United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711 '/ ^ x\
Research and Development
EPA-600/S2-83-070 Jan. 1984
Project Summary
Evaluation of the Efficiency of
Industrial Flares: Background—
Experimental Design—Facility
D. Joseph, J. Lee, C. McKinnon, R. Payne, and J. Pohl
The U.S. EPA has contracted with
Energy and Environmental Research
Corporation to conduct a research
program which will result in the quanti-
fication of emissions from and efficien-
cies of industrial flares. The study is
being conducted in four phases: I—
Experimental Design, II—Design of
Test Facilities, III—Development of
Test Facilities, and IV—Data Collection
and Analysis. This report summarizes
results of Phases I and II.
The report discusses the technical
literature on the use of flares and
reviews available emission estimates.
Technical critiques of past flare efficien-
cy'studies are provided. The parameters
affecting flare efficiency are evaluated
and a detailed experimental test plan is
developed. The design o/ a flare test
facility is discussed, including flare tips,
fuel and steam supply/flow control and
measurement, emissions sampling and
analysis, and data acquisition and
processing.
Results of the testing program (Phases
III and IV) will be provided in a later
report.
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 docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
A flare is a device that allows the
economic safe disposal of waste gases by
combusting them. The waste gases are
injected into the open air through a tip,
designed to promote entrainment of the
ambient air and provide a stable flame
with a wide range of throughputs in high
crosswinds. To reduce flame radiation at
ground level, the flare tip must be
elevated (its height will depend on flame
size). If the heating value of the waste gas
is too low to sustain a flame, auxiliary fuel
may be added. Small flares may utilize
fans to provide some air premixing before
injection, but most large flares are
natural draft with optional steam injection
to promote fuel/air mixing. Flares are
used extensively to burn purged and
waste products from refineries, excess
production from oil wells, vented gas
from blast furnaces, unused gas from
coke ovens, and gaseous wastes from the
chemical industry.
An estimated 16 million tons/year of
gas may be flared in the U.S. The amount
is difficult to estimate because through-
puts fluctuate widely with time and are
seldom measured. The normal, time-
averaged throughput ranges from zero to
5 percent of design capacity, which is
exceeded only during emergencies or
upsets. The flared gases fall into three
categories:
• Low heating value gas produced in
blast furnaces which account for
60 percent of the weight and 19
percent of the heating value of the
estimated annual flared gases.
• Medium heating value gases pro-
duced in coke ovens and in the pet-
rochemical industry.
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• High heating value gases flared in
refineries which account for 18
percent of the weight and 32
percent of the heating value of the
estimated annual flared gases.
Pollutant emissions from flares result
from a failure to completely combust the
flared gases. The pollutant species are
normally carbon monoxide, hydrocarbon,
and soot; total emissions are assessed
based on an estimate of flare efficiency.
The efficiency of combustion of a flare,
which is a measure of its ability todestroy
the flared gas, is difficult to measure;
consequently, estimates of pollutant
emission indices vary. Estimates of flare
efficiencies vary widely: some are very
high, in excess of 99 percent; whereas,
others are as low as 70 percent. This leads
to the conclusion that emission factors
necessary for accurate quantification of
mass emissions are unknown. If flares
were 90 percent efficient, then emissions
of carbon monoxide and hydrocarbons
would be approximately 12 percent of
those emitted by all stationary sources.
More important are the contributions of
flares as localized sources because of
their concentration in refineries and steel
plants where they could be among the
most significant sources of pollutants if
the efficiencies are relatively low.
It can only be concluded that pollutant
emissions from flares are unknown. This
is due to a combination of uncertainties
in the quantity of gases being flared and
their composition, together with the
uncertainties in flare efficiency. Before a
decision can be made whether pollutant
emissions from flares are of concern, an
accurate assessment of flare efficiency
Table 1. Previous Flare Emission Studies
must be made. Theoretical estimates of
flare efficiency cannot be made, emission
measurements from operating flares are
difficult, and previous pilot scale studies
are contradictory or incomplete. Thus,
there is a need for a study to accurately
assess flare efficiency as a function of:
flared gas composition; throughput; flare
design and operation (steam injection,
etc.); ambient conditions; and scale.
Data from this study can be used to
provide an accurate assessment of
pollutant emissions from flares.
Deficiencies in Previous Flare
Emission Studies
Relatively few investigations, concerned
with pollutant emission or efficiency of
flares, have been reported in the open
literature.
Table 1 summarizes the most recent,
known studies, each of which addressed
one or more of the following topics:
• The emissions of incompletely
burned material.
• The distance required to burn the
flared gases.
• The impact of steam injection on
pollutant emissions.
• The effect of ambient conditions on
pollutant emissions.
Although these studies have made
valuable contributions to the knowledge
of flare performance, they neither allow
accurate determination of pollutant
emissions nor provide adequate informa-
tion on the effects of scale or flared gas
composition.
A review of the previous studies
indicates that data acquisition and
manipulation are common problems that
prevent accurate assessment of flare
efficiency. These problems are discussed
below in four main areas:
• Inability to close a material balance.
• Measurement of soot concentration.
• Difficulties caused by flare "inter-
mittency."
• Lack of scaling methodology.
Closure of Material Balance
The global (overall) efficiency of a flare
flame can be calculated if the inlet fuel
composition and mass flux are known,
together with the mass flux of all
hydrogen- and carbon-containing species
of flared material at a height above the
flame where all reaction has ceased.
There is more interest in that fraction of
the fuel flux that becomes air polluting
species rather than harmless CO2 and
Had. It is usual to concentrate on the
carbon in the fuel because all of the
ultimate air polluting species contain it
(i.e., CO, HxCy, soot). If the carbon fraction
of all product gas flux species is summed,
the result should equal the carbon
fraction of fuel mass flux. This isthe usual
mass balance concept and is an account-
ing check on the pollutant species
measurements. It is easy to state but
rather difficult to implement. Of the
studies in Table 1, only Siegel attempted
to close the mass balance. Generally he
was able to account for only about half of
the fuel carbon in the off-gas flux. Siegel
stated that the largest errors were
associated with the velocity measure-
ments needed to determine the mass
flux. Siegel circumvented the need for
Investigator Flare Tip Design
Flared Gas
Throughput
JO6 Btu/hr
Flare Efficiency
Palmer*
Lee & Whipp/e"
0.5 in. dia.
Discrete Holes in 2 in.
dia. cap.
Ethylene
Propane
0.4 - 2.1
0.3
>97.8
96 - WO
SiegeF
Howes et a/.d
Commercial Design
(27.6 in. dia. steam)
Commercial Design
(6 in. dia. air assist)
Commercial Design H.P.
(3 tips @ 4 in. dia.)
=50% Hz
plus light hydro-
carbons
Propane
Natural Gas
49- 178
44
28 (per tip)
97 - > 99
91 - WO
>99
"Palmer, P.A. "A Tracer Technique for Determining Efficiency of an Elevated Flare." E.I. du Pont de Nemours and Co., Wilmington, DE (1972).
bLee, K.C. and G.M. Whipp/e. "Waste Gas Hydrocarbon Combustion in a Flare." Union Carbide Corporation, South Charleston, WV(1981).
cSiegel, K.D. "Degree of Conversion of Flare Gas in Refinery High Flares," Ph.D. Dissertation, University of Karlsruhe (German), February (1980).
"Howes, J.E.. T.E. Hill, R.N. Smith, G.R. Ward, W.F. Herget, "Development of Flare Emission Measurement Methodology, Draft Report," EPA Contract
68-02-2686, Task 118(1981).
-------
further mass balance by using "local
burnout" efficiency, and showing that
errors in the resulting global efficiency
values are minor.
A material balance requires time-
averaged concentration, velocity, and
temperature measurements at some
plane normal to the mean direction of
flow. These measurements are made
above the flame when total emissions are
being assessed, requiring integration of
the species flux across the total jet. Major
errors which prevent adequate material
balance closure are:
• Material escapes undetected, be-
cause at the flame extremities
dilution lowers its concentration
below the detectability limit of the
analytical equipment.
• All the species are not measured.
• The time-averaged velocity is diffi-
cult to measure in and near turbu-
lent flames.
A tracer in the fuel can be used to aid in
obtaining a mass balance by yielding a
double check on the dilution factor in the
product gases. However, the use of a
tracer does not eliminate the need for
velocity measurements in determining
mass flux.
Measurement of Soot
Concentration
Soot represents uncombusted fuel
carbon which should be included in flare
flame efficiency calculations. Siegel
measured soot concentrations between
20 and 80 mg/m3 in an intentionally
smoking flame, estimating that those
dilution conditions reduced flare efficiency
by 3 to 4 percent. More recently, Howes
measured soot in a smoking propane
flame. Using a dilution factor obtained
from the COz concentration, the 18
mg/m3 of soot measured represented a
decrease in combustion efficiency of 0.4
percent. Note that these local efficiencies
are not equivalent to global efficiency,
since they were samples collected at one
sampling point.
Flare Intermittency
"Intermittency" essentially means
that, at one fixed point above the flare, the
flame is not present all of the time. Even
in calm winds, the turbulence induced by
the combustion process causes the flame
to undulate and appear unsteady. This
usually causes corresponding fluctuations
in measured quantities at fixed points
above the flame. Using sufficient sampling
times is one way to time-average data to
avoid this intermittency. An objective of
the proposed experimental plan is to
determine sampling times so that charac-
teristics of the flame are measured,
unmasked by intermittency.
Scaling Methodology
The studies listed in Table 1 did not
provide a methodology whereby the data
from these pilot-scale or small plant-
scale flares could be used to assess
emissions from the total population of
flares. A methodology is required which
will allow data to be obtained
economically at pilot-scale and used to
determine performance of full-scale
systems. The current state-of-the-art of
turbulent flame structure precludes the
use of predictive models. Thus the
experimental plan must provide data
which will allow both the effects of scale
to be determined and, in conjunction with
developing theories of turbulent flame
structure, extrapolation to full scale.
Technical Approach
Current information on flare combus-
tion is fragmentary and inconclusive. This
program attempts to answer several
questions:
• What are the combustion efficien-
cies of small flare flames?
• How are these efficiencies influ-
enced by operational parameters,
flare design, fuel composition, and
scale?
• What are the mechanisms of these
influences?
• How can the efficiencies of large
industrial flares be estimated?
A research program with emphasis on
experimental measurements on a pilot-
scale flare is the most cost-effective way
to approach answering these questions.
It must fulfill these requirements:
• Representativeness - The hard-
ware and operational conditions
must relate to full-scale practice.
• Data Accuracy - The measurement
methods must be developed and
verified satisfactorily to eliminate
the uncertainties that plagued
previous experiments.
• Basic Understanding-Experiments
must be designed to develop an
understanding of the underlying
controlling processes that take
place in flare-flames.
• Extrapolation - Information must be
generated to extend the applicability
of the small-scale data to full-scale
flares.
The design of valid experiments
involving flares must consider the fact
that flare flames are different from other
combustion processes (e.g., enclosed
boiler flames) in that they are buoyancy
dominated, are affected by ambient air
movements, and lose heat to a much
colder environment. It is commonly
accepted that, if sufficient air is mixed
with the fuel and if the resultant mixture
is kept above the reaction temperature,
combustion will go to near 100 percent
completion. However, these two condi-
tions are not necessarily maintained in
flare combustion systems, particularlyfor
the fuel eddies that are separated from
the main flame body. Because of the
geometry of the eddies, they tend to be
quenched at a higher rate than the main
flame body and hence are more likely to
be extinguished before all the fuel is
burned.
The presence of oxygen next to the fuel
is essential for continuation of
combustion. In a flare flame, air may be
entrained into the fuel jet by natural
convection and exhausted by forced
convection through air- or steam-assist.
The effectiveness of these mixing pro-
cesses directly affects the combustion re-
action. If the mixing is not completed be-
fore the burning fuel elements are
quenched below the reaction tempera-
ture, the flame will be extinguished.
Therefore, the research program must
develop the basic understanding of the
mixing and eddy behavior of flare flames.
This may be aided by modeling.
The main emphasis of the research
program, therefore, will be the measure-
ment and characterization of emissions
and flame structures. The program
includes studies of:
• Four flare sizes (11/2, 3, 6, and 12 in.
in diameter), linearly scaled
replicas of each other, including
features of commercial flares.
• Detailed measurements throughout
and beyond the visible flame
envelope to determine profiles of
temperature and species concentra-
tion.
• Tracers injected and measured to
assess air entrainment.
• Photographically recorded flame
structures.
The experiments will start with the
smaller flares to develop and verify the
measurement methods. Once the base-
line flare behavior is defined, the effects
of operational parameters and scale will
be studied.
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The experimental test program is
divided into four tasks.
Task 1 involves generation of a data
base of gross flame parameters as a
function of the complete range of all input
parameters. This will be a rapid screening
process on all flare sizes to assess the
major effects of fuel rate, wind level,
steam rate, and gas composition. The
output measurements will be limited to
visual and photographic observations of
flame length, form and structure, and
sooting tendency. Video recordings can
supplement the photographic technique.
The utility of this task lies in its
identification of those regimes of the
original test plan that need greater
emphasis in the succeeding tasks.
Task 2 relates to the development and
verification of all measurement tech-
niques. This can most effectively be done
using the smaller flare sizes. The
measurements will consist of species
concentration measurements in and near
the flame, including a tracer. Develop-
ment of an integrating hood will be in-
cluded. A major objective of this task is
verification of an adequate carbon mass
balance to provide confidence in the suc-
ceeding task.
Task 3 concerns detailed measurements
according to the test plan revised by Task
1. The major effort will be on the smaller
sizes, with the knowledge gained indicat-
ing the most important test conditions to
be used for the limited number of large-
size tests.
Task 4 relates to continuous evaluation
of test data, development of modeling and
scaling parameters, and documentation.
Information on the range of experimen-
tal parameters to be evaluated during the
testing is provided in Table 2.
Test Facility Design
The test facility will simulate the
important features of the flare process.
The objective of the design is not to build
an optimized flare tip in terms of combus-
tion efficiency and steam utilization, but
to include sufficient flexibility to simulate
the flaring process so that the measure-
ments can describe flare flame characte-
ristics. The experimental flare system dis-
cussed here is based on flare heads
which retain all the important features
but are scaled down in size. An array of
different size flare heads provide in-
formation on the effects and parameters
of scale.
The parameters that are fundamental
to the design are: flare size, flare gas
properties, nozzle gas exit velocity, wind
condition, air entrainment, and measure-
ments.
The full report provides details of the
designs of the various subsystems: flare
stack and tips; fuel supply and handling
(propane, methane, nitrogen, and/or
COa); tracer supply and handling (SOa);
steam supply; flow controls and meter-
ing; ambient conditions measurement
and control; extractive species sampling;
photography; and control, data
acquisition, and processing.
Table 2.
Experimental Parameters and Fuel Costs for Pilot-scale Flare Tests
Dia.
in.
1.5
3.0
6.0
12.0
Case
1
2
3
4
5
6
7
8
9
10
11
12
Velocity
ft/sec
0.25
1.41
10.0
05
2.0
10.0
1.0
2.83
10.0
20
4.0
10.0
Flow
Rate
ft3/hr
11
62
442
88
353
1767
707
2000
7069
5655
11310
28274
rjn
"O
T~V0
500x1 0~3
89x10'3
13x10'3
500x1 0'3
125xW~3
25x1 0~3
500x1 03
177x10'3
50x1 0~3
500x1 0~3
250x1 0~3
100x10~3
Reynolds
Number*
83
471
3343
334
1337
6687
1337
3785
13373
5349
10699
26747
d
g—
U*
Richardson
Number
64
2.013
00403
3220
2.013
0.081
16 1
2.013
0.161
8.05
2.013
0.322
Propane
Flow Rate
Ib/hr"
0.7
4.0
28
56
23
113
45
128
451
361
722
1806
Propane
Cost
$/hr
012
0.7
5.0
1.0
4 1
20.0
80
22.6
798
638
127.7
3194
Methane
Flow Rate
lb/hrc
045
2.56
18.3
364
146
73.0
292
82.6
292 1
2337
4677
1168.0
Methane
Cost
$/hr
0.07
0.4
3.0
0.6
24
120
4.8
13.5
47.9
388
766
191.4
Nitrogen Nitrogen
Flow Rate
Ib/hr"
0.7
4.1
29.3
5.8
23.4
117.2
469
132.6
468.7
375
750
1875
Cost
$/hr
0.15
0.86
6.15
1.2
4.9
24.6
9.8
27.8
98.3
78.6
157.3
393
(a) Reynolds number based on 56% propane, 44% nitrogen mixture
(b) Propane diluted to 1350 Btu/ft3{56 volume %)
(c) Methane fired without dilution.
(d) Nitrogen used to dilute propane to 175 Btu/ft3 (92 7 volume %).
-------
D. Joseph, J. Lee, C. McKinnon, R. Payne, and J.Pohl are with Energy and
Environmental Research Corp., Irvine, CA 92714. *
Bruce A. Tichenor is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of the Efficiency of Industrial Flares:
Background—Experimental Design—Facility," (Order No. PB 83-263 723; Cost:
$23.50, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
US' \
mi., ,, \
un "3 '•"
"•-..OH
PS 0000329
* U.S. GOVERNMENT PRINTING OFFICE: 1984-759-102/835
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