14 May 2024

The compressor station: the beating heart of Porthos

From the completion of the Porthos project, four companies will connect their captured CO2 to the gathering pipeline through the Rotterdam port area. Upon arrival at the compressor station, one compressor unit increases pressure in six stages to enable subsea transport and storage of the gas. It is thus an essential part of the infrastructure. The design and construction of this compressor station are complex processes that require great expertise. The system must meet the highest reliability and safety requirements.

Compressing CO₂
Jannes Hofman has been involved in the Porthos project as a project engineer from Gasunie, one of Porthos’ three parent companies, for six years. In the 30 years he has worked for Gasunie, he has already realised several compressor stations. Together with his teammates, he ensures a reliable and safe system. The design and construction of the compressor station are complex matters, even more so than the other compressor stations the compressor team has previously engineered. ‘Nobody is used to compressing CO₂. Generally, you see CO₂ compression at the CO₂ emitter and you basically only have one compressor unit. Porthos is a ‘first of its kind’ project: here we work with multiple parallel compressors that have to be able to respond to the different flows of the emitters. The system must therefore be able to handle large variations in flows. Switching on and off is going to occur frequently as a result.’

Increasing pressure
Three compressors provide the required pressure increase. This is done inside the compressor by rotating the CO₂ at high speed. The kinetic energy is then converted into pressure, increasing the pressure from 30 bar to 90- 130 bar. These are therefore extremely high pressures. Hofman: ‘This increase in pressure is necessary to transport the CO₂ through the sea pipeline to eventually store it in the depleted gas field some 20 kilometres off the coast to a final pressure of around 300 bar. With the realisation of the Porthos project, the compressor station will immediately be prepared for future connections from, for example, the Aramis and Co2next projects.

Cooling with water from the Yangtze Canal
The technical installation for the compressor station is made up of several components. The compressor itself provides the pressure boost to enable transport and storage of the CO₂. ‘The closed cooling water system cools the compressor and is essential for the compressors to operate efficiently and effectively,’ Hofman explains. ‘It is important that the temperature of the cooling water system is at least 35 degrees Celsius so that the CO₂ gas does not enter the liquid phase. Any liquid will be able to damage the compressor. Seawater from the Yangtze Canal is used to cool the closed cooling water system. This is brought to the compressor station by the cooling water pumps and cools the closed cooling water system in a number of plate heat exchangers. It is then discharged into Yukon Harbour. Heat from the closed cooling water system is also used to heat the main building.’

Building in redundancy
The compressor station is laid out for six compressors. Three compressors will be installed for Porthos first; three more may be installed in a subsequent phase. All compressors are in one building with six compartments, which means each compressor is separate from the others. ‘The moment one compressor malfunctions, the others have to keep running. For this reason, redundancy has been built in. This means that one machine is spare for every other. Should a problem occur at a compressor, the relevant compartment can be shut down and cleared of CO₂ for any repair work. The compressor station is designed in such a way that, in principle, it does not need to be manned. Monitoring and control takes place mainly from Gasunie’s headquarters in Groningen.’

Lessons learned
The biggest challenge of a ‘first of its kind’ project like Porthos is that you learn by doing. Designing the different variants is therefore typical of the engineering process. Hofman: ‘At the very beginning, we focused too much on the CAPEX, the capital expenditure, and later on the OPEX, which resulted in three compressors with different capacities (especially energy costs at low flows). It later turned out that the low flows were underestimated and we switched to machines of the same capacity. Following the various safety studies, we added additional safety measures and increased the design pressures where necessary. In hindsight, it would have been easier to do this right away but the station investment costs would have been higher. For the next project, we know that it is better to do the analysis at the initial stage.´