Investigating the effect of oxy-fuel combustion and light coal volatiles interaction: A mass spectrometric study

Combustion and Flame 204 (2019) 320–330

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Investigating the effect of oxy-fuel combustion and light coal volatiles interaction: A mass spectrometric study Martina Baroncelli a,∗, Daniel Felsmann a, Nils Hansen b, Heinz Pitsch a a b

Institute for Combustion Technology, RWTH Aachen University, Templergraben 64, Aachen 52062, Germany Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551, USA

a r t i c l e

i n f o

Article history: Received 31 August 2018 Revised 28 January 2019 Accepted 12 March 2019

Keywords: Coal combustion Molecular-beam mass spectrometry Chemical kinetics Oxy-fuel

a b s t r a c t Given the multi-physical nature of coal combustion, the development and validation of detailed chemical models reproducing coal volatiles combustion under oxy-fuel conditions is a crucial step towards the advancement of predictive full-scale simulations. During the devolatilization process, a large variety of gases is released and undergoes secondary pyrolysis and oxidation reactions. Therefore, the ability to capture their interactions is a prerequisite for each chemical model used in its detailed or reduced form to simulate these processes. In this work, a high-resolution time-of-flight molecular-beam mass spectrometer was employed to enable fast and simultaneous detection of stable and unstable species in counterflow flames of typical light volatiles. Following an approach of increasing complexity, carbon dioxide and methane were progressively added to an argon diluted acetylene base flame. For the three flames investigated here, results showed a significant increase in the concentration of C2 and C3 hydrocarbons and oxygenated compounds caused by methane addition to the acetylene flame. By hindering the production of the butadienyl radical, the addition of methane induces the reduction of benzene which triggers the decrease of aromatic species. Conversely, CO2 addition did not have significant effects on intermediates. To guide and interpret the measurements, numerical simulations with two existing chemical models were performed and the results were found to be consistent with the experimental data for small hydrocarbons. Some discrepancies were found between the two model predictions and between simulations and experiments for C4 and C5 species. Additionally, numerical simulations were found to overestimate the role of the methyl radical in aromatics formation. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Despite the efforts towards low carbon combustion technologies for power generation, coal is still widely used and is expected to play a significant role in the next 30 years [1]. Nevertheless, combustion of coal produces the highest amount of CO2 /O2 by mass among fossil fuels. Therefore, the development of cleaner coal combustion strategies is of primary importance. As a major climate change mitigation solution, carbon capture and storage (CCS) was recognized in the Paris Agreement and, among the proposed CCS technologies, oxy-fuel combustion appears one of the most promising [2,3]. Coal combustion is a complex multiphase process characterized by three main steps: devolatilization, char burnout, and gas-phase combustion [4,5]. Among these processes, devolatilization accounts for up to 70% of coal weight loss during combustion [6]. It is

∗

Corresponding author. E-mail address: [email protected] (M. Baroncelli).

therefore of crucial importance to understand the effect of an oxyfuel atmosphere on volatiles combustion chemistry and assess the predictivity of existing detailed kinetic models. For a stringent testing of such models in simplified configurations, spatially resolved quantitative measurements of species profiles in laboratory flames are regularly employed [7–9]. To additionally address the complexity arising from the significant variety of volatiles products, a comprehensive understanding of coal volatiles combustion can be achieved by defining surrogate fuels consisting of mixtures of hydrocarbons and inorganic species. To date, despite the wide range of technical applications involving hydrocarbon mixtures in CO2 atmospheres, detailed speciation measurements are still scarce for these conditions. Most of the studies, which are extensively listed in the work of Giménez-López et al. [10], focus on the impact of an oxy-fuel atmosphere on global parameters such as laminar burning velocity, extinction strain rate, and soot related quantities of a single fuel component [11–15]. So far, just Gimenez et al. [10] and Köhler et al. [16] presented detailed speciation measurements on mixtures under oxy-fuel conditions. In both studies, the authors compared their measurements with existing or updated chemical

https://doi.org/10.1016/j.combustflame.2019.03.017 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

M. Baroncelli, D. Felsmann and N. Hansen et al. / Combustion and Flame 204 (2019) 320–330

models but focused on premixed configurations, i.e. the nature of coal combustion was not captured. To remedy the scarcity of species concentration data for nonpremixed oxy-fuel combustion, we present the results of a detailed speciation study performed on a counterflow burner coupled to a time-of-flight molecular-beam mass spectrometer (TOF-MBMS). Counterflow laboratory flames are excellent candidates to reproduce the non-premixed conditions of volatiles combustion in a furnace and allow for comparison with detailed kinetic models with reduced computational efforts. The choice of the fuels was motivated by numerous studies on coal devolatilization which identified methane and acetylene as light volatiles species [5,6]. The important role of acetylene in promoting the growth of polycyclic aromatic hydrocarbons (PAHs) during coal tar secondary reactions was identified by Zeng et al. [17]. Additional pyrolysis experiments performed by Heuer et al. [18] revealed that in a CO2 atmosphere, acetylene was the only detected light hydrocarbon. Therefore, two acetylene flames under conventional and oxy-fuel conditions respectively, were investigated in this work. To gradually take into account the effect on flame composition due to C1 -C2 species interactions, a third flame with a methane-acetylene mixture under oxy-fuel conditions was investigated. This flame also allowed for the possibility to explore the effect of methane addition on the formation of aromatic hydrocarbons, which are known to be soot precursors [19]. Especially, methyl-addition pathways, also known as HAMA pathways, have already been addressed by several authors [20–24] as an alternative route to molecular growth chemistry provided by the hydrogen abstraction acetylene addition (HACA) mechanism [25]. Therefore, in this study we provide a test case to assess the effect of the HACA-HAMA routes competition, which is intended to be valuable data for modelling purposes. The presented work is structured as follows: In Section 2, the design concept and the technical details of the experimental facility are introduced. In Section 3, the experimental conditions and the characterizing parameters of the flames are presented. In Section 4, results are discussed and compared against two existing models. Conclusions follow in Section 5. 2. Description of the experimental setup The experimental facility consists of a newly designed counterflow burner and an electron ionization molecular-beam time-offlight mass spectrometer. The full experimental apparatus is shown in Fig. 1. In the following sections, a description of the two setup components and their verification tests is provided. 2.1. The counterflow burner The burner consists of two co-axial nozzles placed in counterflow configuration. The overall setup is designed to produce the same velocity and scalar fields described mathematically by the similarity solution implemented in several 1D codes [26–29]. The two nozzles have an outlet diameter of 20 mm and an area contraction ratio of 9. To reduce the development of the boundary layer and prevent the occurrence of Görtler–Taylor instabilities, the profile of the two co-axial contoured nozzles was defined according to a numerical procedure based on Thwaites method [30]. The conditioning of the flow entering the burner is ensured by inserting a 5 cm long honeycomb upstream the nozzle contraction section. Additionally, two couples of rings of stainless steel mesh are arranged in two layers at the nozzle exits. In order to shield the flame from the surrounding atmosphere, the design includes a coflow stream, which flows through a 5 mm wide annulus separated from the main nozzle by a 2 mm wall. The burner position can be vertically adjusted by a system of manual translators with a measurable stepsize of about 1/100 mm to allow for probing

321

Fig. 1. Schematic of the experimental setup.

studies along the centerline. Under normal operation, the oxidizer stream is supplied from the bottom nozzle while the fuel is introduced from the top and both streams are kept in momentum balance. A water jacket around the coflow annulus prevents excessive heating of the reactants and ensures the control of the inflow temperatures during the measurements. The assumption that the scalar and velocity fields produced by this burner are consistent with the 1D similarity solution has been verified using two different methods. First, measurements for extinction strain rates of ethylene and propane counterflow diffusion flames were compared to measurements by Humer et al. [31], whose counterflow burner setup has been carefully verified in many studies and for which the nozzle exit velocity profile was shown to obey a nearly perfect plug flow profile [32]. The comparison shown in Fig. 2 shows very good agreement. Second, Mittal et al. [33] have analyzed the effects of non-homogeneous inletvelocity in a counterflow burner. They found, at least for premixed flames, that as long as the temperature profile has no radial curvature at the centerline, the one-dimensional similarity solution is observed. Radial profiles of temperature and some species were measured in presence of the flame and are shown in Fig. 3. Here, the data were normalized against the value measured at the centerline and presented as a function of the nondimensional radial coordinate r/R (where r is the radial coordinate with origin at the centerline and R is the nozzle radius). As it can be seen, for all quantities almost perfectly flat profiles are observed. 2.2. The mass spectrometer The operating principle of electron ionization molecular-beam mass spectrometry in combination with time-of-flight detection

322

M. Baroncelli, D. Felsmann and N. Hansen et al. / Combustion and Flame 204 (2019) 320–330 Table 1 Flame conditions.

2.4

YFuel x 10 [-]

2.2 2 1.8

C3H8

C2H4 This study

1.2 Humer et al.

1 50

100

150

200 250 -1 a ext [s ]

300

350

Fig. 2. Comparison between extinction strain rates of C3 H8 and C2 H4 measured at the ITV facility, versus those measured by Humer et al. [31].

T 0

SO , 0 2

T 0.4

SAr, 0.4

4 Scalar profiles

3.5 3 2.5 2 1.5 1 0.5 0

0.5

1

Normalized distance from the centerline r/R Fig. 3. Normalized temperature T and species signal profiles S measured in presence of the flame. The data were normalized against the value measured at the centerline;  represents the distance from the fuel inlet normalized against the separation distance of the two nozzles L. To avoid overlap of the profiles, the normalized data were shifted.

together with the data evaluation procedure have been described in detail in several works [34,35]. In this section, a short description of the apparatus (Kaesdorf) and the quantification procedure will be given. Further details can be found in the supplementary material. Briefly, species are sampled in a quartz probe with a 30 μm diameter opening and expanded into the first pumping stage operating at 10−4 mbar to form a molecular beam. A conical copper skimmer guides the sample towards a chamber where it is ionized by electrons produced by thermionic emission of a tungsten filament. The ionized analyte is then accelerated into the flight tube by a Wiley–McLaren two stage pulsed ion extraction section [36]. The flight tube is equipped with a double stage reflectron to improve the energy focusing and allow for a mass resolution m/m of 40 0 0. Ions are detected by a multichannel plate (MCP) and the respective flight times are recorded by utilizing a multichannel scaler (Fast P7887). Given the energy of the ionizing electrons E, mass spectra are recorded as a function of their flight time. Here, their quantification relies on the following relationship between the integrated signal Sj (E) of species j and the respective mole fraction xj (E):

S j (E ) = c · F KT (x ) · ϕ (E ) · SW · D(M j ) · x j (E )



xCO2

Zst

Teq [K]

0 0 0.06

0 0.1 0.2a

0.50 0.50 0.49

2262 2224 2213

where c is an instrument factor, FKT(x) is a quantity which includes sampling-position-specific effects, ϕ (E) is the electron current, SW is the number of sweeps and D(Mj ) is the mass discrimination parameter; the two quantities inside the convolution integral, which is integrated over all possible electron energies, are the energydependent ionization cross section σ j (E) and the probability density function of the electron ionization energy f(E) for a given value of the energy E. If the mole fraction xj (E) is evaluated with respect to a reference species whose mole fraction is known, e.g. argon, Eq. (1) simplifies to:

=

-0.5

xCH4

0.080 0.093 0.080

 D (M j ) S j (E ) x j (E ) σ j ( τ ) · f ( E − τ )d τ = · · Sref (E ) xref (E ) D(Mref ) σref (τ ) · f (E − τ )dτ

4.5

-1

xC2 H2

AcAir AcOxy AcMeOxy

a For AcMeOxy , this value refers to the fuel stream while, on the oxidizer side xCO2 was 0.3. This adjustment was necessary to decrease the flame temperature.

1.6 1.4

Flame

σ j (τ ) · f (E − τ )dτ , (1)

x j (E ) ·k j (E ), xref (E )

(2)

where kj (E) is the calibration factor of species j at a given ionization energy. If, given the nature of the species, a direct calibration is not possible, indirect methods like Eq. (1) or relative ionization cross-section (RICS) method [37] can be applied, but then a full evaluation of the quantities presented in Eq. (1) is necessary. In this work, different methods have been employed to determine the calibration factor and species-specific parameters such as the ionization cross section: calibration factors for reactants can be determined at the burner inlet, while for other species they were directly measured or indirectly derived. The full explanation of the evaluation procedure, as well as the characterization of the machine-specific parameters can be found in the supplementary material (S1-3). Considering probe effects, molecular fragmentation, scarce data on species cross sections and assumptions related to the data quantification process [38], the uncertainty of the quantified mole fractions is on the order of 15% for the main species (fuels, O2 , CO, CO2 , H2 O, H2 ) and about 30% for directly calibrated intermediates species; the latter increasing to a factor of 2-3 when kj (E) is not obtained via direct calibration. However, in this work, we compare the effect of oxy-fuel atmosphere and the influence of methane addition on acetylene combustion. In this case, considering that the evaluation procedure is the same, the uncertainty in the relative comparisons reduces to 10-15% also for intermediates [16]. 3. Numerical and experimental procedures Numerical simulations were performed with two different well validated chemical models whose differences are discussed in Section 4. The first model, referred to as ITV model, is documented in series of publications [39–41]. In its first version [41], which still constitutes the base chemistry of the model, also benzene and toluene formation steps were included and a validation against GC measurements in an acetylene counterflow flame was presented. Furthermore, the model was validated using ignition delay times and burning velocities of acetylene, methane, and many other species. In this work, the updated version published in [39] was used and modified by adding the two consumption pathways for the indenyl radical suggested by Mueller and Pitsch [42]. The second model, here referenced as CRECK model, was developed by Faravelli and co-authors and has recently been updated

M. Baroncelli, D. Felsmann and N. Hansen et al. / Combustion and Flame 204 (2019) 320–330

after comparing it with a vast amount of experimental data [43]. The computational framework for this work was provided by the in-house code FlameMaster [29]. Numerical simulations were performed including mixture-averaged transport parameters and thermodiffusion, while radiation was simulated with an optically thin grey gas model. The choice of suitable flame conditions took into account practical and theoretical aspects related to the structure of counterflow flames. This type of flame is characterized by steep species and temperature gradients in a mixing layer of thickness δ that scales as

δ≈



D/a ∝ ( pa )−1/2 ,

(3)

where D is the thermal diffusivity, p the pressure, and a is the global strain rate [44]. This last parameter represents the inverse of the flow characteristic time scale and can be expressed as

 √  |V1 | ρ1 2|V2 | a= 1+ , √ L |V2 | ρ2

(4)

where subscripts i = 1, 2 identify the fuel and oxidizer sides respectively, ρ i and Vi are the density and the velocity of the respective inlet streams, and L is the nozzle separation distance [45]. Eq. (3) implies that in contrast to burner-stabilized premixed flames, the flame thickness in counterflow flames can be adjusted by the strain rate, which is an important advantage of this configuration. The global strain rate plays a fundamental role in determining the flame structure and the kinetics of non-premixed flames [46]. In this work, a strain rate of 70 s−1 was chosen to ensure a sufficient flame thickness (see Eq. (3)) and thereby, a significant number of sampling points in the reactive zone of the flame. Further important descriptive parameters of a counterflow flame are the equilibrium flame temperature and the stoichiometric mixture fraction Zst , which links the fuel and the oxygen content [47]. To allow for a meaningful comparison among the three flames, the concentration of fuel and oxygen as well as CO2 were appropriately adjusted to give a similar Zst and keep the flame equilibrium temperatures at very comparable values. Equilibrium temperatures were computed with both chemical models, which are compared in this work: results shown in Table 1 were obtained from the ITV model since the computations with the CRECK model showed negligible differences. The results show that the maximum differ-

323

ence in the equilibrium temperatures of the three flames is smaller than 50 K. As elucidated in the following sections, this difference is small enough to enable a straightforward analysis on the kinetic effect of CO2 and CH4 on acetylene chemistry decoupled from temperature effects. To guide the reader, Table 1 summarizes the flame respective names and conditions: AcAir consists of a counterflow acetylene-oxygen flame diluted with argon (the subscript Air indicates the absence of an oxy-fuel atmosphere), AcOxy includes the presence of CO2 on both sides; finally in AcMeOxy methane was added to the fuel. Before the measurements, the burner was first operated for roughly 30 minutes, during which the system was thermally stabilized. After this time, the quartz probe of the spectrometer was inserted at the fuel inlet to perform a burner scan by axially moving the burner towards the oxidizer side. The nozzle separation distance was set to 10 mm and the step size of the quartz probe was set to 0.25 or 0.15 mm; the smaller value being used to better capture the shape of the species’ profiles in the 3 mm zone where intermediates are formed. For all three flames, the measured temperatures at the oxidizer and fuel inlets were between 303 and 310 K. Calibrated Alicat mass flow controllers (MC and MCS series) were used to meter the unburnt gases. Argon was chosen as co-flow gas to shield the flames. To estimate the relative errors on the imposed strain rate and the stoichiometric mixture fraction arising from the uncertainty of the mass flow controllers, a Monte Carlo simulation was performed whose results were post-processed with a confidence interval of 95%. For the AcMeOxy flame, which requires the maximum number of mass flow controllers, the error on both parameters is below 2.0%. Burner scans for main species were performed with an electron energy of 17.4 eV, while intermediate species were measured at three different electron energies of 11, 12 and 13 eV respectively. The measurements at 11 and 13 eV were repeated twice for each flame to confirm the repeatability of the experiments and the given uncertainty intervals. 4. Results and analysis 4.1. Main species Measured and simulated profiles of the main species for the three flames discussed in this work are presented in Fig. 4. Regarding these species, the two models did not exhibit significant differ-

Fig. 4. Measured and computed profiles of fuels, O2 , Ar, and major products in the three counterflow flames. The axial coordinate x represents the distance from the fuel inlet. To improve readability, the hydrogen mole fraction was multiplied by a factor of 20.

324

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ences. Therefore, only the comparison with the ITV mechanism is presented. A comprehensive comparison between the two models and the experimental results can be found in Section 3 of the supplementary material. Given the necessity to keep a small gap between the fragile quartz capillary at the probe tip and the burner outlet, the distance covered with the experimental measurements was shorter in comparison to the simulated domain. Considering the practical difficulty in determining the axial distance between the probe tip and the burner, a shift (of 0.63 mm for AcAir , 0.5 mm for AcMeOxy , and 0.52 mm for AcOxy ) was applied to the experimental profiles. To take into account acetone, which constitutes the main impurity of acetylene, the ITV model was modified by adding the respective subset of reactions which is already included in the CRECK mechanism. Further details about the purity of the gases are given in Section 2.3 of the supplementary material. Considering the uncertainty limits discussed in Section 2.2, the overall agreement between the model and the measurements is very good. For all three cases, the fuel consumption, which is completed after roughly 4.5 mm, is captured well. The oxygen and the products

AcAir ITV AcAir CRECK AcAir Exp

AcMe Oxy ITV AcMe Oxy CRECK AcMe Oxy Exp 8

CH4

(a)

Ac Oxy ITV Ac Oxy CRECK Ac Oxy Exp C2H4

(b)

4

(c)

C2H6

(f)

C3H4

(i)

C4H6

4

Mole Fraction × 10 [-]

4.5

profiles also agree well with the predicted values. The slight broadness of the hydrogen profile in the AcMeOxy flame is consistent with similar findings obtained in different experimental studies with probing methods [7,48,49], and is attributed to the presence of the sampling probe. Consistently with Struckmeier et al. [49], we observed a larger broadening of the measured species profiles for microprobes with a larger outer diameter. Interestingly, despite this aspect, both the asymmetric shape and the change in the slope profile of hydrogen and carbon monoxide can be observed in measurements and simulations. It is worth noting, that for all the three flames, the relative difference between the measured and the computed mole fraction of argon is not more than 5%. This result is very important considering that the evaluation procedure of the intermediates relies on a correct prediction of the mole fraction of the reference species, in this case argon. Consistently with the major conclusion of previous studies [10], higher levels of CO were measured in the AcOxy flame in comparison to AcAir . Specifically, a 23% relative increase in the maximum level of CO was measured while, despite an increase of

6 4

4

Mole Fraction × 10 [-]

2

1.5

0

Mole Fraction × 10 4 [-]

3

3

2

1

0

0

C2H2O

(d)

CH2O

(e)

0.9

1.5

4.5

0.6

1

3

0.3

0.5

1.5

0

0

0

6

6

C3H6

(g)

0.6

C4H4

(h)

4

4

0.4

2

2

0.2

0

0

0

2

3

4 x [mm]

5

6

2

3

4 x [mm]

5

6

2

3

4

5

6

x [mm]

Fig. 5. Comparison between measured and computed light intermediate species profiles in the counterflow flames. The axial coordinate x represents the distance from the fuel inlet.

M. Baroncelli, D. Felsmann and N. Hansen et al. / Combustion and Flame 204 (2019) 320–330

Ac Air ITV AcMe Oxy ITV Ac Oxy ITV

Mole Fraction × 10 6 [-]

For small hydrocarbons and oxygenated species, the comparison between experiments and numerical simulations is shown in Fig. 5. With the exception of a small shift ( ≤ 0.5 mm) and considering the experimental uncertainty range, the overall agreement between the measurements and the simulations is satisfactory for C1 , C2 , and C3 species. Conversely, for C4 species, both models show noticeable differences from the experimental values. One reason for larger uncertainties might be the choice of calibration factors for species quantification. Electron-ionization time-of-flight mass spectrometry does not allow discrimination between isomers. Therefore, calibration factors have been chosen on the basis of the most probable isomer according to the models here investigated and the available literature. Information about this choice is given in Table S2 of the supplementary material, where a comparison of calibration factors for different isomers is provided. The experimental uncertainty arising from this limitation depends on the specific species and the ratio among different isomers for a given flame. An estimation of this uncertainty is provided in Table S3 where a comparison among computed calibration factors of different isomers is provided. In the following, the simulated species profiles include all the isomers currently present in the two mechanisms. As Fig. 5 shows, the addition of methane has a boosting effect on oxygenated species, C3 H4 (here calibrated as propyne), and particularly on C2 species, whose amounts increase by a factor of four. In contrast, adding CO2 to the AcAir flame does not produce a significant change except for ketene (C2 H2 O). This could be due to the fact that in the AcOxy flame, the mole fractions of fuel and oxygen were increased to keep the same stoichiometric mixture fraction and control the flame temperature, thus enhancing the flame reactivity. For a better understanding of the results, pathway analyses were performed for the species shown in Fig. 5. In this section, only the most significant conclusions will be presented. As observed for the profile of ethylene, ethane, and C3 H4 , the presence of a larger quantity of methyl radical coming from the addition of methane, favors different pathways involving CH3 directly or indirectly. In the case of ethylene for example, the reaction C2 H5 + M → C2 H4 + H + M constitutes the main production pathway in both mechanisms for the AcMeOxy flame. A further analysis shows that ethane, from which the ethyl radical is formed, is almost entirely produced from CH3 recombination reactions. Similarly for C3 H4 , it is found for both mechanisms that once methane is added, the H addition to the propargyl radical plays a secondary role compared with methyl addition to acetylene. Generally, the analysis of both models leads to the conclusion that the concentration of all reactants involved in the main formation pathways

3

Ac Air CRECK AcMe Oxy CRECK Ac Oxy CRECK

C3 H5

(a)

Tf 1464K

Tf 1374 K 2

Tf 1388K

1

0 Mole Fraction × 10 4 [-]

4.2. Small intermediate species

of the aforementioned species increases or remains equal once methane is added. The profiles of CH2 O and CH2 CO in Fig. 5d and e, show the effect of oxy-fuel atmosphere which is captured by both models. In the pure acetylene flame, the amount of formaldehyde does not change very much with CO2 addition which enhances the production of CH2 CO via the reaction 1 CH2 + CO2 → CO + CH2 O. Conversely, in the AcMeOxy flame, the increase of the methyl radical, which takes part in the OH+CH3 → H+ CH2 O reaction, shows a strong effect. Also, in the AcMeOxy flame, the amount of OH radical is more than double compared to the other two flames, thus enhancing the main production pathway of ketene through the reaction C2 H2 + OH → CH2 CO + H. In contrast to the previously discussed intermediates, the amount of propene in the AcMeOxy flame (Fig. 5g) diminishes in comparison with the other two flames. The reason for this decrease seems to be related to a shift in the peak position of the allyl radical, which participates in the reaction C3 H5 +H → C3 H6

8

(b)

CH3

(c)

C 2H3

6 4 2 0

Mole Fraction × 10 6 [-]

15% for the fuel molar fraction, the molar fraction of carbon due to the fuel contribution decreases in the AcOxy flame. This effect is caused by the CO2 , which actively inhibits CO oxidation through the CO+OH → CO2 +H reaction and competes for H radicals with the chain branching reaction H+O2 → OH+O. This competition, besides reducing the overall reactivity, also increases the overall amount of CO. A pathway analysis reveals that, for the oxyfuel case, the CO concentration is further enhanced by the reaction CH2 +CO2 → CH2 O + CO, which doubles its contribution to the species formation compared with the air case. The comparison with the AcMeOxy flame reveals a further increase in the CO content due both to the methane and the larger amount of CO2 added to the reactants. Additionally, as it is also confirmed by the simulations, the experimentally measured molar fraction of molecular hydrogen increases for AcMeOxy .

325

10 8 6 4 2 0 2

2.5

3

3.5

4

4.5

5

5.5

x [mm] Fig. 6. Computed molar fraction profiles of allyl (C3 H5 ), methyl (CH3 ), and vinyl (C2 H3 ) radicals for the three different flames. For allyl, the flame temperature in correspondence of the peak of its molar fraction computed with the ITV mechanism is shown.

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AcAir ITV AcAir CRECK AcAir Exp

AcMe Oxy ITV AcMe Oxy CRECK AcMe Oxy Exp

2.1

3.5

1.8 Mole Fraction × 10 5 [-]

Ac Oxy ITV Ac Oxy CRECK Ac Oxy Exp

3

C4H8

(a)

1.4 1.2

C5H6

(b)

1.5

2.5

1

1.2

2

0.8

0.9

1.5

0.6

0.6

1

0.4

0.3

0.5

0.2

0

0 1

2

3

4

5

6

C5H8

(c)

0 1

2

x [mm]

3

4

5

6

x [mm]

1

2

3

4

5

6

x [mm]

Fig. 7. Comparison between measured and simulated profiles of C4 H8 , C5 H6 , and C5 H8 in the counterflow flames. The axial coordinate x represents the distance from the fuel inlet.

constituting the faster production pathway for propene. This shift can be seen in Fig. 6a, which also shows the flame temperature in correspondence of the peak of the radical mole fraction. For the AcMeOxy flame, the allyl production is shifted towards the oxidizer side in a region of higher temperatures causing the allyl to be in an unfavorable position. In fact, the rate constant of the reaction responsible for propene formation includes a temperature dependence with a negative exponent leading to a lower reaction rate for the AcMeOxy flame. Fig. 6b and c show the profiles of methyl and vinyl radical and explain why the decrease of propene cannot be compensated by the reaction C2 H3 + CH3 → C3 H6 + H, which is included in both mechanisms. The drastic reduction of vinyl (C2 H3 ) can be traced back to the radical scavenging nature of the methyl radical, which is abundant at rich conditions in the AcMeOxy flame. As a consequence, the H radical concentrations for AcMeOxy at rich conditions is largely reduced compared with the AcOxy flame, thus hindering the main vinyl production reaction C2 H2 +H+M → C2 H3 +M. This fact had two main consequences: first, the formation of a second peak towards the oxidizer side (corresponding to the flame region where methyl starts to decrease), which explains why species directly formed from vinyl such as allyl, vinylacetylene, and 1.3-butadiene (Fig. 5g–i) show slightly shifted peaks; second, the diminution of the amount of vinyl radical, which explains the decrease of vinylacetylene. This species is in fact mainly formed via the reaction C2 H3 +C2 H2 → C4 H4 +H that benefits from large concentrations of acetylene as the main fuel. This effect is particularly strong for the CRECK model, which seems to overestimate the amount of this species for the two acetylene flames. Figure 7a–c show three species of molar mass 56, 66, and 68 g/mol that were detected in this study. Considering the nature of the fuels and the existing literature, these masses have been calibrated as: 1-butene (C4 H8 ), cyclopentadiene (C5 H6 ), and 1,3pentadiene (C5 H8 ), respectively. The detection of a species with molar mass of 56.0626 u, which was identified as a butene isomer, is consistent with the acetylene flame measurements of Skeen et al. [7], where a butene isomer was identified as well. This species was detected with a maximum mole fraction of about 1.4 × 105 in AcAir and AcOxy and a higher mole fraction of 1.8 × 105 for AcMeOxy . The fact that the concentration of butene increases once methane is added to the flame can suggest a formation pathway were the methyl radical is involved; for example a combina-

tion of allyl and methyl, which has been reported in the literature [50]. This reaction is included in both models and constitutes the main butene production pathway in the CRECK mechanism, while in the ITV mechanism the production of butene mostly occurs via the reaction C2 H3 +C2 H5 . Despite this, the significant discrepancy between the model and the measurement suggests other routes which are still not included, or an underestimation of the reaction rates. Concerning C5 H6 , the ITV model matches the experimental values better than the CRECK model. Nevertheless, the measurements do not show a difference in the concentration of this species between the AcMeOxy and the AcAir flames. This experimental evidence seems to be consistent with the current knowledge of the cyclopentadiene chemistry, which is not related to C1 species [51,52]. Finally, the presence of an isomer of C5 H8 is confirmed by preparatory measurements on acetylene flames performed by the authors at the Combustion Research Facility at Sandia National Laboratories. These measurements, performed with a electron ionization TOF-MBMS in a counterflow burner were not quantified but show a clear signal peak in correspondence of massto-charge ratio m/z = 78 u. Also in this case, the signal spectrum showed a clear signal peak at a flight time corresponding to a species with the mass of a C5 H8 isomer. The attribution to 1,3-pentadiene follows the work of Moshammer et al. [8] on 1,3butadiene counterflow flames. In that study, the authors discuss several production pathways for C5 H8 , which involve also allyl and vinyl radicals. A closer look at the models reveals that for the CRECK mechanism the main formation pathway passes through these two radicals, but the predicted amount of C5 H8 is almost zero. The ITV mechanism also includes other isomers of C5 H8 such as isoprene and cyclopentene. For both models, the profiles depicted in Fig. 7 correspond to the sum of all available isomers of C5 H8 . 4.3. Soot precursors Although it is accepted that most of the soot produced in coal combustion comes from tar [53], the role of light species in secondary reactions was underlined by Zeng et al. [17]. In that work, the authors pointed out how acetylene can participate in HACA sequences together with primary tar. At the same time, as previously stated in this work, the methyl radical can induce HAMA sequences, which often proceed with methyl substituted aromatic

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58% 15%

40% 82%

16% 7%

11% 12%

C3H3

327

+C2H2

C4H5

8% 31%

AcOxy AcMeOxy

53% 31% 92% 91% +H

+C2H2 +OH

5% 4%

2%

C4H3

Fig. 9. Benzene formation pathways according to the CRECK model. The same routes are followed in the ITV model. Green figures refer to AcOxy while those in red to AcMeOxy .

Fig. 8. Propargyl radical and benzene profiles. Experiments and simulations are shown as a function of the distance from the fuel inlet.

species. Interestingly, in this study we did not observe a significant role of CO2 in inhibiting ring formation. As mentioned earlier, this could be due to the increase in fuel and O2 concentration necessary to reduce the temperature influence. On the other side, methane addition to the acetylene flame drastically reduces the concentration of aromatic species, which are commonly considered soot precursors. Given the reduced amount of fuel necessary to avoid the clogging of the probe, the mole fractions of the detected species are lower compared to other studies [48]. Nevertheless, species up to C10 H8 (here calibrated as naphthalene) were detected. In the remaining part of this section, computed and measured results will be supported by a pathway analysis. For the sake of clarity, aromatics formation pathways of AcAir are omitted, since they did not show significant differences from AcOxy . Considering the possible reactions that the two fuels could undergo, the aromatic species which initiates the PAH growth process, is expected to be benzene (C6 H6 ), whose measured and computed mole fractions are shown in Fig. 8 together with the propargyl radical. In this case, both experiments and simulation show that the methane addition to the fuel hinders the formation of benzene. This result is particularly interesting, since in this case, methane was not introduced to replace acetylene, but added to it. As shown in Fig. 9, in the AcOxy flame, the production of benzene

occurs via H addition to the phenyl radical, fulvene isomerization, acetylene addition to the butadienyl radical, and propargyl recombination. Neglecting the interconversion with propargyl in the first place, the main production pathway is the acetylene addition to butadienyl which is also the preferred route to form fulvene. In the AcMeOxy flame, the sequence C2 H2 +C4 H5 is replaced by the recombination of propargyl radical, whose mole fraction increases in the AcMeOxy flame as can be seen in Fig. 8a. The change is mostly due to the reduction of the vinyl radical, see Section 4.2, which is responsible for the formation of C4 H5 . Limiting the amount of butadienyl and thereby the possibility for the fuel to participate in growth reactions to aromatic species, hinders the production of benzene and fulvene. Ultimately, it was noticed that the interconversion of phenyl and benzene is favored in the AcOxy flame. The maximum measured mole fractions of the remaining aromatics are shown in Fig. 10, along with a comparison with the two models. Concerning the production of toluene (C7 H8 ) and naphthalene (C10 H8 ) some improvements for the ITV model are proposed and results are compared with the old values. For the sake of clarity, the scope of this paper lies outside a full update of the ITV model. Here we limit our analysis to the identification of critical aspects which deserve further investigation. As a general comment, the two models predict a lower amount of aromatic species compared to the measurements. The effect of methane addition is fully captured by the recently updated CRECK model, which significantly benefits from the recent updates, while the ITV mechanism seems to overestimate the contribution of the methyl radical. By extending the pathway analysis to higher aromatics and following the CRECK model, the measured trends can be further clarified as shown in Fig. 11. Concerning toluene (C7 H8 ), the ratio between its experimentally measured maximum mole fraction and that of benzene, interestingly falls in the range measured by Hansen et al. [20,54]. The ratio was later verified by Ruwe et al. [55] in measurements of premixed low-pressure premixed flames, suggesting a correlation between the chemistry of these two species. This value was also compared against the results from other counterflow low-and-atmospheric pressure flames [7,48] and it was found that for all these dataset the ratio C7 H8 /C6 H6 falls in the range between 0.1 and 0.2. The data from Moshammer et al. [8] reveals a somewhat lower ratio of 0.05. The comparison of the two models and the experimental values is presented in Fig. 10a. As the CRECK model is concerned, the agreement between the measurements and the simulations is

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Fig. 10. Maximum values of the measured and computed concentrations of toluene (C7 H8 ), ethylbenzene (C8 H6 ), styrene (C8 H8 ), indene (C9 H8 ), and naphthalene (C10 H8 ). The solid triangles in subfigures (a), (d), and (e) correspond to the un-modified version of the ITV model [39].

Fig. 11. Aromatics formation pathways as described by the CRECK model. Green figures refer to AcOxy while those in red to AcMeOxy .

quite good especially for AcMeOxy while for the other two flames the model underpredicts the amount of toluene. The ITV model in its original version [39] does not differ much from the CRECK results for the two flames without methane, while the effect of methane is observably not caught (solid symbols in Fig. 10a). The reason for this difference is that in the CRECK mechanism, toluene mostly originates from the benzyl radical. This radical, substantially comes from phenyl, which decreases in the AcMeOxy flame because of the decreased concentration of benzene compared with the AcOxy flame. The reaction C6 H5 + CH3 → C7 H8 , which was extensively discussed by Klippenstein et al. [56], has only a minor

contribution to toluene in the CRECK mechanism, but appears to be the main production pathway for toluene in the ITV model, which explains the increase of toluene by methane addition to the fuel. Also, for the ITV mechanism, the decrease of benzene caused by methane addition is much less than for the CRECK mechanism. In addition to this, in the ITV model, the pressure dependence of this pathway was not included. To demonstrate its importance, Fig. 10a also shows the results of the ITV model once the pressure dependence for this reaction is added. As a consequence of this change, the amount of predicted toluene decreases. Nevertheless, the model is still not able to capture the trend of

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the measurements. The role of the methyl radical in the CRECK model is further clarified by analyzing the formation pathways of benzyl. For the AcMeOxy , 17% of the benzyl radical are produced via CH3 + C6 H5 → C7 H7 + H. Two other important formation routes contributing to benzyl formation are methylphenyl isomerization and the reaction C6 H8 + OH → C7 H7 + CO. The important role of methylphenyl was recently shown by Dames and Wang [57], but this radical is not included in the ITV model. The reduced phenyl concentration explains also the trend of phenylacetylene (Fig. 10c), which is mostly produced by the C2 H2 +C6 H5 sequence and, which is also responsible for the formation of the styryl radical which in turn converts into phenylacetylene via H abstraction. The last observed relevant HACA sequence, concerns acetylene addition to the ethynylphenyl radical to form naphthyl. This pathway, shown in Fig. 11 with a dashed line, is then responsible for a series of reactions, decreasing the molecule size which are not shown in the figure and which pass through the formation of indene and indenyl to ultimately form styrene and again the ethynylphenyl radical. As shown in Fig. 11, there are several other important reactions involving acetylene, propargyl, and the methyl radical whose role is clearly enhanced by methane addition. Figure 10e finally shows a very good agreement between the measured and the simulated maximum of naphthalene. The decrease of the naphthalene amount in the AcMeOxy flame can be explained by the fact that the concentrations of the reactants involved in almost all its formation pathways, except for one reaction, are at least two times higher in the flames without methane. Nevertheless, the presence of the methyl radical is considered in the model and the two main formation pathways of naphthalene in the AcMeOxy flame involve the methylnaphthyl radical and the CH3 C10 H6 O radical. As compared to AcOxy and AcAir , the methyl addition to benzyl is also responsible for 7% of naphthalene formation. On the other hand, the ITV mechanism was found to significantly overestimate naphthalene concentration, which appeared to follow the same trend of indene and toluene since it was almost entirely produced via methyl addition to the indenyl radical. This aspect was already observed by Mueller and Pitsch [58], who added two consumption reactions for the C9 H7 radical. According to the reference cited in the model [39,40], the rate for the reaction C9 H7 + CH3 → C10 H8 + 2H was derived by applying a rate rule to the reaction CH3 + C5 H5 → C5 H4 CH2 + 2H, which was discussed by Sharma and Green [59]. In this work, substituted this global reaction with the full pathway proposed by Laskin and Lifshitz [60], which was already applied by Chernov et al. [61] and Lindstedt et al. [62]. This subset, corresponding to reactions 60-63 of [60] is here added to facilitate the reading:

which means via acetylene addition to phenyl, to form styryl radical (C6 H5 C2 H2 ), with a subsequent acetylene addition which leads to naphthalene. As expected, given the larger amount of benzene (Fig. 8b), the same trend is reflected on the phenyl radical for simulations performed with the ITV mechanisms. This fact would then lead to a higher mole fraction of naphthalene as well. On the other hand, the consumption sequences of styryl radical which do not lead to naphthalene are much faster in the ITV model compared to CRECK, thus compensating the effect induced by the overprediction of benzene. An accurate investigation on the reactions consuming styryl radical and their rates goes beyond the scope of this work but is certainly and interesting aspect which deserves attention.

C9 H7 + CH3 → C9 H7 − CH3

(5)

C9 H7 − CH3 → C9 H6 − CH3 + H

(6)

C9 H6 − CH3 → C9 H6 = CH2

(7)

C9 H6 = CH2 → C10 H8

(8)

The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft within the framework of the collaborative research center SFB/Transregio 129 “Oxyflame”. The Aachen group additionally thanks Miss Vanessa Derichs for the measurements of the diameter of the probe orifice. NH acknowledges support from the U.S. DOE, Office of Science, Office of Basic Energy Sciences. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE National Nuclear Security Administration under contract DE-NA0 0 03525.

The results of this modification are presented in Fig. 10, where the full triangular symbol corresponds to the original value obtained with the ITV model. As it is shown, the prediction of naphthalene concentration for the ITV model is now very close to the measured value and as a consequence also to the value predicted by the CRECK mechanism. Considering the differences of the two models in benzene and other aromatics prediction, especially for the AcMeOxy flame, an explanation seems necessary. The pathway analysis on the models revealed that in both cases naphthalene is formed via the first and the second HACA sequences,

5. Conclusions Understanding combustion of volatiles species released during coal heat-up represents a key aspect for the development of more efficient oxy-fuel facilities. To this end, an experimental assembly consisting of a counterflow burner operating at atmospheric pressure and a time-of-flight mass spectrometer was applied in the context of oxy-fuel combustion. The main goal of this study is to support the development of chemical models in simulating coal combustion in this unconventional atmosphere. To assess the kinetic effect of CO2 , we performed detailed speciation measurements on two non-premixed flames fueled by acetylene, which was identified as a coal light volatile in several studies. A comparison with two existing chemical models shows very good performance of both mechanisms for the light hydrocarbons and oxygenated components, while for C4 to C10 species some discrepancies could be identified. In addition, a third flame was studied and the effect of methane addition in the oxy-fuel flame was investigated with the purpose of testing the model response in a mixture of coal devolatilization products. The experimentally quantified species concentrations show an intensification of the C2 − C3 chemistry, while aromatic components are drastically reduced. This reduction appears to be mostly due to the weakening of C4 chemistry, which in acetylene flames can initiate HACA routes that can benefit from the consumption of the fuel. A second important aspect arising from the study is the connection between naphthalene decomposition and smaller aromatics formation such as indene and styrene. This aspect, which was already observed in previous studies [8], here finds a further confirmation. Additionally, some species like 1-butene and cyclopentene are significantly underpredicted suggesting further investigation in this direction.

Acknowledgments

Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2019.03. 017.

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