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Show mixing of the recirculating fluegas with fresh air and their subsequent distribu~o~ to the burners. There is additional cost and downtime involved due to the need to tIe 1nto the existing stack and connect ducting to and from the recirculating fan. Plot size might also become a limiting constraint. Thus, an FGR system is usually not considered, not only due to economics but also from the standpoint of reliability. Staged-Fuel Burner with Steam Injection Low NOx burners with additional steam injection into the fuel have been considered in a number of applications. This is an attractive option since it can, under the proper operating conditions (clean fuels and low firebox temperatures), meet the emissions requirement of less than 40 ppmvd while requiring minimal furnace floor modifications. A major requirement is that slightly superheated steam be available. If the steam to be injected is not sufficiently superheated it will condense in the line when mixed with the ambient-temperature fuel. The steam must also have enough time to mix with the fuel prior to entering the burner. In a_ attempt to ensure this a homogenizer has been developed to enable the user to mix the fuel and steam at the burner. However, this can be an expensive item. Therefore, the best candidate for this option is when the firebox and fuel conditions are such that the emission requirements are achievable with the addition of steam injection and when superheated steam is also available. Staged-Fuel Burner with Internal FGR The most advanced low NOx burner presently available is the recently developed internal fluegas recirculation staged fuel burner (IFGR), whose schematic is shown in Figure 4. This burner easily achieves the 40 ppmvd emission limit without external fluegas recirculation. This eliminates the use of the hot fan, controls, and fluegas duct. In addition, depending on the firing conditions, this burner has the ability to use steam injection to achieve even lower emissions limits of 25 ppmvd NOx. The burner will, in most cases, require floor modifications due to its larger size (for the same firing rate). The extent of modifications depends on the burner manufacturer and the burner model used. The larger size requirement of the IFGR burner often leads to another problem. Since the tube location in an existing heater is fixed, a larger burner results in a smaller burner-tube distance, with the potential for higher heat fluxes at certain tube locations. Good practice and the conformance to industry standards [5] requires that there be a minimum distance (which is a function of the maximum firing rate) between the burner center and the process tube center to avoid any flame impingement on the tube. However, if computer simulations indicate that the heat flux at the tubes is not a problem under all future firing conditions, a smaller distance may be used. In addition, burner tests can provide guidance regarding flame shape. This is an instance where careful computer simulation and not blind adherence to standards should result in the successful application of a promising technology. TECHNOLOGIES FOR RULE 1109 COMPLIANCE For Rule 1109 heaters, requiring lower NOx emissions limits for compliance, the two major options are the reduction of NOx formation at the burner (source reduction) and the removal of NOx already formed (post-combustion reduction). The low NOx burners discussed above are examples of source reduction. Additional techniques of source reduction, including flameless ceramic fiber burners and post-combustion NOx reduction technologies, are discussed below. In many cases, a combination of source 6 |