FLAME STABILITY LIMITS AND HYDROCARBON DESTRUCTION EFFICIENCIES OF FLARESBURNING WASTE STREAMS CONTAINING HYDROGEN AND INERT GASES

Peter M. Walsh*, David K. Moyeda, W. Steven Lanier, C. Michael Booth, E. E. (Gary) Folk, and John MaxwellGE - Energy and Environmental Research Corporation

Morrisville, NC and Irvine, CA

Wilfred K. WhitcraftE. I. du Pont de Nemours & Company

Wilmington, DE

Roger K. Noble†, Jeff M. Clopton‡, Tracy Rogers§, and Chris Nicely‡Callidus Technologies Inc.

Tulsa, OK

Robert E. Rosensteel and C. Andrew MillerU.S. Environmental Protection Agency

Research Triangle Park, NC-------

American Flame Research Committee, 2002 Fall MeetingCombustion Emission Improvements: Technologies, Requirements, and Trends

Hosted by ChevronTexaco, Bellaire, TX, November 5-6, 2002_______*Present address: Univ. of Alabama at Birmingham and Southern Research Institute, Birmingham, AL 35294.†Present address: Continuum Engineering, LLC, Tulsa, OK 74136.‡Present address: John Zink Company, LLC, Tulsa, OK 74116.§Present address: 222 South First, Morris, OK 74445.

Motivation for the Work

Regulation for flares requiredcombinations of heating valueand velocity in the region atthe upper left.

Flares of waste gasescontaining ~15 vol%hydrogen, having heatingvalues of only 50 -100 Btu/scf,were known to be stable andexpected to have highdestruction efficiencies atvelocities of ~100 ft/s.

Supplemental fuel required tomeet regulation.

EER/EPAFlare Test Facility

J. H. Pohl, R. Payne, and J. Lee,“Evaluation of the Efficiency ofIndustrial Flares: Test Results,”U.S. EPA 600/2-84-095, NTISPB84-199371, May 1984.

Criterion for High Combustion Efficiency

J. H. Pohl, R. Payne, and J. Lee, “Evaluation of the Efficiency of Industrial Flares:Test Results,” U.S. EPA 600/2-84-095, NTIS PB84-199371, May 1984.

Flares burning propane/nitrogen mixtures with no pilot flame

Measurements

Stability limits:

Establish stable flame at a givenvelocity, then reduce H2 content ofH2-N2 mixture to observe lift off andblow out. With and without pilot.

Destruction efficiency:

Add known concentration ofhydrocarbon to flare gas andmeasure its concentration in thecombustion products.

(The blue color is due to emissionfrom electronically excited CO2: CO + O + M Æ CO2* + M)

Flare Characteristics

Tip i.d. 2.9 in. Re = 105

Stable flames were firmlyanchored at the pipe tip.

Lift off: permanent separation ofany part of the flame from the tip.(No stable flames completelyseparated from the tip wereobserved.)

Blow out in absence of pilot:flame extinguished.

Stabilizing tabs had no significanteffect on the H2 contents at lift offand blow out.

Stability in the Absence of Pilot

Even without pilot, flares arecompletely stable (no lift off)down to:

7.6 vol% H2 at 20 ft/s 15.4 vol% H2 at 120 ft/s

Blow out occurs at H2 contentsapproximately 0.8 vol% lower.

Interaction between Pilot and Flare

Lift off: (same as before)permanent separation of any partof the flame from the tip.

Blow out: no propagation offlame from the pilot into oraround the jet.

¨ Completely stable

Lifted ↑

Effect of Pilot Flame on Stability

Three pilot burners were tested:

1. Burner from propane handtorch. Heat output 120 - 240 W;0.05 - 0.1% of flare heat input.

2. Natural gas-fired venturi burner.Heat output 700 - 2700 W;0.3 to 1.0% of flare.

3. Industrial pilot with reducednatural gas orifice. Heat output3000 - 12000 W; 1 - 10% offlare.

Heat input to DuPont pilots is 0.05to 0.6% of the flare heat input.

Stability in the Presence of Pilot

Blow out occurs at H2 contentapproximately 2 vol% lowerthan that at lift off.

Influence of pilot heat inputgreatest at high velocity.

H2 content at blow outextrapolated to zero velocity(4.3 vol%) in good agreementwith lean flammability limit forH2-N2-O2 mixtures (4.8 vol%).

No Effect of Pilot on Lift Off

Conditions at lift offin the presence andabsence of the pilotare indistinguishable.

Pilot for Destruction Efficiency Tests

Venturi burner chosen as thepilot for the destructionefficiency tests.

2680 Btu/hour; 0.27 to 0.48%of flare heat input.

Destruction efficiency alsomeasured without pilot.

Difficulty of Destruction

Use Union Carbide correlation (Leeet al., 1979) to compare difficulty ofdestruction of typical compounds tobe destroyed and candidates forthe measurements.

Correlation gives the temperaturerequired to achieve 99%destruction in a specified residencetime.

Parameters: Numbers of C, N, Oatoms, C=C groups, vinyl chloridegroups, whether aromatic or not,H/C ratio, and autoignitiontemperature.

Benzene 1384oFAcetonitrile 1383Ethylene 1383Hydrogen cyanide 1366Acrylonitrile 1353Propylene 1340Propane 13191,3-butadiene 1314iso-butane 1302n-butane 1247Formaldehyde 1246Carbon monoxide 1244Acetylene 1116

Determination of Destruction Efficiency

Hood used to capturecombustion products.

0.19 - 1.66 vol%ethylene for destructionmeasurements.

10 vol% CO2 added astracer to determinedilution by excess air.

Pilot heat input 0 - 0.5%of flare.

Tip velocity 96 - 120 ft/s.

13 - 19 vol% H2.

Measurements

Ambient ConditionsRelative humidityTemperatureBarometric pressureWind speedCarbon dioxide in airFlare and PilotNitrogen flowHydrogen flowCarbon dioxide in flare gasEthylene in flare gasPilot flame natural gas flowFlare gas temperatureProductsCarbon dioxide in stackEthylene in stackStack temperatureStack flow

Measurement of Stack Flow Rate

Traverse stack usingPitot/static probe.

Velocity distribution is well-behaved and symmetric.

Integrate profiles todetermine flow rate.

Monitor Pitot/static DP onstack centerline during allruns.

Assume ratio of average tocenterline velocity is thesame for small deviationsfrom the calibrationcondition.

Effect of H2 on Response of Flame Ionization Detector

Hydrocarbon analyzer withflame ionization detectorused to monitor ethylenecontent of flare gas.

There is a well known effectof hydrogen on the responseof FID.

Effect is small over the rangeof conditions investigated.

Neglect of lower response isconservative (ethylene feedis in the denominator of thedestruction efficiency).

Destruction Efficiencies

Meaurements repeated 4 to 7times.

Error bars indicate minimumdestruction efficiency at 95%confidence level.

Destruction efficiencies areabove 98% at a level of 95%confidence in all cases.

Destruction efficiencyincreases with hydrogen andethylene content.

Correlation of Destruction Efficiencies

Behavior of hydrogen flaresdominated by hydrogen contentrather than heating value.

Adapt stability ratio of Pohl andcoworkers to hydrogen flaresusing H2 content.

Choose reference condition asH2 content at lift off, because itis well-defined and independentof pilot.

Destruction efficiencies are wellabove 98% at stability ratios of1 and higher.

Conclusion

Hydrogen contents and velocities at lift off of the flame from the 2.9-inch diameterflare were recommended as criteria guaranteeing greater than 98% destructionefficiency in hydrogen flares.

Recommended that any flare having a combination of hydrogen content and tipvelocity making it more stable than at lift off be considered a hydrogen flare,irrespective of the concentrations of other fuel gases.

General Provisions for New Source Performance Standards (40 CFR 60.18) andGeneral Provisions for National Emission Standards for Hazardous Air Pollutants forSource Categories (40 CFR 63.11) amended to include separate specification forhydrogen-fueled flares, effective June 23, 1998 (Federal Register, May 4, 1998).

Acknowledgments

This work was supported by E. I. Du Pont de Nemours & Company.

Thanks also to:

Van Brawley, Ellen Lane, Mary Nuxoll, Robert Perry, and Walter Schrimper ofDuPont

Terry Harrison and Gene Riley of U.S. EPA

Brian Duck, Mike Keller, Roger Poe, Rick James, Chad Ogden, Jim Stockton, andScott Guthrie of Callidus Technologies

Frank Stevens, Bob Elliott, Peter Maly, Roy Payne, Jerry Johnson,John Pohl (now at the University of Queensland, Australia), and

Jerald Cole (now an independent consultant on hydrogen power) of GE - EER