Over the past few years we have heard a lot about “clean coal” technologies. Clean coal refers, supposedly, to methods of burning coal without producing greenhouse gases.
Coal is a material that is almost pure carbon. There are some impurities, of which the one of greatest concern is sulphur. If we burn sulphur we get sulphur dioxide (SO2), a gas. The sulphur dioxide will combine with atmospheric moisture to produce sulphurous acid (H2SO3) or sulphuric acid (H2SO4). These are both undesirable, because they produce “acid rain” that destroys vegetation.
[Aside: the word “sulphur” is nowadays more commonly seen with its American spelling “sulfur”. I'm old-fashioned, so I'll stick with the spelling I'm used to.]
A few years ago, the term “clean coal technologies” would have referred to the use of coal with low sulphur content; to methods of removing sulphur from coal; to methods of removing sulphur dioxide from power station exhaust gases; and related issues. By now the sulphur problem has largely been resolved, but a new problem has arisen. If we burn pure carbon, with no impurities, the result is energy plus carbon dioxide. The relevant chemical equation is
C + O2 → CO2
where C means carbon, O2 means oxygen, and CO2 means carbon dioxide. The problem we now face is that carbon dioxide is a greenhouse gas: a gas whose presence causes global warming. Global warming threatens our quality of life, and perhaps even the survival of humanity on earth, so we worry about it.
The temperature of the earth, at ground level, depends on multiple factors, and I can give only a broad overview here. Broadly, the earth is heated by radiant energy from the sun, and it is cooled by re-radiation of energy back to outer space. Obviously there are local effects – the heating is greatest at the tropics, least at the poles, and the heat is redistributed by winds and ocean currents – but for our present purposes it is sufficient to think in terms of a global total and a global mean temperature.
The heat that matters to us corresponds to solar radiation in the infrared part of the electromagnetic spectrum. We can't see infrared light, because our eyes don't work properly at infrared frequencies; but we can feel it, because a lot of materials, including our skin, are good at absorbing radiant energy at infrared frequencies.
The term “greenhouse gas” comes from the observed fact that the temperature inside a greenhouse is (intentionally) higher than it would be if the greenhouse were not enclosed. This rise in temperature is mainly due to an elevated level of carbon dioxide inside the greenhouse. Similarly, if the amount of carbon dioxide in the earth's atmosphere is increased, the mean global temperature is going to rise. Some local temperatures might fall, but the average will still increase.
Some materials absorb and retain heat, while others are either transparent to it or reflect it. (And most materials lie somewhere between these extremes.) The retention depends on the relationship between the energy states of molecules and the wavelength of the incident radiation. In the case of atmospheric gases, we can observe that some gases are transparent to solar radiation, while others absorb it (and therefore increase their temperature). The gases that contribute most significantly to global temperature increases are (in order of importance) water vapour, carbon dioxide, methane, and ozone. The other important gases in the atmosphere – oxygen, for example, or nitrogen – are less of an issue because of their molecular size: they are not important absorbers in the infrared spectrum.
Water vapour is pretty much uncontrollable. Our oceans provide an almost inexhaustible supply of water, and the amount of that water that turns into water vapour depends on the atmospheric temperature. We could conceivably hope to control the temperature, but we can't hope to control the amount of available water.
Ozone is not an enormous issue because there just isn't a great deal of it in our atmosphere. In any case, we don't want to reduce the amount of atmospheric ozone because the ozone plays a valuable role in shielding us from ultraviolet radiation from the sun.
That leaves carbon dioxide and methane as the gases that we need to worry about. Methane is the product of many biological processes – the belching of cattle being a particularly important source – and molecule-for-molecule it's actually a more important greenhouse gas than carbon dioxide. Despite this, carbon dioxide is the bigger factor because there's more of it in the atmosphere.
More importantly, carbon dioxide levels are more sensitive to human activity. If we burn organic materials, such as wood and oil and coal, we will increase the production of carbon dioxide. The reason why this matters is that we, as a species, use a lot of electricity, and that means a lot of carbon dioxide produced by power stations that burn fossil fuels.
We humans have evolved in an atmosphere that contains about 0.038% (by volume) of carbon dioxide. We actually rely on it. If there were no carbon dioxide in the air, we might even die, because our breathing reflex depends in part on carbon dioxide levels. It is not at all a “bad” gas.
Carbon dioxide is part of the natural respiration cycle of most earthly animal and plant species. Most animals, including humans, breathe in oxygen and expire carbon dioxide. Plants do something similar, but plants also use a photosynthesis operation that consumes carbon dioxide – converting the carbon to organic matter – and releases oxygen.
The overall balance of gases is tied in with the mix of species. An increased carbon dioxide level will probably lead to increased plant growth, partly because of the increased level of available carbon and partly because of an increase of temperature. (Because more carbon dioxide means more absorption of solar radiation.) At the same time, there is likely to be a change in the mix of animal species, because of the combined effects of the change in breathability of the atmosphere and (probably more importantly) the mean temperature of the atmosphere. As a first approximation, we would expect that an increase in CO2 levels would mean a growth in reptile numbers and a drop in mammalian populations. If the change is big enough, some mammals – perhaps including humans – would go extinct.
If the change is not too abrupt, the species mix would change smoothly to adapt to the change in conditions. For those who believe in the Gaia hypothesis, Gaia would survive and adapt. It is important to understand, however, that humans do not occupy a privileged position in the mixture of life on earth. It is certain that Earth can survive even a major change in the concentration of greenhouse gases. It is less certain that humanity would be one of the surviving species.
If we decide that burning coal is a dangerous thing to do, then we need also to decide what to do about our coal-burning power stations.
The “clean coal” philosophy would, if taken literally, lead for a search for a way of burning coal that did not produce carbon dioxide as a product. This in turn suggests that we need to find a way of removing all of the carbon from coal. Since good-quality coal is almost pure carbon, this is unlikely to work.
We can expect, therefore, that the clean coal advocates will accept that the production of CO2 is inevitable, and that they will instead look for ways of stopping the CO2 from escaping into the atmosphere. Most of the obvious ways – for example, the conversion of CO2 to liquid carbonic acid – create further more tricky problems, so the clean coal researchers have a difficult road ahead of them. The difficulty is, however, mitigated by the fact that mine owners are very strongly motivated to pour money into clean coal research, and do not particularly care whether that research is likely to yield any results, provided only that it keeps going.
The most promising approach, as of the date of this article, is carbon sequestration. Sequestration refers to the approach of putting the product of combustion – whether it be carbon dioxide, or something else – into a place where it is not released into the atmosphere. This might, for example, be some sort of underground reservoir. When the stored gas is finally released – by earthquake, or melting of sea ice, or some similar process – it will have a much higher concentration, to the point where it could cause mass species extinctions. The expectation, however, is that this will probably not happen until after the expiration of the term of office of all current governments.
There is already a natural way of removing carbon dioxide from the air. Plants consume carbon dioxide, and turn it into solid carbon compounds. Some of the plants are consumed by animals, including humans, and again much of the carbon remains locked as solids inside the body. (Although we do have to worry about gaseous by-products of digestion. Methane, in particular, is a worse greenhouse gas than carbon dioxide, and it enters the atmosphere when animals fart and belch.) The remaining plants die, and if we're lucky they retain their carbon as solids rather than gases. We just have to be careful not to burn the wood, because that would again release undesirable things into the atmosphere.
In the really long term, some of that decayed vegetable matter is trapped underground, and gets transformed into coal and oil. This is nature's purest form of carbon sequestration. It has finally turned atmospheric carbon compounds into liquids or solids that are trapped underground and will not evaporate.
Of course, we have to make sure that we don't remove this oil and coal from the ground. That would undo the work of millions of years.
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