Capturing carbon from a process is energy-intensive. Depending on the technology employed, this can be due to various process steps. Besides the need to compress the CO2 to the transport and storage pressure for all three methods of carbon capture, special mention must be made of:
• air separation in the oxyfuel process and in the IGCC to provide a nitrogen-free oxygen carrier;
• desorption in CO2 scrubbing processes.
One future task will be to make CCS technology more efficient. The main objective here is to lower fuel consumption. Higher efficiency also reduces the expenditure on equipment needed (e.g., number of power plants). Another aspect is the additional lowering of CO2 emissions.
The main causes of the energy consumption mentioned are found in the separation processes. When oxygen is produced, air is separated; in CO2 scrubbing processes, components in the flue gas flow or the fuel gas flow are separated from one another. For these separation jobs, concept ideas exist that may be able to get along with much less energy than conventional cryogenic air separation or the CO2 scrubbing processes:
• chemical looping combustion (CLC);
• carbonate looping.
For membranes, suitable materials must be developed that are able to perform the separation function with sufficient efficiency (selectivity) and limit the expenditure on equipment (throughput, space requirement), and are resistant to the disruptive substances in the gas flow. In addition, constructive solutions must be developed for the membrane modules, since mechanical stresses occur owing to the differences in pressure and temperature.
Chemical looping is used to provide pure oxygen for combustion. The oxygen is added to combustion by way of a metal oxide. The process involves a link- up between two fluidized-bed reactors. In the first fluidized bed, a metal absorbs the oxygen from the air. The metal oxide that emerges is circulated to the second fluid-
ized bed where it releases the oxygen again directly in the combustion zone. The metal that is now available again is recirculated to the first fluidized bed where it absorbs more oxygen.
Carbonate looping is a post- combustion process for capturing carbon from a flue gas flow. For this purpose, the CO2 is brought into contact with CaO; CaCO3 forms. This process, too, involves two fluidized-bed reactors. The flue gas, which has a temperature of approx. 120 °C downstream of the power plant, is freed from CO2 in the first fluidized bed (carbonator) in a reaction with CaO. The exothermal reaction to form CaCO3 occurs in the fluidized- bed at a temperature of some 650° C. At this temperature, the waste gas free of CO 2 is conducted to a heat exchanger to use its heat, while the CaCO3 reaches the second fluidized bed (calciner). There, it is heated to a temperature of about 900 °C, which again drives out the CO2 from the CaCO3. In this way, CaO forms again and can bind CO2 once more in the first fluidized bed. Also, the CO2 is available in a separated form. Still, in the second fluidized bed, a considerable amount of heat must be added to the process. For this, coal is combusted in the fluidized bed. To ensure that the separated CO2 does not mix again with inert substances (especially nitrogen), the coal is combusted together with oxygen from an air separation unit. So, at this point, an oxyfuel process is implemented. The coal fired in this oxyfuel section of the carbonate-looping process amounts to about 1/3 of the entire coal input in such a power plant. The main positive effect on energy consumption is due to the fact that:
• compared with post-combustion CO2 scrubbing, no desorption heat is lost, since all heat expended can ultimately be used to generate steam at high temperatures;
• compared with the oxyfuel process, only 1/3 of the pure oxygen is needed, which significantly reduces the energy requirements for air separation.
The separation methods cited, besides their own direct development, also require a re-design and adaptation of the basic process, since their inclusion needs certain process parameters -above all high temperatures and increased pressures in places-which are usually not available at the points considered in the basic process.
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