Method for operating blast furnace

A method for operating a blast furnace, characterized by: adding a prescribed volume of blast furnace gas to an air for blast fed to a hot stove; oxidizing, in the hot stove, CO contained in the blast furnace gas into CO.sub.2 and H.sub.2 contained therein into H.sub.2 O to prepare a blast containing CO.sub.2 and H.sub.2 O and additionally heated by the above-mentioned oxidation; blowing the resultant blast containing CO.sub.2 and H.sub.2 O into a blast furnace through tuyeres thereof; thereby controlling the calorific value at the portions around the tuyere exits as a result of the endothermic effect produced by the reactions at the portions around the tuyere exits of the CO.sub.2 and H.sub.2 O contained in the blast with red-hot coke in the blast furnace wherein the amount of the blast is substantially equal to that of a blast without addition of a blast furnace gas.

FIELD OF THE INVENTION 
The present invention relates to a method for operating a blast furnace, 
which permits maintenance of stable furnace conditions, prevention of 
production costs of pig iron from increasing, and efficient operation of 
the blast furnace. 
BACKGROUND OF THE INVENTION 
In operating a blast furnace, a lower-rate operation may sometimes be 
carried out which comprises decreasing the production from that in the 
usual operation. Such a lower-rate operation can be conducted by 
decreasing the volume of blast blown as a reducing gas into the blast 
furnace through the tuyeres thereof. In other words, a decreased volume of 
air blast fed to the blast furnace leads to a decelerated reduction 
reactions of iron ore, thus resulting in a decreased production of pig 
iron. 
However, when the volume of reducing gas flowing through the blast furnace 
decreases to below a certain limit through the above-mentioned decrease in 
the volume of blast, the flow of the blast in the blast furnace becomes 
non-uniform in the cross-sectional direction of the furnace. More 
particularly, the distribution of burden raw materials such as iron ore 
and coke charged into the blast furnace is not uniform, but there are 
portions rich and poor in voids between pieces of burden raw materials. 
Therefore, when the volume of blast flowing in the blast furnace decreases 
to below a certain limit, the blast passes through only the portions rich 
in voids between pieces of burden raw materials, and not through the 
portions poor in voids between pieces of burden raw materials. As a 
result, the reduction reactions of iron ore in the blast furnace cannot 
proceed uniformly, resulting in the furnace conditions becoming unstable 
and production of deposits onto the furnace wall. Such deposits onto the 
furnace wall, if produced, causes partial stagnation of the burden descent 
through the blast furnace in the form of the phenomenon known as hanging, 
which in turn causes slips in which broken hanging results in sudden 
descent of the burden. Consequently, the descending speed of burden 
through the blast furnace becomes non-uniform and the furnace conditions 
are extremely deteriorated. 
A lower-rate operation of pig iron in a blast furnace is possible also by 
discontinuing blowing of the above-mentioned blast to be blown through the 
tuyeres into the blast furnace periodically for a prescribed period of 
time. However, when blowing of blast to be blown into the blast furnace is 
discontinued, the burden in the furnace is not heated during the blowoff. 
The interior of the blast furnace is therefore cooled, and this may result 
in the impossibility of discharging pig iron and slag from the blast 
furnace. Blowoff also causes unstable furnace conditions. 
With a view to solving the above-mentioned problems, the following methods 
are known, which comprises, in a method for operating a blast furnace, 
adding to air for blast to be blown into the blast furnace nitrogen gas as 
the inert gas not contributing to the reduction of iron ore: 
(1) a method for operating a blast furnace, disclosed in Japanese Patent 
Publication No. 19,481/72 filed June 3, 1972, which comprises: 
to a blast to be blown into tuyeres of a blast furnace, adding nitrogen gas 
in an amount of up to 40 vol. % of said blast; and, 
(2) a method for blowing N.sub.2 gas into a blast furnace, disclosed in 
Japanese Patent Provisional Publication No. 76,104/78 filed July 6, 1978, 
which comprises: 
directing N.sub.2 gas containing at least 3% O.sub.2 discharged from an 
oxygen generating plant into a blast main connected to the inlet of a 
blower for a blast furnace, and blowing said N.sub.2 gas into the blast 
furnace, together with a blast. 
According to the above-mentioned methods (1) and (2), by blowing a blast 
enriched with nitrogen gas in a prescribed volume per unit period of time 
into the blast furnace through the tuyeres, the reduction reactions of 
iron ore are decelerated even the volume of blast blown into the blast 
furnace is the same as the volume of blast in an operation not applied 
with a lower-rate production. As a result, this method allows a decrease 
in pig iron production while preventing furnace conditions from becoming 
unstable. 
The above-mentioned methods (1) and (2) are however disadvantages in that 
they invalve higher production costs of pig iron because of the high price 
of nitrogen gas to be added to the air for blast. These methods are also 
disadvantageous in that the increase in the nitrogen content in the blast 
largely reduces the calorific value of blast furnace gas recovered from 
the top thereof. Since the aforementioned blast furnace gas is utilized as 
a fuel for heating furnaces in an iron works, the decrease in the 
calorific value of blast furnace gas adversely affects the operation of 
the iron works as a whole. 
It has been the usual practice, on the other hand, in the operation of a 
blast furnace, to inject an auxiliary fuel such as heavy oil as a fuel and 
a reducing agent, through the tuyeres, together with the blast, with a 
view to reducing the consumption of coke per ton of produced pig iron (the 
coke rate). Recently, however, petroleum oils such as heavy oil are in 
short supply and prices thereof are only increasing. It is therefore 
becoming more common not to inject such an auxiliary fuel as heavy oil as 
mentioned above as fuel and reducing agent in the operation of a blast 
furnace, but employ coke only. 
In the conventional operation with injection of an auxiliary fuel such as 
heavy oil, the auxiliary fuel blown through the tuyeres is decomposed at 
the portion near the tuyere noses into such reducing gases as carbon 
monoxide and hydrogen which reduce iron ore. The decomposing reaction of 
the above-mentioned auxiliary fuel such as heavy oil at the portion near 
the tuyere exits, being an endothermic reaction, causes decrease in the 
temperature at the portion in the blast furnace near the tuyere exits. In 
the conventional blast furnace operation, therefore, the portion around 
the tuyere exits is kept at an appropriate temperature by the temperature 
decreasing effect of the above-mentioned endothermic reaction. However, in 
the operation based on coke only without employing an auxiliary fuel such 
as heavy oil as mentioned above as fuel and reducing agent, an endothermic 
reaction does not take place at the portion around the tuyeres as that 
caused by an auxiliary fuel such as heavy oil as mentioned above. The 
temperature of the portion around the tuyere exits therefore becomes 
excessively high, resulting in unstable furnace conditions. The furnace 
conditions becoming unstable under the effect of temperature rise at the 
portion around the tuyere exits are attributable to the fact that the ash 
in the coke burnt in the portion around the tuyere exits causes production 
of slag rich in silicon oxide (SiO.sub.2) which floats up in a gaseous 
form and condenses at the bosh of the furnace, thus impairing gas 
permeability through the blast furnace. 
Therefore, when carrying out the operation based on coke only without using 
an auxiliary fuel such as heavy oil, it is necessary to reduce the 
temperature of the portion around the tuyere exits in the blast furnace. 
For this purpose, the following methods have been applied to reduce the 
temperature of the portion around the tuyere exits: 
(1) decreasing the temperature of the blast to be blown through the tuyeres 
into the blast furnace; and, 
(2) adding steam to the blast to be blown through the tuyeres into the 
blast furnace, thereby causing an endothermic reaction between steam and 
coke at the portion around the tuyere exits, and decreasing the 
temperature of the portion around the tuyere exits under the effect of 
this endothermic reaction. 
In the above-mentioned methods, however, the calorific value for reducing 
iron ore in the blast furnace is decreased either by the reduction of the 
blast temperature or by the endothermic reaction caused by steam at the 
portion around the tuyere exits. It becomes therefore necessary to 
compensate the above-mentioned decrease in the calorific value by 
increasing the quantity of coke charged into the blast furnace. When 
carrying out the operation based on coke only without using an auxiliary 
fuel such as heavy oil, therefore, there would be such difficulties as a 
considerable increase in the coke consumption per ton of pig iron (coke 
rate) and the rise in production costs of pig iron. 
Even when operation is conducted by injecting an auxiliary fuel such as 
heavy oil, an operation carried out with the quantity of injected 
auxiliary fuel decreased as compared with that in an ordinary operation 
involves the same problems as in the above-mentioned operation without 
using an auxiliary fuel. 
For these reasons, there has been a keen demand for the development of a 
method for operating a blast furnace, which does not lead to deteriorated 
furnace conditions, permits prevention of the iron production costs from 
increasing, and does not cause decrease in the calorific value of blast 
furnace gas recovered from the furnace top, when conducting a lower-rate 
operation of pig iron, or an operation based on coke only without using an 
auxiliary fuel such as heavy oil as fuel and reducing agent, or an 
operation with a decreased quantity of such an auxiliary fuel as heavy 
oil. However, such a method for operating a blast furnace is not as yet 
proposed. 
SUMMARY OF THE INVENTION 
Another object of the present invention is to provide a method for 
operating a blast furnace, which, when conducting a lower-rate operation, 
prevents the iron production costs from increasing, and does not result in 
a decrease in the calorific value of blast furnace gas recovered from the 
furnace top. 
An object of the present invention is to provide a method for lower-rate 
operation a blast furnace, which, when conducting an operation of a blast 
furnace based on coke only as fuel and reducing agent without using an 
auxiliary fuel such as heavy oil as fuel and reducing agent, does not lead 
to deteriorated furnace conditions. 
Another object of the present invention is to provide a method for 
lower-rate operation of a blast furnace, which does not result in a 
decrease in the calorific value of blast furnace gas recovered from the 
furnace top. 
In accordance with one of the features of the present invention, there is 
provided a method for operating a blast furnace, which comprises: 
adding a prescribed volume of blast furnace gas to air for blast which is 
fed to a hot stove; and oxidizing, in said hot stove, CO contained in said 
blast furnace gas into CO.sub.2 and H.sub.2 contained therein, into 
H.sub.2 O to prepare a blast containing CO.sub.2 and H.sub.2 O; and then 
blowing said blast containing CO.sub.2 and H.sub.2 O and additionally 
heated by the above-mentioned oxidation into a blast furnace through the 
tuyeres thereof; and controlling the calorific value at the portion around 
the tuyere exits under the endothermic effect produced from the reaction 
at said portion around the tuyere exits of CO.sub.2 and H.sub.2 O 
contained in said blast with red-hot coke in the blast furnace.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
From the above-mentioned point of view, we carried out extensive studies to 
solve the aforementioned problems encountered when conducting a lower-rate 
operation, or an operation based on coke only without causing an auxiliary 
fuel such as heavy oil as fuel and reducing agent, or an operation with a 
consumption of said auxiliary fuel such as heavy oil decreased as compared 
with that in an ordinary operation. As a result, we have successfully 
developed a method for operating a blast furnace, which comprises: 
adding a prescribed volume of blast furnace gas to an air for blast fed to 
a hot stove; and oxidizing, in said hot stove, CO contained in said blast 
furnace gas into CO.sub.2 and H.sub.2 contained therein, into H.sub.2 O to 
prepare a blast containing CO.sub.2 and H.sub.2 O; and then blowing said 
blast containing CO.sub.2 and H.sub.2 O and additionally heated by the 
above-mentioned oxidation into a blast furnace through the tuyeres 
thereof; and controlling the calorific value at the portion around the 
tuyere exits under the endothermic effect produced from the reaction at 
said portion around the tuyere exits of CO.sub.2 and H.sub.2 O contained 
in said blast with red-hot coke in the blast furnace. 
Now, the method for operating a blast furnace of the present invention is 
described with reference to examples of the lower-rate operation. 
When conducting a lower-rate operation by the method of the present 
invention, blast furnace gas recovered from the top of a blast furnace is 
added in a prescribed ratio to an air for blast before being fed into a 
hot stove to be heated therein. The air for blast thus containing blast 
furnace gas is fed into the hot stove to be heated therein. Table 1 covers 
an example of the chemical composition of blast furnace gas. 
TABLE 1 
______________________________________ 
N.sub.2 CO CO.sub.2 H.sub.2 
H.sub.2 O 
______________________________________ 
53.5 wt. % 
21.5 wt. % 
22.5 wt. % 2.5 wt. % 
20 g/Nm.sup.3 
______________________________________ 
By adding a blast furnace gas having the chemical composition mentioned 
above to air for blast prior to heating in a hot stove, the air for blast 
contains CO, CO.sub.2 and H.sub.2 in the composition thereof. This air for 
blast containing CO, CO.sub.2 and H.sub.2 is fed to the hot stove and 
heated to a prescribed temperature, CO and H.sub.2 react with O.sub.2 so 
that CO is oxidized into CO.sub.2, and H.sub.2, into H.sub.2 O. The air 
for blast is thus converted into a blast containing CO.sub.2 and H.sub.2 O 
and additionally heated by the above-mentioned oxidation. When the blast 
thus heated to a prescribed temperature in the hot stove and containing 
CO.sub.2 and H.sub.2 O is blown through the tuyeres into a blast furnace 
in a prescribed volume per unit time, the blast blown into the blast 
furnace reacts with red-hot coke in the blast furnace at the portion 
around the tuyere exits, so that CO.sub.2 in the blast is converted into 
CO, and H.sub.2 O in the blast, into H.sub.2. These reactions cause an 
endothermic effect at the portion around the tuyere exits. 
Table 2 shows changes in the chemical composition and in the volume 
(Nm.sup.3) at the inlet and the exit of the hot stove and at the portions 
around the tuyere exits in the blast furnace, of the conventional air for 
blast and a blast (hereinafter referred to as the "1% BF gas-containing 
blast") prepared by feeding a gas for blast comprising 99 vol.% air for 
blast, and 1 vol.% blast furnace gas to a hot stove and converting CO 
contained in said blast furnace gas into CO.sub.2 and H.sub.2, into 
H.sub.2 O in the hot stove. 
In Table 2, the column "A" covers the chemical composition and the volume 
per Nm.sup.3 by constituents of the conventional blast at the inlet of a 
hot stove; and the column "A'", the chemical composition and the volume 
per Nm.sup.3 by constituents of the 1% BF gas-containing blast at the 
inlet of a hot stove. The volume (Nm.sup.3) of the conventional blast, at 
the hot stove exit (the column "B") and at the tuyere exits of the blast 
furnace (the column "C"), is expressed in the ratio to the volume of the 
blast at the inlet of the hot stove given in said column "A". The volume 
(Nm.sup.3) of the 1% BF gas-containing blast, at the hot stove exit (the 
column "B'") and at the tuyere exits of the blast furnace (the column 
"C'"), is expressed in the ratio to the volume of the blast at the inlet 
of the hot stove given in said column "A". 
TABLE 2 
______________________________________ 
1% BF 
Conventional blast gas-containing blast 
"A" "B" "C" "A'" "B'" "C'" 
at hot at hot at at hot at hot 
at 
stove stove tuyere stove stove tuyere 
inlet exit exits inlet exit exits 
______________________________________ 
Nm.sup.3 Nm.sup.3 
N.sub.2 0.7841 0.7841 0.7841 
0.7815 0.7815 
0.7815 
Nm.sup.3 Nm.sup.3 
O.sub.2 0.2084 0.2084 -- 0.2063 0.2052 
-- 
Nm.sup.3 
CO -- -- 0.4243 
0.0021 -- 0.4269 
Nm.sup.3 
CO.sub.2 
-- -- -- 0.0022 0.0043 
-- 
Nm.sup.3 
H.sub.2 -- -- 0.0075 
0.0002 -- 0.0079 
Nm.sup.3 Nm.sup.3 
H.sub.2 O 
0.0075 0.0075 -- 0.0077 0.0079 
-- 
Nm.sup.3 Nm.sup.3 
Total 1.0000 1.0000 
Volumic 
ratio to 
volume of 
1.0000 1.0000 1.2159 
1.0000 0.9989 
1.2163 
blast at "a" "a'" 
hot stove 
inlet 
______________________________________ 
As is clear from Table 2 above, in the case of the conventional blast, the 
volume "a" (Nm.sup.3) at the tuyere exits is 1.2159 times the volume at 
the hot stove inlet, whereas, in the case of the 1% BF gas-containing 
blast, the volume "a'" (Nm.sup.3) at the tuyere exits is 1.2163 times the 
volume at the hot stove inlet. Therefore, when using the 1% BF 
gas-containing blast, the volume (Nm.sup.3) of the blast at the tuyere 
exits would be, as expressed in the ratio "a'/a", 1.0003 times as large as 
that in the case of the conventional blast, as is evident from the 
increasing rate of volume (Nm.sup.3). 
Table 3 shows calorific values at the tuyere exits in the operation using 
the conventional blast and in the operation using the 1% BF gas-containing 
blast as expressed in the total heat balance produced in all the relevant 
reactions. 
TABLE 3 
______________________________________ 
Calorific value 
Calorific value 
in operation 
in operation 
with convent- 
with 1% BF gas- 
ional blast 
containing blast 
(Kcal/ton) 
(Kcal/ton) 
______________________________________ 
C + 1/2 O.sub.2 = CO 
(exothermic reaction) 
+491.23 +483.69 
H.sub.2 O + C = H.sub.2 + CO 
(water gas reaction) 
-9.51 -10.02 
CO.sub.2 + C = 2CO 
(carbon dioxide reduction 
-- -7.91 
reaction) 
Heat held by blast 
+415.20 +415.20 
Calorific value in hot 
stove -- +6.40 
Total calorific value 
+896.92 +887.36 
______________________________________ 
As is evident from Table 3 given above, the calorific value at the tuyere 
exits in the blast furnace is 887.36 Kcal/ton in the case of the operation 
with the 1% BF gas-containing blast, which is smaller than 896.92 Kcal/ton 
in the case of the operation with the conventional blast. This is 
attributable to the fact that CO.sub.2 and H.sub.2 O contained in the 
blast react with red-hot coke at the tuyere exits, and the endothermic 
effect produced from the conversion of CO.sub.2 into CO and H.sub.2 O into 
H.sub.2 causes a decrease in the calorific value. 
Accordingly, in order to supply, by using the 1% BF gas-containing blast, a 
calorific value equal to that obtained with the use of the conventional 
blast when operating a blast furnace, it would be necessary, to judge from 
Table 3, to blow blast in a volume, as expressed in the ratio "b/b'", 
1.0108 times that of the conventional blast. When the 1% gas-containing 
blast is blown in a volume 1.0108 times the volume of the conventional 
blast, the volume of gases produced at the tuyere exits would be 1.0111 
times as large as that produced in the case with the conventional blast as 
shown in the following equation: 
EQU 1.0108.times.(a'/a)=1.0111 (1) 
Therefore, when the operation is conducted by blowing the 1% BF 
gas-containing blast in the same volume as the conventional blast into the 
blast furnace, the iron production can be reduced by about 1.1% as 
compared with the case with the conventional blast, as is calculated by 
the following equation (2): 
##EQU1## 
Now, the case is described in which the adding ratio of blast furnace gas 
added to the air for blast before heating in the hot stove was altered and 
the operation was conducted with the blast thus obtained. Table 4 gives 
the calorific values at the tuyere exits and the production decrease 
ratios as compared with the conventional operation, in cases respectively 
with 1 vol.% (air for blast: 99 vol.%, blast furnace gas: 1 vol.%), 3 
vol.% (air for blast: 97 vol.%, blast furnace gas: 3 vol.%), 5 vol.% (air 
for blast: 95 vol.%, blast furnace gas: 5 vol.%), and 8 vol.% (air for 
blast: 92 vol.%, blast furnace gas: 8 vol.%) blast furnace gas added to 
the air for blast before heating in the hot stove. 
TABLE 4 
______________________________________ 
Adding ratio of blast furnace 
gas to air for blast 
1 vol. % 
3 vol. % 5 vol. % 8 vol. % 
______________________________________ 
Calorific value 
at tuyere exits 
887.36 867.99 848.96 819.84 
(Kcal/ton) 
Production 
decrease ratio 
as compared with 
1.1 3.2 5.3 8.6 
conventional 
operation 
______________________________________ 
In Table 4, the production decrease ratio as compared with the conventional 
operation was calculated by comparing with the calorific value at the 
tuyere exits in the operation with the conventional blast shown in Table 
3. 
As is clear from Table 4, it is possible to increase the production 
decrease ratio according as the adding ratio of blast furnace gas to the 
air for blast is increased. 
As is evident from the description given above, when operating a blast 
furnace, it is possible to reduce the iron production without impairing 
the furnace conditions by feeding an air for blast added with blast 
furnace gas to a hot stove and oxidizing, in the hot stove, CO contained 
in the blast furnace gas into CO.sub.2 and H.sub.2 contained therein into 
H.sub.2 O to prepare a blast thus containing CO.sub.2 and H.sub.2 O and 
additionally heated by the above-mentioned oxidation in said hot stove, 
and then blowing this blast into a blast furnace in a prescribed volume 
per unit time. The above-mentioned additional heating of the blast allows 
reduction of the consumption of coke in the blast furnace, or decrease of 
the heating temperature of the blast in the hot stove. The blast furnace 
gas to be added to the air for blast, being recovered from the blast 
furnace top and used in recycle, leads to only a very slight increase in 
the iron production costs. Furthermore, in this method of operation, the 
blast furnace gas recovered from the furnace top, having a smaller 
nitrogen content than in the blast furnace gas in the conventional 
operation with a blast added with nitrogen, gives a high calorific value 
and is therefore superior in the utilization efficiency as a fuel. 
Now, the following example covers the case in which, by the method for 
operating a blast furnace of the present invention, operation is 
conducted, not with an auxiliary fuel such as heavy oil as fuel and 
reducing agent, but with coke only. 
When operation is conducted, not using an auxiliary fuel such as heavy oil, 
but using coke only, as in the lower-rate operation mentioned above, the 
method of the present invention comprises adding a prescribed volume of 
blast furnace gas to an air for blast to be fed to a hot stove, and 
oxidizing CO contained in the blast furnace gas into CO.sub.2 and H.sub.2 
contained therein into H.sub.2 O, and then blowing the blast thus 
containing CO.sub.2 and H.sub.2 O and additionally heated by the 
above-mentioned oxidation into a blast furnace through the tuyeres 
thereof. Said blast blown into the blast furnace reacts with red-hot coke 
at the portions around the tuyere exits, and thus, CO.sub.2 in the blast 
is converted into CO, and H.sub.2 O, into H.sub.2. The endothermic effect 
produced from these reactions causes a decrease in the temperature at the 
portions around the tuyere exits. The temperature of the blast blown into 
the blast furnace through the tuyeres thereof can therefore be increased 
to a higher level by the extent of the temperature decrease at the 
portions around the tuyere exits. As a result, the increase in the coke 
consumption per ton of pig iron, which is necessary in the operation with 
coke only, can be minimized. The above-mentioned additional heating of the 
blast allows reduction of the consumption of coke in the blast furnace, or 
decrease of the heating temperature of the blast in the hot stove. 
Table 5 gives operating results in the operation with injection of heavy 
oil as the auxiliary fuel, the operation based on coke only with the 
conventional blast and the operation based on coke only with the 1% BF 
gas-containing blast. 
TABLE 5 
______________________________________ 
Operation 
Operation with 
based on coke only 
oil injection 
with con- With 1% BF 
(oil rate; 
ventional gas-containing 
50 kg/ton) 
blast blast 
______________________________________ 
Coke rate 
(kg/ton) 400 480 476 
Oil rate 
(kg/ton) 50 0 0 
Blast temp. 
(.degree.C.) 
1,300 1,050 1,090 
Blast humidity 
(g/Nm.sup.3) 
10 20 20 
Combustion temp. 
at tuyere exits 
2,420 2,300 2,300 
(.degree.C.) 
______________________________________ 
As is evident from Table 5, when the operation based on coke only is 
carried out with the conventional blast, it is necessary to decrease the 
blast temperature by 250.degree. C. or by 19.3% for the reasons as 
mentioned above as compared with the operation with oil injection. To 
compensate the resulting decrease in the calorific value, the coke rate 
should be increased by 80 kg/ton or by 20%. When using the 1% BF 
gas-containing blast, in contrast, it is possible to increase the blast 
temperature by 40.degree. C. or by about 4% as compared with the operation 
using the conventional blast, and hence to decrease the coke rate by about 
0.84% as compared with the operation using the conventional blast. 
When, in the above-mentioned operation based on coke only with the use of 
the 1% BF gas-containing blast, the blast volume is made equal to the 
volume of the conventional blast, the iron production decreases as 
described in the above-mentioned lower-rate operation. Therefore, in order 
to give the same iron production as in the operation with the conventional 
blast, the blast volume should be increased over that of the conventional 
blast. 
Even when the operation with injection of an auxiliary fuel such as heavy 
oil is conducted with a decreased consumption of the auxiliary fuel, for 
example, heavy oil consumption per ton of pig iron (oil rate), the method 
of the present invention permits inhibition of the increase in the coke 
rate. More specifically, when the operation with oil injection is 
conducted with a decreased oil rate, it is necessary to increase the 
consumption of coke per ton of pig iron by an amount corresponding to the 
decrease in the oil consumption for the same reasons as in the 
above-mentioned operation based on coke only. However, when the BF 
gas-containing blast is blown into the blast furnace through the tuyeres 
in accordance with the present invention, the increase in the coke rate 
can be prevented as in the above-mentioned operation based on coke only. 
Table 6 shows operating results of operation with heavy oil injection, in 
the case with the conventional blast at an oil rate of 50 kg/ton, in the 
case with the 1% BF gas-containing blast at an oil rate of 40 kg/ton, and 
in the case with the 1% BF gas-containing blast at an oil rate of 30 
kg/ton. 
TABLE 6 
______________________________________ 
Operation 
at oil rate 
Operation at 
Operation at 
of 50 oil rate of oil rate of 
kg/ton 40 kg/ton 30 kg/ton 
(convent- 
(1% BF gas-con- 
(1% BF gas-con- 
ional blast 
taining blast) 
taining blast) 
______________________________________ 
Coke rate 
(kg/ton) 400 412 423 
Blast temp. 
(.degree.C.) 
1,300 1,300 1,300 
Blast humidity 
(g/Nm.sup.3) 
10 10 10 
Combustion temp. 
at tuyere exits 
2,420 2,457 2,494 
(.degree.C.) 
______________________________________ 
As is clear from Table 6, even in the operation with a 20% decreased oil 
rate of 40 kg/ton, when using the 1% BF gas-containing blast, the increase 
in the coke rate is only 3% over that in the ordinary operation with an 
oil rate of 50 kg/ton. Similarly, even in the operation with a 40% 
decreased oil rate of 30 kg/ton, when using the 1% BF gas-containing 
blast, the increase in the coke rate is only 5.7% over that in the 
ordinary operation with an oil rate of 50 kg/ton. In order to achieve the 
same iron production in this operation as in the operation with the 
conventional blast, it is necessary to increase the blast volume over that 
of the conventional blast, as mentioned above. 
Now, the following description covers an example of the operation based on 
coke only as mentioned above, conducted with blasts obtained with various 
adding ratios of blast furnace gas added to the air for blast before 
heating in the hot stove. Table 7 gives operating results obtained with 
the adding ratios of blast furnace gas to the air for blast of 1 vol.%, 3 
vol.%, 5 vol.% and 8 vol.% as previously described in the example of the 
lower-rate operation. 
TABLE 7 
______________________________________ 
Adding ratio of blast furnace 
gas to air for blast 
1 vol. % 
3 vol. % 5 vol. % 8 vol. % 
______________________________________ 
Coke rate 
(kg/ton) 476 474 471 464 
Oil rate 
(kg/ton) 0 0 0 0 
Blast temp. 
(.degree.C.) 
1,090 1,130 1,170 1,270 
Blast humidity 
(g/Nm.sup.3) 
20 15 10 8 
Combustion temp. 
at tuyere exits 
2,300 2,300 2,300 2,300 
(.degree.C.) 
______________________________________ 
As is evident from Table 7, it is possible to increase the degree of 
decrease in the coke rate according as the adding ratio of blast furnace 
gas to the air for blast is increased. 
As described above, when a blast containing CO.sub.2 and H.sub.2 O, which 
is obtained by adding a prescribed volume of blast furnace gas to an air 
for blast to be fed to a hot stove, is blown into a blast furnace through 
the tuyeres it is possible to conduct an economical operation without 
impairing the blast furnace conditions in a lower-rate operation, or in an 
operation based on coke only as fuel and reducing agent, or in an 
operation with a decreased consumption of an auxiliary fuel such as heavy 
oil. In this case, the volume of blast furnace gas added to the air for 
blast should preferably be up to 10 vol.% relative to the total volume of 
the air for blast and the blast furnace gas. When the volume of blast 
furnace gas added to the air for blast is over 10 vol.% relative to the 
total volume of the air for blast and the blast furnace gas, the 
combustion temperature in the hot stove becomes considerably higher, thus 
causing such problems as the breakage of bricks of the hot stove and the 
occurrence of an explosive combustion. The blast furnace gas may be added 
to the air for blast at any position before the supply of the air for 
blast to the hot stove, and may be added at an appropriate position in the 
pipe before or after the blower. However, the blast furnace gas should be 
well mixed with the air for blast prior to feeding thereof into the hot 
stove to avoid accident. 
According to the method of the present invention, as described above in 
detail, an efficient operation of a blast furnace can be conducted without 
impairing the furnace conditions, while preventing the coke rate and the 
iron production costs from increasing and while minimizing the decrease in 
the calorific value of blast furnace gas recovered from the furnace top. 
Particularly when a lower-rate operation is conducted for the adjustment 
of iron production, or when carrying out an operation based on coke only 
without using an auxiliary fuel such as heavy oil as fuel and reducing 
agent or an operation with a decreased consumption of an auxiliary fuel 
such as heavy oil to cope with the increasing oil price, industrially very 
useful effects are provided.