OCR Text |
Show Secondary reactions of other combustion products with ammonia must be considered to assure that new pollution problems are not created. Exxon has reported that nitrous oxide (N.O), which is present from natural sources and is not considered a pollutant, is formed to the extent of 1% to, 2% of the NO reduced. The ammonia also inhibits further oxidation of CO to CC* , 2 but CO is normally already oxidized in the flame zone. Cyanide (HCN) formation has been detected when hydrocarbons are present where the ammonia is injected. As with CO, hydrocarbons should be burned prior to the ammonia injection point, and injection of hydrocarbons with the NH is not planned. The laboratory tests on coal firing showed no cyanide formation resulting from ammonia injection. Ammonia in the combustion gases will also tend to react with S0_ formed from sulfur in the fuel to form ammonium sulfate and/or ammonium bisulfate depending on concentrations and temperature. The sulfates are less corrosive than sulfuric acid. The only potential problem is condensation and collection of ammonium bisulfate in the cooler sections of the boiler backpass or air preheater leading to increased pressure drop. This can be alleviated by intermittent water washing to remove the highly soluble sulfate. This represents a potential operational requirement for the owner rather than an air pollution problem. The ammonia is injected into the flue gas along with a carrier gas to improve mixing characteristics. The carrier is usually either steam or air. Use of low pressure steam minimizes capital costs but results in higher operating costs than air. It is anticipated that a refinery or oil producer might use steam as a carrier while utilities would probably use air as a carrier. 2.2.2 Development Status The Thermal DeNOx process is considered by Exxon to be a developed, commercially available process based on the Japanese installations. The Exxon technology applications, which yielded 40% to 60% NOx reductions, were on process heaters and refinery boilers up to about 90 MW equivalent. The process has yet to be demonstrated in the U.S. on a large utility boiler. Two installations are proceeding in Southern California refinery application. 4-17 P-233 |