Advantages of Low Volatile Coals for PCI

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Advantages of LowVolatile Coals for

PCIReport No: 970005

Date: 2 August, 1997Revision: May, 2000

Prepared for: Jellinbah Resources Attention: Mr R . Stainlay

By: Philip A. BennettPrincipal Consultant

This report was prepared by Energy Tactics -trading name of CoalTech Pty Ltd. . Energy Tactics or any of its sub-contrac tors do no t: a) ma ke an y warr anty, exp ressed or im plied, with respect to th e accu racy, co mpleten ess, or usefu lnessof the information contained in this re port, or that the u se of any informa tion, appar atus, metho d, or process disclosed inthis report may not infringe privately owned rights; or b) assum e any liabilities with respect to use of, or for damagesresulting from th e use of, any inform ation, appa ratus, meth od, or process disclosed in th is report.

Table of Contents

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Pulverised Coal Injection (PCI) Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Issues at High Injection Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Impact of Injected Coal Quality on Blast Furnace Operation . . . . . . . . . . . . . . . . . . . . . . 6Replacement Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Coal Ash Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Combustibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Soot Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Char Reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Coke Degradation in the Raceway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Summary of the Impact of Coal Quality on Blast Furnace Operation . . . . . . . . 13

Impact of Coal Quality on Mill Performance and Handability . . . . . . . . . . . . . . . . . . . . 14Mill Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Mill Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Mill Power Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Product Fineness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Blockages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Economic Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Executive SummaryPulverised coal injection has become a standard practice in most major steelworks withinjection rates varying from 100 to over 200 kg/tHM. The injection rates and number ofinstallations are expected to continue to increase result ing in a coal demand for injectionreaching around 55 million tonnes per year by the year 2005.

A major factor limiting PCI rates, while maintaining stable blast furnace operation, is thepermeability of the coke bed surrounding the raceway. As injection rates increase the levelof unburnt char entering the raceway and the blast momentum both increase, leading to:

C changes in the size of the raceway,

C mechanical degradation of coke in the raceway,

C reduction of permeability of the coke surrounding the raceway, and

C changes in temperature distribution in the raceway.

All of these influence raceway stability and the distribution of gas flow through the lowersections of the blast furnace, both of which impact on furnace stability and therefore blastfurnace productivity.

A general overview was developed showing how the volatile content of the injected coalscan impact on char formation, blast momentum and generation of coke fines in the raceway. Although, this overview was based on the idealised properties of coal and individual coalswill vary from these properties. This overview does however highlight two importantaspects of how coal properties can influence blast furnace performance. These are:

C Coke replacement ratio increases with the dry ash free carbon content of the coalup to approximately dry ash free carbon content of 91% where the replacementratio seems to level out.

C The blast momentum decreases with increasing carbon content of the coal when thedry ash free carbon content is greater than 85%; the blast momentum directlyinfluences the raceway depth and coke degradation within the raceway.

At high injection rates and injecting a coal with a carbon content less than 85% daf, anyfluctuations in the coal rate delivered to a tuyere will result in fluctuations in the racewaydepth and the generation of coke fines. Fluctuations in raceway depth and the generationrate of fines will reduce blast furnace stability and therefore productivity. By choosing acoal with a dry ash free carbon content greater than say 88%, the impact of variations incoal feed rate on blast furnace stability will be reduced and furnace productivity will bemaintained.

The main economic benefit of coal injection is the replacement of high cost coking coals. The coke replacement ratio of a coal can be shown to be dependent on the energy or carboncontent of that coal, with low volatile coals having the highest coke replacement ratio. Thisis shown by steelworks tending to buy PCI coals based on cents per percent carbon which issimilar the purchase of iron at cents per % iron. At high injection rates and using lowvolatile coals for injection the saving in total coal costs will be about US$8 per tonne of hotmetal produced.

Low volatile coals also generally are softer coals with Hardgrove Grindability Index around80. As these coals are very easy to mill there will be savings from reduced mill powerconsumption for a given coal throughput.

Low volatile coal also allows the mill capacity to be increased up to 40% above that of a 50Hardgrove Grindability Index coal, thus allowing the steelworks to increase injection rateswithout further capital expenditure in further milling capacity.

Advantages of Low Volatile Coals for PCICOALTECH PTY LTD 24 May, 2000 Page 1

Pulverised Coal Injection (PCI) TechnologyThe past improvements in productivity, coke consumption and fuel use within thesteelworks have been hastened due to the steel industry being a very competitive globalindustry. It can be expected that further improvements in these areas of operations to reducecosts will continue, but the technological change within the integrated steelworks that ishaving the largest impact on the costs is that of pulverised coal injection (PCI). It is for thisreason that PCI has become a standard practice in most major steelworks with injectionrates varying from 100 to over 200 kg/tHM. The injection rates and number of installationsare expected to continue to increase.

The use of PCI assists blast furnace operators through benefits such as:

C lower costs through the substitution for higher cost coking coals and energysavings;

C marked increase in productivity;

C extended coke oven life due to decreased coke demand, and;

C consistent quality of hot metal with a relatively low silicon content.

Most injection systems use coal ground to approximately 75% minus 75m, as coal millingsystems for this size are commonly used in power plants for electricity generation. Agranulated coal injection system has been developed by British Steel, though this system hasnot been widely accepted by the international steel community. The main elements of a PCIsystem are:

C a mill to pulverise the coal to around 70% minus 75m. The mill is normallyswept with hot gases (low in oxygen) to dry and transport the coal to the storagebins;

C a distribution system which meters and transports the coal from the storage hoppersto the blast furnace and evenly distributes the coal to the 15 to 38 tuyeres,depending on the size of the furnace.

Coal is injected directly into the raceway region through the tuyeres with the hot blast. Thehot blast can be enriched with oxygen to improve combustion efficiency or furnace stability.Uniform distribution of the coal to each tuyere is important for effective operation of thefurnace and has been addressed by all suppliers of PCI systems.

The coal absorbs heat, devolatilizes, the volatile matter combusts in approximate 10 msbefore exiting the blow pipe and the tuyere. The coal char starts to combust as soon as itleaves the tuyere. At low injection rates the distance from the point of injection to the exitof the tuyere is great enough to ensure almost complete combustion of the coal. At highinjection rates the combustion of the coal is not completed and some unburnt char enters thecoke bed. Some operators have decreased the distance from the injection point to the end ofthe tuyere to reduce the pressure drop through the tuyere or to reduce ash build-up on theinside surface of the tuyere (Yoshida and others, 1991). Figure 1 shows the important zonesaround the tuyere.

The influence of the design of the lance on combustion of the coal has been investigated byThyssen Stahl (Joksch and others, 1993) and Japanese researchers (Mati and others, 1996). They found that coaxial lances improve the mixing of coal and oxygen leading to bettercombustion. Other means of enhancing combustion efficiency such as fuel blending,


Figure 1 Schematic representation of tuyere level cokestructure

applying external electric fields, finer grinding of the coal and chemical additives have beeninvestigated by Babich and others (1996). Flierman and others (1996) argued that a simplesingle lance is sufficient as there is little benefit to be gained by more advanced designs.

Current practice in European and Japanese steelworks has shown that lance designsimprove combustion performance of the coal can contribute significantly to the ability toachieve stable operations at high injection rates (NKK 1999)

Issues at High Injection RatesPulverised coal injected at the tuyeres can fulfil two functions of the coke charge. These areto provide a source of heat and, at high injection rates, assist in the reduction of iron ore. PCI cannot provide a permeable bed in the furnace through which molten iron can descendand gases can ascend.

As coal injection rates increase coke/ore ratios decrease leading to thinner coke layers whichimpact on furnace permeability. The raceway adiabatic flame temperature (RAFT)decreases due to the cooling effect of the injected coal and oxygen needs to be added toensure that the RAFT does not go below about 2200 C. Also, the residence time of thecoke increases which impacts on the coke strength and size. All of these changes in blastfurnace operation can impact on furnace stability leading to a reduction in furnaceproductivity. Furnace stability is determined by the:

C variation in hot metal temperature and silicon content,

C uniform burden descent,

C tapping behaviour,

C cooling losses and

C dust content of top gas.

Therefore operators of steelworks need to pay greater at tention to improvement in cokequality, increase in coke size and greater oxygen enrichment, which will help to reducefurnace stability problems and therefore ensure furnace productivity is not reduced withhigh injection rates.

A major factor limiting PCI rates, that can be achieved with stable operation, is the


Figure 2 Comparison of PCI rate and corrected coke rate for Japanese and Europeanblast furnaces (Holcombe & Coin, 1993)

permeability of the coke bed surrounding the raceway. As injection rates increase the levelof unburnt char entering the raceway and the blast momentum both increase, leading to:

C changes in the size of the raceway,

C mechanical degradation of coke in the raceway,

C reduction of permeability of the coke surrounding the raceway, and

C changes in temperature distribution in the raceway.

All of these influence raceway stability and the distribution of gas flow through the lowersections of the blast furnace, both of which impact on furnace stability. The first two, sizeof the raceway and degradation of coke, are depended on the blast momentum which isdetermined by the amount of combustion of the injected coal, level of oxygen enrichment,blast temperature and tuyere diameter.

At ultra-high injection rates it has been estimated (Lngen and Poos, 1996) that for aninjection rate of 250 kg/tHM (coke rate of around 250 kg/tHM) the amount of coke gasifiedat the tuyeres can be as low as 20% of the total coke. The remaining coke is gasified ordissolved by reactions occurring in the shaft and the bosh areas i.e., before the coke reachesthe tuyeres. The residence time of the coke, at high PCI rates, is 3 to 5 times that of an allcoke operation leading to a size reduction of 40% of the original charged coke. Theincreased residence time of the coke impacts on the cokes ability to cope with the harshenvironment in the raceway, this together with the decrease in coke size will effect theamount of coke fines and therefore coke bed permeability.

Yamaoka & Kamei (1992) estimated that the maximum injection rate of about 375 kg/tHM,and a coke rate of 180 kg/tHM, could be possible with 60% oxygen in the blast but thiswould require an ore/coke ratio of 9.0, exceeding the currently achievable ore/coke ratio fedto operating blast furnaces. They predicted that the maximum PCI rate, without exceedingthe achievable ore/coke ratio, would be 250kg/tHM. As seen in Figure 2, no blast furnaceshas operated for extended periods above about 215 kg/tHM which has been attained atBritish Steels's Scunthorpe works (Maldonado and others, 1993) and Hoogovens' IJmuiden


works (Koen and others, 1993). Peters (1994) summarised the maximum average PCI ratesachieved for a period of one month and these are given in Table 1. The coke rate for theseoperations is about 300kg/tHM giving total fuel rates of between 481 and 496 kg/tHM. The Chinese achieved in 1966 injection rates of up to 360 kg/tHM but with coke rates ofaround 300 kg/tHM giving a total fuel rate of over 620 kg/tHM (Liu, 1994). The most recent highest average monthly PCI rates given in the literature are:

C 201 kg/tHM at Kobes Kakogawa No 1 blast furnace during 1994 (Kadoguchi andothers, 1996).

C 218 kg/tHM at NKKs Fukuyama No. 4 blast furnace in October 1994 (Maki andothers, 1996).

266 kg/tHM at NKKs Fukuyama No. 3 blast furnace in July 1998 (NKK, 1999).

The fuel rate (large coke, small coke and coal), for any given blast furnace or steelworks,seems to decrease for increasing PCI rates up to 120 kg/tHM, remain mostly constant forPCI rates from 120 to 160 kg/tHM and then rises slightly for rates from 160 to 200kg/tHM.

At PCI rates above 180 kg/tHM Koen and others (1993) feels that there is need for furtherinvestigations as interactions within the blast furnace are more complicated than just thereplacement of coke with coal. Some European works are currently investigating ultra highinjection rates of above 200 kg/tHM to determine the feasibility of such operations andwhether or not it is necessary to revamp aging coke ovens(Flierman and others, 1996). Theauthor of this report knows of one European steelworks that has recently reduced PCI ratedown to about 140 kg/tHM to improve furnace permeability and therefore increaseproductivity.


Table 1: High Pulverised Coal Injection Rates (Peters,1994)

Works ThyssenStahl AG

Sollac HoogovensGroup BV

British Steel Kobe Steel

Blast Furnace Schwe lgern1

Dunkerque4

IJmuiden 6 ScunthropeQueen

Vict oria

Kobe 3

Hearth Diameter m 13.6 14.0 11.0 9.0 9.5

Production tHM /d 9102 8726 5850 3522 3729

CokeConsumption

kg/tHM 297 287 272 294 308

CoalConsumption

kg/tHM 197 194 212 202 188

Total FuelConsumption

kg/tHM 494 481 484 496 496

Sinter % 61 81 49 75 86.5

Pellets % 27.6 - 49 - 0.5

Lump O res % 11.4 19 2 25 13

Slag Volume kg/tHM 248 313 236 266 270

Volatile Matter % 20 10 35 22 35

BlastTemp erature

C 1230 1189 1183 1125 1185

OxygenEnrichment ofBlast

% 3.3 2.2 5.6 8 3.1

Top GasTemp erature

C 160 187 154 122 184


Table 2 : Changes in RAFT an d Replaceme nt Ratio with co al types (Hunty and others, 199 0).

Coal Type C/H C/O kg injected per10 0 K cha nge inRAFT

Rep lacem ent Rat io

Anthracite 43.7 44.2 122 0.99

LV Bituminous 18.9 29.5 100 0.90

MV Bituminous 13.7 12.7 86 0.86

HV Bituminous 15.6 5.5 67 0.72

Sub-bituminous 16.0 3.6 65 0.54

Lignite 15.0 2.8 58 0.50

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Impact of Injected Coal Quality on Blast FurnaceOperationReplacement RatioThe choice of coal for use as the injected fuel does impact significantly on the cost benefitthat can be obtained by pulverised coal injection. The primary factor that influences thecost benefit of PCI is the amount of coke that can be replaced by the injected coal.

The replacement ratio normally quoted in the literature is the metallurgical corrected cokerate where the coke ra te is corrected for furnace parameters, such as hot metal siliconcontent, blast temperature, etc., to give coke rates under standard conditions. Thetheoretical coke replacement ratio is between 0.8 and 1.0 kg coke/kg coal depending on theenergy and carbon content of the coal. Actual replacement ratios achieved in blast furnaceoperations with low to moderate injection rates tend to be slightly higher due to reducedheat losses and some increase in reduction efficiency, at higher rates over 150 kg/tHm heatlosses can increase which may lead to replacement ratios that are lower than theoretical.

Hutny and others (1990) have reported a general increase in replacement ratio with the C/Hratio of coal. Their results have be summarised as given in Table 2. They also derived a

relationship between the calorific value of the injected coal and replacement ratio which isgiven in Figure 3. More recently Brouwer and Toxopeus (1991) in summarising the PCIoperating results at Hoogovens IJmuiden blast furnace derived a relationship betweenreplacement ra tio and the properties of the coal injected. This relationship, based on thedry carbon, hydrogen and ash content , is given below:


Figure 3 Variat ion in replacement ratio with the rank of the injected coals showingcorrelations based on actual (*) and theoretical (**).

The above relationship reinforces the conclusion reached by Hutny and others thatreplacement ra tio increases with the rank of the coal. Though the positive affect of ash onreplacement ratio is not what is expected. The expected increase in replacement ratio withrank of the coal is shown in Figure 3, in this figure the data from several sources has beenplotted against the dry mineral free (dmmf) carbon content which is a measure of the rankof a coal.

As shown by Hur and others (1998) the replacement rat io is not only related to the

properties of the injected coal but also can be influence by other factors such as oxygenenrichment. They showed that with oxygen enrichment the replacement ratio chnaged to0.99 from 0.88.

Coal Ash ChemistryAsh chemistry is important for PCI coals in terms of furnace operation and iron productquality. Generally the same considerations apply as for coke ash chemistry.

Snyder and Fletcher (1991), based on experience at Armco Steel, suggested that 5.5kilograms of extra carbon must be injected to compensate for every kilogram of extra ash,and it can be deduced that this would correspond to a reduction in replacement ratio byapproximately 0.05 per 1% absolute increase in ash. These authors acknowledged that thiscarbon penalty was considerably higher than most would expect. Experience at BritishSteel (Maldonado & others, 1985) on the other hand suggested that there was no carbonpenalty for extra ash, and allowing that this extra ash is inert there would therefore be areduction in replacement ratio of approximately 0.01.

High levels of sodium or potassium can cause coke degradation, while sulphur orphosphorus impact on hot metal quality. Sulphur can be removed from the iron in the blastfurnace by the introduction of extra limestone flux, while phosphorus is removed inprocessing of the product subsequent to the blast furnace, but in either case there are costsinvolved. Absolute limits are not normally specified on the concentrations of these elementsin the ash, and a blast furnace operator would normally take into consideration thecombined quantities in coal and coke.

A low ash fusion temperature is favourable for slag fusibility, however the majority ofoperators have favoured high ash fusion temperatures to avoid deposition of the ash in theblowpipe or tuyere. There appears to be a trend for coal injection lances to be positioned


closer to the tuyere than previously, thus reducing the extent of impingement of ash in theblowpipe; it may be expected that in future this trend will allow a relaxation of therequirement for high ash fusion temperature.

Some Asian PCI coal users favour coals with low SiO2 content in the ash. The reasons forthis are not clear, however it may be part of the blast furnace operating strategy of limitingthe silicon content of the iron product. An additional reason may be related to slagviscosity, and the increased flux requirements of high silica ash.

Provided that a coal does not possess any outstanding faults, many steel producers willevaluate it on the basis of dollars per unit energy or alternatively Cents per carbon unit..

Combustibility The role of PCI has traditionally been viewed as providing heat to drive the blast furnacereactions. At moderate PCI rates, up to around 160 kg/tHM, there is sufficient oxygen inthe blast to consume all of the coal in the initial combustion reaction producing carbondioxide and water. However, the coke in and around the raceway competes for the oxygenand hence there may be coal remaining after the oxygen is consumed. The dust formed bythis unburnt char may lower the permeability of the coke bed to the movement of gases andliquids, or the entrained dust containing unused carbon may exit the BF with the top gas.

This perceived need for rapid combustion of the injected coal has prompted blast furnaceoperators in the past to pulverise the coal very fine and to choose highly reactive coals. Itis now recognised that complete combustion of the coal is not possible at high PCI rates,and some operators are introducing practices which are counter-productive to goodburnout, such as the use of coarser pulverised coal (Kuwano, 1993) or of GCI (Jukes,1993) to reduce milling costs and improve handling behaviour.

Based on the widely accepted assumption that the coal devolatilises in the tuyere then theamount of char that enters the raceway depends on the amount of volatiles that are releasedat the high temperature within the blow pipe. Several researchers have examined thevolatile release at high temperatures with most finding that the amount of volatiles releaseddepends on the temperature and the rank of the coal and var ies between 1.2 to 2.0 times thevolatile content as measured by the standard ASTM method. The ratio between the actualvolatiles and the ASTM volatiles is usually termed the Q factor.

Figure 4 summarizes the findings of some researchers relating the Q factor to the rank ofthe coal. Researchers evaluating the characteristics of low volatile coals (Tromp andothers, 1992 & Wall and others, 1987) found the Q factor increased significantly withthe rank of the coal. This sharp increase in the Q factor with higher rank coals may bedue to some combustion of the char occurring in the favourable conditions of entrainedflow test furnances.

Standard correlations (Van Krevelen, 1961 & Callcott and others, 1990) relating specificenergy and volatile matter to carbon content of the coal were used in the construction ofFigure 4 and the best curves fitted to the data of the respective authors are given.

Soot FormationThe possibility of soot formation from injected coal was studied by de Lassat and others(1990) through the recovery of pyrolysis products and separation of the char, mixed tars


Figure 4 Variation of the Q factor with the rank of the injected coal

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and soot. They showed that the quantity of soot forming media produced at 1000 and1200C increases with the amount of volatile components up to a level of about 40%. Although, it is expected that the soot yield will decrease for coals with a volatile contentgreater than 30% daf, as a greater fraction of the volatile components released from thecoal are gases for these coals.

Based on the estimated volatile yield as determined by Tromp and others (1992) andassuming 30% of the volatiles form soot then an estimate can be made of the char and sootthat enters the raceway. Figure 5 indicates how this char and soot will vary with the rankof the coal.

Char ReactivityChar reactivity does increase with the volatile content of the coal, however at the elevatedtemperatures pertaining to char combustion in the raceway, chemical reactivity has verylittle significance since combustion rates are limited by the rate of diffusion of oxygen tothe part icle, and burnout times depend more on part icle size and oxygen concentration(Field and others,1967). The particle size depends on the swelling and/or fracturingbehaviour of the char particles. This is supported by the work of Bachhofen and others(1998) who found that the volatile matter of the injected coal had no influence on the extentof coal conversion within the furnace.

Stanmore(1992) evaluated the impact of coal properties on the swelling characteristics and,for the coals tested, he found that the particle diameter (dp) could be related to the burnout(u), the initial particle diameter (do) and a swelling factor (") by the following equation:

Stanmore found that " increased linearly with increasing volatile content of the coal.


Figure 5 Variat ion of char, volatiles and soot produced in the tuyere with the rankof the injected coal.

Using this relationship it can be shown that there is no significant swelling of the coalparticle in the early stages of combustion.

Therefore, since char combustion is diffusion controlled with a short residence time andreduced oxygen level in the raceway, there will be no great differences in the rate of charcombustion for different coals. Steeghs and others (1994) using a coke filled test furnacefound for a low volatile coal approximately 10% less coal is combusted near the tuyerethan for a high volatile coal at the same injection rates. This agrees with the predicted charand soot yields given in Figure 5.

Coke Degradation in the RacewayAt increasing injection rates there has been observed an increase in carry over of finesfrom the top of the blast furnace as well as increasing physical raceway depth andinstability, Willmers & Poultney (1992). This was considered to be partly due to theaddition of unburnt coal to the existing coke fines leading to lower permeability in thedeadman zone resulting in reduced coke bed stability. However, these effects increasedwith increasing volatile content and finer size range of the injected coal. The explanationgiven was that as the combustibility of the coal was increased (increased volatile contentand/or finer grind) there was greater combustion within the tuyere giving a greater volumeof gases being injected into the raceway, ie greater blast momentum.

This greater blast momentum caused the increase in raceway depth and increased thedegradation of raceway coke which led to increased coke fines carry-over. Peters andothers (1991) reviewed the findings of several researchers examining the influence of theblast energy on the formation of the raceway, all researchers found a linear relationshipbetween blast energy and the depth of the raceway. The increase in deadman instabilityand coke degradation due to increased blast momentum has also been modelled by Aokiand others (1993) and Tamura and others (1991). These models show that the depth of the


Figure 6 Effect of raceway depth on fine coke deposit near core surface,(Tamura and others, 1991)

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raceway increases linearly with blast velocity for a constant coke strength, which issupported by the results of Negro and others (1996) when they examined the racewaydepth of three blast furnaces.

Using the data of Negro and others the raceway depth (m) can be estimated from the blastvelocity (m/s) by the following equation:

Tamura and others cited their earlier work where, in a two-dimensional cold test rig, theamount of coke fines increased sharply with raceway depth, see Figure 6. They postulatedthat the sharp increase in -1mm fine coke was due to the increase in the amount and thevelocity of the coke circulating in the raceway, which in turn increases the frequency atwhich coke particles collide and the energy involved in those collisions.

Blast velocity, and therefore raceway depth, depends on the tuyere diameter , blast volume,blast temperature, blast pressure and the amount of the coal combusted within the tuyere. The fraction of coal combusted within the raceway will be the volatile content of the coal,after allowing for any soot formation, as the residence time within the raceway isinsufficient for any significant char or soot combustion to occur. The heat liberated fromthe combustion of the volatiles can be approximated using the correlations developed by


Figure 7: Variation of blast velocity with the rank of the injected coal

Figure 8: Variation of coke fines with the rank of the coal injected

Unsworth and others (1991). Using the previous relationships between char , volatile andsoot yields and the rank of the coal it is possible to estimate how the blast velocity maychange with the rank of the coal as shown in Figure 7 for several coal injection rates. Thecurves in this figure were determined for constant blast air rate, oxygen enrichment andblast temperature.

The maximums in blast velocity at carbon content of around 83 % dmmf is due to highertemperatures being obtained with in the tuyere due to the combustion of the volatiles. Forcoals of higher rank than 83 % dmmf carbon, the volatile matter release is lower and forlower rank coals there is a greater amount of moisture associated with the coal and theenergy content of the volatiles is lower.

An estimate can also be made on how the amount of coke fines may change due to the rankof the injected coal. This estimate was determined based on the data in Figure 7 todetermine blast velocity, by using equation (3) to determine raceway depth, and the fittedcurve to the data of Tamura and others (1991) to estimate the amount of coke fines. Figure 8 indicates how the rank of the injected coal may impact on the generation of cokefines.


Summary of the Impact of Coal Quality on Blast Furnace OperationA general overview of how coals of various rank can impact on the operation of a blastfurnace has been given above. The performance of any individual coal depend on thepropert ies of that coal. These properties can differ from the idealised properties used in thegeneration of Figures 3, 4, 5, 7 and 8.

This general overview does however highlight two important aspects of how coalproperties can influence blast furnace performance, these are:

C Coke replacement ratio increases with the dry ash free carbon content of the coalup to approximately 91% carbon where the replacement ratio seems to level out.

C The blast momentum decreases with the carbon content of the coal when thecarbon content is greater than 83% dmmf; the blast momentum influences theraceway depth and coke degradation within the raceway.

As seen by Figures 7 and 8 at high injection rates and injecting a coal with a carboncontent less than 83% dmmf, any fluctuations in the coal rate delivered to a tuyere willresult in fluctuations in the raceway depth and the generation of coke fines. Fluctuations inraceway depth and the generation rate of fines will reduce blast furnace stability andtherefore productivity. Due to the design of a ll coal injection systems, it is to be expectedthat there will be significant variations in coal feed rate at each tuyere.

By choosing a coal with a dry ash free carbon content greater than say 88%, the impact ofvariations in coal feed rate on blast furnace stability will be reduced and furnaceproductivity will be maintained.


Figure 9: Mill capacity correction factor for Hardgrove Grindability Index

Figure 10: Mill capacity correction factor for fineness

Impact of Coal Quality on Mill Performance andHandabilityMill CapacityThe performance characteristics of a particular mill are determined by a series of capacitycorrection curves which are normally supplied by the mill manufacturer. The maximumthroughput of a mill can be determined by multiplying the mill capacity by the millcorrection factors. Typical vertical spindle mill capacity correction curves for HardgroveGrindability Index (HGI) and product fineness are given in Figures 9 and 10.


Figure 11: Variation in mill specific power with Hardgrove Grindability Index(Bennett & Holcombe, 1994)

Mill DryingThe processes which occur in pulverising mills are drying and size reduction of the coal.

Drying is necessary in order to obtain a product which will be transportable through thepneumatic handling systems and in the storage bins. Typically, the coal is dried to producea moisture level which is somewhat less than the air dried moisture measured as part of theproximate analysis. According to Brouwer & others (1991) the moisture content of coalleaving the mill is two thirds the air dried moisture level, so that the quantity of moisturewhich must be removed in the mill is given by :

Moisture removed in mill = (as received moisture) - b(air dried moisture)

High moisture coals require a higher inlet air temperature and/or a higher air flow. Theserequirements can reduce the mill capacity or cause mill fires.

The throughput of the mill may be limited because, as the temperature of the inlet air isincreased, the capacity of the inlet air fan is reduced. In some other cases there may not besufficient heating capacity to produce the required temperature. Mill fires can occur withhigh moisture coals as a result of the higher mill inlet temperatures and also because thesecoals tend to be more prone to spontaneous combustion.

Mill Power ConsumptionFor a given coal mill power consumption varies with the fineness of the coal, so there isscope for reducing the power consumption if the mill motor is being overloaded byreducing fineness. This may be necessary when PCI rates are increased above the level forwhich the mills were designed, or when using difficult coals.

Mill power consumption depends on the mill design, mill settings, the required fineness andthe properties of the coal. However, mill power can be estimated from the HardgroveGrindability Index of the coal , as shown in Figure 11. This figure is based on pilot scalevertical spindle mill test results when the mill was set at the optimum settings to achieve70% minus 75m product.


Product FinenessSize reduction is normally required in order to maximise the burnout of the coal in theraceway area. A typical requirement on fineness is 70% passing 75 m, but there are largevariations in the perceived requirements, the extreme case being the use of granular coalinjection (


reaches a minimum at a flow rate which is defined as the boundary between dense anddilute phase flow. If the air flow is increased above this point in the dilute phase region thepressure drop along the pipe increases as a result of the increased air flow. As the air flowis decreased below the boundary in the dense phase region the pressure drop increases upto a point where blockage occurs. Studies of different coals, and the same coals atdifferent fineness, indicated that the boundary line at which blockage occurred for differentsolids flow rates was similar for all coals. However, the steepness of the pressure/air flowlines approaching this boundary was generally greater for those coals which were eitherfiner overall or which contained a higher percentage of very fine material. It seems likelythat most coals should be able to operate at the same air and solids flow rates, although thefiner coals may require a slightly higher driving pressure. The finer coals which possessthe steeper characteristic curves may be more sensitive to inevitable fluctuations in coalfeed rate or in back pressure, and therefore are more likely to depar t from the designoperating point and to reach the blockage point.

Bench scale tests, also performed on the same coals at Wollongong University, tend toconfirm that the handling properties of hard coals have values at one extreme while thoseof soft coals which have been pulverised using the same mill settings as for the hard coals,and which are therefore finer overall, will have propert ies at the other extreme. If, on theother hand, the soft coals are pulverised using modified mill settings to make a productwhich is less fine overall, there will still be a relatively high proportion of very finematerial in the products, and the handling properties will have intermediate values.

In summary, softer coals may be more susceptible to blockages in storage bins and indense phase transport systems than harder coals if milled under the same conditions, butthe risks of blockages will be reduced by matching the mill settings to the characteristics ofthe softer coal to reduce the amount of fine coal. By ensuring the grinding pressure isreduced and the classifier is adjusted for softer coals not only are handling problemsreduced but the capacity of the milling system is significantly increased.


Figure 12: Savings in total coal costs from the replacement of coking coals

Economic ImpactThe main economic benefit of PCI use within steelworks is principally due to thesubstitution of coking coal with lower cost non-coking coals. Flierman and others (1996)noted that the operational costs of coking and injection were similar, therefore an earlyclosing of coke batteries which are in good condition and construction of injection plantwill increase hot metal costs. The level of PCI rate within a given steelworks depends onthe current hot metal demand, capacity of coking plant, oxygen plant capacity and theavailability of other fuels, eg coke oven gas, to preheat the blast.

Where a steelworks has an existing PCI plant of a capacity tha t will not meet the expectedfuture requirements of the steelworks then the use of a high Hardgrove Grindability Index(HGI) coal, i.e. a softer coal, will allow the milling capacity of the PCI plant to beincreased without investing in new plant. The extent of the increase in mill capacity willdepend on the HGI of the coal and the required size distribution of the pulverised coal,Figure 9 shows the impact of HGI on the capacity of a vertical spindle mill producing thesame size of grind. For example, by using a coal with a HGI of 80 about a 40% increasein capacity can be obtained while producing the same fineness. Considering theconstruction costs of a new 45 t/h PCI plant in Japan has been given as 5 billion Yen (TexReport, 1997) this increase in capacity by the use of a softer coal can be significant.

The economic benefit of the coke replacement ratio can be illustrated in the followingsimplified example using Brouwer & Toxopeus (1991) data to calculate replacement ratio(Figure 3) and assuming:

C a CIF coking coal price of US$63/t,

C a CIF PCI coal price of US$50/t,

C 1.43 tonnes of coking coal is required for each tonne of coke, and

C the total fuel rate is 490 kg/tHM.

Then the coal cost savings can be calculated as shown in Figure 12. This figure shows upto $8/tHM can be saved at high injection rates using a low volatile coal.


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Advantages of Low Volatile Coals for PCI

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