Possibility Study in CO₂ Free Hydrogen Production Using Dodecane (C₁₂H₂₆) from Plasma Reaction

Abstract

Turquoise hydrogen refers to hydrogen produced through a fossil-fuel-based process in which carbon is separated into solid carbon and no carbon dioxide is produced. In this study, dodecane was selected as a simulated oil for waste plastic pyrolysis recovery oil, and the turquoise hydrogen production characteristics through the thermal cracking reaction using an arc plasma torch were investigated. The plasma was stably discharged at 2 to 4 kW. Hydrogen in the produced gas was analyzed through an online IR gas analyzer, and hydrocarbons from C₁ to C₅ were analyzed through GC-FID. As a result of the experiment, the hydrogen yield tended to increase as the plasma power increased, and a maximum of 11.5% based on mass was obtained. On the other hand, carbon oxides such as CO and CO₂ were not generated. Along with hydrogen, the valuable by-products of this process are solid carbon and gaseous hydrocarbons. The solid carbon yields also increased up to 66% as the plasma power increased. On the other hand, the yield of gaseous hydrocarbons showed an opposite trend to that of hydrogen and carbon and consisted mainly of C₂ series (average content of 77%) and olefins (average fraction of 0.67). Consequently, it can be considered that the plasma thermal cracking is a promising technology for the CO₂-free hydrogen production, as well as solid carbon and C₂-olefin.

1. Introduction

In the last few centuries, humans have made significant technological advances in a short period of time through the use of fossil fuels, but the pollution of the earth due to the increased use of fossil fuels has reached a situation that can no longer be ignored. The most notable of these is climate change. From the point of view of society, climate change is essential for the benefit of mankind, and to solve this problem, many scientists are emphasizing the low-carbon transition of the energy system. The low-carbon transition of energy systems plays an important role in global carbon mitigation [1].

Hydrogen is expected to be the most important energy source in the future, and unlike fossil fuels such as oil, coal, and gas, it has the strength of being an infinite clean energy that can be used anywhere on the earth. To date, steam methane reforming is the most competitive hydrogen production method, but a solution is needed for high CO₂ emissions. On the other hand, water electrolysis technology using renewable energy sources is not widely used yet due to the high cost of hydrogen production.

Due to the recent COVID-19 pandemic, social distancing and telecommuting have led to a rapid increase in plastic waste, mostly due to delivery. Along with the increase in waste plastics, social and environmental problems are emerging, and the necessity of recycling waste plastics is being raised. In particular, in the case of the pyrolysis oil production process using waste plastic, it has the advantage that it can be used as an energy resource at the same time as waste treatment in that it can produce fuel without requiring a high-level screening process. As mentioned earlier, the use of pyrolysis oil in hydrogen production is expected to be a new alternative to solving energy and environmental problems.

The IEA energy-related international organization gives a color-coded range from gray to green for hydrogen production technology as a measure of greenhouse gas [2].

Unlike blue hydrogen, turquoise hydrogen production through methane pyrolysis does not have to go through CCUS, and has the advantage of being able to produce hydrogen with less energy than that of green hydrogen production. In addition, it can be expected to achieve a greenhouse gas negative effect that reduces greenhouse gas emissions, and solid carbon, a by-product of the turquoise hydrogen production process, can also be used as a high value-added product through the research and development process. The direct decomposition of methane into hydrogen and carbon is an endothermic process with a single-step reaction [3,4,5,6].

CH₄(g) → 2H₂ + C, ΔH_o = 74 kJ/mol

Turquoise hydrogen production from fossil fuels can be classified into pyrolysis, plasma-assisted cracking, and catalytic reactions, depending on the energy supply method and reaction conditions.

In the plasma-assisted cracking process, the energy required for thermal decomposition of raw materials such as methane is supplied in the form of electrical energy. Through this, the raw material is decomposed into hydrogen and by-product carbon. In particular, the plasma decomposition reaction has the advantage of being faster than the thermal decomposition and catalytic reaction processes. When energy is continuously applied to the gas to reach a high energy level, the gas molecules are dissociated into atoms and ionized into electrons and positively charged ions. At this time, if the kinetic energy of the electrons and ions is greater than the recombination energy, the electrons and ions are not reduced to electrical neutrality and continue to maintain the form of electrically charged electrons and ionized gas. The quasi-neutral ionized gas in which these electrically ionized gases and neutral atoms are mixed is called plasma, and it can promote reactions at low temperatures and in smaller volumes [7]. The types of plasma can be largely classified into DC, AC, RF, and electromagnetic radiation methods according to the generation source [8]. In addition, plasma can also be classified into non-thermal and thermal plasma according to its thermal equilibrium state. In the case of non-thermal plasma, the temperature of electrons is thousands of degrees, while the temperatures of gases and ions can be maintained at room temperature conditions [9]. On the other hand, in the case of thermal plasma, electrons and gas can maintain the same temperature in thermal equilibrium. Recently, a technique for producing turquoise hydrogen by utilizing thermal plasma technology has been investigated and was initially operated by Kværner by heating it to 1650 °C utilizing an argon plasma [10].

Monolith Materials started researching the methane pyrolysis process using a plasma heat source in 2012. In 2020, 14,000 ton/yr of carbon black and 2500 ton/yr of hydrogen were produced, and in 2020, a process capable of producing 194,000 ton/yr of carbon black and 40,000 ton/yr of hydrogen was operated. The plasma source used by Monolith has a plasma (2000 °C)-based hydrogen and carbon black simultaneous production technology using 100% renewable power in the form of arc discharge [11].

In addition, the hydrogen production process through the decomposition reaction of methane is advantageous in terms of the amount of heat required for the reaction. That is, the heats required for the steam methane reforming and water electrolysis reactions to produce 1 mol of hydrogen are 63.4 kJ and 285.8 kJ, respectively, while the methane decomposition consumes less energy of 37.7 kJ.

Like the methane decomposition reaction, high-density hydrocarbons such as plastic pyrolysis oil are decomposed into low-carbon hydrocarbons through arc plasma, and at the same time, CO₂ free hydrogen can be produced. If the pyrolysis catalytic reactor is connected after the plasma reaction, the low-carbon hydrocarbon generated through the plasma reaction can produce a large amount of hydrogen and carbon as a by-product. In this study, dodecane (C₁₂H₂₆) was selected to simulate the waste plastic pyrolysis oil, and the plasma reaction characteristics were studied. To this end, the yield and composition of hydrogen and hydrocarbons in the gaseous product generated through the arc plasma reaction and the solid carbon yield were measured and analyzed, and based on this, the possibility of designing a catalyst-linked complex process system was confirmed.

2. Materials and Methods

2.1. Plasma Chamber Reactor

To build a thermal cracking system using plasma of waste plastic pyrolysis oil, a 10 kW arc plasma torch was manufactured, and a discharge characteristic test was performed for stable plasma discharge operation. The arc plasma torch’s discharge characteristic test system consists of a plasma torch, a plasma power supply, a plasma transfer gas supply, and a cooling water supply, as shown in Figure 1. Plasma efficiency and discharge stability were evaluated by measuring inlet/outlet temperature changes. The plasma chamber reactor is made of STS (stainless steel)-304 material and is 363 mm long to match the plasma torch inlet size ø 60 mm. A three-way union valve is installed in the middle to supply fuel. In consideration of material deformation, a chamber was designed/manufactured in a clamp-type fastening method for fastening the cylindrical refractory material under the gas outlet line and checking the inside after discharge.

2.2. Instrumentation and Set-Up

In the system for the arc plasma reaction experiment, gaseous nitrogen for arc plasma discharge and methane for reaction was quantitatively supplied by a Mass Flow Controller (MFC, Brooks). Figure 2 is the result of the FT-IR analysis of a typical waste plastic pyrolysis oil, and it can be seen that the oil is mainly composed of C₉–C₁₂ hydrocarbons. Therefore, in order to model the waste plastic pyrolysis oil, dodecane (C₁₂H₂₆, Sigma Aldrich, St. Louis, MO, USA), one of the main components of the pyrolysis oil, was chosen as a liquid fuel.

Liquid fuel dodecane (C₁₂H₂₆, Sigma Aldrich) was supplied by a liquid supply syringe pump, and C₁₂H₂₆ was vaporized through a pre-heater and supplied to the plasma reactor. Properties of dodecane are shown in

Table 1. Properties of dodecane.

Sample		Dodecane
Chemical formula		CH3(CH2)10CH3
Molecular weight	g/mol	170.34
Melting point	°C	−10~−9.3
Boiling point	°C	214~218
Flash point	°C	71
Viscosity	mPa·s	1.34

.

In the arc plasma discharge, plasma discharge starts when a power supply button is pressed while supplying nitrogen as a discharge gas to a plasma torch. For gas analysis generated through plasma discharge, the concentrations of hydrogen and methane were checked in real time with an online IR (A&D system 2000, A&D Company, Ltd., Seoul, Republic of Korea) and FT-IR (C₁–C₄ analysis, Gasmet DX4000, Vantaa, Finland), and gas sampling was conducted at the same time. Through GC-FID (Gas Chromatography, YL6500 GC, Young In Chromass, Kyeonggi-do, Republic of Korea), C₁–C₅ hydrocarbons were analyzed. The generated gas flow rate was measured through a dry gas meter, and all measured values were measured and collected in real time through a data logger (midi Logger GL840, Graphtec, Yokohama, Japan) and used to organize the experimental results. During the reaction, differential pressure sensors were installed at the front and rear of the plasma to check the pressure change. The product gas was first cooled and then discharged through a carbon capture filter (Teflon cartridge filter, Grymon, Kyeonggi-do, Republic of Korea), and solid carbon particles generated during the reaction were captured by the filter (Figure 3). The amount of solid carbon production was calculated by measuring the weight and the change in weight before and after the reaction. All weights of the filter were measured after the drying process. The carbon mass balance can be calculated based on the amount of raw material fed and gas produced and the amount of carbon captured.

3. Results

3.1. Characteristics of Plasma Discharge

Plasma power could be controlled through the applied current and plasma transfer gas flow rate, and characteristics of the plasma discharge were investigated under low-power conditions using a 10 kW scale plasma torch. Under the constant current condition, the plasma discharge shows stable and relatively constant voltage and coolant temperature change, as shown in Figure 4.

The effects of the applied current and the flow rate of the plasma transfer gas on the plasma power and coolant temperature are shown in Figure 5 and Figure 6, respectively.

As the applied current increased, the voltage slightly decreased, but the power tended to increase, and the plasma power also increased according to the flow rate of the transfer gas. In addition, as the plasma power increased, the temperature rises of the cooling water increased, especially under the condition of keeping the transfer gas flow rate low. Based on the results, dodecane thermal cracking experiments were performed by changing the plasma power to 2–4 kW under the conditions of an applied current of 20–50 A and a plasma transfer gas flow rate of 8–20 L/min, and under these conditions, the plasma could be stably discharged.

3.2. Plasma Thermal Cracking of Dodecane in a Downward Direction

The dodecane thermal cracking experiments were carried out under the condition that a plasma torch was installed at the top of the thermal cracking reactor and a plasma discharge was formed in a downward direction. After vaporization, dodecane was also supplied from the top of the reactor at a constant feed rate of 4 mL/min to form a co-current flow with the plasma. After cooling the product gas at the bottom of the reactor, the gas flow rate and composition were measured through the aforementioned online dry gas meter and IR gas analyzer (Figure 7).

The product gas consists only of hydrogen and hydrocarbon gases, and as a result, it can be confirmed that turquoise hydrogen can be produced through this process without generating carbon oxides such as CO and CO₂. After the gas flow rate and composition were stabilized, the generated gas was sampled, and the composition of gaseous hydrocarbons was analyzed from C₁ to C₅. The GC-FID peak analysis results and reference retention time are shown in Figure 8.

The flow rate and composition of the product gas according to the plasma power are shown in Figure 9.

As the plasma power increased, the product gas flow rate tended to increase, and the composition of hydrogen also tended to increase in the range of 20–55%. On the other hand, the total hydrocarbon content decreased from approximately 80% to 40% with increasing plasma power. The main component of the hydrocarbon gas is C₂-based gas, which accounts for about 80% of the total hydrocarbon gas, and ethylene was found to be the most abundant. The yield of component can be defined by mass as in Equation (1), and the yields of hydrogen, total hydrocarbons, and C₂-based hydrocarbons are compared in Figure 10.

$$\mathrm{Yield} \mathrm{of} \mathrm{component}, \mathrm{i} \left({\mathrm{Y}}_{\mathrm{i}}\right)=\frac{\mathrm{Amount} \mathrm{of} \mathrm{component}, \mathrm{i}}{\mathrm{Amount} \mathrm{of} \mathrm{dodecane}}\times 100\%,$$

(1)

As the plasma power increased, the hydrogen yield increased up to 5.8%, while the total hydrocarbons and C₂ yields tended to decrease from 96% to 62% and from 76% to 53%, respectively. The olefin content of the total hydrocarbons was determined to be about 50 to 90%, and the yield of ethylene, a C₂-based olefin, corresponded to approximately 25 to 58%, which was similar to the results of the existing literature [12,13]. As a result, it is possible to confirm the production of olefins that can be used as the main raw materials for various chemical processes along with the production of clean turquoise hydrogen.

The carbon conversion can be defined as the ratio of the carbon content of gaseous hydrocarbons to the carbon content of dodecane, as shown in Equation (2), and the yield of solid carbon can be determined by the carbon mass balance of the carbon supply and emission. The calculation results are shown in Figure 11.

$$\mathrm{Carbon} \mathrm{conversion} \left(\mathrm{CC}\right)=\frac{\mathrm{Amount} \mathrm{of} \mathrm{carbon} \mathrm{in} \mathrm{gaseous} \mathrm{product}}{\mathrm{Amount} \mathrm{of} \mathrm{carbon} \mathrm{in} \mathrm{liquid} \mathrm{fuel}}\times 100\%,$$

(2)

The carbon conversion tended to decrease with increasing plasma power, while the yield of solid carbon increased from about 8% to 35%. The decrease in the amount of hydrocarbons in the product gas and increases in the hydrogen and solid carbon yields can be due to the increase in an additional thermal decomposition of gaseous hydrocarbons as the plasma power increases.

The average product gas composition and average hydrogen yield according to the applied current under the constant plasma power of 3.8 kW are compared in

Table 2. Comparison of product content and yield according to the applied current.

Applied Current	A	35 (Low)	50 (High)
Plasma power	kW	3.78	3.77
Voltage	V	107.66	75.14
Plasma transfer gas	L/min	20.92	9.20
H2	%, N2 free	55.82	26.31
Hydrocarbon	%, N2 free	44.18	73.69
YH2	%	5.77	2.15
YHydrocarbon	%	67.87	90.61
(olefin fraction)		(0.89)	(0.82)
YC(s)	%	30.45	12.21
Carbon conversion	%	64.06	85.59

.

In order to keep the plasma power constant, the plasma transfer gas flow rate was increased to maintain a high voltage when the applied current is low, and when the current is high, the gas flow rate was reduced to operate in a relatively low voltage condition. As shown in Table 2, when the applied current is low and the voltage is kept high, hydrogen composition and yield increased relatively, while hydrocarbon yield and carbon conversion decreased. Consequently, under constant plasma power conditions, turquoise hydrogen production is advantageous for high-voltage operation, and the production of hydrocarbons mainly composed of C₂-olefin is advantageous for low-voltage operation.

3.3. Plasma Thermal Cracking of Dodecane in an Upward Direction

In order to examine the effect of the plasma supply direction, the plasma and dodecane supply position was modified from the top of the reactor to the bottom, and the experiments were conducted under the condition that plasma was formed in an upward direction. As in the previous experiment, the dodecane supply flow rate was kept at 4 mL/min, and the gas flow rate and the composition of the product gas were determined using an online dry gas meter, IR analyzer, and GC-FID.

The flow rate and composition of the product gas according to the plasma power are shown in Figure 12.

As the plasma power increased under the plasma upward supply condition, the flow rate and hydrogen content of the product gas tended to increase and maintained relatively high levels of about 4.4 L/min and 87%, respectively, compared to the downward supply condition. On the other hand, the total hydrocarbon content decreased from about 40% to 13% with increasing the plasma power. Product yield distributions and carbon conversion are shown in Figure 13

Hydrogen and carbon yields increased, while hydrocarbon yields and carbon conversion rates decreased, similar to previous experimental results. On the other hand, the yield value of each product showed a difference according to the plasma supply direction, and in particular, it can be seen that the yields of hydrogen and solid carbon increased relatively greatly. The product content and yield distribution according to the plasma supply direction are compared in

Table 3. Comparison of product content and yield according to plasma supply direction.

Plasma Direction		Down Ward	Upward
Plasma power	kW	3.78	3.20
H2	%, N2 free	56.61	87.05
Hydrocarbon	%, N2 free	43.39	12.95
YH2	%	5.85	11.51
YHydrocarbon	%	66.49	24.32
YC(s)	%	31.56	65.33
Carbon conversion	%	62.74	22.87

.

Under the plasma upward supply condition, the gaseous hydrocarbon yield was relatively decreased, and consequently, the carbon conversion was also reduced. In general, the thermal decomposition of hydrocarbons such as dodecane proceeds in several stages [14]. At the first stage, hydrocarbons break down quickly into mainly hydrogen and hydrocarbon fragments. Thereafter, the hydrocarbon fragments are decomposed into low molecular weight fragments through a secondary pyrolysis reaction to generate additional hydrogen and solid carbon, and the reaction is terminated. In this study, it can be seen that the secondary pyrolysis reaction of the primary hydrocarbon product increased under the plasma upward supply condition, and as a result, the high yield of hydrogen and solid carbon could be maintained. The effect of the secondary reaction according to the plasma supply conditions can be explained through the flow characteristics and the heat/mass transfer phenomenon inside the reactor, and will be confirmed through additional experiments in the future.

Energy efficiency of the process can be defined as the ratio of the energy of gas produced or hydrogen produced to the total amount of energy supplied by liquid fuel and plasma power, as in Equation (3).

$$\mathrm{Energy} \mathrm{efficiency} \left({\mathsf{\eta }}_{\mathrm{Energy}}\right)=\frac{\sum {\mathrm{m}}_{\mathrm{i}}\times {\mathrm{LHV}}_{\mathrm{i}}}{{\mathrm{m}}_{\mathrm{Dodecane}}\times {\mathrm{LHV}}_{\mathrm{Dodecane}}+{\mathrm{P}}_{\mathrm{Plasma}}}\times 100\%,$$

(3)

where m_i and P_Plasma mean the mass flow rate of component i and plasma power, respectively.

The energy efficiencies based on the total gas and hydrogen produced in this study are 24% and 13%, respectively, which were relatively low compared to other processes such as water splitting and conventional conversion processes shown in

Table 4. Comparison of the energy efficiency of this study and other processes [4, 15, 16].

Technology	Energy Efficiency
	Hydrogen	Fuel Gas
This study in a downward direction	5.9%	32.4%
This study in an upward direction	12.9%	23.6%
Methane pyrolysis	58% (42% stored in solid carbon)
Thermochemical water splitting	20~45%
Water electrolysis	50~70%
Steam methane reforming	60% (w/o CCS)~75% (with CCS)
Plasma-assisted steam reforming	9~85%
Chemical-looping steam reforming	65~75%
Biomass gasification	35% (w/o CCS)~50% (with CCS)
Coal gasification	43% (w/o CCS)~60% (with CCS)

.

The main energy efficiency reduction factors are the reduced plasma torch efficiency and energy conversion to solid carbon. The thermal efficiency of the plasma torch can be calculated as the ratio of the net power, i.e., amount of power supplied from the power supply minus heat losses due to the cooling water, to the power supply, as in Equation (4) [17].

$$\mathrm{Thermal} \mathrm{efficiency} \left({\mathsf{\eta }}_{\mathrm{Plasma}}\right)=\frac{{\mathrm{P}}_{\mathrm{Plasma}}- \mathrm{Heat} \mathrm{of} \mathrm{cooling} \mathrm{water}}{{\mathrm{P}}_{\mathrm{Plasma}}}\times 100\%,$$

(4)

The power consumption and thermal efficiency of the torch were 2.3~4.3 kW and 53~64%, respectively. This is because of the low-load conditions for the laboratory experiment despite the design and optimization of the plasma torch for the 10 kW class in this study.

In addition, in terms of product energy distribution, the energy of the supplied liquid fuel is converted into gaseous fuel and solid carbon. At this time, solid carbon conserves approximately 23% to 48% of the liquid fuel energy, consequently affecting energy efficiency.

4. Conclusions

In this study, a plasma thermal cracking experiment was performed using as a simulated oil for waste plastic pyrolysis recovery oil. Hydrogen is the main product of this process, and it was confirmed that turquoise hydrogen was produced without CO₂ generation. In addition, solid carbon and gaseous hydrocarbons as by-products can also be used as valuable chemicals.

The plasma power was controlled in the range of 2 to 4 kW, where stable discharge was confirmed. As the plasma power increased, the hydrogen content and yield in the product tended to increase, and could be obtained up to 11.5%. At the same time, the solid carbon yields also increased, reaching a maximum yield of 66%. On the other hand, gaseous hydrocarbons tended to decrease as the plasma power increased, and it was confirmed that they were mainly composed of C₂-based olefins. The composition and yield of the gas produced depended on the plasma operation method. Under the constant plasma power condition, the hydrogen yield was relatively high, increasing by about 33% during high-voltage operation, while the production of olefinic hydrocarbons increased under low-voltage conditions. In addition, the yield of products such as hydrogen was also greatly affected by the plasma supply method. Under the condition of supplying plasma downward, both the hydrogen and carbon yields were approximately doubled, while the carbon conversion and gaseous hydrocarbon yield decreased by about 70%. As a result, it is possible to produce solid carbon and olefinic hydrocarbons along with the production of clean hydrogen through process optimization according to the use of the final product.