3.2 Separation techniques
During the combustion of coal, oil, gas and biomass, the fuel reacts with oxygen in the air and forms CO2. In the ambient air, oxygen is mixed with other gases, meaning that waste gases also will become mixed with nitrogen and other gases, from the air as well as from the fuel. The concentration of CO2 in flue gases from natural gas combustion amounts to 3-4 %, from coal 13-15 %, and from 36 The compression and geological storage of CO2 requires it to be separated into a pure stream. As biomass has such a high density of CO2 in its flue gas, it is easier to capture the CO2 in the flue stream. This fact partly offsets the smaller scale of the biomass facilities.14-17 %.
There are essentially three techniques for removing CO2 from combustion gases. CO2 can either be separated prior to combustion, so-called pre-combustion, or after, post-combustion. The third technique is to burn fuel with pure oxygen, known as oxy-fuel, whereby pure CO2 is formed, see Figure 10.
There are different technology variants to each method. For the post-combustion techniques,may be used, or chilled ammonia. The Norwegian company Sargas has developed a method based on pressurized combustion and post-combustion separation, so called PFBC technology (Pressurized Fluidized Bed Combustion). Typically, 85-90 % of the CO2 can be separated, but with some methods such as oxy-fuel or PFBC technology, a separation of more than 95 % is achieved.
In the pulp and paper industry, calcium hydroxide is being used for the cleansing of exhaust gases to form PCC (Precipitated Calcium Carbonate). During that process CO2 is removed. For example, 40 - 50 000 tonnes of CO2 per plant, most of which is of biotic origin, is separated and bound in this way at two Swedish pulp plants. PCC is used as a component in the production of paint, glue, sealants, plastics, rubber and pharmaceuticals. When these products decompose through degradation or combustion, the previously bound CO2 returns to the atmosphere within months or a few years, and is not permanently sequestered as it would have been with geologic storage. For this reason, the process does not provide any long term climate benefit.37
Figure 10 The four main technology concepts for CO2 capture
Apart from combustion, there are processes that create relatively pure streams of CO2 at levels of 9599 % concentration. One example is the fermentation of ethanol; another is the upgrading process for biogas, whereis refined to transport fuel quality. Yet another process that provides pure streams of biogenic CO2 is black liquor gasification, a process in which a by-product from pulp mills is converted into automotive fuel. In all three cases, the cost of setting up a carbon storage system can be greatly reduced, as CO2 is already separated as part of the underlying processes.
36 Grönkvist et al., 2008
37 Karlsson et al., 2010