Shifts in Product Distribution in Microwave Plasma Methane Pyrolysis Due to Hydrogen and Nitrogen Addition

Abstract

Methane pyrolysis can produce many valuable products besides hydrogen, e.g., C₂ compounds or carbon black. In the conditions provided by microwave plasma, the distribution of these products might be shifted by the addition of hydrogen and nitrogen. In this work, different ratios of H₂:CH₄, ranging from 0:1 to 4:1, were tested. The most unambiguous and promising result was obtained for the highest H₂:CH₄ ratio. For this ratio, a significant improvement in methane conversion rate was observed (from 72% to 95%) along with the increase in C₂H₂ and C₂H₄ yield and selectivity. The results support the hypothesis that the H radicals present in the plasma are responsible for improving methane conversion, while the presence of molecular hydrogen shifts the product distribution towards C₂ compounds. Based on the carbon balance, the increase in the output of C₂ compounds was obtained at the cost of solid carbon. At the same time, the addition of hydrogen resulted in the formation of bigger carbon particles. Finally, with the addition of both nitrogen and hydrogen, the formation of carbon was completely inhibited. Hydrogen cyanide was the main product formed instead of soot and some of the acetylene.

1. Introduction

Methane pyrolysis is often considered a possible process for CO₂-free hydrogen production [1,2]. However, this process would require a sustainable source of heat necessary for methane decomposition. One of the solutions would be plasma that can be sourced with renewable energy (i.e., wind or solar). Moreover, plasma technologies usually provide an instant on/off procedure, with no need for heating up or cooling down the reactor, thus fitting well with time-variable energy sources.

While methane pyrolysis for hydrogen production has strong competition in water electrolysis, and may be considered a supplementary solution, the process is producing other valuable components, i.e., C₂ compounds (acetylene and ethylene) and carbon black [1,3]. C₂ compounds can be considered one of the main goals of methane pyrolysis as they are important precursors of many organic compounds [4]. Conventionally, these compounds are produced from crude oil in a process (usually steam cracking) that produces high quantities of CO₂. Considering the limited availability and resources of crude oil and the inevitable production of CO₂, plasma methane pyrolysis seems a much more sustainable solution. While the conventional methane resources, i.e., natural gas, are also limited, especially now, there are other possible sources. One of them is biomethane. The other one might be synthetic CH₄ produced from CO₂ [5]. While the latter possibility is to be available in the future rather than right now, it suggests an interesting combination. CO₂ sequestrated from the atmosphere can be reduced to methane and further transformed into C₂ compounds that can be used in the production of polymers. As a result, CO₂ would be bound in a plastic material that can be used for decades or recycled.

While the methods most commonly considered for methane pyrolysis involve catalysts or molten metals and salts [6,7], they are usually focused on hydrogen and carbon black production, not C₂ compounds. In the case of plasma methods, the warm plasma (e.g., gliding arc plasma or microwave plasma) seems most suitable [8]. This type of plasma provides high temperatures (1000–6000 K) and the presence of reactive species (e.g., radicals) [9,10]. Both factors are crucial for the conversion of hydrocarbons. On the contrary, typical cold plasmas, e.g., dielectric barrier discharge (DBD), work usually at low temperatures resulting in low conversion. On the other hand, hot plasmas (e.g., arc plasma) provide temperatures much higher than necessary (eg, 11,000 K [11]) resulting in energy losses.

Methane decomposition requires a high temperature and is a gradual process that may lead to many products, as presented in a simplified reaction (1):

$$2{\mathrm{CH}}_4\to {\mathrm{C}}_2{\mathrm{H}}_6+{\mathrm{H}}_2\to {\mathrm{C}}_2{\mathrm{H}}_4+2{\mathrm{H}}_2\to {\mathrm{C}}_2{\mathrm{H}}_2+3{\mathrm{H}}_2\to 2\mathrm{C}+4{\mathrm{H}}_2$$

(1)

The distribution of the products depends on the temperature and reaction time. With the increase in these two parameters, acetylene, soot, and hydrogen start to dominate [12,13], with the latter two products being the final if the conditions are severe enough. Regardless of the final products, the initial step in methane decomposition is the detachment of H atom (2), which has high activation energy and determines the rate of the reaction [14,15]:

$${\mathrm{CH}}_4\to {\mathrm{CH}}_3+\mathrm{H}$$

(2)

The released H radicals play a key role [9] in the further decomposition of methane, accelerating the process (3) [13,16]:

$${\mathrm{CH}}_4+\mathrm{H}\to {\mathrm{CH}}_3+{\mathrm{H}}_2$$

(3)

Interestingly, some of the research proved that mixing hydrogen with methane may be beneficial to the conversion of the latter and may affect the selectivity of the products. It should be noted that since methane decomposition produces hydrogen, overall the process could be considered a quasi-autocatalytic process in which part of the produced hydrogen could be returned to the process to improve its efficiency. This effect has been reported or suggested only in the case of warm plasma (e.g., microwave (MW) or gliding arc plasmas) [17,18,19,20]. In the case of traditional pyrolysis or strictly non-equilibrium plasmas, the observed effect is the opposite—the addition of hydrogen decreases the conversion rate due to reaction reversal of reaction (3), which is favored in temperatures lower than ca. 2000 K [21,22].

The effect of hydrogen addition on methane pyrolysis in warm plasmas is still an object of debate. Most researchers suggest that the key role is played by the H radicals generated in plasma [18,19,20]. It is speculated that their presence increases the conversion rate of methane (as in Equation (3)). Another observed effect of hydrogen addition is increases in the yield and selectivity of C₂H₄ and C₂H₂. However, some discrepancies are indicated when the effect of specific H₂:CH₄ ratio is considered. For instance, Ref. [20] shows that the beneficial effect of H₂ addition is observed for the maximum H₂:CH₄ ratio of 2. Above that ratio, methane conversion rate and C₂ compound yield start decreasing. In opposite, work [19] shows that the increase in conversion rate and yield is observed even at the H₂:CH₄ ratio of 5. Additionally, it should be noted that none of these works presents results of plasma diagnosis. Yet the results of such a diagnosis could be very valuable, as knowing the temperature of plasma and identifying plasma radicals should support any speculation on the hydrogen influence mechanism.

In the context of the aforementioned articles, this work aims to analyze the process of H₂/CH₄ plasma pyrolysis in a well-diagnosed atmospheric microwave plasma reactor. The reactor in this work has a typical construction that is used by many researchers [23,24,25]. Most importantly, it was tested in a wide range of conditions [10,26,27]. In distinction from other work, we were able to proof the presence of H radicals and to determine the temperature profile in the reactor (See: Section 3 Materials and Methods). This data allows the analyzing the hypotheses for conversion and selectivity changes due to H₂ addition. Moreover, this work provides an initial insight into the hydrogen impact on the soot particle size. Another novel aspect of this work is the investigation of nitrogen’s potential to inhibit the process of soot formation.

2. Results and Discussion

Table 1. Inlet and outlet concentrations of the main gaseous compounds.

H2:CH4 Ratio	${\left[\mathbf{C}{\mathbf{H}}_4\right]}_{\mathit{i}\mathit{n}}$	${\left[\mathbf{C}{\mathbf{H}}_4\right]}_{\mathit{o}\mathit{u}\mathit{t}}$	${\left[{\mathbf{H}}_2\right]}_{\mathit{i}\mathit{n}}$	${\left[{\mathbf{H}}_2\right]}_{\mathit{o}\mathit{u}\mathit{t}}$	${\left[{\mathbf{C}}_2{\mathbf{H}}_4\right]}_{\mathit{o}\mathit{u}\mathit{t}}$	${\left[{\mathbf{C}}_2{\mathbf{H}}_2\right]}_{\mathit{o}\mathit{u}\mathit{t}}$
	%
4:1 N2 *	4.83 ± 0.04	0.39 ± 0.04	20.6 ± 1.7	27.9 ± 1.2	0.10 ± 0.02	1.11 ± 0.10
4:1 Ar *	5.12 ± 0.04	0.22 ± 0.04	19.0 ± 1.2	22.7 ± 1.2	0.11 ± 0.02	1.74 ± 0.10
3:1 Ar	5.03 ± 0.04	1.41 ± 0.04	14.2 ± 1.2	16.1 ± 1.2	0.09 ± 0.01	1.24 ± 0.10
2:1 Ar	4.99 ± 0.04	1.69 ± 0.04	8.7 ± 1.2	11.3 ± 1.2	0.08 ± 0.01	1.05 ± 0.10
1:1 Ar	5.08 ± 0.04	2.35 ± 0.04	4.4 ± 1.2	6.8 ± 1.2	0.08 ± 0.01	0.85 ± 0.10
0:1 Ar	4.92 ± 0.04	1.32 ± 0.04	0.0 ± 0.0	4.6 ± 1.2	0.08 ± 0.01	0.99 ± 0.10

* Indicates the diluent.

presents the volumetric concentrations of the inlet and outlet gases during MW plasma treatment for runs with different H₂:CH₄ ratios. Based on these results, conversion rate, selectivities, and yield were calculated, according to Equations (17)–(21). In addition to C₂H₂, C₂H₄, and hydrogen, another main product was carbonaceous material. This material was deposited on the reactor walls as well as carried with the gas. The exact reaction mechanisms and the characteristics of solid carbon are still debated. For simplicity, the material will be referred to as soot. Methane decomposition in MW plasma can additionally result in other products, mainly C₃-C₄ unsaturated compounds as detected on GC/MS analysis within our previous research [26]. However, the concentration of these species was negligible.

2.1. Methane Conversion

The influence of H₂:CH₄ ratio on methane conversion (for the test with Ar as diluent) is presented in Figure 1 We observe a considerable decrease in X_CH4 when H₂:CH₄ ratio increases from 0:1 to 1:1. This is in opposition to what we reported in our previous work [18] where the addition of hydrogen had positive effect for H₂:CH₄ ratio of 1:1 and even 1:3. Moreover, in work [19], a gradual increase in H₂:CH₄ ratio from 1:5 to 1:1 also showed positive effects on CH₄ conversion. The reason for these different results is probably the diluent. In case of the two aforementioned works, no diluent was applied. In the present work, Ar was used. While Ar does not undergo chemical reaction, it may take part in energy-transition-initiating decomposition reactions. In fact, long-living metastable excited Ar particles carrying high energy are known to initiate the decomposition of organic compounds [27], as in Equation (4):

$${\mathrm{Ar}}^{*}+{\mathrm{CH}}_4\to \mathrm{Ar}+{\mathrm{CH}}_3+\mathrm{H}$$

(4)

The addition of hydrogen may lead to collision with hydrogen molecules instead of CH₄, resulting in the quenching of Ar-excited molecules [28], as in Equations (5) and (6):

$${\mathrm{Ar}}^{*}+{\mathrm{H}}_2\to \mathrm{Ar}+{\mathrm{H}}_2$$

(5)

$${\mathrm{Ar}}^{*}+\mathrm{H}\to \mathrm{Ar}+{\mathrm{H}}^{*}$$

(6)

Therefore, it may be speculated that with the presence of hydrogen and argon, two decomposition pathways compete, one involving H radical (Equation (3)) and the other metastable Ar particle (Equation (4)). With the H₂:CH₄ ratio of 1:1, the amount of hydrogen is enough to quench some of the Ar* particles. At the same time, the number of produced H radicals is not enough to compensate the loss in methane conversion due to metastable argon quenching. Therefore, a drop in CH₄ conversion is observed when compared with Ar-CH₄ mixture. With the increase in H₂ concentration, more H radicals are produced, resulting in the improvement of methane conversion when the applied ratio is 4:1. It should be emphasized that the presence of H radicals was proven in the upper plasma zone (in the place of plasma ignition) in previous research on the same reactor with very similar process conditions [10,26]. Therefore, it can be assumed that in the high-temperature upper part of the reactor (see Section 3 Materials and Methods), the addition of hydrogen results in H radical generation that can enhance methane decomposition.

2.2. C₂ Compounds Selectivity and Yield

In addition to influencing methane conversion, the addition of hydrogen also influences products’ yield and selectivity. Figure 2 presents the changes in the C₂H₂ yield and selectivity in correlation to H₂:CH₄ ratio. Opposite to X_CH4, the trend is more straightforward when considering C₂H₂. The drop in case 1:1 is a result of a significant decrease in CH₄ conversion (see Figure 1). In all other cases with H₂ addition, the C₂H₂ yield increases mainly due to increases in its selectivity but also thanks to a higher conversion rate (case 4:1). The increase in C₂H₂ selectivity is probably caused by the inhibition of acetylene conversion to heavier products, e.g., soot and aromatics [18,26]. In the case of C₂H₄, the results are not as linear as in the case of C₂H₂ (Figure 3). In a high temperature of the plasma, C₂H₂ production is favored over C₂H₄, resulting in a much lower concentration of the latter. As a result, the measurements are much less reliable and burdened with a high error. Therefore, it is hard to analyze the obtained trends in detail. Nevertheless, there is a noticeable and quite clear increase in both the selectivity and yield of C₂H₄ when H₂ is added. It should be noted that the formation of C₂ compounds takes place in the lower part of the reactor where the temperature varies in range of 1000 K (See Section 3 Materials and Methods). In this region, the role of hydrogen addition is different than in the upper part of the reactor. Instead of methane conversion due to the presence of H radicals, the H₂ molecules shifts the distribution of the products towards C₂ compounds.

2.3. Soot

The effect of hydrogen addition on the soot quantity can be evaluated based on the balance of carbon. In the discussed case carbon lack (C_lack—Equation (20)) can be equated with soot yield. This assumption neglects possible small amounts of aromatics and C₃-C₄ products. Carbon lack along with its selectivity (which can be treated as soot selectivity) are presented in Figure 4. C_lack drops significantly when hydrogen is added. The precise impact of different H₂:CH₄ ratios on C_lack is hard to determine as the error of these parameters (resulting from the carbon balance that involves many variables) is relatively high. However, the trend of the selectivity drop is much more reliable. Its gradual decrease is an outcome of a quite stable C_lack and increase in CH₄ conversion. In other words, with the addition of hydrogen, the amount of produced soot is more or less the same regardless of H₂:CH₄ ratio. On the other hand, with increasing ratio, more methane is converted, which should produce more soot. Since it is not, the selectivity of soot drops, suggesting the inhibition of soot formation. As mentioned, the addition of hydrogen inhibits processes leading to the transformation of acetylene into aromatic and soot [18,26]. This can be explained by the termination of aromatic radicals (A_i−), as in Equation (7), that otherwise would react with acetylene (Equation (8)), leading to the densification of aromatic structure that eventually results in soot formation (as in HACA mechanism).

$${\mathrm{A}}_{\mathrm{i}-}+{\mathrm{H}}_2\to {\mathrm{A}}_{\mathrm{i}}+\mathrm{H}$$

(7)

$${\mathrm{A}}_{\mathrm{i}-}+{\mathrm{C}}_2{\mathrm{H}}_2\to {\mathrm{A}}_{\mathrm{i}}{\mathrm{C}}_2{\mathrm{H}}_2$$

(8)

The decrease in the amount of soot formed during the process was also observed visually as the reactor’s quartz tube was remaining transparent longer and the amount of the soot sample that was collected from the tube was the smallest for the runs with the highest H₂:CH₄ ratio. The influence of hydrogen addition on lowering soot output was also confirmed in other research [18,19]

It is hypothesized that the addition of hydrogen can also affect the soot characteristics. In the previous work [18], this assumption was based on the calculated C and H lack as well as H₂ selectivity. Since hydrogen may inhibit the formation of soot derived from aromatics (as in HACA mechanism), the produced carbonaceous material should include a higher share of carbon material that was formed directly from CH₄ decomposition in high-temperature plasma region. Within this experiment, we performed SEM analysis to investigate possible differences in the particle sizes of the formed soot depending on the addition of H₂. Two exemplary SEM pictures along with the particles size distributions are shown in Figure 5 and Figure 6. Analyzing the particle size distribution, it is clear that the addition of hydrogen results in a higher share of bigger particles. For a H₂:CH₄ ratio of 4:1, a larger share of large particles (size up to $r_p\approx$400 µm) was detected. On the other hand, if no H₂ is added to the inlet gas stream, smaller particles are more dominant (59% for the ratio of 0:1 compared to 54% for ratios from 1:1 to 3:1 and 43% for the ratio of 4:1). We hypothesize that the increase in the particles size (due to hydrogen addition) might be caused by the aforementioned changes in the carbonaceous material nature and origin. While this is yet another premise suggesting that the hydrogen addition may impact the characteristics of soot, more analyses and data are necessary as the reason for this phenomenon is still unknown. However, the beneficial outcome of this phenomenon would be an easier separation of the particles from the gas stream, regardless of whether it would be a valuable product or an unwanted byproduct.

2.4. Hydrogen

It should be noted that the hydrogen added to the process is not consumed. However, since it shifts products distribution towards C₂ compounds, less hydrogen might be produced when compared with the most-harsh condition pyrolysis aiming for H₂ production and leading to the co-production of carbon black (see Equation (1)). This phenomenon is shown in Figure 7 presenting yield and selectivity of hydrogen (the selectivity of hydrogen should be considered separately to the carbon-based selectivities, i.e.,: S_C2H2, S_C2H4, C_lack). It should be noted that in the case of H₂:CH₄ ratio of 4, the yield of H₂ is close to the case with no hydrogen addition. This is due to the increase in methane conversion. In other words, while the selectivity of H₂ drops with hydrogen co-feeding, the wastage of the amount of produced hydrogen might be compensated by the improved methane conversion and higher output of the products.

2.5. Influence of Nitrogen Addition

The addition of nitrogen can affect soot formation, as proven in one of the previous works considering MW application in producer gas valorization [26]. In this work, this issue is given more attention by comparing two runs with different diluents: Ar or N₂, with both cases applying the H₂:CH₄ ratio of 4:1. Results from these tests are compared in

Table 2. Nitrogen impact on the process parameters.

Run	XCH4	YC2H2	YC2H4	SC2H2	SC2H4	Clack	SClack
4:1 N2	91.5 ± 2.4	49 ± 13	4.5 ± 1.4	53 ± 14	4.9 ± 1.5	38.5 ± 7.4	42.1 ± 5.2
4:1 Ar	95.4 ± 2.1	72 ± 10	4.7 ± 1.2	76 ± 11	5.0 ± 1.3	18.3 ± 5.8	19.2 ± 1.4

. With N₂ addition, a slight decrease in methane conversion was observed. A much more significant drop was observed for C₂H₂ selectivity and yield. On the other hand, C₂H₄ production was not affected by the N₂ addition. However, the main difference between the two tests is the fact that with N₂ addition, no soot was produced. Any soot deposition was quite easily observed on the quartz tube of the reactor, and this happened in all the cases with Ar as the diluent. With N₂ testing, no deposit was observed. This is visualized in Figure 8. It should be noted that with no soot, the C lack and its selectivity describe the carbon–nitrogen compounds that are produced due to nitrogen addition. It should be noted that this is in contrary to the tests with Ar. These carbon–nitrogen compounds were confirmed in our previous research with almost identical conditions applied (1800 W, 30 L/min, CH₄—4%, H_2—8%, N_2—rest) [10]. The results from the GC/MS analysis of these compounds are presented in Figure 9. It should be noted that this results present only area of the chromatogram’s peaks. The correlation between the peak’s area and the real concentration is usually different for different compounds. However, the size of the peaks is enough to estimate which compounds dominate and if their amount is within the same range. Therefore, while the analysis confirmed presence of C₂-C₃ nitrogen-containing compounds (i.e., cyanogen, acetonitrile, acrylonitrile) their concentration is negligible when compared to HCN. Besides nitrogen compounds, the figure also includes acetylene. This allows estimating that the concentration of C₂H₂ and HCN is more or less within the same range and such a conclusion is in good agreement with C_lack (indicating yield of carbon-nitrogen compounds that were not quantified) and Y_C2H2 (Table 2).

The formation of HCN can be explained by the interaction of acetylene and C_xH_y radicals with nitrogen molecules and radicals [29,30,31] as in reactions (9)–(12):

$$\mathrm{N}+{\mathrm{C}}_2{\mathrm{H}}_2\to \mathrm{HCN}+\mathrm{CH}$$

(9)

$$\mathrm{N}+{\mathrm{CH}}_3\to \mathrm{HCN}+{\mathrm{H}}_2$$

(10)

$$\mathrm{CH}+{\mathrm{N}}_2\to \mathrm{HCN}+\mathrm{N}$$

(11)

$${\mathrm{CH}}_2+{\mathrm{N}}_2\to \mathrm{HCN}+\mathrm{NH}$$

(12)

It should be noted that nitrogen radicals were confirmed within the plasma in similar conditions [10]. This would explain why the concentration of acetylene decreased so significantly when N₂ was added. The consumption of C₂H₂ and hydrocarbon radicals in HCN formation might be competitive with soot formation and could have resulted in a decrease in the amount of produced soot. However, previous research showed that in CH₄/N₂ mixtures both soot and HCN were observed [26]. Therefore, it might be concluded that both N₂ and H₂ are necessary to inhibit soot formation to a higher degree. In that case, one of the possible explanations would involve NH radicals and their reaction with CH₂ and CH radicals [29], as in (13)–(14):

$$\mathrm{CH}+\mathrm{NH}\to \mathrm{HCN}+\mathrm{H}$$

(13)

$${\mathrm{CH}}_2+\mathrm{NH}\to \mathrm{HCN}+2\mathrm{H}$$

(14)

The NH radicals are formed in a high-temperature region of N₂/H₂ MW plasma [26]. The interaction between NH and C_xH_y radicals could have been another way of inhibiting soot formation due to C_xH_y radicals consumption that otherwise would take part in the chain growth of soot. Finally, the other possible explanation might be a transformation of the already produced soot into HCN. This phenomenon was observed in the work of Orikasa and Tomita [32] and is explained by the interaction between nitrogen contained on the surface of carbonaceous material and H₂ and/or H, as presented in reaction (15):

$$\mathrm{C}\left(\mathrm{N}\right)+{\mathrm{H}}_2/\mathrm{H}\to \mathrm{C}\left(\mathrm{H},\mathrm{N}\right)\to \mathrm{HCN}$$

(15)

It should be noted that the soot formed from hydrocarbons in nitrogen MW plasma does include nitrogen in its structures [33], which is necessary for this mechanism to occur. Moreover, in opposite to the two previous mechanisms that would inhibit soot formation, the latter one would convert already produced soot. Finally, the three discussed mechanisms do not exclude each other, and it is possible that all of them have their share in the discussed phenomenon.

3. Materials and Methods

The tests were carried out in the MW plasma reactor that is described in detail in previous research [10,27,28]. Briefly, the power of microwaves initiating and sustaining the plasma is ca. 1800 W. The processed gas acts as a plasma agent and is introduced into the reactor tangentially resulting in a swirl flow. The plasma is generated in the reactor’s quartz tube. It should be noted that while the plasma was not diagnosed for the purpose of the presented results, it was thoroughly investigated previously for very similar conditions [10,27,28]. It was shown that the temperature of the plasma core near the spot of plasma ignition is close to 4000–6000 K, and it drops along the reactor, reaching 1000–1200 K at the bottom of the quartz tube. Therefore, two temperatures zones can be distinguished: a short (up to 10 cm) high-temperature zone and a long (ca. 50 cm) lower-temperature zone. It should be noted that the gases are introduced above the ignition zone and their pass through both of the temperatures zones.

The whole experimental setup is shown in Figure 10. The tests were done with a fixed total gas flow rate of 30 L/min and a fixed microwave power of ca. 1800 W. The process gas consisted of a mixture of methane, a dilution gas and optionally H₂. While the tests were normally performed with an Ar dilution, another analyzed variation involved N₂ instead of Ar. Both of these gases were necessary to sustain the plasma that tends to quench when a high concentration of H₂ and CH₄ is applied (due to increased reflection of microwaves). The ratio of H₂:CH₄ was changed in the range of 0:1 (no hydrogen) to 4:1, with the CH₄ concentration being constant and equal to ca. 5% v/v. The flow rates of all the gases (H₂, CH₄, Ar, and N₂) were controlled with the use of rotameters. The concentration of Ar was used to determine changes in the volumetric gas flow rate due to gas processing and resulting chemical reactions. As argon does not undergo chemical reactions, changes of its concentration are result of the changes in the volumetric gas flow rate. Therefore, a small stream of Ar (ca. 7% v/v) was used even when nitrogen was the main diluent. At the outlet of the reactor, the gas samples were probed into Tedlar’s bags and analyzed by GC (gas chromatography) techniques with the use of Agilent 7820 (TCD detector) and HP 6890 (FID detector). More details on the GC analyses are given in

Table 3. Parameters of the GC analyses.

Analyzed Compound	Detector	Column	Temperature
CH4, C2H2, C2H4	FID	RT-Alumina BOND/KCl	100 °C (3 min)
H2, CO, Ar	TCD	HP-Molesieve	$45°\mathrm{C}(4\min )\stackrel{10℃/\min }{\to }$ 100 °C

.

The soot produced during the tests was collected from the reactor’s quartz tube. Three samples were collected: from the test with the H₂:CH₄ ratio of 0:1, from the test with the ratio of 4:1, and a collective sample from tests with H₂:CH₄ ratio from 1:1 to 3:1. Scanning microscopy images of the carbon powder were captured with a Hitachi TM3030Plus, at 15 kV, a working distance of 10.70 mm, and with a mixed signal from the backscattered and secondary electrons. Particle size distribution was determined using ImageJ. In the first step, the image was binarized using a threshold selected visually to correctly capture particles while ignoring the background. Then the particle analyzer feature of ImageJ was used to give a list of detected particles and their area. The corresponding particle radius was calculated as $r_p\approx \surd A$ (assuming spherical particles).

As toxic hydrogen cyanide was expected to be produced in the tests with nitrogen addition, the outlet of the reactor was connected to the vent. Additionally, an HCN detector (Dräger Pac 8000) was available at the test rig.

To evaluate the process of methane pyrolysis, the following parameters were determined:

Outlet volumetric gas flow rate:

$$v_{out}(\frac{\mathrm{L}}{\min })=v_{in}\times \frac{{\left[\mathrm{Ar}\right]}_{in}}{{\left[\mathrm{Ar}\right]}_{out}}$$

(16)

where $v_{in}$—inlet volumetric gas flow rate (L/min), ${\left[\mathrm{Ar}\right]}_{in}$—inlet argon concentration (% v/v), ${\left[\mathrm{Ar}\right]}_{out}$—outlet argon concentration (% v/v).

Methane conversion rate:

$$X_{{\mathrm{CH}}_4}(\%)=\left(1-\frac{{\left[{\mathrm{CH}}_4\right]}_{out}\times v_{out}}{{\left[{\mathrm{CH}}_4\right]}_{in}\times v_{in}}\right)\times 100$$

(17)

where ${\left[{\mathrm{CH}}_4\right]}_{out}$—outlet methane concentration (% v/v), ${\left[{\mathrm{CH}}_4\right]}_{in}$—inlet methane concentration (% v/v).

Selectivity (carbon-based) of C₂ compounds (acetylene, ethylene):

$$S_{\mathrm{C}x\mathrm{H}y}(\%)=\left(\frac{x\times {\left[{\mathrm{C}}_x{\mathrm{H}}_y\right]}_{out}\times v_{out}}{{\left[{\mathrm{CH}}_4\right]}_{in}\times v_{in}-{\left[{\mathrm{CH}}_4\right]}_{out}\times v_{out}}\right)\times 100$$

(18)

where $x$—number of molecule’s carbon atoms, ${\left[{\mathrm{C}}_x{\mathrm{H}}_y\right]}_{out}$—outlet concentration of C₂ compounds.

Yield of C₂ compounds (acetylene, ethylene):

$$Y_{\mathrm{C}x\mathrm{H}y}(\%)=\frac{X_{\mathrm{CH}4}\times S_{\mathrm{C}x\mathrm{H}y}}{100}$$

(19)

Carbon lack, which is defined as the difference between the quantified amounts of inlet and outlet carbon in gaseous species:

$$C_{lack}(\%)=\left(1-\frac{v_{out}\ast \times {{\displaystyle \sum }}^{}\left(x\times {\left[{\mathrm{C}}_x{\mathrm{H}}_y\right]}_{out}\right)+v_{out}\times {\left[{\mathrm{CH}}_4\right]}_{out}}{{\left[{\mathrm{CH}}_4\right]}_{in}\times v_{in}}\right)\times 100$$

(20)

Selectivity of carbon lack:

$$S_{Clack}(\%)=\frac{C_{lack}(\%)}{X_{\mathrm{C}\mathrm{H}4}(\%)}\times 100$$

(21)

Hydrogen selectivity:

$$S_{{\mathrm{H}}_2}(\%)=\left(\frac{1}{2}\times \frac{{\left[{\mathrm{H}}_2\right]}_{out}\times v_{out}-{\left[{\mathrm{H}}_2\right]}_{in}\times v_{in}}{{\left[{\mathrm{CH}}_4\right]}_{in}\times v_{in}-{\left[{\mathrm{CH}}_4\right]}_{out}\times v_{out}}\right)\times 100$$

(22)

Hydrogen yield:

$$Y_{{\mathrm{H}}_2}(\%)=\frac{X_{{\mathrm{CH}}_4}(\%)\times S_{{\mathrm{H}}_2}(\%)}{100}$$

(23)

4. Conclusions

The paper presents an investigation of the impact of hydrogen addition on methane pyrolysis in microwave plasma with the main focus given to C₂ compounds and soot products rather than hydrogen. It shows that hydrogen presence influences all aspects of methane pyrolysis. Applying H₂:CH₄ ratio of 4:1 results in the increase of methane conversion from ca. 72% (for the case with no hydrogen addition) to 95%. The yields of C₂H₂ and C₂H₄ increase from ca. 41% to 72%, and from 3.2% to 4.7%, respectively. Similarly, the selectivity increases from ca. 56% to 75%, and from 4.3% to 5.0%, for C₂H₂ and C₂H₄ respectively. At the same time, addition of hydrogen leads to decreases in the yield (from ca. 28% to 18%) and selectivity (from ca. 40% to 18%) of soot and increase the size of soot particles. Interestingly, while an increasing trend of CH₄ conversion towards high ratios of H₂:CH₄ is observed a minimum is reached at H₂:CH₄ 1:1. This conversion drop is probably caused by quenching of metastable Ar particles that otherwise play an important role in initiating methane decomposition. Further investigations are necessary to determine the global impact of hydrogen addition on the process efficiency. Nevertheless, the results show that hydrogen addition might be a promising approach for a methane pyrolysis process focused on C₂ compounds formation. Moreover, one aspect is of high technical relevance—the soot formation. The inhibition of soot formation due to hydrogen presence might be important for methane pyrolysis. The amount of soot produced if methane pyrolysis becomes a full-scale commercial process would be far above the global demand, resulting in a stream of problematic waste rather than a valuable product [1]. In that context, this paper shows that one possible solution is the addition of nitrogen. With the presence of nitrogen and hydrogen, the formation of soot was completely stopped. Instead of soot, hydrogen cyanide formed. The possibility of switching between soot and HCN as one of the products, along with improved CH₄ conversion and higher selectivity of C₂ compounds, would surely make methane pyrolysis more flexible and attractive.