Note: Descriptions are shown in the official language in which they were submitted.
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A BOILER AND A SUPERHEATER, AS WELL AS A METHOD
Field of the invention
The invention relates to a boiler for producing thermal energy. The invention
also relates to a boiler superheater. Furthermore, the invention relates to a
method in a boiler for producing thermal energy.
Background of the invention
In boilers, a lot of compounds are formed which are detrimental to the
materials of the heat exchange surfaces of the boiler. Especially when
burning biofuel and refuse fuel, corrosion of the heat exchange surfaces of
the boiler has been detected, especially corrosion of superheaters and their
heat exchange surfaces. In addition, it has been detected that ash produced
during combustion deposits on the heat exchange surfaces, which reduces
the heat transfer and thereby the recovery of thermal energy.
The above-mentioned biofuels include botanical materials from nature, such
as wood chips, bark, agro-biomass, sawdust, black liquor, and the like.
Refuse fuels include, for example, sorted household refuse, industrial waste
and waste from businesses, as well as demolition wood. These fuels include
significant amounts of chlorine. Together with sodium and potassium
released from fuel they form gaseous alkaline chlorides in flue gases, which
are condensed and deposited on heat exchange surfaces, especially on
superheater surfaces. Deposition and condensation takes places especially
in places where the surface temperature of the heat exchange surfaces is
below 650 C. When the surface temperature of a heat exchange surface is
above 450 C, the alkaline chlorides cause chlorine corrosion.
Supplying various additional materials to the furnace has been suggested in
order to eliminate corrosion problems caused by chlorides. Publication WO
2006/134227 Al discloses the spraying of a liquid sulphate-containing to the
superheater area of a steam boiler, to bind the alkaline chlorides formed in
the furnace. According to publication WO 02/059526 Al, a liquid sulphate
compound or sulphuric acid is added to flue gases before the superheaters.
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Publication EP 2071239 A2, in turn, discloses that additional material needed
for preventing corrosion is fed to the flue gases of a boiler by means of at
least one cooled pipe.
It is also known to decrease the nitrogen oxide emissions of different types
of
boilers by supplying into their furnace various additional materials which
decrease the amount of nitrogen oxides in the flue gases formed during
combustion. This kind of a solution is presented, for example, in publication
WO 9813649 Al, in which cooled pipe panel surfaces are installed in the
furnace, which include separate additional material channels for the
additional material.
According to the prior art, the superheaters of the boiler are placed either
in
the furnace of the boiler, typically at the top of the furnace, or in the flue
gas
duct downstream of the furnace, where the flue gases from the furnace are
led. The superheaters are placed in the flue gas flow, and the thermal energy
of the flue gas is transferred to the superheater by means of both thermal
radiation and convection of heat, in which case one can refer to combination
superheaters. It is also possible to use special radiant superheaters, whose
application is primarily based on utilizing the thermal radiation of the
flame,
and special convection superheaters, whose application is primarily based on
the convection of thermal energy by means of contact between the super-
heater and flue gases. The radiant superheater is normally placed at the top
of the furnace, for example suspended in the furnace, and it is in direct
contact with the thermal radiation from the flame. Thus, there is a direct
line
of sight between the flame and the superheater.
The superheaters are used as heat exchangers which typically comprise a
construction of pipes connected to each other, by means of which thermal
energy is transferred to a medium, that is steam, flowing inside the pipes.
Significant advantages would be achieved by raising the superheating
temperature of the steam, but the required development has been hampered
by the restricted corrosion resistance of the material of the superheater.
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Brief summary of the invention
The aim of the present invention is thus to provide a system to avoid the
above mentioned problems which relate particularly to the corrosion and
chemical attacks caused expressly by flue gases. In particular, the aim is to
improve the resistance of the superheaters.
A boiler for producing thermal energy according to the invention is presented
in claim 1. A boiler superheater according to the invention is presented in
claim 7. A method in a boiler for producing thermal energy according to the
invention is presented in claim 9.
A principle of the invention is to prevent the detrimental effects of flue
gases
on the superheater, wherein the aim is not to utilize heat transfer by means
of
convection of the flue gases.
Another principle of the invention is that the superheater placed on the wall
of
the boiler is protected from the effects of the flue gases by means of a
protective gas curtain.
The principle of an embodiment of the invention is that the superheater
placed on the wall of the boiler is protected from the effects of the flue
gases
by means of a protective gas curtain. The aim is to prevent or strongly
restrict
the entry of the flue gas as well as the detrimental compounds, corrosive or
aggressive substances contained in it, onto the heat exchange surfaces of
the superheater.
In one example, the gas or gas mixture of the gas curtain is air. In one
example, the gas or gas mixture may be a gas of prior art, known as such,
which is used to reduce corrosion problems, or, for example, a gas that is
free from corrosive substances. It may also be gas from the boiler.
In one example, inhibitors, such as sulphur dioxide or sulphite, are mixed
into
the gas or gas mixture, to prevent corrosion problems.
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The superheater is placed on the wall of the boiler, for example at the bottom
of the furnace. Preferably, the superheater and its heat exchange surfaces
also have a direct line of sight to the flame of combustion in the furnace, to
recover the thermal energy on the basis of thermal radiation. Convection of
heat via the flue gases is to be avoided.
In one example, a large number of supply devices are provided in the area
covered by the superheater, to enable the supply of gas to the front of the
heat exchange surface of the superheater. Said supply devices may be an
orifice extending through the superheater, gas being supplied through said
orifice, or a separate pipe or duct, or a nozzle attached to the superheater.
In
one example, the nozzle is arranged to direct the gas flow in parallel with
the
heat exchange surface of the superheater, preferably upwards.
The gas supply to the supply device is performed, for example, by means of
a separate pipe or duct.
The walls of the furnace of the boiler are made by using pipes to convey a
medium, to recover the thermal energy from the furnace by means of
radiation and/or convection.
Brief description of the drawings
In the following, the invention will be described in more detail with
reference
to the appended drawings, in which:
Fig. 1 shows a schematic view of a fluidized bed boiler seen from the
side, provided with a superheater,
Fig. 2 shows a schematic view of another example of a superheater
placed in a fluidized bed boiler, and
Fig. 3 shows a schematic cross-sectional view of a superheater
according to one embodiment.
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Detailed description of the invention
Figure 1 shows an example of a steam boiler applying the above-presented
superheater configuration. As the steam boiler and the location for the
5 superheater configuration, it is possible to apply a boiler based on
fluidized
bed combustion, particularly a bubbling fluidized bed boiler (BFB), as shown
in Fig. 1, or a circulating fluidized bed boiler (CFB). In bubbling fluidized
bed
boilers, a fluidized bed is produced by means of a gas flow. As the location,
it
is also possible to apply a soda recovery boiler which is based on the
combustion of black liquor, or a boiler in which the fuel is burnt on top of a
grate, or another steam boile, in which the protection of the superheaters is
needed, among other things, because of the high temperatures.
Figure 1 shows the boiler 1 comprising a furnace 2 limited by the walls. The
walls of the furnace are formed of water-cooled pipes, which are attached to
each other by fins. In the lower part of the furnace, nozzles 3 are provided
for
supplying fluidizing air and combustion air, i.e. primary air from an air box
4
to the furnace 2. By the effect of the fluidizing air, the fluidized bed 5 in
the
lower part of the furnace is fluidized, i.e. brought into continuous movement
in the furnace 2. Fuel is supplied into the furnace from fuel supply devices
6,
and combustion air is supplied from secondary air nozzles 7. In this boiler,
combustion air is also supplied into the furnace from tertiary air nozzles 8.
The fuel used is, for example, biofuel and/or refuse fuel. The flame 12
produced in connection with the combustion of the fuel is placed above the
fluidized bed and extends, for example, above the secondary air nozzles 7
and often also up to the tertiary air nozzles. The combustion of fuel by means
of oxygen-containing gas in the lower part of the furnace 2 is the primary
source of thermal energy.
The upper part of the furnace comprises superheaters 9 and 13, whose
function is to provide superheated steam that is typically used in a turbine
(not shown in the figure). The figure also shows the rear wall 2b and the
front
wall 2a of the furnace, including a nose 10 for guiding the flue gases. In the
figure, the superheaters are drawn in a reduced manner to illustrate the
circulation of the medium.
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The flue gases 19 formed in the furnace are directed further via a flue gas
duct 11 in connection with the furnace. The flue gas duct may be provided
with heat exchange surfaces or heat exchangers 14.
In Fig. 1, a superheater 15 is fixed or placed on the rear wall 2b, wherein it
is
a so-called wall superheater. In the figure, the superheater 15 is drawn in a
reduced manner by illustrating the circulations of the medium, and
configurations known as such can be applied in the superheater. The
superheater 15 has a direct line of sight 16 to the frame 12, to utilize the
thermal radiation produced in the combustion.
The area covered by the superheater 15 is also provided with several supply
devices 17, which make it possible to supply gas to the front of the heat
exchange surface of the superheater. The gas forms a barrier layer 18
between the superheater and the rising flue gases, to insulate the
superheater 15 from the flue gases as well and as extensively as possible.
Simultaneously, the barrier layer 18 guides the flow of flue gases. Gas is
supplied continuously from the supply devices 17.
The gas is led to the supply devices 17 from a desired source, for example
along a duct or pipe 20 shown in Fig. 3 or 4. The duct or pipe 20 is placed,
for example, between the pipes 22 of the superheater, or it is conveyed from
between the pipes of the superheater to the front side of the superheater.
The pipes are connected to each other by means of, for example, one or two
fins. By means of gas supplied from the duct or pipe 20, the barrier layer 18
is formed in front of the heat exchange surface 21 of the superheater.
Figure 5 shows part of the panel structure formed by the superheater 15.
The supply device 17 determines the place or location from where the gas is
supplied. The gas supply device 17 is an orifice 24, pipe or duct 20, or a
separate nozzle 23, for example, in the heat exchange surface 21 of the
superheater. By means of the nozzle, the gas is dispersed or blown in a
desired direction, preferably in parallel with the plane defined by the heat
exchange surface 21. The nozzle 23 may blow the gas in one or more
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directions. Nozzles of prior art, known as such, can be used as the supply
device 17.
The superheater 15 can be placed on the front wall 2a or the rear wall 2b of
the furnace. Figure 2 shows a superheater 15 and supply devices 17 placed
on the side wall of the furnace 2. The supply devices constitute a supply
device matrix comprising, for example, two or more rows, each having two or
more supply devices. In this way, a larger uniform barrier layer 18 is formed.
The supply device matrix is placed in the area limited by the superheater,
and if necessary, also in the direct vicinity of the superheater, so that the
protective barrier layer that comprises gas or a gas mixture would cover the
entire superheater.
The presented superheater configuration can also be applied in a circulating
fluidized bed boiler as well as in a soda recovery boiler or in a boiler
applying
combustion on a grate. In a circulating fluidized bed boiler, the furnace is
supplied with fuel, which may be biofuel, refuse fuel or coal, from fuel
supply
devices, and with combustion air from air nozzles.
In the recovery boiler, the fuel used consists of cooking chemicals produced
in pulp manufacture, as well as liquid that contains parts dissolved from
wood, i.e. black liquor. The boiler does not have a fluidized bed but a
furnace
supplied with black liquor from the fuel supply devices and with combustion
air from air nozzles at different height levels. During the combustion of the
liquor, smelt is produced on the bottom of the furnace, which smelt is
discharged from the furnace to be processed further. In boilers equipped with
a grate, the fuel burns at the bottom of the furnace, on the grate, and
combustion air is also supplied, for example, through the grate.
The invention is not intended to be limited to the embodiments presented as
examples above, but the invention is intended to be applied widely within the
scope of the features defined in the appended claims.
