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Show advantages: (i) the preheated air would be distributed to all three heaters resulting in an overall system efficiency of over 91 % as compared with the original individual heater efficiencies of 83.5 % (a significant fuel cost savings); (ii) since boiler permits would no longer be an issue, unnecessary shutdowns were not a problem and therefore longer runtimes could be expected; (iii) the absorbed heat duty could be increased without exceeding the permitted heat release of the heaters. The equipment list and costs for this option are shown in Tables 4 and 5 respectively. It should be noted that although option B was less expensive on a total installed cost basis as compared to option C, the zero value of steam invalidates that option. Along with the determination to use combustion air preheat as a means of efficiency enhancement was the issue of burner replacement. The issue here was whether to replace the burners with standard staged fuel burner (source reduction) thus lowering the NOx into the SCR unit or leaving the existing burners in place and thus increasing the size of the SCR. Two factors ultimately led to the replacement of the burners. Firstly, the addition of air preheat (approximately 500 F) to the existing burners would have increased their NOx emissions to above 200 ppmvd. With a SCR NOx reduction of 90 percent (the guaranteed maximum by the vendor), the NOx emissions to the atmosphere would have been, at least, 20 ppmvd. With Rule 1109 requiring less than 25 ppmvd, this leaves very little, if any, margin before non-compliance is achieved. However, with the standard staged fuel burners, the NOx emitted with air preheat is only 60-70 ppmvd. Thus a design for 10 ppmvd NOx to the atmosphere would required a 86 percent efficient SCR unit, which is not uncommon. The second reason for changing the burners was an observation made in the heater evaluation step. It was noted that the flame quality was somewhat erratic. Occasionally, the flames lengths were extremely long with a tendency to "lick" the roof tubes. In coker heaters this results in less runtime on the heaters due to coke formation inside the tubes leading to higher maintenance and possible tube failures. The addition of forced draft (as required by air preheat) burners would definitely help this problem due to their more easily controlled shorter and tighter flame patterns. In summary, the fmal selection was a common SCR unit with combustion air preheat and new staged fuel burners. This option reduced the total NOx emitted by the three heaters from 1375 pounds/day (95 ppmvd) to 130 pounds/day (10 ppmvd). At the same time flame quality would be greatly enhanced allowing for longer heater runtimes. Without going into extensive detail, it is hoped that the preceding case studies have provided the reader with a feeling for the kinds of trade-offs that are inherent in any heater retrofit situation. In a sense, each retrofit design is an optimization problem and it is a challenge to arrive at the optimal design answer. 7. CONCLUSIONS The issues related to the retrofit design of petroleum refmery process fired heaters to meet the stringent NOx emissions requirements of the South Coast Air Quality Management District in Southern California were discussed. Some of the major constraints inherent in retrofit situations were discussed followed by a discussion of the types of presently available control technologies that can help achieve the mandated NOx emissions limits. Each technology was discussed in the context of its suitability in retrofit situations. Two case studies were described to provide the reader with a feeling for the types of problems and challenges that must be overcome before a retrofit situation can be successfully implemented. While no single technology is superior in 12 |
