Role of methane in ammonia combustion in air: From microscale to macroscale

Ammonia (NH 3 ) has gained increasing recognition as a carbon-free fuel. To enhance NH 3 combustion, reactive gases, like methane (CH 4 ), are usually added to the combustion system. In this work, the role of CH 4 in NH 3 combustion is systematically studied. A series of reactive force field molecular dynamic (ReaxFF MD) simulations are implemented to investigate effects of CH 4 addition on the consumption of NH 3 and the yields of nitrogen oxides (NO x ) from the atomic perspective: CH 4 accelerates the consumption of NH 3 by shortening the decomposition time of the first NH 3 molecule and increasing the translational kinetic energy of the system; CH 4 modifies the yield of NO x by complicating the elementary reactions and introducing additional intermediates. The fuel ratio of CH 4 and NH 3 between 0.5 and 1 is suggested for a cleaner and enhanced NH 3 combustion. By summarising the findings from the latest publications and the present work, the role of CH 4 in NH 3 combustion is comprehensively analysed from the macroscale and microscale perspectives: CH 4 accelerates the progress of NH 3 combustion flame, activates chemical reactions, and aggravates NO x emissions at a low CH 4 content. Taking the NH 3 /CH 4 combustion as an example, this study provides an exclusive perspective to understand combustion phenomena from the microscale events to macroscale observations.


Introduction
Ammonia (NH 3 ) is increasingly recognized as a green and sustainable fuel in resolving the global carbon emission issues.NH 3 is a promising hydrogen carrier, but the low flame speed and high autoignition temperature limits its wide applications for direct usage in power systems [1].Previous research showed that by adding auxiliary gases/fuels, like methane (CH 4 ) [2], to the NH 3 combustion system, the combustion performance of NH 3 can be effectively improved.
Various aspects of NH 3 /CH 4 combustion were reported in previous studies.Okafor et al. [3] pointed out that high CH 4 volume fraction increases the flame speed in micro gas turbine.Henshaw et al. [4] and Kurata et al. [5] reported a nonlinear relation between the content of CH 4 and NO x formation in NH 3 /CH 4 combustion.Li et al. [6] measured the yields of NO x , CO, and CO 2 in NH 3 /CH 4 combustion under different equivalent ratios.Skreiberg et al. [7] analysed the combustion products of NH 3 /CH 4 under the actual working condition (1400 K) and pointed out that different reaction conditions would influence the types of products.
Kinetic models for modelling NH 3 /CH 4 combustion under varying conditions have also been a hot topic for researchers.Early mechanisms included Konnov's mechanism [8], GRI-Mech 3.0 mechanism [9], Mendiara's mechanism [10] and Tian's mechanism [11].Accuracy of these reaction mechanisms were reviewed in Ref. [12].Afterwards, mechanisms with more accurate predictions for certain species, reduced mechanisms and mechanisms for specific conditions were successively proposed.For example, Okafor et al. [13] simulated the flame characteristics of NH 3 /CH 4 combustion, and pointed out the large deviations in the prediction of laminar flame speed and NO generation by Tian [11] and GRI Mech 3.0 mechanisms [9]; the researchers then developed a set of reaction mechanism that can produce more accurate predictions in flame speed and NO generation.Li et al. [14], Jiang et al. [15] and Mikulčić et al. [16] reduced the NH 3 /CH 4 mechanisms for different combustion conditions.Shu et al. [17] proposed a reaction mechanism for NH 3 /CH 4 combustion particularly for high pressure conditions.
Reactive force field molecular dynamics (ReaxFF MD) simulations play an indispensable role in manifesting the performance of NH 3 combustion.Guo et al. [18] studied the co-combustion behaviour of NH 3 /CH 4 at different temperatures, CH 4 mixing ratio and O 2 equivalence ratios.Zhang et al. [19] investigated the effects of CH 4 reactivity on NH 3 oxidation in an O 2 /N 2 environment.In our previous study [20], the ReaxFF MD method was used to scrutinise the behaviours of the reacting molecules/radicals, and reaction pathways at high temperatures and pressures were proposed by tracking the atomic trajectories.The reaction pathways proposed by the ReaxFF MD method were compared with popular kinetic models, and new pathways and intermediates were identified.According to our previous study, the addition of CH 4 could lower the activation energy of NH 3 combustion and thus accelerate the consumption of NH 3 .Yet, how the CH 4 could modify the NH 3 combustion chemistry from the atomic perspective has not been clarified in such a study.Additional efforts are still needed to reveal the effects of CH 4 on reaction mechanisms for NH 3 combustion at the atomic level.Indeed, the flame properties of NH 3 combustion observed at the macroscale are intimately related to microscale behaviour of the reacting radicals.However, to our best knowledge, there is no paper by far bridging the gaps between the macroscale observations and the atomic behaviours in NH 3 combustion.
The aim of the present study is to present a thorough understanding of the role of CH 4 in NH 3 combustion from the atomic behaviour to macroscale combustion phenomena.To achieve this goal, we first reveal the effects of CH 4 on microscale mechanisms for NH 3 combustion via a series of reactive force field (ReaxFF) molecular dynamics (MD) simulations.Then, we summarise the latest publications on NH 3 /CH 4 combustion.Finally, the macroscale observables and microscale indicators are linked and the role of CH 4 in NH 3 combustion is encapsulated.

Reactive force field molecular dynamics simulations
The ReaxFF MD is a type of MD method which uses a set of welltrained force field parameters to simulate chemical reactions with acceptable accuracy and computational costs.The potential of an MD system is calculated by the sum of bonded and non-bonded energies, as shown in Eq. ( 1).
where E bonded comprises bond energy (E bond ), overcoordination energy penalty (E over ), undercoordination stability (E under ), lone pair energy (E lp ), valence angle energy (E val ) and torsion angle energy (E tor ); E non- bonded includes van der Waals interaction (E vdWaals ) between atoms and electrostatic energy (E Coulomb ) [21].Details about the ReaxFF MD method can be found in Refs.[22,23].

Case set-ups
In the present ReaxFF MD simulations, NH 3 , CH 4 , N 2 and O 2 molecules were randomly placed in a cubic simulation domain with the aid of PACKMOL codes [24].To study the effects of CH 4 on NH 3 combustion, the number of CH 4 molecules was varied and the numbers of NH 3 , N and O 2 were fixed.To facilitate analysis, the fuel ratio of CH 4 and NH 3 , α, was defined in Eq. ( 2).
where n(CH 4 ) and n(NH 3 ) are the numbers of CH 4 and NH 3 molecules, respectively.Herein, α varied from 0 to 3 to mimic NH 3 combustion from a pure NH 3 condition to a mixed gas condition.The cases used in the present study are listed in Table 1.
According to our previous study [20], a high temperature could accelerate reaction rate and significantly reduce computational costs, and thus a temperature of 3000 K was selected in the present study.The same density of molecules at the beginning of the reactions, indicating the same initial pressure, was selected for all simulations as well.To keep a constant density, the side length of the simulation box was slightly changed.A typical initial configuration of the simulation box is demonstrated in Fig. 1.For each case, five simulations with different initial configurations were conducted, and data analysis was implemented based on 35 simulations.

Simulation details
A C/H/N/O ReaxFF parameter set [25,26] was used in the present system, and the force field parameters file can be found in our previous publication [20].The ReaxFF parameters were well trained by quantum chemistry and carefully validated by experiments, and its effectiveness and reliability of parameters for calculating systems with C, H, O and N elements were demonstrated in previous studies [20,27,28].
Before reaction simulations, equilibration simulations were performed to minimize the system energy for a physical time of 40 ps at K. The system was then heated from 300 K to 3000 K for 500 ps.Afterwards, the temperature remained constant at 3000 K and the reaction simulation lasted for 8000 ps until NH 3 was fully consumed.
The time step was 0.1 fs, and the bond order cut-off was 0.3 [29].A canonical (NVT) ensemble with a Nosé-Hoover temperature controlling algorithm was selected for all simulations with a damping constant of 100 fs.The periodic boundary conditions [30] were used in all three directions.All the simulations were implemented with the REAXC package in the LAMMPS (Large-scale Atomic/Molecular Massively     Parallel Simulator) software [31].The visualization was conducted in VMD [32].Post-processing of the MD results was accomplished using inhouse PYTHON (Python Software Foundation, Wilmington, De) scripts.The reaction pathways were analysed using the ChemTraYzer [33] script.Unless otherwise indicated, data with error bars represent mean ± SD (standard deviations).

Time-evolutions of species, start time of reactions and kinetic energy
The time evolutions of numbers of NH 3 at different fuel ratios are shown in Fig. 2a.As shown in Fig. 2a, the addition of CH 4 accelerates the consumption of NH 3 molecules, which agrees with previous experimental studies [34][35][36][37].The consumption rate of O 2 molecules also increases with α (Fig. 2b), as the oxidation of CH 4 molecules requires additional O 2 molecules as well.Time evolutions of numbers of CH 4 and N 2 are shown in Fig. 2c and d.In the rich O 2 environment, CH 4 is rapidly consumed and N 2 is formed as product of NH 3 combustion.
The numbers of NO x at the end of reactions were compared among cases of different fuel ratios, as shown in Fig. 2e.A non-linear trend of NO x generation with the addition of CH 4 was observed: the yield of NO x reached a peak at α = 0.5; afterwards, the addition of CH 4 inhibits the generation of NO x significantly.The α = 0.25 case has the fewer NO x emissions than those in the cases of α = 0.1 and 0.5.The number of reactions relating the formation of NO x was counted and the number of NO x formation pathways at α = 0.25 is fewer than those in α = 0.1 and α = 0.5 cases.Such a non-linear trend was also reported in a previous study [38].The non-linear tread narrows down the content of CH 4 that can be added to the NH 3 combustion: on the one hand, the NO x emissions should be limited and a larger α value is preferable; on the other hand, the number of carbon oxides (CO and CO 2 ) increases with α (Fig. 2f) and a smaller α value is favoured.With these considerations, the fuel ratio of CH 4 and NH 3 is suggested to be between 0.5 and 1 for cleaner and enhanced combustion of NH 3 .
A scrutiny of the time that the first NH 3 molecule starts to decompose suggests that the addition of CH 4 expedites the NH 3 combustion, as shown in Fig. 3a.For example, in the α = 0 case the first NH 3 is consumed at an average instant of 827 ps, whilst in the α = 3 case NH 3 starts to consume at as early as 146 ps.The early start time of NH 3 decomposition in the presence of CH 4 was also reported in previous observations [39,40].In the α = 3 case, the earliest NH 3 consumption was observed, which implies a highly reactive state of the system.The highly reactive state may be due to a high content of reactive radicals.Since results at such a CH 4 content were lacking in previous studies, further validation work is still needed.The effects of CH 4 on NH 3 combustion are two-fold: the addition of CH 4 could accelerate the consumption of NH 3 by generating radical s like H; on the other hand, CH could compete with NH 3 to consume O 2 which impedes the consumption of NH 3 .In Fig. 3a, when α changes from 0.5 to 1, the consumption of NH 3 is retarded, which may be attributed to the competition for O 2 from CH 4. The kinetic energy of these systems was also calculated.The kinetic energy of the system was calculated from the temperature of the system with 1/2k B T of energy for each degree of freedom.Fig. 3b shows kinetic energy of the system increases with the addition of CH 4 , accounting for the accelerated NH 3 consumption in the presence of CH 4 in Fig. 2a.

Intermediates and elementary reactions
The numbers of intermediates and elementary reactions under different fuel ratio conditions are illustrated in Fig. 4a.Both the numbers of intermediate species and reaction pathways increase with the fuel ratio, indicating that the addition of CH 4 complicates the NH 3 combustion as it accelerates the reaction.
Furthermore, the species of intermediates were compared among different fuel ratio conditions.Common intermediates among these cases and unique intermediates in each situation were identified.As shown in Fig. 4b, H   , on the other hand, produces carbon-containing species.The common carbon-containing species were listed in Fig. 4b.Fig. 4b also suggests that the varieties of the carboncontaining species differ when the fuel ratio is changed.For example, C 2 H 3 O and C 2 H 5 were only observed in the α = 3 cases while C 2 O was only produced in the α = 2 case.The variations in species at different fuel ratio conditions further suggest that the presence of CH 4 modifies the reaction mechanisms as it enhances the combustion of NH 3 .
Among all the intermediates and radicals, hydrogen atom (H) and hydroxyl (OH) were observed most frequently.Time-evolutions of the numbers of H and OH radicals under different fuel ratio conditions were investigated, as shown in Fig. 4c and d To further reveal how CH 4 modifies the reaction mechanisms of NH combustion, the reaction pathways leading to the formation of NO and N 2 were compared between the pure NH 3 combustion (i.e., the α = 0 case) and the NH 3 /CH 4 mixed combustion (The α = 0.5 case was used).
In Fig. 5, colour fonts were used to distinguish whether the reaction was from the α = 0 or α = 0.5 case; the digitals represent the probabilities that individual elementary reactions occur among all the elementary reactions.As shown in Fig. 5a and b, the reaction channels and the probabilities individual elementary reactions occur can be completely different, although common intermediates were formed between the pure NH 3 combustion and the mixed gas combustion.For example, NH can be directly converted to HNO in the α = 0.5 case, whilst NH 2 should be formed before HNO in the α = 0 case; O 2 , OH and O radicals could all react with NH 3 in the α = 0 and α = 0.5 cases, however, the probabilities of detecting these elementary reactions vary when α changes.

Reaction pathways of NO x
To reveal the mechanism for the effects of CH 4 on NO x emissions, the reaction pathways were scrutinized.The numbers of NO x formation and Fig. 6.Numbers of NO x formation and consumption reaction types.

Table 2
NO x -related elementary reactions at different fuel ratio conditions.consumption reactions were counted, as shown in Fig. 6.The yield of NO x depends on the difference in fluxes between the formation and the consumption reactions.As shown in Fig. 6, the difference between the numbers of NO x formation and consumption first increases until α reaches 0.5 and drops afterwards, which explains for the trend of NO emissions in Fig. 2e.
The elementary reactions relating to the formation and consumption of NO x were further examined, and forty elementary reactions that frequently occurred were listed in Table 2. Common and unique reactions at different fuel ratio conditions were identified.As shown in Table 2, more reaction pathways for the production of NO than those of NO 2 were observed, which explains the number distribution of NO x in Fig. 2e (more NO molecules than NO 2 were observed in Fig. 2e).As revealed by Table 2, carbon-containing radicals can participate in NO formation directly when α is >1 (R20, R34, R35 and R38).Notably, prompt NO x is formed in the α = 3 case (R38).R1 ~ R4 in Table 2 were four most frequently observed reactions for NO formation.The probabilities that individual reactions occur were quantified by calculating the proportions of reaction frequency of each elementary reactions to the aggregate reaction frequencies of all the reactions.As shown in Fig. 7, the probability of individual reactions varies with the CH 4 content.In Fig. 7, the proportion of HNO + O 2 → NO + HO 2 decreases when CH 4 is added to the NH 3 combustion.CH 4 may compete with NH 3 to consume O 2 , which inhibits the conversion from HNO to NO in the HNO + O 2 → NO + HO 2 reaction.

Discussionrole of CH 4 in NH 3 combustion in air: from the microscale to macroscale
In the past five years, an increasing number of research papers have been published to discuss the combustion properties of NH 3 as an alternative energy vector to fossil fuels.Herein, we have collected the latest papers about the effects of CH 4 on NH 3 combustion and categorised these references in terms of scales (either microscale or macroscale) in Table 3.
As listed in Table 3, the properties or terminologies usually discussed in macroscale studies of NH 3 /CH 4 combustion include ignition delay time or ignition enhancement, laminar flame speed, flammability limit, combustion rate, adiabatic flame temperature, flame propagation, NO x emissions, flame stability, combustion efficiency and chemical kinetics, just name a few.Such macroscale observations or properties could all be understood from the microscale behaviour of atoms, molecules and radicals as revealed by the present study and related work by our group [20].For example, the ignition delay time usually discussed in the macroscale studies is related to the start time that the first NH 3 molecule is decomposed; the laminar flame speed relies on microscale properties like kinetic energy and collision frequency (both affecting the reaction rate); the microscale understandings of the chemical kinetics are indeed the reaction pathways formed by tracking the trajectories of atoms/ molecules/radicals.To bridge the gap between macroscale and microscale understandings, a diagram linking the macroscale observations and their corresponding microscale events was proposed, as shown in Fig. 8a.
Considering all the macroscale and microscale findings available, the role of CH 4 in NH 3 combustion in air is summarised in Fig. 8b: CH accelerates the progress of flame, activates chemical reactions, and aggravates NO x emissions when the CH 4 content is low.

Conclusion
In this study, a series of ReaxFF MD simulations were conducted to simulate the NH 3 /CH 4 combustion in air under different fuel ratio conditions.The results suggest that the addition of CH 4 enhances the combustion of NH 3 by reducing the start time of NH 3 decomposition and increasing the kinetic energy of systems.CH 4 can inhibit NO x emissions at a higher content of CH 4 (α > 1) but aggravate NO x emissions at a lower CH 4 content.The fuel ratios of CH 4 and NH 3 between 0.5 and 1 (i.e., 0.5 < α < 1) are suggested to achieve cleaner and enhanced NH combustion.To better understand the reaction mechanism, the intermediates and reaction pathways were revealed.The presence of CH complicates NH 3 combustion by introducing additional intermediates and reaction pathways, leading to the formation of NO x were compared among different fuel ratio conditions.By summarising the latest publications of NH 3 /CH 4 combustion in air, together with the findings revealed by the present work, the role of CH 4 in NH 3 combustion in air was summarised: CH 4 is an accelerator for the progress of NH 3 combustion flame, an activator for the combustion reactions and an aggravator for NO x emissions when a low content of CH 4 is added to the combustion system.
The present study provides a comprehensive understanding of the effects of CH 4 on NH 3 combustion.Practical suggestion regarding the content of CH 4 addition is provided.In future work, the activation energy of individual elementary reactions will be calculated to form a complete set of reaction mechanisms for NH 3 /CH 4 combustion and to facilitate the prediction of the combustion behaviour.NO x emissions 300 K / 1 atm Using of NH 3 /CH 4 blends extends the stable operational range of the system, in terms of both working temperatures and equivalence ratios, with respect to pure NH 3 .On the other hand, blends produce higher NO x emissions, with respect to both the pure NH 3 and CH 4 cases.

MPa
The relationship between NO emission and NH 3 /CH 4 concentration is nonlinear.
[48] 1-3 bar At higher NH 3 /CH 4 ratios, NO emissions are suppressed as the reduction in free radical mass fraction facilitates the NO reduction reaction.
[49] 900-1100 K / 20, 40 bar The emissions of CO and NO x increase with the increasing CH 4 content in the fuel mixture.
[50] 300 K / 1 atm NH 3 flame properties can be promoted through adding a small amount of CH 4 without increasing the NO x emission level.For the 10% CH 4 flames, NO emission is higher than that of the NH 3 flames when equivalence ratio < 1.05.At fuel rich condition, NO emission seems to reach the same amount as the NH 3 flames.
[51] 300-500 K / 0.10 -0.50 MPa Compared to rich-lean pure NH 3 combustion, the addition of CH 4 increases the flame speed and reduces nitrogen oxides in the secondary combustion zone.[3] Chemical kinetics 293-1500 K The research simulated carbon chemistry with Konnov, Okafor and San Diego mechanisms, and showed that San Diego mechanism proved to be the best in terms of emissions, especially in terms of NO emissions.
[16] 298 K / 0.1 MPa A shortened San Diego mechanism involves 66 species and 380 elementary reactions, and it can accurately predict the ignition delay time.
[14] 298 ± 3 K / 0.10-0.50MPa This study proposed a reaction mechanism that involves 42 species and 130 reactions based on the mechanisms of GRI-Mech 3.0 and Tian.The mechanism models the unstretched laminar burning velocity of and species in CH 4 -NH 3 -air flames with high precision.
[52] 200-6000 K / 4 kPa This research presented a mechanism which includes 84 species and 703 reactions.The mechanisms can correctly predict the concentration profiles of the major species and intermediates.[11]

Fig. 2 .Fig. 3 .
Fig. 2. Effects of CH 4 on NH 3 combustion under different fuel ratios.a.Time evolution of the number of NH 3 (Data were time-averaged every 200 ps).b.Time evolution of the number of O 2 .c. Time evolution of the number of CH 4 .d.Time evolution of the number of N 2 .e. Number of NO x (NO and NO 2 ) changes with fuel ratios.f.Number of CO x (CO and CO 2 ) changes with fuel ratios.

Fig. 4 .
Fig. 4. Intermediates and elementary reactions.a. Numbers of intermediate species and elementary reactions at varying fuel ratio conditions.b.Comparisons of intermediates among cases at different fuel ratio conditions.c.Time evolution of the number of OH radicals.d.Time evolution of the number of H radicals.

Fig. 5 .
Fig. 5. Comparisons of the representative elementary reactions between NH 3 and NH 3 /CH 4 combustion.a. NO formation pathways.b.N 2 formation pathways.Blue font: reactions in the α = 0 case, orange font: unique reactions in the α = 0 case, green font: reactions in the α = 0.5 case, and red font: unique reactions in the α = 0.5 case.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) , respectively.OH is the predominant radical in the NH 3 /CH 4 combustion.Reaction pathways for the generation of OH radicals include CH 4 + O → CH 3 + OH and CH 3 + O → CH 2 + OH.As more CH 4 molecules are added to the combustion system, more OH radicals are produced as shown in Fig. 4c.CH 4 is a source of H atoms, and the number of H radicals increases rapidly at a high α value, as shown in Fig. 4d.Reaction pathways for the formation of H radicals with the addition of CH 4 include CH 3 + O → CH 2 O + H, CHO → CO 2 + H, CHO → CO + H and CO + OH → CO 2 + H.

Fig. 8 .
Fig. 8. Role of CH 4 in NH 3 combustion.a.A diagram linking the macroscale observations and microscale events.b.Role of CH 4 in NH 3 combustion.

Table 1
Cases in the present study.

Table 3
Recent papers on effects of CH 4 on NH 3 combustion.correlation between laminar flame speed and CH 4 volume fraction.The flammability limit varies almost linearly with the CH 4 content.The adiabatic flame temperature increases with CH 4 proportion.strong ignition enhancement to NH 3 ; at higher CH 4 ratios, the ignition delay time approaches that of pure CH 4 and the ignition enhancement tends to level off.Flame sustainability to high stretch rate, temperature, heat release rate and important intermediate radicals of the flames, carbon oxides and nitrogen oxides emissions increase with the CH 4 content in the NH 3 /CH 4 fuel blends.
937 K -1773 K The Mendiara and Glarborg's mechanism involves 97 species and 779 elementary reactions.It can effectively predict NO and CO 2 emissions.addition affects the collision frequency, reaction rate and reaction pathways of NO x formation in the NH 3 combustion.[20] 2400-3200 K CH 4 addition affects the oxidation of NH 3 reaction, number of intermediate molecules (O 2 , CO, CH 3 , H 2 , N 2 and H 2 O) and elementary reactions.