OCR Text |
Show to the mass production rate of NOx from each source. Identification of potential NOx sources becomes easier if the mass production rate is expressed in terms of an integral over the source volume as . -MLV.l d[NOx]d mNOx - v d v v t (2) where V is the volume in the flow over which NOx is produced, and [NOx] is in terms of moles per unit volume. Note that, by convention, JvJvV is chosen as the molar weight of N02 . For simplicity, the scaling model takes conditions within each NOx-producing region to be uniform, so that . d[NOx] mNOx = MWN02 . dt . V (3) The equilibrium thermal NO formation rate in (3) is then (Bowman [14]) mol (4) cc· sec Close inspection of the temperature fields and chemical species concentration fields from the in-flame data [8-13], examples of which are shown in Fig. 9, indicate several possible thermal NO formation zones in these burners and furnaces. Of particular interest are regions in which oxygen concentration levels and temperatures are high enough, and in which the volume is large enough, so that their product as given in (4) is large enough for significant rates of thermal NO production. Simple order-of-magnitude estimates show that there are four distinct regions in the flow that satisfy these requirements for NO formation. These regions, shown schematically in Fig. lOa, consist of: 1. The near-burner region. This region, shown in Fig. lOb, consists of the near burner recirculation zone and the adjoining high temperature region near the burner, and is characterized by intermediate temperatures and relatively high oxygen concentrations over a significantly large volume for the net contribution to the total NO production to be significant. 2. The flame-sheet region. Within the thin and highly wrinkled flame-sheet region shown in Fig. 10c, temperatures are sufficiently high so that, even though the volume is very small, the resulting contribution to the NO mass flux in the flue can be comparable to that from the other regions. 3. The furnace region. In the interior of the furnace, shown in Fig. 10d, temperatures and oxygen concentrations are relatively low, but the volume is very large, so that the net contribution to the total NO production can be significant. 4. The staging-jet region. As shown in Fig. lOa, staged fu el injected into the relatively high oxygen concentration zone emanating from the combustion air duct forms a jet-like flame, the tip region of which is characteri zed by moderate temperatures and oxygen concentrations over a relatively large torroidal volume, shown in crosssection in Fig. -10 e. 7 |