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Criteria For Steam Flare Combustion Efficiency

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Analysis of flare combustion efficiency based on mass and energy balances confirm two operating criteria for steam flare operation

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Criteria For Steam Flare Combustion Efficiency

  1. 1. L. Douglas Smoot & Robert E. Jackson Combustion Resources, Inc Provo, UT Joseph D. Smith Systems Analysis and Solutions, LLC Owasso, OK IFRC International Pacific Rim Combustion Symposium 26-29 September 2010 Maui, Hawaii 1
  2. 2. • Environmental Protection Agency – Mr. Brian Dickens - Technical Discussions – Financial Support • Eastern Research Group, Inc. – Mr. Paul Buellesbach • Technical Review – Dr. Ahti Suo-Att ha – Mr. Larry Berg • Mr. Scott Smith, Zeeco – Flare Photograph 2
  3. 3. • Identify & Quantify Generalized Flare Performance Parameters for Allowing High Flare Combustion Efficiency – Open, single-stage, steam-assisted flares – How do Vent LHV and Flare LHV affect flare performance? – How much steam can be effectively added to reduce smoke? – For various fuels, purge gases • Apply Mass and Energy Balances • Approach Applicable to other Flare Systems 3
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  5. 5. • Vent Gas = • Flare Gas = vent gas + pilot fuel/air + steam • Adiabatic Temperature = maximum temperature of a flare gas – air mixture • Flammability Ratio = volume fraction of fuel in the flare gas – air mixture (includes steam, purge gas) • Lower Heating Value = heat of combustion of a stoichiometric, fuel–air mixture with water vapor product 5 Waste Fuel Gas + Supplemental Fuel Gas + Purge Gas
  6. 6. • Structural – Diameter – Length • Operational – Flow Rates: waste fuel purge gas supplemental fuel steam pilot fuel/air combustion air • External – Wind velocity, ambient air conditions 6
  7. 7. ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ++ Δ = S S P P F F FCF f MW W MW W MW W Vol HW LHV )( ))(( )( ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ++ Δ = S FS P FP F FC MW WW MW WW MW Vol H )/()/(1 )( )( (LHV)f independent of mass flow rates, flare diameter and height (ΔHC)F near-constant for typical hydrocarbons (weight basis) For small purge flow, value of steam/fuel to maintain (LHV)f ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ =⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − Δ =⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ≈⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + VG S MW MW VolLHV MWH W W WW W F S F SCF F S PF S )( Increasing purge flow adds to supplemental fuel requirement 7
  8. 8. Kanary, Glassman Fuel Mol. Weight LHV kcal/g Stoich vol. % Lean Flam Limit, % stoich. (avg = 54) Benzene 78.1 9.56 0.0277 48 1, 3-Butadiene 54.1 10.87 0.0366 54 n-Butane 58.1 10.92 0.0312 58 2-Butene 56.1 10.82 0.0377 53 Cyclohexane 84.2 10.47 0.0227 57 Cyclopentane 70.1 10.56 0.0271 55 n-Decane 142.3 10.56 0.0133 56 Ethane 30.1 11.34 0.0564 53 n-Heptane 100.2 10.62 0.0187 56 n-Hexane 86.2 10.69 0.0216 56 Kerosene 154.0 10.30 --- 53 Methane 16.0 11.95 0.0947 58 n-Nonane 128.3 10.67 0.0147 58 n-Octane 114.2 10.70 0.0165 58 n-Pentane 72.1 10.82 0.0255 55 l-Pentene 70.1 10.75 0.0271 42 Propane 44.1 11.07 0.0402 51 Propene 42.1 10.94 0.0444 54 Toluene 92.1 9.78 0.0227 53 xylene 106.0 10.30 --- 56 Gasoline 73 octane 120.0 10.54 --- --- 8
  9. 9. Fuels Methane Propane Butadiene n-octane Purge Gas Mass Flow Rate 3 levels Steam/Vent Gas 1, 2.5, 4 Flare Gas LHV 50 – 400 9 (Tad, LHVflare, LHVvent, FR)
  10. 10. 0 500 1000 1500 2000 2500 3000 3500 4000 0 50 100 150 200 250 300 350 400 450 500 Flare Gas Lower Heating Value (BTU/scf) AddiabaticFlameTemperature(F) Methane Propene Butadiene n-Octane Log (all data) R 2 = 0.9816 Flare Gas Lower Heating Value (BTU/scf) AdiabaticFlameTemperature(F) 10 Purge Gas (1, 2, 3X), Steam/Vent Gas (1, 2, 3)
  11. 11. 0 500 1000 1500 2000 2500 3000 3500 4000 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Vent Gas Lower Heating Value (BTU/scf) AddiabaticFlameTemperature(F) Methane Propene Butadiene n-Octane Vent Gas Lower Heating Value (BTU/scf) AdiabaticFlameTemperature(F) 11
  12. 12. )]/()/[( )]/()/()/[( )/()( PPFF SSPPFF Fv MWWMWW MWWMWWMWW LHVLHV + ++ = • Required Vent (LHVv) Heating Values always greater than flare LHVF • With no steam, values equal • Flare temperature does not correlate with vent LHV 12
  13. 13. 0 500 1000 1500 2000 2500 3000 3500 4000 4500 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Steam/Vent Gas Ratio FuelFlow(lbm/hr) Methane Propene Butadiene n-Octane Steam/Vent Gas Ratio RequiredFuelFlow(lbm/hr) 13
  14. 14. 0 2 4 6 8 10 12 14 16 18 20 0 50 100 150 200 250 300 350 400 450 Flare Gas Lower Heating Value (BTU/scf) LimitingSteam/VentGasRatio Methane Propene Butadiene n-Octane Flare Gas Lower Heating Value (BTU/scf) MaximumSteam/VentGasRatio 14 (Without Purge Gas)
  15. 15. • Fuel – Air Only Fuel/(Fuel + Air) = Stoichiometric Ratio • Flare Gas – Air (Combustion Zone Gas) Fuel/(Flare Gas + Air) = Flammability Ratio (FR)m • FRm includes inert diluents Purge Gas Carbon Dioxide (from pilot fuel/air) Steam 15
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  17. 17. 17 AdiabaticFlameTemperature(F) Mixture Flammability Ratio FRm
  18. 18. For Steam-assisted Flares: • Flare Operating criteria established via application of mass and energy balances and hydrocarbon flammability limits – Recommended standards not dependant on stack diameter or capacity – Flare gas lower heating value (LHV)f appropriate energy standard for setting flare gas energy level (correlates with adiabatic flame temperature, Tad) – Vent gas heating value (LHVg) not appropriate energy standard (low correlation to Tad) – Minimum LHVf (ca. 200 Btu/ft3) required to maintain efficient combustion above lean flammability limit – LHVf values above ~300 Btu/ft3 restrict steam use, S/V mass ratio < 2 – Appears to be a “Maximum Steam/Vent mass ratio” based on LHVf – Fuel requirements to maintain flare gas LHV reach extreme levels for steam rates above S/V ~ 3/1 – With negligible purge rate, maximum steam rate can be calculated directly to maintain LHVf ≥ 200 Btu/ft3 • Recommended Standards for Steam-assisted flares – 200 ≤ LHV(flare gas) ≤ 300 Btu/ft3 – (S/V)max ≤ 3 18
  19. 19. • Currently, these standards are only applicable to steam- assisted flares • Only apply to hydrocarbon fuels (not hydrogen, acetone, ammonia, alcohols) • These do not guarantee high combustion/destruction efficiency • Applicable to “similar” waste and supplemental fuels • Considers “inert” purge gas • Correlation of flare combustion efficiency data required to to establish flare standards • Fuel tendencies of soot formation not considered Generalized Application of this method can reduce or eliminate limitations! 20
  20. 20. Recent Flare Test Data • Work by EPA with John Zink and Marathon illustrate applicability of these methods • Next few slides were taken from a recent report issued by Marathon Oil Company
  21. 21. Draft - Enforcement Confidential Comparison of Recent data t Pohl’s data for 97-98% Combustion Efficiency Points E D A11 B A19 MPC TxCity 9/09
  22. 22. Combining Recent Marathon Data with earlier Pohl Data, is there an equation which relates “Exit velocity” to “Flare Gas Heating value” for Combustion Efficiency > 98%? Pohl & MPC Data y = 326.2x0.1635 0 100 200 300 400 500 600 700 800 0.1 1 10 100 Exit Velocity (ft/s) HeatingValue(BTU/scf) Series1 Pow er (Series1)
  23. 23. What about Flare Flame Shape as effected by wind and Combustion Efficiency?

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