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.L. Douglas Smoot, Ph.D., Combustion Resources, Inc.

Joseph D. Smith, Ph.D., Idaho National LaboratoryJoseph D. Smith, Ph.D., Idaho National Laboratory

Robert E. Jackson, Combustion ResourcesRobert E. Jackson, Combustion Resources

IFRF 16IFRF 16thth International Members’ ConferenceInternational Members’ Conference

Combustion and Sustainability: New Technologies, New Fuels, New ChallengesCombustion and Sustainability: New Technologies, New Fuels, New Challenges

88--10 June, 2009 10 June, 2009 –– Boston Massachusetts, USABoston Massachusetts, USA

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

Slide 2

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


Slide 3

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.

In 2007, a Curacao court threatened to close Isla if it cannot

meet emissions standards, citing a study estimating that 18

people die prematurely every year from contaminant

exposure.

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.

Industrial flares burn off pressurized

gases but also can shoot out massive

amounts of noxious emissions. The

Houston area has about 400 flare

stacks, and they are among the largest

and least- understood sources of pollution

in the region, researchers said.


Slide 4

Air egression into stack tip can lead to internal burning

and possible explosive conditions

Operating/Design Issues for Pipe Flares

Cross winds cause air egression into flare stack

Flame bends over in wind

and licks downwind side of

flare stack

P. Gogolek, CANMET Energy

Technology Centre – Ottawa Natural

Resources Canada

Wind shortens Flame and

causes soot formation on

back side of flame


Slide 5

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 andlengthen overall flare flame

o Adjacent rows compete for ambient air (longer flames near flare center)


Slide 6

� 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

What is a Flare or Why is there a flame on top of that stack?

†API RP-521, 4th ed., American Petroleum Institute, Washington D.C., March (1997).


Slide 7

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)


Slide 8

Testing on Flame Stability and Heating Value

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)

� 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


Slide 9

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)


Slide 10

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).


Slide 11


� 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).


Slide 12


� 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)


Slide 13Factors Important in Flare Performance

� 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


Slide 14

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


Slide 15

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


Slide 16

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)


Slide 17

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)


Slide 18

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

Purge

Flow (N2)

Steam

Flow

Fuel

Flow

Vent

Flow

Vent

Gas

LHV

Flare Gas

LHV

Steam/Flare

Gas Mass

ratio

Tip Exit

Velocity

Adiabatic

Flame Temp FRm

Reynolds

No.

Froude

No.

Case # (lbm/hr) (lbm/hr) (lbm/hr) (lbm/hr) (BTU/ft3) (BTU/ft3) (lbm/lbm) (ft/min) (ºF) ( - ) ( - ) ( - )

1 28 43 14.9 43 552 200 1.00 6.3 2893 0.702 1005 0.0162 28 208 55.2 83 1199 200 2.50 10.8 2856 0.702 1716 0.0283 28 5554 1360.6 1389 2050 200 4.00 156.0 2843 0.702 24760 0.3974 74 113 39.3 113 552 200 1.00 16.7 2893 0.702 2656 0.0435 74 549 145.8 220 1199 200 2.50 28.6 2856 0.702 4536 0.0736 74 14679 3595.9 3670 2050 200 4.00 412.3 2843 0.702 65438 1.0497 148 227 78.6 227 552 200 1.00 33.5 2893 0.702 5311 0.0858 148 1099 291.5 440 1199 200 2.50 57.1 2856 0.702 9071 0.1459 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 0.576CMA53 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


Slide 19

Correlation of Steam to Hydrocarbon Flow vs Adiabatic Flame Temp (Tad) at constant LHV

2500

2600

2700

2800

2900

3000

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Steam to Vent Gas Ratio

Com

pute

d A

dia

batic F

lam

e T

em

pera

ture

, oF

Propylene


Slide 20

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

Purge

Flow (N2)

Steam

Flow

Fuel

Flow

Vent

Flow

Vent

Gas

LHV

Flare Gas

LHV

Tip Exit

Velocity

Steam/Flare

Gas Mass

ratio

Adiabatic

Flame Temp FRm

Reynolds

No.

Froude

No.

Case # (lbm/hr) (lbm/hr) (lbm/hr) (lbm/hr) (BTU/ft3) (BTU/ft

3) (ft/min) (lbm/lbm) (ºF) ( - ) ( - ) ( - )

10 74 81 7.3 81 130 50 13.2 1.00 1619 0.353 2091 0.03411 74 223 15.3 89 256 50 14.1 2.50 1586 0.353 2233 0.03612 74 397 25.1 99 390 50 15.2 4.00 1574 0.353 2406 0.03913 74 90 16.0 90 265 100 14.1 1.00 2283 0.528 2244 0.03614 74 280 38.0 112 539 100 16.6 2.50 2243 0.528 2633 0.04215 74 594 74.4 148 847 100 20.6 4.00 2229 0.528 3275 0.05316 74 151 76.6 151 862 300 20.9 1.00 3165 0.789 3314 0.05317 74 330 145.8 220 1199 300 28.6 1.50 3150 0.789 4536 0.07318 74 814 332.8 407 1584 300 49.4 2.00 3140 0.789 7838 0.12619 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 0.576CMA53 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


Slide 21

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

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 500 1000 1500 2000 2500

Vent Gas Lower Heating Value (BTU/scf)

Adia

batic F

lam

e T

em

pera

ture

(oF

) Propylene


Slide 22

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

R2 = 0.9941

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 50 100 150 200 250 300 350 400 450

Flare Gas Lower Heating Value (BTU/scf)

Adia

batic F

lam

e T

em

pera

ture

(oF

) "Propylene"

Log. ("Propylene")


Slide 23

Correlation of Flammability Ratio (~%FL/%SR) versus Adiabatic Flame Temp (Tad ) for Propylene

R2 = 0.9963

0

500

1000

1500

2000

2500

3000

3500

4000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

FRm (%Flammability/%SR)

Adia

batic F

lam

e T

em

pera

ture

(oF

)

Propylene

Linear (Propylene)

.4

(LHV)f = 50

(LHV)f = 100

(LHV)f = 200

(LHV)f = 400

(LHV)f = 300

Inflammable

Flammable


Slide 24

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


Slide 25LES 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


Slide 26

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)


Slide 27ISIS-3D Simulation of Multi-Tip Ground Flare (no wind)

Fla

me h

eig

ht ~

11 m

(~36’)

~16 m (52 ft)

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)


Slide 28

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



Slide 29

Wind Effect on Radiative Flux



Slide 30

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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