An introduction to fouling in fired heaters: Part three burner fouling
Burner fouling: To keep a fired heater operating optimally it is important to distribute heat as evenly as possible.
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A maldistribution of heat will result in a poor absorbed heat flux profile and local hot spots. Elevated tube metal temperatures can accelerate fouling inside the tubes and reduce coil life. Burner fouling is a frequent cause of heat maldistribution. The most frequent location of burner fouling is in the gas tips. It is mostly reaction type fouling, so it depends on fuel composition and residence time, as well as the local gas tip temperature.
Impact of fuel composition
The main fouling precursors in the fuel are unsaturated components like olefins (i.e., propylene, butene), polyolefins (for example butadiene) and aromatics (benzene, styrene, toluene). Unsaturates form polyaromatics that condense to form aerosols that coalesce into larger droplets. The droplets hit the hot gas tip walls and form coke. The dienes are the most reactive, but olefins are known to cause problems too.
The severity of the fouling problem depends on the concentration of unsaturated components in the fuel, i.e. olefins, poly-olefins, and aromatics.
At first glance, the typical fuel shown in Table 1 does not seem to present a problem; the total concentration of olefins is 6.5 vol%, but this is equivalent to 16 wt%. To put that in perspective, a 10 MMBtu/h burner will see 80 lb/h of C4 olefins. Compare this to the amount of coke that causes problems inside gas tips: it is measured in ounces. The worst component in the fuel in Table 1 is butadiene. The amount of 0.018 vol% does not seem like much but a 10 MMBtu/h burner sees 2000 lb per year of this highly fouling component.
To prevent the formation of the aerosols it is important to keep a fuel gas like this at elevated temperature (>120°F) to prevent aerosol condensation.
Impact of burner type
Conventional process burners usually have a single gas gun positioned inside the air stream. The gas gun is kept cool by the incoming combustion air. Since all the fuel arrives through a single gun, the fuel ports are relatively large. On the contrary, an Ultra-Low NOx (ULN) burner has many gas tips located outside the burner tile in order to entrain and mix as much flue gas into the flame as possible. These tips are much hotter and the fuel ports are smaller than in a conventional burner. Ignition ports on these tips are as small as 1/16 in.
Impact of burner design
We have already seen that the difference between conventional and ULN burner designs has an impact on burner fouling propensity. There are other burner design factors to consider for fouling concerns as well.
The tip metallurgy has been shown to influence catalytic coking. Typical 310 stainless steels (or CK-20) contain 19-22% nickel which promotes catalytic coke formation. Changing the tip material to ceramic, for example, will all but eliminate this type of coking inside the gas tip due to the absence of nickel. The low thermal conductivity of the ceramic material reduces the inside wall temperature reducing coke formation.
Some burner designs use an orifice plate inside the manifold to keep the gas ports above a minimum size. The adiabatic expansion causes a large temperature drop in the fuel and subsequent condensation of heavy components.
Example: In extreme cases, freezing/hydrate formation has occurred in a natural gas fuel containing water, where the double drop caused the burner manifold temperature to drop below 40°F.
Design of the gas tip
The design of the gas riser and tip itself has an impact on the fouling behaviour. For example, an improperly designed gas riser/tip with high residence time or dead zones inside the riser will provide more time and opportunity for aerosols to dehydrogenate and attach to internal surfaces.
Gas tips that have large, exposed surface areas will absorb more radiant heat from the firebox and be much hotter than small diameter tips.
Burners that use heavy oil fuels suffer from substantial fouling when the atomising steam flow is too low, or too cold. Poor atomising results in oil droplets that are too large for efficient volatilisation, which causes a build-up of tar and ash inside the regen tile and on top of cones and swirlers.
Hoar frost/rime ice
Hoar frost and rime ice can be very problematic in cold conditions. The name hoar frost originates from Old English where hoar is defined as ‘showing signs of old age’. It was thought to make trees resemble a white beard though its feathery or hairy appearance. Rime ice occurs when supercooled droplets freeze and attach onto an exposed surface.
Other sources of fouling are dust and sand entrained by the ambient air, refractory or burner noise suppression lining that has come loose from the walls and ceiling, and rust from the upstream fuel piping.
Symptoms of burner fouling
Burner fouling can have severe consequences. In ULN burners, the smallest ports are typically the ignition ports. This means that the fuel ports that are the most important for flame stabilisation are also the most prone to plugging. Plugged gas ports can result in a variety of symptoms:
• Glowing gas tips. Once the flow of the fuel is reduced to such an extent that it stops cooling the gas tip, the tip assumes the firebox temperature and will start to glow. If left untreated, the gas tip will be completely destroyed.
• Reduction of burner tile temperature. The thermal inertia of burner tiles is often used to anchor a flame to the top or a ledge of the tile. A plugged gas port or tip can result in a local dark colour of the burner tile, where it should be bright yellow/white.
• Lifted flame. At locations where ports are plugged and tiles become cold, flames lose their anchoring point and ‘float’ above the tile.
• Flame huffing. The lifted flame may attempt to re-attach itself to the tile or the cone. The repeated resulting changes in flame temperature and air side pressure drop can excite other parts of the burner or even other flames. In extreme cases, the flame may be extinguished.
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