Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to cement calcining apparatus.
Calcining furnaces are known in which the material to be
calcined is carried in suspension in hot combustion gas to be
efficiently preheated by heat exchange with the gas so that the
material granules are well dried and heated. In the cement
industry, cement calcining apparatus with a suspension preheater
has been generally utilized in which material is carried in
suspension in hot gases drawn from the kiln exhaust for such
preheating.
Lately cement calcining apparatus has come to be put in
practice in which a calcining device is provided in the preheater
section to simultaneously effect combustion of fuel and
calcining of the material with the use of heavy oil as fuel.
Calcining in such a device has the advantages that the
material and fuel are in a mixed state and combustion and heat
conduction are performed in the shortest possible time, and the
material is effectively precalcined for most effective subsequent
processing, e.g. in cement production the material reaches the
kiln entrance at a line standard of 85 - 90%.
It is known that the combustion of pulverized coal is such
that firstly volatile fractions are released and burnt and then -
the carbon is burnt. In the past, the speed of blowing a
suspension of pulverized coal into a furnace was generally at -
least 25 - 35 m/s for avoiding blow-back of flame, however,
(1) when pulverized coal suspension is blown with this speed
into a caicining furnace, combustion may be incomplete because
of insufficient generation of volatile fractions in the furnace;
(2) part of the pulverized coal may be driven in a state of
incomplete combustion from the furnace into the material supply
cyclones and there tends to make secondary combustion, this
causes clogging of cyclones;
(3) as incompletely burnt pulverized coal may enter mixed with
the material into the kiln, temperature distribution in the
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kiln varies, tending to produce caking.
For these reasons pulverized coal has not hitherto been
conveniently used as fuel for calcining cement material in this
type of device. Further, combustion control using pulverized
coal is more difficult as compared with fluid fuels (heavy oil
or gas), because of the following.
(1) The combustion of the carbon in pulverized coal takes longer
than with heavy oil or gas, thus it is necessary to provide more
effective combustion in the calcining device.
(2) When starting up or with the plant under little or no load,
it is necessary to lessen the amount of pulverized coal, however,
for the sake of security, flow in the coal blast tube has to be
maintained at 25 - 35 m/s. As a result, the density of
pulverized coal becomes thin and combustion in the calcining
device becomes very low. For this reason, for the combustion of
pulverized coal in the furnace of such system it has been
proposed to provide a plurality of burner units, changes in load
or output being met by some one or more of the burner units being
turned on or off individually for control, while if using heavy
oil or gas load control is more simple as it can be carried out
not only by varying the amount of fuel blown in suspension.
The object of the present invention is to provide a
calcining device in which pulverized fuels can be used while
providing for a wide range of load control, enabling efficient
combustion in a stable manner while avoiding clogging of cyclones
due to secondary combustion, so as to obtain effectively calcined
material ready for subsequent processing.
According to the invention there is provided calcining
apparatus including a furnace into which material to be calcined
is fed, a source for operatively providing a flow of pulverized
fuel in suspension, a constant-feed burner in said furnace,
constant-feed mechanism supplied with fuel from said flow in use
and operable to feed it in suspension at a predetermined constant
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rate to said burner, and a load-control burner in said furnace
having connection to said source independently of the constant-
feed mechanism, the supply of fuel to the latter burner being
selectively adjustable to control the total amount of
pulverized fuel supplied to the furnace to meet variations in
load.
Other features and objects of the invention will be better
understood from the detailed description of a typical embodiment
illustrated in the accompanying drawings, in which:
Figure 1 is a diagrammatic front view of a cement plant
using pulverized coal as fuel,
Figure 2 is a longitudinal sectional view of a cement
calcining furnace of said plant, and
Figure 3 is a graph of gas analysis related to the speed
of flow of pulverized coal in suspension in the calcining -
furnace.
In Figure 2, numeral 4 generally designates a calcining --
furnace. 13, 14 are hot air entrances from a heat exchanger.
15 are raw material chutes for feeding in material to be calcined.
22 is a coal feed tube leading directly to a load control burner
24. 26 is a coal hopper with a constant rate mechanical feeder
27 for feeding coal by way of a coal blast tube 30 to a constant-
feed burner 32. 23, 31 are flexible hoses for permitting
adjustment of the burner nozzle positions to avoid damage of the
furnace wall caused by contact of coal flame thereto or to avoid
depositing of ash thereon.
In the cement apparatus including said calcining furnace 4
shown in Figure 1, material supplied from a feeder 10 passes
through cyclone 9, cyclone 8 and cyclone 7 successively, is
30 preheated by heat exchange with hot gas, and then enters the - -
furnace 4. For combustion, hot air is supplied through
entrances 13, 14 from a heat exchanger 3 at the exhaust from
rotary kiln 2. A fuel source comprises a feeder 16 from which
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coal is fed to be pulverized in a coal crusher 17 and led via a
bucket elevator 18 to enter a grading separator 19 in granulated
form. Air is supplied by a fan 20 to convey the pulverized
fuel flow in suspension by way of a flow separator 21 where it
is divided into two streams, one for load control burner 24 and
the other for constant-feed burner 32. The former stream passes
directly through coal feed tube 22 to be blown in suspension
into the furnace 4 by the load control burner 24. The latter
stream is collected in a cyclone 25 and stored in a hopper 26.
Coal is drawn from hopper 26 by the constant rate feeder 27 and
is conveyed with a fan 28 utilizing exhaust gas from cyclone 25
to enter in suspension through the constant-feed burner 32 into
the furnace 4. The material calcined in the furnace 4 is
advanced to a mixing chamber 5 wherein the material is mixed
with exhaust gas from main kiln 2. The material thus further
calcined is passed to cyclone 6. Then the material is supplied
to the main rotary kiln 2.
When cement material is calcined with pulverized coal fuel
in suspension from the constant-feed burner even at a minimum
rate of about 20% of the coal fuel supplied by both burners at
their maximum load, it is found that a good result can be
obtained, thus adjustment to suit the load of the plant can be
widely changed by regulating only the fuel supplied to the load
control burner. Division of the coal for constant-feed blasting
from that for load control is controlled by selective adjustment
of the flow separator 21 independently of the constant-feed
mechanism associated with burner 32, the suspension of fuel for
the latter being provided separately from the suspension formed
at the fuel source by fan 20.
The coal blasting tubes 24, 32 of the burner adjacent to
the furnace may be made wide to slow the speed of flow of the
suspension and to allow time for the generation of volatile
fractions in the calcining furnace to promote the full
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combustion of the coal in the calcining furnace. This is
illustrated in Figure 3. By slowing the speed of the coal fuel
suspension (shown on the graph as "blasting speed") to 8 - 10
m/s, it shows in gas analysis 8 - 12% increase in carbon dioxide
(C02) in the furnace. By providing the supply of fuel to burner
32 at a predetermined rate above the minimum needed for stability
of combustion in furnace 4 and independently from that supplied
to the load control burner 24 which can be adjusted to meet
variations in load, stable combustion may be maintained even at
times of low load when burner 24 may be shut down completely.
In the above description reference is made only to
pulverized coal. However, it is to be understood that other
solid fuels having the same properties as pulverized coal, for
example, F.O.C. (fluid oil cokes) by-produced as sludge in
refining heavy oil may also be used.
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