U.S. patent number 3,918,374 [Application Number 05/538,123] was granted by the patent office on 1975-11-11 for method for the disposal of garbage by multi-stage thermal decomposition.
This patent grant is currently assigned to Nippon Kokan Kabushiki Kaisha. Invention is credited to Sho Matsumi, Minoru Yamamoto.
United States Patent |
3,918,374 |
Yamamoto , et al. |
November 11, 1975 |
Method for the disposal of garbage by multi-stage thermal
decomposition
Abstract
In a method for the disposal of garbage by thermal
decomposition, garbage charged into a first incinerator is
thermally decomposed by external heat requiring no oxygen to obtain
generated gas with high calorific value. The whole quantity of said
generated gas is introduced into a second incinerator. Garbage is
charged into said second incinerator in a quantity about four times
that of the garbage charged into said first incinerator, and the
whole quantity of said generated gas introduced from said first
incinerator is burnt with pure oxygen supplied in an amount
corresponding to the theoretical quantity of oxygen necessary for
this combustion. The garbage charged into said second incinerator
is thermally decomposed by the resulting combustion heat to obtain
generated gas with high calorific value in a quantity about four
times that of the generated gas obtained in said first incinerator.
The number of incinerators is increased when necessary, and thermal
decomposition of garbage is carried out in additional
incinerator(s) in the same manner as in said second incinerator,
whereby the quantity of disposed garbage and the quantity of
generated gas are increased in the geometrical series. Generated
gas obtained from the final incinerator is externally used as a
heat source, but part of said product gas is fed back as a heat
source necessary for said incinerators.
Inventors: |
Yamamoto; Minoru (Yokohama,
JA), Matsumi; Sho (Komae, JA) |
Assignee: |
Nippon Kokan Kabushiki Kaisha
(Tokyo, JA)
|
Family
ID: |
11863714 |
Appl.
No.: |
05/538,123 |
Filed: |
January 2, 1975 |
Foreign Application Priority Data
|
|
|
|
|
Feb 6, 1974 [JA] |
|
|
49-14531 |
|
Current U.S.
Class: |
110/346; 110/208;
110/250; 110/229; 110/342 |
Current CPC
Class: |
F23G
5/027 (20130101) |
Current International
Class: |
F23G
5/027 (20060101); F23G 005/12 () |
Field of
Search: |
;110/7R,8R,8C,8E,11,17,23,31 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sprague; Kenneth W.
Attorney, Agent or Firm: Flynn & Frishauf
Claims
What is claimed is:
1. A method for the disposal of garbage by multi-stage thermal
decomposition, characterized by: thermally decomposing garbage at
the first stage by external heat requiring no oxygen; and thermally
decomposing garbage at the second and following stages by
combustion heat, said combustion heat being generated by combustion
of the whole quantity of generated gas obtained by the thermal
decomposition of garbage at the stage directly before the relevant
stage with pure oxygen supplied in an amount corresponding to the
theoretical quantity of oxygen for this combustion.
2. The method of claim 1, wherein said thermal decomposition of
garbage comprises three stages.
3. The method of claim 1, wherein said thermal decomposition of
garbage at said first stage is applied by external heat generated
by an electric heating element.
4. The method of claim 2, wherein said thermal decomposition of
garbage at said first stage is applied by external heat generated
by an electric heating element.
Description
FIELD OF THE INVENTION
This invention relates to a method for the disposal of garbage by
multi-stage thermal decomposition which uses garbage as a heat
source effectively and causes no environmental pollution.
BACKGROUND OF THE INVENTION
Garbage and waste produced in communities must be disposed in some
ways. Especially, the disposal of various kinds of garbage and
waste in large quantities in large cities is one of the important
administrative problems.
A methods for the disposal of garbage, discharging garbage into sea
for reclamation and burying garbage under the ground are known.
However, problems, such as pollution of sea water and difficulty in
getting land, are associated with these methods, and besides the
usefulness of garbage is not exploited as yet.
Therefore, general trend at present is directed toward the disposal
of garbage by the complete incineration. However, in the prevailing
disposal of garbage by incineration, a method is used which burns
garbage on fire grates with large quantities of air supplied, thus
creating such problems as described below:
1. Use of large quantities of air produces large quantities of
exhaust gases, thereby causing air pollution. Further, since the
combustion temperature of garbage is relatively low, the residue of
burnt garbage cannot be completely made harmless.
2. Environmental pollution is caused by effluents.
3. Since the combustion of garbage on fire grates is unstable, the
efficiency of heat recovery is low; that is, it is difficult to use
effectively the heat generated by combustion of garbage.
4. In the fire grate combustion method, vast space must be occupied
by the fire grates, requiring a large area for the site.
Accordingly, the land utility efficiency of the incinerating plant
is low.
5. Difficulty in getting sites for the construction of large
incinerating plants has been increasingly encountered.
Further, as a method for the disposal of garbage, a method for the
disposal of garbage by thermal decomposition has so far been
proposed. The basic principle of this known method comprises
supplying garbage charged into an incinerator with heat necessary
for thermal decomposition and melting the garbage by this heat to
obtain generated gas and slag. In this conventional method, there
are available two processes - a process which uses external heat as
an intense heat source necessary for thermal decomposition and a
process which uses heat generated by partial oxidation of garbage
with air or oxygen supplied. In the former process, since external
heat is used, the problem lies in economy. In the latter process,
since combustion gas gets mixed with generated gas, the calorific
value of generated gas is decreased, disadvantageously making the
usefulness of generated gas inferior to that in the former process.
Accordingly, there are still many problems to be solved in the
methods for the disposal of gabage by thermal decomposition.
SUMMARY OF THE INVENTION
The principal object of this invention is to provide a method for
the disposal of garbagge by multi-stage thermal decomposition by
improving the conventional method for the disposal of garbage by
thermal decomposition.
An object of this invention is to provide a method for the disposal
of garbage by multi-stage thermal decomposition which does not
substantially use air and produces minimum combustionn gas.
Another object of this invention is to provide a method for the
disposal of garbage by multi-stage thermal decomposition which has
a high thermal efficiency and is econominal.
Still another object of this invention is to provide a method for
the disposal of garbage by multi-stage thermal decomposition which
causes no environmental pollution.
A further object of this invention is to provide a method for the
disposal of garbage by multi-stage thermal decomposition which can
be carried out on a relatively narrow site.
This invention is characterized by: thermally decomposing garbage
at the first stage by external heat requiring no oxygen an
thermally docmposing garbage at the second and following stages by
combustion heat, said combustion heat being generated by combustion
of the whole quantity of generated gas obtained by thermal
decompositon of garbage at the stage directly before the relevant
stage with pure oxygen supplied in an amount corresponding to the
theoretical quantity of oxygen necessary for this combustion.
Accordingly, in thermal decomposition of garbage at all stages, use
of air is substantially avoided and combustion of garbage does not
substantially occur.
BRIEF DESCRIPTION OF THE DRAWING
The accompanying drawing is a schematic illustration showing the
application of this invention.
DETAILED DECRIPTION OF PREFERRED EMBODIMENTS
In the method of this invention, thermal decomposition of garbage
is carried out at several stages and the quantity of garbage to be
thermally decomposed is increased in the geometrical series; that
is, said quantity is larger in the second stage than in the first,
larger in the third stage than in the second, and so forth. In the
thermal decomposition of garbage at the first stage, generated gas
with high calorific value which is not diluted with combustion gas
is obtained by using external heat requiring no oxygen, e.g.,
electric heat such as plasma jet heat, arc heat and resistance heat
and such as plasma jet heat, arc heat and resistance heat and solar
heat, as a heat source for thermal decomposition. In the thermal
decomposition of garbage at the second stage, the whole quantity of
generated gas obtained by the thermal decomposition of garbage at
the first stage is used as a heat source for thermal decomposition
and pure oxygen is supplied in an amount corresponding to the
theoretical quantity of oxygen necessary for the combustion of said
generated gas, whereby the amount of combustion gas produced is
minimized and, at the same time dilution of generated gas obtained
by the thermal decomposition of garbage at the second stage with
combustion gas is reduced to a minimum extent. The quantity of
garbage that can be disposed is increased in the geometrical series
by increasing the number of the same stages of thhermal
decomposition of garbage at the second stage. Large quantities of
generated gas obtained by the thermal decomposition of garbage at
the final stage are externally used in an effective manner, but
part of said generated gas is used as energy sources for a pure
oxygen producer used in this invention and also for external heat
for the thermal decomposition at the first stage.
The method of this invention is described below in detail with
reference to a case in which three-stage thermal decomposition of
garbage is used. Supposing:
Lower calorific value per kg of garbage: R kcal,
Calorific value required for thermal decomposition of 1 kg of said
garabage: E kcal,
Quanity of generated gas obtained by thermal decomposition of 1 kg
of said garbage (the water content is excluded; same applied
hereafter): G kcal,
Calorific value of said generated gas obtained by thermal
decomposition of 1 kg of said garbage: Q kcal, and
Theoretical quantity of oxygen necessary for combustion of 1 kg of
said generated gas: 0 kg
Then, in thermal decomposition of ordinary town garbage, the
following equations was found to hold roughly between R, E, G, Q,
and O:
G = 0.4 kg (1) Q = 4E (2) Q = 0.8R (3) O = 1.7 kg (4)
In the thermal decomposition of garbage at the first stage, the
quantity of generated gas obtained by thermal decomposition of 1 kg
of garbage is calculated to be about 0.4 kg by Eq. (1), and the
calorific value possessed by said genereated gas is calculated to
be about four times the calorific value required for thermal
decomposition by Eq. (2). According to Eq. (3), about 80% of the
lower calorific value of garbage becomes the calorific value of
generated gas and the remaining about 20% remains in slag formed by
thermal decomposition of garbage as fixed carbon, etc. In the
thermal decomposition of garbage at the first stage, as mentioned
above, since external heat, such as electric heat and solar heat,
is used as a heat sourc for thermal decomposition, oxidation, i.e.,
combustion of garbage does not occur at this stage. Accordingly
generated gas obtained by the thermal decomposition of garbage at
the first stage will not be diluted by combustion gas.
In the thermal decomposition of garbage at the second stage, the
whole quantity of generated gas obtained by the thermal
decomposition of garbage at said first stage is used as a heat
source for thermal decomposition. Therefore, the total calorific
value of the heat source for thermal decomposition at the second
stage is about four times the total calorific value of the heat
source for thermal decomposition at the first stage. More
specifically, if the total calorific value of external heat used as
a heat source for thermal decomposition at the first stage is
expressed by E.sub.1 kcal, the total calorific value of generated
gas obtained by the thermal decomposition at the first stage is
expressed by Q.sub.1 kcal, and the total calorific value of the
heat source for thermal decomposition at the second stage is
expressed by E.sub.2, the following equations hold according to Eq.
(2):
Q.sub.1 = 4e.sub.1
e.sub.2 = q.sub.1 = 4e.sub.1 (5)
therefore, the quantity of garbage that can be disposed by the
thermal decomposition at the secocnd stage is about four times the
quantity of garbage thermally decomposed at the first stage. The
thermal decomposition at the second stage yield generated gas
having a total calorific value about 16 times that of the heat
source for thermal decomposition at the first stage; that is, if
the total calorific value of generated gas obtained by the thermal
decomposition at the second stage is expressed by Q.sub.2 , the
following equation holds according to Eqs. (3 ) and (5 ):
Q.sub.2 = 4e.sub.2 = 4q.sub.1 = 16e.sub.1 (6)
in order to use generated gas obtained by the thermal decomposition
at the first stage as a heat source for thermal decomposition at
the second stage, said generated gas is burnt, at the second stage,
with pure oxygen supplied. Combustion gas produced by this
combustion is mixed with generated gas obtained by the thermal
decomposition at the second stage. It is necessary, however, to
prevent said generated gas from being diluted by said combustion
gas to the greatest possible extent. For this purpose, the
quantitty of said pure oxygen is limitetd to the theoretical
quantity of oxygen necessary for combustion of said generated gas,
thereby causing only said generated gas to be burnt. Accordingly,
oxidation, i.e., combustion of garbage does not occur also at the
second stage. Said theoretical quantity of oxygen is calculated to
be about 1.7 kg per kg of generated gas by Eq. (4).
In the thermal decomposition of garbage at the third stage, a gas
mixture of the whole quantity of generated gas obtained by the
thermal decomposition of garbage at said second stage and
combustion gas is used as a heat source for thermal decomposition.
Therefore, the quantity of garbage that can be disposed by the
thermal decomposition at the third stage is about four times the
quantity of garbage disposed at the second stage, or about 16 times
the quantity of garbage disposed at the first stage. The thermal
decomposition at the third stage yields generated gas having a
total calorific value about 16 times that of the heat source for
the thermal decomposition at the second stage, or 64 times that of
the heat source for the thermal decomposition at the first stage.
More specifically, if the total calorific value of the heat source
for thermal decomposition at the third stage is expressed by
E.sub.3 and the total calorific value of generated gas obtained by
the thermal decomposition at the third stage is expressed by
Q.sub.3, the following equations hold according to Eqs. (2) and
(6):
E.sub.3 = q.sub.2 = 16e.sub.1 (7)
q.sub.3 = 4e.sub.3 = 64e.sub.1 (8)
in order to use a gas mixture of generated gas obtained by the
thermal decomposition at the second stage and combustion gas as a
heat source for thermal decomposition at the third stage, only said
generated gas is burnt, at the third stage, by supplying said gas
mixture with pure oxygen in an amount corresponding to the
theoretical quantity of oxygen necessary for combustion of said
generated gas. Accordingly, oxidation, i.e., combustion of garbage
does not occur also at the third stage. Also generated gas obtained
by the thermal decomposition of garbage at the third stage is mixed
with combustion gas.
The proportion of combustion gas mixing with generated gas
increases with increasing number of stages of thermal decomposition
of garbage, resulting in deterioration of generated gas. Therefore,
it is not advisable to increase the number of thermal decomposition
unlimitedly. The above-mentioned three-stage thermal decomposition
is preferable from the standpoint of economy and practical use.
The foregoing is summarized in the following table:
Stage of thermal decomposition 1st 2nd 3rd stage stage stage Item
__________________________________________________________________________
Relative ratio of the quan- tity of garbage disposed by thermal
decomposition at 1 4 16 the respective stages Relative ratio of the
total calorific value of heat source for thermal decom- E.sub.1
4E.sub.1 16E.sub.1 position at the respective stages Relative ratio
of the total calorific value of generated 4E.sub.1 16E.sub.1
64E.sub.1 gas at the respective stages Relative ratio of the total
quantity of generated gas 0.4 1.6 6.4 at the respective stages
Relative ratio of the theor- etical quantity of oxygen 0 1.7
.times. 0.4 1.7 .times. 1.6 required at the respective = 0.68 =
2.72 stages Relative ratio of the total quantity of combustion gas
0 0.4 + 0.68 1.08 + 1.6+ at the respective stages = 1.08 2.72 = 5.4
Generated gas/combustion 1.6/1.08 6.4/5.4 gas ratio at the respec-
-- = 1.48 = 1.18 tive stages
__________________________________________________________________________
As is apparent from the table, since the total calorific value of
generated gas obtained by the thermal decomposition of garbage at
the third stage corresponding to 64E.sub.1 is by far larger than
the total calorific value E.sub.1 of the heat source for thermal
decomposition of garbage at the first stage, part of said total
calorific value of generated gas obtained by the thermal
decomposition of garbage at the third stage can be easily fed back
as an energy source for the heat source for the thermal
decomposition at the first stage.
If n-stage thermal decomposition is to be carried out, the quantity
of garbage disposed at the n-th stage and the total calorific value
of generated gas at n-th stage are respectively given by the
following equations:
Quantity of garbage disposed at the n-th stage = (Quantity of
garbage disposed at 1st stage).times. 4.sup.n.sup.-1
Total calorific value of generated gas at the n-th stage = (Total
calorific value necessary for thermal decomposition at 1st
stage).times.4.sup.n
EXAMPLE
The method of this invention is described in more detail with
reference to the accompanying drawing.
A plant comprising three incinerators with a capacity for disposing
garbage by thermal decomposition as mentioned below:
First incinerator: 1t/hr
Second incinerator: 4t/hr
Third incinerator: 16t/hr
Therefore, this plant has a garbage disposal capacity of 21t/hr and
of 504t/day for 24-hours operation. This scale is large enough as a
unit for incinerating plant.
External heat requiring no oxygen, e.g., electric heat such as
plasma jet heat, arc heat and resistance heat and solar heat, is
preferably used as a heat source for the thermal decomposition of
garbage in the first incinerator 1, in order to prevent generated
gas obtained in the first incinerator 1 from being diluted by
combustion gas, etc. In this example, as hown in the figure, a
plasma torch 3 is provided in the first incinerator 1 as an
electric heating element. Garbage 2 charged into the first
incinerator 1 by taking off a cover 1' of said first incinerator 1
is thermally decomposed by intense heat generated by said plasma
torch 3 to yield generated gas and slag. Said generated gas and
slag are spouted through an outlet 4 of the first incinerator 1
down into the second incinerator 5.
An oxygen feeding aperture 7 is provided near an opening of said
outlet 4 extending into the second incinerator 5. Said generated
gas introduced from the first incinerator 1 into the second
incinerator 5 is burnt in the second incinerator 5 with pure oxygen
supplied through said oxygen feeding aperture 7 in an amount
corresponding to the theoretical quantity of oxygen. The intense
heat generated by this combustion thermally decomposes garbage 6
charged into the second incinerator 5 by taking off a cover 5' of
said second incinerator 5 to yield generated gas and slag. Said
generated gas is fed, together with combustion gas produced in the
second incinerator 5, from a gas outlet 8 provided in the lower
part of the side of the second incinerator 5 through a bottom feed
opening 11 of the third incinerator to the third incinerator 10. On
the other hand, slag formed from garbage in the first incinerator 1
and in the second incinerator 5 is discharged through a slag
discharge opening 9 provided at the bottom of the second
incinerator 5 therefrom.
Another oxygen feeding aperture 7' is provided near said bottom
feed opening 11 of the third incinerator 10. Said generated gas
introduced from the second incinerator 5 into the third incinerator
10 is burnt in the third incinerator 10 with pure oxygen supplied
through said oxygen feeding aperture 7' in an amount corresponding
to the theoretical quantity of oxygen. The intense heat generated
by this combustion thermally decomposes garbage 15, charged into
the third incinerator 10 by taking off a cover 12 of said third
incinerator 10, to yield generated gas and slag. Said generated gas
is fed, together with combustion gas produced in second incinerator
5 and in the third incinerator 10, from a gas outlet 13 provided in
the upper part of the third incinerator 10 to, e.g., an electric
power facility, where said generated gas and said combustion gas
are used to obtain utility power and electric power for the heat
source for the thermal decomposition in the first incinerator 1. On
the other hand, slag formed from garbage in the third incinerator
10 is discharged through a lag discharge opening 14 provided in the
lower part of the side of the third incinerator 10 therefrom.
The above-mentioned example is further described in accordance with
concrete numerical values.
The lower calorific value R per kg of garbage used in this example
is about 2,000 Kcal. The total calorific value E.sub.1 of plasma
jet heat of said plasma torch 3 as a heat source for thermal
decomposition of said first incinerator 1 is roughly calculated by
Eqs. (2) and (3) as follows:
E.sub.1 = q.sub.1 /4 = 0.8r .times. 1,000/4 = 4 .times. 10.sup.5
kcal/h =465 kW
The total calorific value Q.sub.3 of generated gas obtained in said
third incinerator 10 is roughly calculated by Eq. (8) as
follows:
Q.sub.3 = 64e.sub.1 = 64 .times. 4 .times. 10.sup.5 = 25.6 .times.
10.sup.6 kcal/h = 30,000 kW
Generated gas obtained in said third incinerator 10 is used as fuel
for a waste head boiler to generate electricity by driving a
generator by means of a steam turbine. Supposing in this case the
efficiency of the boiler to be about 80%, the efficiency of the
turbine to be about 40%, the efficiency of the generator to be
about 90%, and the total efficiency of these installations to be
about 30%, the electric power produced from generated gas obtained
in said third incinerator 10 is roughly calculated as follows:
30,000 kW .times. 0.3 = 9,000 kW
The total heat balance from the above-mentioned calculations shows
that the electric power of external heat as a heat source for
thermal decomposition of garbage in the first incinerator 1 is 465
kW, whereas the electric power produced from generated gas obtained
in the third incinerator 10 is 9,000 kW. Therefore, it is possible
to carry out the thermal decomposition of garbage in the first
incinerator 1 by feeding back only about 5% of the electric power
obtained.
Further, the quantity of pure oxygen corresponding to the
theoretical quantity of oxygen necessary for combustion of
generated gas obtained by the thermal decomposition of garbage in
the first incinerator 1 and in the second incinerator 5 is
calculated to be about 3,400 kg/h, i.e., about 2,500 Nm.sup.3 /h by
Eqs. (1) and (2) as follows:
Quantity of pure oxygen for the combustion of generated gas in the
first incinerator:
1.7 kg .times. 0.4 .times. 1,000 = 680 kg/h
Quantity of pure oxygen for the combustion of generated gas in the
second incinerator:
1.7 kg .times. 1.6 .times. 1,000 = 2,720 kg/h
Since electric power of about 0.7 kWh per Nm.sup.3 of pure oxygen
is required to produce pure oxygen by an air separator, the
electric power required for said quantity of pure oxygen is
calculated as follows:
0.7 kWh/Nm.sup.3 .times. 2,500(Nm.sup.3 /h) = 1,750 kW
Accordingly, if part of the obtained electric power is fed back
also to said air separator, the electric power that can be supplied
to external utility is calculated as follows:
9,000 kW - (465 kW + 1,750 kW) = 6,785 kW
An example of the composition of generated gas obtained in this
plant is in the case of common urban garbage as follows: H.sub.2
40% by volume CO 20% by volume CH.sub.4 10% by volume C.sub.2
H.sub.4 5% by volume C.sub.m H.sub.n 3% by volume CO.sub.2 10% by
volume Others 12% by volume
The composition of said generated gas is largely dependent upon the
thermal decomposition temperature of garbage. Therefore, the
thermal decomposition temperature of garbage at all incinerators in
preferably regulated to the range between about 700.degree.C and
about 900.degree.C.
Further, the purity of oxygen used for the combustion of generated
gas in the second incinerator 5 and in the third incinerator 10 may
be such purity as can be obtained by a usual oxygen separator. Slag
from garbage discharged through slag discharge opening 9 and 14 of
the second incinerator 5 and of the third incinerator 10 is a melt
comprising glass, metals, earth, sand, etc. in which noxious
matters such as heavy metals are fixed in a chemically stable
condition and has a high strength after solidification.
Accordingly, said slag can be used as reclamation material,
pavement material, concrete aggregate, etc.
According to this invention as amplified above, thermal energy of
garbage is obtained as generated gas by thermal decomposition, and
said generated gas is used as a heat source for thermal
decomposition of garbage at the next stage. Repetition of this
cycle enables large quantities of garbage to be disposed
economically and effectively. Moreover, the area of site for the
incinerating plant is relatively narrow. Further, since thermal
decomposition of garbage is carried out substantially with no air,
nitrogen oxides are scarcely produced and environmental pollution
is not caused by the disposal of garbage, thus producing
industrially useful effect.
* * * * *