Coal for Pulverized Coal Injection in Blast Furnace

Coal for Pulverized Coal Injection in Blast Furnace

Injection of pulverized coal in the blast furnace (BF) was initially driven by high oil prices but now the use of pulverized coal injection (PCI) has  become a standard practice in the operation of a BF since it satisfy the requirement of reducing raw material costs, pollution and also satisfy the need to extend the life of ageing coke ovens. The injection of the pulverized coal into the BF results into (i) increase in the productivity of the BF, i.e. the amount of hot metal (HM) produced per day by the BF, (ii) reduce the consumption of the more expensive coking coals by replacing coke with cheaper soft coking or thermal coals, (iii) assist in maintaining furnace stability, (iv) improve the consistency of the quality of the HM and reduce its silicon (Si) content, and (v) reduce greenhouse gas emissions. In addition to these advantages, use of the PCI in the BF has proved to be a powerful tool in the hands of the furnace operator to adjust the thermal condition of the furnace much faster than what is possible by adjusting the burden charge from the top. Schematic diagram of a BF tuyere showing a pulverized coal injection lance is at Fig 1.

Fig 1 Schematic diagram of a BF tuyere showing a pulverized coal injection lance

Several types of coals are being used for PCI in the BF. In principle, all types of coals can be used for injection in BF, but coking coals are not used for injection since they are costly, have lower availability and are needed for the production of coke. Also, if coking coals are used for injections in BF, They lead to tuyere coking. Hence, coals used for injection in BF are usually weak coking coals or non-coking coals. These coals can have low, medium, or high content of volatile matter (VM). The commonly used coals for injection in BF are bituminous coal, anthracite coals, or a blend of different coals.

A wide range of coals, ranging in rank from high volatile (HV) lignite to low volatile (LV) anthracite, have been successfully used for injection in BF. Coal types for PCI are often categorized by their VM content. Coals which have between 6 % and 12 % of VM are generally classified as low volatile (LV), those between 12 % and 30 % as medium volatile (MV) and those over 30 % are as high volatile (HV). While the coal type seems to have little significant impact on BF operation at low injection rates, that is, below 100 kg/tHM, coal properties become more important as injection rate increases.

Selection of coals for injection is a complicated process that often involves adjustments. The performance of a given coal is largely judged based on cost savings, and this depends on coal costs and on the chemical and physical properties of the coal. The required properties are normally site specific. A number of different operational factors determine which properties the BF operator views as essential. These properties can relate to both BF operation and PCI preparation.

The relative importance of different aspects of PCI coal quality has varied, as the technology for injection has improved and the rate of injection increased. In the initial phase during the introduction of PCI technology, the combustibility of coal was considered to be of importance and at that time thermal coals which were readily available and had a much lower cost than hard coking and semi-soft coking coals were used for PCI. But as the understanding of the impact of coal quality on BF performance increased the operators have become conscious over the last few years with respect to the coal quality to be used for injection in the BF. Today, there are many criteria used to measure the coal quality to determine the performance of coal used for injection in the BF, both in terms of economic and technical benefits.

Incomplete combustion of injection coal in the raceway leads to char generation and this results in accumulation of unburned particles in the cohesive zone, dead man and slag affecting the furnace gas permeability, dirtying of the dead man, slag viscosity and therefore its desulphurization ability, and finally a decrease in the furnace productivity, HM quality, and increase in the coke rate.

General requirements of the qualities of pulverized coal for BF coal injection are low ash and sulphur (S) content, good grindability and combustibility, strong reactivity and etc. With the increasing amount of injection of coals in the BF day by day, quality standards for pulverized coal keep on increasing.

The suitability of coal for its injection in the blast furnace is also determined by its proximate analysis and ultimate analysis. Though both types of coals with low VM and high VM are being used for injection, but the coals with higher VM provide a stronger devolatilization and therefore reach a higher burnout. Higher fixed carbon (C) of the coal is a desirable property of the coal. Higher ash content of the coal results into increased slag volumes in the furnace. Similarly high percentages of S and P (phosphorus) in the coal result in increase in the S and P levels of the HM.

Moisture in coal used for injection in the BF increases transportation costs and affects the handleability of the coal. Coals with poor handling properties can cause blockages during transport to the BF. Normally, as the surface moisture increases, so does the difficulty in handling of the coals, especially when combined with high coal fines content. Blockages during transport to the injection lances have also been linked to the moisture content of the pulverized coal. Moisture in coal also affects both the energy consumption and output of the pulverizer by increasing the volume and temperature of the air needed for adequate coal drying. It also influences the raceway adiabatic flame temperature (RAFT). A higher content of moisture tends to lower the RAFT and requires more energy for evaporation of the moisture. Although HV coals may have better combustibility than LV ones, they typically have higher moisture contents. Hence, they need drying before being pulverized, adding to the operating costs, or they can be blended with lower moisture coals. Moisture content is considered to be additive in case of coal blend. In general, a total moisture content of less than 10 % is preferred for coals used for PCI.

At high rates of coal injection, the injected coal/blend becomes a major source of ash and other impurities. An ash content of less than 10 % is normally desired since (i) high levels of coal mineral matter can reduce the performance of pulverizer performance and throughput, and increase wear in the pulverizer and conveying pipelines, (ii) there are lower slag volumes and hence there is a better thermal efficiency, (iii) less energy is needed to melt the ash in lower ash coals, (iv) high levels of ash can cause lance blockage, (v) low level of ash reduces flux requirements, (vi) a higher coke replacement ratio (RR) is achieved with low ash, though the value is relatively small (the reduction in the RR is around 0.01–0.05 for each 1 % increase in coal ash content which arises from the requirement to add additional C to compensate for the extra ash), and (vii) to limit undesirable constituents present in the ash, such as silica (SiO2), alumina(Al2O3), and chlorine (Cl). Some care is needed in the application of the additivity rule for ash content when blending coals of widely different rank.

The constituents in the injection coal ash can influence furnace operation and the quality of the HM. They can affect ash viscosity. Coal ash with high viscosity at high temperatures (around 1600 deg C) can cause permeability problems in the lower part of the BF, mainly in the neighbourhood of the combustion zone, or in the active coke zone or on the deadman surface. The inorganic constituents of interest include (i) Al2O3, which is considered to be responsible for the largest increases in flux requirements, (ii) SiO2 which is desirable to be at a low level in the ash so as to ensure that the slag formed can be easily tapped from the furnace, (iii) alkalis which are generally sodium (Na) and potassium (K) containing compounds can contribute to coke degradation, sinter disintegration and deterioration of the refractory furnace lining (the combined upper limit for sodium and potassium oxides is usually 0.1 % after drying), (iv) Cl, which, mostly in the form of alkali chlorides, is associated with refractory deterioration, (v) P, as it affects product quality (P content of coal below 0.05 % is usually preferred), and (vi) S, because of its effect on the furnace S loading and HM quality. BF slag is a good desulphurizer. However, if coal injection increases the amount of S in the furnace, additional operating costs are incurred associated with greater slag volumes, modifying the slag basicity and/or taking additional HM desulphurization measures outside the BF. It is difficult to remove S and alkalis simultaneously within the BF as S removal requires a basic slag and alkalis an acidic slag. The limit for coal S is typically less than 0.6 %. Ash composition values, including Cl and S contents, are probably additive for coal blends.

VM consists of combustible gases (such as H2, CH4 and CO), incombustible gases (such as CO2 and steam) and condensable volatiles, mainly tar. The coal volatile content can affect char formation, blast momentum and coke fines generation in the raceway. This is due to coal devolatilization in the hot blast and the action of the volatiles liberated in the tuyeres. A higher volume of gases injected into the raceway creates a greater blast momentum, and increases the raceway depth. These, and other factors, need to be considered before deciding whether a low or high volatile coal is suitable for injection.

LV coals give higher coke replacement ratios, and hence lower coke rates, coupled with minimum cooling (VM production is endothermic). They produce a lower volume of off gas with a lower calorific value (CV), less hydrogen (H2) for iron ore reduction, and a higher RAFT as well as have a lower combustion efficiency than HV coals (although there are exceptions). On the other hand, HV coals generally have superior combustion performance due to higher volatile release, a lower ignition temperature and produce more reactive chars (hence better burnout) than LV coals. However, inertinite rich LV coals can also produce reactive chars. Unburnt char can reduce bed permeability and lead to C losses through the off gas. Good combustibility is particularly desirable at high injection rates because of the short residence time available for combustion in the raceway; burnout typically decreases as injection rate increases. HV coals also contribute more H2 for reducing the iron ore. Though, the higher gas volume can lead to back pressure problems in the tuyere. HV coals are more susceptible to spontaneous combustion affecting the ground handling system.

The blast temperature and/or the oxygen (O2) enrichment rate can be adjusted to suit the injected coal. The amount of VM in coal, though, is an issue at plants which have limited O2 enrichment facilities. Medium volatile coals are often perceived as the optimal solution. A blend of low and high volatile coals though can optimize the respective strengths of the two types of coal, although caution is needed for blends when applying the additivity rule for VM for the blend. Also, it has been found that the proximate VM of a coal blend is not a reliable guide to its combustion behaviour if the blend contains coals of widely differing VMs.

The other important properties for coals used for injection in BF include CV, melting characteristics of the coal ash, grindability, ignition point, combustion rate, explosibility, length of back fire, and reactivity.

Calorific value – The CV of coal influences the coke replacement ratio (in general, RR increases as coal CV increases) and the stability of the furnace. Higher CV coals increase the heat flux in the raceway and therefore, the RAFT. Typically, CV increases with the coal rank (decreasing VM content) and is additive for blends.

Melting/softening characteristics the coal ash – Since the coal ash has no fixed melting point, hence it is normally expressed by initial deformation temperature (IDT), softening temperature (ST), and flowing temperature (FT). IDT is an important characteristic of the coal ash. If the IDT value is too low, then ash deposition in the injection lance and tuyeres can occur. Due to design limitations, some BFs need a low IDT to help ensure that the slag formed in the furnace is easily tapped. High IDT coals can block the region of deadman in the furnace if the ash does not melt with the deadman slag. The ST or hemispherical temperature (HT), both higher than the IDT, is then generally specified instead. The IDT is a reflection of the coal ash composition. The presence of alkaline oxides acts as fluxes, lowering the melting temperatures, especially in the presence of excess SiO2. High S (from pyrite) can result in a lower IDT. HV coal ash is often highly alkaline, and thus melting temperature is normally lower. Hence, HV coal is more likely to give ash deposition problems than higher rank LV coals. IDTs are non-additive for coal blends.

Grindability – The grindability of coal refers to its behaviour during the coal grinding. It is a useful parameter for the selection of the coal grinding equipment. It is measured by Hardgrove Grinding Index (HGI).  Grindability is an index and therefore, it has no unit. HGI is a measure for the grindability of coal and is traditionally used to predict the capacity, performance and energy requirement of pulverizers, as well as determining the particle size of the grind produced. The smaller the HGI, the harder is coal texture and less grindable is the coal. Conversely, the higher the HGI, the easier the coal is to grind, with consequent lower power consumption and higher throughput of coal in the pulverizer. The resultant size distribution of the coal can affect its combustibility and coal handleability in the bins and transfer lines. HGI increases to a maximum as coal rank increases from subbituminous to medium-rank coals and thereafter decreases as rank increases to anthracite. Soft coals can produce a high proportion of fines which can clog transport lines, whilst hard (low HGI) coals are difficult to grind, leads to increased operating and maintenance costs. Hence coals with an HGI between 40 and 70 are generally preferred. This also helps to minimize breakage during handling and injection. HGIs are generally not additive unless the blend contains petrographically similar coals with similar HGI values. Furthermore, HGI is not always a good indicator of grinding performance since coals with similar HGI values may not, in practice, perform identically.

Ignition point – Ignition point of coals is that temperature at which the coal combusts. Coals having low ignition points are unsafe for pulverizing and injection. Desired ignition point for coals used for PCI is around 400 deg C or higher.

Combustion rate – Decrease in combustion rate under the condition of high coal rate injection is not an unusual problem in BF with high rate of injection of coal. Although some unburned pulverized coal gets consumed by means of direct reduction in the BF, excessive unburned pulverized coal can cause a decrease in permeability in the furnace, inducing undesirable gas and temperature distribution problems. Also, excessive unburned char discharge with the slag and in top gas, causes decrease in RR of coke. Therefore, high coal combustion efficiency (also termed burnout) is desirable for high PCI operation as it affects the amount of coal which can be injected. Coals with higher VM has improved combustion rate.

Coal explosibility – Explosibility of coals is determined by measuring the length of flame when the dry pulverized coal of one gram with size under 0. 074 mm is injected to a fire source at 1050 deg C. If there is no flame near the fire source then the coal has no explosibility. If the length of flashback in glass tube is less than 400 mm, then the coal is combustible and explosive, otherwise the coal has strong explosibility. Coal explosibility affects the safety during the injection of coal in the BF.

Reactivity – The reactivity of coal refers to reacting ability of coal with carbon dioxide (CO2) or water vapour (H2O) under certain conditions. After pulverized coal injection in the BFe, part of the unburned coal (char) reacts with CO2 while ascending with the gas. This results into improvement in CO to CO2 ratio and in turn protects the coke in getting dissolved as it moves down the BF. Hence it helps in the smooth operation of the BF. Coals having high reactivity improve the combustion rate, increase the injection rate, and improve coal to coke substitution ratio.

Coals for injection in BF are often blended to meet the requisite specification. Blending can optimize the relative strengths of the constituent coals, diluting unfavourable properties, and reduce raw material costs since cheaper coals can be incorporated. The quality of the blend is to be consistent to ensure stable BF operation. However, blending different types of coal, such as low and high volatile coals, can lead to problems. The blends may not behave as an average of their components, but may be affected disproportionately by one coal with problem characteristics. Factors which need to be considered include (i) grinding behaviour of the blend because of the likely occurrence of the preferential grinding of the softer coal, and (ii) combustion behaviour since the individual coals can combust at different temperatures and at different times, and burn out at varying rates.