FROM: Jeff Coburn, RTI International 

TO: Andrew Bouchard and Brenda Shine, EPA/OAQPS 

DATE: December 12, 2013 

SUBJECT: Petroleum Refinery Sector Rule: Operating Limits for Flares 

1. Purpose 

This memorandum summarizes the data and analysis used to identify appropriate operating limits 

for flares. 

2. Background 

Flares used as a control device subject to national emissions standards for hazardous air 

pollutants (NESHAP) are required to meet certain design and operating requirements as specified 

in 40 CFR 63.11. The available data at the time suggested that flares meeting these design and 

operating requirements would achieve a minimum destruction efficiency of no less than 

98 percent emissions control. With the advent of activities to reduce the amount of flaring at 

refineries, more refinery flares are commonly operated at high turndown (minimum flow 

conditions). Recent evidence through Passive Fourier Transform Infrared spectroscopy (PFTIR) 

testing suggests that steam- and air-assisted flares can have a significant degradation in 

destruction efficiency when operated at high turndown or at other times when steam- and airassist rates are too high (EPA, 2012). As many of the flares operated at petroleum refineries are 

steam- or air-assisted, concerns of poor flare destruction efficiency are particularly significant for 

petroleum refinery flares. 

The U.S. Environmental Protection Agency (EPA) supported a study to better understand flare 

performance and to identify key operating/monitoring parameters that would ensure adequate 

performance of flares at all times (EPA, 2012). The EPA is also performing a risk and 

technology review of the petroleum refinery NESHAP (40 CFR part 63, subparts CC and UUU) 

as part of their requirements under the Clean Air Act (CAA) sections 112(d)(6) and (f)(2). 

Particularly, under CAA section 112(d)(6), the EPA is required to review these technology-based 

NESHAP and to revise them “as necessary (taking into account developments in practices, 

processes and control technologies)” no less frequently than every 8 years. Many of the organic 

hazardous air pollutant (HAP) emission controls specified in the refinery NESHAP require 95 to 

98 percent control efficiency (or reduce to 20 parts per million by volume (ppmv) total organic 

carbon) and flares meeting the general provisions requirements for flares as control devices were 

deemed acceptable control alternatives. However, given the growing body of data indicating that 

flares that are over-assisted are likely to not meet these target control levels, the EPA is 

considering revising the flare operating and monitoring requirements to the refinery NESHAP to 

ensure compliance with the CAA section 112(d)(2) and (d)(3) standards. This memorandum 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 2 

summarizes the data from the flare performance studies and the data analysis performed to select 

appropriate operating limits for flares. 

3. Combustion Theory 

Combustion can be defined as “the sequence of exothermic chemical reactions between a fuel 

and an oxidant accompanied by the production of heat and conversion of chemical species” 

(Wikipedia, 2013). Combustion generally requires three ingredients: fuel, an oxidizing agent 

(typically oxygen in air), and heat (or ignition source). The need for heat or an ignition source to 

start the combustion process is the reason why pilot flames and maximum exit gas velocity limits 

have been required on flares for the past 30 years. As flares have flames that are external to a 

combustion chamber and surrounded by ambient air, they are rarely oxygen limited. For a 

refinery flare, the fuel is primarily light hydrocarbons that are either not recovered during normal 

refinery operations or gases produced during sudden shifts in operations, such as during startup, 

shutdown, or process upsets. 

Generally, fuels must contain combustible gases and combustible gases generally have an upper 

and lower flammability limit. The upper flammability limit is the highest concentration of a gas 

in air that is capable of burning. Above this flammability limit, the fuel is too rich to burn. The 

lower flammability limit (LFL) is the lowest concentration of the gas in air that is capable of 

burning. Figure 1 shows a typical combustion path (often referred to as a “nose plot”) for a fuel 

as it is mixed with air, as typically occurs in a flare. The flammability region is the area inside 

the “nose.” As the fuel exits the flare, it may be too rich to combust, but as the fuel mixes with 

ambient air, it moves from Point A (fuel mixture concentration) towards 100 percent air 

following the path of the red arrows (combustion path). For combustion to occur, this 

combustion path must pass through the flammability region. If the fuel mixture contains lots of 

inerts, such as nitrogen and water/steam, it is possible that Point A may be further down and to 

the right on the fuel line so that the combustion path passes through the flammability region for 

only a very short period or does not pass through the flammability region at all. In this case, 

which may occur if too much steam is added to the fuel prior to combustion, little to no 

combustion of the flare gas would be expected to occur. Thus, the combustion efficiency of a 

flare is expected to be a function of how long the fuel and air mixture lies within the 

flammability region. 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 3 

Path of Combustion Zone Gas

(Methane as an example)

Fuel

Vol %

Inert (steam or Nitrogen)

Vol %

“B” point of  insufficient air

“C” point of insufficient fuel, low NHV

Vent gas out of the flare, moving toward 100%

air as fuel/inert mixture diffuses.

26%

Flammability

Region

100% air

100% Inert

100% Fuel

“A” Exit Tip

Figure 1. Typical combustion path for a diffusion flame. 

When considering the combustion of hydrocarbon fuels, “complete combustion” would be the 

conversion of all hydrocarbons components to carbon dioxide (CO2) and water. Complete 

combustion rarely occurs. Often, the combustion process may be incomplete, producing carbon 

monoxide (CO) or other products of incomplete combustion, such as formaldehyde or unburned 

fuel components. Flare performance is commonly characterized by the combustion efficiency. 

PFTIR measurements are used to determine the relative concentrations of CO2 released from the 

flare to the concentrations of CO and hydrocarbons. The combustion efficiency can be estimated 

as: 

 



 





  

 



n

i

2 i i

2

CO CO CMN HC

CO %CE

1

[ ] [ ] [ ]

[ ] 100% (Eqn. 1) 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 4 

where: 

 %CE = Combustion efficiency of the flare, percent. 

 [CO2] = Concentration of CO2 in the offgas from the flare (i.e., above the flare flame), 

parts per million by volume (ppmv). 

 [CO] = Concentration of CO in the offgas from the flare (i.e., above the flare flame, 

ppmv. 

 i = Individual hydrocarbon component in the offgas from the flare. 

 CMNi = Carbon mole number of hydrocarbon component i in the offgas from the flare, 

mole carbon atoms per mole of compound. E.g., CMN for ethane (C2H6) is 2; 

CMN for propane (C3H8) is 3. 

 [HC]i = Concentration of hydrocarbon component i in the offgas from the flare, ppmv. 

While combustion efficiency looks at the simple chemical reaction of burning hydrocarbons and 

transforming them into CO2 and water, there is another common metric also used to describe the 

performance of flares known as destruction efficiency. The destruction efficiency can be 

evaluated for a particular pollutant, i.e., mass rate of a particular component exiting the flare 

flame divided by the mass rate of a component entering the flare. However, it is difficult to 

measure the mass rate of a single component exiting the flare. Alternatively, one could consider 

conversion of hydrocarbons to CO as destruction, so the hydrocarbon destruction efficiency from 

PFTIR measurements could be determined as follows: 

 

 



 





  

  



n

i

2 i i

2

CO CO CMN HC

CO CO %DE

1

[ ] [ ] [ ]

[ ] [ ] 100% (Eqn. 2) 

where: 

 %DE = destruction efficiency of the flare, percent. 

Thus, it is important to note that there is a difference between combustion efficiency and 

destruction efficiency and that destruction efficiency will always be greater than (or equal to) 

combustion efficiency. 

Combustion Indicators for Fuels 

In order to have good combustion, it is important that the fuel air mixture pass through the 

flammability region (see Figure 1). The concept of upper and lower flammability limits was 

discussed, but other indicators (properties of the fuel) have been used historically to indicate 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 5 

whether a fuel contains a sufficient amount of combustibles in order to provide good combustion 

in a flare. The key fuel (or “vent gas”) properties that have been used as indicators that the fuel 

is sufficiently rich to support combustion include the net heating value, the lower flammability 

limit, and the combustible gas concentration. The appropriate equations to determine the net 

heating value, the lower flammability limit, and the combustible gas concentration in the vent 

gas when multiple components are present follow. 





n

i

vg iNHVi NHV x

1

 (Eqn. 3) 

where: 

 NHVvg = Net heating value of flare vent gas, British thermal units per standard cubic 

foot (Btu/scf). 

 i = Individual component in flare vent gas. 

 n = Number of components in flare vent gas. 

 xi = Concentration of component i in flare vent gas, volume fraction. 

 NHVi = Net heating value of component i determined as the heat of combustion where 

the net enthalpy per mole of offgas is based on combustion at 25 degrees 

Celsius (°C) and 1 atmosphere (or constant pressure) with water in the 

gaseous state from values published in the literature, and then the values 

converted to a volumetric basis using 20 °C for “standard temperature.” 

Table 1 summarizes component properties including net heating values. 

 













  n

i i

vg

LFL

χ

LFL

i

1

1

 (Eqn. 4) 

where: 

 LFLvg = Lower flammability limit of flare vent gas, volume fraction. 

 n = Number of components in the vent gas. 

 i = Individual component in the vent gas. 

χi = Concentration of component i in the vent gas, volume percent (vol %). 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 6 

 LFLi = Lower flammability limit of component i as determined using values 

published by the U.S. Bureau of Mines (Zabetakis, 1965), vol %. All inerts, 

including nitrogen, are assumed to have an infinite LFL (e.g., LFLN2 = ∞, so 

that χN2/ LFLN2 = 0). LFL values for common flare vent gas components are 

provided in Table 1. 

Table 1— Individual Component Properties 

Component 

Molecular 

Formula 

MWi

(lb/lb-mol)a

CMNi

(mole/mole) 

NHVi

(Btu/scf) 

LFLi

(vol%)a

Acetylene C2H2 26.04 2 1,404 2.5 

Benzene C6H6 78.11 6 3,591 1.3 

1,2-Butadiene C4H6 54.09 4 2,794 2.0 

1,3-Butadiene C4H6 54.09 4 2,690 2.0 

iso-Butane C4H10 58.12 4 2,957 1.8 

n-Butane C4H10 58.12 4 2,968 1.8 

cis-Butene C4H8 56.11 4 2,830 1.6 

iso-Butene C4H8 56.11 4 2,928 1.8 

trans-Butene C4H8 56.11 4 2,826 1.7 

Carbon Dioxide CO2 44.01 1 0 ∞

Carbon Monoxide CO 28.01 1 316 12.5 

Cyclopropane C3H6 42.08 3 2,185 2.4 

Ethane C2H6 30.07 2 1,595 3.0 

Ethylene C2H4 28.05 2 1,477 2.7 

Hydrogen H2 2.02 0 274 4.0 

Methane CH4 16.04 1 896 5.0 

Methyl-Acetylene C3H4 40.06 3 2,088 1.7 

Nitrogen N2 28.01 0 0 ∞

Oxygen O2 32.00 0 0 ∞

Pentane+ (C5+) C5H12 72.15 5 3,655 1.4 

Propadiene C3H4 40.06 3 2,066 2.16 

Propane C3H8 44.10 3 2,281 2.1 

Propylene C3H6 42.08 3 2,150 2.4 

Water H2O 18.02 0 0 ∞

a

 MWi = molecular weight of component I; lb/lb-mol = pounds per pound-mole; LFLi = Lower flammability limit of 

component i; vol% = volume percent. 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 7 

100%

1 3 



 



  



 



 





h

n

i

i i

vg

χ

CMN χ

C (Eqn. 5) 

where: 

 Cvg = Total volumetric fraction of hydrogen (H2) and combustible organic 

components present in the flare vent gas, fraction as propane. For the 

purposes of Cvg, CO2 is not considered to be a combustible organic 

component, but CO may be included in Cvg. 

 n = Number of individual combustible organic components in flare vent gas. 

 i = Individual combustible organic component in flare vent gas. 

χi = Concentration of combustible organic component i in flare vent gas, vol %. 

 CMNi = Carbon mole number of combustible organic component i in flare vent gas, 

mole carbon atoms per mole of compound. E.g., CMN for ethane (C2H6) is 2; 

CMN for propane (C3H8) is 3. 

χh = Concentration of H2 in flare vent gas, vol %. 

 100% = Constant, used to convert volume percent to volume fraction. 

Impact of Assist Gas on Combustion Indicators for Fuels 

To improve mixing and to reduce soot production (smoking) in the flare flame, steam and/or air 

may be mixed with the flare vent gas. This gas addition is commonly referred to as “assist gas” 

(either assist steam or assist air). Typically, assist air is introduced around the perimeter of the 

flare to support mixing of air and fuel in the combustion zone. Representatives of John Zink 

noted that they have investigated aspirating air into the assist steam line as a means to provide 

both assist air and assist steam (Coburn, 2013); this method of adding assist air is fairly new and 

it is expected to be used in very few petroleum refinery flares, if any. 

According to the combustion path theory represented in Figure 1, the impact of adding assist air 

is expected to have a different impact on the combustion path. The addition of steam is expected 

to shift Point A down and to the right on the fuel line toward 100 percent inerts. On the other 

hand, the addition of air is expected to move Point A towards 100 percent air following the path 

of the red arrows. No combustion efficiency studies have been conducted on a flare using steamentrained assist air, but if the air is mixed prior to the flame zone or pilot gas ignition point, this 

assist air may act in much the same manner as inert gases (shifting point A so that the 

combustion path begins near the lower region or below the flammability region) so that 

combustion efficiency could be negatively impacted. Consequently, a distinction is made 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 8 

between “premix assist air” and “perimeter assist air.” Premix assist air is assist air that is mixed 

with the flare vent gas prior to the outlet of the flare, such as assist air entrained with center 

steam. This premix assist air is treated in the same manner as inerts (steam) added prior to or 

within the flame combustion zone. Perimeter assist air includes all other assist air, such as 

traditional assist air introduced around the perimeter of the flare as well as assist air entrained in 

upper and lower steam. 

Considering combustion path theory, the properties of the fuel mixture entering or within the 

combustion zone is expected to be a better indicator of flare performance than the properties of 

the fuel (flare vent gas) alone. As measurement data are typically made for the flare vent gas 

properties (and because direct sampling of the combustion zone gas is infeasible), the net heating 

value, the lower flammability limit, and the combustible gas concentration in the combustion 

zone must be calculated based on vent gas properties and steam/inert gas addition rates. The 

following equations were used to determine the combustion zone properties. 

  vg s a,premix

vg vg

cz Q Q Q

Q NHV

NHV

 

  (Eqn. 6) 

 

vg

vg vg s a,premix

cz Q

LFL Q Q Q LFL

    (Eqn. 7) 

  vg s a,premix

vg vg

cz Q Q Q

Q C

C

 

  (Eqn. 8) 

where: 

 NHVcz = Net heating value of combustion zone gas, Btu/scf. 

 NHVvg = Net heating value of flare vent gas, Btu/scf. 

 LFLcz = Lower flammability limit of combustion zone gas, volume fraction. 

 LFLvg = Lower flammability limit of flare vent gas, volume fraction. 

 Ccz = Combustibles concentration in the combustion zone gas, volume fraction. 

 Cvg = Combustibles concentration of flare vent gas, volume fraction. 

 Qvg = Cumulative volumetric flow of flare vent gas, standard cubic feet per minute 

(scfm). 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 9 

 Qs = Cumulative volumetric flow of total steam, scfm. 

 Qa,premix = Cumulative volumetric flow of premix assist air, scfm. 

Note that in all performance tests, there was no premix assist air, so Qa,premix = 0. 

4. Performance Data for Flares Using Assist Steam 

The vast majority of the recent flare performance data are for steam-assisted flares (EPA, 2012). 

In 2011, the EPA conducted a detailed Information Collection Request (ICR) of the petroleum 

refining industry (EPA, 2011). Under Component 1 of this ICR, the EPA required petroleum 

refineries to provide information about specific process units and emission sources present at the 

facility, including information about flares. Based on the ICR data, approximately 80 percent of 

refinery flares are steam-assisted, 10 percent are air-assisted, and 10 percent are non-assisted. 

Therefore, the preponderance of data for steam-assisted flares is understandable. 

Performance Indicators Flares Using Assist Steam 

The flare performance studies used PFTIR methods to determine the relative concentrations of 

combustion products (i.e., CO2 and CO) and unburned hydrocarbons in the plume above the flare 

flame. The combustion efficiency was subsequently calculated based on complete oxidation of 

the hydrocarbons to CO2 (i.e., Equation 1 of this memorandum). Figure 2 presents the measured 

combustion efficiency as a function of the combustion zone net heating value (NHVcz). Figure 3

presents the measured combustion efficiency as a function of the combustion zone lower 

flammability limit (LFLcz), and Figure 4 presents the measured combustion efficiency as a 

function of the combustion zone “combustibles” concentration (Ccz), which is the total 

hydrocarbon concentration as propane (C3) plus the H2 concentration.
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 10 

Figure 2. Flare combustion efficiency as a function of the net heating value of the gas within the comb

0.30.40.50.60.70.80.9

1

50 100 150 200 250 300 350 400 

Combustion Efficiency

Combustion Zone Net Heating Value (NHV_cz), Btu/scf

No H2‐Olefin InH2‐Olefin Inter
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 11 

Figure 3. Flare combustion efficiency as a function of the lower flammability of the gas within the comb

0.30.40.50.60.70.80.9

1

0 0.05 0.1 0.15 0.2 0.25 0.3 0

Combustion Efficiency

Combustion Zone Lower Flammability Limit (LFL_cz), volume fraction

No H2‐Olefin InteractionH2‐Olefin Interaction
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 12 

Figure 4. Flare combustion efficiency as a function of the combustibles concentration within the com

0.30.40.50.60.70.80.9

1

0 0.05 0.1 0.15 0.2 0.25 0.3 

Combustion Efficiency

Combustion Zone Combustibles Concentration (C_cz), volume fraction

No H2‐Olefin InteraH2‐Olefin Interactio
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 13 

The destruction (or control) efficiency of a flare is generally higher than the combustion 

efficiency as some hydrocarbons may be partially oxidized to CO rather than fully converted to 

CO2. In the flare performance study, it was estimated that a combustion efficiency of 

96.5 percent was equivalent to a destruction efficiency of 98 percent (EPA 2012). For this 

analysis, we considered primarily the individual, time-stamp specific measurement data, i.e., 

individual data measurements recorded every minute or two during a test run; the average values 

across a given run (typically a run was about 15 minutes in duration) were also evaluated as part 

of the failure analysis. We expect a little more variability in the minute-by-minute recordings, 

but we also expect to see better correlation of the data as waste gas flow rates or steam addition 

rates vary during a given run. Typically, only a single compositional analysis value for the waste 

gas is available during a run, since the gas chromatography (GC) cycle time is approximately 

15 minutes. Therefore, changes in combustion zone compositions were calculated based on 

minute by minute changes in waste gas flow rates and steam addition rates. Figures 2 through 4 

are plots of the individual, time-stamp specific measurement data. 

Hydrogen-Olefin Interaction Affect on Flare Performance 

In looking at Figures 2 through 4, there appears to be certain test conditions that showed 

significantly poorer combustion efficiencies than observed in other test runs. Upon 

investigation, these test runs appear to have considerable concentrations of both olefin (alkenes) 

and H2 in the waste gas. Although not well-understood, it is suspected that there are special 

reactions that occur between olefins and H2 during the combustion process which competes with 

the direct oxidation reactions, making it more difficult to achieve high combustion efficiencies at 

a given combustion zone parameter level as compared to waste gases that do not have a mixture 

of olefins and H2. Figures 2 through 4 indicate the data points that were expected to have 

H2-olefin interactions; these data points were selected using the following criteria: 

1) The H2 concentration in the combustion zone was greater than 1.2 mole percent. 

2) The sum of the olefin concentrations in the combustion zone was greater than 2.5 mole 

percent. 

3) The H2 concentration in the combustion zone plus the sum of the olefin concentrations in 

the combustion zone was greater than 7.4 mole percent. 

All three of these criteria had to be met in order for the data point to be considered as “having 

H2-olefin interactions.” There were numerous runs that had high H2 concentrations but no/low 

olefins content and several runs that had high olefin concentrations but no/very low H2

concentrations. As seen in Figures 2 through 4, the combustion efficiency for the data points 

associated with conditions conducive to H2-olefin interactions are significantly different than for 

data points without H2-olefin interactions, particularly when using NHVcz or LFLcz as the 

indicator of good combustion. Based on these observations, separate operating limits were 

evaluated for flares with H2-olefin interactions and for flares without H2-olefin interactions. 

Failure Analysis of Assist Steam Flare Performance Indicators 

The ultimate goal of this failure analysis is to ensure a flare achieves a destruction efficiency of 

98 percent (expected to be equivalent to a flare combustion efficiency as measured during the 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 14 

test of approximately 96.5 percent) so that the flare achieves an equivalent performance to other 

thermal combustion devices that can be used to comply with the refinery NESHAP. These other 

thermal destruction devices have enclosed stacks so that the control efficiency of the device can 

be readily determined, and their performance is typically averaged over the period of the 

performance test, which is typically 3 hours (i.e., three 1-hour runs). Due to the variability of 

flow to most flares, a long averaging period is not practical. For example, if a flaring event lasts 

only 1 hour, how does one determine a 3-hour average operating limit? Additionally, the data to 

assess flare performance is available only for short durations, so it is important to consider the 

time scale of the data used to derive an operating limit when determining an appropriate 

averaging time for the operating limit. In this analysis, we consider individual time-stamp data 

(i.e., minute-by-minute performance). However, we are also constrained by the availability of 

compositional data. Thus, while we may be evaluating minute-by-minute data, we must 

generally assume a fixed waste gas composition over the test run (which is typically about 

15 minutes) when performing the minute-by-minute calculations. Typically, GC compositional 

analysis of refinery gases requires 10 to 15 minutes to complete. Therefore, a 15-minute block 

average is considered the shortest time from over which a flare operating limit can be reasonably 

evaluated in practice. 

Even though a 15-minute block average is considered the shortest time frame for which a flare 

operating limit can be reasonably evaluated for regulatory purposes, the performance test data 

provide minute-by-minute combustion efficiency measurements and we also evaluated flare 

performance using this minute-by-minute data. Therefore, it is possible that a flare that has a 

measured combustion efficiency less than 96.5 percent for a given minute to still meet a 

combustion efficiency of 96.5 percent on average over a 15-minute period. Consider an example 

in terms of destruction efficiency. If a flare was achieving 99 percent destruction, for 14 minutes 

in a 15-minute block average, that flare would meet the 98 percent destruction efficiency 

provided that the destruction efficiency for the 15th minute was at least 84 percent. However, 

1 minute at 70 percent destruction efficiency would make it impossible for the flare to achieve 

98 percent destruction efficiency across the 15-minute average even if the destruction efficiency 

for the other 14 minutes is 99.9 percent. Therefore, while it may be acceptable to have some 

minutes with combustion efficiencies less than 96.5 percent (expected to be equivalent to 

98 percent destruction efficiency), it is important that the combustion efficiency does not fall 

below 85 or 90 percent in any one minute; otherwise, it becomes extremely difficult, if not 

impossible, for the flare to achieve the desired 98 percent destruction efficiency. 

With this in mind, a failure analysis was conducted using both the minute-by-minute data and the 

run average data to help identify appropriate operating limits for the three different performance 

indicators. A Type 1 error occurs when the operating limit is achieved, but the performance is 

below a specified lower combustion efficiency limit. We conducted the Type 1 error analysis 

considering lower combustion efficiency values of 85 and 90 percent when using the minute-byminute analysis and 95 and 96.5 percent when using the run average data. A Type 2 error occurs 

when the performance indicators suggest non-compliant performance when the combustion 

efficiency is definitely compliant. We conducted the Type 2 error analysis using 98 percent 

combustion efficiency as the “definitely” compliant performance level for both the minute-byminute and run average data analyses. The flare performance dataset was divided into two: one 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 15 

for minute-by-minute data that were not expected to have H2-olefin interactions and one for 

minute-by-minute data that were expected to have H2-olefin interactions (i.e., met all three 

H2-olefin concentration conditions outlined previously). A separate failure analysis was 

conducted for each of these datasets. Operating limits were adjusted by trial and error to 

minimize or eliminate Type 1 errors while also attempting to minimize the occurrence of Type 2 

errors. Table 2 presents the recommended operating limits and summarizes the results of the 

failure analysis for the time-stamp data analysis considering those operating limits; Table 3

summarizes the results of the failure analysis for the run average data analysis considering the 

recommended operating limits. 

Key observations of the failure analysis results suggest: 

 The target operating limits presented in Tables 1 and 2 result in very few Type 1 errors 

(i.e., instances where the operating limit was achieved but performance was well under 

96.5 percent combustion efficiency), even when the facility can elect to comply with any 

of the limits at any time. At the 85 percent combustion efficiency target, only one data 

point was not identified by the performance indicators and target operating limits (only 

for the H2-olefin interactions group and this data point is likely an outlier). None of the 

minute-by-minute data points without H2-olefin interactions and only 0.5 percent of the 

minute-by-minute data points with H2-olefin interactions did not have a combustion 

efficiency of at least 90 percent while compliant with any one of the performance 

indicators at the selected target operating limits. Considering the run average data, all 

test runs compliant with the selected target operating limits had average combustion 

efficiencies exceeding 95 percent, and 97 percent of those had combustion efficiencies at 

or above 96.5 percent. 

 Allowing compliance with any of the three performance indicators and target operating 

limits at any time helps to minimize the number of Type 2 errors (non-compliance with 

the operating limit, while having high combustion efficiencies). Requiring compliance 

with a single operating limit would be more likely to identify a facility as non-compliant 

while they have high combustion efficiencies than allowing compliance with any one of 

the operating limits at any time. 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 16 

Table 2. Failure Analysis of Steam-Assisted Flare Performance Indicators Based on 

Minute-by-Minute Measurement Data 

Data Set/Performance 

Indicator 

Target 

Operating 

Limit 

No. of Type 1 Errors with: No. of Type 2 

Errors with 

CE < 85% CE < 90% CE ≥ 98% 

Does not meet H2-Olefin interaction criteria (2,918 data points) 

 NHVcz ≥270 0 0 347 

 LFLcz ≤0.15 0 0 337 

 Ccz ≥0.18 0 0 305 

 NHVcz, LFLcz , or Ccz See above 0 0 195 

Meets all three H2-Olefin interaction criteria (945 data points) 

 NHVcz ≥380 0 4 122 

 LFLcz ≤0.11 1 1 88 

 Ccz ≥0.23 1 1 112 

 NHVcz, LFLcz , or Ccz See above 1 5 87 

Table 3. Failure Analysis of Steam-Assisted Flare Performance Indicators Based on Run 

Average Measurement Data 

Data Set/Performance 

Indicator 

Target 

Operating 

Limit 

No. of Type 1 Errors with: No. of Type 2 

Errors with 

CE < 95% CE < 96.5% CE ≥ 98% 

Does not meet H2-Olefin interaction criteria (180 data points) 

 NHVcz ≥270 0 2 18 

 LFLcz ≤0.15 0 3 21 

 Ccz ≥0.18 0 2 23 

 NHVcz, LFLcz , or Ccz See above 0 4 15 

Meets all three H2-Olefin interaction criteria (56 data points) 

 NHVcz ≥380 0 2 5 

 LFLcz ≤0.11 0 3 4 

 Ccz ≥0.23 0 2 5 

 NHVcz, LFLcz , or Ccz See above 0 4 4 

5. Performance Data for Flares Using Assist Air 

There are very limited data on flares using perimeter assist air and no data on flares using premix 

assist air. As indicated in Equations 6 though 8, premix assist air is effectively treated as assist 

steam when calculating the combustion zone properties. Therefore, premix assist air is not 

considered further in this section. There were a series of test conducted in 1985 on a very small 

1.5-inch diameter flare (EPA, 1985) and a series of tests conducted on a 2-foot diameter flare by 

the Texas Commission on Environmental Quality (TCEQ) that used perimeter assist air (EPA, 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 17 

2012). A third test was conducted on a flare that used perimeter assist air in 1983, but this test 

did not report air assist rates and, therefore, could not be used to identify “over-assisted” 

conditions. 

Performance Indicators for Flares Using Perimeter Assist Air 

The TCEQ study used different mixtures of natural gas (predominately methane), propane, 

propylene, and nitrogen to assess flare performance at the three different flare vent gas net 

heating values: 2000 Btu/scf; 575 Btu/scf; and 350 Btu/scf. The 1985 study evaluated mixtures 

of nitrogen and propane covering a range of flare gas net heating values ranging from 440 to 

2,350 Btu/scf. Figure 5 shows the measured combustion efficiency as a function of the NHVvg. 

As seen in Figure 5, these tests indicate that the initial NHVvg is not a critical performance 

parameter (at least within the range of net heating values investigated). The flare performance 

was primarily dependent on the air-assist rates and the combustion efficiency for a given flare 

gas net heating value could be high or low, depending on the amount of assist air. 

Figure 5. Flare combustion efficiency as a function of the net heating value of the flare 

vent gas (NHVvg). 

30%

40%

50%

60%

70%

80%

90%

100%

0 500 1000 1500 2000 2500

Combustion Efficiency (%)

Net Heating Value Vent Gas

Air‐assistedFlare:  Net Heating Value in Vent Gas

TCEQ; NHVvg=2000

TCEQ; NHVvg=575

TCEQ; NHVvg=350

EPA, 1985
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 18 

In the Parameters for Properly Designed and Operated Flares report (EPA, 2012), the 

stoichiometric ratio was identified as a potential performance indicator for air-assisted flares. 

However, the correlation of combustion efficiency and stoichiometric ratio is very different for 

the two different studies (see Figure 6). 

Figure 6. Flare combustion efficiency as a function of stoichiometric air assist rates. 

In a diffusion flame, combustion takes place at the flame surface only, where the fuel 

(combustibles in the waste gas) meets oxygen within the flammability region. The interior of the 

flame contains unburned fuel. This type of combustion is the simplest form, and one that occurs 

at the tip of a non-assist flare. Graphically, one can envision the path of the combustion starting 

initially on the zero air line (Point A), symbolizing the quality of the gases in the flare just prior 

to it leaving the flare tip as shown previously in Figure 1. As the vent gases mix with air and 

burn, they proceed on a path from point A, through the flammability region, and eventually go to 

the origin (infinite dilution in air). If the flare vent gas and assist media mixture are already near 

the LFL when the gas mixture is released into air, the concentration could quickly fall below the 

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50 60 70 80

Combustion Efficiency (%)

Stoichiometric Ratio

Air‐assistedFlare:  Stoichiometric Ratio Correlation

TCEQ; NHVvg=2000

TCEQ; NHVvg=575

TCEQ; NHVvg=350

EPA, 1985
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 19 

mixture’s LFL (point C) before having a chance to ignite and burn completely; and therefore, it 

is possible to dilute the flammable gas mixture to a mixture with no combustible characteristics. 

Consistent with a combustion theory and as demonstrated from the analysis of the steam-assist 

data, there needs to be a minimum heat content (or other performance indicator) of flare gas 

within the combustion zone in order for the combustion path to travel through the flammability 

region as air is introduced. All of the air assist runs were compliant with at least one (if not all 

three) of the suggested operating limits based on the flare vent gas conditions. However, it is 

clear from Figure 5 that the initial flare vent gas properties alone are not sufficient to ensure 

good combustion. 

For air-assisted flares, the combustion path can still be expected to follow the path indicated in 

Figure 1, but the rate of travel towards 100 percent air would be much faster than for flare that 

does not use perimeter assist air. If the rate is too fast, the combustible mixture will not exist 

long enough to allow near complete combustion. Therefore, the “dilution rate” caused by the 

addition of assist air must be limited to some extent in order to ensure that the flare gas is not 

diluted so fast that combustion efficiency is compromised. 

Based on the data in Figure 6, it was postulated that the diameter of the flare is a significant 

parameter impacting the allowable dilution ratio of a flare with perimeter assist air. For small 

diameter flares, it is easier for the assist air to fully penetrate and dilute the combustion zone gas 

before significant combustion can occur. Therefore, the “dilution factor” was thought to be a 

function of the flare diameter and act on the combustion zone gas. Using Equations 6 through 8, 

the combustion zone gas properties can be replaced with the vent gas properties, which are 

actually measured. The following dilution factor, as a function of vent gas properties, was 

derived and subsequently evaluated as a means to predict flare performance for flares using 

perimeter assist air. 

vg s a premix a perimeter

vg

Q Q Q Q

Q Diam

DF

  ,  ,

  (Eqn. 9) 

where: 

 DF = Dilution factor, feet (ft). 

 Qvg = Volumetric flow rate of the flare vent gas, scfm. 

 Qs = Volumetric flow rate of the assist steam, scfm. 

 Qvg = Volumetric flow rate of the premix assist air, scfm. 

 Qair = Volumetric flow rate of the perimeter assist air, scfm. 

 Diam = Diameter of the flare vent gas release stack, ft. 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 20 

The following air-assist dilution parameters were defined: 

 Net heating value indicator: NHVdil = NHVvg × DF 

 Lower flammability limit indicator: LFLdil = LFLvg /DF 

 Combustibles concentration indicator: Cdil = Cvg × DF 

Figures 7, 8, and 9 present the correlation between combustion efficiency and these air-assist 

dilution parameters. As seen in these figures, both the TCEQ and EPA data sets exhibit similar 

performance curves when correlated using the air-assist dilution parameters. 

Failure Analysis of Perimeter Assist Air Flare Performance Indicators 

Similar to the failure analysis conducted for steam-assisted flares, a failure analysis was 

conducted to help select appropriate operating limits for the air-assist dilution parameters. The 

air-assisted flare data were not available minute-by-minute. Individual run data for the TCEQ 

tests were typically 5 to 15 minutes long; individual run data for the EPA 1985 tests were 

20 minutes long. As these data represent longer averaging periods, the minimum target 

combustion efficiencies used in the air-assist failure analysis were selected to be much closer to 

the target combustion efficiency of 96.5 percent. When looking at minute-by-minute data, one 

minute of 90 percent combustion efficiency did not necessarily indicate that the target 

combustion efficiency could not be met on a 15-minute average basis. Since the air-assisted 

flare data typically represent average comparable to 15-minute averages, the air-assist dilution 

parameters were compared directly to the target combustion efficiency of 96.5 percent 

combustion efficiency. Table 4 presents the results of the air-assist dilution parameter failure 

analysis. 

Table 4. Failure Analysis of Perimeter Assist Air Flare Performance Indicators 

Data Set/Performance 

Indicator 

Target Operating 

Limit 

No. of Times 

Operating Limit 

was Achieved but 

CE < 96.5% 

(Type 1 Error) 

No. of Times 

Operating Limit was 

Not Achieved but 

CE ≥ 96.5% 

(Type 2 Error) 

Runs that do not meet H2-Olefin interaction criteria (79 runs) 

 NHVdil ≥22 0 1 

 LFLdil ≤2.2 0 1 

 Cdil ≥0.012 0 2 

 NHVdil, LFLdil , or Cdil See above 0 1 

Runs that meet all three H2-Olefin interaction criteria (0 runs) 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 21 

Figure 7. Correlation of combustion efficiency with the net heating value dilution parameter 

30%40%50%60%70%80%90%

100%

0 5 10 15 20 25 30 35 40 

Combustion Efficiency (%)

Net Heating Value Vent Gas × DilutionFactor (Btu/ft2)

Air‐assistedFlare: Net Heating Value Dilution Parameter

TCEQTCEQTCEQEPA, 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 22 

Figure 8. Correlation of combustion efficiency with the lower flammability limit dilution parame

30%40%50%60%70%80%90%

100%

0 2.5 5 7.5 10 12.5 15 17.5 

Combustion Efficiency (%)

Lower Flamability Limit/ Dilution Factor (1/ft)

Air‐assistedFlare: Lower Flamability Limit Dilution Parameter

TCEQ; NHTCEQ; NHTCEQ; NHEPA, 1985
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 23 

Figure 9. Correlation of combustion efficiency with the combustibles concentration dilution par

30%40%50%60%70%80%90%

100%

0 0.005 0.01 0.015 0.02 0.025 0.03 

Combustion Efficiency (%)

Combustible Concentration of Vent Gas × DilutionFactor (ft)

Air‐assistedFlare: Combustible Concentration Dilution Parameter

TCEQ; NHVvg=TCEQ; NHVvg=TCEQ; NHVvg=EPA, 1985
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 24 

Based on the failure analysis, the air-assist dilution parameter target operating limits presented in 

Table 3 provide an excellent indication of when the combustion efficiency will be greater than or 

less than 96.5 percent (assumed to be equivalent to a destruction efficiency of 98 percent) when 

no H2-olefin interaction is expected to be present. No data are available for air-assisted flares 

that meet H2-olefin interaction criteria. It is reasonable to conclude from the steam-assisted data 

analysis that air-assisted flares that meet the H2-olefin interaction criteria (in the combustion 

zone gas) should meet the more stringent H2-olefin interaction operating limits (in the 

combustion zone gas). It is also reasonable to expect that the perimeter assist air dilution 

parameter operating limits would need to be similarly more stringent to ensure good combustion 

when H2-olefin interactions are present. Table 5 shows one method of estimating “similarly 

more stringent” dilution parameter operating limits for flares using perimeter assist air when 

H2-olefin interactions are present in the combustion zone gas. 

Table 5. Projected Dilution Parameter Operating Limits for Flares with Perimeter Assist 

Air Meeting the H2-Olefin Interaction Criteria 

Parameter 

Family 

Target Vent 

Gas 

Operating 

Limit – No 

Olefin 

Interaction 

Target Air 

Assist Dilution 

Operating 

Limit– No 

Olefin 

Interaction 

Apparent 

Dilution 

Factor 

Target Vent 

Gas 

Operating 

Limit – Olefin 

Interactions 

Present 

Estimated Target 

Air Assist Dilution 

Operating Limit – 

Olefin Interactions 

Present 

NHV ≥270 ≥22 22/270 = 

0.0815 ≥380 ≥31 

LFL ≤0.15 ≤2.2 0.15/2.2 = 

0.068 ≤0.11 ≤1.6 

C ≥0.18 ≥0.012 0.012/0.18 = 

0.067 ≥0.23 ≥0.015 

6. Performance Data for Non-Assisted Flares 

There are very limited data on non-assisted flares. The EPA conducted a study in 1983 

evaluating a steam-assisted flare burning propylene or propylene/nitrogen mixtures (EPA, 1983). 

For some of the runs, the assist steam was turned off so the flare was effectively non-assisted. 

During the steam-assisted runs, the combustion efficiency of the flare was definitely best 

correlated with the combustion zone properties. For non-assisted flares, the vent gas 

composition is the same as the combustion zone composition. Figure 10 provides the correlation 

of the NHVcz to the combustion efficiency of the flare. While it is understandable that a NHVvg

limit of 200 Btu/scf was established based on the results of this single study, it is clear from the 

steam-assisted data that there is variability between flares and variability based on the 

composition of the flare gas (particularly when olefins and H2 are both present). While the 1983 

EPA study has limited data points in the transition area between good and poor combustion 

efficiencies, the data suggest that the same transition point between good and poor combustion 

for the non-assisted flares is essentially the same as for the steam-assisted flares when the 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 25 

combustion zone properties are considered. This common transition point is also observed in 

LFLcz and Ccz plots, and it is understandable since a steam-assisted flare will become unassisted 

if the assist steam is turned off. Consequently, it is reasonable to assert that the combustion zone 

operating limits established for steam-assisted flares are also applicable the non-assisted flares 

with the understanding that, for non-assisted flares, the vent gas properties are equivalent to the 

combustion zone properties. 

. 

Figure 10. Correlation of combustion efficiency with combustion zone net heating value 

(NHVcz) from the EPA 1983 study (EPA, 1983). 

As noted previously, the data for steam assisted flares covers a much broader range of operating 

conditions than the data for non-assisted flares. Therefore, the operating limits established for 

steam-assisted flares are expected to ensure compliance over this broader range of operating 

conditions. Based on the data for steam-assisted flares, it is evident that a 200 Btu/scf NHVcz

limit would not ensure compliance with the 98 percent destruction efficiency requirement at all 

times. Using the same failure analysis as conducted previously for steam-assisted flares, the 

impact of maintaining an NHVcz limit of 200 Btu/scf for non-assisted flares can be assessed. 

Table 6 summarizes the results of the failure analysis when using the run average data set. 

60%

65%

70%

75%

80%

85%

90%

95%

100%

100 150 200 250 300 350 400 450 500

Combustion Efficiency

Combustion Zone Net Heating Value (Btu/scf)

EPA, 1983 Flare Study:  Combustion ZoneNet Heating Value

SteamAssist

Non‐Assisted
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 26 

Allowing the use a 200 Btu/scf NHVcz limit would fail to identify poor flare performance 

approximately 20 percent of the time. That is, 20 percent of the runs where the average flare 

combustion efficiency was less than 95 percent (not including runs projected to have H2-olefin 

interactions) met the 200 Btu/scf NHVcz limit. This is in sharp contrast with the suggested 280 

Btu/scf NHVcz limit where none of the runs where the average flare combustion efficiency was 

less than 95 percent met a 270 Btu/scf NHVcz limit. The 200 Btu/scf NHVcz limit is even worse 

when one considers runs projected to have H2-olefin interactions. While it is unlikely a flare that 

receives vent gas that is high in olefins and H2 would be non-assisted, it is clear that a 200 

Btu/scf NHVcz limit for flares (whether non-assisted or steam-assisted) is not sufficient to ensure 

compliance with the target 98 percent destruction efficiency. 

Table 6. Comparison of Failure Analysis Results for Selected NHVcz Operating Limits 

Based on Run Average Measurement Data 

Data Set/Performance 

Indicator 

Target 

Operating 

Limit 

No. of Type 1 Errors with: No. of Type 2 

Errors with 

CE < 95% CE < 96.5% CE ≥ 98% 

Does not meet H2-Olefin interaction criteria (180 data points) 

 NHVcz ≥200 18 30 2 

 NHVcz ≥270 0 4 15 

Meets all three H2-Olefin interaction criteria (56 data points) 

 NHVcz ≥200 7 13 0 

 NHVcz ≥380 0 2 5 

The steam-assisted flare data also suggest that the 8 percent H2 content provision for non-assisted 

flares in 40 CFR 63.11(b)(6)(i) may not ensure proper flare destruction efficiency in all cases. 

There are several runs where the combustion zone H2 concentration exceeded 8 percent, but the 

average run combustion efficiency was well under 95 percent. These poor performing runs 

included runs that had low olefins concentration (i.e., runs not meeting the H2-olefin interaction 

criteria outlined previously in this memorandum). Thus, the poor performance of the flares with 

high hydrogen content in the combustion zone cannot be attributed to H2-olefin interactions. As 

these hydrogen concentrations were assessed in the combustion zone, they are expected to be 

analogous to vent gas properties for non-assisted flares. As such, the 8 percent H2 content 

provision for non-assisted flares in 40 CFR 63.11(b)(6)(i) is not expected to be sufficient to 

ensure proper flare performance. 

7. References 

Coburn, J. 2013. Meeting Minutes for February 13, 2013; Meeting Between U.S. EPA and 

Representatives from John Zink Hamworthy Combustion. Memorandum from Jeff Coburn, 

RTI International, to Andrew Bouchard and Brenda Shine, U.S. EPA. February 18, 2013. 
Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for Flares 

December 12, 2013 

 27 

EPA (U.S. Environmental Protection Agency). 1983. Flare Efficiency Study. Prepared for U.S. 

Environmental Protection Agency, Industrial Environmental Research Laboratory, Research 

Triangle Park, NC. July 1983. EPA-600/2-83-052 

EPA (U.S. Environmental Protection Agency). 1985. Evaluation of the Efficiency of Industrial 

Flares: Flare Head Design and Gas Composition. Prepared for U.S. Environmental 

Protection Agency, Air and Energy Research Laboratory, Research Triangle Park, NC. 

September 1985. EPA-600/2-85-106. 

EPA (U.S. Environmental Protection Agency). 2011. Petroleum Refinery Database, based on 

Refinery Information Collection Request (ICR) for Calendar Year 2010. September 2011. 

Docket Item Nos. EPA-HQ-OAR-2010-0682-0061 through 0069. 

EPA (U.S. Environmental Protection Agency). 2012. Parameters for Properly Designed and 

Operated Flares. Prepared for U.S. Environmental Protection Agency, Office of Air Quality 

Planning and Standards, Research Triangle Park, NC. April 2012. 

Wikipedia. 2013. See http://en.wikipedia.org/wiki/Combustion. Last accessed December 12, 

2013. 

Zabetakis, M.G. 1965. Flammability Characteristics of Combustible Gases and Vapors. U.S. 

Department of the Interior, Bureau of Mines. Bulletin 627, Appendix A, pp. 114-116.