Global Pathway Selection/Analysis (GPSA) algorithm helps in analyzing the chemical kinetics of complex combustion systems by identifying important global reaction pathways that connects a source and a sink species. The present work aims to extend the application of GPSA to plasma assisted combustion systems in order to identify the dominant global pathways that govern the plasma and combustion kinetics at various conditions. The reaction cycles involving the excitation of nitrogen to its vibrational and electronic states and the subsequent de-excitation to its ground state are found to control the reactivity of plasma assisted systems. Provisions are made in the GPSA algorithm to capture the dominant reaction pathways and cycles of plasma assisted combustion (i.e., p-GPSA). Further, the analysis of plasma assisted ammonia combustion are presented as an example, which includes the results obtained using both the traditional path flux analysis and p-GPSA. The dominant pathways for the plasma assisted combustion of ammonia are identified along with the dominant excitation--de-excitation loops and their importance are ascertained and verified using path flux analysis.
more »
« less
This content will become publicly available on July 5, 2024
Plasma-based global pathway analysis to understand the chemical kinetics of plasma-assisted combustion and fuel reforming
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.
more »
« less
- Award ID(s):
- 2002635
- NSF-PAR ID:
- 10481922
- Editor(s):
- Pepiot, Perrine
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Combustion and Flame
- Volume:
- 255
- Issue:
- C
- ISSN:
- 0010-2180
- Page Range / eLocation ID:
- 112927
- Subject(s) / Keyword(s):
- ["Global pathway analysis (GPA)","Plasma assisted combustion (PAC)","Fuel reforming","Ammonia combustion","Methane reforming","Global reaction pathways"]
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Natural gas associated with oil wells and natural gas fields is a significant source of greenhouse gas emissions and airborne pollutants. Flaring of the associated gas removes greenhouse gases like methane and other hydrocarbons. The present study explores the possibility of enhancing the flaring of associated gas mixtures (C1 – C4 alkane mixture) using nanosecond pulsed non-equilibrium plasma discharges. Starting with a detailed chemistry for C0 – C4 hydrocarbons (Aramco mechanism 3.0 – 589 species), systematic reductions are performed to obtain a smaller reduced mechanism (156 species) yet retaining the relevant kinetics of C1 – C4 alkanes at atmospheric pressure and varying equivalence ratios (φ = 0.5 – 2.0). This conventional combustion chemistry for small alkanes is then coupled with the plasma kinetics of CH4, C2H6, C3H8, and N2, including electron-impact excitations, dissociations, and ionization reactions. The newly developed plasma-based flare gas chemistry is then utilized to investigate repetitively pulsed non-equilibrium plasma-assisted reforming and subsequent combustion of the flare gas mixture diluted with N2 at different conditions. The results indicate an enhanced production of hydrogen, ethylene and other species in the reformed gas mixture, owing to the electron-impact dissociation pathways and subsequent H-abstractions and recombination reactions, thereby resulting in a mixture of CH4, H2, C2H4, C2H2, and other unsaturated C3 species. The reformed mixture shows an enhanced reactivity as exhibited by their shorter ignition delays. The reformed mixture is also observed to undergo increased methane destruction and higher equilibrium temperatures compared to the original mixture as the gas temperature increases, thereby exhibiting a potential for reducing the unburnt emissions of methane and other hydrocarbons.more » « less
-
This work aims at comparing the accuracy and overall performance of a low-Mach CFD solver and a fully-compressible CFD solver for direct numerical simulation (DNS) of nonequilibrium plasma assisted ignition (PAI) using a phenomenological model described in Castela et al. [1]. The phenomenological model describes the impact of nanosecond pulsed plasma discharges by introducing source terms in the reacting flow equations, instead of solving the detailed plasma kinetics at every time step of the discharge. Ultra-fast gas heating and dissociation ofO2 are attributed to the electronic excitation ofN2 and the subsequent quenching to ground state. This process is highly exothermic, and is responsible for dissociation of O2 to form O radicals; both of which promote faster ignition. Another relatively slower process of gas heating associated with vibrational-to-translational relaxation is also accounted for, by solving an additional vibrational energy transport equation. A fully-compressible CFD solver for high Mach (M>0.2) reacting flows, developed by extending the default rhoCentralFoam solver in OpenFOAM, is used to perform DNS of PAI in a 2D domain representing a cross section of a pin-to-pin plasma discharge configuration. The same case is also simulated using a low-Mach, pressure-based CFD solver, built by extending the default reactingFoam solver. The lack of flow or wave dominated transport after the plasma-induced weak shock wave leaves the domain causes inaccurate computation of all the transport variables, with a rather small time step dictated by the CFL condition, with the fully-compressible solver. These issues are not encountered in the low-Mach solver. Finally, the low-Mach solver is used to perform DNS of PAI in lean, premixed, isotropic turbulent mixtures of CH4-air at two different Reynolds numbers of 44 and 395. Local convection of the radicals and vibrational energy from the discharge domain, and straining of the high temperature reaction zones resulted in slower ignition of the case with the higher Re. A cascade effect of temperature reduction in the more turbulent case also resulted in a five - six times smaller value of the vibrational to translational gas heating source term, which further inhibited ignition. Two pulses were sufficient for ignition of the Re = 44 case, whereas three pulses were required for the Re = 395 case; consistent with the results of Ref. [1].more » « less
-
null (Ed.)Chemical Looping Reaction is a key strategy to achieve both emission reduction and carbon utilization while producing various value-added chemicals, through redox reactions. Here we study the effect of nanoshape ceria supported Ru catalysts for plasma assisted Chemical Looping Reforming reduction step coupled with water splitting oxidation step reactions in the temperature range 150 ⁰C to 400 ⁰C at 1 atm pressure. The oxygen carrier/catalyst combination materials used are Ru/CeO2 nanorods (NR), Ru/CeO2 nanocubes (NC), Ru/SiO2 nanospheres (NS), and Ni-based perovskite mixed with CeO2. NRs and NCs showed the best catalytic performance followed by Ni-based perovskite and NS. Differences in the selectivity and reactivity for the NRs and NCs were noticed. The NCs showed slightly higher selectivity towards H2 formation during reduction step and lesser carbon deposition. From the analysis of data and literature, it is proposed that the spillover of species such as H adatoms and CHx radicals after activation at Ru sites into the CeO2 supports and lattice O mobility may be slightly faster in the case of NCs. During the oxidation step, the NR and NC materials showed increased H2 production by a factor of more than 4 when compared to Ni based perovskite material.more » « less
-
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.more » « less
