WO1999060306A1 - Premix burner for liquid fuels - Google Patents

Abstract

The invention relates to a premix burner for liquid fuels comprised of a burner tube (2), a fuel distribution device, preferably one atomizer nozzle (1), an air turbulator (3), a preheating body (10) and of ignition electrodes (16). An oil vaporizer (5) is coaxially arranged downstream from the atomizer nozzle (1). Said oil vaporizer comprises outlet openings (7) which are arranged radially in the casing or axially in the base (6). The diameters of the outlet openings are smaller than 5 mm. Downstream from the outlet openings (7), the oil vaporizer (5) itself or heat conductive parts (9, 22), said heat conductive parts being connected to the oil vaporizer (5), project(s) into the combustion chamber (12), or a flame tube (15) is connected to the oil vaporizer (5) in a heat conductive manner.

Description

Premix burner for liquid fuels

State of the art

The principle of the premix burner has long been known for gas. However, premix burners for heating oil are not yet on the market. Burners for gaseous and liquid fuels differ fundamentally in the configuration. Liquid fuels must first be evaporated before they can be mixed with the combustion air and burned.

The standard oil burner, as it has been used for heating buildings for many years, is the so-called pressure atomizer. The fuel is atomized by means of a nozzle, and the combustion air is mixed into the spray mist via a turbulator. After ignition, a diffusion flame is created, in which the areas of mixing, vaporization and combustion merge. Although very small droplets are formed when the heating oil is atomized, a certain amount of time is required for the evaporation of these droplets. The mixture in this flame is extremely inhomogeneous and the pollutant formation correspondingly high. Since the fuel is processed and mixed with the combustion air in the flame itself, the flame volume is correspondingly large. This inhomogeneity is also the reason why a diffusion flame is very sensitive if the firebox is too cold; in this case soot is formed.

Another path is taken with the so-called gasification burner. The fuel is evaporated relatively quickly downstream of the nozzle by recirculating exhaust gases. A compact, gaseous flame is created. But even in this case it is not a real premix burner, because partial combustion also takes place here, even before the fuel has completely evaporated. Such a flame is much less sensitive to cold heating surfaces. Unfortunately, gasification burners produce a very noisy flame. When it comes to ignition, each blue-burning gasification burner is initially a yellow burner, since gasification can only start when the exhaust gases are available for recirculation. During this time, the exhaust emissions are also stronger.

The overlapping preparation steps also make it difficult to achieve functional changes in only one area. Every constructive change always affects all areas. The normal pressure atomizer also has the disadvantage that the minimum powers of less than 10 kW required today cannot be achieved, since the dimensions of the nozzles used cannot be reduced further; the risk of constipation would be too great. Other principles have also been tried to develop low-power burners. So the old compressed air atomizer was brought back to life, but it could not prevail on the market any more than an evaporation burner with a rotating evaporation beaker, which is described in patent applications DE-OS 2649669 and in EP 0283435.

Another principle is described in DE-P 4401799. Here a burner is presented, in which the fuel is led into a porous metal plate, where the fuel then evaporates. In this way, an even distribution of the heating oil is not possible, so that coking can occur.

A burner, which is described in EP-0405481 and which is actually a combination of evaporative burner and, also works in a similar manner Represents compressed air atomizer. The mixture is at least partially generated outside of the body from sintered metal, so that the principle of the premix burner has not been implemented here.

An evaporative burner is also described in D-OS 2356769. Experiments have shown that the evaporator quickly cokes. Such a burner also tends to repel the flame into the mixing chamber. In this construction, a return of the heat from the combustion zone into the evaporator is also not provided, so that the permanently required electrical heating of the evaporator would cause very high electricity costs.

The construction described in D-AS 1 265906 comes closest to a functional premix burner. But even here the problems were not recognized, the solution of which is the prerequisite for a reliable device.

The situation is similar for combustion chambers of gas turbines which are operated with liquid fuels. Dr. In his dissertation, Peter Walzer describes a combustion chamber based on the premix principle. Attention is also drawn to the difficulties which arise from the flame returning into the mixing zone. It is described that outflow velocities from the mixing chamber of more than 50-80 m / s were required to prevent the flame from striking back into the mixer.

Prechambers are also known from diesel engines, but they have not realized the premixing principle, just as a proposal for such a construction for a two-stroke engine in DE-OS 4305468.

All inventors of the designs mentioned have apparently not recognized the requirements of fuel evaporation, provided that it is ultimately an evaporative burner and the fuel is applied to a surface. Only with the two burners, which are described in patent applications DE-OS 2649669 and EP 0283435, was the unconscious choice of the right way to vaporize the fuel without leaving any residue. Leave the usual evaporation burners, especially the pot burners just run the fuel onto the hot pot bottom, where it slowly spreads. The same effect occurs when the heating oil is introduced into a body made of sintered material. However, heating oil can only be evaporated without leaving any residue if a thin oil film is spread quickly, which immediately changes to the vapor state. If this film is applied with an atomizing nozzle, for example, the heating oil sublimes immediately. There is no liquid film on the surface at all, the heating oil is immediately converted into the vapor state. It must be avoided in any case that drops form, because above a temperature of approx. 360 ° C the Leidenfrost phenomenon occurs with heating oil; the drop surrounded by an insulating layer of vapor literally dances on the hot surface and it takes a long time for the drop to evaporate. During this time, cracking processes take place within the drop, which cause oil coke to form on the evaporator surface.

Advantages of the invention

The premix burner according to the invention with the characterizing features of the main claim has the advantage over the prior art that even with insufficient atomization by the thermal treatment, a hot fuel vapor-air mixture is formed, which can be burned like a gas according to the premixing principle. Since the evaporator is preheated for this purpose at the start, the ignition is also easier. With a hot fuel vapor-air mixture, the ignition also generates far fewer pollutants than is the case with a cold mixture.

The flame of the premix burner has the lowest volume in relation to all other mixing principles. For this reason, less nitrogen oxides can form because the combustion products stay in the hot area for a shorter time. In addition, there are no mixing processes in the flame, because the mixture is homogeneous during combustion; In this way, so-called hot nests are avoided, which are also responsible for the formation of pollutants.

The low flame volume also enables the combustion chambers of the heaters to be designed with a smaller volume that can. In this way, the dimensions of the heat exchangers are also reduced and they are lighter in weight. The principle of the premix burner also includes other options. So z. B. the shape of the flame can be chosen freely. It is only the configuration of the burner head that determines whether a flame should have a cylindrical shape or whether the combustion should take place over a larger area. In this way, the energy density of the flame can also be selected, which has an effect on the residence time of the combustion products in the hot area and thus reduces the formation of nitrogen oxides. Of course, infrared emitters can also be implemented. In this case, in particular, a suitable coating is appropriate so that a catalyst is integrated in the burner at the same time.

drawing

Four embodiments of the invention are shown in the drawings and described in more detail below.

Description of the invention examples

The atomizer nozzle 1 is arranged coaxially in the burner tube 2, which is closed off by the turbulator 3. Downstream of the turbulator is the mixing chamber 4 of the oil evaporator 5, which in this case consists of a tube. The oil evaporator 5 is closed off from the perforated base 6 with the bores 7. The perforated plate 6 is delimited upstream by the pipe socket 8. The perforated base 6 widens radially over the pipe socket 8 into the pipe section 9, in which the tubular heating element 10 is cast. Mantelrofir 11 establishes the connection to the fan housing, not shown. The tip of the thermal sensor 13 is seated in the pipe socket 8. The pipe section 9 forms the combustion chamber 12 in which the flame turbulator 14 is located.

At the start of the burner, the tubular heater 10 is switched on. As soon as the thermal sensor 13 has reached a temperature of approx. 300 ° C., the command is given to the control device, not shown here, which gives voltage to the ignition electrodes 16. Fuel is injected into the mixing sprayed chamber 4 and admixed air via the turbulator. The arrows in the burner tube 2 show the direction of the air flow from the fan (not shown here). The fuel is evaporated on the hot perforated plate 6 and a fuel-vapor / air mixture enters the flame turbulator 14 downstream from the holes 7 in the perforated plate 6 Combustion chamber 12 forms a stabilizing vortex. At this point, the mixture is ignited by the ignition electrodes 16. After the ignition, the electrical preheating is switched off and the perforated base 6 receives the temperature required for evaporation via the heat conduction of the pipe section 9, which is located in the region of the vortex of flame formed on the turbulator. The pipe section 9 and the perforated base 6 must be made of a good heat-conducting material. Aluminum and copper or their alloys are suitable for this.

In the pipe section 9, the flame tube 15 is shrunk. Since the flame tube glows downstream at the free end, a sufficient amount of heat is introduced into the tube section 9 and thus into the perforated base 6. The flame tube 15 also prevents the parts made of highly conductive material from cooling the flame too much at this point, as a result of which unnecessary CO would otherwise be generated. The flow path of the mixture through the perforated plate 6, the bores 7 and the flame turbulator 14 is represented by arrows. The flame monitoring is not shown here, it can be carried out in the usual way for blue burners.

In principle, the process for a premix burner was shown, which despite the existing nozzle is no longer a typical pressure atomizer. The nozzle is only used to apply a film of fuel to the surface of the perforated base 6, which then evaporates. For this purpose, other methods can also be used to prepare even the smallest amounts of fuel, as is not possible with conventional pressure atomization.

It is also possible to operate a nozzle with pressure pulses of approx. 50 Hz. This atomization process is not suitable for burners with diffusion flames because the clock frequency is converted into flame noise. With some gasification burners, however, this noise can be greatly reduced so that it is portable in practice. This sound is not audible in real premix burners because after atomization steaming process takes place and the clocking is thereby equalized.

Film evaporation has been used with great success for a long time in direct-injection diesel engines. For gasification burners with a mixing tube, a combination of droplet evaporation and film evaporation is practiced. It has been shown that residue-free evaporation only depends on the time. The evaporation process must take place so quickly before oil coke can form. Therefore, no deposits are formed in the mixing tubes of the gasification burner, any more than in the piston bowls of the diesel. In the premix burner according to the invention it can be seen that when the burner is switched off the flame goes out without any delay. With the heating oil currently used, the boiling point is around 420 ° C, but it is sufficient if the evaporator is preheated to approx. 300 ° C at the start. If the burner is designed correctly, the flame heats the oil evaporator to the desired temperature. The film evaporation offers the invaluable advantage that it is impossible to blow the flame back into the mixing room at the correct temperatures and discharge speeds.

In drawing no. 2, a somewhat different configuration is shown. Similar parts are designated by the same reference numerals as in drawing no. 1.

The atomizer nozzle is arranged coaxially in the burner tube 2, which is closed downstream by the orifice 21 with the air nozzle 20. If the cross section of the oil evaporator is small, as in the example shown, an orifice with an air nozzle is also sufficient, since in this case the fuel does not have to be distributed in such a wide jet. Downstream of the air nozzle 20 is the mixing chamber 4 of the oil evaporator 5, which is designed here as a tube. The bores 7 are arranged radially in the tubular jacket of the oil evaporator 5. The tubular heater 10 is shown here as a cylindrical heating cartridge. The thermal sensor 13 is also located in the jacket of the oil evaporator 5. The flame tube 15 is formed at its upstream end as a flange 17, which is connected in a heat-conducting manner to the thicker part 18 of the oil evaporator 5, which is also upstream. If from the holes 7 of the oil evaporator 5 a finished, hot oil vapor-air mixture emerges, it is ignited by the ignition electrodes 16. Since the bottom and part of the oil evaporator 5 lie in the area of the flame represented by arrows, a large part of the heat of vaporization required for the fuel is distributed in the oil evaporator. The flame tube 15 becomes glowing at its downstream end during operation and conducts the heat via the flange 17 to the oil evaporator 5. The burner can be operated in the form shown, but the flame produces some noise. In the drawing 3, the same burner principle is shown with an additional, radial flame turbulator 14, which is pushed onto the downstream part of the oil evaporator 5.

This flame turbulator reduces the speed of the mixture emerging from the bores 7 of the oil evaporator and at the same time generates a stabilizing vortex, which also favors heat conduction into the oil evaporator. The drawing 3 shows a radial section through the bores 7 of the oil evaporator 5 and the entire flame tube 15.

In this section, the nozzle 1 can be seen from the front, the wall of the oil evaporator 5 with the bores 7. The flame turbulator 14 has a shape which is similar to the wheel of a radial fan. The mixture emerges in the space between the blades of the flame turbulator 14 and forms a stabilizing vortex in the annular space 19, which is delimited radially by the flame tube 15. The two ignition electrodes 16 are also visible in the front view. In this embodiment, an axially arranged flame turbulator, as shown in drawing 1, can also be used, which is pushed downstream of the bores 7 over the oil evaporator 5. In this case, the ignition electrodes 16 can be guided through the wings of the axial flame turbulator 14. Instead of the flame turbulator 14, a glow element made of metal or ceramic can also be arranged, which distributes the mixture emerging from the bores of the oil evaporator (5) over the entire cross section, the outflow rate being reduced so far that the flame can hold onto the surface . An infrared heater is created in this way.

In versions 2 and 3 of the burner according to the invention, the heat for evaporating fuel is both from the Part of the oil evaporator 5 passed upstream, as well as from the flame tube 15, which is connected to the oil evaporator 5 in a heat-conducting manner via the flange 17. Likewise, the burner with a radial flame can also be operated without the flame tube 15. In this case, additional heat-conducting elements must be attached to the oil evaporator. The oil evaporator 5 is shielded by the insulation 24 in its upstream part.

Another embodiment is shown in the drawing 4. In the schematic representation, the nozzle 1 is again arranged in the burner tube 2. The oil evaporator 5 is formed by the perforated base 6 with the holes 7, in which the tubular heater 10 is cast. The flame turbulator 14 is located downstream. The bolt 22 projects centrally into the flame and conducts heat into the perforated base 6. However, several small bolts can also be distributed over the flame turbulator. This version has the lowest thermal inertia, since only a small amount of material has to be heated to start the burner. After ignition, the bolt 22 ensures that sufficient heat is conducted to the perforated base 6 so that the evaporation functions even when the preheating is switched off. Heat is also conducted to the perforated base 6 via the flame tube 15, which is connected to the perforated base 6 via the flange 17. Of course, the preheating at the start can also be carried out using heated air. It is also possible that with larger burners - for example in gas turbines - the heat required for the fuel evaporator is supplied through heat pipes. In this way, the energy from the combustion process can be extracted at a particularly favorable point, since heat conduction is not a problem over long distances.

Staged combustion can also be implemented. In this case, the first stage burns with a lack of air, so that larger amounts of CO are generated. In the second stage, the remaining air is mixed in so that the combustion can take place with the optimal amount of air. If it is necessary, exhaust gases can also be added to the combustion air in the known manner. According to this principle, multi-stage or modulating burners can also be built with the known means. In some cases it can also be advantageous to provide the internal surface of the oil evaporator with a structure or to coat it so that the Surface has a certain roughness.

The prechamber of an internal combustion engine can be constructed analogously, but the functional sequence is somewhat different.

The prechamber is arranged centrally in the cylinder head. The inlet and outlet valves are outside the prechamber. During the compression stroke, the intake combustion air is displaced into the prechamber. At the start of the work cycle, fuel is injected into the prechamber, and the mixture flowing out of the prechamber is also ignited outside. A short flame now emerges from the antechamber, which forms between the chamber and the piston. No soot is produced in this way, even when using diesel oil. However, the compression temperature must be kept lower so that no auto-ignition occurs within the prechamber.

The holes 7 shown in the drawings have a diameter of 1.5-2.0 mm. It has been found that this reliably prevents the flame from striking back in the evaporator. These diameters also allow moderate mixture speeds of around 1 - 3 m / s to be operated, so that the pressure of the normal fans is sufficient for oil burners. It is also important that the material of the oil evaporator 5 also consists of a good heat-conductive material for this reason, because this also makes it more difficult for the flame to bounce back through the bores (7). The stated values were determined for a fuel throughput of 0.3-1.0 l / h. It would be possible to choose even smaller cross-sections, but this increases the risk of clogging and the required air pressure increases disproportionately. When the bores 7 are enlarged to 4 mm in diameter, the security against a backlash of the flame is severely impaired if the outflow speed is not increased enormously, which becomes problematic in the case of small oil burners because of the required fan pressure. Larger holes also increase the likelihood that unevaporated fuel will leave the evaporator. The holes 7 do not necessarily have to have a circular shape, it is also possible to choose slot-shaped openings which can be designed as gills. The stated dimensions for the diameter in this case relate to the slot width.

Claims

claims 1. premix burner for liquid fuels with a burner tube (2), the axially arranged fuel distribution device, preferably an atomizing nozzle (1), an air turbulator (3) or other air mixing device, a preheater (10), the ignition electrodes (16), characterized in that A coaxial oil evaporator (5) is arranged downstream of the atomizing nozzle (1) and has outlet openings (7) whose diameter is less than 5 mm or a slot width of less than 5 mm radially in the jacket or axially in the bottom (6) and that downstream of the outlet openings (7) the oil evaporator (5) itself, or heat-conducting parts (9, 22), which form part of the oil evaporator (5) or are connected to it, protrude into the combustion chamber (12) Flame tube (15) with the oil evaporator (5) is thermally connected. 2. Premix burner according to claim 1, characterized in that a turbulator (14) is arranged downstream of the outlet openings (7). 3. premix burner according to claim 1, characterized in that downstream of the oil evaporator (5) there is a transverse to the flow direction in the combustion chamber (12) arranged incandescent body, which consists of sintered metal, porous ceramic, or metallic tissue. 4. premix burner according to claim 1 and 3, characterized in that this incandescent body is larger in diameter than the flame tube (15) 5. premix burner according to claim 1 and 2, characterized in that the oil evaporator (5) consists of aluminum, copper or an alloy in which both metals are contained in a proportion of more than 10%.