Underground as a Controversial Mitigation Of Climate Change Option: Example Of Carbon Capture And Storage (CCS) implementation

Presenter: Jonas PIGEON, jonas.pigeon@yahoo.com, IDEES, Université du Havre, France, Congress of the Society for Social Studies of Science; 4 th -7 th September 2019.

The idea of capturing CO2 from industry to solve global warming issue was first detail in a Marchetti paper in 1977 [START_REF] Marchetti | On geoengineering and the CO2 problem[END_REF]. In his paper Marchetti highlight the critical impact of CO2 emissions on global warming. According to him, managing these emissions is a needed action to maintain economical model. According to Marchetti store CO2 on ocean ground to dissolve it and consequently reduce it impact on global warming is the most relevant option. He also envisioned geological storage but stressed the limited capacity of this option. However, after some controversial experimentation of oceanic storage of CO2 [START_REF] De Figueirdo | TheHawaii Carbon Dioxide Ocean Sequestration Field Experiment: A Case Study in Public Perceptions and Institutional Effectiveness[END_REF] the application of the technology evolved and only geological storage was considered as a relevant option.

In this paper we will describe various CCS actor-networks in focusing on the role assigned to the underground. Our research material is composed of scientific paper, newspaper articles, but also case studies in various European countries that implement CCS. In order to understand the role assigned to underground reservoirs, these materials were analysed according to narratives analysis principles applied by actor network scientist [START_REF] Deuten | The Narrative Shaping of a Product Creation Process[END_REF]. This analysis will allows us to show issues related to CO2 underground storage and consequently draw the actor network of Carbon Capture an storage technology (CCS). Finally, we will be able to clarify the role of underground and then of CCS in climate change mitigation

Carbon Capture and Storage: a sociotechnical description

The concept of Carbon Capture and Storage first appears in a Marchetti paper in 1977(Marchetii 1977) he shares the view that increase of CO2 emissions will lead to global warming and the need to act against this phenomenon and its economic consequences (p.60). In this paper he displays a vision of the global warming issue with a geophysicist view: as problem of global carbon cycle.

According to him the main issue related to the carbon cycle is the loss of concentration during the combustion process. As he notes:

"We start from a very concentrated form of carbon; We go to a dilution of two or three order in magnitude in the combustion gases; We dilute it again perhaps by three or four orders of magnitude by dispersing these gases into the atmosphere; We have to wait this very dilute stuff to diffuse through the surface of the sea into the thin mixed upper layer of the ocean". (p. 61).

After the problem description Marchetti describes the solution to solve the CO2 emissions responsible of global warming. He formulated it as follow:

"The obvious line of attack would be to avoid the whole chain of dillutions interfaces and to put CO2 directly into the deep ocean. This may seem more easily said than done but I will show later the basis for a technologically and economically feasible operation does exist". (p.61)

According to Marchetti the oceanic mass is large enough to keep its equilibrium after dissolving atmospheric CO2. Therefore to manage this plan CO2 have to be collected, transported and disposed into deep ocean or exhausted gas field. The following pages of its paper are dedicated to describe the system. In 1997 after Kyoto agreement signature, Japan, Canada and Australia are willing to test CO2 oceanic disposal. The depiction of this first attempt is based on report of three scientists involved in this project (Herzog, De Figueirdo and Reiner 2003). The main goal of this first experience is to monitor the evolution and impacts of CO2 disposed on oceanic floor. In order to manage this experience scientists need to find a testing site nearby the coast to have a CO2 source but with relatively deep ocean (800 to 1000 meters deep) [START_REF] De Figueirdo | TheHawaii Carbon Dioxide Ocean Sequestration Field Experiment: A Case Study in Public Perceptions and Institutional Effectiveness[END_REF]. The Keahole point site in Kona island in Hawaii archipelago suits these expectations. In addition, a scientific institution -Natural Energy Laboratory of Hawaii Authority -could host the project. shows that the testing site was currently a refuge for whales. After these two articles the CO2 disposal project developers made a website to communicate on CO2 disposal but the opposition against the project was more and more organized and the coalition against CO2 dumping was funded. It gathers fishermen and natives to denounce the impacts of the project on environment but also on cultural and tourism aspects.

The developers of the projects answered to these opponents with scientific arguments but didn't convinced them. Finally the project aborted. After this first attempt the CO2 oceanic disposal was progressively abandoned because of controversies related to its impacts (Gough and Schackley 2005). In addition the OSPAR convention1 regulating international cooperation and environmental protection in North East Atlantic forbade the CO2 disposal.

This first experience reveals new actants in the actor network that Marchetti had not identified. It illustrates that oceanic wildlife but also socio-cultural dimensions have to be taken into account in order to dispose CO2 on oceanic floor. This experience shows that the enrolment process [START_REF] Callon | Éléments pour une sociologie de la traduction, La domestication des coquilles Saint-Jacques et des marins pêcheurs dans la baie de Saint-Brieuc[END_REF]) of all the actors of the network had not been undertook by project developers but had a strong impact on this technological option.

Build the Actor-Network of CO 2 Deep underground storage: bring together CO 2 & underground reservoirs

After the abandon of the deep ocean disposal of CO2 underground storage remains the main CO2 storage option. But despite the long history of the CCS concept the Actor Network related to this technology remains to consolidate. The following paragraphs describe the "interessment process" [START_REF] Callon | Éléments pour une sociologie de la traduction, La domestication des coquilles Saint-Jacques et des marins pêcheurs dans la baie de Saint-Brieuc[END_REF]) of underground rocks to build the CCS Actor Network. Indeed the main issue pursue by scientists is to fix CO2 permanently in geological rock reservoirs. Cap rocks of geological reservoirs or fault and seismicity as well as injection processes are all agents to gathers in order to build an efficient CCS.

What kind of rocks are reliable allies?

In order to manage an efficient CO2 storage, scientists involved in carbon capture storage implementation have to prove the reliability and permanency of the geological storage. The main sources of uncertainties related to CO2 geological storage are leakages of CO2 on drinkable water as well as on the surface through faults for instance. To prevent CO2 leakage scientists designs pilot projects, CO2 tracking methodologies or big data analysis on analogues natural CO2 storage reservoirs. According to the literature two kind of rocks are mainly dedicated to permanent storage (without other activities combination) : Saline aquifers and basaltic rocks.

Saline aquifers: an agent to enroll

Saline aquifers are deep underground very high salted water reservoir rocks. This type of geological reservoirs don't enter in competition with other economic activities. Experts estimate these reservoirs are able to store safely and permanently CO2 (IPCC 2005). However various uncertainties remains around CO2 behaviour inside saline aquifers. International Panel on Climate Change special report on CO2 storage shows that after its injection CO2 stay liquid and move inside the reservoir. After hundred of years it mineralized and become stable. Therefore the main issues for scientists is to ensure that CO2 is contained in the reservoir as well as to forecast evolution of the gas for hundreds of years.

In order to efficiently track CO2, scientist had identified other gases that could be efficient trackers of the presence of CO2 in the rock. "A major obstacle to the implementation of CCS is the uncertainty over the long-term fate of the CO2 which impacts regulation, insurance, and who assumes the responsibility for maintaining CO2 storage sites. Our study demonstrates that geological carbon storage can be safe and predictable over many hundreds of thousands of years."

Finally to complement their data scientists also design instrumented pilot projects The goal of the Aquistore project in Canada2 is to validate the efficiency and the safety of the CO2 storage in saline aquifers as well as to collect data that could be reused in other project (CCSJ-49, 2016, p.32).

The previous paragraphs shows that in a long terms saline aquifers are relevant allies of CCS. However, in a short and middle terms the addition of another agents (noble gases) is needed to ensure the reliability or this type of rocks.

Basaltic rocks: an efficient ally of CCS?

Despite saline aquifers other kind of rocks are also tested to store CO2 as only purpose. This is for instance the case of basaltic rocks (CCSJ-52, 2016, p.31). A news published in the CCS Journal described the project CarbFix that in Iceland. The following extract summarize the main results of the project:

"We find that over 95% of the CO2 injected into the CarbFix site in Iceland was mineralized to carbonate minerals in less than two years. This result contrasts with the common view that the immobilization of CO2 as carbonate minerals within geologic reservoirs takes several hundreds to thousands of years".

According to the previous extract, basaltic rocks are more reliable allies compared to saline aquifers. Indeed most part of the CO2 injected in this kind of rocks is mineralized quickly and then play the role they are expected by scientists and CCS developers. Saline aquifers as well as basaltic rocks are according literature two complementary reliable geological reservoirs to store CO2. However, in order to be develop at a large scale CCS and attains its goal of climate mitigation also have to allies with other social and economic entities.

Injection process: a key component of the alliance between Rock and CO2?

The injection process is a key step in order to build an reliable alliance between CO2 and the rock of geological reservoirs. The scientific literature surveyed shows CO2 injections in reservoir rock is surrounded by uncertainties related to its effects the reservoir especially for CO2 storage in saline aquifers. According to CO2 storage developers each storage reservoir will face during its history various injection rates caused by various delivery of CO2, or well maintenance operation. However, they don't know the effect of various injections rate on the storage safety. A survey managed by the university of Edinburgh showed that:

[The] storage security would be enhanced because interruptions have the effect of increasing the amount of CO2 trapped within the pore spaces of the rock; the efficiency of storage is increased because trapped CO2 is less mobile than free flowing CO2 and so its migration within the reservoir is more contained; injection pressure would rise due to the increasingly trapped CO2 acting as a barrier to flow and this effect would need to be managed by storage site operators (CCSJ-68, 2019, p. 24).

The previous extract shows that conversely to the expectations of CO2 storage developers Various injection rates has positive effect on safety. However pressure in reservoir is still to manage. Therefore, injection agent is not as strong as expected by CCS developer to destabilise the overall Actor Network of CCS.

How interest Economy & societies to CO 2 storage

Previous paragraphs shows that scientists have quite reliable methods to "interess" and "enrol" [START_REF] Callon | Éléments pour une sociologie de la traduction, La domestication des coquilles Saint-Jacques et des marins pêcheurs dans la baie de Saint-Brieuc[END_REF]) various technical and natural agents. However to implement successfully Carbon Capture and Storage, an alignment of social, economic and political agents are also needed.

Followings paragraphs shows various narratives as well as arguments used by CCS proponents to interest and enrol these social, economic and political forces.

Reaffirm CCS benefits in Paris agreement context

In order to interest social, political and economic agents proponents of CCS first mention the critical role of this technology in order to reach quickly greenhouse gas mitigation targets as defined in the Paris agreement in 2015 as illustrate the following extracts.

"The UN Paris agreement has committed the world to limiting climate warming to well below 2°C from pre-industrial levels. This requires huge reductions in the amount of the greenhouse gas, carbon dioxide, which is released to the atmosphere from industry, electricity generation, heating and transport. Capturing these emissions and ensuring that carbon dioxide can be safely trapped underground is crucial for the successful protection of the atmosphere". (CCSJ-64, 2018, p. 25).

"Fossil fuels are likely to be a significant energy source for the foreseeable future. CCUS is the only currently available technology that can solve the carbon problem while still allowing fossil fuels to be used. All current projections of a low-carbon energy future include significant amounts of CCUS." Princeton University researcher (CCSJ-66, 2018 p.25).

Paris agreement allows CCS proponents to actualise the orignal narrative related to CCS. Societies still facing greenhouse gas emissions growth as well as the dependency to fossil fuels. However limit these emissions is critical and CCS could provide a real support to manage this task quickly compared to other renewable energy and social transformation they implies. The previous paragraphs only depict the role of CCS in climate change mitigation. Describing that way the situation is unfair regarding other renewable energy. The last part of this paper wil give a more accurate overview of the competition between CCS narrative and other renewable narratives.

Show the immediate profitability of CCS

The main argument of Paris agreements has not enough strength to support the implementation of all the CCS system. Indeed international agreements are not strongly prescriptive and some states could easily withdraw from these agreements. Therefore, to implement successfully the technology the proponent of CCS insist on its profitability.

CCS proponents shows that CO2 storage is not only a source of economic costs but can also contributes to improve oil and gas production in current fields or extract shale gas. This framing of the technology is designed to involve economic stakeholder in CCS development. Researchers of Princeton university researchers insist for instance on the improvement of oil & gas production through Enhanced Oil Recovery (EOR) as illustrates the following extract.

Conventional recovery from an oil well yields about 40 percent of the total oil within the rock, .

Injecting carbon dioxide into the reservoir enables recovery of more oil, typically around an additional 15 percent, he said. The carbon dioxide is separated from the oil at the surface, then the gas is returned into the ground and ultimately trapped there. Princeton University researcher (CCSJ-66, 2018, p. 25) In addition according to some experts re-use infrastructures of exhausted oil & gas field could be a right start to develop CCS. The director of the Scottish Carbon Capture and Storage affirm the economic benefit to start this way to develop CCS as illustrates the following extract.

"The recent announcement from OGA heralds a new future for the North Sea. The Acorn site could launch a new multi-billion offshore industry, which progressively and systematically takes the place of oil and gas extraction as fields end production from now to the 2030s. By unlocking access for re-use of existing oil and gas infrastructure, this allows the first CO2 storage projects to reduce capital costs by 20-50%." Stuart Haszeldine Director of SCCS (CCSJ-67, 2019, p. 25)

The CCS plays in this configuration a double role of climate mitigation system but also an enhancing oil & gas production technology. As detailled in the last part of the paper this double role of CCS as consequence in its perception as an energy transition technology.

Demonstrate the value of geological storage to convince local population

Store CO2 in geological reservoirs located onshore is not an easy business. Case studies shows that local populations contest CO2 storage projects. The most contested case of onshore CO2 storage is the project of Barendrecht led by Shell in Netherlands [START_REF] Brunsting | Stakeholder Participation Practices and Onshore CCS: Lessons from the Dutch CCS Case Barendrecht[END_REF]). This city is located in the vicinity of Rotterdam and the main goal of this project was to store CO2 from an hydrogen plant in geological reservoir located under a Barendrecht residential district. The choice of the storage location was motivated because its suits risks evaluation but also by its proximity from the emitting source (16.5 km) [START_REF] Eijs | Containment demonstration for the Barendrecht CO2 storage project[END_REF]. However, after the first public meetings the project was contested locally. The project was both contested regarding its impacts on environment, health as well as real estate market. In addition this project was also regarding its general ambition giving subsidies to CO2 and impacting local population. After this local opposition the project was finally abandoned.

Scientific literature often shows a great concern on safety of the CO2 storage that is a key issue. However as illustrated by the Barendrecht case, local conflict goes most of the time beyond safety concerns. Therefore, convincing people need to undertake an overall approach of a project.

Ask for the design of relevant regulations and subsidies to develop CCS

According to the CCS experts the implementation of CCS infrastructures at a large scale needs a political support. Two regulations tools could contribute to the the implementation: the establishment of tax and tax credits as well as the public funding of CCS infrastructures. Indeed regarding the history of CCS technology Tax systems are key drivers. In Norway the introduction of a tax in 1991 on the CO2 emissions of offshore oil & gas infrastructures (Kongsjorden, Kårstad and Torp, 1998) contribute to stimulate the implementation of CCS.

Princeton university researchers showed that in the U.S tax credits are a relevant tool to enhance scale CCS development as illustrates the following extract.

"The new tax credits are the most significant policy incentivizing carbon capture, utilization and storage (CCUS) in the world today, (...) This is the first time we've had a policy in front of us that lets us seriously consider deployment on a large scale," Ryan Edwards -Princeton University (CCSJ-66, 2018, p.25) In addition, according to Ryan Edwards tax credit only are not efficient to ensure CCS large scale implementation. Infrastructure funding by national states is also a key issue. The following extract illustrates this idea.

"In order for CCUS to have any chance for large-scale development, the necessary infrastructure needs to be built (...) Similar large-scale infrastructures such as the Interstate Highway System and electric-power grids have been built with government financing and coordination (...) Our analysis shows that additional public support beyond the tax credits will be necessary to enable large-scale deployment in the near-term, (...) That's how these things are done -governments play a key role. An excellent recent example is the leadership on clean-energy infrastructure by the Texas state government that enabled the big wind energy boom there."

In previous extract researchers compared CCS infrastructure to Interstate Highways System to insist on general need related to such type of infrastructures. This argument attribute to state government a key role in CCS implementation and some states like Texas seems willing to play this role.

The previous paragraphs shows that the large scale CCS implementation largely depend from regulation as well as state policies. In some states like Norway regulation and/or subsidies make easy the development of CCS, in making it profitable. In other countries regulations didn't fit with CCS expectations. Political stakeholders are still to interest and enroll.

An international standard to legitimate the CO2 storage implementation as a climate change mitigation option.

Another

Conclusion: Is underground belongs to the climate mitigation narrative?

This paper summarize the building of Carbon Capture and Storage (CCS) actor network in order to identify various role assigned to underground storage reservoir. According to the various sources surveyed the "non-human" part of this actor network have been successfully enrolled in the system. In the contrary we also noticed that the CCS proponents have to undertake a strong conviction works with political and various societal stakeholders to fully establish the Actor Network of CCS as a climate change mitigation technology. According to us this difficulty faced by CCS proponents is related to the narrative that frame CCS technology. This last part of this paper will illustrates why CCS is in some way in contradiction with climate mitigation. First, main ideas of climate mitigation narratives will be summarized and then mismatch with CCS narrative will be identified Narratives related to climate mitigation and more broadly ecological transition often mention the idea of a decentralize renewable energy technology but also larger behavioural changes. Most discourses of Environmental NGO (e.g. Greenpeace), but also in some expert reports3 , the need to focus on renewable decentralized energy and to change our behaviours to successfully mitigate climate change. Therefore all these narratives support an ecological imaginary based on the ideas of transformation as well as "small scales" energy and consumption systems [START_REF] Manceron | L'imaginaire écologique[END_REF] where life is based on local activities.

Narratives detailed in this paper shows that contrary to this ecological imaginary CCS narratives promote a quick cut of CO2 emission without behavioural changes. In addition this technology is also strongly related to Oil & Gas industry and draws part of its profitability from enhanced oil recovery. CCS narrative is thus strongly attached to "old" energy model that doesn't match with the ecological imaginary and may be not convincing enough for some political stakeholders incentivized by experts, NGO, but also citizen to engage in an ecological transition.

However CCS proponents had also adapted their narratives and including the idea of CO2 re-use. Circa 2013, Carbon Capture and Storage became Carbon Capture Utilization and Storage.

According to experts [START_REF] Valentin | Le stockage d'énergie à partir de méthanol[END_REF][START_REF] Pierre | Production, valorisation et stockage de l'Hydrogène le projet GRHYD[END_REF] about 1% of total CO2 emissions could be reuse easily. Nevertheless, various CO2 programs are already engaged. For instance CCS journal mentioned various CO2 re-use project. In addition, with the development of hydrogen energy as well as synthetic fuel Carbon Capture could also plays a critical role.

As a conclusion CCS has not yet a strong enough Actor Network. Its proponents still have to convince political stakeholders as well as society of the relevance of CCS but also to connect CCS with new energy narrative to make inscribe CCS in energy transition.

Figure

  Figure 1: CCS as an obligatory passage point

Figure 2 :

 2 Figure 2: Sketch of the network of entities involved in Hawaii Kona Island CO2 oceanic disposal Initially the developers of the project planned to communicate with local populations. However fund dedicated to the communication campaigned were allocated one year after the start of the project. During this period a first newspaper article entitled "Feds to test impact of Dumping CO2 in Kona water " was published in the west Hawaii today in the 18th of March 1999 (De Figueirdo 2003). This article based on data of the United State Department of Energy stresses the impact of CO2 on ocean chemical composition and raises concerns among local population regarding on oceanic life. A second newspaper article in the Hawaii Star Bulletin published the 10th of August 2000shows that the testing site was currently a refuge for whales. After these two articles the CO2 disposal project developers made a website to communicate on CO2 disposal but the opposition against the project was more and more organized and the coalition against CO2

Figure 3 :

 3 Figure 3: Sketch of entities involved in CO2 storage in saline aquifer case.

Figure 4 :

 4 Figure 4: Sketch of entities involved in basaltic rock storage

  Marchetti thinks that apply CO2 capture on power plant or in energy production plants are the most relevant solution to collect easily a large amount of. According to him compressed CO2 until it become liquid is and transport it by pipeline is in 1977 the best way to carry it to disposal reservoirs or deep oceans. (p.64).Finally regarding the CO2 disposal Marchetti imagined to options: exhausted gases & oil fields and oceanic disposal. Regarding the storage in exhausted oil & gas field he considered that

"This possible storage of liquid CO2 at pressure lower to than 100 atm guarantees a large storage capacity than for the original methane, although the solubility of CO2 in water and the solubilization of carbonate rocks do not guarantee a similar stability over the time. The same could be said for exhausted oil fields and other types of natural or artificial cavities. I did not go into the details of this system on the grounds that the capacity of natural structure which is available at reasonable cost may be insufficient" (p.64-65)

The previous extract shows that if Marchetti recognized the CO2 storage potential of underground he does not considered it as relevant in a first time and focuses on deep ocean storage. Initially the Carbon Capture and Storage consider mainly the deep Ocean CO2 disposal. The following graph represent the ANT network of the first CCS system as think by

Marchetti.

  The CCS journal refers to the work led by the school of geoscience at Edinburgh University(CCSJ-58, 2017 p. 26). These Researchers uses nobles gases to track CO2 unplanned migration in the subsurface. These Gases are selected because they move inside the rock quicker than CO2. Therefore they could allow CO2 Storage developers to managed leakage. According to these researchers accurately identifying leaks is a key issue regarding the successful implementation of CCS. The following extract illustrates this idea "Securely storing captured CO2 is critical to its success and our method of identifying any leaks should give assurance to local communities. Our work provides a simple way to easily and unambiguously spot leaks from future storage sites, using the fingerprint of noble gases that the CO2 picks up during storage.

" Dr Stuart Gilfillan

In addition of tracking methodology to ensure CO2 containment, scientists also managed data analysis related to natural CO2 storage and design statistical models. The article "CO2 stored ten times longer than needed" (CCSJ-53, 2016, p.33) refers for instance to a survey managed by a Cambridge university laboratory. Based on data related to natural CO2 reservoir the statistical model designed by researcher shows that stored CO2 in deep underground is more safe than expected. The CO2 storage in deep underground is more predictable over long periods. According to the Professor Mike Bickle, that takes part to the survey the result of this survey are critical for the success of CCS. The following reference illustrates this idea :

  way to strengthen the Actor Network of Carbon Capture and storage use by industry in favour of CCS development was to design a body of standards related to these technologies. Since 2016, five ISO standards related to CCS were passed and three ISO technical reports on technological specifications related to various parts of this carbon capture and storage were edited(ISO/TR 27912:2016, ISO 27913:2016, ISO 27914, ISO/TR 27915, ISO 27916:2019, ISO 27917, ISO/TR 27918, ISO 27919-1). According toMallard (2000) standards plays a key role in a technology development process. He shows that standards bring together various sociotechnical networks and establish a consensus between various measurement methodologies or indicators used to assess the performances of a technology. In addition Mallard also show that standard assigned role to all the actants of a technological system.Regarding the CCS technology the edition of a body of standards legitimate the CCS and especially the CO2 storage as an option recognized internationally to tackles the global warming issue. Indeed through standardization process this technology is no more just a pilot technology but become an industrial reality and therefore, contributes to derisk investment in these technologies. Through standards it is easier to distinguish what are the projects that follow standardized methodologies and instrument that could benefit from insurance and those that won't.

https://www.ospar.org/convention visit 14-08-2019.

https://ptrc.ca/projects/aquistore/ http://aquistore.ca/index

IPCC last report from August 2019 6th illustrates the need to change our food production and consumption systems.


Suphaporn Arunthanayothin

Alessandro Stagni

Yu Song

Olivier Herbinet

Tiziano Faravelli

Frederique Battin-Leclerc

Frédérique Battin-Leclerc

Ammonia-methane interaction in jet-stirred and flow reactors: An experimental and kinetic modeling study

Keywords: Ammonia, Jet-stirred reactor, Flow reactor, Oxidation, Detailed kinetic modeling

come   L'archive ouverte pluridisciplinaire

Introduction

Nowadays, a great attention is being paid to the gas-phase oxidation of ammonia, either as pure or mixed with other hydrocarbons. Ammonia is one of the components of biogas, a potentially renewable fuel produced via the anaerobic digestion of organic feedstocks. Biogas is primarily composed of methane and carbon dioxide, with smaller amounts of water, nitrogen, oxygen, ammonia and hydrogen sulfide. Its relative composition varies according to the original source [START_REF] Strik | A pH-based control of ammonia in biogas during anaerobic digestion of artificial pig manure and maize silage[END_REF][START_REF] Arazo | Bio-oil production from dry sewage sludge by fast pyrolysis in an electrically-heated fluidized bed reactor[END_REF], which may have an influence on the reactivity and the pollutant formation when biogas is used as a fuel. In addition, ammonia is a potentially carbon-free fuel, which could be used, like hydrogen, as a flexible energy carrier, with applications in several industrial sectors [START_REF] Kobayashi | Science and technology of ammonia combustion[END_REF].

Therefore, it has the potential to store excess energy, and unlock the vast potential of intermittent renewable energy (e.g. wind, sun [START_REF] Ikäheimo | Power-to-ammonia in future North European 100 % renewable power and heat system[END_REF]). Due to its weak combustion properties, co-firing of ammonia with a combustion enhancer (e.g. hydrogen, methane or coal) is one of the proposed solutions to allow its use in gas turbines, engines or industrial furnaces [START_REF] Kobayashi | Science and technology of ammonia combustion[END_REF][START_REF] Xiao | Study on premixed combustion characteristics of co-firing ammonia/methane fuels[END_REF][START_REF] Reiter | Combustion and emissions characteristics of compression-ignition engine using dual ammonia-diesel fuel[END_REF]. Whatever the application domain, a comprehensive understanding of ammonia combustion characteristics and its mutual interaction with species like methane is essential for designing efficient industrial processes and controlling undesired emissions. Indeed, the presence of nitrogen in the molecule makes ammonia kinetics intrinsically related to the formation of Nitrogen Oxides (NOx), either as fuel or intermediate species (e.g. organic fuels containing nitrogen).

The literature is very rich in experimental data for neat methane and neat ammonia [START_REF] Stagni | An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia[END_REF], but much less is available with regard to the oxidation of ammonia-methane mixtures, in spite of the growing interest. Most of ammonia-methane oxidation studies were performed in premixed flames (e.g., [START_REF] Konnov | Probe Sampling Measurements of NO in CH4+O2+N2 Flames Doped with NH3[END_REF][START_REF] Li | Measurements of NO concentration in NH3-doped CH4+air flames using saturated laser-induced fluorescence and probe sampling[END_REF]) with a focus on the detection of flame intermediates [START_REF] Williams | Radical species profiles in low-pressure methane flames containing fuel nitrogen compounds[END_REF][START_REF] Rahinov | Absorption spectroscopy diagnostics of amidogen in ammoniadoped methane/air flames[END_REF][START_REF] Garo | Experimental Study of Methane-Oxygen Flames Doped with Nitrogen Oxide or Ammonia. Comparison with Modeling[END_REF], or on the measurements of adiabatic burning velocities [START_REF] Han | Experimental and kinetic modeling study of laminar burning velocities of NH3/air, NH3/H2/air, NH3/CO/air and NH3/CH4/air premixed flames[END_REF][START_REF] Okafor | Measurement and modelling of the laminar burning velocity of methane-ammonia-air flames at high pressures using a reduced reaction mechanism[END_REF]. Previous studies were also carried out in flow reactors [START_REF] Mendiara | Ammonia chemistry in oxy-fuel combustion of methane[END_REF][START_REF] Wargadalam | Homogeneous formation of NO and N2O from the oxidation of HCN and NH3 at 600-1000°C[END_REF] where the oxidation of NH3/CH4 mixtures in N2 or CO2 environments was investigated, at atmospheric pressure and intermediate/high-temperatures with NO, CO and CO2 quantification. Other studies on the oxidation of methane doped with ammonia using CO2 as bath gas were performed in flames [START_REF] Konnov | Probe Sampling Measurements of NO in CH4+O2+N2 Flames Doped with NH3[END_REF][START_REF] Barbas | Experimental and kinetic modeling study of CO and NO formation under oxy-fuel conditions[END_REF][START_REF] Mathieu | Shock-tube study of the ignition of multi-component syngas mixtures with and without ammonia impurities[END_REF][START_REF] Liu | Experimental and numerical study of laminar flame speeds of CH4/NH3 mixtures under oxy-fuel combustion[END_REF]. Several detailed kinetic models were proposed to account for the oxidation chemistry of ammonia/methane mixtures, and to investigate the mutual interaction between the two fuels [START_REF] Konnov | Probe Sampling Measurements of NO in CH4+O2+N2 Flames Doped with NH3[END_REF][START_REF] Mendiara | Ammonia chemistry in oxy-fuel combustion of methane[END_REF][START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF][START_REF] Glarborg | Modeling nitrogen chemistry in combustion[END_REF].

In order to better understand ammonia-methane interaction in a wider range of operating temperature conditions, we performed experimental campaigns using two different configurations, respectively a Jet-Stirred Reactor (JSR) and a Flow Reactor (FR), such as to cover low-, intermediate-and high-temperature conditions. A detailed analysis of reaction products and intermediates was performed, and the results were interpreted with a comprehensive kinetic model, developed through a first-principles approach, describing the oxidation of methane and ammonia, as well as both fuel interaction and NOx formation.

Experimental methodology

The co-oxidation of methane and ammonia was experimentally investigated using two different reactors (see additional details in Supplementary Material (SM1)) working close to atmospheric pressure, with helium as carrier gas. Mass flow controllers were used for reactor feeding (relative uncertainty of ±0.5% in flow). In a first set-up, experiments were performed in a fused silica JSR, a type of continuous stirred-tank reactor usually operated at steady state. JSRs were used for several gas-phase kinetic studies and the present JSR setup was already described in previous works [START_REF] Song | The sensitizing effects of NO2 and NO on methane low temperature oxidation in a jet stirred reactor[END_REF][START_REF] Marrodán | Effects of Bath Gas and NOx Addition on n-Pentane Low-Temperature Oxidation in a Jet-Stirred Reactor[END_REF][START_REF] Pelucchi | An experimental and kinetic modelling study of n-C4C6 aldehydes oxidation in a jet-stirred reactor[END_REF]. It consisted of a spherical vessel with injection of the fresh mixture through four nozzles located at the center of the reactor, creating high turbulence resulting in homogeneity in composition and temperature. As a result, the JSR can be modeled as a perfectly stirred reactor. Its residence time was fixed at 1.5 s. The heating of the reactor was achieved though Inconel resistances, and the reaction temperature was measured with a K-type thermocouple located in a glass finger close to the center of the reactor (uncertainty of ±5 K).

A second setup was used to investigate the co-oxidation of CH4 and NH3 at higher temperature (up to ∼2000 K) in a flow reactor (FR), consisting in an alumina tube (inner diameter of 4 mm and 100 cm in length) designed to approximate plug flow conditions with the Peclet numbers >50 (see SM1). The reactor was located horizontally in an electrically heated oven (Carbolite Gero). Temperature profiles were measured with a R-type thermocouple (profiles are provided in SM2). The isothermal reaction zone is located between 36 and 58 cm with a uniform temperature profile (± 30 K). For each experiment, the residence time was fixed to about 25 ms in the central zone where the temperature can be considered constant. It must be noted that undesirable catalytic effects of the reactor wall were first observed in FR when using non recrystallized alumina.

The reactants and reaction products were analyzed using four diagnostics: • Two gas chromatographs (GCs) were used for the quantification of carbon-containing species such as methane, carbon monoxide, carbon dioxide and the three C2 hydrocarbons (the carrier gas was helium). They were equipped with GSQ (JSR setup) and PlotQ (FR setup) capillary columns, respectively. Both included a Flame Ionization Detector (FID) preceded by a methanizer (for the reduction by H2 of oxygenated and nitrogenated functional groups over a heated nickel catalyst) allowing a better sensitivity for CO, CO2 and HCN detection than a thermal conductivity detector. Calibrations were performed using gaseous standards or using the "effective carbon number" method for carbon containing species [START_REF] Pelucchi | An experimental and kinetic modelling study of n-C4C6 aldehydes oxidation in a jet-stirred reactor[END_REF].

• Nitric oxide (NO) and nitrogen dioxide (NO2) were detected in JSR and FR experiments using a dedicated NOx analyzer (Thermo Scientific Model 42i), based on chemiluminescence. This analyzer had two channels, one for the detection of nitric oxide in a direct and independent way, and a second one for the measure of the total NOx concentration. Calibrations were performed using gaseous standards.

• A continuous-wave Cavity Ring-Down Spectroscopy (cw-CRDS) cell coupled to the JSR by a sonic probe was used for ammonia quantification. This technique was previously successfully used for the detection of species, such as hydrogen peroxide during the oxidation of alkanes [START_REF] Bahrini | Quantification of Hydrogen Peroxide during the Low-Temperature Oxidation of Alkanes[END_REF] and HONO during the oxidation of n-pentane doped with nitric oxide [START_REF] Marrodán | First detection of a key intermediate in the oxidation of fuel + NO systems: HONO[END_REF]. Ammonia has strong absorption lines in the wavelength range 6637-6643 cm -1 , making the quantification accurate and providing a good sensibility. Cross sections were deduced from data recorded under non-reactive conditions.

• On-line mass-spectrometry was used to detect NH3, N2, H2O, CH4, CO2 and O2 during the cooxidation of CH4 and NH3 in FR. Sampling was achieved through a capillary tube directly connecting the FR outlet and the analyzer. This technique requires the calibration of each species as there is no obvious relationship between their structures and their calibration factors. Gaseous standards were used except for CO2 and water, which were calibrated considering the reaction complete at the highest temperature.

• Fourier Transform InfraRed Spectroscopy was used to highlight the possible presence of N2O as this species has a strong absorption in the 400-4000 cm -1 wavelength range. However, no N2O was observed during experiments, likely due to concentrations below the detection limit (estimated to 50 ppm at 2237.5 cm -1 ).

Relative uncertainties in mole fractions are ±5% for species detected by gas chromatography and NO. It is ±10% for ammonia quantified by CRDS, NO2 and the species detected by online mass spectrometry.

Kinetic model

The kinetic model describing ammonia and methane oxidation and their mutual interaction was obtained by following a hierarchical methodology, i.e. the founding principle of the CRECK framework [START_REF] Ranzi | Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels[END_REF]. The mechanism relied on a core C0-C2 module developed by Metcalfe et al. [START_REF] Metcalfe | A Hierarchical and Comparative Kinetic Modeling Study of C1 -C2 Hydrocarbon and Oxygenated Fuels[END_REF], on top of which the C3 mechanism of Burke et al. [START_REF] Burke | An experimental and modeling study of propene oxidation. Part 2: Ignition delay time and flame speed measurements[END_REF] was added. Thermodynamic properties were taken from the database of Burcat and Ruscic [START_REF] Burcat | Third millenium ideal gas and condensed phase thermochemical database for combustion (with update from active thermochemical tables)[END_REF].

NOx submechanism leveraged the recent work of Song et al. [START_REF] Song | The sensitizing effects of NO2 and NO on methane low temperature oxidation in a jet stirred reactor[END_REF], where its capability to represent the sensitizing effects of both NO and NO2 on CH4 oxidation at lower temperatures had been demonstrated. This served as a basis for the development of this new ammonia oxidation mechanism, which was performed via an ab-initio evaluation of the ammonia decomposition and H-abstraction reactions. A full description of the mechanism construction and its wide-range validation can be found in the paper by Stagni et al. [START_REF] Stagni | An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia[END_REF].

The complete mechanism is made up of 157 species and 2444 reactions, and is available under CHEMKIN format in SM3-5, along with thermodynamic and transport properties. For the sake of completeness, a wide-range validation in the conditions of interest is also reported in Figures S4-S12 in SM1.

Experimental results

In this work, the equivalence ratio (Φ) was defined considering CO2, water and NO as final combustion products from methane and ammonia oxidation. NO was considered rather than N2, since non-negligible amounts of NO were formed under the conditions of this study.

Jet-stirred reactor

JSR experiments were performed over the temperature range 600-1200 K, at a residence time of 1.5 s, a pressure of 106.7 kPa, with inlet methane and ammonia mole fractions of 10,000 and 500 ppm, respectively, and at three equivalence ratios (0.5, 1 and 2). Two particular phenomena were observed during experiments, making the oxidation study of methane/ammonia mixtures tricky. The first was the occurrence of oscillation regimes under specific conditions (at the highest temperatures for the lean and stoichiometric mixtures). Mole fractions were not constant in time, which was not compatible with the diagnostics used in this study. For this reason, the investigated temperature range is limited for some conditions in the results displayed hereafter. This phenomenon has been previously reported in literature for the oxidation of neat methane [START_REF] Lubrano Lavadera | Oscillatory Behavior in Methane Combustion: Influence of the Operating Parameters[END_REF], and its kinetic foundations were explained in [START_REF] Stagni | The role of chemistry in the oscillating combustion of hydrocarbons: An experimental and theoretical study[END_REF]. The second phenomenon was the occurrence of wall reactions strongly enhancing the ammonia consumption, although the reactor was made of fused silica. This problem was solved by treating the surface of the JSR before each experiment by flowing all gases but ammonia under reactive conditions. Figure 1 displays the mole fractions of both reactants and main products. The temperature of the reactivity onset is very sensitive to the equivalence ratio: it is about 1000 K for the lean condition, 1075 K for the stoichiometric ones, and 1150 K in rich mixtures, according to the temperature dependence of the methane and ammonia mole fractions shown in Figure 1a-b. Figure 1a also displays CH4 mole fractions in the case of the oxidation of neat methane for comparison. Mole fractions are comparable to those of methane when doped with ammonia for the rich conditions, whereas the reactivity onset is anticipated (∼100 K) by ammonia addition under lean conditions, and conversion is slightly enhanced in stoichiometric conditions. The main carbon containing products are carbon monoxide, carbon dioxide, ethylene and ethane (Figure 1). Under lean conditions, carbon monoxide mole fractions peak at 1025 K. Above this temperature, a decrease of the CO mole fraction is observed with a simultaneous increase of that of CO2. The mole fractions of ethylene and ethane remain low (less than 500 ppm). Larger equivalence ratios favor the formation of unburnt species.

Only two nitrogen-containing species were detected during this study: nitric oxide (NO) and nitrogen dioxide (NO2), but more N-containing species should be formed in significant amounts because the N-atom balance is not closed. This is likely due to the formation of N2, which was expected, but its detection was not possible because of interference in GC analyses. NO and NO2 mole fractions are displayed in Figure 1e-f. Under lean conditions, the mole fractions of these last products increase simultaneously to ammonia depletion. NO2 mole fraction already peaks at 1025 K and afterward decreases whereas that of NO continue to increase. Note that NO behaves closely to CO2 whereas NO2 behaves closely to CO.

Under stoichiometric conditions, oscillation behavior was detected above 1025 K. Beyond this temperature, data for ammonia are not accurate because oscillations are not compatible with the CRDS spectrum acquisition time of ammonia (about 30 min) and the in-situ sampling. Data recorded with the NOx analyzer and by GC, with a sampling strategy based on the injection of a defined volume accumulated on the larger time scale, can be considered as time averages.

Oscillations seem to have a minor impact on species like methane, carbon monoxide and carbon dioxide. This is because the mole fractions of these species are larger compared to those of nitric oxide and nitrogen dioxide.

Flow reactor

FR experiments were performed over a temperature range significantly higher than in the JSR thanks to the use of a recrystallized alumina tube and an oven heating up to temperatures as high as ∼2000 K. The two fuels, the oxidizer, and 11 reaction products were followed. Reaction products can be divided as follows:

• Species specific to methane oxidation: CO, CO2, C2H2, C2H4, C2H6 and C3H6.

• Species specific to ammonia oxidation: N2, NO and NO2.

• Species common to both fuels: water and HCN.

Note that HCN is the only fuel cross-product whose formation was observed during these high temperature experiments. Mole fraction profiles are displayed in Figure 2. C-, N-, H-and O-atom balances were calculated for each temperature and are all in the range of 0.99±0.05 (see values in Table S4 in SM1).

The consumption of methane and ammonia becomes significant starting from ∼1500 K and they are already totally consumed at ∼1600 K. The two main products from the oxidation of methane are CO and CO2. CO peaks at 1573 K and is then progressively converted to CO2 when the temperature increases. Ammonia is mainly converted to NO and N2, and NO/N2 ratio slightly increases with temperature. Note that under these conditions, equilibrium calculations predict the formation of N2 quasi exclusively. Small amounts of NO2 were also observed on a very narrow range (between 1550 and 1600 K). A part of O2 remains not consumed even at the highest temperatures because of the formation of N2 from NH3 (the NH3/O2 inlet ratio was calculated considering only NO as product to be sure to have enough oxygen for fully consuming methane).

Other reaction products (C2 species, C3H6 and HCN) are produced in small amounts and over a narrow temperature range as for NO2 (Figure 2e). Lines: data computed with the model.

Comparison between experimental and simulated results

Figures 1 and 2 compare the experimental data and model results. The kinetic model well predicts the temperature of the reactivity onset for both reactors as it can be seen for methane and ammonia in Figures 1 and2. The temperature dependence of the conversion of both fuels is also well predicted, although the residual presence of methane after T = 1025 K is underestimated. In the JSR, at Φ=1, where oscillation behavior was detected above 1025 K, the mole fractions of methane (GC detection) reasonably agree with averaged computed mole fractions (dashed lines in Figure 1). The model predicts the anticipated reactivity of methane in presence of ammonia under lean conditions.

As far as JSR reaction products are concerned, mole fractions of carbon monoxide and of carbon dioxide are well predicted by the model at all equivalence ratios (even for the stoichiometric case for which oscillating computed mole fractions were averaged). Mole fractions of ethane and NO are also reproduced fairly well by the model. Discrepancies are observed for nitrogen dioxide for which the model under-estimates the formation, especially under lean conditions. This issue was also previously observed by Song et al. in the CH4 oxidation doped with NO [START_REF] Song | The sensitizing effects of NO2 and NO on methane low temperature oxidation in a jet stirred reactor[END_REF]. For the rich case, the signal detected for NO2 under these high temperature conditions is likely due to an interference with that of HCN, which is a typical high temperature product under rich conditions (whereas NO2 is usually produced at low-temperature oxidation), as already observed in [START_REF] Song | The sensitizing effects of NO2 and NO on methane low temperature oxidation in a jet stirred reactor[END_REF].

In the FR, the agreement is also quite satisfactory for the main reaction products such as water, NO and N2. Surprisingly, the model predicts a more abrupt increase of CO2 and decrease of CO mole fractions than in the experiments. This trend is also observed when studying the oxidation of neat methane (open symbols in Figure 2a), indicating that it is not due to the presence of ammonia in the feed of the reactor. According to a kinetic analysis performed at 1600 K, the second most sensitive reaction is CO+OH=CO2+H (after the branching reaction H + O2=OH+O) the kinetic parameters of which are relatively well known. With the current mechanism there is no way to explain this deviation, as well as the pre-reaction of NH3 before ignition (which was also observed in [START_REF] Mendiara | Ammonia chemistry in oxy-fuel combustion of methane[END_REF] with different inlet mixtures). The model predicts negligible amounts of NO2, C3H6 and HCN compared to the experiment.

Kinetic analysis

Rate-of-production and sensitivity analyses were performed to highlight the chemistry involved during the co-oxidation of methane and ammonia, and their mutual interactions. Figure 3 presents a rate analysis for fuel consumption (Figure 3a) and a sensitivity analysis for methane mole fraction (Figure 3b) performed under JSR conditions (944 K, 𝜑 = 0.5, corresponding to conversions of 18% and 9% of methane and ammonia, respectively). Figure 4 displays a rate analysis for fuel consumption (Figure 4b) and a sensitivity analysis for methane mole fraction (Figure 4a) performed under FR conditions (∼ 1560 K, 𝜑 = 1, and an abscissa of 46 cm (length from the tube inlet) corresponding to methane and ammonia conversions of ∼ 50% and ∼ 20%, respectively). At low temperatures and under JSR conditions, the analysis shows that the consumption of methane starts with the classic H-atom abstraction reaction, CH4+OH=CH3+H2O. The main consumption route of CH3 is CH3+NO2=CH3O+NO (as in the case of the oxidation of methane doped with NO and NO2 [START_REF] Song | The sensitizing effects of NO2 and NO on methane low temperature oxidation in a jet stirred reactor[END_REF]), NO2 coming from the reaction NO+HO2=NO2+OH. Minor channels are CH3+HO2=CH3O+OH and the recombination reaction forming ethane. Afterwards, the reaction series leading to CO2, via CH3O, CH2O, CHO, CO, is not directly affected by the presence of ammonia.

The central role of the HO2 radicals in converting CH3 to CH3O is noteworthy, through both direct reaction with CH3 and indirectly by regenerating NO2 from NO. The main sources of HO2 radicals are the reactions HCO+O2=CO+HO2 and H+O2(+M)=HO2(+M).

In these conditions, ammonia is mainly consumed by H-abstraction by HO2, which is present in significant amounts at low temperatures. The high amounts of HO2 also cause the termination of NH2 radical, such as the reverse reaction prevails over the forward H-abstraction, and acts as a termination. This effect had already been noticed by Stagni et al. [START_REF] Stagni | An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia[END_REF] in the oxidation of pure ammonia, although in this setup it does not affect reactivity to a significant extent (Figure 3b) because of the presence of methane in higher amounts. NH2 holds a key role in modifying the reactivity of pure methane: it reacts with NO and NO2, and each of these two paths owns a branching and a terminating channel. The sensitivity analysis (Figure 3b), carried out at different Φ for the same CH4 conversion (1%), highlights the importance of H2NO, formed by both branching channels and by NH2 reaction with HO2. H2NO is then further oxidized to HNO and finally to NO (the chemistry of H2NO is one of the biggest challenges in ammonia kinetics [START_REF] Glarborg | Modeling nitrogen chemistry in combustion[END_REF][START_REF] Stagni | The role of chemistry in the oscillating combustion of hydrocarbons: An experimental and theoretical study[END_REF]). The interplay between NO2 and NO is also evident in the sensitivity analysis, since NO2 formation via NO+HO2 enhances reactivity, and vice versa for its disappearance via its reaction with H. Indeed, NO2 is the key molecule in sensitizing methane chemistry: the formation of the reactive methoxy radical (CH3O) triggers the oxidation process of methane, as already shown in [START_REF] Song | The sensitizing effects of NO2 and NO on methane low temperature oxidation in a jet stirred reactor[END_REF]. Therefore, the two reactions CH3+NO2=CH3O+NO and NO+HO2=NO2+OH act as a catalytic cycle enhancing the consumption of methane. Conversely, with a richer mixture, the importance of ammonia chemistry becomes less and less important, and the lower amount of HO2 stops the catalytic cycle. Thus, with increasing Φ, the usual CH4 oxidation mechanism prevails, and sensitivity analysis shows that H-abstraction reactions and methyl conversion to methoxy govern the system reactivity. Under FR conditions, there are fewer direct interactions between methane and nitrogenated species. Methane is still mainly consumed through H-abstractions by H, O and OH radicals. CH3 radicals mainly react with O-atoms to yield CH2O+H, i.e. bypassing CH3O chemistry, crucial at lower temperatures. The second most important CH3 reaction is the termination to ethane, and the third one is its interaction with NH forming CH2NH+H. However, this does not have a significant impact on reactivity (Figure 4a). Formaldehyde reacts following the usual sequence to give CO2. CH2NH chemistry is not discussed here as it is not determining. The sensitivity diagram in Figure 4a shows that the reactivity is mainly governed by the chemistry of methane, whereas the impact of ammonia is minor, and limited to i) H-abstraction on ammonia itself, slowing down the reactivity since it subtracts active radicals for branching, and ii) N2H2 formation from NH2 and NH, with consequent release of a H radical. Therefore, the oxidation paths of the two fuels is mostly governed by their independent interactions with the radical pool (H, O, OH).

As far as ammonia specific chemistry is concerned, the intermediate HNO plays a minor role under these high temperature conditions. The NH2 radical mainly gives NH. It also reacts back to ammonia and leads to N2H2 by combination. N2H2 is converted into N2, one of the major reaction products, through NNH. The NH radical mainly reacts with CH3 yielding CH2NH, while its second most important consumption route is to produce N2H2. It also yields N2 via N2O and NO, another major reaction product, via N. Rate of production analysis shows that N2O mostly acts as an intermediate species rather than as a final product, due to the high temperatures involved, causing its quick decomposition to N2 and O. Anyway, the main NO formation pathway is still through HNO, which is formed from NH2, although it is a minor consumption pathway for this last species.

Conclusions

In this work, the co-oxidation of methane and ammonia was experimentally and theoretically studied to investigate their mutual interactions in combustion processes. Experiments were performed in a jet-stirred reactor and in a newly developed flow tube reactor working up to ∼2000 K. Comparison of experimental data with simulations using a novel detailed kinetic model showed a satisfactory agreement for the reactivity and for the mole fractions of most reaction products. The kinetic analysis of the system shed light on the underlying causes of the NH3 promoting effect, and on the major role played by NO in anticipating methane reactivity at low temperature. Although the effect of NO as a reactivity enhancer had been already established in previous works [START_REF] Song | The sensitizing effects of NO2 and NO on methane low temperature oxidation in a jet stirred reactor[END_REF], it was observed in the present work that even as an intermediate species in the ammonia oxidation path, it affects methane oxidation to a major extent, shifting the reactivity onset by up to ∼100 K, especially in the leanest conditions. This effect is not present at higher temperatures where the reactivity is mainly governed by fuel H-abstractions by OH, O and H. In this case, as soon as methane reacts, the radical pool necessary to trigger ammonia oxidation becomes available, such as the reactivity onset of the two fuels occurs at the same temperature, and is slightly anticipated with respect to pure CH4 mostly because of the higher amount of fuel.

Figure 1 .

 1 Figure 1. Mole fractions of reactants and main reaction products at a variable temperature and equivalence ratio in the JSR. Symbols: experiments. Lines: data computed with the model (dashed lines are averaged mole fractions due to oscillation regime).

Figure 2 .

 2 Figure 2. Mole fractions of the fuels and main reaction products recorded during the oxidation of methane and ammonia in the flow tube reactor (Φ=1). Symbols: experiments (close symbols in panel a) are for methane-ammonia co-oxidation, open symbols are for neat methane oxidation).Lines: data computed with the model.

Figure 3 .

 3 Figure 3. a) Consumption pathways of methane and ammonia, and b) sensitivity analysis for methane mole fraction at variable Φ for ∼1% CH4 conversion. Sensitivity coefficients are normalized with respect to the value of H + O2 O+OH (not included).

Figure 4 .

 4 Figure 4. a) Sensitivity diagram for methane under the FR conditions. b) and c) Formation routes to NO and N2 (T = 1560 K, Φ=1, x = 46 cm (length from the tube inlet)). Arrow thickness is proportional to the reaction flux.

Acknowledgments

This work has received funding from the European Union H2020 (H2020-SPIRE-04-2016) under grant agreement n°723706.

Supplementary material

• A word file named SM1 containing additional information about the experimental setups, atomic balance calculations, and additional model validations.

• A spreadsheet named SM2 with jet-stirred reactor and flow reactor experimental data.

• Three text files named SM3-5 containing thermodynamic properties, transport data and reactions under the Chemkin format.