Conference Information
American Fuel & Petrochemical Manufacturers (AFPM) Environmental Conference, October 20
– 22, 2013, New Orleans
Title
IR Technology for Automatic Flare Steam Control
Authors
John D. Ditmore – CVR Energy, Inc.
Troy M. Boley – Sage Environmental Consulting, L.P.
Herman Holm – Sage Environmental Consulting, L.P.
Derick G. Kopp – Sage Environmental Consulting, L.P.
Section No. 1
Title: How is CVR Energy, Inc. using infrared technology to automate flare steam control?
Text: In October 2011, CVR voluntarily undertook an initiative to install a Williamson Flare
Monitor Model # FM-17-N4 on their Alky, Coker, and Cold Pond flares, each of which is a
steam-assisted flare. The purpose of installing the instrument was to assess the potential for
automatically controlling the flow rate of steam to the flare tip based upon infrared analyses of
the combustion zone.
The Williamson Flare Monitor uses infrared technology to analyze the combustion zone.
Other manufacturers of similar infrared devices that could be used for automated flare steam
addition include the following:
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Powertrol SLX-302 Flare Monitor
LumaSense E2T Quasar Flare Monitor
Section No. 2
Title: What’s the big deal about controlling the flow rate of steam to a flare?
In theory, complete combustion of hydrocarbon molecules is a simple chemical process.
Complete combustion of a hydrocarbon molecule occurs when the proper amount of oxygen is
present within the combustion zone. The products of complete combustion of a hydrocarbon
molecule from any combustion source are carbon dioxide (CO2) and water vapor (H2O).
Complete combustion of a hydrocarbon molecule.
In reality, attaining complete combustion in a flare is quite complicated. Many dynamic factors
affect the degree at which combustion occurs. The most important factors are the amount and
distribution of oxygen within the combustion zone. For complete combustion to occur, at least
the stoichiometric amount of oxygen must be provided in the combustion zone. The theoretical
amount of oxygen required is proportional to the molecular weight of the gas burned.
Steam is often injected at the flare tip to create turbulence and increase momentum within the
combustion zone, which entrains air within the flame. This results in an increased mixing rate of
oxygen, which leads to increased combustion efficiency. However, the flow rate of steam must
be carefully controlled in order to maintain optimal combustion efficiency because both an
excess amount of steam and lack of steam can lead to poor combustion efficiency.
Poor combustion efficiency as a result of excess steam results because the flame is cooled below
optimum temperatures for combustion. Simply put, pouring water on a flame causes the flame to
go out. An over-steamed flare is characterized by a lack of a visible flame present at the tip.
Poor combustion efficiency as a result of insufficient steam results because the flame is oxygendeprived. The gases entering the base of the flame are preheated by the heat of the combustion
zone, causing larger hydrocarbon molecules to crack and form hydrogen, unsaturated
hydrocarbons, and carbon. The carbon particles may escape further combustion and thus cool
down to form soot or smoke. Olefins and other unsaturated hydrocarbons may also cool and
polymerize to form larger molecules that partially crack, in turn forming more carbon (soot). An
under-steamed flare is characterized by a trail of smoke downwind of the flame.
There are multiple methods by which to automate the flow of steam to ensure that optimum
combustion efficiency is maintained. A device, such as the Williamson FM-17-N4 or similar
flare monitor devices, may be used to assist in the steam flow rate control scheme.
Section No. 3
Title: How does the Williamson Flare monitor FM-17-N4 work?
Infrared technology is widely used to determine composition of gases, liquids, and solids by
relying upon the characteristic absorption of certain wavelengths that are unique to individual
molecules. When molecules are exposed to an infrared source, the molecules selectively absorb
radiation of specific wavelengths, producing an infrared spectrum, which can be analyzed to
determine the type and concentration of molecules that comprise a gas volume, such as a flame
envelope.
Characteristic absorption of methane at various wavelengths (Image courtesy of www.photonics.com).
Based upon the same principle, a single wavelength can be transmitted from an infrared source,
and the concentration of the molecule that corresponds with the absorption of that wavelength
can be determined.
The Williamson Flare Monitor uses dual-wavelength infrared technology to monitor the ratio of
carbon to available oxygen deep within the hot flare flame. This ratio can be correlated to
combustion efficiency and may be used to adjust the flow of steam to steam-assisted flares.
Williamson FM-17-N4 specifications.
Output Scale (ratio
of carbon to oxygen)
Accuracy
Repeatability
Optical Resolution
Maximum Distance
Response Time
0-2000 (Dimensionless)
0 – 700 = Over-steamed condition
700-1295 = Optimum combustion efficiency
1300-2000 = Smoking condition
+/-0010 at 1292
+/-0005 at 1292
D/17
1800 feet (550 m)
Adjustable 0.1 sec to 24 sec (1 sec default
setting)
The wavelength that is transmitted to determine the concentration of carbon within the
combustion zone is based upon a proprietary determination of the hydrocarbon molecule that
most closely represents the absorptive characteristic of that wavelength by all hydrocarbon
molecules.
Because the oxygen molecule (O2) is a homonuclear diatomic molecule, it is not infrared active,
meaning that it does not absorb infrared radiation. Therefore, it is not possible to directly
measure the concentration of O2 within the combustion zone. The Williamson Flare Monitor
determines available oxygen by the surrogate detection of carbon monoxide (CO) and hydroxide
(OH) within the combustion zone.
Based upon the ratio of carbon to available oxygen, the Williamson Flare Monitor may be able to
determine whether the flare is over-steamed or when the flare is smoking. The device operates on
a scale (ratio) of 0 – 2000. For a generic flare, good combustion occurs between approximately
700 and 1300. A value greater than approximately 1300 indicates that the carbon-to-oxygen ratio
is high, which is characteristic of a smoking flare since the flame is oxygen-deprived.
Conversely, a value less than approximately 700 indicates a low ratio, which is characteristic of
an over-steamed flare.
While specific details regarding the operation of similar flare monitor devices may be different,
the other devices operate under the same general principle of infrared absorption of specific
wavelengths.
A smoking flare has a high carbon-to-oxygen ratio, while an over-assisted flare has a low
carbon-to-oxygen ratio.
Section No. 4
Title: How does the flare monitor control the steam flow rate?
Text: The flare monitor is incorporated into the refinery’s Distributed Control System (DCS) that
controls the flow rate of steam. The flare monitor continuously analyzes the carbon-to-oxygen
ratio and instantaneously stores the data in the DCS. The DCS calculates a rolling average
carbon-to-oxygen ratio, which provides a signal to the steam control valve to automatically
increase or decrease the flow rate of steam.
For example, if the averaged carbon-to-oxygen ratio is low, the flow rate of steam will be
reduced until the averaged carbon-to-oxygen ratio is within the acceptable range. Conversely, if
the carbon-to-oxygen ratio is high, the flow rate of steam will be increased until the averaged
carbon-to-oxygen ratio is within the acceptable range.
However, if the flare monitor is not responding correctly or in a timely manner, a control room
operator may manually override the flare monitor control loop with a remote switch in the
control room. The steam addition system defaults to the automated flare monitor control loop
after a certain period of time has passed following the initial switch into manual control mode.
*Note: Although the flare monitors are installed and integrated into the DCS system, CVR is
continuing to validate the operation of the instruments prior to full implementation into the
automated steam control system.
Section No. 5
Title: What potential challenges exist?
Text: As with the implementation of any instrumentation or analytical system, there are certain
challenges that must be addressed in order to maximize the system’s utility. In addition, when
control logic, such as automated steam addition, is based upon that instrumentation or analytical
system, additional challenges may be encountered. Implementation of infrared technology to
automate steam control may present one or more of the following challenges:
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Recent studies using infrared technologies have demonstrated that the temperature of the
combustion zone may affect the absorption of the transmitted wavelengths for a known,
constant gas composition. For the most accurate utilization of infrared technology in a
combustion zone, the results must be temperature-corrected. A temperature correction
technique for infrared technology has been recently patented by IMACC.
Each hydrocarbon molecule requires the stoichiometric amount of oxygen for complete
combustion to occur; therefore, the optimum carbon-to-oxygen ratio at which complete
combustion occurs is different for each hydrocarbon molecule. Because the flare monitor
devices do not speciate individual hydrocarbon molecules, the carbon-to-oxygen ratio at
which optimum combustion occurs for any given gas composition can, at best, only be
approximated.
The advanced techniques by which to control combustion efficiency (see recent Flare
Consent Decrees) require a frequent calculation of the net heating value of the
combustion zone, which necessitates a steam flow rate. However, because the flare
monitor devices measure a relative value (a ratio), it is difficult to infer or correlate a
value related to steam flow rate that can be used to calculate the net heating value of the
combustion zone.
Depending on the distance and angle between the instrument and the flare tip, the “view”
from the instrument may not always be centered on the combustion zone if the direction
and velocity of the wind cause the flare stack to sway or cause the physical flame to
“bend” out of view of the instrument. Additionally, physical objects, such as birds,
cooling tower exhaust, or tall or elevated construction equipment may completely
obstruct the instrument’s “view” of the combustion zone.
Section No. 6
Title: How do you validate infrared technology for automated steam control?
Validation of a flare monitor device should be conducted prior to fully implementing and relying
upon the instrumentation system. Identifying the optimal carbon-to-oxygen ratio that yields the
highest combustion efficiency can be determined with a Passive Fourier Transformation Infrared
(PFTIR) test using a typical gas composition for that flare. The PFTIR testing can clearly
demonstrate the correlation between the reported carbon-to-oxygen ratio and combustion
efficiency of the combustion zone. Additionally, analysis of the steam flow meter data and/or the
steam flow control valve position during the PFTIR testing can identify whether the flare
monitor device is properly operating the steam control valve. Once these site-specific tests have
been completed, the flare monitor device may be used as part of the flare instrumentation suite.
PFTIR testing can be conducted to develop site-specific validation for flare monitor devices.
CVR has not yet completed their validation of the Williamson Flare Monitor to determine
whether the flare monitor device can be used as a substitute for a steam flow meter. However,
there is sufficient data to indicate that the device may be used to supplement the steam flow
meter and steam addition control logic as a redundant method to verify that the flare is neither
over-steamed nor smoking.