U.S. patent application number 13/062704 was filed with the patent office on 2011-08-25 for continuous steam generator.
Invention is credited to Martin Effert, Joachim Franke.
Application Number | 20110203536 13/062704 |
Document ID | / |
Family ID | 41796032 |
Filed Date | 2011-08-25 |
United States Patent
Application |
20110203536 |
Kind Code |
A1 |
Effert; Martin ; et
al. |
August 25, 2011 |
Continuous steam generator
Abstract
A continuous steam generator is provided. The continuous steam
generator includes a combustion chamber having a number of burners
for fossil fuels, downstream of which a vertical gas duct is
mounted, on the hot gas side, in an upper region above a horizontal
gas duct. The outside wall of the combustion chamber is formed, in
a lower region, from evaporation tubes welded together in a
gas-tight manner and mounted upstream of a water separator system,
on the flow medium side, and in an upper region, from superheater
tubes welded together in a gastight manner and mounted downstream
of the water separator system on the flow medium side. The boundary
between the regions of the evaporation tubes and the superheater
tubes is essentially horizontal around the combustion chamber, in a
region of the bottom of the horizontal gas duct.
Inventors: |
Effert; Martin; (Erlangen,
DE) ; Franke; Joachim; (Nurnberg, DE) |
Family ID: |
41796032 |
Appl. No.: |
13/062704 |
Filed: |
September 1, 2009 |
PCT Filed: |
September 1, 2009 |
PCT NO: |
PCT/EP2009/061239 |
371 Date: |
March 8, 2011 |
Current U.S.
Class: |
122/7R |
Current CPC
Class: |
F22B 19/00 20130101;
F22B 29/06 20130101; F22B 29/067 20130101; F22B 29/08 20130101 |
Class at
Publication: |
122/7.R |
International
Class: |
F22B 1/18 20060101
F22B001/18 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 9, 2008 |
EP |
08015863.7 |
Claims
1.-3. (canceled)
4. A continuous steam generator, comprising: a combustion chamber
including a plurality of burners for fossil fuel; a vertical gas
duct, disposed downstream of the combustion chamber and mounted in
an upper region on a hot gas side above a horizontal gas duct; the
horizontal duct; a plurality of evaporator tubes; a plurality of
superheater tubes; and a moisture separation system, wherein a
surrounding wall of the combustion chamber is formed, in a lower
region, from the plurality of evaporator tubes welded together in a
gas-tight manner and mounted upstream of a moisture separation
system on a flow medium side and in an upper region, from the
plurality of superheater tubes welded together in a gas-tight
manner and mounted downstream of the moisture separation system on
the flow medium side, wherein part of the surrounding wall facing
the vertical gas duct is inclined inward below the horizontal gas
duct, thereby forming with a first bottom of the adjacent
horizontal gas duct a projection extending into the combustion
chamber, and wherein a boundary between the regions of the
evaporator tubes and the plurality of superheater tubes is disposed
directly above the projection in an essentially horizontally
circumferential manner around the combustion chamber in a region of
the bottom of the horizontal gas duct.
5. The continuous steam generator as claimed in claim 5, wherein
the boundary between the regions of the plurality of evaporator
tubes and the plurality of superheater tubes is disposed in an
essentially horizontally circumferential manner around the
combustion chamber at a level of an edge formed by the surrounding
wall and a second bottom of the horizontal gas duct.
6. The continuous steam generator as claimed in claim 5, wherein
the second bottom of the horizontal gas duct is formed of the
plurality of evaporator tubes welded together in a gas-tight manner
upstream of the moisture separation system on the flow medium side.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the US National Stage of International
Application No. PCT/EP2009/061239, filed Sep. 1, 2009 and claims
the benefit thereof. The International Application claims the
benefits of European Patent Office application No. 08015863.7 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-though ("continuous") steam
generator with a combustion chamber having a number of burners for
fossil fuel, downstream of which a vertical gas duct is mounted in
an upper region on the hot gas side above a horizontal gas duct,
the surrounding wall of the combustion chamber being aimed, in a
lower region, from evaporator tubes welded together in a gas-tight
manner and mounted upstream of a moisture separation system on the
flow medium side and, in an upper region, from superheater tubes
welded together in a gas-tight manner and 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 be 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 here
usually constitute the combustion chamber wall by being welded
together in a gas-tight manner. 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: steam generators may in
general 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. 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.
[0006] 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.
[0007] 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 circulatory 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.
[0008] However, as the superheater tubes mounted downstream of the
evaporator tubes of the once-through steam generator and usually
only receiving flow medium after 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.
[0009] Based on the flow direction of the gas stream, steam
generators can also 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.
[0010] 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 above the combustion chamber on the flue gas side.
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.
[0011] 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.
[0012] In a steam generator implemented as a two-pass boiler, the
walls of the first pass, i.e. the combustion chamber, are usually
implemented entirely as an evaporator. The moisture separation
system downstream of the evaporator tubes on the flow medium side
is accordingly disposed at the upper end of the combustion
chamber.
[0013] However, because of differences both in the geometry of the
individual tubes and in the heating thereof, different mass flows
and temperatures of the flow medium occur in parallel tubes. These
so-called asymmetries must be limited for the following
reasons:
[0014] On the one hand, the evaporator heating surfaces must be
sufficiently cooled over the entire load range of the steam
generator. The mass flow required for cooling must be reliably
supplied to each individual tube. In addition, the stresses
occurring due to the thermal expansion of the individual tubes must
not exceed the permissible values between adjacent tubes. The
temperatures of the flow medium must be limited both in absolute
terms and in terms of the difference with respect to the adjacent
tubes, as otherwise damage to the combustion chamber wall could
arise.
[0015] To reduce temperature asymmetries in the evaporator tubes,
mixing points can for example be installed in the combustion
chamber walls configured as evaporators. In this case the flow
medium is diverted from the evaporator tubes, mixed and
re-distributed to the other evaporator tubes. Such a system must be
placed downstream of the mixing point for an even distribution of a
water and steam mixture. A design of this kind accordingly involves
a high degree of technical complexity and considerably increases
manufacturing costs.
SUMMARY OF INVENTION
[0016] The object of the invention is therefore to specify a
once-through steam generator of the above-mentioned type which has
a comparatively simple design while providing a particularly long
service life.
[0017] This object is achieved according to the invention by
disposing the boundary between the regions of the evaporator tubes
and the superheater tubes in an essentially horizontally
circumferential manner around the combustion chamber in the region
of the bottom of the horizontal gas duct.
[0018] The invention is based on the ideal that a simple design
combined with a comparatively long service life would be achievable
if comparatively slight temperature asymmetries in the steam
generator tubes were achievable without an additional mixing point
being disposed in the evaporator tubes.
[0019] The moisture separation system present in the steam
generator also collects the water exiting the evaporator tubes in
circulation mode and separates it from the steam. In once-through
operation, the incoming steam is mixed and distributed to the
superheater tubes located downstream on the flow medium side. This
considerably reduces temperature asymmetries. Based on the
knowledge that the moisture separation system thus basically
fulfils the function of a mixing point, by placing it lower down,
e.g. in the region of the bottom of the horizontal gas duct, this
system can therefore be used as a mixing point within the
combustion chamber wall, without an additional mixing system being
required.
[0020] In addition, this position of the moisture separation system
means that the boundary between the regions of the evaporator tubes
and the superheater tubes is disposed in an essentially
horizontally circumferential manner around the combustion chamber
in the area of the bottom of the horizontal gas duct.
[0021] In an advantageous embodiment, the boundary between the
regions of the evaporator tubes and the superheater tubes is
disposed in an essentially horizontally circumferential manner
around the combustion chamber at the level of the edge formed by
the surrounding wall and bottom of the horizontal gas duct. By
means of such an arrangement, all the combustion chamber tubes
welded to the tubes of the walls of the horizontal gas duct are
likewise designed as superheater tubes. In the existing design with
a combustion chamber formed entirely of evaporator tubes,
evaporator and superheater tubes were welded in parallel at this
point. This creates problems particularly for hot-starting of the
steam generator, as the filling of the evaporator tubes with cold
flow medium produces considerable temperature differences with
respect to the unfilled superheater tubes. Disposing the moisture
separation system at the level of the edge formed by the combustion
chamber wall and the bottom of the horizontal gas duct ensures that
such a vertical interface no longer occurs and altogether more
reliable operation of the steam generator can be achieved while at
the same time providing a comparatively long service life.
[0022] In the case of two-pass steam generators, to improve gas
flow, a section of the surrounding wall facing the vertical gas
duct in inclined inward below the horizontal gas duct, thereby
forming, with the bottom of the adjacent horizontal gas duct, a
projection extending into the combustion chamber. In steam
generators of this kind, the boundary between the regions of the
evaporator tubes and the superheater tubes is advantageously
disposed in an essentially horizontally circumferential manner
around the combustion chamber directly above the projection.
[0023] In another advantageous embodiment, the bottom of the
horizontal gas duct is formed of evaporator tubes welded together
in a gas-tight manner upstream of the moisture separation system on
the flow medium side. The bottom of the horizontal gas duct is
actually suitable to be designed as an additional evaporator
heating surface, as its tubes are not welded parallel with the
vertically tubed horizontal gas duct walls configured as
superheaters and therefore the stresses caused by differential
thermal expansion remain comparatively low.
[0024] The particular advantages of the invention are that dual use
of the moisture separation system as a mixing point for reducing
temperature differences between parallel tubes is made possible by
disposing the boundary between the regions of the evaporator tubes
and the superheater tubes in an essentially horizontally
circumferential manner around the combustion chamber in the region
of the bottom of the horizontal gas duct. In addition, one of the
main disadvantages of the two-pass boiler, namely the vertical
interface between wall heating surfaces configured as evaporators
and those configured as superheaters, are eliminated. Particularly
for hot starting of the steam generator, during which high
temperature differences and stresses occur at said interface when
the evaporator tubes are filled with comparatively cold flow
medium, particularly reliable operation and a longer service life
of the steam generator can be achieved by avoiding such
stresses.
[0025] The lower positioning of the moisture separation system and
therefore of the boundary between evaporator and superheater tubes
in the combustion chamber also allows reduced superheating at the
moisture separation system and altogether more material-conserving
startup of the steam generator, which in turn results in a longer
service life of the steam generator and enables less expensive
materials to be used for the manufacture thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] 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
[0027] 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.
[0028] 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 surrounding
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 9 (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
asymmetries between parallel tubes are comparatively lower than
with a vertically tubed combustion chamber 2.
[0029] The steam generator tubes in the lower part 10 of the
combustion chamber 2 are designed as evaporator tubes. The flow
medium is first evaporated therein and fed via pipework 14 to a
moisture separation system (not shown in greater detail). In the
moisture separation system, not yet evaporated water is collected
and drained off. The steam produced is fed into the walls of the
combustion chamber 2 and distributed to the superheater tubes
disposed in the upper region 4 and in the walls of the horizontal
gas duct 6. Such removal of not yet evaporated water is
particularly necessary in startup mode when a larger amount of flow
medium must be pumped in for reliable cooling of the evaporator
tubes than can be evaporated in one evaporator tube pass.
[0030] To improve flue gas flow, the once-through steam generator 1
shown also comprises a projection 16 forming a direct transition to
the bottom 18 of the horizontal gas duct 6 and extending into the
combustion chamber 2. In addition, a grid 20 of further superheater
tubes is disposed in the transition region from the combustion
chamber 2 to the horizontal gas duct 6 in the flue gas duct.
[0031] Particularly in the case of a vertically tubed combustion
chamber 2, temperature differences between parallel evaporator
tubes may now occur which can compromise the operation of the steam
generator as the result of differential thermal expansion. In order
to achieve mixing of the flow medium from different tubes and
therefore temperature equalization without using additional
components, the boundary 22 between evaporator tubes and
superheater tubes is disposed directly above the projection 16 at
the level of the bottom 18 of the horizontal gas duct 6. The
moisture separation system therefore acts not only as a separator
during startup operation, but also as a mixing point in continuous
operation, as the entire flow medium from the evaporator tubes is
collected, mixed and redistributed to the superheater tubes in the
moisture separation system.
[0032] As now both the upper part 4 of the combustion chamber 2 and
the walls of the horizontal gas duct 6 are configured as
superheater tubes, there is also no vertical interface in the
region of the grid 20 between parallel welded evaporator and
superheater tubes. Instead, only the lower part 10 of the
combustion chamber 2 and the bottom 18 of the horizontal gas duct
are configured as evaporator tubes, as a result of which only
superheater tubes are welded together in parallel in this area.
* * * * *