Mathematical model of the SL/RN direct reduction process

Abstract

A mathematical model has been developed to predict the operating behavior of an SL/RN direct reduction kiln from a knowledge of the main process variables. The model is based on steady state principles and is capable of quantitatively describing the different chemical reactions in the kiln such as reduction, Boudouard reaction, coal devolatilization, combustion in the freeboard together with the mass and heat flows. Results of the model, which are in the form of axial temperature profiles in the freeboard gas, solids bed and wall, and axial concentration profiles in the gas and solids, are in good agreement with measurements made on a 200 ton per day pilot plant at the Steel Company of Canada. The model has been used both to investigate the influence of process variables on the kiln performance and to elucidate operating features of the process. The model has shown that highly reactive coals such as subbituminous and lignite, and highly reducible pellets are most suitable for the SL/RN process. It has also demonstrated that the most important process variable from the standpoint of control is the air profile in the kiln; and consequently that heat transfer is the rate controlling step. The model has also been employed to examine scale-up of the SL/RN process by calculating the operating behavior of a large commercial kiln. It has shown that the large kiln should run cooler than the pilot-size unit so that accretion problems may be significantly reduced.