Numerical Investigation on Effects of Nonequilibrium Plasma on

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Numerical investigation on effects of non-equilibrium plasma on laminar burning velocity of ammonia flame Akira Shioyoke, Jun Hayashi, Ryuichi Murai, Noriaki Nakatsuka, and Fumiteru Akamatsu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02733 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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Numerical investigation on effects of non-equilibrium plasma on laminar burning velocity of ammonia flame Akira Shioyoke,*, † Jun Hayashi,†,‡ Ryuichi Murai,† Noriaki Nakatsuka,† Fumiteru Akamatsu†

†

Combustion Engineering Laboratory, Department of Mechanical Engineering, Osaka

University, 2-1, Yamadaoka, Suita, Osaka, Japan

‡

Combustion and Power Engineering Laboratory, Department of Energy Science, Kyoto

University, Yoshida Honmach, Sakyo-ku, Kyoto, Japan

KEYWORDS: plasma-assisted combustion, ammonia combustion, plasma chemistry, kinetic modeling, laminar burning velocity, flame thickness

ABSTRACT: Direct burning of ammonia is required to reduce the consumption of fossil fuels. However, ammonia has an exceedingly low laminar burning velocity. Therefore, it is necessary to improve the burning velocity of ammonia. In this study, the effect of

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non-equilibrium plasma on the problems of ammonia combustion is investigated. To reduce a calculation load from the difference of timescale, simulation is conducted in two stages; a section of non-equilibrium plasma discharge on mixture and a section of combustion. The reactions of plasma discharge in a flow reactor are simulated as a perfect stirred reactor (PSR) using the SENKIN code in the CHEMKIN-II package. Secondly, the laminar flame located downstream of the flow reactor is simulated using the PREMIX code in the CHEMKIN-II package. From the simulation results, as the time of the application of the electric field increases, the amount of decomposed ammonia in the unburned gas increases and the laminar burning velocity becomes considerably fast. Moreover, it could be concluded that the increase of H atom by the non-equilibrium plasma discharge has a dominant effect on increasing the laminar burning velocity.


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1.

INTRODUCTION To reduce the consumption of fossil fuels, the development of sustainable fuels are

required. Even though hydrogen has been recognized as one of the promising fuels, implementing a global hydrogen-based economy is, at present, a not feasible approach unless suitable mediums for storage and transportation could be found1. Furthermore, the volumetric energy density is four times less than that of gasoline if hydrogen is stored in liquid state at 235°C (this storage is not possible for long term). As a way to solve the problem regarding the storage performance of hydrogen, it is drawing attention to carrying hydrogen as different chemical substances. For example, methylcyclohexane (MCH) and hydrogen absorbing alloys attract attention as a carrier of hydrogen. Ammonia can also be used

as a clean energy. Ammonia is liquefied in a similar state as propane (i.e., 8.5 bar at room temperature). In terms of the energy density, ammonia is comparable to gasoline2. As explained above, ammonia can be a promising substance as a hydrogen carrier. Here, a conversion process is required to subtract hydrogen from ammonia to use hydrogen. By directly burning ammonia, there is the merit that there is no cost of conversion and no substance as a carrier of hydrogen remains.


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However, there are two problems in direct burning of ammonia: emission of NOx and low combustion characteristics. Firstly, the density of NOx which produced in the ammonia combustion is an order of magnitude greater than that of hydrogen flame. To reduce the nitrogen oxides is the key of the ammonia combustion use. Hayakawa et al. reported that ammonia flame in conditions of atmospheric pressure in the equivalence ratio is 1.0 emit about 300 ppm NOx3. However, the amount of NOx is decreased by the increase of the equivalence ratio or pressure. As a result, NOx emitted by the ammonia combustion has been found to decrease to about 100 ppm. As described above, there is room for further investigation into the reduction of the NOx emission in the ammonia combustion. Thus, reduction of the NOx emissions in practical level is considered to be possible. Secondly, ammonia has a very low laminar burning velocity (LBV). The maximum value of LBV for NH3/Air premixed mixture is approximately 6 to 8 cm/s at equivalence ratio equals to 1.14. Therefore, it is necessary to improve the burning velocity of ammonia. Takeishi et al. investigated the LBV of ammonia under oxygen enriched conditions, and indicated that the LBV of ammonia increases with increasing the concentration of oxygen5. They also showed that the maximum value of LBV can be reached to the same level of that of hydrocarbons. Li et al. demonstrated the effect of hydrogen addition to the ammonia/air


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flame and showed improvement of burning velocity of ammonia to a favorable level with hydrogen addition6. In this study, the effect of non-equilibrium plasma on the problem of ammonia combustion is investigated. Plasma, which is known as the fourth state of matter, provides an unprecedented opportunity for combustion and emission control owing to its unique performance in producing active species and heat and modifying transport processes7. New reaction pathways, such as atomic O production from the collisions between high energy electrons/ions and oxygen molecules, can be introduced into combustion systems to modify the fuel oxidation pathways considerably. The experimental studies on effect of the non-equilibrium plasma on flame were performed with a focus on ignition8,9, flame propagation10 and extinction11 for gaseous fuel and liquid fuel. On the other hand, most of the numerical studies presented in the literatures are limited in ignition12,13. Nagaraja et al. performed one-dimensional simulation of ignition by pulsed nanosecond dielectric barrier discharges, and showed excellent agreement of ignition delay time with experimental value12. Moreover, Castela et al. constructed the plasma reaction model by analyzing channels through which the electric energy deposition, and simulate plasma assisted ignition in turbulent flows by solving multi-dimensional-flow


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balance equations and combustion kinetics mechanism and plasma reaction model13,14. However, a few studies on the effect of discharge on flame propagation have been performed. Effect of plasma generated by electrical discharge on flame propagation was experimentally indicated in the literatures15–17. It is reasonable to expect that reactions and species effected by discharge increase LBV. Therefore, it is necessary for clarifying optimum discharge conditions to know which reactions and which species in the non-equilibrium plasma has a greatest effect on the LBV. Many Studies on plasma assisted combustion for hydrocarbon fuels have been conducted in the past. For hydrocarbon fuels, almost energy of discharge in CH4/N2/O2 mixture is consumed in electronic and vibrational excitation of N213,14. In addition, it is showed that quenching of electronically excited N2 by O2 play an important role in reduction of ignition delay time by ultrafast gas heating and ultrafast O2 dissociation. Moreover, for lean combustion, it is showed that species like H2 and CO generated in reaction by radicals produced in discharge play an important role in flame stabilization18. While, for plasma assisted combustion of hydronitrogen fuels, several studies have been conducted in the past. One focused on the removal of NO19, another focused on hydrogen


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production from reforming of ammonia20. Improvement of LBV of NH3/Air by dielectric belier discharge was reported10. However, a detailed discussion which focuses on the reactions is still open. Thus, it is worth to investigate from the point of view of reaction characteristics and reaction mechanisms of plasma assisted combustion of ammonia by numerical simulation. Considering numerical simulation of plasma assisted combustion, difference of time scale can lead to large calculation loads. In the simulation of auto-ignition of NH3/Air (

= 0.5,

P = 0.17 MPa) by Nozari et al. when changing the initial temperature from 1250 to 2000 K, the ignition time is required from 2 to 700 ? ?s for auto-ignition21. On the other hand, fast N2 excitation and relaxation of plasma kinetics are occurring within the discharge characteristic time scale or time scale of a few tens of nanoseconds on atmospheric pressure air preheated to 1000 K for values of E/N in the range 120 to 300 Td14,22. Since the time scale of combustion kinetics is several orders of magnitude greater than that of plasma chemistry, it needs to divide the reactions of plasma chemistry and combustion chemistry. The purpose of this paper is to clarify promoting effect of flame propagation and the promotion mechanism of non-equilibrium plasma discharge on hydronitrogen fuel by numerical analysis. In this paper, the results of simulation on the effect of non-equilibrium


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plasma discharge on LBV are shown using the kinetic mechanism which was created by coupling of plasma chemistry17,23–25 and combustion kinetics26. We provide discussion about the effect of non-equilibrium plasma application time of electric field on LBV and degree of ammonia decomposition.


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2.

SIMULATION MODEL

2.1. Model Concept The simulation of plasma assisted combustion is difficult due to the difference of time scale between plasma chemistry and combustion kinetics mentioned in section 1. For this reason, in this study, chemical reaction mechanism is divided into that of plasma chemistry and that of combustion chemistry for simplify of the calculation. More specifically, the simulation is performed in two stages; a section of non-equilibrium plasma discharge on mixture and a section of combustion. This method enables us to obtain LBV after the non-equilibrium plasma discharge easily. Details about the method are described below. In existing simulation of non-equilibrium plasma assisted combustion, plasma chemistry reactions are considered on source term of the balance equation for chemical species12,13. This source term can be obtained by reaction rate coefficients of each reaction. Expressions of reaction rate coefficients are classified into three types according to the kind of reactions. For excited molecules related reactions, reaction rate coefficients k are obtained from Arrhenius form which is function of gas temperature ܶ௚ like following equation

݇ = ‫ܶܣ‬௚?? exp ቆ

‫ܧ‬ ቇ ܴܶ௚


(2.1)

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where the pre-exponential factor A, the temperature exponent ? ?, and the activation energy E are specified for each reaction. For ion related reactions, reaction rate coefficients are function of electron temperature ܶ௘ like following equation.

‫ܧ‬ ݇ = ‫ܶܣ‬௘?? exp ൬ ൰ ܴܶ௘

(2.2)

At the time of discharge, electron temperature is determined from reduced electric field E/N and gas composition. For electron related reactions, reaction rate coefficients are determined from collision cross-sections and reduced electric field27. The problem in solving this source term is the aforementioned difference of timescale between plasma chemistry and combustion kinetics. So, simulation time step used to be set less than the order of nanoseconds to adapt for plasma chemistry. However, small time step can lead to large computational cost. In order to avoid the difficulty of calculation due to difference of the time scale, the target system is divided into “domain 1” and “domain 2”. Figure 1 shows a schematic illustration of the physical configuration. The physical configuration is refer to the experiment in 10. In the experiment, electric field which is applied in the “domain 1” is sin wave as the peak-to-peak voltage is 22-30 kV.

The gap distance of applied voltage is 4 mm in

atmospheric pressure. From dis parameter, reduced electric field of the peak voltage is


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obtained as about 200 Td. However, only one reduced electric field is used. Therefore, 100 Td is used in the simulation as the time-averaged value. Moreover, residence time is 1 ms in the experiment. In the simulation, although application of the electric field is a few tens of ns, sufficient enhancement of the combustion is found. It is thought that this discrepancy is because of the incorrect discharge condition and reaction mechanisms. I am dealing with this problem now. Simulation domain is divided into two parts and each simulation domain marked using a dashed line in Fig. 1. Simulation on flat flame which is formed at the downstream of the non-equilibrium plasma discharge in a flow reactor is performed. Firstly, reactions of plasma discharge in a flow reactor are simulated as perfect stirred reactor (PSR) using SENKIN code28 in the CHEMKIN-II package. Secondly, laminar flame which located downstream of the flow reactor is simulated using PREMIX code29 in the CHEMKIN-II package, and obtained LBV at various plasma application time of electric field.

2.2. PSR for chemical reaction of Plasma Non-equilibrium plasma discharge on the premixed mixture is modeled in “domain 1” as zero-dimensional simulation for isothermal process by using SENKIN code28 in the


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CHEMKIN-II package. Table 1 shows the kinetic mechanisms which are taken into account in this study. As for the plasma chemistry, mechanisms of ammonia and nitrogen from Kambara et al.23,24, a mechanism of oxygen from Meeks et al.25, a mechanism of ozone from Ombrello et al.30 are applied in this section. The mechanisms of inelastic collision are also considered in “domain 1”. Since reaction rate coefficients of electron collision reactions are a function of electron temperature, mean electron temperature in the discharge was obtained using BOLSIG+ (Boltzmann equation analysis software). Here, collision cross section data of N2, O2 and NH3 as the input to BOLSIG+ is from 31. Reaction rate coefficients of electron collision reactions and ions related reactions are obtained by the BOLSIG+ calculation. However, the reaction rate coefficient expression of CHEMKIN-II is restricted only to function of gas temperature as Eqs. (3.1). Thus, reaction rate coefficients are given as a constant by setting A to the reaction rate coefficients calculated in advance and setting β and E to zero. Table 2 shows the reaction mechanisms used in the “domain 1” In this mechanism NH3+, N2+, N+, H+, and O2+ is included as the positive ions. O2– and O– are included in this mechanisms as the negative ions. As the electronically excited species, O(1S) which is expressed as O* is included.


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The parameters of the calculation condition of flow reactor are flow residence time and discharge voltage. In this study, flow residence time is changed while discharge voltage is constant. Pai et al. showed that the electric field is uniformly distributed in the inter-electrode region and that the development of the discharge is uniform along the gap32. So, spatial distribution of reduced electric field is set to be constant. Moreover, in order to simplify the discharge process, the reduced electric field was assumed to be a square-wave pulse. Reduced electric field is set to 100 Td so that it is the same order as reduced electric field of dielectric barrier discharge7.

2.3. PREMIX for chemical reaction of combustion The simulation of flat flame in “domain 2” is performed as one-dimensional simulation using PREMIX code29 in the CHEMKIN-II package given the inlet species composition as obtained from the simulation of “domain 1”. As shown in Table 1, we applied the mechanism

considering

combustion

chemistry

and


de-excitation,

recombination,

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attachment on “domain 2”. We used the mechanism from Lindstedt et al.26 as an ammonia oxidation mechanism. Figure 2 shows the comparison of the laminar burning velocity of the ammonia flame in oxygen mole fraction in the oxidizing agent is 30% between experiment performed by Takeishi et al.5 and simulation with the mechanism from Lindstedt et al. As shown in Fig. 2, the trend of LVB as a function of equivalence ratio was shown good agreement. Therefore, we consider that it is possible to qualitatively discuss the effect of gas composition change on LBV using the mechanism of Lindstedt et al. As for the plasma chemistry, same mechanisms were applied as in “domain 1”. Thermodynamic data and transport coefficient data of each species are necessary to simulate flat flame using CHEMKIN-II. However, thermodynamic data and transport coefficient data of excited species and ion which is not used in conventional combustion analysis are not included in database of CHEMKIN-II. Thermodynamic data of excited species and ions used in the analysis are not the same as that of the ground state species. The thermodynamic data that Burcat distributes33 is applied to the analysis. Description of this point has been corrected in my paper. On the other hand, transport coefficient data of excited species and ions used in the analysis is treated in the same way as the ground state species. However, transport property


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is used only the “domain 2”. There are no electric field in the “domain 2”. Therefore, it is thought that the transport property of the ions have no significant effect on the result of simulation. Electrons in the non-equilibrium plasma discharge are in states of higher energy than ordinary electrons. Therefore, we set the value of enthalpy in consideration of mean electron energy obtained when calculating mean electron temperature. As initial condition, mole fraction of electrons is given as 10-10. In order to avoid destabilization in convergence of solution due to ammonia’s difficulty in burning, we set oxidant composition to oxygen: 30% and nitrogen: 70%.


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3.

RESULTS AND DISCUSSION

3.1. Effects of non-equilibrium plasma discharge on LBV Species mole fractions in “domain 1” are predicted in zero-dimensional simulation for isothermal process by non-equilibrium plasma using SENKIN code287. The mechanism used in this simulation includes the reactions mentioned in Section 2. Figures 3 shows temporal profile of mole fractions of NH3, O2, N2, H2 and H, N, O atoms and NH, NH2 radicals and electrons. Here, Time equals 0 means the moment when the voltage is first applied to the mixture. As shown in Fig. 3, on one hand, mole fraction of ammonia decrease with the increase in the time of the application of the electric field. Thus, it is clear that ammonia decomposed as a result of the application of electric field. On the other hand, mole fractions of NH2 radical, NH radical, N atom and H atom were increased as a result of the decomposition of ammonia. Furthermore, it was found in Fig. 3 that O atom and N atom were generated by the decomposition of oxygen and nitrogen. NH3 fastest decomposition is consistent with a trend of ionization energy. Ionization energy of O234, N235, and NH336 are 12.07 eV, 15.58 eV and 10.02 eV. Thus, electrons can be said predominantly generated by the ionization of ammonia. As shown in Fig. 3, plasma related reactions in “domain 1” proceed at an accelerated pace with application time of electric


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field. To represent the progress of plasma related reactions, the degree of ammonia decomposition

was defined as following equation (3.1)

where ܰ‫ܪ‬ଷ ሺ0ሻ, ܰ‫ܪ‬ଷ ሺ‫݌‬ሻ are amount of substance of the initial and the discharged condition. The simulation of flat flame in “domain 2” is performed using PREMIX code29 given the inlet species composition as obtained from the simulation of “domain 1”. Since reduced electric field is constant in space in “domain 1”, time of the prediction by SENKIN code is regarded as application time of electric field in the flow reactor. Figure 4 shows temperature and mole fractions of H, O and N atoms and OH radical obtained by simulation of flat flame using PREMIX code29 when the degree of ammonia decomposition is 20%. As mentioned above, the initial gas composition supplied from the inlet of PREMIX code is determined by the state after non-equilibrium plasma discharge obtained by SENKIN code28. Consequently, the initial gas contains a large amount of N atom and H atom. As shown in Fig. 4, a relatively small temperature rise (~78 K) occurs in the vicinity of burner inlet (distance from the inlet x = 0 ~ 0.02 cm) compared to temperature rise in flame zone (x = 0.07 ~ 0.14 cm). This rapid temperature increase by


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non-equilibrium plasma in the early stage of the ignition was reported by Leonov et al.38,39. They performed the numerical simulation on the ignition phenomena with applying non-equilibrium plasma discharge in the upstream of fuel injectors in the scramjet engine. The proposed model has two zones. Those are the relatively small heat release and fuel reforming by non-equilibrium plasma (zone 1) and the complete combustion accompanied by large heat release (zone 2). Although difference in presence of fuel in discharge region and fuel type, the same phenomenon can be observed in our model as in the two-zone model proposed by Leonov et al. Figure 5 shows reaction path analysis over a length of 0.02 cm from the inlet. Fig. 5 indicates how N, O and H atoms and OH radical which are generated in "Domain 1" react. N, O and H atoms and OH radical are respectively colored in green, blue, yellow and red. Moreover, species such as O3 and HO2 are coloring to orange. A thin arrow represents the reaction of 10-7 cm-2 s-1 order, the thick arrows indicate reaction of 10-6 cm-2 s-1 order. As shown in Fig. 5, N atoms generate H and O atoms. Generated O atoms generate H atoms and O3. H atoms react with O3 and generate OH radicals. On the other reaction path, H atoms generate HO2. Moreover, H atoms react with HO2 and generate OH radicals. Finally, generated OH radials accelerate decomposition of


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ammonia via a following reaction which is a chain initiation reaction of ammonia combustion26. (R1) This reaction progresses the fuel reforming in ammonia combustion. Figure 6 shows relations of fraction of ammonia at x = 0 and rate of increase in LBV (velocity at x = 0) in “domain 2” to application time of electric field in “domain 1”. In the Simulation of the “domain 2”, the phenomenon that the simulation is diverge has confirmed between 80 ns and 100 ns. Therefore, the results between 80 ns and 100 ns are not shown in Fig. 6. As application time of electric field becomes longer, the amount of decomposed ammonia in the unburned gas becomes larger and the LBV becomes much faster. As mentioned above, OH radicals in the unburned gas are generated when non-equilibrium plasma discharge on ammonia. The OH radicals promote the combustion reaction at a low temperature. Furthermore, the adiabatic flame temperature rises by the energy of the non-equilibrium plasma discharge, to promote radical formation in the flame zone. LBV is increased by these two effects. Therefore, it is expected that to produce the active species in unburned region by non-equilibrium plasma discharge is effective.


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Figure 7 shows relation between degree of ammonia decomposition and ratio of increase in LBV. It is clear that degree of ammonia decomposition is proportional to LBV in Fig. 7. In order to give an explanation of proportional increase in rate of increase in LBV with degree of ammonia decomposition, species generated in “domain 1” were examined. Figure 8 shows the relation between degree of ammonia decomposition and major radicals produced by non-equilibrium plasma discharge in “domain 1”. As shown in Fig. 8, with the progress of decomposition of ammonia, mole fraction of O atom shows little increase, that of N atom increases at an exponential manner, that of NH2 radical and H atom linearly increase. NH2 is generated by the collision of free electron with ammonia represented in following equation. (R2) Moreover, ionization and recombination reactions are occurred in the discharge on NH3. (R3)

(R4) Collisions of free electrons with NH3 (R3) generate NH3+ and collision of free electrons with this NH3+ (R4) generate NH223. On the other hand, for ammonia combustion,


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Lindstedt et al. claimed breakdown of NH3 is predominantly change to NH226. Major reactions of NH2 production from NH3 are shown below. (R5) (R1) (R6) (R7) Therefore, it can be thought that NH2 is not a molecular which have enhancement of combustion but a molecular which formed by the decomposition of ammonia. However, as mentioned above, when H atom reacts with O2 and NH3, O atom and OH radical are formed. OH radical accelerates reaction (R1) which is chain initiation reaction of ammonia combustion. So, it can be concluded that increase of H atom by non-equilibrium plasma discharge have a dominant effect on rising LBV. Figure 9 and 10 show relations of rate of increase in LBV to application time of electric field and degree of ammonia decomposition in equivalence ratio: from 0.80 to 1.30. As shown in Fig. 9, the relations between application time of electric field and rate of increase in LBV follow a similar trend in any equivalence ratio. As the equivalence ratio becomes high, rate of increase in LBV becomes high in same application time of electric field. The


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reason is that fraction of ammonia in gas composition is increased as equivalence ratio increases. As mentioned above, ionization energy of NH3 is higher than that of O2 and N2. H atoms which are generated by the decomposition of NH3 strongly correlate with the rate of increase of LBV. Therefore, more energy of non-equilibrium plasma discharge is consumed in ammonia decomposition and more active species needed for increase of LBV are generated in higher equivalence ratio. In addition, as shown in Fig. 10, linearity of rate of increase in LBV to degree of ammonia decomposition is not affected by equivalence ratio.

As shown in Fig. 3, it can be seen that H atom are generated the most in other

atoms. In addition, in Fig. 5, H atom is directly contributing to the generation of OH radical, it becomes important for exothermic reactions due to decomposition of ammonia (R1). Therefore, the generation of H atom is considered to play an important role in the increase of the laminar burning velocity of NH3. Also, H atom is generated as a result of electron collision reaction to NH3. From the above contents, how NH3 is decomposed much by electron collision reaction contribute to the increase of the laminar burning velocity. Therefore, electron impact to oxidizing agent is presumed to be non-critical. As a result, it is considered that even though the equivalent ratio is changed upward trend of the laminar burning velocity is not changed.


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3.2. Comparison to preheating combustion. As mentioned in Section 3.1, when non-equilibrium plasma discharges on the unburned premixed air, combustion is promoted by increasing the adiabatic flame temperature than the combustion reaction under low temperature. Increase in the adiabatic flame temperature can be relatively easily achieved by preheated combustion. Accordingly, by the present section that compares the effect to promote combustion of the preheated combustion and non-equilibrium plasma-assisted combustion, showing the effect of non-equilibrium plasma for chemical reaction effect rather than thermal effect. Figure 11 shows the comparison of LBV between non-equilibrium plasma assisted combustion for the degree of ammonia decomposition: from 0 to 20% and preheating combustion for temperature of the unburned gas: from 312 to 353 K in equivalence ratio is 1.00. Here, horizontal line is the adiabatic flame temperature so as to plot the relation with input energy. As shown in Fig. 11, effect of increase in LBV in preheating combustion is higher than that of non-equilibrium plasma assisted combustion. The main reason is that unburned gas expands in preheating combustion by virtue of temperature raise. So, the calorific value per the volume of unburned gas in preheating combustion is relatively low.


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Hence, we compare mass fluxes as calorific value of supplied gas between the two combustion techniques. Figure 12 shows comparison of mass flux between non-equilibrium plasma assisted combustion for the degree of ammonia decomposition: from 0 to 20% and preheating combustion for temperature of the unburned gas: from 312 to 353 K in equivalence ratio is 1.00. As shown in Fig. 12, effect of increase in mass flux in non-equilibrium plasma assisted combustion is higher than that of preheating combustion. It can be thought that the effect of non-equilibrium plasma assisted combustion mentioned in section 3.1 make it possible to input more ammonia and to extract more energy per unit of time than preheating combustion. In non-equilibrium combustion rather than preheating combustion, amount of input fuel was found to increase by improved reactivity. Flame shape should be changed by improved reactivity. Here, we defined flame thickness ? ? as an indicator of flame shape as shown in Figure 13 and calculated by following equation

(3.2)


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where, ܶ௨ is temperature of unburned gas, ܶ௕ is adiabatic flame temperature. As shown in Fig. 4, a relatively small temperature rise occurs in the vicinity of burner inlet in the analysis of the non-equilibrium plasma assisted combustion. Therefore, the temperature at x = 0.02 cm is used as T௨ in non-equilibrium plasma assisted combustion. Figure 14 shows comparison of flame thickness between non-equilibrium plasma assisted combustion for the degree of ammonia decomposition: from 0 to 20% and preheating combustion for temperature of the unburned gas: from 312 to 353 K in equivalence ratio is 1.00. Here, the horizontal axis is adiabatic flame temperature. As shown in Fig. 14, non-equilibrium plasma assisted combustion has more decrease ratio of flame thickness than preheating combustion.


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

4.

CONCLUSION Ammonia can be a promising substance for a hydrogen carrier, and it is reasonable to

burn ammonia directly. However, ammonia has a very low laminar burning velocity (LBV). Therefore, it is necessary to improve LBV of ammonia. In this study, the effect of non-equilibrium plasma on the problem of ammonia combustion is investigated. The purpose of this paper is to clarify promoting effect of flame propagation and the promotion mechanism of non-equilibrium plasma discharge on ammonia by numerical analysis. In this paper, the simulation is performed in two stages; a section of non-equilibrium plasma discharge on mixture and a section of combustion. This method enables us to obtain LBV after the non-equilibrium plasma discharge simply. From the simulation results, small gas temperature rise after non-equilibrium plasma discharge and increased adiabatic flame temperature were observed. Furthermore, from the results of the reaction path analysis, it was found that H and O atoms generated by non-equilibrium discharge generate OH radical. Generated OH radical accelerates decomposition of ammonia at a low temperature via chain initiation reaction of ammonia combustion. In addition, as the equivalence ratio becomes high, rate of increase in LBV becomes high in same application time of electric field. By comparing non-equilibrium combustion and preheating combustion, increase in


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mass flux in non-equilibrium plasma assisted combustion is higher than that of preheating combustion, and non-equilibrium plasma assisted combustion have more decrease rate of flame thickness than preheating combustion.


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 1. Schematic of the configuration and simulation domains; domain 1: non-equilibrium plasma discharge in a flow reactor and domain 2: flat flame.


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Figure 2. The comparison of the laminar burning velocity of the ammonia flame in oxygen mole fraction in the oxidizing agent is 30% between experiment performed by Takeishi et al.5 and simulation with the mechanism from Lindstedt et al.


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

Figure 3. Mole fractions of O2, N2, NH3, H2 and H, N, O atoms and NH, NH2 radicals and electron in “domain 1” predicted in zero-dimensional simulation for isothermal process by non-equilibrium plasma for equivalence ratio is 1.0, E/N = 100 Tb.

Figure 4. Temperature and mole fractions of H, N, O atoms and OH radical in “domain 2” predicted in simulation of flat flame when the degree of ammonia decomposition is 20%.

Figure. 5. Reaction path analysis of dominant radicals consumption routes over a length of 0.02 cm from the inlet in “domain 2” when the degree of ammonia decomposition is 20%.


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Figure 6. Relations of mole fraction of ammonia at x = 0 and rate of increase in LBV in “domain 2” to Application time of electric field in “domain 1” for equivalence ratio is 1.0.

Figure 7. Relation between degree of ammonia decomposition and rate of increase in LBV for equivalence ratio is 1.0.


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. Relations between degree of ammonia decomposition and mole fractions of major radicals produced by non-equilibrium plasma discharge in “domain 1” for equivalence ratio is 1.0.

Figure 9. Relations between rate of increase in LBV and Application time of electric field in equivalence ratio: from 0.80 to 1.30.


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Figure 10. Relations between rate of increase in LBV and degree of ammonia decomposition in equivalence ratio: from 0.80 to 1.30.

Figure 11. Comparison of LBV between non-equilibrium plasma assisted combustion for the degree of ammonia decomposition: from 0 to 20% and preheating combustion for adiabatic flame temperature: from 2262 to 2288 K in equivalence ratio is 1.00.


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 12. Comparison of mass flux between non-equilibrium plasma assisted combustion for the degree of ammonia decomposition: from 0 to 20% and preheating combustion for adiabatic flame temperature: from 2262 to 2288 K in equivalence ratio is 1.00.

Figure 13. Schematic of the definition of the flame thickness.


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Figure 14. Comparison of flame thickness between non-equilibrium plasma assisted combustion for the degree of ammonia decomposition: from 0 to 20% and preheating combustion for adiabatic flame temperature: from 2262 to 2288 K in equivalence ratio is 1.00.


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

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Table 1. Mechanisms used in the simulation of “domain 1” and “domain 2”.

Combustion kinetics

NH3

Plasma O2 chemistry N2 O3

Domain 1

Domain 2

–

Lindstedt’s ammonia oxidation model26

Ionization, recombination, dissociation23

Recombination, dissociation, attachment, charge exchange23

Ionization, excitation, dissociation, attachment25

Ionization, de-excitation, attachment, charge exchange25

–

De-excitation24 Reaction related with ozone17

Table 2. Mechanism used in the simulation of “domain 1”; T is gas temperature, R is universal gas constant, units are molecules, centimeters and Kelvin. Reaction 1. 2. 3. 4. 5. 6. 7. 8. 9.

H +e→H ା

Hା + e → H

NHଷା + e → NHଶ + H NHଷା

Rate coefficient

Reference

1.14 × 10 ܶ

ଵସ ି଴.଻

23

3.39 × 10ଶ଻ ܶ ିଶ.ହ

23

1.35 × 10ଵହ

+ e → NH + 2H

2.54 × 10

ଵସ

NHଷ + e → NHଶ + H + e

5.55 × 10ଵଷ

NHଷ + e → NH + Hଶ + e

1.97 × 10

ଵଷ

NHଷ + e → NH + 2H + e

1.39 × 10ଵହ

NHଷ + e → NHଷା + 2e

1.97 × 10ଵହ

H+e→H+e

1.74 × 10

ଵହ

10. H + e → H ା + 2e

6.38 × 10ଵସ

11. N + e → N ା + 2e

9.19 × 10ଵସ


23 23 23 23 23 23 23 23 23

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12. N ା + 2e → N + e 13.

Nଶା

+ e → 2N

14. Oଶ + e → O + O∗ + e ଶା

15. Oଶ + e → O

1.82 × 10 ܶ ଶ଴

ି଴.ଽଷ

expሺ−820/ܴܶሻ

1.43 × 10

ି

ଵଵ

17. O + e → Oା + 2e

2.81 × 10଻

18. O + e → O + 2e ∗

3.03 × 10ଵଶ 2.77 × 10଼

+ 2e

16. O + e → O + O ଶ

2.61 × 10ଵଽ ܶ ି଴.ହ expሺ−27878/ܴܶሻ

5.39 × 10

ା

଼

19. Oି + e → O + 2e

2.44 × 10ଵସ

21. Nଶ + e → 2N + e

6.39 × 10

20. O + 2e → Oି + e

3.63 × 10ଵ଻ ଵଷ

22. Oଷ + Oଶ → Oଶ + O + Oଶ

1.54 × 10ଵସ expሺ98759/ܴܶሻ

24. Oଷ + Oଷ → Oଶ + O + Oଷ

4.40 × 10 expሺ96500/ܴܶሻ

23. Oଷ + O → Oଶ + O + O

25. Oଷ + Nଶ → Oଶ + O + Nଶ 26. Oଶ + O + Oଶ → Oଷ + Oଶ

2.48 × 10ଵହ expሺ95090/ܴܶሻ ଵସ

4.00 × 10ଵସ expሺ94839/ܴܶሻ 3.26 × 10 ܶ ଵଽ

ିଶ.ଵ

27. Oଶ + O + Nଶ → Oଷ + Nଶ

1.60 × 10ଵସ ܶ ି଴.ସ expሺ−5820/ܴܶሻ

29. Oଶ + O + Oଷ → Oଷ + Oଷ

1.67 × 10 ܶ

28. Oଶ + O + O → Oଷ + O

2.28 × 10ଵହ ܶ ି଴.ହ expሺ−5820/ܴܶሻ ଵହ

ି଴.ହ

expሺ−5820/ܴܶሻ

30. Oଶ + Oଶ → 2O + Oଶ

9.80 × 10ଶସ ܶ ିଶ.ହ expሺ493988/ܴܶሻ

32. Oଶ + Oଷ → 2O + Oଷ

1.20 × 10 ܶ

31. Oଶ + O → 2O + O

33. Oଶ + Hଶ O → 2O + Hଶ O 34. 2O + Oଶ → Oଶ + Oଶ

3.50 × 10ଶହ ܶ ିଶ.ହ expሺ493988/ܴܶሻ ଵଽ

ିଵ.଴

expሺ493988/ܴܶሻ

1.20 × 10ଵଽ ܶ ିଵ.଴ expሺ493988/ܴܶሻ 1.50 × 10 ܶ

ଵ଺ ି଴.ସ

35. 2O + Nଶ → Oଶ + Nଶ

6.00 × 10ଵଷ expሺ−7489/ܴܶሻ

37. 2O + Oଷ → Oଶ + Oଷ

1.30 × 10 expሺ−7489/ܴܶሻ

36. 2O + O → Oଶ + O 38. 2Oଶ → Oଷ + O 39. Oଷ + O → 2Oଶ

40. Oଷ + H → Oଶ + OH 41. Oଶ + OH → H + Oଷ

42. Oଷ + OH → HOଶ + Oଶ

43. Oଷ + HOଶ → OH + 2Oଶ

5.34 × 10ଵ଺ ܶ ି଴.ସ

ଵସ

1.20 × 10ଵଷ expሺ420120/ܴܶሻ 4.82 × 10ଵଶ expሺ17142/ܴܶሻ 6.87 × 10 expሺ3640/ܴܶሻ ଵଷ

4.40 × 10଻ ܶ ିଵ.ସ expሺ329440/ܴܶሻ 9.60 × 10 expሺ8322/ܴܶሻ ଵଵ

1.66 × 10ଵଽ ܶ ି଴.ଷ expሺ8322/ܴܶሻ


23 23 25 25 25 25 25 25 25 24 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17

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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

AUTHOR INFORMATION

Corresponding Author Akira Shioyoke

e-mail: [email protected]

Funding Sources Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Energy carriers”. (Funding agency: Japan Science and Technology Agency (JST)).

ACKNOWLEDGMENT This work was partly supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Energy carriers”. (Funding agency: Japan Science and Technology Agency (JST)).


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Numerical Investigation on Effects of Nonequilibrium Plasma on

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