**1. Introduction**

Many countries have tried to accomplish a successful transition of an energy system to hydrogen (H2) based on various political strategies such as 'The National Hydrogen Strategy' (2020) in Germany [1], 'EU Hydrogen Strategy' (2020) in the EU [2], 'Basic Hydrogen Strategy' (2017)', 'Strategic Energy Plan' (2018), and 'The Strategic Road Map for Hydrogen and Fuel Cells' (2019) in Japan [3–5], 'Hydrogen in a Low-carbon Economy' (2018) in the UK [6], 'H2@Scale' (2021) in USA [7], and 'National Hydrogen Roadmap' (2018) in Australia [8]. These active approaches to an H2-based energy system come from the diverse advantages of H2 as a clean energy carrier: it can be utilized in various energy sectors and easily combined in already constructed infrastructure, and, even though its volumetric energy density is relatively low, it shows a very high energy density of 120–142 MJ kg−<sup>1</sup> in the compressed state [9–15]. For the conventional production of H2, energy-intensive processes such as reforming, partial oxidation, and auto-thermal reforming of carbon-based fuels such as methane (CH4) and hydrocarbon have been mainly used. However, these conventional methods, including steam methane reforming (SMR), have led to negative environmental effects due to large emissions of carbon dioxide (CO2) [16–19]. Due to the

**Citation:** Cheon, S.; Byun, M.; Lim, D.; Lee, H.; Lim, H. Parametric Study for Thermal and Catalytic Methane Pyrolysis for Hydrogen Production: Techno-Economic and Scenario Analysis. *Energies* **2021**, *14*, 6102. https://doi.org/10.3390/en14196102

Academic Editor: Dmitri A. Bulushev

Received: 13 August 2021 Accepted: 16 September 2021 Published: 24 September 2021

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environmental issues of conventional methods, a lot of recent research has suggested water electrolysis (WE) powered by renewable energy as an alternative eco-friendly solution to produce H2 because there is no CO2 emission in the procedure [20–22]. However, there are still technical and economic challenges to be immediately utilized [23–25]; most H2 production still depends on conventional methods with additional processes to reduce CO2 such as SMR with CO2 capture and storage (SMR with CCS) [26–28].

To overcome the limitations of current H2 production methods, the concept of thermal methane pyrolysis (TMP), where H2 and carbon (C) are directly produced in the gas phase (Equation (1)), has been paid attention as an alternative, novel H2 production method owing to several technical, economic, and environmental benefits as follows: (a) there is no oxygen (O2) in the reaction leading to no CO2 emissions or additional separation process, theoretically; (b) the process can be relatively simplified and lower energy is required than other methods such as reforming or partial oxidation; (c) reactant of the process, methane, is abundant and cheap leading to a cost effective operation of the process; (d) C products can be marketed because they are usually used as raw materials in various valuable materials such as rubber, tires, and pigments, etc.; (e) the separation of C is much easier than the separation of CO2; (f) it requires a lower amount of heat compared to SMR (Equation (2a–c)) and WE (Equation (3)), which are the most common and novel H2 production methods (Equations (1)–(3)) [29–38].


Because of its endothermicity and strong C–H bonding, TMP is usually operated at over 1373 K to obtain reasonable yields of H2 and C leading to cost ineffectiveness and a large amount of energy being required [39,40]. These problems of having to use high temperature can be reduced by catalytic methane pyrolysis (CMP) where various types of catalysts (non-supported, metal supported, metal oxide supported, and carbonaceous, etc.) are adopted. Among the various catalysts used in CMP, the metal-based catalyst has very critical systematic limitations such as high toxicity of metal and rapid deactivation of the catalyst due to encapsulation of the active metal sites with C product [41]. Thus, carbon-based CMP has a lot of attention owing to the properties of carbon catalysts such as lower cost, higher stability, temperature resistance, and their ability to be safely stored due to their non-toxicity.

Based on the benefits of the concept of MP, many kinds of research have been conducted: Nishii et al. [42] carried out MP with different carbon-based catalysts (activated carbon, carbon black, mesoporous carbon, and carbon nanofiber) and found that all of these catalysts continued to maintain a CH4 conversion of about 17% for longer than 600 min by catalyzing the produced C. It was reported that the produced C covered the catalyst surface, resulting in a specific surface area of 10 m2 g−<sup>1</sup> and an intensity of D-Raman peak and G-Raman peak (Id/Ig) from 1.5 to 1.57 irrespective of the original structures of C. Tezel et al. [43] designed an experiment using CMP with a calcium silicate-based Ni–Fe catalyst with different Fe loading by using the co-impregnation method. It was revealed that the addition of Fe can delay the deactivation of the Ni catalyst and an increase in the CH4 flow rate can decrease the initial reactant conversion and lifetime of the catalyst. It was reported that the highest methane conversion of 69% is obtained at 973 K with the catalyst that has the highest Fe addition. Quan et al. [44] investigated the optimization of a fluidized bed reactor (FLBR) for CMP using 40 wt% Fe/Al2O3 catalyst, and catalyst activity and stability were investigated after optimization in terms of the catalyst bulk density, bed height, and particle size, etc. It was reported that the reaction conditions of 12 L (gcat h)−<sup>1</sup> feed dilution of 20% H2-CH4, and CO2-regeneration of deactivated catalysts are the best conditions for MP. Patzschke et al. [45] investigated promising catalysts for particle suspension in molten

NaBr-KBr and reported that mixed Co–Mn catalysts can be optimal candidates for methane pyrolysis in molten salts owing to their fast kinetics and stability. The authors reported that increasing the ratio of molar Co–Mn from 0 to 2 improved the conversion of CH4 from 4.8% to 10.4% at 1273 K for the smallest catalyst particle size range, which shows that closeness between the catalytic surface and the gas phase can improve conversions. Karaismailoglu et al. [46] investigated the effect of the doping of yttria on a nickel catalyst synthesized by the sol–gel citrate method and reported CH4 conversion of 50% with this type of catalyst. It was reported that the addition of Yttria can improve the stability and activity of catalysts at elevated temperatures and that a lower nickel ratio in the catalyst reduces the formation of carbon. Not only experimental studies but also systematic approaches using process simulations and works for economic feasibility have been reported. Chen et al. [47] designed the vacuum promoted methane decomposition with carbon separation (VPMDCS), which include a reactor of MP continuously generating H2 and a C separation reactor converting carbon into CO. It was reported that VPMDCS showed CH4 conversion of 99.2% and produced high-purity H2 and CO with concentrations of both 99.6%. By economic analysis, the unit hydrogen cost of EUR 5.4 kg−<sup>1</sup> was reported. Riley et al. [48] simulated two concepts of CMP that used H2 combustion and CH4 combustion by Aspen Plus® comparing CO2 emissions and H2 production cost. It was revealed that the quality of produced C and its selling price are major factors in H2 selling price, and H2 production cost in the capacity of 216 ton d−<sup>1</sup> is less than USD 3.25 kgH2 <sup>−</sup><sup>1</sup> without considering the sale of C. Perez et al. [49] designed an MP process using a quartz bubble column including molten gallium, which is used for catalyst and heat transfer agents, with a porous plate distributor. The authors found that a maximum CH4 conversion of 91% was achieved at a reactor temperature of 1392 K where gallium occupied 43% of the total reactor volume with a residence time for a bubble of 0.5 s. Additionally, by techno-economic analysis, it was concluded that a molten metal system can be competitive with SMR if a CO2 tax of EUR 50 ton−<sup>1</sup> is imposed and produced C is marketed. Kerscher et al. [50] designed two concepts of MP using electron beam plasma, which was generated from renewable electricity. The techno-economic assessment reported that levelized costs of H2 for the electron beam plasma method ranged from 2.55 to 5.00 € kgH2 <sup>−</sup>1, and CO2 emission ranged from 1.9 to 6.4 kgCO2 eq. kgH2 <sup>−</sup><sup>1</sup> from a carbon footprint assessment, which shows a high potential for reducing life cycle emissions. Zhang et al. [51] investigated the CO2 mitigation costs of CMP and the integrated power generation process in a fuel cell comparing a combined-cycle gas turbine power plant system with and without CCS. It was revealed that CMP shows low life cycle emissions per unit of electricity output of 0.13 tCO2 eq MWh−<sup>1</sup> but shows a high levelized cost of electricity of EUR 177 MWh<sup>−</sup>1, concluding that it has high potential when assumed that produced C can be sold at current prices. Timmerberg et al. [52] assessed the levelized hydrogen production costs and life cycle greenhouse gas (GHG) emissions from MP in three systems where molten metal, plasma, and thermal gas reactors were used. It was reported that the plasma-based system using electricity from renewable sources shows the lowest emissions of 43 gCO2 MJ<sup>−</sup>1, and the molten metal and thermal gas system shows relatively higher GHG emissions due to the additional combustion and natural gas supply chain.

Even though many types of research have been conducted on the concepts of TMP and CMP, very few studies revealing both technical and economic viability of those technologies are reported, to the best of our knowledge. Therefore, in this study, a preliminary technoeconomic parametric study is conducted to comprehensively investigate the feasibility of the concept of methane pyrolysis (MP). Firstly, a process simulation using Aspen Plus® for various MP processes, namely TMP and CMP, and with additional carbon gasification (TMPG and CMPG) are performed with detailed reaction kinetics under various technical parameters of reaction temperature, ratio of fuel combusted, and ratio of reactants for gasifier (C-Air-H2O) (Figure 1). Based on the technical performance from the process simulation, yields of H2 and C, and the amount of fuel required to supply heat to the MP reactor and gasification unit are obtained, and then, economic feasibility in terms of unit H2

production cost is reported. In addition, to suggest future economic guidelines of this novel concept when this is commercialized, sensitivity and scenario analysis regarding various H2 production scales and different C selling price scenarios are conducted revealing the cost competitiveness compared to the conventional H2 production methods of SMR and SMR with CCS.

**Figure 1.** Schematic diagram of techno-economic parametric study for investigated systems for methane pyrolysis (MP).

#### **2. Methods**

#### *2.1. Process Simulation*

In this study, four systems for MP, classified as TMP-S1, CMP-S2, TMPG-S3, and CMPG-S4, were simulated in Aspen Plus® (Aspen Technology, Inc., Bedford, MA, USA) with detailed reactor validation based on kinetics reported by Keipi et al. [53] for TMP and Kim et al. [54] for CMP. As a result, Figure 2 shows the closeness of methane conversion between experimental and simulated methods at each investigated operating conditions validating proper insertion of reported kinetics to Aspen Plus®.

**Figure 2.** Results of kinetic validation for (**a**) thermal methane pyrolysis (TMP) and (**b**) catalytic methane pyrolysis (CMP) reactions.

For all systems, CH4 entered the validated reactor in different temperature ranges of 1073–1373 K for TMP-S1 and TMPG-S3, and 1023–1173 K for CMP-S2 and CMPG-S4, then, product stream containing remained CH4 and produced H2 and C passed through the units of cyclone and pressure swing adsorption (PSA) for separating solid C and H2, respectively (Figure 3). We assumed the pressure drop of cyclone as 0.01 bar and number of cyclones as only one and assumed separation efficiency of PSA as 100%. Especially for CMP-S2 and CMPG-S4, purified C entered the gasification unit to produce additional H2 and carbon monoxide (CO) with different ratios of C, air, and water (H2O) (1:1:1, 1:1:2, 1:1:3, 1:2:1, and 1:3:1), and water-gas shift (WGS) (Equation (2b)) reactor was followed to convert the produced CO to H2. Additionally, for the heat supply system, various heat supply scenarios were assumed and classified as 100% electricity-based and different ratios of H2 combusted (0%–100% matched with 100%–0% CH4 combusted). Based on the result of the process simulation, material balance was obtained at the temperature of 1273 K for TMP-S1 and TMPG-S3 and 1173 K for CMP-S2 and CMPG-S4, with the ratio of H2 combusted of 40%, and the ratio of reactants for the gasifier of 1:1:2 (C-Air-H2O) (Table 1).

**Figure 3.** Block flow diagrams for methane pyrolysis (MP) systems of (**a**) thermal methane pyrolysis (TMP-S1), (**b**) catalytic methane pyrolysis (CMP-S2), and systems with additional gasification and WGS reaction of (**c**) TMPG-S3 and (**d**) CMPG-S4.

**Table 1.** Material balance for methane pyrolysis (MP) systems of (**a**) thermal methane pyrolysis (TMP-S1), (**b**) catalytic methane pyrolysis (CMP-S2), and systems with additional gasification and WGS reaction of (**c**) TMPG-S3 and (**d**) CMPG-S4.



**Table 1.** *Cont.*
