The major atmospheric gases, nitrogen and oxygen, each have unique properties vital to most living organisms and they alone do not explain our comfortable average temperatures, because they are essentially ‘transparent’ to long-wave electromagnetic radiation.
The so-called greenhouse gases have a temperature stabilising effect because, they intercept radiated heat energy and radiate it out again in all directions. In fact, it is estimated that the average temperature on earth without this ‘greenhouse effect’ would be around -18°C.
Some 51% of the radiant energy over a range of wavelengths that arrives from the sun is absorbed by the oceans, land and surface vegetation, where it is converted into heat – raising the ambient temperature.
This energy is re-radiated but as longer wavelength, invisible infrared rays that cannot escape through the atmosphere, because it is absorbed by certain gas molecules including: water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3).
The difference between greenhouse and other gases lies in their molecular structure; having more than two component atoms that are relatively loosely bound allows energy to be stored as molecular vibration, before being released again as radiation of longer wavelength. In bi-atomic molecules like oxygen and nitrogen, the bonds are too tight to permit molecular vibration, so heat radiation is neither intercepted nor absorbed.
Why is CO2 so important?
Of course, the word that’s so negatively portrayed in the media is CO2. So how does CO2 enter the equation? And why is it so important?
There is no short answer to this question, because the issue has become highly politicised and a fierce debate rages about the validity of computer-generated models of global warming, that predict catastrophic outcomes.
One argument suggests that global warming caused by cyclical climate variation is the real cause of increased atmospheric water and CO2 levels measured today, rather than the reverse. The contribution of each of the various gases in the atmosphere to the overall greenhouse effect depends on the efficiency of their molecules in absorbing heat radiation and their residence time in the atmosphere.
In fact atmospheric water, in the form of clouds (droplets) and vapour (gas), is so effective that it completely overwhelms all the other greenhouse gases and its concentration is virtually unaffected by human activity on a global scale.
However, CO2 is a very effective greenhouse gas, generated in large and increasing volumes by human activity and already posting measurable effects on the atmosphere composition and ocean acidity. Although CH4 and N2O are far less efficient greenhouse gases than CO2 may be, their pre-industrial concentration was far weaker and after adjusting for the relative global warming potential (GWP), 72.4% of the overall greenhouse effect (including both natural & anthropogenic emissions) is attributed to CO2.
CCS – What’s it all about?
Carbon capture and storage (CSS) or carbon sequestration refers to processes under development now, to reduce the global emission of CO2 by separating it from the flue gas of industrial and power generation plants and placing it in secure long-term storage.
The motive being to halt, or if possible even reverse, the accelerating rate of emissions and therefore avert the predicted climate changes resulting from continued CO2 concentration in the natural environment.
CO2 capture and transport
The industrial gas industry has captured and purified CO2 successfully for many years, usually from the exhaust of fossil fuel combustion by amine absorbers and cryogenic cooling, prior to compression and liquefaction. These are energy intensive processes resulting in an extraction cost that is unaffordable for CCS given the scale of operation required.
The cost of capturing CO2 is estimated to represent 75% of the overall cost of any CCS system. Commercial deliveries of liquefied CO2 are routinely made either by road or rail tanker, but cost imperatives require that captured CO2 be transported by pipeline to the intended storage sites.
Several alternative combustion processes have been mooted as a means of avoiding the dilution of CO2 by nitrogen; the inevitable result of combusting fuels with atmospheric oxygen. Separation processes and therefore costs can be reduced if the exhaust gas is already a highly concentrated stream of CO2.
Oxy-fuel combustion has been promoted for many years by the industrial gas industry to promote plant productivity and fuel efficiency. Chemical looping combustion uses metal oxides to supply the oxygen for fluidised bed combustion.
Pre-combustion extraction relies on gasification of fossil fuels to produce syngas, from which CO2 can be extracted prior to entering the combustion process. This alternative is already widely used in plants producing fertiliser, chemicals, gaseous fuels and electric power.
In terms of storage, considerable knowledge has been gained about the possible underground storage of CO2, through its use by the oil industry to extend the recovery of oil after the initial pressure is depleted.
Although developed for completely different reasons, the understanding gained of how geologic structures might trap and contain vast amounts of CO2 is invaluable.
Unfortunately, the same cannot be said for the long-term security of proposed storage opportunities of saline aquifers, un-minable coal seams or undersea storage options. Some estimates suggest that 99% retention of stored CO2 can be expected after 1000 years that suitable sites exist, to accept all the emissions during the next few hundred years.
The risk of leakage from geologic storage into the ocean and atmosphere may not yet be well understood, but the consequences of catastrophic failure could be disastrous.