FROM: Jeff Coburn, Leslie Pearce and Kevin Bradley 

TO: Brian Cook, EPA/OAR/CCD 

DATE: July 10, 2015 

SUBJECT: Proposed Changes to Flare Pilot Gas Reporting Requirements under the 

Greenhouse Gas Reporting Program (GHGRP) 

1. Purpose 

The purpose of this memorandum is to provide recommendations to clarify the reporting 

requirements for greenhouse gas (GHG) emissions associated with flare pilot gas use under 

Subparts Q, X and Y. 

2. Background 

Flares are used in a variety of industries to dispose of waste gases generated at the facility either 

during normal operations or due to startup, shutdown, or malfunction (SSM) events. Flares can 

emit significant quantities of carbon dioxide (CO2) as a result of hydrocarbon combustion. They 

may also emit methane (CH4), primarily as a result of methane in the waste gas that remains 

uncombusted, and small amounts of nitrous oxide (N2O) as a combustion byproduct. On 

October 30, 2009, the U.S. Environmental Protection Agency (EPA) promulgated mandatory 

greenhouse gas reporting requirements (74 FR 56260; 40 CFR part 98) for a variety of GHG 

emission sources. Flares were specifically excluded from the General Stationary Fuel 

Combustion Sources source category (40 CFR part 98, subpart C); however, other industrial 

source categories could require reporting of GHG emissions from flares and could refer to the 

calculation methods in subpart C, if deemed appropriate. The following subparts (in 40 CFR part 

98) require reporting of emissions, at least to some extent, from flares: subparts Q, W, X, Y, 

HH, II, and TT. The EPA is proposing to amend the rule to allow, but not require, facilities to 

exclude pilot gas from the flare gas GHG emissions calculations in Subparts Q, X, and Y. 

Purge/sweep gas would still be included in the flare GHG emissions calculations. This change 

would not impact reporting of emissions from flares under other Subparts. 

3. Flare Terminology and Operation 

Flares can be used to dispose of gases that are consistently generated and have relatively constant 

flow, but they are often used to dispose of gases that are generated due to transient operations, 

such as batch processes or SSM events. Consequently, flares may often operate with little to no 

waste gas flow. To ensure oxygen does not enter into the flare stack or flare gas header system, 

some amount of gas is added to the flare header or to the base of the flare stack; this gas is often 

referred to as purge or sweep gas. To ensure the waste gases exiting the flare ignite once they do 

mix with ambient air, flares operate with pilot flames at the perimeter of the flare tip. Large 

diameter flares may have numerous pilot flames at different points around the perimeter of the 
Technical Memorandum – Proposed Changes to Flare Pilot Gas Reporting Requirements 

July 10, 2015 

2 

flare tip. Many industries, such as refineries and petrochemical facilities are required to 

continuously operate the flare pilots, regardless of waste gas flow. Other industries may use 

flares with automated flare ignition systems, in these systems, a spark ignition source is fired 

(similar to an automatic lighting system for a gas grill) to ignite the flare gas either at regular 

intervals or during times when a significant flare waste gas flow is detected. 

To clarify the discussion of flares and flare emissions sources, we will use the following 

terminology to describe potential sources of flare GHG emissions. 

• Flare waste gas. This is the excess gas generated at the facility that needs disposal. This 

is the gas the flare was installed to treat/destroy. 

• Purge/sweep gas. This is the gas that must be added to the flare header system or to the 

base of the flare in order to prevent oxygen ingress during periods of low waste gas flow. 

• Pilot gas. This is the gas used to maintain a pilot flame at the flare tip. 

The amount of purge/sweep gas needed is dependent on the complexity of the flare gas header 

system and the flare diameter and tip design. Flares generally require at least 0.1 to 0.2 foot per 

second (ft/s) flow velocity at the tip to prevent oxygen ingress (Bai et al., 2014), but can be 

significantly higher for flares with complex header systems. For a 2 foot diameter flare, this 

translates to a minimum flow of 1,100 to 2,200 cubic feet per hour or 1 to 2 million British 

thermal units per hour (mmBtu/hr). The recommended minimum heat rate for industrial flare 

pilots is 0.045 mmBtu/hr (Bellovich et al., 2007), suggesting flare pilots would not contribute 

significantly to emissions even during long periods of standby (no direct waste gas flow). 

4. Relative Impact of Excluding Flare Pilot Gas from GHG Reporting Requirements 

GHG emissions from flare pilots is expected to be small in most cases. In efforts to estimate the 

relative contribution of flare pilot gas to flare emissions, we developed engineering estimates of 

flare emissions specifically for petroleum refineries as additional information is available for 

petroleum refinery flares through a detailed information collection request (ICR) of petroleum 

refineries conducted by the EPA (EPA, 2011). 

Flare Pilot Gas Usage Estimates 

As noted previously, the recommended minimum heat rate for industrial flare pilots is 

approximately 0.045 mmBtu/hr (Bellovich et al., 2007). For costing purposes, the EPA 

recommends an average flare pilot use rate of 70 scf/hr (EPA, 2002), which is essentially 

0.07 mmBtu/hr. The number of pilot burners required depends on flare size and, possibly, on 

flare gas composition and typical wind conditions. The EPA provides the following general 

guide for the number of pilot burners by flare size (EPA, 2002): 

• 1 pilot for flare tip diameters of 1 to 10 inches 

• 2 pilots for flare tip diameters of 12 to 24 inches 

• 3 pilots for flare tip diameters of 30 to 60 inches 

• 4 pilots for flare tip diameters greater than 60 inches 
Technical Memorandum – Proposed Changes to Flare Pilot Gas Reporting Requirements 

July 10, 2015 

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The GHGRP does not collect data on flare tip diameter, but this information was collected as 

part of the refinery ICR (EPA, 2011). We developed model flares of 0.5, 1.5, 3.5, and 6 feet to 

correspond with the flare tip diameter ranges used to estimate the number of pilots. We used the 

refinery ICR data to develop a proportionate number of flares for each size range. In the ICR 

database, there were numerous flares with flare diameters well over 12 feet. In reviewing these 

data, we expect that several facilities reported their flare diameter in “inches” rather than in 

“feet”. Consequently, we divided the reported flare diameter by 12 for all flares that reported a 

diameter of 9 feet or more to correct data that appeared to be reported in “inches” rather than in 

“feet.” We also evaluate the size distribution of flares that were reported to be “routine flow” 

flares and compared that to the size distribution of flares used only for startup, shutdown, or 

upsets. The size distribution was very similar, so we developed a single weighting factor for each 

model flare size: 10 percent very small, 36 percent small, 52 percent medium and 2 percent large 

for model flare diameters of 0.5, 1.5, 3.5 and 6 feet, respectively. 

The CO2 emissions from flare pilot usage was estimated using an average heat rate of 

0.07 mmBTU/hr per pilot times the number of pilots for a given model flare size times the 

number of flares of that size times the default flare CO2 emissions factor of 60 kg/mmBtu. 

Flare Purge Gas Usage Estimates 

As described in the Section 3 of this memorandum, flares generally require at least 0.1 to 0.2 ft/s 

flow velocity at the tip to prevent oxygen ingress (Bai et al., 2014). There are flare tip designs 

with internal gas seals or “molecular seals” (also known as density or labyrinth seals) that allow 

much lower flare tip velocities ranging from 0.001 to 0.04 ft/s (EPA, 2002; John Zink, 1998). No 

information is available from the GHGRP regarding the prevalence of use of molecular seals. A 

molecular seal system adds weight to the flare tip, requiring more structural support for the flare 

stack (Coburn, 2013). The molecular seal and additional structural support can add significantly 

to the costs of the flare. Consequently, molecular seal systems are most commonly used for 

larger flares that were originally expected to have a limited hours of waste gas flow (i.e., flares 

where the monetary savings in purge gas usage reductions exceeds the additional capital costs of 

the flare with a molecular seal). 

The GHGRP does collect data on flare service type (general facility flare, unit flare, emergency 

only flare or back-up flare). We expect that the larger flares with no routine gas flow 

(emergency only or back-up flares) would be the most likely to use molecular seals to reduce 

fuel gas or natural gas consumption. We consider it very unlikely that a molecular seal would be 

used for a 6 inch (0.5 feet) diameter flare, but for all other model flare sizes, we assumed a purge 

flare tip velocity of 0.02 ft/s for emergency only and back-up flares. For all general facility and 

unit flares and for the very small emergency flares, we used a purge flare tip velocity of 0.15 ft/s. 

The CO2 emissions from flare purge gas was estimated as the product of the cross-sectional area 

of the flare, the flare tip velocity, the number of flares of that size, an assumed heat content of the 

gas (1,000 Btu/scf), and the default flare CO2 emissions factor of 60 kg/mmBtu. 

Refinery Flare CO2 Emissions 

The GHGRP data for the 2013 reporting year (RY2013) was queried to determine the number of 

flares and the corresponding CO2 emissions. Table 1 provides a summary of the results as well 
Technical Memorandum – Proposed Changes to Flare Pilot Gas Reporting Requirements 

July 10, 2015 

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as a summary of the CO2 emissions estimated for each of the model flares. The proportion of 

emissions from flare pilot gas was compared to the emissions estimated for flare purge gas and to 

the total flare emissions reported in Subpart Y. As seen in Table 1, for larger flares, CO2

emissions for purge gas are significantly higher than from pilot gas. Overall, including flare pilot 

gas may increase the total flare emissions reported under Subpart Y by 1 to 2 percent. 

Table 1. Summary of CO2 Emission Estimates for Flares and Flare Pilot and Purge Gas 

Model Flarea

Flare 

Diameter 

(ft) 

Number of 

Flaresb

CO2

Emissions 

From Pilot 

Gas Usage 

(Mg/yr) 

CO2

Emissions 

From Pilot 

Gas Usage 

(Mg/yr) 

Pilot Gas 

CO2 Ratio to 

Total Pilot 

and Purge 

Gas CO2 (%) 

Very small, routine flare 0.5 38 1,398 2,118 40% 

Small, routine flare 1.5 136 10,007 68,212 13% 

Medium, routine flare 3.5 197 21,744 537,949 4% 

Large, routine flare 6 8 1,177 64,199 2% 

Very small, emergency flare 0.5 8 294 446 40% 

Small, emergency flare 1.5 30 2,208 2,006 52% 

Medium, emergency flare 3.5 44 4,857 16,020 23% 

Large, emergency flare 6 2 294 2,140 12% 

Model Flarea

CO2

Emissions 

From Pilot 

Gas 

Usage 

(Mg/yr) 

CO2

Emissions 

From Pilot 

Gas 

Usage 

(Mg/yr) 

Pilot Gas 

CO2 Ratio 

to Total 

Pilot and 

Purge Gas 

CO2 (%) 

Total CO2

Emissions 

Reported in 

RY2013 

(Mg/yr) 

Pilot Gas 

CO2 Ratio to 

Total 

GHGRP CO2 

(%) 

All routine model flares 34,327 672,478 4.9% 2,639,710 1.3% 

All emergency model flares 7,653 20,612 27.1% 162,323 4.7% 

All model flares 41,980 693,091 5.7% 2,802,033 1.5% 

a

 Routine flares are those designated as either general facility flare or unit flare in the GHGRP; emergency flares are 

those designated as either emergency only flare or back-up flare in the GHGRP. 

b Number of flares reported in the GHGRP classified as routine (379) or emergency (84) times the proportion of 

flares of that size based on the data reported in the refinery ICR (EPA, 2011). 

5. References 

Bai, Yong-Zhong, P. Wang, and J. Jiang. 2014. “Determination of the minimum safe purge gas 

flow rate in flare systems with a velocity seal.” Chemistry & Technology of Fuels & Oils, 

Vol. 49 Issue 6, p. 474. January. 

Bellovich, J., J. Franklin and B. Schwartz. 2007. “Flare pilot system safety.” Process Safety 

Progress, Vol. 26, No. 1, p. 10-14. March. Available at http://www.johnzink.com/wpcontent/uploads/Flare-Pilot-Safety.pdf. 
Technical Memorandum – Proposed Changes to Flare Pilot Gas Reporting Requirements 

July 10, 2015 

5 

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

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

RTI International, to Andrew Bouchard and Brenda Shine, EPA/OAQPS/SPPD. Dated 

February 18, 2013. Docket ID No. EPA-HQ-OAR-2010-0682-0219. 

EPA (U.S. Environmental Protection Agency). 2002. EPA Air Pollution Control Cost Manual. 

Sixth Edition. EPA/452/B-02-001. Office of Air Quality Planning and Standards, Research 

Triangle Park, North Carolina. Vol. 49 Issue 6, p. 474. January. 

EPA (U.S. Environmental Protection Agency). 2011. “Non-CBI compiled database of all 

Component 1 submittals” based on Refinery Information Collection Request (ICR) for 

Calendar Year 2010. Available at: http://www.epa.gov/ttn/atw/petref/petrefpg.html. 

John Zink Company. 1998. “Purge Conservation Systems.” Bulletin 5085B 2M298. Available 

at http://www.johnzink.com/wp-content/uploads/flar_purge.pdf.