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Show reduction followed by post-combustion treatment is required to achieve Rule 1109 compliance. Ceramic Fiber Radiant Burner Ceramic fiber radiant burners (Figure 5), characterized by uniform, flame~ess heat transfer, low NOx, and low noise, are attractive to the refining industry. ~~ burner can achieve emissions limits as low as 20-25 ppmvd. Although it can be slgruficantly cheaper than an SCR system, the ceramic burner results in a vastly altered combustion environment in the heater. Having a flame different from that of a standard burner and having a much lower burner surface temperature, the ceramic burner has to be placed much closer to the process tubes in order to achieve the same heat flux. Unfortunately, the alteration of the heater firebox geometry can be very expensive and require significant downtime. This limits the applicability of such burners in many retrofit situations. This technology is far more suitable fQr new heaters. Another drawback of such a burner is its fragile surface. The ceramic layer, which is coated on either a carbon steel or stainless steel mesh, is easily damaged. Further, the fiber layer is easily clogged with impurities and therefore an air filter is required. Special transportation, handling, and installation procedures are needed. Ceramic fiber burners also have higher pressure drop on the air side as compared to standard burners. Therefore a forced draft fan is required. Unlike conventional burners, air preheat (APH) seems to have little effect on NOx formation. Selective Catalytic Reduction (SCRl The most feasible technology at the present time to achieve compliance with Rule 1109 is Selective Catalytic Reduction (SCR). Figure 6 shows a schematic of the SCR approach which, by itself, can achieve NOx reductions in the range 80 to 90 percent. It is not uncommon to fmd furnaces combining a SCR unit with standard low NOx burners to achieve outlet NOx concentrations below 10 ppmvd. Among the advantages of SCR is its range of applicability. Also it is higbl y reliable when used in gas fired heaters. Among some of its principal disadvantages are cost, complexity, and fluegas temperature requirements. An SCR unit requires significant amount of extra equipment. Since the fluegas flow through the catalyst bed causes additional pressure drop, an extra fan or increased fan power is required on the fluegas side. Practically, this means that furnaces that are presently operating under natural draft will require an induced draft fan. Furnaces that do have induced draft systems may require upgrades to provide for the extra fluegas pressure drop. SCR units also require ammonia injection into the fluegas upstream of the catalyst bed. Ammonia is used in either aqueous or anhydrous form. While an anhydrous ammonia system requires only that an ammonia injection grid (and an associated carrier gas blower) be installed in the flue gas duct upstream of the catalyst, refineries are loath to transport, store, and handle this hazardous substance. It should be pointed out that the ammonia injection grid also causes additional flue gas pressure drop. Aqueous ammonia, on the other hand, is less hazardous and therefore easier to handle. However, if aqueous ammonia is used, an ammonia vaporizer is required. Aqueous ammonia can be vaporized in many ways: direct steam atomizing; indirect heating using a heat exchanger; fluegas slip stream vaporization; direct injection into the fluegas; and electric heater vaporization. 7 |
