The shipping industry has reached a higher level of maturity in terms of its knowledge and awareness of decarbonization challenges. Carbon-free or carbon-neutralized green fuel, such as green hydrogen, green ammonia, and green methanol, are being widely discussed. However, little attention has paid to the green fuel pathway from renewable energy to shipping. This paper, therefore, provides a review of the production methods for green power (green hydrogen, green ammonia, and green methanol) and analyzes the potential of green fuel for application to shipping. The review shows that the potential production methods for green hydrogen, green ammonia, and green methanol for the shipping industry are (1) hydrogen production from seawater electrolysis using green power; (2) ammonia production from green hydrogen + Haber–Bosch process; and (3) methanol production from CO₂ using green power. While the future of green fuel is bright, in the short term, the costs are expected to be higher than conventional fuel. Our recommendations are therefore as follows: improve green power production technology to reduce the production cost; develop electrochemical fuel production technology to increase the efficiency of green fuel production; and explore new technology. Strengthening the research and development of renewable energy and green fuel production technology and expanding fuel production capacity to ensure an adequate supply of low- and zero-emission marine fuel are important factors to achieve carbon reduction in shipping.

The high-pressure SCR system is suitable for low-speed marine engines that burn high-sulfur fuel, but it will cause the increase of the exhaust back pressure of main engine, which will affect the main engine’s performance and fuel consumption. At the same time, due to the large weight and size of the SCR reactor, the overall heat storage performance of the SCR reactor is large, which makes the temperature difference between the front and back of the turbine in the start-up and shutdown of the main engine and transient conditions, affecting the transient response performance of the turbine. In this paper, 6S46ME marine low-speed diesel engine and its high-pressure SCR system are taken as the research object, the co-simulation model is constructed by using GT-Suite coupled with a programming software, and the accuracy of the test model is verified. The influence of high-pressure SCR system on the performance of main engine is analyzed, and it finds that the fuel consumption of the main engine at low load increases significantly greater than that at high load, and the exhaust temperature of the main engine changes differently at different loads. On this basis, the co-simulation model was used to analyze the matching performance of high-pressure SCR system under the transient conditions such as the main engine Tier II/Tier III mode switching process, fast-loading, normal-loading and fast-unloading. The results show that the slow cut-in/cut-out of SCR reactor is beneficial to the stable operation of the main engine system, and the cut-in/cut-out time of high-pressure SCR system greatly affects the transient fuel consumption of the main engine, and the maximum increase of fuel consumption is about 3.6 g/kW·h; In addition, the thermal inertia of the reactor has a greater impact on the performance of the main engine at low load, but not at high load.

Injection timing variations have a significant effect on the performance and pollutant formation in diesel engines. Numerical study was conducted to investigate the impact of injection timing on engine performance and pollutants in a six-cylinder turbocharged diesel engine. Diesel fuel with different amounts (5%, 15%, and 25% by volume) of n-butanol was used. Simulations were performed at four distinct injection timings (5°, 10°, 20°, 25°CA bTDC) and two distinct loads of brake mean effective pressure (BMEP = 4.5 bar and 10.5 bar) at constant engine speed (1800 rpm) using the GT-Power computational simulation package. The primary objective of this research is to determine the optimum injection timing and optimum blending ratio for improved efficiencies and reduced emissions. Notable improvements in engine performance and pollutant trends were observed for butanol-diesel blends. The addition of butanol to diesel fuel has greatly diminished NO_X and CO pollutants but it elevated HC and CO₂ emissions. Retarded injection timing decreased NO_X and CO₂ pollutants while HC and CO₂ emissions increased. The results also indicated that early injection timings (20°CA bTDC and 25°CA bTDC) lowered both CO₂ and unburned hydrocarbon emissions. Moreover, advanced injection timing slightly improved brake thermal efficiency (BTE) for all engine loads. It is concluded that retarded injection timing, i.e., 10°CA bTDC demonstrated optimum results in terms of performance, combustion and emissions and among the fuels 15B showed good outcome with regard to BTE, higher heat release rate, and lower pollution of HC, CO, and NOx.