Optimum liquefaction fraction for boil-off gas reliquefaction system of semi-pressurized liquid CO2 carriers based on economic evaluation

This investigation studied the optimum reliquefaction fraction of a boil-off gas reliquefaction system for semi-pressurized liquid CO2 carriers. To prevent the overpressure of cargo tanks, the boil-off gas should be properly treated in suitable ways: to be vented into the atmosphere or to be liquefied in a dedicated liquefaction system. Because the former accompanies the penalty of carbon tax and the latter requires energy consumption, there is an optimum reliquefaction fraction minimizing the total treatment expense. To estimate the optimum reliquefaction fraction, the total treatment expense was composed of the three terms: the reliquefaction expense, the emission penalty, and the benefit of CO2 sale, which were expressed by the reliquefaction fraction and other variables. The reliquefaction expense increased rapidly with the reliquefaction fraction, while the other two showed a linear dependence. Even though each term of the total treatment expense varied with the crude price, the CO2 sale price, and the carbon tax, the optimum reliquefaction fraction was found to stay within a narrow region, roughly between 95.7% and 98.2% for all the plausible scenarios for the variables.

► The boil-off gas (BOG) could be hazardous to the cargo containment system for CO2 carriers.
► A new reliquefaction unit to treat BOG is designed.
► The total treatment expense is a combination of the reliquefaction expense, emission penalty, and benefit of CO2 sale.
► The optimum reliquefaction fraction can be founded by the derivative of the equation of the total treatment expense.