The researchers from D.I. Mendeleev Russian University of Chemical Technology, N.I. Lobachevsky Nizhny Novgorod State University and Moscow State University developed a new scheme of capturing carbon dioxide from flue gases at power plants. This is post-combustion CO₂ capturing, when emissions are treated after combustion, which is especially important for operating CHPs and CHPPs, where drastic restructuring of equipment is impossible.
Today chemical absorption by amine solutions remains the main industrial method of CO₂ capturing. This technology is tried-and-true, but it requires huge energy consumption to regenerate the solvent, as well as other costs – corrosion, chemical agents degrading and waste generation. Membrane methods, when gases are separated going through semi-permeable material, seem to be more workable and eco-friendlier: they do not use chemicals, they are space-effective and modular-type. However, for flue gases, where CO₂ concentration is not high, and the pressure is close to the atmospheric pressure, such systems often turn out to be economically unfeasible, as they require either huge membrane surfaces or energy-intense gas compression.
The team of Russian scientists proposed an alternative – an improved membrane scheme based on the continuous membrane column principle. In such arrangements membranes operate as a single whole, and two gas flows are constantly circulating inside the unit. One of them is gradually purged of carbon dioxide and released, and the second one is accumulating CO₂ step after step. Due to this, the separation is enhanced without abrupt pressure boost or creating an extreme vacuum.
The scientists tested the efficiency of this scheme using Aspen Plus computer simulation. They reviewed two versions of the unit and compared them with the most advanced membrane solutions proposed earlier. Actual flue gases parameters of 600 MW power plant containing carbon dioxide, nitrogen, oxygen and steam vapor were used in the calculations. The objective was to capture at least 90% CO₂, to receive the product with at least 95% purity and to decrease the carbon dioxide content in the emissions down to 2% and less.
Both new schemes demonstrated good results, but the optional one was the configuration providing for collecting all gas flows after the membranes and channeling them to CO₂ cooling and liquefication. This approach allowed for minimizing the unit size significantly, which immediately decreases the cost of equipment and makes the system more space-effective. The simulation confirmed: both new schemes allow for achieving these parameters with certainty. At the same time, the configuration in which all the flows are combined before feeding them into the condenser proved to be the most efficient one. The minimal aggregate surface of membranes is required for its operation – circa 2.8 mln m², which is almost one-third less than of the closest analogues. This directly reduced the unit’s CAPEX and simultaneously allows for keeping the energy consumption at the competitive level.
The value engineering showed that in this configuration capturing one ton of CO₂ costs approximately USD 34. This is significantly cheaper than the classic two-stage vacuum configuration and is more beneficial than other modern membrane solutions. At the same time, the new unit is characterized by better balanced expenses: the money is spent not only on power supply, but a rationally distributed between the equipment and OPEX.
