Staged burners

Staged Burners As was pointed out earher under ToUutant Formation and Control in Flames, the proper staging of fuel or air in the combustion process is one technique for minimizing NO emissions. Gas burners that achieve such staging are available. 

Fuel-Staged Burners Use of fuel-staged burners is the preferred combustion approach for NO control because gaseous fuels typically contain little or no fixed nitrogen. Figure 27-36 illustrates a fuel-staged natural draft refineiy process heater burner. The fuel is spht into primaiy (30 to 40 percent) and secondary (60 to 70 percent) streams. Furnace gas may be internally recirciJated by the primaiy... 

FIG. 27-36 Low-NOj fuel-staged burner for a natural draft refinery process beater. (Callidus Technologies, Inc. )... 

A butterfly valve in the line to the first stage seal drum limits the maximum flow to the first stage burner. The valve is set by observing the burners while flaring at design capacity. Once adjusted, the valve should be locked in position. 

Piping to Burners - First and second stage piping and headers, as well as the burner lines themselves, are sized to minimize pressure drop and velocity effects. Thus, maldistribution of flow to the burners will be minimized. The burner lines are fabricated from standard 1(X) mm pipe, and are arranged in a split grid layout with distribution headers and split feed lines on opposite sides, for both first and second stage burners. First and second stage headers must be sloped so that any condensate will drain back to the seal drums. However, the burner lines must be accurately installed in a horizontal plane. 

LIMB [Lime/limestone injection into a multi-stage burner] A flue-gas desulfurization process used in Germany and Finland. Dry, ground limestone is injected directly into the combustion chamber. This reacts with the sulfur dioxide, and the dry particulate product is collected downstream together with the ash. The process is suitable only for those systems which limit the maximum combustion temperature by staging, in order to minimize the production of oxides of nitrogen. 

Low-NO burners are designed to delay and control the mixing of coal and air in the main combustion zone. A typical low-NO air-staged burner is illustrated in Fig. 24-16. This combustion approach can reduce NO emissions from coal burning by 40 to 50 percent. Because of the reduced flame temperature and delayed mixing in a low-NO burner, unburned carbon emissions may increase in some apphcations and for some coals. Overfire air is another technique for... 

Air-staged burners Low-NO air-staged burners for firing gas (or oil) are shown in Fig. 24-28. A high-performance, low-NO, burner for high-temperature furnaces is shown in Fig. 24-32. In this design, both air-staging and external flue-gas recirculation are used to achieve extremely low levels of NO emissions (approximately 90 percent lower than conventional burners). The flue gas is recirculated by a jet-pump driven by the primary combustion air. 

NOx stands for a variety of nitric oxides. Many heaters in the United States have been retrofitted with staged burners in the last decade or so. These staged burners combust the fuel in two or three stages. For example, in the burner shown in Fig. 21.6, 50 percent of the fuel is burned with 100 percent of the air. The flame produced by this first stage of combustion radiates heat to the process tubes and refractory walls. Next, the remaining 50 percent of the fuel is added around the circumference of the first-stage burner. This second stage of combustion also liberates radiant heat. But because the radiant heat is liberated in two steps, the maximum flame temperature is reduced. This has two favorable results ... 

The first example is a small-scale laboratory combustor using an aeroengine gas turbine burner (power 30 kW) while the second one corresponds to a laboratory-scale staged burner in which self-excited instabilities can be easily triggered by changing the outlet acoustic boundary conditions. In staged combustors, fuel and air are premixed but they are introduced into the chamber at different locations and different equivalence ratios so that partially premixed flames are found inside the burner. All combustors are operated at atmospheric pressure. 

Case 2 Self-Excited Staged Burner Configuration... 

Experimental furnace with air-staged burner and instrumentation. (From Hernandez, R. and Ballester, Combustion and Flame, 155, 509-28, 2008. With permission from Elsevier.)... 

As in the previous section, the results with the air-staged burner are used to explain the method in detail, whereas its application to the premixed combustor is summarized in the last subsection. 

Section through the model of gas fuel/air-staged burner. 

For low power oil applications, as used in residential appliances, it is still too expensive to modulate heat power. This is why the technology improved just from a single stage burner to double stage burners, where power is modulated only on two levels. The air-fuel control has no closed loop feedback except for specific applications where O2 or CO sensors on the chimney are used. Regarding the CO sensor, it is interesting to note that there isn t any univocal correlation between CO emissions and excess of air. In this case, closed loop control reduces excess of air until CO starts to increase and then comes back a few steps, controlling combustion on the borders of the safety limit. ... 

---