Marine selective catalytic reduction (SCR) technology is an efficient NO _X removal technology, which has been widely applied in shipbuilding industry. However, the marine SCR technology also needs to be adjusted with the continuous improvement of the environmental requirements and the changes in marine energy structure. Nowadays, most marine SCR technology uses Urea-SCR technology, which is greatly affected by impurities in the marine fuel oils and the temperature of the exhaust. The high level of sulfur and alkali metals in the marine fuel oils has significant toxicity on catalysts, limiting the use of marine SCR. The diesel engine that mainly used on ships determines the range of the temperature change of the exhaust, which finally affects the denitrification of the exhaust gas through the influence on the effect of urea hydrolysis to produce ammonia, the layout of the SCR rectors and the catalyst activity. The above situation indicates that the design of an LP-SCR system with exhaust gas heating will be an important direction of the design of the SCR for the ships using marine fuel oils as fuel. It is critical to find safe source or method of supplying ammonia with high production efficiency and precise control for the development of marine SCR systems. Improving the fuel quality and reducing the toxicity effect in the smoke can provide a foundation for the research and development of new low-temperature and anti-toxic catalysts. The low-carbon and zero-carbon fuels become a global trend due to carbon neutrality, resulting in different exhaust emissions after combustion and then the change of SCR control technology. The zero-carbon and low-carbon fuels have single and reducible composition, and have the potential to be SCR reducing agents. The integrated processing SCR technology that uses the fuels itself for NO _X removal will become an important direction for the development of marine SCR under the goal of carbon neutrality.

The application status of main marine refrigeration units and the refrigerant has been analyzed in order to promote the process of the ship greenhouse gas emission reduction. It summarizes the usage and restrictions of the marine refrigerants by the environmental labels of the classification societies, the development of the marine refrigerants and the marine low global warming potential (GWP) refrigerant with application potential. The results show that the main marine refrigeration system currently uses the steam compression refrigeration, and the commonly used refrigerants are R134a, R404A, R410A, R407C and R407F. The environmental labels of CCS, DNV GL, ABS, BV and LR require the Ozone depletion potential (ODP) of the marine refrigerants to be zero, and the environmental labels with the higher level require the GWP of the marine refrigerants less than 2 000. Special consideration of flammability and toxicity are required for the refrigerants with flammability and toxicity. The marine low GWP refrigerants mainly include the hydrous ferric oxide (HFO) refrigerant R1234yf, the HFO mixed refrigerant series of R449A, R513A and R454, and the natural refrigerants of NH ₃ and CO ₂. The results can be used as references for the design and study of the marine refrigeration system.