But it won’t be possible to make do without petrol and coal in the near future, the latter emitting twice as many greenhouse gases than natural gas by kWh product. Each time we burn a kilo of coal, it is around 3 kilos of CO2 that we emit into the atmosphere ( C + O2 >>> CO2 that is 12 g C + 32 g O2 >>>> 44 g CO2 or 1 kg C + 2.67 kg O2 >>> 3.67 kg CO2. However, in quite standard coal, such as is imported into Belgium for example, there is 75% of carbon, in other words 0.75 kg of carbon in 1kg of coal. From which we get the equation 0.75 kg C + 2 kg O2 >>> 0.75x3.67 = 2.75 kg CO2 ). ‘However, we are witnessing a dazzling return of coal, not only in China, but also in the United States and Europe. King Coal is back,’ states Professor Mathieu. And it is here that Carbon Capture and Storage technologies intervene. Or, if you prefer, zero-emission technologies. And it is the concept of a natural gas powered electricity power station with no emissions which was in effect offered around twelve years ago under the name of the ‘MATIANT cycle’. A name which is the result of a simple contraction of the names of the two designers of such a power station, Philippe Mathieu and the Russian Evgeny Yantovsky (that of the Russian researcher can be written with both a Y or an I). ‘This model is still much studied today and has given birth to similar cycles which have been developed the whole world over,’ underlines Philippe Mathieu.
Three carbon capture pathways
Capturing CO2 means separating it from the other components it is mixed with. Today there exist three carbon capture pathways.
The first is the de-carbonisation of steam. The principal constituent of steam is nitrogen, which typically represents around 75% of its volume, in a mixture with 13% CO2, 8% H2O and 3% O2. For a coal powered power station the operation consists of isolating CO2 by injecting into the steam a cleaning molecule (a solvent) which swallows the CO2 but not the nitrogen, which releases this CO2 when it is heated and which is then recycled to once again play its role as an ‘eater’ of CO2. The operation consumes heat, which involves an energy penalty as far as the system’s performance goes.
The second is the de-carbonisation of fuel (coal, petrol and natural gas), either through gasification or through regeneration. Hydrocarbons being composed of carbon and hydrogen, it is a question of freeing the hydrogen and keeping the carbon in the form of CO2. The chemical formula of the process of the regeneration of natural gas is overall summarised by a simple reaction: CH4 + 2 H2O → CO2 + 3 H2. A process which is thus at the same time a source of hydrogen, which can be used in fuel cells, in transport for example.
Another possible source of hydrogen without carbon is water electrolysis, but on condition that the process is powered by a carbon-free source of electricity, which means either nuclear or renewable energy.
The third is oxy-combustion, the design of which was inspired by the MATIANT cycle. Here the intervention takes place beforehand, before combustion, by injecting pure oxygen instead of air as a combustive. For example the reaction for methane is: CH4 + 2O2 → CO2 + 2H2O. To capture the CO2, it thus suffices to condense the water by cooling the mixture. The advantage of this third de-carbonisation path is that it rests on longstanding proven technology, that of separating oxygen by cryogenics (the Air Liquide business company). ‘It is the most mature, that is to say the nearest to being applied because in technological terms it seems easier to put into practice,’ says Philippe Mathieu. It is also the most efficient since, contrary to the first two capture options, which only keep around 90% of the carbon, oxy-combustion keeps practically 100% of it.