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Show 6 In some situations it is also possible to fire a burner into a secondary combustion air tile where this tile would be operating at a much higher temperature than the rest of the environment inside the furnace. This situation may arise in some thick wall installation such as brick kilns. Although the overall ambient temperature of the furnace may be relatively low, because the time at temperature criterion we have applied to NOx formation have been altered such that the flame retains its heat much longer, we would expect to see much higher NOx levels from this type of geometry than we would if the burner would be allowed to fire into a free open chamber. If you take the example of the large reheat furnace for instance, the application of an intelligent programming system which would shut the furnace onto intermediate low fire settings in the situation that the production has been stalled for some reason, will give two net benefits. The first is obvious in fuel that is saved. The second is less obvious in that a reduction in the chamber temperature often means a reduction in the rate at which NOx is formed. The point I am trying to make is that many simple things may be done to furnaces to make them operate much more efficiently and to make them operate much cleaner and it is these items which must be addressed first in any retrofitting application where NOx reduction is the object of the project. The second method of NOx reduction i.e. burning with reducing flame, is only an option where a controlled composition reducing gas is required for the process. Or in other situations where the resultant reducing gas can be handled efficiently cleanly and safely from the process such as in a dedicated atmosphere burn off or after burner. It should be noted however, that the situation where the after burner is the consequence of firing the burner with a reducing mixture there may be no net NOx decrease in that NOx will tend to be generated in the afterburner rather than in the process. The third option and one which has consumed a lot of our time recently has been reduction flame 1eJnPer.ature. Wit:b our FGR burner we r~ally have a combination of effects. However in its simplest form the reduction of the flame temperature can be achieved in a number of ways. It may be decreased by increasing the concentration of inert gases travelling with the combustion air such that the overall heat capacity of the combusting gases is increased and therefore flame temperature is decrea~ed. It may also be decreased by doping the flame with very small amounts of very high heat capacity materials such as water, or other similar compounds. One of the simplest methods of NOx control that we have been examining is that of returning flue gas to the burner mixed with the combustion air supply to increase the specific heat capacity of the inert fraction of the gases being heated by the flame. |