Experimental Measurement of Laminar Burning Velocity and

Subscriber access provided by GAZI UNIV

Article

Experimental Measurement of Laminar Burning Velocity and Flammability Limits of Landfill Gas at Atmospheric and Elevated Pressures Mohammad Hossein Askari, and Mehdi Ashjaee Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02941 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Experimental Measurement of Laminar Burning Velocity and Flammability Limits of Landfill Gas at Atmospheric and Elevated Pressures Mohammad Hossein Askari∗, Mehdi Ashjaee Mechanical Engineering Department, Tehran University, Tehran, Iran

Abstract Landfill gas (LFG) produced from depletion of biological waste, has the potential to become one of the main energy resources in the future. In this study, laminar burning velocity (ul), Markstein length and flammability limits of different compositions of Landfill gas (LFG) is measured using Schlieren flame front visualization method in an eleven liter constant volume combustion chamber. Three common compositions of LFG with carbon dioxide (CO2) volumetric fraction of fuel ranging from 0.3 to 0.5 are examined. Pressure has changed from atmospheric pressure to 5 bar with the increment of 2 bar. The effect of equivalence ratio, pressure and CO2 content of fuel on laminar burning velocity is investigated and rich and lean burn limits of different compositions of fuel are obtained. Numerical investigation is also performed using CHEMKIN package via GRI3.0 and UBC2.1 chemical kinetic mechanisms. The results indicated that increasing the pressure reduces laminar burning velocity, while it increases adiabatic flame temperature as a result of alternation in equilibrium point of combustion. Pressure considerably increases rich burn limit equivalence ratio and expands flammability range. Markstein length increases by increasing equivalence ratio and its maximum value occurs at rich condition. The results also indicated that, increasing carbon dioxide content of fuel, reduces laminar burning velocity at all pressures and equivalence ratios, mainly due to reduction in adiabatic flame temperature. Keywords: Laminar Burning velocity; Markstein Length; Landfill Gas; Combustion Nomenclature: κ CO2 LFG Lb

Flame stretch rate Carbon dioxide Landfill Gas Markstein length Radius of burned gas

ul ܵ௨ LHV A ߪ

Laminar burning velocity Stretched laminar burning velocity Lower heating value Flame front surface area Gas expansion ratio

Sb

Flame speed

CNG

Compressed natural gas

L

Litter

mw

Milli Watt

Rf

∗

Corresponding author. Present address: Optics and laser laboratory, Mechanical Engineering School, Tehran University, Tehran, Iran. Tel.: +98 9112430179. E-mail address: [email protected]

1 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction Many studies have been concentrated to investigate the combustion characteristics of alternative renewable environmentally-friendly fuels. Among alternative fuels, biogas has definite advantages due to its abundant available resources. Because it is produced from depletion of biomass and biological waste such as urban sewage and garbage. The composition of landfill gas which differs reliant to the manufacturing procedure and resources, is mainly two-thirds to fifty percent methane, and the rest is carbon dioxide. This changeability of the fuel mixture causes variability in combustion characteristics of fuel and therefore brings many difficulties to its usage. It is essential to determine the accurate values of fundamental combustion characteristics of these fuels at high pressures for industrial combustor design such as gas turbines, spark ignited and dual fuel engines. LFG has the potential to become one of the main energy resources in the future. Regarding to methane, biogas has lower heating value, lower burning velocity, higher auto-ignition temperature and narrower flame-stability limits[1]. One of the most important characteristic of biogas is laminar burning velocity which is a vital parameter for the characterization of a fuel and oxidizer mixture. Laminar burning velocity depends only on the reactant composition, temperature, and pressure and therefore, is a fundamental property of a fuel. This makes it a key kinetic parameter for assessing fuel reactivity and for confirming chemical kinetic mechanisms[2]. It also, represents the important information on diffusivity and directly controls the amount of energy released through the combustion of a specified flammable combination[3]. Furthermore, laminar burning velocity is often used to describe the combustion of various fuel-oxidizer mixtures and in determining flammability limits. Many essential flame characteristics such as stability, extinction and flashback are all reliant on the burning velocity[4]. For industrial gas turbines, laminar premixed burning velocities are important for predicting flash-back, blow-off, and dynamic instabilities. Additionally, the measurements of laminar premixed flames serve as fundamental target data for chemical kinetics mechanism development and validation [5]. Several studies have been conducted to study the flame characteristics of different compositions of landfill gas fuel and to clarify the effects of diluent on laminar burning velocity of methane in order to explore methods for more efficient utilization of theses fuels. Hinton and Stone [6], investigated the laminar burning velocity of methane and carbon dioxide mixtures at pressures up to 4 bar and temperatures up to 450 K. They used constant volume cylindrical vessel with Schlieren system to detect burning velocity of methane and landfill gas. They only studied one common composition of landfill gas which consist of 40 percent CO2 and 60 percent methane. In their work, pressure was ranging from 1 bar to 4 bar and equivalence ratio ranging from 0.7 to 1.4. No discussion about flammability limits were presented in their work. Dai et al. [7] experimentally investigated flame stability limits of premixed biogas flame in a reference test burner at atmospheric pressure. They used six composition of biogas with CO2 volume fraction ranging from 30 to 45 percent which stay on a narrow range of possible landfill compositions. Chan et al. [8] studied the effect of CO2 dilution on the laminar burning velocity of premixed methane/air flames using flat-flame burner at atmospheric condition. They tested three fuel composition with CO2 dilution ranging from 0 to 15 percent which is not a landfill fuel composition. Galmiche et al. [9] investigate the effects of carbon dioxide, nitrogen, water vapor, 2 ACS Paragon Plus Environment

Page 2 of 21

Page 3 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

helium and argon dilution on laminar burning velocities of premixed methane/air combustion using experimental and numerical methods. They performed their experiments at atmospheric pressure and 393 K with CO2 content of fuel ranging from 0 to 20 percent. Their numerical simulations were carried out using the GRI mechanism and the Premix code of the Chemkin package at stoichiometric condition. Halter et al.[10] studied the effect of carbon dioxide and nitrogen dilution on laminar burning speed of methane and iso-octane air flames at atmospheric condition using experimental method and numerical simulation by PREMIX code of the CHEMKIN package. They also performed their experiments at atmospheric pressure and temperature with CO2 content of fuel ranging from 0 to 20 percent at stoichiometric condition. Qiao et al. [11] experimentally and numerically computed laminar burning velocity and flammability limits of premixed methane/air flames in the existence of various inert gases containing helium, argon, nitrogen and carbon dioxide. They measured burning velocity using outwardly propagating spherical flame visualization. Their calculations were performed using the steady, one-dimensional laminar PREMIX flame code. In their experiments, CO2 content of fuel was varied from 0 to 22 percent and tests were performed at normal temperature and pressure. Stone et al. [12] derived correlations for the laminar-burning velocity of methane/diluent/air mixtures. They utilized carbon dioxide, nitrogen, and a combination of carbon dioxide and nitrogen as diluents. They only had reported calculated data of laminar burning velocity of CO2 and methane mixtures at atmospheric pressure. Wang et al. [13] numerically studied the flame structure of a CH4/CO2/H2O counter-flow diffusion flame at atmospheric temperature and pressure. They had changed the mole fraction of CO2 in fuel mixture from zero to 0.6. Hu et al. [14] Numerically investigated the effects of diluents (He/Ar/N2/CO2) on the laminar burning velocity of Methane−Air mixtures using the Chemkin package. They had changed the CO2 content of fuel mixture from zero to 20 percent. Although many studies are performed to investigate combustion characteristics of Methane and CO2, most of them are concentrated on mixtures that are far apart from common compositions of landfill fuel. On the other hand, many industrial combustion devices, such as internal combustion engines, work based on premixed flame propagation on elevated pressures. Therefore, understanding the effect of pressure, on combustion characteristics of intended fuel is essential. However, there is a void on adequate provided data of combustion characteristics of Landfill gas at high pressures and most studies are performed using atmospheric burners or numerical simulations. As a result, this investigation is focused on evaluation of the laminar burning velocity of landfill/air flames over a wide range of unburned mixture compositions and initial pressures up to 5 bar utilizing experimental method of Schlieren in a high pressure combustion chamber. This study has provided new data on laminar burning velocity values of Landfill gas at a wide range of pressures, equivalence rations and fuel composition. The lean and rich burn flammability limits of different compositions of fuel are measured using analyzing flame propagating images. Three different composition of landfill utilized in this study are shown in Table 1. The CO2 content of fuel has changed from 30 percent in LFG30 to 50 percent in LFG50. As is shown in this table, landfill gas heating value and Wobbe Index of fuel decreases considerably by increasing CO2 content of fuel.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 21

Table 1. Different fuel contents used in this study

LFG30 LFG40 LFG50

CH4 mole fraction

CO2 mole fraction

0.7 0.6 0.5

0.3 0.4 0.5

LHV (MJ/m3) @ 1 bar & 300 K 24.92 21.36 17.80

Wobbe Index 27.2 22.1 17.5

2. Experimental setup In this study the experiments are performed in a steel cylindrical combustion chamber with inner diameter of 225 mm and length of 280 mm with approximate capacity of 11L. The combustion chamber and its connected equipment along with an obtained image of flame propagation are shown in Figure 1. Spark ignition electrodes utilized for ignition system are made of stainless steel rods with diameter of 2 mm. The spark plugs are mounted in two parallel concentric holes on two sides of combustion chamber. The flame is ignited using a capacitive spark discharge circuit. Discharge voltage of 10kV is used to generate a spark in the gap between two electrodes which was adjustable from 0.5 mm to 2 mm. Two quartz windows with diameter of 100 mm and length of 80 mm are placed in both sides of cylindrical combustion chamber in order to provide optical access. Two pair of parallel flanges hold quartz windows which make 80 mm optical access for Schlieren visualization of the flame propagation. Three holes are being placed at the top of combustion chamber for inlet flow, outlet flow and safety valve. Safety valve operates if the pressure in combustion chamber exceeds 50 bar. Chamber is separated from inlet and outlet flows through high pressure-temperature valves. Two pressure sensors are located before inlet high pressure-temperature valves for monitoring the inside combustion chamber pressure. A ktype thermocouple is also passed through exhaust port to the combustion chamber in order to accurate determination of inside initial temperature. Vacuum pump is connected to combustion chamber using a valve considered at the gas discharge path. Before each experiment, the combustion chamber was evacuated using vacuum pump and then the favorite compositions of gases were filled using the method of partial pressure. After each experiment, the combustion chamber was filled with air and then discharged for several time in order to evacuate the combustion products and cooling the inside walls of the chamber. Two pressure sensors were used for accurate measurement of in-cylinder pressure. The first one was an absolute sensor operating at the range of 0 to 1.1 bar and the second one was a gage sensor operating at the range of 0.87 (ambient pressure) to 10 bar. This strategy allowed us to measure partial pressure accurately and therefore precisely determine the mixture composition. Maximum error in pressure measurement limited to less than 0.5 percent of reading value. After closing the valves, the mixture was left for an adequate time (around 20 minutes) to ensure a homogeneous mixture is formed and to allow the gas to come to rest. The same methodology for mixing gases have been mentioned in the references [5, 6, 15-19]. The adequate time was calculated using analyzing the obtained results in the way that the flame speed won’t change so much by increasing resting time. In this work, experiment for every condition are repeated for three to five times and average obtained result is reported in each case.


Page 5 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 1. Combustion chamber and its attached equipment

The experimental setup and Schlieren system is shown in Figure 2. Two gas cylinder containing Methane and carbon dioxide were used to produce different compositions of Landfill gas with Methane volumetric fraction ranging from 50 to 70 percent. All gases has at least the purity of 99.99%. Gas regulators were placed after cylinder in order to reduce high pressure of gases to pipeline pressure. Combustion air was provided using an air compressor. Air was dehumidified using a water trap. Gases were charged to combustion chamber individually using a gas splitter. The Schlieren system has been adjusted using a 1000 mw green diode laser with a wavelength of 520 nm as a light source. A microscope objective with conical length of 6 mm and a pinhole with the hole diameter of 4 µm is used to expand beam light and produce point light source. A doublet with diameter of 100 mm and focal length of 500 mm with the windows perpendicular to the laser beam utilized to produce parallel light beam. After doublet, the 11 litter combustion chamber is placed. Another doublet with the same characteristics was mounted after combustion chamber for concentrating light beam into a pinhole with adjustable diameter. Flame propagation was displayed on a screen and a high speed camera was used to record propagation images. High-speed camera recorded flame front propagation images at a rate of 4000 frames per second with 960 × 540 resolution.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 21

Figure 2. Schematic of the experimental setup 1. CH4 cylinder 2. CO2 cylinder 3. CH4 regulator 4. CO2 regulator 5. flashback 6. ball valve 7. flow splitter 8. valve to chamber 9. gage pressure sensor 10.pressure display

11.valve 12.absolute pressure measuring device 13.valve 14.high pressure valve 15.air compressor 16.air compressor valve 17.water trap 18.spark ignition plug 19.high voltage power supply 20.voltage stabilizer

21. safety valve 22. exhaust valve

31.light beam 32.doublet lens

23.vacuum valve 24.vacuum pump 25.high pressure valve 26.thermocouple 27. data logger 28.laser 29.microscope object 30.source light pinhole

33.combustion chamber 34.doublet lens 35.pinhole 36.screen 37.high speed camera

3. Numerical simulation A freely propagating adiabatic premixed flame is simulated using CHEMKIN PREMIX [20], a steady laminar one-dimensional code. PREMIX code uses a hybrid time integrating/Newton iteration method to solve the steady state conservation equations of mass, species and energy[21]. The solver procedure uses an automatic coarse-to-fine mesh refinement to enhance the convergence speed of the solution by providing optimal mesh settlement. In this study results are obtained using 6 continuations with the gradient and curvature parameters set to 0.005 and 0.01 respectively resulting 2000 to 3000 grid points. Adiabatic flame temperature is also obtained by means of EQUI codes of CHEMKIN package. Two well-known chemical kinetic mechanisms of GRI3.0[22] and UBC2.1 [23] are used for detail simulation of combustion phenomena. The GRI3.0 mechanism includes 53 species and 325 reactions and UBC2.1 mechanism includes 40 species and 194 reactions.

4. Methodology to determine laminar burning velocity 4.1. Mathematical model 6 ACS Paragon Plus Environment

Page 7 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

After spark ignites at the center of combustion chamber, the flame expands spherically from ignition point. In order to calculate flame front radius from flame propagation images, a circle should be fitted on flame front at each time as shown in Figure 3. In order to eliminate the effect of spark rods on calculated data, 30 degrees of circle area around the rods was excluded from data reduction. The stretched flame propagation speed (Sb) can be derived from the data of flame radius versus time using equation (1): (1) dRf

Sb =

dt Where R f illustrated in Figure 3 is the flame front radius at any time. The stretched laminar flame speed is related to burning speed ( S u ), by equation (2): (2) S Su = b

σ

ఘ

Where ߪ = ఘೠ is the gas expansion ratio and ߩ௨ and ߩ௕ are densities of the unburned and burned ್

gases respectively. The general expression for the flame stretch rate of a spherical flame ( κ ) is [24]defined using equation (3):

κ=

1 dA A dt

(3)

Where A is the surface area of the flame front. In the case of a spherically expanding flame, the stretch rate can be calculated as equation (4): 2 (4) 1 dA 1 dR f 2 dR f . κ = = 2. = A dt

Rf

dt

Rf

dt

After calculating the flame front radius at each time step, in order to obtain the laminar flame speed values, it is essential to use modification on raw data to remove the effect of stretch from obtained results. There are linear and nonlinear methods for calculating unstretched laminar flame speed. At linear method that is based on asymptotic studies [25-27], the flame speed stretch correction is obtained by equation (5): (5) S b = S b0 − σ L b .κ And at nonlinear method proposed by Kelly and Law [28] this correction is obtained by equation (6): 2 (6)  Sb   Sb  Lb κ  0  ln  0  = − 0 Sb  Sb   Sb  Where ܵ௕ is the stretched laminar flame speed, ܵ௕଴ is the unstretched laminar flame speed, Lb is the Markstein length. The Markstein length, is a constant value pronouncing influences of flamestructure on the flame speed[29]. The Markstein length, is on the order of the flame thickness[2]. In this study in order to provide more accurate results, the non-linear methodology has been used to deduce the unstretched flame propagation speed.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Flame front visualization image

4.2. Burning velocity measurement In this section, experimental flame propagation data and the method of calculating stretch burning velocity and Markstein length from this data is presented. Time-evolution of the flame radius obtained using Schlieren photographs at stoichiometric condition for different pressures and CO2 mole fractions of fuel are indicated in Figure 4. Each set of data shown in Figure 4 corresponds to a single flame front propagation versus time in combustion chamber. At the beginning of flame propagation, its radius increases non-linearly with respect to time. After 8 to 10 mm, flame radius increases almost linearly with respect to time. It can be seen that, rate of flame expansion increases by reduction in pressure and CO2 fraction. CO2 mole fraction has greater effect on flame propagation speed of the fuel than that of pressure. Generally speaking, adding CO2 in the fuel mixture or increasing the pressure slows down the propagation of the flame. Reduction effect of the diluent gas and initial pressure of combustion is related to each other in a manner that effect of CO2 on flame propagation speed increases by increasing pressure. Effect of pressure also, increases by increasing amount of CO2 in fuel mixtures. Gradient of the flame radius versus time curve which reflects the stretching effectiveness of the flame, increases with radius. This increasing gradient is a characteristics of a stable flame[30]. To minimize the effect of the inner wall of the combustion chamber on burning velocity, data are not recorded after the flame reached 40 mm. Furthermore, to eliminate the effect of the ignition kernel, the first 5 to 9 mm of the flame front radius in each data set was excluded in data reduction for calculation of laminar burning velocity. The derivative of the flame radius versus time in Figure 4 is the stretched burning velocity, which represents the flame front moving speed in combustion chamber. To determine unstretched burning velocity, the stretched burning velocity versus the flame stretch rate (κ) should be considered. The unstretched burning velocity can be calculated by continuing the tangent line to the experimental results of stretched burning velocity versus stretch rate, to zero flame stretch rate (κ=0). The slope of this line determines the value of the Markstein length. This process is applied to all the experimental results in order to find the unstretched burning velocitys and Markstein lengths.


Page 8 of 21

Page 9 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 4. Variation of flame radius with time

5. Validation For validation of experimental setup and data reduction method, results are compared with experimental data reported in the literature [10-12] and simulated resulted obtained by using two different well-known landfill reaction mechanism (UBC2.1 and GRI3.0). This comparison is carried out at atmospheric condition (1bar and 300K) for different content of CO2 ranging from 0 to 50 percent in the fuel mixture. The Figure 5 illustrated that measured results in this study showed good agreement with previous reported data.

Figure 5. Comparison between obtained experimental and numerical results and literature


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 21

6. Results and discussion The focus of this study is to investigate the effect of initial pressure on laminar burning velocity and Markstein length of landfill gas over a wide range of fuel composition and equivalence ratio. Experimental and numerical results are presented in this section. At firs validation of obtained results with other works and numerical calculation using UBC2.1and GRI3.0 mechanisms is presented. After that, effect of equivalence ratio, pressure and CO2 mole fraction of fuel on laminar burning velocity is investigated. Finally rich and lean burn limits of different compositions of landfill gas are obtained using experimental measurements.

6.1. Markstein length Markstein length (Lb) for different CO2 content of fuel mixture at 1 and 5 bar is illustrated in Figure 6. In general, Markstein length indicates sensitivity of the flame to the effect of stretch rate on the surface of propagating flame, and based on reactant's mixture characteristics and initial condition, it can be positive or negative. The Markstein length varies with the equivalence ratio, mixture composition and initial pressure. For all the cases shown in Figure 6, maximum amount of Lb takes place at rich condition. Lb generally grows with equivalence ratio while the rate of increasing rises by CO2 content of fuel and equivalence ratio. Increasing the pressure leads to an intense decrease in Lb. Due to the combined effects of initial pressure and CO2 content, high-pressure CH4-CO2 flames are characterized by the lowest values of Lb. The obtained results confirm the selection of linear procedure for data analysis because all values of Makstein length remains below 1 mm. At lean and stoichiometric condition, Lb tends to zero and negative values which results in an increase of the burning velocity with stretch. Positive value of Lb is corresponding to a burning velocity which increases as κ decreases. While, a negative value illustrates that burning velocity increases along with an increase in the flame stretch rate. During the flame propagation, flame is stable if Lb is positive, while it is unstable with Lb is negative. At unstable condition (Lb0, the flame front instabilities are suppressed, which leads to stabilization of the flame[31].


Page 11 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 6. Markstein length vs equivalence ratio

6.2. Effect of Equivalence ratio Laminar burning velocity and gas expansion ratio versus equivalence ratio at 1 bar for different CO2 content of fuel is illustrated in Figure 7. As is shown in this figure, simulation using UBC2.1 mechanism has better prediction of laminar burning velocity at stoichiometric and rich conditions, while at lean condition, results obtained using GRI3.0 mechanism shows better consistency with experimental results. For LFG30, UBC2.1 illustrates better prediction of laminar burning velocity. Good agreement between numerical result using detail chemical kinetic mechanisms and experimental result is observed at all operating conditions. By increasing CO2, laminar burning velocity value at phi=1.2 decreases relative to its value at phi=0.8. For LFG30 laminar burning velocity at rich condition (phi=1.2) is 21 percent higher than at lean condition, while for LFG50 this value is 5 percent lower. Expansion ratio which is the density ratio of unburned gas to burned gas, reduces by increasing CO2 content of fuel and at off-stoichiometric condition. This reduction is mainly due to drop in adiabatic flame temperature of combustion products. Expansion ratio correlates unstretched burned burning velocity to laminar burning velocity through equation 5. Therefore, lower values of expansion ratio corresponds to the closer value of flame propagation speed and laminar burning velocity. Another reason for alternation of expansion ratio at different condition is variation in burned gas composition and therefore gas constant (R) which directly affects density through ideal gas law.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 21

a b Figure 7. a) Laminar burning velocity and b)gas expansion ratio at 1 bar

6.3. Effect of carbon dioxide mole fraction Effect of carbon dioxide volumetric fraction of fuel on laminar burning velocity and adiabatic flame temperature is indicated in Figure 8. As is shown in this figure, increasing carbon dioxide fraction of fuel decreases laminar burning velocity and adiabatic flame temperature at all pressures and equivalence ratios with the same trend. Adiabatic flame temperature through Arrhenius rate of reaction exerts an influence on laminar burning velocity. The main reason for reduction in flame temperature and therefore laminar burning velocity by increasing the CO2 content of fuel is reduction in heat of combustion. The second reason is that CO2 will, to some extent, contribute the flame through the elementary chemistry of CO2 decomposition [7]. Increasing the CO2 volumetric fraction of fuel from 30 to 50 percent, decreases laminar burning velocity between 30 to 40 percent. As is represented in Figure 8, ul decreases linearly by increasing carbon dioxide while Tad reduction rate increases by increasing CO2 content of fuel. Another significant result indicated in this figure is that, adiabatic flame temperature increases by pressure at all fuel mixtures. This is mainly due to alternation in equilibrium point of combustion process, especially at higher temperatures (near stoichiometric condition). By increasing the pressure, Gibbs free energy changes and therefore, equilibrium point and enthalpy modifies. As a result, adiabatic flame temperature increases by increasing the pressure.


Page 13 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 8. Effect of CO2 volumetric fraction on laminar burning velocity. Dashed lines:GRI3.0, dotted line:UBC2.1

Flame propagation contours (Schlieren photographs of the flames) at different amount of CO2 mole fraction of fuel at stoichiometric condition and 5 bar are illustrated in Figure 9. As is shown in this figure, flame propagates spherically from the center of the combustion chamber as the time increased from 10 ms to 30 ms; however, the flame tend to offset upward as time elapses. This ascending movement occurs due to effect of buoyancy and increases with time. Therefore flames with slower burning rates show more tendency to ascending of the center of flame front. Figure 9 also indicates that flame propagation speed reduces significantly by increasing CO2 mole fraction of fuel.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Time (ms)

X_CO2 = 0.3

X_CO2 = 0.4

Page 14 of 21

X_CO2 = 0.5

10

20

30

Figure 9. Contours of flame propagation at stoichiometric condition and 5 bar

6.4. Effect of pressure Effect of pressure on laminar burning velocity of different composition of landfill gas at stoichiometric condition is demonstrated in Figure 10. By increasing pressure from 1 to 5 bar, laminar burning velocity decreases between 50 to 56 percent at LFG30 and LFG50 respectively. Main drop in ul (approximately 40 percent) occurs from 1bar to 3 bar. Reduction in ul by pressure has the same trend in all amount of CO2 volumetric fraction. Two important parameters which change by pressure and affect burning rate of fuels are thermal and mass diffusivity. Thermal diffusivity coefficient definition is described using equations 8. The dependence of the mass diffusion coefficient on temperature and pressure for gases is also expressed using Chapman–Enskog theory [32] in equation 9. Where, 1 and 2 index the two kinds of molecules present in the gaseous mixture, T is the absolute temperature (K), M is the molar mass (g/mol), p 14 ACS Paragon Plus Environment

Page 15 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

is the pressure (atm) and σ 1 2 = 1 (σ 1 + σ 2 ) is the average collision diameter and Ω is a 2 dimensionless number indicating temperature-dependent collision integral. As is indicated in these equations, there is reverse correlation between pressure and diffusivity coefficients. Therefore, increasing the pressure, leads to drop in thermal and mass diffusivity coefficient which have direct influence on burning rate of fuel. In fact, burning rate of fuel is a representative of its mass and thermal diffusivity coefficient. Therefore, increasing the pressure results in decrease in laminar burning velocity. Thermal diffusivity

α=

kR T T k k = = u u = f (k , Ru , Cp ) × u Pu ρuCp ρuCp PuCp

(8)

3

Mass diffusivity

D=

1.858 × 10 −3 × Tu 2 ×

1

M1

+ 1

Puσ 122 Ω

3

M2

(9)

T2 = f (M ,σ , Ω) × u Pu

Figure 10. Effect of pressure on laminar burning velocity

Experimental results of obtained laminar burning velocity at different fuel/air equivalence ratio ranging from 0.8 to 1.2 for LFG30 and LFG50 at different pressures is shown in Figure 11. As is illustrated in this figure, at atmospheric pressure, for LFG30 flame at rich condition propagates with higher speed than lean condition. By increasing the pressure from 1 to 5 bar and CO2 mole fraction from LFG30 to LFG50, laminar burning velocity at lean mixture increases regard to rich one. At 5 bar for LFG50 fuel, lean mixture laminar burning velocity is 50 percent more than its value at rich condition. 15 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 21

Figure 11. Effect of equivalence ratio on laminar burning velocity

6.5. Flammability limits Any fuel has a lean and a rich flammability limit at which flame extinction occurs. From theoretical viewpoint, flammability limits arise due to domination of phenomena such as chaintermination reactions, heat losses and preferential diffusion on energy-releasing from chemical reactions[11]. Flame propagation contours (Schlieren photographs of the flames) at different values of equivalence ratio at 3 bar are illustrated in Figure 12. These contours are illustrated in order to explain the flammability limits of the fuel. Four conditions including lean and rich flammability limits, unstable flame and no burning condition are shown in this figure. As is shown, after spark ignition, flame propagates spherically from the center of the combustion chamber as well as its center starts moving upward as a result of buoyancy effect. The speed of flame center ascending is almost constant for different conditions presented in this figure. Therefore flames with slower burning rates show more tendency to ascending. At low burning velocity such as at flammability limits, the upper part of the flame forms a hemisphere while the lower part of the flames formed the mushroom shape because the flame could not propagate downwardly. At unstable condition shown in Figure 12, misfiring occurs. It means that after spark ignites, all mixture in the vicinity of ignition point doesn’t burn. This condition is considered as unstable flame and fuel/oxidizer mixture just before this condition are reported as flammability limits. At no burning condition, no flame front propagating is observed.


Page 17 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Tim e (ms)

Lean burn limit Phi = 0.65

Rich burn limit Phi = 1.4

Unstable flame Phi = 1.45

No burning Phi = 1.55

10

30

50

70

Figure 12. Contours of flame propagation at 3 bar for LFG30

Flammability limits at different pressures and carbon dioxide volumetric fraction in fuel are illustrated in Figure 13. Tests are carried out at equivalence ratio interval of 0.05 in the vicinity of obtained result from numerical simulations. As is shown in this figure, lean burn flammability limit doesn’t change by pressure so much, while rich burn flammability limit is completely reliant on pressure. Increasing the pressure from 1 to 3 bar doesn’t affect rich burn flammability limit considerably while from 3 to 5 bar it increases by a factor of 1.2. As is depicted in Figure 13, flammability range (the difference between rich and lean flammability limits) increases by increasing pressure and by decreasing carbon dioxide content of fuel. On the other hand, by increasing pressure, as well as decreasing carbon dioxide, flame becomes more stable at very high and low level of oxidizer content. High lean burn flammability limit has some advantages like capability towards reduction of NOx and designing more stable industrial combustion 17 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

chamber. It also has some drawbacks like safety and possibility of explosion when it leaks to the environment.

Figure 13. Flammability limits at different pressures and CO2 mole fraction

Figure 14 demonstrates the laminar burning velocity at flammability limits. As is shown in this figure, by increasing the pressure from 1 to 3 bar, burning velocity decreases much more that, by increasing it from 3 to 5 bar. Value of burning velocity at rich burn limit at 1 bar is higher than lean burn limit, while at higher pressures, burning velocity at lean burn limit is higher. Another consequence obtaining from Figure 14, is the low values of burning velocity at flammability limits at 5 bar pressure. Burning velocity at this condition is less than 4 centimeter per second.

Figure 14. Laminar burning velocity at flammability limits at different conditions


Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

7. Conclusion Landfill gas is a biogas generated from digestion of biomass and organic waste that has different composition based on different sources and production procedures. Exact determination of combustion characteristics of different composition of LFG is essential for industrial application of this fuel. In this work the laminar burning velocity of landfill/air flames over a wide range of unburned mixture compositions and initial pressure is investigated using Schlieren system and numerical simulation. CHEMKIN package using GRI3.0 and UBC2.1 chemical kinetics mechanism has been used to numerically investigate the flame. The most significance results of this investigation are summarized as follows: • Increasing pressure reduces laminar burning velocity. • Increasing carbon dioxide fraction, decreases burning velocity and adiabatic flame temperatures at all pressures and equivalence ratios. • Flammability range increases by increasing pressure and decreases by increasing CO2 volumetric fraction of fuel. • There is a strong relationship between adiabatic flame temperature and laminar burning velocity. • Increasing pressure, increases adiabatic flame temperature as a result of alternation in equilibrium point. In this study, fundamental combustion characteristics of Landfill gas over a wide range of equivalence ratio and pressure is investigated experimentally and numerically. Good understanding of the effect of fuel composition and initial pressure on combustion characteristics of LFG is obtained. The interaction between various flame characteristics such as adiabatic flame temperature and Markstein length laminar burning velocity is investigated. These results can be useful for industrial design of combustion chambers and burners. References [1]

[2] [3]

[4] [5] [6] [7] [8]

[9]

C. A. Cardona and A. A. Amell, "Laminar burning velocity and interchangeability analysis of biogas/C3H8/H2 with normal and oxygen-enriched air," International Journal of Hydrogen Energy, vol. 38, pp. 7994-8001, 2013. J. Beeckmann, L. Cai, and H. Pitsch, "Experimental investigation of the laminar burning velocities of methanol, ethanol, n-propanol, and n-butanol at high pressure," Fuel, vol. 117, pp. 340-350, 2014. Z. Al-Hamamre and J. Yamin, "The effect of hydrogen addition on premixed laminar acetylene–hydrogen– air and ethanol–hydrogen–air flames," International Journal of Hydrogen Energy, vol. 38, pp. 7499-7509, 2013. Y. He, Z. Wang, L. Yang, R. Whiddon, Z. Li, J. Zhou, et al., "Investigation of laminar flame speeds of typical syngas using laser based Bunsen method and kinetic simulation," Fuel, vol. 95, pp. 206-213, 2012. J. de Vries, W. B. Lowry, Z. Serinyel, H. J. Curran, and E. L. Petersen, "Laminar flame speed measurements of dimethyl ether in air at pressures up to 10atm," Fuel, vol. 90, pp. 331-338, 2011. N. Hinton and R. Stone, "Laminar burning velocity measurements of methane and carbon dioxide mixtures (biogas) over wide ranging temperatures and pressures," Fuel, vol. 116, pp. 743-750, 2014. W. Dai, C. Qin, Z. Chen, C. Tong, and P. Liu, "Experimental studies of flame stability limits of biogas flame," Energy Conversion and Management, vol. 63, pp. 157-161, 2012. Y. L. Chan, M. M. Zhu, Z. Z. Zhang, P. F. Liu, and D. K. Zhang, "The Effect of CO2 Dilution on the Laminar Burning Velocity of Premixed Methane/Air Flames," Energy Procedia, vol. 75, pp. 3048-3053, 2015. B. Galmiche, F. Halter, F. Foucher, and P. Dagaut, "Effects of Dilution on Laminar Burning Velocity of Premixed Methane/Air Flames," Energy & Fuels, vol. 25, pp. 948-954, 2011.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[10]

[11]

[12]

[13] [14] [15]

[16]

[17]

[18]

[19] [20]

[21] [22] [23] [24]

[25] [26] [27] [28] [29] [30]

[31]

[32]

F. Halter, F. Foucher, L. Landry, and C. Mounaïm-Rousselle, "Effect of Dilution by Nitrogen and/or Carbon Dioxide on Methane and Iso-Octane Air Flames," Combustion Science and Technology, vol. 181, pp. 813-827, 2009. L. Qiao, W. J. A. Dahm, and G. M. Faeth, "Burning Velocities and Flammability Limits of Premixed Methane/Air/Diluent Flames in Microgravity," presented at the Aerospace Sciences Meeting and Exhibit, Reno, Nevada, 2008. StoneA, R. , A. ClarkeA, and P. BeckwithB, "Correlations for the Laminar-Burning Velocity of Methane/Diluent/Air Mixtures Obtained in Free-Fall Experiments," Combustion and Flame, vol. 114, p. 10, 1998. L. Wang, Z. Liu, S. Chen, C. Zheng, and J. Li, "Physical and Chemical Effects of CO2and H2O Additives on Counterflow Diffusion Flame Burning Methane," Energy & Fuels, vol. 27, pp. 7602-7611, 2013. E. Hu, X. Jiang, Z. Huang, and N. Iida, "Numerical Study on the Effects of Diluents on the Laminar Burning Velocity of Methane–Air Mixtures," Energy & Fuels, vol. 26, pp. 4242-4252, 2012. E. Hu, J. Fu, L. Pan, X. Jiang, Z. Huang, and Y. Zhang, "Experimental and numerical study on the effect of composition on laminar burning velocities of H2/CO/N2/CO2/air mixtures," International Journal of Hydrogen Energy, vol. 37, pp. 18509-18519, 2012. E. Hu, Z. Huang, J. He, C. Jin, and J. Zheng, "Experimental and numerical study on laminar burning characteristics of premixed methane–hydrogen–air flames," International Journal of Hydrogen Energy, vol. 34, pp. 4876-4888, 2009. W. S. Song, S. W. Jung, J. Park, O. B. Kwon, Y. J. Kim, T. H. Kim, et al., "Effects of syngas addition on flame propagation and stability in outwardly propagating spherical dimethyl ether-air premixed flames," International Journal of Hydrogen Energy, vol. 38, pp. 14102-14114, 2013. X. Wu, Z. Huang, X. Wang, C. Jin, C. Tang, L. Wei, et al., "Laminar burning velocities and flame instabilities of 2,5-dimethylfuran–air mixtures at elevated pressures," Combustion and Flame, vol. 158, pp. 539-546, 2011. V. V. Zamashchikov, V. A. Alekseev, and A. A. Konnov, "Laminar burning velocities of rich near-limiting flames of hydrogen," International Journal of Hydrogen Energy, vol. 39, pp. 1874-1881, 2014. J. F. G. R. J. Kee, M. D. Smooke, J. A. Miller, and E. Meeks, "PREMIX: AFORTRAN Program for Modeling Steady Laminar One-Dimensional Premixed Flames," Reaction Design, 11436 Sorrento Valley Road San Diego, CA 921211998. R. J. Kee, F. M. Rupley, and E. Meeks. (1989). CHEMKIN-II: a Fortran chemical kinetics package for the analysis of gas-phase chemical kinetics. Gregory P. Smith, David M. Golden, Michael Frenklach, Nigel W. Moriarty, Boris Eiteneer, Mikhail Goldenberg, et al. GRI-Mech 3.0. Available: http://www.me.berkeley.edu/gri_mech/ J. Huang, P. G. Hill, W. K. Bushe, and S. R. Munshi, "Shock initiated ignition in homogeneous methanehydrogen-air mixtures at high pressure," International Journal of Chemical Kinetics, vol. 38, p. 12, 2006. S. D. Dowdy DR, Taylor SC, Williams A., "The use of expanding spherical flames to determine burning velocities and stretch effects in hydrogen/air mixtures," proceeding of combustion institute, vol. 23, pp. 325-332, 1990. S. GI., "On a distorted flame front as a hydrodynamic discontinuity," Acta Astronautica, vol. 3, pp. 889918, 1976. M. B. Matalon M, "Flames as gasdynamic discontinuities," Journal of Fluid Mechanics, vol. 124, pp. 239259, 1982. P. Clavin, "Dynamic behavior of premixed flame fronts in laminar and turbulent flows," Progress in Energy and Combustion Science, vol. 11, pp. 1-59, 1985. A. P. Kelley and C. K. Law, "Nonlinear effects in the extraction of laminar flame speeds from expanding spherical flames," Combustion and Flame, vol. 156, pp. 1844-1851, 2009. W. FA., Combustion Theory (2d Ed). Menlo Park (CA): Addison-Wesley, 1985. D. Pugh, A. P. Crayford, P. J. Bowen, T. O’Doherty, R. Marsh, and J. Steer, "Laminar flame speed and markstein length characterisation of steelworks gas blends," Applied Energy, vol. 136, pp. 1026-1034, 2014. T. M. Vu, W. S. Song, J. Park, D. S. Bae, and H. S. You, "Measurements of propagation speeds and flame instabilities in biomass derived gas–air premixed flames," International Journal of Hydrogen Energy, vol. 36, pp. 12058-12067, 2011. E. L. Cussler, Auteur, Diffusion, mass transfer in fluid systems New York: Cambridge University Press, cop, 1997.


Page 20 of 21

Page 21 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels


Experimental Measurement of Laminar Burning Velocity and

Recommend Documents