EP0906545B1 - Surface-combustion liquid-fuel burner and method of combustion - Google Patents

Abstract

The invention concerns a method by which a (starter) burner can be combined with a porous element to make the surface combustion of liquid fuels possible. The burner is started up by means of an electric heater or by activating the starter burner. The transition to surface combustion is carried out by means of a short interruption in the fuel flow. With the same flow-rate settings, a fuel-vapour/air mixture which will burn on the porous element is then produced in the vicinity of the starter burner. The energy required to vaporize the fuel in steady-state operation can be provided in various ways. The surface can have a layered structure, thus enabling the energy flow to be regulated to the required rate. The surface can also have a special shape, enabling secondary air to be supplied, under certain conditions, without any danger of ignition before combustion at the surface.

Description

The basis of the patent is a burner for liquid fuels (in the following example and called oil for short), with which the combustion with air stabilized on a porous surface the required concept for mixture preparation and the mode of operation of the Burner.

The technique of surface combustion is known from the combustion of gases and is already there for radiant burners, for example for heating industrial halls and used for heating buildings (gas wall heaters).

The advantages of surface burning are based on the stabilization of the flame in close proximity to the surface or in the material used Surface structure. The solid heats up strongly due to the proximity of the flame and can emit a large part of the heat of reaction to the environment through heat radiation. This will be an effective way of flame cooling and thereby lowering the thermal nitrogen oxide formation reached. Because the heat transfer through to a large extent Heat radiation occurs, the heat exchanger can be made very compact. by virtue of the low velocity of the gases on the surface and the associated low turbulence, flame noises are largely avoided.

In addition, the structure of the porous surface offers the possibility of being catalytic bring effective substances into contact with combustion gases and thus direct influence to take on the reduction of pollutants in the reaction zone.

In order to burn the surface, the mixture must form before the surface largely completed. While the mixture formation in gases is relatively easy is to be implemented, the premixing of liquid fuels requires a higher technical Expenditure. Aspects of segregation through condensation and agglomeration must be taken into account. The risk of self-ignition of the mixture below the Surface must be excluded.

The problem of homogeneous mixture formation in liquid fuels is that Heating phase, which is necessary for the evaporation or evaporation of the fuel. at Heating oil carry concepts for the evaporation or evaporation of liquid films, dic either run down hot surfaces or be sucked in by capillary forces, mostly on the problem of the formation of deposits on the surfaces of the evaporator. The deposits are reaction products from cracking reactions in the fuel oil occur at temperatures above 400 ° C. Such high temperatures up to above the Boiling end of the heating oil (380 to, 400 ° C) are for a sufficient heat transfer required from the evaporator wall to the liquid medium. The deposits lead to a deterioration in the heat transfer and thus the susceptibility to failure of the system.

From JP 57041508 a burner for liquid fuels is known, in which in the starting phase the fuel supplied is evaporated in an evaporation tube. This is done by an electric heater. The vaporized fuel is injected through injected into a mixing chamber. Pre-heated air is also introduced into this. The mixture from preheated fuel and preheated air burns during the start phase at the Surface of the porous body. After the start phase has ended, the power supply to the electric heater ended. The fuel is now evaporated by the heat of the combustion taking place on the porous body.

A water heater is known from JP 56012908. The combustion takes place with this burner of liquid fuel by injecting the fuel directly into the air. The mixture is heated in an evaporation zone and then on the surface of a porous body burned. In a starting phase, the application of the Evaporation energy in the evaporation zone through a spark plug a starting flame generated. The starting flame is extinguished after a minimum operating temperature has been reached. This is followed by the evaporation of the fuel on the surface of the porous body.

The object of the invention is to improve such burners.

According to the invention, the object is achieved by a method for combustion liquid fuels according to claim 1 or by a burner for liquid fuels according to claim 6.

The mixture is prepared using the concept of Atomization and evaporation of the fuel in an air stream. The required system start-up capability (when the burner is cold started, it is available Mixture formation (evaporation) necessary thermal energy not yet available) is solved by the concept of combining a start burner (swirl-assisted flame tube stabilization) with a surface burner, and the Possibility of using the start burner units to process a fuel-air mixture to use for the surface burner.

The required heat input takes place via various mechanisms, which in their Total ensure complete evaporation and overheating of the Avoid fuel mixture. This eliminates the risk of self-ignition of the mixture in the mixture formation zone avoided. In contrast to the previous state of knowledge the ignition temperature of the fuel, for example Heating oil EL (220 ° C) occurs a spontaneous combustion of the fuel-air mixture under the Conditions in this burner concept only at much higher temperatures (> 500 ° C) on. Only then can the homogeneous mixture formation (complete evaporation and over-stoichiometric mixture with atmospheric oxygen) can be achieved.

To avoid the formation of deposits, the contact of the liquid fuel be avoided with hot walls. This applies both to the mixture formation process as well as for the transport of the mixture to the surface of the burner. The glowing Surface for flame stabilization is not a critical zone, as this is due to its high temperature is not wetted. Deposits that may have formed at the start can be are even burned off here without residue.

The invention further relates to a burner for liquid fuels, with a central one Fuel lance, an air supply, which can be provided with a swirl device with an ignition device, with an optional flame tube, with an optional Heat exchanger for air preheating and with a porous body to stabilize one large or large-volume flame.

The burner works in two operating states. State I, the start mode, serves that Preheat the burner to a minimum operating temperature. With the aim of minimizing the electrical Energy consumption - for example in the case of clocking small combustion systems - allows Construction and mode of operation according to the invention, however, also the heating by a Start burner operation. The burner works as a flame tube-stabilized burner inside the surrounding surface. This operating state only serves to heat the System to the minimum operating temperature of state II and is designed to be short in time. The construction according to the invention allows this burner to be used with little effort (Minimization of the necessary burner units and the electrical energy for Supply to the units).

The smoke from the start burner in operating state I can pass directly through the porous one Body. This improves the heat transfer of the gases to the body to be heated, so that in comparison to a gas flow past Shortening the start phase can be achieved. A mechanical blockage of a the second routing of the flue gases from the start mode is also eliminated.

To abort operating state I (start operation), it is sufficient to have the combustion of the Interrupt fuel-air mixture for a short time. This is conveniently done through a very short interruption in the oil supply. After the flame has extinguished, the Do not create a new flame zone if the igniter is inside the burner is not activated. An oil vapor-air mixture is then in the area of the flame tube generated that can be supplied to the area of the porous body.

The surface of the mixture can then be ignited conventionally by ignition electrodes become. However, the developed concept also allows for the outside Ignore ignition device and self-ignition of the mixture on the surface to effect. This works if the temperature of the surface is at least one point is high enough. The start burner allows the area around the end of the flame tube heated up sufficiently and made to glow easily. Through the relative large ignition area on the (partially or completely glowing) porous body the starting emissions of operating mode II of the combustion on the porous body in comparison for selective ignition sources (e.g. ignition electrodes) are kept low.

Through a suitable flow control (aim of a quick homogenization of the Mixture) and limitation of the temperature (in particular the heat radiation by the glowing surface) can self-ignite the ignitable oil vapor-air mixture be avoided within the burner. When using the burner in processes that have a include high air preheating due to necessary exhaust gas cooling (chemical High temperature processes, Stirling engine, u. a.), or other required Boundary conditions can also be achieved by coupling flue gases into the mixture formation zone and / or by dividing the air flow into a primary and a secondary air flow can be avoided. Both methods cause the Mixture (reduction of the oxygen partial pressure) and thus an increased Ignition delay.

The flue gases can be fed through openings in the area of the mixture formation. The dosage of the flue gases can be changed by changing the area of the opening Example by a slide or a plate also during operation. A Temperature control of the change in area can be done efficiently by temperature sensors or Bimetallic or fluidic.

Flue gases can be drawn in through the impulse of the air in the start burner (short: primary air) become. The primary air can also be passed through heat exchangers (8) before entering the combustion chamber be preheated. By introducing the oil into the hot flue gas / primary air mixture the oil evaporates quickly and mixes homogeneously.

A partial chemical conversion of the oil can be done through the choice of process parameters (Temperature, air volume, flue gas volume) can be reached. The space in the flame tube of the Startbrennner then acts as a pre-mixing and pre-reaction room.

If the air flow is divided (for the purpose of inerting the mixture formation zone), this takes place Addition of additional air (short: secondary air) downstream of the flame tube. The Secondary air can also be preheated by heat exchangers (8) before mixing become. The impulse of the secondary air can overcome the pressure loss of the matrix of the surface burner can be used.

The surface burner consists of a porous body (e.g. stainless steel, ceramic). This can be at the flue gas outlet behind the start burner or around it be attached around.

To re-ignite the fuel mixture in the area of the secondary air admixture (in front of the surface burner) through the hot surface, the Surface burners can be cooled by the secondary air itself. Preheating the Secondary air then takes place partially or entirely in the surface burner.

A particularly even addition of secondary air without the risk of premature Ignition of the mixture is possible through the mixture in the surface matrix itself.

The surface burner can be used to influence the chemical conversion of the oil surface-enlarging substances and / or coated with catalytically active substances.

The supply of the fuel to evaporate and heat the combustion air necessary energy can be done in different ways. One can be efficient Combination of the ways described.

By heating the air to a temperature corresponding to the boiling point of the used fuel (with heating oil approx. 400 ° C) the back condensation of evaporated fuel avoided. If the air is heated further, the Energy requirements for the evaporation of the fuel from the air flow are met. The The fuel vapor is then expediently generated by direct injection of fuel in the hot air.

By means of a heat exchanger, the combustion air can be mixed with the Fuel mist can be heated. The heat exchanger can be in the form of tubes made by the air flows through, be arranged outside or inside the burner. By the arrangement within the burner can overheat the area of the fuel-air mixture can be avoided by heat dissipation.

If the heat exchanger is arranged outside the burner, it can also be used Low temperature heat (through flue gas cooling) in high temperature processes (chemistry, Stirling, u. a.) are coupled.

If flue gases are removed from the combustion at a high temperature level, it works above it the supply of part or all of the heat, which evaporates the fuel is required. The flue gases also cause an inertization of the Mixture formation zone and additional cooling of the flame zone.

The heat is emitted by radiation through the very hot porous one in operating state II Body takes place both outside (to the medium to be heated, e.g. boiler water) as well also inside. This effect can be used to heat the premix zone if this lies within an area enclosed by the porous body. The Evaporation of the fuel then takes place not only through convective heat transfer hot air to the oil droplets, but also by the direct radiation on the individual Drops. Limiting the temperature in the premix zone can protect against Auto-ignition of the mixture through a layered construction of the porous body respectively. The outer layer of the surface is in support of the Combustion optimized (material, materials, structure). The inner layer is related to the exact coupling of the necessary heat optimized (heat conduction and Radiative properties).

By sucking in superheated oil vapor-air mixture in the area of Fuel and air supply can enter heat for evaporation in the premix zone become. When the oil vapor is applied to the porous body, it comes through Radiation of the body inwards to further warm the mixture. Part of the Mixture flow can be caused by the negative pressure of the air supply in the area Premixing chamber are sucked back. This is done on the same principle as that Intake of smoke gases for flame cooling in conventional burners. Will that Flame tube for the burner in operating state I operated with recirculation openings, see above Superheated oil vapor is automatically sucked back through these openings.

Claims (10)

1. Method for combusting liquid fuels, wherein fuel and air are mixed by direct injection of the fuel into the air, heated up in an evaporation zone and subsequently combusted, with a stable flame forming at the surface of a porous body (7), and
   wherein in a start-up phase a start-up flame is produced for applying evaporation energy in the evaporation zone, the start-up flame being extinguished when the minimum operating temperature has been reached,
   characterized in that
   at least a portion of the heat required for evaporating the fuel is supplied by returning the mixture of hot air and fuel vapour into the evaporation zone.
2. Method according to claim 1, characterized in that the heat required for evaporating the fuel is additionally supplied by taking one or a plurality of the following measures:

   a) coupling-in of the combustion heat via heat exchangers (8),

   b) coupling-in of hot flue gases, and/or

   c) heat radiation from hot surfaces.
3. Method according to claim 1 or 2, characterized in that the transition from the start-up phase to operation with superficial combustion is effected by temporarily stopping the fuel flow when a minimum operation temperature has been reached.
4. Method according to one of claims 1-3, characterized by the supply of flue gases into the evaporation zone.
5. Method according to one of claims 1-4, characterized by splitting an air flow, by means of which the air to be mixed with the fuel in supplied, in

   a primary air flow supplied via an air nozzle of the start-up burner for producing the start-up flame, and

   a secondary air flow supplied near the porous body (7) or in this porous body (7).
6. Burner for liquid fuels, comprising
   a central fuel gun (1),
   an air supply device (2),
   a mixing chamber for mixing the supplied fuel with the supplied air and for evaporating the fuel, and
   a porous body (7) surrounding an evaporation zone, the porous body (7) defining the mixing chamber and forming a surface burner, wherein in the mixing chamber a start-up burner is arranged which produces a start-up flame in a start-up phase and is subsequently used for preparation of the fuel-air mixture,
   characterized in that
   around the evaporation zone a flame tube (6) is arranged above which arises the porous body (7), and
   a recirculation distance for the mixture of hot air and fuel vapour is provided from the porous body (7) and the flame tube (6) to the evaporation zone.
7. Burner according to claim 6, characterized in that a radiation transmission distance from the porous body (7) to the evaporation zone is provided.
8. Burner according to claim 6 or 7, characterized in that the flame tube (6) comprises recirculation openings.
9. Burner according to one of claims 6-8, characterized in that for supply of the energy required for evaporating the fuel a heat exchanger (8) is additionally provided which supplies a portion of the het released during combustion via heated-up air into the evaporation zone.
10. Burner according to one of claims 6-9, characterized in that the porous body (7) is configured as porous honeycomb body through whose honeycombs air and fuel mixture alternately flow.

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