School of Process, Environmental and Materials Engineering Computational study of combustion in flares: structure and emission of a jet flame in turbulent cross-flow GERG Academic Network Event Brussels 3-4 June 2010 Shariff Lawal, M. Fairweather, D. Ingham L. Ma, M. Pourkashanian and A. Williams
Introduction Flares: safe burning of waste hydrocarbons Oilfields, refinery, LNG Pollutants: NO x, CO 2, CO, unburned hydrocarbons, greenhouse gases Global flaring >100 billion m 3 /year Operating conditions have significant effect on flare flame stability, efficiency and emissions Important to predict this effect to control emissions (stringent regulation / taxes ) Solution gas flare on an off-shore oil production platform
Motivation Accurate emission measurement: Unresolved in large-scale flares Passive Fourier-transform infra-red spectroscopy failed validation testing 1 Experiment /Computational fluid dynamics (CFD) Few CFD study on emissions pattern from high momentum jet flares Challenging aspect of flares CFD : (3-D, turbulence, complex chemistry, unsteadiness, partial-premixing e.t.c.) 1. TCEQ PFTIR I Testing Report, URS(2004): http://www.tceq.state.tx.us Elevated refinery flare (source: John Zink CO. U.S.A)
Research Objectives Identify range of fuel jet and wind velocities for sustainable operations (refinery and oil-field flares) Investigate effect of: fuel jet and cross-flow velocities Improved prediction of important physical phenomenon (turbulence, unsteadiness, partial-premixing and coherent structures) Study unresolved issues associated with scaling from small-scale to industrialscale flares: minimum burner diameter appropriate dimensionless parameter
CFD Validation study Laboratory-scale, high jet momentum flares 2 ( Bandaru and Turns (2000)) Computational domain: 1.67 x 0.68 x 0.34m 3 ; burner diam.: 4.12 and 5.54 mm X Boundary conditions: Inlet, outflow, walls and symmetry Flow conditions: Fuel: pure methane u j = 10-85 m/s, u cf = 2.3 and 5.0 m/s Re cf = 103,500-193,500 1.0 m Jet flow (CH 4 ) Y 0.17 m Cross-flow (Air) Schematic of the Jet flame in cross-flow (JFICF) Z 2. Bandaru, R.V. and Turns, S.R. (2000). Combustion and Flame, 121, 137-151.
Mathematical models Reynolds average Nervier-Stokes (RANS) transport equations for: continuity, momentum, enthalpy, mixture fraction/variance Turbulence: realizable and model with buoyancy Radiation: discrete ordinate method Soot : Brookes and Moss (1990) Combustion: steady / unsteady flamelet model (Peters, 1986) Multiple flamelet profile, scalar dissipation rate (SDR) ranges: 0.01 39/s; 0.0001 39/s Reaction mechanism: GRI Mech. 3.0 Fluent 12.0 code κ ε
Results Temperature profile (K) (a) X=0.3 m Mean flame temperature(k) on symmetric plane. (a) Contours, (b) Radial profile at axial location, x = 0.3 m 3. Birch, A.D., Brown, D.R., Fairweather, M., and Hargrave, G.K. (1989). Combust. Sci. and Tech., 66, 217-232
Results Flame length M = ρ u ρ u 2 2 j j cf cf Curvilinear distance from the pipe exit to the flame tip (1200 K) Longer flame at low and high M M =336, u cf /u j = 0.04 Increase in M cause decrease and increase in flame length Trend of flame length with M is similar Varying jet exit velocity Varying cross-flow velocity Momentum ratio, M Predicted and measured flame length at different momentum ratios.
Results Flame length Increase in flame height due to lower effect of cross-flow on fuel jet Reduce interaction, mixing and air entrainment (a) (b) (c) Mean flame temperature contour on the symmetric plane for: (a) M = 332, (b) M = 192, and (c) M = 116
Results Flame length M = ρ u ρ u 2 2 j j cf cf Curvilinear distance from the pipe exit to the flame tip (1200 K) Longer flame at low and high M M =336, u cf /u j = 0.04 Increase in M cause decrease and increase in flame length Trend of flame length with M is similar Varying jet exit velocity Varying cross-flow velocity Momentum ratio, M Predicted and measured flame length at different momentum ratios.
Results Radiant fraction Peak radiant heat flux with soot higher by only 2.2% Lower radiant fraction at high M Trend correlate with decrease in flame volume and residence time due to higher Reynolds number Radiant fraction (%) 20 18 16 14 12 10 8 6 4 2 0 100 200 300 400 500 600 700 800 Momentum flux ratio, M Predicted radiant fractions at different momentum flux ratios
Results Emission index (NO x ) Emission index quantity of pollutant in g per kg of fuel burned 4.5 Measured(Bandaru and Turns, 2000) sdr=0.0001 sdr = 0.01 Higher EINO x at low momentum ratio No significant changes in EINO x with increase in M Improved species prediction for model with small SDR EINOx(g/Kg) 3.5 2.5 1.5 0 100 200 300 400 500 600 700 800 Momentum ratio, M Predicted and measured NOx emission index at different momentum ratios.
Computational details Results Emission index (CO) Much higher effect of small SDR on CO emission index (EICO) CO emission index higher at high M No significant changes in EICO with increase in M Inconsistency in CO prediction possibly due to formation of coherent structures not resolved by RANS turbulence model EICO(g/Kg) 13 12 11 10 9 8 7 6 5 4 Measured(Bandaru and Turns, 2000) sdr=0.0001 sdr=0.01 0 100 200 300 400 500 600 700 800 Momentum ratio,m Predicted and measured CO emission index at different momentum ratios.
Computational details Results NO 2 /NO x ratio NO 2 in flames depicted by brownyellowish colouration in fumes NO 2 /NO x trend similar to CO species NO 2 /NO x ratio increases slightly at higher M NO 2 //NO x No significant changes in NO 2 /NO x with increase in M Can same conclusion be made for large-scale flares? Momentum ratio, M Predicted and measured NO 2 /NO x ratio at different momentum ratios.
Current work Large-scale industrial flare modelling (CANMET Flare test Facility, Ottawa, Canada) : Computational domain: 1.83 x 1.2 x 8.3 m 3, burner diameters: 2 to 6 Fuel: pure CH 4 and mixture of CH 4 and N 2 Range of jet / wind velocities: 0.23 to 22 ms -1 / 2.0 to 8.5 ms -1 Low momentum flux ratios: 0.2 < M < 15 Second-order turbulence closure resolves secondary flow features Unsteadiness RANS resolves unsteady mean flow structures Preliminary results
Current work preliminary results Unsteady calculation necessary for buoyancy dominated flow regime (a) RSM Reynolds stress turbulence model improved prediction of velocity field and turbulence kinetic energy Better prediction of recirculation region and flame temperature (b) SKE SKE Similar prediction for the species? (inconclusive: work-in-progress) Comparison of predicted temperature profile: (a) Reynolds stress (b) standard κ ε turbulence model
Conclusion Longer flame in the high and low momentum ratio regime Lower radiant heat flux and NO x at high momentum ratio Higher CO and NO 2 /NO x ratio at high M, no significant increase as M increases Flare operation is more likely acceptable at high momentum ratio ranges Unsteady calculation important in large-scale buoyancy dominated flares Future Work Improved accuracy of emission prediction through advance turbulence (LES) and combustion models (transported PDF )
Acknowledgement Nigerian Government funding for PhD research through the PTDF overseas scholarship scheme