METALS AND MATERIALS, Vol. 2, No. 1 (1996), pp. 1-7

Coke Properties at Tuyere Level in Blast Furnace with Pulverized Coal Injection

Jin Kyung CHUNG, Jeong Whan HAN* and Jeong Ho LEE

Kwangyang Iron & Steelmaking Research Team, Technical Research Laboratories POSCO, 699 Kumho-dong, Kwangyang, Cheonam 544-090, Korea

*Department of Materials Engineering, Kyonggi University, Suwon 440-270, Korea

Coke in the blast furnace experiences great changes in their properties during the blast furnace operation. Pulverized Coal Injection (PCI) into the blast furnace through tuyeres together with oxygen enrichment af- fect coke properties and blast furnace operation. Using coke sampler at tuyere level, coke samples were col- lected and analysed at various coal injection rates in the both conventional and co-axial oxygen enrichment. With the help of information obtained from coke sampling experiments, gas permeability resistance index at lower part of blast furnace (LK) and mean size variation of coke were predicted and matched well with the measurements.

1. I N T R O D U C T I O N

The blast furnace still covers major portions of hot me-

tal production and there is a world wide tendency to in- ject pulv.erized coal into blast fumace tuyere. Recently, the prime concern of the ironmakers has become a max- imization of Pulverized Coal Injection Ratio (PCR) while maintaining the blast furnace in good operating

condition [1]. In order to figure out what happens in the blast fur-

nace with high PCR, various experiments are being car-

fled out. The properties of coke such as mean size (MS), coke

strength after reaction (CSR) and degradation index (DI) have been known to be changed at lower part of blast furnace with coal injection.

The state of coke at tuyere level in the blast furnace has not yet been quantitatively understood [2]. The role of the raceway as combustion space is manifold; pro- ducer of reducing gas, distributor of gas and heat source. Since the raceway is very important for stable operation of a blast furnace, the role of coke in the neighborhood of raceway, as supporting structure, has become more

and more important [3]. This paper focuses on the ef- fects of coal injection rate and methods of oxygen en- richment on the shape of the raceway and its vicinity in the blast furnace. Coke mean diameter at tuyere level

and the gas permeability resistance index (LK) at lower part of blast furnace are predictect based upon the test

results.

2. E X P E R I M E N T A L

For a better coal combustion efficiency in blast fur- nace, oxygen enrichment is required when coal is in- jected. As shown in Fig. 1, there are two methods of ox- ygen enrichment with different coal/oxygen lance con- figurations. One is oxygen enrichment in blast, the other is oxygen enrichment with co-axial lance. In the center of the inner pipe of each lances, coal is being injected.

Co-axial lance is known to be more effective on the coal combustion than the conventional one, because cold air, which is injected through the outer pipe of the con- ventional lance, prevents turbulent mixing of pure ox- ygen and coal [4]. In order to investigate the raceway behaviors with these different oxygen enrichments and PCRs, a movable tuyere coke sampler by Compair-Hol- man [1] was introduced into the blast furnaces at Kwan- gyang Works, all four having a hearth diameter of 13.2 m and inner volume of 3800 m ~. Since 1991, 80-130 Kg/t-p PCR applied to No. 1~3 BF and No. 4 BF had 150 Kg/t-p PCR. Total 10 times of coke samplings have been carried out along with the change of the operation conditions such as PCR, oxygen enrichment, lance con-

2 Jin Kyung CHUNG, Jeong [~Ttan HAN and Jeong Ho LEE

Fig. 1. Schematic diagram showing oxygen enrichment.

figuration, tuyere diameter and burden distribution etc.

as shown in Table 1. During furnace stoppage, a hollow pipe with 150

mm~ was pushed through a tuyere and advanced up to

one third of the hearth radius. The pipe was cut open and coke was divided at every 200 mm for screen analysis. Because coke sampled at the tuyere level of the blast furnace is much less than 10 Kg, minimum value for CSR or DI determination specified in JIS at each sec- tion, coke strength at tuyere level was modified and rede-

fined as follows; sampled coke of 100 g sized in the range of 15.9 and 24.7 mm was reacted with carbon dioxide at t 10ff'C for I hr. The reacted coke is mechan- ically rotated at a speed of 20 rpm for 30 min in I-type drum tester. Then, weight percentage of coke size which is over 10 mm was measured as the CSR'. Dr of coke

was also re-determined by the weight of coke over 10 mm after rotating at speed of 20 rpm for 30 min in l-

type drum tester.

3. R E S U L T S A N D D I S C U S S I O N

3.1. Characteristics of coke properties at tuyere level As reported before [5], four distincitive parts could be

identified; bosh, raceway, bird's nest and deadman as shown in Fig. 2(a). The bosh coke which is located a- bove the raceway generally consists of large angular coke lumps with a dark sooty surface resulting from car- bon solutioning somewhere above the raceway in the fur- nace.

The birds nest is the most compact zone and contains coke, slag and iron. The fine coke tends to fill the voids of the larger coke lumps and, if present in excess, it results in poor drainage of liquid slag and iron. The size of both raceway and bird's nest vary. with PCR, oxygen enrichment method and other conditions, as illustrated in

Fig. 2(b). It is interesting to note that the coke size distribution

at tuyere !evel shows different pattern with different PCRs and oxygen enrichment. As shown in Fig. 3, the

Table 1, Coke sampling dates and operating conditions

No. of sample No.1 No.2 No.3 No.4 No.5 No.6 No.7 No.8 No.9 No.10

Date 91/8/2 91/9/10 93/6/2 93/6/30 94/2/4 94/3/8 94/4/27 94/5/4 95/2/6 95/3/20

Blast Funace 2BF 3BF 3BF 3BF 4BF 2BF 4BF 1BF 4BF 4BF

Dia.(mm) 140 140 140 150 150 140 150 14Cl 15(1 150

canyon- conven- conven- co- co- conven- co- conven- co- co- Lance type tional tional tional axial axial tional axial tional axial axial

PCR(Kg/t-p) 86 97 129.9 124 132.1 108 144 86.2 142 150

PD(ton/day) 8360 8588 8684 8788 8632 8767 8816 8311 8833 8800

VB(Nm3/min) 6190 6255 5789 5847 6095 6243 5973 6196 6008 5975

Oxen(Nm~/qar) 7120 4800 9600 10000 8000 7248 12000 4570 13560 14900

TB,('C) 1170 t197 1224 1227 1179 1167 1185 1166 1197 1196

HMT(~ 1510 1504 1514 1515 1509 1523 1518 1521 1521 1520

MS(mm) 49.1 50.2 49.8 49.8 49.9 52 51.3 48.9 50.4 49.8

DI(%) 87.3 87.5 87.5 87.4 87.3 87.4 87.4 86.7 87.7 87.6

CSR(%) 66.9 67.1 67.1 65.1 67.3 63.8 67.4 68.2 67.0 63.9

Coke Properties at Tuyere Level in Blast Furnace with Pulverized Coal hzjection 3

Fig. 3. Cumulative coke size distribution at tuyere level (a) PCR: 86 Kg/t-p and (b) PCR: 144 Kg/t-p.

Fig. 2. Schematic diagram showing coke sampling and state of coke at tuyere level with various PCRs, lance configurations and oxygen enrichment.

coke size over 25 mm in bosh, raceway and birds nest were larger at 86 Kg/t-p PCR than at 144 Kg/t-p.

The weight fraction of liquid (metal + slag) over coke varies along the radius as shown in Fig, 4. With the con- ventional lance, the weight fraction of metal and slag in mixture increased with the increase of PCR from 86 to 130 Kg/t-p. With co-axial lance, however, the weight

fraction of metal and slag returned to the level of 86 Kg/ t-p even at 132 Kg/t-p. When PCR was again increased to 144 Kg/t-p, the weight fraction increased as well.

The fraction of fines, say - 3 ram, may be regarded as the indicator of combustion characteristics of injected coal. As shown in Fig. 5 with conventional lance, that

the fines sized under 3 mm increases with PCR, while the oxygen enrichment with co-axial lance decreases the fines at the same PCR and even at 144 Kg/t-p it shows

similar level as 86 Kg/t-p. Since the raceway zone acts as a distributor of gas as-

cending through the bosh and the shaft of furnace, a complete combustion of injected coal is desirable even

at higher PCRs. If we define the raceway depth as the distance from

tuyere tip to the point where the 3 mm fines reach

Fig. 4 Weight fraction of (metal-slag) over coke along

the radius of blast furnace at tuyere level.

10% as we previously suggested [6], the raceway depth decreased at higher PCR with conventional lance as

shown in Fig. 6. When co-axial lance is used, however, the raceway

depth increased in absolute values, The unbumt coal in the raceway is blown into the coke bed to form the birds nest between the end of raceway and the surface of dead- man. Since the birds nest makes the temperature of dead- man to decrease, disturbs gas flow into the center of deadman and makes the permeability worse, and it could

4 Jin Kyung CHUNG, Jeong Whan HAh~ and Jeong Ho LEE

Fig. 5. Difference in -3 mm size fraction with various

PCRs, lance configurations and oxygen enrichment at tuyere level.

Fig. 7. Change of mean diameter of coke along the ra- dius of blast furnace at tuyere level.

Fig. 6. The size variation of raceway and birds nest

with the change of PCRs.

change the direction of gas and cause heat losses through the furnace wall [7], as discussed elsewhere, a large birds nest is not desirable. There are several rea- sons for the increase of fines in the deadman besides the unburnt coal. It is well known that the extention of resis- dence time with PCR in blast furnace makes coke di- sintegrate. The properties of charged coke must be re- lated to disintegration of coke as well [8].

3.2. Coke size changes with PCR The results of analyses in Fig. 7 indicate that the vari-

ation of coke mean size along the radius of blast furnace at tuyere level shows a pattern that the mean coke size first decreases, and, then increases in some distance lt~

Fig. 8. Variations of bosh, raceway coke properties with various PCRs.

have a peak at the birds nest. It represents that the large coke is introduced into the raceway vacancy, then the coke is burnt in the raceway with oxygen in the blast. Coke strength index CSR' and strength index DI' were also deteriorated with PCR as shown in Fig. 8.

The relationship between PCR and mean diameter of coke at tuyere level in Fig. 9 also show a clear tendency of decreasing mean diameter of coke at higher PCR. A higher reactive coal in the raceway is gasified more readily than coke, and at higher coal injection the ex-

Coke Properties at 7~vere Level in Blast Furnace with Pulverized Coal Injection 5

Fig. 9. Change of mean coke size at tuyere level with various PCRs.

tended residence time of coke in bosh gives more chances of mechanical disintegration of coke, making coke size smaller [9]. Accordingly, the accumulation of coke fines in deadman as birds nest is expected to in- crease with PCR, which could result in an increase of gas permeability resistance index.

3.3. Prediction of coke size at tuyere level The charged ore and coke keep their original shape

and size above the cohesive layer. Here, the burden spe- ed (SP') is calculated as follows;

SP' (mm/min) = (C,,TJD~ + C,~ko/D~,,k,)/A' (1)

The residence time RT' (rain) = HJSP' (2)

Below the cohesive layer, ore layer is smelted down into a liquid resulting in negligible volume and coke size is drastically changed along the furnace height by carbon solution reaction [2].

The burden speed (SP") is calculated as follows;

SP" (mm/min) = (C~,,ko/I)~,,~o)/A" (3)

The residence time here would be RT" (rain) = Htv'SP" (4)

The total residence time is the sum:

RT(min) = RT' + RT" (5)

Since, the increase of PCR requires less charged coke, the burden speed will be reduced to increase the resi- dence time.

CSR, DI, MS of the charged coke, had influences on the mean diameter of coke as shown in Fig. 10.

Fig. 10. Relationship between RT, CSR, DI, MS of charged coke and coke mean diameter at tuyere level.

Through multiple regression analysis of these points an equation to calculate the mean size of bosh coke from RT, CSR, DI and MS of charged coke was obtained;

Dp = { 0.0423RT + 1.2MS + 1.5DI + 0.48CSR

-196}/1000 (6)

R= 0.9

As was well known, gas permeability resistance index (K) in blast furnace is influenced by the pore size of bur- dens and gas volume passing through the pore [1].

Besides the properties of burden materials, the pore size is affected by ore/coke ratio. Since ore is smaller than coke and the increase of pig iron production rate means a higher ore/coke ratio, K value would increase at a high production rate. Likewise, the permeability resis- tance will increase with bosh gas volume. The effects of coke properties on permeability tests were carried out by charging different cokes on two consecutive days. Results are shown in Fig. 11; RT calculated from charg- ed ore, coke and PCR, is constant and the bosh gas volume also becomes constant for all three days.

On 25th Aug. 1994, CSR increased and DI and MS decreased more than those on 24th Aug., but LK was not changed because of nearly constant mean diameter of coke obtained from equation (6). LK increase on 26th Aug. may be attributed to the change of coke properties, since coke charged had lower CSR, and DI. As shown in Table 2 the change of coke mean diameter calculated

6 Jin Kyung CHUNG, Jeong Whan HAN and Jeong Ho LEE

Fig. 11. Changes of permeability resistance index (LK) at lower part of blast furnace with coke properties.

by equation (6) was 0.0107 m, when CSR and DI of charged coke dropped from 68.7 to 66.5, and from 88.6 to 88.3, respectively, although MS increased by 0.3 ram. The gas permeability resistance at lower part of blast fur- nace (LK) measured on 26th Aug. was 2.0 up by 0.2 from measured value on 25th Aug.

To test the predictability of LK values, Ergun equation [10] which can estimate the change of the gas permeability resistance index through the calculation of coefficients in the first and second term of the Eq. (7), was adopted and the results are listed in Table 2.

ziP = 150 ~_~_ u + 1.75 pB u2 (711 L gc gc

(1 e) 2 _ (l-e) where, A ~ I ~ ' B - (~Dv) e3

Table 2 was obtained based on the relationship of equation (6) between the coke mean size and other properties. The void fraction (e) was obtained from the assumption of uniform diameter and the sphericity (~) of coke was calculated according to equatuion (811 [11].

= 0.390 loglo(Dp) + 1.331 (8)

According to the change of coke mean size (Dp) es- timated by equation (6), B in equation (7) may be con-

Table 2. Gas permeability resistance index predicted and measured according to operating conditions

24th Aug. 25th Aug. 26th Aug.

Dp(m) 0.0262 0.02627 0.0252

(26th-25th Aug.) of Dp - 0.00107

e 0.465 0.465 0.467

0 1.1041 1.1046 1.0975

in Eq. (6) I A 3402 3380 3646 B 184 183 ] 189

(26th/25th Aug.) of A 1.08

permeability resistance 1.8 1.94

predicted

(26th-25th Aug.) of permeability resistance

0.16 predicted

permeability resistance 1.8 2.0

measured on operation

(26th-25th Aug.) of permeability resistance

0.2 measured on operation

sidered to be nearly constant. The change in AWL is then determined by the change of A. As shown in Table 2, there was little change between A on 24th and 25th Aug., while A on 26th Aug., 5470 is much larger than 5071 on 25th Aug. It means that AWL increased by 1.08 times for decrease of 0.0107 m in coke size. In other words, LK increased by 0.16 from 1.8 to 1.94. There was good agreement between the prediction and measur- merit.

4. C O N C L U S I O N S

(1) It was found from the coke sampling experiments that as PCR increases the raceway depth decreases and the size of birds nest increases, resulting in bad gas per- meability. Coke degradation at tuyere level with PCR is the major cause as indicated by increase of the fines und- er 3 mm.

(2) Oxygen enrichment with the co-axial lance in- creased the raceway depth and reduced birds nest thick- heSS.

(3) The mean diameter of coke at tuyere level had a good relationship with the coke residence time (RT) and properties of charged coke like MS, CSR, DI in the

Coke Properties at Tuyere Level in Blast Furnace with Pulverized Coal Injection 7

form of

Dp = { - 0.0423RT + 1.2MS + 1.5DI

+ 0.48CSR -- 196}/1000 The strength of sampled coke at tuyere level deteriorated with PCR.

(4) Change of gas permeability resistance obtained from the variation of coke properties was well matched with the prediction.

N O M E N C L A T U R E

Dia. : tuyere diameter (mm) VB : blast volumetric rate (Nm3/min) PD : production rate of pig iron (ton/day) Oxen : amount of oxygen enrichment ratio (Nm~/hr) TB : temperature of blast air ("C) HMT : hot metal temperature (~ PCR : pulverized coal injection ratio (Kg/t-p) LK : permeability resistance index in lower part of

blast furnace (-) K : permeability resistance index in blast furnace (-) MS : charged coke mean size (ram) CSR : coke strength index after reaction with CO2 ac-

cording to JIS 2151 (%) DI : coke strength in air according to JIS 2151 (%) CSR' : strength index of coke which is sampled from

blast furnace, after reaction with CO2 (%) DI' : strength index of coke which is sampled from

blast furnace, in air (%) Dp : estimated coke diameter in lower part of blast

furnace (m) AP : pressure drop (Pa) L : thickness of particle bed (m) u : gas velocity in vacancy (m/s), 0 : sphericity (-) p : gas density (Kg/m 3)

la : gas Viscosity (Pa - s) : Newton's law proportionality factor (Kg �9 m/N -

s) C .... : amount of charged ore (Kg/day)

Cc,,ko : amount of charged coke (Kg/day) D .... : bulk density of charged ore (Kg/m 3) D~,,k~ : bulk density of charged coke (Kg/m ~) A' : cross sectional area at above the cohesive layer

(m 2) A" : cross sectional area at below the cohesive layer

(m-) SP' : burden speed at above the cohesive layer (ram/

rain) SP" : burden speed at below the cohesive layer (ram/

min) SP : total burden speed in blast furnace (mm/min) Hs : height of BF above cohesive zone (mm) Ha : height of BF below cohesive layer (ram)

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