2010 Multi Tip Flare Ignition Presentation
1 like887 views
The document presents a study on the prediction and measurement of flare ignition using a computational model developed by Idaho National Laboratory and its collaborators. Various tests were conducted to analyze ignition phenomena, including pressure wave measurements and flame velocity assessments, which indicated critical factors affecting ignition delays and combustion characteristics. The findings underscored the model's accuracy in simulating different operational scenarios and its potential for improving safety and efficiency in combustion technology.
2010 Multi Tip Flare Ignition Presentation
- 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