Reaction Mechanisms And Characteristics Of Supported Potassium Carbonate Sorbents For Ambient Low-concentration CO₂ Removal From Confined Spaces

Large amounts of anthropologic CO2 emissions from fossil-fuel-fired power plants are considered as the main contributor of interest to the severe global warming and climate change that has received increasing attention recently.By contrast,air quality degradation induced by the accumulation of CO2 in local scenarios of confined spaces such as submarines and space-crafts has not gained enough concerns.Due to ventilation limitation,build-up of the low-concentration CO2 derived from respiratory metabolism,machinery operation and material degradation could be hazardous and would exert adverse effect on long-term task performances therein.Hence,continuous removing CO2 from confined spaces at ambient temperature to maintain its concentration at an allowable level is one significant subject in the field of safety science and engineering.Potential technology options suitable for removing CO2 from confined spaces should meet several requirements such as high CO2 removal efficiency,excellent regeneration capability,low energy consumption,superior long-term working stability,preferable moisture tolerance,minimize in system volume and mass and considerable safety and reliability performances.Currently prevailing options for ambient low-concentration CO2 removal in confined spaces could involve the CO2 chemisorption process using lithium hydroxide(LiOH),soda lime,liquid amines and solid amines,and physical adsorption over molecular sieves.Despite that these technology options own their merits for this purpose,they are also expected to suffer several imperfections such as non-regenerability for LiOH,moisture sensitivity for soda lime,corrosion problem,intensive energy consumption and large overall system volume and mass for aqueous amine scrubbing technology,amine degradation and loss in cyclic operations for solid amines and poor moisture tolerance for molecular sieves.Recently,the use of supported potassium carbonate sorbents for capturing CO2 from post-combustion flue gas at low temperature has been deemed as a promising technology option,when their merits including high capture capacity,preferable regeneration capability,low energy penalty and cost,considerable cyclic stability,no corrosion and no secondary pollution problems are considered.Hitherto,there have been few reports concerning the application of the supported potassium carbonate sorbents for ambient low-concentration CO2 removal from typical confined spaces.Compared to the flue gas in power plants,the environmental conditions in confined spaces could be different.Besides,the requirements for the goal and function of the CO2 removal system could also be varied.Whether supported potassium carbonate sorbents are applicable to ambient low-concentration CO2 removal in confined spaces deserves further demonstration.Besides,there are also critical issues need to be addressed before the application can be approved.In the current thesis,K2CO3 is supported on activated carbon(AC),Al2O3,zeolites 5A and 13X and silica gels(SG)to prepare different supported potassium carbonate sorbents as K2CO3/AC,K2CO3/Al2O3,K2CO3/5A,K2CO3/13X and K2CO3/SG.The work mainly focuses on reaction mechanisms and characteristics of the synthesized supported potassium carbonate sorbents for ambient low-concentration CO2 removal from confined spaces.Firstly,CO2 sorption and sorbent regeneration performances of several supported potassium carbonate sorbents are investigated in a TGA reactor with orthogonal test method and a fixed-bed reactor system,aims to screen the optimal candidate and the optimum operation conditions.Secondly,the reaction mechanisms of the sorbents are revealed by N2 adsorption-desorption,X-ray diffraction(XRD),field emission scanning electron microscopy(FESEM)and Fourier transform infrared spectroscopy(FT-IR).Thirdly,CO2 sorption and sorbent regeneration behaviors,long-term stability and abrasion characteristics are further studied in detail.Lastly,sorbent deactivation pathways in acid impurities of SO2 and NO2 are revealed,and potential means for inhibiting the sorbent deactivation process are also proposed.The main contents and conclusions are listed as follows:1.Optimization of support and operation conditions for supported potassium carbonate sorbentsThe main factors affecting the carbonation and regeneration process of supported potassium carbonate sorbents are studied using the TGA system combined with the orthogonal testing method,and the reaction conditions are optimized.Supporting material is determined as the principal factor amongst various operation parameters that significantly affects their carbonation and regeneration performances.Amongst various supported potassium carbonate sorbents,K2CO3/AC presents the optimal carbonation and regeneration performances.The maximum carbonation conversion(81.1%)and average carbonation rate(8.0%/min)could be achieved under the optimal carbonation condition of 20℃,1%CO2 and 2.5%H2O.Besides,the excellent regeneration conversion(73.1%)and average regeneration rate(17.6%/min)could be obtained under the optimum regeneration condition of 200℃ and 10℃/min,CO2 sorption and sorbent regeneration capabilities of the sorbents are investigated using a fixed-bed reactor to select the optimum sorbent.K2CO3/Al2O3 presents the highest CO2 sorption capacity of 1.18 mmol CO2/g,while the stable byproduct of KAl(CO3)2(OH)2 requires intensive energy for regeneration.CO2 sorption capacities of K2CO3/5A,K2CO3/13X and K2CO3/SG are far from expectation.By contrast,K2CO3/AC shows a high CO2 sorption capacity of 0.87 mmol CO2/g and the greatest carbonation conversion of 96.4%.Moreover,the spent K2CO3/AC sorbent can be completely regenerated at medium temperature,which means the process could be cost-effective and energy efficient.Therefore,K2CO3/AC can be considered as the optimal candidate for further investigation.2.CO2 sorption and sorbent regeneration mechanisms and characteristics of K2CO3/AC for ambient low-concentration CO2 removalCO2 sorption and sorbent regeneration mechanisms of K2CO3/AC for ambient low-concentration CO2 removal are revealed,based on several physical and chemical characterization techniques.The low-concentration CO2 removal is depended upon the carbonation process of the active component K2CO3 and the physical adsorption process over the porous AC support.The carbonation process of K2CO3 consists of two steps as that K2CO3 is firstly converted to the intermediates of K2CO3·1.5H2O and K4H2(CO3)·1.5H2O through hydration,then the unstable intermediates are converted to KHCO3 rapidly through carbonation.The effects of operation conditions on CO2 sorption and sorbent regeneration performances of K2CO3/AC are studied.Long-term working stability and abrasion behaviors of the sorbent are also investigated.CO2 sorption capacity of K2CO3/AC increases first and then decreases,with the increasing K2CO3 loading amount,H2O pretreatment duration and gas flow rate.Whereas,it decreases with increasing temperature and particle size,and increases with increasing molar ratio of H2O concentration over CO2 concentration.Sorbent regeneration performances would increase with the increase in the final temperature and slightly decrease with the increasing ramping rate.Changing gas flow rate shows no obvious effect on sorbent regeneration performances.K2CO3/AC shows considerable long-term working stability during multiple cyclic operations.The change of average particle diameter versus circulation time follows an exponential decay tendency.More K2CO3 will be converted to KHCO3 for the sorbent with lower K2CO3 loading amounts or at higher temperatures or under lower H2O concentrations.More K2CO3 ·1.5H2O and K4H2(CO3)3·1.5H20 will be formed for K2CO3/AC with higher K2CO3 loading amounts or at lower temperatures or higher H2O concentrations.CO2 concentration exert no significant effect on the carbonation reaction paths,whereas the relative humidity plays an important role in this process.3.CO2 sorption and sorbent regeneration kinetics performances of K2CO3/AC for ambient low-concentration CO2 removalCO2 sorption kinetics performances over a single K2CO3 particle are investigated in TGA system using single particle kinetics models.The results based on the double exponential model shows that the carbonation process of K2CO3 particles involves two stages as H2O diffusion-hydration and CO2 diffusion-carbonation,and the former is the rate-limiting step.The results based on the shrinking core model indicates that the carbonation process can be divided as the surface chemical reaction-controlled region and the internal diffusion-controlled region,and the latter is the rate-limiting step.The results based on the deactivation model shows that the initial reaction rate and deactivation rate of K2CO3/AC would increase with the increasing temperature,CO2 concentration,H2O concentration,H2O pretreatment duration and gas hourly space velocity(GHSV).With the increase in K2CO3 loading,the initial reaction rate increases first and then decreases,while the deactivation rate presents the opposite tendency.The increase in particle size will reduce the initial reaction rate and deactivation rate.CO2 sorption kinetics performances over K2CO3/AC sorbent are also studied in a fixed-bed reactor using bed kinetics models.The results based on the mass transfer kinetics model indicates that the integrated mass transfer rate increases first and then decreases with the elevating temperature,and it increases with the increase in CO2 concentration,H2O concentration,H2O pretreatment duration and GHSV.CO2 physical adsorption kinetics performances and sorbent regeneration behaviors of K2CO3/AC are studied.The results based on the linear driving force(LDF)model shows that the adsorption quantity for multicomponents over K2CO3/AC decreases while the adsorption rate increases with the increasing temperature.The adsorption capacity for multicomponents over K2CO3/AC increases first and then decreases with the increase in K2CO3 loading.The results based on the Avrami-Erofeev model indicates that the activation energy for the regeneration process of K2CO3/AC would increase with the increasing ramping rate and decrease with the elevating final temperature.4.Deactivation pathways and reaction characteristics of K2CO3/AC for ambient low-concentration CO2 removal in the presence of impuritiesThe deactivation pathways and reaction characteristics of K2CO3/AC for ambient low-concentration CO2 removal in impurities are studied.The presence of acid trace impurities of SO2 and NO2 would aggravate sorbent deactivation by reducing its CO2 sorption and sorbent regeneration performances as well as the long-term stability.SO2 and NO2 react with K2CO3 and the unstable intermediate K4H2(CO2)3·1.5H2O under a moist condition to form byproducts of K2SO3 and KNO3.The stable byproducts cannot be thermally regenerated,and they retain on the surface and in the pores,and eventually cause coverage of active sites and blockage of pore structures,therefore to reduce K2CO3 utilization efficiency.Efficient means such as H2O pretreatment,KOH addition and PEI modification are proposed for inhibiting sorbent deactivation of K2CO3/AC in acid impurities,and the mechanisms for sorbent deactivation inhibition are revealed.With sufficient H2O pretreatment to increase relative humidity,the active component K2CO3 can be hydrated into K2CO3·1.5H2O,and the latter is stable enough with strong SO2 and NO2 resistance.KOH addition and PEI loading could provide considerable alkaline and amine groups.On the one hand,these active groups would react with SO2 and NO2 to form either K2SO3·H2O,KNO3 and KNO2 or SO2-amine adducts and nitramine or nitrosamine compounds,therefore to mitigate sorbent deactivation.On the other hand,these groups would also react with CO2 directly to form either K2CO3 or carbamates or protonated amines/imides,therefore to improve its CO2 sorption capacity.The effects of proposed means on sorbent deactivation inhibition performances are demonstrated.With the increasing H2O pretreatment duration,KOH addition and PEI loading amount,CO2 sorption capacities of the pretreated and modified sorbents increase first and then decrease.Compared to the sorbents modified with KOH and PEI,K2CO3/AC pretreated with H2O shows better long-term stability during the multiple cyclic operations.Therefore,H2O pretreatment could be an effective means for inhibiting sorbent deactivation.The suggested ways in practical operation for this purpose could be to pretreat K2CO3/AC with sufficient H2O prior to CO2 sorption and to regenerate the spent sorbent in humidity to convert the active K2CO3 phase or the product of KHCO3 to stable K2CO3·1.5H2O,therefore to improve its SO2/NO2 resistance and K2CO3 utilization efficiency as well as CO2 sorption performances.