OCR Text |
Show Injection of ammonia and mixing with the flue gas must be quickly and effectively accomplished to take advantage of the limited residence time available. Basically, a multiple injector system is designed for each application. The design procedures and know-how are proprietary. Exxon makes performance guarantees for the system based on their experience. The mechanical and operational impact on the boiler is slight. The only change to the existing boiler is addition of the injection grid. The ammonia supply system components (tank, vaporizer, and controls) are all external and independent so a system failure would not compromise the boiler. The only adverse effect anticipated would result from the reaction of a few ppm of SO in the flue gas (most of the sulfur in the fuel will be converted to SO ) with part of the ammonia to form ammonium sulfate [(NH ) SO ] or ammonium bisulfate (NH HSO ). Ammonium sulfate is a dry crystalline compound which will emerge from the stack with the flue gas. Ammonium bisulfate condences at about 520 °F. Although sulfates are emitted, they may or may not add to the quantity of particulate emissions, depending on the measurement procedure. In California's South Coast air basin, SO. is already counted as particulate (and calculated as sulfuric acid dihydrate, [H2S04.2H2Q]) by the AQMD. The emitted material is also partially neutralized and therefore less environmentally damaging than sulfuric acid. The condensed ammonium bisulfate can accumulate on relatively cool surfaces such as the preheater wheel. To prevent increasing preheater pressure drop, the Japanese have utilized soot blowing and intermittent water washing of the preheater. The deposits are very water soluble but may be difficult to remove by soot blowing. The ammonium bisulfate is mildly corrosive, but less so than sulfuric acid. For all except base load units, it will be necessary to accommodate for changes in boiler load as system demand varies. This can be achieved either by installing more than one injection grid at locations appropriate for different loads or by adding hydrogen in varying amounts to adjust the optimum reaction temperature to match the actual temperature. Since the mass emissions (lb/hr) from a unit are roughly proportional to the firing rate, a reduction in load will automatically reduce total poundage of NOx emissions compared to high load conditions. 4-19 p-233 |