EXPERIMENTAL AND NUMERICAL SIMULATION ON NO x EMISSION IN LIQUID FUEL SPRAY FLAMES

Transcription

1 MCS 7 Chia Laguna, Cagliari, Sardinia, Italy, Setember 11-15, 2011 EXPERIMENTAL AND NUMERICAL SIMULATION ON x EMISSION IN LIQUID FUEL SPRAY FLAMES Vahid Nejati, Yasamin Khazraii*, Keyvan Daneshvar and Shirin Khazraii** vnejati@yahoo.com *Assistant Prof., Islamic Azad University, Mashhad Branch, Khorasan Razavi, Iran; Islamic Azad University, Bandar Lenge Branch, Hormozgan, Iran **Islamic Azad University, Shahrood Branch, semnan, Iran; Ferdowsi University of Mashhad, Khorasan Razavi, Iran Abstract In this aer roduction and emission of x ollutant in a cylindrical furnace liquid fuel, for various angles and atterns of fuel sray was studied through the exerimental and numerical method. Pollutants measurement has been done for fuel sray angles 45, 60 and 80, sray attern of hollow and solid cone. Numerical simulation of two hase flow and combustion modelling for ollutants formation are done with Fluent6.32 software. Comarison of numerical and exerimental results shows very good agreement. The results show that by increasing in sray angle x emission increases. When the sray angle increases, the diameter of exhausted fuel articles from nozzle declines, which as a result the contact between fuel and air rises and also the air-fuel mixing increases. Therefore with increasing in the mentioned sray angle, a more erfect combustion haens and the maximum flame temerature increases. Also, the concentration of x which is affected by maximum temerature increased. Introduction Combustion rocesses, usually lead to roduction of ollutants to the environment. In otimization of these rocesses not only the combustion efficiency and reduction fuel consumtion, but also the environment and reducing certain ollutants should be considered. For this urose, extensive researches have been done to save energy and achieve high efficiency and control and reduce ollution caused by combustion. A significant roortion of major air ollutants are roduced from the combustion of liquid fuel. Liquid fuel is widely used in the boilers, industrial furnaces and gas turbine combustion chambers. Sulfur oxide, carbon monoxide, soot and nitrogen oxides are most imortant contaminants which sread in the liquid fuel flames. Produced ollution by nitrogen oxides has remained harmful effects on human health and the environment and also lays an imortant role in the formation of acid rain, chemical smog and ozone layer hole. Laboratory measurements lay an imortant role in engineering researches and because of the comlex combustion rocess, numerical techniques and mathematical modeling of combustion rocess still is not fully reliable and laboratory results are needed. In the other hand because of secial exerimental condition and sensitive measurement devices, the laboratory results should be verified. Liquid fuels are categorized to light and heavy fuels deending on the amount of carbon. Liquid fuel burners are usually non-remixed and the fuel and oxidizer are mixed in the combustion chamber. Gas fuels are widely categorized as remixed or non-remixed. In remixed burner smaller and stronger flame is roduced, in comarison with the nonremixed tye. Flame adiabatic temerature is higher and radiation heat transfer and also the rate of the formation of nitrogen oxides are increased. But in non-remixed tye, flame is taller, the temerature at the center of flame and x roduction are reduced [1]. Although the chemical rocesses of the both remixed and non-remixed combustion are the same but

2 additional hysical rocesses regarding to the non-remixed combustion (evaoration and mixing) are needed. For examle, overall mixing can be stoichiometric, but there are some areas in combustion chamber with large or low amount of fuel. This asect of the nonremixed combustion can cause to more comlex formation of ollutants [2]. Liquid fuel combustion is controlled by the known arameters such as atomization of the jet liquid fuel, drolets fuel sray evaorating, mixing of fuel and oxidizer. Seed, angle and attern of fuel sray can be controlled by the burner nozzle. Very extensive researches have been done for the effect of the nozzle design, working conditions and fuel roerties on the combustion characteristics and on sraying the hydrocarbon fuels [3-5]. Most conducted research has focused on the nozzles used in the combustion gas turbines and boilers. In gas turbine combustion, getting the high thermal efficiency and very small emissions of ollutants are very imortant. By increasing the turbine inlet temeratures it can reach to higher efficiency and combustion chamber design should be imroved for lower combustion ollutants. In liquid fuel combustion in the gas turbine, there is the roblem of the formation of soot and x emissions and hysical henomena such as fuel atomization or evaoration, drolets disersion, structure of sray and mixing with air, have widely effected on combustion. In the ast, research was often erformed based on techniques and methods, to imrove combustion efficiency and quality of atomization. Imrovement of the burner efficiency requires the study of sray nozzle characteristics. Recently, most research is focused to review and to study sray characteristics such as length of segregation, sray angle and drolet size and distribution in the burner nozzle [6-7]. The urose of this research is to study the roduction and emission of air ollutants, which roduced from liquid fuel burner and to consider fuel sray angle effects on the emissions. In this study, two different methods as laboratory measurement and numerical simulations are used. To calculate the amount of x ollutant, two mechanisms as thermal and fuel are emloyed and the obtained results are comared in two methods. Case study and geometry of liquid fuel furnace Laboratory furnace which is used in this study, includes a horizontal cylindrical with 1800 mm length and ratio of L/D= Inner surface of furnace is comletely smooth to revent resistance of hot gases movement. Furnace body is made from AISI316 steel to resist high temeratures. Around the furnace is covered by high temerature thermal insulation with the roer thickness so that the heat transfer from the furnace body can be almost considered zero. For measuring the temerature and combustion gas concentration in different arts of the furnace including the body hull and the chimney, some laces are embedded with the ca to get samle when it is necessary. Schematic exerimental system is shown in Figure 1. Three commercial nozzle sray angles of 45, 60 and 80 degrees with two sray atterns as hollow cone sray and solid cone sray are selected for testing. Furnace burner with maximum ower of 240 W is ressure tye and fuel inlet ressure to the nozzle equals to 1.5 MPa. Fuel and air flow rate entering to the combustion chamber are adjustable by the fuel um and air throttle valve. Mixing fuel and air is rovided with intake air swirl. Fuel is injected into the furnace through the nozzle. Burner characteristic are resented in Table 1. Tests are done for the diesel fuel with inut temerature 40 o C and viscosity is centistokes. Other fuel rofile is given in Table 2. Laboratory furnace is designed in such a way that the fuel sray nozzle can be easily relaced. Since the surface roughness of

3 comonents inside nozzle is effective to atomize the fuel articles, those nozzles with available calibration and verification evidence for their erformance are used. All measurements are erformed after the furnace temerature reaches steady state because the temerature changes can cause to alter the fuel concentration and viscosity and these factors lay an imortant role on size of fuel drolets. During the tests the inlet temeratures of air and fuel are under control and are ket constant. A samling device has been located within 160 cm from the furnace vent in order to analyze exhaust gas in the chimney. Flow of roduced combustion gases such as CO 2, CO, x, and combustion efficiency are measured every 5 minutes by using a gas analyzer system (Testo350XL). Figure 1. Schematic of exerimental furnace for liquid fuel. Table 1. Features of burner. Power source Motor Oil um tye Pum ressure Nozzle fuel rate Sray angle AC 220V / Hz 240 W Gear um 1.5 Ma gal/hr 45, 60, 80 degree

4 Table 2. Proerties of gasoline at a temerature of 295 K. Density C Vaorization Temerature Boiling Point N 2 Percentage 830 kg/m J/Kg*K 373 *K K 462 K 0.09 % Governing equations Starting oint for comuting the flows without combustion is to solve Navier Stocks equations. For the flows including with the heat transfer, the energy equation is added. Comleting of these conservative equations is equation of state and fluid roerties characteristics. Combustion flows are included of the release of thermal energy because of conversion of chemical secies. In this kind of combustion flows, the combustion rocess modeling and adding to the Navier Stocks equations are needed. Two-hase flow Modeling In many ractical combustion rocesses, fuel is solid or liquid and during the combustion, firstly the fuel is converted to the gas hase and then is burned by oxidation gas. Liquid fuel combustion is usually done by injecting liquid fuel into the gas hase combustion environment. Turbulence within the liquid flow is created inside the injector. This turbulent fluid flow gets out of the nozzle in the form of comlex mix of strings and then crushes into small dros and aears as dense clouds of drolets that launches through the gas into the flame zone. Heat transfer to dro, increases the vaor ressure and therefore the fuel evaorates into the gas, so burning of gas hase is began. Non-remixed flame surrounds the grou dros or drolets and eventually fuel vaor burns. This set of rocesses is called combustion sray. By assuming that sherical articles of liquid fuel is disersed in the gas hase and their hits due to their quick evaoration is negligible, in the Lagrangian System, continuity and energy conservation equations governing to the articles, are written as follows: dt dt dd dt 6k C 2d b Re 1/ 2 2 1/ Re 1/ Pr T TP 2 d C P P, P (1) (2) In equation (1), d is dro diameter Cb is evaoration constant which is function of hysical roerties of the environment and fuel that is determined as follows: C b 8k C l ln 1 C / L T T P (3)

5 In this equation k and C are thermal conductivity coefficients, and the secific heat in constant ressure for gas mixtures, resectively. T, T and L are gas temerature, the dro temerature and the latent heat of vaorization of fuel resectively. By using equation (1), the article diameter change rate, after reaching to the boiling temerature, and from equation (2) the temerature change rate of the fuel articles can be calculated [8]. Combustion modeling In this study chemical formula for gasoline is assumed to be C 16 H 29 and one ste Magnussen- Hiertager model with eddy dissiation is used for combustion modeling [9]. In this simulation firstly, the liquid fuel converts into a gas tye and then combustion is erformed. Evaoration of fuel articles is started when the articles temerature reaches to the boiling temerature, and will be continued until all their mass becomes finish. Gas fuel is done in one ste and according to the selected Magnussen-Hiertager combustion model; the effective factor to the reaction rate is the flow dynamics. Chemical reaction between oxygen and gasoline is written as follows: C16H O2 16CO H2O. (4) Nitrogen oxide modeling To estimate, the solution of the mass transort equation (Y ) is required. This equation is solved after determining the main flow field and their main secies. Mass transort equation for is written as follows, which includes influence convection term, roduction and consumtion of. Y Y Y ui D S t xi x i x (5) i Where source term (S) in this equation is calculated according to three forms mechanism : thermal, romt and fuel. Since in liquid fuels combustion the amount of romt comared with the other two tyes are almost neglected, in this study only thermal and fuel are calculated. Thermal, which is formed at high temeratures by nitrogen oxidation contained in combustion air, is exressed using develoed Zeldovich mechanisms [10]. k 1 O N2 N (6) k2 N O2 O (7) k3 N OH H (8) Where k + and k - are reaction forward and backward Constants. Assuming that consumtion rate of free nitrogen atoms is equal to its roduction rate; concentration is obtained from following equation:

6 2 k 1k2 1 d[ ] k 1 N2 k2 O2 2k O N (9) 1 2 dt k 1 1 k 2 O2 k3 OH Constants of value of equilibrium reaction rate in the above equations are obtained in the reference [11]. Considering that the formation rate of is much lower than the original hydrocarbon oxidation rates, more thermal is formed after comleting combustion. In above equation, concentrations of N 2 and O 2 are determined by combustion calculation and the radical concentration for [O] and [OH] is obtained from the following relations [12]. / 2 O 36.64T O 1/ 2 ex (10) 2 T (11) 2 T.57 2 OH 212.9T ex O 1/ H O 1/ 2 Therefore some source term in the art of the equation (5) is calculated from Zeldovich mechanism as follows: S d, th M (12) dt In which M is molecular mass of gas. roduction fuel is a very comlex henomenon and it is strongly deendent to the flame stoichiometric, local combustion characteristics and initial concentration of the nitrogen comounds. Because of heating and evaoration of fuel drolets and nitrogen-containing radicals such as HCN, CN and NH, nitrogen-containing comounds are decomosed, which can be converted to x. Considering that nitrogen cyanide (HCN) is redominant radicals, acceted mechanism for the formation of fuel is include of the formation of HCN from nitrogen in the fuel, and then erformance of two oxidation reactions to get and combination with some art of and formation of N 2 [13]. O HCN 2 (13) HCN (14) N 2 Reaction rates of these two related based on measurements in De Soete work [13] are exressed as follows: a E1 R1 A1 X HCN X O2 ex (15) RT E2 R2 A2 X HCN X ex (16) RT

7 In the above equations, X is mole fraction and 10, 12, and. A / s A2 310 E cal / mol E Considering that is roduced in the reaction (13), and is used in reaction (14), source term from fuel in equation (5) is obtained from the following equation: S M R1 R. (17) RT 2 Numerical calculation Dimension of comutational mesh, is very effective on accuracy of results in the numerical calculations and on erformance time. Although by increasing the number of nodes in mesh generation and comutational field, the accuracy of results is increased, but much more time and memory are needed. In this study, the area near the entrance and walls that the flow roerties changes are higher, smaller meshes are selected. Comuting meshes in the combustion chamber is shown in Figure 2. Results have shown that alying the smaller meshes than 150 * 400 cells cannot make results better and this matter roved the meshindeendent solution. for discretization of convection terms in the governing equations, first order uwind scheme, and to correct the ressure field SIMPLE algorithm are used. For convergence, under relaxation factor of 0.7 for the flow field and 0.9 for the chemical secies including ollutants are used. The convergence criterion of equations is 1*10-6. Numerical solution method is based on finite volume and steady state condition. Boundary conditions Due to axisymmetric flow, half of the combustion chamber in cylindrical coordinates in the form of two-dimensional is solved. Nozzle inlet diameter of liquid fuel, 1 mm and diameter of the fuel drolets, 50 to 100 microns are assumed. Fuel and air inut temerature, 300K and inlet ressure, 1 atm has been considered. Central air inlet velocity is 2.5 m/s and the equivalence ratio is Furnace wall temerature is assumed constant and equals to 750 K. Figure 2. Comutational mesh in solution domain. Results and discussion The flow streamline attern inside the furnace, for the resent numerical solution is shown in Figure 3. As shown in Figure 3, the vortex flow and backflow are informed near the furnace wall and entrance exansion area. Figure 3. Stream lines inside the combustion chamber.

8 Temrature[K]]].] Temerature contours in the combustion chamber for different fuel sray angles of 45 o, 60 o and 80 o degree is shown in Figure 4. Increasing the fuel sray angle, the maximum temerature is increased. At sray angle of 45 o, 60 o, 80 o degree, maximum temerature are increased u to 1442 K, 1466 K, 1592 K resectively. a) Sray Angle: 45 o T max = K b) Sray Angle: 60 o T max = K c) Sray Angle: 80 o T max = K Figure 4. Temerature contours in combustion chamber for different fuel angles. In Figure 5 the effect of the fuel sray angle on the temerature at the central axial of furnace is shown. Results show that with increasing the fuel sray angle, the amount of maximum temerature in the central axial furnace is also increased. By increasing the fuel sray angle, the time of staying articles inside the combustion chamber is increased. Due to extension of flame surface, axial velocity of articles is reduced and therefore the location of occurrence of maximum temerature in the central line of furnace with increasing the angle of fuel sray is closer to the inut area Fuel Sray Angle: 80[deg] Fuel Sray Angle: 60[deg] Fuel Sray Angle:45[deg] Axial Distance from Nozzle[m] Figure 5. Effect of fuel angle on the temerature rofiles on the central axial of furnace.

9 x [m] n In Figure 6 x Contour based on m has been shown for three fuel sray angle 45 o, 60 o and 80 o. It should be note that in the fluent software the outut of x concentration are given based on mass or mole ratio or mole concentration not in terms of m, which is common in the industry. An emirical function is defined and therefore the Fluent software can determine the amount of x, in terms of m instead of mass or mole ratio. a) Sray Angle: 45 o b) Sray Angle: 60 o c) Sray Angle: 80 o Figure 6. x contour [m] at different fuel sray angles inside the combustion chamber. The effect of sray angle on the x emissions on the central axial furnace is shown in Figure 7. The results show that concentration along the axis is firstly increased from the burner to a maximum and then after slightly decreasing, remains constant almost to the end of the furnace. Three sraying angles comarison show that with increasing the angle of sraying, x concentration is increased near the fuel nozzle. Fuel injection at low sray angles causes to longer flame length, thus the location of occurrence of maximum x moves to farther distance in furnace from the inut area Fuel Sray Angle: 45 [deg] 130 Fuel Sray Angle: 60 [deg] 120 Fuel Sray Angle: 80 [deg] Axial Distance from Nozzle [m] Figure 7. Effect of fuel sray angle on the x concentration of in the central axial of furnace.

10 In Figure 8 the laboratory results for the effect of fuel sray angle on and 2 emissions in furnace outut, with two different sray atterns as hollow and solid with equivalence ratio of 0.66 is shown. Increasing the sray angle causes to incensement of emission. Higher concentration is due to increasing the fuel retention time in the furnace and increasing of temerature because of its comlete combustion, and thus creating suitable conditions for the formation of thermal. As shown in Figure 8 with increasing the nozzle sray angle, 2 emissions in the furnace outut is increased. However, 2 concentration comared with concentration is minimal, because aroximately 90 to 95 ercent of roduction of x in combustion rocess is in form of. x rofile strongly deends on the flame temerature. By increasing angle of sraying, the fuel articles diameter in the outut nozzle becomes smaller. This increases the contact surface between fuel and air and also the better mixing between them. Thus increasing the sray angle mines more comlete combustion with higher maximum flame temerature [14]. The results show that ollutant emissions for the hollow cone sray attern are aroximately 10% less than solid cone. Nozzles with the hollow sray attern are used for better fuel atomization, due to the more efficient radial distribution of fuel. In the hollow cone, the concentration of fuel drolets in the outer of sray edge is high, thus there is no fuel, even very low levels, in the center of sray. Solid cone sray attern naturally roduces a very long flame and it is not used in square or cylinder combustion chamber. Hollow cone, under inconsistent conditions, is more stable in sray angle and drolet distribution comared to solid cone. This matter for the fuel with high viscosity is remarkable, which may lead to reduce the effective sray angle and to increase the drolet size [15]. Figure 8. Effect of fuel sray angle on and 2 emissions in two different sray atterns (exerimental results). Effect of sray angle on x emissions at furnace outut in the form of laboratory and numerical comutations are shown in Figure 9. Comarison of comutational and exerimental results shows that tendency of both methods for x is similar, but comutational results are slightly more than exerimental results. High calculated temerature level is one of the reasons for this increasing. According to the thermal mechanism, high calculated temerature level is affected to the amount of simulated. This is more remarkable at the sray angle of 80 o because the calculated temerature comared with angle

11 of 45 is aroximately increased 150K. Other reasons for reducing the amount of x in the furnace outut comared with numerical results is, reducing the temerature due to conduction heat transfer from the furnace wall, radiation heat transfer and back flow of cool gas in the end of furnaces, which is because of rotational combustion gases. Figure 9. Effect of fuel sray angle on x concentration in the furnace outlet, the comarison between numerical and exerimental results. Conclusion Exerimental and comutational results for the ollutant emissions resulting from combustion of liquid fuels are evaluated. Results of laboratory measurements for three different fuel sray angles of 45, 60 and 80 and two sray atterns are resented. Numerical simulations are done using fluent software version Regarding to the existence of nitrogen in liquid fuels, the concentration of gas are calculated using two methods of thermal and fuel mechanisms. Comarison of comutational results with exerimental results shows good agreement. The results show that the amount of emissions is influenced of both sray angle and nozzle sray attern. Increasing the fuel sray angle causes to increase x in furnace outut. Pollutant mission rate in solid cone sray attern, at the equivalence ratio 0.66 is aroximately 10 ercent more than the hollow cone sray attern. Numerical results also show that on the central axial furnace, X gas concentration and temerature are in maximum value and by increasing the fuel sray angle, the maximum amount of temerature and x in the central line are increased and move closer to the burner. References [1] Kermes, V., Belohadsky, P., and Stehlik, P., Testing of Gas and Liquid Burners for Power and Process Industries. Journal of Energy. 33: (2008). [2] Turns, S.R., An Introduction to Combustion: Concets and Alications, McGrawHill Book Co., 2000.

12 [3] Rhim, J.H., and No, S.Y., Break u Length of Conical Emulsion Sheet Discharged by Pressure-Swirl Atomizer, Conference of ILASS, Jaan,( 2000). [4] Tasi, R.F., and Lee, C.f, Sray Unsteadiness in Swirl-Stabilized Flames. Conference of ILASS-Asi, Korea,( 2001). [5] Leroux B., Lacas F., Recourt P., and Delabory, O., Couling between Atomization and Combustion in Liquid Fuel-Oxygen Flames, International Combustion Symosium, Hawaii, (2001). [6] Laryea, G.N., and No, S.Y., Sray Angle and Break u Length of Charged Injected Electrostatic Pressure Swirl Nozzle. Journal of Electrostatics, 60: (2004). [7] Laryea, G.N., and No, S.Y., Sray Characteristics of Electrostatic Pressure-Swirl Nozzle for Burner Alication. KOSCO Symosium, Korea, (2002). [8] Kuo, K.K.Y., Princiles of combustion, John Wiley and Sons, New York, [9] Magnussen, B.F., Hiertager, B.H., Olsen, J.G. and Bhaduri, D., On the Mathematical Modeling of Turbulent Combustion with Secial Emhasis on Soot Formation and Combustion, Seventeenth Symosium (International) on Combustion, : , The Combustion Institute, Pittsburg, PA, (1978). [10] Zeldovich, Y.B., Sadovnikov, P.Y., Oxidation of Nitrogen in Combustion, Science Academy of USSR, (1974). [11] Rake, M.C., Correa, S.M., Pitz, R.W., Shyy, W. and Fenimore, C.P., Suer Equilibrium and Thermal Nitric Oxide Formation in Turbulent Diffusion Flames, Combustion and Flame. 69 : (1987). [12] Saario, A., Rebola, P.J., Heavy Fuel Combustion in a Cylindrical Laboratory Furnace, Fuel. 84: ( 2004). [13] De Soete, G.G., Overall Reaction Rates of and N 2 Formation from Fuel Nitrogen, 15th Symosium (International) on Combustion, : , [14] Bashirnezhad, K., Moghiman, M., Amoli, M.J., and Zabetnia, S., Effect of Fuel Sray Angle on Soot Formation in Turbulent Sray Flames. WASET, 31, ,( 2008). [15] Nasr, G.G., Yule, A.J., and Bendig, L., Industrial Srays and Atomization Design, Analysis and Alication, London, Sringer, 2002.