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10 Burner Operation
The safety of fired heaters is determined by the operators and the automated functions installed. API 556 provides guidance on the instrumentation applied to fired heaters. API 556 also provides the minimum consideration for burner light off for both natural and forced draft heaters.
10.1 Special Safety Reminders
Always purge the firebox before any light off attempt is made.
Do not use adjacent burner flames or hot brickwork as an ignition source for a pilot or main burners. Always ensure that the burner to be light off has a pilot or an appropriate portable igniter is used to light the burner directly.
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Safety interlocks should be in service.
Re-initiate the start-up from the appropriate step if immediate ignition does not occur. Always shut off fuel valves and open steam purge to oil guns after failure of oil burners. Fuel line blinds should be reinstalled if there will be an extended delay prior to relighting.
Do not remove a portable igniter unless certain that the burner will remain lit.
Do not close stack damper completely. During the initial light-off, it may be necessary to close the burner register completely on that specific burner to maintain the igniter in service.
Always use safety glasses as a minimum or better still a face and eye shield (when oil firing) when observing furnace flame pattern(s).
10.2 EXCESS AIR CONTROLS 10.2.1 Optimum Excess Air Levels
The highest energy efficiency is achieved when the combustion takes place using the exact stoichiometric requirement of air. However, a certain level of excess oxygen is required to prevent the emission of unburned hydrocarbons and to account for fluctuations in operating conditions such as fuel composition, ambient conditions and firing rate.
There is an optimum level of excess oxygen in the flue gas for each type of heater, burner and fuel used. Typical excess air and oxygen levels are shown in Table 15. The sophistication of the burners or the presence of only one burner may allow reduced levels. The user may find their own optimum excess air level by conducting CO breakthrough tests. Figure 19 may be used to adjust excess air to a minimum. By alternatively setting draft at the bridgewall and measuring both O2 and CO optimum excess air can be set.
Table 15 Optimum Excess Air Levels
Burner Type
A completely sealed heater containing a few burners and automatic oxygen and draft controls may allow a reduction in these O2 levels. An existing heater with significant casing leakage may not achieve the oxygen levels in Table 15 without significant CO emissions.
As well as determining the CO break point, operational settings must consider the cyclical firing nature of the process (heater at design rates or turndown), speed of response of the O2 analyser and control system to affect change, speed and compositional changes of the fuel gas that may be encountered.
10.2.2 Disadvantages of High Excess Air
Higher excess air than design will increase NOx emissions for most types of burners.
High excess air will reduce heater efficiency for the following reasons:
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More fuel is required to the heat the additional air entering the burners.
Additional air lowers the flame temperature resulting in a lower radiant thermal efficiency.
Increased flue gas flow rate and raise the flue gas pressure differential. This may result in a positive pressure in the heater forcing a reduction in capacity.
10.2.3 Advantages of Increased Excess Air
Increased excess air allows for bigger fluctuations in process operations and ambient conditions such as wind velocity and direction.
Increased excess air can lower CO formation at low firebox temperatures, although too much air will chill the flame increasing CO emissions.
Increased excess air can improve the flame quality and length in for fuel gases containing heavy hydrocarbons as well as oil fired burners. Again too much excess air on oil fired heaters may lead to ‘stripping’ where there is insufficient heat recirculated back to the flame to complete burn all the fuel. Oil droplets may be emitted from the fired heater.
Increased excess air will increase the convection section duty at the expense of the radiant duty. Radiant section tube metal temperatures may fall while the convection section tube metal temperatures will increase. Increasing the convection duty may be of value if a greater duty is desired from a waste heat coil (steam, reboiler, hot oil, etc.).
10.2.4 Excess Air Adjustment
Excess air and draft are inter-related. By adjusting the excess air using the air dampers or registers this changes the flow of flue gases through the heater, consequently affecting the draft. Correcting the draft by means of the stack damper or induced draft fan suction damper (or variable speed drive) affects the flow of air through the burners as the pressure at the burner changes. It will be necessary to readjust the air control registers/dampers and stack damper or induced fan setting when the draft and excess air are properly set.
As mentioned earlier in this practice, a negative pressure must be maintained throughout a heater. A positive pressure inside the heater will cause flue gas leakage and damage to the furnace casing and structure. It can also be a safety hazard to operating personnel. Figure 20 shows a typical draft profile within a fired heater. A draft reading of 1.5 mmH2O to 2.5 mmH2O (0.05 inH2O to 0.10 inH2O) at the radiant arch is desired. Too much draft will increase air leakage and result in undesirable effects explained earlier.
The user should always ensure there is sufficient excess air available to combust all the fuel. Combustion air should be increased prior to an increase in heater duty or fuel flow. Efficient operation is achieved when an optimum excess air level for combustion without producing a positive pressure at the heater arch.
It is not recommended to use the stack damper or burner air register alone for draft and excess air control without additional control applications. Using the stack damper without draft constraint applications on draft and O2 is not advisable. A combination of the two adjustments is necessary to obtain the optimum draft and excess air.
Figure 19 is a draft adjustment chart. For a given heater, with constant duty and fuel composition, closing the stack damper will, in general, have the following effects:
1) Reduced oxygen to the burners and in the flue gases.
2) Decreased draft at the radiant arch.
3) Increased flue gas temperature leaving radiant section. (For an all radiant heater, the radiant flux density is constant. When the excess air level is reduced, the bridgewall temperature will be reduced.)
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4) Decreased stack temperature.
5) Increased radiant heat flux density (an all-radiant process coil will have an unchanged radiant heat flux density).
6) Decreased convection heat flux density.
7) Increased heater efficiency.
Closing burner registers has the same effect on performance as closing the stack damper except the draft at the radiant arch will increase due to the reduction in pressure drop through the system with the flue gas flow.
Figure 19 Natural Draft Heater Adjustment Flow Chart
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Figure 20—Typical Draft Profile in a Natural Draft Heater