Note: Descriptions are shown in the official language in which they were submitted.
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LOW COZ THERMAL POWERPLANT
Technical field
The present invention relates a method for generation of electrical power
mainly from a
coal based fuel, where the combustion gas is separated into a COZ rich stream
which is
exported e.g. for safe deposition, and a CO2 poor stream that is released into
the
surroundings. The invention additionally relates to a plant for performing the
method
and a part of the plant.
Background
The concentration of CO2 in the atmosphere has increased by nearly 30 % in the
last
150 years, mainly due to combustion of fossil fuel, such as coal and
hydrocarbons.
The concentration of methane has doubled and the concentration of nitrogen
oxides has
increased by about 15 %. This has increased the atmospheric greenhouse effect,
something which has resulted in:
= The mean temperature near the earth's surface has increased by about 0.5 C
over the last one hundred years, with an accelerating trend in the last ten
years.
= Over the same period rainfall has increased by about 1 %
= The sea level has increased by 15 to 20 cm due to melting of glaciers and
because water expands when heated up.
Increasing discharges of greenhouse gases is expected to give continued
changes in the
climate. The temperature can increase by as much as 0.6 to 2.5 C over the
coming 50
years. Within the scientific community, it is generally agreed that increasing
use of
fossil fuels, with exponentially increasing discharges of C02, has altered the
natural
CO2 balance and is therefore the direct reason for this development.
It is important that action is taken immediately to stabilise the CO2 content
of the
atmosphere. This can be achieved if COZ generated in a thermal power plant is
collected
and deposited safely. It is assumed that the collection represents three
quarters of the
total costs for the control of CO2 discharges to the atmosphere.
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Discharge gas from thermal power plants typically contains 4 to 10 % by volume
of
C02, where the lowest values are typical for gas turbines, while the highest
values are
only reached in combustion chambers with cooling, for example, in production
of
steam.
Capturing of CO2 from CO2 containing gas by means of absorption is well known,
see
e.g. EP 0 551 876. The CO2 containing gas is here brought into contact with an
absorbent, usually an amine solution which absorbs CO2 from the gas. The amine
solution is thereafter regenerated by heating the amine solution. The
absorption is,
however, dependent on the partial pressure of COZ. If the partial pressure is
too low,
only a relatively small part of the total CO2 is absorbed. Normally the
partial pressure of
CO2 in combustion gas is relative low, for gas turbines a value of 0.04 bar is
typical.
The energy consumption in such a plant is about 3 times higher per weight unit
CO2
than if the partial pressure of CO2 in the feed gas is 1.5 bar. The cleaning
plant becomes
expensive and the degree of cleaning and size of the power plant are limiting
factors.
Therefore, the development work is concentrated on increasing the partial
pressure of
CO2. According to WO 00/48709, the combustion gas that has been expanded over
a
gas turbine and cooled, is re-pressurized. The pressurized gas is then brought
in contact
with an absorbent. In this way, the partial pressure of CO2 is raised, for
example to 0.5
bar, and the cleaning becomes more efficient. An essential disadvantage is
that the
partial pressure of oxygen in the gas also becomes high, for example 1.5 bar,
while
amines typically degrade quickly at oxygen partial pressures above about 0.2
bar. In
addition, costly extra equipment is required.
Another possibility to raise the partial pressure of CO2 is air separation. By
separating
the air that goes into the combustion installation into oxygen and nitrogen,
circulating
CO2 can be used as a propellant (for gas turbines) or as a cooling gas (for
coal fired
boilers) in gas turbine combined cycle or coal fired power plants,
respectively. Without
nitrogen to dilute the CO2 formed, the CO2 in the exhaust gas will have a
relatively high
partial pressure, approximately up to 1 bar. Excess CO2 from the combustion
can then
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be separated out relatively simply so that the installation for collection of
CO2 can be
simplified. However the total costs for such a system becomes relatively high,
as one
must have a substantial plant for production of oxygen in addition to the
power plant.
Production and combustion of pure oxygen represent considerable safety
challenges, in
addition to great demands on the material. This will also most likely require
development of new turbines.
From WO 2004 /001301 it is known to let the combustion take place under
elevated
pressure, cool down the combustion gas by generation of steam, split the
combustion
gas in a CO2 rich stream for deposition, and a CO2 poor stream, and expanding
the CO2
poor stream over a turbine before it is released into the atmosphere. The
plant in
question is however a gas powered plant, and there is no mentioning of the use
of coal
asafuel.
WO 2004 /026445 relates to a method for separation of combustion gas from a
thermal
gas fired power plant into a COZ rich stream and a CO2 poor stream. The
combustion
gas from the power plant is here used as an oxygen containing gas in a
secondary
combined power plant and separation plant.
The methods described above mostly relates to natural gas fired power plants.
Today,
however, coal is a more widely used fuel for thermal power plants than natural
gas.
Coal fired thermal power plants do, additionally, produce more CO2 per unit of
electrical power than plants based on natural gas. Additionally, coal is an
easy available
and compared with natural gas, less expensive fuel.
Introduction of a coal based fuel, such as pulverized coal, into a pressurized
combustion
chamber is connected with technical challenges. Using air as a propellant for
the coal
dust will give an explosive mixture that will cause the combustion to start
before
entering the combustion chamber, an may even result in an explosion in the
means for
mixing air and coal or in connecting lines or in the combustion chamber. Using
an inert
gas as nitrogen would be another possibility but purification of nitrogen
would add
unacceptable cost to the plant. Additionally, addition of nitrogen would
increase the
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total gas flow and result in a reduced partial pressure of CO2 in the
combustion gas,
which is disadvantageous for the separation of CO2.
According to the so-called PFBC process, pulverized coal is mixed with water
to give a
paste-like mixture that is squeezed into the combustion chamber. The water -
coal paste
mixture is required in order to pump the fluid and thereby overcome the boiler
combustion pressure. The water in the paste will vaporize with resulting loss
of
efficiency. In order to fire the water - coal paste, a fluid bed combustor is
required.
This is large and expensive equipment. In addition, the fluid bed gives a
significant
pressure drop, in the order of 2 bar. This reduces the downstream turbine
power.
Accordingly, there is a need for a cost effective method for generation of
electrical
power from a coal based fuel where the combustion gas is split into a CO2 rich
stream
for deposition and a CO2 poor stream that may be released into the atmosphere.
According to a first aspect, the present invention relates to a method for
generation of
electrical power mainly from a coal based fuel, where the coal based fuel and
an oxygen
containing gas is introduced into a combustion chamber and combusted at an
elevated
pressure, the combustion gases are cooled down in the combustion chamber by
generation of steam for production of electricity, the combustion gas is
further cooled
down and separated into a CO2 rich stream and a CO2 poor stream in a CO2
capturing
unit, the CO2 poor stream is reheated and expanded over a turbine to produce
electrical
power, before the CO2 poor stream is released into the surroundings, wherein
the CO2
rich stream is split into a stream for deposition or export, and a stream that
is recycled to
the combustion chamber. In the combustion chamber, the recycled CO2 is used to
bring
the pulverized coal into the combustion zone. If the pulverized coal is fed
into the
boiler by air instead of CO2, there is severe explosion hazard. By use of CO2
instead of
air, the explotion hazard is removed. Additionally, the pressure drop
mentioned for
fluidized bed reactors, is eliminated.
According to a preferred embodiment, at least a portion of the CO2 rich stream
that is
recycled to the combustion chamber is mixed with the coal based fuel before
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introduction to the combustion chamber and is injected into the combustion
chamber
together with the coal based fuel. The CO2 rich stream that is recycled to the
combustion chamber may be used to fluidize the fuel in the tanks in the
intermediary
storage means, to avoid that settled coal fuel may hinder the injection into
the
5 combustion chamber. Additionally, the CO2 rich stream may be used as a
propellant for
the fuel to force the fuel from the tank into the combustion chamber.
The CO2 poor stream is preferably heated by heat exchanging against combustion
gas
from a secondary combustion chamber fired by gas, before the CO2 poor stream
is
expanded over a turbine. This is done to optimize the energy output from the
plant and
increase the part of the electricity that is produced by expansion of this
stream before it
is released into the surroundings.
The pressure in the combustion chambers may be from 5 to 35 bar, preferably 10
to 20
bar, more preferably from about 12 to about 16 bar. The absorption of CO2 in
the CO2
capturing device is more effective at an elevated pressure than at a lower
pressure.
Combustion at an elevated pressure delivers combustion gas at an elevated
pressure to
the capturing device without energy consuming compressors. By keeping the
combustion chamber nearly fully fired, the mass flow of flue gas to be
purified is
minimized, and the conceritration and hence the partial pressure of CO2 are
thus
maximized.
It is preferred that the temperature in the combustion gas leaving the
combustion
chamber, is reduced to below about 350 C by production of steam. By keeping
the
temperature in the combustion gas leaving the combustion chamber below 350 C,
normal quality steel may be used in the equipment for further handling of the
gas.
Additionally, a high energy output is taken out as steam that is used for
production of
electric energy.
According to an embodiment, natural gas in introduced into the combustion
chamber to
support the combustion. The combustion becomes more effective when supported
by
addition of natural gas.
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According to a second aspect, the invention relates to a thermal power plant
mainly
fired with a coal based fuel, the thermal power plant comprising a combustion
chamber,
means for introducing the coal based fuel and an oxygen containing gas into
the
combustion chamber, cooling means for cooling the combustion gas in the
combustion
chamber and means for separation of the combustion gas into a COZ rich stream
and a
CO2 poor stream, wherein the power plant additionally comprises a line for
recirculation
of a part of the CO2 to the combustion chamber and a COZ line for delivering
the
remaining COZ rich stream for deposition or export.
The cooling means are preferably cooling coils inside the combustion chamber,
where
the cooling coils are cooling the combustion gas by generation of steam.
Cooling coils
inside the combustion chamber are effective in cooling the combustion gases at
the
same time as steam for generation of electric power is produced.
Preferably, the thermal power plant further comprises a steam turbine
connected to a
generator for the production of electrical power.
According to a preferred embodiment, a secondary combustion chamber fired by
gas,
for generation of heat for heating the CO2 poor stream, and turbine for
expanding of the
heated CO2 poor stream before it is released into the surroundings, are
employed.
Heating of the stream before it is released into the surroundings, adds energy
to the gas.
As a result the production of electrical power from the expansion of the CO2
poor
stream over a turbine becomes more efficient and improves the total efficiency
of the
plant.
It is preferred that the turbine for expansion of the CO2 poor stream is
connected to a
generator for production of electrical power.
According to a third aspect the invention relates to an injector for a coal
based fuel and
an oxygen-containing gas into a pressurized combustion chamber, comprising a
central
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pipe for injection of a mixture of pulverized coal based fuel and CO2 gas,
surrounded by
a plurality of injectors for oxygen containing gas. The construction of the
injector
having a central tube for injection of the coal and CO2 surrounded by
injectors for
oxygen containing gas ensures rapid and intimate mixing of the coal based fuel
and the
oxygen containing gas. This rapid and intimate mixing of the fuel and oxygen
containing gas ensures optimal combustion in the combustion chamber.
According to a preferred embodiment, the injector additionally comprises one
or more
gas injectors for injection of natural gas. Addition of additional fuel in the
form of
natural gas may be used both in starting up the combustion and for maintenance
of the
combustion.Combustion of natural gas in the combustion chamber results in a
better and
more optimal combustion of the coal as the additional heat ensures that
lighter
components in the coal evaporates and are more effectively combusted.
Helically ribs may additionally be provided inside the central pipe. The
helical ribs will
cause the mixture of coal based fuel and CO2 have a vortex motion out of the
central
tube. This motion ensures even better mixing of the coal based fuel, the
oxygen
containing gas and any added natural gas.
According to one embodiment, the gas injectors are orientated so the gas
rotates the
opposite way relative to the coal powder. Rotating of the gas and coal powder
opposite
relative to each other ensures optimal mixing of gas and coal powder.
Brief description of the drawings
Figure 1 is a schematic diagram of a preferred embodiment of the invention;
Figure 2a) illustrates a longitudinal section through an injector according to
the
invention;
Figure 2B illustrates the section A-A in figure 2a;
Figure 3 illustrates an exemplary grinding and intermediate fuel storage
device for the
plant according to the invention;
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Figure 4 is a longitudinal section through a combined heat exchanger and
secondary
combustion chamber for plant according to the invention;
Figure 5 is a schematic diagram of an intermediate fuel storage device and
means for
taking care of C02; and
Figure 6 is a schematic diagram of an exemplary CO2 capturing unit.
Detailed description of the invention
An exemplary embodiment of a thermal power plant fired by natural gas and coal
is
illustrated in figure 1. Coal and optionally limestone, are introduced into a
coal mill 12
through a coal line 10 and a lime stone line 11, respectively. The coal and
the optional
limestone, are milled to a ground mixture in the coal mill 12 to a particle
size suitable
for feeding into a combustion chamber.
The ground coal and optional limestone are carried on a conveying means 13 to
intermediate storage means 14. The intermediate storage 14 in the illustrated
embodiment comprises two or more storage units, each unit operated in a batch
wise
manner. Two or more units are necessary to give a continuous operation of a
combustion chamber.
Each intermediate storage unit comprises an inlet valve 15, a storage tank 16
and an
outlet. valve 17. Additionally, each unit comprises one or more inlets for COZ
coming in
from a C02-line 18. The ground mixture from the coal mill is conveyed to the
intermediate storage device and filled into one storage tank at a time. The
inlet valve 15
for the tank 16 to be filled is opened and the outlet valve 17 is closed.
During or after
filling of a tank 16, air is preferably purged from the tank by means of CO2
from the
C02-line 18 to avoid creation of dangerous mixtures of air and coal dust.
The CO2 is controlled by means of a CO2 valve 19. After filling the tank and
purging air
from the tank, the inlet valve 15 is closed. Before the mixture in the tank is
to be
introduced into a combustion chamber 25, CO2 is filled into the tank to give a
pressure
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in the tank that is higher than the pressure in the combustion, for example
0,5 to 1 bar,
such as 0,7 bar, higher.
According to one embodiment, the CO2 inlets in the tank are placed so that the
mixture
in the tank is at least partly fluidized by the incoming stream of CO2. The
outlet valve
17 is thereafter opened and the mixture is led to an injector 21 through a
line 20. The
mixture is introduced into the combustion chamber 25 by the injector 21
together with
C02, compressed oxygen containing gas from an air line 23 and optionally
natural gas
from a gas line 22. The injector 21 is described in more detail below with
reference to
figure 2. Gas from the gas line 22 is used to promote the combustion in the
combustion
chamber and to adjust internal combustion therein.
The oxygen containing gas may be air, oxygen enriched air or oxygen. The terms
air
and oxygen containing gas in the description and claims, used as synonyms to
describe
these possibilities.
The combustion in the combustion chamber 25 occurs at an elevated pressure,
for
example from 5 to 25 bar, more preferred from about 10 to about 20 bar, and
most
preferably about 15 bar.
Solid matter in the combustion chamber, such as non-combustible residues from
the
coal and calcium sulphate produced in binding of sulphur compounds in the
combustion
gases, is collected in the bottom of the combustion chamber and removed
through a
solids removal line 24.
The above described combustion chamber 25 is a presently preferred combustion
chamber. The skilled man in the art will, however, understand that other
constructions
and principles of operation are possible. The described combustion chamber
may, e.g.
be substituted by a fluidized bed combustion chamber.
A substantial amount of the heat produced from the combustion is removed from
the
combustion chamber by producing steam in cooling coils 9 inside the combustion
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chamber. Most of the heat is removed from the top of the combustion chamber to
reduce the temperature of the combustion gas leaving the combustion chamber 25
through a combustion gas line 35.
5 The steam produced in the cooling coi19 is removed from the combustion
chamber
through a steam line 26 and is expanded over a turbine 28 to produce
electricity in a
generator 27. The expanded steam is led in a line 29 to a condenser 30, where
the
expanded gas is cooled and condensed. The condensed water is pumped by a pump
31
and pre-heated by heat exchanging in a pre-heater 32 before the water again is
10 introduced through a line 33 into the cooling coil 9 in the combustion
chamber 25. It
must be noted that this circuit may be far more complex. The cooling coi19 may
be
divided into two or more cooling coils each taking out a part of the heat to
one or more
steam turbines.
The combustion gas leaving the combustion chamber 25 through the combustion
gas
line 35 has preferably a temperature of about 350 C, or lower. A temperature
of less
than 350 C in the combustion gas leaving the combustion chamber makes it
possible to
use relatively inexpensive steel in the construction of lines and processing
equipment,
and reduces the building cost.
The combustion gas in line 35 contains dust from the combustion chamber. This
dust
may be harmful for the further processing of the combustion gas. Accordingly,
the dust
has to be removed in a dust removal unit 36 comprising a plurality of cyclones
and/or
filters 38.
The illustrated dust removal unit 36 comprises two lines in parallel each
comprising a
number of cyclones and or filters in series. The unit may, however, comprise
of more
than two lines in parallel. To allow continuous operation of the dust removal
unit, one
or more of the parallel lines may be shut down for cleaning and service as
long as at
least one of the parallel lines are open and in operation at all times.
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The inlet side of one of the parallel lines may be closed by means of an
upstream valve
37, whereas the other side of the parallel lines, may be closed by a
downstream valve
40. Dust, separated in the cyclones and/or filters, is removed through dust
removal lines
39.
From the dust removal unit, the dust free combustion gas is led via a line 41
to a
selective catalytic reduction unit (SCR unit) for substantial reduction of NOx
produced
in the combustion chamber. In the SCR unit 42, NOx can be removed with NH3,
according to the reaction 3N0 + 2NH3 = 2.5 N2 + 3H20. This cleaning has up to
90 %
efficiency at atmospheric pressure, but is assumed to be much better at the
working
pressure which is typically above 10 bara. It will therefore be possible to
clean NOx
down to a residual content of 5 ppm or better. By adapting the heat
exchangers, the gas
can be given a temperature that is optimal for this process. Other known
methods for
NOx removal without using NH3 may also be used. The NH3 method has the
disadvantage that it gives some NH3 "slip".
The cleaned gas, is leaving the SCR unit in a line 43 and is cooled in a heat
exchanger
44. From the heat exchanger 44, the gas is led into a condenser 47 in a line
46. In the
condenser, the gas cooled further down and condensed water is removed from the
gas.
The gas leaving the condenser is led to a CO2 capturing unit 49 in a line 48.
Alternatively, a gas scrubber may be provided upstreams of the condenser. In
the
optional gas scrubber the gas is saturated with water vapor, and the gas is
cooled by
countercurrent contact with water at suitable temperatures. The scrubber may
employ
chemicals to oxidize and / or absorb multiple flue gas stream residuals
including NOx,
SOx, other acids or gases, and particulates. Such chemical may be the NH3
"slip" from
the SCR system which provides an alkaline solution, or a special chemical with
alkaline
and / or oxidizing properties. In the latter case, the scrubber may replace
the SCR unit
42 completely.
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The purification of the flue gas is essential to minimize the formation of
heat stable salts
in the CO2 capturing absorbent, and to minimize the degradation of CO2
capturing
performance with time.
The CO2 capturing unit typically comprises' an absorber where the flue gas
flows
countercurrent to an absorbent such as an amine, hot carbonate or a physical
absorbent.
The amount of CO2 in the flue gas is typically reduced by 90 to 99% in the
absorber
before the flue gas leaves the absorber as a CO2 poor stream. The absorbent
with
absorbed CO2 (rich absorbent) is heated in a solvent / solvent heat exchanger
and
regenerated in a stripper column. The regenerated solvent is cooled in the
solvent /
solvent exchanger, cooled in a trim cooler and returned to the C02 absorption
tower,
whereas the COZ is removed from the stripper column as a CO2 rich stream.
Figure 6
illustrates an exemplary COZ capturing unit. The detailed design the unit
will, however,
depend on the type of solvent used.
The CO2 capturing unit 49 may be any kind of unit capable of splitting the
partly
cleaned combustion gas in a C02-rich stream leaving the unit through a CO2-
line 51,
and a C02-poor stream leaving the unit through a line 50. The C02-rich stream
in line
51 is compressed to a pressure of about 100 bar in a compressor 52 powered by
a motor
53. A part of compressed CO2-rich stream is leaving the compressor in line 54
and is
recycled as a source of COZ for the intermediate storage means 14. The
remaining COZ
is compressed further and is removed from the plant in a C02-line 55.
The C02-poor stream leaving the CO2 capturing unit 49 through line 50 is
introduced
into a re-humidifier, where the gas is heated and saturated with water before
it is led
through a line 57 to the heat exchanger 44 where the CO2-depleted gas is
heated against
the hot gas in line 43. Preferably, air or another suitable gas is introduced
into line 57
(or alternatively line 50) through an air line 73 to make up for the mass of
the COz that
has been removed from the combustion gas so that the heat capacity of the C02-
poor
stream is approximately the same as the heat capacity of the combustion gas in
line 43.
The air is taken into the system through an air intake 70 and is compressed by
means of
a compressor 71 powered by a motor 72. As an alternative, some air from the
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compressor 78 may be by-passed the combustor 25 and downstream equipment, and
introduced in line 50 or line 57. (This is not shown in Figure 1).
The heated C02-poor stream leaves the heat exchanger 44 through a line 58 and
is
introduced into a heat exchanger 59 where the C02-poor stream is heated
against
combustion air entering the heat exchanger in a line 82 from a secondary
combustion
chamber 81. The secondary combustion chamber 81 is fired by natural gas from a
gas
inlet line 80. Oxygen for the combustion in the secondary combustion charnber
81 is
introduced into the secondary combustion chamber through a line 87.
The cooled down gas from the heat exchanger 59 leaves the heat exchanger in a
line 86
that is introduced into the line 41 for CO2-removal. A part of the gas in line
86 may be
taken out in a line 83 and recycled into line 82 by means of a fan 84 and a
line 85. The
recirculation through line 83 is used to increase the mass flow of heated gas
through the
heat exchanger 59 from line 82. If the heat exchanger is built of material
that stand high
temperature, such as up to 2000 C, the recirculation is superfluous.
The heated C02-poor stream leaving the heat exchanger 59 in a line 60, is
expanded
over a turbine 61. The expanded C02-poor stream leaving the turbine 61 through
a line
62 is cooled further in heat exchangers 63 before the gas stream is released
into the
atmosphere through a line 64. The heat exchanger(s) 63 may be identical to the
preheater 32, preheating the water entering the cooling coils in the
combustion chamber
so that energy in the expanded COZ-poor stream is used to heat the water in
the
preheater 32.
Air for both the combustion chamber 25 and the secondary combustion chamber 81
is in
the illustrated embodiment introduced to the system through an air intake 75.
The air in
air intake 75 is compressed, preferably in a two step compressor, having two
compressors 76 and 78 and an intercooler 77. The compressed gas leaving the
compressor 78 in a line 79, is split into two streams into the air line 23
leading to the
injector 21, and into the second air line 87 leading into the secondary
combustion
chamber 81. A leakage in the compressors 76, 78 and/or the turbine 61 is
illustrated by
a leakage line 88. The compressors at the illustrated embodiment is placed on
a shaft 66
that is common to both the compressors 76, 78, the turbine 61 and a generator
65 for
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generation of electric power. As an alternative, there may be a two stage
compressor
76, 78 (as shown) and a two stage turbine 61 (low pressure stage and high
pressure
stage) - not shown - such that the low pressure turbine drives the low
pressure
compressor 76, and the high pressure turbine drives the high pressure
compressor 78
plus the generator 65.
Figure 2a represents a length section through the combustion chamber and a
preferred
embodiment of an injector 21. The injector 21 is supported by a collar 101
welded to the
wall of the combustion chamber. The injector is inserted into the collar 101
and fastened
to the collar by means of a holding plate 100. The injector comprises a
central tube 102
for injection of coal, air injectors 103 and gas injectors 104 surrounding the
central tube.
The collar 101 is preferably cooled down by means of air from air inlet 109
circulating
in a cooling jacket 106 surrounding the collar. Preferably the air heated by
cooling the
collar in the cooling jacket is led in a line 107 and is introduced into the
air injectors
103 and injected into the combustion chamber.
The mixture of coal, CO2 and optionally lime stone entering the injector 21
through line
20, is introduced into a central pipe 102. The mixture is blown through the
tube by
means of pressurized CO2 and injected into the combustion chamber. By using
nozzles,
as indicated in the figure, to inject the air into the combustion chamber, the
venturi
effect caused by the nozzles will cause an additional drag of material from
the central
pipe into the combustion chamber.
The hot and burning gas / coal mixture leaving the injector 21 may be harmful
to the
wall of the combustion chamber and steam heating coils 9. To avoid damage to
the wall
of the combustion chamber and steam heating coils 9, a reflector plate 111 is
arranged
opposite the injector 21 for reduction of velocity of remaining unburned
particles and
avoid or reduce damages to the inner wall of the combustion chamber.
Preferably, the
reflector is cooled by means of CO2 delivered through a gas line 110 being
circulated
trough cooling channels 112 at the rear side of the reflector plate. Normally,
one
reflector plate is arranged per injector if more than one injector is arranged
in the wall
of the combustion chamber. Alternatively, the reflector may be frustoconical
having
openings for the injectors.
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Figure 2b illustrates the cross section A-A in figure 2a. The central pipe 102
is
surrounded by a plurality of air injectors 103. The gas injectors, for
injection of natural
gas introduced into the injector in gas line 22, are in the illustrated
injector, situated
inside one or more of the air injectors. A plurality of helically shaped ribs
105 at the
5 inner wall of the central pipe, causes the coal mixture to rotate and
accordingly create
turbulence in the combustion chamber. The creation of turbulence is important
to assure
proper mixing of the injected coal, gas and air to promote optimal conditions
for
combustion.
Figure 3 illustrates a combined mill and intermediate storage device 14. Coal
and lime
10 stone are transported on conveying means 10, 11, 13 into a funnel 150
leading to a mill
12. The funnel 150 has a plurality of internal flaps 151 for reduction of the
coal /
limestone feeding velocity into the mill 12. The reduced feeding velocity will
allow for
optimum abatement of air. The mill 12 preferably comprises more than one mill,
where
the incoming coal and limestone firstly are introduced into a mill and
thereafter into a
15 fine mill to give the preferred particle size.
The mill and lower part of the funnel is preferably purged by CO2 entering
from a purge
line 152 to reduce the amount of oxygen or air that is carried with the coal
and
limestone, as a mixture of coal dust and oxygen may be explosive. The stream
of CO2
in the purge line is controlled by a valve 153.
From the mill, the coal and limestone dust is vertically fed by an Archimedes
screw 13
to the tank 16. A valve 15 inserted between the conveyor 13 and the tank 16 is
used to
close the inlet of the tank when the tank is full of coal and limestone dust.
When the
tank 16 is to be emptied into the combustion chamber, the valve 15 is closed,
CO2 is
introduced into the tank at the top of the tank through a CO2 line 154
controlled by a
valve 155, and/or through a CO2 line 157 controlled by a valve 158. The
introduction of
CO2 either through the line 154 or line 157 will boost the pressure in the
tank. The
pressure in the tank is increased to a pressure that is higher than the
pressure in the
combustion chamber. Preferably, the pressure in the tank is from 0,5 to 1 bar
higher
than in the combustion chamber. Introduction of CO2 through line 157, close to
the
bottom of the tank, will at least partly fluidize the content of the tank. The
valve 17 in
line 20 is then opened, and the mixture of CO2, coal dust and limestone is
forced
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through the line 20, through the injector 21 and into the combustion chamber
as
described above. After the tank 16 is emptied, the valve 17 is again closed,
valve 15 is
opened, and the tank again filled with dust as described above.
Figure 4 illustrates a combined secondary combustion chamber and heat
exchanger 200
to substitute for the secondary combustion chamber 81, heat exchanger 59 and
lines
connecting them. This combination is more heat efficient and avoids or reduces
the use
of connection lines.
Air and natural gas are introduced through an air line 203 and a gas line 202,
respectively, into a combustion chamber 201. CO2 is introduced from a CO2 line
204
through a cooling jacket 205 to cool down the upper part of the combustion
chamber,
and is released into the combustion chamber to adjust the gas composition in
the
combustion chamber. The burning gas in the combustion is forced downwards in
the
combustion chamber and through openings 206 near the bottom of the combustion
chamber. The warm flue gas from the combustion chamber is circulated in a flue
gas
chamber surrounding the combustion chamber. The hot flue gas in the flue gas
chamber
is cooled by heat exchange against the C02-poor stream from line 58 entering
the
device through an inlet 212. The CO2 poor stream circulates in the circulation
space
defined between the outer wall of the flue gas chamber 207 and a heat
exchanger shell
210.
The flue gas from the secondary combustion chamber 201 leaves the device
through a
flue gas outlet 208 and is introduced into line 86. The heated COZ poor stream
leaves the
device through a heat exchanger outlet 213 into line 60. The air to be
introduced into air
line 203 is preferably preheated by heat exchanging against the COZ poor
stream, as the
air is introduced into an air inlet to a jacket 216 surrounding at least a
part of the heat
exchange shel1210. The heated air is removed through an air outlet 217 and is
introduced into air line 203.
This combined combustion chamber and heat exchanger gives a more compact
construction of the combined device. A high temperature difference over the
wall
separating the combustion chamber and the heat exchange part of the device,
results in
the need of a relatively small heat exchange area.
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Figure 5 illustrates an embodiment of the intermediate storage means 14,
including
storage means 250 for CO2. The CO2 storage means 250 comprises a CO2 storage
tank
255, a compressor 259 run by a motor 263, a dust filter 252 and connecting
lines 257
and 261, and several valves 253, 254, 258, 260 and 262, controlling the flow
in the
system. The CO2 storage means 250 may be closed of from the intermediate
storage
means 14 by means of an optional valve 251.
When CO2 under pressure in the tank 255 is to be filled into one of the tanks
16, 16' or
16", the valve connected to the tank 16, i.e. 248, 248' or 248" is opened. The
valves
256 and 262 are then opened to allow the gas in tank 255 flow through the
lines 256,
261 and 249, 249' or 249". When the flow from tank 255 into tank 16, 16' or
16"
declines due to lower pressure difference, valve 256 is closed, valves 254,
260 and 258
are opened and the CO2 from the tank 255 is compressed by the compressor 259
until
the pressure in the tank 255 is about atmospheric pressure. All valves 253,
254, 256,
258, 260, 262 and 248 are subsequently closed.
To fill excess CO2 from a tank 16, 16' or 16", into the tank 255, the
corresponding
valve 248, 248' or 248" is opened. The COZ is then allowed to flow through the
filter
252 from the tank 16, 16' or 16" into the tank 255 by opening valves 253 and
254. As
soon as the flow decreases due to reduced difference in pressure between the
tanks,
valve 254 is closed, the valves 260, 258 and 256 are opened and the gas from
the tank
16, 16' or 16" is compressed and led to tank 255 for temporary storage. When
the
pressure in the tank 16, 16' or 16" is about atmospheric pressure, all the
valves 248,
248', 248", 253, 254, 256, 258, 260 and 262 are closed.
It is obvious for the skilled man that CO2 may be introduced or memoved from
the tank
16 through any CO2 lines into the tank, such as line 154, 157 or 18 and that
line 249 is
illustrative and may cover any of the mentioned lines alone or in combination.
Figure 6 illustrates an exemplary and somewhat simplified CO2 capturing unit
49. The
cooled down combustion gas enters the unit 49 through line 48 and is
introduced into an
absorber 300 near the bottom. The cleaned combustion gas leaves the absorber
300 in
line 50 close to the top of the absorber. An absorbent, such as an amine or
hot
carbonate solution, is introduced into the absorber through a line 301 close
to the top of
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the absorber, and leaves the absorber as a rich absorbent (rich in C02)
through a line
302 close to the bottom of the absorber. The countercurrent flow of gas to be
cleaned
and absorber through the absorber ensures optimal conditions for absorption of
CO2.
The rich absorbent in line 302 is heated in a heat exchanger 303 against
regenerated
(lean) absorbent before the rich absorbent is introduced into a stripping
column 305
close to the top thereof. The temperature in the stripping column is higher
and the
pressure is lower than in the absorber 300, causing CO2 to be released from
the
absorbent. CO2 released from the absorbent is removed from the stripping
column
through a CO21ine 306. The COz in line 306 is cooled in a reflux condenser 307
to
remove humidity in the CO2 rich stream leaving the CO2 capturing unit through
line 51.
Humidity that is condensed in the reflux condenser 307 is returned to the
stripping
column in a reflux line 308.
The stripped or lean absorbent is taken out close to the bottom from the
stripping
column 305 in line 301. The lean absorbent in line 301 is cooled in heat
exchanger 303
and cooler 311 before it is reentered into the absorber 300. A part of the
lean adsorbent
may be taken out in a heating circuit 309 where it is heated in a reboiler 310
before the
heated lean absorbent is reintroduced into the stripping column 305.
In an exemplary plant according to figure 1, key figures for temperature,
pressure and
mass flow may be as follows:
Table 1 Pressure, temperature, mass flow and effect for different units / at
different
locations in a 400 MW plant
Ref. No. Pressure (bara) Temperature Mass flow aC (kg/s) Effect (MW)
13 1,013 30 21 (coal)
22 20 15 2,3
23 16 300 300
26 300 600 272
27 428
35 16 350 323
46 120 - 130
48 40- 90
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55 100 30 78
58 15 330 385
60 15 850 385
65 80
73 16 145 50
75 1,013 15 400
80 20 15 5
82 870
86 15 330 90
87 16 300 85
88 16 300 15
The skilled man in the art will understand the mentioned heat exchangers,
turbines,
compressors and the like may represent two or more parallel and/or serially
connected
devices. Additionally, where two or more parallels are mentioned, the number
of
parallels may be different from the exemplified embodiment.
