Note: Descriptions are shown in the official language in which they were submitted.
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This application relates to processes for reducing
nitrous oxide, NO, sometimes referred to as NOx, in
combustion-zone effluent gas streams.
More particularly, the invention concerns a
selective catalytic reduction (~~SCR~~) process which,
under limited circumstances, employs pretreatment of a
combustion zone effluent gas stream by a selective non-
catalytic reduction (~~SNCR~~) process, i.e.; only when the
NO content oz the SCR effluent exceeds a preselected
maximum value.
In another respect the invention pertains to an
integrated SCR-SCNR NO reduction process which minimizes
the cost of chemical reducing agents.
Carbonaceous fuels can be made to burn more
completely, with reduced emissions of carbon monoxide and
unburned carbon and/or hydrocarbons, when the air/fuel
ratio employed-causes a high flame temperature. When
fossil fuels are used in suspension fired boilers e.g.
large utility boilers, flame temperatures above
2000°F, to 3000°F, are generated. Such high
temperatures, as well as hot spots of higher
_2_
temperatures, cause theproduction of thermal NO, the
temperatures being so high that atomic oxygen and
nitrogen are formed and chemically combine as nitrogen
oxides. -Nitrogen oxides, i.e., NO or NO, ("NO"), can also
be formed as the result of. oxidation of nitrogen-
containing species in the fuel, e.g. those found in
heavy fuel oil, municipal solid waste and coal. NO
derived from nitrogenous 'compounds contained in the. fuel
canform even in circulating fluidized bed boilers which
operate at temperatures that typically range from 1300°F
to 1700°F.
NO is a troublesome pollutant found in the
combustion effluent stream of boilers and other
combustion equipment. Nitrogen oxides contribute to
tropospheric oaohe, a known threat to health, and can
undergo a process known as photochemical smog formation,
through a series of reactions in the presence of sunlight
and hydrocarbons. Moreover, NO is a significant
contributor to acid rain and has been implicated as
contributing to the undesirable warming of the
atmosphere, commonly referred to as the 'greenhouse
effect!' .
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Recently, various-processes for reducing NO in
combustion effluents have been developed. They can
generally be segregated into two categories: selective
and non-selective. The selective processes include SCR
and SNCR processes: ' -
SCR processes involve passing--the combustion zone
effluent across or through a catalyst bed in the presence
of NH3, to achieve NO reductions as high as 50%-95% or
higher. SNCR processes involve introducing NO reducing
agents e.g. ~.. NF-I3 into the effluent at higher
temperatures than SCR processes, to-achieve NO reductions
of up to 50% or greater.
SCR processes for reducing NO are well known and
utilize a variety of catalytic agents. For instance, in
European Patent Application WO 210,292, Eich cltz and
Weiler,disclose the catalytic removal of nitrogen oxides
using activated charcoal or activated coke, as a
catalyst, with the addition of NH3' Kato et al., in U.S.
Patent No. 4,138,469, and Henke in U.S. Patent No.
4,393,0'31, disclose the catalytic reduction of NO with
NH3, using platinum group metals and/or other metals e.cj.
titanium, copper, molybdenum, vanadium, tungsten, or
oxides thereof to achieve the desired Catalytic
reduction.
-4-
Another catalytic reduction process is disclosed by
Canadian patent No. 1,100,292 to , which discloses
the use of a platinum group metal, gold, and/or silver
catalyst deposited on a refractory oxide. Mori et ai ,
in U.S. PatentNo_ 4,107,272 disclose the catalytic
reduction of NO using oxysulfur, sulfate, or sulfite _
compounds of vanadium, chromium, manganese, iron, copper,
and nickel with the addition of NH, gas.
In a mufti-phase catalytic system, Gincrer, in-U. S.
Patent No. 4,268,488, discloses treating a NO containing
effluent to a first catalyse comprising a copper compound
e.g. copper sulfate and a second catalyst comprising
metal combinations e.g. sulfates of vanadium and iron
or tungsten and iron on a carrier, in the presence of NH,.
- SNCR processes were also proposed to remove NO from
combustion gas efflueht streams by injecting NH3 or an NH3
precursor in the presence of oxygen, without using
catalysts. For example, such processes are disclosed in
U.S. Patent No. 3,900,554 and in U.-S. Patents Nos.
4,777,024; 5,057,293; and 4,780,289.
In addition, combination SNCR-SCR processes have
been proposed, e.g. the processes disclosed in U.S.
Patents Nos. 4,978,514; 5,139,'764; 4,286,467; 4,302,431
and 5,233,934.
_5_
Prior combined SNCR-SCR processes sought to avoid
handling NH3 and/or sought to reduce catalyst costs,
focusing on SNCR as the primary NO reduction stage. SCR
waa relegated to a secondary role, i.e., to remove other
- pollutants from the SNCR effluent and to minimize
catalyst consumption.
However, we have determined that the use of SNCR as
the primary stage for NO removal is not cost effective
and results in substantially increased chemical
consumption which more than offsets any savings from
reduced catalyst consumption. Moreover, overfeeding the
reducing chemicals to the SNCR stage to produce excess NH3
in the feed to the SCR stage, as in certain prior
processes, causes poorchemical utilization and severe
NO/NH3 stratification at the inlet to the SCR stage. This
stratification greatly diminishes the effectiveness of
SCR, with poor NO removal and high NH, breakthrough from
the SCR stage.
We have now discovered that the overall economics of
NO removal from combustion zone affluent gas streams can
be significantly improved and the technical limitations
caused by NO/NH3 stratification can be significantly
reduced by an improved integrated SCR/SNCR process. Our
process is an improvement on prior combination SNCR/SCR
processes. These prior processes included the steps of -
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contacting the effluent gas stream in an SNCR zone with
NH; to reduce part of the NO in the stream and then
contacting the SNCR zone effluent in an SCR zone with NH3
and a NO reduction catalyst, to further reduce the NO
content of the gas stream. Our improvement in such prior
combined SCR/SNCR processes comprises the steps of
injecting NH3 into the gas stream upstream of the SCR zone
to provide a mixed gas/ammonia stream, providing
sufficient catalyst in the SCR zone to reduce the NO
content of the SCR zone effluent to a preselected maximum
value at a design total NO throughput and at a design
total gas stream throughput, and injecting NH3 into the
SNCR zone only when the NO content of the SCR effluent
exceeds this preselected maximum value.
As used herein, the term ~~preselected maximum value"
means the desired or target maximum limit of the
concentration of NO in the SCR zone effluent. The
desired maximum concentration ar target maximum
concentration limit is commonly established by reference _
to maximum NO emissions standards set by regulations,
laws or recommendations promulgated by cognizant
governmental authorities, quasi-governmental authorities
or-industry associations. Based on such laws,
regulations and recommendations or other considerations,
the designer and/or operator of combustion equipment
preselects a specific target or desired maximum NO
~1~~4'~"~
concentration limit value which the combustion facility
is not to exceed and that "preselected maximum value" is
then used in operating the facility according to the
claimed process. This preselected maximum value can be
the same as the maximum value set by the law,
regulations, or recommendation or, to provide a safety
margin, the preselected maximum value can be-less than
the maximum value set by the law, regulation, or
recommendation. Further, the preselected maximum value
can be changed from time totime in response to changes
in the laws, regulations or recommendations or in
response to historical experience with the same or
similar combustion systems. Prior to any such change
however, the preselected numerical value.
The term "design total NO throughput" refers to the
total NO flow rate-(e.g., pounds per hour) in the
combustion zone effluent, as determined by material
balance: calculations made by the combustion system
designer. These calculations are based on specific
"design" parameters, e.g. measured NO concentration in
the combustion zone effluent, fuel composition, fuel flow
rates, air flow rates, combustion efficiency, etc.
The term "design total gas stream throughput" means
the total gas stream flow rate (e. g., pounds per hour,
cubic foot per minute, etc.) of the combustion zone
' ~~~44~r
_8_
effluent gas as determined by material balance
calculations made by the combustion system designer,
based on specific "design" parameters, s.g. fuel
composition, fuel flow rates, ai r flow rates, combustion
efficiency, etc.
The amount ofcatalyst provided in the SCR zone is
sufficient, at design total gas stream throughput and at
design NO throughput, to provide the necessary contact
time under the ambient reaction conditions to achieve a
- preselected maximum NO concentration in the SCR effluent
at any condition of gas stream throughput and/or NO
throughput up to these design maximum rates. When total
gas stream throughputs or NO throughputs to the SCR stage
which are above these design maxima, the NO content of
the SCR effluent will rise above the preselected maximum,
because the catalyst quantity is insufficient-to
accommodate throughputs above these maxima..
If the NO content of the SCR effluent exceeds the
preselected maximum value, and only if this occurs, NH, is
injected into the SNCR zone. NH; is injected into the
SNCR zone at the minimum Normalized Stoichiometric Ratio
mole ratio ("NSR") to reduce the NO throughput in the SCR
zone to below the design maximum throughput. (Aa used
herein, "NSR" means the actual NH3:N0 mole ratio in the
SNCR stage divided by the stoichiometric NH,:NO ratio in
21'744?7
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this stage.) This minimizes or entirely prevents NHS
breakthrough from the SNCR zone.
Furthermore, NH3 is injected into the SNCR stage at
the optimum temperature for maximum overall NH;
utilization. As will be explained below, there is an
optimum temperature range for NH3 injection into the SNCR
stage which will maximize the overall NH3 utilization of
the combined SCR/SNCR process. In a preferred embodiment
of the inventipn, hydrogen (H2 is also injected into the
SNCR stage and, optionally at the inlet to the SNCR
stage, to broaden the optimum temperature range for NH3
utilization in this stage.
In a further preferred embodiment of the invention,
rapid mixing baffles are positioned in the gas stream
upstream of the SNCR stages and the SCR stage, to further -
reduce-the possibility of NH3/NO stratification by
turbulent mixing of the components of the SNCR effluent
and turbulent mixing of the NH3 and/or hydrogen injected
into the SNCR effluent upstream of the SCR zone.
In the accompanying drawings:
Fig. 1 is a flow sheet which schematically depicts a typical installation;
Fig. 2 is a graph of maximum NOx reduction in % as abscissa and temperature
in °F as ordinate;
Fig. 3 is a graph of NH3 ufilization factor as abscissa and temperature in
°F
as ordinate;
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Fig. 4 and Fig. 5 are graphs of (NH;)i/(NO,~i mode ratio as abscissa and
(NO)x removal, in percent as ordinate;
Fig. 6 is a dual graph with each of chemical utilization, percent and ammonia
slip, in PPM as abscissa and normalized stoichiometry ratio as ordinate;
Fig. 7 is a graph of NOx reduction in % as abscissa and temperature in
°F as
ordinate;
Fig. 8 is a graph of NH3 slip, in PPM as abscissa and temperature in
°F as
ordinate;
Fig. 9 is a graph of Ei2/NH3 ratio for max.NOx reduction as abscissa and
temperature in °F as ordinate; and
Fig. 10 is a graph of NH3 utilization factor as abscissa and temperature in
°F
as ordinate.
As seen in Fig. 1, a carbonaceous fuel 10 is mixed with air
20
~ ~'~ 4 4'~'~
-1~-
I1 and burned in a combustion zone 12. The fuel 10 is
typically a fossil fuel which is burned to fire electric
utility and industrial steam boilers. The fuel 10 may
include coal, fuel oil or natural gas. In addition to
utility and industrial boilers, the invention has
application in controlling NO emissions from fluidized
bed boilers, fuel fired petrochemical furnaces, cement
kilns, glass melting furnaces, hazardous waste
incinerators and even in methanation plants and landfill
gas removal applications.
The high temperature effluent gases 13 from the
combustion zone are, optionally passed through rapid
mixing baffles 14 to increase the chemical and thermal
homogeneity of the combustion zone effluent 13. At total
gas stream and total NO throughputs which are below the
design throughputof tl2e system, the combustion zone
effluent 13 passes through the SNCR zone-15 without
injection of NH3 16 or hydrogen 17. When the gases have
cooled to an appropriate temperature for SCR processing,
NH3 18 and, optionally, hydrogen 19 are injected,
preferably at or from the edges of the rapid mixing
baffles 21, located just upstream of the SCR zone 22.
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The mixed combination SNCR zone effluent, NH3 and hydrogen
are contacted in the SCR zone 22 with the NO reduction
catalyst 23. The SCR zone effluent 24, with NO content
at or below the design maximum, is then vented to the
atmosphere through stack 25.
As previously noted our process employs SCR as the -
primary stage for NO removal, because SCR provides
essentially 100 percent utilization of injected NH3 and,
thus, offers the highest NO removal with the minimum
chemical consumption. The temperature of the combustion
gas at which contact with the catalyst is effected in the
SCR stage will suitably range from 300.°F to 1200°F.
SNCR is only used when the maximum NO removal
capability of the SCR stage is reached and exceeded. The
combustion gas temperature at which NH3 is injected in the
SNCR stage will range between 1200°F and 2200°F.
Chemical utilization is maximized in the SNCR stage by
minimizing the NSR in this stage and by using hydrogen to
expand the temperature window for the selective reduction
of NO with NH3. H~ injection is used for achieving
maximum NO removal and high NH3 utilization in the SNCR
stage at low NSR. By selecting the Hz/NH3 mole ratio and
the chemicals injection temperature, high NO removal and
high total NH3 utilization can be simultaneously achieved
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in the SNCR stage as depicted in Figs. 2 and 3. H~ also
functions to reduce NH3 breakthrough from the SNCR stage
In the preferred embodiment of this invention, rapid
mixing baffles 14, 20, and 21 are used in the integrated
system prior-to the injection of chemicals, during the
injection of chemicals, and after the injection of
chemicals. The-purpose of using the baffles 14 prior to
the injection of chemicals 16, 17, 18, 19 is to ensure
uniform temperature and gaseous species distribution,
thus maximizing the NO reduction-by the injected
chemicals. Baffles 21 are used during the injection o~
chemicals to enhance chemical mixing with the flue gas.
Baffles, 20, installed downstream of the injection of
chemicals, further reduce temperature and chemical
species'stratification upstream of the SCR stage 22.
In a preferred embodiment of this invention,
chemicals are injected in three locations within the
integrated low NO system. The first and preferred
location is upstream of the SCR system. NH3 only is
injected in this location. The sole purpose for the
injection of NH3 in this location is to effect the
reduction of NO on the surface of the catalyst. The
amount of NH3 injected is increased to maintain a stack NO
value and/or NH3 breakthrough below a certain threshold.
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NH3 or A2/NH3 are Injected in the SNCR-Stage 2 only
if the stack NO threshold is not achieved or stack NH3
threshold is exceeded. It is expected that up to three
discretely separate injection manifolds will be used.
Each of these manifolds may inject NH3 only, H, only, or a
combination of the two chemical-s. NSR would range from 0
to 2, and H2/NH3 mole ratio would range from 0 to 5.
In the preferred embodiment of this invention, the
NH3 injection would be-staged, thus reducing the NSR ratio
for each stage and improving chemical utilization. The
amount and location of Hz injection would depend on
injection temperature and NH3-Ha mole ratio.
Prior processes used SNCR as the primary method for
NO reduction. Certain of these processes increased the
NSR in the SNCR-stage and used NH3 breakthrough from this
stage to feed NH3 to the SCR stage. It was disclosed that
higher NSR in the SNCR stage increases the NO removal in
this stage and the excess NH, was used to effect further
reduction of NO in the SCR stage. However, experience
does not demonstrate the economic attractiveness of this
approach.
The data in Fig. 4 are for low injection temperature
conditions, and data in Fig. 5 are for high injection
temperature conditions. The solid lines show the mole
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ratio and the corresponding NO removal obtained with the
use of a nitrogenous reducing chemical in the SNCR stage.
The dotted lines depict the overall mole ratio required
to achieve a specific overall NO removal with an
integrated SNCR/SCR system.
Thus, increasing the NO removal achieved in the SNCR
stage increases the mole ratio required to achieve a
specific overall integrated system NO removal. For an
overall integrated system NO removal of 90 percent, the
mole ratio is between two to four times the mole ratio
for a stand alone SCR eystem.
The penalty in chemical consumption is less for low
temperature injection conditions because of the reduced
tendency for NH3 oxidation at low temperatures, as shown
in Fig. 4.-- Nevertheless, chemical consumption can still
more than double that used in our integrated SCR/SNCR
system, in which SCR is employed as the primary stage for
NO removal
Typical curvesillustrating NH3 utilization and
breakthrough in an SNCR stage are depicted in Fig. 6.
The data show that increasing the NSR reduces chemical
utilization and increases NH3 breakthrough. For this
reason, in our process we limit the mole ratio in the
SNCR stage to control NH3 breakthrough to below 20 percent
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of the initial NO concentration at the inlet to the SNCR
stage.
Reducing the mole ratio in the SNCR stage not only
improves chemical utilization but to also reduces NH3
concentration as well as NH3/NO concentration
stratification at the discharge of the SNCR stage.
In addition to the NSR, chemical utilization in the
SNCR stage is greatly influenced by reaction temperature
Fig. 7 depicts the change in NO reduction with
temperature as generated by analytical modeling.. As
discussed above, the NO reduction/temperature
relationship has the shape of a bell curve with maximum
NO removal obtained at a specific temperature.-
Increasing or reducing the reaction temperature results
in reducirig the NO removal. The injection of Ha with NH3
reduces the optimum reaction temperature and the
magnitude of this temperature reduction depends upon the
amount of H2 used. An increase in H,/NH3 mole ratio
produces a larger reduction in optimum temperature as
shown in Fig. 7.
NH3 breakthrough from the SNCR stage is also
dependent upon reaction temperature.- The dependence of
NH3 breakthrough on temperature is shown in Fig. 8. Fig.
8 shows that NH3 breakthrough rapidly increases with a
-16-
reduction in temperature. Fig. 8 shows that, for a given
temperature, injection of H2 with NH3 reduces NH;
breakthrough. For example, for a reaction temperature of
1500°F, increasing A2/NH3 mole ratio from 0.125 to 2.5
reduces NH3 breakthrough from-150 ppm to 40 ppm. Ha
injection can thus be used to control NH3 breakthrough as
well as to improve NH3 utilization in-the SNCR stage.
Maximum NO reduction as a function of temperature
can be achieved by changing Ha/NH3 mole ratio. These
data, determined by analytical modeling, are depicted in
Fig. 9. The data show that at temperatures between
1800°F and 2000°F, little or no Ha injection is needed_
As the reaction temperature is reduced, Ha requirements to
maintainoptimum NO reduction is increased.
As discussed above, our invention provides improved
chemical utilization. This is achieved in the SNCR stage
by reducing the NO removal requirement in this stage and,
in turn, the required NSR. It is also achieved by
improving total NH3 utilization, i.e., the sum of NH3 used
for NO reduction and NH, breakthrough. In an integrated
SCR/SNCR system, maximizing total NH3 utilization will
ultimately result in the maximum efficiency of use of
chemicals. NH3 utilization factor as a function of
temperature is presented in Fig. 10. Fig. 10. shows
that, depending on H,/NH3 mole- ratio, an optimum
~~~~4'~'~
-17-
temperature can be defined to achieve 100 percent total
NH3 utilization. Such temperature, for example, is 1800°F
for H2/NH3 mole ratio of 0.0, and 1600°F for H2/NH3 mole
ratio of 0_.125.
However, optimum temperature for maximum total NH3
utilization does not produce the maximum NO removal in
the SNCR stage. As shown in Fig. 7, injecting NH3 at
optimum temperature for maximum total NH3 utilization
results in less than one-third the NO removal achieved by
injecting NH3 at the optimum temperature for maximum NO
removal.
Preferably, Ha is injected to achieve maximum NO
reduction in the SNCR stage at all temperature
conditions. The use of Ha maintains essentially a
constant NO removal over a broad temperature range. The
amount of H, used changes with temperature as shown in
Fig. 9. H, injection is also used to maximize total NH3
utilization. While maintaining maximum NO removal in the
SNCR stage, the use of Ha allows NH, utilization to range
between 70 and 90 percent.
In addition to the chemical factors discussed above,
the NO removal performance of integrated system is
improved in our invention by reducing the stratification
of temperature-and chemical species concentration within
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the flue gas. Flue gas temperature stratification exists
in a utility boiler application for a variety of reasons.
Burners malfunction can result in extended flames and the
creation of hot furnace regions in the convective section
of the furnace.- Heat transfer surface fouling can lead
to non-uniform heat absorption profile resulting in
temperature atratifications. Many boilers are designed
with divided furnaces to bypass portions ofthe flue gas
around the reheat and/or superheat sections of the
boiler, allowing the control of steam temperature with
minimal use of stream attemperation. The use of flue gas
bypass results in temperature stratification at the
discharge of the economizer or at the inlet .to the SCR
system. Temperature--stratification in the SNCR stage
influences NH3 reduction of NO (Fig. 7). Localized high
temperatures due to severe stratification can result in
the oxidation of NH3 to NO. Localized low temperatures
result in high NH3 breakthrough.
2~.'~44'~'~
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Like any other chemical reaction system, gaseous
species stratification in a NO reduction system is
detrimental. As discussed above, NH3 utilization in the
SNCR stage is greatly dependent upon NSR. The
stratification of NO may result in "localized" high NSRa,
leading to poor NH3 utilization: Reaction time of Ha with
NH3 is in the order of milli-seconds-. For the injected Ha
to be fully utilized it is important that other gaseous
species (NO and NH3) are uniformly distributed.
Preferably, stratification is further minimized in
the beat mode-of our system with the use of rapid mixing
baffles, e.g., the °delta wings" developed by Balcke-Durr
of-Germany and others-for a broad range of flue gas
mixing applications. The rapid mixing baffles are
positioned in the flow path ofthe flue gas. The size
and shape of the baffles and their orientation with
respect to the flue gas flow direction are selected to
induce a large downstream recirculation field. The
recirculation enhances gas mixing, thus reducing or
almost totally eliminating temperature or gaseous species
stratification.
In the best mode of our system, the baffles are
installed upstream of the NH3 and Ha injection location to
ensure uniformity of flue gas temperature and gaseous
species distribution. Preferably, the rapid mixing
. ~1~44~7
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baffles are also used for the injection of chemicals.
The injection of chemicals downstream of the baffles thus
provides for mixing the chemicals with the flue gas.
Finally, the baffles are used to mix the flue gas
downstream of the SNCR stage to reduce or substantially
eliminate temperature and chemical species stratification
at the inlet to the SCR stage.
In the preferred practice of our invention, the NH3
is injected into the SNCR zone at multiple discretely
separate injection locations to reduce the NSR for each
injection location and improve chemical utilization. In
the beat mode of our invention there are at least two of
these discrete separate injection locations. Likewise,
hydrogen is injected into the SNCR zone at multiple
discretely separate injection locations. This expands
the temperature range for ammonia NH3 reduction of NO,
improves overall chemical utilization and reduces NH3
breakthrough from the SNCR stage. The NH3 and hydrogen
can be injected separately into the SNCR stage or as
mixtures in each injection location.
Thus, according to the preferred practice of our
invention, the NSR in the SNCR stage-ranges between 0 and
2 and the H2:NH; mole ratio in the SNCR stage ranges
between 0 and 5. Under these conditions NH3 breakthrough
from the SNCR zone is maintained at or below 20 percent
-zl-
of the inlet NO concentration of the gases eritering the
SNCR stage. The NH3:N0 mole ratio of the process is
selected to maintain NH3 in the effluent from the SCR
stage below 20 ppm.
