BURNER DEVICE FOR THE INDIRECT HEATING OF A STOVE ROOM

Description

[0001] The invention relates to a burner device for liquid or, in particular, gaseous fuels for the indirect heating of a furnace chamber, comprising a combustion chamber with a combustion chamber opening and a radiant tube surrounding the combustion chamber at least in the region of the combustion chamber opening, and an inner tube arranged inside the radiant tube, preferably extending coaxially to the combustion chamber opening, with an inner tube inlet pointing towards the combustion chamber opening and an inner tube outlet directed towards a closed end of the radiant tube, wherein the outer circumferential surface of the inner tube forms an annular gap with the inner circumferential surface of the radiant tube for returning the gas mixture flowing through the inner tube from its inlet to the inner tube outlet, and wherein a recirculation passage is formed between the inner tube inlet and the combustion chamber opening.

[0002] When using industrial burners to heat furnace chambers, it is common practice to reduce emissions, particularly nitrogen oxide emissions, by supplying combustion air in stages and recirculating a comparatively large proportion of exhaust gas. This recirculated gas, along with the secondary air flowing in through the burner head, is mixed with the primary flame exiting the combustion chamber. This results in a so-called "mild combustion" or flameless oxidation, in which the gas-air mixture reacts without a visible flame. Compared to stable combustion of the fuel with flame formation, flameless oxidation is characterized by very low nitrogen oxide emissions. Achieving high recirculation rates and thus ensuring flameless oxidation is now a very common method in the direct firing of furnaces.

[0003] In many industrial heat treatment processes, it is essential to prevent the material being introduced into the furnace for heat treatment from coming into contact with the exhaust gases from the combustion process. In such treatment processes, the burners are operated within radiant tubes that transfer their energy to the furnace interior in the form of solid-state radiation. Combustion thus takes place inside the radiant tube of the burner assembly, with the tube conducting the heat generated within it to its outer surface. The radiant tube extends into the furnace chamber and emits the heat there as radiant heat. Generally, an inner tube is arranged concentrically to the radiant tube, connecting at the inlet or outlet of a combustion chamber.The gas-air mixture exiting the combustion chamber is guided through the inner tube to the front, sealed end of the jet tube, where it exits the inner tube and then flows back towards the burner head through the radially outer annular gap between the outer surface of the inner tube and the inner surface of the jet tube. The heat generated during the reaction is transferred by the hot gas mixture to the jet tube wall. A portion of the exhaust gas flowing back through the annular gap from the combustion process is intended to be recirculated to reduce emissions, but this proves difficult in known systems due to the boundary conditions inside the jet tube and the inner tube.

[0004] EP 0 875 716 A2 discloses a method and a device for the multi-stage combustion of fuel, wherein the combustion takes place in a closed space, preferably in a radiant heating tube. EP 0 875 716 A2 discloses a burner device with the features of the preamble of claim 1.

[0005] EP 2 910 855 A1 discloses a recuperator burner with an additional heat exchanger that encloses the recuperator. US 4 894 006 discloses a burner system in which the exhaust gases are discharged at high velocity.

[0006] The object of the invention is to further develop a burner device of the type mentioned above in such a way that, even with the comparatively small dimensions of the radiant tube (in relation to a furnace chamber with direct heating), reliable flameless oxidation can be ensured inside the device by means of a sufficiently high degree of recirculation of exhaust gases, so that the emission values ​​of such a radiant tube burner device are comparable to those that are possible with the direct heating of a furnace chamber when flameless oxidation is used there.

[0007] This problem is solved by the invention by means of a recuperator surrounding the combustion chamber for preheating combustion air by means of exhaust gas routed from the burner device, wherein the recuperator has a centering nozzle on the exhaust gas inlet side pointing towards the annular gap, which forms the inlet for the inner tube or interacts with it and on which the recirculation passage is formed, wherein the ratio of the flow cross-section of the inner tube to the flow cross-section of the annular gap is selected such that the static pressure component of the gas mixture flowing through the inner tube and the annular gap in the area of ​​the inner tube inlet is smaller than in the annular gap in the area in front of the recirculation passage.

[0008] This design results in a pressure differential of static pressure at the recirculation port between its inlet, which faces outwards towards the annular gap, and its outlet, which faces inwards towards the inner tube. This pressure differential causes a correspondingly high proportion of the exhaust gas flowing back towards the burner head through the annular gap to be drawn in through the recirculation port and, together with the gas-air mixture exiting the combustion chamber, flow again through the inner tube. This ensures that the reaction between gas and fuel occurs without the formation of a visible flame as it travels through the inner tube and back through the annular gap. The formation of nitrogen oxides is thus reliably minimized. In practice, the (static) pressure difference, and therefore the high recirculation rate of exhaust gas through the recirculation port, is achieved through a specific design of the burner and jet tube system.In particular, the pressure difference is achieved by reducing the flow cross-section of the inner pipe relative to the flow cross-section in the annular gap, so that the flow velocity of the gas mixture flowing through the inner pipe is greater than in the annular gap. Accordingly, the dynamic pressure component in the inner pipe is higher than in the annular gap, so that the pressure gradient determined by the static pressure components is established at the desired level, thus promoting the backflow of exhaust gases through the recirculation passage.

[0009] The flow behavior in a burner device of the type presented here is essentially as follows: the flame exiting the burner at its orifice, or, in flameless operation, the reactive mixture exiting the burner, enters the inner tube located inside the jet tube and flows through it to its end. At the end of the inner tube, the gas mixture then exits the outlet of the inner tube and is deflected into the annular gap between the inner tube and the jet tube. The gas mixture then flows back through the annular gap towards the burner, where a portion of the exhaust gases is to be fed back to the flame or the reactive mixture exiting the burner orifice and thus recirculated. The proportion of exhaust gas that is mixed with the gas mixture supplied for the combustion process describes the so-called recirculation ratio.This is usually expressed as the ratio of the recirculated mass flow to the total mass flow (sum of exhaust gas and recirculated exhaust gas).

[0010] While in the past the inner tube of a burner of this type was apparently considered only in terms of guiding the exhaust gases and thereby achieving the smallest possible temperature gradient along the length of the tube, the invention recognized that the achievable recirculation rate is decisively determined by the velocity and pressure fields in the jet tube, and these in turn are significantly influenced by the inner tube. In previously known burner systems, the flow cross-section of the inner tube is larger than the flow cross-section in the annular gap. This leads to the situation that, in the steady state of the system, i.e., at a constant recirculation rate, the average flow velocity in the annular gap is greater than the average flow velocity in the inner tube. In known systems, the factor between the two velocities is approximately 1.3, meaning the average velocity in the annular gap is approximately...30% above the mean velocity in the inner tube. If one determines the dynamic pressure ratios based on these velocity ratios to estimate the static pressure difference between the annular gap and the inner tube, one finds – even when considering pressure losses in the system – that in known systems with comparatively large-diameter inner tubes – apart from local areas near the burner outlet – the mean static pressure difference between the annular gap and the inner tube is negative, i.e., it points in the wrong direction. A significant recirculation rate is therefore not achievable, or only by utilizing very high velocities in the burner outlet area. By using inner tubes with smaller diameters, the calculated value of the static pressure difference is reversed, i.e.,The difference in static pressure between the annular gap and the inner pipe becomes positive, which allows for a significantly higher recirculation rate of exhaust gas from the annular gap back into the inner pipe and thus ensures flameless operation.

[0011] It is particularly advantageous in the invention if the flow cross-section of the inner tube is at least substantially constant along its length between the inner tube inlet and the inner tube outlet. In particular, the arrangement can be advantageously configured such that the ratio of the flow cross-section of the inner tube to the flow cross-section of the annular gap is at least substantially constant. The pressure gradient of the static pressure at the recirculation passage, as desired by the invention, is thus achieved according to this particularly preferred embodiment without any special cross-sectional changes along the inner tube, i.e., in particular without the installation of orifices or nozzle-like constrictions inside the inner tube, such as a Laval nozzle or the like.Rather, the desired pressure gradient, which enables the high recirculation rate, is achieved essentially solely through the appropriate selection of the flow cross-sections in the inner pipe and the annular space, thus minimizing pressure losses that are otherwise regularly associated with the installation of additional flow-related components. "Essentially constant" in this context means that the flow cross-section of the inner pipe does not have to be exactly the same along its entire length, but that, within the scope of the invention, it is possible to allow minor cross-sectional changes along the length of the inner pipe. Such minor cross-sectional changes can occur, for example, at socket joints of an inner pipe consisting of several interconnected pipe sections, or in the area of ​​support feet that are integrally molded onto the inner pipe to support it against the inner circumferential surface of the jet nozzle.

[0012] Preferably, the jet tube and the inner tube each have annular cross-sections and are arranged concentrically to each other. The ratio of the flow cross-section of the inner tube to the flow cross-section of the annular gap is preferably in the range of 0.4 to 0.8, preferably over the entire length of the inner tube from its inlet to the inner tube outlet, where the gas flow is deflected into the annular gap. In the preferred concentric arrangement of the inner tube and the jet tube with a circular cross-section, the ratio of the inner diameter of the inner tube to the inner diameter of the jet tube concentrically surrounding it can preferably be in the range of 0.55 to 0.65 (± 0.1).

[0013] In a further, advantageous embodiment of the invention, the combustion chamber opening can be arranged at a burner tip that projects a short distance into the inner tube in front of the recirculation passage. This ensures that the gas-air mixture exiting the combustion chamber opening, or the flame resulting from the primary reaction, enters the inner tube completely and cannot flash back through the recirculation passage.

[0014] In a preferred further embodiment of the invention, the burner device has at least one secondary air supply device with at least one secondary air outlet. The at least one secondary air outlet preferably opens into the recirculation passage in the direction of flow in the region upstream of the combustion chamber opening. According to this embodiment, only a portion of the total combustion air supplied to the combustion process enters the combustion chamber (primary air), where it is mixed with the fuel in a substoichiometric ratio (λ < 1.0). Due to the substoichiometric mixing ratio, in which the amount of oxygen supplied is insufficient for a complete reaction, the formation of nitrogen oxides is suppressed in this first combustion stage.

[0015] The substoichiometric flame exits the combustion chamber at a relatively high velocity and mixes with the secondary air supplied via the secondary air supply device and the recirculated exhaust gas. While the recirculated exhaust gas does not participate in the further reaction that takes place in the inner tube and partially during the return of the gas mixture through the outer annular gap, it ensures a more uniform combustion process and prevents locally occurring temperature spikes during further oxidation with the consumption of the supplied secondary air, thus ensuring complete fuel conversion with low emissions.

[0016] The at least one secondary air outlet can be designed as an annular gap opening surrounding the combustion chamber or as several air pipe openings distributed around the circumference of the combustion chamber, preferably not interconnected. If several air pipe openings are provided independently of one another for the supply of secondary air, not only the quantity of secondary air supplied, but also its location on the circumference of the combustion chamber can be specifically varied, thereby influencing the combustion process.

[0017] According to the invention, the device comprises a recuperator surrounding the combustion chamber for preheating combustion air by means of exhaust gas discharged from the burner device. The arrangement is preferably such that the recuperator has a recuperator tip facing the annular gap on the exhaust gas inlet side, which is located at a distance from the inner pipe inlet and limits the recirculation passage. The at least one secondary air outlet can then open into the recirculation passage at the recuperator tip. With the aid of the recuperator, the combustion air is preheated for the combustion process in a known manner, by the exhaust gas discharged from the burner device transferring at least some of its residual heat to the combustion air via heat exchange surfaces of the recuperator.

[0018] The combustion chamber outlet preferably has an outlet cross-section for the gas mixture exiting the combustion chamber, which has a cross-sectional area of ​​2–10 mm² / kW, preferably 3–8 mm² / kW, relative to the burner output. The combustion chamber outlet expediently forms a sharp-edged nozzle opening or a "tripping hazard" of, for example, approximately 4–10 mm in height for the gas mixture to exit the combustion chamber. Due to the sharp-edged nozzle opening or the tripping hazard and the associated abrupt change in cross-section at the gas mixture's outlet from the burner, a higher recirculation rate and better mixing of the primary flame with the secondary air and / or the recirculated exhaust gas are achieved than with a nozzle with a continuously changing cross-section. This effect can be assumed to be due to the formation of separations and eddies in the gas flow.

[0019] Empirical investigations have also shown that the ratio of the combustion chamber outlet cross-section to the cross-section of the at least one secondary air outlet - in each case based on the power output - is preferably in the range of 1.5 to 3.0 in order to achieve the best possible result of flameless combustion in the inner and radiant tube.

[0020] Further features and advantages of the invention will become apparent from the following description and the drawing, in which a preferred embodiment of the invention is illustrated and described by way of example. It shows: Fig. 1 shows a burner device according to the invention in longitudinal section; and Fig. 2 shows the object of the Fig. 1 in a perspective view, partly in section.

[0021] The drawing shows a burner device designated as 10 in total, which is suitable for the indirect heating of a (not shown) furnace chamber.

[0022] The burner device comprises a recuperator burner 11 with a combustion chamber 12, which is connected on the inlet side to a fuel gas supply line 13, and has air inlets 14 through which a portion of the combustion air supplied to the burner 11 via an air connection 15 enters the combustion chamber 12 (primary air) to mix with the fuel gas there.

[0023] The combustion chamber 12 has a combustion chamber opening 16 at its outlet, from which the fuel gas mixed with the primary air exits and is ignited in a known manner, e.g., by means of an ignition lance. At least the front end of the burner 11, with the combustion chamber 12 located therein, is surrounded by a jet tube 17, inside of which an inner tube 18 is arranged, running coaxially to the combustion chamber opening 16. The inner tube 18 is supported by a series of support feet 19 on the inner circumferential surface of the jet tube 17. The inner tube 18 has an inner tube inlet 20 pointing towards the combustion chamber opening 16 and an inner tube outlet 22 directed towards a closed end 21 of the jet tube 17. An annular gap 23 is formed between the outer circumferential surface of the inner tube 18 and the inner circumferential surface of the jet tube 17, the cross-section of which is only very slightly and locally restricted by the support feet 19.In the illustrated embodiment, the inner tube consists of several individual tube sections connected by socket joints. The inner cross-section of the inner tube (flow cross-section), through which the gas mixture flowing through the burner device passes, is essentially constant along its length. Minor variations in the diameter of the inner tube, for example in the area of ​​the socket joints and the support feet, therefore do not significantly affect the effective flow cross-section. In particular, there are no relevant cross-sectional constrictions inside the inner tube, such as orifices, nozzles, or the like, which could significantly contribute to the pressure loss of the flowing gas mixture.

[0024] In the illustrated embodiment, the inner tube inlet 20 is equipped with an associated centering nozzle 24, which is fixed to the front end of the burner 11 by three support feet 25 evenly distributed around its circumference. This centering nozzle 24 ensures a permanently coaxial alignment of the inner tube inlet relative to the combustion chamber opening 16. The recessed areas of the centering nozzle between the three support feet 25 form a recirculation passage 26 for a portion of the exhaust gas resulting from the combustion process, as will be described in detail below.

[0025] Turning our attention back to the combustion chamber 12, we see that a secondary air supply device 27 is provided around it. This device supplies additional air (secondary air) to the mixture of fuel gas and primary air to ensure complete combustion of the fuel gas. While the primary air enters the combustion chamber 12 from the air connection through the interior of the recuperator burner housing at the rear end of the combustion chamber 12 via the primary air inlets 14 located there, and mixes with the fuel gas, the secondary air flows around the outside of the combustion chamber 12 and reaches secondary air outlets 29, which are located at the front end of the burner 11 at its burner tip 30, via several air channels 28 distributed around its circumference.The secondary air initially flows along the outside of the combustion chamber 12 and mixes at the inlet 20 to the inner tube 18 with the mixture of fuel gas and primary air exiting from the combustion chamber opening 16.

[0026] The reactive gas mixture, ignited upon exiting the combustion chamber opening 16, enters the inner tube 18 together with the added secondary air through the inner tube inlet 20 and flows through it to the inner tube outlet 22. There it exits the inner tube 18 and is deflected at the closed end 21 of the jet tube 17 into the outer annular gap 23, in order to be returned through this in the direction of the recuperator burner 11 located at the inlet-side end of the device 10.

[0027] To achieve the best possible and most constant recirculation rate of the exhaust gas resulting from the combustion of fuel gas and air back into the inner tube 18, in order to achieve stable, flameless oxidation of the fuel in this tube and the subsequent annular gap, with a resulting reduction of undesirable combustion products (NOx), the invention implements specific fluid-mechanical conditions. For this purpose, the ratio of the flow cross-section AI of the inner tube 18 to the flow cross-section AR of the annular gap 23 is selected such that the static pressure component of the gas mixture flowing through the inner tube and the annular gap is lower in the inner tube inlet 20 than in the annular gap 23 in the region upstream of the recirculation passage 26.

[0028] A smaller flow cross-section, with a constant volume flow rate of the gas mixture flowing through it, leads to an increase in the flow velocity of the gas mixture and thus also in the dynamic pressure component of the gas flow. Correspondingly, the static pressure component decreases. If one ensures that the static pressure component is greater outside at or just before the recirculation passage 26, i.e., in the region of the outer annular gap through which the combustion gas flows from the nozzle tip back to the burner, than inside in the region of the inner tube inlet, a positive (static) pressure gradient is obtained between the exhaust gas flowing outside the recirculation passage 26 and the combustion gas / air mixture entering the inlet 20 of the inner tube 18. As a result, a significant portion of the exhaust gas is drawn into the inner tube and thus recirculated.This allows for stable, flameless oxidation in the jet tube, largely preventing the formation of nitrogen oxides.

[0029] The desired (static) pressure gradient is achieved by relatively increasing the flow velocity in the inner tube compared to the flow velocity prevailing in the outer annular gap at the recirculation passage. Experiments have shown that the desired effect is particularly effective when the ratio of the flow cross-section of the inner tube (AI) to the flow cross-section of the annular gap (AR) is in the range of AI / AR = 0.4 to 0.8. If, as in the illustrated and described embodiment, circular cross-sections are used for the inner tube and the surrounding jet tube, the ratio of the inner diameter (DI) of the inner tube to the inner diameter (DS) of the concentrically surrounding jet tube is preferably in the range of DI / DS = 0.55 to 0.65 ± 0.1. The flow cross-section of an inner tube with a circular cross-section is its cross-sectional area, i.e., AI = π DI 2 < 4.The flow cross-section in the concentric annular gap is accordingly AR = π (DS 2< -DA 2< ) / 4. Here, DI represents the inner diameter of the inner tube, DA the outer diameter of the inner tube, and DS the inner diameter of the surrounding jet tube.

[0030] As can be seen in the drawing, the combustion chamber opening 16 at the front end of the combustion chamber 12 forms a sharp-edged nozzle opening 31. This ensures good turbulence and thus mixing of both the primary air with the fuel gas and the secondary air flowing along the outside of the combustion chamber with the substoichiometric gas-air mixture exiting the combustion chamber. A cross-sectional area of ​​2 to 10 mm² / kW, preferably 3 to 8 mm² / kW, has proven particularly advantageous as the outlet cross-section for the gas mixture exiting the combustion chamber, relative to the burner output.

[0031] The burner is equipped in a manner known per se with a recuperator 32, which is formed on the outside of the burner housing, surrounding the combustion chamber 12. The recuperator 32 preheats the combustion air flowing inside the burner housing by heat exchange with the portion of the exhaust gas that is not recirculated and passes over cooling fins 33 of the recuperator. At the front end of the recuperator 32, i.e., on the exhaust gas inlet side, the centering nozzle 24, which has the recirculation passage 26, is arranged on the burner. The front end of this nozzle is inserted into the inner tube 18 to center it at its inlet 20, while also allowing for changes in the length of the inner tube due to thermal expansion.

[0032] The invention, through the interplay of various modifications in the geometry of the burner device, in particular by altering the flow conditions in the jet tube and its inner tube itself, enables stable flameless oxidation and thus a drastic reduction in emissions. Particularly advantageous are the adaptation of the geometry (area and specific design) of the combustion chamber outlet (first combustion stage) and its position relative to the actual burner opening (recuperator tip), the adaptation of the secondary air outlet depending on the combustion chamber outlet area, the position of the inner tube in the jet tube relative to the combustion chamber opening or the recuperator tip, and, in particular, the adaptation of the diameter of the inner tube in the jet tube relative to the inner diameter of the jet tube itself.In flameless operation, fuel can be supplied via the usual gas nozzle in the combustion chamber. Alternatively, a separate, central gas lance designed specifically for flameless operation can be used. This lance typically passes centrally through the combustion chamber and extends to its outlet. The gas lance can also be routed laterally along the inner or outer wall of the combustion chamber. In flameless operation, the fuel gas can then be supplied via this lance.

Claims

1. Burner device for liquid or gaseous fuels for the indirect heating of a furnace chamber, comprising a combustion chamber (12) with a combustion chamber opening (16), with a jet pipe (17) surrounding the combustion chamber (12) at least in the area of the combustion chamber opening (16) and an inner pipe (18) arranged inside it, preferably running coaxially to the combustion chamber opening (16), which has an inner pipe inlet (20) facing the combustion chamber opening (16) and an inner pipe outlet (22) facing a closed end (21) of the jet pipe (17), wherein the outer circumferential surface of the inner pipe (18) forms an annular gap (23) with the inner circumferential surface of the jet pipe (17) for recirculating the gas mixture flowing through the inner pipe (18) from its inlet (20) to the inner pipe outlet (22), wherein a recirculation passage (26) is formed between the inner pipe inlet (20) and the combustion chamber outlet (16), and with a recuperator (32) surrounding the combustion chamber (12) for preheating combustion air by means of exhaust gas discharged from the burner device, wherein the ratio of the flow cross-section (Ar) of the inner pipe (18) to the flow cross-section (AR) of the annular gap (23) is selected such that the static pressure component of the gas mixture flowing through the inner pipe (18) and the annular gap (23) in the region of the inner pipe inlet (20) is smaller than in the annular gap (23) in the region upstream of the recirculation passage (26), and characterised in that the recuperator has a centering nozzle (24) facing the annular gap (23) on the exhaust gas inlet side, which forms the inlet (20) for the inner pipe (18) or cooperates with it and on which the recirculation passage (26) is formed.

2. Device according to claim 1, characterised in that the flow cross-section (AI) of the inner pipe (18) is at least substantially constant over its length between the inner pipe inlet (20) and the inner pipe outlet (22).

3. Device according to claim 1 or 2, characterised in that the ratio of the flow cross-section (AI) of the inner pipe (18) to the flow cross-section (AR) of the annular gap (23) is at least substantially constant.

4. Device according to one of claims 1 to 3, characterised in that the jet pipe (17) and the inner pipe (18) each have substantially circular cross-sections and are arranged concentrically to each other.

5. Device according to one of claims 1 to 4, characterised in that the ratio of the flow cross-section (AI) of the inner pipe (18) to the flow cross-section (AR) of the annular gap (23) is in the range of AI / AR = 0.4 to 0.8.

6. Device according to claim 4 or 5, characterised in that the ratio of the inner diameter (DI) of the inner pipe (18) to the inner diameter (DS) of the jet pipe (17) surrounding it concentrically is in the range of DI / DS = 0.55 to 0.65 ± 0.1.

7. Device according to one of claims 1 to 6, characterised in that the combustion chamber opening (16) is arranged in the area of the recirculation opening (26) or the inlet (20) into the inner pipe (18) or a short distance behind it.

8. Device according to one of claims 1 to 7, characterised by at least one secondary air supply device (27) with at least one secondary air outlet (29), which preferably opens into the recirculation passage (26) in the area upstream of the combustion chamber opening (16) in the direction of flow.

9. Device according to claim 8, characterised in that the at least one secondary air outlet (29) is designed as an annular gap opening surrounding the combustion chamber (12) or as a plurality of air pipe openings distributed around the circumference of the combustion chamber (12), preferably not connected to one another.

10. Device according to claim 8 or 9, characterised in that the at least one secondary air outlet (29) opens into the recirculation passage (26) and / or the centring nozzle (24).

11. Device according to one of claims 1 to 10, characterised in that the combustion chamber outlet (16) has an outlet cross-section for the gas mixture emerging from the combustion chamber (12) which has a cross-sectional area of 2 to 10 mm2 / kW, preferably 3 to 8 mm2 / kW, relative to the burner output.

12. Device according to one of claims 1 to 11, characterised in that the combustion chamber outlet (16) forms a sharp-edged nozzle opening (31) for the discharge of the gas mixture from the combustion chamber (12).

13. Device according to one of claims 1 to 12, characterised in that the ratio of the outlet cross-section of the combustion chamber opening (16) to the cross-section of the at least one secondary air outlet (29) is in the range of 1.5 to 3.0.