The present invention relates to a power plant including a two-stage
steam turbine and a steam generator plant having a fluidized bed combustion system
that includes a fluidized bed combustor, at least one separator, and a gas flue
comprising a reheater and superheater.
The present invention relates also to a method of controlling reheater
temperatures in a steam generator having a fluidized bed combustion system that
includes a fluidized bed combustor, at least one hot separator, and a reheater
in a gas flue.
Several methods are presently known for controlling reheat steam
One method of reheater temperature control is the use of a system
for gas bypass over the reheater. Two separate flue gas passes are provided in
the convection pass of the boiler (one for superheater and one for reheater), with
means such as dampers downstream of each to vary the amount of flue gas flow over
each section. The outlet steam temperature of the reheater can be controlled by
varying the amount of flue gas flow between the convection pass sections. The
main disadvantage of this system is that the dampers are located in a higher temperature
(260-371°C) dust laden flue gas path making them susceptible to erosion and mechanical
failure. Also, the steam temperature control range is limited with this type of
Another method of reheater outlet steam temperature control is by
the use of external heat exchangers. With this approach, a portion of the recirculated
solids within the circulating fluidized bed system is diverted to an externally
mounted fluidized bed heat exchanger, i.e. external heater exchanger (EHE), in
which a section of or complete reheater is located. By varying the amount of solids
flow to the EHE, the quantity of heat transfer to the reheater and the reheater
outlet steam temperature is controlled. The main disadvantages of this system are
that the solids flow control valve is a high maintenance item and the reheat tube
surface within the EHE is subject to erosion. This effects the availability of
It has also been suggested in US 4,748,940 to arrange first reheater
heating surfaces in a flue gas passage of a circulating fluidized bed combustor
and to connect to this first reheater a second reheater disposed in an external
heat exchanger (EHE). An adjustable by-pass line is connected in parallell to
the reheater heating surfaces. The outlet temperature of the reheater is controlled
by controlling the solids flow in the external heat exchanger and by controlling
the steam slow in the two reheaters by means of the by-pass line.
A further different arrangement of first and final stages of reheaters
has been suggested in EP-A- 0 274 637. EP-A-0 274 637 shows a power plant including
a two-stage turbine and a steam generator plant having a fluidized bed combustor,
particle separators, first and second or final stages of reheaters and a superheater.
The first and final stages of reheaters are according to EP-A- 0
274 637 sequentially disposed. Two stages of reheaters are disposed in separate
external heat exchangers for cooling discharged ash. The final stage of reheater
is disposed in a gas flue connected to the gas outlet of the fluidized bed combustor.
The quantity of heat transferred to the first stages of reheaters and the reheater
outlet temperature is controlled by varying the amount of solids flow to the external
heat exchangers. This arrangement is not favored due to solids flow control valve
maintenance problems as already mentioned earlier.
A further approach to the control of the reheater outlet steam temperature
is by the use of spray desuperheater. This approach utilizes spraying water for
desuperheating and thereby controlling reheater outlet steam temperature. This
is a simple approach, but not gererally accepted because it degrades the cycle
Still another approach is by the use of excess air. Excess air supplied
to the boiler can be used for reheat steam temperature control. This approach,
however, is not favored because of its negative affect on boiler efficiency.
A still further approach is by the use of gas recirculation. By this
approach, large quantities of flue gases are recirculated to achieve the rated
reheater outlet steam temperature. This approach, however, requires the use of
a gas recirculation fan for handling a hot dust laden gas and requires additional
power consumption, which makes this approach disadvantageous.
Accordingly, the present invention is directed to an improved method
and system for reheat steam temperature control.
SUMMARY AND OBJECTS OF THE INVENTION
It is the primary object of the present invention to provide an improved
system and method for controlling the reheater (outlet) steam temperature in circulating
fluidized bed boilers.
In accordance with a primary aspect of the present invention, a steam
generator is provided having a fluidized bed combustion system that includes a
fluidized bed combustor, at least one separator, and a reheater in a flue gas pass
and is characterized by
- a first stage of reheater and a second or final stage of reheater sequentially
disposed in a common gas flue,
- means for dividing cold steam from a turbine into selective first and second
portions and directing said first portion through the first stage of reheater,
- means for recombining the first and second portions and directing same through
the second stage of reheater. Preferably the steam generator includes means for
controlling the temperature of the second or final stage of the reheater and comprises
means for by-passing a selected portion of cold steam around said first stage reheater
directly to said second or final stage reheater.
A method according to the present invention is characterized by
BRIEF DESCRIPTION OF THE DRAWINGS
- dividing the reheater into a first and second or final stage reheater and sequentially
disposing the first and second stages of the reheater in a common gas flue,
- dividing cold steam returning to the reheater into selective first and second
portions and directing the first portion through the first stage of reheater and
- recombining the first portion and the second portion and directing the same
through the second or final stage of reheater.
The above and other objects and advantages of the present invention
will become apparent from the following description when read in conjuntion with
the accompanying drawings wherein:
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
- Fig. 1 is a schematic diagram illustrating a typical circulating fluidized
bed boiler system embodying the present invention;
- Fig. 2 is a schematic diagram illustrating another embodiment of the present
- Fig. 3 is a schematic diagram illustrating an arrangement of two typical boilers
connected to a single turbine.
Referring to Fig. 1, a power plant embodying a typical circulating
fluidized bed boiler with superheater and reheater is illustrated with the system
incorporating a preferred embodiment of the present invention. The boiler system,
designated generally by the numeral 10, comprises a fluid bed combustor 12 having
a combustion chamber 14 into which combustible material, non-combustible material,
possibly additives or recirculated material, primary air and secondary air are
fed. In the combustion chamber, the bed is maintained in fluidized state by having
the correct inventory of bed material and flow of air. The combustion chamber
is provided with a bottom 16 having a grid-like construction through which fluidizing
air is introduced. The combustion chamber walls are preferably constructed with
membrane type tube walls, with or without a refractory covering.
First and second stages of superheaters 18 and 20 are located within
the combustion chamber. The combustion chamber materials are carried from the combustion
chamber by way of flues 22 to a hot separator 24 wherein the solids are separated
from the flue gases for return by way of particle recycling system 26, 28 and 30
to the bottom of the combustion chamber for recirculation. These may be passed
through fluidized bed coolers or the like prior to return to the combustion chamber.
The details of circulation circuit for the feed water and the primary
superheaters are not illustrated as they do not form an essential part of the present
Flue gases from the hot separator pass along by way of flue 32 to
a convection pass 34. A single stage superheater 38 is placed or located in the
convection pass with reheaters 40 and 42 located downstream of the superheater
38 and upstream of an economizer surface 44. The reheater is illustrated as two
stages with 42 being a first stage and 40 being a second or final stage. The reheater
may have more than two stages, with the final stage just down stream from superheater
38 such as 40 is located. These are arranged as counter flow heat exchangers with
the gas flow direction down and the reheat steam flow direction up. The placement
of the superheater 38 within this pass helps keep the temperature of the gas flow
to reheater 40 below the critical temperature. This arrangement together with
the bypass feature as will be explained enables a unique and effective control
of the temperatures within the reheater sections.
When the steam temperature leaving the particular section (in a counter
flow heat exchanger arrangement) is close to the gas temperature entering that
section, reducing the steam flow to that section will result in a considerable
reduction in 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 principle used for the reheat temperature control system in
accordance with the present invention.
The generating system as illustrated in Fig. 1 is supplying steam
to a two-stage turbine. In the illustrated arrangement, steam from superheater
38 flows via an outlet header 46 and supply line 48 by way of valve 50 to the inlet
side of the high pressure turbine (HPT) 52. Cold steam leaving the turbine 52
returns by way of return line 53 to the reheaters 42 and 40. At the reheater 42,
a bypass line 54 joins the return line 53 at 55 and bypasses a portion of the
cold steam with the remaining portion of the steam going by way of differential
control valve 56 to the inlet header 58 of the first stage reheater 42.
The steam passing through the reheater 42 exits by way of a header
60 and rejoins or combines with the bypass portion of the cold steam at 62. A flow
control valve 64 is provided in the bypass line 54 for control of the flow between
the inlet manifold of the first stage reheater 42 and the bypass line. The recombined
steam at 62 flows into inlet header 66 of the second or final stage reheater 40
where it is further heated and flows by way of outlet header 68, supply line 70
and valve 72 to the second stage or lower stage of the turbine (IPT) 74. The selective
proportioning of the cold steam between the bypass line 54 and the first stage
of the reheater 42 provides an effective and efficient means of controlling the
temperature in the reheater stages.
The location of the first stage reheater 42 along the flue gas path
is so chosen that bypassing the required portion of the cold reheat steam directly
to the second stage reheater 40 cannot increase the steam temperature leaving
the first stage reheater to more than the allowed metal temperature for the reheater
tube material. A limit will be set to protect the first stage reheater materials
from exceeding their allowable metal temperature. The value of 566°C is a typical
limit and may vary depending upon the actual design conditions. The purpose of
the system is such that the maximum tube outside surface temperature will not
exceed the allowable metal temperature limit for the material selected.
The arrangement of the control valves 56 and 64 is so chosen that
controllability is achieved throughout the steam temperature control range and
permits all reheater surfaces to be placed in the convection pass of the boiler,
eliminating the need for in furnace reheater surfaces. This also makes feasible
a simplified start-up scheme when more than one boiler for example is connected
to a common turbine system. In this arrangement, the set of valves provide a means
for reheat steam flow balancing under various operating conditions.
In the circulating fluidized bed boiler, the combustion takes place
in a fluidized bed of inert material. The fluidized bed material leaving the combustor
is returned by means of a hot collector (such as a hot cyclone) through suitable
sealing device. In operation, air and fuel are delivered to the combustion chamber
14 wherein the bed material is maintained in a fluidized state by having the correct
flow of air and bed material. The fluidizing air is introduced through a grid-like
grating or construction at 16 in the bottom of the chamber. The flue gas and combustion
products, along with the carry over solids, first convey heat to the superheaters
18 and 20 and are conveyed by way of flue 22 into the hot separator 24 wherein
the solids are separated and returned to the combustion chamber through the recycling
arrangement 26, 28 and 30.
The hot flue gases are then conveyed from the hot separator(s) by
way of flue 32 to the convection pass section 34 wherein the final stage superheater
38 and the reheater stages 40 and 42 are located.
Three superheater stages are disposed in the described system, these
being 18 and 20 and 38, with 38 being in the flue gas convection pass. Desuperheaters
may be positioned between the superheater stages for steam temperature control
if necessary. The two stages 40 and 42 of the reheater are positioned in the convection
pass 34 and in conjunction with the control valves and interconnecting piping so
that precise control of the reheater outlet steam temperature is possible. The
piping system is such that cold steam reentering this system at pipe 53 is selectively
divided into two streams at the juncture 55 thereof with the bypass line 54. One
stream passes to the first stage reheater and is distributed through inlet header
58. The other steam goes to the second stage reheater by way of valve 64 and inlet
header 66. The selective division of the stream will be in proportion to the temperature
control necessary, which is accomplished by the valves 56 and 64.
The hot steam leaving the first stage reheater from the outlet header
60 is mixed with the cold steam via the bypass line 54 after or down stream of
the flow control valve 64 and the blended stream enters the second stage reheater
by way of the inlet header 66. The flow through the first stage reheater is controlled
by proper manipulation of the two control valves 56 and 64, which in turn control
the steam temperature leaving the second stage reheater 40. Hot steam from the
second or final stage reheater is directed back to the turbine by way of the hot
reheat steam line 70.
A differential pressure responsive control unit 80 controls the setting
of valve 56 for controlling the pressure differential available for the control
valve 64. The control unit 80 is responsive to the pressure differential between
the cold steam return line 53 and the outlet pressure at juncture 62 of the outlet
of reheater 42 and the bypass line 54. This is indicated by phantom line 84 in
Fig. 1. The control unit 80 is set to control the valve 56 as a function of load
on the boiler.
The valve 64 in the bypass line 54 is controlled by temperature responsive
control unit 82 which responds to the temperature of the outlet steam from the
second or final stage reheater 40. This is indicated by phantom line 86 in Fig.
1. In the illustrated embodiment, as an example the temperature of reheater 40
is maintained within the limit of about 538°C, plus or minus 10 °C. As the temperature
of the steam leaving reheater 40 begins to increase above 543°C, the valve 64
is opened to bypass additional cold steam directly to reheater 40. As the temperature
begins to fall below 532°C, the valve 64 is closed to reduce the flow of bypass
cold steam to the second stage 40.
In Fig. 2, a system identical to Fig. 1, but with superheater 38
located between the reheaters 40 and 42, is disclosed. A single staged superheater
38 is placed in the convection pass, with second stage reheater 40 located upstreams
and first stage reheater 42 downstream of the superheater. This is in contrast
to what was shown in Fig. 1. An economizer 44 is located downstream of the superheater
38. The placement of the second stage reheater 40 upstream of the superheater
38 allows it to pick up more heat at lower loads. This gives it the potential to
extend its steam temperature control range, while having little if any effect
on the superheater control range. This potential extension of the reheat steam
temperature control range will enhance the coupling of two units to one turbine
easier as to temperature matching capabilities.
The present arrangement, with second stage reheater 40 upstream of
superheater 38, gives even greater control over the temperature in the reheater
stages. As the gas now passes reheater 40 before it passes superheater 38, it
may not be below the critical temperature for reheater 40 up to some load of the
boiler. Thus, with superheater 38 in the pass behind the reheater 40, the gas temperature
will be below the critical temperature for reheater 40 only until after about
25% to 30% load is reached. At this time cold steam is available for control of
the temperature in accordance with this invention. If a higher load point is required,
the tube metal materials could be upgraded to allow a maximum load of about 35%
to 40%. This point of not requiring flow through the reheater until the unit is
at 25% to about 40% load is another advantage of this invention.
Referring to Fig. 3, a system identical to Fig. 1, but with a duplicate
boiler, is disclosed. In this system, the components of the first and the second
boiler arrangements are identified by the same reference numerals as in Fig. 1,
with the second boiler arrangement being identified with the same numbers primed.
Therefore, in this arrangement, a boiler turbine system is disclosed wherein two
boilers are supplying steam to a single turbine. One essential feature required
for this type of system is that means be provided for controlling the amount of
reheat steam flow to each boiler, so that the steam temperature at reheat outlet
is within limits at all possible operating conditions. In the illustrated system,
duplicate controls and piping are provided for the two boilers.
The control valves 56 and 64 for the reheat steam temperature control
can be used for flow balancing and maintaining the reheater outlet temperature
within limits under both normal and abnormal operating conditions. In this arrangement,
pressure reducing valves 80 and 82, along with desuperheaters 76 and 78, provide
for flexibility during cold start-up, hot start-up and also when starting the
second unit while the first one is on line. This simple system eliminates the need
for a sophisticated steam blending 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 the introduction of fuel and combustion air. As heat is generated
as a result of the combustion, the hot gases of combustion move upward in the
combustion chamber transferring heat to the water in the combustion chamber walls
and to superheaters 18 and 20. The hot gas, combustion products, and solids pass
from the combustion chamber along flue 22 into the hot separator 24 where the
solids are separated for return to the combustion chamber. The hot flue gas passes
along flue 32 into the convection pass 34 where the heat is transferred in sequence
to the superheater 38, the second or final stage reheater 40, and the first stage
reheater 42. The flow of hot gas through the system begins before the flow of cold
steam. The boiler is fired and fuel burns for a period of time providing hot gas
before steam is generated and starts the turbine. Cold reheat steam does not start
flowing until after the turbine starts.
As the hot gas give up their heat to the water and steam in the water
walls, in superheaters and reheaters, the temperature drops so that it is less
at each successive stage. It should be noted that the gas temperature leaving
the combustion chamber outlet at full load will be in the range of 843 to 927°C.
The greater the temperature differential between the gas and the water, the greater
the heat transfer will be, and the cooler the gas will be as it passes from the
Therefore, as the gas passes superheater 38, it will be below the
critical temperature for reheater 40 up to some load of the boiler. Thus, with
superheater 38 in the gas pass ahead of the reheater 40, the gas temperature will
be below the critical temperature for reheater 40 until after about 40% to 50%
load is reached. At this time cold steam is available for control of the temperature
in accordance with this invention. This point of not requiring flow through the
reheater until the unit is at 50% load is another advantage of this invention.
Most standard systems require flow through the reheater during the earlier stages
of start-up (hot or cold), to protect some from burn out. Thus, an expensive by-pass
system must be utilized. However, with this system's physical layout, a bypass
is not required and system start-up periods can be shortened.