Dynamic Behaviors in Methane MILD and Oxy-Fuel Combustion

Subscriber access provided by MICHIGAN STATE UNIVERSITY | MSU LIBRARIES

Article

Dynamic behaviors in methane MILD/Oxyfuel combustion. Chemical effect of CO2. Pino Sabia, Giancarlo Sorrentino, Alfonso Chinnici, Antonio CAVALIERE, and Raffaele Ragucci Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef501434y • Publication Date (Web): 17 Feb 2015 Downloaded from http://pubs.acs.org on February 19, 2015

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 28

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

Dynamic behaviors in methane MILD/Oxy-fuel combustion. Chemical effect of CO2. P. Sabia°*, G. Sorrentino°°, A. Chinnici°, A. Cavaliere °°, R. Ragucci° ° Istituto di Ricerche sulla Combustione - C.N.R., Naples, Italy °° D.I.C.M.A.P.I. - Università Federico II, Naples, Italy KEYWORDS: temperature oscillations, adiabatic/non-adiabatic systems, CSTR, kinetics.

ABSTRACT The oxidation process of CH4/O2 mixtures diluted in CO2 under MILD/Oxy-fuel conditions was numerically investigated in a perfectly stirred flow reactor at atmospheric pressure. The analysis aimed to investigate the kinetics involved in fuel oxidation in systems highly diluted and strongly pre-heated. Particular attention was focused on the effects of CO2 on oxidation routes because it can significantly alter the kinetic pathways participating directly in key reactions, or indirectly in termolecular reactions as a third body species. Furthermore, adiabatic flame temperatures are lower with respect to air conditions because of the higher thermal heat capacity of CO2 in comparison to N2, thus modifying the kinetics promoted by temperature in combustion processes. The analyses were realized as a function of main system parameters, systematically changing inlet temperatures and mixture compositions. Results showed the establishment of complex dynamic behaviors in terms of temperature oscillations for both lean and rich fuel mixtures, in both non-adiabatic and adiabatic conditions. Further numerical analyses were performed to highlight the kinetic aspects of the problem. Simulations suggested that in fuel lean conditions, the dynamics observed are related to the H2/O2 sub-system reactions independently of diluent nature, while, for fuel rich mixtures diluted in carbon dioxide, the CO2 decomposition to CO and CH3 recombination to

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 2 of 28

ethane are key reactions for the onset of temperature oscillations.

1. INTRODUCTION New combustion technologies are designed to be more efficient and environmental friendly with respect to conventional combustion systems in order to face the increasing energy demand and issues concerning combustion-generated pollutions. In this framework, MILD and Oxy-fuel combustion have been identified as promising techniques to address these issues. MILD (Moderate or Intense Low-oxygen Dilution)

1

refers to a combustion mode that uses

mixtures highly diluted, beyond conventional flammability limits, and pre-heated above their self-ignition temperature, to promote fuel oxidation in a homogeneous manner. Such conditions are attained by means of exhausted gases (mainly CO2 and H2O) that, at the same time, dilute and enhance the sensible enthalpy of fresh mixtures. As a result, a flame front is no longer identifiable, and a volumetric enlargement of the reaction zone up to the entire combustion chamber is observed with respect to conventional flames

1-4

. The modest

temperature increase, due to the high concentration of diluent species, implies very low emission of pollutants (i.e. NOx, soot, CO)

3-8

, while the high inlet temperatures and the

homogeneity of the process, coupled with high concentration of irradiative species (namely CO2 and H2O), induces an increase of the net radiation flux

2- 9

. Weber et al.

2

performed

experiments in a furnace operated under MILD conditions. They showed that a substantial improvement (up to 60%) in net flux of the thermal radiation was achievable in comparison to conventional combustion systems. CO2 from recycled gases can facilitate the stabilisation of the MILD combustion process with respect to systems diluted in N2. As matter of fact, recently Li et al.4 performed experiments in a laboratory-scale furnace using several gas mixtures externally diluted in N2 and CO2. They demonstrated that the MILD O2/CO2 combustion is easier to reach with respect to ACS Paragon Plus Environment

Page 3 of 28

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

MILD O2/N2 mixtures. In Oxy-fuel combustion 10, 11, fuels are burned in a nitrogen-deficient and carbon dioxide-rich environment, obtained by feeding the combustion chamber with an oxygen-rich stream and recycled flue gases. The recycled flue gases replaces the N2 in the combustion air and are used to lower the flame temperature

12, 13

. The combustion process generates a high CO2

concentration stream suited to sequestration, after flue-gas pretreatments

14

. Oxy-fuel

combustion coupled with CO2 capture and storage technologies, is one of the most promising approaches for reducing global CO2 emissions 15. The oxy-combustion process features an initial oxygen concentration up to 35% in comparison to the MILD combustion concept, in which the reactive process evolves with low oxygen levels (less than 5-10%) 4. This generally leads to an increase of the flame temperature and combustion intensity in comparison to a MILD combustion process, with several disadvantages occurring with the pollutant emissions and flame stabilization 7. In this perspective, to avoid efficiency loss in oxy-fuel combustion processes, it would be reasonable to increase CO2 dilution levels by increasing exhausted gas recirculation levels, thus moving from oxy-fuel to MILD operative conditions (MILD/oxy-fuel combustion) 7. The use of a high level of CO2 in the reaction zone can significantly alter the oxidation kinetics involved during the combustion process. More specifically, CO2 can modify the chemical pathways involved in oxidation processes because of thermal and kinetic effects 1635. In fact, such a species has a higher heat capacity with respect to N2, thus changing the adiabatic flame temperature 4 and the kinetic pathways. Consequently, it can then participate in chemical reactions as an active species in bi-molecular reactions, or in third-molecular reactions as a third body species. Glarborg and Bentzen

16

studied the oxidation of CH4/O2/CO2 mixtures in an atmospheric-

pressure flow reactor under highly diluted conditions in CO2, investigating fuel-lean and fuelrich conditions across a wide range of temperature (1200-1800 K). In such working 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

conditions, they found a strong increase of CO levels in comparison to traditional air combustion. Kinetic analysis showed that CO2 competes with O2 for H atoms and leads to CO formation through the reaction CO2 + H = CO + OH. Moreover, the CO2 decomposition reaction, as well as reactions of CO2 with hydrocarbon radicals (CH, CH2), also contributes to CO formation. They found that the most important reactions are those of singlet and triplet CH2 radicals with CO2. Le Cong et al. 20 experimentally and numerically studied the oxidation of methane-based fuels diluted by CO2 in a Jet Stirred Reactor operating at 1-10 atm, over a temperature range of 900-1450 K, from fuel-lean to fuel-rich conditions under “Flameless Oxidation” 21 conditions. Their results demonstrated that CO2 has an inhibiting effect on fuel oxidation under highly diluted conditions via the reaction CO2 + H = CO + OH. This reaction competes with the Habstraction reaction CH4 + H = CH2 + H2, decreasing the consumption rate of CH4. Such an effect is more pronounced under fuel-rich conditions since the concentration of H radicals increases with the equivalence ratio. Furthermore H radicals, consumed in the CO2 decomposition reaction, are not available for the main chain-branching reactions H + O2 = OH + H and H + HO2 = OH + OH, resulting in a strong alteration of the O/H radical pool that controls the evolution of oxidation processes. Yuan et al.

22

numerically investigated the oxidation of methane with pure oxygen in the

presence of CO2. They showed that the inhibition effect of CO2 addition on CH4 oxidation is significant at high temperature (> 1200 K) and it increases by increasing the temperature. Recently, Chen et al.

36

numerically studied the oxidation of methane in a one-dimensional,

opposed-flow flame. They performed preliminary thermodynamic calculations showing that from a chemical perspective, CO2 affects the concentration of CO at the equilibrium state under rich/lean mixture conditions. One-dimensional, counter-flow diffusion flame simulations revealed that the reaction H + CO2 = OH + CO, enhances CO formation in the presence of high CO2 concentrations, leading to a significantly higher CO concentration under ACS Paragon Plus Environment

Page 4 of 28

Page 5 of 28

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

oxy-fuel combustion conditions. Furthermore, CO2 decomposition impacts on the reaction OH + H2 = H + H2O via OH radicals leading to lower H2 and higher H2O concentrations in the flame profile. Although the chemical and thermodynamic effects of CO2 on the oxidation processes of fuels have been discussed in the literature, these studies are mainly relate to those done under air or standard combustion conditions. The aim of this study is to numerically exploit the oxidation of CH4/O2/CO2 mixtures under MILD conditions, where high dilution levels and lower working temperatures with respect to standard combustion conditions can reveal other features of the effects of CO2 on the oxidation process. Previous experimental and numerical studies

36-39

were devoted to understand the effect of high dilution and pre-heating levels on

the oxidation of CH4/O2 mixtures diluted in N2 in an atmospheric, non-adiabatic, perfect stirred flow reactor. Several oxidation regimes were identified as a function of mixture inlet temperatures and gas compositions, and complex dynamic behaviors were recognized. Kinetic analyses suggested that oscillations were related to a strong coupling between oxidation kinetics and heat loss mechanisms to the surroundings. In this paper the attention has been focused on mixtures of CH4/O2 highly diluted in CO2. Numerical simulations have been performed in a perfectly-stirred flow reactor under nonadiabatic and adiabatic conditions. In both conditions the results have showed the onset of complex oscillation behaviors. While in non-adiabatic conditions, dynamic behaviors have a thermo-kinetic nature, as widely discussed elsewhere

37-41

, in adiabatic conditions, the onset

of instabilities appears only related to the competitions between various kinetic routes promoted under the MILD/Oxy-fuel operative conditions. Therefore they are kinetically driven. In contrast to previous works

37-40

, the numerical analyses have been extended to systems

under adiabatic conditions to understand the kinetic role of CO2 on methane oxidation and on the establishment of dynamic behaviors. These aspects represent a further advance in the 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 6 of 28

study of oxidation kinetics under MILD conditions with respect to previous works 37-40.

2. NUMERICAL TOOLS Simulations were performed by means of the AURORA code of the Chemkin Package 42 and the detailed kinetic mechanism C1-C3

43

. AURORA allows simulating the transient and the

steady-state solutions of complex kinetics in a Continuous Stirred Flow Reactor (CSTR). The reliability of the C1-C3 scheme in predicting the behavior of CH4 oxidation process under MILD conditions has been widely discussed in other works

33, 37-40

and in the supporting

information section. The results were compared to those obtained by means of other kinetic mechanisms 44-47 available in the literature to verify the consistency of numerical data. Numerical simulations were carried out at atmospheric pressure for different inlet temperatures and mixtures compositions in both adiabatic and non-adiabatic conditions. The fuel/oxygen ratio was identified by means of the carbon/oxygen feed ratio (C/O), considering just the fuel carbons and the oxygen atoms in the form of O2 (i.e. CO2 is not included in the calculation of the C/O ratio). To convert this reference parameter to the equivalence ratio value, it is just needed to divide the C/O feed ratio by 0.25. In the numerical simulations the carbon/oxygen ratio was varied from values close to zero up to 0.5. The mixture inlet temperature (Tin) was changed in the range 1000 - 1500 K. Following indications from previous works 37-40, the reactor volume was set to 100 cm3 with an inner surface of 104 cm2. For the non-adiabatic study, the heat transfer coefficient was set to 0.02 cal/(cm2 K s). Considering the operative conditions in which experiments and numerical simulations were realized in previous works 37-40, the concentration of the diluent species (dCO2) was set equal to 90% (by volume) while the flow residence time (τ) to 0.5 s. The maximum integration time was 5 s.

3. NUMERICAL RESULTS ACS Paragon Plus Environment

Page 7 of 28

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

Numerical simulations were carried out under non-adiabatic conditions, following the same methodology used for CH4/O2/N2 mixtures in previous works

37-40

. The temporal temperature

profiles were considered as the reference numerical outcome to identify combustion regimes. Typical profiles are summarized in Fig. 1a) and 1b). They were obtained for a CH4/O2/CO2 mixture with varying C/O ratio and inlet temperature (Tin). Temperature profiles reported in Fig. 1a) refer to a CH4/O2/CO2 mixture with a C/O ratio to equal 0.1 and for a Tin = 1200 K (gray curve) and 1300 K (black curve), respectively.

Figure 1. Temporal temperature profiles for different combustion behaviors. a) steady stationary (black), periodic oscillation (gray); b) damped oscillation (gray), multiple ignition (black). Figure 1a) shows the “steady stationary” (black) and the “periodic oscillation” (gray) behaviors. The “steady stationary” profile first increases slowly for a short time, then shows an abrupt temperature increase up to a maximum value. Thereafter, the temperature decreases down to a steady stationary value, due to heat loss to the surroundings. In general, for the


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

cases identified with “periodic oscillation”, the temperature profile reaches a steady state characterized by oscillations with a constant frequency and amplitude over time. Fig. 1b) shows the temperature profiles obtained for a CH4/O2/CO2 mixture at the same Tin = 1200K and a C/O ratio equal to 0.45 (fuel-rich mixture), and 0.25 (stoichiometric mixture), respectively. In the first case, a “damped oscillation” behavior (gray profile) occurs, while in the second case, a “multiple ignition” profile (black) is numerically predicted. The “damped oscillation” profile is similar to the “periodic oscillation” profile, but the amplitude of the oscillations decreases with time, finally reaching a steady stationary value. In the case of “multiple ignition”, the temperature first increases slowly, then abruptly, reaching a maximum value. Afterwards it decreases down to the inlet temperature value. This behavior repeats over time. At the maximum temperature the fuel is completely converted. This oxidation regime is characterized by oscillations with a period longer than the nominal flow residence time inside the reactor ( = 0.5 s). On the basis of such characteristic profiles, it was possible to present the results in the form of a Tin - C/O map of behavior (Fig. 2), and therefore identify combustion regime regions as a function of the operative conditions for the non-adiabatic condition. The region relative to “no reactivity” conditions extends from 1000 K up to about 1125 K for fuel-lean (C/O < 0.25) and fuel-rich mixture compositions (C/O > 0.4). Under such conditions, no temperature increase was numerically predicted.


Page 8 of 28

Page 9 of 28

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 2. Tin-C/O map for CH4/O2/CO2 system in non-adiabatic conditions.

“Damped oscillations” were detected in two different conditions: for fuel ultra-lean mixtures (0.025 < C/O < 0.04) and for Tin in the range (1050 K – 1125 K), and for rich conditions (0.4 < C/O < 0.5) and for Tin in the range 1125 and 1350 K. The “multiple ignition” behavior extends in the region between the two damped oscillations areas. For C/O = 0.07, it ranges from Tin = 1060 K up to 1125 K. Increasing the C/O feed ratio, the inlet temperature range relative to “multiple ignition” shifts towards higher values. For C/O = 0.4, it is comprised in the range about 1125 and 1300 K. Periodic oscillations were predicted in two different regions of the map. The former corresponds to fuel-lean mixtures, namely from C/O values close to zero up to 0.2 and intermediate Tin (1150 -1215 K), while the latter occurs for fuelrich mixtures (0.3 < C/O < 0.4) and Tin higher than 1250 K. For inlet temperatures higher than 1250 K and mixtures with a feed ratio lower than 0.3 and higher than 0.4, “steady combustion” conditions were numerically identified (Fig. 2). As explained elsewhere

37-40

, the establishment of dynamic (multiple ignition, periodic and

damped oscillations) behaviors are related to a strong interaction between chemistry involved


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 28

in the methane oxidation process and the heat exchange to the surroundings. The shift among branching kinetic routes involved during the methane oxidation process, is relatively slower under MILD conditions with respect to traditional flames, because of quasi-isothermal conditions due to high dilution levels of the mixture. Therefore the kinetics are not characterized by the net prevalence of one particular mechanism over any other, but by a delicate competition between them. Under such conditions, external perturbations (i.e. heat loss mechanisms) to the system can have a huge impact on the evolution of the oxidation process. This feature makes the system prone to instabilities. The same simulations were also performed under adiabatic conditions. As such, because of the absence of heat loss mechanisms to the surroundings, instabilities due to system thermokinetic interactions would not be expected. Numerical results under adiabatic conditions showed that dynamic behaviors were identified, underpinning the strong kinetic nature of such phenomenologies. Therefore, they are related to the kinetic competitions between chemical routes coupled with the energy balance among endothermic and exothermic reactions (kinetically driven oscillations). For non adiabatic systems, heat loss mechanisms from the reactor to the surroundings strongly interact with this kinetic mechanism and emphasize the establishment of dynamic behavior (thermo-kinetic oscillations). Also under adiabatic conditions, several combustion regimes were identified. Likewise the non-adiabatic case, “non-reactive” conditions, “periodic” and “damped” oscillations along with “steady stationary state” areas were identified. Instead, the “multiple ignition” behavior was not predicted. Following the same methodology described earlier

37-40

, data were re-determined in a

reactivity map (Fig. 3).


Page 11 of 28

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 3. Tin-C/O map for CH4/O2/CO2 system for adiabatic conditions. Areas for periodic and damped oscillations were identified for fuel-lean and fuel-rich conditions. In particular, they are numerically detected for the following parameters: C/O values close to zero up to 0.1; low-intermediate Tin (1075-1215 K); for fuel rich mixtures (0.35 < C/O < 0.45), and Tin higher than 1150 K. Small damped oscillations areas are positioned on both the sides of the lean periodic oscillation area, as well as for the following parameters: 0.4 < C/O < 0.5, and 1125 < Tin < 1325 K. The non-reactive area is slightly less extended than the region obtained for non-adiabatic conditions, while the “steady-stationary combustion” region is wider. In order to verify the reliability of the numerical results, the same simulations were performed using several different kinetic detailed mechanisms

44-47

available in literature. Results are

reported in Fig. 4. They were performed with respect to periodic oscillations for both fuellean (upper part), and fuel-rich (lower part) mixtures, for the adiabatic condition. In general, most of the considered mechanisms were able to predict the oscillatory behavior identified by the C1-C3 mechanism, even though ignition delay times, oscillations frequencies and amplitude were different.


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 4. Temporal temperature profiles with different kinetic mechanisms. Figure 4a) shows the temporal temperature profile, for a mixture with C/O = 0.05 and Tin =1100 K. The “San Diego”

44

behavior, while the “Konnov”

and the C1C3 mechanisms, predicted a “periodic oscillation” 44

scheme resulted in a damped oscillation profile. The other

two kinetic schemes (GUI 46, RAMEC 47) resulted in a stationary steady state profile for this reference condition. It is worth noting that on increasing the inlet temperature, these two mechanisms also predicted periodic oscillations. Figure 4b) shows the temporal temperature profiles obtained for a mixture with C/O = 0.4 and Tin = 1200 K. Under such conditions, all the cited mechanisms predicted an oscillatory behavior. The aim of fig. 4 was not to identify the best mechanism available in the literature, but to show that most of the kinetic mechanisms (recently updated) can reproduce the oscillatory behaviors, even though ignition times along with the oscillation frequency and amplitude, are relatively different. Such results suggest that it should be possible to repeat the numerical analyses with other kinetic mechanisms, obtaining similar results, thus not compromising the generality of the


Page 12 of 28

Page 13 of 28

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

results presented and discussed in this paper. Temperature oscillations for the adiabatic case are promoted by methane oxidation chemistry for CO2-diluted mixtures and they are not influenced by heat losses towards the surroundings, thus the implementation of a kinetic study in such conditions can give important information on CO2 interactions with methane oxidation routes. Kinetic analyses were performed for CH4/O2/CO2 mixtures in an adiabatic system to clarify the chemical role of CO2 on the establishment of dynamic behavior. The results relative to Tmax and the main species trends for a fixed inlet temperature varying the C/O feed ratio, are reported in Fig. 5. The data show the main features of the oxidation process along with the identification of oscillatory conditions. Figure 5a) shows the maximum temperature and the CO2 concentration trend as a function of the C/O feed ratio for an inlet temperature of 1200 K. The system temperature increases as the C/O feed ratio increases from 0.01 up to the stoichiometric condition (C/O = 0.25), with the Tmax = 1610 K. Subsequently, Tmax decreased down to 1400 K for C/O = 0.5. With regard to the temperature oscillations, two different values were reported and these are represented by dashed lines and are highlighted in the figure by light gray areas. For fuel-lean mixtures, the maximum temperature oscillation was 15 K, while for fuel-rich mixtures it was about 30 K. The temperature gradient decreased when moving towards either richer conditions in relation to fuel-rich mixtures, or to leaner conditions for lean-fuel mixtures. The decrease in temperature oscillation amplitude was accompanied by an increase of frequency.


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 5. Tmax and CO2 trend as a function of the C/O feed ratio (a), CH4, CO and O2 as function of the C/O feed ratio (b).

The CO2 trend as a function of the C/O ratio was similar to that reported for the temperature gradient (Figure 5a). It increased as the C/O feed ratio was increased, up to a maximum value, which occurred for mixtures slighter lower than the stoichiometric condition, and then decreased. For fuel-rich mixtures (C/O > 0.25), the CO2 molar fraction decreased below the inlet concentrations, implying that CO2 decomposition reactions maybe occurring. Figure 5b) shows the O2, CO and CH4 mole fractions profiles as a function of C/O. Methane was completely converted for C/O ratios between 0.05 and 0.35. For C/O > 0.4, the methane concentration slightly increased. Oxygen concentration decreased as the C/O was increased, and approached zero for C/O equal to about 0.3. In between 0.35 and 0.4, the oxygen concentration oscillated between zero and about 0.005. Afterwards it slightly increased. It is worth noting that for fuel rich mixtures, although oxygen was depleted, its mole fraction was not equal to zero. CO mole fraction slightly increases with C/O up to C/O< 0.25, then it increases rapidly in ACS Paragon Plus Environment

Page 14 of 28

Page 15 of 28

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

correspondence of CO2 prompt decrease. For C/O higher than 0.4 it slightly increases.

4. DISCUSSION A kinetic analysis was performed to understand the role of CO2 in the establishment of temperature oscillations for both lean and rich fuel conditions. Previous works reported for CH4/O2 mixtures diluted in N2 37-40 have highlighted the role of kinetic mechanisms involved during temperature oscillations. In particular, two main kinetic mechanisms involved in methane combustion were identified. The former is the oxidation channel through the sequence CH3 -> CH2O -> HCO -> CO, the latter envisages the formation of C2H6 and its dehydrogenation to vinyl radicals

32

. Vinyl radicals can be oxidized to acetaldehyde, or

alternatively, dehydrogenized to acetylene. Acetaldehyde is dehydrogenated to CH3CO which then decomposes to CH3 and CO, thus feeding the oxidation channel. Where the temperature exceeds 1200 K, acetylene is formed and stabilized. In all cases, the recombination channel lowers system reactivity. The competition between the oxidation channel and methyl recombination/acetaldehyde channels is active during temperature oscillations. In fuel-rich conditions and high temperatures, the stabilization of acetylene leads to the system achieving a stationary steady state. Flux diagrams, temperature sensitivity analyses and net reaction rates (reported as Supporting Information) were determined for two oscillatory cases under adiabatic conditions, these being representative of lean (C/O = 0.05 and Tin =1100 K), and rich (C/O = 0.4 and Tin =1200 K) conditions, respectively. The analyses suggested that the main reactions involved in the methane oxidation under MILD conditions are those reported in Table 1. Table 1. Main reactions involved during oscillatory regimes. r1 r2 r3 r4 r5

H + O2 = OH + O H + O2 + M = HO2 + M OH + HO2 = H2O + O2 O + HO2 = OH + O2 H2O + O = OH + OH


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

r6 r7 r8 r9

CO + OH = CO2 + H OH + H2 = H2O + H CH4 + OH = CH3 + H2O CH3 + CH3 + M = C2H6 + M

The numerical study was extended by performing Rate Of Production analyses (ROP) of key species and Heat Release (HR) analyses per reaction. Such analyses confirmed that the key reactions involved during the dynamic behavior are the ones identified by flux diagrams and sensitivity analyses. Furthermore, the ROP and HR analyses clearly showed the relative weight of such reactions over time, during periods of oscillation. For the lean-fuel mixture, the ROP is shown in Fig. 6 along with the temporal temperature profile. At the minimum temperature (T approximately 1221 K), r1 is the most important reaction to produce OH radicals, while r3 consumes them. The conversion of CO to CO2 (r6) and the recombination channel, identifiable with the reaction r9, is not relevant.

Figure 6. Rate Of Production (ROP) analysis and temperature profile for C/O = 0.05 and Tin = 1100 K. After the minimum value, the reactor temperature increased. For T > 1224 K, it increases abruptly reaching a maximum value of about 1230 K. All the ROP values exhibit a maximum


Page 16 of 28

Page 17 of 28

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

absolute value of T of approximately 1225 K. In such a condition, r1 and r5 produce OH radicals that are consumed mainly by reactions r6, followed by r3. Reaction r2 produces HO2 radicals that react with OH forming stable compounds by means of r3. Afterwards, all reaction rates decrease rapidly to values close to zero, while the temperature decreases. Other information which may help in the understanding of the kinetics responsible for oscillations is obtained by analyzing the heat release rates for the same reactions considered in Fig. 6. Such analyses are reported in Fig. 7 along with the temporal temperature profile. Reactions r1 and r5 are endothermic, while r2, r3, r4 and r6 are exothermic. In particular, r1 is the most endothermic reaction of the system.

Figure 7. Heat Release Rate and temperature profiles for C/O = 0.05 and Tin = 1100 K.

The temperature increases with the maximum slope when all the heat release curves show a maximum value. In such conditions, r3 is the most exothermic reaction, followed by r2 and r6. Afterwards the heat releases per reaction rapidly decrease to values close to zero in concert with the temperature maximum value during oscillations. The heat needed for supporting r1 is provided by r2 and r3. In particular, r2 and r3 are the


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

most exothermic reactions of the system and they provide for the 70% of the total heat release. HO2 radicals are formed in r2 and then consumed by r3. The heat released by such reactions is able to increase the working temperature by a few degrees and promote high temperature branching reactions (initiated by r1) in the temperature oscillation range. Thus the system reactivity rapidly increases and methane is fully converted. Afterwards, the system goes to a “freeze state” and the incoming fresh CH4/O2/CO2 mixture causes a decrease of the working temperature and lowers the concentration of the radicals. In the meantime, termination reactions also occur and contribute to the consumption of the radical species. This mechanism is cyclically repeated during temperature oscillations. Therefore, the results show that for lean conditions, the dynamic behavior is related to the competition between the branching reaction r1 and r2 and r3. It should be noted that, even though r1 is faster than r2, the high-temperature branching reactions are limited by r2 and r3. Reaction 2 consumes H radicals and the O2 limiting reaction r1, and produces HO2 radicals that consume OH radicals through r3 leading to stable species, thus lowering system reactivity. Conversely as, r2 and r3 are the most exothermic reactions, they can increase system temperature and promote reaction r1. Therefore r2 and r3 kinetically inhibit r1, but thermally promote it. For inlet temperatures higher than 1200 K, analyses of the rate of production and the heat release rate indicate that r1 is supported by the high temperatures thus the system reaches a steady stationary state. In support of the hypothesis that such instabilities are related to the reactions of the H2/O2 system, independently of CO2, numerical simulations were carried out by varying the nature of the diluent species. In particular, CH4/O2 systems were diluted in He, N2, Ar and a fictitious inert species X (with the same thermodynamic properties of CO2). The periodic oscillatory behavior occurs in a well-defined range of system working temperatures, between 1180 and 1280 K. Therefore, for fuel lean mixtures, diluents slightly change the C/O-Tin ACS Paragon Plus Environment

Page 18 of 28

Page 19 of 28

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

values for which the system adiabatic temperature falls within the mentioned range, because of different specific heat capacities. Such results indicate that CO2, for lean mixtures, does not interact in the kinetic mechanism that promotes temperature oscillations for lean mixtures. Further simulations were carried out also including heat transfer towards the surroundings of the system. They confirm that, independent of Tin and the heat-transfer coefficient values, the dynamic behaviors are numerically observed if the working temperature is within the range indicated above. To highlight the role of r2 and r3, such reactions were alternatively deleted from the detailed kinetic mechanism. By means of such modified schemes, no temperature oscillations were predicted. Therefore, numerical results suggest that oscillations are due to a delicate kinetic/heat released balance among reactions r1 and r2 + r3. The same ROP analysis was performed for fuel rich mixtures in conditions relative to the oscillatory behavior. In particular Fig. 8 refers to a mixture with C/O = 0.4 at Tin = 1200 K. It shows that, when the temperature is approaching the minimum value, OH radicals are produced by r1, r5 and the reverse of r6, while being consumed by r7 and r8. When the temperature increases, r1 and r5 become dominant for producing OH, while r7 and r8 consumes OH. In this condition, it has to be noted that as r6 reverses, its rate becomes the most negative one among the reactions reported in Fig. 8, and consumes OH radicals leading to an increase in the CO2 molar concentration.


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 8. Rate Of Production (ROP) analysis for OH radicals and temperature profile for C/O = 0.4 and Tin = 1200 K. For fuel-rich mixtures, the role of the recombination channel becomes relevant and therefore the ROP analysis for the radical CH3 was undertaken. Fig. 9 shows that methyl radicals are produced by the methane dehydrogenation reactions (r11 CH4 + OH = CH3 + H2O, r12 CH4 + H = CH3 + H2, r13 CH4 + O = CH3 + OH), and that they mainly react through r9 and r10 (CH3 + OH = CH2(S) + H2O). It is also of note that the methyl recombination reaction mainly presents a maximum absolute value which corresponds with minimum temperature occurring during oscillations. Afterwards, this reaction decreases and is overtaken by the other reactions. The fate of CH2(S) radicals is to feed the oxidation channel. Therefore the recombination channel slows down system reactivity inhibiting the oxidation channel. In the case of CO2 dilution, such an effect is more marked with respect to N2, because of the higher collisional efficiency in the reference termolecular reaction.


Page 20 of 28

Page 21 of 28

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 9. ROP analysis for the radical CH3 and temperature profile for C/O = 0.4 and Tin = 1200 K.

The heat release analysis per reaction was numerically performed for C/O = 0.4 and Tin = 1200 K (results not showed). The results suggest that the recombination channel is the most exothermic reaction corresponding to the minimum temperature, thus, while inhibiting the oxidation channel, it becomes the most significant reaction in sustaining the high temperature branching reactions and the CO2 decomposition reaction. Following the same approach used for the fuel-lean conditions, simulations were performed for inlet conditions within the dynamic behavior region, substituting the CO2 with N2 and with a fictitious species (with the same thermodynamic properties of CO2). No oscillations were predicted for both the fictitious species and N2, implying that CO2 decomposition is responsible for the dynamic behavior of rich mixtures through the above-reported kinetic routes. Further integrations were performed for mixtures with C/O = 0.3 and 0.5 at 1200 K, in order to value the controlling kinetics for these two conditions and highlight the role of CO2 by comparing such results. In both the two cases, reactions achieve a steady stationary value, as


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

indicated in the Tin-C/O map in fig. 2. The rate of production analyses (not reported here) suggested that for C/O = 0.3 the controlling kinetic is similar to the one occurring at the temperature maxima obtained at C/O = 0.4 during oscillations, while for C/O = 0.5 it is similar to the reaction corresponding to the minimum temperatures at C/O = 0.4. At C/O = 0.3, the methane oxidation reactions are very active, being close to the stoichiometric condition (C/O = 0.25), and the reaction r1 is the main branching reaction. The CO2 behaves as an inert species and does not decompose through the reverse of r6. For C/O = 0.5, the methyl recombination reaction to ethane and C2 species dehydrogenation reactions become important, thus the oxidation channel is relatively depressed. The branching mechanism is sustained by reaction r1, but its reaction rate is lowered by the CO2 decomposition reaction, that consumes H radicals. In particular for C/O = 0.5 the net reaction rate of CO2 decomposition is faster than reaction r1. In the steady stationary combustion regimes established for fuel rich mixtures above the dynamic phenomenology, the net rate of the CO2 decomposition reaction is always faster than the one relative to reaction r1. For such mixture compositions, O2 is mainly consumed by reaction r1. Being H radicals consumed by CO2 decomposition reactions, O2 cannot be completely consumed in the system and it remains in the flue gases as Fig. 5 shows. Further simulations were realized for C/O = 0.5 diluting the system in N2. In this case, the molecular oxygen concentration at the stationary condition was equal to zero. Such a result suggests that the O2 at the outlet for the system diluted in CO2 is exclusively correlated to the CO2 decomposition reaction. It is possible to infer that CO2 decomposition alters the radical pool production and consumes H radicals. When the net reaction rates of reaction r1 and the reverse of r6 are comparable, their competition for H radicals promotes instabilities. For very fuel rich conditions, the CO2 decomposition overcome reaction r1 and becomes the dominant mechanism for H radical ACS Paragon Plus Environment

Page 22 of 28

Page 23 of 28

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

consumption. When such a condition is satisfied, the system reaches a stationary steady state.

5. CONCLUSION Transient calculations of a perfectly-stirred flow reactor were performed for methane/oxygen mixtures highly diluted in CO2 in order to study the chemical effects of CO2 on the evolution of the combustion process. The results showed the occurrence of temperature oscillations and the establishment of several combustion regimes in both adiabatic and non-adiabatic systems. The onset of dynamic phenomenologies for hydrocarbons in model reactors in non-adiabatic conditions has been widely recognized in the literature, and such behaviors have been related to a strong interaction between oxidation kinetics with heat exchange mechanisms towards the surroundings. Furthermore, under such operating conditions, the modest temperature increase, caused by elevated dilution levels, implies lower reaction rates with respect to traditional flames, stressing the competition among several kinetic pathways involved in hydrocarbon oxidation. This kinetic aspect, coupled with heat loss mechanisms from the system to the surroundings, induces instabilities. In contrast to previously published results, such dynamic behaviors were obtained under adiabatic conditions, so that they are related exclusively to the combustion kinetics. This study provides a better understanding of the oxidation chemistry of fuels under MILD and Oxy-fuel conditions. Dynamic behaviors were numerically identified for fuel-lean and fuelrich mixtures. By means of several numerical analyses, the kinetic pathways responsible for the onset of the dynamic phenomenologies were identified for both fuel mixtures. In summary, the following conclusions can be made under adiabatic conditions: 1) For fuel-lean mixtures, temperature oscillations are related to a strong chemical interaction among reactions of the H2/O2 sub-mechanism. Furthermore, the establishment of the dynamic behavior is not dependent on the nature of the diluent. The results of the simulations undertaken with different bath gases, showed that temperature oscillations occur when the 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

reactor adiabatic temperature is between 1180 and 1280 K, and is independent of the mixture inlet temperatures and compositions. Furthermore, when the heat exchange value was enabled in simulations, oscillations were detected in the same working temperature range. 2) For fuel-rich mixtures, temperature oscillations were primarily related to CO2 reactions. In particular, carbon dioxide significantly alters both the CH4 oxidation mechanism via the CO/CO2 equilibrium reaction, and the CH3 recombination reaction to C2H6. The CO2 decomposition reaction is significant at high inlet temperatures (> 1200 K), where the competition between CO2 and O2 for H radicals is significantly strong. The reaction, CO2 + H = CO + OH, inhibits the high-temperature branching reactions of the H2/O2 sub-mechanism, lowering system reactivity. Simultaneously, CO2 acts as a third-body species with higher collisional efficiency with respect to N2 on the CH3 oxidation/recombination routes, promoting the formation of C2 compounds, thus lowering system reactivity. At the same time, the methyl recombination reactions mainly provide the heat needed to promote the branching reactions, while the decomposition of CO2 consumes it. The complex kinetic equilibrium among these reactions makes the system prone to instabilities.

ACKNOWLEDGMENTS This work is financially supported by Ministero dello Sviluppo Economico within the – Accordo di Programma CNR-MSE, Gruppo Tematico Carbone Pulito – Fondo per il Finanziamento Attività di Ricerca e Sviluppo di Interesse Generale per il Sistema Elettrico Nazionale (http://www.ricercadisistema.cnr.it/images/det/libroMSE_2anno.pdf). The authors would like to acknowledge Dr. Marco Mehl (Lawrence Livermore National Laboratory) and Dr. Alberto Cuoci (Politecnico di Milano) for their help and valuable suggestions.


Page 24 of 28

Page 25 of 28

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

AUTHOR INFORMATION Corresponding Author E-mail address: [email protected]

REFERENCES

(1)

Cavaliere, A., de Joannon, M. Prog. Energ. Combust. Sci. 2004, 30 (4), 329-366.

(2)

Weber, R., Orsino, S., Lallemant, N., Verlaan, A. Proc. Combust Inst. 2000, 28 (1),

1315-1321. (3)

Wunning, J. A., Wunning, J. G. Prog. Energ. Combust. Sci. 1997, 23 (1), 81-94.

(4)

Li, P., Dally, B. B., Mi, J., Wang, F. Combust. Flame 2013, 160(5), 933–946.

(5)

Szego, G., Dally, B., Nathan, G. J. Combust. Flame 2008, 154 (1), 281-295.

(6)

Dally, B., Riesmeier, E., Peters, N. Combust. Flame 2004, 137 (4), 418-431.

(7)

Li, P., Wang, F., Mi, J., Dally, B. B., Mei, Z. Energy Fuels 2014, 28 (3), 2211–2226.

(8)

Li, P., Mi, J., Dally, B. B., Wang, F., Wang, L., Liu, Z., Sheng, C., Zheng, C. Sci. China

Tech. Sciences 2011, 54(2), 255-269). (9)

de Joannon, M., Sabia P., Cavaliere A. Handbook of Combustion. New technologies,

WILEY-VCH Verlag GmbH&Co. KGaA 2010, vol.5, 237-256. (10) Chen, L., Zheng, S., Ghoniem, A. Prog. Energ. Combust. Sci. 2012, 38 (2), 156-214. (11) Buhre, B. J., Elliott, L.K., Sheng, C. D., Gupta, R. P., Wall, T. F. Prog. Energ.

Combust. Sci. 2005, 31 (4) 283-307. (12) Seepana, S., Jayanti, S. Fuel 2012, 102 (1), 32-40. (13) Scheffknecht, G., Al-Makhadmeh, L., Schnell, U., Maier, J. Int. J. of Greenh. Gas Con. 2011, 5, S16-S35. (14) de Joannon, M., Chinnici, A., Sabia, P., Ragucci, R. Chem. Eng. J. 2012, 211-212, 318326.


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 26 of 28

(15) Wall, T. F. Proc. Combust Inst. 2007, 31 (1) 31-47. (16) Glarborg, P., Bentzen, L. L. B. Energy Fuels 2008, 22 (1) 291-296. (17) Amato, A., Hudak, B., D’Carlo, P., Noble, D., Scarborough, D., Seitzman, J., Lieuwen, T. J. Eng. Gas Turb. Power 2011, 133 (6): 061503. (18) Seepana, S., Jayanti, S. Fuel 2012, 102 (1), 32-40. (19) Park, J., Hwang, D. J., Choi, J. G., Lee, K. M., Keel, S. I., Shim, S.H. Int. J. of Energ.

Res. 2003, 27 (13), 1205-1220. (20) Le Cong, T., Dagaut, P., Dayma, G. J. Eng. Gas Turb. Power 2008, 130, 041502– 041512. (21) Weber R.; Smart, J. P.; Kamp, W. Proc. Combust Inst. 2005, 30, 2623-2629. (22) Yuan, T., Liu, T., Su, Y., Huang, J. “A kinetic analysis of CO2 addition in CH4/O2 Combustion”, in: European Combustion Meeting, Lund, Sweden (2013). (23) Liu, F., Guo, H., Smallwood, G., Gulder, O. Combust. Flame 2001, 125, 778–787. (24) Liu, F., Guo, H., Smallwood, G. Combust. Flame 2003, 133, 495–497. (25) Renard, C., Musick, M., Van Tiggelen, P.J., Vandooren, J. “Effect of CO2 or H2O addition on hydrocarbon intermediates in rich C2H4/O2/Ar flames”, in: European

Combustion

Meeting,

Orléans

(France),

2003

(www.gfcombustion.asso.fr/ecm/2003/203_Vandooren.pdf). (26) Mazas, A. N., Lacoste, D. A., Schuller, T. “Experimental and numerical investigation on the laminar flame speed of CH4/O2 mixtures diluted with CO2 and H2O”. In ASME Turbo

Expo 2010: Power for Land, Sea, and Air, 2010, (pp. 411-421). (27) Watanabe, H., Arayi, F., Okazachi, K. Combust. Flame 2013, 160 (11), 2375-2385. (28) Le Cong, T., Dagaut, P. Proc. Combust. Inst. 2009, 32, 427–435. (29) Heil, P., Toporov, D., Forster, M., Kneer, R. Proc. Combust. Inst. 2011, 33, 3407– 3413. (30) Sevault, A., Dunn, M., Barlow, R., Ditaranto, M. Combust. Flame 2012, 159 (1), 3342ACS Paragon Plus Environment

Page 27 of 28

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

3352. (31) Abian, M., Gimenez-Lopez, J., Bilbao, R., Alzueta, M. Proc. Combust. Inst. 2011, 33, 317-323. (32) Mardani, A., Tabejamaat, S., Hassanpour, S. Combust. Flame 2013, 160 (9), 16361649. (33) Sabia, P., de Joannon, M., Picarelli, A., Chinnici, A., Ragucci, R. Fuel 2012, 91 (1), 238–245. (34) de Joannon, M., Sabia, P., Cozzolino, G., Sorrentino, G., Cavaliere, A. Comb. Sci. Tech. 2012, 184 (7-8), 1207-1218. (35) Sabia, P., de Joannon, M., Lubrano Lavadera, M., Giudicianni, P., Ragucci, R.

Combust. Flame 2014, article in press. http://dx.doi.org/10.1016/j.combustflame.2014.06.006. (36) Chen L., Ghoniem, A. F. Comb. Sci. Tech. 2014, 186 (7). (37) de Joannon, M., Cavaliere, A., Faravelli, T., Ranzi, E., Sabia, P., Tregrossi, A. Prog.

Combust Inst., 2005, 30 (2), 2605-2612. (38) de Joannon, M., Sabia, P., Tregrossi, A., Cavaliere, A. Comb.Sci. Tech. 2004, 176 (5). (39) de Joannon, M., Sabia, P., Tregrossi, A., Cavaliere, A. “Periodic regimes in low molecular weight paraffin oxidation”, Proceedings of the European Combustion Meeting, (2003). (40) de Joannon, M., Sabia, P., Tregrossi, A., Cavaliere, A. Int. J. of Energ. for a Clean Env. 2007, 7(2), 127-139. (41) Wada, T. , Jarmolowitz, F., Abel, D., Peters, N. Comb. Sci. Tech. 2010, 183 (1), 1-19. (42) Rupley, F.M., Kee, R.J., Miller, J.A., Coltrin, M.E., Grcar, J.F., Meeks, E., Moffat, H.K., Lutz, A.E., Dixon-Lewis, G., Smooke, M.D., Warnatz, J., Evans, G.H., Larson, R.S., Mitchell, R.E., Petzold, L.R., Reynolds, W.C., Caracotsios, M., Stewart, W.E., Glarborg, P., Wang, C., Adigun, O., Houf, W.G., Chou, C.P., Miller S.F. CHEMKIN Collection, Release 3.7, Reaction Design, Inc., San Diego, CA, (2003). 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

(43) Ranzi, E., Frassoldati, A., Grana, R., Cuoci, A., Faravelli, T., Kelley, A.P., Law, C.K.

Prog. Energ. Combust. Sci. 2012, 38 (4) 468-501. http://creckmodeling.chem.polimi.it/. (44) “Chemical-Kinetic Mechanisms for Combustion Applications", San Diego Mechanism web page, Mechanical and Aerospace Engineering (Combustion Research), University of California at San Diego, http://maeweb.ucsd.edu/˜combustion.cermech. (45) Konnov, A. A., “Development and validation of a detailed reaction mechanism for the combustion of small hydrocarbons”, in: Twenty-Eighth Symposium (International) on

Combustion, Edinburgh, Abstr. Symp. Pap. 2000, p. 317 (46) Healy, D., Kalitan, D. M., Aul, C. J., Petersen, E. L., Bourque, G., Curran, H. J. Energy

Fuels 2010 24 (3) 1521-1528. http://c3.nuigalway.ie/mechanisms.html (47) 290.

Petersen, E. L., Davidson, D. F., Hanson, R. K. Combust. Flame 1999, 117 (1), 272-


Page 28 of 28

Dynamic Behaviors in Methane MILD and Oxy-Fuel Combustion

Recommend Documents