Note: Descriptions are shown in the official language in which they were submitted.
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Title: Method for removing nitrogen oxides from an oxygen-containing
gas stream
This invention relates to a catalytic conversion of nitrogen oxides to
molecular nitrogen, which nitrogen oxides are formed upon combustion of
hydrocarbons and/or synthesis gas (H~/CO). More particularly, it relates to
the conversion of nitrogen oxides in the presence of oxygen such as these are
formed, for instance, in the operation of units such as combustion engines
under so-called poor or 'lean-burn' conditions, that is: under combustion
conditions where an excess of oxygen is present. The invention further
relates to the conversion of nitrogen oxides which may be formed in
industrial processes, such as nitric acid production.
In the combustion of hydrocarbons with molecular oxygen (for
instance originating from air), oxides of nitrogen may be formed due to the
prevailing temperature and pressure during the combustion process. These
nitrogen oxides, among which NO and N02 (usually denoted by NOx) are
very harmful to the environment. Nitrogen oxides axe held responsible inter
alia for the formation of acid rain and photochemical smog.
Different methods for reducing NOx emission are known and a
number of them are already being applied in practice.
Reducing NOX emission in stoichiometrically running engines is
often accomplished by making use of a so-called three-way catalyst system.
The NOx conversion catalyst in such systems is capable of converting
nitrogen oxides to harmless compounds by reacting them with the reducing
combustion products present in the exhaust gas, such as hydrocarbons and
CO, to form N2.
Generally, the known three-way catalysts which effect the
reduction of nitrogen oxides are incapable of performing this conversion in
the presence of a considerable amount of oxygen.
This is a problem in particular in the removal of nitrogen oxides
from the exhaust gases of the above-mentioned lean-burn engines, such as
lean-burn gas turbines, diesel engines, gas engines and off-gases of
industrial processes, since in such gases, in addition to nitrogen oxides, a
considerable amount of oxygen is present. Moreover, hydrocarbons and/or
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CO are not present, or present in an insufficient amount, which is a bar to
the successful operation of the above-mentioned three-way catalyst systems.
In installations where a considerable amount of oxygen is present
in the exhaust gas, therefore, often an amount of reluctant is added. The
nitrogen oxides can then be converted by the reluctant in the presence of a
suitable catalyst (deNOx catalyst). This method is known as the so-called
Selective Catalytic Reduction (SCR).
Widely used reductants for the SCR reaction are ammonia and
urea. Also, it is known from the literature to use hydrocarbons such as
ethylene, propylene and propane as reluctant (see, for instance, G.P. Ansell
et al., 'Mechanism of the lean NOx reaction over Cu/ZSM-5', Appl. Catal. B,
2 (1993), pp. 81-100). Other possible reductants are, for instance, CO, H2
and CH4, ethanol, hydrocarbons, in particular fuels such as gasoline and
diesel oil.
Used most by far as reluctant in practical SCR applications,
however, are ammonia or urea, or an aqueous urea solution. The use of
these agents entails a number of disadvantages. Dosage is extremely
critical. If too large an amount of ammonia or urea is charged to the deNOx
catalyst (i.e., more than is necessary to convert the nitrogen oxides), this
will lead to so-called ammonia slip. The emission of ammonia from such
installations is actually even more harmful from an environmental point of
view than the emission of NOx. Also, it is possible that through oxidation of
ammonia, this excess dosage leads to the production of NOX, which is the
very opposite of the object contemplated, viz. reducing the NOx emission.
Another disadvantage of the use of ammonia or urea is the necessary
storage thereof and the necessity of periodically supplementing the supply if
this is not produced within battery limits. In particular ammonia is very
dangerous and noxious and the transport thereof involves great safety and
environmental risks. As a consequence of all this, both the investment costs
and the operational costs of this technique are high.
Although the choice of other reducing agents, such as, for instance,
the above-mentioned hydrocarbons, might partly solve these problems,
there still remain disadvantages, such as the necessity of separate transport
and storage. The attendant safety and environmental hazards are often
unacceptable.
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This problem would be solved by using as a reducing agent the
same fuel that is used or is present in the engine, the (gas) turbine or the
combustion process in industrial processes. The hydrocarbons which are
present in, for instance, diesel oil and gasoline, however, prove not to be
sufficiently active to convert NOx under process conditions with an
acceptable rate and selectivity.
It is known to start from an SCR catalyst, with the required
reducing agent being manufactured from an available source of
hydrocarbons, such as methanol, LPG and natural gas, optionally under
addition of H2 which has been obtained from electrolysis or which is stored
in storage tanks.
In DE-A-44 04 617 a technique is described whereby, using an
electrically heated reactor, a hydrocarbon-containing fuel is catalytically
cracked at200 to 700°C and the cracking products are further activated
with air before they are added as reducing agent to the exhaust gas,
whereupon the total gas stream is passed over an SCR catalyst.
According to DE-A-196 00 558, also cracked hydrocarbons are used
as reducing agent. These hydrocarbons have been obtained from diesel.
According to this known method, under SCR conditions, hydrogen is added
to the exhaust gas to reduce NOx over the SCR catalyst where the cracked
hydrocarbons are not sufficiently active. The hydrogen originates from a
storage tank or is made through electrolysis or methanol reforming.
The addition of hydrogen as reducing agent for the reduction of
NOx in the exhaust gases of continuous combustion processes is described
in DE-A-42 30 408. There, too, the hydrogen can be obtained by electrolysis
or from reforming through steam reforming or partial oxidation (PO) of
hydrocarbon containing fuel. To obtain a hydrogen stream with as little
carbon monoxide as possible, two shift reactors are placed behind the
reformer to convert the CO for the most part with water vapor to form
hydrogen and C02.
The use of in situ produced hydrogen as a reducing agent for the
reduction of NOx from exhaust gas of internal combustion engines is
described in EP-A-0 537 968. In this document, the technique of reforming
(steam reforming and partial oxidation) of hydrocarbon containing fuels is
described. According to this publication, the conditions must be chosen such
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that the amount of CO in the hydrogen is so low that the CO concentration
cannot yield any problems regarding emissions. The temperature needed for
reforming is achieved by making use of the heat of the exhaust gases. The
reduction of NOx is carried out over an SCR catalyst. As oxidizing agent for
the partial oxidation, air is mentioned.
According to the present invention, in a reductant-forming step,
hydrocarbons are converted to a reductant stream under suitable
conditions, optionally by contacting them with a reductant forming catalyst.
The hydrocarbons in the product stream of the reductant forming step can
be unreacted hydrocarbons from the feedstock of this step, but may also be
smaller hydrocarbons which have been formed by cracking reactions during
the reductant forming step.
The reductants can be prepared, by the use of a reductant forming
catalyst, from, for instance, residues of hydrocarbons which are contained in
the exhaust gas of the unit in which the combustion takes place. It is also
possible to draw these hydrocarbons from a different source, for instance the
fuel for the combustion unit, which is already available in situ.
Combinations of effluent and such a different source are naturally also
possible. The oxygen needed for this step at least partly originates from the
off-gas to be treated, i.e., the nitrogen oxide- and oxygen-containing gas.
Preferably, substantially all oxygen present in the part of the off-gas used
for the reductant formation is used for forming the reducing gas stream.
The hydrocarbons present in this portion of the off gas can also be converted
to reducing compounds in this step. The hydrocarbons present in the off gas
can then also be converted to reducing compounds in this step.
Through the use of a suitable reductant forming process, optionally
utilizing a catalyst, the reductants necessary for the catalytic reduction of
NOx, in the form of CO and/or H2, optionally supplemented with
hydrocarbons, can be prepared from hydrocarbons in situ, so that the
above-outlined drawbacks in removing NOx under oxygen-rich conditions
can be eliminated at least partly.
In addition to the above-mentioned reductants, CO and/or H2,
optionally supplemented with hydrocarbons, also ammonia (NHa) can be
formed in the presence of hydrogen and nitrogen, under suitable process
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conditions, because of the position of the chemical equilibrium
3H2 + N2 = 2NHs. As mentioned above, ammonia is a good reductant.
The use of a part of the exhaust gases as a source of hydrocarbons
and certainly as an oxidation source for the (catalytic) partial oxidation
5 and/or steam reforming has as a major advantage over the existing
technology that the emission of uncombusted hydrocarbons can be
minimized, the oxygen content in the total exhaust gas stream decreases, so
that the conditions for deNOx-ing improve and the energy present in the
form of heat in the exhaust gases can be used directly for the catalytic
process without making use of extra heat exchanging surface.
Another advantage is that no pure H2 or H2 with small amounts of
carbon monoxide needs to be produced. As a consequence, it is possible to
omit shift reactors and membrane technology.
The reductant forming step can be a partial oxidation step, in
which use can be made of a partial oxidation catalyst. In addition, it is
possible to carry out the partial oxidation without catalyst, for instance by
supplying a controlled amount of energy to a fuel stream, for instance by
means of an electrical discharge. Also, the reductant forming step can be a
steam reforming step in which use is made of a steam reforming catalyst.
A combination of partial oxidation and steam reforming is also possible.
A suitable reductant forming catalyst is, for instance, a partial
oxidation catalyst. In the presence of such a catalyst, the partial oxidation
of
hydrocarbons takes place. As mentioned, according to the invention, the
oxygen needed for this partial oxidation is derived from the effluent of the
combustion unit, optionally supplemented with oxygen obtained from
elsewhere, coming, for instance, from added air. The product stream of the
partial oxidation step is highly suitable for use as reductant stream.
Another possibility of preparing a stream comprising H2 and/or CO,
and optionally hydrocarbons, from a stream comprising hydrocarbons, is the
3o use of a so-called steam reforming. In steam reforming, in addition to
hydrocarbons, water is to be added to the steam reforming step. This water
may originate from the effluent of the combustion engine, from a separate
stock, or from a combination of these two sources. In steam reforming,
hydrocarbons are converted with water (steam) to a mixture of
hydrocarbons, such as methane, and/or HZ and C02. As a consequence of
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chemical equilibria, in addition to these components, CO may also be
present. The mixture thus formed is very suitable to be used as a reductant
stream.
Next, the reductant stream, together with the effluent of the
combustion unit, is contacted with a deNOx catalyst so that the desired
conversion of nitrogen oxides takes place.
The in situ production of the reductant offers a number of
important advantages. Thus, according to the invention, the supply of
reductants can take place continuously, in the case of natural gas, or in any
case simultaneously with the supply of the fuel for the combustion unit, and
it is no longer necessary to have and maintain a separate stock thereof. This
can be practical, for instance, when used in mobile combustion units, such
as trucks or passenger cars, because then no separate storage tanks for the
reducing agent need to be present. This may also be an important
advantage in stationary units. The fact that no ammonia and urea (whether
or not as a urea solution) are used, at least do not need to be drawn from
elsewhere, is also an advantage because the above-outlined disadvantages
associated with the use of these reductants no longer occur.
Another advantage of the invention arises if hydrocarbons from the
effluent of the combustion unit are used as feedstock for the reductant
forming step, as a result of which the amounts of hydrocarbons in the
exhaust gas will then be lowered in that they are used for the reaction with
NOx. Such a reduction is favorable, since the emission of hydrocarbons is
undesirable from an environmental point of view. Another advantage of the
use of the effluent of the combustion unit for the formation of the reducing
gas is the decrease of the amount of hydrocarbons needed for the reductant
formation.
Accordingly, the present invention is characterized by a method for
reducing the content of nitrogen oxides in a nitrogen oxide- and oxygen-
containing gas stream by contacting this gas stream in the presence of a
deNOx catalyst with a reducing gas stream, which comprises CO, H2 and
possibly NHa and which reducing gas stream has been obtained by
converting hydrocarbons with the oxygen from the nitrogen oxide- and
oxygen-containing gas stream, optionally in the presence of a reductant
forming catalyst.
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According to a preferred embodiment, the nitrogen oxide- and
oxygen-containing gas stream is the effluent of a step for combusting fuels,
which comprises the steps of feeding at least a stream a) comprising one or
more fuels and a stream b) containing excess oxygen with respect to the
fuel, which further comprises nitrogen, wherein the effluent, together with
the reducing gas stream c) which further optionally comprises one or more
hydrocarbons, is contacted with the deNOx catalyst, wherein to stream c) no
ammonia and no urea is added and wherein stream c) has been
substantially obtained by contacting streams d) and e) with the reductant
forming catalyst, stream d) comprising one or more hydrocarbons and
stream e) comprising oxygen and water.
The combustion unit is suitable for generating heat, and optionally
energy. The combustion unit can work on the basis of a flame, but the
combustion in the combustion unit can also proceed by catalytic route.
Preferably, the combustion unit is a gas engine, gas turbine, diesel engine
or gasoline engine.
A stream c) which is substantially free from externally added
ammonia and urea is understood to mean that to this stream, in accordance
with the invention, no reductants of this type need to be added. Still,
ammonia and possibly compounds derived therefrom, such as urea, can be
present as a result of the above-mentioned equilibrium reaction of N2
and H2.
According to the present invention, the NOX is reduced with a
mixture of H2, CO and possibly NHs. In addition, hydrocarbons which have
not been (entirely) converted can be present in the reducing gas stream.
These hydrocarbons also work as reductants. The reducing gas mixture is
obtained by converting the hydrocarbons from a part of the exhaust gas
stream with optionally extra added hydrocarbons, by means of (catalytic)
partial oxidation, steam reforming or a combination of both technologies by
means of the oxygen and water vapor present in the same part of the
exhaust gas stream and optionally externally added air and/or water vapor.
Optionally, extra reducing agent such as hydrogen can be added to the
reducing gas mixture obtained.
Particularly preferred is the method and the apparatus suitable
therefor according to the invention, wherein the engine comprises a heat
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exchanger, such that at least a portion of the heat released upon the
combustion can be usefully employed, for instance for heating, as of
greenhouses or other spaces. Such units, in which at the same time both
heat and energy are generated, the energy being typically in the form of
electrical power, are also referred to as combined heat and power units or
total energy plants.
The invention can also be applied in various means of transport
such as vessels, airplanes, trucks and passenger cars and trains provided
with a hydrocarbon-fired locomotive engine.
The method according to the invention and the apparatuses
suitable therefor is suitable in particular for operating combustion units
under so-called lean-burn conditions, i.e., conditions where the ratio
between the streams a) and b) is selected such that the amount of oxygen is
at least the amount needed for the complete combustion of the fuels in
stream a).~These are the conditions under which oxygen is present in the
effluent of the combustion unit and wherein, with advantage, the deNOx
reaction with the effluent of the catalytic partial oxidation step can be
carried out.
Suitable fuels for the feedstock a) are hydrocarbons and/or
synthesis gas (CO/H2 mixture).
Preferably, the hydrocarbons for the reductant forming step are at
least partly drawn from the same source as the fuel for the combustion unit.
In this case, stream a) and stream d) comprise the same components.
In order to reduce the content of hydrocarbons in the emission of
the apparatus for the method according to the invention, the hydrocarbons
that are present in the effluent of the combustion unit are at least partly
used as feedstock for the reductant forming step, which may or may not be
supplemented with a hydrocarbon stream drawn from elsewhere.
In order to keep the oxygen content low when contacting the
streams with the deNOx catalyst, the effluent of the combustion unit is used
as oxygen source, which may or may not be supplemented with an oxygen
stream drawn from elsewhere.
As fuel for the combustion unit and/or as feedstock for the
reductant forming step, besides synthesis gas, in principle all hydrocarbons
suitable therefor can be used. It is practically preferred, however, when the
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hydrocarbons from stream a) and d) are selected independently from the
group consisting of natural gas (which comprises substantially methane),
methane, diesel oil, gasoline, fuel oil, methanol, ethanol, naphtha, kerosene,
ethane, propane, butane, LPG, derivatives and mixtures thereof.
The catalyst for converting nitrogen oxides can be selected from the
group of catalysts which catalyze the reduction of NOx, such as the
conventional catalysts for removal of NOx. Preferably, these are selected
from the group comprising zeolites, metal-exchanged zeolites, such as Co-,
Cu- and/or Ce-exchanged zeolites, Pt, Rh and/or Ir catalyst, optionally
provided on a support such as a washcoat which may further comprise Ba,
La, Y, Sr, Pr, Ce, Si, Ti, A1 and/or Zr.
The catalyst for the partial oxidation of hydrocarbons can be
selected from the group consisting of Pt, Rh, Ru, Pd, Co and Ni, if desired
provided on suitable supports such as Al2Oa, Si02, Ti02, Zr02,
silica/alurnina-zeolites and mixtures thereof, optionally stabilized with, for
instance, Si, La, Ba or Y and mixtures thereof.
The steam reforming catalyst which is capable of converting a
mixture of hydrocarbons and water to a mixture of H2, CO, C02 and/or
hydrocarbons can be any conventional steam reforming catalyst, which may
or may not be supported, according to conventional techniques, as known to
the skilled person. Preferably, the steam reforming catalyst is a supported
catalyst comprising Ni, Rh and/or Pt.
For operating an apparatus according to the invention, factors such
as hydrocarbon/oxygen ratio, temperature, pressure, residence time and/or
amount of catalyst for the partial oxidation of hydrocarbons should be
selected such that no complete oxidation takes place. The molar
hydrocarbon/oxygen ratio is expressed in ~,PO, such that for stoichiometric
ratios (i.e., precisely suffcient oxygen to effect the complete combustion of
the fuel), ~,PO = 1. According to the invention, ~,PO < 1. Preferably, 0.2 <
~,PO <
0.7. ~,PO is controllable by adjusting the air/fuel amount and is dependent on
the hydrocarbons used.
The temperature for the reductant forming step is generally
between 250 and 1100°C. The residence time for the reductant forming
step
is generally between 200 en 150,000 h-1. Although the pressure will also
have an influence, it is generally dictated by the other process conditions.
In
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general, the pressure will be atmospheric or slightly higher and not higher
than 50 bars.
The use of an SCR catalyst as a method of converting NOx with a
reducing agent to N2 often has the limitation that sufficient conversion of
5 NOX can be achieved in a limited temperature frame. Thus, in DE-A-
196 00 558, as a practical example, a conversion curve of NOx as a function
of the temperature is given, in which a conversion of 40% is achieved. This
is typical of SCR systems that do not work with NHs or urea as reducing
agent. Selecting the process conditions of the reductant forming step such
10 that also NHa is generated is therefore advantageous because then a higher
NOX conversion can be achieved.
When a high conversion of NOx is required, as, for instance, in the
application of exhaust gases as fertilizing gas in horticulture, it is better
for
the method according to the present invention to be carried out with an NOx
storage system, instead of an SCR system. If the deNOx step is carried out
with such an NOX storage system (also referred to as NOx Storage and
Reduction Catalyst, NSR), such as described, for instance, in N. Takhashi
et al., Environmental Catalysis, p. 45, (1995), a very high conversion in
NOx can be obtained. According to this method, nitrogen oxides are
absorbed from the nitrogen oxide- and oxygen-containing gas stream to a
suitable absorbent and subsequently, for instance by switching, the
reducing gas stream is contacted with the absorbent. The deNOx step is
thus operated discontinuously. A very effective NOx removal can thus be
obtained.
With this NOx storage system, it is possible to absorb NOx in an
oxidizing medium (~, < 1), and in a reducing medium (~, < 1) to convert both
the NOx in the exhaust gas and the adsorbed NOx to nitrogen. The catalyst
in the NOx storage system can very suitably consist of platinum provided on
a barium-containing and/or zeolite-comprising alumina washcoat. The
barium present can react with NOx to barium nitrate. This nitrate salt
decomposes in a reducing medium to barium and N2.
The NOx storage system can be operated according to the invention
by passing the exhaust gases through the NOx storage system until the
system is saturated with NOx. Thereafter, regeneration can be done with a
reducing agent which has been obtained as described above. Optionally, this
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reducing agent can be supplemented with a reduction agent drawn from
elsewhere.
Preferably, this NOx storage system is designed with minimally
two parallel beds. One bed is used to absorb NOx, while the other bed is
regenerated. As soon as the former bed is saturated and/or the second bed is
sufficiently regenerated, the streams are switched, so that the regenerated
bed can proceed to absorb NOx and the loaded NOx bed is regenerated in
that the absorbed NOx is converted to nitrogen.
The effluent of the regeneration step of the NOx storage bed can be
1o recirculated with advantage and, together with the inlet air, be passed to
the inlet of the combustion unit (for instance a gas engine). This provides at
least two advantages. In the first place, in this way no CO-containing gas
needs to be discharged. In the second place, according to this embodiment, it
is of less importance to obtain a complete conversion of hydrocarbons to
CO/Hz in the preceding preparation step of the reducing gas stream.
Both the deNOx catalyst and the reductant forming catalyst can be
present in the method according to the invention in the forms known to
those skilled in the art, as in the form of a bed of granules, extrusions,
granules, andlor pellets, or provided on ceramic or so-called metal
monoliths, or differently structured forms.
The use of the catalyst in structured form is preferred because this
allows other relevant factors for the method, such as pressure drop, mixing,
contact time, heat management, mechanical strength and life, to be tailored
to the prevailing conditions through suitable choices, and the method can
thereby be optimized.
The invention is therefore characterized by the use of a catalyst
suitable for converting nitrogen oxides in combination with either a catalyst
suitable for the partial oxidation of hydrocarbons, or a catalyst suitable for
steam reforming, for converting nitrogen oxides in generating heat and
optionally energy from hydrocarbons, without this requiring ammonia or
urea to be added externally.
