Geosequestration
Sequestration of CO2 underground: below the seabed; in depleted oil or gas reservoirs; or in deep saline aquifers is technically possible and already practiced on a small scale.
Yet the scale required, to sequester the just the current annual increase in world CO2 emissions (around 220 million tonnes) would be an enormous engineering challenge. It is one thing to land a man on the Moon; it is another to relocate the Great Pyramid (of Cheops) there.
Disposal of liquid or compressed gaseous CO2: underground; below the seabed; in depleted oil or gas reservoirs; or in deep saline aquifers is technically possible and is already practiced in some oil fields to improve oil extraction. But the scale required for meaningful sequestration of coal sourced carbon dioxide is an enormous engineering and environmental challenge of quite a different magnitude.
Most current work is directed to finding appropriate deep leak proof geological strata below land for the purpose. Undersea sequestration would be an engineering challenge on an even greater scale, and potentially very damaging to ocean organisms coral reefs and fisheries.
As a result of simple chemistry the underground volume required to dispose of carbon dioxide, is substantially greater than that of the coal or the oil that produced it.
For example, the CO2 produced by burning a tonne of coal is well over twice the weight of the coal used to generate it and the volume (even after being compressed to a liquid) is around five times greater (see the box below):
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(Those of you who are allergic to this kind of thing can skip this and just take my word for it) The atomic weight of carbon (C) is 12 and oxygen (O) 16 so: C + O2 → CO2 and: The specific gravity of carbon is around 2.15 (1 cubic metre weighs 2.15 tonnes). The specific gravity of liquid CO2 is 1.18 (one cubic meter weighs 1.18 tonnes) Different coals have considerable variability in ash (6.5% to 30%) and volatiles (20.8% to 37.9%) depending on grade and purpose. Let's estimate that steaming coal is 75% carbon by weight. Thus after adjusting for carbon content, one cubic metre of coal going into a power station will produce about five cubic metres (kilolitres) of CO2, if compressed to a liquid. |
Thus transporting the CO2 from the furnace is undertaking that is two to five times the scale (by weight and volume) of delivering the coal that generated it.
Under CCS, getting the CO2 from a power station to the sequestration site and injecting it is a much bigger job than mining the coal and transporting it to the power station. This means that the existing power stations are the wrong technology in the wrong place for CCS. They would need to be located on good sequestration sites; as opposed to being close to the source of coal.
But there are additional reasons to doubt that CCS can be generally applied, even if a new generation of plants make capture economically feasible.
As I have official data for New South Wales Australia (NSW) I will use these as a hypothetical CCS project to illustrate the issues.
The annual production of CO2 from stationary electricity generation in NSW in 2017 was 66.2 Mt CO2e.
To put this into perspective:
- These coal burning power stations contributed less than 12% of Australia's annual production of CO2 in 2017.
- In the same period Australia contributed just 1.16% of global world CO2 production.
- So if the following CCS example could be fully implemented it would reduce annual, global fossil sourced CO2 emissions by 0.14%
Let's assume the whole 66 million tonnes of CO2 is liquefied. We have seen (above) that it would compress to around 56 million cubic metres (56 GL). Pumped underground this would occupy a volume of about five and a half thousand hectares (13,480 acres) in area one meter thick - every year. Of course it would be dispersed through the strata but unless the strata had many voids the surface would rise in a reverse of the subsidence caused by long-wall underground mining. It would damage property and seriously disrupt any underground aquifers.
To be more realistic it's unlikely that the entire output of CO2 could be captured. Yet to have any impact at all on global carbon emissions and thus climate change, hundreds of thousands of tonnes of CO2 would need to be sequestered over the life of each new fossil fired power station. Apart from the mass and volumes involved there are handling difficulties.
At less than 5 times atmospheric pressure liquid CO2 undergoes a phase transition and becomes a solid, dry ice. To keep it liquid it needs to be kept at a pressure well in excess of five atmospheres[6] at -56.4o C. At ambient temperature it needs to be kept at above 60 atmospheres to remain liquid. To be pumped across the countryside in uninsulated pipelines it needs to be above the critical point pressure of 73 atmospheres. Any loss of pressure, due to a rupture or a loss of power, may well result in its boiling to gas followed by solidification of sections of the pipeline and/or damage the pumps.
Ordinary carbon steel corrodes in the presence of moist CO2 and the pipelines need to be lined or made of stainless steel. A very significant and entirely novel infrastructure of large diameter high pressure pipelines and pumping stations will be required. The pumps need to move large tonnages and volumes of liquid, comparable to a good sized city’s water supply, at very high pressures.
This will consume a lot of energy. The initial compression of the gas to liquid needs to overcome the latent heat of liquefaction of CO2 (approx 160 kWh/tonne).
Continuing with the NSW example, according to its Wikipedia entry[7]Liddell power station releases an estimated 14.7 million tonnes of CO2 to generate 17,000 GWh per year. Assuming no inefficiencies, compressing this gas would consume 14.7 x 160 = 2,350 GWh per year or about 14% (and probably a lot more) of its electricity output.
As it is expected that this liquid would be pumped into porous strata, where it will fill interstitial voids to perhaps 10% of the volume, several thousand square kilometres of strata would be required annually. These volumes would also require hundreds of kilometres of high pressure distribution pipeline and hundreds of injection bore holes the diameter and depth of oil wells.
Within a few years, the underground sequestration site (or sites) required for CO2 would underlie hundreds of thousands of hectares of countryside with high pressure liquid/solid phase CO2 that would pose, probably insurmountable: geological; engineering; environmental; aesthetic; safety; and cost issues.
To this needs to be added the unknown, but substantial, energy required for transportation to the sequestration sites and for underground injection, as well as vast additional infrastructure; capital and running costs. Existing gas pipelines burn some of the gas to run pressure booster pumps along the string but these would need to be electric for CO2, involving additional high voltage lines and inevitable grid losses.
Clever integrated design may potentially reduce some of these overheads (for example some of the heat released during compression may be recoverable at the compressing power station) but it is probable that the additional infrastructure and energy overheads required for CCS would make any future coal fired station so inefficient and resource consuming as to be impractical.
Pumping a few thousand litres down a hole at a test site proves little. Small scale return of to oil wells has been employed for many years.