6 CO2 transport
The safe CO2 transport requires maintaining the CO2 in a stable phase, selected depending on boundary conditions and transport and storage section optimisation design, avoiding risks associated with the change in CO2 phase, because of temperature and pressure variations.
Flexible operation of the upstream units, frequent start-up or shutdown and load changes lead to fluctuation of the captured CO2 flowrate. This shall be taken into account in the design of the pipeline, in order to avoid change of the CO2 physical state.
6.1 Flexible operation
During shutdown of a CO2 capture plant, the pressure in the pipeline tends to drop, while approaching the external conditions.
In this case, a two-phase flow or a significant change of the physical properties could occur in the pipeline during shutdown or cycling mode operation of the capture plant, unless the pipeline is properly designed to maintain the pressure above the critical conditions of the CO2. For this reason, the pipeline shall be adequately designed with proper heat insulation and TSO valves.
6.2 CO2 pipeline start-up
Start-up process of the CO2 pipeline consists in filling and pressurisation of the entire pipeline volume. The entire process can take several days to be completed, depending on the starting pressure and density condition.
During the start-up process, the CO2 physical state changes from gas phase to the final liquid or supercritical phase, depending on the design conditions selected. Filling and pressurisation process is slow when the CO2 is in gas and two-phase condition, while becomes much quicker when the pipe fluid is entirely in the dense liquid phase.
As CO2 pipeline start-up may have a significant duration, plant shall be started-up independently and CO2 shall be fed into the pipeline progressively, achieving its pressurisation while keeping closed the connection to the storage. Once the line pressure is at the required values, the downstream block valve can be open and CO2 flow to storage.