Boiler Outlet Steam Attemperation System
Technical Field
This invention relates to the steam cycle thermal power generation system with either pulverized coal firing or fluidized bed combustion technology. It is equally applicable to process steam generation system and combined cycle power generation system having fired or unfired boiler or waste heat recovery steam generator having natural or forced circulation system with a steam separating drum in the boiler feed water-steam cycle operating in sub critical regime. This invention relates specifically to the improved control arrangement of boiler output steam temperature and quality by staggered attemperation of superheated steam with cooler steam from lowest temperature superheater and simultaneously high pressure feed water heating in the associated regenerative feed heating system including reducing extraction of steam from turbine.
Background Prior Art
In typical steam boiler of thermal power generation system final outlet steam temperature is dependent on boiler steam generation load. Steam temperature control is required even with constant steam generation load due to variation in fuel quality, combustion process, effectiveness of heat transfer to tube surface damaged by slagging, fouling or internal scaling due to poor boiler feed water quality.
Many operating variables and quality as well as quantity of the input resources change the quality and temperature of the boiler outlet superheated and reheated steam (in reheat boiler) which need suitable measures for best performance of steam turbine as well concerning overheated tube failure.
Prior art available to control the quality and temperature of boiler outlet steam are usually of the following types:
1. Separately Fired Super heater - The super heater is isolated from boiler furnace and fired independently. Therefore, the final steam temperature is independent of boiler operating temperature. It has limited application with higher capital and running costs, less efficiency and more pressure loss in the system.
2 Excess Air - The supply of combustion air in furnace varies heat absorption in radiant and
convective passes of suerheater and reheater. A change to less excess air than optimum results
partial combustion reducing superheater/reheater temperature, whereas, an increase in excess air above optimum increases stack loss with reduced heat absorption. As such, this arrangement has a limited application as emergency control to bring down the furnace temperature or to maintain superheater/reheater steam temperature at part load operation. This system has an inherent associated problem of either excess dry flue gas loss or carbon in ash loss or incomplete combustion loss resulting boiler efficiency loss.
3 Flue Gas Recirculation or Tempering - In this process flue gas from boiler economizer outlet is
recirculated back either at furnace bottom (Gas Recirculation) or near the furnace outlet to
subsequent pass (Gas Tempering) by suitably located gas recirculation fans and associated ducts The convection heat transfer depends on upon gas mass flow and gas temperature in boiler. The percentage of total flue gas at the back end recirculated back into the furnace determines the superheat section heat absorption thereby controlling the outlet superheat/reheat temperature. However, installation of Gas Recirculation fans involves capital cost and frequent maintenance due to handling abrasive coal ash laden gas.
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4. Divided Back Pass with Damper Control - In this process the flue gas flow over primary superheater and reheater elements, if placed vertically at back pass of the boiler in divided sections separated by baffle walls, is controlled by back end louver damper positioned at each section in the flue gas path usually before economizer. A bypass duct is also provided with damper control to stop flue gas flow in any or both the superheater and reheater tube elements. This system increases boiler pressure part tonnage with respect to available heat transfer area. The damper has little control over flue gas flow above 60% opening. Besides, dust laden gas frequently result control damper seizure.
5 Burner Selection and Tilting - Sometimes selective burner operation at various burner elevation
at part load operation of boiler controls furnace exit gas temperature thereby redistributing heat
absorption in the superheater, reheater tube elements placed at radiant and convective zone of
flue gas passage. Shifting fuel firing from bottom to top burner elevation level or vice versa
(provided a multi elevation fuel firing arrangement exists in the boiler) during part load boiler
operation affect superheater and reheater outlet temperature due to variation of heat transfer in
evaporating and preheating (sensible heating) water tube elements. However, the system cannot
be deployed at full load boiler operation. Another available boiler outlet steam temperature
control method is tilting the burners (+ 300 upward or downward) with respect to normal
horizontal position of firing. However, application of this system is only available with corner
firing arrangement and not suitable for any other mode of front firing or opposed firing
arrangement due to combustion or flame instability.
6 Attemperation - Unlike aforesaid boiler outlet steam temperature control systems working upon
variation of heat absorption at various heat traps placed in the boiler, prior art of attemperation
(cooling) method involves diluting high temperature steam with low temperature boiler feed
water taken from any point after boiler feed water pump upstream to economizer. The direct
contact spray attemperator also known, desuperheater, is located in the upstream of
superheated/reheated steam circuit between the steam inlet to superheater/reheater elements and
final outlet from boiler. Superheater/reheater outlet steam temperature is controlled (and cooled)
by injection of water spray through nozzle in a conventional attemperator. With existing
demineralizing water preparation system in use for a steam cycle power plant or for feeding in a
process steam generating boiler, this boiler feed water spray could result contamination of
boiler outlet steam by introduction of unpurified water into the attemperator.
The prior art of steam attemperator circuit as applied to combined cycle WHRSG has limitation of a typical application associated with steam-cooled gas turbine and applicable independently to a steam cycle power plant boiler with multistage superheating having an arrangement of short circuiting of final stage of superheater with initial stage of superheater through an attemperation conduit not exposed to heat transfer zone and low temperature superheater is placed near the hot gas entry to WHRSG. This arrangement is also having the limitation of overheating of intermediate superheater tube elements with steam generation load variation and inlet gas temperature variation in WHRSG as set forth in U.S. Pat. No. 5628179. Besides, this arrangement requires more modification for application to conventional boiler superheating arrangement with a lesser stability in the control mechanism at varied steam generating load.
The prior art of steam mixing for attemperating superheated steam in WHRSG with high gas turbine exhaust gas temperature observed the temperature controller bypassing a large amount of steam flow from steam drum to mixing point that is located at downstream of superheater thereby reducing flow through steam cooled superheater tubing which may result tube failure for overheating. Besides, variation of cooling steam flow through a control valve
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upstream to attemperator will result condensation in the cooling steam line. Thus, the configuration requires a continuous drain to remove the condensate.
Summary of the Invention
The principal objective of this invention is to meet these requirements and remove aforementioned drawbacks with appropriate system improvements such that the boiler outlet steam temperature and quality is maintained with a suitable accurate, precise and economical arrangement without use of gas bypass, separately fired external heat exchanger, spray water attemperation, excess air or flue gas recirculation and simultaneously minimize steam tube failures with a distributed steam temperature control mechanism as well as help to maintain the inlet temperature of boiler feed water to economizer at various steam generation load thereby optimize thermal efficiency of the system. The invention minimizes the problem of condensation of cooling steam due to control valve operation from no flow limit to full flow limit by adopting a mechanism of distribution of a specific quantity of total cooling steam with respect to steam generation load at two different points of attemperation where cooling steam flow is controlled by temperature controller associated with control valve in the individual attemperator cooling steam line.
The boiler outlet steam temperature control system can be introduced in a conventional steam cycle power or process steam plant having a fuel fired boiler or unfired waste heat recovery steam generator (WHRSG) with at least a steam drum and multistage superheating arrangement wherein low temperature superheating elements are placed at back end of flue/waste heat gas pass and high temperature superheating elements are placed at furnace radiant zone of a boiler or first pass of WHRSG.
A part of the steam coming out of lowest temperature superheater in convective zone or rear side of the boiler located downstream to steam drum (carrying out steam separation, washing and drying), is extracted as cooling medium for successive stages of superheated steam attemperation which eliminate chance of contamination of superheated steam with relatively impure boiler feed water injected through attemperator. The cooling steam is passed through a heat exchanger for further reducing or conditioning the steam to saturation temperature at that working pressure in non-contact heat exchanger prior to introduction to the various stages of attemperation. The rejected heat of cooling steam in the heat exchanger is absorbed by the boiler feed water passing upstream to the economizer.
The heat exchanger can be most conveniently located just before inlet of boiler feed water to the economizer which is also nearest point from cooling steam tapping. The introduction of this heat exchanger compensates the efficiency losses with attemperation. In other way, this heat exchanger work as a part of regenerative feed heating system reducing the size of high-pressure heater working with extraction steam from HP turbine thereby reducing extracted steam consumption and improving overall steam turbine efficiency of a pure steam cycle power plant. It also help to maintain the inlet temperature of boiler feed water to economizer at various steam generation load thereby optimize thermal efficiency of the boiler system
The cooling steam is continuously injected through two attemperation points, one located after first stage of primary superheating and another located before last stage of final superheating. As the cooling steam supply to individual attemperator is taken from a common line, the mixing quantity is varied with the control valve fitted into cooling steam line to each attemperator. The common line of cooling steam is bifurcated to the attemperating stages after heat exchanger and fitted with a steam trap for minimal condensate collection. There is very little possibility of condensation in the cooling steam circuit as there will be a specific and continuous flow of cooling steam with respect to boiler steam generation load. As the attemperation system is staged in two different heat transfer zones i.e. one in the first pass or the furnace radiant zone and another in the second pass or the convective zone thereby eliminating the chance of overheating of
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superheater tube elements as well as balancing heat absorption in the superheater elements placed in radiant zone or first pass and convective zone or second pass with the variation of boiler steam generation load.
Another important aspect of this boiler outlet steam temperature control system is its adaptability with existing boiler arrangement with least modifications as most of the boilers pressure part layout arrangements or heat transfer area distributions are similar in nature (with low temperature superheater at convective pass and high temperature superheater at radiant zone or first pass) thereby making it more economical under renovation, modernization and up gradation project.
List of Preferred and Optional Features
In another aspect, this invention relates to reheat steam cycle configuration of a reheat boiler of a steam cycle power or process plant. It also relates to the reheat cycle arrangement of WHRSG operating in a combined cycle system where WHRSG output is used as either process steam or input to a steam turbine for power generation. For both of these reheat cycle arrangements the aforesaid outlet steam temperature control system can be incorporated provided the boiler is having a multistage reheat system with low temperature reheater placed at backend of the boiler and high temperature reheater tubing is in radiant zone of furnace along a steam drum and the WHRSG is having the multistage reheat steam circuit with low temperature reheater at exhaust pass/back end and high temperature reheater at front end/first pass along a high pressure evaporator, upstream the cooling steam circuit. The reheat steam attemperation is done in two phases in first pass and second pass by cooling steam taken from the attemperating steam header downstream to non-contact heat exchanger which preheat the boiler feed water before entering economizer and thereby reduces the cooling steam temperature towards the saturation temperature at the working steam pressure in water pre-heater.
In another aspect, this invention includes within its scope, a fluidized bed power plant or process plant boilers. Such a plant having a combustor for burning fossil fuel or other synthetic fuels or organic fuels or combination of these fuels with similar layout of multistage superheater heating elements with at least one steam separating drum and primary superheater elements are positioned in convective zone at back end of flue gas pass and final superheater elements are positioned in the vicinity of fluidizing bed and attemperation is done in two phases in first pass and second pass by cooling steam taken from the steam drum after passing through a non-contact heat exchanger which preheat the boiler feed water prior entry to economizer and thereby reduces the cooling steam temperature towards the saturation temperature.
In another aspect, the boiler steam temperature control system relates an arrangement of attemperation by low temperature superheated steam, which is used after heat exchange with boiler feed water.
Yet, in another aspect, this boiler outlet steam temperature control system relates to the improvement in regenerative feed water heating system where heating is done by a source other than steam turbine extraction steam.
Still, in another aspect this invention relates to a method to control the quality and temperature of final superheated or reheated steam coming out of a fired or unfired steam generator associated with a steam cycle power plant or combined cycle power plant or a process steam plant running within sub critical temperature of working fluid, in natural circulation or forced circulation mode which includes a steam separating drum and multistage steam heating arrangement wherein low temperature steam heating elements are placed at back end of flue/waste
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heat gas pass and high temperature steam heating elements are placed at furnace radiant zone fitted with distributed attemperation system at both first pass/hotter flue gas zone and second pass/ colder flue gas zone using saturated steam as final attemperating medium.
Brief Description of the Drawings
The mentioned and other derivable features, aspects, and advantages of the present invention is better understood with following drawings where:
FIG - I is a schematic diagram of a system within the scope of the present invention. Therefore, FIG - I illustrates boiler outlet steam temperature control system with multistage attemperation at various passes of boiler with cooling steam arrangement including conventional layout of steam heating elements rather heat traps across the heat absorption passage within boiler and other heat exchangers in a schematic representation for best description of the invention.
FIG - II is a schematic representation of the control aspects and methodology for best practicing the invention which is to be considered in conjunction with the schematic representation of FIG -1.
Detail Description of the Embodiment for Best Mode of Practice
Referring to FIG - I, an embodiment of the present invention depicts, a steam generator operating within sub critical temperature regime, having conventional demineralized water as working fluid fed through a boiler feed water regulating valve 01 downstream of high pressure regenerative feed water heater 55 operating with heating steam 58 extracted from HP turbine and increasing the feed water temperature from discharge of boiler feed water pump 56, in a water tube boiler arrangement, with natural or forced circulation system, where heat input is either combustion of fossilized or other form of fuel source like organic or synthetic fuels or a combination of multiple fuel through appropriate fuel firing arrangement 38, or unfired and heat supplied by gas turbine exhaust of a combined cycle power plant, including the heat traps across the flue gas path, such that high temperature steam heaters in multiple stages are placed in hotter region or radiant zone or first pass of the boiler 37, and low temperature steam heaters in multiple elements are placed at colder region or convective zone or second pass 51- The boiler or steam generator of the above embodiment essentially consists a high-pressure steam-separating drum 04 which may or may not be placed within heat transfer zone of the boiler.
The boiler feed water supplied by boiler feed water pump 56, is entering boiler at rear pass 51 in economizer 03 after passing through the high pressure regenerative feed heater 55, where heating medium is high pressure steam turbine extraction steam 58, and subsequently passing through a feed water regulating valve 01 followed by an external non-contact feed water pre-heater 02, where preheating is done by part of low temperature superheated steam coming through 23 from the outlet header 36 of first stage of low temperature superheating element. The principle objective of this feed water preheater 02 is to improve regenerative feed heating system or to maintain feed water inlet temperature to boiler at various loading with reduced amount of water heater 55 requirement resulting less extraction from high pressure turbine and improving turbine performance. At the same time, the heating medium of this non-contact feed water preheater 02 is extracted steam 23 from outlet header 36 of first stage low temperature superheater 41, which is getting cooler to saturation temperature at that particular pressure before further use as attemperating steam 24 entering attemperating steam header 25. The attemperating steam header 25 is attached with a steam trap 49 for time-to-time discharge of condensate of steam flow line. However, as the total flow of attemperating steam through attemperating steam header 25 with
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respect to boiler steam generation load is fixed, the only variation is in the distribution of cooling steam into the two phases of attemperation, one cooling steam line 26 supplying towards convective zone attemperation system 28 in second pass 51 and another cooling steam line 29 supplying towards final or second stage attemperating system 31in radiant zone or first pass or hotter gas zone 37.
The preferred economizer 03 is non-steaming type where sensible heat gain occur within the forced flow section placed before flue gas outlet 39 to air preheater (outside the purview of the embodiment) of the boiler. Outlet from economizer 50 flows to steam drum 04 and recirculating into the furnace water walls 07 after passing through downcomers 05 and distributed to various water wall tube panels from furnace bottom feed water header 06 before hotter steam-water mixture within the nucleate steam boiling regime is returning back to steam drum 04 through furnace roof header 08. The steam from water-steam mixture is being separated while returning to steam drum 04 through swirl or vortex creating cyclone separator 10. Steam drum 04 is working as a vessel for steam separation followed by moisture washing and drying prior discharge to superheating system 11. It facilitate the continuous steam separation and recirculation of separated water along the incoming feed water from economizer 03 across the boiler furnace water walls which may occur naturally due to thermo-siphonic action or may be assisted with a forced circulation pump located at point 06 of FIG -1.
The dry steam 11 from steam drum 04 is entering the lowest temperature superheating system 41 through inlet header 12. This first stage superheater element 41 may be in the form of horizontal superheater tube panel placed upstream to economizer 03 in the flue gas flow path 40 -"39 as shown in FIG - II or low temperature zone / second pass roof tube superheater panel or second pass wall tube panels located within convective zone 51. The superheated steam from the lowest temperature superheater 41 outlet header 36, is distributed into two parts. While major portion is flowing into the next phase of low temperature superheater 32 through inlet header 13, a part is extracted as cooling steam 23 (which may also referred as hot cooling steam before heat exchanger) for attemperation eliminating the possibility of contamination by impure spray water attemperation device. The aforesaid cooling steam is further conditioned through a non-contact heat exchanger (previously referred as boiler feed water pre-heater) 02, making it cooler 24 (which may be referred as cold cooling steam after heat exchanger) to saturation temperature at that particular operating pressure of the heat exchanger.
The steam entering second stage low temperature superheater 32 through inlet header 13 is coming out through outlet header 14 and taken to first stage attemperator 28 placed outside the boiler heat transfer zone through steam conduit 15. The attemperated steam then passed to convective primary superheater 33 entering through inlet header 16 and going out from outlet header 17. From the convective primary superheater 33 the superheated steam is passed to final superheating elements 34 & 35 located within radiant zone of furnace or hot gas zone of boiler 37. The first phase of final superheater is a platen superheater 35 in which primary superheated steam is entering inlet header 18 and going out through outlet header 19 before passing to second stage attemperator 31. The attemperated superheated steam then flows through the last stage of superheating in the superheater element 34 entering through inlet header 20 and going out through outlet header 21 and subsequent discharge to final superheated steam outlet line 22.
The cooling steam from feed water preheater 02 flows to attemperating steam header 25 attached with a steam condensate trap 49 for condensate collection. From attemperating steam header 25 cooling steam is distributed into the two stages of attemperation. The convective superheater steam attemperator 28 is placed between intermediate low temperature superheater 32
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outlet header 14 and convective superheater 33 inlet header 16. The convective superheater steam attemperator receives attemperating steam through cooling steam line 26 fitted with a control valve 27. Since the convection heat transfer rates are almost a direct function of boiler output, the cooling steam flow in the convective attemperator 28 is controlled by control valve 27 based on a differential temperature controller 42 receiving pre-attemperation 44 and post-attemperation 45 temperature measurements across the second stage of attemperation 31 with appropriate functioning arrangement 46, that is not shown, for superheated steam entering inlet header 20 of final superheating arrangement 34 and subsequent discharge of attemperated final superheated steam through outlet header 21 to main steam line 22.
The other part of cooling steam 29 coming out of attemperating steam header 25 is flowing to second stage attemperator 31, after passing through a control valve 30. The controller of this control valve 30 receives control signal 48 through a temperature controller 43 that receives input temperature 47 at inlet header 18 of platen secondary superheater 35, which may also be referred as first leg of secondary superheating. However, due to fluctuation of boiler loading and combustion parameters, situation may arise when the final superheated steam temperature 57 of boiler outlet superheated steam 22 dictates a further emergency attemperation, for this, steam from boiler steam drum 04 outlet point 52 is taken directly as cooling steam and flowed through a control valve 53 to a charging point 54 downstream of control valve 30 in the cooling steam supply line 29 for second stage attemperator 31. The steam-cooled attemperators 28 & 31 are placed outside the heat transfer zone of boiler for the stated embodiment.
As detailed, the arrangement is practicable for described configuration and layout of heat traps within the heat transfer zone of boiler. Besides, the system is such that it can retrofit the existing spray water attemperation system in boiler with this invention conveniently.
As already pointed out the system is equally applicable for reheater attemperation arrangement in a reheat boiler with the similar attemperating arrangement. Therefore, the invention has been described in a view of most practical and preferred embodiment but its application can cover a wide range within the spirit and scope of the appended claims.
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What I claim is as stated below:
1. A method for the boiler outlet steam attemperation by attemperation (cooling) steam taken from a boiler feed water preheater where the system is associated with a steam cycle power plant or combined cycle power plant or steam generator of a process plant or a fluidized bed combustor including the following features:
• Where the boiler is having at least a steam separating drum and steam
heating arrangement above steam saturation temperature.
• The boiler is having the two passes. At front side, the hot combustion
product is generated or introduced from other source. The low temperature
combustion product is taken away from backside.
• The super heating elements are in multiple passes downstream to steam
drum. The low temperature super heaters are placed in various segments in
the back end, convective section of boiler. The high temperature super
heating arrangement is placed at front side of the boiler with hotter
combustion product.
• The attemperation of boiler outlet steam is done in two stages, one located
at primary superheating zone and another before completion of final
superheating.
• Temperature controller is attached to individual control valve for low
temperature or high temperature superheater attemperator. Heat absorption
in super heating tube elements is balanced by attemperation steam, which is
distributed such a manner that first stage attemperator cooling steam flow is
varied according to differential temperature across second stage
attemperator and second stage attemperator cooling steam flow is according
to temperature of super heated steam at inlet header of platen superheater or
the first stage of radiant zone super heating or first super heater element
receiving the hottest flue gas as depicted in most preferred embodiment.
The system comprises the steps of:
• The two stage attemperation system, that improves super heated steam flow
across all the steam super heating tube element with cooling steam from a
convenient location of low temperature super heating section with a pre-
cooling arrangement in a non-contact heat exchanger in the regenerative
feed heating circuit thereby eliminates the possibility of tube damage due to
overheating
• The cooling medium being lower temperature steam eliminates the
possibility of contamination problem associated with spray-water
attemperation system. The cooling steam taken from low temperature super
heated steam is farther conditioned to saturated steam temperature before
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used for attemperation by passing over a feed water pre-heater, an external non-contact heat exchanger placed upstream of economizer, i.e. before the feed water flows to heat transfer zone of boiler.
• Incorporation of the feed water preheater improves in regenerative feed water heating system where heating is done by a source other than steam turbine extraction steam resulting performance improvement in steam turbine and maintaining feed water inlet temperature to boiler at various steam generation load. The system also incorporate an emergency arrangement as per preferred embodiment for correction of final boiler outlet steam temperature for mismatch of boiler steam generation load and combustion guidelines.
2. The system of claim 1 and including a flow control valve in the line of super
heating elements of the said configuration.
3. System of claim 1 and including an arrangement to use cooling steam from
regenerative feed heating system for attemperation of superheated steam.
4. The system of claim 1 in the reheat steam cycle configuration of a reheat boiler of a
steam cycle power plant or of a combined cycle power plant WHRSG or of a
process steam generator or of fluidized bed combustor where cooling steam with
the preferred or logical embodiment is introduced in reheater attemperation system.
5. A method of improvement of regenerating feed water-heating system of steam
cycle power plant where heating source is other than extraction steam from turbine.
6. A system of claim 4 including a method to maintain feed water inlet temperature to
boiler at various steaming load.
7. A method for attemperation in multiple stages using steam as cooling medium.
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In a steam cycle power or process plant having a fuel fired boiler or unfired waste
heat recovery steam generator (WHRSG) with at least a steam drum and multistage superheating arrangement wherein low temperature superheating elements placed at back end, colder gas pass of boiler and high temperature superheating elements placed at boiler furnace or gas inlet of WHRSG, the improvement in superheated steam quality and temperature control (attemperation) is conceptualized having distributed attemperating arrangement to both pass of boiler or at hotter and cooler end of WHRSG with extracted cooling steam from downstream of steam drum, passing through a non-contact feed water heater placed outside boiler, reducing cooling steam temperature to saturation temperature, and injected to hot superheated steam in attemperator and improving boiler feed water temperature upstream to economizer and thereby reducing high pressure feed water heating with steam turbine extraction in the regenerative feed heating system.