The Potential of Thermal Plasma Pyrolysis for Clean Hydrogen Gas and High Quality Solid Carbon Production from Methane

In the contemporary landscape, the demand for natural gas (NG) as a fuel has surged owing to its high combustion efficiency and lower pollutant emissions in comparison to oil and coal [1]. However, the direct utilization of NG in the energy sector has resulted in a substantial increase in greenhouse gas emissions [2]. A statistical study reveals that the energy sector is accountable for 46% of CO2 and 82% of SO2 total emissions [1]. Given the current scenario, a projected rise in CO2 emissions from the energy sector is anticipated, escalating from 50% in 2030 to a concerning 80% in 2050 [1]. Recognizing the environmental repercussions, there is a growing interest in exploring alternatives to address energy vulnerability [1].Hydrogen emerges as a pivotal element in the pursuit of a clean and sustainable energy system. One notable characteristic of hydrogen is its diverse range of sources for production [1]. However, achieving a carbon-neutral energy source requires overcoming challenges related to the efficiency of hydrogen production technology and its economic feasibility [1]. Presently, steam reforming and coal gasification, constituting approximately 66% of the production methods, dominate hydrogen production [2]. Both methods, unfortunately, release CO2 into the atmosphere, undermining efforts to reduce emissions, with steam reforming generating about 10 kg of CO2 for every kg of hydrogen produced [2].

An alternative approach involves pyrolysis, a process that involves the decomposition of methane to produce hydrogen. The energy required for pyrolysis can be sourced through various means, such as electrical heating furnaces, solar furnaces, and the utilization of electricity to create plasmas [2]. This method offers a potential avenue to mitigate CO2 emissions associated with traditional hydrogen production methods.

A process concept needs to be meticulously developed for the viable pyrolysis of CH₄, ensuring the production of enriched H₂ gas and high-quality carbon. This process design should prioritize achieving maximum stability for continuous production while optimizing utilization and reaction rates. The valuable carbon byproduct holds the potential to offset production costs. Figure 2 illustrates a comprehensive study matrix for the project, highlighting the key focus areas mentioned above.

The Thermal Plasma Lab facility, featuring a DC transferred arc, played a pivotal role in our experiments. The reactor allows the introduction of various gas components such as Ar, CH₄, and N₂ through the hollow cathode positioned at the top. The electric discharge occurs between the cathode and the graphite anode pin. Notably, the flexible arc distance, spanning 1 to 6 cm, enables a power supply ranging from 3.5 to 16 kW. Figure 3 provides a visual representation of the facility and the process layout.

The pyrolysis test procedure unfolds as follows:

1. Purging the entire reactor with Ar.
2. Shifting the cathode to the arc ignition position, initiating a short circuit.
3. Igniting the arc using Ar as the plasma gas and promptly returning the cathode to a stable arc distance position.
4. Monitoring the stable arc through the camera installed on the top window of the lid.
5. Introducing CH₄ to the transferred arc by transitioning the gas from pure Ar to a mixture of Ar and CH₄.
6. Monitoring the reaction using the gas analysis device installed on the off-gas pipeline.
7. Halting the CH₄ flow upon completion of the intended test and transitioning to 100% Ar.
8. Powering off and discontinuing the plasma gas flow.
9. This systematic approach allows for precise control and observation throughout the pyrolysis test, facilitating a comprehensive understanding of the process dynamics.

The experimental study underscores the technical feasibility of methane plasma pyrolysis. However, a meticulous and comprehensive examination is imperative to pinpoint and monitor the influential process parameters. A refined understanding of the process is essential for its progressive development.

Building on insights from the pre-trials, several key enhancements emerge:

1. Vertical Design: Facilitating a clean reaction zone.
2. Water-Cooled Reactor: Preventing refractory contamination.
3. External CH₄ Inlet: Extending electrode life.
4. Adjustable Electrodes: Facilitating the use of different electrode materials.
5. Moving Electrodes: Enabling in situ shifting.
6. Various Gas Components: Conducting different trials.
7. High Power Input: Resulting in a higher production rate.

These strategic modifications not only address existing limitations but also open avenues for further exploration and refinement, paving the way for an optimized and efficient methane plasma pyrolysis process. A schematic representation of the conceptual design is presented here.