Specialized metallurgical processes – compositions for use therei – Processes – Process control responsive to sensed condition
Reexamination Certificate
2000-05-11
2002-11-12
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Process control responsive to sensed condition
C075S414000, C075S460000, C266S047000, C266S081000, C266S085000, C266S182000
Reexamination Certificate
active
06478846
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device for injection of reducing agents into a shaft furnace, in particular for injection of pulverised coal into a blast furnace during production of pig iron.
To save high-quality reducing agents such as coke in the production of liquid metals in shaft furnaces, a portion of these reducing agents can be replaced by pulverised coal. The pulverised coal is obtained from raw coal in a preparation plant. The raw coal is crushed and dried and subsequently stored temporarily in coal silos. For introduction into the shaft furnace the temporarily stored pulverised coal is loosened, pressurised and injected into the shaft furnace pneumatically by a carrier gas via conveying lines. Injection is generally effected by several injection lances, which terminate in the blast tuyeres of the shaft furnace, so that introduction takes place simultaneously at various points of the shaft-furnace.
To achieve the largest possible saving of reducing agent costs by the injection of pulverised coal, the injected pulverised coal must be converted as completely as possible in the blast tuyere air duct, so that only residual coke need be gasified in the eddy zone. The term “complete conversion” here means that all carbon atoms combine with oxygen, carbon monoxide and/or carbon dioxide being formed. If the conversion in this zone is not complete, which may be the case in particular at high injection rates, conversion residues are concentrated in the shaft furnace, which leads to unstable furnace conditions.
The important causes of the defective pulverised coal conversion in the eddy zone lie firstly in the small dimensions of the actual reaction space and secondly in the high speeds of the media and flow properties of the hot blast air in the tuyere. Within the short time consequently available for the complete conversion of the coal particles at the lance outlet the pulverised coal flow emerging from the lance must be mixed with the hot-blast air, the individual pulverised coal particles must be heated to such an extent that their ignition temperature is achieved,. Released pyrolysis gases must be mixed with the available oxygen, ignite and the solid residue after conclusion of the pyrolysis must enter into an oxidation reaction with any still free or bonded oxygen.
To improve the reaction conditions for the conversion of pulverised coal in the injection zone and thus accelerate the reaction kinetics of the conversion, various measures have been proposed, e.g. increase of the oxygen concentration in the hot blast air, the local increase of the oxygen concentration by simultaneous injection of oxygen via coaxial lances or double lances or the minimisation of the pulverised coal outlet pulse at the tip of the lance by enlargement of the outlet cross-section. Although these measures, some of which are already used, whereas others are only in the testing or optimisation phase, bring about a certain improvement in the reaction conditions, the acceleration of the pulverised coal conversion which is achieved by these measures has proved to be still inadequate.
SUMMARY OF THE INVENTION
Consequently, the task of the present invention is to propose a method and device for injection of coal into a shaft furnace which substantially accelerate the reaction kinetics of pulverised coal conversion, so that the latter begins immediately after the injection and is essentially concluded on entry into the eddy zone.
According to the invention this problem is solved by a method for injection of reducing agents into a shaft furnace according to claim
1
and by a device according to claim
6
.
In the method according to the invention, in which a reducing agent such as pulverised coal is conveyed in a pneumatic flow to the blast furnace, the conveying flow is divided into several partial flows, which are led through a heating device, the reducing agent in the individual partial flows being heated inside the heating device by a heat supply. The individual partial flows are subsequently preferably combined into a common conveying flow again for homogenisation of the temperature conditions prior to a possible apportionment. of the conveying flow to the individual injection lances distributed around the shaft furnace.
By heating the pulverised coal inside the heating device heat can be fed to the pulverised coal in a controlled manner, so that its temperature can be set to a value which is favourable for use of the pulverised coal conversion in the shaft furnace. Consequently the pulverised coal preheated in this way needs to absorb significantly less heat from the hot blast air after injection into the shaft furnace in order to achieve its ignition temperature, and its conversion in the shaft furnace clearly starts more quickly than with pulverised coal injected “cold”, so that the short available reaction time can be fully utilised for the conversion.
In addition the cooling of the reaction space is reduced by the smaller heat transfer of the hot blast air to the pulverised coal with the result that it can be ensured that the temperature in the entire reaction space remains high enough to permit reduction of the carbon dioxide formed during conversion of the pulverised coal to carbon monoxide. Consequently a further increase in the pulverised coal conversion is achieved, because the oxygen atoms present can bond substantially more carbon atoms. On the other hand the higher carbon monoxide proportion has a highly favourable effect on the operation of the shaft furnace, because it serves as a reducing agent for the actual metal recovery.
In a preferred embodiment the reducing agent is heated in several stages, i.e. subdivision of the conveying flow into several partial flows, conduction of the individual partial flows through a heating device and heating of the reducing agent in the individual partial flows inside the heating device are repeated several times, the individual partial flows between two successive stages being combined into a common delivery flow for homogenisation of the temperature conditions.
To improve the heat transfer to the reducing agent in a partial flow, the latter is preferably made to rotate about its direction of flow. The rotary motion and the associated turbulence in the partial flow produce rearrangement of the material in the partial flow, so that the latter is thoroughly mixed. In addition the velocity of the individual particles in the partial flow and their exchange path length in the heat-exchanger increase. Consequently the heat transfer to the reducing agent in the heat-exchanger can be clearly improved. Hence all partial flows are preferably made to rotate in this way.
Finally cold reducing agent can be fed to the common conveying flow for control of the injection temperature of the reducing agent before the injection into the shaft furnace.
A device according to the invention for injection of reducing agent into a shaft furnace comprises a conveying line for pneumatic conveyance of the reducing agent to the shaft furnace, several heat-exchanger tubes integrated in the conveying line with parallel connection, so that the pneumatic conveying flow is divided into several partial flows, and a heating device for transfer of thermal energy to the individual partial flows. With this device heat can be fed in a controlled manner to the reducing agent during its conveyance between a storage tank and the injection lances on the shaft furnace.
The heating takes place inside the conveying line, i.e. it can be carried out immediately before the injection of the pulverised coal into the shaft furnace. Consequently the pulverised coal is discharged to the consumer immediately after the heating, so that safety problems do not occur during temporary storage of a heated fuel. In addition the heat losses to the environment are very small, not least because of the reduced radiation surfaces.
Subdivision of the pneumatic conveying flow into several partial flows produces the required effective heat transfer surface in an arrangement with small dimensions
Langner Karl
Mack Raymond
Schmit Louis
Schott Klaus
Andrews Melvyn
Smith, Gambrell & Russell L.L.P.
Wurth Paul
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