Experimental study of the combustion properties of methane/hydrogen mixtures Gersen, Sander

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1 University of Groningen Experimental study of the combustion properties of methane/hydrogen mixtures Gersen, Sander IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2007 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Gersen, S. (2007). Experimental study of the combustion properties of methane/hydrogen mixtures s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date:

2 RIJKSUNIVERSITEIT GRONINGEN Experimental study of the combustion properties of methane/hydrogen mixtures Proefschrift ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit Groningen op gezag van de Rector Magnificus, dr. F. Zwarts, in het openbaar te verdedigen op vrijdag 7 december 2007 om 14:45 uur door Sander Gersen geboren op 2 oktober 1976 te Gouda

3 Promotor: Copromotor: Prof. dr. H.B. Levinsky Dr. A.V. Mokhov Beoordelingscommissie: Prof. dr. ir. R. Baert Prof. dr. H.C. Moll Prof. dr. ir. Th.H. van der Meer ISBN ISBN (electronic version)

4 Experimental study of the combustion properties of methane/hydrogen mixtures Sander Gersen

5 The work described in this thesis was performed in the Laboratory for Fuel and Combustion Science at the University of Groningen, Nijenborgh 4, 9747 AG Groningen, the Netherlands. This project is supported with a grant of the Dutch Program EET (Economy, Ecology, Technology) a joint initiative of the Ministries of Economic Affairs, Education, Culture and Sciences and of Housing, Spatial Planning and the Environment. The program is run by the EET Program Office, SenterNovem. S.Gersen, Experimental study of the combustion properties of methane/hydrogen mixtures, Proefschrift Rijksuniversiteit Groningen (2007)

6 Table of contents Introduction 5 Chapter 1 : Combustion properties of homogeneous reacting gas mixtures 1.1 Motivation to study the combustion properties of CH 4 /H 2 gas mixtures Governing equations for a homogeneous reacting gas mixture in a closed gas system Laminar premixed flames Governing equations for a one-dimensional laminar flame Chemical mechanisms 22 Chapter 2: The Rapid Compression Machine, Experimental Techniques, Procedures and Setup 2.1 Background Design and Operations Experimental System Gas filling system and filling procedure Instrumentation and data acquisition Determination autoignition delay time Temperature determination 36 Appendix A.1 40 Appendix A.2 41 Appendix A.3 42 Appendix A.4 43 Appendix A.5 44 Chapter 3: High-pressure autoignition delay time measurements in methane/hydrogen fuel mixtures in a Rapid Compression Machine 3.1 Introduction Experimental approach Numerical simulation and analysis of experimental data 52 I

7 3.3.1 Chemical mechanisms Numerical simulations Results and discussion Comparison of experimental results with numerical simulations Summary and conclusions 68 Chapter 4: One-dimensional laminar flames, Experimental Techniques, Procedures and Burner Setup 4.1 General introduction Burner Gas handling system Extractive probe sampling system Estimate of the conversion of C 2 H 2 and HCN during sampling Laser absorption spectroscopy Theory Wavelength Modulation Absorption Spectroscopy (WMAS) Experimental setup for Tunable Diode Laser Absorption Spectroscopy Experimental procedure TDAS measurements of acetylene Experimental procedure WMAS with second harmonic detection HCN measurements 90 Chapter 5: Extractive Probe Measurements of acetylene in atmospheric pressure fuel-rich premixed methane/air flames 5.1 Introduction Experimental Results and discussion Conclusions 107 II

8 Chapter 6: HCN formation and destruction in atmospheric pressure fuel-rich premixed methane/air flames 6.1 Introduction Experimental Results and discussion Conclusions 120 Chapter 7: The effect of hydrogen addition to rich stabilized methane/air flames 7.1 Introduction Experimental Results and discussion HCN profiles C 2 H 2 profiles Conclusion 132 Summary 136 Samenvatting 140 Dankwoord 144 III

9 IV

10 Inroduction Introduction Combustion is mankind s oldest technology. Nowadays the combustion of fossil fuels provides more than 80% of the world s energy, and is used for electric power generation, domestic heating, transportation and many other processes. A negative aspect of fossil fuels is that during combustion not only heat is generated, but also pollutants such as soot and NO x. Moreover, the combustion of fossil fuels disturbs the atmospheric CO 2 balance, which is believed to contribute to global warming. Stringent emission regulations and the expectation that the known fossil fuel reserves will be exhausted within this century, forces combustion researchers to find methods to reduce pollutant emission, improve the efficiency of combustion equipment and to utilize renewable energy sources, such as biogas and hydrogen, as alternative fuels. However, the currently available renewable energy sources are insufficient to satisfy the world s energy consumption. In a sustainable economy, hydrogen, either from electrolysis of water from sustainable generated electricity (wind, water) or from biomass, can fulfill a role as energy carrier. Yet at present, there is no sustainable hydrogen production, nor is there widespread energy conversion technology to utilize hydrogen as a fuel. To avoid the necessity of large investment in new hydrogen utilization equipment, the addition of hydrogen to natural gas could be a first step towards the wide-scale introduction of hydrogen into the energy infrastructure. However, since the combustion properties of hydrogen differ in many respects from those of natural gas, the allowable fraction of hydrogen in natural gas may be limited by the deteriorating performance of gas combustion equipment such as spark-ignited engines, burners and turbines to hydrogen-enriched natural gas. For example, increased knock in gas engines, causing extensive damage to the machines, or unacceptable increases in NO x formation from combustion equipment, both caused by the presence of hydrogen in the fuel, are clearly unwanted side-effects and must be avoided. To investigate these practical consequences of the changes if fuel composition effectively, it is necessary to study the changes in the underlying physical and chemical processes that are responsible for the combustion behavior of natural gas when hydrogen is added. Gaining fundamental insight into these consequences using practical combustion devices is difficult, since the experimental conditions are generally poorly defined, complicating the interpretation of the data. For this reason, 5

11 Introduction devices as shock tubes, rapid compression machines (RCM) and one-dimensional flame burners have been developed to enable the study of combustion under welldefined conditions. The insights gained from such studies permit the analysis of the behavior of broad groups of practical combustion equipment, and are also indispensable for the design of new combustion equipment. Furthermore, data from these well-defined studies aid the development of methods for modeling complex combustion phenomena. The objective of this thesis is to investigate potential changes in the combustion properties of methane caused by the addition of hydrogen to the fuel. Specifically, the ignition properties of methane/oxygen and methane/hydrogen/oxygen mixtures are studied by measuring auto ignition delay times in a rapid compression machine (RCM) at conditions relevant to knock in gas engines (950<T<1100K and 10<P<70bar). In addition, insight into changes in soot and NO x formation in methane flames is gained by measuring the spatial profiles of C 2 H 2 and HCN in atmospheric pressure, one-dimensional CH 4 /air and CH 4 /H 2 /air flames. All measurements are compared with the results of numerical calculations designed to predict the behavior of these experimental systems. 6

12 Chapter 1 CHAPTER 1 Combustion properties of homogeneous reacting gas mixtures 7

13 Chapter Motivation to study the combustion properties of CH 4 /H 2 gas mixtures Consider a homogeneous (premixed) fuel/oxidizer gas mixture. A premixed mixture is characterized by the equivalence ratio, ϕ, which expresses the ratio of fuel and oxidizer the unburned mixture. This is given by, [ Fuel] 1 ϕ =., (1.1) [ Oxidizer] f st where the amounts [fuel] and [oxidizer] can be expressed in molar, volume or mass units, and f st is the ratio of fuel to oxidizer under stoichiometric conditions using the same units. Here we will generally use moles or mole fraction as units. A mixture is said to be stoichiometric (ϕ=1) when fuel and oxidizer are present in the ratios prescribed by the balanced chemical reaction for combustion: C H n n + ( x + ) O 2 xco 2 + H O, (R1.1) 4 2 x n 2 If the oxidizer in the unburned mixture is in excess, the mixture is said to be fuel-lean (ϕ<1), while the mixture is called fuel-rich (ϕ>1) when an excess of fuel in the unburned mixture is present. The gas mixture can remain unreacted, such as in fuel-air mixtures at room temperature in the absence of an ignition source, or the fuel and oxidizer react (combust) to form products. Combustion can take place either in a flame (a reaction front propagates subsonically through the mixture) or in a non-flame mode ( homogeneous combustion, reaction occurs simultaneously everywhere in the mixture). To understand which mode of combustion takes place under a given set of conditions (temperature, pressure, equivalence ratio), it is necessary to study the chemical processes in the system in detail. The overall combustion process can be described by reaction (R1.1). However, it is unrealistic to think that combustion proceeds via this single reaction because it would require breaking high-energy bonds, which at room temperature makes this reaction extremely slow. Instead, combustion occurs in a sequential process involving many reactive intermediate species. To illustrate this process, we first consider a H 2 -O 2 gas mixture. The conversion of hydrogen and oxygen to water starts with the formation of reactive species (radicals) 8

14 Chapter 1 to initiate a chain of reactions [1]. The reactions in which radicals are formed from stable species are called chain-initiation reactions, and an example of a chaininitiation reaction is the (endothermic) dissociation reaction: H + M = 2H + M 436kJ / mole, (R1.2) 2 The H radicals formed in reaction (R1.2) can react further with oxygen molecules, forming two new radicals, OH and O, H + O2 = OH + O 70.6kJ / mole. (R1.3) This reaction (R1.3), in which two radicals are created for each radical consumed is called a chain-branching reaction and is crucially important in combustion processes. The formation of the radicals OH and O can lead to further chain branching via O + H 2 = OH + H 8.2kJ / mole. (R1.4) In addition, there are reactions in which the number of radicals does not change, such as OH + H = H O + H 63.21kJ / mole, (R1.5) which are called chain-propagating reactions. The reactions in which radicals react to stable species without forming another radical are called chain-terminating reactions, as in H + OH + M = H 2 O + M kJ / mole. (R1.6) Although not strictly chain terminating, since HO 2 is a radical, the reaction H + O + M = HO + M kJ / mole (R1.7)

15 Chapter 1 is often considered chain terminating because, compared with the flame radicals, H, O and OH, the HO 2 radical is relatively unreactive. At low pressures, reaction (R1.7) is an important chain-terminating reaction because the mildly reactive HO 2 radicals diffuse to the wall, where they react at the surface. Summing the chain-branching and chain-propagating reactions (R1.3+R1.4+R1.5+R1.5) results in H + H + O 3H + 2H O 47.62kJ / mole, (R1.8) from which we can see that starting with one radical, three radicals are formed from the reactants in this simplified mechanism. For quantitative description of chemical processes the rate of change of the species concentrations (formation and consumption) should be determined. For species A in an arbitrary bimolecular elementary reaction, k f aa + bb cc + dd, (R1.9) kr this is expressed as: da a b c d = k f [ A] [ B] + kr[ C] [ D], (1.2) dt where A,B,C,D denote the different species in the reaction, a,b,c,d are the stoichiometric coefficients of species A,B,C,D, respectively, and k f and k r represent the forward and reverse rate coefficient of the reaction. For example, the rate of change of the oxygen radical in reaction (R1.3) is expressed as, d[ O] R1.3 R1. 3 = k [ H ][ O2 ] k r [ OH ][ O] dt f +. (1.3) The rate coefficients k f and k r are connected through the equilibrium constant K w. The reaction rate constant k of a reaction is generally assumed to have a modified- Arrhenius temperature dependence, b E k AT exp A = RT, (1.4) 10

16 Chapter 1 where A is the pre-exponential factor, b the temperature exponent, T the temperature, R the universal gas constant and E a the activation energy, which corresponds to the energy barrier that has to be overcome during reaction. Here should be mentioned that the activation energy is always higher than the heat of the formation. Generally, the more exothermic a reaction is, the smaller the activation energy. As can be seen from reactions (R1.3-R1.5), the rate of formation of the important free radicals ( H + OH + O) is proportional to the concentration [n] of the radicals with some coefficient α. The rate of consumption of radicals (chain termination) is proportional to the concentration [n] as well (R1.7), with some rate β; the rate of chain initiation, denoted as γ, is independent of the concentration [n] (R1.2). Thus, in generalized form, the rate of change of the concentration of free radicals can be expressed as, d[ n] = γ + ( α β )[ n], (1.5) dt From equation (1.5) three different scenarios can be derived, which are presented schematically in figure 1.1 a [2]. Figure 1.1a) Schematic of the growth of the concentration of free radicals [n] in time. b) Schematic of the growth of free radicals [n] in time for the cases with and without heat release. 11

17 Chapter 1 For the condition α>β the concentration [n] increases exponentially in time, and ignition takes place. When α<β, d[n]/dt becomes zero, and no exponential growth of free radicals occurs (no ignition). The condition α=β results in a linear growth of the concentration of free radicals, and defines the ignition limit. Equation (1.5) shows that the growth of the concentration of free radicals is determined by the competition between the chain branching (R1.3) and chain terminating reactions (R1.7). A very important parameter in this competition is the temperature, since the chain branching reaction (R1.3) has large activation energy E a [3] while that of the chain terminating reaction (R1.7) is small [4]. Thus, the value of α (chain branching) is strongly dependent upon the temperature, and β (chain terminating) is more or less independent of the temperature. At low temperatures the endothermic chain branching reaction will not proceed rapidly, so the value of α is much smaller than β (α<β), and ignition does not occur. Increasing the temperature results in an increase in α, while β remains unchanged; at sufficiently high temperatures α will be larger than β and ignition occurs. The temperature during the early period of the ignition process remains more or less constant because the heat release from the branching and propagating reactions (R1.8) is small, but as the radical concentration grows, exothermic reactions such as (R1.6) and (R1.7) will produce substantial quantities of heat. If the heat produced by the exothermic reactions in system exceeds the rate of heat loss to the surrounding, the temperature in the system will rise. Since the rate of reaction, and thus the rate of heat release, grows exponentially with temperature, the overall reaction will auto-accelerate, that is, the system will explode. The time before explosion takes place is called autoignition delay time (figure 1.1b). In this example, if no heat accumulates in the system (T=constant), no auto-acceleration of the reaction rate takes place; in this case we speak of non-explosive reactions, both situations are shown in figure 1.1b. As described above, the dominance of chain branching reaction (R1.3) characterizes the high temperature regime, while at low and intermediate temperatures the in essential chain terminating reaction (R1.7) competes effectively with reaction (R1.3) [5]. Since the rate of a three-body reaction increases with pressure much faster than the rate of a two-body reaction, there exists a pressure above which reaction (R1.7) exceeds the rate of the competing reaction (R1.3). Reaction (R1.7) is only a chain terminating reaction when the produced HO 2 radicals will diffuse to the wall 12

18 Chapter 1 and recombine to stable molecules without having undergone a reaction. At high pressures, species collide much more frequently. Therefore, at sufficiently high pressures the HO 2 radicals will be frequently interrupted in its path to the walls by reacting with H 2 molecules to produce H and H 2 O 2 (R1.10) [5], HO + H = H O + H 72.45kJ / mole. (R1.10) The H radicals so produced can contribute to chain branching via (R1.3) or will generate more HO 2 via reaction (R1.7) and H 2 O 2 itself can contribute to branching via [5], H O + M = 2OH + M 214.6kJ / mole. (R1.11) 2 2 Thus at high pressure and moderate temperatures reaction (R1.7) will dominate over (R1.3), and the number of active centers will grow (α>β) via the sequence of reactions (R1.7), (R1.10), and (R1.11). Instead of heating the cold gas mixture by the heat release of exothermic reactions as in this example of a closed homogeneous system, in flames, the mixture is heated by conduction from the hot flame gases. Furthermore, radicals needed to decompose the fuel are also transported from the high temperature region of the flame. As in the closed system described above, the rate of formation of radicals controls the overall rate of reaction in flames. For flames, however, the chain initiation and chain-terminating reactions are less important in the formation/destruction of radicals (α>>β), and the reactions (R1.3)-(R1.5), responsible for the growth of the radical pool, dominate the overall reaction rate in H 2 -O 2 flames [6]. The combustion chemistry of hydrocarbon fuels is much more complicated than that of hydrogen. As an example, we consider a CH 4 -O 2 mixture. Under flame conditions (α>>β), the most important chain branching reaction in the oxidation of methane is also reaction (R1.3) [6]. After radicals are transported into the unburned gas mixture, methane is attacked by the radicals, as in CH + H CH + H 13kJ / mole. (R1.12)

19 Chapter 1 As can be seen from figure 1.2, the rate coefficient of reaction (R1.12) [7] is much larger than that for reaction (R1.3) [3]. Thus reaction (R1.12) competes effectively with reaction (R1.3) for H atoms and reduces the chain branching rate. Figure 1.2. Reaction rate expressions for reaction R1.3 [3] and reaction R1.12 [7]. Furthermore, the very reactive H radical is replaced in reaction (R1.12) by the unreactive CH 3 radical. These two processes result in a slow conversion of methane and contributes to the low burning velocity of methane (40 cm/s) as compared to hydrogen (340 cm/s) [8]. If the CH 4 -O 2 mixture under consideration is at moderate or low temperature (T below roughly 1100K), chain branching reaction (R1.3) is too slow to provide a sufficient branching rate for autoignition, and a different reaction path dominates [9]. These paths are extremely complex and strongly dependent upon temperature and pressure [10]. At sufficiently high pressures, reactions involving the radical HO 2 become important in the low temperature regime, for example [9], CH + HO CH + H O 85.5kJ / mole. (R1.13) The oxidation of the CH 3 formed, and the development of the radical pool, is complicated and slow. This process dominates most of the ignition delay period and is 14

20 Chapter 1 characterized by the accumulation of significant amounts intermediate species such as H 2, C 2 H 6, CH 2 O and H 2 O 2. The decomposition of H 2 O 2 via reaction (R1.11) and the oxidation of CH 2 O via a sequence of reactions lead to a sharp increase in the concentration of free radicals [10] and ultimately ignition occurs (α>β) [10]. The H (R1.12) and HO 2 (R1.13) scavenging reactions compete effectively with R1.3 and R1.10 respectively, and thus effectively reduce the chain branching rate in the CH 4 -O 2 system. This, together with the formation of the relatively unreactive CH 3 radical, contributes to the fact that CH 4 -O 2 mixtures tend to auto ignite slower than H 2 -O 2 mixtures [6], illustrated in figure 1.3. Figure 1.3. Computed autoignition delay times for stoichiometric H 2 /air and CH 4 /air mixtures at P=30 bar. Calculations were made using the GRI-Mech 3.0 mechanism [11]. Besides their effects on the combustion properties, such as burning velocity and ignition, the differences in the combustion chemistry of methane and hydrogen have significant consequences for pollutant formation. One of the main consequences is that during the combustion of methane (and other hydrocarbons) carbon containing pollutant species like soot, HCN and CO are formed, while the only pollutant from hydrogen combustion is NO x. Moreover, the formation of NO in methane combustion is different than that from H 2 combustion; in methane flames an additional mechanism exist that produces NO via the hydrocarbon intermediate CH [12]: 15

21 Chapter 1 CH + N 2 products NO. (R1.14) ( NCN?, HCN?) This mechanism is particularly important under fuel rich conditions. A challenging task is to understand the possible changes in the combustion chemistry caused by addition of hydrogen to methane and how this affects combustion properties like pollutant formation and ignition delay. Since there is a clear distinction between the chemistry in flames and ignition, it is necessary to study both. To gain understanding in the underlying chemical kinetics and physical processes involved in these two kinds of combustion process, it is necessary to analyze the combustion processes quantitatively by solving the governing equations with detailed chemical mechanisms. 1.2 Governing equations for a homogeneous reacting gas mixture in a closed system The time-dependent behavior of a closed system containing a reacting gas mixture is described by the system of the conservation equations for mass and energy. Since no mass can be formed or destroyed by chemical reaction, the total mass of the closed systems remains constant over time: d( ρv) d K = ρvy k = 0, (1.6) dt dt k = 1 where ρ is the overall mass density, V is the system volume and Y k is the mass fraction of k-th component in the gas mixture. The mass fractions of individual species change in time according to dy ρ k = ωk Wk, k = 1.K (1.7) dt 16

22 Chapter 1 where ω k and W k are the molar chemical production rate per unit of volume and molecular weight of the k-th species, respectively. The density is related to temperature T, pressure p and composition through the ideal gas equation of state: p ρ = W, (1.8) RT K Y k where W = 1/ is the average molecular weight of the mixture. The system k = 1 M k (1.6) (1.8) consists of (K+1) linearly independent equations and contains (K+3) unknown parameters: ρ, p, T, V and Y k. Since the system contains more unknowns than equations, it can be solved only when two unknown parameters (for example, the measured temperature and pressure) are used as input. The number of input parameters can be decreased to one if the energy conservation equation is added to the system. The energy conservation equation can be derived from the first law of thermodynamics, which states that heat δq added to the system is equal to the sum of the change of its internal energy du and the work PdV of the system against an external force: du + PdV = δq. (1.9) Equation (1.9) can be rewritten as du dt dv dq + P = = Q dt dt loss. (1.10) The internal energy of the mixture is given by U K = ρvykuk, (1.11) k= 1 where u k is a specific energy of the k-th component. After differentiating expression (1.11) and substituting in (1.10) one receives 17

23 Chapter 1 1 d dt ρ 1 K. Cv + p + ukωkwk = qloss, (1.12) dt dt ρ k= 1 where q loss = Q /(ρv) is the heat loss per unit of mass and C v is the specific heat of loss system at constant volume. For an adiabatic mixture of inert gases (ω k = 0 and q loss = 0), equation (1.12) can be easily integrated, resulting in the following expression T CvW R T0 dt T ρ = ln. (1.13) ρ 0 It is common to use the ratio of molar heat capacities at constant pressure and constant volume, γ, and a specific volume v, instead of C v and ρ. In this case, equation (1.13) can be rewritten as T 1 ( γ 1) T 0 dt T V = ln 0. (1.14) V Several simulation programs have been developed to solve the set of governing equations. The program used in this study is SENKIN [13], and runs in conjunction with pre-processors from the CHEMKIN library [14], which incorporate the chemical mechanism and thermodynamic properties. 1.3 Laminar premixed flames Flat laminar premixed flames are ideally suited for combustion research, since the one-dimensional character offers great advantages for modeling and unambiguous model-experiment comparison. Moreover, the structure of these flames is representative for many practical flames. The structure of laminar premixed flames 18

24 Chapter 1 can be divided in three zones (figure 1.4), the preheat zone, the flame front (reaction zone) and the post-flame zone (burned-gas zone). In the preheat zone the unburned gas mixture is heated by conduction and diffusion of species from the flame front; this zone can be considered as chemically inert. The flame front, located downstream of the preheat zone is a thin zone (in the order of 1 mm at atmospheric pressure) in which the fuel is rapidly oxidized by radicals from the post-flame zone as described above, leading to a steep gradients in temperature and species concentrations. The flame front is rich in radicals and intermediate species. Although the temperature and major species in the post-flame zone are close to their equilibrium value, the concentrations of minor species can differ substantially from their equilibrium value. In the post-flame zone, the system goes to equilibrium predominantly via radicalrecombination reactions such as (R1.6). Figure 1.4. Schematic illustration of the structure of a premix one-dimensional flame. Premixed flat flames can be characterized by the free-flame laminar burning velocity, v L. In the laboratory system, where the cold gas moves with velocity v u, the flame front propagates with velocity v u -v L. We can consider three situations regarding the stability of a idealized one-dimensional flame. If the cold gas velocity is larger than the laminar burning velocity, v u >v L, the flame front propagates upstream. When 19

25 Chapter 1 the burning velocity, v L is equal to the velocity of the unburned gasses, (v=v L ) the flame front is stationary in space, and if v u <v L the flame front will be convected downstream. The laminar burning velocity and the temperature of the burned gas are completely determined by the properties of the unburned mixture, such as the equivalence ratio, temperature and the identity of the fuel [2]. Figure 1.5 shows the interaction of the idealized 1-D flame with a porous-plug burner. Figure 1.5a illustrates a flat flame stabilized on a burner where the unburned gas velocity is set equal to the free-flame laminar burning velocity (v u =v L ). In this situation, all heat generated during combustion is transferred completely into the gas mixture and the flame is essentially adiabatic (neglecting flame radiation). Lowering the unburned gas velocity v u causes propagation of the flame front towards the burner surface. Since the porous plug is too dense to allow propagation of the flame into the burner, the flame is stopped in its upstream propagation. In this case, the flame transfers heat to the burner by conduction, lowering the flame temperature and thus lowering the actual burning velocity of the flame v L'. The flame transfers enough heat to the burner to reach a stationary situation (v u = v L' ), illustrated in figure 1.5b. This type of flame is called a burner-stabilized flame. Figure 1.5. a) Adiabatic flat flame (freely propagating flame) b) Burner-stabilized flat flame Further decrease in the unburned gas velocity results in increasing heat loss and drop in temperature; ultimately the temperature drops to such a level that α<β and the flame extinguishes. 20

26 Chapter Governing equations for a one-dimensional laminar flame The description of one-dimensional laminar flames is based on the conservation equation for mass, species mass fraction and energy. Using the assumptions that onedimensional laminar flames are: (1) stationary (all flame parameters are independent of time), (2) the system is at constant pressure and (3) effects due to viscosity, radiation and external forces are negligible [2,15], the conservation equations governing the behavior of these flames can be summarized as follows: overall conservation of mass d( ρv) dx dm = dx = 0, (1.15) where v is the mass averaged flow velocity, x is the distance along the line normal to the burner surface and M is called the mass flux. conservation of species d( ρyk( Vk + v)) = ω k W k, k=1.k (1.16) dx where Y k is the mass fraction and V k is the diffusion velocity, which accounts for the effect of molecular transport due to concentration gradients of the k th species [2,16]. Since mass can neither be destroyed nor formed in chemical reactions it follows from (1.15) and (1.16) that, K d( ρy ( )) K k Vk + v d( ρv) = ω k W k = = 0 dx dx k= 1 k= 1 k=1 K. (1.17) Addition of the ideal gas equation of state (1.8) to the system of equations (1.15, 1.16) results in a system containing (K+1) linear independent equations. Assuming that the diffusion velocity V k is a known function of temperature and species concentrations, 21

27 Chapter 1 the system contains (K+2) unknown parameters (T, ρ, v and Y k ). Therefore an additional equation should be introduced to solve the system of equations: conservation of energy d dt ρyk( v+ Vk) Hk λ = 0, (1.18) dx dx k where H k the specific enthalpy of species k and λ the thermal conductivity coefficient. With the proper choice of the boundary conditions for one-dimensional flames, it is possible to solve the governing equations [2,16]. Various software packages have been developed, which are able to calculate the one-dimensional flame structure in only a few minutes by solving the set of governing equations. The simulation program used in this study is the PREMIX code [17]. This code is included in the CHEMKIN II simulation package [13]. This package operates using a reaction mechanism data file as input, along with thermal and transport properties of the species involved in the mechanism. The program is able to calculate temperature- and mole fraction profiles in both burner-stabilized and free flames. 1.4 Chemical mechanisms In the last decades chemical kinetic mechanisms have been developed to model combustion of hydrogen (for example, see [18]) and hydrocarbon mixtures ([11,19], among many others). These mechanisms, used to describe the transformation of reactants into products, may contain hundreds of species and thousands of elementary reactions. Improvement of the mechanisms currently in use is necessary, since none of them can be regarded as comprehensive [20], i.e. accounting for all combustion phenomena and the predictive power is only accurate for a small range of parameters. In order to improve the existing chemical mechanisms, they should be validated against experimental data, where parameters are varied in a well-defined manner. Sensitivity and rate-of-production analyses are used to design and optimize models. Using these methods rate-limiting steps and characteristic reaction paths can be identified [2]. 22

28 Chapter 1 The experimental data obtained in this study have been modeled using different mechanisms. One of these mechanisms is GRI-Mech 3.0 [11], which is widely used and has arguably become the industry standard for methane in the research community. This mechanism is optimized to model natural gas combustion and contains 325 reactions and 53 species, including reactions that describe NO formation and reburn chemistry. 23

29 Chapter 1 Literature 1. G. Dixon-Lewis., D. J. Williams., Comprehensive Chem. Kin. 17, (1977). 2. J. Warnatz, U. Maas, R. W. Dibble, Combustion, (Springer, Berlin, 1996). 3. C.-L. Yu, M. Frenklach, D. A. Masten, R. K. Hanson, C. T. Bowman, J. Phys. Chem. 98 (1994) M. Frenklach, H. Wang, M. J. Rabinowitz, Prog., Energy Combust. Sci. 18: (1992) B. Lewis, Q. von Elbe, Combustion Flames and Explosions of Gases, (Third edition 1987). 6. C. K.Westbrook, F. L. Dryer, Prog. Energy. Combust. Sci. 10 (1984) J. M. Rabinowitz, J. W. Sutherland, P. M. Patterson, R. B. Klemm, J. Phys. Chem. 95 (1991) B. E. Milton, J. C. Keck, Combust. Flame 58 (1984) C. K.Westbrook., Proc. of the Combust. Inst. 28 (2000) J. Huang, P. G. Hill, W. K.Bushe, S. R. Munshi, Combust. Flame 136 (2004) G. P. Smith, D. M. Golden, M. Frenklach, N. W. Moriarty, B. Eiteneer, M. Goldenberg, C. T. Bowman, R. K. Hanson, S. Song, W.C. Gardiner, V. Lissanski, Z. Qin, C. P. Fenimore, Proc. Combust. Inst. 13 (1971) A. E. Lutz, R. J. Kee, J. A. Miller, SENKIN: A FORTRAN program for predicting homogeneous gas phase chemical kinetics with sensitivity analysis. Sandia Report SAND Sandia National Laboratories, (1987). 14. R.J. Kee, F.M. Rupley, J.A. Miller, CHEMKIN II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics., Sandia National Laboratories, (1989). 15. R. M. Fristrom and A. A. Westenberg, Flame Structure, (McGraw-Hill, New York, 1965). 16. R. J. Kee, F. M. Rupley, J. A. Miller, M. E. Coltrin, J. F. Grcar, E. Meeks, H. K. Moffat, A. E. Lutz, G. Dixon-Lewis, M. D. Smooke, J. Warnatz, G. H. Evans, R. S. Larson, R. E. Mitchell, L. R. Petzold, W. C. Reynolds, 24

30 Chapter 1 M. Caracotsios, W. E. Stewart, P. Glarborg, C. Wang, and O. Adigun, CHEMKIN Collection, Release 3.6, Reaction Design, Inc., San Diego, CA, (2000). 17. R. J. Kee, J. F. Grcar, M. D. Smooke, J. A. Miller, Fortran program for modelling steady one-dimensional premixed flames. Sandia Report SAND Sandia National Laboratories, (1985). 18. O. M. Conaire, H. J. Curran, J. M. Simmy, W. J. Pitz, C. K. Westbrook, Int. J. Chem. Kin. 36 (2004) J. M. Simmie, Prog. Energy Combust. Sci. 29 (2003)

31 Chapter 2 Chapter 2 The Rapid Compression Machine Experimental Techniques, Procedures and Setup 26

32 Chapter Background Over the years, several facilities have been used to investigate autoignition under strictly controlled experimental conditions, including flow reactors, shock tubes and rapid compression machines (RCM). While each of these facilities has its merits, their utility is restricted to certain ranges of pressure, temperature and ignition time. Flow reactors: In a flow reactor, fuel is injected into a flowing air stream at high temperature and/or pressure. The combustible mixture propagates through the reactor and, depending on the velocity ignites at some distance downstream the fuel injector location. Because the reaction zone is spread over a large distance, the flow reactor offers the advantage of relatively simple measurements of the evolution of species concentrations during the ignition process. The main drawback is that pressures achievable in flow reactors are relatively low; further, since flow reactors makes use of electric heaters, the maximum air temperature is limited, on the order of 1000K. One of the most advanced flow reactors was developed at Princeton University, and provides pressures up to 20 bar and temperatures up to 1200K [1,2]. Shock tubes: A shock tube uses the compressive heating of a shock wave to bring a premixed combustible mixture to high temperature and pressure in a very short time. A shock tube is ideal for studying ignition phenomena with short characteristic times (order of tens of microseconds) under the conditions obtained. A limitation of this technique is that the well-controlled test conditions persist for less than 5 ms [3]. Rapid Compression Machines (RCM): The operating principle of the RCM is to compress a homogeneous fuel/oxidizer mixture to moderate temperatures (T max 1200K) and high pressures (P max 70bar) in a cylinder by the motion of a piston. The RCM offers the advantage that the temperature and pressure of the compressed mixture can be sustained for times longer than 10ms [4]. Moreover, it provides a simple method of simulating the processes that take place in practical devices such as spark engines and Homogeneous Charge Compression Ignition (HCCI) engines. The time needed to compress the test gas mixtures limits the minimum characteristic time of investigation to 1ms. Several rapid compression machines have been developed and used to study autoignition. The RCM developed at the University of Science and Technology at 27

33 Chapter 2 Lille is a right angle dual piston design RCM [5]. One of the pistons is air driven and is connected by way of a cam to the other piston that compresses the mixture. The cam controls the length of the stroke, the initial and final position of the compressing piston, and prevents piston rebound after ignition. The maximum compression ratio achievable with this machine is 10. Maximum pressures and temperatures after compression are reported around 17 bar and 900K, with total times of compression of ms. Minetti et al. used this RCM to study autoignition and two-stage ignition of several hydrocarbon fuels [6-8]. In addition, this group performed measurements of the temperature distribution in their RCM [4] and found that the gas temperature is homogeneous for 15ms after compression, which is then distorted due to heat loss to the wall. Griffiths et al. (University of Leeds) studied autoignition behavior of several fuels [9-12] using an RCM that consists of a pneumatically driven piston. In support of their experimental work they examined the development of the temperature field in the combustion chamber of their machine [13,14] and observed that piston motion during compression causes a roll-up vortex that moves cold gas from the wall into the core. This resulted in a region with adiabatically heated gas directly after compression containing a plug of colder gas. The Leeds RCM has a maximum compression ratio around 15 and is able to compress the mixture in 22 ms. Final pressures up to 20 bar and temperatures up to 1000K have been reported in this machine. Park and Keck (MIT) developed an RCM [15,16] that consists of a hydraulically operated piston-cylinder assembly. They also used a piston head with a special crevice, designed to capture vortices created during compression [15,16], to improve the homogeneity of the core gas. Lee and Hochgreb (MIT) optimized this piston design for the suppression of the vortices [17,18], using results of detailed modeling. Compression ratios of 19, maximum peak temperatures of 1200K and maximum peak pressures of 70 bar can be achieved in this RCM. The gas mixture can be compressed within 10 to 30 ms. Several autoignition experiments have been performed with this machine [19,20]. Simmie and coworkers (National University of Ireland, Galway) used an RCM, originally built at Shell laboratories [21] that uses two horizontally opposed pneumatically driven pistons to rapidly compress the gas mixture. For this machine, the maximum compression ratio reported is 13, the compression time is 10 ms, the maximum peak compression pressure is 44 bar and temperatures up to 1060 K have been reported. The machine has been used to study 28

34 Chapter 2 methane ignition [22], among other fuels. Also, this group confirmed the importance of the crevice in the piston head [22,23]. Recently, a free-piston RCM had been developed by Donovan et al. (University of Michigan). Compression ratios between 16 and 37, peak pressures around 20 bar and peak temperatures of 2000K have been reported [24] using this RCM. Given the wide range of pressures and temperatures achievable, we chose the MIT design to study autoignition of CH 4 /H 2 /oxidizer mixtures. For this reasons a replica of the MIT RCM was built in our laboratory and used to study autoignition. 29

35 Chapter Design and Operation Conceptually, the rapid compression machine simulates a single compression event of an internal combustion engine. It is designed to compress a gas mixture in a short time to high temperature and pressure while maintaining a well-defined uniform core temperature in the reaction chamber (adiabatic core). Fast compression is necessary to prevent substantial heat losses and radical build up before the end of the compression. The piston is driven pneumatically and is decelerated smoothly by a special hydraulic damper to reduce the impact velocity at the end of the compression. The machine contains only two moving parts, the fast acting valve and the piston. Both are made of aluminum, while all other parts are made from steel. The piston used is hollow, to reduce its mass and to have uniformly distributed stress throughout the body [17]. The piston head is removable and can be replaced by heads with other crevice configuration. Figure 2.1. Sketch of the cross sectional view of the RCM. 30

36 Chapter 2 The RCM, shown schematically in figure 2.1, and in detail in Appendix A.1, includes a nitrogen-filled driving chamber, a speed-control oil chamber, an oil reservoir chamber, a fast acting valve, a combustion chamber and a piston. The speedcontrol oil chamber and the oil reservoir chamber, alternately connected and separated by the fast acting valve are part of the hydraulic system that controls the movement of the piston. The detailed sequence of operation for the RCM is given in Appendix A.2. Briefly, after the RCM is triggered, the piston is in the down position and the fast acing valve is in the up position, thus connecting the speed control oil chamber and the oil reservoir chamber. To prepare for the next run, the piston is moved up until it hits the stroke stop by pressurizing the oil reservoir chamber with 3 bar nitrogen. The fast acting valve is than moved down by pressurizing the chamber above the fast acting valve with 7 bar nitrogen, and locked in down position using 70 bar oil pressure. The speed control and the oil reservoir chamber are now no longer connected hydraulically. By pressurizing the speed control oil chamber with 48 bar high-pressure oil, the piston is firmly locked in place against the stroke stop. After loading the combustion chamber with the test gas mixture, the driver chamber is pressurized with 35 bar nitrogen. The force on the piston created by the 35 bar nitrogen pressure in the driving chamber is lower than the opposing force of oil on the hydraulic piston, and hence the piston assembly is held in position by the stroke stop. By opening the solenoid valve, the 70 bar oil pressure on the fast acting valve is released and the fast acting valve will be pushed up by the 48 bar oil pressure in the speed control chamber. The forces between the driving chamber and speed control chamber are no longer balanced, and the pressure in the driving chamber causes the piston to accelerate downward, compressing the test gas in the combustion chamber. Subsequently, the piston s acceleration slows, and the piston moves with constant velocity until it is smoothly decelerated by a hydraulic damper [16]. In the final stage, the deceleration force and velocity are reduced to zero, so that the final stop of the piston at the bottom plate occurs without rebound. The piston is held firmly by the force of driving nitrogen, which is greater than the force of the compressed gas mixture in the reaction chamber. This allows combustion to take place at constant volume. Since the area ratio of the piston on the driving side compared to the side of combustion chamber is 4:1, the pressure inside the combustion chamber may be a factor of 4 larger than the maximum pressure in the driving chamber before the piston 31

37 Chapter 2 will move. In the present construction, gas mixtures can be compressed with total compression times of ms up to pressures around 70bar. The piston speed can be controlled, by varying the pressure in the driving chamber. The characteristics of the RCM are presented in Table 2.1. To cover a wide range of compression ratios, the rapid compression machine was designed with adjustable piston stroke and clearance height. The piston stroke, which is determined by the initial position of the piston, can be varied by turning the stroke adjustment screw, see appendix A.1.. The clearance height can be changed by replacing the clearance ring in the combustion chamber. To simulate temperatures and pressures realistic for gas engines, different combinations of compression ratios, initial pressures of the test gas and heat capacity of the diluent gases are used in this study. Table 2.1 RCM Characteristics Cylinder bore Maximum stroke Maximum compression ratio Clearance height Piston length Maximum driving pressure Maximum compression pressure Compression time 50.8 mm 160 mm mm 172 mm 35 bar 70 bar ms Experimental System Appendix A.3 shows the overall diagram of the RCM experimental system, containing all main lines, pressure meters, oil drums, valves and the oil reservoir. All lines are made from stainless steel with an inner diameter of 11 mm. The orifice diameter of the solenoid valve has the same inner diameter (11 mm) as the lines, to allow maximum throughput of oil. The high-pressure oil was supplied to the speed control chamber ( 48 bar) and to the fast acting valve ( 70 bar) from two high- 32

38 Chapter 2 pressure oil accumulators. These oil accumulators contain oil and a bladder filled with nitrogen to prevent mixing of nitrogen with oil. After 25 runs the accumulators were refilled by oil from the main oil reservoir. Compressed nitrogen from five fifty-liter, 200 bar nitrogen bottles provided the required pressures in all parts of the system. The operation pressures used, recommended by Park [16], are given in Appendix A Gas filling system and filling procedure All test gas mixtures were prepared in advance in a 10-liter gas bottle and used to charge the combustion chamber at the required pressure. The gas filling system is shown in figure 2.2. Before preparing the gas mixture, the gas bottle and the gas lines connected to it are evacuated to less than 0.5 mbar using a vacuum pump. After adding an individual component the bottle is closed. Subsequently, the gas lines are again evacuated and the next mixture component is added to the bottle. The test mixtures thus prepared were allowed to mix 24 hours to ensure homogeneity. Before filling the combustion chamber, the poppet valve and the solenoid valve are opened, and the whole system is evacuated to a pressure below 0.5 mbar. The combustion chamber was filled with the gas mixture to the desired initial pressure, by opening the bottle that contains the test gas. The poppet valve is then closed and the mixture is ready for compression. After each run, the compressed gases in the RCM were vented to the outside air, and the chamber was evacuated again before preparing the next run. The solenoid valve in the gas-filling line was included for safety purposes, and electrically connected such that when the solenoid valve used to trigger the fast-acting valve is open, the solenoid in the gas-filling line is always closed. This prevents flame propagation back to the gas-mixture bottle when the poppet valve is not properly closed. All test gases used in this study have purity greater than 99.5%. The composition ratios of the gas mixtures are calculated from the measured partial pressures of the individual gases. 33

39 Chapter 2 Figure 2.2. Gas handling system Instrumentation and data acquisition An MKS baratron diaphragm pressure gauge (type 722A) was used for measuring all partial pressures of the components in the test mixture, and all other pressures in the gas filling system. This pressure meter has an operating range from mbar with an accuracy of 0.5% of reading. The dynamic pressures in the combustion chamber during compression and throughout the post-compression period were measured using a Kistler 6025B piezoelectric pressure transducer (range bar, linearity ± 0.1%) placed at the bottom of the combustion chamber. The signal from the transducer was amplified by a 5010B Kistler charge amplifier, recorded digitally by an oscilloscope with a sample rate of 500 khz and 16-bit resolution, and processed by a PC. The initial temperatures of the mixture were measured by a Pt-Rh thermocouple with an accuracy of ± 0.2K, located at the wall of the combustion chamber. The data acquisition was triggered simultaneously with the opening of the solenoid for the fast-acting valve. 34

40 Chapter Determination autoignition delay time A typical measured pressure trace is presented in figure 2.3. The gas mixture is compressed in 20ms, to a peak pressure that indicates the end of the compression event. The majority of the pressure rise in the compression period takes place in a very short time (<3 ms). During this rapid compression heat losses and radical built up are not substantial. After the peak compression pressure is reached, the pressure drops gradually due to heat transfer to the walls. Subsequently, heat release due to exothermic reactions causes a slight increase in pressure, followed by a sharp increase in pressure indicating ignition. Figure 2.3. Typical measured pressure trace for a stoichiometric CH 4 /H 2 //O 2 /N 2 /Ar mixture, with the definitions of autoignition delay time, peak pressure and compression time. The dotted line shows the calculated adiabatic peak pressure for the given compression ratio. The auto ignition delay time is defined in this study as the time interval between the peak pressure P c that marks the end of compression and the time of maximum pressure rise during ignition. 35

41 Chapter Temperature Determination To study autoignition behavior, one must know the instantaneous temperature in the combustion chamber, since chemistry is very sensitive to temperature. However, measuring the temperature in the reaction chamber directly by optical methods is problematical, given the characteristic test times (order of a millisecond) and the difficulty of making the reaction chamber optically accessible. The use of thermocouples also permits in-situ temperature measurements, but the presence of the thermocouple in the combustion chamber can significantly influence the measurements, since the test gas can interact with the surface of the thermocouple. For example, the surface of the thermocouple may act as a catalyst for chemical reactions, and unintentionally induce ignition. The most straightforward method is to calculate the temperature from the instantaneous measured pressure, assuming the existence of an adiabatic core in the RCM chamber [18]. When an adiabatic system goes from one state (P 1,T 1,V 1 ) to another state (P 2,T 2,V 2 ), initial and final parameters are related to each other by the isentropic relation (1.13) of an ideal gas, which can be rewritten as, T2 V1 1 ln( ) = d lnt, V2 γ 1 T1 (2.1) T2 1 P ln ln 2 d T =, γ 1 P1 T1 (2.2) where γ(t) is the ratio of temperature-dependent heat capacities of the mixture at constant pressure and constant volume, γ(t)=c p (T)/C v (T). The heat capacities used in this study are taken from [25]. In figure 2.4, the temperature dependence of γ for two inert gases, N 2 and Ar, and the mixture H 2 /CH 4 /O 2 /Ar/N 2 (0.95/0.05/1.85/3/4.3) used in our experiments is illustrated (for a pressure trace for this mixture, see figure 2.3). The figure shows that γ for Ar is much larger than that for both N 2 and the combustible mixture. Moreover, as expected for a monatomic gas, γ for Ar is constant while those for N 2 and the combustible mixture show a slight dependence upon the temperature. 36

42 Chapter 2 Figure 2.4, Temperature dependence of γ for the gas mixtures N 2, Ar and CH 4 /H 2 /O 2 /N 2 /Ar. The relations (2.1) and (2.2) show that the pressure and temperature of the gas only depend on the volumetric ratio and the specific heat capacities and the initial conditions (Pi, Ti) of the gas mixture. Thus for an ideal RCM, the temperature (T c ) and pressure (P c ) after compression can easily be calculated from equation (2.1) and (2.2) by measuring only the initial temperature (T i ) and pressure (P i ), and the mechanical compression ratio C R, which is defined as the ratio of the initial volume (V i ) to the final volume (V c ) of the reaction chamber. As an example, the pressures and temperatures are calculated, based on the mechanical compression ratios that can be obtained in our RCM, for the three aforementioned gases/mixtures using the relations (2.1) and (2.2), and presented in figure 2.5a and 2.5b. The calculations show that the larger γ(t) for Ar, in comparison to N 2 and the combustible mixture (figure 2.4), results in a much higher temperature and pressure after compression at identical compression ratio. In reality, the temperatures and pressures will be lower than those calculated due to heat losses during compression. As an example, for the compressed CH 4 /H 2 /O 2 /N 2 /Ar mixture with C R =22.5 and P i =0.49 bar, we measured a peak pressure of 32 bar (figure 2.3). However, simple adiabatic calculations show that at C R =22.5 the calculated temperature is 1050K (figure 2.5a) and P c /P i =80 (figure 2.5b), this results in a calculated compressed pressure of 40 bar at P i =0.49bar. 37

43 Chapter 2 Figure 2.5a) Temperature calculated as function of compression ratios for different gas mixtures at T i =295K, using the relation (2.1). b) Pressure ratio (Pc/Pi) calculated as function of temperature for different gas mixtures at initial temperature T i =295K, using the relation (2.2). It is more realistic to make the assumption of the existence of an isentropically compressed core region [11] that is unaffected by heat and mass transfer. In this case, the relations (2.1) and (2.2) are valid for the pressures, temperatures and volumes in the adiabatic core. To avoid the difficulty of calculating an effective compression ratio based on the unknown volume of the core gas within the combustion chamber, the ratio of measured pressures is used to calculate the temperature of the adiabatic core gas using equation (2.2). The uncertainty of the calculated core gas temperatures (T c ) is less than ±3.5K for all measurements. (Appendix A.5). 38

44 Chapter 2 Several studies [26,17] have indicated that vortices created by the motion of the piston causes unwanted mixing of cold boundary-layer gas into the compressed core gas, destroying the adiabatic core. It has been shown, both numerically [19,23,27] using CFD calculations and experimentally [27] by temperature mapping using the planar laser-induced fluorescence of acetone, that the incorporation of a specially designed crevice on the piston head successfully suppresses the vortex formation, and preserves the well-defined homogeneous core region intact. In this study, the creviced piston head based upon the best design from the MIT RCM [17,18] was used. 39

45 Chapter 2 Appendix A.1 Stroke adjustment Driving chamber Speed control chamber Piston Piston lock chamber Oil reservoir chamber Fast acting valve Combustion chamber Poppet valve Pressure transducer Figure A.1. Cross Sectional view of the Rapid Compression machine (RCM). 40