CO2 levels in the atmosphere are at the highest ever recorded caused mainly, we are informed, by the combustion of fossil fuels to produce energy in the power stations worldwide.
At present the CO2 produced in this process goes unhindered into the atmosphere. Also with the supply of hydrocarbons predicted to run out relatively quickly, coal would seem to be the fuel to take over from the hydrocarbons of oil and gas. But coal has the highest CO2 emissions of all the fossil fuels, so we need to clean up the end result. To this end Carbon Capture and Storage (CCS) has been developed, with government legislation prohibiting any new coal-fired power stations being built without CCS.
We covered the capture of the CO2 in the previous article and here we have examined the different locations for the storage of CO2.
I have listed these below with their advantages and disadvantages, which we must weigh up in our own minds, against the very real threat of global warming.
- Depleted Oil and Gas Reservoirs
Advantages - These are readymade storage locations for the CO2, and provided the original oil and gas export pipelines are still located on the seabed and are in serviceable condition, can be used to pipe the CO2 to the location.
Disadvantages - CO2 reacts with water in the long term to produce carbolic acid. This could corrode any cement caps used to seal the old reservoir wells, in time allowing the CO2 to escape to the atmosphere. Existing steel well lining pipes would also be liable to corrode, but this can be overcome by lining these pipes internally with a non-corrosive metal such as nickel.
Advantages - Again we have a readymade storage facility although the mine will need to be capped to prevent escape of the CO2 to the atmosphere should it vaporize. There is also the advantage of the byproduct of useful methane gas, which is displaced from the coal seams and cracks by the injection of CO2.
Disadvantages - Again there will CO2 contact with the water gathered at the bottom of the mine and although this can be pumped dry it will seep back in again through natural means. Also the coal tends to expand and swell in contact with CO2 and this can lead to problems.
Advantages - The Ocean already continually absorbs quantities of CO2 from the atmosphere with no adverse effects. The oceans are the largest potential storage locations for CO2. At below 3000m, the CO2 becomes heavier than the seawater, sinking to the bottom.
Disadvantages - Some scientists are concerned about the possibility of the CO2 somehow converting back to gas and floating to the top of the ocean, thus escaping to the atmosphere in large quantities.
Advantages - These aquifers contain brine absorbed in layers of soluble rocks. It is thought that the introduction of CO2 into the aquifer will result in a carbonate limestone–like rock, which will provide a lasting storage location.
Disadvantages - Not enough information is known about the effects of CO2 on the rock formation in saline aquifers, so more R & D is required. However Statoil’s natural gas field off the Norway coast has been pumping liquid CO2 into a saline aquifer under the sea since the mid-nineties with no known adverse effects.
In all the above cases there are pros and cons, but during my investigation, a couple of items stood out – capturing the CO2 was relatively easy, but expensive. Transporting either by pipeline or ship was again straightforward, but might cause a few problems due to the CO2’s liquid, solid, and gas phases being very close to each other. But one thing was certain– more research and development is required before we can safely store the captured CO2 in any of the proposed long-term locations.