*3.2. Parametric Study—Fuel Consumption*

To investigate the amount of fuel combusted to cover the total heat required in each system, the required amount of fuel (H2 and CH4) was obtained (Figures 5 and 6).

**Figure 5.** Amount of fuel (CH4 and H2) consumption for methane pyrolysis (MP) systems of (**a**) thermal methane pyrolysis (TMP-S1) and (**b**) catalytic methane pyrolysis (CMP-S2) in temperature of 1073–1373 K and 1023–1173 K.

Figure 5 shows the required amount of fuel in TMP-S1 and CMP-S2 according to temperature and the ratio of H2 combustion (0%, 20%, 40%, 60%, 80%, and 100%). From the process simulation, the total amount of heat required of 3425, 5739, 10,380, and 11,079 cal s−<sup>1</sup> for TMP-S1 were needed at 1073 K, 1173 K, 1273 K, and 1373 K, respectively, and that of 3016, 3941, 5868, and 9246 cal s−<sup>1</sup> for CMP-S2 were needed at 1023 K, 1073 K, 1123 K, and 1173 K, respectively, showing the higher amount of heat is needed for TMP-S1 than CMP-S2 due to its higher reaction temperature and the technical benefit of the catalyst-based MP.

For TMP-S1, the amounts of CH4 consumption were estimated as 0.07, 0.11, 0.21, and 0.22 kmol h−<sup>1</sup> and the amounts of H2 consumption of 0.22, 0.37, 0.67, and 0.72 kmol h−<sup>1</sup> were obtained when CH4 and H2 covered the total amount of heat required, respectively, showing an increasing trend as temperature increased. Similarly, for CMP-S2, 0.06–0.18 kmol h−<sup>1</sup> of CH4 consumption and 0.20–0.60 kmol h−<sup>1</sup> of H2 consumption were estimated when each type of fuel totally covered the required heat.

**Figure 6.** Amount of fuel (CH4 and H2) consumption for methane pyrolysis (MP) systems of thermal and catalytic methane pyrolysis with gasification and WGS reaction ((**a**) TMPG-S3 and (**b**) CMPG-S4) in temperature of 1073–1373 K and 1023–1173 K.

In Figure 6, the amount of required fuel to supply heat for TMPG-S3 and CMPG-S4 is shown. In TMPG-S3 and CMPG-S4, much larger amounts of heat of 2691–4330, 725–19852, 1047–20,821, and 1464–21,245 cal s−<sup>1</sup> for TMPG-S3 and 2503–3643, 2442–5737, 1925–10,514, and 745–18,954 cal s−<sup>1</sup> for CMPG-S4 were obtained as the temperature increased, showing much greater increase than previous systems due to the additional endothermic process of C gasification. For both systems in particular, the ratio of reactants of 1:1:3 showed the highest amount of heat required compared with other ratios due to its high reaction extent, represented by high H2 and C production rates in Figure 4. In addition, the very high impact of H2O in the amount of heat required was confirmed again with trends of the required fuel at different ratios of reactants for the gasifier.

For TMPG-S3, the amount of CH4 fuel required of 0.01–0.42 kmol h−<sup>1</sup> when it covers total heat required can be replaced by the amount of H2 combusted of 0.05–1.39 kmol h<sup>−</sup>1; for CMPG-S4, the H2 consumption range of 0.05–1.24 kmol h−<sup>1</sup> was estimated to replace the amount of CH4 required of 0.01–0.37 kmol h<sup>−</sup>1.

#### *3.3. Itemized Cost Estimation*

Based on the results from the process simulation, the itemized cost estimation for each MP system using only CH4 and H2 as fuel was conducted to investigate unit H2 production cost 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 3).

**Table 3.** Results of itemized cost estimation 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.


For TMP-S1, unit H2 production cost of USD 2.14 kgH2 <sup>−</sup><sup>1</sup> was estimated considering the capital cost of the MP reactor, PSA, cyclone, and supplement, and the operating cost of reactant, fuel, labor, PSA operating cost, maintenance, and other costs. In this estimation, the costs of the MP reactor and reactant account for 31% and 28% of the production cost with no consideration of the C selling price (USD 4.17 kgH2 <sup>−</sup>1), respectively, showing its high importance in the economic feasibility. For CMP-S2, unit H2 production cost of USD 3.66 kgH2 <sup>−</sup><sup>1</sup> was estimated with additional items related to a catalyst such as the cost of the regenerator and catalyst, and its operating cost. Among economic parameters, it is clear that the costs of the MP reactor and reactant are the most influential economic factors, showing high ratios of 23% and 22% of the production cost without considering the C selling price (USD 5.69 kgH2 <sup>−</sup>1). In both TMP-S1 and CMP-S2, where units of the gasifier and WGS reactor were not constructed in the process simulation, the cost of the MP reactor and reactant and the C selling price have a very high economic impact on H2 production.

For TMPG-S3 and CMPG-S4, additional economic parameters related to the gasification of C and the WGS reaction means that the costs of the WGS reactor, gasifier, and water were considered and compared to both TMP-S1 and CMP-S2. For both systems, the relatively increased unit H2 production costs of USD 3.53 and 3.82 kgH2 <sup>−</sup><sup>1</sup> for TMPG-S3 and CMPG-S4, respectively, were estimated compared to those of USD 2.14 and 3.66 kgH2 <sup>−</sup><sup>1</sup> for TMP-S1 and CMP-S2, respectively. In addition, similar to TMP-S1 and CMP-S2, costs of the MP reactor and reactant were found to be the most influential economic parameters showing portions of 30% and 19%, and 28% and 19% for TMPG-S3 and CMPG-S4, respectively.

Our results indicated that the selling of C can be a very effective way to obtain economic competitiveness through the concept of MP, and showed the importance of the cost of the MP reactor and reactant for economic feasibility.

#### *3.4. Parametric Study—Economic Aspects*

To investigate the effects of the important parameters of reaction temperature, the ratio of H2 combusted to supply the heat required in the process, and the ratio of reactants composed of C, Air, and H2O for the gasifier on economic feasibility, a comprehensive parametric study revealing trends of unit H2 production cost was conducted (Figure 7).

**Figure 7.** *Cont*.

**Figure 7.** Unit H2 production cost for methane pyrolysis (MP) systems of (**a**) thermal methane pyrolysis (TMP-S1), (**b**) catalytic methane pyrolysis (CMP-S2), and the systems with additional gasification and WGS reaction of (**c**) TMPG-S3 and (**d**) CMPG-S4.

In the case of low temperature use where 1073 K for thermal-based systems (TMP-S1 and TMPG-S3) and 1023 K for catalyst-based systems (CMP-S2 and CMPG-S4) were considered, there were no economic benefits in either system. The minimum unit H2 production costs of USD 46.09 kgH2 <sup>−</sup><sup>1</sup> (TMP-S1) and USD 29.48–38.06 kgH2 <sup>−</sup><sup>1</sup> (TMPG-S3) for thermal-based systems and USD 83.40 kgH2 <sup>−</sup><sup>1</sup> (CMP-S2) and USD 43.11–55.56 kgH2 −1 (CMPG-S4) for catalyst-based systems were reported. In addition, for the case of using electricity as the heat source, the cost was slightly higher but almost the same as the lowest cost using fuel combustion for both the thermal-based and catalyst-based system. Therefore, it is advantageous to use electricity as a heat source at low temperatures, but it still seems it would be difficult to gain economic benefits because of the high production costs.

In cases of high temperature, where 1173–1373 K for thermal-based systems and 1073–1173 K for catalyst-based systems were studied, cost reductions as temperature increased were shown. For TMP-S1, as temperature increased unit H2 production costs of USD 2.20–2.29, 2.09–2.21, and 2.10–2.23 kgH2 <sup>−</sup><sup>1</sup> and a cost reduction of 5.03% for each minimum cost were reported, which are much cheaper than the minimum costs for TMPG-S3 of USD 3.40, 3.29, and 3.30 kgH2 <sup>−</sup>1, proving the economic weakness of adopting the additional H2 production processes of C gasification and WGS reaction. Interestingly, the lowest hydrogen production cost was found at temperatures of 1273K, even though it was not the highest temperature. In addition, for the case of using electricity, the cost for TMP-S1 is much higher at USD 2.43, 2.49, and 2.53 kgH2 <sup>−</sup><sup>1</sup> than in the combustion case as temperature increased. However, in TMPG-S3, it shows values of USD 3.78–3.97, 3.69–3.86, and 3.70–3.87 kgH2 <sup>−</sup><sup>1</sup> that are close to the average for combustion; thus, electricity can be beneficial as a heat source depending on the ratio of H2 combusted.

Compared to the trend of unit H2 production cost for the thermal-based system, dramatic cost reductions in CMP-S2 were obtained for the catalyst-based system showing decreased unit H2 production costs of USD 28.77–114.35, 10.17–16.28, 3.43–4.16 kgH2 −1 due to its technical improvement as temperature increased. In addition, except at the temperature of 1173 K, the economic benefit of the processes for C gasification and WGS reaction was reported showing lower minimum unit H2 production cost ranges of USD 16.34–20.52 and 7.02–8.42 kgH2 <sup>−</sup><sup>1</sup> for CMPG-S4 than those of USD 28.77 and 10.17 kgH2 −1 for CMP-S2 at 1073 K and 1123 K, respectively. In the case of electricity as the heat source, CMP-S2 shows costs of USD 29.68, 10.69, and 3.81 kgH2 <sup>−</sup><sup>1</sup> and CMPG-S4 shows costs of USD 17.10–20.95, 7.55–8.55, and 3.93–4.20 kgH2 <sup>−</sup><sup>1</sup> as temperature increased, showing slightly higher costs than the lowest cost using fuel combustion.

From these results, very critical effects, especially for the catalyst-based MP process, of temperature on economic feasibility and the need for the proper adoption of additional H2 production processes can be revealed.
