(310f) A System-Level Analysis of the Toluene-Methylcyclohexane System for Long-Distance Hydrogen Transport: Australia-Korea As a Case Study

Fossil fuels, which have been primary energy source in recent decades, have caused environmental issues such as global warming and resource depletion. To replace fossil fuels, the commercialization of renewable energy has been actively underway. However, there is a temporal and spatial discrepancy between the demand and supply of renewable energy, which leads to energy loss and increases the environmental impact and the cost of renewable energy. Additionally, the energy production rates vary over time (e.g. solar and wind). These issues need to be addressed to establish renewable energy as a major energy source that can replace fossil fuels. Therefore, a proper energy carrier is required.

H₂ is one of the viable solutions for energy carriers due to its free of CO₂ emission and relatively high gravimetric energy density (120 MJ/kg). However, the low volumetric energy density of H₂ (10.9 kJ/L) presents an obstacle to economical transportation under ambient conditions. Therefore, a proper storage method is required. H₂ storage methods are divided into physical and chemical storage. Physical storage such as high-pressure compression (350~700 bar) or cryogenic liquefaction (-253 K) is simple and reliable but requires relatively morer energy than chemical storage. Liquid organic hydrogen carriers (LOHCs) are potential H₂ storage methods as they can chemically store H₂ with high H₂ storage capacity. Furthermore, LOHCs are liquid at ambient conditions, allowing them to be transported with relatively little energy consumption. Additionally, since LOHCs have similar properties to petroleum, pre-existing petrochemical infrastructures can be utilized for LOHC transportation.

Among the LOHC candidates, such as toluene (TOL), methanol, and dibenzyltoluene, TOL–methylcyclohexane (MCH) system was selected for this study because of its high stability, high hydrogen storage capacity (6.2%), relatively low toxicity, and affordable price. In this study, we designed a looping process consisting of hydrogenation, transportation, and dehydrogenation parts. However, TOL and MCH have high volatility, and the accumulative byproducts occur during hydrogenation and dehydrogenation reactions. To address these drawbacks, an extraction column and pressure swing adsorption. Subsequently, the energy requirement of the designed process was reduced by integrating the heat networks.

Furthermore, the designed process was comprehensively analyzed at the system-level to investigate the economic and environmental viability, using the example of transporting H₂ from Australia to Korea. Techno-economic analysis was carried out to determine the major cost driver and the minimum H₂ transportation cost (MHTC), which is the cost where the net present value of the designed process is zero. It was found that the dehydrogenation part, accounting for 61.8% of the total cost, is more cost-intensive than the hydrogenation part, accounting for 36.3% of the total cost. Specifically, 76% of hydrogenation and 54% of dehydrogenation costs are derived from their reaction subsystems. For hydrogenation, the high cost of the reaction subsystem is due to the need to make up for the TOL loss that occurred during the separation of chemically similar benzene, while for dehydrogenation, it is due to the use of expensive noble metal catalyst. Moreover, MHTC was also investigated to be $2.08/kg-H₂. The influence of the economic and environmental parameters was investigated through sensitivity analysis, and the price stability was stochastically investigated using Monte Carlo simulation. Lastly, the environmental impacts (EIs) of the three parts were quantified using life-cycle assessment. Since the dehydrogenation part is more energy-consuming, it has generally larger EI than the hydrogenation part. However, the hydrogenation part has 35% higher human toxicity, 14% higher ozone depletion, and 39% higher fossil depletion mainly due to the H₂ and TOL makeup. Furthermore, changes in EI depending on the energy sources are compared. With nuclear or renewable energy, EIs related to climate change and fossil depletion can be reduced by up to 96% and 53%, respectively. The outcomes of this study can offer insights for improving and making informed choices regarding H₂ supply chains, thus contributing to the revitalization of the H₂ economy.