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Show AFRC 2018 Title Ultra-Low Steam Consumption, High Capacity Smokeless Flaring Authors Rex Isaacs, Chief Technology Officer, Zeeco, Inc. Clayton Francis, Aftermarket Applications Manager, Zeeco, Inc. Abstract This paper will introduce the technology behind, test data for, and industry challenges addressed by a new Ultra-Low Steam Consumption, High Capacity Smokeless Flare design developed by Zeeco, Inc. This flare technology is designed to further improve flaring efficiency and reduce steam consumption while continuing to meet the EPA Code of Federal Regulations, Chapter 1, Subchapter C, Part 63, Subpart CC requirements. We will focus on how the design addresses known industry challenges in high capacity, low steam consumption flaring, such as needing the ability to operate at low flare gas pressure since many applications have a maximum flare gas pressure at the flare tip of 3 psig. The paper will detail how this new design can achieve as low as 0.17 lbs. steam / lbs. flare gas at 20% of maximum flow rate, with the maximum flow rate achieved at a flare gas pressure of 3 psig. The above data is based upon a 5 mph wind with less than Ringlemann 1 opacity and a flare gas that is 100% propylene. For smokeless operation with propylene, other current steam assisted flare designs require approximately 0.5 lbs. steam / lbs. flare gas and / or a much higher flare gas pressure at maximum flaring capacity. Ultra-Low Consumption Steam Assisted Flaring is very important since any reduction in the required steam flow rate saves not only money, but also reduces the emissions produced from the production of the required higher steam flows. A key feature of this technology is that the air and steam mixture leave the flare at the same elevation as the flare tip exit, meaning no pre-mixing of air into the flare stream. Other current industry designs mix the air and steam with the flare gas prior to exiting the flare tip, negatively impacting the NHVcz according to the new calculation parameters required by MACT CC. Zeeco's design more efficiently mixes the steam and air together and then mixes the resulting stream with the flare gas, creating a final mixture with a significantly increased volume of air. When the resulting mixture interacts with the flare gas at the tip exit, the increased air volume is readily available for combustion, meaning the flare is less likely to smoke. Since the design more efficiently mixes the air and steam together, less steam is required to achieve smokeless operation. Furthermore, the inherent efficiency of the mixing delivers a design less dependent upon using flare gas pressure to achieve smokeless operation. The flare can successfully operate at lower gas pressures at maximum flow rate. Exploring New Ultra-Low Steam Consumption, High Capacity Smokeless Flaring Introduction The SteamForce HC Low Consumption Steam Assisted Flare from Zeeco lowers the amount of steam consumption during normal operation, reduces steam consumption during required smokeless flaring, achieves longer flare life with less chance of capping, uses no pre-mixing of flare gas with the air / steam mixture before the tip exit, provides a high stability flame, and is designed for easy retrofit of existing applications. Decades ago steam flare technology was improved by adding internal steam/air tubes (S/A tubes) as shown in Figure 1. These original bent S/A tubes took air educted by steam and transported it to the center of the gas bundle at the flare tip exit. The bent S/A tubes provide a matrix of air injection and enhance smokeless performance. Newer steam injection technologies are enhancing the S/A tubes by making them straight on the injection axis. Figure 1: Original bent S/A tube method of propelling combustion air into core of flare stream exit pictured on left (Type II). Newer technologies employ straight S/A tubes as pictured on right (Type III). Image credit: Parameters for Properly Designed and Operated Flares; U.S. EPA Office of Air Quality Planning and Standards, April 2012. Flares are a safety device and are generally not operated at significant rates nor used for routine flaring. Most of the time, flares are operated in purge mode, ready to be used in the case of a flaring event. To meet overall design efficiency objectives, engineers must ensure the tip operates efficiently during the normal operation case of purge mode, since this will generally dictate steam usage throughout a year's operation. Therefore, the amount of cooling / warming steam required during purge rate conditions is a significant design consideration. The amount of air educted into the combustion zone is a critical design feature, since for any particular flare gas composition, a certain amount of air is required in the combustion process to ensure smokeless operation. To decrease the total steam requirement for smokeless operation, the amount of air entrained for a specific amount of steam must therefore increase. One proven way to increase the amount of air entrained by a given amount of steam is to use a true venturi as opposed to the traditional method of straight or bent S/A tubes. Since the venturi is more efficient than straight or bent S/A tubes on a steam injection basis, and more air is educted into the system, significantly less steam is required to prevent the flame from burning inside the tip and / or inside the venturi (see Figures 2 and 3). Figure 2: Venturi versus straight S/A tube air entrainment devices. In general, the momentum from the steam ejected from the steam nozzle (on both devices) pulls surrounding air into the inlet bell. The mixture of air and steam then flows through the Straight Section with the flow becoming developed. In the venturi design, the flow then moves through the outlet bell which gradually increases in outlet area, lowering the pressure, allowing more flow to move through the system. Figure 3: For a steam flow rate of 3,427 lbs./hr, the venturi design with no pre-mixing can pull in 49,572 lbs./hr of air compared to 27,468 lbs./hr of air for a straight S/A tube device with pre-mixing of flare gas, steam, and air. Furthermore, the venturi device achieves a mass ratio of 14.47 lbs. air / lbs. of steam as compared to 8.06 lbs. of air / lbs. of steam for the straight S/A tube device. Based upon these CFD results, the venturi flows approximately 80% more air versus a straight S/A tube device with pre-mixing using the same amount of steam. Another key design consideration in purge operation is that the flare gas flows through an annulus around the steam / air venturi (see Figure 4). The smallest area of the flare gas annulus is the flare tip exit. As the flare gas flows toward the flare gas annulus exit, it is accelerated since it is flowing through a gradually decreasing area. Furthermore, the flare flame tabs located at the outlet of the flare gas annulus form a low-pressure zone to enhance flame stability. The combination of the flare flame tabs and decreasing annulus area create a throat - much like what is used on a burner - to significantly increase flame stability at low purge rates and to help prevent burn back into the flare gas zone and / or the venturi section. The same design approach of using a gradually decreasing flare gas area with the smallest area at the exit, in combination with a steam jet ring and the venturi inlet S/A tubes, significantly decreases the chance of "capping." Capping can occur when steam flares operate at purge rates. During this type of operation, the disparity in the purge rate versus maximum design flow is responsible for much of the damage inflicted on flare tips. The velocity, momentum, and pressure of the flare gas purge at turndown are negligible while the continuous steam injection rates in previous steam flare technologies dominate the forces at the flare tip exit. These forces ultimately "cap" the exit, induce internal burning inside the tip, and degrade it. Generally, this occurs in lower-efficiency injection technologies since they require significant amounts of steam flowing through the injection nozzles. In contrast, the SteamForce HC utilizes a steam jet ring designed to not only use the momentum of the steam to pull in air from the outside, but also to simultaneously induce the flare gas out the annular flare gas throat exit section. That induction action in combination with the controlled, limited flow to the steam jet ring reduces "capping" the flare gas as commonly encountered with previous technologies. The SteamForce HC steam jet ring is located below the flare gas exit, with ports properly spaced to fully utilize Free-Jet mixing technology - used successfully in Zeeco burners for more than two decades. Figure 4: The enhanced venturi inlet bell reduces the entrance pressure loss to maximize the entrainment of air generated by the momentum of the steam exiting the supersonic steam nozzle. The SteamForce HC tip uses the following key design features (see Figure 4): • Flare gas flows through the flare gas connection into the reduced surface area head to reduce the amount of area that is exposed to radiation. • From the head, the flare gas flows into multiple nozzles. The number of nozzles depends on the amount of flare gas flow required and the desired steam consumption at the designed smokeless rate. • Each nozzle is comprised of a small cross-sectional elbow that is connected to a straight flare gas exit section. • No pre-mixing of air and flare gas occurs, ensuring zero oxygen concentration in the fuel gas stream at the flare gas tip exit. • Further improves flaring efficiency and reduces steam consumption while meeting the EPA Code of Federal Regulations, Chapter 1, Subchapter C, Part 63, Subpart CC requirements. Reduced Steam Consumption during Normal Low Flow Operation As previously discussed, reducing the amount of cooling and / or warming steam used under normal conditions when upset flaring is not occurring can significantly reduce operating costs and the emissions generated to produce steam. Since the venturi design is more efficient than typical straight S/A tube designs which premix the flare gas with steam and air, much less steam is required for cooling and or warming. In the example below, the steam per nozzle is reduced from approximately 280 lbs./hr in a straight S/A tube with pre-mixing of fuel, air, and steam; to 100 lbs./hr in a Zeeco SteamForce HC nozzle. This saves 180 lbs./hr/nozzle. In Table 1, each flare uses four (4) nozzles. Therefore, the total steam savings per hour for the SteamForce HC flare is 720 lbs./hr when compared to the straight S/A tube design and 1,510 lbs./hr when compared to the bent S/A tube design.
Table 1 shows the Zeeco SteamForce HC flare reduces steam consumption during normal operation at purge rates with minimum cooling steam from 1,120 lbs./hr to 400 lbs./hr. versus a straight S/A tube design. When compared to a bent S/A tube design, consumption drops from 1,910 lbs./hr to 400 lbs./hr at the same fuel rate.
Table 2: The above table illustrates the savings in steam consumption and money when comparing the Zeeco SteamForce HC to a straight S/A tube with pre-mixing flare and to a conventional bent S/A tube flare. Smokeless Flaring with less Steam Consumption Another prime objective for the SteamForce HC design team was to achieve smokeless flaring with less steam consumption than current technologies. The venturi design plus supersonic steam used to pull in even more air achieved the necessary design advantages versus available alternatives. This increased inherent efficiency made the design significantly less dependent upon fuel gas pressure. The development case was based on a maximum flare gas pressure of 3 psig at maximum flare gas flow and smokeless operation at 20% of maximum flow. The resulting pressure at the smokeless flare gas flow rate was 0.12 psig. This pressure level is more widely applicable and is lower than other designs which require higher flare gas pressures of up to 5 psig for smokeless operation (See Tables 3 and 4). Table 3: Above is the data for smokeless operation with different wind speeds at a flare gas pressure of 0.12 psig (3" WC). Wind conditions impact the smokeless performance with most flares; however, the data shows that very low steam to flare gas ratios can still be achieved in higher wind conditions.
Table 4: Above is a comparison table for a 24" SteamForce HC Flare compared to both a 24" straight S/A tube with pre-mixing flare and a 24" conventional bent S/A tube flare, all operating at 0.12 psig (3" WC). Longer Flare Life Flare tip life is prolonged because the design inherently reduces the chances of capping. With SteamForce HC, most of the air and steam for smokeless flaring is introduced vertically on the same axis as the flare gas. This, in combination with the reduction of steam injection at purge rates, reduces the capping forces normally present in other designs that are detrimental to the flare tip lifespan. Existing steam flare technologies can require multiple injection lines with unique control parameters, which often result in the incorrect application of steam. Most significantly, it is possible to over-apply the perimeter steam and force combustion into the tip and through S/A tubes, thus causing irreparable damage to the flare tip. The SteamForce HC utilizes a single steam injection control line to eliminate the possibility of incorrect steam application. This improved control maximizes steam efficiency and ensures the longevity of the tip. And as previously shown in Figure 4, the tip uses a reduced surface area head and small cross-sectional distributors to significantly reduce the adverse effects of thermal radiation upon the flare tip. Another feature that increases the life of the flare tip is the use of the steam jet rings. The steam ejected from the ports in the ring are spaced to provide improved mixing of the steam with the air and also add to the momentum of the flare gas moving upward and away from the sides of the flare tip. An additional feature which helps keep the flame upright and away from the tip is the flare gas acceleration section which maximizes the forward momentum of the flare gas at the flare tip exit. In general, the momentum of the steam and the momentum of the flare gas are used to enhance the mixing energy in the combustion zone and keep the flame moving upward and away from the tip. Flare tip life is increased by decreasing the surface area for radiation, reducing the chance of capping, and maximizing the upward momentum of the flame. No Pre-Mixing of Fuel, Air, and Steam Before Exiting the Flare Tip There is no pre-mixing of the air and steam mixture before exiting the flare tip. Therefore, the flare tip design complies with EPA Code of Federal Regulations, Chapter 1, Subchapter C, Part 63, Subpart CC requirements without premixing air with flare gas. While pre-mixing is technically allowable, when the flare gas and induced air are mixed prior to the flare tip exit, combustion performance based on EPA NHVcz calculations is negatively impacted. Because the volume of air premixed with flare vent flow must be included in the NHVcz calculation, the equation significantly "dilutes" the zone compared to separate streams of vent gas and perimeter air. In order to achieve the EPA target density of 270 BTU/SCF, a flare utilizing premixed air must offset the dilution with an enriching stream, which comes at a significant cost to the operator. To truly provide the lowest cost of operation to the end-user, the SteamForce HC was developed to meet requirements and efficiency targets without the use of premix air. No pre-mixing is also important since at low flare gas flows in a pre-mixed design, the possibility exists that air could flow backward into the flare gas annulus. Not only does the SteamForce HC design comply with EPA requirements, but the flare is inherently more efficient with improved flame characteristics and a longer tip life. Emissions Reduced Because Less Steam Must Be Generated Emissions savings for the facility directly correlate to the reduced demand for steam generation since boiler emissions for steam production are reduced (See Table 5).
Table 5: Table 5 results are based upon the example given in Table 4 of 24" steam assisted flares of different designs and their resulting requirements for steam consumption over the period of a year. The new design not only enhances efficiency and reduces cost, it also has a significant impact on overall emissions reduction. Boiler emissions that would be reduced would include NOx, CO, CO2, Particulate PM10, UHC, and VOC emissions. Conclusion The SteamForce HC venturi design is inherently and significantly more efficient than existing straight S/A tube or bent S/A tube designs for inducing air with the momentum from steam flow. This efficiency translates into less steam required at normal purge rates and for smokeless operation cases. Using less steam significantly reduces emissions and operating costs. The flare design accelerates the flare gas through the flare gas nozzle to drastically reduce the probability of burn back and flare capping. The design uses steam and air on both sides of the flare gas annulus, creating more efficient mixing, maximizing the momentum to produce a flame that moves away from the tip exit, and reducing the chances of flame roll over. The flare gas is not mixed with air or steam until the flare exit, thus meeting EPA requirements in the most efficient manner with regard to enrichment and utility consumption. In summary, many improvements have been made in the development of the SteamForce HC flare design, resulting in a longer lifespan for the flare tip, lower operating costs, lower cost of ownership, and reduced overall emissions. |