Out the flue, out of mind – in fact, carbon capture requires more than a big magnet– New technologies could help to separate climate-damaging carbon dioxide directly from exhaust gas. Now researchers place great hopes on chemical absorption processes in the smokestack.
Extract, separate, compress – CO2 shall be removed from the exhaust. And, if possible, put somewhere far, far away. Carbon capture and storage (CCS) is the name of the process with which industrial flue gases could be made virtually CO2-free. Given a large scale application, CCS could achieve between 10 % and 55 % of the global emission targets by 2100, the world climate council IPCC estimates. Individual power stations could even reduce the CO2 content in flue gas by 80 % to 90 %.
Since the 1990s, carbon dioxide from gas fields has been separated and pumped back. This works well as the CO2 in natural gas is of high purity. In the smokestack, however, the gas is mixed with other combustion products and atmospheric nitrogen – the partial pressure of CO2 in power station exhausts is around 15 %.
Today, development for CCS concentrates on three essential processes: scrubbing out of the exhaust (post-combustion), CO2 reduction by coal gasification (pre-combustion), or the Oxyfuel process. In the latter, fuel is burnt in an atmosphere of pure oxygen and fed-back flue gas. The relatively clean, highly concentrated carbon dioxide can be easily separated by conventional methods.
An Oxyfuel process at a 30 MW pilot installation in Germany achieved separation efficiencies of up to 90 %, while extracting a total of 10,650 tonnes of CO2. Alternatively, it is possible to remove a portion of the carbon even before combustion by coal gasification. In a second step, the resulting carbon monoxide is split catalytically to a mixture of CO2 and hydrogen.
At high pressures, the CO2-partial-pressure is high enough here to enable physical absorption at high efficiency. The advantage of this so-called pre-combustion technology is its lower efficiency loss (below 10 %).
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