Research On Preparation Of Coal-based Microporous Activated Carbon With CO₂ As Activating Agent

Microporous activated carbons were prepared from Taixi anthracite by the procedures: blending, extruding, carbonization and activation, which can be used for separating and concentrating CO₂ or CH₄. The properties of the activated carbons were characterized by adsorption capacty, iodine adsorption, pore stuctural parameters,pore size distribution, surface functional groups, separation factor etc. The influence of process conditions on the properities and adsorption capacity of the activated carbons was studied.Orthogonal experiment and single factor experiment method were employed to research the CO₂ adsorption properties of the activated carbons. The orthogonal results show that the influence of additive ratio and activation time on CO₂ adsorption capacity is very significant; the influence of activation temperature, the interaction of additive ratio and carbonization temperature on CO₂ adsorption capacity is significant;the influence of carbonization temperature and carbonization heating rate on CO₂ adsorption capacity is weak; additive ratio and activation temperature, activation temperature and corbonization temperature have no interaction with each other. The optimum preparation conditions for CO₂ adsorption activated carbons are 3% of additive ratio, 920 ℃ of activation temperature, 3h of activation time, 600℃ of carbonization temperature and 11℃·min^-1 of carbonization heating rate. CO₂ adsorption capacity of the activated carbon prepared at optimum condition reaches59.30 m L·g^-1 under 298 K,0.10 MPa. The linear correlation of CO₂ adsorption capacity with iodine adsorption, specific surface area, pore volume is not high while it has great relevance to the pore size distribution. Pores smaller than 0.93 nm play a key role in CO₂ adsorption. The results of single factor experiments show that additive can promote to produce more micropores. As a result, the CO₂ adsorption capacity of the activated carbons is improved. What is more, additive can reduce the activation time effectively. The suitable additive ratio is 3%. High activation temperature is not suitable for the development of micropores and the suitable activation temperature is900-920 ℃. The activated carbons have narrower pore size distribution when treated with CO₂ as activating agent other than H2 O. The volume of pores smaller than 0.79 and 0.93 nm of CO₂ treated activated carbon is respectively 17.35% and 12.28%higher than that of H2 O treated activated carbon and CO₂ adsorption capacity of CO₂ treated activated carbons is higher. The properties of the activated carbons treatedwith mixed agent are between pure CO₂ and pure H2 O. The influence of activating agent flow rate and carbonization temperature on pore size distribution and adsorption capacity is weak and they can be 330 m L·min^-1 for per 40 g carbonized materials and600℃ respectively. The specific surface area, total pore volume, microporosity and CO₂ adsorption capacity of the activated carbon when activation temperature is 920℃and burn-off 40% reaches 1071m2·g^-1, 0.4470 m L·g^-1, 89% and 59.44 m L·g^-1respectively. The surface functional groups have a significant reduction after carbonization process. The influence of activation process and different activating agent on the surface functonal groups is weak. The pore size distribution other than surface functional groups is the key factor for adsorption properties of activated carbons.The orthogonal experimental results are analyzed to investigate the CH₄ adsorption capacity of the activated carbons. The results show that the influence of activation time, additive ratio, the interaction of additive ratio and carbonization temperature, the interaction of additive ratio and activation temperature on CH₄ adsorption capacity is very significant; the influence of carbonization temperature and carbonization heating rate on CH₄ adsorption capacity is weak; the carbonization temperature and activation temperature has no interaction with each other. The optimum preparation conditions of activated carbon for CH₄ adsorption are the same as that of CO₂ adsorption activated carbons and CH₄ adsorption capacity of the activated carbon reaches 24.54 m L·g^-1 under 298 K, 0.101 MPa. The linear correlation of CH₄ adsorption capacity and iodine adsorption, specific surface area, pore volume is not high. The pore size distribution is a key factor for CH₄ adsrption and volume of pores smaller than 0.79 nm play a key role.The adsorption isotherms obtained at 298 K of the activated carbons are fitted by Langmuir equiation to calculate the separation factor. The values of separation factor of CO₂/CH₄、CO₂/N₂、CH₄/N₂ are above 4, 20, 4.8 respectively.