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Show 3. NEW SYSTEM DESIGN (cont'd) Injection Methods Once the grade of ammonia has been determined, the injection method must be chosen. Anhydrous ammonia injection is usually accomplished by the use of a distribution grid, whereas aqueous can be done by either a grid or direct injection. The distribution grid method is the most reliable but requires more equipment, hence controls than direct injection. Direct injection is often desirable due to lower cost, but more importantly, for ease of control. Direct injection is only recommended with aqueous ammonia due to the greater volume and thus better spraying characteristics it poses. The quantity of anhydrous ammonia to be injected is usually so small (onefourth that of aqueous) that a carrier gas, such as air, is required making the advantages of a grid unavoidable. The one major requirement that must be checked and satisfied for either injection method, is mixing time prior to the SeR bed. SeR manufacturers generally require less than one half of a second of mixing time for grids and 1-2 seconds for direct injection. With fluegas velocities approaching 40 ftl sec, that translates to about 20 feet of duct downstream of the grid to the SeR or 40-80 feet for the direct injection point. For a typical SeR system, the SeR is located in the downleg of the ductwork to save plot (Figure 4). For grid injection, this requires that the grid be installed upstream of the return bend causing possible distribution problems. Further, the plot requirement for direct injection is impractical. To combat these problems, turning and mixing vanes can be installed in the duct to ensure proper mixing. The design of these vanes, as should be obvious, becomes the critical item in the system. Flow modelling programs along with engineering ingenuity are combined to achieve the best mixing in the shortest distance. Control Method Now that the ammonia grade and injection technique are chosen, the question, "How do we know and control the amount of ammonia to inject?", must be answered. This is a critical issue in that an unstable injection system makes consistent NOx reduction almost impossible. Three control methods have been devised: • Feedforward • Feedback • Feedforward with Feedback Reset 9 |