Claims
- 1. A fluidized bed furnace, particularly for burning partly dehydrated sludge, comprising a furnace housing having a bottom for supporting a fluidized bed, means for directing heated gases including combustion air upwardly from said bottom into the fluidized bed for maintaining said bed, said furnace having side walls extending upwardly from said bottom above said bed and defining a combustion chamber above said bed, a substantially horizontal furnace roof extending between said side walls defining a top of said combustion chamber and having at least one sludge charging opening therein for charging the sludge downwardly into said combustion chamber and also having a post combustion chamber opening, post combustion chamber wall means defining a post combustion chamber extending upwardly from said post combustion chamber opening and being substantially smaller in cross-section than the cross-section of said combustion chamber, and means for supplying secondary air into said post combustion chamber.
- 2. A fluidized bed furnace, according to claim 1, including a conduit connected to said sludge charging opening having a sludge treatment device.
- 3. A fluidized bed furnace, according to claim 1, wherein said combustion chamber is of a cross section which includes a central rectangular portion with an end portion on each side of the rectangular portion of a semi-circular configuration, said post combustion chamber opening being located adjacent at least one of the semi-circular portions.
- 4. A fluidized bed furnace, according to claim 1, wherein the combustion chamber has a cross-section which includes a central elongated rectangular portion with a semi-circular portion at each end, said post combustion chamber being located centrally of said rectangular portion, said sludge opening being located adjacent an end semi-circular portion.
- 5. A fluidized bed furnace, according to claim 1, wherein said side walls enlarge upwardly from said bottom at an angle of from 15.degree. to 20.degree. relative to the vertical.
- 6. A fluidized bed furnace, according to claim 1, wherein the upper portion of said side walls extends obliquely inwardly to said furnace roof, and a burner in each obliquely extending portion extending downwardly toward said combustion chamber.
- 7. A fluidized bed furnace, according to claim 1, wherein said post combustion chamber comprises a radiation recuperator for heating the combustion air directed through the fluidized bed.
- 8. A fluidized bed furnace, according to claim 1, wherein said means for supplying said secondary combustion air comprises a conduit connected tangentially into said post combustion chamber.
- 9. A fluidized bed furnace, according to claim 1, wherein said post combustion chamber is of rectangular configuration.
- 10. A fluidized bed furnace, according to claim 1, wherein said post combustion chamber includes two rectangular side portions opposite to each other which are mounted on said furnace roof.
- 11. A fluidized bed furnace according to claim 1, wherein the interior of said furnace has a width at least as great as the height.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 2356058 |
Nov 1973 |
DT |
|
FIELD AND BACKGROUND OF THE INVENTION
This invention relates in general to the construction of furnaces and, in particular, to a new and useful fluidized bed furnace for burning partly dehydrated sludge and to a method of burning such sludge in the furnace.
The present invention is particularly directed to a fluidized bed furnace for burning partly dehydrated sludge. With such a furnace, the fluidized bed includes a fill of refractory grains and means for supplying air from below for producing the fluidized bed and partly for supplying the necessary combustion air. It is known that particularly sewage sludge can be successfully burned in a fluidized bed furnace in which partly dehydrated sludge is employed so that the burning operation takes place without the necessity of supplying appreciable amounts of heat from the outside. The process is controlled so that the gases which are annoying by their odor are securely burned so that the odor of the resultant discharge is eliminated. This is ensured by maintaining a temperature of from 800.degree. to 900.degree.C in the combustion zone. The ashes which are produced during the combustion of the sludge should not be melted down but they should be carried out by the flue gases to the greatest possible extent in order that the ashes may be subsequently separated from the flue gases. In such arrangements, the sludge to be burned is fed in either from a bin which is located above the fluidized bed furnace through a gravity tube extending centrally through the fluidized bed furnace down to the level of the combustion zone, or they are fed laterally directly into the fluidized bed by means of a feeding screw. With the first manner of feeding, it is assumed that the sludge may be supplied into the furnace from a level which corresponds to the fluidized bed level. However, this is not always possible since, for hydromechanical reasons, the sludge is mostly available at the ground level or even near the water surface level. Therefore, the sludge must first be transported into a bin located above the furnace for which purpose, in view of the nature of the sludge, only a very expensive device, such as a bucket elevator or the like, are suitable for such operation. With the second method of feeding directly into the fluidized bed, the pressure is very irregular so that the performance of the fluidized bed is correspondingly low. Also, in this case, the fluidized bed is intensely cooled down at the feed inlet and this may result in interruptions of the operation.
The present invention provides a fluidized bed furnace which is designed such that the transportation costs for the sludge are minimized without causing an excessive throughput in the fluidized bed. This advantage, as well as others are obtained in accordance with the invention by providing a furnace roof above the combustion chamber which is substantially horizontal and which is provided with an opening for attaching a post combustion chamber thereabove which has a substantially smaller cross-section than the combustion chamber. A supply connection for secondary combustion air is connected into the post combustion chamber. The furnace roof is also provided with another opening for feeding the sludge downwardly into the combustion zone.
With the inventive arrangement of fluidized bed furnace, it is assumed that the sludge to be burned is transported only to the level of the furnace roof. From this level, the feeding is possible without the use of a gravity tube so that the expense for such a tube and its replacement during the operation are avoided. A particular advantage is that the distribution of the feed in sludge within the combustion chamber is such that the contact with the fluidized bed in which the supply sludge is dried, pulverized and partly burned and subsequently gasified and incinerated is approximately central and relatively uniform. The relatively voluminous combustion chamber of the inventive arrangement ensures an extensive burning out of the combustible gases escaping from the fluidized bed and air is supplied at the bottom to maintain the bed. The post combustion takes place in a relatively small post combustion chamber. Since the flow velocity is higher in the post combustion chamber because it has a substantially smaller cross-section than the main combustion chamber, the preponderant part of the ashes is entrained at such location. This can be advantageously utilized by designing the post combustion chamber as a radiation recuperator because the dust particles permit a great heat transfer by radiation. At the same time, the drop of the flue gas temperature entails an improvement of the conditions for the final dust separation.
In view of the reduction of sludge transportation costs with the invention, it is particularly advantageous to provide sludge treatment devices on the roof of the furnace so that only at this location is the sludge brought into a state more suitable for feeding. Consequently, the sludge can be easily pumped in a still relatively very fluid state to the roof level at which pumping may be easily carried out since the roof level is at a height which is not above the normal pumping height at atmospheric pressure. By appropriate further treatment in centrifuges, filter presses, or the like, the sludge can be dehydrated to the necessary extent and thereby made suitable for feeding directly at the furnace roof.
Since with the invention, the difficulties in the transportation of sludge which is feedable from above are largely eliminated, the post combustion chamber can be designed, as to its length, as a recuperator, so that a large part of the heat produced by the sludge combustion may be recuperated. This is of great importance particularly for sludge combustion because the water content allowable in the sludge to be burned can be higher the more the combustion air supplied into the fluidized bed and the combustion chamber is preheated. Thus, the novel fluidized bed furnace simultaneously provides conditions for an appreciable reduction of the sludge dehydration costs.
By making the cross-section of the combustion chamber substantially larger than the post combustion chamber, in accordance with the invention, and by also designing the chambers so that the combustion chamber has semi-cylindrical ends and intermediate rectangular portions, an optimal design is effected. With the invention, therefore, the usual circular cross-section of the fluidized bed may be abandoned. The central rectangular portion of the combustion chamber may be elongated or made relatively short and the inlet of the post combustion chamber is provided either in the zone of the end half-cylindrical portions or in the center of the rectangle. In the case where the post combustion chamber connects into the center of the central rectangular portion, a feed opening for the sludge may be provided in the zone of each of the two semi-cylindrical portions so that, even with an elongated fluidized bed, a uniform contact is ensured. In a variation of the invention, oval or even triangular cross-sections may be provided for the combustion chamber so that the conditions for a regular operation of the fluidized bed can be obtained also in larger furnace units.
Accordingly, it is an object of the invention to provide a fluidized bed furnace, particularly for burning partly dehydrated sludge, which includes a furnace housing which has a bottom for supporting a fluidized bed with means for directing heated gases, including combustion air, upwardly through the bed, and which includes side walls extending upwardly from the bottom above the bed and defining a combustion chamber above the bed with a substantially horizontal roof extending over the combustion chamber, and having at least one sludge charging opening therein for charging the sludge downwardly into the combustion chamber and a connection for a post combustion chamber which extends upwardly from the main combustion chamber and it is substantially smaller in cross-section than the main combustion chamber and which is provided with means for supplying secondary air thereto.
A further object of the invention is to provide a method of operating a fluidized bed furnace which comprises a bottom over which a fluidized bed is formed and a post combustion chamber connected into a main combustion chamber above the fluid bed, comprising directing heating gases, including combustion air, upwardly through the bottom to maintain a fluidized bed above the bottom and to maintain a combustion of gases above the fluidized bed, directing the gases which are formed through a relatively small diameter post combustion chamber, continuously maintaining post combustion in the post combustion chamber by adding additional combustion air thereto and charging the furnace with sludge from the roof thereof downwardly into the combustion chamber.
A further object of the invention is to provide a fluidized bed furnace which is simple in design, rugged in construction, and economical to manufacture.
For an understanding of the principles of the invention, reference is made to the following description of typical embodiments thereof as illustrated in the accompanying drawing.
US Referenced Citations (6)
Patent Grant
3941065
