The HCN pathway

In the presence of fuel-bound nitrogen in the form of pyrrole- or pyridine-type compounds HCN is formed; this compound is also generated to some extent through the reaction of CH radicals with molecular nitrogen (prompt NO). The consecutive reactions in the flame system are very complex, as shown in Figure 9.3.

N2O is formed mainly by the reaction of NCO radicals with NO:

and to a lesser extent by the reaction:

Figure 9.3 Scheme of the most important reactions during the formation of NO and N2O through fuel-bound nitrogen and prompt NO through the reaction CH + N2

Source: After Becker et al (2000a)

Figure 9.3 Scheme of the most important reactions during the formation of NO and N2O through fuel-bound nitrogen and prompt NO through the reaction CH + N2

Source: After Becker et al (2000a)

The formation of nitrous oxide is, however, counterbalanced by its very fast destruction by hydrogen radicals, according to the reaction:

Calculations performed by Kilpinen and Hupa (1991) demonstrated a decrease in the N2O concentration with increasing temperature. Above 1200K, NCO radicals are converted almost completely into NO, whereas with decreasing temperature N2O formation through NCO radicals increases.

From the competition between N2O formation and destruction reactions, it follows that any changes in conditions that substantially decrease the hydrogen atom concentration in the N2O formation zone can be expected to increase gas-phase N2O emissions. Examples are (1) lowering the gas-phase combustion temperature, which might well provide a partial explanation of relatively high N2O emissions measured from fluidized bed combustors; and (2) air or fuel staging, i.e. where secondary air or air/fuel is introduced into low-temperature combustion zones. For the same reasons, an increase of N2O emissions should be expected when - as mentioned above - ammonia, urea or other amine or cyanide species are injected into combustors, especially at relatively low temperatures.

In conclusion, non-negligible N2O emissions may arise from gas-phase combustion when:

• mixture inhomogeneities are created;

• temperatures of oxidation zones are low;

• the oxygen concentration is increased.

In addition to gas-phase chemistry, N2O can be formed or destroyed during combustion by heterogeneous reactions, defined here as reactions either occurring between a gaseous reactant and a solid reactant, or between two gaseous reactants but taking place at the gas/solid interface, where the solid may play the role of a catalyst.

As far as formation and destruction of nitrous oxide is concerned, the following heterogeneous reaction mechanisms have been identified and, at least, partly described:

• In reactions taking place during combustion:

• reduction of N2O on char- and soot-bound carbon atoms;

• N2O formation from char-bound nitrogen atoms.

• In reactions taking place during catalytic after-treatment of combustion products:

° formation/destruction of N2O during selective catalytic reduction (SCR);

• formation/destruction of N2O during NO reduction in engine exhaust by three-way catalysts.

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