For carbon dioxide capture and storage (CCS), various specifications of the water content in CO2 have been given. These specifications range from 40 to 500 ppm. Unfortunately, little has been published on the rationale behind these concentration limits. The present lack of clarity on the dryness requirements is undesirable, because eventually, we must come to a water content standard for CCS that ensures cost-efficient CO2 transport. The work presented here aims at analyzing CO2 transport to provide some basic input for this standard.
CO2 captured from power plants always contains moisture. The water can be removed to a certain extent at the capture plant, but a small amount of water will remain. When the water is in solution in the CO2, there is no problem, but free water combined with CO2 is very acidic. The corrosive nature of wet CO2 poses a threat to the transport system integrity. Economical considerations lead to the use of regular carbon steel, which is commonly used for most pipelines. Corrosion resistant steel would inhibit corrosion, but it would be prohibitively expensive to build CCS pipelines with this type of steel.
On one hand, using regular carbon steel requires corrosion tests to quantify the destructive effects of free water in case it is present in the CO2. On the other hand, the occurrence of free water must be excluded as much as possible. No free water anywhere in the CO2 transport system would be the most straightforward way of protecting it. Drying captured CO2 costs both money and energy and reduces flexibility in the CCS chain, so a water concentration limit should not be more stringent than necessary.
A quick overview of the solubility of water in CO2 is given to enable a discussion of the operational limits of the CCS transport chain.
For normal operation, the pressure range of dense phase CO2 in a pipeline transmission system is between 85 and 150 bars onshore and between 85 and 200 bars offshore. The lower limit is determined by the critical point of CO2 (73,8 bars for pure CO2, somewhat different for CO2 with impurities). A pressure of 85 bars ensures the CO2 remains in the dense phase in case of a temporary shutdown. The upper limits of 150 and 200 bars are chosen with regard to safety and economical optimization.
Expected CO2 characteristics in the transport network include a minimum temperature of 0 °C (onshore) or 4 °C (offshore) and a maximum temperature of over 30 °C immediately after a compressor. This leads to a water solubility of at least 1500 ppm during normal operation.
Commissioning of a CO2 pipeline and blowdown scenarios are discussed. The relation between the CO2 conditions during planned blowdowns and the water content should be investigated. Unplanned blowdown could involve a rapid decompression and temperature drop, for which there are no validated models available. Therefore it is difficult to determine the right water concentration limit.
It was found that for a good technical and economical basis for determining the required water concentration limit some questions remain to be answered. Cost data of drying installations are needed. It should be found out what are acceptable blowdown conditions as a function of water concentration. Some thermodynamical issues are brought up as well. Finally the impurities present in the captured CO2 will need to be taken into account.
Although in the USA, no serious problems seem to have surfaced with around 500 ppm water in CO2, several research questions need to be addressed to arrive at a sound and cost efficient water concentration limit.
This work is carried out within CO2EuroPipe, an EU research project under the 7th Framework Programme. This project, which runs for 2, 5 years, until November 2011, aims at paving the road towards large-scale, Europe-wide infrastructure for the transport and injection of CO2 captured from industrial sources and low-emission power plants.