Method of injecting ammonia into a flue for waste gases

In a method of injecting a gaseous mixture of ammonia and air into an exit flue, connected to the flue gas discharge end of an air heater receiving waste hot gas from an oil-fired boiler and having an air supply conduit and effecting heat exchange between the hot gases in a flue, leading from the boiler to the air heater, and the cooler air in the air conduit, from a number of spaced nozzles arranged in the exit flue, downstream of the air heater, in spaced groups distributed substantially uniformly across the exit flue cross-section and each including at least one nozzle, to neutralize sulfuric acid components in the flue gas, with the exit flue having positioned therein, downstream of the nozzles, structures including precipitators, the improvement comprising measuring the amounts of sulfuric acid components at uniformly spaced points of a rectangular grid extending across the exit flue cross-section downstream of the air heater;

• measuring the temperature at such uniformly spaced points;
• setting a distribution pattern of the gaseous mixture discharged from at least one nozzle of each group of nozzles to conform the distribution of the injected ammonia to the measured sulfuric acid component distribution;
• and compensating for the non-uniform distribution of the sulfuric acid components and the non-uniform temperature distribution over the cross-section of the exit flue downstream of the air heater by controlling total flow of ammonia, without modifying such distribution pattern, so as to adjust individually the molar ratio of the injected ammonia to the sulfuric acid components at each point in accordance with the measured temperature at the respective point, to a value such that the resulting reaction products will not melt and adhere to any structure connected to the exit flue downstream of the air heater.