I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
2.2 Process Oil Burner
Process burners are primarily used in the hydrocarbon, petrochemical, and chemical industries. These burn- ers usually have smaller fired duties than those found in power generation or steam generation boiler burners.
Typically, process burners can be either natural draft or forced draft. The most common process oil burner will be designed with a single oil gun located in a central oil tile and burner throat tile. The oil gun is usually designed as an independent assembly for ease of maintenance. The flame stabilization mechanism is positioned around the oil gun.
2.2.1 Oil gun Type
There are many different types of oil guns. Each of these types has features that make them more or less desir- able for different applications. Typical oil guns are illus- trated in Figure 2.1 and described in detail in Chapter 1, Section 1.5.
Process burner oil guns are mainly designed for highly viscous, residual fuel oils. These fuel oils are commonly called No. 6 and can widely range in heating value, spe- cific gravity (SG), and viscosity. While these oil guns are most commonly designed for heavy oil applications,
they can also be used for light oil applications. The oil gun capacity will vary between oils, especially when the oil characteristics change.
The heat content, lower heating value (LHV), or higher heating value (HHV) on a volume basis varies directly with the hydrogen to carbon ratio. Low-viscosity fluids require less pressure to flow the same volume. Heavy oils (more viscous oils) will require a temperature increase for viscosity reduction. Light oils can be oper- ated at atmospheric temperature.
If steam is used as the atomizing medium, heat transfer between oil and steam will affect the oil gun capacity. Typically, for any specific oil gun, the capacity for light oil is higher than that for heavy oil.
Figure 2.2 shows that at the same oil pressure, the oil gun delivers more heat release for light oil than for heavy oil.
As covered in Volume 1, Chapter 10, a twin-fluid oil gun can also use compressed air as the atomiz- ing medium. Different atomizing media will result in different capacity curves. For a specific oil gun, its capacity using steam atomization is usually larger than that using air atomization as indicated in Figure 2.3. The figure illustrates that for the same fuel pressure, the oil gun delivers more fuel flow
using steam atomization than that using air atomiza- tion. The reason is that air density is much smaller than steam density. In other words, air specific vol- ume is much larger than steam specific volume. This results in air occupying more space than steam and delivering less liquid fuel.
Viscosity can be defined as resistance to separating liquid into droplets. Oil viscosity is a function of temper- ature. Generally, raising the oil temperature acts to lower its viscosity. When using compressed air to atomize heavy oil, heat transfer from the high-temperature oil to the low-temperature air can reduce the oil temperature.
This results in increasing the oil viscosity, reducing the level of atomization of the oil, and degrading the flame quality. The preferred medium for heavy oil atomiza- tion is usually steam. For light oil, air atomization is less problematic since the viscosity of light oil at atmospheric temperature is low.
Selection of the oil gun type and the mechanical design of the atomizing system must take into account the type of atomizing medium. Using steam to atomize light oil or a highly volatile fuel may not be advisable.
High-temperature steam can vaporize the volatile com- ponents in light fuel oils and cause vapor lock resulting in a pulsating oil flame.
(a)
(b) (c)
Figure 2.1
Typical liquid fuel atomizer-spray tip configurations: (a) HERO gun, (b) MEA oil gun, and (c) EA oil gun.
2.2.2 Oil Flame Stabilization
Stabilization of the oil flame is a primary consideration when designing an oil burner. In low airside pressure drop burners, such as process heater burners, a mechan- ical stabilizer is used (see Figure 2.4). The function of a mechanical stabilizer is as a bluff body within the
combustion airflow, creating recirculation zones. These recirculation zones bring combusting fuel/air mixtures and hot combustion products back to the exit ports of the oil gun dispersion tip. These hot gases aid in vapor- izing the oil droplets and the combusting fuel/air mix- ture ignites the new vapors. This continually renewing
0 2 4 6 8 10 12
0 10 20 30 40 50 60 70 80 90 100
Heat release MMBtu/h
Note: recommended fuel oil operating viscosity = 200 SSU (Max.) Steam pressure differential = 30 psi above oil pressure
No. 2
No. 6
Pressure (psig) Figure 2.2
Oil gun capacity curves for heavy oil and light oil for one specific oil gun.
6
5
4
3
2
1
00 10 20 30 40 50 60
Fuel flow rate (gpm)
Steam atomizing Air atomizing
Fuel pressure (psig) Figure 2.3
Oil gun capacity curves for steam atomizing and air atomizing for one specific oil gun.
circulation keeps combustion continuous and results in a stable oil flame. With a mechanical stabilizer, the burner tile throat is for air metering and flame shape molding. The tile throat opening size depends on the heat release and available furnace draft or burner pressure loss.
Another form of mechanical stabilizer is the “dual block” form, using an oil tile. In this form, a sec- ond burner tile is designed with a central ignition air source and a ledge. This design functions similarly to the mechanical bluff body discussed earlier. The low- pressure zone created by the ledge provides for the recirculation of the combusting fuel/air mixture. This design, using refractory in the bluff body, is a design enhancement for very heavy oils. The tile refractory provides high heat transfer back to the atomized oil (see Figure 2.5). Again, the burner tile throat is for air metering and flame shape molding, and the tile throat opening size depends on the heat release and available burner pressure loss.
2.2.3 Flame Shape
Due to the multitude of process heater designs, the flame shape required from process burners can vary greatly. In general, flame shapes are categorized as round or flat, but this is truly an oversimplifica- tion. Round flames can vary from long and narrow, pencil-like, to short and wide, basket-like. Flat flames can vary from wide to narrow fans. In addition, there
are considerations for both flames when the burner is mounted either vertically (up or down fired) or hori- zontally. Horizontal mounting of the burner takes on another dimension when the flame pattern is flat. In that case, it is necessary to know if the “fan” of the flat flame is in the horizontal plane or the vertical.
In low airside pressure drop burners, like process burners, the oil flame shape is directly affected by the design of the final dispersion tip. For round flames, the number of exit ports, the spacing between ports, and the spray angle are as important to flame shape as hav- ing a round tile (see Figure 2.6). For flat flames, the best tile arrangement is rectangular (see Figure 2.7). In addi- tion, the exit ports of the oil gun should be arranged in a linear pattern with little or no spread in the “narrow”
direction and with a spread in the “fan” direction that represents the required flame shape. In the case of flat flame drillings, the number, size, and proximity of the ports are also of importance.
2.2.4 emission Considerations
Since the 1970s, environmental concerns have been added to the functions that need to be addressed by combustion systems (see Volume 1, Chapter 14). In addi- tion to the aforementioned functions of flame stabili- zation, air metering, and flame molding, burners must also meet emissions regulations. This includes oil- fired burners. This has resulted in a change in burner designs from high-efficiency fuel/air mixers to staged
Figure 2.4
Oil flame cone-type stabilizer. Figure 2.5
Oil tile to stabilize the oil flame.
combustion mixers. As discussed in Chapter 1, for gas fuel burners, both staged-air and staged-fuel combus- tion systems have been developed. For combination burners, the design selection becomes more challenging.
The pollutants presently being regulated most closely for process burners are NOx, CO, and particulates. The first two, NOx and CO, are common to both oil firing and gas firing and can be reduced through efficient staged combustion burner designs. For oil or combi- nation firing, particulates can be a challenge. Because emissions have to be met for both gas firing and oil fir- ing, not only NOx and CO, but also particulate emis- sions control must be designed.
The NOx source contains thermal NOx, prompt NOx, and fuel NOx (see Volume 1, Chapter 15). Thermal and prompt NOx are generated from high-temperature
combustion reactions, while fuel NOx comes from the fuel content, that is, fuel-bound nitrogen. The oil fuel sometimes contains high amount of fuel-bound nitro- gen, which contributes a significant amount of fuel NOx to the total NOx emission. In other words, NOx emis- sions for oil firing or combination firing also depend on the fuel property, that is, nitrogen content.
There are many ways to reduce burner NOx. The popular technologies are air-staging, fuel-staging, and flue gas entrainment. In the combination burner, gas firing can use any of the aforementioned technologies.
However, for oil firing, since oil fuel is delivered by an oil gun, it is difficult to employ fuel-staging technol- ogy on the burner. Hence, most oil burners use air- staging technology to lower the NOx emissions. The patented DEEPstar burner1,2 uses staged-fuel/staged- air and flue gas entrainment to reduce NOx emissions when firing gas, and staged-air and flue gas entrain- ment to abate NOx emissions during oil firing. The detailed description of the DEEPstar burner design is covered in Chapter 1.
Particulate emissions are controlled by the oil gun and burner designs. An oil gun with good atomization
Figure 2.6
Round burner tile provides round oil flame.
Figure 2.7
Rectangular burner tile provides flat oil flame.
produces low particulates. This is discussed in Volume 1, Chapter 10. Poor air distribution in the burner design will also increase particulate emissions.
Poor air distribution may create fuel-rich pockets, which enhance soot generation. Soot is a small solid carbon particle and is generated under fuel-rich envi- ronments. Air-staging technology can lower oil NOx emissions, but has a negative impact on particulate emissions. This is a common situation, where a tech- nology that lowers NOx emissions creates a negative impact on CO and/or particulate emissions. Burner design engineers are required to pay extra attention on burner designs to achieve an optimum balance between NOx, CO, and particulate emissions.