JPH0716430A - Treatment waste gas in methane fermentation system - Google Patents

Abstract

PURPOSE:To provide the method for treating waste gas capable of preventing the air polution due to CO2, SOx and NOx generated by burning of gaseous methane and attains the pH control of a methane fermentation tank and the acceleration of the nitrification in a water treating system. CONSTITUTION:The basis of the method is that after removing hydrogen sulfide from the gaseous methane generated from the methane fermentation tank 1 by a desulfurizing apparatus 2, the hydrogen sulfide is burned by a burning device 3 and 3a, and combustion waste gas is subjected to the selective reduction of NOx while mixing with the gaseous methane under the presence of the catalyst at a denitrification catalyst reactor 4. Moreover, the treating method in which the waste gas passing through the denitrification catalyst reactor 4 is introduced to a carbon dioxide absorption apparatus 5 under supplying alkali soln. to fix the carbon dioxide as water soluble sodium carbonate is provided. Some of the fixed sodium carbonate aq. soln. is fed to a water treating system 8 to accelerate nitrification and on the other hand, the other part of the aq. soln. is returned to the methane fermentation tank 1 to be used as a pH controlling agent.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention performs denitration of combustion exhaust gas using methane gas in a methane fermentation treatment system,
The present invention relates to an exhaust gas treatment method for fixing carbon dioxide, adjusting pH of a methane fermentation tank, and promoting nitrification of a water treatment system.

[0002]

2. Description of the Related Art In anaerobic digestion, a means for burning methane gas generated for self-heating of a fermenter is generally used. The methane gas becomes carbon dioxide by burning, sulfur oxides (SO _X) Toka nitrogen oxides (N
O _X ) is a byproduct. It is necessary to efficiently remove these sulfur oxides and nitrogen oxides in terms of emission control, and further, emission of carbon dioxide (CO ₂ ) tends to be regulated. As a CO ₂ immobilization method for reducing the concentration of CO ₂ in the atmosphere, a method of liquefying this CO ₂ and confining it in the deep sea floor, a method of immobilizing it as biomass by photosynthesis, and the like can be considered. Further, a method of performing a single treatment for immobilizing sulfur oxides, nitrogen oxides, and carbon dioxide in exhaust gas is known, but a method of performing a batch treatment is not known.

As a method of removing carbon dioxide from the exhaust gas, there is a method of reacting with Ca (OH) ₂ or NaOH to produce CaCO ₃ and Na ₂ CO ₃ , respectively.
Among them, Ca (OH) ₂ is produced by the reactions shown in the following formulas (1) and (2).

CaCO ₃ → CaO + CO ₂ (1) CaO + H ₂ O → Ca (OH) ₂ (2) As a raw material, as shown in the above formula CaCO ₃ (calcium carbonate, limestone) is used, and when this raw material is heated to CaO (calcium oxide, quick lime) and further reacted with water, Ca (OH) ₂ (calcium hydroxide, slaked lime)
Is generated.

On the other hand, the methane gas generated in the methane fermentation tank usually contains hydrogen sulfide. Since this hydrogen sulfide becomes SO _X by combustion, it is sulfurized by using a wet or dry desulfurizing agent before combustion. Need to remove hydrogen,
Therefore, the concentration of SO _X contained in the exhaust gas is low. Of flue gas denitration technology is currently in development, the NO _X
There is known a method of reacting hydrogen peroxide with ammonia or methane in the presence of V ₂ O ₅ or zeolite to selectively reduce N ₂ gas and water.

Particularly, when the methane fermentation system is operated with a relatively low concentration of waste water as a substrate, the adjustment of the pH is important. Also, when it is divided into two stages of liquefaction (acidic fermentation) and gasification (alkaline fermentation) like a two-phase methane fermentation tank, the pH decreases due to liquefaction, so an alkaline agent is added to neutralize it. Gasification may be performed later. Further, even in the case of a single-phase methane fermentation tank, an alkaline agent may be added to prevent the pH from decreasing.

That is, since the nitrification reaction consumes alkalinity, if the alkalinity is insufficient, the pH is lowered and the reaction is stopped. Therefore, a method of injecting an alkaline agent is used to adjust the pH. In this case, as the alkaline agent, NaOH, Ca (OH) ₂ , and Na ₂ CO ₃ are used, but among them, since NaOH is a strong alkali,
Although the absolute amount required for pH adjustment is small, it is difficult to control the injection amount. Ca (O
H) ₂ is inexpensive, but CaCO ₃ precipitates by reacting with carbonic acid, which increases the amount of chemicals required for pH adjustment.
There is a problem that the amount of solids increases.

On the other hand, Na ₂ CO ₃ has the advantages that the injection amount can be easily controlled and the buffering property of the liquid phase can be enhanced, and Na ₂ CO ₃ is most suitable as the alkaline agent among the above three agents. However, there is a problem that the price is higher than the other two.

[0009]

However, in such a conventional methane fermentation system, carbon dioxide, sulfur oxides (SO _x ), and nitrogen oxides (NO _x ) produced as by-products when methane gas generated in the fermenter is burned. There was a problem that the atmosphere was polluted by _X ).

That is, as a method of removing carbon dioxide from the exhaust gas, as shown in the above equations (1) and (2), C
There is a method of reacting a (OH) ₂ or NaOH with exhaust gas to form CaCO ₃ and Na ₂ CO ₃ , respectively. Consider the case of using Ca (OH) ₂ produced in this way for removing CO _2. When, as is clear from equation (1), because the CO ₂ is already at the manufacturing process is released into the atmosphere, the effect of reducing the emissions even CO ₂ to remove CO ₂ from the flue gas is observed Absent. On the other hand, since NaOH is produced by electrolysis of saline solution, when a power source other than fossil fuel is used, the effect of reducing CO ₂ emission is recognized. However, since Na ₂ CO ₃ has high solubility,
When CO ₂ is absorbed and removed using an OH solution, Na ₂ CO
₃ solutions are obtained, and this Na ₂ CO ₃ solution is inconvenient to handle and there is no way to reuse it, so there is a problem that disposal is difficult.

In order to further promote the reaction in the methane fermentation tank, it is necessary to adjust the pH in the fermentation tank as appropriate, and in particular, the relatively expensive virgin Na ₂ CO which is most suitable as an alkaline agent for methane fermentation. _{Since 3} must be used, there is a problem that running cost becomes high.

Therefore, the present invention has been made based on the above point of view, and prevents the atmospheric pollution due to carbon dioxide, sulfur oxides and nitrogen oxides due to the combustion of methane gas, and adjusts the pH of a methane fermentation tank. It is an object of the present invention to provide an exhaust gas treatment method capable of promoting nitrification of a water treatment system.

[0013]

In order to achieve the above-mentioned object, the present invention removes hydrogen sulfide from a methane gas generated from a methane fermentation tank by a desulfurizer and then burns it to obtain C
A methane fermentation treatment system in which combustion exhaust gas containing O ₂ and NO _x is sent to a denitration catalyst reaction tower, and NO _x is selectively reduced and decomposed in the presence of a catalyst while mixing the methane gas and the combustion exhaust gas. Based on the exhaust gas treatment method, according to claim 2, the exhaust gas that has passed through the denitration catalyst reaction tower is introduced into a carbon dioxide absorption device, and carbon dioxide in the exhaust gas is dissolved in water under the supply of an alkaline solution. A treatment method of immobilizing as sodium is provided.

Further, according to claim 3, a part of the immobilized sodium carbonate aqueous solution is fed into a water treatment system to promote nitrification, while the other part is returned to the methane fermentation tank to serve as a pH adjusting agent. An exhaust gas treatment method in a methane fermentation treatment system adapted to be used.

[0015]

According to the exhaust gas treatment method in the methane fermentation treatment system, hydrogen sulfide is removed from the methane gas generated from the methane fermentation tank by the desulfurization device, and the combustion treatment is performed in the combustion device to produce the combustion exhaust gas containing CO ₂ and NO _x. The gas is sent to the NOx removal catalytic reaction tower together with a part of the methane gas generated from the methane fermentation tank and mixed there, and selectively reduced and decomposed into nitrogen gas and carbon dioxide in the presence of a catalyst.

Further, the exhaust gas that has passed through the denitration catalyst reaction tower is introduced into a carbon dioxide absorption device, and CO ₂ in the exhaust gas is fixed as a carbonate soluble in water under the supply of the alkaline solution. Part of this fixed Na ₂ CO ₃ solution is sent to the water treatment system to promote nitrification, while the other part is returned as a sodium carbonate solution to the methane fermenter to generate p.
Used as an H regulator.

[0017]

EXAMPLE An example of a method for utilizing exhaust gas in a methane fermentation treatment system according to the present invention will be described in detail below with reference to the schematic view of FIG. First, the structure of the apparatus will be described. In the figure, 1 is a methane fermentation tank as a digestion tank, 2 is a desulfurization apparatus, 3 is a self-heating combustion apparatus, and 3a is another combustion apparatus.
Further, 4 is a denitration catalyst reaction tower, 5 is a carbon dioxide absorption device, and 6
Is an alkaline solution supply device, 7 is a sodium carbonate solution storage tank, and 8 is a water treatment system.

The operation of the above embodiment is as follows. First, when the substrate 10 is put into the methane fermentation tank 1 as a digestion tank and digested, the methane gas generated from the methane fermentation tank 1 is dehydrogenated (H ₂ S) by the desulfurization device 2 and self-heated. Combustion processing is performed in the in-use combustion device 3 and the other combustion device 3a. The other combustion device 3a includes a combustion device using a heat source other than methane gas, and the CO discharged from the self-heating combustion device 3 and the combustion device 3a.
Combustion exhaust gas 9 containing ₂ and NO _X is sent to the denitration catalyst reaction tower 4. A part of the combustion exhaust gas of the self-heating combustion device 5 is returned to the methane fermentation tank 1.

On the other hand, a part of the methane gas 11 generated from the methane fermentation tank 1 is directly sent to the denitration catalytic reaction tower 4 and mixed with the combustion exhaust gas 9, and in the denitration catalytic reaction tower 4, a part of the methane gas as a catalyst is removed. In the presence of vanadium oxide (V ₂ O ₅ ) or zeolite, selective reductive decomposition is performed as shown in the following formula (3).

2NO + CH ₄ + O ₂ → N ₂ + CO ₂ + 2H ₂ O (3) Next, the exhaust gas passing through the denitration catalyst reaction tower 4 is passed to the carbon dioxide absorption device 5 of the next stage. be introduced. The NaOH solution is supplied from the alkaline solution supply device 6 to the carbon dioxide absorption device 5, and CO ₂ in the exhaust gas is fixed as carbonate as shown in the following formula (4).

2NaOH + 2CO ₂ → Na ₂ CO ₃ + H ₂ O (4) This immobilized Na ₂ CO ₃ solution is soluble in water,
Carbon dioxide absorber 5 to sodium carbonate solution storage tank 7
And a part thereof is sent to the water treatment system 8 to promote nitrification, while another part is returned to the methane fermentation tank 1 as a sodium carbonate solution 12 and used as a pH adjuster.

In the methane fermentation, CO ₂ + 4H ₂ → CH ₄ + 2H ₂ O ... (5) ) Of the reaction formula, but increasing the soluble carbonic acid concentration increases the concentration of the substrate, which is a secondary effect that the gas generation amount per unit volume of the methane fermentation tank increases. Is obtained.

When, for example, an activated sludge treatment method or a circulation type nitrification denitrification method is adopted as the water treatment system 8, the soluble carbonic acid is ingested by the autotrophic nitrifying bacterium and is used for microbial cell synthesis. Therefore, the effect of promoting the growth of nitrifying bacteria and increasing the nitrification reaction rate of the water treatment system 8 can be obtained.

[0024]

As described in detail above, according to the exhaust gas treatment method in the methane fermentation treatment system according to the present invention, hydrogen sulfide of methane gas generated from the methane fermentation tank is removed and burned CO ₂ and Combustion exhaust gas containing NO _X is sent together with other methane gas to the denitration catalytic reaction tower, is selectively reduced and decomposed into nitrogen gas and carbon dioxide in the presence of a catalyst, and the exhaust gas passing through the denitration catalytic reaction tower absorbs carbon dioxide. In the apparatus, CO ₂ in the exhaust gas can be converted to a water-soluble carbonate under the supply of an alkaline solution. Thus the carbon dioxide by-produced methane gas generated in the fermentation tank when burned, sulfur oxides (SO _X) is advantageously and nitrogen oxides (NO _X) never atmosphere is contaminated by obtained.

In particular, CO ₂ is finally partially immobilized as a biomass on nitrifying bacteria or other autotrophic bacteria.
Since O ₂ can be fixed as a carbonate, it becomes possible to cope with the regulation of carbon dioxide.

A part of the immobilized Na ₂ CO ₃ solution is sent to a water treatment system, where soluble carbonic acid is taken up by autotrophic nitrifying bacteria and utilized for cell synthesis.
Since the growth of nitrifying bacteria is promoted to increase the nitrification reaction rate, and as a result, the HRT (residence time) in the nitrification reaction tank is shortened, it is possible to reduce the volume of the tank and the construction cost.

Further, another sodium carbonate solution can be returned to the methane fermentation tank and used as a pH adjusting agent, and can be effectively used as a substitute for expensive sodium carbonate of virgin to reduce the running cost. You can

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an apparatus configuration for implementing an exhaust gas treatment method in a methane fermentation treatment system according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Methane fermentation tank 2 ... Desulfurization apparatus 3 ... Self-heating combustion apparatus 3a ... Other combustion apparatus 4 ... Denitration catalyst reaction tower 5 ... Carbon dioxide absorption apparatus 6 ... Alkaline solution supply apparatus 7 ... Sodium carbonate solution storage tank 8 ... Water treatment system

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location B01D 53/86 ZAB C02F 3/28 ZAB Z 11/04 ZAB A 7446-4D B01D 53/34 ZAB Z 135 Z

Claims (3)

[Claims] 1. A methane gas generated from a methane fermentation tank is subjected to a combustion treatment after removing hydrogen sulfide by a desulfurization device,
Combustion exhaust gas containing O ₂ and NO _X is sent to a denitration catalyst reaction column, and selective reduction decomposition of NO _X is performed in the presence of a catalyst while mixing the methane gas and combustion exhaust gas with each other. Exhaust gas treatment method in system. 2. A methane gas generated from a methane fermentation tank is subjected to a combustion treatment after removing hydrogen sulfide with a desulfurizer.
Combustion exhaust gas containing O ₂ and NO _x is sent to a denitration catalytic reaction tower, NO _x is selectively reduced and decomposed in the presence of a catalyst while mixing the methane gas and the combustion exhaust gas, and passed through the denitration catalytic reaction tower. The exhaust gas treatment method in a methane fermentation treatment system, comprising introducing the exhaust gas into a carbon dioxide absorption device to immobilize carbon dioxide in the exhaust gas as water-soluble sodium carbonate under the supply of an alkaline solution. 3. A part of the fixed aqueous solution of sodium carbonate is fed into a water treatment system to promote nitrification, while another part is returned to the methane fermentation tank to be used as a pH adjuster. The exhaust gas treatment method in the methane fermentation treatment system according to claim 2.