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Show Proceedings of the AFRC 2011 Annual Meeting September 18-21, Hyatt Regency Hotel, Houston, Texas Ultra-low NOx Burner Arrangements in Furnace Revamps: Utilization of CFD to Prevent and Mitigate Adverse Furnace Flow Patterns Shirley Chen, Jaime A. Erazo, Jr., Bailey Hendrix and Kirk Wendel John Zink Company, LLC Tulsa, OK, 74116 USA Abstract Introduction As NOx emission regulations continue to become more stringent, the need to retrofit existing furnaces with Ultra-low NOx burner technology will continue to rise. Ultra-low NOx emissions from process burners are typically achieved by combining staged combustion with internal flue gas recirculation. As a result, the flames from an Ultra-low NOx burner are longer and exhibit lower peak flame temperatures compared to the flames from a conventional type of burner. These characteristics make the performance of Ultra-low NOx burners more susceptible to furnace flow patterns than conventional burners. In order to mitigate the effects o f adverse furnace flow patterns, tools such as Computational Fluid Dynamics (CFD) can be used to reveal the flue gas flow patterns inside of a particular furnace. This knowledge can then be used to optimize the burner design. The John Zink Company frequently uses CFD in this fashion and several successful case studies are presented. Conventional style, raw gas, process burners have characteristics of short, intense flame envelopes (on the order of 1 - 1.5 ft/MMBtu/hr flame lengths). There is no fuel gas or combustion air staging in this style of burner. The fuel and air mix together and combust near the throat of the tile to produce a small flame envelope resulting in high flame temperatures and consequently what are now considered high oxides of nitrogen (NOx) emissions. Process heaters designed to utilize conventional burners were therefore optimized for process performance based on this small flame volume. Currently, regulations have become more stringent to lower the amount of NOx emissions. This has been achieved in process burners by staging the fuel gas or combustion air and adding internal flue gas recirculation (entraining heater flue gases into fuel jets) to reduce the flame temperature which results in lower NOx emissions. The staging of the fuel gas or combustion air results in longer flames (1.5 - 2 ft/MMBtu/hr) and the addition 1 of internal flue gas recirculation requires the burners to be spaced properly to facilitate proper flue gas entrainment. If, or when, a heater designed for conventional burners is revamped with Ultra-low NOx burners, problems may arise due to the larger flame envelope and spacing required for these burners to operate as designed. These potential problems can be foreseen and possibly prevented by modeling the burners using Computational Fluid Dynamics (CFD) and solutions proposed through either heater or burner modifications. John Zink Co. has used CFD to model several different types of heaters and burner arrangements. Three examples are detailed below: single row cabin, staggered double row cabin, and single circle vertical cylindrical heater. A crude heater requiring a burner upgrade from conventional raw gas/oil burners to Ultra-low NOx burners was modeled using CFD to check for any possible problems that may occur when changing the style of burner. The heater consists of four cells connected to a common convection section (see Figure 1). The ducting into the convection section from the top of the radiant cell is located on one side of the radiant cell. Tubes run horizontally along the length of the radiant cell on each wall and are suspended from the top. The burners are installed in the floor (firing vertically upward) as a single row over the length of the heater centered between the tubes. The conventional burners were installed in the heater in the early 1970's. Recently, new emission requirements necessitate changing the conventional burners to Ultra-low NOx burners. The heater dimensions, process constraints, and operating conditions were modeled with the new burners. Figure 2 shows that the flame was longer than expected (greater than 2/3 of the radiant cell height) and leaned significantly at the top of the radiant cell towards the convection section inlet to the point that it was Figure 1: CFD geometiy of a Single Row Cabin (one cell of the four cell crude heater). Figure 2: Standard ULNB flame impinging on heater tubes. Single Row Cabin 2 extending from the inlet of the convection section to the center of the firebox, effectively moving the flue gas outlet directly above the burner. This model showed no leaning flame; illustrating that the problem was caused by the asymmetrical heater configuration and not the burner (see Figure 4). Since modifying the heater was not in the interest of the end user, changes were made to the burner to counteract the flue gas currents. Several modifications to the burner were implemented together including: reduced burner size, fuel biasing on the gas tips, and an optimized gas tip drilling for shorter flame length. These modifications straightened and significantly shortened the flame envelope (see Figure 5) but with some trade-offs accepted by the end user, such as, reduced capacity and slightly higher NOx emissions. The reduced burner size served to increase the momentum of the air flowing through the burner as well as provide more space between the burners for more efficient internal flue gas Figure 3: Pathline view of heater currents. impinging on the process tubes. The asymmetric firebox layout also caused a flue gas recirculation zone without oxygen but with large amounts of carbon monoxide (CO) (see Figure 3) next to the burner. This grossly extended the flame envelope. A simple CFD analysis of the heater was done with a baffle ooo o o o o 0 o o o o o o o o o o o Figure 4: Flow pathlines used to show recirculation caused by heater configuration with baffle present. Figure 5: Flame envelope of modified ULNB. 3 recirculation. Fuel biasing compensated for flue gas that was oxygen deficient in some of the heater currents. Finally, the short flame drilling pattern kept the flame from impinging on the tubes at the top of the heater. Staggered Double Row Cabin CFD modeling was also used to examine an older heater design that was being constructed for a new process with Ultra-low NOx burners. This heater consists of three separate cells with vertical tubes around the perimeter of each cell (no walls between the cells, see Figure 6 ). Above the cells is the inlet for the convection section which runs across the three cells and is perpendicular to the burner rows. The burners in each cell are arranged in two rows, staggered, to fit all the burners in the cell while still meeting API recommended burner centerline to tube centerline spacing. The CFD model showed two problems Figure 7: Flame envelope of burner leaning toward center of cell and merging together. in the heater performance: 1 ) the flames were interacting between adjacent burners, and 2 ) the flames from the burners the end of each row leaned towards the center of the heater (see Figure 7). This modeling showed that the overall configuration and corresponding burner spacing prevent proper internal flue gas recirculation to each burner. As all the burners were competing to entrain the flue gas between the two rows, the burners were not able to perform as designed. Also, in Figure 8 it can be seen from the flow pathlines that the flue gases tended to circulate parallel with the burner rows at the end o f the row instead o f radially around each burner. This caused the center burners to only entrain flue gas from the other burners which was oxygen deficient. The lack of oxygen for combustion then caused the flames to Figure 6: CFD model of a Staggered Double Row Cabin. 4 2/3 Furnace Height Figure 8: Flow pathlines show recirculation taking place at the ends of the burner rows instead of around each burner. Figure 9: Flame envelope of shortened flames. Flames still merge together but do not reach the process tubes. elongate and results in the flames merging together into one large flame that exceeded the maximum flame length requirements. To mitigate this problem the burners were altered by changing the burner fuel distribution; the staged fuel was reduced to decrease the amount of flue gas recirculation. This shortened the flame length by 2 0 % in single burner testing. The changes were added to the CFD model and the calculations were re-run. Since the burner spacing was not changed, the flames of the burners still merged together. However, the shorter single burner flame length helped reduce the combined flame to an acceptable height (see Figure 9). selecting a proper burner circle diameter (BCD) (see Figure 10). The initial BCD of the heater resulted in a ratio of burner Single Circle Vertical Cylindrical Heater A vertical cylindrical heater with five Ultra-low NOx burners in a single circle was modeled to show the effect of Figure 10: CFD model of a Single Circle Row Vertical Cylindrical Heater. 5 Figure 11: Flame envelope of burners with initial BCD. Figure 12: Flame envelope of burners with enlarged BCD. spacing (tip to tip) to burner liberation of 1.12 inches per MMBtu/hr. When the CFD model was complete it was found that the zone in the middle of the heater was oxygen deficient and that the gas tips in the middle could not properly entrain heater flue gas into the burner flames. This caused the flames to elongate and merge together creating an undesired flame length (see Figure 11). The heater was then modeled with a BCD that was increased to provide 2.37 inches per MMBtu/hr spacing between burner tips, double the original case. The model of this configuration showed an individual flame for each burner that was significantly shorter than the initial model (see Figure 12). The model also showed higher oxygen concentration between the burners as well as in the center of the heater (see Figure 13). Using the CFD model in this case showed the possibility of problems with efficiency as well as tube impingement. Figure 13: Mole fraction of oxygen increased between the burners; a) initial BCD, b) enlarged BCD. 6 Conclusion The Ultra-low NOx burner technology required to meet more stringent emission regulations has changed the flame envelopes of burners. Compared to conventional raw gas burners, these ULNBs have longer flames and require more burner to burner spacing to maximize their usage of flue gas recirculation for NOx control. This change has made CFD modeling a valuable tool to prevent potential problems when installing ULNBs in a heater designed for conventional raw gas burners. Such problems include: flame impingement on tubes, higher NOx emission than predicted, and undesired flame envelope. Once these problems are recognized, the burner or heater can be modified as a preventative action. 7 |
