EPA 

600/ 

2-

86/(80 

... 

¥> .. 

United States 

Erwironmemal Pro~ectrC>n 

Agency 

... 

Research and 

Development 

EVALUATION OF THE 

EFFICIENCY OF INDUSTRIAL 

FLA~ES: H2S GAS MIXTURES AND 

PILOT ASSIST ED FLARES 

Prepared for 

EPA-600/2-86-080 

September 1986 

Office .of Air Quality Planning and Standards 

Prepared by 

Air and Energy Engineering Research 

Laboratory 

Research Triangle Park NC 27711 
r . 

RESEARCH REPORTING SERIES 

Research reports of the Office of Research and Development. U.S. Environmental 

Protection Agency. have been grouped into nine series. These nine broad categories were established to facilitate further development and application of environmental technology. Elimination of traditional grouping was consciously 

planned to fost8f technology transfer and a maximum interface in related fields. 

The nine series are: 

1. Environmental Health EHeets Research 

2. Environmental Protection Technology 

3. Ecological Research 

4. Environmental Monitoring 

5. Socioeconomic Environmental Studies 

6. Scientific and Technical Assessment Reports (STAR) 

7. Interagency Energy-Environment Research and Development 

8 ... Special" Reports 

9. Miscellaneous Reports 

This report has been assigned to the ENVIRONMENTAL PROTECTION TECHNOLOGY series. This series describes research performed to develop and dem· 

onstrate instrumentation, equipment, and methodology to repair or prevent en· 

vironmental degradation from point and non-point sources of pollution. This work 

provides the new or improved technology required for the control and treatment 

of pollution sources to meet environmental quality standards. 

U.;:>. Environnl:'.rt'il Prnt,.ct1oo Agi..l::l01 

:cn-··~·-y, R"~ · ~-~n ~ F'M<Hl-A 

10: M Street, S.W. 

\>i;,:.'das\•n. DC 20460 .. ·.:.:;1 

EPA REVIEW NOTICE 

This report has been reviewed by the U.S. Environmental Protection Agency, and 

approved for publication. Approval does not signify that the contents necessarily 

reflect the views and policy of the Agency, nor does mention of trade names or 

commercial products constitute endorsement or recommendation for use. 

This document is available to the public through the National Technical Information Service, Springfield, Virginia 22161. 
t> \ / \ 

• ' i ') 

: \ 

',. ) ••. ·J 

v' 

EPA-600/2-86-080 

September 1986 

EVALUATION OF THE EFFICIENCY OF 

INDUSTRIAL FLARES: H2S GAS MIXTURES 

AND PILOT ASSISTED FLARES 

Prepared by: 

J. H. Pohl and N. R. Soelberg 

• 

ENERGY AND ENVIRONMENTAL RESEARCH CORPORATION 

18 MASON 

IRVINE, CALIFORNIA 92718 

EPA Contract No. 68-02-3661 

EPA Project Officer: Bruce A. Tichenor 

Air and Energy Engineering Research Laboratory 

Hazardous Air Technology Branch 

Research Triangle Park. North Carolina 27711 

Prepared for: 

U.S. Environmental Protection Agency 

Office of Research and Development 

Washington, D.C. 20460 

U.S EPA Headquarters Library 

Mail code 3404T 

1200 Pennsylvania Avenue NW 

Washington, DC 20460 

282-566-0556 
ABSTRACT 

The U.S. EPA has contracted with Energy and Environmental Research 

Corporation to conduct a research program which will result in quantification 

of emissions from, and efficiencies of, industrial flares. The program is 

divided into four phases: Phases I and II (Experimental Design and Design of 

Test Facilities, respectively) have been reported in report EPA-600/2-83-070; 

and Phases Ill and IV (Development of Test Facilities and Data Collection) have 

been reported in report EPA-600/2-84-095. Further data collection (part of 

Phase IV) is reported in report EPA-600/2-85-106. 

Initial results (EPA-600/2·84-095) were limited to tests conducted burning 

propane/N2 mixtures in pipe flares without pilot flare stabilization. Further 

results (EPA-600/2-85-106) reported the w.fluence of the flared gas and flare 

head design on destruction and combustion efficiency without stabilization by 

pilot flares. The current report is the fourth in the series and gives test data 

on the combustion efficiency and destruction efficiency 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: 

o Gas mixtures of•H2S/N2 can be stably flared at much lower volumetric 

gas heating values than can propane/N2 mixtures. 

o Destruction and combustion efficiencies greater than 98% are obtained 

for gas mixtures of H2S/N2 and H2S/propane/N2 when the gas heating 

value is at least 1. 2 times the level required to produce a stable flame. 

0 

0 

0 

0 

For mixtures containing both H2S and propane, H2S destruction efficiency 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 aerodynamic flame stabilization devices. 

Combustion efficiencies greater than 98% for pilot assisted flares are 

achieved 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 1 to 3 (on 3- and 6-in. open pipe flares without other flame stabilization) could decrease the heating value of the gas required for stability 

by about 10-20%. 

• 

ii 

----·---·------------------------------------------
TABLE OF CONTENTS 

Section 

LIST OF FIGURES • • • . . . . . . . . . • • • • • v 

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . • • vi 

1.0 

2.0 

3.0 

4.0 

INTRODUCTION • 

1.1 Previous Flare Results 

1.2 Objectives •••• 

1.3 Approach •••••• 

1.4 References •.•• 

SUMMARY AND CONCLUSIONS 

2.1 Summary ••••••• 

2. 2 Conclusions . • • • 

2.3 References ••••• 

GAS MIXTURE TEST OF H2S 

. . . . . . . . . . . . . . 

• I • 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . 

. . . . . . . 

• • 1-1 

• • 1-2 

• • 1-7 

• • 1-7 

• • 1-10 

• 2-1 

• • 2-1 

• 2-25 

• • 2-26 

3-1 

3.1 Measurement Techniques . 3-2 

3.2 Laboratory Scale Tests • • • • • • 3-3 

3.3 Pilot Scale Tests ••••• 

3.4 References ••••••• 

PILOT ASSISTED TEST RESULTS . . . 4.1 Flame Stability ••••••• . . . 4.2 Combustion Efficiency • 

4.3 References •.•••••••••• 

• • • • 3-8 

• 3-19 

•• 4-1 

. . . . • 4-3 

• 4-12 . . . • 4-14 

APPENDICES 

A 

B 

EPA FLARE TEST FACILITY AND PROCEDURES 

A.l Flare Test Facility •.••••• 

A.2 FTF Procedures .•••••••• 

A. 3 References • • • • • • • • • • • 

. . . . 

FLARE SCREENING FACILITY AND TEST PROCEDURES 

B.l Flare Screening Facility ••• 

8.2 FSF Test Procedures •••••••••• 

B. 3 Reference • • • • • • • • • • 

Hi 

• A-1 

• •• A-1 

• • • A-6 

• A-7 

. . . • • • • B-1 

• • • B-1 

• B-3 

• • • B-3 
Section 

c 

D 

E 

F 

TABLE OF CONTENTS (CONTINUED) 

FLAME STABILITY LIMIT •• 

C.1 Stability Limits 

C.2 References •.• . . . . . . . 

DATA ANALYSIS ••••• 

D.1 FTF Data Analysis •• 

D.2 FSF Data Analysis 0.3 References •••.•• 

QUALITY ASSURANCE •••. 

E.1 Flowrate Measurement. 

E.2 Sample Analysis E.3 References ••. 

CONVERSION FACTORS • 

. . . 

. . . . . . 

. . 

• 

1v 

• • 

. . . . 

. . 

. . . . . . . . . . 

• C-1 

• • • • C-1 . . .. . • • C-1 

. . . • • • D-1 

•• D-1 .. • • • • • • D-4 

•••••• D-8 

• E-1 

. . . . . . • • • E-1 . E-2 

• E-4 . . . . . . . . . . • F-1 

• 
LIST OF FIGURES 

~igure 

2-1 Flare Screening Faci 1 i ty ( FSF) • • • • • • • • • • • • • • 2-3 

2-2 Flame stabilfty curves for propane/N2 and HzS/propane/Nz 

gas mixtures flared using a 0.042 inch ID nozzle • • • • • • • 2-5 

2-3 EPA Flare Test Facility (FTF) at EER • • • • • • • • • • • 2-6 

2-4 Flame stability limits for H2S gas mixtures flared using 

a 3-inch diameter open pipe flare. • • • • • • • • • • • • 2-8 

2-5 Propane combustion efficiency of H2S/propane/N2 gas mixtures 

flared from an unassisted 3-inch open pipe flare • • • • • 2-10 

2-6 Destruction efficiency of H2S gas mixtures flared from 

an unassisted 3-inch open pipe flare • • • • • • • • • • • 2-11 

2-7 Correlation between H2S destruction efficiency and propane combustion efficiency for H2S/propane/N2 mixtures fiared 

from an unassisted 3-inch open pipe flare. • • . . • • • 2-12 

2-8 Flame stability curves for 3-inch pipe flare with a single pilot at a pilot gas flowrate of 2.1-2.2 scfm natural gas. 2-17 

2-9 Flame stability curves for 6-inch open pipe flare with a 

single pilot at a pilot gas flowrate of 1.0-1.1 scfm 

natural gas • • • • • • • • • . • • . • • • • • • • • • • • • 2-18 

2-10 Flame stability curves for 6-inch open pipe flare with a 

single pilot at a pilot gas flowrate of 2.1-2.4 scfm 

natural gas •.•....•...... - • . • • • . • • • • 2-19 

2-11 Flame stability curves for 6-inch open pipe flare with a 

single pilot at a pilot gas flowrate of 3.9-5.4 scfm 

natural gas . . . . . . . . . . . . . . . . . . . . . . . . . 2-20 

2-12 Flame stability curves for 6-inch open pipe flare with 

multiple pilots at a total pilot gas flowrate of 

2.0-2.3 scfm natural gas • • • . • • • • • • • • • • 2-21 

2-13 Calculated adiabatic flame temperature vs. limiting stable 

gas exit velocity for propane/Nz gas mixtures flared 

using pilot-assisted 3-inch and 6-inch open pipe flares. • 2-23 

2-14 Combustion efficiency of pilot assisted flares • • • • • • 2-24 

3-1 Flame stability curves for propane/N~ and H2S/propane/N2 

gas mixtures flared using a 0.042 1nch ID nozzle • • • • • • • 3-6 

3-2 Flame stability curves for H2S gas mixtures flared using 

a 3-inch diameter open pipe flare • • • • • • • • • • • • • • 3-9 

3-3 Flame stability curves using heating value per mass 

relationship • • • • • • • • . . • • • • • . • • • • • • 3-10 

3-4 Calculated adiabatic flame temperature versus limiting stable 

gas velocity from an unassisted 3-inch open pipe flare • • • • 3-13 

3-5 Propane combustion efficiency of HzS/propane/N2 gas mixtures flared from an unassisted 3-inch open pipe 

f1 are . . . . . . .. . . . . . . . . . . . . . . . . 3-16 

3-6 Destruction efficiency of HzS gas mixtures flared from 

an unassisted 3-inch open pipe flare • • • • • • • • 3-17 

3-7 Correlation between HzS destruction efficiency and propane combustion efficiency for H2S/propane/N2 mixtures flared 

from an unassisted 3-inch open pipe flare. • • • • • • 3-18 

4-1 Schematics of single, double, and triple pilots • • • • • • 4-2 

v 
Figure 

4-2 

4-3 

4-4 

4-5 

4-6 

4-7 

4-8 

A-1 

A-2 

A-3 

8-1 

D-1 

Table 

1-1 

2-1 

2-2 

2-3 

3-1 

3-2 

3-3 

E-1 

FIGURES (CONTINUED) 

Flame stability curve for 3-inch pipe flare with a single pilot at a pilot gas flowrate of 2.1-2.2 scfm natural gas ••• 

Flame stability curves for 6-inch open pipe flare with a 

single pilot at a pilot gas flowrate of 1.0-1.1 scfm 

natura 1 gas . . . • • . . . . . • . . . . • . .. . . • . . . . Flame stability curves for 6-inch open pipe flare with a 

single pilot at a pilot gas flowrate of 2.1-2.4 scfm 

natura 1 gas . . . . . . • . . . . . . . . . . . . . . . . . . Flame stability curves for 6-inch open pipe flare with a 

single pilot at a pilot gas flowrate of 3.9-5.4 scfm 

natural gas . . . . . . • . • .. . . . . . . . • . . . . . • . Flame stability curve for 6-inch open pipe flare with 

multiple pilots at a total pilot gas flowrate of 

2. 0-2. 3 scfm natura 1 gas • • o • • o o o o o • • • • 

Calculated adiabatic flame temperature vso limiting stable 

4-4 

4-6 

4-7 

4-8 

4-9 

gas exit velocity for propane/Nz gas mixtures flared 

using pilot-assisted l-inch and 6-inch open pipe flares •• 

Combustion efficiency of pilot assisted flares •• 

• • 4-11 

EPA Flare Test Facility (FTF) at EER • o •••• o 

Fuel flow control and metering system schematic •• 

• 0 4-13 

•• A-2 

• • A-3 

Flare Test Facility sample system. • • • •• o • A-5 

Flare Screening Facility (FSF) ••••••••• 

Schematic of integration geometry ••••••••• 0 • • • 8·2 • 0 • • D·S 

LIST OF TABLES 

Summary of Previous Flare Combustion Efficiency Studies •• 

Flare Screening Facility HzS Gas Mixture Test Results. 

Nozzle ID • Oo042 Inches • o •••••••••••••• 

H2S Gas Mixture Combustion and Destruction Efficiency Test 

Results ........................ . Pilot Assisted Flare Head Test Results •••••••••• 

Measurement Techniques for H2S and SOz •••••••••••• 

Flare Screening Facility HzS Gas Mixture Test Results. 

Nozzle ID • 0.042 Inches •••••••••••••• 

Physical Properties of HzS and Propane at 60F, 

1 Atlnosphere • •••••••••••••••••••••.•• 

Flare Facility Analytical Methods ••••••••••• • • 

vi 

Page 

1-3 

2-4 

2-7 

2-13 

3-4 

3-7 

3-14 

E-3 

• 
1.0 INTRODUCTION 

Large amounts of waste and purge gases are flared annually in the United 

States and worldwide. Exact figures are uncertain because of the limited 

measurement and control of flare gas flow. An estimate of the amount of gas 

flared in the United States in 1974 was 12 million tons 1 {1.1); this estimate 

was extrapolated to 16 million tons of gas flared in 1980 (1.2). Sources of 

flared gases include oil refineries, oil and gas production, blast furnaces, 

coke ovens, and chemical plants. By volume, the largest contributor is from 

blast furnaces which were estimated to release 9.6 million tons {69 x 1012 

Btu) in 1980 •. By energy released however, the major contributors are oil 

refineries and oil and gas production wells which were estimated to release 

5.0 million tons (219 x 1012 Btu) in 1980. Commonly flared gases include a 

wide variety of compounds, such as aliphatic hydrocarbons, aromatic 

hydrocarbons, and chlorinated, oxygenated, nitrogen-bearing, or sulfurbearing compounds. 

Industrial f'Jare operating conditions and emissions are frequently very 

difficult to measure. Flow control and measurement is often limited or nonexistent. Many flares burn mixtures of leaked or purged gases where the gas 

composition is largely unknown. Generally, flares are elevated to decrease 

ground level noise and radiation and to enhance dissipation of heat and 

combustion products. For this reason collection of plume samples from 

commerci61 flares is difficult. Various methods to determine plume 

composition and flare emissions by remote means are still being developed and 

are not yet available. 

The most flexible, economical and accurate method of determining flare 

emission and combustion efficiency is to use a pilot-scale facility dedicated 

to accurate flare gas flowrate measurement and control with reliable plume 

sampling to determine flare emissions. Scaling is then required to apply 

1 English units are generally used throughout this report. Appendix F 

provides conversion factors from English to Metric units. 

,_, 

• 
pilot-scale results to full-scale flare operations. Several studies have 

been conducted in which scaling criteria have been evaluated (1.2- 1.7). 

Flare research has been conducted at Energy and Environmental Research 

Corporation (EER) since 1980 (1.2, 1.4). A pilot-scale flare test facility 

was constructed for the u.s. EPA in 1982. This research has been sponsored 

by the u.s. EPA as part of an effort to provide data upon which to base 

regulations for industrial flaring practices. 

1.1 Previous Flare Results 

In the past 15 years there has been increased interest in research on 

flare combustion efficiency, due largely to increased governmental and 

industrial environmental awareness. Since flare research using full-scale 

industrial flares is difficult and expensive, most recent research has been 

conducted on pilot-scale flares 12 inches in diameter or less. Industrial 

flares are as large as 60 inches in diameter, so application of pilot-scale 

test results to these large flares requires scaling, using fundamental energy 

and mass transfer principles. Scaling is difficult, however, because of {1) 

basic aerodynamic differences, such as incompatible Reynolds number and 

Richardson number relationships, between small and large flames, (2) nonlinear flame envelope and combustion zone characteristics, (3) the effects of 

the wide variety of industrial flare types and designs on flare performance, 

and (4) differences in wind and weather conditions. Scaling criteria have 

been investigated at EER by testing different sized flares (0.042-inch to 12-

inch diameter) for comparison of fundamental differences (1.2, 1.3, 1.4, 1.6 

1.7). Noble, et al (1.5) have also studied scaling criterial. 

Flare combustion efffciency research up to 1983 has been reviewed by 

Dubnowski and Davis (1.8). Since 1972, flares and nozzles with 0.042-inch to 

47-inch diameters have been tested. As Table 1-1 shows, flare combustion 

efficiency testing after 1983 has been conducted by Pohl, et al. (1.3) and 

Pohl and Soelberg {1.6). A wide variety of commercial heads have been 

evaluated, and a data base of results from open pipe flares and nozzles has 

been accumulated. Gas exit velocities have ranged between 0.15 to 891 ft/ 

1-2 
~ 

I 

w 

STUDY 

PAIAIIr (1.9) 

Herget (1.10) 

StrAitz (1.11) 

Siegel (1.12} 

Lee & Whipple (1.13) 

HDN~s, et tl. (1.14) 

McDill lei ( 1. 15, 1.16} 

Pohl, et AI (1.2) 

TABlE 1-1. SUMMARY OF PREVIOUS FLARE COMBUSTION EFFICIENCY STUDIES1 

fLAil£ GAS EXIT GAS MEASUR~g,. COfiiUSTJ ~12E ~[LOCITY HEATIMG EffltlENtY 

YEAR (In} Dt:SIGH (fls} VALUf (Btu/ft~ GAS FLARED ' COfKNTS 

1972 0.5 SteUI Ass is ted 50 - 250 1448 Ethylene >97.8 Hellu. tracer for full e~per1-.ntA1 nozzle stze flare evtluatlon 

1971 47 full she IIA IIA CArbon blAck 2500:1 EPA ROSE reaote sensing viByl •onomer reduction systetR . In CO 

1978 2-6 Steu And pi lot 1000 - 2350 H1t11rAl 1145, 75 - 99 Results of limited validity 

propene due to Instrument rAnge sensitivity 

Refinery gas2 . 1980 17. Coamerc It I fl.regu 0.7 - 16 1500 97 - 99 Multtposltton plume extrtcthe sampling 

1981 2 Holes 1~ 2• CIP 1.8 2190 - 2385 Propllle 96 - 100 

(1.1 In open aret} 

1981 ,13) to.erct&l Air 40 - 60 2385 PropAne 92 - 100 8oth extrtcttve and EPA 

assist. Zlnk STF-LH ROSE pluae sampling 

1981 3at4(4) eo.-rdAI lt.P. NeAr Sontc 1000 NaturAl gas >99 

(estl••te) 

1983 8 Cclaerci&J Ztnk 0.03 - 62 209 - 2183 Propylene/112 67 - 100 Both extrActive And EPA 

STF-S-8 ROSE plume S.lmpllng 

1983 6(3) eo..erctll •lr J.4 - 218 83 - 218l Propylene/112 55 - 100 us 1st 

Ztnk STF-LH-457-5 

1984 3-12 Open pipe And 0.2 - 420 291 - 2350 Propane/112 90 - 99.9 Multlprobe plume extrac- cllllllll!rclal tive s.ampl tng 

-
~ 

~ • 

·TABlE 1·1. SUMMARY OF PREVIOUS FlARE COMBUSTION EFFICIENCY STUOIES1 (CONTINUED) 

fLAil£ 

SIZE 

STUDY YlAR (tn) 

Polll and Soelberg .( 1.6) ·- 0.042 

---

1!185 1.5-12 

1985 0.042-

2.5 

1985 3 

-----·----

MA • lot Available 

1M • Not Measured 

I:ESUil 

Nozzle 

Co..erc:lal coanda 

ste• lnjectton, 

pressure assisted, 

air assisted, open pipe, pilot assisted 

louie 

Open pipe 

-

1. In pert froa Oubnowskl and Davis (1.8) 

GASEUT 

VELOCITY 

(ft/HC) 

31 - 854 

0.2 • 5!11 

5.6 - 891 

0.15- ll!l 

-··· 

MEASUIIEO 

GAS CCMWSTIOH 

HEATING EFFICIENCY 

VALUE (Btu/ scf) GAS FLARED ( I) 

i23 - 3320 2i different gas >98 

•btu res (80 - 99.9!1 

destruction 

effie: Ieney) 

122 - 2350 Propane/N2 l6- !I!U 

588- 2350 Propane/12 Ill 

145 - 877 HzS/propane/112 92 • !1!1.7 

Nlllpropane/12 (92 - 99.9 

1,3-butedlene/12 

destruction 

Ethylene Oxtde/112 

efficiency) 

- -·-

2. 501 hydrogen plus light hydrocarbons 3. Supplied through spiders; high 8tu gas through 1rea Is 5.30 ln2 ~low Btu gas through 11.24 tn2 

4. Three spiders, each with an open area of 1.3 In 

COitlENTS 

co.paratlve screening tests 

COAiparat1ve connercttll 

flare type evaluation 

fl..e aerodynaatc tests 

&is alxture testing 

--
sec. Gases and gas mixtures flared in pilot-scale tests include natural gas, 

ethylene, propane, propylene, refinery gas (50 percent hydrogen plus light 

hydrocarbons), hydrogen sulfide, ammonia, 1,3-butadiene, and ethylene oxide. 

Results of these studies are being used as a data base for the 

promulgation of industrial flare practices and regulations. The key findings 

are: 

• Flares can be operated with combustion and destruction efficiencies 

exceeding 98-99 percent. 

• Flare efficiency depends on flame stability. A flare operated 

within the envelope of stable operating conditions will exhibit 

high efficiency unless too much steam or air assist is used. 

• A flare operated outside its stable flame envelope becomes 

unstable; this can result in combustion and destruction efficiency 

below 98 percent. 

• The stable flame operating envelope is specific to flare head 

design and gas composition. 

• Operating conditions that have the largest influence on flame 

stability for a given flare head are'the gas exit velocity and 

heating value. Depending on flare type, levels of steam, air, or 

pilot assist can also affect flame stability and destruction and 

combustion efficiency. Results also show that flare gases of 

equivalent heating value but different composition can have 

different stable flame operating envelopes when flared from the 

same fl ar·e. 

Flare efficiency depends on flame stability, which in turn depends on 

flare head design and flare gas exit velocity, heating value, and 

composition. There are practically as many different combinations of these 

variables as there are industrial flares. Therefore further research is 

1-5 
needed on flare flames to improve and extend scaling factors, and to develop 

methods correlating the influences of gas mixture, flare head ~pe, and 

operating conditions on the combustion and destruction efficiency for 

commercial flares. 

EER was commissioned by the EPA in 1980 to conduct flare research. For 

this research, a pilot-scale Flare Test Facility was designed and constructed 

at EER's test site in El Toro, California (1.2) Flare research at EER is 

briefly summarized in Table 1-1. A data base has been developed which 

includes flare performance and efficiency measurements for (1) open pipe 

nozzles and flares ranging in size from 0.042 to 12 inches in diameter, with 

and without flame retention rings, (2) seven commercial flare heads ranging 

in size (based on open area for gas flow) from 1.5 to 12 inches in diameter 

-- representative of coanda steam injection, pressure, and air assist flare 

heads, and (3) limited steam and natural gas pilot assisted tests on both 

open pipe and commercial flare heads. Propane, blended with nitrogen diluent 

to alter gas heating value, was used as the flare gas for these tests. 

Scaling parameters investigated include exit velocity, residence time, 

Reynolds number, and Richardson number. Flare flame aerodynamics, including 

lift-off and flame length, were also studied. Additional tests have been 

conducted to generate a data base for flaring different gases and gas 

mixtures. Combustion and destruction efficiencies of 25 different gases and 

gas mixtures representing aliphatic, aromatic, chlorinated, oxygenated, 

sulfur-bearing, nitrogen bearing and low heating value compounds have been 

tested for comparative performance using a 0.042-inch nozzle. Pilot-scale 

flare performance of four gases, ammonia, 1,3-butadiene, ethylene oxide, and 

hydrogen sulfide, has been measured using an unassisted 3-inch open pipe 

flare. 

Throughout the EER flare research program, advice and consultation has 

been provided by a Technical Advisory Committee. Serving on this committee 

are representatives from EER, EPA, California Air Resources Board, flare 

manufacturers {Peabody Engineering, McGill, Inc., John Zink, and Flaregas 

Corporation) and industrial flare users (Exxon Chemical Company, Exxon R&E, 

Union Carbide, Getty Refining and Marketing Co., Chevron USA, and Dow 

1-6 
Chemical Company}. Among the industrial user representatives are members of 

the Chemical Manufacturers Association {CMA) and the American Petroleum 

Institute (API). This committee has provided a review and critique of test 

plans and results, has ensured relevance of the ongoing study to current 

regulatory and industrial concerns, and has facilitated efficient information 

transfer. 

1.2 Objectives 

Objectives for the research reported herein were: 

1) Additional HzS gas mixture testing to evaluate flame stability and 

combustion and destruction efficiencies. 

2) More extensive pilot assisted flare testing to measure the effects 

of single and multiple pilots at high and low flowrates on flare 

performance. 

1.3 Approach 

Accurate measurement of flare emissions and combustion efficiency is 

difficult. Experimental difficulties encountered by previous researchers 

include: 

1 Inability to close mass balances due to large amounts of plume 

materia1 lost. 

1 Inability to measure soot emissions, which may be significant in 

smoking flare situations. 

1 Sampling only on the plume centerline, not obtaining measurements 

representative of the entire plume. 

, ... ] 
• Flare flame fluctuations due to turbulence and/or wind, causing 

erroneous data by destabilizing or blowing out the flame, or 

consistently blowing the plume away from the sample probe. 

The Flare Test Facility at EER has been designed and built in order to 

minimize such problems. The following procedures have been developed to 

verify the accuracy of combustion efficiency measurements (1.3): 

• Material balance closure was verified using a hood to capture the 

entire flare plume for s~ll flames, and by using S02 as a tracer 

for large flames. 

• Soot concentration was measured for all tests. 

• The average concentration of completely and incompletely burned 

combustion species from the flare flame was determined for the 

entire plume by (1) using a hood to completely capture small 

flames, and (2) simultaneous sampling using separate probes at five 

radial positions in the plume for large flames.· These local 

measurements, combined with velocities calculated from jet theory, 

were integrated to calculate overall combustion efficiency. 

• The effects of flare flame fluctuations were limited by using five 

radially located sample probes and by collecting time-integrated 

samples over a 20 minute time period. This time span was 

experimentally determined to be sufficient to time-average flame 

f1 uctuati ons. 

Previous EER studies have established accurate flame combustion 

efficiency test methodology, developed a pilot-scale test facility, and 

established a data base of combustion efficiency test results for 0.042 

through 12 inch diameter flare heads and nozzles burning a variety of gas 

mixtures. This work has provided the experience necessary to qualitatively 

estimate flare flame combustion and destruction efficiency and has provided 

1-8 
data for selection of scaling criteria to extrapolate the pilot-scale test 

results to full-scale operations. 

This report is the fourth in a series describing the EPA/EER flare 

research program, and provides results from the following tasks: 

• Task 1. Determine flame stability limits for gas m.ixtures of 

increasing H2S concentration. 

• Task 2. Develop an accurate technique for measuring H2S 

concentrations in plume samples containing relatively high SOz 

levels. 

• Task 3. Measure overall combustion efficiency and H2S destruction 

efficiency measurements at and above the limit of flame stability 

as a function of gas mi_xture strengths and gas exit velocities. 

• Task 4. Determine the flame stability limits for a flare assisted 

by single, double, and triple pilots at constant pilot gas 

flowrate, and by a single pilot at different pilot gas flowrates. 

• Task 5. Measure overall combustion efficiency at and above the 

flame stability limits for the flare with each of the pilot assist 

conditions identified in Task 4. 

• Task 6. Reduce, analyze, and report the data. 

Open pipe flares were used for these tests, to eliminate any 

complicating effects from flame holding or stabilization devices and for 

~omparison with the majority of previous EER data. Aerodynamic flame 

stabilization devices are commonly used industrially, and depending upon 

design, may be expected to affect the relationship between exit velocity, 

1eat content, and flame stability. A nominal 3-inch (ID = 3.125 inches) head 

~tas be used for the H2S gas mixture tests and for the tests with a single 

1-9 
pilot at a single pilot gas flowrate. For multiple pilot and pilot gas 

flowrate tests, a nominal 6-inch (ID = 6.065 inches) head was used. 

Commercial gases are mixed at the Flare Test Facility prior to delivery 

to the flare. Holding tanks are available at the facility for commercial 

liquified propane and nitrogen. Liquified hydrogen sulfide and sulfur 

dioxide {used as a tracer) are provided in portable cylinders. Natural gas 

is· provided by the local utility company. A flow regulating and measuring 

system is provided for each gas. The pure flare gas constituents are 

supplied to a header where they are mixed, and then delivered through a 

series of baffles for blending, before entering the flare head. Mixtures of 

various amounts of hydrogen sulfide, propane, and nitrogen were used for the 

H2S gas mixture tests. Propane and nitrogen mixtures were used for the pilot 

assisted tests. Natural gas was used for the pilot flames. 

1.4 References 

1.1 Klett, M. G., and J. B. Galeski, Flare Systems Study, EPA Report No. 

600/2-76-079, (NTIS Report No. PB-251664), March 1976. 

1.2 Joseph. o., J. Lee, C. McKinnon, R. Payne, and J. Pohl, 11Evaluation of 

the Efficiency of Industrial Flares: Background-Experimental DesignFacilityN, EPA Report No. 600/2-83-070, NTIS No. PB83-263723, August 

1983. 

1.3 Pohl, J. H., R. Payne, and J. Lee, "Evaluation of the Efficiency of 

Industrial Flares: Test Results .. , EPA Report No. 600/2-84-095, NTIS 

No. P884-199371, May 1984. 

1.4 Pohl, J. H., N. R. Soelberg, and E. Poncelet. "The Structure of Large 

Buoyant Flames .. , American Flame Research Committee Fall Meeting, 

Livermore, CA, 16-18 October 1985. 

1-10 
1.5 Noble, R. K., M. R. Keller, and R. E. Schwartz, '

1

An Experimental 

Analysis of Flame Stability of Open Air Diffusion Flames 11 , presented at 

American Flame Research Committee, 1984 International Symposium on 

Alternative Fuels and Hazardous Wastes, Tulsa, Oklahoma, October, 1984. 

1.6 Pohl, J. H. and N. R. Soelberg, 11 Evaluation of the Efficiency of 

Industrial Flares: Flare Head Design and Gas Composition11 , EPA Report 

No. 600/2-85-106, NTIS No. PB86-100559/AS, 1985. 

1.7 Pohl, J. H., J. Lee, R. Payne and B. Tichenor, 11 The Combustion 

Efficiency of Flare Flames 11 , 77th Annual Meeting and Exhibition of the 

Air Pollution Control Association, San Francisco, CA., June 1984. 

1.8 Dubnowski, J. J. and B. c. Davis, '

1

Flaring Combustion Efficiency: A 

Review of the State of Current Knowledge, 11 The 76th Annual Air 

Pollution Control Association Meeting, Atlanta, GA, 1983. 

1.9 Palmer, P. A., ''A Tracer Technique for Determining Efficiency of an 

Elevated Flare," E. I. duPont de Nemours and Co., Wilmington, DE, 

1972. 

1.10 Herget, w. F., "An Overview of the EPA Programs for Groundbased Remote 

Sensing of Air Poll ution1', 23rd SPIE Symposium, San Diego, CA, August 

27-30, 1979. 

1.11 Straitz, J. F., 11 Flaring for Gaseous Control in the Petroleum 

Industry .. , 1978 Annual APCA Meeting, June 19, 1978. 

1.12 Siegel, K. o., "Degree of Conversion of Flare Gas in Refinery High 

Flares, "Ph.D. Oi ssertati on, University of Karlsruhe (Federal Republic 

of Germany), February, 1980. 

1.13 Lee, K. c. and G. M. Whipple, "Waste Gas Hydrocarbon Combustion in a 

Flare," Union Carbide Corporation, South Charleston, WV, 1981. 

1-11 
1.14 Howes, J. E., T. E. Hill, R.N. Smith, G. R. Ward, and W. F. Herget, 

"Development of Flare Emission Measurement Methodology, Draft Report," 

EPA Contract No. 68-02-2686, Task 118, U.S. Environmental Protection 

Angency, 1981 (Ora ft Report) • 

1.15 McDaniel, M., "Flare Efficiency Study," EPA Report No. 600/2-B3-052, 

NTIS No. PB83-261644, August 1983. 

1.16 Keller, N. and R. Noble, "Ract for VOC- A Burning Issue", Pollution 

Engineering, July 1983. 

1-12 
2.0 SUt-f.1ARY ,,NO CONCLUSIONS 

The test program "Evaluation of the Efficiency of Industrial Flares" has 

been funded by the U.S. EPA and conducted at the Energy and Environmental 

Research Corporation (EER) El Tore Test Site. This program has been 

conducted in phases. The first phase involved construction of a pilot-scale . flare test facility. During the second phase, combustion efficiency tests 

were conducted on eight different commercial and EER prototype flare heads 

ranging in size between three and twelve inches in diameter. During the 

third phase, effects of flare head design and gas composition on flare 

combustion and destruction efficiencies were studied. Commercial coanda 

steam assisted heads, pressure heads, and an air-assisted flare head were 

tested. Also, different gas mixtures containing ammonia, 1,3-butadiene, 

ethylene oxide, and hydrogen sulfide were tested. 

Throughout this test program a Technical Advisory Committee has provided 

consultation. Committe-e members included representatives from EER, EPA, 

California Air Resources Board, flare manufacturers, and industrial flare 

users. Each phase of this test program has been designed to provide test 

data relevant to current regulatory and industrial concerns as defined by 

this co11111ittee. 

2.1 Summary 

This phase of the work had two objectives: 

• Evaluation of H2S destruction efficiency for H2S-containing flare 

gases 

• Evaluation of the effects of pilot assist on flare combustion 

efficiency 

In order to determine the limits of stable flare operation for these gas 

1uixtures and pilot assisted flares, and key operating conditions that affect 

flame stability and efficiency, some conditions with poor stability and low 

2-1 
combustion efficiencies were measured. Such results merely indicated flare 

operating performance at or beyond the edge of the operating envelope, and 

are not indicative of normal commercial flare operation. 

2.1.1 Destruction Efficiency of H2S 

Before HzS destruction efficiency could be evaluated, it was necessary 

to develop techniques to accurately and reliably measure HzS at plume 

concentration levels of 0-1000 ppm, without interference from S02, present in 

levels between 0-10,000 ppm. Methods successfully adapted for this 

measurement were the methylene blue method and Draeger tubes. For higher HzS 

gas concentrations (25 ppm or greater}. gas chromatography was also used. 

Tests were conducted with H2S gas mixtures on the Flare Screening 

Facility CFSF) shown in Figure 2-1. These tests verified the applicability 

of HzS and SOz measurement methods and safely and inexpensively determined 

H2S destruction efficiency for HzS gas mixtures flared from a nozzle 0.042 

inches fn diameter. Results of these tests are shown in Table 2-1. Very 

high H2S destruction efficiency (99.7 to >99.99 percent) and propane 

combustion efficiency {98.6 ·to 99.99 percent) were measured for tests 

conducted with gas heating values at and above the flame stability limits as 

shown in Figure 2-2. This stability limit curve defines the minimum heating 

value for a given gas exit velocity which will maintain a flame. At heating 

values below this limit, flame blowout may occur. 

Destruction efficiency tests of H2S were also conducted using a 3-inch 

diameter open pipe flare at the Flare Test Facility (FTF) shown in Figure 

2-3. These tests aerodynamically approximate full-scale industrial 

operations while those of the laboratory-scale Flare Screening Facility tests 

do not. Test results are tabulated in Table 2-2. Flame stability limit 

curves for these tests are shown in Figure 2-4, where gas heating value on a 

volumetric basis is related to gas exit velocity. Gas heating value on a 

mass bas~s may also be correlated to gas exit velocity, but is less effective 

in correlating flame stability for different gas mixtures. There is good 

agreement in Figure 2-4 between the current 1985, ·5 percent H2S gas mixture 

2-2 
MIXING CHAMBER 

1116-1/8 IN I NOZZLE 

GLASS BEADS 

HEATED LINE 

SYRINGE DRIVE ~==GAS 

8 

AIR FAN 

Figure 2-1. Flare Screening Facility (FSF). 

2-3 
N 

I 

.,.. 

--

Test 

No •. 

1 

2 

3 

4 

5 

TABlE 2-1. FlARE SCREENING FACILITY H2S GAS MIXTURE TEST 

RESULTS. NOZZLE ID = 0.042 INCHES 

Gas C011pos1t1on Gas Nozzle A1r H2S Propane 

(Remauder N2l Heaung Exft Stofchfo Destruct1on CORibus t1 on 

H2S Propane Value Veloc1 ty metrfc Eff1c1ency Eff1clency (Btu/scf) (f/s) Ratfo (I) . (I) (%) (%) 

4.31 45.2 1086 27.5 3.15 >99.99 99.9 

34.8 52.0 1427 17.4 2.90 >99.99 99.99 

31.9 55.0 1480 16.5 2.90 >99.99 99.9 

4.28 45.5 1093 27.0 3.17 >99.7 99.9 

69.5 30.5 1125 8.52 4.02 >99.99 98.6 

--- ---

' 

Mass Balance 

Closure 

Mass In/Mass Out 

c 0 s 

0.567 0.944 0.446 

0.706 0.939 1.12 

0.878 0.852 0.794 

0.952 0.918 0.715 

0.729 0.891 o. 774 
.... 

~ 

u 

~ 

~ 

~ 

~ 

CQ 

~ 

~ 

... 

~ 

=-

~ 

... = 

~ 

~ 

Q 

:c 

~ 

~ 

U) 

2400~--------------~------------~------------~ 

1200 

1000 

800 

600 

400 

1984 Tests, Propane/N2 Mixtures (2.1 

1985 Tests, Propane/~ Mixtures 

Tests, 4-5% H2 S 1 Mixtures 

Tests, 30-41% H2s 1 Mixtures 

Tests, 50-70% H2S Mixtures 

I 

~ ' 

I 

I 

I 

I 

I 

I 

I 

, 

I 

I 

e' 

I ' I 

I 

p 

' 

200~ ______________ ._ ____________ ~~------------~ 

1.0 

Figure 2-2. 

10.0 100.0 

Exit Velocity (f/s) 

Flame stability curves for propane/N and 

H2S/propane/N2 gas mixtures flared u~ing a 0.042 inch rD nozzle. 

1. Remainder is propane and N2 

2-5 

1000.0 
N 

I 

01 

Adjustable Rake for 

Sa.ple Probes, Filters, Driers, 

Tenax Traps, and Bubblers 

for HzS, S02 & NHJ 

Flare Gu Line 

Support Structure 

S~le Collect1on 

& Analys1s 

for Oz 

co 

C02 

Hydrocarbon 

SOz 

112$ 

110-IICJ,, 

NHJ 

Figure 2~3. EPA Flare Test Fac11tty (FTF) at EER. 
N 

I 

....... 

TEST 

NO. 

2" 

2n 

278 

2, 

]00 

101 

102 

]0] 

TABLE 2-2. H2S GAS MIXTURE COMBUSTION AND DESTRUCTION EFFICIENCY TEST RESULTS1 

GAS 

GAS COMPOSITJON2 UlJ IIJ HEATING 

VELOC lY rALIE (f/s) 

. ,,,s 

8.88 

0.48 

O.S4 

l2.S 

,,46 

0., '·" 

PROPANE u2s ca u/scn 

1S.1 }." 117 

20.1 S.11 S02 

14.4 4.81 

4.11 ,,,s "' S10 

0 42.0 247 

0 u.s 2U 

0 s, .• }]1 

0 28., 170 

1. l-INCK PIP£ flARE, NO PILOT 

2. REMAINDER IS NITROGEN 

l. HEIGHT ABOVE FLARE TIP 

4. NOT APPLICABLE 

.~~~~} WIND 

SPEED 

(f) (ltPfl) 

11 s 

12 

l " 

4 4 " 

15 4 

8 4 

8 4 

J l 

OBSERVATIONS 

f~AME Cl~Vfuc,t~ L NCTH hiP EFFICIENCY 

(f) UnJ COLOR SNOICE SOUND (I) 

' } ll ctt T lltAHGE NONE NONE '"·' 1' 2-S YELLOW ORANGE SLIGHT NONE ''·" 1.5 o.s DIM YEllOW HillE NONE 77.7 

2.S 0-2 liGHT PIJIPlE NONE ,,] 

10 6-24 liGHT PlRPLE PROFUSE GREY BLUE HISS .. , .. s· 0 BUf YELUIII GREY BLUE HONE NA 

1 0 PIIIPLE YELLOW GREEN GREY BLUE NONE NA 

1 0 INVISIBLE I I GREY BlUE MilNE NA 

~fS~iUcHON I 

FFICJEIICY 

(I) 

"·2 

,,8 

,,2 

100 

78.] 

"·' ,8.1 

"·' 
N 

I 

oa 

-

til 

:J 

~ 1984 -51 u2s 1n Propane/N2 Mixtures (2.1) 

17 1985 -ss H2S 1n Propane/N2 Mixtures 

C> H2StN2 Mixtures 

~- v ____ .......,.'"'V 

• 

100~----------------------------------------------~----------------~ 0.1 1.0 ; 10.0 100.0 1000.0 

Exit Velocity (f/s) 

figure 2-4. Flame stability curves for H2S gas mixtures flared using a 3-inch 

diameter open pipe flare. 
tests and the 1984, -5 percent gas mixture tests. The stability limit curve 

for the H2S/N2 ga.s mixture tests is much lower than that for t~e tests for 

the -5 percent 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 volumetric H2S concentration in an H2S/N2 mixture than propane in 

a propane/N2 mixture of equivalent heating value, {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 apparently enhances flame stability of 

H2S gas mixtures. 

Propane combustion efficiency and H2S destruction efficiency were 

correlated to flame stability as shown in Figures 2-5 and 2-6. Destruction 

efficiencies greater than 98 percent were attained when the gas heating value 

was at least 1.2 times the minimum gas heating value required for stability. 

Both propane combustion efficiency and H2S destruction efficiency can rapidly 

decrease below 98 percent when the gas heating value ratio decreases below 

1.2. 

Destruction efficiency of H2S is compared_to pro~ane destruction 

efficiency in Figure 2-7 for tests of gas mixtures containing both H2S and 

propane. H2S destruction efficiency was consistently higher than propane 

combustion efficiency. 

2.1.2 Influence of Pilot Flares 

Tests were also conducted using a pilot assisted 3-inch open pipe flare 

at the Flare Test Facility. These tests were conducted to measure the 

effects of pilot assist on combustion efficiency. The flare gas for these 

tests was propane diluted with nitrogen to reduce the heating value. The 

pilot gas was utility-supplied natural gas. Test results are shown in Table 

2-3. Parameters tested were {1} flare size (3-inch and 6-inch), {2) pilot 

number from one to three, and (3} pilot gas flowrate, from one to five scfm. 

For these tests, the flare gas heating value includes the contribution of the 

pilot gas. 

2-9 
99.99 Reg1 on for hydrocarbon cCII'Ibustfoa 0.01 

effiei.enc:y for prev;ousTy testect 

propa.ne/NI' mixtures flared, frca 3 ,~ -c: through 1 ·~· fnclt beads,.; and· for-'· t~,.;; . 0 

.,Q 

butad'fenellt2• ethylene oxide/~. ~ s.. 

,., 

propane/It¥'" and ~~ropane/ffi: gas · u 

flf,xtures 1 arect , . m unass :sted .• ·.·· - .~fncla'' ope&'f pfpe: fhrt· (Z'.t l ,., 

-

~ 

99.9 ~ - 0.1 - i" r.:: 

--~ -Itc: 0 

4J 

y ~ 

s.. c: 

cu 

ca. • 4J 

- ~ -" ~ y 

s.. ~ . QJ 

(J Q. 

c: 

QJ [.7 ~ - - c: 

y ' Q 

- 99.0 I"'" - 1.0 .,Q 

It- s.. 

It- ,., 

"" (.,) 

c: t. ~ 

0 4J 

- c: ...., s.. 

II' :::s 

::I 

~ = j 1984 5% H S in >. 

u 7 Propane/N2

2

Mixtures (2. 1) QJ 

...., 

17 1985 5% H S in 4J 

f Propane/N2

2

Mixtures ..-

90.0 ~ -10.0 ca. 

5 

c:::=' - 70% ~S in Propane/ (J 

c: 

N2 Mixt res -

o~----------~·~----------~·~--------~100.0 0 1.0 2.0 

Heating Value/Minimum Heating Value for Stability 

Figure 2-5. Propane combustion efficiency of H,SI 

propane/N'- gas mixtures flared from an uriassiste~ 3-inch open pipe flare. 

1. Scale is log (100 - Combustion Efficiency) 

2·10 
..-

-..., 

c 

Q,l 

u 

'-

Q,l 

Q.. 

>, 

u 

c 

Q,l 

-u 

.... 

~ 

~ 

LU 

c 

0 

-..., 

u 

:::1" 

..., '-

Ill 

41 

Q 

99.99 

99.9 

99.0 

90.0 I 

Region of destruction 

efffc..ienc:y for anaonia, 

1 .3•butacU ene., ethylene 

oxide,. and propane. (2.1) 

17 1985 - 5% H2S in Propane/N2 Mixtures (2.1) 

c:::' -70~ H2S in Propane/N2 Mixtures 

C> H2S!N2 Mixtures 

0.01 

0.1 

1.0 

10.0 

0 '-------......a. ______ _,_ ____ __. 100.0 

0 1.0 2.0 

Heating Value/Minimum Heating Value for Stability 

Figure 2-6. Destruction efficiency of H2S gas mixtures flared from an unassisted 

3·fnch open pipe flare. 

1. Scale is log (1Qo.·oestruction Efficiency) 

2·11 

-IG 

.... 

..., 

-c 

-
.... 

~ 

c 

~ 

~ 

~ 

~ 

~ 

~ 

~ 

c 

~ 

... 

~ 

... 

~ 

~ 

~ 

c 

0 

~ 

~ 

~ 

~ 

~ 

~ 

~ 

~ 

~ 

~ 

:c 

100 

C? 51 H2S in Propane/N2 Mixtures 

~701 H2S in Propane/N2 Mixtures 

90._ ____ ._ ____ ~----~----_.----~ 

94 96 98 100 

Propane Combustion Efficiency (Percent) 

Figure 2-7. Correlation between H S destruction efficiency and propane combustio~ efficiency for H2Stpropane/ N1 mixtures flared from an unassisted 3-inch open plpe flare. 

2-12 
N 

I 

...... 

w 

TEST 

NO. 

304 

305 

306 

307 

308 

309 

310 

311 

312 

313 

114 

115 

316 

111 

318 

319 

320 

321 

322 

323 

124 

325 

326 

FLARE NlltBER 

SIZE OF 

CJO) PILOTS 

3.125 1 

3.125 1 

3. 125 1 

3.125 1 

3.125 1 

3.125 1 

3.125 1 

3.125 1 

3.125 1 

J. 125 1 

1.125 1 

3.125 1 

J. 125 1 

3.125 1 

3.125 1 

3. 125 1 

3.125 l 

6.065 1 

6.065 1 

6.065 1 

6.065 1 

6.065 1 

6.065 1 

--

fLARE 

PilOT GAS 

GAS EXIT FLOWRATE VElOCITY 

(scfa) (f/s) 

2.1 19.5 

2.1 78.5 

2.1 116. 

2.1 )4.2 

2. 1 JJ.O 

2.1 17.2 

2.1 18.5 

2.1 &1.8 

8.1 6 0.9 

2.2 no. 

2.2 121. 

2.1 142. 

2.1 148. 

2.1 JJ.O 

2. 1 U.J 

2.1 58.9 

2.1 17.0 

2.1 19.7 

2.1 19.4 

2.1 19.1 

2.1 5.61 

2.1 6. 7J 

2.1 7.}] 

T~BLE 2-3. PILOT ASSISTED HEAD TEST RESULTS 

(PAGE 1 OF 3) 

GAS OBSERVATIONS 

HEATI~G PROBE} Ill NO FLAME liFT I VALU£ tiEIGHl SPEED ENGTH4 OFF 

PROPANE 1 (8tu/scf) (f) (lllph) (f) (Jn) COLOR 

"' 11.0 28) ORANGE FLICKERS 

15.8 377 ORANGE FLICKERS 

8.60 206 I TRANSPARENT 

13.4 326 ~ STABILITY TESTS ORANGE fliCKERS 

10.1 257 ORANGE FLICKERS 

10.5 275 ORANGE FLICKERS 

9.53 251 IJ ORANGE FLICKERS 

6.93 174 15 4 385 3 TRANSPARENT 

4. 77 146 15 6 385 1 TRANSPARENT 

8,88 211 25 4 405 0 TRANSPARENT 

8.47 204 ' TRANSPARENT 

6.99 168 TRANSPARENT 

6.56 158 TRA~SPARENT 

9.08 229 ORANGE FLICKERS 

6.61 168 TRANSPARENT 

5.10 110 TRANSPARENT 

3.89 '' STAB Ill JY JESTS TRANSPARENT 

9.25 224 ORANGE FLICKERS 

7.12 1" ORANGE FliCKERS 

7.16 l76 TRACE ORANGE 

10.24 264 ORANGE FLICKERS 

8.44 219 FAINT ORANGE 

7.76 201 

) TRANSPARENT 

CliiBUSTION 

EFFICIENCY 

SMOKE SOUND (I) 

I 

NONE lOll ROAR 99.2 

NONE HISS 75.0 

NONE ROAR 99.3 
N 

I 

..... 

~ 

TEST NO. 

)27 

328 

329 

)]0 

331 

312 

331 

334 

}}5 

}}6 

3J7 

338 

}39 

340 

341 

342 

343 

344 

345 

346 

347 

348 

FlARE 

SIZE 

Un> 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

6.065 

PILOT 

tatER L£4.\n PILOTS Cscf•> 

1 2.2 

1 2.4 

t 2.1 

1 2.1 

1 1.1 

1 1.1 

1 1.1 

1 1.1 

1 S.J 

1 5.4 

1 5.3 

1 5,} 

1 1.1 

1 1.1 

1 1.1 

1 1.1 

t 1.1 

1 1. 1 

1 1.1 

1 1.1 

1 5.1 

1 3.9 

TABlE 2-3. PILOT ASSISTED HEAD TEST RESUlTS 

(PAGE 2 OF 3) 

FlARE GAS OBSERVATIONS GAf vEfftHn HEAT~ PRDBf WI~ (LAME I VALUE HUCit IJP 3 SPED NGTH4 

Cfls) PRDPANE1 (Btu/sen (f) C•ph) (f) c Jnl COLOR 

19.6 7.01 172 28 l 15 15 ORANGE FliCkERS 

44.7 5.67 117 TRANSPARENT 

34.8 6.49 1S1 18 3 17 2 TRANSPARENT 

11.8 4.22 112 

' 7 

4 5 12 TRANSPARENT 

36.} 6.97 1" TRACE ORANGE 

n.t 7.96 187 ORANGE FLICKERS 

36.9 5.12 121 TRANSPARENT 

36.7 4.58 110 TRANSPARENT 

}6.7 4.58 118 FAINT ORANGE 

16.9 5.23 1}} ORANGE FLICkERS 

36.3 3.28 88.} TRANSPARENT 

36.5 },67 97.1 TRANSPARENT 

10.3 6.28 155 STABILITY TEST~ TRANSPARENT 

10.1 4.52 114 TRANSPARENT 

10.1 3.72 95.8 TRANSPARENT 

10.5 7.57 185 ORANGE FLICKERS 

10.5 5." 141 TRANSPARENT 

14.0 4.22 105 TRANSPARENT 

1l.9 3.19 40.8 TRANSPARENT 

9.61 6.16 lo;l TRANSPARENT 

33.1 1.21 40.8 TRANSPARENT 

34.8 1.72 48.9 I) TRANSPARENT 

SMOKE SOUND 

CCJ48USTIOH EffiCIENCY 

(I) 

NONE RIJIBLE/HISS 99.3 

NONE ROAR 98.2 

NOHE RIJIBLE/HISS 81.3 
N 

I 

...... 

U1 

TABLE 2-3. PILOT ASSISTED HEAD TEST RESULTS 

(PAGE 3 OF 3) 

r FLARE GAS I GAS R · OBSE VATIONS 

FLARE NUMBER P~kRT EXIT I HEATI~G PROBE WIND flAME liFT TEST SIZE OF FLOWRATE VELOCITY I · VALUE HEI&HT1 SPEED LENCTH4 Off 

NO. <lnl PilOTS <scf•l U/sl PROPANE 1 lBtu/scf) (f) (llph) (f) (fl I COLOR 

349 6.0&5 

350 6.0&5 

351 6.065 

352 6.065 

353 6.065 

354 6.065 

155 6.065 

356 6.065 

351 6.065 

358 6.065 

359 6.065 

360 6.065 

361 6.065 

362 6.065 

363 6,065 

364 6.065 

365 6.065 

366 6.065 

1 

1 

1 

1 

2 

2 

2 

2 

2 

2 

2 

3 

3 

3 

3 

3 

J 

] 

4.1 

1.0 

1.1 

4.0 

2.0 

2.0 

2.0 

2.0 

2.0 

2.1 

2.3 

2.0 

2.0 

2.1 

2.0 

2.0 

2.0 

2.2 

20.4 

20.4 

20.5 

20.1 

20.2 

10.2 

28.3 

35.0 

37.4 

47.6 

20.1 

13.3 

12.9 

30.3 

34.9 

48.3 

49.6 

20.2 

1.63 

5.15 

6.96 

5.21 

2.44 

4.91 

6.81 

4.98 

6.84 

.5. 31 

5.22 

6.34 

2.85 

6.39 

3.88 

6.07 

4.81 

5.21 

100 

125 

167 

138 

65.1 

130 

165 

121 

164 

128 

U1 

160 

79 

155 

95 

146 

116 

130 

1. REMAINDER OF flARE GAS IS N2 AND S02 (TRACER> 

9 

n 

17 

14 

II 15 

> 

15 

3 4 

) 4 

) 17 

4 14 

~lABILITY TESTS 

3 I 13 

lsTABILHV TESTS 

3 I 13 

2. INCLUDING THE HEATING VALUE CONTRIBUTION OF THE PILOT GAS 

3. ABOVE flARE TIP 

18 TRANSPARENT 

18 TRANSPARENT 

12 ORANGE FliCKERS 

1 TRANSPARENT 

TRANSPARENT 

TRANSPARENT 

TRANSPARENT 

TRANSPAREMT 

TRANSPARENT 

TRANSPARENT 

0 !TRANSPARENT 

FAINT ORANGE 

TRANSPARENT 

TRANSPARENT 

TRANSPARENT 

TRANSPARENT 

TRANSPARENT 

12 !TRANSPARENT 

SNOKE! ·sooND 

NONE ROAR 

NONE LIN RUMBLE 

NONE RUMBLE 

NONE RUMBLE 

NONE I LOW ROAR 

NONE I LOW ROAR 

4. flAME LENGTH DETERMINED BY LENGTH OF VISIBLE FLAME COLORING AND TRANSPARENT HEAT WAVES (liGHT DIFFRACTION) 

5. INCLUDES HEAT WAVES ABOVE FlAME ENVELOPE 

6. USED WRONG ORIFICE METER 

COMBUSTIOH 

EFFICIENCY 

(I) 

92.0 

94.3 

98.5 

97.2 

95.7 

96.6 
The flame stability limit for the pilot assisted tests was difficult to 

determine, because 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 percent combustion 

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 J•stability limit". This stability limit is specific to these 

tests burning propane/nitrogen mixtures. 

Stability curves for the pilot assisted flares are shown in Figures 2-8 

through 2-12. Use of pilot assist greatly enhances flame stability. For 3 

and 6-inch unassisted open pipe flares, operated with a propane-nitrogen gas 

exit velocity of 40 f/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 becomes transparent and, by definition, the stability limit is 

reached. For the 6-inch flare, the same heating value reduction can be 

attained with the pilot at only 1 scfm. Additional pilot assist, however, 

only marginally increases flame stability. Increasing the pilot gas to 5 

scfm only ·reduces the heating value to 120 Btu/scf for the 6-inch flare. 

Increasing the number of pilots to two or three while keeping the total pilot 

gas rate constant at 2 scfm decreases the limiting heating value to 130 Btu/ 

scf for a 6-inch pipe flare. 

The quantitative results of these stability tests are limited to open 

pipe flares in the 3 - 6 inch diameter size range. By scaling the relative 

number of pilots, the pilot and flare gas flowrates and velocities, 

Richardson numbers, and Reynolds numbers, these results could also be applied 

to 12-inch or larger open pipe flares (4.1). The results of improved 

stability with pilot-assisted flares can be only qualitatively applied to 

flares that also have aerodynamic or other stability enhancing devices. 

2-16 

• 
N 

I 

...& 

....... 

-.... 

0 

"' ....... 

:::11 

... 

al 

...... 01 

:::11 

..... 

tel 

> 

Dl 

c 

~ 

't 

01 

:::1: 

"' c1 300 

200 

100 

Visual Observations 

--- o Faint orange flicker in flame envelope (stable) 

---- Q Disappearance of faint orange flicker ( stabt lity limit) 

• • • • • •• • Unstable 

~ 3-inch open pipe flare with stn9le pilot at 2.1 

scfm pilot gas rate (1984 tests)(2.1) 

Fa tnt orange flicker~ ~ $) 

1n fla!De envelope (stable) ~ $lat' 

Disappearance of fat nt orange r--..tfi,>-- •• ._.. 

flicker (stability ltmtt) ~ ~ ~-~······ ~ 7 • 

1. Includes pilot gas contribution Unstable~ 

0~---------------L----------------L---------------~----------------L---J 0.1 1.0 10.0 100 1000 

Flare Ex1t ·velocity (f/s) 

Figure 2-8. Flame stability curve for 3-fnch pipe flare with a single pilot at a pilot 

gas flowrate of 2.1 - 2.2 scfm natural gas. 
-..... 

u 

Ill 

-:I 

~ 

Gel 

.-

-

.. 

:I 

.-

.. 

> 

N ICft I IC ...... .,... co ~ 

.. 

.. 

:z:: 

(D = 

120~--------------~--------------~--------------y---------------~---

- 0 

...... - ~ 

Visual Observations 

Faint orange flicker 1n flame envelope (staple) 

Disappearance of faint orange flicker (stabtltty ltmtt) 

uvv~- ••-•• • Unstable 

200.-

100 

1. Includes ptlot gas contribution 

Faint orange flicker (stable) 

-o I <> ·-~-·-Q-.~- Stability 11mtt 

······•··•·····~·-unstable 

0~------------~~------------~~------------~~------------~~~ 0.1 1.0 10.0 100 

Exit Velocity (f/s) 

Figure 2-9. Flame stability curves for 6-inch open pipe flare with a single pilot at a 

pilot gas flowrate of 1.0- 1.1 scfm natural gas. 
1 

......... 

.... 

u 

Ill 

....... 

::I 

~ 

to 

..... -

Ql 

:I 

..... .., 

> 

g) 

c 

·~ N 

"g I 

..... .., 10 GJ 

::z:: 

Ill 

.., 

(JI 

a 

·-·-·- Q 

-------- . ----

Visual Observations 

Stability curve for 3-inch 

() ~nt ~e flicker (stable) 

-<Q ,. . 

Estimated stability limit J·-·-·-·-•••••• 

•..Q- - •• ~' 1 ............... Stab111ty curve for l-inch 

f pilot assisted open pipe Unstable nozzle 

1. 

0.1 10.0 100 1000 

Flare Gas Exit Velocity (f/s) 

Figure 2-10. Flame stability curves for 6-inch open pipe flare with a single pilot at a 

pilot gas flowrate of 2.1 - 2.4 scfm natural gas. 
-.... 

u 

lA 

...... 

~ 

.,a 

CD 

..... 

-

~ ,... 

• > 

1\) CJ) 

I c: 

N ~ 

0 .,a 

• 

:z: • 

lA 

(Jt • 

V1sua1 Observations 

D 

·-·- • ~ 01sappearance of faint orange flicker 

• • • • • • ••• • Unstable 

100 

. 

Ol 1. Includes p11ot gas . . ._JL. ~ Stability lf•tt 

.1 I cootrtbutton .......... ·~ ... 'jo•• 1.0 1 Unstilble · 

10.0 I 

• octty (f/s) I 1 

100 1000 

Exit v 1 

Figure 2-11. Flame stability curves for 6-inch open pipe flare with a single pilot 

at a pilot gas flowrate of 3.9 - 5.4 scfm natural gas. 
-.... 

u 

en 

....... 

:J 

.... 

QO 

r-., -

:J 

.-

_, 

> 

N ltJ' 

c I ..... N .... ..... _, 

4U 

::c 

en 

_, 

(D 

-.-

~·-· ~ 

••••••• • 0 

--·-· ~ 

•• •• • •• • 

Visual Observations 

Double pilot. disappearance of orange flicker (stability limit) 

Double pilot. unstable 

Triple pilot. faint orange flicker in flame envelope (stable 

Triple pilot. disappearance of orange flicker (stability 

Triple pilot. unstable limit) 

'--ll-·~-· •• 

Stability 11mit .........__ 0 ~r.l: 

1. Includes pilot gas contribution ~··1···· 01 1 Un~table 

0.1 1.0 10.0 100 

Exit Velocity (f/s) 

1000 

Figure 2-12. flame stability curve for 6-fnch open pipe flare wfth multiple pilots 

at a total pilot gas flowrate of 2.0 - 2.3 scfm natural gas. 
Adiabatic flame temperature can be used as an indicator of flame 

stability for specific gases. The flame stability limit is approached when 

the flame speed approaches the relief gas velocity. Flame speed depends on 

mixing and flame reaction rates. Flame reaction rates are functions of flame 

temperature, frequency factors, and activation energies for flame reactions. 

Figure 2-13 shows that the limiting stable gas exit velocity can be 

correlated with an estimated adiabatic flame temperature of the gas mixture 

burned in a stoichiometric air mixture. However, the curves for different 

gas mixtures and for the pilot assisted flares are different. This indicates 

that differences in activation energies, frequency factors, mixture 

strengths, and mixing rates for different compounds may affect flame · 

stability differently for different flare gas mixtures and pilot assisted 

flares. 

Combustion efficiency for the pilot assisted flares is correlated to 

flame stability in Figure 2-14. Combustion efficiency is greater than 

98 percent for heating value stability ratios greater than about 1.2. 

Combustion efficiency can rapidly decrease when the heating value ratio 

decreases below about 1.2. 

There are some subtle differences in combustion efficiency performance 

of the pilot assisted flames: 

• Combustion efficiency greater than 98 percent is maintained for the 

3-fnch head down to a heating value stability ratio of 1.0. This 

could be because the impact of the pilot on the flame for the 3-

inch flare is greater than on the flame from the 6-inch flare. 

Also, tests for the 3-inch head were conducted at slightly higher 

gas heating values than for the 6-inch head, because the stability 

limit curve for the 3-inch head requires higher heating value than 

the curves for the 6-inch head. 

• Below a heating value stability ratio of 1.0, however, the decrease 

in combustion efficiency for the 6-inch head at the high pilot gas 

rate and for the double and triple pilot is less rapid than for the 

2-22 

-------------------------------'""" 
-lit 

....... 

.... 

-,., 

.... 

-

--

~ 

oa 

.... 

V) 

I. 

0 

.... 

,., 

.... 

-u 

o-

,... 

cu 

:> 

Ql 

s:: 

-.... 

-E 

.,.. 

..... 

1000 

() l-inch flare with single pilot at 2 scfm pilot gas rate '84 

o 3-inch flare with single pilot at Z scfm pil·ot gas rate 

() 6-inch flare with single pilot at 1 scfm pilot gas rate 

CJ 6-inch flare with single pilot at 2 scfm pilot gas rate 

() 6-inch flare with single pilot at 4-5 scfm pilot gas rate 

() 6-inch flare with double pilot at 2 scfm pilot gas rate 

Cj 6-inch flare with triple pilot at 2 scfm pilot gas rate 

100 

10 

1.0 

Ethylene oxide/N2 gas mixtures flared from 

unassisted l-inch 

open pipe flares 

H2S/N2 gas mixtures flared 

from unassisted 3-inch open pipe flares 

Propane/N dominated gas mixtures flared from 

3-inch a~d 6-inch open pipe flames (unassisted) 

Figure 2-13. Calculated adiabatic flame temperature vs. limiting 

stable gas exit velocity for propane/N2 gas mixtures 

flared using pilot-assisted 3-inch and 6-inch open pipe flares. 

2-23 
0 

0 

---{ () 

CJ 

······{ D 

D 

0 

100 

-.... 

c: 

Qa 

1:.1 

'" Qa 90 0.. 

-::00. 

1:.1 

c: 

u 

-u 

-~ 

. ~ 

IA.I 

c: 

Q 

-.... 

~ 

~ 

1 

y 80 

3-fnch flare with single pilot at 2 scfm pilot gas rate (1984) (2.1) 

3-fnch flare with single pilot at 2 scfm pilot gas rate 

6-inch flare with single pilot at 1 scfm pilot gas rate 

6-inch flare with single pilot at 2 scfm pilot gas rate 

6-inch flare with single pilot at 4-5 scfm pilot gas rate 

6-inch flare with double pilot at 2 scfm pilot gas rate 

6-inch flare with triple pilot at 2 scfm pilot gas rate 

' .. : . . :: ... ; ... 

Region of combustion efffciency 

• measurements for previously tested 

.. - propane/N mixtures flared from 

• 3, 6, and212-f nch open pipe flares ( 2. ) .. 

.. 

-

• • 

• 

• • • • • CJ 

70~--------------._ ______________ ._ ____________ __. 

0 1.0 2.0 

Heating Value/Minimum Heating Value Required for Stability 

Figure 2-14. Combustion efficiency of pilot-assisted flares. 

2-24 
6-inch head with the single pilot medium and low pilot gas rates, 

and the 3-inch pilot assisted head. This may be because, even at 

or below the stability conditions, high relative pilot gas rates or 

multiple pilots can improve combustion efficiency. 

2.2 Conclusions 

The following conclusions were made based on the results of this report: 

• There are viable met~ods for accurate and reliable measurement of 

H2S and S02 in flare plume samples. 

• Flame stability depends not only on flare head design and flare gas 

volumetric heating value, but also on compounds present in the 

flare gas. Gas mixtures of H2S/N2 can be stably flared at much 

lower volumetric gas heating values than can propane/N2 or -5 

pePCent H2S in propane/N2 gas mixtures. 

• High H2S destr~ction efficiency is achieved for H2S/N2 and -5 -

70 percent H2S in propane/N2 gas mixtures when the gas heating 

value is at least 1.2 times the level required for flame stability. 

• For gas ~ixtures containing both H2S and propane, H2S destruction 

efficiency was consistently higher than propane combustion 

efficiency. 

• The total gas heating value required for a stable flame, including 

pilot contribution, is much lower for pilot assisted 3 and 6-inch 

open pipe flares than for the same unassisted flares. 

• High combustion efficiency is achieved for the pilot assisted tests 

when the gas heating value is at least 1.2 times the level required 

for flame stability. • 

2-25 
• There are subtle differences f n the flare combustion efficiency 

performance of the different pilot assist configurations tested in 

this program. Combustion efficiency greater than 98 percent is 

maintained for the 3-inch head even down to a heating value 

stability ratio of 1.00. Below a heating value stability ratio of 

1.00, however, combustion efficiency decreases more rapidly for the 

3-inch and 6-inch head with the single pilot at medium and low 

pilot gas rates, than for the 6-inch head with the single pilot at 

the high pilot gas rates and with the double and triple pilots. 

2.3 References 

2.1 Pohl, J. H. and N. R. Soelberg, 11Evaluation of the Efficiency of 

Industrial Flares: Flare Head Design and Gas Composition,'' EPA Report 

No. 600/2-85-106, NTIS No. PB86-100559/AS, 1985. 

2-26 
3.0 GAS MIXTURE TESTS OF HzS 

Unburned hydrogen sulfide emissions from industrial flares are of great 

concern because of the large amounts of highly poisonous and noxious hydrogen 

sulfide gases flared in the petroleum industry. Work was conducted during 

the test program to evaluate the overall combustion efficiency and 

destruction efficiency of H2S in gas mixtures. This work repeats limited 

testing conducted at EER in 1984 (3.1) on gas mixtures containing propane, 

nitrogen, and 5 percent H2S. Measurement errors for H2S prevented evaluation 

of HzS destruction efficiency for those tests. 

Testing of H2S gas mixtures proceeded in phases. First, HzS and SOz 

measurement techniques were evaluated for accuracy and adaptability to 

measurements of plume samples containing both H2S and S02 at high and low 

concentrations. After verifying the accuracy of measurement techniques using 

gas standards, tests were conducted using the laboratory-scale Flare 

Screening Facility. These tests were conducted to determine at what 

concentrations H2S could be burned efficiently. These tests were also used 

to verify the usefulness of H2S and SOz sampling and analysis. 

Although the flames produced on the 0.042 inch ID nozzle of the Flare 

Screening Facility are not similar to pilot scale and full sized flares, use 

of this facility has several advantages. Its low capital and operating costs 

provided an economical way to quickly evaluate if HzS could be burned 

efficiently. The nozzle is enclosed in a reactor shell, isolated from the 

environment which allowed undiluted.~lume samples to be collected. The small 

size and reactor shell enclosure increase safety for flaring toxic gases. 

The isolating enclosure also gives the ability to close inlet and exit mass 

balances, for verification of sampling techniques. 

After the review of measurement techniques and laboratory scale tests 

were completed, pilot scale H2S gas tests were conducted using the Flare Test 

• Facility. These tests repeated the incomplete and partially uncertain HzS 

gas mixture tests in EER's 1984 test program, and extended those results to 

3-1 
include efficiency measurements for flare gases with higher H2S 

concentrations. 

3.1 Measurement Techniques 

Accurate measurement of HzS and S02 levels in plume samples is essential . to estimating H2S destruction efficiencies. It is important that 

measurements are made before HzS and SOz levels in the samples can 

significantly change due to reaction with other sampled species or with the 

sample apparatus or containers. It is also important that either the H2S or 

S02 species be physically separated (such as by gas chromatography) before 

analysis, or that the presence of SOz in the sample does not interfere with 

H2S measurement, and vice versa. The H2S destruction efficiency measurements 

conducted at EER in 1984 were inconclusive because so2 in the samples 

interfered with H2S measurements. 

Prior to HzS gas mixture testing in this test program, a thorough 

investigation of H2S and S02 sample techniques was conducted. Analysis 

methods for H2S included: 

• Concentration of gaseous HzS gas scrubber bottles containing 

aqueous cadmium sulfate solution, followed by iodometric titration 

• Concentration of gaseous H2S gas scrubber bottles containing 

aqueous zinc acetate solution, followed by colorimetric analysis 

using the methylene blue method 

• Reaction of gaseous HzS with lead acetate, causing darkening of 

lead acetate impregnated paper tape 

• Gas chromatography 

• Reaction with color indicators in Draeger tubes 

3-2 
Methods for S02 sample analysis that were investigated were: 

• Concentration of gaseous S02 gas scrubber bottles containing 

aqueous hydrogen peroxide solutions followed by barium perchlorate 

titration 

• Gas chromatography 

• Reaction with color indicators in Draeger tubes 

Tests were conducted using standardized gases and gas mixtures. These 

tests were conducted to verify app 1 i cabil ity of the techniques to measure 

plume gas concentration of H2S and 502 by evaluating accuracy, repeatability, 

reliability, and measurable ranges. Results of these tests are shown in 

Table 3-1. The fodometric titration and lead acetate methods for H2S 

measurement gave very inaccurate and unreliable results and were not used for 

further analyses. The methylene blue and Draeger tub~ methods proved to be 

most accurate and reliable in the required sample range of 0-100 ppm. Where 

possible, both methods were used for H2S sample analysis. Gas chromatography 

was also used when the H2S concentration was within the GC detection range of 

25 - 1000 ppm. 

For 502 measurement, the barium perchlorate titration method was most 

accurate. Occasionally, however, low measurements were obtained, possibly 

due to sample system leaks or analytical errors. Draeger tubes and gas 

chromatography were used to verify results from the titration measurements. 

3.2 Laboratory Scale Tests 

Laboratory scale tests were conducted to determine concentrations at 

which H2S could be burned efficiently and to verify the usefulness of H2S and 

S02 sample procedures. The Flare Screening Facility, equipped with a 0.042 

inch 10 nozzle, was used for these 

safer to operate than the larger FTF. 

• 

tests, because it is less expensive and 

Complete inlet and outlet mass balance 

3-3 
c...l 

I 

~ 

TABLE 3-1. MEASUREMENT TECHNIQUES FOR H2S AND so2 

I£ASUREO IWtGE HOLDING TIME 

I£TifOD SPECIES REf ,.,., ACCURACY SENSITIVITY COffQTS 

lodoletrtc Titration H2S EPA Method 11 1 6 - 520 !.1001 NM1 Large errors due to presence of 

40 CFR Ch. 1 oxtdtzlng or redutlng agents. (7-1-85 Ed.) leaks. or H2s loS$ 

'ethylene Blue Method ~s MSI/ASlM 0.5-20 .!. 501 0-301 No measurable Interference from 

D 2725-70 Increase In other species (Rea~proved 111easured I eve Is 

1975 In 24 hours 

lead Acetate Method HzS MSI/ASTM 100-2000 + lOOS NM Considerable Interference fro. 

(Tracor-Atlas ~S D 2420-66 - so2, reducing H2S aeasure.ents 

Mter) (Rearroved 

1976 

Dr1eger Tubes HzS Detector Tube 0.5-2000+ +2~ Gradual color Some Interference fro. SO 

HindbOOE, Uli boundary blurring presence. but thts 1s the2best and 

ta. AUg. 1979 after 5 •lnutes to easiest .. thod for H2

s .. asurements 

1 llour 

so2 Detector Tube 0.5-8000+ !.201 Good holding ttme HiS also detected tn 50108 tube 

Rana60oE • :Uii exceeding 48 hours r nges Ea •. AUg. 1979 

&as Chra.atography H2S Supelco 25-1000+ !.201 Sll .. t .,.,..,. } (Perktn-El .. r SigMa 2 Chromatography after 24 hours 

&as Chra.atography Suppltes No H2S-S0f cross-senstttvtty. but 

not usefu at low concentrations 

so2 Supelco 10-5000 !.201 Slight decrease Chro~~~tography after 24 hours 

Supplies 

· Bart1111 Perchlorate SOz EPA Method 6 1-2000+ +151 Good holdtng tt• Occastona1 low values from 1eats 

Titration 40 CFR Ch.l - exceeding 48 hours tn systeM or analytical errors, but 

(7•1w85 Ed.) th1S ts the best Method for S~ 

•asure~~ents. ~-

--·----- --~ 

1. NM • Not Measure4 
closure could also be determined with this facility to provide verification 

of sample procedures. 

Figure 3-1 compares flame stability limit results for the H2S gas 

mixtures to those for propane/N2 mixtures flared using the 0.042-inch nozzle 

and 3, 6, and 12-inch flare heads. The flame stability limit is defined as 

the operating condition where an increase in gas exit velocity or decrease in 

gas heating value results in flame blowout. The curves in Figure 3-1 were 

generated by determining the minimum gas heating value attainable before 

flame blowout for different gas exit velocities. Much higher gas heating 

values are required to maintain a flame on the 0.042 inch nozzle than for the 

3, 6, and 12-i nch heads. This is because of the aerodynamic differences 

between small and large diameter jets. Reynolds number (Re) indicates the 

degree of turbulence in a fluid stream. For jets from the 0.042 inch nozzle, 

Re ranged between 102 to 104, indicating that the flow was in the laminar to 

turbulent transition regime. Re for jets from the larger flare heads ranged 

from 103 to 106, denoting transition to turbulent flow. Richardson number 

(Ri) indicates relative influence of buoyant forces on aerodynamic flow. 

Richardson numbers greater than one indicate dominance by buoyant forces. 

The jets from the 0.042-inch nozzle had Ri ranging from to-6 to 10-J, 

signifying inertial force dominance over buoyant forces. For the 3 through 

12-inch heads, Ri was between l0-4 and 103. Some of these flames were 

dominated by inertial forces and some by buoyant forces. Most commercial 

flare flames are dominated by buoyant forces. 

The destruction efficiency of H2S and mass balances for these tests are 

shown in Table 3-2. The H2S gas content was 4.31 to 69.5 percent for these 

tests and the nozzle exit velocities ranged from 8.52 to 27.5 f/s. Greater 

than 99 percent H2S destruction efficiency was measured for all test 

conditions. Mass balance closure for carbon, oxygen, and sulfur was less 

than 1.00 for all but one case. The best closure was with oxygen. Carbon 

and sulfur closure ratios were generally less than the corresponding oxygen 

closure ratios, but satisfactory for measurement of flare combustion and 

destruction efficiency. 

3·5 
.... 

~ 

u 

~ 

" ~ 

~ 

GQ 

.... 

~ 

~ 

... 

4 

=-

~ 

~ 

..-

~ 

4 

~ 

:c 

~ • «D 

2400~------------~------------------------------

1400 

1200 

1000 

~0 

600 

1984 Tests, Propane/N2 Mixtures (3.1 

1985 Tests, Propane/~ Mixtures 

Tests, 4-5% H2 S 1 Mixtures 

Tests, 30-41% H2s 1 Mixture 

Tests, 50-701 H2S Mixtures 

0 

I 

' ? 

I ' 

I 

I 

I 

I 

I 

I 

I 

I 

~ l 

I 

I 

I 

p 

' 

roo~~--------------------------~--------------~ 1.0 10.0 100.0 1000.0 

Exft Velocity (1/s) 

Figure 3-1. Flame stability curves for propane/N2 and H2S/propane/ N2 gas mixtures flared using a 0.042 inch ID nozzle. 

1. Remainder is propane and N2 

3-6 
w 

I 

...... 

Test 

No. 

1 

2 

3 

4 

5 

TABLE 3-2. FLARE SCREENING FACILITY H2S GAS MIXTURE TEST RESULTS~ NOZZLE ID = 0.042 INCHES 

Gas Compos1t1on Gas Nozzle A1r H2S Propane 

(Remcu nder N2) Heaung Exit Stofchfo Oestrucuon C011bus t1 on 

H2S Propane Value Velocity metric Efhc1ency _Eff1c1ency 

(Btu/scf) (f/s} Ratio (I) (S) 

(') (') 

4.31 45.2 1086 27.5 3.15 >99. 99 99.9 

34.8 52.0 1427 17.4 2.90 >99.99 99.99 

31.9 55.0 1480 16.5 2.90 >99.99 99.9 

4.28 45.5 1093 27.0 3.17 >99.7 99.9 

69.5 30.5 1125 8.52 4.02 >99.99 98.6 

Mass Balance 

Closure 

Mass In/Mass Out 

c 0 s 

I 

0.567 0.944 0.446 ' 

0.706 0.939 1.12 

0.878 0.852 0.794 

0.952 0.918 0.115 

0.729 0.891 0.774 
3.3 Pilot Scale Tests 

Pilot scale tests of H2S gas mixtures were conducted using a nonassisted 3-inch open pipe flare head at the Flare Test Facility. These tests 

were conducted to measure flame stability, combustion efficiency, and 

destruction efficiency for H2S gas mixtures. Results were used to verify 

incomplete results of the EER 1984 test program and to extend the"test data 

to include gas mixtures with higher H2S concentrations. 

Flame stability limit measurements are shown in Figure 3-2. These 

curves were generated by determining the minimum gas heating value attainable 

before flame blowout for a range of different gas exit velocities. Good 

reproducibility was attained between the 1984 and 1985 tests with -5 percent 

H2S in prop«ne/N2 mixtures. Since the dominant combustible gas in these 

mixtures is propane (14.4 - 20.1 percent), it is not surprising that the 

stability curve for these mixtures is withi~ boundaries previously determined 

for propane/Nz mixtures flared on the 3, 6, and 12-inch heads. 

On~ other method of.depicting the flame stability limit is to correlate 

the minimum gas heating value on a mass basis to the gas exit velocity. The 

difference between volumetric heating value and mass heating value for 

gaseous compounds depends on the compound molecular weight. Compounds with 

high volumetric heating values (1,3-butadiene, for example, having 2,730 Btu/ 

scf) may have low mass heating values (19,500 Btu/lb for 1,3-butadiene) 

compared to the heating values of other compounds (hydrogen, for example, 

having only 275 Btu/scf, but 51,000 Btu/lb). In Figure 3-3, the stability 

curve shown in Figure 3-2 is presented showing heating value on a mass basis. 

The correlation with exit velocity for different gases is no better than in 

Figure 3-2. Since the majority of the flare gas in all these mixtures is 

nitrogen (50 - 95 percent), the overall molecular weight and density of these 

gas mixtures only changes slightly from mixture to mixture. 

The flame stability limit curve for H2S/N2 gas mixtures in either 

Figure 3-2 or Figure 3-3 is much lower than for the H2S/propane/N2 or 

propane/N2 mixtures, but the volumetric heating value for H2S gas (588 Btu/ 

3-8 
w 

I 

10 

-...... 

u 

Ul 

';;' 800 ~ 

CD 

.... 

ld 

> 

"r'" 

~ 

ld 

.. 

::z: 

Ill 

ld 

tD 

'\) 1984 -s% H2s in Propane/N2 Mixtures (3.1) 

[7 1985 -5% H2S in Propane/N2 Mixtures 

C> H2StN2 Mixtures 

-""""-d>- 'f 

-------------------.--------~----------------------------------------------

100~--------------~~------------~~------------~~~----------------~ 0.1 1.0 10.0 100.0 1000.0 

Exit Velocity (f/s) 

Figure 3-2. Flame stability curves for H2S gas mixtures flared using a 3-inch diameter 

open pipe flare. 
-.0 

.... 

..... 

:;, 

+' 

II) 

-.. 

:;, 

.. 

=-

en 

c 

- w 

• +' Ill .... 

0 .. 

~ 

.,. 

Ill 

cD 

8000• Flared Using 3-Inch Op~n Pipe Without Pilot Assist 

100 

() 1,3-butadtene/N2 Mixtures 

(J Ethylene Oxtde/N2 Mixtures 

,, ... ~e., ,.,, "' .. 1-... ~ '\o"" \~· 

[> Hydrogen Sulf1de/N2 Mixtures ._, ~' 

~~e\~',Jt.o~'<.. ~'(Q~b, ... 

,o'( ~eti 

() 

'\ .......... , , .... t~o" 

... ~ ... "'"""' b ,'l.' ... ~~ , fi.t' 

, .... "- b• ~ A~· a ,~.., 

~0~ ~, 

~e~~eb , .... ~ 

Ethylene Ox1de/N2 

[> 

_l __ ~()-

J 1.3-butadiene 

H2SJN2 

~o.-1--------------~1~.o--------------~1o--------------~1o~o----------~ 

Extt Velocity (f/s) 

Figure 3-3. Flame stability curves using heating value per mass relationship. 
scf) is much lower than that of propane (2350 Btu/scf). There is actually a 

higher concentration of H2S (28.9 - 49.5 percent) in the H2S/N2 mixtures at 

the stability limit than the combined propane and H2S in the 5 percent H2S 

mixtures {18.8 - 25.2 percent). The presence of higher H2S concentrations, 

and/or better combustion kinetics for H2S than for propane, may help to 

maintain a stable flame at lower heat input. 

The flame stability limit is approached when the flame propagation speed 

in the gas approaches the relief gas velocity. The determination of flame 

speed is very complex, involving reaction kinetics and mixing patterns. The 

inherent complexities discourage a direct evaluation of flare flame kinetic 

and mixing rates. However, the higher levels of H2S concentration in a 

mixture near the stability limit could increase the flame speed (unless the 

flame reactions are kinetically limited) by providing a greater number of H2S 

- 02 molecular interactions relative to the total number of molecular 

interactions. 

However, the flame speed may be limited by reaction kinetics, and not 

affected by mixing. Kinetic rates of flame reactions are dependent on flame 

temperature and other parameters according to the exponential Arrhenius 

equation: 

-E/RT 

k = Ae (3-1) 

where k is the rate constant for a given reaction, A is the frequency factor, 

E is the activation energy, R is the ideal gas constant, and T is 

temperature. If all parameters except temperature in Equation 3-1 are 

maintained constant as the temperature is increased, the reaction rate 

constant exponentially increases. If the flame speed is limited by reaction 

rates, the flame speed (and limiting maximum gas exit velocity) should 

increase exponentially as the flame temperature increases. 

The actual flare flame temperature is however difficult to determine 

accurately. Since the flame is in the open air, subject to wind and eddy 

conditions, both in-flame and optical temperature measurement methods are 

3-11 
subject to errors. Also, within the flame envelope, thereis a wide 

temperature range due to varying degrees of air dilution and reaction 

completion. As a basis for comparison, a pseudo flame temperature can be 

used for showing the dependence of flame stability on temperature for 

different flare gases. This pseudo flame temperature may be calculated 

assuming (1) adiabatic, complete combustion of the flare gas with a 

stoichiometric amount of air, or (2) adiabatic, complete combustion of a 

flare gas-air mixture with stoichiometry between the upper and lower 

flammability limits of the mixture, or (3) non-adiabatic, complete combustion 

of a flare gas-air mixture, assuming a certain amount of heat loss. In 

previous studies (3.1 and 3.2), pseudo adiabatic flame temperatures based on 

stoi chi ometri c combustion of flare gas with a 1 r were .. compared to flame 

stability. This pseudo adiabatic flame temperature was also combined with 

both upper and lower flammability limits and rel6ted to flame stability. 

Within the scope of this report, and fn order to make comparisons with 

previous EER research ( 3.1), p_seudo adiabatic flame temperatures based on 

stoichiometric combustion were related to flame stability in Figure 3-4. 

The limiting maximum stable gas exit velocity is shown to correlate well 

with adiabatic flame temperature for individual gas mixtures. However, there 

is poor agreement between the correlations for the different gas mixtures. A 

much higher flame temperature is required to burn propane mixtures at a given 

velocity than other gas mixtures. Gas mixtures of H2S exhibit the lowest 

flame temperature for a given gas exit velocity. Therefore factors other 

than flame temperature also affect the flame stability of different gases. 

Such differences in flame stability of different gas mixtures shown in 

Figure 3-4 is expected, considering the structural, physical, and chemical 

differences of the different compounds. Each of the compounds shown in 

Figure 3-4 has a different structure and properties distinctive of different 

classes of materials -alkanes (propane), conjugated dienes (1,3-butadiene), 

oxygenated compounds (ethylene oxide), and sulfur compounds {hydrogen 

sulfide). Some comparative physical properties for HzS and propane are shown 

in Table 3-3. H2S has a wider flammable range, a lower adiabatic flame 

temperature, and a lower minimum ignition temperature than does propane. 

3-12 
rn 

....... 

1000 Flare Gas Composition 

o Propane N2 a Ammonia/Propane/N2 0 1,3 Butadiene/N2 D Ethylene Oxide/A2 

~ 100 

~ Hydrogen Sulf1de7Propane/N2 (1984) ~ Hydrogen Sulffde/Propane/N2 (1985) ~ Hydrogen Sulfide/N2 

~ 

~ 

..... 

-·-.c 

IU 

~ 

V') 

s.. 

Q 

.... 

~ 

~ 

..... 

u 

Q 

-:: 

01 

c 

..... 

~ 

..... 

e 

..... 

...J 

1,3-Butadiene 

~~-

~ 

10 

Propane-Dominated 

Gas Mixtur ---- 1.0 Ethylene Oxide 

0 

0.1----~----~--------------~----------~-----

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 

Adiabatic Flame Temperature (1/R) x 104 

Figure 3-4. Calculated adiabatic flame temperature versus 

limiting stable gas exit velocity from an 

unassisted 3-fnch open pipe flare. 

3-13 
TABLE 3-3. PHYSICAL PROPERTIES1 OF H2

S AND PROPANE AT 60°F, 1 ATMOSPHERE 

Property ijzS Propane 

Molecular Weight 34.076 44.097 

Gas Density, (scf/lb) 11.4 8.77 

Lower Heating Value ~Btu/sc~) 588 2350 

Btu/lb 6700 20600 

Flammability Limits fn Air (') 

Lower 4.3 2.1 

Upper 45.5 10.1 

Adiabatic Flame Temperature for 

Stofchiometfc Combustfon'Wfth Air (R) 

Calculated2 3338 3838 

Literature NA3 4055 

Ignition Temperature (F) 558 871-898 

Flame Velocity (f/s) NA3 0.95-1.3 

1. Data From: Balzhiser, R. E., M. R. Samuels, and J.D. Eliassen, 

Chemical Engineering Thermodynamics. 1972 

CRC Handbook of Tables for Applied Engineering Science, 2nd Ed., 1976 

CRC Handbook of Chemistry and Physics, 53rd Ed., 1972-73 

Howard, H. F. and G. w. Jones, 11Lim1ts of Flannability of Gases and 

Vapors•, USBM 8ullet1n 503, 1952 

GPSA Engineering Data Book, 1972 

Gas Engineers Handbook, lst Ed., 1965 

North American Combustion Handbook, 1st Ed., 1952 

2. Calculated by integration of heat capacity data 

3. Not Ava1lable 

3-14 
These differences indicate differences in other factors in Equation 3-1 which 

could influence kinetic rates for reactions in H2S and propane flames. 

The relationship between flame stability and flare combustion efficiency 

(see Appendix 0 for efficiency definitions) has been developed in previous 

studies. Figure 3-5 relates propane combustion efficiency for the H2S/ 

propane/N2 gas mixtures to the ratio of actual gas heating value and the 

minimum heating value required to produce a stable flame at that velocity. 

This correlation is within the range previously developed for a variety of 

gas mixtures and flare head types and designs. Combustion efficiency is 

high, exceeding 98 percent, for stability conditions greater than about 1.2. 

When stability conditions decrease below 1.2, combustion efficiency can 

rapidly decrease below 98 percent. 

A similar correlation for H2S destruction efficiency is seen in 

Figure 3-6. All data points except one fall within or above the previously 

determined destruction efficiency region for ammonia, 1,3-butadiene~ ethylene 

oxide, and propane. High destruction efficiency, greater than 99 percent, is 

attained for stability conditions greater than about 1.2. Destruction 

efficiency rapidly drops below 98 percent when the stability criteria 

decreases below 1.2. The presence of the single low efficiency point outside 

the shaded area is from a test that may have been influenced by ambient 

conditions or an upset or discontinuity in flow control or sample procedures. 

Since these tests were predominantly conducted very near the limit of flame 

stability, any slight change radically affected flare performance and 

efficiency. 

Destruction efficiency of H2S is compared to destruction efficiency of 

propane in Figure 3-7. For the H2S/propane/N2 mixtures and operating 

conditions of these tests, HzS destruction efficiency was consistently higher 

than propane combustion efficiency. This indicates that H2S flames may be 

more stable and react more quickly than propane flames. Figure 3-2 shows 

that H2S flames are more stable than propane flames. 

3-15 
"'"" -~ 

c:: 

Q.l 

u 

s.. 

cu 

c. 

->, 

u 

c:: 

Q.l 

.... 

u 

.... 

ItIt1.1.1 

c: 

0 

-

~ 

"' ~ 

.,Q 

5 

u 

99.99 0.01 

99.9 0.1 

.. ' .·.··. 

·. ~.:~-t;.,< .. 

99.0 .>:,:-~ 1.0 ' 

, 1984 ss H2

s in 

Propane/N2 Mixtures 1) 

1985 51 H S in 

Propane/N2 90. 2

Mixtures 10.0 

-101 ~sin Propane/ 

Nz Mixt res 

0 ..._-----~~----~~----_.. 100.0 0 1.0 

Heating Value/Minimum Heating Value for Stability 

Figure 3-5. Propane combustion efficiency of H2S/ 

propane/N?. gas mixtures flared from an 

uriassistea 3-inch open pipe flare. 

1. Scale is log (100- Combustion Efficiency) 

3-16 

-c:: 

0 

.Q 

~ 

te 

u 

. 

-te 

-

~ 

.... 

c: 

-It0 

~ 

c: 

cu 

u 

s.. 

Q.l 

~ 

-

c: 

0 

.Q 

~ 

te 

u 

~ 

cu 

c: 

s.. 

~ 

= 

>, 

-cu 

~ 

cu 

-c. 

8 

u 

c: 

-
~ 

c 

CLI 

u 

seLl 

c.. 

-

~ 

u 

c:: 

CLI .,.. 

u 

.... 

99.99 

99.9 

~ 

~ 99.0 

c:: 

0 

..... 

90.0 

l> 

.:., '· 

:, ... · .. : ... 

Regf aa of destruc:tfOft' 

efficfenq"for ... 1a:. 111 ~butacl1ene., ethylene 

oxide-., ami propane. C3•1·JC7 1985 -ss H2s in Propane/N2 Mixtures (3.1) 

C7 -70% H2S in PropanetN2 Mixtures 

C> H2StN2 Mixtures 

0.01 

0.1 

1.0 

10.0 

0 ...._ _____ _... ______ ...._ ____ _.100.0 

0 1.0 2.0 

Heating Value/Minimum Heating Value for Stability 

Figure 3-6. Destruction efficiency of H S gas mixtures flared from an una~sisted 3-inch open pipe flare. 

1. Scale is log {100-0estruction Efficiency) 

3-17 

-

V')t 

N ::c 

.... .., 

..... 

~ 

.... 

c:: 

-

~ 

0 

~ 

c:: 

cv 

u 

seLl 

c.. 

-

V') 

N ::c 

en 

c:: 

.,... 

c:: 

-.., 

w 

a: 
100 

.., 

c:: 

41 

u 

So. 

41 

Q. 

-

>, 

u 

c:: 

41 

-u 

-~ 

~ 

.., 

c:: 

0 

.... 

.., 

u 

:s 

So. 

.., 

Ill 

41 

Q 

~ 92 

N 

:: 

90 

Figure 3-7. 

f7.1' 

, 

/ *" 

, 

""' r7"_ 

51 H2S in Prapane/N2 Mixtures 

~70" H2s ; n PrapanetN2 Mixtures 

90 92 94 96 98 100 

Propane Combustion Efficiency (Percent) 

Correlation between H S destruction efficiency and propane combustio~ efficiency for H

2

S/propane/ N2 mixtures flared from an unassisted 3-inch open 

pTpe flare. 

3-18 
3.4 References 

1. Pohl, J. H. and N. R. Soelbel"'g, 11 Evaluation of the Efficiency of 

Industria 1 Flares: Flare Head Des i gn and Gas Composition, 11 EPA Report 

No. 600/2-85-106, NTIS No. PB86-100559/AS, 1985. 

2. Noble, R. K., M. R. Keller. and R. E. Schwartz. 11 An Experimental 

Analysis of Flame Stability of Open Air Diffusion Flames 11 , AFRC 

International Symposium on Alternative Fuels and Hazardous Wastes, 

Tulsa, OK, 1984. 

3-19 

4.0 PILOT ASSISTED TEST RESULTS 

Very little testing of pilot assisted flares has previously been 

conducted at EER or reported in the literature. Most previous work at EER 

has been on unassisted flares, in order to simplify the test programs and to 

evaluate operating parameters independent of pilot assist. Industrially, 

however, continuous gas pilots are commonly used to ensure safe, continuous 

and emergency flaring. 

Upon recommendation of the flare Technical Advisory Committee, more 

extensive pilot assisted testing has been conducted at EER. For these tests, 

the flare gas was a mixture of propane blended with nitrogen to vary the gas 

heating value. Utility-supplied natural gas was used for the pilot gas. 

Open pipes of 3 and 6-inch diameter were used as the flare heads. Single, 

double, and triple pilots were tested. The single pilot was a John Zink Co. 

2-inch gas pilot equipped with a flame front generator. The double and 

triple pilots were manufactured by EER and are similar to the Zink pilot. 

Orientation of the pHots to the flare as suggested by the Advisory Panel is 

shown in Figure 4-l. 

Operating parameters evaluated were the number of pilots and the pilot 

gas flowrate. Tests were conducted on the 3-inch and 6-inch flares with a 

single pilot and a nominal pilot gas flowrate of 2 scfm. Using the 6-inch 

flare, tests were also conducted with the single pilot at pilot gas flowrates 

ranging from 1-5 scfm. Tests were also conducted on the 6-inch flare with 

single, double, and triple pilots, maintaining the total pilot gas rate at 

2 scfm. 

The flare gas exit velocity range for all these tests was limited to the 

range between 8-150 f/s for the 3-inch head and 5-50 f/s for the 6-inch head. 

Lower velocities were not used because of the high pilot gas rate relative to 

propane flowrate in the flare gas. Higher velocities were limited by the 

maximum nitrogen flowrate capacity of the Flare Test Facility. 

4-1 
A. Single pilot 

B. Double pilot • Radial 

angle between pilots 1s 

180 degrees. 

C. Triple pflot. Radial 

angle between pilots 

120 degrees. 

,,. \ 

I I \ I 

'*' I I , I 

I 

I 

, 

,,, 

I I \ 

I~\ I . I \ 

Dimensions: 

A. 30 degrees 

B. 2.5 inches 

C. 1 inch 

Figure 4-1. Schematics of single, double, and triple pilots. 

---------- ----------~ 
4.1 Flame Stability 

For all combinations of pilot assist. flame stability limit curves were 

de.termined. The flame stability limit is defined in Appendix Cas the 

condition where a decrease in flare gas heating value or an increase in flare 

gas exit velocity results in flame blow-out. The previously used flame 

stability limit determination procedure was to incrementally decrease the 

flare gas heating value at constant gas exit velocity until flame 

destabilization and blowout occurs. This definition and procedure could not 

be applied in the p;lot assisted tests because the presence of a pilot 

prevented flame blowout, even at very low flare gas heating values. 

Consequently, the definition and determination of the flame stability limit 

were more subjective. The gas heating value required for 98 percent 

combustion efficiency at a given flare gas velocity was observed to be the 

operating condition where the last faint flickers of orange color completely 

disappeare and the flame envelope becomes completely transparent, except for 

blue-orange cones near the pilot and flare tips. This limit is determined by 

gradually decreasing the flare gas heating value, maintaining constant gas 

exit velocity and pilot gas flowrate and observing the flame, as orange and 

yellow color in the flame decreases and eventually disappears. This 

definition applies to specific pilot assisted tests of this stu~ burning 

propane/nitrogen mixtures. 

Flame stability for the 3-inch pilot assisted head is shown in Figure 

4-2. Curves defining stable, stability limit, and unstable flames are shown. 

The gas heating value includes the added heating value contribution of the 

pilot gas. Some scatter in the data points is due to the subjectivity of the 

visual measurements and to slight fluctuations in the ambient conditions, 

such as lighting and wind variations. For a given gas exit velocity, the 

stability curves occur at significantly lower gas heating values than the 

region of stability limit curves for unassisted 3 through 12-inch flares. 

Clearly, this pilot-assisted open pipe flare can be operated stably at.much 

lower total heating values (pilot gas included) than the same flare without 

pilot assist. Again, it should be noted that these data are for very simple 

open pipe flares. In commercial practice a variety of aerodynamic 

4-3 
-Itu 

-

"' :I 

+' 

IQ 

-.... cu 

::11 

.-

> • 

"' I: 

.... ~ ... • I 

~ IU 

% 

"' • "' 

100 

Visual Observations 

~ Faint orange flicker tn flame envelope (stable) 

~---. Disappearance of faint orange flicker (stability ltiDit) 

• • • • •••' Unstable 

~ 3-tnch open pipe flare with single pilot at 2.1 

scfm pilot gas rate (1984 tests)(4.1) 

Faint orange fl tcker ~ j) 

tn flame envelope (stable) 6) j;J~' 

Disappearance of fa tnt orange r---fl>-- ...... flicker (stab111ty limtt) _./" • 4p •••••••• 

1. Includes ptlot gas contribution Unstable~ • 

0 ~o-.,~----------~1~.o~-------------1~o-.o-------------1-00---------------1·ooo~-J 

Flare Extt ·velocity (f/s) 

Ffgure 4-2. Flame stability curve for 3-1nch p1pe flare w1th a single pilot at a pilot 

gas flowrate of 2.1 - 2.2 scfm natural gas. 
stabilization devices are commonly used. Flares with such features may 

exhibit differences between piloted and non-piloted performance which are 

smaller than for the simple system studied here. 

Flame stability curves for the 6-inch flare with a single pilot at low, 

medium, and high pilot gas rates are shown in Figure 4-3 through 4-5. For 

each pilot gas flowrate series, the curves of total gas heating value 

(including pilot) vs flare gas exit velocity for the stability limit, as 

defined by the disappearance of orange coloring in the flame envelope. are 

shown. These stability limit curves depend only slightly on velocity in the 

range tested. For low and medium pilot gas flowrate, the limiting total gas 

heating value is about 150 Btu/scf for flare gas exit velocities between 

5.6-45 f/s. For comparison, Figure 4-4 shows that this limiting heating 

value is the same as that for the 3-inch pilot assisted flare with a medium 

pilot gas rate, at a flare gas exit velocity of 35 f/s. The limiting heating 

value for the tests with high pJlot gas rate is also constant, but slightly 

lower at about 120 Btu/scf. Tests conducted below this level were in the 

unstable regime. 

Flame stability curves for the 6-inch flare with multiple pilots are 

shown in Figure 4-6. The stability limit for the double and triple pilot 

tests is flat at a constant heating value of about 130 Btu/scf, for gas exit 

velocities between 10-30 f/s. This is slightly lower than for the single 

pilot assisted tests at equal or lower pilot gas rates, but slightly higher 

than for the single pilot tests at higher pilot gas rates. At gas exit 

velocities above 40 f/s, the stability limit for the multiple pilot tests 

gradually increases as it does for the 3-inch pilot assisted flare tests. 

Use of pilot assist greatly enhanced flame stability during these tests. 

For 3 and 6-inch unassisted open pipe flares, operated with propane-nitrogen 

gas exit velocity of 40 f/s, the minimum gas heating value required to 

maintain a flame is about 540 Btu/scf. If a 2 scfm natural gas pilot used, 

the total heating.value (including pilot contribution) required for a stable 

flame can be reduced to 150 Btu/scf. At this condition. the flame envelope 

becomes transparent and by definition. the stability limit is reached. For 

4-5 
-.... 

u 

Ill 

....... 

:::J 

... 

m 

.-

-., 

:::J 

.-

'If 

> 

""' ~ 

c: 

• .... 0\ ~ .. 

:c 

Ill 

CD • 

....... f 

200.-

100 

- ~ 

·-·~ ~ 

---~ • 

Visual Observations 

Faint orange flicker in flame envelope (staple) 

Disappearance of faint orang~ flicker (stability limit) 

Unstable 

Faint orange flicker (stable) 

-o I o- ·-~·-Q-.~ -stability limit 

........... ••••t•-unstable 

1. Includes pHot gas contrtbut ton 

0~--------------~--------------L---------------~--------------~--~ 0.1 1.0 10.0 100 1000 

Exit Velocity (f/s) 

Figure 4-3. Flame stability curves for 6-1nch open p1pe flare with a single pilot 

at a pilot gas flowrate of 1.0- 1.1 scfm natural gas. 
10 

-.... 

u 7 Ill 

..... 

::t 

~ 

aa 

..... -

Q.l 

::t 

..-

11:1 

> 

CJl 

c: 

..... ~ "0 4 • 11:1 ..... Q.l 

:z: 

Ill 

11:1 

UJ 

0 

·-·-·· Q 

·----·--- . -----

Visual Observations 

~ ~nt orange flicker (stable} 

-G. g ,. 

_7·-·-·-·-·.....Qo- -.~' Estimated stability limit ti•••••J•••········~··stab111ty curve for 3-1nch 

f pilot assisted open pipe Unstable nozzle 

1. at contribution 

0.1 1.0 10.0 100 1000 

Flare Gas Exit Velocity (f/s) 

Figure 4-4. Flame stability curves for 6-inch open pipe flare with a single pilot 

at a pilot gas flowrate of 2.1 - 2.4 scfm natural gas. 
-Po 

I 

CX) 

-ltu 

Ill 

..... 

:1 

.,a 

co 

! 

.-

til 

> 

m 

c 

.... 

.... 

til 

., 

:z: 

:; 

"' 

• 

Visual Observations 

D (stable) 

·-·-. i) 

• • • • • • •• • • Unstable 

·-~· ~ Stabntty limit 

1. Includes pilot gas contribution: .. , •••••• ·~ • ••• ,.-.-- Unstable 

01~------------~~------------~~------------~--------------~~ 0.1 1.0 10.0 100 1000 

Exit Velocity (f/s) 

Figure 4-5. Flame stability curves for 6-1nch open pipe flare with a single pilot at 

a pilot gas flowrate of 3.9 - 5.4 scfm natural gas. 
-I 

1.0 

-.... 

u 

Ill 

....... 

:::1 

~ 

m 

-

cu 

:::1 

rIll 

> 

Cl 

c 

.... 

~ 

fll 

11.1 

::1: 

In 

fll 

CD 

-·-· E) 

••••••• • qJ 

-·-· I) 

•• •• • •• • 

300 

Visual Observations 

Double pilot. disappearance of orange flicker (stability limit) 

Double pilot, unstable 

Triple pilot. faint orange flicker in flame envelope (stable 

Triple pilot. disappearance of orange flicker (stability 

Triple pilot. unstable limit) 

Stability 11m1t .........._. 0 IQJ ~ 

~ol>-·--0>-·~ ..... 

1. Includes pilot gas contribution ~-~· ••" 1 

Ol 1 Unstable• 1 1 

·o.1 1.0 10.0 100 1000 

Exit Velocity (f/s) 

Figure 4-6. Flame stability curve for 6-inch open pipe flare with multiple pilots at a 

total pilot gas flowrate of 2.0 - 2.3 scfm natural gas. 
• 

the 6-inch flare, the same heating value reduction can be attained with the 

pilot at only 1 scfm. Added pilot assist, however, only marginally increases 

flame stability. Increasing the pilot gas rate to 5 scfm only reduces the 

limiting heating value to 120 Btu/scf for the 6-inch flare. Increasing the 

number of pilots to two or three while keeping the total pilot gas rate 

constant at 2 scfm decreases the limiting heating value to 130 Btu/scf. 

Another indicator of improved flame stability with pilot assist is the 

calculated adiabatic flame temperature of the gas mixtures flared at or near 

the stabi 1 i ty limit. In Figure 4-7, the maximum stable exit velocity is 

compared to the adiabatic flame temperature for pilot assisted and non 

assisted flares tested at EER. The pilot assisted flares could stably burn 

propane/N2 gas mixtures with much lower adiabatic flame temperatures than non 

assisted flares could burn HzS/Nz, ethylene oxide/N2, and especially propane/ 

N2 gas mixtures. The scatter in the data for the pilot assisted tests fs 

apparently due to the increased difficulty in determining the flame stabiHty 

limit due to fluctuations in ambient conditions and because the flame cannot 

be extinguished with pilot assist. 

4.1.1 

The quantitative flame stability results discussed above are applicable 

to flares of similar size and geometry (open-pipe, no aerodynamic or other 

stabilization devices). There is however a question with regard to the 

extent that these data are scalable to flares of a larger size. In an 

earlier study, Pohl and Soelberg (4.1) have reported aerodynamic and flame 

stability similarities for flare heads sized from 2-1/2 to 12 inches in 

diameter. Extrapolation of the pilot-assisted results for the 3 and 6-inch 

heads. with one to three pilots at a pilot gas flowrate range between 1-5 

scfm, may be used to predict flame stability for pilot-assisted open pipe 

flares up to 12 inches in diameter. A conservative prediction would be 

further l.imited by (1) using the minimum pilot gas to flare gas volumetric 

flowrate ratio of these tests (0.002) as the minimum allowable ratio for 

operation, {2) using three pilots on a 12-inch flare (even though little 

dependence of stability on number of pilots was observed on the 6-inch head) 

4-10 
-..Q 

.., 

.... 

V1 

I1000 

100 

• 

() 3-inch flare with single pilot at 2 scfm pilot gas rate '84 

() 3-inch flare with single pilot at 2 scfm pilot gas rate 

() 6-inch flare with single pilot at 1 scfm pilot gas rate 

CJ 6-inch flare with single pilot at 2 scfm pilot gas rate 

C> 6-inch flare with single pilot at 4-5 scfm pilot gas rate 

C) 6-inch flare with double pilot at 2 scfm pilot gas rate 

CJ 6-fnch flare with triple pilot at 2 scfm pilot gas rate 

.e 10 

>, 

Ethylene oxide/N2 gas mixtures flared from 

unassisted 3-fnch .... open pipe flares -v 

0 

G) 

> 

.... 

.... 

-E 

.... 

..J 

1.0 H2

S/N2 gas mixtures flared 

from unassisted 3-fnch open pipe flares 

Propane/N dominated gas mixtures flared from 

3-fnch a~d 6-inch open pipe flames (unassisted) 

Figure 4-7. Calculated adiabatic flame temperature vs. limiting stable gas exit velocity for propane/N2 gas mixtures 

flared using pilot-assisted 3-inch and 6-inch open pipe flares. 

4-11 
for improved mixing, and (3) limiting the maximum exit velocities to ranges 

tested. 

With these limitations, a conservative prediction of minimum heating 

value is 150 Btu/scf for a stable flame on a 12-inch open pipe flare with 

three natural gas pilots. The maximum flare gas exit velocity for stability 

would be about 35 f/s. For velocities above 35 f/s, results from the 3-inch 

pilot-assisted head tests show increasing heating value limits with 

increasing velocity, up to a minimum heating value of 240 Btu/scf at 200 f/s. 

Maintaining the minimum experimental pilot gas/flare gas ratio of 0.002, the 

minimum total pilot gas rate would be about 20 scfm when the flare gas exit 

velocity is 200 f/s. 

Predictions such as these, although conservative, are based on 

extrapolations of the pilot scale data. Verification of these predictions by 

further, larger scale pilot-assisted stability testing is however required 

before these predictions can be considered valid. Only qualitative results 

of the test data reported herein can be applied to flares that have 

aerodynamic or other stability enhancing devices. 

4.2 Combustion Efficiency 

Previous EER studies have shown that flare combustion efficiency can be 

related to flame stability. This correlation also holds for the pilot 

assisted flares with the flame stability limits defined as above, as shown in 

Figure 4-8. Measured combustion efficiencies for the pilot assisted flares 

are within approximately the same region of efficiency vs stability ratio 

already developed for unassisted 3, 6, and 12-inch flares. 

Combustion efficiency is greater than 98 percent for stability ratio 

conditions greater than about 1.2. Combustion efficiency rapidly decreases 

when the ratio of (heating 1alue)/(minimum heating value required for 

stability) decreases below about 1.2. The major difference between the pilot 

assisted and unassisted flares efficiency results is that measurements for 

the assisted flares were made at very low gas heating values, ranging from 

4-12 
0 

0 

---{ () 

a 

······{ D 

D 

0 

100 

-.... 

c 

cU 

u 

~ 

ou 90 a.. 

>. 

u 

c 

ou ..,... 

u 

..,... 

~ 

~ 

161 

c 

0 

..,... 

.... 

Ill 

;, 

1 

y 80 

3-inch flare with single pilot at 2 scfm pilot gas rate (1984) (4-1) 

3-inch flare with single pilot at 2 scfm pilot gas rate 

6-inch flare with single pilot at 1 scfm pilot gas rate 

6-inch flare with single pilot at 2 scfm pilot gas rate 

6-fnch flare with single pilot at 4-5 scfm pilot gas rate 

6-inch flare with double pilot at 2 scfm pilot gas rate 

6-inch flare with triple pilot at 2 scfm pilot gas rate 

• 

• 

-~ Region of combustion efficiency .. measurements for previously tested 

• propane/N mixtures flared from 

• - 3, 6, and212-1nch open pipe flares (4-1 ) & 

• • 

• • 

• 

• • 

• • • a 

70~--------------~--------------~--------------~ 0 1.0 2.0 

Heating Value/Minfmge Heating Value Required for Stability 

Figure 4-8. Combustion efficiency of pilot-assisted flares. 

4-13 
100·211 Btu/scf while measurements for the unassisted flares made at higher 

gas heating values of 273-2350 Btu/scf. The stability limits for the pilot 

assisted flares are much lower than the stability limits at the same gas exit 

velocities compared to the unassisted flares. However, high combustion 

efficiency is maintained for the pilot assisted flares until the gas heating 

value, including the pilot contribution, decreases to within 1.2 times the 

stability limit heating value. 

There are some subtle differences betw~en combustion efficiency 

performance of the pilot assisted heads. Combustion efficiency greater than 

98 percent for the 3-inch pilot assisted flare is maintained even down to a 

stability ratio of 1.0, demonstrating higher combustion efficiency between 

1. 0 and 1. 2 stabi 1 i ty ratio than for the 6-inch pi 1 ot assisted head. This 

could be due to the fact that, for the 3-inch head, the differences between 

the pilot (and pilot gas flow} are less than for the 6-inch head. Therefore, 

the impact of the pilot flame .is greater on the flame from the 3-inch head ~ 

than on the flame from the 6-inch head. Also, tests for the 3-fnch head were 

conducted at slightly higher gas heating values than for the 6-inch head, 

because the stability limit curve for the ~-inch head was not as low as the 

curves for the 6-inch head. 

Below a stability ratio of 1.0, however, the decrease in combustion 

efficiency for the 6-inch head at the high pilot gas rate and for the double 

and triple pilot is less rapid than for the 6-inch head with the single pilot 

at medium and low pilot gas rates, and the 3-inch pilot assisted head. This 

is because, even at conditions near and below the stability limit, high 

relative pilot gas rates or multiple pilots can improve otherwise very low 

combustion efficiency. 

4.3 References 

4.1 Pohl, J. H. and N. R. Soelberg, "EvaluatioJ1 of the Efficiency of 

Industrial Flares: Flare Head Design and Gas Composition,•• EPA Report 

No. 600/2-85-106, NTIS No. PB86-100559/AS, 1985. 

4·14 
APPENDIX A 

EPA FLARE TEST FACILITY AND PROCEDURES 

A.l Flare Test Facility 

The EPA Flare Test Facility (FTF), shown in Figure A-1, was designed and 

built by EER at their El Toro Test Site for the U.S. Environmental Protection 

Agency, under EPA Contract No. 68-02-3661. The facility was completed in 

1982. The facility is reviewed briefly below, and is described in more 

detail by Joseph, et al. (A.l) and Pohl, et al. (A.2). 

For wind protection, the FTF is located in a box canyon surrounded by 

70-foot cliffs. The facility includes gas delivery systems, a flare head 

mount enclosed in a framework structure supporting (1) screens for additional 

wind protection, and (2} plume sample probes, and a building containing 

delivery system controls and analytical instruments. The facility is 

designed for relief gas flows ranging from 10 to over 40,000 scfh. The 

maximum flow depends on gas composition. 

Gases are delivered to the flare and auxiliary equipment through 

parallel manifolds shown in Figure A-2. Propane, natural gas, and nitrogen 

manifolds each have three orifice meters and one small rotameter, each with 

its own control valve. These manifolds were designed to accurately measure 

and control a wide range of flowrates. One additional manifold of two 

parallel orifice meters (not shown) is used to measure the flow of one 

additional flare gas. The propane, natural gas, and nitrogen manifolds and 

flow lines are constructed of carbon steel. The flow line and manifold 

system for the additional flare gas is constructed of stainless steel, for 

compatability with corrosive gases such as ammonia and hydrogen sulfide. 

There are also similar supply systems for steam, sulfur dioxide (tracer) 

and air. Steam is used (1) for steam-assisted flare tests, (2) in steam heat 

exchangers for ~aporizing sulfur dioxide and flare test gases. and (3) for 

A-1 
f 

N 

AdjustAble like for 

Sa.ple Probes, Filters, Driers, 

Ten•x Trips, •nd Bubblers 

for H2S, S02 I NH3 

............ _. 

Figure A~l. EPA Flare Test Facility (FTF) at EER . 

• 
.lquld 

llltrGIJMI 

1--t:Xl-<...,.---n•rw 

Stack 

Figure A-2. Fuel flow control and metering system schematic. 

A-3 

• 
sample probe heating. Sulfur dioxide is used as a tracer for flare test mass 

balances. Air is used during air-assisted flare tests. 

Flare and tracer gases (except natural gas) are supplied from cylinders 

or tanks. Propane is stored as a liquid in a 2,100 gallon tank. At low 

flowrates, natural vaporization is sufficient to supply propane gas for 

flaring; at higher flowrates, propane-fired vaporizers are used to increase 

the propane flowrate up to 15,000 scfh. Natural gas is supplied by the local 

utility at a maximum flowrate of 7,000 scfh. Nitrogen is delivered from 

either a liquid nitrogen tank or liquid nitrogen cylinders to banks of 

finned-tube atmospheric vaporizers capable of providing a maximum nitrogen 

flowrate exceeding 24,000 scfh. Other liquified flare gases such as hydrogen 

sulfide are delivered from cylinders and vaporized in a steam heat exchanger. 

This system can deliver over 4,000 scfh of gas, depending upon the compound. 

Steam is produced using a lS hp ~as-fired boiler. The boiler can supply 

up to 400 lbs/hr of 100 psig saturated steam. Sulfur dioxide, used as an 

inert tracer, is fed from liquid S02 cylinders and vaporized through a steamheated vaporizer at a flowrate of up to 7 scfh. Air is supplied by a forceddraft fan at a maximum flowrate of 60,000 scfh, at a static pressure of 17.6 

inches H2o. 

The sample collection and analysis system is shown fn Figure A-3. Plume 

samples are collected--usin~ fl~st~inless-steel, steam-heated probes mounted 

on a movable rake. Samples are collected concurrently from five different 

radial locations in the plume. Pumps draw the soot- and moisture-laden 

samples into the probes, where filters collect the soot for subsequent weight 

measurement. Membrane tube bundles in Permapure dryers are used to 

selectively remove water vapor from the sample stream. The dried gas samples 

are collected in Tedlar bags for analysis of 02, CO, C02, total hydrocarbons, 

H2S, S02, and NO/NOx content. Other species such as S02, H2S or NHJ are 

concentrated into aqueous solutions in absorption bubblers. 

A-4 
~ 

Vent 

ll•h fiON Helen 

II 8r)'er 

-fj;T. 

R•ke 

12 

1•9 fill 

112 •• n 0111 .!!!..!!!:.' . . .... 

u ~ 

~ 

lrpu.s tar 

I 

fiON llelers 

.... ~-~ 

• a tl Jot II ~ llet :IIIIPIIFf" I ~ flow Heter )::o 14 I 

U'l 

lUke 

15 

••u flU 

~8yp•u 

figure A-3. Flare Test Facility sample system. 
A.2 FTF Procedures 

The Flare Test Facility {FTF) test procedure includes measuring 

background conditions, igniting the flame, establishing test conditions, 

sampling, and analysis. Tests are not conducted in rainy weather or when the 

wind speed is greater than 5 mph. Most testing is done in the morning, when 

the weather is calm. A typical test requires about 4 hours, although the 

actual sample period is only 20 minutes. 

Before each test, the ambient air is sampled and analyzed for background 

levels of 02. co, C02, hydrocarbons, S02, NO/NOx. soot, and for H2S or NHJ, 

if applicable. The flame is then ignited using a hand-held spark igniter or 

a Zink flame front propagating igniter, and the test conditions are set by 

adjusting the gas flowrates. Most of the tests were conducted near the 

stability limit of the flame. The flame stability limit is determined by 

adjusting the flowrates until the flame becomes unstable and is eventually 

extinguished. 

After test conditions are set, plume samples are collected for 20 

minutes in order to time-average perturbations and collect sufficient amounts 

of sample for analysis. Samples are collected from five different radial 

locations, at a height above the flame experimentally determined to be beyond 

the flame envelope. If the probes are too high, air dilution of the samples 

reduces combustion product measurement accuracy. If the probes are located 

too low, inside the flame envelope, incompletely burned samples may be 

collected, which would result in artificially low combustion efficiency 

measurements. 

While the plume is being sampled, the flame structure and other 

characteristics such as color are recorded visually and photographically. 

After sample collection and flame observations are complete, the flame is 

shu~ down. Sample analysis is then conducted to measure levels of 02, CO, 

C02, HC, NO/NOx, soot, and other species in the plume samples. 

A·6 
A.3 References 

A.l Joseph, 0., J. Lee, c. McKinnon, R. Payne, and J. Pohl, "evaluation of 

the Efficiency of Industrial Flares: Background-Experimental DesignFacility", EPA Report No. 600/2-83-070, NTIS No. PB8J .. 263723, August 

1983. 

A.2 Pohl, J. H., R. Payne, and J. Lee, "Evaluation of the Efficiency of 

Industrial Flares: Test Results", EPA Report No. 600/2-84-095, NTIS No. 

PB84-199371, May 1984. 

A .. 7 

APPENDIX 8 

FLARE SCREENING FACILITY AND TEST PROCEDURES 

8.1 Flare Screening Facility 

The laboratory-scale Flare Screening Facility (FSF) is used to 

inexpensively, quickly, and easily identify potential difficulties in flaring 

a wide variety of compounds. Advantages of the FSF over the Flare Test 

Facility are its small size, low operating cost for gases and materials, the 

ability to obtain complete, undiluted samples of flare combustion products, 

the ability to close mass balances, and the increased safety for flaring 

toxic gases. This facility is reviewed briefly below, and is described in 

more detail by Pohl and Soelberg (8.1). 

Figure B-1 shows the FSF schematic. The facility was adapted to burn 

either liquid or gaseous compounds supplied from pressure cylinders and 

metered through calibrated rotameters. Combustion air is injected to the 

reactor co-axially with the fuel stream, through a flow straightening screen. 

By maintaining a very low air velocity relative to the fuel velocity, effects 

of the co-current air stream on the fuel stream are minimized. Test results 

verify that the flame behavior is similar to that of a jet in a quiescent 

atmosphere. 

The flame is completely enclosed in a water-cooled reactor shell, with 

sample probes located at the reactor outlet. The shell isolates the flame 

from the environment and prevents air dilution of the flare products. This 

allows complete mass balance closure over the system. A single sample probe 

is located at the reactor shell outlet to enable sampling of a well-mixed 

plume sample. 

B-1 
1116-1/8 IN. NOZZLE 

GLASS BEADS 

HEATED LINE 

~==GAS SYRINGE DRIVE 

AIR FAN 

Figure B-1. Flare Screening Facility (FSF). 

B-2 
8.2 FSF Test Procedures 

The FSF test procedures are simpler than the FTF test procedures. Since 

the facility is enclosed, it is not subject to environmental conditions such 

as wind or rain. The system is smaller and hence, more easily monitored. 

·Probe positioning is unnecessary, since the sample probes are located 

permanently at the reactor outlet. 

For each test, all the instruments are zeroed and calibrated. The flame 

is ignited by hand, and flow conditions are set. There is some time lapse, 

depending on the gas flowrates, for flare steady state to be achieved after 

gas flowrate or mixture changes because of gas residence time in the 

flowlines. After steady state is reached, sample collection is initiated and 

visual and photographic flame observations are recorded. Plume samples 

collected in Tedlar bags are analyzed for Oz, CO, COz, total hydrocarbons, 

HzS, S02, NO/NOx content, and other species using the instruments shown in 

Figure A-3. Plume samples are also bubbled through gas scrubber bottles for 

concentration and subsequent analysis of such species as HzS, S02, and NH3. 

8.3 Reference 

8.1 Pohl, J. H. and N. R. Soelberg, "Evaluation of the Efficiency of 

Industrial Flares: Flare Head Design and Gas Composition'', EPA Report 

No. EPA-600/2-85-106, NTIS No. PB86-100559/AS, September 1985. 

B-3 

APPENDIX C 

FLARE FLAME STABILITY LIMIT 

C.l Stability Limits 

Previous work (C.l through C.4) has related flare combustion and 

destruction efficiency to flame stability. A stable flame exhibits high 

efficiency. However, as flame conditions approach the stability limit, 

efficiency can rapidly decrease. 

The flame stability limit is defined as the operating condition where an 

increase in flare gas exit velocity or a decrease in flare gas heating value 

results in flame blowout. Theoretically, this occurs when the gas velocity 

is not reduced to the flame velocity before the gas becomes diluted {by air 

!ntrainment and mixing) below its lower flammability limit or when the flame 

speed is less than the imposed velocity. 

A characteristic exit velocity vs heating value curve, maintaining all 

ather conditions constant, can be generated for each flare head and gas 

mixture combination. This curve is generated by determining the minimum gas 

heating value attainable before flame blowout, for a range of different gas 

·~xit velocities. At exit velocity and heating value combinations above this 

c:urve, a flame will be present; at conditions below the curve, there will be 

no flame. 

C:.2 References 

C:.l Pohl, J. H., R. Payne, and J. Lee, ••Evaluation of the Efficiency of 

Industrial Flares: Test Results", EPA Report No. 600/2-84-095, NTIS No. 

PB84-199371, May 1984. 

C.2 Pohl, J. H., J. Lee, R. Payne and B. Tichenor, "The Combustion 

Efficiency of Flares", 77th Annual Meeting and Exhibition of the 

Air Pollution Control Association, San Francisco, CA, June 1984. 

C-1 
C.3 Pohl, J. H. and N. R. Soelberg, "Evaulation of the Efficiency of 

Industrial Flares: Flare Head Design and Gas Composition••; EPA Report 

No. EPA-600/2-85-106, NTIS No. PB86-100559/AS, September 1985. 

C.4 Pohl, J. H., N. R. Soelberg and E. Poncelet, "The Structure of Large 

Buoyant Flames", American Flame Research Committee Fall Meeting, 

Livermore, CA, 16-18 October 1985. 

C·2 
APPENDIX D 

DATA ANALYSIS 

0.1 FTF Data Analysis 

Data reduction and analysis must be conducted on the flare test results 

to determine flare combustion efficiency. Results of FTF tests must be 

:orrected for background levels of sampled species and air dilution of the 

Jlume. Also, numerical integration must be conducted using the local probe 

1neasurements and velocities calculated from jet theory. The development and 

1etails of the Flare Test Facility data analysis procedures are already 

~eported (D.l. 0.2) so only a summary is provided here. 

Data reductio~ is conducted on the FTF plume sample results to determine 

local air dilution of the combustion products. local combustion and 

~testruction efficiencies, and integrated overall average combustion and 

·riestructi on efficiencies. The 1 oca 1 di 1 uti on factor is: 

Ym - Yaf 

OF=--- D-1 

where OF = dilution factor = volume of air in the local sample divided 

by the volume of stoichiometric combustion products. 

Y = local concentration of 02, C02, or S02 (tracer} 

m = measured in plume 

af = air-diluent-free, stoichiometric basis 

b = background 

Combustion efficiency is defined as the degree to which all fuel 

naterials have been completely oxidized. The local combustion efficiency is 

based upon local probe measurements of plume constituents, whereas the 

D-1 
integrated average combustion efficiency is calculated by integrating the 

local plume fluxes to obtain average compositions of plume species. Since 

local plume measurements are diluted by ambient air, they must be corrected 

for the background levels of plume species in the ambient air. These 

corrections are made using equation D-2: 

0-3: 

where h = plume species 

c = corrected 

0-2 

Local combustion efficiency (CE) can then be calculated using equation 

x; v; Y;,c 

CE • 1 - D-3 

Xj llj Y j ,c 

where v·= stoichiometric coefffcfent 

i = incompletely burned species 

j = completely and incompletely burned species 

Local destruction efficiency is similar to local combustion efficiency, 

but is a measure of the degree of destruction of the particular fuel 

material. It is equal to the combustion efficiency for that species only 

when there are no incompletely burned intermediates, such as CO or soot for 

hydrocarbon species. Local destruction efficiency (DE) for a fuel species is 

calculated usfng equation D-4: 

0-2 
DE = 1 D-4 

where k = fuel species 

1 = completely and incompletely burned species fr.om the fuel 

species. 

Integrated average combustion and destruction efficiencies are computed 

by ·first combining the local corrected plume composition with the local plume 

velocity to obtain a local corrected mass flux for each plume constituent. 

The local corrected plume species concentrations are found using equation 

D-2, and the local plume velocity is calculated from jet theory using 

equatio~s D-5 and D-6: 

Vr,x = Ymax exp 

where V = velocity 

[ 

Rr 2] -5-

x 

R = radial distance from plume centerline 

X = probe axial distance above flare head 

r • radial pos1tion 

max = maximum 

o = flare head outlet 

D-5 

D-6 

llumerical integration of the local fluxes is used to calculate average 

c:ombustion and destruction efficiencies using equations D-7, D-8, and D-9: 

D-3 
Xr x; v; Y;,c Yr Ar 

CE a 1 • -------- Xr Xj Vj Yj,c Yr Ar 

D·7 

D-8 

D·9 

where Ar = radial area sampled by probe r (Figure D-1). 

D.2 FSF Data Analysis 

Data analysis for the Flare Screening Facility (FSF) test results is 

much simpler than for the Flare Test Facility (FTF) test results. The FSF 

flare flame is completely enclosed within a steel reactor shell. The inlet 

fuel and combustion air flowrates are metered, so the plume flowrates of 

excess air and air-free combustion products can be directly calculated based 

upon the combustion stoichiometry of the gas: 

Ye.a. = (SR-1) Vr.a. 

where v = volumetric flowrate, scfh 

e.a. • excess air 

S.R. a stoichiometric ratio 

r.a. = required air for 100~ combustion 

D-4 

D-10 

D-11 
~ 

T , 

I 

I 

I 

I 

\ A4 

X \ 

' ' ' ' ... .. ..... 

Figure 0-1. Schematic of integration geometry. 

D-5 
where p = stoichiometric products of combustion with air (air-free 

basis, 0 percent 02) 

g = inlet gas 

v = stoichiometric coefficient 

i = combustion product species ''i" 

where t = total plume 

This approach assumes 100 percent combustion, in order to determine the 

excess air, combustion product, and total plume flowrates. The same 

assumption was used in data reduction of the pilot-scale tests~ Where 

combustion is only slightly le.ss than 100 percent, flowrate errors due to 

th~s assumption are small. Even where combustion is significantly less than 

100 percent, the error in total plume flowrate fs small, because a majority 

of the plume gas is nitrogen, unaffected by combustion efficiency 

( discounting N2 - NOx reactions and combustion of nitrogen-containing fuel 

species, 0.2). 

The plume is sampled at the reactor exit, where the plume is well mixed. 

This eliminates the need for collection of multiple local plume samples 

across the plume radius, assumptions of local velocities at radial locations 

in the plume, and the integration of local species fluxes to calculate total 

plume species flowrates, because species concentrations in the plume sample 

are representative of average plume concentrations. Species flowrates in the 

plume are calculated using the measured concentrations and the plume flowrate 

found from equation D-13: 

D-13 

0-6 
where Y = mole fraction 

In cases of high combustion efficiency, the plume concentration levels 

of incompletely combusted species such as CO and hydrocarbons are near 

background levels. The plume species flowrates must then be corrected by 

subtracting the background contribution: 

where c = corrected 

b = background 

D-14 

Combustion and destruction efficiencies are calculated using equations 

D-15 and D-16. 

. 

CE = 

X; v; V; ,c 

1- --~-

Z:j Vj V; .c 

D-15 

where i = incompletely burned species 

j = incompletely and completely burned species 

DE 1 vk,c 

= D-16 x, 111 v, ,c 

where k = fuel species 

1 = incompletely and completely burned species that came from 

the fuel species 

0-7 
0.3 References 

0.1 Pohl. J. H., R. Payne. and J. Lee, ••Evaluation of the Efficiency of 

Industrial Flares: Test Results 11 , EPA Report No. 600/2-84-095, NTIS No. 

PB84-199371, May 1984. 

0.2 Pohl, J. H. and N. R. Soelberg, 11 Evaluation of the Efficiency of 

Industrial Flares: Flare Head Oesi gn and Gas Composition", EPA Report 

No. EPA-600/2-85-106, NTIS No. PB86-100559/AS, September 1985 • 

• 

D-8 
APPENDIX E 

QUALITY ASSURANCE 

E.1 Flowrate Measurement 

The flare gases used at both the Flare Test Facility and the Flare 

Screening Facility were prepared by mixing the pure components. Accurate 

measurement and flow control of the pure gases was required to prepare the 

des1red flare gas composition and operate the flare at the desire~· gas exit 

velocity. Accurate flowrate measurement and control over a very wide range 

was achieved by using banks of calibrated parallel-flow square-edged orifice 

plates at the Flare Test Facility. For the smaller-scale Flare Screening 

Facility. banks of parallel-flow rotameters were used for accurate flowtate 

:neasu rements • 

Each of the orifice plates used at the Fare Test Facility was calibrated 

Jsing the working gas for that orifice and a standard pre-calibrated laminar 

flowmeter. dry gas meter, or wet test meter to obtain the flow coefficient 

for that particular orifice and gas. This orifice coefficient was used in 

,:!quation E-1 for flowrate measurement: 

• (P.4P ) 1/2 

V = KMWT 

uhere V = fl owrate, sc fm 

K = orifice coefficient 

P = static orifice pressure, psia 

~P = orifice differential pressure, feet H20 column 

MW • gas molecular weight 

T = orifice temperature, R. 

E-1 

E-1 
The standard deviation of K for 13 different orifices used in this test 

program was less than 5.4 percent. and less than 3.0 percent for the 

majority. 

A similar procedure was used to calibrate the rotameters used for 

flowrate measurement in the Flare Screening Facility. Each rotameter was 

calibrated using the working gas and pre-calibrated dry gas meters, wet test 

meters, or water displacement columns. Variations in pressure and 

temperature between the calibrated and actual operating values were corrected 

for using equation E-2: 

• 

E-2 

where subscript 1 is for conditions during the calibration, and subscript 2 

is for operating conditions. 

E.Z Sample Analysis 

Accurate sample analysis is critical for determining reliable combustion 

and destruction efficiency results. Table E-1 shows the analytical methods, 

instruments, and accuracies used in this test program. The listed accuracies 

are for the concentration ranges most tYPically encountered at the Flare Test 

Facility. Accuracy for a specific method may change if concentration levels 

for the sampled species are outside the ranges listed in Table E-1. 

Many of the analytical methods of Table E-1 were developed in previous 

EER flare studies (E.l. E.Z). but methods for HzS and S02 measurement were 

evaluated in this study. Results of these evaluations are reported in 

Section 3 of this report. Only the most reliable of these methods were used 

for HzS and SOz measurements in the typical ranges observed at the Flare test 

Facility shown in Table E-1. 

E-2 
f I 

1"11 

I 

w 

SPECIES 

jlr*a:s::::aa•••=-= 

02 ' 

co 

C02 

Total 

Hydrocarbons 

HzS 

so2 

NH3 

NO/NOx 

Part1culate 

TABLE E-1. FLARE FACILITY ANALYilt;AL Mt::IHODS 

II&STRUMENT PRINCIPLE RANGE ACCURACY 

·····--···======·· ·==···===·=·8···=== ~·=-=:;;:••.;a:::a:::z:a:s P.•=====·=·===· Taylor 570A Paramagnetism 0-1001 ;t0.2S of 

Measured 

Beck•an 315A NOIR 0-21 ;tD.1 ppm 

Beckman 3158 NOIR 0-20S :tD.021 of 

·Measured 

Beckman 400 flO 0-5000 ppm ±0.5 PPII 

Draeger Tubes Color-lnd1cat1ng o. 5-2000 ppm ±20'1. of 

Chemtcal Reactfon Measured 

Color1111eter Methylene blue 0.5-20 ppm ±.501 of 

React ton Measured 

Draeger Tubes Color-lndtcatfng o. 5-5000 ppll ±20'1. of 

Chemtcal Reactfon Measured 

Bartu11 Perchlor- Bartu• Perchlor- 1-2000 PP• ±151 of 

ate T1tratton ate Reaction Measured 

Ion-Spec1f1c lon1c Potential 0.1-10 ppll ±151 of Electrode ~asured 

Teco 14B-E Chemtlumtnescence 0.05-10 ppm ±51 of 

Measured 

ftl ter Timed o-to-6 lb/cf ±lOS of 

Collection Measured 

MEASURED I 

CONCENTRATIONS 

=======·=·==2=~=·• 

18-211 

' 

3-500 ppm 

0.05-21 

3-1300 ppm 

0-1000 ppll 

0-1000 ppm 

0.1-0.6 PPII· 

0.02-10 ppm 

10-8-to-6lb/cf 

.. ·-
E.3 References 

E.l Joseph, D., J. Lee, c. McKinnon, R. Payne, and J. Pohl, "Evaluation of 

the Efficiency of Industrial Flares: Background-Experimental DesignFacility", EPA Report No. 600/2-83-070, NTIS No. PB83-263723, August 

1983. 

E.2 Pohl, J. H., R. Payne, and J. Lee, 11 Evaluation of the Efficiency of 

Industrial Flares: Test Results 11 , EPA Report No. 600/2-84-095, NTIS No. 

PB84-199371, May 1984. • 

E-4 
APPENDIX F 

CONVERSION FACTORS 

To Convert From To l·1ul tip ly 

English Metric By 

Btu B ri ti sh Therma 1 Unit kJ Kilojoule 1.055 

c:fm Cubic Feet per Minute m3;h Cubic Meters per Hour 1.700 

·in Inch m Meter 0.0254 

·in H2o Inches Water Column Pa Pascal 249 

psi Pounds per Square Inch Pa Pascal 6893 

.:t Foot m Meter 0.3048 

<:f Cubic Foot m3 Cubic Meter 0.02832 

·:b Pound k.g Kilogram 0.4536 

nph Miles per Hour km/h Kilometers per Hour 1.609 

Degrees Rankine {R) is converted to degrees Celsius {C) via the following 

formula: 

C = 5/9 (R - 492) 

Degrees Fanrenheit (F) is converted to degrees Celsius (C) via: 

C = 5/9 (F - 32) 

F-1 
TECHNICAL REPORT DATA (Pleut~l'tltld l•~~n~ctiont on the Tevenl befon complttittl} 

1. REPORT NO. ,2. 3. RECIPIENT'S ACCESSIO .. NO. 

EPA-600/2-86-080 

4. TITLi ANO SUBTITLE 5, REPORT DATE 

Evaluation of the Efficiency of Industrial Flares: H2S S~tember 1986 

Gas Mixtures and Pilot Assisted Flares 6. PERFORMING ORGANIZATION CODE 

7. AUTHOAISI 8. PERFORMING ORGANIZATION REPORT NO. 

J. H. Pohl and N. R. Soelberg 

9. PERFORMING OAOANIZATION NAME ANO ADDRESS 10. I"ROGAAM ELEMENT NO. 

Energy and Environmental Research Corporation 

18 Mason 11. ~!WIN I t1ACT/GAANT NO. 

Irvine. California 92718 68-02-3661 

12. SPONSORING AGENCY NAME ANO AOORESS 13. TYPE OF REI"ORT AN,p PERIOD COVERED 

EPA, Office of Research and Development Final; 4/85 - 7/86 

14. SPONSORING AGENCY CODE 

Air and Energ:y Ezu;(ineering Research Laboratory 

Research Triangle Park, NC 27711 EPA/600/13 

15• suPPLEMENTARY NOTes AEERL project officer is Bruce A. Tichenor. Mail Drop 54. 919/ 

541-2991. Earlier reports in this series are EPA-600/2-85-106 .. -84-095. and -83-oio 

1

6. ABsTRACT The report is the fourth in a series on a research program which will result in quantification of emissions from. and efficiencies of. industrial flares. The 

report gives test data on the combustion efficiency and destruction efficiency of (1) 

gas mixtures containing H2S. and (2) flare flames with pilot flame stabilization. 

(NOTE: Previous reports were limited to tests burning propane/N2 mixtures on pipe 

flares without pilot flare stabilization. and discussed the influence of the flared gas 

and flare head design on destruction and combustion efficiency without stabilization 

by pilot flares.) The latest tests,. conducted on 3- and 6-in. open pipe flares without 

aerodynamic flame stabilization devices, led to conclusions including: (1) gas mixtures of H2S/N2 can be stably flared at much lower volumetric gas heating values 

than can propane/N2 mixtures; (2) destruction and combustion efficiencies greater 

than 98% are obtained for gas mixtures of H2S/N2 and H2S/propane/N2 when the gas 

heating value is at least 1. 2 times the level required to produce a stable flame; (3) 

for mixtures containing both H2S and propane. H2S destruction efficiency was consistenUy higher than propane combustion efficiency; and (4) combustion efficiencies 

greater than 98% for pilot assisted flares are achieved when the heating value is 

greater than 1. 2 times that required to stabilize the flame. 

17. KEY WORDS AND DOCUMENT ANALYSIS 

a. DESCRIPTORS b. IDENTIFIERS/OPEN ENDED TERMS c. COSA Tl Field/Group 

Pollution Pollution Control 13B 

Flames Stationary Sources 21B 

Hydrogen Sulfide Industrial Flares 07B 

Nitrogen Flame Stabilization 

Propane 07C 

HI. OISTRIEIUTION STATEMENT 19. SECURITY CLASS (This Report) 21. NO. OF PAGES 

Unclassified 103 

Release to PubUc 20. SECURITY CLASS (This pep) 22. PRICE 

Unclassified 

EPA Fo•m Z220•1 (S•73) F-2 
·~---- ---~~~-------------. 

DATE DUE 

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