U.S. patent application number 13/062738 was filed with the patent office on 2011-07-07 for continuous steam generator.
Invention is credited to Martin Effert, Joachim Franke, Frank Thomas.
Application Number | 20110162592 13/062738 |
Document ID | / |
Family ID | 41820262 |
Filed Date | 2011-07-07 |
United States Patent
Application |
20110162592 |
Kind Code |
A1 |
Effert; Martin ; et
al. |
July 7, 2011 |
CONTINUOUS STEAM GENERATOR
Abstract
A continuous steam generator is provided. The continuous steam
generator includes a number of burners for fossil fuels, the
outside wall thereof being fully or partially formed from steam
generator tubes welded together in a gas-tight manner. The burners
are arranged in a combustion chamber, and a vertical gas duct is
mounted downstream of the combustion chamber above a horizontal gas
duct on the hot gas side. A first part of the steam generating
tubes forms a system of evaporation tubes mounted upstream of a
water separator system, on the flow medium side, and a second side,
and a second part of the steam generating tubes forms a system of
superheater tubes mounted downstream of the water separator system
on the flow medium side. Superheater tubes adjacent and parallel to
evaporation tubes are mounted directly downstream of the water
separator system on the flow medium side.
Inventors: |
Effert; Martin; (Erlangen,
DE) ; Franke; Joachim; (Nurnberg, DE) ;
Thomas; Frank; (Erlangen, DE) |
Family ID: |
41820262 |
Appl. No.: |
13/062738 |
Filed: |
September 4, 2009 |
PCT Filed: |
September 4, 2009 |
PCT NO: |
PCT/EP2009/061468 |
371 Date: |
March 8, 2011 |
Current U.S.
Class: |
122/406.4 ;
122/324; 122/460; 122/488 |
Current CPC
Class: |
F22B 29/06 20130101;
F22B 37/26 20130101; F22B 21/345 20130101 |
Class at
Publication: |
122/406.4 ;
122/324; 122/488; 122/460 |
International
Class: |
F22B 29/06 20060101
F22B029/06; F22B 37/26 20060101 F22B037/26; F22B 21/34 20060101
F22B021/34; F22G 3/00 20060101 F22G003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 9, 2008 |
EP |
08015871.0 |
Claims
1.-4. (canceled)
5. A continuous steam generator, comprising: a plurality of burners
for fossil fuel; a plurality of steam generator tubes; a combustion
chamber; a vertical gas duct; a horizontal gas duct; and a moisture
separation system, wherein a surrounding wall of the plurality of
burners is completely or partially formed from steam generator
tubes welded together in a gas-tight manner, wherein the plurality
of burners are disposed in the combustion chamber downstream of
which the vertical gas duct is mounted above the horizontal gas
duct on a hot gas side, wherein a first part of the plurality of
steam generator tubes is implemented as a system of evaporator
tubes mounted upstream of a moisture separation system on a flow
medium side, and wherein a second part of the steam generator tubes
is implemented as a system of superheater tubes mounted downstream
of the moisture separation system on the flow medium side, and
wherein a plurality of superheater tubes in parallel contiguity
with the plurality of evaporator tubes are mounted immediately
downstream of the moisture separation system on the flow medium
side.
6. The continuous steam generator as claimed in claim 5, wherein a
combustion chamber wall is formed from the plurality of evaporator
tubes and a sidewall of the horizontal gas duct is formed from the
plurality of superheater tubes, and wherein the plurality of
superheater tubes adjacent to the combustion chamber is mounted
immediately downstream of the moisture separation system on the
flow medium side.
7. The continuous steam generator as claimed in claim 5, wherein a
top of the once-through steam generator is formed from the
plurality of superheater tubes which are mounted immediately
downstream of the moisture separation system on the flow medium
side.
8. The continuous steam generator as claimed in claim 5, wherein
vertically disposed superheater tubes in parallel contiguity with
the plurality of evaporator tubes are designed such that the flow
medium flows through the plurality of superheater tubes from top to
bottom.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the US National Stage of International
Application No. PCT/EP2009/061468, filed Sep. 4, 2009 and claims
the benefit thereof. The International Application claims the
benefits of European Patent Office application No. 08015871.0 EP
filed Sep. 9, 2008. All of the applications are incorporated by
reference herein in their entirety.
FIELD OF INVENTION
[0002] The invention relates to a once-through ("continuous") steam
generator comprising a number of burners for fossil fuel, the
surrounding wall thereof being completely or partially formed from
steam generator tubes welded together in a gas-tight manner. Said
burners are disposed in a combustion chamber downstream of which a
vertical gas duct is mounted above a horizontal gas duct on the hot
gas side, a first part of the steam generator tubes being
implemented as a system of evaporator tubes mounted upstream of a
moisture separation system on the flow medium side, and a second
part of the steam generator tubes being implemented as a system of
superheater tubes mounted downstream of the moisture separation
system on the flow medium side.
BACKGROUND OF INVENTION
[0003] In a fossil fired steam generator, the energy of a fossil
fuel is used to produce superheated steam which in a power plant,
for example, can then supplied to a steam turbine for power
generation. Particularly at the steam temperatures and pressures
prevalent in a power plant environment, steam generators are
normally implemented as water tube boilers, i.e. the water supplied
flows in a number of tubes which absorb energy in the form of
radiant heat of the burner flames and/or by convection from the
flue gas produced during combustion.
[0004] In the region of the burners, the steam generator tubes are
usually welded together in a gas-tight manner to form the
combustion chamber wall. In other areas downstream of the
combustion chamber on the flue gas side, steam generator tubes
disposed in the waste gas duct can also be provided.
[0005] Fossil fired steam generators can be categorized on the
basis of a large number of criteria: based on the flow direction of
the gas stream, steam generators can be subdivided, for example,
into vertical and horizontal types. In the case of fossil fired
steam generators of vertical design, a distinction is usually drawn
between single-pass and two-pass boilers.
[0006] In the case of a single-pass or tower boiler, the flue gas
produced by combustion in the combustion chamber always flows
vertically upward. All the heating surfaces disposed in the flue
gas duct are located above the combustion chamber. Tower boilers
offer a comparatively simple design and simple control of the
stresses produced by the thermal expansion of the tubes. In
addition, all the heating surfaces of the evaporator tubes disposed
in the flue gas duct are horizontal and can therefore be completely
dewatered, which may be desirable in frost-prone environments.
[0007] In the case of the two-pass boiler, a horizontal gas duct
leading into a vertical gas duct is mounted in an upper region
downstream of the combustion chamber on the flue gas side. In said
second vertical gas duct, the gas usually flows vertically from top
to bottom. Therefore, in the two-pass boiler, multiple flow
baffling of the flue gas takes place. Advantages of this design
are, for example, the lower installed height and the resulting
reduced manufacturing costs.
[0008] Steam generators may also be designed as natural
circulation, forced circulation or once-through steam generators.
In a once-through steam generator, the heating of a number of
evaporator tubes results in complete evaporation of the flow medium
in the evaporator tubes in one pass. Once evaporated, the flow
medium--usually water--is fed to superheater tubes downstream of
the evaporator tubes where it is superheated. Strictly speaking,
this description is valid only at partial loads with subcritical
pressure of water (P.sub.Kri.apprxeq.221 bar) in the evaporator--at
which there is no temperature at which water and steam can be
present simultaneously and therefore also no phase separation is
possible. However, for the sake of clarity, this representation
will be used consistently in the following description.
[0009] The position of the evaporation end point, i.e. the location
at which the water content of the flow is completely evaporated, is
variable and dependent on the operating mode. During full load
operation of a once-through steam generator of this kind, the
evaporation end point is, for example, in an end region of the
evaporator tubes, so that the superheating of the evaporated flow
medium begins even in the evaporator tubes.
[0010] In contrast to a natural or forced circulation steam
generator, a once-through steam generator is not subject to
pressure limiting, so that it can be designed for main steam
pressures well above the critical pressure of water.
[0011] During light load operation or at startup, a once-through
steam generator of this kind is usually operated with a minimum
flow of flow medium in the evaporator tubes in order to ensure
reliable cooling of the evaporator tubes. For this purpose,
particularly at low loads of e.g. less than 40% of the design load,
the pure mass flow through the evaporator is usually no longer
sufficient to cool the evaporator tubes, so that an additional
throughput of flow medium is superimposed in a circulating manner
on the flow medium passing through the evaporator. The operatively
provided minimum flow of flow medium in the evaporator tubes is
therefore not completely evaporated in the evaporator tubes during
startup or light load operation, so that unevaporated flow medium,
in particular a water-steam mixture, is still present at the end of
the evaporator tubes during such an operating mode.
[0012] However, as the superheater tubes mounted downstream of the
evaporator tubes of the once-through steam generator and usually
not receiving flow medium until it has flowed through the
combustion chamber walls are not designed for a flow of
unevaporated flow medium, once-through steam generators are
generally designed such that water is reliably prevented from
entering the superheater tubes even during startup or light load
operation. To achieve this, the evaporator tubes are normally
connected to the superheater tubes mounted downstream thereof via a
moisture separation system. The moisture separator is used to
separate the water-steam mixture exiting the evaporator tubes
during startup or light load operation into water and steam. The
steam is fed to the superheater tubes mounted downstream of the
moisture separator, whereas the separated water is returned to the
evaporator tubes e.g. via a circulating pump or can be drained off
via a flash tank.
[0013] However, particularly in startup mode, the above mentioned
concept causes high temperature differences between evaporator
tubes and superheater tubes: during cold starting, as yet
unevaporated flow medium flows in the evaporator tubes at
saturation temperature, while steam at higher temperature is still
present in the superheater tubes. During hot starting, on the other
hand, the evaporator tubes are filled with cold feedwater, while
the superheater tubes are still at operating temperature level.
This can result in overloading and damage of the materials due to
the differential thermal expansion.
SUMMARY OF INVENTION
[0014] The object of the invention is therefore to specify a
once-through steam generator of the above mentioned type requiring
comparatively low repair costs and having a comparatively long
service life.
[0015] This object is achieved according to the invention by
mounting superheater tubes in parallel contiguity with evaporator
tubes immediately downstream of the moisture separation system on
the flow medium side.
[0016] The invention is based on the idea that it would be possible
to reduce repair costs and increase the service life of the
once-through steam generator if damage caused by differential
thermal expansion of welded-together steam generator tubes could be
minimized. The differential expansion is the result of high
temperature differences between the steam generator tubes. Said
temperature differences are caused by differential cooling of the
steam generator tubes and different temperatures of the flow medium
flowing therein and therefore occur in particular at the interface
between welded-together evaporator and superheater tubes, as these
exhibit a different throughput of flow medium with different
temperatures through the intervening moisture separation system
particularly during cold and hot starting.
[0017] Particularly in the case of once-through steam generators of
the two-pass type, the design means that an interface between
parallel-welded evaporator and superheater tubes is typical. In
order to minimize as far as possible the temperature differences
between evaporator and superheater tubes, the steam temperature in
the superheater tubes welded parallel with the evaporator tubes
must be minimized. This can be achieved by mounting said
superheater tubes immediately downstream of the moisture separation
system, so that there is no increase in the temperature of the flow
medium flowing therein due to additional intervening superheater
tubes. This consistently minimizes temperature differences as a
cause of damage at the interface.
[0018] In an advantageous embodiment, the combustion chamber wall
of the once-through steam generator is formed from evaporator tubes
and a sidewall of the horizontal gas duct is formed from
superheater tubes, the superheater tubes adjacent to the combustion
chamber being mounted directly downstream of the moisture
separation system on the flow medium side. This effectively
minimizes the temperature differences at the vertical interface
between evaporator tubes of the combustion chamber and superheater
tubes of the horizontal gas duct in the case of the two-pass
boiler.
[0019] Advantageously, the top of the once-through steam generator
is found from superheater tubes which are disposed immediately
downstream of the moisture separation system on the flow medium
side. This means that the superheater tubes of the top are mounted
parallel with other superheater tubes adjacent to the evaporator
tubes. Due to the paralleling of the heating surfaces, such an
arrangement is advantageous in respect of the pressure loss to be
expected.
[0020] In a once-through steam generator in which superheater tubes
in parallel contiguity with evaporator tubes are disposed
vertically, these are advantageously designed such that the flow
medium flows through the superheater tubes from top to bottom. This
means that, in the event of overfeeding of the moisture separation
system resulting in unevaporated flow medium being applied to the
superheater tubes, this can be drained off at the outlet header of
the superheater tubes, thereby enabling any flow stagnation to be
effectively prevented.
[0021] The advantages achieved with the invention are in particular
that by mounting superheater tubes in parallel contiguity with
evaporator tubes immediately downstream of the moisture separation
system on the flow medium side, the temperature differences between
said tubes are consistently minimized. As a result, the
differential thermal expansion is minimized and damage and
overloading are prevented, in turn resulting in fewer repairs and a
longer service life of the once-through steam generator.
[0022] Such an arrangement is advantageous particularly in the case
of once-through steam generators without circulating pump. The
absence of circulation results in lower inlet temperatures to the
evaporator, smaller steam mass flows and an increase in the firing
capacity required at startup. Simulations have shown that
particularly for these systems, impermissible temperature
differences can occur at the interface between evaporator and
superheater tubes if--as hitherto usual--the superheater tubes at
the interface are mounted downstream of other superheater tubes,
e.g. of the top. Mounting said superheater tubes directly
downstream of the moisture separation system effectively prevents
these temperature differences.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] An exemplary embodiment of the invention will now be
explained in greater detail with reference to the accompanying
drawings in which the FIGURE schematically illustrates a
once-through steam generator of two-pass design.
DETAILED DESCRIPTION OF INVENTION
[0024] The once-through steam generator 1 according to the FIGURE
comprises a combustion chamber 2 implemented as a vertical gas
duct, downstream of which a horizontal gas duct 6 is disposed in an
upper region 4. The horizontal gas duct 6 is connected to another
vertical gas duct 8.
[0025] In the lower region 10 of the combustion chamber 2, a number
of burners (not shown in greater detail) are provided which combust
liquid or solid fuel in the combustion chamber. The wall 12 of the
combustion chamber 2 is formed of steam generator tubes welded
together in a gas-tight manner into which a flow medium--usually
water--is pumped by a pump (not shown in greater detail), said flow
medium being heated by the heat produced by the burners. In the
lower region 10 of the combustion chamber 2, the steam generator
tubes can be oriented either spirally or vertically. In the case of
a spiral arrangement, although comparatively greater design
complexity is required, the resulting heating differences between
parallel tubes are comparatively lower than with a vertically tubed
combustion chamber 2.
[0026] To improve flue gas flow, the once-through steam generator 1
shown also comprises a projection 14 forming a direct transition to
the bottom 16 of the horizontal gas duct 6 and extending into the
combustion chamber 2.
[0027] The steam generator tubes of the combustion chamber 2 are
designed as evaporator tubes. The flow medium is first evaporated
therein and fed via outlet headers 20 to the moisture separation
system 22. In the moisture separation system 22, not yet evaporated
water is collected and drained off. This is particularly necessary
in startup mode when a larger amount of flow medium must be pumped
in to ensure reliable cooling of the evaporator tubes than can be
evaporated in one evaporator tube pass. The steam produced is fed
to the inlet headers 24 of the downstream superheater tubes which
form the top 26 of the once-through steam generator 1 and the walls
of the horizontal gas duct 6. The transition from the sidewalls of
the vertical gas duct to the sidewalls of the horizontal gas duct 6
constitutes the interface 18 between evaporator tubes of the
combustion chamber wall 12 and superheater tubes in the walls of
the horizontal gas duct 6.
[0028] In addition to the two-pass boiler shown in the FIGURE,
other configurations for fossil fired boilers are self-evidently
also possible.
[0029] In order to prevent damage due to differential thermal
expansion caused by temperature differences at the interface 18
between the evaporator tubes of the combustion chamber wall 12 and
the superheater tubes in the walls of the horizontal gas duct 6,
these superheater tubes are mounted directly downstream of the
moisture separation system 22 via a connecting line 28. As a
result, said superheater tubes are only subject to saturated steam
and not higher-temperature superheated steam, thereby reducing the
temperature.
[0030] The superheater tubes in the walls of the horizontal gas
duct 6 are parallel to those of the top 26 and are flowed through
from top to bottom. Thus, in the event of overfeeding of the
moisture separation system 22, unevaporated flow medium in the
outlet headers 30 of the superheater tubes can be drained off and
flow stagnation cannot occur.
[0031] The arrangement described minimizes the temperature
differences at the interface 18 between the evaporator tubes of the
combustion chamber wall 12 and the superheater tubes in the walls
of the horizontal gas duct 6, thereby enabling damage to be
effectively prevented. This results in comparatively fewer repairs
and a longer service life of the once-through steam generator
1.
* * * * *