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CCS: Taking out the trash

Carbon carriers - a whole new type of fleet

In part three of our carbon capture and storage (CCS) series, Engineering At Sea examines the new types of ship which will be used to carry CO2 to where it can be stored and re-used

So far, gas pipelines feature heavily in most carbon capture (CCS) discussions. In areas where this infrastructure already exists, such as sites of previous oil and gas extraction, it can easily be repurposed to suit a carbon economy.

But tankers and LNG carriers exist for a reason; pipelines are just not practical in many scenarios. The same would be true if humanity were to switch to a carbon-capture economy; a 2011 study into CCS by Daewoo Shipbuilding and Marine Engineering (DSME) and Seoul National University estimated that for any distance over 1,000km, carriage by ship would be the more cost-effective option by far.

CO2 is easier to store than LNG; at the sort of temperatures at which natural gas is liquefied (−160°C), CO2 is a solid, known as dry ice, sublimating (that is, turning directly from a solid into a gas) only when the temperature rises above -78°C.

However, by increasing the pressure, CO2 can be stored as a liquid (LCO2). Though alien-sounding, this is in fact quite a familiar concept – fire extinguishers, for example, store LCO2 under a pressure of 55 bar. A Daewoo Shipbuilding and Marine Engineering (DSME) design from 2009 features vertically-stacked pressure cylinders for CO2, amounting to 100,000m3 of CO2 capacity. Using pressure cylinders would avoid the need to cool the CO2, meaning that it could be carried at ambient temperatures; but overall capacity, proportionate to ship size, would be reduced.

A new horizon

In terms of design, approaches to storing LCO2 differ. Currently, it has almost no capacity; a niche trade up to this point, the very few CO2 carriers in the world, many of them operated by Larvik Shipping and Yara, are limited to 3,600m3 or less.

Recently Denmark’s Evergas and Ultragas agreed to form a joint-venture, Dan-Unity CO2, which would ship CO2 using a specially-designed carrier fleet, comprised of three far larger ship sizes – 50,000m3, 20,000m3 and 7,000m3. Dan-Unity CO2 has since garnered a grant from the Danish Maritime Fund.

“The technology and experience are in place. Capture, transport and storage are all proven concepts,” said Evergas CEO Steffen Jacobsen, in May. “Thus, what it takes to start building the needed vessels is to secure the regulatory framework including CO2 taxation are in place.”

One pitch by Mitsubishi Heavy Industries (MHI) is for a design which would refrigerate the CO2 to a temperature of -50°C, allowing it to be stored at a pressure of just 7 bar – about seven times atmospheric pressure.

In evidence to suggest MHI is aiming to be a pioneer in the carbon-trading economy, in January, it invested in Infinium, based in the US, an electrofuels company which would refine CO2 into drop-in electro-fuels, using input of renewable energy – a circular-economy method which would offset the use of virgin hydrocarbons.

A few months later, MHI announced a partnership with Klaipedos nafta and Larvik Shipping, to perform a feasibility study into loading LCO2 at the former’s facility at the Lithuanian seaport of Klaipeda, and shipping it across Europe to sequestration facilities.

Another design by Norway’s Høglund Marine Solutions and Gemrany’s HB Hunte Engineering would increase capacity using a bi-lobe “Type C” tank, some of which are already in use on LNG, liquid ethylene gas (LEG) and LPG carriers. It would store LCO2 at around -35°C, under 15 bars of pressure.

Using bi-lobe tanks would eliminate many of the size and weight concerns of mono-lobe tanks, the designer claims, and could afford vessels capacity of up to 8,000m3 for a single tank.

Keeping costs down

A quirk of CO2 carriers is they can burn any fuel their owner desires, assuming the right equipment is present. Given that a number of companies have been investigating the possibilities of performing CCS aboard ships, it is likely that a small fraction of the ship’s cargo holds will be left free to store CO2 generated by their own propulsion.

Not only would this keep costs low, but it would maximize cargo space, since hydrocarbon-based fuels have high energy density. In CCS – a field which represents pure cost – reigning in cost, while building up new infrastructure, will be essential.

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