Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR REMOVING CONTAMINANTS FROM GAS STREAMS
[0001] The present application claims priority from US Provisional Patent
Application 60/897,003 filed January 23, 2007.
BACKGROUND OF THE INVENTION
[0002] The present invention provides for processes for removing
contaminants from gas stream emissions. More particularly, the present
invention provides for removing contaminants such as nitrogen oxides from
gas streams in sulfuric acid production processes.
[0003] Sulfuric acid is used in a wide spectrum of process industries.
Sulfuric acid is believed to be the world's largest chemical produced. Over
past few decades, worldwide, most of the sulfuric acid is produced by a
contact process, which involves generating a sulfur dioxide containing gas
stream from variety of sulfur sources. Examples include burning elemental
sulfur, or process of roasting metal ore or burning H2S arising from
industrial
operations such as hydrodesulfurization of petroleum products or simply
burning waste containing sulfate or sulfuric acid or combusting spent sulfuric
acid all generate SO2 in the gas stream. If the source of sulfur is dirty,
flue
gas is conditioned and oxidized to convert almost all SO2 to SO3 over a V205
catalyst in a multi pass converter. The oxygen required for oxidation is
either
present or supplemented in the form of additional air or oxygen. This SO3
containing gas stream is absorbed in sulfuric acid solution, which results in
the H2SO4 product as a >95 A wt acid or oleum of desired strength.
[0004] Since sulfuric acid is a very low cost product, and reactions are
exothermic, heavy emphasis is put on heat integration and therefore generally
most exothermic heat that is recovered is used within the process for captive
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requirement of energy and any net surplus is exported in the form of steam.
Nitrogen oxides (NOx) are generally formed during the SO2 generation step in
varying quantities based on a variety of factors. When an SO3 containing gas
stream is absorbed into sulfuric acid solution, some of the NOx reacts with a
circulating solution of sulfuric acid forming a complex which is referred in
industry as niter (nitrosyl sulfuric acid) and some of its homologs. Niter in
the
product is an undesirable impurity in many applications and also imparts
some color to the product.
[0005] Some of the NOx which leaves the scrubber passes through much
of the process equipment and is finally exhausted to the environment. It is
often noted that the plume arising from the sulfuric acid production facility
is
correlated with SOx emissions, NOx emissions, niter, types of mist eliminating
devices and various process parameters. Some of these environmental
problems are alleviated in the modern plant by a dual stage absorption
process, choosing effective mist elimination devices followed by a caustic
scrubber. Selective catalytic reduction (SCR), selective non-catalytic
reduction (SNCR) type of processes have been suggested for NOx removal.
However, the problems of NOx emissions, acid plume, deterioration of
product quality due to niter and nitrogen containing compounds in sulfuric
acid still exists at varying levels in the industry. With increasing
environmental
concern and government oversight, the present levels of NO controls are not
adequate.
[0006] Sulfuric acid is a high production volume but a low cost and low
margin chemical. The cost of a plant producing sulfuric acid is relatively
high.
The relationship of the capital cost and the plant capacity is not linear.
Therefore, plants with a larger production capacity achieve much better
scales of economy compared to plants with smaller capacities. Sulfuric acid is
a highly reactive chemical and therefore transporting it over long distance is
not only expensive but also increasingly hazardous. For a smaller plant
operator, it makes good economic sense to boost the capacity of sulfuric acid
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by employing oxygen enrichment in the SO2 generation and or oxidation
stage.
[0007] Oxygen enrichment when done to the SO2 generation stage, not
only increases throughput, but also can improve thermal efficiency thereby
reducing fuel requirements, increasing SO2 concentration in the process gas
stream, and exporting more steam and reducing unit product cost. Replacing
some of the combustion/oxidation air with gaseous oxygen not only improves
capacity of the furnace but also increases SO2 content of the process gas
stream exiting the furnace. Generally downstream equipment such as
catalytic converters, waste heat recovery equipment, fans, etc. operate more
effectively at higher concentration of SO2 and lower process gas flow rates.
Typical sulphuric acid processing equipment has adequate processing
capacity to handle 30 to 40 A additional SO2 load. In the case when SO2 is
arising from a metal roasting furnace, oxygen enrichment not only improves
sulfuric acid throughput but also enhances ore processing capacity.
[0008] With all these positive aspects of oxygen enrichment with respect to
capacity and costing, there is a major down side. Oxygen enrichment
produces higher combustion temperatures in the furnace with greater 02
concentration resulting in higher amount of NOx formation. Without
addressing issues regarding higher environmental emissions and increased
niter content of the product, full potential or benefits of oxygen enrichment
can not be achieved. Figure 1 depicts the difficulty in economically
justifying
smaller size plants due to longer payback period. However with 02
enrichment, this payback period can be significantly reduced.
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SUMMARY OF THE INVENTION
[0009] The present invention provides for a method for removing
contaminants from a waste gas stream in a sulfuric acid production process
comprising the steps:
a) contacting said waste gas stream with ozone;
b) directing said waste gas stream to a particulate scrubber;
c) directing said waste gas stream to a gas dryer; and
d) recovering said waste gas stream.
[0010] The present invention also provides for a method for removing
contaminants from a gas stream from a candle mist eliminator in a sulfuric
acid production process comprising the steps:
a) contacting said gas stream with ozone;
b) directing said gas stream to an environmental scrubber; and
c) recovering said gas stream.
[0011] The present invention further provides for a method for producing
sulfuric acid comprising the steps:
a) recovering a gas stream from a sulfuric acid recovery furnace;
b) directing said gas stream to a particulate scrubber;
c) injecting ozone into said gas stream prior to it entering said
particulate scrubber;
d) directing said scrubbed gas stream to a gas drying tower;
e) directing said dried, scrubbed gas stream to a catalyst bed to
convert sulfur dioxide present in said dried, scrubbed gas stream to sulfur
trioxide;
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directing said gas stream containing sulfur trioxide to a sulfuric
acid absorption tower; and
g) recovering sulfuric acid.
[0012] The invention relates to de-bottle-necking capacity of sulfuric acid
production plant particularly when SO2 gas is derived from sources other than
elemental sulfur. Examples of SO2 gas derived from sulfur sources are:
1.) Metal ore processing furnace where SO2 is produced as a result of the
reaction between Metal sulfide and Oxygen.
MS + 2 024 MO2 + SO2
2.) H2S generated by refinery processes
2 H2S + 3 02 4 2 SO2 + 2 H20
3.) Sulfate containing waste or spent sulfuric acid furnace
SO4 S02 + 02
2 H2SO4 4 2 SO2 + 2 H20 +02
[0013] The spent sulfuric acid stream is generally weak or contaminated
sulfuric acid which often has water and some by-products of the main reaction
and needs to be purged. One of the major sources of spent sulfuric acid is
the alkylation process of refinery gas where C4 (butenes, isobutenes)
containing gas stream is subjected to an alkylation reaction to produce iso-
octane containing petroleum feedstock. Other important processes that use
sulfuric acid are esterification, nitration, oxidation, and sulfonation of
organic
molecules. Some specific examples where the spent sulfuric acid stream is
purged in manufacturing are production of oxalic acid, Nylon 66 (adipic acid),
and dioctyl, diethyl, dimethyl pthalates, etc.
[0014] Although theoretically most of the spent sulfuric acid can be
regenerated, economically it makes sense to recover sulfuric acid from
streams that are generated in large quantities with low water content;
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especially where cheaper means of disposal or treatment are not viable or
practical.
[0015] There are a number of methods to remove, reduce and prevent
NOx formation in sulfuric acid regeneration and production systems. Most of
these methods are either not very effective, are capital intensive,
complicated
and/or require significant amount of energy.
[0016] The present invention provides for a novel approach to removal of
NO at two different locations based on the required needs of the plant
operator. If the primary need is to reduce niter content of the product
sulfuric
acid, the first option is more suitable and if the NO in the flue gas in the
stack
is the concern, the second option may be an optimal choice.
[0017] In both options, ozone is injected into the gas stream to oxidize
insoluble NO to highly soluble oxides of nitrogen.
NO + 03 4 NO2 + 02
NO2 + 03 4 NO3 + 02
NO3 + NO2 = N205
N205 is very soluble compared to NO2 and NO and therefore can be
very easily scrubbed with water.
N205 + H20 4 2 HNO3
[0018] Ozone is generated on site and as needed by using either an up
to 25 psig dry instrument air to produce 2.7% by wt ozone or 93% or higher
purity oxygen to produce 10% by wt or higher concentration ozone.
[0019] In the second option, the environmental scrubber not only removes
NOx, but also is intended to remove unconverted SO2. Therefore, the
scrubbing solution consists of sodium hydroxide or carbonate solution.
Absorption of SO2 in the sodium carbonate or hydroxide solution forms
sodium sulfite and bisufite in situ.
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SO2 + NaOH 4 NaHS03
NaHS03 + NaOH = Na2S03 + H20
The presence of sulfite is essential in Option 2 to deplete excess ozone if
NOx concentration in the treated gas stream is to be reduced below 20 PPM
by volume. Unreacted ozone in the scrubber is depleted in the following
reaction:
Na2S03 + 034 Na2SO4 +02
[0020] In addition to Ozone oxidizing Sulfite, oxygen present in the gas
stream also oxidizes sulfite in the scrubbing solution to sulfate.
2 Na2S03 + 024 Na2SO4
[0021] NO2 in the gas stream also is known to deplete sulfite in the
aqueous stream. Therefore, if ozone emission via the treated gas stream to
the stack is a concern, supplementary sulfite may be added to the
environmental scrubber. Sodium thiosulfite or reduced sulfur may be added in
the environmental scrubber to maintain the required level of sulfite for
depletion of ozone.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Figure 1 is a graph representing plant capacity versus years to
recovery for capital cost.
[0023] Figure 2 is a graph showing the increase in NOx and niter as
oxygen content in the feed increases.
[0024] Figure 3 is a schematic representation of a sulfuric acid
regeneration (SAR) process integrated with the NOx reduction schemes per
the present invention.
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[0025] Figure 4 is a graph representing the increase in revenue versus
capacity increase in oxygen enrichment.
DETAILED DESCRIPTION OF THE INVENTION
[0026] A Sulfuric Acid Regeneration (SAR) plant and acid recovery system
on a metal ore roaster furnace is a slightly modified form of a sulfur burning
sulfuric acid plant. In all three types of plants, a source of sulfur is
converted
to SO2 in the process gas. In the first two types of plants, SO2 containing
streams have particulate matter and need to be washed and dried prior to
oxidizing to S03. The clean and dry SO2 containing stream is passed through
a series of heat exchangers and beds of V205 catalyst to convert it to SO3 at
about 700 C. Typically 3 to 4 catalyst beds called converters are used. The
heat from the process gas stream exiting the final converter bed is used in
heating the gas entering the converter by series of cascaded heat exchanger.
SO3 is absorbed in the sulfuric acid absorber to form oleum or 98% sulfuric
acid and some product is continuously removed.
[0027] In the newer sulfuric acid plants, the flue gas stream from the
absorber is again heated and passed through a V205 bed to oxidize residual
amounts of SO2 and then subjected to another absorber to remove almost all
of sulfur as S03. The exhaust gas from the 2nd absorber is passed through a
candle mist eliminating device to remove H2SO4 mist and finally scrubbed with
caustic soda in an environmental scrubber before exhausting through the
stack. Environmental scrubbers are not always employed and mostly
configured in the train to meet the local regulations governing SO2 emissions.
[0028] The main difference between a traditional sulfur burning sulfuric
acid and an acid recovery or SAR unit is how the sulfur source is converted
to SO2. A SAR unit as shown in Figure 3 uses a furnace to convert spent
sulfuric acid to SO2. Since decomposition of H2SO4 is endothermic and
favored by raising the temperature, natural gas or hydrocarbon feedstock is
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required to raise temperature of the furnace. Generally finely atomized
sulfuric acid is held at 650 C or higher for a sufficient time to obtain 99.5
%
conversion. A supplemental feed stream of H2S can be fed to this furnace for
three primary reasons, 1) H2S has a calorific value 2) it is a good source of
sulfur and 3) there is a monetary benefit in taking care of H2S. The exhaust
from SAR furnace, in addition to SO2, has other contaminants, such as fly
ash, etc. After recovering the heat in the waste heat boiler, the exhaust gas
is
around 230 C to 260 F. This process gas is subjected to an aqueous wash
to remove particulate matter, fly ash and other impurities. The gas is then
dried by scrubbing with sulfuric acid and forwarded to a series of heat
exchangers and converters.
[0029] In a conventional sulfuric acid plant, molten elemental sulfur is
burnt in a furnace to form sulfur dioxide. In contrast to SAR, SO2 produced
from elemental sulfur is relatively free from dust, fly ash and other
contaminants and does not require "washing" or scrubbing. The SO2
containing gas stream from the furnace can be directly led to series of waste
heat boilers, converters and heat exchangers. Therefore sulfur burning
sulfuric acid plants export as much as 1.4 tons of steam per ton of sulfuric
acid produced.
[0030] Some NO is always produced in furnaces where SO2 is generated.
The sulfuric acid decomposition reaction in the SAR process, in particular, is
favored by higher furnace temperature which in turn causes some of the
nitrogen to convert to nitric oxide in the furnace. Some organic nitrogen
content in the spent sulfuric acid converts to nitric oxide in the furnace. To
assure adequate destruction of organic contamination in spent sulfuric acid, a
certain residence time is required at furnace temperature. To increase SAR
unit throughput (up to 30%) the furnace is often supplemented with pure
oxygen stream. All these lead to formation of NO in the furnace.
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[0031] NOx formed consists mainly of NO and NO2. Both nitric oxide (NO)
and nitrogen dioxide (NO2) are sparingly soluble gases. They are not
significantly removed in the particulate scrubbers and pass along with process
gas through converters to the sulfuric acid absorber. Some of the NO reacts
with the sulfuric acid and forms nitrosyl sulfuric acid "niter" and imparts a
violet coloration to the sulfuric acid product. Industrial sulfuric acid users
are
sensitive to concentrations of "nitrogen" or "niter" in the sulfuric acid. The
exhaust from the sulfuric acid absorber still has an equilibrium concentration
of NO, some of which further condenses in the candle mist eliminator as niter.
Finally the remainder of NO exits the sulfuric acid plant with exhaust gas
which is emitted to the atmosphere via the stack.
[0032] In order to increase the production capacity in the existing SAR
furnace or metal ore kiln, the feed air can be supplemented with oxygen.
Figure 2 depicts the effect of 02 enrichment on stack emissions and product
quality. As shown in Figure 2 with an increase in 02 concentration in the
feed, NOx content in the flue gas through stack rapidly increases and so does
the niter content of the product acid.
[0033] Therefore it is very likely that the enrichment that provides up
to 30 % more throughput can cause issues with the environment and product
quality. In addition, although exact reasons are not known but higher niter
content in the product acid is also associated with visible plume at the
stack.
[0034] Many geographical regions in the United States such as the North-
East, Houston-Galveston and California regions fall under ozone non-
attainment area rules and regulations. The control of NOx emissions is a
primary concern for local, state and federal environmental protection
authorities.
[0035] The Clean Air Act of 1990 and the Interstate Air Quality Rules
(IAQR) mandate the USEPA, state and local air-quality management
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authorities to implement tougher standards to improve air quality. Most
existing refineries that generate spent sulfuric acid are on the east coast,
gulf
coast and along the west coast of the United States. The amount of spent
sulfuric acid generated by an individual refinery is not large enough for an
economically viable SAR unit. Therefore a separate unit that can process
spent sulfuric acid streams from more than one refinery is more preferable.
Such a unit becomes a new and independent source and therefore is outside
the bubble permit of any one refinery.
[0036] Sulfuric acid is a very low value commodity and is hazardous cargo
to haul. There is also increasing pressure on refineries to reduce sulfur
content of liquid fuels (diesel). It is therefore of interest to set up a
spent
sulfuric acid plant in the vicinity of refineries where spent and product
sulfuric
acid can be exchanged via pipeline. In addition, SAR units can also
advantageously process additional amounts of H2S generated by these
refineries. However, such a location as mentioned above invites close
scrutiny in environmental permitting and mandates industry to pursue gas
pollution control devices that meet MACT standards.
[0037] Turning to Fig. 3, a furnace A is fed through line 1 with fuel gas.
Spent acid is fed through line 2 and oxygen and hydrogen sulfide are fed
through lines 3 and 4 respectively. Waste gas from the furnace A will leave
through line 13 and enter waste heat boiler B. Steam from the waste heat
boiler B will exit through line 15. The cooler waste gas exits waste heat
boiler B through line 14 and enters air heater C which is fed air through line
5.
Hot air from the air heater C will also be directed through line 1A into line
1 for
the fuel gas being fed to the furnace. In an alternate configuration lines 3
and
lines 4 can be also directed into line 1.
[0038] The waste gas stream will leave air heater C through line 16 and be
directed into the particulate scrubber D. The first option of the present
invention begins here with the introduction of ozone through line 16 such that
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the waste gas stream and ozone are mixed together prior to the waste gas
stream entering the particulate scrubber D. If the waste gas temperature
entering the particulate scrubber D exceeds 135 C, flue gas may be
quenched prior to mixing with ozone. The scrubbed gas stream will exit the
particulate scrubber through line 18. The scrubbing solution is pumped out of
particulate scrubber D through pump 17 and directed into the spray header
assembly through line 17A.
[0039] The wet gas stream in line 18 has air injected into it through line 6
and this stream now enters the gas drying tower E. The solution used in the
gas drying tower E (generally H2SO4) is pumped out through pump 19 and
reenters the tower via liquid distributor through line 19A. Some circulating
H2SO4 from this tower is exchanged with Sulfuric Acid absorption tower J.
This circuit is not depicted in the diagram. Dry gas leaves the gas drying
tower E through line 20 and this gas stream is at about 65 C. This dry gas
stream will enter a series of heat exchangers, in this example F, G and H
respectively through line 20 before entering the converter I. Converter I has
through separate converters present therein containing catalytic materials
which will convert the clean and dry sulfur dioxide gas stream entering the
converter I into sulfur trioxide.
[0040] The sulfur
trioxide generated by the catalytic conversion will exit the
converter I through line 23 and be directed to the first heat exchanger H
where it will be cooled and reenter the converter I at a point lower than when
it was removed. The same holds true with sulfur trioxide withdrawn through
line 22 where it will enter the second heat exchanger G and reenter the
converter I at a point lower than where it was removed. Lastly the converted
sulfur trioxide is withdrawn from the bottom of the converter I through line
21
and will pass through the third heat exchanger before it enters the sulfuric
acid absorption tower J. The heat exchange system may also have the
provision to produce steam. Oxidation of SO2 to SO3 is highly exothermic and
occurs at high temperatures in industrial applications. Normal practice is to
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carry it out in the temperature range in excess of 550 C. There are many
configurations practiced in meaningful recovery and use of heat. The present
invention is applicable to all the configurations. For the sake of brevity we
have described only one of them in this example.
[0041] Sulfuric acid is fed into the sulfuric acid absorption tower through
line 7 and the absorbing solution will exit through pump 24 through line 24A
which feeds the absorbing solution into the liquid distributor at the top of
the
sulfuric acid absorption tower J. Oleum or sulfuric acid as product is
withdrawn through line 8. The gas stream which has much of its sulfuric acid
content removed will leave the absorption tower J through line 25 and enter
the final heat exchanger K before entering the final converter L. The final
converter L will contain catalytic material which will again convert any
residual
sulfur dioxide into sulfur trioxide.
[0042] The gas stream exiting the converter which now contains little sulfur
dioxide is directed through line 26 into the final sulfuric acid absorption
tower M. Sulfuric acid is circulated into this tower. The scrubbing solution
(sulfuric acid) is recovered through pump 9A and fed back to the liquid
distributor through line 9B. Some sulfuric acid (product) is also withdrawn
from 9A via line 10. A sulfuric acid solution is added to absorption Tower M
through line 9. The scrubbed gas will leave the final sulfuric acid absorption
tower through line 28 and will enter the candle mist eliminator N. The candle
mist eliminator N will contain mesh or other gas filtering devices to separate
the gas mixture entering the eliminator which contains sulfur dioxide, some
sulfuric acid, nitrogen oxides, carbon dioxide and oxygen and nitrogen. The
residual sulfuric acid which is separated from the gas mixture will leave the
candle mist eliminator through pump 29 and be directed into feed line 10..
Sometimes, the collection from the mist eliminator is not mixed with the
product acid (in line 10) and separately processed as it may have higher
concentration of niter.
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[0043] The separated gas stream which still contains nitrogen oxides will
leave the candle mist eliminator N through line 30. Ozone is injected into
this
line through line 12 so that it mixes with the gas stream containing the
nitrogen oxides before entering the environmental scrubber 0. The ozone
injection spot in the line 30 is so chosen as to provide adequate residence
time for ozone to mix and oxidize NOx prior to entering the Scrubber 0.
Ozone is injected via nozzle(s) or perforated tube to ensure thorough mixing
within bulk of the gas stream.
[0044] In the scrubber 0 the solution will scrub the nitrogen oxides and
sulfur oxides remaining in the gas mixture. Scrubber solution is drawn from
the environmental scrubber 0 through pump 31 and bled from the system
through scrubber bleed line 11. What solution is not bled off is directed back
into the environmental scrubber 0 into its spray headers through line 31A.
The gas that is now substantially free of nitrogen oxides and sulfur oxides
will
leave the environmental scrubber 0 through line 32 to be directed to the
stack. The pH of the environmental scrubber is maintained by feeding caustic
soda or alkaline carbonates which is not depicted in the figure.
[0045] When S0x, present in the line 30 is low, sulfite generated in-situ
in
the environmental scrubber 0 may not be enough to deplete the unreacted
ozone. A small feed of sodium sulfite, thiosulfate or reduced sulfur may also
be fed to maintain sulfite concentration in the environmental scrubber
necessary to deplete ozone.
[0046] The first inventive option is to treat the process gas exiting the
waste heat boiler downstream of the SAR furnace at a low temperature
(preferably less than 132 C) to selectively oxidize NOx to higher water
soluble oxides such as N205, which reacts with moisture in the flue gas to
form nitric acid. Extensive testing at various facilities has indicated this
technology does not oxidize SO2 to generate any measurable amount of S03.
The wet scrubber to remove fly ash and other particulate matter in the
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process gas stream also removes this oxidized form of NOx, namely N205
and nitric acid. This option produces an SO2 containing process gas that is
substantially free from NOx. Therefore, if this alternative is elected, the
sulfuric acid product will not contain objectionable quantities of niter or
nitrosyl
sulfuric acid. Since an excess of ozone in the process flue gas stream is of
little consequence in SAR processes, NO levels as low as 2 ppm can be
maintained with proper engineering and process controls. If the quencher is
used to reduce temperature of the flue gas, care must be taken to reduce or
minimize water droplets in the ozone oxidation zone.
[0047] The second inventive option is to treat the process flue gas exiting
the candle mist eliminator. The SO2 content of the flue gas varies from 50
to 2000 PPM depending on the plant design. This flue gas also contains
some sulfuric acid mist, between 10 to 1000 ppm of NON, 4-12 % CO2, and
the remainder oxygen and nitrogen. NOx can be selectively oxidized to N205
and nitric acid vapor by mixing ozone into the flue gas. An environmental
scrubber with aqueous solution of caustic soda or alkaline
carbonate/bicarbonate recirculation can reduce substantially both SOx and
NOx at the same time prior to exhausting flue gas to the stack. To neutralize
nitric acid, there will be a slight increase in the consumption of scrubber
alkali.
[0048] Both options depicted above are capable of delivering a wide range
of desired NO removal efficiencies with large variations in process conditions
and irrespective of increase in feed nitrogen oxides or load swings.
Accordingly, the present invention can be used either to treat process or flue
gas for quality/environmental compliance (first option) or solely for
environmental compliance (second option).
[0049] As seen in the above example, there is no new equipment to be
added and existing equipment can be modified or retrofitted with 03/02
injection skid.
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[0050] The cost impact of such a NOx control solution is a small fraction of
the benefit that could be achieved by 02 enrichment.
[0051] Figure 4 depicts a clear representation of incremental revenue due
to capacity enhancement including increased operating costs.
= [0052] While this invention has been described with respect to particular
embodiments thereof, it is apparent that numerous other forms and
modifications of the invention will be obvious to those skilled in the art.
The scope of the claims should not be limited by the preferred embodiments
set forth in the examples, but should be given the broadest interpretation
consistent with the description as a whole.
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