CA2185964A1

CA2185964A1 - Process for the thermal disposal of loose refuse

Info

Publication number: CA2185964A1
Application number: CA002185964A
Authority: CA
Inventors: Hans Ruegg; Thomas Ungricht; Ruedi Frey; Stefan Forsberg; Ernst Hugentobler; Patrick Muller
Original assignee: Von Roll Umwelttechnik AG
Current assignee: Hitachi Zosen Innova AG
Priority date: 1995-10-06
Filing date: 1996-09-19
Publication date: 1997-04-07
Also published as: HU9602730D0; NO964229L; HUP9602730A1; JPH09126428A; CH691404A5; CZ286435B6; EP0767342A1; EP0767342B2; PL316406A1; CZ290496A3; ATE198790T1; JP3049210B2; EP0767342B1; HU216861B; NO964229D0; DE59606339D1

Abstract

Loose refuse is pyrolysed without pretreatment in a first stage with turning and transportation and with feed of a gas containing at least 40% oxygen, a substoichiometric amount of oxygen - based on the com-bustible constituents of the refuse - being fed. In a second stage, the solid pyrolysis product is melted by combustion of its combustible constituents and, if appropriate, by combustion of pyrolysis gases by means of impingement with a gas containing at least 40% oxygen. As much oxygen is fed as is necessary to produce the heat of fusion.

Description

-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.

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 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 2 1 8~964 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 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 - 2 1 85q64 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 2~ 85~64 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.

2 ~ 8~964 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 g 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 21 ~5964 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.

Claims

1. Process for the thermal disposal of loose refuse, in which at least some of the combustible constituents of the refuse are burnt and incombustible solid constituents of the refuse are melted, characterized in that, in a first stage, the loose refuse is pyrolysed with turning and transportation and with feed of a gas containing at least 40% oxygen, a substoichiometric amount of oxygen - based on the combustible constituents of the refuse - being fed, and a solid pyrolysis product and combustible pyrolysis gases being produced, and, in a second stage, the solid pyrolysis product is melted by combustion of its combustible constituents and, if appropriate, by combustion of pyrolysis gases by means of impingement with a gas containing at least 40% oxygen, as much oxygen being fed as is necessary to produce the heat of fusion.

2. Process according to Claim 1, characterized in that, in the second stage, additional heat of fusion is supplied by partial combustion of combustible pyrolysis gases from the first stage.

3. Process according to Claim 2, characterized in that the combustion of combustible pyrolysis gases in the second stage proceeds with additional feed of oxygen into the space above the melt, the heat produced in the gas combustion being radiated on to the melt.

4. Process according to one of Claims 1 to 3, characterized in that combustible pyrolysis gases being subjected to an afterburning with heat recovery.

5. Process according to Claim 4, characterized in that the temperature in the afterburning is 850 to 900 degrees C.

6. Process according to Claim 4 or Claim 5, charac-terized in that the afterburning is carried out in an afterburning chamber (14) functioning according to the principle of a circulating fluidized bed.

7. Process according to one of Claims 1 to 6, characterized in that the melt from the second stage is post-treated in a third stage under reducing conditions, a glass melt and a metal melt being obtained separately.

8. Process according to Claim 7, characterized in that the post-treatment of the melt in the third stage is carried out in two successive part-stages, the glass melt from the first part-stage being fed to the second part-stage and post-treated and obtained there, the metal melt being obtained separately from both part-stages.

9. Process according to one of Claims 1 to 8, characterized in that, in the first stage, the refuse is transported, with turning, on a grate (5) through a combustion compartment (4) of a stationary pyrolysis furnace (1) and is pyrolysed in the course of this.

10. Process according to one of Claims 1 to 8, characterized in that, in the first stage, the refuse is pyrolysed in a rotary kiln combustion compartment con-structed as a rotary drum.

11. Process according to one of Claims 1 to 10, characterized in that the oxygen-containing gas used is technical-grade oxygen containing at least 90% oxygen.

12. Process according to one of Claims 1 to 11, characterized in that the solid pyrolysis product is impinged by the gas containing at least 40% oxygen at at least the velocity of sound.

13. Apparatus for carrying out the process according to Claim 1, characterized by a pyrolysis furnace (1) for carrying out the first stage, which has means for trans-porting and turning the refuse through a combustion compartment (4), into which opens an inlet orifice for the gas containing at least 40% oxygen, and by a melting furnace (10), which follows the pyrolysis furnace (1), for carrying out the second stage which is provided with means for impinging the solid pyrolysis product from the first stage with the gas containing at least 40% oxygen.

14. Apparatus according to Claim 13, characterized in that the melting furnace (10) has a vertical, cylindrical furnace compartment (12) into which open the means for impinging the solid pyrolysis product from the first stage with the gas containing at least 40% oxygen, preferably in the form of lances directed downwards at an angle.

15. Apparatus according to Claim 14, characterized in that the lances open into the furnace compartment (12) tangentially to an imaginary circle lying at a distance from the wall.

16. Apparatus according to Claim 13, characterized in that the pyrolysis furnace (1) has a stationary combus-tion compartment (4), the means for transporting and turning the refuse being formed by a grate (5) arranged in this combustion compartment (4).

17. Apparatus according to Claim 13, characterized in that the pyrolysis furnace is constructed as a tubular kiln, the means for transporting and turning the refuse being formed by a combustion compartment constructed as a rotary drum.