Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF T~IE INVENTION
The present invention relates to a process for the
production of a metal-oxide aerosol and a process for
utilizing the metal-oxide aerosol as an effluent sorbent.
As is well known, acid gas effluents such as SO2,
SO3, NO, NO2, H2S and HCl w~ich are present in off
gases from numerous chemical reactions represent primary
atmospheric pollutants. ~eretofore, reduction of acid
gas effluents in these gaseous streams to
environmentally acceptable levels has proven to be
extremely costly.
For example, one of the commercial processes used
to control SO2 emissions by commercial power plants is
in-furnace limestone injection. In accordance wit'n this
commercial process, limestone is injected into the
commercial furnace where it reacts with sulfur oxides to
form solid calcium sul-fate. The solid calcium sulfate
particles are thereafter separated from the flue gases
by conventional particulate control devices. The major
drawback oE the limestone injection process for
in-furnace SO2 capture is its low calcium
utilization. While the amount of sulfur removed from
the combustion products by in-furnace limestone
injection is in the order of 50%, calcium utilization is
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in the order of only L5 to 25%. As a result, extremely
large quantities of limestone must be injected per unit
mass of sulfur contained in the fuel. This has proven
to be quite costl~.
Naturally, it would be highly desirable to provide
a mechanism for removing effluents Erom industrial
combustion streams in an economic manner.
AccordingLy, it is a principle object of the
present invention to provide a process for the
production of an effluent sorbent aerosol material.
It is a further object of the present invention to
provide a process for the production of an effluent
sorbent-oxide aerosol for removing acid gas effluents
from a gaseous stream which is highly effective.
Further objects and advantages of t'ne present
invention will appear hereinbelow.
SUMMARY OF THE INVENTION
In accordance with the present invention, the
foregoing objects and advantages are readily obtained.
The present invention is drawn to a process for the
production of a metal-oxide aerosol from a corresponding
metal and a process for utilizing the metal-oxide
aerosol as an effluent sorbentO The process of the
present invention comprises vaporizing a metal'of
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desired sorbent in an oxidant free environment and,
preferably, in a gaseous stream o-E an inert gas under
vaporizing temperature conditions at a temperature T~
in a first zone. The vaporized metal gaseous stream is
thereafter passed from the first zone to a second zone
wherein the metal vapor stream is contacted with an
oxidant so as to oxidize the metal vapor thereby forming
an aerosol consisting of solid metal-oxide particles in
a gaseous carrier stream. By controlling various
parameters of the process, the size of the metal-oxide
particles may be controlled so as to produce an
optimized metal oxide aerosol. The metal oxide aerosol
is thereafter fed to a gaseous stream containing acid
gas effluents and is contacted therewith at a
temperature suitable for the reaction between the metal
oxide aerosol and the effluent to be captured so as to
form a solid metal compound of the effluent.
BRIEF ~ESCRIPTION OF THE DRAWINGS
Figure l is a schematic illustration of the process
of the present invention for producing a metal-oxide
aerosol and for removing effluents from a gaseous stream
using the metal-oxide aerosol.
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Figwre 2 is a graph illustrating the eefluent
absorption capabilities of the process of the present
invention empl.oying calcium as the sorbent metal and
sulfur as the effluënt.
Figure 3 is a graph illustrating the effluent
absorption capabili-ties of the process of the present
invention employing magnesium as the sorbent metal and
sulfur as the effluent.
DETAILED DESCRIPTION
The present invention relates to a process for the
production of a metal-oxide aerosol and a process for
utilizing the metal-oxide aerosol so produced as an
effluent sorbent. The process is particularly useful in
removing acid gas effluents such as SO2, SO3, NO,
NO2, H2S and HCl which are by~products from various
chemical reactions from the off gases of the chemical
reactions.
The process of the present invention will be
described in detail with reference to Figure l and -the
schematic illustrations shown therein.
The process of the present invention comprises the
production of a metal-oxide aerosol and its s~bsequent
formation into a solid metal compound of the effluent
being captured. ~hile the process will be described and
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illustrated employing sulfur as the effluent in a
combustion gas stream, the process is useful in
capturing all the acid gas effluents mentioned above
which are by-products of numerous chemical reactions.
With reference to Figure l, the metal-oxide aerosol
is produced by vaporizing a metal of the desired sorbent
under vaporizing temperature conditions in a vaporizing
zone which is an oxidant free environment and thereafter
passing the metal vapor so produced to an oxidizing zone
wherein the metal vapor is contacted with an oxidant so
as to produce a metal oxide aerosol.
In accordance with the process of the present
invention, suitable sorbent metals for use in the
process of the present invention are the metals selected
from the group consisting of alkaline metals, alkaline
earth metals, metals having a valence greater than or
equal to the alkaline earth metals and mixtures
thereof. Particularly suitable metals are magnesium and
calcium with calcium being preferred. It is preferred
in the process of the present invention to vaporize the
desired metal sorbent in the vaporizing zone in a
gaseous stream. It is necessary for the gaseous stream
to be an inert gaseous stream and may be of any of the
following gases: argon, helium, nitrogen, ma,hane,
etc. Preferred gases would be aryon, nitrogen.' Inert
89-399 2~20~
gases are required because t'ne vaporiza-tion step must
take place in a substantial]y oxi-lant free environment
in order -to insure complete vaporization of the metal
sorben-t. The use of an inert gaseous stream is
desirable because it increases the amount of metal vapor
in the gaseous stream, that is metal vapor load, which
has a positive effect on the particle size on the metal
oxide produced. The flow rate of the gaseous stream
should be adjusted so as to produce the desired aerosol
characteristic for a suitable particle size of the metal
oxide aerosol (smaller than a 0.1 mircon). The required
inert gas flow should be adjusted to produce a metal
vapor load of 5 g/Nm to 250 g/Nm and preferably
about 50 to 150 g/Nm .
The inert gas flow rate depends on several factors
such as metal vaporization temperature (Tl), t~vpe of
metal to be vaporized, ~uenching rate used in the
aerosol formation step, desired aerosol primary particle
size, among others. The inert gas flow rate used in the
process oE the present invention is preferably chosen to
provide a suitable metal vapor load that will
subsequently produce an adequate aerosol primary
particle size of about 0.05 microns mean diameter which
is ideally suitable for obtaining a high metal
utilization in the effluent absorption step.
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In accordance wl-th the present invention, it is
necessary for the vaporization of the metal in the
gaseous stream in the first zone of the furnace to be
conducted at controlled temperature conditions Tl at a
pressure Pl. The temperature T1 is the temperature
that is necessary in order to obtain complete metal
vaporization, that is, the melting point of the metal,
and will vary depending on the metal sorbent to be
vaporized. In addition, in order to obtain complete
metal vaporization, the vaporizing zone must be
substantially oxidant free. Further~ore, the
temperature employed for vaporization is preferably
significantly higher than the melting point of the metal
sorbent employed. This is desirous because an increase
in temperature Tl increases vapor load in the stream
fed to the oxidation zone which, as noted above, has a
positive effect on the metal oxide particle size. For
cost reasons, the temperature of vaporization should be
below 2000C. ~n accordance with the present inven-tion
the vaporization temperature is about between o50 to
1200C, preferably 1000 to 1300C for calcium and
between 450 to 1150C, preferably 750 to 950C for
magnesium at atmospheric pressure.
Once the metal vapor is produced in the vaporizing
zone, the metal vapor in the gaseous stream with or
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without the inert gaseous stream is passed from the
first zone of t~e furnace to a second zone. The metal
vapor in the gaseous stream is contacted in the second
zone of the furnace with an oxidant such as oxygen, air,
C2 and the llke so as to oxidiæe the metal vapor to
form the aerosol o~ the present invention which
comprises the metaL-oxide particles in the gaseous
stream of carrier gas. In accordance wit'n the present
invention, the formation of the aerosol stream in the
second zone should be controlled as to produce the
required particle size. It has been found tha-t
submicron particle sized metal oxide particles are
desirable in order to obtain effective sorbent
utilization and correspondingly good effluent capture.
A mean particle size of less than O.l microns is
preferred with particle sizes of less than 0.05 being
ideal.
As noted above, the aerosol stream fed from the
second zone is contacted with an effluen-t carrying
gaseous stream in a third zone under controlled
temperature conditions. The temperature at which the
metal oxide aerosol contacts the effluent carrying
gaseous stream must be within the temperature range
suitable for the reaction between the metal oxide
aerosol and the effluent to be captured so as tb form a
solid metal compound of the effluent. For example, in
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the case of sulfur, the ef~luent absorption step is
carried out under the foLlowing conditions: 650 to
1250~C and preferably between about 950c to 1200C for
calcium oxide aerosol and about between 350 and 850C
and preferably between about 700 and 800C for
magnesium. These temperature ranges represent the
practical and/or thermodynamic limits for the sulEation
of calcium and magnesium oxides.
In accordance with the process of the present
invention, sorbent utilization is greater than or equal
to about 90% and 50% for CaO aerosol and MgO aerosol,
respectively, for a gas stream containing about 2000 ppm
o-f SO2.
The following examples illustrate specific features
of the process of the present invention but in no way
are intended to be limiting.
EXAMPI.E I
With re-ference to Figures 1 and 2, 3.5 g of calcium
metal was fed to a vaporizing zone. Argon gas was fed
to the zone at a gas flow rate of 15.~ ml/sec., a
temperature of 25C and 1 atm. pressure. The zone was
heated to a temperature of 1000C. Under these
conditions a steady production of an aerosol of about 5
mg/min. was measured. The aerosol stream was ~ontacted
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with a stream of 54 l/min. of heatecl dry air at a
temperature of 950C in an oxidation zone so as to
æroduce a metal oxide aerosol o~ CaO. The aerosol was
injected into a gaseous stream having a measured amount
f S2 and was contac-ted with the aerosol stream in an
effluent ahsorption zone. Five separate runs were made
with SO2 concentrations measured in volume parts per
million in the air of 250, 500, 750, 2000 and 3500. An
electrostatic precipitator sampling probe was located at
the end of the absorption zone to capture the aerosol
samples. Aerosol samples were taken in order to
determine the amount of calcium utilized in sulfur
absorption. The results are shown in Figure 2. It is
important -to note that the residence time used in this
experiment, about 0.5 seconds, is well within the time
span spent by the flue gases inside industrial boilers
to drop from 1200 to 950C. Therefore, the results
presented are well within the required industrial time
frame. As can be seen from Figure 2, the ca~cium
utilization is greater than 80~ at SO2 concentrations
of above 1500 ppm which is far superior to that
utilization obtained from in-furnace limestone injection
processes. In addition to the foregoing, the
temperature at which effluent absorption takes place,
25 that is, 950C for calcium and 800C for magnesium, are
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89-399
much lower than those used in in-furnace limestone
injection processes thereby making the process of the
present invention even more attractive. Finally, the
average particle aiameter of calcium oxide was measured
and found to be 0.015 Jum.
EXAMPL.E II
The process of Example II was similar to that set
forth above with regard to Example I but employed
magnesium rather than calcium. The magnesium oxide
aerosol was generated and then fed to the five S02
containing air streams set forth above with regard to
Example I. In order to vaporize magnesium, the
temperature in the first zone was adjusted to 850C.
The results are set forth in Figure 3. It can be seen
that an aerosol containing magnesium oxide particles is
not as effec-tive as an aerosol containing calcium oxide
particles at low SO2 concentrations of S02, however,
the magnesium oxide aerosol is still superior to known
in-furnace limestone injection processes. The particle
size of the magnesium oxide particle obtained in -the
aerosol in accordance wit'n this example was 0.020 ~mO
This invention may be embodied in other forms or
carried out in other ways wit'nout departing from the
spirit or essential characteristics thereof. The
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89-399
present embodiment is therefore to be considered as in
all respects illustrative and not restrictive, the scope
of the invention being indicated by the appended c:Laims,
and all changes which come within the meaning and range
of equivalency are intended to be embraced therein.
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