Comparison of the energy intensity of the selected CO₂-capture methods applied in the ultra-supercritical coal power plants

Limiting the emission of pollutants, including carbon dioxide, to the atmosphere has recently become an important element of global energy policy. This policy is a result of new and existing commitments, such as the Energy-Climate Package and the Emission Trading Scheme, introduced by Directive 2009/29/EC. These obligations impose the requirement for a significant reduction of CO₂ emissions from power generation processes on the member countries of the European Union. Such reduction is a difficult task, especially in countries in which the share of coal in the electricity generation process is significant. There are several methods for the reduction of emissions from energy production processes. One of the methods of increasing the efficiency of power systems is to increase the steam parameters. However, limiting emissions only by increasing the efficiency of energy production is not always sufficient. Thus, it is important to consider the implementation of carbon-dioxide-capture installations in supercritical power plants. The greatest disadvantage of existing separation methods is the high-energy requirements of the capture processes. In the case of amine-based chemical absorption, this requirement causes a decrease of the efficiency of the electricity production process by even as much as 10 percentage points. Therefore, new methods of CO₂ capture are required that will allow for the reduction of the energy consumption of the process. In this paper, the two methods of post-combustion carbon dioxide capture from the flue gas of an ultra-supercritical coal unit are described and the processes are compared. The selected methods are chemical absorption with the use of an aqueous solution of monoethanolamine (MEA) and membrane separation with the use of ceramic-polymer membranes that have an ideal selectivity coefficient equal to 100. To conduct these analyses, adequate models of the systems were built in the Aspen Plus software. A comparison of these methods was made primarily from the perspective of the energy intensity of the particular processes, but also the possibility of decreasing the energy intensity of that process was applied. The calculations for the separation process were made in such a way as to obtain the same CO₂-purity and CO₂-capture rate levels. The results of this analysis show that, in terms of energy intensity, both separation processes perform at a similar level, although the performance of membranes is slightly better. When the compression process of the captured CO₂ before transport to a storage place is considered, the absorption process is significantly less energy-intensive than membrane separation. However, the development of membrane techniques allows for the assumption that this technology could be competitive with other separation methods in the future, especially from the perspective of effectiveness and cost.