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‘Clean coal’ process is not so clean cut

By John Harborne - posted Friday, 16 January 2009

For several years now, "clean" coal technology, involving carbon capture and storage (CCS), has been promoted as the only realistic means of substantially reducing, or even eliminating, emissions of carbon dioxide (CO2) from fossil-fuel-burning power stations. But this necessarily comes at a considerable price increase for electricity, because of the large capital outlay for infrastructure and the energy requirements needed for the CCS process. Carbon dioxide, of course, is alleged to be the main cause of anthropogenic global warming (AGW).

CCS, in its ultimate form, involves a sequence of processes: the combustion of coal, such that a concentrated stream of almost pure CO2 is produced; the capturing of this CO2; the liquefaction of the captured CO2; and the injection of the liquefied CO2 into a suitable rock formation at least 800m underground, where it will be stored, or sequestered.

All but the final step in the process are relatively simple and for the most part are mature technologies, albeit with notable variations.


In old-style coal-fired power stations, pulverised coal is burnt using air, which means that the resultant CO2 emitted is heavily diluted with nitrogen - since nitrogen forms 78 per cent of the atmosphere. To increase the concentration of CO2, thereby aiding its capture, the coal in proposed new-generation power stations is first burnt in a mixture of commercially pure oxygen and water vapour, producing so-called “syngas”. The syngas is then reacted with steam, resulting in CO2 and hydrogen gases. The hydrogen is burnt and the resultant steam drives the power station’s turbines, while the CO2 is collected for burial underground.

A few possibilities exist, or are under consideration, for CO2 burial (geosequestration), including unmineable coal seams, but the preferred option is sedimentary rock formations.

Of course, storage in depleted oil or gas fields should present few problems, because ready-made suitable geological formations already exist. However, such sites are few in Australia and are remote from existing or potential power stations.

Under room-temperature conditions CO2 exists only in two forms: solid (“dry ice”) and gas. Liquid CO2 can only be obtained under substantially increased pressure; however, if the temperature is above 31.4°C, it is not at all possible to obtain liquid CO2. Instead, what is obtained is supercritical-fluid CO2 - a “near-liquid” phase which exhibits combined characteristics of both the liquid and gaseous phases. Not unexpectedly, temperatures prevailing in the CCS process are above 31.4°C.

Of the stages in the CCS process, storage underground of the near-liquid CO2 presents considerable difficulties:

  1. the rock formation must be porous (or permeable);
  2. the geological umbrella-like formation above the porous rock bed must be impervious (“caprock”) and geologically stable; and
  3. the stored CO2 must not be allowed to contaminate aquifers for potential human or animal consumption.

An example of a suitable storage bed is saline-water-bearing sandstone, where the interstices, or matrix, contain brine - which is displaced by the liquefied CO2.

The criterion for storage of CO2 underground is that leakage to atmosphere is a maximum of 1 per cent per 1,000 years. This is a major ask, given that burial is on a continuing basis, and that no one can guarantee geological stabilities over thousands of years. One of the properties of near-liquid CO2 is a very low viscosity. This means that the injected CO2 can infiltrate the porous rock bed with relative ease, and it necessarily follows that it can readily escape to atmosphere through any microscopic fissures in the caprock, or major crack in the event of an earthquake. Moreover, its tendency to migrate upwards is aided by its low density, being only two-thirds that of water.

Near-liquid CO2 is a very reactive chemical. Being acidic, it reacts very quickly with carbonates and other mineral substances, such as heavy metal species, in the absorption bed. These, if left unchecked, have the potential to contaminate potable water aquifers. Sandstones, depending on type, have clayey binders that often contain organic materials, and the CO2 reacts with these too.

So, pumping near-liquid CO2 deep underground requires very careful consideration of known potential hazards that may arise.

Less well-known - and certainly not advertised - is that every cubic metre of (solid) coal that is burnt produces about six cubic metres of near-liquid CO2. (The actual amount of near-liquid CO2 is based on complete combustion of the coal, its complete capture, and the actual carbon content of the coal ... an 80 per cent carbon coal yields six cubic metres of near-liquid CO2.)

Some may wonder how one volume of coal is, apparently miraculously, transformed into six volumes of near-liquid CO2. Without describing in detail the chemical mathematics (which are quite simple anyway), suffice to say that 1kg of carbon, when completely combusted, produces 3.67kg of CO2, as is well known. Factoring in the conversion of mass to volume for an 80 per cent carbon coal (typical specific gravity 1.35) and for near-liquid CO2 (SG 0.65) results in the around-sixfold volume increase.

It doesn't take an Einstein to realise the immense logistics and difficulties of dealing with the considerable increase in volume from coal to near-liquid CO2. Unless power generators have a ready nearby sink, such as a depleted oil well, in which to inject the voluminous CO2, it won't take long before multiple injection points have to be created, because the CO2 will readily exhaust the brine-filled pores of a deep, geologically acceptable rock body.

Also,if the geosequestration injection points are well away from the power station, huge costs in infrastructure to transport the large volumes of near-liquid CO2 (pipelines or tankers) will be inevitable.

As an example, the combined annual coal consumption of just two Hunter Valley (NSW) power stations exceeds 10 million tonnes. Assuming an 80 per cent carbon content and complete combustion, this 10 Mt (equivalent to 7.7 million cubic metres of solid coal) would convert to 46 million cubic metres of near-liquid CO2. Transportation and storage underground for this volume would be required year in, year out. After 100 years, at the same rate of coal consumption, the volume of near-liquid CO2 to be “swept under the geological carpet” becomes nearly half a cubic kilometre.

Perhaps this figure doesn’t seem high, but the pores in sandstone occupy typically only about 20 per cent of its volume, so the amount of suitable absorption rock inflates to about 2½ cubic kilometres. Obviously, the storage facility for CO2 cannot be viewed as some huge, fixed underground containment pond or cistern, as is sometimes employed to store LPG.

Australia’s annual consumption of black coal by power stations has been reported as 60 Mt per annum. Over a period of 100 years at this rate of consumption, the near-liquid CO2 needing burial becomes more than 30 cubic kilometres, with the result that absorption beds totalling more than 150 cubic kilometres would become exhausted. Not only that, but the containment of the CO2 would have to be constantly and meticulously monitored for leakage to potable or brackish water supplies, and to the atmosphere.

Considering its high costs and potential hazards, can the implementation of CCS be justified? Its need, according to environmental activists, is to mitigate, or even stop, global warming. However, despite claims to the contrary, the science of global warming is very far from settled. It is hard to see how one molecule of anthropogenic carbon dioxide in 10,000 molecules of the air we breathe - amounting to only 0.01 per cent, or 100 ppm - can influence world temperatures.

Activists point out that average global temperatures have risen concurrently with atmospheric CO2 levels since industrialisation began, but the correlation is quite weak (approximately 25 per cent), and, in any case, world temperatures have trended downwards over the past decade - in fact temperatures in the Northern Hemisphere are currently at record low levels.

As the evidence stands, it can well be argued that CCS will have insignificant influence on climate, will present both known and unforeseen hazards, and will unnecessarily raise electrical power costs.

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About the Author

John Harborne is a retired investigative metallurgist. He became interested in global warming “science” through CCS (CO2 geosequestration) when the proposed process came to the fore only a few years ago. Through his academic training in metallurgy he quickly realised that experts in the field were not divulging physical data on the transition from coal to CO2 storage.

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