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:
- the rock formation must be porous (or permeable);
- the geological umbrella-like formation above the porous rock bed must be impervious (“caprock”) and geologically stable; and
- 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.