The Basic Idea of Carbon Capture

Proceeding from the above task description of capturing carbon from the gas mixture CO - , N2 and H- O, the oxyfuel process starts by removing the nitrogen. This is done prior to combustion. In conventional firing systems the nitrogen reaches the flue gas via the combustion air. An upstream air-separation unit removes the nitrogen from the combustion air, and only the oxygen is used to burn the coal. Hence the process name: oxyfuel -combustion with pure oxygen.

Air separator»

Flue gas: C02> yf2

Figure 11.5 Basic idea of carbon capture based on the oxyfuel process.

Flue gas: C02> yf2

Temperature decrease, condensation

Figure 11.5 Basic idea of carbon capture based on the oxyfuel process.

So the flue gas now only consists of C02 and H20. The second step in carbon capture consists in the removal of the H20, which is separated from the C02 by lowering the temperature and condensing out. The basic idea of carbon capture based on the oxyfuel process is illustrated in Figure 11.5. Technological Implementation

In a real situation, however, special heed must be paid in the oxyfuel process in particular to the other components of the flue gas, since they have a considerable influence on the quality of carbon capture. This specifically concerns gaseous components like 02 or N2, which can enter the process and, hence, the flue gas via various routes. Such gaseous components impair the condensing out of the water, since they lower the partial pressure of the water in the flue gas, while making it more difficult to obtain the necessary purity of the C02.

The gaseous substances of relevance here include above all the air components oxygen, nitrogen and argon. These gases can access the flue gas path at several points.

• The oxyfuel process ideally requires pure oxygen. Air separation produces an oxygen that still contains the other two main air components as well. To limit the investment and operating costs for air separation, an oxygen content of 95% is usually chosen. Higher purities, for example, 99%, make C02 treatment at the end of the process easier, but make air separation much more costly.

• The next point at which the gases can reach the process is combustion. If combustion is to be efficient, complete and low-polluting-this specifically concerns the formation of NOx and CO as well as unburnt matter in the ash-oxygen must be added hyperstoichiometrically, that is, in excess. The excess oxygen can be found in the flue gas.

• Gaseous substances also reach the process via leaking units. The steam generator has, for example, sliding points to offset heat expansion in operations. In addition, the flue gas pass is operated in vacuum mode, so that the hot flue gases cannot escape. Conversely, however, it is possible in this way for air to reach the flue gas pass at the leaks. So, the oxyfuel process requires heavy outlays to seal the steam generator and the other components and, above all, to keep them sealed over the years of operation.

This being so, one focus of attention when designing the oxyfuel process is on the optimal balance between the outlays for avoiding gaseous substances at the front end of the process and the outlays for treating the CO 2 at the end of the process chain.

Combustion in the oxyfuel process requires special measures. Since combustion is carried out with pure oxygen, there is no atmospheric nitrogen, which would act in various ways in conventional combustion systems. During combustion, nitrogen as inert substance limits the temperature; in the flue gas, nitrogen accounts for the largest share of the mass flow. In addition, the lack of atmospheric nitrogen in the oxyfuel process shifts the entire thermal engineering. To counter these effects, cooled2off flue gas, that is, CO 2 and possibly H2O is re-circulated from the end of the oxyfuel process chain and added to the combustion and in this way replaces atmospheric nitrogen. In this way, process conditions are obtained again that are similar to air combustion, although it must be borne in mind that CO 2 has physical properties that are different from those of N 2 . For instance, CO2 has a stronger damping effect on the reaction rate of combustion. Also, CO2 has different heat radiation properties and a different effective heat capacity which is important for the heat transfer process in the steam generator. Determining the re-circulated CO2 quantity, therefore, is of central importance in designing the oxyfuel process.

In large steam power plants, pulverized-coal combustion has gained acceptance for the burning of coal. With few exceptions, all new coal-fired power plants for the public power supply are based on this technology in which the coal is finely ground and combusted in a large number of burners. As alternative, fluidized-bed combustion, by contrast, is only used where special marginal conditions apply. For the oxyfuel process, too, pulverized-coal combustion is at the center of developments. Fluidized-bed combustion has special advantages for the oxyfuel process, however, so that it is attracting renewed attention. In fluidized-bed combustion it is possible, for example, to dimension for a lower amount of re - circulating CO 2 than in the case of pulverized-coal combustion. This has a positive effect on the auxiliary power requirement and reduces the volumetric flow in the overall flue gas pass. Components can be built correspondingly smaller. Also, some progressive developments to reduce energy consumption in carbon capture are based on fluidized^ed combustion, which will be explained in greater detail in Section 11.3.4 on further developments in carbon capture.

To obtain the necessary purity of the CO2 using the oxyfuel process, additional outlays are needed in the CO2 treatment. The nuisance components are, above all, O2 and H2O, but other gaseous companion substances should be removed from the CO2 flow as well. On the other hand, air pollutants that also have to be taken into account in principle in the transportation and storage of CO2 are sufficiently contained, thanks to the usual flue - gas cleaning facilities.

Even where the oxyfuel process is optimally designed, O2 cannot be avoided entirely owing to the oxygen excess needed for the combustion and owing to the leaks, while H- O is still present in the CO- flow even after the lowering of the temperature and the condensing out in line with the temperature- dependent partial pressure. Additional process steps must be taken to separate O2 and H2O from the CO2 flow. In earlier concepts, H2O was bound by a hygroscopic solvent. Due to the patently rigorous requirements to be met by CO2 purities, however, more recent concepts are based on costly distillation methods, so that both H2O and O2 as well as other gases are removed.

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