| It is well accepted that the gradually increased atmospheric concentration of greenhouse gases caused by human activities have resulted in the serious greenhouse effect and climate change. As the major greenhouse gas, carbon dioxide (CO2) is currently responsible for over 60% of the enhanced greenhouse effect. So, in order to restrain the continued deterioration of climate and environment, emissions of carbon dioxide in the coal-fired flue gases must be controlled. Several technologies can be selected to control CO2 emissions, such as post-combustion CO2 capture, pre-combustion CO2 separation and oxy-fuel combustion. But in the short-term, the conventional chemical absorption or novel membrane gas absorption technology is considered as the most promising technology. However, when heating regeneration system is adopted, the industrial applications of these technologies will be restricted due to their gigantic regeneration energy consumption. Based on it, new blended solvents and novel vacuum regeneration process will be developed and explored in this paper to reduce the regeneration energy consumption.During the course of CO2 absorption and regeneration experiments of the single absorbents, three key factors such as CO2 absorption capacity, the utilization percent of CO2 absorption capacity and the initial CO2 absorption rate were selected to evaluate the comprehensive CO2 absorption performance of solvents. In addition, the regeneration capacity, initial regeneration extent and regeneration energy consumption were adopted to evaluate the regeneration performance. The CO2 absorption performance of nine typical absorbents was ranked as the following:PZ>MEA> PG≈PT>DEA> DIPA>AMP>TEA>MDEA. The regeneration performance was also ranked as:TEA≈MDEA> PT>DEA>AMP>DIPA>PG>PZ>MEA. Based on these results, the mixing method to form the blended absorbents with higher CO2 absorption and regeneration performance was put forward, which is that using the absorbents with higher CO2 absorption performance and the absorbents with higher regeneration performance to form the blended absorbents may get the comprehensive CO2 absorption and regeneration performance, like MEA/MDEA or MDEA/PZ blended absorbents. When MEA/MDEA (main absorbent/additive) blended absorbents were used to absorb CO2 and then regenerated, the relative mass concentration ratio of MDEA to MEA (β) can be determined to 0.2 in order to get the optimal CO2 absorption and regeneration performance. As for MDEA/PZ solvents, the relatively optimal mass concentration ratio of PZ to MDEA (β) is about 0.4.MEA/MDEA and MDEA/PZ blended solvents were selected to capture CO2 using the hollow fiber membrane contactors and packed column. The results show that whatever operating conditions were selected, the blended solvents with the optimal concentration ratio have the higher CO2 absorption performance than others. In addition, it is worthy to be noticed that compared to the single MEA solution, MEA/MDEA (β=0.2) and MDEA/PZ (β=0.4) can reduce the regeneration energy consumption by above 10% in the well-designed system.In order to get the lower regeneration energy consumption, the novel regeneration technology using vacuum technology was put forward. Firstly, the vacuum regeneration feasibility and mechanism of CO2-rich solution were investigated in a fixed experimental device. The results show that vacuum regeneration may be viable if the bulky CO2-rich solution can be easily partitioned into the continuous single thin-layer solutions with an appropriate thickness. In addition, the regeneration performance is also strongly depended on the CO2 amount contained in the rich solution.The hydrophobic hollow fiber membrane contactors were adopted to regenerate the rich solution. Additionally, the effects of regeneration pressure, temperature, absorbent concentration, flow rate and CO2 loading of rich solution on vacuum regeneration performance were experimented. After the estimation of regeneration energy consumption, it is interesting to find that vacuum regeneration can reduce the regeneration energy consumption by above 50% compared to the conventional MEA heating regeneration process. Two mathematical models were developed to simulate the mass transfer coefficient of membrane vacuum regeneration. The predicted results show that when the regeneration pressure is less than the water vapor pressure over the rich solution at the regeneration temperature, the simplified model where gas phase resistance can be ignored can be recommended to predict the mass transfer coefficient, and the deviation was found to be within±10%. In addition, the effect of wetting ratio of membrane pores on the total liquid phase mass transfer coefficient was predicted by using the model. The theoretically predicted results show that the regeneration performance decreases considerably with the increase of wetting ratio, and the regeneration mass transfer will be finally controlled by membrane phase mass transfer.Effect of molecular structure of absorbent on the regeneration performance was experimented by using membrane vacuum regeneration. It can be founded that the increase of the active hydrogen atom numbers in the amino group will decrease the regeneration performance, but the increase of carbon chain length, numbers of hydroxyl group (OH) and amine group will contribute to improve the regeneration performance. In addition, the gigantic groups having the steric hindrance effect or carboxylate group around the amine group will also improve the vacuum regeneration performance.Finally, the comparative analysis of CO2 separation from coal-fired flue gas by membrane gas absorption technology (MAS) and chemical absorption technology (CAS) was carried out in this paper. Results show that when fresh membranes were used, MAS can be considered as the promising alternative to CAS to capture CO2 from flue gas because of its higher CO2 absorption performance. But, when all the membrane pores were wetted or 50% of pores were plugged, the experimental results inversely imply that the superiorities of MAS over CAS disappear. In addition, a newly-built ultra supercritical PC power plant with 840-MWe-gross-output was selected to act as the reference base to evaluate the effect of CO2 separation and compression on the power plant performance. Three CO2 separation technologies were adopted, such as conventional chemical absorption and heating regeneration technology (CAS+HRS), novel membrane absorption combined with heating regeneration technology (MAS+HRS) and novel membrane absorption combined with membrane vacuum regeneration technology (MAS+MVR). The results show that if the membrane price is less than RMB (?) 35/m2 and membrane real lifetime is longer than 5 years, the cost of CO2 avoided using MAS+MVR (about RMB Y158/tCO2) can be reduced by about RMB(?)96/tCO2 compared to CAS+HRS, and the cost of CO2 avoided using MAS+HRS (about RMB(?)217/tCO2) can be reduced by about RMB (?)40/tCO2. However, when the membrane is more expensive or membrane real lifetime is very shorter, the cost of CO2 avoided of CAS+HRS will be lowest among the three technologies, which suggests that CAS+HRS may be more suitable for this condition. It can be concluded that MAS may be more suitable for the larger-scale CO2 separation projects or the future projects in which membrane has the very lower price and longer lifetime. So, the statement that MAS is prior to CAS will be somewhat arbitrary unless the membrane wetting and plugging prevention technologies are very mature and low-cost in the future. In addition, using the blended solvents developed in this paper to replace the conventional single MEA can lead to the reduction of the cost of CO2 avoided by about 5%～10%:So, it can be concluded that the energy consumption of CO2 chemical absorption process may be considerably decreased when the new blended solvents and membrane vacuum regeneration process are adopted.
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