Process for the aftertreatment of an exhaust gas from a pyrolysis reaction

Abstract

Process for the aftertreatment of an exhaust gas from a pyrolysis reaction, in which hydrocarbon compounds contained in the respective exhaust gas stream (5) are thermally decomposed, by guiding an exhaust gas stream (5) released during a pyrolysis within a first interval through at least one open-cell structure (3) by means of a pressure difference between a device in which the respective pyrolysis is carried out and an exhaust gas discharge connected to the environment, and in the process a temperature of at least 600 °C is maintained at the at least one open-cell structure (3), so that at least some of the hydrocarbon compounds contained in the exhaust gas stream (5) are thermally decomposed and the carbon obtained is deposited on surfaces of the at least one open-cell structure (3) and after expiry of a predeterminable operating time and / or the reaching of a predeterminable threshold value of a pressure difference of a pressure P1, which is determined before the exhaust gas flow (5) enters the at least one open-cell structure (3) and a pressure P2 in the flow direction after the exit from the at least one open-cell structure (3) or a predeterminable threshold value of the pressure P1, which is determined before the exhaust gas flow (5) enters the at least one open-cell structure (3) and of a pressure P2 in the flow direction after exiting the at least one open-cell structure (3), the supply of the exhaust gas stream (5) to the at least one open-cell structure (3) is stopped and then, in a second interval, instead of the exhaust gas stream (5), oxygen or an oxygen-containing gas mixture is passed through the at least one open-cell structure (3) and, at a temperature of at least 600 °C, the carbon deposited on surfaces of the at least one open-cell structure (3) is oxidized and the oxide(s) formed and exhaust gas residues are removed.

Description

[0001] The invention relates to a method for the aftertreatment of an exhaust gas from a pyrolysis reaction.

[0002] Pyrolysis reactions are typically used for the thermal decomposition of organic chemical compounds that are components of materials used in the manufacture or modification of products. These are often organic binders. These binders are required in a wide variety of processes, such as sintering processes and additive manufacturing processes for components in semi-finished products or green bodies, or even materials used in additive manufacturing, to enable shaping. After shaping or locally defined solidification, these organic components can be removed because they are no longer required, and a finished component or component region, for example, is then formed solely from inorganic components.

[0003] Thermal decomposition takes place under vacuum conditions or in inert atmospheres, so that complete thermal decomposition cannot be achieved and the exhaust gas then contains undecomposed or only partially decomposed organic components in the form of hydrocarbon compounds. These organic components of the exhaust gas, which are usually released to the outside and then into the environment, settle on the surfaces of the respective exhaust gas discharge, on valves and even on elements of a compressor that is connected to an exhaust gas discharge on the intake side, usually after condensation, and thus accumulate. This can make cleaning the inside of the exhaust gas discharge and in particular the elements of compressors used to discharge the respective exhaust stream to the outside, and thus protect them from lasting damage, very complex.It may even be necessary to repair damage caused by the adhesion of hydrocarbon compounds to compressor components. This is a very costly and time-consuming process due to the strong adhesive properties of hydrocarbons on surfaces to which they have adhered. Furthermore, the correspondingly long downtime required for such cleaning or repairs is a disadvantage.

[0004] For example, DE 2 317 267 A discloses a process for the separation of substances in porous bodies.

[0005] WO 94 / 01202 A1 relates to a method and a device for the thermal treatment of gas, in particular thermal and / or catalytic afterburning of exhaust gas.

[0006] It is therefore an object of the invention to provide possibilities for avoiding or reducing the amount of hydrocarbon compounds contained on surfaces in the exhaust gas tract after pyrolysis has been carried out and which settle on surfaces there.

[0007] According to the invention, this object is achieved by a method having the features of claim 1. Advantageous embodiments and further developments of the invention can be realized with features defined in the dependent claims.

[0008] In the method for aftertreating an exhaust gas from a pyrolysis reaction, hydrocarbon compounds contained in the respective exhaust gas stream are thermally decomposed by passing the exhaust gas stream released during pyrolysis within a first interval through at least one open-cell structure by means of a pressure difference between a device in which the respective pyrolysis is carried out and an exhaust gas discharge connected to the environment. A temperature of at least 600°C, preferably at least 800°C, particularly preferably at least 1000°C, is maintained at the at least one open-cell structure, such that at least some of the hydrocarbon compounds contained in the exhaust gas stream are thermally decomposed and the resulting carbon is deposited on surfaces of the at least one open-cell structure.

[0009] After a predeterminable operating time has elapsed and / or a predeterminable pressure difference of a pressure P1, which is determined before the exhaust gas flow enters the at least one open-cell structure and a pressure P2 in the flow direction after the exit from the at least one open-cell structure, has been reached, the supply of the exhaust gas flow to the at least one open-cell structure is stopped. This can also be achieved by only taking into account the pressure P1, as is also the case in the description of Fig. 1 will be explained.

[0010] Then, in a second interval, instead of the exhaust gas stream, oxygen or an oxygen-containing gas mixture is passed through the at least one open-cell structure, and the carbon deposited on the surfaces of the at least one open-cell structure is oxidized at a temperature of at least 600 °C, preferably at least 800 °C, and particularly preferably at least 1000 °C. The oxide(s) formed (CO and / or CO2) as well as exhaust gas residues, which may still contain minor residues of hydrocarbon compounds, are then removed.

[0011] The mentioned pressure difference between P1 and P2 can be achieved with at least one compressor, with which the exhaust gas flow is sucked through the at least one open-cell structure.

[0012] An open-cell structure can be an open-pore body (foam body) or a textile structure made of filaments, made of or with an electrically conductive material. A textile structure can be, for example, a braided fabric, a knitted fabric, a scrim, or a woven fabric of filaments.

[0013] The open-cell structure can be connected to an electrical voltage source for electrical resistance heating, and the respective minimum temperature can be maintained by means of electrical resistance heating or by means of an electrical AC voltage source and at least one electrical coil. One or more electrical coils can enclose the respective open-cell structure from the outside.

[0014] To achieve this, at least one open-cell structure made of a metal or an electrically conductive ceramic material, or one with a metal on the surfaces of an open-cell structure made of a ceramic material, can be used. The respective metal or a corresponding alloy should have a melting temperature that is above the temperature to be reached during heating of the respective open-cell structure in the intervals. Particularly suitable metals are FeCrAl, NiCrAl, nickel-based, or cobalt-based alloys.

[0015] For example, SiC can be used as an electrically conductive ceramic, as it contains a carbon content that places the SiC material above the percolation threshold. To influence electrical conductivity and other parameters, a silicon nitride material can also be modified with molybdenum silicide. A mixture or a composite material can be used.

[0016] An open-cell structure can also be formed with a ceramic material coated on its surfaces with an electrically conductive material, particularly a metal or metal alloy. The surface coating should be designed in such a way that sufficient and as homogeneous as possible heating can be achieved by electrical current.

[0017] The coating can be carried out using a PVD or CVD process, but also by coating with a suspension containing metal particles, whereby the organic components of the suspension are decomposed (pyrolyzed) during a thermal treatment after the coating and the metal particles are then sintered together.

[0018] First and second intervals can be repeated several times in succession, alternating between one another. However, it is also possible to pass the exhaust gas flow alternately through several open-cell structures connected in parallel, so that a first interval is carried out in at least one open-cell structure and, at the same time, a second interval is carried out in at least one second open-cell structure connected in parallel. After a predeterminable operating time has elapsed, a change of operating mode can then be initiated in the open-cell structures, so that after a first interval has been carried out, a second interval is carried out with one open-cell structure and a first interval is carried out with the other open-cell structure after a second interval has been carried out.

[0019] Alternatively, exhaust gas released during the execution of a second interval, which contains hydrocarbon compounds with an increased concentration after pyrolysis, can also be fed to a buffer container and stored therein before it is fed to the at least one open-cell structure for the execution of a first interval after the completion of the respective second interval.

[0020] In this way, at least a nearly continuous process can be achieved.

[0021] The predetermined operating time selected for carrying out the first intervals can be adjusted after at least a first and a second interval have been carried out and can preferably be shortened. This means that it can be taken into account that when the second intervals are carried out, residual carbon deposits on a respective open-cell structure that have not been fully oxidized, thus reducing the effectiveness of the first and possibly also second intervals. If the pressure difference between the pressures P1 and P2 is determined, the length of the time intervals in which the first and possibly also the second intervals are carried out can be adjusted accordingly, depending on the determined pressure difference, based solely on this pressure difference determination. Analogously, only the pressures P1 in the flow direction upstream of an open-cell structure can be taken into account accordingly.

[0022] The open porosity of the open-cell structures used in the invention should be at least 50% when heated electrically by resistance. When heated by induction, the open porosity should be greater than 10%.

[0023] The exhaust gas flow can be guided through at least one open-cell structure which has a decreasing porosity or pore size or a reduction of the voids between filaments of a textile structure in the flow direction of the exhaust gas flow.

[0024] Alternatively, several open-cell structures can be arranged in the flow direction of the exhaust gas flow and flowed through by the exhaust gas flow, the porosity and / or pore size of which or the cavities between filaments of which are successively reduced in the flow direction of the exhaust gas flow.

[0025] This can result in a smaller specific surface area of ​​material of the respective open-cell structure in the front areas in the direction of flow of the exhaust gas stream, while simultaneously providing larger cavities, than in the downstream areas in the direction of flow of the exhaust gas stream. There is more space available for deposited carbon to accumulate on the surface, as experience has shown that a larger amount of carbon accumulates there during thermal decomposition, which could otherwise lead to premature clogging of the open-cell structure. Consequently, significantly shorter times for the first and possibly second intervals would have to be accepted.

[0026] Advantageously, carbon oxide(s) formed during the second intervals can be used for further processing, preferably separately after separation as carbon monoxide and carbon dioxide. Separation of carbon monoxide and carbon dioxide can be achieved with suitable process control during the second intervals, provided the conditions of the Boudouard equilibrium can be taken into account.

[0027] The implementation of a method according to the invention can be influenced by an electronic control and regulation device. In particular, the respective composition of the exhaust gas to be thermally treated, i.e., which hydrocarbon compounds are contained in which quantity, its volume flow, the formation of open-cell structures (material, dimensions, porosity, number of operating cycles already performed in which first and second intervals have already been carried out), the pressures P1 and P2, the temperature or the temperature distribution of the exhaust gas stream or at the at least one open-cell structure, and / or the electrical resistance at the respective open-cell structure can be taken into account.

[0028] Depending on the situation, the power used to heat the at least one open-cell structure and the respective length of time with which the first and second intervals are carried out can be influenced, for example, by controlling or regulating the respective electrical voltage source and the valves V1 and V2.

[0029] The invention will be explained in more detail below by way of example.

[0030] Showing: Fig. 1 in schematic form the process of post-treatment of an exhaust gas stream of a pyrolysis reaction and Fig. 2 a sectional view through an example of a device in which an exhaust gas stream from a pyrolysis reaction is passed through a heatable open-cell structure in order to thermally decompose hydrocarbon compounds contained in the exhaust gas stream and convert them into more harmless chemical compounds.

[0031] How to Fig. 1, a pyrolysis reaction can be carried out in a furnace 7. The exhaust gas released thereby passes through a line and through an open-cell structure 3 by means of a compressor 8 connected to the suction side into the environment, after which a thermal post-treatment has been carried out with the open-cell structure 3.

[0032] In the line between the furnace 7 and the open-cell structure 3, a valve V1 is arranged, which is open during the execution of a first interval and closed during the execution of a second interval.

[0033] In the area of ​​the line in which valve V1 is located, another line 9 is connected between valve V1 and open-cell structure 3. This line 9 can be opened and closed with valve V2 in exactly the opposite direction to valve V1. Oxygen or an oxygen-containing gas mixture is supplied to open-cell structure 3 through this line 9 during the execution of second intervals in order to oxidize the carbon deposited on the surfaces of open-cell structure 3 during the execution of the first intervals.

[0034] Before the exhaust gas stream 5 enters the open-cell structure 3, a pressure P1 exists which is greater than the pressure P2 prevailing after flowing through the open-cell structure 3. As already explained in the general description, the pressure difference P1 to P2 can be used to influence the process with regard to the initiation and termination of the execution of first and second intervals. For this purpose, the execution of first intervals can be terminated when this pressure difference has exceeded a predetermined larger threshold value and then the initiation of the execution of a second interval can be initiated. The execution of second intervals can be terminated when the pressure difference has reached a smaller predetermined threshold value and then a first interval can be carried out again.

[0035] In the case of essentially constant conditions, ie in particular with the same operation of the compressor 8, instead of the pressure difference, the determination and consideration of the pressure P1 alone may be sufficient to influence the process control in such a way that a larger threshold value for P1 is used to terminate the first intervals and a smaller threshold value for P1 is used to terminate the second intervals by opening or closing the valves V1 and V2 accordingly.

[0036] In the schematic representation of Fig. 2 shows how an open-cell structure 3, in this case a structure of an open-pore metal body, is arranged in a housing 1 and how an exhaust gas stream 5 can flow through the open-cell structure 3. In the flow direction of the exhaust gas stream 5, pressure P1 acts upstream of the open-cell structure 3, and pressure P2 acts downstream.

[0037] The open-cell structure 3 is connected to an electrical voltage source 4 so that it can be resistance heated with electrical current to achieve the required and appropriate temperature for performing the first and second intervals. Electrical and preferably also thermal insulation 2 is arranged between the housing 1 and the open-cell structure 3.

[0038] The housing 1 can be additionally cooled.

Claims

[1] Process for the aftertreatment of an exhaust gas from a pyrolysis reaction, in which hydrocarbon compounds contained in the respective exhaust gas stream (5) are thermally decomposed, by guiding an exhaust gas stream (5) released during a pyrolysis within a first interval through at least one open-cell structure (3) by means of a pressure difference between a device in which the respective pyrolysis is carried out and an exhaust gas discharge connected to the environment, and in the process a temperature of at least 600 °C is maintained at the at least one open-cell structure (3), so that at least some of the hydrocarbon compounds contained in the exhaust gas stream (5) are thermally decomposed and the carbon obtained is deposited on surfaces of the at least one open-cell structure (3) and after expiry of a predeterminable operating time and / or the reaching of a predeterminable threshold value of a pressure difference of a pressure P1, which is determined before the exhaust gas flow (5) enters the at least one open-cell structure (3) and a pressure P2 in the flow direction after the exit from the at least one open-cell structure (3) or a predeterminable threshold value of the pressure P1, which is determined before the exhaust gas flow (5) enters the at least one open-cell structure (3) and of a pressure P2 in the flow direction after exiting the at least one open-cell structure (3), the supply of the exhaust gas stream (5) to the at least one open-cell structure (3) is stopped and then, in a second interval, instead of the exhaust gas stream (5), oxygen or an oxygen-containing gas mixture is passed through the at least one open-cell structure (3) and, at a temperature of at least 600 °C, the carbon deposited on surfaces of the at least one open-cell structure (3) is oxidized and the oxide(s) formed and exhaust gas residues are removed. [2] Method according to claim 1, characterized by that an open-porous body or a textile structure formed with filaments, which is formed from or with an electrically conductive material, is used as at least one open-cell structure (3). [3] Method according to one of the preceding claims, characterized bythat the open-cell structure (3) is connected to an electrical voltage source (4) for carrying out electrical resistance heating and that the temperature of at least 600 °C is maintained by means of electrical resistance heating or by means of an electrical alternating voltage source and at least one electrical coil inductively heated to the temperature of at least 600 °C. [4] Method according to one of the preceding claims, characterized by that the first and second intervals are repeated alternately one after the other. [5] Method according to one of claims 1 to 3, characterized by that during the implementation of a first interval, the exhaust gas flow (5) is carried out through a first open-cell structure (3) and, in parallel thereto, a second interval is carried out by means of a second open-cell structure (3) which is connected and operated in parallel to the first open-cell structure (3). [6] Method according to one of the preceding claims, characterized by that an open-cell structure (3) consisting of a metal, which consists of an electrically conductive ceramic material or an open-cell structure (3) with a metal coating is used on surfaces of an open-cell structure (3) made of a ceramic material. [7] Method according to one of the preceding claims, characterized by that the predetermined operating time selected for the execution of first intervals is adjusted, preferably shortened, after the execution of at least a first and second interval. [8] Method according to one of the preceding claims, characterized bythat exhaust gas released during the execution of a second interval is fed to a buffer container and stored therein before it is fed to the at least one open-cell structure (3) after the completion of the execution of a respective second interval for the execution of a subsequent first interval. [9] Method according to one of the preceding claims, characterized by that the exhaust gas flow is guided through at least one open-cell structure which has a decreasing porosity or pore size or a reduction in the voids between filaments of a textile structure in the flow direction of the exhaust gas flow, or several open-cell structures (3) are arranged in the flow direction of the exhaust gas flow and are flowed through by the exhaust gas flow, the porosity and / or pore size or the voids between filaments of which are successively reduced in the flow direction of the exhaust gas flow. [10] Method according to one of the preceding claims, characterized by that carbon oxide(s) formed during the implementation of second intervals are used for further purposes, preferably separately after separation as carbon monoxide and carbon dioxide.

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