(2ck) Dual-Modal Flexible Operation of on-Site Hydrogen Refueling Station

The direct use hydrogen as a fuel in transportation sector is expected to mitigate greenhouse gas (GHG) emissions which account for the majority of the total emissions. While hydrogen fuel cell electric vehicles (FCEV) as an end-user have reached a stage of real deployment from technical and economic perspectives, hydrogen infrastructure, from hydrogen production to distribution, is not fully developed to meet a huge demand in transportation sector. In this study, we propose an energy efficient and economic hydrogen refueling station, which includes two distinct hydrogen production processes for a flexible dual-modal operation: steam methane reforming (SMR) and autothermal reforming (ATR) processes. Integrating SMR and ATR as a base and flexible production option, respectively leads to the reduction of a hydrogen storage capacity of hydrogen refueling station. We first developed a simulation model of the proposed process using a simulator from natural gas processing, synges production, hydrogen purification and hydrogen storage. We then developed a new optimization model to identify the optimal design and operating strategies by adjusting the capacity of SMR and ATR production as well as detailed production scheduling of ATR according to demand profiles. In addition, a techno-economic evaluation was performed to analyze the CAPEX and OPEX of the proposed process, compared to the conventional hydrogen production process using SMR or ATR. We confirmed that the hydrogen storage cost can be remarkably reduced through the flexible operation of the proposed process, thereby leading to lower hydrogen supply cost than two others.

One of most important global environmental issues is the fuel depletion and climate caused by fossil fuel-based energy systems. The development of a sustainable energy system has been one of urgent global issues, by developing promising alternatives of a fossil fuels-based energy system. Especially, to address the greenhouse gas (GHG) emissions from the transportation sector, hydrogen has been regarded as the most attractive alternative to fossil fuel in transportation sector. The direct use of hydrogen in the transportation sector is expected to contribute to solving environmental problems resulting from the use of fossil fuels, despite relatively low economics. In order to build a hydrogen economy in the transportation sector, the hydrogen infrastructure, including fuel stations for hydrogen supply, should be accompanied. The current hydrogen infrastructure consists of two distinct routes: transporting hydrogen from centralized hydrogen production process, and on-site production in distributed hydrogen fuel stations. The most widely used technology for mass hydrogen production is steam methane reforming (SMR). However, In the case of a distributed hydrogen fuel station, since SMR has a constant production rate due to the technical difficulties on repeated startup and shutdown, a large storage tank capacity is required to respond to changes in fuel demand over time. In this work, we propose a novel hydrogen production process by combining ATR and SMR, which enable a flexible operation of the distributed hydrogen fuel station. Especially, the cyclidic operation of ATR that repeats continuously a start-up and shut-down with a short make-ready time enables the on-site process to manipulate the production rate according to a temporal demand profile.

As the first step, this study developed process models using Aspen plus V9: SMR based hydrogen production, ATR based hydrogen production and combined SMR and ATR hydrogen production. All the processes are assumed to use natural gas as a feed from a gas grid. In order to supply directly to the fuel station, hydrogen purity is targeted >99.999 % and the production rate is 1200 kg per day. The proposed hydrogen production process includes the main technologies of; 1) the reforming and shift technologies for hydrogen production, 2) CO₂ capture process using monoethanolamine (MEA), 3) hydrogen pressure swing adsorption (PSA) process. In all the process schemes, pre-reforming technology is adopted to convert C₂-C₅ range hydrocarbon in LNG to syngas. SMR and ATR technologies are well-known process for producing hydrogen from natural gas. The CH₄-rich gas mixture from pre-reforming process is fed to the SMR and ATR reactor. After the reforming process, the stream contains CO, which can be converted to hydrogen using the water gas shift (WGS) technology. To maximize hydrogen production using CO, a two-stage WGS reactor is considered: high- and low- temperature WGS. The operating condition of reforming and shift reactor is determined using process optimization algorithm where the objective function is to maximize hydrogen production rate. In the separation and purification technologies, the amine-based CO₂ absorption and PSA are adopted for hydrogen purification process. The CO₂ absorption process contributes to the environmental performance of the process by capturing CO₂ generated by the WGS process. The absorption process recovers 85% of the CO₂ contained in the stream after the shift reaction. In order to ensure the target purity of hydrogen, the PSA process is adopted. Hydrogen from PSA process is compressed to 900 bar for high-pressure hydrogen storage or directly supply to vehicle.

As the second step, we developed a new optimization model to determine the capacities of SMR and ATR as well as the optimal scheduling and dispatching of ATR using a mixed integer linear programming (MILP). To minimize the total CAPEX of the on-site hydrogen fuel stations, including the hydrogen storage cost, the operation of ATR is flexibly adjusted according to a hydrogen demand profile; for instance, ATR should be fully operated during the daytime when the demand is high. Basically, SMR-based hydrogen production contributes to 60% of the total hydrogen production, and the ATR is set to produce the rest from 9 am to 6 pm (10 hours). The hourly hydrogen demand was calculated based on the energy consumption of the current vehicle fuel demands. The required capacity of the hydrogen storage tank was calculated using the hourly hydrogen production and demand data, ant it was assumed that the cost of 1540 $ per kg was consumed.

As a result, the flexible operation of the proposed process was analyzed to consume 11 kmol/hr of LNG for a daytime operation and 5 kmol/hr for a nighttime. By using such LNG as a feed, the proposed process produces 39 kmol/hr during the daytime and 16 kmol/hr at nighttime, which well corresponds to the real demand profile. The techno-economic and environmental performance evaluation was also performed. The result reveals that the unit production cost of the proposed process is lower by around 10% than the compared SMR and ATR-based processes.