Procedure for improving the efficiency of a steam power plant process

The invention concerns a procedure for improving the efficiency of a steam power plant process wherein damp fuel is used, which is dried by conducting at least part of the flue gas flow to a fuel drying process. In addition to the feed water the combustion air is preheated in steam power plants with tapped steams from the turbine, in counter-pressure power plants with tapped steams and/or counter-pressure steams, and in remote heating power plants with tapped steams and/or remote heating water. As taught by the invention, the temperature of the flue gas going to the damp fuel dryer installation (10), or the cooling of the flue gas flow by-passing it, is controlled by taking thermal energy from the flue gas and transferring it to the combustion air and/or the feed water, their quantity and/or temperature being controlled in accordance with the cooling requirements of the flue gases determined by the drying requirements of the damp fuel.

BACKGROUND OF THE INVENTION 
The present invention concerns a procedure for improving the efficiency of 
a steam power plant process, in the process damp fuel is used which is 
dried by conducting at least part of the flue gas flow to dry the fuel. 
Through the application DE No.-3 111 011 is known a procedure applied in a 
steam power plant process, in which preheating of the combustion air is 
effected with a calorifier. Cooling of the flue gases is effected with a 
flue gas air preheater (extra economizer). The aim of the procedure 
disclosed in this reference is to achieve good protection of the flue gas 
air preheaters (12) against sulphuric acid corrosion. The efficiency of 
the power plant process cannot be significantly increased with the 
apparatus design of the reference. No possibilities of control for 
controlling the ultimate flue gas temperature are disclosed in the 
reference. On the other hand no need of such control exists in the process 
described in the reference. 
In the application DE No.-2 243 380 a power plant process is disclosed in 
which preheating of the combustion air is with the aid of water 
circulation, the feed water going to the water circulation being preheated 
with tapped steam. Moreover, flue gas cooling has been implemented with a 
partial feed water flow in this process. The aim in the process of the 
reference is primarily to separate the air and flue gas passages as 
regards lay-out. No significant improvement of the efficiency of the power 
plant process is achieved in the process of the reference. The reference 
also fails to disclose any so-called flue-gas air preheater ("Luvo"), 
i.e., a means in which the thermal energy carried by the flue gases is 
used to preheat the combustion air for the fuel of the boiler. 
It is thus known in the prior art, and also through the references cited 
above, to improve the efficiency of a steam process by heating the feed 
water with steam tapped from the turbine. It is also known, though perhaps 
not as common, that preheating of the combustion air is also performed 
with tapped steam. 
SUMMARY OF THE INVENTION 
The procedure of the invention is mainly characterized in that in addition 
to the feed water the combustion air is also preheated in steam power 
plants with steams tapped from turbines, in back-pressure power plants 
with tapped steams and/or back pressure steams, and in area heating 
installations with tapped steams and/or remote heating water, and that the 
temperature of the flue gas going to the damp fuel dryer installation or 
the cooling of the flue gas flow by-passing it is controlled by drawing 
thermal energy from the flue gas and transferring it to the combustion air 
and/or to the feed water, the quantity and/or temperature of these being 
controlled in accordance with the cooling requirements of the flue gases 
determined by the drying requirements of the damp fuel. 
Applying the teachings of the invention, a procedure has been implemented 
in which the temperature of the flue gas going to the dryer installation 
is controlled in accordance with the drying requirements of the wet fuel. 
Need of such control is particularly encountered when the fuel is dry 
peat. In the case of peat, the temperature of the flue gas going to the 
dryer varies in the range 220.degree. C. to 650.degree. C. The temperature 
normally required is over 300.degree. C. In the case of said temperatures 
exceeding 300.degree. C., flue gas cooling causes no problems. In that 
case all of the air as well as all of the feed water can be heated with 
steam tapped from the turbine or with back pressure steam of a back 
pressure power plant, or with remote heating water in an area heating 
power plant. If exceptionally high flue gas temperatures are needed, this 
is normally arranged such that part of the flue gases bypass a 
considerable part of the boiler's cooling heat surfaces. The possibilities 
of control afforded by the present invention are only needed when 
exceptionally low temperatures, i.e. under 300.degree. C., are required. 
It is then necessary to forego part of the air or feed water preheating in 
order that cold enough air or feed water might be available in the last 
flue gas cooling step. 
If the dryer installation is only dimensioned to use part of the flue 
gases, in this case, too, an arrangement for cooling the remainder is 
required in which there is available a given quantity of cold air or feed 
water, typically having a temperature below lOO.degree. C. The amount of 
this cooling requirement has to be controllable in direct proportion to 
the variation in quantity of the gas that has to be cooled. 
In the procedure of the invention for improving the efficiency of a steam 
power plant process provided with damp fuel drying apparatus, at least 
part of the combustion air is preheated, in addition to the feed water, 
with tapped steam from the turbine without any increase of flue gas losses 
resulting therefrom. In the procedure of the invention the temperature of 
the flue gas going to the damp fuel dryer or the cooling of the flue gas 
partial flow by-passing the dryer is controlled. As taught by the 
invention, the control of the temperature of the flue gas going to the 
dryer can be carried out in a way which is optimal as regards energy 
householding. This control is carried out on the basis of the requirements 
imposed by the boiler loading and by the moisture content of the peat. 
The invention is described in the following, referring to certain 
advantageous embodiments of the invention, presented in the figures of the 
attached drawings, yet to which the invention is not meant to be 
exclusively confined.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, a modern mill dryer installation is depicted. The fuel is 
conducted from a peat silo 1 to the mill 2. From the mill 2, the fuel is 
transported to cyclone separators 3 and 4. The combustion air is carried 
to the boiler 5 by the duct 8, to the vicinity of the grate 6 of the 
boiler 5. The greater part of the combustion air is branched off by the 
duct C, through a calorifier and air preheater 7 to the boiler 5. The flue 
gases are removed from the boiler by the ducts D and E. The duct D carries 
the flue gases through the air preheater 7 to electric filters 8, and 
further to the stack 9. The hot drying gas is carried by the duct E to the 
duct of the cold drying gas F, and these gases are carried to join the 
material flow G entering the mill 2. The fuel G is separated from the 
cyclone separators 3 and 4 and carried by the duct H to the boiler 5. From 
the cyclone 3, the flue gas is carried to the cyclone 4 and further to the 
duct I, from which a partial flow I' is branched off, back to join the 
material flow H going to the boiler, and the part I" is directed to go 
straight to the stack 9. 
In FIG. 2, a modern flash dryer is depicted. The damp peat is introduced in 
the flash dryer 2a by the duct 1a. The dryer gas is introduced by the duct 
3a. The peat dust is separated from the cyclone M.sub.5 by the duct 4a. 
The peat dust and carrier air go by the duct 5a to the boiler M.sub.1. The 
blower M.sub.2 is used to transport the combustion air by the duct 6a to 
the air preheater and further to the boiler M.sub.1, or part of the flow 
is by-passed through the by-pass 7a on the air side, to the combustion air 
duct 6a going straight to the boiler M.sub.1. The reference numeral 8a in 
the figure indicates the gas-side by-pass of the air preheater. The 
by-pass of the superheater M.sub.3 and economizer M.sub.4 on the gas side 
is indicated by reference numeral 9a. 
In FIG. 3 are depicted the procedure and apparatus design of the invention 
wherein all flue gases from the boiler go to the dryer installation. In 
the figure the dryer installation is indicated with reference numeral 10, 
the boiler with 11, and a steam turbine or equivalent with 12. The flue 
gases from the boiler 11 are conducted by the duct 13 to the dryer 
installation 10, from which they are further conducted out through the 
stack. The fuel is introduced in the dryer installation 10, and the dried 
fuel is carried by the path 16 or equivalent to the boiler 11. The 
combustion air for the fuel is introduced by the duct 13 in the boiler 11. 
The feed water is carried to the boiler 11 by the duct 15, and the steam 
duct between boiler and turbine is denoted with reference numeral 14 in 
the figure. In the figure are also depicted, indicated with common 
reference numerals 20-25, combustion air preheaters, and the feed water 
preheaters have similarly been indicated with common reference numerals 
30-35. The means causing air circulation is indicated with 18, and the 
action means producing feed water circulation are indicated with 19. In 
the process chart, the temperature control point in the flue gas flow is 
indicated with P. The feed water going to the boiler 11 is preheated with 
tapped steams 30', 31', 32', 33', 34', 35' from the turbine 12. The 
function of actual flue gas temperature control unit is provided by the 
combustion air preheating means 26 (Luvo) inserted in the flue gas duct 
and by the steam preheaters 20, 21, 22, 23, 24, 25 for air. The combustion 
air is by the aid of a blower 18 or equivalent carried through the heat 
exchangers 25-20 to the heat exchanger 26, or Luvo, disposed in the flue 
gas duct. The heat is recovered from the hot flue gas in the heat 
exchanger 26. In the heat exchangers 20-25 the temperature of the 
combustion air has been stepwise elevated with the aid of tapped steam 
flows 20'-25' from the turbine. The temperature at the points P of the 
flue gas flow going to the dryer installation 10 is controlled by 
controlling the temperature of the combustion air going to the air 
preheater 26. The regulating dampers K.sub.1 and K.sub.2 are used to 
control the quantity of combustion air going through the heat exchanger 
26. At the same time the temperature of the flue gas in the flue gas duct 
13, at the point P, is also controlled. By closing the damper K.sub.2 and 
opening the damper K.sub.1, the desired amount of air, depending on the 
opened position of the valve K.sub.1, is caused to flow through the heat 
exchanger 26 by the duct 17b and the rest is caused to by-pass it through 
the by-pass duct 17c. A duct 17d leads from the branching point 27 after 
the air preheater 26, to the boiler 11. 
Control in order to increase the cooling may, for instance, be accomplished 
in the following way. The damper K.sub.1 is first closed and the damper 
K.sub.2 opened until all air passes through the air preheater 26. The flue 
gas cooling is then increased by throttling the valve V.sub.1 regulating 
the tapped steam flow 20' from the turbine 12. Hereby the temperature of 
the air going to the heat exchanger, or air preheater 26 in the flue gas 
duct 13 is lowered, and this increases the cooling of the flue gas. After 
the valve V.sub.1 has been fully closed, the operation is continued by 
throttling and closing the valves V.sub.2, V.sub.3, V.sub.4, V.sub.5 and 
V.sub.6, i.e., by taking the air preheaters out of operation one after the 
other, starting with the tapped steam branch carrying the highest 
pressure. When the fuel is peat, manipulation of three valves, V.sub.1, 
V.sub.2 and V.sub.3, already suffices for adequate control. 
The mode of control just described affords the advantage of good energy 
economy. The factors effecting some cost increase include the need of a 
heat exchanger, or air preheater, 26 and, particularly, the need to 
provide control means K.sub.1 and K.sub.2 in conjunction with said air 
preheater 26. 
In FIG. 4 is depicted another mode of control, and apparatus assembly, 
employed in the procedure of the invention. FIG. 4 presents a procedure 
and apparatus design in which likewise the total flue gas flow is 
conducted to the dryer installation 10 by the duct 13. The control aiming 
at regulating the temperature at the point P is carried out with the aid 
of feed water preheaters, or economizers, 36 and 37 disposed in the flue 
gas duct. The flue gas flow is in contact with the heat exchange surfaces 
between feed water and flue gases in the economizers 36 and 37, and the 
feed water has thus been arranged to take up heat from the flue gas. For 
controlling the flue gas temperature to be such as is desired, in view of 
the drying process, at the point P in the flue gas duct, one has to be 
able to control the water flow passing through the economizers, or feed 
water preheaters, 36 and 37 and/or its temperature. 
This control may be implemented, for instance, as follows. The feed water 
valve V.sub.2 is kept open to begin with, and the valves V.sub.1 and 
V.sub.3 are kept closed. This control is applied when a high flue gas 
temperature is needed at the point P, i.e., when high power is needed to 
dry the wet fuel. The economizers 36 and 37 operate in series, and all 
feed water passes through them at its maximum temperature. When the 
temperature at the point P has to be lowered, the requisite flue gas 
cooling effect is increased by closing the valve V.sub.2 and opening 
valves V.sub.1 and V.sub.3 according to the cooling requirements. The 
cooling rate control in question can be steplessly accomplished by 
manipulating the valves V.sub.1, V.sub.2, V.sub.3. In such control, part 
of the feed water by-passes the high pressure preheaters through the valve 
V.sub.3, and the water in question arrives at the feed water preheater 36 
with considerably lowered temperature. This enhances the cooling of the 
flue gases. 
The feed water duct 15a divides at the point 43 into a duct section 15c and 
duct section 15b. The duct section 15b contains the valve V.sub.3. The 
duct section 15c contains the steam preheaters 30, 31 and 32. The output 
of the steam preheater 30 communicates by the duct 15d with the valve 
V.sub.2 and with the branching point 39 between duct 15d and duct 15b. A 
duct leads from the branching point 39 to the first economizer 36, from 
which a connecting duct 15f leads to the second economizer part 37, these 
economizer parts thus being connected in series. The duct 15g leads from 
the second economizer part 37 to the boiler 11. The duct 15f between the 
economizer parts 36 and 37 is joined at the branching 44 by the duct 
between branching points 44 and 42, which contains the valve V.sub.1. 
The advantage of the control system of FIG. 4 is its simple air and gas 
duct lay-out. No combustion air preheater operating on flue gas (Luvo) is 
needed. The design of FIG. 3 is less expensive in investment than that of 
FIG. 3, but it is inferior to the design of FIG. 3 in energy economy. 
Preheating of the combustion air is in the design of FIG. 4 as taught by 
the invention, also by means of tapped steams. The combustion air is 
carried through the heat exchangers 25, 24, 23, 22, 21 and 20, in which 
the temperature of the combustion air is stepwise increased by carrying 
the tapped steams 20', 21', 22', 23', 24' and 25' from the turbine to said 
heat exchangers. 
In FIG. 5 is depicted an embodiment of the invention in which only part of 
the flue gases is carried through the dryer installation 10 and in which 
only part of the flue gas is cooled in the dryer installation 10. The rest 
is carried through a side duct and cooled to temperature at removal about 
115.degree. C. The control of the dryer implies varying the flow rate of 
the gas flow A in the side duct 13b. Thermodynamically best results are 
obtained when the rate of cooling material flow, which is thus excluded 
from the preheating taking place with tapped steams, is controlled in 
proportion to the gas flow A that has to be cooled. In the design of the 
figure, part of the gas flow is carried by the flow duct 13a to the dryer 
installation 10, and the rest of the gas flow is carried by the duct 13b 
past the dryer installation 10 and further out through the stack. In the 
duct 13b has been inserted, between the boiler 11 and the branching point 
13b joining the flue gas duct 13c leading out from the dryer installation, 
a combustion air 26'. The feed water is preheated with the tapped steams 
30', 31' , 32', 33', 34', 35' by carrying the tapped steams from each 
stage of the turbine to the respective heat exchangers 30, 31, 32, 33, 34, 
35. 
The combustion air is similarly carried by the aid of a blower 18 or 
equivalent apparatus through the combustion air preheaters 25, 24, 23, 22, 
21 and 20 to the output branch of the air preheater 26', to its branching 
point 27'. Preheating of the combustion air is accomplished with the 
tapped steams 20', 21', 22', 23', 24' and 25' from the turbine in similar 
fashion to feed water preheating. Between the combustion air preheaters 24 
and 23 has been disposed a branching point 27" for the combustion air duct 
17b going to the air preheater 26'. The dampers K.sub.1 ' and K.sub.2 ' 
control the combustion air passing through the air preheater 26', and 
therefore said dampers K.sub.1 ' and K.sub.2 ' control that combustion air 
flow which comes into contact with the heat surfaces of the air preheater 
26' in the flue gas duct 13b and to which combustion air thermal energy of 
the flue gas is transferred. 
In FIG. 6 is depicted an embodiment of the invention in which only part of 
the gas flow is carried through the dryer 10 and in which the whole 
combustion air quantity is preheated with the tapped steams 20', 21', 22', 
23', 24', 25' from the turbine. Cooling of the flue gas flow A is 
accomplished with feed water in the economizers 40, 41 in the flue gas 
duct branch 13b. Part of the flue gas is conducted by the duct 13b past 
the wet fuel dryer installation 10, and the rest of the flue gas is 
carried by the duct 13a to the dryer installation 10, where thermal energy 
of the flue gas is utilized in drying the wet fuel. From the dryer 
installation 10 the flue gas is removed by the duct 13c in that the duct 
13b joins the duct 13c over the branching point 13b ' . Thus, both the 
fuel gases coming from the duct 13b and those coming from the duct 13c are 
removed through one single duct. In the flue gas duct 13b has been 
disposed at least one feed water preheater; advantageously two are 
provided: the preheaters 41 and 42. The feed water is conducted, pumped by 
a pump 19a, 19b or equivalent, through the feed water preheaters 35, 34, 
33, 32, 31, 30. The temperature of the feed water is raised in said heat 
exchangers with the tapped steams 35', 34', 33', 32', 31' and 30 from the 
turbine. The valves V.sub.a and V.sub.b are used to control that feed 
water fraction which goes to the economizers 41 and 42, and therefore it 
is possible by controlling the feed water quantity passing through the 
economizers 41 and 42 also to control that thermal energy which transfers 
from the flue gas to the feed water, in other words, one may thus control 
the flue gas temperature and/or the feed water temperature to be as 
desired. The economizer 40, which is first in the direction in which the 
feed water travels, has been so disposed in the flue gas duct 13b that the 
economizer 40 has been connected to the input side and output side of the 
feed water preheater 33 through the duct 15b, in which further has been 
disposed a valve V.sub.b opening and closing the flow. The second 
economizer 41 is similarly disposed in the flue gas duct branch 13b in 
such manner that a by-pass branch 15c has been conducted from the steam 
preheating circuit 15a past the steam preheaters 30, 31, 32 and directly 
to the duct going to the boiler. In the by-pass branch 15c has been 
provided the valve V.sub.a closing and opening the flow. 
By the procedure of the invention, e.g. in a condensing power plant savings 
amounting to about 6 MW are achieved when the size of the power plant is 
150 MW. This is a relative saving of about 4%. This is based on the 
assumption that a flash dryer is used and that the fuel is damp peat. A 
further assumption is that full air preheating with tapped steam is 
applied, as presented in the foregoing figures. 
In FIG. 7 is depicted an embodiment of the invention in which in an area 
heating power plant the preheating of the combustion air has been arranged 
to take place not only with tapped steam 20', 21', and 20' from the 
turbine 12' but also with remote heating water L.sub.1. From the boiler 
11, the flue gas is conducted to the dryer installation 10, and the flue 
gas temperature is controlled in a way similar to that in FIG. 3, with the 
aid of an air preheater 26". 
In FIG. 8 is depicted an embodiment of the invention in which once again 
the combustion air is preheated with remote heating water L.sub.2 and with 
tapped steams 20', 21' and 20" from a turbine. In this embodiment, all 
combustion air is conducted to the boiler 11 past the flue gas ducts, and 
the temperature at the point P of the flue gas going to the dryer 
installation 10 is controlled with the aid of economizers 36' and 37' in a 
way similar to that in the embodiment of FIG.4. 
In the embodiment of FIG. 9, air preheating is, in addition to heating with 
tapped steam from a counter-pressure power plant turbine 12, also with 
counter-pressure steam L. In the embodiment of the figure, all preheated 
air is conducted directly past the flue gas duct to the boiler 11. The 
temperature of the flue gas going to the dryer installation is controlled 
in a manner similar to that in the embodiment of FIG. 4, with the aid of 
economizers 36', 37' disposed in the flue gas duct and by controlling the 
feed water flow with the aid of valves V.sub.1 " and V.sub.2 " and V.sub.3 
'". 
In FIG. 10 is depicted an embodiment of the invention in which air 
preheating is carried out with tapped steam from the turbine 12 of a 
counter-pressure power plant and, in addition, with counter-pressure steam 
L.sub.4. In this embodiment the preheated combustion air is conducted to 
an air preheater 26'", with the aid of which the temperature of the flue 
gas going to the dryer is controlled in similar manner as in the 
embodiment of FIG. 3. 
Also conceivable are those embodiments of the procedure of the invention in 
which the apparatus arrangement is otherwise like that in FIGS. 5 and 6 
except that the combustion air is preheated with tapped steam from a 
turbine and/or counter-pressure steam or with tapped steam from a turbine 
and/or remote heating water, as in the embodiments of FIGS. 7-10.