Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention relates to sorbents and processes
for removing pollutants from gas streams using such
sorbents. More particularly, the sorbents of the present
invention are resistant to physical degradation which
results from recurring adsorption and regeneration. More
specifically, the invention is directed to removing nitrogen
oxides, sulfur oxides and hydrogen sulfide from gas streams.
The nitrogen oxides which are pollutants are nitric
oxide ~NO) and nitgrogen dioxide (NO2 or N2O4). The
relatively inert nitric oxide is often only difficulty
removed, relative to NO2. The lower oxide of nitrogen, N2O
(nitrous oxide), is not considered a pollutant at the levels
usually found in ambient air, or as usually discharged from
effluent sources. Nitrous oxide, however, degrades
(decomposes) in the atmosphere to produce nitric oxide and
thus eventually becomes a polluting component.
Sulfur oxides considered to be pollutants are sulfur
dioxide and sulfur trioxide.
Particularly obnoxious sources of nitrogen and sulfur
oxide pollutants are power plant stack gases, automobile
exhaust. gases, heating plant stack gases, and various
industrial process effluents such as smelting operations and
nitric and sulfuric acid plants.
Power plant emissions represent an especially
formidable source of nitrogen oxides and sulfur oxides, by
virtue of the very large tonnage of these pollutants in such
emissions discharged into the atmosphere annually.
Moreover, because of the low concentration of the pollutants
in such emissions, typically 0.05% or less for nitrogen
oxides and 0.3~ or less for sulfur dioxide, their removal is
difficult because very large volumes of gas must be treated.
Hydrogen sulfide is a pollutant in the effluents of the
following operations: coal gasification, coal liquefaction,
oil shale processing, tar sands processing, petroleum
processing and geothermal energy utiliziation. -
Of the few practical systems which have hitherto been
proposed for the removal of nitrogen oxides from power plant
flue gases, all have certain disadvantages. One such
process entails scrubbing the gas with a slurry of magnesium
hydroxide or carbonate; the slurry is regenerated by
treatment with ammonia. This process, however, produces
by-product ammonium nitrate which is difficult to dispose
of, and also requires cooling and reheating of the flue gas
stream.
Processes for the removal of nitrogen oxides from gases
using various sorbents are discussed in the following: U.S.
Patent 2,684,283 to Ogg, Jr. et al (sorbent: mass of ferric
oxide and sodium oxide); U.S. Patent 3,382,033 to Kitagawa
(sorbent: porous carrier impregnated with FeS04 + H2S04,
FeSO FeS04(NH)4S04, PdS04, KMnO4, 4 2 4
2 + NaO~, Na2MoO4, X2S23~ Na2S23 + NaOH
NaPH04, Na202, As202 + NaOH, CuC12, or ICI3 + NaOH); U.S.
Patent 3,498,743 to Kyllonen (use of a bed of finely divided
solid sodium carbonate); and U.S. Patent 3,864,450 to
Takeyama et al (use of a catalyst consisting essentially of
carbon impregnated with sodium or potassium hydroxide).
Various methods have been proposed for the removal of
sulfur dioxide from power plant flue gases, but all of these
have disadvantages. For example, wet scrubbing systems
based ~n aqueous alkaline materials, such as solutions of
sodium carbonate or sodium sulfite, or slurries of magnesia,
lime or limestone, usually necessitate cooling the flue gas
to about 55C in order to establish a water phase. At these
temperatures the treated gas requires rehea~ing in order to
develop enough bouyancy to obtain an adequate plume rise
from the stack. Moreover, such processes create products
involving a solid waste disposal problem.
Various solid phase processes for the removal of sulfur
dioxide which have hitherto been proposed also have
disadvantages. The use of limestone or dolomite, for
example, to adsorb sulfur dioxide creates a waste disposal
problem because the solid is not regenerated.
_ 3 _ ~ 133L4
. Processes for the removal of sulfur oxides from gases
- using various sorbents are discussed in the following: U.S.
Patent 2,992,884 to Bienstock et al (sorbent: alkali metal
oxide dispersed on a carrier such as alumina or chromia);
U.S. Patent 3,411,865 to Piipers et al ~sorbent: alkali
metal oxide and iron oxide dispersed on a carrier such as
alumina, magnesia or chromia); U.S. Patents 3,492,083 and
3,669,617 to Lowicki et al (sorbent: oxide, hydrated oxide
or hydroxide of aluminium, zinc, iron or manganese and an
oxide or hydroxide of an alkali metal or alkaline earth
metal); U.S. Patent 3,589,863 to Frevel (porous alkali metal
bicarbonate aggregates); U.S. Patent 3,755,535 to Naber
(sorbent: activated alumina or magnesia impregnated on
inert carrier); U.S. Patent, 3,948,809 to Norman et al
(sorbent: bauxite and al~ali metal carbonate); U.S. Patent
3,959,952 to Naber et al (sorbent: alumina carrier
-v impregnated with copper and aluminum, magnesium, titanium or
zirconium) and United Kingdom 1,154,009 (sorbent: vanadium
compound and an alkali metal compound).
U.S. Patent 3,~80,618 to McCrea et al concern the
simultaneous removal of sulfur and nitrogen oxides from
gases using alkalized alumina or alkali metal carbonate or
oxide.
U.S. Patent ~,071,436 to Blanton, Jr. et al describes
the removal of sulfur oxides using reactive alumina.
Alkalized alumina is discussed in the following: D.
Bienstock, J.H. Fields and J.G. Myers, "Process Development
in Removing Sulfur Dioxide from Hot Flue Gases, 1.
Bench-Scale Experimentation, Report of Investigations 5735,
United States Department of the Interior, pp. 8-17; V.S.
Patent 3,551,093 to J.G. Myers et al and U.S. Patent
3,557,025 to Emerson et al. As discussed hereinbelow in
greater detail, alkalized alumina sorbents, heretofore
utilized for flue gas treatment have exhibited severe
degradation of their attrition resistance due to the
chemical processes of adsorption and regeneration.
_ 4 ~
The alkalized alumina sorbent is manufactured by
~precipitating dawsonite (NaAl(OH)2CO3) from a solution of
Al(SO4)3 and Na2CO3 at 90C. The resulting solid is then
,heated to 130C to dry the residue moisture and crushed to a
small size. Since the dawsonite is formed through
precipitation, it has a very tight solid structure with
little room to absorb SO2. Therefore, the chemically bonded
H2O and CO2 have to be removed through calcination at high
temperatures in order to form a porous sorbent.
NaAl(OH)2CO3~s) heat> NaAlO2s) + H2O(g) ~ CO2(g)
The calcinated sorbent (NaAlO2), known as alkalized
alumina, is thereafter useable in a flue gas treatment
process.
Sodium is an integral part of the whole crystal
structure of alkalized alumina. The concentration of sodium
in alkalized alumina is about 25% by weight.
The chemical process of adsorption produces changes in
the sorbent and creates internal forces that cause sorbents
of a type similar to those of the present invention, e.g.,
alkalized alumina sorbent, to attrite (crumble) rapidly.
The sorbents of the present invention do not suffer from
this attrition problem which has been associated with
sorbents of a similar type, such as alkalized alumina.
As adsorption proceeds, the sulfite/sulfate product
layer growth takes place in both directions from the initial
pore boundary, however, the growth into the substrate
material is limited to only a very thin layer for the
impregnated sorbent. As the product layer grows into the
alkalized alumina material itself, it disrupts and distorts
the crystal structure. The product molecule (Na2SO3 and
Na2SO4) volumes are much larger than the unreacted molecules
(Na2O) so the product layer produces a very disturbed and
weakened material. As the growth continues, the product
layer buckles and cracks producing pathways even deeper into
the substrate body,. The effect of this process is to create
physical stresses that dramatically increase sorbent
attrition. The growth proceeds with both,impregnated and
coprecipitated sorbent until all the sodium is consumed or
until all the void space within the pore is occupied. Most
- of the surface area, and consequently the sodium, exists in
the many very small pores of the impregnated sorbent. The
dimension of these pores decreases continuously to si~es
orders of magnitude smaller than the average pore diameter.
In fact, many of the pores are of the size of the product
molecule.
U.S. Patents 4,323,544 and 4,426,365 both assigned to
the assignee of the present invention, concern processes for
the removal of nitrogen oxides using a sorbent comprising
alumina having a surface area of about 20 m2/g and an
alkaline component comprising at least one salt of a Group
IA (alkali metal) or Group IIA (alkaline earth metal).
As pointed out above, a major drawback of heretofore
used sorbents for removal of sulfur oxides and/or nitrogen
oxides is that such sorbents suffer from attrition. The
sorbents of U.S. Patents 4,323,544 4,426,365, which are
quite effective in removing pollutants from waste gas
streams, begin to suffer irreversible attrition at 175C.
Accordingly, it would be quite advantageous to have a
sorbent which is not only effective in removing gaseous
pollutants such as sulfur oxides and nitrogen oxides, but is
also able to withstand high temperatures without undergoing
attrition.
The present invention provides sorbents that do not
unduly degrade (does not unduly attrite) as a result of
chemical use.
$he present invention further provides a method of
removing nitrogen oxides and, optionally, sulfur oxides,
from waste gas streams simultaneously, in a single process.
Moreover, in the present invention it is possible to treat
waste gas streams at temperatures at which the streams still
have adequate buoyancy to obtain good plume rise from the
stack. The sorbents of this invention remove NO2, as well
as the relative inert N0, in an efficient manner.
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The present invention also provides for thç removal of
nitrogen oxides and sulfux oxides from waste gases (which
- process produces elemental nitrogen and elemental sulfur)
without producing solid waste product which would create a
disposal problem. The process of the present invention
utilizes only relatively small quantities of natural gas or
other hydrocarbon fuel.
The present invention also provides for the removal of
hydrogen sulfide.
The present invention concerns a sorbent for removal of
gaseous nitrogen oxides, sulfur oxides and hydrogen sulfide
from waste gas streams containing one or more of gaseous
nitrogen oxides, sulfur oxides and hydrogen sulfide. The
sorbent of the invention includes an alumina substrate,
preferably a gamma-alumina substrate, and an alkali or
alkaline earth compound, i.e., alkali metal (a Group IA
metal) or an alkaline earth metal (a Group IIA metal). The
alkali or alkaline earth metal is contained in an amount
between 50 and 400 ~g per m2 of substrate and preferably
between 100 and 350 ~ug per m2 of substrate, and most
particularly between 150 and 250 ~g per m2 of substrate.
The process of the present invention comprises
contacting a waste gas stream containing oxides of nitrogen
and, optionally, oxides of sulfur with a sorbent comprising
alumina and a alkali or alkaline earth component to sorb at
least part of the nitrogen oxides and sulfur oxides. The
sorbent having the alkali or alkaline component contained
therein in an amount between 50 and 400 ~g per m2 of
substrate and preferably between 100 and 350 ~g per m2 and,
particularly between about 150 and 250 ~g per m2. The
nitrogen- and sulfur-laden sorbent is then regenerated by
heating the sorbent in a reducing atmosphere, e.g., hydrogen
or hydrogen sulfide-containing gas stream, at temperatures
up to about 650C, whereby nitrogen is removed as elemental
fl
-- 7 --
nitrogen and sulfur is removed as elemental sulfur.
Alternatively, regeneration is conducted by heating the
sorbent in an inert atmosphere at temperatures up to about
350C to 650C, whereby the nitrogen oxide is removed as
nitric oxide, and then contacting the hot sorbent with a
reducing agent, whereby the sulfur is removed as elemental
sulfur. The sulfur produced in regeneration may be
partially used to produce hydrogen sulfide, while the
remainder of the sulfur is recovered. The regenerated
sorbent is then used for further removal of oxides of sulfur
and nitrogen.
The present invention includes a process for removing
hydrogen sulfide from a gas stream. This process for
hydrogen sulfide removal involves contacting the gas stream
containing hydrogen sulfide with the above described sorbent
at temperatures ranging from 300C to 650C. Regeneration
of the spent sorbent for such process is conducted using
steam, whereby the hydrogen sulfide is displaced from the
sorbent and removed from the sorbent surface. The excess
steam is subsequently condensed producing a stream of high
hydrogen sulfide concentration for direct use or for further
processing.
~ BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a summary of attrition test results comparing
a prior sorbent with sorbents according to the present
nvention .
Fig. 2 is a plot of surface area (m2/g) versus Na2CO3
content, weight %, demonstrating the effect of sodium
loading on the surface area of gamma alumina substrate.
Fig. 3 is a flow diagram depicting a process of the
present invention for the simultaneous removal of sulfur
oxides and nitrogen oxides from flue gas.
Fig. 4 is a flow diagram depicting a process according
to the present invention for the removal of nitrogen oxides
from flue gas.
- 8 ~
Fig. 5 is a flow diagram depicting a process according
to the present invention for the removal of hydrogen sulfide
from a H2S laden gas stream.
~ ig. 6 shows the results of a mercury porosity test to
determine the pore volume distribution in gamma alumina
substrate.
Fig. 7 is a schematic diagram showing macro and micro
pores of a sorbent according to the present invention.
DETAILED DESCRIP~ION OF THE INVENTION
Alumina in the present invention means a form of
alumina with an extended surface area, usually above about
100 square meters per gram, and often as high as 400 or 500
square meters per gram. For NOx removal alone, surface
areas of 200 m2/g are operative. Many methods are known in
the art whereby such forms of alumina may be prepared. For
exmaple, high surface area alumina may be precipitated from
a sodium aluminate solution or sol by the addition of an
acidic material such as carbon dioxide, mineral acid, or an
acidic salt such as aluminum sulfate. Other methods of
producing high surface area aluminas involve the dehydration
of aluminum hydroxides such as aluminum hydrate (Al(OH)3) or
bauxite~ Activated bauxite is a particularly useful source
of alumina for the present invention because of its low
cost. A further useful source of high-surface-area alumina
for purposes of the present invention are the by-products
from the hydrolysis of aluminum alkoxides such as aluminum
tri-isopropoxide. Such aluminas which are characterized by
extremely high purity in terms of the absence of other
metallic elements, have recently become commercially
available at relatively low cost.
Gamma-alumina is the preferred form of alumina for the
substrate of the sorbent of the present invention.
The alumina substrate of the present invention has
pores therein for receiving the alkali or alkaline earth
component. In an embodiment of the present invention, the
.
- 9 -
substrate is prepared by adhering individual particles of
gamma-alumina to each other. Such particles having pores
therein. Accordingly, such substrate has internal particle
pores of a cPrtain small diameter, dl, and pores between
each particle, i.e., interstices, of a certain small
diameter, d2. The pores dl have an average pore diameter of
between 30 and 400 Angstroms, preferably between 60 and 200
Angstroms, and more particularly, between 80 and 100
Angstroms. The pores d2 have an average particle diameter
of between 80 and 3000 Angstroms, preferably between 100 and
1000 Angstroms, and more particularly, between 200 and 500
Angstroms.
The overall pore volume of the alumina substrate of the
invention is between 0.4 and 0.8 cc/g, preferably between
0.6 and 0.8 cc/g. The surface area of the alumina substrate
is between 100 m2/g and 500 m2/g.
It is also within the present invention io utilize as a
sorbent solely gamma-alumina (having no further component
such as sodium). Such gamma alumina having a pore volume of
20 between 0.4 and 0.8 cc/g and a surface area between 100 m2/g
and 500 m2/g. Such sorbent can be utilized in the processes
described herein in the same manner, i.e., same temperatures
and residence times, as sorbents containing an alkali metal
or alkaline earth metal.
The alkali or alkaline earth component of the sorbent
of the present invention is an alkali metal, i.e., Group IA
metal, namely, Li, Na, K, Rb, Cs or Fr, or an alkaline earth
metal, i.e., Group IIA metal, namely, Be, Mg, Ca, Sr, Ba or
Ra. The preferred components are sodium, potassium and
calcium, with sodium being particularly preferred.
The alkali or alkaline earth component of the sorbent
of the present invention may be advantageously incorporated
as the hydroxide, carbonate, nitrate, acetate, or other
soluble salt of a Group IA metal, or of a Group IIA metal.
It will be understood that mixed salts may b~ used as
i.e., as mixture of salts (1) having the same metal but
different anion portions, or (2) having the same anion but
- 10 - ~ 3~4~
different metal portions, or (3) having different metal and
anion portions, may be used. For instance, a m~xture of
- sodium acetate and carbonate, or a mixture of potassium and
sodium carbonates, or a mixture of potassium acetate and
sodium carbonate may be advantageously employed.
The sorbent according to the present invention can be
prepared by the "dry impregnation" technique. The alkali or
alkaline earth component, e.g., sodium, is loaded onto the
substrate, e.g., gamma-alumina, by spraying the substrate
with a solution of a salt of the alkali or alkaline earth
component, e.g., a sodium carbonate solution. The
impregnated sorbent is subsequently heated to dry the
residue moisture. It has been found by preparing the
sorbent as described above, that the alkali or alkaline
earth component is evenly distributed inside the pores of
the substrate. The chemical reactions involved in the
sorbent preparation as described above are as follows:
Na2CO3( )~A123(S)+H2(l) ~Na2 3(s) 2 3 2 (s)
~NaAlO2(s)+NaAl(OH)2CO3(s)
The dry impregnated sorbent is the final product and
can be readily used in flue gas cleaning processes. In
constrast thereto, alkalized alumina sorbent such as
developed by the U.S. Bureau of Mines requires calcination
at high temperature before use in such processes.
In further contrast to alkalized alumina wherein the
sodium is distributed throughout the entire solid matrix, in
the sorbent of the present invention, the alkali or alkaline
component is coated only on the internal surface, i.e., on
the porous structure of the substrate.
- The sorbent of the present invention can be further
characterized in that thè layer of alkali or alkaline earth
salt component, e.g., Na2CO3, on the suface of the porous
structure does not exceed approximately one molecule
thickness.
Without wishing to be bound by any particular theory of
~operability, it is believed that there is a critical level
o~ alkali or alkaline earth component, e.g., sodium loading
per pore volume, surface area of substrate beyond which the
physical strength of the sorbent particle is severely
weakened, thus leading to attritionO
Fig. 1 shows a series of bar graphs of percentage
weight loss versus number of cycles for three sorbents
subjected to attrition testing. Each bar represents an
incremental percentage weight loss over a ten minute period
in the attrition test.
The sorbents tested had the following physical
characteristics:
Physical Characteristics Sorbent A' Sorbent 8' Sorbent C'
15 ~ Sodium 3.50 3.46 6.55
Compact3 Bulk Density,
lbs./ft 50.0 42.4 46.3
Surface Area, m2/gm 225 222 144
Pore Volume by H2O, cc/gm 0.22 0.78 0.62
Sorbents B' and C' are according to the present
invention; Sorbent A' is a prior art sorbent.
The s~rbents were tested in an Accelerated Air Jet
Attrition ~AAJA) test apparatus for a period of 30 minutes.
The AAJA apparatus was developed by W. R. Grace & Co. AAJA
tests measure the attrition strength of sorbents exposed to
various operating conditions over a number of chemical
cycles.
A typical AAJA test apparatus is described as follows:
`~ A 50 gram sample of sorbent previously screened to +10,
-20 mesh is placed in an inverted one liter Pyrex, wide
mouth (45mm diameter), Erlenmeyer flask. The flask has a
6.2 cm diameter hole centered in its bottom which is covered
by a 40 mesh screen. The mouth of the flask is fitted with
a nylon stopper having a concave bottom roughly 1.1 cm deep.
l;~le ,~larl~
3~
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A 1/4" O.D. (1/8" I.D.~ stainless steel tube is inserted
~through a hole in the stopper to a point even with the
concave bottom and the tube opening is covered by a small
piece of 60 mesh screen. The stopper is held tight to the
flask mouth by means of two rubber "O" rings fitted into
groo~es carved into the side of the stopper.
Air is obtained from a pressurized gas cylinder
equipped with a pressure regulator. The air pàsses through
3/8" flexible, "TEFLON" tubing to a drying tube, a valve, a
10 rotometer (0-6.43 ACFM at 21C, 1 atm.) and into the 1/4"
tube fitted through the stopper inserted into the mouth of
the inverted Erlenmeyer flask. The flask is supported by a
ring clamp attached to a stand and placed within a
laboratory fume hood. The flask is levelled on the stand
prior to the start of the test.
Sor~ent B' was tested at four different adsorption
temperatures ranging from 107C to 290C. In each case, the
sample was subjected to five cycles of
adsorption/regeneration. The first adsorption was performed
in a fluidized bed. The sample was then transferred to a
fixed bed reactor for the remainder of the test. A single
sample of Sorbent B' was also tested over 15 cycles of
operation at a constant adsorption temperature of 120C.
From Fig. 1, it can be seen that the attrition rate of this
2S sorbent appears to be unaffected by adsorption temperature
or by the number of chemical cycles. The percentage weight
loss in the second and third ten minutes of the attrition
test on each sample in this series was essentially the same,
in most cases less than 1 percent of sample weight.
The apparent differences in percentage weight loss
during the first 10 minutes of attrition tests on Sorbent B'
are believed to be attributable to different methods of
sample preparation. All samples in the series, other than
the five cycle test at 290C, were initially prepared by
screening roughly 250 grams of the sorbent in a mechanical
shaker for 20 minutes and then extracting a 150 gram sample
of the 10 x 20 mesh size fraction. For the test at 290C,
'
K
- 13 -
small quantities o the sorbent were carefully screened by
hand to eliminate all particles smaller than 10 x 20 mesh
prior the test. As seen in Fig. 1, the weight loss in the
first ten minutes of the test on this sample was
considerably less than all others in the series. It is
believed that a significant portion of the loss in the first
ten minutes for all other samples in this series is due to
the presence of particles smaller than 20 mesh in the
original sample placed in the fixed bed reactor.
Fig. 1 also shows the results of attrition tests
performed on Sorbent A' and Sorbent B. The attrition rate
of Sorbent A' was considerably higher than Sorbent B' in
tests cn both the fresh and the cycled material. The sample
of Sorbent A' tested after five cycles at 177C yielded a
weight loss of 41.5% after the first ten minutes of the
attrition test, as compared ~ith a corresponding 1 to 2~
loss for all samples of Sorbent B' tested. The major
difference between these two sorbents is in pore volume;
each has roughly the same surface area and sodium loading.
The pore volume of Sorbent B' is more than three times
greater than that of Sorbent A'.
Sorbent C' has twice the sodium loading and roughly 65
percent of the surface area of Sorbent B'. Sorbent C' was
also te~sted after five cycles at 290C.
The sorbents according to the present invention
(Sorbents B' and C'~ exhibited very little percentage weight
loss even at temperatures up to 290C and with 15 cycles
(one absorption - one regeneration per cycle). In contrast
the sorbent with the 0.22 cc/g pore volume (Sorbent A')
experienced a dramatic increase in attrition rate at 177C.
It is believed that reduced pore volume limits the space
available for product layer expansion, thereby resulting in
a significant increase in stress in the sorbent particle and
an increased attrition rate. The run at 177C for Sorbent
A' was aborted after the first ten minutes due to the
excessively large initial weight loss.
1;~ 1L3~
- 14 -
The effect of sodium loading on the surface area of
gamma-alumina substrate is illustrated in Fig. 2. When the
- sodium loading is low, the sodium salt, ire~, Na2CO3, layer
ic approximately one molecule thick. When the loading
increases to the extent that most of the internal surface is
covered, the molecules of Na2CO3 begin to pile up. As a
result, the thickness of the Na2CO3 layer will increase
proportional to the loading. Fig. 2 indicates that when the
Na2CO3 loading is below about 8~, the surface area remains
fairly constant, because it is only partially covered with a
mono-molecular layer of Na2CO3. ~owever, when the Na2CO3
loading increases above 8%, (e.g., 12 to 20%) the thickness
-of the Na2CO3 layer rapidly expands. The expansion of the
Na2CO3 layer results in reduced pore diameters and
consequently reduced surface areas.
A comparison of the typical characteristics of a
sorbent according to the present invention and alkalized
alumina is as follows:
Sorbent According To Alkalized
Present Invention Alumina
_
Total Sodium Loading, wt% 3.5 20-25
Surface Area, m /g 222 47
Pore Volume, cc/g 0.69 0.69
Average Pore Diameter, nm 12.4 58.7
The exact chemical or crystallographic form of the
sorbent is not narrowly critical in the present invention.
It is believed that the alumina in the prepared sorbent is
poorly crystalline and exists as gamma-A12o3 and
gamma-A12O3H2O. When sodium is employed as Na2C03, it
exists in the sorbent as Na2CO3, Na2CO3H2O~ gamma-NaAlO2,
beta-NaAlO2 and NaAl(OH)2CO3. The sorbent may change in
structure after regeneration as compared to its fresh
condition. The alkaline component may be present as the
oxide, hydroxide, carbonate, or aluminate, or mixtures of
15;~
these compounds, when the sorbent is freshly prepared or
after it has been regenerated. ~arious amounts of sulfur or
nitrogen containing salts may also be present, such as
nitrates, nitrite, sulfate, sulfite, or sulfide.
Various other metallic oxides, such as copper, iron,
vanadium, zinc, molybdenum, or rare earth elements, may also
be present in amounts up to about 10 atom percent, based on
the total atoms of aluminum, alkaline component, and other
metal(s).
The waste gas stream containing nitrogen oxides and
sulfur oxides is contacted with the sorbent at temperatures
of 85C to about 200C, and preferably about 90C to 150C.
At higher temperatures the efficiency of nitrogen oxide
removal is decreased, while at lower temperatures the waste
lS gas stream would require reheating or recompression to
develop adequate stack plume.
The sorbent and waste gas may be contacted in a fixed
bed, fluid bed, or moving bed, according to methods which
are known in the art. If the contacting is in a fixed bed
the gas residence time is in the range of 0.1 to about 10
seconds but a wider range is possible in fluid bed
operation.
After the sorbent has become laden with nitrogen and,
optionally, sulfur, preferably to a level corresponding to
greater than about one equivalent of nitrogen plus sulfur
for each five equivalents of alkaline component, it is
regenerated. For this purpose one equivalent of sulfur is
taken as one-half of a gram-atom, one equivalent of nitrogen
is one gram-atom, one equivalent of alkali metal is one
gram-atom, and one equivalent of alkaline earth metal is
one-half of a gram atom. The sorbent is regenerated by
contact with a regenerant gas stream containing at least
0.01 atmosphere partial pressure of reducing gas such as
hydrogen or hydrogen sulfide, at temperatures of about 350C
to about 700C, for a period of time sufficient to recover a
substantial portion of a sorbent's capacity for nitrogen
oxide and sulfur oxide sorption. The minimum time required
14
- 16 -
for regeneration depends strongly on the temperature and
partial pressure of hydrogen ~ulfide in the regenerant gas,
and may vary from a few minutes at 750C to 12 hours or more
at lower temperatures.
The regenerant gas preferably contains carbon dioxide
or water vapor, and, more preferably, contains both carbon
dioxide and water vapor. Alternatively, the sorbent is
treated with carbon dioxide and/or water vapor after
contacting with the hydrogen-sulfide containing regenerant
gas. When carbon dioxide and/or water vapor are used, such
are preferably employed in total amounts corresponding to at
least about one mole of carbon dioxide and/or water vapor
per mole of oxide gas sorbed before regeneration.
A convenient means of obtaining a suitable regenerant
gas containing carbon dioxide is by the catalytic vapor
phase reaction of stream, sulfur, and a hydrocarbon such as
` methane, essentially according to the following reaction:
CH4 + 2H~O + 4S ~ 4H2S + CO2.
For the purpbse of this invention, the use of hydrogen
sulfide in the regenerant gas should be taken to include the
use of other compounds which will essentially form hydrogen
sulfide under the conditions of regeneration, viz. carbon
disulfide or carbon oxysulfide in the presence of steam,
such as by the following reactions:
CS2 + 2H2O ~ ~ CO2 + 2H2S, or
COS + H2 ` C2 + H2S.
During regeneration sulfur forms in the regenerant
stream and is condensed by cooling downstream from the
sorbent. During this process at least part of the hydrogen
sulfide is converted to elemental sulfur. Any unconverted
hydrogen sulfide can be readily recycled after the sulfur
has bee condensed.
- 17 - ~t~3~
After the sorbent has been regenerated, it is cooled to
~the sorption temperature, for example, by contacting with a
cooler water gas stream. The sorbent is then re-used for
removing sulfur oxides and nitrogen oxides.
In an em~odiment of the present invention, a
concentrated stream of nitrogen oxides removed by a sorbent,
such as the sorbent described herein, from waste gas streams
containinq the same can be recycled back to the source of
such nitrogen oxides, e.g., a steam boiler, after
regenerating the sorbent, i.e., driving off the nitrogen
oxides from the sorbent. In this embodiment, the nitrogen
oxide level n the boiler reaches a certain equilibrium,
e.g., 600 ppm, and thus the recycled nitrogen oxides will be
broken down in the steam boiler without increasing the
nitrogen oxides ppm in the stack effluent gas.
Referring to Fig. 3, wherein like numerals indicate
like elements, there is shown a flue gas stream 12
containing both Sx and NOX from a power plant (not shown)
which is passed through a fluid bed adsorber 14 containing
sorbent according to the present invention. Adsorber 14 has
a fluidizing grid 15. The sulfur oxides and nitrogen oxides
are adsorbed on the surface of the sorbent and removed from
the flue gas stream.
The saturated sorbent 16 is subsequently transported to
a staged, fluid bed heater 18 wherein the sorbent
temperature is raised above 532C (lOOO~F) using high
temperature air 20 supplied by air heater 22 to which a
stream of ambient air 24 and a fuel stream 26, e.g., natural
gas, enter. Simultaneously, the sorbed NOX is stripped from
the sorbent and carried away in the hot gas stream which
passes through cyclone 28 and via stream 30 is mixed with
the power plant combustion air stream (not shown).
The hot sorbent is removed from the sorbent heater 18
into a moving bed regenerator 32 via line 34. In the moving
bed regenerator 32, the sorbent is contacted with a
regenerate gas stream 36. The regenerant gas 36 reacts with
the sorbed sulfur oxides to produce elemental sulfur.
.
3~
- 18 -
Off-gas stream 38 containing elemental sulfur is transported
into a sulfur condenser and mist eliminator 45 wherein a
steam stream 42, water stream 44 and elemental sulfur stream
46 are produced. A stream 40 from sulfur condenser and mist
eliminator 45 is returned to regenerator 32.
The regenerated sorbent is transported via stream 48
past valve 50 to a staged, fluid bed sorbent cooler 52,
where it is contacted with atmospheric air supplied via line
54 to reduce its temperature to about 120C (250F). The
heated atmospheric air 56 subsequently is transported to gas
heater 22 where its temperature is increased well above
532C (1000F) for use as the heated medium in fluid bed
heater 18.
Cooled sorbent via line 58 is transported by air in
line 54 to a pneumatic lift line 60 into cyclone separator
62 via stream 64. Cyclone separator 62 separates stream 64
into air 66 and sorbent 68. Sorbent stream 68 enters
absorber 14. The discharge gas from absorber 14 exists via
line 70.
In Fig. 4, a flue gas stream 72 containing NOX from a
combustion facility (not shown) which used a fuel free of
sulfur is passed through a fluidized bed sorber 74
containing sorbent and having a fluidizing grid 76. The NOX
free sorbent is returned to the sorber 74 for reuse via line
108 and the air that was entrained therewith is separated by
cyclone 104 and exists via line 110. The NOX is removed by
sorption on the surface of the sorbent. The NOX free flue
gas ~clean gas stream) is subsequently dischraged to the
atmosphere via line 78.
As the sorbent becomes saturated wit NOX, it is
transported via line 80 to a staged, fluid bed heater 82,
wherein it is contacted with a stream of hot air 84. The
hot air 84 is generated by an air heated 86 wherein fuel 88
for example, natural gas and ambient air 90 enter. The hot
air strips the sorbent NOX from the surface sorbent. The
hot gas stream containing NOX exists heater 82 via line 92
and is directed to a cyclone 94. Air 96 from cyclone 94 is
- / q -
used in the combustion facility (not shown). The NOX free
`sorbent stream 98 from fluid bed heater 82 is then
transported via a pneumatic lift 100 by air supplied by line
102 to cyclone 104 via line 106 according to the present
invention.
In Fig. 5, a gas stream 112 containing hydrogen sulfide
as a contaminant, as from, for example, a oil shale retort
or a coal gasification unit is passed through a fluidized
bed sorber 114 containing sorbent according to the present
invention. The sorbent rests on grid 116 in sorber 114.
Hydrogen sulfide is removed from the gas stream by sorption
on the surface of the sorbent. The cleaned gas stream
leaving the sorber 114 via line 118 is used for its intended
purpose. A hydrogen sulfide saturated sorbent stream 120
from sorber 114 is transported into a fluidized bed
regenerator 122 where it is contacted with steam stream 124.
In regenerator 122, the sorbent rests on grid 126. A steam
and hydrogen sulfide stream 128 from regenerator 122 is then
transported into a water condenser 130, where the excess
steam is removed producing a high concentration hydrogen
sulfide gas stream 132 and a water stream 134. The stripped
sorbent from regenerator 122 is returned to sorber 114 via
line 136 for reuse.
Fig. 6 shows the results of a mercury porosity test to
determine the pore volume distribution in the gamma-alumina
substrate of the sorbent of the present invention. "D" in
Fig. 6 refers to diameter. The distribution is bimodal and
may be separated into a group of micropores having a
probable pore radius ranging from 10 to lO0 Angstroms, and a
group of macropores with radii at least one and possibly two
orders of magnitude larger than the micropore radii.
Fig. 7 is a schematic diagram showing macro and micro
pores in the substrate of the sorbent of the present
invention. The solid circles consist of tightly packed
uniform spherical grains. The macropores correspond to the
space in the interstices between grains, while the
micropores correspond to the pores within each grain. A
- 20 ~
greater portion of the sorbent's surface axea resides in the
micropores, but a greater portion of pore volume is in the
macropores. It is believed that the micropores, with high
surface area, are where most of the chemical reaction takes
place. The size of the micropores, 10 to 100 Angstroms, is
similar to the size of the olecules produced on adsorption
of SO2. As SO2 is adsorbed, the products expand into the
void space available in the pore. A mechanical stress may
be placed on the solid structure under conditions of high
SO2 loading and/or low micropore volume. The stress results
in an unacceptably high rate of sorbent attrition.
The invention will now be described with reference to
the following non-limiting examples:
Example 1
Gamma-alumina prepared by the Davison Division of W. R.
Grace & Co. according to U.S. Patents 4,154,812 and
4,279,779 was treated with a surface coating of Na2CO3
equivalent to 8 weight percent Na2CO3. To accomplish this,
the required amount of Na2CO3 was dissolved in a solution
which was applied to the surface via an incipient wetness
technique. The solid was then heated to drive off moisture,
leaving. a layer of Na2Co3 on the surface. The physical
properties of the resulting sorbent were as follows:
weight percent sodium 3.5
N2 surface area, m2/g 222
Pore volume by H2O, cc/g 0.78
Compacted bulk density, g/cc 0.68
Example 2
A lOOg sample of sorbent prepared according to Example
1 ~only gamma-alumina) was placed in a 2 foot by 2 inch
diameter fixed bed reactor heated by a
temperature-controlled tube furnace. The sorbent was
- 21 ~
contacted at 120C with actual flue gas from a coal-fired
boiler having the following approximate volume composition:
74% N2~ 12% CO2, 9% H2O~ 4% 2~ 0-23~ SO2, 0.05% NO, and
0.0025% NO2. The flue gas flowrate was 10 liters per
minute, measured at 25C and 1 atmosphere pressure.
Samples of the reactor effluent were analyzed for
nitrogen oxides ~NO NO2, expressed as NOX) and for sulfur
dioxide with the following results:
Time On Stream
_ (minutes) % NOX Removal*
100
98
58
Time On Stream
(minutes) % SO2 Removal*
6 100
12 100
18 100
24 95
82
(NO outlet concentration)
* Percent NO removal=100[1 - x ~%:
(NOX inlet concentration)
similarly for percent SO2 removal.
Example 3
A 100g sample of sorbent prepared according to Example
1 (containing weight 8% Na2CO3) was contacted with actual
- 22 -
flue gas, using the same operating conditions and feed
stream composition as in Example 2. The results. were as
- follows:
Time On Stream
(minutes)~ NOX Removal*
100
97
94
94
93
88
105 65
Time On Stream
(minutes)% SO Removal*
--2
100
94
83
120 57
150 42
By comparing the results of Example 2 (only
gamma-alumina) with the results of Example 3 ~gamma-alumina
plus 8% NA2CO3), it is clearly seen that with increasing
time on stream, the sor~ent of Example 3 resulted in better
removal of NOX and SOx.
Example 4
A 100g sample of sorbent prepared according to Example
1 ~containing weight 8% NA2CO3) was heated to 570C and
contacted with a gas containing 30% ~2S and 70~ N2 by
volume. The gas flow rate was 60 liters per hour, measured
at 25C and 1 atmosphere pressure. Samples of the reactor
- 23 -
effluent were continuously analyzed for H2S with the
following results:
Time On Stream
(minutes) % H S Removal*
- -- 2
2.5 100
100
7.5 100
12.5 60
(H S outlet concentration)
* Percent H2S remoal = 100[1 - 2 ]%;
(H2 inlet concentration)
Example 5
The spent sorbent of Example 3 was regenerated by
heating to 550C in N2 gas, and then introducing a gas
stream containing 30% H2S in N~ for 50 minutes at 60 liters
per hour. The sorbent was then contacted with steam and
cooled to 120C. After a second sorption cycle similar to
Example 3, the results were as follows:
20Time On S~ream
(minutes) ~ NOX Removal
100
98
96
2560 95
94
go 88
105 63
~l~6~
- 24 -
Time On Stream
(minutes % Sx Removal
- O 100
96
120 35
Similar results were obtained after this sample had been
adsorbed/regenerated a total of 15 times.
Example 6
10 Two samples of sorbent according to the present
invention were subjected to five cycles of adsorption
regeneration and then tested in an Air Jet Attribution
Apparatus (A~AA) to determine if there was any change in
attrition properties with chemical cycling. The physical
properties of the two sorbents, prior to chemical cycling,
were as follows:
SORBENT Sorbent A Sorbent B
Sodium content, wt. % 4.4 3.5
N~ surface area, m2/g 225 222
20 Pore volume by H2O, cc/g 0.22 0.78
Compacted bulk density, g/cc 0.8 0.68
One type of sorbent according to the present invention
is composed of a layer (or layers) of Na2CO3 applied to the
surface of a gamma-alumina substrate. The sorbents used in
Example 6 were made from different substrates, but the
method of depositing the Na2CO3 on the surface of the
substrate was the same in each case. Electron micrographs
showed the surface of the Sorbent B using a gamma-alumina
substrate supplied by the Davison Di~ision of W.R. Grace &
Co. to consist of similarly shaped, platelike structures
with large areas of common bonding. Examination of the
- 25 - ^l~ ~ 31~
surface of the Sorbent A using a substrate of Reynold's
- gamm-alumina was depicted by electron micrograph to consist
of tiny irregularly shaped particles with small areas of
common bonding.
Sorbents A and B were placed in 2 foot by 2 inch
diameter fixed bed reactor heated by a
temperature-controlled tube furnace. Sorbents A and B were
contacted at a temperature of 177C with actual flue gas
from a coal-fired boiler containing, on average, 74% N2, 12%
CO2, 9% H2O~ 4~ 2~ 0.21% SO2, 0.05~ NO, and 0.0025~ NO2.
The flue gas flowrate in each case was 10.0 liters per
minutes, measured at 25C and 1 atmosphere pressure.
Sorbents A and B were then regenerated by heating the fixed
bed reactor to 500C in N2, and then introducing a stream
containing 30~ H2S and 70~ N2 at 60 liters per hour,
measured at 25C and 1 atmosphere pressure. After
regeneration, Sorbents A and B were contacted with steam,
cooled at 120C, and then reused in the adsorption process.
After five cycles of adsorption, a 50g sample of
Sorbent A and Sorbent B were extracted and tested for
attrition strength in an Air Jet Attrition Apparatus (AJAA).
The results of this test were as follows:
AJAA
Adsorption Weight
No. Of Temperature Length of Test Loss
25 Sorbent Cycles (C) (minutes) (%)
Sorbent A Fresh NA 30 24.3
5 Cycles 177 10 41.5
Sorbent B Fresh NA 30 4.3
5 Cycles 177 30 3.0
~ttrition losses measured in this test for fresh
(unreacted) Sorbent A were over five times greater than
fresh Sorbent B. In addition, the performance of Sorbent A
deteriorated substantially with chemical cycling, while that
of Sorbent B did not. Sorbent loading was similar for
Sorbents A ar.d B, corresponding to 5 grams SO2 adsorbed per
- 26 -
100 grams sorbent. The relatively poor performance of
Sorbent A vis-a-vis Sorbent B is believed to be attributed
- to differences in the surface structure of the two
substrates.
Example 7
Sorbent ~ used in Example 6 was tested to determine if
the attrition rate would be affected by increases in the
temperature of adsorption or number of cycles of adsorption
and regeneration. The methods of adsorption and
regeneration were the same as that described in Examples 3
and 5. The average sorbent load varied with adsorption
temperature, ranging from 5 to 2.5 grams SO2 adsorbed per
100 grams sorbent, at 88C and 300C, respectively. The
sorbent load in a series of 5, 10 and 15 cycle tests at
120VC was constant at 5 grams SO2 adsorbed per 100 grams
sorbent. The results were as follows:
AJAA
Adsorption Weight
No. OfTemperature Length of Test Loss
20 Cycles (C) (minutes) (~)
Fresh NA 30 4.3
5 Cycles 105 30 3.9
5 Cycles 120 30 3.2
5 Cycles 177 30 3.0
25 5 Cycles 285 30 1.7
10 Cycles 120 30 3.8
15 Cycles 120 30 5.2
The above results show that the attrition rate of
Sorbent B is unaffected by increased temperature of
adsorption of increased number of chemical cycles.
- 27 ~ 3~4
Example 8
The AJAA test is commonly used in industry to assess
the relative attrition strengths of different solid
catalysts. To predict attrition losses in large scale
processing, howe~er, the test data must be correlated with
actual measured attrition losses in pilot or commercial
scale systems. Samples of fresh (unreacted) Sorbent B were
submitted to the United States Department of Energy ("DOE")
for testing on their AJAA apparatus. The results were as
follows:
Cumulative
% Weight Loss Cumulative
Time of Test (AJAA as Used % Weight Loss
~minutes) In Example 6)_ (DOE AJAA)
~.6 0.12
3.8 0.27
4.2 0.49
Obviously, the AJAA according to Example 6 subjects the
sample to a more severe tests of attrition strength. Such
differences are not uncommon in AJAA testing where the
results are very sensitive to the design of the test
apparatus, i.e., the air nozzle design (small differences in
the air nozzle design can cause differences in the test
results). Nevertheless, all results reported in Examples 6,
7, and 9 (below) were generated through identical procedures
using the same test apparatus. The data may therefore be
compared to assess the relative attrition characteristics of
the samples tested.
The Department of Energy reports AJAA test losses
similar to those cited above for fresh Sorbent B have been
obtained on samples from batches that exhibited a
steady-state attrition rate of 0.02 to 0.03% of the
circulating solids inventory measured in pilot-scale tests
on a fluidized bed adsorber.
- 28 ~ 3~
Example 9
A sample of Sorbent B was coated with Na2CO3 equivalent
to 21 weight ~. The physical properties of this sorbent,
designated Sorbent C, are as given hereinbelow. The
physical properties of the substrate and Sorbent B are shown
for comparison. Note that Sorbent C has considerably less
surface area and pore volume than either the substrate or
Sorbent B.
Substrate
(gamma-alumina) Sorbent B Sorbent C
~ sodium 0 3.5 9.1
N2 surface area, m2/g 216 222 122
Pore volume by H2O, cc/g 0.78 0.78 0.48
Compacted bulk density,
g/cc 0.62 0.68 0.86
Sorbent C was subjected to five cycles of adsorption in
flue gas at 260C in a procedure identical to that of
Example 3. Sorbent C was regenerated by heating the reactor
to 650C in N2, then introducing a stream containing 30% H2
and 70% N2 at about l liter per minute for 40 minutes. The
results of this test are given hereinbelow. An identical
series of tests using H2S as the reducing gas is shown for
comparison. It should be noted that treatment with H2S does
not fully regenerate the sorbent. Furthermore, as
SO2 loading increases on regeneration with H2, loss on
attrition increases substantially.
Regenerant H2 2
Number of cycles 5 5
- Average SO2 loading*13 6
Time of tests (minutes) 20 30
AJAA weight loss (~)19.4 5.0
* Average So2 Loading c grams So2 adsorbed
100 grams sorbent
- 29 ~ 3~
The unacceptably high rate of sorbent attrition
~exhibited by Sorbent C is believed to be caused by a
combination of low surface area and high S02 loading. When
Sorbent B with twice the surface area and 40% of the S02
loading shown above was subjected to a similar test (see
Example 8), measured attrition loss was 2 to 5 weight
percent in a 30 minute testO Sorbent surface area, pore
volume, and S02 loading are critical factors in obtaining
economically acceptable rates of sorbent attrition.
The present invention may be embodied in other specific
forms without departing from the spirit or essential
attributes thereof, and accordingly, reference should be
made to the appended claims, rather than to the foregoing
specification, as indicating the scope of the invention.
