Flare Performance And Analysis Smoot Smith Jackson

TECHNICAL FOUNDATIONS TO ESTABLISH NEW
CRITERIA FOR EFFICIENT OPERATION OF INDUSTRIAL
STEAM-ASSISTED GAS FLARES

L. Douglas Smoot, Ph.D., Combustion Resources, Inc.
Joseph D. Smith, Ph.D., Idaho National Laboratory
Robert E. Jackson, Combustion Resources

IFRF 16th International Members’ Conference
Combustion and Sustainability: New Technologies, New Fuels, New Challenges

8-10 June, 2009 – Boston Massachusetts, USA

Discussion Outline
• Basic Flare Operation and Control
– Operating/design issues for Pipe Flares and Ground Flares
• Historical Performance Analysis
– EPA and CMA Data
– API Guidelines
• Proposed Metrics for Flare Operation
– Adiabatic Temperature vs. LHV
– Combustion Efficiency vs. Steam Ratio
• BAT for Flare Performance Analysis
• Conclusions and Recommendations

IFRF 16th International Members’ Conference:

Flares in Recent News!

Decades of contamination of the water and soil from oil and gas operations mean most food
must now be imported, says JNN, and the practice of "gas flaring" has put a toxic
pall over many villages.

WILLEMSTAD (Reuters) - A refinery on Curacao operated by
Venezuela's state oil company is damaging people's health
and must cut emissions or face multi-million dollar fines, a
court on the Caribbean island ruled on Thursday. Industrial flares burn off pressurized
gases but also can shoot out massive
In 2007, a Curacao court threatened to close Isla if it cannot amounts of noxious emissions. The
meet emissions standards, citing a study estimating that 18 Houston area has about 400 flare
people die prematurely every year from contaminant stacks, and they are among the largest
exposure. and least- understood sources of pollution
in the region, researchers said.
Van Unen also took into account excessive flaring that
sometimes runs for days and weeks. The amounts to be
paid were decided with the serious health consequences and
the earnings of PDVSA in mind, the judge stated.


Operating/Design Issues for Pipe Flares

Wind shortens Flame and
causes soot formation on
back side of flame

Flame bends over in wind
and licks downwind side of
flare stack

Air egression into
stack tip can lead
to internal burning
and possible
explosive
conditions

Cross winds cause air P. Gogolek, CANMET Energy
egression into flare stack Technology Centre – Ottawa Natural
Resources Canada


Operating/Design Issues for Low Profile Flares
High tip velocity increases air
entrainment
o Tip design controls air entrainment
(fuel/air mixing)
o High jet velocities = High Jet Noise
Pressure Assist improves
mixing and combustion
o Smoke below certain tip pressure
(D-stage pressure) = ambient air
entrainment
Tip spacing critical
o Tips must cross light
o Individual tip flames can merge and
lengthen overall flare flame
o Adjacent rows compete for ambient
air (longer flames near flare center)


What is a Flare or Why is there a
flame on top of that stack?
Flares are Safety Devices used to prevent catastrophic events in
systems with highly flammable gases
Flares provide safe discharge point for relief devices or in case of loss
of containment
API RP-521† describes proper flare design and operation but does not
define them as “combustors with routine emissions” (e.g., incinerators
or process heaters)
Plants required to have “Flare minimization plan” as part of air permit –
recovery devices operate with quasi-steady flows but can’t operate over
large flow ranges possible in hydrocarbon refineries or chemical plants
Pollution control devices (i.e., incinerators) combust process emissions
Safe and efficient flare design must handle very low hydrocarbon flows
(due to fuel cost) and high hydrocarbon flows (due to safety constraints)
under variable ambient conditions (i.e., wind and rain) for non-uniform
gas compositions
†API RP-521, 4th ed., American Petroleum Institute, Washington D.C., March (1997).


Flare Efficiency Studies (EER)
Flare Screening Facility and Flare Test Facility used to analyze
flame stability and combustion/destruction efficiency for
different fuels and different flare tips (captured/analyzed flare
plume)
Used 3”, 6”, and 12” Open Pipe Flares plus Air-assisted,
Pressure- assisted and Steam-assisted Flares
Considered Pilot vs non-pilot operation
Considered saturated/unsaturated hydrocarbon, H2S, and NH3
Correlated Flame stability vs Gas heating value and Tip exit
velocity
o Flame stability defined as Tip velocity > Flame velocity
o Assisted flares more stable, Piloted flares more stable,
unsaturated fuels more stable (but require more air)
Pohl, J.H. & N.R. Soelberg, Evaluation of the Efficiency of Industrial Flares: Flare Head
Design and Gas Composition, EPA-600/2-85-106, September (1985)
Pohl, J.H. & N.R. Soelberg, Evaluation of the Efficiency of Industrial Flares: H2S Gas
Mixtures and Pilot Assisted Flares, EPA-600/2-86-080 September (1986)


Testing on Flame Stability and Heating Value
Identified Min HV for Gas where Flame Speed equal Exit Velocity
Characterized flows for different fuels and tip sizes
Pilot and Assist media Improved Flame Stability

Pohl, J.H. & N.R. Soelberg, Evaluation of the Efficiency of Industrial Flares: H2S Gas Mixtures and Pilot Assisted
Flares, EPA-600/2-86-080 September (1986)


Flare Efficiency Studies by CMA/EPA/JZ (1983)Ŧ

Tested Air-Assisted and Steam-Assisted Flares with emphasis on
determination of combustion efficiency and factors that affect it
Utilized commercial scale steam and air-assisted flares while burning
propylene and nitrogen in varying compositions. Factors considered:
o Flow rate of the relief gas,
o Heating value of the relief gas, and
o Steam/relief gas ratio
Showed flares achieve >98% destruction efficiency if properly
operated
Optimal Combustion efficiency with steam to relief gas mass ratio of
0.4 – 1.5 lbm/lbm
Over-steaming appeared to occur for steam/relief gas ratio greater
than 4x recommended steam rates

Ŧ McDaniel, M., Flare Efficiency Study, EPA-600/2-83-052, July (1983)


Factors Important in Flare
Performance

Smokeless operation¥
o Black Smoke = Soot (fv< 10-5 Vol Frac, dprim< 40 nm)
o Smoke burns up in high temperature region (smokes if
soot exits this region before being consumed)
o Smoke forms when C-C bonds in hydrocarbons crack and
aromatic structures grow into multi-ring molecules (>3 ring
= primary soot particle)
o Other Poly-aromatic hydrocarbons (PAH) form along
reaction route to soot
¥ Moss, J.B., Stewart, C.D., and Young, K.J., “Modeling Soot Formation and Burnout
in a High Temperature Laminar Diffusion Flame Burning under Oxygen-Enriched
Conditions,” Combustion and Flame, 101: 491-500 (1995)

Gollahalli and Parthasaronthy, R.P., "Turbulent Smoke Points in a Cross-Wind,"
Research Testing Services Agreement No. RTSA 3-1-98, University of Oklahoma,
Norman, OK, August (1999).


Factors Important in Flare
Performance
High Exit velocity: use
“momentum ratio” to account
for cross wind effect
o Flare gas entrains surrounding air to
enhance mixing/combustion efficiency
o Assist media (steam/air) enhances entrainment
o Purge conditions (low momentum ratio) results in flame
deflection and poor mixing
o Flare gas momentum < 10% wind momentum allows flame
stabilization downwind of tip‡
‡Pohl, J., Gogolek, P., Schwartz, R., and Seebold, J., “The effect of Waste Gas Flow & Composition Steam Assist &
Waste Gas Mass Ratio Wind & Waste Gas Momentum Flux Ratio Wind Turbulence Structure on the Combustion
Efficiency of Flare Flames”

Gogolek, P.E.G., and A.C.S. Hayden, “Efficiency of Flare Flames in Turbulent Crosswind,” Advanced Combustion
Technologies, Natural Resources Canada, American Flare Research Committee, Spring Meeting, May (2002).


Factors Important in Flare Performance
Flare Gas Heat Content > Minimum Heating Value for
Stability
o Lower Heat value provides less energy to local reaction zone
o Combustion at lower flame temperature
o Reaction zone more susceptible to flame shearing (quenching)

Pohl, J.H., R. Payne & J. Lee, Evaluation of the Efficiency of Industrial Flares: Test Results, EPA-600/2-84-095, May (1984)


Factors Important in Flare Performance Slide 13

Flare Gas Heat Content > Minimum Heating Value for
Stability
o Saturated HC’s: stable flame at
increasing exit velocities for higher
gas HV (increased mixing dilutes
reactants before combusted –
requires more fuel to compensate)
o Unsaturated HC’s: stable flame at
increasing exit velocities for constant/
decreasing gas HV (increased mixing
with fast kinetics and reduced O2 demand for unsaturated HC)
o Assist media increases stability (enhanced mixing for flare gas
with pilots)
o Practical operation: increasing C/H ratio (unsaturation) requires
more assist media


Quantifying Flare Performance Using Flammability Ratio
and Steam/Fuel Mass Ratio

General Straight Chain Saturated (Aliphatic) Hydrocarbons
have Heating Values ~20,000 Btu/lbm
Flammability Limit represents “mixture” that has sufficient
oxidant and fuel to react
Other factors affecting “reaction” include:
o Fuel/Oxidant Mixing (turbulent versus laminar)
o Ignition source (create radicals to promote reaction)
o “Stability” (radicals to propagate reaction)

Example Calculation:
o Propylene (fuel), Nitrogen (purge), Natural Gas (pilots and
supplemental fuel), and Steam


Definitions Used in Proposed Performance
Metrics:¥
m = molar, st = stoichiometric, F = Fuel, M = mixture of fuel plus air

o Vent gas = Waste Fuel Gas + Supplemental Fuel Gas + Purge gas
o Flare gas = Vent gas + Pilot fuel gas + Steam

o (FR)m = (F/CZG) / (F/M)st
o CZG = Combustion Zone Gas
= Flare gas plus added Combustion Air to reach
Stoichiometric Oxygen
o (F/M)st = Fuel / (Fuel + Stoichiometric Air) (molar basis)
o (FR)m = Flammability Ratio (molar basis)
~ % Lean Flammability Limit / % Stoichiometry

¥ Proposed Metrics by Mr. Brian Dickins, EPA Engineer, Chicago, Illinois


Proposed Performance Metrics for Steam Flares:
Standard 1 - Steam/Vent Gas Mass Ratio Limit:
• Optimum = manufacturer’s recommendation or constant
• Maximum ≤ 4 times manufacturer’s recommended value
• All flows defined by operating conditions – need Purge Gas
flow
• Purge gas flow rate can be set based on manufacturer value
or estimated by Husa criteria:
Purge Gas (scfh)‡ = 0.0035 D3.46K (1)
where: D = flare stack diameter (inches)
K = Purge gas constant for specific gases
= 2.33 for methane;
= 1.71 for N2 with wind
= 1.07 for N2 without wind

Standard 2 – Minimum Gas Heat Content Limit:
• If Flare Gas, then LHV ≥ 200 BTU/scf (2A)
‡
• If Vent Gas, then LHV ≥ 300 BTU/scf (2B)
Berg, L.D., Smith, J.D., Suo-Anttila, A., Price, R., Modi, J., Smith, S., “Flare Purge Rates: Comparison of CFD and
Husa”, AFRC Symposium, Houston, TX (2006)


Application of Metrics: Required Information

Input Parameters:
• Flare diameter (ft) and tip design
• Waste fuel to be flared, including moisture
• Wind velocity (ft/hr)
• Pilot Gas flow rate (scfh)
• Combustion Air flow into tip zone (lb/hr)

Operational Parameters (to be Specified)
• Purge gas flow rate (lb/hr)
• Supplemental fuel flow rate (lb/hr)
• Steam flow rate to tip for air entrainment/mixing (lb/hr)


Example: Calculated Adiabatic Flame Temperature for
Selected Steam Flow at constant LHV

Vent Steam/Flare
Purge Steam Fuel Vent Gas Flare Gas Gas Mass Tip Exit Adiabatic Reynolds Froude
Flow (N2) Flow Flow Flow LHV LHV ratio Velocity Flame Temp FRm No. No.
3 3
Case # (lbm/hr) (lbm/hr) (lbm/hr) (lbm/hr) (BTU/ft ) (BTU/ft ) (lbm/lbm) (ft/min) (ºF) (-) (-) (-)
1 28 43 14.9 43 552 200 1.00 6.3 2893 0.702 1005 0.016
2 28 208 55.2 83 1199 200 2.50 10.8 2856 0.702 1716 0.028
3 28 5554 1360.6 1389 2050 200 4.00 156.0 2843 0.702 24760 0.397
4 74 113 39.3 113 552 200 1.00 16.7 2893 0.702 2656 0.043
5 74 549 145.8 220 1199 200 2.50 28.6 2856 0.702 4536 0.073
6 74 14679 3595.9 3670 2050 200 4.00 412.3 2843 0.702 65438 1.049
7 148 227 78.6 227 552 200 1.00 33.5 2893 0.702 5311 0.085
8 148 1099 291.5 440 1199 200 2.50 57.1 2856 0.702 9071 0.145
9 148 29359 7191.7 7340 2050 200 4.00 824.5 2843 0.702 130876 2.098

CMA52 2.14 201 0.452 2.592 251 112 2330 0.576
CMA53 1.41 201 0.226 1.636 197 112 2323 0.575

Notes:
• Flare Gas LHV = (Fuel / Combustion Zone Gas) / (Molar Stoichiometric Ratio Based on Air)
• Steam and Fuel Flows adjusted to achieve Desired Steam/Flare Gas Mass Ratio and Adiabatic Flame
Temp calculated for mixture


Correlation of Steam to Hydrocarbon Flow vs
Adiabatic Flame Temp (Tad) at constant LHV
Computed Adiabatic Flame Temperature, F
o

3000
Propylene

2900

2800

2700

2600

2500
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Steam to Vent Gas Ratio


Example: Calculated Adiabatic Flame Temperature for
Selected LHV at different Flare Operating Conditions

Vent Steam/Flare
Purge Steam Fuel Vent Gas Flare Gas Tip Exit Gas Mass Adiabatic Reynolds Froude
Flow (N2) Flow Flow Flow LHV LHV Velocity ratio Flame Temp FRm No. No.
3 3
Case # (lbm/hr) (lbm/hr) (lbm/hr) (lbm/hr) (BTU/ft ) (BTU/ft ) (ft/min) (lbm/lbm) (ºF) (-) (-) (-)
10 74 81 7.3 81 130 50 13.2 1.00 1619 0.353 2091 0.034
11 74 223 15.3 89 256 50 14.1 2.50 1586 0.353 2233 0.036
12 74 397 25.1 99 390 50 15.2 4.00 1574 0.353 2406 0.039
13 74 90 16.0 90 265 100 14.1 1.00 2283 0.528 2244 0.036
14 74 280 38.0 112 539 100 16.6 2.50 2243 0.528 2633 0.042
15 74 594 74.4 148 847 100 20.6 4.00 2229 0.528 3275 0.053
16 74 151 76.6 151 862 300 20.9 1.00 3165 0.789 3314 0.053
17 74 330 145.8 220 1199 300 28.6 1.50 3150 0.789 4536 0.073
18 74 814 332.8 407 1584 300 49.4 2.00 3140 0.789 7838 0.126
19 74 69 63.4 137 768 400 19.4 0.50 3339 0.841 3082 0.049
CMA52 2.14 201 0.452 2.592 251 112 2330 0.576
CMA53 1.41 201 0.226 1.636 197 112 2323 0.575

Notes:
• Vent Gas adjusted to achieve desired (LHV)f level and Adiabatic Flame Temp calculated for mixture
• Flare Gas LHV = (Fuel/Combustion Zone Gas) / (Molar Stoichiometric Ratio Based on Air)
• (LHV)f includes Fuel + Steam + Purge
• CMA data with Low LHV included for comparison purposes


Correlation of Vent Gas LHV versus Adiabatic Flame
Temp (Tad ) for Selected Flare Operating Conditions
5000
Propylene
Adiabatic Flame Temperature (F)

4500
o

4000
3500
3000
2500
2000
1500
1000
500
0 500 1000 1500 2000 2500
Vent Gas Lower Heating Value (BTU/scf)


Correlation of Flare Gas LHV versus Adiabatic Flame
Temp (Tad ) for Select Flare Operating Conditions
5000

"Propylene"
o

4500 Log. ("Propylene")

4000
R2 = 0.9941
3500
3000
2500
2000
1500
1000
500
0 50 100 150 200 250 300 350 400 450
Flare Gas Lower Heating Value (BTU/scf)


Correlation of Flammability Ratio (~%FL/%SR) versus
Adiabatic Flame Temp (Tad ) for Propylene
4000
(LHV)f = 400
3500 (LHV)f = 300
o

(LHV)f = 200
3000 Inflammable
(LHV) f = 100
2500
Flammable
2000 (LHV)f = 50
R2 = 0.9963
1500
.4
Propylene
Linear (Propylene)
1000

500

0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
FRm (%Flammability/%SR)


Deficiencies and Limitations
Need to account for wind effects (i.e., wake stabilized mixing
versus momentum mixing)
o If wind effects added, need good wind data to facilitate
incorporation into metrics
Additional testing useful in understanding previous data that
show low LHV and high combustion efficiency
o CMA data indicates “stable” combustion at low heat
values?
o Calculation basis, definition of LHV, etc.
Metrics do not account for tip geometry effects (i.e., internal
burning, flame stabilizer rings, etc.)
o Impossible in simplified approach to account – CFD useful
for detailed analysis


LES Based CFD Represents “BAT” for
Flare Flame Analysis
CFD captures dynamics of flare flame
o Momentum vs. Buoyancy
o Wind Effect on Flame Structure
o Chemical Reactions
o Radiation to/from flame
Approaches
o Eddy Breakup (EBU) Chemistry in Reynold’s Averaged
Navier-Stokes (RANDS) Formulation (averaged flame
structure)
o Large-Eddy Simulation (LES) with Detailed Chemistry
and Radiation (accurate flame structure)
o LES with EBU Chemistry and Diffuse Radiation


Isis-3D Flare Model:
• Provide reasonably accurate estimates of the total
heat transfer to objects from large fires
• Predict general characteristics of temperature
distribution in surrounding objects (i.e., Fence)
• Accurately assess impact of flare operations in
given ambient conditions (effect of wind speed, fuel
flow, fuel composition, surrounding equipment for
given flare geometry)
• Reasonable CPU time requirements using
“standard” desktop LINUX workstations (i.e., P4
processor, 1 GByte RAM)


ISIS-3D Simulation of Multi-Tip Ground Flare (no wind)

~16 m (52 ft)

Flame height ~11 m (~36’)

12 m (39 ft)
Non-luminous region ~1 m (3’)

Tip Height~3 m (10’)

Smith, J.D., Suo-Ahttila, A., Smith, S., and Modi, J., “Estimation of the Air-Demand, Flame Height, and Radiation Load from Low-Profile Flare using ISIS-
3D,” American – Japanese Flame Research Committees International Symposium, Advances in Combustion Technology: Improving the Environment
and Energy Efficiency, Marriott Waikoloa, Hawaii - Oct. 22 –24 (2007)


ISIS-3D Validation: Radiation Predictions
Compared to Experimental Data

Smith, J.D., Suo-Ahttila, A., Smith, S., and Modi, J., “Estimation of the Air-Demand, Flame Height, and Radiation Load from Low-Profile Flare using
ISIS-3D,” American – Japanese Flame Research Committees International Symposium, Advances in Combustion Technology: Improving the
Environment and Energy Efficiency, Marriott Waikoloa, Hawaii - Oct. 22 –24 (2007)


Wind Effect on Radiative Flux

Smith, J.D., Suo-Ahttila, A., Smith, S., and Modi, J., “Estimation of the Air-Demand, Flame Height, and Radiation Load from Low-Profile Flare using
ISIS-3D,” American – Japanese Flame Research Committees International Symposium, Advances in Combustion Technology: Improving the
Environment and Energy Efficiency, Marriott Waikoloa, Hawaii - Oct. 22 –24 (2007)


Conclusions and Recommendations
• Flare performance effected by cross winds
– ratio of flare gas to wind momentum
– combining oxidant with fuel
– shearing reaction zone
– Cooling reaction zone
• Flare performance effected by steam rate
– steaming adds additional air to combustion zone
– over steaming quenches (cools) reaction zone
– over steaming dilutes reaction zone
• Flare performance effected by flare gas heating value
– flare gas (vent + purge + pilots + steam) must be flammable when mixed
with air to burn
– Flammability ratio (fuel/flare gas) / (fuel/fuel+air)stio captures
“combustibility” of flare gas with steam addition to combustion zone
• LES based CFD represents BAT for Flare Performance analysis
– includes combustion/soot reactions plus detailed radiation
– results compare well to test data


Flare Performance And Analysis Smoot Smith Jackson

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