EP0699289B1 - Water heater - Google Patents

Abstract

The invention relates to a water heater with an inlet (8) for liquid fuels, several outlets (41, 46) for fresh air, an inlet for a fluid (2) to be heated, at least two combustion stages (16, 20) through which the fuel-air mixture flows, with catalytic combustion chambers which are at least partly surrounded by at least one fluid chamber (4) filled with the fluid (2) and with an exhaust gas heat exchanger for the fluid (2) to be heated, through which the exhaust gas from the combustion chambers flows. The first combustion stage (16) has an evaporation chamber (40) having at least partially the catalyst coating (13) on the outside of its edge (31).

Description

The invention relates to a water heater according to the preamble of claim 1, as e.g. is known from DE-A-33 32 572. Furthermore, the applicant's patent 42 04 320.4 also describes a water heater which has, in particular, a first advantageous combustion stage. For the rest, reference is made to this application for further understanding, in particular of the first combustion stage, and to the detailed explanations of the second combustion stage. For better clarity, the reference numbers in this application correspond in part. those of the patent 42 04 320.

1 Introduction

When fossil fuels are burned, other pollutants such as sulfur dioxide and nitrogen oxides are created in addition to the greenhouse gas carbon dioxide. In conventional flame burners, the reduction options, mainly nitrogen oxides, are limited by the flame stability and the formation of carbon monoxide. A significant reduction in nitrogen oxide emissions can be achieved with flameless combustion on oxidation catalysts (eg Pt) due to the low reaction temperature. Catalytic burners also offer the advantage that mixtures of fuels with different energy densities can be stably implemented in a wide range of the mixing ratio.

2. State of the art

Burners for gasoline, diesel or e.g. Today, methanol is only available as a conventional flame burner. Because of the high reaction temperature (flame temperature), such burners have high nitrogen oxide emissions. There are ways to reduce emissions even with such burners, e.g. Flame cooling or changing the air ratio, however, this leads to a decrease in flame stability and an increase in carbon monoxide emissions.

This prior art has the disadvantage that it is not particularly suitable for liquid fuels. The object of the invention is therefore according to the water heater. develop the preamble of claim 1 so that liquid fuels can be used without substantial cracking. This object is achieved by the water heater according to claim 1. The invention makes it possible to thermally couple the first stage of the two-stage catalytic burner to the evaporation chamber.

An advantageous further development for the catalytic cracking burner is described in claim 2.

According to claim 3, the evaporation chamber is designed as a combustion chamber, for which purpose it has an ignition device. If necessary, e.g. a bypass of the feed for the liquid fuel. In addition, the water heater has a supply of primary air to the combustion chamber for this purpose.

According to claim 4, the fuel is supplied in isolation so that the fuel reaches the evaporation space without cracking.

Furthermore, it is advantageous to provide a nozzle or other devices for atomizing the fuel (claim 5).

It can be advantageous to return part of the exhaust gas from the first stage to the evaporation space, since the liquid fuel is then more easily evaporated and the water or water vapor produced during the combustion process also minimizes possible cracking reactions.

According to claim 8, it can be advantageous to provide devices for directing the gas flow in the evaporation chamber, in particular these devices can be thermally insulated if they are not heated by the first combustion stage.

Furthermore, it can be advantageous to design the evaporation chamber to be rotationally symmetrical and to allow it to rotate, since the fuel is then pressed against the wall and comes into better contact with the wall there, which is on the rear side of the first combustion stage as a result of the reaction of the combustion gas. Air mixture is heated on the catalyst layer.

3. Description of exemplary embodiments

The subject of the application is a two-stage catalytic burner for liquid fuels and their mixtures with internal evaporation or gasification. In the interior of the burner, the fuel is vaporized or gasified, possibly with an air supply (primary air). The energy required for this is provided by the heat of combustion. The fuel gas / air mixture (with added secondary air, which can be the sole air supply after the start phase) flows over a catalytic surface and reacts there to approx. 80 - 85%. The reaction temperatures are around 800 - 900 ° C. Radiation, heat conduction and convection emit heat to the cooling medium and the evaporation zone. In the second stage, the remaining fuel is converted in a monolith catalyst. The narrow channels ensure good mass transport and thus a high power density. This means that temperatures of approx. 1000 ° C are reached, which enable complete conversion. Part of the heat for preheating the primary air can be extracted from the monolith, which e.g. is advantageous for intermittent operation.

Description:

The catalytic burner (Fig. 1) consists of two stages 16, 20. The first stage consists of a metal tube 31 or ceramic tube coated on the outside with catalyst 13. This catalyst tube is surrounded by a ceramic or metal tube and a cooling jacket, so that a gas gap is formed between the catalyst tube and the ceramic tube 11. The mixture of vaporized, gaseous fuel and air flows in this gas gap and reacts on the catalyzed surface of the tube 31. The second stage, which is arranged above the first, consists of a ceramic honeycomb structure (monolith), which is coated with catalyst. The exhaust gas from the first stage with the remaining fuel flows through this monolith and reacts completely. The supply line for the primary air and the liquid fuel mixture is arranged centrally in the monolith.

Two concentrically arranged pipes 8 and 41, which are passed through the monolith from above, form the feed line for the liquid fuel and the primary air. The primary air, which is preheated by the adjacent monolith, flows in the outer tube 41. The liquid fuel flows in the inner tube. This is only slightly preheated, since the gas gap between this pipe and the monolith has an insulating effect, so that no evaporation or cracking can occur in the feed pipe. These concentric tubes end at the top of the first burner stage. The liquid fuel is finely atomized by means of a nozzle 42 and introduced into the interior of the catalyst tube of the first burner stage, which thus forms the evaporation chamber 40 or combustion chamber. The concentrically supplied primary air, which has been preheated by the monolith, also flows into the interior of the catalytic converter tube through holes drilled in a ring around the fuel feed line. A high proportion of primary air ensures that the liquid fuel can be evaporated far below its boiling temperature. In the favorable case, the addition of primary air can be switched off after the start phase. Furthermore, the fine atomization of the fuel achieves a large evaporation surface and good mass transfer numbers through the flow of the primary air. The evaporation energy is provided by the heat of the preheated air and by the supply of heat (heat conduction, convection and radiation) from the catalytic tube.

The fuel gas / air mixture flows downwards inside the evaporation chamber or burner chamber. A cone 45 built on the bottom of the burner guides the gas at the lower end of the catalyst tube into the annular gap between the ceramic and catalyst tube. At this point, the secondary air is added, which flows directly into the ring gas gap from below. The cone has two main functions. Its task is to allow the fuel gas / air mixture to flow evenly into the annular gap. Without this cone, dead space areas could easily form on the bottom of the burner, from which crack products, for example, could collect. Another important point is that the cone is emitted by the radiation from the evaporation space (rear of the catalyst room) is heated. This can prevent parts of the fuel from condensing out again at the bottom of the burner or when deflected into the gas gap.

• 1. das Katalysatorrohr wird aufgeheizt bzw. auf der Reaktionstemperatur gehalten
• 2. das Reaktionsgas wird aufgeheizt
• 3. es wird Wärme an das Innere des Katalysatorrohrs abgegeben; diese wird zur Verdampfung des flüssigen Brennstoffgemisches benötigt; die Wärme wird durch Konvektion, Wärmeleitung und Strahlung übertragen
• 4. vom Katalysatorrohr wird ebenfalls durch Konvektion, Wärmeleitung und Strahlung Wärme an das Keramikrohr abgegeben; von dort wird die Wärme durch Wärmeleitung weiter nach außen an den mit Kühlmedium (Wasser, Luft) durchflossenen Doppelmantel abgegeben.
  Das Katalysatorrohr hat eine Temperatur von ca. 700-900°C. In dieser ersten Stufe werden ca. 80% des Brennstoffs umgesetzt.

The fuel gas / air mixture with the added secondary air flows upwards in the annular gap between the ceramic and catalyst tubes. Some of the fuel reacts on the catalytic surface. The energy released is distributed as follows:

• 1. the catalyst tube is heated or kept at the reaction temperature
• 2. the reaction gas is heated
• 3. heat is given off to the inside of the catalyst tube; this is required for the vaporization of the liquid fuel mixture; the heat is transferred by convection, heat conduction and radiation
• 4. Heat from the catalyst tube is also given to the ceramic tube by convection, heat conduction and radiation; From there, the heat is dissipated further to the double jacket through which cooling medium (water, air) flows.
  The catalyst tube has a temperature of approx. 700-900 ° C. In this first stage, approximately 80% of the fuel is converted.

• 1. der Stofftransport zum Katalysator ist aufgrund der engen Kanäle sehr gut
• 2. durch die geringen Wärmeverluste aus der Wabe und die Wärmeproduktion aufgrund der Reaktion erreicht die Wabe Temperaturen von ca. 900-1000°C; die Reaktionsgeschwindigkeit ist bei dieser Temperatur so hoch, daß bei der relativ großen Verweilzeit (niedrige Strömungsgeschwindigkeit) der Brennstoff vollständig umgesetzt werden kann. Obwohl durch die schlechte Wärmeleitfähigkeit des keramischen Monolithen nur sehr wenig Wärme abgeführt wird, kann damit doch die Primärluft, deren Zufuhr im Zentrum der Wabe angebracht ist, ein we nig vorgeheizt werden. All zu hoch darf die Vorheiztemperatur der Primärluft ohnedies nicht sein, da sonst beim Zusammentreffen mit dem verdüsten Brennstoff Crackreaktionen auftreten könnten.
  Die Abluft aus der zweiten Verbrennungsstufe wird dann in einem (nicht dargestellten) Wärmetauscher benutzt, das in der ersten Verbrennungsstufe erwärmte Wasser bzw. Fluid 2 weiter zu erwärmen.

From the annular gap of the first stage, the gas mixture flows upwards into the expanded space below the honeycomb coated with catalyst (eg Pt). The widening of the cross-section leads to a slowdown of the flow speed and to thorough thorough mixing again before the second burner stage. The gas now flows through the narrow channels of the catalyst honeycomb, the remaining fuel being fully converted. The good sales in this second stage are due to the following conditions:

• 1. The mass transfer to the catalyst is very good due to the narrow channels
• 2. due to the low heat losses from the honeycomb and the heat production due to the reaction, the honeycomb reaches temperatures of approx. 900-1000 ° C; the reaction rate is so high at this temperature that the fuel can be completely converted with the relatively long residence time (low flow rate). Although very little heat is dissipated due to the poor thermal conductivity of the ceramic monolith, the primary air, the supply of which is located in the center of the honeycomb, can be a little be preheated. The preheating temperature of the primary air must not be too high anyway, otherwise cracking reactions could occur when it comes into contact with the atomized fuel.
  The exhaust air from the second combustion stage is then used in a (not shown) heat exchanger to further heat the water or fluid 2 heated in the first combustion stage.

The entire burner is started by igniting a flame in the evaporation chamber. The primary air flow is so large that complete combustion is guaranteed. The flame heats the inside of the catalytic converter tube through radiation, heat conduction and convection. The hot exhaust gases flow downwards, are led through the cone at the bottom into the gas gap and flow through this upwards and through the honeycomb. The hot exhaust gas gives off the heat and thus heats the burner with the honeycomb. If the burner has reached a temperature level at which the catalytic reaction can proceed at a correspondingly high reaction rate (approx. 600 ° C), the flame is switched off. This can be done by briefly switching off the primary air and / or the fuel supply.

Possibly. Crack products formed, which are deposited on the hot inside of the catalyst tube, can be eliminated by igniting a flame in the interior of the catalyst tube at certain time intervals. This flame is operated with excess air so that the cracked products can be burned out on the surfaces.

• 1. durch seitliche Zuströmung bzw. Ansaugung des Abgases aus der ersten Stufe erfolgt eine gute Durchmischung und weitere Verdünnung des Brennstoff/Luft-Gemisches im Inneren des Katalysatorrohrs; dies führt zu einer schnelleren Verdampfung
• 2. das heiße Abgas bringt zusätzliche, für die Verdampfung benötigte Wärme in den Verdampferraum
• 3. der im rezirkulierten Abgas vorhandene Wasserdampf aus der Verbrennung bewirkt, daß Teile des Brennstoffes zu Kohlenmonoxid bzw. Kohlendioxid und Wasserstoff reformiert werden, und somit evtl. auftretende Crackreaktionen minimiert werden können
• 4. bei ausreichender Rezirkulation des Abgases kann auf die Primärluft verzichtet werden.

The same burner is shown again in FIG. 2, but here openings 44 are made from the gas space between the first and second stage to the interior of the catalyst tube (evaporation space). These openings, which can be designed as nozzles, cause part of the exhaust gas from the first burner stage to be able to recirculate through the evaporation chamber. This has the following advantages:

• 1. by lateral inflow or suction of the exhaust gas from the first stage, thorough mixing and further dilution of the fuel / air mixture takes place inside the catalyst tube; this leads to faster evaporation
• 2. The hot exhaust gas brings additional heat required for the evaporation into the evaporator space
• 3. The water vapor from the combustion present in the recirculated exhaust gas causes parts of the fuel to be reformed to carbon monoxide or carbon dioxide and hydrogen, and thus any cracking reactions that may occur can be minimized
• 4. With sufficient recirculation of the exhaust gas, the primary air can be dispensed with.

Figure 3 shows a burner in which a highly porous structure 43 is attached to the inside of the catalyst tube. This structure means that some of the injected fuel drops, especially the larger ones, are deposited on the porous body and thus do not come into contact with the hot wall of the catalyst tube. Because the temperatures are kept low there can be no cracking. The porous structure can consist of ceramic or metal and can be configured as a cuboid, cylinder or also as a tube. The structure can also be coated with catalyst material to accelerate the evaporation reaction.

According to the invention, the first stage of the two-stage catalytic burner is thermally coupled to the evaporation chamber. The evaporation chamber also serves as a combustion chamber for preheating. The thermal coupling between the first catalyst stage and the evaporation chamber enables a heat flow from this space, which is then the combustion chamber, to the first catalyst stage and, in the case of catalytic burner operation, conversely a heat flow from the first catalyst stage to the evaporation chamber in order to provide the required enthalpy of vaporization there.

The basic structure of the burner according to the invention is not limited to the tube geometry outlined, but can also be transferred to rectangular channels or plate-shaped arrangements.

The water heater can advantageously also be used to heat warm air or another fluid to be heated.

Claims (9)

1. Hot water heater with an inlet (8) for liquid fuels, a plurality of inlets (41, 46) for fresh air, an inlet for a fluid (2) to be heated, at least two combustion stages (16, 20) traversed by the fuel-air mixture with catalytic combustion chambers surrounded at least partially by at least one fluid chamber (4) filled with fluid (2) and with an offgas heat exchanger for the fluid (2) to be heated,said heat exchanger being traversed by the offgas escaping from the combustion chambers,
   characterised in that
   the first combustion stage (16) has an evaporation chamber (40) which has on the outside of its margin (31), at least partially, the catalyst layer (13) of the catalytic combustion chamber(11) of the first combustion stage (16).
2. Hot water heater according to Claim 1, characterised in that the first catalytic combustion chamber (11) of the first combustion stage (16) is designed as a catalytic cracking burner (11).
3. Hot water heater according,to Claims 1-2, characterised in that the evaporation chamber (40) is designed as a combustion chamber for starting the hot water heater and a source (41) of primary air and an ignition device, possibly with its own source of combustion gas, or a heated inlet for liquid fuel.
4. Hot water heater according to Claims 1-3, characterised in that the supply of fuel to the evaporation chamber is thermally insulated.
5. Hot water heater according to Claims 1-4, characterised in that a nozzle (42), for example a piezocrystal, porous ceramic, or vorticisation nozzle, and/or a porous structure (43) for atomistion and/or evaporation of the fuel are provided.
6. Hot water heater according to Claims 1-5, characterised in that an opening (44) is provided for offgas recirculation from the outlet of the first combustion stage to the evaporation chamber.
7. Hot water heater according to Claims 1-6, characterised in that a device (45) is provided in the combustion chamber for guiding the gas stream.
8. Hot water heater according to Claims 1-7, characterised in that the combustion chamber is made movable, e.g. rotatable.
9. Hot water heater according to Claims 1-8, characterised in that the margin (31) of the evaporation chamber (40) is a cylinder, which has the catalyst layer (13) at least partly on the outer jacket surface.

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