Researchers at TU Graz have evaluated various carbon capture technologies for use in shipping. These technologies will be necessary to achieve the climate targets, but they will realistically not enable complete capturing of greenhouse gases on board.

By 2050, maritime shipping should be climate-neutral. The International Maritime Organisation has set itself this ambitious goal. As green fuels for the propulsion of ships and the engines required for this will not be available to a sufficient extent in the near future, the extraction of CO₂ from the ships’ exhaust gas stream on board is considered an important building block for achieving the climate targets. For this reason, researchers at Graz University of Technology (TU Graz) and the COMET K1 Centre LEC (Large Engines Competence Center) have evaluated the feasibility of current CO₂ capture technologies for different types of ships in the “CCS on Ships” research project.

High space requirement as a limitation

For its investigation, the team used the LEC ENERsim software developed at the LEC, which simulates complex energy systems and enables the optimisation of energy and mass flows. “We have assessed the consequences of capturing CO₂ from a ship’s exhaust gas stream. Depending on the capture technology, the investment costs vary, as does the additional energy consumption in the form of electricity, process heat or both,” explains Andreas Wimmer from the Institute of Thermodynamics and Sustainable Propulsion Systems at TU Graz. What all capture technologies have in common is the considerable amount of space required for CO₂ storage, as the combustion of diesel produces around three times as much CO₂ by mass. “The purpose of the ships is particularly decisive for the size of the storage facility, as this determines the possibilities for unloading the CO₂ in the harbours.” Large-volume storage systems have a negative impact on transport capacity. “For this reason alone, complete CO₂ capture from the exhaust gas stream is unrealistic. In addition, energy consumption increases disproportionately if the separation rates are increased,” says Andreas Wimmer.

Many factors influence separation rates

How much carbon dioxide can ultimately be captured on board the ships depends on a number of factors – first and foremost the type of ship. LNG tankers, for example, have better prerequisites than container ships or general cargo ships due to the existing infrastructure for cooling and storing liquefied gases. Expected future fuel prices, freight rates and CO₂ taxes will also influence how much cargo volume shipping companies can forego for interim CO₂ storage. However, route planning is also a very important factor, so that the ships can unload the CO₂ in appropriately equipped harbours before the storage facilities on board are at full capacity. Andreas Wimmer believes that separation rates of around 30 per cent make sense for initial retrofits to establish the technology.

However, it is important to develop the technology to market maturity. On the one hand, it serves as a transitional solution for a few decades, during which the existing fleet of ships can remain in service with retrofits. “On the other hand, we will need the technology in the long term in order to operate future ships powered by carbon-based e-fuels in a climate-friendly way,” says Andreas Wimmer.

The “CCS on Ships” project was funded as a German-Austrian industry research project as part of the CORNET programme and by the innovation and transfer network FVV (Forschungsvereinigung Verbrennungskraftmaschinen [Research association for combustion engines]).