Staged down draught combustion device for alternative fuels

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

[0001] General awareness on world-wide level for the need to reduce energy squandering, to minimize and to fight pollution sources and to protect the environment has led, among others, to a considerable improvement of technologies for recycling of solid domestic waste and of other similar refuse resulting from human acitivities. While, on the one hand, technologies have been optimized to transform putrescible organic fractions of solid waste into compost, other sophisticated techniques are being developed to separate the fractions with high energy content from huges masses of refuse. These fractions are internationally known under the name of R.D.F. or Refuse Derived Fuel.

[0002] R.D.F. thus permits to overcome the obsolete technologies of total incineration, everywhere under accusation and topical subject of debate, dispute and contestation because of the many problems related to its possible emission of dioxins and its pollution of the environment. R.D.F. may be produced in 3 quality standards resulting from three production stages, i.e.

a) "Fluff" R.D.F. which is a raw product, sorted from the mass of refuse and consisting of "fluffy" shreds of paper, plastic (mostly polythelene) film, fabric, wood, etc. This Fluff has a 35% - 38% average moisture content and 30 - 40 kg/m³ bulk density. This product cannot be stored since it is too bulky; its high moisture content would soon cause aerobic fermentation with the serious risk of spontaneous ignition. Therefore, this Fluff must be immediately burnt in suitable furnaces designed for com­bustion in turbolent air suspension;

b) Densified R.D.F., i.e. Fluff after compression and extrusion in pelletizing presses.
Its bulk density is thus increased to about 450 - 500 kg/m³. The product is easier to handle but cannot be stored for a long time because of the problems already mentioned for Fluff;

c) Dried and pelletized R.D.F. when the Fluff is dried before being compressed to a moisture content not exceeding 11 - 10% so that it can be pelletized. The final fuel will thus be stable and can be stored for long periods, provided it is kept in sheltered and dry storage spaces.

[0003] The densified as well as pelletized R.D.F. may be burnt in grate furnaces or in rotary furnaces provided they are equipped with an afterburning chamber associated with other fuels (coal, lignite, industrial waste etc.).

[0004] The thermocombustor subject matter of this application for Letters Patent is filling a gap and constitutes a specific equipment for densified or pelletized R.D.F. combustion.

[0005] The special configuration of the combustor, its reverse-­flame combustion in two separate stages, as described in detail in this Specification, will prevent the onset of the problems usually arising when using these fuels in known combustion systems, i.e. clogged gratings, lumping, caking, formation of prefential flame channels, "hot Spots", sagging of refractories and so on.

[0006] Obviously, R.D.F. is only one of the fuels that can be burnt in the thermocombustor subject matter of this Invention, since this furnace can be used for combustion of numerous small sized industrial waste, of combustible products having a high content of toxic or noxious pollutants, while ensuring neutralization of acid combustion products.

[0007] This invention is illustrated in one of its practical and exemplifying implementations in the enclosed drawings in which:

Fig.1 shows the structural arrangement of the thermocom­bustor in a vertical central section ;

Fig.2 shows the precombustion air feeder according to X - X of fig.1;

Fig.3 shows the final air feeder according to Y - Y of Fig.1;

Fig.4 shows the structural arrangement of the fuel and limestone feeder systems of the thermocombustor;

Fig. 5 and 6 show the structures supporting the refractory material according to the K-K and W-W sections respectively of fig.1 .

[0008] According to these figures, the thermocombustor is consisting of four coaxially arranged, cylindrical metal shells 1,2,3,4, so that they are forming four chambers, A, B, C, and D having well defined and differentiated functions.

A) First continuous fuel feeding chamber:

[0009] This chamber is consisting of a hollow space inside the shell 1, which is the innermost of the four shells. This chamber A is closed at the top by a capping in which openings have been made for charging of the fuel 6 and lime 7.

[0010] The arrangement of the feeder system shown in fig.1 is only indicatory for exemplification purposes. Indeed, fuel and limestone can be charged with suitable equipment apt to ensure preliminary batching and intensive mixing of the two products. One of these solutions is illustrated in fig.4.

[0011] Two storage bins 38 and 39 in metal construction are provided for storage of small sized fuel and limestone. The fuel is drawn from the bin in prefixed quantities through star-valve controlled apron gates (40) which not only release the fuel in the required quantities but also prevent air from entering the first chamber A, thus providing a perfect seal.

[0012] Limestone is drawn from the storage bin 39 by an electromagnetic measuring hopper 41, the discharge rate of which can be continuously remote controlled and adjusted to the feed rate of the fuel measuring hopper (40). The measured fuel and limestone are both dumped on a dust-tight vibratory feeder conveyor where the two products are batched before they are introduced together into the first chamber A.

[0013] Obviously, the capacity of the vibratory feeder conveyor 43 shall be greater than the total capacity of the two measuring hoppers 40 and 41.

[0014] To prevent air from entering the first chamber A, all measuring and feeding devices are interconnected by rubberized fabric sleeves 42, secured by metal clips. Furthermore, low-level probes 44 and 45 will be mounted inside the fuel and limestone storage bins which will stop discharging of these two products when the minimum level in either bin is reached.

[0015] Fuel and limestone are discharged into the chamber A by variable delivery measuring hoppers which are efficiently sealed to prevent air from entering and gas from escaping.

[0016] In this way, the fuel inside the chamber cannot ignite due to lack of combustion air.

[0017] The covering structure (5) has also a central manhole shaped inspection door (17). This door is consisting of a metal anular frame to which a lightweight disc in asbestos cement or similar material is secured. In case of explosion, this disc will be shattered by the pressure wave which is thus released without damaging the furnace structure.

[0018] The first chamber A will always be kept filled with fuel to which a suitable proportion of limestone will be added.

[0019] The level probes 19, 19′ will control the measuring equipment so that the fuel level in chamber A will never drop below the minimum. This will permit to reduce the free volume in the chamber A while minimizing explosion hazard.

[0020] The shell 1 is coaxially enclosed by the shell 2 which has of course a greater diameter. A hollow space is thus generated forming:

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

[0021] This primary combustion air is supplied by the fan 8 by means of the volute 28 and the guide vanes 29. The air flow and pressure are regulated by the valve 9.

[0022] This valve 9 is actuated by servo controls and driven by suitable analyzers of the gas developed during primary combustion.

[0023] As illustrated in fig. 1, the shell 2 is externally attacked by the hot gas thus heating the primary combustion air flowing through the hollow space. This arrangement also prevents excessive heating of the wall 1 and will prevent clotting or caking of the feed mass enclosed in the first chamber A.

[0024] The first and second chamber both lead into

[0025] C) the third chamber in which primary precombustion and neutralizing take place with gasification and combustion of the fuel, limestone dissociation generating CO₂ and CaO and hence neutralizing the acid containing combustion products (Cl - SO₂ - SO₃ - NO_x - etc.)

[0026] This chamber is delimited by the shell 3 in reinforced refractory construction and/or metal elements, internally cooled by water circulation.

[0027] The lower end of the third chamber C is delimited by a rotary disc 11 driven by a variable speed gear motor. The third chamber (C) is kept filled with fuel batched with a measured quantity of CaCO₃ coming from the chamber 1, located on top of chamber A.

[0028] Primary combustion air, which has been preheated in chamber B, is let into chamber C through a truncated cone 10 connecting the two shells 2 and 3.

[0029] As already explained before, the air flow rate is regulated so as to provide approximately the stoichiometric quantity of oxygen required for combustion. The air entering the third chamber thus gets in touch with the fuel, co-flowing from top to bottom through the fuel, causing its progressive ignition, re-­kindling and combustion. The temperature too rises gradually as the fuel moves downward from the mouth of the shell 3. Three zones can be identified inside the precombustion chamber C, from top to bottom, cor­responding to three progressive combustion stages:

I) Ignition and drying zone

[0030] i.e. the upper zone in which the fuel gets in touch with the hot combustion air which has been preheated in the second chamber B, causing ignition of the fuel. The temperature in this zone is still rather low due to the strong evaporation of the imbibition water which now vaporizes.

[0031] In downward direction is located:

II) the distillation zone.

[0032] Upon vaporization, the vola­tile substances of the fuel are distilled by thermal cracking. The heat causes dissociation of polythene and other plastic materials, paints, man-made fabrics, wood etc. releasing combustible gas which in turn will ignite. The limestone temperature will now be high enough to cause dissociation with release of CO₂ and residual CaO. The above reactions (except for combustion) are all of endothermal nature. They are absorbing heat so that the temperature, though rising, will not exceed 550° - 750°C in this zone.

[0033] This zone is followed by:

III) Reaction and gasification zone:

[0034] As the volatile substances are released through dissociation of the plastic materials, resins etc. contained in the fuel, the temperature rises further causing combustion of the carbonacious products, thus producing a large amount of CO since there is no excess air. Limestone dissociation is completed and the resulting CaO is highly reactive. While the fuel is gradually depleted, the CaO content in the burning mass will increase during its downward journey, thus forming a reactive and porous CaO bed interacting with the gaseous combustion products passing through this bed. The maximum temperature values required in the third precombustion chamber (C) are reached in the center of this zone III, ranging between 850° and 900°C, then to drop slightly. Combustion is controlled and moni­tored by acting on the flow rate of the primary air supplied by the fan 8, thus preventing a further tempera­ture rise for several and well defined reasons.

[0035] First of all, it is necessary to prevent slag and ashes from sticking to the refractory material of the third chamber C which might clog the slots 13 forming the gas outlets.

[0036] Residue ashes from R.D.F. combustion are mostly neutral or slightly acid due to the presence of kaolin in the paper charge, i.e. the main component of Refuse Derived Fuel. Therefore care should be taken not to reach temperature values at which the ashes scorify in the presence of CaO and to keep the ashes loose, so that they may be easily discharged when moving the rotary disc 11.

[0037] To keep the combustion temperature within the preset limits, combustion air must be reduced while limiting excess air to the strictly necessary stoichiometric quantity.

[0038] This expedient will limit the gas volume and hence the speed at which the gas flows through the slots 13. This in turn, will limit the quantity of flying ashes that will be entrained in the afterburning chamber (D). The porous reactive CaO bed mentioned above is directly supported by the rotary disc 11. During its slow rotation which can be stepless adjusted to need, this disc will prevent the formation of preferential channels inside the combustion mass, thus keeping the mass loose and porous, in close contact with the CaO of the granulate concen­trated in the porous reactive CaO bed . Total fuel depletion is thus achieved, while the ashes will be trapped in the interstices between the reactive CaO gra­nulate, moving downwards with the rotating disc to be directly discharged into the quenching basin 12.

D) Fourth afterburning chamber

[0039] The virtually neutral gas developed in the third precom­bustion chamber C during the complex chemical-physical reactions, due to its intensive and protracted contact with the quicklime in the porous reactive CaO bed, but still having a high CO and partially burnt hydrocarbon content, flows through the radial slots 13 made in the bottom section of the shell 3 of the third (precom­bustion) chamber 3, into the fourth (afterburning) cham­ber D. The latter is delimited on the outside by the shell 4 and on the inside by the outer walls of the shell 3 and of the primary air preheater 2.

[0040] At its lower end, the afterburning chamber is delimited by the edge of the rotary disc 11 and at its upper end by an anular flange fitted with several manholes closed by explosion doors 37.

[0041] One or more sets of nozzles 15 are located in this afterburning chamber D which will supply turbulent air to complete gas combustion (fig.3).

[0042] In this properly dimensioned fourth chamber, the temperature rises to 1000° - 1050°C and is kept at this value for at least 3 seconds which is sufficient for total dissociation of any dioxin.

[0043] The secondary air for afterburning is supplied by an external medium-pressure pneumophore unit which obviously is no part of this Patent since it is already known.

[0044] The very hot and completely dissociated waste gas leaves the fourth chamber D of the thermocombustor through one or more vacuum controlled ducts 16, connecting the fourth chamber D to the equipment apt to utilize the sensible heat of this exhaust gas (boilers, driers, heat exchan­gers etc.) . After its utilization in the users, the now sufficiently cooled waste gas is released through a stack into the atmosphere by suction fans, ensuring a suffi­cient vacuum at the thermocombustor outlet.

[0045] The ring-shaped ash quenching basin is housing the rotary disc 11 supporting and driving unit 23.

[0046] Two anular sealing skirts 24 and 25 are immersed in the basin, one of which is fixed and directly connected to the bearing structures 22 and to the cylindrical outer wall 4, whereas the second 25, is directly connected to the rotary disc 11 and is slowly revolving on its own axis. These skirts 24 and 25 provide a perfect hydraulic seal thus preventing false air from entering the fourth (afterburning) chamber D.

[0047] Ashes and mineral residues of the combustion process as well as salts resulting from the reaction between CaO and the acid content of the waste gas removed by the disc 11 from the precombustion chamber C, are dumped into the water in the basin 12. They are gravity collected on the bottom by scrapers 20 secured to the rotary disc and slowly pushed towards the outlet connecting the quenching basin to the worm conveyor for thickening and removal of the ashes 21.

[0048] This worm conveyor is slowly revolving and partially immersed in the quenching water, and will discharge the sludge into appropriate tanks for disposal.

[0049] No detailed description is given of other important equipment, because it is not deemed essential for this specification of the invention; however these items will be briefly mentioned for completeness sake:
- removable start-up burners 27 Diesel oil fuelled for cold start-up of the combustor;
- Level probes to control and guarantee filling of the first (charging) chamber A.
- Recording Pyrometers to measure the temperature in various spots of the precombustion chamber;
- of the primary air
- of the gas entering the afterburning chamber
- in the middle and at the outlet of the afterburning chamber
- Waste gas analyzers to determine waste gas costituents in percentage: at the outlet of the slots 13
at gas outlets to users
- Servomechanisms driving the secondary air supply system
- Floating and continuous discharging devices for make-­up water to keep the quenching water level constant;
- Electric and electronic alarm and safety systems
- Equipment for measuring the fuel and limestone charge complete with sealing devices to prevent false air from entering the combustor.

[0050] Furthermore, no detailed description is provided of the bearing and connecting structures since they are not essential for the efficiency of this invention.

Claims

1) Two-stage reverse-flame thermocombustor, specifically designed for combustion of Refuse Derived Fuels obtained from solid urban waste, small-sized combustible industrial waste, "fluff" or pelletized fuels having a high ash and sulphur content etc.for simultaneous neutralization of acid combustion products on a reactive porous CaO bed directly achieved in the primary combustion chamber, cha­racterized by the facts that the combustor is consisting of:

a) four coaxial shells (1,2,3,4) lined with refractory material, if necessary, thus forming four chambers (A,B,C,D) each having the following precise and differen­tiated functions, i.e.:
- the first chamber (A) enclosed in the first shell (1) is acting as a properly supplied storage bin for a batched fuel/limestone mixture;
- the second chamber (B) delimited by the first and second shell (1,2) is acting as preheating chamber for the primary combustion air supplied by a fan (8) and let into the truncated cone shaped zone (10) connecting the first to the third chamber (A,C);
- the third chamber (C) located below the first chamber (A) is delimited by the third shell (3) and is acting as primary combustion chamber while neutralizing the acid combustion products;
- the fourth and outer chamber (D) delimited by the fourth shell (4) is surrounding the air preheating (B) and primary combustion (C) chambers and is acting as final combustion chamber of the waste gases exhausted through the slots (13) of the third chamber (C), thus producing hot gas which is conveyed to the users through one or more outlet ducts (16);

b) a batching feeder system, feeding a measured quantity of fuel and limestone into the first chamber (A), consisting of:
- fuel and limestone storage bins (38, 19)
- fuel and limestone batchers (40, 41)
- a feeder system (43) for the fuel/limestone mix
- sealed sleeves (42) connecting the various facilities
- level probes (19, 19′) in the first chamber (A);

c) Discharging system of ashes and solid residues, consisting of a rotary disc (11) provided with scoops (36) located below the primary combustion chamber (C);

d) an ash quenching system (12) provided with two anular hydraulic sealing skirts (24, 25), one of which is fixed to the fourth shell (4) and the other is applied to the rotary disc (11);

e) a screw feeder (21) for thickening and dis­charging of the ashes by means of scrapers (20) acting on the bottom of the tank (12); so as to prevent clogging of the gratings, caking or lumping of the material, the formation of preferential flame channels with production of noxious gas as happens in present combustion plants.

2°) Thermocombustor as described in claim 1, cha

2°) Thermocombustor as described in claim 1, cha­racterized by the fact that the first chamber (A) has a manhole shaped safety door (17);

3°) Thermocombustor as described in claim 1, cha­racterized by the fact that the second chamber (B) is closed at the top by a volute shaped air scoop fed by the primary air fan, the air flow being regulated by the valve (9) and connected by a truncated cone (10) to the underlying third chamber (C).

4°) Thermocombustor as described in claim 1, cha­racterized by the fact that the third shell (C) is in reinforced refractory material and/or in metal construction (31) cooled by internal water circulation and supported by metal brackets (32) also sustaining the burners (27) for combustion start-up.

5°) Thermocombustor as described in claim 1, cha­racterized by the fact that the slots (13) through which the gas is discharged from the third chamber (C) have rounded tips and are flared inwards in order to keep back the fuel and limestone in the Chamber (C).

6°) Thermocombustor as described in claim 1, cha­racterized by the fact that the fourth chamber (D) features at gas inlet level (13) a larger dimensioned anular zone (14) to facilitate elimination of flying ashes which will drop and settle on the rotary disc (11) from which they are discharged by adjustable scoops (36) into the quenching tank (12).

7°) Thermocombustor as described in claim 1, cha­racterized by the fact that the fourth and final combustion chamber receives suitably pressurized and preheated secondary air through adjustable nozzles (15) located tangential to the median circumference (t) of this chamber (D) from a bustle main (33) so as to generate a whirling flow and thorough air/gas mixing;

8°) Operation of a thermocombustor as described in claim 1, characterized by the following phases:
- airtight feeding of fuel and limestone batched and mixed in the first chamber (A)
- progressive flow of the material from the first (A) to the third precombustion and neutralizing chamber (C);
- heating of the primary combustion air in the second chamber (B) with the utilization of exhaust gas and feeding of the third chamber (C) in which gasification and reverse combustion takes place at a temperature not higher than 550°C - 750°C with dissociation of limestone and neutralization of acid combustion products,
- gas flow from the third (C) to the fourth final combustion chamber (D) in which the gas is mixed with preheated and flow controlled air at a 1000° - 1050°C temperature range so as to ensure dissociation of any dioxin.
- the hot combustion gases are then piped to the users and after further cooling down, they are then discharged into the atmosphere.

Drawing