JP2532750B2 - System and method for reheat steam temperature control in a circulating fluidized bed boiler - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention comprises a two-stage steam turbine and a fluidized bed combustion system including a fluidized bed combustion device, at least one separator and a gas flue having a reheater and a superheater. And a power generator including a steam generator.

The present invention also relates to a method of controlling reheater temperature in a steam generator having a fluidized bed combustion system, a fluidized bed combustion system including at least one hot separator and a reheater in a gas flue.

Several methods are currently known for controlling reheat steam temperature.

One way to control the reheater temperature is to use a system for gas bypass to the reheater. Two separate flue gas paths are provided in the convection path of the boiler (one for the superheater and one for the reheater) to vary the amount of flue gas flow for each section A means such as a damper is provided on the downstream side of each unit.
The reheater outlet steam temperature can be controlled by varying the amount of flue gas flow between the convective path sections. The main drawback of this system is that the damper has a relatively high temperature (260-371
C) dust, and is located in the flue gas path that causes corrosion and mechanical failure. Also, the steam temperature control range is limited by this type of system.
Another way to control the reheater outlet steam temperature is through the use of an external heat exchanger. According to this method, a portion of the recirculated solids in the circulating fluidized bed system is an externally mounted fluidized bed heat exchanger, or external heat exchanger (EHE), within which the reheater Converted to the one in which the section or whole is located. By varying the flow rate of solids to the EHE, the amount of heat exchange to the reheater and the reheater outlet steam temperature are controlled. The main drawback of this system is that the solids flow control valve is a high maintenance component and the reheat tube surface in the EHE is unavoidable for corrosion. This affects the availability of units.

Also, in U.S. Pat. No. 4,748,940, a first reheater heating surface is disposed in the flue gas passage of a circulating fluidized bed combustor and an external heat exchanger ( EHE)
And connecting a second reheater disposed therein. An adjustable bypass line is coupled in parallel with the reheater heating surface. The reheater outlet temperature is controlled by controlling the solids flow in the external heat exchanger and by controlling the vapor flow in the two reheaters by bypass lines.

Reheater Outlet Temperature Another way to control steam temperature is through the use of a spray superheat reducer. This method employs spraying water to reduce superheat and thereby controlling the reheater outlet steam temperature. This is a simple method, but it is not commonly adopted because it reduces cycle efficiency.

Yet another alternative is by using excess air.
Excess air supplied to the boiler can be used for reheat steam temperature control. However, this method is unwelcome because of its negative impact on boiler efficiency.

Yet another alternative is by using gas recycle. According to this method, a large amount of flue gas is recirculated to obtain the rated reheater outlet steam temperature. However, this method requires the use of a gas recirculation fan to handle the gas entrained with hot dust, and requires additional power consumption which makes this method disadvantageous.

Accordingly, the present invention is directed to improved methods and systems for reheat steam temperature control.

APPLICATIONS AND OBJECTS OF THE INVENTION A primary object of the present invention is to provide an improved system and method for controlling reheater (outlet) steam temperature in a circulating fluidized bed boiler.

According to a main aspect of the invention, a steam generator having a fluidized bed combustion system with a fluidized bed combustion device, at least one separator and a reheater located in the flue gas path is: continuous in a common gas flue; A first stage of the reheater and a second or final stage of the reheater, arranged to selectively divide the low temperature steam from the turbine into first and second parts and to divide said first part. The first of the reheater
Means for guiding through the stages, and-means for recombining the first and second parts and for guiding them through the second stage of the reheater.

Preferably said steam generator comprises means for controlling the temperature of the second or final stage of the reheater and a selected portion of the cold steam is bypassed by said second stage reheater. Means for direct bypass to the stage or final stage reheater.

The method according to the invention comprises: dividing the reheater into first-stage and second-stage or last-stage reheaters and continuously reconstituting the first and second stages of the reheater into a common gas flue. Arranging, -dividing the cold vapor returning to the reheater into selective first and second parts and directing the first part through the first stage of the reheater, -said first part And re-joining the second part and directing them through the second or final stage of the reheater.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and advantages of the present invention will become apparent from the following description when considered in conjunction with the accompanying drawings. In the drawings: FIG. 1 is a schematic diagram illustrating a typical circulating fluidized bed boiler system in which the present invention is practiced.

FIG. 2 is a schematic diagram illustrating another embodiment of the present invention. And, FIG. 3 is a schematic diagram illustrating an arrangement of two typical boilers coupled to a single turbine.

Detailed Description of the Preferred Embodiment Referring to FIG. 1, a power plant implementing a typical circulating fluidized bed boiler having a superheater and a reheater is illustrated with a system incorporating a preferred embodiment of the present invention. It
A boiler system, generally designated by the numeral 10, is a fluidized bed combustor having a combustion chamber 14 into which combustibles, non-combustibles, possibly additives or recirculations, primary air and secondary air are delivered. Having twelve. Within the combustion chamber, the bed is maintained in a fluidized state by having proper inventory of bed materials and air flow. The combustion chamber is provided with a bottom 16 having a grid-like structure through which fluidized air is introduced. The combustion chamber wall is preferably constituted by a membrane-type tube wall with or without a refractory cover.

Superheater first and second stages 18 and 22 are located in the combustion chamber. Combustion chamber material is conveyed from the combustion chamber through a plurality of flues 22 to a hot separator 24, where solids are separated from the flue gas and removed from the combustion chamber for recirculation through particle recirculation systems 26, 28 and 30. Returned to the bottom. These particles are passed through a fluidized bed cooler or the like prior to return to the combustion chamber.

The details of the circulation circuit for the water supply and the primary heater are not illustrated since they do not form an essential part of the invention.

Flue gas from the hot separator passes by flue 32 to convection pass 34. A single stage superheater 38 is placed or positioned in the convection path and reheaters 40 and 42
Is located downstream of the superheater 38 and upstream of the economizer surface 44. The reheater 42 is the first stage and 40 is the second
Alternatively, the final stage is illustrated as a two-stage type. The reheater may have more than two stages, in which case the last stage is 40
Is located immediately downstream of the superheater 38. These are arranged as counterflow heat exchangers with the gas flow direction facing down and the reheat steam flow direction facing up. The placement of the superheater 38 in this pass helps keep the temperature of the gas stream to the reheater 40 below the critical temperature. This arrangement, in combination with bypass features that will be explained later, allows for unique and effective control of temperature within the reheater section.

When the steam temperature leaving a particular section (in a counterflow heat exchanger) is close to the gas temperature entering the section, reducing the steam flow rate to that section has the effect of significantly reducing heat absorption. As the steam temperature approaches the gas temperature, the effective thermal heat available for heat transfer is reduced. This provides the basis for the principles used for the reheat temperature control system according to the invention.

The generation system illustrated in Figure 1 supplies steam to a two-stage turbine. In the illustrated arrangement, steam from superheater 38 flows through outlet header 46 and feed line 48 through valve 50 to the inlet side of high pressure turbine (HPT) 52. The cold steam leaving turbine 52 returns to reheaters 42 and 40 through return line 53. In the reheater, the bypass line 54
At 55, it connects with the return line 53 to bypass a portion of the low temperature steam and the remaining portion of the steam passes through the differential control valve 56 to the inlet header 58 of the first stage reheater 42.

Steam passing through reheater 42 exits via header 60 and rejoins or joins at 62 with the bypass portion of the cold steam. Flow control valve 64 is a bypass line
Located at 54 to control the flow between the inlet manifold of the first stage reheater 42 and the bypass line. The steam rejoined at 62 is the inlet header of the second or final stage reheater 40.
It flows into 66, is further heated in reheater 40, and flows through outlet header 68, feed line 70 and valve 72 to the second or low pressure stage of the turbine (IPT). Bypass line 54
The selective allocation of low temperature steam between the first stage reheater 42 and the
Provide effective and efficient temperature control means in the reheater stage.

The location of the first stage reheater 42 along the flue gas path allows bypassing a required portion of the cold reheat steam directly to the second stage reheater 40 leaving the first stage reheater. Is selected so that it cannot be increased above the allowable metal temperature of the reheater tube material. Limits are set to prevent first stage reheater materials from exceeding their allowable metal temperatures. 566 ° C
The value of is a typical limit and varies depending on the actual design conditions. The purpose of the system is to ensure that the maximum tube outer surface temperature does not exceed the permissible metal temperature of the selected material.

The arrangement of control valves 56 and 64 allows controllability to be gained over the entire steam temperature control range and allows all reheater surfaces to be located in the convection path of the boiler, and the in-core reheater. Selected to eliminate the need for faces. This also makes it possible to carry out a simplified start-up plan, for example when more than one boiler is combined in a common turbine system. In this arrangement, the valve set provides reheat vapor flow balancing means under various operating conditions.

In a circulating fluidized bed boiler, combustion occurs in a fluidized bed of inert material. Fluidized bed material leaving the combustor is returned by a hot collector (eg, a hot cyclone) through a suitable sealing device. During operation, air and fuel are delivered to the combustion chamber 14 where the bed material is maintained in a fluidized state by having the correct flow rates of air and bed material. Fluidized air is introduced through a grid or structure in the bottom 16 of the combustion chamber. Flue gas and combustion products, along with any solids that remain, are first removed from the superheater.
Heat is transferred to 18 and 20 and conveyed via flue 22 into a hot separator 24 where solids are separated and returned to the combustion chamber through recirculation devices 26, 28 and 30. The hot flue gas is then conveyed from the hot separator through the flue 32 to the convection path section, where the final stage superheater 38 and reheater stages 40, 42 are located.

In the system described, three superheater stages are arranged, they are 18, 20 and 38, and 38 is in the flue gas convection path. If desired, desuperheaters can be placed between the superheater stages for control of steam temperature. The two stages 40, 42 of the reheater are located in the convection path 34 with control valves and interconnect piping to allow fine control of the reheater outlet steam temperature. The piping system is configured such that the cold steam re-entering the system at pipe 53 is selectively split into two streams at its connection point 55 with the bypass line 54.
One stream goes to the first stage reheater and is distributed through the inlet header 58. The other stream goes through valve 64 and inlet header 66 to the second stage reheater. Selective splitting of steam is proportional to the required temperature control achieved by valves 56 and 64.

The hot steam leaving the first stage reheater from the outlet header 60 is mixed with the cold steam or downstream flow of the flow control valve 64 through the bypass line 54, and the mixed flow is passed through the inlet header 66 to the first stream. Enter the two-stage reheater. The flow through the first stage reheater is controlled by the proper operation of two control valves 56 and 64, which in the future controls the temperature of the steam leaving the second stage reheater 40. The hot steam from the second or final stage reheater is directed back to the turbine by hot reheat steam line 70.

The differential pressure responsive control unit 80 controls the adjustment of valve 56 to control the pressure differential that can be utilized for control valve 64. The control unit 80 responds to the pressure difference between the cold vapor return line 53 and the outlet pressure at the connection 62 between the outlet of the reheater 42 and the bypass line 54. This is represented by the dashed line 84 in FIG. The control unit 80 is set to control the valve 56 as a function of the load on the boiler.

The valve 64 on the bypass line 54 is controlled by the temperature reaction control unit 82, which responds to the temperature of the steam discharged from the second or final stage reheater 40. This is the first
It is represented by a broken line 86 in the figure. In the illustrated embodiment, as an example, the temperature of the reheater 40 is maintained within limits of about 538 ° C ± 10 ° C. When the temperature of the steam leaving the reheater 40 begins to rise and exceeds 543 ° C, the valve 64 is opened to bypass additional cold steam directly to the reheater 40. When the temperature begins to drop below 532 ° C, valve 64 is closed and the second
Reduce the flow rate of bypass cryogenic steam to stage 40.

In FIG. 2 there is disclosed the same system as in FIG. 1, but with the superheater 38 located between the reheaters 40 and 42. The single stage superheater 38 is located in the convection path, the second stage reheater 40 is located upstream of the superheater, and the first stage reheater is
42 is located downstream of the superheater. The economizer 44 is located downstream of the superheater 38. This is in contrast to that shown in FIG. The placement of the second stage reheater 40 upstream of the superheater 38 allows it to absorb more heat at lower loads. This gives it the possibility to extend its steam temperature control range, while its effect on the superheater control range is insignificant, if any, such as the reheat steam temperature control range mentioned above. The expansion of 2 more easily enhances the coupling of 2 units to 1 turbine in terms of temperature matching capability.

This arrangement, in which the second stage reheater 40 is located upstream of the superheater 38, also provides greater control over the temperature in the reheater stage. Since the gas now passes through the reheater 40 before it passes through the superheater 38, it is not below the critical temperature for the reheater 40 until it is related to the particular load of the boiler. Thus, when the superheater 38 is after the reheater 40 in its pass, the gas temperature is below the critical temperature for the reheater 40 at a load of about 25% to 30%. Only after being done. At this point, low temperature steam can be used for temperature control in accordance with the present invention. If higher load points are required, the tube metal material can be graded to allow a maximum load of about 35% -40%. This is another advantage of the present invention in that it does not require flow through the reheater until the unit is at 25% to about 40% load.

Referring to FIG. 3, a system similar to that of FIG. 1 but with a dual boiler is disclosed. In this system, the components of the first boiler system are identified by the same reference numerals as in FIG. 1 and the second boiler system is identified by the same primed numbers. Thus, in this scheme, a boiler turbine system is disclosed in which two boilers supply steam to a single turbine. One essential feature required for this type of system is the amount of reheated steam to each boiler so that the steam temperature at the reheater outlet remains within limits at all possible operating conditions. A means for controlling is provided. In the illustrated system, dual control means and piping are provided for the two boilers.

Control valves 56 and 64 for reheat steam temperature control may be used to balance flow rates and maintain reheater outlet temperatures within limits under normal and abnormal operating conditions. In this arrangement, the pressure reducing valves 80 and 82, along with the superheat reducers 76 and 78, provide flexibility when starting the second unit during cold start, hot start, and during operation of the first unit. This simple system eliminates the need for an elaborate vapor mixing system. It provides a simple and effective system and method for reheat outlet steam temperature control under varying load conditions.

In operation, from a cold start, combustion is initiated in the combustion chamber 14 with fuel and air introduced. As heat is generated as a result of combustion, the hot combustion gases move upwards in the combustion chamber to the water in the combustion chamber wall and to the superheater 18 and
Transfers heat to 20. Hot gases, combustion products and solids enter the combustion chamber along the flue 22 into a hot separator 24 where the solids are separated for return to the combustion chamber. The hot flue gas enters the convection path 34 along the flue 32 where heat is sequentially transferred to the superheater 38, the second or final stage reheater and the first stage reheater 42. The flow of hot gas through the system begins prior to the flow of cold steam. After the boiler is burned and the fuel burns for a time interval that provides hot gas, steam is generated and the turbine is started.

As the hot gases transfer their heat to the water and steam in the water wall, the heat drops in the superheater and reheater and is therefore lower in each successive stage. It should be noted that at full load the temperature of the gas leaving the combustion chamber outlet is in the range 843-927 ° C. The greater the temperature difference between the gas and the water, the greater the heat transfer and the lower the temperature as the gas passes from each superheater.

Therefore, as the gas passes through the superheater 38, it will be below the critical temperature of the reheater 40 until a particular temperature in the boiler is reached. Thus, when the superheater 38 is in front of the reheater in the gas path, the gas temperature is below the critical temperature of the reheater 40 until after a load of about 40% -50% is reached. At this point, low temperature steam may be utilized for temperature control according to the present invention. This is another advantage of the invention in that it does not require flow through the reheater until the unit is 50% loaded. Most standard systems require flow through the reheater during the initial stages of start-up (hot or cold) to prevent loss due to overheating. Therefore,
Expensive bypass systems must be used.
However, due to the physical design of this system, no bypass is needed and system start-up time can be reduced.

Other modifications and changes are possible in the above disclosure, and in some cases some features may be used without a corresponding use of other features. Accordingly, while the present invention has been illustrated and described with respect to particular embodiments, numerous changes and modifications therein may be made without departing from the spirit and scope of the invention as limited by the appended claims. It should be understood that it can be applied.

Front Page Continuation (72) Inventor Ruskin, Nail, Earl. USA 92009 Carlsbad, CA Maslolane 7322 (56) References JP 58-164903 (JP, A) JP 59-219603 (JP, A) Japanese Unexamined Patent Publication No. 54-151743 (JP, A) Actually developed 61-175504 (JP, U) US Pat. No. 4473032 (US, A)

Claims (5)

(57) [Claims] 1. A two-stage steam turbine and a steam generator, the steam generator comprising a fluidized bed combustor (12), at least one separator (24), and at least reheats arranged in series. In a power plant having a first and second or final stage of the reactor and a fluidized bed combustion system having a superheater (38), the steam generator is continuously in a common gas flue (34). Arranged reheater first stage (42) and reheater second or final stage (40)
And the first and second selective low temperature steam from the turbine (52).
Means (54, 64) for dividing the first part through the first stage (42) of the reheater and rejoining the first and second parts and reconnecting them. A means (62) for guiding through the second stage (40) of the reheater and a first reheater located at the inlet of the first reheater stage (42).
A power plant having at least one pressure control valve (56) responsive to a difference between a stage outlet steam pressure and the pressure of the cold steam. 2. A power plant according to claim 1, wherein the means (54, 64) for splitting the cold steam directs a selected portion of the cold steam from the turbine to a first stage (42) of the reheater. A) and a bypass line (54) with at least one flow control valve (64) for bypassing directly to the second or final stage (40) of the reheater. And a power plant. 3. A method for controlling reheater temperature in a steam generator having a fluidized bed combustion system, a fluidized bed combustion system having at least one hot separator and a reheater, the method comprising: Splitting the first and second or re-final stage reheaters and placing the first and second stages of the reheater consecutively in a common gas flue; Dividing the first and second parts into selective ones and directing the first part through the first stage of the reheater; recombining said first and second parts and re-connecting them. Directing through the second or final stage of the heat exchanger and the differential pressure between the pressure of the cold steam and the outlet steam pressure of the first stage of the reheater to split the cold steam into first and second parts. Controlling the reheater temperature in the steam generator. How to control the. 4. A method according to claim 3, wherein a second portion of the split cold steam is conducted through a bypass line extending between the inlet and the outlet of the first stage of the reheater. Of controlling the reheater temperature in a steam generator. 5. A method according to claim 3 in which the steam temperature of the second or final stage of the reheater is adjusted in order to react the temperature to split the cold steam into first and second parts. A method of controlling a reheater temperature in a steam generator, the method comprising controlling.