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Response of high ash Indian coals to precarbonization techniques
Response of high ash Indian coals to precarbonization techniques Amit
Chatterjee
and H. N. Prasad
Research and Development Division, Tata Iron and Steel Company, Jamshedpur 831 007, India (Received 3 December 1986: revised 20 January 7988)
PO Burma
Mines,
An investigation was carried out on the response of inherently high ash Indian coals to precarbonization techniques, such as preheating, briquette blending, tar addition, stamp charging and selective crushing, both
individually and in conjunction wherever possible. Extensive tests were conducted in pilot ovens followed in some cases by confirmatory tests in commercial ovens. Based on the results obtained in this study and the analysis of data available from commercial installations recommendations are made for the selection of the most suitable technique for countries faced with the problem of poor quality and/or scarce reserves of coking coal. (Keywords: coal; carbonization;
ash)
Unlike European and American coals, Indian coking coals’ are generally difficult to wash, because of their very high ash contents. Furthermore, intimate dispersal of the mineral matter within the coal substance results in an appreciable loss of combustibles in the form of middlings and rejects during washing. Generally attempts are made to obtain 17.5 + 1% ash in the clean coal and then the yield of washed coal seldom exceeds 60%. Any further decrease in the ash content becomes uneconomic because of the higher costs of washing and a sharp drop in the yield of clean coal. Not only is the ash content of Indian coal high, but the reserves of coking coal are also limited. However, higher ash improves the bulk density of the oven coal charge2 because of the greater affinity of mineral components for water, but it certainly adversely affects the coking properties. The problem of coking coal shortage and unsuitability of the conventional coke-making technology to produce coke of acceptable quality from inferior coals is, therefore, a subject of concern in India. Accordingly the response of inherently high ash Indian coals to precarbonization techniques3-‘, such as preheating *-lo, briquette blending11ei5, tar addition’6v’7, stamp charging’ 8-2o and selective crushing2’, both individually and in conjunction wherever possible, have been studied in detail at Tata Steel. A 200 kg and a 600 kg pilot oven were used extensively followed in some cases by confirmatory tests in 19 tonne commercial ovens. The results may be of interest to other countries faced with the problem of poor quality and/or scarce reserves of coking coal. In any case, the results are noticeably different from those for European and American coals. EXPERIMENTAL The conditions for each set of experiments are given in Table 1. All the coals tested were normally pulverized in a
hammer mill to 80f 2% through 3.2 mm, except for the stamp charging tests in which the coal was crushed liner. 001~2361/88/101417-11$3.00
each case at least two oven tests for every blend composition were carried out to check the reproducibility of the results, and the results from individual tests are plotted in the figures given later. Details of all the coals used, comprising broadly prime, medium and blendable” types, are summarized in Tables 2 and 3. For comparison, conventional top coal charge tests were also carried out with coal containing 5 to 6 % moisture and 0.2 y0 light diesel oil (LDO), in accordance with the normal practice at Tata Steel. The + 50 mm coke produced from each test was first stabilized by subjecting it to two drops in the standard shatter unit and 25 revolutions in the Micum drum as per Indian Standard IS:7189-7423. The coke strength was then assessed by the Micum test on + 50 mm stabilized coke (as per BS :10168024/IS:1354-6425) and wherever possible also by the Irsid test26, in which + 20 mm stabilized coke is rotated in a Micum drum for 500 revolutions, and by the Japanese Drum test on + 50 mm stabilized coke using the standard Japanese drum and rotating it for 30 and 150 revolutions to obtain the D30 and D:z” indices (as per JIS, K 215 l15 197227). In
Preheating
A batch type steel drum preheater of 1200 mm diameter Table 1
Details of different ovens used Commercial
Parameters
Pilot oven (600 kg)
Oven dimensions (mm) 1370 x 1070 x Length x height x width 450 Mean flue temperature 1080 (“C) Total carbonization time 18 (h) Temperature at centre of coke mass (“C) tOOcl
FUEL,
1988,
Pilot oven
oven
(200 kg)
(19t)
960 x 1065 x 300
13590x4500x 400
110&1120
1250
12
15t-16
920
103&1050
Vol 67, October
1417
Response of high ash Indian coals to precarbonization techniques: A. Chatterjee and H. N. Prasad Table 2 Characteristics of coals used in the tests: proximate analysis and caking index Coals used Proximate analysis (% dry basis)
Sources Nomenclature
Coalfield
Colliery
Ash
VM
FC
Caking index”
Jamadoba washed (Jam) Prime coal
Jharia
Jamadoba and Sijua Group
17.3
27.0
55.7
14
West Bokaro washed (W.B.) Medium coal
West Bokaro
West Bokaro
16.9
26.7
56.4
13
Blendable coal (B.C.): No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9
Raniganj South Karanpura Raniganj West Bokaro Raniganj Raniganj Raniganj Raniganj Jhari
Seethalpur Lapanga Girimint (Koithee seam) Kuju Chinakuri Girimint (Poniati seam) Ranipur Dhemomain Sijua
16.0 19.9 17.3 19.3 15.3 13.9 16.9 24.9 19.5
33.6 33.0 36.3 27.8 35.9 34.5 34.9 31.6 20.5
50.4 47.1 46.4 52.9 48.8 51.6 48.2 43.5 60.0
11 4 9 6 12 10 10 8 10
@Bureau of Indian Standards, IS:1353-59, Method B (if determined by BS:705-1936 the CI value would be l-5 times this value) Table 3 Characteristics of coals used in the tests: petrographic analysis and reflectance distribution of prime, medium and some blendable coals used in the tests Petrographic analysis (~01%) Coal
Vitrinite
Semi-vitrinite
Exinite
Inertinite
Mineral matter
Jam. W.B. B.C. No. 1 B.C. No. 6 B.C. No. 8
63.1 48.3 69.8 68.2 72.6
6.2 6.5 2.8 1.5 1.8
0.7 4.5 4.3 4.2 2.6
22.9 34.9 14.0 12.3 12.1
7.1 5.8 9.1 13.8 10.9
Reflectance distribution (%) Coal Jam. W.B. B.C. No. 1 B.C. No. 6 B.C. No. 8
v5
V6
v7
V8
v9
VlO
Vll
v12
v13
v14
_ 2 -
_ 21 19
_
8 47 54 3 7
44 35 14 1 2
8 18 _ _
24 _ _ _ _
10 _ _ _ _
3 _
3 _ _ _ _
30 75 72
-
Average ( %)b 1.06 0.92 0.83 0.73 0.74
“Vitrinites are classified by reflectance, the number referring to the reflectance range in which the sample falls, thus, V5 has a reflectance of0.50-0.59% *Maximum reflectance of vitrinite in oil
and 1500 mm length was used, which was heated by gas burners and driven at a speed of 7 to 8 rpm. It took about 2 h to heat each batch of 700 kg of pulverized coal (80% - 3.2 mm) to about 200-250°C. After the coal mass had attained the desired temperature it was transferred to a hopper, charged into the 600 kg pilot oven and carbonized. Briquette blending
The experimental equipment comprised a drum for mixing the coal under test with a binder, a 1.5 t h-’ briquetting press with grooved rolls, a drum mixer for blending fine coal and briquettes and the same 600 kg pilot oven. To make the briquettes, pulverized coal and 10 % binder (8 % pitch + 2 % tar) were mixed thoroughly and heated to 100°C in an externally heated drum mixer. The mixture was then allowed to cool to 70-8O”C and fed to the briquetting machine to produce oval shaped briquettes (50 x 25 x 19 mm). The briquettes were ultimately blended with the prepared fine coal mixture in a predetermined ratio, before charging into the oven.
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FUEL, 1988, Vol 67, October
Pilot oven (600 kg) tests were carried out with a charge containing Jamadoba (Jam.) and West Bokaro (W.B.) coals (keeping the ratio of prime to medium coking coal as either 55:35 or 45:55) in admixture with various proportions of blendable coals only in briquette form, and also with charges containing various amounts of fine coal, with the remainder briquettes made from the same blend, a procedure often referred to as partial briquetting. Encouraging results were obtained in the pilot plant, and commercial scale oven tests were carried out in the 19 tonne ovens with different briquette blended charges. Some tests were carried out to ascertain any benefits accruing from coupling preheating with briquette blending; the fine coal charge was first heated to 250°C in the drum type preheater and then mixed with the briquettes before charging the total mixture to the oven. Direct addition of tar to coal
Direct tar addition was studied by pilot oven (600 kg) tests, batch type full scale oven tests and continuous plant trials in a full battery (54 x 19 t ovens) over a limited
Response
of high
ash Indian
coals
to precarbonization
period. To prepare the charge for each pilot oven test, 600 kg of coal blend containing the constituent coals in predetermined proportions was mixed in a pan mixer. To arrive at 5~3% moisture in the final blend, 4-S% water was added during mixing along with first 0.2% LDO (light diesel oil) and second 1.25 % tar. Direct addition of tar was precluded by its high viscosity and the options open were either to heat the tar or to add a solvent to lower its viscosity at room temperature. For ease of operation at room temperature in any coking plant, the latter alternative was chosen. It was found that addition of one part of anthracene oil to 5 parts of tar lowered its viscosity sharply. The optimum amount of this mixture for addition to coal was found to be 1.5 % (1.25 % tar and 0.25 % anthracene oil), and this was added to the coal blend in all relevant tests. For individual commercial oven tests, 19 t of the charge was prepared in the same way as mentioned above. In view of the very promising results of pilot oven and full scale oven tests, a full battery trial was conducted with 1.5% tar--anthracene oil mixture addition, with the prime objective of confirming the coke strength improvement on a sustained basis. To prepare the 5: 1 tar-anthracene oil mixture, a recirculating geared pump was used for 2 days continuously and the mixture prepared was sprayed onto coal. For 150 t h-’ coal flow, the spraying rate was controlled at 4045 1min-‘. The total trial comprised 2 weeks without tar mixture addition followed by 1 week of operation with tar mixture addition. Stamp charging
The coals chosen for the tests were crushed to three levels of crushing fineness (78-80x, 88-90x, 93-95 % below 3.2 mm) in a hammer mill. To prepare the stamp charge 250 kg of coal mixture containing the constituent coals in the required proportions were mixed in a pan mixer. During mixing 8-12 o/0water was added, to arrive at 12-15% moisture in the final blend. Stamping of the prepared charge was carried out manually in a steel chamber (711 mm long, 915 mm high and 267 mm wide internally), which rested on a trolley provided with a mechanical device for inserting the cake into the 200 kg oven chamber. The quantity of prepared coal compacted was ascertained and the dimensions of the stamped cake were measured to determine its bulk density before introduction into the oven chamber. The prepared cake resting on the trolley was positioned in front of the oven, introduced mechanically into the chamber through the oven door, and firmly held in position before the supporting plate was withdrawn. The door was placed and luted with clay before starting the carbonization cycle. Some tests were also carried out to ascertain the benefits accruing from coupling stamp charging with briquette blending. Selective crushing
A pilot plant utilizing progressive crushing and screening (developed by Longway Burstlein) was available at Tata Steel, and was employed for controlled crushing. The plant, of 2 t h-’ capacity, consisted of a feed hopper, screw conveyor, bucket elevator, electrically heated double deck screen, a controlled crusher and belt conveyors. Different coals were separately subjected to progressive crushing and screening until the entire
techniques:
A. Chatterjee
and H. N. Prasad
quantity of each coal passed through 3 mm. Pilot oven (600 kg) tests were carried out with coals and blends crushed by this so-called ‘Sovaco’ process and also crushed normally in a hammer mill to various finenesses, for comparative assessment of the resultant coke properties. RESULTS AND DISCUSSION Preheating Bulk density of the charge. The bulk density of the charge is an important factor affecting the operation of an oven, particularly its throughput and the coke quality. Pre-heating eliminates moisture and the dry particles flow more easily and pack together more closely. The determination of the bulk densities (ASTM D291-60)2s of some wet charges vis-&is preheated charges (Table 4) showed that the latter had consistently higher bulk densities, ranging from 905 to 920 kg rnp3, than the wet charges (containing 5 % moisture and 0.2 % light diesel oil), maximum 756 kg rnm3. Attempts were also made to determine the effect of the preheating temperature on the bulk density; on increasing the preheating temperature to 250°C the bulk density increased, but beyond 250°C it again decreased. Conflicting observations have been reported on the influence of preheating temperature on bulk density. Perch and Russe129, as well as Malek and Walsh3’ found that coals with zero moisture had the highest bulk densities, and that the bulk density decreased with increasing temperature of preheating, whereas Smith et aL31 observed that bulk densities increased appreciably on preheating. However, the effect of preheating in improving coke strength is due mainly to increase in the bulk density of the charge, and it can be assumed that with any preheated charge, the bulk density and size distribution within the oven will be more uniform than with corresponding conventional wet charges. Coke characteristics. The qualities of cokes produced from prime and medium coking coals, and from blends of 8 different blendable coals with Jamadoba coal and West Bokaro coal in the normal proportion of 55:35, were assessed. The optimum proportion of blendable coal (B.C.) in any ternary blend was ascertained by limiting the M 1o index of the stabilized coke to a maximum of 11,
Table 4 charges
Comparative
bulk
densities
of some
wet and
Bulk density
Coal/coal
blend 100 40 100% 507;
Jam. W.B. Jam. W.B. 55 Jam.:35 W.B. mixture B.C.No. 1 55 Jam.:35 W.B. mixture B.C. No. 1 55 Jam:35 W.B. mixture B.C. No. 1
-
50 “/:, 90 7: 10% SS”,; 157; 80 9” 20 I:/,
preheated
(kg mm3)
Wet, 5 to 6% moisture and 0.24;; LDO
Preheated to 250°C
756 752
913 905
750
905
745
905
745
913
745
921
FUEL. 1988, Vol 67, October
1419
Response of high ash Indian coals to precarbonization Table 5 Typical examples of quality of coke produced from wet and preheated charges Coke strength (micum indices) Charge preheated to 2oo-25O”c
Wet charge Coal/coal blend
M40
Ml0
M40
Ml0
Jam.
100%
82.4
10.2
82.2
W.B.
100%
82.6
10.6
82.3
8.2 1.7
17.6
16.7
79.6
13.0
80.0
13.8
80.6
17.0
13.9
79.1
55 Jam.:35 W.B. mixture B.C. No. 2 55 Jam.:35 W.B.mixture B.C. No. 3 55 Jam.:35 W.B.mixture B.C. No. 4
L2
38
90% ~ 10% 90% ~ 10% 90% __ 10%
16.
/
techniques:
A. Chatterjee and /i. N. Prasad
were 80 %, 25 %, 50 %, 40 y0 and 20 % respectively with preheating, while with wet charging only 20 % of coal No. 1 and O-5% of the others could be used. These findings emphasize the efficacy of preheating in extending the range and amounts of coals which can be blended with prime and medium coking coals and produce metallurgical coke of acceptable quality. In view of the meagre and poor quality reserves of coking coals and relatively abundant deposits of inferior grade coals in countries like India, this finding could affect the future of the iron and steel industry in these countries. Briquette blending Bulk density of charges. The size distribution of a briquette blended charge is influenced by the percentage of briquettes, whose apparent density ranges from 1.2 to 1.3 gcmm3, 0.3 to 0.5gcme3 greater than that of fine coal. Figure 4(a) shows that as thecharge was increasingly
.lOO
16-
-
PRE-HEATED
---*
WET
CHARGE
CHARGE
-90
It-----l
I
-70,
x
26 -
-60
22-
-50
f o
2
-LO
2618-
0
10
20
30
% ADDITION OF BLENDABLE (55 JAM: 35 W.B.)
i.0 COAL
50
60
No 1 TO BASE
70
80
COAL MIX
Figure 1 Effect of addition of blendable coal No. 1 on coke properties after pre-heating (average values from repeated tests); ‘% addition’ refers to the amount of added blendable coal as briquettes calculated as a percentage of the final carbonization charge mixture
22 -
18-
0 10 20 30 % ADDITIONOF BLENDABLE BASE
COAL
MIX (55
I LO
COAL JAM : 35 W.B.)
I 50
I 6i0
No.6
TO
Figure 2 Effect of addition of blendable coal No. 6 on coke properties after pre-heating (average values from repeated tests)
which was then considered to be the bare minimum requirement for blast furnace operations in India. Preheating of a particular coal, rather than a coal blend, led to cokes in which the M, 0 values improved by 2 to 5 points. The improvement was more marked in the case of the poorer coal, which itself could be of considerable significance. The results of the tests (Table 5) indicated, for example, that 10 % of blendable coals No. 3 or 4 could only be used with the basic coal mixture (55 Jamadoba:35 West Bokaro) to produce metallurgical coke if such charges were preheated to 200-250°C. These blends would otherwise be totally unsuitable since the M,, values for wet charges were all above 13. The influence on coke quality of addition of blendable coals 1, 5, 6, 7 and 9, to the base coal mixture, was studied in detail; some results are given in Figures 1-3. It was apparent that in all cases, if the criterion was that the coke quality should not deteriorate beyond a certain level as a result of blending, higher proportions of blendable coals could be used in preheated charges. However, it is interesting to note that the percentage of blendable coal which would be tolerated in each case was distinctly dependent on the type of blendable coal, e.g. the permissible proportions of blendable coals 1,5,6,7 and 8
1420
FUEL, 1988, Vol 67, October
42-
18
I
I
PRE-HEATED .-I WET CHARGE 38-
16-
6
I CHARGE WY / i’
-100
-90
-90
-80
,;,,
1 I I ALO 0 10 20 30 40 % ADDITION OF BLENDABLE COAL No. 7 TO BASE COAL MIX (55 JAM: 35 W.B.)
A30
Figure 3 Effect of addition of blendable coal No. 7 on coke properties after pre-heating (average values from repeated tests)
Response
0
10
of high ash Indian
20 30 40 O/o ADDITION
coals to precarbonization
50
60
Figure 4 Bulk density of the charge as a function of: a, the percentage of briquettes in a charge of coal blend, 55 JAM.:35 W.B.:lO B.C. No. 1; b, the percentage of B.C. No. 1 added to basecoal mixture, 55 JAM.:35 W.B., B.C. No. 1 in briquette form; c, as in b, but with B.C. No. 1 in tine form (average values from repeated tests)
briquetted, the bulk density increased almost linearly from 745 kg m - ’ for the crushed charge to 820 kg m - 3 with 50% briquettes. Figure 4(b) shows that when only the poorly coking constituent of the oven charge was briquetted, the bulk density again increased with the proportion of briquettes; however, when the poorly coking coal was introduced as line coal instead of briquettes, the corresponding increase in bulk density was marginal (Figure 4(c)). Coke characteristics. As with preheating, the most notable feature of briquette blending was an improvement in the quality of coke. The properties of coke produced from briquette blended charges containing blendable coals Nos. 5 or 7, in admixture with Jamadoba and West Bokaro coals in proportions of either 55:35 or 45155, were assessed. As is evident from Figures 53, the introduction of the poorer quality blendable coal (BC 5 or 7) immediately resulted in deterioration of the coke properties compared with the base blend. Nevertheless, the introduction of the blendable coal in line form had a more pronounced deleterious effect than when the same blendable coal was briquetted. In some cases, the extent of deterioration was greater when part of the coal charge containing the blendable coal was briquetted rather than the blendable coal alone being briquetted and added to the rest of charge in the same proportion. This clearly showed a major advantage of briquetting and its applicability to poorer coking coals which are normally less expensive and more abundantly available. The improvement in the coke quality as a result of briquette blending was also reflected in an increase of about 2 % in the yield of blast furnace coke.
techniques:
A. Chatterjee
and H. N. Prasad
The fact that briquetting of only the poorer quality coals, and their subsequent addition to the finely divided base coal blend, had a more beneficial effect on the ultimate coke strength than partial briquetting of the entire coal blend, prompted examination of the possibility of coupling briquetting and preheating. While possibly only of academic interest at present, this combination could involve briquetting of poor quality coals as an ‘off line’ activity, either within or even outside a coking plant, and preheating of the better quality coals only, before ultimate mixing of the briquetted and preheated components in the charging car. When this was attempted in the pilot oven by adding blendable coal briquettes to the coking coal constituent of the total charge preheated to 250°C encouraging results were obtained; the M,, and M,, indices were found to be superior in all cases to those obtained by total preheating of the charge, which itself was superior to briquetting alone for all the coals tested. Thus a combination of preheating and briquetting appears worthy of further investigation. The results obtained in the pilot oven were extended by commercial scale oven tests which were carried out with different briquette blended charges (Table 6). From the test results, it will be seen that the blending of 30% briquettes, made from a charge containing 55% Jam, 35% W.B. and 10% B.C. No. 1, improved the coke
83
18
81
16
X iii z -
79
5 =
77
75
73 0 10 O/oADDITION
20
8 30
OF BRIQUETTES IN COAL BLEND (BLEND UsED AS FINE COAL AND BRIQUETTES 40 JAM :50W.B. YOB.C.Nob5)
Figure 5 Effect of addition of briquettes on coke properties using tine coal and briquettes of the same composition (average values from repeated tests)
FUEL, 1988, Vol 67, October
1421
Response
of high
83
coals to precarbonization
.-4B.C.No.5
techniques:
A. Chatterjee
and H. N. Prasad
on its way to the cdal service bunker and no special mixing facilities, such as the pan mixer used in the other tests, were provided.
I
I
81 E n z
ash Indian
Coke characteristics. The effects of tar and LDO addition on coke strengths in the pilot oven tests are shown for B.C. No. 1 in Figure 9. The results for all the three blendable coals, mixed separately with the 45 Jam.:55 W.B. coal mixture, proved that, compared with
IN
79
s = 77
81 ts !z -
79
0 u
75
=
75
73
0
10
O/oADDITION
20
Figure 6 Effect of addition of blendable coal No. 5 on coke properties after briquetting (average value from repeated tests)
strength in terms of the MI0 index by about 2 points and M,, by 4 points, while the addition of 20% briquettes made from B.C. No. 1 only improved the M,, value by 0.8 points only. In the case of B.C. No. 5, both briquetting of a part of the blend and briquetting of the poor quality blendable coal only improved the M,, values by about 2 points. B.C. No. 7 did not appear to respond favourably to either briquetting system; blending of 40 % briquettes made from this coal could improve the M,, value by 1 point only compared with a charge containing 10% of this coal in tine form. Briquettes made from B.C. No. 9 were found to improve the M 1o value by about 2 points. These tests confirmed that it is preferable to briquette only the poorer quality coal rather than a part of the total coal blend to achieve the best coke quality. tar addition
-10
30
OF BLENDABLE COAL No.5 TO THE BASE COAL MIX (45 JAM: 55 W. B. )
Direct
77
to coal
Determination of the bulk density of coal blends containing blendable coals No. 1,7 or 9, added at levels of lo,20 and 30x, to the 45 Jam:55 W.B. binary showed that addition of 1.5 % coal mixture tar fanthracene oil mixture increased the bulk density in most cases by a maximum of 20-25 kg me3 compared with addition of 0.2 y0 LDO. However, during full battery trials, tar addition varying from 0.68 to 1.83 % appeared to have no marked influence on the bulk density of the charge (Table 7), possibly because of inadequate mixing in this case since the tar was merely sprayed on crushed coal
73
1 I 1 I -8 10 20 30 40 50 % ADDITION OF BRIQUETTES IN COAL BLEND (BLEND USED AS FINE COAL AND BRIQUETTES 55JAM: 35 W.B: lOB.C.No.7)
0
Figure 7 Effect of addition of briquettes on coke properties using fine coal and briquettes of the same composition (average values from repeated tests)
83
--B.C. ---.B.
I
No,7 C. No.7
I
I
IN FINE FORM IN BRIQUETTE
18
81 X 5
79
-0 r”
77
75
Bulk density.
1422
FUEL, 1988, Vol 67, October
73
0 10 20 30 40 O/o ADDITION OF BLENDABLE COAL No.7 TO THE BASE COAL MIX (55 JAM : 35 W.B. )
Figure 8 Effect of addition of blendable coal No. 7 on coke properties after briquetting (average values from repeated tests)
Response of high ash Indian coals to precarbonization Table 6
techniques: A. Chatterjee and H. N. Prasad
Strengths of cokes from commercial oven tests with briquette blended charges Characteristics of coke
3. No.
1.
Composition of charge 55 Jam.:35 W.B. mixture
90%
B.C. No. 1 55 Jam.:35 W.B. mixture
10% 80%
B.C. No.1 55 Jam.:35 W.B. mixture
20% 80%
B.C. No. 1 briquettes 55 Jam.:35 W.B. and 10 B.C. No. 1
20%
2. 3.
4.
Briquettes from the same blend Repeat of test 4 45 Jam.:55 W.B. mixture
5. 6.
B.C. No. 5 40 Jam.50 W.B. and 10 B.C. No.5
7.
Briquettes from the same blend
8. 9. 10.
11.
12. 13. 14.
Table 7
70%
Irsid indices
Bulk density (kg mT3)
Micum indices Ma0
MI0
816
76.4
9.3
816
79.2
10.8
832
79.0
10.0
890
80.3
868
Japanese indices D'S0 15
I 20
I 10
DSO IS
66.4
28.5
_
_
-
_
_
66.4
28.0
92.0
74.0
7.5
71.3
22.1
94.0
80.0
80.4
7.7
70.8
22.4
94.0
80.0
752
80.9
12.2
60.8
36.0
92.0
72.0
880
19.6
10.0
63.6
30.8
90.5
70.5
880
79.6
10.2
61.6
28.8
88.5
68.0
744
79.4
10.0
57.8
35.6
89.0
66.0
832
80.6
11.8
62.8
32.8
90.0
70.0
904
76.6
10.6
66.4
26.8
92.0
68.0
864
77.5
9.1
64.0
28.6
94.0
72.0
836
80.6
10.2
62.8
34.0
94.0
74.0
892
80.6
8.4
70.7
24.2
95.0
75.0
30%
70% 30% 70% 30%
45 Jam.:55 W.B. mixture
70%
B.C. No. 5 briquettes 45 Jam.:55 W.B. mixture
30% 90%
B.C. No. 7 45 Jam.:55 W.B.mixture
10% 80%
B.C. No. 7 40 Jam.50 W.B. and 10 B.C. No. 7
20%
Briquettes frim the same blend 45Jam.:55 W.B.mixture
30% 60%
B.C. No. 7 briquettes 45 Jam.:55 W.B. mixture
40% 70%
B.C. No. 9 45 Jam.:55 W.B. mixture
30% 70%
B.C. No. 9 briquettes
30%
70%
Summary of results of tar spraying in full battery tests Fortnight period before tar spraying (with LDO spraying)
One week period with tar spraying
Parameters
Max.
Min.
Avg.
Max.
Min.
Avg.
Crushing fineness, % < 3.2 mm Bulk density (kg mm3) Plastic layer thickness in mm % LDO/tar % Blend composition : Prime coal Medium coal Blendable coal Proximate analysis of coal blend (% dry basis) Ash V.M. F.C. Proximate analysis of coke (% dry basis) Ash VM FC Coke strength: Micum-M 10 Micum-M40
82.1 823 19 0.35
64.1 772 17 0.1
70.7 783 18 0.23
77.4 788 23 1.83
63.4 771 22 0.68
72.1 778 23 1.41
48.7 73.6 17.4
36.3 21.8 4.6
42.3 46.3 11.4
51.4 66.0 18.9
35.6 23.8 7.0
44.1 43.8 12.1
21.5 25.7 55.1
19.4 23.9 52.3
20.3 25.1 54.6
21.5 26.3 55.0
19.5 24.2 53.1
20.3 25.7 54.0
26.8 1.2 74.7
25.4 0.4 72.8
25.9 0.7 73.4
26.2 3.0 74.3
25.1 0.5 72.7
25.7 1.1 73.2
10.8 80.9
9.9 79.2
10.2 79.1
9.0 81.2
8.7 78.9
8.8 80.3
FUEL, 1988, Vol 67, October
1423
Response of high ash Indian coals to precarbonization techniques: A. Chatterjee and H. N. Prasad TaMe 8
Full scale oven tests with blends containing tar +anthracene
oil Drum indices of coke
% Tar+ % LDO in the blend
anthracene oil in the blend
Jam. 40 50 W.B. B.C. No. 7 10
_ 0.2
1.5 _
Jam. W.B. B.C.No.7
_
1.5
Blend
36 44
Jam. W.B. B.C.No.
20
0.2
40 50
_
Micum
_
1.5
Irsid
Japanese
I 20
I 10
D30 IS
D’S0 15
8.6
65.2
29.6
11.0
57.8
35.6
94.0 89.0
78.0 66.0
M,,
M,,
81.6 79.4 82.4
9.0
64.4
31.2
92.0
76.0
80.6
11.8
62.8
32.8
90.0
70.0
80.3
9.9
63.0
32.0
90.0
72.0
81.0
12.0
60.8
34.0
90.0
70.0
1
10
0.2
36
_
1.5
80.6
12.2
56.8
40.8
90.0
70.0
W.B. B.C.No.
1
44 20
0.2
-
79.2
14.0
60.8
36.0
90.0
68.0
Table 9
Effect of crushing fineness and moisture level on bulk density of stamped charges and coke strength (Jamadoba washed coal)
Jam.
Screen analysis (%) Fineness aimed at (% <3.2mm)
-0.21 mm
Moisture (%)
Bulk density of cake (kg rne3)
M,,
M,,
15.6 15.3 19.0 19.0 16.0 16.1
27.0 26.8 28.0 30.9 23.9 24.3
10.2 10.5 12.0 12.2 15.0 15.0
1297 1268 1303 1303 1303 1303
80.0 79.2 79.0 78.0 77.6 78.4
8.4 8.4 8.8 8.8 9.0 8.8
88.1 88.5 89.8 89.3 89.5 88.3
16.0 15.9 18.1 18.2 18.3 17.2
32.3 32.2 35.9 33.5 34.2 33.0
10.5 10.2 12.0 12.0 14.5 15.2
1326 1326 1320 1326 1326 1326
82.2 81.8 81.6 81.2 80.0 80.8
8.0 8.0 8.0 8.2 8.4 8.2
94.6 94.5 94.8 94.7 95.6 95.4
18.7 18.8 18.6 18.7 17.7 17.7
42.2 42.1 41.3 41.0 46.2 44.3
10.2 10.2 12.2 11.5 15.5 15.2
1351 1357 1326 1326 1326 1326
82.0 82.4 82.8 82.4 82.0 82.0
7.6 7.6 8.0 8.0 8.2 8.2
-3.2mm
-0.42 +0.21 mm
80
80.1 79.1 80.4 80.8 78.8 79.3
90
95
LDO, the addition of 1.5% tar-anthracene oil mixture improved the coke quality by a minimum of 2 points in the M r0 index, irrespective of the proportion of blendable coal, while the M,, index was improved by at least 1 point. The Irsid and the Japanese drum indices showed similar improvements in coke strength. In eight similar tests with full-scale ovens (Table 8), the ovens were charged with blends containing 10 or 20% B.C. No. 1 or 7 mixed with the 45 Jam.:55 W.B. coal mixture. The results clearly showed that there was an improvement of more than 2 points in the M 1o values of cokes from tar-treated coal blends, with similar improvements in the other drum indices also. The pilot oven coke strength indices did not match numerically those from the large ovens, presumably an effect of scale difference; nevertheless, the results fully confirmed the findings from the pilot oven regarding the relative improvement in coke strength due to tar addition. The coke strength results for the full battery trial showed that the daily average M,, value of coke in the fortnight before tar spraying was started was 10.2, while
1424
FUEL,
1988,
Vol 67, October
Micum indices of coke
during the tar addition it fell to 8.8. The M,, value thus improved by 1.4 points on account of tar mixture spraying, while the improvement in the M,, index was only 0.6 points. In general, the trial agreed well with the pilot oven and full-scale oven tests conducted earlier. Stamp charging Bulk density. The principal factor in recent techniques for improving coke strength is increase in the bulk density of the charge, a factor which is influenced markedly by the stamping energy required for compaction as well as other parameters such as size distribution and addition of moisture, oil, tar, etc. The improvement in the bulk density of coke with increasing crushing fineness at a level of 10-12x moisture, following stamp charging, may be seen in Table 9. It is often pointed out that the density relative to stamped cake is not comparable with the bulk density of conventional top charges, since the oven volume is not completely filled by the stamped cake. To ensure smooth charging the cake is approximately 3 mm smaller than the
Response
I
of high ash Indian
I
Cloals
to precarbonization
I
-0.2% L. D.O. + --d.25%~AR+0.2S”/( ANTHRACiZNE OIL
73
205
h 2: n
15 f r 2 r
techniques:
A. Chatterjee
and H. N. Prasad
Stamping of a particular coal, rather than a coal blend, resulted in cokes with M,, values improved by 2 to 4 points. The improvement was, as expected, more marked in the case of poor grades of coal. Typical results of the effect of addition of blendable coals to the base coal
82
----
I
I CONVENTIONAL CHARGING STAMP CHARGING
81-
-16
7115
-lL
IO
------eML0
--------?
i?i n
45-s 43:
-;112 % i
41 g
- 11
3gz n 37 z
CY
H
1:
/
72
70
50 fYL”rn
;8 n’f5 w -2 i6 WX i4 s2
7L
0 10 20 308 % ADDITION OF BLENDABLE COAL No. 1 TO THE BASE COAL MIX ( L5 JAM : 55 W. B. 1
Figure 10 Effect ofaddition ofblendablecoal after stamping (average values from repeated
ww
82
aQz
81 -
Zn
Q-
----
I
No. 1 on coke properties tests)
I CONVENTIONAL CHARGING STAMP CHARGING
-16
..a I 80~ ’ 50 10 20 30 ADDITION OF BLENDABLE COAL No.1 TO THE BASE COAL MIX, ( O/o) Figure 9 Typical effect of tar versus LDO addition (average values from repeated tests)
on coke strength
clear width of the oven. Since in most cases the cake maintains its shape in the oven, this gap remains until the beginning of the plastic expansion, and is gradually filled during coking, depending on the properties of the coal. Thus, the density relative to the stamped cake is more informative. Coke characteristics. The qualities were assessed of cokes produced from stamped cakes of single coals (Jam., W.B. and B.C. No. 9) and of blends of B.C. No. 1,5,7 or 9 with 45 Jam :55 W.B. mixture. The optimum proportion of blendable coal to be used in any-ternary-blend was ascertained in the same way as for preheating and briquette blending.
0 10 20 36 % ADDITION OF BLENDABLE COAL No. 5 TO THE BASE COAL MIX (L5 JAM : 55 W. B. ) Figure 11 Effect ofaddition ofblendable coal No. 5 on coke properties after stamping (average values from repeated tests)
FUEL, 1988, Vol 67, October
1425
Response of high ash Indian coals to precarbonization techniques: A. Chatterjee and H. /V. Prasad
mixture are shown in Figures 10 and 11. The results indicated that stamping permitted the use of 10, 20, 30 and 40% respectively of B.C. No. 1, 5, 7 and 9 in admixture with Jamadoba and West Bokaro coals, while with conventional charging only 10% of B.C. No. 7 and less than 10% of other blendable coals could be used. Based on the findings on briquette blending, the possibility of coupling briquetting and stamping was examined. This was attempted in the pilot oven by adding blendable coal briquettes to the binary coking coal mixture in tine form and stamping the total briquette blended charge; the M 1o index was found to be improved by 1 to 1.5 points in all cases compared to briquette blending alone, the extent of the improvement depending on the characteristics of the blendable coal used for briquetting. Thus, a combination of briquetting and stamping appears possible. Lask and Petak3’ have reported that blends of coals without dilation properties and with a swelling index of <2 can be converted into good quality coke by stamping of the preheated coal charge. The improved quality of coke as a result of stamping may also account for an increased mean size of the coke produced from stamped charges compared with conventional charges. It was found that irrespective of the charge composition, the mean size of coke from stamped charges was higher and breeze generation was lower, allowing an increased yield of blast furnace coke (+50 mm) from stamp charges compared with conventional charging.
marked differences in coke strength were observed irrespective of the crushing process. Sovaco process crushing improved the M 1,, values by 1 to 6 points when cokes produced from the coals/blends crushed to 80% fineness (normally obtained in a hammer mill) were compared with those from charges pulverized to 100 % fineness (normally obtained in the Sovaco process). The Sovaco process allowed the use of poorly coking blendable coals in the coking coal mixture without adversely affecting the coke strength. With increasing fineness the amount of ‘zero fines’ (below 0.2 mm) produced by hammer mill crushing increased, but at 100 y0 fineness the percentage of ‘zero lines’ generated by the Sovaco process and the hammer mill were similar, which is unusual. CONCLUSIONS
1. With increase in fineness, the quality of coke improved in terms of M,, values, but at 100 ‘A - 3.2 mm no
On the basis of the present studies, it can be concluded that high-ash Indian coals tend to respond favourably to any of the pre-carbonization techniques preheating, briquette blending, stamping or direct tar addition, in so far as improvement in coke quality is concerned. Success with selective crushing was limited, though the beneficial effects of finer crushing were demonstrated. Before choosing a particular technique for commercial exploitation, other features have also to be examined. For example, shortage of tar, often used as a fuel in developing countries such as India, may preclude adoption of tar addition, while high capital cost, as well as operational and maintenance problems, are likely to prohibit the commercial exploitation of preheating, at least in these countries. An important factor in favour of stamping, which was used in Germany earlier, and briquette blending, currently popular in Japan, could be low capital expenditure.
Table 10
crushing
Selective crushing
Some typical results are summarized in Table 10. The test results showed that:
Typical
strengths
of cokes after Hammer
Hammer
Coal/blend, coke quality, ‘zero fines’ (% < 0.2 mm), and screen analysis Jamadoba Micum
washed indices
process
Mill crushing
80
of coal charge
(% - 3.2 mm)
85
Sovaco process crushing (100x-3.2mm)
100
95
(Jam.) M,, MI,
80.0 10.2 20.0
80.2 10.6 22.1
81.0 10.2 28.2
80.8 10.2 28.0
80.5 10.0 32.6
80.7 9.9 33.7
80.2 9.0 34.9
80.5 9.1 34.7
82.2 9.4 30.0
83.2 9.2 31.7
M,, MI,
78.3 14.3 20.8
77.7 14.7 19.4
80.1 13.2 22.9
79.0 13.6 24.0
80.4 12.6 33.3
81.4 12.9 32.8
79.4 10.2 35.9
80.2 10.4 34.0
82.8 9.8 35.7
82.6 10.2 35.4
W.B.) mix 77.5 12.8 19.6
78.0 12.6 21.1
81.4 12.4 28.8
80.8 12.2 28.8
81.2 11.4 32.8
80.6 11.4 33.1
79.8 9.8 32.8
80.6 9.4 37.9
81.4 11.4 30.9
79.8 11.8 30.0
W.B.) mix. 78.8 12.7 25.4
79.0 12.8 24.2
80.8 11.0 28.3
81.0 10.6 27.9
82.6 10.6 34.4
83.0 10.2 33.1
79.4 9.8 34.3
79.6 9.8 34.7
82.9 10.0 33.7
83.0 10.1 31.5
% <0.2mm West Bokaro washed Micum indices
(W.B.)
% <0.2mm 10% B.C. No. 7,90x Micum indices
(45 Jam.55 MU MI,
% <0.2mm 10% B.C. No. 9,90x I&cum indices
(45 Jam.:55 M,, M,,.-
% <0.2mm Range of screen analysis of charge +3.2 mm -3.2 mm f1.6 mm -1.6mm +0.42mm -0.42 mm +0.2 mm -0.2 mm
1426
Mill or Sovaco
(%)
FUEL, 1988, Vol 67, October
18-20 15-17 18-23 19-27 19-25
14-15 16-19 13-17 20-26 23-29
46 1618 1418 26-33 32-34
Nil 18-21 17-21 27-33 33-38
1-2 18-23 2428 23-33 3CL36
Response Table 10
Summary
of high ash Indian
of assessment
of precarbonization
coals to precarbonization
techniques:
A. Chatterjee
and H. N. Prasad
techniques
Parameters
Conventional top charging
Briquette blending
Preheating
Stamp charging
Selective crushing
Crushing fineness ( - 3.2 mm) (“/,) Additive (%) Moisture in coal (‘7”) Bulk density of coal charge (kg m-‘) M,, M,, Carbonization time Oven throughput Maintenance requirement
78-80 0.2 LDO 68 7o(f750 I@11 77-80 100 100 100
78-80 8-10 pitch and tar
78-80 _ _ 800-850 8-9 793 1 8035 110-115 150-175
89-91 _ 8-11 1050-1150 7-9 80-8 I 110-115 112-115 110-115
98-100 0.2 LDO 6-8 650-700 9-10 78-79 100 96-98 105-l 10
750~00 8.5-9.5 7941 105-l 10 105-l 10 105
In countries such as India, briquette blending would also require an input of tar, and although it is a technologically and economically viable method in view of its simplicity and favourable impact on poor quality coals, which could be installed in any battery at any time, tar availability could be a major problem. With stamp charging, though the energy utilization is not optimum, since moisture to the extent of lo-12 % has to be added to make a strong enough cake and has to be removed later in the coking cycle, the improvement in coke properties is better and no other binder is required. Stamp charging could thus offer distinct possibilities where the maximum improvement in coke properties, with a given type of coal mixture, is required. Unlike briquette blending, stamp charging does not require added tar or pitch, and can be incorporated as soon as the stamping hardware becomes available. However, whether stamp charging can be accommodated in an existing coking plant, has to be examined in individual cases since some design changes may be necessary, possibly only justified when a battery is due for renewal. Table 11 summarizes the relative merits of the various pre-carbonization techniques.
14 15 16 17 18 19
20 21
22 23
REFERENCES Mazumdar, B. K., Chakrabarty, M. and Mukherjee, D. K. ‘Problems and prospects of coal utilisation in India’, paper presented at the 37th Annual Session of IIChE and Joint Symposium with AIChE, Delhi, India, December 1984 Stuchlite, V. Gliickauf 1970, 31 (4), 204 Flockenhans, C. and Wagener, D. Proc. AIME, Ironmaking Conf., 1981,40,49 Chatterjec, A. and Prasad, H. N. Ironmaking and Steelmaking 1983, 10(I), 1 Chatterjee, A. and Prasad, H. N. Journal Institution ofEngineers (India) 1982, 63, (MM-l), 1 Chatterjee, A., Prasad, H. N. and Dinker, M. R. Trans. Indian Insritute of Metals 198 1, 34(6), 465 Beck, K. G. and Meckel, J. F. Ironmaking and Steelmaking 1980, 7(3), 111 Munson, Jr. J. G., Mettenry, W. L., Brayton, W. E. and Shoenberyer, R. W. in ‘Coal, Coke and the Blast Furnace’, 1978, London, The Metals Society, 12&131 Graham, J. P. in ‘Coal, Coke and Blast Furnace’, 1978, London, The Metals Society, 117-123
24 25 26 27
28
29 30 31
32
Peter, V. J. and Warford, F. K. in ‘Year book of the Coke Oven Managers Association’, 1980, 134-160 Akamatsu, K., Nire, H., Miyazaki, T. and Nishoka, N. in ‘Coal, Coke and the Blast Furnace’, 1978, The Metals Society, 51-65 Nakamura,N.,Togino,Y.andAdachi,T.in’Coal,Cokeand the Blast Furnaces’, 1978, The Metals Society, 93-106 Higuchi, M., Lizuka, M., Kuroda, K., Nakayama, N. and Saito, H. in ‘Coal, Coke and the Blast Furnaces’, 1978, The Metals Society, 19-28 Nakamura, N., Noguchi, N., Matsuoka, H. et al. ‘ILAFA’ Coal Congress, Bogota, Colombia, 1979 Yoshinaya, M., Sanada, M. and Utsunomiya, M. Proc. AZME Ironmaking Converence 1976, 35,256 Chatterjee, A. and Prasad, H. N. Fuel 1983,62, 591 Brown, N. A. BHP Technical Bulletin 1979, 23(2), 59 Leibrock, K., Grab, H., Petak, H. and Harion, E. Proc. AIME lronmaking Conference 1982,41, 341 Harion, E. and Mazumdar, A. K. Proc. International Symposium on ‘Blast Furnace Ironmaking’, November 1985, Jamshedpur, India, 69 Leibrock, H., Petak, W., Harion, E. and Schlebusch, D. ILAFA Coal Congress, Bogota, Columbia, 1979 Rao, R. A. ‘Report on selective crushing of coal’, Central Research and Development Organization, Hindustan Steel Ltd, India, 1970 Prasad, H. N., Mitra, A. N. and Chakrabarty, P. K. Trans. lndian Institute of Metals 1977, 30(5), 355 Bureau of Indian Standards, Specification for Blast Furnace Coke, IS:7189, 1974 British Standards Institution, Methods for analysis and testing of coal and coke (tests special to coke), BS 1016, Part 13, 1980 Bureau of Indian Standards, Methods of test for coke-special tests, IS:1354, 1964 Elliott, M. A. (Ed.) ‘Chemistry of coal utilization’, Volume 2, John Wiley and Sons, Chichester, 1981, 945 Japanese Testing Method for Coke Strength, Japanese Industrial Standard; JISK 2151-1972, Translation by Okuyama, Y., Nippon Kokan K.K., Technical Research Centre, Kawasaki, Japan American Society for Testing and Materials, standard test method for cubic foot weight ofcrushed bituminous coal, ASTM D291-60 Perch, M. and Russel, C. C. Coke hens Blast Furnace and Steel Plant 1959, 47(6), 591 Malek, M. A. and Walsh, J. H. ‘Coke in Iron and Steel Industry’, Proc. Charleroi Conference, 1966, 261-271 Smith, F. W., Birge, G. W., Wolfson, D. E. and Reynolds, D. A. ‘Better Coke by Thermal Pre-treatment of Coal’, R 5418, US Bureau of Mines, Washington DC, 1958, 26 Lask, G. W., Petak, H. and Echterhoff, J. Metallurgical Plant and Technology 1980, 3 (5/80), 4/15
FUEL,
1988,
Vol 67, October
1427
Chatterjee
and H. N. Prasad
Research and Development Division, Tata Iron and Steel Company, Jamshedpur 831 007, India (Received 3 December 1986: revised 20 January 7988)
PO Burma
Mines,
An investigation was carried out on the response of inherently high ash Indian coals to precarbonization techniques, such as preheating, briquette blending, tar addition, stamp charging and selective crushing, both
individually and in conjunction wherever possible. Extensive tests were conducted in pilot ovens followed in some cases by confirmatory tests in commercial ovens. Based on the results obtained in this study and the analysis of data available from commercial installations recommendations are made for the selection of the most suitable technique for countries faced with the problem of poor quality and/or scarce reserves of coking coal. (Keywords: coal; carbonization;
ash)
Unlike European and American coals, Indian coking coals’ are generally difficult to wash, because of their very high ash contents. Furthermore, intimate dispersal of the mineral matter within the coal substance results in an appreciable loss of combustibles in the form of middlings and rejects during washing. Generally attempts are made to obtain 17.5 + 1% ash in the clean coal and then the yield of washed coal seldom exceeds 60%. Any further decrease in the ash content becomes uneconomic because of the higher costs of washing and a sharp drop in the yield of clean coal. Not only is the ash content of Indian coal high, but the reserves of coking coal are also limited. However, higher ash improves the bulk density of the oven coal charge2 because of the greater affinity of mineral components for water, but it certainly adversely affects the coking properties. The problem of coking coal shortage and unsuitability of the conventional coke-making technology to produce coke of acceptable quality from inferior coals is, therefore, a subject of concern in India. Accordingly the response of inherently high ash Indian coals to precarbonization techniques3-‘, such as preheating *-lo, briquette blending11ei5, tar addition’6v’7, stamp charging’ 8-2o and selective crushing2’, both individually and in conjunction wherever possible, have been studied in detail at Tata Steel. A 200 kg and a 600 kg pilot oven were used extensively followed in some cases by confirmatory tests in 19 tonne commercial ovens. The results may be of interest to other countries faced with the problem of poor quality and/or scarce reserves of coking coal. In any case, the results are noticeably different from those for European and American coals. EXPERIMENTAL The conditions for each set of experiments are given in Table 1. All the coals tested were normally pulverized in a
hammer mill to 80f 2% through 3.2 mm, except for the stamp charging tests in which the coal was crushed liner. 001~2361/88/101417-11$3.00
each case at least two oven tests for every blend composition were carried out to check the reproducibility of the results, and the results from individual tests are plotted in the figures given later. Details of all the coals used, comprising broadly prime, medium and blendable” types, are summarized in Tables 2 and 3. For comparison, conventional top coal charge tests were also carried out with coal containing 5 to 6 % moisture and 0.2 y0 light diesel oil (LDO), in accordance with the normal practice at Tata Steel. The + 50 mm coke produced from each test was first stabilized by subjecting it to two drops in the standard shatter unit and 25 revolutions in the Micum drum as per Indian Standard IS:7189-7423. The coke strength was then assessed by the Micum test on + 50 mm stabilized coke (as per BS :10168024/IS:1354-6425) and wherever possible also by the Irsid test26, in which + 20 mm stabilized coke is rotated in a Micum drum for 500 revolutions, and by the Japanese Drum test on + 50 mm stabilized coke using the standard Japanese drum and rotating it for 30 and 150 revolutions to obtain the D30 and D:z” indices (as per JIS, K 215 l15 197227). In
Preheating
A batch type steel drum preheater of 1200 mm diameter Table 1
Details of different ovens used Commercial
Parameters
Pilot oven (600 kg)
Oven dimensions (mm) 1370 x 1070 x Length x height x width 450 Mean flue temperature 1080 (“C) Total carbonization time 18 (h) Temperature at centre of coke mass (“C) tOOcl
FUEL,
1988,
Pilot oven
oven
(200 kg)
(19t)
960 x 1065 x 300
13590x4500x 400
110&1120
1250
12
15t-16
920
103&1050
Vol 67, October
1417
Response of high ash Indian coals to precarbonization techniques: A. Chatterjee and H. N. Prasad Table 2 Characteristics of coals used in the tests: proximate analysis and caking index Coals used Proximate analysis (% dry basis)
Sources Nomenclature
Coalfield
Colliery
Ash
VM
FC
Caking index”
Jamadoba washed (Jam) Prime coal
Jharia
Jamadoba and Sijua Group
17.3
27.0
55.7
14
West Bokaro washed (W.B.) Medium coal
West Bokaro
West Bokaro
16.9
26.7
56.4
13
Blendable coal (B.C.): No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9
Raniganj South Karanpura Raniganj West Bokaro Raniganj Raniganj Raniganj Raniganj Jhari
Seethalpur Lapanga Girimint (Koithee seam) Kuju Chinakuri Girimint (Poniati seam) Ranipur Dhemomain Sijua
16.0 19.9 17.3 19.3 15.3 13.9 16.9 24.9 19.5
33.6 33.0 36.3 27.8 35.9 34.5 34.9 31.6 20.5
50.4 47.1 46.4 52.9 48.8 51.6 48.2 43.5 60.0
11 4 9 6 12 10 10 8 10
@Bureau of Indian Standards, IS:1353-59, Method B (if determined by BS:705-1936 the CI value would be l-5 times this value) Table 3 Characteristics of coals used in the tests: petrographic analysis and reflectance distribution of prime, medium and some blendable coals used in the tests Petrographic analysis (~01%) Coal
Vitrinite
Semi-vitrinite
Exinite
Inertinite
Mineral matter
Jam. W.B. B.C. No. 1 B.C. No. 6 B.C. No. 8
63.1 48.3 69.8 68.2 72.6
6.2 6.5 2.8 1.5 1.8
0.7 4.5 4.3 4.2 2.6
22.9 34.9 14.0 12.3 12.1
7.1 5.8 9.1 13.8 10.9
Reflectance distribution (%) Coal Jam. W.B. B.C. No. 1 B.C. No. 6 B.C. No. 8
v5
V6
v7
V8
v9
VlO
Vll
v12
v13
v14
_ 2 -
_ 21 19
_
8 47 54 3 7
44 35 14 1 2
8 18 _ _
24 _ _ _ _
10 _ _ _ _
3 _
3 _ _ _ _
30 75 72
-
Average ( %)b 1.06 0.92 0.83 0.73 0.74
“Vitrinites are classified by reflectance, the number referring to the reflectance range in which the sample falls, thus, V5 has a reflectance of0.50-0.59% *Maximum reflectance of vitrinite in oil
and 1500 mm length was used, which was heated by gas burners and driven at a speed of 7 to 8 rpm. It took about 2 h to heat each batch of 700 kg of pulverized coal (80% - 3.2 mm) to about 200-250°C. After the coal mass had attained the desired temperature it was transferred to a hopper, charged into the 600 kg pilot oven and carbonized. Briquette blending
The experimental equipment comprised a drum for mixing the coal under test with a binder, a 1.5 t h-’ briquetting press with grooved rolls, a drum mixer for blending fine coal and briquettes and the same 600 kg pilot oven. To make the briquettes, pulverized coal and 10 % binder (8 % pitch + 2 % tar) were mixed thoroughly and heated to 100°C in an externally heated drum mixer. The mixture was then allowed to cool to 70-8O”C and fed to the briquetting machine to produce oval shaped briquettes (50 x 25 x 19 mm). The briquettes were ultimately blended with the prepared fine coal mixture in a predetermined ratio, before charging into the oven.
1418
FUEL, 1988, Vol 67, October
Pilot oven (600 kg) tests were carried out with a charge containing Jamadoba (Jam.) and West Bokaro (W.B.) coals (keeping the ratio of prime to medium coking coal as either 55:35 or 45:55) in admixture with various proportions of blendable coals only in briquette form, and also with charges containing various amounts of fine coal, with the remainder briquettes made from the same blend, a procedure often referred to as partial briquetting. Encouraging results were obtained in the pilot plant, and commercial scale oven tests were carried out in the 19 tonne ovens with different briquette blended charges. Some tests were carried out to ascertain any benefits accruing from coupling preheating with briquette blending; the fine coal charge was first heated to 250°C in the drum type preheater and then mixed with the briquettes before charging the total mixture to the oven. Direct addition of tar to coal
Direct tar addition was studied by pilot oven (600 kg) tests, batch type full scale oven tests and continuous plant trials in a full battery (54 x 19 t ovens) over a limited
Response
of high
ash Indian
coals
to precarbonization
period. To prepare the charge for each pilot oven test, 600 kg of coal blend containing the constituent coals in predetermined proportions was mixed in a pan mixer. To arrive at 5~3% moisture in the final blend, 4-S% water was added during mixing along with first 0.2% LDO (light diesel oil) and second 1.25 % tar. Direct addition of tar was precluded by its high viscosity and the options open were either to heat the tar or to add a solvent to lower its viscosity at room temperature. For ease of operation at room temperature in any coking plant, the latter alternative was chosen. It was found that addition of one part of anthracene oil to 5 parts of tar lowered its viscosity sharply. The optimum amount of this mixture for addition to coal was found to be 1.5 % (1.25 % tar and 0.25 % anthracene oil), and this was added to the coal blend in all relevant tests. For individual commercial oven tests, 19 t of the charge was prepared in the same way as mentioned above. In view of the very promising results of pilot oven and full scale oven tests, a full battery trial was conducted with 1.5% tar--anthracene oil mixture addition, with the prime objective of confirming the coke strength improvement on a sustained basis. To prepare the 5: 1 tar-anthracene oil mixture, a recirculating geared pump was used for 2 days continuously and the mixture prepared was sprayed onto coal. For 150 t h-’ coal flow, the spraying rate was controlled at 4045 1min-‘. The total trial comprised 2 weeks without tar mixture addition followed by 1 week of operation with tar mixture addition. Stamp charging
The coals chosen for the tests were crushed to three levels of crushing fineness (78-80x, 88-90x, 93-95 % below 3.2 mm) in a hammer mill. To prepare the stamp charge 250 kg of coal mixture containing the constituent coals in the required proportions were mixed in a pan mixer. During mixing 8-12 o/0water was added, to arrive at 12-15% moisture in the final blend. Stamping of the prepared charge was carried out manually in a steel chamber (711 mm long, 915 mm high and 267 mm wide internally), which rested on a trolley provided with a mechanical device for inserting the cake into the 200 kg oven chamber. The quantity of prepared coal compacted was ascertained and the dimensions of the stamped cake were measured to determine its bulk density before introduction into the oven chamber. The prepared cake resting on the trolley was positioned in front of the oven, introduced mechanically into the chamber through the oven door, and firmly held in position before the supporting plate was withdrawn. The door was placed and luted with clay before starting the carbonization cycle. Some tests were also carried out to ascertain the benefits accruing from coupling stamp charging with briquette blending. Selective crushing
A pilot plant utilizing progressive crushing and screening (developed by Longway Burstlein) was available at Tata Steel, and was employed for controlled crushing. The plant, of 2 t h-’ capacity, consisted of a feed hopper, screw conveyor, bucket elevator, electrically heated double deck screen, a controlled crusher and belt conveyors. Different coals were separately subjected to progressive crushing and screening until the entire
techniques:
A. Chatterjee
and H. N. Prasad
quantity of each coal passed through 3 mm. Pilot oven (600 kg) tests were carried out with coals and blends crushed by this so-called ‘Sovaco’ process and also crushed normally in a hammer mill to various finenesses, for comparative assessment of the resultant coke properties. RESULTS AND DISCUSSION Preheating Bulk density of the charge. The bulk density of the charge is an important factor affecting the operation of an oven, particularly its throughput and the coke quality. Pre-heating eliminates moisture and the dry particles flow more easily and pack together more closely. The determination of the bulk densities (ASTM D291-60)2s of some wet charges vis-&is preheated charges (Table 4) showed that the latter had consistently higher bulk densities, ranging from 905 to 920 kg rnp3, than the wet charges (containing 5 % moisture and 0.2 % light diesel oil), maximum 756 kg rnm3. Attempts were also made to determine the effect of the preheating temperature on the bulk density; on increasing the preheating temperature to 250°C the bulk density increased, but beyond 250°C it again decreased. Conflicting observations have been reported on the influence of preheating temperature on bulk density. Perch and Russe129, as well as Malek and Walsh3’ found that coals with zero moisture had the highest bulk densities, and that the bulk density decreased with increasing temperature of preheating, whereas Smith et aL31 observed that bulk densities increased appreciably on preheating. However, the effect of preheating in improving coke strength is due mainly to increase in the bulk density of the charge, and it can be assumed that with any preheated charge, the bulk density and size distribution within the oven will be more uniform than with corresponding conventional wet charges. Coke characteristics. The qualities of cokes produced from prime and medium coking coals, and from blends of 8 different blendable coals with Jamadoba coal and West Bokaro coal in the normal proportion of 55:35, were assessed. The optimum proportion of blendable coal (B.C.) in any ternary blend was ascertained by limiting the M 1o index of the stabilized coke to a maximum of 11,
Table 4 charges
Comparative
bulk
densities
of some
wet and
Bulk density
Coal/coal
blend 100 40 100% 507;
Jam. W.B. Jam. W.B. 55 Jam.:35 W.B. mixture B.C.No. 1 55 Jam.:35 W.B. mixture B.C. No. 1 55 Jam:35 W.B. mixture B.C. No. 1
-
50 “/:, 90 7: 10% SS”,; 157; 80 9” 20 I:/,
preheated
(kg mm3)
Wet, 5 to 6% moisture and 0.24;; LDO
Preheated to 250°C
756 752
913 905
750
905
745
905
745
913
745
921
FUEL. 1988, Vol 67, October
1419
Response of high ash Indian coals to precarbonization Table 5 Typical examples of quality of coke produced from wet and preheated charges Coke strength (micum indices) Charge preheated to 2oo-25O”c
Wet charge Coal/coal blend
M40
Ml0
M40
Ml0
Jam.
100%
82.4
10.2
82.2
W.B.
100%
82.6
10.6
82.3
8.2 1.7
17.6
16.7
79.6
13.0
80.0
13.8
80.6
17.0
13.9
79.1
55 Jam.:35 W.B. mixture B.C. No. 2 55 Jam.:35 W.B.mixture B.C. No. 3 55 Jam.:35 W.B.mixture B.C. No. 4
L2
38
90% ~ 10% 90% ~ 10% 90% __ 10%
16.
/
techniques:
A. Chatterjee and /i. N. Prasad
were 80 %, 25 %, 50 %, 40 y0 and 20 % respectively with preheating, while with wet charging only 20 % of coal No. 1 and O-5% of the others could be used. These findings emphasize the efficacy of preheating in extending the range and amounts of coals which can be blended with prime and medium coking coals and produce metallurgical coke of acceptable quality. In view of the meagre and poor quality reserves of coking coals and relatively abundant deposits of inferior grade coals in countries like India, this finding could affect the future of the iron and steel industry in these countries. Briquette blending Bulk density of charges. The size distribution of a briquette blended charge is influenced by the percentage of briquettes, whose apparent density ranges from 1.2 to 1.3 gcmm3, 0.3 to 0.5gcme3 greater than that of fine coal. Figure 4(a) shows that as thecharge was increasingly
.lOO
16-
-
PRE-HEATED
---*
WET
CHARGE
CHARGE
-90
It-----l
I
-70,
x
26 -
-60
22-
-50
f o
2
-LO
2618-
0
10
20
30
% ADDITION OF BLENDABLE (55 JAM: 35 W.B.)
i.0 COAL
50
60
No 1 TO BASE
70
80
COAL MIX
Figure 1 Effect of addition of blendable coal No. 1 on coke properties after pre-heating (average values from repeated tests); ‘% addition’ refers to the amount of added blendable coal as briquettes calculated as a percentage of the final carbonization charge mixture
22 -
18-
0 10 20 30 % ADDITIONOF BLENDABLE BASE
COAL
MIX (55
I LO
COAL JAM : 35 W.B.)
I 50
I 6i0
No.6
TO
Figure 2 Effect of addition of blendable coal No. 6 on coke properties after pre-heating (average values from repeated tests)
which was then considered to be the bare minimum requirement for blast furnace operations in India. Preheating of a particular coal, rather than a coal blend, led to cokes in which the M, 0 values improved by 2 to 5 points. The improvement was more marked in the case of the poorer coal, which itself could be of considerable significance. The results of the tests (Table 5) indicated, for example, that 10 % of blendable coals No. 3 or 4 could only be used with the basic coal mixture (55 Jamadoba:35 West Bokaro) to produce metallurgical coke if such charges were preheated to 200-250°C. These blends would otherwise be totally unsuitable since the M,, values for wet charges were all above 13. The influence on coke quality of addition of blendable coals 1, 5, 6, 7 and 9, to the base coal mixture, was studied in detail; some results are given in Figures 1-3. It was apparent that in all cases, if the criterion was that the coke quality should not deteriorate beyond a certain level as a result of blending, higher proportions of blendable coals could be used in preheated charges. However, it is interesting to note that the percentage of blendable coal which would be tolerated in each case was distinctly dependent on the type of blendable coal, e.g. the permissible proportions of blendable coals 1,5,6,7 and 8
1420
FUEL, 1988, Vol 67, October
42-
18
I
I
PRE-HEATED .-I WET CHARGE 38-
16-
6
I CHARGE WY / i’
-100
-90
-90
-80
,;,,
1 I I ALO 0 10 20 30 40 % ADDITION OF BLENDABLE COAL No. 7 TO BASE COAL MIX (55 JAM: 35 W.B.)
A30
Figure 3 Effect of addition of blendable coal No. 7 on coke properties after pre-heating (average values from repeated tests)
Response
0
10
of high ash Indian
20 30 40 O/o ADDITION
coals to precarbonization
50
60
Figure 4 Bulk density of the charge as a function of: a, the percentage of briquettes in a charge of coal blend, 55 JAM.:35 W.B.:lO B.C. No. 1; b, the percentage of B.C. No. 1 added to basecoal mixture, 55 JAM.:35 W.B., B.C. No. 1 in briquette form; c, as in b, but with B.C. No. 1 in tine form (average values from repeated tests)
briquetted, the bulk density increased almost linearly from 745 kg m - ’ for the crushed charge to 820 kg m - 3 with 50% briquettes. Figure 4(b) shows that when only the poorly coking constituent of the oven charge was briquetted, the bulk density again increased with the proportion of briquettes; however, when the poorly coking coal was introduced as line coal instead of briquettes, the corresponding increase in bulk density was marginal (Figure 4(c)). Coke characteristics. As with preheating, the most notable feature of briquette blending was an improvement in the quality of coke. The properties of coke produced from briquette blended charges containing blendable coals Nos. 5 or 7, in admixture with Jamadoba and West Bokaro coals in proportions of either 55:35 or 45155, were assessed. As is evident from Figures 53, the introduction of the poorer quality blendable coal (BC 5 or 7) immediately resulted in deterioration of the coke properties compared with the base blend. Nevertheless, the introduction of the blendable coal in line form had a more pronounced deleterious effect than when the same blendable coal was briquetted. In some cases, the extent of deterioration was greater when part of the coal charge containing the blendable coal was briquetted rather than the blendable coal alone being briquetted and added to the rest of charge in the same proportion. This clearly showed a major advantage of briquetting and its applicability to poorer coking coals which are normally less expensive and more abundantly available. The improvement in the coke quality as a result of briquette blending was also reflected in an increase of about 2 % in the yield of blast furnace coke.
techniques:
A. Chatterjee
and H. N. Prasad
The fact that briquetting of only the poorer quality coals, and their subsequent addition to the finely divided base coal blend, had a more beneficial effect on the ultimate coke strength than partial briquetting of the entire coal blend, prompted examination of the possibility of coupling briquetting and preheating. While possibly only of academic interest at present, this combination could involve briquetting of poor quality coals as an ‘off line’ activity, either within or even outside a coking plant, and preheating of the better quality coals only, before ultimate mixing of the briquetted and preheated components in the charging car. When this was attempted in the pilot oven by adding blendable coal briquettes to the coking coal constituent of the total charge preheated to 250°C encouraging results were obtained; the M,, and M,, indices were found to be superior in all cases to those obtained by total preheating of the charge, which itself was superior to briquetting alone for all the coals tested. Thus a combination of preheating and briquetting appears worthy of further investigation. The results obtained in the pilot oven were extended by commercial scale oven tests which were carried out with different briquette blended charges (Table 6). From the test results, it will be seen that the blending of 30% briquettes, made from a charge containing 55% Jam, 35% W.B. and 10% B.C. No. 1, improved the coke
83
18
81
16
X iii z -
79
5 =
77
75
73 0 10 O/oADDITION
20
8 30
OF BRIQUETTES IN COAL BLEND (BLEND UsED AS FINE COAL AND BRIQUETTES 40 JAM :50W.B. YOB.C.Nob5)
Figure 5 Effect of addition of briquettes on coke properties using tine coal and briquettes of the same composition (average values from repeated tests)
FUEL, 1988, Vol 67, October
1421
Response
of high
83
coals to precarbonization
.-4B.C.No.5
techniques:
A. Chatterjee
and H. N. Prasad
on its way to the cdal service bunker and no special mixing facilities, such as the pan mixer used in the other tests, were provided.
I
I
81 E n z
ash Indian
Coke characteristics. The effects of tar and LDO addition on coke strengths in the pilot oven tests are shown for B.C. No. 1 in Figure 9. The results for all the three blendable coals, mixed separately with the 45 Jam.:55 W.B. coal mixture, proved that, compared with
IN
79
s = 77
81 ts !z -
79
0 u
75
=
75
73
0
10
O/oADDITION
20
Figure 6 Effect of addition of blendable coal No. 5 on coke properties after briquetting (average value from repeated tests)
strength in terms of the MI0 index by about 2 points and M,, by 4 points, while the addition of 20% briquettes made from B.C. No. 1 only improved the M,, value by 0.8 points only. In the case of B.C. No. 5, both briquetting of a part of the blend and briquetting of the poor quality blendable coal only improved the M,, values by about 2 points. B.C. No. 7 did not appear to respond favourably to either briquetting system; blending of 40 % briquettes made from this coal could improve the M,, value by 1 point only compared with a charge containing 10% of this coal in tine form. Briquettes made from B.C. No. 9 were found to improve the M 1o value by about 2 points. These tests confirmed that it is preferable to briquette only the poorer quality coal rather than a part of the total coal blend to achieve the best coke quality. tar addition
-10
30
OF BLENDABLE COAL No.5 TO THE BASE COAL MIX (45 JAM: 55 W. B. )
Direct
77
to coal
Determination of the bulk density of coal blends containing blendable coals No. 1,7 or 9, added at levels of lo,20 and 30x, to the 45 Jam:55 W.B. binary showed that addition of 1.5 % coal mixture tar fanthracene oil mixture increased the bulk density in most cases by a maximum of 20-25 kg me3 compared with addition of 0.2 y0 LDO. However, during full battery trials, tar addition varying from 0.68 to 1.83 % appeared to have no marked influence on the bulk density of the charge (Table 7), possibly because of inadequate mixing in this case since the tar was merely sprayed on crushed coal
73
1 I 1 I -8 10 20 30 40 50 % ADDITION OF BRIQUETTES IN COAL BLEND (BLEND USED AS FINE COAL AND BRIQUETTES 55JAM: 35 W.B: lOB.C.No.7)
0
Figure 7 Effect of addition of briquettes on coke properties using fine coal and briquettes of the same composition (average values from repeated tests)
83
--B.C. ---.B.
I
No,7 C. No.7
I
I
IN FINE FORM IN BRIQUETTE
18
81 X 5
79
-0 r”
77
75
Bulk density.
1422
FUEL, 1988, Vol 67, October
73
0 10 20 30 40 O/o ADDITION OF BLENDABLE COAL No.7 TO THE BASE COAL MIX (55 JAM : 35 W.B. )
Figure 8 Effect of addition of blendable coal No. 7 on coke properties after briquetting (average values from repeated tests)
Response of high ash Indian coals to precarbonization Table 6
techniques: A. Chatterjee and H. N. Prasad
Strengths of cokes from commercial oven tests with briquette blended charges Characteristics of coke
3. No.
1.
Composition of charge 55 Jam.:35 W.B. mixture
90%
B.C. No. 1 55 Jam.:35 W.B. mixture
10% 80%
B.C. No.1 55 Jam.:35 W.B. mixture
20% 80%
B.C. No. 1 briquettes 55 Jam.:35 W.B. and 10 B.C. No. 1
20%
2. 3.
4.
Briquettes from the same blend Repeat of test 4 45 Jam.:55 W.B. mixture
5. 6.
B.C. No. 5 40 Jam.50 W.B. and 10 B.C. No.5
7.
Briquettes from the same blend
8. 9. 10.
11.
12. 13. 14.
Table 7
70%
Irsid indices
Bulk density (kg mT3)
Micum indices Ma0
MI0
816
76.4
9.3
816
79.2
10.8
832
79.0
10.0
890
80.3
868
Japanese indices D'S0 15
I 20
I 10
DSO IS
66.4
28.5
_
_
-
_
_
66.4
28.0
92.0
74.0
7.5
71.3
22.1
94.0
80.0
80.4
7.7
70.8
22.4
94.0
80.0
752
80.9
12.2
60.8
36.0
92.0
72.0
880
19.6
10.0
63.6
30.8
90.5
70.5
880
79.6
10.2
61.6
28.8
88.5
68.0
744
79.4
10.0
57.8
35.6
89.0
66.0
832
80.6
11.8
62.8
32.8
90.0
70.0
904
76.6
10.6
66.4
26.8
92.0
68.0
864
77.5
9.1
64.0
28.6
94.0
72.0
836
80.6
10.2
62.8
34.0
94.0
74.0
892
80.6
8.4
70.7
24.2
95.0
75.0
30%
70% 30% 70% 30%
45 Jam.:55 W.B. mixture
70%
B.C. No. 5 briquettes 45 Jam.:55 W.B. mixture
30% 90%
B.C. No. 7 45 Jam.:55 W.B.mixture
10% 80%
B.C. No. 7 40 Jam.50 W.B. and 10 B.C. No. 7
20%
Briquettes frim the same blend 45Jam.:55 W.B.mixture
30% 60%
B.C. No. 7 briquettes 45 Jam.:55 W.B. mixture
40% 70%
B.C. No. 9 45 Jam.:55 W.B. mixture
30% 70%
B.C. No. 9 briquettes
30%
70%
Summary of results of tar spraying in full battery tests Fortnight period before tar spraying (with LDO spraying)
One week period with tar spraying
Parameters
Max.
Min.
Avg.
Max.
Min.
Avg.
Crushing fineness, % < 3.2 mm Bulk density (kg mm3) Plastic layer thickness in mm % LDO/tar % Blend composition : Prime coal Medium coal Blendable coal Proximate analysis of coal blend (% dry basis) Ash V.M. F.C. Proximate analysis of coke (% dry basis) Ash VM FC Coke strength: Micum-M 10 Micum-M40
82.1 823 19 0.35
64.1 772 17 0.1
70.7 783 18 0.23
77.4 788 23 1.83
63.4 771 22 0.68
72.1 778 23 1.41
48.7 73.6 17.4
36.3 21.8 4.6
42.3 46.3 11.4
51.4 66.0 18.9
35.6 23.8 7.0
44.1 43.8 12.1
21.5 25.7 55.1
19.4 23.9 52.3
20.3 25.1 54.6
21.5 26.3 55.0
19.5 24.2 53.1
20.3 25.7 54.0
26.8 1.2 74.7
25.4 0.4 72.8
25.9 0.7 73.4
26.2 3.0 74.3
25.1 0.5 72.7
25.7 1.1 73.2
10.8 80.9
9.9 79.2
10.2 79.1
9.0 81.2
8.7 78.9
8.8 80.3
FUEL, 1988, Vol 67, October
1423
Response of high ash Indian coals to precarbonization techniques: A. Chatterjee and H. N. Prasad TaMe 8
Full scale oven tests with blends containing tar +anthracene
oil Drum indices of coke
% Tar+ % LDO in the blend
anthracene oil in the blend
Jam. 40 50 W.B. B.C. No. 7 10
_ 0.2
1.5 _
Jam. W.B. B.C.No.7
_
1.5
Blend
36 44
Jam. W.B. B.C.No.
20
0.2
40 50
_
Micum
_
1.5
Irsid
Japanese
I 20
I 10
D30 IS
D’S0 15
8.6
65.2
29.6
11.0
57.8
35.6
94.0 89.0
78.0 66.0
M,,
M,,
81.6 79.4 82.4
9.0
64.4
31.2
92.0
76.0
80.6
11.8
62.8
32.8
90.0
70.0
80.3
9.9
63.0
32.0
90.0
72.0
81.0
12.0
60.8
34.0
90.0
70.0
1
10
0.2
36
_
1.5
80.6
12.2
56.8
40.8
90.0
70.0
W.B. B.C.No.
1
44 20
0.2
-
79.2
14.0
60.8
36.0
90.0
68.0
Table 9
Effect of crushing fineness and moisture level on bulk density of stamped charges and coke strength (Jamadoba washed coal)
Jam.
Screen analysis (%) Fineness aimed at (% <3.2mm)
-0.21 mm
Moisture (%)
Bulk density of cake (kg rne3)
M,,
M,,
15.6 15.3 19.0 19.0 16.0 16.1
27.0 26.8 28.0 30.9 23.9 24.3
10.2 10.5 12.0 12.2 15.0 15.0
1297 1268 1303 1303 1303 1303
80.0 79.2 79.0 78.0 77.6 78.4
8.4 8.4 8.8 8.8 9.0 8.8
88.1 88.5 89.8 89.3 89.5 88.3
16.0 15.9 18.1 18.2 18.3 17.2
32.3 32.2 35.9 33.5 34.2 33.0
10.5 10.2 12.0 12.0 14.5 15.2
1326 1326 1320 1326 1326 1326
82.2 81.8 81.6 81.2 80.0 80.8
8.0 8.0 8.0 8.2 8.4 8.2
94.6 94.5 94.8 94.7 95.6 95.4
18.7 18.8 18.6 18.7 17.7 17.7
42.2 42.1 41.3 41.0 46.2 44.3
10.2 10.2 12.2 11.5 15.5 15.2
1351 1357 1326 1326 1326 1326
82.0 82.4 82.8 82.4 82.0 82.0
7.6 7.6 8.0 8.0 8.2 8.2
-3.2mm
-0.42 +0.21 mm
80
80.1 79.1 80.4 80.8 78.8 79.3
90
95
LDO, the addition of 1.5% tar-anthracene oil mixture improved the coke quality by a minimum of 2 points in the M r0 index, irrespective of the proportion of blendable coal, while the M,, index was improved by at least 1 point. The Irsid and the Japanese drum indices showed similar improvements in coke strength. In eight similar tests with full-scale ovens (Table 8), the ovens were charged with blends containing 10 or 20% B.C. No. 1 or 7 mixed with the 45 Jam.:55 W.B. coal mixture. The results clearly showed that there was an improvement of more than 2 points in the M 1o values of cokes from tar-treated coal blends, with similar improvements in the other drum indices also. The pilot oven coke strength indices did not match numerically those from the large ovens, presumably an effect of scale difference; nevertheless, the results fully confirmed the findings from the pilot oven regarding the relative improvement in coke strength due to tar addition. The coke strength results for the full battery trial showed that the daily average M,, value of coke in the fortnight before tar spraying was started was 10.2, while
1424
FUEL,
1988,
Vol 67, October
Micum indices of coke
during the tar addition it fell to 8.8. The M,, value thus improved by 1.4 points on account of tar mixture spraying, while the improvement in the M,, index was only 0.6 points. In general, the trial agreed well with the pilot oven and full-scale oven tests conducted earlier. Stamp charging Bulk density. The principal factor in recent techniques for improving coke strength is increase in the bulk density of the charge, a factor which is influenced markedly by the stamping energy required for compaction as well as other parameters such as size distribution and addition of moisture, oil, tar, etc. The improvement in the bulk density of coke with increasing crushing fineness at a level of 10-12x moisture, following stamp charging, may be seen in Table 9. It is often pointed out that the density relative to stamped cake is not comparable with the bulk density of conventional top charges, since the oven volume is not completely filled by the stamped cake. To ensure smooth charging the cake is approximately 3 mm smaller than the
Response
I
of high ash Indian
I
Cloals
to precarbonization
I
-0.2% L. D.O. + --d.25%~AR+0.2S”/( ANTHRACiZNE OIL
73
205
h 2: n
15 f r 2 r
techniques:
A. Chatterjee
and H. N. Prasad
Stamping of a particular coal, rather than a coal blend, resulted in cokes with M,, values improved by 2 to 4 points. The improvement was, as expected, more marked in the case of poor grades of coal. Typical results of the effect of addition of blendable coals to the base coal
82
----
I
I CONVENTIONAL CHARGING STAMP CHARGING
81-
-16
7115
-lL
IO
------eML0
--------?
i?i n
45-s 43:
-;112 % i
41 g
- 11
3gz n 37 z
CY
H
1:
/
72
70
50 fYL”rn
;8 n’f5 w -2 i6 WX i4 s2
7L
0 10 20 308 % ADDITION OF BLENDABLE COAL No. 1 TO THE BASE COAL MIX ( L5 JAM : 55 W. B. 1
Figure 10 Effect ofaddition ofblendablecoal after stamping (average values from repeated
ww
82
aQz
81 -
Zn
Q-
----
I
No. 1 on coke properties tests)
I CONVENTIONAL CHARGING STAMP CHARGING
-16
..a I 80~ ’ 50 10 20 30 ADDITION OF BLENDABLE COAL No.1 TO THE BASE COAL MIX, ( O/o) Figure 9 Typical effect of tar versus LDO addition (average values from repeated tests)
on coke strength
clear width of the oven. Since in most cases the cake maintains its shape in the oven, this gap remains until the beginning of the plastic expansion, and is gradually filled during coking, depending on the properties of the coal. Thus, the density relative to the stamped cake is more informative. Coke characteristics. The qualities were assessed of cokes produced from stamped cakes of single coals (Jam., W.B. and B.C. No. 9) and of blends of B.C. No. 1,5,7 or 9 with 45 Jam :55 W.B. mixture. The optimum proportion of blendable coal to be used in any-ternary-blend was ascertained in the same way as for preheating and briquette blending.
0 10 20 36 % ADDITION OF BLENDABLE COAL No. 5 TO THE BASE COAL MIX (L5 JAM : 55 W. B. ) Figure 11 Effect ofaddition ofblendable coal No. 5 on coke properties after stamping (average values from repeated tests)
FUEL, 1988, Vol 67, October
1425
Response of high ash Indian coals to precarbonization techniques: A. Chatterjee and H. /V. Prasad
mixture are shown in Figures 10 and 11. The results indicated that stamping permitted the use of 10, 20, 30 and 40% respectively of B.C. No. 1, 5, 7 and 9 in admixture with Jamadoba and West Bokaro coals, while with conventional charging only 10% of B.C. No. 7 and less than 10% of other blendable coals could be used. Based on the findings on briquette blending, the possibility of coupling briquetting and stamping was examined. This was attempted in the pilot oven by adding blendable coal briquettes to the binary coking coal mixture in tine form and stamping the total briquette blended charge; the M 1o index was found to be improved by 1 to 1.5 points in all cases compared to briquette blending alone, the extent of the improvement depending on the characteristics of the blendable coal used for briquetting. Thus, a combination of briquetting and stamping appears possible. Lask and Petak3’ have reported that blends of coals without dilation properties and with a swelling index of <2 can be converted into good quality coke by stamping of the preheated coal charge. The improved quality of coke as a result of stamping may also account for an increased mean size of the coke produced from stamped charges compared with conventional charges. It was found that irrespective of the charge composition, the mean size of coke from stamped charges was higher and breeze generation was lower, allowing an increased yield of blast furnace coke (+50 mm) from stamp charges compared with conventional charging.
marked differences in coke strength were observed irrespective of the crushing process. Sovaco process crushing improved the M 1,, values by 1 to 6 points when cokes produced from the coals/blends crushed to 80% fineness (normally obtained in a hammer mill) were compared with those from charges pulverized to 100 % fineness (normally obtained in the Sovaco process). The Sovaco process allowed the use of poorly coking blendable coals in the coking coal mixture without adversely affecting the coke strength. With increasing fineness the amount of ‘zero fines’ (below 0.2 mm) produced by hammer mill crushing increased, but at 100 y0 fineness the percentage of ‘zero lines’ generated by the Sovaco process and the hammer mill were similar, which is unusual. CONCLUSIONS
1. With increase in fineness, the quality of coke improved in terms of M,, values, but at 100 ‘A - 3.2 mm no
On the basis of the present studies, it can be concluded that high-ash Indian coals tend to respond favourably to any of the pre-carbonization techniques preheating, briquette blending, stamping or direct tar addition, in so far as improvement in coke quality is concerned. Success with selective crushing was limited, though the beneficial effects of finer crushing were demonstrated. Before choosing a particular technique for commercial exploitation, other features have also to be examined. For example, shortage of tar, often used as a fuel in developing countries such as India, may preclude adoption of tar addition, while high capital cost, as well as operational and maintenance problems, are likely to prohibit the commercial exploitation of preheating, at least in these countries. An important factor in favour of stamping, which was used in Germany earlier, and briquette blending, currently popular in Japan, could be low capital expenditure.
Table 10
crushing
Selective crushing
Some typical results are summarized in Table 10. The test results showed that:
Typical
strengths
of cokes after Hammer
Hammer
Coal/blend, coke quality, ‘zero fines’ (% < 0.2 mm), and screen analysis Jamadoba Micum
washed indices
process
Mill crushing
80
of coal charge
(% - 3.2 mm)
85
Sovaco process crushing (100x-3.2mm)
100
95
(Jam.) M,, MI,
80.0 10.2 20.0
80.2 10.6 22.1
81.0 10.2 28.2
80.8 10.2 28.0
80.5 10.0 32.6
80.7 9.9 33.7
80.2 9.0 34.9
80.5 9.1 34.7
82.2 9.4 30.0
83.2 9.2 31.7
M,, MI,
78.3 14.3 20.8
77.7 14.7 19.4
80.1 13.2 22.9
79.0 13.6 24.0
80.4 12.6 33.3
81.4 12.9 32.8
79.4 10.2 35.9
80.2 10.4 34.0
82.8 9.8 35.7
82.6 10.2 35.4
W.B.) mix 77.5 12.8 19.6
78.0 12.6 21.1
81.4 12.4 28.8
80.8 12.2 28.8
81.2 11.4 32.8
80.6 11.4 33.1
79.8 9.8 32.8
80.6 9.4 37.9
81.4 11.4 30.9
79.8 11.8 30.0
W.B.) mix. 78.8 12.7 25.4
79.0 12.8 24.2
80.8 11.0 28.3
81.0 10.6 27.9
82.6 10.6 34.4
83.0 10.2 33.1
79.4 9.8 34.3
79.6 9.8 34.7
82.9 10.0 33.7
83.0 10.1 31.5
% <0.2mm West Bokaro washed Micum indices
(W.B.)
% <0.2mm 10% B.C. No. 7,90x Micum indices
(45 Jam.55 MU MI,
% <0.2mm 10% B.C. No. 9,90x I&cum indices
(45 Jam.:55 M,, M,,.-
% <0.2mm Range of screen analysis of charge +3.2 mm -3.2 mm f1.6 mm -1.6mm +0.42mm -0.42 mm +0.2 mm -0.2 mm
1426
Mill or Sovaco
(%)
FUEL, 1988, Vol 67, October
18-20 15-17 18-23 19-27 19-25
14-15 16-19 13-17 20-26 23-29
46 1618 1418 26-33 32-34
Nil 18-21 17-21 27-33 33-38
1-2 18-23 2428 23-33 3CL36
Response Table 10
Summary
of high ash Indian
of assessment
of precarbonization
coals to precarbonization
techniques:
A. Chatterjee
and H. N. Prasad
techniques
Parameters
Conventional top charging
Briquette blending
Preheating
Stamp charging
Selective crushing
Crushing fineness ( - 3.2 mm) (“/,) Additive (%) Moisture in coal (‘7”) Bulk density of coal charge (kg m-‘) M,, M,, Carbonization time Oven throughput Maintenance requirement
78-80 0.2 LDO 68 7o(f750 I@11 77-80 100 100 100
78-80 8-10 pitch and tar
78-80 _ _ 800-850 8-9 793 1 8035 110-115 150-175
89-91 _ 8-11 1050-1150 7-9 80-8 I 110-115 112-115 110-115
98-100 0.2 LDO 6-8 650-700 9-10 78-79 100 96-98 105-l 10
750~00 8.5-9.5 7941 105-l 10 105-l 10 105
In countries such as India, briquette blending would also require an input of tar, and although it is a technologically and economically viable method in view of its simplicity and favourable impact on poor quality coals, which could be installed in any battery at any time, tar availability could be a major problem. With stamp charging, though the energy utilization is not optimum, since moisture to the extent of lo-12 % has to be added to make a strong enough cake and has to be removed later in the coking cycle, the improvement in coke properties is better and no other binder is required. Stamp charging could thus offer distinct possibilities where the maximum improvement in coke properties, with a given type of coal mixture, is required. Unlike briquette blending, stamp charging does not require added tar or pitch, and can be incorporated as soon as the stamping hardware becomes available. However, whether stamp charging can be accommodated in an existing coking plant, has to be examined in individual cases since some design changes may be necessary, possibly only justified when a battery is due for renewal. Table 11 summarizes the relative merits of the various pre-carbonization techniques.
14 15 16 17 18 19
20 21
22 23
REFERENCES Mazumdar, B. K., Chakrabarty, M. and Mukherjee, D. K. ‘Problems and prospects of coal utilisation in India’, paper presented at the 37th Annual Session of IIChE and Joint Symposium with AIChE, Delhi, India, December 1984 Stuchlite, V. Gliickauf 1970, 31 (4), 204 Flockenhans, C. and Wagener, D. Proc. AIME, Ironmaking Conf., 1981,40,49 Chatterjec, A. and Prasad, H. N. Ironmaking and Steelmaking 1983, 10(I), 1 Chatterjee, A. and Prasad, H. N. Journal Institution ofEngineers (India) 1982, 63, (MM-l), 1 Chatterjee, A., Prasad, H. N. and Dinker, M. R. Trans. Indian Insritute of Metals 198 1, 34(6), 465 Beck, K. G. and Meckel, J. F. Ironmaking and Steelmaking 1980, 7(3), 111 Munson, Jr. J. G., Mettenry, W. L., Brayton, W. E. and Shoenberyer, R. W. in ‘Coal, Coke and the Blast Furnace’, 1978, London, The Metals Society, 12&131 Graham, J. P. in ‘Coal, Coke and Blast Furnace’, 1978, London, The Metals Society, 117-123
24 25 26 27
28
29 30 31
32
Peter, V. J. and Warford, F. K. in ‘Year book of the Coke Oven Managers Association’, 1980, 134-160 Akamatsu, K., Nire, H., Miyazaki, T. and Nishoka, N. in ‘Coal, Coke and the Blast Furnace’, 1978, The Metals Society, 51-65 Nakamura,N.,Togino,Y.andAdachi,T.in’Coal,Cokeand the Blast Furnaces’, 1978, The Metals Society, 93-106 Higuchi, M., Lizuka, M., Kuroda, K., Nakayama, N. and Saito, H. in ‘Coal, Coke and the Blast Furnaces’, 1978, The Metals Society, 19-28 Nakamura, N., Noguchi, N., Matsuoka, H. et al. ‘ILAFA’ Coal Congress, Bogota, Colombia, 1979 Yoshinaya, M., Sanada, M. and Utsunomiya, M. Proc. AZME Ironmaking Converence 1976, 35,256 Chatterjee, A. and Prasad, H. N. Fuel 1983,62, 591 Brown, N. A. BHP Technical Bulletin 1979, 23(2), 59 Leibrock, K., Grab, H., Petak, H. and Harion, E. Proc. AIME lronmaking Conference 1982,41, 341 Harion, E. and Mazumdar, A. K. Proc. International Symposium on ‘Blast Furnace Ironmaking’, November 1985, Jamshedpur, India, 69 Leibrock, H., Petak, W., Harion, E. and Schlebusch, D. ILAFA Coal Congress, Bogota, Columbia, 1979 Rao, R. A. ‘Report on selective crushing of coal’, Central Research and Development Organization, Hindustan Steel Ltd, India, 1970 Prasad, H. N., Mitra, A. N. and Chakrabarty, P. K. Trans. lndian Institute of Metals 1977, 30(5), 355 Bureau of Indian Standards, Specification for Blast Furnace Coke, IS:7189, 1974 British Standards Institution, Methods for analysis and testing of coal and coke (tests special to coke), BS 1016, Part 13, 1980 Bureau of Indian Standards, Methods of test for coke-special tests, IS:1354, 1964 Elliott, M. A. (Ed.) ‘Chemistry of coal utilization’, Volume 2, John Wiley and Sons, Chichester, 1981, 945 Japanese Testing Method for Coke Strength, Japanese Industrial Standard; JISK 2151-1972, Translation by Okuyama, Y., Nippon Kokan K.K., Technical Research Centre, Kawasaki, Japan American Society for Testing and Materials, standard test method for cubic foot weight ofcrushed bituminous coal, ASTM D291-60 Perch, M. and Russel, C. C. Coke hens Blast Furnace and Steel Plant 1959, 47(6), 591 Malek, M. A. and Walsh, J. H. ‘Coke in Iron and Steel Industry’, Proc. Charleroi Conference, 1966, 261-271 Smith, F. W., Birge, G. W., Wolfson, D. E. and Reynolds, D. A. ‘Better Coke by Thermal Pre-treatment of Coal’, R 5418, US Bureau of Mines, Washington DC, 1958, 26 Lask, G. W., Petak, H. and Echterhoff, J. Metallurgical Plant and Technology 1980, 3 (5/80), 4/15
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