Carbon dioxide that is captured from industrial or power plant emissions will in most cases have to be transported across long distances to the storage site. At Statoil's LNG production site on the island Melkøya (near Hammerfest in northern Norway), for example, captured CO2 is transported in a 153 km pipeline out to the Snøhvit field, where it is injected for storage into a geological formation below the sea floor.
Rotvoll in Trondheim, Statoil has established two test rigs to perform studies on the entire carbon dioxide transport chain, with specific emphasis on the interfaces between capture and transport and between transport and storage. As part of the research project "CO2 Interface Transport Interface Storage", which receives support from the CLIMIT programme, Statoil is studying what happens in the event of sudden pressure loss and how impurities affect the transport of CO2.
− The project aims at increasing knowledge about thermodynamics, depressurization of pipelines and other transient operations, heat transfer, impurities, vertical CO2 transport and natural gas with high CO2 concentrations, says Project Manager Gelein de Koeijer, principal researcher at Statoil. His team is comprised of about ten persons from Statoil and SINTEF. These issues are vital for safe transport of CO2 in pipelines.
− As operator of three facilities that include CO2 transport, Statoil has considerable experience, but the design of CCS systems can be improved, says de Koeijer. The three plants that process natural gas with high concentrations of CO2 are Snøhvit and Sleipner in Norway and In Salah in Algeria. The main challenge faced by the operators is how to avoid sudden losses of pressure. With more knowledge, designers can optimise the type of material and pipeline dimensions, which in turn will increase safety and reduce costs.
The properties of CO2 differ considerably from those of natural gas and oil. Above the so-called critical point (31 °C, 74 bar, liquid CO2 gradually evaporates without boiling, as it would at lower temperature and pressure. Below the so-called triple point (-56.6 ºC, 5.2 bar, the gas transforms directly to the solid state without going via the liquid state when pressure and temperature fall. In the event of leakage and sudden pressure loss, CO2 would directly form dry ice. Increasing our knowledge of how CO2 behaves in the event of sudden pressure loss shall contribute to improved routines and safer operations.
Statoil has constructed two laboratory rigs for conducting research on CO2 transport in connection with the development of the Snøhvit field. They are located at Statoil's research centre in Trondheim. One of the rigs can test pressure release: liquid CO2 is contained and subjected to a sudden drop in pressure. The pipe, with a length of 130 metres and a diameter of 1 cm, lies between two pressure tanks, thereby enabling researchers to test pressure drops between various pressures, and not only between the pipeline and the atmosphere.
The rig is also used to test two-phase transport, during which CO2 occurs as both liquid and gas. It is very complicated to handle this mixture, which is something operators thus want to avoid. However, the two-phase state can occur in the event of sudden pressure drops and when CO2 is compressed. The other rig is a large box, within which is fastened a pipeline section of the same type as used at the Snøhvit site. The pipe is filled with liquid CO2, whereas the box can be filled with water. This enables the researchers to measure heat transfer between the surrounding water and the CO2 in the pipeline at different pressures and temperatures. The transfer of heat can vary considerably.
− For instance, ice may form on the pipe, which in turn affects the heat transfer coefficient, says de Koeijer. The results of these studies will be used to, among other things, design and operate the pipelines so that the build-up of ice on the outside of the pipes can be avoided.
Prior to transport, operators will remove impurities such as water, natural gas and other substances to avoid problems in the pipeline system. To remove water, the CO2 is dehydrated, using either glycol washing or zeolite adsorption. The dehydration process is generally integrated in the compression facility used to liquify the CO2 prior to transport. Natural gas is another impurity. Methane affects carbon dioxide's critical point, and therewith whether or not CO2 remains in the liquid state. The project shall also simulate the transport of CO2 through the vertical pipes down into the storage formation. Towards the end of the project period, the researchers will gain more insight into the behaviour of natural gas with high concentrations of CO2.
– It is important to increase our knowledge about pipeline transport of carbon dioxide before we start implementing large-scale CCS in Norway. This project helps us to understand all aspects of CO2 transport, thereby enabling it to become safe and cost-efficient, underlines Special Adviser Trygve U. Riis in the Research Council of Norway.
ABOUT THE PROJECT
Name: CO2 Interface Transport Interface Storage
Project manager: Statoil
Partner: SINTEF Energiforskning
Budget: Maximum NOK 34 million
CLIMIT co-funding: Ca NOK 5 million