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Show 3. NEW SYSTEM DESIGN (cont'd) Fan Control and Reliability (cont'd) A lost fan or driver would either put the burners out instantaneously or depending on the shutdown system, shut the heater down. Neither case is desirable, the former a serious safety issue, the latter, loss of production opportunities. In an attempt to protect the system from unnecessary problems, different fan configurations can be considered. The use of these different configurations are widespread with a single fan dual drive being the most economical and recommended. Fan control is accomplished with either parallel inlet louver dampers (when motor driven), or by speed (when turbine driven). Pressure control for fans is usually selected over flow due to the close relationship between the two and the ease of measuring pressure over air or flue gas flow. Ammonia Injection Control In a previous section it was noted that no matter how good a LoNOx burner is at reducing NOx, it is only as good as the oxygen level it is controlled at. The same is true with an SCR unit and ammonia injection. The concept of an SCR unit is to mix ammonia (in a 1:1 mole ratio) with the NOx bearing fluegas prior it passing through a catalyst bed. The problem that arises is that while the main concern is to minimize the amount of NOx going out the stack, there are also regulations that limit the amount of ammonia that leaves the stack (ammonia slip). Any ammonia injected into the fluegas that doesn't convert the NOx simply passes through to the stack. Ammonia Grades The choice of which ammonia grade (aqueous versus anhydrous) to inject is a difficult one. Whereas anhydrous ammonia is much simpler to inject, the safety issues associated with the storage of it makes most consider aqueous ammonia injection. The problems with the control of aqueous ammonia injection are that it requires that a solution (usually 25%) be made and unlike and inherent to anhydrous, it requires that this solution be vaporization. In the case where the aqueous ammonia is made on site, a poor quality solution can be expected unless great care is made to only use extremely clean ammonia and water. If the solution is of poor quality, it results in plugged instrumentation and injection nozzles. 8 |
