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Show Taken together, the simultaneous readings of C O and N O x are the most precise metric in identifying the accuracy of flame fit. As C O is reduced, combustion efficiency increases. However, the lowest equilibrium levels of C O are not practical to achieve in a highly diffusion limited flame surrounded by cold evaporator walls. In addition, as C O is reduced, the rise in unburned hydrocarbons and other volatile organic compounds reaches a point where any further reduction in C O produces no net efficiency gain. As a practical measure in industrial boilers, four-hundred parts per million by volume of C O in the flue gas is equivalent to one one-hundredth of one percent of the gross heat input from the fuel. C O concentrations less than 400 ppmv are practical to achieve with quite low levels of other pollutants. 100 ppmv can also be achieved with low V O C s and particulates, but with an increase in N O x . Ten ppmv of C O can also be achieved along with acceptable V O C s and particulates in a proper installation, but with an additional increase in N O x . In addition, achievement of low C O requires furnaces which do not leak, symmetrical combustion air flow entering the burner and no asymmetric burner exit plane distortions. The human eye cannot see the difference in the flame size for a flame which produces four-hundred ppmv C O , and a flame which produces one-hundred ppmv C O . C O measurement is essential to determining the flame fit. The design analysis method does accurately provide the repeatable analytical means to shape flames that provide the desired concentration of C O in the equilibrium products FLAME FIT EFFECT ON EMISSIONS As the data base has grown for flames designed in advance of installation using the now validated analytical methods, the emissions measurements have been used to determine the precision of the flame fit, whereas the human eye is limited to more global observations. Flames designed by analysis, which have very high C O at initial commissioning, usually have a problem, or problems, which are imposed on the flame. Such problems include skewed air flowing into the burner, incorrectly installed equipment, leaking furnaces, and incorrect operation. The human eye can detect some of these gross changes in flame shape and provide accurate guidance in remedying them. In flames which are otherwise a good fit, the C O measurements provide the accuracy required in confirming the analytical flame design results, and for minimizing NOx. C O is a valuable diagnostic tool in flame design, as is N O x . Figure 4 shows the behavior of C O in three different cases. C O is plotted on the ordinate, versus load on the abscissa [9]. The upper curve is characteristic of a flame which lengthens as furnace volumetric heat release increases with load. The middle curve is indicative of a flame which is too wide or too close to the floor or roof. The bottom curve is characteristic of a precisely fitting flame at all loads. The C O remains low at all loads until the highest rated load. As the furnace volumetric heat input reaches the maximum rated value, the C O will begin to increase in a furnace which has not been over designed with respect to enclosed volume. Figure 4. CO VARIATION WITH LOAD. Figure 5 is a plot of NOx versus boiler load [9]. The LONG curve monotonically increases until a load at which the N O x concentration begins to decrease. The N O x reduction is characteristic of a flame which stretches out too far and impinges on the target wall or enters the superheater or convective tube bank. The emissions measurements provide the diagnostic tools to assess flame shape accuracy. The PRECISE curve is characteristic of an accurate flame fit. This is the curve to be expected from a flame designed using the analytical methods which are the subject of this paper. The next higher curve is the result of a flame which has more clearance than necessary to all interior furnace surfaces. The highest curve is characteristic of multiple burner flames overlapping. The shape and location of N O x curves provides valuable insight in diagnosing flame shape problems. Curves in Figure 5 are related to those in Figure 4. As N O x concentration is reduced, C O increases. T wo examples are evident in Figures 4 and 5. Comparing both curves for the longest flames shows that as the flame is lengthened to the point where some of the heat release in the flame occurs at and along cold boiler tube |