Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process for reducing NO~
emissions in the thermal reaction of mass streams con-
taining nitrogen oxides, for example Nz0 (laughing gas) and
No (nitrogen monoxide), with fuel components or in the
oxidation of nitrogen-containing fuels with oxygen or air
by a multi-stage reaction, all the nitrogen oxides present
or spontaneously formed being completely reduced in the
first stage and then reacted in one or more following
stages until all the reduction components have been com
pletely oxidized.
Tn the combustion of nitrogen-bound fossil fuels and
organic residues, for example organic liquids containing
organically bound nitrogen, and in the post-oxidation of
waste gases of vapors already containing high levels of
nitrogen oxides, the emission of nitrogen oxides is con-
siderably higher than in the combustion of the relatively
nitrogen-free fuels, natural gas and light heating oil,
with an excess of air or oxygen. Thus, nitrogen oxides are
normally formed in a concentration of 5000 ppm although the
permitted limit is only 100 ppm. This is because, where
the fuels are burnt with an excess of oxygen, the nitrogen
present in the fuel is completely or partly converted into
nitrogen oxides and any nitrogen. oxide components already
present pass through the reaction as quasi-inert components
which produces the high emission levels.
Tt is known that NOX emissions can be reduced by so-
called combustion in stages which is understood to be the
division of the combustion process as a whole into a par-
tial oxidation, i.e. incomplete combustion in the substoi-
chiometric range with partial release of the heat of reac-
tion and another or multistage after-reaction until all the
oxidizable reaction components have been completely reac-
ted.
Tie A 28 282 1
In the case of gas mixtures containing high percen-
tages of nitrogen oxides, far example laughing gas or
nitrogen monoxide, fuel has to be introduced in at least
such a quantity - commensurate with these percentages by
volume - that the entire oxygen component of the nitrogen
oxides can be reduced. Since the enthalpies of formation
of the nitrogen oxides are released in addition to the
calorific value of the fuel components and since a stoichi-
ometric reaction always occurs during the oxidation of the
fuel component with the oxygen of the nitrogen oxides,
relatively little nitrogen being available for reducing the
theoretical reaction temperature in contrast to the reac-
tion with air, an extremely high temperature is established
which, in the presence of free oxygen, would lead to high
NOx values in accordance with the equilibrium. Accordingly,
steps have to be taken to ensure that, as in partial
oxidation, a reducing atmosphere is maintained. This can
only be achieved by the introduction of additional fuel or
reducing components. Although endothermic decomposition
reactions do reduce the temperature, depending on the
quantity of fuel, the excess of fuel has to be kept very
small for economic reasons which in turn leads to a low
chemical potential and, in the absence of kinetic support,
would require a very long residence time to complete the
reaction.
In the partial oxidation of fuels, for example in the
gasification of coal or oil, few - in any - nitrogen oxides
are formed because, in these reactions, a reducing gas
phase always limits the formation of nitrogen oxides to a
considerable extent on account of the low partial pressure
of oxygen atoms and, at the same time, the nitrogen oxide
molecules already formed are reduced in statu nascendi.
This situation is determined by a number of reactions
which, although together tending to establish a certain
equilibrium, are all influenced to different extents and,
he A 28 282 2
in some cases, even in different directions by the thermo-
dynamic parameters of state, such as temperature, pressure
or chemical potential. Because the heat of reaction is
only partly released, in conjunction with endothermic
reactions, the partial oxidation takes place at lower
temperatures than in complete stoichiometric combustion so
that, on the one hand, atomic oxygen is formed to only a
relatively limited extent through the dissociation of
reaction products, with the result that the NOx equilibrium
concentration itself is relatively low at that temperature.
In addition, the high chemical potential of the reducing
components ensures that the probability of a reaction to
molecular oxygen is very high.
Complete oxidation of the reduction components, such
as carbon monoxide and hydrogen, still present from the
partial oxidation then takes place in the after-reaction
stage following the partial oxidation. These reduction
components are formed in the first stage and form the
reactants for the reduction of NOX to molecular nitrogen and
water.
Thus, where burning is carried out in stages, all the
reaction stages take place at a thermodynamically lower
level so that the kinetic reaction to complete conversion
is impaired or takes place at a reduced rate in each reac-
tion stage compared with single-stage combustion because
the percentage of activated molecules decreases in accor-
dance with the temperature level. To correct this defici-
ency, a considerably longer reaction time is required for
establishing an equilibrium between all the reaction prod-
ucts. Unfortunately, however, this cannot be economically
achieved. Since both exothermic and endothermic reactions
take place simultaneously in the first stage and, together,
determine a reaction temperature for the equilibrium, each
element of space throughout the reaction zone should
contain an equal quantity of reactants. However, this is
Le ~ 28 282 3
only inadequately achieved, even by introducing the reactants
into the reaction zone in one or more channels. Although this
primary mixing can be optimized through the design of the
inflow channels, this unfortunately does not meet the stringent
requirements which the course of the reaction has to satisfy.
Accordingly, the problem addressed by the present
invention was to improve the course of the reaction and, hence,
economy in a process of the type mentioned at the beginning for
reducing NOX emissions. Thus, the present invention provides a
process for reducing NOX emissions in the thermal reaction of
nitrogen oxide-containing product streams with fuel components
or in the oxidation of nitrogen-containing fuels with oxygen or
air by a multistage reaction, all the nitrogen oxides present
or spontaneously formed being completely reduced in a first
reaction stage fed with less than the stoichiometric quantity
of oxygen and then being reacted in one or more following
reaction stages until all the reduction components have been
completely oxidized, wherein oxidation of the reduction
components takes place in at least one after-reaction stage
kinetically supported by activation regenerators in which the
reactants are intensively mixed and a uniform temperature is
established over the entire flow cross-section.
Another problem addressed by the invention was to
provide a simple arrangement for carrying out this process.
Thus, the present invention also provides an arrangement for
carrying out the process consisting of a furnace comprising a
fuel feed pipe or a waste gas feed pipe and an oxygen or air
feed pipe, wherein the furnace is divided by at least one
activation regenerator which consists of a wall with
throughflow openings into a first reaction stage and at least
one following after-reaction stage, the first reaction stage
comprising the fuel feed pipe or the waste gas feed pipe and an
4
CA 02069218 2001-07-05
oxygen or air feed pipe and only one oxygen or air feed pipe
opening as sole feed pipe into the after-reaction stage.
According to one aspect of the invention, the first
of these two problems has been solved by a process which is
characterized in that oxidation of the reduction components
takes place in an after-reaction stage kinetically supported by
activation regenerators, wherein an activation regenerator is
defined by a plurality of throughflow channels or openings
through which the gaseous reactants are conveyed and in which
an intensive mixing of the reactants and a homogenization of
the temperature across the total cross-section of flow are
obtained.
The additional activation of the reactants enables
the kinetic conditions under which the reaction takes place in
the corresponding reaction stage to be improved to such an
extent that substantially optimal reaction conditions are
established. NOX emission can thus be almost completely
suppressed or reduced.
In a preferred embodiment, the reactants are
additionally activated in the first stage of the reaction.
In another embodiment of the process according to the
invention, however, the reactants are additionally activated in
all stages of the reaction. This optimizes the course of the
combustion process as a whole because optimal conditions can be
established in each stage of the reaction.
In another embodiment of the process according to the
invention, the reactants are additionally activated after
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the particular reaction stage.
Another preferred embodiment of the process according
to the invention is characterized in that the reactants are
mixed together in the corresponding arrangement used for
their activation, the reactants preferably being mixed
together both in the macro range and in the micro range.
In addition or alternatively, however, heat flow is equal-
ized by the corresponding arrangement used for activation.
In this embodiment of the invention, therefore, the
course of the reaction in the reaction zone - supported by
an activation regenerator - is influenced to the extent
that the reactants may be mixed (forcibly) both in the
macro range and in the micro range. The resulting homogen
eous distribution of all the reactants thus serves to
accelerate the reaction and hence to shorten the residence
time of the reactants in the reaction zone. In addition,
the activation regenerator provides for rapid equalization
of heat flow which is essentially achieved by an intensive
exchange of heat by radiation. After-reactions can thus be
influenced to the extent that they can still take place
despite the low potential towards the chemical equilibrium
which can be established at the reaction temperature. To
be able to achieve the result of low NOX emission, it is of
crucial importance to the invention spontaneously to
activate the possible reactants under these relatively
unfavorable thermodynamic conditions, which is achieved by
mixing of the reactants in accordance with the invention
and by the equalization of heat flow. Since the partial
oxidation in the first stage of the reaction is determined
by other reactions than the stoichiometric combustion
reactions, different thermodynamic conditions have to be
established accordingly. The course of the reaction is
crucially determined in this regard by the heterogeneous
and homogeneous water gas reaction, the Boudouard reaction
and, in addition, the decomposition of all the hydrocarbons
present in the gas. Since all the reaction equations are
interdependent, the desired equilibrium between the result-
Le A 28 282
~~~~~~~_d
ing reaction products at a certain temperature can be
influenced by controlled introduction of the reactants. To
this end, spontaneous activation conditions for the reac-
tion also have to be established in the after--reaction
stage. According to the invention, this is achieved in
relatively short residence times by rapid mixing with the
oxidation partner air or possibly or even oxygen, accom-
panied by temperature equalization. In this post-oxidation
stage, the activation regenerator as a mixer and heat
reservoir also provides the kinetic support required in
accordance with the invention. Although the exothermic
oxidation reactions release heat, only slight increases in
temperature occur locally because a large inert gas stream
is always available for absorbing the heat. Accordingly,
the temperature level remains in a range in which nitrogen
oxides can only be formed as thermal BOX in the permitted
limiting concentrations.
Accordingly, the success of the process according to
the invention lies in the homogeneous distribution of all
the reactants and in the establishment of the same tempera
ture, which should be above 100°C, in each element of
space.
In another very important embodiment of the process
according to the invention, steam is additionally supplied
as reactant to the reaction stages with substoichiometric
combustion. This steam may be supplied either by injection
of water into the reaction zone or directly as steam, the
equilibrium concentrations being displaced towards a better
reduction potential. Tn addition, the presence of steam in
the reaction phase can prevent the separation of carbon
crystals, thus avoiding a significant fall in the effective
reduction potential.
Another embodiment of the process according to the
invention is characterised in that, depending on the
optimal reaction temperature, the reactants are used in the
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corresponding ratio to one another in the particular
reaction stage.
Finally, a further embodiment of the process according
to the invention is characterized in that, in each reaction
stage, the temperature is regulated with recycled, cold
waste gas.
The arrangement for carrying out the process according
to the invention is characterized in that at least one
activation regenerator is arranged in the furnace, prefer
ably having a wall with throughflow openings.
This wall is preferably arranged behind a reaction
stage and extends over the entire cross-section of the
furnace so that the reactants have to pass through the
throughflow openings in the wall. If, for example, two
reaction stages are provided, a wall of the type in ques-
tion is arranged between these two reaction stages. In
addition, a wall is also provided after the second reaction
stage.
This wall forms a technically very simple activation
regenerator and is designed in such a way that it acts as
a static mixer for macromixing of the reactants and also
performs the function of a heat reservoir. By virtue of
its high heat capacity and the favorable exchange of heat
by radiation, this wall establishes the thermal reaction
conditions in a very short time and, in the case of exo-
thermic reactions, re-absorbs heat which is than released
again as and when required.
The activation regenerator and, more particularly, the
wall may consist either of a temperature-resistant ceramic
material or of a temperature-resistant metal, more par°
ticularly steel and, above all, special steel. These
materials are capable of satisfying the requirements which
an activation regenerator has to meet, in addition to which
an activation regenerator of ceramic material may also act
as a catalyst.
Le A 28 282
The wall is preferably in the form of checkerwork,
particularly. where it is made of metal.
To enable it to be produced in a technically simple
manner, the activation regenerator preferably consists of
individual regenerator elements.
Where the wall is made in particular of ceramic
material, it preferably consists of individual bricks or
tiles which are joined firmly to one another in vertical
layers. The wall can thus be made in a technically simple
manner.
The bricks or tiles are preferably square in shape
with throughflow openings.
In another embodiment, the bricks or tiles have
additional protuberances or the Tike on their surfaces.
The resulting increase in surface area constantly influ
ences boundary layer flows in such a way that the reactants
undergo intensive micromixing which in turn provides for
rapid and uniform heat and mass transfer.
Finally, in another embodiment, several activation
regenerators are arranged one behind the other. This
ensures that the reactants are present in optimally acti
vated form in each of the stages so that the reaction
follows an optimal course. Where the activation regenera
tors are arranged one behind the other, several walls in
the form of static mixers are preferably provided.
One embodiment of an arrangement according to the
invention for reducing NOx emission in the oxidation of
nitrogen-containing fuels or in the reduction of waste
gases or vapors containing nitrogen oxides is described in
detail in the following with reference to the accompanying
drawings, wherein:
Figure 1 is a purely schematic side elevation of the
arrangement for reducing NOx emission.
Figure 2 is another purely schematic side elevation of
the arrangement shown in Fig. 1 in the form of a static
Ls A 28 282 8
mixer.
Figure 3 is an elevation on a somewhat larger scale of
an activation regenerator in the form of a wall provided
with throughflow openings of the arrangement shown in Figs.
1 and 2.
A furnace 1 for the oxidation (combustion) of nitro-
gen-containing foals or far the reduction of waste gases or
vapors containing nitrogen oxides comprises a fuel supply
pipe 2 for the nitrogen-containing fuels or reduction fuels
~.0 and a feed pipe 3 for the waste gases or vapors captaining
nitrogen oxides. The furnace 1 further comprises a steam
feed pipe 4 and an air feed pipe 5 which branches into the
air feed pipes 5°, 5°' opening successively into the
furnace 1.
The furnace 1 is a so-called multistage combustion
furnace. It comprises a first reaction stage 51 and a
following after-reaction stage S2. The air feed pipe 5'
opens in the region of the reaction stage S1 while the air
feed pipe 5 " opens in the region of the after-reaction
stage S2. Finally, the furnace 1 comprises a waste gas
outlet 6 at its exit end.
An activation regenerator 7 in the form of a wall
occupying the entire cross-section of the furnace is
arranged after the reaction stage S1 and the after-reaction
stage S2. Each of these walls consists of individual
regenerator elements 8 in the form of bricks which are
substantially square in shape and which have openings
underneath. These openings define throughflow openings 9
in the wall. Finally, the bricks have additional protuber
ances 10 on their surfaces.
The arrangement operates as follows:
Either nitrogen-containing fuel and reduction fuel or
waste gases or vapors containing nitrogen oxides are fed to
the furnace 1 through the fuel feed pipe 2 or through the
waste gas or vapor feed pipe 3. Whereas the nitrogen-
Le A 2~ 2~2
containing fuels are to be oxidized, i.e. burnt, the waste
gases or vapors are subjected to reduction.
The combustion or post-oxidation reaction takes place
in stages. Air is fed to the first reaction stage S1
through the air pipe 5' in such a quantity that partial
oxidation takes place in the substoichiometric range in
this first reaction stage S1. In view of the incomplete
combustion in this first reaction stage S1, oxidation is
not complete so that carbon monoxide and hydrogen are
l0 formed. The carbon monoxide and hydrogen in turn form the
reactants for the reduction of NOx to nitrogen and water in
the after-reaction stage S2.
The activation regenerator 7 in the form of the wall
between the reaction stage S1 and the after-reaction stage
S2 and the wall following the after-reaction stage S2 cause
the reactants to be mixed both in the macro range and in
the micro range. This is schematically illustrated in Fig.
2. In addition, the walls, which may be made bath of
ceramic materials and of metals, provide for rapid equaliz-
ation of heat flow which is essentially achieved by inten-
sive exchange of heat by radiation.
Sy virtue on the one hand of the intensive mixing of
the reactants, resulting in homogeneous distribution
thereof, and by virtue on the other hand of the establish-
ment of the same temperature in each element of space
through the high heat capacity of the walls and the result
ing intensive exchange of heat by radiation, the reaction
taking place in the furnace 1 is accelerated through that
NOX emission in the waste gas outlet 6 is reduced in an
economically favorable manner.
The introduction of steam through the steam feed pipe
4 displaces the equilibrium concentration towards a better
reduction potential. In addition, the presence of steam in
the reaction phase can prevent the separation of carbon
crystals, so that a significant fall in the effective
reduction potential is avoided.
lLe A 28 282 10
