Experimental Study On CO₂ Capture From Flue Gas Of Coal-Fired Power Plant By Regenerated Aqua Ammonia

The researches on CO₂ capture in coal-fired power plants are essential in dealing with the global warming incurred by anthropogenic greenhouse gases emissions. Chemical absorption has been proved to be one of the most effective ways in removing CO₂ from flue gas after combustion. The available studies indicate that, comparing with MEA method, aqua ammonia has demonstrated significant advantage as a CO₂ capture agent, and has attracted more and more attentions. Considering the vast amount of CO₂ emitted from power plants, the regeneration and recycle of CO₂ capture agent would be a more appropriate approach.In this research, an atmospheric semi-batch absorption/desorption apparatus has been developed, and the whole process of CO₂ capture by regenerated ammonia has been thoroughly investigated. The study has been conducted from both the mass transfer and the chemical reaction kinetics'points of view. The factors affecting the CO₂ removal efficiency and desorption ratio have been discussed. Both the experimental results and theoretical analysis revealed that, in CO₂ capture by aqua ammonia, the process was diffusion controlled if the CO₂ loading of absorption solution was lower than 0.2 molCO₂/molNH₃. Reaction kinetics would take control if the loading is higher than 0.8 molCO₂/molNH₃. A region of transition existed between 0.2 and 0.8 molCO₂/molNH₃.Under the experimental conditions, the CO₂ absorption capability of ammonia was 0.7¹ molCO₂/molNH₃, whilst the averaged CO₂ removal efficiency might be above 90%. In CO₂ desorption stage, the CO₂ desorption ratios from carbonated ammonia solutions increased with increasing desorption temperature and solute concentration, although the increasing trends were slightly different. Between 60⁹0°C, the decomposition of ammonium bicarbonate was a 2nd-order reaction with reaction constant k=4.4 ×1013exp(-13541/T). The CO₂ loading of regenerated low ammonia concentration solution was above 0.4 molCO₂/molNH₃, however, for high ammonia concentration solution, the CO₂ loading of regenerated solution was lower than 0.2 molCO₂/molNH₃ if the ammonia concentration remained constant. Taking into account both the absorption and desorption efficiency, it is recommended to set the initial ammonia concentration at 7.25 mol/L and CO₂ loading at 0.45⁰.55 molCO₂/molNH₃ to achieve 90% CO₂ removal efficiency. The CO₂ desorption ratio was about 80% when the desorption stage set at 87.5°C. The recovered NH₃ from the desorption stream was capable of reducing the CO₂ loading of recycled regenerated solution to lower than 0.2 molCO₂/molNH₃.By simulating the application of CO₂ capture by regenerated aqua ammonia to a 300MW power plant, it has been estimated that the capital cost is 32.1% of the total capital investment of the power plant. The energy consumption of decarbonization agent regeneration is 1.67 GJ/tCO₂. If the energy were provided by extracting steam from the steam cycle of the utility, the overall electricity output would have decreased by 16%. This research revealed that ammonia method had the advantage over MEA method. It was economically feasible and should be further studied.