OCR Text |
Show NO Formation in Flames In natural gas combustion, two processes are thought to be imponant in NO fonnation. The fIrst, postulated by Fenimore, l is so-called prompt NO, formed near the flame zone. CH radicals, produced by chemical attack of H and OH on methane, react with N2 from the air to form HCN and N atoms. The N atoms react directly with O2 and OH to produce NO; the HCN undergoes a series of oxidation steps to later form another NO molecule. Typical amounts produced in premixed, laminar flames are several parts per million to tens of parts per million, although the amounts found in diffusion and/or turbulent flames are often more. The second mechanism 2 is thennal NO, also called Zeldovich NO. 0 atoms present in hot flame gases react with N2 in the air to form NO. The first mechanism produces significant NO at relatively moderate temperatures, around 1500K, but the second process becomes imponant only for temperatures above 1800K. All of the species involved in NO fonnation are minor constituents in flame environments. In each case, the production process is governed by chemical kinetics; even in the Zeldovich thermal NO case the 0 atoms are usually present at chemically controlled super equilibrium amounts. Thus a proper description requires detailed chemical models and measurements of high sensitivity . Laser-Induced Fluorescence In the method of laser-induced fluorescence (LIF), a laser is tuned to the same wavelength as an absorption line of the molecule of interest. The absorption process creates an electronically excited state which is able to radiate photons. Those fluorescent photons are then detected to form the signal. LIF is quite sensitive and can be used in flames to measure many species at the ppm or even ppb level, so it is eminently suitable for an investigation of the chemistry of the fonnation of NOx• The method has other attributes that are also appealing for combustion measurements. Due to the narrow bandwidth of the laser, LIP can be very selective for molecules with distinct spectra. Even if more than one molecule absorbs at the same wavelength, these can be distinguished by the spectral characteristics of the fluorescence. The size of the laser beam can be made less than 0.1 mm to resolve the structure of flames even at atmospheric pressure. When properly applied, so that photochemically induced interferences are avoided,3 laser methods do not disturb either the flows or chemistry of the flame, and can be used in hostile environments. LIP may be employed in several ways to study flames. In one approach, measurements can be made of distributions of chemically active species in turbulent flows. These can be pointwise measurements furnishing distribution functions, or two-dimensional images providing an 2 |