WO1989012787A1

WO1989012787A1 - An arrangement for supplying pre-heated combustion air to a combustion chamber

Info

Publication number: WO1989012787A1
Application number: PCT/SE1989/000336
Authority: WO
Inventors: Karl Sigurd Herman Hultgren
Original assignee: Hultgren Karl S H
Priority date: 1988-06-14
Filing date: 1989-06-13
Publication date: 1989-12-28
Also published as: AU3836189A; SE8802224L; SE8802224D0

Abstract

The present invention relates to an arrangement for pre-heating combustion air supplied to a combustion chamber. In accordance with the invention, there is provided around the wall of the combustion chamber (3) a shell structure (22) which extends from the bottom of the combustion chamber along the whole or part of the vertical extension thereof and which is spaced from the combustion chamber wall so as to form a peripheral gap between the inner surface of the shell structure and the outer surface of the ocmbustion chamber wall, one end of the peripheral gap communicating with the ambient atmosphere and the opposite end of the gap being connected to a connection which discharges into the interior of the combustion chamber.

Description

An arrangement for supplying pre-heated combustion air to a combustion chamber.

The present invention relates to an arrangement for supplying pre-heated combustion air to a combustion chamber. The invention is intended particularly for application in a combustion chamber in which the combus¬ tion air supplied thereto is divided into a primary flow and a secondary flow, which are subsequently mixed together in a combustion chamber secondary zone in which the combustion of gassified solid fuel takes place.

A combustion chamber of this kind is known from WO 83/00373 claiming priority from Applicant's earlier Swedish patent application 8104516-3. Reference is made to the descriptive part of the aforesaid application for a better understanding of the desired combustion process and the constructive fundamental principles by means of which this process is achieved, while in order to obtain an understanding of the invention it is sufficient to know that the conditions required for combustion in the so-called blue phase, i.e. in the present case combus¬ tion of gassified solid fuel, is favoured when the combustion air delivered to the combustion chamber is pre-heated and that the flue gases resulting from the combustion process have a higher temperature than the flue gases generated by conventional combustion. In order to enable the walls of the known combustion cham^¬ ber to withstand the high temperatures concerned, the walls are either water-cooled or constructed of a thick, insulating refractory material. Walls constructed of insulating material, optionally combined with water- cooling, are preferred, however, since cold wall sur^¬ faces in the combustion chamber negatively affect the conditions for the desired blue-phase combustion. In the case of the known arrangement, the combustion air is pre-heated by recycling flue gases and mixing said gases with the fresh air delivered to the combustion chamber.

The object of the present invention is to provide an arrangement in which the combustion air is further pre¬ heated and in which cooling of the combustion chamber wall can be effected without the use of thick, insulat¬ ing walls of refractory material and without the use of water-cooling.

These objects are achieved in accordance with the inven¬ tion by means of a shell structure which surrounds the combustion chamber wall and extends from the bottom of the combustion chamber along the whole or part of the vertical extension thereof and at a distance from said chamber such as to form a peripheral gap between the inner surface of the shell structure and the outer surface of the combustion chamber wall, one end of said peripheral gap communicating with the ambient atmosphere and the opposite end of said gap having joined thereto a connection which discharges into the interior of the combustion chamber. Thus, when the arrangement is in operation, fresh air will flow into the peripheral gap and be heated by the hot combustion chamber wall, which therewith delivers heat to the air flow. This arrange¬ ment will enable the wall of the combustion chamber to be constructed from commercially available, high-alloy steel plate material. The shell surrounding the combus- tion chamber wall and defining said peripheral gap together with said wall is preferably constructed from conventional insulating material which is held in place between thin covering plates and which prevents the transfer of heat between the air flowing in the gap and the ambient air. In accordance with an alternative embodiment of the invention, the interior of the combustion chamber com¬ municates with the peripheral gap through a row of openings provided peripherally around the upper part of the combustion chamber wall. In the case of this alter¬ native embodiment of the inventive arrangement, flue gases are sucked into the peripheral gap by the ejector effect engendered by the air flow passing through said gap, and are mixed with the fresh air in said gap.

A particularly advantageous application of the invention in a boiler into which solid fuel is fed and which is constructed for combustion of the fuel in the so-called blue phase will now be described with reference to the accompanying drawings, in which:

Figure 1 is a partial sectional view of a boiler provi¬ ded with an inventive arrangement for supplying pre¬ heated combustion air to a combustion chamber; and Figure 2 illustrates in larger scale the boiler of Fig. 1 provided with an alternative embodiment of the inven¬ tive arrangement.

The boiler illustrated in Fig. 1 includes an ash box 1, a fan means 2, a combustion chamber 3, the bottom of which comprises a grate 10 operative in permitting air to flow into the combustion chamber, a convection part 8, in which water is heated by heat delivered by the hot flue gases, and a flue gas outlet 9 provided at the top of the convection part 8.

The combustion chamber 3 is encircled by a shell struc¬ ture 22, which is spaced from the chamber 3 so as to form a peripheral gap between the inner surface of the shell structure and the outer surface of the wall of the combustion chamber. The top of the gap is open to atmos¬ phere and the lower part of the gap is connected with one or more conduits 27, which in turn are connected to the suction side of the fan means 2. A conduit 19 ex- tending from the flue gas outlet 9 is also connected to the suction side of the fan means. The pressure side of the fan means is connected to the suction side thereof by means of a shunt conduit 25, and valves or dampers 24, 26, 28 are mounted in the conduits for the purpose of controlling the flow of gas therethrough.

The combustion chamber wall 35 is manufactured from high-alloy steel plate, which may be chosen from commer¬ cially available steel plate varieties, and the walls of the convection part 8 are preferably made of a simpler material, such as SIS 1312 or corteen. The shell struc¬ ture 22 includes a core of insulating material, e.g. rockwool, enclosed between covering plates.

Fuel is fed into the combustion chamber 3 by means of a fuel feed system 4. In the case of the illustrated embodiment, this system comprises a conveyer belt 7 by means of which solid fuel is fed to a storage tank 6, from which the fuel falls gravitationally down through a feed conduit which discharges into the combustion cham¬ ber. The quantity of fuel falling through the feed conduit can be controlled by means of a control device 5. The combustion chamber 3 also has mounted therein three inserts 11, 13, 15 which will be described in more detail hereinafter with reference to Fig. 2.

The boiler illustrated in Fig. 2 differs from the boiler illustrated in Fig. 1, in that the conduit 19 for re¬ cycling flue gases from the flue gas outlet 9 is omitted in the Fig. 2 embodiment. Instead, the combustion cham- ber wall 35 has provided therein openings 16 which form a connection between the interior of the combustion chamber and the peripheral gap defined between the shell structure and the wall of the combustion chamber. The openings 16 preferably comprise a peripheral row of openings formed in the wall 35, by first cutting-out the side and bottom contours of the intended openings and the bending-out the thus freed metal sections to form metal tongues, as illustrated in Fig. 2. Naturally, the openings 16 may have a different configuration. For instance, they may have the form of a small number of peripheral slots formed without leaving outwardly bend- able metal tongues, although the embodiment of outwardly bendable tongues is preferred, since it is ensured thereby that the fresh air flowing in the peripheral gap will generate a satisfactory ejector effect.

In other respects, the boiler illustrated in Fig. 2 corresponds to the boiler illustrated in Fig. 1.

The grate 10 is intended to permit combustion air to pass into the combustion chamber, and includes to this end a multiple of mutually parallel air distributing tubes 41, the ends of which are connected to collecting chambers 42, which also comprise tubes connected to the pressure side of the fan means 2, by means of one or more tubular connectors 33. The tubes 41 are open up¬ wardly towards the interior of the combustion chamber 3 and the uppermost parts of respective tubes have a nozzle-like configuration in cross-section. In order to ensure that the air will exit from the nozzles at a uniform flow rate, the tubes 41 will preferably taper or narrow in the direction of air flow. Narrow, spacer bars 40, e.g. in the form of edge-standing iron flats, extend centrally over the nozzle-like openings of the tubes 41 and centrally between said tubes, for the purpose of supporting the fuel and of holding said fuel spaced from the nozzle openings. In order to enable the bars to expand linearly, said bars are not firmly mounted but instead rest, for instance, in grooves or channels formed in the wall of the combustion chamber or on the upper surface of the collecting chambers. When small- lump fuel is used, a steel net, perforated plate or the like may be placed on the bars, so as to prevent fuel from falling through the grate 10 and into the ask box 1.

Located above the grate 10 and in the immediate vicinity thereof is a first tubular insert 11. This insert pre- ferably has the form of a truncated cone with the cone base facing upwards, and has an inwardly curved upper edge surface. Mounted on the bottom of the insert is a beam-cross 39 by means of which the insert is carried by a rotatable shaft 32, which is driven for rotation by a motor 29 in some suitable fashion, for instance by; means of a sprocket chain 30 connected to a sprocket wheel mounted on the shaft 32. The insert will also preferably support against rollers 37 which project out from the combustion chamber wall 35 and which coact with shoul- ders 36 or the like provided on the outer surface of the insert 11. Because the insert 11 is rotatable and has sloping walls, the fuel supplied to the combustion chamber from the fuel feed system 4, which discharges the fuel at a location above the insert 11, can be uniformly distributed on the grate 10.

It will be understood, however, that the insert may be rotated in some other way, for example by rotating one or more of the rollers 37 in co-action with a friction ring or the like mounted on the insert, in those cases when the combustion chamber is equipped with such rollers. A gear drive is also a conceivable alternative to a friction drive.

Mounted above the first insert 11 is a second tubular insert 13. In the case of the illustrated embodiment, the second insert has the form of a straight cylinder with inwardly curved top and bottom edge surfaces and a diameter which is larger than the base of the first insert. When seen in the vertical extension of the boiler, the beginning of the inwardly curved bottom edge surface of the second insert 13 is located on the same level as the beginning of the inwardly curved top edge surface of the first insert 11, so as to form an an- nular, relatively narrow inlet gap for secondary air of combustion, the flow cross-section of which gap is defined by the curvature of respective inwardly curved edge surfaces and by the height position of the second insert. The second insert may optionally be attached to the wall of the combustion chamber in a manner which will enable its height position, and therewith the gap width, to be varied.

The diameter of the second insert is smaller than the diameter of the combustion chamber 3, and consequently an annular space is formed between the wall 35 of the combustion chamber and the outer surfaces of the first and second inserts.

The combustion chamber and inserts of the described embodiment have a circular cross-section, which is the preferred cross-section. It will be understood, however, that other cross-sectional shapes are possible, although shapes other than circular are less favourable from the aspect of air flow, since rotation of the first insert would then disturb the flow homogenity of the arrange¬ ment.

Finally, a third tubular insert 15 is mounted in the combustion chamber above the second insert 13. Similar to the first insert, the third insert has the form of a truncated cone, although the base of the third insert faces downwardly, as distinct from the base of the first insert. Furthermore, the bottom edge surface of the third insert extends fully to the wall 35 of the com¬ bustion chamber, and the third insert thus constitutes an upper limitation of the annular space defined between the wall 35 of the combustion chamber and the outer surfaces of the first and second inserts. The wall of the third insert co-acts with the inwardly curved top edge surface of the second insert in a manner to form an inlet gap for tertiary air.

The walls of the convection part 8 have formed therein gas channels 20 in accordance with Swedish patent speci¬ fication 8090799-0, thereby obtaining highly effective heat transfer to the water present in the convection part, as a result of the vortex-laminar gas flow occurr¬ ing in said channels.

The modis operandi of the boiler illustrated in Fig. 1 will now be described. The occurrent gas flows are indicated by arrows in the Figures.

Fresh air is drawn by suction from the peripheral gap between the inner surface of the shell structure 22 and the outer surface of the wall 35 of the combustion chamber by the fan means, through the conduit 29. Prior to entering the fan means, the fresh air is admixed with flue gases sucked in from the flue gas outlet 9, through the conduct 19, and with an air-gas mixture entering the shunt conduit 25 and optionally taken from the pressure side of the fan means. The fresh air flowing in the peripheral gap will cool the combustion chamber wall 35, which results, of course, in pre-heating of the fresh air prior to its entry into the fan means. The air flow in the peripheral gap can be adjusted in a relatively simple manner, by suitable dimensioning of the gap width, the cross-section of the inlet and outlet, and the fan capacity, such as to achieve suitable cooling of the combustion chamber wall 35, i.e. so that the tem¬ perature of the inner surface of the wall is suffic¬ iently high to prevent the combustion process from being negatively influenced, while ensuring that the tem- perature is not sufficiently high to jeopardize the mechanical strength of the wall.

The admixture with flue gases will further pre-heat the fresh air. Thus, an air-gas mixture flows from the fan means into the collecting chambers 42 and into the air distributing tubes 41, whereby the grate 10 is cooled and the air-gas mixture heated to a higher temperature. The air-gas mixture then flows out of the nozzle-like openings of the tubes 41, and is deflected locally by the bars 40 and as it enters the combustion chamber 3 is divided into two mutually separate flows by the wall of the insert 11, namely a primary flow within the insert 11 and a secondary flow in the annular space located externally of the insert 11. This primary flow shall constitute about 60% of the total flow.

Since the air surplus during operation of the arrange¬ ment is kept low, when seen in total, it will be under¬ stood that the oxygen carried solely by the primary flow will not be sufficient to ensure that stoichiometric combustion conditions are achieved. This contributes greatly to the fact that the desired gassification of the solid fuel resting on the grate 10 and present within the insert 11 will take place in the absence of true combustion to any appreciable extent. Thus, gas¬ sified solid fuel mixed with the primary flow will flow upwardly from the lower part of the insert 11.

Part of the secondary flow will enter the inserts through the gap defined between the first and the second inserts 11, 13. The proportion of air-gas mixture which flows into said gap is determined essentially by the relationship between the flow resistance in the inlet gaps between the inserts 11, 13 and 13, 15 and when the vertical or height positions of the inserts 13 and 15 can be adjusted can be adapted to suitable values for mutually different types of solid fuels.

A pre-heated mixture of fresh air and recycled flue gases thus flows at high velocity into the central part of the combustion chamber defined by the inserts, at a location which is level with the upper edge surface of the first insert 11. This secondary air-gas mixture is guided by the inwardly curved edge surfaces of the first and second inserts 11, 13 forming the inlet gap, and the mixture will flow radially inwards from the top edge surface of the insert 11. As indicated in the figure, this will result in a local vortex flow which is effec¬ tive in mixing the primary flow containing the gassified fuel effectively with the inflowing secondary air-gas mixture. This inflow ensures that sufficient oxygen is supplied to the gassified fuel to achieve the desired oxidation of carbon to carbon monoxide without generat¬ ing free carbon. The effective mixing and recycling process generated by this vortex motion greatly assists in generating favourable conditions for the primary combustion process.

The remainder of the secondary flow exiting from the grate 10 and passing through the gap defined between the first and the second inserts continues upwards along the annular space between the combustion chamber wall 35 and the second insert 13, and enters the gap located between the third insert 15 and the inwardly curved top edge surface of the insert 13, from where it passes radially into the central part of the combustion chamber. Similar to the case of the secondary air-gas mixture at said inlet part, the inflow of this tertiary air-gas mixture creates a local vortex flow, resulting in a subsequent mixing and recycling process. The oxygen supplied by this tertiary flow engenders a final combustion process comprising substantially the oxidation of carbon mon¬ oxide to carbon dioxide.

The sole difference between the modis operandi of the boiler illustrated in Fig. 2 and that of the boiler illustrated in Fig. 1 is that the admixture of flue gases with fresh air takes place in the peripheral gap instead of in the fan inlet conduit. The ejector effect generated by the fresh air flowing in the peripheral gap can be modified by modifying the extent to which the tongues are bent out, as accomplished in the preferred embodiment of the opening 16.

It will be understood that the described and illustrated arrangement can be modified within the scope of the invention. For example, the shell structure 22 may have the form of a single-piece structure including insula¬ tion for the convection part 8 and provided with pas- sageways for the intake of fresh air. The invention is therefore only restricted by the contents of the accom¬ panying claims.

Claims

l. An arrangement for supplying pre-heated combustion air to a combustion chamber, characterized by a shell structure (22) which peripherally encircles the wall (35) of the combustion chamber (3) and which extends from the bottom of the combustion chamber along the whole or part of the vertical extension of said chamber and which is spaced from the wall (35) such as to form a peripheral gap between the inner surface of the shell structure and the outer surface of said wall, one end of the peripheral gap being in communication with the ambient atmosphere and the opposite end of said gap being connected to a connection (27) which discharges into the interior of the combustion chamber.

2. An arrangement according to Claim 1, characterized in that the shell structure (22) extends along the full height of the combustion chamber (3) ; and in that the top of the peripheral gap is open to the ambient atmos¬ phere.

3. An arrangement according to any one of Claims 1 and 2, characterized by a unit (2), preferably a fan means, operative in generating forced circulation of the com¬ bustion air delivered to the combustion chamber.

4. An arrangement according to Claim 3, characterized in that the circulation unit (2) is mounted in the con¬ nection (27) leading from the peripheral gap to the interior of the combustion chamber.

5. An arrangement according to any one of Claims 1-3, characterized by at least one conduit (19, 16) for recycling flue gases and mixing said gases with fresh air supplied to the combustion chamber (3) .

6. An arrangement according to any one of Claims 1-5, characterized in that a shunt conduit (25) connects the outlet side of the circulation assembly (2) with the inlet side thereof.

7. An arrangement according to Claim 5 or 6, charac¬ terized in that the flue gas recycling conduits (19) discharge into the connection (27) leading from the peripheral gap to the interior of the combustion chamber and on the inlet side of the circulation assembly (2) mounted in said connection.

8. An arrangement according to Claim 5 or 6, charac¬ terized in that the flue gas recycling conduits com- prise conduits (16) which extend from the interior of the combustion chamber to the peripheral gap.

9. An arrangement according to Claim 8, characterized in that the upper part of the combustion chamber wall (35) has a row of openings (16) formed in the periphery thereof.

10. An arrangement according to any one of the preceding claims, characterized in that regulating means (k24, 26, 28) for regulating the flow of combustion air supplied to the combustion chamber (3) are provided at least in the connection extending from the peripheral gap to the interior of the combustion chamber.