I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
1.4 Radiant Wall Burners .1 Premix radiant Wall Burners
1.4 Radiant Wall Burners
(a) (b) Figure 1.35
(a) Modified RTW burner and (b) CFD-calculated result.
(a) (b)
Figure 1.36
(a) PXMR-DS burner and (b) flame photo.
ethylene-cracking applications since the late 1950s.
These burners have evolved over time from their origi- nal low-capacity, low-efficiency premix assemblies. The older designs used sand castings for the venturi and tips. These castings, with their rough internal surfaces, the venturi inlet, and tip outlet configurations were not optimum. The present-day design of the PMS burner can reach higher capacities with higher premix stoi- chiometries. This increase in capacity and premix effi- ciency is due to the new spin-cast or extruded venturis and higher-quality, higher-efficiency outlet premix gas tips. The new venturis have smoother interior surfaces, reducing frictional losses, and better inlet configurations, reducing inlet losses. The new tips are higher-quality castings, are smoother, and have rounded surfaces in all flow passages for reduction of losses. Computer model- ing or CFD (see Volume 1, Chapter 13) and cold flow test- ing (see Volume 1, Chapter 11) have been used extensively to ensure minimal losses and maximum conversion of the fuel gas motive energy into mass of air entrained.
The basic PMS burner, shown in Figure 1.38, is com- prised of five parts: (1) the venturi, (2) the primary pre- mix tip, (3) the fuel orifice, (4) the primary air door, and (5) the secondary lighting and sighting port(s). Burner tile assemblies including tile mounting plates, high- temperature hot face, and an insulating backup block are also available. The primary insert starts with a ven- turi, or fuel/air mixer, and the premix distribution tip.
The fuel gas is metered and injected into the venturi through the fuel orifice that is positioned at the entry to the venturi. The PMS burner fuel metering is located externally to the heat-affected furnace zone. The fuel is used to provide the motive energy to draw ambient air
into the venturi for premixing with the fuel prior to the ignition zone. Primary air can be manually adjusted by opening or closing the primary air door assembly, also located at the entry to the venturi. If required, secondary air can be introduced to the burner, outside the premix system, through the lighting and sighting port(s) located in the annular space around the venturi and tip assembly.
The PMS burner is a high-efficiency premix burner that provides a flat, radially projected flame pattern (see Figures 1.39 and 1.40). This burner is typically mounted horizontally but can be mounted in any direction. The burner is mounted flush through a furnace refractory- lined casing, with its tip projecting a short distance into the radiant chamber. The radial flame then heats the surrounding refractory, which then radiates to the process. Due to the high efficiency of its premixing of combustion air with the fuel, PMS burners are relatively immune to furnace draft variations. These burners can also operate through a range of firing rates with only small changes in stoichiometry.
The PMS burner can be sized for 100% premix capac- ity including excess air (all air through the venturi and tip). Capacities can range from 0.07 MW (240,000 Btu/h) up to 0.88 MW (3.0 × 106 Btu/h). As a premix burner, it is limited in fuel composition, range of fuel calorific value, and range of fuel molecular weight (see Volume 2, Chapter 6). Typically, a fuel with 80 mol% or greater hydrogen content will severely limit the range of operation due to flashback. Additionally,
Figure 1.37
Premix radiant wall burner array.
Figure 1.38
PMS premix gas burner assembly.
if a burner is designed for a typical midrange calorific value fuel of 23.6–47.3 MJ/N-m3 (600–1200 Btu/scf), attempting to operate on higher calorific gases (such as propane or butane) will result in a limit to the maximum capacity. This limited capacity is due to the loss in air entrainment due to the lowered pres- sure required for fuel supply. A typical turndown
capacity for these burners is 25% of burner maximum design capacity. This is often increased (higher per- cent of design rate/less turndown), dependent on the maximum design rate, the fuel composition, and the design fuel pressure.
1.4.1.2 Hamworthy Walrad Burner
Hamworthy’s Walrad series burner (Figure 1.41) has been designed for the process industry where an aspi- rating radiant wall burner is required. The burner fuel pressure is used for aspiration and mixing of the combustion air with the fuel. Due to this, these burn- ers are generally accepted as limited in gas composi- tion range to less than 60 mol% hydrogen. Beyond this point, the possibility of flashback (preignition within the burner insert) is vastly increased. The momen- tum of the fuel gas is used to entrain atmospheric air using a jet and venturi. The standard Walrad burner utilizes the fuel to entrain 100% of the total combus- tion air; therefore, the gas injector is designed for the highest pressure available to maximize the momen- tum of the fuel gas. Secondary air ports are typically closed or slightly open to allow a cooling air path and have been included in the base design to allow small changes in the burner design operation or for operational flexibility. The low-NOx version uses the staged-fuel principle of NOx reduction taking approximately 20% of the total gas through a single auxiliary fuel injector.
Over the years, the burner insert design has been improved to allow a greater flexibility with regard to performance. During the development of the new design (in particular the main gas nozzle), full use was
Figure 1.39
PMS flame (front view).
Figure 1.40
PMS flame (side view).
Figure 1.41
Hamworthy Walrad burner assembly.
made of Hamworthy’s flow modeling and test facilities.
Improvements were made to the internal gas/air flow patterns, eliminating the air pockets and dead zones that were present due to internal recirculation.
In addition, during the “hot” firing tests, it was seen that the nozzle produced skin temperatures in excess of 870°C (1600°F) in what was considered criti- cal areas. The outlet nozzle was therefore redesigned to reduce its temperature. This redesign included the following:
1. Reduction in total mass
2. Redesign of the nozzle drilling pattern—creating a flame front further from the nozzle, reducing direct heating
3. Reduction of the setting dimension (protrusion into the furnace)
4. Improved flow characteristics through internal profile change
The old, heavy cast components have been replaced with a fabricated stainless steel construction. This results in a significant weight reduction and increased corrosion resistance. This construction offers the following ben- efits to the end user:
• Total weight of the burner insert of 15 kg (33 lb)
• No necessity for complex paint systems and continued repainting
• Reduction in burner standoff from the furnace The standard Walrad burner is comprised of five parts: (1) the burner body (venturi), (2) the main gas nozzle (primary premix tip), (3) the fuel gas jet, (4) silencer and damper, and (5) the secondary air port(s). The primary insert starts with a venturi, or fuel/air mixer, and the premix distribution tip.
Burner tile assemblies including tile mounting plates, high-temperature hot face, and insulating backup block are also available.
The burner can operate with a wide range of fuel gases from 60 mol% hydrogen to a highly inert purge gas thro ugh propane. It should be noted that high gas pressures are required at the extreme cases. Lower fuel gas pressures will limit the burner’s ability to fire higher-hydrogen fuels. The Walrad burner can be designed for duties ranging from 0.1 to 0.7 MW (341,000 Btu/h to 2.39 × 106 Btu/h). A typical turndown would be 33% of design (3:1) and higher fuel gas pressures can result in greater turndown.
1.4.1.3 LPMW Burner
The John Zink series lean premix wall (LPMW) burner shown in Figure 1.42 is a gas-fired, radiant wall,
ultralow-NOx burner. As with other radiant wall burn- ers, it is mounted through the refractory-lined casing of a radiant chamber and provides a minimal projection radial flame to heat the surrounding refractory. It uses lean premix primary fuel technology and high-temper- ature diffusion flames for stability. It uses a centrally located, radially fired staged-fuel injector in conjunction with the lean premix primary to achieve reduced NOx emissions.
The LPMW fuel metering orifices are located exter- nal to the furnace, placing the smallest orifices outside of the heat-affected zone. The venturi and primary premix distribution tip are “sized” dependent on the capacity required, the fuel composition, and the fuel pressure available. The primary fuel orifice is typically sized for 50%–65% of the burner’s design capacity. This reduced quantity of the total fuel must entrain all of the combustion air required for both the primary and staged fuel, including any design excess air. The staged insert provides the remaining fuel required to meet the design capacity. The staged fuel is introduced into the combustion zone in such a way that it mixes with the low-oxygen furnace gases prior to being incorporated into the flame.
There are three basic designs of LPMW burner that can be provided5–8:
1. The first design (Figure 1.43) provides the ven- turi offset at an angle from the primary premix and staged tips. In this design, the staged-fuel insert is mounted through an elbow. This con- figuration provides access to the staged insert,
Figure 1.42
LPMW radiant wall burner array.
enabling maintenance of the staged tip without dismounting the main premix burner from the tile or furnace. Full shutdown of the burner is required, but the burner does not have to be dis- mantled, and the main fuel piping to the burner does not have to be disconnected.
2. The second design (Figure 1.44) provides a more compact in-line venturi and tip configuration.
In this design, John Zink has developed an in- line adapter that allows the venturi to remain in direct line of flow with the primary tip, increas- ing the premix efficiency. This in-line adaptor incorporates flow vanes and provides the flow
channel for supply of fuel to the center-mounted staged-fuel tip. Maintenance of the staged tip now requires the disconnection of the fuel sup- ply and the dismounting of the burner insert from the tile or furnace.
3. The third design (Figure 1.45) is similar to the second. In this design the venturi remains in- line with the primary premix tip. However, the adaptor and the staged-fuel tip are omitted.
This design is specifically for the purposes of integrating John Zink’s patented RFS (remote fuel staging) insert. The primary fuel remains the same sub-100% of the design capacity.
The premix stoichiometry remains the same as if the burner had staged fuel, in the range of 170%–220%. The staged fuel required to complete the burners’ design capacity will be injected into the combustion zone from a totally separate injection point not contained within this burner.
1.4.2 raw gas radiant Wall Burners 1.4.2.1 FPMR Burner
The FPMR burner, seen in Figures 1.46 and 1.47, is designed to operate with cold or preheated combustion air under forced-draft conditions. The FPMR burner is designed in such way that fuel and air are intro- duced into the furnace at separate locations. The fuel is injected into the furnace through a central gas tip.
The combustion air is provided in an annular region around this tip and is injected in a radial pattern into the furnace, parallel to the furnace wall. Prior to the initiation of the combustion reaction, both the fuel and the air mix with high-temperature, inert furnace gases.
Figure 1.43
LPMW with elbowed venturi.
Figure 1.44
LPMW with in-line venturi and staged-fuel adaptor/tip.
Figure 1.45
LPMW with in-line venturi for RFS integration.
The fuel is finally oxidized in a quasi-flameless com- bustion environment.
In order to ensure that the distribution of the air is as even and as parallel as possible, the exit velocity of the air tip slots is kept higher than the velocity of the air in the annulus.
The furnace gases are entrained into the combustion zone by the high exit velocity of the air leaving the air tip and by the fuel gas. By entraining inert gases into the combustion zone, the burner reduces NOx emis- sions by reducing the actual flame temperature that, in turn, reduces the amount of thermal NOx forma- tion. Additionally, by keeping the air and fuel streams separate for as long as possible, the combustion surface is increased, and the heat per unit volume produced is decreased, further reducing the flame temperature.
The burner can be designed to use high-pressure gas (typical 2 barg or 29 psig), low-pressure gas (typical, PSA at 0.2 barg or 2.9 psig), and vaporized heavy fuels such as propane, butane, pentane, and naphtha. FPMR burners have been supplied for use in steam/methane reforming furnaces and could also be applied in ethyl- ene dichloride crackers (EDC).