US2029576A

US2029576A - Apparatus for burning low-grade fuels, especially refuse

Info

Publication number: US2029576A
Application number: US718486A
Authority: US
Inventors: Kolb Viktor
Original assignee: American Lurgi Corp
Current assignee: American Lurgi Corp
Priority date: 1933-05-20
Filing date: 1934-03-31
Publication date: 1936-02-04
Anticipated expiration: 1953-02-04

Description

Feb. 4, 1936. v. KOLB 2,029,576

APPARATUS FOR BURNING LOW GRADE FUELS, ESPECIALLY REFUSE Filed March 31,- 1934 wAv/w/w/w A N WWA WAWAYWQYN Patented Feb. 4, 1936 Ti' @FEICE APPARATUS FOR BURT-ENG MOW-GRADE FUELS, ESPECIALLY REFUSE Application March 31,

1934, Serial No. 718,486

In Germany May 30, 1933 2 Claims.

This invention relates to apparatus for burning low-grade fuels, especially refuse.

The burning of refuse and the utilization of the resulting heat of combustion for steam raising, is attended with special difficulties on account of the low calorific value, low combustibility, low ignitability, irregular composition and usually very high moisture content of this material. For this reason, numerous apparatus have been proposed for burning refuse, in which these unfavourable properties have been taken into consideration by the employment of suitable expedients.

For instance, it has been proposed to dry the refuse before delivering it to the furnace in which it is to be burned. Another method is to introduce a charge of the undried material into a furnace in which it is ignited and blasted with air until sufficiently burned and then expelled by means of a pusher and replaced by a new charge.

A suitable manner of quickly and evenly igniting the new charge consists in the provision of a starting fire, which is most simply made by leaving a residue of the cake of cinders in the furnace during expulsion by the pusher. Before admitting the fresh charge, this residue is evenly distributed over the grate, either by hand or by a spreader attached to the pusher plate, so that the new charge lies on a thin layer of glowing cinders and begins to burn as soon as the blast is turned on.

The burned cake from the furnace is preferably passed into a secondary combustion chamber, in which the air is passed over or through it in order to complete the burning of any combustible matter that may still remain.

The object of the present invention is the improvement and acceleration of the combustion of refuse in shaft or hearth furnaces equipped with a blast grate.

According to the present invention, the refuse is thrown loosely on to the grate of a main furnace, on which a starting fire is already in position. When combustion is in progress, the refuse is compressed, to a certain extent, by a pusher plate, but not discharged from the furnace, and after the pusher plate has been withdrawn, a new charge is placed in the free space resulting from said compression. The charging of the grate and the compressing of the burning charge are repeated several times. The entire charge is then left to burn out completely, and the whole of the burnt mass is discharged from the furnace by opening the far wall and (Cl. 1l015) advancing the pusher plate close up to the outlet. The ejected clinker is passed, for example, into a truck, bunker or secondary-combustion chamber.

Alternatively, the wall facing the pusher plate is opened during, at least, the greater portion of the time the charge is being compressed. During the forward movement of the pusher plate, it compresses the nearest portion of the charge, the compression being gradually propagated, as the plate continues to advance, through the bed of fuel, until the latter is finally moved forward, in its entirety, by the plate. As the result, that portion of the charge that is nearest the outlet and, having been longest in the furnace, is already well burned and sintered, is forced out of the furnace. When sufficient quantities of the burnt charge have been discharged, which-according to the method of workingis, for example, the case, when the pusher plate has advanced a longer or shorter distance along the grate, the pusher is then withdrawn, as previously described, and a fresh charge of refuse is introduced.

Finally, if the combustion is to be carried on as a practically continuous operation, the forward movements of the pusher must be short in comparison with the length of the grate. Between each successive forward stroke of the pusher, the free space that has been formed in front of it is filled with fresh refuse. The interval .between the successive strokes of the pusher, and therefore between the successive admission of the several charges, depends on the condition that, before each fresh charge is introduced,

the preceding charge must have ignited and be burning freely. The refuse situated on the various portions of the grate in the main furnace are in different stages of combustion. The portion of refuse immediately in front of the pusher is ignited and begins to burn, whilst that more remote from the pusher is in a state of active combustion. At the far end of thegrate, the combustion has progressed to such an extent that clinkering has begun. At each forward stroke of the pusher, the refuse in its immediate vicinity on the grate which has not yet started to clinker, is not only pushed forward but is also compressed and thus rendered more suitable for further combustion. At the same time, a portion of the already clinkered refusewhich is consequently incapable of further compressionat the discharge end of the grate is pushed off the grate and drops, for example, into a secondary-combustion chamber.

naturally Combinations of these various methods of operating are also feasible.

In all cases the manner of effecting, in accordance with the present invention, the compression of the loose, deep bed of refuse lying on the grate of the main furnace, produces a series of specially advantageous effects.

By pushing a portion of the charge before it, the pusher plate disperses the cavities, nests and other irregularities which were initially present in the charge, or have been formed therein, during combustion, owing to irregularities in the degree and rate at which combustion proceeds in the several portions of the charge. At the same time, the more and less readily combustible portions become intermingled and brought into closer proximity. The formation of gaps through which the air can escape freely is thereby pre vented, and the distribution of the air of cornbustion in the charge is rendered highly uniform. This results in a substantial acceleration of the combustion, which affects all portions of the charge more uniformly and leads to stable conditions in respect of the exhaust gases, (temperature, dew point, CO2 content and so forth) thereby improving the evaporation value and erhciency of a succeeding steam boiler.

On the pusher plate being returned'into its original position, the layer of fuel breaks down along its free face and slopes down towards the exposed portion of the grate, so as to cover the latter with a more or less shallow layer of the burning: refuse, thus serving as a starting fire, the production of :which therefore needs no further manual labour orany auxiliary mechanical appliances.

The method of operating according to the presentinvention also improves the texture of theclinker, which becomes more compact, its

- contentof unconsumed constituents being also substantially lessened.

Moreover,-apparatus according to the present invention can now be constructed in large units. If desired, a plurality of grates, with separate pusher plates, can be disposedside by side in a common combustion chamber. The direction in which the pressureis exerted upon the bed of fuel may vary. For example, given suitably modifieddevices the refuse may be compressed laterally by pushing it transversely across .thegrate,

- along a diagonal or in other ways.

'To enable thewall of the combustion chamber, at the end opposite the pusher plate, to Open and close properly, it is advisable that the closure member should take the form, for example, of a slide and be provided. with'cutting edges which will displace and remove any obstructing clinker when the member is being closed.

According to the present invention, it is also advisable, as in theknown process, to employ well preheated air for combustion.

The various portions-of the bed of fuel require different amounts of air for combustion. According to the invention, this circumstance can be taken into consideration, for example by subdividing the device' for admitting the air below the grate, and supplying different quantities of air to the several compartments. Very hot air is required for igniting the freshly charged material. In many cases a high blast temperature is no longer of. such importance half way through the combustion process, but the supply of air must be particularly abundant at that stage. Finally, towards the end of the cornbustion, both the amount and temperature of the air can often be reduced, without inconvenience. When a grate in which the air is admitted in zones is employed, each section of the grate can be supplied with the proper amount of air, at suitably controlled temperature, for carrying out the combustion of the refuse in that section.

Similar results can also be obtained in other ways, for example by varying the apertures of the nozzles in the several ections of the grate.

The efficiency of the furnace can also be affected by pre-drying the refuse. This, however, is successful only when the drying gases are passed through a relatively thin layer of the refuse.

The best method is to combi c the drying with the charging device by d ling 1e shaft through which the fresh refuse is charged on to the grate so as to serve, at the same time, as a pre-drying apparatus, portion of the gases of I fro is a blast grate or slotted grate, constructed, for

example, of refractory material. This has the advantage of considerably throttling the gases that are drawn through it, so that the gases enter the refuse situated in the drying shaft, in the form of fine, penetrating jets. drawn through the rier by means of a special fan which exhausts the gases. or a mixture consisting of exhaust gases from the combustion furnace and air, through the drier.

In drying the refuse, the drying gases flowing through the drier become laden with malodorous constituents. For this reason, according to the present invention; the gases coming from the drier are not passed direct to the combustion chamber of the furnace, but are led to the underside of the grate and through the layer of burning refuse. This procedure is particularly favourable in the case of a grate provided with zonal admission of air. In such case, the gases from the pro-drying apparatus are conducted to certain zones of the grate, for example to those in which the combustion has already passed the maximum stage and clinkering has commenced. In this manner, the malodorous gases from the pre-drier are compelled to pass through the incandescent and partly incinerated layer of refuse. In order to prevent the combustion from being adversely affected in these zones, a certain amount of fresh air isintroduced into the gases, before or after their entry into the predrier, which air may be pre-heated if desired. Contact with the glowing clinker completely and effectually destroys all the malodorous constituents in the gases.

With the apparatus according to the invention, such a high concentration of heat can be attained that no ignition arch, or the like-such as otherwise serve for improving the heat concentration-is needed above the furnace. The grate carriessuch a deep layer of burning refuse, and

The gases are 1 the combustion according to the invention is so intensive, that radiation, from above, on to the fire plays a merely unimportant part. On this account, the height of the space above the grate in the combustion chamber may be considerable, and if the height be sufiicient, a steam boiler, such as is often employed for the direct utilization of the heat of combustion in refuse-burning plants, may be located in the space directly above the grate, without any risk of adversely cooling the fire or damaging the boiler. In such case, the boiler tubes form part of the boundary of said combustion chamber and constitute a cooling surface, which is in nowise injurious, but entirely beneficial, in action. Large quantities of flue dust are formed in the burning of refuse and are often liable to fuse and incrust the walls of the combustion chamber, as the result of the high heat concentration due to the (often very high) temperatures of combustion. If, however, a portion of the walls of the combustion chamber be bounded by boiler tubes, and thereby cooled, this inconvenience is precluded, inasmuch as the fine particles of ash carried up into the vicinity of the heating surface of the boiler, are cooled down to such an extent that they can no longer fuse, but are deposited in the boiler fines or drop back on to the grate.

In order more clearly to understand the nature of the invention, reference is made to the accompanying drawing, which illustrates diagrammatically and by way of example, one embodiment of refuse furnace according to the invention.

In said drawing the figure shows a diagrammatical section through a refuse furnace provided with a pre-drier.

22 is the combustion chamber of the furnace, with the blast grate 23 under which are located the

twyer boxes

24, 25, 25 with which the air pipes 2'1, 28 and the drying-gas pipe 29 communi cate. A pusher plate 3d, adapted to be moved, in short strokes, by mechanical or preferably hydraulic means, is located on one side of the grate, the clinker pit 35 being on the opposite side. The furnace wall next the clinker pit is provided with cleaning and servicing doors 32. In front of the grate, and in a separate masonry chamber, is the drying shaft 33, which is preferably of rectangular cross section and widened from the top downwards. The walls of this are provided with a plurality of openings, the wall 34 nearest the combustion chamber being preferably of refractory material and provided with narrow nozzles or slots, whereas the opposite wall 35 of the shaft may consist of perforated sheet metal. The drier is charged by means of a device 36, preferably a rocking chute. The charge material first passes through a magnetic separator 3'3, in which coarse pieces of iron are removed and are delivered on to a conveyor 38, the rest of the charge material passing into the drying shaft The gases for pre-drying said material are drawn in from the combustion chamber 22 through the opening 3bprovided with a control device lil enabling the amount of gas employed for drying to be variedand the further opening it, the fan 32 being employed for this purpose. The gases that have passed through the drier are led, by the pipe 43, under the last zone or box 29 of the grate 23. The refuse is continuously removed from the drying shaft by some suitable appliance, preferably a worm conveyor i t. The dried refuse delivered charging shaft 33 arranged as a drier.

grate t5, from which it is dislodged by the ram 56 on to the main grate every time the pusher plate 3i] has returned to its original position. The burning refuse is gradually moved onward over the grate 23 by the pusher plate 35!, and the clinkers fall into the pit 3i, from which they are discharged, through the breaker rollers ll, to a conveyor it. The gases of combustion pass through the combustion chamber to the boiler 45, the tubes 59 of which are arranged so as to limit the combustion chamber at the upper end. After passing the boiler, the flue gases are conveyed to an air heater (not shown) by the pipe 55. To enable hot gases to be supplied separately to the air heater, if desired, a damper 52 is providcd, through which a portion of the gases of combustion can be led direct from the first boiler fiue to the air heater.

In other respects, a furnace of with means, according to vention, operates approximately as follows:-

The refuse is introduced into the bunker 53, for example by means of grabs, and the heap of refuse is broken up into a Wide and comparatively shallow stream by the rocking chute 36, forming th bottom of the open-front charging bunker, this stream being delivered into the Before reaching the drier, the refuse can be freed from this type, fitted coarse particles of iron by an electromagnetic I separator 31 The refuse then gradually descends, by gravitation, through the drying shaft 33, and is dried, during its passage, by fire gases, or mixtures of these gases with air or the like, drawn transversely through the perforated walls 3% and of the drier and the column of refuse in the shaft. It is then removed from the bottom of the shaft by a discharging device, which may consist, for example, of a worm 44. Each time the pusher plate 38 has completed a stroke, the

refuse is delivered, by the ram 45, from the grate E5 on to the grate 23, in front of the pusher. After the loose, deep pile of refuse on the grate has ignited, the pusher moves a short distance along the grate, compressing the freshest refuse and finally pushing the entire charge before it. The refuse that is furthest away from the pusher and is already in a clinkered condition, then drops from the grate into a clinker pit 3! which,

if desired, may be designed as a secondarycombustion chamber. The pusher now returns to its original position, thus leaving a free space in front of it. The now freely exposed face next the pusher-of the pile of refuse that has been compressed and moved along the grate 001- i lapses, being no longer supported by the pusher, and comes to rest at its natural angle, facing the pusher, so that between the refuse and the pusher, the grate is covered with a thin layer of glowing refuse. A fresh charge of refuse is then delivered in front of the pusher. These operations are repeated at short intervals. Each fresh charge passing from the drier into the furnace is immediately brought into a state of active combustion by the pre-heated air blown into the first twyer box, the bottom layer of burning refuse from the preceding charge igniting the new charge very quickly. After compression, intense heat is present in the refuse on the midway por-. tion of the grate, and an abundant supply of air is admitted to that section from the second twyer box. The gases from the drier are blown-in, with a little auxiliary air, through a third twyer box. If desired, the clinker pit at the further end of by the worm drops, in the first place, on to thethe grate can be supplied with secondary air,

the present infor the purpose of effecting a certain secondary combustion, for which purpose, and for cooling, the clinker may be left for a further short period in the pit, being finally discharged into a conveying device. The discharge is preferably continuous, through hedgehog rollers 41 or similar breaking appliances, which break up the large fragments of clinker and deliver uniform mediumsize pieces to a conveyor 48.

I claim:

1. A furnace for burning refuse, comprising a rectangular combustion chamber, a grate closing the combustion chamber at the bottom and hav ing a plurality of air chambers therebelow for supplying air for combustion, a drying shaft disposed laterally above the grate and having certain of its walls perforated, a blower arranged to draw gases of combustion from the combustion chamber above the grate, through the perforated walls of said drying shaft and through the refuse disposed therein and to deliver the resulting gases into the air chamber furthest away from said drying shaft, a device for charging the drying shaft and devices for moving the dried refuse out of the bottom end of the drying shaft on to the grate.

2. A furnace for burning refuse, comprising a rectangular combustion chamber, a grate closing the combustion chamber at the bottom and having a plurality of air chambers therebelow for supplying air for combustion, a pusher plate in one wall of the combustion chamber arranged to compress the burning refuse and push it at intervals over and off said grate, a drying shaft disposed laterally above the grate and having certain of its walls perforated, a blower arranged to drawgases of combustion from the combustien chamber above the grate, through the perforated walls of said drying shaft and through the refuse disposed therein and to deliver the resulting gases into the air chamber furthest away from said drying shaft, a device for charging the drying shaft and devices for moving the dried refuse out of the bottom end of the drying shaft on to the grate.

VIKTOR KOLB.