I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
1.5 Combination Gas and Oil Firing
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).
becomes complicated. Gas/oil combination burners are discussed in Volume 2, Chapter 6, and are discussed in more detail in Chapter 2 of this book. As follows, dif- ferent designs of atomization systems and combination burners are introduced.
1.5.1 atomization Systems (Oil guns)
Most oil guns in industry are of a concentric tube design. In most oil guns designed with concentric tubes, the oil flows within the inner tube, while the atomiz- ing medium, most commonly steam, flows through the annular area formed by the inner and outer tubes (see Figure 1.48).
The concentric tube design is well suited for the high- viscosity residual oil commonly called No. 6 oil, or other high-viscosity liquid fuels. Viscosity is a function of temperature. Higher temperature results in lower liquid hydrocarbon oil viscosity. Low-viscosity liquid is much easier to atomize. A concentric tube arrangement atom- ized with steam has the advantage of heating the oil and keeping the fuel at low viscosity.
Concentric tube oil gun designs can be used for light oil, or low-viscosity distillate oils. The fuel pressure versus fuel flow characteristics will be different, as dis- cussed in detail in Chapter 2. However, if the light oil contains high vapor pressure (volatile or low boiling point) components, concentric tube oil guns utilizing steam for atomization are not recommended. The heat imparted by high-temperature steam can vaporize vola- tile components and cause vapor lock resulting in pulsa- tion or instability of the oil flame. In this case, a dual-tube oil gun design as shown in Figure 1.49 is recommended.
1.5.1.1 John Zink EA Oil Gun (Internal Mixing Chamber)
The JZ EA oil gun with the concentric tube design is shown in Figure 1.50. In this process oil gun, the atom- izing medium pressure is controlled at a positive dif- ferential above the oil pressure. In an internal mixing chamber type of oil gun, the high-pressure steam used as the atomizing medium is injected through several atomization ports into the mixing chamber where the metered oil is flowing. For heavy oils requiring heat- ing to achieve the correct viscosity, this differential is 20–30 psig (2.4–3.1 barg) higher than the oil pressure. The steam-oil mixture is then introduced into the furnace through the tip. In the EA series oil atomizer, the exit from the atomizer mixing chamber is axial. This design is especially good for residual oils with suspended par- ticulates. It is also beneficial in locations where the oil viscosity is difficult to maintain due to inconsistent fuel supply or lack of oil temperature control. The exit ports on the tip can be arranged at various spray angles and different spray shapes, that is, hollow cone shape or fan shape, depending on the application. A detailed descrip- tion can be found in Volume 1, Chapter 10 on oil atomi- zation and in Chapter 2 of this book.
1.5.1.2 John Zink MEA Oil Gun (Internal Mixing Chamber)
The MEA oil gun design is similar to the EA gun with the exception of a modified mixing chamber outlet and tip design as indicated in Figure 1.51. Before reach- ing the tip, the steam-oil mixture is split into multiple
Figure 1.48
Oil gun with concentric tube design.
Figure 1.49
MEA oil gun with dual (parallel)-tube design.
Figure 1.50
John Zink EA oil atomizer and tip.
Figure 1.51
John Zink MEA oil atomizer and tip.
streams and turned 90°. The exiting atomized/mixed oil-steam stream impacts the walls of the tip exit cham- ber prior to exiting. These additional changes in flow direction and separation into multiple streams prevent large droplets from reaching the exit ports. This design is intended to reduce particulate emissions generated by low atomization quality.
1.5.1.3 Hamworthy SAR (Steam Atomized Residual) Oil Gun
The Hamworthy SAR oil atomization system is an internal mix atomization system used where sufficient atomizing pressure is available (see Figure 1.52). The gun is designed as a concentric tube arrangement. The fuel travels through the inner pipe to the core (mixing chamber) where it flows through the oil orifice into the outer tip chamber. The atom- izing medium, most commonly steam, passes along the outer pipe to enter the core. Within the core and the outer tip, a thorough emulsification of the oil with the atomizing medium occurs. This mixed stream passes through at a high velocity to the cone tip, where it is ejected in a highly
atomized state. The flame pattern generated is controlled by the exit port drilling pattern of the cone tip.
1.5.1.4 Hamworthy DS (Dual Stage) Oil Gun
The Hamworthy DS oil atomization system utilizes a fixed atomizing pressure; hence, it is an ideal choice when the atomizing medium pressure is limited (see Figure 1.53). The first stage of atomization is by fuel pressure. The oil entering the dual stage gun is forced through a mechanical atomizing assembly consist- ing of a fuel body, disk, and tip. The second stage of atomization utilizes the momentum of the atomizing medium, usually steam, but can be compressed air. The mechanically atomized oil is sprayed into the outer tip chamber where the atomizing medium exits, churning the oil into a complete emulsion and initiating vapor- ization. Finally, expansion of the two phase flow of atomized oil, oil vapor and atomizing medium through the cone tip completes the spray. The flame pattern gen- erated is controlled by the exit port drilling pattern of the cone tip.
Outer tip
Cone tip
Discharge holes
Mixing chamber Atomizing
medium
Fuel
Fuel hole Mixing core
Atomizing medium holes Figure 1.52
Hamworthy SAR oil atomizer and tip.
Fuel tip Outer tip
Cone tip Discharge holes
Mixing chamber Fuel atomizing holes
Fuel Atomizing
medium
Fuel body
Fuel discharge hole Fuel disc
Figure 1.53
Hamworthy DS oil atomizer and tip.
1.5.1.5 John Zink PM Oil Gun (Port Mix Atomization) The John Zink PM oil atomization system is one form of the port mix, or Y-jet, design common throughout industry. This type of oil gun utilizes multiple “sets”
of converging ports to break the oil down into small droplets (see Figure 1.54). Other similar atomizing system oil guns are supplied by COEN, TODD, and Hamworthy.
This design utilizes intersecting oil metering and atomizing medium (typically steam) ports. The angle of intersection is designed to develop shear forces on the oil. The point of intersection occurs at the entrance to an expanded area exit port. This impinging flow of oil and atomizing medium at an abrupt expansion results in a shearing of the viscous liquid, breaking it up into very fine droplets for dispersion. This atomi- zation occurs almost immediately prior to exiting the tip. The exit port is limited in length to prevent re-combining of the droplets. The length of the exit port is only as long as required to provide the desired direction of the jet.
Since the oil metering is accomplished through multiple ports, this type of oil gun is typically used only for larger heat releases. John Zink will typically limit its use to heat releases greater than 5.9 MW (20 MM Btu/h).
The PMA, a dual-tube design of the PM oil gun, is also available.
1.5.1.6 HERO® Oil Gun
The HERO gun is an acronym of High Efficiency Residual Oil gun. It is a patented atomization device9,10 with advantages of low steam consumption, good atomization, short flame length, low pollutant emis- sions including NOx and particulates. The HERO gun design is shown in Figure 1.55. It is a combination of a Y-jet11 and the EA oil gun. The detailed descrip- tion of HERO gun design and performance can be found in Volume 1, Chapter 10 on oil atomization and elsewhere.12–14
1.5.2 Combination Burner
A combination burner can be viewed as really two burners in one, or maybe even three in one. This is because, first, the combination burner must perform using oil exclusively as the fuel. Second, it must per- form using gas exclusively as the fuel, and third using both gas and oil simultaneously as fuels. Good perfor- mance on one fuel does not imply any trend toward good performance or even adequate functioning on the other fuel or the combination. The design strategies that work well for either gas-only or oil-only burners actually end up conflicting in a combination burner.
1.5.2.1 Conventional Combination Burner
A typical combination burner contains primary oil tile (also called the regen tile) and secondary tile as indi- cated in Figure 1.56. The oil gun is located at the center of the regen tile and the gas tips are located between the regen tile and the secondary tile.
1.5.2.2 PLNC Burner
The PLNC is a first generation low-NOx combination burner (see Figure 1.57). Air or steam atomization of liq- uid fuels is difficult to stage into primary and second- ary fuel zones. Therefore, to reduce NOx emissions, it is more practical to stage the combustion into multiple zones by using staged-air technology. The gas firing is the same as a conventional combination dual block burner. The oil gun is centrally located inside a primary tile, also called “regen tile.” The gas firing is provided through multiple tips located around the oil tile, in the secondary tile zone. Staged air is provided through an
Figure 1.54
John Zink PM atomization system (port mix).
Figure 1.55 The HERO gun.
open area around the burner tile. Different air dampers are used to control the various levels of air through each channel (see Section 1.2.2.1).
1.5.2.3 DEEPstar® Burner
The DEEPstar burner is a patented combination burner15 (Figure 1.58) designed to achieve low NOx emissions for gas firing, oil firing, and combined gas/oil firing. The design uses staged fuel and furnace gas entrainment to reduce NOx emissions when firing gas.
Fuel gas firing is accomplished using two primary gas tips located adjacent to the ledge in the primary tile throat and four staged gas tips located in the refrac- tory-shielded secondary tile sections positioned in the secondary air zone around the outer circumference of the primary tile. This refractory shielding of the staged tips creates the staged-fuel gas combustion. In addi- tion, the staggered secondary air and shielded fuel gas injection allows a greater secondary air and furnace gas interface for mixing prior to the secondary com- bustion zone.
Oil firing is designed as staged air with furnace gas entrainment in the second combustion zone for reduc- tion of NOx emissions.16 Air for the first combustion zone is supplied through the primary tile throat. The oil gun is located at the center of the primary tile within
the first air zone, the primary tile throat. Air for the second combustion zone is via the four slots formed by the secondary (staged) gas tip refractory shield tiles.
The furnace gas entrainment occurs due to this stag- gered arrangement of the air slots and the gas tip shield blocks. This staggered secondary air arrangement allows the furnace gas to be entrained by the primary oil flame with minimal interference from the secondary air. It also allows greater air and furnace gas interface for mixing prior to entering into the secondary combus- tion zone.
Oil and gas flames from a DEEPstar burner are shown in Figure 1.59. The bright yellow color from the oil flame is due to soot particles and small droplets in the oil flame. These factors of liquid hydrocarbon com- bustion radiate in the visible light spectrum. The more transparent flame from the gas flame is due to a lower occurrence of solid particles raising the predominant radiation above the visible spectrum. It is often quite difficult to see a well-mixed gas flame in an operating furnace environment.