Study On Characteristics Of CO₂ Adsorption And Recovery From Wet Flue Gas And Process Optimization

Under the current background of carbon neutrality,it is of great significance to complete the "low carbon" work efficiently.Carbon capture by using adsorption method is a simple,environmentally friendly and effective "low carbon" measure.As the main energy source of human society at present,fossil fuels are the main source of carbon dioxide emissions worldwide by the form of flue gas released after combustion.The moisture content of flue gas brings a challenge to carbon capture by using adsorption method.Current researches about carbon capture and recovery by using adsorption method are mainly based on dry flue gas.Effects of water on recovery process remains to be further explored.New adsorption equilibrium models for wet flue gas on hydrophilic/hydrophobic adsorbents has been propsed in this paper.At low humidity,hydrophobic adsorbent activated carbon can adsorb a certain amount of H2O due to the objective existence of a few hydrophilic groups on the surface.In this paper,Langmuir model is used to describe the surface adsorption of H2O,CO2 and N2 on activated carbon.The pore size distribution of activated carbon characterized by an exponential fraction function combined with the Kelvin formula are used to describe the capillary condensate adsorption of H2O at high humidity.On this basis,the Langmuir-arc-tangent adsorption equilibrium model of H2O on activated carbon in the full humidity range is derived,and then the adsorption equilibrium model of multi-component wet flue gas(H2O/CO2/N2)on activated carbon is obtained.Al2O3 and 13X will also adsorb more H2O at low humidity because their hydrophilic characteristics.In this paper,Sips model is used to describe the adsorption of CO2,N2 and that part of H2O on Al2O3 and 13X.The effective pore size distribution of Al2O3 and 13X characterized by Gaussian distribution function combined with the Kelvin formula are used to describe the capillary condensate adsorption of H2O at high humidity.On this basis,the Sips-Gaussian adsorption equilibrium model of H2O on Al2O3 and 13X in the full humidity range is derived,and then the adsorption equilibrium model of multi-component wet flue gas(H2O/CO2/N2)on Al2O3 and 13X is obtained.Above models can accurately describe the adsorption behavior of wet flue gas on different adsorbents.Different adsorbents have their own advantages and disadvantages in carbon capture from wet flue gas.On the basis of the adsorption equilibrium model,carbon capture and recovery process from wet flue gas by using hydrophilic/hydrophobic adsorbent fixed bed were numerically studied.The adsorption and desorption characteristics of each component of wet flue gas were analyzed in detail.The influence of water vapor on the process of carbon capture and the heat/mass transfer law in adsorption bed were explored and revealed.In process of CO2 recovery from wet flue gas by using hydrophobic adsorbent(activated carbon),for the condition of this paper,the average bed temperature increases from 340.63 K to 368.34 K,the average bed temperature change during an adsorption and desorption period increases from 17.21 K to 34.58 K,and the CO2 throughput amount decreases from 0.133 mol to 0.100 mol with the inlet relative humidity increasing from 0 to 100%at 333 K.The recovery rate of carbon dioxide decreases with the increasing of relative humidity when the inlet temperature is constant.In the high humidity section,due to the large H2O adsorption amount of capillary condensation,large adsorption heat released and the recovery rate decreased more obviously.The recovery is stable at first and then decreases with the increasing of inlet temperature when the inlet H2O concentration is constant.It is because that the relative humidity is high and H2O adsorption amount of capillary condensation is large at low feed temperature,and the adsorption heat released by capillary condensation is the main factor affecting the carbon recovery effect.At high feed temperature,the relative humidity decreases and H2O adsorption amount of capillary condensation decreases,and the heat carried by the feed gas is the main factor affecting the carbon recovery effect.In process of CO2 recovery from wet flue gas by using hydrophilic adsorbent(Al2O3+13X),alumina is used as the pretreatment layer mainly to adsorb water vapor,and 13X is used as the main adsorption layer mainly to adsorb carbon dioxide.For the condition of this paper,the average bed temperature increases from 342.98 K to 445.94 K,the average bed temperature change during an adsorption and desorption period increases from 16.30 K to 30.15 K,and the CO2 throughput amount decreases from 0.149 mol to 0.105 mol with the inlet relative humidity increasing from 0 to 100%at 333 K.Due to the release of H2O adsorption heat,an obvious temperature peak appears in the pretreatment layer.The maximum adsorption temperature at the end of the adsorption in the pretreatment layer at 20%,40%and 60%relative humidity reaches 425.81 K,510.60 K and 545.38 K,respectively,Al2O3 will also absorb more water at low humidity,which makes the bed temperature rise.The better recovery effect in low humidity condition is mainly caused by the strong adsorption of 13X to CO2 in condition of without H2O.At high humidity,the H2O adsorption amount of capillary condensation is large in the pretreatment layer,which leads to the significant temperature rise of the whole bed,and the carbon capture performance is reduced.H2O always has a negative effect on carbon capture of traditional hydrophilic/hydrophobic adsorbents.K2CO3/Al2O3 can use H2O to complete CO2 capture.In this paper,the hydration reaction and carbonation reaction mechanism of potassium based supported particles were analyzed,the adsorption equilibrium and adsorption kinetics model of CO2/H2O on K2CO3/Al2O3 adsorbent were established,and the CO2 recovery characteristics from wet flue gas in K2CO3/Al2O3 fixed bed under different humidity and temperature conditions were studied.For the condition of this paper,with the increase of inlet humidity from 10%to 100%at 333 K,the CO2 throughput amount increased from 0.0289 mol to 0.1250 mol,respectively.CO2 recovery and dry purity increases with the increasing of relative humidity.The recovery and dry purity at 100%relative humidity were 4.28 and 1.29 times of that at 10%relative humidity,respectively.K2CO3/Al2O3 has obvious advantages in carbon capture under high humidity conditions.Based on the characteristics of K2CO3/Al2O3,the process is optimized.The CO2 recovery can be effectively improved by pre-supplementing of H2O.The smaller the H2O concentration of feed gas,the more obvious the improvement effect will be after increasing the appropriate time of pre-wetting operation.Increasing temperature can promote regeneration of K2CO3/Al2O3.The VTSA process can obtain better CO2 recovery effect under higher vacuum pressure and reduce the energy consumption of vacuum pump.Suggestions for rational selection of adsorbents were further provided.CO2 recovery,purity and recovery energy consumption were used as evaluation indexes.The vacuum swing adsorption(VSA)process of K2CO3/Al2O3,activated carbon and Al2O3+1 3X adsorbent beds for CO2 recovery from wet flue gas was compared.The results showed the selection of activated carbon or double layered adsorbent bed is more favorable at middle and low humidity.If we focus on obtaining CO2 with high purity,activated carbon should be selected.If we focus on obtaining high recovery of CO2,double layered adsorbent should be selected.At high humidity,K2CO3/Al2O3 should be selected,which can obtain higher recovery and dry purity with lower energy consumption.Finally,this paper made a preliminary study on integrated process of CO2+H2O heat/mass recovery from wet flue gas.Combined process of activated carbon adsorption bed and ceramic porous membrane is proposed.This process can reduce energy consumption,save resources,and make carbon capture more efficient.