BF Coke and its Role in Blast Furnace
BF Coke and its Role in Blast Furnace
The raw materials for a blast furnace (BF) normally consist of three types of materials namely (i) metallic materials (sinter, pellets, and calibrated ore lumps or briquettes), (ii) fuels (coke, pulverized coal, oil, and natural gas) and fluxes (limestone, dolomite, and quartzite). Metallic materials, fluxes and coke are charged from the top, whereas auxiliary fuels (pulverized coal, oil, or natural gas) are injected through the BF tuyeres.
The combustion of BF coke and injected fuel occurs in front of tuyeres, from which pre-heated oxygen (O2) enriched air blast at temperature around 1200 deg C is blown in. Temperatures in the combustion area reach above 2000 deg C. The high kinetic energy of the hot air blast creates a cavity in front of each tuyere which is known as raceway. After combustion at the raceway, the formed CO2 (carbon di-oxide) and H2O (water vapour) gases are reduced to CO (carbon mono-oxide) in the active coke zone, which also quickly reduces the temperature of the gas. Behind the raceway, there exists a tightly packed zone. This zone is known as the bird’s nest and contains small sized coke, unburned char and soot, and liquids. In the centre of the furnace exists a zone called deadman, in which a pillar of coke is very slowly diluted to liquid iron. During the BF operation, it takes around 5 hours to 6 hours for the charged material to reach the tuyere level and 5 seconds to 10 seconds for the gas to reach the top from the tuyere zone. Both are dependent on furnace size and process conditions.
Coke plays a very important role for the smooth operation of the BF. Hence, the quality of the coke is very important. There are some important properties of the coke which influences the smooth running of the BF. Degradation mechanisms of coke and effects of coke quality on BF performance is shown in Fig 1.
Fig 1 Degradation mechanisms of coke and effects of coke quality on BF performance
Assessment of coke properties
There are several important properties of coke which are needed to be evaluated for the smooth running of the BF. Major among them are (i) cold strength, (ii) hot strength, (iii) coke reactivity, (iv) reaction mechanism and post reaction strength, (v) mineral matter, and (vi) degree of graphitization. These are described below.
Cold strength – The ability of the coke to maintain its size distribution during handling, charging, and its descent in the BF is determined by its strength. Hence, the strength of the coke is considered to be one of the most important properties for the efficient operation of the BF. Besides the coke hot strength, it is important to know the factors which affect the coke strength at low temperatures since many of the same properties, such as pore structure, also affect strength at high temperatures besides some new factors which also get introduced.
The pore structure of coke is one of the most important characteristics regarding its strength. Under compressive stress, small micro cracks extend from one pore into the cell wall and to an adjacent pore. The total porosity is linearly connected with coke strength basically due to the reduction of solid matter. As per the results of 3D modelling, it is seen that the effect of the total porosity on coke strength is the foremost compared to the C (carbon) matrix bonding strength. It has been shown that both the total porosity and pore shape are important for stress concentration in coke and thus for its mechanical strength and the desired pore shape is circular.
Pore size distribution is also considered important, since it affects the distribution of stresses. It has been established that small pore size and a high density of pores produce higher strength compared to a small number of large pores. As per theory, thick walls of coke cell are expected to be beneficial for the strength. However, in practice a high density of small pores tends to produce a thin cell wall. Hence, it is considered that large pores control the tensile strength of coke since they act as stress concentration points for crack initiation. However, the 3D modelling has shown that regular pore distribution is a dominant factor regarding coke strength, whereas the number or mean size of pores is not as important, since cracks can easily propagate through connected pores. It has also been shown that the connectedness of pores is a key factor regarding coke strength.
Coke texture can be classified to inert maceral and reactive maceral derived components (isotropic, anisotropic, banded, etc.) based on the optical properties. The elastic modulus of inert maceral is larger than that of reactive maceral. Hence, inert maceral can be treated as a reinforcing material. A high amount of inert texture in coke is a factor which has been linked with good strength. Inert macerals have also been stated to decrease the mean pore size, which can partially explain their positive effect on the coke strength. A decrease of the size of inert macerals can improve coke micum strength upto a certain limit.
Differences exist in the hardness of the reactive coke textures. However, despite some efforts to link the textural distribution of coke with the mechanical strength, correlations between the two are generally not strong and it is likely that the effects of texture distribution are over shadowed by other factors. Pre-existing fissures and cracks formed during the carbonization process and handling are also important for the coke strength.
Hot strength – It is the strength of coke at high temperatures and is perhaps one of the most important characteristics of the coke. Despite this, there is little information available regarding the coke strength at temperatures applicable to the lower part of the BF shaft. The hot strength is normally not measured either at the coke oven batteries or at the BF. The test for CSR (coke strength after reaction) is sometimes misleadingly called a hot strength test, but the actual strength measurement is performed at room temperature after gasification at a temperature of 1100 deg C. Only limited scientific studies have been carried out regarding the coke hot strength, mainly because of the difficulties in creating an experimental setup.
During one of the early studies, it has been reported that some of the grades of coke cause (i) disturbed BF operation, (ii) a rise in pressure, and (iii) plugging of blow pipes roughly around 6 hours to 8 hours after charging. This has indicated failure of coke in the lower BF in spite of good stability and hardness values. This study has led to the increased interest towards the study of coke hot strength.
Another study has shown that the coke drum strength is increased at 800 deg C, maybe due to the annealing of cracks, but it begins to decrease as the temperature is increased from 800 deg C to all the way upto 1300 deg C. During the study, 12 types of coke qualities have been tested for the room temperature strength after heat treatment at 1450 deg C. It has been found that strength has decreased in the most of the qualities of the coke, although strength has increased for some qualities of the coke. Tensile strength, however, was higher at 1200 deg C compared to the room temperature strength.
One of the study which has been carried out for the strength of seven qualities of the coke at 1400 deg C, it has been found that there is increase in the strength for six of the seven qualities of the coke and a decrease of strength for one quality of the coke. In another study, it has been observed that coke strength generally increased at 1300 deg C, although for some coke qualities, there is decrease in the strength. When the temperature is increased to 2000 deg C, a strong decrease in strength has been observed for all the qualities of the coke. This is because of the deformation behaviour being plastic at 2000 deg C and at 2300 deg C the strength is not measurable because of the plastic deformation.
These early studies have shown a mixed result with respect to the development of coke strength and both the increases and decreases of strength have been obtained as temperature has been increased above the coking temperature. These mixed results have been obtained since (i) the studies have been carried out by wide ranging methods and analysis equipment, (ii) different coke qualities behave very differently at high temperatures, (iii) compressive strength of coke can increase in a certain temperature range (such as upto 1300 deg C to 1400 deg C), but it declines when temperature is further raised, and (iv) the number of test samples has not been sufficient, which casts uncertainty on the reliability of these results. The properties of coke which define its hot strength are largely unknown.
Coke reactivity – The coke reactivity can be defined as the rate at which coke is consumed in the presence of an oxidizing gas, such as O2, CO2, or water vapour(H2O). It is primarily determined by the ash composition, pore structure, the surface area, and the rank of the coal from which coke has been produced. When coke is heated, it begins to approach the atomic structure of graphite. The degree of organization of the C matrix of the coke is called graphitization. A higher degree of graphitization normally leads to lower coke reactivity. During the process of coking, coal particles maintain their inherent structure. Hence, a higher coalification (rank) of the coal generally leads to a higher graphitization degree of the coke, although coking temperature also increases the degree of graphitization.
Isotropic coke textures are generally more reactive with CO2 gas compared to anisotropic textures. This leads to selective solution-loss and a weakening of coke strength in the BF. Normally lower rank coals have a greater content of isotropic textures present in the coke. As coal rank is increased, the amount of anisotropic coke texture increases and the reactivity of the coke decrease. During gasification, isotropic and inert regions are preferentially gasified. The rate of gasification of anisotropic material is slower and decreases with the increase in the size of the anisotropic regions. Normally, flow-type anisotropy is the most resistant to gasification.
The threshold temperature of coke is determined by both the catalytic components and the structure of the C lattice. Accumulation of alkali in the BF reduces further the threshold temperature. Also, the threshold temperature of coke gasification correlates well with the temperature of thermal reserve zone.
BF gas composition also has influence on then coke reactivity. The main components of the BF shaft gas includes N2 (nitrogen), CO, CO2, H2 (hydrogen), and H2O, of which CO2 and H2O can participate in solution-loss reactions, but CO and H2 also play a role in constraining the reactions. A large number of studies have been carried out on coke reactivity with only CO2 as a gasifying component, but only a few studies have been done on the effects of H2O. This is because CO2 is more prevalent in the BF shaft gas and quite possibly because of the fact that the widely used CRI (coke reactivity index) test is carried out in 100 % CO2.
The presence of H2O in the BF is rarely deliberated and its content in the top BF gas is usually not reported since it is difficult to measure it. However, some of the studies on the coke gasification which included H2O, it has been found that the presence of H2O can both increase coke reactivity and alter the reaction mechanism.
Also, the mass balance calculations combined with thermal calculation shows that both H2 and H2O are always present in both the top and shaft gasses in different amounts. Their content largely depends on the type of injectant fuel and injection rate. Other sources of H2 in the BF are coke, moisture in the injectant, and moisture in the hot air blast.
According to theory, the gasification reaction of coke occurs in specific atoms in the C lattice, called active sites. The reaction path can be shown by the following equations.
CO2 + Cf <–> Co + CO
H2O + Cf <–> Co + H2
Co <–> CO + n Cf
In the above equations, Cf denote a free active site capable of a reaction, Co denotes an occupied site possessing an O2 atom, and ‘n’ denote the number of free active sites after reaction. Reaction sites can detach an O2 atom from a gaseous CO2 or H2O molecule which collides with them and produces either CO or H2, while an O2 molecule can be adsorbed to the solid C structure occupying the reaction site. Further, in another step, which is considered to be the slowest step of the reaction, the O2 atom at the occupied site is released into the gas phase as CO. The number of free active sites after reaction can have a value of 0, 1, or 2 depending on the structure of the reacting C.
For coke, only few studies have been carried out compared the reactivity in CO2 and H2O. However, multiple such studies have been done for the gasification of coal. The reactivity of C is strongly correlated with the number of active reaction sites. CO and H2 have been found to constrain reaction by decreasing the number of active sites, while inert gases, such as N2, do not affect reactivity if the partial pressure of gasifying component is sustained.
In a gas mixture with both CO2 and H2O, two theories have been suggested depending on the reactions with C occurring on (i) common active sites, or (ii) separate active sites. These theories have been tested for coal char gasification with different results, while in some of the tests, results have been found to be in favour of common active sites, whereas in other tests, the results have been found to be in favour of separate reaction sites theory.
When comparing the reaction rate of C in the presence of H2O and CO2, both studies carried out with coke gasification and with coal gasification have shown that gasification in H2O increases reaction rate compared to CO2. It has been shown that the relative number of reaction sites per unit weight of coke is around 60 % higher in H2O than in CO2. This can be explained by either H2O being able to react with more types of sites than CO2, or by the concentration gradient in the gas phase inside particles. In another study it has been seen that the reactivity of coke in H2O is higher than in CO2 at temperatures less than 1500 deg C, but no difference is seen at temperatures higher than 1500 deg C.
It has been found that H2O is more reactive than CO2 due to the weak bonds of H2 in the water molecule compared to double bonds forming CO2 molecules. Gasification with H2O has been found to increase the coal surface area during gasification and hence also increase the number of reaction sites and reaction rate, while reaction with CO2 does not have a similar effect. Also, besides the number of reactive sites, reactivity is also determined by the access of the reacting gas to the reaction sites. H2O has the advantage over CO2 in pore diffusion due to its smaller molecule size. It has also been seen that coal char gasification with H2O occurs primarily on micro-porous surfaces with a diameter larger than 6 Å (angstrom). Gasification of coal with CO2, on the other hand, has been observed to occur outside the micro-porous network on the surfaces of larger pores. It has been proposed that this is most likely due to the active sites (graphite crystallite edges) or catalytic components being concentrated in larger pores, and not due to micro-pore inaccessibility to CO2. For anthracitic coals, the reactivity is upto ten times higher in H2O compared to CO2, which is due to the small sized micro-porosity present in the anthracite coals.
Another important property of coke which affects its reactivity is the catalyzing minerals in coke ash. Iron (Fe), calcium (Ca), sodium (Na), and potassium (K) which have a catalyzing effect on coke gasification, although only in certain forms. For anthracite coal gasification, it has been found that the catalyzing effect of minerals is having higher impact in CO2 gasification compared to H2O.
Reaction mechanism and post-reaction strength – Since the total amount of solution-loss in the BF is independent of coke reactivity, the degree to which the solution-loss weakens coke strength depends on two factors namely (i) reaction kinetics, and (ii) the competition between chemical reaction and diffusion into pores. The factor which constitutes the limiting step can bring completely different results in terms of porosity development and post-reaction strength. If chemical reaction is the rate limiting step, gasification occurs throughout the coke matrix and the strength degradation affects the entire coke uniformly. If diffusion into pores is the limiting factor then the reaction occurs mostly on the surface and the strength of the inner core is unaffected. In the BF, the situation lies between these two extremes and is strongly dependent on coke properties and operating conditions.
The gasification mechanism of coke can be divided into three different temperature regions by ascending temperature. These regions are (i) region 1 which is limited by chemical reaction, (ii) region 2 which is limited by pore diffusion and chemical reaction, and (iii) region 3 which is limited by film diffusion. It has been found that the complete gasification of CO2 takes place upto 1100 deg C in region 1, upto 1350 deg C to 1450 deg C in region 2 and above 1500 deg C in region 3. These regions have not been found for simulated BF gases or in 100 % H2O. The temperature region in which gasification occurs has significant implications on the post-reaction strength.
A large number of studies have been carried out on the correlation between gasification temperature and post-reaction strength in CO2 gasification. In one study, it has been found that the higher the temperature, the more solution-loss reactions weaken the surface, but leave the inner core intact. In another gasification study, it has been shown that coke is weaker after gasification at 970 deg C compared to 1070 deg C. Yet another study found coke gasification to occur uniformly at 1000 deg C and only on the surface at 1300 deg C. Another study found that CO2 gasification gradually changes from uniform reaction at 1100 deg C to surface reaction at 1500 deg C, whereas gasification with H2O occurs mostly on the surface already at 1100 deg C.
In the BF, the decrease of coke strength is generally much lower compared to the values obtained in the CSR (coke strength after reaction) test. This is an indication of the coke solution-loss being controlled by chemical reaction in the CRI (coke reactivity index)/CSR test, but mainly by diffusion in the BF.
Mineral matter – Various mineral compounds in the form of coke ash are available in BF coke. Coke ash usually has around 10 % of the total mass of the coke. The main components of coke ash are SiO2 (silica) and Al2O3 (alumina). Smaller quantities of CaO (calcium oxide), Fe, MgO (magnesia), S (sulphur), K2O (potassium oxide), and Ti (titanium) are also present in the coke ash. Temperatures in the BF are extremely high, capable of exceeding 2000 deg C at the tuyere level. Mineral matter in coke undergoes changes in the BF due to its melting and chemical reactions. At the lower portion of the BF shaft, chemical reaction in the coke ash is the main reason for increasing the porosity and decreasing the coke strength. As regards coke strength, the most important reaction is the transformation of SiO2, which is the biggest component of the coke ash. During heat treatment, the most notable changes occur between 1300 deg C and 1500 deg C. During this temperature range, quartz and cristobalite (both SiO2) are reduced to silicon carbide (SiC). Above 1500 deg C, no SiO2 is generally present.
In a study done on the mineral matter of coke sample taken at the tuyere level, it has been found that mineral matter can migrate from inside the coke matrix and form mineral spherules inside the pores of coke. This migration can leave voids which can affect the strength of the coke. This migration is expected to occur due to the increase of the temperature followed by melting and the tendency of the liquids to form larger droplets. As temperatures increase, coke gradually transforms into a two phase system namely (i) mineral matter, and (ii) a mineral-free C matrix. This is expected to cause changes in both the reactivity and physical strength of the coke.
In another study, it has been found that the mineral matter in coke migrates at high temperatures and can in some cases cause swelling and even cracking in the coke. The composition of mineral matter in coke changes significantly in various parts of the BF. In an EBF (experimental blast furnace), it has been found that the amount of alkali (Na2O and K2O) in the coke is increased 10 times from the top (less than 1 %) to the cohesive zone (around 4 %). It has also been seen that the alkali is distributed evenly inside the coke matrix and not just on the periphery. Alkali build-up in coke is followed by reactions with other mineral compounds. Alkalis are high at the lower level of BF shaft in the bird’s nest and deadman areas, but completely absent near the tuyere zone as a result of high temperature vaporization.
Degree of graphitization – A graphitic structure of C atoms can be defined by regular, vertical stacking of hexagonal aromatic layers. Carbons produced by solid-state pyrolysis of organic matter can be categorized into graphitizing and non-graphitizing. Graphitizing carbons, such as coke, begin to approach the atomic structure of graphite when heated to high temperatures. The height of graphite crystallites (Lc value) in coke starts to grow as coke is heat treated at temperatures above the coking temperature, which is generally around 1100 deg C. Graphitization is made possible by the plastic phase during carbonization, during which C layers are structured in near-parallel orientations. As the temperature of solid coke is increased, it enables the continuous rearrangement of the layer-planes to take place by small stages.
The temperature history of coke can be assessed by measuring the Lc value with an X-ray diffraction analysis (XRD). The Lc value is shown to be directly related to the highest temperature experienced by a coke. The Lc value is a linear function of temperature experienced by the coke and has no influence from the chemical reaction. XRD technique is normally being used to assess temperatures in a BF from coke samples excavated from the BF.
The graphitization degree also affects coke strength. In the structure of graphite, each C atom within an aromatic layer is linked through covalent bonds to three C atoms. However, bonding between aromatic layers is weak, easily broken by external forces. Non-graphitic carbon, however, contains cross-linking between the aromatic layers and a much higher force is required to dissociate them. During heat treatment, non-organized C is presumably attached to the edge atoms of the graphite-like layers, which enables the growth of organized layers but decreases the cross-linking between the layers. It has been stated that coke strength becomes higher with a non-graphitic structure and this is usually associated with coke made from lower rank coals.
Coke grades have been found to have differing degrees of graphitization after treatment in similar annealing temperatures and hence it is important to measure the graphitization tendency of the coke during hot strength testing.
Coke properties measurements
Coke properties which are normally measured include strength (stability, hardness), reactivity (CRI), strength after reaction (CSR), coke size distribution and ash composition. The coke quality requirements have been summarized in different standards, and estimates of coke quality requirements in the future with low coke rate and high rates of PCI have been predicted in many studies. High PCI rates require higher strength, lower reactivity, increased mean size of coke and lower contents of harmful components. Different analyses of BF coke are described below.
Strength – Coke strength is normally described by drum strength measurements performed at room temperature. There are 4 standards (Micum, ASTM, JIS, and IRSID) which are generally used. The drum strength indices describe the ability of a weighed amount of sized coke to resist degradation in a tumbler equipped with lifters for a specific amount of rotations. However, it is often being debated whether the drum indices can actually simulate the stresses which the coke is experiencing in the BF operating conditions.
The drum strength indices are usually classified to measure two different properties. These are (i) capability of coke for abrasion resistance, and (ii) capability of coke to resist volume breakage. The number of rotations and the measured size distribution are selected to highlight these strength properties. As an example, indices for abrasion are M 10 for Micum, and indices for volume breakage are M 40 for Micum.
Improvements in coke cold strength (stability) are often being suggested for the decrease of BF coke rate, for improving permeability, for improving the stability of BF, for allowing higher flame temperature, and for lowering the number of tuyere failures. In bigger BFs, the improvement of cold strength has only limited effect since in these BFs the mechanical demands are more. Based on the requirements for coke cold strength needed for bigger BFs, strength needs to be sufficient to reduce size degradation, however, after a certain point further increase in strength yields diminishing benefits.
Coke reactivity and post-reaction strength – The reactivity of coke and its post-reaction strength are usually measured through the CRI and CSR tests. The CRI test is the percentage of weight loss to the original coke mass after reaction in 100 % CO2 at 1100 deg C for 2 hours. The test is done for 200 grams (g) of coke with a size of 19 mm to 21 mm. High reactivity of BF coke causes increased coke size degradation, increased fines at the raceway, and decreased raceway depth. Normally the same correlations can be found between coke CRI and CSR values and BF performance, because the results of these tests are generally linked together.
The CSR test is the percentage of +10 mm coke obtained after 600 revolutions in a drum. It is performed after gasification in the CRI test. Improvement in the coke CSR values has multiple improvements on the BF performance. These include lower coke rate, increased productivity, increased permeability, decreased coke degradation, and higher PCI injection rates. It has also been reported that an increase of CSR leads to an increase of average coke size in the lower part of the BF due to the lower degradation between the throat and tuyere level.
In some of the BFs, good value of coke CSR has been linked with improved raceway length. Most of the improvements produced by increasing coke CSR have been derived with CSR values below 60 and further improvements generally bring diminishing benefits. There are mixed reports on the benefits of coke CSR on the BF operation. The process improvements achieved as a result of increased CSR value in one BF are not always seen in other BFs.
In spite of the historical usefulness of the CSR test, there are many opinions which question the validity of CSR to actually simulate the post-reaction strength of coke in the BF. The criticism of the CSR test is based on the reasons as described below.
The total amount of gasification in the CRI/CSR tests is not controlled, unlike in the actual BF process where it is limited to roughly 25 % to 35 % depending on the operating conditions. The unlimited amount of gasification results in a roughly linear correlation between the CRI and CSR tests. Due to it, measuring coke strength after a varying degree of gasification does not accurately reflect the BF operation. The gas atmosphere in the CRI/CSR tests is also considered to be too intensive and completely different from the actual BF gas atmosphere. The conditions of the test result in a significant increase of porosity and the degradation of coke strength throughout the coke matrix, whereas in the actual BF operation the inner core of coke has been found to be mostly unreacted. Further, the drum strength of coke taken both from an operating BF and an EBF has shown much higher than coke CSR. This suggests that the CSR test is much more degrading the coke than the degradation in the BF process. Also, correlation has not been found between coke CSR value and the post-reaction strength of coke excavated from the EBF.
It is also to be noted that in addition to the reactivity, abrasion resistance is another factor which has been found to correlate with CSR. BF coke with very low or very high abrasion resistance can deviate from the linear correlation between CRI and CSR tests.
In spite of the differences of opinions, the benefits of the CSR test are indisputable. However, it is not entirely clear why increasing coke CSR produces improvements in BF operation. It has been suggested that the improvements can be derived more from the lower reactivity of coke instead of the actual post-reaction component of the CSR test. Really, the effects of CSR on BF performance can be possibly found also for coke CRI, since the two values are normally correlated.
Role of BF coke in the blast furnace
BF coke has multiple roles. It acts as a reducing agent, a source of reducing CO gas, a source of heat, a filter of dust and soot, a carburizer of hot metal (HM) and as a structural support material. The role of BF coke as a structural support material is especially important, particularly in the lower parts of the BF. No other material can replace BF coke in this respect.
A lack of permeability of the BF burden restricts blowing rates and lead to poor gas distribution in the shaft area. The flow of fluids in the lower part of the BF is also strongly influenced by mean particle size and voidage of the material bed. The most important properties of BF coke with respect to the permeability are an optimal size, good pre-reaction strength and post-reaction strength, and a narrow size range. Poor coke quality leads to excessive size degradation and the formation of fines, both of which normally impair the permeability of the BF. The permeability of the coke bed in the lower part of the BF determines the technological limits of the BF which include maximum driving rates, best fuel efficiency, and long campaign life.
Poor quality of BF coke can have several adverse effects in the BF. These include increased flue dust, changes to the shapes of the raceway and cohesive zones, increased heat losses, the channeling of the flow of liquids and solids, increased pressure loss, and decreases in the drainage ability of the deadman. The quantity of BF coke charged depends on the amount of auxiliary fuels used and the performance of the BF. In modern BFs, high PCI (pulverized coal injection) rates are generally used which reduces the coke rate in the BF.
Coke gasification in the shaft area of the BF
Coke gasification in the BF shaft, also known as solution-loss, is one of the main mechanisms for the degradation of the strength of the BF coke. The total quantity of the BF coke consumed by solution-loss is mostly independent of the reactivity of the BF coke. In the BF, the total quantity of solution-loss of BF coke is mainly due to the result of operational conditions, such as blast temperature, rate of auxiliary fuel injection, and O2 enrichment etc. Normal estimate of solution-loss of the BF coke is in the range of 20 % to 30 %, which is strongly dependent on process conditions. However, with high PCI injection rates, the degree of coke solution-loss can be in the range of 30 % to 40 %. The gasification of the BF coke in the shaft of BF can occur with either CO2 or water vapour (H2O).
The threshold-temperature of the gasification of the BF coke in combination with the reducibility properties of the iron burden determines the temperature of the thermal reserve zone. The threshold-temperature of the gasification of the BF coke is generally to be in the range of around 900 deg C to 950 deg C. However, it is strongly dependent on the quality of BF coke. Accumulation of catalyzing constituents in the BF, such as alkali, can also significantly decrease the threshold-temperature to a range of around 760 deg C to 810 deg C. The rate of solution-loss reaction intensifies when temperature or the content of oxidizing gases is increased. The probable time duration of BF coke in the main temperature zone of gasification (from 950 deg C to the end of the cohesive zone) is in the range of 2 hours to 3 hours and depends on the injection rate.
The size of the coke is not significantly affected by only the solution-loss reaction. It has been shown that the removal of 20 % to 30 % of BF coke from an average sized lump (say 55 mm) only reduces the size of the coke lump by 1 mm to 2 mm, but the solution loss reaction weakens the coke structure making it more vulnerable to breakage under mechanical stresses. Solution-loss reaction can degrade the coke surface and subject it to the formation of fines under grinding stresses. It can also degrade the entire coke matrix and subject it to volume breakage. The strength decrease caused by solution loss reaction is dependent on the reaction mechanism.
Degradation of BF coke
Several studies have been done on size degradation and quality changes of BF coke in the BF by the BF excavations and drilling studies in Europe and Japan. Degradation of BF coke is determined by both the properties of the feed coke and the conditions existing in the furnace. Hence, the degree of degradation of BF coke varies based on the furnace. Degradation of the BF coke size takes place steadily during its descent, but substantial size changes are limited to the lower part of the BF.
The degradation of BF coke during its descent in the BF occurs due to multiple different types of stresses. These stresses are of many types namely (i) mechanical stresses such as impact during charging, abrasion, compressive load, and high velocity impact etc., (ii) thermal stresses such as graphitization, and thermal shock etc., and (iii) chemical stresses such as gasification, alkali attack, combustion at raceway, and erosion by slag and hot metal etc.
In the upper part of the BF shaft, at temperatures upto 1000 deg C, the size of the coke decreases slightly due to abrasion, but the strength remains unaffected. The strength of coke lump starts to degrade in the middle part of the shaft (temperature around 1000 deg C) due to the selective solution loss reaction and it continues down to the cohesive zone (temperature around 1400 deg C). However, the size of the coke does not change. This is perhaps due to weak mechanical stresses in this area, which are not adequate to peel off the reacted surface layer. The degradation of coke size in the bosh area is found to be of only a few millimeters. The extent of the decrease in the strength depends on whether the gasification occurs on the surface of coke or throughout the C (carbon) matrix.
Coke is subjected to alkali attack and to liquid attack after the gasification zone, below the cohesive zone. Due to the alkali attack, the coke is significantly more susceptible to size reduction by abrasion. From the lower shaft of the BF (temperature around 1400 deg C) to just above the tuyeres (temperature around 1600 deg C), there is quick decrease in the mean size of the coke. However, the strength of coke in this area does not decrease since the weakened surface layer is peeled off. At the level of the raceway, the degree of degradation in the coke size and the overall decrease in the strength depends on the radial position of the coke.
Coke near the walls descends to the raceway and is subjected to thermal shock due to (i) the temperatures which are higher than 2000 deg C, (ii) combustion with the hot air blast, and (iii) high velocity impact from the hot air blast. The porosity of coke in the raceway is high with its strength low and size distribution is the lowest in the BF. Coke near the midline descends to the deadman area and is slowly consumed by dissolution into liquid iron. The strength and porosity of coke in the deadman area is only slightly lower than that of feed coke since the thermal and mechanical stresses in the deadman area are low. The median size of the coke decreases from top of the BF to the tuyere level varies depending on the positioning along the radius with minimum decrease in the raceway and maximum decrease at the deadman.
The chemical reactions in the coke ash play a major role in increasing porosity and decreasing the strength of coke at the lower part of the BF. Possibly the most important reaction is the transformation of SiO2. Other significant reactions include vaporization of alkali at the raceway.
The degradation of coke size is more significant in large BFs due to the higher abrasive forces and mechanical load. Hence, coke quality requirements are higher for large BFs. In small BFs, the mechanical stresses are not strong enough to peel away the reacted surface layer of coke.
The increase of injection rates has considerably lengthened the residence time of coke in the BF. Hence, it has also increased the chemical and physical stresses on coke. With the injection rates of 200 kg/tHM to 250 kg/tHM of PCI, the residence time of coke is twice as compared to a case without PCI injection. The increase of coke porosity and the decrease of strength in the lower parts are also noticed when there is an increase in the PCI rates. Hence, the requirements for coke quality are considerably higher in the present day BFs with high injection rates of PCI.
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