Study On Pellet Drying Process And Heat Matching In Updraft Drying Section Of Grate

Pellets have good strength,high productivity and low production energy consumption,and are widely used in iron and steel smelting.The grate is an important equipment for preheating and increasing the strength of the pellets.The drying condition of the raw balls on the updraft drying section of the grate directly affects the quality and yield of the pellet production.In this paper,the drying process of pellets in the updraft drying section of the grate is taken as the research object.Based on the theoretical analysis,numerical simulation and experimental verification methods,the drying rate and the cracking rate of the pellets are revealed.Strengthen the drying effect of the pellets in the blast drying section and increase the utilization rate of heat energy.1.Based on heat transfer,porous media theory and moisture migration mechanism during pellet drying,internal heat conduction of pellets,gas-solid convective heat transfer,water migration during pellet drying,air flow,pellet crack under steam pressure,etc are combined.Based on reasonable assumptions,a mathematical model of the drying process of the pellets in the updraft drying section is established,including the control equations of the gas and solid phases,the drying rate and the cracking rate model of the pellets.2.Based on the similarity principle,the physical model of the blow drying section of the grate is established to discretize the area.Based on the FLUENT-EDEM coupling interface,UDF and API are used to define the source terms of the equation and the properties of the humidity and drying rate of the pellet respectively.The CFD-DEM numerical simulation method based on the Euler-Lagrange model is used to simulate the drying process of the pellet.The temperature distribution characteristics of the air drying section of the chain grate,the changing laws of the drying rate and the breaking rate of the pellets are obtained: there are increasing temperature gradients and decreasing humidity gradients in the moving direction and height direction of the grate bed in the air drying section;The overall drying rate of the pellet layer is fluctuating and rising,and some pellets will pass through the surface drying stage and enter the internal drying stage;according to the selected pellet raw materials and the set thermal parameters,the overall cracking rate of pellets is less than 10%.After the pellet drying is mainly through the surface drying stage into the internal drying stage,the steam pressure inside the pellet is greater than the tensile strength which causes cracking.3.The heat energy matching in the blast drying section is explored.The heat energy matching model of the updraft drying section was established.By changing the temperature and speed of inlet air,the thickness of pellets layer and the speed of grate bed,the orthogonal test method was used for numerical simulation and the comprehensive balance method was used for the result analysis.The results showed that : Under the condition that the pellet raw material is determined,the main factors affecting the pellet breakage rate are the inlet air temperature,the thickness of the pellets layer,the inlet air velocity and the speed of grate bed;the main factor order that affects the efficiency of thermal energy use is the inlet air temperature,the thickness of the pellets layer,the speed of grate bed and inlet air speed;the optimal heat energy supply parameters for the blast drying section are inlet airflow temperature 473 K,inlet airflow speed 1.45 m / s,the speed of grate bed 0.04 m / s,the thickness of the pellets layer 50 mm.4.A test platform for pellet drying process in the updraft drying section was developed,a test plan was designed,and a test for pellet drying process and heat energy matching in the updraft drying section was carried out.The results show that the maximum temperature error between the numerical simulation and the test is 8.66%,the maximum error between the pellet humidity and the cracking rate is 9.3%,the heat energy utilization rate after the heat energy matching is increased by 5.3%,the rationality of established mathematical models of updraft drying section and numerical simulation methods is verfied.