PREDICTING T H E EFFECTS OF HYDROCARBON INJECTION ON BLAST FURNACE OPERATION H
. E.
W. B U R N S I D E , Esso Research
and Engineerinf Co., Linden, N. J .
The results of tests of hydrocarbon injection in the blast furnace indicate that the amount of coke displaced relative to the quantity of hydrocarbon injected can vary widely. A basis for explanation of the wide variation in hydrocarbon to coke displacement ratios is developed. It is shown that there are three process paths for hydrocarbon injection, and the displacement ratio of a given tert depends on the path or combination of paths followed. An added factor which can influence the results appreciably is the effect of injection of the blast additive on gas utilization in the furnace. The principles utilized in the study of displacement ratios are applied to evaluation of eight blast additives comprising heavy fuel oil; natural gas, coke-oven gas, hydrogen, carbon monoxide, nit;ogen, oxygen, and carbon dioxide.
two or three years, the practice of injection of hydrocarbons into the blast furnace has been growing rapidly throughout the world. Many articles have been presented giving the results of natural gas and coke-oven gas injection tests ( 7 , 2, 6-8). Results are beginning to become available from operations with Bunker C fuel oil injection (5, 9, 7 7 ) . One fact which has become evident from the data presented is that the amount of coke displaced by hydrocarbon injection does not necessarily bear a constant relationship to the amount of hydrocarbon injected. In fact, a wide variation in results have been obtained. For example, the weight of coke displaced by 1000 SCF of natural gas has been claimed to be as little as 40 pounds and as much as 140 pounds. The purpose of this report is to provide a basis for a t least partial explanation of the wide variety of hydrocarbon to coke displacement ratios experienced. I n addition, a study is made of the coke displacement values of various potential blast additives. Most of the studies discussed here are based on unchanging fractional utilization of reducing gas in the furnace as hydrocarbon injection increases and coke is displaced. Insufficient data are available to allow quantitative prediction of the effect of hydrocarbons on reduction kinetics. However, in the study of displacement ratios, examples are shown of the effects of assumed changes in reducing gas utilization resulting from hydrocarbon injection. Data from tests of hydrocarbon and steam injection at the experimental furnace of the Bureau of Mines, Bruceton, Pa., serve as a guide in setting the direction of the changes in reducing gas utilization assumed. URING THE PAST
Heat Sources in a Blast Furnace
There are four major controllable sources of heat which provide the blast furnace its thermal needs. These are: Air Preheat. This represents heat in the air which is available above the temperature a t which its components leave the top of the furnace. Some of the heat in the air is lost to decomposition of part of the moisture in the air. 2
I&EC PROCESS DESIGN AND DEVELOPMENT
Coke. The formation of CO and COz and a small amount of HzO from the carbon and hydrogen in the coke normally provides the major part of the heat required by the furnace. Some of the heat thus produced is lost as sensible heat in the top gas. Also, 5 to 7% of the carbon in the coke is absorbed by the hot metal product. Hydrocarbons. When a hydrocarbon is injected at the tuyeres of a blast furnace, its conversion to C O , COS, and H20 leaving in the top gas provides heat which allows removal of coke from the burden. The net heat provided is equal to that available from formation of the top gas constituents decreased by their sensible heat and by the heat of decomposition of the hydrocarbon. Preheat i n Blast Additives. Certain blast additives, such as oxygen and steam. can be preheated along with the blast air to provide sensible heat. Other additives, such as natural gas or coke-oven gas, can be preheated separately to increase the amount injectable. The amounts of heat provided by the first three sources have been calculated on the basis of certain assumptions which are representative of average good furnace operation. The assumptions used are listed in Table I. Also shown are the compositions and heats of formation taken for the fuels, coke, Bunker C, and natural gas, which are evaluated. Table I1 shows the details of the calculations of the heat available in each case. The thermodynamic values used in Table I1 are found in Table 111. The values were obtained from smoothed correlations of the data from the American Petroleum Institute Project 44 (10). Heat from Air. The calculated result is equivalent to roughly 0.7 to 0.8 million B.t.u. (M1MB.t.u.) per ton of hot metal (THM), or about 8 to 9% of the heat requirement of the furnace. The value of incremental air preheat is quite dependent on the effect of reduced coke rate. and perhaps gas temperature gradient, on the reaction :
c + coz
=
2co
(11
or its equivalent. in the stack of the furnace. If constant hydrogen and CO utilization are assumed, the coke saving is about 21 pounds per THXl per 100' F. air preheat. However, this assumption requires a n increase in the amount of Reaction 1. If it is assumed that there is no change in the amount of
Reaction 1 and the relationship:
H e a t from Hydrocarbon Gasification. T h e calculated values for heat available from gasification of Bunker C fuel oil and natural gas amount to 46 and 41%, respectively, of the lower heating values of these fuels. T h e relationship of these values, of course, results directly from the top gas ratios (Ht/HzO and CO/COt) which have been taken as a basis. Equivalence of H e a t Sources. I t might seem. from the fuel heating values in Table 11, that about 0.83 pound of Bunker C or 0.78 pound of natural gas should always displace 1 pound of coke in the hypothetical blast furnace. However, these equivalents ignore the need for control of the RAFT. As pointed out above, if this temperature becomes too high, the furnace can have "hanging" difficulties-i.e., the burden does not move smoothly. If the temperature is too low, in addition to poor operation, heat transfer from the gas to the hot metal and coke may cease, and the furnace may tend to freeze. Therefore, injection of steam and hydrocarbons into the air entering the raceway should be guided by the calculated RAFT. Inasmuch as the hot metal and slag temperatures must usually range from 2600' to 2800' F., the coke temperature a t the raceway is generally assumed to be 2800" to 3000' F. T o provide the required high level heat, temperature of the raceway gases must be above these coke temperatures. I t is apparent that if the R A F T is to be held constant, the temperature of the air entering the raceway must be increased when steam or hydrocarbon injection is increased. The only exothermic gas reaction in the raceway is the formation of CO from hot coke or from the carbon in the cold hydrocarbon. The heats of decomposition of the steam or hydrocarbon and the additional sensible heat load must be provided by the increase in air preheat. The additional sensible heat load results from the displacement of hot coke by cold carbon of the hydrocarbon and the additional hydrogen from steam or hydrocarbon which must be heated to raceway temperature. This added heat in the air, which is necessarily made available when injecting hydrocarbons. reduces considerably the weight of hydrocarbon which must be injected to displace 1 pound of coke when holding RAFT constant. Largely because higher air temperatures are required to inject a given weight of natural gas (because of its higher heat of formation and hydrogen content) than are required for Bunker C, the displacement ratios for natural gas (pounds of gas/pound of coke) are lower than for Bunker C. For a given air tempera-
in the top gas is held constant, the calculated coke saving is about 39 pounds per THM per 100' F. air preheat. Some data in the literature (3) indicate that coke saving by increase in blast air temperature is even greater; this means that Reaction 1 is actually reduced by increased air preheat in such cases. Coke saving by increased air preheat is obtained a t the expense of increased racew'ay flame temperature (RAFT). ("Racewa)" is the term used for the combustion zone in the furnace at the point of air injection through the tuyeres. The gases formed in the raceway are assumed to be limited to N?,CO, and Ht. .411 C02 and H20 are reacted to CO and H2. The adiabatic flame temperature calculated for the rraction of the hot blast air with the combustibles in the raceway is termed R A F T in this discussion.) I n general. furnace movement can become rough or even stop entirely after the temperature gradient reaches a certain limit. This limit appears to vary with furnace burden quality and operating practice. At any rate, it is common practice to control R A F T by the addition of steam to the blast air. T h e reaction: HgO
+C
=
CO
+ HZ
(2)
is endothermic and lowers RAFT. However, because only part of the H ? formed in the raceway from Reaction 2 is utilized in reduction, the furnace must bear a n added heat load when blast moisture is increased. For example, if R A F T must be maintained constant by moisture injection when raising air temperature. coke saving is reduced from 21 to 3.9 pounds per THM per 100' F. air preheat for the constant top-gas ratios case. Heat from Coke Gasification. Calculation of the heat contributed by coke gasification in the furnace shows that about 43% of the oxygen utilized in fuel gasification comes from reduction of the burden components which appear in the hot metal. The remainder, of course, comes from the oxygen and decomposition of part of the H20 in the air. There is evidence in some cases that chemically bonded moisture in the burden enters into the reactions; however, this reaction is not considered in these calculations. The heat calculated for coke gasification amounts to about 51% of the heating value of the coke.
Table 1. Ore:
Hypothetical Blast Furnace Operating Data no COz evolution in the furnace
Fully preAuxed and calcined)
To) Gas ( a t Dry Burden Level) Temp. = 400' F. Ratios: Hz/HzO = 1.5 = 407, H2 utilization CO/COz = 2.0 = 337'0 CO utilization Air
Preheat temp. = 1150" F. Moisture content = 7.0 grains/SCF (dry) Calculated RAFT* = 3400' F.
Fuels Coke Bunker C
a
Temp., O F . 100 250 Natural gas 100 RAFT = raceway adiabatic flame temperature.
Formulab
Carbon, Wt.
B.t.u./Lb. Atom Carbone Heat formation Sensible (700" F . ) 0 0 - 935 -7,000 0 -29,500
70
90.0 85.4 Mol. wt. = 17.6 Ignoring S, ash, etc.
c
Negative aalues are exothermic.
VOL. 2
NO.
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JANUARY
1963
3
~
Table II. Basis:
Heat Contributed by Sources in Blast Furnace 0 from 02 in air Other 0 for fuel garificotion:
from air moisture reduction from ore and slag reduction Total 0
Heat Source Components
Lb. moles
Sensible (400'F.)
= 1 Ib.
atom
= 0.0302 = 0.776 = 1.8062 Ib. atom
Enthalpy," B.t.u./Lb. Mole Formation (700' F.)
.Vet Heat B.t.u. llb. fuel (or SCF)
B.t.u
t
Air
0 ( l 1 5 O o + 400OF.) Y?
0.5 1.881 0,0503
H20
0.j151' 0.8991 0.4495 0.0121 0,0081
H? H20
Bunker C gasificationf 0 2 ( 4 0 0 ° + 100" F.)
0.51jle 0.7372 0,3686 0.4976 0,3317 1.1058
co coo
Natural gas gasification0 0 9 (4OOO-t 100" F.)
...
- 5870 - 5497 - 6694
- 21 55
2102 2937 2088 2456
-5,800 -5.497 56,187
62,k1
-2,155 -45,418 - 166.360 2,088 -101.636
- 47, i 2 0
- 169.297
- 104,092
-935
-2,155 -45,418 - 166: 360 2,088 - 101,636 6,065
7.000
0,5151" 0,5769 0.2884 0.9787 0.6524 0,8653
-2,935 - 10,340 2,826 -10,449 -1,110 -40,835 -74 779 25 - 823 - 117,522
-6530
-1,110 -33,482 -61:320 1,039 -33.713 6>707 -121,879
-7837
-2,155 -45,418 -166: 360 2,088 - 101,636 29 500
-1,110 -26$202 - 47,978 -2,044 -66,307 29,500 25 526 -8335( -386) - 114,027 a Thermodynamic values from Table III. Product of lb. moles and net enthalpy. .VegatiLe values are exothermic. Allows f o r f a c t that net air moisture reduction is 607& 7.8062 0 1.3486 CHo.oa = 0.8997 CO 4- 0.4495 C0z 0.0727 H? 0.0081 H 2 0 . e Assumes the 0 f r o m ore and slag enters the blast furnace at 700" F. f 7.8062 0 -I- 1.7058 CHI.&= 0.7372 CO 0.3686 CO? 0.4976 H 2 0.3317 H20. 0 7.8062 0 0.8653 CHa.7, = 0.5769 C O 0.2884 COz 0.9787 H P 0.6524 HZO. CO
~
+
+
+
++
+
U
"
u 0
I 8oo
1 r
600
4
\ AIR MOISTURE G RA INS/S.C.F. WAR IES)
---STEAM
TEMPERATURES = RAFT
,
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
I
+-
+
~
+
+
ture, the higher the RAFT which is desired. the smaller the amount of steam or hydrocarbon which can be injected. In most cases it is desirable, from the standpoint of economy, to operate the blast furnace with the highest air temperature and lowest air moisture content consistent with operability. This usually results in the highest capacity and lowest coke rate when operating without hydrocarbon injection and should form the base case from which hydrocarbon injection is evaluated. For the purpose of this study, it is assumed that the maximum operable R A F T in a hypothetical furnace is 3400" F. when operating without hydrocarbon injection. One of the most important observed results of hydrocarbon injection is an improvement in smoothness of furnace operation and a consequent extension of the maximum RAFT. For rhis reason. in this study the operable R A F T with hydrocarbon injection is assumed to range not only 400' F. below, but 400' F. above the base of 3400' F. Hydrocarbon injection tests with appropriate adjustment of burden have shown that blast furnace operation with variation in RAFT over several hundred degrees Fahrenheit can be experienced tvithout affecting hot metal temperature or quality. Figure 1 gives the correlations between coke requirement and air temperature for operation without hydrocarbon injection a t 3400' F. RAFT and also for natural gas and Bunker C injection a t 3000', 3400°, and 3800" F. RAFT. The assumed bases are those given previously in Table I. Figure 1 sho\vs the increased
VOL. 2
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JANUARl
1963
5
coke displacement obtainable with Bunker C relative to that by natural gas. The coke requirement curves for the two fuels intersect, of course, at the 7 grains per SCF air moisture point of the coke-only curve, or “steam line,” for the R A F T involved. I t is evident that, if high temperature air is available and if the furnace can operate with low RAFT, a large fraction of the coke can be displaced, particularly by Bunker C injection. The question naturally arises: What is the amount of hydrocarbon required to accomplish the coke displacement? The following discussion provides some guidance toward the answer.
Table IV.
RAFT, F. 3000 3200 3400 3600 3800
V. Hydrocarbon/Coke Displacement Ratios at
Table
Coke Displacement Ratios
Hydrocarbon/Coke Displacement Ratios at Constant RAFT and Air Moisture Path 7 Displacement Ratio Bunker C ~VaturalGas Lb. coke/ Lb. coke/ DR bbl. DR MSCF 0.617 555 0,476 94.1 0.625 548 0,481 92.8 0.631 543 0.487 91.7 0.637 538 0.491 90.9 0.642 533 0.495 90.0
Constant RAFT and Air Temperature
No Effect of Hydrocarbon Injection on Top Gas Characteristics. The coke in a blast furnace can be displaced by hydrocarbons along three different process paths. The d splacement ratios (pounds of hydrocarbon/pound coke) (DR’s) along these paths have different values. The DR’s evaluated by steel industry tests often involve a combination of displacement paths. T h e DR’s obtained, therefore, vary to a n extent depending upon the amount of each path involved. Because of these and other factors brought out below, there appears to be confusion regarding interpretation of DR’s obtained in hydrocarbon injection tests. The following table gives the process characteristics of the three paths for coke displacement by hydrocarbons :
7 Constant Variable Constant
RAFT Air temperature Air moisture
Path 2 Constant Constant Variable
3
Variable Constant Constant
With reducing gas utilization and top gas temperature held constant, the DR’s by path 1 are independent of air temperature but increase slightly with RAFT. Values calculated for this study are given in Table IV. Also given are the equivalent weights of coke displaced per unit volume of oil or gas. Path 2 might be considered the most desirable and logical approach to injection of hydrocarbons. The furnace is first
Path 2 Displacement Ratio, at R A F T , a F. Bunker C Natural Gas
Air Temp., F. 900 1200 1500 1600 1700 1850 2000 Coke lb. /bbl. lb.’MSCF
3000
3400
3800
0.663
3000
3400
3800
0,534
0.544
0,685 0.668
0.539 0.676
0.543 0.685 0.690 0.687
0.680 -496-5
0.547
0.540 0.556 0.552
17 -80-84-
brought to its maximum operable air temperature and R A F T using steam injection to control RAFT. Hydrocarbon is then gradually substituted for steam. R A F T is thus maintained while coke is displaced from the burden. The displacement ratios obtained on the bases of this study appear to vary only slightly with RAFT and air temperature, as shown in Table V.
0.9
0.8
1
3000°F.
-
3200’F.
I
I
I
I
I
0.7-
340OOF.
0’51
BUNKER C INJECTION BASE = STEAM LINE AT RAFT = 3400’F.
0.4
800-
0.3
1
600
0.2
-
J
g V
~
-OPERABLE
---
li’
INOPERABLE TEMPS, = RAFT
I
I
TEMPERATURES = RAFT
400 -
-OPERABLE
- --
0.1 -
1200
1400
1600
1800
2000
The basis chosen is important in evaluation of
DR’s 6
I
I&EC PROCESS DESIGN AND DEVELOPMENT
I
I ,
d
I I
I I
-
I I
I
BLAST AIR TEMPERATURE,’F.
Figure 2.
I
I
INOPERABLE 0
1000
I I
I
I
1
I
The third path, that of constant air temperature and fixed moisture content with R A F T variable, gives displacement ratios independent of air temperature and RAFT. The values calculated are as follows : Hydrocarbon Natural Gas Bunker C
DR 0.78 0.83
= =
Coke 57 lb./MSCF 410 lb./bbl.
,4s might have been expected, the above values are the same as the fuel equivalents calculated previously. EVALUATION O F TESTDATA. The above discussion shows why consideration of test data on hydrocarbon injection must obviously take into account the basis upon which the evaluation is made. A wide range of DR’s can result from a combination of displacement paths. Reference to Figure 2 will make this clear. Figure 2 is a duplication of the steam line and Bunker C curves of Figure 1. The hydrocarbon lines of constant R A F T and air moisture are GH, AF, A’I. Line AB is the base steam line a t 3400’ F. R A F T ; air moisture increases from 7.0 a t point A to about 38 grains per SCF a t 2000’ F. air temperature. Line AA’ represents the decrease in coke which would be obtained by increasing air temperature only a t constant air moisture content. The line A’D would represent operation with coke and steam a t 3800’ F. RAFT. However, AA‘ and AID represent inoperable conditions under the assumptions of this study. Part of the hydrocarbon line for 3800’ F. RAFT, section A’E, is also inoperable. A certain minimum quantity of hydrocarbon, represented by the point E, must be injected to provide operability at this high RAFT. As was pointed out previously, the possibility of operating with a higher R A F T when injecting hydrocarbons than when operating with only steam injection is one of the important indications found in hydrocarbon injection tests. From the discussions in the previous section and from consideration of the meaning of the areas of Figure 2, the following tabulation can be made of displacement ratios along a constant air temperature path: Section of Path in Figure 2 B to C c to E E to F to H
D R , Lb. Hydrocarbon/Lb. Coke Bunker C ‘Vatural gas 0
0
0,685-0.690
0.540-0.556
0.83
0.78
I t is apparent from Figure 2 that the average D R determined between certain points, such as B and E, depends on the air temperature; this sets the amount of coke, C-E, displaced by hydrocarbon substitution for steam relative to that, B-C, displaced only because of a decrease of air moisture content and accompanied by increase in RAFT. The average D R for point E can theoretically range from 0 to a large fraction of the D R between C and E, although values near 0 would necessarily lie within the region taken to be inoperable. On the other hand, DR’s for points a t the same R A F T as the base steam line, such as B and F or C and E, do not involve a partial displacement of coke by reducing air moisture (and increasing R.4FT) without hydrocarbon injection. Therefore, there is little effect of air temperature on these DR’s. In the case of coke displacement by hydrocarbons a t RAFT’s below that of the base steam line, the direction of the effect of air temperature on displacement ratio is the opposite of that for RAFT’s above the base. This results from the fact that the
steam line for the lower R A F T would lie above the base steam line (AB). Thus, essentially, some hydrocarbon injection is involved in lowering coke requirement to that of the base steam line. Inasmuch as D R is taken relative to the base line, no credit for this part of the coke displacement is taken. Therefore, particularly a t low air temperatures where coke displacement is low, D R rises considerably. Figure 3 shows the relationship between average DR’s (for Bunker C) and R A F T and air temperature for air moisture content of 7 grains per SCF. The extreme range of displacement ratios, particularl). for RAFT’s above the base 3400’ F., is apparent. Similar relationships are obtained for natural gas. Eflects of Variation in Top Gas Characteristics. T h e results of tests in the Bureau of Mines experimental blast furnace indicate the direction of the effects of hydrocarbon injection on top gas composition. Operation with hydrocarbon injection a t higher R A F T than in the base operation appears to improve reducing gas utilization. I n other words, C O to CO1 ratios are lower and more coke is displaced than would be calculated for unchanged ratios. The opposite appears to result when R A F T is lower than in the base operation. Also, top gas temperature tends to be lower for the high R A F T operation and higher for low R.4FT. relative to base temperature. These effects act in the same direction as R A F T on coke displacement ratios, and the combination of eflects extends the variation in DR’s beyond that discussed in the previous section and illustrated in Figure 3. The following comparison gives examples of the effects of assumed variations in top gas characteristics as related to changes in R A F T and air temperature. Changes in both HZ to HzO and C O to COz ratios are taken. The resulting DR’s in column A are compared with DR’s. in column B, obtained when no changes in top gas characteristics are assumed.
RAFT, ’ F. 3800 3000
Air Temp., a F. 2000
1500
Top Gas Characteristics Lb. Bunker C/ Temp., Lb. Coke ’ F. Hz/HzOCO/COz A B 300 1.0 1.8 0,404 0.515 600 2.0 2.2 0.930 0.762
Thus, a blast furnace operator has the opportunity for economical coke displacement, even when coke is cheap, by operation a t high RAFT. On the other hand, where coke is expensive relative to other fuels, he may save more money by displacement of larger amounts of coke by operation at low RAFT. Blast Furnace Capacity and Top Gas B.t.u.
No Effect of Additives on Top Gas Characteristics. T h e effects of hydrocarbon injection on furnace capacity, as measured by air requirement, are shown in Figure 4. The top gas heating values are plotted in Figure 5. These correlations show that in the case where hydrocarbon injection has no effect on reducing gas utilization or top gas temperature, the following conclusions regarding hydrocarbon injection are indicated : At constant RAFT, a n optimum air temperature is found for maximum capacity. This optimum is a t a lower air temperature for natural gas than for Bunker C. Decreasing RAFT, a t a given air temperature, by increasing hydrocarbon injection increases capacity in the case of Bunker C but decreases it when injecting natural gas. Figure 5 shows that the heating values of top gas, B.t.u./ SCF, are essentially identical for natural gas and Bunker C. The increase with air temperature is slightly less than that for VOL. 2
NO. 1 J A N U A R Y 1963
7
1.05
~
Table VI.
Commercial Furnace Operating Data Base Operation Coke, Ib./THM = 1,294 Air, SCF/THM = 62,317 Temp., O F. = 1,563 HpO, grains/SCF = 18.38
-BUNKER C ---NATURAL GAS NUMBERS = RAFT, OF,
1.04
Top Gas Temp., F. Ha/H20 CO/CO? Calculated RAFT, F. Heat loss, B.t.u./THM
1.03 0 I-
% n 0
:: 1 02 z 0 E 0 LL
Table VII.
1.01
2
+ 4
400 1 5 2.013 = 3,458 = 975,350
=
-
Fuels and Blast Additives Used to Study Blast Additive Values wt.
w K
1.00
0.98 11
I
I
I
I
I
1200
1400
1600
1800
2000
B L A S T AIR TEMP.,OF.
Figure 4. At a given air temperature, injection of hydrocarbon tends to produce opposite effects on capacity when Bunker C or natural gas i s used
Bunker C fuel 85.4 10.9 0.5 2.5 0.3 0.1 0.3 100.0 8.105 18,430 -7,697
Coke 89.0
Analysis for
0.99
7%
C H 0 S N Ash H?O Total Lb./gal. HHV,a B.t.u./lb. Heat of formati0n.b B.t.u./atom C
...
0.6 0.6 0.6 7.0 2.2 100.0 12,614
...
Vol.
Yo Coke oven gas 27.8 3.4
Natural gas 94.45
...
CdHio H2
... ... ... ...
co
CO? N?
a
w
e
Higher heating value.
b
hlegative values are exothermic.
I
1200
I
1400
I
1600
1800
Top Gas Characteristics Temp., H ? / C O / F. H ? O COz 400 1 . 5 2.0 2.2 600 2.0 3800 2000 400 1.5 2.0 1.8 300 1.0 a Relative to value obtained for base top air temperature and R A F T . Temperatures, F. Blast RAFT air 3000 1500
2000
BLAST AIR TEMPERATURE,"F.
Figure 5. Injection of Bunker C and natural gas increases top gas heating value but tends to lower B.t.u./THM 8
56.5 7.3 1.9 3.1 100.0 -23,804
steam injection. Under the assumed top gas ratios, the top gas heat content. B.t.u./THM, is slightly greater for natural gas than for Bunker C. The values fall with increasing air temperature and RAFT. Effect of Variation in Top Gas Characteristics. The magnitude and/or direction of the effects of the variables of Figures 4 and 5 can be revised considerably by changes in reducing gas utilization \vhich may result from hydrocarbon injection. The following comparison shows the calculated effects of decrease in gas utilization at RAFT below base and increase at R.4FT above base. respectively, when injecting Bunker C fuel:
90
1000
...
100.0 Heat of formation,* - 30,570 B.t.u./atom C Other blast additives: pure HzO, 0 2 , H,, CO, X?,Con.
BUNKER C A N D NATURAL C A S NUMBERS - RAFT, 'F.
0.801
... ...
4.21 1.14 0.20
I & E C P R O C E S S DESIGN A N D D E V E L O P M E N T
Top Gas Heating Value RelaRelattvea tivea Furnace B.t.u./ B.t.u./ Capacity S C F THM 1.00 1.000 9 9 . 2 0,909 102.6 1.14 I .OO 1.000 96.4 1,053 9 2 . 1 0.90 gas characteristics at the same
. :0 00 0 3
0--N
mr-mLn 0
.% r.300
m. O. 0 . Q .
8 :
N O - 0 - N
x-0
m .w m. m. ,. 0. 30
mr0 0
'
O -
,:
Ln
c",*Or?
m. m. r .- 0. Or-ocr-
z. 0
:
N u -- N 0
-Om-
r-mor-
Nu-* . . . .
-
0 0 00 e x
uxx0 N O 3 N
r--00 . . . O--d
8 : 0 . X
u
0
CI
d
6 3 - m * .* o . r.0 0 hl
.
=omm N O -3N0
: .
, XCC 'c)
0 0
3
: .
NU,N3 3 X d N
N
. . . .
0 0 0 0
Lnu3m
m,
3
N C 0 X
3
Ll ri
n
Q h
0
N 0
: .
s
UxNu
u. -. 0.x . N5oflrci
Fr-NC
3 N
3
L d
a
VOL. 2
NO.
1
JANUARY
1963
9
The magnitude of the effects of additives on top gas characteristics may differ from furnace to furnace depending on coke and ore qualities. Much commercial data are required for development of a method for more definitive predictions; however, the present prediction methods have been found to be excellent for guiding and analyzing hydrocarbon injection operations in commercial blast furnaces.
Table X.
$M M / Y e a r / 2 0 0 0
Blast .4dditiue
Bunker C Bunker C with 0 2 Natural gas 100 a F. gas 100' F. eas with 0. 1200 FPgas 1800 F. cracked gas Coke oven gas
Evaluation of Blast Additives
I t is informative to compare the effects of various potential blast additives on the coke requirement and capacity of a blast furnace. A simplifying assumption which can be applied in making such a study is that the additives have no effect o n the percentage utilization of reducing gas in the furnace or on top gas temperature. Holding R A F T constant in such a study tends to decrease the degree of potential effects on top gas characteristics, but the possibility of unusual effects remains. Data obtained from tests of hydrocarbon injection indicate that this basis is adequate for guidance of such tests. However, insufficient data are available a t present to determine what basis is valid for prediction of the effects of variations in oxygen concentration in the blast air, as well as the effects of unusual additives such as hydrogen in large quantities. I n spite of these reservations, it is still of interest to consider the relative value of blast additives determined on the basis of the assumption that their use does not change top gas characteristics relative to the base coke-only operation. I n Table VI are given the pertinent furnace operating data upon which this study is based. Table VI1 gives the characteristics of the blast furnace coke and of the Bunker C, natural gas. and coke-oven gas used as blast additives. Other additives studied are pure H 2 0 , oxygen, hydrogen, CO, nitrogen, and COZ. In addition, the effects of preheating and of cracking natural gas to C Hz are evaluated. Table VI11 shows the results of computer calculations on the effects of all of the above additives. I n these calculations it is assumed that more air preheat has become available and blast air temperature can be increased from the 1563' F. of the actual furnace data (Table VI) to 1800' F., and R A F T is held constant at 3458' F. by injection of the various blast
+
Table IX.
Break-even Values of Blast Additives Calculated for Two Coke Prices (Dollars per Ton) Furnace J'apacity placed, Increase,b - ..
Blast Additive
Bunker C with 0 z c 100" F. nat. with 0 2 1200'F. nat. gas 1800'F. cracked
Value" 70 20 $/Bbl. 2 . 4 2 4.92 1 . 5 2 3 80 kr //MS. - - - - C. - - F. 40.4 82.3 15.3 50.6 40.6 83.1 42.8 88.6
Vah" 70 20
%
70
d/.ZI.MB.t.u.d 40 8 8 3 . 0 2 8 . 4 2 5 . 7 64.0 4 3 . 2
3.1 3 4 . 68
85.7 52.7 86.5 92.3
1.4 30.26 3.5 28.04
42.1 15.9 42.3 44.5
17.9 25.7 20.8 45.0
4.0 19.5 39.2 28.7 57.7 16.8 9.78 11.3 22.4 41.2 8 1 . 8 17.4 10.8' 12 1 23 1 3 7 . 6 7 1 . 9 1 6 . 0 CO 2.1 6.0 , . . ... 1 0 . 7 13.2* N? 0.3 0.3 . . . ... 0.0 - 0 . 8 CO? Value relative to base operation zeith steam injection at 1800' F. air temp.; heat from combustion of surplus top gas taken at 20d/MMB.t.u. b Assuming furnace air rate can be held constant. Air enriched to 26%; Lower heating ualue basis. e In these cases O? priced at $8.00/ton. capacity increase may be limited to much lower aalues by pressure drop or dust loss. See Table VIZZ.
Cokeovengas
H,
~~
10
I&EC
PROCESS DESIGN A N D DEVELOPMENT
Annual Blast Furnace Fuel Cost Savings Calculated for Two Coke Prices (Dollars per ton)
a
Assumed additive prices at furnace: Bunker C h'atural gas and coke oven gas 0 2
Top gas credit
Tons/
D a y of Pig Irona 70 20 -0.04 1.29 -0.87 1.16
0.14 -0.63 0.16 0 42 0.03
0.98 0.58 1.14 2.54 0.82
$2.50/bbl. 35klMMB.t.u. B8/ton 20$/MMB.t.u.
additives. Table VI11 includes the potential increases in furnace pig iron capacity as limited by each of three possible critical gas volumes, blast air, top gas, or raceway gas. Using the results in Table V I I I , the unit values of the blast additives used were calculated from the cost of the coke displaced and the change in surplus top gas heating value. I t should be understood that all evaluations were made relative to the calculated operation with steam injection a t the air temperature of 1800' F. and not relative to the lower air temperature operation of Table VI. This is felt to represent the proper basis, inasmuch as the use of steam to control R A F T is standard furnace practice and would have been used in the furnace of Table V I if the 1800' F. air temperature had been available. Table I X summarizes the break-even values determined for the blast additives for coke costs of $10 and $20/ton. Also shown are the percentage of the coke displaced and the increase in furnace capacity a t constant air rate. Table X gives example? of annual fuel savings in a 2000-ton-per-day blast furnace for the more important blast additives when the latter are given a certain set of reasonable prices. The following ccnclusions can be drawn from Tables I X and X. LVhen coke is $10 per ton, fuel oil and natural gas must be priced below $2.40 per barrel and 40 cents MMB.t.u. to be of interest. When coke is priced as high as $20 per ton, almost all additives should be of interest. I t should be noted that for the examples given in Table X, the relative advantage of the various additives can change markedly with coke price. If used only as a means of increasing coke displacement, oxygen at $8.00 per ton results in a lower value for blast additives. However, in certain situations the increased pig iron capacity obtainable with oxygen may make its use of considerable value. Preheating natural gas appears to be justifiable if equipment costs are small. If natural gas can be cracked to carbon and hydrogen and fed to the blast furnace at 1800' F., a large increase in coke displacement and in the amount of gas injectable is obtained relative to uncracked natural gas. Coke oven gas is not as valuable as natural gas, even on a heating value basis. Hvdrogen and CO have slightly less value than natural gas on a heating value basis and would be of interest as blast furnace additives only if available at a price close to fuel value, except where coke is costlv. Seither nitrogen nor COS have appreciable value as blast additives \+Then it is assumed their injection has no effect on top gas ratios. Preheated nitrogen would tend to decrease the air requirement, but blast furnace pressure drop or dust loss limitations could eliminate even this advantage.
Conclusion
T h e considerations of this report, although based on certain limiting assumptions, point the way to explanations of the wide range of coke displacement ratios obtained in blast furnaces by the use of blast additives. T h e potential improvements in blast furnace economics can be affected appreciably by the influence of the chosen furnace operating conditions on the effect of the additives. I t is hoped that the economic value of obtaining much more commercial data on the effect of additives, particularly a t limiting furnace conditions, is emphasized by this discussion. literature Cited
(5) Knepper, W. A., Woolf, P. L., Sanders, Am. Iron & Steel Inst. Meeting, Chicago, Ill., September 1961. (6) Kobrin, C. L., Iron Age 187, 107-9 (Feb. 9, 1961). (7) Negomir, J. M., Pearson, E. F., Assoc. Iron & Steel Engrs. Meeting, Cleveland, Ohio, September 1960. (8) Ostrowski, E. J., Melcher, N. B., Kesler, G. J., J . Metals 13, 25-30 (January 1961). (9) Rombough, LV. R., AIME Blast Furnace, Coke Oven, and Raw Materials Conference, Philadelphia, Pa., 1961. (10) Rossini, F. D., Pitzer, K. S., .4rnett. R. L., Braun, R. M., Pimentel, G. C., “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds,” Comprising the Tables of the American Petroleum Institute Research Project No. 44 (extant as of Dec. 31, 1952), pp. 464, 557-610, Carnegie Press, Pittsburgh, Pa., 1953. (11) Taylor. H. C., Rombough, W. R., .4nn. Joint Meeting, Eastern and [Vestern States Blast Furnace and Coke Oven Assoc., Pittsburgh, Pa., No\ ember 1961.
(1) AIME Blast Furnace, Coke Oven, and Ra\\ hlaterials Conf., panel discussion, Philadelphia, Pa., April 1961, AZ,ME Proc. 20,
RECEIVED for review April 5, 1962 A C C E P T E D November 9, 1962
540-604 (1961). (2) Baily, T. F., Iron Age 184, 104-5 (July 16, 1959). ( 3 ) Burnside, H. E. W., Esso Research and Engineering Co., Linden, N. J., unpublished commercial blast furnace data. (4) “Chemical Engineers’ Handbook,” J. H. Perry. Ed., 3rd ed.,
p. 220, McGraw-Hill, h’ew York, 1950.
Symposium on Process Metallurgy, Division of Industrial and Engineering Chemistry, 141st Meeting, ACS, Washington, D. C., March 1962.
APPLICATION OF HEAT-TRANSFER PRINCIPLES T O A METALLURGICAL PROCESS PROBLEM Relationsh$ of Ladle Preheating t o Temperature Losses W.
M . D A N V E R , J.
K. M c C A U L E Y , A N D F . C. L A N G E N B E R G
Crucible Steel Co. of America, Pittsburgh 73, Pa. The chemical engineer plays an important role in process research and development activities in the steel industry. The use of material and energy balances, the concepts of unit operations, and the principles of heat and mass transfer are being applied to an increasing number of metallurgical process problems. This paper presents a simple example of the application of heat-transfer principles to a metallurgical process problem. The conclusions are applicable to many other high-temperature heat-transfer studies.
all the steel made in the United States is in open-hearth furnaces, electric furnaces, or oxygen converters. The metal is removed or tapped from the furnace into a refractory-lined ladle. When the ladle is filled, it is transported to the pouring-pit platform and the metal is poured or teemed into molds. The quality of steel is strongly influenced by the temperature of the liquid metal entering the ingot molds. Investigators have shown that the as-cast qrain size is related to the pouring or teeming temperature; transverse ingot cracks have been traced to heats poured too rapidly a t high temperature; and ingots poured cold often exhibit shell or double skin. Production yields also suffer when the metal cools excessively in the ladle. I n such cases. part of the molten metal freezes in the ladle, and the resulting skulls represent lost production and increased operating cost. Therefore, the temperature losses during tapping. holding the ladle, and teeming must be carefully controlled. This requires accurate temperature measurements in the metal in the range of 2600’ to 3200” F. and a knowledge of the heat losses betlreen the furnace and molds. RACTICALLY
p melted
Heat loss Calculation
The calculation described here \vas undertaken to obtain a relationship between ladle preheating and steel temperature drop. Heat is lost from steel during tapping, holding, and teeming by the three mechanism-radiation, convection, and conduction. Radiation and convection occur a t the exposed liquid surface? and conduction takes place at the ladle brickmetal interface. The exact values of these heat losses are difficult to calculate; holiever, it is possible to show the relative importance of the individual mechanisms and the effect of ladle preheating on them. Radiation and convection losses subtract heat from steel during tapping, but neither is a function of ladle preheating. The resulting steel temperature drop from these sources is comparative1)- small for large quantities of steel. -4fter tapping, the slzg or oxide layer, which is present on liquid steel, tends to minimize the temperature drop created by radiation and convection losses. I n fact, most of the heat loss from the dark slag cover is supplied by the fusion and sensible heat of this material and not the steel. VOL. 2
NO.
1
JANUARY
1963
11
