| The chemical looping combustion technology uses a recyclable oxygen carrier to provide reaction heat and lattice oxygen for the system,which can greatly reduce energy consumption and pollution,and reduce the cost of coal consumption.Therefore,it is of great significance to screen and evaluate suitable oxygen carriers for large-scale industrialization.In this paper,natural manganese ores and hematite,which are widely used in industrial production,are used as oxygen carriers.Based on the new experimental platform of fluidized bed thermogravimetric analyzer（FB-TGA）,the redox of manganese ores and hematite oxygen carriers is carried out.The reaction characteristics were studied and explored the mathematical model of gas-solid reaction.Aiming at the problem that the traditional fluidized bed reactor and thermogravimetric analyzer（TGA）cannot satisfy the fluidization and real-time quality measurement of the bed material at the same time,this paper builds a new FB-TGA based on the micro-fluidized bed theory.Manganese ore particles were used as the bed material.The reactor stability,error analysis and gas switching experiments were carried out in the FB-TGA reactor,and the effects of different fluidization numbers（U/Umf）and gas switching on the reactor were determined.The results show that:under the condition of 900℃,when the fluidization number is3.0-3.6,fine particles are blown out of the reactor,and there is precipitation,and the quality data fluctuates greatly;when the fluidization number is reduced to 2.4,no precipitation is observed.However,vigorous fluidization is still maintained in the reactor;when the fluidization number is between 1.2 and 1.8,the reactor as a whole obtains a relatively stable fluidization.The mass change at this stage is kept within 1 mg,the mass error is small and the stability is high.Then,N2was used as the switching gas,and the flow rate of100 m L/min was passed into the reactor.Compared with the mass signal when no gas was introduced,the mass fluctuation of the switching gas was within 1mg,and the mass fluctuation was stable;when the N2flow rate increased to 200 m L/min,the maximum mass change caused by switching the gas is about 2 mg;as the flow continues to increase to 500 m L/min,a mass signal fluctuation of about 5 mg can be observed,and the error compared with the actual bed mass is within 5%.It is proved that the reactor can guarantee the high accuracy（0.1 g）,fast response（0.1 s）and transmission speed of the oxygen carrier sample mass measurement,and can also reduce the influence of gas mass transfer and temperature gradient on the mass measurement.Based on the new FB-TGA reactor platform,444 redox reaction cycles of manganese ore oxygen carrier were carried out,and the redox performance and agglomeration of the oxygen carrier before and after the reaction were studied.The results show that the redox reaction cycle of manganese ore oxygen carrier can be divided into four stages,stage I:Mn2O3begins to release oxygen;stage II:Mn3O4and Mn O undergo mutual conversion;stage III:oxygen carrying under the influence of cyclic thermal stress The bulk particles are obviously worn;Stage IV:The redox process is affected by the deep reduction,and the wear is aggravated and the flow is lost.With the increase of the number of cycles,the reduction rate and oxidation rate gradually decreased,and the active phase in the manganese ore changed from Mn2O3to Mn3O4,and the resistance of the reduction process was greater than that of the oxidation process.After prolonging the reduction time,the mechanical strength of the particles is reduced and the wear of the particles is accelerated due to the opening effect of H2in the reduction process of manganese ore.According to the characterization analysis of the oxygen carrier particles in different beds before and after the reaction,after a long period of redox cycles,the particles will be broken and worn,and at a constant fluidization number,the particles with small particle size are easier to be transported to the upper bed;the particles are melted,and the surfaces are in contact with each other to form a particle mass,so that the large particle size（>500μm）in the upper part of the bed gradually decreases,and the proportion of small particle size（<300μm）increases.The difference in the surface properties of the particles in the upper and lower beds affects their final fluidized state in the reactor:the agglomeration of the particles in the lower bed is severe,which leads to the formation of porous channels in the bed,which further strengthen the reaction gas and carrier.In the gas-solid contact between the oxygen particles,the reaction gas gradually penetrates from the particle surface to the inside of the particle,forming a gas-solid reaction channel on the particle.The particle agglomeration gradually diffuses inward and upward,eventually leading to loss of flow in the fluidized bed.Due to the exothermic heat of the particles caused by the redox reaction,combined with the formation of manganese silicate in the characterization results,it is speculated that the low melting point manganese silicate generated by the local temperature peak is the main factor causing the sintering and agglomeration of the particles at the bottom of the bed.Based on the new FB-TGA reactor,139 redox reaction cycles of the hematite oxygen carrier were carried out.Using the two-phase fluidized bed model proposed by Kunii and Levenspiel,combined with the experimental results,the circulation of the oxygen carrier in the fluidized bed was analyzed.The characteristics of reaction kinetics in different stages of the reaction determine the range of reaction kinetic parameters in different stages of the cycle.The results show that the whole redox process can be divided into 3 stages:the interconversion between Fe2O3,Fe O and Fe3O4（the first stage）,the wear stage（second stage）which is aggravated by the cyclic thermal stress on the oxygen carrier particles and deep reduction stage（third stage）.The typical cycle data were selected for model calculation in three stages respectively.At 900°C,the oxidation reaction rate constant（ks）selected by the apparent refinement model in the 1st,60th,and 120th cycles was（2.65±0.02）×10-2m3/（mol·s）,（2.83±0.02）×10-2m3/（mol·s）,（8.40±0.02）×10-2m3/（mol·s）;As the reaction progresses,the hematite oxygen-carrying particles accelerate the gas-solid reaction rate between the reaction gas and the particles due to surface wear,and the rate constants（ks）of the oxidation and reduction reactions selected by the model increase with the progress of the reaction.,that is,the oxidation and reduction reactions are faster,and the model can better match the experimental results of the whole process of the redox reaction of hematite.The method combining the refined model with the experimental data can accurately describe the reaction kinetics of the oxygen carrier in different stages of the whole redox reaction stage. |
PDF Full Text Request