Melter Gasifier Research Articles

Experimental Study of the Effects of Operation Conditions on Burden Distribution in the COREX Melter Gasifier

Experimental Study of the Effects of Operation Conditions on Burden Distribution in the COREX Melter Gasifier

Experimental determination of the strength and reactivity of lumpy and briquetted chars from non-coking coals under melter-gasifier conditions

ABSTRACTWith the advent of alternative smelting-reduction processes of ironmaking like COREX® and FINEX®, the demand for non-coking coals as a major raw material has increased. In these processes, char is formed from non-coking coals inside the melter-gasifier (MG) by pyrolysis. This char, either in lumpy or briquetted form, is the major reductant and fuel of the smelting-reduction process. Just like the coke in blast furnace, the char should also have certain chemical and mechanical properties suitable for operational requirements of the MG. However, there is a lack of systematic works related to the properties of chars from non-coking coals for ironmaking. This work aims to define a standardised testing method to determine the reactive and mechanical properties of lumpy and briquetted chars comparable to existing ISO norms. It also demonstrates the effect of different operational conditions on char properties. The testing method described in this work should be useful to the industry personnel to evaluate the quality of chars from non-coking coals for their usage in the MG.

Experimental Study of Burden Distribution in the COREX Melter Gasifier

The COREX melter gasifier (MG) is a reduction reactor applied to produce liquid iron from non‐coking coal and directly reduced iron. The burden distribution in its operation plays an important role in achieving high stability and energy efficiency. This work establishes a 1/7.5 scale experimental model to study the burden distribution with a mixed charging. First, the relationship between the angle of distributor and the burden falling point is investigated. Then, the effects of the initial burden surface profile and the descending burden are discussed in terms of the radial ore‐to‐coal ratio and voidage distribution. The results show that the burden flow widths of coal and pellet increase along the radial direction, and the burden flow width of coal is larger than that of coal in a modern blast furnace. The ore‐to‐coal ratio near the wall is significantly larger than that of the center, possibly leading to worse reduction of burden near the wall. Compared to results with a horizontal initial burden surface, the results under the conditions of a naturally‐formed initial burden surface and a descending burden are more realistic helpful for the operation of the MG.

Characteristics and analyses of COG injection from tuyere in COREX melter gasifier

ABSTRACTCoke oven gas (COG), as an environment-friendly source, is projected to be introduced into the COREX process to reduce solid fuel consumption. In this paper, a static model has been developed based on mass and heat balance, which can calculate characteristics of melter gasifier, such as the raceway adiabatic flame temperature (RAFT), volume and component of bosh gas. The results showed that compared with N2, the COG injection from tuyere is more effective on reducing the RAFT and improving the bosh gas volume. The quantity of COG injected is limited for the RAFT, and without other thermal compensation, the largest injection quantity is about 150 Nm3 t−1. The quantity of COG injection can be increased by preheating tuyere oxygen, adjustment of fuel structure and addition of tuyere oxygen. COG injection can promote the reduction and hearth permeability, decrease the RAFT and protect the tuyere, which is beneficial to COREX operation.

Research on the Combustion Characteristics and Kinetic Analysis of the Recycling Dust for a COREX Furnace

Thermogravimetric analysis of recycling dust (RD) from the melter gasifier of COREX, coke1 (C-1), coke2 (C-2) and coal char (CC) under 70% oxygen atmosphere was carried out using thermal balance. The chemical composition and physical structure of the samples were investigated. The characteristic temperatures and comprehensive combustion characteristic indexes were calculated and kinetic parameters during the combustion process were calculated as well using a distributed activation energy model (DAEM). The results show that the carbon in the recycling dust originates from unconsumed CC and coke fines, and the average stacking height of carbon in RD is larger than that of C-1, C-2 and CC. The conversion curves of RD are different from those of C-1, C-2 and CC, and there are two peaks in the RD conversion rate curves. The combustion profiles of RD moves to a higher temperature zone with increasing heating rates. The average activation energies of their combustion process for RD, C-1, C-2 and CC range from 191.84 kJ/mol to 128.31 kJ/mol. The activation energy for recycling dust increases as the fractional conversion increases, but the value for C-1, C-2 and CC decreases with increasing conversion, indicating different combustion mechanisms.

Effect of screw casing structure on descending of burdens in COREX shaft furnace

COREX shaft furnace (SF) is a typical screw feeder with a storage container coupled with eight screw casings and screws. The structure of screw casing plays an important role in the moving behavior of burdens, stress distribution, abrasive wear of screws, and energy consumption during the operation of SF. Therefore, a three-dimensional semi-cylindrical model of actual size of COREX-3000 SF was established based on discrete element method to investigate the influences of screw casing structure. The results show that the increase in the gap between the outside of screw flight and screw casing is beneficial for the smooth operation of SF, resulting in uniform descending velocity along the radius of SF in the lower part, decreasing the size of recirculation region, and alleviating stress concentration in the screw casing. Moreover, raising the gap appropriately is also beneficial to weaken screw abrasive wear, decrease energy consumption, and then prolong the service life of the screws. However, enlarging the gap also leads to more undesired high temperature reduction gas into the SF from melter gasifier, thereby deteriorating the operation of SF. Thus, an ideal distance exists between the outside of the screw flight and the screw casing, which is suggested to be equal to the average of particle diameter.

Discrete particle simulation of solid flow in a melter-gasifier in smelting reduction process

Smelting reduction process is one of the promising technologies that produce liquid iron with advantages in the reduction of costs and adverse effect on environment. In this process, the main reactor of Melter-Gasifier (MG) involves complicated multiphase flow, heat and mass transfer phenomena, in which solid flow plays a dominant role. In this work, the solid flow behavior in an MG is investigated by discrete particle simulation under different conditions. The results show a good agreement with the observations in experiments. It reveals that three stagnant zones are formed: one is at the MG central bottom, and the other two are located on the side walls. The analysis indicates that charging method, MG geometry, and particle-wall frictional force are factors affecting the size of the stagnant zones on the side walls. Rolling friction does not affect solid flow much. Particle scale analysis of solid flow and force structures is also conducted, showing that strong forces exist in the regions with low porosity. The analysis of particle mass fraction shows that at the MG upper part, HCI (hot compacted iron) particles dominate the MG center, and coal particles tend to distribute to the side walls.

Three-dimensional DEM Study of Coal Distribution in the Melter Gasifier of COREX

COREX is one of the commercialized smelting reduction ironmaking processes. It mainly includes two reactors: a reduction shaft and a melter gasifier (MG). Lump coal is used in the MG to generate reducing agents, and hence its distribution plays an important role in determining the temperature distribution and the reduction of iron ores. To understand the formation of burden profiles, a three-dimensional discrete element method (DEM) model is established and validated against experimental tests. The effects of key operational parameters such as the chute angle and the rotational speed are investigated, and the underlying mechanisms in the formation of an asymmetrical pile are analyzed. The results show that the asymmetrical pile is intrinsic to the discharge system due to the outward motion of lump coal. The results also reveal a significant size segregation of lump coal along the radial direction. Small particles tend to accumulate in the inner side of the pile while the large ones have large fractions in the outer side. The effect of the rotational speed is minor within the range from 3 to 12 rpm. These findings are helpful for the control and optimization of the MG to improve its energy efficiency.

Effects of Operation Parameters on Cohesive Zone in COREX Melter Gasifier

The cohesive zone plays very important role in the operation of COREX melter gasifier. A two-dimensional 1/30 scale thermal dynamic model, in which paraffin and corn are used to simulate DRI, coke and lump coal respectively, has been established to study the cohesive zone of COREX melter gasifier. A set of different operation parameters, such as discharging rate (melting rate), DRI to lump coal and coke volume ratio, blast temperature as well as blast volume, were taken into account. The effects of these mentioned parameters on cohesive zone position and thickness in COREX melter gasifier have been analyzed in detail.

Estimation of Energy Consumption in COREX Process Using a Modified Rist Operating Diagram

Fuel consumption in the COREX-3000 process run in Baosteel is currently higher than the design index. Therefore, mass and heat balance equations for the COREX process were established using the basic principles included in the Rist operating diagram for blast furnace (BF) as a reference. Thermodynamic calculations were then used to modify the Rist operating diagram so that it was suitable for the COREX process. The modified Rist operating diagram was then applied for the evaluation of metallization rate (MR) and fuel structure to reduce the energy consumption in the COREX process. The modified Rist operating diagram for the shaft furnace (SF) provided a nearly ideal value for the restriction point W when the metallization rate was increased, while the point P on the operating line for the melter gasifier (MG) moved upward due to reduction in the heat required in hearth. The feasibility of reducing the energy consumption during the COREX process by changing the fuel structure was also demonstrated.

Modelling of an ironmaking melter gasifier unit operation with multicomponent/multiphase equilibrium calculations

The blast furnace is the traditional ironmaking process for the production of hot metal with high iron content. The Corex® and Finex® processes are the main alternative technological solutions in this field of industrial activity. Their main advantages are focused in better economic and environmental characteristics. The melter gasifier is the most important unit operation of both processes. The aim of this paper is to describe the development and implementation of a predictive mathematical model of a multizone melter gasifier. The process modelling and simulation software used for this task is the gPROMS ModelBuilder. For the calculation of the hot metal/slag mixture chemistry, the Gibbs minimisation routines for multicomponent/multiphase systems were implemented through FactSage and ChemApp. A communication scheme between gPROMS and FactSage/ChemApp was established. The validation of this methodology took place by comparing the basic iron making reactions against literature. Special attention was given in the numerical stability by designing a dedicated initialisation strategy. The agreement between simulation results and plant data proved to be successful for the main components (iron, carbon, calcium oxide, magnesium oxide, aluminium oxide, sulphur). Deviation appeared for the silicon and manganese components. This work contributes in making accurate predictions for the hot metal and slag chemistry based on thermodynamic first principles. The reliable calculation presents the opportunity to apply rigorous optimisations that can lead to higher process efficiency. This reflects the potential of reducing fuel consumption, thus leading to lower environmental emissions.

Numerical simulation of raceway phenomena in a COREX melter–gasifier

The raceway of the COREX melter–gasifier where the interaction between gas phase and particle phase appears to be violent plays an important role in the melting process. In order to describe the behaviors of the particle phase and the gas phase in the raceway, a coupled simulation approach of discrete element method (DEM) and computational fluid dynamics (CFD) is performed at a microscopic level. The particle motion, particle phase volume fraction, velocity vector field of particles, velocity vector field of the gas phase and scalar velocity contours of particles in front of tuyere are calculated by the coupled model. In this work, as a result of considering the multiple particle–gas interaction forces (including drag force, virtual mass force, lift force and pressure gradient force) and a well-balanced computational time step, an almost stable cavity is formed in front of the tuyere, and the particles follow a rotating path in this cavity which can be defined as raceway. The scalar velocity contours of particles show that the particle velocity is greatest in the raceway central area, and it will decrease gradually with increasing distance from the raceway center. Furthermore, the calculation result is consistent with results from a physical experiment.

Influence of Cohesive Zone Shape on Solid Flow in COREX Melter Gasifier by Discrete Element Method

Based on the principle of discrete element method (DEM), a 2D slot model of a COREX melter gasifier was established to analyze the influence of cohesive zone shape on solid flow, including mass distribution, velocity distribution, normal force distribution and porosity distribution at a microscopic level. The results show that the cohesive zone shape almost does not affect the particle movement in the upper shaft and deadman shape. The particles in the lower central bottom experience large normal force to support the particles above them, while particles around the raceway and in the fast flow zone exhibit weak force network. The porosity distribution was also examined under three kinds of cohesive zones. Like the velocity distribution, the whole packed bed can be divided into four main regions. With the increase of cohesive zone position, the low porosity region located in the root of cohesive zone increases. And the porosity distribution becomes asymmetric in the case of biased cohesive zone.

Experimental Study on Burden Descending Behavior in COREX Shaft Furnace with AGD Beams

COREX is an industrially and commercially proven smelting reduction process. It consists of two reactors, the shaft furnace (SF) and the melter gasifier and the SF is placed above the melter gasifier. In the new design of COREX‐3000 in Baosteel, China, a new technique called Areal Gas Distribution (AGD) has been adopted. With the installation of two AGD beams, the cross‐sectional area varies in a complicated manner which will affect the burden descending behaviour. This work uses a cold model to investigate solid particle behaviour in the SF of COREX‐3000. The effect of gas flow, discharging rate and abnormal conditions are characterised in terms of solid flow pattern. The results show that gas flow has a minor effect on burden descending behaviour. Increasing the discharging rate has an effect of reducing the quasi‐stagnant zone. A gas channel is observed downstream of AGD beam and the triangle shaped free area increase with the increase of burden discharging rate. For asymmetric flow, particles descend from the top and move to the discharging outlet and a very big stagnant zone is formed on the opposite side of the discharging outlet. The effect of AGD beams on solid flow is evaluated under normal and abnormal conditions. The investigation reveals that the AGD beams affect the solid flow significantly and lead to poor adaptability of furnace to abnormal operations.

Definition of Raceway Boundary Using Fractal Theory

The particle velocity contours were obtained by tracking the tracer particles in the raceway region of the COREX melter gasifier model and the contours were irregular. According to the fractal theory, the fractal dimensions of different particle velocity contours were determined. Through the analysis of the fractal dimensions, a new method for precise determination of the raceway boundary was proposed. The results show that, when the velocity is less than 0.18 m/s, the particles are located in the stagnant zone and the fractal dimensions of particle velocity contours are almost constant as 1.41; when the velocity increases from 0.18 to 0.83 m/s, the particles are located in the rapid movement zone and the fractal dimensions decrease gradually from 1.41 to 1.05; when the velocity is greater than 0.83 m/s, the particles are located in the cavity zone and the fractal dimensions are again almost constant as approaching to 1.00. Therefore, the velocity contour of 0.18 m/s, which is critical to distinguish the rapid movement zone and stagnant zone, can be used to define the raceway boundary. Based on this method, the effect of blowing rate on raceway size was calculated and the results show that the penetration depth and height of the raceway increase with the increase of blowing rate.

Investigation of coal properties to avoid lump coal fines generation in COREX process

Fines generation of lump coal is a most important limiting factor for efficient production in the COREX process. In this paper, the fines generation of lump coal has been examined based on simulation experiments of the relationships of coal char conversion with temperature and heat absorption. According to the results, in the early stage of coke formation in the melter gasifier, lump coal absorbs large amounts of heat but without significant increase in its high temperature strength. Strength was only improved at later stages of the coke forming process. Residual coal was also found in most tuyere coke samples illustrating that the lump coals added to the melter gasifier were not completely transformed into coke when they arrived at tuyere region. An improvement in the high temperature performance of lump coal is speculated be limited in the melting gasifier due to the low coke forming rate which could become the core factor of fines generation of lump coal in the melting gasifier. Thus, we suggest preheating the lump coal before it is added to the melting gasifier as a solution.

Analysis on Inherent Characteristics and Behavior of Recycling Dust in Freeboard of COREX Melter Gasifier

The generator gas of melter gasifier inevitably carries a lot of dust, which would be transported to the freeboard for recycling, as a result of the existence of the fluidized bed. The recycling dust, with an attractive concentration of C and Fe, is considered as a useful secondary material, thus it is important to investigate the inherent characteristics and behavior of recycling dust in the freeboard. In the present work, the recycling dust characteristics are investigated through the granulometry, chemical, mineralogical and micro analysis. The recycling dust has an irregular shape and an uneven granulometric distribution with the following primary minerals: Fe, FeO, SiO2, 2CaO·SiO2 and 2CaO·Al2O3·SiO2. Furthermore, a three-dimensional mathematical model at steady state is developed to describe the recycling dust combustion and the freeboard temperature distribution, which significantly influences the recycling dust behavior. The region of high temperature above 3000 K can be observed in front of DB due to the combustion of C in the recycling dust, with a combustion ratio of almost 100%. Based on the above analysis, the recycling dust behavior is complex. Fe and FeO in the recycling dust may be oxidized and finally smelts into liquid iron, while other minerals smelt into molten slag. It is worth noting that the phenomena of circulation and accumulation of alkali metals, ZnO and S could occur, despite that most of them are discharged out of the melter gasifier through the generator gas and molten slag.

Modeling the Gas-solid Flow in the Reduction Shaft of COREX

COREX is a promising alternative to blast furnace ironmaking. It includes two main reactors: a reduction shaft (RS) for the direct reduction of iron ore and a melter gasifier for the melting reduction of directly reduced iron. This work uses a two-dimensional slot model to investigate the gas-solid flow in the RS by a combined computational fluid dynamics and discrete element method approach. The three-dimensional flow of cohesive solids is then examined for three RS designs by the discrete element method. The effects of gas flow, the stickiness between particles, the rotational speed of screws, and different designs are depicted in terms of gas-solid flow pattern, overall bed pressure drop and solid flowrate. The results show that the effect of gas flow is insignificant on gas-solid flow pattern due to the small gas-solid interaction forces under the considered conditions. Solid flow varies in a complex manner with the rotational speed of screws and the sticking force, and a correlation is formulated for predicting the solid flowrate based on the simulated results. It is also shown that the effect of geometrical design on solid flow is complicated and significant. Caution should be taken for any changes in the design. The findings should be useful for the design, control and optimization of the RS operation.

Influence of Operation Parameters on Dome Temperature of COREX Melter Gasifier

Statistical analysis and theoretical calculation have been carried out to study the parameters affecting dome temperature of COREX melter gasifier by using actual plant data. The relationship between parameters and dome temperature has been realized qualitatively and quantitatively. It is found that the dome temperature is influenced significantly by metallization, fuel rate, coal rate, coke rate, and oxygen volume. High volatile matter and low moisture coal are beneficial to maintaining dome temperature. Regression analysis was carried out and equations have been developed to predict and regulate dome temperature.

High temperature pyrolysis behaviour and kinetics of lump coal in COREX melter gasifier

The COREX process is an industrially and commercially proven smelting reduction process producing hot metal of the same quality as blast furnace ironmaking. The high temperature pyrolysis behaviour of lump coal is crucial important in the COREX melter gasifier, which determines energy utilisation and gas composition, as well as metallurgical property and particle size of semicoke produced. In the present study, an experimental study was carried out to simultaneously investigate the pyrolysis behaviour of two Chinese lump coals used in the COREX process at different temperatures, and the effect of pyrolysis temperature on microstructure of semicoke produced was analysed by X-ray diffraction technology. In addition, the pyrolysis mechanics and the kinetics parameters of two lump coals were obtained. The results show the pyrolysis rate of lump coals is very rapid, the required time to completion of pyrolysis decreases with increasing temperature, and the contents of moisture and volatile have a remarkable effect on the maximum weight loss rate. The pyrolysis process of lump coals includes three stages: the first is water release accompanying a small quantity of devolatilisation, the second is devolatilisation, and the third is stabilisation. It is found the stacking height of graphite-like crystallite layer along the c-axis for the produced semicokes gradually increases with increasing temperature, the micro graphite spacing decreases and the crystallite size increases while raising the pyrolysis activation temperature. Three-dimensional diffusion is the primary restrictive link in the whole pyrolysis process of lump coals, the expression 3/2(1−α)2/3[1−(1−α)1/3]−1 is the optimal kinetic mechanical function. The apparent activation energy decreases with volatile increasing.