U.S. patent application number 11/524339 was filed with the patent office on 2007-01-18 for process and device for cleaning combustion flue gases.
This patent application is currently assigned to tesa Aktiengesellschaft. Invention is credited to Hansjorg Herden, Robert Mergler, Barbara Roth, Harald Sauer, Berthold Stegemann.
Application Number | 20070014706 11/524339 |
Document ID | / |
Family ID | 7691599 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070014706 |
Kind Code |
A1 |
Herden; Hansjorg ; et
al. |
January 18, 2007 |
Process and device for cleaning combustion flue gases
Abstract
The invention relates to a process and a device for the removal
of dust, HF, HCl, SO.sub.2, SO.sub.3, heavy metals, heavy-metal
compounds, polyhalogenated hydrocarbons and polycyclic hydrocarbons
from combustion flue gases by treating the pollutant-containing
combustion flue gases with a sorbent in a circulating fluidized
bed. The process is characterized in that the sorbent used is a
mixture of Ca(OH).sub.2, at least one naturally occurring zeolite
and a carbon-containing substance, in that the treatment of the
pollutant-containing combustion flue gases with the: sorbent is
carried out at from 120 to 180.degree. C. in the presence of
water/steam, in that the reactor (3) of the circulating fluidised
bed is operated at a gas velocity of from 2 to 10 m/s, a mean
residence time of the solids particles in the case of a single pass
of from 1 to 10 seconds, and a solids circulation rate of from 10
to 100, where the gas/solid suspension present in the reactor (3)
has a mean suspension density of from 1 to 10 kg of solid/Nm.sup.3
of flue gas, and in that the removal of the loaded sorbent is
carried out by filtration.
Inventors: |
Herden; Hansjorg; (Rodgau,
DE) ; Roth; Barbara; (Brussel, BE) ; Mergler;
Robert; (Eschborn, DE) ; Stegemann; Berthold;
(Eschborn, DE) ; Sauer; Harald; (Frankfurt am
Main, DE) |
Correspondence
Address: |
Norris, McLaughlin & Marcus P.A.
18th Floor
875 Third Avenue
New York
NY
10022
US
|
Assignee: |
tesa Aktiengesellschaft
Hamburg
DE
|
Family ID: |
7691599 |
Appl. No.: |
11/524339 |
Filed: |
September 20, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10190644 |
Jul 8, 2002 |
|
|
|
11524339 |
Sep 20, 2006 |
|
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Current U.S.
Class: |
422/168 |
Current CPC
Class: |
B01D 53/508 20130101;
F23J 2219/50 20130101; B01D 53/12 20130101; F23J 2219/30 20130101;
B01D 2257/206 20130101; F23J 15/006 20130101; B01D 2257/2047
20130101; B01D 2257/60 20130101; B01D 2253/102 20130101; B01D
2257/602 20130101; B01D 2257/2045 20130101; B01D 53/68 20130101;
B01D 2257/302 20130101; B01D 53/64 20130101; B01D 2253/108
20130101 |
Class at
Publication: |
422/168 |
International
Class: |
B01D 53/34 20060101
B01D053/34 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 12, 2001 |
DE |
101 33 991.7 |
Claims
1-38. (canceled)
39. An apparatus for the removal of dust, HF, HCl, SO.sub.2,
SO.sub.3, heavy metals, heavy metal compounds, polyhalogenated
hydrocarbons and polycylic hydrocarbons from combustion flue gases,
said apparatus comprising: a flue gas supply line for guiding said
combustion flue gases, the flue gas supply line being connected to
a first supply line for supplying a fresh sorbent and a second
supply line for supplying a recycled sorbent into said flue gas
supply line; a reactor comprising a Venturi tube and a diffuser
arranged at the base of the reactor, the Venturi tube being
connected to one end of the flue gas supply line; at least one
nozzle for introducing water, arranged above the diffuser inside
the reactor; a separator being in fluid connection with the reactor
via a connection line for guiding a generated gas-solid suspension
from the reactor to the separator; a first discharge line for
discharging cleaned combustion flue gases from the separator; a
second discharge line for recycling at least partly separated-off
solid containing sorbent; and a fluidization channel having a first
end and a second end, wherein the first end of the fluidization
channel is connected to the second discharge line and the second
end of the fluidization channel is connected to the second supply
line.
40. An apparatus for the removal of dust, HF, HCl, SO.sub.2,
SO.sub.3, heavy metals, heavy metal compounds, polyhalogenated
hydrocarbons and polycylic hydrocarbons from combustion flue gases,
said apparatus comprising: a reactor, a separator and a return
line, wherein the base of the reactor is a Venturi tube, wherein
one or more nozzles for the introduction of water are disposed
immediately above a diffuser of the Venturi tube and distributed
over the reactor cross-section, wherein the separator is a bag
filter containing a low-pressure jet pulse device which is operated
at an excess pressure of 0.6 to 0.8 bar, wherein the return line is
a first inclined fluidization channel which opens into a flue gas
duct connected to the reactor, and wherein the base has a
gas-permeable fabric through which a gas serving for fluidization
of a recycled sorbent is introduced into the first inclined
fluidization channel, and a second inclined fluidization channel,
which serves for feeding the fresh sorbent wherein the base has a
gas-permeable fabric through which the gas serving for fluidization
of a fresh sorbent is introduced into the second fluidization
channel, is introduced into the flue gas duct.
41. The apparatus as claimed in claim 1, wherein a back-flushing
gas used in the low-pressure jet pulse device is pre-warmed air or
a part-stream of the cleaned combustion flue gas.
42. The apparatus as claimed in claim 1, wherein a line for
introducing the gas/solid suspension into the bag filter opens into
a lower part of the bag filter, disposed therein is at least one
inertial separator acting as a baffle-plate separator which the
gas/solid suspension strikes.
43. The apparatus as claimed in claim 1, wherein a loaded sorbent
accumulating at a base of the bag filter is discharged via at least
one metering roller.
Description
[0001] This application is a divisional of U.S. Ser. No. 10/190,644
filed Jul. 8, 2002
[0002] The invention relates to a process for the removal of dust,
HF, HCl, SO.sub.2, S0.sub.3, heavy metals, heavy-metal compounds,
polyhalogenated hydrocarbons and polycyclic hydrocarbons from
combustion flue gases by treating the pollutant-containing
combustion flue gases with a sorbent in a circulating fluidized bed
consisting of a reactor, a separator and a return line, where the
pollutants are bound in the reactor by the sorbent suspended in the
gas stream, and the pollutant-loaded sorbent is separated off from
the gas/solid suspension in the separator and in part returned to
the reactor via the return line and in part discharged. The
invention furthermore relates to a device for carrying out the
process. The invention finally relates to a preferred use of the
process and the device.
[0003] In the combustion of fossil fuels, which is carried out with
the aim of energy recovery, and in the incineration of refuse,
sewage sludge and industrial waste, which is carried out with the
aim of waste disposal and energy recovery, flue gases are formed
which are contaminated by dust, HF, HCl, SO.sub.2, SO.sub.3, heavy
metals, heavy-metal compounds, polyhalogenated hydrocarbons and
polycyclic hydrocarbons, the contaminants being present in the flue
gas in different amounts depending on the particular combustion
process, and the concentration of the contaminants undergoing
certain variations during each combustion process. For example, the
different and varying composition of refuse, industrial waste and
sewage sludge means that the flue gases formed on combustion of
this waste are also contaminated by different amounts of
environmentally polluting substances. However, all contaminants
must be substantially removed from the flue gases before the latter
can be released into the atmosphere, since very many contaminants
exert toxic effects on humans, animals and plants even in low
concentration.
[0004] The dust present in the combustion flue gases in an amount
of up to 50 000 mg/Nm.sup.3 is separated off in cyclones,
electrostatic filters, fabric filters or scrubbers, it also being
possible for the flue gas from which dust is to be removed to be
passed through a plurality of these apparatuses. The known
dust-removal processes today allow residual dust contents of <5
mg/Nm.sup.3, even on a large industrial scale. The most substantial
dust removal possible is therefore necessary because the dust
adsorbs, in particular, toxic heavy metals, heavy-metal compounds,
polyhalogenated dibenzodioxins and dibenzofurans, as well as
polycyclic hydrocarbons.
[0005] SO.sub.2 and HCl are each present in the combustion flue
gases in an amount of up to 7000 mg/Nm.sup.3; in addition, HF and
S0.sub.3 are each present in amounts of up to 100 mg/Nm .These
gaseous compounds react with the water vapour present in the
atmosphere to form acids, which are very frequently in the form of
aerosols and have a toxic action. They are therefore substantially
separated off, with the known cleaning processes on an industrial
scale enabling residual S0.sub.2 contents of <20 mg/Nm.sup.3,
residual HCl contents of <5 mg/Nm.sup.3 and residual HF and
S0.sub.3 contents of <1 mg/Nm.sup.3 to be achieved. SO.sub.2,
HCl, HF and S0.sub.3 are separated off using dry, quasi-dry and wet
cleaning processes, it also being possible for a plurality of
processes to be connected in series. The reactants used in these
processes are, in particular, Ca(OH).sub.2, CaO, CaCO.sub.3, NaOH
and Na.sub.2C0.sub.3, these compounds reacting with the gaseous,
acidic pollutants with salt formation. Particular importance has
been achieved by spray absorption, in which an aqueous suspension
of Ca(OH).sub.2 reacts with the acidic pollutants SO.sub.2,
SO.sub.3, HCl and HF, the water is evaporated, and a solid reaction
product is formed which also contains dust and other pollutants. In
addition, scrubbing processes are known which allow very
substantial removal of the above-mentioned acidic pollutants.
[0006] The heavy metals and the heavy-metal compounds, in
particular mercury, cadmium and arsenic and their compounds, and
the polyhalogenated and polycyclic hydrocarbons are present in the
flue gases in a lower concentration. However, these substances are
extremely toxic and therefore have to be removed from the flue
gases virtually quantitatively, which is preferably carried out in
accordance with the prior art by adsorption and/or scrubbing
processes. Adsorbents which have proven successful are, in
particular, activated carbon and zeolites, while the scrubbing
processes operate both in the acidic and in the alkaline ranges.
The polyhalogenated hydrocarbons can also be removed from the flue
gases by catalytic decomposition, while the polycyclic hydrocarbons
can also be oxidized by reaction with the oxygen present in the
combustion flue gas on an oxidation catalyst to give water and
carbon dioxide. The polyhalogenated hydrocarbons present in
combustion flue gases are polyhalogenated dibenzodioxins and
dibenzofurans, polychlorinated biphenyls, polychlorinated phenols
and polychlorinated aromatic compounds, all of which are extremely
toxic.
[0007] In addition to the above-mentioned pollutants whose removal
is a subject-matter of the invention, the combustion flue gases
also contain the oxides of nitrogen, i.e. N.sub.2O, NO and
NO.sub.2, which are likewise toxic. These pollutants also have to
be removed from the combustion flue gases, which is carried out in
accordance with the prior art by, in particular, reaction with
NH.sub.3, this reaction proceeding either at high temperatures or
at lower temperatures on a catalyst with formation of N.sub.2 and
H.sub.2O.
[0008] The combustion flue gases themselves consist of N.sub.2,
CO.sub.2, H.sub.2O and O.sub.2. The source of the nitrogen is
predominantly the nitrogen content of the combustion air, while the
oxygen present in the combustion flue gases in an amount of from 2
to 12% by volume results from the fact that the combustion is
carried out with a stoichiometric excess of oxygen. CO.sub.2 and
H.sub.2O form on combustion of the carbon or hydrogen present in
the fuels with the oxygen present in the combustion air.
[0009] The industrial suitability of the known flue gas cleaning
processes depends, in particular, on them causing the lowest
possible investment and operating costs and that they supply
process products which are formed in the lowest possible amount and
can either be sent to landfill without serious difficulties or can
be recycled into the cleaning processes after regeneration. In
order to achieve the highest possible degree of separation for the
individual contaminants mentioned above, it is usual for a number
of cleaning processes to be combined with one another. For example,
a dust-removal process can be linked to a process for the quasi-dry
separation of gaseous, acidic pollutants (spray absorption), which
can then be followed by an adsorption process for the removal of
heavy metals and hydrocarbons and a nitrogen removal process.
However, the present invention has the aim of removing as many of
the above-mentioned pollutants as possible virtually quantitatively
from the combustion flue gases in a single reaction step, since
only then is it possible to minimize the investment and operating
costs for the cleaning of combustion flue gases. Flue gas cleaning
processes which pursue this aim with varying degrees of success are
already known from the prior art.
[0010] Thus, for example, EP-B 0 253 563 discloses a process for
the elimination of mercury vapour and vapour-form organic compounds
as well as acidic constituents from a hot flue gas stream which
also contains fly ash leaving a incinerator. In this known process,
the flue gas stream to be cleaned is passed at a temperature of
from 135 to 400.degree. C. into a spray absorption chamber in which
an aqueous liquid containing a basic absorbent is atomized. The
flue gas cools in the spray absorber to from 180 to 90.degree. C.
due to evaporation of the water, and at the same time the acidic
constituents SO.sub.2 and HCl from the flue gas are bound, with a
particulate material being formed which contains the products of
the reaction of the basic absorbent and the acidic constituents of
the flue gas as well as unreacted absorbent. The particulate
reaction product, together with any fly ash present, is separated
from the flue gases in a particle separator downstream of the spray
absorption chamber. In the known process, it is also provided that
powder-form activated carbon in an amount of from 1 to 800
mg/Nm.sup.3 is introduced into the flue gas stream at least at one
point upstream of the spray absorption chamber, in the spray
absorption chamber or downstream of the spray absorption chamber,
but upstream of the particle separator. The powder-form activated
carbon is separated off in the particle separator together with the
particulate reaction products. From today's point of view, a
particular disadvantage of this known process is that both a spray
absorber and a flow injection reactor, i.e. two process steps, are
used to remove the pollutants.
[0011] DE-A 4415719 discloses a process for the removal of HF, HCl,
SO.sub.2, polyhalogenated hydrocarbons, mercury, mercury compounds
and dust from a flue gas, in which the contaminated flue gas is
brought into contact with an adsorbent in a reactor at a
temperature of from 70 to 180.degree. C. above the dew point, the
gas/solid suspension is subsequently passed into a pre-separator,
in which the majority of the solids are removed, the pre-cleaned
gas/solid suspension is then passed into a final separator, into
which the entire amount of the adsorbent is introduced and in which
the suspended solids are removed virtually quantitatively, the
solids obtained in the final separator are fed back into the
reactor, and a sub-stream of the solids obtained in the
pre-separator is fed back into the reactor, while a second
sub-stream of the solids obtained in the pre-separator is
discharged and sent to landfill. In this process, the flue gas is
cooled to a temperature of from 70 to 180.degree. C. before entry
into the reactor or in the reactor by mixing with water, where the
reactor is designed as a fluidizedbed reactor, flow injection
reactor or spray absorber. The adsorbent consists of Ca(OH).sub.2
and/or CaO and has a mean particle diameter d.sub.50 of from 5 to
500 .mu.m. The adsorbent may contain from 2 to 20% by weight of
activated carbon and/or zeolites. From today's point of view, this
known process has the disadvantage that a pre-separator and a final
separator are required to carry out the removal of the
pollutant-loaded solids, and in addition the separation of the
solids from the gas stream is particularly difficult if they have a
relatively high content of CaCl.sub.2 since this compound is
hygroscopic and therefore results in solid encrustations in the
separators since a sufficient amount of water, which is adducted by
the CaCl.sub.2, is present in the flue gas stream.
[0012] DE-A 4403244 discloses a process for cleaning
oxygen-containing flue gases formed in the combustion of refuse,
industrial waste and sewage sludge, in which mercury, mercury
compounds and polyhalogenated hydrocarbons are removed from the
flue gases by adsorption on zeolites. This process is characterized
in that the flue gases are reacted with a mixture of naturally
occurring zeolites for a reaction time of from 0.5 to 10 seconds in
a gas/solid suspension above the dew point at a temperature of from
80 to 180.degree. C. and a gas velocity of from 3 to 20 m/s, where
the mean particle size d.sub.50 of the zeolite mixture is from 5 to
50 .mu.m and the mean suspension density of the gas/solid
suspension is from 0.02 to 10 kg of solid/Nm.sup.3 of flue gas. The
mixture of naturally occurring zeolites may contain from 10 to 30%
by weight of CaC0.sub.3, CaO and/or Ca(OH).sub.2. This known
process can be carried out in the circulating fluidized bed, with
the gas velocity being from 3 to 8 m/s and the mean suspension
density of the gas/solid suspension being from 2 to 10 kg of
solid/Nm.sup.3 of flue gas. This process has, in particular, the
disadvantage that it is not able to remove the acidic pollutants
HF, HCl, SO.sub.2 and SO.sub.3 substantially and that to this
extent a further process step is absolutely necessary. This has the
consequence that the combustion flue gas firstly has to be
subjected to dust removal in a suitable device and then has to be
substantially freed from the acidic, gaseous pollutants in a spray
absorber before the heavy metals and polyhalogenated hydrocarbons
and the remaining acidic pollutants can be removed.
[0013] The invention is based on the object of providing a process
for the removal of dust, HF, HCl, SO.sub.2, SO.sub.3, heavy metals,
heavy-metal compounds, polyhalogenated hydrocarbons and polycyclic
hydrocarbons from combustion flue gases which effects the pollutant
removal in only one process step, which furthermore produces a
cleaned flue gas which does not exceed the limits prescribed by the
17th Ordinance for Implementation of the German Federal Emission
Protection Act (Ordinance Regarding Incinerators for Waste and
Similar Combustible Substances--17.BImSchV) of 23.11.1990, amended
by the ordinance of 23.02.1999, and which, finally, produces a
pollutantloaded solids mixture which remains flowable and
transportable and does not result in encrustations and caking
either in the solids separator or in the transport systems. The
invention is furthermore based on the object of providing a device
for carrying out the process and of indicating a preferred use of
the process and the device.
[0014] The object on which the invention is based is achieved by a
process of the type mentioned at the outset, which is characterized
in that the sorbent used is a mixture of Ca(OH).sub.2, at least one
naturally occurring zeolite and a carbon-containing substance,
where the mean particle size d5o of the sorbent is from 2 to 50
.mu.m, in that the treatment of the pollutant-containing combustion
flue gases with the sorbent is carried out at from 120 to
180.degree. C. in the presence of water/steam, in that the reactor
is operated at a gas velocity of from 2 to 10 m/s, a mean residence
time of the solids particles in the case of a single pass of from 1
to 10 seconds, and a solids circulation rate of from 10 to 100,
where the gas/solid suspension present in the reactor has a mean
suspension density of from 1 to 10 kg of solid/Nm.sub.3 of flue
gas, and in that the removal of the loaded sorbent in the separator
is carried out by filtration.
[0015] The Ca(OH).sub.2 present in the sorbent has a purity of
>98% and reacts with or absorbs the acidic pollutants HF, HCl,
SO.sub.2 and SO.sub.3 with formation of the corresponding calcium
salts. The salt formation proceeds relatively quickly since the
water introduced into the reactor in liquid form firstly wets the
sorbent particles and only evaporates thereafter, with the
Ca(OH).sub.2 particles being activated with respect to their
reactivity by the wetting with liquid water. The invention
therefore expressly uses the term "water/steam". The naturally
occurring zeolites and the carbon-containing substance adsorb the
heavy metals and heavy-metal compounds present in the combustion
flue gases as well as the polyhalogenated and polycyclic
hydrocarbons. However, the natural zeolites present in the sorbent
also do another job, i.e. at from 120 to 180.degree. C. they adsorb
water since their adsorption capacity for water at 20.degree. C. is
from 30 to 50 g/kg, while they are unable to adsorb any further
water at from 220 to 230.degree. C., meaning that their adsorption
capacity for water at from 120 to 180.degree. C. is still
considerable. The adsorption processes proceed significantly more
slowly than the salt-formation reactions. This has the consequence
that the naturally occurring zeolites adsorb water from the
gas/solid suspension, in particular in the upper part of the
reactor, which has the very advantageous effect that the water is
no longer adducted by the hygroscopic CaCl.sub.2 and that the
solids leaving the reactor do not bake on or cake, in particular in
the separator; instead, they remain flowable. The solids
circulation rate of from 10 to 100--the individual solids particles
thus pass through the reactor from 10 to 100 times--in accordance
with the invention ensures that a large amount of sorbent is always
present in the reactor, meaning that the sorbent is in excess over
the pollutants. This has the consequence that from 85 to 99% of the
Ca(OH).sub.2 employed react with the gaseous, acidic pollutants and
that by far the majority of the adsorption capacity of the
naturally occurring zeolites and of the carbon-containing substance
is utilized. The good flowability of the solids particles enables
them to be separated off from the gas/solid suspension in the
separator simply by filtration and means that, in particular, their
partial recycling; into the reactor is possible without problems.
The combustion flue gas cleaning according to the invention in a
circulating fluidized bed enables the pollutant limits prescribed
in 17.BImSchV to be observed without problems and for the most part
even bettered. The advantage of the process according to the
invention is thus, in particular, that all the above-mentioned
pollutants can be removed virtually completely in one process step,
and with minimal equipment complexity.
[0016] In addition, it is particularly advantageous that the
separated-off sorbent contains no CaSO.sub.3, since this originally
formed reaction product is oxidized quantitatively to CaSO.sub.4 by
the oxygen present in the combustion flue gas owing to the
catalytic activity of the heavy metals present; no SO.sub.3.sup.2-
has been detected in the discharged sorbent. The discharged sorbent
thus contains the pollutants SO.sub.2 and SO.sub.3 in the form of
sulphates of calcium, where the calcium sulphates predominantly
consist of CaSO.sub.4.0.5 H.sub.20. This hemihydrate of CaSO.sub.4
solidifies on contact with water to form CaSO.sub.4.2 H.sub.20. The
dust present in the discharged sorbent is therefore firmly bound if
the discharged sorbent comes into contact with water, for example
in a landfill site.
[0017] The circulating fluidized bed is designed as a circulation
system and is distinguished by the fact that--in contrast to the
classical fluidized bed, in which a dense phase is separated from
the gas space above it by a clear density jump--distribution states
without a defined boundary layer are present. A density jump
between the dense phase and the gas space above it is non-existent
in an expanded circulating fluidized bed; however, the solids
concentration decreases constantly from bottom to top within the
reactor. This has the consequence that absorption and adsorption
processes are able to proceed throughout the reactor, since an
adequate number of sorbent particles is always present, even in the
upper part of the reactor. It has been found that the respirable
dust particles--i.e. small dust particles having a mean particle
diameter d.sub.50 of <1 .mu.m--are substantially adsorbed in the
circulating fluidized bed by the relatively large particles of the
sorbent through the action of van der Waal's forces. These
adsorbates are advantageously not destroyed by the shear forces
prevailing in the circulating fluidized bed, while relatively large
solids agglomerates are broken down into the individual solids
particles by the shear forces of the circulating fluidized bed,
which likewise has an advantageous effect on the course of the
process.
[0018] It is particularly advantageous in accordance with the
invention for the sorbent to consist of from 75 to 96 parts by
weight of Ca(OH).sub.2 having a mean particle diameter d.sub.50 of
from 2 to 5 .mu.m, from 3 to 15 parts by weight of a
carbon-containing substance having a mean particle diameter
d.sub.50 of from 10 to 30 .mu.m, and from 1 to 10 parts by weight
of at least one naturally occurring zeolite having a mean particle
diameter d.sub.50 of from 10 to 25 .mu.m. A sorbent nature of this
type effects optimum pollutant removal and ensures fault-free,
continuous operation of the circulating fluidized bed.
[0019] The naturally occurring zeolite used in accordance with the
invention is advantageously analcime, chabasite, clinoptilolite,
faujasite, harmotome, mordenite or natrolite, it also being
possible to use a mixture consisting of two or more of these
substances. The carbon-containing substance used in accordance with
the invention is advantageously open hearth coke or activated
carbon, the open hearth coke firstly being inexpensive and secondly
having a sufficiently high adsorption capacity for heavy metals,
heavy-metal compounds and toxic hydrocarbons.
[0020] In a further embodiment of the invention, it is provided
that the naturally occurring zeolite and/or the carbon-containing
substance are doped with from 2 to 10% by weight of sulphur and/or
at least one sulphur-containing compound. The doping is carried out
by mixing the naturally occurring zeolite and/or the
carbon-containing substance with sulphur or at least one
sulphur-containing compound, where all substances have
approximately the same particle diameter, or by impregnating the
naturally occurring zeolite and/or the carbon-containing substance
with a solution containing at least one sulphur-containing
compound, and subsequently drying the solids impregnated in this
way. The sulphur-containing compounds used are polysulphides of
sodium, potassium or calcium, dithiocarbamates, trithiocarbonates
or the sodium salt of trimercapto-S-triazine. The sulphur and the
sulphur-containing compounds react with the heavy metals present in
the combustion flue gas at elevated temperature with formation of
sulphides.
[0021] In accordance with the invention, the recycled,
pollutant-loaded sorbent is introduced into the
pollutant-containing combustion flue gases upstream of the reactor,
i.e. in the flue gas channel which runs into the reactor. In
accordance with the invention, the pollutant-loaded, discharged
sorbent is replaced by fresh sorbent, where the fresh sorbent is
introduced into the pollutant-containing combustion flue gases
upstream of the reactor--i.e. in the flue gas channel--and where
the fresh sorbent and the recycled sorbent are introduced
separately from one another into the pollutantcontaining combustion
flue gases.
[0022] By means of the two measures, intimate mixing of the fresh
and recycled, loaded sorbent is advantageously achieved even before
entry into the reactor. It has been found that the separate feed of
the fresh and recycled sorbent into the pollutant-containing
combustion flue gas upstream of the reactor enables avoidance of
the undesired agglomeration of the solids that occurs when fresh
and recycled sorbent are introduced together into the combustion
flue gas and are already mixed before their introduction. It has
furthermore been found that all solids particles in the stream of
pollutantcontaining combustion flue gas are heated to a temperature
above the working temperature prevailing in the reactor before they
are introduced into the reactor, which results in solids
agglomeration being prevented and the recycled zeolite particles
being dried so that they are again able to take up water in the
reactor. Finally, it has been found that a well-mixed gas/solid
suspension is already formed in the combustion flue gas upstream of
the reactor and then effects very uniform solids distribution over
the entire reactor cross section in the reactor itself.
[0023] In a further embodiment of the invention, it is provided
that the amount of fresh sorbent supplied is regulated in
accordance with the total amount of the pollutants HCl and SO.sub.2
and the amount of Ca(OH).sub.2 which is stoichiometrically
necessary for their binding. Carrying out the process in this way
in accordance with the invention enables, in an advantageous
manner, very rapid reaction to changes in the amount and
composition of the pollutant-containing combustion flue gases,
which means that an adequately large amount of fresh sorbent is
always present in the reactor.
[0024] The process according to the invention operates particularly
advantageously if the pollutant-containing combustion flue gases
have a temperature of from 200 to 280.degree. C. upstream of the
reactor and are cooled to a temperature of from 120 to 180.degree.
C. in the reactor by addition of water, where the addition of water
takes place in the lower part of the reactor, and the; water is
introduced into the reactor in the form of drops having a diameter
of <40 .mu.m. The feed of the water in finely divided form into
the lower part of the reactor crucially accelerates the
salt-formation reaction of the acidic, gaseous pollutants with the
Ca(OH).sub.2, since the reactivity of the Ca(OH).sub.2 is activated
by the water. An adequate reaction time is then available for
adsorption of the excess water by the naturally occurring zeolite
present in the sorbent, so that the hygroscopic properties of the
CaCl.sub.2 are suppressed and cannot have an adverse effect with
respect to the operation of the circulating fluidized bed. It is
particularly advantageous for the pollutant-containing combustion
flue gases to be cooled to a temperature of from 130 to 160.degree.
C. in the reactor.
[0025] In a further embodiment of the invention, it is proposed
that the loaded sorbent present in the return line and the fresh
sorbent are each fluidized and transported by addition of a
sub-stream of the cleaned combustion flue gases. This measure
effects problem-free introduction of the sorbent into the
combustion flue gas upstream of the reactor without the significant
operating costs for this, in particular for a fluidization gas,
being entailed. However, pre-warmed air can also be used for the
fluidization and for the transport of the fresh and loaded,
recycled sorbent.
[0026] In order to maintain the operating conditions pre-specified
in accordance with the invention in the circulating fluidized bed,
it is provided, in the case of partial-load operation of the
incinerator in accordance with the invention, that the operating
conditions in the circulating fluidized bed are kept constant in
the case of a change in the amount of combustion flue gases to be
cleaned (partial-load operation) by addition of a sub-stream of the
cleaned combustion flue gases. Carrying out the process in this way
safeguards continuous operation and prevents undesired frequent
start-up and shut-down of the cleaning plant.
[0027] Finally, it is provided in a further embodiment of the
invention that the cleaned combustion flue gases leaving the
separator are either released into the atmosphere or fed to a
process for removal of the oxides of nitrogen. The cleaned flue
gases can be released into the atmosphere if the combustion process
is carried out in such a way that only small amounts of N.sub.2O,
NO and/or NO.sub.2 are formed. By contrast, it is necessary to
remove the oxides of nitrogen from the cleaned combustion flue
gases if they have been formed in larger amounts during the
combustion process. Removal of the oxides of nitrogen can be
carried out by known processes, which should be installed
downstream of the process according to the invention (for example
nitrogen removal by an SCR process, which operates at relatively
low temperatures and with catalysts).
[0028] Alternatively, it is provided in a further embodiment of the
invention that the pollutant-containing combustion flue gases are
fed to a process for the removal of the oxides of nitrogen, for
example a known SNCR process, which operates at high temperatures
and without catalysts, immediately after leaving the incinerator
and before entering the flue gas channel.
[0029] The object on which the invention is based is furthermore
achieved by the provision of a device for carrying out the process
according to the invention, which consists of a reactor, a
separator and a return line and which is characterized in that the
base of the reactor is designed as a Venturi tube, in that one or
more nozzles for the introduction of the water arc arranged
immediately above the diffuser of the Venturi tube and distributed
over the reactor cross section, in that the separator is designed
as a bag filter containing a low-pressure jet pulse device, in that
the return line is designed as an inclined fluidization channel
which runs into the flue gas channel connected to the reactor and
whose base has a gas-permeable fabric through which the gas serving
for fluidization of the recycled sorbent is introduced into the
fluidization channel, and in that a second inclined fluidization
channel which serves for the feed of the fresh sorbent and whose
base has a gas-permeable fabric through which the gas serving for
fluidization of the fresh sorbent is introduced into the second
fluidization channel runs into the flue gas channel.
[0030] It has been found that a Venturi tube is particularly
suitable for the production of the gas/solid suspension present in
the reactor and favours transport of the fresh and recycled sorbent
from the two fluidization channels into the reactor since the
stream of pollutant-containing combustion flue gas readily sucks
the solids panicles out of the fluidization channels through the
action of the Venturi tube (water jet pump principle) and conveys
them. The Venturi tube has an aperture angle of from 30 to
50.degree. to the diffuser, and the diffuser extends over the cross
section of the reactor. In the case of large reactor cross
sections, a plurality of bundled Venturi tubes can run into the
diffuser. The arrangement of the nozzles for the introduction of
water selected in accordance with the invention effects uniform and
rapid distribution of the water in the lower part of the reactor,
which means that a local temperature decrease in the reactor cannot
occur. The bag filter serving as solids separator facilitates
virtually quantitative removal of all solids particles, i.e. both
the dust particles introduced with the combustion flue gas and the
sorbent particles. It is therefore not necessary to use a plurality
of different solids separators, which has an advantageous effect
both with respect to the investment costs and with respect to the
operating costs. The bag filter is cleaned in accordance with the
invention by means of a low-pressure jet pulse device, which does
not result in operational interruptions since the solids mixture
filtered off does not cake and can easily be detached from the
filter bags. The low-pressure jet pulse device is only operated at
an excess pressure of from 0.6 to 0.8 bar, which prevents the
disadvantageous results of the Joule-Thomson effect (cooling of the
gas on its decompression and partial condensation of the water
present in the gas). The return line in the form of an inclined
fluidization channel ensures problem-free recycling of the
sub-stream of pollutant-loaded sorbent, and the fresh sorbent is
also readily introduced into the flue gas stream via the second
inclined fluidization channel.
[0031] In accordance with the invention, the low-pressure jet pulse
device is operated with pre-warmed air or with a sub-stream of the
cleaned combustion flue gas, these gases serving for cleaning the
bag filter (back-flushing gas).
[0032] It has proven particularly advantageous in accordance with
the invention for the line for the introduction of the gas/solid
suspension into the bag filter to run into the lower part of the
bag filter, where at least one baffle-plate separator, which acts
as inertial separator and which the gas/solid suspension hits, is
arranged. This results in a large part of the solids particles
being removed from the gas/solid suspension even before the actual
filtration operation, which enables the filter area to be reduced.
The baffle-plate separator changes the flow direction of the
gas/solid suspension.
[0033] In a further embodiment of the invention, it is furthermore
provided that the loaded sorbent arising at the base of the bag
filter is discharged via at least one metering roll. The metering
roll has proven particularly successful for solids recycling since
it enables a rapid change in the amount of solid discharged, and a
proportional amount of solid matched to the varying crude-gas
stream can therefore be recycled. In the case of a small design,
the metering roll enables high solids throughput.
[0034] Finally, the object on which the invention is based is
achieved in that the process according to the invention and the
device according to the invention are used for cleaning flue gases
arising in the combustion of refuse, hazardous waste, industrial
waste and sewage sludge. This is because both the process and the
device are so flexible in the way they are operated that their
operation can be matched to the rapidly changing flue gas amounts
and flue gas compositions in waste combustion without the
pre-specified pollutant limits in the cleaned combustion flue gas
being exceeded.
[0035] The subject-matter of the invention is explained in greater
detail below with reference to the drawing and an illustrative
embodiment.
[0036] Domestic and industrial refuse is burned in the incinerator
1, with a pollutant-containing combustion flue gas being formed.
The heat of combustion is removed from the pollutant-containing
combustion flue gas in a heat exchanger, which is not shown in the
drawing, to the extent that the flue gas leaves the incinerator 1
at a temperature of about 230.degree. C. The pollutant-containing
combustion flue gas is discharged from the incinerator 1 via the
flue gas channel 2 and enters the reactor 3.
[0037] The reactor 3 is in the form of a cylindrical tube whose
base is designed as a Venturi tube 24 with a diffuser 25. One or
more nozzles through which the water 5 carried in the line 4 is
introduced into the reactor 3 in finely divided form, are arranged
above the diffuser 25 over the reactor cross section, depending on
the size of the cross section. The amount of water is set in such a
way that the pollutant-containing combustion flue gas is cooled to
a temperature of about 145.degree. C. In the reactor 3, the gas has
a flow velocity of about 5 m/s. The recycled sorbent is fed to the
flue gas channel 2 via line 23, where it is in fluidized form, and
is entrained by the stream of pollutant-containing combustion flue
gas carried in the flue gas channel 2 and is introduced into the
reactor 3 via the Venturi tube 24. The fresh sorbent is fed to the
flue gas channel 2 via line 27, where it is in fluidized form. The
fresh and recycled sorbent are mixed with one another in the flue
gas channel 2. A gas/solid suspension forms, with a mean suspension
density of about 5 kg/Nm.sup.3 in the reactor 3. The gas/solid
suspension has a mean residence time of about 8 seconds in the
reactor 3. During this time, the pollutants present in the
combustion flue gas are bound virtually quantitatively by
absorption and adsorption onto the sorbent. In addition, the
naturally occurring zeolite present in the sorbent takes up some of
the water 5 in the reactor 3 by adsorption. The naturally occurring
zeolite used is preferably a mixture of from 10 to 30% by weight of
mordenite and from 90 to 70% by weight of clinoptilolite. The
carbon-containing substance used is open hearth coke.
[0038] The gas/solid suspension leaves the reactor 3 via line 6 and
is introduced into the lower part of the bag filter 7, where it
firstly hits the baffle-plate separator 8. A large part of the
solids is separated off at the baffle-plate separator 8 and flows
off into the lower part of the bag filter 7. The gas/solid
suspension then hits the bag filter elements 9, the solids
particles being filtered off and accumulating on the bag filter
elements 9, where they form a filter cake. The combustion flue gas
freed from the solids particles and the gaseous pollutants passes
through the bag filter elements 9 and enters the upper part of the
bag filter 7, which is separated from the lower part of the bag
filter 7. The cleaned combustion flue gas leaves the upper
clean-gas part of the bag filter 7 via line 10.
[0039] The low-pressure jet pulse device 11, which serves for
cleaning the bag filter elements 9, is arranged in the upper part
of the bag filter 7. The low-pressure jet pulse device 11 is a line
system whose exit apertures project into the bag filter elements 9.
The low-pressure jet pulse device 11 is supplied via line 12 with
pre-warmed air, which is produced in the compressor 29 and acts, in
the form of a pulse, on the inner surfaces of the bag filter
elements 9 in such a way that the filter cake located on the
outside of the bag filter elements 9 is detached and falls into the
lower part of the bag filter 7. The cleaning process is repeated at
time intervals corresponding to the increase in filter resistance,
enabling the bag filter 7 to be operated continuously. A
temperature of from about 135 to 140.degree. C. prevails in the bag
filter 7, which means that the temperature cannot fall below the
dew point of the combustion flue gas. When the cleaned combustion
flue gas carried in line 10 has a dust content of <3
mg/Nm.sup.3, a sub-stream of this clean gas can be used instead of
the pre-warmed air as back-flushing gas for the low-pressure jet
pulse device 11.
[0040] The separated-off solid, which consists of the dust present
in the combustion flue gas and the pollutant-loaded sorbent,
accumulates at the base of the bag filter 7. Some of the solid is
removed from the bag filter 7 via the star wheel discharge device
14 and line 16 and enters the storage bunker 17. Instead of the
star wheel discharge device 14, it is also possible to use a
metering roll. The discharged solid is sent to landfill as
hazardous waste. A further part of the solid is removed from the
bag filter 7 via the metering roll 15 and enters the fluidization
channel 19 via line 18. The fluidization channel 19 is inclined at
an angle of 10.degree.. A sub-stream of the cleaned combustion flue
gas, which is carried in line 22, is introduced above the base of
the fluidization channel 19 as fluidization gas. This fluidization
gas converts the solids carried in the fluidization channel 19 into
a fluidized, flowable state. This solid enters the flue gas channel
2 via fine 23.
[0041] The fresh sorbent enters the second fluidization channel 26,
which has an inclination angle of 10.degree., from the storage
bunker 20 via line 21. A sub-stream of the cleaned combustion flue
gas, which is carried in line 28, is introduced as fluidization gas
above the base of the second fluidization channel 26. The
fluidization gas converts the fresh sorbent into a fluidized,
flowable state. The fluidized, fresh sorbent passes through line 27
into the flue gas channel 2, where it is mixed with the recycled
sorbent with formation of the gas/solid suspension, which enters
the reactor 3.
[0042] The device consisting of the reactor 3, the bag filter 7 and
the fluidization channels 19 and 26 is operated in such a way that
the circulation rate of the recycled sorbent is between 40 and 50.
This results in the sorbent collected in the storage bunker 17
being substantially loaded with pollutants, since more than 90% of
the Ca(OH).sub.2 have reacted with the acidic, gaseous pollutants
with salt formation. The cleaned combustion flue gas carried in
line 10 is virtually free from dust and gaseous pollutants; it can
subsequently be fed to a nitrogen removal plant, in which the
oxides of nitrogen are removed.
[0043] If the refuse incinerator 1 has to be reduced to
partial-load operation, a sub-stream of the cleaned combustion flue
gas is introduced into the flue gas channel 2 via line 13 in order
to maintain the operating conditions of the circulating fluidized
bed. This enables continuous operation of the circulating fluidized
bed to be continued under optimum fluid-dynamic conditions even in
the case of partial-load operation of the refuse incinerator 1.
[0044] The process shown in the drawing has been employed on an
industrial scale for cleaning the flue gas from a refuse
incinerator over an extended period. During the trials of the
process according to the invention, the operating results shown in
the table were maintained continuously for an operating time of
1200 hours. During this time, no operating faults occurred. In
particular, a sufficiently cleaned flue gas and a solids mixture
which could be handled readily and transported to a landfill site
without problems, especially as it contained no CaS0.sub.3, were
produced during the entire operating time. TABLE-US-00001 1)
Pollutant-containing flue gas from a refuse incinerator Amount 100
000 Nm.sup.3/h Temperature 230.degree. C. Dust 2000-10 000
mg/Nm.sup.3 HF 5-60 mg/Nm.sup.3 HCl 500-2500 mg/Nm.sup.3 S0.sub.2
100-800 mg/Nm.sup.3 S0.sub.3 5-100 mg/Nm.sup.3 Hg 0.1-1 mg/Nm.sup.3
Cd 0.1-1 mg/Nm.sup.3 Dioxins, furans .sup.1) 1-10 ng TE/Nm.sup.3 3)
PCB, PCP, PCA .sup.2) 1-10 .mu.g/Nm.sup.3 Organic carbon .sup.4)
<20 mg/Nm 2) Cleaned flue gas from a refuse incinerator
Temperature 140.degree. C. Dust <5 mg/Nm.sup.3 HF not detectable
HCl <4 mg/Nm.sup.3 SO.sub.2 <15 mg/Nm.sup.3 SO.sub.3 not
detectable Hg <0.02 mg/Nm.sup.3 Cd not detectable Dioxins,
furans .sup.1) <0.1 ng TE/Nm.sup.3 3) PCB, PCP, PCA .sup.2)
<0.1 .mu.g/Nm.sup.3 Organic carbon .sup.4) <0.1 mg/Nm.sup.3
Footnotes: .sup.1) Total content of polyhalogenated dibenzodioxins
and dibenzofurans .sup.2) Total content of polychlorinated
biphenyls (PCBs), polychlorinated phenols (PCPs) and
polychlorinated aromatics (PCAs) .sup.3) TE = toxic equivalent as
defined in 17.BImSchV, annex .sup.4) Includes polycyclic
hydrocarbons
* * * * *