Feasibility of a Plasma Furnace for Methane Pyrolysis: Hydrogen and Carbon Production

Abstract

The imperative to achieve a climate-neutral industry necessitates CO₂-free alternatives for H₂ production. Recent developments suggest that plasma technology holds promise in this regard. This study investigates H₂ production by methane pyrolysis using a lab-scale plasma furnace, with the primary objective of achieving a high H₂ yield through continuous production. The plasma furnace features a DC-transferred thermal plasma arc system. The plasma gas comprises Ar and CH₄, introduced into the reaction zone through the graphite hollow cathode. The off-gas is channeled for further analysis, while the plasma arc is recorded by a camera installed on the top lid. Results showcase a high H₂ yield in the range of up to 100%. A stable process is facilitated by a higher power and lower CH₄ input, contributing to a higher H₂ yield in the end. Conversely, an increased gas flow results in a shorter gas residence time, reducing H₂ yield. The images of the plasma arc zone vividly depict the formation and growth of carbon, leading to disruptive interruptions in the arc, hence declining efficiency. The produced solid carbon exhibits high purity with a fluffy and fine structure. This paper concludes that further optimization and development of the process are essential to achieve stable continuous operation with a high utilization degree.

1. Introduction

In contemporary times, the escalating demand for natural gas (also called methane gas) as a fuel is driven by its superior combustion efficiency and reduced pollutant emissions compared to traditional sources such as oil and coal [1]. Yet, the direct utilization of natural gas in the energy sector precipitates a significant increase in greenhouse gas emissions [2]. Statistical studies underscore the energy sector’s substantial contribution, accounting for 46% of CO₂ and 82% of SO₂ total emissions [1]. In 2022, global CO₂ emissions from energy combustion and industrial processes increased by 0.9%, amounting to 321 million metric tons, reaching a record-breaking 36.8 billion metric tons. Given the current circumstances, CO₂ emissions from the energy sector are projected to rise further [3]. This environmental challenge has spurred heightened interest in exploring alternative energy sources to mitigate energy vulnerability [1]. Hydrogen gas (H₂) can be considered a key component in developing a clean and sustainable energy system. Its integration in transportation, power generation, and mobility holds significant potential. H₂ can be efficiently stored and transported, offering flexibility in energy distribution. Another feature of H₂ is the diversity of sources through which it can be produced [1]. However, to attain a carbon-neutral energy source, hurdles related to the efficiency of H₂ production technology and economic feasibility must be surmounted [1]. At present, steam methane reforming (SMR) dominates H₂ production, accounting for approximately 48% of the total production methods for converting methane gas (CH₄) to H₂ [2]. Other methods, such as oil reforming and coal gasification, contribute to H₂ production using different feed materials, representing 30% and 18% shares, respectively.

Unfortunately, all of these methods release CO₂ into the atmosphere, hindering efforts to reduce CO₂ emissions. In particular, SMR results in the production of approximately 10 kg of CO₂ for every kg of H₂ produced [2]. Electrolysis, a method involving water molecule splitting for H₂ generation, is a green alternative but suffers from high energy demand and low efficiency, representing only 4% of total H₂ production [2,4]. Nevertheless, electrolysis remains an evolving technology, progressing towards economic feasibility. In this context, pyrolysis emerges as a promising alternative for converting CH₄ into H₂ and solid carbon. Furthermore, from a thermodynamic standpoint, pyrolysis requires 38 kJ of energy per mole of produced H₂, compared to electrolysis, which demands seven-times more energy, totaling 285 kJ/mol H₂ [5]. Pyrolysis, occurring at temperatures exceeding 1200 °C, can be energized through various means, including electrical heating furnaces, solar furnaces, and plasma formation using electricity [2].

For the process concepts with temperatures below 1200 °C, catalysts are essential for CH₄ decomposition. Consequently, techniques for CH₄ decomposition can be classified into two categories: catalytic and non-catalytic [6,7]. However, the use of catalysts presents a challenge due to the deactivation effect caused by the deposition of carbon produced during the process. On the other hand, plasma technology does not require catalysts due to the high temperatures that can be attained, reaching up to several thousand Celsius. Under these conditions, even the most vital material bonds are broken [8]. The advantages of plasma pyrolysis extend beyond carbon-neutral H₂ production. Unlike the SMR process, which consumes water and CH₄ while generating CO₂ emissions, plasma pyrolysis offers the advantage of producing valuable solid carbon as a by-product, also referred to as carbon black (CB). This aspect is particularly crucial as a significant portion of methane, constituting 75% by mass, is carbon. Addressing the major carbon markets is essential for optimizing hydrogen production. It is estimated that more than fifty types of CB are commercially available and can be classified according to their manufacturing process [9]. The use of plasma in the production of carbon is relatively new and allows the obtaining of new grades of CB, such as carbon nanotubes, graphene nanosheets, and other carbon structures of high quality, with the possibility of variations in its parameters [9]. Adding to these features, if the electricity for plasma is supplied by renewable sources and the resulting carbon is fully utilized in other sectors, the process achieves the goal of zero carbon footprint.

2. Advanced H₂ Production: Utilization of Plasma Pyrolysis

Plasma, the fourth state of matter, comprises ions, electrons, and neutral particles, maintaining electrical neutrality [8]. Plasma technology harnesses high-voltage electricity to create an arc stabilized by a gas stream between electrodes [10]. High-voltage electricity, either direct current (DC) or alternating current (AC), is used to supply this energy, creating an arc stabilized by a gas stream between two electrodes. Thermal plasma, a subset of hot plasma, can be a transfer or non-transfer type, with the former achieving higher temperatures due to flowing electrical current [11]. Apart from its capacity to generate H₂ without CO₂ emissions, thermal plasma technology offers a compact design, high efficiency, and minimal energy requirements [4,12,13].

Numerous studies have focused on plasma pyrolysis for the production of multiple components, including H₂, acetylene (C₂H₂), and different grades of carbon with varying structures and morphologies. A commercial example is the well-known Hüls process, dating back to the 1940s in Germany, which utilizes thermal plasma to split hydrocarbons and produce C₂H₂. The process involves reactors with a 100 mm diameter and 1.5 m length, operating at 8 MW power level [14,15].

In equilibrium, solid carbon and molecular hydrogen are the preferred products within the temperature range of 1000 to 2500 °C. If the temperature exceeds 2500 °C, hydrogen dissociates, forming acetylene with a peak concentration of around 3000 °C. Acetylene serves as a precursor to benzene rings and higher aromatics, with the addition of acetylene leading to the formation of polycyclic aromatic hydrocarbons. These compounds increase in molecular weight, becoming hydrogen-deficient and ultimately forming primary carbon. The growth of the carbon nucleates is facilitated by the decomposition of C₂H₂ over time. The key to bypassing this stage and producing C₂H₂ is rapid cooling. Despite the flake-like shape of carbon produced in the Hüls process, it was often deemed waste due to hydrocarbon contamination [15,16].

Some decades later, in the 1990s, responding to the increasing demand for CB, the Norwegian company Kvaerner endeavored to produce high-grade and pure CB. A production plant was established in Canada in 1997, boasting a yearly production capacity of 20,000 tons of CB. Unfortunately, due to the unsolved problem with the product quality, the process was discontinued in 2003 [17]. Despite encountering setbacks, recent advancements in plasma technology and the growing demand for sustainable energy solutions have revived interest in thermal plasma pyrolysis to generate carbon-free hydrogen gas and high-quality CB by-products. In 2012, Monolith Materials, a US-based company, together with the research group Fulcheri et al. at MINES ParisTech, France, initiated a project to produce carbon as its primary output using plasma pyrolysis. However, the plant was dismantled in 2018 for undisclosed reasons. Nonetheless, the company rebounded in 2020 by commissioning a new commercial plant for CB production, which remains operational today. Monolith Materials has ambitious plans to expand the plant’s capabilities to include the production of green hydrogen and ammonia in addition to CB. This strategic move underscores the potential of plasma pyrolysis to contribute significantly to a sustainable future. Since the process is under development and still growing, detailed information and data on the principles are unavailable. However, some results of related study works are published by Fulcheri et al. [5,15,18,19,20,21,22,23,24].

Fulcheri et al. conducted an experimental study utilizing a thermal plasma device for methane pyrolysis [5]. Their approach involved a three-phase plasma torch with a power capacity of up to 250 kW. Each experiment had a runtime of 40 min, achieving a highly satisfactory conversion rate exceeding 99%, with H₂ and solid carbon as the primary products. A comparison was made between a commercial production facility of the Monolith process, achieving 4600 tons annual H₂ production at approximately 25 kWh/kg H₂, and electrolysis at 60 kWh/kg H₂. The study explained that scaling up enhances process efficiency, concluding that plasma pyrolysis is more efficient, requiring only 42% of the energy intensity of water electrolysis. The potential for further improvement in efficiency is highlighted, considering the evolving nature of thermal plasma pyrolysis technology. Additionally, the study emphasized the added value of the produced solid carbon.

Considering the recent advancements in plasma technology, there is a need to formulate a process concept for CH₄ pyrolysis using thermal plasma that can offer scientific insights into what different process parameters influence the outcomes, such as the H₂ yield rate or the solid carbon characterization. Such an endeavor would contribute to bridging the existing knowledge gap and further enhancing our comprehension of the potential of plasma pyrolysis as an environmentally sustainable approach to generating H₂ and solid carbon.

The objective of the current paper is to qualitatively establish correlations between selected parameters, such as plasma gas and power input, and the H₂ yield rate through experimental work. Additionally, this paper aims to delve into the behavior and stability of the plasma arc.

3. Methodology

A series of experiments were conducted to screen the pyrolysis process with different parameters. The aim was to see if the process concept works and which parameters or conditions play a critical role. It is of significant importance to produce H₂ and reach the highest process stability, regarding the plasma arc behavior, with the highest possible H₂ yield.

3.1. Experimental

For the production of H₂ and solid carbon, a plasma furnace in the laboratory of the Chair of Ferrous Metallurgy located at the Montanuniversitaet Leoben (MUL) was used. The facility layout is shown in Figure 1. Various research articles used the current facility for investigating H₂-based steelmaking [25,26,27,28]. The facility is slightly modified for the aim of this study, which is explained further.

The furnace comprises two distinct components: the upper lid and the main body, both lined with refractory material, as illustrated in Figure 2(B-5). Additionally, they are equipped with an efficient water cooling system. To prevent gas leakages, the reactor incorporates multiple layers of sealing rings, denoted by number 6 in Figure 2.

Figure 2A illustrates the top lid, featuring various openings, with the central orifice serving as the entry point for the hollow cathode into the reactor. This hollow cathode facilitates the introduction of gas into the reaction chamber. The hollow cathode is shifted down and positioned at a predetermined distance from the anode pin at the reaction chamber’s base. The anode pin is meaningful in initiating and sustaining the arc discharge. The gas flow is adjusted by mass flowmeters. Crafted from graphite, the reaction chamber is specifically designed to maximize the capture of solid carbon within the reaction zone, minimizing impurities in the deposited carbon, as depicted in Figure 2(B-4).

The arc is vertically ignitiated through physical contact between the anode pin and the hollow cathode, powered by a DC supply with an average input of 5 kVA, supplied by Messer Grießheim GmbH, Ludwigshafen am Rhein, Germany. Subsequently, the arc length will be manually increased using the spindle drive engine. The current flow is controllable through an implanted thyristor (silicon-controlled rectifier or SCR) connected to a three-phase transformer, supplying a power range of 1 to 16 kVA in two adjustable levels. The thyristor trigger can be manually adjusted within a range of up to 100%. Amperage and voltage are recorded during operation.

Both the hollow cathode and anode pin, shown in Figure 3, are constructed from graphite. The hollow cathode’s dimensions include an inner diameter of 5–7 mm (d_in), an outer diameter of 24–27 mm (d_out), and a length of 120–150 mm (L_c). The anode pin, on the other hand, has a diameter of 28 mm (d) and a length of 40 mm (L_a).

Observation of the arc, facilitated through the opening indicated in Figure 2(A-2), is conducted using a camera system (Axis-Q1775) from Pieper GmbH, Schwerte, Germany. Details of the camera setup are illustrated in Figure 4.

The gas input consisting of argon gas (Ar) and CH₄ is controlled by two EL-Flow PRESTIGE F-210C mass flow controllers produced by Bronkhorst High-Tech BV, Ruurlo, Netherlands. The CH₄ used in the test is supplied by Methane 2.5 cylinder with a purity higher than 99.5. Moreover, the Ar had a purity of over 99.99%.

The steam produced in the reaction chamber exits through the designated opening in the top lid, as illustrated in Figure 2(A-3). Subsequently, it undergoes a dust removal by passing through a ceramic hot gas filter, specifically the FE2 model from ABB Ltd., Zurich, Swiss, followed by a fine filter. The treated off-gas then undergoes analysis using a gas mass spectrometer, precisely the GAM 200 type produced by InProcess Instruments Gesellschaft für Prozessanalytik mbH, Bremen, Germany.

The pyrolysis test procedure unfolds as follows and demonstrated in Figure 5, indicating different steps:

• Purging the entire reactor with Ar to provide an inert atmosphere and ensure safe operation.
• Moving the cathode to the arc ignition position, where it comes into contact with the anode, causing a brief short circuit.
• Igniting the arc using Ar as the plasma gas and promptly returning the cathode to a pre-defined stable arc length position. Adjusting to a 5 kW power input as an example.
• Monitoring the plasma arc through the camera installed on the top window of the lid. This stage is active until halting the power supply, hence the plasma arc.
• Introducing CH₄ to the transferred arc by transitioning the gas from pure Ar to a mixture of Ar and CH₄, taking 60% Ar and 40% CH₄ as an example.
• Monitoring the reaction using the data from the gas mass spectrometer device installed on the off-gas pipeline. The off-gas analysis is watched during the entire test.
• Halting the CH₄ flow upon completion of the intended test and transitioning to 100% Ar. Powering off and discontinuing the plasma gas flow simultaneously.
• Purging the entire reactor with Ar before dismantling it.

Various tests were conducted to assess the viability of the process across different parameters. In a test with the same setup, the testing conditions were varied in different phases to study the influencing factors and the correlation to the resulting changes in the outcome. The experiments involving variable adjustments are categorized into three areas, each targeting specific influences, as detailed in

Table 1. The testing conditions of different phases with listed variables.

Pretest	Target	Influencing Parameter	Phase No.	Gas Total [Nl/min]	Methane Content [%]	Arc Length [mm]	Thyristor Level [%]
Area 1	Plasma arc stability	Gas composition Arc length Power input	1-1	5	0	20	75, 80, 85, 90, 95, 100
			1-2	5	0, 20, 40	20, 25, 30, 35, 40, 45
Area 2	H2 yield	Gas composition Gas volume	2-1	3	33	20	85
			2-2	5	40	20	85
			2-3	3.5	43	20	85
Area 3	H2 yield	Power input	3-1	5	40	20	75, 80, 85, 90

. The findings corresponding to the different study areas, outlined in Table 1, are expounded upon in Section 4.1, Section 4.2 and Section 4.3. The testing parameters were selected because they showed the greatest influence on the process during the pretesting.

3.2. Evaluation

The mass spectrometer provides the volume fraction of the defined gas components, which are Ar and H₂. The volume fraction is recorded in cycles, each 5–6 s. The off-gas analysis was used for thermodynamic calculations of Ar and H₂ volume in the off-gas. To calculate the absolute volume of each gas in the cycles, the initial step involves computing the constant volume of Ar. This is achieved by multiplying the Ar flow rate per second by the cycle time in seconds, as per Equation (1). Subsequently, employing Equation (2), the absolute volume of produced H₂ in each cycle can be determined. The calculation considers the ratio percentage to Ar for H₂, recognizing Ar as an inert gas with an unchanging volume.

$$V_{Ar}=t_{cyc}·{\stackrel{·}{V}}_{Ar}$$

(1)

$$V_{H_{2-produced}}=C_{V,H_2}·\frac{V_{Ar}}{C_{V,Ar}}$$

(2)

$V_{H_{2-produced}}$ and $V_{Ar}$ represent the gas volume for the corresponding gas in each measuring cycle, [Nl]; $C_{V,H_2}$ and $C_{V,Ar}$ stand for the gas concentration in the off-gas in each cycle, [vol. %]. ${\stackrel{·}{V}}_{Ar}$ is the volume flow rate of the input gas, [Nl/min]; $t_{cyc}$ is the time of a measuring cycle, [min].

Maintaining a steady CH₄ flow rate into the reactor gas, the absolute volume of the CH₄ inlet can be calculated similarly to that of Ar, according to Equation (3).

$$V_{{CH}_{4-in}}=t_{cyc}·{\stackrel{·}{V}}_{{CH}_{4-in}}$$

(3)

${\stackrel{·}{V}}_{{CH}_{4-in}}$ stands for the volume flow gas of CH₄ input, [Nl/min]; $V_{{CH}_{4-in}}$ refers to the gas volume in the measuring cycle, [Nl].

Consequently, the H₂ yield can be determined by establishing the ratio of H₂ in the off-gas to the CH₄ inlet. The division by two is essential because each CH₄ molecule can split stoichiometrically into two H₂ molecules.

$${\mathrm{H}}_2 yield=\frac{V_{H_{2-produced}}}{2·V_{{CH}_{4-in}}}·100$$

(4)

4. Results and Discussion

As mentioned before, the main objective of this conceptual study is to attain a comprehensive understanding of the selected process parameters and their influence on the overall efficacy of methane plasma pyrolysis. In alignment with Table 1, the impacts of the chosen testing parameters are scrutinized and closely monitored. These influences are elaborated upon individually in the next three subsections. In the area 1 test series, various stability fields are identified for different gas compositions, used in the current setup, and subsequently compared with that of pure Ar gas. In the areas 2 and 3 test series, the focus is on the H₂ yield rate during the pyrolysis tests, exploring the influences of the plasma gas and power input, respectively. Nevertheless, the successful detection of H₂ gas in the off-gas analysis, combined with the production of high-purity solid carbon, represents a significant achievement.

The presence of solid carbon in the reaction chamber was observed upon dismantling the facility, as depicted in Figure 6. It appears that initial solid particle deposition on the inner surface of the graphite chamber is followed by the formation of aggregates and agglomerates. While the hot gas filter effectively trapped the fine carbon carried with the off-gas, the solid carbon remaining in the reaction zone disrupted the electric discharge focused on the defined spot: the anode pin. Consequently, tests could not be sustained for more than a few minutes, with the first 10–15 min yielding the most favorable results. Subsequently, gradual drops in process stability and efficiency were observed. Despite the challenges encountered during the test, it was imperative to proceed. This was a means of reaching the target of this study and evaluating the influencing process parameters. Consequently, even with a decrease in H₂ yield, certain process parameters were intentionally varied to elucidate their correlation with the H₂ yield. Another significant hurdle involved the consumption of both the graphite cathode and anode throughout the test. Furthermore, carbon deposition on the cathode tip during the test resulted in a narrower gas inlet gap. Concurrently, the consumption of the anode had implications for the plasma arc, leading to changes in arc length.

4.1. Influence of Gas Composition, the Arc Length, and the Power Input on the Arc Stability—Area 1

In light of a prior investigation into the stability of the plasma arc, it was established that arc stability hinges on factors such as power supply, gas composition, cathode geometry, and the magnetic field generated by the circuit. Typically, the arc is not perfectly centered, and the temperature field is unevenly distributed. Understanding arc stability is crucial for ensuring a stable process and optimizing efficiency [27].

Various variables were systematically tested to delineate the relationships between gas composition, power input (including current and voltage range), and arc length. These tests encompassed different CH₄ contents in the plasma gas, varying arc lengths, and adjustments to electrical current, hence power input.

In one set of experiments, at a fixed length and gas composition, the current was gradually decreased with the thyristor until the arc disconnected. The data points should be traced from higher to lower current values. The results are presented as voltage versus current values in Figure 7, illustrating trends of stable arcs with different gas compositions and arc lengths. The gray-indicated horizontal field represents 100% Ar in the plasma gas with a 20 mm arc length. Conversely, the gray-indicated vertical field depicts a gas mixture of 60% Ar and 40% CH₄ with varied arc lengths ranging from 20 to 35 mm in 5 mm increments. When the current is reduced for 100% Ar, the voltage remains relatively unchanged, indicating a stable arc with a lower power supply. However, transitioning to the gas mixture with 40% CH₄ content shortens the stability field and does not allow for low amperage values. In other words, decreasing amperage for 100% Ar leads to nearly unchanged voltage, requiring lower power input for plasma state development. In contrast, with the introduction of 40% CH₄ in the gas mixture, voltage fluctuations increase, necessitating higher power input. For instance, when maintaining a consistent arc length of 20 mm for both gas variants, it becomes evident that the same amperage requires 20 V higher voltages for the gas with 40% CH₄ content. However, higher voltage ranges were unattainable with this experimental setup, limiting the use of higher CH₄ content.

Consequently, a gas mixture with 40% CH₄ content compromised arc stability, ultimately interrupting the process. This is attributed to the higher atomization and ionization energy required for the five atomic CH₄ compared to the monoatomic Ar. The 40% CH₄ content was the maximum limit for conducting the test, as any increase led to an immediate disruption of the arc.

In the transitional range between these two critical states, the effects of adding 20% CH₄ to the plasma gas are illustrated in Figure 8. In this scenario, the arc length is incrementally increased from 20 mm to 45 mm in 5 mm steps. A lower CH₄ content enhances the stability field, allowing for longer arc lengths and underscoring the confirmed impact of CH₄. Nevertheless, the stability field is still shorter at low amperage values, requiring higher voltages. Arc lengths exceeding 45 mm resulted in an unstable arc and are therefore not feasible for processing. Higher arc lengths lead to steeper trends for the gas with 20% CH₄. It is important to note that a higher arc length corresponds to a broader heat source and an enlarged reaction zone, ultimately resulting in a higher H₂ yield rate. This aspect is further elaborated in the subsequent subsection. In summary, adding CH₄ introduces voltage fluctuations, and compared to pure Ar it necessitates a higher power supply.

4.2. Influence of Gas Composition and Volume on H₂ Yield Rate—Area 2

As mentioned before, in the overall decomposition reaction, for each mole of CH₄ gas, two moles of H₂ are produced, resulting in a pressure increase within the reactor. For safety considerations and to prevent gas leakage, a maximum gas flow of 5 Nl/min was tested. Gas flows lower than 3 Nl/min are also detrimental to the stability of the plasma arc, as the test cannot be reliably operated at such low gas flows. Therefore, two different operational gas flows were tested: Phase 1, with a total gas flow of 3 Nl/min, and Phase 2, where the total gas flow increased to 5 Nl/min. The results are presented as H₂ yield vs. time in Figure 9. It is essential to note that the gas mass spectrometer device detects H₂ gas with a 4 min delay, indicating that it takes approximately 4 min for the gas concentration to stabilize. Therefore, the initial substantial increase in the H₂ yield is primarily due to the response delay in the gas mass spectrometer analysis and should not be interpreted kinetically. This delayed response is also apparent after stopping the test in the later phase. The reason for that delay is the response time of the equipment. Examining the calculated curves of the H₂ yield for both phases, it becomes evident that a higher total gas flow with a slight increase in CH₄ content does not lead to a higher H₂ yield; instead, it lowers it. Put simply, the majority of the CH₄ introduced into the furnace exits without being decomposed to H₂. The rationale behind this observation lies in the fact that a higher flow rate reduces the gas residence time in the reaction zone, and a higher CH₄ content decreases arc stability, as previously discussed.

The curve fluctuations stem from the arc zone’s dynamic nature, which switches positions, creating a dynamic hot spot in the reaction zone. Consequently, the stability of the arc plays a critical role in determining process efficiency, as previously discussed in this paper.

In another test conducted in two phases, Phase 1 introduced 2 Nl/min of Ar and 1 Nl/min of CH₄, followed by a switch to 1.5 Nl/min of CH₄ while maintaining the Ar flow, resulting in an increased CH₄ content from 33% to 43%. As depicted in Figure 10, it is evident that a higher CH₄ content in the gas results in a 20-to-30% decrease in H₂ yield.

4.3. Influence of Power Input on H₂ Yield Rate—Area 3

In another trial involving varied power input, the investigation focused on the H₂ yield, as depicted in the corresponding H₂ yield rate shown in Figure 11. It is crucial to emphasize that the low H₂ yield rate is primarily attributed to the contaminated chamber and the pre-existing solid carbon, both exerting an impact on the arc quality and constraining the reaction. The current and voltage are recorded every second, while the gas analysis is conducted in a cycle of about 5 s. To align the data from the power input with the H₂ yield, the average power values over 5 s are used, corresponding to that cycle.

Evidently, increasing the power input from 4 to 7 kW results in an approximately 10% increase in the H₂ yield.

4.4. The Plasma Arc Behavior

The evolution of the plasma arc and the formation of solid carbon is documented in Figure 12. The plasma arc is denoted by an oval, and a comparison between Figure 12a and 12b reveals the concurrent deposition of carbon, as highlighted by the straight arrows. Additionally, Figure 12c showcases carbon deposited on the edges of the anode, denoted by a rectangle, with further growth evident in Figure 12d.

An additional test involving an arc jump and interruption is depicted in Figure 13. The initial phase is halted in Figure 13d, and the second phase resumes in Figure 13e. The challenge arises as the deposited carbon interferes with the arc plasma, presenting new surfaces that divert the discharge from the anode, as discussed earlier. The jump in the arc can be observed by comparing its position, indicated by an oval in Figure 13b, with that in Figure 13c. The same comparison holds true for Figure 13e,f. The curved arrows in Figure 13b,e illustrate the arc’s movement, while the straight arrows in the rest of the images indicate the carbon deposition.

The dendritic growth of carbon is more effectively illustrated in Figure 14. It commences with a coral-shaped carbon structure hanging from the cathode edge, as observed in Figure 14a. Figure 14b–d depict the progression of this growth, extending and contaminating the reaction zone, ultimately resulting in the interruption of the arc, as evident in Figure 14d.

4.5. Characterization of the Produced Solid Carbon

The collected carbon samples underwent thorough examination through morphological and elemental analyses, employing a scanning electron microscope (SEM). Figure 15 provides a visual insight into the microstructure of the carbon products. These images vividly depict the dendritic morphology of the carbon agglomerates, comprising numerous smaller primary particles and aggregates.

The energy-dispersive X-ray (EDX) analysis reveals highly pure carbon samples, as depicted in Figure 16. Nonetheless, additional methods must be employed to further investigate and deliver more precise characteristics.

5. Summary and Conclusions

The current study has concluded that thermal plasma is a feasible and efficient method for H₂ production, along with the generation of valuable solid carbon by-products. Several crucial influencing parameters were systematically investigated using a lab-scale plasma furnace. This study’s findings are summarized as follows:

• The experimental study demonstrates the technical feasibility of methane plasma pyrolysis with an H₂ yield ranging between 60 and 100%, depending on the process conditions. An advanced setup, combined with optimized conditions, can substantially elevate the process efficiency, ensuring a consistently superior and increased H₂ yield.
• Gas composition, consisting of CH₄, imposes limits on power input, with pure Ar exhibiting optimal arc stability. Beyond the factors explored in this study, arc stability’s nuanced characteristics are intricately influenced by various parameters such as geometry, magnetic field, and facility setup.
• A 10% increase in CH₄, from 33% to 43%, in the plasma gas resulted in a substantial 20-to-30% decrease in H₂ yield. It is essential to acknowledge that establishing a direct correlation and quantifying results is challenging, given that H₂ yield depends on the intricate plasma process influenced by a multitude of factors.
• Introducing CH₄ presents challenges, leading to difficulties in sustaining arc discharge and occasional interruptions. Initial trials indicate that a mixture of 60% Ar and 40% CH₄ balances process stability and productivity. With a power supply that provides a higher voltage level, the content of CH₄ may be increased even further, maintaining stable operation.
• A higher gas flow results in reduced gas residence time, more unreacted CH₄, and, consequently, decreasing H₂ yield. An increase of 3 to 5 Nl/min plasma gas led to a 10-to-20% decrease in H₂ yield.
• Solid carbon rapidly deposits on the inner surface of the reaction chamber during pyrolysis. Initially deposited carbon evolves into aggregates, agglomerates, and eventually larger particles. Carbon deposition in the reaction zone complicates plasma arc discharge, acting as a conductive material that decreases plasma arc stability and H₂ yield over time. The produced solid carbon exhibits a fine, fluffy structure with high purity, exceeding 99% C, according to SEM and EDX analysis.
• Increasing the power input by 3 kW, achieved by enhancing amperage, promotes the pyrolysis reaction, leading to a 10%-higher H₂ yield.
• The consumption of both cathode and anode alters plasma arc conditions, affecting arc length and destabilizing plasma arc discharge.

6. Outlook

Significant improvements are necessary for the testing device in response to the challenges identified during the trial. A higher power input with a broader range of voltage options can stabilize the plasma arc with CH₄, facilitating a continuous process. Adopting a vertical reactor design and expanding the reaction zone are key strategies to enhance process stability, ensuring a constant discharge of solid carbon and providing a more extensive and cleaner area for reactions and the plasma arc. An improved design should be tailored to streamline the process.

Developing a new design for a demonstration plant is underway and actively progressing at Montanuniversitaet Leoben (MUL). This will facilitate a systematic study for process optimization and elevate the process’s technology readiness level (TRL), ultimately realizing it on a commercial scale. Meanwhile, a deeper and more comprehensive understanding of the influencing factors is essential and will be gained in subsequent studies.