Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
930423CRR Attorney Docket No. 92A227
0312G
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RECOVERY OF CARBON DIOXIDE EROM ~ILD GAS
FIELp OF ~ lNVEN~IQN
This invention relates to the recovery of carbon dio~ide
from gas streams, and more particularly to the recovery of
carbon dioxide from gaseous effluents from kilns.
BACKGROUND OF THE INVENTION ~-
Certain inorganic substances, such as limestone (calcium
carbonate) and dolemite ~magnesium carbonate) contain
significant quantities of carbon dioxide in the form of
carbonates. When these substances are subjected to high
temperatures, such as in lime and cement manufacturing
processes, the carbon dio~ide is liberated from the mineral and
is generally released to the atmosphere. As part of the
manufacturing process the feed material is heated, usually by
directly firing a fuel, such as coal or coke or a liquid or
gaseous hydrocarbon or misture of hydrocarbons into a kiln
containing the mineral substance, thereby raising the
temperature of the mineral substance to as high as 3000 F.
(about 1650 C.) or more, and the carbon dioside formed during
the combustion is released to the atmosphere with the carbon
dioside liberated from the mineral. Since the release of
carbon dioside to the atmosphere contributes to the already
overburdening ~greenhouse efect~, it would be desirable from
an environmental standpoint to reduce the volume of carbon
dioside ~ischarqes, such as those described above, to the
atmosphere.
0312G ~ 2 0 8 ~ 8 Attorney Docket No. 92A227
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Since high purity carbon dioxide is a valuable commodity,
recovery of carbon dio~ide from kiln exhaust gases can provide
an economic benefit, provided that the value of the recovered
carbon dioxide exceeds the recovery cost. In general, it is
economically feasible to recover carbon dioside from waste gas
streams only when the concentration of carbon dio~ide in the
waste gas streams is fairly high, ~uch as in the neighborhood
of 40% by volume or more. The eshaust gas stream e~iting
furnaces such as lime kilns contains up to 40% carbon dio~ide
when the osidant is air, and up to 50% or more by volume carbon
dio~ide when osygen-enriched air or pure osygen i8 used as the
oxidant. Accordingly, it is worthwhile to undertake recovery
of carbon dioxide from these gas streams.
E~haust gases from kilns contain ash and other fine solids
which must be removed from the gases prior to their discharge
from the plant. The fine solids are typically removed from the
exhaust gas by filtration using fine filtering equipment such
as baghouses. The baghouses contain glass fiber bags through
which the eshaust gases pass, which results in the removal of
fine particulates from the gases. However, because the kiln
exhaust gases are very hot, typically as hot as 1000 C. or
more, it is necessary that they be cooled prior to introduction
into the filtering equipment. The hot kiln gases are usually
cooled by injecting cool air into them soon after they exi~ the
kiln. This rapidly cools the waste gases to temperatures at
which they can be further cooled by passage through a water
spray or directly introduced into a baghouse. Unfortuna~ely
however, the introduc~ion of air into the kiln eshaust gas
dilutes the carbon dioside therein, often to ~oncentrations of
20~ by volume or less. Cooling the exhaust gas with water is
possible but is typically more espensive than direct gas
cooling.
~ ince direct injection of a cool gas into a hot gas is an
economical procedure for rapialy cooling the hot gas it would
be desirable to continue to use this cooling method, but
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930423CRR Attorney Docket No. 92A227
'0312G 21208~8
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~ithout decreasing the concentration of carbon dio~ide in the
stream, so that recovery of the carbon dio~ide from the waste
gas stream will remain economically attractive. The present
invention accomplishes this goal by using cool carbon dioside
as the heat exchange medium.
It is known to recover carbon dioside from waste gas
streams from kilns. For e~ample, in Romanian Patent No. 91001,
Barau et al. describes the recovery of carbon dioside from the
eshaust gas of a rotary lime kiln. Steps to prevent
unnecessary dilution of carbon dio~ide produced in lime kilns
are also known. For instance, in Hungarian Patent No. T36257,
Body et al. discloses a lime kiln furnace with a feed throat
containing alternately operated locks.
~MMA~Y OF T~E_I~v~TIQ~
According to a broad embodiment of the invention high
purity carbon dioside i8 produced by calcining a
carbonate-containing mineral by combusting a high carbon
content fuel with an oxidant in the presence of the mineral,
thereby producing a hot carbon dio~ide-rich exhaust gas;
cooling the hot exhaust gas by injecting a cool carbon
dioside-rich gas into the hot e~haust gas; optionally further
cooling the gas by passing it through a water spray or by
indirect heat exchange with a cool fluid; purifying the cooled
eshaust gas to remove particulate impurities therefrom and
separating carbon dio~ide ~rom the other components of the
purified gas stream.
The invention is most advantageously applied to the
production of carbon dio~ide by the calcination of a calcium
carbonate-containing material, such as limestone or dolemite,
in a kiln by burning a high carbon content solid, liquid or
gaseous fuel, such as coal or coke, liquid hydrocarbons, or
natural gas. Preferred fuels are coal and natural gas.
930423CRR 2~ 2 ~8~ Attorney Docket No. 92A227
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Any oxygen-inert gas mi~tures, such as air, oxygen-enriched
air, or oxygen-inert gas mi~tures can be used in the
invention. The preferred oxidant is oxygen-enriched air.
Recovery of the high purity carbon dioxide from the
purified exhaust gas may be accomplished by any procedure
effective for separating carbon dioside from gas ~treams.
Typical carbon dioside separation procedures are adsorption,
absorption, condensation and membrane separation. Preferred
separation procedures are adsorp~ion and a~sorption, and the
most preferred separation technique is adsorption, particularly
pressure swing adsorption, using a zeolite adsorbent such as
13 X ~eolite.
In a preferred embodiment moisture is removed from the
e~haust gas prior to carbon dio~ide recovery. The method of
moisture removal is not critical, but in preferred embodiments
moisture is removed by passing the e~haust gas through an
adsorbent such as silica ~el or activated alumina, or mistures
of these.
According to the most preferred embodiment of the invention
high purity ~arbon dioside is produced by calcining limestone
or dolemite, either alone or in the presence of other minerals,
in a kiln using coal or natural gas as fuel and osygen-enriched
air as osidant; the hot e~haust gas issuing from the ~iln is
cooled by injecting previously cooled and purified eshaust gas
into the hot e~haust gas; the cooled gas is purified to remove
particulate impurities therefrom; and high purity carbon
dioside is separated from the cooled and purifued gas stream by
pressure swing adsorption using a zeolite adsorbent. If
desired, moisture can be removed from the high purity carbon
dio~ide product stream by, for eYample, passing it through a
bed of silica gel or activated alumina adsorbent.
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~ 93~0423CRR Attorney Docket No. 92A227
0312G 21 2 0 ~ .~ 8
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f BRIEF DESCRIPTION OF THE DRAwINGS
Z
f Fig. 1 is a schematic representation of one embodiment of a
Z system for producing high purity carbon dioxide by the process
Z of the invention; and
Z
Fig. 2 is a schematic representation of an alternate
embodiment of a system for producing high purity carbon dioside
by the process of the invention.
DETAILED DE$~RIPTION QF~ THE INVENTION
The invention can be practiced with any lime or cement
manufacturing process in which zn inorganic carbonate-
fnfntaining substance is calcined to remove carbon dio~ide by
heating the substance by combusting a carbon-containing fuel
with an oxidant in the presence of the inorganic substance.
fS'fuch processes include calcination of limestone or dolemite in
the manufacture of lime and cement.
The process of the invention can be more completely
understood from the following description considered witfh the
appended drawings. Valves, lines and equipment not necessary
for an understanding of the invention have been omitted from
the drawings. Like reference characters desiqnate like or
corresponding parts in different ~iews.
~ urning now to Fig. 1, illustrated therein iZfs a system for
processing a carbonate-containing mineral substance by a
preferred embodiment of the invention. The equipment
comprising the ~ystem of Fig. 1 inncludes kiln 2, secondary
eshaust g~fs cooler 4, gas purifier 6 and carbon diofside
separator 8.
Xiln 2 may be any type of furnace in which mineral
æubstances are calcined, fsuch as a rotary or vertical kiln.
Kiln 2 is equipped with carbonate-containing fill port, which
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930423CRR Attorney Docket No. 92A227
0312G
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may be a hopper or the like, fuel line 12, o~idant feed line
14, carbonate-depleted outlet port 16 and e~haust gas line 18.
Exhaust gas line 18 connects to the gas inlet to eshaust
gas cooler 4. Cooler 4 may be any type of cooler used for
cooling hot gases. Preferred coolers are water spray coolers
wherein a spray of cool water washes the hot gas passing
through the cooler. During the waterwash some o the
particulates entrained in the gas stream are washed from the
qas and removed from cooler 4 through a cooling water discharge
line (not illustrated). Cooler 4 may also be a heat exchanger
¦ through which a coolant, such as water passes in indirect
I contact with the hot gases that are to be cooled. Cooler 4 is
equipped with a cooled gas discharge line 20, which is
I connected to gas purifier 6.
s
i Gas purifier 6 may be any of the well-known types of filter
means used to remove particulate matter from kiln eshaust
gases, such as a bag house or an electrostatic precipitator.
Purifier 6 is provided with a clean gas discharge line 22,
which connects to the inlet to carbon dioside separator 8. In
~ the embodiment illustrated in Fig. 1, a cooling gas recycle
3 line connects line 22 and line 18.
Carbon dioside separator 8 may be any unit conventionally
used for the separation of carbon dioside from a gas stream.
Typical carbon ~io~ide separator~ include absorption systems
containing an absorbent which absorbs carbon dioside but does
not appreciably absorb the other gases present in the effluent
stream, such as mono- di- and triethanolamine; adsorbent
systems containing an adsorbent which adsorbs carbon dioside in
preference to the other gas in the e~haust gas ~tream, such as
zeolite and carbon molecul~r sieves, etc.; and condensing
systems, which cool the gas sufficiently to condense carbon
dio~ide but not the other components of the gas strPam. When
separator 8 is an adsorber or a condenser it is preferable to
remove moisture from the gas stream prior to carbon dio~ide
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separation; otherwise the water would be separated from the gas
stream with the carbon dioside. Water removal can be effected
by means of a dryer (not shown) which contains an adsorbent,
such as silica gel or alumina which ~dsorbs moisture in
preference to carbon dioside and the other components of the
gas stream. When carbon dioside separator 8 is an adsorption
system it may comprise a single adsorption bed, but usually
comprises two or more adsorption beds operated in parallel and
out of phase to provide substantially continous operationO
Carbon dio~ide separator 8 is provided with line 26 for
removing the carbon dioxide-rich stream from the system and
line 28 for removing the gas stream containing the other
components of the kiln gas effluent, (predominantly nitrogen
and argon). If nitrogen oxides or sulfur osides are present in
the carbon dioside-rich gas stream from the kiln, they can be
removed, if desired, prior to separation of carbon dioside from
the exhaust gas stream.
The system of Fig. 1 is a preferred embodiment of the
invention in that it permits relatively large guantities of
cooled gas to be recirculated without diluting the kiln
effluent with respect to carbon dioside content. The recycled
qas can be introduced into line 18 by any suitable device, such
as a dispersing nozzle.
In operation, a carbonate-containing mineral is calcined in
kiln 2 by burning a fuel with an osidant in the presence of the
mineral. The spent mineral is removed from the kiln through
port 16 a~d a hot carbon dioside-rich eshaust gas is removed
~ through line 18. Upon esiting kiln 2 the hot exhaust gas is
A mi~ed with cool recycle gas from line 24, and is thereby
sooled. The cooled eshaust gas is passed through cooler 4 (if
such a unit is employed) and then through filter 6, wherein
particulate matter, such as flyash, is removed from the gas
stream. The cleaned gas is nest divided into a recycle stream
and a carbon dioside separator feed stream. A ~ubstantially
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930423CRR 2 12 0 8 ~i 8 Attorney Docket No. 92A227
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pure carbon dio~ide product gas is withdrawn from separator 8
through line 26 and the remaining gas stream is vented to the
atmosphere through line 28.
The system illustrated in Fig. 2 is similar to the system
of Fig. 1, except that recycle line 30 replaces recycle line 24
of the Fig. 1 system. In the system of Fig. 2 a substantially
pure carb~n dio~ide stream is recycled to line 18 to serve as a
coolant for the hot gaseous effluent from kiln 2. This
embodiment is particularly applicable when separator 8 is an
adsorber or a condenser and has the advantage that the carbon
dio~ide from separator 8 has additional cooling capacity
because it is cooler than the gas in line 22 when separator 8
is an adsorber or condenser.
The fuel used to provide the heat for calcination of the
carbonate-containing material can be any solid, liquid or
gaseous fuel. The fuel is preferably a substance that is low
in sulfur and nitrogen to avoid the production of sulfur and
nitrogen oxideæ. Preferred fuels are coal, particularly low
sulfur coal, and methane, because of their low cost, purity
and ready availability.
Preferred osidants include air, osygen-enriched air or
osygen-inert gas mixtures. Osygen-enriched air containing high
concentrations of o~ygen is preferred because of its ready
availa~ility, low cost and low nitrogen content. ~
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Although the invention has been described with reference to -
specific e~amples, variations of these are contemplated. For
e~ample, other ~teps, ~uch as sulfur oside scrubbing, may be
included in the process of the invention. Furthermore,
zupplemental cooling with other gases, such ~ air, is within
the purview of the invention. The scope of the invention is
limited solely by the breadth of the appended claims.
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