TORCH SYSTEM FOR BURNING TWO TYPES OF GAS

Description

Field of invention

[0001] The invention relates to a method for the thermal disposal of exhaust gases generated during synthesis gas production and / or synthesis gas processing, with at least two different levels of carbon monoxide, comprising a high-level flare system. State of the art

[0002] A gas flare, or simply flare, is a device for the controlled flaring, i.e., the controlled burning of flammable gases that are not intended or cannot be used for energy or material purposes. Gas flares are frequently used when large quantities of such gases are expected to be released intermittently, as can be the case, for example, during start-up and shutdown procedures and operational disruptions. With gas flares, exhaust gas components are converted into the less environmentally and climate-damaging carbon dioxide through combustion.

[0003] Essential components of a gas flare are the actual burner, one or more pilot burners for its ignition, pipelines for supplying the gases to be flared, possibly a support structure, and control and safety devices for safe flare operation.

[0004] Besides the less common ground flares, the most frequent type of flare system currently in use is the elevated flare. In these systems, the flare tip, which contains the burner, is mounted at the top of the flare system at a certain height above the ground or the installation surface. This reduces heat radiation to the ground or the underlying surface and improves the dispersion profile for the flared pollutants and their combustion products. The feed pipes are designed as risers. The entire assembly of risers, burner, and any supporting structure is usually referred to as a flare stack or simply a flare, although the distinction between these terms is not always clear-cut.

[0005] The state of the art for flare systems is described in API standards 521 and 537.

[0006] Basically, there are the following types of flares: a) Self-supporting flare stacks are often used for lower heights with limited installation space or ground area. They are usually limited to a single flare stack or riser pipe. Their height can reach up to 100 m. A separate liquid separator for condensate removal is usually not required. b) Cable- or wire-supported flare stacks generally require more ground area than self-supporting or derrick-supported flare stacks. Their height can reach up to 250 m. The flare is usually limited to a single riser pipe, and a special liquid separator is often required. c) Flare stacks with a tripod or tripod as a support device are only suitable for smaller, simple flares.d) Derrick flares with drilling rig-like support structures or frames are used only for larger flare stacks due to the increased complexity. These larger flare stacks are not practical or feasible, and a tension-cable-fixed flare stack is not possible due to space constraints. Very high installation heights are achievable. Some derrick designs allow the flare stack and flare tip to be lowered for inspection and maintenance. This is particularly useful when multiple flare stacks are installed on the same derrick due to space limitations (so-called multiflare derrick). A separate liquid separator for condensate removal is usually required.

[0007] Flare systems of the type described are used not only in petrochemical plants but also in synthesis gas production plants to dispose of gas product streams or exhaust gas streams by combustion when these cannot be temporarily processed further within the process chain or for which no other disposal option currently exists. Synthesis gases are gas mixtures containing hydrogen and carbon oxides, which are used in various synthesis reactions. Technical details of the synthesis gas production processes are explained in detail, for example, in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, entry "Gas Production".

[0008] The production of raw synthesis gas, for example by steam reforming, autothermal reforming, or partial oxidation of hydrocarbon-containing feedstocks, is usually followed by several process steps to remove unwanted gas components, such as methane, through cryogenic gas separation in a so-called CO cold box. In this box, carbon monoxide can be obtained as a pure product through steps such as methane scrubbing, partial condensation, and CO scrubbing, or a synthesis gas stream with a controlled hydrogen / carbon monoxide (H₂ / CO) ratio can be obtained. Due to the extremely low temperatures resulting from cryogenic gas separation, the pure CO product is discharged from the CO cold box as a cryogenic gas stream.

[0009] In synthesis gas production plants, flaring of synthesis gas (20 to 300 °C) and cryogenic CO (-180 °C) is typically required to avoid disruptions or planned shutdowns of the entire synthesis gas production plant or parts thereof, and to dispose of gas product streams or exhaust gas streams by burning that cannot be temporarily processed further within the process chain or for which there is currently no other disposal option.

[0010] Various technical solutions currently exist for the high-level flare systems used in practice for burning synthesis gas and cryogenic CO (CO gas). Their designs depend, among other things, on the H₂ / CO ratio of the synthesis gas to be combusted. In synthesis gas production plants, a type 1 flare system can be used if the plant's main product is synthesis gas with a high H₂ / CO ratio or exclusively hydrogen. The small amount of CO gas that needs to be processed in the flare system in this case can be fed to the top of the main carbon steel (C steel) flare stack via a small, external riser pipe made of a corrosion-resistant material, such as a high-alloy steel like stainless steel. This riser pipe is located on the outside of the main flare stack.

[0011] It is particularly advantageous to manufacture the pipe that carries the cryogenic CO gas from the cryogenic gas separation process from a fully austenitic stainless steel, as unalloyed and low-alloy steels become extremely brittle at the typical low temperatures of CO gas. This significantly increases the probability of failure, especially when subjected to cyclic mechanical stresses (e.g., chimney vibration due to wind, pressure surges, etc.). Fully austenitic stainless steels possess very high low-temperature toughness compared to unalloyed and low-alloy steels. Therefore, their toughness at very low temperatures is similar to that at room temperature. The determining factor for using such low-temperature tough materials is not the CO content, but the temperature of the gas. As a guideline, a low-temperature tough, high-alloy material should generally be selected for temperatures of -50°C or lower.

[0012] At the top of the main flare stack, synthesis gas and CO gas are combusted in separate, dedicated burners. To compensate for the different thermal expansion rates resulting from the use of different construction materials for the main flare stack and the CO riser, as well as the high pressure drop, the riser design must incorporate a sufficient number and size of expansion joints. This design places additional loads on the main flare stack, limiting the maximum possible overall height for a self-supporting system before a derrick flare becomes necessary.

[0013] The advantage of a Type 1 flare is that each burner can be optimized for maximum safety and destruction of pollutants in the exhaust gas, taking into account the gas being flared or burned. For example, to ensure a high degree of pollutant removal or combustion while simultaneously preventing the flame from detaching from the burner tip, the exhaust velocity for many flare systems should not exceed 25 to 30 m / s. For synthesis gas, this value can be greater than 150 m / s.

[0014] The second type of high-flare system (Type 2) is similar to Type 1, but incorporates a combined burner design in which the cold, CO-rich gas from cryogenic gas separation is injected into the common main burner at the same level as the synthesis gas. When the H₂ / CO ratio of the synthesis gas produced in the synthesis gas generation plant decreases, resulting in synthesis gas containing more CO than hydrogen, a separate CO burner becomes too large to be mounted at the top of the flare stack next to the main burner. Therefore, a combined burner design of Type 2 offers advantages in such cases.

[0015] However, this design reaches its limits once the CO gas stream becomes the main flow, making the CO riser pipe too large and heavy. Furthermore, in some cases, the additional injection of CO gas into the common burner, while simultaneously combusting the synthesis gas, can cause local gas velocities that pose a risk of flame detachment from the burner tip.

[0016] A third type of high-level flare system (Type 3) is used in cases where the CO gas flow rate is very high compared to the synthesis gas flow rate. As with Type 1, dedicated burners are provided for each flare gas stream. A disadvantage of this design is the need for a stable support structure, such as a derrick design, to accommodate the increased loads from the two completely separate supply and burner systems. This also requires additional floor space.

[0017] In summary, it can therefore be concluded that there is still a need for a simply constructed high-level flare system that allows several types of gas from a synthesis gas production plant, for example CO gas and synthesis gas with a wide H2 / CO ratio, to be flared side by side and preferably simultaneously, and thus safely disposed of thermally.

[0018] DE 10 2007 027819 A1 discloses a process for cooling a cryogenic gas separation unit in which, during normal operation, a carbon monoxide fraction is separated from synthesis gas containing carbon monoxide. The synthesis gas can be obtained from hydrocarbon-containing feedstocks by various production methods such as catalytic steam reforming or partial oxidation. In a gas separation unit, the carbon monoxide is separated from other synthesis gas components by process steps such as cooling, liquefaction, and distillation. During the cold start-up phase of the gas separation unit, none of the gas fractions produced from the synthesis gas meet the product requirements, which is why these gas fractions are, for example, flared off.

[0019] US 3,822,984 A discloses a flare system comprising a first exhaust pipe made of a first material, arranged perpendicular to the horizontal, and a supply line for exhaust gas with a first CO content to the first exhaust pipe, as well as a second exhaust pipe made of a second material, arranged perpendicular to the horizontal, and a supply line for exhaust gas with a second CO content to the second exhaust pipe. The first and second exhaust pipes are arranged coaxially and terminate at their upper ends in a common burner. Description of the invention

[0020] The object of the present invention is to provide a method for the thermal disposal of exhaust gases generated during synthesis gas production and / or synthesis gas processing, comprising a high-level flare system and overcoming the disadvantages of the prior art. In particular, the aim is to provide a method comprising a high-level flare system with a self-supporting flare stack that can be applied across the entire range of H₂ / CO ratios occurring in the product gases or exhaust gases of a synthesis gas production plant that is relevant in practice.

[0021] This problem is solved by a method with the features of claim 1. Further embodiments of the invention are described in the respective dependent claims.

[0022] A synthesis gas production plant or synthesis gas production process encompasses all processes in which synthesis gas, i.e., gas mixtures containing hydrogen and carbon oxides, is initially obtained as a raw product gas. In principle, all known synthesis processes used in industrial production are suitable for the production of synthesis gas, including, in particular, steam methane reforming (SMR) of hydrocarbons, especially natural gas, or, for example, naphtha or refinery residues; non-catalytic partial oxidation of hydrocarbons (POX); and autothermal reforming, a hybrid of the two aforementioned processes. Technical details of these processes are well-known to experts and are explained in detail, for example, in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, entry "Gas Production".

[0023] The raw synthesis gas produced in this way is usually subjected to a multi-stage processing procedure. These processing steps include, in particular, one or more cooling steps with or without steam generation, heat exchange between the raw synthesis gas being cooled and the flue gas produced by the burners to preheat process media, CO conversion (CO shift) to maximize the hydrogen content, steps to remove carbon dioxide, e.g., by gas scrubbing with amine-containing detergents, and measures to separate other gas components, such as methane, traces of higher hydrocarbons, or carbon monoxide as a pure product through cryogenic gas separation in a so-called cold box. This process primarily uses liquid methane or liquid nitrogen to absorb higher-boiling gases such as carbon monoxide and thus separate them from the hydrogen.

[0024] A coaxial arrangement of two tubes is understood to be an arrangement in which the axes of rotation of the two tubes coincide in the longitudinal direction.

[0025] Steels are classified as low-alloy if the total content of alloying elements does not exceed 5% by mass. If this content is exceeded, the steels are classified as high-alloy. Examples include carbon steel (C-steel) as a low-alloy steel and stainless steel, especially rust-resistant stainless steel, as a high-alloy steel.

[0026] Qualitative information, such as low or lower or high or higher CO content, is always to be understood qualitatively in relation to the ratio of two or more material flows under consideration.

[0027] In one embodiment, the flare system comprises a flare stack with a concentrically arranged stainless steel riser pipe as an inner tube, the outer wall of which forms a radially uniformly spaced annular space with the inner wall of the outer tube. Inside the stainless steel riser pipe, the cold CO gas, originating from cryogenic gas separation, is directed to the upper end of the flare stack, where the burner is located.

[0028] The synthesis gas flows through the annular space between the inner pipe and the outer pipe, which can be made of an unalloyed or low-alloy steel, e.g. carbon steel, and is thus also directed to the top of the flare stack.

[0029] This design allows the flare stack to be used for a wide range of H₂ / CO ratios, in particular all H₂ / CO ratios occurring in product gases or exhaust gases from a synthesis gas production plant. The diameter of the inner pipe is designed for the maximum CO gas flow rate. The inner diameter of the outer pipe is then determined by the outer diameter of the inner pipe and the ratio of the annular area to the acceptable pressure differential required to allow the transport of the synthesis gas flow within the flare stack.

[0030] Furthermore, the outer pipe is statically designed to support the load of the entire flare stack assembly, i.e., outer pipe, inner pipe, and burner. An external support structure is not required.

[0031] The inner tube is attached to the lower end of the flare stack, guided upwards inside the outer tube, and can preferably be centered along its length at one or more levels by spacers. More preferably, the spacers are designed as sliding bearings at the end opposite the attachment point. This, along with the coaxial arrangement, allows for free expansion of the inner tube relative to the outer tube, which is necessary when the inner and outer tubes are manufactured from materials with different coefficients of thermal expansion, without requiring additional compensation measures.

[0032] Due to the design of the flare stack, the burner receives CO gas via a central nozzle into which the inner tube opens at its upper end. The synthesis gas enters the burner from the upper end of the annular space, arranged around the CO nozzle. This symmetrical design results in particularly uniform combustion.

[0033] This arrangement also prevents the formation of streaks and thus locally excessive gas velocities at the burner when a gas stream to be flared is introduced into the burner at several discrete points around its circumference. Such streaking can occur due to inconsistencies at the individual injection points, potentially leading to excessively high gas velocities at the burner that are too high to safely combust the gases being flared. Furthermore, the calculation of the resulting mixed gas velocities within the burner is simplified due to their areal distribution.

[0034] Since the choice of burner size is not mechanically limited, but can be relatively freely selected by choosing a compatible outer tube size or a compatible ratio of outer tube to inner tube, and by appropriately dimensioning external piping and manifolds, the inner tube can in any case be designed to be large enough to ensure an arbitrarily low CO gas flow velocity. If necessary for reliable combustion, a further reduction in the CO gas flow velocity can be achieved by selecting a correspondingly large burner diameter.

[0035] Further advantages result from the coaxial routing of the inner tube within the outer tube. The two gases to be flared have different CO contents and temperatures. Synthesis gas, as a final or intermediate product to be flared, typically has temperatures between 20 and 300 °C. In contrast, CO gas from cryogenic gas separation, which is also to be flared, is often in a cryogenic state with temperatures around -180 °C. The arrangement of the inner and outer tubes therefore also acts as a co-flow heat exchanger, reducing the temperature difference between the two gases in the flare stack before they exit the burner. This results in more uniform combustion and a more homogeneous flame profile across the burner cross-section.

[0036] Another aspect of the high-level flare system is its self-supporting design and installation. This eliminates the need for support structures and frameworks, resulting in reduced investment costs. Furthermore, the flare stack's location can be changed more quickly should this become necessary due to site redevelopment within the plant.

[0037] Another aspect of the flare system is characterized by the fact that the first and second exhaust pipes have a circular cross-section, and the second exhaust pipe runs coaxially as an inner pipe inside the first exhaust pipe, which is an outer pipe. A radially uniformly spaced annular space is formed between the outer surface of the inner pipe and the inner surface of the outer pipe. This results in a more uniform distribution of the flared gas flowing through the annular space, leading to more even combustion in the burner and reduced corrosion, particularly on the inner wall of the outer pipe.

[0038] Another aspect of the flare system is characterized by the fact that the arrangement of the inner and outer tubes is enabled by a plurality of spacers, wherein at least two, preferably at least three, spacers are arranged radially at a specific height within the flare system and are attached to the inside of the outer tube or to the outside of the inner tube. In this way, the inner tube can be reliably centered relative to the outer tube. It is recommended to provide such groups of spacers at at least different heights. Particularly preferably, three spacers are provided, arranged in the form of a triangle or a planar tripod perpendicular to the longitudinal axis of the tubes.

[0039] Another aspect of the flare stack system is characterized by the fact that the spacers are designed as sliding bearings at the end opposite the mounting point. Since the inner and outer pipes are made of different materials with generally different coefficients of thermal expansion, and the temperature differences between the annular space and the interior of the inner pipe (central chamber) are considerable and vary significantly along the length of the flare stack, it is advantageous to ensure that the relative change in length of the two pipes can occur undisturbed, preventing mechanical stresses that could lead to deformation. This is achieved by designing the spacers as sliding bearings.

[0040] Another aspect of the flare system is characterized by the fact that the outer tube is made of a low-alloy or unalloyed steel, preferably carbon steel, and the inner tube is made of a high-alloy steel, preferably stainless steel, in particular low-temperature resistant stainless steel. The CO gas from cryogenic gas separation has a low temperature and therefore places higher demands on the material of the inner tube with regard to its embrittlement under the given operating conditions compared to the other gas. It is therefore advantageous to manufacture the inner tube, through whose interior the CO gas is fed to the burner, from a particularly low-temperature resistant material. This is not absolutely necessary for the outer tube, the inside of which is in contact with the other gas, so that a low-alloy or unalloyed steel can be used here as a more cost-effective material.

[0041] Another aspect of the flare system is characterized by the fact that the first exhaust gas has a lower CO content than the second exhaust gas and is routed through the annular space between the inside of the outer pipe and the outside of the inner pipe. This offers the advantages previously mentioned in connection with the discussion of the aforementioned aspect: The exhaust gas with the higher temperature and / or lower corrosion potential is routed through the annular space.

[0042] Another aspect of the flare system is characterized by the fact that the second exhaust gas has a higher CO content than the first and is routed through the inner pipe. This offers the advantages previously mentioned in connection with the discussion of the aforementioned aspect: The exhaust gas with the higher temperature and / or lower corrosion potential is routed through the annular space.

[0043] In one embodiment, the process according to the invention is characterized in that the first product stream or exhaust gas stream comprises or is formed from a synthesis gas stream to be disposed of. The CO content of synthesis gas streams that occur as intermediate or end products in a synthesis gas production plant is lower than that of CO gas from cryogenic gas separation, so that the corrosion potential of the first product stream or exhaust gas stream is lower than that of the second product stream or exhaust gas stream. It is advantageous to pass the exhaust gas with the lower corrosion potential through the annular space, as explained in connection with the sixth aspect of the invention.

[0044] In a further embodiment, the process according to the invention is characterized in that the second product stream or exhaust gas stream comprises or is formed from an exhaust gas stream with a high CO content from the cryogenic gas separation plant. The CO content of CO gas from the cryogenic gas separation is higher than that of synthesis gas streams that occur as intermediate or end products in a synthesis gas production plant, so that the corrosion potential of the second product stream or exhaust gas stream is higher than that of the first product stream or exhaust gas stream. It is also advantageous to route the colder exhaust gas with the higher corrosion and / or embrittlement potential through the central chamber, as explained in connection with the sixth aspect of the invention. Example of implementation

[0045] Further developments, advantages, and possible applications of the invention will also become apparent from the following description of exemplary embodiments and the drawings. All features described and / or illustrated, whether individually or in any combination, constitute the invention, irrespective of their compilation in the claims or their cross-references.

[0046] It shows the only figure: Fig. 1 A schematic representation of an exemplary embodiment of a high-level flare system.

[0047] In the schematic representation in Fig. 1The flare system 1 comprises a flare stack with an outer tube 2 made of carbon steel. The outer tube is cantilevered on a flat base 3 and sealed against it. It has a circular cross-section and is cylindrical or, preferably, frustoconical with a slight upward taper. The wall thickness of the outer tube is dimensioned to support the loads of the entire flare system, including the main components: outer tube, inner tube, and burner. Synthesis gas, which is temporarily generated as exhaust gas in a synthesis gas production plant (not shown), is introduced into the flare stack at its base via a supply line 4. The temperature of the synthesis gas exhaust gas can range from 20 to 300 °C. It depends primarily on the location or assembly within the synthesis gas production plant from which the exhaust gas is to be discharged.Thus, at one point downstream of the raw synthesis gas production stage, for example a steam reformer, the temperature is approximately 300 °C. During the subsequent treatment stages, it decreases continuously and is typically between 20 and 50 °C, for example 40 °C, before being introduced into the final pressure swing adsorption stage for pure hydrogen production.

[0048] Inside the outer tube is the inner tube 5, which is also placed on the flat base 3 and sealed against it. It is made of a high-alloy steel, for example, low-temperature tough stainless steel, and has a circular cross-section. CO gas, which is temporarily generated as exhaust gas in a cryogenic decomposition system (not shown) for the production of raw synthesis gas or already treated synthesis gas, is introduced into the underside of the flare stack via supply line 6 and then fed into the inner tube 5. Supply line 6 passes through the outer tube 2, with the point of entry being sealed against the environment. The CO gas has a temperature of -180 °C. Preferably, it is passed through a liquid separator (not shown) before being introduced into the inner tube to remove any condensates.

[0049] In the present example, the inner pipe is fixed relative to the outer pipe by means of spacers 7, which are provided at three different heights of the flare stack. The spacers are attached to the inner wall of the outer pipe and are designed as sliding bearings on the side facing the inner pipe. Three spacers are provided at each height, spaced at angular intervals of 120°.

[0050] The inner pipe diameter is designed for the maximum CO gas flow rate. The outer pipe's inner diameter is then determined by the inner pipe's outer diameter and the ratio of the annular space area to the acceptable pressure differential for transporting the synthesis gas flow within the flare stack. When selecting the diameters, it is essential to ensure that even at maximum flow rates of both exhaust gases, the flow velocity at the burner outlet remains low enough to prevent flame detachment. The maximum flow velocity under these conditions may need to be determined through preliminary tests.

[0051] In the flare stack, the synthesis gas exhaust is guided from bottom to top through the annular space formed between the inside of the outer tube and the outside of the inner tube, and finally introduced into the common burner 8. The CO gas exhaust flows upwards within the interior of the inner tube (central chamber) and also enters the common burner. There, the exhaust gases are ignited and combusted by means of pilot burners (not shown). The oxygen required for this is drawn from the ambient air. To assist combustion, additional steam can be injected into the burner. The resulting combustion products are released into the environment. Reference sign

[0052] [1]High-level flare system [2]Outer pipe [3]Subsoil [4]Supply line [5]Inner pipe [6]Supply line [7]Spacer [8]Common burner

Claims

1. A method for the thermal disposal of exhaust gases resulting from syngas generation and / or syngas treatment, the exhaust gases having at least two different carbon monoxide contents (CO content), comprising a high flare system (1), which comprises: (a) a first exhaust gas pipe (2) made of a first material and arranged vertically relative to the horizontal and a feed line (4) for an exhaust gas having a first CO content to the first exhaust gas pipe (2), (b) a second exhaust gas pipe (5) made of a second material and arranged vertically relative to the horizontal and a feed line (6) for an exhaust gas having a second CO content to the second exhaust gas pipe (5), (c) wherein the first and the second exhaust gas pipes are arranged coaxially and open at their upper end into a common burner (8), wherein the first and the second exhaust gas pipes have a circular cross-section and the second exhaust gas pipe (5) extends coaxially as an inner pipe (5) inside the first exhaust gas pipe (2) acting as an outer pipe (2), wherein a radially uniform annular space is formed between the outer side of the inner pipe (5) and the inner side of the outer pipe (2), the method comprising the following steps: (a) producing a raw syngas containing hydrogen and carbon oxides from a feedstock containing hydrocarbons by means of a syngas generation process, (b) multi-stage conditioning of the raw syngas to form a pure syngas, comprising a cryogenic gas separation unit as one of the conditioning stages, (c) discharging a first product stream or exhaust gas stream having a low CO content and introducing it into the annular space between the inner side of the outer pipe (2) and the outer side of the inner pipe (5) of the high flare system, (d) discharging a second product stream or exhaust gas stream having a high CO content and introducing it into the inner pipe (5) of the high flare system, (e) combusting the first product stream or exhaust gas stream and the second product stream or exhaust gas stream in the common burner (8), and wherein the first exhaust gas has a lower CO content than the second exhaust gas and is guided through the annular space between the inner side of the outer pipe (2) and the outer side of the inner pipe (5), and the second exhaust gas has a higher CO content than the first exhaust gas and is guided through the inner pipe (5).

2. The method according to claim 1, wherein the first product stream or exhaust gas stream comprises a syngas stream to be disposed of or is formed from the latter.

3. The method according to claim 1 or 2, wherein the second product stream or exhaust gas stream comprises an exhaust gas stream with high CO content from the cryogenic gas separation unit or is formed from the latter.