International Journal of Greenhouse Gas Control 10 (2012) 46–55 Contents lists available at SciVerse ScienceDirect International Journal of Greenhouse Gas Control j our na l ho me p age: www.elsevier.com/locate/ijggc Optimum liquefaction fraction for boil-off gas reliquefaction system of semi-pressurized liquid CO 2 carriers based on economic evaluation Bongsik Chu, Daejun Chang ∗ , Hyun Chung Division of Ocean Systems Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea a r t i c l e i n f o Article history: Received 9 June 2011 Received in revised form 16 May 2012 Accepted 21 May 2012 Available online 13 June 2012 Keywords: CO2 carrier Boil-off gas reliquefaction system Reliquefaction fraction Total treatment expense Optimization a b s t r a c t This investigation studied the optimum reliquefaction fraction of a boil-off gas reliquefaction system for semi-pressurized liquid CO 2 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 CO 2 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 CO 2 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. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction The climate change arising from increases in CO 2 emissions has recently become a major concern throughout the world. The International Energy Agency (2006) stated that carbon capture and storage (CCS) should be considered as an important option to reduce green house gas emissions. Even though geological for- mations are indispensible for CCS, public anxiety over potential leaks due to stored CO 2 makes it difficult to locate onshore storage sites that are acceptable to the public. Naturally, seabed geological formations have arisen as promising storage candidates. For transportation of the green house gas to the seabed sites, there are two options: ship-based and pipeline-based CCS. The latter is suitable to handle a large amount of CO 2 without inter- mediate storage. Its capital investment is much higher than the former when the distance from CO 2 source to storage is stretched (Svensson et al., 2004). Safety and economic considerations for pipeline transport have been well studied by many researchers (Skovholt, 1993; Gale and Davison, 2004). The ship transporta- tion is more flexible than pipeline transportation in terms of transportation capacity and storage location. Ozaki and Ohsumi (2011) proposed the hub-type CCS by shuttle ships and socket buoys. They focused their attention on flexibility of ship-based ∗ Corresponding author. Tel.: +82 42 350 1514; fax: +82 42 350 1510. E-mail address: djchang@kaist.edu (D. Chang). CCS for plan change such as route, injection capacities, water depth, and transport distance. The FINNCAP project, though it was canceled recently, conducted a comparative feasibility study, leading to the conclusion of transportation by ships of 20,000 m 3 with approximately 2000 km of transportation distance (Rauramo, 2010). They assumed that ship transportation was suitable for long transportation distances and in the case of using CO 2 as enhanced oil recovery (EOR) with multiple offshore storage sites. Yoo et al. (2011) presented a new CCS system integration with CO 2 carrier and liquefaction process. They expected that CO 2 shipping for large amounts of CO 2 transport could be more competitive than pipeline transportation in the new total CCS chain consisting of capture, liquefaction, ship transportation, injection, and storage. Cost effectiveness is the key to the realization of commercial scale CCS. In their extensive study on carrier-based transportation, Aspelund et al. (2006) considered the supply chain from onshore liquefaction to offshore unloading and analyzed the chain in view of cost, energy utilization, exergy and CO 2 emission. They indi- cated that the triple point was the optimum condition for the supply chain, and the energy requirement over the whole chain was 142 kWh/ton-CO 2 with a transportation cost of about 25 $/ton-CO 2 for a 1500 km roundtrip distance. Aspelund and Gundersen (2009) proposed a novel transport chain for liquefied CO 2 , liquefied natu- ral gas, and liquefied nitrogen including an offshore installation, a combined gas carrier, and an onshore integrated receiving terminal. In their concept, the cold exergy both in the offshore and onshore processes was utilized to make the whole chain energy-efficient 1750-5836/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2012.05.016