Note: Descriptions are shown in the official language in which they were submitted.
2 1 85964
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Process for the thermal disposal of loose refuse
The invention relates to a process for the
thermal disposal of loose refuse in accordance with the
preamble of Claim 1.
Processes for the thermal disposal of loose
refuse have been disclosed, for example, by Swiss Patent
482 988 or Swiss Patent 432 703, in which combustible
constituents of the refuse are burnt in a melting furnace
and incombustible constituents are taken off as melt. In
all previous processes of this type, the inhomogeneity of
the refuse and the fluctuation in performance and pre8sure
resulting therefrom are a great problem, which has made
pretreatment of refuse essential to date. The pretreatment
usually comprises sorting, prel;m;nAry comminution, m;Y;ng
and homogenization of the refuse. However, even using these
measures, it is not possible to ensure that, for example,
relatively large, refuse-bound quantities of water are not
suddenly charged into the melting furnace, which lead to
great fluctuations in pressure there owing to explosive
evaporation. In the processes disclosed by the above-
mentioned publications, the refuse is therefore predried in
a separate compartment, in order to be subsequently burnt
and melted in a melting compartment with feed of additional
fuels. The use of external fuels to produce the necessary
melting temperature makes the process more expensive and is
therefore uneconomic. The object underlying the present
invention is to propose a more economic process of the type
mentioned at the outset, in which, without complex special
pretreatment of the refuse and without use of external
energy, manageable, preferably constant, pressure
conditions are created.
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This object i8 achieved according to the
invention by the features specified in the characterizing
part of Claim 1.
Surprisingly, it has been found that the exhaust
gases from the process according to the invention contain
only a small amount of nitrogen oxides, so that the
denitration, which is otherwise necessary in the CleAn; ng
of exhaust gases from refuse incineration plants and is
complex, can be omitted. A further advantage of the process
according to the invention is m;n;~; zing the amount of flue
gas, as a result of the use of gases having a high oxygen
content. The considerable reduction in the amount of flue
gas means that downstream flue gas cle~n;ng and flue gas
cooling equipment can be built 80 as to be small and thus
inexpensive. Moreover, a small amount of exhaust gas leads
to low pollutant emissions, when existing limit values are
complied with.
The process according to the invention is
described in more detail below with reference to the
drawing.
A pyrolysis furnace is designated 1 in the
drawing. The refuse to be treated, in particular domestic
refuse, is charged without special pretreatment, in
particular without homogenization, into a feed hopper 2 in
a manner not shown in more detail and, using a propor-
tioning plunger 3 arranged at the lower end of the feed
hopper 2, is pushed on to a grate 5, which is arranged in a
combustion compartment 4 of the pyrolysis furnace 1. With
each forward stroke of the propor~tioning plunger 3, the
same amount of refuse is fed to the grate 5. Grates of thi~
type are known, for example, from refuse combustion. The
refuse, in a first process stage, is transported through
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the combustion compartment 4 on the grate 5 and dried and
pyrolysed in the course of this. With feed of a gas
containing at least 40% oxygen, combustible gases from the
refuse are burnt above the refuse layer and the radiant
heat from this combustion operation effects the pyrolysis.
The oxygen is fed in a substoichiometric amount - based on
the combustible constituents of the refuse. To feed the gas
having the high oxygen content into the combustion
compartment 4, a plurality of elements 7 which are
distributed with respect to location open into the
pyrolysis furnace 1 above the refuse layer situated on the
grate 5. The elements 7 can preferably be constructed as
gas lances, nozzles or tubes provided with radial bore
holes. The oxygen-cont~;n;ng gas used can be technical-
grade oxygen having an oxygen content of about 90%, oxygen-
enriched air or pure oxygen. The feed of the gas cont~;n;ng
at least 40% oxygen can be controlled on the basis of the
temperature development in a desired manner, 80 that
manageable temperature conditions are ensured in the
pyrolysis furnace 1.
The oxygen introduced into the combustion com-
partment 4 above the refuse layer situated on the grate 5,
together with the combustible volatile substances escaping
from the refuse, forms flames. The refuse layer is heated
by the thermal radiation until pyrolysis occurs. The
volatile substances burn partially. The solid pyrolysis
product is slag containing combustible constituents. The
combustible constituents are principally carbon. In the
process, drying the refuse and thfe volatilization of as far
as possible all volatile constituents of the refuse and
generation of a solid, dry pyrolysis product having as high
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as possible a content of combustible constituents is sought
after.
The grate 5, in addition to the transport func-
tion, also ensures constant turning of the waste, 80 that
new waste surfaces are continuously exposed to the thermal
treatment in the combustion compartment 4. For this
purpose, a grate pathway has a plurality of grate block
tiers arranged sequentially in a stepped manner which are
known per se and therefore not shown in detail, stationary
and movable grate block tiers alternately following one
another respectively. The refuse situated on the grate 5 is
advanced and simultaneously turned by a thrust motion of
the movable grate block tiers. The inclined grate pathway
can, moreover, be composed along its length of a plurality
of grate elements 6, which have movable and immovable grate
block tiers. In the drawing, three such grate elements 6,
6', 6", which can be driven separately, are indicated dia-
grammatically. Furthermore, a plurality of grate pathways
can be mounted adjacently, which form the width of the
grate. The number of the grate elements 6 and the grate
pathways depends on the preset throughput rate of the
refuse.
The solid pyrolysis product and the unburnt
volatile substances are fed to a melting furnace 10
connected downstream of the pyrolysis furnace 1.
In the melting furnace 10, a second process
stage i8 carried out. The combustible constituents of the
~olid pyrolysis product are burnt with supply of a gas
containing at least 4~% oxygen an~d the incombustible
constituents are melted. The injection of the oxygen-
cont~;n;ng gas, preferably technical-grade oxygen
cont~;n;ng approximately 90% oxygen (this could
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alternatively be oxygen-enriched air or pure oxygen,
however), is indicated only diagrammatically by arrow 11 in
the drawing and is effected in reality via lances or
nozzles directed on to the surface of the melt bath or on
to the solid pyrolysis product burning and melting on the
melt bath. The injection is preferably performed at a
velocity which corresponds at least to the speed of sound.
This achieves sufficient vortexing and mixing of the refuse
layer.
The melting furnace 10, in a preferred embodi-
ment, has a vertical, cylindrical furnace compartment 12,
into which lances directed downwards at an angle in a
manner not shown open tangentially to an imaginary circle,
80 that the impingement with the gas having the high oxygen
content sets the melt into a rotary movement, which ensures
good m; ~; ng, rapid melting and uniform combustion. It is
likewise of advantage, if the gas-feeding nozzles or lances
open into the furnace compartment 12 at a distance from the
wall lining; the movement of the melt in the vicinity of
the wall is then ~;n;m~l, which also m;n;~;zes the
thermal/mechanical loading of the wall lining. The
combustible constituents of the solid pyrolysis product can
burn smoothly and without pressure fluctuations directly on
the rotating melt bath, the heat energy obtained by this
means being sufficient to melt the incombustible
constituents without additional fuel being required. Since
the combustible gases produced in the pyrolysis furnace 1
can, in the preferred embodiment shown, likewise be passed
through the melting furnace 10 (however, they could
alternatively be subjected directly to afterburning with
external heat energy utilization), some of these pyrolysis
gases can also be conjointly burnt and can likewise
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contribute to the production of heat energy for the melting
operation. However, dep~n~;ng on the heat requirement,
additional gas combustion can be performed by additional
injection of oxygen into the upper area of the furnace
compartment 12 above the melt bath. This thermal
supplementation effects radiation downwards on to the melt
of the heat generated in the gas combustion and increases
the efficiency of the melting process.
In the second process stage, sufficient oxygen
0 i8 fed 80 that the combustion process tAking place in the
melting furnace 10 generates the required heat of fusion,
but no oxygen excess is present, 80 that no undesired
oxidation reactions proceed and no undesirably high
temperatures result.
Dividing the process into two stages smooths the
pressure and temperature peaks, as a result of which the
process becomes more manageable and the loading of the
plant is reduced.
The gases from the melting furnace 10 which are
still combustible are subjected to afterburning in an
afterburning chamber 14 with thermal energy recovery. The
afterburning chamber 14 is constructed as a fluidized-bed
reactor functioning on the principle of a circulating
fluidized bed, to which the gases are fed from the melting
furnace lOa as fluidizing gases and are subjected to
afterburning by feed of oxygen and/or combu~tion air
(indicated by arrow 13 in the drawing). As fluidized bed
solids, use can be made of quartz sand, lime or other
materials.
The fluidized-bed reactor i8 operated at a gas
velocity such that at least some of the solid particles are
discharged from the afterburning chamber 14 together with
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the flue gas stream. Having arrived via a line 15 in a dust
separator 16, the solids are separated from the flue gas
stream. The dust separator 16 can be constructed, for
example, as a cyclone. The separated solids are preferably
recycled into the afterburning chamber 14 via an external
fluid-bed cooler 17, 80 that the circulating fluidized bed
is formed. In the fluid-bed cooler 17, the solids removed
in the dust separator 16 are cooled in a stationary
fluidized bed (fluid bed) by direct or indirect heat
transfer and are then reintroduced into the afterburning
chamber 14 via a line 18. In the afterburning chamber 1~,
these solids absorb the heat from the hot gases from the
melting furnace 10 and heat up to the ~;~; ng temperature
prevailing in the afterburning chamber 14. However, it
would also be possible to construct the walls of the
afterburning chamber 14 as cooling or heat-transfer
surfaces or to arrange other heat-transfer surfaces
directly in the fluidized bed. These heat removal measures,
alone or in combination with the external fluid-bed cooler
17, would be suitable in order to be able to operate the
afterburning chamber 14 below the flue dust melting point
at an optimum temperature of about 900 degrees C.
The turned and rolled over solids produce a
highly homogeneous temperature distribution in the
afterburning chamber 14. This creates optimum and uniform
reaction conditions for the afterburning. The circulating
fluidized bed permits very efficient cooling of the hot
gases.
As indicated in the draiwing by arrow 20, oxygen-
cont~;n;ng gases are fed to the fluid-bed cooler 17 as
fluidizing gases, which are taken off again in a manner not
shown in detail above the fluid bed for further use.
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The flue gases which are completely burnt, freed
from solids and cooled flow via a line 19 to further flue
gas cle~n;ng or flue gas cooling devices which are not
shown, before they pass to the atmosphere. Since the amount
of flue gas, in comparison with previously known refuse
disposal processes, is considerably reduced by the use of
oxygen both in the first and in the second process stage
and also in the afterburning, the size of such devices and
their complexity can be decreased. The melt from the
melting furnace 10 is further treated under reducing
conditions in a third process stage. The melt flows
continuously from the melting furnace 10 into a downstream
first slag treatment furnace 23, where the heavy metal
oxides present in the melt are converted into their
metallic form by reduction. The temperature setting in this
further treatment of the melt is dependent on the desired
reduction rate. For this purpose, for example, carbon
electrodes can be used in the slag treatment furnace 23 as
heating electrodes which simultaneously act as reducing
agents. Correspon~;ng processes for treating solid residues
from refuse combustion plants and apparatuses for carrying
out the process are subject-matter of European Patent
Application No. 95116647.9 and International Patent
Application No. PCT/CH/95/00204.
The low-volatility reduced heavy metals collect
in a liquid phase at the bottom of the slag treatment
furnace 23 and can be tapped off from there (cf. tap hole
25 and pan 26 in the figure).
In a preferred embodime~nt shown in the figure,
the first slag treatment furnace 23 is connected in the
upper area to a second slag treatment furnace 24. The glass
melt from the first slag treatment furnace 23 flows into
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the second slag treatment furnace 24, where renewed
settling of metal still present in the glass melt proceeds.
The metal melt collected in the slag treatment furnace 24
is removed via the tap hole 27 and collected in the pan 28.
The glass melt - then substantially free from
environmentally harmful heavy metals - flowing from the
second slag treatment furnace 24 can, after cooling and
granulation in a water bath 29, be used as building
material, for example clinker subætitute, in the building
industry.
To carry out the first process ~tage, the refuse
pyrolysis, instead of the pyrolysis furnace 1 provided with
a grate and shown diayLa~atically in the drawing, a
tubular kiln can also be used, in which the combustion
compartment is designed as a rotary drum, through which the
refuse is transported with simultaneous turning and is
pyrolysed in the course of this with feed of the gas
cont~;n;ng at least 40% oxygen. A process of this type and
a tubular kiln of this type are subject-matter of a
20 European Patent Application No. 95 112 657.2.
The process of the invention is explained in
more detail below by a working example.
Example:
In a plant for carrying out the process of the
invention, 6000 kg/h of refuse are fed to the pyrolysis
furnace 1 and pyrolysed with feed of 1300 m3 (S.T.P.)/h of
technical-grade oxygen (93% oxygen content). The solid
pyrolysis product is l~ntroduced i~to the melting furnace
30 10, where it i8 impinged by an amount of 1000 m3 (S.T.P.)/h
of technical-grade oxygen and melted. The afterburning of
the combustible pyrolysis gases in the afterburning chamber
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14, operating on the principle of the circulating fluidized
bed, proceeds with addition of 1130 m3 (S.T.P.)/h of
technical-grade oxygen (93%). 3000 m3 (S.T.P.)/h of
fluidizing air are fed to the external fluid-bed cooler 17.
(m3 (S.T.P.) denotes cubic metre~ at st~n~rd temperature
and pressure.)
In the melting furnace 10, melt i6 produced at a
rate of 1180 kg/h. From this melt, 1100 kg/h of granules
are produced as building material, for example clinker
substitute, and 80 kg/h of metal alloy are obtained.
