Hydrocarbon Processing - March 2021 - 58

Heat Transfer
the tile. Those tips may inject fuel up
along the outside of the tile to delay mixing for NOx reduction,4 and inject some
fuel through holes in the side of the tile
into the main or primary flame zone. The
holes may be straight through the tile or
they may be at an angle to impart some
swirl to the flame. In either case, they
are used to help mix the fuel and air. A
burner tile can have secondary air outlets
built into the tile. The primary air comes
through the middle of the burner throat.

FIG. 3. A burner tile with " teeth " for flame
stabilization.

FIG. 4. Example of an inner regen tile
for a combination burner capable of firing on
gas or liquid fuel.

FIG. 5. Burner with a rectangular tile.

MARCH 2021 | HydrocarbonProcessing.com

The purpose of the secondary air is for
staging to reduce NOx emissions.
Maintain ignition. An important safety
consideration in any combustion system is
to maintain ignition when fuel is flowing.
Otherwise, that fuel may ignite somewhere
else, which can prove to be dangerous.
The tile ledges may be designed to
help anchor the flame and sustain combustion, as previously described for mixing. Many process burners are diffusion
or raw gas (also known as nozzle-mix)
burners, which means the fuel and air
are mixed at the burner outlet. This is in
contrast to premix burners, which mix
some or all of the fuel and air inside the
metal parts of the burner. While some
advantages of premix burners exist, a significant disadvantage is the possibility of
flashback. The tile used in raw gas burners is specifically designed to mix the fuel
and air at the outlet to anchor the flame
throughout the design firing rate range.
The visible flame may start a short distance away from the tile, but it should not
be a very long distance, regardless of the
firing rate. If the gap between the end of
the tile and the start of the visible flame
is too long, then the condition is referred
to as a " lifted flame. " Lifted flames are undesirable, as they are much more susceptible to lifting off completely, causing the
flame to be extinguished. If fuel continues
to flow to that burner and components
in the heater-such as the burner tile or
heater walls-are above the autoignition
temperature for the fuel/air mixture, then
a re-ignition of the mixture is likely, which
can lead to an explosion. An obvious sign
of an unstable flame is pulsing or huffing (where the flame is bouncing up and

down). This is a very dangerous condition that must be corrected immediately.
FIG. 3 shows an example of a burner tile
with a specially-designed edge that has the
appearance of teeth, for flame stabilization.
Another important function of the tile
when firing liquid fuels is to produce a hot
chamber where the liquid fuel is atomized.
The higher temperatures help promote
and maintain liquid vaporization. Some
burners are designed to recirculate hot
combustion products inside the tile to aid
in the vaporization. These tiles are sometimes called " regen tiles, " which is short
for " regenerative tiles " (FIG. 4).
Mold the flame. The shape of the tile
outlet helps determine the cross-section
of the flame: round tiles make round
flames and rectangular tiles make rectangular flames (FIG. 5). These are by far the
two most popular tile cross-sections for
process burners.5
Minimize emissions. In some burner

designs, the tile helps reduce NOx (e.g., a
burner tile where secondary fuel tips are
in the tile wall). The purpose is to stage
the secondary fuel into the flame to reduce hot spots, reducing NOx emissions.
FIG. 6 shows an advanced burner tile
designed to help minimize NOx formation. The tile has two different alternating
slopes with dividers in between. Primary
tips inject some fuel through the tile into
the primary flame zone inside the tile.
Those primary tips also inject some fuel
along the outside of the tile. The staged
tips only inject fuel along the outside of
the tile. The overall purpose of the design
is to minimize the amount of fuel in the
primary zone and maximize the amount
of fuel in the secondary flame zone. This
fuel staging reduces hot spots in the flame,
minimizing NOx formation.
Another important aspect of this tile
design are the holes going through the

FIG. 6. Advanced burner tile design to help
minimize NOx formation.

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Hydrocarbon Processing - March 2021

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