FIELD OF THE INVENTION:
The present invention generally relates to a steam generation system which
produces steam at high pressure and high temperature by a chemical conversion
of calorific value of blast furnace gas (BFG). More particularly the invention
relates to an improved steam generation system operating with blast furnace-gas
(BFG) providing increased cycle efficiency.
BACKGROUND OF THE INVENTION
A prior art boiler system operating with blast furnace gas (BFG), the fuel (BFG)
being supplied at ambient temperature to the furnace. A Combustion of the fuel
produces a gas temperature of around 220°C at the inlet of an air heater which
in turn leads to a final boiler efficiency of around 86.5-87.5 %. The efficiency of
the system is proportional to the quantity of heat that can be retrieved from the
flue gas and put back into the system.
US patent US 4492568 entitled "Process and apparatus for preheating the
combustion mediums used for firing blast furnace stoves" describes that
Combustion air and fuel gas used for firing the blast furnace stoves, is preheated
by utilizing the heat of waste gases resulting from such firing. The combustion
air and fuel gas are passed through respective heat exchangers in heat exchange
relationship with the waste gases, thereby preheating the combustion air and
fuel gas. The combustion air, after passage thereof through the respective heat
exchanger, is further preheated by an externally fired recuperator. The flue
gases from the externally fired recuperator are fed to the waste gases before
preheating of the combustion air by such waster gases. The external firing of the
recuperator is controlled as a function of the final preheated temperature of the
thus twice preheated combustion air.
Patent titled "Waste heat recovery apparatus, waste heat recovery system, and
method of recovering waste heat" deals about a heat exchanger filled with a
catalyst used for reforming DME is provided in an exhaust system of a
combustion apparatus such as a furnace, an internal combustion engine, or
electrical power generation equipment. A mixed gas of DME and H2O is fed to the
heat exchanger, thereby pyrolyzing the DME and recovering the sensible heat of
an exhaust gas of the furnace of the internal combustion engine as H2O fuel.
Since the waste heat is recovered not as steam hot water, heated air, or the like,
but as H2 fuel H2 and co fuel, the application of the recovered energy is not
limited. The current invention is entirely different from the one described above.
Other related patents known to the present inventors are furnished below;
a. Patent titled "Blast furnace and heater combination" (Patent number :
2,832,681), explains about the shaft furnace, such as blast furnace, or a
wet-bottom producer, in combination with a heating device, such as a
steam boiler, waste heat boiler, gas burner, and other industrial heating
devices.
b. Patent titled "Air Preheater" (Patent number: 1,834,579), describes the
improved means for utilizing available heat in the heating gases leaving a
boiler furnace for preheating the air supplied to the furnace for
combustion process.
c. Patent titled "Combustion air heater for furnaces arranged to minimize
corrosion by flue gases" (Patent number : 2,511,647), shows the
improved means for controlling the metal temperature at the gas outlet
end of an air heater so as to maintain a high efficiency of heat recovery.
d. Patent titled "Method of heat exchange" (Patent number: 1,970,534),
explains about the operation of heat exchanging installations, and has
particular reference to heat exchange systems for hot gases containing
excessive amount of solid impurities.
e. Patent titled "Recuperative heat exchanger" (Patent number: 4,625,792),
deals with a recuperative heat exchanger for gas-gas heat exchange at a
temperature above 700°C, comprising a refractory lined vessel having a
vertically extending steel shell closed at top and bottom.
In another prior art system, operating with blast furnace gas, the fuel is pre
heated by tapping some steam from the steam turbine outlet and using the
steam to heat the BFG in various stages or phases before admitting into the
furnace. This is far more effective than the first type of prior art.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose an improved steam
generation system operating with blast furnace gas (BFG), which increases cycle
efficiency.
Another object of the invention is to propose an improved steam generation
system operating with blast furnace gas (BFG), which is enabled to recover the
maximum amount of heat after the economizer stage.
A still object of the invention is to propose an improved steam generation system
operating with blast furnace gas (BFG), which allows the recovered heat to be
inputted back to the system to increase cycle efficiency.
A further object of the invention is to propose an improved steam generation
system operating with blast furnace gas (BFG), which reduces the operating cost
including the weight of the boiler.
SUMMARY OF THE INVENTION
According to the invention, high efficiency of the boiler is achieved by heating
the blast furnace gas at ambient temperature, to higher temperature before
allow the BFG to enter the combustion chamber. The improvement is
accomplished by introducing a BFG heater, which is essentially a single block
tubular heat exchanger picking up heat from the flue gas and transferring the
acquired heat to the ambient temperature Blast Furnace Gas fuel so that the
BFG is allowed to enter the furnace itself with a higher temperature, which inter
alia foster the combustion process. Since temperature of the flue gas entering
the air heater is comparatively lower, the heat difference (heat picked up for
heating the BFG and input heat of the air heater) is fed back into the system.
Accordingly, the efficiency of the steam generation system is enhanced.
According to the invention, the fuel is heated in the steam generation system- by
the heat of the flue gas as compared to the heat from the turbine steam in the
prior art system.
The economiser and air heater in the prior art are retained in the flow process,
wherein the economiser is eliminated from the flow path according to the
invention to achieve heat transfer feasibility.
The proposed invention differs from prior art due to introduction of an additional
component called BFG heater which heats the fuel (BFG) to attain heat transfer
feasibility.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS :
Figure 1 represents a flow pattern in prior art steam generation system.
Figure 2 shows an alternate embodiment of steam generators of prior art using
blast furnace gas as the fuel.
Figure 3 shows a schematic arrangement of a BFG heater incorporated in the
improved steam generation system of prior art.
Figure 4 shows the arrangement of tubes.
Figure 5 shows a flow diagram of the inventive system wherein the major
devices of prior art systems are retained with the addition of a BFG heater (a
tubular air to air heat exchanger.)
DETAILED DESCRIPTION OF THE INVENTION
Since the Blast Furnace Gas (BFG) is highly toxic in nature due to high presence
of CO in itf no leakage in the BFG heater or BFG transporting system can be
allowed. Accordingly, a rigid all-welded design, of a heater has been considered.
The inventive system results in easy maintenance due to rearrangement of
existing components including incorporation of the new BFG heater.
Figure 1 show a furnace (1) where the combustion takes place. The fuel, i.e.,
blast furnace gas is allowed to enter through burners. From the furnace, the
calorific value of the gas is utilised and the resultant of the combustion, i.e. flue
gas flow through a superheater (2) and an economiser (3), which are essentially
air-liquid tubular heat exchangers where the flue gas heats up the boiler water &
feed water respectively. From there the flue gas goes to an air pre heater (4)
where it will give up the remaining heat to raise the ambient temperature air to a
higher temperature before it is fed either by natural draft or induced draft into
the atmosphere through chimney (5).
Figure 2 shows an alternate system with higher cycle efficiency. In this system,
the furnace (24) is the device where the combustion of the blast furnace gas
takes place and the water flowing in the water walls get converted into
superheated steam and goes into steam turbine (27) and then back to the boiler.
A part of the steam is then bled from the turbine (27) to heat in fuel in a steam
BFG heater wherein the fuel is admitted from a fuel source (29) at a particular
head for the flow into the combustion chamber.
Figure 3 shows a schematic diagram of a proposed BFG heater (9) of the
invention, comprising an array of headers (22) and distribution tubes (23) inside
which the BFG flows from the BFG source (29) to the furnace (24). The hot flue
gas from the outlet of the economiser (3) flow transversely thereby heating the
BFG inside the tube (23).
Figure 4 shows an arrangement of the tubes (23). The number of tubes (23) in a
single assembly as well as the number of the total assemblies in the total heater
component is decided on the basis of the boiler capacity and the quantity of the
fuel to be admitted into the furnace (24).
Figure 5 shows a flow process of the proposed system retaining the conventional
components like economiser and air heater, and further incorporating a new
tubular air to air heat exchanger, constituting the BFG heater (9) in the system
after the economiser (3) where the flue gas coming from the economiser (3) is
heating the blast furnace gas fuel.
The invention relates to a steam generator or boiler operating on blast furnace
gas, which being a by-product of steel industries, as the fuel. The present
system allows admission of the fuel i.e., blast furnace gas into the combustion
chamber at ambient temperature itself or by heating it using a part of the steam
bled off from the turbine outlet. The flue gas formed on combustion of this fuel
follows the conventional path, i.e., through the superheater, the economiser and
then into the air heater. The proposed system comprises a furnace, superheater,
an economiser, a BFG heater, an air heater and a chimney. The fuel gas from a
fuel source is supplied through the headers under a pressure into the tubular
BFG heater common header. From there, the fuel gas passes through the
distribution tubes and then into the collection header. By this time, the fuel gas
is heated to a difference of around 20- 30 °C from the ambient temperature by
the hot flue gas which flows over the tube in a transverse manner. The heated
fuel is then admitted into the furnace where it undergoes combustion to from the
flue gases. The water flowing through the water wall panels get vaporized
partially, and in the drum, the vapours are separated and fed in to the
superheaters where the flue gas heat, the vapours again to a superheat
temperature. After heating the feed water also in the economises the flue gas
now passes into the BFG heater to heat the fuel. Finally like in the conventional
system, it passes into the air heater where to heat the combustion air also
before being emitted into the atmosphere through the chimney.
The proposed invention incorporates an all welded tubular heat exchanger heater
for pre heating the BFG fuel prior to admitting it to the furnace using the waste
heat from the flue gas The BFG heater have tubes running down from a common
header and ending in another common header. The tubes contain BFG gas. The
number of assemblies of the tubes forming the final heater component is decided
based on the boiler capacity and the quantity of the fuel that is to be admitted.
This also results in the flue gas temperature at the air heater entry to be lower
than that having no BFG heater thereby reducing the environmental impacts
also.
ADVANTAGES OF THE INVENTION
The present invention enables the flue gas temperature at the outlet of the air
heater (4) which is the final heat exchanger component in the inventive system
to come to the range of 130°C to 135°C when compared to the range of 220° to
225°C in the prior art systems where only the econorniser (3) and air heater (4)
are present in addition to the superheater (2) to extract heat from the flue gas.
The new system also brings about an overall cycle efficiency of the boiler in the
range of 89% to 90% when the conventional system with the same process
parameters offers efficiency in the range of 86% to 88%. However it has to be
noted that those variants of the alternative prior art systems which use a BFG
heater using steam (as opposed to BFG) from the steam turbine has efficiencies
comparable to the proposed system.
The fuel gas, i.e., BFG is a by-product of steel industries existing above 350°C.
But due to constraints in transportation of such huge volume of hot gas through
the pipeline over a distance from the blast furnace to the boiler inlet,
necessitates the gas to be pre cooled to ambient temperature before being
transported from the blast furnace region to the boiler region. Hence in prior art
systems, the fuel is made available at the boiler inlet only at ambient
temperature. However in the proposed invention, the fuel temperature is
increased in the range of 60°C in the BFG heater (9) before being admitted into
the combustion chamber.
Even though the proposed invention involves the addition of a new component, it
drastically reduces the existing weight of the econorniser by the reduction of the
temperature gradient prevailing across the component. Hence by virtue of this,
an overall reduction in weight of the boiler by around 10-15% is possible
including a space reduction of around 10% as compared to the conventional
systems.
The service life of the system is more as the temperature gradient across the
various components is reduced due to incorporation of the BFG heater unit (9)
by around 10% from the conventional system. However the pressure loss across
the fuel line with heater is around 30-35 mmwc whereas there won't be any
considerable pressure loss in the fuel line if there is no all welded tubular indirect
heat exchanger. Also the increase in the length of the fuel line and the resultant
pressure loss result in the additional power consumption by around 35-40watts
due to a head inducing fan prior to the fuel line.
WE CLAIM :
1. An improved steam generation system operating with blast furnace gas
(BFG), the hot BFG outputting from the blast furnace boiler traversing a
substantial distance through pipeline to be inputted to a boiler which
forcing the hot BFG to be initialled cooled at the exit of the blast furnace
to ambient temperature before being transported, the improvement is
characterized in that the air heater is provided and integrated with a blast
furnace gas (BFG) heater constituting a tubular air to air heat exchanger
enabled to increase the temperature of the ambient temperature BFG to
around 20° to 30°C by the waste flue gas flowing transversely over the air
preheater such that the BFG is input to the combustion chamber at a
temperature higher than the ambient thereby increase the cycle
efficiency.
2. The system as claimed in claim 1, wherein a plurality of headers for
distribution of gas in the heat exchanger is provided.
3. The system as claimed in claim 1 wherein the BFG heater is in rectangular
or any other shape.

ABSTRACT

The invention relates to an improved steam generation system operating with
blast furnace gas (BFG), the hot BFG outputting from the blast furnace boiler
traversing a substantial distance through pipeline to be inputted to a boiler which
forcing the hot BFG to be initialled cooled at the exit of the blast furnace to
ambient temperature before being transported, the improvement is characterized
in that the air heater is provided and integrated with a blast furnace gas (BFG)
heater constituting a tubular air to air heat exchanger enabled to increase the
temperature of the ambient temperature BFG to around 20° to 30°C by the waste
flue gas flowing transversely over the air preheater such that the BFG is input to
the combustion chamber at a temperature higher than the ambient thereby
increase the cycle efficiency.