METHOD AND INSTALLATION FOR THE COMBUSTION OF A GAS MIXTURE

Abstract: Method and installation for the combustion of a combustible gas mixture containing a hydrocarbon or hydrogen gas, sufficient air for a complete combustion of the hydrocarbon or hydrogen gas, and an essentially non-combustible ballast gas, such as air. The gas mixture is supplied to a pressure space (27) bounded by a burner plate (1; 6; 12; 15; 18) with an inlet side in the pressure space and an outlet side lying opposite the inlet side for the gas mixture flowing through at right angles to the plane of the burner plate. A burner plate is used in which between the inlet side (2: 7) and the outlet side (3; 8) one or more regions containing a large number of narrow channels for the gas throughput are formed, which regions (4; 20) have a predetermined low flow resistance, and are bounded by regions (5; 19) with a high flow resistance. The regions with a low flow resistance lie at least at such a distance from each other that on combustion of the gas mixture at the outlet side of the burner plate from each region with a low flow resistance a flame occurs which is at least almost completely separate from the flames of the other regions with a low flow resistance.

Description: Method and Installation for the combustion of a gas mixture. The invention relates to a method and installation for the combustion of a combustible gas mixture containing a hydrocarbon or hydrogen gas, sufficient air for a complete combustion of the hydrocarbon or hydrogen gas, and an essentially non-combustible ballast gas, which gas mixture is supplied under pressure to a pressure space which is at least partially bounded by a burner plate with an inlet side in the pressure space and an outlet side lying opposite the inlet side for the gas mixture flowing through essentially in a direction at right angles to the plane of the burner plate. In order to be able to continue meeting the present- day stricter environmental standards, it is necessary to lower the flame temperature in burners. In particular, it is desirable to have a flame temperature which at any arbitrary point in the flame lies between 600°C and 800°C, since on the one hand the NO x emission can be kept limited if the flame temperature is lower than 800°C and, on the other hand, the emission of carbon monoxide and hydrocarbons can be kept low by selecting a flame temperature higher than 600°C Various principles are known for obtaining a lower flame temperature in a burner. A first principle is that heat is drawn from the flame by a surface which is placed in or near the flames, and which thereby begins to glow and transfers the heat drawn from the flame by means of radiation to an element to be heated, for example a heat exchanger. However, only a limited output per unit surface area can be obtained in this way, and therefore a compact burner cannot be achieved. Moreover, in a burner operating on this principle the stability of the combustion is a problem during output modulation. A second principle for obtaining a lower flame temperature is to increase the air factor to a value greater than 1, i.e. the combustible gas mixture contains more air than can react with the combustible component of the gas during complete combustion. However, the use of high air factors in burners according to the prior art is not possible just like that. In particular, in the case of air factors greater than 1.4 it is difficult to obtain a stable flame, due to the fact that the low flame temperature and low temperature of the burner surface resulting from the high air factor are insufficient to ignite the gas mixture stably, and the high gas velocity also resulting from the high air factor leads to blowing away of the flame. The object of the invention is to provide a method and an installation which permit combustion of a combustible gas mixture containing an essentially non- combustible ballast gas, in particular a gas mixture with an air factor greater than 1.4 or a gas mixture to which through a process of recirculation exhaust gas is added, while retaining a high specific output and a stable, resonance-free combustion. Another object of the invention is to provide the possibility of a great modulation of the output over a large range. These objects are achieved in the afore-mentioned method through the use of a burner plate in which between the inlet side and the outlet side one or more regions containing a large number of narrow channels for the gas throughput are formed, which regions have a predetermined low flow resistance, viewed from the inlet side to the outlet side of the burner plate, and are bounded by regions with a high flow resistance, wherein the regions with a low flow resistance lie at least at such a distance from each other that on combustion of the gas mixture at the outlet side of the burner plate from each region with a low flow resistance a flame occurs which is at least almost completely separate from the flames of the other regions with a low flow resistance. Such a burner plate is known in itself from EP-A-0 267 671, and is used in an atmospheric gas burner for a solid fuel effect gas fire. Quite surprisingly, experiments have shown that with such a burner plate, when used in a burner in which a combustible gas mixture containing an essentially non-combustible ballast gas is supplied under pressure to the burner plate, a stable and resonance-free combustion can be attained. The burner plate is used in an installation comprising a pressure space to which the combustible gas mixture is supplied through a feed duct, and a combustion space in which the gas mixture is burned, both spaces being at least partially bounded by the burner plate, and compression means for generating a pressure in the pressure space which is higher than the pressure in the combustion space, the burner plate having an inlet side in the pressure space and an outlet side in the combustion space for the gas mixture flowing through essentially in a direction at right angles to the plane of the burner plate, and ignition means which are fitted at the outlet side of the burner plate in the combustion space. In such an installation according to the invention, regions with a significantly lower gas velocity than elsewhere in the flames are found between the flames and near the edges of the foot of the flames. These regions with low flow velocity remain intact up to a great distance from the burner plate and ensure a stable ignition along the edge of the flame. Such regions are absent in a burner in which the flames do fuse together, so that in such a burner other stabilisation means, e.g. glowing areas or separate glowing elements, are needed. In addition, in a burner with a burner plate according to the invention the regions with a large number of channels for the gas throughput have a low, but not negligible flow resistance. This flow resistance causes a pressure drop in the gas flow, with the result that pressure fluctuations over the burner plate, which occur particu¬ larly in closed appliances, during combustion have less effect on the gas flow velocity and gas distribution through the burner plate, and flame resonances are thus suppressed. An installation according to the invention therefore permits a stable combustion, and the output can be modulated over a large range. As a result of the use of a ballast gas, which absorbs heat during combustion and thus makes the flame temperature relatively low, the burner plate has a low surface temperature. The burner plate can therefore have a long service life, and no strict standards as regards mechanical properties need to be met. The ballast gas can be air, leading to an air factor of the combustible gas mixture of more than one, or part of the gas mixture after combustion thereof can be added to the combustible gas mixture to serve as a ballast gas. In the last-mentioned case, the combustion space of the installation is connected to the feed duct for the combustible gas mixture. Another suitable ballast gas is water, and for special burner applications other ballast gases may be used. It will be clear that a ballast gas may in itself be a mixture of different gases. In an advantageous embodiment of the installation according to the invention the narrow channels in the burner plate are labyrinth-shaped in the regions with a low flow resistance, i.e. the axis of a channel in general does not form a straight line, and channels can be interconnected. These regions can be formed by a porous material, such as ceramic material. In another embodiment, the narrow channels in the burner plate are straight and run parallel to each other in a direction at right angles to the plane of the burner plate. In this way it is possible to make the channels by cutting with a laser beam or water jet or the like. In order to obtain sufficient flow resistance, a hydraulic diameter of the channels which is smaller than 0.4 mm, and is preferably smaller than 0.2 mm, is envisaged. The burner plate is preferably designed in such a way that the smallest cross-section of each region with a low flow resistance between the inlet and outlet side, viewed in the plane of the burner plate, is at least about 5 mm 2 , and the sum of these smallest cross-sections of all regions with low flow resistance is at most 70% of the burner plate surface area at the outlet side. In particular, the burner plate is designed in such a way that the abovementioned sum of the smallest cross-sections is at least about 10% and at most about 50% of the burner plate surface area at the outlet side, and more particularly these lowest and highest percen¬ tages can be 20 and 40, respectively. The regions in the burner plate with a low flow resistance advantageously have an essentially round cross-section, viewed in the plane of the burner plate, with the result that flames with a natural shape can develop. A very advantageous embodiment is obtained if the plate consists of a base plate of an essentially gastight material, which base plate is provided with holes of which the edges determine the boundaries of the regions with a low flow resistance, and also consists of a gas- permeable structure extending over at least the cross- section of the holes. In particular, the gas-permeable structure can be made in the form of an essentially flat plate covering the abovementioned base plate. In this way the burner plate can be assembled in a particularly simple way from two plates. It is also possible to fill the holes in the base plate at least partially with the gas-permeable struc¬ ture. This makes a thin burner plate possible, in par¬ ticular if the gas-permeable structure covering the cross-section of the holes is situated completely in these holes. It is advantageous as regards price and delivery time to select a standard perforated metal plate as the base plate. Another possible embodiment comprises a gas- permeable plate which is locally densified in order to form the regions with a high flow resistance there. This again makes it possible to produce a thin burner plate, and in addition the plate forms one self-supporting entity. The gas-permeable plate can be densified locally in a simple way by impregnating it with a filling material. The densification can also be obtained by compressing the plate at the places where regions with a high flow resistance have to be formed. The structure of the gas-permeable plate in this embodiment can be a foam structure, so that a simple manufacturing process is possible. The gas-permeable plate can also be two foam structures with different degrees of gas permeability. In the abovementioned embodiments the regions with a low flow resistance can advantageously be made of metal fibres or aluminium oxide fibres, on account of the resistance of these materials to the prevailing tempera¬ tures. Metal fibres can also be impregnated, which is advantageous, inter alia, in the abovementioned embodi¬ ment, in which the burner is one plate densified locally by impregnation. Aluminium oxide fibres can be applied in an excellent way by spraying onto a carrier, which makes them very suitable for use in combination with a base plate according to an earlier described embodiment. The invention is explained with reference to the drawing, in which: Fig. 1 shows a schematic view of an embodiment of an installation according to the invention; Fig. 2 shows a partially cut-away perspective view of a first embodiment of a burner plate for use in an installation according to the invention; Fig. 3 shows a partially cut-away perspective view of a second embodiment of a burner plate for use in an installation according to the invention; Fig. 4 shows a partially cut-away perspective view of a third embodiment of a burner plate for use in an installation according to the invention; Fig. 5 shows a cross-section of a fourth embodi¬ ment of a burner plate for use in an installation according to the invention; and Fig. 6 shows a cross-section of a fifth embodi- ent of a burner plate for use in an installation according to the invention. Fig. 1 shows a boiler 21, comprising an air inlet duct 22 and a gas inlet duct 23, which open out in a mixing chamber 24 for mixing the air and gas supplied to it. In the figure arrows indicate the direction of flow of media. The mixing chamber 24 is connected to a feed duct 25, in which a fan 26 which can pressurize the combustible gas/air mixture is situated. The feed duct 25 ends in pressure space 27, which is bounded by a burner plate 6. At the other side of burner plate 6 a combined igniter and temperature sensor 28 is situated in a combustion space 29. Adjacent to this combustion space 29 is a heat exchanger 30, through which the hot combustion gases coming from separate flames can flow and transfer heat to another medium also flowing through the heat exchanger 30, following which the combustion gases can flow away through a discharge duct 31. This boiler 21 can be used, for example, as a central heating boiler, and can then be fired by natural gas. To lower the flame temperature in the burner, excess air may be supplied to the mixing chamber 24, or a part of the exhaust gases may be recirculated by means of a recircula ion duct 32 shown in dashed lines connecting the discharge duct 31 to the feed duct 25. In this way essentially non-combustible ballast gas is added to the combustible gas mixture in feed duct 25. The recirculation duct 32 may comprise a control valve 33 for setting the flow in the duct 32. In the burner shown in Fig. 1 a mixing chamber 24 strictly speaking is unnecessary, since the fan 26 can perform the same function. Further, instead of a fan 26 in the feed duct 25, a fan may be situated in the discharge duct 31 for generating the same gas flows in the boiler 21. The recirculation duct 32 may, instead of being connected to the feed duct 25, be connected to the mixing chamber 24, the air inlet duct 22 or the gas inlet duct 23. Fig. 2 shows a rectangular burner plate l with an inlet side 2 and an outlet side 3 for a gas mixture which can flow in the direction indicated by arrow A through the regions formed by narrow parallel, straight channels with a low flow resistance 4. The reference number 5 indicates the regions with a high flow resistance. The burner plate can be a metal plate in which the channels are made by laser cutting. Fig. 3 shows a rectangular burner plate 6, again provided with an inlet side 7 and an outlet side 8, through which a gas mixture can flow in the direction indicated by an arrow B. This burner plate is composed of a metal plate 9, which is perforated with square holes 10, and a porous plate 11 which forms a gas-permeable structure and is made of sprayed-on aluminium oxide fibres. The gas flow direction can also be selected opposite to the direction of arrow B. In that case reference numeral 8 indicates the inlet side, and refer¬ ence numeral 7 indicates the outlet side. Fig. 4 shows a rectangular burner plate 12, consisting of a perforated base plate 13 which is pro- vided with a porous layer 14 covering the top and bottom side of the plate and filling up the perforations in the base plate. Fig. 5 shows a burner plate 15 similar to the burner plate 6 of Fig. 3, and differing from it in that round conical holes 17 are provided in the metal plate 16. Fig. 6 shows a burner plate 18 which consists of a metal fibre mat which is densified to form regions with high flow resistance 19, which bound regions with low flow resistance 20. A burner plate made up of a metal plate perfor¬ ated with round holes and covered with a porous plate was tested. The configuration of this plate and the test conditions are given in Table l, in which the emission of harmful substances is also stated, for an output which is held constant, and as a function of the air factor. It can be seen clearly that a considerable reduction of this emission can be achieved compared with known combustion methods, where the air factor generally was lower and never more than 1.4. Table 2 further shows the influence of the output on the harmful emission, for a combustion method according to the invention. It can be seen from this that the output can be varied over a large range, while the harmful emission remains low and virtually constant. The burner plates shown in the drawing are all made flat. Of course, the plates can also be a different shape, such as a curved, ribbed or bent shape. It is, however, essential that the burner plates according to the invention should be designed in such a way that during use in a burner flames which are at least almost completely separate from each other occur at the outlet side of the burner plate. Table 1: Tesr results with a burner plate made up of a metal plate perforated with round holes and covered with a porous plate, the air factor being varied. No recirculation of exhaust gases. configuration width of metal plate 160 mm length of metal plate 200 mm thickness of metal plate 2 mm number of holes 150 hole diameter 8 mm surf, with low flow resistance % total surface 0.24 porosity of gas-permeable plate 80% pore size in gas-permeable plate 0.1 - 0.3 mm thickness of gas-permeable plate 1 mm test conditions calorific upper value natural gas 33.56 MJ/m : 3 output 24 k burn-out height (flame length) 100 mm measuring results Table 2: Test results with a burner plate made up of a metal plate perforated with round holes and covered with a porous plate, the output being varied. No recirculation of exhaust gases. configuration width of metal plate 160 mm length of metal plate 200 mm thickness of metal plate 2 mm number of holes 160 hole diameter 6 mm surf, with low flow resistance % total surface 0.14 porosity of gas-permeable plate 80% pore size in gas-permeable plate 0.1 - 0.3 mm thickness of gas-permeable plate 1 mm test conditions calorific upper value natural gas 33.56 MJ/m 3 air factor 1.82 burn-out height (flame length) 150 mm measuring results NO. ppm 4.0 2.8 1.9 1.7 1.7 1.5 1.8 1.8

Claims: CLAIMS 1. Method for the combustion of a combustible gas mixture containing a hydrocarbon or hydrogen gas, sufficient air for a complete combustion of the hydrocarbon or hydrogen gas, and an essentially non- combustible ballast gas, which gas mixture is supplied under pressure to a pressure space (27) which is at least partially bounded by a burner plate (1; 6; 12; 15; 18) with an inlet side in the pressure space and an outlet side lying opposite the inlet side for the gas mixture flowing through essentially in a direction at right angles to the plane of the burner plate, characterized in that a burner plate (1; 6; 12; 15; 18) is used in which between the inlet side (2; 7) and the outlet side (3; 8) one or more regions containing a large number of narrow channels for the gas throughput are formed, which regions (4; 20) have a predetermined low flow resistance, viewed from the inlet side to the outlet side of the burner plate, and are bounded by regions (5; 19) with a high flow resistance, wherein the regions with a low flow resistance lie at least at such a distance from each other that on combustion of the gas mixture at the outlet side of the burner plate from each region with a low flow resistance a flame occurs which is at least almost completely separate from the flames of the other regions with a low flow resistance. 2. Method according to claim 1, characterized in that the ballast gas is air. 3. Method according to claim 1, characterized in that part of the gas mixture after combustion thereof is added to the combustible gas mixture to serve as a ballast gas. 4. Installation for the combustion of a combustible gas mixture containing a hydrocarbon or hydrogen gas, sufficient air for a complete combustion of the hydrocarbon or hydrogen gas, and an essentially non- combustible ballast gas, the installation comprising a pressure space (27) to which the combustible gas mixture is supplied through a feed duct (25) , and a combustion space (29) in which the gas mixture is burned, both spaces being at least partially bounded by the burner plate (1; 6; 12; 15; 18), and compression means (26) for generating a pressure in the pressure space (27) which is higher than the pressure in the combustion space (29) , the burner plate having an inlet side in the pressure space (27) and an outlet side in the combustion space (29) for the gas mixture flowing through essentially in a direction at right angles to the plane of the burner plate, and ignition means (28) which are fitted at the outlet side of the burner plate in the combustion space (29), characterized in that between the inlet side (2; 7) and the outlet side (3; 8) of the burner plate (1; 6; 12; 15; 18) one or more regions containing a large number of narrow channels for the gas throughput are formed, which regions (4; 20) have a predetermined low flow resistance, viewed from the inlet side to the outlet side of the burner plate, and are bounded by regions (5; 19) with a high flow resistance, wherein the regions with a low flow resistance lie at least at such a distance from each other that on combustion of the gas mixture at the outlet side of the burner plate from each region with a low flow resistance a flame occurs which is at least almost completely separate from the flames of the other regions with a low flow resistance. 5. Installation according to claim 4, characterized in that the combustion space (29) is connected to the feed duct (25) for adding part of the gas mixture after combustion thereof to the combustible gas mixture to serve as a ballast gas. 6. Installation according to claim 4 or 5, characterized in that the narrow channels in the burner plate form a labyrinth. 7. Installation according to claim 4 or 5, characterized in that the narrow channels in the burner plate (1) are straight and run parallel to each other in a direction at right angles to the plane of the burner plate. 8. Installation according to any of claims 4-7, » characterized in rhat the smallest cross-section of each region (4; 20) in the burner plate (1; 6; 12; 15; 18) with a low flow resistance between the inlet side (2; 7) and outlet side (3; 8) , viewed in the plane of the burner 5 plate, is at least about 5 mm 2 , and the sum of these smallest cross-sections of all regions with low flow resistance is at most about 70% of the burner plate surface area at the outlet side. 9. Installation according to any of claims 4-7, 10 characterized in that the smallest cross-section of each region (4; 20) in the burner plate (1; 6; 12; 15; 18) with a low flow resistance between the inlet side (2; 7) and outlet side (3; 8) , viewed in the plane of the burner plate, is at least about 5 mm 2 , and the sum of these 15 smallest cross-sections of all regions with low flow resistance is at least about 10% and at most about 50% of the burner plate surface area at the outlet side. 10. Installation according to any of claims 4-7, characterized in that the smallest cross-section of each 0 region (4; 20) in the burner plate (l; 6; 12; 15; 18) with a low flow resistance between the inlet side (2; 7) and outlet side (3; 8) , viewed in the plane of the burner plate, is at least about 5 mm 2 , and the sum of these smallest cross-sections of all regions with low flow 5 resistance is at least about 20% and at most about 40% of the burner plate surface area at the outlet side. 11. Installation according to any of claims 4-10, characterized in that the regions (4; 20) in the burner plate (1; 6; 12; 15; 18) with a low flow resistance have 0 an essentially round cross-section, viewed in the plane of the burner plate. 12. Installation according to any of claims 4-11, characterized in that the burner plate consists of a base plate (9; 13; 16) of an essentially gastight material, 5 which base plate is provided with holes (10; 17) of which the edges determine the boundaries of the regions (4; 20) with a low flow resistance, and also consists of a gas- permeable structure (11; 14) extending over at least the cross-section of the holes. 13. Installation according to claim 12, characterized in that the gas-permeable structure (11; 14) is in the form of an essentially flat plate covering the base plate. 14. Installation according to claim 12 or 13, characterized in that the holes (10; 17) in the base plate (9; 13; 16) are at least partially filled with the gas-permeable structure (11; 14) . 15. Installation according to any of claims 12-14, characterized in that the base plate (9; 13; 16) is a perfora^d metal plate. 16. Installation according to any of claims 4-11, characterized in that the burner plate (18) is a gas- permeable plate which is locally densified. 17. Installation according to claim 16, characterized in that the burner plate is a gas-permeable plate which is locally densified by impregnation with a filling material. 18. Installation according to claim 16, characterized in that the burner plate (18) is a gas- permeable plate which is locally densified by partially compressing the plate. 19. Installation according to any of claims 16-18, characterized in that the burner plate (18) has a foam structure. 20. Installation according to any of claims 16-18, characterized in that the burner plate (18) consists of two foam structures with different gas permeabilities. 21. Installation according to any of claims 4-18, characterized in that the regions (4; 20) in the burner plate with a low flow resistance are made of metal fibres. 22. Installation according to any of claims 4-18, characterized in that the regions (4; 20) in the burner plate with a low flow resistance are made of aluminium oxide fibres.

Inventors: IN HET VELD ANDRE, VLOON PAULUS JACOBUS, IN HET VELD, ANDRE, VLOON, PAULUS, JACOBUS
Applicants: FASTO NEFIT BV, NEFIT FASTO B.V.
Application year: 1992
Main classification: F23D14
IPC codes: F23D14