Evolution Mechanism And Control Methods Of Structure In Preparation Of Unformed Activated Carbons From Coals

Coal consumption saw a rapidly increasing trend over the past years,of which 70% is used for direct combustion,leading to distinct contrast of serious SO2 pollution and a shortage of sulfur resource in China.The activated carbons from coal are produced by high temperature flue gas from boilers according to the practical situation of power plant.The activated carbon as adsorbent based flue gas desulfurization with advantages like sulfur resources recovery is promising in the future.Therefore,preparing high performance and low cost activated carbon is the key in the development of flue gas desulfurization for application.It has been a common sense that high-performance activated carbon for SO2 removal should hierarchical pore structures and high specific surface area(SBET).However,the activation process for the development of pores to increase the SBET,leading to results in external excessive carbon consumption,and thus low pore porduction efficiency and high cost of activated carbon preparations.These problems can not be solved only by changing activation conditions.This thesis focused on the control of structural evolution of coal chars during pyrolysis to obtain the suitable precursors,which changed the pore development and position of carbon loss during the activation process.There are three key scientific questions in this thesis,namely,“mechanism on structural evolution of coal char in pyrolysis process”,“mechanism of air preoxidation on structural control of coal chars”,and “mechanism of iron-based catalysts on structural control of coal chars”.Based on these studies,this paper achieved in understanding the correlation of char structure with pore structure and caorbon loss,what's more proposing a cooperative control method of char structure to promote pore porduction efficiency.By picking Huolinghe lignite,Jixi and Datong bituminous and Jincheng anthracite coal as the feedstocks,this paper firstly analysed the effect of coal rank on structural evolution of coal chars at slow heating speed(8 ?/min)and wide temperature range(?1600 ?).Based on relevant research results,we found when the pyrolysis temperature is lower than 1000 ?,the effect of coal rank on char structure is more obvious that the production of liquid tar and the release of volatile matters control macrostrucure development,and the plasticity and condensation characteristics control microstrucure development.When the pyrolysis temperature is higher than 1000 ?,the structural development of different chars was similar.Furtherly,by picking Jixi bituminous coal owing to easy regulation as the feedstocks,effect of different pyrolysis conditions including rates,temperatures and holding times on the structural evolution of coal char was studied.The results show that high pyrolysis rates reduce the crosslinking condensation at 800 ?,and facilitate the formation of pores.Crosslinking condensation reaction is strengthened by extension of holding time,leading to the production of more micropores.A carbonized char that produced by slow pyrolysis at 800 ? has a low burn off/carbon loss ratio of 12.37 m2/(g·%)during activation process,however,a carbonized char that produced by rapid pyrolysis at 800? has a higher burn off/carbon loss ratio of 34.09 m2/(g·%),indicating a higher pore porduction efficiency.Then,the effect of air pre-oxidation on the structure evolution of coal char was studied by the mean of different oxidation times at 200 ?.The results show that in the pre-treatment stage,the aliphatic structure is removed to produce the stable oxygen-containing structure,resulting in an open structure with the increase of oxidized time.In the pyrolysis stage,the evolution of different oxygen containing structures plays a more important role than the loss of plastic behavior on physicochemical structure of coal char.Oxygen containing structures with different thermal stability promotes the production of different pores and active sites.At the early stage of activation,many active sites promoted the the etching of crystallites to produce the more pores,however,it is impossible to fundamentally change the reaction pathways of actvation gas and active sites.After activation process,a carbonized char that produced by air oxidization at 48 h has a high burn off / carbon loss ratio of 23.78 m2/(g·%).Based on loading the different amounts of Fe Cl3 catalyst to analyze the competition between two catalytic paths of catalysts,the effect of iron-based catalysts on structural evolution of chars in different preprartion stages was studied.The results show that in the pyrolysis stage,the distribution form of catalyst within char particles determines the catalytic reaction.The distribution form of catalyst can be changed from dispersion to agglomerate state,weakening the catalytic splitting and strengthening the graphitization.In the activation stage,the reaction pathway of activation gas and original active sites can be changed by the existence of FeCl3 catalyst,consistently promoting theetching of crystallites to produce the more pores.After activation process,a carbonized char that produced by loading 3 wt% Fe Cl3 has a high burn off/carbon loss ratio of 18.05 m2/(g·%).What's more,,the relation between char structure and pore development and carbon loss was studied.The results show that the internal diffusion of the activating gas within char particlescould affect the pore development.For the etching process over the crystallites using activated gas,the number of pore formation was controlled by initial microstructure of precursors and its conversion during activation.The position of carbon loss was changed collectively by two above factors.Based on the above findings,we propoesd the cooperative control method of air preoxidation and iron-based catalysts on the evolution of char structure.The results show that in the pyrolysis stage,oxygen-containing functional groups can improve the distribution of catalyst within char particles,hindering the agglomerate of catalysts.In the activation stage,the rapid consumption of original active sites is hindered by the existence of iron-based catalysts,strenthening the etching of crystallites to produce the more pores.After activation process,a carbonized char that prepared by air oxidation at 15 h and loading 3 wt% amount of Fe Cl3 has a high SBET of 1274.64 m2/g at a burn-off of 22.5% and a higher burn off/carbon loss ratio 56.65 m2/(g·%).The results of desulfurization experiment show this activated carbon has a persistent and high-performance for SO2 removal.