GB2030273A

GB2030273A - >Apparatus for treating combustible wastes

Info

Publication number: GB2030273A
Application number: GB7928246A
Authority: GB
Original assignee: Hitachi Zosen Corp
Current assignee: Hitachi Zosen Corp
Priority date: 1978-09-12
Filing date: 1979-08-14
Publication date: 1980-04-02
Also published as: DK160365B; JPS5538859A; DK379079A; JPS5940184B2; GB2030273B; DE2934831C2; DE2934831A1; FR2436175A1; CH641830A5; SE7907530L; DK160365C; FR2436175B1; SE434273B

Abstract

A shaft furnace for treating combustible wastes by pyrolysis has a drying and preheating zone C, a pyrolysis zone B and a char gasification and oxidation zone A. The waste is dried and pyrolyzed with the combustion gas produced in the oxidation zone. The pyrolysis zone B is defined by a furnace wall 11b tapered to give the pyrolysis zone B a cross sectional area gradually decreasing downward to maintain the pressure between the furnace wall 11b and the waste in the pyrolysis zone B at a level not lower than a specified value and thereby prevent channeling. A furnace wall 11a continuously extending downward from the lower end 12 of the pyrolysis zone B is partly flared to give the oxidation zone A a cross sectional area gradually increasing downward and a combustion space D below the oxidation zone may have vertical or downwardly flared walls. <IMAGE>

Description

SPECIFICATION Apparatus for treating combustible wastes The present invention relates to an apparatus for treating combustible wastes by pyrolyzing the wastes to recover useful gas and oil.

Various apparatus for treating combustible wastes have heretofore been proposed which comprise a shaft furnace providing a moving bed.

With such apparatus, wastes are charged into the shaft furnace at its top and incinerated with oxygen or air, and steam supplied at the bottom of the furnace. The shaft furnace provides, as arranged from top to bottom in its columnar interior space, three zones which distinctly differ from each other in operating temperature and in the composition of the resulting product, namely a drying and preheating zone, a pyrolysis zone and a char gasification and oxidation zone. While the interior of the shaft furnace must be maintained in a stable condition during the treatment of the wastes, one of the serious problems encountered against stability is that the wastes are liable to form a bridge while descending the interior of the furnace, because the bridging phenomenon, when taking place, could lead to complete cessation of the furnace operation in an extreme case.To avoid this problem, a furnace has already been proposed which has a gradually decreasing cross sectional area from the bottom of the furnace to its top.

With this furnace, however, a hollow combustion gas channel extending from the char gasification and oxidation zone to the drying and preheating zone will be formed in the pyrolysis zone without permitting the combustion gas to pass through small interstices between the wastes and come into full contact with the wastes for pyrolysis. A reduced pyrolysis efficiency will therefore result.

The channeling phenomenon poses a serious problem and is difficult to overcome especially when treating wastes, such as municipal refuse, which have widely varying compositions.

The main object of the present invention is to provide an apparatus for treating combustible wastes which comprises a shaft furnace and in which channeling can be prevented effectively.

To fulfil this object, this invention provides a treating apparatus comprising a shaft furnace including a furnace wall defining at least a lower portion of a pyrolysis zone and tapered to give the pyrolysis zone a cross sectional area gradually decreasing downward, the shaft furnace including a char gasification and oxidation zone having a larger cross sectional area than the lower end of the pyrolysis zone.

With the treating apparatus, the wastes in the pyrolysis zone can be treated with the pressure between the wastes and the wall of the furnace maintained at least at a specified level, so that pyrolysis can be effected free of channeling, giving a useful gas in an increased amount and with an improved composition, hence an enhanced treating efficiency. Since a proper combustion space is available in the char gasification and oxidation zone, the wastes can be incinerated effectively and thus utilized advantageously.

According to a preferred embodiment of the invention, the pyrolysis zone is defined by a furnace wall tapered downward over the entire pyrolysis zone, and at least an upper portion of the char gasification and oxidation zone is defined by a furnace wall which flares to give this zone a cross sectional area gradually decreasing downward.

Other features and advantages of the invention will become apparent from the following description of the preferred embodiments with reference to the accompanying drawings, in which: Fig. 1 is a view in vertical section schematically showing the construction of a first embodiment of the invention; Fig. 2 is a view similar to Fig. 1 and showing a second embodiment of the invention; Fig. 3 is a graph showing variations in the temperature of the furnace wall of the apparatus of the invention at the lower end of its drying and preheating zone; and Fig. 4 is a graph similar to Fig. 3 and representing a conventional apparatus.

With reference to Fig. 1, a shaft furnace 1 has a top portion 2 extending from its upper end and provided with a produced gas outlet 2a. At the upper end of the top portion 2, there is provided a feeder 3 for charging wastes into the furnace 1 while sealing the interior of the furnace. The feeder comprises a hopper 4 having a funnelshaped upper end, a feeding duct 5 for keeping the lower end opening of the hopper 4 in communication with the interior of the top portion 2, and a pair of slidable dampers Sa, 6b disposed at two locations spaced apart axially of the feeding duct 5. The furnace 1 is chargeable by opening the dampers 6a and 6b in succession, while the interior of the furnace 1 can be sealed off by closing one damper 6a or 6b when the other damper 6b or 6a is opened at all times.The shaft furnace 1 has a columnar interior space 10 which is divided into a char gasification and oxidation zone A at a lower portion, a pyrolysis zone B at an intermediate portion above the zone A and a drying and preheating zone C at an upper portion.

The waste placed into the furnace 1 is eventually incinerated in the oxidation zone A at the lower portion, producing a combustion gas, which pyrolyzes the waste in the pyrolysis zone B above the zone A, causing the waste to evolve a useful gas. The combustion gas further removes an excess of waterfrom another portion of the waste in the preheating zone C above the zone B to permit smooth reaction in the pyrolysis zone B.

Accordingly the oxidation zone A, the pyrolysis zone B and the preheating zone C are not distinctly bordered as predefined space areas but are formed with some overlapping portions under a given condition. However, the interior space of the furnace 1 is so divided that the boundaries between the zones can be approximately located.

The shaft furnace 1 has a furnace wall 1 1 b defining the pyrolysis zone B and tapered to give the zone B a cross sectional area which gradually decreases downward. The oxidation zone A is defined by a furnace wall 11 a, which is flared downward at the portion where the wall 1 la is continuous with the boundary 12 between the pyrolysis zone B and the oxidation zone A, thus giving the zone A a cross sectional area which gradually increases downward and affording a sufficient combustion space. The other portion of the furnace wall 11 a defining the oxidation zone A and a furnace wall 1 a defining the preheating zone C extend straight vertically, giving a constant cross sectional area to the corresponding portion of the zone A and to the zone C at any level.The furnace wall 1 lib defining the pyrolysis zone B has an angle of inclination, relative to a phantom vertical line, which will not permit the waste to form a bridge while the waste descends the interior of the furnace. The angle varies with the size of the furnace.

With the construction described above, the waste placed into the furnace 1 through the feeder 3 is dried while passing downward through the preheating zone C and then reaches the pyrolysis zone B which is formed by the downwardly tapered furnace wall 11 b. Because the zone B is thus defined, a pressure of not lower than a specified level is maintained between the waste and the furnace wall during the whole period of passage of the waste through the pyrolysis zone B, consequently preventing channeling. The gas produced in the pyrolysis zone B flows through the preheating zone C and is drawn off through the outlet 2a at the top portion 2. The oil contained in the produced gas is separated from the gas at room temperature for recovery.The waste then flows from the pyrolysis zone B through the boundary 12 into the oxidation zone A with a sufficient combustion space and is effectively incinerated. The oxidation zone A yields a residue containing some char. The residue is discharged from the furnace through a layer D below the oxidation zone.

Figs. 3 and 4 show the results of experiments conducted to substantiate the advantages of the invention. Fig. 3 shows the results achieved by the invention, and Fig. 4 those achieved by a conventional apparatus having a cylindrical furnace wall with no tapered inner surface. The experiments were conducted with use of testing furnaces having an inside diameter d of 1,500 mm, a height L of 5,500 mm (see Fig. 1) and a capacity of 20 tons/day. In the case of the furnace of Fig. 3, the furnace wall 1 b defining the pyrolysis zone had an angle of inclination of 1 20 with respect to a phantom vertical line, while Fig.

4 shows the case in which the furnace had no inclination as stated above. The temperature of the furnace wall close to the lower end of the preheating zone C was measured every hour with four thermometers T, toT4 arranged around the furnace at angular spacing of 90 deg. The occurrence and degree of channeling can be detected from the difference between temperatures measured by the thermometers.

With reference to Fig. 3, the thermometers T1 toT2 indicated no high peaks in the temperature variations measured, thus revealing absence of channeling, whereas Fig. 4 shows that channeling took place between 14:00 and 1 7:00 at or near the location of the thermometerT4.

When the angle of inclination was 100 or smaller under the foregoing test conditions, channeling occurred in the furnace, which therefore failed to fully achieve the effects of the invention. However, when having an altered inside diameter, the furnace will invariably have an altered angle of inclination, while it is very likely that the furnace operation will be affected by the size and composition of the waste, the height of the furnace, etc. Thus the invention is characterized in that the furnace wall defining the pyrolysis zone is tapered downward at a suitable angle. The invention is not limited to a particular angle of inclination, therefore.On the other hand, other experiments have revealed that the most suitable angle of inclination is 120 when the ratio of the size g of the waste to the furnace inside diameter d, namelyg/d, is in the range of 0.015 to 0.05 and the ratio of the height I of the charge in the furnace to the furnace inside diameter d, namely lId, is in the range of 2.6 to 2.0.

Fig. 2 shows a second embodiment in which the furnace walls 11 c and 11 bare tapered downward over the combined height of from the upper end of the preheating zone C to the lower end of the pyrolysis zone B, while the furnace wall 1 a extending from the boundary 12 is flared downward entirely from the upper end of the oxidation zone A to its lower end. The second embodiment thus constructed achieves the same effects as the first embodiment.

Although not shown, both the furnace walls 1 C and 1 b defining the preheating zone C and the pyrolysis zone B respectively may be tapered downward, with the furnace wall 1 C of the preheating zone C inclined at a smaller angle than the furnace wall 11 b of the pyrolysis zone B.

Claims

1. An apparatus for treating combustible wastes by pyrolysis comprising a shaft furnace having a columnar interior space divided into a drying and preheating zone, a pyrolysis zone and char gasification and oxidation zone as arranged from an upper portion of the furnace toward its lower portion, the shaft furnace including a furnace wall defining at least a lower portion of the pyrolysis zone and tapered to give the pyrolysis zone a cross sectional area gradually decreasing downward, the shaft furnace further including a furnace wall defining the char gasification and oxidation zone and giving the oxidation zone a cross sectional area larger than the cross sectional area of the lower end of the pyrolysis zone.

2. An apparatus as defined in claim 1 wherein the pyrolysis zone is defined by a furnace wall tapered downward over the entire pyrolysis zone.

3. An apparatus as defined in claim 2 wherein the preheating zone is defined by a furnace wall extending straight vertically and giving a constant cross sectional area to the preheating zone at any level.

4. An apparatus as defined in claim 2 wherein the preheating zone is defined by a furnace wall tapered to give the preheating zone a cross sectional area gradually decreasing downward, the furnace wall of the preheating zone having a lower end continuous with the upper end of the furnace wall of the pyrolysis zone.

5. An apparatus as defined in claim 4 wherein the furnace wall of the preheating zone has substantially the same inclination as the furnace wall of the pyrolysis zone.

6. An apparatus as defined in claim 1 wherein the furnace wall of the oxidation zone flares downward at least at the portion thereof continuous with the lower end of the furnace wall of the pyrolysis zone and gives the oxidation zone a cross sectional area gradually increasing downward.

7. An apparatus as defined in claim 6 wherein the portion of the furnace wall of the oxidation zone lower than the upper flaring portion thereof extends straight vertically and gives a constant cross sectional area to the oxidation zone at any level below the flaring portion.

8. An apparatus as defined in claim 6 wherein the entire furnace wall defining the oxidation zone is flared.

9. Apparatus for treating combustible wastes by pyrolysis substantially as described herein with reference to and as illustrated in Figures 1 and 3 or Figure 2 of the accompanying drawings.