A transportation infrastructure that carries CO2 in large enough quantities to make a significant contribution to climate change mitigation will require a large network of pipelines spanning over hundreds of kilometres. Given that the most economical means of transporting CO2 is in the supercritical state due to its low viscosity and high density, a typical 100 km, 0.8 m diameter CO2 pipeline under such conditions would contain approximately 9000 tonnes of inventory.
In the event of pipeline failure, for example a full bore rupture, based on the hyperbolic discharge behaviour synonymous with such failures, a significant proportion of the inventory would be discharged in the first few minutes. At a concentration of 10%, an exposed individual would lapse into unconsciousness in 1 min. Furthermore, if the concentration is 20% or more, the gas is instantaneously fatal. The ability of CO2 to collect in depressions in the land, in basements and in other low-lying areas such as valleys near the pipeline route, presents a significant hazard if leaks continue undetected. Hydrocarbons will eventually ignite or explode in such areas if, and when, conditions are 'right', but CO2 can remain undetected for a very long time. Also, CO2 will be mixed with potentially toxic substances whose natural dispersion might be impeded by the dense CO2 vapour layer close to the ground, further increasing hazards. In 1986 a cloud of naturally-occurring CO2 spontaneously released from Lake Nyos in Cameroon killed 1,800 people in nearby villages.
There are several other hazards associated with the accidental release of CO2. It can act as an ignition source for nearby combustible materials due to friction induced static discharge. In 1953, such an incident resulted in 29 fatalities. CO2 also reacts with water to form carbonic acid leading to the corrosion of carbon steel pipelines. Supercritical CO2, widely considered to be the most economical state for pipeline transportation is a powerful solvent giving possible toxic contamination and sealing problems. Its release may lead to low temperatures resulting in brittle fracture of surrounding equipment. High velocity solid CO2 discharge may pose the risk of erosion impact (supercritical CO2 with solid CO2 pellets is used commercially as a cutting media).
It is clear that the hazards associated with CO2 pipelines are quite different compared to those posed by hydrocarbon pipelines, presenting a new set of challenges. As such, any confidence that existing experience with operating hydrocarbon pipelines can be wholly extended to CO2 pipelines is dangerously misplaced. Without a clear understanding of the hazards associated with the failure of CO2 pipelines, CCS can not be considered as a viable proposition for tackling the effects of global warming. The development of reliable validated pipeline outflow and dispersion models are central to addressing this challenge.