Use of Nut Coke in a Blast Furnace
Use of Nut Coke in a Blast Furnace
Metallurgical coke also called blast furnace (BF) coke plays an important role in the stable operation of the BF. BF coke has typical size of 30 mm to 60 mm (some furnaces use BF coke of size 40 mm to 80 mm) and it constitutes a big component of the production cost of the hot metal (HM). The high cost is because of the generation of a large percentage of minus fractions of coke during the production of BF coke at the coke ovens. These minus fractions are known as coke breeze (-10 mm) and nut coke (10 mm to 30 mm). The entire quantity of coke breeze generally is consumed in sinter plant during the sintering of ore fines. Earlier there was practically no use of nut coke in an integrated steel plant and it was sold to other users. Prof. V. I. Loginov suggested in 1960s to charge the nut coke into the BF mixed with sinter. Though this idea was successfully tested, yet there was initial resistance to use nut coke in the BF.
However, the use of nut coke in the BF as a substitute of a part of BF coke is now considered as a proven technology and the addition of nut coke in BF ore burden has almost become a standard practice. The usage of nut coke is highly dependent on its availability. Monthly average consumption of nut coke over 100 kilograms per ton of hot metal (kg/tHM) has been successfully achieved in some BFs. The statistical analysis of the average annual indices of some of the European BFs has shown that the introduction of nut coke in the BF burden gave the coefficient replacement factor close to 1.0.
In conventional BF ironmaking practice, ferrous burden (lump ore, sinter and pellets) and BF coke are charged in alternate layers. Nut coke is normally charged in the BF mixed with the ferrous burden. Addition of the nut coke into the ferrous burden layer of the BF (i) helps in the effective utilization of low value product generated during carbonization of coking coal, (ii) reduction in BF coke consumption at the BF, (iii) improvement in the BF productivity, (v) optimizes the gas dynamic regime of the smelting operation in the BF, (v) improves the techno-economic indices of the BF because of the reduction in the production cost of HM, and (vi) reduces to the CO2 emissions since lesser coke is to be produced in the coke ovens. Fig1 shows comparison of conventional charging and nut coke mixed charging of the ore burden in the BF.
Fig 1 Comparison of conventional charging and nut coke mixed charging of the ore burden
Several studies have been done and based on these studies many process changes have been carried out to reduce the consumption of the BF coke During the HM production in the BF. Different studies have pointed out that the nut coke mixed charging improves the permeability of the softening and melting layer, as well as the direct reduction. With the nut coke, there is a superior effect for this phenomenon. Operation of many BFs has proved the possibility of coke saving and increase in the productivity of BF when using nut coke mixed with iron burden but reasons and mechanism of this phenomenon has not been very clear until the recent time. Basically, three reasons can affect the decrease in coke rate. These are (i) improvement of gas permeability in ‘dry zone’ of the BF, (ii) improvement of reduction conditions of iron burden, and (iii) ‘protection’ of metallurgical coke from the solution loss reaction in the BF shaft due to the higher reactivity of the nut coke.
In recent past, it has been discovered that small amount of nut coke mixed with ferrous burden leads to better reduction kinetics, lower reductant consumption, and better gas and liquid permeability. Nut coke is charged as a replacement of the regular BF coke. Its utilization affects the ferrous burden to coke layer thickness ratio and permeability. Its utilization varies from BF to BF. Nut coke interaction with burden at both low and at high temperatures is important. If nut coke is charged more than the optimum quantity then it causes unconsumed nut coke descending to the lower part of the BF. This has a detrimental effect on the BF hearth as the unconsumed nut coke fines accumulate and choke the deadman zone. Choked deadman disturb the quality of the HM and productivity of the BF. This effect is more severe when the BF operates with high coal injection rate. Ideally nut coke is to get consumed completely before the cohesive zone. Fig 2 shows a comparison of BF operation without and with nut coke in the BF burden.
Fig 2 Comparison of BF operation without and with nut coke in the BF burden
A study carried out on the influence of sinter and coke layers thickness and sinter-nut coke mixture on the gas permeability at temperatures in the range of 1,100 deg C to 1,600 deg C has shown that as the thickness of sinter and ore layers is decreased, the pressure drop is decreased. Mixing of 90 grams (g) of nut coke in 1,400 g of sinter is able to decrease the pressure drop at 1,400 deg C from 380 mm WC (water column) to around 50 mm WC. The conclusion from the study has been that the application of high ratio nut coke mixed charging technology in the BF results into the improvement of the gas permeability in cohesive zone.
In another study, nut coke segregation and radial distribution in a charging system before the entering the BF has been simulated using Discrete Element Method. It has been reported that the circumferential balance of nut coke mass in the charging hopper is very important factor which affects the nut coke distribution. It has been found that the application of stabilizer on the tip of charging chute is an efficient method to avoid nut coke segregation.
In several BFs, the change in reduction processes by using nut coke-ore mixed charging has been studied. It has been found that direct reduction can be promoted in the cohesive zone and inhibited in the hearth and, hence, the heating of the hearth is improved. Nut coke effect on coke consumption is dependent on the characterization of both nut coke and the BF coke. The influence of two separate roles of nut coke as a reducing agent and as a bed spacer has been studied. It has been found that because of the size of the nut coke in ore burden layer enables the nut coke to be mainly consumed to regenerate CO gas, and the BF coke gets protected from degradation and hence it improves the bed permeability. The preferential consumption of nut coke by the solution loss reaction is dependent on the nut coke size. Fig 3 gives comparison of conventional charging and nut coke mixed charging of the ore burden in BF.
Fig 3 Comparison of conventional charging and nut coke mixed charging of the ore burden in BF
Higher degree of BF coke replacement with nut coke is always desirable, but with very high usage of nut coke, there is always a risk of the unconsumed nut coke going to the lower part of the BF, which can result in the choking of the deadman and the hearth. In a study based on mathematical modelling and experiments, it has been concluded that when the nut coke rate is low, it is completely consumed by the gasification reaction and it leads to better permeability. But when the nut coke rate is very high, it does not get consumed completely by the gasification reaction. It continues to exist in the lower part of the BF and causes increase in a pressure drop of the coke packed bed. It has been found based on the BF operational experience that increase in the nut coke utilization beyond certain optimum concentration affects the hearth drainage capacity, and results in poor smelting rate and lower BF productivity.
In another simulation study of the deadman zone in the BF hearth, it has been noticed that voidage is of greater importance than the coke diameter in the deadman zone. The low voidage in the deadman zone can cause lower penetration of the hot gases into lower BF region and develops low temperature zone in the deadman area. Fines generation and its accumulation are not desirable for the permeable deadman zone. It has also been seen that the coke fines present in the lower part of the BF causes increase in the pressure drop and this increase in pressure drop is higher than the magnitude of the pressure drop decrease in the cohesive zone due to the nut coke. Thus, overall pressure drop increases in the BF.
Above findings clearly indicate that there is an optimum quantity of the nut coke above which the benefits of using nut coke are little, moreover this can also cause some abnormalities in the BF behaviour and hampers the production.
Effects of charging nut coke with the ferrous burden
The charging of nut coke with the ferrous burden in the blast furnace has the following effects on the BF working.
Improvement in the permeability – Permeability is a parameter controlled by blast volume and pressure drop of the shaft column. Under the constant condition of blast volume, the permeability can be improved by decreasing the pressure drop. Bed permeability is one of most important factor in the operation of BF. The BF productivity can be enhanced by higher blast intake and hence needs adequate gas permeability. The gas permeability of the bed determines the flow of the reducing gas in the BF. The way the reducing gas flows in the BF has an influence on the productivity. It is believed that the pressure drop can be decreased by adding the nut coke into the ferrous burden.
The use of nut coke with ferrous burden has the advantage of better permeability in the dry zone of the BF. The addition of nut coke in the ferrous burden has a positive effect on the gas permeability. The beneficial effect of the nut coke addition to the ferrous burden on permeability is less significant in case of low nut coke mixing ratio. By the use of around 10 % and 20 % nut coke in the ferrous burden, the BF productivity can be improved by 1.5 % and 2.5% respectively.
A study was made during two periods in a BF. The first period was operated with conventional charging with no nut coke and the second period was operated with charging of ferrous burden with nut coke. In both the periods the ore quantity is kept the same. It has been found that the pressure jump of BF gas increased and varied with the amount of the nut coke. The operation of the BF has been more even with increasing percentage of the nut coke. The BF output also increased and the main reason for the higher output has been the reduced consumption of the BF coke. The heating and reducing capacity of the gas flow is completely utilized due to the better distribution of the furnace gas and the more even operation of the furnace.
In an another study of the BF operation using a large quantity of nut coke mixed with the ore burden, it has been concluded that addition of nut coke into the ore burden layer prevented the permeability deterioration in the lower part of the BF. It has been inferred from the study that the normal BF coke remains large in the lower part as the result of selective solution loss reaction of nut coke and the permeability of the cohesive layer remains is good because of the charging of the nut coke with the ferrous burden.
Another study has been made on the properties of the ore burden layer consisting of ore and nut coke. Two methods for mixing layers have been used in the study. The first one is changing height and to have more number of layers. The second one is changing the quantity of nut coke into the ore layer. It has been found out that the pressure drop decreases with the decrease of layer height (increase of layer number) and the peak disappeared under the conditions of more than 3 layers. The pressure drop lowers rapidly with increase of the quantity of nut coke. It has been thought that carburization occurs at the interface of the sinter and coke. When metal melting at the boundary, gas mainly passes through coke and its surroundings and the permeability of the packed layer is maintained enough. Generally it is believed that the bigger is the voidage in the burden layer, the higher is the permeability. However, through a gas dynamics study, it has been found that when nut coke is mixed in the burden layer, voidage is decreased, but the permeability is improved when compared with conventional charging of ore without nut coke. Hence, both the voidage and the voidage structure is to be considered when studying burden permeability.
The addition of the nut coke into the ore layer can reduces the gas resistance of the cohesive zone. The lowering of the gas permeability resistance is because the mixed coke adds a new void to the ore layer.
Ferrous burden in the BF mainly consists of Fe2O3 and Fe3O4. Removing oxygen (O2) from the ore burden is called reduction. The BF process is based on the reduction behaviour of the ferrous burden materials. Reduction rate and reduction degree influence the production of the BF directly. The thermal reserve zone temperature in the BF is approximately consistent with the starting temperature of the Boudouard reaction (solution loss) of coke (C + CO2 = 2CO), which involves intensive endothermic reactions. The Boudouard reaction controls the overall reaction inside the BF. If the starting temperature of the thermal reserve zone can be lowered, the equilibrium concentration of the FeO-Fe reduction reaction (FeO + CO = Fe + CO2) is shifted to higher CO gas utilization efficiency. This results in improved CO gas utilization efficiency at the BF top and a decrease in the consumption of reducing agents.
Higher reduction rate of iron oxide is desirable for higher BF productivity. In a study, based on experimental analysis on the reduction of iron oxide from 900 deg C to 1200 deg C, it has been suggested that the rate controlling reaction for reduction is oxidation of carbon. The reduction of iron oxide takes place in two stages. In the first stage it is reduced from Fe3O4 to FeO and in the second stage it is reduced further from FeO to Fe. The rate of first reduction reaction is faster than the second reaction. In the final stage of FeO reduction by carbon (C), the reduction rate further decreases due to the formation of fayalitic (FeO.SiO2) slag. The ‘reduction retardation’ phenomena especially occur during and after softening of the ferrous burden. A study on the basis of experimental observation confirms that at high temperature (higher than 1100 deg C) ferrous burden without nut coke suffers with ‘reduction retardation’, but in ferrous burden mixed with nut coke this phenomenon is not observed. For higher reduction degree, it has been found that the size of nut coke is to be comparable or smaller than that of the ferrous burden
Softening and melting behaviour – The area where ore starts to soften and melt is known as the cohesive zone. Softening and melting are physical phenomena and chemical changes in the cohesive zone behaviour are related to the location and shape of the cohesive zone and the gas flow, and have an important influence on the BF operation.
Thinner cohesive zone is desired for lower pressure drop and better permeability in the BF. This can be achieved with the ferrous burdens which have less temperature difference between their softening and melting. The cohesive zone thickness can also be altered by mixing nut coke in the ferrous burden. By performing high temperature experiments with nut coke mixed with ferrous burden in a study, it has been found that softening and melting temperature is increased by 86 deg C and 15 deg C respectively. The softening and melting temperature difference is squeezed by 71 deg C. This indicates the formation of thinner cohesive zone with the nut coke mixed ferrous burden.