| OCR Text |
Show within wide variations in the F G R and excess air (EA). At the same time, pulsation is almost unavoidable as the velocity of the flow nears the flame speed. The velocity of flame propagation decreases as the adiabatic temperature of the combustion reduces with the increased amount of F G R or excess air. In order to avoid pulsation at low loads, the burner had to operate with even higher F G R or EA. With the reduced flame temperature, the N O x is reduced below the target point up to 1.7 to 2 ppm. At this point, even small increases in F G R or EA greatly reduces combustion efficiency (i.e., high C O and hydrocarbon emissions) and the flame is on the verge of blow out. At this N O x level, blow out can happen without a single pressure pulse in the system. Consecutive Modifications of the Burner Design and the Burner Performance. The understanding of the pulsation of the pulsation phenomena allowed refinement of the burner design and widen the burner stability limits. At first, the gas and air mixing section of the simplistic first prototype was refined towards higher uniformity mixing. This modification improved burner performance. The minimum N O x emissions of this "Prototype 2" were now 1.7 to 2 ppm. The amount of F G R in the at single digit N O x levels was also reduced by a few percentage points. The burner performance as different "Iso-NOx" lines is shown in Figure 1. Figure 1 also outlines operating limits of the burner at high fire and low flammability limit of methane mixed with air and F G R in a well stirred reactor. This limit was computed based as lower flammability limit of methane adjusted to the elevated temperature of the mixture when 70°F air is mixed with 350°F flue gas [Flammability Characteristics of Combustible Gases and Vapors. U.S. Bureau of Mines, Bulletin 627]. As seen in Figure 1, there is relatively small difference between the flammability limit and the edge of the excess air and F GR limits defined by the burner performance envelope. However, Prototype 2 of the burner was capable of only 3:1 turndown, even with manual controls. This was a small setback compared with the 4:1 turndown performance of the first prototype where mixing of fuel and air were less uniform. In the next developmental step, the burner was redesigned with several important features added, including modulation of the bluff body position with load. With a specially shaped burner throat and modulating center part of the burner and other changes, the discharge velocity of the mixture of fuel, air, and F G R was increased at low loads by a factor of three. The burner was also equipped with secondary gas injectors for injection of a minor portion of fuel into the periphery of the flame and oil firing capabilities. A schematic of the burner shown in Figure 2. 5 |
