It has come to be pointed out that one of the causes of the global warming is a greenhouse effect of CO2, and it has became an urgent task, also internationally, to provide a countermeasure for CO2 to protect the global environment against the warming. CO2 is generated by any human activities combusting fossil fuels, and there are increasing demands for suppressing CO2 emissions. Along with such an increasing demand, researches are energetically investing a method for reducing and collecting CO2 in combustion flue gas, and storing therein collected CO2 without releasing it to the air, by bringing combustion flue gas emitted from a steam generator in contact with aqueous alkanolamine solution and the like, for applications in a power plant that consumes a large amount of fossil fuels, such as a thermal plant. In addition, it has been suggested that the aqueous alkanolamine solution can reduce acid gas such as H2S (hydrogen sulfide), as well as CO2 (carbon dioxide).
Examples of the alkanolamines include monoethanolamine (MEA), triethanolamine (TEA), N-methyldiethanolamine (MDEA), diisopropanolamine (DIPA), and diglycolamin (DGA), and usually monoethanolamine (MEA) is preferably used. In addition, it has been suggested to use a cyclic amine such as piperazine as an auxiliary absorbing agent in these alkanolamines (Patent Document 1).    [Patent Document 1] U.S. Pat. No. 3,233,809    [Patent Document 2] Japanese Patent Application Laid-open No. H06-343858    [Patent Document 3] Japanese Patent Application Laid-open No. H08-257354    [Patent Document 4] Japanese Patent Application Laid-open No. H08-252430    [Patent Document 5] Japanese Patent Application Laid-open No. H06-343858