Method and plant for reheating flue gases behind a wet flue-gas desulfurization plant

Reheating flue gas behind a wet flue gas desulfurization plant, in which the desulfurized flue gas is admixed with fresh air which had been warmed up first together with the fresh air intended for the combustion, in a heat exchanger by a third medium, then by the flue gas air preheater, by the flue gas flowing into the flue gas desulfurization plant without the temperature falling below the dew point temperature of the flue gas before it enters into the flue gas desulfurization plant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method and a plant for reheating flue gas behind 
a wet flue-gas desulfurization plant. 
2. Description of the Prior Art 
In known plants for wet flue-gas desulfurization of flue gas containing 
gaseous sulfur compounds, the flue gas is cooled down to a flue gas exit 
temperature of about 45.degree. C., depending on the method. This 
temperature is distinctly lower than that temperature which is 
indispensible for maintaining the required buoyancy in the chimney. It is 
also below the dew point. The purified flue gas, also called "pure gas", 
which flows out of the flue-gas desulfurization plant carries with it 
small acidulated water droplets which lead to corrosion in all following 
components. 
In order to improve the buoyancy in the chimney and to evaporate the water 
droplets in the flue gas it is known to reheat the purified gas flowing 
out of the flue-gas desulfurization plant at a temperature of about 
45.degree., with energy from an external source. In this process 
considerable sums must be spent for energy cost. 
It is also known to heat the purified gas flowing out of the flue-gas 
desulfurization plant by bypassing part of the hot undesulfurized flue gas 
which normally flows through the flue-gas desulfurization plant and mixing 
the hot bypassed gas with the purified gas which leaves the flue-gas 
desulfurization plant. This method can be performed at relatively low 
costs. However, it leads to SO.sub.2 emission values which are above the 
permissible emission value for most coal types. 
A plant for reheating flue gas behind a wet flue-gas desulfurization plant 
has also become known, in which the flue gas is first conducted through a 
flue gas-air preheater, subsequently to a flue gas dust separation plant 
and, before it is conducted into a flue-gas desulfurization plant, through 
a raw gas/purified gas heat exchanger. In the latter, it is first cooled 
down to about 60.degree. to 80.degree. C. and, after the flue gas sulfur 
separation, is reheated to about 90.degree. C. This plant operates with 
only small thermal losses and also generates a sufficiently strong 
buoyancy in the chimney. However, due to the fact that the temperature is 
below the dew point, corrosion and contamination in the raw gas/pure gas 
heat exchanger is considerable. 
It has also been proposed to conduct the flue gas through a flue gas air 
preheater, an electrofilter, a cold air preheater into the flue-gas 
desulfurization plant and to admix part of the fresh air which had been 
preheated in the cold air preheater and then in the flue gas/air preheater 
to the purified gas flowing into the chimney. This plant, which brings 
with it good utilization of the thermal energy of the flue gas, requires 
considerable investment for the fabrication of the different additional 
components. Because the temperature is below the dew point, considerable 
corrosion must be expected here in the cold air preheater. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a method for reheating the flue 
gas behind a flue-gas desulfurization plant with a minimum of losses and 
without subjecting the components used for this purpose to corrosive 
influences. 
With the foregoing and other objects in view, there is provided in 
accordance with the invention a method for reheating flue gas which flows 
via a flue gas line into a wet gas desulfurization plant wherein the flue 
gas is desulfurized and cooled which comprises, admixing fresh air with 
the desulfurized flue gas, said fresh air, prior to said admixing, 
together with fresh air to be used for combustion to form flue gas is 
preheated by a third heating medium in a heat exchanger, the preheated 
fresh air from the heat exchanger thereafter flows through a flue gas-air 
preheater wherein preheated fresh air is further heated by heat exchange 
with the flue gas flowing into the flue gas desulfurization plant without 
the temperature falling below the dew point temperature of the flue gas 
before it enters the flue gas desulfurization plant. 
In accordance with the invention there is further provided a plant for 
reheating flue gas behind a wet flue-gas desulfurization plant comprising 
a flue gas line through which flue gas flows, interposed in the flue gas 
line and connected in series are a single flue gas-air preheater wherein 
air passes in heat exchange with hot flue gas cooling the flue gas to 
maximally the dew point, a wet flue gas desulfurization plant for removal 
of sulfur compounds in the flue gas, and a chimney for the discharge of 
desulfurized flue gas into the atmosphere; also connected in series are a 
fresh air blower for suctioning in air from the atmosphere, a heat 
exchanger wherein said fresh air is preheated by a third medium, said 
flue-gas-air preheater wherein the preheated air is further heated, and a 
fresh air line from the flue gas-air preheater to a combustion chamber for 
supplying air for combustion; a hot air line branches off from the fresh 
air line between the flue gas-air preheater and the combustion chamber and 
opens into the flue gas line between the flue gas desulfurization plant 
and the chimney to supply hot air to the desulfurized flue gas. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
method and plant for reheating flue gases behind a wet flue-gas 
desulfurization plant, it is nevertheless not intended to be limited to 
the details shown, since various modifications may be made therein without 
departing from the spirit of the invention and within the scope and range 
of equivalents of the claims.

DETAILED DESCRIPTION OF THE INVENTION 
In one method of the type mentioned at the outset, fresh air is admixed to 
the desulfurized flue gas. However, this fresh air for admixture with the 
desulfurized flue gas was preheated together with fresh air intended for 
combustion in a heat exchanger by a third medium. The thus preheated air 
from this heat exchanger flows through a flue gas-air preheater in heat 
exchange with the flue gas flowing into the flue-gas desulfurization plant 
without the temperature of the flue gas falling below the dew point 
temperature of the flue gas before it enters into the flue-gas 
desulfurization plant. This procedure avoids the need that the purified 
flue gases provided with water droplets are warmed up by heating surfaces 
which, according to experience, get contaminated and corroded quickly. At 
the same time, the cold air preheater or the raw gas/pure gas preheater 
previously used are unnecessary and their non-use represents a saving in 
capital investment. 
An increase of the overall efficiency in the reheating of purified flue gas 
can be achieved if, in a practical further embodiment of the invention, 
part of the heated fresh air is returned from the exit of the flue gas/air 
preheater to the input of the heat exchanger heated by the third medium. 
This results in a higher air entrance temperature of the air-side input of 
the flue gas/air preheater and/or in savings of the heat which has 
otherwise to be supplied by the third medium. A higher air entrance 
temperature of the air-side input of the flue gas-air preheater permits 
more effective cooling of the flue gas without the temperature falling 
below the dew point in parts of the flue gas-air preheater. 
Further details of the invention will be explained with the aid of 
embodiment examples shown in the drawings. 
In the schematic presentation of FIG. 1, it is seen that, in the flue gas 
line 2 leading to the chimney 1 there are arranged in the flow direction 
of the flue gas, in series with each other, a rotary air preheater 3 as 
the flue gas-air preheater, a flue gas dust separation plant 4, a suction 
blower 5, a flue gas desulfurization plant 6, and a steam heated auxiliary 
heat exchanger 7 which can alternatively also be heated by flue gas. In 
the fresh-air line 9 leading to the combustion chamber 8, a fresh air 
blower 10, a steam-heated air preheater 11 and the flue gas-air preheater 
3 are connected in series. From the fresh air line 9, between the rotary 
preheater 3 and the combustion chamber 8, a hot airline 12 is branched off 
which opens into the flue gas line 2 between the flue gas desulfurization 
plant 6 and the auxiliary heat exchanger 7. 
In this plant for reheating flue gas behind (downstream-based on the flow 
of the flue gas to the chimney) a wet flue gas desulfurization plant, the 
flue gas is cooled-down in the rotary air preheater 3, for instance from 
about 410.degree. to about 120.degree.. This cooled-down flue gas is 
transported by the suction blower 5 through the dust separation filter 4 
and the wet flue gas desulfurization plant 6. In the wet flue gas 
desulfurization plant, the flue gas is cooled-down to about 45.degree. C., 
but may vary somewhat depending on the particular desulfurization method. 
This value is far below the dew point and also below that temperature 
which is required for the development of a sufficient buoyancy in the 
chimney 1. The fresh air, simultaneously drawn-in from the outside 
atmosphere by the fresh air blower 10 is heated in this plant in the 
steam-heated air preheater 11 to approximately 60.degree. to 70.degree. C. 
and conducted into the rotary air preheater 3. There, it is heated to 
about 350.degree. C. and is transported at this temperature into the 
combustion chamber 8 for the purpose of burning a fuel, frequently to heat 
a boiler. The fuel is usually derived from fossil fuel and may be solid, 
liquid or gaseous such as coal, liquid petroleum fraction or natural gas. 
The products of combustion contain sulfur compounds such as H.sub. 2 S and 
SO.sub.2, derived usually from the fuel burned, and it is these sulfur 
compounds which are to be removed by wet flue gas desulfurization before 
sending the flue gas to the chimney and discharging it into the 
atmosphere. Approximately 11% of this heated fresh air are admixed via the 
hot air line 12 to the purified flue gases leaving the flue gas 
desulfurization plant 6 and heat up the latter to about 75.degree. C. This 
value is above dew point and makes possible the buoyancy in the chimney 1 
which is just sufficient. 
To improve the buoyancy in the chimney, the flue gas can be heated further 
via the auxiliary heat exchanger 7. For this purpose, the exhaust steam of 
a steam tubrine or another steam consumer or preheated water or hot raw 
gas can be fed into the auxiliary heat exchanger 7. With this auxiliary 
heat exchanger there is no danger of corrosion or contamination because 
the temperature of the inflowing flue gas is already above the dew point 
due to the admixture of hot air. This auxiliary heat exchanger 7, in which 
the flue gas can be heated by the exhaust heat from the power generating 
station to above 100.degree. C., ensures a sufficient buoyancy in the 
chimney 1 during the starting up of the plant and at times of low load. 
FIG. 2 shows the temperature .theta. as a function of the transferred 
amount of heat Q in the rotary air preheater 3 of FIG. 1. In this latter, 
the flue gas 13 flows in a direction opposite that of the fresh air 14. In 
the normal case, i.e. if only the combustion air is heated in the flue gas 
air preheater, which flows toward the combustion chamber 8, see dashed 
curve 14' of FIG. 2 and curve 30' of FIG. 4, then the temperature 
difference between the flue gas and the air on the hot side of the flue 
gas-air preheater is always distinctly smaller than that on the cold side. 
The fresh air heats up faster because of a lower water value, i.e. the 
product of mass flow and specific heat, then does the flue gas 13 because 
of its greater water content. In the embodiment example of FIG. 1, 111% of 
the combustion air are conducted into the rotary air preheater 3 because 
of the admixture of hot air. There, flue gas 13 enters the rotary air 
preheater 3 with a temperature t.sub.RE =410.degree. C. and leaves it with 
a temperature t.sub.RA =120.degree. C. The fresh air 14 is introduced into 
the rotary air preheater 3 with a temperature of t.sub.LE =60.degree. and 
leaves it with an exit temperature t.sub.LA =350.degree. C. The slopes of 
the two temperature curves 13 and 14 are still different. 
t.sub.B in FIG. 2 indicates the sheet metal temperature which is important 
with respect to the corrosion on the cold side of the rotary air 
preheater. It is approximately in the middle between the air input 
temperature t.sub.LE and the flue gas exit temperature t.sub.RA and 
therefore follows approximately the equation 
EQU t.sub.B =(t.sub.LE +t.sub.RA)/2 
This sheet metal temperature must not be below the dew point if corrosion 
and contamination to be avoided, and should therefore not be lower than in 
general 90.degree. C. As a result, the flue gas must not be cooled down 
below 120.degree. C. for an air entrance temperature of 60.degree. C. If 
the air would be allowed to enter into the rotary air preheater 3 without 
preheating at about 45.degree. C., then the flue gases must not be 
cooled-down below 135.degree. C. because otherwise, the sheet metal 
temperature t.sub.B at the cooled end of the rotary air preheater would 
fall below 90.degree. C. The exhaust gas losses, however, are smaller, as 
is well known, as more heat is removed from the flue gases before they 
enter into the flue gas desulfurization plant. For this reason, the air 
preheater 11 heated by a third medium is used for the purpose of removing 
more heat from the flue gases in the rotary air preheater 3, without 
falling below the dew point. 
FIG. 3 shows a plant, somewhat changed from the embodiment example of FIG. 
1, for reheating the flue gas behind a wet flue gas desulfurization plant. 
In this plant, as in the plant of FIG. 1 discussed above, are arranged in 
the flue gas line 16 leading to the chimney 15, in series, a rotary air 
preheater 17 as the flue gas air preheater, a flue gas dust separation 
plant 18, a suction blower 19, a flue gas desulfurization plant 20 and a 
steam or flue gas-heated auxiliary heat exchanger 21. Also on the fresh 
air side, there are arranged in the fresh air line 23 leading to the 
combustion chamber 22, in series with a fresh air blower 24, a steam 
heated air preheater 25 and the rotary air preheater 17. From the fresh 
air line 23 is branched off between the rotary air preheater and the 
combustion chamber, a hot air line 26 which opens into the flue gas line 
16 leading from the flue gas desulfurization plant 20 to the auxiliary 
heat exchanger 21. In contrast to the plant according to FIG. 1, the hot 
air line 26, however, contains a hot air return line 27 which opens into 
the suction channel 28 of the fresh air blower 24. Furthermore, a second 
dust separation plant 29 is built into the hot air line 26. The hot air 
which is separated from the combustion air is about 8 to 25% by volume 
based on the volume of combustion air. 
In this plant for reheating flue gas, heat is first removed from the flue 
gas in the rotary preheater 17, similarly to what has been described with 
the aid of the embodiment example of FIG. 1. For this purpose, hot fresh 
air is admixed to the pure gas later, after dust and sulfur separation, 
and thus its temperature is raised above the dew point. Further heat for 
increasing the buoyancy in the chimney can be transferred to this reheated 
pure gas via the auxiliary heat exchanger 21. 
Differing from the embodiment example of FIG. 1, part of the hot air 
generated, however, is returned into the suction channel 28 of the fresh 
air blower 24. In this manner, the fresh air on the suction side is 
previously warmed up. The fresh air throughput through the steam-heated 
air preheater 25 and the rotary air preheater 17 is increased by this 
recirculating portion of hot air. The result of this, as the diagram in 
FIG. 4 shows, is that the fresh air 30, now flowing into the rotary air 
preheater 17 is increased by about 11% as compared to the embodiment 
example of FIGS. 1 and 2 and is heated up less rapidly for an unchanged, 
i.e. constant quantity of flue gas. This means that the slope of the fresh 
air temperature curve in FIG. 4 is less steep than in FIG. 2. With a 
suitable increase of the recirculating fresh air stream, it becomes 
equally steep or even somewhat flatter than the temperature of the flue 
gas 31 which flows in the opposite direction. 
Assuming in the embodiment example of FIG. 4 again a flue gas entrance 
temperature of t.sub.RE =410.degree. C. and a fresh air exit temperature 
t.sub.LA of 350.degree. C., the flue gas exit temperature t.sub.RA and the 
fresh air entrance temperature t.sub.LE are closer together. With a sheet 
metal temperature t.sub.B =90.degree. C., this leads to an air entrance 
temperature of t.sub.LE =70.degree. C. and to a flue gas entrance 
temperature t.sub.RA =110.degree. C. This means more heat can be removed 
from the flue gas than in the embodiment of FIGS. 1 and 2, without the 
temperature being lower than 90.degree. C. and therefore below the dew 
point. This greater cooling-down of the flue gas in the rotary air 
preheater 17, however, leads to an increased efficiency of the overall 
plant, because the heating of air preheater 25 by the third medium can be 
reduced due to the better utilization of the flue gas heat. The flue gas 
dust separation plant 29 built into the hot air line 26 serves to remove 
the ash transported in the rotary air preheater 17 from the flue gas side 
to the air side. 
In the plant shown in FIG. 3 for reheating flue gas, the hot air return 
line 33 which is equipped with a blower 32 and shown by dashed lines can 
be used instead of the hot air return line 27. In this procedure, an 
improved heat transfer in the air preheater 25 is counter-acted by the 
increased cost of the blower 32. 
The foregoing is a description corresponding, in substance to German 
application No. P 33 24 388.3, dated July 6, 1983, international priority 
of which is being claimed for the instant application, and which is hereby 
made part of this application. Any material discrepancies between the 
foregoing specification and the specification of the aforementioned 
corresponding German application are to be resolved in favor of the 
latter.