Rate of reduction of ore-carbon composites: Part II. Modeling of reduction in extended composites

Abstract

A new process for ironmaking was proposed using a rotary hearth furnace and an iron bath smelter to produce iron employing wood charcoal as an energy source and reductant. This paper examines reactions in composite pellet samples with sizes close to sizes used in industrial practice (10 to 16 mm in diameter). A model was constructed using the combined kinetic mechanism developed in Part I of this series of articles [1] along with equations for the computation of pellet temperature and shrinkage during the reaction. The analysis of reaction rates measured for pellets with wood charcoal showed that heat transfer plays a significant role in their overall rate of reaction at elevated temperatures. The slower rates measured in pellets containing coal char show that the intrinsic kinetics of carbon oxidation is more significant than heat transfer. Model calculations suggest that the rates are highly sensitive to the thermal conductivity of pellets containing wood charcoal and are less sensitive to the external conditions of heat transfer. It was seen that the changes in pellet surface area and diameter due to shrinkage introduce little change on reaction rates. The model developed provides an adequate description of pellets of wood charcoal up to circa 90 pct of reduction. Experimentally determined rates of reduction of iron oxide by wood charcoal were approximately 5 to 10 times faster than rates measured in pellets with coal char.

Carbon Gasification in Self-reducing Mixtures

“…Fortini and Fruhan 17,18) analysing the reduction of wustite/carbon composites claim that reduction of wustite has a significant effect on the overall rate of selfreduction at the temperature above 1 086°C. The conclusions of Srinivassan and Lahiri 16) go even further because they assert that the decrease of apparent activation energy from about 400 kJ/mol at the beginning (20% of reduction) to 56 kJ/mol at the late stage of reaction (80% of fractional reduction) proves that the reaction limiting overall selfreduction rate changes from the carbon gasification to the reduction of wustite.…”

“…[11][12][13][14][15] Others propose a mixed control regime with the influence of both reactions (gasification and reduction)or a mixed gasification and heat transfer control regime. [16][17][18][19][20] Pure heat transfer control has also been postulated and modelled. 21) These findings are based on different experimental approaches such as the continuous weight loss of composite sample, the reactor output gas analysis and the temperature profiles inside the composite piece.…”

“…[28][29][30] The mechanism and kinetics of self-reduction has already been modelled to better understand the rate phenomena and to optimize the iron oxide/carbon composite technology by better selection of constituents and their processing conditions. 10,[18][19][20][21]24) These models take into consideration not only the kinetics of gasification and reduction reactions but also the mass and heat transfer phenomena and balances. They need precise measures of the involved reactions rate constants and some structural parameters (porosity, specific surface, etc.)…”

Reduction Kinetics of Iron Ore-Graphite Composite Pellets in a Packed-Bed Reactor under Inert and Reactive Atmospheres

“…Some mathematical models [29][30][31][32][33][34][35] based on first-order rate equations and one-dimensional heat and mass-transfer equations have also been developed to predict the degree of reductions for single-composite pellet/mixtures. Sun and Lu [29,30] developed a rigorous mathematical model involving first-order rate equations, and equations of mass, momentum, and heat conservation for reduction of a ore-coal mixture in a packed bed.…”

Reduction in packed bed of iron ore and coal under one-dimensional heating: experimental results and modelling

“…8. However, the unknown extent of the strongly endothermic backreaction of H 2 O with the char (water gas reaction; heat 7 Temperature profile through samples of 40 mm thickness, at end of 9 min reaction at 1400uC furnace temperature, for mixture containing standard sizes of ore and coal ('base case'), and mixtures containing either larger or smaller particles of either coal or ore 8 Relationship between the total heat transferred to the sample, and the extent of reduction, averaged over the sample. Data as for Fig.…”