&EPA 

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United States Industrial Environmental ~' f' -?' ..- Environmental Protection Research laboratory ~-- . :: 

Agency Research Triangle Park NC 27711 ... <r-4. -=--~----~----------------------~~=~~~~~~~~~~~~--~<~,~f,\, Research and Development EPA-600/S2-84-095 July 1984 

Project Summary 

Evaluation of the Efficiency of 

Industrial Flares: Test Results 

J.H. Pohl, R. Payne, and J. Lee 

The report gives results of Phases 3 

and 4 of a four-phase research program 

to quantify emissions from, and efficiencies of, industrial flares. Phase 1 

involved the experimental design; 

Phase 2, the design of the test facilities; 

Phase 3, development of the test 

facilities; and Phase 4, data collection 

and analysis. (NOTE: Report EPA600/2-83-070 gives results of Phases 

1 and 2.) 

The combustion efficiency of large 

pilot-scale flares was measured. The 

flame structure and combustion efficiencies were correlated with operating 

conditions of the flare, size of the flare 

head, and properties of the flared gases. 

The combustion efficiency was correlated with the ratio of heating value of 

the gas flared to the heating value 

.required to maintain a stable flame, and 

was independent of the flame head size. 

In tum, the heating value required to 

maintain a stable flame was correlated 

with the reciprocal of an estimated 

flame temperature based on properties 

of the flared gas. The length of the 

flame, entrainment into the flame, and 

liftoff distances were also correlated, 

using combinations of the Richardson 

Number, jet theory, and properties of 

the flared gas. 

This Project Summary was developed 

by EPA's Industrial Environmental 

ReseiiTch Laboratory, Research Triangle 

Parle, NC, to announce key findings of 

the research project that is fully documented in a separate report of the same 

title (see Project Report ordering 

information at back). 

Test Facility 

EPA's Flare Test Facility (FTF) was 

constructed at Energy and Environmental 

Research Corporation's (EER'S) El Toro 

test site. The FTF (Figure 1 ) includes a pad 

and structure for installing and testing 

flare heads, screens to shield the flame 

from the wind, parallel delivery systems 

to accurately meter the wide range of gas 

flows to the. flare, a hood to sample the 

entire plume, a movable rake probe to 

simultaneously sample five radial positions. high-speed movie and photographic 

equipment to record the structure of the 

flare flame, and a room from which to 

control the flare and analyze gas samples. 

Techniques were developed to operate, 

sample, analyze, and reduce the data. 

Analysis included visual and photographic 

observation of the flare flame structure, 

and samples of soot, 02. CO, C02• total 

hydrocarbon, and S02. which was used as 

a tracer. The data were corrected for the 

measured background of combustion 

species and for dilution of the flare plume 

by ambient air. Dilution and local combustion efficiencies were calculated at 

each probe position, and the maximum 

potential error in the dilution and 

combustion efficiencies were estimated 

for each data point. The local combustion 

efficiencies were integrated, using 

velocity profiles estimated by jet theory to 

yield a global combustion efficiency for 

each flare flame. 

The combustion efficiencies were 

measured for a wide range of operating 

conditions typical of commercial flares: 

• Head type-

- 3-in.* EER prototype. 

- 6-in. EER prototype. 

- 12-in. EER prototype. 

- 3-in. EER prototype with convergent ring. 

*To convert to metric equ111alents, please use the 

factors at the end of th1s Summary. 
C Propane ) 

___ ran---~ 

Vaporizer 

City 

Natural ------t 

Gas 

Nitrogen 

Tanks 

Sulfur Dioxide 

Tank 

Steam 

Generator 

Flow 

Controls 

&Meters 

Flow 

Controls 

&Meters 

Flow 

Controls 

& Meters 

Figure 1. EPA's flare test facility at EER. 

- 3-in. EER prototype with divergent 

ring. 

- 12-in. commercial (three manufacturers-A, B, and C). 

• Gas-

- Propane/nitrogen mixtures. 

- Natural gas. 

• Heating value of the flared gas -

270 - 2350 Btu/ft3

• 

• Flow rates-

- Velocities. from 0.2 - 428 ft/sec. 

- Reynolds Numbers. from 337 to 

217,000. 

- Richardson Numbers, from 2.9 x 

10-5 to 8 x 102

. 

Results 

Table 1 summarizes combustion efficiencies. In addition to combustion 

efficiencies. other calculations were 

made: 

• Combustion intensity was found to 

be 90.000 Btu/hr/ft3

, independent 

of flare or flame conditions. 

• The flame length was correlated with 

the Richardson number. 

• The liftoff distance was correlated 

with ratios of velocities and concentrations of combustible gas. 

2 

Induction 

Fan 

c::::x:::> Hood 

Probe 

Hood 

Rake Probe 

Flarehead 

Electrically 

Heated Lines 

.------. 

1 Tedlar t 

1 Bags 1 "l 

•- ----J I 

Analyzers 

----1~· Oa CO. C02 

HC. S02 

Sample 

Drie1s 

I 

I 

Sampling 

Control 

Panel 

Tedlar 

Bags 

Table 1. Combustion Efficiencies of Various Flare Heads 

3-in. 

Purpose 3-in. EER 6-in. 12-in. 12-in. 12-in. 12-in. 

of Test EER Hi-Vel EER EER Indus A lndusB Indus C 

No Retaining 95.11-

Ring 99.66 

1 00"/o C :.He 99.74-

99.87 

77% C:.Ha 99.73-

99.88 

56% C:.Ha 98.37- 97.27- 9847- 98.29- 99.12- 99.48- 99.08 

98.95 99.33 99.76 99.50 99.78 99.65 99.65 

50% C:.Ha 99.72-

99.87 

Stable Flame 99.81-

Limit 99.88 

Law-Btu 90.19- 92.24- 94.89- 98.49 99.21- 91.16 

99.92 99.36 99.73 99.72 99.52 

Effect of 98.94- 99.89-

Steam 99.96 99.92 

Steam. 99.32 99.84 

Smokeless 

City Gas 99.91 

• The flame stability was correlated a flame is maintained; flame instability 

with the reciprocal of an estimated occurs when the jet velocity exceeds the 

flame temperature. flame velocity and the flame goes out. 

• The combustion efficiency correlated Figure 2 shows the gas heating value 

with a dimensionless heating value versus the gas exit velocity at the point of 

of the gas fired. instability (i.e., at the point where the 

The term "flame stability" means that flame starts to "go out"). This point is 
1100P---------~--------~------~~------~~-------, perturbed, can easily produce high emissions of unburned material. 

1000 

900 

600 

500 

400 

300 

200 

0.1 

0 3-/n. Flare Head 

() 3-/n. Flare Head. Divergent Ring 

e 3-ln. Flare Head. Convergent Ring 

6 6-/n. Flare Head 

0 12-ln. Flare Head 

0 12-ln. Industrial Head C 

~ 12-ln. Industrial Head A 

0 12-/n. Industrial Head 8 

c' 

~s\ 

0 

96o/o 

0 ~ej \)~ 

0 

1.0 

0 

10.0 

181C/ = Confidence Interval. 

1

b

1

Based on flares burning propane/ 

nitrogen mixture with no pilot 

flame. 

100.0 1000.0 

All conclusions are based on the data of 

this study and are limited to head geometries, gases, and variables examined. 

Head geometries were limited to: 

• Simple pipe flare of 3-, 6-, and 12-in. 

in diameter. 

• Three commercial 12-in. flare heads 

of different design and manufacture. 

The gases studied were limited to: 

e Propane/nitrogen mixtures with 

hel;lting valu~s of 270- 2350 Btu/fe. 

• One test with natural gas. 

The variables examined were: 

• Velocities from 0.2 to 420ft/sec. 

• Reynolds numbers from 340- 217,000. 

• Richardson numbers from 2.9 x 1 o-s 

to 8 x 102

. 

• Steam flow from 0 to 1 lb steam/lb 

fuel. 

The flare flames were shielded from the 

wind, and combustion efficiencies were 

not measured in winds greater than 5 

mph. 

Exit Velocity, ftlsec 

The following conclusions are based on 

study results: 

• Flares operating with unstable flames 

can have low combustion efficiency. FiguN 2. Region of flame instability. tbl 

determined by establishing a propane/ 

nitrogen flame at a given velocity and 

then decreasing the flow of propane untH 

the flame goes out. The combustion 

efficiency is then measured at the 

conditions just prior to flameout. 

The region shown in Figure 2 indicates 

the minimum gas heating value required 

to produce a stable flame at the gas exit 

velocity within the 95 percent confidence 

limits of the mean. For any given velocity, 

a gas with a heating value above this 

region produces a stable flame; a gas with 

a heating value below this region produces an unstable flame. Velocity/gas 

combinations in or below the region tend 

to produce flames with lower combustion 

efficiency. Thus, for any given test velocity, the minimum gas heating value for a 

stable flame can be determined. By 

dividing the actual gas heating value by 

the minimum value required for stability, 

a ratio is obtained which is greater than 1 

for stable flames, and less than 1 for unstable flames. Figure 3 plots combustion 

efficiencies versus this ratio and shows 

that high combustion efficiencies are 

achieved when the ratio exceeds 1. When 

the ratio is 1 or less, lower combustion 

efficiencies are often obtained. Note that 

even at a ratio less than 1, high combustion efficiencies are sometimes achieved, 

demonstrating the uncertainty associated with the stability measurements. In 

general, however, stable flames are efficient, and unstable flames can be inefficient. Flames near the stability limit are 

very sensitive to perturbations and, when 

• Combustion efficiency does not 

depend on flare size or geometry. 

• Successful correlations were developed for flare flames: 

- Flame length was correlated with 

a modified Richardson number. 

100r---~~~-r~~~~~~-,J~~--~r----r----,----------, 

98 

~ 96 lb 

.. ... 

lb 

Cl. 

::.:. 

.. 

c:: 

·!!! 

·!:! 

~ c: 

·~ 

"' :;, 

~ C) 

(.,) 

Figure 3. 

2 3 

0 3-/n. EER 

f:l 6-ln. EER 

0 12-ln. EER 

~ 12-/n. Commercial A 

0 12-ln. Commercial 8 

0 12-ln. Commercial C 

fa) Reason for low combustion efficiency is unknown. 

(b) Based on flares burning propane/ nitrogen mixture 

with no pilot flame. 

4 5 6 7 8 

Heating Value/Minimum Heating Value for Stability1

b

1 

Combustion efficiency near the lower limits of flame stability. 

3 
- Liftoff distances were correlated 

with ratios of velocities and concentrations. 

- Flame stability was correlated with 

a pseudo flame temperature. 

- Entrainment was correlated with 

ratios of distances and the Richardson number or with velocity. 

• Combustion efficiency was high for 

flares with high velocities, provided 

the heating value of the gas was in 

the region of stability. . 

• Steam injection completely suppressed 

soot production but did not appreciably alter combustion efficiency unless the flame was oversteamed 

(>0.5 lb steam/lb fuel), and then 

combustion efficiency decreased. 

Conversion Factors 

To convert nonmetric units used in this 

Summary to their metric equivalents, 

please use the following factors: 

Nonmetric Multiplied by Yields Metric 

Btu 1.055 kJ 

ft 0.305 m fe 0.028 m3 

in. 2.54 em 

lb 0.454 kg 

mi 1.609 km 

J. H. Pohl. R. Payne, and J. Lee are with Energy and Environmental Research 

Corporation. Irvine, CA 92714. 

Bruce A. Tichenor is the EPA Project Officer (see below). 

The complete report. entitled "Evaluation of the Efficiency of Industrial Flares: 

Test Results," (Order No. PB 84-199 371; Cost: $17.50, subject to change) will 

be available only from: 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

Telephone: 703-487-4650 

The EPA Project Officer can be contacted at: 

Industrial Environmental Research Laboratory 

U.S. Environmental Protection Agency 

Research Triangle Park, NC 27711 

1:r US GOVERNMENT PRINTING OFFICE; 1984- 759·015/7742 

\ 

United States 

Environmental Protection 

Agency 

Center for Environmental Research 

Information 

Cincinnati OH 45268 

Official Business 

Penalty for Private Use $300 

•