| Chemical looping technology is a new type of clean and efficient flameless combustion technology,which uses oxygen carriers(OCs)to alternately redox between the fuel reactor and the air reactor to achieve oxygen and heat transfer.It has a wide application prospect.As a by-product of coal pyrolysis gas production and carbon production process,coal tar is an important raw material for the preparation of carbon black(CB)and other high value-added products such as phenol.According to the research of chemical looping gasification,the research group proposed a process for preparing CB by liquid fuel chemical looping pyrolysis.By controlling the reaction temperature and the ratios of OC to fuel to improve the carbon yield of the product.In order to accurately study the pyrolysis reaction law and the OC surface reaction mechanism,the characteristics of coal tar chemical looping pyrolysis process in fluidized bed reactor and the deactivation mechanism of OC are studied in detail.The main research contents and results are as follow:Firstly,the numerical simulation was carried out to determine the thermal state operating conditions of the small fluidized bed reactor.Analyzing the yield and influence factors of the pyrolysis products in the reactor is helpful to determine the reasonable operating conditions and the theoretical guidance for further thermal experiments.The mechanisms of chemical looping pyrolysis of coal tar and the evolution characteristics of particle size of OC particles were initially studied.It was found that the evolution of the oxygen carrying capacity of the OC during the redox reactions can be expressed with the particle size change rate,they are linear dependence.Fe/Al compound OC was prepared by impregnation method.Applying the operation parameters based on the simulation results,the thermal experimental study of the CB preparation by coal tar chemical looping pyrolysis was carried out in a small fluidized bed reactor.The effects of different coal tar/OC ratio,reaction temperature and CB residence time on CB yield,energy conversion efficiency and CB quality were investigated to determine the optimum reaction conditions.TEM,flue gas analyzer and gas chromatography were used to analysis the CB and gas product.The reaction paths of various main elements in coal tar's chemical looping pyrolysis process were summarized.The carbon black yield arrives the highest when the coal tar / OC molar ratio is of 1:3 and the reaction temperature is at 950°C.The higher the reaction temperature and the shorterthe residence time,the smaller the particle size of the CB,and the more developed the spatial structure of the obtained CB.The structural characteristics of OC in the multi-cycle process were analyzed by XRD,BET,SEM and other characterization techniques to study the evolution of morphology,structure and physical properties of OCs during cyclic reaction to explain the oxygen release mechanism of iron-based OC.It was found that the continuous reduction of the oxygen carrying capacity and the local sintering results in the destruction of the porous structure are the main two reasons for the decline of the OC performance in multi-cycle reactions.In order to improve the multi-cycle performance of iron-based OC,Calcium oxide was added as the catalyst and the pore-forming agent to improve the reaction performance,and to improve the stability of OC under multi-cycle conditions.It was found that calcium oxide has catalytic effect on coal tar pyrolysis,promoting the C-H bond break,causing more free carbon atoms to polymerize into developed chain CB forming.After 20 cycles of reaction,CB yield can still achieve 59%.The addition of calcium oxide also promotes the hydrogen production reaction,significantly increasing the hydrogen concentration in the gas products,improving the energy conversion efficiency of the entire reaction.Calcium oxide carbonation-thermal decomposition can effectively regenerate the reaction area and pore volume of OC particles.After 20 cycles of reaction,OC particles can still maintain a good porous structure.The addition of calcium oxide effectively delays the sintering caused by the local redox reduction,and has promoting effect on maintaining the reactivity of OC particles. |
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