JP2012211213A - Method for refining biogas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a biogas refining technology suitable for the case where a biogas is injected into a city gas conduit.SOLUTION: A gas from a methane fermentation tank 4 is once stored in a storage tank 9, and thereafter mixed with a gas containing hydrogen from a storage tank 3a, and introduced to a refining apparatus 5. The mixed gas is brought into counterflow contact with water sprayed from an absorption tower (not shown) upper part in a high pressure state. The mixed gas in the absorption tower is separated according to the differences in solubility to water. Carbon dioxide and sulfur-based impurities are absorbed in water, and methane, hydrogen and oxygen are not absorbed but exit the refining apparatus 5 as gases. Siloxane compounds condense in the high-pressure state, and accompany the high-pressure water and are collected in the lower part of the absorption tower and removed. A biogas from the refining apparatus 5 is introduced to an oxygen removal apparatus 6. Hydrogen and oxygen are catalytically burned in a reactor and removed, only methane is introduced to a heat quantity regulator 7, and the heat quantity regulation is carried out according to the city gas supply rule. The heat-regulated gas is pressure-regulated and then injected in a gas conduit L0 and injected in conduit.

Description

  The present invention relates to a biogas purification method, and more particularly to a biogas purification method suitable for injecting purified biogas into a city gas conduit.

2. Description of the Related Art In recent years, as a new renewable energy, attention has been paid to a technique of fermenting biomass such as sewage sludge with anaerobic microorganisms and using methane obtained by refining the produced biogas as power generation or boiler fuel. Furthermore, techniques for injecting and supplying biogas into city gas conduits have also been proposed (for example, Patent Documents 1 and 2).
When injecting into city gas conduits, it is necessary to keep the impurity components other than methane below the allowable concentration for supply. In the biogas supply system 100 of Patent Document 1, the supply gas branched and introduced from the natural gas conduit 104 as shown in FIG. 4 is mixed with the biogas purified and adjusted in the purification device 101 and the calorific value adjustment device 102. It is temporarily stored in the mixing tank 103 and injected into the conduit 104. Corresponding to the impurity concentration of the purified biogas, the mixing ratio of the introduced supply gas and the purified biogas is adjusted by the valve 105 to control the impurity concentration of the injected gas to be equal to or lower than a predetermined reference value.

Biogas generally contains carbon dioxide as a main component in addition to methane, and contains trace amounts of hydrogen sulfide, siloxane, oxygen, etc., so carbon dioxide is removed by chemical absorption, PSA, high-pressure water absorption, etc. . Hydrogen sulfide can be removed using activated carbon or iron oxide. Siloxane is particularly contained in biogas derived from sewage sludge methane, and can be removed by activated carbon or high-pressure water absorption.
Oxygen is derived from dissolved oxygen in the purification process by high-pressure water absorption or leakage from pipes, etc., and a method of adding hydrogen from outside the system and removing it by catalytic combustion reaction is disclosed (for example, patent document) 2).

  Regarding the hydrogen fermentation process in biogas production, the hydrogen fermentation process and methane fermentation process of food waste are promoted in two stages, so that hydrogen / acid production and methane production are efficiently advanced, and the gasification efficiency of organic matter is improved. Has been proposed (Patent Document 3).

JP 2010-59416 A JP 2004-300206 A JP 2006-255538 A

However, hydrogen addition from outside the system requires a hydrogen supply facility such as a hydrogen supply device or a water electrolysis device, and the facility cost and running cost become problems.
Moreover, although the technique of patent document 3 is accompanied by hydrogen production | generation in a system, it aims at using produced | generated hydrogen for boiler fuel, fuel for fuel cells, etc., and the impurity oxygen contained in biogas It is not intended for removal.

The present invention provides an efficient and low-cost biogas purification technique by using hydrogen generated by hydrogen fermentation of biomass for oxygen removal.
The gist of the present invention is as follows. That is, the biogas purification method according to the present invention comprises:
(1) A method for purifying biogas, in which raw material biomass is fermented with hydrogen to produce a gas mainly composed of hydrogen gas and carbon dioxide, and biomass that has undergone the hydrogen fermentation process is fermented with methane. , A methane fermentation process for producing biogas mainly composed of methane and carbon dioxide, a hydrogen gas addition process for adding a gas containing hydrogen gas that has undergone a hydrogen fermentation process to a biogas that has undergone a methane fermentation process, and hydrogen gas addition An impurity removal step of removing carbon dioxide and other impurities from the biogas that has undergone the process, and an oxygen removal step of removing oxygen, which is an impurity, by reacting hydrogen and oxygen in the biogas that has undergone the impurity removal step. It is characterized by including.

(2) The residence time of the hydrogen fermentation process is controlled corresponding to the amount of biogas generated through the methane fermentation process.

(3) In the oxygen removal step, when the residual oxygen concentration in the purified biogas after removal exceeds a predetermined reference value, in addition to the hydrogen gas obtained by the hydrogen fermentation step, further hydrogen gas from outside the system Is added.

  ADVANTAGE OF THE INVENTION According to this invention, in the oxygen removal in biogas, the hydrogen supply from the outside can be made unnecessary or minimized, and there exists an effect that an installation cost and a running cost can be reduced.

It is a figure which shows the structure of the biogas purification system 1 which concerns on 1st embodiment. It is a figure which shows the flow of hydrogen fermenter residence time control of 1st embodiment. It is a figure which shows the structure of the biogas purification system 20 which concerns on 2nd embodiment. It is a figure which shows the structure of the conventional biogas purification system.

  Hereinafter, embodiments of the present invention will be described in more detail with reference to FIGS. Needless to say, the scope of the present invention is described in the claims and is not limited to the following embodiments.

<First embodiment>
FIG. 1 is a diagram showing a configuration of a biogas purification system 1 according to an embodiment of the present invention.
The purification system of the biogas purification system 1 includes a pretreatment tank 2, a hydrogen fermentation tank 3, a methane fermentation tank 4, a purification apparatus 5, an oxygen removal apparatus 6, a calorific value adjustment apparatus 7, a storage tank 3a, and a storage tank. A tank 9 is provided as a main component. Each apparatus is connected by piping L1-L8. Furthermore, the piping L8 for mixing the hydrogen gas produced | generated in the hydrogen fermenter 3 with the methane fermentation tank 4 output gas, and the piping L8 for inject | pouring refined gas into the city gas conduit L0 are provided.
In addition, the control system includes a flow meter S1 that measures the flow rate of the methane fermentation tank output gas, and a feeding device 10 (for example, for feeding the liquid or slurry fermentation residue in the hydrogen fermentation tank 3 to the methane fermentation tank 4). The main components are a slurry pump and the like, and a control unit 8 that commands the operation of the feeding device 10 based on the measurement value of the flow meter S1 and controls the residence time in the hydrogen fermenter 3.

Biomass that has been subjected to physicochemical treatment (foreign matter removal, heating, crushing, acid treatment, alkali treatment, etc. in the pretreatment device 2) is introduced into the hydrogen fermenter 3. In the hydrogen fermentation tank 3, the raw material biomass is decomposed in the presence of anaerobic microorganisms in the tank, and a gas mainly composed of hydrogen and carbon dioxide is generated. The generated gas is temporarily stored in the storage tank 3a.
The raw material is supplied batchwise or continuously. As hydrogen fermentation conditions, the temperature is preferably 30 ° C. to 65 ° C., the pressure is normal pressure, and the pH is preferably in the range of 4-8.
The amount of hydrogen and carbon dioxide generated in the product gas is determined by the residence time of the raw material in the tank, and the longer the residence time, the more hydrogen gas is generated. The residence time is determined according to the flow rate of the methane fermentation tank output gas, as will be described later.

  The hydrogen fermentation residue in the hydrogen fermenter 3 includes organic acids (acetic acid, formic acid, butyric acid, etc.), unreacted organic substances, and the like, and these are fed into the methane fermenter 4 by the operation of the feeding device 10. The hydrogen fermentation residue is decomposed by anaerobic microorganisms in the methane fermentation tank 4 to generate a gas mainly composed of methane and carbon dioxide. The fermentation residue is collected in a treatment tank (not shown), processed, and used as fertilizer. As methane fermentation conditions, the temperature is preferably between 35 ° C.-60 ° C. and pH 6-8.

The gas output from the methane fermentation tank 4 is temporarily stored in the storage tank 9 and then mixed with the gas containing hydrogen output from the storage tank 3 a and introduced into the purifier 5. The introduced gas contains methane, carbon dioxide and hydrogen as main components, and contains trace amounts of hydrogen sulfide, siloxane, oxygen and the like as impurities.
In the purifier 5, the mixed gas is pressurized to a predetermined pressure by a booster (not shown), and in countercurrent contact with water sprayed from the upper part of the absorption tower (not shown). The mixed gas in the absorption tower is separated by the difference in solubility in water. That is, carbon dioxide and sulfur-based impurities (such as H ₂ S) are absorbed by water, and methane, hydrogen, and oxygen exit the purifier 5 without being absorbed. Further, the siloxane compound is condensed in a high pressure state, and is collected and removed at the bottom of the absorption tower accompanying the high pressure water.

  Biogas from the refining device 5 containing methane, hydrogen and trace oxygen is introduced into the oxygen removing device 6. Here, hydrogen and oxygen are removed by catalytic combustion in a reactor (not shown). As a catalyst to be used, a catalyst in which Pt, Pd, Ru, Rh, Ag is supported on a support can be used. Biogas having an allowable concentration or less with respect to impurity components other than methane is introduced into the calorific value adjusting device 7. Here, the amount of heat is adjusted according to the city gas supply regulations. The heat-adjusted gas is pressure-adjusted and then injected into the gas conduit L0 to be used as part of the supply gas in the conduit.

Next, the specific contents of the residence time control in the hydrogen fermenter 3 executed by the command of the control unit 8 will be described with reference to FIG. In the initial state, the residence time Ts is set to a default value (Td) (S101). During the fermentation process, the flow rate Fm of the methane fermentation tank output gas is measured (S102).
If Fm is within the upper and lower threshold ranges (Fmax ≧ Fm ≧ Fmin) (YES in S103), the current residence time is maintained (S105). When Fm> Fmax, the amount of methane produced is excessive, so that the amount of hydrogen fermentation gas generated is increased, and the methane production ratio expressed by the energy ratio (calorie base) of the generated methane and hydrogen is reduced. The time is extended by one step (S104). When Fm <Fmin, the amount of methane produced is too small, so the residence time is shortened by one step in order to reduce the amount of hydrogen fermentation gas produced and increase the methane production ratio (S106).
Usually, since the oxygen concentration of the purified gas 5 is 0.2-1.0%, the amount of hydrogen addition can be increased or decreased according to the oxygen concentration by repeatedly performing the above control every predetermined time. The residual oxygen concentration of the biogas injected into the gas conduit L0 can be controlled to a value that is acceptable for supply.

  In the present embodiment, the example in which the carbon dioxide, the sulfur-based impurities and the siloxane compound in the mixed gas are removed by high-pressure water absorption in the purification apparatus 5 has been shown. Can be removed by activated carbon or iron oxide, followed by removal of siloxane compounds by activated carbon, and subsequent removal of carbon dioxide by PSA or an absorbent.

  Moreover, in this embodiment, although the example which controls the residence time of the hydrogen fermenter 3 based on the flow volume of the methane fermentation tank 4 output gas was shown, the hydrogen fermenter 3 based on the residual oxygen concentration of the oxygen removal apparatus 6 output gas The residence time may be controlled.

  Moreover, although the example which once stores the hydrogen fermentation tank 3 output gas in the storage tank 3a was shown, it is good also as an aspect made to mix with the methane fermentation tank 4 output gas directly, without storing.

<Second Embodiment>
Next, another embodiment of the present invention will be described with reference to FIG. The configuration of the biogas purification system 20 according to the present embodiment is different from the above-described biogas purification system 1 in the configuration of the oxygen removing device. That is, the oxygen removing device 21 of the present embodiment includes a hydrogen gas addition device 23 including a hydrogen cylinder 23 a and a flow rate control device 23 b on the upstream side of the oxidation catalyst reactor 22, and the gas introduced into the catalyst reactor 22. Is that hydrogen is added. Furthermore, an oxygen concentration meter S2 is provided on the downstream side of the catalyst reactor 22, and the residual oxygen concentration of the gas output from the catalyst reactor 22 can be measured. Since the other configuration is the same as that of the biogas purification system 1, a duplicate description is omitted.
With the above configuration, in the biogas purification system 20, the oxygen concentration of the gas output from the catalytic reactor 22 is controlled to a predetermined allowable concentration or less based on the measurement value of the oxygen concentration meter S2. That is, when the oxygen concentration of the gas output from the catalyst reactor 22 is equal to or greater than a predetermined threshold, hydrogen from the hydrogen gas addition device 23 is added to promote the oxidation reaction in the catalyst reactor 22. As a result, the residual oxygen concentration of the biogas injected into the gas conduit L0 can be controlled below the allowable reference value.

Hereinafter, the component ratio and energy recovery efficiency of the produced biogas when the raw material residence time of the hydrogen fermenter was changed were measured. The test conditions are as follows.

Table 2 shows the hydrogen and methane generation ratio with respect to the total biogas generation amount for each residence time condition. Here, the recovery efficiency is the ratio of the energy of the recovered gas (hydrogen, methane) to the energy of the raw material (calorie base).

  From the above results, it was found that the ratio of the recovery efficiency of hydrogen and methane can be changed by changing the residence time of the hydrogen fermenter.

  The present invention can be widely applied not only to sewage sludge but also to biogas purification systems using various biomass as raw materials such as raw garbage, food waste, waste materials, and wood chips.

DESCRIPTION OF SYMBOLS 1, 20 ... Biogas purification system 2 ... Pretreatment tank 3 ... Hydrogen fermentation tank 4 ... Methane fermentation tank 5 ... Purification apparatus 6, 21 ... Oxygen Removal device 7 ... Heat amount adjusting device 8 ... Control unit 9 ... Storage tank 10 ... Delivery device 22 ... Oxidation catalyst reactor 23 ... Hydrogen gas addition device L0 ... ・ Gas conduit S1 ... Flow meter S2 ... Oxygen concentration meter

Claims (3)

A method for purifying biogas,
A hydrogen fermentation process in which raw material biomass is fermented with hydrogen to generate a gas mainly composed of hydrogen gas and carbon dioxide;
A methane fermentation process in which biomass that has undergone a hydrogen fermentation process is subjected to methane fermentation to produce biogas mainly composed of methane and carbon dioxide;
A hydrogen gas addition process for adding a gas containing hydrogen gas that has undergone a hydrogen fermentation process to biogas that has undergone a methane fermentation process;
Impurity removal process for removing carbon dioxide and other impurities from the biogas that has undergone the hydrogen gas addition process,
Oxygen removal step of reacting hydrogen and oxygen in biogas that has undergone the impurity removal step to remove oxygen as an impurity;
A method for purifying biogas, comprising:   The biogas purification method according to claim 1, wherein the residence time of the hydrogen fermentation process is controlled in accordance with the amount of biogas generated through the methane fermentation process. In the oxygen removal step, when the residual oxygen concentration in the purified biogas after removal exceeds a predetermined reference value, hydrogen gas from outside the system is further added in addition to the hydrogen gas obtained by the hydrogen fermentation step. The method for purifying biogas according to claim 1 or 2, wherein

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