Note: Descriptions are shown in the official language in which they were submitted.
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-~: -,VO 91/01174 2 ~ ~ 3 2 ~ ~ PCT/Fl90/00184
A process for the purification of flue gases, and an apparatus
for it
The present invention relates to a process for the purifica-
tion of flue gases. It relates in particular to a process for
the purification of flue gases wherein a dust-like substance
which absorbs the gaseous sulfur compounds present in flue
gases, primarily sulfur dioxide, or becomes absorbent while in - ~ .
the furnace, is fed into the furnace of a boiler, the partly
reacted dust-like absorbent is separated from the flue gases
at a point after the furnace, is hydrogenated and returned to
the flue gas flow at a point after the separation point, but
bèfore the reaction zone, in which additional water or steam
is fed into the flue gas, whereafter the dust which contains
solid sulfur compounds is ultimately separated from the flue
gases thus purified.
The invention also relates to an apparatus for use in the
process referred to above, for activating at a point before
the reactor the partly reacted dust-like absorbent traveling
together with the hot flue gases emerging from the boiler fur-
nace, and in particular to a separation device, intended to be
connected to the flue-gas duct between the boiler furnace and ~
the reactor, such as a cyclone having means for hydrogenating :
the partly reacted absorbent separated from the flue-gas duct
and for returning it to the flue gas duct at a point after the
separation apparatus, before the reactor.
In the prior art there are known numerous processes for puri-
fying flue gases with respect to gaseous sulfur compounds pre-
sent in them. In the so-called LIFAC sulfur removal process
developed by Tampella Oy and described in GB Patents 2 169
887, 2 174 081 and 2 174 082, a limestone powder is injected
in the form of a fine dust into the boiler furnace, in which
the calcium carbonate present in the limestone breaks down to
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WO91/01174 -- PCT/F190/0018~ ~-
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calcium oxide, which reacts with the sulfur dioxide present in
the flue gases, thereby forming a calcium sulfate crust on the
surfaces of the calcium oxide particles. This crust may hamper
the penetration of sulfur dioxide into a particle, thereby im-
pairing the conversion of the lime.
In this process the removal of sulfur dioxide and the conver-
sion of lime are improved by moistening the lime and by in-
creasing the moisture content of the flue gases by spraying
either steam or water into them. Thus a calcium hydroxide
forming reactlon is triggered, which improves the degree of
sulfur removal and the degree of utilization of the limestone.
It is known that the formation of calcium hydroxide begins at
a temperature below 570 C, and that the rate of formation is
greater the higher the temperature. In practice, steam or
water has been sprayed into the flue gases after the flue
gases have first been used for preheating, by indirect heat
exchange, the air to be ~ed into the boiler furnace, whereupon
the temperature of the flue gases has usually been below lS0
C, and often even below 120 C. This system has been found to
be the most advantageous in terms of the heat economy of the
boiler.
~,
From FI Lay-open Print 76931 there is additionally known a
process for the purification of flue gases, wherein a dust in
which the partly reacted dust-like calcium oxide has concen-
trated is separated from the hot flue gases emerging from the
boiler furnace, whereafter the dust thus separated is direct-
ed, together with flue gases, into a separate reactor, in
which the flue gases are moistened either with water or with a
- slurry which has been produced when moistening in a separate
moistening system the above-mentioned dust separated from the
flue gases. In terms of apparatus this process is inconven-
ient, since its implementation requlres a large number of de-
vices which include both moving par.s and parts which wear,
and may in practice cause stoppages.
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The object of the present invention is therefore to provide a
process for the purification of flue gases, and an apparatus
for use in it, by means of which an absorbent present in the
dust traveling together with the flue gases emerging from the
boiler furnace is activated before it is fed into the actual
reactor. By the use of the method and apparatus according to
the invention, the removal of sulfur dioxide and the degree of
utilization of the absorbent are improved.
According to the present invention, this is accomplished by
dividing, in a separation zone, the hot, dust-bearing flue gas
emerging from the furnace into a dust-poor main flow and a
dust-rich secondary flow, into which steam and/or water mist
is fed at a point at which the temperature of the secondary
flow is still so high that the reaction among the sulfur co~-
pounds, the absorbent and water is still drastic enough to
break up the surface layer of the partly reacted dust par- -
ticles, thereby activating them.
.
In the method according to the present invention, the flue
gases are thus not entirely separated from the dust at a point
before the actual reactor, as is done in the above-mentioned
FI Lay-open Print 76931; instead, the hot flue gases which
contain partly reacted dust-like absorbent are divided into
two fractions, namely a dust-poor main flow which, in a manner
known from the LIFAC process, is directed, possibly via an air
preheater, into the actual reactor, and a dust-rich secondary
flow into which steam and/or water mist is fed. Thereby the
problematic solids-moistening systems which include moving
parts (cf. FI Lay-open Print 76931) are entirely avoided, and
the entire flue gas volume need not be moistened with steam or
water at a point before the actual reactor, as is necessary in
the above-mentioned LIFAC process.
According to the present invent'on, an oxide, hydroxide or
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WO91/01174 ~ PCT/F190/00184
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carbonate of calcium or magnesium, preferably limestone, which
is known as an inexpensive raw material, can be fed into the
furnace of a boiler. In the furnace the limestone breaks down
into calcium oxide, which partly reacts with the gaseous sul-
fur compounds, mainly sulfur dioxide, present in the flue
gases, whereby a gypsum crust is formed on the calcium oxide
particles, preventing the calcium oxide in the inner part of
the particle from reacting with sulfur dioxide. In order to
activate the calcium oxide partlcles, their surface layer is
broken according to the invention by rapidly increasing the
moisture content of the flue gas at a temperature at which the
- chemical equilibrium is on the side of calcium hydroxide for-
mation, but nevertheless so high that the calcium hydroxide
and hydrous calcium sulfate forming reactions will take place.
The spraying of steam or atomization of water into the dust-
rich secondary flow of flue gases makes possible an effectiye
mixing and a rapid formation of calcium hydroxide and hydrous
calcium sulfate, causing a sudden swelling of the particles
and their breaking up into small particles which have reactive
calcium hydroxide on their planes of fracture.
According to the present invention, steam or water mist is fed
into the dust-rich secondary flow of flue gases, either after
it has been separated from the main flow or, preferably, into
the separation zone itself, at a point at which the flue gases
already have a significant concentration of dust but at which
there, nevertheless, still prevails effective mixing.
The dust-rich secondary flow is made to join the dust-poor
main flow of flue gases, either in the reactor or at a point
before it, but preferably only after the main flow has been
brought into an indirect heat-exchange contact with the air to
- be fed into the boiler furnace. Part of the secondary flow
can, however, be introduced into the main flow even before the
above-mentioned indirect heat-exchange contact.
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oll74 PCT/F190/00184
29g~2~
A portion of the dust-poor main flow separated from the flue-
gas flow can be introduced into the tail end of the actual re-
actor in order to raise its temperature. Likewise, a portion
of the preheated air may be introduced into this section of
the reactor.
In the process according to the present invention, the separa-
tion device used is preferably a cyclone, the structure of
which is, however, such that the dust traveling together with
the flue gases emerging from the boiler furnace will not sepa-
rate out from the gas phase as a separate solids phase, but
the flue gases will divide into two separate gas fractions,
i.e. a dust-poor main flow and a dust-rich secondary flow,
which secondary flow is removed from the lower end of the cy-
clone, there being in the lower section of the cyclone, in the
vicinity of the secondary-flow outlet, nozzles for feeding in
steam and/or for atomizing water into the cyclone, the -~
secondary-flow outlet in the lower end of the cyclone being
connected by means of a connecting duct to the flue-gas duct
which is between the cyclone and the actual reactor and in
which the main flow travels, in order to make the activated,
dust-rich secondary flow to join the dust-poor main flow at a
point after the separation device.
The invention is described below in greater detail with refer-
ence to the accompanying drawing, which depicts a partly cross
sectional side elevation of an apparatus suitable for the im-
plementation of the process according to the invention.
.
In the drawing, the furnace of the boiler is generally indi-
cated by reference numeral l. A sulfur-containing fuel 8 is
burned in the boiler furnace l, usually with air 9. The flue
gases, which contain gaseous sulfur compounds, mainly sulfur
dioxide, cool on the heat surfaces 20 in the upper section of
the boiler and in the air 9 preheater lO located at a point
after the boiler. At a point after the boiler and the pre-
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WO91/01174 PCT/F190/00184 :
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heater lO the flue gases are directed into an oblong reactor
4, which is followed by a dust-separation apparatus 5 and a
flue 6.
The finely-divided reagent for absorbing the gaseous sulfur
compounds present in the flue gases, or its precursor, prefer-
ably calcium carbonate, is fed from a storage container 2 by
means of batching feeders 17 along a pipe 16 into the air flow
of a pneumatic air-conveying blower 18, and it travels to-
gether with this air flow along a pipe 7 to the upper section
of the boiler. In addition, water is sprayed into the oblong
reactor 4 via nozzles 15, which are installed at the beginning
of the reactor 4. In addition, it is possible to feed into the
reactor 4, at an appropriate point, preferably its tail end,
warm air 9' or hot flue gases ll' derived from the boiler, in
order to raise the temperature at the tail end of the reactor,
at a point before the dust separator 5.
According to the present invention, the hot, dust-bearing flue
gases emerging from the boiler are divided in a cyclone 3 into
two fractions, namely a dust-poor main flow ll of flue gases,
which is directed via an air preheater lO into the reactor ~,
and a dust-rich secondary flow 12, which is made to join the
main flow at a point after the air preheater lO, preferably at
a point before the reactor 4. However, a portion of this dust-
rich secondary flow 12 can be directed along a pipe 13 branch-
ing from the pipe 12 and be introduced into the main flow even
at a point before the air preheater lO.
In order to activate the adsorbent traveling together with the
dust-rich secondary flow 12, 13, water as a finely-divided
mist is sprayed through nozzles l9 installed in the lower sec-
tion of the cyclone 3 and connected to a water pipe l4. A
strong turbulence prevails in the lower section of the cyclone
3, and thus the finely-divided drops of water will mix well
with the dust-enriched flue gas before it flows out along the
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pipe 12 connected to the lower section of the cyclone 3. Al-
ternatively, or in addition to this, water can be atomized
directly into the pipe 12 at a point before the branch pipe
13. Instead of water, or in addition to water, the reagent
present in the dust can be activated using steam. At the point
of spraying water or steam, the temperature of the dust-rich
flue gases is approx. 200-600 C, at which temperature the
formation of calcium hydroxide and hydrous calcium sulfate is
very drastic, causing the dust particles to swell rapidly and
to break up into smaller particles with reactive calcium hy-
droxide on their planes of fracture.
By using the process and apparatus according to the invention,
the following advantages are gained in comparison with the
state of the art: the need for limestone is reduced, the de-
gree of sulfur removal is improved, the steam quantity re-
quired for the moistening is low and can well be replaced by
water spraying, and in addition, the apparatus required is
simple and does not include parts which move and are therefore -
prone to wear and disturbances.
The invention is described below with the aid of an example.
ExamPle
Limestone powder is fed into the furnace of a boiler in order
to remove sulfur dioxide from the flue gases by using a lime-
to-sulfur ratio of two. In order to improve the removal of
sulfur dioxide, the moisture content of the flue gases leaving
the boiler is increased, whereby the reactivity of the lime
remaining in the flue gases is improved through the formation
of calcium hydroxide.
.
According to the present invention, a secondary flow is sepa-
rated by means of a solids-separating apparatus from the flue
gas flow leaving the boiler, 50-80 % of the dust present in
the flue gases concentrating in this secondary flow. This dust
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WO91/01174 PCT/F190/00184-;
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consists of fly ash and of lime which has formed from the
limestone powder blown into the boiler and on the surface-of
which a calcium sulfate layer has formed in the boiler fur-
nace. The temperature t(in) of the flue gas entering the dust
separation system is usually approx. 400 C after the heat
surfaces of the boiler. No substantial cooling takes place in
the dust-separation apparatus, and thus the main flow of the
flue gases returns at the same temperature to the flue-gas
duct. Water and/or low-pressure steam is atomized into the
secondary flow of the flue gases, and its temperature is low-
ered. Under the effect of the moistening, the calcium sulfate
layer on the particle surface, preventing the binding of sul-
fur dioxide, breaks up, and the more active lime returning to
the process together with the secondary flow of the flue gas
improves the capacity of the entire process to bind sulfur
dioxide.
Table
V(tot) V(in) Ca/S H20 t(in) CaO(tot) CaO(in) XSO2 XSO2
Nm3/s Nm3/s g/Nm3 C 5/Nm3 g/Nm3 %t65-t60C
l00 20 2.0 - 400 l0 40 70 75
l00 20 2.0 l00 200 l0 40 75 80
l00 l0 2.0 l00 200 l0 80 75 80
l00 20 2.0 l50 120 l0 40 85 90
l00 l0 2.0 150 120 l0 80 85 90
l00 20 l.5 - 400 7.5 30 60 65
l00 20 l.5 l00 200 7.5 30 65 70
l00 20 l.5 150 120 7.5 30 70 75
l00 l0 l.5 150 120 7.5 30 70 75
In can be seen from the table that by moistening the dust-rich
secondary flow of flue gas, the sulfur-binding capacity of the
LIFAC process can be improved substantially or the consumption
of lime can be decreased.
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