First of all, I am not really qualified to write about this (whenever I hear the word “capture” followed by some explanation why which chemical process is better than any other my brain takes a long break). However, during a recent Winter School I had the opportunity to listen to the work other PhD students do on Capture technology so I will give it my best!
Carbon Capture systems
The basic idea behind Carbon Capture is, as you probably have guessed already, to capture CO2 that is generated when fossil fuels are combusted. Now while this sounds quite simple (e.g. so just capture the stuff (a.k.a. flue gas) that comes out of the big chimneys), it is not that easy and there are three basic systems to capture CO2 from power plants: Post combustion, pre combustion and oxyfuel (see process flow chart from the IPCC report on CCS (2005)). Using post combustion systems CO2 is separated from flue gases that originate from the combustion of fossil fuels in the boiler. Pre combustion uses a different approach: Here CO2 is separated from the fossil fuel before the actual combustion process. For Oxyfuel capture systems the air used for the combustion process is nearly only oxygen, resulting in a flue gas that is nearly pure CO2. Here I will only write about post combustion capture (not necessarily the best nor the cheapest, but the one I somehow understand!).
Fossil fuels such as coal and gas are burned in a boiler to produce heat (and electricity). The flue gas derived from the combustion is then cooled (to around 40-60 °C) and stripped of its sulphur and other metal and trace elements before it enters the part where the actual CO2 capture takes place: The absorber. The absorber is basically a big vessel filled with CO2 -absorbing material (e.g. chemical solvents such as amines) through which the flue gas has to flow. About 85-95% of the CO2 will be absorbed by the solvents. Now the first step is done, the CO2 has been “captured” in the absorber. However, it would not be very inefficient if the absorbing material had to be replaced once it has done its job. Hence the CO2 has to be stripped from the absorbent. For most materials used in post combustion this stripping process (or regeneration) takes place at temperatures ranging from 100-140 °C. These temperatures are higher than the ones used for the absorption process and thus require additional thermal energy. Some of the energy can be derived from the cooling of the flue gas earlier in the sequence but the heating is in general quite energy intensive. But on the bright side the absorbents can be reused! During the “stripping” process CO2 is released, can be easily captured and is more or less ready for transport.
Building a capture unit for a power plant is quite expensive (around 1 billion $) but it is expected that with the rising number of units build the prices will go down. Theoretically most coal or gas power plants could be retrofitted with a capture unit. Practically there are a lot of challenges, one of them is the actual size of a capture unit. It needs about the space of a football field next to the power plant to be build! At the image below you can see the power plant of the ROAD project to the left and the capture unit (well what has been build already) to the right (image by E.ON).
Much research is still ongoing to figure out what materials/processes are the best for CO2 capture. We are searching for a material that absorbs as much CO2 as possible and does not need much energy input to release (or get stripped of) the absorbed CO2 . It should be reusable many times, cheap in large quantities and non-toxic. If you happen to have a material like that you could make many people very happy (and save the world)!
Sorry about this poor entry, I just cannot go into more detail without making up things! Hope it helped nonetheless 🙂 Next entry will be about the Transport part of CCS.