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Show burners will reduce the NOx emissions from the heater from its present value of 95 pounds/day (125 ppmvd) to 31 pounds/day (40 ppmvd). Case; Rule 1109 Compliance A set of three identical coker heaters were chosen as the subject of this case study. Relevant data are shown below. Number of identical heaters: Service Type Draft Firing rate, total (design) Current NOx emissions Required -NOx emissions Original thermal efficiency 3 Coker Box Natural 492 MMBtu/hr (HHV) 95 ppmvd < 25 ppmvd 83.5 It was not possible to attain the emissions limit for NOx, under all operating conditions simply by using low NOx burners alone. The ceramic fiber burners were briefly considered. However, due to geometry restrictions, their implementation would have necessitated extensive redesign of the interior of the heaters. Therefore, the only real choice remaining was SCR. While SCR technology could definitely attain the NOx limit, it was not immediately clear as to how many units should be used and where they should be placed. The following options were considered. Option A: Maintain the present independent operation of the three coker heaters and use a SCR for each heater. Split the convection section of each heater and place the SCR between the process coils and a steam coil. Since this option essentially amounted to three separate systems (see Table 4 for equipment details), it was not cost-justified (Table 5). It should be noted that cost of individual equipment such as the CEMS and, to a certain extent, the SCR unit, is basically fixed within a range of operating conditions. Option B: Combine the fluegases from all three heaters and treat the combined stream in a single common SCR unit. Due to the lack of duplication of major equipment (Table 4), substantial savings result as compared to option A. However, placement of the common SCR posed a major design problem. To attain the necessary temperature window, a steam coil in the convection section could be moved; however, combining the fluegases to the SCR unit then redistributing them back to their respective convection sections downstream of the SCR was not practical. Therefore, the installation of a common steam generation coil was considered. The relative cost for this option is also shown in Table 5. This option, although a relatively sound design, was not acceptable for the following reasons. Firstly, having more than adequate steam supply in the plant, the refinery had no need for additional steam, i.e. the economic value of additional steam was zero. The second disadvantage was that, as with any steam system, boiler permits (pressure vessels) have to be renewed periodically, which requires that the unit be shutdown for inspections. As previously discussed, these shutdowns result in high revenue losses. Option C: This option was similar to option B above. However, instead of recovering the waste heat via the steam coil, it was proposed that a combustion air preheater be installed downstream of the SCR unit. The addition of air preheat had the following 11 |
