Gas treatment method

Abstract

Provided is a method for efficiently decomposing N2O contained in a treatment target gas containing CO2 and N2O.　This method comprises: a step (a) for causing a treatment target gas containing CO2 and N2O to flow through a first adsorption column in which an adsorptive material capable of adsorbing CO2 and N2O is accommodated; a step (b) for measuring the concentration of N2O in a first gas at a position downstream of the adsorptive material in the first adsorption column in the flow direction; and a step (c) for sending the first gas into a second adsorption column in which an adsorptive material capable of adsorbing CO2 and N2O is accommodated. The execution of the step (c) is started in a time zone in which the concentration of N2O in the first gas as measured in the step (b) indicates an increasing tendency.

Description

Gas Treatment Method

[0001] The present invention relates to a gas treatment method, and more particularly to a method for treating a gas containing a mixture of carbon dioxide (CO2) and nitrous oxide (N2O).

[0002] Since the Industrial Revolution, the average temperature of the Earth has been rising, making global warming a pressing issue. Greenhouse gases known to cause global warming include CO2, methane (CH4), N2O, and chlorofluorocarbons. Of these gases, CO2 is the most abundant, followed by CH4, and then N2O.

[0003] On the other hand, when we look at the global warming potential (GWP), it has been reported that the GWP of CH4 is 25 times that of CO2, and that of N2O is 298 times that of CO2. For these reasons, the impact of N2O emissions on global warming cannot be ignored.

[0004] As a method for reducing NO emissions, for example, Patent Document 1 listed below discloses a technology in which exhaust gas containing NO emitted from an engine is irradiated with ultraviolet laser light from an excimer laser device to decompose the NO.

[0005] Japanese Patent Application Laid-Open No. 2021-088964

[0006] NO is emitted during processes such as the management of farm soil and livestock waste, the incineration of waste, and the treatment of wastewater. In particular, because NO is generated by microorganisms, the composition of the emitted gas is mostly atmospheric components, and NO is often present in low concentrations of around several hundred ppm.

[0007] In consideration of irradiating a gas to be treated with ultraviolet light to decompose NO, it is expected that the efficiency of decomposing NO will be extremely low if the concentration of NO contained in the gas to be treated is low. That is, as described in Patent Document 1, when irradiating the gas to be treated with excimer laser light, the irradiation dose of the excimer laser light must be extremely high. This means that the amount of energy required to decompose the NO contained in the gas to be treated, in other words, the amount of power input to the excimer laser light source, increases, which raises concerns about an increase in CO2 emissions due to increased power consumption.

[0008] In other words, even though the treatment is carried out with the aim of decomposing the NO contained in the gas to be treated from the viewpoint of curbing global warming, the treatment may induce an increase in emissions of CO, a greenhouse gas. Therefore, for gas to be treated that contains a low concentration of NO, the method of irradiating the gas with ultraviolet light with the aim of decomposing NO from the viewpoint of preventing global warming cannot be said to be effective.

[0009] One method for efficiently decomposing N2O contained in a gas to be treated at low concentrations is to adsorb N2O from the gas to be treated to increase its concentration, and then decompose the N2O.

[0010] Considering the above-mentioned NO emission sources, the gas to be treated typically contains CO at a higher concentration than NO. One possible method for concentrating NO involves passing the gas to be treated through an adsorbent to temporarily adsorb NO onto the adsorbent, and then desorbing NO from the adsorbent with a certain amount of NO adsorbed. This method allows the concentration of NO contained in the desorbed gas to be higher than that of NO contained in the gas to be treated. Therefore, it appears that photodecomposition of NO by irradiating the desorbed gas with, for example, ultraviolet light may improve the efficiency of NO decomposition compared to direct irradiation of ultraviolet light on the gas to be treated.

[0011] However, as described above, when the gas to be treated is passed through an adsorbent to adsorb the NO contained in the gas, not only NO but also CO is adsorbed by the adsorbent. There is a limit to the amount that can be adsorbed by the adsorbent. As described above, the gas to be treated usually contains a higher concentration of CO than NO. The amount of NO and CO adsorbed by the adsorbent depends on the concentration ratio of NO to CO in the gas to be treated. Therefore, when the gas to be treated is passed through the adsorbent, it is expected that a large amount of CO will be adsorbed by the adsorbent, and it will not be possible to ensure a sufficient amount of NO adsorption.

[0012] In order to ensure the amount of NO adsorption under these circumstances, one possible method would be to prepare an adsorption tower equipped with a large adsorbent and introduce the gas to be treated into the adsorption tower. However, this would require an extremely large device, and the effect relative to the size of the device would be limited, making the introduction of this method unrealistic.

[0013] As described above, even if a method of passing the gas to be treated through an adsorbent is adopted, it is difficult to efficiently increase the concentration of NO contained in the gas to be treated, and as a result, the effect of improving the decomposition efficiency of NO can be said to be extremely limited.

[0014] In view of the above problems, an object of the present invention is to provide a method that enables efficient decomposition of N2O contained in a gas to be treated that contains CO2 and N2O.

[0015] The gas treatment method according to the present invention comprises the steps of: (a) passing a gas to be treated, which contains CO2 and N2O, through a first adsorption tower containing an adsorbent capable of adsorbing CO2 and N2O; (b) measuring the concentration of N2O in the first gas located downstream of the adsorbent in the first adsorption tower in the flow direction; and (c) sending the first gas to a second adsorption tower containing an adsorbent capable of adsorbing CO2 and N2O, wherein step (c) is initiated during a time period when the concentration of N2O in the first gas measured in step (b) shows an increasing trend.

[0016] Since the first adsorption tower contains an adsorbent capable of adsorbing CO2 and N2O, when the gas to be treated containing CO2 and N2O is passed through the first adsorption tower (step (a)), at an initial stage, the CO2 and N2O contained in the gas to be treated are sufficiently adsorbed by the adsorbent, and therefore the gas (first gas) that has passed through the adsorbent of the first adsorption tower has lower concentrations of both CO2 and N2O compared to the gas to be treated.

[0017] As the step (a) is continued, the adsorption performance of the adsorbent gradually decreases. When the adsorption performance of the adsorbent decreases significantly, the concentrations of both CO and N O contained in the gas (first gas) that has passed through the adsorbent increase.

[0018] As a result of intensive research by the present inventors, it has been confirmed that when the adsorbent approaches or reaches its adsorption limit, comparing CO and NO adsorbed by the adsorbent, NO tends to desorb from the adsorbent first. When this desorption occurs, an adsorbable area is secured in the adsorbent, and new CO is adsorbed. In other words, when the concentrations of CO and NO in the first gas are compared, the timing at which the NO concentration begins to increase arrives before the timing at which the CO concentration begins to increase.

[0019] In other words, focusing on the first gas that has passed through the adsorbent in the first adsorption tower, the CO concentration is relatively low while the NO concentration is relatively high during a time period after the NO concentration starts to increase and before the CO concentration increases to a certain level. Therefore, by sending the first gas during this time period to a second adsorption tower separate from the first adsorption tower, the adsorbent housed in the second adsorption tower can adsorb NO while reducing the amount of CO adsorbed.

[0020] As a result, the adsorbent of the second adsorption tower increases the amount of NO adsorbed in the gas to be treated, which contains CO and NO. For example, by desorbing the adsorbed substance from the adsorbent of the second adsorption tower at a timing when the adsorption of NO by the adsorbent of the second adsorption tower has progressed sufficiently, the gas discharged from the adsorbent side of the second adsorption tower by the desorption step will contain a high concentration of NO, and gas containing a high concentration of NO can be produced from the gas to be treated.

[0021] The adsorbent housed in the first adsorption tower and the adsorbent housed in the second adsorption tower are made of adsorbents capable of adsorbing CO2 and NO. One example of such an adsorbent is an adsorbent whose main component is zeolite. Among zeolites, those having a skeletal structure of MOR, HEU, LTA, CHA, Beta, or FAU are particularly preferred, and hydrophilic zeolites with a Si / Al ratio of 1 or more and 8 or less are even more preferred. Another example is an adsorbent whose main component is MOF (Metal Organic Frameworks), CMS (Carbon Molecular Sieve), or the like.

[0022] When carrying out step (c), a NO concentration sensor may be installed, for example, in a flow path connected to the outlet of the first adsorption tower or at a position downstream of the adsorbent within the first adsorption tower, and the measurement results of the sensor may be monitored over time to detect a time period during which the NO concentration of the first gas shows an increasing tendency.

[0023] In the gas treatment method according to the present invention, the source of the gas to be treated containing CO and NO may be any source. For example, the gas to be treated may be gas generated from farm soil, livestock waste management sites, waste incineration plants, wastewater treatment plants, etc.

[0024] The step (c) may be stopped during a time period when the concentration of N2O in the first gas measured in the step (b) shows a decreasing trend.

[0025] As a result of intensive research by the present inventors, it has been confirmed that when the NO adsorption performance of the adsorbent contained in the first adsorption tower reaches its limit, the NO concentration of the first gas that has passed through the adsorbent rises sharply to a value that exceeds the NO concentration of the gas to be treated, and then decreases to a value close to the NO concentration of the gas to be treated. It has also been confirmed that during the time period when the NO concentration of the first gas shows a decreasing trend, the CO concentration of the first gas that has passed through the adsorbent shows a rising trend.

[0026] If the CO2 concentration in the first gas delivered to the second adsorption tower increases, there is a possibility that the NO already adsorbed in the adsorbent contained in the second adsorption tower will be desorbed and the CO2 will be adsorbed. Therefore, by stopping the delivery of the first gas that has passed through the adsorbent in the first adsorption tower to the second adsorption tower during a time period when the NO concentration in the first gas is showing a decreasing trend, in other words, during a time period when the CO2 concentration in the first gas is expected to be showing an increasing trend, the amount of NO adsorbed in the adsorbent in the second adsorption tower can be maintained at a high value.

[0027] In view of the above, it is also possible to monitor the CO2 concentration of the first gas over time, and during the period when this value shows an increasing trend, stop sending the first gas that has passed through the adsorbent in the first adsorption tower to the second adsorption tower.

[0028] That is, the step (b) may include a step of measuring the concentration of CO2 in the first gas, and the step (c) may be stopped when the concentration of CO2 in the first gas measured in the step (b) reaches or exceeds a predetermined first threshold value.

[0029] The gas treatment method includes a step (d1) of desorbing an adsorbed substance adsorbed to the adsorbent in the first adsorption tower, and the step (a) may be stopped during a time period in which the step (c) is stopped, and the step (d1) may be performed during a time period in which the step (a) and the step (c) are stopped.

[0030] This allows the adsorbent in the first adsorption tower to be reused. For example, step (d1) can be performed using a method including at least one of a process of heating the first adsorbent and a process of creating a negative pressure in the first adsorption tower.

[0031] The method includes a step (e) of measuring an index value indicating the adsorption state of a substance in the adsorbent in the second adsorption tower, and a step (d2) of desorbing the adsorbed substance adsorbed to the adsorbent in the second adsorption tower, wherein the step (c) is stopped after the index value reaches a predetermined second threshold value or more, and the step (d2) may be started after the step (c) is stopped.

[0032] The point at which the index value reaches or exceeds the predetermined second threshold corresponds to the point at which it is assumed that the adsorbent in the second adsorption tower has sufficiently adsorbed NO. Therefore, even if the first gas is fed into the second adsorption tower for a longer period of time, it is unlikely that the amount of NO adsorbed by the adsorbent in the second adsorption tower will increase significantly, and it is expected that the NO in the first gas will pass through the second adsorption tower unchanged. Therefore, by stopping step (c) of feeding the first gas into the second adsorption tower when the index value reaches or exceeds the predetermined second threshold, and then performing step (d2) of desorbing the adsorbed substance adsorbed by the adsorbent in the second adsorption tower, the gas discharged from the adsorbent side of the second adsorption tower will contain a high concentration of NO, and a gas containing a high concentration of NO can be obtained.

[0033] The index value may be any one of the values ​​of the NO concentration of the second gas located downstream of the adsorbent in the second adsorption tower in terms of the flow direction, the CO concentration of the second gas, and the weight of the second adsorption tower.

[0034] That is, the step (e) can be any one of the steps of measuring the concentration of NO in the gas that has passed through the adsorbent in the second adsorption tower, measuring the concentration of CO in the gas that has passed through the adsorbent in the second adsorption tower, and measuring the weight of the adsorbent in the second adsorption tower. Note that the step (e) can be a combination of two or more of the steps listed above.

[0035] The gas treatment method may include a step (f) of decomposing N2O contained in the second gas obtained after the start of the step (d2).

[0036] This allows NO, which has a high GWP, to be converted into N or O, which are not greenhouse gases. The step (f) can be performed by irradiating the second gas with ultraviolet light or by passing the second gas through a plasma generation region. That is, the step (f) may be performed by feeding the second gas obtained after the start of the step (d2) into a decomposition space formed by at least one of an ultraviolet light irradiation region and a plasma generation region.

[0037] Two or more second adsorption towers may be arranged in a state where they are connected to each other. In this case, the gas treatment method may include a step (g) of feeding the second gas obtained after the start of step (d2) in any of the second adsorption towers other than the second adsorption tower located furthest downstream in the gas flow direction to a subsequent second adsorption tower, and the step (f) may be a step of decomposing NO contained in the second gas obtained after the start of step (d2) in the second adsorption tower located furthest downstream in the gas flow direction.

[0038] By adopting the above-described method, the adsorbent housed in the second adsorption tower located at the most downstream side is brought into a state in which the adsorbent has a relatively higher NO adsorption amount than the other adsorbents, and therefore, by carrying out the step (d2) of desorbing the substance adsorbed by the adsorbent housed in the second adsorption tower located at the most downstream side, the second gas obtained by the desorption has an extremely high NO concentration.

[0039] The gas treatment method may include a step (h) of operating a decomposition device that generates at least one of the ultraviolet rays and the plasma during the execution of the step (f), and after the index value reaches a predetermined third threshold value that is smaller than the second threshold value, the execution of the steps (d2), (f), and (h) for the second adsorption tower located furthest downstream in the flow direction may be stopped.

[0040] The point in time when the index value reaches or is less than a predetermined third threshold value, which is smaller than the second threshold value, corresponds to the point in time when it is assumed that a certain amount of the substance adsorbed by the adsorbent in the second adsorption tower has been desorbed due to the continuous execution of step (d2) of desorbing the substance adsorbed by the adsorbent. In other words, after this point in time, it is assumed that the NO concentration in the second gas flowing downstream from the adsorbent in the second adsorption tower has decreased to a certain extent. Therefore, even if step (f) of decomposing NO contained in the second gas is executed after this point in time, high decomposition efficiency may not be achieved.

[0041] Therefore, after the index value reaches or exceeds the third threshold, the desorption step (d2) for the second adsorption tower located furthest downstream in the flow direction, the step (f) of decomposing NO contained in the second gas, and the step (h) of operating a decomposition device for decomposing NO are stopped. This is expected to improve the operating efficiency of the decomposition device and extend the operating life of the device. As the decomposition device, for example, at least one of an excimer lamp using a dielectric barrier discharge and an atmospheric pressure plasma generation device can be used.

[0042] The gas treatment method may also use two or more first adsorption towers. In this case, the step (a) may include a step (a1) of passing the gas to be treated through one of the first adsorption towers, and a step (a2) of passing the gas to be treated through a first adsorption tower other than the first adsorption tower through which the gas to be treated was passed in the step (a1) after the step (a1) is completed, and the step (c) may be a step of feeding the first gas exhausted from the first adsorption tower through which the gas to be treated was passed in the step (a1) and the first gas exhausted from the first adsorption tower through which the gas to be treated was passed in the step (a2) into the same second adsorption tower.

[0043] According to the above method, after the NO adsorption capacity of the adsorbent housed in one first adsorption tower reaches its limit, NO in the gas to be treated can be adsorbed using the adsorbent housed in another first adsorption tower. Furthermore, the second adsorption tower is connected to multiple first adsorption towers. Therefore, the first gas passing through multiple adsorbents installed in each first adsorption tower can be sent to the second adsorption tower, thereby increasing the NO concentration of the first gas supplied to the second adsorption tower compared to when a single first adsorption tower is connected to the second adsorption tower. Therefore, as described above, when NO adsorption by the adsorbent in the second adsorption tower has progressed sufficiently, the gas (second gas) obtained by desorbing the adsorbed substance from the adsorbent in the second adsorption tower contains a high concentration of NO. In other words, according to the above method, the rate at which a gas containing a high concentration of NO is produced from the gas to be treated is increased.

[0044] According to the present invention, it is possible to efficiently decompose N2O contained in a gas to be treated that contains CO2 and N2O.

[0045] 1 is a block diagram schematically showing an example of a gas processing system capable of executing an embodiment of a gas processing method according to the present invention. FIG. 2 is a flowchart schematically showing an execution procedure of an embodiment of a gas processing method according to the present invention. FIG. 3 is a flowchart schematically showing an execution procedure of an embodiment of a gas processing method according to the present invention. FIG. 4 is a graph showing changes over time in the NO concentration and CO concentration in a gas obtained after a gas to be processed containing NO and CO is passed through an adsorbent. FIG. 5 is a cross-sectional view schematically showing an example of the configuration of a decomposition tower. FIG. 6 is a schematic side view of an excimer lamp constituting a decomposition device. FIG. 7 is a schematic cross-sectional view of the excimer lamp shown in FIG. 6 taken along line VII-VII. FIG. 8 is a schematic cross-sectional view of an excimer lamp having a different structure constituting a decomposition device. FIG. 9 is a schematic cross-sectional view of the excimer lamp shown in FIG. 9 taken along line X-X. FIG. 11 is a schematic side view of an atmospheric pressure plasma generation device constituting a decomposition device. FIG. 12 is a schematic cross-sectional view of the excimer lamp shown in FIG. 11 taken along line XII-XII. 1 is a block diagram schematically illustrating another example of a gas processing system; 2 is a block diagram schematically illustrating another example of a gas processing system; 3 is a block diagram schematically illustrating another example of a gas processing system;

[0046] The present invention relates to a gas processing method for increasing the concentration of N2O in a gas to be processed containing CO2 and N2O, to a concentration higher than that of the gas to be processed. Hereinafter, an embodiment of this method will be described with reference to the drawings as appropriate.

[0047] 1 is a block diagram showing a schematic example of a system (hereinafter referred to as a "gas processing system 1") capable of implementing an embodiment of a gas processing method according to the present invention. The gas processing system 1 shown in FIG. 1 includes a gas supply source 3, a first adsorption tower 10, a second adsorption tower 20, and a decomposition tower 30.

[0048] The gas supply source 3 is a supply source of the gas Gt to be treated that contains CO and NO. As an example, the gas supply source 3 is composed of cylinders, piping, etc. that collect gases generated from farm soil, livestock waste management areas, waste management areas, waste incineration plants, wastewater treatment plants, etc.

[0049] The first adsorption tower 10 contains an adsorbent 11 therein and is connected to the gas supply source 3 via a pipe 51. The adsorbent 11 is made of a material capable of adsorbing CO and NO, and examples of the material include zeolite, MOF, and CMS as its main component.

[0050] The second adsorption tower 20 contains an adsorbent 21 therein and is connected to the first adsorption tower 10 via a pipe 52. The adsorbent 21 is made of a material capable of adsorbing CO and NO, and examples of the material include zeolite, MOF, and CMS as its main components.

[0051] The decomposition tower 30 contains a device (referred to as a "decomposition device") capable of generating a decomposition space for decomposing NO, and is connected to the second adsorption tower 20 via a pipe 53. The decomposition device will be described in detail later.

[0052] 2 and 3 are flow charts showing a typical procedure for carrying out the gas processing method of this embodiment. In the following description, step numbers shown in the flow charts of FIGS. 2 and 3 will be referred to as appropriate.

[0053] (Step #1) The gas to be treated Gt is supplied from the gas supply source 3 to the first adsorption tower 10 via the pipe 51. Note that an on-off valve (not shown) may be provided at the connection between the gas supply source 3 and the pipe 51 or in the middle of the pipe 51, and the on-off valve may be controlled to supply the gas to be treated Gt from the gas supply source 3 to the first adsorption tower 10.

[0054] As described above, the first adsorption tower 10 contains the adsorbent 11 capable of adsorbing CO and NO. Therefore, when the gas to be treated Gt is supplied to the first adsorption tower 10, the adsorbent 11 installed in the first adsorption tower 10 starts adsorbing CO and NO contained in the gas to be treated Gt.

[0055] As described above, the first adsorption tower 10 and the second adsorption tower 20 are connected via the pipe 52. When step #1 is performed, the flow of gas through the pipe 52 is blocked. As an example, as shown in FIG. 1 , if an on-off valve 42 is provided in the pipe 52, the on-off valve 42 may be controlled to a closed state.

[0056] The first adsorption tower 10 is connected to a pipe 71 separate from the pipe 52. When step #1 is performed, the gas that has passed through the first adsorption tower 10 can flow through the pipe 71. As shown in FIG. 1 , if an on-off valve 41 is provided in the pipe 71, the on-off valve 41 may be controlled to an open state.

[0057] That is, during the execution of step #1, the gas to be treated Gt passes through the adsorbent 11 in the first adsorption tower 10, and the gas after CO and NO are adsorbed by the adsorbent 11 flows out from the first adsorption tower 10 via the pipe 71. Hereinafter, the gas after the gas to be treated Gt has passed through the adsorbent 11 in the first adsorption tower 10 will be referred to as the "first gas G1."

[0058] This step #1 corresponds to process (a).

[0059] (Step #2) In step #2, the concentration of NO in the first gas G1 is measured. As an example, as shown in Fig. 1 , the concentration of NO in the first gas G1 flowing out from the first adsorption tower 10 is measured by a gas sensor 61. The gas sensor 61 may be installed inside the first adsorption tower 10 and downstream of the adsorbent 11 in the first adsorption tower 10.

[0060] At the initial point when execution of step #1 is started, the NO concentration of the first gas G1 is sufficiently low because the NO contained in the gas to be treated Gt is adsorbed by the adsorbent 11 in the first adsorption tower 10. However, as time passes and the adsorption performance of the adsorbent 11 in the first adsorption tower 10 approaches its limit, the amount of NO contained in the gas to be treated Gt being supplied becomes greater than the amount of NO that can be adsorbed by the adsorbent 11 per unit time, and as a result, the NO concentration of the first gas G1 that has passed through the adsorbent 11 begins to show an increasing trend.

[0061] In step #2, the NO concentration of the first gas G1 is monitored over time by the gas sensor 61. This step #2 corresponds to process (b).

[0062] (Step #3) Step #3 is a step of detecting whether a predetermined first timing has arrived based on the measurement result of the gas sensor 61. If the arrival of the first timing is detected (Yes in step #3), step #4 is executed. If the first timing has not arrived, steps #1 and #2 are executed again.

[0063] The first timing here corresponds to a time period when the N2O concentration of the first gas G1 shows an increasing trend, more preferably, a time period when the N2O concentration of the first gas G1 starts to show an increasing trend.

[0064] (Step #4) When the arrival of the first timing is detected in step #3 (Yes in step #3), the first gas G1 that has passed through the first adsorption tower 10 is sent to the second adsorption tower 20 via the pipe 52. As shown in FIG. 1 , if the on-off valve 42 is provided in the pipe 52, the on-off valve 42 may be controlled to an open state. Note that the on-off valve 41 may be controlled to a closed state at the timing when the on-off valve 42 is controlled to an open state. In this case, the first gas G1 that has passed through the first adsorption tower 10 is controlled to flow into the pipe 52 without flowing into the pipe 71.

[0065] As described above, since the first timing has arrived, the first gas G1 that has passed through the first adsorption tower 10 contains NO that was not completely adsorbed by the adsorbent 11 in the first adsorption tower 10. In other words, the first gas G1 in a state containing NO is sent to the second adsorption tower 20. As a result, the NO contained in the first gas G1 is adsorbed by the adsorbent 21 in the second adsorption tower 20.

[0066] This step #4 corresponds to process (c).

[0067] (Steps #5 and #6) In step #5, the adsorption state of the adsorbent 11 in the first adsorption tower 10 is measured. Step #5 is performed for the purpose of detecting whether or not the timing to stop the execution of step #4 (hereinafter referred to as the "second timing") has arrived.

[0068] Several methods can be employed to execute step #5. One example is a method in which the arrival of the second timing is detected by the gas sensor 61 when the NO concentration contained in the first gas G1 that has passed through the adsorbent 11 in the first adsorption tower 10 shows a decreasing trend. Another example is a method in which, if the gas sensor 61 is capable of detecting the CO concentration contained in the first gas G1, the arrival of the second timing is detected when the CO concentration of the first gas G1 that has passed through the adsorbent 11 in the first adsorption tower 10 shows an increasing trend and reaches or exceeds a predetermined first threshold. The predetermined first threshold here depends on the target gas Gt, but is, for example, approximately 2% when the target gas Gt is flue gas from a sludge incinerator. The first threshold may be set appropriately depending on the CO concentration contained in the target gas Gt.

[0069] 4 is a graph showing the time-dependent changes in the NO concentration and CO concentration of a first gas G1 that has passed through the first adsorption tower 10, measured with a gas sensor 61, when a mixed gas obtained by mixing 5% CO and 200 ppm NO with N as a base gas is supplied to the first adsorption tower 10 as the gas to be treated Gt. In FIG. 4, the left vertical axis represents the CO concentration of the first gas G1, the right vertical axis represents the NO concentration of the first gas G1, and the horizontal axis represents the elapsed time.

[0070] 4, the concentrations of NO and CO contained in the first gas G1 were both below the detection limit until approximately 2,340 seconds had elapsed since the gas to be treated Gt began to be supplied to the first adsorption tower 10. In other words, it is estimated that during this time, the adsorbent 11 contained in the first adsorption tower 10 was able to almost completely adsorb the NO and CO contained in the gas to be treated Gt.

[0071] Thereafter, as the target gas Gt continued to be supplied to the first adsorption tower 10, the NO concentration in the first gas G1 began to increase, while the CO concentration in the first gas G1 remained below the detection limit.

[0072] The NO concentration contained in the first gas G1 showed an increasing trend until about 2,500 seconds had elapsed since the start of supplying the first gas G1 to the first adsorption tower 10, and then showed a decreasing trend. At approximately the same time that the NO concentration contained in the first gas G1 started to show a decreasing trend, the CO concentration contained in the first gas G1 showed an increasing trend.

[0073] From this result, it is considered that the timing when the NO concentration contained in the first gas G1 starts to show a downward trend or the timing when the CO concentration contained in the first gas G1 starts to show an upward trend is a time period when the NO concentration in the first gas G1 is relatively high. Then, after this timing, it is considered that the NO concentration in the first gas G1 starts to relatively decrease, and the CO concentration in the first gas G1 starts to relatively increase.

[0074] Therefore, by designating the above timing as the "second timing" and sending the first gas G1 up to this timing to the second adsorption tower 20, a gas (first gas G1) having a relatively high concentration of NO compared to the gas Gt to be treated is sent to the second adsorption tower 20.

[0075] 4, the reason why the timing at which the NO concentration of the first gas G1 shows an increasing trend arrives before the timing at which the CO concentration of the first gas G1 shows an increasing trend is presumably because the NO adsorbed by the adsorbent 11 in the first adsorption tower 10 is pushed out and desorbed by the adsorption force of CO in the gas to be treated Gt onto the adsorbent 11. In other words, when an adsorbable amount of NO has been adsorbed onto the adsorbent 11 in the first adsorption tower 10, if the gas to be treated Gt is further supplied to the adsorbent 11, the NO adsorbed by the adsorbent 11 is pushed out by the adsorption force of CO in the gas to be treated Gt onto the adsorbent 11, and as a result, CO is newly adsorbed in the region where NO was previously adsorbed.

[0076] The above phenomenon continues for a certain period of time, but during this time period, the adsorbent 11 in the first adsorption tower 10 is nearly at its adsorption limit, so the adsorbent 11 cannot completely adsorb the CO2 in the gas Gt to be treated, and the CO2 begins to pass through the adsorbent 11. As a result, the concentration of CO2 in the first gas G1 also begins to show an increasing trend.

[0077] If the gas to be treated Gt continues to be supplied to the adsorbent 11 thereafter, the NO adsorbed to the adsorbent 11 by the CO in the gas to be treated Gt is sufficiently desorbed, and thereafter, it becomes difficult to secure an adsorption area within the adsorbent 11. As a result, the NO concentration of the first gas G1 decreases to approach the NO concentration in the gas to be treated Gt, and the CO concentration of the first gas G1 increases to approach the CO concentration in the gas to be treated Gt.

[0078] In other words, the gas sensor 61 measures the change over time in the NO concentration or CO concentration contained in the first gas G1, and by detecting that the NO concentration is showing a downward trend or the CO concentration is showing an upward trend, the adsorption state of the adsorbent 11 in the first adsorption tower 10 can be measured.

[0079] (Step #7) In this way, in step #5, the adsorption state of the adsorbent 11 in the first adsorption tower 10 is measured, and when the arrival of the second timing is detected (Yes in step #6), the supply of the gas to be treated Gt to the first adsorption tower 10 is stopped. As long as the arrival of the second timing is not detected (No in step #6), the delivery of the first gas G1 to the second adsorption tower 20 continues.

[0080] As a specific example, the arrival of the second timing can be detected when the change in the NO concentration in the first gas G1 over time measured by the gas sensor 61 becomes equal to or less than a predetermined negative value. As another example, the arrival of the second timing can be detected when the CO concentration in the first gas G1 measured by the gas sensor 61 becomes equal to or greater than the first threshold value.

[0081] After the second timing arrives, it is expected that the NO concentration in the first gas G1 will decrease. In other words, from the time when the first timing arrives and delivery of the first gas G1 to the second adsorption tower 20 is started until the time when the second timing arrives and delivery of the first gas G1 to the second adsorption tower 20 is stopped, the first gas G1 supplied to the second adsorption tower 20 exhibits a relatively high NO concentration. Therefore, according to this method, the first gas G1 containing an extremely high concentration of NO compared to the gas to be treated Gt is supplied to the second adsorption tower 20.

[0082] (Step #8) Thereafter, a desorption process is performed on the adsorbent 11 in the first adsorption tower 10. Specifically, this is performed by heating the adsorbent 11, reducing the pressure of the adsorbent 11, or the like. As a result, the substances adsorbed to the adsorbent 11 in the first adsorption tower 10 are desorbed from the adsorbent 11, and the adsorption performance of the adsorbent 11 is restored.

[0083] Step #8 corresponds to step (d1). When performing this desorption step, if the substances adsorbed to the adsorbent 11 are exhausted in an airflow, for example, the on-off valve 41 may be opened and the target gas Gt, air, or buffer gas may be temporarily introduced into the first adsorption tower 10. In this case, the first gas G1 containing the substances adsorbed to the adsorbent 11 flows through the pipe 71. Step #8 may be achieved by applying negative pressure (typically a vacuum) to the first adsorption tower 10 and drawing the substances adsorbed to the adsorbent 11 to the pipe 71 side.

[0084] (Step #9) After step #7, the supply of the gas to be treated Gt from the gas supply source 3 to the first adsorption tower 10 is stopped. At this point, if there is a need to continue treating the gas to be treated Gt (Yes in step #9), for example, if there is remaining gas to be treated Gt in the gas supply source 3, the process returns to step #1. On the other hand, if the treatment of the gas to be treated Gt is to be terminated or interrupted (No in step #9), the process is terminated.

[0085] (Steps #10 and #11) In step #10, the adsorption state of the adsorbent 21 in the second adsorption tower 20 is measured. Step #10 is performed to detect whether the timing to start execution of step #12 (described later) (hereinafter referred to as the "third timing") has arrived. Step #10 corresponds to process (e).

[0086] Several methods can be employed to execute step #10. One example is a method in which the gas sensor 62 detects the arrival of the third timing when the concentration of NO contained in the second gas G2 that has passed through the adsorbent 21 in the second adsorption tower 20 reaches a predetermined value or higher. Another example is a method in which, if the gas sensor 62 is capable of detecting the concentration of CO contained in the second gas G2, the third timing is detected when the concentration of CO in the second gas G2 that has passed through the adsorbent 21 in the second adsorption tower 20 reaches a predetermined value or higher. Another example is a method in which the second adsorption tower 20 is provided with a weight sensor (not shown) capable of measuring the weight of the adsorbent 21, and the third timing is detected when the measured value of the weight of the adsorbent 21 by this weight sensor reaches a predetermined value or higher. These predetermined values ​​may be set appropriately based on the concentration of NO or CO contained in the gas to be treated Gt, the initial weight of the adsorbent 21, and the like.

[0087] As described above, during the time period from step #4 to step #7, the on-off valve 42 is open, and the first gas G1 that has passed through the first adsorption tower 10 is sent to the second adsorption tower 20. During this time period, NO contained in the first gas G1 is adsorbed by the adsorbent 21 in the second adsorption tower 20. Note that, for the reasons described above, although the first gas G1 has a relatively high concentration of NO, it is inevitably mixed with substances other than NO, and these mixed substances are also adsorbed by the adsorbent 21 in the second adsorption tower 20.

[0088] 4, it is expected that the adsorption performance of the adsorbent 21 will also deteriorate over time. Therefore, if the adsorbent 21 reaches its NO adsorption limit, even if the first gas G1 is subsequently delivered from the first adsorption tower 10 to the second adsorption tower 20, the NO contained in the first gas G1 will not be adsorbed by the adsorbent 21 and will be delivered from the second adsorption tower 20 as the second gas G2 containing a large amount of NO.

[0089] When the gas sensor 62 confirms that the concentration of NO or CO contained in the second gas G2 has reached a predetermined value or higher, this means that the adsorbent 21 in the second adsorption tower 20 is close to or has reached its limit for adsorbing NO. Similarly, when the weight sensor detects that the weight of the adsorbent 21 in the second adsorption tower 20 has reached a predetermined value or higher, this also means that the adsorbent 21 in the second adsorption tower 20 is close to or has reached its limit for adsorbing NO.

[0090] That is, the concentration of NO contained in the second gas G2, the concentration of CO contained in the second gas G2, or the weight of the adsorbent 21 in the second adsorption tower 20 measured in step #10 corresponds to an index value indicating the adsorption state of the substance in the adsorbent 21 in the second adsorption tower 20. Depending on whether this index value has reached or exceeded a predetermined value (corresponding to the "second threshold value"), it becomes possible to determine whether the adsorbent 21 in the second adsorption tower 20 is close to or has reached its limit for adsorbing NO.

[0091] As described above, when the process returns to step #1 after step #9, after the supply of the gas Gt to be treated from the gas supply source 3 to the first adsorption tower 10 has started, the first gas G1 is delivered from the first adsorption tower 10 to the second adsorption tower 20 from step #4 onwards. As will be described in a later embodiment, a configuration is also envisioned in which the first gas G1 is delivered from each of the multiple first adsorption towers 10 to the same second adsorption tower 20. That is, the adsorbent 21 in the second adsorption tower 20 will eventually reach its limit of adsorption performance, and even if the first gas G1 is delivered to the second adsorption tower 20 at a timing thereafter, it will not be adsorbed by the adsorbent 21 in the second adsorption tower 20, and NO will likely flow out of the second adsorption tower 20.

[0092] In step #10, an index value indicating the state of adsorption of a substance in the adsorbent 21 in the second adsorption tower 20 is measured, and when it is detected that the index value has reached a predetermined threshold value (second threshold value) or more, i.e., that the third timing has arrived (Yes in step #11), the next step #12 is executed. On the other hand, in the embodiment shown in the flowchart of FIG. 3 , step #12 is not executed until the third timing arrives (No in step #11).

[0093] (Step #12) Since the third timing has arrived, it is considered that the adsorbent 21 in the second adsorption tower 20 has reached its NO adsorption limit. Therefore, in step #12, a desorption process is performed on the adsorbent 21 in the second adsorption tower 20. Specifically, this is performed by a method such as heating the adsorbent 21 or reducing the pressure of the adsorbent 21. As a result, the substance adsorbed to the adsorbent 21 in the second adsorption tower 20 is desorbed from the adsorbent 21, and the adsorption performance of the adsorbent 21 is restored.

[0094] In performing this desorption step, when the substances adsorbed to the adsorbent 21 are exhausted in an airflow, for example, the on-off valve 42 may be opened and the gas to be treated Gt, air, or buffer gas may be temporarily introduced into the second adsorption tower 20 via the first adsorption tower 10. As another example, air or buffer gas may be introduced into the second adsorption tower 20 through a gas inlet (not shown) provided in the second adsorption tower 20. When these modes are performed, the second gas G2 containing the adsorbable substances such as NO adsorbed to the adsorbent 21 is delivered to the decomposition tower 30 via the pipe 53. This step #12 may be realized by applying a negative pressure (typically a vacuum) to the second adsorption tower 20 and drawing the substances adsorbed to the adsorbent 21 to the pipe 72 side.

[0095] This step #12 corresponds to process (d2). In the gas treatment method according to the present invention, step #12 may be performed to desorb the substances adsorbed by the adsorbent 21 before the adsorption performance of the adsorbent 21 in the second adsorption tower 20 reaches its limit, in other words, even when the adsorbent 21 still has a margin of adsorption capacity.

[0096] (Step #13) The second gas G2 exhausted from the second adsorption tower 20 in a state in which the substances adsorbed on the adsorbent 21 in the second adsorption tower 20 have been desorbed is sent to the decomposition tower 30 via the pipe 53. In this step #13, NO contained in this second gas G2 is decomposed in the decomposition tower 30. This step #13 corresponds to the process (f).

[0097] 4 , the time period during which the first gas G1 that has passed through the first adsorption tower 10 is fed to the second adsorption tower 20 corresponds to the time period during which the NO concentration in the first gas G1 is relatively high. Therefore, by supplying the first gas G1 to the adsorbent 21 in the second adsorption tower 20 during this time period, a relatively large amount of NO is adsorbed by the adsorbent 21 per unit time compared to the adsorbent 11.

[0098] That is, as a result of the adsorbent 21 in the second adsorption tower 20 being desorbed in step #12, the second gas G2 supplied from the second adsorption tower 20 to the decomposition tower 30 contains NO at a higher concentration than the gas Gt to be treated. Therefore, by decomposing the NO contained in this second gas G2 in the decomposition tower 30, NO with a high GWP can be efficiently decomposed.

[0099] Fig. 5 is a cross-sectional view schematically illustrating an example of the configuration of the decomposition tower 30. In the example shown in Fig. 5, the decomposition tower 30 is equipped with a decomposition device 80 consisting of an excimer lamp. Fig. 6 corresponds to a schematic side view of the excimer lamp constituting the decomposition device 80 in this example, and Fig. 7 corresponds to a cross-sectional view taken along line VII-VII in Fig. 6.

[0100] This excimer lamp includes a tube 81 and a pair of electrodes (82a, 82b). In this example, the tube 81 has a double-tube structure. More specifically, as shown in Fig. 7, the tube 81 has a cylindrical outer tube 81a located on the outside, and a cylindrical inner tube 81b located inside the outer tube 81a and coaxially with the outer tube 81a, with an outer diameter smaller than the inner diameter of the outer tube 81a.

[0101] 6 and 7 show an example of an excimer lamp (decomposition device 80) in which the tube axis direction of the tube body 81 is arranged along the flow direction d1 of the second gas G2. However, the tube axis direction of the tube body 81 does not necessarily need to be parallel to the flow direction d1 of the second gas G2, and these drawings are merely an example. Hereinafter, for convenience, the flow direction d1 of the second gas G2 and the tube axis direction of the tube body 81 are assumed to be parallel, and in order to avoid an increase in the number of symbols, the same symbol "d1" as the flow direction is used for the tube axis direction.

[0102] The outer tube 81a and the inner tube 81b are both sealed at their ends in the tube axis direction d1 (not shown), and a light-emitting space having a ring shape (here, a circular ring shape) when viewed from the tube axis direction d1 is formed between them. A light-emitting gas 83G that forms excimer molecules by discharge is sealed in this light-emitting space.

[0103] An electrode 82a is disposed on the outer wall of the outer tube 81a. In this embodiment, the electrode 82a has a mesh or linear shape. A rod-shaped electrode 82b extends along the axial direction of the tube body 81 and is inserted into the inner tube 81b.

[0104] The outer tube 81a and the inner tube 81b are made of a dielectric material such as synthetic quartz glass that is transparent to the ultraviolet light L1. The electrodes (82a, 82b) are made of a metal material such as stainless steel, aluminum, copper, tungsten, titanium, or nickel.

[0105] When a high-frequency AC voltage of, for example, about 1 kHz to 5 MHz is applied between the electrodes (82a, 82b) via a power supply (not shown), the voltage is applied to the light-emitting gas 83G via the tube body 81. At this time, a discharge plasma is generated in the discharge space in which the light-emitting gas 83G is sealed, and the atoms of the light-emitting gas 83G are excited to an excimer state, and excimer light emission occurs when these atoms transition to the ground state. In other words, ultraviolet light L1 derived from excimer light is generated due to the dielectric barrier discharge.

[0106] The material of the light emitting gas 83G determines the wavelength of the ultraviolet light L1 emitted from the tube 81. When a gas containing xenon (Xe) is used as the light emitting gas 83G, the excimer light emission becomes ultraviolet light L1 having a main peak wavelength in the vicinity of 172 nm.

[0107] The wavelength of the ultraviolet light L1 can be changed by changing the substance used as the luminous gas 83G. For example, ArBr (main peak wavelength near 165 nm), ArCl (main peak wavelength near 175 nm), F (main peak wavelength near 153 nm), etc. can be used as the luminous gas 83G. Here, a case where the luminous gas 83G is a gas containing Xe will be described.

[0108] As described above, in the excimer lamp of this embodiment, the electrode 82a disposed on the outer wall of the outer tube 81a has a mesh shape. Therefore, gaps exist in the electrode 82a, and the ultraviolet light L1 is extracted toward the outside of the outer tube 81a through these gaps. This ultraviolet light L1 is irradiated onto the second gas G2 flowing through the decomposition tower 30.

[0109] When the ultraviolet light L1 is irradiated onto N2O contained in the second gas G2, the reaction of the following formula (1) occurs. Note that in formula (1), O( 1 D) is excited atomic oxygen. The left side of equation (1) conveniently expresses that ultraviolet light with a wavelength λ (frequency ν) is absorbed by N2O. The same applies to the following equations. N2O + hν → N2 + O( 1 D) ...(1)

[0110] Theoretically, the decomposition reaction of the above formula (1) occurs when ultraviolet light L1 with a wavelength λ of 340 nm or less is used. However, since the absorption cross section of NO for light with a wavelength of 200 nm or more is small, it is preferable to use light with a wavelength of less than 200 nm, which has a relatively large absorption cross section, in order to promote the decomposition reaction of the formula (1).

[0111] When the wavelength λ of the ultraviolet light L1 is less than 175 nm, the ultraviolet light L1 is also absorbed by the O2 contained in the second gas G2, and the reactions of the following formulas (2) and (3) proceed. 3 P) is atomic oxygen in the ground state. The reactions of formulas (2) and (3) occur depending on the wavelength component of the ultraviolet light L1. Specifically, formula (2) is a reaction that occurs when the shorter wavelength component of the ultraviolet light L1 is absorbed by oxygen (O2). O2 + hν → O( 1 D) + O( 3 P) ‥‥(2) O2 + hν → O( 3 P) + O( 3 P) ... (3)

[0112] Some of the O atoms generated in the formulas (2) and (3) react with oxygen (O2) contained in the second gas G2 to generate ozone (O3) according to the following formula (4). In formula (4), M represents a third body (the same applies below). In formula (4), O( 1 D) and O( 3 O + O2 + M → O3 + M (4)

[0113] When ultraviolet light L1 is absorbed by the ozone (O3) obtained by the above formula (4), the reaction of the following formula (5) proceeds: O3 + hν → O( 1 D) + O2 (5)

[0114] Excited state atomic oxygen O( 1 Since NO) is a highly reactive substance, when it comes into contact with NO contained in the second gas G2, the reaction of formula (6) or (7) occurs. NO + O ( 1 D) → O2 + N2 ‥‥(6) N2O + O( 1 D) → NO + NO (7)

[0115] As described above, when the second gas G2 is supplied to the decomposition tower 30, NO in the second gas G2 is decomposed by the reactions of the above-described formulas (1), (6), and (7). As a result, the treated gas Gd (see FIG. 5) that has passed through the decomposition tower 30 contains substances with low GWP, and can be discharged to a subsequent stage (or the atmosphere) via the pipe 73 (see FIG. 1).

[0116] 5, the decomposition tower 30 includes a decomposition device 80 formed of an excimer lamp, which generates a decomposition space 31 corresponding to the ultraviolet irradiation region, and the second gas G2 passes through this decomposition space 31, thereby decomposing NO in the second gas G2. For example, the operation of the decomposition device 80 may be started at the timing when execution of step #13 is started (corresponding to process (h)). More specifically, if the decomposition device 80 is formed of an excimer lamp, the excimer lamp may be energized at this timing.

[0117] By executing step #12, the substances adsorbed to the adsorbent 21 in the second adsorption tower 20 are desorbed and sent to the decomposition tower 30 as a second gas G2 containing the substances, where decomposition processing is performed in the decomposition tower 30. In other words, when the execution of step #12 progresses to a certain extent, the substances adsorbed to the adsorbent 21 in the second adsorption tower 20 are sufficiently desorbed.

[0118] Therefore, similarly to step #10, when an index value indicating the state of adsorption of substances in the adsorbent 21 in the second adsorption tower 20 is measured, this index value shows a gradual downward trend from the start of execution of step #10. For example, when it is detected that the index value has reached a predetermined threshold value (third threshold value) or less, it may be determined that the substances adsorbed by the adsorbent 21 in the second adsorption tower 20 have been sufficiently desorbed, and operation of the decomposition device 80 may be stopped. In particular, when the decomposition device 80 is composed of an excimer lamp, when it is detected that the index value has reached the third threshold value or less, power supply to the excimer lamp may be stopped. This makes it possible to extend the life of the excimer lamp and reduce the amount of energy consumed by the excimer lamp when decomposing NO.

[0119] When the decomposition device 80 is configured with an excimer lamp, the structure of the excimer lamp illustrated in FIGS. 6 and 7 is merely an example.

[0120] For example, as shown in Fig. 8, an excimer lamp may have a single tubular body 81. Fig. 8 is a schematic cross-sectional view of an excimer lamp illustrated similarly to Fig. 7. The tubular body 81 is sealed at its end in the longitudinal direction, i.e., in the tube axis direction d1 (not shown), and a light-emitting gas 83G is sealed in the internal space. A mesh-shaped or linear electrode 82a is disposed on the outer wall surface of the tubular body 81, and a rod-shaped electrode 82b is disposed inside (inside) the tubular body 81.

[0121] As another example, as shown in Figures 9 and 10, a configuration can be employed in which both electrodes (82a, 82b) are disposed on the outer wall surface of a tubular body 81 of an excimer lamp. Figures 9 and 10 are drawings that schematically show the structure of an excimer lamp of this alternative configuration example, with Figure 9 corresponding to a plan view and Figure 10 corresponding to a cross-sectional view taken along line XX in Figure 9. The excimer lamp shown in Figures 9 and 10 has a single tubular body 81, and both electrodes (82a, 82b) are disposed on the outer wall surface of the tubular body 81 so as to face each other across the tubular body 81. The electrodes (82a, 82b) are both mesh-shaped or linear-shaped so as not to interfere with the extraction of ultraviolet light L1 generated within the tubular body 81 to the outside of the tubular body 81.

[0122] Furthermore, the decomposition device 80 for generating the decomposition space 31 as described above is not limited to an excimer lamp. As another example, the decomposition device 80 may be configured by an atmospheric pressure plasma generator, as illustrated in Figures 11 and 12.

[0123] 11 and 12 are drawings showing a structural example of an atmospheric pressure plasma generator as a decomposition device 80, with Fig. 11 corresponding to a side view of the atmospheric pressure plasma generator and Fig. 12 corresponding to a cross-sectional view taken along line XII-XII in Fig. 11. Unlike an excimer lamp, this atmospheric pressure plasma generator does not have a light-emitting gas 83G sealed in the tubular body 81. Also, in Fig. 11, a second gas G2 is caused to flow through the tubular body 81. Other configurations are similar to those of the excimer lamp described above with reference to Figs. 6 and 7.

[0124] When a voltage is applied between the two electrodes (82a, 82b) from a power supply (not shown), a dielectric barrier discharge occurs within the tubular body 81. As a result, a dielectric barrier discharge occurs in the second gas G2 flowing through the decomposition space 31 formed within the tubular body 81, and the second gas G2 is converted into plasma.

[0125] When NO contained in the second gas G2 flows through the atmospheric pressure plasma space, the reaction of the following formula (8) occurs. In formula (8), AP means that energy is added by atmospheric pressure plasma. This also applies to the following formulas. In formulas (8) to (10) below, O( 1 D) and O( 3Both NO and AP are expressed as atomic oxygen O. NO + AP → N + O (8)

[0126] The oxygen (O2) contained in the second gas G2 undergoes the reaction shown in formula (9) below when flowing through the atmospheric pressure plasma space: O2 + AP → O + O (9)

[0127] A portion of the atomic oxygen O generated in formula (9) reacts with oxygen (O2) contained in the second gas G2 to generate ozone (O3) according to formula (4) above. Formula (4) is shown below again: O + O2 + M → O3 + M (4)

[0128] When energy from atmospheric pressure plasma is applied to the ozone (O3) obtained by the above formula (4), the following reaction (10) occurs: O3 + AP → O + O2 (10)

[0129] That is, when the second gas G2 flows through the decomposition space 31 formed by the atmospheric pressure plasma generator, ozone (O3) is introduced into the second gas G2, and the O ( 1 D) is generated. As a result, O( 1 When D) comes into contact with N2O contained in the second gas G2, N2O is decomposed via the reaction of formula (6) or (7).

[0130] Fig. 13 is a block diagram schematically illustrating another configuration example of the gas processing system 1. The gas processing system 1 illustrated in Fig. 13 differs from the gas processing system 1 illustrated in Fig. 1 in that the second adsorption tower 20 has a multi-stage configuration (second adsorption towers 20a, 20b), but is otherwise common to the gas processing system 1 illustrated in Fig. 1.

[0131] In this case, the upstream second adsorption tower 20a essentially functions the same as the first adsorption tower 10, except that the gas supplied is the first gas G1 rather than the target gas Gt. That is, during a time period when the NO concentration of the second gas G2a passing through the adsorbent 21a is relatively high, the on-off valve 44a is opened, and the second gas G2a is sent to the downstream second adsorption tower 20b. In the second adsorption tower 20b, the adsorbent 21b adsorbs the NO contained in the second gas G2a. When the adsorption performance of the adsorbent 21b approaches or reaches its limit, the on-off valve 44b is controlled to an open state, the adsorbed material of the adsorbent 21b is desorbed, and the second gas G2b discharged from the downstream second adsorption tower 20b is sent to the decomposition tower 30. In the decomposition tower 30, the NO contained in the second gas G2b is decomposed. Other aspects are similar to those of the gas treatment system 1 shown in FIG. 1 , and therefore further description will be omitted.

[0132] Although FIG. 13 illustrates an example in which the second adsorption tower 20 has a two-stage (20a, 20b) configuration, it may have three or more stages.

[0133] Fig. 14 is a block diagram schematically illustrating another configuration example of the gas processing system 1. The gas processing system 1 illustrated in Fig. 13 differs from the gas processing system 1 illustrated in Fig. 1 in that a plurality of first adsorption towers 10 (10a, 10b) are arranged, but the rest is common to the gas processing system 1 illustrated in Fig. 1.

[0134] In this case, it is possible to switch the first adsorption tower 10 (10a, 10b) to which the gas supply source 3 supplies the target gas Gt. For example, in step #1, the on-off valve 60a is opened and the on-off valve 60b is closed, and the target gas Gt is supplied from the gas supply source 3 to the first adsorption tower 10a (corresponding to process (a1)). Then, steps #2 to #7 are executed. Next, the on-off valve 60b is opened and the on-off valve 60a is closed, and the target gas Gt is supplied from the gas supply source 3 to the first adsorption tower 10b (corresponding to process (a2)). Then, steps #2 to #7 are executed. Thereafter, steps #10 to #13 are executed.

[0135] As described above, the timing at which the first gas G1 is supplied from the first adsorption tower 10 to the second adsorption tower 20 occurs between the detection of the first timing in step #3 and the detection of the second timing in step #6. As described above with reference to Fig. 4 , this time is relatively short, and therefore the time required for the adsorbent 21 in the second adsorption tower 20 to reach its limit in adsorption performance is expected to be longer than the time required for the adsorbent 11 in the first adsorption tower 10 to reach its limit in adsorption performance. Therefore, by configuring the gas processing system 1 shown in Fig. 14 in such a way that the first gas G1 is supplied to the second adsorption tower 20 while being switched from a plurality of first adsorption towers 10, it is possible to efficiently process the target gas Gt from the gas supply source 3.

[0136] 14 , the target gas Gt from the gas supply source 3 can be fed into the first adsorption tower 10b at the same time that desorption is being performed on the adsorbent 11 in the first adsorption tower 10a. That is, the process for restoring the adsorption performance of the adsorbent 11 in the first adsorption tower 10a and the process for adsorbing the substances contained in the target gas Gt onto the adsorbent 11 in the first adsorption tower 10b can be performed in parallel. This increases the processing capacity for the target gas Gt from the gas supply source 3 and enables continuous processing.

[0137] 14 illustrates a case in which two first adsorption towers 10 (10a, 10b) are connected to the same second adsorption tower 20, but three or more first adsorption towers 10 may be connected to the same second adsorption tower 20. Furthermore, in the gas processing system 1 shown in FIG. 14, the second adsorption towers 20 may be connected in multiple stages, as in FIG. 13.

[0138] Another embodiment will now be described.

[0139] 14 is configured such that the target gas Gt is supplied from the same gas supply source 3 and the supply destination first adsorption towers 10 (10a, 10b) are switchable. In contrast, as shown in FIG. 15, a configuration may be adopted in which the target gas Gt is supplied from different gas supply sources 3 (3a, 3b) to two first adsorption towers 10 (10a, 10b).

[0140] <2> The gas processing system 1 shown in Fig. 1 may be provided as a plurality of independent systems. The same applies to the gas processing systems 1 shown in Figs. 13 to 15.

[0141] <3> In the gas processing system 1 described with reference to the above drawings, a single second adsorption tower 20 is directly connected immediately after the first adsorption tower 10. However, a plurality of second adsorption towers 20 may be connected in parallel immediately after the first adsorption tower 10.

[0142] <4> The configurations of the gas processing system 1 described above with reference to the respective drawings can be combined as appropriate as necessary.

[0143] <5> The present invention is not limited to the above-described embodiments and includes various modifications. For example, the above-described embodiments have been described in detail to provide a better understanding of the present invention, and the present invention is not necessarily limited to those having all of the configurations described. The scope of the present invention is defined by the claims, and it is intended to include all modifications within the meaning and scope of the claims.

[0144] REFERENCE SIGNS LIST 1: Gas treatment system 3: Gas supply source 10, 10a, 10b: First adsorption tower 11: Adsorbent 20, 20a, 20b: Second adsorption tower 21, 21a, 21b: Adsorbent 30: Decomposition tower 31: Decomposition space 41, 42, 43, 44: On-off valve 43a, 43b, 44a, 44b: On-off valve 51, 52, 53: Pipe 60a, 60b: On-off valve 61, 62, 62a, 62b: Gas sensor 71, 72, 73: Pipe 72a, 72b: Pipe 80: Decomposition device 81: Tube body 81a: Outer tube 81b: Inner tube 82a, 82b: Electrode 83G: Luminous gas G1: First gas G2, G2a, G2b: second gas Gd: processed gas Gt: gas to be processed L1: ultraviolet light

Claims

1. A gas treatment method comprising: (a) a step of passing a gas to be treated, which contains CO2 and N2O, through a first adsorption tower containing an adsorbent capable of adsorbing CO2 and N2O; (b) a step of measuring the concentration of N2O in the first gas located downstream of the adsorbent in the first adsorption tower in the direction of flow; and (c) a step of sending the first gas to a second adsorption tower containing an adsorbent capable of adsorbing CO2 and N2O, wherein step (c) is initiated during a time period when the concentration of N2O in the first gas measured in step (b) shows an increasing trend.

2. The gas treatment method according to claim 1, wherein step (c) is stopped during a time period when the N2O concentration of the first gas measured in step (b) shows a downward trend.

3. The gas processing method according to claim 1, wherein step (b) includes a step of measuring the CO2 concentration of the first gas, and step (c) is stopped when the CO2 concentration of the first gas measured in step (b) reaches or exceeds a predetermined first threshold value.

4. A gas treatment method according to claim 2 or 3, characterized in that it comprises a step (d1) of desorbing the adsorbed substance adsorbed on the adsorbent in the first adsorption tower, the step (a) being stopped during a time period in which the step (c) is stopped, and the step (d1) being carried out during a time period in which the step (a) and the step (c) are stopped.

5. The gas treatment method according to claim 1, comprising: a step (e) of measuring an index value indicating the adsorption state of a substance in the adsorbent in the second adsorption tower; and a step (d2) of desorbing the adsorbed substance adsorbed to the adsorbent in the second adsorption tower, wherein the step (c) is stopped after the index value reaches a predetermined second threshold value or more, and the step (d2) is started after the step (c) is stopped.

6. The gas treatment method described in claim 5, characterized in that the index value is one of the values ​​of the N2O concentration of the second gas located downstream of the adsorbent in the second adsorption tower in terms of the flow direction, the CO2 concentration of the second gas, and the weight of the second adsorption tower.

7. The gas treatment method according to claim 6, further comprising a step (f) of decomposing N2O contained in the second gas obtained after the start of the execution of step (d2).

8. The gas treatment method according to claim 7, wherein the step (f) includes a step of feeding the second gas obtained after the start of the execution of the step (d2) into a decomposition space constituted by at least one of an ultraviolet ray irradiation region and a plasma generation region.

9. A gas treatment method according to claim 7, characterized in that two or more of the second adsorption towers are arranged in a state of being connected to each other, the method comprises a step (g) of feeding the second gas obtained after the start of execution of step (d2) for the second adsorption towers other than the second adsorption tower located furthest downstream in the flow direction to the second adsorption tower located in the subsequent stage, and the step (f) is a step of decomposing N2O contained in the second gas obtained after the start of execution of step (d2) for the second adsorption tower located furthest downstream in the flow direction.

10. A gas treatment method according to claim 8, further comprising a step (h) of operating a decomposition device that generates at least one of the ultraviolet rays and the plasma during the execution of the step (f), wherein, after the index value reaches a predetermined third threshold value that is smaller than the second threshold value, the execution of the steps (d2), (f), and (h) for the second adsorption tower located furthest downstream in the flow direction is stopped.

11. The gas treatment method according to claim 10, wherein the decomposition device is at least one of an excimer lamp using a dielectric barrier discharge and an atmospheric pressure plasma generation device.

12. A gas treatment method according to claim 1, characterized in that two or more of the first adsorption towers are provided, the step (a) comprises: a step (a1) of passing the gas to be treated through one of the first adsorption towers; and a step (a2) of passing the gas to be treated through a first adsorption tower other than the first adsorption tower through which the gas to be treated was passed in step (a1) after completion of step (a1), and the step (c) is a step of feeding the first gas exhausted from the first adsorption tower through which the gas to be treated was passed in step (a1) and the first gas exhausted from the first adsorption tower through which the gas to be treated was passed in step (a2) into the same second adsorption tower.

13. The gas treatment method according to claim 1, wherein the adsorbent contained in the first adsorption tower and the adsorbent contained in the second adsorption tower are mainly composed of zeolite.

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