Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BOILER ENERGY RECOVERY SYSTEM
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of US provisional patent application
61/558,629 filed on November 11,2011.
BACKGROUND
(a) Field
[0002] The subject matter disclosed generally relates to an energy
recovery system and a boiler system. More particularly, the subject matter
disclosed generally relates to an energy recovery system designed to integrate
a
boiler system including an economizer, and a deaerator.
(b) Related Prior Art
[0003] Fossil fuels represent a significant energy source for the production
of electrical energy in many countries, such as Canada, United States and the
like. It is available throughout much of the countries. Moreover, the plants
which
convert fossil fuels energy into electrical energy are efficient and, in
comparison
to hydroelectric projects and other plants, they are easy and inexpensive to
construct.
[0004] The actual industry provides steam power plants including a boiler
equipped with one or a plurality of deaerators to remove oxygen and other
gases
from the boiler feedwater prior to admission in boilers. On the other hand,
the
condensate tank is often used as a thermal deaerator in some small plants.
These gases, such as oxygen, carbon dioxide and other gases, being in
effervescence at high temperature, the deaerator is generally maintained at
pressures and temperatures slightly above the boiling point of water, which is
around 5 PSIG and 227 F.
[0005] The means commonly used to maintain these pressures and
temperatures are to inject steam to control the pressure in the deaerator.
Usually,
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the steam comes from the boilers via a control valve to reduce pressure and
adjust the flow. However, this mean do not provide with an efficient energy
recovery system or plant. In these configurations, the net energetic yield of
the
boiler may decrease.
[0006] As noted, the feedwater requires deaeration with a deaerator to
remove dissolved gases from the feedwater to prevent corrosion and erosion of
the system. Deaerators represent mechanical devices that remove dissolved
gases from boiler feedwater. A deaerator protects the steam system from the
effects of corrosive and erosive gases. It accomplishes this by reducing the
concentration of dissolved oxygen and carbon dioxide to a level where
corrosion
is minimized. A dissolved oxygen level of about 7 parts per billion or lower
is
needed to prevent corrosion in most boilers. While oxygen concentration of up
to
43 parts per billion may be tolerated in low-pressure boilers, equipment life
is
extended at little or no cost by limiting the oxygen concentration to 7 parts
per
billion. Dissolved carbon dioxide is essentially completely removed by the
deaerator.
[0007] The design of an effective deaeration system depends upon the
amount of gases to be removed and the final oxygen gas concentration desired.
This, in turn, depends upon the ratio of boiler makeup to condensates returns
and the operating pressure of the boiler. Deaerators use steam to heat the
water
to the full saturation temperature corresponding to the pressure in the
deaerator
and to scrub out and carry away dissolved gases. Steam flow may be parallel,
cross, or counter to the water flow. The deaerator consists of a deaeration
section, a storage tank, and a vent. In the deaeration section, steam bubbles
through the water, both heating and agitating it. Steam is cooled by incoming
water and condensed at the vent condenser, if there is one (none in 99% of
cases). Non-condensable gases and some steam are released through the vent.
Steam fed to the deaerator provides physical stripping action and heats the
mixture of condensates returns and condenses makeup to saturation
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temperature. Most of the steam condensates, but a small fraction (usually 1%
to
5%) must be vented to accommodate the stripping requirements.
[0008] Normal design practice is to calculate the steam required for
heating and then make sure that the flow is sufficient for stripping as well.
If the
condensates returns rate is high (higher than about 80%) and the condensates'
pressure is high in comparison to the deaerator's pressure, then very little
steam
is needed for heating and provisions may be made for condensing the surplus
flashed steam.
[0009] It is well known that the deaerator steam consumption is equal to
the steam required to heat incoming water to its saturation temperature, plus
the
amount vented with the non-condensable gases, less any flashed steam from hot
condensates or steam losses through failed traps. The heat balance calculation
is made with incoming water at its lowest expected temperature. The vent rate
is
a function of deaerator type, size (rated feedwater capacity), and the amount
of
makeup water. The operating vent rate is at its maximum with the introduction
of
cold, oxygen-rich makeup water.
[0010] A deaerator provides the water storage capacity and the net
positive suction head necessary at the boiler feed pumps' inlet. Condensates
returns are mixed with makeup water within the deaerator. Operating
temperature range from 215 F to more than 350 F, which reduces the thermal
shock on downstream preheating equipment and the boilers.
[0011] It is well known that to reduce the energy consumption needed for a
deaerator, the deaerator section and storage tank and all piping conveying hot
water or steam should be adequately insulated to prevent the condensation of
steam and loss of heat as well as personal safety.
[0012] Additionally, to reduce the energy consumption, it is well known that
deaerator steam requirements should be reexamined following the retrofit of a
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steam distribution system, condensates returns system, or heat recovery energy
conservation measures.
[0013] On the other hand, many plants include an economizer in their
design to provide an efficient steam system. More particularly, economizers
are
mechanical devices intended to reduce energy consumption. In simple terms, an
economizer is a heat exchanger.
[0014] In boilers, economizers are usually heat exchanger devices that
heat fluids, usually water, up to but not normally beyond the boiling point of
that
fluid at boiler pressure. Economizers are so named because they can make use
of the enthalpy in fluid streams that are hot, but not hot enough to be used
in a
boiler, thereby recovering more useful enthalpy and improving the boiler's
efficiency. They are a device fitted to a boiler which saves energy by using
the
exhaust gases from the boiler to preheat the water used to feed it.
[0015] There is therefore a need in an energy recovery system having a
boiler system with an economizer, to integrate a deaerator.
SUMMARY
[0016] According to an embodiment, there is provided a system for
installation on a boiler for providing energy to a deaerator comprising a heat
exchanger for positioning at an exhaust of the boiler and for connecting to
the
deaerator for decreasing combustion gases temperature from a temperature T1
to a temperature T2 before evacuation of the combustion gases to atmosphere
for providing energy to the feedwater and the deaerator.
[0017] According to another embodiment, the deaerator is for operating at
a temperature Tdeaerator.
[0018] According to another embodiment, the energy provided by the heat
exchanger heats a deaerator condensates returns from a returned temperature
to the temperature Tdeaerator by connecting the heat exchanger to the
deaerator
condensates returns of the deaerator.
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[0019] According to another embodiment, the energy provided by the heat
exchanger heats a deaerator fluid inlet from a lower temperature to the
temperature Tdeaerator by connecting the heat exchanger to the fluid inlet of
the
deaerator.
[0020] According to another embodiment, the energy provided by the heat
exchanger heats a deaerator condensate returns from a returned temperature to
the temperature Tdeaerator and heats a deaerator fluid inlet from a lower
temperature to the temperature Tdeaerator by connecting the heat exchanger to
the
fluid inlet of the deaerator and by connecting the heat exchanger to the
deaerator
condensates returns of the deaerator.
[0021] According to another embodiment, the deaerator fluid inlet is a
make-up water inlet.
[0022] According to another embodiment, the lower temperature of the
deaerator fluid inlet is lower than the returned temperature of the
condensates
returns.
[0023] According to another embodiment, the lower temperature of the
deaerator fluid inlet is lower than the returned temperature of the
condensates
returns and the returned temperature of the condensates returns is lower than
the temperature Tdeaerator.
[0024] According to another embodiment, the make-up water inlet further
comprises oxygen.
[0025] According to another embodiment, the heat exchanger is an
economizer.
[0026] According to another embodiment, the economizer is an indirect
contact economizer.
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[0027] According to another embodiment, decreasing from a temperature
T1 to a temperature T2 the combustion gases before their evacuation to
atmosphere increases combustion efficiency of the boiler.
[0028] According to an embodiment, there is provided a boiler loop system
for providing energy to a deaerator comprising: a boiler having an exhaust for
evacuation of combustion gases; a heat exchanger positioned at the exhaust of
the boiler and connected to the deaerator for decreasing combustion gases
temperature from a temperature T1 to a temperature T2 before evacuation of the
combustion gases to atmosphere providing energy to the feedwater and the
deaerator.
[0029] According to another embodiment, the deaerator is for operating at
a temperature Tdeaerator.
[0030] According to another embodiment, the energy provided by the heat
exchanger heats a deaerator condensates returns from a returned temperature
to the temperature Tdeaerator by connecting the heat exchanger to the
deaerator's
condensates returns of the deaerator.
[0031] According to another embodiment, the energy provided by the heat
exchanger heats a deaerator fluid inlet from a lower temperature to the
temperature Tdeaerator by connecting the heat exchanger to the fluid inlet of
the
deaerator.
[0032] According to another embodiment, the energy provided by the heat
exchanger heats a deaerator condensates returns from a returned temperature
to the temperature Tdeaerator and heats a deaerator fluid inlet from a lower
temperature to the temperature Tdeaerator by connecting the heat exchanger to
the
fluid inlet of the deaerator and by connecting the heat exchanger to the
deaerator
condensates returns of the deaerator.
[0033] According to another embodiment, the deaerator fluid inlet is a
make-up water inlet.
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[0034] According to another embodiment, the lower temperature of the
deaerator fluid inlet is lower than the returned temperature of the
condensates
returns.
[0035] According to another embodiment, the lower temperature of the
deaerator fluid inlet is lower than the returned temperature of the
condensates
returns and the returned temperature of the condensates returns is lower than
the temperature Tdeaerator.
[0036] According to another embodiment, the make-up water inlet further
comprises oxygen.
[0037] According to another embodiment, the heat exchanger is an
economizer.
[0038] According to another embodiment, the economizer is an indirect
contact economizer.
[0039] According to another embodiment, decreasing from a temperature
T1 to a temperature T2 the combustion gases before their evacuation to
atmosphere increase combustion efficiency of the boiler.
[0040] According to another embodiment, the temperature Tdeaerator is in a
range from about 200 F to about 300 F.
[0041] According to another embodiment, the temperature Tdeaerator is
about 227 F.
[0042] According to another embodiment, the combusted gases are
carbon dioxide, nitrogen, water vapor, oxygen or a combination thereof.
[0043] According to another embodiment, the temperature T1 is about
450 F.
[0044] According to another embodiment, the temperature T2 is about
350 F.
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[0045] According to another embodiment, the returned temperature of the
deaerator condensates returns is from about 150 F to about 200 F.
[0046] According to another embodiment, the lower temperature of the
deaerator make-up water is about 50 F.
[0047] According to an embodiment, there is provided a heat recovery
process comprising the system as described above.
[0048] The following terms are defined below.
[0049] The term "economizer" is intended to mean any mechanical device
intended to reduce energy consumption, or to perform another useful function
like preheating a fluid. The economizer may be an indirect contact economizer
(ICE) or any other types of economizers.
[0050] The term "deaerator" is intended to mean a mechanical device that
removes dissolved gases from boiler feedwater and thereby protects the steam
system from the effects of corrosive and erosive gases.
[0051] Features and advantages of the subject matter hereof will become
more apparent in light of the following detailed description of selected
embodiments, as illustrated in the accompanying figures. As will be realized,
the
subject matter disclosed and claimed is capable of modifications in various
respects, all without departing from the scope of the claims. Accordingly, the
drawings and the description are to be regarded as illustrative in nature, and
not
as restrictive and the full scope of the subject matter is set forth in the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] Further features and advantages of the present disclosure will
become apparent from the following detailed description, taken in combination
with the appended drawings, in which:
[0053] Fig. 1 is a common arrangement of a heat recovery boiler system
as commonly seen in the prior art;
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[0054] Fig. 2 is a flowsheet of a heat recovery boiler system according to
one embodiment; and
[0055] Fig. 3 is a flowsheet of a heat recovery boiler system according to
another embodiment.
[0056] It will be noted that throughout the appended drawings, like
features are identified by like reference numerals.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0057] Referring now to the drawings, and more particularly to Figs. 2 and
3, there is shown a boiler loop system 10. The system 13 is for installation
on a
boiler 12 for providing energy to a deaerator 14. The system 13 comprises a
heat
exchanger 16 positioned at an exhaust 20 of the boiler 12 for decreasing from
a
temperature T1 to a temperature T2 the combustion gases before their
evacuation
to atmosphere for providing energy to the deaerator 14. In the system 13, the
heat exchanger 16 is for connecting to the deaerator 14.
[0058] Moreover, in the system 13 as described above, the deaerator 14 is
for operating at a temperature Tdeaerator. In the embodiments presented in
Figs. 2
and 3, the temperature Tdeaerator of the deaerator 14 is about 227 F at a
pressure
of 5 PSIG. In the system 13, the energy provided by the heat exchanger (0 =
ritCp(T2 ¨ T1)) heats a feedwater 36 from a temperature Tdeaerator to a
temperature
Tdeaerator + T. Also the energy provided by the heat exchanger 16 may heat a
deaerator fluid inlet 18 from a lower temperature to the temperature
Tdeaerator +AT.
In the other hand, the energy provided by the heat exchanger 16 heats
deaerator
condensates returns 30 from a returned temperature to the temperature
Tdeaerator
+AT. In the system 13 as described above, the deaerator fluid inlet 18 may be
a
make-up water inlet, but any other fluid, such as a glycol loop, and the like.
[0059] In the system 13, the temperature of the make-up water inlet 18 is
lower than the returned temperature of the condensates returns 30.
Additionally,
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the returned temperature of the condensates returns 30 is lower than the
temperature Tdeaerator=
[0060] Moreover, the make-up water inlet 18 of the system 13 may further
comprise oxygen and other gases such as carbon dioxide. It is important to
note
that in embodiments presented by Figs. 2 and 3, the heat exchanger 16 of the
system 13 is an economizer, and more particularly, an indirect contact
economizer, or ICE. However, it is important to note, that the heat exchanger
16
may be any type of well known heat exchanger in the industry.
[0061] Also, in the system 13, decreasing from a temperature T1 to a
temperature T2 the combustion gases before their evacuation to atmosphere
increases the combustion efficiency of the boiler 12.
[0062] Still referring to Figs. 2 and 3, there is shown an economizer 16 in a
boiler loop system for providing energy to the feedwater 36. The economizer 16
transfers heat from a first fluid to a second fluid, in which the first hot
fluid is the
exhaust gas 22 and the second cold fluid is the feedwater 36. Indeed, the
temperature of the exhaust gas 22 decreases from about 450 F to about 350 F,
while the temperature of the feedwater 36 increases from about 227 F to about
252 F. The boiler loop system comprises a boiler 12 having an exhaust 20 for
evacuation of combustion gases and a heat exchanger 16 positioned at the
exhaust 20 of the boiler 12 for decreasing from a temperature T1 to a
temperature T2 combustion gases before their evacuation to atmosphere and
then providing energy to the feedwater 36. In the boiler loop, the heat
exchanger
16 is connected to the deaerator 14.
[0063] As noted before, the deaerator of the boiler loop system is for
operating at a temperature Tdeaerator= The energy provided by the heat
exchanger
16 of the boiler loop system heats deaerator condensates returns 30 from a
returned temperature to the temperature Tdeaerator ,t,T. The energy provided
by the
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heat exchanger 16 may also heat a make-up water inlet 18 from a lower
temperature to the temperature Tdeaerator
[0064] In the boiler loop system as described above, the temperature
Tdeaerator may be in a range from about 200 F to about 300 F. More
particularly, in
the embodiments presented in Figs. 2 and 3, the temperature Tdeaerator is
about
227 F. In the boiler loop and in the boiler system 13, the combustion gases
may
be carbon dioxide, nitrogen, water vapor, oxygen or a combination thereof.
[0065] Moreover, in the boiler loop system as described above, the
temperature T1 may be about 450 F and the temperature T2 is about 350 F.
Also, the temperature of the deaerator's condensates returns 30 is about 160 F
and the lower temperature of the make-up water 18 is about 50 F.
[0066] Moreover, still referring to Figs. 2 and 3, there is shown a flowsheet
of a boiler loop system 10 according to different embodiments. More
particularly,
there is provided a boiler loop system 10 for heating and deaerating make-up
water 18 of a boiler 12 having an exhaust 20 for evacuation of combustion
gases.
The boiler loop system 10 comprises the step of transferring an amount of
energy from the combustion gases provided by the exhaust 20 of the boiler 12
to
the economizer (or heat exchanger) 16 having a convection surface area, the
economizer 16 (or heat exchanger) being positioned at the exhaust 20 of the
boiler 12.
[0067] Additionally, the boiler loop system 10 also includes the step of
directing combustion gases which energy is transferred by the economizer 16 to
feedwater 36. Furthermore, the boiler loop system 10 includes the step of
directing the make-up water inlet 18 into a deaerator 14 to remove gases, such
as oxygen and/or carbon dioxide, from the make-up water inlet 18 directed to
the
boiler 12; and where the convection surface area of the economizer 16 provides
an heat transfer from the combustion gases evacuated by the exhaust 20 to the
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feedwater 36, thereby providing a heat recovery from the combustion gases of
the boiler 12 to the deaerator 14.
[0068] The steam admitted to the deaerator 14 heats the condensates
returns 30 from its returned temperature to the deaerator temperature
Tdeaerator.
More particularly, the steam admitted to the deaerator 14 may heat the
condensates returns from its returned temperature, about 150 F, to the
deaerator
temperature Tdeaerator, about 227 F. Also, the steam admitted to the deaerator
may heat the make-up water inlet 18 from a lower temperature to the
temperature of the deaerator Tdeaerator. More particularly, the steam admitted
to
the deaerator may heat the make-up water inlet 18 from a lower temperature,
about 50 F, to the temperature of the deaerator Tdeaerator, about 227 F.
Additionally, the steam admitted to the deaerator 14 leads the non-condensable
gases that need to be eliminated outside the deaerator 14 via a vent 26.
Generally, from about 1% to 2% of the water or fluid mass flow comes out from
the vent 26 of the deaerator 14. The addition of these needs in steam is not
negligible. It may present 12% of the steam production of the boiler 12. The
steam needs are provided by the economizer 16 installed on the boiler 12. More
particularly, the economizer 16 is an indirect contact economizer.
[0069] The boiler loop system 10 replaces a part or the totality of the
steam provided by the boiler 12 by the steam provided by the economizer 16.
The economizer 16 may be an indirect contact economizer (ICE) or any other
type of economizer. The economizer 16 may be installed on the exhaust 20,
where the gases are expelled from the boiler 12. The installation of the
economizer 16 decreases the temperature of the combustion gases before their
evacuation to the atmosphere. The combustion efficiency of the boiler system
13
is than increased.
[0070] The present invention will be more readily understood by referring
to the following examples which are given to illustrate the invention rather
than to
limit its scope.
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EXAMPLE 1
Heat recovery system with a 600HP boiler
[0071] In a first example, the boiler loop system 10 includes a 600 HP
boiler (20,000, 000 BTU/hr). The operating pressure of the boiler 12 is up to
120
PSIG at 350 F. The deaerator 14 connected to the boiler 12 operates at a
temperature of 227 F and at a pressure of 5 PSIG. When the indirect contact
economizer (ICE) 16 is not installed at the exhaust 20 of the boiler 12, the
temperature of the combusted gases is about 450 F. The combustion efficiency
of the boiler without the indirect contact economizer (ICE) 16 is about 80.6%.
On
the other hand, when the indirect contact economizer (ICE) 16 is installed at
the
exhaust 20 of the boiler 12, the temperature of the combustible gases is about
350 F. The combustion efficiency of the boiler 12 with the indirect contact
economizer (ICE) 16 is about 84.2%.
[0072] In this example, 25 000 #/hr of combusted gases are cooled from
about 450 F to about 350 F via the indirect contact economizer (ICE) 16. This
energy heats 20 000 #/hr of the feedwater 36 from about 227 F to about 252 F.
This feedwater 36 at 252 F is admitted in the deaerator 14, as shown in Fig.
3.
Also, this feedwater 36 at 252 F may be admitted to a reservoir 34 for
flashing,
as shown in Fig. 2.
[0073] While preferred embodiments have been described above and
illustrated in the accompanying drawings, it will be evident to those skilled
in the
art that modifications may be made without departing from this disclosure.
Such
modifications are considered as possible variants comprised in the scope of
the
disclosure.
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