Method for heat reintegration of thermal energy gained from a
flue gas stream
The invention is aimed at a method for reintegration of thermal energy, gained from the flue gas stream of a steam boiler - fired with carbonaceous fuel, especially coal, preferably dried brown coal or hard coal, and run in oxyfuel mode - of a power plant, into the water/steam cycle of the power plant, wherein a first partial stream which gathers the carbonaceous fuel is branched off from the flue gas stream and recirculated to the burners of the steam boiler, and a second partial stream is branched off from the flue gas stream and, flowing through a heat transfer device which is exposed to throughflow, preferably in counterflow, by at least portions of the flue gas stream, is recirculated to the burners of the steam boiler.
For quite some time now, ever since signing of the Kyoto Protocol, intensive attempts have been undertaken to reduce the emission of CO2 gas, which is produced during the combustion of fossil fuels, into the atmosphere in order to reduce this greenhouse gas in the atmosphere, which is responsible for climate warming. In fossil-fired, especially coal-fired, power plants, the integrated separation of CO2 is available in this case as one process route. This integrated separation is realized during the so-called oxyfuel process. In this case, a flue gas stream with a high concentration of carbon dioxide {CO2) is produced as a result of combusting the fossil fuel, especially coal, with pure oxygen instead of air and after condensation of the water vapour portion can be directly disposed of without additional scrubbing.
The oxyfuel running mode of a steam boiler of a power plant is impaired with regard to its efficiency in comparison to a
conventional power plant which is operated with air combustion. Therefore, the most diverse ways are sought with which an efficiency increase can be achieved in a steam boiler, run in the oxyfuel mode, of a power plant. Also, in the event that such a power plant is equipped with integrated CO2 separation, an additional energy cost for a CO2 compression system for liquefying the carbon dioxide is necessary.
The invention is therefore based on the object of creating a solution which enables an improved utilization of the thermal energy which is generated in the oxyfuel mode of a fossil-fired steam boiler.
With a method of the type referred to in more detail in the introduction, this object is achieved according to the invention by thermal energy, in the form of a partial heat flow, being extracted from the flue gas stream and/or from a partial stream of the flue gas stream, and being injected into the water/steam cycle of the power plant in the region of the high-pressure preheating system and/or of the low-pressure preheating system.
As a result of this, it is possible to save steam which otherwise is usually extracted from the high-pressure turbine, intermediate-pressure turbine and/or low-pressure turbine of the water/steam cycle and fed to the high-pressure preheating system and/or low-pressure preheating system of the water/steam cycle. As a result of this recovery of thermal energy from the flue gas stream into the region of the low-pressure preheating system and/or high-pressure preheating system of the water/steam cycle of the power plant, less steam has to be extracted within the water/steam cycle so that the overall efficiency of the power plant can be increased as a result. It is also possible, however, to
use the consequently "saved" steam somewhere else in the power plant, for example for air separation in the air separation plant for producing the oxygen which is required in the oxyfuel mode so that at this location a corresponding energy supply can then be saved.
A particularly favourable development of the invention is that a bypass stream, which flows around the heat transfer device and downstream of the heat transfer device in the flow direction of the flue gas re-enters the flue gas stream, is branched off from the flue gas stream, wherein the bypass stream flows through a second heat transfer device, by means of which the partial flow of thermal energy is extracted from the bypass stream and injected into a heat transfer medium which is fed to the region of the high-pressure preheating system of the water/steam cycle and is injected directly into the water/steam cycle there, or wherein thermal energy is extracted from the heat transfer medium there and injected into the water/steam cycle.
For an approximately equal adiabatic flame temperature, in hard coal-fired power plants with CO2 separation and an operating regime in oxyfuel mode, about 2/3 of the flue gas has to be recirculated. So that the gas preheater which is arranged in the flue gas stream, and in the case of conventional operation designed as an air preheater, can be used for preheating the recirculated first flue-gas partial stream, the mass flow of flue gas which discharges from the boiler and enters the gas preheater must be reduced. The bypass stream which bypasses the gas preheater is designed for this. So as not to increase the flue gas temperature downstream of the gas preheater in the flow direction, a heat transfer device, which is designed as a heat exchanger, is located in the bypass stream. This heat transfer device or this heat exchanger is designed as a gas/water heat
exchanger, wherein the water serves as heat transfer medium, into which the thermal energy extracted from the bypass flue gas partial stream is injected and fed in the region of the high-pressure preheating system of the water/steam cycle. Since acidic constituents, such as sulphur, are contained in the flue gas stream which flows through the gas preheater, the temperature downstream of the gas preheater in the flue gas flow direction lies at least 10 Kelvin above the acid dew point. This, on the other hand, is contingent upon the fact that the temperature level at which the water enters the bypass heat exchanger or the bypass heat transfer device is to be selected so that the temperature of the bypass stream which re-enters the flue gas stream also has such a level that downstream of the entry point in the flow direction the temperature level of the flue gas stream also lies at least 10 Kelvin above the acid dew point. The temperature of the hot water from the heat transfer device or from the bypass heat transfer device which is fed to the high-pressure preheating system as heat transfer medium results from the predetermined mean logarithmic temperature difference of the heat transfer device. The heated water is integrated into the high-pressure preheating system of the water/steam cycle. Therefore, less steam has to be provided from the high-pressure turbine and/or from the intermediate-pressure turbine for the high-pressure preheating system in the water/steam cycle.
A further possibility for improving the heat balance of a steam boiler which runs in oxyfuel mode is that according to a further development of the invention the flue gas stream, downstream of the entry of the bypass stream in the flue gas flow direction, is fed to a third heat transfer device which extracts a further partial heat flow from the flue gas stream and injects it into a further heat transfer medium which flows to -the region of the low-pressure preheating system of
the water/steam cycle, wherein thermal energy is extracted from the further heat transfer medium in the region of the low-pressure preheating system and injected into the water/steam cycle, or wherein the further heat transfer medium is injected into the water/steam cycle in the region of the low-pressure preheating system.
In the case of this embodiment, downstream of the gas preheater in the flue gas flow direction, in combination with a downstream electrostatic precipitator a third heat transfer device is formed and arranged here, and in combination with the electrostatic precipitator acts as a SO3 sink. The third heat transfer device is arranged downstream of entry of the bypass stream into the flue gas stream in the flue gas flow direction and upstream of an electrostatic precipitator. In this heat transfer device, the flue gas is cooled to a temperature which lies appreciably below the acid dew point, preferably 20 Kelvin below the acid dew point. In order to achieve this, the temperature of the water which flows as heat transfer medium to this third heat transfer device is selected in such a way that the desired cooling of the flue gas to the temperature below the acid dew point, preferably of up to 2 0 Kelvin below the acid dew point, is achieved by means of a corresponding heat flow of thermal energy from the flue gas and injection into the heat transfer medium. The heat transfer medium/water which is heated as a result of injection of this thermal energy is then fed to the low-pressure preheating system of the water/steam cycle so that at this point additional steam, which would otherwise have to be made available from the water/steam cycle, is also saved. On account of falling short of the acid dew point, in the flow gas stream SO3 condenses on the ash particles which are contained in the flue gas and act as basic condensation nuclei. These ash particles, laden with SO3, are then subsequently removed from the flue gas in the electrostatic
precipitator. The temperature of the flue gas in the region of the third heat transfer device on the one hand is now adjusted so that downstream of the third heat transfer device in the flue gas flow direction the temperature is cooled down to 20 Kelvin below the acid dew point, but on the other hand the flue gas temperature at this point still lies about 20 Kelvin above the water dew point.
Furthermore, in the case of an intended removal of the CO2 from the flue gas, provision is made, if desired, for a CO2 compression system, connected downstream to a flue gas drying plant, for liquefying the CO2. In this case, it is advantageous according to a further development of the invention that the flue gas stream is fed to a multistage CO2 compression plant, in which downstream of each pressure stage in the flue gas flow direction an additional partial heat flow is extracted from the flue gas stream by means of subsequent heat transfer devices in each case and thermal energy of the respective additional partial heat flow is injected into the water/steam cycle in the region of the low-pressure preheating system. It is also possible to feed the thermal energy which is extracted from the flue gas in the CO2 compression plant to the partial stream before entry into the gas preheater in order to increase the flue gas substance stream in the bypass and therefore to increase the amount of heat which is yielded to the heat transfer medium.
Also, as a result of this, heat can be recovered and reintegrated into the water/steam cycle. For the waste heat in the CO2 compression plant or CO2 purification unit, an additional cooling water circuit is especially constructed in order to protect the water/steam cycle against contamination. The cooling water which is heated as heat transfer medium in the subsequent heat transfer devices there is fed to the region of the low-pressure preheating system where the
thermal energy is extracted from this in water/water heat exchangers and as a result the cooling water is cooled down, as a result of which this thermal energy is injected in the water/steam cycle in the region of the low-pressure preheating system. These water/water heat exchangers are arranged in the bypass of a plurality of low-pressure preheaters.
In addition to using the steam volume or steam mass which is saved as a result of the procedure according to the invention either for increasing the overall gross output and therefore the overall efficiency of the power plant, or for oxygen preheating, this steam can also be used for driving an air compressor. The invention is therefore finally also characterized in that a steam volume corresponding to the reintegration of thermal energy is used for driving an air compressor, especially of the air separation plant, and/or for preheating the oxygen which is added to the recirculated flue gas. Within the scope of this air separation for splitting off the oxygen, the air must first be compressed. This can be carried out both adiabatically and isothermally. In this case, the saved steam can be used for driving the air compressor. In the case of adiabatic air compression, the oxygen can be preheated by means of heated air. In the case of isothermal compression, saved steam can be used for preheating the oxygen.
-The invention is subsequently exemplarily explained in more detail with reference to the drawing. In the drawing
Fig. 1 shows in schematic view the flue gas recirculation cycle of a coal-fired power plant run in oxyfuel mode,

Fig. 2 shows in schematic view a CO2 compression plant for CO2 liquefaction connected downstream to the flue gas recirculation cycle according to Fig. 1, and
Fig. 3 shows in schematic view an alternative oxygen preheating system.
The plant which is provided for carrying out the method according to the invention features a steam boiler 1, fired by dried brown coal TBK or hard coal, of a power plant which is run in oxyfuel mode. The flue gas leaves the steam boiler 1 along the flue gas path 2. In this flue gas path 2, a gas preheater 3, an electrostatic precipitator 4, a flue gas desulphurization plant 5 and a flue gas drying plant 6, especially a flue gas cooler, are arranged in series in the flow direction. In the exemplary embodiment, a first partial stream 7 gathering carbonaceous fuel, as so-called primary gas recirculation partial stream, branches downstream of the flue gas drying plant 6 from the flue gas stream which flows along the flue gas path 2. In a device which is schematically represented and designated 8, the first partial stream 7 picks up coal as carbonaceous fuel and is then fed to the burners of the boiler 1 at 9. A second partial stream 10, a so-called secondary gas recirculation partial stream, also branches from the flue gas stream downstream of the flue-gas drying plant 6 and, flowing through the gas preheater 3, is also fed to the burners of the steam boiler 1 at 9. Downstream of the gas preheater 3 and upstream of the inlet into the steam boiler 1 in the flow direction, oxygen originating from an air separation plant 11 in the second partial stream 10, is added to the second partial stream 10 at 12.
Upstream of the gas preheater 3 in the flue gas flow direction, a bypass stream 13 branches from the flue gas path
2, and downstream of the gas preheater 3 in the flue gas flow direction re-enters the flue gas path 2. In the bypass stream 13, a second heat transfer device is arranged and exposed to throughflow by the bypass stream. A heat transfer medium, moreover, in the present case being water 15, is ducted in the second heat transfer device 14 and in the second heat transfer device 14 gathers thermal energy by way of extraction from the bypass stream and injection into the heat transfer medium flow 15. The heat transfer medium 15 which is heated here is fed to the high-pressure preheating system of the water/steam cycle of the power plant which is associated with the boiler 1. In the region of the high-pressure preheating system, thermal energy which is present in the heat transfer medium 15 is injected into the water/steam cycle. The heat transfer medium 15 which flows to the second heat transfer device 14 is extracted from the water/steam cycle downstream of the feedwater pump in its flow direction.
Downstream of the inlet of the bypass stream into the flue gas path 2 in the flow direction, a third heat transfer device 16 is arranged in this flue gas path. This, as a result of the cooling down of the flue gas stream, which is described further above, to a temperature below the acid dew point, in combination with the downstream electrostatic precipitator 4, acts as an SO3 sink. Water is also fed as heat transfer medium 17 to the third heat transfer device, and thermal energy, which is extracted from the flue gas stream by means of the third heat transfer device, is injected into the water. From the third heat transfer device, the heat transfer medium 17 is fed to the low-pressure preheating system of the water/steam cycle. From this region, the water is also fed to the third heat transfer device for the gathering of thermal energy.
Also shown, and represented by broken lines, are other possibilities of branching off the second partial stream 10 from the flue gas path 2, which can be applied if necessary depending upon process control.
In a way not shown, an injection of CaCO3 into the flue gas
stream, with subsequent SO3 removal, can also be provided
downstream of the electrostatic precipitator 4 and upstream
of the induced-draft fan which follows it, in the flue gas
flow direction.
In a way not shown further, the oxygen can also be added to the partial stream 7. Furthermore, preheating of the partial stream 7 in the gas preheater 3 is also possible, which is not shown here either.
Fig. 2 shows the altogether seven-stage CO2 compression system, which overall is designated 18. Here, subsequent heat transfer devices 19, associated with, and downstream of, each stage, are arranged in each case in the flue gas path 2. For .injection and extraction of thermal energy, these subsequent heat transfer devices 19 are exposed to throughflow by water 2 0 as heat transfer medium which from the subsequent heat transfer devices is fed for low-pressure preheating of the water/steam cycle, and from there is fed back again to the subsequent heat transfer devices. It is possible, for example, to add the thermal energy extracted from the flue gas in 18 to the partial stream 10 before entry into the gas preheater 3 in order to increase the flue gas substance flow in the bypass 13 and therefore to increase the amount of heat which is yielded to the heat transfer medium 15.
Fig. 3 shows an alternative oxygen preheating system in the case of adiabatic air compression. The heat which is
generated as a result of the compression is used for preheating the O2 which is produced in the air separation plant 11. However, it is also possible to provide an oxygen preheating system, which is not shown here, in the case of isothermal air compression. In the case of isothermal compression, saved steam can be used for preheating the oxygen.

Patent claims
1. Method for reintegration of thermal energy, gained from
the flue gas stream (2) of a steam boiler (1) - fired
with carbonaceous fuel, especially coal, preferably dried
brown coal or hard coal, and run in oxyfuel mode - of a
power plant, into the water/steam cycle of the power
plant, wherein a first partial stream (7) which gathers
the carbonaceous fuel is branched off from the flue gas
stream and recirculated to the burners of the steam
boiler, and a second partial stream {10) is branched off
from the flue gas stream and, flowing through a heat
transfer device (3) which is exposed to throughflow,
preferably in counterflow, by at least portions of the
flue gas stream, is recirculated to the burners of the
steam boiler,
characterized in that
thermal energy, in the form of a partial heat flow, is extracted from the flue gas stream (2) and/or from a partial stream (13) of the flue gas stream and injected into the water/steam cycle of the power plant in the region of the high-pressure preheating system and/or of the low-pressure preheating system.
2. Method according to Claim 1, characterized in that a
bypass stream (13), which flows around the heat transfer
device (3) and downstream of the heat transfer device in
the flue gas flow direction re-enters the flue gas stream
(2), is branched off from the flue gas stream, wherein
the bypass stream flows through a second heat transfer
device (14), by means of which the partial flow of
thermal energy is extracted from the bypass stream and
injected into a heat transfer medium (15) which is fed to
the region of the high-pressure preheating system of the
water/steam cycle and injected directly into the
water/steam cycle there, or wherein thermal energy is
extracted from the heat transfer medium there and
injected into the water/steam cycle.
3. Method according to Claim 1 or 2, characterized in that the flue gas stream, downstream of the entry of the bypass stream in the flue gas flow direction, is fed to a third heat transfer device (16) which extracts a further partial heat flow from the flue gas stream and injects it into a further heat transfer medium (17) which flows to the region of the low-pressure preheating system of the water/steam cycle, wherein thermal energy is extracted from the further heat transfer medium in the region of the low-pressure preheating system and injected into the water/steam cycle, or wherein the further heat transfer medium is injected into the water/steam cycle in the region of the low-pressure preheating system.
4. Method according to one of the preceding claims, characterized in that the flue gas stream is fed to a multistage CO2 compression plant (18), in which downstream of each pressure stage in the flue gas flow direction an additional partial heat flow is extracted from the flue gas stream by means of subsequent heat transfer devices (19) in each case and thermal energy of the respective additional partial heat flow is injected into the water/steam cycle in the region of the low-pressure preheating system.
5. Method according to one of the preceding claims, characterized in that a steam volume corresponding to the reintegration of thermal energy is used for driving an air compressor, especially of the air separation plant
(11), and/or for preheating the oxygen which is added to the recirculated flue gas.