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Energy & Fuels 2008, 22, 1698–1706
Computational Fluid Dynamics for Simulation of Wind-Tunnel Experiments on Flare Combustion Systems David Castiñeira and Thomas F. Edgar* Department of Chemical Engineering, UniVersity of Texas at Austin, 1 UniVersity Station C0400, Austin, Texas 78712-0231 ReceiVed September 11, 2007. ReVised Manuscript ReceiVed January 10, 2008
Flaring is used extensively in the energy and petrochemical industries to dispose of unwanted combustion gases by burning them in an open flame. However, these units may represent an important source of gas emissions due to inefficient operation under certain conditions such as high crosswind velocities. Several experimental studies have previously focused on flames burning in a fixed volume by using wind tunnels. In these experiments, the entire plume of combustion products was collected, sampled, and analyzed to calculate the combustion efficiency. Present work simulates these wind-tunnel experiments by using the commercial computational fluid dynamics (CFD) software package Fluent 6.2. Several three-dimensional (3D) computational models are developed, and suitable turbulence and chemistry models are applied to simulate the complex combustion phenomena and flame downwash. The computational work was greatly reduced by applying the laminar flamelet model, which assumes that a turbulent flame is an ensemble of small laminar structures called flamelets. Inefficient combustion is observed at high crosswinds, and simulation results are in very good agreement with experimental data. These results show that CFD can successfully simulate these wind-tunnel flare experiments. The resulting simulation models could be used to estimate the hydrocarbon emissions from chemical and petrochemical flares at crosswind conditions, an environmental issue of great importance in air pollution models.
1. Introduction Industrial flares are combustion systems designed to safely dispose of waste gases from chemical and petrochemical plants. Theoretically, these units convert effectively all hydrocarbons to CO2 and H2O through high temperature oxidation reactions. When operating properly, industrial flares achieve a 98–99% combustion efficiency, so only a few percent of unburned hydrocarbons would be released to the atmosphere under these conditions. Unfortunately, there are many situations in which this high combustion efficiency may be compromised. For example, steam-assisted flares are designed to provide smokeless combustion by adding steam into the combustion zone. However, some experimental studies have shown that addition of steam into the flare may affect the resulting combustion efficiency.1,2 While adding turbulence and promoting beneficial chemical interactions with the carbon particles, excessive steam addition can decrease the temperature of the flame to the point where inefficiency becomes a concern. Another variable that affects flame destruction efficiency is the wind speed surrounding the flare. Flames at crosswind conditions may exhibit a stripping away and dilution of part of the fuel stream before it encounters a source of ignition to begin its reaction.3 Moreover, crosswind reduces the flame size, * To whom correspondence should be addressed. Telephone: (512) 4713080. Fax: (512) 471-7060. E-mail:
[email protected]. (1) McDaniel, M. Flare Efficiency Study; EPA-600/2-83-052; July 1983. (2) Pohl, J. H.; Soelberg, N. R. EValuation of the Efficiency of Industrial Flares: H2S Gas Mixtures and Pilot Assisted Flares; EPA-600/2-86-080; September, 1986. (3) Johnson, M. R.; Kostiuk, L. W. Combust. Flame 2000, 123, 189– 200.
resulting in lower combustion efficiency because less oxygen is entrained into smaller flames, and flare combustion efficiency may decrease rapidly as wind speed increases from 1 to 6 m/s. Experimental evidence suggests that flare combustion efficiencies typically may be in the range of 70% at low wind speeds and could be even lower at higher wind speeds. Unfortunately, there is still great uncertainty about flare efficiency and the resulting emissions. Published research on large-scale jet diffusion flames burning in an open atmosphere is limited.4,5 However, collecting representative samples from flames burning in an open atmosphere is very difficult. In addition, online measurements on these systems is complicated by the size and turbulence of the flames, which are typically located at the tip of stacks anywhere from 10 to over 100 m tall to prevent dangerous conditions at ground level. For all these reasons, two different approaches have been proposed to analyze the effect of crosswind velocity on industrial flare performance and the resulting emissions. One of these approaches involves the experimental study of reduced scale, turbulent diffusion flames located within a wind-tunnel facility. Typically, a closed-circuit wind tunnel is used so samples can be easily collected and analyzed. The second approach to understand industrial flare behavior is to use computational fluid dynamics (CFD). CFD is based on the application of fundamental physics for the prediction of reacting flow phenomena, and it relies on the numerical solution (4) Kuipers, E. W.; Jarvis, B.; Bullman, S. J.; Cook, D. K.; McHugh, D. R. Combustion efficiency of natural gas flares; effect of wind speed, flow rate, and pilots; Internal report; Shell Research and Technology Thorton and British Gas Research Centre, 1996. (5) Strosher, M. InVestigations of Flare Gas Emissions in Alberta; Environmental Technologies, Alberta Research Council: Calgary, Alberta, Canada, 1996.
10.1021/ef700545j CCC: $40.75 2008 American Chemical Society Published on Web 04/18/2008
CFD on Flare Combustion Systems
of the governing transport equations for mass, energy, species, and momentum. Even though research has been done on the subjects of CFD and industrial combustion, there is very little discussion of industrial flares, except by Baukal et al.,6 which discusses CFD applications in industrial combustion. CFD has been recently applied to simulate the effect of steam addition and air addition for several laboratory-scale, turbulent nonpremixed flames.7 Detailed finite-rate chemistry models were applied to predict the species concentrations in the flame, while species mass balances were set up in order to compute the resulting flame combustion efficiency. Simulation results showed that incomplete combustion of hydrocarbons may occur at high steam/fuel and air/fuel ratios up to the point where these flames become extinguished. The computational work was greatly reduced by assuming two-dimensional (2D) axisymmetric models. Reynolds numbers of these laboratory scale flames were comparable to those for industrial flares. CFD has also been used to develop 3D simulations of the effect of crosswind on a laboratory-scale, turbulent combustion flame.8 The flame was simulated at the exit of a vertical burner that was perpendicular to the air flow, a configuration that is relevant to continuous gas flaring in the atmosphere. Simulations clearly showed that for high momentum flames moderate velocities may significantly reduce the resulting combustion efficiency. Unfortunately, direct application of CFD to simulate largescale industrial flares is very difficult. First of all, industrial flares are clearly turbulent, and direct numerical simulation of turbulent flows is not possible because of the wide range of time and length scales. Thus, some type of turbulence model must be applied. Second, realistic chemical mechanisms for hydrocarbon combustion cannot be described by a single reaction equation. Such models may include tens of species and hundreds of reactions that are known in detail for only a limited number of fuels. Hence, some chemistry simplification must be made. Furthermore, it is necessary to deal with complex turbulencechemistry interaction due to the sensitivity of reaction rates to local changes. It is very challenging to perform combustion simulations for large-scale flares. The total number of grids needed to capture all the combustion details makes the computational work almost prohibitive for large flares. Employing three-dimensional (3D) models significantly increases the computational work. Even so, the lack of experimental data for industrial flares makes it very difficult to validate potential simulation results. Thus, CFD is restricted to the simulation of wind-tunnel experiments in this work, in order to compare model predictions with experimental data. This allows us to validate our results by direct comparison with experimental data. Validated simulation models could be used to estimate the actual hydrocarbon emissions from chemical and petrochemical plants. The commercial software Fluent 6.2 is used in this work. As a first approach, we have selected the wind-tunnel experiments of low-momentum jet diffusion flames of Johnson and Kostiuk,3 where natural gas was burned at crosswind velocities ranging from 1.0 to 11.0 m/s in a 0.0221 m diameter burner. In addition, a new set of wind-tunnel experiments that have been experimentally studied at CANMET Energy Technology Centre, Ottawa, are used for simulation of larger scale flares. These (6) Baukal, C. E.; Gershtein, V. Y.; Li, X. Computational Fluid Dynamics in Industrial Combustion; CRC Press LLC: Boca Raton, FL, 2000. (7) Castiñeira, D.; Edgar, T. F. Energy Fuels 2006, 20, 1044–1056. (8) Castiñeira, D.; Edgar, T. F. CFD for simulation of crosswind on the efficiency of high momentum jet turbulent combustion flames. J. EnViron. Eng., submitted for publication.
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new experiments are characterized by larger burner diameters, in some cases comparable to industrial flare diameters. 2. Governing Equations CFD relies on solving conservation or transport equations for mass, momentum, energy, and participating species. If the flow is turbulent, model equations for specific turbulent quantities have to be solved in addition. Since even with today’s super computers resolving turbulent length scales directly results in tremendous effort, Reynolds averaged equations are typically applied to include the physics of turbulence. Hence, the basic model equations for a fluid in turbulent flow are the Reynoldsaveraged Navier–Stokes (RANS) equations. For steady state, these equations are given below: 2.1. Continuity Equation. ∇(Fν) ) 0
(1)
where F is the density of the fluid and νj is its ensemble-averaged velocity vector, defined on a 3D domain. 2.2. Momentum Conservation Equation. ∇(Fνν) ) - ∇ p + ∇ (µ(∇ν + (∇ν)T) - Fν′ν′)
(2)
where ν′ is the turbulent fluctuation of the velocity vector, µ is the dynamic molecular viscosity of the fluid, and F is the pressure. The overbars denote mean values. The Reynolds stresses, Fνj′νj′, are extra terms that stem from decomposing solution turbulent variables into the mean and fluctuating components; these terms must be modeled in order to close eq 2. A common approach employs the Boussinesq hypothesis to relate the Reynolds stresses to the mean velocity gradients:
(
-ν′ν′ ) υt
)
∂νi ∂νj 2 + - (υt(∇ν) + k)δij ∂xj ∂xi 3
(3)
where the Einstein summation notation is being used; that is, δij is the Krönecker delta, νt is the eddy kinematic viscosity, and k is the kinetic energy of turbulence, defined by 1 (4) k ) ν′iν′i 2 The Boussinesq approach is used in the k-ε model, which is the turbulence model applied in this work. The advantage of this approach is the relatively low computational cost associated with the computation of the kinematic viscosity. For the k-ε model,theeddyviscosityisobtainedfromthePrandtl-Kolmogorov relation: υt )
Cµk2 ε
(5)
where ε is the rate of turbulent kinetic energy dissipation. Robustness, economy, and reasonable accuracy for a wide range of turbulent flows explain the popularity of this model in industrial flow and heat transfer simulations. 2.3. Energy Conservation Equation. When considering heat transfer within the fluid and/or solid regions of the problem, Fluent also solves the energy equation. This equation is given below in a very general form: ∂ (FE) + ∇ (ν b(FE + p)) ) ∇ (keff ∇ T ∂t
∑ h bJ + (τ i i
b)) + Sh effV
j
(6) where keff is the effective conductivity, Ji is the diffusion flux of species i, and h is enthalpy. Notice that radiation effects are
1700 Energy & Fuels, Vol. 22, No. 3, 2008
Castiñeira and Edgar Table 1. Cases Used for Simulation and Corresponding Jet Exit Velocity (Vj), Crosswind Velocity (U), and Experimental Fuel Jet to Crosswind Momentum Flux Ratio (R)
Figure 1. Schematic of a closed-loop wind tunnel facility (all dimension in meters) at the University of Alberta. Reprinted by permission of Elsevier Science from Johnson and Kostiuk,3 Copyright 2000 by The Combustion Institute.
Figure 2. Sketch of flow structures in a low-momentum jet diffusion flame in a crosswind.
not considered in this work, so the corresponding term has been removed from eq 6. 2.4. Species Transport Equations. Finally, for reacting systems the species transport equations must be solved. In general form, this equation is given by ∂ (FY ) + ∇ (FνYi) ) - ∇ Ji + Ri ∂t i
(7)
where Ri is the net rate of production of species i by chemical reaction. Fluent applies the finite volume method to discretize and solve the governing flow equations described above. 3. Wind Tunnel Configuration A set of low-momentum, natural gas diffusion flames located in a closed-loop wind tunnel were used for simulation. These flames were studied experimentally at the University of Alberta and the National Research Council of Canada. Measurement of experimental combustion efficiencies was reported by Johnson and Kostiuk.3 A detailed description of the experimental setup and results was given as a final report by Kostiuk et al.9 A schematic representation of the wind tunnel is given in Figure 1. The basic information about the experimental setup is given below; refer to refs 3 and 9 for a more detailed description. The experimental flames were established at the exit of a burner tube mounted vertically in the wind tunnel and perpendicular to the airflow. In the vertical section downstream of the flame and in the upper section of the tunnel, a series of (9) Kostiuk, L.; Johnson, M.; Thomas, G. Flare research project, final report, University of Alberta, September, 2004.
case
Vj (m/s)
U (m/s)
R
B C D E F G
2.08 2.10 2.09 2.11 2.11 2.09
1.33 2.76 4.09 5.49 8.27 11.05
1.43 0.34 0.15 0.085 0.038 0.021
supplementary fans were used to ensure that the plume of combustion products was fully mixed into the wind-tunnel air before sampling. The wind tunnel was sufficiently large that, during a typical 5-10 min test, the concentration of hydrocarbons in the tunnel remained small, and the effects of reburning were completely negligible. From a simulation point of view, only the test section around the burner (e.g., the box that followed the contraction section) is strictly relevant. The dimensions of this box are 2.44 m in width by 1.22 m in height by 11.8 m in length. However, a 5 m long box is enough to capture the flame behavior, so unnecessary computation work could be avoided by reducing the simulation domain. The floor of the wind tunnel was constructed with 19 mm thick plywood, while downstream of the flare the tunnel was covered with 30 gauge aluminum sheeting to protect it from possible direct flame impingement. The walls along the test section were primarily Plexiglas. The ceiling upstream of the flare was constructed with 19 mm thick plywood, but downstream of the flare the ceiling was made of 19 mm thick ceramic panels that could safely resist the accidental impingement of the flame or hot combustion products. The diffusion flames were established at the exit of a 24.6 mm o.d. (22.1 mm i.d.) pipe that extended 47 cm into the wind tunnel. The experimental setup also included a 65% blockage ratio perforated plate “turbulence plug” with 3 mm diameter holes, which was placed inside the pipe three diameters upstream of the exit. The purpose of this plug was to create velocity profiles similar to the turbulent pipe flow expected in full-scale industrial flares, independent of the actual flow velocity in the laboratory-scale flares. However, computer simulation of that perforated plate is very difficult to perform and may introduce numerical errors in the solution process. Thus, the turbulence of the fuel gas was adjusted in our simulation to match the experimental turbulence intensity measured 5 mm above the exit plane of the burner tube (see Kostiuk et al.9). For these experiments, the jet exit velocity of the fuel, Vj, was held approximately constant at 2 m/s, and the crosswind speed, U, was varied from 1 to 11 m/s. The external cold-flow Reynolds number (Re) ranged from 1570 to 17 270 as the crosswind increased from 1 to 11 m/s. Under these conditions, the flow regime on the outside of the pipe flare could be considered to be in the regime of having a laminar boundary layer separation. The turbulent fluctuation in the core flow of the tunnel was found to be consistently less than 0.4% except at low wind speeds (
