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Show feasibility standpoints. The addition of APH does, however, require a larger induced draft fan as well as a forced draft fan and ducting to the burners. The induced draft fan now must overcome not only the pressure drop from the SCR unit but also from the APH system. In most cases, the addition of an APH system will increase the size of the fan to approximately twice that required for a SCR unit alone. Since the APH system also requires a motive force to transfer the air though the APH and burners, a forced draft fan will also be required. However, given the lower volumetric flow requirement as compared to the induced draft fan, this fan is generally two-thirds the size of the induced draft fan. Air preheat may cause problems such as higher heat fluxes in the firebox (with the possibility of lowering tube life) and increased NOx formed at the burners. Low Temperature SCR One of the most promising developments in the NOx reduction retrofit applications is the Shell low temperature SCR. The low temperature unit whose main component is the lateral flow reactor (LFR) as shown in Figure 8, so called for its patented flow distribution, can achieve NOx reduction percentages similar to those of a conventional SCR unit at temperatures as low as 350 F. The obvious advantage to this unit would be its ability to be installed downstream of most refinery heater heat recovery sections, with no convection section coil modifications except duct tie-ins. While an induced draft fan will still be required, no modification of the heat recovery section is required. This method potentially eliminates plot space requirements since the SCR unit and fans can be located above grade. 6. CASE STUDIES To illustrate some of the choices that must be made in implementing the technologies discussed above, two case studies will now be described. The first deals with burner selections in heaters for compliance with Rule 1146 while the second discusses NOx reduction controls for Rule 1109 heaters. Case: Rule 1146 Compliance This case study is derived from a retrofit design that was implemented at a major Los Angeles basin refinery. The following are the most pertinent design data: Type of heater Service Firing rate (maximum) Firing rate (design) Current NOx emission Required NOx emission Existing burner circle dia Tube circle diameter Existing tube-burner distance Recommended tube-burner dist. [5] Firebox height Vertical Cylindrical Hot oil heater 32 MMBtu/hr (HHV) 26 MMBtu/hr (HHV) 100-125 ppmvd 40 ppmvd 31 inches . 111 inches 40 inches 44 inches 18 feet Since the NOx emissions from this heater were required to comply with Rule 1146 the control technology choices were limited to the different types of low NOx bu~ers discussed earlier. This implied that the existing standard burners would have to be removed and the new burners put in their place. The user's principal concern was that 9 |