Note: Descriptions are shown in the official language in which they were submitted.
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Method and system for utilising biomass
and block-type thermal power plant
The invention relates to a method for utilising biogenic mass,
in particular sewage sludge, in which the material to be
utilised is first dried and then thermally decomposed in a
pyrolysis reactor for the purpose of generating pyrolysis gas.
The invention further relates to a system for utilising
biogenic mass.
For years, the utilisation of biogenic mass, in particular its
use as an alternative energy carrier, has been the subject of
intensive research. The generic term "biogenic mass" includes
"biomass" in accordance with the German Biomass Ordinance,
that is to say vegetable residues, waste and by-products of
vegetable and animal origin, biowaste, waste wood, etc. and
also recycled process waste and domestic and industrial sewage
sludges.
In particular, the utilisation and disposal of sewage sludges,
which is possible in different ways, has recently proven to be
problematic. In principle, utilisation is possible by
depositing the sewage sludges over arable land (agricultural
utilisation). Although this is permitted in accordance with
the provisions of the German Sewage Sludge Ordinance, in the
long term the use as a fertiliser is associated with a
contamination and burdening of the soil, wherein the food
produced from crops cultivated on this soil is enriched with
harmful substances. The deposition of sewage sludges over
arable land is also continually associated with a high
transport volume, and therefore high costs and CO2 emissions
emerge as a further drawback.
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Although the co-incineration of sewage sludges in central
power plants is possible in principle, it is likewise again
associated with a high transport volume since the sewage
sludge has to be transported to the power plant with a solid
fraction of just approximately 25 % (the other 75 % being
moisture). At the power plant, the sludge is to be subjected
to a complex drying procedure which is energy intensive such
that the thermal energy additionally obtained during the
subsequent co-incineration of the dried sewage sludge is
consumed more or less completely for this purpose. There are
thus no advantages in terms of energy for the energy supplier.
The incineration of sewage sludge in decentralised power
plants also leads only to little success in terms of reduced
process costs. Although the transport routes are generally
shortened, the energy obtained from the incineration process
is still too low compared to the energy to be consumed for the
drying process. Furthermore, the incineration process has to
be maintained using additional fuels, wherein it is
approximately only these which account for an ultimately
positive energy balance.
A further possibility of utilisation of sewage sludge lastly
consists of fermentation in biogas systems. However, a central
drawback in this instance is that the yields of biogas are too
low and therefore the efficiency of the method is also too
low.
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Starting from this prior art, the object of the invention is
to provide a method and a system for utilising biogenic mass,
in particular sewage sludge, which method and system can be
operated with high efficiency based on the energy content of
the introduced biogenic mass in relation to the energy
generated by the utilisation.
In accordance with the invention the object is achieved with a
method for utilising biogenic mass, in particular sewage
sludge, according to the preamble of claim 1, in that the
material is thermally dried in at least two dryer stages
arranged in succession, wherein the waste heat of the dryer
stage arranged downstream in the direction of transport of
material is used as process heat for the dryer stage arranged
upstream.
The specific advantage of the method according to the
invention is that, owing to the use of the waste heat of the
downstream dryer stage as useful heat in the upstream dryer
stage, the energy to be provided for the necessary drying of
the biogenic mass can thus be minimised overall, and therefore
the energy obtained for example during a combustion of the
pyrolysis gas obtained during the pyrolysis process is much
greater than the energy to be applied for the drying of the
biogenic mass, which was not previously possible in comparable
methods of the prior art. In this regard the applicant's
calculations have demonstrated that an energy recovery from
biogenic mass of approximately up to 80 % is possible based on
the energy content thereof.
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A further advantage of the method according to the invention
is that it can be operated in a fully decentralised manner,
for example since the biogenic mass can be dried and pyrolysed
in the vicinity of the location of its origin, that is to say
in the case of sewage sludge in the vicinity of a sewage
treatment plant, wherein the pyrolysis gas can optionally then
be used in a fuel cell or in a heat engine connected to a
generator, for example a gas turbine, a combustion engine or a
Stirling engine to produce electricity. In the case of use in
a block-type thermal power station, useful heat can be
obtained in addition to electricity, and therefore the method
according to the invention is also significant in terms of the
desired increased use of power-heat-cogeneration.
The at least two dryer stages preferably comprise at least one
low-temperature dryer as an upstream dryer stage and at least
one high-temperature dryer as a downstream dryer stage. In
this case, in a cascaded drying process the waste heat of the
high-temperature stage is thus made available to the low-
temperature stage as process heat, and is utilised in the
system, that is to say in the process inherent to the system.
It is understood that further dryer stages may be provided in
addition to a low-temperature stage and a high-temperature
stage, and therefore a dryer cascade formed of a plurality of
dryer stages can also be formed, in which each next highest
dryer stage arranged downstream in the direction of transport
of the material to be dried makes its waste heat available as
process heat to the upstream dryer stage or to the upstream
dryer stages which is/are at a lower temperature.
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In accordance with a further embodiment of the invention, the
heat of the waste gases of the auxiliary burner firing the
pyrolysis reactor is used as process heat in the upstream
dryer stage and/or the downstream dryer stage. In this case
5 the principle upon which the invention is based is thus
extended to the effect that the waste heat of the pyrolysis
reactor, which is also still at a much greater temperature
level compared to a high-temperature dryer stage, is made
available to one or more dryer stages as process heat. Quite
generally, in a cascade of process stages of increasing
temperature the waste heat of a downstream process stage is
thus made available as process heat to the upstream process
stages of lower temperature.
Moving on, the heat of the hot pyrolysis gas produced in the
pyrolysis reactor can also be used as process heat in the
upstream dryer stage and/or in the downstream dryer stage. The
high thermal energy content of the pyrolysis gas can thus be
fed as process heat to the dryer stages arranged upstream of
the pyrolysis reactor, the efficiency of the entire process
thus being further increased.
In accordance with a further embodiment of the invention the
pyrolysis gas produced in the pyrolysis reactor is fed to an
energy converter unit for conversion of the energy content of
the pyrolysis gas into electricity.
A fuel cell which converts the chemical energy content of the
pyrolysis gas directly into electricity can be considered an
energy converter unit, as can a heat engine driving a
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generator, in particular a gas turbine, a combustion engine or
a Stirling engine.
In the case of a heat engine, in accordance with a further
embodiment of the invention for further increasing the process
efficiency, the heat of the waste gases of the heat engine can
be used as process heat in the upstream dryer stage and/or in
the downstream dryer stage.
In accordance with a further embodiment of the invention, each
of the at least two dryer stages is supplied with process heat
via its own heat transfer circuit, in particular a thermal oil
circuit. In practice, the waste heat can thus be conveyed to
the downstream dryer stage, in particular in the form of
exhaust vapours, that is to say in the form of a steam-air
mixture, by a heat exchanger integrated into the heat transfer
circuit of the upstream dryer stage so as to be used in
accordance with the invention as process heat for the upstream
dryer stage.
If, in the case of a heat engine operated with the pyrolysis
gas, the waste heat of the waste gases thereof is used as
process heat for the upstream dryer stage and/or the
downstream dryer stage, this may take place in practice in
that the waste gases of the heat engine are conveyed through a
waste gas heat exchanger integrated into the heat transfer
circuit of the respective dryer stage. In particular it is
possible to first convey the waste gases through a waste gas
heat exchanger integrated into the heat transfer circuit of
the downstream dryer stage, whereupon it is then conveyed
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through a heat exchanger integrated into the heat transfer
circuit of the upstream dryer stage.
The exhaust vapours flowing off from a dryer stage may also
supply, at least in part, the necessary process heat for this
dryer stage itself, in that at least some of the exhaust
vapours are first compressed with the addition of energy,
heated and then condensed in a heat exchanger integrated into
the heat transfer circuit of the respective dryer stage,
wherein the condensation enthalpy is delivered to the heat
transfer circuit and the heat transfer medium is heated. In
this case the waste heat of the dryer stage is thus raised to
a higher temperature level by compression in the manner of a
heat pump and is then fed in the form of useful heat via a
heat exchanger acting as a condenser back into the heat
transfer circuit supplying the dryer stage with process heat.
In accordance with a further advantageous embodiment of the
invention, some of the pyrolysis gas produced in the pyrolysis
reactor is used as fuel for the burner of a boiler, in
particular a thermal oil boiler, integrated into the heat
transfer circuit of the upstream and/or downstream dryer
stage. The boiler is preferably arranged in the heat transfer
circuit of the downstream dryer stage and the waste gases of
the boiler burner are then guided through a heat exchanger
integrated into the heat transfer circuit of the upstream
dryer stage. In this case the energy content of the branched
off pyrolysis gas is used in a particularly efficient manner
for both of the at least two dryer stages.
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it may further be provided for some of the pyrolysis gas
produced in the pyrolysis reactor to be used as fuel for the
auxiliary burner of the pyrolysis reactor itself. As a result,
the system can be operated basically independently of further
fuels.
It may further be provided for the pyrolysis coke produced
during pyrolysis of the dried material to be fed to a gasifier
and for the lean gas produced there by gasification to be fed
as fuel to the auxiliary burner for the pyrolysis reactor.
This constitutes a further possibility to increase the process
efficiency, since in particular the energy of pyrolysis
products which generally go unused, in this case the pyrolysis
coke, is used directly in the method.
In terms of device the object mentioned at the outset is
achieved with a system for utilising biogenic mass, in
particular of sewage sludge, according to the preamble of
claim 17, in that the dryer device comprises at least two
dryer stages arranged in succession in the direction of
transport of the material and coupled to one another in such a
way that the waste heat of the dryer stage arranged downstream
in the direction of transport of the material can be used as
useful heat for the dryer stage arranged upstream.
The comments made above apply accordingly with regard to the
advantages of the system according to the invention.
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The invention will be described hereinafter in greater detail
with reference to drawings illustrating an embodiment, in
which:
Fig. 1 is a block diagram of a system for generating
electricity from sewage sludge;
Fig. 2 shows of a block diagram of the low-temperature dryer
comprising a thermal oil circuit, according to a
preferred embodiment;
Fig. 3 shows a preferred embodiment of the pyrolysis reactor
of the system from Fig. 1; and
Fia. 4 is a flow chart illustrating a method for utilising
sewage sludge.
The system illustrated schematically in the form of a block
diagram in Fig. 1 for generating electricity from sewage
sludge as biogenic mass comprises a dryer device 1, through
which the sewage sludge introduced into the system at a feed
point la is transported and dried. The dryer device is divided
into two dryer stages, that is to say a low-temperature dryer
3 and a high-temperature dryer 4. Further dryer stages may be
added (not shown in this case).
A pyrolysis reactor 2 is arranged behind the high-temperature
dryer 4 in the process direction and is fired by an auxiliary
burner 2a. During the pyrolysis process, the sewage sludge
dried in the dryer device 1 is thermally decomposed, wherein
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pyrolysis gas (normally consisting of nitrogen, carbon
dioxide, hydrogen, carbon monoxide and higher carbon atoms)
and, as further products, pyrolysis coke and ash which cannot
be utilised further are produced.
The pyrolysis gas escapes from the pyrolysis reactor 2 via a
line 25 and arrives in a heat engine, in this case a gas
turbine 5, which in turn is connected to a generator 5a for
generating electricity. A combustion engine, a Stirling engine
or a fuel cell which converts the chemical energy of the
pyrolysis gas directly into electricity may also be provided
instead of a gas turbine.
The low-temperature dryer 3 as well as the high-temperature
dryer 4, as individual dryer stages of the dryer device 1,
each comprise a heat transfer circuit, in this case a thermal
oil circuit 30, 40 which supplies the respective dryer stage
3, 4 with process heat. The thermal oil circuits 30, 40 can be
coupled to one another (not shown in Fig. 1), which is
advantageous in particular when starting up the system so as
to achieve rapid drying of the sewage sludge until a steady
operating state is reached.
A thermal oil boiler 41 for heating the thermal oil and a heat
exchanger 42 are arranged in succession in the thermal oil
circuit 40 of the high-temperature dryer 4. For its part, the
thermal oil boiler 41 comprises an auxiliary burner 41a, of
which the fuel feed line 43 is connected to the pyrolysis gas
line 25. The auxiliary burner 41a is accordingly operated
directly with the pyrolysis gas produced in the pyrolysis
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reactor 2 as fuel. In the heat exchanger 42, the thermal oil
circulating in the thermal oil circuit 40 is additionally
heated by the hot waste gases flowing off via a waste gas line
52 from the gas turbine 5.
In the present case a total of five heat exchangers 31 to 35
are arranged in succession in the thermal oil circuit 30 of
the low-temperature dryer 3. The waste gases of the burner 41a
of the thermal oil boiler 41 arranged in the thermal oil
circuit 40 flow through the heat exchanger 31. The residual
heat of the waste gases flowing out from the heat exchanger 31
escapes as lost heat. In turn, the waste gases of the gas
turbine 5 flow through the heat exchanger 32 once they have
already passed through the heat exchanger 42 arranged in the
thermal oil circuit 40. For the sake of clarity, the
connection of the heat exchanger 42, 32 is merely indicated in
Fig. 1 by the symbols C-C. The residual heat of the waste
gases of the gas turbine escapes, again as lost heat, after
passing through the heat exchanger 32, wherein the thermal oil
circulating in the thermal oil circuit 30 is further heated.
The thermal oil of the thermal oil circuit 30 is further
heated by the waste gases of the auxiliary burner 2a of the
pyrolysis reactor 2. For this purpose these waste gases flow
through the waste gas line 23 and into the heat exchanger 33
integrated into the line.
Furthermore, the line 44 through which the exhaust vapours
exiting from the high-temperature dryer 4 flow is connected to
the heat exchanger 34 of the thermal oil circuit 30, in such a
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way that the exhaust vapours flow through the heat exchanger
34 and deliver some of their thermal energy to the thermal
oil.
Finally, the heat exchanger 35 is arranged in the thermal oil
circuit 30 of the low-temperature dryer 3. The hot pyrolysis
gases exiting from the pyrolysis reactor 2 flow through said
heat exchanger, wherein some of their heat is delivered to the
thermal oil.
Fig. 2 shows a block diagram of a particularly preferred
embodiment of the low-temperature dryer 3. A further heat
exchanger 37 is integrated into the thermal oil circuit 30 of
the low-temperature dryer 3. For the sake of clarity, the heat
exchangers 31 to 35 described above are not shown in Fig. 2.
The exhaust vapours flowing out from the low-temperature dryer
3 are compressed in a compressor 36 in accordance with the
arrangement of Fig. 2, wherein they are raised to a higher
temperature level and then flow as a compressed exhaust vapour
flow through the line 38 into the heat exchanger 37, which
acts as a condensation heat exchanger. The exhaust vapours are
accordingly liquefied as they pass through the heat exchanger
37, wherein the condensation heat is delivered to the thermal
oil circulating in the thermal oil circuit 30. Owing to this
structure corresponding roughly to the operating principle of
a heat pump, further process heat for the drying process in
the low-temperature dryer 3 can be provided in a very
efficient manner by the use of additional energy in the
compressor.
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Fig. 3 shows a block diagram of a particularly preferred
embodiment of the pyrolysis reactor 2 of the system of Fig. 1.
The components already known from the block diagram of Fig. 1
bear corresponding reference numerals. The specific feature of
the arrangement illustrated in Fig. 3 is that the pyrolysis
coke produced during the pyrolysis process is recovered from
the reactor via a line 24 and is fed to a gasifier stage 26,
where the pyrolysis coke is gasified in ways known per se from
the prior art. The lean gas produced is cleaned in a cleaning
stage 27 and is then fed to the auxiliary burner 2a of the
pyrolysis reactor 2 as additional fuel. The efficiency of the
entire method is thus further increased since further
pyrolysis products, in this instance the pyrolysis coke, are
utilised as an energy source in the process.
The method taking place in the system of Fig. 1 to generate
electricity from sewage sludge will now be explained in
greater detail in conjunction with Fig. 1 and the schematic
flow chart of Fig. 4.
In a first step the sewage sludge to be dried having a dry
substance content of normally approximately 25 % (the
remaining 75 % is formed by water) is fed into the system and
transported into the low-temperature dryer 3 and pre-dried.
Here, it is dried until it has a dry substance content after
leaving the low-temperature dryer 3 of approximately 40 %. The
low-temperature dryer 3 is supplied with the necessary process
heat by the thermal oil circuit 30. The pre-dried material is
then conveyed into the high-temperature dryer 4 and is dried
until reaching the final degree of dryness. The exhaust
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vapours produced in the high-temperature dryer 4 are conveyed
via the line 44 into the heat exchanger 34 provided in the
thermal oil circuit 30 of the low-temperature dryer 3, where
they deliver some of their heat to the thermal oil circulating
in the thermal oil circuit 30. As a result, the waste heat of
the dryer stage arranged downstream in the direction of
transport of the material, that is to say the waste heat of
the high-temperature dryer 4, is thus used as process heat for
the dryer stage arranged upstream, i.e. the low-temperature
dryer 3.
The material dried to a dry substance content of approximately
85 % is then fed into the pyrolysis reactor 2, where the
material is preferably thermally decomposed in a two-stage
pyrolysis process in the absence of oxygen, as is known per se
from the prior art. The heat necessary for this is generated
by the auxiliary burner 2a. The burner waste gas produced is
fed via the line 23 to the heat exchanger 33 provided in the
thermal oil circuit 30 of the low-temperature dryer 3, and
therefore the heat of the burner waste gases is also used as
process heat in a dryer stage, in this case in the low-
temperature dryer 3.
The pyrolysis gas produced in the pyrolysis reactor 2 leaves
the pyrolysis reactor 2 via the line 25 and first passes
through a dust separator 21, where any dusts still contained
in the pyrolysis gas flow are separated. As can be seen in
Fig. 1, the pyrolysis gas then flows through the heat
exchanger 35 so that the heat of the pyrolysis gas produced in
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the pyrolysis reactor 2 is again fed as process heat to this
dryer stage.
Before passing though the heat exchanger 35, some of the
pyrolysis gas flow is branched off from the line 25 into the
lines 22, 43. The pyrolysis gas fed into the line 22 is used
as fuel to fire the auxiliary burner 2a of the pyrolysis
reactor 2, whilst the fraction fed into the line 43 is fed as
fuel to the auxiliary burner 41a of the thermal oil boiler 41
arranged in the thermal oil circuit 40 of the high-temperature
dryer 4. The chemical energy contained in the pyrolysis gas
produced in the pyrolysis reactor 2 is thus used in a
particularly efficient manner to maintain the entire process.
The pyrolysis gas flowing through the line 25 is then fed into
the gas turbine 5, where it is combusted, wherein the gas
turbine 5 drives a generator 5a. The waste gases of the gas
turbine are fed through the line 52 to the heat exchanger 42
arranged in the thermal oil circuit 40 of the high-temperature
dryer 4 and are then fed to the heat exchanger 32 arranged in
the thermal oil circuit 30 of the low-temperature dryer 3 so
that the heat contained in the waste gas of the gas turbine is
again made available to the two dryer stages 3, 4 as process
heat.
The principle of providing, in the form of process heat and in
a multi-stage process of increasing process temperature, the
waste heat of a process step of specific temperature to one or
more upstream process steps of lower temperature in order to
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increase the overall efficiency of the entire process is thus
implemented with the method described above.
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