SE461295B - DEVICE FOR COMBUSTION OF FIXED FUEL, PROVIDED WITH INSTALLATION ORGANIZED IN THE BRAENN CHAMBER FOR CONTROL OF SUPPLY OF COMBUSTION AIR - Google Patents

Description

The desired process in the combustion of the solid fuel can be divided into three stages, namely a first stage in which the solid fuel is gasified, a second stage in which the gasified fuel is combusted with formation of combustion products, which do not contain freely heated carbon but the carbon is constantly bound to other substances, among other things by the formation of carbon monoxide, and a third step, in which a final oxidation of the combustion products takes place. formed.

Such combustion can be maintained with an air supply which closely corresponds to a combustion reaction with stoichiometric proportions between fuel and oxidizer, which is advantageous both in terms of pressure drop and heat transmission.

In addition, such combustion produces cleaner exhaust gases and is thus beneficial from an environmental point of view. which means, among other things, that no nitrogen oxides are formed.

The present invention aims to provide a device for burning solid fuels. in which the combustion is further improved compared with the result obtained in the above-mentioned publication.

According to the invention, this is achieved by a device of the kind mentioned in the introduction, which is characterized in that at least one further tubular insert is arranged in the combustion chamber above the two other inserts. which additional insert or inserts have such a configuration that an air inlet gap is formed between the lower edge of each additional insert and the upper edge of an underlying insert. and that the lower edge of the top insert extends to the inside of the combustion chamber wall. The additional insert or inserts thus create a tertiary zone in which the final oxidation mainly takes place. Thereby, the two oxidation steps in the desired combustion process are separated from each other, which makes it possible to get even closer to stoichiometric proportions in the combustion process than in the device known from the above-mentioned publication without compromising the final oxidation. which further improves combustion. In tests with a device according to the invention, the carbon dioxide values of the final products of 17-20 2 have been measured, which values are close to the theoretically optimally achievable value.

Further features and advantages of the invention will become apparent from the following detailed description of a preferred embodiment of a device according to the invention with reference to the accompanying drawings, of which: Fig. 1 shows, partly in section. a device according to the invention and Fig. 2 shows, partly in section. the lower part of the device in Fig. 1.

The device shown in Fig. 1 comprises an ash tray 1, a fan device 2. a combustion chamber 3, to the bottom of which a grate 10 is connected to allow air access to the combustion chamber, and a convection part 8 in which water is heated by the heat from the hot flue gases and which has a flue gas outlet 9.

A jacket 22 of insulating material surrounds the combustion chamber 3 at some distance therefrom so that a peripheral gap is formed between the inside of the jacket and the outside of the combustion chamber wall. This gap is open upwards towards the atmosphere and in its lower part connected to one or more lines 27. which are connected to the suction side of the fan device 2. A line 19 from the flue gas outlet 9 is also connected to the suction side of the fan device. Furthermore, a shunt line 25 connects the pressure side of the fan to its suction side and valves or dampers 24. 26, 28 are arranged in the lines to control the gas flows through them.

Through a fuel supply system 4, fuel can be fed into the combustion chamber 3. In the example shown, this consists of a conveyor belt 7, on which the solid fuel enters a storage tank 6, from which the fuel falls down by gravity through a feed pipe. which opens into the combustion chamber. The amount of falling fuel can be regulated by a regulator S. Furthermore, three inputs ll. 13. arranged in the combustion chamber 3, which will be described in more detail below with reference to Fig. 2. 15 15 25 25 35 461 295 'In fig. Fig. 2 shows the lower part of the device in Fig. 1 on a larger scale. The grate 10 should, as previously stated. allow 'combustion air to be supplied to the combustion chamber. and for this purpose comprises a number of parallel air distribution pipes 41, which at their ends are connected to collecting chambers 42, which can also be constituted by pipes. connected to the pressure side of the fan device 2 by one or more pipe sockets 33. The pipes 41 are open upwards towards the combustion chamber 3 and have a nozzle-shaped cross section in their upper part. In order to achieve an even outflow velocity in the nozzles, the tubes 41 are preferably tapered in the air flow direction. as shown in Fig. 1. Further, narrow spacer beams 40 extend. for example in the form of upright flat iron, opposite the nozzle openings of the pipes 41 and between these pipes to support the fuel and keep it at a distance from the nozzle openings. To allow length extension of these beams, these are not fixedly clamped but rest, for example, in grooves formed in the combustion chamber wall or on the upper side of the collecting chambers. In addition, if small pieces of fuel are used, a steel mesh can be arranged on top of the beams to prevent fuel from falling through the grate 10 and into the ash box 1.

Above and directly adjacent to the grate 10, a first tubular insert 11 is provided. This is preferably in the form of a truncated cone with the base facing upwards and inwardly bent upper edge.

Furthermore, the insert is supported via a beam cross 39 arranged in its bottom by a rotatable shaft 32, which is suitably connected, for example by a gear chain 30, to a gear arranged on the shaft 32. Rotationally driven by a motor 29. Preferably, the insert also supports against rollers 37 projecting from combustion chamber road 35, which cooperate with circumferential lugs 36 or the like formed on the outside of the insert 11. Because the insert 11 is rotatable and has sloping walls, a uniform distribution of fuel introduced from the feeding device 4 opening above the insert 11 can thus be achieved.

In the case where the insert rests on rollers 37, in a variant these can be used for the rotational drive of the insert by one or more of these being designed as drive rollers and suitably cooperating with the lugs 36, e.g. so that the insert is rotated due to the frictional forces between the roller and the heel.

A second tubular insert 13 is arranged above the first. In the example shown, this has the shape of a straight cylinder with net-curved upper and lower edges and has a larger diameter than the base of the first insert. In height, the beginning of the inwardly curved inner edge of the second insert 13 is flush with the beginning of the inwardly curved upper edge of the first insert 11 so that an annular relatively narrow inlet gap for secondary air is formed. whose flow cross-section is defined by the curvature of resp. net-curved edge and the height position of the second insert. Optionally, the second insert can be attached to the combustion chamber wall in such a way that its height position and thus the gap width can be varied.

Furthermore, the diameter of the second insert is smaller than the diameter of the combustion chamber 3 so that an annular space is formed between the combustion chamber wall 35 and the outsides of the first and second inserts.

In the described embodiment, the combustion chamber and the inserts have circular cross-sections. which is preferred. Other cross-sectional shapes are, of course, possible, but are less advantageous from an air flow point of view. since the rotation of the first insert can then interfere with the flow homogeneity of the device.

Finally, a third tubular insert 15 is arranged in the combustion chamber above the second insert 13. The third insert has, like the first shape of a truncated cone but has, in contrast to the first insert, the base facing downwards. Furthermore, the lower edge of the third insert extends all the way to the combustion chamber wall 35 and the third insert thus constitutes an upper limitation of the annular space between the combustion chamber wall 35 and the outsides of the first and second inserts. The wall of the third insert cooperates with the inwardly curved upper edge of the second insert to form an inlet gap for tertiary air.

In the convection part 10, the walls of the gas ducts 20 are designed in accordance with the Swedish patent 80900799.0, whereby a very efficient heat transfer to the radiator water is obtained by the vortex laminar gas movement arising in these ducts.

The function of the described device will now be described. The gas flows that occur are indicated by arrows in the figures.

The fan device 2 sucks in fresh air via the line 27 from the peripheral gap between the inside of the jacket 22 and the outside of the combustion chamber wall 35- Before entering the fan, the fresh air is mixed with hot flue gases sucked in through the line 19 from the flue gas outlet 9 and air-gas mixture. The fresh air flowing in the peripheral gap cools the combustion chamber wall 35, which at the same time naturally means that the fresh air is preheated before entering the fan.

From the fan, therefore, combustion air in the form of an air-flue gas mixture flows to the collecting chambers 42 into the air distribution pipes 41., whereby the grate is cooled and the combustion air is further preheated.

The combustion air then flows out of the nozzle openings of the pipes 41. is deflected locally by the beams 40 and enters the combustion chamber 3 in two flows separated from the wall of the insert 11, namely a primary flow inside the insert 11 and a secondary flow in the annular outside the insert 11 the space.

In this case, the primary flow must be about 60% of the total flow to ensure a sufficient amount of oxygen for the desired gasification. 10 15 20 25 30 35 -7 461 295 Since the excess air during the operation of the device is kept low overall, it is realized that the oxygen content of the primary flow is not sufficient for stoichiometric combustion conditions to occur in the fuel core itself. In this case, therefore, gasification of the solid fuel mainly takes place. Thus, gasified solid fuel mixed with the primary flow flows upwards from the lower part of the insert 11.

A 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 flowing into this gap is essentially determined by the ratio of the flow resistances in the inlet gaps between the inserts 11. 13 and 13, 15 and can thus - if the height positions of the inserts 13 and 15 are adjustable - be adapted to suitable values for different solid fuels.

A preheated mixture of fresh air and recirculated flue exhaust gases thus flows at high speed into the central part of the combustion chamber limited by the inserts at the height of the upper edge of the first insert 11. This secondary combustion air is controlled by the inlet gaps between the first and second inserts 11. 13 forming inwardly curved edges of these inserts and the mixture flows radially inwardly from the upper edge of the insert 11. Thereby arises. as indicated in the figures. a local eddy current, which provides a very efficient mixture of the gasified fuel containing the primary flow and the inflowing secondary combustion air. This inflow means that sufficient oxygen is added to the gasified fuel for the desired oxidation of carbon to carbon monoxide to take place without the formation of free carbon. The efficient mixing and recirculation process effected by the vortex motion greatly contributes to creating favorable conditions for this primary combustion process. The remaining part of the secondary flow flowing past the gap between the first and the second insert flows upwards along the annular space between the annulus. The fire chamber wall 35 and the second insert 13. are deflected into the gap between the third insert 15 and the bent-in upper edge of the insert 13 and flow from this edge substantially radially into the central part of the combustion chamber. In the same way as with the inlet gap for the secondary combustion air, a local eddy flow with accompanying mixing and recirculation processes is produced by this inflow of tertial combustion air. The oxygen supplied by the tertialine flow then allows a final combustion, which mainly consists of carbon monoxide being oxidized to carbon dioxide.

It thus appears that the relationship between the secondary combustion air flow and the four-month combustion air flow depends on the oxygen demand for the different combustion stages. In tests with a device according to the invention, it has been found that the device works well with tertial combustion air flows between 30 to SO 2 of the total secondary flow coming from the grate 10. The relationship between these flows naturally depends on the composition of the fuel and the desired oxidation processes in the primary and final combustion, and since the tests were carried out with the combustion of forest pellets, other values can be obtained when burning solid fuels other than normal biofuels.

Since the secondary flow flowing along the outside of the insert walls cools these walls through so-called film cooling avoids inlays, which is favorable from an operational point of view. and furthermore, less demands are placed on the material for the inserts.

The rotation of the first insert takes place at a speed which is adapted to the combustion cycle of the fuel. so that a quantity of solid fuel which falls down from the feed device along a generator of the wall of the first insert is completely burned (gasified) after a revolution when this generator again ends up in the middle of the outlet of the feed device in the combustion chamber.

When burning pellets in a combustion chamber designed in accordance with the invention with a nominal value of 500 kw, the following combustion values were achieved with a substantially continuous feed: CO 2 17-20% soot 0-1 flue gas temperature 1500-1700 ° C. with successive closing of the damper 24, the carbon dioxide content decreased by one or a few units while the soot number was unchanged. The load could be changed down to about 20 1.

The samples were run with forest pellets with a diameter of about 12 mm. but also straw pellets with a diameter of 17 mm have the sample. whereby the latter showed a certain tendency to sintering, ie. a certain baking tendency of the residual products directly on the grid provided with grids was noted. There was no such tendency in forest pellets. during the tests, so-called scaling in certain parts of the combustion chamber. The combustion chamber material used was a general structural steel (SIS 1312) and this should therefore be replaced in some parts of the combustion chamber with a material with a higher scaling temperature.

The measured values obtained indicate that the desired steps in the combustion process are to a large extent achieved with a device in accordance with the invention.

The achieved values of the soot. which is graded on a scale from 0-10 and is a measure of the extent to which free carbon is formed during the combustion process. shows that most of the fuel is burned in gasified form.

The carbon dioxide content is directly proportional to the excess air during combustion and is theoretically between 20-21 l during combustion under stoichiometric conditions. The values obtained on the carbon dioxide content thus show that the combustion in a device according to the invention takes place with a very small excess of air and thus under almost stoichiometric conditions.

The spatial separation achieved by the invention of the three steps included in the combustion process - gasification of solid fuel. primary combustion and final combustion - thus means that the primary and final combustion can take place under almost stoichiometric conditions and thus the combustion as a whole can be operated with a smaller excess of air than with the device known from WO ~ 83 / O0373.

The described device can of course be modified in a number of ways within the scope of the invention. For example, the first insert may be static instead of rotating. wherein the feeding device must in that case be arranged to distribute the fuel evenly over the grate, which can be achieved, for example, by the fuel falling down on a distribution cone arranged in the central part of the combustion chamber. However, a rotating first effort is preferred. since conventional type distribution devices tend to disturb the flow in the combustion chamber too much. The final combustion can of course be achieved in several final stages by introducing additional gap-forming inserts between the second and third inserts shown. The curvature of the bent-in edges of the inserts can be varied to achieve the desired gap cross-sections so that the combustion air flowing into the inner part of the combustion chamber is given the desired direction and speed.

The invention is therefore to be limited only by the content of the appended claims.

Claims (7)

10 15 20 25 461 295 11 Patent claims Device for burning solid fuels, such as wood chips. pellets. charcoal or finely chopped wood, which device comprises a combustion chamber (3). a grid (10) adjacent to the bottom of the combustion chamber. a device for supplying combustion air to the combustion chamber through the grate. a flue gas outlet (9) at the upper end of the combustion chamber and a device (4) for supplying fuel to the combustion chamber. which fire chamber by means of a first and a second insert (ll. 13). which inserts are tubular and arranged one above the other. is divided into an inner and an outer part in that the inserts have a smaller diameter than corresponding parts of the combustion chamber so that an annular space is formed between the combustion chamber wall (35) and the outside of resp. in- sats. wherein the inserts in the facing ends have such a configuration that a gap for supplying secondary combustion air to the inner part of the combustion chamber is formed between them. characterized in that at least one further tubular insert (15) is arranged in the combustion chamber above the other two inserts, which additional insert or which additional inserts have such a configuration that an air inlet gap is formed between the lower edge of each additional insert and the upper edge of an underlying insert, and that the lower edge of the top insert extends to the inside of the combustion chamber wall (35) - Device according to claim 1, characterized in that the sum of the smallest cross-sectional areas of the air inlet slots constituted by the additional inserts is equal to or less than the minimum cross-sectional area of the secondary combustion area of the secondary combustion air inlet gap. Device according to claim 1, characterized in that three inserts are arranged in the combustion chamber and that the uppermost insert (15) has the shape of an upwardly facing, truncated cone with a cone angle which is between 150-90 °. BV 81212 Device according to claim 1, characterized in that the wall of the upper insert (15) cooperates with an inwardly curved UI 10 15 461 295 12 inner edge of an underlying insert to form an air inlet gap and that the lower edge of each between the upper and the the bottom insert located insert is inwardly curved and correspondingly cooperates with an inwardly curved upper edge of an underlying insert. Device according to claim 1, characterized in that the first insert (II) is rotationally driven. Device according to claim 5. Characterized by the fact that the first insert (II) has a circular cross-sectional shape. 7. Device according to claim 1, characterized in that the inserts (13. 15) apart from the first insert (ll) are arranged height-adjustable.