(353a) Closing the Carbon Cycle in the Shipping Industry

The shipping industry is highly dependent on fossil-based fuels like heavy fuel oil (HFO), and marine diesel oil (MDO), causing around 2.5% of the global GHG emissions [1]. Hence, keeping its emissions low is key to ensuring the sustainable global trade of goods [2]. Notably, decarbonization efforts set by the International Marine Organization anticipate a GHG-reduction of 70% by 2050 [3]. Hence, adopting alternative fuels i.e., low carbon CO₂-based fuels or zero-carbon fuels, is imperative. In this context, ammonia, hydrogen, and electricity are zero-carbon fuels that hold a promise to halt the use-phase CO₂ emissions from the shipping industry. However, they require new engines and retrofits, which are still under development [4]. Consequently, precedence should be given to CO₂-based fuels.

Motivated by the use of CO₂-based fuels, this work applies circular economy principles in the shipping industry to reduce CO₂ emissions drastically. Notably, since CO₂-based fuels will still emit CO₂ at their use phase, we investigate a new way of reducing the combustion CO₂ emissions by coupling the shipping industry with carbon capture and utilization to close the carbon loop in marine fuels. Following this approach, CO₂ generated on the ship from the engine and steam generation is captured and stored on-board while the ship is moving. Upon arrival at the next port, the CO₂ is unloaded and sent to a chemical plant located near the port. At the chemical plant, CO₂ reacts with renewable hydrogen, generating a CO₂-based fuel which will then be sent to the port to be bunkered by the arriving ships. In this way, the carbon cycle is closed.

A wide range of low-carbon fuels potentially suitable to decarbonize the shipping industry has been identified. The most well-known alternatives to replace HFO suitable for the concept of circular marine fuels are CO₂-based methanol and synthetic natural gas (SNG). Notably, for a CO₂-based fuel to be appropriate for usage in ships, it should contain a high specific energy, low production cost, and should be easily scalable. At the same time, available bunkering infrastructure in most ports will be necessary. The analysis considers three scenarios; CO₂-based methanol for propulsion and steam generation, CO₂-based methanol for propulsion and natural gas for steam generation, and synthetic natural gas for propulsion and steam generation.

The carbon capture technology of choice is post-combustion capture with monoethanolamine (MEA) as the amine-based solvent. For all the scenarios, the process flowsheets were developed in Aspen Hysys v.11. For the carbon capture part, the composition and flow rate of the engine exhaust were calculated using the lower heating value of each fuel and considering a 23000 kW engine. The two streams of flue gases, one coming from the engine exhaust and one from the furnace, are mixed, cooled, pumped, and fed to the post-combustion capture section, which captures CO₂. The fuel demand for the furnace were calculated from the steam requirements in the stripper column. Lastly, the CO₂ is liquefied and stored on-board.

For the liquefaction of CO₂, an ammonia refrigeration cycle is included for the methanol-fueled ship. In contrast, CO₂ is cooled down in the evaporator that heats up SNG for the natural gas fueled ship, while for additional cooling, the ammonia refrigeration cycle is used. For the fuel production plants, methanol production was based on González-Garay et al. [5], and synthetic natural gas on Chauvy et al. [6]. The fuel production of the chemical plants was based on the marine fuel demand of Hamburg's' port.

An economic and environmental assessment was performed as well as a technical feasibility analysis for all the scenarios. For the technical feasibility analysis, the weight and space taken by the equipment added on-board the ship for a week trip were considered and compared to the actual cargo weight (DWT – deadweight tonnage) and space (TEU – twenty-foot equivalent units). In order for these scenarios to be feasible and profitable, the added equipment should not take up most of the space of the cargo, hindering their purpose. The percentage that added equipment takes is up to 5% of the cargo. Regarding the economic assessment, the total cost of each scenario includes capture on-board and fuel production, covering capital and operating expenditures.

The environmental assessments were implemented in SimaPro v9.2.0.2 software [7], following the ISO 14040/14044 standards. The inventories integrate data of the foreground and background systems. For the background system, data available in Ecoinvent v3.5 were used [8]. The mass and energy flows in the foreground system were taken from process simulations complemented with literature data. For the production of the steam generation fuel used in the furnace, CO₂ from direct air capture was modelled according to Keith et al. [9]. The hydrogen used in the chemical plants is produced from polymer electrolyte membrane electrolysis powered by offshore wind electricity [10]. The inventory of the ship construction was also taken from Ecoinvent v.3.5. The goal of the LCA was to compare CO₂-based methanol and SNG as circular marine fuels with the business-as-usual (BAU) based on IPCC 2013 GWP 100a impact assessment method. The scope of the analysis is cradle-to-cradle, and the functional unit (FU) is 1 tonne ·kilometer.

Our results show that the proposed concept can be technically and environmentally feasible, whereas much more economical compared to other proposed decarbonisation scenarios. More precisely, concerning technically feasible, we find that additional equipment will only take up a small percentage of the actual cargo transported. Moreover, regarding the environmental results, CO₂-based methanol for propulsion and steam generation can generate the most emission reductions. Lastly, considering the costs, we find that the cost of capture on-board is minimal in comparison to the production of the fuel, where the economic feasibility depends strongly on the cost of electrolytic hydrogen. Our work opens up a new pathway for tackling the shipping industry's emissions by coupling carbon capture offshore with utilization onshore.

Acknowledgments: This work was created as part of the NCCR Catalysis, a National Centre of Competence in Research funded by the Swiss National Science Foundation.

References

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