CO₂ Capture From Industrial Flue Gases By Adsorption Process

With increasing worldwide demand for energy,fossil fuels will retain their dominance as a global energy source because of their availability and relatively low cost.Therefore,reducing CO2 emissions from fossil fuel use is urgently required to reduce their potential negative impact on the environment.CO2 capture and storage(CCS)is one of the strategies that are able to alleviate and control the CO2 emissions from large exhaust sources such as fossil fuel power plants,cement industries,iron and steel mills and other industry sectors which account for about 60%of total CO2 emissions.Within the field of CCS,a range of technology options for CO2 capture is being evaluated and developed,and the number of demonstration-scale projects in operation or plan is growing.Among the various options,CO2 capture by adsorptiontechnology has attracted some interest,considering its simplicity of facility,relatively low energy requirement,and ease of retrofitting to existing facilities.However,the current drawbacks of adsorption technology are undesirability separation performance of absorbents and relatively high cost for capture,which restrict its large-scale industrial application.The crucial solution is the development of new adsorbents and the optimization of adsorption process.Thus,this thesis was focus on two aspects:on one hand,the novel adsorbent materials were prepared to improve the adsorbent’s nature of CO2 capture;on the other hand,adsorption processes were designed and optimized according to the properties of selected adsorbent for the goal of improving the system separation performance and reducing the total energy consumption.Firstly,microporous carbons with high BET surface and porosity were synthesized by a simple pyrolysis method using PVDF-HFP as carbon resource.The effects of carbonization temperature on pore structures,surface functional groups and CO2 adsorption capacity were investigated.With the increase of carbonization temperature,more micropores are generated due to the releasing of HF.The maximum BET surface area of the carbons is 1175 m2·g-1 with a micropore volume of 0.51 cm3·g-1 at 800℃.The textual of carbon will deteriorate because of collapse of pore if the temperature is more than 800 ℃.The CO2 capacity on the micropore carbon(800 ℃)isup to 5.12 mmol·g-1 at 273 K and 100 kPa withCO2/N2 dynamic separation factor of 7.125.Secondly,CO2 capture by vacuum swing adsorption process from flue gases of different industrial sectors was evaluated.The relatively high adsorption pressure was applied in our procedure for the purpose of enhancing practical applicability and economic feasibility of VSA process.Parameter analysis of inlet CO2 concentration,adsorption temperature,feeding time,vacuum pressure and desorption time was assessed to optimize the CO2VSA process condition.CO2 product purity and recovery are significantly affected by the vacuum pressure,operating temperature and inlet CO2 concentration.Rapid evacuation results in high pressure drop which leads to inadequate desorption of the adsorbed CO2.High desorption flow rates also limit CO2 release from the solid phase to the gas phase due to mass transfer limitations.On the basic of parameter analysis,suitable process configurations for effectively separating CO2 from flue gases from different industrial sectors were specially designed to meet the target of separation(purity>95%,recovery>80%).It was found that for a feed gas containing 15%CO2(representing flue gas from power plants),high CO2 purities and recoveries could be obtained for one stage capture,but with more stringent conditions such as deeper vacuum pressures(≤3 kPa).2-stage VSA process operated in series allowed us to use simple process steps and operate at more realistic vacuum pressures.For feed gases containing>30%CO2,a single-stage VSA capture process operating at moderate vacuum pressure(15-25 kPa)can achieve very high product purities and recoveries.Moreover,three types of commercial zeolites 13X:APGI,APGIII and PSAO2 HP were investigated and compared in terms of their performance for CO2 capture.The VSA processing performance for the compared zeolites 13X in the temperature range of 20-100 ℃ follows:PSA02 HP>APGⅢ>APGI.A novel 16 step VSA cycle was employed with this adsorbent to produce a high purity of CO2(over 97%)at a recovery of 65%at a temperature of 60℃ and a vacuum pressure of 8 kPa.The development of adsorbent should focus on reducing the N2 adsorption capacity as well as CO2 loading capacity,so as to achieve high purity product.Adsorption heat can significantly affect the adsorbent separation performance,so the adsorption heat is supposed to be in areasonable range.Finally,a novel temperature swing adsorption(TSA)process in which the recovered CO2 product is heated and used as regeneration purge gas was investigated,using a new zeolite NaUSY as adsorbent.Impact factors such as adsorption time,regeneration temperature and purge/feed ratio(P/F)were determined.Prolonging feeding time benefits the product purity but reduces CO2 recovery;high regeneration temperature can greatly improve the recovery of carbon dioxide,but it also increases the specific energy consumption The design of cycle configuration is also vitally important for the capture performance.It has been shown that product purity of 95.44%could be obtained with recovery of 71.05%and productivity of 40.516 gCO2·kgads-1·h-1 for regeneration temperatures of 250 ℃ by 1-bed/5-step TSA process.The productivity can be improved by changing cooling method or increasing adsorption beds.The processing specific energy consumption is 4.55 MJ·kgcO2-1,which is roughly equivalent to most of published TSA results.