Reheat type waste heat recovery boiler and power generation plant

A power generation plant comprising a gas turbine and a waste heat recovery boiler in which feed water is heat-exchanged with exhaust gas from the gas turbine to produce vapor. A steam turbine system incorporates a reheat turbine and a high pressure turbine to which the vapor is supplied from the waste heat recovery boiler, with a generator being driven by the steam turbine system and condenser for condensing vapor from the stream turbine system into condensate and for supplying the condensate to the waste heat recovery boiler as the feed water. The waste heat recovery boiler is provided with a secondary reheater, a super heater, a primary reheater reheating vapor to be supplied to the secondary reheater and an evaporator for evaporating the feed water from the condenser into vapor and for supplying the vapor to the super heater, in order with respect to the exhaust gas flow. The vapor produced in the super heater is introduced into the high pressure turbine and the vapor produced in the secondary reheater is introduced into the reheat turbine.

FIELD OF THE INVENTION AND THE RELATED ART STATEMENT 
The present invention relates to a combined power generation plant 
combining a gas turbine means, waste heat recovery boiler means, a steam 
turbine system and a generator means, and, more particularly, to a reheat 
type combined power generation plant in which a reheat cycle is applied to 
the steam turbine system. 
As compared with the common thermal power generation plant, the combined 
power generation plant incorporating the gas turbine means therewith is 
capable of conducting load changes and starting-up/stopping rapidly and 
effectively. In order to operate such combined power generating plant more 
effectively, it is performed to apply the reheat cycle thereto. Namely, 
not only the evaporator and the super heater, but also the reheater is 
incorporated with the waste heat recovery boiler which recovers thermal 
energy from the exhaust gas from the gas turbine means and generates 
vapor. The vapor generated in the evaporator is superheated in the super 
heater into superheated vapor which is to be introduced into the high 
pressure turbine. The superheater vapor does the work on the high pressure 
turbine and thereafter is exhausted therefrom. The exhaust vapor from the 
high pressure turbine is reheated by the reheater in the recovery boiler 
means to high temperature vapor which is to be introduced into the medium 
or the low pressure turbine. The reheat cycle is effective in improving 
the efficiency in the combined power generation plant in which combustion 
temperature in the gas turbine means is high. The reheat type combined 
power generation plant can obtain a higher efficiency than that of the 
conventional combined power generation plant, for example, one 
incorporating multi-pressure turbine system. 
A prior art waste heat recovery boiler incorporated in the reheat type 
combined power generation plant is disclosed in, for example, 
JP-A-61289201. In such recovery boiler, the exhaust gas from the gas 
turbine means is once separated into two flows. In one flow, a secondary 
super heater and a primary reheater are disposed in order with respect to 
such flow. On the contrary, in the other flow, a secondary reheater and a 
primary super heater are disposed in order with respect to such flow. 
Evaporator means are disposed in a combined exhaust gas flow which is 
integrated by the separated exhaust gas flows. The vapor generated in the 
evaporator means flows through the secondary super heater, the primary 
super heater, high pressure turbine means, the secondary reheater, the 
primary reheater and the medium/low pressure turbine means. 
Such prior art waste heat recovery boiler causes the following 
disadvantages. 
At first, since any consideration is not paid to the change of the 
temperature of exhaust gas which has been heat exchanged in the super 
heaters and the reheaters, there is a difference between the exhaust gas 
temperatures in the respective separated exhaust gas flows. Accordingly, 
the amounts of vapor generated in the respective evaporators differ from 
each other, which are disposed down stream side of the super heaters and 
of the reheaters. In order to make the temperature of the exhaust gas 
which has flowed through one of the separated exhaust gas flows identical 
to that through the other one, it is required to change the thermal load 
distribution on the respective super heaters and the reheaters in 
accordance with the difference between such temperatures. However, it is 
impossible in fact to change such distribution in proper within all over 
load range. 
Secondary, in the combined plant in which heat is recovered from the 
exhaust gas from the gas turbine means, and in which vapor is generated by 
such recovered heat, as different from the thermal plant (e.g., steam 
plant), it is difficult to maintain the temperature and the pressure of 
the vapor in high level. Further, the vapor has a tendency to readily 
become wet vapor after conducting the work on the steam turbine. In 
particularly, the reheat type turbine has the thermodynamic disadvantages 
and the problems of erosion generated in the final stage of the steam 
turbine due to the wetness therein. Further, in a partial load operation 
of the gas turbine, the temperature of the exhaust gas decreases. Such 
decrease exerts adverse influences on the above disadvantages. 
OBJECT AND SUMMARY OF THE INVENTION 
An object of the present invention is to provide a reheat type combined 
power generation plant incorporating a heat recovery boiler capable of 
supplying primary vapor and reheated vapor to the corresponding steam 
turbines, respectively. 
To this end, in the present invention, in order to produce vapor by means 
of using the exhaust gas from the gas turbine means and to suply such 
vapour into steam turbine system incorporating therein high pressure 
turbine means and reheat turbine means, disposed in the exhaust gas flow 
are a secondary reheater, a super heater and a primary reheater for 
reheating vapor to be supplied to the secondary reheater, in order with 
respect to the exhaust gas flow. The vapor generated in the super heater 
is supplied to the high pressure turbine means and the vapor generated in 
the secondary reheater is supplied to the reheat turbine means. 
According to the present invention, the reheat cycle is applicable to the 
combined power generation plant combining the gas turbine means and the 
steam turbine system, whereby improving the thermal efficiency over wide 
range of load of the gas turbine means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawing wherein like reference numerals are used 
throughout the various views to designate like parts and, more 
particularly, to FIG. 1, according to this figure, a power generation 
plant according to one embodiment of the present invention includes a 
waste heat recovery boiler generally designated by the reference numeral 1 
with a gas turbine system including a compressor C, a gas turbine GT and a 
combustor B. The waste heat recovery boiler 1 exchanges heat in exhaust 
gas F from the gas turbine GT with feed water flowing through the waste 
heat recovery boiler 1. The boiler 1 includes a secondary reheater 2, a 
secondary super heater 3, a primary reheater 4, and a primary super heater 
5, which are disposed in order with respect to a flow direction of the 
exhaust gas. F. The boiler 1 further includes an economizer 6 and vapor 
generator means (evaporator) 7 disposed between the economizer 6 and the 
primary super heater 5. During load operation of the gas turbine GT, the 
vapor generated in the evaporator 7 flows into a hih pressure turbine 
(HPT) of a steam turbine system 10 through the primary super heater 5 and 
the secondary super heater 3. Such vapor does the work on the high 
pressure turbine (HPT) and then is introduced into a medium pressure 
turbine (MPT) of the steam turbine system 10 through the primary reheater 
4 and the secondary reheater 2. After the vapor does the work on the 
medium pressure turbine (MPT), the vapor is introduced into a low pressure 
turbine (LPT) and does the work thereon. Thereafter, the vapor is 
condensed into condensate in a condenser 11. Since the boiler 1 is 
plural-stage recovery boiler, the boiler 1 further includes a lower 
pressure side economizer 8 and a lower pressure side evaporator 9. A part 
of feed water flowing through the lower pressure side economizer 8 is 
evaporated into vapor of lower pressure in the lower pressure side 
evaporator 9 and is introduced into the low pressure turbine (LPT). The 
lower pressure vapor together with the vapor from the medium pressure 
turbine (MPT) does the work on the low pressure turbine (LPT). The rest of 
feed water bypasses the lower pressure side evaporator 9 and is returned 
back to the higher pressure side evaporator 7 through the economizer 6. In 
generally, an exhaust vapor temperature from the high pressure turbine 
(HPT) is about 300.degree. C. The exhaust vapor from the high pressure 
turbine (HPT) is reheated by the reheaters 2 and 4, so that a wetness 
fraction of the vapor in a final stage of the low pressure turbine (LPT) 
can be reduced. 
Referring to FIG. 2, the work done on the steam turbine system 10 will be 
described more fully here and below. 
A line 1s is a saturation line. Under the line 1s, the lower region the 
vapor resides, the higher the wetness fraction thereof becomes. To the 
contrary, the higher region the vapor resides, the higher the dryness 
fraction thereof becomes, i.e. the vapor becomes a superheated vapor. A 
line 1i is an intake pressure line which is determined by the pressure of 
vapor introduced into the steam turbine system 10. The values of specific 
enthalpy h and of specific entropy s of the vapor introduced into the high 
pressure turbine (HPT) from the secondary super heater 3 shown in FIG. 1 
are represented by the point a in FIG. 2. A line 1e is an exhaust pressure 
line which is determined by the pressure of vapor introduced into the 
condenser 11 from the low pressure turbine (LPT). The point e" represents 
values of specific enthalpy h and of specific entropy s of the vapor 
without employing a reheat cycle. The point f represents values of 
specific enthalpy h and of specific entropy s of the vapor with employing 
the reheat cycle. 
In FIG. 2, the segment abe represents change of vapor condition in the 
steam turbine without reheating the vapor. Accordingly, as the work done 
on the steam turbine increases, the vapor condition becomes under the 
point e, so that the vapor includes the wet vapor. If the steam turbine 
exhausts the vapor at the point e' on a characteristic line 1e' which does 
not reside in the wet vapor region, it is impossible to recover heat from 
the exhaust vapor sufficiently. On the contrary, if the steam turbine 
exhausts the vapor at the point e" on the exhaust pressure line 1e in the 
wet vapor region, it become possible to sufficiently recover heat from the 
exhaust vapor. However, the steam turbine may be damaged by wet vapor. 
The segment abcdf represents change of vapor condition in the steam turbine 
with reheating the vapor. The vapor at the point a does the work on the 
high pressure turbine (HPT) and declines to the point b. Thereafter, the 
vapor is reheated to rise to the point c. Accordingly, such reheated vapor 
can do the work cdf on the medium (MPT) and the low pressure turbine 
(LPT). Even though, under the saturation line 1s, i.e. between the point d 
and the point f, the vapor includes wet vapor, the metal material 
selection in the final stage in the turbine makes it somewhat resistant 
against such wet vapor. Since the vapor is reheated and can be maintained 
in a higher temperature level, the wetness fraction in the final stage of 
the steam turbine can be reduced. Namely it can be possible to reduce the 
wetness fraction from a degree e--e" to a degree d-f. 
In a power generation plant combining a steam turbine system and a gas 
turbine means, the temperature in exhaust gas from the gas turbine means 
is low on a lower load operation of the gas turbine means. In the plant 
shown in FIG. 1, since the reheater is disposed in an upper most stream 
side of the exhaust gas, the vapor can be reheated to a high temperature 
close approximate to the exhaust gas temperature. Accordingly, optimum 
vapor can be obtained, which is optimized with respect to wetness fraction 
in the final stage of the steam turbine. In this regard, the reheat cycle 
is effective. However, since the flow rate of the primary vapor is 
substantially identical to that of the reheated vapor, if the reheater is 
simply disposed upstream side of the super heater, the temperature of 
vapor in such super heater is lowered. 
To avoid this, according to the present invention, the separate reheaters 
are so disposed as to interpose the super heater therebetween in order to 
balance the heat absorption. Accordingly, the heat exchange 
characteristics shown in FIG. 3 can be obtained, so that outlet 
temperatures of vapor in the super heater and the reheater are balanced. 
FIG. 3 shows the relationship between the vapor/feed water temperature in 
the respective apparatus in the heat recovery boiler 1 and the exhaust gas 
temperature therein. As shown in FIG. 3, the exhaust gas from the gas 
turbine means exchanges heat with the apparatus in the heat recovery 
boiler 1 and then the temperature of the exhaust gas is lowered. On the 
contrary, the vapor/feed water is heated in the economizers 8, 6, the 
evaporator 7, the primary super heater 5 and the secondary super heater 3, 
in order. Namely, the feed water of temperature To is heated and then 
changed into the vapor of temperature T1. The vapor of temperature T1 from 
the secondary super heater 3 does the work on the high pressure turbine 
(HPT) and then is changed into the vapor of temperature T2. The vapor of 
temperature T2 is introduced into the primary reheater 4 and is reheated 
in the primary reheater 4 and the secondary reheater 2 to be changed into 
the reheated vapor of temperature T3. Accordingly, it is possible to 
obtain the reheated vapor of the highest temperature and it is also 
possible to maintain the primary vapor in a high temperature level by 
means of selecting heat capacities of the super heaters and the reheaters 
proper. The vapor generated in the lower pressure side evaporator 9 has a 
temperature corresponding to the saturation temperature T4 in FIG. 3. 
To the contrary, in the prior art, since the super heater and the reheater 
are parallel to flow of exhaust gas from the gas turbine so as to balance 
the temperatures in the super heater and the reheater, the energy in the 
exhaust gas is substantially separated into two halves. If the 
heat-transfer area is changed, the exhaust gas energy is separated into 
two parts which are not identical to each other. However, when the 
temperatures in the super heater and the reheater are balanced, the 
exhaust gas energy is separated into two parts which are approximately 
equal to each other. Further, in the prior art, the flow rate of the vapor 
flowing through the super heater is substantially identical to that 
through the reheater. Accordingly, when the temperature of the exhaust gas 
from the gas turbine is lowered, or the gas turbine operates in a low load 
level, the reheater can only use no more than a half of exhaust gas flow 
rate due to countermeasure for wet vapor in the final stage of the steam 
turbine. Accordingly, in the prior art, in order to obtain a heat balance 
between the super heater and the reheater the same as that of the present 
invention, it is required to prepare the super heater and the reheater, 
each of which has a cross section with respect to the exhaust gas flow 
direction twice that of the present invention. Further, in order to 
restrict the exhaust gas speed in the heat transfer surface of each of the 
super heater and the reheater within a predetermined range, i.e. in 
generally from 20 m/s to 30 m/s, the exhaust gas receiving part (the super 
heater and the reheater) of the waste heat recovery boiler must be 
considerably enlarged, so that disadvantages with respect to the 
arrangement and the cost are increased. The present invention can solve 
such disadvantages. 
FIG. 4 shows the temperature characteristics of the exhaust gas in the 
respective parts of the waste heat recovery boiler with respect to the 
load of the gas turbine. 
The temperature of the exhaust gas from the gas turbine means decreases in 
accordance with the load decrease. The reheater is disposed in an 
upper-most stream side with respect to the exhaust gas flow direction. The 
reheated vapor can be readily maintained in a maximum temperature level 
close to the exhaust gas temperature, and is most effective with respect 
to the wet vapor in the final stage in the steam turbine. Further, the 
temperature characteristics in the super heater and the evaporator which 
are disposed in a down stream side are also improved the same as the 
reheater, so that the temperature distribution is well balanced. 
As apparent from the above-mentioned explanation, since the reheater is 
disposed in the upper-most stream side, the reheated vapor having a 
maximum temperature can be obtained even though the temperature of the 
exhaust gas from the gas turbine is lowered, or the gas turbine operates 
in a low load level, so that it is possible to avoid the erosion in the 
final stage of the steam turbine due to wet vapor. 
Further, referring back to FIG. 4, the following explanation will be made 
to the vapor in the outlet of the high pressure turbine (HPT) in the plant 
in which the super heater is interposed between the separate heaters. 
Generally, it is required to maintain the super heat degree of the vapor in 
the outlet of the high pressure turbine in a predetermined level. e.g. 
50.degree. F, in order to prevent the erosion due to the wet vapor from 
generating in the final stage of the turbine, and so on. Further, when the 
temperature of the exhaust gas from the gas turbine is low or the gas 
turbine operates in a low load, the temperature distribution in a down 
stream side of the exhaust gas flow is flat and the temperature difference 
is smaller than that on the high load operation. The degree of temperature 
decrease in the down stream side becomes small. Namely, due to decrease in 
the exhaust gas temperature, the heat exchange in the secondary reheater 
is not fully performed, and then the amount of vapor generation therein is 
reduced. However, the heat exchange in the secondary super heater, which 
is disposed in a down stream side of the secondary reheater, is performed 
fully to supplement the heat exchange loss. The lower the load, the 
smaller the temperature decrease in the down stream side becomes, thereby 
maintaining the temperature. In other words, when the temperatures in the 
outlets of the reheater and the super heater on full (100%) load operation 
are setted to any desired temperatures, the temperature change in the 
super heater becomes gradual on the load decrease, which is disposed in 
the down stream side of the reheater, which is disposed in the down stream 
side of the reheater, thereby being capable of fully maintaining the 
degree of super heating in the outlet of the high pressure turbine. 
The other embodiment of the present invention shown in FIG. 5 additionally 
includes a bypass passage 100 for controlling the vapor temperature, which 
bypasses the secondary reheater 2 and connects the inlet of the secondary 
reheater 2 to the outlet thereof. Further, any other varieties of the 
recovery boiler can be readily obtained by providing the bypass passage 
similar to the bypass passage 100 to the super heater for controlling the 
vapor temperature therein, which is disposed between the separate 
reheaters. 
According to the present invention, it can be possible to apply the reheat 
cycle to the plant combining the steam turbine system and the gas turbine 
means, whereby obtaining a safe operation of the plant having a high 
thermal efficiency over a wide range of the load. 
First, the reheater is disposed in an upper most stream side in the waste 
heat recovery boiler, so that the vapor in the reheater can be reheated up 
to a temperature close to the temperature of the exhaust gas from the gas 
turbine. Therefore, it becomes possible to maintain the wetness fraction 
of the vapor in the final stage of the low pressure turbine within a 
proper range. 
Secondly, the separate reheaters are so provided that the super heater is 
interposed between such reheaters, so that the primary vapor is maintained 
in a high temperature level even though the gas turbine operates in a 
partial load. Therefore, it is also possible to maintain the wetness 
fraction of the vapor in the outlet of the high pressure turbine within a 
proper range.