Direct Capture Of CO₂ From Ambient Air For Facility Agriculture

For more than a century,the burning of fossil fuels have increased the concentration of CO₂ in the atmosphere,which have led to the greenhouse effect and global climate change.Climate change has a great adverse effect on agricultural production and crop harvest.Therefore,the capture and utilization of CO₂ has aroused widespread concern in the academic community.CO₂ is not only greenhouse gas but also a raw material for photosynthesis.Providing high concentrations of CO₂（application of CO₂ fertilizer）to crops can greatly increase crop yield,and this method has been widely used in developed countries in Europe and the United States.Among the many methods of CO₂ fertilizer application in greenhouses,only the pure CO₂ fertilizer application method can effectively control the CO₂ concentration in greenhouse,which conform the requirements of modern facility agriculture.However,the high cost of CO₂ capture is not in line with Chinese basic national conditions.Our team proposed to capture CO₂ in the air directly for greenhouse crop planting.Direct capture of CO₂ from the air is not limited by the location,and avoids the adverse effects of pollutants in industrial gas sources on crops and adsorbents（SOx,NOx,Hg,etc.）.For the CO₂ adsorption and desorption process,the performance of the adsorbent and the optimization of the adsorption and desorption process are the most critical,which directly determines the economics of the process.In the development and screening of adsorbents,the goal of energy conservation is mainly achieved by reducing the desorption temperature,which makes the solid amine adsorbents have advantages over other adsorbents.In view of the extremely low concentration of CO₂ in the air,a large amount of air needs to be circulated through the adsorbent bed.Reducing the resistance of the bed under the premise of ensuring sufficient contact between the air and the adsorbent material is the primary problem.To develop a new type of integrated honeycomb carrier to reduce the pressure drop of the adsorbent bed,it is necessary to combine the preparation of the adsorbent powder with the molding and design and optimization of the adsorbent bed.The main contents and conclusions of this study are as follows:（1）For the solid amine adsorbent,its CO₂ adsorption capacity is not only related to the chemical adsorption capacity of the organic amine itself,but also related to the pore structure of the carrier.The solid amine adsorbent was prepared by impregnating PEI onto porous silica gel with PEI as adsorbent.The effects of adsorption flow rate,temperature,humidity and PEI load on the adsorption performance of porous silica gel adsorbent were studied.The adsorption capacity of CO₂ increases with the increase of PEI load,but when PEI load is more than 40%,the adsorption capacity of CO₂ will decrease.At 298K,when the flow rate is 100ml/min,the optimum load of PEI is 40%,and the maximum adsorption capacity of adsorbent CO₂ is 1.68mmol/g.In addition,due to the large bed resistance of particle adsorbent,the adsorption capacity of CO₂ decreases with the increase of intake flow.This is related to the residence time of the gas in the adsorption bed.The longer the residence time is,the more fully CO₂ reacts with the adsorbent.The CO₂adsorption capacity of the adsorbent will increase with the increase of adsorption temperature and humidity.With the increase of temperature,the fluidity of PEI was enhanced,the active sites of the adsorbent were exposed to CO₂,and the utilization ratio of amino group was increased.The existence of water changes the reaction mechanism of CO₂ capture process.Under the dry condition,1mol carbon dioxide can react with 2mol amino group to form carbamate,while under the condition of water,1mol carbon dioxide reacts with 1mol amino group to form carbonate and bicarbonate.Therefore,the presence of water improves the CO₂ capture ability of amino group.At 298K,the adsorbent can maintain the initial CO₂ adsorption capacity,the attenuation is relatively weak,the fluctuation of adsorption capacity is small,the adsorbent is relatively stable and the regeneration capacity is strong.In addition,in addition to porous silica gel,a variety of porous materials were synthesized,such as mesoporous materials（MCM-48,SBA-15,halloysite）and microporous materials（DDR）,on which PEI was loaded to make solid amine adsorbent.At 298K,when the flow rate is 100ml/min,it is found that the CO₂adsorption capacity of these porous materials has the trend of porous silica gel adsorbent>mesoporous material adsorbent>microporous material adsorbent.The solid amine adsorbent made of porous silica gel as carrier with PEI has good capability of CO₂ capture and stands out among various solid amine adsorbents.（2）With stainless steel wire mesh as the support framework,ceramic fiber cotton and silica gel powder were loaded onto the surface of the honeycomb carrier by impregnation method,and the new structured low resistance adsorption bed was developed to reduce the bed resistance.Honeycomb carrier has the characteristics of high mechanical strength,small bed resistance,large gas-solid contact area,and silica gel powder can enrich the pore structure of honeycomb carrier.PEI was loaded on honeycomb carrier by impregnation method to prepare honeycomb amine adsorbent.At 298K,the adsorption capacity of CO₂ is 0.40mmol/g when the intake flow is 600ml/min.Compared with the powder adsorbent,its CO₂ adsorption capacity is only 40% of porous silica gel amine adsorbent.However,the utilization degree of honeycomb carrier adsorbent bed is high.The bed saturation of powder adsorbent and honeycomb adsorbent is 0.500 and 0.625 respectively.Secondly,the honeycomb carrier is easy to prepare and low cost,which can replace the expensive ceramic fiber paper and has certain commercial value.（3）On the basis of the traditional formula of DDR zeolite,the inorganic alkali was used as mineralizer,and the toxic reagents such as fluoride and ethylenediamine were eliminated.It was found that the DDR zeolite with double crystal pyramid,rhombus,hexagonal prism and sphere morphology could be prepared by controlling the kinds of metal ions,basicity,template concentration,synthesis temperature and time.In the experiment,the synthesis of DDR zeolite by microwave heating method not only shortens the synthesis time,but also can obtain the micro DDR crystal with high crystallinity and high yield.In addition,the synthesis process avoids the use of toxic reagents such as ethylenediamine and fluoride,which meets the requirements of green chemistry.Inorganic alkali as mineralizer can reduce the cost of synthesis.