Balancing the variability of flow in carbon dioxide transport and storage networks: low-carbon electricity systems in Great Britain

Abstract

The variability of flows can cause challenges to the long term integrity of the carbon dioxide (CO2) transport and storage infrastructure. In particular, repeated cycling and periods of low flow in injection wells can lead to a range of deleterious effects that impair the integrity of injection wells and reduce their lifetimes. How flow variability will be managed cost-effectively across clusters of CO2 sources or CO2 sinks is still unclear. At the time of writing, CCS deployment has indeed focused on single source-to-sink projects operating continuously, and, in most cases, supplying CO2 continuously for the purpose of enhanced oil recovery. Using a combination of economic dispatch models of power stations, including wind, solar, nuclear and combined cycle gas turbines with CO2 capture, with hydraulic models of dense phase CO2 pipelines, a method to characterise the magnitude and frequency of the variability of flow changes in CO2 transport networks and at injection wells is presented for the first time. A case study of the electricity system of Great Britain shows that CO2 flow variability and repeated periods of low flow are prevalent at high levels of deployment of wind power and at carbon intensity of electricity generation of approximately 60g/kWh, and very likely beyond. The effects on flow variability of the deployment of large levels of solar capacity are, however, limited. It is possible to reduce the number of occurrences of periods of low flow at injection by increasing the diameter of dense phase CO2 pipelines, and by extension their line-packing capabilities. For an offshore pipeline of 100 km and 24 inch (36 inch) diameter, this number is reduced from 202 to 32 per year (7) in the reference electricity system scenario of this study. The use of solvent storage in combined cycle gas turbines with post-combustion CO2 capture can balance flow variability, within the boundaries of CCS power plants, reducing further the need for injection wells to operate flexibly. A rigorous state-of-the-art process model determines operating limits for an 804MW power plant in terms of output of electricity and CO2, and the extent and duration to which electricity and CO2 production can be decoupled. An additional solvent inventory of 6200m3 (7.8 m3/MWe) allows decoupling electricity and CO2 production for up to 3hrs, with a fixed pressure operating strategy for steam extraction from the combined cycle. Smoothing of flows with solvent storage, sized to deliver at least 10% of nominal pipeline flow, further reduces the number of occurrences of low flow periods to 2 per year (2) representing an overall 99% reduction for the reference electricity system scenario.

# # # # # ADDITIONAL FILE: Pipeline model example for calculating line-depacking times in reference pipeline scenario of 100km, 610mm outer diameter, 150bar maximum operating pressure