Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
_ 2121285
Docket No.: GK-ZM-623
METHOD FOR BURNING FUELS. PARTICULARLY
FOR INCINERATING GARBAGE
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention relates to a method for burning fuels,
particularly for incinerating garbage, in incinerators with a
stoker grate, for which the primary air is supplied into the
fuel layer below the stoker grate and the secondary air is
supplied above the fuel layer.
b) Background Art
Methods of the above-described general type have
been known for a long time.
Because of political objectives, garbage must be
incinerated so that contamination of the environment is
largely avoided. The previously customary procedure for
achieving this objective is the optimization of the
incinerating process on the stoker grate by supplying
combustion air selectively with respect to amount and
distribution along the stoker grate and also in the region of
the secondary combustion zone, as well as by the use of
constantly more extensive and more expensive facilities for
downstream gas cleaning. Admittedly, this has already led to
drastic reductions in emissions from such systems. However,
it also brings about a great increase in the disposal costs,
which are associated with the operation of these systems and
an out-of-proportion expansion of the gas cleaning section
compared to the conventional part of the facility consisting
of the furnace and the energy utilization, such as the
A
2lz~zg~
generation of steam. In the meantime, a standard has been
attained for flue gas cleaning systems, which permits further
improvements in the deposition performance to be anticipated
only if very high additional costs are accepted, since
cleaning efficiencies of 99% are being attained. Experience
has shown that there is an exponential relationship between
expenditure and cleaning performance, so that technical
exertion no longer is justifiable for increasing the cleaning
efficiency further. With the presently known measures, the
technically justifiable limit of cleaning flue gases has been
reached. Particularly in recent years, additional attempts
have been made to achieve further improvements by modifying
the combustion process.
From the disclosure of DE 39 15 992 A1, it is known
how the formation of nitrogen oxides can be prevented, in
order to make possible therewith savings in downstream
cleaning systems. The basic principle of this known method
consists primarily of modifying the oxygen content of the
primary air by admixing recirculated flue gases from the
combustion process, so that the combustion on the grate
proceeds in a damped form with the objective of keeping the
temperature at the tips of the flame below 1300C, because it
is known that the formation of the nitrogen oxide sets in to a
greater degree above this temperature. For this known method,
the secondary air is introduced in two stages, one above the
other. In the lower stage, essentially recirculated flue gas
is introduced, in order to generate a turbulence, while air
from the surroundings is supplied to the second stage, in
order to achieve the necessary combustion of the flue gases.
In the case of the selectively retarded combustion reactions,
however, there is a distinct deterioration in the ignition of
solid garbage components on the incinerator grate. As a
-2-
pr
-z~2~zs5
direct consequence, the proportion of uncombusted materials in
the combustion residue increases and there is a greater
possibility that pollutants will be bound in the residue, that
is, in the ash, because of the lower combustion bed
temperatures. Admittedly, the proportion of nitrogen oxides
is reduced by this method. However, the proportion of
uncombusted pollutants is increased because of the decrease in
the oxidation potential.
For a different known method described in DE 40 27
908 A1, the proportion of oxygen in the primary combustion air
as well as in the secondary combustion air is varied, this
variation extending from an increase in the proportion of
oxygen above that of the surrounding air to a decrease in the
proportion of oxygen below that of the surrounding air. For
this purpose, the oxygen, obtained in an air fractionation
plant and the nitrogen, also obtained there, are used in order
to increase the proportion of oxygen in the combustion air by
supplying oxygen and to decrease the proportion of oxygen in
the combustion air by supplying nitrogen. This procedure can
improve the course of the combustion in important areas of the
furnace or the combustion chamber. However, it does not lead
to a decrease in the amount of flue gas flowing through the
installation as a whole and especially also not in the amount
of gas flowing through the flue gas cleaning system.
Until now, the concentration of pollutants in the
flue gas has generally been the starting point for evaluating
the relevance of garbage incinerators to the environment. The
presently valid emission guidelines, namely the 17.
Bundesimissionsschutzverordnung (17th Federal Emission
Protection Regulation) of Germany or the EC Guidelines for
Incinerating Garbage, as well as the Provisional Regulations
hereto, govern the conditions imposed on such installations by
-3-
A
2121295
limiting concentration values. This mode of consideration
reflects the pollutants emitted into the environment only
incompletely, since only relative and not actual amounts,
which leave an emission source, can be identified. On the
other hand, the evaluation of the pollutant loading describes
the ecological significance of an emission source more
comprehensively.
Experience from operating modern flue gas cleaning
systems show that the attainable concentrations of pollutants
l0 and purified gas practically do not depend any more on the
concentrations of pollutants in the flue gas entering the
cleaning system. Instead, the concentrations at the end of
the flue gas cleaning remain constant. This is explained by
the exponential relationship, already alluded to above,
between the degree of deposition and the expenditure for or
the dimensioning of aggregates to be used.
OBJECTB AND SUMMARY OF THE INVENTION
It is therefore an object of the invention to
achieve a significant decrease, beyond the previously obtained
findings with respect to the attainable cleaning efficiencies,
in the pollutant loading emitted to the environment, with a
higher technical efficiency and smaller dimensioning of the
aggregates connected downstream from the incineration.
Pursuant to the invention, this objective is
accomplished, starting out from a method of the initially
mentioned type for combusting fuels, owing to the fact that
the combustion intensity of the fuel on the stoker grate is
increased at least partially by increasing the proportion of
oxygen in the primary air and that the combustion intensity in
the secondary combustion zone is choked by decreasing the
proportion of oxygen in the secondary air. Preferably, flue
-4-
A
2121295
gas recirculated from the combustion process is used to
decrease the proportion of oxygen in the secondary air.
The increase in the proportion of oxygen in the
primary combustion air depends on the reaction zones, that is,
on whether it is the warming-up zone, the main combustion zone
or the final combustion zone. Due to this partly significant
increase in the proportion of oxygen in the primary combustion
air, the proportion of nitrogen is necessarily decreased. As
a result, the intensity of the combustion is increased
significantly and the volume of air required is decreased
greatly, since the nitrogen content, which acts only as a
ballast, is decreased. With that, there are already
significant savings in the volume of combustion air required.
These savings make it possible not only to reduce the
dimensioning of the primary air blower and ducts, but also to
decrease in size the downstream aggregates, such as the steam
generator, the flue gas cleaning facility, the induced-draft
blower and the chimney. The appreciable increase in the
intensity of the primary combustion usually leads to an
increase in the temperature of the combustion bed on the
combustion grate and of the flame in the secondary combustion
zone. The higher combustion bed temperatures have the
advantageous effect of an improved total combustion of the
solid parts of the fuel and an improved tying up of the
pollutants in the ash because of sintering. On the other
hand, the higher temperature of the secondary combustion
results in the increased formation of nitrogen oxides (thermal
NOx). In the case of the known method, which has been
mentioned above, the combustion process is choked appreciably
for this reason already on the stoker grate. The invention
thus initially follows a path, which should be regarded as in
the wrong direction according to previous findings, in that
-5-
A
- 212295
the combustion initially is fanned beyond the usual extent on
the stoker grate, in order to achieve thereby a considerable
reduction in the amount of primary air, an improvement in the
ash quality and a decrease in the combustible, that is,
oxidizable pollutants, and then corrects this path in the
secondary combustion zone, so that there is no increased
formation of nitrogen oxides, the removal of which would
entail a considerable expense. The further measure of the
invention thus takes place here. This measure consists of
choking the combustion process above the combustion layer at
the start of the secondary combustion zone so that the
formation of nitrogen oxides, which usually occurs, does not
take place or takes place only to a greatly reduced extent.
This damping measure thus commences at a point in time at
which the elimination of oxidizable pollutants is already
largely concluded due to the strongly fanned combustion
process. Because the oxygen content of the primary air has
been increased greatly, this air still contains enough oxygen
after passing through the combustion layer so as to make the
desired, throttled combustion in the secondary combustion
region possible. This throttled combustion is to be
controlled so that those temperatures at which nitrogen oxides
are formed, do not arise. Pursuant to the invention,
preferably flue gas from this combustion process is used to
achieve the choking. Flue gas, the oxygen content of which is
greatly reduced, is used in the inventive method to dampen the
after-burning. Due to the use of flue gas, it is thus no
longer necessary to supply additional, oxygen-deficient gas or
inert gas. For this reason, the amount of gas does not have
to be increased despite adequate combustion in the secondary
combustion region, whereas it does have to be increased in the
case of the previously known method, for which additional
-6-
A
2121299
combustion air, in the form of air from the environment, was
supplied over secondary air nozzles in the region of the
secondary combustion zone. Even in the case of methods for
which no pure combustion air was supplied, the amount of gas
was nevertheless increased, because a mixture of the
surrounding air and flue gas or an inert gas, such as
nitrogen, was supplied, which increased the total amount of
gas. The damping measures in the secondary combustion zone
are thus produced for the present invention preferably with
the help of flue gases, which arise during the primary
combustion, that is, during the combustion of the combustion
layer on the stoker grate, so that additional amounts of gas
either are no longer required or are required to a lesser
extent.
Modern garbage incinerators usually are operated
with a division of the combustion air into 65o primary air and
35% secondary air. If, for example, the entire secondary air
is replaced by recirculated gas in order to achieve the
desired damping of the combustion process in the secondary
combustion zone, then 35% of the total amount of gas flowing
through the installation is saved by this measure. Together
with the measure of increasing the oxygen content of the
primary air, the amount of gas flowing through the downstream
aggregates can be reduced by these means by two thirds.
However, if, as noted above, the concentrations of the
pollutants at the end of the flue gas cleaning system are
independent of the inlet pollutant concentrations, the amount
of pollutants arising, that is, the amount of pollutants
emitted from one source, can be decreased by the reduction in
the total gas volume by up to two thirds compared to
previously operated plants in a manner that was not possible
by means of the traditional mode of operating incinerators
A
2121295
because of the exponential increase in costs mentioned above.
Starting out from the general, basic teachings
explained above, a special further development of the
invention consists therein that the oxygen content of the
primary air is increased locally, above the oxygen content of
the surrounding air, as a function of the combustion behavior
or the burning intensity of the fuel layer on the stoker
grate, that the oxygen content of the secondary air is
decreased below the oxygen content of the surrounding air by
admixing recirculated flue gas, and that the secondary air is
adjusted locally with respect to the composition, the amount,
the supplying site and a high turbulence in the secondary
combustion zone, so that the formation of nitrogen oxides
above the fuel layer is largely avoided. By adaptation to the
local circumstances, that is, by taking into consideration the
different states of the fuel along the stoker grate, the
primary air can be adapted to the particular circumstances and
adjusted so that the desired, intensified combustion effect
and, with that, the combustion of oxidizable pollutants takes
place. For this purpose, for example, known thermocouple
elements are used to measure the temperature or infrared
cameras are used to determine the solid-state radiation which
is emitted directly from the combustion layer and permits
conclusions to be drawn concerning the temperature of the
combustion bed, that is, the temperature of the fuel mass in
the process of combustion, in order to adjust the oxygen
percentage. This is necessary to bring about a greater
combustion intensity, at the particular site in the combustion
grate, which is possible, for example, by appropriately
subdividing the underblast guiding system for the combustion
grate. For this purpose, the disclosure in DE 38 25 931 C2
makes a relevant proposal. In this connection, what is of
_g_
A
2121295
decisive importance is not the known fact that the percentage
of oxygen basically can be controlled, but rather that the
oxygen percentage is increased in the case of the present
invention to such an extent, that a combustion intensity and
the decrease in oxidizable pollutants associated therewith is
attained, which previously was not pursued because of the
anticipated disadvantages. These disadvantages, which are to
be seen, for example, in the thermal formation of nitrogen
oxides and in the overheating of the protective lining of the
combustion chamber, are, however, avoided in the case of the
present invention owing to the fact that selective damping
measures with respect to the temperature development and the
adjustment of other conditions are initiated over the
combustion layer, so that the anticipated disadvantages do not
occur. These damping measures include basically the reduction
in the oxygen percentage, as was already explained further
above. In this connection, the amounts of secondary air and
the place in relation to the combustion layer or the
combustion chamber and the turbulence, with which the amount
of gas referred to as secondary air is supplied over the
secondary air nozzles, are of great importance. The
generation of a high turbulence has a particularly
advantageous effect, since a frequently nonuniform
distribution of oxygen is levelled out by these means and the
oxygen that is present can be utilized better. As a result,
the desired course of the combustion can be achieved at
relatively low temperatures in the secondary combustion
region. It is possible to adjust the conditions in the
secondary combustion region with the help of temperature
measurements and observations by means of infrared cameras or
of flame monitors in such a manner, that temperatures harmful
for the lining of the combustion chamber as well as for the
-9-
A
2121299
formation of nitrogen oxides are prevented, since it is known,
for example, that a temperature of 1,300C represents a
limiting value, above which increased formation of nitrogen
oxides must be expected.
Depending on the type of material to be combusted,
it may be advantageous to increase the oxygen content of the
primary air only in the main combustion zone of the stoker
grate, because the fuel in other regions initially is
preheated or already has too low a calorific value, as is the
case, for example, in the final combustion zone.
The oxygen content of the primary air, which can be
adjusted locally differently in relation to the length of the
combustion segment on the stoker grate, can be adjusted to a
value of 25 to 50% by volume and preferably of 35% by volume.
By these means, the amount of primary air can be reduced in
accordance with a diagram, which is explained in greater
detail below and is referred to as Figure 2.
Since the combustion intensity on the stoker grate
within the primary combustion zone can be increased
particularly greatly pursuant to the present invention,
sufficient oxygen for the secondary combustion still remains
in the air that has passed through the combustion layer, so
that, pursuant to a further development of the invention, the
flue gases, in the secondary combustion zone, constitute 20%
to 65% and preferably 35% of the total amount of air and gas
supplied to the combustion process. For example, the total
amount of gas, which is introduced through the secondary air
nozzles, may consist of recirculated flue gas if it is assumed
that preferably 35% of the total amount of air and gas is used
as recirculated flue gas and that the secondary air
constitutes 35% of the total amount of gas. Since it is a
question of recirculated flue gas, more than a third of the
-10-
A
212125
total amount of gas is saved by this method.
Preferred parameters for controlling the oxygen
content are, in relation to the primary combustion air, the
surface temperature of the material being combusted and, in
relation to the secondary air, the temperature existing in the
secondary air combustion zone or the length of the flame in
the vertical direction in the combustion chamber.
The recirculated flue gas is preferably exhausted
after it has passed through the boiler. As a result, the
decrease in the volume of gas, arising out of the
recirculation of the flue gas, becomes noticeable only in the
flue gas cleaning system. In particular situations, it may
also be possible to exhaust the recirculated flue gas directly
above the combustion layer.
The adjustment of the oxygen content of the primary
air and, with that, the deliberate increase in the combustion
intensity in the primary combustion zone can be advanced so
far without regard to the later consequences, which are
eliminated by the inventive damping measures, that the
combustion process on the stoker grate is limited essentially
by conditions which arise out of the operating ability of the
combustion grate. An increase in the temperature of the
combustion layer by up to 300C above that of conventional
combustion processes is possible. Since the combustion bed
temperature can be up to 300C higher than in the case of a
combustion with normal outside air, the pollutants are bound
into the ash to a higher degree. The thus decreased
leachability of the pollutants improves the possibility of
using the ash as a building material or lowers the
requirements that a residue garbage dump must meet.
Basically, various important advantages arise out of
enriching the oxygen content of the primary combustion air.
-11-
A
2~2~295
It has already been emphasized in detail that the total amount
of gas volume can be reduced up to two thirds, as a result of
which a smaller dimensioning of the installation as a whole
and, with that, a considerable reduction in costs arise. Due
to the increase in the oxygen content in the primary air,
there necessarily is a drastic reduction in the percentage of
nitrogen supplied. This nitrogen is to be regarded merely as
ballast and, in the past, reduced the thermal efficiency
considerably. This effect of reducing the thermal efficiency
decreases correspondingly due to the decrease in the
percentage of nitrogen, so that the thermal efficiency is
improved. The increase in the percentage of oxygen has its
limits where a thermal overloading of the individual grate
elements could occur. In practice, it has turned out that the
percentage of oxygen can constitute up to 50% by volume of the
primary air supplied without damage to the grate elements. By
decreasing the volume flow of gas, the entraining effect on
light and small garbage particles is reduced, so that these
burn on or immediately above the grate and do not reach the
purification installation as uncombusted parts. Dust
precipitators, in addition to the possible reduction in size
due to the lower volume of flue gas, can therefore also be
dimensioned smaller. The clearly more intensive combustion
resulting from the invention does not only lead to higher
temperatures of the fuel particles on the grate itself, but
also brings about an increase in the gas temperatures
immediately above fuel layer, which leads to a decrease in the
concentrations of carbon monoxide and residual hydrocarbons.
Furthermore, the concentration of the so-called precursor
compounds is lowered, which in the region of the later cooling
of the combustion gases can lead to the formation of dioxins
and furans.
-12-
A
212 ~29~
Due to the drastic choking of the combustion process
in the secondary combustion region preferably by recirculating
flue gas, disadvantages are avoided, which have arisen above
the grate due to the previous oxygen enrichment. By supplying
air from the environment to the secondary combustion region,
overheating has been detected in this region. The result is
expressed in a higher percentage of thermally formed nitrogen
oxides and a long-term, unavoidable destruction of the
protective ceramic lining of the combustion chamber region.
These disadvantages are avoided by drastically choking the
course of the combustion above the combustion layer, that is,
in the secondary combustion region. The thorough mixing,
required in the secondary combustion region in order to avoid
an oxygen concentration gradient between recirculation gas and
combustion gas from the primary combustion zone, is achieved
by supplying the recirculation gas to the secondary combustion
zone, the amount of gas supplied here being sufficient for
ensuring the required turbulence. The lesser swirling up of
dust in and above the combustion layer leads, in addition to
the already mentioned smaller design of the dust filters, also
to less contamination of the heating surfaces in the boiler
and thus also to more favorable operating conditions with
respect to boiler availability, elapsed time between two
heating-surface cleanings and boiler efficiency. Due to the
greatly reduced amount of gas while the amount of fuel is kept
constant, there is necessarily a proportional increase in the
concentration of the different pollutants, which are to be
deposited by the flue gas cleaning system. This is evident
from the diagrams in Figures 3 to 6. In the event that
valuable materials are to be recovered in the flue gas
cleaning process, the effectiveness and costs of this process
are distinctly improved, for example, for the recovery of
-13-
A
2121299
hydrochloric acid and gypsum from precipitated pollutants.
In accordance with the present invention there is
provided a method for burning fuels, particularly garbage, in
incinerators with a stoker grate, for which the primary air is
supplied into the fuel layer below the stoker grate and the
secondary air is supplied above the fuel layer, the
improvement comprising the steps of:
increasing the intensity of the combustion of the fuels
on the stoker grate at least partly by increasing the
percentage of oxygen in the primary air; and
choking the intensity of the combustion in the secondary
combustion zone by decreasing the percentage of oxygen in the
secondary air.
The invention is explained in the following by way
of example by means of a furnace for carrying out the method
in conjunction with drawings and diagrams, which show various
experimental results.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a longitudinal section through a
furnace;
Figure 2 shows a diagram relating to the reduction
in the amount of flue gas at different oxygen concentrations
in the primary air and different percentages of recirculation
gas based on the amount of secondary air;
Figure 3 shows an experimental result from the
operation of a furnace using outside air as primary air and
without any recirculation of the flue gases into the secondary
air;
Figure 4 shows an experimental result from the
operation of a furnace with an oxygen concentration of 35$ by
volume without any recirculation of the flue gases;
- 14 -
25986-26
zr2 X295
Figure 5 shows an experimental result from the
operation of a furnace with an oxygen concentration of 35~ by
volume in the primary air and complete recirculation of the
flue gases; and
Figure 6 shows an experimental result from the
operation of a furnace with an oxygen concentration of 50~ by
volume in the primary air and 100~s recirculation of the flue
gases.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As is evident from the drawing, a furnace for
carrying out the method described has a charging hopper 1 with
- 14a -
25986-26
A
._ z~21295
connecting charging chute 2 for charging the material to be
burned on a charging table 3, on which charging rams 4, which
can be moved back and forth, are provided, in order to feed
the material to be burned and coming from the charging chute
to a stoker grate 5, on which the combustion of the material
takes place. It is immaterial here whether the grate is
inclined or horizontal, no matter what principle is involved.
Below the stoker grate 5, facilities for supplying
primary combustion air are disposed. These facilities, as a
whole, are labeled 6 and can comprise several chambers 7 to
11, to which primary combustion air is supplied over a
pipeline 13 by means of a fan 12. Due to the arrangement of
the chambers 7 to il, the stoker grate is divided into several
underblast zones, so that the primary combustion air can be
adjusted differently depending on the requirements on the
stoker grate. Depending on the width of the stoker grate, the
underblast zones can also be divided in the transverse
direction, so that the supply of primary air can be controlled
depending on local circumstances.
Above the stoker grate 5, there is the combustion
chamber 14, which in the upper part goes over into a flue gas
15, adjoining which there are aggregates that are not shown,
such as a waste-heat boiler and a flue gas cleaning system.
In the rear region, the combustion space 14 is bounded by a
cover 16, a rear wall 17 and side walls 18.
The combustion of the material labeled 19 takes
place on the front part of the stoker grate 5, over which
there is the flue gas flue 15. In this region, most of the
primary combustion air is supplied through the chambers 7, 8
and 9. On the rear part of the stoker grate 5, there is only
completely burned material, that is, the ash and primary
combustion air is supplied to this region over chambers 10 and
-15-
A
2121295
il essentially for the purpose of cooling this ash.
The combusted parts then fall onto an ash
discharging unit 20 at the end of the stoker grate 5. In the
lower region of the flue gas flue 15, rows of secondary air
nozzles 21 and 22 are provided which supply the secondary
combustion air to the rising flue gas, in order to bring about
afterburning of the combustible parts in the flue gas.
The rows 21 and 22 of secondary air nozzles are
connected to air collectors 27a and 27b, which are supplied
over a pipeline 26 by a blower 25, which takes in, on the one
hand, flue gas over a pipeline 28 from a part of the
incinerator lying further to the rear and, on the other, if
necessary, air from the environment over a pipeline 29.
From an air fractionation plant, which is not shown, either
pure oxygen or oxygen-enriched air from the environment is
taken by a blower 30 over pipeline 31 and supplied over
pipeline 32 to pipeline 13 for the primary combustion air.
The amounts of air in chambers 7 to 11 are adjusted by control
facilities 33, which control the supply of the air-oxygen
mixture to the individual chambers 7 to 11. Of course, each
chamber 7 to il can have a separate oxygen pipeline, in order
to be able to adjust the oxygen content of the primary air
differently in the individual chambers.
For controlling the combustion process, a video
camera or a thermographic camera 34, a monitor 35, a freely
programmable computer 36 and a controller unit 37 are
provided. The video camera or thermographic camera 34 is
aligned, so that it can view the material 19 burning on the
stoker grate 15 from above through the combustion chamber 14.
The video camera is connected to the monitor 35 and the freely
programmable computer 36, which resolves the image received
appropriately and compares the digital values obtained, which
-16-
A
221295
are a measure of the brightness on the respective combustion
zone, with specified standard values and, in the event of a
deviation, initiates an appropriate control process over the
controller unit 37, which adjusts the control facilities 33,
which are constructed as shutters or slides, in the individual
chambers 7 to 11. On the basis of solid body radiation
received and emanating from the combustion layer 19, the video
camera 34 supplies values, which permit conclusions to be
drawn concerning the temperature of the combustion layer 19.
The amount of primary air can be adjusted by adjusting the
control devices 33 and the oxygen content of the primary air
is adjusted over the controller unit 37 by adjusting the
control device 38 in the pipeline 31. The amount of primary
air and the oxygen content are adjusted as a function of the
burning behavior of the combustion layer 19 on the stoker
grate 5 in the respectively monitored zone of the stoker
grate. In the region of the secondary combustion zone,
thermocouple elements are provided, of which only one
thermocouple element 39 is shown in the drawing. The
thermocouple element 39 serves to monitor the temperature in
the secondary combustion zone and is connected with the freely
programmable computer 36, which, in turn, is connected with
the controller unit 37. Over the control devices 40 and 41,
this controller unit 37 adjusts the amount of flue gas which
is to be supplied to the secondary air nozzles 21 and 22 and
optionally the amount of air from the surroundings, which is
adjusted over the controlling device 41. Instead of using
thermocouple elements 39, the temperature can also be measured
by means of a sound pyrometer (pyrosonic) or by the laser
Doppler method.
With the help of this control device and the
corresponding monitors, it is possible to control, on the one
-17-
A
2121295
hand, the intensive burning on the stoker grate by controlling
the amount of primary air and the percentage of oxygen in the
combustion air and, on the other, the behavior in the
secondary combustion zone by adjusting the secondary air,
which consists largely of recirculated flue gas. By these
means, an excessive increase in the temperature in the
secondary combustion region is avoided and the amount of gas
passed through the installation as a whole, is reduced. In
this connection, the reduction in the amount of gas is at a
maximum, when no additional air is taken in from the
surroundings. Compared to previously known incinerators, the
amount of primary air is reduced owing to the fact that the
oxygen content of the primary air is increased over that of
the surrounding air and, with that, the percentage of
nitrogen, which acts as a ballast, is decreased.
The diagram of Figure 2 shows the achievable
reduction in the amount of flue gas as a function of the
oxygen concentration of the primary air on the one hand and
the recirculated amount, as a percentage of the secondary air,
on the other. The relative volume of flue gas is plotted from
0% to 100% on the abscissa and the recirculated amount, as a
percentage of the secondary air, is plotted from 0% to 100% on
the ordinate. The four curves drawn reproduce the reduction
in the amount of flue gases as a function of the oxygen
concentration of the primary air.
Various experimental results are assembled in
Figures 3 to 6. To help with understanding these results, the
formulas and abbreviations used there are explained below:
02-Conc. - oxygen concentration
NOX-Conc. - nitrogen oxide concentration
HCL-Conc. - hydrogen chloride concentration
-18-
A
212129
S02-Conc. - sulfur dioxide concentration
HF-Conc. - hydrogen fluoride concentration
RGR - flue gas cleaning
PL - primary air
SKL - secondary air
Rezi-Anteil - recirculated amount, as a percentage of the
secondary air
Lambda - air excess factor
In all the experiments described below, the
installation is charged with fuel consisting of garbage, the
hourly amount charged and the calorific value being kept
constant in all experiments, so that the effects of the
inventive measures become clear under constant starting
conditions with respect to the fuel.
In all the experiment diagrams, the amounts of
garbage charged hourly and the calorific value are given in
field a and the amount of primary air, the primary air as a
percentage of the total combustion air, the lambda value, the
oxygen concentration in the primary air and the percentage of
flue gas recirculated are given in field b. The amount of
flue gas arising in the grate region, the oxygen concentration
and the concentration of the important pollutants are listed
in field c. The amount of and the oxygen concentration in the
secondary air are given in field d. The amount of flue gas,
the oxygen concentration in the flue gas, as well as the
amounts of the different pollutants are given in field e. The
amount of flue gas flowing through the boiler, the air excess
factor measured in the boiler, the oxygen concentration as
well as the concentrations of the different pollutants in
mg/Nm3 are assembled in field f. The amount of flue gas, the
oxygen concentration as well as the concentrations of the
-19-
A
212129
different pollutants, in each case before and after the flue
gas cleaning, are assembled in fields g and h. The pollutant
loadings in g/h, which leave the chimney after the cleaning of
the flue gas, are given in field i. Below field i, the
reduction in the amount of flue gases is given as a
percentage.
The experimental results during the normal operation
of an incinerator are given in Figure 3. During this normal
operation, the incinerator is operated with normal outside air
as primary air and with normal outside air as secondary air.
The ratio of primary air to secondary air is 65 . 35. Since
normal outside air is used for the secondary air, there is no
recirculation of flue gas.
In the first experiment, which is summarized in
Figure 3, the normal operation is reproduced. The achievable
reductions in the amounts of flue gas as a percentage compared
to normal operation and the pollutant loads arising as
important results of the inventive combustion process are
given in Figures 3 and 4.
For the experimental results given in Figure 4, the
installation was operated with an oxygen content of 35% by
volume in the primary air, a ratio of primary air to secondary
air of 49.3 . 50.7 without recirculation of the flue gas,
normal outside air being used for the secondary air. Compared
to the normal operation of Figure 3, it was possible to
achieve a reduction of 31% in the amount of flue gas.
Compared to the normal operation, it was possible to reduce
appreciably the pollutant loading of oxidizable pollutants
leaving the chimney every hour, as is evident from a
comparison of the numbers given in field i. On the other
hand, the proportion of NOX increased slightly in comparison
to Figure 3. This can be attributed to the increased oxygen
-20-
21 2 1 2 95
content in the primary air. However, the NOX loading is less
because of the decreased amount of flue gas.
From a further experimental result assembled in
Figure 5, a decrease in the amount of flue gas of 62~ compared
to the normal operation and a further drastic reduction of the
pollutant loading leaving the chimney can be observed. For
this, the installation was operated with an oxygen content of
35~ by volume in the primary air, a ratio of primary air to
secondary air of 65 . 35 and a recirculation of 1000. The
great reduction in the portion of NOX despite the high oxygen
percentage in the primary air can be attributed to the damping
measures achieved by recirculating the flue gas.
For the experiment recorded in Figure 6, the
installation was operated with an oxygen concentration of 50%
by volume in the primary air, a ratio of primary air to
secondary air of 50 . 50 and with 100% recirculation. A
reduction in the amount of flue gas of 75o compared to normal
operation was achieved. If the pollutant loading of this
experiment, given in field i, is compared with that of the
normal operation given in Figure 3, it is observed that the
HC1 loading has decreased from 34.4 g/h to 8.5 g/h; this
corresponds to a decrease of about 75% compared to the normal
operation. For the other pollutant loadings, the reduction is
also around 75% compared to the normal operation. In other
words, not only is the amount of flue gas in the experiment of
Figure 6 reduced by 75~ compared to that of a normal
operation, but a correspondingly high reduction in the
pollutant loading has also been achieved. If the pollutant
concentrations in the flue gas after the cleaning of the flue
gas, for example in the case of the experiment of Figure 3,
that is, of a normal operation, and the pollutant
concentration in the experiment of Figure 6 are considered,
these values in each case being given in field h, it is noted
-21 -
A
z~z~z95
that the values are identical, whereas the corresponding
values before the cleaning of the flue gas, based on the
standard cubic meter of flue gas, are almost four times as
high in the experiment of Figure 6 than in the experiment of
Figure 3, that is, during normal operation. From this it can
be seen that, due to the reduction in the amount of flue gas
and based on the standard cubic meter of flue gas, there is a
significantly higher, that is, about four times as high a
concentration of pollutants in the flue gas before the
cleaning of the flue gas. As a result, this flue gases offers
significantly better preconditions, for example, for the
recovery of hydrochloric acid and gypsum than does the flue
gas from normal operations. For normal operations, the
expenditure for equipment is very high if hydrochloric acid
and gypsum are to be recovered starting out from the
relatively low concentrations of these pollutants.
While the foregoing description and drawings
represent the preferred embodiments of the present invention,
it will be obvious to those skilled in the art that various
changes and modifications may be made therein without
departing from the true spirit and scope of the present
invention.
-22-
A
