Achieving Environmental Compliance through Proper Destruction Efficiency of Low-Profile Flare Systems

Paper from the AFRC 2014 conference titled Achieving Environmental Compliance through Proper Destruction Efficiency of Low-Profile Flare Systems by J.D. Smith.

To meet evolving environmental regulations, petrochemical facilities like those found in the Gulf Coast must remain adapt to the changing regulatory requirements. To ensure environmental compliance, the proper design of multi-tip low profile (MTLP) flare systems is critical for achieving proper destruction efficiency. The MTLP flare system is responsible for safely and efficiently burning significant amounts of flammable gases during plant upsets. Also, the flare system must perform its designed capacity in an environmentally acceptable manner. A flare system operating at less than peak efficiency during design conditions could potentially vent unburned hydrocarbons to the environment. Large low-profile flare fields pose significant design challenges including elongated flames, air supply to inner burner tips, and high radiation flux to the surrounding wind fence. To ensure the best design, MTLP flares have been analyzed using advanced CFD modeling techniques. Validation of the CFD tool has been used to confirm the accuracy of the modeling results. The validated CFD tool has been used to evaluate MTLP flare designs to ensure proper destruction efficiency and effective operation. Zeeco in conjunction with Systems Analyses and Solutions will review proper MTLP flare design considerations and present several examples of how CFD was used to analyze proper design for safe and clean operation at design conditions.

American Flame Research Committees 2014 - INDUSTRIAL COMBUSTION SYMPOSIUM Hyatt Regency Hotel Houston, Texas - September 7 -10, 2014 Achieving Environmental Compliance through Proper Destruction Efficiency of Low-Profile Multi-Tip Flare Systems Joseph D. Smith, Ph.D., Robert Jackson and Ahti Suo-Anttila, Ph.D., Systems Analyses and Solutions Inc., Tulsa, Oklahoma, USA Scot Smith, Doug Allen, Zeeco, Inc. Zeeco Inc. Broken Arrow, Oklahoma, USA ABSTRACT To meet evolving environmental regulations, petrochemical facilities like those found in the Gulf Coast region must adapt to changing regulatory requirements. To ensure environmental compliance, the proper design of low-profile multi-tip (LPMT) ground flares is critical for achieving high destruction efficiency. The LPMT flare system must safely and efficiently burn large amounts of flammable gases during plant upsets. Also, the flare system must operate at its design capacity with low impact on the surrounding environment. A flare system operating at less than peak efficiency during normal operating conditions could emit unburnt hydrocarbons that could adversely affect the environment. Large LPMT flare fields pose significant design challenges including elongated flames, air supply to inner burner tips, and high radiation flux to the surrounding wind fence, equipment and personnel. To ensure the best design, LPMT flares have been analyzed using advanced CFD modeling techniques. Validation of the CFD tool has been used to confirm the accuracy of the simulation results. The validated CFD tool has been used to evaluate several LPMT flare designs to ensure proper destruction efficiency through safe and effective operation. Work performed by Zeeco and Systems Analyses and Solutions has focused on establishing proper LPMT flare design considerations which are demonstrated by several examples presented in this paper. This work illustrates how CFD coupled with testing has been used to analyze and develop effective flare designs that allow safe and clean operation at design operating conditions. Low Profile Multi-Tip ground flares operated under various operating conditions including no-wind and cross wind conditions have been analyzed using SAS' proprietary modeling technology. Examples presented below consist of large flare fields with hundreds of pressure assisted flare tips some with steam and air assist as well. These LPMT flare systems are capable of firing over a million kilograms per hour of flare gas with high destruction efficiency. The detailed CFD flare model used to perform these analyses approximates the detailed combustion processes and estimates soot formation and radiation between flames and surroundings. Wind effects are captured using transient large eddy simulation techniques. Flare performance under both no-wind and wind conditions have been successfully compared to results from both single and multi-tip to estimate full flare field operating behavior. The technical approach has helped lead flare Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas design and operating decisions to ensure proper air/fuel mixing at all tips to achieve high destruction efficiency for these large LPMT flare systems. LOW PROFILE MULTI-TIP FLARE DESIGN Low profile multi-tip flares (LPMT) represent a special class of flares to safely and cleanly process large amounts of flare gas. Originally, these flares involved "unstaged candle flares" and "pipe burners" (see Figure 1) with staging developed and added in the early 1970's to broaden their turn down ratio and allow them to operate at larger flare gas flow rates (see Figure 2). Over the past 35 years, many improvements have been made to these LPMT flares including: 1) proper tip spacing and row spacing required for cross lighting from a single pilot, 2) advanced fence design to supply adequate air to the entire flare field and to protect surrounding equipment and personnel from flame radiation, 3) use of air and steam assist media to support low pressure operation, and 4) advanced tip design for high combustion efficiency. Of particular importance is the ability of staging to allow large turn down ratio while maintaining highly efficient smokeless combustion. Figure 1 - Legacy multi-tip ground flare designs over the years Page 2 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Figure 2 - Multi-Point Ground flare is part of larger production sites Improvements in LPMT flares have addressed several important design issues which include among other things: 1. Deviation from normal flare operation for non-standard flow rates and flaring duration, 2. Clean, efficient combustion for varied flare gas composition, flow rates, tip pressures/ temperatures for non-standard flaring scenarios, 3. Relative spacing between flare field and nearby equipment and personnel for design and non-design flow rates with associated radiation flux from the flame, 4. Wind fence design to efficiently protect surrounding equipment and personnel from flame radiation, 5. Wind fence design that allows sufficient air to promote clean efficient combustion for standard and non-standard gas composition, operating pressure/ temperature and external wind, 6. Noise emissions from operating flares and their impact on workers and the surrounding community, and 7. Flare burner tip design to promote local air/fuel mixing to support clean, efficient combustion. Today's LPMT flares utilize highly engineered burner tips that promote local air/fuel mixing that results in clean efficient combustion (see Figure 3). These tips have variable area arms, are constructed out of 310 SS cast materials using investment casting to promote longer life and are pressure tested at the factory to ensure safe operation. These flares also use extensive fuel staging to provide high destruction efficiency over a wide range of flare gas flow rates (see Figure 4). Of particular focus is clean operation at low flow rates where air/fuel mixing is difficult due to a lack of mixing energy. Well-designed LPMT flares include wind fences that safely shield surrounding equipment and personal from flame radiation while allowing sufficient air inside the flare field for high Page 3 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas destruction efficiency. These fences are optimized to reduce the amount of steel required in their fabrication but with enough structural integrity to withstand the stress caused by high winds. Finally, these fences are design to protect the flare flames from cross winds effects (see Figure 5). Figure 3 - Low profile flare tips use arms connected to gas riser to ensure proper mixing of air and flare gas Figure 4 - Modern ground flare system include complex piping systems to stage flare gas to accommodate wider operating ranges with high combustion efficiency Multi-tip ground flare design has also been optimized for safe operation. Besides the radiation issues mentioned earlier, plume dispersion has been studied to minimize its impact on overhead aircraft by Gerhold, 1999 [1] and Smith et.al., 2012 [3]. Ignition of multi-tip flares as a function of tip and row spacing has also been addressed by Smith et.al., 2007 [4] and Smith et.al., 2010 [5]. Page 4 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Figure 5 - Effective wind fence designs protect surrounding equipment and personnel from high radiation flux while providing necessary air for efficient combustion Environmental and safety issues related to reduced flaring operations requires reliable and efficient ignition systems which has been a focus at Statoil as discussed by Bjorkhaug, 2008 [6]. Tip and row spacing to ensure effective cross lighting is optimized during initial testing (see Figure 6) and is implemented in full scale operation (see Figure 7). Figure 6 - Time shots for cross lighting a multi-point flare from top down at Zeeco test facility Figure 7 - Cross lighting during staging in a large multi-point ground flare Lights at top of flame and moves down to tip Initial time ~0.01 sec later ~0.02 sec later Page 5 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas RECOMMENDED PRACTICE AND REGULATION Today's flare design and operation is regulated by both state and federal law. In addition, the American Petroleum Institute has published recommended practices and guidelines for safe and efficient flare operation. Most flare regulations focus on "Utility" and "Assisted" flares with little attention given to multi-tip ground flares. In addition, flare design and performance guidelines have been established for general flares while ground flares have additional performance metrics to consider (i.e., tip pressure/temperature, tip/row spacing, etc.). As with elevated flares, LPMT ground flare performance is difficult to quantify given the very large flame with its high radiation flux as well as questions of what to measure (i.e., air flow vs fuel flow) and where to measure it (edge of flare field at ground level vs middle of flare plume at a reasonable distance above the flame). According to 40 CFR 60.18(b) and 63.11(b), flares must have a pilot flame at all times, operate with sufficiently low exit velocities to ensure stable combustion at the tip, burn flare gas with a heating value high enough for flammable operation, and have adequate mixing energy to effectively mix the flare gas with the surrounding air. In addition, 40 CFR 60.11(d), 60.18(d), 60.482-10(e), 63.6(e), 63.11(b) and 63.172(e) state that the flare design must adhere to vendor supplied design specifications and use "good" air pollution control practice. Proposed regulations state flares are to be designed and operated with no visible emissions using EPA Method 22 (except for periods not to exceed 5 minutes in 2 hours). The regulations state that flares will be operated with a flame present at all times, confirmed by a thermocouple and that the flare will be "used only when the net heating value of the flare gas is 300 BTU/scf or greater (for steam or air assist) or 200 BTU/scf for non-assisted flares. Also, the flare should be designed and operated with an exit velocity less than 60 fps or less than 400 fps for flare gas with sufficiently high net heating value.1 To meet these regulations, questions related to LPMT flare design and operations remain: 1. Where does combustion air required for high flare combustion efficiency originate? 2. How do cross winds affect flare combustion efficiency? 3. How does individual tip geometry, tip/row spacing, and flare gas flow rate/ composition affect flame shape and optimum tip/row spacing? Answers to these questions and other design/ operating questions require the use of both virtual and physical testing. Virtual testing uses Computational Fluid Dynamics (CFD) to analyze flame interactions between the hundreds of flare tips given a specific burner design as well as specific tip and row spacing for a particular flare field (see for example Figure 8) capable of processing millions of kg/hr of saturated and unsaturated hydrocarbons in a clean and efficient fashion. The CFD tool also accurately represents the full flare field dimensions which may exceed 100 x 100 meters. The CFD tool must also accurately approximate all of the governing physics (and their interactions) for general flare operation (i.e., turbulent fluid mechanics, radiation heat transfer, high temperature chemical kinetics, etc.). Predictions must be validated to provide 1 Enforcement Alert, "EPA Enforcement Targets Flaring Efficiency Violations", EPA-325-F-012-002, August (2012). Page 6 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas confidence in the estimated flame shape and height from which wind fences are designed and built. Figure 8 - Sample configurations of a LPMT ground flare [1, 2] Page 7 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Systems Analyses and Solutions (SAS) engineers have developed and applied detailed computational fluid dynamics (CFD) models for several LPMT ground flares using SAS's proprietary CFD software called C3d. SAS has also used this tool to analyze performance for air and steam assisted flares, enclosed flares, and elevated multi-tip flares. Analysis of LPMT flares has focused on establishing the expected air flow through the wind fence for a given design and the expected flame shape and height during maximum flaring rates. Modeling has been validated using single and multiple burner tests conducted at Zeeco's test facility using propane, propylene, ethylene and xylene. Tip spacing has been established for several burner designs to ensure efficient safe cross-lighting for general saturated and unsaturated hydrocarbons. Features of the C3d flare modeling tool will be discussed below. Examples of model validation will also be presented. Lastly, several analyses of LPMT flares will be discussed to demonstrate their high destruction efficiency. Information presented below describes work performed over the past several years in conjunction with Zeeco and concludes with design and operating recommendations for LPMT flares. CFD FLARE TOOL The C3d flare modeling tool used to analyze LPMT flare systems has been described in detail elsewhere. [7, 8] Validation work for this tool, performed at the Zeeco test facility has also be described elsewhere. [9, 10] The combustion model, central to modeling LPMT flares, has also been described elsewhere. [11, 12, 13] Flare model specifics The C3d flare tool simulates turbulent reaction chemistry coupled with radiative transport from buoyancy driven fires (i.e., pool fires, gas flares, etc.) and surrounding objects (i.e., wind fence, process equipment, etc.). The tool "reasonably" estimates the effects of cross winds, flame shape and height, and thermal fatigue for a given flare configuration. Typical simulations require CPU times ranging from hours to a few days on a "standard" desktop workstation. The tool, originally developed at Sandia National Laboratory, is based on ISIS-3D originally built to simulate pool fires and the thermal performance of nuclear transport packages. ISIS-3D has been adapted and modified to analyze large gas flare performance. C3d predicts flame shape and size, estimates the smoking potential for a flare firing unsaturated and saturated hydrocarbon gases, and estimates the radiation flux from the flame to surrounding objects. Combustion Model The C3d combustion model is a variant of what was originally developed by Said et.al. [14]. Species involved in the combustion process include Fuel vapor (F) from the flare tip, oxygen (O2) from the air, products of combustion (PC) including water vapor and carbon dioxide, radiating carbon soot (C) produced from hydrocarbon combustion, and non-radiating intermediate species (IS). Reactions take the general form of: 1kg F + (2.87-2.6S1) kg O2 → S1 kg C + (3.87-3.6S1) kg PC + (50-32S1) MJ Eq. 1 Page 8 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Eq. 1 describes incomplete combustion of fuel (F) that produces radiating soot (S) and products of combustion (PC) plus energy. The standard combustion soot stoichiometric parameter (S1), normally set as 0.05, is adjusted based on fuel type (i.e., for natural gas, S1 is set to 0.005). The soot producing cracking reaction consumes fuel (F) and energy and produces radiating carbon (C) plus the intermediate species (IS): 1kg F + 0.3 MJ → S2 kg C + (1-S2) kg IS Eq. 2 Eq. 2 requires the Cracking Parameter (S2) be specified. This is normally set to 0.15 but can also be adjusted for fuel type. Soot combustion is described by: 1kg C + 2.6 kg O2 → 3.6 kg CO2 + 32 MJ Eq. 3 which consumes soot (C) and oxygen and produces carbon dioxide plus some energy. Combustion of Intermediate Species (IS) is described by: MJS-1S3250 kgS-1S 6.387.3 kgS1S6.287.2 kg 12222222−+−→−−+PCOIS Eq. 4 where coefficients are selected to complete combustion of the intermediate species (IS) and produces Products of Combustion (PC) and thermal energy. Coefficients for these formulas are in mass weights as opposed to moles. The first reaction has a low activation energy which allows partial burning and heat release of the flare gas. This maintains combustion since the partial heat released allows the second reaction, which produces most of the heat and all of the soot, to occur. The flare gas Arrhenius combustion time scale is combined with the turbulence time scale to yield an overall time scale for the reaction rate. The characteristic time from the kinetics equation was combined with the characteristic turbulence time scale as: =Δ2 Eq. 5 where is the characteristic cell size, C is a user input constant (0.2x10-4), is the eddy diffusivity from the turbulence model and tturb is the characteristic time required to mix contents in a computational cell (i.e., turbulence time scale). Reaction rates are combined using simple addition of time scales. The combustion model approximates turbulent reacting flows the same as the standard eddy dissipation model does and includes local equivalence ratio effects. Combustion reactions are based on Arrhenius kinetics with: 1𝑑 =−􀵣􆎠Π𝑖 𝑁𝑁𝑖𝑖􀵧􆜵−⁄ Eq. 6 where coefficient C and the Activation Temperature TA are supplied for all reactions. The Arrhenius kinetics and turbulent mixing are from the Eddy-Breakup type combustion model. The kinetics and turbulence models are combined by summing the characteristic time scales. In addition to these dynamic models, sequences of irreversible chemical reactions that describe the combustion chemistry are required. To minimize computational load, a minimum number of chemical reactions are used that fulfill the requirements of total energy yield and species Page 9 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas consumption and production. From the basis of heat transfer, flame size, and air demand, details of the chemical reactions are not critical as long as the oxygen consumption is correctly balanced for a given fuel. To this end, a multi-step chemical reaction model for flare gas is used to approximate the global reaction mechanism as shown: 1𝑑 =𝑏 12−⁄ Eq. 7 where Ak is the pre exponential coefficient, X1 is the mole fraction of fuel, X2 is the mole fraction of oxygen, Ea is an activation energy, T is the local gas temperature, and b, c and d are global reaction parameters. For this approach it is noted that when global combustion kinetics are implemented in the combustion model, care must be taken since reaction coefficients are based on a specific experiment so using coefficients published elsewhere in the global reaction mechanism may produce misleading results. In summary, the C3d flare modeling tool: 1. Simulates the transient behavior of LPMT ground flare operation, 2. Includes interactions between combustion chemistry and radiation heat transfer, 3. Accounts for interactions with neighboring burners, the wind fence, and surrounding buildings, 4. Uses fuel specific soot kinetics which contributes to radiation transfer, 5. Captures momentum vs. buoyancy effects, cross wind effects on flame structure, detailed chemical reactions, radiation flux to/from the flame and surrounding surfaces, and 6. Assesses various flaring scenarios for specified ambient conditions and flare gas flow, composition, temperature/pressure. Key assumptions required for C3d flare models include: 1. Flame emissivity is a function of flare gas composition, soot volume fraction, flame size and shape, 2. Participating media is included in radiation attenuation, 3. Pressure boundary conditions are used to match specified wind conditions, 4. Flame radiation heat transfer is approximated by an optically thick flame with time dependent view factors, and 5. Surrounding surfaces contribution to the net radiation heat transfer (i.e., ground re-radiation). FLARE TESTING Virtual testing of LPMT flares allows a design engineer to assess various design features for different flaring scenarios under varying ambient conditions. Before using virtual results, the simulation tool must be validated. The C3d flare tool has been extensively validated at the Zeeco flare test facility as described below. Page 10 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas The flare testing conducted at Zeeco used the configuration shown in Figure 9 and the test stand shown in Figure 10. Tests were conducted burning Tulsa Natural Gas, Ethylene, Propane, Propylene and Xylene to cover a wide range of saturated and unsaturated hydrocarbons to allow model validation for subsequent ground flare simulation. During the tests, radiation measurements were also taken at two locations (see Figure 11). Figure 9 - Zeeco multi-flare burner test setup [2]. Figure 10 - Test stand for single and multi-tip experiments Page 11 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Figure 11 - Relative location of radiometers for flare testing Figure 12- Single burner tests to validate predictions of flame shape and height. Observed flame lift off represents non-luminous (non-sooting) combustion near tip Radiation meters at 25 ft and 50 ft from flames Page 12 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Figure 13- Three burner tests to validate prediction of flame shape and height LPMT FLARE SIMULATION As discussed earlier, C3d requires a combustion model for different flare gas compositions. Examples of this work are described in detail elsewhere. For propane combustion, a two-step chemical reaction is used with propane and oxygen reacting to produce water, carbon dioxide and soot. For ethylene and mixed gases, a three-step reaction is used. The first reaction burns half the hydrogen in the hydrocarbon fuel and any free hydrogen. The second reaction burns the remaining hydrogen in the hydrocarbon fuel plus most of the carbon and produces some soot. The third reaction burns the soot produced in the second reaction. The two step reaction for propane is less stable than the three step reaction used for ethylene and mixed gases since the first reaction has a relatively low activation energy which allows partial combustion at low temperatures releasing approximately 20% of the heat of combustion which keeps the fire burning and common in most hydrocarbon fires. The specific reaction mechanisms, shown below, are discussed in more detail elsewhere. [4, 5, 7, 10] Propane Combustion model The two-step chemical reaction developed for propane combustion includes: C3H8 + 3.6 O2  3 CO2 + 1.6 H2O + 0.024 Soot + 46 MJ/kg propane Eq. 8a Soot + 2.66 O2  3.66 CO2 + 32 MJ/kg Soot Eq. 8b Page 13 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas with the stoichiometric coefficients used in the formulas listed in terms of mass instead of moles. The Arrhenius kinetics equation and parameters for this reaction is: Propane Rate (moles/m3/sec) = XC2H8 * XO2 *A Exp(-Ta/T) Eq. 9 Here X is the molar concentration of the species (moles/m3), A is the pre-exponential factor (3.29e10 and 8e11 for the 1st and 2nd reactions), Ta is the activation temperature (15,922 K and 26,500 K), and T (K) is the local temperature. The reaction rates are combined by simple addition of the time scales. Ethylene Combustion Model The more stable three step combustion model developed for Ethylene combustion includes: C2H4 + 0.57 O2  0.93 C2H2 + 0.64 H2O + 9.4 MJ/kg ethylene Eq. 10a C2H2 + 2.58 O2  2.7 CO2 + 0.7 H2O + 0.2 Soot + 34.1 MJ/kg intermediate Eq. 10b Soot + 2.66 O2  3.66 CO2 + 32MJ/kg Soot Eq. 10c where again the coefficients listed are in mass instead of mole. The Arrhenius kinetics equations and parameters for these reactions are: First Reaction Rate (moles/m3/sec) = XC2H4 * XO2 *1.0e15 Exp(-10,500/T) Eq. 11a Second Reaction Rate (moles/m3/sec) = XC2H2 * XO2 *1.0e11 Exp(-15,500/T) Eq. 11b Third Reaction Rate (kg/m3/sec) = YC * YO2 *1.0e11 Exp(-20,500/T) Eq. 11c Again, X is the molar concentration and Y is a mass concentration. As discussed above, the three-step mechanism includes a reaction with a low activation energy which allows partial combustion and associated heat release and provides heat for the second reaction to occur. As with the propane combustion model, the ethylene Arrhenius combustion time scale is combined with the turbulence time scale to yield an overall time scale for the reaction rate. Mixed Gas Combustion Model The same approach using a three step mechanism has also been used for mixed gas combustion. For a mixture of approximately 32% ethylene, 20% ethane and 34% hydrogen the reactions become: 0.572 C2H4 + 0.383 C2H6 + 0.043 H2 +0.982 O2  0.53 C2H2 + 0.34 C2H3 + 1.1 H2O + 14.2 MJ/kg Eq. 12a 0.61 C2H2 + 0.39 C2H3 + 2.66 O2  2.66 CO2 + 0.813 H2O + 0.181 Soot + 34.4 MJ/kg Eq. 12b Soot + 2.66 O2  3.66 CO2 + 32 MJ/kg Eq. 12c with the Arrhenius kinetics equation and parameters as: First Reaction Rate (moles/m3/sec) = Xfuel * XO2 *1e15 Exp(-10500/T) Eq. 13a Second Reaction Rate (moles/m3/sec) = Xmix * XO2 *1e12 Exp(-15500/T) Eq. 13b Page 14 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Third Reaction Rate (kg/m3/sec) = YC * YO2 *1e11 Exp(-20500/T) Eq. 13c Flare Model Computational Mesh To model LPMT flares, three separate models are prepared - one for a single burner, one for a group of three burners and one for the full flare field with hundreds of burners. Each computational mesh includes a different degree of refinement with the single burner model having the most refined mesh to capture flow details for individual jets from each arm in the flare burner (see Figure 14). A less resolved mesh is used for multi-tip simulations to reduce the overall computational demand required to obtain a converged solution in a reasonable amount of time (see Figure 15 for the three burner model and Figure 16 for the full flare model). All meshes use rectangular orthogonal cells which also speed up the overall computation. Figure 14 - Single Burner Mesh with 110,000 cells for a domain size of 6 x 6 x 30 m Page 15 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Figure 15- Three-Burner geometry for a domain of 30m X 30m X 25m including radiation meters (green boxes) located 15 m and 50 m respectively from the flare tips Figure 16- Full flare field Mesh, 1.2 million cells (domain extends 10 m beyond fence in all directions and 25 m high) Page 16 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Single Flare Burner Simulations Single burner simulations for propane have been performed for the "no-wind" condition and compared to the single burner tests discussed earlier. For the "no-wind" case (see Figure 17) the simulation appears to predict the tight "pencil-like" flame shape observed in the test and correctly predicts the flame height of approximately 15 m. For a 3 m/s wind condition, the single burner test shows that the flame bends over about 10º from vertical with a height of about 10 m. Comparison to simulation results shows good agreement for the "wind" condition. Figure 17 - Single burner results for the "no-wind" condition Figure 18- Single burner results for a 3 m/s "wind" condition Single BurnerNo wind produces tight "pencil-like" flames15 m (49 ft)20 m (66 ft)Flare tip located 2 m (~7 ft) above gradeMeasured flame length wo/ wind13.8 -15.3 m (48' -53')Non-luminous Flame ~1 m10.5 m (34 ft)~8-10˚ flame tiltMeasured flame length w/ windShorter flame with ~10º flame tilt Page 17 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas The three burner simulations for propane have also been performed for both a "no-wind" and a "windy" condition. Comparing simulation results for the "no-wind" case to the test results shows the model is able to capture the key features for the 3 burner case (see Figure 19). In this case, the flames appear as three independent flames near the tip flame interaction near the top. In this case, the simulation correctly predicts the flame height and overall shape. Figure 19 - Three burner results for a no-wind condition Radiation Measurements and Predictions A key issue for flare performance is the radiation flux from the flame to the surrounding equipment and personnel. The C3d flare modeling tool has been used to predict flame radiation for the three burner test to validate the approach for full scale flare modeling. The radiation flux from the flare flame depends upon several issues including combustion chemistry, soot production, atmospheric conditions, and fluid dynamics for flame size, flame shape, flame and atmospheric temperature and gas and wind velocities. Thus, of all flare operating variables, radiation has the greatest sensitivity to errors or inadequacies in the model. Therefore, a direct comparison between radiation measurements and model predictions was completed to validate the predictions. Results of this work is summarized below but is discussed in much more detail elsewhere. [4] Validation testing was completed for a three burner flare test shown above (see Figure 19) for a no-wind condition with ground re-radiation accounted for in the predictions. Results include comparison between predicted radiation flux to measured radiation flux for several tips and several firing rates. Tip Height~ 3 m (10')Non-luminous region ~1 m (3')Flame height ~11 m (~36')~16 m (52 ft)12 m (39 ft) Page 18 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Table 1- Predicted and Measured Radiation flux comparison for three-burner flare test [4] As shown during the experimental testing (see Figure 18), wind both reduces flame height and changes flame location and shape. Thus, the flare model has been used to assess the effect of cross wind on radiation flux. Previous work considered wind speeds between 0 and 4 m/s with the wind direction 45 degrees toward the radiation meters. As shown in Figure 20, wind reduces the amount of radiation flux from a flare flame. Figure 20 - Wind speed effect on radiation flux from flare flame Full Flare Analysis Several full flare field calculations have been completed and discussed elsewhere [3, 4, 5] using C3d with and without wind. These full LPMT flare calculations consider large flare fields Page 19 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas with dimensions on the order of hundreds of feet so a coarse mesh must be used where possible to reduce the overall computational time required. Mesh coarseness must be balanced with the need to accurately resolve the flame above the flare burners and the resolution required to capture the actual burners. To do this, a non-uniform mesh density is used (see Figure 16 and Figure 21) mesh sizes on the order of between 1,000,000 and 4,000,000 are routine. Results from this work illustrate the ability to predict both radiation fluxes (see Table 2) and flame shape and combustion products (Figure 22) in the full flare. To accurately capture the wind effect with the fence design, detailed analysis of the wind fence has also been completed and is briefly described below. Figure 21 - Final mesh size was 360 x 175 x 68 for 4,284,000 cells with fine mesh included near the flare tips to resolve ignition/combustion phenomena Table 2 - Radiation flux from full flare for "no-wind" cases WALL CASE Left Wall (W/m2) Right Wall (W/m2) Flame Optical Thickness Peak Flow Case No Wind 61,000 35,000 0.275 Sustained Flow Case No Wind 6,600 6,600 0.280 Page 20 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Figure 22 - Predicted flame size/shape (colored iso-surfaces) and products of combustion (transclentant iso-surfaces) A key issue discovered in earlier LPMT flare analysis is the potential impact on the direction of wind coming into the flare field. As shown in Figure 23, wind approaching the flare field at an angle can create regions in the flare field with low air flow which can result in taller flames. In addition, wind coming over the wind fence can push flames below the flare tips potentially leading to high temperature impact on flare tips. The C3d model was used to check this issue and confirmed no expected problems for the proposed flare design (see Figure 24). Figure 23 - Flames represented by fuel iso-surfaces with streamlines indicating areas in the field with low air flow resulting in longer flames Low air flow Low air flow Page 21 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas Figure 24 - Flames represented by fuel iso-surface (volume fraction = 0.02) colored by temperature indicating wind does not push flames down below flare tips Wind flow through the wind fence is analyzed using CFD model of a 40 meter section considering the detailed fence design. This allows us to determine the pressure drop through the wind fence (see Figure 25). Using this approach, the average predicted pressure drop determined for the detailed fence model was 540 Pa. Based on this, the fence porosity was established as 0.489. Using this porosity, the porous plate discharge coefficient was modified in a series of porous plate fence simulations to match the flow and pressure drop from the detailed fence model to the porous plate fence model. Once the porous plate fence model parameters were determined the full flare field simulation was set up using this porous plate model for the exterior fences as well as for the fencing between flare fields. Each of the flare tips was modeled as a jet with the appropriate exit area and velocity based on the actual flare tip geometry. CONCLUSIONS LPMT flare systems are the largest gas flare used in the Petrochemical industry to provide safe, clean and efficient ways to dispose of significant amounts of flammable hydrocarbon gases. Work completed over the years has resulted in advanced designs for these flares that include flare burners that promote effective mixing of flare gas with local air to support high destruction efficiency, staging that allows large turn down ratios for flare gas flow rate while maintaining proper tip velocity to ensure smokeless operation, and efficient wind fences that allow sufficient air to enter the field but protect the flame from cross wind effects. The work described in this paper illustrates both testing and simulations carried out by Zeeco to ensure their LPMT flare designs are capable of operating at large turn down ratios for both wind and no-wind conditions. The C3d flare modeling tool used by SAS has been routinely applied to analyze specific operating Page 22 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas scenarios for LPMT flares to ensure the resulting flames are properly shielded from surrounding equipment and personnel while allowing the flares to operate efficiently with high destruction efficiencies. The modeling tool, validated by direct comparison to experimental test results, has been used to identify potential operating issues related to wind speed and direction relative to flare field orientation to minimize mal-distributed flare flames. The modeling tool has also been used to assess air demand for a given flare field configuration including multiple flare fields combined in the flaring system. Figure 25 - Fence configuration modeled with C3d showing streamlines through fence Page 23 of 25 Environmental Compliance in Multi-Tip Ground Flares September 7 -10, 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Houston, Texas REFERECNES 1. Gerhold, B., "Computing for Profit: Corporate CFD", Phillips Petroleum, November (1999). 2. Smith, S., Modi, J., McDonald, J. M., Little, C., Smith, J. D., Berg, L., "Environmental Testing of an Advanced Flare Tip for a Low-Profile Flare Burning Ethylene", AFRC-JFRC 2007 Joint International Combustion Symposium, Marriott Waikoloa Beach Resort, Hawaii, October 21-24, (2007). 3. Smith, J.D., Jackson, R., Berg, L., Suo-Anttila, A. and Smith, S., "Plume Predictions from Gas Flares", American Flame Research Committee, Salt Lake City, Utah, September 5-7 (2012). 4. 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