TECHNIQUE DE SEPARATION DES POLLUANTS GAZEUX DES GAZ CHAUDS DE PROCESSUS DE FABRICATION

METHOD FOR SEPARATING GASEOUS POLLUTANTS FROM HOT PROCESS GASES

Abrégé français

Les gaz chauds de processus de fabrication passent par un four de contact (10) dans lequel des particules absorbantes, qui réagissent avec les polluants gazeux contenus dans les gaz de fabrication, sont introduites dans ces gaz pour transformer les gaz polluants en poussière séparable. Les gaz de fabrication passent ensuite par un séparateur de poussière (5). La plus grande partie de la poussière séparée dans le séparateur (5) passe alors dans un mélangeur (11), où elle est mélangée et humidifiée avant d'être recyclée comme matière absorbante par introduction dans les gaz de fabrication en même temps qu'un absorbant frais. L'absorbant frais ajouté est de la chaux vive. La poussière reste si longtemps dans le mélangeur et elle est recyclée tellement de fois que la durée de séjour de la chaux vive à l'état humide dans le mélangeur suffit à ce que celle-ci réagisse de façon assez complète avec l'eau alimentant le mélangeur pour se transformer en chaux éteinte.

Abrégé anglais

Hot process gases are passed through a contact reactor (10), in which a
particulate absorbent material, which is reactive with gaseous
pollutants in the gases, is introduced into the gases to convert the gaseous
pollutants into separable dust. The process gases are then passed
through a dust separator (5). The major part of the dust separated in the dust
separator (5) is passed to a mixer (11), in which it is mixed
and moistened, whereupon it is recirculated as absorbent material by being
introduced, together with fresh absorbent, into the process gases.
Burnt lime is added as fresh absorbent. The dust is kept so long in the mixer
and recirculated so many times that the total residence time
of the burnt lime in the mixer in moistened state is sufficiently long for the
burnt lime to react substantially completely with water supplied
to the mixer and form slaked lime.

Revendications

7
CLAIMS
1. A method for separating gaseous pollutants from
hot process gases, in which method the process gases are
passed through a contact reactor (10), in which a
particulate absorbent material, which is reactive with the
gaseous pollutants, is introduced in a moistened state into
the process gases for converting the gaseous pollutants
into separable dust, whereupon the process gases are passed
through a dust separator (5), in which dust is separated
from the process gases and from which the process gases are
discharged, part of the dust separated in the dust
separator (5) being passed to a mixer (11), in which it is
mixed and supplied with water so as to be moistened,
whereupon it is recirculated as absorbent material by being
introduced into the process gases together with an addition
of fresh absorbent, characterised in that burnt lime is
added as fresh absorbent, and that the major part of the
dust separated in the dust separator (5) is supplied to and
discharged from the mixer (11) in a substantially
continuous flow, the dust supplied to the mixer (11) is
kept so long in the mixer (11) and is recirculated so many
times that the total residence time of the burnt lime in a
moistened state in the mixer (11) is sufficiently long for
the burnt lime to have time to react substantially
completely with water supplied to the mixer (11) so as to
thereby form slaked lime.
2. The method as claimed in claim 1, characterised
in that the fresh absorbent in the form of burnt lime is
supplied to the mixer (11).

8
3. The method as claimed in claim 1, characterised
in that the fresh absorbent in the form of burnt lime is
added to that part of the dust separated in the dust
separator (5) which is supplied to the mixer (11).
4. The method as claimed in claim 1, characterised
in that the fresh absorbent in the form of burnt lime is
introduced directly into the process gases in the contact
reactor (10).
5. The method as claimed in claim 1, characterised
in that an air flow is supplied to the mixer (11) in order
to fluidize the dust mixed therein.

Description

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METHOD FOR SEPARATING GASEOUS POLLUTANTS FROM
HOT PROCESS GASES
The present invention relates to a method for sepa-
rating gaseous pollutants, such as sulphur dioxide, from
hot process gases, such as flue gases; in which method
the process gases are passed through a contact reactor,
in which a particulate absorbent material, which is reac-
tive with the gaseous pollutants, is introduced in mois-
tened state into the process gases for converting the
gaseous pollutants into separable dust, whereupon the
process gases are passed through a dust separator, in
which dust is separated from the process gases and from
which the cleaned process gases are discharged, part of
the dust separated in the dust separator being passed to
a mixer, in which it is mixed and supplied with water so
as to be moistened, whereupon it is recirculated as ab-
sorbent material by being introduced into the process
gases together with an addition of fresh absorbent.
The above-described method for separating gaseous
pollutants from hot process gases is known.
To this end use is preferably made of slaked lime (calcium
hydroxide) in particle form as fresh absorbent. This
absorbent is mixed with dust which has been separated
from the process gases in the dust separator, whereupon
the~mixture is supplied with water to be introduced, in
moistened state, into the flue gases in the contact reac-
tor. The slaked lime is comparatively expensive, and so
various experiments have been made to use burnt lime
., (calcium oxide) instead, which is considerably less ex-
pensive. In these experiments, a plant had to be used,
in which the burnt lime has first been slaked, i.e. the
calcium oxide has been caused to react with water to
form calcium hydroxide, before being introduced into the
cleaning process. Such a lime slaking plant is expensive,

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which means that the expected profit when changing from
slaked lime to burnt lime failed to appear.
The object of the present invention is to provide a
method for separating gaseous pollutants from hot process
gases, in which the slaked lime can be replaced by burnt
lime, without necessitating an expensive, separate plant
for slaking the burnt lime.
According to the invention, this object is achieved
by a method which is of the type mentioned by way of in
troduction and is characterised in that burnt lime is
added as fresh absorbent, and that the major part of the
dust separated in the dust separator is supplied to and
discharged from the mixer in a substantially continuous
flow, the dust being kept so long in the mixer and being
recirculated so many times that the total residence time
of the burnt lime in the mixer in moistened state is suf-
ficiently long for the burnt lime to have time to react
substantially completely with water supplied to the mixer
and form slaked lime.
The fresh absorbent in the form of burnt lime is
supplied preferably to the mixer, but it may also be
added to that part of the dust separated in the dust
separator which is supplied to the mixer. Alternatively,
the burnt lime can be introduced directly into the flue
gases in the contact reactor.
An air flow is suitably supplied to the mixer in
order to fluidise the dust mixed therein and thus improve
the mixture.
The invention will now be described in more detail
with reference to the accompanying drawing, which schema-
tically illustrates a plant for cleaning flue gases from
r
a coal-fired central heating plant, the cleaning plant
being provided with equipment for carrying out the method
according to the present invention.
The drawing schematically illustrates a plant for
cleaning flue gases from a coal-fired central heating
plant 1, said flue gases containing dust, such as fly

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ash, and gaseous pollutants, such as sulphur dioxide. A
preheating device 2 is arranged to transmit heat from the
hot flue gases to combustion air, which via a duct 2a is
supplied to the central heating plant 1 by means of a fan
3.
The hot flue gases are conveyed through a duct 4 to
a dust separator 5, which in the embodiment shown is an
electrostatic precipitator having three successive preci-
pitator units, through which the flue gases ate passed to
be cleaned. The flue gases cleaned in this manner are via
a duct 6 passed to a flue gas fan 7, which via a duct 8
feeds them on to a chimney 9 to be emitted into the at-
mosphere. The dust separator can also be a bag.filter.
The duct 4 .comprises a vertical portion which forms
a contact reactor 10. A mixer 11. communicates with the
contact reactor 10 in the lower part. thereof. The mixer
11 introduces.a particulate absorbent.material, which is
reactive with the gaseous pollutants in the flue gases,
in moistened state into the flue gases in the lower part
of the contact reactor l0. This absorbent material con-
verts the gaseous pollutants into dust, which is separat-
ed in the precipitator 5.
The dust particles separated in the precipitator
5 are collected in the hoppers 12 of the precipitator
units. The major part of the collected dust particles
is recirculated in the system in a manner which will be
described in detail below. The remainder of the collect-
ed dust particles are transported away in a manner which
will not be described in detail, for instance by means of
a worm conveyor.
The mixer 11 is of the type that
has essentially the shape
of a double-bottom box. Between the two bottoms, the
upper of which consists of a tensioned fluidising cloth
l3 of polyester, there is a chamber 14, to which air for
fluidising the particulate absorbent material in the
mixer.ll is conducted through an air supply conduct 15.

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Water is supplied to the mixer 11 through a water supply
conduit 16 and nozzles 17, which are arranged in the
upper portion of the mixer. The particulate material ,~
,.
which is to be mixed, is supplied to the mixer 11 through
two material inlets 18 and 19 at the inlet end of the
,.
mixer. The mixer 11 further comprises a mechanical mixing
mechanism 20 consisting of two cooperating, parallel agi-
tators (of which only one is shown in the drawing), each
having a horizontal shaft and, mounted thereon, a plura-
lity of inclined elliptic plates. The outlet end of the
mixer 11 extends into the contact reactor 10 in order to
continuously supply thereto through an overflow 21 well-
mixed, moistened absorbent material.
That part of the dust particles collected in the
hoppers 12 of the precipitator units which is to be
recirculated in the system is supplied to the mixer 11
through the inlet 19. Particulate burnt lime (calcium
oxide) is supplied to the mixer 11 through the inlet 18
to be mixed with the dust particles supplied through the
inlet 19. The mixture is moistened with water supplied
through the nozzles 17. Water is supplied through the
nozzles 17 also in order to slake the burnt lime supplied
to the mixer 11. Owing to the construction of the mecha-
nical mixing mechanism 20 and the fluidisation of the
material particles supplied to the mixer 11, the mixer
produces a homogeneously moistened, homogeneous mixture
of material particles, which are, through the overflow
21 of the mixer 11, continuously supplied to the contact
reactor 10 as absorbent material. The residence time of
the material particles in the mixer 11 is in the order of
5-60 s, especially 10-20 s.
The residence time as stated above (10-20 s) of the
material particles in the mixer 11, i.e. the time during
which the lime particles are in moistened state, is in-
sufficient for the burnt lime to have time to react com-
pletely with the water added for slaking and form slaked

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lime. Such a. reaction is relative~.y show and takes seve-
ral minutes.
The invention will now be described in more detail
with the aid. of the theoretical Example below. The Exam-
5 ple states the conditions prevailing at points A, B and C
in the drawing, i.e. in the duct 4 before the mixer 11,
in the reactor ZO after the mixer 11 at the inlet of the
precipitator 5 and at the outlet of the precipitator 5,
respectively.
A B C
Gas flow (Nm3/h) 100,000 103,993 103,993
Gas temperature (C) 125 65 65
S02 concentration (ppm) 1,150 280 172
Dust concent;x-ation ( g/Nm3 ) 20 1, 000 < 0 . 03
The duet: at A is substantially fly ash, whereas
the dust at E3 is fly ash and absorbent material.
At D, 2,930 kg of dust are discharged per hour,
2,000 kg being fly ash.
In thi~~ Example, the particulate absorbent mate-
rial, includ~~ng the burnt lime, which is being slaked,
is thus circulated on the average about 35 times
(1.0 x 103,9;33 / 2,930 ~ 35) in the system before being
discharged e31~ D. The total residence time of the absor-
bent materia:L in the mixer 11 thus is 350-700 s, i.e. in
the order o:E 6-12 min, which is sufficient for the burnt
lime to have time to be slaked.
The total consumption of water in the above Example
is 3,366 1/h, of which 152 1/h is required for the slak-
ing of lime. When this amount of water is consumed, the
moisture content of the absorbent material discharged
from the mi:x~~r 11 is about 6$. However, the moisture con-
tent can, a~~~~ording to the composition of the mixture,
suitably be 'varied in the range 2-15$.
If, in 'the above Example, the fly ash content of the
flue gases at point A is zero, i.e. the discharge at D is

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930 kg/h, the circulation number will, by analogy with that
stated above, be about 110 (0.980 x 103,993 / 930 ~ 110),
which in turn yields a residence time of 1,100-2,200 s,
i.e. in the order of 18-37 min.
,.

Dessin représentatif