Note: Descriptions are shown in the official language in which they were submitted.
The present inven~ion relates to a process of coal
grinding, drying and coal firing which is easy on the environment
and saves energy by minimizing thè amount of primary air even at
high moisture contents oE the coal.
In process furnaces ~lame formation has a substantial
effect on energy consumption. For a low energy consumption a
fast combustion with favourable heat transfer by a short and hot
flame is required. Apar~ from other factors important parameters
for this purpose are: the proportion of primary air, the
temperature of both the primary air and the secondary air, and
the injection impulse ratios. The proportion of primary air
should be as small as possible and the temperatures of both the
primary air and the secondary air as high as possible. The rate
of injection and the torque component of the combustlon in the
furnace should be freely selectable within an optimal range.
The following methods ~fcoal firing are known.
1. lndirect firlng
2. direct firlng
3. semidirect firing with and without an additional
filter.
In indirect firing the coal ~rinding drying unit is
operated completely independently of the furnace firing. In
fact in this process the determining factors for flame formation
can be optimized independently of the grinding drying unit.
However, a separate filter for purifying the dust-containing
grinding mill exhaust air and an intermediate storage bin for
the ground dry coal are required. Such a process is hazardous
because of coal-dust explosions and fires. Moreover, the
expenditures for the required capital, maintenance and operation
are high.
In direct firing the entire coal-dust containing
grinding millexhaust air is injected into the process furnace.
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The capital costs are substantially lower since, for example,
a separate filter for purifying the dust-containing grinding
mill exhaust air and an i~ltermediate storage bin for the ground
dry coal are avoided. Because of the absence of these intermediate
uni~cs and of the required intermediate steps the danger of
coal-dust explosions and fires is substantially reduced. High
proportions of primary air are a disadvantage. Therefore,
optimal injection impulse ratios at the furnace burner cannot
be attained.
In semidirect firing, the coal dust contained in the
ex~aust air of the grinding mill is first removed in a cyclone.
A portion of this air is returned to the mill unit as recycled
air and serves as additional carrier air in the mill while the
residual air together with the coal dust separated in the cyclone
is injected into the furnace. At high coal moisture and mill
heating with low -cemperatures the amount of exhaust air
resulting from the drying procedure is greater than that correspond-
ing to the re~uired amount of carrier air. Therefore, in these
cases a high amount of primary air must be accepted or an
~0 additional filter must be ins~alled, thus resulting in disadvantages
similar to those in direct and indirect firing. Thus a high
amount of prima~y air prevents the injection impulse ratios
at the furnace burner from being optimized and an addifcional filter
means additional capital costs and increased danger of coal dust
explosions and fires.
The present invention provides a process in which the
proportion of primary air can be kept selectively small irrespec-
tive of the coal moisture content and the heating of the grinding
mill and where the injection ratios at the burner nozzle are
adjustable independently of the grinding drying process, without
having to accept a separate coal dust filter or an intermediate
s-torage bin and the safety risks and additional costs associated
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therewith, as for example, in indirect firing, and in which the
heat of the exhaust air of the ~rinding mill can be recovered.
According to the present invention there is provided a
method for using solid fuel to heat a kiln which discharges heated
materials to a cooler, comprising: pulverizing the fuel in a mill;
pneumatically conveying the pulverized fuel to a separator for sepa-
rating fuel particles from fuel-dust-laden air; pneumatically con-
veying the separated fuel particles to -the kiln for burning; and
conveying at least a portion of the fuel-dust-laden air to the
cooler.
Thus, according to a preferred embodiment of the inven-
tion the proportion of coal-dust containing exhaust air of the
grinding mill which is not required as primary air is used for heat-
ing the secondary air and/or for cooling the heated material.
In another preferred embodiment of the present invention
the portion of the mill exhaust air which is not required as pri-
mary air and from which the dust has been removed by means of a pre-
liminary purification, particularly with the aid of a cyclone,
is used for cooling the heated material and/or for heating the
secondary air so that the heat from the heated material can also
be recovered for heating the exhaust air of the grinding mill.
~ n another preferred embodiment of the present inven-
tion, the hot coal-dust containing exhaust air of the grinding
mill is injected into the hot front end of the clinker cooler so
that the heat is also recovered in this zone and that moreover,
the residual coal dust ignites in this hot zone and thus is sub-
stantially removed and also advantageously heats the secondary air.
In yet another preferred embodiment of the present inven-
tion the exhaust air of the grinding mill, from which the dust
has been remo~ed by means of a preliminary purification, is inject-
ed below the grate of the clinker cooler.
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In a further preferred embodiment of the present
invention the exhaust air of the grinding mill, from which the
dust has been removed by means o~ a preliminary purification,
is injected above the ~rate of the clinker cooler.
In another manner of carry1ng out the process accordir.y
to the invention hot air from the clinker cooler or from the
top of the furnace is used for drying the coal.
In a further embodiment of the present invention
hot exhaust gas of the furnace unit is used for drying the air.
However, in a further special embodiment separate
firing is used for drying the coal.
The process according to the invention is suitable
particularly for burning cement clinker, lime or expanded clay
in a rotary furnace system with a series-connected planetary,
flash, tunnel or fluidized hed cooler. Said process is also
suitable for burning cement clinker, lime or expanded clay when
using a shaft or carriage furnace. Moreover, the process
according to the invention is suitable when using a roll mill
or a ball mill for the grinding drying operation. Finally the
~0 process according to the invention is also suitable in cases
in which pure coal firing or mixed firing with coal and other
fuels is used. The above possibilities of using the process
according to the invention are only examples.
Thus, in accordance with the present invention, it is
possible to reduce the amount of primary air for the furnace
burner to the minimum required for an optimal combustion,
irrespective of the grinding drying system and the moistu~e
content of the coal while simultaneously using the residual
amount of yrlnding mill exhaust air without a separate filter
system in a manner which is easy on the environment. ~oreover
the process according to the invention saves energy since the heat
from the hot exhaust alr of the mill is reused for heating
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the secondary air. Furthermore, the residual coal dust of the
exhaust air of the grinding mill is fed to the secondary air
so that it need not be filtered out. More specifically this
residual coal dust can be burned in the secondary air flow
according to a special embodiment of the process of the invention.
This means a more direct and complete utilization of the entire
coal dust of the exhaust air o~ the mill so that not only does
the process accordlng to the invention save energy and raw
material but it also is easy on the environment and less
costly since the coal dust is removed and a separate filter
system is no lon~er required.
Furthermore because of the absence of both an inter-
mediate storage bin for the coal dust and the filter unit and
since the corresponding gr~nding drying svstem can be operated
and rendered ~nert with low hot air temperatures even at high
coal moistures without heat economy l~sses the process according
to the invention provides a hi~h de~ree of safety against coal
dust explosions and fires, and even high coal moisture contents
can be overcome without any problems by the extensive use of
waste heat of the furnace and/or of the clinker cooler.
The process according to the invention is further
illustrated by way of the accompanying drawing which diagram- ~ ~
matically shows a system operated by means of the process accord-
ing to the invention. The system comprises particularly a
cement-making furnace with a thrust-grating clinker cooler.
The coal dr~ed and ground in a coal mill 1 is separated
in a high-speed cyclone 2 and is fed via a shut-off valve 3 into
the primary air flow or int~ a partial air flow of the primary
air 4.
The amount of gas or air required for drying and as
the carrier air is sucked by the blower system 5 through the coal
mill 1 and the high-speed cyclone 2. As the conveying medium
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and dryin~ medium upstrea~ of the coal mill 1 either hot cooler
air through the cooler ~ix pipe 6 or hot air ~rom the furnace
top through the furnace top air pipe 7, furnace exhaust gas
through the furnace exhaust gas pipe 8, hot ~as from a
combustion chamber 9, or combinations of cooler air, ~urnace
top air, furnace flue gas and hot gas are used, the cooler air
valve 10, the furnace air choke 11, the furnace flue gas choke
12 and the combustion chamber choke 13 or the cold air valve
30 being used for adjusting the flows. The dust-containing
hot air is subjected to preliminary purification in a cyclone
14. The coal-dust containin~ exhaust air of the mill 1, from
which the dust has been removed in the high-speed cyclone 2,
is divided downstream of the blower system 5 and can be conveyed
to the primary air blower 15, to the inlet of the mill 1 via
the connecting pipe valve 19 through the connecting pipe 18 or
to a cooling air blower 16 o~ the clinker cooler 17. The
proportions o~ the various amounts can be optimized depending on
the requirements of the system, the coal moisture content and
the flame formation and can be adjusted in the range from 0 to
~0 100~. For this purpose the connecting pipe valve 19, the
cooling air supply valve 20 and the auxiliary valve 21 as well
as the primary air valve 22 and the valve 23 for admixing cold
air are used.
By these adjusting means the proportion of primary
air can be reduced optionally and the most varied burner nozzles
24 and combinations of firing, for example, with additional
fuel 36, can be used without having to take into account the
coal moisture content and the conditions at the grinding drying
unit.
Thus, for example, particularly at high coal moisture
contents a higher proportion of exhaust air is injected into the
clinker cooler. The injection procedure is carried out, for
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example, with an existing cooling air blower 16, like that shown
in the Figure, or directly into one of the cooling chambers 31,
the latter procedure is not shown in the Figure. ~lowever, the
injection pr~cedure can also be distributed over several cooling
air blowers 16 or it can even be carried out with a separate
cooling air feed blower 34 into one or several cooling air
chambers 31 or directly into the hot air end of the clinker,
cooler 37 via the grate 25, but preferably into the cooling air
chambers 31 closest to the hot air end of the clinker coolex.
The thermal heat of the exhaust air from the mill 1, can thus be
fully utilized for heating the secondary air. Furthermore, the
portion of non-thermal heat still contained in the mill exhaust
air in the form of the residual coal dust content is i~nited on
passing through the hot clinker layer and further heats the
secondary air. The excess exhaust air from the cooler is passed
via the exhaust air pipe 32 to the exhaust air dust collector
27, where~the dust is removed. With the aid of both the exhaust
air blower 28 and the contrQl valve 29 the exhaust air, as a
function of the pressure ln the furnace top, is released into the
atmosphere.
In the l~rocess according to the invention the entire
heat of the exhaust air, i.e., both the thermal heat and the heat
contained in the residual coal dust of the exhaust air is recovered
in contrast to the indirect firing. ~oreover a coal filter is
no longer required because of the combustion of the residual
coal dust content. This means a substantial saving and an
increased operating safety. Furthermore, coal dust is no longer
released into the free atmosphere so that the process according
to the invention is much easier on the environment than the process
known heretofore. Intermediate storage of coal dust is not
required either so that the danger of coal dust explosions and
fires is substantlally reduced. Of cause the overall capital,
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maintenance and operating costs are lower.
In contras-t to the direct and semidirect processes the
amount of primary air can be ~reely selected. Thus, the flame
can be optimized and the amount of primary air can be kept
subs~antially lower. This results in savings of fuel which can
be as high as approximately 100 kcal per kg of clinker (0.42 GJ
per ton of clinker) for high coal moistures.
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