Chemical Engineering November 2014 - 42

Cover Story
Burner throat tiles. Located at
the burner edge are the throat tiles
of the burner. Throat tiles help stabilize
combustion and shape the
flame. The burner tile is usually
shaped so that one section has a
minimum cross-sectional flow area.
This area, called the throat, acts
as a Venturi component in the airstream.
The opening in most burner
tiles is circular. In some cases, the
tile and other components are designed
to produce a non-circular
flame shape. A rectangular-shaped
tile opening can be used to produce
a flat flame, which is needed in
some furnace arrangements.
Swirlers. The function of swirl in
burners is to increase flow turbulence.
Turbulence facilitates the
mixing of fuel and air, and various
elements within the burner can
induce a swirl phenomenon. Some
burners contain tangentially disposed
doors in the air registers,
which increase turbulence. Other
burners contain axially disposed
spin vanes to impart swirl in the
combustion air. Other burners may
use adjustable swirl vanes to increase
turbulence.
Fired equipment
Burners are utilized in fired equipment
in the CPI to combust fuel.
The energy created from combustion
is mainly used to convert water
into steam for various processes,
or to heat up a process to a desired
temperature. The main fired
equipment that utilize burners are
boilers, heaters and heat-recovery
steam generators (HRSGs).
* Fired boilers use forced-draft
burners to combust either oil or
gas to heat water and convert it
to steam. Boiler tubes can contain
water (water-tube boilers),
or there may be flames in the
tubes (fire-tube boilers)
* Heaters use natural-draft burners
that rely on the draft (negative
pressure) in the furnace's
radiant box to induce the air required
for combustion
* HRSGs use duct burners for the
supplementary firing to produce
more steam to satisfy the
operation requirements
Sulfur recovery units
Sulfur recovery units (SRUs)
consist of many different firedequipment
elements, so they
merit special mention here. The
fired equipment and burners
found in SRUs are detailed in the
following section.
Air and acid preheater. Multiple
burners and continuous pilots
are incorporated into air and acid
preheaters. The pilots and burners
are individually supervised,
each with its own flame monitor.
The pilots are lit via integrated
spark igniters.
Reaction furnace. Typically
equipped with one high-intensity
burner, reaction furnaces are an
important element in an SRU. The
high-intensity burner is a forceddraft
burner that requires a relatively
high air-pressure drop. The
burner carries out a thermal conversion
process to convert acid gas
into sulfur. This conversion operates
sub-stoichiometrically, meaning
that there is insufficient air to
allow complete combustion of the
acid gas. The burner is usually ignited
by a high-energy, direct-spark
ignition system. Optical flame
monitors are used to check for the
presence of the main flame.
Reheater burner. Reheater burners
use a single forced-draft, fuelgas
or fuel-gas/acid-gas fired burner
to reheat the process stream above
the liquid-sulfur dewpoint prior to
entering the next catalytic conversion
stage. Burners operate at near
stoichiometric fuel-to-air ratios.
The burner usually ignites by a
high-energy, direct-spark ignition
system. Optical flame monitors are
used to check for the presence of
the main flame.
Thermal oxidizer. Thermal oxidizers
use either natural- or forceddraft
burners to provide the heat
input required to combust the SRU
tailgas in the incinerator chamber
to form sulfur dioxide (SO2). Thermal
oxidizers can use either single
or multiple burners with or without
continuous pilots. Optical flame
monitors can be ultraviolet (UV) or
infrared (IR), and are used to check
for the presence of the main flame.
42 ChemiCal engineering www.Chemengonline.Com noVemBer 2014
FIGURE 4. A burner in a thermal oxidizer
in a sulfur recovery unit combusts
the incinerator gas to form SO2
Figure 4 shows a typical SRU thermal-oxidizer
burner.
Flare system burners
Flare systems form an important
part of the emergency relief systems
in CPI plants, allowing for
safe dissipation and disposal of
certain gases by combustion. The
flare burner's tip can be located
at ground level or elevated on a
support structure. Figure 5 shows
a typical flare burner with an
elevated tip.
There are various types of
flares and burner configurations,
and their applications depend on
the requirements of the process.
An awareness of smoke in the
flare system is especially important
for combustion operations.
The various type of flares include
the following:
* Pipe flares are used where the
gases to be burnt do not create
smoke, or there is no requirement
for smokeless combustion
* Steam-assist flares inject steam
into the flare tip through a
series of nozzles to promote
entrainment of air to improve
combustion and reduce
smoke formation
* Air-assist flare tips introduce
air directly into the flare tip to
improve combustion and reduce
smoke formation
* High-pressure (sonic) flare tips
use the high exit velocity of the
gas to be burnt to entrain air,
improving combustion and reducing
smoke formation
Burn pits. The burn pit of a
flare system is equipped with a
burner to safely combust hydro
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Chemical Engineering November 2014

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