Method and apparatus for generating heat

Abstract

The present invention relates to a method and device for generating heat. In the method for generating heat, a mixture (3) is prepared containing a gaseous fuel (1) and a gaseous oxidizer (2). The fuel (1) is oxidized. The mixture (3) is outside the explosion range, more precisely, the oxygen content of the mixture is below the critical oxygen concentration. In this way, a particularly safe and technically uncomplicated method is provided.

Description

[Technical Field]

[0001] The present invention relates to a method and apparatus for generating heat.

[0002] Exothermic reactions, such as combustion, can be used to generate process heat. x H y , N x H y Hydrogen-containing compounds, such as hydrogen or hydrogen itself, H2, are usually burned with an oxidizer, such as atmospheric oxygen. The combustion gases reach high temperatures, sometimes exceeding 1000°C. This temperature is determined by the air-fuel ratio (or air-fuel equivalence ratio) λ. The air-fuel ratio λ, also known as the combustion air ratio, indicates the mass ratio of air to fuel compared to the stoichiometrically ideal ratio for a complete combustion process. Combustion can take place in a combustion chamber. The hot gases produced can distribute heat to a secondary circuit (e.g., water, thermal oil, process steam) via a heat exchanger. The hot medium transfers the heat required for each process, is cooled, and then reheated during the process. Due to the high temperatures involved, materials must be correspondingly heat-resistant. Heat transfer oils, for example, are subjected to high stresses due to high film temperatures.

[0003] Fuels form explosive mixtures in an oxidizing atmosphere depending on the fuel's specific concentration range. Processes using explosive mixtures require special safety equipment and monitoring, which entails increased technical effort. Explosion protection measures are outlined, for example, in the Technical Regulations on Hazardous Substances.

[0004] During exothermic processes, the mixture may reach a temperature at which it will ignite by itself (autoignition temperature). For this reason, it is particularly important that explosive mixtures are not created.

[0005] In mixtures below the lower explosive limit, the fuel is present in low concentrations. There is correspondingly a large amount of excess air. This results in low efficiency. If the temperature is increased, for example by preheating, the lower explosive limit is further reduced, resulting in even lower efficiency. In addition, a large amount of air flow is required, so that the blower and heat exchanger must be very large. The object of the present invention is to provide a method and an apparatus with which heat can be generated in a particularly efficient and safe manner. This problem is solved by a method according to claim 1 and by a device according to the additional claims. Advantageous embodiments are set out in the subclaims.

[0006] The problem is solved by a heat-producing method in which a mixture is prepared. The mixture includes a gaseous fuel and a gaseous oxidizer. The mixture may further include one or more inert gases. The fuel is oxidized, particularly by the oxidizer. The oxygen concentration of the mixture is below a critical oxygen concentration.

[0007] The explosive range or explosion range (flammability limit or explosion limit) is the range of mixture ratios of fuel, oxidizer, and optionally inert gas, in which the mixture becomes explosive. The mixture is outside the explosive range (above the upper flammability limit), i.e., the oxygen concentration of the mixture is below the critical oxygen concentration. The critical oxygen concentration indicates the oxygen content below which the entire mixture is no longer explosive. Insufficient oxygen is present to allow an explosion. This means that independent flame spread, unrelated to the ignition source, is no longer possible. In particular, explosive mixtures are not present at any stage of the method. In this way, the technical effort can be significantly reduced. At the same time, safety can be guaranteed in a simple and effective manner.

[0008] The fuel is a gaseous fuel that can be oxidized with oxygen or air. For example, the fuel can be hydrogen, a compound of the general formula C x H y compounds of the general formula N x H yThe oxidizer may be, for example, oxygen or may contain oxygen. The oxidizer may also be present in any ratio with one or more other gases, which may be inert. Mixtures are also possible. The fuel may be present in any ratio with one or more other gases, which may be inert.

[0009] One possibility consists of introducing the fuel or oxidizer in stages, which prevents the formation of explosive mixtures. For hydrogen, complete conversion with atmospheric oxygen at an overall air-fuel ratio λ=1 requires about 10 stages, with each stage consuming roughly 2% of the oxygen. However, the capital costs are very high.

[0010] By using a mixture with an oxygen content below the critical oxygen concentration, the present invention manages to achieve an overall air-fuel ratio λ of 1. This means that only the amount of air specified by the stoichiometry of the oxidation reaction is required in the process. In one embodiment, the oxidation is carried out as a heterogeneous catalysis using a catalyst.

[0011] In heterogeneous catalysis, the catalyst and reactants of a chemical reaction exist in different phases. Specifically, the catalyst is a solid. Specifically, the catalysis is a gas-phase catalytic reaction. The fuel and oxidant exist as gases. Catalytic oxidation is also called catalytic combustion. Catalytic oxidation is an exothermic reaction that produces usable heat.

[0012] In catalytic heat transfer units, the nature of the equipment means that the fuel and oxidant mixture occupies a relatively large volume, and if an explosive mixture is created, ignition and explosion can have devastating effects. Therefore, mixtures with an oxygen content below the critical oxygen concentration are particularly advantageous in conjunction with catalysis.

[0013] In particular, three-dimensional structures are used that are typically coated with a catalyst, preferably optimized to minimize pressure loss, and preferably optimized to allow for easy dissipation of the generated heat. In one embodiment, the catalyst is in thermal contact with the radiator, thus allowing all generated heat to be transferred directly to the radiator.

[0014] There is no need to use or dissipate heat from the hot gas or combustion chamber. No heat transfer medium, such as water, thermal oil, or process steam, is required. Therefore, a secondary circuit is not required to dissipate and transfer heat. Heat can be transferred directly from the catalytic converter to a radiator. The radiator can be a catalyst-coated structure or can be thermally connected to such a structure. In particular, the catalytic action occurs directly on the surface of the radiator. Therefore, heat can be dissipated and transferred directly from the catalyst surface. In addition, significantly lower temperatures are achieved during the catalytic reaction compared to conventional combustion processes. This significantly reduces the technical complexity of the device. The system does not need to be designed to the temperatures that occur during conventional combustion.

[0015] A radiator is an object that dissipates heat. For example, the method can be used to heat a room. In particular, the radiator transfers heat directly to the area and / or device to be heated.

[0016] In particular, the radiator is a heat exchanger. The heat exchanger is configured to transfer heat from the catalyst to another location. The heat is thus dissipated directly from the catalyst, which allows direct control and / or regulation of the process temperature. In addition, the heat can be transported to the location where it is needed without the need to change the heat transfer medium. This ensures, on the one hand, a very simple heat transfer in terms of the equipment technology, and, on the other hand, a heat transfer with very low losses. The method is very efficient due to the low heat transfer.

[0017] In particular, the catalyst is solid. In particular, the catalyst is in mechanical contact with the heat sink, which may be direct or indirect mechanical contact, so that heat can be transferred from the catalyst to the heat sink by thermal conduction. In one embodiment, recycled product gas is also used to make the mixture.

[0018] The product gas is a gas produced during the oxidation process. In particular, the product gas is a combustion product. Typically, the product gas is inert. This is possible especially when complete stoichiometric conversion occurs during the oxidation. In particular, the concentration of the oxidant and / or fuel is reduced by recycling. The mixture therefore comprises fuel, oxidant, and product gas. Preferably, the oxygen concentration is reduced by recycling to below the limiting oxygen concentration. The recycled product gas is a product gas resulting from the oxidation process, in particular produced as a product of the oxidation process, and recycled to be fed back into the oxidation process. In one configuration, the preparation of the mixture and / or the oxidation process is continuous. The process is typically a continuous process. In one embodiment, the product gas stream is split: a first partial stream is recycled and a second partial stream delivers heat to the fuel and / or oxidant.

[0019] Thus, the first partial stream is added to the feed stream for the oxidation process, and the second partial stream is used to preheat the fuel and / or oxidant. In particular, at least one heat exchanger is provided for this purpose. In particular, the preheating occurs before the mixture is formed. After preheating, the second partial stream is discharged as waste gas. In one embodiment, the first partial flow is conveyed by a blower. In this way, the recirculation flow can be directed and / or controlled in a targeted manner.

[0020] In one embodiment, the first partial flow is fed into the oxidizer supply line. In this way, the oxidizer can be initially diluted so that an explosive mixture is not formed. This allows, for example, a safe start-up of the reactor in which the method is carried out, in which fuel is gradually added with sufficient supply of oxidizer and sufficient recirculation to completely consume the oxygen.

[0021] In one embodiment, both the fuel and the oxidant are heated by heat from the product gas. In particular, the heat treatment of the fuel is spatially separated from the heat treatment of the oxidant. In other words, two separate heat treatment steps are performed. In one embodiment, a second partial stream of the product gas stream is diverted to heat the fuel and the oxidant with the separate product gas streams.

[0022] In one embodiment, the mixture is prepared after heat treatment of the fuel and oxidizer. In one embodiment, air is used as the oxidizer. This minimizes the technical effort.

[0023] In one embodiment, the oxidation treatment is carried out at temperatures below 500° C., in particular below 400° C. and / or above 100° C., in particular above 200° C. Particularly efficient catalytic combustion can be carried out at these temperatures. In one embodiment, the oxidation process is at least approximately stoichiometric, resulting in the production of a substantially inert product gas.

[0024] This provides the additional advantage that the heat of condensation of water can be used due to the high water vapor content in the exhaust gas, thus further increasing efficiency. In particular, the apparatus includes a device for collecting and / or discharging condensed water.

[0025] A further aspect of the present invention is an apparatus for producing heat, comprising a gaseous fuel supply, a gaseous oxidant supply, an oxidation reactor for oxidizing the fuel with the oxidant, and / or a product gas outlet, the apparatus further comprising a mixing device configured to produce a mixture of fuel and oxidant, the mixture having an oxygen concentration outside the explosive range, in particular below the critical oxygen concentration. The device is particularly suitable for carrying out the method according to the invention. All features, properties and advantages of the method described above also apply to the device and vice versa. In one embodiment, the oxidation reactor comprises a catalyst, in particular a catalyst for heterogeneous catalysis.

[0026] In one configuration, the method and / or apparatus is used to provide heat for an endothermic reaction separate from heat production, for example, for cracking ammonia. In another configuration, the method and / or apparatus is used to remove fuel.

[0027] Exemplary embodiments of the invention will now also be described in more detail with reference to the drawings. The features of the exemplary embodiments may be combined individually or in multiple combinations with the subject matter recited in the claims, unless otherwise indicated. The scope of protection claimed is not limited to the exemplary embodiments. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a diagram showing the efficiency of complete combustion for various air-fuel ratios. [Figure 2] Triangular diagram of explosion radius. [Figure 3] 1 is a schematic diagram of an apparatus according to the present invention;

[0029] Figure 1 shows the different efficiencies of the complete combustion of hydrogen with air via the reaction H2 + (1 / 2)O2 → H2O. For different air-fuel ratios λ, the efficiency η relative to the calorific value is shown as a function of the exhaust gas temperature T (°C). These efficiencies are independent of the type of combustion and apply to both conventional and catalytic combustion processes. Efficiencies above 100% are due to the condensation of water vapor contained in the exhaust gas. Calculations apply to an ambient temperature of 20°C.

[0030] The lower explosive limit (LEL) of hydrogen in air at 1 bar is 4.0% at 20°C, 3.4% at 100°C, 2.9% at 200°C, 2.1% at 300°C and 1.5% at 400°C.

[0031] To obtain a "lean" non-explosive hydrogen-air mixture, the hydrogen concentration at 20°C must be less than 4%. In this case, the air-fuel ratio λ must be at least 10. As can be seen from the above values, the LEL decreases at higher temperatures, resulting in the need for higher air-fuel ratios λ. For example, preheating of the fuel and / or oxidizer leads to higher temperatures. Figure 1 shows that even an air-fuel ratio λ of 10, and especially higher air-fuel ratios λ, result in a significant decrease in efficiency η. At an exhaust gas temperature of 50°C, the theoretical efficiency for λ = 20 is only 82% of the heating value. This demonstrates the significant disadvantages of excessively lean mixtures below the LEL.

[0032] Figure 2 shows a ternary diagram, also known as a ternary diagram, in which the explosion limits 30 for a mixture of hydrogen, air, and an inert gas, e.g., recycled product gas, are plotted. For example, the inert gas may consist of 35% water vapor and 65% N2, which corresponds to the composition of the product gas in a stoichiometric reaction. The hydrogen content 31, the air content 32, and the percentage of inert gas 33 are plotted on the axes.

[0033] Most of the area within the triangle is occupied by the explosive region 30. Only a narrow strip on the right and a very narrow strip at the bottom lie outside the explosive region 30. A particularly advantageous process window 35 is shown at the bottom right. In the advantageous process window 35, the air content 32 is typically less than 15%, the hydrogen content 31 is less than 5%, and / or the inert gas content 33 is greater than 80%.

[0034] According to the present invention, the oxygen concentration of the mixture is below the critical oxygen concentration. In other words, the critical safety parameter is not the fuel concentration relative to the upper or lower explosive limit, as in conventional methods, but the oxygen concentration, typically relative to the critical oxygen concentration. For example, a stoichiometric ratio of 4% H2 and 2% O2 from air can exist at 300°C without entering the explosive range. In this case, a maximum of 2.1% H2 would be possible in air. It can be seen that the low-oxygen mixture according to the present invention allows for higher fuel concentrations outside the explosive range compared to a simple mixture of fuel and air.

[0035] 3 shows a flow diagram of an apparatus 8 for producing heat according to the invention. An oxidation reactor 10 is designed as a catalyst 11. An oxidant 2, e.g., air, is transported to the oxidation reactor 10 via a first channel by an optional first blower. Fuel 1 is introduced into the first channel via a second channel, so that a mixture 3 is formed from fuel 1 and oxidant 2 at the point where the first channel intersects with the second channel. The intersection thus serves as a mixing device.

[0036] Oxidation at the catalyst 11 results in a product gas 4, which is particularly inert. The product gas stream is divided into a first partial stream 15 and a second partial stream 16. The first partial stream 15 is recirculated, or fed back into the first path by an optional second blower 25, preferably before the first path intersects with the second path. The incoming oxidant 2 is therefore first diluted with the inert product gas 4 before the fuel 1 is added. In this way, a constant low oxygen concentration can be ensured.

[0037] Preferably, the second partial stream 16 of the product gas 4 is further diverted. A first portion of the second partial stream 16 is used to heat the oxidant 2 in a first heat exchanger 21. In the configuration shown here, the first heat exchanger 21 is located between the first blower 24 and the inlet to the first pass of the recycled product gas 4. However, it is also possible to arrange the first heat exchanger 21 upstream of the first blower 24. In other words, the heat generated during the catalytic reaction is used to heat the oxidant. For example, air, particularly an air stream, is heated. In particular, this is done in a heat exchanger. Heating the fuel 1 and the oxidant 2 separately helps ensure that an explosive mixture does not exist at any time.

[0038] A second portion of the second partial stream 16 is used to heat the fuel 1 in a second heat exchanger 22. Typically, the second heat exchanger 22 is located before the point where the first path intersects with the second path. It can be seen that a process with very high efficiency can be realized without significant additional technical effort, while at the same time meeting high safety requirements. <Reference code list> fuel 1 Oxidizing Agent 2 mixture 3 Product gas 4 equipment 8 Oxidation reactor 10 Catalyst 11 Heatsink 12 First partial flow 15 Second partial flow 16 First heat exchanger 21 Second heat exchanger 22 First Blower 24 Second Blower 25 Exhaust gas 27 Efficiency η Product gas temperature T Explosion radius 30 Hydrogen content 31 Air content 32 Inert gas fraction 33 Process Window 35

Claims

1. 1. A method for producing heat, comprising: forming a mixture (3) comprising a gaseous fuel (1) and a gaseous oxidant (2) to oxidize the fuel (1), and the oxygen concentration of the mixture (3) is less than a threshold oxygen concentration.

2. 2. The method according to claim 1, characterized in that the oxidation is carried out as heterogeneous catalysis using a catalyst (11).

3. 3. The method according to claim 2, characterized in that the catalyst (11) is in thermal contact with the radiator (12), so that the generated heat is transferred directly to the radiator (12).

4. 4. The method according to claim 1, wherein recycled product gas (4) is also used to form the mixture (3).

5. 5. The method according to claim 4, characterized in that the product gas stream (4) is divided, a first partial stream (15) being recycled and a second partial stream (16) delivering heat to the fuel (1) and / or the oxidant (2).

6. 6. The method according to claim 5, characterized in that the first partial flow (15) is conveyed by a blower.

7. 7. The method according to claim 5, wherein the first partial flow (15) is fed to a feed for the oxidant (2).

8. 8. The method according to any one of claims 1 to 7, characterized in that both the fuel (1) and the oxidant (2) are heated with heat from the product gas (4).

9. 9. A method according to claim 8, characterized in that the mixture (3) is made after heating the fuel (1) and the oxidizer (2).

10. 9. The method according to claim 5 and claim 8, characterized in that a second partial flow (16) of the product gas stream (4) is diverted for heating the fuel (1) and the oxidant (2) by means of a separate product gas stream (4).

11. 11. The method according to any one of claims 1 to 10, characterized in that air is used as the oxidizing agent (2).

12. 12. The method according to any one of claims 1 to 11, characterized in that the oxidation is carried out at a temperature below 500°C, in particular below 400°C and / or above 100°C, in particular above 200°C.

13. 13. The method according to any one of claims 1 to 12, characterized in that the oxidation is carried out at least substantially stoichiometrically, resulting in the production of a substantially inert product gas (4).

14. 1. An apparatus for producing heat, comprising a supply line for a gaseous fuel (1), a supply line for a gaseous oxidant (2), an oxidation reactor (10) for oxidizing the fuel (1) with the oxidant (2), and an outlet line for product gas (4), the apparatus further comprising a mixing device configured to be able to produce a mixture (3) from the fuel (1) and the oxidant (2), the mixture having an oxygen concentration below a limit oxygen concentration.

15. 15. Apparatus (8) according to claim 14, characterized in that the oxidation reactor (10) contains a catalyst (11) for heterogeneous catalysis.

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