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2010 Multi Tip Flare Ignition Presentation

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Analysis of elevated flare ignition and resulting pressure wave generated as a function of ignition delay and flow rate.

Analysis of elevated flare ignition and resulting pressure wave generated as a function of ignition delay and flow rate.

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  • 1. www.inl.gov Prediction and Measurement of Flare Ignition Using the LES based C3d J. D. Smith, Ph.D., Idaho National Laboratory A. Suo-Anttila, Ph.D., Systems Analyses and Solutions S. Smith and N. Philpot, Zeeco, Inc. American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton, Maui, Hawaii – September 26-29, 2010
  • 2. OUTLINE • Background and Introduction • Flare Tests • Model Setup and Methodology • Simulation Results – Low Flow Conditions – High Flow Conditions • Observations and Conclusions Slide 2 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Maui, Hawaii September 27 - 29, 2010
  • 3. Slide 3 Elevated Multi-Tip Gas Flare Ignition Slide 3 • Nominal Firing Rate = 350 Tons Per Hour (TPH) • Max Firing Rate – 1350 TPH • Mostly Natural Gas (Mwt = 18) • Experienced Pressure Wave during ignition • Conducted Tests to quantify ignition phenomena: − Microphones used to measure pressure wave − High Speed Video used to capture flame during ignition • Test results reported elsewhere (summarized below) • Test video shows ignition behavior American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Maui, Hawaii September 27 - 29, 2010
  • 4. Slide 4 Test Layout Slide 4 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Maui, Hawaii September 27 - 29, 2010
  • 5. Slide 5 Flame during Ignition Slide 5 Maui, Hawaii September 27 - 29, 2010 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
  • 6. Slide 6 Test Results: Sound Level (pressure wave) and Flame speed estimate Slide 6 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton • Flame speed estimate from tests is 45 m/s (Test 1) and 50 m/s (Test 2) • Maximum pressure generated by spherical flame propagating at 50 m/s would be ~48 mB (AIChE correlation*) * Center for Chemical Process Safety, Guidelines for Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires and BLEVEs. AIChE (1994). Maui, Hawaii September 27 - 29, 2010
  • 7. Model Setup: General Comments • Transient conservation equations with radiative heat transfer and combustion chemistry • Considers soot formation and other multi-phase systems using Eulerian/Eulerian formulation • Accurately assess different operation scenarios (wind, flow rate, fuel type, surroundings) • Reasonable CPU time requirements on “standard” workstation • Trade offs for “Engineering” Approach – Sacrifice generality (large fires only) in favor of quick turnaround with quantitative accuracy – Reaction rates and radiation heat transfer models apply only to large fires – Models intended to make predictions “good-enough” for industrial use – Model validation for each application to establish accuracy of results Slide 7 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Maui, Hawaii September 27 - 29, 2010
  • 8. Combustion Model • Variant of Said et al. (1997) turbulent flame model • Relevant Species (model includes relevant reactions)  F = Fuel Vapor (from evaporation or flare tip)  O2 = Oxygen  PC = H20(v) + CO2  C = Radiating Carbon Soot  IS = Non-radiating Intermediate Species • Eddy dissipation effects and local equivalence ratio effects • Reactions based on Arrhenius kinetics  C and TA determined for all reactions Slide 8 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Maui, Hawaii September 27 - 29, 2010
  • 9. Reaction Rate Model • Arrhenius rate model – Consumption of primary reactant increases on reactants mass fraction fRi and temperature T in volume – Coefficients C and Activation Temperatures (TA) determined for all reactions – Where: Ak = Pre-exponential Factor X1 = Natural Gas Mol Frac X2 = O2 Mol Frac Ea = Activation Temperature T = Local Gas Temperature b, c, d = Global Exponents American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Slide 9 Maui, Hawaii September 27 - 29, 2010
  • 10. Approach (1) American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Slide 10 • Laminar flame global mechanism used as starting point – Used Activation temperature + mol frac exponents (based on reaction) – Pre-exponential (Ak) factor adjusted to match turbulent combustion rxn rates • Turbulent mixing effect on combustion included via LES – Two coefficients adjust effect (ε = turbulence intensity scale factor; δ = combustion species mixing time delay) • Parameters adjusted to to match experimental results Maui, Hawaii September 27 - 29, 2010
  • 11. Approach (2) • Other model variations considered: – Computational grid size & cell number – Turbulence model (zero equation and one-equation LES) – Nozzle structure (jet cone vs nozzle surface) – Numerical upwind differencing – Time to ignition • Boundary Conditions – Hydrostatic pressure on all external boundaries (adjusted to account for wind) – Initial temperature and composition set to 300 K and air • Other Assumptions – Flare gas combustion approximated as described above – Thermal radiation calculated w/ standard radiation models – Wind conditions, flare gas inlet temperature and pressure, and radiation effects set to match measured value – Flame emissivity = F (gas comp, soot fv, flame size/shape, temp) Slide 11 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Maui, Hawaii September 27 - 29, 2010
  • 12. Slide 12 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Nozzle Approximation Two approaches considered for detailed nozzle structure: 1.Mass Sources on Nozzle Cone: • Place source terms on cone surface and inject natural gas at correct velocity and mass rate as if resolved using fine cells • Individual nozzle flow kept exactly identical (in absence of any flow mal-distribution) 2.Mass Inlet on Nozzle Surface • Inject fluid through cells representing nozzles • Total inlet adjusted for correct mass flow • Individual rates varied (nozzle sizes varied due to overlap of square cells on circular tips) 3.More general “mass source” approach selected Maui, Hawaii September 27 - 29, 2010
  • 13. Slide 13 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Computational Mesh o Flare dimensions approximated as 3.35m square, computational volume set as 20m long X 20m wide X 15m high (domain extended ~9m beyond flare edge) o Domain bottom set at top of elevated flare exit (reduce mesh size) o Domain separated into two regions Region 1: Near Tip Region just above nozzle and 7m square by 8m high • Fixed horizontal cells with equal spacing (80 cells 0.0875m on a side) • Vertical dimension slowly varied with 0.05m at nozzle face to 0.14m at top of region (90 cells) • High resolution region had 576,000 cells Region 2: Buffer Region surrounding Near Tip Region • Course, stretched cells to provide buffer between boundaries and near tip • Both horizontal dimensions included 14 cells; vertical dimension included 12 cells • Provided large distance from edge of domain (pressure boundary) and flame surface to prevent estimated pressure in igniting flame ball o During analysis, mesh refined several times to improve calculation results Maui, Hawaii September 27 - 29, 2010
  • 14. Slide 14 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Pressure Monitoring Locations Note: x and z are horizontal positions (x = 0 and z = 0 is flare center) and y is height above flare tip as shown in graphic) Maui, Hawaii September 27 - 29, 2010
  • 15. Model Tuning (1) • Over 60 CFD runs indicated pressure wave magnitude mostly dependent on ignition time (combustion kinetics and turbulence had secondary effects) • Typical pressure pulse of +30 to +40 mB wave followed by negative wave of -10 to -20 mB • Runs with ignition delay exhibited higher pressures waves – Combustion parameters varied over significant range but had little effect on predicted peak pressure wave • Ignition delay accomplished by: – Natural gas jets turned on for 0.25 sec prior to igniting pilot – After ignition, pilot flame grew and ignited flare gas at approximately 1 sec – Resulting flame ball had significantly higher pressures than nearly all other cases considered – Cases #3 and #41 had overpressures ~0.5 atm American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Slide 15 Maui, Hawaii September 27 - 29, 2010
  • 16. Predicted Pressure for All Cases at Low Flow Conditions American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Slide 16 Maui, Hawaii September 27 - 29, 2010
  • 17. Model Tuning (2) • Final chemical kinetics coefficients selected as providing “best” fit to ignition tests: – Ak = 5.0e16, Ta = 20098, b = 0.5, c = 1, and d = 1 • Turbulence parameters selected: – ε = 0.2; δ = 1e-5 • Kinetics and turbulence parameters not highest values tested (i.e. fastest kinetics and most rapid mixing) • Cases with higher values not always result in higher pressures since high values also leads to combustion in non-ideal mixtures • Increasing turbulence scale improves mixing and suppresses natural fluid oscillations in turbulent jet (scale factor not allowed to exceed 2x recommended value of 0.2) American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Slide 17 Maui, Hawaii September 27 - 29, 2010
  • 18. Slide 18 Low Flow Results: Filtering Effect Slide 18 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton • Time history plot of local gas pressure for typical case @ 4m elevation above nozzle • LHS figure has each point representing average of 4 time steps (slight filtering) • Without filtering (RHS figure), isolated pressure peaks for single time steps (< 0.1 ms) predicted considered not representative of experimental measurements (too fast for test equipment to accurately monitor) • Filtering used to insure pressure histories representative of large regions and times more consistent with pressure histories inferred from flame velocity measurements • Pressure change (max – min) reaches approximately 50 mB (or more) – same as reported in flare tests • Time between max and min pressure is on order of 16 ms (~60 Hz sound frequency) Case 18 Case 37 42 mB (0.62 psig) -10 mB (0.15 psig) 0.77 psig Maui, Hawaii September 27 - 29, 2010
  • 19. Slide 19 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Low Flow Results: Ignition Delay Effect • Pressure histories from two delayed ignition cases (Case 3 and Case 41) • Highest Pressure observed on outer edge of growing fire ball • Minimum pressure observed at center of growing fire ball after high pressure propagates outward Case 41Case 3 425 mB (6.25 psig) 125 mB (1.84 psig) -130 mB (1.91 psig) -60 mB (0.88 psig) 2.72 psig 8.16 psig Maui, Hawaii September 27 - 29, 2010
  • 20. Slide 20 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Low Flow Results: Pressure Spikes from Ignition Delay • Highest Pressure on outer edge of growing fire ball • Minimum pressure at fire ball center after high pressure region propagates outward Maui, Hawaii September 27 - 29, 2010
  • 21. Slide 21 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Low Flow Results: Horizontal temperature contour through flame ball at 4m Beginning of Ignition Mid-Point of Ignition Near End of Ignition • Spatial distance between tick marks on plots is 1m; temporal distance between plots is 30 ms • Dividing flame propagation distance by time between frames yields flame velocity of 33 m/s • Experimental flame propagation velocity ~50 m/s (examining video data indicated they failed to subtract initial ball diameter). Correcting test results yields actual growth rate of 40 – 44 m/s Maui, Hawaii September 27 - 29, 2010
  • 22. Slide 22 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Low Flow Results: Flame ball growth for normal and delayed ignition Normal Ignition Delayed Ignition Maui, Hawaii September 27 - 29, 2010
  • 23. Slide 23 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Low Flow Results: Normal Ignition Video Maui, Hawaii September 27 - 29, 2010
  • 24. Slide 24 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Low Flow Results: Delayed Ignition Video Maui, Hawaii September 27 - 29, 2010
  • 25. Slide 25 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton High Flow Results: Ignition Delay Effect • Pressure history from non-delayed (RHS) and delayed (LHS) ignition pressure wave (4m above flare tip) • Higher flow wo/ ignition delay caused slightly higher pressure wave (3.53 vs 0.77 psig) • With Ignition delay, pressure builds until calculation becomes unstable (detonation) High Rate w/ Ignition DelayHigh Rate wo/ Ignition Delay 160 mB (2.35 psig) 1700 mB (24.98 psig) -80 mB (1.18 psig) 3.53 psig Maui, Hawaii September 27 - 29, 2010
  • 26. Slide 26 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton High Flow Results: Normal Ignition Video Maui, Hawaii September 27 - 29, 2010
  • 27. Slide 27 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton High Flow Results: Delayed Ignition Video (only two frames at sampling rate) Maui, Hawaii September 27 - 29, 2010
  • 28. Conclusions • Natural Gas Flare Gas Fired through multi-burner tip: – C3d flare model based on LES mixing model – Combustion model used EBU type reactions (includes soot) – 2-zone computational mesh (adjusted to optimize grid) – Final mesh size ~1.2MM cells • Simulated low flow (200-350 TPH) and high flow (1350 TPH) conditions • Compared results to test results – Pressure wave estimated by AIChE correlation + flame speed estimated from high speed video (pressure measurements via microphone – not sensitive enough) – Predictions compared well to data for flame speed and pressure wave – from 12 tests (2 tip sizes, 3 operating pressures, 2 radiation sample locations) • Estimated Pressure wave – Low flow, no ignition delay < 0.75 psig, flame speed ~33 m/s (measured 40 m/s) – Low flow, ignition delay ~ 8 psig possible! – High flow, no ignition delay > 3.5 psig – High flow, ignition delay resulted in explosion! Slide 28 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Maui, Hawaii September 27 - 29, 2010
  • 29. Backup Slides American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Slide 29 Maui, Hawaii September 27 - 29, 2010
  • 30. Radiation Inside Large Fires • High soot volume fractions make large fires non-transparent (optically thick) which causes flame to radiate as a cloud (radiatively diffuse) • Fire volume defined where soot volume fraction (fi) greater than minimum volume fraction (fsoot > fmin) • Flame edge (fflameedge) where soot volume fraction = 0.05 ppm Calculated flame surfaces from 3 time steps from validation against test Slide 30 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Maui, Hawaii September 27 - 29, 2010
  • 31. • When fsoot < fflameedge then outside “flame” (participating medium considered) • View factors from fire to surrounding surfaces calculated at each time step (includes attenuation by gas and soot media for flames) • Re-radiation from surroundings also calculated at each time • Fire considered black body radiator (εfiresurface = 1) • Radiation from flame to surroundings assumes Tsurround = constant Radiation Outside of Large Fires Slide 31 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Maui, Hawaii September 27 - 29, 2010
  • 32. Diffuse Radiation Within Fire • Calculated indirectly using a Rossland effective thermal conductivity – σ = Stefan-Boltzman Constant – T = local temperature – βR= local extinction coefficient. Dependent on local species concentrations • Radiation transport model: – Predicts radiant flux on external (and internal) surfaces – Provides source/sinks terms to overall energy equation Air R R k T k >>= β σ 3 16 3 Slide 32 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Maui, Hawaii September 27 - 29, 2010
  • 33. Reactions Involving Fuel • Incomplete Fuel Combustion (soot producing) 1 kg F + (2.87-2.6S1) kg O2 → S1 kg C + (3.87-3.6S1) kg PC + (50-32S1) MJ – Combustion Soot Mass Parameter, S1 = 0.005 • Endothermic Fuel Pyrolysis (soot producing) 1 kg F + 0.3 MJ → S2 kg C + (1-S2) kg IS – Cracking Parameter, S2 = 0.15 American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Slide 33 Maui, Hawaii September 27 - 29, 2010
  • 34. Reactions Not Involving Fuel • Soot Combustion 1 kg C + 2.6 kg O2 → 3.6 kg CO2 + 32 MJ • Combustion of Intermediate Species – Coefficients chosen so that complete combustion of C and IS produce same species and thermal energy as direct combustion of fuel American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in Combustion Technology: Improving the Environment and Energy Efficiency Sheraton Slide 34 Maui, Hawaii September 27 - 29, 2010