(86f) Cracking Heater Convection Retrofit with Integral Scr

When faced with a state imposed mandate to reduce NOx emissions from a steam cracker, ChevronPhillips determined that the most advantageous solution would be a retrofit of new convection sections on the existing 13 cracking furnaces, incorporating both integral Steam Superheating coils and Integral SCR for NOx removal. This allowed ChevronPhillips to reduce stack heat losses as well as NOx emissions in the cracking heaters, while completely idling a low efficiency Steam Superheater and avoiding the NOx generated therein.

Technical and design issues were addressed by a team incorporating members of CP Chem, Lummus Heat Transfer, who provided the heater thermal and mechanical design, and KBR, who engineered the ammonia delivery system and erected the new convection modules.

Significant design issues needed to be addressed, in terms of the design of the Ammonia Delivery System and the Ammonia Injection Grid (AIG). As the revamp of the convection sections required additional heating surface for higher thermal efficiency and also new services (Steam Superheat), the height of the convection section would increase by 30 ft. over the existing design. The associated increases in dead weight and wind load imposed on the structural members and foundations were concerns that ultimately required some additional bracing and fortifications of the supporting steel.

In order to minimize the overall height, a very compact design was developed for the AIG that allowed it to be located a short 6 ft. below the catalyst face. Heat transfer surface was located between the AIG and catalyst face to further reduce the height and promote mixing for the dilute ammonia with flue gas. Computational Fluid Dynamic (CFD) modelling allowed the grid design to be optimized for ammonia mixing, while external controls on the ammonia distribution allowed the ammonia distribution to be biased to match any potential NOx maldistribution.

Project execution planning for a major revamp in an operating unit, while minimizing lost production due to heater downtime is key to any cracking heater SCR retrofit. Detailed and specific execution planning allowed 12 of the 13 cracking furnaces to be revamped in a 122-day period surrounding a plant turnaround, with 8 of these furnaces revamped during the 52-day turnaround while the plant was down.

The Ethylene Plant Steam and Fuel balances were impacted significantly by the revamp, and shutdown of the low efficiency Steam Superheater. The overall thermal efficiency of the cracker was improved from 83% to 92% LHV. Net fuel consumption was decreased by 200 MM Btu/hr firing avoided, at equal throughput.

Plant operating data is presented for one heater (recycle heater) that was revamped first, and has been in operation since 3Q 2003.