Description
The growing demand for low-carbon hydrogen has intensified research into alternative methane conversion routes beyond conventional steam methane reforming. Among these, thermal decomposition of methane in microwave plasma sources (MPS) has been widely investigated due to the high energy density, fast start-up, and atmospheric-pressure operation of microwave plasmas in compact and decentralised systems. However, one of a persistent limitation of such systems is the method of methane introduction, which strongly affects discharge stability, energy efficiency, and device lifetime.
In conventional configurations, methane is injected together with the carrier gas directly into the plasma initiation region of the MPS. This approach promotes intensive carbon deposition inside the plasma source, leading to progressive discharge instability, reduced microwave energy coupling efficiency, and shortened operational lifetime. The novelty of the present work lies in a direct methane injection strategy designed to overcome these limitations. In the proposed configuration, microwave plasma is generated exclusively in the carrier gas (nitrogen), while methane is injected directly into the nitrogen plasma flame beneath the MPS waveguide, outside the plasma initiation region. This arrangement shifts the main conversion reactions away from the waveguide, mitigating carbon deposition inside the plasma source and preserving stable plasma generation.
The experimental study was conducted at atmospheric pressure using a metal-cylinder-based, nozzleless MPS operating at a microwave frequency of 2.45 GHz. The influence of absorbed microwave power, total gas flow rate, and the methane-to-nitrogen ratio on hydrogen production efficiency and discharge stability was systematically investigated. The results show that methane conversion increases with rising absorbed microwave power; however, this trend is accompanied by a decrease in hydrogen energy yield. Increasing the methane flow rate improves the hydrogen production rate, energy yield, and hydrogen concentration in the post-plasma gas, whereas higher nitrogen dilution produces the opposite effect. Overall, our preliminary results demonstrate that the proposed direct methane injection into the plasma flame shows strong potential for efficient hydrogen production in microwave plasma systems.