Study On Preparation Of Algae-Based Porous Biochars And Adsorption Of CO₂ From Coal-Fired Flue Gas

In recent years,global warming caused by the greenhouse effect is one of the greatest threats to mankind.This will have serious impacts on the environment and human body,such as desertification of land,increasing ocean acidity,rising sea level,increasing diseases and insect pests,affecting people’s normal work and rest,making people feel dizzy and nauseous,etc.As the main component of greenhouse gases,the excessive emission of carbon dioxide（CO₂）has led to the continuous rise of the earth’s temperature.Thermal power plants are the largest anthropogenic source of CO₂emissions,accounting for more than 1/3 of global CO₂emissions.Although clean energy is vigorously developed nowadays,it will not replace fossil fuels in a short time to reduce CO₂emissions.Therefore,studying how to reduce the CO₂emissions of coal-fired power stations is still the focus of current work in the field of carbon capture.Activated carbon adsorption is considered to be one of the most effective methods to capture CO₂.However,some traditional materials（e.g.,coal）used to prepare activated carbon are relatively expensive.In order to further reduce the industrial cost of CO₂capture,researchers have begun to turn their attention to biochars,which have a wide range of sources and low cost.However,the cost of traditional biomass materials（e.g.,terrestrial biomass）is still relatively high due to the limitation of plant yield,growth cycle and geographic location.The algal biomass（e.g.,seaweed and microalgae）that grows in the oceans is of many species and rapid reproduction,which is not affected by these factors,and is very suitable as raw material of adsorbents.In addition,the process of using algae biomass to produce biofuels by pyrolysis will produce large amounts of by-products（biochars）.If these by-products are not handled properly,they may cause solid waste pollution.On the contrary,if they are applied to CO₂adsorption,which not only can reduce the preparation cost,but also can improve the utilization rate of algae biomass and reduce the solid waste pollution.Therefore,this study uses algae biomass（seaweed and microalgae）as raw materials and urea as nitrogen source to prepare nitrogen-doped porous biochars by KOH high temperature activation method and the KOH microwave activation method,respectively.In the fixed bed adsorption system,the effects of the preparation parameters,adsorption temperature,CO₂concentration and other flue gas components on the CO₂adsorption performance of the nitrogen-doped porous biochars were studied,and the regeneration performance of the nitrogen-doped porous biochars was investigated by ten cycles of adsorption/desorption tests.In addition,this study also investigated the CO₂adsorption mechanism of the nitrogen-doped porous biochars based on CO₂adsorption performance test,adsorbent characterization,thermodynamic calculation and kinetic model analysis.Using seaweed biomass（sargassum and enteromorpha）as raw materials,nitrogen-doped seaweed-based porous biochars were prepared by urea treatment/KOH high temperature activation method,and used to adsorb CO₂.Various characterization methods were used to analyze the physical and chemical properties of nitrogen-doped seaweed-based porous biochars.Then various parameters that may affect CO₂adsorption performance were studied in the fixed bed adsorption system,and the CO₂adsorption mechanism of nitrogen-doped seaweed-based porous biochars was explored.The results show that:（1）After urea treatment and KOH high temperature activation,the structural characteristics and surface functional groups（such as C-N,N-H and C=O）of nitrogen-doped seaweed-based porous biochars are significantly improved,and the CO₂adsorption capacities are significantly increased;（2）The CO₂adsorption capacities of the nitrogen-doped seaweed-based porous biochars decrease with increasing adsorption temperature（the optimal adsorption temperature is 25°C）but increase with increasing CO₂inlet concentration;（3）The O₂and NO in the flue gas have no effect on the CO₂adsorption performance of nitrogen-doped seaweed-based porous biochars,while SO₂and H₂O（g）have inhibitory effects;（4）Nitrogen-doped seaweed-based porous biochars have good regeneration performance.After ten regeneration cycle tests,the CO₂adsorption capacities only slightly decrease（less than 10%）;（5）The pseudo-first-order kinetic model can better describe the CO₂adsorption process of nitrogen-doped seaweed-based porous biochars,and external mass transfer is the key control step for CO₂adsorption;（6）Physical adsorption is dominant in the CO₂adsorption process of nitrogen-doped seaweed-based porous biochars.Using microalgae（chlorella and spirulina）as raw materials,two kinds of nitrogen-doped microalgae-based porous biochars were prepared by urea treatment/KOH microwave activation method（KOH high temperature activation method as a comparison）,and their CO₂adsorption performance was explored.Various characterization methods were used to analyze the physical and chemical properties of nitrogen-doped microalgae-based porous biochars.Then,various parameters that may affect CO₂adsorption performance were studied in the fixed bed adsorption system,and the CO₂adsorption mechanism of nitrogen-doped microalgae-based porous biochars was explored.The results show that:（1）After urea treatment,nitrogen-containing functional groups（such as C-N and N-H）are successfully added to the surface of nitrogen-doped microalgae-based porous biochars;（2）Time of KOH microwave activation is only 1/6 of high temperature activation,but KOH microwave activation can better improve the pore structure of biochars,and the prepared nitrogen-doped microalgae-based porous biochars have better CO₂adsorption performance;（3）The CO₂adsorption capacities of the nitrogen-doped microalgae-based porous biochars decrease with increasing adsorption temperature（the optimal adsorption temperature is 25°C）but increase with increasing CO₂inlet concentration;（4）The O₂and NO in the flue gas have no effect on the CO₂adsorption performance of nitrogen-doped microalgae-based porous biochars,while SO₂and H₂O（g）have inhibitory effects;（5）Nitrogen-doped microalgae-based porous biochars have good regeneration performance.After ten regeneration cycle tests,the CO₂adsorption capacities only slightly decrease（less than 10%）;（6）Physical adsorption is dominant in the CO₂adsorption process of nitrogen-doped microalgae-based porous biochars,and external mass transfer is the key control step for CO₂adsorption.