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

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. • 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. • 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

4. 4

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. • 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. ⎥
⎦
⎤
⎢
⎣
⎡
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
++
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. 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. 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. 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. 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. )]/()/[(
)]/()/()/[(
)/()(
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. 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. 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. • 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

16. 16

17. 17
AdiabaticFlameTemperature(F)
Mixture Flammability Ratio FRm

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. • 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. 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

27. 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

28. 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)

29. What about Flare Flame Shape as effected by wind and Combustion Efficiency?