JP2012215376A - Offgas combustion system and combustion method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an offgas combustion system that can subject methane contained in an offgas to combustion disposal while suppressing unburned part from occurring without using auxiliary fuel or special equipment, thereby reliably preventing the methane contained in an offgas from being diffused in the atmosphere, and to provide a combustion method for the same.SOLUTION: The offgas combustion system 100 for burning the offgas emitted when separating methane from a biogas, includes: a methane separating device (PSA) 1 for separating the methane from the biogas; a buffer tank 3 for storing the offgas emitted from the PSA 1; a pressure governor 5 for regulating the pressure of the offgas discharged from the buffer tank 3 before burning the offgas; and a combustion tower 6 for burning the offgas whose pressure is regulated by the pressure governor 5.

Description

  The present invention relates to an off-gas combustion system and a combustion method thereof, and more particularly to an off-gas combustion system suitable for burning off-gas emitted when separating and purifying methane from biogas and a combustion method thereof.

  Biogas is a gas obtained by anaerobic fermentation of biomass (organic resources) such as sewage sludge, food waste, and livestock excrement. This biogas is a non-depleting renewable resource. From the viewpoint of effective use of renewable energy, this biogas is expected as a fuel for micro gas turbines, fuel cells, natural gas automobiles, etc. Development is progressing rapidly.

Biogas is mainly composed of about 55 to 65% methane (CH ₄ ) and about 45 to 35% carbon dioxide (CO ₂ ), and contains trace amounts of impurity components such as hydrogen sulfide, siloxanes, ammonia, and methyl mercaptan. include. For this reason, in order to use biogas effectively, for example, in order to use biogas for power generation facilities, it is necessary to remove hydrogen sulfide and siloxanes, which are harmful components, as well as natural gas automobile fuel and urban In order to use it as a gas fuel, it is necessary to separate methane and carbon dioxide or to concentrate methane in biogas.

  As technology for separating methane from methane-rich biogas generated from such anaerobic fermentation process and purifying high-purity methane, for example, separation technology by PSA (Pressure Swing Adsorption) method and membrane separation method are used. Such a separation technique is mentioned.

  Here, FIG. 10 shows an outline of the methane separation apparatus by the PSA method, and this methane separation apparatus is composed of an unnecessary component (for example, other than methane in biogas by the adsorbent packed in the tower). By adsorbing and removing components such as carbon dioxide, a predetermined component (for example, methane) can be separated.

In the methane separation apparatus P shown in FIG. 10, a plurality of towers Pa and Pb (two in FIG. 10) filled with an adsorbent are provided, and each of the towers Pa and Pb includes a biogas supply line LA, Biogas is supplied through the flow path switching device Va and the biogas supply branch lines LBa and LBb. In this way, the biogas is supplied to the towers Pa and Pb packed with the adsorbent, and components other than methane in the biogas are adsorbed and removed by the adsorbent, thereby increasing the methane concentration in the remaining gas. Can do. For example, if the biogas contains only methane and carbon dioxide (CO ₂ ), it is not adsorbed by the adsorbent by supplying the biogas into the towers Pa and Pb packed with a catalyst that adsorbs CO _2. In addition, the methane concentration (or purity) of the gas discharged from the towers Pa and Pb can be increased, and a high concentration of methane can be stored in the methane storage tank.

  If only one tower filled with an adsorbent is provided, it is necessary to supply biogas only during adsorption while repeating adsorption and desorption. Therefore, separation of methane (enhancement of methane concentration) must be processed in a so-called “batch type”.

Therefore, as shown in FIG. 10, a plurality of towers are provided, the biogas is supplied to the tower filled with the regenerated adsorbent, and the supply of the biogas is stopped for the tower having a reduced adsorbent adsorption capacity. By performing desorption (regeneration) of the adsorbent, methane can be continuously separated or purified. That is, in one column, a component other than methane (for example, CO ₂ ) is adsorbed by an adsorbent, and in another column, a component other than methane (for example, CO ₂ ) is desorbed to regenerate the adsorbent, thereby regenerating methane from biogas. Other components can be continuously adsorbed.

In the methane separation apparatus P shown in FIG. 10, for example, when the tower Pa is in the desorption process, the tower Pa filled with the adsorbent to be desorbed (regenerated) is decompressed and adsorbed by the adsorbent. A component (for example, CO ₂ ) is separated from the adsorbent and discharged out of the tower Pa. The gas discharged (sucked) out of the tower Pa by the pressure reduction in this desorption process is supplied to the combustion tower, for example, via the offgas discharge line LCa, the flow path switching device Vb, and the offgas supply line LD as offgas. At that time, the tower Pb other than the tower Pa is in the adsorption step, and adsorbs components other than methane (for example, CO ₂ ) by the adsorbent. When the desorption process of the tower Pa is completed and the tower Pb is in the desorption process, the offgas discharged (sucked) out of the tower Pb by the pressure reduction in the desorption process is the offgas discharge line LCb, the flow path switching It is discharged out of the system via the device Vb and off-gas supply line LD.

Here, in order to increase the purity of methane during the adsorption step, care must be taken so that components other than methane (for example, CO ₂ ) in the biogas do not enter the methane. Therefore, in order to prevent the adsorbent from adsorbing components other than methane and preventing components other than methane from being output to the methane storage tank side together with methane, in general, when the adsorbent has a sufficient adsorbing capacity, that is, in the tower Before the adsorbent reaches the limit for adsorbing components other than methane, the adsorption process is switched to the desorption process (regeneration process) for regenerating the adsorbent.

  However, even if switching to a desorption process that regenerates the adsorbent when the adsorbent has sufficient capacity, the adsorbent in the tower is slightly mixed with methane, and the offgas discharged outside the tower It is known that methane is included in a trace amount.

  In addition, as described above, in the desorption process, the inside of the tower is decompressed and discharged, so when the process is switched to the desorption process, the biogas remaining in the tower is also discharged to the outside of the tower by the depressurization and becomes off-gas It is processed. That is, biogas containing methane (the methane concentration of biogas is about 60%) is also discharged out of the tower as offgas, and methane is necessarily included in the offgas.

  Although only the PSA method has been described with reference to FIG. 10, it is known that similar problems may occur in other methane separation apparatuses. For example, the membrane separation method is a method of separating a specific gas from a mixed gas of a plurality of types of gas by controlling fine pores of an inorganic membrane such as a polymer membrane or zeolite, and the size and shape of the target gas molecule. The desired gas to be separated from the mixed gas can be separated using the difference. However, it is well known that even in such a methane separation apparatus using a membrane separation method, there is a possibility that methane gas is mixed into the off-gas.

As described above, the off-gas from which methane is separated from the biogas by the methane separator generally has a relatively low methane concentration as compared to the biogas, but generally about 10 to 35% methane (CH ₄ ) and about 90%. It is a gas containing ˜65% carbon dioxide (CO ₂ ) as a main component. In addition, the gas composition of this off gas changes with the gas composition of the biogas which becomes the origin, and the separation capability of a gas separation apparatus.

Conventionally, there have been cases where off-gas containing such a low concentration of methane is released into the atmosphere. However, methane has a warming potential 20 to 30 times that of CO _{2 when} compared in the same number of moles, that is, methane emissions to the atmosphere are on the order of 20 to 30 times greater than CO ₂ emissions. It is known to adversely affect the global environment. Therefore, in recent years, from the viewpoint of global warming and safety, it is considered that it is not preferable to dissipate directly into the atmosphere even for such an off-gas containing a low concentration of methane.

  In order to solve the above problem, there is a conventional technique that suppresses the amount of methane emitted to the atmosphere and reduces the environmental load by burning off-gas including methane while supplying auxiliary fuel (for example, city gas). ing. Further, Patent Document 1 includes a detection device that detects the combustion state of the burner, and preheats the air supplied to the burner according to the combustion state of the burner, so that no auxiliary fuel is required and in the off-gas state. An off-gas combustion apparatus capable of combusting methane is disclosed.

  In addition, as another conventional technique, Patent Document 2 discloses a digestion gas storage provided with a desulfurization device, a methane concentration device, and a gas holder filled with a methane storage agent in order to increase the methane concentration and the methane storage amount. Equipment is disclosed.

  Furthermore, Patent Document 3 discloses a combustible gas processing system that performs lean gas combustion that can be combusted even with a low concentration of combustible gas, and can generate the power by the combustion heat generated during the combustion, thereby enabling the system to operate independently. Is disclosed.

JP 2010-230299 A JP 2001-949 A JP 2002-257313 A

  In the off-gas combustion apparatus disclosed in Patent Document 1, methane in the off-gas can be combusted relatively stably without adding auxiliary fuel (for example, city gas), and the running cost can be reduced. Can do. However, for example, when the offgas is discharged from the methane separation apparatus by the PSA method, since the pressurization and the pressure reduction as described above are repeated, the discharged offgas also includes pressure fluctuations and concentration fluctuations resulting from the pressure fluctuations. When off-gas is directly combusted in a combustion tower, there is a problem that the combustion tower is strongly affected by the pressure fluctuation and concentration fluctuation of the off-gas, and unburned components are generated.

  In addition, in the digestion gas storage facility disclosed in Patent Document 2, by providing a gas holder filled with a methane storage agent having a large methane storage capacity, the amount of methane stored per volume is increased compared to gas storage. The facility can be downsized, and stable operation can be achieved by the buffer effect. On the other hand, pressure fluctuation may still be included in the methane gas released from the gas holder, and unburned components may be generated when the methane gas released from the gas holder is directly combusted. Therefore, even when the gas holder is applied to the off-gas after the methane is separated and purified from the biogas by the gas separation device, the methane gas released from the gas holder may still contain pressure fluctuations.

  Furthermore, in the combustible gas processing system disclosed in Patent Document 3, biogas is appropriately processed for a small-scale combustible gas generation source with a relatively small gas generation amount and a relatively low gas concentration. However, it is not disclosed at all that it can be applied to an off-gas having a lower concentration and a relatively large concentration fluctuation or pressure fluctuation.

  The present invention has been made in view of the above-described problems, and when off-gas discharged when separating methane from biogas contains low-concentration methane, the methane in the off-gas is burned. An off-gas combustion system that can suppress the generation of unburned components without using auxiliary fuel or special equipment and reliably prevent methane in the off-gas from being released into the atmosphere. It aims at providing the combustion method.

  In order to achieve the above object, an off-gas combustion system of the present invention is an off-gas combustion system that burns off-gas discharged when methane is separated from biogas, and a methane separation device that separates methane from biogas; A buffer tank for storing off-gas discharged from the methane separator, a pressure regulator for adjusting the pressure of the off-gas discharged from the buffer tank before the combustion of the off-gas, and the pressure adjusted by the pressure regulator And a combustion tower for burning off-gas.

  According to the aspect described above, fluctuations in methane concentration in the offgas can be reduced by temporarily storing the offgas discharged when the methane is purified and separated from the biogas by the methane separation apparatus in the buffer tank. . In addition, as a factor of the concentration fluctuation | variation of the component in this offgas, the density | concentration fluctuation | variation etc. which can arise in offgas when it discharges | emits from the biogas obtained by anaerobic fermentation, for example from a methane separator, etc. can be mentioned. .

  Furthermore, by adjusting the pressure of the offgas by the pressure regulator before the combustion of the offgas, it is possible to reduce the pressure fluctuation of the offgas to a predetermined range. For example, a gas having a substantially constant pressure with little pressure fluctuation is supplied to the combustion tower. Can be supplied.

  Therefore, it is possible to supply both off-gas with stable characteristics to the combustion tower by reducing both concentration fluctuation and pressure fluctuation of the off-gas after methane purification, and to maintain stable off-gas combustion without using auxiliary fuel. The production | generation of the unburned component which can be generated at the time of combustion can be suppressed reliably. Note that the capacity of the buffer tank and the pressure of the off-gas supplied to the combustion tower can be appropriately set according to the combustion state of the combustion tower, and the concentration fluctuations are within a predetermined range according to the combustion performance of the combustion tower. If pressure fluctuation can be suppressed, generation of unburned components that may occur during combustion can be suppressed.

  Here, as the methane separation apparatus, a separation method such as a PSA method or a membrane separation method can be applied. In particular, in the case of the methane separation apparatus by the PSA method, the gas is desorbed from the adsorbent by repeating the pressurization and depressurization as described above. Concentration fluctuations and pressure fluctuations are likely to increase. Therefore, particularly when off-gas discharged from a methane separation apparatus by the PSA method is burned, it is necessary to reliably reduce off-gas concentration fluctuations and pressure fluctuations. Also, the concentration fluctuation and pressure fluctuation of the off-gas change depending on the separation performance of PSA. Therefore, even when such a PSA separation apparatus is used, both the buffer tank and the pressure regulator are provided, and their capacities and pressures are within a certain range, so that they are affected by the separation performance of PSA. It has been found by the present inventors that concentration fluctuations and pressure fluctuations are reduced to a predetermined range.

  In a preferred aspect of the off-gas combustion system of the present invention, the biogas is supplied to the combustion tower. Furthermore, in another preferred aspect of the off-gas combustion system of the present invention, the off-gas combustion system includes a switching valve that switches between supply of off-gas to the combustion tower and supply of biogas.

  In general, the off-gas discharged from the methane separator has a low methane concentration (about 20%), so it is difficult to reliably ignite the combustion tower (burner) with this off-gas. Therefore, it is possible to guarantee stable off-gas combustion by first igniting the combustion tower using biogas having a high methane concentration (about 60%), preheating the combustion tower to a predetermined temperature, and switching to off-gas combustion. it can.

  Therefore, according to the aspect described above, by switching the switching valve, for example, the biogas before the methane separation treatment can be supplied to the combustion tower, and the combustion tower is first ignited reliably with this surplus biogas, and The combustion tower can be preheated to a predetermined temperature. In addition, even when the temperature of the combustion tower decreases during off-gas combustion, the biogas with a high methane concentration can be supplied to the combustion tower by switching the switching valve to the biogas supply side, which stabilizes the combustion tower. Guaranteed combustion.

  Here, it is preferable that the switching valve is provided between the buffer tank and the pressure regulator, so that the pressure of the biogas supplied to the combustion tower can be adjusted. Since the pressure of the biogas supplied to the combustion tower can be adjusted, the biogas after pressure adjustment can be supplied to the combustion tower even when the biogas contains pressure fluctuations. It can be burned stably and the amount of unburned components that can be generated during the combustion of biogas can be suppressed.

  Further, the off-gas combustion method of the present invention is an off-gas combustion method for burning off-gas discharged when methane is separated from biogas, and includes a methane separation step for separating methane from biogas, and the methane separation step. An off-gas storage step for storing the off-gas discharged, an off-gas pressure adjusting step for adjusting the pressure of the off-gas after the off-gas storage and before combustion, and an off-gas combustion step for burning the off-gas whose pressure has been adjusted It is characterized by becoming.

  According to the combustion method described above, by providing an off-gas storage step for storing off-gas discharged in the methane separation step, it is possible to reduce off-gas concentration fluctuations and adjust the off-gas pressure after storage and before combustion. By providing the off-gas pressure regulating step, off-gas pressure fluctuation can be reduced. Therefore, in the off-gas combustion process of burning off-gas, it is possible to burn off-gas having stable characteristics with reduced concentration fluctuations and pressure fluctuations, and unburned components that can be generated during off-gas combustion without using auxiliary fuel Can be reliably suppressed. The concentration and pressure of off-gas used for combustion can be appropriately set within a range suitable for the combustion state of the combustion process.

  Here, although the methane separation step can be performed using a PSA method or a membrane separation method, particularly according to the PSA method, as described above, there is a high possibility that the concentration fluctuation and the pressure fluctuation of the off-gas increase. Thus, by performing both the off-gas storage process and the pressure regulation process, the concentration fluctuation and pressure fluctuation of the off-gas can be effectively reduced.

  In a preferred aspect of the off-gas combustion method of the present invention, a biogas combustion step of burning the biogas is included as a step preceding the offgas combustion step. According to this aspect, for example, before the off-gas combustion step of burning off-gas in the combustion tower (burner), the combustion tower (burner) can be reliably ignited by burning the biogas having a high methane concentration. Therefore, stable combustion in the subsequent off-gas combustion process can be realized.

  In the biogas combustion step, it is preferable to burn the biogas after pressure adjustment. By burning the pressure-adjusted biogas, it is possible to stably burn the biogas with little pressure fluctuation, and it is possible to suppress the amount of unburned components that can be generated during the combustion of the biogas.

  As can be understood from the above description, according to the off-gas combustion system and the combustion method of the present invention, the generation of unburned components is suppressed without using auxiliary fuel when methane in the off-gas is combusted. An off-gas combustion system and a combustion method thereof are obtained.

1 is a block diagram showing an embodiment of an off-gas combustion system according to the present invention. The flowchart explaining the combustion method of the off-gas combustion system shown in FIG. The figure which shows the outline of the combustion tower used in Example 1 and Comparative Examples 1 and 2. FIG. The block diagram which shows one Embodiment of the off-gas combustion system of the comparative example 1. FIG. The block diagram which shows one Embodiment of the off-gas combustion system of the comparative example 2. FIG. FIG. 4 illustrates each measurement result of Example 1, (a) is a diagram showing the internal pressure of the buffer tank, (b) is the pressure of off-gas supplied to the combustion tower, carbon monoxide and methane in the exhaust gas The figure which shows the relationship of a density | concentration. It explains each measurement result of Comparative Example 1, (a) is a diagram showing the pressure of off-gas discharged from the PSA, (b) is a diagram showing the amount of off-gas heat before and after the buffer tank. The figure which shows the relationship between the pressure of the offgas supplied to the combustion tower of the comparative example 1, and the density | concentration of carbon monoxide and methane in waste gas. The figure which shows the measurement result of the carbon monoxide and methane density | concentration in the waste gas of Example 2. FIG. The figure explaining the outline | summary of the methane separation apparatus by PSA method.

  Hereinafter, a description will be given based on an embodiment of the present invention with reference to the drawings. FIG. 1 shows an embodiment of an off-gas combustion system according to the present invention. FIG. 2 explains the combustion method of the off-gas combustion system shown in FIG.

  An off-gas combustion apparatus 100 shown in FIG. 1 includes a methane separation apparatus (PSA) 1 by a PSA method, a methane storage tank 2, a buffer tank 3, a switching valve 4, a pressure regulator 5, a combustion tower 6, and a control unit 10 that is a control means. It has.

  The PSA1 communicates with a biogas supply source (not shown) via the biogas supply line L1. The PSA 1 communicates with the methane storage tank 2 through the methane gas transfer line L2, and the biogas supplied from the biogas supply source to the PSA1 is purified and separated by, for example, PSA1 as shown in FIG. The later high-concentration methane is stored in the methane storage tank 2 via the methane gas transfer line L2.

  Further, the PSA 1 communicates with the buffer tank 3 through an off-gas discharge line L3, and off-gas containing low-concentration methane discharged from the PSA 1 is temporarily stored in the buffer tank 3. The capacity of the buffer tank 3 can be appropriately set according to the off-gas combustion conditions and the like.

  The buffer tank 3 communicates with the combustion tower 6 through an off gas supply line L4, and a switching valve 4 and a pressure regulator 5 are interposed in the off gas supply line L4. Here, in the methane gas supply line L1, a bypass line L5 is branched at a branch point B1, and the bypass line L5 communicates with the switching valve 4 interposed on the off-gas supply line L4. Thereby, in the off gas supply line L4, the gas supplied to the combustion tower 6 can be switched between the off gas discharged from the buffer tank 3 and the surplus biogas supplied from the bypass line L5. The gas passing through the off-gas supply line L4 is supplied to the combustion tower 6 after the pressure is adjusted by the pressure regulator 5.

  Further, an exhaust gas line L6 is connected to the combustion tower 6. The exhaust gas line L6 is open to the atmosphere, and the exhaust gas after burning off-gas and biogas is diffused into the atmosphere via the exhaust gas line L6.

  The control unit 10 can control the start and stop of the PSA 1 and the start and stop of the combustion tower 6 and can control the desorption process of the PSA 1 and the like. Further, based on information such as the concentration and pressure of gas components in the buffer tank 3 and information on the combustion state of the combustion tower 6, the buffer tank 3 is opened and closed, the switching valve 4 is switched, the pressure regulator 5 is pressure controlled, and combustion is performed. The combustion state of the tower 6 can be controlled.

  Next, a combustion method of the off-gas combustion system shown in FIG. 1 will be described with reference to FIG.

  First, methane contained in biogas obtained from organic resources or the like (biomass) is separated and purified by PSA1 (S11). The separated and purified high-concentration methane is stored in the methane storage tank 2 (S12).

  The off-gas discharged from the PSA 1 after methane is separated from the biogas is stored in the buffer tank 3 (S13). The off-gas temporarily stored in the buffer tank 3 is released from the buffer tank 3 based on, for example, a signal from the control unit 10, and the pressure is adjusted by the pressure regulator 5 within a predetermined range (for example, substantially constant) ( S14). At this time, the switching valve 4 on the offgas supply line L4 shown in FIG. 1 supplies the offgas to the combustion tower 6.

  Then, the pressure-adjusted off-gas is burned in the combustion tower 6 (S15), and the burned exhaust gas is released to the atmosphere (S16).

  In addition, when the combustion tower 6 is ignited at the time of starting the system (that is, before combustion of off-gas), the switching valve 4 supplies biogas to the combustion tower 6. Burn the gas. Since the biogas flowing through the bypass line L5 is in a state before methane is separated by the PSA1, the methane content is higher than that of the offgas. Therefore, the biogas which flows via the bypass line L5 is supplied to the combustion tower 6, the combustion tower 6 is ignited steadily, and the combustion tower 6 is preheated. The pressure of the biogas supplied to the combustion tower 6 is adjusted by the pressure regulator 5. When the combustion tower 6 is preheated to a predetermined temperature, the information is transmitted to the control unit 10, and a switching signal is transmitted from the control unit 10 to the switching valve 4, and the off gas is sent to the combustion tower 6 as described above. Supply. Even when off-gas is combusted, the switching valve 4 can be switched according to the combustion state of the combustion tower 6 to supply biogas to the combustion tower 6.

[Experiment and results of carbon monoxide and methane concentrations in exhaust gas after off-gas combustion]
The present inventors, for Examples 1 and 2 having the configuration shown in FIG. 1 and Comparative Examples 1 and 2 having the configurations shown in FIGS. 4 and 5 below, the concentration and pressure of the offgas flowing in the offgas combustion system. The carbon monoxide and methane concentrations in the exhaust gas after off-gas combustion were measured. In Example 1 and Comparative Examples 1 and 2, the concentrations of carbon monoxide and methane in the exhaust gas after burning off gas mainly composed of about 35% methane and about 65% carbon dioxide were measured. In Example 2, the concentrations of carbon monoxide and methane in the exhaust gas after burning off-gas mainly composed of about 18% methane and about 82% carbon dioxide were measured.

  First, the combustion tower used in Examples 1 and 2 and Comparative Examples 1 and 2 will be described. In Examples 1 and 2 and Comparative Examples 1 and 2, the concentration of methane gas contained in the off-gas is low, and the off-gas pressure during combustion is also low. A combustion tower capable of burning sex gases was used. An outline of the combustion tower is shown in FIG.

  The combustion tower 6 shown in FIG. 3 is generally composed of a burner 62 disposed on a base 61 and a hollow cylindrical combustion cylinder 63. The combustion cylinder 63 includes an inner cylinder 65, an outer cylinder 66, and a partition member 67. The partition member 67 includes a cylindrical partition 71 and a flange 72 fixed to one end of the partition 71.

  The burner 62 is disposed substantially at the center of the inner cylinder 65, and the inner space of the inner cylinder 65 constitutes an exhaust gas line L61 for discharging exhaust gas after combustion. Further, the inner cylinder 65 and the outer cylinder 66 are arranged concentrically, and a cylindrical space 64 is formed by the inner cylinder 65 and the outer cylinder 66. The inner cylinder 65 and the outer cylinder 66 are connected to an annular lid 68 at the upper end, and the upper end of the cylindrical space 64 is closed by the lid 68. Further, a notch is formed in a part of the lower end portion of the inner cylinder 65 and the outer cylinder 66, and as shown in the drawing, the upper edge portion 65 A of the notch of the inner cylinder 65 is formed by the notch of the outer cylinder 66. It is provided above the upper edge portion 66A. This notch allows combustion air to flow from the outside air into the inner cylinder 65.

  Here, the diameter of the cylindrical partition 71 of the partition member 67 is set to be larger than the outer diameter of the inner cylinder 65 and smaller than the inner diameter of the outer cylinder 66. The partition member 67 is disposed concentrically with the inner cylinder 65 and the outer cylinder 66, and is installed on the upper surface of the base 61 by a flange 72. In addition, the height of the partition 71 of the partition member 67 is relatively lower than the height of the inner cylinder 65 and the outer cylinder 66, as shown in the figure.

  Next, the flow of air and fuel in the combustion tower 6 shown in FIG. 3 will be described.

  First, the air outside the combustion cylinder 63 passes through the notch provided at the lower end of the outer cylinder 66 (arrow X1), flows between the outer cylinder 66 and the partition 71 (arrow X2), and passes through the upper end of the partition 71. Rotates and flows between the partition 71 and the inner cylinder 65 (arrow X3), passes through a notch provided at the lower end of the inner cylinder 65 (arrow X4), and is supplied to the burner 62 in the inner space of the inner cylinder 65 Is done. The air supplied to the burner 62 is preheated by the combustion exhaust gas of the burner 62 while flowing between the inner cylinder 65 and the outer cylinder 66.

  A spiral guide 69 is formed on the outer peripheral surface 65B of the inner cylinder 65, and the spiral guide 69 allows the air flowing between the partition 71 and the inner cylinder 65 to pass through the spiral flow path. By flowing, the time during which the air supplied to the burner 62 is preheated becomes longer, and uniform air is supplied to the burner 62.

The base 61 is provided with two adjustment air lines L62 and L63 in order to prevent incomplete combustion due to the shortage of combustion air described above. Therefore, if carbon monoxide (CO), methane (CH ₄ ), etc. in the exhaust gas discharged when the off-gas or biogas is burned by the burner 62 is detected and determined to be incomplete combustion, adjustment is performed. Further adjustment air can be supplied to the burner 62 via the air lines L62 and L63.

  Note that the outer peripheral side of the outer cylinder 66 is covered with a heat insulating material 70. The combustion cylinder 63 is provided with a viewing hole 60 for observing the combustion state of the burner 62.

  Next, configurations of the off-gas combustion systems of Examples 1 and 2 and Comparative Examples 1 and 2 using the combustion tower will be described.

[Examples 1 and 2]
In Examples 1 and 2, an off-gas combustion system having the configuration shown in FIG. 1 was used. Here, the gas obtained from organic resources etc. (biomass) was used as biogas. Further, as the PSA, a two-column PSA methane separation apparatus (maximum processing capacity 12 m ³ / h) was used, and adsorption and desorption were repeated for each column at an adsorption cycle of 160 seconds (see FIG. 10). The flow rate of biogas flowing into the PSA was 10 m ³ / h, and the flow rates of concentrated methane stored in the methane storage tank and off-gas discharged from the PSA were 5 m ³ / h, respectively. In addition, the maximum value of the discharge pressure of concentrated methane at that time was 0.3 MPaG, and the discharge pressure of off-gas was about 0.03 to 6.0 kPa (see FIG. 7A). The capacity of the buffer tank for storing the off gas is 0.5 m ³ .

[Comparative Example 1]
As shown in FIG. 4, the off-gas combustion system of Comparative Example 1 is different from Examples 1 and 2 in that no pressure regulator is provided. Other configurations are the same as those in the first and second embodiments, and the performance of the PSA and buffer tank to be used is the same. That is, the offgas after the methane is separated by the methane separation device 1 ′ is temporarily stored in the buffer tank 3 ′, and the stored offgas is supplied to the combustion tower 6 ′ via the switching valve 4 ′ and burned. Is done. In addition, illustration is abbreviate | omitted about the control unit shown in FIG.

[Comparative Example 2]
As shown in FIG. 5, the off-gas combustion system of Comparative Example 2 is different from Examples 1 and 2 in that it does not include a buffer tank. Other configurations are the same as those in the first and second embodiments, and the performance of the PSA used is also the same. That is, the pressure of the off-gas after the methane is separated by the methane separation device 1 ″ is adjusted by the pressure regulator 5 ″ via the switching valve 4 ″, and the off-gas after the pressure adjustment is the combustion tower 6 ″. To be burned. In addition, illustration is abbreviate | omitted about the control unit shown in FIG.

[Relationship between concentration and pressure of off-gas flowing through off-gas combustion system and concentration of carbon monoxide and methane in exhaust gas after off-gas combustion]
Hereinafter, the concentration and pressure of off-gas flowing through the off-gas combustion system and the concentration of carbon monoxide and methane in the exhaust gas after off-gas combustion, measured by the present inventors, in Examples 1 and 2 and Comparative Examples 1 and 2, respectively. The relationship will be described.

  First, measurement results of Example 1 and Comparative Examples 1 and 2 in which off-gas having a relatively high methane concentration (about 35% methane and about 65% carbon dioxide as main components) are burned will be described.

[Example 1]
FIG. 6 illustrates each measurement result in Example 1. FIG. 6 (a) shows the internal pressure of the buffer tank, and FIG. 6 (b) shows the pressure of off-gas supplied to the combustion tower and the exhaust gas. It explains the relationship between carbon monoxide and methane concentrations. Note that the concentration variation of the buffer tank will be described based on the off-gas heat quantity measured in Comparative Example 1 shown in FIG.

First, as shown to Fig.7 (a), it was confirmed that the pressure of the off gas discharged | emitted from PSA fluctuates periodically between about 0.03-6 kPa. Further, as shown in FIG. 7 (b), the amount of heat before the off-gas stored in the buffer tank, it was confirmed that varies periodically between 0~28MJ / Nm ^3. This fluctuation cycle was equivalent to the PSA adsorption cycle (160 seconds). Further, since the combustible component in the off gas is only methane, the off gas calorific value and the methane concentration have a correlation. Therefore, it was confirmed that the off-gas methane concentration before being stored in the buffer tank fluctuates periodically. On the other hand, it was confirmed that the amount of off-gas heat released from the buffer tank was 14 to 15 MJ / Nm ³ and substantially constant. That is, it was proved that by providing the buffer tank, the concentration fluctuation of the methane concentration was reduced and the methane concentration was made uniform.

  However, as shown to Fig.6 (a), the internal pressure of the buffer tank in Example 1 is fluctuate | varied periodically between 30-60 kPa. This fluctuation cycle of the internal pressure is also related to the PSA adsorption cycle, and it was confirmed that the tank internal pressure increased immediately after the PSA switching, and the tank internal pressure decreased immediately before the PSA switching (PSA adsorption cycle 160 seconds). ). As described above, it has been proved that the pressure fluctuation of the off-gas cannot be reduced with the buffer tank alone by switching the PSA.

  FIG. 6B shows the relationship between the pressure of off-gas supplied to the combustion tower and the concentrations of carbon monoxide and methane in the exhaust gas. In the region R1 up to 8 minutes, the biogas was burned for ignition and preheating of the combustion tower.

  As shown in the figure, in Example 1, the off gas was burned by adjusting the pressure of the off gas supplied to the combustion tower to 0.21 kPa with a pressure regulator in the region R2 from 10 to 23 minutes. In this region R2, the methane concentration, which is an unburned component during combustion, was reduced to about 400 ppm, and the carbon monoxide concentration was reduced to about 40 ppm. In the region R3 from 23 to 42 minutes, the offgas pressure was further reduced and adjusted to 0.16 kPa or less by a pressure regulator to burn offgas. In this region R3, the methane concentration was reduced to about 30 ppm, and it was demonstrated that almost no carbon monoxide was discharged.

[Comparative Example 1]
FIG. 8 illustrates the relationship between the off-gas pressure supplied to the combustion tower and the concentrations of carbon monoxide and methane in the exhaust gas in Comparative Example 1. As in Example 1, in the region R1 ′ up to 12 minutes, the biogas was burned for ignition and preheating of the combustion tower.

  As described with reference to FIG. 7B, also in the comparative example 1, by providing the buffer tank, it is possible to reduce the fluctuation of the methane concentration in the off-gas supplied to the combustion tower. However, as described with reference to FIG. 6A, the pressure of the off-gas in the buffer tank has a periodic fluctuation due to the switching of the PSA.

  Therefore, as shown in FIG. 8, it is confirmed that the pressure of off-gas supplied to the combustion tower during off-gas combustion (region R2 ′ after about 13 minutes) also varies periodically between 0 and 8.2 kPa. It was done. Since the off-gas pressure fluctuates greatly in this way, combustion in the combustion tower becomes unstable, and in Comparative Example 1, it was demonstrated that 2000 ppm or more of methane and 1000 ppm or more of carbon monoxide are generated in the exhaust gas. The reason why the peaks of methane concentration and carbon monoxide concentration are flat is that methane and carbon monoxide having a concentration exceeding the measurement limit of the measuring device was detected.

[Comparative Example 2]
In Comparative Example 2, the combustion tower was misfired immediately after the gas supplied to the combustion tower was switched from biogas to off-gas, and combustion could not be continued. The reason for this is that since the buffer tank was not provided, as shown in FIG. 7B, the methane concentration in the off-gas discharged from the PSA greatly fluctuated, and the methane concentration in the combustion tower also fluctuated greatly. This is probably because the combustible component could not be stably supplied to the combustion tower.

  Next, measurement results of Example 2 in which off-gas having a relatively low methane concentration (about 18% methane and about 82% carbon dioxide as main components) are burned will be described. In the off-gas combustion system of the second embodiment, as in the first embodiment, by providing a buffer tank, the concentration fluctuation of the methane concentration is reduced and the methane concentration becomes substantially uniform. Since it has been demonstrated that the pressure of the gas is maintained substantially constant, the offgas was burned in a state where the methane concentration and the offgas pressure were adjusted to predetermined values.

[Example 2]
FIG. 9 shows the measurement results of the carbon monoxide and methane concentrations in the exhaust gas of Example 2. As shown in the figure, when off gas having a relatively low methane concentration is burned, it is demonstrated that the carbon monoxide concentration is reduced to about 5 to 10 ppm, and methane is hardly discharged. In particular, it was confirmed that the methane concentration in the exhaust gas was further reduced as compared with the case of burning off-gas having a high methane concentration.

  In this way, by providing both the buffer tank and the pressure regulator, it is possible to effectively suppress concentration fluctuations and pressure fluctuations of methane in the off-gas to a predetermined range according to the combustion state. It was proved that the off-gas with low methane concentration can be burned stably. Moreover, it was demonstrated that the offgas can be stably burned while suppressing the generation of unburned components over a wide range of the offgas produced by separating methane from the biogas.

DESCRIPTION OF SYMBOLS 1 ... Methane separator (PSA), 2 ... Methane storage tank, 3 ... Buffer tank, 4 ... Switching valve, 5 ... Pressure regulator, 6 ... Combustion tower, 10 ... Control unit, 100 ... Off-gas combustion system

Claims (9)

An off-gas combustion system for burning off-gas emitted when methane is separated from biogas,
A methane separator for separating methane from biogas,
A buffer tank for storing off-gas discharged from the methane separator;
A pressure regulator for adjusting the pressure of the offgas discharged from the buffer tank before the combustion of the offgas;
An off-gas combustion system comprising: a combustion tower that burns the off-gas whose pressure is adjusted by the pressure regulator.   The off-gas combustion system according to claim 1, wherein the methane separation device separates methane using a PSA method or a membrane separation method.   The offgas combustion system according to claim 1 or 2, wherein the biogas is supplied to the combustion tower.   The off-gas combustion system according to claim 3, further comprising a switching valve that switches between the supply of the off-gas to the combustion tower and the supply of the biogas.   The off-gas combustion system according to claim 4, wherein the switching valve is provided between the buffer tank and the pressure regulator, and is capable of adjusting the pressure of the biogas supplied to the combustion tower. An off-gas combustion method for burning off-gas emitted when separating methane from biogas,
A methane separation process for separating methane from biogas,
An off-gas storage step for storing off-gas discharged in the methane separation step;
An off-gas pressure regulating step of adjusting the pressure of the off-gas after storage of the off-gas and before combustion;
An off-gas combustion method comprising: an off-gas combustion step of burning the off-gas whose pressure is adjusted.   The off-gas combustion method according to claim 6, wherein the methane separation step is performed using a PSA method or a membrane separation method.   The off-gas combustion method of Claim 6 or 7 including the biogas combustion process which burns the said biogas as a process before the said offgas combustion process.   The offgas combustion method according to claim 8, wherein the biogas after pressure adjustment is burned in the biogas combustion step.

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