OCR Text |
Show work covers the wavelength range 290-400 nm. This spectral region has the advantage of being free of background contributions from wall radiation that is predominately in the visible and IR. The U V spectral features that are particularly useful is the emission from O H radicals and the appearance of a continuum that may be attributed to soot radiation and to C O ; * emission resulting from C O + O -> CO;* recombination reaction [16]. By monitoring the intensity of the OH (0-0) emission band of the (A:X-»X:n) transition at 306.4 nm, information on fuel/oxidizer mixing, stoichiometry, firing rate, and pollutant formation can be obtained. The O H emission intensity is observed to be much higher in oxygen flames than in air flames. In air flames, the source of O H radiation results essentially from exothermic reactions involving radicals (chemiluminescence). It must be noted that the thermal excitation, which is generally considered negligible in air flames, can be for oxy-fuel flames up to 6000 times stronger. A detailed study on the contribution of both chemiluminescence and thermal emission is underway at Air Liquide. A number of examples using the O H emission for monitoring stoichiometry and firing rate are presented elsewhere by Von Drasek et al. [1.2]. In these examples O H emission is monitored by a P C spectrometer board that uses a neural network to relate the observed flame emission to stoichiometry and power. The technique shows potential for monitoring dynamic changes in such processes as batch, waste-incineration, and oscillating combustion. The visible radiation from the flame (400 to 800 nm) mainly appears as a broad continuum resulting soot radiation towards the red with a contribution from CO;* hot bands towards the blue region of the spectrum. In general. monitoring this spectral region is difficult because of the background contribution from the hot furnace walls. Close to the burner nozzle emission from the radicals C H (432 n m ) and C; (473 n m and 516 n m ) can be detected [16]. The emission intensity of these radicals can be related to the burner firing rate or changes in the fuel composition [1,2]. Farther downstream, emission from these radicals quickly diminishes due to lower radical concentrations from dilution effects and flame temperature reduction, as shown in the series of spectra in Figure 11. For this case a 26 k W burner was used with a conventional tube-in-tube design. At large distances from the burner nozzle, only O H appears along with a strong continuum due to soot and CO;* emission. The time-averaged spectra were collected at various distances from the burner nozzle (X/D. where D is the burner diameter and X is the distance from the nozzle along die centerline) using a 0.5 m scanning monochromator. Background contributions from the furnace walls are negligible since the walls were cold. OH Imaging Collecting flame emission data using conventional spectrometers, as discussed above, provides only local path averaged spatial information. Use of such techniques to obtain a complete flame mapping is often a laborious procedure and is not practical on industrial scale flames. To overcome this difficulty, imaging techniques are implemented to collect spatial flame emission information over the entire combustion region, thus providing insight into flame and aerodynamic behavior. All of the spectral regions discussed in the previous section can be adapted for imaging using special cameras and filters. However, O H imaging of oxy-fuel flames in furnaces is particularly useful because its free of background wall radiation. In addition, the O H intensity observed from oxy-fuel flames is much greater compared with air-fuel flames due to the higher O H concentration and higher temperature. Because of the high emission intensities observed from oxy-fuel flames, conventional C C D cameras (with their U V absorbing components removed) can be used. A n interferometric filter centered on 308 n m and with a bandwidth of 10 n m is adequate to discriminate the O H emission from other emitting sources. Figure 12 displays as an example the time-averaged OH emission field of a 30 kW burner with an annular natural gas stream between two coaxial oxidant outlets [17]. This image was obtained by averaging 50 background subtracted O H images. The high emission intensity areas along the interface between the fuel and central oxygen stream correspond to high heat release zones. In this example, 4 0 % of the total oxygen is injected from the center tube at 41 m/s. The high oxygen velocity increases the mixing with the fuel resulting in high emission intensity. For the outer oxygen flow, the exit velocity is only 5.2 m/s, thus resulting in weaker mixing and weaker emission intensity. 1 Estimated from Boltzmann distribution factor and Einstein transition probability [18] for adiabatic stoichiometric O2/CH4 and air/CfL flames. 6 |