Exhaust stack, exhaust stack assembly, and gas carburizing furnace

Abstract

The present invention provides an exhaust stack for a gas carburizing furnace capable of improving the conversion rate, which indicates the efficiency of soot deposition, and a gas carburizing furnace using the same. [Solution] The exhaust stack 1 is for discharging exhaust gas EG discharged from the gas carburizing furnace 2. The exhaust stack 1 has a carbon recovery unit 10 that reacts CO and H2 contained in the exhaust gas EG to produce solid carbon and H2O, and recovers the solid carbon produced. The exhaust stack 1 is located downstream of the carbon recovery unit 10 and has a downstream moisture recovery unit 12 that recovers moisture contained in the exhaust gas EG after it has passed through the carbon recovery unit 10. A portion of the exhaust gas EG after it has passed through the downstream moisture recovery unit 12 is configured to be recirculated to the carbon recovery unit 10.

Description

[Technical Field]

[0001] This invention relates to an exhaust stack, an exhaust stack group, and a gas carburizing furnace. [Background technology]

[0002] Conventionally, gas carburizing is a process in which steel parts such as automotive gears are heated to a high temperature (around 950°C), allowing carbon (C) to penetrate from the surface, and then quenched in oil to induce martensitic transformation, thereby improving wear resistance and fatigue strength. A gas carburizing furnace is used during this gas carburizing process.

[0003] Generally, a modified gas, which has been modified in a transformation furnace, is fed into a gas carburizing furnace. For example, the modified gas is produced by introducing a mixture of hydrocarbons (e.g., propane, butane, city gas, etc.) and air into a transformation furnace filled with a catalyst such as a nickel catalyst heated to a high temperature (around 1050°C), causing an endothermic reaction to transform it into a gas mainly composed of CO, H2, and N2. In addition to the modified gas, enrichment gases such as propane and butane are sometimes added to the gas carburizing furnace in amounts of a few percent to adjust the equilibrium carbon concentration (CP).

[0004] It should be noted that while Patent Document 1 is not a technology related to gas carburizing furnaces, it describes a technology for recovering carbon dioxide in exhaust gas emitted from fuel cells by using hydrogen contained in the exhaust gas and controlling the temperature to precipitate it as carbon. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 4227775 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In the field of gas carburizing furnaces, the exhaust gas after gas carburizing is discharged through an exhaust stack connected to the furnace. At this time, the exhaust gas flowing from the furnace into the exhaust stack contains CO, H2, and N2 due to the transformation of the gas. Of these, CO cannot be released directly into the external environment. Therefore, currently, from a safety and environmental perspective, the exhaust gas is burned at the outlet of the exhaust stack to convert CO to CO2, which is then released into the external environment. In other words, conventional gas carburizing furnaces are forced to emit a large amount of CO2 from the exhaust stack due to the atmospheric gas essential for carburizing.

[0007] In view of the above, the applicants have developed an exhaust stack that has a carbon recovery section that reacts CO and H2 contained in the exhaust gas to produce solid carbon and H2O, and recovers the solid carbon produced, and has the technology to reduce CO2 emissions by applying this to a gas carburizing furnace.

[0008] However, simply installing a carbon recovery unit in the exhaust stack will only result in a plateau in the conversion rate, which indicates the efficiency of soot deposition, leaving room for improvement.

[0009] This invention has been made in view of the above problems, and aims to provide an exhaust stack, exhaust stack group, and gas carburizing furnace for a gas carburizing furnace that can improve the conversion rate, which indicates the efficiency of soot deposition. [Means for solving the problem]

[0010] The exhaust stack, exhaust stack group, and gas carburizing furnace according to the present invention are as follows.

[0011] [1] An exhaust stack for discharging exhaust gas from a gas carburizing furnace, A carbon recovery unit that reacts CO and H2 contained in the exhaust gas to produce solid carbon and H2O, and recovers at least the solid carbon produced, The system includes at least one of the following: an upstream moisture recovery unit provided upstream of the carbon recovery unit to recover moisture contained in the exhaust gas before it flows into the carbon recovery unit, and a downstream moisture recovery unit provided downstream of the carbon recovery unit to recover moisture contained in the exhaust gas after it has passed through the carbon recovery unit. When the downstream water recovery unit is included, The exhaust gas, after passing through the downstream moisture recovery section, is configured to be recirculated to the carbon recovery section, or The exhaust gas, after passing through the downstream moisture recovery section, is configured to be sent to the carbon recovery section located in a second exhaust stack separate from the aforementioned exhaust stack. Exhaust pipe. [2] The downstream water recovery unit is, The system is configured to condense and recover moisture contained in the exhaust gas after it has passed through the carbon recovery unit, or The system is configured to recover moisture contained in the exhaust gas after it has passed through the carbon recovery section by adsorbing it onto an adsorbent. [1] The exhaust pipe described above. [3] The carbon recovery unit described above is configured to maintain a temperature between 450°C and 700°C. The exhaust pipe as described in [1] or [2]. [4] The system is configured to allow heat exchange between the exhaust gas before it flows through the carbon recovery section and into the downstream moisture recovery section, and a portion of the exhaust gas that passes through the downstream moisture recovery section and is circulated back to the carbon recovery section. The exhaust pipe described in any one of [1] through [3]. [5] A group of exhaust pipes, each consisting of multiple exhaust pipes described in any one of [1] through [4]. [6] A gas carburizing furnace having an exhaust stack described in any one of [1] to [4], or a group of exhaust stacks described in [5]. [Effects of the Invention]

[0012] The exhaust stack according to the above [1] has the above configuration. In the exhaust stack according to the above [1], when an upstream-side water recovery unit is provided on the upstream side of the carbon recovery unit, water contained in the exhaust gas before flowing into the carbon recovery unit can be recovered. Then, the exhaust gas after this water is recovered is supplied to the carbon recovery unit. Further, in the exhaust stack according to the above [1], when a downstream-side water recovery unit is provided on the downstream side of the carbon recovery unit, water contained in the exhaust gas after passing through the carbon recovery unit can be recovered. Then, part of the exhaust gas after this water is recovered is circulated to the carbon recovery unit, or the exhaust gas after the water is recovered is sent to a carbon recovery unit included in another second exhaust stack. Therefore, according to the exhaust stack according to the above [1], it is possible to improve the conversion rate indicating the soot precipitation efficiency (hereinafter, may be simply referred to as conversion rate. Details of the conversion rate referred to in the present specification will be described later.) as compared with an exhaust stack having only a carbon recovery unit.

[0013] In the exhaust stack according to the above [2], water contained in the exhaust gas can be recovered by dew condensation or adsorption in the downstream-side water recovery unit. Therefore, according to the exhaust stack according to the above [2], the above-described effect of improving the conversion rate can be ensured.

[0014] In the exhaust stack according to the above [3], the carbon recovery unit is configured to have a temperature of 450°C or higher and 700°C or lower. Therefore, in the exhaust stack according to the above [3], the reaction of converting CO into solid carbon easily occurs in the carbon recovery unit, and CO can be efficiently fixed as solid carbon. Therefore, according to the exhaust stack according to the above [3], in addition to the effects of the exhaust stack according to the above [1] or the above [2], it becomes easier to suppress CO₂ emissions.

[0015] In the exhaust stack according to the above [4], heat exchange can be performed between the exhaust gas that has passed through the carbon recovery unit and before flowing into the downstream-side water recovery unit, and a part of the exhaust gas that has passed through the downstream-side water recovery unit and is circulated to the carbon recovery unit. Therefore, according to the exhaust stack described in the above [4], in addition to the effects of the exhaust stack described in the above [1] to the above [3], energy loss is further reduced and energy saving can be achieved.

[0016] In the exhaust stack group according to the above [5], a plurality of the exhaust stacks according to any one of the above [1] to the above [4] are connected. Therefore, the exhaust stack group described in the above [5] can achieve a higher conversion rate than a single exhaust stack.

[0017] The gas carburizing furnace according to the above [6] includes the exhaust stack according to any one of the above [1] to the above [4], or the exhaust stack group according to the above [5]. Therefore, according to the gas carburizing furnace described in the above [6], it is possible to obtain a gas carburizing furnace capable of improving the conversion rate that indicates the soot precipitation efficiency. Brief Description of the Drawings

[0018] [Figure 1] Figure 1 is an explanatory view schematically showing the exhaust stack of Embodiment 1 and a gas carburizing furnace using the same. [Figure 2] Figure 2 is an explanatory view schematically showing the exhaust stack of Embodiment 2 and a gas carburizing furnace using the same. [Figure 3] Figure 3 is an explanatory view schematically showing the exhaust stack of Embodiment 3 and a gas carburizing furnace using the same. [Figure 4] This is a diagram showing the internal temperature (°C), which is the temperature at the central part of the catalyst layer, and the changes in the gas composition (CO concentration, CO₂ concentration, CH₄ concentration, and H₂ concentration) (volume%) of the test gas before and after the reaction test, as well as the conversion rate, obtained in Test 2 of the experimental example. [Figure 5] This is a diagram showing the changes in the gas composition (CO concentration, CO₂ concentration, CH₄ concentration, and H₂ concentration) (volume%) of the test gas before the reaction test, in the reference example, and in the example, as well as the conversion rate, obtained in Test 2 of the experimental example. [Modes for carrying out the invention]

[0019] The exhaust stack, exhaust stack group, and gas carburizing furnace according to the present invention will be described in detail below using embodiments. However, the exhaust stack, exhaust stack group, and gas carburizing furnace according to the present invention are not limited to the examples given in the following embodiments. Furthermore, the lower and upper limits of the numerical ranges shown below can be arbitrarily combined, including the values ​​described in the experimental examples (details omitted below).

[0020] (Embodiment 1) The exhaust stack and gas carburizing furnace of Embodiment 1 will be described with reference to Figure 1.

[0021] As illustrated in Figure 1, the gas carburizing furnace 2 of this embodiment has an exhaust pipe 1 of this embodiment, which will be described later. The gas carburizing furnace 2 is a furnace that can be used not only for gas carburizing but also for gas carbonitriding. The gas carburizing furnace 2 is configured so that modified gas MG is supplied into the furnace from a modification furnace (not shown). The modification furnace is configured to be able to introduce a mixed gas of hydrocarbons such as propane, butane, or city gas and air, and the furnace is filled with a catalyst such as a nickel catalyst. The modification furnace can cause an endothermic reaction by heating the catalyst to a high temperature (for example, about 1050°C) and modify the mixed gas into modified gas MG containing CO, H2, and N2. In this embodiment, the modified gas MG contains N2, but the modified gas MG does not have to contain N2.

[0022] In this embodiment, a modified gas MG containing CO, H2, and N2 is introduced into the gas carburizing furnace 2. In addition to the modified gas MG, the gas carburizing furnace 2 may also be enriched with a few percent of enrichment gas RG such as propane or butane to adjust the equilibrium carbon concentration (CP). By maintaining the gas carburizing furnace 2 at a predetermined temperature and for an appropriate time under a gas atmosphere with an adjusted equilibrium carbon concentration (CP), carburizing can be performed on steel parts W such as automotive gears. The exhaust gas EG generated in the furnace has a temperature of approximately 900°C and contains CO, H2, and N2 due to the composition of the modified gas MG.

[0023] As illustrated in Figure 1, the exhaust stack 1 of this embodiment is for discharging exhaust gas EG discharged from the gas carburizing furnace 2. In other words, the exhaust stack 1 can also be described as an exhaust gas flow path member for discharging exhaust gas EG.

[0024] The exhaust stack 1 is cylindrical, and one end of the gas inlet 1i is configured to be connectable to the exhaust gas outlet 20 of the gas carburizing furnace 2. In this embodiment, the exhaust gas EG that has flowed through the inside of the exhaust stack 1 is discharged to the outside from the gas outlet 1o at the other end.

[0025] The exhaust pipe 1 has a carbon recovery unit 10. In this embodiment, the exhaust pipe 1 has a carbon recovery unit 10 at some intermediate point between the gas inlet 1i and the gas outlet 1o of the exhaust pipe 1. The carbon recovery unit 10 is configured to react CO and H2 contained in the exhaust gas EG to form solid carbon (C(s)) and H2O, and to recover at least the solid carbon 11 produced. That is, the exhaust pipe 1 can fix the CO in the exhaust gas EG as solid carbon 11 by causing at least the reaction CO + H2 → C(s) + H2O in the carbon recovery unit 10. Note that the reaction in the carbon recovery unit 10 is not necessarily limited to the above reaction, and for example, a reaction in which CO and CO react to form solid carbon 11 and CO2 may occur. In other words, the carbon recovery unit 10 can recover not only the solid carbon 11 produced by the reaction CO + H2 → C(s) + H2O, but also solid carbon 11 produced by other reactions. Furthermore, the carbon recovery unit 10 can recover solid carbon 11 as soot, which is an aggregate of solid carbon 11.

[0026] In the exhaust stack 1 illustrated in Figure 1, the carbon recovery unit 10 is located in the section where the exhaust gas EG discharged from the gas carburizing furnace 2 flows from bottom to top within the exhaust stack 1. More specifically, in Figure 1, the first cylindrical section 1a through which the exhaust gas EG discharged from the gas carburizing furnace 2 flows is connected to the lower end side of the carbon recovery unit 10. In addition, one end of the second cylindrical section 1b through which the exhaust gas EG that flows from bottom to top within the carbon recovery unit 10 and is discharged from the carbon recovery unit 10 flows is connected to the upper end of the carbon recovery unit 10.

[0027] In this embodiment, the carbon recovery unit 10 may have a catalyst layer 101 from the viewpoint of promoting the solid carbonization of CO. The catalyst layer 101 can be fixed at a desired position within the carbon recovery unit 10 by a holding member (not shown), such as a retaining member. Furthermore, in the exhaust pipe 1, the catalyst layer 101 may include a catalyst having a shape such as fibrous or particulate, from the viewpoint of ensuring gas permeability.

[0028] As the catalyst constituting the catalyst layer 101, iron-based catalysts, cobalt-based catalysts, molybdenum-based catalysts, etc., can be suitably used. Here, an iron-based catalyst is a catalyst composed of a material containing 50% by mass or more of iron, or a material in which that material is supported on a carrier such as ceramics. A cobalt-based catalyst is a catalyst composed of a material containing 50% by mass or more of cobalt, or a material in which that material is supported on a carrier such as ceramics. A molybdenum-based catalyst is a catalyst composed of a material containing 50% by mass or more of molybdenum, or a material in which that material is supported on a carrier such as ceramics. These can be used individually or in combination of two or more. Specifically, examples of catalysts include steel wool, iron particles, Co particles, Mo particles, or at least one of these metal particles supported on a carrier such as ceramic particles. These can be used individually or in combination of two or more. Of these, steel wool can be suitably used from the viewpoint of catalytic activity, gas permeability, etc.

[0029] The carbon recovery unit 10 may have a carbon emission mechanism 102 for recovering solid carbon 11 from the catalyst layer 101 from which solid carbon 11 has been recovered and for discharging the solid carbon 11 to the outside. Specifically, the carbon emission mechanism 102 can have a configuration that includes, for example as illustrated in Figure 1, a carbon storage section 102a located below the carbon recovery section 10 for temporarily storing solid carbon 11 that has fallen from the catalyst layer 101, a carbon emission section 102c located below the carbon storage section 102a for sending the solid carbon 11 flowing in from the carbon storage section 102a downwards, a carbon receiving section 102e located below the carbon emission section 102c for receiving the solid carbon 11 emitted from the carbon emission section 102c, an upper carbon shutter section 102b that can be opened and closed to partition the space between the carbon storage section 102a and the carbon emission section 102c, and a lower carbon shutter section 102d that can be opened and closed to partition the space between the carbon emission section 102c and the carbon receiving section 102e.

[0030] In this configuration, solid carbon 11 that has detached from the catalyst layer 101 and fallen free within the carbon recovery section 10 is stored in the carbon storage section 102a. When the upper carbon shutter section 102b is opened from a closed state, the solid carbon 11 flows from the carbon storage section 102a to the carbon discharge section 102c. The solid carbon 11 that has flowed into the carbon discharge section 102c is transported to the lower carbon shutter section 102d. In this embodiment, the carbon discharge section 102c has a carbon rotating blade 102f inside, and as the carbon rotating blade 102f rotates, the solid carbon 11 is transported from near the upper carbon shutter section 102b to the lower carbon shutter section 102d. When the lower carbon shutter section 102d is opened from a closed state, the solid carbon 11 falls downward from the carbon discharge section 102c and is received by the carbon receiving section 102e. The upper carbon shutter section 102b and the lower carbon shutter section 102d may be configured to be opened and closed manually or automatically. With this configuration, the solid carbon 11 generated in the carbon recovery section 10 can be separated from the carbon recovery section 10 and discharged to the outside for recovery, using the detachment and downward fall of the solid carbon 11 within the carbon recovery section 10 with a simple configuration. The carbon rotating blade 102f may be configured to function not only as a transporter of solid carbon 11 but also as a carbon shutter that separates the environment between the carbon storage section 102a and the outside of the device. In this case, the upper carbon shutter 102b, etc., can be omitted.

[0031] More specifically, it is preferable that the carbon recovery unit 10 is configured to be at a temperature of 450°C to 700°C. In this case, the exothermic reaction CO + H2 → C(s) + H2O is more likely to occur in the carbon recovery unit 10, allowing CO to be efficiently fixed as solid carbon 11, thus further reducing CO2 emissions. Figure 1 shows an example in which the temperature of the carbon recovery unit 10 can be adjusted by a heating element 103 arranged around the carbon recovery unit 10.

[0032] The temperature of the carbon recovery unit 10 is preferably 450°C or higher, more preferably 500°C or higher, and even more preferably 550°C or higher, in order to promote the above reaction. Furthermore, the temperature of the carbon recovery unit 10 is preferably 700°C or lower, more preferably 680°C or lower, and even more preferably 650°C or lower, in order to further promote the reaction and improve carbon recovery efficiency. Note that the temperature of the carbon recovery unit 10 is the internal temperature of the exhaust pipe 1 in the region of the exhaust pipe 1 where the carbon recovery unit 10 is provided.

[0033] In this embodiment, the exhaust pipe 1 has a downstream moisture recovery section 12. The downstream moisture recovery section 12 is located downstream of the carbon recovery section 12. The downstream moisture recovery section 12 is a section for recovering moisture 14 contained in the exhaust gas EG after it has passed through the carbon recovery section 10. The moisture contained in the exhaust gas EG after it has passed through the carbon recovery section 10 mainly originates from H2O produced by the reaction CO + H2 → C(s) + H2O. In other words, the exhaust gas EG after it has passed through the carbon recovery section 10 contains moisture produced by the above reaction, and the downstream moisture recovery section 12 mainly recovers this moisture.

[0034] In this embodiment, the downstream moisture recovery unit 12 is specifically configured to recover moisture contained in the exhaust gas EG after it has passed through the carbon recovery unit 10 by condensation, as illustrated in Figure 1. For example, a cold trap can be used as such a downstream moisture recovery unit 12. Figure 1 shows an example in which the downstream moisture recovery unit 12 is configured as a cold trap. The configuration of the cold trap is not particularly limited. Here, the downstream moisture recovery unit 12 as a cold trap has a coolant storage unit 121 in which a coolant 121a is stored, and is configured so that moisture condenses inside the downstream moisture recovery unit 12 when the exhaust gas EG containing moisture supplied into the downstream moisture recovery unit 12 comes into contact with the wall surface of the coolant storage unit 121. The downstream recovery unit 12 may also have a structure in which the coolant 121a is cooled by an external coolant cooling unit (not shown) and circulated with the coolant storage unit 121 to maintain a temperature suitable for cooling. In Figure 1, reference numeral 14 denotes water droplets due to condensation.

[0035] The downstream moisture recovery unit 12 may have a moisture discharge mechanism 122 for recovering moisture that has condensed inside the downstream moisture recovery unit 12 and discharging the moisture to the outside. Specifically, the water discharge mechanism 122 can have a configuration that includes, for example as illustrated in Figure 1, a water storage section 122a provided below the downstream water recovery section 12 to temporarily store water droplets 14 that have fallen from the wall surface of the coolant storage section 121, a water discharge section 122c provided below the water storage section 122a to send the water droplets 14 flowing in from the water storage section 122a downward, a water receiving section 122e provided below the water discharge section 122c to receive the water droplets 14 discharged from the water discharge section 122c, an upper water shutter section 122b that can be opened and closed to partition the water storage section 122a and the water discharge section 122c, and a lower water shutter section 122d that can be opened and closed to partition the water discharge section 122c and the water receiving section 122e.

[0036] In this configuration, water droplets 14 that fall free from the wall surface of the coolant storage section 121 within the downstream water recovery section 12 are stored in the water storage section 122a. When the upper water shutter section 122b is opened from a closed state, the water droplets 14 flow from the water storage section 122a to the water discharge section 122c. The water droplets 14 that flow into the water discharge section 122c are transported to the lower water shutter section 122d. In this embodiment, the water discharge section 122c has a rotating water vane 122f inside, and as the rotating water vane 122f rotates, the water droplets 14 are transported from near the upper water shutter section 122b to the lower water shutter section 122d. When the lower water shutter section 122d is opened from a closed state, the water droplets 14 fall downward from the water discharge section 122c and are received by the water receiving section 122e. The upper moisture shutter section 122b and the lower moisture shutter section 122d may be configured to be opened and closed manually or automatically. With this configuration, by utilizing the detachment and downward fall of water droplets 14 within the downstream moisture recovery section 12, the water droplets 14 generated in the downstream moisture recovery section 12 can be separated from the downstream moisture recovery section 12 and discharged to the outside for collection, using a simple configuration.

[0037] In the case of the exhaust pipe 1 of this embodiment, where the exhaust pipe 1 has a downstream moisture recovery section 12, the exhaust pipe 1 can be configured so that a portion of the exhaust gas EG that has passed through the downstream moisture recovery section 12 is recirculated to the carbon recovery section 10.

[0038] In the exhaust stack 1 illustrated in Figure 1, specifically, the other end of the second cylindrical section 1b, through which the exhaust gas EG discharged from the carbon recovery section 10 flows, is connected to the upper end of the downstream moisture recovery section 12. Furthermore, one end of the third cylindrical section 1c, through which the exhaust gas EG discharged from the downstream moisture recovery section 12 flows, is connected to the side surface of the upper end of the downstream moisture recovery section 12. The other end of the third cylindrical section 1c is connected at the branching section 1d to one end of the fourth cylindrical section 1e, through which a portion of the exhaust gas EG discharged from the downstream moisture recovery section 12 flows. The other end of this fourth cylindrical section 1e is connected to an intermediate portion of the first cylindrical section 1a. Furthermore, one end of the fifth cylindrical section 1f, through which the remaining portion of the exhaust gas EG discharged from the downstream moisture recovery section 12 flows and which leads to the gas outlet 1o of the exhaust stack 1, is connected to the branching section 1d. In this embodiment, the other end of the fifth cylindrical section 1f serves as the gas outlet 1o of the exhaust pipe 1, and the exhaust gas EG can be burned at the gas outlet 1o by a burner B or the like.

[0039] In this embodiment, the exhaust pipe 1 has a downstream moisture recovery section 12 located downstream of the carbon recovery section 10, so that moisture contained in the exhaust gas EG after passing through the carbon recovery section 10 can be recovered. Then, a portion of the exhaust gas EG after this moisture has been recovered is recirculated to the carbon recovery section 12. Therefore, with the exhaust pipe 1 of this embodiment, it is possible to improve the conversion rate, which indicates the efficiency of soot deposition, compared to an exhaust pipe having only a carbon recovery section 12.

[0040] In this specification, the conversion rate, which indicates the efficiency of soot precipitation, is defined as follows: The precise conversion rate R, which serves as an indicator for evaluating the carbon recovery efficiency at a given moment, is given by the following equation (1).

[0041]

number

[0042] However, in equation (1), m c投入 : Rate of carbon content in the input gas being introduced into the exhaust stack [mol / min] mc回収 :Rate at which carbon components contained in the input gas become solid carbon in the exhaust stack [mol / min] P in x :Partial pressure of gas component x at the inlet of the exhaust stack [atm] P out x :Partial pressure of gas component x at the outlet of the exhaust stack [atm] V in :Volumetric inflow rate of gas into the exhaust stack [L / min(nor)] V out :Volumetric outflow rate of gas from the exhaust stack [L / min(nor)]

[0043] However, m c回収 and V out are difficult to directly measure experimentally, so a simplified formula R' of the following formula (2) is defined.

[0044]

Equation

[0045] Here, the change ΔV in the volumetric flow rate of the gas before and after passing through the exhaust stack is represented by the following formula (3). V out / V in =ΔV ···(3) From formula (1), formula (2), and formula (3), the following relational expression (4) between the strict conversion rate R and the simplified formula R' is derived. (1-R) / (1-R')=ΔV ···(4) The reactions accompanied by precipitation of solid carbon (symbol C(s)) in the exhaust stack are the following formula (5) and formula (6), which are accompanied by a decrease in the volume of the gas. CO+H2=C(s)+H2O ···(5) 2CO=C(s)+CO2···(6) Other reactions include those shown in equations (7) and (8), but equation (7) does not involve a change in gas volume. Also, equation (8) shows that when carbon is recovered from CH4, the gas volume increases, but within the scope of this disclosure, only a small amount of CH4 is generated in the exhaust stack, and the gas volume decreases. H2 + CO2 = CO + H2O ... (7) CH4=C(s)+2H2···(8) In general, the gas changes in the exhaust stack are considered to be due to a continuous reaction involving a decrease in volume as long as the precipitation of solid carbon continues, thus yielding equation (9). ΔV<1 ···(9) From equations (9) and (4), equation (10) can be derived, and it can be seen that the simplified formula R' is smaller than the exact conversion rate R, indicating that the evaluation is on the safe side. R>R' ···(10) Furthermore, when H2O is removed from the gas by the moisture recovery unit, the volume of the gas corresponding to the removed H2O decreases. Therefore, the simplified formula R' is evaluated on the safe side, and even with the moisture recovery unit, the exact conversion rate R will not be overestimated. Accordingly, in this specification, the simplified formula R' is used as the conversion rate that indicates the efficiency of soot deposition.

[0046] In this embodiment, the downstream moisture recovery unit 12 has been described using an example in which moisture contained in the exhaust gas EG after passing through the carbon recovery unit 10 is recovered by condensation. The downstream moisture recovery unit 12 may also be configured to recover moisture contained in the exhaust gas EG after passing through the carbon recovery unit 10 by adsorption onto an adsorbent, although this is not shown in the figures. Examples of adsorbents used in such a downstream moisture recovery unit 12 include synthetic zeolites. Examples of synthetic zeolites include molecular sieves.

[0047] (Embodiment 2) The exhaust stack and gas carburizing furnace of Embodiment 2 will be described with reference to Figure 2. Note that, among the reference numerals used in Embodiment 2 and later, those that are the same as those used in the previously described embodiments represent the same components as those in the previously described embodiments, unless otherwise specified.

[0048] As illustrated in Figure 2, the exhaust pipe 1 of this embodiment is configured to send the exhaust gas EG, after passing through the downstream moisture recovery section 12, to a carbon recovery section 10 located in a second exhaust pipe 1', which is separate from the exhaust pipe 1.

[0049] In Figure 2, specifically, one end of the third cylindrical section 1c, through which exhaust gas EG discharged from the downstream water recovery section 12 flows, is connected to the upper side of the downstream water recovery section 12. The other end of the third cylindrical section 1c is connected to an intermediate portion of the first cylindrical section 1a, through which exhaust gas EG discharged from another gas carburizing furnace (not shown) flows.

[0050] Furthermore, the gas carburizing furnace 2 of this embodiment has the exhaust stack 1 of this embodiment.

[0051] In this embodiment, the downstream moisture recovery unit 12 can recover moisture contained in the exhaust gas EG after it has passed through the carbon recovery unit 10. The exhaust gas EG, from which moisture has been recovered, is then sent to the carbon recovery unit 10 of another second exhaust pipe 1'. Therefore, according to this embodiment, unreacted CO and H2 can be further reacted in this carbon recovery unit 10. Thus, this embodiment also makes it possible to improve the conversion rate, which indicates the efficiency of soot deposition.

[0052] Other configurations and effects are the same as in Embodiment 1.

[0053] (Embodiment 3) The exhaust stack and gas carburizing furnace of Embodiment 3 will be described with reference to Figure 3.

[0054] As illustrated in Figure 3, the exhaust pipe 1 of this embodiment is configured to allow heat exchange between the exhaust gas EG before it passes through the carbon recovery unit 10 and flows into the downstream moisture recovery unit 12, and a portion of the exhaust gas EG that passes through the downstream moisture recovery unit 12 and is circulated back to the carbon recovery unit 10.

[0055] In Figure 3, specifically, a heat exchange section 15 is provided between the second cylindrical section 1b, through which the exhaust gas EG flows before passing through the carbon recovery section 10 and entering the downstream water recovery section 12, and the fourth cylindrical section 1e, through which a portion of the exhaust gas EG that passes through the downstream water recovery section 12 and is circulated back to the carbon recovery section 10 flows. The heat exchange section 15 is configured to transfer heat received from the exhaust gas EG flowing through the second cylindrical section 1b to the exhaust gas EG flowing through the fourth cylindrical section 1e.

[0056] Furthermore, the gas carburizing furnace 2 of this embodiment has the exhaust stack 1 of this embodiment.

[0057] In this embodiment, heat exchange can be performed between the exhaust gas EG before it passes through the carbon recovery unit 10 and enters the downstream moisture recovery unit 12, and a portion of the exhaust gas EG that passes through the downstream moisture recovery unit 12 and is circulated back to the carbon recovery unit 10. Therefore, according to this embodiment, energy loss is reduced and energy conservation is achieved.

[0058] Other configurations and effects are the same as in Embodiment 1.

[0059] (Embodiment 4) The exhaust stack and gas carburizing furnace of Embodiment 4 will be described with reference to Figure 1.

[0060] The exhaust pipe 1 of this embodiment has the carbon recovery unit 10 described above. Furthermore, the exhaust pipe 1 of this embodiment does not have the downstream moisture recovery unit 12 described above, and instead has an upstream moisture recovery unit (not shown) in its place.

[0061] The upstream moisture recovery section is located upstream of the carbon recovery section 10. The upstream moisture recovery section is a part for recovering moisture contained in the exhaust gas EG before it flows into the carbon recovery section 10. Exhaust gas EG discharged from the gas carburizing furnace 2 may also contain moisture. The upstream moisture recovery section primarily recovers this moisture. For example, a cold trap can be used as the upstream moisture recovery section.

[0062] Furthermore, the gas carburizing furnace 2 of this embodiment has the exhaust stack 1 of this embodiment.

[0063] In this embodiment, the upstream moisture recovery unit can recover moisture contained in the exhaust gas EG before it flows into the carbon recovery unit 10. The exhaust gas EG from which this moisture has been recovered is then supplied to the carbon recovery unit 10. Therefore, this embodiment also makes it possible to improve the conversion rate, which indicates the efficiency of soot deposition.

[0064] Other configurations and effects are the same as in Embodiment 1.

[0065] (Embodiment 5) The exhaust stack group and gas carburizing furnace of Embodiment 5 will be described with reference to Figure 1.

[0066] The exhaust pipe group (not shown) of this embodiment is composed of multiple exhaust pipes 1 of Embodiment 1 connected together. Specifically, the exhaust pipe group of this embodiment can be configured, for example, by connecting multiple exhaust pipes 1 in series or in parallel.

[0067] Furthermore, the gas carburizing furnace 2 of this embodiment has the exhaust stack group of this embodiment.

[0068] This embodiment also makes it possible to improve the conversion rate, which indicates the efficiency of soot deposition. In particular, this embodiment makes it possible to improve the conversion rate compared to the case of a single exhaust pipe 1 alone.

[0069] Other configurations and effects are the same as in Embodiment 1.

[0070] The descriptions of each embodiment described above may be referenced to one another as needed, and each configuration described in each description may be applied individually or in any combination as necessary.

[0071] (Example of experiment) -Test 1- A straight glass tube with an inner diameter of 70 mm was prepared to simulate an exhaust stack for discharging exhaust gas from a gas carburizing furnace. Next, a region was formed in the middle of the glass tube by filling it with 20 g of steel wool to a height of 50 mm as a catalyst. Then, this glass tube was set up so that the axis of the tube was vertical. Next, a test gas simulating exhaust gas was flowed from the lower end opening of the glass tube to the upper end opening. In this experiment 1, a gas mainly composed of CO, H2, and N2 was used as the test gas. The main components mentioned above mean that the total volume percentage of CO, H2, and N2 relative to the total gas is 50 volume% or more. Specifically, the test gas was a mixed gas with the gas composition of CO: 25.1 volume%, H2: 33.0 volume%, CO2: 0.503 volume%, CH4: 0.994 volume%, and the remainder: N2. In this experiment 1, the temperature of the catalyst layer in the center of the glass cylinder, i.e., the internal temperature, was controlled to a predetermined temperature (450°C, 500°C, 550°C, or 600°C) by heating with a heater installed on the outer circumference of the glass cylinder. Next, 2 L / min of test gas was supplied, and the proportion of components in the test gas discharged from the upper opening of the glass cylinder was measured approximately 2 hours after the start of the test gas flow.

[0072] As a result of the above test 1, soot was generated on the steel wool at all internal temperatures. In this experimental example, it is thought that a reaction occurred in which CO and H2 contained in the test gas supplied into the glass cylinder to produce solid carbon C and H2O (CO + H2 → C(s) + H2O), and that the resulting solid carbon was fixed as soot.

[0073] Figure 4 shows the changes in the internal temperature (°C), which is the temperature at the center of the catalyst layer, and the gas composition (CO concentration, CO2 concentration, CH4 concentration, and H2 concentration) (volume %) of the test gas used as simulated exhaust gas, as well as the conversion rate, obtained in Test 1 above. The conversion rate is calculated using the simplified formula R' described above and is expressed as a percentage in the figure.

[0074] As shown in Figure 4, the reference example of Test 1, which simply involves reacting CO and H2 contained in the test gas to produce solid carbon and H2O, and then recovering the resulting solid carbon as soot, showed the highest conversion rate of 26.7% at an internal temperature of 550°C. Therefore, Test 2 was conducted with an internal temperature of 550°C.

[0075] -Test 2- As the test gas, a mixed gas with the following gas composition was prepared: CO: 24.9 vol%, H2: 31.0 vol%, CO2: 0.497 vol%, CH4: 1.01 vol%, and the remainder: N2. In Test 2, the exhaust pipe was made of a stainless steel pipe (SUS316) with an inner diameter of 44.8 mm, and the same amount of steel wool as in Test 1 was packed inside the stainless steel pipe as a catalyst layer to a volume of approximately 0.2 liters. Heating was controlled to reach 550°C in the center of the catalyst layer inside the stainless steel pipe by heating with a heater installed on the outer circumference of the stainless steel pipe, as in Test 1. Next, 0.5 L / min of the test gas was supplied, and the proportion of components in the test gas discharged from the upper opening of the stainless steel pipe was measured approximately 2 hours after the start of the test gas flow. This was used as a reference example in Test 2. In other words, the reference example in this test 2 simply involves reacting CO and H2 contained in the test gas to produce solid carbon and H2O, and then recovering the resulting solid carbon as soot.

[0076] Meanwhile, moisture was collected from the test gas discharged from the upper opening of the stainless steel cylinder by condensation using a cold trap (manufactured by AS ONE, "H2SO5"). The temperature of the cold trap was maintained at 0°C using ice water. Next, the test gas from which this moisture had been collected was pressurized using a circulation pump (not shown) and sent to the lower opening of the stainless steel cylinder, where it was combined with the newly introduced test gas. The combined test gas was then circulated again towards the upper opening of the stainless steel cylinder. The overflow portion of the test gas was diverted to the outlet of the stainless steel cylinder, and the proportion of components in the test gas discharged from the outlet of the stainless steel cylinder was measured. The flow rates of the circulating gas and the overflowed gas were approximately 4.5 L / min and approximately 0.5 L / min, respectively. However, the measurement of the flow rates of the circulating gas and the overflowed gas is not necessarily accurate because the gas composition fluctuates. This will be presented as the example in Test 2. In other words, the example in Test 2 involved reacting CO and H2 contained in the test gas to produce solid carbon and H2O, and recovering the resulting solid carbon as at least soot. Furthermore, after carbon recovery, water contained in the test gas was recovered, and the unreacted CO contained in this test gas was reacted again, and the resulting solid carbon was recovered as at least soot. The results are shown in Figure 5.

[0077] As shown in Figure 5, the example showed a greater decrease in both CO concentration and H2 concentration compared to the test gas before the reaction compared to the reference example. This means that H2, which had not previously been able to contribute to the solid carbon deposition reaction, was now able to contribute to the solid carbon deposition reaction, and the conversion rate of the example improved significantly from 24.0% in the reference example to 54.1% in the example. In comparison to the reference example in Test 1 and the reference example in Test 2, although the flow rate of the test gas was reduced in Test 2 and the contact opportunity between the test gas and the catalyst was increased, no significant difference in the conversion rate was observed between the two. This is because, along with the deposition of solid carbon inside the catalyst layer, H2O is generated inside the catalyst layer, and this generated H2O hinders the solid carbon deposition reaction downstream of the catalyst layer, causing the conversion rate to plateau.

[0078] Based on these results, it can be seen that the conversion rate, which indicates the efficiency of soot deposition, can be improved by recovering carbon from exhaust gas from which moisture (such as moisture produced by the reaction) has been removed.

[0079] Based on the results described above, it can be said that the exhaust stack and gas carburizing furnace specified in this disclosure make it possible to realize an exhaust stack and gas carburizing furnace that can improve the conversion rate, which indicates the efficiency of soot deposition.

[0080] The present invention is not limited to the embodiments and experimental examples described above, and various modifications are possible without departing from the spirit of the invention. Furthermore, the configurations shown in the embodiments and experimental examples described above can be combined in any way. [Explanation of Symbols]

[0081] 1 Exhaust stack 10. Carbon Recovery Unit 11 Solid carbon 12 Downstream water recovery section 14 water drops 2. Gas Carburizing Furnace EG exhaust gas 1' Second exhaust stack

Claims

1. An exhaust stack for discharging exhaust gas from a gas carburizing furnace, CO and H contained in the exhaust gas ２ Reacting with solid carbon and H ２ Let O be a carbon recovery unit that recovers at least the solid carbon generated, The system includes at least one of the following: an upstream moisture recovery unit provided upstream of the carbon recovery unit to recover moisture contained in the exhaust gas before it flows into the carbon recovery unit, and a downstream moisture recovery unit provided downstream of the carbon recovery unit to recover moisture contained in the exhaust gas after it has passed through the carbon recovery unit. When the downstream water recovery unit is included, The exhaust gas, after passing through the downstream moisture recovery section, is configured to be recirculated to the carbon recovery section, or The exhaust gas, after passing through the downstream moisture recovery section, is configured to be sent to the carbon recovery section located in a second exhaust stack separate from the exhaust stack. Exhaust pipe.

2. The downstream water recovery unit is, The system is configured to condense and recover moisture contained in the exhaust gas after it has passed through the carbon recovery unit, or The system is configured to recover moisture contained in the exhaust gas after it has passed through the carbon recovery section by adsorbing it onto an adsorbent. The exhaust pipe according to claim 1.

3. The carbon recovery unit described above is configured to maintain a temperature between 450°C and 700°C. The exhaust pipe according to claim 1.

4. The system is configured to allow heat exchange between the exhaust gas before it flows through the carbon recovery section and into the downstream moisture recovery section, and a portion of the exhaust gas that passes through the downstream moisture recovery section and is circulated back to the carbon recovery section. The exhaust pipe according to claim 1.

5. A group of exhaust pipes, wherein multiple exhaust pipes described in claim 1 are connected.

6. A gas carburizing furnace having an exhaust stack according to any one of claims 1 to 4, or a group of exhaust stacks according to claim 5.

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