Paradigm Shift Regarding Flare Combustion Efficiency
Derick G. Kopp, Sage Environmental Consulting, L.P.
Marcus Herald, Marathon Petroleum Company, L.P.
Troy M. Boley, Ph.D., Sage Environmental Consulting, L.P.
ABSTRACT
Historically, flare testing demonstrated that routine refinery flare operation could safely assume a
98% VOC Combustion Efficiency (CE). The operational parameters associated with flame
stability and required by current EPA regulations to assess the veracity of the CE assumption are
limited. Because of a shift in the awareness of proper flare operation regarding in part the use of
assist gas, a continuous CE of 98% is no longer an automatic presumption. The degree of CE
control cultivated over the past 30 years was based upon a few, simple, fixed parameters.
However, new monitoring and measurement techniques are providing further insight, and an
evolution of operational parameters, for consideration. Due to safety concerns and cost, it is not
practical to continuously measure CE directly from open flame flares. Therefore, testing has
been conducted to determine suitable system parameters to indicate or suggest the degree of CE
in the combustion zone of the flare. A varying degree of correlation to CE exists among these
parameters. For example, combustion zone net heating value (NHVcz) is one parameter that has
been recently demonstrated to closely correlate to CE for steam-assist elevated flares. This paper
is designed to introduce the evolution of flare performance parameters that have been considered
over the past decade.
OVERVIEW
Although measurement of the combustion efficiency of the flare is possible, long-term
implementation of these technologies for continuous monitoring is currently both costprohibitive and impractical. Therefore, it has been necessary to identify predictive or surrogate
parameters which can be correlated to combustion efficiency to indicate the degree of
combustion efficiency at the flare tip. These parameters can be integrated into the control logic
of the Distributed Control System (DCS) for the flare in order to optimize flare performance,
increase combustion efficiency, and potentially minimize flare emissions. These parameters
must address the following factors, which have been identified as critical components to the
combustion efficiency of the flare: over-steaming, excess aeration, flare gas composition, high
cross winds, and flame lift-off.
Many surrogate parameters have been introduced since the mid-1980s. These parameters can be
grouped into three broad categories and include the following:

Tip velocity-based parameters
o Tip velocity, and
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

o Momentum flux ratio (MFR).
Assist gas-based parameters
o Steam-to-vent gas ratio (S/VG), and
o Stoichiometric air ratio (SR).
Heating value-based parameters
o Static net heating value (NHV) calculated in the flare header, and
o Lower flammability limit (LFL) and Dynamic NHV calculated at the flare tip.
The current federal regulations for flares promulgated in 40 CFR 60.18 and 40 CFR 63.11, which
were derived from studies conducted in the early- to mid-1980s, provide no clear guidance or
restriction on the utilization of assist gas. Current United States Environmental Protection
Agency (USEPA) flare regulations in §60.18 and §63.11 include basic requirements generally
for two parameters, static flare gas heating value and tip velocity. Flare design guidance from
industry trade groups and professional organization in the 1990s and early flare Consent Decree
(CD) requirements (mid- to late- 2000s) included recommendations and limitations on the use of
assist gas-based parameters. Most notably, a continuously changing combustion zone heating
value-based parameter is explained within the most recent flare Consent Decrees. Furthermore,
the inclusion of an in-depth analysis of a variable combustion zone heating value-based
parameter in an April 2012 publication by the USEPA Office of Air Quality Planning and
Standards (OAQPS) is an indication that such a complex scheme to analyze and regulate flares
using a variable parameter may be included in future rulemaking for flares.
Discussion
Tip Velocity-Based Parameters
Tip velocity is one of the two parameters promulgated in the original flare regulations.1,2 Tip
velocity is the volumetric flow of the flare gas divided by the cross-sectional area of the flare tip.
This parameter addresses the concern of flame lift-off, which can occur at high exit velocities
when the flame and the flare tip become separated, creating a void directly above the flare tip in
which it appears that combustion does not occur. The parameter is based upon research
conducted by Marc McDaniel4 and John Pohl5 in 1983 and 1984, respectively. However, at the
time, the understanding of the relationship between tip velocity and combustion efficiency was
limited by the boundaries of the data sets which were collected by McDaniel and Pohl. The
regulatory limits for tip velocity published in the Federal Register1,2 are simply the upper bounds
of the exit velocities tested by McDaniel and Pohl, at which optimal combustion efficiencies
were still being observed. The research did not indicate how combustion efficiency would be
affected at higher exit velocities, and therefore, the body of knowledge regarding the relationship
between tip velocity and combustion efficiency was limited.
MFR is one of the parameters introduced by the recent flare Consent Decrees8,10 and is espoused
by the OAQPS publication, Parameters for Properly Designed and Operated Flares.3 MFR is
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the ratio of the momentum of the flare gas to the momentum of the cross wind. This parameter
addresses the concern of high cross winds which may result in a discontinuous wake-dominated
or wake-stabilized flow at very low MFR. Discontinuous wake stabilized flow appears as a
flame that is immediately adjacent to the exterior of the flare tip body and below the exit plane of
the flare tip such that pockets of the flame are detached from the portion of the flare that is
immediately adjacent to the exterior of the flare tip body, resulting in low combustion efficiency.
The basis for validation of this parameter is research conducted by Gogolek and Hayden.6
Assist Gas-Based Parameters
In recent years, the acknowledgement that assist gas can significantly affect overall CE has
gained wider acceptance. S/VG is the ratio of steam to flare gas and can be calculated on a mass
or volumetric basis. SR is the ratio of air to flare gas, calculated on a mass basis. These
parameters address the concern of over-steaming in steam-assisted flares and over-aerating in
air-assisted flares, each of which results in the dilution of the heating value of the flare gas.
Large amounts of research and data have been collected and compiled regarding these
parameters. The OAQPS publication referenced above3, 2010 Texas Commission on
Environmental Quality (TCEQ) Flare Study Project Final Report7, and flare CDs8,9 have all
published values for S/VG and SR that are expected to indicate the degree of combustion
efficiency at the flare tip. Additionally, the American Petroleum Institute (API) Recommended
Practice 52110 published a table of suggested steam-to-vent gas ratios for a range of gases being
flared, with increasing steam suggested for heavier molecules. While the published values for
S/VG and SR have been demonstrated to correlate to combustion efficiency for simplecomposition gas during research testing7, the complex and dynamic composition of industrial
flare gas prohibits reliance solely on the published static values.
Heating Value-Based Parameters
The NHV of the flare gas in the flare header is one of the parameters promulgated in the original
flare regulations.1,2 This parameter addresses the concern regarding low combustion efficiency
of gases with low calorific value. The original research for this parameter was conducted by
McDaniel.4 From his research, a static value was extracted and published in the Federal
Register.1,2 However, recent testing by the TCEQ has demonstrated that not all gases exhibit the
same combustion profile7, such that not all gases combust with the same efficiency at a static
NHV. Furthermore, the focus of the location at which the NHV is calculated has shifted in
recent years.11 Many gases may be injected into the flare gas stream (steam, nitrogen, pilot gas,
etc.) downstream of the point at which the NHV in the flare header is calculated and may
significantly alter the combustion profile of the flare gas at the flare tip. Both the OAQPS
publication referenced above3 and the recent flare CDs8,9 incorporate a variable NHV parameter
that is calculated at the flare tip. The variable NHV is calculated based upon the individual
lower flammability limits (LFL) of each gas that comprises the gas that is combusted at the flare
tip. This ensures that the NHV is appropriate for the specific gas composition based upon each
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specific gas that is combusted in the flare tip. There is strong evidence that of all the parameters
that have been developed since the mid-1980s, this variable NHV parameter most closely
correlates to combustion efficiency and thus currently provides the best indication of the degree
of combustion efficiency.6 However, operating a flare based upon a variable NHV parameter
requires a significant amount of advanced instrumentation and a robust DCS.
SUMMARY
Many parameters have been either incorporated into federal flare regulations or specific flare CD
requirements over the past 30 years. Parameters that are enforceable only to those with whom
the EPA has negotiated the recent flare CDs may provide greater correlation and control of the
combustion efficiency at the flare tip. Inclusion of these “new” parameters in federal rulemaking
may prove to be that on which the combustion efficiency paradigm of the future is built.
However, it is not yet clear which, if any, of the new parameters will be included in the next
proposed rule making for flares. Research continues to be conducted with respect to identifying
additional parameters upon which to develop the combustion efficiency paradigm to ensure
greater than 98% combustion efficiency at the flare tip.
REFERENCES
1. 51 FR 2701, General Control Device and Work Practice Requirements. January 1986.
2. 59 FR 12430, Control Device and Work Practice Requirements. March 1994.
3. US EPA Office of Air Quality Planning and Standards (OAQPS), Parameters for
Properly Designed and Operated Flares. Report for Flare Review Panel. April 2012.
4. U.S. Environmental Protection Agency. Flare Efficiency Study, EPA-600/2-83-052. July
1983.
5. U.S. Environmental Protection Agency. Evaluation of the Efficiency of Industrial Flares:
Test Results, EPA-600/S2-84-095, July 1984.
6. Gogolek, P.E.G., and Hayden, A.C.S. Efficiency of Flare Flames in Turbulent
Crosswinds. Presentation given to American Flame Research Committee, Maui, HI.
September 2010.
7. The University of Texas at Austin. 2010 TCEQ Flare Study Project Final Report,
DRAFT. PGA No. 582-8-862-45-FY09-04. May 2011.
8. United States of America, and the State of Indiana, and the Sierra Club, Save the Dunes,
The Natural Resources Defense Council, The Hoosier Environmental Council, the
Environmental Integrity Project, The Environmental Law and Policy Center, Susan
Eleuterio and Tom Tsourlis v. BP Products North America Inc. 2:12 CV 207. May 2012.
9. United States of America v. Marathon Petroleum Company LP, and Catlettsburg
Refining, LLC. April 2012.
10. American Petroleum Institution. Guide for Pressure-Relieving and Depressuring
Systems, Publication 521. Fourth Ed. March 1997.
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11. Dickens, Brian. US EPA Flare Activity Overview. Presentation given to National
Petroleum and Refining Association, October 2011.
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