Coal Gasification based Production of Direct Reduced Iron
Coal Gasification based Production of Direct Reduced Iron
Direct reduced iron (DRI) is one of the major input materials for the production of steel through electric furnace route. DRI is produced predominantly by gas based processes around the world in vertical shaft furnaces utilizing natural gas. DRI is also used in some countries by rotary kiln process or by rotary hearth process with the use of thermal coal. The capacities of coal based rotary kiln process and rotary hearth process are quite small compared to gas based DRI production processes. Gas based processes are more popular in those regions of the world where there is abundant availability of natural gas at low price.
With the development of cost effective coal gasification processes, DRI production has now become feasible in gas based processes in those regions where there is good availability of thermal coal and there is no availability and/or low price of natural gas. Synthesis gas (syngas) which is an acceptable reducing gas for the production of DRI in a shaft furnace can be produced in coal gasification plant. Coal gasification plant can be coupled with the DRI production process. In this way coal can replace natural gas for the production of DRI in shaft furnaces.
There are two main competing processes for the gas based DRI production in the shaft furnace. They are Midrex process and Energiron process Based on HYL technology. The shaft furnaces of these two processes exhibit some differences (mostly in gas composition and pressure, size, and internal equipment details) but their basic characteristics are similar.
Coal gasification technologies
Coal gasification technologies exist both for underground gasification and over ground gasification of coal. Technologies for over ground gasification of coal are of interest for production of DRI in shaft furnaces. It is well known that full combustion of coal does not produce the combination of CO (carbon mono oxide) and H2 (hydrogen)which is mainly desired in syngas for its further use as fuel and reducing gas. Coal gasification process is hence, based on the fundamental principle of partial combustion of coal. The gas which comes out after partial combustion is called raw syngas which is purified further in processing units before it is used for DRI production.
Coal gasification is carried out with limited amount of oxygen (O2) which is around one-fifth to one-third of the theoretically O2 required for complete combustion. Only a fraction of carbon (C) is burned for generation of heat. H2 and CO are the main products of gasification. CH4 (methane) and CO2 (carbon di-oxide) are other two major products and their content goes up with increasing pressure with H2+CO content going down. In coal gasifiers two physicochemical processes take place. They are (i) pyrolysis or devolatilization process, and (ii) gasification process.
In the pyrolysis process, as the coal enters into the gasifier, it is first dried by the hot gases present in the gasifier. A series of complex physical and chemical process start slowly at temperature less than 350 deg C and accelerate as temperature exceeds 700 deg C. The composition of the released products of pyrolysis is dependent on the temperature, pressure and gas composition during pyrolysis.
The pyrolysis process can be represented by the reaction Coal –> Heat –> Char –> Gases –> Vapours or liquid.
The three products which are produced by pyrolysis are (i) light gases such as CO, H2, CO2, CH4, and H2O (water vapour), (ii) tar which is a corrosive and viscous liquid composed of heavy inorganic and organic molecules, and (iii) char which is a solid residue mainly contains C.
The gasification process involves a series of endothermic reactions which are supported by the heat produced from the combustion reactions occurring inside the gasifier. These reactions are represented by the equations (i) C + O2 = CO2 with delta H = -94.05 kcal/mol, and (ii) H2 + 0.5 O2 = H2O with delta H = – 68.3 kcal/mol. The major gasification reactions which take place are (i) water gas shift reaction, (ii) Boudouard reaction, (iii) shift conversion, and (iv) methanation.
In water gas shift reaction where the partial oxidation of C by steam occurs and is represented by the equation C + H2O = H2 + CO with delta H = 28/3 kcal/mol. During the Boudouard reaction, the char present in the gasifier reacts with the CO2 and produces CO. The reaction is represented by the equation CO2 + C = 2CO with delta H = 38 kcal/mol. The shift conversion is an endothermic reaction and is known as water–gas shift reaction. Due to this reaction there is increase in the ratio H2 to CO in the gas. This reaction is used during the production of syngas. The reaction is CO + H2O = CO2 + H2 with delta H = – 10.1 kcal/mol. For methanation, nickel based catalyst is used. This catalyst at 1100 deg C and at a pressure of 6 kg/sq cm to 8 kg/sq cm accelerate the reaction of the formation of methane which is preferred in IGCC (integrated gasification combined cycle) applications because of its high heating value. The reaction involve is given by the equation C + 2H2 = CH4 with delta H = 17.8 kcal/mol.
The complete gasification reactions are carried out in the gasifiers which are required to be operated at certain temperature in order to drive certain endothermic C – steam and C – CO2 reactions. The required temperature is maintained by heat evolved from exothermic reaction between O2 and coal.
Depending upon the medium of gasification, gasifiers are classified into two categories namely (i) air blown, and (ii) O2 blown. In air blown gasifiers, air is used as gasification medium while in O2 blown gasifiers pure O2 is used as gasification medium. When air is used as gasification medium, the N2 is simultaneously brought into the process which results in the product gas dilution. As a result product gas has a lower calorific value.
Depending upon the contact between gas and fuel, there are three types of gasifiers. These are namely (i) moving or fixed bed gasifier, (ii) fluidized bed gasifier, and (iii) entrained bed gasifier. All the three technologies are based on partial oxidation (gasification) of a carbonaceous (C containing) feed material (coal). While each of these technologies can make an acceptable reducing gas for the production of DRI, the fixed bed and fluidized bed technologies are the preferred choice for high ash coals.
The general partial oxidation reaction is 2CHn + O2 = 2CO + nH2. The consumption of O2 in the process depends on the ash content and calorific value (CV) of the coal. Insufficient supply O2 ensures partial oxidation of coal. This reaction produces a mixture of gases namely H2, CO, CH4, and CO2. The end product is syngas. The mixture’s composition changes with the pressure.
Moving bed gasification technology is the oldest technology and is being used widely. The gasifier is also known as fixed bed gasifier. Gasification medium slowly flows through a fixed bed of solid particles. The two possible configurations of this type of gasifiers are up-draft and down-draft depending upon the direction of flow of gasification medium. The up-draft configuration is more commonly used since there is low tar content. The preferred feed coal size is 5 mm to 80 mm. The combustion zone attains a maximum temperature of around 1500 deg C to 1800 deg C and for the slagging and dry ash gasification zone a maximum temperature of around 1300 deg C. The temperature profile is formed over the bed, so that the feed coal is successively preheated, dried, pyrolyzed, gasified and combusted. Lurgi gasifier is the oldest moving bed gasifier technology.
The fluidized bed gasifier has the bed of solid particles which behaves as a fluid. In this type of gasifier, the particle size of the feedstock is less than 5 mm and the particles are suspended in the O2 rich gas. The rising gas reacts with the feedstock and maintains the fluidized state of the coal particles. A uniform temperature distribution is achieved in this type of gasifiers. Also, in this type of gasifier, the clinker formation and de-fluidization of the bed is avoided since the operating temperature is in the range of 800 deg C to 1050 deg C which is well below the ash fusion temperature. Ash discharge can be carried out in the form of either the dry or the agglomerated ash. Dry ash fluidized bed gasifier is traditionally being used for the low rank coals. The agglomerated ash fluidized bed gasifier is being used for any rank of coal.
Entrained flow gasifier uses pulverized coal particles of size less than 0.1 mm which are suspended in a stream of steam and O2 at high speed. Depending upon the method of coal feeding, dry (nitrogen being used as a transport gas used) or wet (carried in water slurry), gasifiers are accepting almost any type of coal. Entrained flow gasifiers ensure high C conversion since they operate at a high temperature range of 1400 deg C to 1600 deg C (well above the ash slagging temperature). These gasifiers are of high capacity since the gas residence time is measured in seconds.
In addition to the desired CO and H2, the syngas leaving a gasifier also contains other compounds. The product of gasification contains desirable components like CO, H2, CH4 and undesirable components like CO2, H2O, ash, entrained soot, tar, particulate matter, certain amount of H2S (hydrogen sulphide), and traces of NH3 (ammonia), HCl (hydrochloric acid), HCN (hydrogen cyanide). Hence, cleaning of syngas is an important aspect of the coal gasification process. The undesirable components need to be removed from product gas. There are number of techniques being used to remove undesirable components.
Product gases (CO, H2, and CH4) of the coal gasification process have fuel value. If a fixed bed gasification technology is used, the syngas also contains aromatic organic compounds. Typically, 1 kg of bituminous coal can be converted into 1.5 cum to 1.7 cum of syngas.
In terms of feedstock flexibility, several gasification plant designs have been developed to utilize various grades of coal. Gasification results in very low gaseous emissions of conventional (non GHG) pollutants, due to the nature of the process operation. It also offers a potentially low marginal cost route for capturing the resulting CO2 by-product for either geological storage or enhanced oil recovery from the oil fields.
In addition, coal gasification processes require significant water use. They are also large emitters of CO2. For I ton of syngas, typical coal consumption is around 2.8 tons, water requirement is around 6.6 tons and CO2 generation is 2.5 tons. The CO2 is released is as a byproduct and can be sold or compressed for conveying to the underground storage. A short description of the present coal gasification technologies are given below.
Gasification technology of GE Energy – GE Energy acquired its gasification technology from Chevron in 2004. The GE coal gasifier comprises a single-stage, downward-feed, entrained-flow refractory lined gasifier to produce syngas. Coal/water slurry is pumped into the top of the gasifier, which together with O2 is introduced through a single burner (Fig 1). The coal reacts exothermically with the O2 at high temperature (1200 deg C to 1480 deg C) to form syngas. The syngas contains mostly H2 and CO, and slag.
Fig 1 Coal gasifier of GE energy
The slag which flows downwards is quenched and then removed from the bottom of the gasifier via a lock-hopper arrangement. The water leaving the lock-hopper is separated from the slag and sent to a scrubbing unit after which it can be recycled for slurry preparation. The raw syngas leaving the gasifier can be cooled by a radiant and/or convective heat exchanger and/or by a direct quench system, where water is injected into the hot raw syngas. The selection from these alternatives is a choice of cost and application.
The radiant cooling design uses a soot-tolerant radiant syngas cooler which generates high-pressure steam. Slag is quenched in a water pool located at the bottom of the reactor vessel, and removed through a lock-hopper. The syngas is further cooled after leaving the gasifier by a water scrubber to remove the fine particulate matter, before the gas is sent on to downstream processing. The direct quench system uses an exit gas water quench where hot gas leaving the gasifier is contacted directly with water via a quench ring. It is then immersed in water in the lower portion of the gasifier vessel. The cooled, saturated syngas is then sent to a scrubber for soot and particulate removal. The quench design is less efficient, but also less costly, and it is commonly used when a higher H2 to CO ratio syngas is needed.
Gasification technology of Shell – Gasification technology of Shell comprises a dry-feed, pressurized, entrained flow, slagging gasifier. The coal-based variant was developed in the 1970s. Coal is pulverized and fed to the gasifier through two sets of horizontally opposed burners using a transport gas (either syngas or nitrogen). Preheated O2 and steam (as a moderator) are mixed and fed to the injector, where they react with the coal to produce syngas consisting mainly of H2 and CO with only small amount of CO2 and no hydrocarbon liquids or gases. The hot product gases flow upward through a vertical membrane cylindrical wall, as shown in Fig 2.
Fig 2 Shell strained flow gasifier
Molten ash entrained with the upward-flowing syngas is deposited on the water walls and flows downwards. It is removed through the base of the gasifier where it is quenched in a water bath. The raw syngas leaves the gasifier in the temperature range 1370 deg C to 1480 deg C and is then treated with lower temperature recycled product gas to convert any entrained molten fly slag to a hardened solid material. It then enters the syngas cooler for heat recovery, generating high-pressure (HP) superheated steam. The bulk of the fly ash contained in the raw syngas leaving the syngas cooler is removed from the gas using either commercial filter equipment or cyclones. Any remaining fly ash is captured downstream with a wet scrubber.
Gasification technology of Siemens – The Siemens gasifier is a dry-feed, pressurized, entrained-flow system, with a top-fired burner through which coal together with O2 and steam is introduced (Fig 3). It can be designed with either a refractory lining, for low ash feed stocks, or with a gas-tight membrane wall structure in the gasification section of the gasifier.
Fig 3 Siemens entrained flow gasifier
The molten slag formed in the gasifier flows down the reactor chamber into the quench section where it solidifies upon contact with water from a ring of quench nozzles and is removed through a lock-hopper arrangement. The gasifier can achieve C conversion rates higher than 99 % and the technology is well suited for all types of coals from anthracite to lignite.
Lurgi dry bottom pressurized coal gasification process – The Sasol Lurgi gasification process comprises the reaction of steam and O2 with lump sized, low or medium caking coals on a rotating grate at pressures of 20 kg/sq cm to 30 kg/sq cm. The gasifier for the dry bottom pressurized coal gasification process is shown in Fig 4.
Fig 4 Lurgi dry bottom pressurized coal gasifier
In the bottom combustion zone at the grate, the coal char is burned with O2 to provide energy for the gasification reactions. As the coal moves down the gasifier, it is heated by the upward-flowing syngas which leaves the gasifier. The heat causes the coal to dry followed by devolatilization. Some of the devolatilized products escape before reacting and leave the gasifier with the raw syngas. As the devolatilized coal moves down, it is gasified with combustion products from the combustion zone below. In the dry ash mode of operation, excess steam is injected with O2 to keep the temperature below the ash fusion temperature. A motor driven rotating ash grate is used to remove ash in a ‘dry’ state and also to support the coal bed.
The counter-current flow of gasification agent and fuel results in a high thermal efficiency of the gasifier to produce a raw gas with heating values of around 2650 kcal/cum to 2850 kcal/cum. Depending on the characteristics of the feed coal, the product gas contains by volume 25 % to 33 % CO2, 15 % to 21 % CO, 35 % to 41 % H2 and 10 % to 13 % CH4. For use as syngas, CH4 and CO2 are required to be removed.
Since the 1960s, the Lurgi process has been improved through increases in reactor size and components, extension of the feed coal slate to include low rank coals, and the use of air instead of O2 as the gasification agent. In addition, the design has been demonstrated for operation at upto 100 kg/sq cm pressure in order to increase the gasifier throughput while at the same time increasing the CH4 content of the raw gas.
The British Gas Corporation, in co-operation with Lurgi, developed a new design of the gasifier bottom in order to avoid the problems associated with rotating equipment in the fuel/ash bed, while simultaneously overcoming the limitation set by the ash softening temperature in the gasification zone.This resulted in the BGL slagging gasifier. The gasifier differs from the standard Lurgi reactor through (i) the replacement of the grate and ash lock by a hearth for liquid slag tapping, (ii) the introduction of the gasification agent O2 and steam by means of tuyeres instead of through the grate, and (iii) the use of refractory lining in the lower part of the reactor body to reduce heat loss.
BGL slagging gasifier also operates at higher gasification temperatures than the standard Lurgi gasifier and, hence, the CO/CO2 ratio in the product gas is higher and the CH4 content correspondingly lower. Typical gas compositions by volume are 2 % to 3 % CO2, 55 % to 60 % CO, 25 % to 28 % H2 and 6 % to 9 % CH4. The high temperature provides for a better steam utilization and, therefore, the amount of water which is needed to be cleaned and processed, is much reduced. Coal ash is converted into slag which forms a non-leachable glass on removal. This requires a low slag viscosity, which is obtained by adding fluxing agents, usually limestone or basic blast furnace (BF) slag.
Synthesis Energy Systems gasification technology
Synthesis Energy Systems (SES) has a worldwide exclusive license for the U-Gas gasification technology, which is a single stage fluidized bed system and which can provide a low-to-medium heating value syngas. The flowsheet of SES gasification technology is given in Fig 5. SES gasification technology is particularly suitable for gasifying low quality fuels, including all ranks of coal.
Dried and ground coal is fed via a lock-hopper into the gasifier, which is fluidized by a mixture of steam and O2. These reactant gases are introduced at the bottom of the gasifier through a distribution grid, and at the ash discharge port in the centre of the distribution grid. The bed is maintained at temperatures ranging from 840 deg C to 1100 deg C depending on the softening temperature of the ash within the fuel. At such conditions, the concentration of fuel ash (mineral content) particles within the gasifier increases such that they begin to agglomerate and form larger particles, which are selectively removed from the fluidized bed by gravity. This design allows for 95 % or more of the fuel’s C getting gasified.
Fig 5 Flowsheet of SES gasification technology
KBR TRIG coal gasification technology
The Transport Integrated Gasification (TRIG) technology was developed by the Southern company and KBR Inc. It is designed to process reactive low rank coals, including those with upto 50 % ash and high moisture content, and can be operated with steam and either air or O2 as the gasification medium. Air-blown operation is preferable for power generation, while O2 blown operation is better suited for syngas production. The simplified layout of the TRIG process is shown in Fig 6.
The system comprises a circulating gasifier, which consists of a mixing zone, riser, disengager, cyclone, standpipe, loop seal, and J-leg. This is designed to operate at high solids circulation rates and gas velocities, resulting in higher throughput, C conversion and efficiency. The raw syngas is formed in the riser portion of the unit, from which laden with unreacted solids it passes through a series of cyclones where the solids are removed. The ash material is recirculated through the riser to allow unconverted C to be utilized and to provide heat to the gasifier. As ash accumulates in the down comer, it is discharged from the unit. The gasifier operates at moderate temperatures and below the melting point of ash, which can increase the equipment reliability and availability. The latter is enhanced by the use of a downstream particulate filter, which eliminates water scrubbing and significantly reduces plant water consumption and effluent discharge.
Fig 6 Simplified layout of the TRIG process
DRI production with syngas
The gas based direct reduction (DR) process is designed to convert iron pellet/lump into metallic iron by the use of reducing gases in a solid-gas moving bed shaft furnace. O2 is removed from the iron ore by chemical reactions based on H2 and CO for the production of highly metallized DRI. Syngas produced through coal gasification has a high percentage of reducing gases such as CO and H2. Some volume of CH4 is also present in the syngas. Hence the use of syngas is possible in the shaft furnace for the production of DRI. Use of syngas does not need reforming which is needed with the use of natural gas based DRI production. All the commercially available gasification technologies can be paired with the shaft furnace to produce any form of DRI (cold DRI, hot DRI, or HBI).
The three important aspects in gas based DRI production process are (i) the molar content of H2 and CO decreases from the reduction zone bottom to its top whereas that of CO2 and H2O increases which means that inversely, the content of iron oxides decreases from top to bottom with mainly iron leaving from the shaft bottom, (ii) the solid and gas temperatures are equal along the shaft furnace except in a thin layer at the top near the solid inlet where a great temperature difference exists, the pellets are charged cold, and the temperatures of both solids and gas increase from the shaft top to its bottom, and (iii) reaction enthalpy is of great importance, namely, the rather endothermic nature of H2 reduction reactions versus the exothermic nature of CO reduction reactions, as well as the endothermic nature of steam CH4 reforming.
In the shaft furnace, the conditions which support in situ reforming are (i) presence of oxidants and hydrocarbons (H2O+CO2 +CH4), (ii) high temperature and (iii) presence of catalyst (DRI). The conditions which supports the reduction of iron oxides are (i) presence of reductants (very high ratio of H2+CO)/(H2O+CO2 ), (ii) high temperature, and (iii) presence of iron oxides. In the shaft furnace reduction zone since the above conditions are present, simultaneous reduction and in-situ reforming becomes possible.
The reactions which are taking place in the shaft furnace are (i) reduction reactions (Fe2O3 +3H2 = 2Fe + 3 H2O and Fe2O3 + 3CO = 2Fe + 3 CO2), (ii) in-situ reforming reactions (CH4 + H2O = CO + 3H2 and CH4 + CO2 = 2CO + 2H2), and (iii) carburizing reactions (3Fe + CH4 = Fe3C + 2H2).
In the shaft furnace, both endothermic and exothermic reduction reactions are taking place. The reduction reaction with H2 (Fe2O3 +3H2 = 2Fe + 3 H2O) is endothermic while the reduction reaction with CO (Fe2O3 + 3CO = 2Fe + 3 CO2) is exothermic. Each of these reactions is reversible and in order to proceed in the reducing state, the ratio of reductant to oxidant must be greater than the equilibrium value.
The degree of metallization is a function of H2/CO ratio. H2 has better reducing properties when compared with CO. Hence, a higher H2/CO ratio gives higher value of metallization in the DRI. The optimum value of H2/CO ratio is around 1.6 when the exothermic heat of reaction with CO is balanced by the endothermic heat of reaction from reduction with H2. Based on the analysis of treated syngas (typically from a Lurgi gasifier), expected DRI characteristics for syngas based direct reduction (DR) plant are 93 % or more of metallization and upto 2 % C.
The typical syngas analysis from the coal gasification plant consists of CO – 26 %, H2 – 55 %, CH4 – 16 %, CO2 – 2 %, and N2 – 1 %. Specific requirements of syngas per ton of DRI correspond basically to the typical make-up requirement of the gas and it is around 685 N cum/t DRI.
The flowsheets of Midrex and Energiron DRI production processes with syngas are shown in Fig 7 and Fig 8.
Fig 7 Flowsheet of Midrex DRI production process with syngas
Fig 8 Flowsheet of Energiron DRI production process with syngas
The main components of syngas based DRI are (i) vertical shaft furnace, (ii) process gas heater, (iii) CO2 removal system, and (iv) product cooler. Depending on particular applications, optional schemes can be incorporated. These optional schemes include (i) in-plant electrical generation by installing a turbo expander in the treated syngas stream before being fed to the DR module, which allows potential power savings by taking advantage of the gasifier high operating pressure, and (ii) recovery of CO2 for sale as by-product.
The important aspects of the syngas based DR process scheme are (i) no major changes are needed in the basic process scheme with the reduction section is incorporated as it is in typical natural gas based DR plants, (ii) syngas is conditioned through shifting and CO2 removal to produce the H2 rich gases which characterize the process, and (iii) recycling of reducing gases, through CO2 removal, minimizes syngas make-up.
Another important aspect of the syngas based DR process is that when the temperature of the bed increases to a point where the individual pellets and lumps begin to fuse together, clustering or sticking occurs inside the furnace. So bustle gas temperature is dictated solely by the maximum temperature which the specific oxide feed can tolerate without sticking. In case of syngas based DRI, H2/CO ratio of bustle gas is around 2.1. Hence endothermic input is more in this syngas based DRI. In order to overcome the energy deficiency due to the endothermic reaction occurring in the DRI shaft furnace, bustle gas temperature is required to be maintained. The requirement of high temperature of bustle gas is a must for the maintenance of the bed temperature. Maintaining the minimum bed temperature is needed for achieving higher metallization values in DRI.
Top gas from DRI shaft furnace is scrubbed, cleaned and passed through CO2 removal unit in order to remove CO2. A part of the top gas after scrubber goes to burners as top gas fuel for combustion and rest of the part goes to CO2 removal system.
In case of Midrex process, lean process gas (also called process gas after CO2 removal) after passing through CO2 removal system, is mixed with fresh syngas in the gas mixer (then gas is a called feed gas) and passed through feed gas saturator and feed gas mist eliminator. H2S injection is also done into the feed gas for avoiding metal dusting of heater tubes. Feed gas is sent to the heater for attaining required reducing gas temperature of 930 deg C (gas, coming out from heater, is called reducing gas). Further increase in temperature is achieved by injecting O2 into the reducing gas. After this the gas is called bustle gas. Fresh syngas is also passed through the heater to increase its temperature upto 150 deg C for using it in turbo-expander which produces upto 10 MW power.
In case of Energiron process, the treated H2 enriched syngas from the gasifier is fed to the DR plant. Adequate CO2 removal from the syngas optimizes the reuse of top recycle gas. The mixture of syngas make-up and recycle gas is preheated in a direct gas heater upto 930 deg C and fed to the shaft furnace. After reduction of iron ore in the shaft furnace, top exhaust gas is passed through a scrubbing unit for dust removal and cooling. The gas is then recycled by the compressor. To further decrease energy consumption, a top gas heat recuperator is normally incorporated. Inside the shaft furnace, hot reducing gas is fed to the reduction zone. The gas moves upward counter-current to the iron ore moving bed. The gas distribution is uniform and there is a high degree of direct contact between gas and solids. The exhaust reducing gas (top gas) leaves the reactor at around 400 deg C and passes through the top gas heat recuperator, where its energy is recovered to produce steam, or alternatively to preheat the reducing gas stream, and then through the quenching/ scrubbing system. In these units, water produced during the reduction process is condensed and removed from the gas stream and most of the dust carried with the gas is also separated. Scrubbed gas is then passed through the process gas recycle compressor, where its pressure is increased. Compressed gas, after being sent to the CO2 removal unit, is mixed with the syngas gas make-up, thus closing the reducing gas circuit.
Important aspects of syngas based DRI production
Due to the counter-current of the solid and gas flows in the shaft furnace, there are several aspects which influence the furnace performance. For example, the gas coming from the transition zone influences the solid in the central zone, which in turn influences the transition zone gas. Likewise, the solid from the transition zone influences the gas from the cooling zone, which in turn influences the solid. Also, the flow rate of the solid in the central zone is considerably smaller than that in the peripheral zone. Moreover, the transition zone has very little impact on the temperature of the descending solid.
It can be also seen that CH4, H2O, and CO2 flow rates decrease above the gas entrance, whereas those of H2 and CO. This is the opposite in the upper half of the shaft furnace, except for CH4, which sees its flow rate reach equilibrium. This inversion can be explained by the preponderance of the iron oxide reduction reactions, as well as the inverse Boudouard reaction (2CO = C + CO2) as evidenced by the C production. The decrease in CH4 can be related mainly to the steam reforming (CH4 + H2O = CO + 3H2) with C deposition from CH4 (CH4 = C + 2H2) having a rather negligible impact. This little impact is emphasized by the decline in C flow rate near the gas inlet, which is due to the direct Boudouard reaction.
In the shaft furnace, hematite is readily reduced into magnetite. The magnetite remains present over most of the height, being slowly reduced or almost not changing until the gas inlet.
The importance of the transition zone is to also be noted. The main reaction in this zone is CH4 decomposition (CH4 = C+ 2H2). C is herein deposited on produced iron, leading to the final C content of the DRI. This C comes in addition to the C possibly deposited via the inverse Boudouard reaction earlier. At the rather low temperature of the transition zone, no iron oxide reduction by solid C occurs. The H2 produced by the CH4 decomposition reaction contributes to the final metallization degree. Moreover, the gas-solid temperature difference is reversed in this zone, the descending solid being hotter than the ascending gas.
Quality of feed material – The main feed material in DRI shaft furnace is DR grade pellets and sometimes calibrated lump ore (CLO) is added in certain percentage in the charge mix. Use of CLO in the charge mix is desirable only with good quality pellets. The quality of pellets is to be monitored closely in order to operate DRI shaft furnace smoothly when syngas is used as reducing gas. The distribution of cold crushing strength (CCS) is important to look at in order to control the amount of fines feeding into the shaft furnace. It is apparent that wide distribution of CCS does not support the smooth operation of DRI shaft furnace. Also important is the percentage of unfired pellets since these pellets get thermally fragmented very easily while entering into the furnace. Temperature of bustle gas also plays an important role in thermal fragmentation. Porosity is also an important factor for achieving good reduction capability in less residence time and increased gas utilization. The percent metallization is also increased with optimized porosity values when the CCS value is higher than 270 kg/pellet. Gangue content consisting percent of SiO2 and percent of Al2O3 in pellets plays an important role for increasing the metallic content of the pellets and reducing the fine generation during transportation and thermal fragmentation inside the DRI shaft furnace.
In case of Energiron process, the low gas velocity inside the shaft furnace, due to the high operating pressure, reduces the fines carry over in the gases.
Importance of reducing gas heating – The syngas contains more of H2 than the CO. The reduction using such gas carryout endothermic reactions inside the shaft furnace and it leads to reduction in the average bed temperature. The temperature of the reducing gas is to be such that it maintains the minimum bed temperature which is more than 800 deg C in the shaft furnace. Apart from H2, the reactions of CH4 are also endothermic inside the shaft furnace when it gets converted into H2 and CO (reforming reactions). Hence for maintaining the minimum bed temperature and overcoming of the heat deficiency created by endothermic reactions, reducing gas is heated up in the heater and to increase the temperature further O2 is injected into the reducing gas. The rise of around 100 deg C can be achieved by O2 injection. After the O2 injection, reducing gas is normally called bustle gas. The temperature increase due to O2 injection is because of the combustion reactions which releases some energy and heats up the bustle gas. Since there are three components in the bustle gas, the combustion energy of CH4 is minimum compare to the combustion energy of H2 and CO.
In the DR plant, CO2 emissions are related to the C content of the fuel used (syngas). There are three outlets for the C. The major portion of the C is captured as CO2 because of a CO2 absorption system for selective elimination of CO2. A part of the C as CO2 is emitted to the atmosphere mainly from the process gas heater. The remaining part of the C goes into the product DRI. The higher C product also has advantages during its use in the steelmaking in the electric arc furnace.