# &EPA**~1//** United States Industrial Environmental

### ~' f' 

# -?'..- Environmental Protection Research laboratory**~--** . :: Agency Research Triangle Park NC 27711

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## <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 ofafour-phase research program to quantify emissions from, and efficiencies of, industrial flares. Phase 

### 1 involved the experimental design; Phase 

### 2, the designofthetestfacilities; Phase 

### 3, development of the test facilities; and Phase 

### 4, data collection and analysis. (NOTE: Report EPA

### 600/2-83-070 gives results of Phases 

### 1 and 

### 2.)

The combustion efficiency of large pilot-scale flares was measured. The flame structure and combustion efficiencieswere correlated withoperating conditions of the flare, size oftheflare head, and propertiesoftheflaredgases. The combustion efficiency was correlated with the ratio of heating value of the gas flared to the heating value .requiredto maintaina stableflame, and was independent ofthe 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 ProjectSummarywas

### developed by EPA's Industrial Environmental ReseiiTch Laboratory, Research Triangle Parle, NC, to announce key findings of the research project that is fully documentedin a separate report ofthe 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 )includesapad and structure for installing and testing flare heads, screens to shield the flame from the wind, parallel delivery systems to accurately meterthewide range ofgas flows to the. flare, a hood to sample the entire plume, a movable rake probe to simultaneously sample five radial positions. high-speed movie andphotographic equipment to record the structure of the flare flame, and a room from which to control the flareandanalyzegassamples.

Techniques were developed tooperate, sample, analyze, and reduce the data. Analysis includedvisual and photographic observation of the flare flame structure, and samples of soot, 02. CO, C02 • total hydrocarbon, and S02. whichwas used as a tracer. The data were corrected for the measured background of combustion species and for dilution oftheflare 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 byjettheoryto 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

SulfurDioxide Tank

Steam Generator Flow Controls &Meters

Flow Controls &Meters

Flow Controls 

### &Meters

Figure 1. EPA's flare test facilityat 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 lengthwascorrelatedwith 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 HeatedLines



# .------. 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 combustionefficiencycorrelated Figure 2 shows the gas heating value with a dimensionless heating value versus the gas exitvelocity atthe 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. IndustrialHead C**~** 12-ln. IndustrialHeadA 

# 0 12-/n. IndustrialHead8



## c'**~s\** 

## 0 96o/o 0**~ej \)~** 


## 0

1.0 0



### 10.0 181C/= Confidence Interval.

1 b 1

Based on flares burningpropane/ nitrogen mixture with no pilot flame.



### 100.0 1000.0 All conclusionsare basedon thedataof this study and are limited to head geometries, gases, and variables examined. Head geometries were limited to:

• Simple pipe flare of3-, 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** of270- 2350 Btu/fe. • One test with natural gas. The variables examined were:

• Velocities from 0.2 to 420ft/sec. • Reynolds numbersfrom 

## 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 operatingwith unstableflames can have lowcombustion efficiency. FiguN 2. Region offlame instability. tbl determined by establishing a propane/ nitrogen flame 

### at a given velocity and then decreasingtheflowof 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 velocitywithin the 95 percent confidence limitsofthe mean. For anygivenvelocity, a gas with a heating value above this region produces astableflame; agaswith a heating value below this region produces an unstable flame. Velocity/gas combinations in or belowthe region tend to produce flames with lowercombustion efficiency. Thus, for any given test velocity, the minimum gas heating valuefor 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 greaterthan 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 whenthe ratioexceeds 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 sometimesachieved, 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 perturbationsand,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. CommercialA 

# 0 12-ln. Commercial8 

### 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 efficiencynear the lower limits offlame stability.
3- Liftoff distances were correlated with ratios of velocities and concentrations.

- Flame stabilitywas correlatedwith

a pseudo flame temperature.

- Entrainment was correlated with

ratios ofdistances andthe 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 butdid notappreciably 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, andJ. Lee are with Energy and Environmental Research Corporation. Irvine, CA 92714. BruceA. Tichenor is the EPA Project Officer (see below). The complete report. entitled "Evaluation ofthe Efficiency ofIndustrial Flares:

Test Results,"(OrderNo. 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 contactedat: Industrial EnvironmentalResearch Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC27711

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



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