DESCRIPTION
RESIDUAL 5XHAU3T GAS TREATMENT METHOD AND TRSATMBNI
SYSTEM
TECHNICAL FIELD
The present invention relates to a treatment method and treatment system for safely burning and dispersing into the atmosphere any exhaust gas remaining in an exhaust gas path including an exhaust gas treatment device when suspending or stopping operation of a shaft furnace.
BACKGROUND ART
In recent years, from the viewpoint of promoting recycling of resources and environmental protection, attention is being paid to the technology for using iron-containing dust pellets etc. and/or iron scraps as a source of iron and reducing and melting them in a shaft furnace to produce pig iron. In such a shaft furnace, the reduction and melting are carried out inside the furnace simultaneously, 30 it is important for the (gas utilization rate) of gas created when burning the solid fuel charged mixed with the iron source to be controlled to an optimum value.
For example, Japanese Patent Publication (A) NO.' iO-36906 discloses the technology for conrrolling the of exhaust gas and securing the most app)ropriate (gss utilization rate) fcr reduction and melting in a method of operation of a shaft furnace reducing and melting a raw m.aterial and fuel in a shaft furnace.
In this way, ccntrol of the composition of the exhaust gas is important in operarion of a shaft furnace, however, treatment of the large amount of generated, exhaust gas (containing corabustible ingredients) is important in terms cf environmental protection and safety
in exhaust gas treatment systemS.
Normally,, during operation, a shaft furnace discharges exhaust gas coinprising 25 to 3C% CO, 2 to 3% of H2, and a balance of N2 at 30,000 to 40,000 NmVhr. The large amount of exhaust gas includes the co.iritoustible ingredient CO, so it is sucked in, cooled, and cleared of dust by an sxhaust gas treatment device, then dispersed into the atmosphere from the combustion dispersion stack.
Conventionally, v;hen dispersing the exhaust gas from the coirilDustion dispersion stack, as shown in FIG. 1, the lower portion of the coinbustion dispersion stack 1 is provided with a water seal apparatus 2 that prevents flashback X so as to secure safety in the exhaust gas treatment system. That is, during the operation of the shaft furnace, the exhaust gas sucked in, cooled, and cleared of dust by an exhaust gas treatment device (not shown) passes through an exhaust gas pipe 6, enters the water seal apparatus 2, and is dispersed, into the atraosphere from a combustion dispersion stack 1 with an opened main shutoff valve 4.
When the operation of the shaft furnace is idled or stopped, the exhaust gas remaining in the exhaust gas treatment device must be treated. The exhaust gas path between the shaft furnace and exhaust gas treatment device is closed, the minimuin necessary amount of nitrogen is fed at 1300 Nm3/hr to the exhaust gas patih at the exhaust gas treatitient device side, the nitrogen is used to dilute the combustible ingredients, then the residual exhaust gas passes through the water seal apparatus 2 and is burned and dispersed into the atmosphere from, a bypass pipeline 3 (by opening the bypass shutoff valve 5 and closing the main shutoff valve 4.) .
At this tiine, even if feeding nitrogen, sometimes air enters the exhaust gas path containing the exhaust gas treatment device and the residual exhaust gas becomes explosive. Therefore, the water seal apparatus 2 has the
function of preventing flashback even in residual exhaust gas treatraent when operation of the shaft furnace is idled or stopped.
However, a water seal apparatus is an apparatus requiring a tank with for exatrple a 5 irieter diameter and 3 xaeter height for constantly holding water and control of the water level by recirculated water. This is set at the exhaust gas path. Its constant operation, maintenance, and inspection raise the operational costs of the shaft furnace.
When treating exhaust gas containing conibustible ingredients, ensuring safety is the highest priority. However, from the viewpoint of promoting recycling of resources, an increase in salvaging costs is undesirable, so there is demand for technology which can. safely treat exhaust gas containing combustible ingredients without having to set a large sized water seal apparatus in the exhaust gas path.
DISCLOSURE OF THE INVENTION
As explained above, in treatment of residual exhaust gas when operation of a shaft furnace is idled or stopped, even if feeding nitrogen gas to the exhaust gas patn, sometimes air enters the exhaust gas treatment device and the gas in the exhaust gas path becomes explosive. Thus, the present invention has as its object to provide a treatment method and treatment system, that can safely burn and disperse into the atmosphere residual exhaust gas containing combustible ingredients even without installing a large sized water seal apparatus.
The reason why the exhaust gas remaining in an exhaust gas path containing an exhaust gas treatm.ent device bscorries explosive ia the entry of air (that is,-oxygen) into ~he exhaust gas r^ath, so the inventors engaged in intensive study regarding techniques to completely prevent the entry of air into the exhaust gas path.
As a result, they dig.':cvered that if, rather than simply feeding nitrogen into the exhaust gas path, controlling the gcs pressure in the exhaust gas r>at,h sixultaneously with nitrogen feed and ccntrolling the gas flow- rate at time of comiDustion and dispersal, it is possible to prevent flashbacks and safely disperse residual exhaust gas into the atmosphere.
The present invention was made based on the above discovery and has as its gist the following:
(1) A residual exhaust gas treatment method
treating exhaust gas containing combustible ingredients
remaining in an exhaust gas path containing an exhaust
gas treatment device when operation of a shaft furnace ts
idled or stopped, comprising
(a) closing the exhaust gas path between the shaft furnace and exhaust gas treatment device,
(b) feeding nitrogen into the exhaust gas path at the exhaust gas treatment device side,
(c) holding the gas pressure in the exhaust gas path at a positive pressure, and
(d) burning and dispersing the gas at a flow rate exceeding flashback rates of CO and H2.

(2) A residual exhaust gas treatment method as set forth in (1), wherein the exhaust gas includes CO: 25 to 30% and H3: 2 to 3%,
(3) A residual exhaust gas treatment method as'set forth in (1) or (2), wherein the exhaust gas treatment device includes a gas cooler and a dust eliminator.
(4) A residual exhaust gas treatment method as set forth in,any one of (1) to (3), wherein the gas in the exhaust gas path is cooled and cleared of dust before combustion and dispersal.
(5) A residual exhaust gas treatment method as set forth in any one of (1) to (4), wherein the flow rate exceeding the flashback rates is 5 m/sec or more.
(6) A residual exhaust gas treatment. method as set forth in any one of (1) to (5), wherein the flow rate
exceeding the flashback rates is 12 m/sec or less. (7) A residual exhaust gas treatment system treating exhaust gas containing corebustible ingradients remaining in an exhaust gas path containing an exhaust gas treatment device when operation of a shaft furnace is idled or stopped, the system provided with
(a) a cutoff valve closing the exhaust gas path between the shaft furnace and exhaust gas treatment device,
(b) a nitrogen feeding means for feeding nitrogen from near the cutoff valve to the inside of exhaust gas path at the exhaust gas treatment device side,
(c) a gas pressure regulator maintaining a gas pressure m the exhaust gas path betv;een the exhaust gas treatment device and a combustion dispersion stack at a positive pressure and
(d) a cutoff valve closing the gas passage of the combustion dispersion stack and a bypass pipeline, branching from the combustion dispersion stack and burning and dispersing the gas in the exhaust gas path.
(8} A residual exhaust gas treatment system as set forth in (7), wherein the exhaust gas includes CO: 25 to 30% and H2: 2 to 3%.
(9) A residual exhaust gas treatment, system as set forth in (7) or (S) , wherein the exhaust gas tre.atment device includes a gas coder and dust eliminator.
(10) A residual exhaust gas treatment system as set forth in any one of (7) to (9), wherein the gas in the exhaust gas path is cooled and cleared of dust before combustion and dispersal.
(11) A residual exhaust gas treatment system as set forth in any one of (7) to (11), which burns and > disperses the gas in the exhaust gas path from an. outlet of the bypass pipeline at a flow rate exceeding the flashback rates of CO and H2.
(12) A residual exhaust gas treatment system as set forth in (ll), wherein the flow rate exceeding the
flashback rates is 5 ,'n/sec or more.
(13) A residual exhaust cas treatinent system as set forth in any one of (7) to (12), wherein the flow rate exceeding the flashback rates is 12 m/sec or less.
According to the present invention, when operation of a shaft furnace is idled or stopped, the exhaust gas containing combustible ingredients remaining in an. exhaust gas path contaming an exhaust gas treatraent device can be burned and dispersed into the atmosphere safely and without fear of flashback.
BRIEF DESCRIPTION OF THE DRAWINGS
PIG. 1 is a view showing a conventional exhaust gas treatment facility provided with a water sealing apparatus -
FIG. 2 is a view showing one mode of an exhaust gas path for carrying out the present invention.
FIG. 3 is a view showing a mode of adjusting an opening degree of a regulator and maintaining a gas pressure in the exhaust gas path at a positive pressure.
FIG. 4 is a view showing a gas pressure distribution in r.he exhaust gas path when the present invention is applied.
BEST MODS FOR CARRYING OUT THE INVENTION
The present invention will be explained based on the drawings.
FIG. 2 shows one mode of an exhaust gas path for carrying out the present invention. On a base 12, a cooler 8, gas suction dust eliminator 9, and mist separator 10 are arranged while connected by an exhaus-gas pipe 6. At the time of operation of the shaft furnace, the exhaust gas generated at the shaft furnace (not shewn) is introduced by the exhaust gas pipe 6 fixed on a support base 13 to the cooler S and is cooled by being sprayed by water inside the cooler 3 .
The cooled exhaust gas is next sucked up and clsared
of dust by the gas suction dust eliminator 9 and cleared of mist (fine water droplets) by the nist separator 10, then, passes through the exhaust gas pipe 6 leading to a coiribustion dispersion stack 1, is burned at the outlet of the combustion dispersion stack 1 (refer to flame 14) and is dispersed in the atmosphere.
At the corabustion dispersion stack 1, a main stutoff valve 4 for opening and closing the gas passage is attached. At the time of norraal operation, is made wide open. Further, the combustion dispersion stack 1 is provided with a bypass pipeline 3 that branches from its middle. This is used at the time of exhaust gas treatment when the operation of the shaft furnace is idled or stopped. At the bypass pipeline 3, a bypass shutoff valve for opening and closing the pipeline is attached. A the time of normal operation, it is fully closed.
Exhaust gas generared at a shaft furnace nor.mally contains CO: 25 to 301; and H2: 2 to 3%. If air (oxygen) enters, the gas becomes explosive, however, the gas suction dust eliminator 9 is subjected to suction by a suction apparatus (not shown), so the exhaust gas quickly passes through the exhaust gas path without retention and is dispersed from the coro.bu3tion dispersion stack. That is, no particular problem occurs in exhaust gas treatment in normal operation of the shaft furnace.
The exhaust gas pipe 6 introducing the exhaust gas to the cooler 8 is provided with a cutoff valve 7 which is actuated when the shaft furnace is idled or stopped. Further, immediately after the cutoff valve 7 is actuated, the exhaust gas pipe 6 is connected with a nitrogen feed pipe 11 of a nitrogen feeding means for feeding nitrogen.
When the operation of the shaft furnace is idled or stopped, upon an order from a control apparatus (not shown), the cutoff valve 7 closes the exhaust gas pipe 6 and the niirogen feed pipe 11 begins to feed nitrogen to the inside of the exhaust gas pipe 6- At this tirrie, simultaneously, the gas suction dust eliminator 9
continues to operate. The opening degree of the regulator arranged at the upper part of the mist separator 10 is adjusted to hold the gas pressure inside the exhaust gas path containing the exhaust gas treatment device at a positive pressure exceeding 1 atmosphere.
In the present invention, when the operation of the shaft furnace is idled or stopped, the suction apparatus applying suction to the gas suction dust eliminator 9 is operated and the opening degree of the regulator arranged at the upper part of the misr. separator 10 is adjusted to maintain the gas pressuce inside the exhaust gas path containing "he exhaust gas treatment device at a positive pressure, so entry of air into the exhaust gas path can be prevented. This point is the characterizing feature of the present invention,
In general, it is known that if maintaining the pressure inside a device at a positive pressure, intrusion of outside air into the device can be prevented. However, in an exhaust gas path that contains an exhaust gas treatment device treating a large amount of exhaust gas of 30,000 to 40, 000 Nm3/hr at time of normal operation and further feeding nitrogen at 1,330 Nm3/hr to dilute the combustible exhaust gas at time of idling or stopping of operation, technical ingenuicy is necessary to prevent air intrusion and secure safety throughout all of the exhaust gas path by holding the gas pressure at a positive pressure exceeding the 1 atraosphere and by maintaining the flew rate, which is dependent on the gas pressure, at a required range of flow rats at the outlet with the combustion dispersion stack.
FIG. 3 schematically shows one mode of adjusting the opening degree of the regulator and holding the gas pressure in the exhaust gas path containing the exhaust gas treatment device at a positive pressure. The top of the mist separator 10 is provided with three furnace top pressure regulators 15a and one in-path pressure
regulator 15'c.
At the tine of normal operation of the shaft furnace, the in-path pressure regulator 15b is closed. The opening degrees of the three furnace top pressure regulators 15a are appropriately adjusted for the large amount of exhaust gas of 30,000 to 40,000 Nm3/hr to regulate the furnace top pressure. Normally, the opening degrees of the three furnace top pressure regulators 15a are adjusted so that the furnace top pressure becomes 0.
Even when idling or stopping operation of the shaft fiirnace operation, the suction apparatus applying suction to the gas suction dust eliminator 9 continues to be operated, then all three furnace top pressure regulators 15a are closed to adjust the opening degree of the in-path pressure regulator 15b,
At the same tirae that the operation, is idled or stopped, the cutoff valve 7 is actuated and 1,300 Nm3/hr of nitrogen begins to be fed from the niirogen feed pipe 11 to the exhaust gas pipe 6. With a feed of 1,3G0 Nir.'/hr of nitrogen, the furnace top pressure regulator 15a regulating the pressure of the large amount of exhaust gas becomes ineffective in pressure regulation action. Therefore, nitrogen is fed to the exhaust gas pipe 6, all of the three furnace top pressure regulators 15a are closed fully, and the opening degree of the in-path pressure regulator 15b of a small diameter suitable for the feed of 1,300 Nm3/hr of nitrogen is adjusted in order tc generate a large positive pressure inside the mist separator 10 and maintain the gas pressure inside the exhaust gas path upstream of the mist separator 10 at a positive pressure.
If the gas pressure in the exhaust gas path becomes too high, conversely the residual exhaust gas will leak outside, so the gas pressure is naade a range where residual exhaust gas does not leak outside.
In the stat with the gas pressure inside the exhaust gas path maintained at a positive pressure, the residual
exhaust gas with r.he ccmbustible ingrediem: dilvited by nitrogen passes through the in~path pressure regulator 15t and flows into the exhaust gas pipe 6 leading tc the cormbustion dispersion stack 1. However^ the inflow amount is inevitably smaller compared with the flow amount during normal operation, so the main shutoff valve 4 of the combstion dispersion stack 1 is closed, the bypass shutoff valve of the bypass pipe 3 is opened, and the residual exhaust gas is dispersed through the bypass pipe 3 from the outlet of the comoustion dispersion stack 1.
Here, FIG. 4 shows an example of the gas pressure distribution in the exhaust gas path when the present invention is applied.
PI tc ?2 in FIG. 4 correspond to ?I to P2 in FIG. 3. P2 is the position of rhe mist separator. In the present invention, the opening degrees of the regulators arranged at nhe top of the mist separator are adjusted. Here, a high positive pressure is formed to maintain the gas pressure inside ths exhaust gas path upstream of the mist separator at a positive pressure.
As shown in FIG. 4, if the pressure formed at the mist separator {P2) is low, a positive pressure cannot be maintained at the inside of the cooler (P1) and a.ir enters the cooler.
It will be understood that since a positive pressure state is maintained even at the position P1 of the cooler 8, the gas pressure inside the exhaust gas path concaining the exhaust gas treatment device is maintained at a positive pressure.
When the residual exhaust gas is dispersed from the combusticn dispersion stack 1, naturally the gas pressure becomes the atmospheric pressure, however, at this time, in order to prevent flashbacks due to the comjDUStible ingredients {CO and H2) in the residual exhaust gas, it is important that the gas flow rate at the time of residual exhaust gas dispersal be made a flow rate exceeding the flashback rates of CO and H2.
In the present invention, the gas flow rate at the time of residual exhaust gas dispersal is made a flow rate exceeding tne CO and H2 flashback rates, so there is no need to install a large water seal apparatus at the coiri-iusrion dispersion stack.
To more reliably prevent flashbacks, the gas flow rate at the time of residual exhaust gas dispersal is preferably 5 m/sec or more.
Raising the gas flow rate at the time of residual exhaust gas dispersal is preferable in that it prevents flashbacks, however, if raised too high, soraetimes the flame blows out, so a rate of 12 m/sec or less is preferable.
EMBODIMENT
Next, an embodiment of the present invention will be explained, however, the conditions of the embodiment are examples of conditions used to confirm the workability and advantageous effects of the present invention. The present invenoion is not limited to these examples ot conditions. The present invention can use various conditions so long as it dees not stray from ths gist of the present invention and achieves r.he object of the present invention.
(EiTibodim.ent 1)
In the exhaust gas path shown in FIG. 2, at the !time of idling of operation of the shaft furnace, the cutoff valve was actuated and nitrogen fed into the exhaust gas pipe at 1,300 Nm^/hr and the furnace top pressure regulators of the mist separator were closed and the opening degree of the in-path pressure regulator appropriately adjusted to hold the gas pressure in the ro.ist separator at 4 3 hPa.
In the cooler, 5 hPa could be secured and antry of air into the gas patn containing the exhaust gas treatment device could be prevented. The residual exhaust gas was dispersed through the bypass pipeline at a flow
rate of 8 m/sec from the outlet of the combustion dispersion stack into the atmcsphare.
During residual exhaust gas treatment, this was continued, whereby the gas pressure of the gas path cculd be maintained at a positive pressure and the gas could be dispersed from the combustion dispersion stack at a flow rate exceeding the flashback rates of the combustible ingredients, so no flashbacks occurred.
INDUSTRIAL APPLICABILITY
As explained above, according to tha present invention, when the operation of a shaft furnace is idled or stopped, the exhaust gas containing combustible ingredients remaining in the exhaust gas path containing a exhaust gas treatment device can be safely burned and dispersed into the atmosphere without fear of flashbacks. Therefore, the present invention has great applicability in secondary industries operating furnaces that release exhaust gas containing combustible ingredients.

CLAIMS
1. A residual exhaust gas treatment method
treating exhaust gas conta.in.ing combustible ingredients
remaining in an exhaust gas path containing an exhaust-
gas treatment device when operation of a shaft furnace is
idlad or stopped, comprising
(a) closing the exhaust gas path between the shaft furnace- and exhaust gas treatment device,
(b) feeding nitrogen into the exhaust gas path at the exhaust gas treatment device side,
(c) holding the gas pressure in the exhaust gas path at a positive pressure, and
(d) burning and dispersing the gas at a flow rats exceeding flashback rates of CO and H2.

2. A residual exhaust gas treatment method as set forth in claim 1, wherein the exhaust gas includes CO: 25 to 30% and H2: 2 to 3%.
3. A residual exhaust gas treatment method as set forth in claim 1 or 2, wherein the exhaust gas treatment device includes a gas cooler and a dust eliminator.
4. A residual exhaust gas treatment method as set forth in any one of claims I to 3, wherein the gas in the exhaust gas path is cooled and cleared of dust before combustion and dispersal.
5. A residual exhaust gas treatment method as set forth in any one of claims 1 to 4, wherein the flow irate exceeding the. flashback rates is 5 ro/sec or more.
6. A residual exhaust gas treatment method as set forth in any one of claims 1 to 5, wherein the flow rate exceeding the flashback rates is 12 m/sec or less.
7 . A residual exhaust gas treatment system, treating exhaust gas containing combustible ingredients remaining in an exhaust gas path containing an exhaust gas treatment device when operation of a shaft furnace is idleci or stopped, the system provided with
(a) a cutoff valve closing the exhaust gas path between the shaft furnace and exhaust cas treatment
device,
(b) a nitrogen feeding means for feeding nitrogen from near the cutoff valve to the inside of exhaust gas path at the exhaust gas treatment device side,
(c) a gas pressure regulator maintaining a gas pressure in the exhaust gas path between the exhaust gas treatment device and a combustion dispersion stack at a positive pressure, and
(d) a cutoff valve closing the gas passage of the combustion dispersion stack and a bypass pipeline branching from the combustion dispersion stack and burning and dispersing the gas in the exhaust gas path.

8. A residual exhaust gas treatment system as set forth in claim 7, wherein the exhaust gas includes CO: 25 to 30% and H2: 2 to 3%.
9. A rasidual exhaust gas treatment system as set forth in claim 7 or 8, wherein the exhaust gas treatment device includes a gas cooler and dust eliminator.
10. A residual exhaust gas treatment system as set forth in any one of claims 7 to 9, wherein the gas in the exhaust gas path is cooled and cleared of dust: before combustion and dispersal.
11. A residual exhaust gas treatment system as set forth in any one of claims 7 to 11, which burns and disperses the gas in the exhaust gas path from an outlet of the bypass pipeline at a flow rate exceeding the i flashback rates of CO and H2.
12. A residual exhaust gas treatment system as set forth in. claim 11, wherein the flow rate exceeding the flashback rates is 5 ra/sec or mere.
13. A residual exhaust gas treatment system as set forth in any one of claims 7 to 12, wherein the flow rate exceeding the flashback rates is 12 m/sec or less.