METHODE POUR LA REDUCTION DES NO.SUB.X POLLUANTS

METHOD OF REDUCING NO.SUB.X-EMISSION

Abrégé anglais

ABSTRACT OF THE DISCLOSURE
A method of reducing NOx emission during combustion
of nitrogen-containing fuels with burners in a closed com-
bustion chamber, according to which the combustion occurs
in a below-stoichiometric primary combustion zone with
direct air supply through the associated burner, and in
an above-stoichiometric secondary combustion zone by ad-
dition of the remaining or balance of the air. The re-
sidue air is supplied to the combustion chamber, regulated
as a function of the load, around the particular burner,
in such a way that after development of the primary com-
bustion zone, the remaining air flows into the outer region
of the flame of the burner while supplying the secondary
combustion zone.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PRO-
PERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of reducing NO emission during com-
bustion of nitrogen-containing fuels with burners in a closed
combustion chamber, said method including the steps of:
effecting combustion in a below-stoichiometric primary
combustion zone with direct air supply through the asso-
ciated burner;
effecting combustion in an above-stoichiometric se-
condary combustion zone with the addition of the balance of
the air;
regulating said balance of the air as a function of
load; and
adding said balance of the air to said combustion
chamber around the associated burner in such a way that
after formation of said primary combustion zone, said
balance of the air flows into the outer region of the flame
while feeding said secondary combustion zone.
2. A method according to claim 1, which includes
the step of adding air directly through the associated
burner in such a quantity that an air number between
n = 0.9 and n = 0.5 is obtained in said primary zone.
3. A method according to claim 2, which includes
the steps of dividing said balance-of-the-air flow into at
least two separated partial streams, introducing said
partial streams to said combustion chamber from a divided

circle which concentrically surrounds the associated burner,
and withdrawing flue gases from said combustion chamber into
said primary zone by the impulse of the flame by means of the
free space between said separated partial air streams.

Description

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The present illvention rela,e lo a method of reducing
NOx emission during combustion of nitrogen-containing fuels
with burners in a closed combustion chamber, according to
which the combustion occurs in a below-stoichiometric primary
combustion zone with direct air supply through the associated
burner, and in an above-stoichiometric secondary combustion
zone by additon of the remaining or balance of the air.
The reaction mechanisms which cause the formation of
nitrogen oxides in technical firing or combustion are mostly
known. At the present, there are essentially two different
formation reactions as follows:
(1) The thermic NOx formation, which is based upon oxi-
dation of molecular nitrogen, which occurs for instance
abundantly in the combustion air. Since the oxidation of
molecular nitrogen requires atomic oxygen or aggressive
radicals (for instance OH, 03, etc.), such oxidation is
strongly temperature dependent, thus thermic NOx; and
(2) the formation of Euel NOx, which occurs by oxi-
dation of nitrogen compounds bound in the fuel. During the
pyrolysis, nltrogen-carbon and nitrogen-hydrogen radicals
(CH, HCN, CH, etc.) form from these nitrogen compounds.
These radicals oxidize into NOx in the presence of oxygen
already at relatively low temperatures because of their
reactivity with molecular oxygen.
A reduction of the thermic NOx-formation is accordingly
primarily obtained by lowering the combustion temperature and
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~hat retention times at high temperatures. Since with the
combustion of fuels with bound nitrogen, however, a large
portion of the total N ~-formation results from the fuel-N0~-
reaction, the aforementioned measures with such fuels are not
sufficient for complying with the emission standards existing
in certain countries. For this purpose, it îs necessary to
reduce the nitrogen compounds into molecular nitrogen (N2)
still during the pyrolysis in the absence of oxygen. Tests
have shown that these reduction reactions to molecular nitro-
gen occur, for example, when the fuels are burned or combusted
at below-stoichiometric conditions, that is, with less oxygen
or air addition than needed for complete combustion. To
achieve optimum results, an air ratio between 0.9 and O.S is
selected for the primary combustion zone as a function of the
limiting or edge conditions (for lnstance wall temperat-lre of
the combustion chamber). Ilowever, to achleve a complete com-
bustion of the hydrocarbon compounds of the fuel, the reaction
products resulting in the below-stoichiometric primary region
must then be afterburned.
Tests have shown that with such a two-stage combustion,
both the fuel-N0x-formation (with simultaneous heat removal
from the below-stoichiometric region) as well as the thermic
Nox-formation can be considerably reduced. In tests, with
the utilization of the two-stage combustion, the N0x-emission
values were reduced approximately up to 70% compared with
unstepped combustion.

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Through tests it was proven that the formation of fuel
NOx could be clearly reduced by operating the burner in the
near-stoichiometric or below-stoichiometric range. To avoid
losses by incomplete combustion, and to avoid increase of
other noxious material emissions (CO, hydrocarbons, and
particles), it is necessary for additonal air to be blown in
above the burners in the combustion chamber during below-
stoichiometric operation of the burners. The disadvantage
of this manner of operation is that in the below-stoichio-
metrically operated lower part of the combustion chamber,scoring and corrosion of the tube walls can occur. Accord-
ingly, the operational certainty of the system is in dangerO
It has furthermore been determined that by slowing the
mixture between air flow and fuel flow, likewise consider-
able reduction of NOx-emission can be achieved. For this
purpose, flow or spray burners, for lnstance, are suitable,
with which both the air stream and the fuel stream are blown
in parallel into the combustion chamber. To achieve a satis-
factory ignition, the burner streams, however, must support
2~ each other, for instance in a corner firing or combustion.
With the arrangement of the burners in a front or counter-
firing or combustion, the mixture of air and fuel can, for
example, be slowed thereby that the secondary air surround-
ing the dust stream is blown-in at substantially the same
speed.
In a known burner, the secondary air stream is added

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separately in two tubes, which are arranged annularly rela-
tive to each other, to permit discharge of, for example, the
outer secondary air flow with higher speed, and of the inner
secondary air flow, with low speed, directly adjoining the
dust stream. DisadvanLageous with this arrangement is that
an extension or lengthening of the flame occurs, which has
as a consequence larger combustion chambers, and that with
the load-conditioned reduction of the secondary air, the
secondary air speed is reduced below the dust-air speed,
whereby the character and shape of the flame change. The
ignition could also be disadvantageously influenced hereby.
Furthermore, it is known to undertake a primary combus-
tion at below-stoichiometric conditions in an antechamber of
the combustion chamber, and to admix the air necessary for
complete combustion with the combustion gases which leave
the antechamber. The disadvantage of this arrangement exists
in the danger of tube wall corrosion of the antechamber,
which is operated at below-stoichiometric conditions.
The object of the present invention accordingly is to
provide a method of reducing N0x-emission during combustion
of nitrogen-containing fuels with burners in closed combustion
chambers, with the method assuring that a low formation of
N0x is obtained by influencing the secondary air flow, and
simultaneously maintaining the certainty of operation against
scoring and corrosion of the pipe walls while simultaneously
maintaining an intensive ignition and a good combustion or

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burning out.
This object, and other objects and advantages of the
present invention, will appear rnore clearly from the follow-
ing specification in connection with the accompanying drawing,
which illustrates the principle of a burner for the present
inventive method, along with the flame generated therewith.
By one aspect of this invention, a method is provided of
reducing N0x emission during combustion of nitrogen-containing
fuels with burners in a closed combustion chamber, said method
including the steps of: effecting combustion in a below-stoichi-
ometric primary combustion zone with direct air supply through
the associated burner; effecting combustion in an above-stoi-
chiometric secondary combustion zone with the addition of the
balance of the air; regulating said balance of the air as a
unction of load; and adding said balance of the air to said
combustion chamber around the associated burner in such a way
that after Eormfltion of said primary combustion zone, said
bfllance of the air flows into the ou~er region o the flame
while feeding said secondary combustion zone.
There has been discovered as especially advantageous
that the secondary air flow supplied directly to the burner
be such that in the primary zone an air number between
n c 0.9 and n = 0.5 is obtained. Additionally, provision
is made according to th~ present invention that the second
partial flow of the secondary air (stepped air), itself be
divided into at least two partial flows, which are introduced

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into the combustion chamber from a divided circle which con-
centrically surrounds the burner, and that smoke or flue
gases be drawn out of the combustion chamber into the primary
zone by the impulse of the flame by means of the free space
located between the stepped air flows~
By dividing the secondary air flow, the following ad-
vantages are obtained with the inventive method: low N0x-
emission, no sintering and corrosion on the tube walls of the
combustion chamber, as well as a certain ignition and satis-
factory combustion over a wide operating range.
While that part of the secondary air which is supplied
directly to the burner can be so influenced in relation to
twist and speed that an intensive ignition is assured over
the entire load range, the second part of the secondary air
flow, which is added to the flame as a stepped-air flow ex-
ternally o the burner, is such that flfter successful igni-
tion and primary combustion there is realized the combustion
in the secondary zone and the object o the present inven-
tion by means of the mixing energy of the stepped-air strearn.
Referring now to the drawing in detail, the burner in-
cludes a core-air pipe or tube 2, a fuel and carrier-air
portion 1, and a mantle-air part 3. With this burner, there
is obtained a partial combustion zone (primary zone) 6, the
air number of which is between 0.9 and 0.5 times the stoi-
chiometric relationship~
The burner is embodied in such a way that by predeter-

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mined measures (twist of the mantle-air, conically widened
burner outlet, closed core-air~ there is generated in the
interior of the flame a zone of intensive return flow or
back-flow 5 from a region of already advanced combustion. In
so doing, the fuel-air mixture is quickly heated up and
ignited. The heating-up and ignition can be influenced by
adding core-air. Accordingly, the best ignition is assured
wllen the core-air is closed.
The air necessary for the burning-out of the remainder
or residue is blown in along the periphery as stepped- or
staged-air 4 by means of several jets or nozzles in such a
way that not until after formation of the primary flame is
the secondary flame or also the afterburner zone 7 supplied
with oxygen. For this purpose, the stepped-air flow 4 is
arranged in a divided circle which corresponds to twice the
diameter of the mantle-air tube. This assures that the
stepped-air 4 only reaches the actual flame downstream from
the burner outlet after a distance of approximately one to
two times the dlameter of the mantle-air tube.
Smoke or flue gases are drawn in from the combustion
chamber by impulse exchange at those segments o~ the
peripheral surface of the flame not adjoining the stepped-
air flow 4. In this manner, the flame temperature is re-
ducedO