CA1221012A - Method and apparatus for the igniting or burning of combustible waste oils, in a combustion dish and method of preparing the cleaning of the combustion dish - Google Patents

Abstract

A b s t r a c t A method and an apparatus for igniting and burning combus-tible waste oils of high residue contents in a furnace in-cluding at its base an upwardly open combustion (21) and, above said combustion dish, an optionally divided -combustion chamber (30), and further including means (2, 7, 8) operative to supply the waste oil in metered fashion to said combustion dish (21). Prior to the igni-tion of the waste oil the empty combustion dish (21) is heated to a temperature above the ignition and evaporation temperature of the waste oil by means of an auxiliary heat source (23, 28) adapted to be turned off. The waste oil is subsequently fed onto the heated combustion dish (21) in metered quantities and ignited thereon.

The fuel residues (bituminous slag) remaining in said combustion dish upon interruption of the fuel supply and burnout of the combustible waste oil volume may be pyro-lyzed with the aid of the additionally energizable heat source (23, 28) heating said combustion dish (21).

Description

~ ~ 1 ~;~2~

Method and apparatus for the igniting or burning of combustible waste oils in a combustion dish, and method of ereparing the cleanlng of the combustion dish The present invention relates to a method of igniting and burning combustible waste oils of high residue conten~ in a furnace including at its base an upwardly open combustion dish and, above said combustion dish, an ~ optionally divided - combustion chamber, and further including means-operative to supply the wasteoil ln a metered fashion to said combustion dish. Further, the present invention relates to a method of preparing ~r the cleaning, required in the combustion of waste oil, of the combustion dish upon interruption of the fuel supply and burn-out of the csmbustible waste oil volume still present ln said combustion dish. Finally, the present invention relates to an apparatus for the burning of combustible waste oils in a combustion dish.

The term "waste oils" includes especially such oil mixtures as are collected ln service station operations.
Although the regeneratlon of these waste oils is in principle possible, such regeneration requires an expenditure of energy and power which is not justified in many instances. Therefore~ such waste oils are frequently burnt. ~uite often, such waste olls contain a high proportion of contamlnants, namely, for example, highly vlscous trans~ission oil residues, grease components, carbon particles, metal particles resulting '

2 122~ 2 1 from wear, and the like~ "Waste oils" of this kind may also contain paint residues, (mechanical) wood pulp particles and water, Still further, waste oils of the abovemelltioned type may also include such oils which contain coal dust, tar oils, bilge oils, waste substances from refining plants, chemical com~ustible wastes as well as carbon-water~oil mixtures. Of importance is that the mixture, when properly heated, automatically maintains the combustion thereof, and that it contains at least parts of a liquid being combustible upon evaporation thereof. The quality of the waste oil, as measured by the calorific value of the fuel, may range between O and 10,000 kcal. A value of O applies substantially to pure water. Accordingly, it is necessary to mix all components or constituents in a reservoir, ln order to provide a combustible waste o1l consistency.

So-cal1ed "waste oil furnaces" are known, for example, 20 from Swiss Patent 368,255. This conventional combustion apparatus includes a drum-shaped housing with a combustion dish being posltioned at the base thereof.
The fuel is supplied to the combustion dish in metered volumes through a supply line~ The supply line termina-tes above the combustion dish, whereby the waste oildrops from its mouth into the combustion dish~

The apparatus is ignited by first supplying priming fuel to the combustion dish and igniting the waste

3~ oil by means of an igniting wick. When the combustion dish is sufficiently heated up, the supply of the waste oil is initiated. In this connection, it has to be noted that a great amount of bl~ck smoke is produced both in the ignition phase and in the combustion phase as such, and that the flame "smokes" for a relatively long time.

3 ~2~2 1 When the waste oil supply is interrupted, the fl~me continues to burn for some period of time until all evaporable residual constituents are burnt out. During the burn-out phase, the temperature of the combustion dish proper increases to about 350 to 400 C. Owing to this temperature highly viscous bituminous xesidues of high carbon contents, "slag" of high carbon contents, etc., are left in the combustion dish, and these substances form an extremely intimate compound with the combustion dish which is difficult to remove.
Therefore, these residues, termed "bituminous slag", must be removed from the dish by scraping or hammering in a troublesome manner, after the combustion of about 30 to 80 liters of waste oil each.
Accordingly, it is the object of the present invention to improve the conventional apparatus by providing a method a~d an apparatus for the combustion of waste oil, which facilitates the igniting process in a controlled manner. The apparatus and a method to be performed by such apparatus, respectively, a 1 s o~ facllitate the cleaning of the combustion dish after the combustion of waste oil.

These objects are attained with the same means which also-ren~erSpossible the combustion of flowable (fluid) waste oils by heating the empty combustion dish, prior to igniting the waste oil, at least partially to a temperature above the lgnition and evaporation tempe-rature of the waste oil by means of an auxiliary heatsource adapted to be turned off. Then, the waste oil is supplied onto the heated dish and ignited in the latter, preferably without resorting to further measures.
However, it is also concei~able to use auxiliary igniting aids, such as, for example, prelgnlters (glow-type igniters~

~2~l~12 1 When ignition has taken place, the auxiliary heat source is turned off; the combustion of the additionally supplied ~uel is then self-sustaining.

Still,further, the combu~tion dish to be heated may be used l:o prepare for the cleaning process which is required in the combustion of waste oil. To this end, upon interruption of the fuel supply and burn-out of the fuel left in the combustion dish, the remaining fuel residues (the 'bituminous slag") is pyrolyzed by means of the re-activated heat source.

Of course, the method of igniting and the apparatus used for carrying out such method, as well as the normal burner operation, may be performed or operated, respectively, also with high-purity fuels, e. g. fuel oil of class EL, which produce almost no ash. On the other hand, the conventional furnaces operated wlth fuel oil can not at all be operated with waste oil.
In view of this, the technological expenditure spent for the apparatus as described in the following is justified primarily only for the combustion of highly contaminated waste oils.

In view of the fact that the temperature existing in the combustion dish must reach both the ignition and the pyrolysis temperatures, it must be possible to bring the combustion dish to a temperature of more than 400 C. For example~ pyrolysts ls carried out at a temperature of from 700 to 800 C~ whereas an ignition and evaporation temperature of from about 350 to 400 C is sufficient for normal waste oils,such~as tnosecollected in service stations. By means of a corresponding thermostate control device, various temperatures may be set With the combustion dish temperature T beln~ between 400 and 800 C, ~ 2~
1 An apparatus for carrying out the methods for the eombustion of combustible waste oils in a combustion dish includes a combustion dish at its base. Positioned above the combustion dish is a - possibl~ divided -combustion chamber, It is well known to let the fueldrop into the dish from above, or to cause the fuel to flow into the dish from a lateral or central point, as seen from the combustion dish surface. On principle, various well-known methods of supplying the fuel may be used~ In order that the abovementioned methods may be carried out auxiliarly to the customary burner operation, at least one additional heaterd~ice is preferably disposed below the combustion dish. Such additional heater devices comprise preferably electri-cally operated heater colls. Also possible is an inductive heating of the combustion dish, For special cases, it is also conceivable to employ a connectible or add-on type gas burner including a burning lance which terminates below the combustion dish. Essential is that the principle of the auxiliary heating system adapted to be turned on and off is ensured.

In order to save energy and to more rapidly heat the dish before the ignitton process, the combustion dish is provided with a trough having a structured (textured) surface and being arranged below the end of the waste oil supply line. This trough which may have, for example, a grooved or wafer-shaped structure, has a slightly thinner bottom and is locally provided with more intense heating means so as to be heated in a shorter period of time than the remaining portion of the combùstion dish. The trough has an oval or kidney-shaped outline, and it assumes about one-fifth to one-third of the bottom surface of the com~ustion dish.

Preferably, the combustion dish has a relatively high ~2~P2 1 heat capacity; therefore, th~ dish is preferably cast from cast iron (DIN 1493) and then faced a~ its lower side. On its upper side (flame side), the combustion dish, similar to the trough portion, is provided with concentric grooves, with a honeycomb (wafer) structure, with bosses or with a similar s~ructure increasing its surface. This structure results ln the prevention of Leiden~rost's phenomenon; further-more , such surf~ce structure provides for very rapidly establishing a good thermal contact between the waste oil droplets and the surface. Size (dimension) and diameter of the combustion dish depend on the capacity of the combustion apparatus.

Preferably, the oil supply line terminates with a spacing of from 2 to 20 cm, advantageously from 7 to 10 ~m, above the combustion dish or the trough.
This arrangement offers the advantage'that the energy contained in the dropping oil droplets disintegrates these droplets thereby provlding for an accelerated evaporating effect. The Leidenfrost's phenomenon which may be observed in the starting phase and which opposes a rapid evaporation of the droplets, is then substantially fully suppressed by the great number of relatively small droplets.

In order to increase the kinetic energy of the impinging droplets, the supply nozzle forming the end portion of the supply line is mounted with an angle of inslination of between 25 and 35 relative to the horizontal. Also, the supply nozzle has a substantially greater open (inner~ cross-sectional area than the oil supply line proper which ends at the supply nozzle. In this way, the waste -oil supplied does not_ fill out the full cross-sectional area in the end portion of the oil supply line, whlch filling effect generally results in undesira~le after-flow phenomena upon interruption , 1 f the oil flow.

Further features which are disclosed in the claims are explained in the course of the description cf exemplary embodiments as illustrated in the drawing, weherein: -Figure 1 is a side elevational viewof an apparatusfor the combustion of waste oils in accordance with the present invention, Figure 2 is a front Yiew of the apparatus of Figure 1, Figure 3 is a det~iled view of a different igniting device, Figure 4 is a plan view of part of the combustion dish including the trough, ~ Figur~s 5 and 6 are a side elevational vlew and a front view, respectively, of an oil supply nozzle including a cleaning device, and Figure 7 shows a specific kind of fresh air supply.
The apparatus for the combustion of waste oil, as shown in Figures 1 and 2, has the exterior outline of a known heatlng device, The base of the apparatus has arranged therein a xeservoir 1 which receives the waste oil~
The reservoir 1 has a volume of, for example, 145 liters. Mounted laterally to the reservoir 1 is a downwardly protruding sump vessel 5 in which coarse deposits are separated. Above the sump vessel 5, but within the reservoir 1, a cylindrical strainer basket 6 is placed around the sump vessel, which basket acts to shield the inlet region of a metering oil pump 2.
Normally, the oil pump 2 is a gear-type pump or a 1 different pump adapted to pump and supply the waste oil in metered fashion.

As can be seen, the suction port of the oil pump 2 is situated within the strainer basket 6, but above the sump level. The oil pump 2 is driven by a motor 3 via a shaft 4, with the motor being mounted on a cover 9 above the reservoir. A supply line 7 connected to the oil pump 2 extends out from the strainer basket 6 1o in upward direction and acts to conduct the pumped oil to a feed or supply nozzle 8. The oil pump 2, i~cluding its motor, can be removed from the strainer basket 6 for cleaning purposes, upon removing the cover 9. In the embodiment shown, the supply (pumping) capacity is between about 0.5 to 3 kg of oil per hour.
The waste oil volume pumped may be continuously metered within this range by means of the pump.

Underneath the reservoir 1 formed as a base, four supporting legs 17 are mounted which may be vertically adjusted for adjusting the liquid level.

The actual apparatus according to the invention, formed as a hot air-producing thermal device 10, has exterior dimensions similar to conventional apparatuses used for the same purpose. The apparatus comprises a housing 11 the base (bottom) side of which is defined by the reservoir 1. A heat exchanger 13 is mounted to the head port~on of the housing 11. An exhaustconduit or duct 44 leads to the atmosphere through a stack. It is possible and expedient to install suitable, conventional filters in said stack.

A burner box 14 is installed ln the lower poxtion of the housing 11, with the rear wall of such burner box forming part of the housing 1~, This portlon of the housing is placed on the reservoir by means of interposed 9 :~22~
1 legs 15. Within the burner box 14, a burner pot or cylinder 18 is provided which has a cylindrical wall being spac~d from the wall of the burner box 14 at a distance equal to about one-half o~ the burner cylinder diameter.

The bottom of the burner pot 18 has embedded therein a heating and insulating plate 19 upon which a combustion dish is placed. The combustion dish 21 has an approxi-mately pan-shaped or dish-shaped configuration. The upright walls of the dish 21 diverge from the bottom 22 thereof, such that the dish has at its upper edge a greater free width than th~t corresponding to the bottom area. The combustion dish 21 is formed of cast iron, with its wall thickness being dimensioned, in accordance with the expert's experience and the heat capacity, such that continuous evaporation and combu-stion of the inflowing liquidwaste oil is secured.
An electrical resistance wire coil 23 is embedded into the heating and insulating plate 19, namely , directly below the bottom of the combustion dish 21, with the leads of said w~re coil be-ng extended to the outside, while observing the relevant thermal insulation and ?rotection requirements. By means of the coil-23, the combustion dish 21 may be heated to a temperature of from 350 to 800 C (red heat). In order to provide for partially (locally) increased heating of the comb-ustion dish, the latter is provided with an auxiliary heater device, namely another high-temperature coil 28, in its portion directly beneath the mounth of the supply nozzle 8. This high-temperature coil 28 is located directly below a bottom trough 27 embedded into the bottom 22 of the combustion dish 21, which trough in the illustxated embodiment has an approxi-mately kidney-shaped configuration and assumes about 25 % of the bottom surface of the combustion dish 21.
The bottom trough 27 is provided with a grooved or 1o 1 riffled structure. This structure is contemplated to directly disintegrate the waste oil droplets falling into the bottom trough 27, so as to be brought into optimum irregular, large-surface contact with the heated surface. These measures are taken in order to avoid the Leidenfrost's phenomenon, i. e. the formation of a steam or vapor laye_ ~round the droplets, which layer would inhibit the rapid evaporation. Upon ignition, the trough portion also is much more rapidly heated than the remaining portion of the combustion dish. In this way, a substantial amount of heat is concentrated onto a small Yolume of waste oil, such that this Yolume evaporates instantaneously. It is in this way ensured that the very first droplet falling onto the combustion dish 21 initiates the ignition, because the droplet hits the area of the trough, to hurst on the trough and evaporate at once. Ignition of this first droplet takes place in an environment where an excess of air exists. No (black) smoke is produced since ignltion takes place in almost an explosion-like manner.

Further, a temperature sensor or detector 37 is installed below the combustion dish 21, which sensor may be connected directly to the combustion dish 2~, too.
This temperature sensor acts to monitor and control the functions to be described below. As mentioned above, the combustion dish 21 is surrounded b~ the burner cylinder 18. Above the edge of the combustion dlsh 21, the wall or jacket of the burner cylinder 18 is provided with a great number of vent holes 20 through which the combustion air enters~ Approximately in the upper one-third of the burner cylinder ~8, a supporting rlm 38 is formed in the wall or jacket, which rim supports the edge of a glow hood 40, This hood 40, formed of cast steel and adapted to be heated to red heat, is likewise provided with perforations 4~, such that the ~.~2~

1 combustion gases, and even the incompletely burned gases, may flow through these perforations 41 to be after-burnt within the hood 40. The hood 40has an approximately frustoconical configuration with a wider opening 42 in its upper end, with a glow converter insert or element 43 being adapted to be placed into said opening. The glow converter element is formed of thin wire coils of a refractory wire. This wir~ glows after a short period of combustion, such that a good after-burning state is obtained also in the vicinity of this wire. These components ensure that all combusti-ble constituents of the waste oil are completely burnt, namely fully into H20 and Co2.

The wall surfaces of the glow hood which define an angle of from about 30 to 60 relative to the horizontal, act to pass the hot gases, partially still burning and partially burnt, through a combustion chamber 30 which joins the burner box 14 directly at its upper portion.
The combustion chamber 30 is also of a cylindrical configuration. The shell 31 of the combustion ch~mber 30 is provided, in vane-like fashion, with a plurality of radiator ribs 32 acting to improve the thermal contact with the air flowing along its outer side.
The upper side of the combustion chamber 30 is closed by a cover 33, and the combustion chamber opens into an outlet or exhaust opening 34 which is followed directly by the labyrinth~type heat exchanger 13.

Figures 1, 5 and 6 illustr~te the supply nozzle 8.
The mouth 50 of the supply nozzle termlnates freely within the burner cylinder ~8 above the burner dish trough 27 at a level of about 15 cm above the surface.
The supply nozzle 8 has an inclination of between about 25 and 35 relative to the horizontal, Further, it has to be noted that the open cross~sectional area of the supply nozzle 8 is substantlally greater than 1 that of the oil supply line 7 which ends at the supply nozzle 8. Besides, the supply nozzle 8 has a constant inner cross-sectional area from the inlet opening 51 to its lower mouth 50. Thus, the open cross-section does not diverge. The oil pumped by the oil pump 2 through the inlet opening 51, thus, flows primarily on the lower inner wall surface of the supply nozzle, with increasing flow velocity, downward towards the mouth 50. At the latter, the oil flows out and is atomized at the edge into fine droplets. This flow principle greatly prevents an incrustation or plugging of the supply nozzle 8. Furthermore, in order to avoid excessive heating of the supply nozzle 8, the latter is arranged such that it passes from the front side of the burner box 14 through the air conducting space 16, and so as to be positioned within the burner pot 18 with a relatively short length only. Also, the inner cavity of the supply nozzle communicates with the atmosphere. Accordingly, the oil is prevented from flowing back within the nozzle to the fuel line 7.

Owing to the greatly different qualities of the waste oils, in extreme exceptional cases depositions may be formed within the supply nozzle 8. To provide for this, a cleaning plunger 52 is provided with is movable within the cavity of the nozzle and which, as shown in Figure 6, has a cross-sectional configuration resembling a maltese (Geneva) cross, with free spaces 54 being defined between the arms 53 of the plunger, through which the waste oil may flow irrespective of the plunger being installed in the nozzle. The cleaning plunger 52 has a rod 55 passing outwards through a cover 56 of the nozzle 8, with the rod terminating in a knob 57. A helical spring 58 is inserted between the knob and the cover. The plunger 52 must be urged inwards against the force of the spring 58; having been urged inwards, the plunger automatically resumes 13 ~ 12 1 its original position (when released). In this way, it is ensured that the region of the mouth 50 is always exposed. The maltese (Geneva~ cross configuration of the plunger has been chosen for the reason that scrapexs or cutting edges may be mounted to each of the heads contacting the inn~r face of the supply nozzle 8.

I~lined position and frequent cleaning cooperate in establishing an unrestricted, accelerating oil flow within the supply nozzle 8, with the oil flow or stream being disintegrated into small droplets and with the vertical velocity component being transmitted to a high degree to the droplets and resulting in in-creasing the kinetic energy of these droplets. Particles scraped off by the plunger5~ likewise drop into the combustion dish to be burnt or pyrolyzed therein.

The automatic program(ming) system of the hot-air thermal device is controlled in such a manner that the combustion dish is heated first, before the oil starts to flow into thls dish. Although a generally automatic ignition takPs place in the region of the trough due to the high temperatures of the combustion dish, it must nevertheless be ensured that waste oils containing components of relatively high flame point can be reliably ignited, too. To this end, an ignition device 39 is provided whichincludes a glow element 45 being installed in concentric fashion into the tubular housing of the ignition device 39. The glow element comprises a perforated ceramic tube having a heater coil installed therein. Prior to admission of the oil, the program control system causes the glow element to assume a red hot state, whereby the fuel-air mixture is ignited instantaneously. The above measures prevent the generation of black smoke.

Instead of using the glow element 45, the fuel may be 1 ignited also with the aid of an infrared mirror reflec-tor igniter 46. Such igniter has a heater coil 48 dis-posed within a reflector casing 47. The heat radiation ls focused onto the trough 27 so as to form an ignition polnt. In addition, it is possible to connect to the mirror reflector igniter 46 an air duct or line 49 which directs a combustion alr stream onto the ignition region at the moment the fue] is ignited.

The apparatuses described abPve will allow the ignition -of waste- oil in a safe manner.

Still further, a photocell system 24 for monitoring the combustion process is mounted in the wall of the lS burner box 14 and so as to extend into the burner cylinder 1B, with the assoclated photocell 25 receiving essentially the radiation emitted from the region of the trough 27 -in the burner dish. The photocell system 24 acts to continuously monitor the combustion process.
The photocell system 24 which comprises essentially an ocular tube, i5 further provided with a cooling air nozzle 26 which is fed with cooling air, and the cooling air is conducted to the end of the ocular tube to be blown into the combustion zone of the trough. In this way, it is provided that the combustion zone is con-stantly fed with combustion air, such that combustion is maintained as long as unburnt waste oil is still present in the combustion dish 21. The flame goes out when the combustible materlal is exhausted, and such extinction is positively detected by the photocell system 24.

The air guiding or conducting system is now explained by referring to Figures 1 and 7. The suction ports for the room air to be heated and for the combustion a~r are located at a height of 1.40 meters above the floor. This arrangement has been chosen in order to ~5 ~L~2~
1 avoid sucking in vapors of low boiling organic com-pounds which might result in a detonation in the com-bustion chamber.

A suction port 70 for the combustion air is shown in Figure 7. The sucked in air is fed, via a duct 71, to a combustion air blower or fan 72 which urges the combustion air, metered in accordance with the respec-tive thermal capacity, into the burner box. The air is distributed within the burner bo~ and pressed through the perforations 20 into the burner cylinder where combustion takes place or where combustion gases are produced. After-burning then takes place in the com-bustion chamber. The burnt gases enter the heat exchan-ger 13 and flow out from the latter as exhaust gasesvia the exhaust line 35.

Simultaneously, room air or fresh air to be heated, respectively, is aspirated through a port 74 of greater cross-section, and this air is supplied via a conduit 75 to a hot-air blower or fan 76 which transports the air to be heated in upward direction through the heat exchanger 13 and in contact with the heat exchanger elements heated by the exhaust gases.
Via the head (portion) 77 of the thermal device 10, the heated air is conducted to a plurality of air outlets:
namely, an air outlet 78 in the upper portion and a lower air outlet 79 in the lower portion. The air conduction path is insulated by means of correspondingly insulated walls of the housing 11. In particular, it is thereby avoided that the hot burner portions in the interior of the apparatus 10 are allowed to come into contact with the environment. Accordingly, accidents by burning from the outer portions of the apparatus are prevented.

Furthermore, as can be seen from Figure 2, a tank 16 ~ 2 1 (liquid) level indicator 80 including an associated float 31 and an oil drain screw 82 at the sump 5 are provided. The COA~bUStiOn dish 18, including the auxiliary heater system, is arranged within a drawer 83 which may be drawn out from the housing.

To provide for safe operation, there is provided a further security measure. In case that the flame goes out in spite of the monitoring by tne photocell system 24, while the oil supply is not interrupted, unburnt fuel will overflow the edge of the combustion dish 21 after some period of time. In order to detect this malfu~ction, an overflow control sensor or detector 87 is desposed in the lower portion of the housing.
The furnace functions are controlled by means of an electronic control unit 88 which is housed within a switch box on the side of housing 11.

Description of functions At the start of the ignition and combustion process, the con~ustion dish 21 is empty, except for ash residues.
Upon turning on the hot-air thermal system, the heater coil 23 and the high temperature coil 28 are supplied with heating pow~r, so as to heat the dish to an average temperature of about 400 C and to a tempera-ture of 800 C in the region of the trough. The tem-perature of the combustion dish 21 is constantly monitored (detected) by the temperature detector 37.
When the temperature required for combustion and ignition is reached, the oll pump 2 is activated.
A thin flow or stream of waste oil is initiated through the line and the supply nozzle 8, so as to flow into the com~ustion dish 27 through the mouth 50 in the form of fine droplets. At this time, the lgnition device 45 or 46 has been actlvatedr too. The first droplet which drops onto the heated trough, bursts 17 :~21~
1 on the trough to become thoroughly evaporated instantaneously. As experiments have shown, ignition takes place at once, such that the further waste oil supplied is also inflamed at once.

The resistance wire coil 23 is left activated during this initial ignition process, but it is kept at a lower temperature. The subsequently impinging oil droplets burst and immediately come into intense contact with the surface of the combustion dish. As the ignition temperature is exceeded, the fuel-air mixture above the combustion dish is instantaneously inflamed. As soon as the photocell system 24 detects the ignition, the fan (blower) 72 is activated to produce a continuous combustion air flow.

After a given, presettable time delay, the flame emitted from the combustion dish has heated the latter to such degree that the combustion becomes self-sustain-ing. It is to be noted that the combustion dish has arelatively high heat capacity because of the material of which it is formed.

In case that ignition does not take place after a given periodJfor example after four seconds, the oil pump 2 is deactivated through a time delay relay, and the fan 72 is also turned off after a further time delay~ Then, the ignition process may be repeated after some period of time. Normally, however, it is not necessary to repeat the ignition process.

During the combustion process, only the oil pump 2 and the combustion air fan or blower 72 are in operation to sustain the flame, The other heating and ignlting means described above are turned off during this period.
The starting hot-air fan or blower urges the room alr or exterior air through the respective heat exchangers.

~2~
~8 1 The heated air is never directly contacted with the exhaust gases.

The shut-down operation of the apparatus is effected by interrupting the oil flow by deactivatlon of the oil pump 2. After a further time delay, the combustion air fan 72 is turned off. As long as combustible waste oil residues are still present within the dish, these residues are further burnt with the aid of the minor air ~olumes sucked through the photocell system 24 and the air fan 72, while the flame is monitored by the photocell 25. The resistance wire coil 23 is energized again so that the combustion dish 21 is heated up to a temperature of the dish from 600 to 700 C, thereby initiating pyrolyzing. At the end of the combustion process, a viscous, solid bituminous slag has formed within the dish because of the relatively high quantities of foreign substances, which slag con-tains a great amount of carbon and of other combustible substances. The combustible residues are pyrolyzed and burnt on the heated dish It can be seen that the solid bltuminous slag is disintegrated into residual dust during this pyrolyzing process, which dust may be removed without problem when the box containing the combustion dish is drawn out. Pyrolysis of the bitumi-nous slag takes about five minutes. After this period, the coil 23 is deenergized.

As the heater aggregate, including the combustion dish, is arranged in a drawer 23, the respective part of the apparatus may be drawn out. This arrangement greatly facilitates maintenance and repair workO

The apparatus described above is suitable both for the heating of rooms and for the generation of steam~ hot water or hot gases. The apparatus can be put into operation in a fully automatic manner. Mainten~nCe of 1 the apparatus can be performed generally without any problem. In particular, the ash quantities are reduced to an amount which was heretofore unknown in so-called waste oil furnaces. Still further, it has been found that ash formed i~n operation and which may contain polluting quantities of heavy metals, can be readily removed by means of fllter devices.

Besides, an intense heat exchange is effected by the 1~ ribbed shell of the combustion chamber 30. These ribs are formed, for example, of copper. The air outlet ports can be controlled with respect to direction and intensity. Measurements have shown that the ex-haust gas temperatures are in the range of from about l5 180 to 200~ C. The C02 contents of the exhaust gases were measured to amount to about 10 % by ~olume. This provides an overall efficiency of 92%. Due to the capa~i-lity of metering the combustion air by means of the combustion air fan 72 and metering the waste oil by means of the oil pump 2~ a substantially stoichiometric combustion may be maintained. The contènt of soot is extremely low in fact, extrem~ly low quantities o~ soot can be observed even at the start of the combustion process.

The apparatus may be safely operated with a waste oil quantity of as small as 0.5 kg per hour. At this low oil volume, it is generally only a sustaining flame that is maintained in the combustion dish. Stoichio-metric conditions may be malntained even with these low flow volumes. The heating power may thus be easily increased in a short tlme.

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of cleaning out a furnace, having an upwardly open combustion dish and a fuel supply nozzle and being used for the combustion of burnable waste oils having high residue contents, said method comprising the following steps:
(a) interrupting the supply of waste oils to said com-bustion dish;
(b) energizing a heater means attached to said combus-tion dish to burn-out the volume of waste oil still remaining in said combustion dish after interrupting said supply of waste oil:
(c) pyrolyzing the fuel residue remaining in said com-bustion dish by continuing to energize said heater means in order to raise the temperature T of said combustion dish to 400°C ? T ? 800°C; and (d) removing the powder-like pyrolyzed residue which results from said pyrolyzing step.

2. The method as claimed in claim 1, further comprising the step of monitoring the temperature of said combustion dish during said heater means energizing and said pyrolyzing steps.

3. The method as claimed in claim 2, further comprising the step of cleaning the nozzle by activating a cleaning plunger adapted to be moved within the nozzle cavity.

4. The method as claimed in claim 3, wherein said step of removing the powder is accomplished by sliding out a drawer in which said combustion dish is mounted.