Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE
A boiler producing steam from flue gases with high electrical efficiency and
improved
slag quality.
TECHNICAL FIELD OF THE INVENTION
The invention relates to a boiler drying, igniting and combusting refuse and
producing
steam by heat exchange with flue gases. Subsequently, the steam is utilised to
pro-
duce electricity.
The refuse to be burnt can be any mixture of household refuse, bark,
industrial waste
and hospital refuse and other kinds of waste.
BACKGROUND OF THE INVENTION
US patent no. 6,269,754 discloses a steam generator for superheated steam for
incin-
eration plants with corrosive flue gases. It essentially comprises a radiation
section
and a convection section, having at least one superheater and having plates
arranged
on the inside of a wall of the radiation section, a space is being provided
between the
plates and the wall of the radiation section. At least a part of the
superheater is being
arranged as a wall superheater in the space in the radiation section. This
space con-
tains a less-corrosive gaseous atmosphere, which is at a higher pressure than
the
pressure of the gases in the combustion chamber. Hereby, it is possible to
reach a
high superheater temperature without corrosion to the final superheater, so
that the
superheater can be made from inexpensive material.
However, US patent no. 6,269,754 does not provide direct contact between the
flues
gases and the mentioned superheaters, and accordingly there is a less
efficient
transfer of energy from the flue gases to the steam.
EP 0536268 131 - from the applicant - discloses a method and apparatus for
incinerat-
ing different kinds of solid and possibly liquid waste material. Solid and
possibly liquid
waste material is incinerated by a) partial combustion on the stepped grates
of the
solid waste material, the latter being delivered to a rotary kiln at such a
high tempera-
ture that a liquid slag is formed at the inlet of the rotary kiln, b) possibly
adding liquid
waste material to the solid waste material being incinerated on the stepped
grates, and
c) collecting the ash products from the combustion process, such as grate
screenings,
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boiler ash, fly ash, and residual products from flue-gas cleaning, and
returning these
products to the input end of the rotary kiln, at which input end these
products are intro-
duced into the liquid slag. In this manner, it is achieved that the slag, fly
ash and other
harmful residual products from the combustion process are fused into a glass-
like
mass, from which salts and heavy metals cannot be leached out.
However, EP 0536268 131 does not provide for an optimised, electrical
efficient output
from the incineration of the solid and liquid waste material.
In a world where natural energy resources, e.g. oil, is increasingly scarce,
there is an
increasing demand for energy supplied from other sources. When refuse is
incinerated
in a boiler, energy can be extracted from the incineration process. Thus it is
important
that the incineration process is optimised to provide steam that is not
condensed and
has a sufficient high temperature to ensure that the steam, when fed in a
steam tur-
bine driving a generator, provides a high electrical power output with high
efficiency.
Such steam can e.g. be superheated steam.
Thus there is a need for a boiler optimised to provide a high electrical power
output
from superheated steam and an end superheater with a higher temperature on the
wall.
Typically, superheated steam comes from a so-called end superheater. However,
in a
boiler some of the gases, e.g. flue gases and the ash particles, are
corrosive, which,
due to their corrosive nature, will attack said end superheater with the
result that the
lifetime of the end superheater is shortened.
Thus, there is also a need for a boiler having an end superheater, where some
means
are provided to extend the lifetime of the end superheater.
These needs are fulfilled when the boiler comprises a reactor, positioned
downstream
of the moving bed furnace and possibly co-fired with firing a secondary fuel,
for gener-
ating a less-corrosive gas flow and an end superheater located in the flow of
said less-
corrosive gas. The boiler dries, ignites and combusts refuse and produces
steam by
heat exchange with flue gases.
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Hereby, the invention has the advantages that the lifetime of the end
superheater of
the boiler is increased and that the boiler also provides a high and efficient
electrical
power output due to the increased steam temperature in the end superheater.
Furthermore the end superheater can be applied at a higher temperature when
sub-
jected to cleaner gases, i.e. less-corrosive gas and ash particles.
The invention will be explained more fully below in connection with preferred
embodi-
ments and with reference to the drawings, in which:
fig. 1 shows an embodiment of the boiler using a reactor in combination with a
burner
applied to generate a non-corrosive gas, and
fig. 2 shows an embodiment of the boiler using a reactor in combination with a
burner
applied to generate a non-corrosive gas with a separator element.
Throughout the drawings, the same reference numerals indicate similar or corre-
sponding features or functions.
In general the terms "superheater" or "end superheater" refers to a device
that heats
the steam generated by the boiler further, thereby increasing the thermal
energy in the
steam and decreasing the likelihood that said steam condenses. Steam, which
has
been superheated, is logically known as superheated steam; conversely non-
super-
heated steam is called saturated steam or wet steam. It is important to avoid
the latter
steam and thus primarily to use the superheated steam. Thus, when this latter
steam
is fed into a steam turbine driving a generator, it will provide a high and
efficient elec-
trical power output, especially if the temperature and the pressure of the
steam are
sufficiently high.
In general bottom ash is called slag. Bottom ash or slag is defined as the ash
removed
from the bottom part of the combustion zone of the boiler. Ashes are defined
as the
residual products from the combustion process.
Figure 1 shows an embodiment of the boiler using a reactor in combination with
a
burner applied to generate a less-corrosive gas. In general, the boiler (1)
dries, ignites
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and combusts refuse. When refuse is incinerated, a gaseous atmosphere, i.e.
flue
gases (3), is in the first place the result of incineration of refuse.
The reactor (16) can be a sintering reactor, a rotary kiln, a fluidised bed or
a spouted
bed. The reactor sinters the bottom ash such that the leaching of heavy metals
is re-
duced and the possibilities of utilising the bottom ash are improved.
The sintering reactor is a reactor that heats the ash/slag so that leaching
properties
are improved. This means that the leaching of heavy metals from the ash/slag
is re-
duced.
In fluid beds and fast fluidised beds, smooth and steady recirculation of
solids through
a dipleg or other solid trapping device is crucial to good operations. In the
spouted bed
it represents a somewhat related contacting mode in which comparatively
coarse,
uniformly sized solids are contacted by gas. In this operation, a high
velocity spout of
gas punches through the bed of solids, thereby transporting particles to the
top of the
bed. The rest of the solids move downward slowly around the spout and through
gent-
ly upward percolating gas. Behaviour somewhere between bubbling and spouting
is
also seen, and this may be called spouted fluidised bed behaviour.
Said reactor (16) in general burns the refuse and turns it into slag and/or
ash. The
refuse to be burnt can be any mixtures of household refuse, bark, industrial
waste and
hospital refuse and other kinds of waste. Typically the refuse is supplied to
the reactor
- from the left to the right in the figure - by means of grate blocks, e.g.
reciprocable
grates (21). In order to convey the refuse, the grates can be combined with
one or
more conveyors.
The reactor is located - in the direction of the flow of the refuse - after
the grate ar-
rangement (21).
The process of refuse-incineration is now to be followed - from left to right
in the figure
- starting at reference numeral 3 with flue gases, proceeding to reference
numeral 7,
and ending at reference numeral 6. At reference numeral 7, a corrosive gas
flow is the
result in the pre-process, and conversely at reference numeral 6, a less-
corrosive gas
flow is the result in the post-process. The less-corrosive gas flow is the
output from
said reactor (16), possibly fired with a fuel (18), typically a secondary fuel
(18). The
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secondary fuel is selected to develop flue gases that are less corrosive than
the flue
gases from said refuse or waste.
In this embodiment the reactor is fired from the left-hand side with a
secondary fuel
(18). The firing follows the direction of the transport of refuse in the
reactor, and as a
result the less-corrosive gas flow as the output from said reactor is co-
current, as indi-
cated with the arrow 6.
Alternatively, or additionally, the firing of the reactor can take place by
means of a
burner (19), which can be fired with said secondary fuel (18), i.e. the burner
can be
fired with any combination of oil, gas, coal, biomass, air and a selected
waste or refuse
fraction. This also applies when said secondary fuel in addition, or as an
alternative, is
supplied directly to the reactor instead. Said firing may take place by means
of a
burner (19) and/or take place in the reactor.
The burner can be a suspension burner, possibly supplied with coal or a gas,
or an oil
burner, etc.
The fuel and air injection into the reactor inlet comes via a separate
housing/chute,
which is separated from the corrosive flue gas (7).
Since the reactor is fired, it reaches a higher temperature as compared to a
reactor
with no firing. This heat is needed to burn out the volatiles and to sinter
the waste slag,
trace and heavy metal species. This can be considered as a post-treatment
process,
i.e. the boiler or waste incinerator is combined with the reactor. As a
result, the final
ash and/or slag from the reactor is - due to its low content of leachable
trace and
heavy metal species, such as one or more of leachable Pb, As, Cd, Cu, Zn, Ni
and Zn
well-suited for reuse in road construction, etc and/or for disposal.
Accordingly, envi-
ronmental harm from ash and slag are minimised.
The ash and/or slag is provided from the reactor by means of a bottom ash or
slag re-
moval device, which e.g. is a water filled through a piston pusher or belt
conveyor.
Thus an integrated bottom ash treatment and improved electrical plant
efficiency may
be obtained by the concepts shown in the figure. It is thus advantageous to
combine
the high-efficient grate firing of e.g. refuse, such as municipal solid waste,
with a post-
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treatment of the bottom ash (by means of the reactor) in a single plant, so
that the final
bottom-ash and/or slag produced will fulfil the current environmental and
technical re-
strictions necessary for reuse, and at the same time this integrated post-
treatment
provides a less-corrosive flue gas (6), which can be applied to increase the
end super-
heater (8) steam temperature and thereby the electrical efficiency of the
waste-fired
boiler of the plant. The total process is integrated, energy-efficient and
contained
within a single plant without the need for transporting, storing, and
subsequent han-
dling/treatment of the bottom ash and/or slag from the grate in another plant.
The electrical efficiency of the waste incineration plant is improved
significantly due to
the generation of the less-corrosive flue gas (6), which will allow higher
steam tem-
peratures (around 500 degrees Celsius) at the outlet 8b of the end superheater
(8).
Furthermore, it makes it possible to dispose of massive amounts of bottom ash
and
slag from waste incineration due to fixation of otherwise leachable trace and
heavy
metal fractions.
Said less-corrosive gas is essentially free from corrosive components, such as
Cl, K,
Na, Zn, Pb, whereas the corrosive gas comprises corrosive components, e.g. com-
prises one or more of Cl, K, Na, Zn and Pb. In essence, less-corrosive gas can
be
understood as gases that provide less corrosion on the end superheater.
Figure 2 shows an embodiment of the boiler using a reactor in combination with
a
burner (19) applied to generate a less-corrosive gas with a separator element.
In this embodiment the reactor is fired from the right-hand side with a
secondary fuel
(18). The firing does not follow the direction of the transport of refuse in
the reactor,
and as a result the less-corrosive gas flow as the output from said reactor is
counter
current, as indicated with the arrow 6. Note that the reactor is located at
the end of the
transportation direction of the refuse, which is discharged directly from the
grate into
the reactor.
Alternatively, or additionally, the firing of the reactor can take place by
means of a
burner (19), which can be fired with said secondary fuel (18), i.e. the burner
can be
fired with any combination of oil, gas, coal, biomass, air and a selected
waste or refuse
fraction. This also applies when said secondary fuel in addition, or as an
alternative, is
supplied directly to the reactor instead.
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Since the reactor is fired, it reaches a higher temperature as compared to a
reactor
with no firing. This heat is needed to burn out the volatiles and to sinter
the waste slag,
trace and heavy metal species. This can be considered as a post-treatment
process,
i.e. the boiler or waste incinerator is combined with the reactor. As a
result, the final
ash and/or slag from the reactor is - due to its low content of leachable
trace and
heavy metal species, such as one or more of Pb, As, Cd, Cu, Zn, Ni and Zn -
well-
suited for reuse in road construction, etc and/or for disposal. Accordingly,
environ-
mental harm from ash and slag is minimised.
At this point of the process, it is important that corrosive and less-
corrosive gases are
not mixed, since these, i.e. reference numerals 6 and 7, are to be treated
differently.
This is because the less-corrosive gas (6) shall be kept separated from the
corrosive
gas (7) rising from the grate combustion.
Consequently, according to the invention, a separation is provided to maintain
separa-
tion of the gases, i.e. in order to protect the end superheater (8) from the
corrosive
gases (7), whereby it is mainly subjected to the non-corrosive gases (6). Said
separa-
tion of the flue gases (3) is maintained by means of a separator element
denoted with
reference numeral 4. This element could in an exemplary embodiment be provided
as
a plate (4a) or in the form of a wall (4b).
The plate (4a) is typically a water filled boiler tube panel extending from
one boiler side
wall, typically also a water filled boiler tube panel, to the other boiler
side wall, and the
plate is suspended on said side walls. The plate may be corrosion-protected on
the
surfaces by e.g. high-alloy Cr-Ni overlay welding or by essentially tight
refractory mate-
rials.
The wall (4b) is typically a reinforced brick or cast refractory wall
extending from the
one boiler sidewall to the other boiler sidewall. The reinforcement may be
hollow, al-
lowing for passage of a cooling medium being e.g. a liquid, a vapour, a gas or
air.
Moreover, the separator element could in another exemplary embodiment be
provided
as a channel, i.e. said plate (4a) and wall (4b) could in various combinations
be used
to form the channel. The channel could also have a tubular shape.
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Thus the separator element secures that the less-corrosive gas flow (6) and
the corro-
sive gas flow (7) are kept separated at this point, and that mainly the less-
corrosive
gas flow (6) from the reactor (16) reaches the end superheater (8). In the
long run, the
optimal position of the separator element may be reflected in a high
efficiency and high
electrical power output from a generator driven by a steam turbine supplied
with steam
from the boiler.
The separator element is adapted to be suspendable on or from the walls of the
boiler.
In an exemplary embodiment, the separator element may be a plate, a wall or
channel
able to pivot at the top in bearings suspended on the opposite boiler side
walls and
e.g. being able to move and fixate in different positions forwards/backwards
at the
bottom onto the boiler side walls.
In general for any embodiment, i.e. it applies both when the firing takes
place from the
left-hand side (figure 1) and from the right hand side (figure 2), said less-
corrosive gas
(6) and the corrosive gas (7) continue in the boiler to the mixing zone (10)
of the boiler
(1).
Also being general for any embodiment is that the steam (2) at between 300 and
450
degrees Celsius, after leaving said one or more superheaters, is fed, by means
of one
or more pipes, to an inlet (8a) of an end superheater (8), through which this
steam (2)
is heated, resulting in a temperature increase of between 25 and 200 degrees
Celsius.
This warmer steam (2a), i.e. the steam with the increased temperature, is e.g.
supplied
from an outlet (8b) of the end superheater (8) to a steam turbine (14). Thus
this steam
(2a) can be utilised to produce electricity. For example said steam can be fed
by
means of piping from said outlet into the steam turbine (14), which drives a
generator
(15), from which generator electrical power then can be generated. Since
warmer
steam (2a) is the output from the boiler, i.e. the output from the end
superheater, the
boiler accordingly also provides high power output efficiency. This is, of
course, higher
than if steam (2) at between 300 and 450 Celsius was the output from the
boiler. Thus
the heating of the steam in said end superheater provides the high electrical
power
and high efficiency output.
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Typically said end superheater (8) is located in proximity to said separator
element (4),
e.g. said plate, wall or into the channel, and in all cases in the flow (6) of
said less-cor-
rosive gas. It is thus an advantage that the end superheater is less subject
to corro-
sion.
It is therefore an advantage - which applies to both figures - that the end
superheater
is located in the flow of said less-corrosive gas (6) as compared to the flow
(7) of said
corrosive gas. If the end superheater was located in the flow (7) of said
corrosive gas -
which is not the case according to the invention - such location of the end
superheater
would result in a short life time of the end superheater, and this location in
the aggres-
sive environment would require excessive and frequent repair work due to being
sub-
jected to corrosive gases during its working life time.
The invention therefore has the advantages that the lifetime of the end
superheater is
increased and that the boiler provides a high electrical power efficiency.
As discussed above, said less-corrosive gas (6) and the corrosive gas (7) are
mixed
together in the mixing zone (10) of the boiler (1). The boiler further
comprises a blow
unit (12). This is adapted to - by blowing secondary air - effectively mix
said less-cor-
rosive gas (6) with said corrosive gas (7), whereby said mix can be
effectively burnt
out before it reaches the top zone (13) of the boiler. Moreover - this
applying to both
figures - the boiler is provided with an Industrial Draught fan, which sucks
the gases,
i.e. flue gases, the less-corrosive and the corrosive gases through the
boiler. Addition-
ally, combustion air can be blown in under the grate arrangement (21).
When said less-corrosive gas (6) and the corrosive gas (7) together reach the
mixing
zone (10) of the boiler, these gases are mixed by injection of secondary
combustion
air for outburning, and the now mixed gases are cooled by means of the
evaporation
wall in the radiation zone and one or more superheaters (11), which produce(s)
steam
(2) at between 300 and 450 Celsius. This (i.e. cooling by means of said one or
more
superheaters) takes place regardless of whether the less-corrosive gas (6) has
been in
contact with the plate, the wall or the channel when it moved in the direction
(6) and/or
whether it was the result of the firing with the burner to the reactor from
the left-hand
side or from the right-hand side.
