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Show cool-bound NITROGEN devobliHsolton I f -/.'the cyonide/omine - pool ////A MNx '<# 4 INx rs,/A clfTCt char combustion \ «Cv i £ • NO .1 & NO N2 Ihernml-NO formation mechanism FIGURE 5: SIMPLIFIED FUEL-NO MECHANISM 27 sl - M/Vx = 7T ) W rw/ 14 where 7vol is the nitrogen content in the volatile matter and is the release rate of volatilcs. (26) vol The reduction of M N x via path 3, is determined by the amount of Ox-species and H-radicals available. Here, this amount is simply approximated by the amount of available 02- Hence, the sink of M N x can be expressed as rv-NO = c ^ [ 0 2 ] [ N 2 ][/,«/]exp(-|^) (25) The constants, C, b and E a lake the following values: C = 6.4 x l()6 s 1 , b = 0.5. E a = 72.5 x 10^ cal/gmole. In the calculations of natural gas flames presented later on, Eq. (25) is used to calculate prompt-NO formation rates. The effect of fluctuating temperature is accounted for by the Beta-pdf function in the same way as for thcrmal-NO (see Equation 18). In calculations of N O emissions from pulverized coal combustion, the prompt N O is omitted. Fuel-NO. The coal-bound nitrogen is released from the pulverized coal cither via dcvolatilizalion or direct char combustion (see Figure 5). In the case of devolalilization (path 1), ihc coal-bound nitrogen enters the gaseous phase mainly in the form of hydrocyanic acid (HCN) or ammonia (NH3). These are the main nitrogen-containing species of the volatile matter, in short, MNx-spccics. Due to very fast reactions with oxygen-containing (Ox-) species (O, O H , 6 2 ...) and H-radicals, the M N are transformed into mainly NCO-, NH}. and N-radicals (path 3) which arc called intermediate nitrogen-containing species, in short, INx-spccies. The whole of MNx-spccies and INx-species is called the cyanide/am inc-pool (De Soete, 1990). In the case of direct char combustion (path 5), the assumption is made that the coal-bound nitrogen is released into the cyanide/am ine-pool in the form of INx-species directly. Depending on the conditions in the flame, a fraction of the INx-spccies reacts with Ox-species to finally form fucl-NO (path 4). Within the mechanism depicted in Figure 5, there are two routes to reduce the N O formed. In the first route, the remaining INX-specics, those which do not react with the Ox-species to form N O are available to react with N-containing species (mainly N O ) to form molecular nitrogen (path 6 in Figure 5). In the second route, the N O formed can, under fuel-rich conditions react with CHX - radicals to form M N x species (path 2 in Figure 5). The former route is frequently named the air staging mechanism while the latter the fuel staging mechanism. The nitrogen-content in the volatile matter of the coal can be established in the coal devolalilization experiments. It is reasonable to presume that the source of MNx-species due to dcvolatilizalion equals: Sr3-MNx = A •p min y»MNx *mNO J (27) In the above expression, it is assumed that as soon as the M N x species are mixed with oxygen they react infinitely fast to form NO. Thus, the above formula is an extension of ihc eddy-brcak-up model (see Eq.(5)). If all the MNx destroyed were transformed to NO, then ihc source of fuel-NO formed via paths 3 and 4, would equal (30/27)Sr3_MNx. However, depending on the conditions in the flame, only a fraction (the so-called yield) is actually transformed. In general, the yield is determined by the local stoichiometry, the temperature, and the amount of N O already present. At this stage of model development, it is assumed that this yield depends on only the local stoichiometry. An expression to roughly approximate the yield, H ^ o has been obtained from graphical data of De Soete (1990). ¥ NO ri.o = A 2.1 Lo.o 175 - 1.25/X if 1/X<0.94 if0.94<l/X<1.74. if l/tel.74 (28) In oxygen rich regions of the flame, the yield T N O equals 1.0, and, in oxygen lean regions, it equals 0.0. Between these two extremes, it varies linearly with the local stoichiomctr X. Hence, the actual source of fuel-NO formed via paths 3 and 4, equals JfM-NO - 3 0 ^ - - - 11 27 NO ^r3-MNx (29) Similar expressions (see Peters and Weber, 1991) can be derived for all the other sources and sinks of the simplified mechanism shown in Figure 5. In the model described above, the complex chemistry for the fate of fuel nitrogen has been drastically simplified. The major chemical reactions are mixing controlled with the rate of mixing calculated using the eddy-break-up model. Oihcr approaches to modeling of fuel-NO assume lhat the MNx-spccics (HCN) arc converted to N O and N 2 via (De Soete, 1975): 11-11 |
