Pulverized Coal Injection in a Blast Furnace
Pulverized Coal Injection in a Blast Furnace
Pulverized coal injection (PCI) is a process which involves injecting large volumes of fine coal particles into the raceway of the blast furnace (BF). Pulverized coal is an important auxiliary fuel used in the BF ironmaking. PCI provides auxiliary fuel for partial coke replacement and has proven both economically and environmentally favourable. It can result in substantial improvement in the BF efficiency and thus contribute to the reductions of energy consumption and environmental emissions.
When the pulverized coal is injected into the BF through blowpipes and tuyeres, the coal is a source of heat and a reductant, because of the reactions of devolatilization, gasification, and combustion as well as the formation of unburned char. In the present day environment, pulverized coal is extensively used in BFs as a partial replacement of the metallurgical coke. PCI is a well-established technology today for the production of hot metal (HM) in a BF. It is practiced in most of the BFs and all the new BFs are normally built with PCI capability. The composition and properties of the coal used for injection can influence the operation, stability and productivity of the BF, the quality of the HM, and the composition of the BF gas.
In the present scenario, there are many criteria which are used to measure the performance of PCI. The first is pulverizing and handleability. The main operating costs in the PCI, other than coal costs, are related to the pulverizing and distribution of the coal to the BF. The second relates to the operation of the BF. The injected coal quality can influence the quality of the HM, stability of the BF, and the top gas composition. The ash from the injected coal (i) can act as an inhibitor for the oxidizing process, (ii) is a main deliverer of undesirable alkalis, and (iii) consumes melting energy. The third relates to the economic benefit. The main cost benefit is the replacement of high cost coking coal plus the coke making operating costs, though other benefits such as improved productivity have also been observed, this improvement is dependent on coke quality. The important economic and operational benefits of using PCI in BF include the following.
- Lower consumption of expensive coking coals. Replacing BF coke with cheaper soft coking or thermal coals reduces reductant costs.
- Extended coke oven life since less coke is required to be produced. This is important as many coke ovens are reaching the end of their useful life and significant investment is needed to replace or maintain them.
- PCI system is less costly than the cost of an additional coke oven battery. Lower capital cost means lower depreciation and interest to be charged on the HM.
- Higher BF productivity, that is, higher amount of HM produced per day (in conjunction with other operational changes).
- Greater flexibility in BF operation. For example, PCI allows the flame temperature to be adjusted, and the thermal condition in the BF can be changed much faster than is possible by adjusting the burden charge at the top of the furnace.
- Improved consistency in the quality of the HM and its silicon content.
- Reduced overall emissions from steel plant, in particular, lower emissions from coke making due to the decreased coke requirements.
Pulverized coal injection was developed in 19th century, but was not implemented for industrial use. In early 1960s, PCI was successfully implemented in AK Steel of USA and Shougang in China. Though trials in several countries at that time had proved that the technology for pneumatic transport and injection of coal were available, but the economics and relative ease of the process was such that oil and natural gas injection became more popular. In the 1980s, interest in PCI escalated, prompted mainly by dramatic increases in the price of fuel oil in 1973 and again in 1979, and its potential as a coke replacement began to be realized. In the early 1980s, coal injection facilities were installed throughout Europe and Japan, with injection rates normally between 40 kg/tHM and 90 kg/tHM and with coke replacement rates of the order of 0.9 kg of coke/kg of coal. This process has developed very fast since then and in the second half of the 1980s there were successful practices of coal injections at rates ranging from 180 kg/tHM to 200 kg/tHM. In nineties PCI technologies became mature. But the real shift to PCI has taken place only when the cost of metallurgical coke started rising due to the increased global demand.
Coal for PCI
The relative importance of different aspects of coal quality for the PCI has varied, as the technology for injection has improved and the rate of injection increased. In the late 1970s, triggered by the oil crisis, interest in PCI was renewed and coal was considered as an economic replacement fuel for oil. As combustibility was considered to be of importance, the coals used for PCI were mostly thermal coals. At that time, thermal coals were readily available and had a much lower cost than hard coking and semi-soft coking coals.
After the initial focus on combustibility of the injected coal, the focus was then directed to the understanding of the impact of coal quality on the coke replacement. During this period, it was seen that lower volatile (LV) coals gave better replacement ratios than high volatile (HV) coals.
As regards pulverizing of coal, the four main operating parameters for the pulverizer are (i) feed rate, (ii) air flow rate, (iii) setting of classifier consisting of cyclone and bag filter, and (iv) grinding pressure (load on roller/ ball). The feed rate, grinding pressure and classifier setting has a direct influence on the pulverizer load and hence the needed power. The air flow rate indirectly influences pulverizer power as it impacts on the efficiency of the classifier.
The grinding characteristics of coals are typically described by the results of a small number of standard analytical tests. Based on these results, predictions are made as to the anticipated grinding behaviour of the coals. There is a general tendency to reduce an assessment of the grinding behaviour of coals to the HGI (Hardgrove Grindability Index), which is a measure for the grindability of coal. Grindability is an index and hence it has no unit. The smaller the HGI, the harder is coal texture and less grindable is the coal. Grindability is an important factor for the pulverizing of coal in the pulverizer. It influences product fineness, power consumption and the throughput.
The moisture content of the coal has an influence on the pulverizing of coal. It is necessary to reduce the total moisture contained in the coal to around the equilibrium moisture level for reducing the handling problems within the pulverizer mill as well as in the bunkers. The moisture content of coal leaving the pulverizer is to be two thirds of the equilibrium moisture level. The quantity of moisture which is to be removed in the pulverizer is given by the equation ‘moisture removed in the pulverizer = (as received moisture) – 2/3(equilibrium moisture)’, where the equilibrium moisture for the sub-bituminous and higher rank coals is approximately given by the equation ‘equilibrium moisture = 0.69 + 0.84 x (air died moisture) + 0.18 x square of the (air died moisture). Equilibrium moisture content varies with the rank of the coal, maceral composition, and ash content.
The type of coal and its surface moisture govern pulverizing drying requirements. The drying capability of a given pulverizer design depends on the extent of circulating load within the pulverizer, the ability to rapidly mix the dry classifier returns with incoming wet coal feed, and the air ratio and air inlet temperature which the particular pulverizer design is to tolerate.
The PCI process system
The PCI process system consists of a number of sub-systems from coal receiving to the injection of pulverized coal at each tuyere for fulfilling the requirements for PCI. It constitutes six numbers of sub-systems namely (i) storage and discharge of the raw coal, (ii) pulverizing and drying of the raw coal, (iii) transportation, storage and supply of the pulverized coal to the injection system, (v) uniform distribution of pulverized coal to each tuyere, (v) combustion of pulverized coal, and (vi) explosion prevention facilities. The important characteristics of the PCI process system are described below.
The reservoir tank is designed to have high capacity so that even if one of the pulverizing lines stops to operate, the coal injection can be continued until corresponding alternation of operational conditions, such as the reduction in the amount of ore charged into the furnace or preparations for blowing down are taken.
There are three feed tanks, one tank from which coal is being discharged, the second tank on stand-by for discharging, and the third tank in the stage of pressure reduction, filling of pulverized coal, and pressurizing after the completion of the charging.
The transport pipeline, consisting of a dense-phase pipeline and a dilute-phase pipeline, ensures smooth transportation of coal with small pressure losses. The high-pressure pipeline between the reservoir tank and tuyeres does not have movable parts, except the shutoff valves, which ensures a high availability and facilitates control and maintenance.
For the pulverizing and storing processes, a reliable system to prevent explosion is established by installing explosion suppressors, and fire extinguishers etc.
For successful operation of the PCI system, attention is to be paid to various phenomena occurring in the BF. Important among these phenomena are (i) combustion degree of injected coal or behaviour of unburned coal fines in the coke bed which affects the gas flow distribution in the burden region, (ii) behaviour of fused coal ash or possibility of its deposition in the blowpipe-tuyere zone, (iii) permeability or gas flow distribution in the burden zone affected by the ore/coke ratio increasing with increasing the coal injection rate, and (iv) thermal properties represented by the bosh gas temperature or the heat flow ratio affecting reactions taking place in the lower part of the BF.
The positioning of the injection lance in the blowpipe is important and is to be such so as to ensure the complete combustion of the injected coal within the raceway zone and simultaneously avoiding the coal ash deposit in the blowpipe end. The typical flow sheet of PCI process is shown in Fig 1.
Fig 1 Typical flow sheet of pulverized coal injection in blast furnace
The process of coal injection
The PCI process is based on the simple concept of carrying the finely ground (pulverized) dried coal by a conveying gas (normally nitrogen) to the BF where it is distributed to different tuyeres and injected through a lance in the blow pipe. In the blow pipe, it is mixed with oxygen enriched hot blast air and then supplied to the BF in the raceway.
Raw coal is received in the receiving hopper. It is screened and processed to remove tramp materials and is stored in raw coal bunkers. Raw coal is pulverized, dried and then pneumatically conveyed to the classifier in a once through system. Coal is dried thoroughly to prevent siltation and compaction. The pulverized coal is deposited in a single reservoir tank where it is stored under inert conditions.
Pulverized coal is gravity fed from the reservoir tank to the feed tanks which are then pressurized with inert gas as part of a batch process in which the feed tanks are filling, feeding, venting or holding in order to provide a continuous flow pulverized coal into the BF. The flow rate of the pulverized coal is regulated by inert gas pressure as a function of feed tank weight change. The single stream of dense phase coal from a feed tank is combined with transport gas (nitrogen) at the mixing tee.
A single transport pipe carries the coal/gas mixture to a coal distributor located at the BF (Fig 2). At the distributor the single stream of coal/gas mixture is divided automatically into multiple equal streams and conveyed by a pipe into each tuyere for injection into the BF. A block detector system guards against tuyere blockage.
Fig 2 Types of distributors
There are several important characteristics of the PCI process. In the closed loop system, the emitted gas from the filter is partly recirculated to the inlet of the pulverizer. This creates an inert condition which minimizes the oxygen content in the system and reduces the chances of coal dust explosion.
BF stove waste gas along with BF gas (in case of necessity) is used as source of energy for coal drying thus reducing the consumption of primary energy. The moisture of the coal can be removed from the system very effectively without the use of a gas cooler or a condenser. The use of inert gas during the operation of the reservoir tank reduces the risk of fire and explosion.
The total weight of the injected coal is controlled precisely by a load cell system which adjusts continuously to meet the set point. The parallel arrangements of the feed tanks ensure continuous flow of the coal into the BF. The coal distribution system is simple and effective with no moving parts.
The injection rate is normally controlled by modulating the position of a metering valve mounted at the discharge of feed tanks. The system ensures even distribution of the injected coal between the total numbers of tuyeres and has inherent capability to redistribute automatically the total coal injected into the BF in case injection is discontinued in one or more tuyeres due to any reason.
Pressurized nitrogen is used for injection of pulverized coal from the feed tank into the transport pipeline. With a small amount of additional transport gas, the pulverized coal is conveyed and injected into the BF under dense phase conditions. In such condition, loading of around 40 kg to 50 kg coal per kg of gas is achieved in the coal conveying pipeline.
The coal is conveyed in the pipeline at a speed of only a few meters per second. Low conveying speed of the coal reduces the wear in the pipeline thus increasing its life. Dense conveying system is powerful and can transport the pulverized coal to long distances.
Uniform distribution of pulverized coal to tuyeres is very important. For operating the BF with high efficiency, the uniform distribution of the burden and gas flow in the circumferential direction is essential. In this sense, the rate of pulverized coal to be injected through each tuyere is to be as uniform as possible. High distribution accuracy of around 1.5 % is desired (the normal distribution accuracy of the hot blast air is around 2.5 %) which is being obtained using a distributor (Fig 2). The schematic of coal injection at the tuyere and the raceway is shown in Fig 3.
Fig 3 Schematic of pulverized coal injection at the tuyere and the raceway
BF operation and PCI
It has been observed that at PCI rates, higher than 140 kg/tHM, changes are occurring in the BF operation. Some of these changes include (i) reduction in coke/ore ratio, (ii) the size of the raceway, (iii) reduction of permeability of the coke surrounding the raceway, (iv) changes in temperature distribution in the raceway, (v) mechanical degradation of coke in the raceway, and (vi) decrease in deadman temperature. All these changes are interdependent and are influenced by the properties and the amount of the injected coal, coke quality and blast conditions.
Permeability – The gas flows, liquid flows and burden descent within the BF are dynamic leading to the appearance and disappearance of non-active zones in the bosh, deadman, or stack. For high productivity and stable BF operation with high PCI rates, a critical requirement is consistent coal flow and coal properties through each of the tuyeres. Consistent coal quality can be controlled by blending PCI coals and coal flow by individual controls on each tuyere. The properties of the coal can impact on the complex gas, liquid and solid flows in the lower zone of the BF (Fig 4), but this impact is not significant if stable flow and coal quality is achieved. The injected coal can influence the permeability in the lower zone through the following reasons.
- Combustibility which affects the amount of unburnt material (char and soot) that exits the raceway.
- Coke fines generation within the raceway is due to the energy imparted to the coke by the momentum of the blast. The amount of fines generated is dependent on the coke quality. Blast momentum is influenced by the degree of combustion which occurs within the tuyere.
- Slag viscosity influences the flow of slag and HM through deadman zone and the penetration of hot gases from the raceway into the deadman zone which further reduces slag viscosity within the deadman zone.
Operational changes can be made to address reduced permeability at high injection rates. For example, central coke charging increases the permeability of the BF shaft and increasing the raceway depth improves permeability and reduces heat load losses. Some of the influences of the PCI on BF operation are shown in Fig 4.
Fig 4 Influence of PCI on BF operation
Impact of unburnt char – At low PCI rates, coke fines generated in the raceway are consumed by combustion, solution-loss, and other reactions in the lower zone of the BF. At increasing PCI rates, it has been observed that there is an increase in carry-over of fines from the top of the BF as well as an increase in the physical raceway depth and the BF instability. However, the increase carbon carry-over is fine coke with some small amounts of soot and negligible unburnt char. The unburnt char can be beneficial as in the presence of char the wear resistance of the coke is increased. This behaviour seems to be limited to low reactivity and high strength coke.
Char combustibility – It is observed that the combustibility of the injected coal at the tuyere level decreases with the increase in the injection rate or decrease in the volatile matter of the coal. However, the combustibility of coal at 700 mm above the tuyere is over 95 % for all coals due to the solution loss reaction of the unburnt char. Unburnt char not consumed by solution loss reaction is trapped in the BF or exit as dust. It has been estimated that the maximum injection rate which can be obtained is 230 kg/tHM at a combustion efficiency of 75 %.
The intense condition of combustion within the tuyere and the raceway leads to higher volatile release. While evaluating the characteristics of low volatile coals, it has been found the ratio of volatiles released to the proximate volatile matter (VM) of the coal increased significantly with the rank (carbon content) of the coal at high heating rates. There is a clear evidence of the fragmentation of burning particles. The flow of unburnt char from the raceway has impact on the permeability of the lower zone and the deadman zone of the BF.
Fragmentation of the coal particle can occur during devolatilization (primary fragmentation) and during char combustion (secondary fragmentation). The primary fragmentation is influenced by the VM of the coal. Large char particles produce far more fly ash particles than small char particles. The coal rank also has a major influence. The extent of fragmentation tends to increase with increasing coal rank and tends to decrease with increasing ash loading.
In the BF, the rapid heating rate, high temperatures, enriched oxygen and generally higher rank coals used for PCI, all contribute to primary fragmentation within a tuyere. Under these conditions, the external surface of a particle can harden due to reaction with oxygen or thermal annealing before the complete devolatilization of the inner core of the particle. This leads to explosive fragmentation of the particle.
Char reactivity increases with the VM content of the coal due to changes in the char morphology, however at the elevated temperatures pertaining to char combustion in the raceway, chemical reactivity has very little significance since combustion rates are limited by the rate of diffusion of oxygen to the particle, and burnout times depend more on particle size and oxygen concentration. High ash content in the char influences the combustion behaviour negatively. The combustibility of LV coal can be enhanced by blending with a HV coal, as the HV coal decreases the ignition time and increases the temperature within the tuyere.
The ash composition of the unburnt char can influence the catalytic effect of the ash to solution loss reaction. Depending on its composition, the ash can also retard the carbon conversion due to blockage of char pores as a consequence of increased proportion of slag formation in char particles. Although much of the char is consumed by in-furnace reactions due to the significantly higher reactivity of the char compared to that of the coke, it is normally believed that the unburnt char plays significant role in the deactivation of the deadman by reducing permeability of the deadman to gas and liquid flow. The reduction in permeability is considered to increase with an increase in injection rate as the combustibility of the coal decreases due to a decrease in excess air ratio. Some of the unburnt char can be captured by the dripping slag, though the amount of this captured char is influenced by the non-wetting behaviour of char with iron rich slags.
The fragmentation of the coal/char and combustion being controlled by diffusion are the main reasons why the VM has little effect on the combustibility of PCI coals.
Deposition of unburnt char in the BF – The deposition of unburnt char and coke fines in the lower zones of the Bf is a complex phenomenon which consists of several generation mechanisms such as reaction, multi-phase flow, accumulation, and re-entrainment. It has been shown both by measurement within a BF and numerically that the shape and size of the raceway changes dynamically with the accumulation and re-entrainment of powder in the lower zone. These changes have roughly two periods, the shorter period is the usual fluctuations within the raceway and the longer period corresponds to the large change with flow due to powder accumulation. It is the large change in gas flow which can lead to high gas velocities near the wall of the BF resulting in higher heat lost and wear of the BF walls.
The accumulation of powder in a packed bed has been examined, experimentally and numerically. The experimental work in a 2-dimensional packed bed using 3 mm particles and 0.075 mm powder has shown the influence of powder loading and superficial gas velocity has on the accumulation of powder in a region corresponding to the floor and bird-nest of the raceway. The influence of different cohesive zone shapes on particle accumulation has also been shown experimentally. The numerical analysis is able to show much of the same accumulation behaviour as the experimental test data. This numerical analysis has also shown the significant influence powder size has on the accumulation of powder in the deadman zone.
Numerical analysis of the influence of fine powder (unburnt char and fine coke) which has been conducted have shown that the unburnt char and the fine coke having different diameters and densities have different flow patterns, areas of accumulation, and reaction zones. The density of the generated powder has a significant effect on the powder flow pattern especially for the larger particles. As powder particles become larger than 1 mm their tendency is to settle from the upward flowing gas and descend into the deadman deteriorating its permeability. Any unburnt char is preferentially carried with the gas flow to the upper region of the BF where it reacts with the dripping HM and / or gas. There is some evidence of the existence of a bird nest with a very high content of fines. The amount of fines depends on coke quality and on the PCI levels. The large and heavier coke fines tend to leave the gas stream and accumulate in the deadman zone having an adverse effect of lower zone permeability.
The recent numerical analysis and physical sample has confirmed that the amount of unburnt char accumulated in the deadman zone is relatively small and does not increase with the increase of PCI rate. Thus, the unburnt char has little influence in the lower part of the furnace and the increase of permeability resistance. It is most likely that the deposition of fine coke has a more significant influence on BF permeability than what the unburnt char has.
Influence of coke properties – It is often stated the need for high quality sinter and coke are essential to achieve high PCI rates and to maintain high productivity. It is most important to improve the overall BF permeability to achieve high productivity at high PCI rates. Normally BFs with high PCI operations use high strength coke (in terms of drum index) and low SiO2 and low AL2O3 sinter with excellent high temperature reducing properties. Several Asian BFs are achieving high productivities with PCI rates exceeding 200 kg/tHM by using high quality raw materials.
Based on industrial experience, it has been seen that the bosh coke size increases with cold strength (I40), increases with hot strength (CSR), increases with coarse coke size, and decreases with PCI rate. Results from tuyere raking at ‘Corus Ijmuiden BF’ have shown that one point I40 results in 1.5 % more bosh coke above 40 mm square. It has been found that there is no relationship between the coal injection rate and the amount of fine coke at the tuyere level. The data for injection and I40 suggests that there is maximum coke degradation at around PCI rate of 120 kg/tHM.
A study examining coke behaviour in the lower zone of the BF under high PCI rates found that char is preferentially consumed. This lowers the CO2 concentration around the coke particle and expands the reaction layer of the coke around the surface and to the inside of the particle. In the presence of injected char, wear resistance of the coke has increased and total pore volume in the coke has increased. This increased abrasion resistance with increased PCI rates can be the reason for the increase bosh coke size at high PCI rates.
Analysis of coke samples taken at tuyere level has shown that, at high PCI rates, the high temperature properties (CSR and CRI) have a larger positive effect on permeability than cold strength properties. It seems that permeability is not improved after a CSR value of around 65 % to 70 %.
In a study with the data of several BFs with PCI rates above 170 kg/tHM, the BFs have a hearth index (HI = CRI – 2.5 x CSR + 100 %) above 97 %, though no clear relationship between HI and injection rate has been found. It has also been observed that productivity increases with HI. It has also been suggested that at high PCI rates the coke is to resist abrasion (low I10) and have high CSR.
In a study examining the influence of the coke quality on the productivity of the BF, a global coke quality index has been developed. This index gives an indication of the heat variation at the bottom of the BF and hence an idea of the hearth and shaft permeability of the BF. This global coke quality index has been defined by the equation Iglobal.coke = 0.5 x [(I40 – 3.42 x I10 + 100) + (CSR – 2.6 x CRI + 100)]. The variation in the heat variation in the bottom of the BF can be explained by the variation in the Iglobal.coke. This index is being used in some plants to determine the operating strategy of the BFs, if Iglobal.coke is low the coal injection rate and production is decreased. It has been considered that the addition of unburnt coal to the existing coke fines lead to lower permeability in the deadman zone resulting in reduced coke bed stability.
However, the permeability is found to decrease with increasing VM content and finer size range of the pulverized coal. An explanation for this is that as the combustibility of the coal is increased (increased volatile content and/or finer grind), there is greater combustion within the tuyere giving a greater volume of gases being injected into the raceway, leading to greater blast momentum. This greater blast momentum causes the increase in raceway depth and increase in the degradation of raceway coke, which leads to increased coke fines carry-over.
In a study examining the influence of the blast energy on the formation of the raceway, a linear relationship between blast energy and the depth of the raceway has been seen. A 2-dimensional model of the raceway shows linear relationship between blast velocity and raceway depth for constant coke size and BF dimensions. The increase in deadman instability and coke degradation due to increased blast momentum has been modeled by in another studies. These models show that the depth of the raceway increases linearly with blast velocity for a constant coke strength.
Using the data of various studies, the variation in blast momentum with the carbon content of the injected coal is estimated. This is able to show how the rank of the injected coal influences coke fine generation. A typical high VM coal produces upto twice as much coke fines as a low VM PCI coal due to the increased blast momentum resulting from the combustion of the volatiles within the tuyere. The operational data show that, at injection rates of around 170 kg/tHM, permeability and productivity increases when the raceway depth is increased (higher blast momentum). Expanding the raceway reduces the deadman area and hence increases the area where HM and slag can descent and where gas flow can ascend.
In Europe, the cold coke strength and in Japan, the hot coke strength is used as the coke parameter to monitor coke quality by many plants operating with high PCI rates. There is evidence that coke strength alone is not sufficient to predict the degradation of coke due to physical and chemical mechanisms.
Influence of coal ash chemistry on slag viscosity – The viscosity of the slag can influence the BF productivity since the productivity is related to the flooding phenomenon which can occur in the lower zone of the BF. The slag viscosity can adversely influence the permeability and the liquid flooding factor. Tuyere samples from a Japanese BF has given a valuable insight into the influence of the ash from injected coal on the physical properties of the dripping slag and the permeability of the region surrounding the raceway for a high productivity and operations with high PCI rates.
In recent years, there have been several studies into to the prediction of slag viscosities due to its importance to the flow behaviour of slag in the BFs. In one of the study, a quasi-chemical viscosity model has been developed for fully liquid slags in the Al2O2-CaO-FeO-MgO-SiO2 system, which has shown good agreement between experimental data and predictions over the whole compositional range. The composition of the slag influences the liquidus temperature in SiO2-CaO-MgO-Al2O3 systems. A slag critical temperature at which the viscosity of the slag abruptly changes has been described. At a basicity of less than 1.3, the critical temperature is around 1340 deg C and at a basicity of over 1.3, the critical temperature is around 1380 deg C. Lowering the MgO content of the slag decreases the slag viscosity but the temperature of the slag is the major factor.
A laboratory study has shown that mixing of pulverized fluxes with ash from coke and coal reduces the melting point of the tuyere slag by more than 200 deg C. At the same time slag viscosity is highly improved. In another study the reactions at the interface of unburnt char and iron have been examined. It has been found that the dissolution of carbon into the liquid iron is influenced more by the chemistry of the ash layer that formed at the interface than other factors. An increase in silica content slows the carbon dissolution. Silicon vaporization can occur in some coals. Further, it has been shown that the formation of ferritic iron at the surface can also slow the carbon dissolution.
The alkalis from coke or PCI which are of the most interest to BF operators are Na2O and K2O. Within the BF there is a re-circulation of the alkalis as the alkali vapours produced in the lower section of the furnace are condensed in the upper region of the furnace. High alkali load can lead to the formations of scabs (skulls) on the BF wall leading to BF irregularities like hanging and slipping. The chlorine content of the injected coal can increase the recirculation of alkalis within the BF. The level of re-circulating load can be controlled by adjusting slag volume and the chemistry.
The results of a study into the influence of alkalis in BFs indicate that decreased slag basicity, decreased hearth temperature, increased top gas temperatures, use of low alkali-burden are important factors in the control of alkalis in the BF. The most effective control is to limit alkali input to 2.0 kg/tHM to 2.5 kg/tHM. The main interaction of alkalis with coke occurs as the coke passes through the re-circulating zone becoming enriched in alkalis. This weakens the coke and makes it more susceptible to breakage. The extent of this effect probably depends on the type of carbon texture. The coke loses its alkalis as it proceeds down the furnace through the high temperature zone near the raceway.
With PCI the alkalis from the injected coal evaporate in the raceway and then due to the temperature drop condense in the deadman zone. The peak values of alkalis contents are around 2 m from the tuyere nose. The build-up of alkalis within deadman zone can lead to permeability problems associated with the HM flow to the hearth. In one of the studies on the effect of chlorine and alkali on BF operation, it has been found that a reduction of chlorine and alkali inputs improved BF permeability.