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.

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.

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.