JP2012246495A - Off-gas burning apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an off-gas burning apparatus which can surely prevent methane in off-gas from being diffused to atmosphere and does not require an auxiliary fuel during burning treatment of the methane in the off-gas.SOLUTION: The off-gas burning apparatus which burns off-gas of a methane separator 1 for separating methane from biogas includes an off-gas line L3 through which the off-gas of the methane separator 1 flows and a combustion catalyst 7 for burning the off-gas. A heat exchanger 5 for adding heat of exhaust gas of the combustion catalyst 7 to the off-gas supplied to the combustion catalyst 7, a heater 6 for heating the off-gas supplied to the combustion catalyst 7 in an area between the heat exchanger 5 and the combustion catalyst 7, off-gas temperature measuring devices 21, 22 for measuring the temperature of the off-gas supplied to the combustion catalyst 7, and a catalyst temperature measuring device 23 for measuring the surface temperature of the combustion catalyst 7 are interposed in the off-gas line L3.

Description

The present invention relates to a technique for separating methane (CH ₄ ) from biogas. More specifically, the present invention relates to a technique for preventing methane in the offgas from being released into the atmosphere when methane is contained in the offgas discharged from an apparatus for separating methane from biogas.

Biogas generated from biomass contains components other than methane. In order to effectively use biogas, it is necessary to separate methane from biogas or to increase the methane concentration in biogas.
As a technology for separating methane from methane-rich biogas generated from an anaerobic fermentation process and purifying high-purity methane, for example, separation technology by PSA (Pressure Swing Adsorption) method or separation according to membrane separation method Technology exists.

  FIG. 6 shows an outline of the PSA method, in which an unnecessary component (for example, a component other than methane in biogas) is adsorbed and removed by an adsorbent packed in the tower, so that a predetermined amount is obtained. An apparatus for separating components (eg, methane).

In FIG. 6, a plurality of towers P (three in FIG. 6) filled with an adsorbent are provided, and each of the towers P has a biogas supply line Lb, a flow path switching device Vc, and a biogas supply branch line. Biogas is supplied through Lb1 to Lb3.
If biogas is supplied to the tower P (PSA) filled with the adsorbent and components other than methane in the biogas are adsorbed and removed by the adsorbent, the methane concentration in the remaining gas can be increased. If the biogas contains only methane and carbon dioxide (CO ₂ ), if the biogas is supplied into the tower P filled with a catalyst that adsorbs CO ₂ , the gas ( The gas supplied to the methane tank) contains only methane, or the methane concentration increases.
That is, if PSA is used as a methane separator, a methane concentration (or purity) can be relatively increased by adsorbing and removing components other than methane (for example, CO ₂ ).

Here, if only one tower P filled with the adsorbent is provided, it is necessary to repeat the adsorption and desorption and supply the biogas only in the case of adsorption. Therefore, the separation of methane (sophistication of methane concentration) must be processed in a so-called “batch type”.
In order to continuously separate or purify methane, a plurality of towers P are provided, biogas is supplied to the tower filled with the regenerated adsorbent, and the adsorption capacity of the adsorbent is reduced. For, the biogas supply is stopped and the adsorbent is desorbed (regenerated). That is, a component other than methane (for example, CO ₂ ) is adsorbed by an adsorbent in a certain column P, and a component other than methane (for example, CO ₂ ) is desorbed by another column P to regenerate the adsorbent. Therefore, the components other than methane are continuously adsorbed from the biogas.

In the desorption step, the tower P filled with the adsorbent to be desorbed (regenerated) is decompressed, and the component adsorbed on the adsorbent (CO _{2 in the} above example) is separated from the adsorbent, To discharge.
In the desorption process, the gas discharged (sucked) out of the tower P by decompression constitutes off-gas.

The tower filled with the adsorbent is generally switched to the desorption process (regeneration process) at a stage before the adsorbent in the tower P reaches the limit for adsorbing components other than methane (for example, CO ₂ ).
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. In order to prevent the adsorbent from adsorbing components other than methane and preventing components other than methane from being output together with methane to the methane storage tank, the adsorbent can be adsorbed at a stage where the adsorbent has sufficient capacity. It is necessary to switch from the adsorption process to a desorption process for regenerating the adsorbent. That is, the desorption process is performed on the tower filled with the adsorbent that can still be adsorbed.

As described above, in the desorption process, the inside of the tower is depressurized and discharged, so when the process is switched to the desorption process, the biogas remaining in the tower is also discharged outside the tower by the depressurization, and the offgas Is processed as
That is, since biogas containing methane (usually the methane concentration of biogas is about 60%) is discharged as off-gas to the outside of the tower, methane is included in the off-gas.

Although only the PSA of FIG. 6 has been described, similar problems exist with other methane separators.
For example, the membrane separation method is a method of separating a specific gas from a mixed gas of a plurality of types of gases by controlling fine pores of an inorganic membrane such as a polymer membrane or zeolite, and the molecular size of the target gas, The gas to be separated is separated from the mixed gas by utilizing the difference in shape.
However, it is well known that even in a methane separation apparatus using such a membrane separation method, methane gas may be mixed into the off-gas.

For off-gas containing methane, there was a case where it was released to the atmosphere before.
At present, when the methane concentration is high to some extent, it cannot be released into the atmosphere. Methane has a warming coefficient 20 to 30 times that of CO ₂ at the same number of moles, and methane emission to the atmosphere has an adverse effect on the global environment on the order of 20 to 30 times that of CO _{2 to} the atmosphere. Because it gives.

On the other hand, there is a conventional technique in which off-gas including methane is burned to reduce damage to the global environment. In the related art, off-gas containing methane is burned in the surplus combustion tower.
However, since the methane concentration in the biogas is unstable, the methane concentration in the off-gas is also unstable. Therefore, it is difficult to continue stable combustion without adding auxiliary fuel (for example, city gas). If there is no stable combustion in the surplus combustion tower, there is a risk of misfire. If misfire occurs in the surplus combustion tower, off-gas containing methane will be discharged into the atmosphere as unburned material. Like, it has a huge negative impact on the environment.
Therefore, in the related art, it is necessary to burn off gas containing methane while supplying auxiliary fuel (city gas or the like), and the cost for supplying auxiliary fuel is required.

As another conventional technique, for example, a digestion gas storage facility including a desulfurization device, a methane concentrator, and a gas holder filled with a methane storage agent has been proposed in order to increase the methane concentration and the methane storage amount. (See Patent Document 1).
In addition, a technique has been proposed in which auxiliary fuel gas and air are mixed in accordance with the supply amount and components of biogas and used as fuel gas (see Patent Document 2).
However, none of these prior arts (Patent Document 1 and Patent Document 2) disclose the treatment of methane contained in off-gas from an apparatus for separating methane from biogas.
JP 2001-949 A JP 2002-226878 A

  The present invention has been proposed in view of the above-described problems of the prior art, and when methane is contained in the offgas discharged when separating methane from biogas, the methane in the offgas is atmospheric. An object of the present invention is to provide an off-gas combustion apparatus that can reliably prevent the emission and does not require auxiliary fuel when methane in the off-gas is burnt.

  The off-gas combustion apparatus of the present invention is an off-gas line (L3) through which the off-gas of the methane separation apparatus (1) flows in the off-gas combustion apparatus that burns off-gas of a methane separation apparatus (eg, PSA) that separates methane from biogas. ) And a combustion catalyst (7) for burning off-gas, and in the off-gas line (L3), heat input to the off-gas supplied to the combustion catalyst (7) is stored in the off-gas line (L3). An exchanger (5), a heating device (for example, an electric heater 6) for heating off-gas supplied to the combustion catalyst (7) in a region between the heat exchanger (5) and the combustion catalyst (7), and a combustion Off-gas temperature measuring devices (first thermocouple 21 and second thermocouple 22) for measuring the temperature of off-gas supplied to the catalyst (7), and a catalyst temperature measuring device for measuring the surface temperature of the combustion catalyst (7) (heat The exhaust gas line (Lx) through which the exhaust gas of the combustion catalyst (7) flows is connected to the first exhaust gas line (Lx1) communicating with the heat exchanger (5), A flow rate adjusting device (solenoid valve V3) that branches to the second exhaust gas line (Lx2) that bypasses the heat exchanger (5) and has a function of adjusting the exhaust gas flow rate is the first exhaust gas line (Lx1) or An air supply device (for example, a blower 8) that is interposed in any of the second exhaust gas lines (Lx2) and supplies combustion air to the combustion catalyst (7) is provided, and the air supply device (8) discharges. The air supply line (La) communicating with the outlet is characterized in that it joins the upstream side (methane separation device 1 side) region of the off-gas line (L3) with respect to the heat exchanger (5).

In the present invention, since the combustion catalyst (7) is arranged, methane contained in the off-gas of the methane separation device (1: PSA, for example) is burned by the combustion catalyst (7). Stable combustion is possible.
When the concentration of methane contained in the offgas is so low that it cannot be burned by the burner, the combustion catalyst (7) can burn the methane contained in the offgas on the combustion catalyst side.

  As a result, the range in which the methane contained in the off gas can be reliably burned (the range of the methane concentration in the off gas) is expanded particularly to the side where the methane concentration is reduced.

Even if the methane concentration in the off-gas is so low that stable combustion cannot be guaranteed by using a burner, the low concentration methane can be reliably burned by the combustion catalyst.
That is, according to the present invention, methane contained in the off-gas is reliably burned without using auxiliary fuel and without misfiring. As a result, atmospheric emission of methane is completely prevented, and the global environment is not adversely affected.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
An embodiment of the present invention will be described with reference to FIGS.

In FIG. 1, an off-gas combustion apparatus denoted as a whole by reference numeral 103 combusts methane contained in a PSA-type methane separator (PSA) 1, a methane storage tank 2, a heat exchanger 5, an electric heater 6, and off-gas. And a control unit 10 as control means.
The PSA 1 communicates with a methane gas supply source (not shown) via the methane gas supply line L1.

The PSA 1 communicates with the methane storage tank 2 through a methane gas transfer line L2, and the PSA 1 that is a methane separator and the combustion catalyst 7 are connected by a methane gas-containing off-gas line L3.
A heat exchanger 5, a first thermocouple (temperature sensor) 21, an electric heater 6, and a second thermocouple 22 are interposed in the methane gas-containing offgas line L3.
A third thermocouple 23 for measuring the surface temperature is attached to the combustion catalyst 7, and a fourth thermocouple 24 is interposed in the combustion exhaust gas line Lx of the combustion catalyst 7.

A combustion exhaust gas line Lx through which combustion exhaust gas from the combustion catalyst 7 flows is branched at a branch point B into a first branch line Lx1 and a second branch line Lx2.
The first branch line Lx1 is opened to the atmosphere via the heat exchanger 5, and an electric valve (flow control valve) V3 is interposed in a region between the branch point B and the heat exchanger 5. Yes.
The second branch line Lx2 is also open to the atmosphere.

In the methane gas-containing off-gas line L3, a junction G is formed between the heat exchanger 5 and the PSA1, and the air supply line La is joined to the junction G.
A flow control valve Va is interposed in the air supply line La, and a blower 8 as an air supply device is connected to the end of the air supply line La.

The first thermocouple 21 to the fourth thermocouple 24 are all connected to the control unit 10 through the input signal line Si.
The PSA 1, the electric heater 6, the electric valve V3, and the flow control valve Va are all connected to the control unit 10 via the control signal line So.

As the combustion catalyst 7, for example, palladium supported on alumina is used.
In order to burn with the combustion catalyst 7, it is necessary to heat to a predetermined temperature or higher. However, the required temperature range differs depending on the type of catalyst. The catalyst used in the illustrated embodiment needs to be heated, for example, within a range of 400 ° C. to 800 ° C. in order to burn methane contained in the off gas. In other words, the lower limit target value of the combustion temperature is 400 ° C., and the upper limit target value is 800 ° C.
As described above, the combustion catalyst 7 is more suitable for the combustion of off-gas having a low methane concentration to the extent that misfire occurs using the burner than the combustion of off-gas having a high methane concentration to the extent that the burner 4 can stably burn. .

In order for the off-gas to burn in the combustion catalyst 7, combustion air is required. Therefore, in FIG. 1, the above-described air supply line La and blower 8 are provided.
Furthermore, in order to prevent sintering (sintering) of the combustion catalyst 7, it is necessary that the temperature of the combustion catalyst 7 does not exceed the upper limit of 800 ° C. That is, it is necessary to control the upper limit value of the temperature in the combustion catalyst 7.
Here, in the combustion catalyst 7 used in the illustrated embodiment, 400 ° C. has some allowance for the lower limit value of the combustion temperature, and 800 ° C. has some allowance for the upper limit value of the combustion temperature. It is selected as a numerical value.
Of course, since the upper limit value and the lower limit value of the combustion temperature differ depending on the type of catalyst, the specific examples of the upper limit target value and the lower limit target value of the above-described temperatures of 800 ° C. and 400 ° C. also differ.

In order to raise the temperature of the gas flowing into the combustion catalyst 7 to, for example, 400 ° C. or higher, the amount of heat held by the combustion exhaust gas of the combustion catalyst 7 flowing through the exhaust gas line Lx (first branch line Lx1) is converted into a heat exchanger. 5, the off gas flowing into the combustion catalyst 7 is introduced.
Here, depending on the combustion state of the combustion catalyst 7, the temperature of the off-gas flowing into the combustion catalyst 7 can be raised to a necessary temperature (400 ° C.) simply by inputting the amount of heat held by the combustion exhaust gas in the heat exchanger 5. There may be a shortage. In order to deal with such a case, in FIG. 1, an electric heater 6 as a heating means is interposed in a region downstream of the heat exchanger 5 (combustion catalyst 7 side) of the methane-containing offgas line L3. Thus, it is possible to raise the temperature of the off gas flowing into the combustion catalyst 7 to a target temperature (for example, 400 ° C.).

The bypass line Lx2 (second branch line Lx2) is combusted by the combustion catalyst 7 so that the surface temperature of the combustion catalyst 7 does not become too high and becomes higher than the target upper limit (800 ° C.). Part (or all) of the exhaust gas is provided to bypass the heat exchanger 5.
When the catalyst temperature becomes too high and it is necessary to lower the temperature of the off-gas flowing into the combustion catalyst 7, the opening degree of the solenoid valve (flow control valve) V3 is reduced (squeezing the solenoid valve V3). The flow rate of the combustion exhaust gas flowing through the heat exchanger 5 is reduced, and thus the amount of heat input to the off-gas flowing into the combustion catalyst 7 is reduced.

As described above, the third thermocouple 23 that measures the surface temperature of the combustion catalyst 7 and the fourth thermocouple 24 that measures the combustion exhaust gas temperature at the outlet of the combustion catalyst 7 are provided. Only one of the thermocouple 23 and the thermocouple 24 may be provided.
Of course, as shown in FIG. 1, both the third thermocouple 23 and the fourth thermocouple 24 may be provided.

Generally, in order to prevent the reaction surface of the combustion catalyst 7 from adhering to foreign matters, a filter is required on the PSA 1 side or the biogas supply source side (not shown: left side in FIG. 1).
However, in the embodiment shown in FIG. 1, no filter is interposed on the upstream side of the combustion catalyst 7.
This is because foreign substances are adsorbed and removed by an adsorbent (not shown) in the PSA 1. In addition, since a filter (not shown) is provided on the outlet side of the PSA 1, foreign matters are also removed there.

Control at the time of starting the off-gas combustion apparatus 103 using the combustion catalyst shown in FIG. 1 will be described with reference to FIG.
In FIG. 2, the electric heater 6 is operated in step S41, and the blower 8 is operated in step S42 to supply combustion air to the methane-containing offgas line L3.
In step S43, the solenoid valve V3 is fully opened, and the process waits until the measured value of the second thermocouple 22, that is, the temperature of the off-gas flowing into the combustion catalyst 7 becomes 400 ° C. or higher (step S44 is a NO loop).

When the temperature of the off-gas flowing into the combustion catalyst 7 (measured value of the second thermocouple 22) reaches 400 ° C. or more (step S44 is YES), the process proceeds to step S45, and the first tower PSA1 is activated.
Then, it progresses to step S46 and the desorption process is started in PSA1 of the 1st tower. By starting the desorption process, the off-gas from the first tower PSA 1 is sent to the combustion catalyst 7.
In step S47, the control proceeds to control when the combustion catalyst 7 is in a steady state (control during steady state: FIGS. 3, 4, and 5).

In the constant control, the control of the electric heater 6 shown in FIG. 3, the control of the blower 8 shown in FIG. 4, and the opening / closing control of the electromagnetic valve V3 shown in FIG. 5 are executed in parallel.
First, control of the electric heater 6 in FIG. 3 will be described.
In FIG. 3, in step S51, the first thermocouple 21 is checked.
The control unit 10 determines whether or not the measured value of the first thermocouple 21 is less than 400 ° C. (step S52).

If the measured value of the first thermocouple 21 is less than 400 ° C. (YES in step S52), it is determined that the temperature rise (heating amount) of the off gas flowing into the combustion catalyst 7 is insufficient, and in step S53. The electric heater 6 is activated and the process proceeds to step S55.
In step S52, if the measured value of the first thermocouple 21 is 400 ° C. or higher (NO in step S52), it is determined that the temperature rise (heating amount) of the off-gas flowing into the combustion catalyst 7 is sufficient, Proceeding to step S54, the electric heater 6 is not operated, and if the electric heater 6 is activated, this is stopped and the process proceeds to step S60.

In step S55 (the measured value of the first thermocouple 21 is less than 400 ° C .: YES in step S52), the second thermocouple 22 is checked, and the control unit 10 determines whether the measured value of step S55 is within a predetermined range. It is determined whether or not (step S56).
If the measured value in step S55 is within a predetermined range (YES in step S56), it is determined that the temperature rise (heating amount) of the off gas flowing into the combustion catalyst 7 is appropriate, and the process proceeds to step S57 where the output of the electric heater 6 is output. The process is maintained as it is, and the process proceeds to step S60.
If the measured value in step S55 is lower than the predetermined range (step S56 is “low”), it is determined that the temperature rise (heating amount) of the off-gas flowing into the combustion catalyst 7 is insufficient, and the process proceeds to step S58. Then, the output of the electric heater 6 is increased and the process proceeds to step S60.
If the measured value in step S55 is higher than the predetermined range (step S56 is “high”), it is determined that the temperature rise (heating amount) of the off-gas flowing into the combustion catalyst 7 is excessive, and the process proceeds to step S59. The output of the heater 6 is decreased and the process proceeds to step S60.
In step S60, the control unit 10 determines whether or not to end the control of the electric heater 6. If the control of the electric heater 6 is to be finished (step S60 is YES), the control is finished as it is. On the other hand, if the control is to be continued (NO in step S60), the process returns to step S51 and repeats step S51 and subsequent steps.

Next, control of the blower 8 of FIG. 4 will be described.
In FIG. 4, in step S61, the third thermocouple 23 measuring the surface temperature of the combustion catalyst 7 is checked. Then, it is determined whether or not the surface temperature of the combustion catalyst 7 is an appropriate temperature (step S62).
If the measured value of the third thermocouple 23 is an appropriate temperature (predetermined range, for example, 500 ° C. to 700 ° C.) (YES in Step S62), it is determined that the air volume of the blower 8 is appropriate, and Step S63 is performed. The flow of the blower 8 is maintained.

  If the catalyst surface temperature measured by the third thermocouple 23 is higher than the appropriate temperature (“temperature high” in step S62), the opening degree of the flow control valve Va is increased and supplied to the combustion catalyst 7. The air flow rate (blower air volume) is increased to lower the catalyst surface temperature (step S64). Here, if a large amount of air is supplied from the blower 8 to the combustion catalyst 7, the surface temperature of the combustion catalyst 7 measured by the third thermocouple 23 falls.

On the other hand, if the catalyst surface temperature measured by the third thermocouple 23 is lower than the appropriate temperature (“temperature low” in step S 62), the flow control valve Va is throttled and the air supplied to the combustion catalyst 7. Reduce the flow rate (blower air volume).
If the flow rate of air supplied to the combustion catalyst 7 (blower air volume) decreases, the surface temperature of the combustion catalyst 7 measured by the third thermocouple 3 rises.

In step S66, the control unit 10 determines whether or not to end the air volume control of the blower 8. If the blower air volume control is to be terminated (YES in step S66), the control is terminated as it is.
If the air volume control of the blower 8 is to be continued (NO in step S66), the process returns to step S61 and repeats step S61 and subsequent steps.

The opening / closing control of the electromagnetic valve V3 in FIG. 5 will be described.
In FIG. 5, in step S71, the process waits until the air volume of the blower 8 becomes maximum (step S71 is NO loop). If the blower air volume becomes maximum (YES in step S71), the third thermocouple 23 and / or the combustion catalyst that measures the surface temperature of the combustion catalyst 7 corresponding to the temperature of the combustion exhaust gas of the combustion catalyst 7 The third thermocouple 24 measuring the combustion exhaust gas temperature at the outlet 7 is checked (step S72).
In step S73, it corresponds to the surface temperature of the combustion catalyst 7 (measured value of the third thermocouple 23) and / or the combustion exhaust gas temperature at the outlet of the combustion catalyst 7 (measured value of the third thermocouple 24). Thus, the opening degree of the electromagnetic valve V3 is controlled to an appropriate opening degree. Here, the surface temperature of the combustion catalyst 7 (measured value of the third thermocouple 23) and / or the combustion exhaust gas temperature at the outlet of the combustion catalyst 7 (measured value of the fourth thermocouple 24), and the electromagnetic valve V3. The relationship with the appropriate opening is preset and stored in the control unit 10 in the form of a characteristic equation, a characteristic table, a characteristic map, or the like.
In step S74, it is determined whether or not the opening / closing control of the electromagnetic valve V3 is to be ended. If the control is continued (NO in step S74), the process returns to step S71. Step S71 and subsequent steps are repeated.

The illustrated embodiment is merely an example, and is not intended to limit the technical scope of the present invention.
For example, in the illustrated embodiment, a methane separation device by the PSA method is used as a device for purifying or separating methane from biogas, but a methane separation device by a membrane separation method may be used.

The block diagram which shows embodiment of this invention. The flowchart which shows the starting procedure of embodiment. The flowchart which shows control of the electric heater at the time of steady state of embodiment. The flowchart which shows the control of the blower at the time of steady state of embodiment. The flowchart which shows the flow control to the heat exchanger of the catalyst combustion exhaust gas in the steady state of embodiment. Explanatory drawing which shows the outline | summary of a PSA separation device.

1 ... Methane separator / PSA
2 ... methane storage tank 5 ... heat exchanger 6 ... electric heater 7 ... combustion catalyst 8 ... blower 10 ... control means / control units 21-24 ... first to first 4 thermocouples L1 ... biogas supply line L2 ... methane gas transfer line L3 ... methane-containing offgas line Lx ... exhaust gas line

Claims (1)

An off-gas combustion apparatus for burning off-gas of a methane separation apparatus (1) for separating methane from biogas, an off-gas line (L3) through which the off-gas of the methane separation apparatus (1) flows, and a combustion catalyst (7) for burning off-gas The off-gas line (L3) includes a heat exchanger (5) that inputs the amount of heat held in the exhaust gas of the combustion catalyst (7) into the off-gas supplied to the combustion catalyst (7), and a heat exchanger (5). And a heating device (6) for heating off gas supplied to the combustion catalyst (7) in a region between the combustion catalyst (7) and an off gas temperature measurement for measuring the temperature of the off gas supplied to the combustion catalyst (7) The devices (21, 22) and the catalyst temperature measuring device (23) for measuring the surface temperature of the combustion catalyst (7) are interposed, and the exhaust gas line (Lx) through which the exhaust gas of the combustion catalyst (7) flows is Heat exchange Branching into a first exhaust gas line (Lx1) communicating with (5) and a second exhaust gas line (Lx2) bypassing the heat exchanger (5), the function of adjusting the exhaust gas flow rate An air supply device for supplying combustion air to the combustion catalyst (7), the flow rate adjusting device (V3) having the flow rate adjusting device (V3) interposed in either the first exhaust gas line (Lx1) or the second exhaust gas line (Lx2) (8) is provided, and the air supply line (La) communicating with the discharge port of the air supply device (8) joins a region upstream of the heat exchanger (5) of the off-gas line (L3). An off-gas combustion apparatus characterized by that.

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