**4. Conclusions**

The promising alternative clean concept of H2 production of methane pyrolysis (MP) was technically and economically investigated by preliminary techno-economic analysis consisting of a process simulation using Aspen Plus®, itemized cost estimation, and sensitivity and scenario analysis for various parameters such as temperature, the ratio of fuel (CH4 and H2 produced from MP) combusted, and the ratio of reactants for C gasification

(C, Air, and H2O). To investigate various H2 production scenarios, thermal and catalytic methane pyrolysis (TMP-S1 and CMP-S2) and systems with additional H2 production processes composed of C gasification and WGS reaction (TMPG-S3 and CMPG-S4) were considered in this study. The results of the process simulation indicated that reaction temperature is the most influential process variable for determining the technical performance of all investigated systems, and catalyst-based MP (CMP-S2 and CMPG-S4) showed much lower net H2 and C production rates than thermal-based systems (TMP-S1 and TMPG-S3), where theoretical maximum yields were obtained, due to different kinetics used in the simulation. In addition, the amount of H2O added in TMPG-S3 and CMPG-S4 was reported as the most important factor to increase the amount of H2 produced. For an aspect of fuel consumption estimated from the amount of heat required for each system, trends similar to the net H2 and C production were obtained showing the amounts of heat of 3425–11,079 cal s−<sup>1</sup> for TMP-S1, 3016–9246 cal s−<sup>1</sup> for CMP-S2, 725–21,245 cal s−<sup>1</sup> for TMPG-S3, and 744–18,945 cal s−<sup>1</sup> for CMPG-S4 matched with the required fuel amounts of 0.068–0.219, 0.060–0.183, 0.014–0.420, and 0.015–0.375 kmol h−<sup>1</sup> and 0.223–0.723, 0.197–0.603, 0.047–1.386, 0.049–1.236 kmol h−<sup>1</sup> for CH4 and H2, respectively. From the itemized cost estimation for each system at 1273 K for TMP-S1 and TMPG-S3 and 1173 K for CMP-S2 and CMPG-S4, with 40% H2 combusted, and the ratio of 1:1:2 for C-Air-H2O, unit H2 production costs of USD 2.14, 3.66, 3.53, and 3.82 gH2 <sup>−</sup><sup>1</sup> for each system, respectively, were obtained showing a very high portion of the costs of the MP reactor and reactant, and the economic benefits of the carbon (C) selling price. To investigate the effects of each parameter of temperature, the ratio of fuel combusted, and ratios of C, Air, and H2O on economic feasibility, a parametric study was conducted proving the economic benefits of high temperature and the additional H2 production process of C gasification and WGS reaction for CMPG-S4 but not for thermal-based MP process. The importance of the costs of the MP reactor and reactant and the C selling price for economic feasibility was calculated again by sensitivity analysis, where the variation of ±20% was assumed, with variations of unit H2 production cost of 19.1% and 11.1% for C selling price in TMP-S1 and CMP-S2, respectively, and 12.1%–5.6% and 10.9%–3.9% for costs of the MP reactor and reactant, respectively, in all investigated systems. In addition, the effects of H2 production scale for all systems and the C selling price for TMP-S1 and CMP-S2 on the unit H2 production costs were investigated suggesting that all systems can compete with SMR with CCS, and especially TMP-S1 and CMP-S2, even with the pessimistic 50% reduced C selling price; for TMPG-S3, economic competitiveness with the commercialized H2 production method of SMR can be achieved when an H2 production scale larger than 1000 Nm<sup>3</sup> h−<sup>1</sup> is assumed.

Conclusively, the techno-economic feasibility of MP processes, classified as the four systems of TMP-S1, CMP-S2, TMPG-S3, and CMPG-S4, was investigated with detailed H2 and C production rates, the amount of fuel required to supply heat in each system, the trends of unit H2 production cost according to temperature, the ratio of H2 combusted, and the ratio of reactants used in the C gasifier, and revealed key economic parameters of the costs of the MP reactor and reactant and the C selling price. Although several technoeconomic enhancements such as scale-up should be researched further to accomplish the economic competitiveness of MP compared to SMR, the environmental benefits of MP are clearly shown in this study based on its theoretical reaction stoichiometry and trends of its consumption of fuels. Based on the results, the potential of both thermal and catalytic MP for promising H2 production is clearly presented.

**Author Contributions:** Conceptualization, H.L. (Hankwon Lim) and S.C.; methodology, S.C. and M.B.; validation, M.B.; investigation, D.L. and H.L. (Hyunjun Lee); writing-original draft preparation, S.C. and M.B.; writing-review and editing, S.C. and M.B.; supervision, H.L. (Hankwon Lim); funding acquisition, H.L. (Hankwon Lim). All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20203020040010) and supported by the 2021 Research Fund (1.210103.01) of UNIST (Ulsan National Institute of Science and Technology).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Nomenclature**


#### **References**

