factors appar-
ently enhances flame stability of H2S gas
mixtures.
Influence of Pilot Flares
Tests were also conducted using a pilot
assisted 3-in. open pipe flare. These tests
were conducted to measure the effects of
pilot assist on combustion efficiency. The
flare gas for these tests was propane di-
luted with N2 to reduce the heating value.
The pilot gas was utility-supplied natural
gas. Parameters tested were (1) flare size
(3- and 6-in.), (2) pilot number from one
to three, and (3) pilot gas flowrate, from
1 to 5 scfm. For these tests, the flare gas
heating value includes the contribution of
the pilot gas.
The flame stability limit for the pilot as-
sisted tests was difficult to determine, be-
cause the presence of a pilot effectively
prevented flame blowout, even at very low
flare gas heating values. Consequently, the
definition and determination of the flame
stability limit became more subjective. The
gas heating value required for 98% com-
bustion efficiency at a given flare gas exit
velocity was found to be the operating
point where the last faint flickers of orange
color disappeared and the flame envelope
became transparent. Such flare flames
usually had blue-orange cones near the
pilot and flare tips. In order to maintain
consistency with previous results reported
under this program, this operating point
was defined as the "stability limit." This
stability limit is specific to these tests
burning propane/N2 mixtures.
Stability curves for the 3-in. pilot assist-
ed flare are shown in Figure 2. Use of pilot
assist greatly enhances flame stability. For
3- and 6-in. unassisted open pipe flares,
operated with a propane/N2 gas exit
velocity of 40 ft/s, the minimum gas
heating value to maintain a flame is about
540 Btu/scf. If a 2 scfm natural gas pilot
is used, the total heating value (including
pilot contribution) can be reduced to 150
Btu/scf, when the flame envelope be-
comes transparent and, by definition, the
stability limit is reached. For the 6-in. flare,
the same heating value reduction can be
attained with the pilot at only 1 scfm.
Additional pilot assist, however, increases
flame stability only marginally. Increasing
the pilot gas to 5 scfm reduces the heating
value to only 120 Btu/scf for the 6-in. flare.
Increasing the number of pilots to two or
three while keeping the total pilot gas rate
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1000
900
800
700
oJ w
•$£400
Visual Observations
—^— o Faint orange flicker in flame envelope (stable)
• --— e Disappearance of faint orange flicker (stability limit)^
• Unstable
<£> 3-in. open pipe flare with single pilot at 2.1
scfm pilot gas rate (1984 tests)
Faint orange flicker
in flame envelope (stable)
Disappearance of faint orangej
flicker (stability limit) )
Unstable
0.1
Figure 2.
1.0
100
10.0
Flare Exit Velocity, ft/s
Flame stability curve for 3-in. pipe flare with a single pilot at a pilot gas
flowrate of 2.1 -2.2 scfm natural gas.
1000
constant at 2 scfm decreases the limiting
heating value to 130 Btu/scf for a 6-in.
pipe flare.
Conclusions
• Flame stability depends on com-
pounds present in the flare gas. Gas
mixtures of H2S/IM2 can be stably
flared at much lower gas heating
values than can propane/N2 or ~5%
H2S in propane/N2 gas mixtures.
• High H2S destruction efficiency is
achieved for H2S/N2 and ~5% H2S
in propane/N2 gas mixtures when
the gas heating value is at least 1.2
times the level required for flame
.-.-,• ,. stability. •• * ••• :- •,-• •-•"• ••••-•••••
• The total gas heating value required
for a stable flame, including pilot con-
tribution, is much lower for pilot
assisted flares than for the same un-
assisted flares.
• High combustion efficiency is achiev-
ed for the pilot assisted tests when
the gas heating value is at least 1.2
times the level required for flame
stability.
Conversion Factors
Readers more familiar with the metric
system may use the following factors to
convert the nonmetric units used in this
Summary.
Nonmetric
Btu
ft3
cfm
ft
in.
Times
1.055
0.0283
1.700
0.305
0.0254
Yields Metric
kJ
m3
m3/hr
m
rn
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J, H. Pohl and N, R. Soelberg are with Energy and Environmental Research
Corporation, Irvine. CA 92718.
Bruce A. Tichenor is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of the Efficiency of Industrial Flares: HzS
Gas Mixtures and Pilot Assisted Flares," (Order No. PB 87-102 372/A S; Cost:
$16.95. subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC27711
United Slates
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use S300
EPA/600/S2-86/080
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