Investigative Studies of the Stability of an Amine Blend in the Presence of Flue Gas Dust and O2 in a CO2 Capture Process

By: Thomas Boakye

This research focuses on examining the stability of an amine blend in the presence of impurities from flue gas. Flue gas from coal-fired power plants, cement, iron and steel, and natural gas or gas-fired power plants mostly include pollutants such as NOx, O2, SOx, and flue dust, which potentially decrease the performance of the amine blend in when CO2 is considered gas of interest. Flue dust comprises of small inorganic oxides particles such as Fe2O3, ZnO, Al2O3, CuO, MgO, MnO, and SiO2. It is important to understand how these oxides dissolve and interact with the amine blend in order to design CO2 capture systems that are more reliable and effective.

In an oxidative amine degradation, these inorganic oxides act as catalysts. These metal oxides dissolve in the amine and release metal ions, which react in a direct electron transfer redox reaction with the amine. Ammonia is the main emission product from amine degradation and is caused by the dissolved metal acting as a Lewis acid by accepting an electron from the amine and forming an amine radical.  The dissolved oxygen re-oxidizes the reduced metal, and this process repeats itself until the amine solvent completely degrades.

Prior to the design of the stability experiments, solubility of various metal oxides including Fe2O3, ZnO, MnO and Al2O3 which were selected based on their dominant amount in a typical iron and steel industry flue gas was first explored. The oxides were put in a 200 ml, 5M and 0.30 mol CO2/mol biblend of MEA/DMAE solvent (Nyarko, 2023). ZnO recorded the highest solubility followed by Fe2O3, MnO and Al2O3. The maximum solubility value measured for ZnO is due to its superior chemical affinity to water molecules that the rest of the metal oxides cannot match up. A 21-day continuous glass-scale degradation experiment was conducted by passing oxygen of varying concentrations (with nitrogen balance) through a defined volume filtrate of the prepared metal oxide dissolved amine solvent in a three-necked flask exposed to varying absorber temperatures. The degradation rate (mmol/goxide.L.hr), accumulated amount of ammonia emissions (ppmv) and ammonia emissions rate (ppmV/hr) show that Fe2O3 has the greatest influence on degradation of the CO2 loaded bi-blend of MEA/DMAE amongs