Advantages of a Tangentially Fired Delayed Coker Heater (Conventional or Flameless) to Increase Run Lengths and Monitor Combustion

Conference Paper

AFRC 2016 Industrial Combustion Symposium Sept. 11-14 2016 Kauai, HI ADVANTAGES OF A TANGENTIALLY FIRED DELAYED COKER HEATER (CONVENTIONAL OR FLAMELESS) TO INCREASE RUN LENGTHS AND MONITOR COMBUSTION Authors: William C. Gibson Marianne Zimola - Sr. Process Engineer-Advanced Technologies Great Southern Group, LLC 2700 N. Hemlock Ct., Suite 100 Broken Arrow, OK 74012 [e-mail] [e-mail] ABSTRACT After three years of continuous operation, observation and testing of the tangentially fired flameless crude heater, it is obvious that tangential firing is perfectly suited for delayed coker heater applications. Whether firing conventionally or flameless, the tangential orientation eliminates flame and hot gas impingement and provides an even radiant heat flux to the tubes. Additionally, tangential firing allows for combustion monitoring with GSF's temperature rate of change monitoring system which has been proven to be more reliable and robust than traditional flame monitoring equipment.   TECHNICAL PAPER BACKGROUND: Great Southern Group (GSG) designed and fabricated a flameless crude heater. This heater has been in operation for three and a half years. 1 #12;    During this time, GSG has learned and documented the many advantages of tangentially firing a refinery heater. The advantages are evident and equally beneficial whether the heater is fired in the conventional mode or in the flameless mode. DESCRIPTION OF TANGENTIAL FIRING LAYOUT FOR THE EXISTING CRUDE HEATER: The GSG tangentially fired crude heater is illustrated in Figure 1. Figure 1 The key components are as follows: 1) Double fired vertical tube radiant coil. 2) Burners oriented in a "head to tail" orientation. 3) GSG textured castable hot face wall construction (patented) to pin combustion against the face of the refractory. 4) Flue gas exits the top of the radiant section after the flue gas in rotation slowly migrates away from the wall and rises to the top of the radiant section. 5) Downstream wall mounted thermocouples to monitor combustion (proven robust and reliable over the past 3.5 years). 2 #12;    DESCRIPTION OF TANGENTIAL FIRING FOR DELAYED COKER HEATERS: Modern delayed coker heaters require the following key design features: 1) Horizontal coil configuration. 2) Double fired radiant coil configuration. 3) Designed to minimize longitudinal flux variations from end to end and from bottom to top of the heater. The design objectives are commonly achieved with heaters that have a radiant section height of approximately 24 feet and have a line of flat flame burners on either side of the double fired coil with a heat release per burner of less than 2.0 MM Btu/hr (LHV). This design has certain drawbacks (with a straight wall or a slanted wall). 1) The up fired burner heat release must be necessarily small (burner to tube clearances versus burner heat release at the bottom elevations of the heater). 2) Small burner heat releases create flames that commonly have a maximum flame height of 7' or less. 3) Longitudinal factor is high due to higher radiant heat flux at the bottom elevations of the heater. GSG TANGENTIALLY FIRED COKER HEATER LAYOUT: The Great Southern Independent's Patented process coil (US Patent 8,771,475 B1 INTERTWINED TUBE COIL ARRANGEMENT FOR A DELAYED COKER HEATER) plus tangential firing eliminate all of the current design limitations imposed by traditional single pass double fired coils and floor firing small flat flame burners. The Loop in Loop design (or Intertwined Tube Coil Arrangement) allows for a much smaller size coker heater footprint compared to traditional delayed coker designs to achieve equivalent capacity. SK-01 shows the layout and configuration of the typical delayed coker design. SK-02 shows the layout of a GSG tangentially fired delayed coker heater which achieves the same capacity as the traditional design depicted in SK-01. 3 #12;4 #12;5 #12;    Important design advantages are as follows: The height of a single radiant cell is nearly doubled to approximately 45 feet. This greatly reduces the real estate plot space required for the heater. Two radiant passes are intertwined (loop in loop) in each radiant cell. This provides even heat flux to both passes improving both the circumferential and longitudinal flux factors. The intertwined loop in loop coil design also further reduces the heater footprint saving valuable refinery real estate. The GSG design has 21% more radiant absorption for the same radiant surface area. In the GSG design each radiant cell has 4 levels of tangential firing and each level has 6 burners (3 per side) for a total of 24 burners per cell in the twin cell design shown in SK-02. The burners fire in a "head to tail" configuration. The fuel jets and combustion airflow momentum maintain a high volume of flue gas in rotation (four times greater than the flue gas volume in traditional heater designs) which stays pinned against the dimpled refractory heater wall. The burners may be either traditional low NOx type flat flame burners or the heater could utilize Flameless Nozzle Groups (FNG's) and operate in the conventional combustion mode. Therefore, for 160 MMBtu/hr process absorbed duty, the total number of burners is reduced from 112 (all floor fired) at 1.8 MM Btu/hr per burner (LHV) in a conventional delayed coker heater to 48 tangentially fired burners in the GSG design with a heat release of 4.2 MM Btu/hr per burner (LHV). ADVANTAGES OF LOOP IN LOOP CONFIGURATION WITH MULTILEVEL TANGENTIAL FIRING: GSG has documented the significant increase in radiant section efficiency and both longitudinal and circumferential radiation factors approaching 1.0. For example, the circumferential factors for the delayed coker designs are as follows: Traditional Double Fired Design = 1.2/1.0 Conventional Tangential Fired GSG Design = 1.1/1.0 Flameless Tangential Fired GSG Design ≈ 1.0/1.0 The GSG design provides for extended run lengths. Conventional coker heaters must be shut down frequently for decoking due to localized high flux rates and high fouling rates caused by hot gas or flame impingement on the process coil. By tangentially firing on 4 levels with the patented GSG refractory design, hot gas and flame impingement are eliminated. Tangentially firing in a loop in loop coil configuration reduces the number of single radiant cells to half of the common number. This represents a significant saving in the heater purchase price and the installation cost as well as the required real estate. 6 #12;    Reducing the number of burners by 57% significantly reduces the burner costs (base quantity of burners purchased, piping, monitoring, maintenance, etc). Additionally, with the GSG FNG nozzles, the need for a filter/coalescer is eliminated due to the large fuel firing ports. (See Figure 2) Typical low NOx burners use very small (1/16") ignition ports which require frequent cleaning along with filter coalescer systems in order to prevent plugging. FIGURE 2 Visibility of infrared thermography is greatly enhanced by having multiple levels of platforms and not having to shoot through the flames (as is common with floor fired delayed coker heaters when trying to shoot the lower 1/3 of the radiant coil elevation). Tube skin temperatures and flux rates are as uniform as possible. Only flameless combustion in the same configuration will provide even more uniform flux rates to a double fired coil. In summary, the GSG design for double fired coker heaters with the radiant coil configured in a loop in loop arrangement with multilevel tangential firing provides far superior performance than is possible with current double fired coker heater layouts. Flameless combustion can be included in the design with the addition of an air preheat system and the uniformity of radiant section flux rates can be further enhanced. The GSG design also eliminates any possibility of flame impingement or hot gas impingement on the radiant coil. This is a significant advantage since many if not most all of the hot spots in delayed coker heaters are the result of hot gas or flame impingement. By eliminating hot spots with tangential firing and pinning the flame against the wall, run lengths can be significantly increased. Levels of flux can also be adjusted in 10' height increments by varying the fuel gas firing rate per level. This feature provides another level of control for extending delayed coker heater run lengths that has never before been possible. And finally, the tangential firing configuration allows the use of GSG's downstream wall mounted thermocouple system to monitor combustion. This flame monitoring system measures the 7 #12;    temperature rate of change to confirm combustion. It has proven to be extremely robust and reliable over the past 3.5 years of operation in the flameless crude heater in both flameless firing mode and the conventional firing mode. Nuisance trips are eliminated by clustering thermocouples in groups of 3 and voting 2 out of 3 to detect a loss of combustion for each of the 48 burners. GSG believes the Intertwined Tube Coil Arrangement with tangential firing will be the preferred technology for the future of Delayed Coker Heaters. 8