Steam power plant with fluidized bed heat source for superheater and method of producing superheated steam

A steam power plant is disclosed in which superheaters for the various turbine stages include heat exchange coils located in the bed portion of high temperature fluidized beds. Improved heat transfer and reduced corrosion of the superheater coils are achieved. A method of producing superheated steam without high temperature corrosion is also disclosed.

BACKGROUND OF THE INVENTION 
In power plants of the type comprising a boiler with an integral 
superheater, a turbine and a separate superheater between the turbine and 
the integral superheater, it is desirable to drive the superheating of the 
steam as high as possible to achieve a high thermal efficiency and good 
heat economy. However, problems have been experienced in prior art plants 
when the temperature of the steam considerably exceeds 500.degree. C. 
Under such conditions, ashes from the flue gases passing through the 
superheaters frequently contain vanadates and other compounds with low 
melting points which are extremely corrosive in the molten condition. Such 
molten compounds cause coatings to form on the tubular walls of the 
superheater and there give rise to high-temperature corrosion. A practical 
upper temperature limit in the case of oil fired boilers has been found to 
be around 540.degree. C. 
OBJECTS OF THE INVENTION 
An object of the invention is to provide a steam power plant having 
superheaters which are protected from the corrosive effects of compounds 
entrained in the gases heating the superheater, whereby higher superheat 
temperatures are attainable. 
Another object of the invention is to provide such a power plant in which 
the superheaters are provided with separate sources of heat in the form of 
fluidized beds. 
A further object of the invention is to provide such a power plant with 
fluidized beds for the superheaters, in which the exhaust gases from the 
beds are conveyed to the conventional boiler of the system for exchanging 
heat therein. 
Yet another object of the invention is to provide such a power plant in 
which the superheaters and their associated fluidized beds may be easily 
removed from the system to permit conventional operation. 
Still another object of the invention is to provide a method of producing 
high superheat steam without high temperature corrosion. 
These objects of the invention are given only by way of example. Thus, 
other advantages and desirable objects inherently achieved by the 
disclosed structure may occur to those skilled in the art. Nonetheless, 
the scope of the invention is to be limited only by the appended claims. 
SUMMARY OF THE INVENTION 
In order to avoid the corrosion problem experienced with prior art and 
thereby to enable superheating in excess of 540.degree. C, the 
superheaters according to the invention are provided with a separate heat 
source in the form of a fluidized bed located separate from the 
conventional boiler. In contrast to conditions in the combustion gases in 
a common steam boiler, it is possible in such a fluidized bed to reduce 
the formation of corrosive compounds on the superheater tubes and to 
capture and essentially neutralize the ashes within the bed material. In a 
fluidized bed used in accordance with this invention, the ashes from the 
flue gases are precipitated in powdered form and continuously mixed with 
the bed materials so that they have little tendency to form coatings on 
the superheater tubes. Should coatings begin to form, they are quickly 
rubbed off due to the movement of the bed material. Thus, the high 
temperature ash has little time to cause the corrosion noted in the prior 
art, so that much higher superheat temperatures become feasible with the 
invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The single FIGURE shows a steam boiler 1 which delivers steam to a high 
pressure turbine 2 and an intermediate pressure turbine 3. From 
intermediate pressure turbine 3 the steam is passed to a low pressure 
turbine (not shown) with an associated condenser (also not shown). The 
condensate is passed through certain apparatus and low-pressure preheaters 
(not shown) and is returned to boiler 1 through high-pressure preheaters 4 
and 5 which are heated by the discharge steam from the intermediate 
pressure turbine 3 and the high-pressure turbine 2. 
In addition to a conventional boiler section 14, steam boiler 1 may 
comprise an economizer 10, a steam dome 11 and an integral superheater 12. 
Steam dome 11 may possibly house a heat-exchanger for cooling auxiliary 
steam which, from an output located downstream of integral superheater 12, 
is conducted to an outlet 13. Alternatively, re-cooled steam may be 
produced by cooling steam from an output located downstream of integral 
superheater 12 by spraying in feed water, in the manner familiar to those 
skilled in the art. 
According to the invention, a separately heated superheater 6 is included 
between steam boiler 1 and high pressure turbine 2; and superheater 7, 
between high pressure turbine 2 and intermediate pressure turbine 3. The 
tubes of superheaters 6 and 7 are immersed in fluidized beds for cooling 
the beds. The fluidized bed used to heat superheater tubes 6 and 7 is of a 
conventional type fired with common fuel oil. Representative examples of 
fluidized beds which may be used in accordance with the teachings of this 
invention are shown in U.S. Pat. Nos. 3,466,012 and 3,924,402. Combustion 
air at pressure usually just above atmospheric and temperature in the 
range of 200.degree.-300.degree. C is admitted to the bed for burning the 
fuel. The flow rate of the combustion air is adjusted as necessary to keep 
the material fluidized without carrying it out of the bed and away from 
the combustion chamber. Bed materials such as silicon oxide with a maximum 
particle size of 3 to 6 mm are suitable for use in the invention. The 
superheater tubes are preferably of a high strength material such as 
Incaloy 800, suitable to withstand pressure of up to 130 atmospheres and 
temperatures of 600.degree.-800.degree. C. Due to the use of a fluidized 
bed for superheaters 6 and 7, it is possible to achieve highly efficient 
heat transfer from the combustion gases via the bed material to the tubes, 
resulting in a superheating to 700.degree. C or more without any of the 
disadvantages, for example in the form of high-temperature corrosion, 
which occur in boilers with common combustion devices and heating 
chambers. Uniform bed temperatures of 900.degree. C are attainable. The 
combustion in the fluidized bed is controlled using familiar techniques so 
that a constant superheating temperature is reached. 
The power plant is constructed so that, in the case of operational 
difficulties, unplanned stoppages or particular operating conditions, the 
superheater 6 and/or the intermediate superheater 7 can be by-passed and 
shut down by closing valves 8A and 9A and opening valves 8B and 9B so that 
steam boiler 1 and the turbine machinery including the high-pressure 
turbine 6 and the intermediate pressure turbine 7 and other auxiliary 
turbines and steam-driven apparatus present in the power plant can be used 
without difficulty as before. 
The exhaust gases from the fluidized beds 6 and 7 have a temperature of as 
much as 850.degree. C and are suitably conducted to steam boiler 1, which 
preferably comprises structure (not shown) permitting optimal use of these 
off-gases. Steam boiler 1 should also have sufficient capacity to be able 
to meet the steam requirements which arise in particular operational cases 
when the fluidized beds 6 and 7 are shut off and thus do not provide any 
additional heat for the steam boiler 1. 
Finally, it is worth emphasizing that the fluidized bed used in the 
invention has very good heat transmission properties, a fact which, 
together with the preferred location of the fluidized bed superheaters 6 
and 7 between the steam boiler and the turbines, gives a minimum 
consumption of high-alloyed steel in the superheaters and the conduits for 
superheated steam to the turbines 2 and 3. Thus, the device produces 
optimum additional superheat and reduced high temperature corrosion at 
optimally minimized additional expense.