(195e) On the Use of Low Carbon Marine Fuels with Options for on-Board Carbon Capture: A Holistic Evaluation Using Life Cycle Analysis and an Integrated Approach

The shipping industry is under increasing pressure to reduce Greenhouse Gas (GHG) emissions due to increasingly stringent emission regulations worldwide. The International Maritime Organization envisages a 40% reduction of CO₂ emissions per transport work across international shipping by 2030 compared to 2008. In order to comply, technologies involving low-carbon and carbon-free alternative fuels must mature to a point of economic viability and longevity. This study deploys Life Cycle Assessment (LCA) methodologies to evaluate the total environmental impact of carbon capture solutions combined with the use of prominent alternative fuels, namely; Liquified Natural Gas (LNG), methanol, biodiesel and ammonia, while adopting a systems engineering approach for energy optimization and general design considerations, with the objective of supporting the design of low environmental impact ROPAX ships. Stakeholders require such evaluations to maintain an accurate view on the technical performance and limitations of combined technologies focused on GHG emissions reduction.

While currently there is increased interest in decarbonization technologies from the shipping sector, significant research on the technologies themselves and on energy system studies is needed to evaluate possible comprehensive solutions for both the short and long term. Various energy efficiency measures have potential to reduce carbon emissions, however widespread adoption of low- or zero-carbon fuels and energy carriers is critical in order to achieve GHG goals while ensuring longevity of investments. A key advantage of alternative fuels is that they offer energy integration and cogeneration opportunities that are different and often additional to those in conventional fuels. Notably, in the case of LNG there is opportunity for cogeneration and auxiliary firing that one could utilize to support the hotel demands on ships such as ROPAX and ferries with larger service areas on board. Additionally, required conditions of fuel storage are similar to those of CO₂ storage, presenting a unique advantage for energy integration, reduction of methane slip and significant OPEX reductions when carbon capture systems are implemented on board. Considered together, alternative fuels combined with carbon capture technologies have the potential for a near-zero environmental impact ship at a reasonable financial cost. For extensive fuel transition, multiple options in fuel choice must be considered as each presents different opportunities and risks.

LNG presents a promising alternative to Marine Diesel Oil because of its increased availability relatively to other alternative fuels, zero Sulphur emissions and lower carbon emissions. In addition, due to its cryogenic storage it is a viable source of cold energy which can be used in the liquefaction of CO₂ from flue gas, resulting in a lower energy penalty for CO₂ capture and short-term storage on board. Another cost-competitive although potentially hazardous alternative marine fuel is methanol, which features reduced air pollutant and GHG emissions, however on a life-cycle basis the production method of methanol (sources range from coal to biomass) determines the carbon intensity of its use. Combined with carbon capture technologies and green hydrogen, methanol usage has the potential to be part of a circular economy cycle, where its carbon emissions are used to produce more methanol. Ammonia, as a carbon-free alternative fuel, has the potential for significant reduction in GHG emissions, while being a balanced solution in terms of volumetric energy density. The main specific risk of ammonia use as marine fuel, which is included in the impact assessment, is its high toxicity and the potential exposure to humans and to the environment. One of the limitations of alternative fuels in terms of viability is the minimum requirement of retrofitting a preexisting ship. This aspect is eliminated when considering biodiesel fuel as an alternative fuel for marine diesel engines applications. Biodiesel is suitable for wide spread use as it is non-toxic, renewable and biodegradable and while it is associated with an increased production of nitrogen oxide gas, it conversely reduces other toxic gas emissions. In this study, the carbon capture technology chosen was membrane air separation technology, with the final product, after extensive cooling, being liquid CO₂, which is unloaded at the port of destination.

The paper supports a systems approach that considers multiple degrees of freedom, that include the type of alternative fuel, the design and efficiency of the cogeneration process, the engine used, as well as CO₂ capture methods, by making use of data from experts in the maritime and energy industries when available and from literature and computational models when unavailable. Possibly varying or dynamic aspects are included, to illustrate the impact of critical parameters on the available options feasibility and environmental benefit. Design parameters include engine efficiency, air to fuel ratio, combined heat and power generation system use, extend of CO₂ separation, and energy use reduction due to process integration. The proposed approach supports an energy and process integration methodology that considers the ship as a system to integrate in order to minimize CO₂ emissions at different levels of CAPEX consideration. The LCA results function as the main criteria of comparison to evaluate the optimal selection of parameters for minimal environmental impact.

With the objective of maximizing the efficiency of energy resources, methods of heat and power integration in all presented cases of fuels are applied including when CO₂ is captured on board when applicable, resulting in uniquely optimized scenarios for each energy carrier and combined CO₂ capture system, within the same energy network of the ship. The methodology for this analysis combines thermodynamic cascade methods and Pitch Analysis while utilizing the synergy potential for further energy optimization between storage requirements of alternative fuels and carbon capture technology.

The functional unit of the LCA is a sea travel roundtrip in Greece involving a ROPAX vessel, with utilization during the summer months, including maneuvering and at-sea travel which is used as a case study in this paper. A comparative assertion Life Cycle Assessment was conducted between the alternate energy carriers and carbon capture technologies that includes all the sub-processes within the vessel that are identified to relevantly contribute (attributional modelling). All processes throughout the life cycle of fuel use are included in the assessment (cradle to grave) however in the primary data of the analysis the impact of capital goods is considered minimal across the lifespan of the vessel and left out, while the databases used for background secondary data about processes beyond the scope of this study (e.g. fuel production and transport to the ship) include the environmental impact of capital goods. Primary data refer to the storage, integration and use of the proposed alternative fuels within the boundaries of the vessel. The assessment results are the basis for a sensitivity analysis to support identification of the main contributing elementary flows as part of the stepwise improvement of relevant inventory data and uncertainty management, thus allowing for quantification and precision improvement of the study. The tools used for this process were Sima Pro v9.4 and the ecoinvent v3 Life Cycle Inventory database for increased precision in data based on geographical location and in traceability of impact.

Preliminary results indicate that for the use of LNG the adoption of energy recovery and cogeneration methods can lead to an increase in thermodynamic effectiveness of up to 38%, depending on the configuration used. Notably, the application of 2-stroke engines combined with a heat recovery steam generator (the produced steam is used for hotel demands) presents a very efficient scenario with an efficiency improvement of 26%, while also reducing methane slip by 67%. With the additional use of turbines, up to 33,6% of total electricity requirements of the ship can be fulfilled. When considering the capture and liquefaction of CO2 from flue gas by 70% to 80%, the potential for an energy efficient ship with low carbon emissions is evident. Further valorization of energy losses from LNG storage to combustion can lead to even higher overall efficiency of fuel use, provided comprehensive heat integration systems.