Study On Interaction Mechanism And Characteristics Of Simultaneous Absorption Of CO₂ And SO₂ By Chilled Ammonia Solution

Environmental pollution and climate change are becoming more and more serious with the development of human economy and industry.Reducing CO₂ emissions from coal-fired power plants is considered as an economic and reliable way to mitigate global climate change.Chilled ammonia solution has attracted extensive attention in recent years due to its excellent CO₂ absorption performance,thermal stability,ability to absorb a variety of acidic gases and low ammonia volatility.However,the presence of SO₂ in flue gas has an important impact on the performance of CO₂ absorption by chilled ammonia process.In this study,the absorption heat of combined capture of CO₂and SO₂,as well as the interaction law between them by chilled ammonia solution were studied systematically and deeply.And then,the interaction mechanism and main influence factors of combined absorption of CO₂ and SO₂ in chilled ammonia solution were clarified.The thermodynamic models of NH₃-CO₂-H₂O,NH₃-SO₂-H₂O and NH₃-CO₂-SO₂-H₂O system were built based on the equilibruim model in Aspen Plus.And the contrabution of each independent reaction,especially the formation of NH₄HCO₃?s?to the overall heat of absorption was deeply analyzed.SO₂ was introduced in the NH₃-CO₂-H₂O system to study the mechanism of the influence of SO₂ on the heat of CO₂ absorption in chilled ammonia in NH₃-CO₂-SO₂-H₂O system.According to the precipitation conditions of solid NH₄HCO₃?s?,the effects of different temperatures and CO₂ loading on the precipitation of solid NH₄HCO₃?s?were studied.A reasonable absorption temperature and CO₂ loading were determined to regulate the precipitation of solid NH₄HCO₃?s?,which reduced the heat of absorption.When temperature is about 288 K,the critical CO₂ loading of solid phase NH₄HCO₃?s?formation is more than 0.8 mol CO₂/mol NH₃ with the minimal absorption heat as low as-43 kJ/mol CO₂.And at this temperature,it can effectively inhibit ammonia volatilization?the ammonia concentration at the outlet is less than 5%?.In addition,increasing SO₂ loading can reduce the amount of precipitation of the solid NH₄HCO₃?s?in the solution.The reaction heat of absorption SO₂ by chilled ammonia solution is higher than that of CO₂,but the overall heat of NH₃-CO₂-SO₂-H₂O system can be reduced by appropriately increasing the SO₂ loading.The mass transfer behavior of CO₂ during the simultaneous absorption of CO₂ and SO₂ by chilled ammonia was experimental determined using a double stirring reactor.Based on the three film theory,the mechanism of the effect of SO₂ on the mass transfer of CO₂ in chilled ammonia solution was proposed:when the flue gas contacts with the liquid phase,SO₂ preferentially reacts with NH₃ to form a SO₂ region at the liquid-gas interface,because the reaction rate between NH₃ and SO₂ is much faster than that with CO₂.This can increase the diffusion distance of CO₂ in liquid phase,thereby inhibiting the mass transfer of CO₂ in chilled ammonia solution.Based on the reaction plane three film theory,a mathematical model is established to prodict the effects of SO₂concentration,temperature and NH₃ concentration on the mass transfer of CO₂ in chilled ammonia solution.Increasing temperature,NH₃ concentration and reducing the SO₂ concentration in the flue gas promote the mass transfer of CO₂ by chilled ammonia prcoess.Using the Termolecular mechanism,the kinetics equation of CO₂ absorption by chilled ammonia solution is obtained:k_NH3^T=1.23×10¹⁶·exp?-9149.7?T?;k_H2O^T=5.09×10⁹·exp?-5687.8??.Based on the RateFrac module in Aspen Plus,a strict Rate-based model of simultaneous absorption of CO₂ and SO₂ by chilled ammonia solution is established.The effect of packed height,NH₃ concentration,CO₂ concentration in flue gas,SO₂ and CO₂ loading,gas and liquid temperature,liquid flow rate on CO₂ absorption performance is analyzed.