Combination burner

There are also burners that can burn either oil or gas. They offer no efficiency advantages, but the ability to switch fuels in the event of a shortage or price differences is an advantage. They are available as combination burner and boiler units. 

Direct-fired furnace. Fired heaters are designed to increase the process temperature of oil and gas streams. This increase of temperature in most every case does not change molecular structure. Thus, temperatures up to 500°F maximum with 400°F design are very common. Designs are usually cylindrical, with vertical radiant tube banks fired by oil/gas combination burners. 

Diagram c in Fig. 114 gives the metal skin temperatures of the vertical tubes for a terrace wall furnace, as well as the temperatures reached by the wa s and supports. The top part of the figure shows the suspension of the coils by hangers, rod and counterweight Heating is provided in this case by combination burners running both on gas and fad oil atomized with steam, in the form of a line burner for the gas. and a tunnel burner for the fuel. 

Combination burners are typically freestanding burners that can fire both gas and liquid fuels. The liquids that are commonly burned are fuels such as No. 2 fuel oil (diesel) or No. 6 fuel oil. These burners also require connections for the medium used to atomize the liquid fuel, which is typically steam or air. The burner can be used to fire only gas, only liquid, or some combination of the two. Figure 18.12 is a rendering of an up-fired combination burner. 

Natural draft burners are supplied with combustion air from their surroundings and they do not allow for fhe preheating of combustion air. They utilize furnace nafu-ral draft (stack effect) and often simultaneous ejection effect of fuel streaming from burner jefs under a high speed. This is a rather frequent method of combustion air supply and is used for bofh gas fuel and liquid fuel burners (and possibly for combined burners). The burners are relatively difficult to operate under a low excess of air since the amount of supplied combustion air can only be controlled by a chimney damper or in some cases by flap valves on individual burners, which is rather demanding on the operators. Due to complicated control they are predestined to be used in operations with constant heat consumption without any sudden changes of heat supply, as it would cause product quality deterioration that is, process furnaces in chemical and pefrochemical industries [2]. 

Exhibits 7-33 and 7 34 present simplified piping and instrumentation diagrams for a circular furnace. The furnace has 6-in inlet and outlet lines, with control valves, combination burners, and soot blowers in the... 

Figure 6.9 Schematic drawing of Siemens combined burner for coai gasification 1 -pulverized coal, 2 - oxygen (and steam if used), 3 - pilot fuel, 4 - pilot oxygen, 5 - ignition device and flame detector, 6 - spiral inlet duct gap, 7 - water-cooled parts, 8 - burner tip [42].

Commercially, the burner chamber and the absorber cooler sections are combined as a single unit for small-scale production. However, in large capacity plants, these units are separated. A typical commercial unit is schematically described in Figure 5 (32). 

The plant is designed to satisfy NSPS requirements. NO emission control is obtained by fuel-rich combustion in the MHD burner and final oxidation of the gas by secondary combustion in the bottoming heat recovery plant. Sulfur removal from MHD combustion gases is combined with seed recovery and necessary processing of recovered seed before recycling. 

NOj Control. NO control limitations are described in both Tide 1 and Tide 4 of the CAAA of 1990. Tide 4 requirements affect only coal-fired boilers and take effect at the same time that the boilers are impacted by CAAA SO2 requirements. As of 1996, EPA had estabHshed Tide 4 NO limits only for tangentially fired and waH-fired, dry-bottom boilers that would be impacted by Phase I of the CAAA SO2 regulations (Tide 4). Limits of 0.22 kg/10 kJ (0.5 lb/10 Btu) and 0.19 kg/10 kJ (0.45 lb/10 Btu) have been set for wall-fired and tangentially fired units, respectively. The EPA based these levels on what was achievable using low NO burners. However, plants can employ a number of different front- or back-end emissions controls, including a combination of options, to achieve these levels. EPA plans to announce Tide 4 NO requirements for 300 additional boilers by late 1996 or eady 1997. 

High Temperature. The low coefficient of thermal expansion and high thermal conductivity of sihcon carbide bestow it with excellent thermal shock resistance. Combined with its outstanding corrosion resistance, it is used in heat-transfer components such as recuperator tubes, and furnace components such as thermocouple protection tubes, cmcibles, and burner components. Sihcon carbide is being used for prototype automotive gas turbine engine components such as transition ducts, combustor baffles, and pilot combustor support (145). It is also being used in the fabrication of rotors, vanes, vortex, and combustor. 

As for oil and gas, the burner is the principal device required to successfully fire pulverized coal. The two primary types of pulverized-coal burners are circular concentric and vertical jet-nozzle array burners. Circular concentric burners are the most modem and employ swid flow to promote mixing and to improve flame stabiUty. Circular burners can be single or dual register. The latter type was designed and developed for NO reduction. Either one of these burner types can be equipped to fire any combination of the three principal fuels, ie, coal, oil and gas. However, firing pulverized coal with oil in the same burner should be restricted to short emergency periods because of possible coke formation on the pulverized-coal element (71,72). 

Oil Burners The structure of an oil flame is shown in Fig. 27-28, and Fig. 27-29 illustrates a conventional circiilar oil burner for use in boilers. A combination of stabilization techniques is used, typically including swirl. It is important to match the droplet trajectories to the combustion aerodynamics of a given burner to ensure stable ignition and good turndown performance. 

FIG. 27-30 Lo v -NO, combination oil/ as forced-draft boiler burner. (Todd Cojiihnsliort, Iric.)... 

Gas Burners Gas burners may be classified as premixed or non-premixed. Many types of flame stabilizer are employed in gas burners (see Fig. 27-32). Bluff body, swirl, and combinations thereof are the predominant stabilization mechanisms. 

Some new features of furnaces available today include variable speed blowers, which deliver warm air more slowly and more quietly when less heat is needed, and variable heat output from the burner, which when combined with the variable speed blower allows for more continuous heating than the typical fixed firing rate. Distribution system features can be sophisticated with zoned heating which employs a number of thermostats, a sophisticated central controller, and a series of valves or dampers that direct airflow or water to different parts of the home only when needed in those areas. 

From a simple cycle standpoint, the combination of intercooling with recuperation eliminates the problem of the reduced combustor inlet temperature associated with intercooled cycles. The simple cycle then gets the benefit of the reduced compressor work and, at all but high pressure ratios, actually has a higher burner inlet temperature than the corresponding nonintercooled, nonrecuperated cycle. This results in a dramatic increase in the simple cycle efficiency. 

For hot water boilers the burner controls will be similar but controlled by a combination of pressure and temperature signals. A single overriding level control will be fitted to the flooded boiler to protect against any accidental low-water condition. 

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