Note: Descriptions are shown in the official language in which they were submitted.
=
CA 02872454 2014-11-03
=
P70029W0_application text 281014 Ki/ez
RWE Power AG
Process for the beneficiation of pit-moist raw brown
coal
The invention relates to a process for the
beneficiation of pit-moist raw brown coal, in
particular for thermal utilization in a power station
boiler, wherein the raw brown coal is firstly
precrushed and subsequently comminuted in at least one
milling apparatus and passed to subsequent drying and
the drying is carried out in a fluidized bed using at
least one indirectly heated fluidized-bed dryer which
is operated using steam as fluidization medium.
A process for drying water-containing brown coal in a
fluidized bed is known, for example, from
DE 29 01 723 02.
In the fluidized-bed drying process described there,
the feed coal is introduced by means of a screw
conveyor into the fluidized-bed dryer.
The process known from the abovementioned document is
said to be suitable for drying lumpy materials having a
size in the range from, for example, 0.3 cm to 10 cm.
For this purpose, the material to be dried, for example
brown coal, is fluidized in a denser material such as
silica sand. The process requires that the dried solid
material is removed together with part of the
particulate fluidizable material and that the solid
material and the fluidizable material are separated
from one another so that the fluidizable material
separated off is returned to the fluidized bed. The
process is complicated and cannot be readily
implemented in industry.
Another process for drying brown coal, in particular
for use in a power station boiler, is known from, for
CA 02872454 2014-11-03
P70029W0_application text 281014 Ki/ez RWE Power AG
- 2 -
example, DE 196 20 047 Al. This process is operated
using exclusively brown coal as solid and steam as
fluidizing medium. The brown coal to be introduced into
the dryer is comparatively finely milled, for example
to an average particle diameter d50 of about 1 mm, to
enable it to be fluidized in the fluidized bed.
Depending on the nature of the brown coal, such a
fluidized-bed drying process can proceed relatively
stably and in a problem-free manner in a steady-state
fluidized bed.
As has already been described in DE 29 01 723 C2, this
depends essentially on the fluidizability of the solid
in the fluidized-bed dryer. The hydromechanical
processes within a steady-state fluidized bed are
extremely complex and can be simulated to only a
limited extent. In a pilot plant operated by the
applicant, disturbances in the mixing dynamics have
been found from time to time, depending on the mass
flow to be introduced. As a result of such
disturbances, deposits of raw brown coal are
preferentially formed on the heat exchangers of the
fluidized-bed dryer. As a result, heat transfer is
impaired, the performance of the dryer decreases
significantly and in the worst case the fluidized bed
collapses.
Experiments using fluidized-bed dryers at a variety of
throughputs have shown that the fluidizability of raw
brown coal depends not only on the loading of the
fluidized-bed dryer as a proportion of the capacity but
also on the type and composition of the raw brown coal.
It is therefore an object of the invention to provide a
process for the beneficiation of pit-moist raw brown
coal using at least one indirectly heated fluidized-bed
81783619
- 3 -
dryer which ensures essentially stable and disturbance-free
fluidized bed operation even at a high throughput and for a
variety of feed coals.
The object of the invention is achieved by a process for the
beneficiation of pit-moist raw brown coal, in particular for
thermal utilization in a power station boiler, wherein the raw
brown coal is firstly precrushed and subsequently comminuted in
at least one milling apparatus and passed to subsequent drying,
where the drying is carried out in a fluidized bed using at
least one indirectly heated fluidized-bed dryer which is
operated using steam as fluidization medium, where the raw
brown coal is introduced as bulk material having an average
particle diameter d50 of not more than 2 mm into the fluidized
bed and a substream is branched off from the dried brown coal
downstream of the fluidized-bed dryer and mixed into the raw
brown coal before drying.
Thus, in one aspect, there is provided process for the
beneficiation of pit-moist raw brown coal, in particular for
thermal utilization in a power station boiler, wherein the raw
brown coal is firstly precrushed and subsequently comminuted in
at least one milling apparatus and passed to subsequent drying,
where the drying is carried out in a fluidized bed using at
least one indirectly heated fluidized-bed dryer which is
operated using steam as fluidization medium, where the raw
brown coal is introduced as bulk material having an average
particle diameter d50 of not more than 2 mm into the fluidized
bed and a substream is branched off from the dried brown coal
downstream of the fluidized-bed dryer and mixed into the raw
brown coal before drying, where the proportion of recirculated
CA 2872454 2019-03-11
81783619
- 3a -
dry brown coal is regulated as a function of the amount of raw
brown coal introduced into the fluidized bed, varied as a
function of the bulk density of the raw brown coal in the
compacted state, or a combination thereof.
The particle size distribution can be ensured, for example, by
variation of the speed of rotation of one or more of the mills
located upstream of the fluidized-bed dryer. Mills employed
are, for example, beater mills which achieve a different
milling fineness as a function of the speed of rotation. The
particle size distribution is checked by sieving and sampling
can for this purpose be provided daily or even per batch. The
samples are monitored in the laboratory by sieve
classification. As an alternative, the particle size
distribution can also be measured volumetrically by means of an
on-line method.
To achieve stability of the fluidized bed, it is, first and
foremost, important that the proportion of oversize particles
(> 2 mm) is not too great since this could otherwise adversely
affect the stability of the
CA 2872454 2019-03-11
CA 02872454 2014-11-03
P70029W0_application text 281014 Ki/ez RWE Power AG
- 4 -
fluidized bed.
The particle size distribution can, as an alternative,
also be ensured by milling of the dried brown coal
being followed by sieve classification, with the
oversize particles being sieved out and subjected to
after-milling.
The invention is based on the recognition that pit-
moist raw brown coal has different cohesive behavior
depending on its origin, and this has a more or less
large influence on the fluidizability of the coal.
It is known that raw brown coal, as natural product
obtained by mining, differs in respect of water
content, proportion of carbon and mineral composition
depending on its origin. Particular properties of the
raw brown coal have to be accepted in use.
The applicant was able to discover experimentally that
the fluidizability of the raw brown coal is closely
related to its flowability and that the flowability of
the raw brown coal can surprisingly be positively
influenced by mixing in dry brown coal of the same
type.
According to the invention, the same type of coal in
the form of dry brown coal is accordingly added to the
raw brown coal to be dried before the drying operation.
The stability of the beneficiation process of the
invention, in particular the stability of the fluidized
bed within the fluidized-bed dryer, can surprisingly be
improved by part of the dried brown coal being
circulated within the process, so that even raw brown
coal which is difficult to fluidize or cannot be
fluidized can in this way be used and dried without
CA 02872454 2014-11-03
P70029W0_application text 281014 Ki/ez RWE Power AG
- 5 -
problems with at the same time a comparatively high
throughput through the fluidized-bed dryer.
The problem of fluidizability of the fluidized bed also
depends on the loading of the fluidized-bed dryer as a
proportion of capacity. Disturbances in the fluidized
bed can, in particular, occur at a high loading of the
fluidized-bed dryer as a proportion of capacity.
The mixture of raw brown coal and recirculated dry
brown coal is, in the process of the invention,
preferably fed via a star feeder into the fluidized-bed
dryer which is under slightly superatmospheric
pressure, introduced above the fluidized bed and
distributed over the fluidized bed.
Particular preference is given to mixing a proportion
of dry brown coal of from 10 to 30% by mass, preferably
from 10 to 20% by mass, based on the total bed, into
the raw brown coal.
The proportion of recirculated dry brown coal is
advantageously regulated as a function of the amount of
raw brown coal introduced into the fluidized bed.
Regulation can be effected by setting a particular
ratio of dry brown coal to raw brown coal and metering
in the dry brown coal accordingly. Depending on the
plant loading set, the amount of recirculated dry brown
coal can automatically be adapted at a constant ratio.
For the purposes of the present patent application,
regulation is automatic regulation for which
appropriate process control instrumentation is
provided.
The raw brown coal can have a fluctuating water
content/moisture content of up to 65% by mass. The
CA 02872454 2014-11-03
P70029W0_application text 281014 Ki/ez RWE Power AG
- 6 -
water content of the dry brown coal taken off from the
fluidized-bed dryer is determined in the hygroscopic
range at constant system pressure by means of the
fluidized-bed temperature or the course of the
desorption isobars. The dry brown coal taken off from
the fluidized-bed dryer can have an average particle
diameter d50 of from 0.4 mm to 0.8 mm, preferably from
0.1 mm to 0.4 mm, possibly also from 0.1 mm to 0.2 mm.
The moisture content of the dry brown coal can be in
the range from 10% by mass to 15% by mass, preferably
about 15% by mass - 18% by mass. The gauge pressure
within the fluidized-bed dryer can be up to 10 bar. The
moisture content of the dry brown coal can also be in
the range from 8 to 20% by mass, preferably from 10 to
16% by mass.
As mentioned above, the fluidized-bed dryer is
preferably indirectly heated by means of steam as
heating medium. Part of the vapor from the dryer or
alternatively external steam from a coupled power
station process can be used for fluidizing the
fluidized bed or the raw brown coal within the
fluidized-bed dryer.
The dry brown coal which has been branched off can be
intimately mixed with the raw brown coal using at least
one mixing apparatus before introduction into the
fluidized-bed dryer. Such mixing is not necessary under
all circumstances, but significantly improves the
adhesion of fine dry coal particles to the particles of
the moist raw brown coal used.
For example, the dry brown coal can be cooled
downstream of the fluidized-bed dryer and subjected to
after-milling, with a substream of dry brown coal being
branched off downstream of after-milling.
CA 02872454 2014-11-03
P70029W0_application text 281014 Ki/ez RWE Power AG
- 7 -
It is of course also possible to branch off the dry
brown coal to be recirculated directly downstream of
the fluidized-bed dryer, but cooling and after-milling
is more advantageous, so that, for example, the dry
brown coal having an average particle size d50 of from
0 mm to 1 mm is added to the raw brown coal. The
particle size distribution can likewise be determined
volumetrically by sieve classification, and the after-
milling can be carried out accordingly in the case of a
deviating particle size distribution.
The recirculated dry brown coal can be mixed into the
raw brown coal between two milling stages or downstream
of a last milling stage. Of course, it is also possible
to add the recirculated dry brown coal to the raw brown
coal before a first milling.
In this case, intimate mixing is ensured simply by
joint milling of raw brown coal and dry brown coal, so
that separate mixing apparatuses may be dispensable.
However, for a relatively effective and energy-
efficient process, it is good practice to circulate as
little as possible dry brown coal and to keep the path
of the dry brown coal to be recirculated or conveyed
concomitantly as short as possible. However, the point
of view of explosion protection should also be taken
into account. For the latter reason, it is advantageous
and sensible to add the recirculated dry brown coal to
the raw brown coal downstream of fine milling of the
raw brown coal and carry out mixing with the raw brown
coal, for example by means of static or dynamic mixing
apparatuses, in the feed line to the fluidized-bed
dryer. Mixing can, for example, also be carried out via
a conventional transport apparatus.
It is advantageous for the proportion of dry brown coal
mixed into the raw brown coal to be varied as a
CA 02872454 2014-11-03
P70029W0_application text 281014 Ki/ez RWE Power AG
- 8 -
function of the adhesion properties of the raw brown
coal and/or the loading state of the fluidized-bed
dryer. As will be again stated below, it has been found
that the flow properties and the adhesion properties of
raw brown coal depend to a significant extent on the
compressibility of the raw brown coal and thus on the
bulk density of the raw brown coal in the compacted
state.
The pouring of the raw brown coal into the fluidized
bed is advantageously carried out by means of at least
one rotating distributor chute arranged above the
fluidized bed, preferably according to a prescribed
distribution based on the cross-sectional area of the
fluidized-bed dryer.
The invention is illustrated below with reference to
the accompanying drawings.
The figures show:
figure 1: a schematic depiction of the process
principle of the beneficiation process
according to the invention, and
figure 2: a graph in which the bulk material strength
of materials of differing flowability is
shown as a function of the consolidation
stress.
Reference will firstly be made to the process principle
depicted in figure 1, which illustrates the flow
diagram of a fluidized-bed drying plant which can be
connected, for example, to a power station boiler for
firing with brown coal.
From an open-cast brown coal mine, precrushed raw brown
CA 02872454 2014-11-03
P7002950 application text 281014 Ki/ez RWE Power AG
- 9 -
coal having an average particle size of from 0 mm to
80 mm is, for example, fed into a raw brown coal hopper
1. From the raw brown coal hopper 1, the raw brown coal
is finely milled to an average particle size (d50) of
about 0 mm - 2 mm in two mills arranged in series. The
raw brown coal is then mixed with dry brown coal, as
described further below, and introduced into a
fluidized-bed dryer 3. The fluidized-bed dryer 3 is,
for example, heated indirectly by means of steam via
appropriate heat-exchange internals. The steam feed to
supply the heat exchanger is denoted by the reference
symbol 4 in the process diagram (fig. 1). The
fluidized-bed dryer 3 is operated in a known way under
slightly superatmospheric pressure, with the raw brown
coal being introduced into the fluidized bed via a star
feeder which is not shown and via a distributor chute
arranged in the upper part of the fluidized-bed dryer .
3. The vapor 5 taken off from the fluidized-bed dryer 3
is, after removal of dust in an electrostatic
precipitator 6, passed to various other uses. It can,
for example, be released into the atmosphere. As an
alternative, it can be condensed, with the low-
temperature heat from the condensation of the vapor
being, for example, integrated into the boiler feed
water preheating of a power station process. In a
further alternative, the vapor can be compressed and
fed back into the fluidized-bed dryer 3 for the purpose
of heating. The energy from the vapor can also be
integrated into an ORC (organic rankine cycle) process.
A substream 7 of the vapor 5 is in any case fed as
fluidizing medium to the fluidized-bed dryer 3. The dry
brown coal 8 taken off from the fluidized-bed dryer 3
is firstly cooled in a cooler 9 and then after-milled
in a mill 10 and fed into a dry brown coal silo 11.
Possible places for taking dry brown coal off from the
brown coal stream intended for thermal utilization are
CA 02872454 2014-11-03
P7002950 application text 281014 Ki/ez RWE Power AG
- 10 -
denoted by El to E4, where El denotes an offtake point
downstream of the fluidized-bed dryer 3 and upstream of
the cooler 9, E2 denotes an offtake point downstream of
the cooler 9 and upstream of the mill 10, E3 denotes an
offtake point downstream of a mill 10 and upstream of
the dry brown coal silo 11. Finally, E4 denotes an
offtake point downstream of the dry brown coal silo 11.
The substream of dry brown coal to be recirculated is
preferably taken off at E4, since because of the
storage provided in the dry brown coal silo 11, better
meterability of the substream to be recirculated is
ensured.
The dry brown coal silo 11 can be provided at the
discharge end with a discharge star feeder which can be
operated at a variable speed of rotation. The speed of
rotation control of the discharge star feeder enables
the amount of dry brown coal to be metered so that it
is possible to set a ratio of amount of dry brown coal
recirculated to the amount of raw brown coal as a
function of the cohesivity of the raw brown coal and/or
as a function of the load as a proportion of capacity
or the load state of the fluidized-bed dryer 3. The dry
brown coal silo 11 can have two separate dry brown coal
offtakes, of which one is provided for dry brown coal
recirculation or dry brown coal backmixing while the
other is the offtake for dry brown coal as usable
product of the drying process. The use of two separate
offtakes has, in particular, the advantage in terms of
regulation that the subsequent transport path can also
be used for filling the fluidized-bed dryer 3 with dry
brown coal before start-up. Before a first introduction
of raw brown coal on start-up of the fluidized-bed
dryer 3, it is necessary firstly to build up a
fluidized bed by means of dry brown coal 8 since the
raw coal is not fluidizable because of its cohesive
CA 02872454 2014-11-03
P7002950 application text _281014 Ki/e7 RWE Power AG
- 11 -
properties.
A further advantage of such an arrangement is that
should the raw coal feed fail, dry brown coal
recirculation can compensate for the discharge of dust
from the fluidized bed and the fluidized bed and all
control circuits of the fluidized-bed dryer can
continue to be operated normally.
The positions R1 to R4 denote possible recirculation
points for the dry brown coal to be recirculated, where
the recirculation point R1 is provided directly
downstream of the raw coal hopper 1, the recirculation
point R2 is provided between a first mill and a second
mill, and the recirculation point R3 is provided
downstream of a second mill and upstream of the
fluidized-bed dryer 3. The recirculation point R4 is
provided directly upstream of the fluidized-bed
dryer 3.
The applicant has surprisingly found that the
flowability of various raw coals is directly related to
their fluidizability in the fluidized bed. Mixing dry
brown coal into the raw brown coal which is difficult
to fluidize enables the flowability of the raw brown
coal to be introduced into the drying process to be
significantly improved.
To demonstrate this relationship, the applicant has
examined various raw brown coals from various open-cast
mines and also each dry brown coal obtained from these
raw coals in respect of their flow properties. The
various coal samples, designated below as samples 1 to
5 in the interests of simplicity, and the dry brown
coals obtained therefrom, designated below as TBK 1 to
TBK 5, were each subjected to flowability tests, with a
flowability being determined as ratio of a
CA 02872454 2014-11-03
=
P70029W0_application text 281014 KI/ez RWE Power
AG
- 12 -
consolidation stress to a compressive stress. The
flowability is given by:
ff, = al to a,, where ff, is the flowability, al is the
consolidation stress and a, is the compressive strength.
Such a flowability can be determined both by means of
the known single-axis pressure test and by means of
commercially available ring shear instruments. Such a
ring shear instrument is commercially available from,
for example, the company Dr. Dietmar Schulze
Schuttgutmesstechnik (ring shear instrument ST-XS).
Various other types of ring shear instruments are
available.
The flowability of bulk material can be classified as
follows:
ff, < 1 non-flowing
1 < ff, < 2 very cohesive to non-flowing
2 < ff, < 4 cohesive
4 < ff, < 10 readily flowing
10 < ff, free-flowing.
The bulk material strength 0, as a function of the
consolidation stress al for regions of different
flowability is shown by way of example in figure 2.
The samples examined by the applicant were examined at
a temperature of about 19 C at an atmospheric humidity
of about 30% relative humidity. The result of the
flowability measurements is shown below in table 1,
where a, denotes the bulk material strength or
compressive strength of the bulk material after it has
been compacted under the stress ol, ffc is the ratio of
al to ac, Pb in kg/m3 is the bulk material density, cPe is
the measure of the internal friction angle of the bulk
material in the case of steady-state flow, (pnn is the
CA 02872454 2014-11-03
P70029W0_application text 281014 Ki/ez RWE Power AG
- 13 -
gradient angle of the linearized flow location
approximated as a straight line and cpsf is the internal
friction angle in steady-state flow.
Table 1:
Sample a [Pa] o, [Pa] if, [-] Pb [kg/m3] (P.
[ 3 [ 1 (Psf [ ]
1 4020 1703 2.4 530 48 37 40
2 4067 2320 1.8 501 53 36 42
3 3890 ___________ 1946 2.0 504 49 35 40
4 4443 2007 2.2 554 49 37 42
5 4104 2096 2.0 529 50 35 41
TBK from 1 4219 514 8.2 543 41 38 38
TBK from 2 4109 514 8.0 604 44 41 39
TBK from 3 4192 403 10.4 574 40 38 38
TBK from 4 4022 524 7.7 650 40 37 37
TBK from 5 4243 534 7.9 607 42 39 39
In the case of the raw brown coal samples 1 to 5, the
flowability ffc in table 1 is from 1.8 to 2.4 for the
consolidation stress studied. The samples without
influence of consolidation over time can thus be
classified as cohesive to very cohesive. The most
unfavorable flowability is in the case of sample 2,
while the most favorable flowability is obtained for
sample 1: based on the bulk material strength 0, of
sample 1, sample 2 has a strength which is greater by
1/3.
As regards the dry brown coal, samples 6 to 10, the
flowability of the dry brown coal is significantly
better than that of the raw brown coal.
The raw brown coal sample 2 displays the most
unfavorable flow properties. Various proportions of the
dry brown coal TBK 2 (dry brown coal from sample 2)
were then mixed in proportions by weight of 5%, 10%,
CA 02872454 2014-11-03
P70029W0_applIcation text 281014 Ki/ez RWE Power AG
- 14 -
15% and 20% into the raw brown coal sample 2. The
mixture was then examined to determine its flowability,
and the measurement result is shown in table 2.
Table 2:
Proportion o_ [Pa: oo [Pa] ffc :-] pl, [kg/m3] (Pe [ ]
(Pfin r] Tsf [01
of TBK
0% 4067 2320 1.8 501 53 36 42
5% 3780 2148 1.8 502 51 33 39
10% 3935 1923 2.0 511 48 33 39
15% 3780 1650 2.3 519 47 35 38
20% 3758 1356 2.8 523 43 33 37 __
100% 4109 514 8.0 604 44 41 39
The bulk density was determined both in the uncompacted
loose state and after consolidation under a stress of
about 4 kPa (values from table 1). The bulk density
both in the uncompacted state and in the consolidated
state of the sample is shown in table 3 below both for
the samples 1 to 5 and for the samples TBK 1 to TBK 5
and also for various mixtures.
CA 02872454 2014-11-03
P70029W0_application text 2l014 Ki/ez RwE Power AG
- 15 -
Table 3:
Sample Bulk density Pb [kg/m3] at a
consolidation stress of
-> 0 kPa approx. 4 kPa
1 433 530
2 380 501
3 410 504
4 467 554
1447 529
1
TBK from 1 453 453
TBK from 2 603 604
TBK from 3 574 574
TBK from 4 640 650
TBK from 5 604 607
2 + 0% of TBK 380 501
2 + 5% of TBK 380 502
2 + 10% of TBK 400 511
2 + 15% of TBK 410 519
2 + 20% of TBK 429 523
100% of TBK 603 604
Sample 2 has a particularly low bulk density. This
corresponds with the unfavorable flowability for this
5 sample, as is indicated in table 1. If a bulk material
has an unfavorable flowability, the individual
particles are not mobile in the bed, so that voids
remain and the bulk density is low. As a consequence,
the samples are compressible and the bulk density
therefore depends on the stress applied. The greatest
compressibility is likewise found in the case of
sample 2.
The dry brown coals also differ appreciably in terms of
the bulk density. However, no relationship is found
between the raw brown coals and the dry brown coals: a
CA 02872454 2014-11-03
=
P70029W0_app1ication text_281014 Ki/ez
RWE Power AG
- 16 -
low bulk density of the raw brown coal does not
necessarily mean a low bulk density of the dry brown
coal. The dry brown coals are only very slightly to
measurably compressible in the consolidation stress
range studied.
Both the uncompacted bulk density and the bulk density
under a consolidation stress of about 4 kPa increase
with increasing proportion of dry brown coal. This can
be explained by the better flowability after addition
of the dry brown coal in that a more favorable
flowability allows closer packing with a smaller
proportion of voids and thus a higher bulk density.
In conclusion, it can be said that mixing in of dry
brown coal, for example into sample 2, has a favorable
effect on the flowability of the mixture.
CA 02872454 2014-11-03
P70029 application text 281014 Ki/ez RWE Power AG
- 17 -
Reference symbols
1 Raw brown coal hopper
2 Mills
3 Fluidized-bed dryer
4 Steam feed
Vapor
6 Electrostatic filter
7 Substream of the vapor
8 Dry brown coal
9 cooler
Mill
11 Dry brown coal silo
El - E4 Offtake points for dry brown coal
R1 - R4 Recycle points for dry brown coal
