Environmental Engineering Reference
In-Depth Information
turbulence. The probability density function of temperature fluctuations, or at least
its intensity, should be taken into account in the calculation of NO.
Heavy oil and coal generally contain significant amounts of organic nitrogen
compounds, and the fuel nitrogen can be converted into fuel NO even at relatively
low temperatures around 1000˚C. The amount of fuel nitrogen converted to NO is
considered high for lean combustion and low in rich combustion. Although the
nitrogen content in fuels varies significantly, no nitrogen content is mostly assured
when gaseous hydrocarbon fuels are used. Prompt NO is first recognized by the
discrepancy between actual NO concentration in flames and the concentration pre-
dicted by the Zel'dovich mechanism even at temperatures as low as approximately
1300˚C. The term prompt NO was initially used because of the rapid formation of
NO in the flame, but it means NO formed in flames by mechanisms other than the
Zel'dovich mechanism in a more general sense.
Prompt NO is now considered to be formed via an HCN group, that is, N atoms
are formed mainly through Reactions R4 through R9. In turn, N atoms are consumed
by Reactions R2 and R3, and converted to HCN through Reactions R10 and R11.
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CH + N
2
⇔ HCN + N
(R4)
C + N
2
⇔ CN + N
(R5)
CH
2
+ N
2
⇔ HCN + NH
(R6)
CH
2
+ N
2
⇔ H
2
CN + N
(R7)
CO + N
2
⇔ NCO + N
(R8)
H + N
2
⇔ NH + N
(R9)
N + CH
3
⇔ H
2
CN + H
(R10)
H
2
CN + M ⇔ HCN + H + M
(R11)
Therefore, the presence of a C atom plays an important role in the scheme.
Consequently, prompt NO must be taken into account only in hydrocarbon flames.
Since thermal NO predicted by the Zel'dovich mechanism exhibits strong dependency
on temperature, the discrepancy between the predicted thermal NO and the actual
emission level has been ascribed to the presence of prompt NO, which becomes large
particularly in lean combustion or low temperature flames. Recently, it has been found
that the Zel'dovich mechanism predicts too much NO also in the higher temperature
range compared with the value obtained by the full kinetic scheme including the
chemistry of prompt NO. The coupling of thermal and prompt mechanisms should
always be important in correct prediction of NO emission.
25
It was proved that both
thermal and prompt mechanisms would produce a large concentration of NO with the
increase of flame temperature. Adding to these facts, we know from long-term expe-
rience that temperature is the most important factor governing the formation of NO
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