Making Waves: Could emissions capture at sea be the answer to low-carbon shipping?

At first glance a story about low carbon shipping is good news (and don’t we all need one of those these days). This one offers an entrepreneurial approach to finding a climate friendly solution to a tricky to decarbonise sector. The key ingredient: quicklime. But is it as easy and neat a solution as it at first appears? Our Senior Research Fellow in Sustainability Policy, Richard Sulley, shares his expert opinions on behalf of our Green Watch team.

Why the world needs low carbon shipping

International shipping accounts for 3% of global greenhouse gas emissions. This could rise to 10% by 2050 as other sectors decarbonise. It is also the foundation of the world economy. At any one time there are over 5,500 container ships on the ocean and tens of millions of containers in the system.

A typical 20,000 TEU (twenty foot equivalent unit) large container ship has a capacity of around 220,000 tonnes.

Many are searching for a way to reduce international shipping’s dependence on heavy fuel oils and marine diesel and move to cleaner, greener alternatives; from returns to sales, to batteries and hydrogen, ammonia and a plethora of others.

Step in Seabound. Its headline grabbing and seemingly relatively simple plan is to capture carbon at sea directly from the exhausts of ships.

The plant takes quicklime (calcium oxide) and reacts it directly with carbon dioxide from the engine exhaust to produce limestone (calcium carbonate). So far so good. There’s no arguing with the carbon dioxide absorbing chemistry on the ship.

CaO(s) + CO2(g) → CaCO3 (s)

Where solid calcium oxide absorbs carbon dioxide gas in a reaction that produces solid calcium carbonate and liberates lots of heat.

But can it really be that simple? 

A question of balance

To fully assess a system’s sustainability credentials requires a full Life Cycle Analysis (LCA). While we aren’t undertaking that here, it’s worth bearing in mind the thoughts of the Grantham Centre’s Head of Sustainability Assessment, Dr Stuart Walker:

There’s a whole world of complexity to LCA, and results are not always what they seem. Transparency and clear definition of scope and boundaries are key first steps. Uncertainty is an unavoidable part of the method, and studies claiming to have resolved the impact of large complex systems into a single unequivocal result either apply only to a very specific case, or are optimistic in their claims.

Dr Stuart Walker

Head of Sustainability Assessment

Within the boundary of the ship and its exhaust, it’s easy to demonstrate that a quicklime carbon capture plant would reduce “tailpipe” emission. However, we need to think bigger than that. Where does the material come from and are there any unforeseen consequences of adding such a process to a container ship?

Let's take it back to the start

The process starts on shore with the reaction occurring on the ship driven in the opposite direction by limestone (calcium carbonate) being heated in a furnace at temperatures of >900°C and the carbon dioxide being released. 

CaCO3(s) → CaO (s) + CO2(g)

The product of this reaction is calcium oxide, commonly known as quicklime.  This is formed into the pellets that Seabound uses in their carbon capture plant that is carried on board the containerships, conveniently, for the pilot scale plants they have developed, in shipping containers.

So if we look at the full cycle of carbon release and capture we see some issues with the overall efficacy of the process in reducing emissions.

In simple terms, from Seabound’s trials they say they can currently absorb 78% of the CO2 in the ship's exhaust., So let’s take a tonne of CO2 in that exhaust. 220kg of it goes straight through the system and out into the atmosphere. To capture the remaining 780kg, we would need around 1,000kg of Quicklime. To produce that we would need 1,780kg of Limestone, which during the calcination process is stripped of the equivalent amount of carbon dioxide.

However the calcination process to produce the quicklime needs energy. This will likely emit carbon, unless it’s from a completely renewable source.  The process itself uses around 5GJ of energy per tonne of limestone. So in our example we’d need 8.9GJ, which if sourced from UK grid electricity would emit 437kg CO2e.

In summary, for every 1,000kg of CO2 the ship’s engine produces, 780kg is captured, 220kg is lost through the exhaust, 780kg is emitted from the calcination of limestone, and 437kg is produced in heating of that process. Not to mention upstream emissions in transportation, fuel production, etc.

This results in a net position of 1,437kg emitted per 1,000kg produced. So not the 78% saving in carbon emissions the story claims, but an increase of 43.7%.

Weighing up the Issues

“Tonnes of Carbon Dioxide” is a phrase we are used to hearing, but perhaps not great at understanding.  As a gas, the weight of CO2 doesn’t really impact us. However, as soon as you try to capture, store and transport it around, it certainly becomes an important consideration.

A typical medium/large container ship emits 600 tonnes CO2e per days travel. A journey from China to the UK takes around 41 days. So during that trip approximately 24,600 tonnes of CO2  would be produced. Where will it all go?

As we’ve seen previously, 1 tonne of quicklime can absorb a theoretical maximum 786kg of CO2. So if you planned to absorb all the CO2 from a journey from China to the UK you’d have to start with 31,287 tonnes of Quicklime on board and would finish with 55,897 tonnes of limestone. That is about a quarter of the capacity of a large container ship.

This doesn’t include any sulphur or water vapour you pick up at the same time.

The role of life cycle assessment

It’s crucial that we continue to investigate and research novel technologies, or novel uses of existing technologies, in the ongoing fight against climate change. But these must be backed up by robust science and transparency. We picked this story about shipping because it highlights some of the issues and practical considerations which need to be addressed through life cycle analysis to develop new solutions. Yes the funnel emissions of the ship are reduced but only because they have been displaced onshore, if we include all the emissions generated to enable this technology the total CO2 emission per tonne of freight will increase.  Seabound may have answers to some of these, but according to our Green Watch team, we still have further to go before we can make low carbon shipping a reality.