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ABSTRACT
Flares are important safety devices for pressure relief; at the same time, flares are a significant point source for soot and highly reactive volatile organic compounds (HRVOCs). Currently, simple guidelines for flare operations to maintain high combustion efficiency (CE) remain elusive. This paper fills the gap by investigating the characteristics of the incipient smoke point (ISP), which is widely recognized as the condition for good combustion. This study characterizes the ISP in terms of 100–% combustion inefficiency (CE), percent opacity, absorbance, air assist, steam assist, air equivalence ratio, steam equivalence ratio, exit velocity, vent gas net heating value, and combustion zone net heating value. Flame lengths were calculated for buoyant and momentum-dominated plumes under calm and windy conditions at stable and neutral atmosphere. Opacity was calculated using the Beer–Lambert law based on soot concentration, flame diameter, and mass-specific extinction cross section of soot. The calculated opacity and absorbance were found to be lognormally distributed. Linear relations were established for soot yield versus absorptivity with R2 > 0.99 and power-law relations for opacity versus soot emission rate with R2 ≥ 0.97 for steam-assisted, air-assisted, and nonassisted flares. The characterized steam/air assists, combustion zone/vent gas heating values, exit velocity, steam, and air equivalence ratios for the incipient smoke point will serve as a useful guideline for efficient flare operations.
Implications: A Recent EPA rule requires an evaluation of visible emissions in terms of opacity in compliance with the standards. In this paper, visible emissions such as soot particles are characterized in terms of opacity at ISP. Since ISP is widely recognized as most efficient flare operation for high combustion efficiency (CE)/destruction efficiency (DE) with initial soot particles formed in the flame, this characterization provides a useful guideline for flare operators in the refinery, oil and gas, and chemical industries to sustain smokeless and high combustion efficiency flaring in compliance with recent EPA regulations, in addition to protecting the environment.
Nomenclature
| A | = | Air assist (lb/MMBTU) |
| Abs | = | Absorbance |
| BTU | = | British thermal unit |
| BC | = | Black carbon or soot yield (lb/MMBTU) |
| c | = | Soot concentration (lb/ft3) |
| C | = | flow rate of combustibles, (lb/hr) |
| I0 | = | Incident intensity (cd) |
| I | = | Transmitted intensity (cd) |
| k | = | Imaginary part (eqs 19–21 in supplemental information) |
| m | = | Refractive index function (eq 19 in supplemental information) |
| n | = | Real part (eqs 19–21 in supplemental information) |
| M | = | Mass flow rate of soot (lb/hr) |
| MMBTU | = | Millions of BTU |
| NHV | = | Net heating value (BTU/scf) |
| q | = | Gross heat release (cal/s) |
| Q' | = | Volumetric flow rate (ft3/hr) |
| S | = | Steam assist |
| scf | = | Standard cubic feet (at 68 °F and 1 atm) |
| T | = | Temperature (Kelvin) |
| T | = | Transmittance |
| u | = | Average cross-wind speed (ft/sec) |
| V | = | Flare tip exit velocity of vent gas includes center steam as per 40 CFR 60.18 (f) (4) (ft/s) |
| χc | = | Fractional completeness of combustion |
| Metric Units | = | |
| d | = | Flare tip diameter (m) |
| DE | = | Destruction and removal efficiency (%) |
| Dp | = | Primary particle diameter (m) |
| L | = | Optical Path Length (m) |
| L1 | = | Height of bottom portion of flame for steam-assisted flares (m) |
| Lf | = | Flame Length (m) |
| mw | = | Molecular weight (g/mol) |
| q' | = | Efficiency (eqs 23–26 in Supplemental Information) |
| Qn | = | Net heat output, MW |
| Q | = | Gross heat release (watt) |
| r | = | Radius, m |
| TNG | = | Tulsa natural gas |
| u | = | Average cross-wind speed (ft/s) |
| Vf | = | Flame volume (m3) |
| Vsample | = | Volume of sample (m3) |
| Greek Symbols | = | |
| α | = | Mass specific coefficient (m2/g) |
| δ Constant | = | |
| ε | = | Extinction coefficient (m−1) |
| λ | = | Wavelength (m) |
| η | = | Constant |
| ρ | = | Density (g/cm3) |
| σ | = | Cross sections |
| τ | = | Residence time (sec) |
| ω | = | Coefficient |
| Subscripts | = | |
| A | = | Air-assist |
| a | = | Ambient |
| abs | = | Absorption |
| cz | = | Combustion zone |
| ext | = | Extinction |
| f | = | Flame |
| isp | = | Incipient smoke point |
| p | = | Products |
| Plume | = | Plume |
| R | = | Reactants |
| rxn | = | Reaction |
| s | = | Stack |
| si | = | Sensible |
| S | = | Steam-assist |
| sca | = | Scattering |
| AR | = | Actual air |
| vg | = | Vent gas |
Acknowledgment
Special thanks are due to Ed Fortner and Scott Herndon of Aerodyne Research, Inc. (ARI), for providing numeric soot data for 2010 John Zink flare campaign. The authors also acknowledge the data contributed by Dr. Darcy Corbin and Dr. Matthew Johnson from their flare study at Carleton University in 2014. Valuable comments from Dr. Peyton Richmond and Dr. Helen Lou during the course of this study are also acknowledged.
Additional information
Funding
Notes on contributors
Daniel H. Chen
Daniel H. Chen, Ph.D., P.E., is University Professor & Scholar, Leland Best Distinguished Faculty Fellow at Lamar University, Beaumont, TX.
Arokiaraj Alphones
Arokiaraj Alphones is a Ph.D. candidate in the Dan F. Smith Department of Chemical Engineering at Lamar University, Beaumont, TX.