Interaction of NH3 on the H2 oxidation chemistry
Session chaired by Dr. Agustin Valera-Madina
The transition of the energy scenario towards no-carbon economy is becoming mandatory within 2050, because of fossil fuels depletion and CO2 strict emissions regulations, while worldwide energy demand is increasing [1-2]. Within this panorama, the role of green and blue hydrogen (from renewable sources) as energy carrier for stationary and transport applications is recognized as the main leading challenge [3]. Hydrogen is considered to be the ideal fuel because of its high heat value (on mass basis), the absence of CO2 emissions and limited NOx emissions, when burned in the appropriate conditions. Nonetheless, the attempt to reach an energy system based on the hydrogen economy is a long-term project, since issues related to NOx emissions and to hydrogen storage/delivery and safety. The H2 conversion to other energy vectors more easily transportable and with high-energy density represents a relatively cheap alternative capable of guaranteeing energy supply within the constraints of the energy trilemma [4]. Within the palette of new fuels [5, 6], ammonia has received a lot of attention because of its high hydrogen density and heating value, the absence of CO2 emissions with already available storage and delivery infrastructures, as well as some well-established production technologies and plants. Nonetheless, its physical/chemical properties represent an hindrance to its practical use both in stationary and transportation systems [7, 8]. MILD Combustion has been proven to allow the use of pure ammonia. The role of third-molecular reactions is emphasized, in virtue of relatively low working temperatures and the massive presence of “strong” colliders (H2O and CO2) [9]. Many authors have attributed the high collisional efficiency of water high to a very strong polar-polar interaction between HO2 radicals and H2O [10]. In this respect, it has to be also accounted for that ammonia is a four-atom molecule with a relatively high polarity, and a dipole moment comparable to water. Its third body effect cannot be ruled out “a priori” in chemical kinetics modeling of ammonia combustion, but it has to be addressed through experimental evidences. Therefore, experimental tests were realized for H2/O2/Ar mixtures as reference cases in a Jet Stirred Flow Reactor (JSFR) [11], in presence of H2O and NH3, for a fuel-lean H2/O2 mixture diluted in Ar at d=94% as a function of mixture inlet temperature (Tin), at a fixed residence time (t=0.5 s) under atmospheric pressure (p=1.2 atm). The experimental tests suggest that ammonia strongly interacts with the H2 oxidation chemistry within the considered operative conditions, with an overall delaying effect on H2 characteristic chemical times. As a direct evidence, the onset of the system reactivity is shifted towards higher inlet temperatures as the ammonia concentration increases. In addition, hydrogen instabilities, identified for the mixtures H2/O2/Ar, disappear as soon as ammonia is added to reactants. This effect is even more evident with respect to the delaying effect of “water” on hydrogen oxidation. All these considerations contribute to conceive a plausible role of ammonia as a “strong” collider in ter-molecular reactions. All these considerations may have a strong impact on the development of detailed kinetic schemes for ammonia oxidation under low-intermediate temperatures. In addition, several practical issues towards the development of a H2 economy system can be deduced, overcoming problems related to an hydrogen safely delivery/storage systems while reducing ammonia production cost through Haber-Bosh processes. References [1] Pacala S, Socolow R. Stabilization wedges: Solving the climate problem for the next 50 years with current technologies. Science 2004;305:968–72. https://doi.org/10.1126/science.1100103. [2] IRENA (2019). Global energy transformation: A roadmap to 2050 (2019 edition). Abu Dhabi: International Renewable Energy Agency; 2019. [3] IEA, The Future of Hydrogen. https://www.iea.org/reports/the-future-of-hydrogen, 2020. [4] Poudineh R, Jamasb T. Smart Grids and Energy Trilemma of Affordability, Reliability and Sustainability: The Inevitable Paradigm Shift in Power Sector. SSRN Electron J 2012. [5] Valera-Medina A, Xiao H, Owen-Jones M, David WIF, Bowen PJ. Ammonia for power. Prog Energy Combust Sci 2018;69:63–102.
[6] Ash N, Scarbrough T. Sailing on solar: Could green ammonia decarbonise international shipping?. London: Environmental Defense Fund; 2019. [7] Ichikawa A, Hayakawa A, Kitagawa Y, Kunkuma Amila Somarathne KD, Kudo T, Kobayashi H. Laminar burning velocity and Markstein length of ammonia/hydrogen/air premixed flames at elevated pressures. Int J Hydrogen Energy 2015;40:9570–8. [8] Mathieu O, Petersen EL. Experimental and modeling study on the high-temperature oxidation of Ammonia and related NOx chemistry. Combust Flame 2015;162:554–70. https://doi.org/10.1016/j.combustflame.2014.08.022. [9] Sabia P, de Joannon M. Critical Issues of Chemical Kinetics in MILD Combustion. Front Mech Eng 2020;6:1–10.
[10] Michael J V., Su MC, Sutherland JW, Carroll JJ, Wagner AF. Rate constants for H + O2 + M → HO2 + M in seven bath gases. J Phys Chem A 2002;106:5297–313. [11] Manna MV, Sabia P, Ragucci R, de Joannon M. Oxidation and pyrolysis of ammonia mixtures in model reactors. Fuel 2020;264. https://doi.org/10.1016/j.fuel.2019.116768.
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