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July
2020
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HYDROCARBON
ENGINEERING
T
he process of flaring is a critical safety and quality
procedure used in refineries and the chemical
processing industry. Flaring occurs most often during
start-up, shutdown, and process upsets such as power
failures. During process operations, excess flammable and
hazardous gases – comprised primarily of hydrogen, methane,
and other hydrocarbons – are emitted by different types of
plant equipment. These excess gases that cannot be recovered
or recycled are burnt, resulting in the emission of carbon dioxide
(CO
2
) and water vapour as an alternative to the direct release of
the hydrocarbons into the atmosphere. The two main
requirements of flares are the complete combustion of the
gases and to ensure that these emissions are not visible.
US Environmental Protection Agency (EPA) Rule 40 CFR
63.670 outlines the requirements for the operation and
monitoring of such flares to ensure complete combustion and
simultaneous non-visible emissions. In order to ensure complete
combustion, a proper fuel/oxygen ratio and a compositional
heating value of the gas must be maintained at 270 Btu/ft
3
. To
ensure that the emissions off the flare are non-visible, steam is
injected into the process gas mix prior to flaring. However, the
addition of steam also results in decreasing the overall Btu value
of the gas mixture and can result in incomplete combustion if
over-steaming occurs. It is imperative that the Btu value is
closely monitored. EPA Rule 40 CFR 63.670 requires at least one
successful/passing Btu measurement every 15 minutes.
Two of the most common techniques for monitoring the
Btu values of flares have been calorimetry and gas
chromatography (GC). Calorimetry utilises a thermocouple to
measure the change in flame temperature from an incinerated
premixed fuel/process gas stream and correlates the change to
the heating value of the process gas. It is a well understood,
inexpensive, and fast technique. However, it is susceptible to
sulfur poisoning and has difficulty measuring hydrogen (H
2
) in
the presence of carbon monoxide (CO), which can lead to the
need for a separate H
2
detector and artificially high Btu values.
Tom Watson, AMETEK, USA,
examines a method to measure component
concentration and monitor Btu values in flaring operations.