Regular Article

Numerical analysis of the carbon containing pellets direct reduction process

¹ School of Energy and Power Engineering, Chongqing University, Chongqing 400044, PR China
² Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing 400044, PR China

^* e-mail: wangfeng@cqu.edu.cn

Received: 22 October 2020
Accepted: 10 February 2021

Abstract

In view of the carbon-containing composite pellets direct reduction process in rotary hearth furnace, a mathematical model coupling heterogeneous chemical reaction kinetics, heat and mass transfer of this process was established. The effects of furnace temperature (from 1273.15 K to 1673.15 K) and pellet radius (from 6 mm to 16 mm) on the direct reduction of carbon-containing composite pellets were studied by adopting computational fluid dynamics software. The pellet temperature and composition changes under different operating conditions were analyzed. CO and CO₂ fluxes, heat fluxes on the pellet surface were especially studied. Total heat absorption by the pellet, CO and CO₂ overflow from the pellet surface together with pellet degree of metallization (DOM) and zinc removal rate (ZRR) were calculated. Results show that with the increasing of furnace temperature or the decreasing of the pellet radius, the temperature difference between pellet surface and its center and the final DOM, ZRR increased. The larger the pellet radius, the smaller the heat absorption, also the smaller CO and CO₂ overflow. But heat absorption and CO overflow per unit volume are higher. There is an optimal pellet radius with high CO utilization efficiency. Pellet porosity decreases at first and then increases with reducing time. It is also found that effective thermal conductivity is a major factor limiting the pellets temperature increasing. The reduction sequence of the pellets is Fe₂O₃→Fe₃O₄→FeO→Fe.