Innovative steam-assisted flare technology

A new steam-assisted flare has been developed which reduces or eliminates some of the potential limitations of conventional steam-assisted flares. The improved performance includes significantly reduced steam requirements (>30% reduction), elimination of two of the three steam supply lines, simplified controls, longer tip life, and reduced installation and operating costs. This innovative new technology improves flare operations, while simultaneously reducing costs.

A rticle fo r Hydrocarbon Engineering, July 2007 In n o v a tiv e S te a m -A ssiste d F la re T e c h n o lo g y J. H ong, Ph.D., C. Baukal, Ph.D., M. Bastianen, J. Bellovich, K. Leary John Zink Company, LLC Tulsa, OK 74116 (U SA ) A new steam-assisted flare has been developed which reduces or eliminates some of the potential limitations o f conventional steam-assisted flares. The improved performance includes significantly reduced steam requirements (>30% reduction), elimination o f tw o o f the three steam supply lines, sim plified controls, longer tip life, and reduced installation and operating costs. This innovative new technology improves flare operations, w hile simultaneously reducing costs. Introduction Flares are devices used to safely and efficiently dispose o f operational or emergency relief o f flammable gases and liquids [1]. They are somewhat unique compared to other com m on combustion devices such as burners because o f the very w ide range o f flow rates and com positions that they often handle. This makes the design very challenging as flares often have to be capable o f safely handling gas flow s from as low as several hundred pounds per hour or less (purge rate) up to as much as a m illion or more pounds per hour. This means a flare must have a very w ide turndown range, where turndown is the ratio o f the highest to low est flow rates o f waste streams that can be safely handled. There have traditionally been six important performance parameters o f interest for most flares [2]. The first is the hydraulic capacity, which is the maximum gas flow rate that can flow through the flare at a given pressure for a given gas molecular w eight and gas temperature. W hile the gases are safely combusted, smoke is often generated. Since this is the maximum design flow that could occur during an emergency, the primary focus is safely Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology disposing o f the gases and not on how much smoke is generated. Then, the second parameter is the sm okeless capacity. This is the maximum flow o f waste gases that can be sent to the flare without producing significant levels o f smoke. It is usually lower than the hydraulic capacity. A flare is typically sized so that the sm okeless capacity is at least as much as the maximum waste gas flow rate expected during normal daily operation. The third performance parameter o f interest is the thermal radiation generated by the flare as a function o f the waste gas flow rate and com position [3]. The radiation levels at specific points o f interest are typically limited to avoid injuring personnel and damaging equipment. The height o f the flare is then determined by how tall the stack needs to be so that the radiation levels at specified locations are maintained at or b elow desired levels. The fourth parameter o f interest in flares is noise. E xcessive noise can injure personnel, equipment, and property both inside and outside o f the plant. The fifth parameter is utility consumption rates at various waste flow rates, typically measured in terms o f steam-to-hydrocarbon mass ratio for steam-assisted flares and horsepower required per unit hydrocarbon mass flow rate for airassisted flares. This parameter reflects how efficient a flare design is in utilizing the momentum o f the steam in a steam-assisted flare, or the air supplied by the blower in an airassisted flare. The sixth parameter is the minimum purge gas flow rate required. The purge gas is supplied for tw o purposes: 1) to prevent air ingression into the stack w hich could produce an explosive mixture leading to severe stack damage; and 2) to prevent internal burning in the fuel plenum that could lead to flare tip failure. The purge rate required to prevent stack damage is typically lower than the purge rate required to prevent internal burning. In order to prevent internal burning in a steam-assisted flare, center steam is often used. A n additional parameter that has received considerable attention recently is pollutant em issions from flares [4]. Page 2 of 13 Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology There are various types o f flares, depending on the specific application. If the waste gases are at a relatively low pressure, then some type o f assist m edia is often used to entrain air into the flam e to increase the sm okeless waste gas flow rate. The tw o m ost com m on assist m edia are steam and air. In the case o f air-assisted flares, one or more blowers supply a portion o f the combustion air to the flare. Air-assisted flares are com m only used where steam is limited, such as in locations where water is limited (e.g., the desert). For steam-assisted flares, steam is used to entrain a portion o f the combustion air to the flare. For either type o f assist medium, m ost o f the combustion air com es from the ambient air surrounding the flare flame. Conventional Steam-Assisted Flares B y far, the m ost popular type o f assisted flare is steam-assisted, which is the type discussed here. Figure 1 shows the effectiveness o f steam-assist for increasing the sm okeless capacity o f a flare. There are many types o f steam-assisted flares that are available. One com m on high efficiency design uses a bundle o f tubes inside the flare stack where the waste gas flow s outside the tubes and a steam/air mixture flow s inside the tubes. A drawing o f a typical design is shown in Figure 2. There is no premixing inside the flare, so the combustion air and flammable waste products m ix at the exit o f the flare tip. A photo o f the tip o f a typical steam flare is shown in Figure 3. There are usually three different steam supply lines for this type o f flare: to the tubes inside the flare, to the upper rim at the top o f the flare tip, and inside the waste gas plenum o f the flare tip (often referred to as center steam). For optimum performance, each o f these lines is independently controlled. The steam supply to the tubes inside the tip is used to entrain combustion air into the interior o f the flam e to increase the sm okeless capacity. The steam supply to the upper rim o f the tip is primarily to m inim ize wind effects, but it also helps entrain combustion air into the flare flame. The center steam supply is primarily to prevent internal burning that can occur during very low Page 3 of 13 Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology waste gas flow rates where air can migrate inside the flare gas plenum due to buoyancy, cross wind and capping. One potential limitation of conventional steam-assisted flares is related to what can happen if the steam supplies are not in the correct proportions. If the steam flare is not operated properly, a detrimental phenomenon called "capping" which can occur if too much steam is supplied to the upper steam ring and too little is supplied to the lower steam ring. This is shown schematically in Figure 4. The upper steam flow s upward and inward toward the center. The collision above the flare tip creates a zone with a relatively high pressure, acting like a fluidic dome or a cap over the flare tip that can force some o f the flame down inside the waste gas plenum, causing internal burning. Depending on the severity, flam es can actually be pushed all the way back through the steam-air tubes, resulting in flame engulfment o f the flare tip. Prolonged capping can damage the flare due to the internal burning and flame engulfment. Figure 5 shows a photo o f a capped flare. Center steam is often used to prevent internal burning at purge rate flow s by increasing the volum e flow rate and therefore the velocity through the tip to prevent air infiltration. This is less expensive than simply increasing the purge flow rate o f a purchased gas such as natural gas. However, the center steam flow still represents a significant cost. U sing center steam in freezing weather conditions can sometim es cause the steam to condense and freeze, plugging up the flare tip. In cold weather conditions, it is com m on to turn o ff the center steam and increase the purge gas flow rate, which typically increases the operating costs o f the flare. New Steam-Assisted Flare Development A development project w as initiated to address some o f the limitations o f conventional steam-assisted flares. An important objective o f designing a new steam-assisted flare w as to reduce the amount o f steam required to achieve sm okeless combustion o f a given waste flow Page 4 of 13 Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology rate. This can be quantified as the ratio o f the mass o f steam needed per unit mass o f flare gas (S/HC ratio), at a flare gas flow rate where smoke is just beginning to be produced (som etim es referred to as the incipient smoking rate). Reducing steam consumption directly reduces utility costs. If the plant is steam-limited, reducing the steam flow to the flare system can also free up steam for other uses in the plant. An extensive development program, utilizing computational fluid dynamic (CFD) m odeling [5], cold flow modeling, and large-scale combustion testing [6] led to a new flare design called the Steamizer XP (patent pending). A drawing o f one version o f the XP is shown in Figure 6. The XP consists o f multiple m odules connected to a com m on waste gas supply header. An important innovation with this design is that the steam/air tubes are straight, instead o f having a bend as in the conventional steam-assisted flare design. This dramatically reduces the flow frictional losses. The increased flow efficiency optim izes air entrainment for a given steam flow rate, increasing the m ixing between the air and the waste gases. Extensive computer m odeling w as done to investigate various aspects o f the new flare design. CFD, including the effects o f flame radiation, was used to study the heat load effects caused by high or low waste gas flow s during high and low winds. Extensive thermal stress m odeling was also done to study the effects o f flame radiation and very high wind loading on the stresses created in the flare tip. A fatigue assessm ent was included based on the A SM E VIII Section II code. The stress analyses showed that the design is very robust under the conditions modeled. Another important design feature o f the XP design is the shape o f the nozzle outlet, w hose purposes are to efficiently m ix the air with the flare gas and to minimize/prevent internal burning. A proprietary geometry and flame stabilizer configuration w as developed with the help o f CFD m odeling [5]. Page 5 of 13 Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology Testing o f various module configurations was conducted in a full scale air entrainment test rig with accurate measurement o f both steam and entrained air flow s (see Figure 7). Bent tubes as used in the conventional design were compared against straight tubes in the XP design. Extensive combustion testing was also performed in John Zink's state-of-the-art flare test facility [6]. The sm okeless capacity, steam consumption rate, thermal radiation output, and other characteristics were experimentally determined. Moderated Shear M ixing The new XP flare design has som e important benefits compared to conventional steamassisted flare designs. The design o f the outlet nozzle is critical to the performance o f the XP flare. One o f the reasons for the special shape is to enhance the m ixing o f the air and the waste gas. Figure 8 shows some simple schematic diagrams o f different types o f mixing. The left diagram shows that m ixing is lo w and shear force is minimal when there are two parallel fluid streams o f approximately the same lo w (laminar, not turbulent) velocities. The middle diagram shows that m ixing is greatly increased where one stream is perpendicular to the other. In that case, the shear between the tw o streams is very high which enhances mixing. The right diagram shows moderated m ixing where there is some shear between the fluids, but not as much as w hen they are perpendicular. The XP flare uses moderated shear m ixing to improve m ixing compared to the conventional steam-assisted flare design. The converging nozzle causes the waste stream to intersect the steam/air stream at a slight angle, unlike the conventional design where the streams are parallel. Extensive m odeling and testing have shown that the angle is critical, otherwise capping can occur, which is why the m ixing is termed "moderated." Also, the optimal moderated design m inim izes pressure losses for the given waste gas flows. The improved m ixing o f the air and waste gas increases the sm okeless capacity for a given set o f conditions compared to the conventional design. Alternatively, a flare requiring Page 6 of 13 Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology less steam could be used for a given sm okeless capacity, compared to the conventional design. The effective turndown is increased because higher sm okeless capacities are possible with the new design. The new nozzle design with moderated shear m ixing eliminates the center steam and upper steam required on the conventional design. This dramatically reduces the steam requirements and capping caused by excessive upper steam flow. Secondary Air Entrainment Another important design feature is dividing the outlet area into multiple nozzles to improve m ixing between the secondary air and the flare gas stream around and along the flame. In the conventional design, the flare gases in the center o f the flare can only m ix with the air that has been entrained into and through the steam/air tubes. Since this is only a relatively small fraction o f the total air needed to com pletely combust the waste gases, air from around the flame must be entrained to make up the balance o f the requirement. Although the air from around the flame is termed "secondary air" and the air entrained into the steam/air tube is termed "primary air", the secondary air actually accounts for the majority o f the air required to com pletely combust the waste gas. In the conventional design, secondary air can not get to the interior o f the flame. The multiple individual nozzles in the XP allow secondary air to go between the nozzles into the center o f the flare before all the individual flam es merge into a continuous ring o f flam e with a hollow center (see Figure 9). Secondary air is entrained into the hollow center to further suppress smoke formation. The tubes in the XP design are optimally spaced to m inim ize the overall size o f the tip, without adversely affecting the secondary air entrainment capability (see Figure 9). If the tubes are too closely spaced, then air flow to the inner portion o f the tip w ould be restricted, thus reducing the sm okeless capacity o f the flare. Page 7 of 13 Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology Improving the secondary air entrainment around the flare effectively either increases the sm okeless capacity for a given steam flow rate, or reduces the steam flow rate for a given sm okeless capacity, compared to the conventional steam-assisted flare design. Improved Eductor Efficiency Another important feature o f the XP flare is the reduced pressure losses or hydraulic resistance in the air/steam passage compared to the conventional design which uses bent tubes. Extensive tests in this study showed that the more angled the steam/air tubes, the less efficient the steam is for entraining air and m ixing it with the waste gases. The less smooth the turn (e.g., mitered vs. contoured elbows), the less the air entrainment for a given steam flow rate. Also, the closer the bend is to the steam/air tube inlet, the lower the air entrainment. The XP flare eliminates the bends in the steam/air tubes w hich makes the steam much more efficient at entraining air and m ixing entrained air with the waste gases. Again, this means increased sm okeless capacity for a given tip design and steam flow rate, or alternatively less steam flow required for a given sm okeless capacity. Conclusions The new Steamizer XP flare has been developed w hich reduces or eliminates the potential limitations o f conventional steam-assisted flares. Extensive CFD modeling, cold flow testing, and full-scale combustion testing were used in the development program. The XP utilizes straight steam/air tubes, proprietary nozzles, and multiple individual m odules to produce moderated shear mixing, increased secondary air entrainment, and improved eductor efficiency. These features provide the follow ing benefits: significantly reduced steam requirements (>30% reduction), tw o o f the three steam supply lines eliminated, controls simplified, longer tip life, and reduced installation and operating costs. This innovative new technology w ill improve flare operations, w hile simultaneously reducing costs. Page 8 of 13 Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology References 1. R. Reed, Flaring and Disposal, Chapter 2 in Furnace Operations, G ulf Publishing, Houston, 1973. 2. R. Schwartz, J. W hite and W. Bussman, Flares, Chapter 20 in the John Zink Com bustion H andbook, edited by C. Baukal, CRC Press, B oca Raton, FL, 2001. 3. J. H ong, J. White, and C. Baukal, Accurately predict radiation from flare stacks, H ydrocarbon P rocessing, Vol. 85, N o. 6, pp. 79-81, 2006. 4. R. Levy, L. Randel, M. Healy, and D. Weaver, Reducing Em issions from Plant Flares, Proceedings o f the Air & W aste M anagement A ssoc. Conf. & Exhibition, N e w Orleans, LA, June 2006, Paper #61. 5. C. Baukal, V. Gershtein, and X. Li (eds.), Com putational F lu id D ynam ics in Industrial Com bustion, CRC Press, B oca Raton, FL, 2001. 6. J. H ong, C. Baukal, R. Schwartz, and M. Fleifil, Industrial-Scale Flare Testing, Chem ical E ngineering P rogress, Vol. 102, N o. 5, pp. 47-54, 2006. Figures Figure 1. Effectiveness o f steam in smoke suppression: (a) without steam, (b) with steam. Page 9 of 13 Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology Figure 2. Schematic o f an internal tube type steam-assisted flare. Figure 3. Photo o f an internal tube type steam-assisted flare. flame t fuel gas Figure 4. Schematic o f capping (green represents fuel gas, blue represents steam, and red represents flame). Page 10 of 13 Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology Figure 5. Photo o f a capped steam-assisted flare. Figure 6. Figure 7. C old flo w air entrainm ent testing. Page 11 of 13 Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology fu el a ir ft I fu el (a) a ir f (b) a ir t\ (c) Figure 8. Exam ples o f mixing: (a) low shear, (b) high shear, (c) moderated shear. Figure 9. Air entrainment around XP modules. Page 12 of 13 fu el Hydrocarbon Engineering, July 2007 Innovative Steam-Assisted Flare Technology Author Contact Information Same address for all authors: 11920 E. Apache Tulsa, OK 74116 Nam e Phone # Fax # Email J. H ong (9 1 8 )2 3 4 -5 8 4 5 [phone] j [e-mail] C. Baukal [phone] [phone] [e-mail] J. B ellovich (9 1 8 )2 3 4 -5 8 1 6 [phone] john.bellovich@ johnzink.com M. Bastianen (9 1 8 )2 3 4 -1 9 1 5 [phone] [e-mail] K. Leary (9 1 8 )2 3 4 -5 7 8 6 [phone] kevin.leary@ johnzink.com Page 13 of 13