CFD Study of Flare Operating Parameters

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Smith Chemical Engineering Department Xianchang Li Mechanical

1 CFD Study of Flare Operating Parameters By Daniel Chen, Helen Lou, & Peyton C. Richmond Dan. F. Smith Chemical Engineering Department Xianchang Li Mechanical Engineering Department Lamar University, Beaumont, Texas AIChE Annual Meeting San Francisco, CA November 5, 2013

2 Introduction

Industrial Flares & Air Quality Flaring is one of the potential under-reported or unreported sources identified by TCEQ Flare emissions account for 60% of the HRVOCs (2007 HRVOC special inventory).

3 Industrial Flares & Air Quality Flaring is one of the potential under-reported or unreported sources identified by TCEQ Flare emissions account for 60% of the HRVOCs (2007 HRVOC special inventory). Flare emissions depend heavily on Destruction & Removal Efficiency (DRE or DE) & Combustion Efficiency (CE) Recent studies/gas imaging IR camera suggest that flares may not be as efficient as claimed (Environ, 2008;) DE may drop below 98% even when the flare operation is in compliance with 40 CFR (Allen, TCEQ Flare Study, 2010) Operators have the tendency to suppress smoke at the expense of DRE/CE. In addition to unburned hydrocarbons, there is the issue of incomplete combustion products VOCs, CO, Soot, HO x, and NO x As a result, EPA and TCEQ are evaluating flare operations (EPA/OAQPS, 2012;

Complexity in Flare Emissions Process Type (Vent Gas Species/Heating Value/Flammability) Refinery, Olefin, Polymer, Landfill, and Exploration fields (H 2 -C 4 or higher; Alkanes vs.

4 Complexity in Flare Emissions Process Type (Vent Gas Species/Heating Value/Flammability) Refinery, Olefin, Polymer, Landfill, and Exploration fields (H 2 -C 4 or higher; Alkanes vs. Alkenes) Operation Mode (Exit velocity or mass flow rate) Startup, Shutdown, Upset, Maintenance, and Standby (Turndown Ratio up to 15000:1) Flare Design/Control Air assisted, Steam assisted (center/upper steam), Non-assisted, Pressure-assisted Elevated, Enclosed Steaming, Aeration Tip Diameter Meteorological condition Cross wind What species are emitted? DRE/CE? ethylene

5 Scope Validation of CFD Model tools with Berkeley flame McKenna flame TCEQ s 2010 controlled flare tests Simulations with various operating Parameters to study the trend and interaction among jet velocity (V), crosswind (U), and combustion zone heating value (CZHV)

6 Methodology

7 New 50-Species Mechanism with NO 2 (LU 1.1) for FLUENT The full 93 species GRI USC mechanism was reduced based on mole fractions, rate constants, & their effect on other major species for use in the EDC model. In 2010 John Zink flare tests, NO 2 was a monitored species. CN was replaced with NO 2 in LU 1.1. Runs more efficiently (in-house UDF available) 7

8 Laminar Flame Speed (cm/sec) Laminar Flame Speed vs. Equivalence Ratio LU 1.0 Mechanism LU 1.1 Mechanism Experimental Equivalence Ratio Adiabatic Flame Temperature (K) Adiabatic Flame Temperature vs. Equivalence Ratio LU 1.0 Mechanism LU 1.1 Mechanism Experimental Equivalence Ratio Ignition Delay Test Ignition Time *10-6 (sec) LU 1.0 Mechanism LU 1.1 Mechanism Experimental /T (K) 8

9 Fluent Model Selection Realizable k-ε turbulence model Turbulence intensity = 15% Turbulence viscosity ratio = 10 Eddy dissipation concept (EDC) turbulencechemistry interaction model. Reduced 50-species mechanism LU 1.1 derived from a full 93-species GRI USC Mechanism. 9

10 Validation with Lab Flames Data 10

67%) NO x chemistry present(includes premixed N 2 from air) Lifted

11 CH 4 /Air Flame (UC Berkley & Sandia Lab) Experimental study (Raman-Rayleigh-LIF measurements) CH 4 (33.33%) / Air (66.67%) NO x chemistry present(includes premixed N 2 from air) Lifted methane-air jet flames in a vitiated co-flow, Combustion and Flame, Volume 143, Issue 5, December

12 2500 Temperature (K) vs. x/d 2.5E-01 CH 4 Mass Fraction vs. x/d E-01 Experimental Temperature (K) Experimental Simulated x/d Mass Fraction (CH4) 1.5E E E E E-02 Simulated x/d 1.0E-01 CO 2 Mass Fraction vs. x/d 5.0E-02 CO Mass Fraction vs. x/d Mass Fraction (CO2) 9.0E E E E E E E-02 Experimental Simulated Mass Fraction (CO) 4.0E E E-02 Experimental SImulated 2.0E E E E x/d -2.8E x/d 12

Ar NO x data absent Experimental Study of Premixed Stoichiometric

13 Ethylene/Oxygen/Argon Flame National Synchrotron Radiation Laboratory Experimental study (MBMS, Molecular-Beam Mass Spectrometry) C 2 H 4 /O 2 /30% Ar NO x data absent Experimental Study of Premixed Stoichiometric Ethylene/Oxygen/Argon Flame, Chinese Journal of Chemical Physics, Volume 19, Number 5, October

14 2500 Temperature vs. Height above burner (mm) 0.6 O 2 Mole Fraction vs. Height above burner (mm) Simulated Experimental Temperature (K) Simulated Experimental O 2 Mole Fraction Height above the burner (mm) Height above burner (mm) C 2 H 4 Mole Fraction C 2 H 4 Mole Fraction vs. Height Above Burner (mm) Simulated CO 2 Mole Fraction CO 2 Mole Fraction vs. Height Above Burner (mm) Simulated Experimental Height Above Burner (mm) Height Above Burner (mm) 14

15 CO Mole Fraction vs. Height Above Burner CO Mole Fraction Experimental Simulated Height Above Burner (mm) CH 2 O Mole Fraction vs. Height Above Burner (mm) 1.00E E-01 Experimental CH 2 O Mole Fraction 1.00E E E-04 Simulated Max mole fraction = 3.2e E E Height Above Burner (mm) 15

16 Validation with John Zink Flare Tests (September 2010, Tulsa, OK) Data 16

17 Air-Assisted Flare, 2010 TCEQ Flare Study Final Report, UT/CEER, TCEQ PGA No FY09-04 and Task Order No. UTA LOAT-RP9, Aug. 1, 2011.

18 18

19 Geometry used for Modeling Air-assisted flares in (Concentrated, localized fuel leads to a sustained flame with EDC) Fuel + Pilot Air-assist Outlet Stack 19

20 Expanded Definition of CZHV for Steam and Air-assisted Flares CZHV = f i f i * H + m + s + i + m* H m a * x eff f i : Volume flow rate of i th component in vent gas m: Volume flow rate of makeup gas a: Volume flow rate of assisted air s: Volume flow rate of assisted steam H i : Heating Value of the i th component in fuel gas (BTU/scf) H m : Heating Value of the makeup gas (BTU/scf) CZHV: Combustion Zone Heating Value (BTU/scf) x eff : Effective fraction (effective fraction of air-assist that causes the dilution), 2% is proposed for 2010 JZ Tulsa tests K. Singh et al, Engineering Applications of Computational Fluid Mechanics, manuscript in revision. 21

21 Parametric Studies on Steam-Assisted Ethylene Flares Effect of Adding Steam to Ethylene Flares on -DRE/CE -Flare temperature -Emissions of VOCs, HRVOCs, NO x, HO x

22 DRE/CE vs. Steam to Fuel (Mass) Ratio 100% 90% Efficiencies vs. S/F Ratio DRE (%) = -3.9(S/F) (S/F) R² = DRE/CE 80% 70% 60% 50% 40% 30% 20% 10% 0% DRE CE CE (%) = -3.4(S/F) 2-0.5(S/F) R² = Steam/Fuel (Mass) Ratio

23 Polynomial Equations Exponential Equations Response Surface Model for Air-Assisted flares * * * * * * Y a X a Y X a Y a X a a Z = c b Y X a Z * * = CE = (a+b*u+c*v+d*u^2+e*v^2+f*u*v+g/v+h/u+i/v^2+j/u^2+k*u/v+p*(u/v)^2)*(x*czhv^m)

24 29

25 Conclusions (I) For the lab scale CH 4 /Air mixture (UC Berkeley) flame, the EDC model accurately predicts the profiles of temperature and concentrations of major species (CH 4, CO 2, CO). The model predicts the mass fraction of trace species NO and OH (at the core of the flame) within a factor of 3.5. In case of C2H4/O2/Ar flame a good agreement was found between the predicted and experimental profiles of temperature, C 2 H 4, CO 2 and other major species concentrations. The EDC model under-predicts the mole fraction of CH 2 O by a factor of 10 and the exiting mole fraction of CH 2 O by a factor of 1.6. The EDC model with new geometry improves DRE of airassisted low LHV/low jet velocity propylene flares with an average of 4.6% compared to 13.3% in the 2011 AQRP report (for A/F mass ratio<28) 30

26 Lamar University CFD Lab Cutting Edge High Performance Computing (HPC) Cluster 3 X 12 core servers; Intel Xeon More than 50 high speed processors Up to 10GBs/second of data transfer speed for faster parallel computing

27 Publications K. Singh, T. Dabade, H. Vaid, P. Gangadharan, D. Chen, H. Lou, X. Li, K. Li, C.Martin, "Computational Fluid Dynamics Modeling of Industrial Flares Operated in Stand-By Mode," Industrial & Engineering Chemistry Research, 51 (39), , October, H. Lou, D. Chen, C. Martin, X. Li, K. Li, H. Vaid, K. Singh, P. Gangadharan, "Optimal Reduction of the C1-C3 Combustion Mechanism for the Simulation of Flaring, "Industrial & Engineering Chemistry Research, 51 (39), , October, H. Lou, C. Martin, D. Chen, X. Li, K. Li, H. Vaid, A. Tula, K. Singh,"Validation of a Reduced Combustion Mechanism for Light Hydrocarbons," Clean Technologies and Environmental Policy, 14 (4), pp , August H. Lou, C. Martin, D. Chen, X. Li, K. Li, H. Vaid, A. Kumar, K. Singh, & D. Bean, "A reduced reaction mechanism for the simulation in ethylene flare combustion," Clean Technologies and Environmental Policy, 14 (2), pp , April Kanwar Devesh Singh, Preeti Gangadharan, Daniel Chen, Helen H. Lou, Xianchang Li, P. Richmond, " Parametric Study of Ethylene Flare Operations and Validation of a Reduced Combustion Mechanism," Engineering Applications of Computational Fluid Mechanics, manuscript is being revised (2013). 33

28 Acknowledgements This project is financially supported by Texas Air Research Center AQRP TCEQ

29 09/18/2008 Chen, Yuan, Lou, Lin, Li 35 35

Air Quality Research Program Project Work Plan. CFD Modeling for UT/TCEQ Low BTU & Low Flow Rate Flare Tests

February 2011, Rev. 3 Air Quality Research Program Project 10-022 Work Plan CFD Modeling for UT/TCEQ Low BTU & Low Flow Rate Flare Tests Prepared by: Daniel Chen, PI Helen Lou, Co-PI Kuyen Li, Co-PI Dan