Experimental Research On Process Optimization And Absorbent Modification In Ammonia-based Carbon Capture

As one of the greenhouse gas,CO₂ from fossil fuel combustion in 2013 reached 32 Gt.Electricity and heating CO₂ emissions generated about 13.7Gt CO₂,of which coal-fired power plants reached 9.9Gt.In order to alleviate global warming,it is necessary to reduce carbon emission in coal-fired power plants.Absorption method,adsorption method,membrane method and cryogenic method are the main methods to capture CO₂ in flue gas in coal-fired power plant.Among them,chemical absorption,organic or ammonia solution,is one of the effective methods to remove CO₂ from flue gas.Compared with organic absorbent,ammonia method has many advantages such as adaptability for the power plant flue gas and its conditions.However the volatile characristic of ammonia goes against the application.Meanwhile ammonia escaped into the environment is the secondary pollution.Therefore to reduce ammonia escape in ammonia-based carbon capture is necessary.In this paper,self-designed absorption and desorption experiment systems with atmospheric pressure were applied to research ammonia escape in ammonia decarburization process.Firstly,simplified mathematical models were established to study mass transfer.Experiments about CO₂ absorption and ammonia escape were carried out in the bubbling reactor and wetted wall reactor.Under condition of 1L/min gas flow,0¹5vol% inlet CO₂ concentration,1⁸wt% ammonia concentration in solution,283³13K,the total volumetric mass transfer coefficients of CO₂ absorption and ammonia escape are 2.86×10-5¹7.9×10-5 mol/?m3·s·Pa?and 1.39×10-5⁴.34×10-5 mol/?m3·s·Pa?,interface area of in bubbling reactor is 75.19²56.41 m2·m-3.Under condition of 1³L/min gas flow,0¹5vol% inlet CO₂ concentration,0²0wt% ammonia concentration in solution,283³13K,the total mass transfer coefficient of ammonia escape in wetted wall reactor obtain es 0.612².556 mol/?k Pa×m2×h?.Based on experiments results,temperature is key control factor to mass transfer of ammonia escape,and promoting factor to mass transfer of CO₂ absorption under 303K;ammonia concentration in solution enhances the driving force of ammonia escape but has a little impact on mass transfer coefficients of ammonia escape,meanwhile ammonia concentration in solution accelerated CO₂ absorption primarily;inlet CO₂ concentration and ratio of liquid to gas have negative correlation to mass transfer of ammonia escape,but are non-critical factors.Then,based on the results from mass transfer research,the experimental studies on process optimization was carried out.Under condition of 1L/min gas flow,15/100vol% inlet CO₂ concentration,0²5wt% ammonia concentration in solution,283 K,crystallization and desorption using sound wave were carried out.It is confirmed that,for flue gas of 12¹5vol% CO₂,crystallization happenes observably in 14.9wt% ammonia solution.Desorption efficiency can be enhanced by 17 k Hz sound wave at 303³53K.However the comprehensive analyses showed that these two methods still had limitations,and could not achieve obvious effect on low ammonia escape.Therefore researches in next section was carried out in absorbent.Finally,various metal ions were used to study performance on ammonia escape reduction.Under condition of 1L/min gas flow,14vol% inlet CO₂ concentration,8wt% ammonia concentration in solution,288 K,the optimal concentration of Ni²⁺ is about 0.05⁰.06 mol/L in absorption?28% reduction efficiency?.Under condition of 1L/min gas flow,inlet N2 gas,1wt% ammonia concentration in solution,0.5mol/L NH4HCO3 concentration,353 K,The optimum concentration of Ni²⁺ is about 0.06 mol/L in desorption?33% reduction efficiency?.Moreover piperazine?PZ?was applied additionly for better performance of absorbent.Under condition of 1L/min gas flow,0/15vol% inlet CO₂ concentration,1⁴wt% ammonia concentration in solution,293 K,,existence of PZ promotes absorption efficiency of CO₂ obviously while increases little ammonia escape.Through various mixture experiments,several schemes were obtained.For example,to reach 70% CO₂ absorption efficiency and low ammonia escape,1wt% ammonia +0.05mol/L PZ +0.05 mol/L Ni²⁺ was available.With XRD,Uv-vis characterization,it is found that ammonia escape reduction by Ni²⁺ is caused by complexing reaction which is impacted by ammonia and Ni²⁺ concentration without PZ.When Ni²⁺ concentration is much higher or ammonia concentration in solution is much lower,Ni?OH?2 appears possibly.In this paper,basic data of the mass transfer,process optimization and absorbent modification were obtained.These data and schemes could be used as important reference for next research.Additive schemes proposed in the thesis could be used as application reference.