Improved Burner Air Inlet Design & Control

Paper from the AFRC 2013 conference titled Improved Burner Air Inlet Design & Control by Erwin Platvoet

PROPRIETARY DOCUMENT © 2013 by JOHN ZINK COMPANY LLC. This document is proprietary. It is to be maintained in confidence. Use of, or copying in whole or part is prohibited and shall may only be granted by written permission of John Zink Company. American Flame Research Committee 2013 Industrial Combustion Symposium Sept. 22-25, 2013 - Kauai, Hawaii Improved Burner Air Inlet Design & Control Erwin Platvoet - John Zink Hamworthy Combustion Introduction When optimizing process burners for refinery and petrochemical applications, most attention is traditionally focused on the burner elements that are inside the firebox and are thought to have the most impact on flame shape and emissions. The part of the burner outside the firebox, which serves to feed and control air to the burner, only gets special attention during the design process when noise limitations become important. In this paper we will demonstrate that this element can have a considerable impact on flame shape and emissions, and an improved air inlet design will be introduced. The improved linear control response of this design will be of great additional benefit to the operators of this burner. Conventional air inlet design Early process burners were constructed with two concentric metal cylinders, each having slots as shown in Figure 1 and Figure 2. One cylinder is stationary while the other can be rotated such that all or a portion of the slot on one cylinder can be aligned with those on the other. This allows air to flow through the slots into the burner. The disadvantage of rotating concentric cylindrical air registers is that in fully closed position leakage rates up to 50 % of the fully-open flow rate are possible (1). Figure 1. Common plenum oil and gas burner with rotating register Figure 2. Gas burner with rotating register PROPRIETARY DOCUMENT © 2013 by JOHN ZINK COMPANY LLC. This document is proprietary. It is to be maintained in confidence. Use of, or copying in whole or part is prohibited and shall may only be granted by written permission of John Zink Company. Other downsides of this design are that it does not dampen noise very well and that the registers can become very difficult to move after having been in service for an extended period of time. In order to improve these issues, burners are now mostly designed with a side entry into the burner plenum, as show in Figure 3 and Figure 4. The air flow rate is typically controlled using a single- or two blade damper system. Figure 3. Staged fuel gas burner with side entry Figure 4. Staged fuel gas burner with side entry (bottom view) Problems with side entry burner designs While the side entry design works well to dampen noise levels and the damper system is easy to control and maintain, there are a number of issues with this particular arrangement. 1. Volume Comparing Figure 1, Figure 2 with Figure 3 and Figure 4, it becomes rapidly apparent that the side entry requires a lot of space. In order to limit the entry pressure losses, the velocity of the air entering the burner cannot be much higher than the plenum velocity. As a result, the muffler-damper system as well as the plenum must be relatively bulky in order to accommodate this. This, in turn, often leads to interference issues with fuel piping, furnace structural components (see Figure 5), and between burners themselves. This is especially difficult in cases where very low noise limits require very long mufflers, or when conventional burners with rotary registers are replaced with modern low NOx burners. PROPRIETARY DOCUMENT © 2013 by JOHN ZINK COMPANY LLC. This document is proprietary. It is to be maintained in confidence. Use of, or copying in whole or part is prohibited and shall may only be granted by written permission of John Zink Company. Figure 5. Example of burner - structure interference. 2. Air flow maldistribution inside the burner When air flows into the burner plenum from one side only, its momentum causes it to "accumulate" on the opposite side of the plenum. The CFD path lines in Figure 6 illustrate this. As a result, the air velocity in the burner throat can show a significantly unequal air distribution, as shown in Figure 7. This maldistribution gets worse at lower burner pressure losses, as the burner throat is usually the only remaining hydraulic resistance that can redistribute the air. Figure 6. CFD Path lines illustrating flow maldistribution caused by the side entry plenum Figure 7. Air velocity plot (m/s) in the throat of a burner with a side entry (on the side with the low air velocity ) The uneven air distribution in the burner throat leads to other, undesired, effects; PROPRIETARY DOCUMENT © 2013 by JOHN ZINK COMPANY LLC. This document is proprietary. It is to be maintained in confidence. Use of, or copying in whole or part is prohibited and shall may only be granted by written permission of John Zink Company. • The flame shape becomes asymmetrical, since one half of the burner is operated with a significantly higher excess air than the other side. This also results in difficulty to achieve the desired flame length on the side of the burner where there is an oxygen deficiency. • In certain cases, the starved side of the flame could be low enough in air velocity to form a sub-stoichiometric fuel-air mixture, leading to elevated CO emissions. This is especially true at turndown conditions, where difficulty in meeting acceptable flame shape and CO emissions usually requires burner operation at much higher levels of excess air than under normal operating conditions. • The uneven distribution of air typically leads to increased levels of nitrogen oxides. • The momentum in the horizontal direction can cause flames to lean, and in the worst scenarios to merge or to impinge on radiant coils. In order to improve the air distribution, burners can be fitted with single or double curved turning vanes as shown in Figure 8 and Figure 9. The downside of this approach is that it adds complexity to the burner and reduces accessibility to the burner throat for the pilot burner, the UV scanner, and the sight and lighting ports. Also, when the overall burner pressure loss is too low the turning vanes are not able to even out the air distribution sufficiently. Figure 8. Air velocity plot (m/s), curved turning vane in burner plenum. Figure 9. Air velocity plot (m/s), double curved turning vanes in burner plenum. 3. Sensitivity to wind speed and direction Natural draft heater operation experience usually shows that when wind changes in speed or direction, changes are needed in burner damper settings in order to avoid poor flame quality or CO emissions. In cases where this happens frequently, heaters are often operated at high draft levels and burner dampers partially closed in an attempt to mitigate these effects. The role of wind on natural draft PROPRIETARY DOCUMENT © 2013 by JOHN ZINK COMPANY LLC. This document is proprietary. It is to be maintained in confidence. Use of, or copying in whole or part is prohibited and shall may only be granted by written permission of John Zink Company. burner operation becomes much clearer in CFD models. In Figure 10 and Figure 11 the static pressure contours around a vertical cylindrical heater are shown while it is subjected to a wind speed of (only) 20 miles per hour. The upwind sections of the heater experience static pressure increases of up to 0.2 inH2O due to stagnation of the air. Conversely, the downwind regions of the heater have low pressure zones in the order of magnitude of -0.2 inH2O. Considering that the average natural heater draft is in the range of 0.3 - 0.6 inH2O, it becomes clear that ambient pressure variations are in the same order of magnitude and can play an extremely important role in the distribution of air between burners when different burners see different ambient pressures. Path lines in Figure 12 and Figure 13 show that, in addition to the static pressure variations, the wind direction causes a very different way of air flowing into each of the different burners. In a number of burners, the tangential component of the air even causes a strong eddy inside the burner. As a result of all the ambient influences, the air distribution between the burners can become very poor and individual damper adjustments will be necessary to correct flame shape or emissions problems. Moreover, these adjustments must be redone when the wind changes direction or velocity. Figure 10. Static pressure (inH2O) variations around a VC heater subjected to a 20 mph wind Figure 11. Static pressure (inH2O) variations around a VC heater subjected to a 20 mph wind PROPRIETARY DOCUMENT © 2013 by JOHN ZINK COMPANY LLC. This document is proprietary. It is to be maintained in confidence. Use of, or copying in whole or part is prohibited and shall may only be granted by written permission of John Zink Company. Figure 12. Path lines of air flowing into the burners Figure 13. Path lines of air flowing into the burners Alternative Design It is obvious that the conventional designs with a radial air inlet as shown in Figure 1 and Figure 2 will result in a better internal air distribution, mitigating many of the problems associated with side entry designs. The challenge, however, is to develop a damper system that does not have the leakage problems of the conventional designs and is also easier to operate and maintain. The solution developed by John Zink Hamworthy Combustion is shown in Figure 14 and Figure 15. The front plate of the burner is stationary as usual, as this is where the pilot and scanner connections are. The air enters the burner plenum radially through a perforated plate. The perforated plate has several different purposes: 1. Bug / trash screen 2. It creates a specific pressure loss to a. help distribute the air evenly around the inlet, b. fine tune the overall burner pressure loss, c. reduce the impact of wind on the air inlet flow, and d. Equalize the air between different burners in a common duct. 3. Personnel protection 4. Noise reduction In the region between the front plate and the burner plenum a damper plate is used to control the effective area of the opening. Its movement is typically controlled by a cam as shown, but can also be accomplished by other slide-and-lock type actuators. PROPRIETARY DOCUMENT © 2013 by JOHN ZINK COMPANY LLC. This document is proprietary. It is to be maintained in confidence. Use of, or copying in whole or part is prohibited and shall may only be granted by written permission of John Zink Company. Figure 14. ARIA Radial register (damper closed) Figure 15. ARIA register 3D image (damper closed) The advantage of this approach is a significantly more compact burner. The volume of the radial inlet plenum is approximately half that of a conventional plenum design. The ARIA™* style COOLstar® burner has been installed in heaters with only 20 inches of space available underneath the heater floor. The radial burner inlet design has resulted in other advantages such as • Lower NOx and CO emissions due improved burner throat air distribution • Better control of the flame shape and dimensions • Tight shutoff • Jackshaft controllable However, the most significant advantage is the improved operability of the burner, specifically at turndown. Turndown control The conventional side entry designs with opposed blade damper systems become difficult to control at turndown conditions. The first reason is that burner throat pressure loss is greatly reduced during turndown. For example, if the air flow is reduced by half, the throat pressure loss reduces with a factor 4. That means that the throat has even less hydraulic resistance to correct any air flow distribution. * Patent pending PROPRIETARY DOCUMENT © 2013 by JOHN ZINK COMPANY LLC. This document is proprietary. It is to be maintained in confidence. Use of, or copying in whole or part is prohibited and shall may only be granted by written permission of John Zink Company. Moreover, in order to limit the excess air during turndown conditions the burner dampers need to be partially closed to create additional pressure loss. Since they must compensate for the reduced pressure loss of all the other burner components (muffler, plenum, throat), they must be closed disproportionally more. For example, in order to maintain the same excess air for a 50% turndown rate, burner dampers typically need to be closed 70 - 80%. This non-linear behavior makes precise air control difficult. Because of the way they are designed, opposed blade dampers create three high velocity jets into the burner plenum as shown in Figure 16. Combined with the low throat losses at turndown conditions, this tends to create a much more skewed air distribution profile in the throat than during normal operation. The resulting poor flame quality makes the air adjustment even more problematic. As a consequence of their difficult controllability, side entry burners are usually operated at high excess air during turndown in order to prevent poor flame shape and CO emissions. The lack of precise air control means that emissions of NOx during turndown are often much higher than they need to be. Figure 16. Velocity vectors around dampers Using the ARIA register, air flow control during turndown conditions is much better and uniform. The most obvious reason for that is that air flows radially into the burner, regardless of damper position. The other reason is that the ARIA damper movement can be designed to control the air flow based on the duty requirement, as shown in Figure 17. In this figure, two designs are compared: 1. The traditional design moves the damper symmetrically; the damper closes 50% when the actuator is moved 50%. 2. The asymmetric design moves the damper disproportionally; the damper closes faster than the actuator setting. PROPRIETARY DOCUMENT © 2013 by JOHN ZINK COMPANY LLC. This document is proprietary. It is to be maintained in confidence. Use of, or copying in whole or part is prohibited and shall may only be granted by written permission of John Zink Company. The burner test results in Figure 17 show that the asymmetric damper tracks the duty very well. At 50% burner duty the damper actuator was set to 50% to achieve 3% O2 at design draft. This approach has several distinct advantages: a. The control of the burner becomes much simpler for the operator once he knows the heater duty. Air leakage into the firebox becomes easy to identify. b. Automation of the heater air control system using a combination of jackshafts and asymmetric ARIA registers allows for precise heater excess air control between 40% and 120% of burner duty. c. As the duty drops below 50%, the asymmetric design still has 50% of its control range left, whereas the symmetric design has reached the end of its useful range and excess air has to go up. Figure 17. Comparison of damper control linearity PROPRIETARY DOCUMENT © 2013 by JOHN ZINK COMPANY LLC. This document is proprietary. It is to be maintained in confidence. Use of, or copying in whole or part is prohibited and shall may only be granted by written permission of John Zink Company. Conclusions Low NOx burners rely on high rates of flue gas recirculation to lower flame temperatures and thermal NOx emissions. This is typically achieved by maximizing the number of fuel jets and fuel gas pressure, while placing the gas tips outside the burner tile. Simultaneously, the lowest NOx is typically achieved with low air side velocities in the burner throat - this reduces the mixing rate with the fuel, slows the combustion rates and lowers thermal NOx production. As a result, ultra-low NOx flames tend to be voluminous, long and ‘lazy' compared to more conventional style burners. It also means that the flames become very sensitive to the prevailing flue gas recirculation patterns. In order to achieve acceptable flame dimensions and emissions over the entire range of burner operation, the air inlet design of the natural draft burner needs to be redesigned from conventional side entry to a radial entry type. Through burner testing and CFD studies the radial entry type has been shown to have several advantages over the conventional type: • Lower NOx and CO emissions due improved burner throat air distribution • Better control of the flame shape and dimensions over the entire duty range • Significantly reduced burner size • Improved operability and excess air control of the burner, specifically at turndown conditions • Reduced sensitivity to (fluctuations in) wind speed and direction • Reduced impact on firebox flue gas flow patterns • Tight shutoff Due to these many advantages, John Zink Hamworthy Combustion has applied for a patent for the ARIA register, as this will be the default register design for the company's natural draft burners. References 1. Burners for Fired Heaters in General Refinery Services - API Recommended Practice 535 2nd Ed., 2006 JOHN ZINK and COOLstar are registered trademarks of John Zink Company LLC in the US and other countries worldwide. JOHN ZINK HAMWORTHY COMBUSTION and ARIA are trademarks of John Zink Company LLC.