Basic And Application Research Of Externally Heated Gas Based Direct Reduction Ironmaking Process

Under the background that the Chinese government has clearly put forward the goal of carbon peak by 2030 and carbon neutralization by 2060,the absolute reduction of CO2 emission from ironmaking process and the demand of high-end special steel for high-quality pure sponge iron raw materials urge enterprises and research institutes to accelerate the development of gas-based direct reduction ironmaking process adapted to China's national conditions.Based on the successful pilot test experience of low rank coal pyrolysis and producing CO+H2 by carbon and steam,this paper further extended the cooperative development of a new type of externally heated gas-based direct reduction ironmaking process.It took ten years to carry out the basic theoretical and experimental research of externally heated gas-based direct reduction process.According to the results of basic research,the technologies and equipment with annual output of 3 000 tons,80 000 tons and 300 000 tons were designed.Two independent internal and external systems are separated by high thermal conductivity materials(hot wall)and set up in the shaft furnace used in the externally heated gas-based direct reduction ironmaking process.The characteristics are that the heat energy required for reduction is provided by the external heat system through the hot wall after combustion of gas,and the reduction gas and pellets are heated to the required reduction temperature by the external heat system in the reduction furnace.The advantages of this process are that pellet and reducing gas entering the furnace are at normal temperature,reducing iron leaving the furnace at low temperature,low pressure loss of reduction gas and low energy consumption of per ton of iron.The external thermal combustion system has an independent regenerator fire channel heating function,which can control the temperature of the reduction system in the furnace accurately,improve the utilization rate of the original gas,and effectively avoid the bonding of the pellets in the high temperature section at the early stage of reduction and carbon evolution at low temperature after obtaining sponge iron at the end.In the high temperature reaction vessel constructed by externally heated gas-based shaft furnace,the multi-phase system composed of reducing gas and iron oxide completes reaches the equilibrium in the furnace.The production efficiency of reduced iron is related to the composition of CO+H2 reducing gas,reduction temperature and external heating energy consumption.The gas utilization limit of reducing gas based on CO + H2 at 900? is only 31.098%?37.181%.The process of heating,cooling and heat exchange will cause a large fluctuation of enthalpy,which will affect the comprehensive energy consumption per ton of iron.It is necessary to combine the characteristics of externally heated gas-based shaft furnace to study the composition of reducing gas and the mechanism of how to improve the utilization rate of reducing gas and reduce energy consumption under what conditions,which is the main purpose of this study.Firstly,according to the principle of minimum free energy,the Gibbs free energy temperature expressions of matter in different temperature ranges are derived by using isobaric specific heat capacity and thermodynamic equation,which are used to compile the calculation program of minimum free energy in corresponding temperature ranges.The initial volume of reducing gas,concentration composition of each component,pressure,temperature and other conditions were set to obtain the corresponding equilibrium composition.The utilization rate of reduction gas,equilibrium concentration,participation degree of reduction gas,reaction enthalpy,gas production ratio and other parameters were extended.According to the energy consumption model of external heated gas based direct reduction ironmaking,the heating enthalpy,reaction enthalpy,solid heating enthalpy,total enthalpy demand,system enthalpy input,recoverable enthalpy and enthalpy recovery rate under different conditions are obtained.It is found that when the reduction is in the stages of Fe2O3?Fe3O4,Fe3O4?Fe0.947o,FOe0.947O?Fe respectively,i.e.the stage of step-by-step reduction of iron oxide,the utilization rate and equilibrium concentration of reduction gas,reduction participation of gas,conversion rate of CH4 in reduction gas and gas production ratio remain unchanged.Using this character,the demand of reducing gas,the gas composition after reduction,the amount of each component and the enthalpy of reaction can be deduced.The multi-phase thermodynamic equilibrium system of reducing Fe2O3 by reducing gas%CO+%H2=100 was studied:1)the evolution law of the initial composition and utilization ratio of the reduction gas with different composition of reducing gas was analyzed.It was found that for the reduction gas with%CO=%H2=50,the reduction gas with different composition was analyzed.The difference between the reaction parameters of Fe3C+C+CH4 system and Fe3C+C+CH4 system in Fe0.947O?Fe stage was studied.It was found that the latter had lower reaction enthalpy because the reaction was endothermic reaction,and CH4 was exothermic reaction,which was superimposed and the heat absorption was reduced;When the initial concentration of CO is more than 50%,the utilization of CO+H2 decreases with the increase of temperature;When the initial concentration of CO is less than 50%,the utilization of CO+ H2 gas increases with the increase of temperature.The higher the initial concentration of CO,the smaller the total utilization rate of CO+H2 gas.At 900?,the pressure has a significant effect on the utilization of CO+H2 gas;When the temperature is above 1000?,the effect of pressure on the utilization of CO+H2 gas can be ignored.The theoretical energy consumption of Fe2O3 reduction by CO+H2 gas was calculated.It is found that when the initial concentration of CO is less than 10%,the comprehensive enthalpy(i.e.the difference between heat income and heat consumption)decreases with the increase of temperature;When the initial concentration of CO is 20%,the range of the comprehensive enthalpy of the system is very small with the temperature change,but the overall trend is that the comprehensive enthalpy of the system decreases with the increase of the initial concentration of CO;When the initial concentration of CO is more than 30%,the comprehensive enthalpy of the system increases with the increase of temperature.When the initial concentration of CO is 65%?71%,the comprehensive reaction enthalpy of the system is zero.In the proportion of the input terms of enthalpy,the main factors from large to small are the enthalpy of gas heating,the enthalpy of solid heating and chemical reaction enthalpy.The pressure of the system has little influence on the energy consumption per ton of iron.The enthalpy input of the system of CO+H2 reduction is in the range of 108.062?122.051 kgce/tFe,and the enthalpy recovery rate varies from 65%to 95%.For the characteristics of the composition diversity of reducing gas,the reaction characteristics and the theoretical energy consumption of the initial reduction gas Fe2O3 were studied by using the minimum free energy model of multi-phase.The results show that:under 900?,6.100-7.337 Nm3 of raw gas demand,1.940-9.666 MJ of the chemical reaction enthalpy,6.058-7.598 MJ of heating reduction gas,and 4.118-17.264 MJ of the input of the enthalpy energy was reduced for every 1 Nm3 CH4 increase in the reduction gas on ton iron production.The necessary conditions for the formation of reducing gas CH4+H2 are studied theoretically,the heat absorption and the heating demand are reduced compared with pure H2 reduction system;for CH4+CO reduction,the heat release and the corresponding process conditions for local overheat possibility were reduced compared with pure CO reduction system.Coke oven gas rich in H2 and CH4 is a typical industrial gas.The study on the thermodynamic balance and energy consumption model of reducing Fe2O3 by coke oven gas shows that coke oven gas has the feasibility of direct application in reducing iron making directly without reforming,but it is necessary to control the coke oven gas quantity,iron oxidation amount,reduction temperature and reduction dynamic conditions accurately.It is possible to precipitate C from coke oven gas with too much,and increase the energy consumption per ton of iron.The utilization rate of coke oven gas increased with the increase of temperature when Fe2O3 was completely reduced.The utilization rate of coke oven is 81.900%and the demand for enthalpy of per ton of iron was 165.416 kgce at 900?,but the two items would be changed with the change of gas composition.Further study on the multiphase thermodynamic equilibrium and energy consumption model of reducing Fe2O3 by%CO+%H2+%CH4=100 reduction gas at 900? shows that:1)With the increase of initial CH4 concentration,the reaction enthalpy and total enthalpy demand increase,the gas heating heat decreases,the energy consumption proportion of reaction enthalpy increases continuously,and the energy consumption proportion of gas heating heat and solid heating heat decreases continuously.2)When%CH4=50,the total enthalpy demand is 275.412?276.433 kgce/tFe.Compared with the reduction gas of%CO+%H2=100,the total enthalpy demand increase is 76.668?98.420 kgce/tFe,which is equivalent to 38.379%?55.629%of the total enthalpy demand of%CH4/(%CO+%H2)=0.Based on the theoretical model calculation,the carbon evolution of Fe3O4 reduction process by%CO=%H2=50 reduction gas was verified.The results showed that the maximum carbon evolution occurred in the range of 400??750?,while there was no obvious carbon black precipitation at 400?.This is consistent with the theoretical calculation of the reduction of Fe2O3 under 750?when the theoretical calculation of%CO?30,but it is found that the carbon will not be decayed under 400?.This is different from the thermodynamic theoretical calculation results,so further research is needed.Therefore,if the CO content in the reduction gas is more than 30%,when the furnace type temperature node of the external heat reduction furnace is controlled,the residence time of the temperature between the reducing gas and sponge iron should be controlled as short as possible in the range of 400-750?;The research on the smelting of sponge iron obtained by gas-based reduction is carried out.By controlling the composition of slag,99.9%pure iron with purity is obtained after 20 minutes the gas-based reduction sponge iron melting.It provides the basis for the purification and smelting of gas-based sponge iron in the future.Finally,based on the theoretical calculation of multi-element and multi-phase minimum free energy for reduction of iron oxide by various possible reducing gases,a new type of externally heated gas-based reduction shaft furnace is designed,which can accurately control independent internal reduction system by external heating,and its material balance,chemical balance,heat balance,pressure loss is theoretically analyzed.It is found that:1)in the process of external heated shaft furnace ironmaking,when the composition of reducing gas and furnace structure is determined,the nodal temperature of reduction furnace is consistent under different output.2)The temperature of internal system can be controlled by external heating,and the gas demand under a certain output can be accurately calculated by heat balance method and heat transfer method in advance.Through repeated correction,the theoretical calculation can be equal to the actual demand,so as to improve the utilization rate of reducing gas,reduce the energy consumption per ton of iron,reduce the temperature of furnace top gas,and reduce the loss of pressure drop.3)The process of reducing iron by external heating gas based shaft furnace can meet the application of reducing gas with different CO+H2 concentration combinations.The higher the H2 concentration is,the higher the energy consumption per ton of iron is,the lower the top gas temperature is,and the less the gas pressure loss is.