Research Of Reduction Reaction And Movement Behavior Of Gas-solid In The Hydrogen Shaft Furnace

Utilization of hydrogen as the reductant to produce sponge iron in the shaft furnace is considered as an effective way to reduce the dependency on coal and cut down the carbon dioxide emission in ironmaking.Reduction process by hydrogen has its own characteristic of high reaction rate,but it is endothermic and in need of excessive gas flow for heat supply,leading to the decrease of utilization ratio and increase of energy consumption.In the present work,multi-step direct reduction,movement behavior together with mass and energy transfer between the burden and reductant in hydrogen shaft furnace are studied through the combination of thermodynamics analysis,experiment and numerical simulation.Major factors affecting on ore consumption are studied through mass balance.Characteristics of low utilization rate and large energy consumption are revealed for pure hydrogen reduction with the consideration of thermodynamics and heat balance.Some effective methods are summarized to increase the efficiency,including proportioning nitrogen and methane,or raising the temperature of gas or solid.Reductions of oxidized pellets in 75%H₂-25%N₂ under the isothermal conditions at temperatures in the range of 760¹000℃are executed respectively.Weight loss during reduction is recorded and the model of un-reacted core is adopted for dynamic analysis.Results show that addition of nitrogen makes reaction rate of reduction by hydrogen slightly lower,but the reduction rate constant remains at the same order of magnitude.The reduction rate was controlled by chemical reaction,and the reaction can be effectively promoted through appropriately raising the temperature.Reductions tests under the non-isothermal conditions are induced to simulate actual atmosphere in the shaft furnace.Morphology of metalized pellets is analyzed through optical microscope and scanning electron microscope.Compressive strength and degradation index are also detected.Reduction behavior of pellets made from converter sludge under hydrogen is also studied in consideration of adaptability to raw materials for hydrogen reduction furnace.Results show that the reaction rate under the hydrogen reduction condition is higher compared with the coal based reduction,and the metallization rate is also larger,however the compressive strength is lower.Based on the above analysis,the two-dimensional axisymmetric numerical model is furtherly established for simulation of hematite pellet reduction with hydrogen step by step in the shaft furnace through the finite volume method according to the gas-solid quality,energy and momentum conservation equation.Results show that the reduction rate in pure hydrogen is much higher and the height of shaft furnace can be shortened compared with the traditional Midrex technology.The effect of nitrogen addition,gas temperature and hot charging temperature on gas utilization rate is further verified.Movement behavior plays a very important role in the uniform drawdown pattern,which directly affects the uniform gas distribution and further the smooth operation in the hydrogen direct reduction shaft furnace.A three dimensional model is established on basis of the discrete element method to simulate the particle descending velocity distribution in furnace with the different height of shaft furnace and the screw flight diameter.Results show that the descending velocity decreases along the radial direction,and the inhomogeneity of velocity is more remarkable in hydrogen furnace compared with traditional Midrex furnace.It is better to decrease the screw flight diameter in the secton inserted into furnace in order to achieve a uniform descending velocity along the radius.This study can provide scientific guide for rational utilization of hydrogen energy in iron making.