Kinetic Study Of Semi-coke Forming Process And Co-combustion With Pulverized Coal

Pulverized coal injection technology is widely used in blast furnace ironmaking to reduce the consumption of metallurgical coke.The mainly anthracite mixed with bituminous is mostly used in this process.The resource of anthracite is in short supply with the annual consumption.In order to seek the substitute of anthracite,the pyrolysis processes of coal and semi-coke co-combustion with pulverized coal are studied in this paper.Firstly,experiments of forming semi-coke are carried out at different terminal temperature(400℃、500℃、600℃、700℃、800℃)by using a hanging high temperature furnace.The results show that with the increase of pyrolysis temperature,the volatile content of semi-coke decreases,the fixed carbon content and the calorific value increases,and the combustion performance become worse.When the terminal temperature reaches 600℃,the volatile fraction of semi-coke is less than 10%.Taking the safety conditions of blast furnace and the combustion performance of the fuel into account,the semi-cokes prepared under 600℃ are selected as the substitute of anthracite to conduct the following experiments.The pyrolysis dynamic experiments of coal are carried out through thermogravimetric analyer.According to the characteristics of coal pyrolysis process,the reaction process is divided into three parts.First of all,the adsorbed gas and crystal water are removed;meanwhile,the weak chemicial bond is decomposed.Secondly,the reactions are dominated by the depolymerization and decomposition reaction with a large amount of coal gas and tar vapors evaporating off,coal is transformed into semi-coke.Thirdly,the semi-coke is further coked with the obvious volume shrinkage.Then,according to the characteristics of each stage,interfacial chemical reaction model,internal diffusion model with the constant volume of resultant,and internal diffusion model with the shrinking volume of resultant are developed to describe three ranges of pyrolysis process,respectively.The results illustrate that the experimental data is fitted well,and all the correlation coefficients exceed 0.9.Taking Anyuan coal as an example,the influence of heating rate on the pyrolysis process of coal is discussed in detail.The results show that there is a thermal hysteresis phenomenon with the heating rate rising.In order to describe the influence of heating rate on the rate constant of coal pyrolysis process,a modified Arrhenius equation,lnki=-Ea0/RT+△Ea(α-1/RT)lnβi+ln(A0),is proposed in this paper.The results show that the modified Arrhenius equation not only provides an expedient way to depict the pyrolysis kinetics at different heating rates,but also presents extrapolation reliability for heating rates from 5℃·min-1 to 90℃·min-1.Taking Shenhua bituminous and Shouyang anthracite used in Shougang Jingtang as example,the combustion kinetics of pulverized coal are studied.The combustion kinetics process is divided into two stages.The complicated chemical reactions occur in the low temperature region,such as:formation of gaseous products,devolatilization,and fixed carbon combustion.In the high temperature region,the primary reaction is the oxidation of carbon.The modified Arrhenius equation has good applicability for coal combustion kinetics at heating rates from 10℃·min-1 to 45℃·min-1.The heating rate has a different influence on the rate constant with varies thermal effects of reactions.In the modified Arrhenius equation,the value of △Ea is negative for the exothermic reaction,while it is positive for the endothermic reaction.The influence of different proportions semi-coke mixed with pulverized coal on the combustion performance is studied.The results show that the ignition temperature(412℃)and burnout temperature(627℃)of blended fuel increase with the ratio of semi-coke rising,while the flammability index Cb(7.06×10-5 min-1·℃-1)and the combustion characteristics index SN(9.07×10-7 min-2·℃-3)are decreased.It indicates that the combustion process of fuel shows a worse performance with the addition of semi-coke rising.Finally,the production index of pulverized coal mixed with powder coke used in blast furnace injection is investigated in Shougang Jingtang.It shows that 6%～7%coke powder added in the blended fuel has a little effect on the normal production of blast furnace.Utilizations of pulverized coal and coke reach 99%.Therefore,it is feasibility that the semi-coke with the superior property than powder coke is used as the part substitute of injection fuel.