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Combustion of residual steel gases: laminar flame analysis and turbulent flamelet modeling☆
Fuel 82 (2003) 983–991 www.fuelfirst.com
Combustion of residual steel gases: laminar flame analysis and turbulent flamelet modelingq Olivier Gicquela, Luc Vervischb,*, Guillaume Joncquetb, Bernard Labegorrec, Nasser Darabihaa b
a Laboratoire EM2C, Ecole Centrale Paris, UPR 288 CNRS, Chatenay Malabry, France INSA de Rouen Campus du Madrillet, UMR-CNRS-6614-CORIA, Avenue de l’Universite´, BP 8, 76801 Saint Etienne du Rouvray Cedex, France c CRCD, Air Liquide, Jouy-en-Josas, France
Received 28 March 2002; revised 23 May 2002; accepted 24 May 2002; available online 29 October 2002
Abstract In recovery combustion systems operating in the steel industry, energy is provided by boilers burning residual gases of blast furnace and coke oven. To help understand combustion of this particular type of fuels, a numerical study is conducted where the major chemical properties of steel gas flames are collected. The chemical composition of representative fuel and oxidizer steel gas is varied over a large range in calculations using detailed chemistry and complex transport properties. The chemical equilibrium compositions, premixed flame speeds and diffusion flame extinction strain rates are determined. The advantages and shortcomings of the use of vitiated air emerge, and its introduction into the boiler appears as an interesting alternative to reduce NOx emission. The detailed information obtained with laminar flame calculations is also introduced in flamelet turbulent combustion modeling. Reynolds Averaged Navier Stokes (RANS) simulations of a test case burner are performed and some comparisons between numerical predictions and experimental results are presented. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Reynolds Averaged Navier Stokes; Laminar flame; Diffusion flamelet modeling
1. Introduction Novel technologies for energy production are under development, as cogeneration or integrated gasification combined cycle. These systems are proposed in the steel industry, where the energy of residual gases issued from a blast furnace, or coke oven, is recovered in boilers for electricity production [1,2]. The requested performance of steel gas burners is increasing, specifically, their capacity to burn various fuels. This leads manufacturers to redesign existing burners, for which the constraints are to comply with present and future emission regulations. It is also sometimes requested to enlarge the range of operating conditions. These objectives should be reached by modern recovery combustion systems, but preserving flame attachment at the burner and avoiding undesirable combustion instabilities. This requires a precise knowledge of the behavior of the reaction zones observed in steel gas combustion. * Corresponding author. E-mail address: [email protected] (L. Vervisch). q Published first on the web via Fuelfirst.com –http://www.fuelfirst.com
The composition of steel gas is complex and not always perfectly known in practice. Fuel and oxidizer may vary strongly; the fuel may be composed of pure BFG, pure coke oven gas, or may result from the mixing of these two fuels. The oxidizer is either essentially composed of pure air, or, air that has been more or less mixed with gas turbine exhaust gas. A set of representative chemical compositions of steel gas was determined by probing a plant. Once this information is available, the exact characterization of the flames that could exist in such boilers motivates basic questions: What are the chemical equilibrium conditions for these gases? What are the orders of magnitude of the burning rate and of the deflagration velocity for such mixtures? What is the amplitude of the local velocity shear that may prevent the reaction zone developing? These questions are addressed in the first part of the study, where laminar flame calculations are performed to collect the relevant information. A full characterization of the two idealized combustion modes usually considered, premixed and nonpremixed, is then obtained for various representative compositions of the steel gas.
0016-2361/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 3 8 4 - 8
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In a second step, nonpremixed turbulent combustion modeling of steel gas is addressed. A large variety of turbulent combustion modeling techniques is now available [3 –6] and the numerical simulation of turbulent flames may be of great help in understanding such devices. The ultimate objective is the development of a numerical tool that will be useful in the design of steel gas burners. Diffusion flamelet turbulent combustion modeling is retained; the result of the previous flame analysis is a large chemical database that is easily introduced into such a combustion modeling approach. The numerical closure accounts for the possible mixing and dilution of different fuels issuing from various inlets. The modeling is introduced into CFD commercial software and a burner test case is computed. In the subsequent sections, the chemical model is first presented. After studying the equilibrium conditions, the behavior of prototype laminar flames is reported for several synthetic steel gases. An approximate formulation is finally discussed to incorporate this information into Reynolds Averaged Navier Stokes (RANS) calculations of steel gas boilers.
2. Chemical model and flame properties 2.1. Introduction The composition of the mixture entering the boiler includes blast furnace gas (BFG) more or less mixed with coke oven exhaust gas (COG). These fuels will be represented by a combination of simple gases, such as CH4, CO and other usual molecules found in hydrocarbon flames. Data have been collected in an industrial plant, the related representative composition of steel gas is given in Table 1. In principle, the burner can be fed by any combinations of BFG and COG. The gas composition may thus vary from 100% of BFG up to 100% of COG. In the case of pure COG, the evolution of the mixture is mainly governed by hydrogen chemistry, whose lower calorific value is of the order of 130,000 kJ kg21. Pure BFG is mostly characterized by CO chemistry, featuring a much lower calorific value, of the order of 10,700 kJ kg21. When BFG and COG are separately injected in the boiler, the local composition of the fuel resulting from turbulent mixing will Table 1 Composition (% in volume) of typical coke oven and blast furnace plant exhaust gas at 300 K
CO H2 CH4 H2 O N2 CO2
Coke oven (COG) (%)
Blast furnace (BFG) (%)
5.6 62 28.8 0 4.1 1.5
22.2 2.2 0 2.4 51 22.2
thus strongly affect the amount of heat released by combustion. This BFG/COG mixture is introduced in burners where the oxidizer is provided in two different ways; either pure air or gas turbine exhaust gas (vitiated air) may act as oxidizer. The influence of the fraction of vitiated air in the oxidizer is analyzed in the next sections; the retained air compositions are summarized in Table 2. Various calculations are reported below. Major properties of premixed and laminar flames are investigated. Equilibrium compositions, laminar premixed flame speeds and extinction strain rates of counterflow diffusion flames are determined. This is done by varying fuel and oxidizer compositions in calculations involving fully detailed chemistry and complex transport properties. Complex chemical schemes are available in the literature to reproduce major trends of flames for mixtures including H2, CH4 and CO, which are the main components of the gases under study. They capture, with a reasonable level of accuracy, auto-ignition delays [7], along with premixed laminar flame speeds and diffusion flame extinction strain rates [8]. Among the various existing schemes, the chemical mechanisms proposed by Lindstedt [9] has been retained. It includes 49 species and a detailed mechanism for nitrogen chemistry that accounts for thermal and prompt NO formation. This detailed chemistry has been validated carefully in other circumstances [10]. 2.2. Thermodynamic equilibrium composition The chemical equilibrium compositions and the related temperatures are first determined. The adiabatic maximum temperature in the boiler and the composition leading to the lower production of NO are then readily obtained. The calculations are performed with EQUILI, provided in the CHEMKIN II package [11] developed by SANDIA National Laboratory. The software returns the thermodynamic equilibrium composition and temperature for any gas mixture. (Notice that only thermodynamic considerations are necessary to collect these equilibrium conditions; chemical reaction schemes are not yet introduced at this stage.) The local composition of the gas varies according to three parameters: the fuel composition, the oxidizer composition and the amount of fuel mixed with oxidizer. All species and thermodynamic variables are accessible. The temperature, the concentration of O2 and of NO are particularly discussed when the oxidizer is composed of pure air at 300 K (case a), hot vitiated air at 823 K (case b) Table 2 Composition of pure, vitiated and mixed air used as oxidizer Case
(a)
(b)
(c)
Composition Temperature (K) O2 (%) N2 (%)
Pure air 300 23 77
Vitiated air 823 12 88
Mixed air 50% (a) þ 50% (b) 570 17.5 82.5
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Fig. 1. Thermodynamic equilibrium temperature in Kelvin. X: volume fraction (%) of air mixed with fuel. Y: volume fraction (%) of BFG in the fuel. (a) Pure air; (b) vitiated air; (c) pure air mixed with vitiated air (Table 2).
and mixed air at 570 K including 50% of pure cold air and 50% of vitiated hot air (case c) (Table 2). The highest equilibrium temperature is observed in the case of high proportions of COG. It is of the order of 2200 K (Fig. 1). Lower temperature levels are found with BFG, of the order of 1500 K. The temperature distribution is thus a direct function of the fraction of BFG in the fuel. A basic question arises in steel gas combustion, which is related to the eventual and undesirable existence of multiple zones of high temperature in recovery boilers, which could be promoted by the interaction between the two types of fuel interaction that would lead to an inhibition of the chemistry. Fig. 1 shows that only one temperature maximum is observed in the composition domain. Hence, when the temperature peaks at more than one location in a practical system, this can hardly be attributed to the complex chemical composition of the fuel. This may simply result from the turbulent mixing properties of the boiler. Fig. 1 shows that the temperature decreases when the fraction of vitiated air in the oxidizer is enhanced.
The temperature is of the order of 1950 K for pure COG and is limited to 1250 K for pure BFG. Because of the high dilution with 88% of nitrogen, vitiated air cannot contribute to very high combustion temperature, even when it is preheated (850 K). The knowledge of the equivalence ratio of the mixture and of the corresponding stoichiometric condition is needed to understand the combustion regime and the average flame location in a boiler. Both fuels involve species with O atoms and may react as oxidizer; it is then not easy to formally determine the stoichiometric isoline. However, this problem can be overcome using the value of O2. The stoichiometric line is a priori defined from the equilibrium calculations, as the border of the zone where the mass fraction of O2 vanishes. Equilibrium O2 mass fractions are presented in Fig. 2. The stoichiometric isoline is located in the zone where the fraction of air is high, a point that is amplified by the use of vitiated air. Since the mean turbulent flame zone is likely to develop in the vicinity of the stoichiometric surface of the mixture, adding vitiated air in practical
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Fig. 2. O2 mass fraction. X: volume fraction (%) of air mixed with fuel. Y: volume fraction (%) of BFG in the fuel. (a) Pure air; (b) vitiated air (Table 2).
systems will move the mean flame zone towards the air injection area. Equilibrium calculations also provide an approximation of the maximum level of pollutants that could be generated for any composition of the mixture. The maximum NO emission is of the order of 472 ppm, obtained with pure air (Fig. 3). When vitiated air is chosen, the maximum NO level is decreased to 175 ppm (Fig. 3). NO production is very sensitive to the temperature drop observed with vitiated air, explaining the lower levels of NO. As a preliminary conclusion, the introduction of vitiated air may be an interesting candidate to help in controlling flame position and NO emission. 2.3. Premixed laminar flame speed The properties of laminar deflagration are obtained for various steel gas composition and are analyzed in this section. The one-dimensional code PREMIX [12] developed by SANDIA is retained to calculate laminar premixed
flames. Simulations were performed for fuels ranging from 20 up to 80% of BFG. For each of the fuel compositions, the lean and rich flammability limits are tracked together with the corresponding flame speed. The whole flammability domain is then determined. Fig. 4 displays the results obtained for the compositions given in Table 2. For all cases, flammability domains are limited to a somewhat reduced area of the mixture composition. The laminar flame speed is proportional to the fraction of hot vitiated air contained in the oxidizer. The maximum values are of the order of 75 cm/s for pure air (case a), 117 cm/s with an oxidizer composed of 50% of pure air and 50% of vitiated air (case c), and, 157 cm/s for pure vitiated air (case b). Because the turbulent burning rate is somehow related to the flame speed [13], this is a result of practical interest. It suggests that situations exist where vitiated air is much more efficient than pure air. The maximum value of the laminar flame speed is extracted from the calculations to refine the analysis; Fig. 5 shows these maxima for the three oxidizer compositions of Table 2. The maximum laminar
Fig. 3. NO mass fraction. X: volume fraction (%) of air mixed with fuel. Y: volume fraction (%) of BFG in the fuel. (a) Pure air; (b) vitiated air (Table. 2).
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Fig. 4. Flame speed. X: volume fraction (%) of air mixed with fuel. Y: volume fraction (%) of BFG in the fuel. (a) Pure air; (b) vitiated air; (c) pure air mixed with vitiated air (Table. 2).
flame speed is reached for mixture close to their stoichiometric condition. This graph confirms that the laminar flame speed increases with the fraction of vitiated air. Part of this increase is simply the result of the much higher temperature of the vitiated air that enhances the laminar flame speed. Flame stabilization is usually achieved through the propagation of partially premixed reacting fronts. The existence of greater flame speeds mean that reaction zones may be expected closer to the burner lips with vitiated air than with pure air. In nonpremixed turbulent combustion, because of the fluctuations of the mixture induced by unsteady turbulent mixing, the fraction of BFG in the fuel may rapidly evolve over a wide range. Accordingly, important variations of the laminar flame speed may exist. For pure air, the velocity ranges between 26 and 76 cm/s, while for vitiated air, the velocity evolves between 64 and 157 cm/s. These variations may promote the development of combustion instabilities within the boiler, instabilities that are triggered by the unsteady rolling up of the shear layers. Therefore,
even though vitiated air may help in flame stabilization, the reaction zones will be much more sensitive to variations of composition with vitiated air than with pure air. The basic response of a flame to an acoustic excitation, or to turbulent velocity and pressure fluctuations, depends on its characteristic time and length scales [14]. The flame thickness is determined in the simulations as the distance between the two points associated to a heat release rate equal to half of its maximum value. The characteristic flame time is estimated as the ratio between flame thickness and flame speed. Results are presented in Fig. 5. The flame thickness increases with the fraction of vitiated air in the oxidizer. The range of time scales is found to be larger for pure air than for vitiated air, moreover these ranges are shifted in time. The control of flame stability is a real challenge. Nevertheless, it may be interesting to take advantage of the very large range of local burning rates and characteristic scales of steel gas combustion to control flame stability. In other words, it may be interesting to adjust the oxidizer composition with vitiated air to insure combustion stability.
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Fig. 5. Laminar premixed flame properties. X: volume fraction (%) of BFG in the fuel. (i) Laminar flame speed (cm/s); (ii) flame thickness (cm); (iii) characteristic flame time (s). Solid: case (a) pure air. Dashdot: case (b) vitiated air. Dashed: case (c) (Table. 2).
2.4. Diffusion flame response to strain rate The response of steady laminar counter-flow diffusion flames to strain rate is helpful to calibrate the robustness of a given nonpremixed reaction zone [4]. Results are presented for different fractions of BFG in the fuel, for both pure and hot vitiated air (Fig. 6). The fuel composition has a strong effect on the flame extinction limits. Flames are much more robust using COG than BFG as fuel. The extinction limits for COG are 3412 s21 with pure air and 4198 s21 using vitiated air, while for BFG, they are of the order of 36 s21 with pure air and 92 s21 using vitiated air. The local reaction zone is, therefore, more likely to be quenched when it is fed with BFG. The introduction of vitiated air increases the strength of the flame, which is an additional factor indicating that vitiated air may be helpful to stabilize a turbulent flame base in a boiler. As computations of laminar diffusion flames have been performed in a continuous manner from a weakly strained reaction zone up to full quenching, a detailed flamelet
database is available. These flamelets were obtained for a wide range of fuel composition. This provides a three entrances flamelet look-up table, whose control parameters are: the local equivalence ratio in the diffusion flame, expressed with a mixture fraction Z (a conserved scalar equals to unity in the fuel stream and that vanishes in the oxidizer stream) [14], a the ratio between COG and BFG in the fuel, and, a the rate of strain imposed to the laminar diffusion flamelet. These tables were generated for both pure air and vitiated air; all the species mass fraction Yi ða; a; ZÞ and temperature Tða; a; ZÞ are then available.
3. RANS calculations with diffusion flamelets 3.1. Introduction It is intended in this section to conduct RANS calculations of an industrial burner operating with steel gas. An IFRF (International Flame Research Foundation)
O. Gicquel et al. / Fuel 82 (2003) 983–991
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Fig. 6. Temperature (Kelvin) response versus strain rate (s21). Solid: 0% of BFG in the fuel. Dashed: 25%. Dashdot: 50%. Dotted: 75%. Longdash: 100%. (a) Pure air; (b) vitiated air (Table. 2).
experiment is retained as a test case [15]. The objective is to perform computations for estimating the eventual prediction capabilities of numerical modeling. To our knowledge, because of the complex composition of the fuel, RANS calculations of this type of steel gas burner have not been reported so far. The mean temperature distribution in the turbulent flow is approximated by coupling the available chemical look-up table with CFD. The simulations of strained laminar diffusion flames discussed earlier are perfectly suited to become an ingredient of flamelet modeling. This approach is thus adopted to account for the effect of the unresolved turbulent fluctuations on the strongly nonlinear behavior of the chemistry. 3.2. Flamelet modeling of steel gas Steady diffusion flamelet modeling is based on an asymptotic view of nonpremixed turbulent combustion [4]. For a given state of mixing in the turbulent flow, flamelet models are derived assuming that the local balance between diffusion and reaction is similar to the one found in a prototype laminar flame. The control parameters of the flamelet are Z, the mixture fraction, and a, the strain rate. In steady flamelets, it is implicitly supposed that the characteristic time required to reach a chemical steady state in nonpremixed diffusive-reactive layers is smaller than any other turbulent flow times and this is a strong hypothesis. The use of the steady flamelet model may then be viewed as an improvement of the infinitely fast chemistry hypothesis, as a first step to incorporate some finite rate chemistry effects in the modeling. In the chemical table, the strain rate a is related to the scalar dissipation rate xs ¼ Dl7Zl2s [4], where l7Zl is measured under stoichiometric condition, D is a diffusion coefficient. The local fluctuations in fuel-oxidizer mixing are ~ p Þ; the probability density accounted for via PðZ function of the mixture fraction that captures the statistical
properties of turbulent mixing. Mass weighted averaging is ~ p Þ from retained. A beta-pdf is used for approximating PðZ the solution of the modeled balanced equations for Z and 00 2; Zg 2 the mean and fluctuations of the mixture fraction [16]. The conditional averaged value of the scalar dissipation rate under stoichiometric conditions, x~s ; is approximated in the RANS calculations from the mean scalar dissipation rate x~ and the distribution of laminar Ð x in a strained ~ p ÞdZ p : A linear diffusive layer, as x~s ¼ x~FðZs Þ 10 FðZ p ÞPðZ relaxation model is chosen to approximate x~ [16], x~ ¼ 00 2=ðk=1Þ; Cx Zg 2 where Cx is a constant equals to 1.2 in the calculations, k is the energy of the turbulent motion and 1 its dissipation rate. The function FðZÞ ¼ expð22½ erfc21 ð2ZÞ2 Þ provides the scalar dissipation rate distribution in the counter-flow diffusion flamelet, xðZÞ ¼ x0 FðZÞ; where x0 ¼ a=p: Further detail concerning this modeling procedure are available in Ref. [4]. The fuel composition being fixed, a is known and the profiles Yi ða; x~s ; ZÞ and Tða; x~s ; ZÞ are retained in the ~ pÞ : diffusion flamelet table for averaging with PðZ ð1 ~ p ; xÞdZ p ð1Þ Y~ i ðxÞ ¼ Yi ðaðxÞ; x~s ðxÞ; Z p ÞPðZ 0
~ ¼ TðxÞ
ð1 0
~ p ; xÞdZ p TðaðxÞ; x~s ðxÞ; Z p ÞPðZ
ð2Þ
The RANS quantities Y~ i and T˜ are then available to characterize the turbulent flame. To complete this study, the turbulent combustion model was introduced into the commercial software FLUENT1 via ‘user define functions’. 3.3. Results A IFRF burner is calculated [15]. It is a three inlets nonpremixed combustion system using a low calorific value 1
http://www.fluent.com
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Fig. 7. (i) Picture of the flame; (ii) temperature profiles on the centerline of the burner (Kelvin). Dotted line: RANS calculations. Square with error bar: measurements; (iii) iso-mean temperature (RANS calculations) (Kelvin).
fuel to mimic steel gas, it operates with a ¼ 0:986: The burner is composed of co-flowing jets: a central jet of primary air diameter ðfAir1 ¼ 70 mm; mean velocity VAir1 ¼ 3 m=sÞ is surrounded by a fuel jet ðfFuel ¼ 133 mm; VFuel ¼ 24:2 m=sÞ and an annular secondary air jet ðfAir2 ¼ 159 mm; VAir2 ¼ 4 m=sÞ: The mean power of the system is of the order of 1 MW. The central air jet impacts on a flame holder to enter the burner perpendicularly to the main stream. The flame holder leads to a complex flow topology that is necessary for flame stabilization. Recirculation zones appear and the stoichiometric line meets the centerline close to the burner exit. A first flame featuring a small length is then observed in the vicinity of the flame holder, this flame consumes most of the primary air of the central jet and acts as a pilot flame (Fig. 7). The burnt gases of the first combustion zone contribute to the ignition of the external mixing layer that develops between the fuel and the secondary jet. Hence, a second combustion zone is observed further downstream. This decomposition of combustion in two separated main reaction zones leads to two peaks of temperature on the burner axis. Detail concerning the burner properties and measurements may be found in Ref. [15].
The expected flame topology is reproduced by the RANS calculations, with a short internal flame followed by a much wider external combustion zone. Temperature measurements have been performed on the centerline with a suction pyrometer. Unfortunately, measurements are not available in the first and short internal flame. A first peak of temperature is observed in the calculations (Fig. 7). Outside of this short flame, the amount of heat released decreases in the computation. As expected, further downstream, the temperature increases again in a second flame zone. This increase of temperature, found at locations where measurements are available, is reasonably captured by the calculations (Fig. 7).
4. Conclusion Steel gas combustion has been investigated. Responses of laminar premixed and diffusion flames to various parameters have been collected. The premixed deflagration speed, the extinction strain rate of diffusion flamelets and, more generally, the major consequences of using hot vitiated air as oxidizer have been determined. The exact impact of
O. Gicquel et al. / Fuel 82 (2003) 983–991
the gases composition on burning rate, NO pollution and possible flame attachment have also been discussed. The information provided by the detailed chemistry was included into RANS turbulent combustion modeling. A burner test case has been computed to assess the prediction capability of the numerical procedure. Previous experimental investigations of this burner have shown a complex flame structure, whose overall properties are reproduced by the numerical results. Acknowledgements The authors have benefited from fruitful discussions with Drs Franc¸ois Lacas and Denis Veynante. References [1] Luessen HP. Gas turbine technology for steel mill gas and syngas applications. International Gas Turbine and Aeroengine Congress and Exhibition, Orlando, Florida; June 2–5, 1997. [2] Edthofer H, Schmidt G. Energy supply and energy conservation at voest-alpine stahl linz works. Seminar on Economic Aspects of Clean Technologies, Energy Waste Management in Steel Industry; 1998.
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[3] Bray KNC. Proc Combust Inst 1996;26:1– 26. [4] Peters N. Turbulent combustion. Cambridge: Cambridge University Press; 2000. [5] Vervisch L. Proc Combust Inst 2000;28:11–24. [6] Veynante D, Vervisch L. Prog Energy Combust Sci 2002;28: 193 –266. [7] Gray P, Griffiths J, Scott K. Proc Combust Inst 1984;20:1809– 15. [8] Vagelopoulos CM, Egolfopoulos F. Proc Combust Inst 1994;25: 1317– 23. [9] Lindstedt P. Proc Combust Inst 1998;27:269– 85. [10] Gicquel O. De´veloppement et Validation d’une Nouvelle Technique de Re´duction de Sche´mas Cine´tiques, Application au Me´thane. PhD Thesis. Ecole Centrale Paris; 1999. [11] Kee RJ, Rupley FM, Miller JA. Chemkin-II: a fortran chemical kinetics package for the analysis of gas phase chemical kinetics. Technical report. Sandia National Laboratories; 1989. [12] Kee RJ, Grcar JF, Smooke MD, Miller JA. A fortran program for modeling steady laminar one-dimensional premixed flames. Technical report. Sandia National Laboratories; 1992. [13] Vervisch L, Veynante D. Proc Combust Inst 2000;28:175– 83. [14] Poinsot T, Veynante D. Theoretical and numerical combustion: R.T. Edwards; Philadelphia, USA; 2001. [15] Study on the burner design characteristics required for the combustion of variable quality lean gases. Technical report. IFRF; 1984. [16] Dopazo C. Recent developments in pdf methods. In: Libby P, Williams F, editors. Turbulent reacting flows. London: Academic Press; 1994. p. 375 –474.
Combustion of residual steel gases: laminar flame analysis and turbulent flamelet modelingq Olivier Gicquela, Luc Vervischb,*, Guillaume Joncquetb, Bernard Labegorrec, Nasser Darabihaa b
a Laboratoire EM2C, Ecole Centrale Paris, UPR 288 CNRS, Chatenay Malabry, France INSA de Rouen Campus du Madrillet, UMR-CNRS-6614-CORIA, Avenue de l’Universite´, BP 8, 76801 Saint Etienne du Rouvray Cedex, France c CRCD, Air Liquide, Jouy-en-Josas, France
Received 28 March 2002; revised 23 May 2002; accepted 24 May 2002; available online 29 October 2002
Abstract In recovery combustion systems operating in the steel industry, energy is provided by boilers burning residual gases of blast furnace and coke oven. To help understand combustion of this particular type of fuels, a numerical study is conducted where the major chemical properties of steel gas flames are collected. The chemical composition of representative fuel and oxidizer steel gas is varied over a large range in calculations using detailed chemistry and complex transport properties. The chemical equilibrium compositions, premixed flame speeds and diffusion flame extinction strain rates are determined. The advantages and shortcomings of the use of vitiated air emerge, and its introduction into the boiler appears as an interesting alternative to reduce NOx emission. The detailed information obtained with laminar flame calculations is also introduced in flamelet turbulent combustion modeling. Reynolds Averaged Navier Stokes (RANS) simulations of a test case burner are performed and some comparisons between numerical predictions and experimental results are presented. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Reynolds Averaged Navier Stokes; Laminar flame; Diffusion flamelet modeling
1. Introduction Novel technologies for energy production are under development, as cogeneration or integrated gasification combined cycle. These systems are proposed in the steel industry, where the energy of residual gases issued from a blast furnace, or coke oven, is recovered in boilers for electricity production [1,2]. The requested performance of steel gas burners is increasing, specifically, their capacity to burn various fuels. This leads manufacturers to redesign existing burners, for which the constraints are to comply with present and future emission regulations. It is also sometimes requested to enlarge the range of operating conditions. These objectives should be reached by modern recovery combustion systems, but preserving flame attachment at the burner and avoiding undesirable combustion instabilities. This requires a precise knowledge of the behavior of the reaction zones observed in steel gas combustion. * Corresponding author. E-mail address: [email protected] (L. Vervisch). q Published first on the web via Fuelfirst.com –http://www.fuelfirst.com
The composition of steel gas is complex and not always perfectly known in practice. Fuel and oxidizer may vary strongly; the fuel may be composed of pure BFG, pure coke oven gas, or may result from the mixing of these two fuels. The oxidizer is either essentially composed of pure air, or, air that has been more or less mixed with gas turbine exhaust gas. A set of representative chemical compositions of steel gas was determined by probing a plant. Once this information is available, the exact characterization of the flames that could exist in such boilers motivates basic questions: What are the chemical equilibrium conditions for these gases? What are the orders of magnitude of the burning rate and of the deflagration velocity for such mixtures? What is the amplitude of the local velocity shear that may prevent the reaction zone developing? These questions are addressed in the first part of the study, where laminar flame calculations are performed to collect the relevant information. A full characterization of the two idealized combustion modes usually considered, premixed and nonpremixed, is then obtained for various representative compositions of the steel gas.
0016-2361/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 3 8 4 - 8
984
O. Gicquel et al. / Fuel 82 (2003) 983–991
In a second step, nonpremixed turbulent combustion modeling of steel gas is addressed. A large variety of turbulent combustion modeling techniques is now available [3 –6] and the numerical simulation of turbulent flames may be of great help in understanding such devices. The ultimate objective is the development of a numerical tool that will be useful in the design of steel gas burners. Diffusion flamelet turbulent combustion modeling is retained; the result of the previous flame analysis is a large chemical database that is easily introduced into such a combustion modeling approach. The numerical closure accounts for the possible mixing and dilution of different fuels issuing from various inlets. The modeling is introduced into CFD commercial software and a burner test case is computed. In the subsequent sections, the chemical model is first presented. After studying the equilibrium conditions, the behavior of prototype laminar flames is reported for several synthetic steel gases. An approximate formulation is finally discussed to incorporate this information into Reynolds Averaged Navier Stokes (RANS) calculations of steel gas boilers.
2. Chemical model and flame properties 2.1. Introduction The composition of the mixture entering the boiler includes blast furnace gas (BFG) more or less mixed with coke oven exhaust gas (COG). These fuels will be represented by a combination of simple gases, such as CH4, CO and other usual molecules found in hydrocarbon flames. Data have been collected in an industrial plant, the related representative composition of steel gas is given in Table 1. In principle, the burner can be fed by any combinations of BFG and COG. The gas composition may thus vary from 100% of BFG up to 100% of COG. In the case of pure COG, the evolution of the mixture is mainly governed by hydrogen chemistry, whose lower calorific value is of the order of 130,000 kJ kg21. Pure BFG is mostly characterized by CO chemistry, featuring a much lower calorific value, of the order of 10,700 kJ kg21. When BFG and COG are separately injected in the boiler, the local composition of the fuel resulting from turbulent mixing will Table 1 Composition (% in volume) of typical coke oven and blast furnace plant exhaust gas at 300 K
CO H2 CH4 H2 O N2 CO2
Coke oven (COG) (%)
Blast furnace (BFG) (%)
5.6 62 28.8 0 4.1 1.5
22.2 2.2 0 2.4 51 22.2
thus strongly affect the amount of heat released by combustion. This BFG/COG mixture is introduced in burners where the oxidizer is provided in two different ways; either pure air or gas turbine exhaust gas (vitiated air) may act as oxidizer. The influence of the fraction of vitiated air in the oxidizer is analyzed in the next sections; the retained air compositions are summarized in Table 2. Various calculations are reported below. Major properties of premixed and laminar flames are investigated. Equilibrium compositions, laminar premixed flame speeds and extinction strain rates of counterflow diffusion flames are determined. This is done by varying fuel and oxidizer compositions in calculations involving fully detailed chemistry and complex transport properties. Complex chemical schemes are available in the literature to reproduce major trends of flames for mixtures including H2, CH4 and CO, which are the main components of the gases under study. They capture, with a reasonable level of accuracy, auto-ignition delays [7], along with premixed laminar flame speeds and diffusion flame extinction strain rates [8]. Among the various existing schemes, the chemical mechanisms proposed by Lindstedt [9] has been retained. It includes 49 species and a detailed mechanism for nitrogen chemistry that accounts for thermal and prompt NO formation. This detailed chemistry has been validated carefully in other circumstances [10]. 2.2. Thermodynamic equilibrium composition The chemical equilibrium compositions and the related temperatures are first determined. The adiabatic maximum temperature in the boiler and the composition leading to the lower production of NO are then readily obtained. The calculations are performed with EQUILI, provided in the CHEMKIN II package [11] developed by SANDIA National Laboratory. The software returns the thermodynamic equilibrium composition and temperature for any gas mixture. (Notice that only thermodynamic considerations are necessary to collect these equilibrium conditions; chemical reaction schemes are not yet introduced at this stage.) The local composition of the gas varies according to three parameters: the fuel composition, the oxidizer composition and the amount of fuel mixed with oxidizer. All species and thermodynamic variables are accessible. The temperature, the concentration of O2 and of NO are particularly discussed when the oxidizer is composed of pure air at 300 K (case a), hot vitiated air at 823 K (case b) Table 2 Composition of pure, vitiated and mixed air used as oxidizer Case
(a)
(b)
(c)
Composition Temperature (K) O2 (%) N2 (%)
Pure air 300 23 77
Vitiated air 823 12 88
Mixed air 50% (a) þ 50% (b) 570 17.5 82.5
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Fig. 1. Thermodynamic equilibrium temperature in Kelvin. X: volume fraction (%) of air mixed with fuel. Y: volume fraction (%) of BFG in the fuel. (a) Pure air; (b) vitiated air; (c) pure air mixed with vitiated air (Table 2).
and mixed air at 570 K including 50% of pure cold air and 50% of vitiated hot air (case c) (Table 2). The highest equilibrium temperature is observed in the case of high proportions of COG. It is of the order of 2200 K (Fig. 1). Lower temperature levels are found with BFG, of the order of 1500 K. The temperature distribution is thus a direct function of the fraction of BFG in the fuel. A basic question arises in steel gas combustion, which is related to the eventual and undesirable existence of multiple zones of high temperature in recovery boilers, which could be promoted by the interaction between the two types of fuel interaction that would lead to an inhibition of the chemistry. Fig. 1 shows that only one temperature maximum is observed in the composition domain. Hence, when the temperature peaks at more than one location in a practical system, this can hardly be attributed to the complex chemical composition of the fuel. This may simply result from the turbulent mixing properties of the boiler. Fig. 1 shows that the temperature decreases when the fraction of vitiated air in the oxidizer is enhanced.
The temperature is of the order of 1950 K for pure COG and is limited to 1250 K for pure BFG. Because of the high dilution with 88% of nitrogen, vitiated air cannot contribute to very high combustion temperature, even when it is preheated (850 K). The knowledge of the equivalence ratio of the mixture and of the corresponding stoichiometric condition is needed to understand the combustion regime and the average flame location in a boiler. Both fuels involve species with O atoms and may react as oxidizer; it is then not easy to formally determine the stoichiometric isoline. However, this problem can be overcome using the value of O2. The stoichiometric line is a priori defined from the equilibrium calculations, as the border of the zone where the mass fraction of O2 vanishes. Equilibrium O2 mass fractions are presented in Fig. 2. The stoichiometric isoline is located in the zone where the fraction of air is high, a point that is amplified by the use of vitiated air. Since the mean turbulent flame zone is likely to develop in the vicinity of the stoichiometric surface of the mixture, adding vitiated air in practical
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Fig. 2. O2 mass fraction. X: volume fraction (%) of air mixed with fuel. Y: volume fraction (%) of BFG in the fuel. (a) Pure air; (b) vitiated air (Table 2).
systems will move the mean flame zone towards the air injection area. Equilibrium calculations also provide an approximation of the maximum level of pollutants that could be generated for any composition of the mixture. The maximum NO emission is of the order of 472 ppm, obtained with pure air (Fig. 3). When vitiated air is chosen, the maximum NO level is decreased to 175 ppm (Fig. 3). NO production is very sensitive to the temperature drop observed with vitiated air, explaining the lower levels of NO. As a preliminary conclusion, the introduction of vitiated air may be an interesting candidate to help in controlling flame position and NO emission. 2.3. Premixed laminar flame speed The properties of laminar deflagration are obtained for various steel gas composition and are analyzed in this section. The one-dimensional code PREMIX [12] developed by SANDIA is retained to calculate laminar premixed
flames. Simulations were performed for fuels ranging from 20 up to 80% of BFG. For each of the fuel compositions, the lean and rich flammability limits are tracked together with the corresponding flame speed. The whole flammability domain is then determined. Fig. 4 displays the results obtained for the compositions given in Table 2. For all cases, flammability domains are limited to a somewhat reduced area of the mixture composition. The laminar flame speed is proportional to the fraction of hot vitiated air contained in the oxidizer. The maximum values are of the order of 75 cm/s for pure air (case a), 117 cm/s with an oxidizer composed of 50% of pure air and 50% of vitiated air (case c), and, 157 cm/s for pure vitiated air (case b). Because the turbulent burning rate is somehow related to the flame speed [13], this is a result of practical interest. It suggests that situations exist where vitiated air is much more efficient than pure air. The maximum value of the laminar flame speed is extracted from the calculations to refine the analysis; Fig. 5 shows these maxima for the three oxidizer compositions of Table 2. The maximum laminar
Fig. 3. NO mass fraction. X: volume fraction (%) of air mixed with fuel. Y: volume fraction (%) of BFG in the fuel. (a) Pure air; (b) vitiated air (Table. 2).
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Fig. 4. Flame speed. X: volume fraction (%) of air mixed with fuel. Y: volume fraction (%) of BFG in the fuel. (a) Pure air; (b) vitiated air; (c) pure air mixed with vitiated air (Table. 2).
flame speed is reached for mixture close to their stoichiometric condition. This graph confirms that the laminar flame speed increases with the fraction of vitiated air. Part of this increase is simply the result of the much higher temperature of the vitiated air that enhances the laminar flame speed. Flame stabilization is usually achieved through the propagation of partially premixed reacting fronts. The existence of greater flame speeds mean that reaction zones may be expected closer to the burner lips with vitiated air than with pure air. In nonpremixed turbulent combustion, because of the fluctuations of the mixture induced by unsteady turbulent mixing, the fraction of BFG in the fuel may rapidly evolve over a wide range. Accordingly, important variations of the laminar flame speed may exist. For pure air, the velocity ranges between 26 and 76 cm/s, while for vitiated air, the velocity evolves between 64 and 157 cm/s. These variations may promote the development of combustion instabilities within the boiler, instabilities that are triggered by the unsteady rolling up of the shear layers. Therefore,
even though vitiated air may help in flame stabilization, the reaction zones will be much more sensitive to variations of composition with vitiated air than with pure air. The basic response of a flame to an acoustic excitation, or to turbulent velocity and pressure fluctuations, depends on its characteristic time and length scales [14]. The flame thickness is determined in the simulations as the distance between the two points associated to a heat release rate equal to half of its maximum value. The characteristic flame time is estimated as the ratio between flame thickness and flame speed. Results are presented in Fig. 5. The flame thickness increases with the fraction of vitiated air in the oxidizer. The range of time scales is found to be larger for pure air than for vitiated air, moreover these ranges are shifted in time. The control of flame stability is a real challenge. Nevertheless, it may be interesting to take advantage of the very large range of local burning rates and characteristic scales of steel gas combustion to control flame stability. In other words, it may be interesting to adjust the oxidizer composition with vitiated air to insure combustion stability.
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Fig. 5. Laminar premixed flame properties. X: volume fraction (%) of BFG in the fuel. (i) Laminar flame speed (cm/s); (ii) flame thickness (cm); (iii) characteristic flame time (s). Solid: case (a) pure air. Dashdot: case (b) vitiated air. Dashed: case (c) (Table. 2).
2.4. Diffusion flame response to strain rate The response of steady laminar counter-flow diffusion flames to strain rate is helpful to calibrate the robustness of a given nonpremixed reaction zone [4]. Results are presented for different fractions of BFG in the fuel, for both pure and hot vitiated air (Fig. 6). The fuel composition has a strong effect on the flame extinction limits. Flames are much more robust using COG than BFG as fuel. The extinction limits for COG are 3412 s21 with pure air and 4198 s21 using vitiated air, while for BFG, they are of the order of 36 s21 with pure air and 92 s21 using vitiated air. The local reaction zone is, therefore, more likely to be quenched when it is fed with BFG. The introduction of vitiated air increases the strength of the flame, which is an additional factor indicating that vitiated air may be helpful to stabilize a turbulent flame base in a boiler. As computations of laminar diffusion flames have been performed in a continuous manner from a weakly strained reaction zone up to full quenching, a detailed flamelet
database is available. These flamelets were obtained for a wide range of fuel composition. This provides a three entrances flamelet look-up table, whose control parameters are: the local equivalence ratio in the diffusion flame, expressed with a mixture fraction Z (a conserved scalar equals to unity in the fuel stream and that vanishes in the oxidizer stream) [14], a the ratio between COG and BFG in the fuel, and, a the rate of strain imposed to the laminar diffusion flamelet. These tables were generated for both pure air and vitiated air; all the species mass fraction Yi ða; a; ZÞ and temperature Tða; a; ZÞ are then available.
3. RANS calculations with diffusion flamelets 3.1. Introduction It is intended in this section to conduct RANS calculations of an industrial burner operating with steel gas. An IFRF (International Flame Research Foundation)
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Fig. 6. Temperature (Kelvin) response versus strain rate (s21). Solid: 0% of BFG in the fuel. Dashed: 25%. Dashdot: 50%. Dotted: 75%. Longdash: 100%. (a) Pure air; (b) vitiated air (Table. 2).
experiment is retained as a test case [15]. The objective is to perform computations for estimating the eventual prediction capabilities of numerical modeling. To our knowledge, because of the complex composition of the fuel, RANS calculations of this type of steel gas burner have not been reported so far. The mean temperature distribution in the turbulent flow is approximated by coupling the available chemical look-up table with CFD. The simulations of strained laminar diffusion flames discussed earlier are perfectly suited to become an ingredient of flamelet modeling. This approach is thus adopted to account for the effect of the unresolved turbulent fluctuations on the strongly nonlinear behavior of the chemistry. 3.2. Flamelet modeling of steel gas Steady diffusion flamelet modeling is based on an asymptotic view of nonpremixed turbulent combustion [4]. For a given state of mixing in the turbulent flow, flamelet models are derived assuming that the local balance between diffusion and reaction is similar to the one found in a prototype laminar flame. The control parameters of the flamelet are Z, the mixture fraction, and a, the strain rate. In steady flamelets, it is implicitly supposed that the characteristic time required to reach a chemical steady state in nonpremixed diffusive-reactive layers is smaller than any other turbulent flow times and this is a strong hypothesis. The use of the steady flamelet model may then be viewed as an improvement of the infinitely fast chemistry hypothesis, as a first step to incorporate some finite rate chemistry effects in the modeling. In the chemical table, the strain rate a is related to the scalar dissipation rate xs ¼ Dl7Zl2s [4], where l7Zl is measured under stoichiometric condition, D is a diffusion coefficient. The local fluctuations in fuel-oxidizer mixing are ~ p Þ; the probability density accounted for via PðZ function of the mixture fraction that captures the statistical
properties of turbulent mixing. Mass weighted averaging is ~ p Þ from retained. A beta-pdf is used for approximating PðZ the solution of the modeled balanced equations for Z and 00 2; Zg 2 the mean and fluctuations of the mixture fraction [16]. The conditional averaged value of the scalar dissipation rate under stoichiometric conditions, x~s ; is approximated in the RANS calculations from the mean scalar dissipation rate x~ and the distribution of laminar Ð x in a strained ~ p ÞdZ p : A linear diffusive layer, as x~s ¼ x~FðZs Þ 10 FðZ p ÞPðZ relaxation model is chosen to approximate x~ [16], x~ ¼ 00 2=ðk=1Þ; Cx Zg 2 where Cx is a constant equals to 1.2 in the calculations, k is the energy of the turbulent motion and 1 its dissipation rate. The function FðZÞ ¼ expð22½ erfc21 ð2ZÞ2 Þ provides the scalar dissipation rate distribution in the counter-flow diffusion flamelet, xðZÞ ¼ x0 FðZÞ; where x0 ¼ a=p: Further detail concerning this modeling procedure are available in Ref. [4]. The fuel composition being fixed, a is known and the profiles Yi ða; x~s ; ZÞ and Tða; x~s ; ZÞ are retained in the ~ pÞ : diffusion flamelet table for averaging with PðZ ð1 ~ p ; xÞdZ p ð1Þ Y~ i ðxÞ ¼ Yi ðaðxÞ; x~s ðxÞ; Z p ÞPðZ 0
~ ¼ TðxÞ
ð1 0
~ p ; xÞdZ p TðaðxÞ; x~s ðxÞ; Z p ÞPðZ
ð2Þ
The RANS quantities Y~ i and T˜ are then available to characterize the turbulent flame. To complete this study, the turbulent combustion model was introduced into the commercial software FLUENT1 via ‘user define functions’. 3.3. Results A IFRF burner is calculated [15]. It is a three inlets nonpremixed combustion system using a low calorific value 1
http://www.fluent.com
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O. Gicquel et al. / Fuel 82 (2003) 983–991
Fig. 7. (i) Picture of the flame; (ii) temperature profiles on the centerline of the burner (Kelvin). Dotted line: RANS calculations. Square with error bar: measurements; (iii) iso-mean temperature (RANS calculations) (Kelvin).
fuel to mimic steel gas, it operates with a ¼ 0:986: The burner is composed of co-flowing jets: a central jet of primary air diameter ðfAir1 ¼ 70 mm; mean velocity VAir1 ¼ 3 m=sÞ is surrounded by a fuel jet ðfFuel ¼ 133 mm; VFuel ¼ 24:2 m=sÞ and an annular secondary air jet ðfAir2 ¼ 159 mm; VAir2 ¼ 4 m=sÞ: The mean power of the system is of the order of 1 MW. The central air jet impacts on a flame holder to enter the burner perpendicularly to the main stream. The flame holder leads to a complex flow topology that is necessary for flame stabilization. Recirculation zones appear and the stoichiometric line meets the centerline close to the burner exit. A first flame featuring a small length is then observed in the vicinity of the flame holder, this flame consumes most of the primary air of the central jet and acts as a pilot flame (Fig. 7). The burnt gases of the first combustion zone contribute to the ignition of the external mixing layer that develops between the fuel and the secondary jet. Hence, a second combustion zone is observed further downstream. This decomposition of combustion in two separated main reaction zones leads to two peaks of temperature on the burner axis. Detail concerning the burner properties and measurements may be found in Ref. [15].
The expected flame topology is reproduced by the RANS calculations, with a short internal flame followed by a much wider external combustion zone. Temperature measurements have been performed on the centerline with a suction pyrometer. Unfortunately, measurements are not available in the first and short internal flame. A first peak of temperature is observed in the calculations (Fig. 7). Outside of this short flame, the amount of heat released decreases in the computation. As expected, further downstream, the temperature increases again in a second flame zone. This increase of temperature, found at locations where measurements are available, is reasonably captured by the calculations (Fig. 7).
4. Conclusion Steel gas combustion has been investigated. Responses of laminar premixed and diffusion flames to various parameters have been collected. The premixed deflagration speed, the extinction strain rate of diffusion flamelets and, more generally, the major consequences of using hot vitiated air as oxidizer have been determined. The exact impact of
O. Gicquel et al. / Fuel 82 (2003) 983–991
the gases composition on burning rate, NO pollution and possible flame attachment have also been discussed. The information provided by the detailed chemistry was included into RANS turbulent combustion modeling. A burner test case has been computed to assess the prediction capability of the numerical procedure. Previous experimental investigations of this burner have shown a complex flame structure, whose overall properties are reproduced by the numerical results. Acknowledgements The authors have benefited from fruitful discussions with Drs Franc¸ois Lacas and Denis Veynante. References [1] Luessen HP. Gas turbine technology for steel mill gas and syngas applications. International Gas Turbine and Aeroengine Congress and Exhibition, Orlando, Florida; June 2–5, 1997. [2] Edthofer H, Schmidt G. Energy supply and energy conservation at voest-alpine stahl linz works. Seminar on Economic Aspects of Clean Technologies, Energy Waste Management in Steel Industry; 1998.
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[3] Bray KNC. Proc Combust Inst 1996;26:1– 26. [4] Peters N. Turbulent combustion. Cambridge: Cambridge University Press; 2000. [5] Vervisch L. Proc Combust Inst 2000;28:11–24. [6] Veynante D, Vervisch L. Prog Energy Combust Sci 2002;28: 193 –266. [7] Gray P, Griffiths J, Scott K. Proc Combust Inst 1984;20:1809– 15. [8] Vagelopoulos CM, Egolfopoulos F. Proc Combust Inst 1994;25: 1317– 23. [9] Lindstedt P. Proc Combust Inst 1998;27:269– 85. [10] Gicquel O. De´veloppement et Validation d’une Nouvelle Technique de Re´duction de Sche´mas Cine´tiques, Application au Me´thane. PhD Thesis. Ecole Centrale Paris; 1999. [11] Kee RJ, Rupley FM, Miller JA. Chemkin-II: a fortran chemical kinetics package for the analysis of gas phase chemical kinetics. Technical report. Sandia National Laboratories; 1989. [12] Kee RJ, Grcar JF, Smooke MD, Miller JA. A fortran program for modeling steady laminar one-dimensional premixed flames. Technical report. Sandia National Laboratories; 1992. [13] Vervisch L, Veynante D. Proc Combust Inst 2000;28:175– 83. [14] Poinsot T, Veynante D. Theoretical and numerical combustion: R.T. Edwards; Philadelphia, USA; 2001. [15] Study on the burner design characteristics required for the combustion of variable quality lean gases. Technical report. IFRF; 1984. [16] Dopazo C. Recent developments in pdf methods. In: Libby P, Williams F, editors. Turbulent reacting flows. London: Academic Press; 1994. p. 375 –474.