Emerging onboard carbon capture technology is explored in a new publication from ABS, launched during the ABS Sustainability Summit, one day before SMM 2022.
n particular, the report, entitled “Insights into Onboard Carbon Capture”, examines the various methods of onboard carbon capture taking into consideration carbon handling and storage and downstream challenges, as well as regulatory issues.
Carbon capture using solvents
Depending on the fuel type and exhaust quality, the first step in many exhaust gas purifying systems for carbon capture is to reduce the impurities and gas species including SOx, Particulate Matter (PM), heavy metals, ash and nitrogen oxides (NOx) that may be present in the exhaust gas. Onboard carbon capture systems may utilize wet scrubbing in the exhaust gas quenching/cooling stage and then be arranged using an absorber unit where the solvent extracts CO2 from the exhaust stream. The CO2-rich solvent is then sent to a desorber, or stripping, unit to both separate CO2 from the solvent and recover the solvent for reuse. Depending on the type of solvent used, they may degrade over time at various rates and require replenishment or replacement, while the spent solvent or residue requires proper handling and disposal.
Supporting systems for the main process stages include water vapor removal, heat exchangers for temperature and constituent phase control, blowers or pumps for circulation, or other systems to achieve the desired quality of captured CO2.
Carbon capture using sorbents in dry scrubbers
Membranes can be implemented as a physical filtration system to absorb various impurities, including carbon gas. Conventional membrane technology consists of filters specific to molecule sizes and can require significant input pressure.
Gaseous membrane CO2 filters are made of a semi-permeable fabric that allows selected molecules to pass through while restricting the flow of others. The efficiency of these systems is negatively affected by the presence of other gasses such as NOx and SOx groups, and moisture.
Advanced membrane technology may also use solid carriers (sorbents) bonded to the surface of a filter to encourage chemical or electrical CO2 separation. These are novel and emerging technologies which involve less commercially available systems but may show potential for the future of carbon capture on board ships.
Cryogenic carbon capture
Cryogenic carbon capture is a process in which carbon is separated from exhaust gas by controlling phase changes via temperature and pressure (thermodynamic) modulations. The effectiveness of cryogenic carbon capture relies on the various chemicals found in the gas stream. The process involves cooling exhaust gas to the solidification point of CO2 (-100 to -135° C). Where conventional distillation processes may prefer liquid products for ease of handling, it has been shown in various studies that vapor-to-solid separation can be more energy-effective. Using the CO2 solidification extraction method to extract gases, including NOx and SOx, results in two exhaust gas streams; one consisting of pressurized pure CO2 (99 percent or higher), and another comprised of the remaining contents in the original exhaust at ambient pressure. This system can be installed on existing ships with a relatively small footprint connected to an exhaust gas input and a power source. The extreme temperatures necessary for the cryogenic carbon capture process require the integration with other systems on board to optimize the heat exchange process.
This method is achieved primarily by a network of heat exchangers, the specific architecture of which can significantly improve the energy efficiency of the installed system. It is estimated that this process can reduce the energy consumption of carbon capture by 50 percent when compared to solvent-based carbon capture systems. While cryogenic carbon capture systems appear to have promising advantages over other systems, research is still ongoing to develop, optimize and implement for application onboard ships.
Due to the low temperature requirements for cryogenic carbon capture, they may be of interest to vessels carrying liquefied natural gas (LNG) (which is stored at temperatures as low as -163° C). There may be opportunities for the LNG cryogenic systems to work harmoniously with the cryogenic carbon capture systems to gain additional efficiencies.
Although there has been increased interest from the industry, the technology and its associated value chains have a long way to mature, and there are many factors to consider such as onboard power supply, fuel types, exhaust characteristics and onboard storage. This Insights document is an important step in moving the conversation forward, supporting shipowners and operators with the latest information from ABS engineering and joint development projects
stated Georgios Plevrakis, ABS Vice President, Global Sustainability.