CARBON CAPTURE AND SEQUESTRATION

The effort to reduce the carbon footprint from shipping has led the industry to explore alternative low- and zero-carbon fuels, technologies and methods to increase vessel efficiency. Exhaust gas aftertreatment systems are among the technologies that can contribute to the reduction of greenhouse gases (GHG) and other regulated emissions.

Carbon Capture and Sequestration (CCS) refers to a set of technologies that can be used to remove carbon dioxide (CO 2 ) from vessel exhaust gas or the atmosphere and store it for subsequent use.

Combustion of zero-carbon fuels, such as ammonia and hydrogen, would result in zero CO 2 formation; however, in all other cases of fuels presented in this report — liquefied natural gas (LNG), liquefied petroleum gas (LPG), methanol, bio or renewable diesel and dimethyl ether (DME), — CO 2 will form as a complete combustion product in proportion to the carbon content of the fuel.

Therefore, with all but the zero-carbon fuels, CCS technology could be used on board ships to further reduce their carbon emissions.

CO 2 can be removed either from the exhaust gas of marine engines or directly from the atmosphere, a method often referred to as “direct air capture.” Both technologies are based on the same fundamental principles, but removing CO 2 from the exhaust gas requires less energy because of its higher CO 2 concentration compared to air.

The separation of CO 2 from any stream requires two steps: capture and desorption/regeneration.

During capture, the CO 2 is absorbed into a solid or liquid by contacting the CO 2 source with the absorber. In the desorption/regeneration step, CO 2 is selectively desorbed from the absorber, resulting in a flow of pure CO 2 gas, and the original capture absorber is regenerated for further use 33 .

Over the last 20 years, many research groups around the world have explored CCS technologies to increase the efficiency of the capture and reduce the volume and cost of the systems.

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Due to their present size, the majority of CCS systems have been designed and demonstrated in electric power plants. However, a recent concept study by Mitsubishi Heavy Industries (MHI) focused on installing a marine carbon capture and storage unit on a very large crude carrier (VLCC).

The system comprised four towers for cooling the exhaust, absorbing CO 2 , treating the exhaust and regenerating the CO 2 , in addition to the required liquefaction and storage facilities. The objective of the project was to investigate onboard production of methane or methanol by combining hydrogen from water electrolysis with the captured CO 2.

MHI reported a CO 2 capture rate of about 86 percent, which is expected to improve with further advances of this technology. However, the capital cost required for the CCS system was about $30M, and the cost for the methane or methanol production system was an additional $15M.

This level of investment would require 20 years to recover, making current CCS technology challenging from an economic perspective. In addition to the economic challenge, MHI reported that the size and weight of the system requires the vessel to be redesigned. Each of the four towers of the system is roughly the same size as a scrubber unit, and the weight of the total system exceeds 4,500 tonnes, or nearly two percent of the vessel’s deadweight.

Despite these technical and economic challenges, carbon capture technology still can be an effective way to reduce the GHG emissions of future vessels, especially in combination with low-carbon fuels. Further technical advances are expected to reduce the scale, cost and complexity of CCS technology.