EP0393161A1 - An arrangement for the combustion of solid fuels, comprising a combustion chamber having mounted therein inserts for controlling the supply of the combustion air - Google Patents

Abstract

The present invention relates to a device for the combustion of solid fuels, comprising a combustion chamber (3) divided into an internal part and an external part by means of inserts (11, 13, 15) which are mutually different. Combustion air is supplied to the combustion chamber from below and is divided into a primary flow, which flows into the internal part of the combustion chamber, and a secondary flow, which flows into the external part of the combustion chamber. Parts of the secondary flow are blown into the internal part of the combustion chamber, through openings defined between the different inserts. The present invention makes it possible to obtain a combustion chamber in which the combustion takes place in the so-called bluish phase.

Description

An arrangement for the combustion of solid fuels, com¬ prising a combustion chamber having mounted therein inserts for controlling the supply of the combustion air.

The present invention relates to an arrangement for combusting solid fuels, such as chips, pellets, coal or finely chopped wood, said an arrangement comprising a combustion chamber, a grate mounted adjacent the bottom of the combustion chamber, means for supplying com¬ bustion air to the combustion chamber through the grate, a flue gas outlet provided at the upper end of the combustion chamber, and means for delivering fuel to the combustion chamber, and further comprising a first and a second insert which are of tubular construction and arranged one above the other, and which divide the combustion chamber into an internal and an external part, in that said inserts have a diameter which is smaller than corresponding parts of the combustion chamber so as to form an annular space between the wall of the combustion chamber and the outer surfaces of respective inserts, wherein the mutually facing ends of respective inserts have a configuration such as to form therebetween a slot or gap through which secondary air of combustion can be delivered to the internal part of the combustion chamber.

An arrangement of this kind is known from the publi¬ cation WO/83 00373 claiming priority from Applicant's earlier Swedish application 8104516-3. Effective com¬ bustion is achieved in this arrangement of apparatus, by recycling flue gases which are admixed with the com¬ bustion air supplied, this air of combustion containing only a small air surplus for the purpose of ensuring oxidation of the resultant combustion products. Those who desire a fuller account of the combustion process taking place in said arrangement are referred to the aforementioned publication. It is sufficient with re¬ spect to the present application to disclose that the combustion process taking place in said arrangement is effected through gassification of the solid fuel fol¬ lowed by almost totalc combustion of gassified fuel and other combustion products.

The process strived for in the combustion of solid fuel can be divided into three stages, namely a first stage in which the solid fuel is gassified, a second stage in which the gassified fuel is combusted to form combustion products which contain no free heated carbon, but in which the carbon is bound constantly to other sub¬ stances, inter alia as a result of the formation of carbon monoxide, and a third stage in which final oxi¬ dation of the combustion products takes place, therewith generating carbon dioxide, among other products. Com- bustion of this nature can be sustained with an air supply corresponding substantially to a combustion reaction of stoichio etric proportions between fuel and oxidizer, which is advantageous both with respect to pressure drop and thermal transmission. Furthermore, combustion of this nature will result in cleaner waste gases and is thus advantageous also from an environ¬ mental aspect, since, inter alia, no oxides of nitrogen are formed.

The object of the present invention is to provide a solid fuel combustion arrangement in which the com¬ bustion achieved is a further improvement on the result achieved with the arrangement according to the aforesaid publication. According to the invention, this object is achieved with an arrangement of the kind described in the introduction which is characterized in that at least one further tubular insert is mounted in the combustion chamber above the other two inserts, this further insert or inserts having a configuration such as to form an air inlet gap between the bottom edge surface of each fur¬ ther insert and the top edge surface of an underlying insert; and in that the bottom edge surface of the uppermost insert extends fully to the inner surface of the wall of the combustion chamber. The further insert or inserts thus form a tertiary zone, in which the final oxidation process mainly takes place. In this way, the two oxidation stages of the combustion process strived for are separated, one from the other, which means that a closer approach can be made towards stoichiometric proportions during the course of combustion than is possible with the arrangement known from the aforesaid publication, without jeopardizing the final oxidation process, which improves combustion still further. When testing an arrangement constructed in accordance with the present invention, measurements were taken of the carbon dioxide values of the end products. These values were found to be 17-20%, which lie close to the theo- retically optimum value capable of being achieved.

Further features of the invention and advantages afforded thereby will be apparent from the following detailed description of a preferred exemplifying embodi- ment of an inventive arrangement, this description being made with reference to the accompanying drawings, in which;

Figure 1 is a partial sectional view of an arrangement constructed in accordance with the invention; and Figure 2 is a partial sectional view of the lower part of the arrangement illustrated in Figure 1.

The arrangement illustrated in Fig. 1 includes an ash box 1, a fan means 2, a combustion chamber 3, a grate 10 which is connected to the bottom of the combustion chamber and operative in permitting air to pass into the combustion chamber, and a convection part 8 in which water is heated by the heat radiated from the hot flue gases. A flue gas outlet 9 is provided on the convection part 8.

The combustion chamber 3 is encircled by a shell 22 of insulating material, said shell being spaced from the combustion chamber so as to form a peripheral gap between the inner surface of the shell and the outer surface of the combustion chamber wall. This gap is open to atmosphere at the top thereof, whereas the lower part of the gap is connected to one or more conduits 27, which are, in turn, connected to the suction side of the fan means 2. A conduit 19 extending 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 con¬ nected 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 arrangement coacts with a fuel supply system 4, by means of which fuel is fed into the combustion chamber 3. In the case of the illustrated embodiment, the fuel supply system comprises a conveyer belt 7 on which solid fuel is conveyed to a storage tank 6, from which the fuel falls gravitationally down through a feed conduit, which discharges into the combustion chamber. The amount of fuel falling through the feed conduit can be regu¬ lated by means of a control device 5. As will be descri¬ bed in more detail hereinafter with reference to Fig. 2, the combustion chamber 3 incorporates three inserts 11, 13, 15.

Fig. 2 illustrates the bottom part of the arrangement shown in Fig. 1 in larger scale. As previously mentio¬ ned, the grate 10 is operative in allowing combustion air to flow into the combustion chamber and, to this end, comprises a multiple of mutually parallel air distributing tubes 41, the ends of which are connected to respective collecting chambers 42, which may also consist of tubes and which are connected to the pressure side of the fan means 2 by means of one or more pipe connectors 33. The tubes 41 are open upwardly towards the combustion 3 and, when seen in cross-section, the uppermost part of respective tubes has a nozzle-like configuration. In order to ensure that the gas exiting from the nozzles will have a uniform flow rate, the tubes 41 preferably taper in the direction of air flow, as illustrated in Fig. 1. Furthermore, narrow spacer bars 40, for instance in the form of edge-standing iron flats, extend centrally above the nozzle openings of the tubes 41 and centrally between said tubes, these flats being intended to support the solid fuel and to hold said fuel spaced from the nozzle openings. In order to allow the bars to expand linearly, the bars are not rigidly attached, but rest, for instance, in grooves or channels provided in the wall of the combustion chamber or on the top surface of the collecting chambers. When small lump fuel is used, a steel net may be placed on top of the bars, so as to prevent fuel falling through the grate 10 and down into the ash box 1. Located above the grate 10 and in the immediate vicinity thereof is a first tubular insert 11. This first insert preferably has the form of a truncated cone, with the cone base facing upwards and having an inwardly curved top edge surface. Furthermore, the insert 11 has mounted on the bottom thereof a beam-cross 39 by means of which the first insert is supported on a rotatable shaft 32, which is driven for rotation by a motor 29 through the intermediary of some suitable system, for instance through a sprocket chain 30 connected to a sprocket wheel mounted on the shaft 32. According to one variant, the insert will also preferably support against rollers 37 which project from the combustion chamber wall 35 and which coact with peripherally extending shoulders 36 or the like formed on the outer surface of the insert 11. The fact that the insert 11 is rotatable and has sloping walls ensures that fuel fed from the feed system 4, which discharges at a location above the insert 11, can be uniformly distributed over the grate.

In that case when the insert rests on rollers 37, one or more of the rollers may have the form of a drive roller and may be arranged, in some suitable manner, to coact with the shoulders 36, for instance such that the insert will be rotated by the frictional forces engendered between roller and shoulder, therewith enabling the rollers 37 to be used for rotation of the insert.

A second tubular insert 13 is mounted above the first insert 11. In the case of the illustrated embodiment, the second insert 13 has the form of a straight cylinder having inwardly curved top and bottom edge surfaces and having a larger diameter than the base of the first insert. Seen in a vertical direction, the beginning of the inwardly bent inner edge surface of the second insert 13 is located on the same level as the beginning of the inwardly bent top edge surface of the first insert 11, such as to form a relatively narrow, annular inlet gap for the passage of secondary air of combus- tion, the flow cross-section of which gap is defined by the curvature of respective inwardly curved edge sur¬ faces and by the height position or vertical position of the second insert. The second insert may optionally be connected to the wall of the combustion chamber in a manner such as to enable the height position of the second insert to be adjusted, and therewith also the width of the inlet gap.

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

In the case of the illustrated embodiment, the combus¬ tion chamber and the inserts have a circular cross- section, which is the preferred cross-section. However, it will be understood that other cross-sectional shapes are possible, although less advantageous from the aspect of air flow, since rotation of the first insert is then liable to disturb the ho ogenity of the flow within the arrangement.

Mounted in the combustion chamber, above the second insert 13, is a third tubular insert 15 which, similar to the first insert 11, has the form of a frustated cone, although with the base facing downwards in contra¬ distinction to the base of the first insert. Further¬ more, the bottom edge surface of the third insert ex- tends fully to the wall 35 of the combustion chamber. and the third insert will thus form an upper limitation of the annular base between the combustion chamber wall 35 and the outer surfaces of the first and second inserts.

The wall of the third insert coacts with the inwardly curved top edge surface of the second insert, such as to form an inlet gap for tertiary air of combustion.

The walls of the gas passages 20 in the convection part 8 are configured in accordance with Swedish patent 80900799.0, the vortex-laminar gas flow in the gas passageways resulting in highly effective heat transfer to the radiator water.

The modus operandi of the aforedescribed arrangement will now be described. The occurrent gas flows are indicated in respective Figures by means of arrows.

The fan means 2 draws in fresh air by suction, through the conduit 27, fresh air from the peripheral gap lo¬ cated between the inner surface of the shell 22 and the outer surface of the combustion chamber wall 35. Prior to entering the fan, the fresh air is admixed with hot flue gases drawn by suction from the flue gas outlet 9, through the conduit 19, and also with an air-gas mixture taken optionally from the pressure side of the fan means, through the shunt conduit 25. The fresh air flowing in the peripheral gap cools the wall 35 of the combustion chamber, and is, of course, thereby pre¬ heated prior to entering the fan means.

The combustion air, in the form of an air/flue gas mixture, passes from the fan means to the collecting chambers 42 and into the air distributing tubes 41, therewith cooling the grate and further heating the combustion air.

The combustion air then flows out through the nozzle- like openings of the tubes 41, and is deflected locally by the bars 40 and enters the combustion chamber 3 in the form of two distinct flows separated by the wall of the insert 11, namely a primarily flow within the insert 11 and a secondary flow in the annulus space lying externally of the insert 11. In this case, the primary flow shall comprise about 60% of the total flow, so as to ensure that the oxygen present will be sufficient to achieve desired gassification of the fuel. Since, when seen in total, there is only a small air surplus during operation of the arrangement, it will be understood that the oxygen content of the primary flow will not suffice to provide stoichiometric combustion conditions in the actual fuel hearth. Conseguently, mainly gassification of solid fuel takes place in the fuel hearth. Thus, gassified solid fuel mixed with the primary flow will flow upwards from the lower part of the insert 11.

Part of the secondary flow will flow into the interior of the inserts through the gap between the first and second inserts 11, 13. The proportion of combustion air which flows in through this gap is determined substan¬ tially by the relationship between the restistance to flow afforded by respective inlet gaps between the inserts 11, 13 and 13, 15 and, when the height positions or vertical positions of the inserts 13, 15 can be adjusted, can thus be adapted to suitable values for mutually different solid fuels. A pre-heated mixture of fresh air and recycled flue gases will thus flow at high speed into the central part of the combustion chamber defined by the inserts, at a height location which is level with the top edge surface of the first insert 11. The secondary air of combustion is guided by the inwardly curved insert edge surfaces defining the inlet gap between the first and second inserts 11, 13, and the mixture will flow radially inwards from the top edge of the insert 11. As indicated in the Figures, this results in local turbulent flow, which produces a highly effective mixture from the gassified fuel contained by the primary flow and the in¬ flowing secondary air of combustion. This inflow is effective in supplyling sufficient oxygen to the gassified fuel to achieve the desired oxidation of carbon to carbon monoxide without forming free carbons. The effective mixing and recycling process engendered by said vortex motion greatly assists in generating fav¬ ourable conditions for the primary combustion process.

The remainder of the flow of secondary combustion air exiting from the grate 10 and passing through the gap defined between the first and the second inserts moves upwards along the annular space between the wall 35 of the combustion chamber and the second insert 13, and is deflected into the gap located between the third insert 15 and the inwardly curved top edge surface of the insert 13, and flows from said edge surface sub¬ stantially radially into the central part of the co - bustion chamber. Similar to the events occurring at the inlet gap for secondary air of combustion, this inflow of tertiary air of combustion results in the occurrence of a local vortex which results in a subsequent mixing and recycling process. The oxygen supplied through the inflow of tertiary combustion air results in a final combustion process comprising essentially the oxidation of carbon monoxide to carbon dioxide.

It will thus be seen that the relationship between the flow of secondary air of combustion and the flow of tertiary air of combustion depends on the oxygen re¬ quirement of the different combustion stages. Tests carried out on an arrangement constructed in accordance with the invention have shown that the arrangement will function satisfactorily with a tertiary combustion air flow of between 30 to 50% of the total value of the secondary air flow entering from the grate 10. Of course, the ratio between these flows will depend on the composition of the fuel concerned and the desired course of oxidation during the primary and final combustion processes, and since the tests were carried out with the combustion of forest pellets other values can be achie¬ ved when combusting solid fuels other than standard biofuels.

Since the secondary air flowing along the outer surfaces of the insert walls will cool said wall surfaces by so- called film cooling, it is not necessary to ginge-in the inserts, which is favourable from an operational aspect and also reduces demands on the material from which the inserts are made.

The first insert is rotated at a speed adapted to the fuel combustion cycle, such that the quantity of solid fuel falling from the fuel infeed system along a gen- eratrice of the first insert wall will be totally com¬ busted (gassified) subsequent to one revolution of the insert, at which point the generatrice will again be located opposite the outlet of the infeed system in the combustion chamber. The following combustion values were achieved when feeding pellets substantially continuously into a com¬ bustion chamber constructed in accordance with the invention and having a nominal rating of 500 kW: C0₂ 17-20% soot index 0-1 flue gas temperature 1500-1700°C

The carbon dioxide content was decreased by one or two units, by successively closing the valve or damper 24, whereas the soot index remained unchanged. The load could be stepped down to about 20%.

The tests were run with forestry pellets having a dia¬ meter of about 12 mm, although straw pellets of diameter 17 mm were also experimented with, these latter pellets showing a certain tendency to sinter, i.e. a given tendency for the residual products to bake onto the net placed on the grate. No such tendency was observed in the case of forestry pellets. So-called scaling was observed in certain parts of the combustion chamber. The combustion chamber was manufactured from a conventional construction steel (SIS 1312) and this material should consequently be replaced with a material of higher scaling temperature in certain parts of the combustion chamber.

The measuring values attained indicate that the desired stages in the combustion process were successfully achieved with an arrangement of apparatus constructed in accordance with the invention.

The soot index values attained show that the greater part of the fuel was combusted in a gassified state. said soot index being graduated in a scale from 0-10 and being a measurement of the extent to which free carbon is formed during the combustion process.

The carbon dioxide content is directly proportional to the air surplus present during the combustion process and is theoretically between 20-21% during combustion under stoichiometric conditions. The resultant carbon dioxide content values thus show that combustion in an inventive arrangement is effected with a very small air surplus and therewith under substantially stoichiometric conditions.

The spacial separation of the three stages of combustion achieved by means of the inventive arrangement, i.e. gassification of solid f el, primary combustion and final combustion, enables the primary and final combus¬ tion stages to be effected under substantially stoichio¬ metric conditions, and consequently the entire combus- tion process can be effected with a smaller air surplus than that possible with the arrangement known from WO 83/00373.

It will be understood that the described and illustrated arrangement can be modified in several ways within the scope of the invention. For instance, the first insert may be stationary instead of rotatable, in which case the fuel feed system must be constructed to distribute the fuel uniformly over the grate, which can be achie- ved, for instance, by causing the fuel to fall onto a distributing cone mounted in the central part of the combustion chamber. However, a rotating first insert is preferred, since conventional distributing arrangements have a tendency to disturb excessively the flow in the combustion chamber. Furthermore, it will be understood that final combustion can be achieved in several final stages, by introducing additional gap-forming inserts between the illustrated second and third inserts. The curvature of the inwardly curved edge surfaces of the inserts can be varied, in order to achieve desired gap cross-sections, such as to impart the desired direction and speed to the combustion air flowing into the interior of the combustion chamber.

The invention is therefore solely restricted by the contents of the following claims.

Claims

1. An arrangement for combusting solid fuels, such as chips, pellets, coal or finely cut wood, comprising a combustion chamber (3), a grate (10) arranged in the immediate vicinity of the bottom of the combustion chamber, means for supplying combustion air to the combustion chamber through said grate, a flue gas outlet (9) located in the upper end of the combustion chamber, and means (4) for supplying fuel to the combustion chamber, said combustion chamber being divided, by means of a first and a second insert (11, 13) of tubular configuration and arranged one above the other, into an internal and an external part by the fact that said inserts have a smaller diameter than corresponding parts of the combustion chamber, so as to form an annular space between the wall (35) of the combustion chamber and the outer surface of respective inserts, the mutu- ally facing ends of said inserts having a configuration such as to form therebetween a gap through which secon¬ dary air of combustion is delivered to the interior of the combustion chamber, characterized in that at least one further tubular insert (15) is mounted in the co - bustion chamber above the two other inserts; in that the further insert or further inserts has, or have, a con¬ figuration such as to form an air inlet gap between the bottom edge surface of each further insert and the top edge surface of an underlying insert; and in that the bottom edge surface of the uppermost insert extends fully to the inner surface of the wall (35) of the combustion chamber.

2. An arrangement according to Claim 1, characterized in that the sum of the smallest cross-sectional areas of the air inlet gaps provided by the at least one further insert is equal to or smaller than the smallest cross- sectional area of the secondary combustion area of the inlet gap for secondary air of combustion.

3. An arrangement according to Claim 1, characterized in that three inserts are mounted in the combustion cham¬ ber; and in that the uppermost insert (15) has the form of a frustated cone having a cone angle of between 150- 90°.

4. An arrangement according to Claim 1, characterized in that the wall of the uppermost insert (15) coacts with an inwardly curved inner edge surface of an underlying insert such as to form an air inlet gap; and in that the bottom edge surface of each insert located between the uppermost insert and the lowermost insert is curved in¬ wardly and, in a corresponding manner, coacts with an inwardly curved top edge surface of an underlying in- sert.

5. An arrangement according to Claim 1, characterized in that the first insert (11) is rotatable.

6. An arrangement according to Claim 5, characterized in that the first insert (11) has a circular cross-sec¬ tional shape.

7. An arrangement according to Claim 1, characterized in that the inserts (13, 15) with the exception of the first insert (11) can be adjusted in the vertical direc¬ tion.