March 2021

Heat Transfer

Getting the most out of your process burner tiles

It is often incorrectly assumed that the burner tile is simply a piece of refractory or firebrick. Nothing could be further from the truth.

Baukal, C., Johnson, B., John Zink Hamworthy Combustion

It is often incorrectly assumed that the burner tile is simply a piece of refractory or firebrick. Nothing could be further from the truth. The burner tile is the “heart” of the burner. Tile that has not been poured correctly, installed correctly or that has not been properly maintained can have a serious impact on burner performance.

Burner tiles, sometimes called quarls, are an important component in burners.1 Tiles are typically made out of some type of ceramic and serve many different purposes, including:

  • Protecting the metal components inside the burner from the heat in the combustion chamber
  • Shaping the flame
  • Radiating heat: these burners are commonly called radiant wall burners (FIG. 1) as they are normally mounted on the side wall of a heater.2
    FIG. 1. A cutaway of a radiant wall burner.

Many potential issues are related to the tile. For example, if the ceramic used to make the tile is improperly cured, the tile may be damaged during startup. If the burner tile is improperly installed, this can adversely affect the operation and performance of the burner, including the emissions. If a burner tile is damaged, burner performance may suffer.

The furnace temperature and composition of the combustion products can affect the materials used in the tile. Burners fired downward, for example in a down-fired reformer, require special mounting considerations. If the tile is oversized, the burner may not operate correctly because the air-fuel mixing and flame shaping may be adversely affected. If the tile is undersized, the pressure drop may be too high and the firing capacity reduced.

These elements are discussed here in detail, as well as general fabrication, installation and troubleshooting issues related to burner tiles.

UNDERSTANDING THE SIX “M”s

Six “M”s generally can be used to describe general burner design principles3 that can be applied specifically to the tile.

Meter the fuel and air. The tiles help meter air flow

The tile throat is sized to allow a certain amount of air into the combustion reaction at a given pressure drop (FIG. 2). If the throat is too small or too large, the burner will not operate with the required amount of excess air. If the tile is too small, then not enough air can be pulled through a natural draft burner, or the pressure drop and power requirements will increase in a forced draft burner.

FIG. 2. A burner tile throat, which creates an orifice to meter the combustion air flow.

It may be tempting to oversize the tile throat in case the burner firing rate needs to be increased at some point in the future. However, an oversized tile reduces the turndown in a natural draft burner. For example, if the tile is sized for a 10 MMBtu/hr-burner capacity and has a 5:1 turndown, then its lowest design firing rate would be 2 MMBtu/hr. If the actual operating conditions today require only 5 MMBtu/hr, then the effective turndown is 5:2 or 2.5:1. Oversizing the tile for a possible firing rate of 10 MMBtu/hr is also detrimental to burner performance because the burner is designed to work optimally at 10 MMBtu/hr. If it is fired at half rate, it may not get as low emissions as it would have at the full design rate, for example.

Mix the fuel and air

Proper mixing of the fuel and air is critical to process burner operation. Burner tiles are often used to help mix the fuel and air using a variety of techniques. One such technique is to have ledges inside and/or at the top of the tile. These ledges are bluff bodies that promote mixing. For example, a ledge at the top of a tile can be used to cause the combustion air inside the tile and the fuel on the outside (from secondary or staged fuel injectors) to form vortices on the ledge to promote mixing. It may be desirable to delay mixing until the end of the burner to prevent flashback. It may also be desirable to reduce nitrogen oxide (NOx) emissions due to the delayed combustion that minimizes hot spots in the flame.

Holes through the side of some tiles help mix fuel and air. Some burner designs have fuel injectors or tips outside 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. 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.

FIG. 3. A burner tile with “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).

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

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

FIG. 5. Burner with a rectangular tile.

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.

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

Another important aspect of this tile design are the holes going through the side of the tile. In addition to the high-speed fuel from the primary tips flowing through those holes, furnace gases are entrained by the fuel and flow into the primary flame zone, as well. That entrainment of furnace gases helps to homogenize the flame temperature and reduce flame hot spots, which helps minimize NOx. The size of those holes is important. If the holes are too small, less furnace gas is entrained and more NOx is produced; if the holes are too big, too much furnace gas could be entrained, which could cause flame instability.

Minimize costs

While many tile shapes could be made, round and rectangular are by far the most common shapes for process burners. They are typically the cheapest to make because the tile molds are simpler to build. Those tile shapes are also usually stronger because they have fewer sharp changes in direction that produce corners, which are more likely to crack.

Tile fabrication, installation and maintenance

Proper tile fabrication is critical to burner performance. Obviously, the shape must be correct, but the actual fabrication process is important, as well. For example, if the ceramic is made from a water-based slurry, it must be slowly dried to prevent steam from rapidly forming inside the mold, which could cause the ceramic to spall apart. If the tile is improperly dried prior to installation in a heater, a rapid startup could also cause steam formation in the tile to spall the refractory. There are chemically-base ceramic materials that do not need to be slowly dried because of the lack of water in the ceramic.

Burner tiles will not operate properly if they are installed incorrectly.6 Some burner tiles consist of four segments or sections that make it easier to lift the tile into position without the use of a forklift. It is important that enough—but not too much—mortar is applied between the segments. The burner drawings and manual should be followed to make sure the proper tile throat area is maintained. Tiles must be properly aligned to allow the design combustion air flowrate.7

Tile maintenance is relatively simple: ensure that any damaged tiles are repaired or replaced. FIG. 7 shows some tiles that have been damaged beyond repair. FIG. 8 shows a tile that has been coated with catalyst. That coating reduces the burner outlet, which reduces the air flow through the tile for a given draft level. Some cracks in the tile are evident, as well.

FIG. 7. Damaged (left) regen tile and (right) burner tiles.
FIG. 8. Catalyst buildup on a regen tile.

Troubleshooting and takeaways

Burner tiles usually degrade over a period of time, although they can become damaged suddenly, as well. For example, refractory falling out of the roof onto a tile can cause damage. Operators should examine burner tiles on a regular basis during heater operation. Burners can be operated with damaged tiles, as long as the damage is not too severe. A much closer inspection should be made during maintenance turnarounds. Typically, turnarounds are somewhat infrequent, so any damaged tiles should be repaired or replaced during turnarounds as another chance may not be available for some time.

The burner tile is an important and integral part of a process burner that may impact the fuel/air mixing, burner stability, flame shape and pollution emissions. It must be properly manufactured, installed and maintained for optimum performance. A variety of possible tile problems exist, such as cracking, buildup, chemical attack and improper installation. Most of these are easy to detect and should be corrected, as appropriate. HP

LITERATURE CITED

  1. Baukal, C., “Introduction,” Industrial Burners Handbook, Chapter 1, CRC Press, Boca Raton, Florida, 2003.
  2. Venizelos, D., R. Hayes and W. Bussman, “Radiant wall burners,” Industrial Burners Handbook, Chapter 15, CRC Press, Boca Raton, Florida, 2003.
  3. Waibel, R. T., M. G. Claxton and B. Reese, “Burner design,” Vol. 2: Design and Operations, Chapter 6, The John Zink Hamworthy Combustion Handbook, CRC Press, Boca Raton, Florida, 2013.
  4. Baukal, C. and W. Bussman, “NOx emissions,” Vol. 1: Fundamentals, The John Zink Hamworthy Combustion Handbook, CRC Press, Boca Raton, Florida, 2013.
  5. Baukal, C., R. Waibel and M. Claxton, “Natural-draft burners,” Industrial Burners Handbook, Chapter 16, CRC Press, Boca Raton, Florida, 2003.
  6. Johnson, W., M. Pappe, E. Platvoet and M. G. Claxton, “Burner installation and maintenance,” Vol. 2: Design and Operations, Chapter 11, The John Zink Hamworthy Combustion Handbook, CRC Press, Boca Raton, Florida, 2013.
  7. Platvoet, E., I. -P. Chung, M. G. Claxton and T. Fischer, “Process burners,” Vol. 3: Applications, Chapter 1, The John Zink Hamworthy Combustion Handbook, CRC Press, Boca Raton, Florida, 2013.

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