Fuel NOx and Prompt NOx

Fuel NOx is produced if nitrogen is chemically bound in the fuel molecule and is primarily a concern with heavy oils and solid fuels. Some gaseous fuels, however, can contain NH3, HCN or amine carry-over as potential sources of fuel bound nitrogen. For fuels with organically bound

nitrogen, the fuel NOx mechanism begins with the decomposition of the organic molecule in the flame zone:

CxHyN → Cx-1Hy-1 + HCN

or CxHyN → CxHy-1 + NH,

depending on the nature of the carbon/hydrogen/nitrogen bonds. The HCN or NH reacts further and may be oxidized to NO.

Prompt NOx is the NOx formed from N2 in the very early portion of the flame zone where the fuel and air are first reacting. It is formed in a part of the flame where little, if any thermal NOx should be formed and, by definition, is that NOx formed from molecular nitrogen which is in

excess of the NOx predicted by the thermal NOx mechanism. There are several reaction paths postulated for forming prompt NOx. One of the more important involves the reaction of molecular nitrogen with hydrocarbon radicals formed during the decomposition of the fuel in the

initial reaction zone. The major reactions are:

CH + N2 ↔ HCN + N,

and C + N2 ↔ CN + N.

The fuel NOx and prompt NOx mechanisms proceed identically after these initiation reactions.