Study On Adsorption Of Carbon Dioxide From Coal-fired Flue Gas By Seaweed-based Modified Biochars

In recent years,the global warming crisis mainly caused by the greenhouse gas carbon dioxide?CO₂?is gradually spreading,and environmental problems such as melting glaciers and frequent extreme weather have caused increasing concerns.80%of the anthropogenic emissions of CO₂ comes from the burning of fossil fuels such as coal.Therefore,the control of anthropogenic sources of CO₂,such as the emission of flue gas from coal-fired power plants,is the key to reducing CO₂ emissions.Activated carbon adsorption is an effective CO₂ separation technology,but its relatively high cost limits its wide application.Biochar is considered to be a very promising substitute for activated carbon due to its good physical and chemical properties.However,most of the traditional terrestrial biomass materials are limited by factors such as their geographic location and yield of cultivated land,and large-scale recovery costs are relatively high.The widely distributed and rapidly growing marine biomass?e.g.seaweed?can get rid of these factors.In addition,the process of using marine biomass to produce bio-oil will produce a large amount of by-products?biochars?,which will form solid waste pollution if not handled properly.In order to utilize these solid wastes as resources,three kinds of seaweed-based modified biochars were prepared from sargassum/enteromorpha and chlorella by using single step KOH carbonization and activation method,and KOH/urea combined modification method,respectively.And in the fixed-bed adsorption system,the main influencing factors of these adsorbents for adsorption of CO₂ in flue gas,adsorption mechanism,renewable performance and adsorption kinetics were systematically studied.The main research contents and results are as follows:Sargassum modified biochars and enteromorpha modified biochars were prepared by using single step KOH carbonization and activation method,and were used to adsorb CO₂ from flue gas.The changes of physicochemical properties before and after modification and adsorption of these two kinds of seaweed-based modified biochars were analyzed by a series of characterization analysis methods.The effects of activation temperature,KOH and seaweed powder mass ratio,adsorption temperature and the concentration of each flue gas component on the CO₂ adsorption performance of seaweed-based modified biochars were investigated in a fixed-bed adsorption system.The adsorption mechanism was revealed by the kinetic model.The results show that:?1?KOH activation significantly improves the physicochemical properties of seaweed-based biochars and enhances their CO₂ adsorption performance.?2?The optimum activation temperature of seaweed-based modified biochars is 800°C,and the best mass ratio of KOH and seaweed powder is 1:1.?3?The CO₂ adsorption capacity decreases with the increase of adsorption temperature and increases with the increase of CO₂ inlet concentration.O₂ and NO do not affect the CO₂ adsorption performance of seaweed-based modified biochars,while SO₂slightly inhibits the CO₂ adsorption.?4?After ten adsorption-desorption cycles,CO₂ adsorption performance of seaweed-based modified biochars only slightly decreases,indicating its good renewable performance.?5?The adsorption of CO₂ by seaweed-based modified biochars is mainly physical adsorption.?6?The pseudo-first-order kinetic model better fits the experimental data,indicating that external mass transfer plays a leading role in the CO₂ adsorption process of the two seaweed-based modified biochars.Modified chlorella biochars were prepared by using the combined modification method of KOH and urea,and were used to adsorb CO₂ from flue gas.The changes of physicochemical properties before and after modification and adsorption of these chlorella biochars were analyzed by a series of characterization analysis methods.The effects of modification method,KOH and nitrogen-doped chlorella biochars?CN?mass ratio,adsorption temperature and the concentration of each flue gas component on the CO₂ adsorption performance of modified chlorella biochars were investigated in a fixed-bed adsorption system.The adsorption mechanism was revealed by the kinetic model.The results show that:?1?The combined modification of KOH/urea significantly improves the physical and chemical properties of chlorella biochar and enhances its adsorption performance for CO₂.?2?The combined modification method of KOH/urea is obviously superior to the separate modification methods of KOH and urea,and the best mass ratio of KOH/CN is 2:1.?3?The CO₂ adsorption capacity decreases with the increase of adsorption temperature and increases with the increase of CO₂ inlet concentration.O₂ and NO do not affect the CO₂ adsorption performance of modified chlorella biochar,while SO₂ slightly inhibits the CO₂ adsorption.?4?After ten adsorption-desorption cycles,CO₂ adsorption performance of modified chlorella biochar only slightly decreases,indicating its good renewable performance.?5?The adsorption of CO₂ by CNK?2?is mainly physical adsorption,and chemical adsorption is a supplement.?6?The pseudo-first-order kinetic model better fits the experimental data,indicating that external mass transfer plays a leading role in the CO₂ adsorption process of CNK?2?.