I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
1.3 Flame Shape: Round versus Flat and Freestanding versus Wall-Fired
The main two flame shapes are round and flat. A round flame is typically associated with freestanding burners, that is, there are individual unsupported flames in the middle of the firebox with radiant tubes mounted against the firebox walls (see Figure 1.27). This is a cost-effective way to build a firebox since the amount of tube surface per unit firebox volume can be quite high, but typically results in the tubes being heated only from one side.
Firing tubes from one side only is therefore restricted to applications where the tube circumferential heat flux dis- tribution is not critical.
A wall-fired burner, on the other hand, typically pro- duces a flat flame. The idea behind it is to heat the refrac- tory wall and use it as a radiating plane toward the tubes that are located in the center of the firebox. Flue gas just by itself is typically a poor radiative emitter as it emits and radiates only in certain wavebands (see Volume 1, Chapter 7). A solid wall does not have this restriction and is therefore capable of a more efficient transfer of radiant heat. This type of firebox arrangement with burners against the walls and tubes in the center is more costly to build since the amount of tube surface per unit
Figure 1.27
Freestanding burners.
Figure 1.26 HALO flame photo.
Coanda tile surface
Primary ring Figure 1.25
Coanda tile surface.
Low pressure zone
Surrounding fluid entrainment
Injection of fuel
Figure 1.24
Entrainment around a Coanda surface.
firebox volume is low. It does allow much better control of heat flux to the tubes and is therefore the preferred solution in those applications where the flux and tem- perature profile, both longitudinal and circumferential, are critical. These are used in, for example, coker heaters and steam-cracking furnaces.
In some firebox designs, a mix of round and flat flames is used. In such cases, the flat flames fire against the wall, and the round flame burners are arranged in rows between rows of tubes.
1.3.1 round Flame Burners
A number of round flame burners have already been presented in the preceding sections. In addition to the low-NOx upward-firing burners, round flames are typ- ically used in applications where horizontal flames are required or where burners are firing in the downward direction. A typical downward-firing application is in steam methane reformers, where a natural gas feed is passed over a catalyst with steam in order to produce
“syngas,” a gas made up primarily of hydrogen and CO.
For more information on this process, refer Volume 1, Chapter 2.
1.3.1.1 MDBP Burner
A typical burner for down-firing reformer service is the John Zink MDBP, shown in Figure 1.28. Waste gas from the pressure swing adsorption (PSA) unit, containing
large amounts of CO2, is mixed with makeup fuel gas and injected into the flame through a center gas gun. The design of the burner tile and internals is aimed at creating a compact flame while yielding low NOx emissions due to the presence of the inert components in the fuel gas.
1.3.1.2 PDSMR Mk-II Burner
Another way to inject the PSA gas into the flame is by individual fuel tips for the makeup and PSA gas. This allows for staging of both these gases and yields addi- tional reductions in NOx emissions. One such burner to employ this concept is the John Zink PDSMR Mk-II, shown in Figures 1.29 and 1.30.
1.3.2 Flat-Flame Wall-Fired Burners 1.3.2.1 PXMR Burner
Flat-flame wall-fired burners, as mentioned in Section 1.1, use the firebox wall to create a certain heating pattern that is compatible with the process. A tradi- tional flat-flame, wall-fired burner for John Zink has been the PXMR burner, shown in Figure 1.31. This relatively simple concept produces good flame pat- terns and low NOx emissions by injecting primary fuel from the sides and staged fuel from the front of the burner. This burner is most suited for applications with low heat release per burner, such as delayed coker heaters.
1.3.2.2 PSFFR Burner
Applications that require higher heat release per burner, such as steam cracking, are better served with the John Zink PSFFR burner, shown in Figure 1.32.
The PSFFR is used for heat releases up to 10 MM Btu/h (2.9 MW) and produces typical NOx emissions of 45 ppm (corr. to 3 vol% O2, dry) in ethylene applications.
By injecting primary fuel into two internal venturis, the burner entrains flue gas into a primary combus- tion zone. The resulting flame produces a heat flux pro- file that is compatible with the most stringent process requirements.
1.3.2.3 LPMF Burner
The LPMF burner (Figure 1.33) is often used in ethyl- ene applications that require even more stringent NOx emissions. This burner uses very lean premixed air/fuel technology to create a quasi-flameless combustion zone.
The resulting NOx levels are typically in the range of 25–30 ppm in cracking furnaces with arch temperatures of ∼2200°F (1200°C). Section 1.2.3.4 offers more detailed information on the LPMF burner design.
Figure 1.28
MDBP burner firing PSA off gas (view looking up at burner).
(a) (b) Figure 1.30
(a) PDSMR Mk-II burner assembly and (b) flame photo.
Tile ledge
PSA center-gas nozzle and stability ring
Tile
PSA gas nozzle
Hole in tile for primary PSA fuel
Staged
makeup fuel Hole in tile for primary makeup fuel Figure 1.29
PDSMR Mk-II tile and fuel tips.
1.3.2.4 RTW Burner
The RTW burner has been designed specifically for wall-fired coker applications. Built for low burner pres- sure drop (less than 0.3 in H2O [0.75 mbar]) and heat releases ranging from 1.5 to 3 MM Btu/h (0.44 to 0.88 MW), it produces very short and wide flames. During the burner design process, CFD was used to ensure optimal air and fuel distribution. With only four gas tips, the burner produces a very uniform heating of the refractory wall, resulting in optimal flux control.
The NOx emission level ranges from 20 to 35 ppm (cor- rected to 3% O2) depending on air preheat temperature.
Figures 1.34 and 1.35 compare photos of the test burner and CFD-calculated result.
1.3.3 Flat-Flame Freestanding Burners 1.3.3.1 PXMR-DS Burner
Applications that require freestanding flat flames, such as steam superheaters in styrene monomer plants, are served well with the John Zink PXMR-DS (Figure 1.36).
This burner is essentially a rectangular version of the John Zink COOLstar burner (Section 1.2.3.6). NOx emissions from this burner are comparable to the COOLstar burner.
Staged fuel gas tip
Fuel manifold
Plenum
Damper
Muffler cap Mounting
plate Primary fuel
gas tip Tile
Pilot
(a) (b)
Figure 1.31
(a) PXMR burner assembly and (b) flame photo.
Figure 1.32
PSFFR burner assembly.
1.4 Radiant Wall Burners