The Physical Activation Preparation Of Coconut Shell Based Activated Carbon And Pore Structure Evolution

As the economy continues to develop, on the one hand, the resource and energy crisis have become increasingly prominent, on the other hand, the environment continuously deteriorates, which restrict sustainable development of a society. Currently, the study on preparation of activated carbon mainly presents in biomass and coal as raw material. The biomass has low-cost and renewable characteristics and activated carbon products with well-developed pore structure prepared by carbonization and activation are of great significance to improve their economic value. This paper examines the impact of two kinds of heating methods (microwave heating and conventional heating) and the different activating agents (steam and carbon dioxide) in two kinds of processing route (two-step activation and one-step activation) on the preparation of coconut shell based activated carbon, and analyzes the effect of different types of heating methods and activator type on pore structure and surface chemical properties of activated carbon. Therefore, the paper realizes the comprehensive utilization of renewable resources so as to meet social demands of energy saving, environmental protection, and high efficiency.The pyrolysis characteristics and kinetics of coconut shells are investigated via thermo-gravimetric analysis (TGA) at different heating rates. The distributed activation energy model is used to study the pyrolysis kinetics. The experimental results reveal that the pyrolysis characteristics and the yield are affected by heating rate. Curves of the activation energies of samples do not monotonously increase, but show "N" shape. The distribution function of activation energy reveals changes of the reaction properties of mass in different stages of pyrolysis.The effect of carbonization temperature on the yield of coconut shell char and its properties are systematically investigated, pore structure of carbonized products, microcrystalline structure and surface chemical properties are analyzed, and carbonization process evolution of the material is revealed. Studies have shown that, as the carbonization temperature increases, the yield of carbonized materials continues to lower, while BET surface area, total pore volume and micropore volume increase, average pore size decreases, and pore size distribution move towards smaller pore size. Meanwhile, with escalation of carbonization temperature, the stacking thickness Lc, crystallite diameter La and graphite crystallite sheet layer number increase, but the average interlayer distance (doo2) of graphitic carbon decrease, so that arrangement between layers in the microcrystalline tends to be more orderly. Oxygen-containing functional groups go down with the carbonization temperature going up.Using different heating methods (microwave heating and conventional heating) and activating agents (steam and carbon dioxide), the impact of activation temperature and activation time on the yield of activated carbon and its performance has been researched, the impact of heating methods and activating agent on the pore structure of activated carbon products and surface chemical properties is analyzed, and pore-forming mechanism by different heating methods and various activating agents is also investigated. The results show that, irrespective of water vapor or carbon dioxide activation, microwave heating can shorten activation time by about 50% and reduce activation temperature by 100-150℃in comparison with conventional heating. Activated carbon prepared by microwave heating has a higher of BET surface area, total pore volume and micropore volume. In the similar yield, BET surface area of activated carbon by the microwave heating are better, while in the similar surface area, activated carbon prepared by the microwave heating has a higher yield. When steam is an activation agent, no matter which heating method is adopted, the activation process is a simultaneous process of creation of new pores and widening of the existing pores. When carbon dioxide is an activation agent, under the conventional heating conditions, the activation process is creation of new pores first and widening of the existing pores second. Under the microwave heating conditions, its activation process, which is different from conventional heating, is that creation of new pores and widening of the existing pores happens simultaneously. Activated carbon prepared by microwave heating contains less oxygen-containing functional groups than conventional heating.Using one-step activation preparation of activated carbon is studied, the impact of heating methods and activating agent on the pore structure of activated carbon products and its surface chemical properties is analyzed, and the effect of heating methods on the structure of activated carbon is also probed. The results show that activated carbon prepared by microwave heating has higher BET surface area than that by conventional heating and CO2 activation has a higher BET surface area than steam activation. One-step activation has advantages of shorter activation time, lower activation temperature and higher yield.Through systematic researches about the influence of char in different carbonization temperature on the pore structure of activated carbon, carbonization with high temperature and activation technology with microwave heating are applied, activated carbon with high BET surface area is prepared, which decrease activation time and increase the yield of activated carbon, and pore structure and surface chemical properties of the samples are analyzed as well. The results show that carbonization temperature has a significant impact on pore structure of activated carbon and its yield. With the activation time extending, BET surface area, total pore volume and micropore volume of activated carbon prepared by carbonized materials at different carbonization temperature increase and the yield constantly drops. BET surface area, total pore volume and micropore volume of activated carbon prepared by carbonized materials at high carbonization temperature is higher than those at low carbonization temperature, and its yield is also higher than that at low carbonization temperature. In microwave heating conditions, using steam, carbon dioxide and their mixture as an activation agent, BET surface area of activated carbons, regardless of the activation agent, exceeds 2000m2/g, activation time can be greatly shortened, and the yield is higher. Activation of carbon dioxide can prepare activated carbon with the largest BET surface area and more consistent pore size distribution. Surface functional groups of the activated carbon prepared by using different activation agents have nothing to do with the type of activating agent but with the nature of carbonized material.In short, this paper examines the impact of two kinds of heating methods (microwave heating and conventional heating) and the different activating agents (steam and carbon dioxide) in two kinds of processing route (two-step activation and one-step activation) on the preparation of coconut shell based activated carbon, and analyzes the effect of different heating methods and activation agent on pore structure and surface chemical properties of activated carbon. Through systematic researches on the impact of carbonization temperature on the carbonized material and activated carbon, applying microwave heating, activated carbon with high surface area is prepared. This method greatly shortens the activation time, raises the yield of products, achieves energy saving, and provides a new processing route for preparing activated carbon with high surface area by physical activation in a short time.