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flame. The number of jets and included angle of these jets is important in
achieving stability and flame shape.
2.4.7 Gas Pilot
Due to the higher air velocities, the pilot has to operate under more arduous
conditions than on a natural draught burner. The type of pilot which has
usually been fitted has its own independent air supply so that the air/gas ratio
will remain constant no matter what the burner load. The stabilizing system
inside the pilot tip normally requires ignition from an internally placed portable
electric igniter to ensure the stabilizing jets ignite. Recently, some burner
manufacturers have developed suitable inspirating pilots, which have been
proven to remain alight with Register Draught Losses (RDLs) up to 250 mm
(10" wc).
2.5 High Intensity Burners
Two of the features, which improve combustion, are rapid uniform air fuel
mixing and a high temperature environment. The high intensity burner sets
out to try and achieve these features by containing the flame in a small
refractory lined chamber (combustor). To promote rapid air fuel mixing the
combustion air is either swirled at the point of entry or introduced through jets
arranged to promote a vortex within the chamber.
This type of burner, when operating correctly, will combust fuel oil and gas at
very low excess air and with low stack solids levels. Unfortunately, it is very
susceptible to coking problems and combustor refractory life can be quite
short. These problems are increased by the use of heavier fuel oils and oil
residues from refinery processes such as visbreaking.
A well-designed high intensity burner, when operating correctly, is a very
impressive sight. However as the burner is very susceptible to fuel and
operating condition changes and unless the manufacturers can improve their
designs, new installation using this type of burner will be rarely seen.
Examples of two high intensity burners, Gulf Vortomax and Urquhart, are
shown in Figure 2.14 and 2.15 respectively.
Figure 2.14 Gulf Vortomax High Intensity Burner
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Figure 2.24 Peabody LNOG Staged Air burner
(3) Exhaust Gas Recirculation
It has been shown in fundamental studies that exhaust gas recirculation is
more effective in reducing the thermal NOX component than fuel derived NOX
emissions. For this reason, the technique is more suited to gas fired
installations where all the NOX emissions are thermally derived. The CCT
(Combustion Contract Technology, now part of Sterling Process Engineering)
FGI series of burner (Figure 2.25) uses a venturi in the primary air stream
inside the windbox to inspirate the mainly inert combustion products from the
hearth area of the furnace. These recirculated products are then mixed with
the primary air before it passes through the swirler. The secondary air, which
is kept separate from the primary air inside the windbox, is introduced through
the annulus between the swirler and the quarl. With this type of burner, it is
essential that there is an effective seal between the segments of the quarl in
order to avoid the possibility of inspirating non-vitiated air from the secondary
air stream. It is also necessary for the quarl to have a large diameter top
surface in order to ensure that the combustion products are drawn from a
position remote from the secondary air annulus. The burner can also be fired
as a conventional non-recirculating burner by raising the conical venturi to
seal off the internal recirculation ducts.
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Figure 2.26 John Zink SMR InfurNOx Burner
2.9.2 Performance of Low NOx Burners
Conventional process burners are designed to create an intense and turbulent
flame in order to achieve good flame stability and complete burnout of the
fuel. The intense flame and rapid mixing of the fuel and air create the ideal
situation for high NOX and low combustible emissions (particulates, smoke,
CO, hydrocarbons etc.). In order to achieve low NOX, the mixing of the fuel
and the air has to be controlled so that the peak flame temperature is reduced
and the oxygen availability is restricted. However, these methods have an
adverse effect on flame stability and fuel burnout and if taken to the extreme,
will result in the loss of the flame. Therefore, it should be evident that Low
NOX burners represent a compromise between acceptable emissions and
flame stability. Generally speaking, any modifications that are made to a
burner to reduce NOX emissions will have an adverse effect on flame stability
and combustible emissions. Environmental legislation invariably calls for low
NOX and low combustibles and whilst either of these parameters is easily
achievable separately, to achieve them concurrently with acceptable flame
stability is difficult. This is particularly so when the burner is required to
operate over a wide range of fuel types and quality.
Despite these limitations, the burner manufacturers have made significant
strides forward in recent years and can offer burners that will operate safely,
efficiently and with reduced emission levels. Figure 2.27 shows the relative
performance of commercially available Low NOX and conventional burners
when firing natural gas and heavy fuel oil respectively. Figure 2.27 also shows
that despite all the advances made in burner design, it is still not possible to
achieve the EU Large Combustion Plant Directive for new equipment on NOX
and particulate emissions (450 mg/m3 NOX, 50 mg/m3 particulates)
concurrently when firing a "normal" (i.e. 40 cSt @ 100 °C) heavy fuel oil.
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6. Combustion
6.1 The Combustion Reaction
Combustion is, in general terms, an exothermic oxidation reaction of fuel and
air during which physical processes such as energy, mass and momentum
transfer are occurring simultaneously. The extent of completion of the
exothermic reaction depends on the interaction of the above processes in
space and time.
The combustion of gaseous fuels is referred to as homogeneous and the
combustion of solid and liquid fuels as heterogeneous. The phenomena
occurring during homogeneous combustion are less complex than those
occurring in heterogeneous combustion. For both oil and gas, the degree of
mixing with combustion air is a critical first step in achieving high combustion
efficiencies.
During homogeneous combustion, chemical reaction commences as soon as
mixing takes place. The mixing of fuel with air is explained in terms of the
turbulent motion of the fluid and the density differences between flame and
surroundings. Heterogeneous combustion requires longer burning times and
liquid fuels need to be atomized prior to combustion.
The combustion of fuel oil is shown in Figure 6.1. Fuel oil combustion
commences with the lighter components being immediately boiled off as the
oil droplets leave the atomizer and enter the hot combustion zone. The
remaining heavy residue then undergoes pyrolysis at the high combustion
temperature and it is these pyrolysis reactions, which cause smoke formation.
Poor atomization of the fuel oil will result in oil drip. The residual carbon or
coke remaining after the pyrolysis reaction then undergoes further burn out.
Ideally, the carbonaceous matter should be completely burnt out to reduce the
remaining solids or particulates to the minimum possible i.e. only inorganic
ashes remaining. In practice, particulates from refinery heaters in particular
and also boilers often contain 90% or more carbon material.
Figure 6.1 Combustion of a Fuel Oil
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The effects on burner performance are now illustrated with examples for the
following burners:
Burner A
- Natural Draught
Burner B
- Forced Draught
Burner C
- Forced Draught
Burner D
- Forced Draught.
Data sheets for these burners are provided in Table 11.1
11.3.1 Flame Length
11.3.1.1 Effect of Liberation
The visible flame size increases with increasing liberation although not
proportionally. This can be seen from the table above e.g. for Burner A a 66%
increase in liberation results in only a 33% increase in visible flame length
when firing heavy fuel oil. The disproportionality is due to the effect of a
change in the RDL.
Visible flame length is usually used as a characteristic because flame volume
is difficult to define due to the shape of the flame envelope. See Figure 11.2
which shows an oil flame. The maximum flame length and diameter are
normally given so that the possibility of tube impingement and interference
with flames issuing from adjacent burners can be avoided.
Figure 11.2 An Oil Flame
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Figure 15.4 Good gas firing
Figure 15.6 Partially blocked
atomizer on oil burner
Figure 15.5 Good oil firing
Figure 15.7 Oil flame lifted from
burner tip
Figure 15.8 Oil burner ‘sparkler’
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