Study On The Absorption And Separation Performance Of Coal-fired Flue Gas Pollutants Using Ionic Liquids

With the increasing attention on environmental pretection,developing high-efficient and environment-friendly flue gas treatment technology with low investment and operation cost is becoming a main trend.As environmentally friendly green solvents,ionic liquids have several unique advantages in the gas absorption and separation,which are regarded as high potential absorbents for coal combustion flue gas treatment.This paper aimed at solving the key problems of the absorption and separation of flue gas using ionic liquids.The dynamic data of absorption enthalpy was obtained.The interaction between these gases and ionic liquids was analyzed.The method to improve the performance of absorption was put forward.It is of great theoretical significance and application value for the application of this technology in the field of flue gas treatment and multi-pollutant control.For the design of power plant flue gas treatment operation,the enthalpy of absorption is very necessary.At the beginning of our work,the ionic liquid with optimized enthalpy of absorption for CO₂ was screened out through quantum chemical calculation.The dynamic enthalpy of absorption was tested on a micro calorimeter.Results showed that the absorption enthalpy of[C₄mim][AC]for CO₂ was-41.5 kJ/mol CO₂ at 30?,which is between the the physical absorption of CO₂ in conventional ionic liquids and the chemical absorption of CO₂ in organic amine absorbents.Compared with the theoretical calculation,the experimental measurement is close to the real system,which provides effective data for the calculation of regenerative energy consumption.Compared with the MEA process,the regeneration energy consumption of the[C₄mim][AC]-CO₂ process is reduced by 53.7%due to the the unique properties of ionic liquids such as low specific heat capacity,optimized enthalpy and good thermal stability.The absorption peformance and the interaction mechanism of acid gases with ionic liquids were studied.The effects of SO₂ in the flue gas on the absorption of CO₂ by ionic liquids were experimentally investigated.The interaction mechanism of CO₂ and SO₂ in the ionic liquid absorption process was analyzed by means of structural characterization and quantum chemical simulation.Results show that CO₂ absorption capacity in[C₂mim][AC]was largely decreased by 25%in the presence of 0.2%SO₂.After five cycles of regeneration,the initial absorption capacity of CO₂ in[C₂mim][AC]remarkably decreased by 48%.The quantum chemical calculation indicated that the negatively charged O atom in acetate anion is the main active site for single CO₂ and SO₂ absorption.There was a competition relationship between CO₂ and SO₂,and CO₂ could hardly compete with SO₂ for the main active sites in[C₂mim][AC].The adverse effects of SO₂ should be fully considered in the application of ionic liquids in CO₂ capture technology.Moreover,ionic liquids can not only absorb CO₂ and SO₂,but also effectively remove NO_x under high pressure.The experimental results show that the removal efficiency of NO_x by ionic liquids is mainly determined by the oxidation rate of NO in the gas phase under high pressure and the absorption capacity of NO₂ in the liquid phase.The simultaneous removal performance of SO₂ and NO_x by[C₄mim][AC]was further studied,providing an interesting possibility for the application of ionic liquids in flue gas desulfurization and denitrification technology.It can be found that 98.0%SO₂ and 93.5%NO_x in the flue gas were removed by[C₄mim][AC]under 1.0MPa and 25?.To improve the absorption performance of functional ionic liquids with high viscosity,supporting ionic liquids on porous materials by physical impregnation method was used.This method can increase the gas-liquid contact area and enhance the absorption and mass transfer,providing a new way for the application of functional ionic liquids with high viscosity.In this work,mesoporous molecular sieve MCM-41 was employed as support material to prepare a novel absorbent?MCM-41-TMGL?.BET and TGA results indicate that the MCM-41-TMGL adsorbents showed high specific surface area and good thermal stability.Using MCM-41 as a support for TMGL greatly shortened the equilibrium time of SO₂ absorption and lead to an enhancement of the absorption rate.MCM-41-TMGL-10%with satisfactory regeneration properties could maintain 95%of the initial absorption capacity after 10 cycles of regeneration.Therefore,the supported ionic liquid adsorbents can solve the problem of low absorption rate and desorption efficiency caused by pure ionic liquids with high viscosity.Finally,the supported ionic liquids adsorbents were attempted for the removal of elemental mercury in flue gas.The adsorbents were prepared by the impregnation of[C₄mim][FeCl₄]on SiO₂.Compared with pure ionic liquids and ionic liquids mixtures,the supported ionic liquid adsorbents can effectively remove elemental mercury from flue gas.The optimized ionic liquid loading is 30%,of which the mercury removal efficiency reached as high as 97%.The reaction mechanism was proposed based on the TPD experiments.The ionic liquid[C₄mim][FeCl₄]served as the mercury oxidant for converting Hg⁰ to HgCl₂ and then the product HgCl₂ was physically adsorbed on the porous adsorbent.