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Geosequestration

Sequestration of CO2: underground; below the seabed; in depleted oil or gas reservoirs; or in deep saline aquifers is technically possible. But the scale required, to sequester just 25% of NSW coal sourced CO2 (for example that produced by coal fired electricity), is 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.

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.

Not only is CO2 over twice the weight of the coal used to generate it; but the volume (after being compressed to a liquid) is around five and a half times greater:

The specific gravity of carbon in black coal is around 2.15 (1 cubic metre weighs 2.15 tonnes).  1 cubic metre produces 7.9 tonnes of CO2 (see Footnote 4).  The specific gravity of liquid CO2 is 1.18 (slightly denser than water) and the volume occupied by 7.9 tonnes is 6.68 cubic metres (6.68 kilolitres). 

Thus after adjusting for carbon content, one cubic metre of coal going into a power station will produce about five and a half cubic metres of liquid CO2 when compressed.

If it was liquefied, the CO2 produced annually by NSW power stations would compress to about 63 GL (gigalitres) = 63 thousand million cubic metres.

If it was liquefied, the CO2 produced annually by NSW power stations would compress to a volume of about a quarter of a thousand square kilometers one meter deep.  As indicated in the introduction, disposal of a volume of this size is an enormous challenge.

It can be seen that under carbon capture and storage, getting the CO2 from a power station to the sequestration site and injecting it is a much bigger job than mining the coal or getting the coal to the furnace.  This means that the existing power stations are the wrong technology in the wrong place for CCS.  They need to be located on good sequestration sites; as opposed to being close to the source of coal.

For these reasons alone, CCS technology is unlikely to be applied (in any but a demonstration or token way) to existing stations in NSW, and for similar reasons is unlikely to be applicable to the majority of current generation coal fired power stations in the World. But there are additional reasons to doubt that CCS can be generally applied even if a new generation of plants make capture economically feasible.

Pumping CO2 underground is a massive undertaking that is well in excess of the coal mining enterprise providing the coal.  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. 

But to have any impact on carbon emissions, hundreds of gigalitres of CO2 would need to be sequestered over the life of each new or converted coal 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 piplines 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).  For 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% of its present electricity output.

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.

 

 


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