Study On Energy Requirement For CO₂ Capture With Aqueous Ammonia Solution

Aqueous ammonia has been proposed as a promising absorbent for CO₂ capture from the flue gas of coal-fired power plants.Its advantages include low solvent cost,high CO₂ carrying capacity,low reaction heat,resistance to thermal and oxidation degradation and the potential for the removal of multiple acid gases in the flue gas.However,the aqueous ammonia based CO₂ capture process still has high energy requirement for CO₂ capture,leading to a remarkable decrease in the power generating efficiency and increase in the costs of the power plant when the CO₂ capture unit was installed.In this work,the energy requirement of aqueous ammonia based CO₂ capture process was studied and methods to decrease the energy requirement were proposed.The operating conditions of ammonia based CO₂ capture process were optimized.The rate-based model for NH₃-CO₂-H₂ O system was developed with Aspen Plus and validated against thermodynamic data and results of pilot plant trials.With this model available,the effects of operating parameters on the energy requirement of ammonia based CO₂ capture process were investigated.Both the energy requirement related to CO₂ regeneration and NH₃ recovery were considered.The optimal conditions were identified through orthogonal analysis method and the minimum energy requirement was detected as 2.85 MJ/kg CO₂ for CO₂ regeneration process and 4.07 MJ/kg CO₂ for the whole process.A novel process for aqueous ammonia based CO₂ capture was developed.With the integration of CO₂ absorption,CO₂ regeneration,product purification,SO₂ absorption,NH₃ abatement and NH₃ recovery,the total energy requirement for CO₂ capture was significantly reduced.The rate-based model for NH₃-CO₂-SO₂-H₂O system was developed and validated.The effects of operating parameters were experimentally studied.The feasibility of this novel process was evaluated with model.It is revealed that this novel process can achieve the ammonia abatement and recovery in an energy-saving way as well as the elimination of solid precipitation in CO₂ stripper with no need of extra heat input.The minimum energy requirement of aqueous ammonia based CO₂ capture process can be reduced to 2.6 MJ/kg CO₂.The NH₃/PZ blended solution was applied for CO₂ capture.A rigorous,rate-based model was developed and validated against the experimental results.The model was adopted to predict the CO₂ capture process under experimental conditions to gain insights into the CO₂ absorption with NH₃/PZ blended solutions.The addition of PZ into the aqueous ammonia was proven to be able to enhance the CO₂ absorption with aqueous ammonia solution remarkably and increase the ammonia loss rate at the same time.From the perspective of energy requirement,it is revealed that the addition of PZ will decrease the energy for CO₂ regeneration and increase the energy for NH₃ recovery,leading to the increase in the total energy requirement under conventional process.However,it was identified that the total energy requirement for CO₂ capture in the novel process can be further reduced to 2.47 MJ/kg CO₂.The aqueous ammonia based CO₂ capture process was integrated with coal-fired power plants and the effects of CO₂ capture unit on the power generating efficiency and costs of the power plant were investigated.The net efficiency of power plant will be decreased from 38.9% to 27.7%,30.3% and 30.9% respectively when the conventional ammonia based CO₂ capture process,the novel ammonia based CO₂ capture process and the novel NH₃/PZ blended solution based CO₂ capture process were adopted,leading to a CO₂ avoided cost of $83,$66 and $62/t CO₂.