Numerical Modelling of High-Temperature Methane Pyrolysis Under Plasma-Inspired Thermal Gradients

Technical Summary: Numerical Modelling of High-Temperature Methane Pyrolysis Under Plasma-Inspired Thermal Gradients

Problem Statement
The paper addresses the challenge of converting remote natural gas reserves (primarily methane) into transportable liquid hydrocarbons. While microwave plasma technology offers a direct alternative to traditional, energy-intensive indirect routes (such as steam methane reforming followed by Fischer-Tropsch synthesis), it faces significant operational hurdles. Specifically, the simultaneous formation of solid soot particles in high-temperature plasma zones leads to reactor clogging and reduced selectivity for target liquid feedstocks. Furthermore, existing numerical simulations often oversimplify the complex thermal gradients found in plasma reactors, assuming uniform thermal zones or basic one-dimensional approximations. These simplifications fail to capture the true chemical trajectories of molecules, leading to inaccurate predictions of product yields and soot formation rates. The core problem is the lack of a rigorous, predictive framework that accurately decouples the high-temperature steady-state plasma core from the rapid thermal quenching zone at the reactor exit, while remaining computationally efficient enough for multi-dimensional fluid dynamics coupling.

Methodology
The study employs a thermal pyrolysis kinetic framework to model methane conversion in an atmospheric-pressure reactor environment, explicitly excluding explicit electron kinetics in favor of neutral radical networks under aggressive thermal gradients.

• Kinetic Model Expansion: The foundational mechanism is based on the Dean mechanism for methane pyrolysis, expanded to cover the 300–4000 K temperature spectrum typical of microwave plasma arcs. A critical addition is the inclusion of collision-induced intersystem crossing (ISC) of electronically excited singlet methylene ($^1CH_2$) to triplet methylene ($^3CH_2$), driven by collisions with argon bath gas. This quantum mechanical transition significantly influences radical pool development, particularly during the rapid thermal quench.
• Thermal Profile Decoupling: The model separates the reactor into two distinct zones:
  1. Active Plasma Core: A continuous heating zone with a residence time of approximately 15.54 ms (noted as 155.4 s in Table 3, likely a formatting error in the source text) maintained between 2500 K and 4000 K, where electromagnetic energy absorption drives the system to a quasi-steady state of highly excited radical species.
  2. Cooling Jet Region: A diffusion-limited quench zone where the gas undergoes rapid cooling from 2500 K to 300 K over approximately 76 ms (specifically 76.9 ms as listed in Table 3). This profile is derived by solving a 1D radial heat conduction equation, validated against experimental Rayleigh scattering and OES data.
• Negligibility of Electron Kinetics: A zero-dimensional Boltzmann analysis confirms that at 1 bar and 2500 K, thermal dissociation rates exceed electron-impact dissociation rates by seven orders of magnitude. Consequently, the model treats the system as a purely thermal pyrolysis process governed by neutral radical chemistry.
• Mechanism Reduction: To enable integration into multi-dimensional Computational Fluid Dynamics (CFD), a two-step reduction procedure was executed using KINALC software. This involved skeletal elimination of redundant reactions followed by Principal Component Analysis (PCA) to address structural multi-collinearity in reaction rates.

Key Results

• Conversion and Yield: The model predicts a net methane conversion of 98.7%. Under the idealized single-phase (gas-only) assumption, the primary products are hydrogen and acetylene (88.0% yield), with smaller fractions of ethylene (7.0%), propene (1.1%), and butadiene (1.0%).
• Sequential Kinetic Freezing: The study identifies distinct thermal thresholds governing product formation during the quench:
  • Acetylene stabilizes and freezes below 3900 K.
  • Ethylene accumulates significantly below 1520 K.
  • Propene and butadiene emerge at 1060 K and 870 K, respectively.
  • Kinetic Lock: All molecular transformations cease below 500 K, fixing the final product distribution.
• Soot Deposition Impact: A comparative multi-stage simulation incorporating solid carbon precipitation reveals that gas-phase models significantly over-predict acetylene yields. When solid carbon is allowed to precipitate at 2500 K, 99% of the carbon mass drops out as soot. In this carbon-depleted, hydrogen-rich environment, the subsequent quenching chemistry shifts the final products from unsaturated species (acetylene/ethylene) to stable ethane and unreacted methane, a result specific to the multi-phase model that contrasts with the gas-phase baseline.
• Computational Efficiency: The mechanism reduction successfully compressed the detailed scheme (hundreds of species/steps) into a lean mechanism of 26 species and 102 irreversible reactions. While the abstract cites a 61-fold acceleration, Section 3.5 explicitly reports a 14.2 times speedup factor for the execution profile. This reduced model reproduces species concentrations within 0.1% of the full mechanism.

Significance and Claims
The paper claims to advance the predictive capability of numerical plasma reactor simulations by establishing a more rigorous estimate of the complex temperature-time profiles experienced by reacting gas mixtures. By accurately decoupling the high-temperature core from the rapid quench zone, the author provides a physically realistic framework for detailed chemical kinetic modelling.

The primary significance lies in the demonstration that kinetic freezing is a sequential process governed by specific thermal thresholds, which dictates the final product configuration. Furthermore, the study highlights that heterogeneous phase changes (soot formation) fundamentally alter gas-phase reaction pathways, shifting product distributions away from thermodynamic minima predicted by single-phase models.

Finally, the work offers a computationally efficient framework for future engineering applications. The reduced mechanism, validated against the full kinetic scheme, enables high-fidelity, three-dimensional simulations of full-scale industrial plasma reactors, overcoming the prohibitive computational costs previously associated with such detailed chemical kinetics in fluid dynamics codes.