Research Of Carbon Capture Process Using Ammonia As Absorbent Based On Antisolvent Crystallization

Global warming caused by large greenhouse gas emissions is a very serious environmental problem,in which CO₂ is the main source of greenhouse gases,accounting for 80%of total human greenhouse gas emissions.The CO₂ emissions generated by thermal power generation in China account for more than 50%of the total emissions.It can be seen that carbon emission reduction is imminent in China.Decarburization by ammonia has the advantages of weak corrosivity,relatively lo w energy consumption for regeneration,low cost and comprehensive removal of various pollutants.Therefore,it has broad application prospects in CO₂ emission reduction.However,at present,decarburization by ammonia still has problems such as slow absorption rate,serious ammonia escaping and regenerative energy consumption to be further reduced,which limits the industrial application of the method.Aiming at above problems,this paper proposes a carbon capture process which enhances the process of mass transfer and low energy consumption by ammonia method.The method of applying static magnetic field strengthens the activity of low concentration ammonia to promote the mass transfer process and improve the absorption efficiency.The crystallization process of carbonized ammonia is enhanced by antisolvent method,and the regeneration of the rich carbonized ammonia is replaced by crystal product regeneration which greatly reduces the regeneration energy consumption.At the same time,the problem of ammonia escape,low absorption efficiency and high regeneration energy consumption in the process of decarburization by ammonia are solved to some extent.Aiming at the problem of ammonia escaping and low absorption efficiency,this paper proposes a method of adding static magnetic field to enhance mass transfer and increase the reactivity of low concentration ammonia.Using ammonia as the absorbent,the semi-continuous absorption system was used to study the mass transfer characteristics of carbon dioxide absorption under static magnetic field condition.Under the experimental research conditions,the bubble reactor can be used as an absorber for efficient mass transfer behavior and the mass transfer process can be strengthened by the magnetic field.This study found that the volumetric mass transfer coefficient of CO₂ during the absorption process can increase by 17.8%due to the presence of magnetic field when the concentration of ammonia is 6 wt.%.It indicates that the reactivity of the low concentration ammonia water itself can be enhanced by the static magnetic field.When the CO₂concentration in flue gas is low or flue gas flow rate is low,the magnetic field promotes the mass transfer process more obviously.The experimental results show that when the CO₂ concentration in flue gas or the flue gas flow rate is low,the ammonia water concentration is 10 wt.%,and the volumetric mass transfer coefficient of CO₂ under static magnetic field can reach 18.7×10^-5 mol/（m³·s·Pa）,which is 15.3%higher than the condition of non-magnetic field.Under some conditions,the carbon dioxide removal efficiency can reach 98.5%.The phase interface area of the bubble reactor can rise from 177.4 m²/m³ to 199.2 m²/m³ due to the presence of a magnetic field.It provides an important reference for the process optimization of static magnetic field enhanced absorption process.In view of the problem of high energy consumption for regeneration,this paper proposes a method strengthening the crystallization process by antisolvent crystallization and replacing the rich liquid by crystal product which can greatly reduce the regeneration energy.The key of this method is to add ethanol to the carbonized ammonia to reduce the solubility of solute to enhance the crystallization process.The key of this study is how to improve the crystallization yield,how to control the crystal size and how to make the rich liquid with lower carbonization ratio to produce crystal.Therefore,it is necessary to understand the thermodynamics and kinetic behavior of solute in carbonized ammonia in mixed solvent of ethanol-water.The crystal product was ammonium bicarbonate mainly determined by XRD method.Therefore,the solubility of ammonium bicarbonate in ethanol-water mixed solvent was determined by the static method.The solid-liquid equilibrium behavior of the binary system is studied in detail,providing thermodynamic basis and related data support for the study of the antisolvent crystallization process.The thermal motion of all molecules in the system and the interaction between the various phases fully reflect the crystallization thermodynamics of the crystallization system.In this paper,the thermodynamic behavior of the solution of ammonium bicarbonate in the crystallization process is discussed.Since the carbonized ammonia is composed of various components,based on the particularity of this study,the effect of ammonia and ammonium carbamate which were the main components in the carbonized solution are further studied.The influence of solid-liquid equilibrium,especially the solubility of ammonium bicarbonate in different ratios of binary mixed solvents,was obtained and related mathematical models were o btained.The results show that the solubility of ammonium bicarbonate decreases with the increase of ethanol content in the mixed solvent,and increases with the increase of temperature,but the effect is not significant when the molar fraction of ethanol is higher than 0.3.The（CNIBS）/Redlich-Kister equation was determined to be the appropriate solubility model for this study and corresponding model parameters were obtained.The addition of ammonium carbamate promotes the crystallization of ammonium bicarbonate,and ammonia has an inhibitory effect on the crystallization of ammonium bicarbonate in the binary mixed solution.In the ethanol-water binary solvent system of ammonium bicarbonate,the dissolution heat of ammonium bicarbonate ranges from 16.41 to 29.17 kJ/mol,which provides thermodynamic basis and related data for the study of the dissolution crystallization process.The crystallization kinetics of the crystal product ammonium bicarbonate in ethanol-water mixed solvent was studied based on a large number of experiments using reasonable assumptions and simplified processes to study the effects of temperature,solvent composition and magnetic field on the induction period.The entropy factor determines the growth mechanism of the crystal and the kinetic model of the crystallization process.The kinetics of ammonium bicarbonate at different operating parameters is studied by intermittent dynamic method and the experimental data is used to calculate the grain number calculation equation using linear least squares regression.The experimental data shows the nucleation rate and growth rate under different working conditions.In order to analyze the factors of crystallization kinetic behavior.Finally,the experimental data is used to calculate the comprehensive rate equation of the crystallization process under different working conditions,which shows the effect of different working conditions on the crystallization process.The results show that at the same temperature,with the increase of the content of antisolvent,the surface tension of the solid liquid increases,and the induction time becomes significantly longer.Under the same solvent composition conditions,as the temperature increases,the surface tension of the solid solution decreases,and the induction period is significantly shortened.The presence of a magnetic field can significantly reduce the induction period of the dissolution crystallization process.The crystal growth under the working conditions in this paper belongs to particle size independent growth.Growth kinetics model based on crystal nucleation rate and growth rate are⁰=2.994×10^81.907.For another key problem crystal regeneration in this study was further analyzed by a thermogravimeter to determine the appropriate regeneration temperature and heating rate.The results show that the crystal product in this experiment begins to decompose at room temperature,and the decomposition rate increases with the increase of heating rate,what’s more,the conversion rate can reach 100%.As the heating rate increases,the activation energy decreases.When the heating rate is15°C/min,the activation energy can be as low as 48.38 kJ/mol.When the constant temperature is 80°C,a better regeneration rate can be obtained,which is suitable for extracting heat from the waste heat of the desulfurization system as a regenerative heat source,further reducing the system energy consumption.Using Aspen plus to establish a simulation model for antisolvent-enhanced crystallization and carbon capture process,the crystallization and regeneration process were simulated and analyzed.From the energy quality,combined with the principle of“energy cascade utilization”,the heat pump technology was used to cycle the wet desulfurization unit.The study shows that the saturated wet flue gas at 56°C can obtain the latent heat of 50.894 MW by reducing 10°C.When the working fluid of heat pump is R245fa and the condensing temperature is 85°C,the heat of unit quality working fluid from a single heat pump is 161.1 kW,the compressor power is37 kW,and the absorbed heat is 129.65 kW.In summary,this thesis proposed a technical route for strengthening the mass transfer process of low-concentration ammonia water by static magnetic field,and provided basic data for the antisolvent crystallization process of low-carbonized ammonia and crystal product regeneration process,what’s more,at the same time the upgraded tail gas as a regenerative heat source can further reduces the energy consumption for regeneration,and by these ways the two technical barriers of low absorption efficiency and high renewable energy in the ammonia-based method were solved to some extent.