JP5202833B2 - Biogas purification system - Google Patents

Description

  The present invention relates to a biogas purification system, and more particularly to a biogas purification system capable of performing efficient biodesulfurization while maintaining the alkalinity of a cleaning liquid for biodesulfurization.

  Conventionally, as a desulfurization method of a gas containing hydrogen sulfide, a method using iron oxide or alkali as a desulfurization agent is known, but many desulfurization agents are required for desulfurization of a gas containing hydrogen sulfide at a high concentration. It is not preferable in terms of economy and maintenance.

  For this purpose, a desulfurization tower is provided with a filler carrying sulfur-oxidizing bacteria. By passing biogas through this filler, the sulfur-oxidizing bacteria are contacted with sulfur-oxidizing bacteria to oxidize sulfur compounds contained in the biogas. A biodesulfurization method for purifying biogas has been proposed (Patent Documents 1, 2, and 3).

  In this biological desulfurization method, sulfuric acid is generated by oxidation of sulfur compounds, but by supplying a cleaning liquid (circulating liquid) to the biological desulfurization tower and spraying it on the filler, sulfuric acid produced by the sulfur-oxidizing bacteria is washed into the cleaning liquid ( In the circulating fluid) and removed. In addition, since the sulfur-oxidizing bacteria group is an aerobic bacterium, the gas to be treated that flows into the desulfurization tower is pre-injected with air to supply oxygen necessary for the bacterium.

  Furthermore, with regard to a system for purifying biogas produced in methane fermentation, a technology has been proposed in which the amount of heat generated by a power generator such as a fuel cell is used for heating methane fermentation in order to improve the overall energy utilization efficiency. (Patent Document 4).

  However, in the system described in Patent Document 4, only a part of the amount of heat generated in the power generation device can be effectively used, and the heat utilization efficiency is low.

As a further heat utilization, as an example of a heat source for heat sterilization of methane fermentation digestive liquid, which is a waste of methane fermentation as in Patent Document 5, or as a process for concentrating and carbonizing methane fermentation digestive liquid as in Patent Document 6. Although there are examples of using as a heat source, all of them are merely used for treatment of digestive juice.
Japanese Patent Laid-Open No. 2006-143779 JP 2006-143780 A JP 2006-143781 A JP 2000-333101 A JP 2004-195308 A Japanese Patent Laid-Open No. 2005-87978

Looking at the biogas purification process using the biological desulfurization method, first, liquid-side absorption of hydrogen sulfide (H ₂ S) occurs, and then proceeds at the stage of oxidation.

Liquid side absorption is rapidly absorbed as hydrogen sulfide ions (HS ⁻ ) at about pH 7.5. Oxidation (H ₂ SO ₄ conversion) proceeds rapidly at a pH of about 6 or higher.

Even if cleaning is performed with the pH of the cleaning liquid set to about 7, if the cleaning liquid is circulated, the sulfuric acid concentration in the circulating liquid increases, so the pH of the circulating liquid decreases. When the pH of the circulating liquid becomes acidic (pH 4 or lower), the sulfuric acid is not oxidized and the oxidation is stopped by sulfur due to the reaction of 2H ₂ S + O ₂ = 2S ↓ + 2H ₂ O. The precipitated sulfur accumulates at the bottom of the filler, clogs the filler, and hinders the distribution of biogas and the flow of cleaning liquid (circulating liquid).

  In the case of hydrogen sulfide desulfurization, in order to efficiently perform desulfurization, it is important to maintain the alkalinity of the circulating fluid that makes gas-liquid contact in order to absorb hydrogen alkali in the gas first (alkali absorption). However, until now, no practical method has been disclosed.

  Accordingly, an object of the present invention is to provide a biogas purification system capable of performing efficient biodesulfurization while maintaining the alkalinity of the cleaning solution for biodesulfurization.

  Other problems of the present invention will become apparent from the following description.

  The above problems are solved by the following inventions.

(Claim 1)
A methane fermentation tank that introduces biomass and performs methane fermentation, and biogas that is biodesulfurized by sulfur-oxidizing bacteria carried on the filler by introducing biogas containing methane gas and sulfur oxide generated in the methane fermentation tank A biological desulfurization tower for performing purification, a circulation tank for storing a washing liquid for washing and removing the biological desulfurization product generated by the biological desulfurization, and a circulation pump for supplying the washing liquid in the circulation tank to the upper part in the biological desulfurization tower And a sprinkling part for sprinkling the cleaning liquid sent by the circulation pump from the upper part of the filler, and a return pipe for returning the cleaning liquid containing the biological desulfurization product in the biological desulfurization tower to the circulation tank In the purification system,
Digestive fluid supply means for supplying digestive fluid containing at least Kjeldahl nitrogen component produced in the methane fermentation tank to the circulation tank;
Cogeneration means for generating electric power and thermal energy using biogas purified in the biological desulfurization tower as an energy source;
Alkalinity that introduces thermal energy generated by the cogeneration means into the circulation tank, and thermally decomposes the Kjeldahl nitrogen component in the cleaning liquid in the circulation tank to convert it into ammonia, thereby maintaining and increasing the alkalinity of the cleaning liquid It has a maintenance means,
The biogas purification system, wherein the digestive fluid supply means has gas-liquid contact means for aerobing the digested liquid while aerating the digested liquid to dissipate carbon dioxide contained therein .

(Claim 2)
Introducing waste gas discharged when biogas is burned by the cogeneration means into the circulation tank to aerate the digestion liquid, to release the carbon dioxide and to thermally decompose the Kjeldahl nitrogen The biogas purification system according to claim 1 .

  ADVANTAGE OF THE INVENTION According to this invention, the biogas refinement | purification system which can maintain the alkalinity of the washing | cleaning liquid of biodesulfurization and can perform biodesulfurization efficiently can be provided.

  Embodiments of the present invention will be described below.

  FIG. 1 is a schematic diagram illustrating an example of a biogas purification system according to the present invention.

  In FIG. 1, 1 is a methane fermentation tank for methane fermentation of organic waste (biomass). Examples of biomass include livestock waste (for example, livestock manure, carcass, processed products thereof) and green farm waste And wastewater treatment sludge. The methane fermentation (anaerobic digestion) can be applied to a so-called medium temperature type (about 37 ° C.), a high temperature type (about 55 ° C.), a slurry (wet type) type, or a dry (dry type) type. .

  Biomass is put into the methane fermentation tank 1 and methane fermentation is performed under anaerobic conditions. Biogas containing methane, carbon dioxide, and hydrogen sulfide generated by methane fermentation is sent to the biological desulfurization tower 2 through the biogas introduction pipe 100 and subjected to biological desulfurization treatment.

  The biological desulfurization tower 2 is filled with a filler 200 carrying sulfur-oxidizing bacteria. As the filler 200, for example, a normal gas-liquid contact filler made of magnetic material or resin, porous soft resin; porous particles such as activated carbon, charcoal, zeolite, ceramics, etc. can be used. A carbon-based material such as activated carbon or charcoal is preferable for supporting sulfur-oxidizing bacteria. However, when gas-liquid contact is made in a countercurrent, a normal resin filler for gas-liquid contact can also be preferably used.

  101 is an air introduction pipe, and the introduced air (oxygen) is used in the biological desulfurization tower 2 for the oxidation reaction of hydrogen sulfide, which is a sulfur compound.

  A cleaning liquid spraying unit 201 (for example, a spray nozzle) is provided in the upper part of the biological desulfurization tower 2, and the cleaning liquid spraying unit 201 is connected to a cleaning liquid supply pipe 202. The cleaning liquid is sprayed from the cleaning liquid spraying unit 201 through the cleaning liquid supply pipe 202 by the circulation pump 300 from the circulation tank 3 that stores the cleaning liquid.

  Hydrogen sulfide is oxidized by the action of sulfur-oxidizing bacteria in the biological desulfurization tower 2 to produce sulfuric acid, which is washed with the sprayed washing liquid. The cleaning liquid absorbs sulfuric acid and is discharged from the lower side of the biological desulfurization tower 2 through the return pipe 203 in a state where the pH is lowered, and is returned to the circulation tank 3.

  In this way, the cleaning liquid is sent from the circulation tank 3 to the biological desulfurization tower 2, returned again, and this is repeated. Accordingly, the cleaning liquid is also referred to as a circulating liquid as necessary.

  In the present invention, the cleaning liquid in the circulation tank 3 is a digestive liquid containing at least a Kjeldahl nitrogen component generated in the methane fermentation tank 1 by a digestive liquid supply means. As the digestive fluid supply means, only the digestive fluid supply pipe 102 is shown in the example of FIG. 1, but a supply pump (not shown) can also be used.

  A pH meter 301 and an ORP meter 302 are installed in the circulation tank 3 so that pH and ORP can be measured. Further, the circulation tank 3 is provided with a drain pipe 303 and a drain valve 304 for discharging a predetermined amount of cleaning liquid.

  4 is a cogeneration means for recovering heat and electric power from the biodesulfurized purified biogas, and the heat and electric power can be obtained by burning the purified biogas. As the cogeneration means, for example, a gas engine is used.

  Part of the power obtained is consumed for the utility of the methane fermentation system. The heat energy generated by the gas engine is heated by a heat exchanger (not shown) to obtain hot water. The obtained warm water (usually about 70 ° C.) is sent to a warm water coil 401 provided in the circulation tank 3 through the warm water line 400 to heat the cleaning liquid (circulating liquid) by heat exchange.

  The circulating liquid returned from the biological desulfurization tower 2 to the circulation tank 3 absorbs sulfuric acid in the biological desulfurization tower 2, and the pH value decreases. In the present invention, the circulation tank 3 is heated to 50-60 ° C. by a hot water coil 401 of about 70 ° C. In the biogas purification system of the present invention, when using a methane fermentation digestive liquid as a circulating liquid for biological desulfurization, the alkalinity of the digested liquid can be maintained and raised by using heat recovered from the cogeneration means. As a result, biodesulfurization is efficiently performed.

  In the present invention, the thermal energy generated by the cogeneration means is introduced into the circulation tank 3, and the Kjeldahl nitrogen component in the cleaning liquid in the circulation tank 3 is thermally decomposed and converted into ammonia, thereby maintaining the alkalinity of the cleaning liquid. The raising means is alkalinity maintaining means, and in the above example, the hot water coil 401 plays the role.

  Methane fermentation digestive juice contains weak acids such as acetic acid and propionic acid in addition to a large excess of carbonic acid as an acidic component, but also contains basic components such as ammonia, which is an active area of methanogenic bacteria under normal conditions. Neutral to weakly alkaline. This liquid is a main component that determines pH by carbonic acid and ammonia, and has a pH buffering property, so that effective and economical biodesulfurization can be achieved by utilizing this effectively.

  The digested liquid sent from the methane fermentation tank 1 to the circulation tank 3 contains Kjeldahl nitrogen components that could not be decomposed by methane fermentation in addition to ammonia. When heated in the circulation tank 3 as described above, the Kjeldahl nitrogen component is decomposed by heating to produce ammonia. The generated ammonia is not vaporized due to the relationship between the partial pressure and the solubility, and remains dissolved in the digestive juice, so that the pH of the digestive fluid rises.

  Therefore, the circulating liquid whose pH of the digested liquid is increased and the alkalinity is maintained or increased is then sent again to the biological desulfurization tower 2 by the circulation pump 300 and used as the circulating liquid for biological desulfurization.

  According to the study by the present inventors, the pH of the circulating digestive fluid is preferably 7.5 or more, and is preferably 9 or less, more preferably 8.5 or less, considering the active pH region of the methanogen. In the present invention, the alkalinity is preferably maintained so as to correspond to such a pH range.

  Next, in the present invention, it is preferable that the digestive fluid supply means has gas-liquid contact means for aerobing the digestive liquid while aerating the digestive liquid to dissipate carbon dioxide contained therein.

  FIG. 2 is an explanatory view of a main part showing a preferred embodiment of the present invention, in which 5 is a screen for removing coarse solids to prevent clogging of the circulation line. 6 is a gas-liquid contact tower, and a plurality of shelves 600 are provided therein. In addition to the plate tower, a packed tower, a wet wall tower, or the like may be used.

  Digested liquid that is a fermentation residue after methane fermentation in the methane fermentation tank 1 is discharged from the transport pipe 103. Part or all of the discharged digested liquid is sent to the screen 5 to separate and remove coarse solids. The screen-treated digestive juice is sent to the spraying section 601 (for example, a spray nozzle or the like) at the top of the gas-liquid contact tower 6 through the introduction pipe 501 and sprayed. An air introduction pipe 602 is connected to the lower part of the gas-liquid contact tower 6, and air is introduced from the air introduction pipe 602. The introduced air and the sprayed digested liquid come into gas-liquid contact (also referred to as aeration in the present invention) to dissipate carbon dioxide and aerobicize the digested liquid. Since carbon dioxide is dissolved in the digestive fluid by the action of partial pressure with methane in biogas, the pH of the digestive fluid can be increased by releasing it in contact with air. Therefore, this embodiment is also preferable because it contributes to maintaining the alkalinity of the circulating fluid.

  In FIG. 2, the same reference numerals as those in FIG.

  Although not shown, an aeration tank is provided without providing a gas-liquid contact tower, and the digestion liquid in the aeration tank is aerated to dissipate carbon dioxide and aerobize the digestion liquid. The pH of the solution can be increased, and the alkalinity of the circulating fluid can be maintained.

  Since the digested liquid can be aerobic by using a gas-liquid contact method with air as the carbon dioxide diffusing means, it is preferable to measure pH and ORP at all times or at any time in the present invention.

It is important to bring the oxidation-reduction potential (ORP value) of the digested liquid supplied and circulated to biological desulfurization and the like into the SO ₄ ²⁻ production equilibrium region (see FIG. 4) by this gas-liquid contact. Since the potential -pH diagram shown in FIG. 4 represents the thermodynamic equilibrium _value, H 2 _S, HS ^- stable region indicates that little progress oxidation to H ₂ SO _4.

  Furthermore, an embodiment as shown in FIG. 3 is also preferable. In FIG. 3, reference numeral 402 denotes an aeration pipe provided in the circulation tank 4. A gas containing the air is supplied to the aeration pipe 402, and the digested liquid in the circulation tank 3 is aerated and stirred. According to this embodiment, carbon dioxide emission from the digestive liquid and aerobic digestion liquid can be performed in one circulation tank, which is preferable.

  As the gas containing air, it is preferable to use waste gas generated from the gas engine employed in the cogeneration means 4. The waste gas is reduced in oxygen and increased in carbon dioxide due to combustion in the gas engine, but the concentration of carbon dioxide is lower than that of the digested liquid in a gas-liquid equilibrium state with the biogas in the methane fermentation tank. Therefore, the effect is not significantly different from the case of aeration by introducing air.

  Furthermore, when the digestive juice is already heated by the hot water coil 401, heating with the thermal energy of the waste gas is preferable because the energy utilization efficiency is increased. In addition, if the heated liquid is aerated, the ammonia produced by the decomposition of Kjeldahl nitrogen may be released, but due to the solubility and partial pressure difference with carbon dioxide, even if released, it is less than a few percent of the carbon dioxide emission. is there.

  In FIG. 3, the parts denoted by the same reference numerals as those in FIGS. 1 and 2 have the same configuration, and thus description thereof is omitted.

  Furthermore, in the present invention, it is also preferable to use the carbon dioxide emission means shown in FIG. 2 and the aeration means (including waste gas utilization shown in FIG. 3) also in the circulation tank.

  Hereinafter, the effect of the present invention is illustrated by examples.

Example 1
Milking cow dung was introduced into the test plant as biomass for biogas purification. The test plant biogas production was 15 t / day.

  The test plant was provided with a gas-liquid contact tower for performing carbon dioxide emission shown in FIG. 2 in the system of FIG. The circulation tank was provided with a hot water coil (warm water 70 ° C.) to heat the circulating liquid, and the liquid temperature was adjusted to 55-60 ° C.

  The biogas composition at the outlet of the methane fermentation tank was a methane concentration of 53%, a carbon dioxide concentration of 47%, and a hydrogen sulfide concentration of 3,000 ppm.

  On the other hand, the pH of the digestive liquid at the outlet of the methane fermentation tank was 8.3, and the pH of the digestive liquid after performing carbon dioxide emission in the gas-liquid contact tower was 9.0. The pH increased due to carbon dioxide emission.

  When this was introduced into the circulation tank and heated with a hot water coil, the pH became 9.2 due to ammonia production.

  This was biogas purified by biodesulfurization with sulfur-oxidizing bacteria in a biodesulfurization tower. The digested liquid was introduced and sprayed into the biological desulfurization tower as a circulating liquid.

  The composition of the biogas after desulfurization was a methane concentration of 65%, a carbon dioxide concentration of 35%, and a hydrogen sulfide concentration of 10 ppm or less. The pH of the circulating liquid at the lower outlet of the desulfurization tower was 8.5.

The results are shown in Table 1.
This shows that the alkalinity is maintained and the desulfurization efficiency is excellent.

Example 2
In Example 1, the same test was performed without providing a gas-liquid contact tower for carbon dioxide emission.

  The pH of the digested liquid introduced into the circulation tank was 8.3, and the pH after heating was 8.9.

The results are shown in Table 1.
This shows that the alkalinity is maintained and the desulfurization efficiency is excellent.

Comparative Example 1
In Example 2, the test was performed in the same manner except that the circulation tank was not heated.

The results are shown in Table 1.
The pH of the digested liquid introduced into the circulation tank was 8.2, and the pH in the circulation tank was lowered to 7.0 at the same gas-liquid contact ratio and residence time as in the examples.
When heating is not performed, the alkalinity is not maintained and the desulfurization efficiency decreases.

Example 3
In Example 1, without providing a gas-liquid contact tower that emits carbon dioxide, an aeration pipe is disposed in a circulation tank, and waste gas generated by a gas engine (engine nozzle outlet temperature of about 500 ° C.) is introduced. Biological desulfurization was conducted.

The results are shown in Table 1.
This shows that the alkalinity is maintained and the desulfurization efficiency is excellent.

Example 4
In Example 3, biodesulfurization was performed in the same manner except that the waste gas introduced into the aeration pipe was replaced with air (normal temperature).

The results are shown in Table 1.
This shows that the alkalinity is maintained and the desulfurization efficiency is excellent.
It can also be seen that there is no significant difference between the case where air is used for aeration and the case where exhaust gas is used.

Reference Example Table 2 shows the test results that confirmed that ammonia was produced when the digestive juice was heated.

  When two types of digestive juice were tested, it was found that the composition as shown in Table 1 contained Kjeldahl nitrogen component (total nitrogen) and ammonia component.

When these solutions were allowed to stand at 65 ° C. for 1 hour, the value of NH ₄ ⁺ -N increased by about 10 to 30%. This is because a part of the organic nitrogen compound was thermally decomposed and ammonia was produced, and it can be seen that the alkalinity of the digested liquid increases.

Schematic showing an example system according to the present invention Schematic showing a preferred form that can be employed in the system according to the present invention. Schematic showing another preferred embodiment that can be employed in the system according to the present invention. Potential-pH relationship diagram for oxidation reaction of hydrogen sulfide

Explanation of symbols

1: Methane fermenter 100: Biogas introduction pipe 101: Air introduction pipe 102: Digestion liquid supply pipe 103: Transport pipe 2: Biological desulfurization tower 200: Filler 201: Cleaning liquid supply section 202: Cleaning liquid supply pipe 203: Return pipe 3 : Circulation tank 300: Circulation pump 301: pH meter 302: ORP meter 303: Drain pipe 304: Drain valve 4: Cogeneration means 400: Hot water line 401: Hot water coil 402: Aeration pipe 5: Screen 501: Introduction pipe 6: Air Liquid contact tower 600: shelf 601: spraying section 602: air introduction pipe

Claims (2)

A methane fermentation tank that introduces biomass and performs methane fermentation, and biogas that is biodesulfurized by sulfur-oxidizing bacteria carried on the filler by introducing biogas containing methane gas and sulfur oxide generated in the methane fermentation tank A biological desulfurization tower for performing purification, a circulation tank for storing a washing liquid for washing and removing the biological desulfurization product generated by the biological desulfurization, and a circulation pump for supplying the washing liquid in the circulation tank to the upper part in the biological desulfurization tower And a sprinkling part for sprinkling the cleaning liquid sent by the circulation pump from the upper part of the filler, and a return pipe for returning the cleaning liquid containing the biological desulfurization product in the biological desulfurization tower to the circulation tank In the purification system,
Digestive fluid supply means for supplying digestive fluid containing at least Kjeldahl nitrogen component produced in the methane fermentation tank to the circulation tank;
Cogeneration means for generating electric power and thermal energy using biogas purified in the biological desulfurization tower as an energy source;
Alkalinity that introduces thermal energy generated by the cogeneration means into the circulation tank, and thermally decomposes the Kjeldahl nitrogen component in the cleaning liquid in the circulation tank to convert it into ammonia, thereby maintaining and increasing the alkalinity of the cleaning liquid It has a maintenance means,
The biogas purification system, wherein the digestive fluid supply means has gas-liquid contact means for aerobing the digested liquid while aerating the digested liquid to dissipate carbon dioxide contained therein . Introducing waste gas discharged when biogas is burned by the cogeneration means into the circulation tank to aerate the digestion liquid, to release the carbon dioxide and to thermally decompose the Kjeldahl nitrogen The biogas purification system according to claim 1 .