DE68903809T2 - STAGE COMBUSTION DEVICE WITH DOWNWARD TOW FOR ALTERNATIVE FUELS. - Google Patents

Abstract

The two-stage reverse flame thermocombustor is consisting of four shells (1,2,3,4,) forming four chambers (A,B,C,D) of which the upper chamber (A) is acting as a storage space for fuel mixed with limestone. The second chamber (B) surrounding the first chamber, preheats the precombustion air,the flow rate of which is regulated by a fan (8). The third chamber 3, located below the first, is closed at its bottom end by a rotary disc (11) for elimination of the ashes and solid residues. The first combustion stage of the fuel and limestone takes place in this third chamber (C). The combustion gases enter the fourth chamber (D) through radial slots (13) where more air is added through a ring-shaped bustle main (33) and nozzles (15) for final combustion at high temperature. Combustion gases are conveyed to the users through the duct (16).

Description

The increasing awareness worldwide of the need to avoid energy waste, reduce or combat pollution and protect the environment has, among other things, led to a significant improvement in the technology for recycling urban solid waste and other wastes resulting from human activities. While on the one hand, techniques have been optimized to convert putrefactive organic components of solid waste into compost, on the other hand, techniques have been developed to separate fractions with a high energy content from large quantities of waste. These fractions are known internationally as P.D.F. (Refuse Derived Fuel).

R.D.F. makes it possible to overcome the outdated technologies of total combustion, which has become a current subject of debate and controversy everywhere due to the possible emission of dioxin and environmental pollution with accusations. R.D.F. can be produced in three qualities, resulting from three production stages, namely:

a) "Flocculent" (FLUFFF) R.D.F., which is a raw product sorted out of the waste mass and consists of "flaky" scraps of paper, plastic film (mostly polythelene), textiles, wood, etc. This flufff has a moisture content in the range of 35% to 38% and a bulk density of 30 to 40 kg/m3. This product cannot be stored because it is too bulky. Its high moisture content would soon cause an aerobic fermentation with the risk of sudden ignition. This flufff must therefore be burned immediately in suitable furnaces with turbulent air distribution;

b) Compacted R.D.F., which is fluff after compaction and extrusion in pelletizing presses.

Its bulk density is increased to about 450 - 500 kg/m3.

This product is easier to handle, but cannot be stored for long periods of time due to the problems with fluff discussed above;

c) Dried and pelletized R.D.F. is fluff dried before pressing with a moisture content not exceeding 11 to 10% so that it can be pelletized. The final fuel is stable and can be stored for a long time provided it is kept in covered and dry storage rooms. Both dried and pelletized R.D.F. can be burned in post-fired furnaces or in rotating furnaces equipped with an afterburning chamber in combination with other fuels (coal, lignite, industrial waste, etc.).

The subject of this patent application is to fill a gap and create a special arrangement for the combustion of dried or pelletized R.D.F.

The special design of the combustion device, its back-flame combustion in two separate stages, as described in detail in this description, avoids the occurrence of problems that usually arise when using these fuels in known combustion systems, e.g. clogged grates, coking, caking, formation of preferential flame channels, "hot spots", sagging of linings, etc.

Of course, RDF is one of those fuels that can be burned in a combustion device according to the invention, because this device is also suitable for the combustion of various small-sized industrial waste and of combustion products that have a high proportion of toxic or non-toxic impurities, because the neutralization of acidic combustion products is ensured.

Essentially, the combustion device according to claim 1 consists of four shells forming four chambers, of which the first upper chamber serves as a storage container for Limestone mixed fuel. The second chamber, which surrounds the first chamber, preheats the preheating air, the flow rate being regulated by a fan. The third chamber, located below the first chamber, is closed off at its lower end by a rotating disc, which takes care of the removal of ash and solid residues.

The first combustion of the fuel and limestone takes place in this third chamber. The combustion gases enter the fourth chamber through radial slots, where more air is supplied through a wind ring and nozzles for final combustion at high temperature. The combustion gases are supplied to the users through a pipe, while the ash discharged through the disc is tipped into a water basin from which it is removed by a screw conveyor.

GB-A-2 002 501 describes a waste incineration furnace which has four different chambers and a system for cooling the ash. According to this patent, however, the air chamber is formed by a central tube with a conical element at its lower end, whereas according to the invention the air chamber is arranged around the storage container in order to prevent the wall of the storage container from heating up too much and thus preventing the mass fed in from caking together.

Furthermore, according to the aforementioned GB-A-2 002 501, it is provided that the supply of air takes place in the combustion chamber, whereas according to the invention this supply takes place between the storage and combustion chambers. Furthermore, according to GB-A-2 002 501, the hot combustion gases from the fourth chamber heat up the waste in the first chamber, whereas according to the invention the combustion gases heat the combustion air contained in the second chamber.

Various other design details distinguish the solutions according to GB-A-2 002 501 and the invention.

GB-A-1 561 938 describes an ash discharge system consisting of a rotating disc and an ash chilling arrangement with two annular hydraulic rims; GB-A-4 68 813 describes a screw conveyor for ash discharge.

The invention is described in the accompanying drawings in one of its practical embodiments. They show:

Fig. 1 is a vertical section through the combustion device; Fig. 2 is a section along X-X in Fig. 1 showing the device for preheating the air;

Fig. 3 in a section along Z-Z in Fig. 1 the supply of the final air;

Fig. 4 shows the arrangement of the fuel and limestone supply systems of the combustion device;

Fig. 5 and 6 Sections along lines K-K and W-W in Fig. 1 showing the structure of the lining.

According to the drawings, the combustion device consists of four coaxially arranged cylindrical metallic shells 1, 2, 3, 4 so as to form four chambers A, B, C and D which have well-defined different functions.

A) First chamber for continuous supply of fuel:

This chamber consists of a hollow space within the innermost shell 1. The chamber A is closed at the top by a cap in which openings are provided for the supply of fuel 6 and limestone 7.

The arrangement of the feeding system according to Fig. 1 is only an example. In fact, fuel and limestone can be fed with various suitable arrangements that allow layering and intensive mixing of the two products. One of these solutions is shown in Fig. 4.

Two metal storage containers 38 and 39 are provided for storing fuel and limestone in small pieces. The fuel is drawn from the storage container in predetermined quantities through inlets 40 of a star valve which not only allow the fuel to pass through in the required quantities but also prevent air from entering from the first chamber A.

Limestone is withdrawn from the storage container 39 by an electromagnetic measuring cylinder 41, whereby the corresponding amount can be remotely adjusted to the amount of fuel fed through the measuring cylinder 40. The measured amounts of fuel and limestone are fed into a dust-tight vibratory conveyor in which the two products are mixed before being introduced into the first chamber A.

Of course, the capacity of the vibrating conveyor 43 should be greater than the total capacity of the two measuring cylinders 40 and 41.

In order to prevent air from entering the first chamber A, all the measuring and feeding devices are connected to one another by rubberized fabric sleeves 42, which are secured by metal clips. Furthermore, measuring sensors 44 and 45 are arranged in the fuel and limestone containers, which stop the discharge of these two products as soon as a minimum height is reached in one of the storage containers.

Fuel and limestone are discharged into chamber 8 through variable discharge measuring cylinders which are sufficiently sealed to prevent inflow of air and outflow of gas.

In this way, the fuel inside the chamber cannot ignite due to the lack of combustion air.

The cover 5 has a central manhole 17 as an inspection opening. This opening consists of a central ring frame on which a light disc made of asbestos cement or similar material is attached. In the event of an explosion, this disc will shatter due to the pressure wave without damaging the furnace structure.

The first chamber A is always kept filled with fuel, to which an appropriate proportion of limestone is added.

The level probes 19-19' control the measuring arrangement so that the fuel level in chamber A never falls below the minimum. This reduces the free volume in chamber A, thereby minimizing the risk of explosion.

The jacket 1 is coaxially enclosed by the jacket 2, which therefore has a larger diameter. This creates a cavity which forms:

B) The second preheating chamber for the primary combustion air.

This primary air is supplied by a fan 8 by means of the spiral 28 and the guide vanes 29. The quantity and pressure of the air are regulated by the valve 9.

This valve 9 is actuated by servo controls and driven by suitable analysis devices for the gas developed during the primary combustion.

As shown in Fig. 1, the jacket 2 is exposed to the outside by the hot gas, which heats the primary combustion air flowing through the cavity. This arrangement also prevents the wall 1 from heating up too much and prevents the masses enclosed in the first chamber A from caking together.

The first and second chambers lead in

C) the third chamber, where the first pre-combustion and neutralization with gasification and combustion of the fuel takes place; the dissociation of the limestone produces CO₂ and CAO and therefore neutralization of the acid contained in the combustion products (CL- CSO₂ - SO₃ -NOX etc.).

This chamber is limited by the shell 3, which consists of a reinforced lining construction and/or metal elements 30-32 with an internal filling by a water circuit 31 exists.

The lower end of the third chamber C is delimited by a rotatable disc 11 driven by a gear motor 23 operable at various speeds. The third chamber C is kept filled with fuel mixed with a measured quantity of CACO₃ coming from the chamber 1 located above the chamber A.

The primary combustion air, which has been preheated in chamber B, is admitted into chamber C via a tapered cone 10 which connects the two jackets 2 and 3.

As already stated above, the amount of air is regulated to approximately reach the stoichiometric amount of oxygen required for combustion. The air entering the third chamber comes into contact with the fuel, flowing from the top to the bottom through the fuel, causing progressive ignition, re-ignition and combustion. The temperature increases with the downward movement from the mouth of the jacket 3. According to the three progressive combustion stages, three zones can be identified from top to bottom within the chamber C:

I. Ignition and drying zone. This is the upper area in which the fuel comes into contact with the hot combustion air, which has been preheated in the second chamber B and causes the fuel to ignite. The temperature in this zone is rather low due to the evaporation of the water it contains.

In the downward direction is arranged:

II. the distillation zone. After evaporation, the volatile components of the fuel are distilled by thermal separation. The heat causes the dissociation of polythene and other plastics, paints, textiles, wood, etc., releasing combustible gases that are ignited again. The temperature of the limestone is is now high enough to cause dissociation with released CO₂ and remaining CAO. The above reactions (except combustion) are all endothermic in nature. Heat is therefore absorbed, so the temperature in this region will not exceed 550 to 750 degrees C.

This zone is connected to:

III. Reaction and gasification zone: After the release of the volatile substances by dissociation of the plastics, resins, etc. contained in the fuel, the temperature continues to rise, causing the combustion of the carbonaceous products and, due to the lack of excess air, producing a large amount of CO. The dissociation of limestone is completed and the resulting CAO is highly reactive. As the fuel is increasingly depleted, the CAO content in the fuel mass will increase during the downward movement, creating a reactive and porous CAO bed that reacts with the gaseous combustion products flowing through this bed. The required maximum temperature values in the third pre-combustion chamber C are reached in the middle of this zone III, and range between 850 and 900 degrees C, after which they decrease slightly. Combustion is controlled by the amount of primary air supplied by the compressor 8, thus preventing a further increase in temperature for various reasons.

Above all, it is necessary to prevent slag and ash from settling on the lining material of the third chamber C and causing clogging of the slots 13 forming gas outlets.

The remaining ash from RDF combustion is mostly neutral or slightly acidic due to the presence of kaolin in the main component, namely paper. Therefore, care should be taken not to reach temperatures at which the ash slags in the presence of CAO; the ash should be kept loose so that it can be easily removed by moving the rotating Disc 11 can be removed.

In order to keep the combustion temperature within the specified limits, the combustion air must be limited to the necessary stoichiometric amount.

This limits the amount of gas and the speed at which gas flows through the slots 13. This also limits the amount of fly ash entering the afterburning chamber D. The porous reactive CAO bed is directly supported by the rotating disc 11. During its slow rotation, which can be infinitely adjusted if necessary, this disc prevents the formation of preferential channels within the combustion mass, thereby keeping this mass loose and porous in close contact with the CAO of the granules in the porous reactive CAO bed. A complete yield of the fuel is thus achieved, the ash being trapped in the spaces between the reactive CAO granules, which move downwards and are discharged directly into the basin 12 by means of the rotating disc.

D. Fourth afterburning chamber.

The actually neutral gas developed in the third pre-combustion chamber during the complex chemical-physical reactions flows into the fourth post-combustion chamber D through the radial slots 13 provided in the bottom part of the shell 3 of the third chamber 3. This gas has a high CO and partial hydrocarbon content due to its intensive contact with the slaked lime in the porous reactive CAO bed. The chamber D is delimited on the outside by the shell 4 and on the inside by the outer walls of the shell 3 and the primary air heater 2.

At its lower end, the afterburning chamber is delimited by the edges of the rotating disk and at its upper end by an annular flange provided with several manholes closed by explosion openings 37.

One or more nozzle sets 15 are arranged in this afterburning chamber D, which supply swirling air for complete gas combustion (Fig. 3).

In this suitably dimensioned fourth chamber, the temperatures rise to 1000-1050 degrees C and are maintained at these values for at least 3 seconds, which is sufficient for the complete dissociation of any dioxin. The secondary air for the afterburning is supplied by an external medium-pressure unit 33, 34, which of course is not part of this invention, since it is already known.

The very hot and completely dissociated exhaust gases leave the fourth chamber D of the combustion device through one or more vacuum-controlled channels 16, which connect the fourth chamber D to a device that can use the heat of these exhaust gases (boiler, dryer, heat exchanger, etc.). After use by the users, the exhaust gases, now sufficiently cooled, are discharged through a chimney with the help of induced draught fans, which can ensure a sufficient vacuum at the outlet of the combustion device.

The ring-shaped quenching basin for the ash contains the rotating disk 11 and the support and drive unit 23.

Two circular sealing aprons 24 and 25 are immersed in the tank, one of which is directly connected to the supporting structure 22 and the cylindrical outer wall 4, while the second apron 25 is directly connected to the rotating disc 11 and rotates slowly about its own axis. These aprons 24 and 25 form a perfect hydraulic seal, preventing the entry of false air into the fourth (afterburning) chamber D.

Ash and mineral residues from the combustion process as well as salts resulting from the reaction between CAO and the acid content of the exhaust gas are removed from the pre-combustion chamber C through the disc 11 and the water in the basin 12 is emptied. They are collected at the bottom by scrapers 20 attached to the rotating disc and slowly pushed to the outlet which connects the quenching basin with the screw conveyor for thickening and discharging the ash 21.

This screw conveyor, which is partially submerged in the quenching water, rotates slowly and empties the sludge into suitable containers.

Other important equipment is not described in detail since it is not essential for the present description of the invention; however, it is briefly described for the sake of completeness:

-Removable starting burners 27, which run on diesel oil and are used for cold starting of the combustion device;

-Level sensor to control and ensure the filling of the first (feed) chamber A;

-Pyrometer for measuring the temperature at various points in the pre-combustion chamber, namely

-the primary air

-of the gas entering the afterburner chamber

-in the middle and at the outlet of the afterburner chamber

-Analyzers for exhaust gases to determine the components in percentages:

at the outlet of slots 13

at the outlet to the users

-Servo mechanisms for the operation of the secondary air system

-Float-operated devices to keep the water level in the quenching tank constant;

-Electrical and electronic alarm and security systems

-Device for measuring the fuel and limestone charge together with sealing devices to prevent the entry of false air.

Furthermore, no detailed description of the bearing and connecting elements is given because they are not essential to the effectiveness of this invention.

Claims (8)

1. Two-stage combustion device, in particular for the combustion of waste-derived fuels obtained from solid urban waste, small combustible industrial waste, "fluff" or pelletized fuels and having a high ash and sulphur content, etc., with simultaneous neutralization of acidic combustion products on a porous CaO bed in the first combustion chamber, the combustion device consisting of: a) four chambers (A, B, C, D) formed by four coaxial cylindrical shells (1, 2, 3, 4) preferably lined with refractory material, wherein - a first chamber (A) is enclosed in the first shell (1) and acts as a delivery and storage container for a fuel/limestone mixture ; - a second chamber (B), delimited on the outside by the second shell (2) and on the inside by the external surface of the first shell (1), acting as a preheating chamber for the primary combustion air supplied by a fan (8) which is guided by outlets (29) into a conical zone (10) connecting the lower part of the second shell (2) to the upper part of the third shell (3) and connecting the first chamber (A) with - a third chamber (C) located below the first chamber (A), which is delimited by the third casing (3), at its upper part by the said zone (10) and at its lower part by a rotating disc (11) which removes the ash, the third chamber acting as a primary combustion chamber when the acidic combustion products are neutralised, and slots (13) are provided in the bottom part of the third chamber which are provided with - a fourth and outer chamber (D) which is delimited on the outside by the fourth casing (4), on the inside by the outer walls of the second and third casings (2 and 3) and at its lower end by the rotatable disc (11), said fourth chamber having nozzles (15) for the introduction of secondary air and acts as a final combustion chamber for waste gases emitted through the slots (13) of the third chamber (C), thereby generating hot gases which are supplied to consumers through one or more outlet channels (16); b) an airtight feed system that supplies a measured quantity of fuel and limestone into the first chamber (A) and consists of: - storage tank (38, 39) for fuel and limestone, - feeding devices (40, 41) for fuel and limestone, - a feed system (43) for the fuel-limestone mixture - closed sleeves (42) for connecting the various devices - level sensor (19, 19') in the first chamber (A); c) a discharge system for ash and solid residues, consisting of the rotating disc (11) provided with blades (36); d) an ash cooling system (12) provided with two annular hydraulically closing edges (24, 25), one of which is mounted on the fourth casing (4) and the other on the rotating disc (11), and a screw conveyor (21) for thickening and discharging the ash by means of scrapers acting on the bottom of the container (12). 2. Device according to claim 1, characterized in that the first chamber has a safety gate (17) designed as a manhole; 3. Device according to claim 1, characterized in that the second chamber (B) is closed off at its top by a spiral-shaped air intake (28) which is fed by the first fan (8), the air flow being adjustable by a valve (9). 4. Device according to claim 1, characterized in that the third shell (C) consists of reinforced heat-resistant material and/or metal, cooled by an internal water circulation and supported by metal supports (32) which also hold burners (27) for initiating combustion. 5. Device according to claim 1, characterized in that the slots (13) through which the gas exits the third chamber (C) have rounded ends and are tapered in order to retain fuel and limestone in the chamber (C). 6. Device according to claim 1, characterized in that the fourth chamber (D) has a larger annular space (14) in the region of the gas inlet (13) in order to achieve the removal of fly ash which settles on the rotating disk (11), from where it is discharged into the cooling container (12) by adjustable blades (36). 7. Device according to claim 1, characterized in that the fourth and last combustion chamber receives secondary air, which has an appropriate pressure and is preheated and is introduced through adjustable nozzles (15) arranged from a wind ring (33) tangential to the central periphery (T) of this chamber (D) in order to obtain a swirling flow and mixing of air and gas. 8. Method for operating a device according to claim 1, characterized by the following steps: - Airtight supply of fuel and limestone, which materials are allocated to the first chamber (A) and mixed therein - flowing the material from the first (A) to the third pre-combustion and neutralization chamber (C); - heating the primary combustion air in the second chamber (B) using exhaust gases and feeding the third chamber (C), in which downward combustion takes place at a temperature not higher than 550 o C to 750 o C with simultaneous decomposition of the limestone and neutralization of acidic combustion products, - Flow of the gas from the third (C) to the fourth final combustion chamber (D) in which the gas is mixed with preheated and flow-controlled air in the range of 1,000 to 1,050 o C to ensure dissociation of all dioxins, - Supply of hot combustion gases to the users and discharge after further cooling into the atmosphere.