[0001] The present invention relates to the control of combustion in coke ovens for reduction
of nitrogen oxides in coke-oven gases, to render said gases less polluting when they
are discharged to atmosphere. More particularly, the invention relates to equipment
that provides said control, and hence reduction of the nitrogen oxides in coke-oven
gases, at far lower cost than the conventional equipment. The systems generally used
for reduction of the nitrogen oxides in coke ovens are: (a) reduction of flame temperature
by recirculating a proportion of the flue gases so as to dilute the fuel gas so as
to lower the combustion temperature somewhat and (b) admission of the combustion air
in various zones along a suitable pipe in order to "stagger" the flame along the pipe
so that a reduction of the temperature of the flame itself is also achieved in this
way, by breaking it up into several flamelets with consequent reduction of the production
of nitrogen oxides.
[0002] A third system is also known for the reduction of nitrogen oxides in coke-oven gases,
described for example in patent US-A-5,017,270 of Krupp Koppers GmbH, which relates
substantially to a combination of these two arrangements. In accordance with this
patent it is proposed to adopt, in combination, a system for circulation, which in
itself is already known, with a system for distribution of air at various levels,
again in itself already known.
[0003] According to this known technology, the two channels leading to one combustion chamber
of the coke oven both become air ducts and no longer flue gas ducts and moreover the
two channels leading to the adjacent chamber become flue gas ducts and extend downwards
to a regenerator which, alternately, in a first time segment, for example in the first
20 minutes, accumulates the heat of the flue gases and in the next, gives up this
heat to the air. This prior patent envisages outlets at various levels and so combines
the effect of recirculation with the effect of distribution of the air at several
levels.
[0004] According to the present invention, the problem of reduction of nitrogen oxides is
solved with a combustion system in which the flame is not staggered physically but
is fractionated owing to an aerodynamic shape. This system has the advantage that
it is simpler than the known systems, since it does not involve the ad-hoc construction
of a channel, made of refractory bricks, which would make the oven design more complicated.
It only requires the construction of a wall in the combustion chambers of the oven,
positioned between the two air inlets, rising to a height of only about one metre
in the oven, and with a hole near its base equipped with a diverting plate, made of
a refractory material, causing the air to go more in one direction than another.
[0005] For a better assessment of the scope of the present invention, it has to be borne
in mind that 600-700 different brick moulds are required for constructing a coke oven
plant of this type. With the present system, about ten shapes of bricks are eliminated,
because it is no longer necessary to build a long, high channel with several small
holes for outlet of the secondary air streams. This constitutes a definite economic
advantage in oven construction, in view of the considerable simplification of oven
design.
[0006] A further advantage of the design according to the present invention relates to supply
of these ovens with gas mixture. It is known that the problem of nitrogen oxides does
not exist when the oven is supplied with gas mixture, whereas the problem does exist
when the oven is supplied with rich gas. For most of the time, coke oven batteries
operate on gas mixture, not on rich gas. Since the gas mixture has low calorific value,
it must be preheated, for which it follows the fate of the air. If a simple wall is
built in the middle of the combustion chamber, the result is that the air leaves on
one side and the gas on the other and in the lower part there is no flame so that
this part is poorly heated. We are therefore faced with the problem of finding how
to carry air from the gas and vice versa to make it burn at the bottom as well otherwise
the device that worked well for the rich gas since there were two air streams, is
limiting when operating on gas mixture. The device according to the invention which
causes diversion of the gas flow meets this objective. The ovens consume 1/3 of gas
for heating and 2/3 for exporting. Since the rich gas is a high-value gas (4000 or
more kilocalories), at iron and steel works the aim is to supply the oven with low-value
gas from the blast furnace, which is called gas mixture or lean gas, so that all the
gas produced by the oven is exported, instead of consuming 1/3. Therefore the coke
ovens at iron and steel works, where there is production of gas mixture of low calorific
value (about a quarter compared to rich gas, i.e. approx. 1000 kcal), are operated
on gas mixture.
[0007] Further aims and advantages of the present invention will become clearer from the
following description of one embodiment of the invention, illustrated as a nonlimiting
example in the appended drawing, in which a portion of a coke oven battery is shown
schematically in longitudinal section.
[0008] Referring to the drawing, it shows the combustion chambers or channels 1, and a distillation
chamber 2. Refractory walls 3 separate the distillation chamber 2 from the combustion
channels 1, and the heat is transmitted through said walls 3 to the coal charge contained
in the distillation chambers. To achieve optimum distillation with minimum production
of nitrogen oxides, it is necessary for the vertical temperature distribution on walls
3 to be perfectly uniform. For this purpose, according to the present invention, a
central wall 4 is erected in the combustion chambers 1, extending to a relatively
small height, of about one metre, and dividing the base of chambers 1 into two zones
101, 201 respectively. Near its base, this wall has a through-hole 5, and a plate
6 is arranged corresponding to the top edge of said hole 5, and projects from said
wall into zone 101 of chamber 1. A pipe for admission of combustion air is labelled
7, and a manifold for feed of fuel gas is labelled 8, and communicates with nozzle
9 with its opening in section 101 of chamber 1. As shown by the dashed-line arrow
F, a portion of the air from pipe 7 is diverted by plate 6 through hole 5 in chamber
201, whereas another portion of the air continues on its way (arrow F1) in chamber
101, meeting the gas coming from nozzle 9, so that a portion of the gas is caused
to burn in chamber 101 i.e. in the bottom part of combustion chamber 1, whereas another
portion of the gas moves upwards, beyond wall 201, where it meets the air diverted
by plate 6, and burns in the top part of chamber 1. Owing to this arrangement, uniform
heating of the whole of combustion chamber 1 is obtained, and owing to the staggering
of the combustion flame the emission of nitrogen oxides in the exhaust gases is greatly
reduced.
[0009] The exhaust gases flow into the regenerators 108 (arrow F2), heating the air fed
to the oven in countercurrent to them, and then to the smokestack.
[0010] Of course, the present invention is not limited to the embodiments that have been
illustrated and described, but comprises all variants and modifications within the
wider scope of the inventive concept, substantially as claimed hereunder.
1. Method of controlling combustion in coke ovens for reduction of nitrogen oxides in
coke-oven gases, characterized in that, in the combustion chambers of the oven, between the two air inlets, a wall is constructed
which rises in said chambers to a reduced height between about 1/3 and 1/5 of the
height of said chambers, said walls being provided with a hole near their base, and
being provided to correspond to the upper limit of said hole, on one side of the said
wall housing the nozzle for supply of the combustion gas, with a diverting element
for diverting a proportion of the combustion air supplied to said chamber onto the
other side of said wall.
2. Method according to Claim 1, in which said diverting element comprises a plate projecting
from said wall.
3. Coke oven for carrying out the method of Claim 1 or 2, in which said wall has a height
of about 1 metre.
4. Coke oven according to Claim 3, for carrying out the method according to either one
of the Claims 1 or 2, substantially as described, illustrated and for the purposes
explained above.
