GB1592391A

GB1592391A - Method and apparatus for treating waste material

Info

Publication number: GB1592391A
Application number: GB3009577A
Authority: GB
Original assignee: Nichols Engineering and Research Corp
Current assignee: Nichols Engineering and Research Corp
Priority date: 1977-07-18
Filing date: 1977-07-18
Publication date: 1981-07-08

Description

(54) METHOD AND APPARATUS FOR TREATING WASTE MATERIAL (71) We, NICHOLS ENGINEERING & BR< RESEARCH CORPORATION, a corporation of the State of Delaware, U.S.A., of Belle Mead, New Jersey 08502, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to method and apparatus for treating waste material and more particularly to the treatment of the exhaust gases from incinerating waste material. The invention is particularly adapted, among other possible uses, for use in treating the exhaust gases from incinerating municipal, industrial or community garbage, trash or refuse, and sewage sludge, for example.

The problem of preventing air pollution in our present environment has become a critical matter. The present invention is particularly directed to substantially reducing the particulate matter and other impurities appearing in the exhaust gas from furnaces.

The seriousness of this problem is such that the National Air Pollution Control Administration Air Criteria (United States Public Health) as well as the Envirmonmental Protection Agency, have constantly been tightening the minimum required standards.

In view of the new and higher standards of air emission, it has become more difficult and expensive to remove the solid particles from the furnace exhaust gases. We have substantially reduced this problem in a new and improved manner, as will become apparent as the description proceeds.

According to the present invention we provide a counter-flow furnace wherein waste material to be processed is introduced at one end thereof and the processed material is discharged from the other end thereof, while simultaneously air is introduced thereto and the gases of combustion are caused to flow in counter-currenet direction with respect to the material being processed and are exhausted at said one end of the furnace and wherein the furnace has a natural tendency to form zones of processing including sequentially from said one end of the furnace, a drying zone, a charring and volatile burning zone, a fixed carbon burning zone and an ash cooling zone, a method comprising the steps of bleeding secondary exhaust gases from the middle of said furnace to an outlet substantially between said fixed carbon burning zone and said charring and volatile burning zone, and adding supplemental air to said furnace in at least one of said last three zones.

According to a further aspect of the invention we provide apparatus for carrying out the above method.

The invention will be more fully understood from the following description given by way of example with reference to the single figure of the accompanying drawing in which there is shown a diagrammatic illustration, partially in axial, sectional elevation of a furnace system for incinerating waste material.

In the drawing there is shown a multiple hearth furnace 10 of generally cylindncal configuration. Such a furnace may be of the type, for example, as described in detail in United States Patent No. 3,905,757 issued September 16, 1975. The furnace is constructed of a tubular outer steel shell 12, which is lined with fire brick or other similar heat resistant material 14. The furnace is provided with a plurality of burner nozzles 16, with one or more being provided on one or more of the hearths, as necessary, for controlling the temperatures within the different regions of the furnace to carry out the particularly processing desired. Any suitable type of fuel may be provided to the burners.

The interior of the furnace 10 is divided, by means of hearth floors 20 and 22, into a plurality of vertically aligned hearths, the number of hearths being preselected depending on the particular process being carried out. Each of the hearth floors is made of refractory material and is preferably of slightly arched configuration to be selfsupporting with in the furnace. Outer penpheral drop holes 24 are provided near the outer steel shell 12 of the furnace, and central drop holes 26 are formed in alternate hearth floors 22, near the center of the furnace. While Figure 1 shows the uppermost, or first, hearth as being an in-flow hearth, it will be appreciated with the concepts of my invention apply equally well to a furnace having an out-flow first hearth.

As illustrated a rotatable vertical centre shaft 28 extends axially through the furnace 10 and is secured by upper bearing means indicated at 30 and lower bearing means 32.

This centre drive shaft is rotatably driven by an electric motor and gear drive 34, provided for the purpose. A plurality of spaced rabble arms 36 are mounted on the center shaft 28, as at 38, and extend outwardly in each hearth over the hearth floor. The rabble arms have rabble teeth 40 formed thereon which extend downwardly nearly to the hearth floor.The rabble teeth are inclined with respect to the longitudinal axis of their respective rabble arms to that as the rabble arms 36 are carried around by the rotation of the centre shaft 28, the rabble teeth 40 continuously rake through the material being processed on the associated hearth floor and gradually urge the material toward the drop holes 24 and 26 in the hearth floors The material to be processed enters at the top of the furnace at an inlet 42 and passes downwardly through the furnace in a generally serpentine fashion alternately inwardly and outwardly across the hearths and is discharged at the bottom of the furnace, as indicated at 44.

In effect, the furnace is divided into four zones. However, the zones are not finely segregated, but vary depending on the characteristics of the material being processed. For example, the first or upper zone 46, consisting of the first several hearths is a drying zone, and the second zone 48 consisting of the next several hearths is a charring or volatile burning zone. The third zone 50 is a fixed carbon burning zone, and the fourth zone 52 is an ash cooling zone.

Heretofore, in order to support combustion, excess air was added at the bottom of the furnace. It will be appreciated that the hottest part of the furnace is in the central portion thereof, i.e. in the lower portion of zone 48 or in the upper portion of zone 50.

Problems were encountered due to the fact that these middle hearths tended to overheat beyond the structural design limits of the furnace. In order to overcome this problem, it was thought necessary to add more air or oxygen at the bottom of the furnace. Thus, such a system frequently operated with as much as 100% excess air (above that required for supporting combustion) being added at the bottom of the furnace in order to cooling the central portion thereof to workable limits. However, such excess air tended to entrain or carry with it particulate matter into the exhaust gases which all exited from the top of the furnace.

In said furnace all of the exhaust gases exited at the top of the furnace and, hence, there is still the possibility that in some installations some particulate matter could be entrained or carried with the exhaust gases, thereby creating a problem for the downstream scrubbers or other cleaning devices.

In the present invention, there is provided a secondary exhaust outlet 54 at the middle of the furnace, i.e. at about the top of the fixed carbon burning zone 50 or the bottom of the charring or volatile burning zone 48.

By manipulation of valves 56, 58 and 60, this exhaust gas may be led via lines 62 and 64 directly to a gas cleaning device, which may be of any suitable conventional type such as a bag collector, hot cyclone, electrostatic precipitator, or mechanical filter, for example. It will be particularly appreciated that this secondary exhaust gas is relatively rich in "heavy metal" particles and/or vapours such as lead, arsenic and antimony just to name a few. Further, the quantity of these secondary exhaust gases is relatively small, as compared to the total quantity of the exhaust gases exiting at the top of the furnace in conventional designs, and hence, it is economical to treat the secondary exhaust gases separately and in a manner particularly suitable to the characteristics thereof.It will be further appreciated that most of this "heat metal" vapour and/or particulate matter is formed in the lower portion of the furnace, i.e. in the fixed carbon burning zone 50, and hence, according to the invention the upper portions of the furnace, will be relatively free of this matter so that the exhaust gases exiting from the furnace at the upper primary exhaust gas outlet 66 may be cleaned or treated in a different manner, that is particularly suitable for its characteristics.

Since the secondary exhaust gases are bled from the furnace in the middle thereof, as indicated at 54 additional air or oxygen is added to the furnace at 68 in order to facilitate the processing occuring in the upper zone of the furnace. In some installations, it is desirable to provide a heat exchanger 70 so that by proper manipulation of the valves 56, 58 and 60, the secondary exhaust gases serve to heat the air entering the furnace through the inlet 68. This provides added efficiency to the system.

Further it has been found desirable to add a minimum quantity of air to oxygen to the furnace at the inlets 72 in the lower portion of the furnace so that there is a minimum quantity of air in the fixed carbon burning zone 50 and then adding relatively larger quantities of air in the charring or volatile burning zone 48 where it is needed, as by the inlet 68 or by the upper air inlets 74.

WHAT WE CLAIM IS: 1. In a counter-flow furnace wherein waste material to be processed is introduced at one end thereof and the processed mater

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. or first, hearth as being an in-flow hearth, it will be appreciated with the concepts of my invention apply equally well to a furnace having an out-flow first hearth. As illustrated a rotatable vertical centre shaft 28 extends axially through the furnace 10 and is secured by upper bearing means indicated at 30 and lower bearing means 32. This centre drive shaft is rotatably driven by an electric motor and gear drive 34, provided for the purpose. A plurality of spaced rabble arms 36 are mounted on the center shaft 28, as at 38, and extend outwardly in each hearth over the hearth floor. The rabble arms have rabble teeth 40 formed thereon which extend downwardly nearly to the hearth floor.The rabble teeth are inclined with respect to the longitudinal axis of their respective rabble arms to that as the rabble arms 36 are carried around by the rotation of the centre shaft 28, the rabble teeth 40 continuously rake through the material being processed on the associated hearth floor and gradually urge the material toward the drop holes 24 and 26 in the hearth floors The material to be processed enters at the top of the furnace at an inlet 42 and passes downwardly through the furnace in a generally serpentine fashion alternately inwardly and outwardly across the hearths and is discharged at the bottom of the furnace, as indicated at 44. In effect, the furnace is divided into four zones. However, the zones are not finely segregated, but vary depending on the characteristics of the material being processed. For example, the first or upper zone 46, consisting of the first several hearths is a drying zone, and the second zone 48 consisting of the next several hearths is a charring or volatile burning zone. The third zone 50 is a fixed carbon burning zone, and the fourth zone 52 is an ash cooling zone. Heretofore, in order to support combustion, excess air was added at the bottom of the furnace. It will be appreciated that the hottest part of the furnace is in the central portion thereof, i.e. in the lower portion of zone 48 or in the upper portion of zone 50. Problems were encountered due to the fact that these middle hearths tended to overheat beyond the structural design limits of the furnace. In order to overcome this problem, it was thought necessary to add more air or oxygen at the bottom of the furnace. Thus, such a system frequently operated with as much as 100% excess air (above that required for supporting combustion) being added at the bottom of the furnace in order to cooling the central portion thereof to workable limits. However, such excess air tended to entrain or carry with it particulate matter into the exhaust gases which all exited from the top of the furnace. In said furnace all of the exhaust gases exited at the top of the furnace and, hence, there is still the possibility that in some installations some particulate matter could be entrained or carried with the exhaust gases, thereby creating a problem for the downstream scrubbers or other cleaning devices. In the present invention, there is provided a secondary exhaust outlet 54 at the middle of the furnace, i.e. at about the top of the fixed carbon burning zone 50 or the bottom of the charring or volatile burning zone 48. By manipulation of valves 56, 58 and 60, this exhaust gas may be led via lines 62 and 64 directly to a gas cleaning device, which may be of any suitable conventional type such as a bag collector, hot cyclone, electrostatic precipitator, or mechanical filter, for example. It will be particularly appreciated that this secondary exhaust gas is relatively rich in "heavy metal" particles and/or vapours such as lead, arsenic and antimony just to name a few. Further, the quantity of these secondary exhaust gases is relatively small, as compared to the total quantity of the exhaust gases exiting at the top of the furnace in conventional designs, and hence, it is economical to treat the secondary exhaust gases separately and in a manner particularly suitable to the characteristics thereof.It will be further appreciated that most of this "heat metal" vapour and/or particulate matter is formed in the lower portion of the furnace, i.e. in the fixed carbon burning zone 50, and hence, according to the invention the upper portions of the furnace, will be relatively free of this matter so that the exhaust gases exiting from the furnace at the upper primary exhaust gas outlet 66 may be cleaned or treated in a different manner, that is particularly suitable for its characteristics. Since the secondary exhaust gases are bled from the furnace in the middle thereof, as indicated at 54 additional air or oxygen is added to the furnace at 68 in order to facilitate the processing occuring in the upper zone of the furnace. In some installations, it is desirable to provide a heat exchanger 70 so that by proper manipulation of the valves 56, 58 and 60, the secondary exhaust gases serve to heat the air entering the furnace through the inlet 68. This provides added efficiency to the system. Further it has been found desirable to add a minimum quantity of air to oxygen to the furnace at the inlets 72 in the lower portion of the furnace so that there is a minimum quantity of air in the fixed carbon burning zone 50 and then adding relatively larger quantities of air in the charring or volatile burning zone 48 where it is needed, as by the inlet 68 or by the upper air inlets 74. WHAT WE CLAIM IS:

1. In a counter-flow furnace wherein waste material to be processed is introduced at one end thereof and the processed mater

ial is discharged from the other end thereof, while simultaneously air is introduced thereto and the gases of combustion are caused to flow in counter-current direction with respect to the material being processed and are exhausted at said one end of the furnace and wherein the furnace has a natural tendency to form zones of processing including sequentially from said one end of the furnace, a drying zone, a charring and volatile burning zone, a fixed carbon burning zone and an ash cooling zone, a method comprising the steps of bleeding secondary exhaust gases from the middle of said furnace to an outlet substantially between said fixed carbon burning zone and said charring and volatile burning zone, and adding supplemental air to said furnace in at least one of said last three zones.

2. The method according to claim 1 wherein said supplemental air is added to said furnace in the charring and volatile burning zone.

3. The method according to claim 1 or claim 2 characterized in that secondary exhaust gases are passed in heat exchange relationship with respect to said supplemental air being added to the furnace.

4. The method according to claim 1 or claim 3 characterized in that air is added to the furnace in one of the last named two zones in a quantity below that theoretically required for complete combustion of material being processed.

5. The method according to any one of claims 1 to 4, characterized in that the counter-flow furnace consists of a multiple hearth furnace having a plurality of vertically spaced hearths, wherein the waste material is introduced to the furnace at the top thereof and moves downwardly in a generally serpentine fashion alternately inwardly and outwardly across the hearths and is discharged at the bottom of the furnace, while simultaneously air is introduced thereto towards the bottom of the furnace and the gases of combustion are caused to flow in counter-current direction with respect to the material being processed and are exhausted at the top of the furnace.

6. The method according to any one of claims 1 to 5, characterized in that said secondary exhaust gases are directed to a gas cleaning device for removal of heavy metals.

7. The method according to claim 6 characterized in that said gas cleaning device comprises one of the class consisting of a bag collector, a hot gas cyclone, electrostatic precipitator, and a mechanical device for removal of heavy metals comprising at least one of a class consisting of lead, arsenic, chromium and antimony.

8. The method according to any one of claims 1 to 7 characterized in that said primary exhaust gases are directed to a gas cleaning device.

9. - Apparatus for carrying into practice the method according to any one of claims 1 to 8, characterized in that it comprises a device for introducing air into the said furnace, means for causing the gases of combustion to flow in a counter-current direction with respect to the material being processed, a primary exhaust gas outlet disposed at said one end of the furnace, means for bleeding secondary exhaust gases from the middle of said furnace substantially between the fixed carbon burning zone and said charring and volatile burning zone, and means for adding supplemenetal air to said furnace in at least one of said last three named zones.

10. Apparatus according to claim 9 for incinerating waste material, characterized in that it comprises means for adding supplemental air to said furnace in the charring and volatile burning zone.

11. Apparatus according to claim 9 or claim 10 for incinerating waste material, characterized in that it comprises means for passing said secondary exhaust gases in heat exchange relationship with respect to said supplemental air being added to the furnace.

12. Apparatus according to claim 9 or claim 11 for incinerating waste material, characterized in that it comprises means for adding air to said furnace in one of the last named two zones in a quantity below that theoretically required for complete combustion of the material being processed.

13. Apparatus according to any one of claims 9 to 12 for incinerating waste material, characterized in that it comprises a counter-flow furnace consisting of a multiple hearth furnace having a plurality of vertically spaced hearths, a rotatable center shaft extending through the center of the furnace and passing through each hearth, a plurality of spaced rabble arms secured to the centre shaft and extending radially outwardly over each hearth, alternate hearths having drop holes disposed towards the centre shaft and the other hearths having drop holes disposed towards the outer periphery thereof, said furnace having an upper material inlet and a lower material dispensing outlet, and said furnace having an upper exhaust gas outlet, means for bleeding secondary exhaust gases from the middle of said furnace substantially between the fixed carbon burning zone and said charring and volatile burning zone, and means for adding supplemental air to said furnace in at least one of said last three named zones.

14. Apparatus according to any one of claims 9 to 13 characterized in that it comprises means for directing the secondary exhaust gases to a gas cleaning device for removal of heavy metals.

15. Apparatus according to claim 14 characterized in that said gas cleaning device comprises one of the class consisting of a bag collector, a hot gas cyclone, electrostatic precipitator, and a mechanical device for removal of heavy metals comprising at least one of a class consisting of lead, arsenic, chromium and antimony.

16. Apparatus according to any one of claims 9 to 15 characterized in that it comprises means for directing the primary exhaust gases to a gas cleaning device.

17. A method of treating waste material substantially as herein described.

18. Apparatus for treating waste material constructed and arranged substantially as herein described and shown in the figure of the accompanying drawing.