Research On CO₂ Adsorption From Flue Gas By Activated Carbons

Post combustion carbon dioxide capture technology was widely expected to be the most effective approach to control CO₂ emission and mitigate climate change in the near future.Adsorption technology are regarded as a promising technology due to simple system process,less investment cost and low energy consumption.However,it has not be applied in large scale CO₂ capture from coal-fired flue gas and the understanding of adsorption process is very important to the development and application of this technology.Among various adsorbents,activated carbon is considered as the most suitable candidate for CO₂ capture with the merits of easy regeneration,good stability,insensitivity to water and low cost.In this thesis,adsorption process characteristics and regeneration performance based on activated carbons were comprehensively studied through combining theoretical analysis and experiments,and the development of low-cost and high performance activated carbon material were studied.Dynamic CO₂ adsorption on activated carbon in a fixed bed with simulated flue gas were conducted.The internal mass transfer in the process is reflected by the temperature variation during adsorption.The influence mechanism of both feed flowrate and adsorption pressure on adsorption was investigated to evaluate the effective utilization of adsorbents in the bed by column efficiency-With the increase of feed flowrate,mass transfer zone of adsorption moves faster resulting in the column efficiency decreased,and with the increase of adsorption pressure,mass transfer zone moves more slowly which results in the column efficiency increased.With the experiments of adsorption in the presence of water,it was confirmed that water has the negative effects on the CO₂ adsorption on activated carbon.The theoretical and experimental studies on the kinetics and thermodynamics of CO₂ adsorption on activated carbon were carried out.Based on the experiments of equilibrium CO₂ adsorption on activated carbon,the equilibrium adsorption process was accurately predicted by the Avrami kinetic model.A series of CO₂ concentrations?5-18%?and four temperatures were applied to investigate adsorption behaviors.A simplified deactivation kinetic model was successfully used to simulate the adsorption process,and the correlation between adsorption kinetic constants and temperature was obtained.According to the simulation,an empirical formula describing the adsorption rate constant and deactivation rate constant with temperature was received.An isotherms modelling was made by the Langmuir isotherm model,and the experimental isotherms data were well fitted.Then,the thermodynamics of CO₂ adsorption were analysed by Clausius-Clapeyron equation.The calculated isoteric heat declined slightly with the adsorbed amount on activated carbon,and had an average value of 16.23kJ/mol.These investigations on kinetics and thermodynamics provide theoretical basis for practical application of activated carbon materials to CO₂ adsorption.Studies on regeneration characteristics of activated carbon were carried out by N₂ purge and vacuum regeneration.The results show that the regeneration efficiency can be increased with the increase of purging flowrate and purging duration.Vacuum pressure is the key parameter to regeneration efficiency,and most of the desorption behavior occurs during the initial stage of vacuum evacuation.The favorable effect on regeneration by integration of temperature swing was confirmed.Assessment of regeneration efficiency and energy consumption in vacuum regeneration process by ASPEN Adsorption simulation software.It was found that the optimal vacuum pressure is 5kPa with the minimum specific energy consumption of 2MJ/kg CO₂ approximately.The high adsorption performance activated carbon,the activated carbon material was developed by using single-step procedure of physical activation of pecan shells,which is an agricultural residue available in Zhejiang province.Activation temperature is such important parameter that with the increase of activation temperature the porosity of activated carbon become more abundant,and CO₂ adsorption capacity increases as well.The influence of the activation temperature is more remarkable when it is higher than 800?.The highest adsorption capacity can reach 2.31mmol/g.Increasing the activation time can also make the porosity become more abundant,and the CO₂ adsorption capacity of activated carbon increases linearly.Moreover,powdered activated carbon was prepared by chemical activation with KOH of tar pitch?TP?,which was produced by coal polygeneration in Zhejiang University.The results show that KOH/TP ratio has a great influence on porosity and CO₂ adsorption capacity.Although the activated carbon has a very high BET surface area?>200m2/g?,the micropre properties decrease with the increase of KOH/TP ratio.The maximum adsorption capacity reaches 2.45 mmol/g at KOH/TP of 2,but it decreases gradually with the increase of KOH/TP ratio.The adsorption capacity of C02 was not determined by the specific surface area,but related to microporosity of the carbon material.Based on the tar pitch,activated carbon monoliths were developed by three different methods.All of them have good pore structure and excellent CO₂ adsorption performance.The successful development of these high-performance activated carbons provides a useful reference for the development and application of efficient activated carbon adsorbent and enhancement of the economic value of raw materials.