Techno-economic optimization of onboard carbon capture systems under escalating emission penalties using surrogate models

Onboard carbon capture systems (OCCSs) are emerging as a medium-term response to progressively tightening greenhouse-gas fuel-intensity (GFI) requirements, motivating assessment of appropriate capacity and operation. This study develops a hierarchical multi-period framework that jointly determines one-time OCCS design and annual operation. Gaussian-process-regression surrogates efficiently replace rate-based process simulations, and the framework is applied to a liquefied-natural-gas-fueled very large crude carrier with a 30 wt% monoethanolamine process. On independent test sets, the surrogates achieved coefficients of determination of at least 0.9992 and mean absolute percentage errors of at most 2.01 %. Without surplus-unit trading, the optimum treats 30.0 % of the highest-load main-engine exhaust and matches capture heat demand primarily to recovered waste heat, reducing life cycle cost by 7.9 % relative to compliance without capture. Tightening the GFI trajectory alone does not necessarily increase capture because capture energy also affects attained GFI. When surplus units are traded, their price determines adoption and scale: installation is economical at 304 USD/ton CO2eq and above but not at 266 USD/ton CO2eq and below, while 342 USD/ton CO2eq drives the designs in all three regulatory scenarios to the maximum available main-engine exhaust flow. At 380 USD/ton CO2eq , gross lifetime capture increases from 199 to 634 kton relative to no trading, but net greenhouse gas avoidance increases only from 147 to 325 kton as parasitic emissions rise. Under the assumed immediate-sale surplus-unit market, the prescribed surplus-unit price and net emissions strongly influence OCCS value alongside the target trajectory.