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1. Plasma-based global pathway analysis to understand the chemical kinetics of plasma-assisted combustion and fuel reforming

   https://doi.org/10.1016/j.combustflame.2023.112927  

   Johnson, Praise N.; Taneja, Taaresh S.; Yang, Suo (July 2023, Combustion and Flame)

   Pepiot, Perrine (Ed.)

   The Global Pathway Analysis (GPA) algorithm helps analyze the chemical kinetics of complex combustion systems by identifying important global reaction pathways connecting a source species to a sink species through various important intermediate species (i.e., hub species). The present work aims to extend GPA algorithm to plasma-assisted combustion and fuel reforming systems to identify the dominant global pathways in such systems at various conditions. In addition, the present study extends the ability of GPA algorithm to identify reaction cycles involving the excitation of high-concentration species (e.g., O2, N2, and fuel) to their vibrational and electronic states and the subsequent de-excitation to their ground state, based on their significance on the reactivity of plasma-assisted systems in terms of gas heating and radical production. Provisions are made in the GPA algorithm to evaluate the reactivity of identified reaction pathways and cycles based on the element-flux transfer (i.e., dominance), heat release, and radical production rate. The newly developed Plasma-based Global Pathway Analysis (PGPA) algorithm is then used to analyze the plasma-assisted combustion of ammonia and reforming of methane. The PGPA analyses elucidated the significance of vibrational-translational cycles on the reactivity of NH3/air mixtures. Further, analyses on the production of NO ascribed the early reforming of NH3 to N2 and H2 in impeding the production of NO during plasma-assisted NH3 ignition. Lastly, the enhanced reforming of CH4/N2 mixtures using plasma has been attributed to electron impact dissociation of CH4 when compared to thermal reforming. In contrast, conventional path-Flux analysis (PFA) was found to require significant manual effort and pre-analysis intuitions from expert knowledge, making it arduous to provide valuable insights into plasma chemistry. The user-friendly and automated nature of PGPA thus provides a valuable tool for assessing the kinetics of plasma-assisted systems helpful in analyzing and, further, a foundation in reducing plasma-assisted chemistry, without the needs of expert knowledge. 

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2. 1D Simulation of Avalanche to Streamer to Spark Transition of Plasma Discharge in Ammonia-Air Combustion

   https://doi.org/10.2514/6.2024-2607  

   Taneja, Taaresh Sanjeev; Yang, Suo; Sitaraman, Hariswaran (January 2024, American Institute of Aeronautics and Astronautics)

   The backward problem of plasma assisted combustion emphasizes evaluating the effect of the evolving thermochemical state on the plasma discharge. This paper investigates the dependence of avalanche to streamer to spark formation dynamics and kinetics on the gas composition and temperature at different points in an ammonia-air premixed laminar flame using a self-consistent multigrid-based 1D plasma solver. Different values of alpha, the coefficient for effective ionization events per unit length, have been reported for electron avalanches in air and stoichiometric NH3-air mixtures. The streamer inception has been shown to obey the Meek’s criterion. An exponential reduction in streamer and spark formation time has been observed from plasma simulations at different points in the unburnt, pre-heat zone, reaction zone and the fully burnt regions of the premixed flame. While the enhancement of the reduced electric field with increasing temperature affects effective ionization, there exists a minimum breakdown field for streamer formation, which does not vary proportionally with the changing number density of the gas. The change in the mixture from reactants (NH3, O2, N2) to products of complete combustion of ammonia in air (N2, H2O) has also been shown to affect the streamer and spark formation. Finally, the major pathways during the streamer and spark phases which are responsible for producing important radicals used in combustion of NH3 are also discussed. 

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3. Numerical Modeling of Plasma Assisted Pyrolysis and Combustion of Ammonia

   https://doi.org/10.2514/6.2021-1972  

   Taneja, Taaresh Sanjeev; Yang, Suo (January 2021, AIAA Scitech 2021 Forum)

   This work aims at studying the combustion and pyrolysis characteristics of ammonia (NH3) using non-equilibrium plasma. The well known challenges of ammonia combustion and the advantages of using non-equilibrium plasma are discussed using results of zero-dimensional and one-dimensional coupled simulations. Periodic nanosecond pulsed discharges of plasma are interspersed with microsecond gaps of combustion to assess the assistance provided by plasma on overall combustion characteristics of ammonia fuel, such as ignition delay and flammability limit. Due to the lack of a reliable plasma mechanism for ammonia, a validated plasma kinetic mechanism of methane and oxygen is transformed to that of ammonia and oxygen, and is coupled with an experimentally validated ammonia combustion mechanism in this work. Another NH3 / O2 / He plasma mechanism that was recently assembled and published is also used to study the discharge and inter-pulse kinetics. A 0D model is developed to compute the rates of the electron impact reactions during the discharge, and ion-electron recombination reactions and quenching reactions along with the combustion reactions during the gap. Finally, the species concentrations and temperatures from this model are compared with those obtained using a detailed 1D model which solves for the transient electric field in addition to the species concentrations and temperature. 

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4. Mapping the performance envelope and energy pathways of plasma-assisted ignition across combustion environments

   https://doi.org/10.1016/j.combustflame.2024.113793  

   Dijoud, Raphael J; Laws, Nicholas; Guerra-Garcia, Carmen (January 2025, Combustion and Flame)

   Nanosecond pulsed plasmas have been demonstrated, both experimentally and numerically, to be beneficial for ignition, mainly through gas heating (at different timescales) and radical seeding. However, most studies focus on specific gas conditions, and little work has been done to understand how plasma performance is affected by fuel and oxygen content, at different gas temperatures and deposited energies. This is relevant to map the performance envelope of plasma-assisted combustion across different regimes, spanning from fuel-lean to fuel-rich operation, as well as oxygen-rich to oxygen-vitiated conditions, of interest to different industries. This work presents a computational effort to address a large parametric exploration of combustion environments and map out the actuation authority of plasmas under different conditions. The work uses a zero-dimensional plasma-combustion kinetics solver developed in-house to study the ignition of CH4/O2/N2 mixtures with plasma assistance. A main contribution of the study is the detailed tracking of the energy, from the electrical input all the way to the thermal and kinetic effects that result in combustion enhancement. This extends prior works that focus on the first step of the energy transfer: from the electrical input to the electron-impact processes. Independent of the composition, four pathways stand out: (i) vibrational-translational relaxation, (ii) fast gas heating, (iii) O2 dissociation, and (iv) CH4 dissociation. Results show that the activated energy pathways are highly dependent on gas state, composition, and pulse shape, and can explain the observed range in performance regarding ignition enhancement. The approach can be used to calculate the fractional energy deposition into the main pathways for any mixture or composition, including new fuels, and can be a valuable tool to construct phenomenological models of the plasma across combustion environments. 

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5. Non-thermal plasma-assisted rapid hydrogenolysis of polystyrene to high yield ethylene

   https://doi.org/10.1038/s41467-022-28563-7  

   Yao, Libo; King, Jaelynne; Wu, Dezhen; Ma, Jiayang; Li, Jialu; Xie, Rongxuan; Chuang, Steven S. C.; Miyoshi, Toshikazu; Peng, Zhenmeng (February 2022, Nature Communications)

   Abstract The evergrowing plastic production and the caused concerns of plastic waste accumulation have stimulated the need for waste plastic chemical recycling/valorization. Current methods suffer from harsh reaction conditions and long reaction time. Herein we demonstrate a non-thermal plasma-assisted method for rapid hydrogenolysis of polystyrene (PS) at ambient temperature and atmospheric pressure, generating high yield (>40 wt%) of C₁–C₃hydrocarbons and ethylene being the dominant gas product (Selectivity of ethylene,S_C2H4 > 70%) within ~10 min. The fast reaction kinetics is attributed to highly active hydrogen plasma, which can effectively break bonds in polymer and initiate hydrogenolysis under mild condition. Efficient hydrogenolysis of post-consumer PS materials using this method is also demonstrated, suggesting a promising approach for fast retrieval of small molecular hydrocarbon modules from plastic materials as well as a good capability to process waste plastics in complicated conditions. 

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