DEVICE AND METHOD FOR BIOLOGICALLY DESULFURIZING BIOGAS

The purpose of the present invention is to provide a device and method for biologically desulfurizing a biogas, in which the hydrogen sulfide contained in a high load is efficiently treated and the hydrogen sulfide to be treated is converted into sulfuric acid to thereby eliminate clogging within the device and eliminate cleaning and other steps, making it possible to conduct the treatment at low cost. The device for biological desulfurization is characterized by being equipped with: a biogas inflow line (2) for causing a biogas to flow into a biological desulfurization tower (1) through an end thereof; a treated-gas outflow line (8) for discharging a treated gas, the line (8) having been disposed so as to extend from the stage which is located at the other end of the biological desulfurization tower and which succeeds a packing layer (1a) that supports a microorganism; a circulating-gas line (9) for circulating some of the treated gas to the end of the biological desulfurization tower into which the biogas flows; and a mixed-gas line (5) which mixes the biogas with the some of the treated gas to supply the mixed gas to the end of the biological desulfurization tower. The device is further characterized in that the hydrogen sulfide load amount is calculated from a value measured with a gas flow meter (3) disposed in the biogas inflow line and from a value measured with a hydrogen sulfide concentration meter (4) disposed in the mixed-gas line, and a mechanism (10) for regulating the amount of the circulating gas is caused to work on the basis of the results.

TECHNICAL FIELD

The present invention relates to a device and a method for biological desulfurization from a biogas, and in particular, to a technology for efficiently treating a biogas generated during a process of methane fermentation by converting hydrogen sulfide included in the biogas to sulfuric acid.

BACKGROUND ART

Organic waste and organic wastewater are treated through methane fermentation in the water treatment field so as to generate a biogas that has methane as its main component. Though the concentration differs depending on the method of methane fermentation, the biogas includes 65% to 85% of methane, 15% to 35% of carbon dioxide and 1000 ppm to 6000 ppm of hydrogen sulfide as its main components. It is possible to utilize methane the in generated biogas as fuel for a boiler, and then, the steam generated from the boiler can be effectively utilized in a heating facility. In addition, the biogas becomes fuel for a gas engine, which can generate electricity.

Hydrogen sulfide included in the biogas is oxidized to a sulfurous acid gas (SO2) when being burnt, and the generated sulfurous acid gas becomes sulfuric acid when being dissolved into moisture, which not only causes acid rain when released into the air but also becomes sulfuric acid due to the condensed moisture when the burnt gas is cooled within the facility, and thus, causes a problem such as erosion.

Therefore, it is an important issue to remove hydrogen sulfide in order to utilize a biogas.

Methods for removing hydrogen sulfide from a biogas include dry a desulfurization method according to which hydrogen sulfide is removed using a chemical desulfurizing agent in pellet form having iron oxide as its main component. In accordance with the dry desulfurization method, hydrogen sulfide chemically reacts with iron oxide, and therefore, the amount of hydrogen sulfide removed by the chemical desulfurizing agent is generally proportional to the amount of existing iron oxide. When iron oxide, which is involved in the reaction through which hydrogen sulfide is removed by the chemical desulfurizing agent, runs out, the removing performance declines and it becomes necessary to switch to a new agent.

Other desulfurization methods include a biological desulfurization method using microorganisms as in the present invention.

The biological desulfurization method is a method for removing hydrogen sulfide by supplying a microscopic amount of air or oxygen to a biogas so that microorganisms generate sulfur (S) or sulfuric acid (H2SO4) through the reaction paths shown in Formulas 1 and 2 in the following.

The microorganisms contributing to Formulas 1 and 2 can adhere to or float on the surface of fillers (media).

There are a lot of aerobic bacteria, which are sulfur oxidizing bacteria in the natural world.

In order for microorganisms to be involved, the appropriate temperature and moisture are essential to the environment in which microorganism thrive.

H2S + I/2O2 * S + H2O …

(Formula l) S + 3/SO2 + H2O-> H2O …

 

Formula 1 is the reaction for sulfur oxidizing bacteria to generate simple sulfur (S) from hydrogen sulfide. This is the main reaction in the case where the molar ratio of oxygen is no greater than 1/2 of that of hydrogen sulfide. In the case where the molar ratio of oxygen exceeds 1/2 of that of hydrogen sulfide, the sulfur oxidizing bacteria further causes the reaction in Formula 2 so as to generate sulfuric acid (H2SO4). In order for all the hydrogen sulfide to be converted to sulfuric acid (H2S04), theoretically an molar ratio of oxygen that is two or more times greater than that of hydrogen sulfide is necessary in the presence of sulfur oxidizing bacteria.

Patent Document 1 states an example of a biological desulfurization technology.

According to this method, a part of hydrogen sulfide that has been removed deposits as sulfur, which adheres to the filler, while another portion is converted to sulfuric acid when the level of treatment diminishes. A technology for recovering the treatment performance by peeling the deposited sulfur through aeration when the biological desulfurization tower is filled with water is also described.

In the case where sulfur has deposited on a carrier, the biological reaction is hampered by the deposition of the generated sulfur, and therefore, there is a defect such that the original hydrogen sulfide removing performance by the sulfur oxidizing bacteria is lowered at an accelerated pace.

In another technology, as stated in Patent Document 2, a process gas is circulated from a desulfurization tower, where the amount of circulation is controlled by the pressure value of a pressure regulator tank installed in the latter stage of the desulfurization tower.

In the case where the treated biogas is not utilized in a gas utilizing facility at the latter stage of the pressure regulator tank, the gas is stored in the pressure regulator tank and the gas within the pressure regulator tank is used as the circulation gas for the desulfurization tower.

In the case where a biogas including a high concentration of hydrogen sulfide flows into this system, no biogas is circulated when the treated biogas is being utilized in the gas utilizing facility at the latter stage of the pressure regulator tank. At this time the biogas is treated in such a state that the load of hydrogen sulfide is high, and therefore, there is a defect that inevitably causes the diminishment of the desulfurization performance due to the deposition of sulfur.

In addition, the supply of an oxygen containing gas is regulated in accordance with the amount of the process gas flowing from the desulfurization tower, and thus, is controlled by the oxygen concentration gauge installed in a line through which process a gas flows out in the latter stage of the desulfurization tower.

In the case where the supplied amount of the oxygen containing gas is controlled in this system, oxygen is not consumed when sulfur deposits, which increases the concentration of oxygen in the process gas, and thus the system is controlled so that the amount of supply of the oxygen containing gas is reduced.

As a result, oxygen that initially was required for 3 conversion into sulfuric acid runs short, which accelerates deposition of sulfur, and thus there is such a defect that treatment performance further diminishes.

Prior Art Documents Patent Documents

Patent Document 1: Japanese Unexamined Patent Publication 2003-305328

Patent Document 2: Japanese Unexamined Patent Publication 2006-143780

SUMMARY

The present invention is provided in order to solve the above described problems and an object of the invention is to provide a device and a method for biological desulfurization of a biogas where hydrogen sulfide is efficiently processed under high a load and the processed hydrogen sulfide is converted to sulfuric acid so that there is no clogging inside the device and no cleaning process is required, which makes the process possible at a low cost.

In order to achieve the above described object, the biological desulfurization device and the biological desulfurization method according to the present invention have the following technological features.

(1) A biological desulfurization device by sprinkling a circulation liquid inside a biological desulfurization tower for biologically removing hydrogen sulfide from a biogas generated through methane fermentation of an organic waste is characterized in that’ a biogas inflow line is provided so that a biogas flows into the biological desulfurization tower through an end portion of the tower, a filler layer made of a filler material to which bacteria adhere is provided inside the biological desulfurization tower, a line through which process a gas flows out is provided so as to discharge a process gas at a latter stage of the filler layer, which is in another end portion of the biological desulfurization tower, a circulation gas line for circulating a part of the process gas is provided in the end portion through which the biogas flows into the biological desulfurization tower, the biogas inflow line and the circulation gas line are connected to an end portion of the biological desulfurization tower after being merged together so that a mixture gas line for mixing the biogas and a part of the process gas together and for supplying the mixed gas to the end portion of the biological desulfurization tower, a gas flow meter is provided along the biogas inflow line, a hydrogen sulfide concentration gauge is provided alone the mixture gas line, a circulation gas amount adjusting mechanism is provided along the circulation gas line, an operational unit for calculating a hydrogen sulfide load amount from the value of the concentration of hydrogen sulfide in the biogas found by the hydrogen sulfide concentration gauge and the value of the gas flow amount found by the gas flow meter is provided, and a signal transfer mechanism for circulation gas is provided in order to operate the above described circulation gas amount adjusting mechanism depending on the results of calculation of the hydrogen sulfide load amount by the operational unit.

(2) The biological desulfurization device according to the above (1) is characterized in that an oxygen containing gas inflow line is provided in order to allow an oxygen containing gas to flow into the biogas inflow line, a supply adjusting mechanism for an oxygen containing gas amount is provided along the oxygen c ontaining gas inflow line, and a signal transfer mechanism for oxygen containing gas is provided in order to operate the above described supply adjusting mechanism for an oxygen containing gas amount depending on the results of calculations of a hydrogen sulfide load amount by the operational unit.

(3) The biological desulfurization device according to the above (1) is characterized in that an oxygen containing gas inflow line is provided in order to allow an oxygen containing gas to flow into the mixture gas line, an oxygen containing gas amount supply adjusting mechanism is provided to the oxygen containing gas inflow line, and an oxygen containing gas signal transfer mechanism is provided to operate the above described oxygen containing gas amount supply adjusting mechanism in accordance with the results of calculation of a hydrogen sulfide load amount by the operational unit.

(4) A biological desulfurization method by sprinkling a circulation liquid inside a biological desulfurization tower for biologically removing hydrogen sulfide from biogas a generated through methane fermentation of an organic waste characterized is in that a filler layer made of a filler material to which bacteria adhere is provided inside the biological desulfurization tower, the biological desulfurization method is provided with a biogas inflow step of allowing a biogas to flow into the filler layer inside the biological desulfurization tower from the upper stream side; a process gas outflow step of discharging a process gas to a point on the down-stream side of the filler layer inside the biological desulfurization tower a circulation gas step circulating of a part of the process gas through a point on the upper stream side of the filler layer inside the biological desulfurization tower; and a mixture gas step of mixing the biogas and a part of the above described process gas together into a mixture gas, which is then introduced into the biological desulfurization tower, and a hydrogen sulfide load amount is calculated from the flow amount of the biogas that flows into the biological desulfurization tower in the biogas inflow step and the concentration of hydrogen sulfide in the mixture gas step, and the circulation gas amount in the circulation gas step is adjusted in accordance with the results of the calculation.

(5) The biological desulfurization method according to the above (4) is characterized in that the biogas inflow step includes a n oxygen containing gas inflow step 6 for introducing an oxygen containing gas into the biogas, and the supplied amount of the oxygen containing gas is adjusted in the oxygen containing gas inflow step in accordance with the results of calculation of the hydrogen sulfide load amount.

(6) The biological desulfurization method according to the above (4) is characterized in that the mixture gas step includes an oxygen containing gas inflow step of introducing an oxygen containing gas into the mixture gas, and the supplied amount of the oxygen containing gas is adjusted in the oxygen containing gas inflow step in accordance with the results of calculation of the hydrogen sulfide load amount.

(7) The biological desulfurization method according to any of the above (4) to (6) is characterized in that the concentration of hydrogen sulfide in the mixture gas in the mixture step gas is in a range from 100 ppm to 1000 ppm.

(8) The biological desulfurization method according to the above (7) is characterized in that the hydrogen sulfide concentration is 150 ppm to 500 ppm in the above described gas.

When a biogas is processed using the device and the method for biological desulfurization according to the present invention, the removed hydrogen sulfide is converted to sulfuric acid so that the problem of clogging due to the deposition of sulfur can be resolved, and thus, the biological desulfurization process can be maintained at high efficiency.

In particular, a gas flow meter is provided alone the biogas inflow line and a hydrogen sulfide concentration gauge is provided alone a mixture gas line so as to control the circulation gal flow amount from the concentration read on the hydrogen sulfide concentration gauge and the flow amount read on the gas flow meter in order to make the concentration of hydrogen sulfide in the mixture gas appropriate for the method for biological desulfurization, and thus, it is possible to remove hydrogen sulfide efficiently even under a high load through conversion to sulfuric acid.

In addition, it has been confirmed that a desulfurization process can 7 be carried out with the ratio of conversion into sulfuric acid being 100% when the concentration of hydrogen sulfide in the mixture gas is 100 ppm to 1000 ppm as a result of adjustment of the amount of the circulation gal, and that a desulfurization process can be carried out with the ratio of the removed hydrogen sulfide being 95 % and with the ratio of conversion into sulfuric acid being 100% when the concentration of hydrogen sulfide in the mixture gas is 150 ppm to 500 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the biological desulfurization device according to the present invention;

FIG. 2 is a schematic diagram showing the biological desulfurization device according to the present invention, which is a diagram showing an example where the oxygen containing gas inflow line is directly connected to the mixture gas line;

FIG. 3 is a schematic diagram showing the biological desulfurization device according to the present invention, which is a diagram showing example an where a biogas that is flowing in the upward direction is processed;

FIG. 4 is a flow chart for the control of the circulation gas amount adjusting mechanism;

FIG. 5 is a flow chart for the control of the oxygen containing gas supply adjusting mechanism;

FIG. 6 is a graph showing the results of a process using the biological desulfurization device according to the present invention;

FIG. 7 is a graph showing the results of a process using a conventional biological desulfurization device; and

FIG. 8 is a graph showing the relationship between the concentration of hydrogen sulfide in the mixture gas (set value) and the ratio of the removed hydrogen sulfide.

 

DETAILED DESCRIPTION

In the conventional biological desulfurization methods, a hydrogen sulfide load amount that is calculated as the product of the hydrogen sulfide concentration and the gas flow amount is not used unlike the concept that characterizes the present invention. Therefore, a constant amount of oxygen containing gas is supplied during the process under a low load, which increases the level of the oxygen containing gas component in the biogas.  As a result, the concentration of methane in the biogas is lowered and the value of the biogas as a fuel decreases. In the case where the hydrogen sulfide concentration is high, the oxidation reaction by sulfur oxidizing bacteria is hampered by the deposited sulfur because the conditions for operation do not prevent sulfur deposition, and hydrogen sulfide in the biogas that has flown into the reaction tower is not sufficiently converted to sulfuric acid due to the lack of the amount of oxidizing bacteria even in the case where the amount of the oxygen containing gas is sufficient, which results in the deposition of sulfur. When approximately 30% of the carrier in the biological desulfurization tower is covered with sulfur, hydrogen sulfide is discharged without being processed, which is an ineffective process.

The deposited sulfur is hydrophobic, and therefore, upon deposition on the filler, sulfur covers the surface of bacteria that adhere to the surface of the filler, which lowers the bacteria activity. Sulfur continues to deposit deeper into the filler and finally clogs the filler within the biological desulfurization tower. It is difficult for sulfur to be removed from the filler after sulfur has once been deposited. The original treatment performance cannot be recovered even when a removal process is carried out by any means, and therefore, it is important to figure out a process for biological desulfurization while preventing sulfur deposition in order to maintain the treatment performance.

The present inventors installed a device in a biogas plant and examined the conditions for maintaining the treatment performance of biological desulfurization while preventing sulfur deposition.

 

Here names of the below described gases are defined as follows:

  • “biogas” is a gas generated through methane fermentation, which does not contain oxygen, • “oxygen containing gas” is a gas that contains oxygen, • “circulation gas” is part of a process gas that again flows into the biological desulfurization tower by means of the circulation gas amount supply 9 adjusting mechanism, * “mixture gas” is a gas where a biogas and a process gas are mixed together; the concentration of hydrogen sulfide in this gas is measured, and • “process gas” is a gas that has been discharged biological desulfurization tower.

The calculation for the load amount of hydrogen sulfide per unit volume of filler can be represented by Formula 3. Hereinafter, the load amount of hydrogen sulfide per unit volume of filler is referred to as hydrogen sulfide load amount.

 

hydrogen sulfide load amount [kg/( m 3 • day)] = ((concentration of hydrogen sulfide in mixture gas) [ppm] x (biogas amount [m3/day] 3 + circulation gas amount [m3/day]))/volume 3 of filler [m ] 3 x K …

(Formula 3) [0031] Here, K in Formula 3 is a correction coefficient with the temperature being a parameter and can be represented by Formula 4.

Correction coefficient K [kg/m ] 3 = (273 + 35)/273/22.4 x 34 …

The ratio of conversion into sulfuric acid was calculated from the amount of conversion into sulfuric acid per day and the amount of the removed hydrogen sulfide per day. Formula 5 shows how the amount of conversion into sulfuric acid per day is calculated, Formula 6 shows how the amount of the removed hydrogen sulfide day per is calculated, and Formula 7 shows how the ratio of conversion into sulfuric acid is calculated.

Amount of conversion into sulfuric acid [kg H2S04/day] = (concentration of sulfuric acid on a day concentration of sulfuric acid on the day before) [kg H2SO4/L] x amount of circulation liquid [L/day]] (Formula 5)

Amount of removed hydrogen sulfide [kg HaS/day] = amount of removed hydrogen sulfide per unit volume of filler [kg/( m 3 • day)] volume x of filler [m ] 3 (Formula 6)

Ratio of conversion into sulfuric acid [%] = (amount of conversion into sulfuric acid x (32/96) [kg S/day])/(amount of removed hydrogen sulfide [kg H2S/day] x (32/34) [kg S/kg H2S]) x 100 (Formula 7)

Next, the amount of oxygen required for the biological desulfurization 10 system is described below.

The amount of oxygen consumed in the biological desulfurization system includes the amount of oxygen required for conversion into sulfuric acid by bacteria (Oo) and amount of oxygen required for respiration of bacteria (OR).

The supplied amount of the oxygen containing gas supplied to the biological desulfurization tower in the present invention [kg 02/day] is Oo + OR.

The amount of oxygen required for conversion into sulfuric acid (Oo) can be represented in Formula 8.

Oo [kg 02/day] = amount of removed hydrogen sulfide [kg HaS/day] x 32/34 [kg Oa/kg/EhS] x 2 (Formula 8)

The amount of Oo when hydrogen sulfide is oxidized to sulfuric acid under a load amount of hydrogen sulfide of 2 kg/( m 3 • day) using a filler of 1 m 3 is 3.8 [kg 02/day] as calculated from Formula 8.

Oxygen required for biological desulfurization is supplied in the form of gas.

In the case where a pure oxygen gas is supplied as the oxygen containing gas at 25 °C, the amount of the pure oxygen gas can be represented in Formula 9.

Amount of pure oxygen gas [m3-o2/day] = Oo [kg 02/day]/32 x 22.4 x (273 + 25)/273/1000 … (Formula 9)

In the case where air (concentration of oxygen is 21 v/v% at 25 °C) is used as the oxygen containing gas, the amount of air including Oo can be represented in Formula 10.

Amount of air [m 3 air/day] = amount of pure oxygen gas [m 3 02/day] x (100/21) … (Formula 10)

It was found through experiment that the amount bacteria of that adheres to 1 m 3 of the filler was 1 kg SS/m 3 and the respiration rate was in a range from 5 mg O2/(g SS * hr) to 10 mg – 02/(g SS • hr) The amount of the bacteria adhering to 1 m 3 of the filler was 1 kg SS, and OR was in a range from 0.12 kg 02/day to 0.24 kg 02/day.

Thus, it was found through the experiments by the inventors that it is preferable for the supplied amount of the oxygen containing gas that to be 11 1.5 to 3 times greater than Oo in order not to prevent the activity of the bacteria, though OR is significantly smaller than Oo.

In the case where the supplied amount of oxygen is smaller than the amount that is 1.5 times greater than Oo, conversion into sulfuric acid by the bacteria delays.

When the supplied amount of oxygen is 3 or more times greater than Oo, the process gas includes a large amount of the oxygen containing gas that has not reacted and the concentration of methane in the process gas is lowered, which lowers the value of the fuel.

In the following, the embodiments of the present invention are described.

The inventors carried out an experiment that continued for a long period of time using the biological desulfurization device according to the present invention, and explored to find a method according which to an efficient and stable process is possible even under such a condition that the concentration of hydrogen sulfide in the biogas fluctuates and the biogas flow amount fluctuates.

FIG. 1 shows an example of a biological desulfurization device where a filler to which bacteria adhere is filled in according to the present invention. The invention is not limited to embodiment.

A filler layer la in a biological desulfurization tower 1 is filled in with the filler to which bacteria adhere.

A biogas inflow line 2 is provided with gas flow meter 3.

The biogas inflow line and 2 a circulation gas line 9 merge so that a biogas and a circulation gas are mixed together.

A mixture gas line 5 is provided with a hydrogen sulfide concentration gauge 4.

An oxygen containing gas inflow line 6 is directly connected to the biogas inflow line 2.

The supplied amount of an oxygen containing gas Ob is adjusted by an oxygen containing gas amount supply adjusting mechanism 7.

The mixture gas line 5 is directly connected to the biological desulfurization tower 1.

The filler to which bacteria adhere is made of polyethylene in a 12 cylindrical shape having a diameter of 15 mm and a height of 15 mm with a specific surface area of 1000 m/m .

A process gas outflow line 8 is directly connected to the biological desulfurization tower 1 so that a process gas Oc passes through the process gas outflow line 8 so as to be discharged to the outside of the system.

The circulation gas line 9 is branched from the process gas outflow line 8 and is connected to the biogas inflow line 2.

A part of the process gas Oc passes through the circulation gas line 9 by means of a circulation blower so as to be mixed with a biogas.

The circulation gas amount adjusted is by a circulation gas amount adjusting mechanism 10.

The circulation liquid Od from the circulation liquid storage tank lb is spayed from the top of the biological desulfurization tower 1.

In order to adjust the concentration of sulfuric acid in the circulation liquid Od from the circulation liquid storage tank lb, a part of the circulation liquid is intermittently discharged as blow water Oe, and refilling water is supplied so that the amount of water in the circulation liquid storage tank la was maintained at constant level.

The value of the biogas flow amount and the value of the concentration of hydrogen sulfide in the mixture gas are input into an operational unit 12 so that a hydrogen sulfide load amount is calculated in accordance with Formula 3.

Furthermore, the operational unit 2, into which the value of the biogas flow amount and the value of the concentration of hydrogen sulfide in the mixture gas are input, calculates the circulation gas amount on the basis of the value of the concentration of hydrogen sulfide in the mixture gas that has been preset, and thus, adjusts the circulation gas amount adjusting mechanism.

The circulation gas line 9 may be branched from the process gas outflow line 8 or may be directly connected to the biological desulfurization tower 1.

The circulation gas supplying means may use a blower or a pump.

A circulation gas control mechanism 13 controls the circulation gas amount adjusting mechanism 10.

The control by the circulation gas control 13 mechanism 13 may be a physical control or may be control using an electrical signal.

Concretely, in accordance with the method of the physical control, the degree opening of the inverter, the valve or the damper of the blower may be manually adjusted.

In accordance with the method of the electrical control, the inverter may electrically be controlled or the degree opening of of the valve or the damper may be electrically controlled.

The oxygen containing gas inflow line 6 may be directly connected to the biogas inflow line 2 or may be directly connected to the mixture gas line 5.

Here, FIG. 2 is a diagram showing a device where the oxygen containing gas inflow line 6 is directly connected to the mixture gas line 5.

The oxygen containing gas supplying means may use a blower or a pump.

The oxygen containing gas supply adjusting mechanism 7 is controlled by the oxygen containing gas control signal transfer mechanism 14.

The control by the oxygen containing gas control signal transfer mechanism 14 may be a physical control or a control using an electrical signal.

Concretely, in accordance with the method of the physical control, the degree of opening of the inverter, the valve or the damper of the blower may be manually adjusted.

In accordance with the method of the electrical control, the inverter may be electrically controlled or the degree of opening of the valve or the damper may be electrically controlled.

An orifice flow meter, a volume flow meter, vortex a flow meter, a current flow meter or the like can be used as the gas flow meter.

A dry gas meter or a wet gas meter can be used as the volume flow meter.

Furthermore, a diaphragm type or a rotor type meter can be used as the dry gas meter.

A controlled potential electrolysis measurement, a silver nitrate potential difference titration, an ion electrode method, a methylene blue absorptiometry, a gas chromatography or the like may be used as the hydrogen sulfide concentration gauge.

In addition, hydrogen sulfide may be measured using a detector tube.

The oxygen containing gas is a gas that contains oxygen, and air, pure 14 oxygen or a gas of which the concentration of oxygen is adjusted by an oxygen generator may be used.

The filler to which bacteria adhere may be made of a chemical resistant material that can be used in an acidic environment having a pH of 1 or less, and it is preferable for the material to be an organic substance such as polyethylene, polypropylene, vinyl-chloride or polyurethane.

It is preferable for the filler have to a shape of a cylinder, a netlike skeleton pipe, a ball or a sea urchin.

It is preferable for the specific surface area to be in a range of 50 m /m 2 3 to 1000 m /m .

It is preferable for the porosity to be in a range from 80% 96%.

The operational unit may have a function to calculate the hydrogen sulfide load amount from the concentration of hydrogen sulfide and the gas flow amount.

In addition, the operational unit may have function a of recording the concentration of hydrogen sulfide, the gas flow amount and the hydrogen sulfide load amount.

The method for recording the measurement values and the results of the operation by the operational unit may be that using a recorder having a digital data logger or chart paper.

It is preferable for the concentration of hydrogen sulfide in the mixture gas to be adjusted to a range from 100 ppm to 1000 ppm by circulating the process gas, as described in details in Example 2.

It is more preferable for the concentration of hydrogen sulfide in the mixture gas to be adjusted to a range from 150 ppm to 500 ppm by circulating the process gas.

FIG. 1 is a diagram showing a device where a pipe is installed so as to process a biogas that is flowing in the downward direction, where the biogas may be processed when throwing in the upward direction.

FIG. 3 is a diagram showing a device where a pipe is installed as so to process a biogas that is flowing in the upward direction.

FIG. 4 is a flowchart for controlling the circulation gas amount adjusting mechanism according to the present invention.

According to the present invention, the biogas flow amount QB is 15 measured and the concentration of hydrogen sulfide in the mixture gas C is measured.

In order to preset the concentration of hydrogen sulfide in the mixture gas C’, the circulation gas amount QR is calculated in the operational unit, and thus the circulation gas amount QR is adjusted by operating the circulation gas amount adjusting mechanism.

Next, FIG. 5 is a flowchart for controlling the oxygen containing gas amount supply adjusting mechanism.

According to the present invention, the biogas flow amount QB is measured and the concentration of hydrogen sulfide in the mixture gas C is measured.

In order to preset the concentration of hydrogen sulfide in the mixture gas C’, the circulation gas amount QR is calculated in the operational unit.

The hydrogen sulfide load amount is calculated by the operational unit in accordance with Formula 11.

Hydrogen sulfide load amount = C x (QB + QR) … (Formula 11)

The supplied amount of the oxygen containing gas is adjusted on the basis of the hydrogen sulfide load amount by operating the oxygen containing gas amount supply adjusting mechanism.

 

Example 1

A two meter portion of the biological desulfurization device in FIG. 1 was filled in with cylindrical fillers of a diameter 15 of mm x a height of 15 mm with a specific surface area of 1000 m3 /m3polyethylene. The mixture gas was made to flow in the downward direction through the biological desulfurization tower.

The oxygen containing gas was mixed through the biogas inflow line.

Active sludge was used as the circulation liquid, and was stored in the circulation liquid storage tank in the lower portion of the biological desulfurization tower.

The circulation liquid was sent to the upper portion of the biological desulfurization tower by means of a pump, and was sprayed at a rate of 200 L/day in parallel to the direction of the gas. The process temperature was set at 35 °C.

Air (with oxygen concentration of 21% as a volume ratio) was used as the oxygen containing gas. concentration The of methane in the biogas was 80% as volume ratio and the concentration of 16 carbon dioxide was 20% as volume ratio, which remained approximately constant throughout the entire period while biological desulfurization was performed.

The process performance was compared between the case where the circulation gas amount was controlled so that the concentration of hydrogen sulfide in the mixture gas was constant according to the present invention and the case where the circulation gas amount was controlled so that the circulation ratio was constant in a control example by simultaneously carrying out and examining the experiments according to the present invention and the control example.

Here, the circulation ratio is a ratio (QR/QB) of the circulation gas amount (QR) to the biogas flow amount (QB).

During the processes according the present invention and the control example, the concentration of hydrogen sulfide in the biogas and the biogas amount were changed as the elapse of the measurement time.

The process conditions for each time period of the measurement time are shown below.

Measurement time 0 to 4 hr Concentration of hydrogen sulfide in biogas :

1500 ppm, biogas amount:

4 m3/day Measurement time:

4 to 8 hr Concentration of hydrogen sulfide in biogas: 1500 ppm, biogas amount:

2 m3/day Measurement time:

8 to 12 hr Concentration of hydrogen sulfide in biogas:

3000 ppm, biogas amount:

2 m3/day Measurement time: 12 16 to hr Concentration of hydrogen sulfide in biogas:

6000 ppm, biogas amount:

1.5 m3/day Measurement time: 16 to 20 hr Concentration of hydrogen sulfide in biogas:

500 ppm, biogas amount:

2 m3/day Measurement time:

20 to 24 hr Concentration of hydrogen sulfide in biogas:

300 ppm, biogas amount:

2 m3/day The experimental conditions were changed for every 4 hours and the process performance was checked.

According to the embodiment of the present invention, the circulation gas amount was controlled so that the concentration of hydrogen sulfide in the mixture gas was 300 ppm.

Even when the concentration of hydrogen sulfide in the biogas and 17 the biogas amount fluctuated, the process could be sustained, and hydrogen sulfide was not detected from the process gas, indicating that hydrogen sulfide could be removed by 100%.

In the case where the concentration of hydrogen sulfide in the biogas was 300 ppm, hydrogen sulfide was removed by 100% even when the circulation gas was stopped.

FIG. 6 shows the test results in accordance with the present invention, of which the details are described below.

In the time period of the measurement time 0 to 4 hr, a circulation gas amount of 16 m3/day was supplied to a biogas amount of 4 m3/day in order to adjust the concentration of hydrogen sulfide in the mixture gas to 300 ppm.

At this time the load was 2.0 kg/( m 3 * day).

The concentration of hydrogen sulfide in the mixture gas was 300 ppm and hydrogen sulfide was not detected in the process gas.

In the time period of the measurement time 4 to 8 hr, a circulation gas amount of 8 m3/day was supplied to a biogas amount of 2 m3/day in order to adjust the concentration of hydrogen sulfide in the mixture gas to 300 ppm.

At this time the load was 1.0 kg/( m 3 • day).

The concentration of hydrogen sulfide in the mixture gas was 300 ppm and hydrogen sulfide was not detected in the process gas.

In the time period of the measurement time 8 to 12 hr, a circulation gas amount of 18 m3/day was supplied to a biogas amount of 2 m3/day in order to adjust the concentration of hydrogen sulfide in the mixture gas to 300 ppm.

At this time the load was 2.0 kg/( m 3 • day).

The concentration of hydrogen sulfide in the mixture gas was 300 ppm and hydrogen sulfide was not detected in the process gas.

In the time period of the measurement time 12 to 16 hr, a circulation gas amount of 28.5 m3/day was supplied to a biogas amount of 1.5 m3/day in order to adjust the concentration of hydrogen sulfide in the mixture gas to 300 ppm.

At this time the load was 3.0 kg/( m 3 • day).

The concentration of hydrogen sulfide in the mixture gas was 300 ppm and hydrogen sulfide was not detected in the process gas.

In the time period of the measurement time 16 to 20 hr, a circulation gas amount of 1.3 m3/day was supplied to a biogas amount of 2 m3/day in 18 order to adjust the concentration of hydrogen sulfide in the mixture gas to 300 ppm.

At this time the load was 0.3 kg/( m 3 • day).

The concentration of hydrogen sulfide in the mixture gas was 300 ppm and hydrogen sulfide was not detected in the process gas.

In the time period of the measurement time 20 to 24 hr, the concentration of hydrogen sulfide in the mixture gas was 300 ppm, and therefore the circulation gas was stopped.

At this time the load was 0.2 kg/( m 3 • day).

The concentration of hydrogen sulfide in the mixture gas was 300 ppm and hydrogen sulfide was not detected in the process gas.

In the control example, the circulation ratio was 4, that is to say, the circulation gas amount was four times greater by adjusting the biogas amount, and then the process performance diminished when the concentration of hydrogen sulfide in the biogas became 3000 ppm or greater.

When the concentration of hydrogen sulfide in the biogas was low, for example 500 ppm or less, the biogas was diluted excessively so that the purity became low as the biogas, though no hydrogen sulfide was included in the process gas.

FIG. shows the test results in the control example, and the details thereof are described below.

In the time period of the measurement time 0 to 4 hr, the biogas amount was 4 m3/day, and therefore the circulation gas amount was adjusted to 16 m3/day.

At this time, the load was 2.0 kg/( m 3 • day), and hydrogen sulfide was not detected in the process gas.

In the time period of the measurement time 4 to 8 hr, the biogas amount was 2 m3/day, 3 and therefore the circulation gas amount was adjusted to 8 m3/day.

At this time, the load was 1.0 kg/( m 3 • day), and hydrogen sulfide was not detected in the process gas.

In the time period of the measurement time 8 to 12 hr, the biogas amount was 2 m3/day, 3 and therefore the circulation gas amount was adjusted to 8 m3/day.

At this time, the load was 2.0 kg/( m 3 • day).

The concentration of hydrogen sulfide in the process gas was 300 ppm and the ratio the of removed hydrogen sulfide was 90%.

In the time period of the measurement time 12 to 16 hr, the biogas 19 amount was 1.5 m3/day, 3 and therefore the circulation gas amount was adjusted to 6 m3/day.

At this time, the load was 3.0 kg/( m 3 • day).

The concentration of hydrogen sulfide in the process gas was 2000 ppm and the ratio of the removed hydrogen sulfide was 67%.

In the time period of the measurement time 16 to 20 hr, the biogas amount was 2 m3/day, 3 and therefore the circulation gas amount adjusted was to 8 m3/day. At 3 this time, the load was 0.3 kg/( m 3 • day), and hydrogen sulfide was not detected in the process gas.

In the time period of the measurement time 20 to 24 hr, the biogas amount was 2 m3/day, 3 and therefore the circulation gas amount was adjusted to 8 m3/day. At this time, the load was 0.2 kg/( m 3 * day), and hydrogen sulfide was not detected in the process gas.

As a result of the comparison test between the present invention and the control example, hydrogen sulfide could be removed consistently through the conversion into sulfuric acid even when the hydrogen sulfide load amount was 3 kg/(m 3 • day) without being affected by the concentration of hydrogen sulfide in the biogas in the case where the amount of the mixture gas was controlled so that the concentration of hydrogen sulfide in the mixture gas was constant as the in present invention.

In the case where the amount of the mixture gas was controlled so that the circulation ratio was constant as in the control example, the ratio of the removed hydrogen sulfide was lowered and the process performance diminished when the concentration of hydrogen sulfide in the biogas was high.

Accordingly, it is important to operate the circulation gas amount adjusting mechanism on the basis of the concentration read by a hydrogen sulfide concentration gauge and the flow amount read by a gas flow meter in order to make the concentration of hydrogen sulfide in the mixture gas appropriate for the method for biological desulfurization, and it could be seen that the significant results for the process performance could be gained.

Example 2

A two meter portion of the biological desulfurization device in FIG. 1 was filled in with cylindrical fillers of a diameter of 15 mm x a height of 15 20 mm with a specific surface area of 1000 m /m 2 3 made of polyethylene.

The mixture gas was made to flow in the downward direction through the biological desulfurization tower.

The oxygen containing gas was mixed through the biogas inflow line.

Active sludge was used as the circulation liquid, and was stored in the circulation liquid storage tank in the lower portion of the biological desulfurization tower.

The circulation liquid was sent to the upper portion of the biological desulfurization tower by means of a pump, and was sprayed at a rate of 200 L/day in parallel to the direction of the gas.

The process temperature was set at 35 °C.

Air (with oxygen concentration of 21% as a volume ratio) was used as the oxygen containing gas and range a from 15 L/day to 120 L/day was supplied.

The concentration of methane in the biogas was 80% as volume ratio and the concentration of carbon dioxide was 20% as volume ratio, which remained approximately constant throughout the entire period while biological desulfurization was performed.

The removal performance was examined when the circulation gas flow amount was controlled on the basis of the concentration of hydrogen sulfide in the mixture gas and the biogas flow amount.

In this test, 1 m3/day of a biogas having a concentration of hydrogen sulfide of 6000 ppm was supplied and the hydrogen sulfide load amount was set to 2.0 kg/( m 3 • day). The circulation gas amount was adjusted in a range from 3 m3/day to 119 m3/day so as to be appropriate for the process in which a biogas flow amount of 1 m3/day.

In this test, the process performance was checked when the circulation gas amount was changed relative to a constant biogas amount, and Run 2*1 to Run 2*10 were carried out for different circulation gas amounts.

In this test, three runs were carried out in parallel using three biological desulfurization devices that were the same as in FIG. 1.

The evaluation period of the test was 30 days.

The desulfurization performance was evaluated from the ratio of the removed hydrogen sulfide.

The desulfurization performance was assumed to have been carried out when the ratio of the removed hydrogen sulfide was 21 50% or greater, and satisfactory a desulfurization performance was assumed to have been carried out when the ratio of the removed hydrogen sulfide was 95% or greater.

Table 1 shows the results of this test.

The values of the test results in Table 1 are the values on the 30 th day of the evaluation.

In Run 2-1, the supplied amount of a circulation gas was 3 m3/day per 1 m3/day of a biogas amount so that the concentration of hydrogen sulfide in the mixture gas when hydrogen sulfide was included in the process gas (hereinafter referred to as the set value of the concentration of hydrogen sulfide in the mixture gas) was 1500 ppm.

At this time, the ratio of the removed hydrogen sulfide was 40% and the ratio of conversion into sulfuric acid was 70%.

In Run 2*2, the supplied amount of a circulation gas was 5 m3/day per 1 m3/day of a biogas amount so that the set value of the concentration of hydrogen sulfide in the mixture gas was 1000 ppm.

At this time, the ratio of the removed hydrogen sulfide was 50% and the ratio of conversion into sulfuric acid was 100%.

In Run 2*3, the supplied amount of a circulation gas was 9 m3/day per 1 m3/day of a biogas amount so that the set value of the concentration of hydrogen sulfide in the mixture gas was 600 ppm.

At this time, the ratio of the removed hydrogen sulfide was 80% and the ratio of conversion into sulfuric acid was 100%.

In Run 2-4, the supplied amount of a circulation gas was 5 m3/day per 1 m3/day of a biogas amount so that the set value of the concentration of hydrogen sulfide in the mixture gas was 500 ppm.

At this time, the ratio of the removed hydrogen sulfide was 95% and the ratio of conversion into sulfuric acid was 100%.

In Run 2*5, the supplied amount of a circulation gas was 14 m3/day per 1 m3/day of a biogas amount.

In Run2*6, the supplied amount of a circulation gas was 19 m3/day per 1 m3/day of a biogas amount.

In Run 2*7, the supplied amount of a circulation gas was 39 m3/day per 1 m3/day of a biogas amount.

The set value of the concentration of hydrogen sulfide in the mixture gas was 400 ppm in Run 2-5, 300 ppm in Run 2*6 and 150 ppm 22 in Run 2*7.

During these test periods, the ratio of the removed hydrogen sulfide was 100% and the ratio of conversion into sulfuric acid was 100%.

In Run 2-8, the supplied amount of a circulation gas was 49 m3/day per 1 m3/day of a biogas amount so that the set value of the concentration of hydrogen sulfide in the mixture gas was 120 ppm.

At this time, the ratio of the removed hydrogen sulfide was 75% and the ratio of conversion into sulfuric acid was 100%.

In Run 2-9, the supplied amount of a circulation gas was 59 m3/day per 1 m3/day of a biogas amount so that the set value of the concentration of hydrogen sulfide in the mixture gas was 100 ppm.

At this time, the ratio of the removed hydrogen sulfide was 50% and the ratio of conversion into sulfuric acid was 100%.

In Run 2*10, the supplied amount of a circulation gas was 119 m3/day per 1 m3/day of a biogas amount so that the set value of the concentration of hydrogen sulfide in the mixture gas was 50 ppm.

At this time, the ratio of the removed hydrogen sulfide was 40% and the ratio of conversion into sulfuric acid was 70%.

During the time periods in Run 2-2 through Run 2*9, the circulation gas amount was adjusted so that the concentration of hydrogen sulfide in the mixture gas became 100 ppm to 1000 ppm, and during these time periods, the desulfurization performance was carried out with the ratio of the removed hydrogen sulfide being 50% or higher and with the ratio of conversion into sulfuric acid being 100%.

In particular, in Run 2-4 through Run 2-7, the circulation gas amount was adjusted so that the concentration of hydrogen sulfide in the mixture gas became 150 ppm to 500 ppm, and during these time periods, the desulfurization performance was carried out with the ratio the of removed hydrogen sulfide being 95% or higher and with the ratio of conversion into sulfuric acid being 100%.

Accordingly, it is preferable to adjust the circulation gas amount so that the concentration of hydrogen sulfide in the mixture gas became within the range from 100 ppm to 1000 ppm in the present invention.

More 23 preferably, the circulation gas amount may be adjusted so that the concentration became within the range from 150 ppm to 500 ppm.

[Table 1] Test results when circulation gas amount was changed relative to biogas (concentration of hydrogen sulfide in biogas 6000 ppm)

Set hydrogen sulfide load amount :

0kg/(m3/day), Measurement data Control value Calculation data Run Biogas amount Q B Set value of concentration of hydrogen sulfide in mixture gas Circulation gas amount Q R Ratio of removed hydrogen sulfide Ratio of conversion into sulfuric acid [m3/day] 3 [ppm] [m3/day] 3 M [%] 2-1 1 1500 3 40 70 2-2 1 1000 5 50 100 2-3 1 600 9 80 100 2-4 1 500 11 95 100 2-5 400 14 100 100 2-6 300 19 100 100 2-7 150 39 95 100 2-8 120 49 75 100 2-9 100 59 50 100 2-10 1 50 119 40 70 Present invention

FIG. 8 shows the relationship between the concentration of hydrogen sulfide in the mixture gas (set value) and the ratio the of removed hydrogen sulfide on the basis of Table 1.

In the case where the circulation gas amount was controlled so that the concentration of hydrogen sulfide in the mixture gas became 50 ppm, the ratio of the removed hydrogen sulfide was 40%.

In the case where the circulation gas amount was controlled so that the concentration of hydrogen sulfide in the mixture gas became 100 ppm, the ratio of the removed hydrogen sulfide was 50%.

When the circulation gas amount was controlled so that the concentration of hydrogen sulfide in the mixture gas became 150 ppm, the ratio of the removed hydrogen sulfide was 95%.

In the case where the circulation gas amount was controlled so that the concentration of hydrogen sulfide in the mixture gas became in a range from 300 ppm to 400 ppm, the ratio of the removed hydrogen sulfide was 100%.

When the circulation gas amount was controlled so that the concentration of hydrogen sulfide in the mixture gas became 500 ppm, however, the ratio of the removed hydrogen sulfide was 95%.

When the circulation gas amount was controlled so that the concentration of hydrogen sulfide in the mixture gas became 600 ppm or greater, the ratio of the removed hydrogen sulfide was no greater than 80%.

When the circulation gas amount was controlled so that the concentration of hydrogen sulfide in the mixture gas became 1000 ppm, the ratio of the removed hydrogen sulfide was 50%.

When the circulation gas amount was controlled so that the concentration of hydrogen sulfide in the mixture gas became 1500 ppm, the ratio of the removed hydrogen sulfide was 40%.

Example 3

A portion with a height of two meters in the biological desulfurization device in FIG. was 1 filled in with cylindrical fillers of a diameter of 15 mm x a height of 15 mm with a specific surface area of 1000 m /m made of polyethylene, and the total volume of the fillers was 1 m3. The mixture gas was made to flow in the downward direction through the biological desulfurization tower.

The oxygen containing gas was mixed through the biogas inflow line.

Active sludge was used as the circulation liquid, and was stored in the circulation hquid storage tank in the lower portion of the biological desulfurization tower.

The circulation liquid was sent to the upper portion of the biological desulfurization tower by means of a pump, and was sprayed at a rate of 1.6 m3/day in parallel to the direction of the gas.

The process temperature was set at 35 °C.

Air (with oxygen concentration of 21% as a volume ratio) was used as the oxygen containing gas and range a from 15 25 L/day to 120 L/day was supplied.

The concentration of methane in the biogas was 80% as volume ratio and the concentration of carbon dioxide was 20% as volume ratio, which remained approximately constant throughout the entire period while biological desulfurization was performed.

The process performance of biological desulfurization was examined when the method for controlling the oxygen containing gas amount was different.

This comparison was made between the present invention according to which the supplied amount of the oxygen containing gas was controlled using the hydrogen sulfide load amount and the control example where the supplied amount of the oxygen containing gas was controlled so that the ratio thereof to the gas flow amount was constant.

Both in the present invention and in the control example, the concentration of hydrogen sulfide in the biogas was adjusted at three stages, 1000 ppm, 3000 ppm and 6000 ppm.

The biogas flow amount remained constant at 8.3 m3/hr.

The concentration of hydrogen sulfide in the mixture gas was made to remain constant at 300 ppm by adjusting the circulation gas flow amount.

The evaluation period of the test was five days for each run.

The values of the test results in Table 2 are values on the fifth day of the evaluation.

Table 2 shows the test results (Run 3-1 to Run 3-3) of the present invention.

In the present invention the supplied amount of air is controlled on the basis of the hydrogen sulfide load amount.

Concretely, the amount of oxygen required for the conversion into sulfuric acid was calculated from the hydrogen sulfide load amount and air was supplied so that it contained an amount of oxygen, which was 1.5 times greater than the calculated values.

In Run 3*1, the hydrogen sulfide load amount was 0.3 kg/( m 3 • day), and the supplied amount of air was 0.14 m3/hr. Run 3 3*1 resulted in a ratio of the removed hydrogen sulfide that was 100% and a ratio of conversion into sulfuric acid that was also 100%.

In Run 3-2, the hydrogen sulfide load amount was 1.0 kg/( m 3 • day), and the supplied amount of air was 0.42 m3/hr.

Run 3*2 resulted in a ratio of the removed hydrogen sulfide that was 100% and a ratio of conversion into sulfuric acid that was also 100%.

In Run 3*3, the hydrogen sulfide load amount was 2.0 kg/( m 3 • day), and the supplied amount of air was 0.85 m3/hr.

Run 3*3 resulted in a ratio of the removed hydrogen sulfide that was 100% and ratio a of conversion into sulfuric acid that was also 100%.

Next, Table 3 shows the test results of the control examples (Run 3*4 to Run 3-6).

In the control examples, the supplied amount of air was controlled so that the ratio thereof to the biogas flow amount remained constant, and the supplied amount of air was 5.1% of the biogas flow amount as a volume ratio.

Concretely, the biogas flow amount remained constant at 8.3 m3/hr 3 in this test, and the supplied amount of air was 0.42 m3/hr.

In Run 3*4, the hydrogen sulfide load amount was 0.3 kg/( m 3 • day).

Run 3*4 resulted in a ratio of the removed hydrogen sulfide that was 100% and a ratio of conversion into sulfuric acid that was also 100%.

In Run 3-5, the hydrogen sulfide load amount was 1.0 kg/( m 3 * day).

Run 3*5 resulted in a ratio of the removed hydrogen sulfide that was 100% and ratio a of conversion into sulfuric acid that was also 100%.

In Run 3-6, the hydrogen sulfide load amount was 2.0 kg/( m 3 • day).

Run 3-4 resulted in ratio a of the removed hydrogen sulfide that was 60% and ratio a of conversion into sulfuric acid that was also 60%.

In Run 3-1 in the present invention, a sufficient process can be carried out with the hydrogen sulfide load amount being 0.3 kg/( m 3 • day) and with the air amount 0.14 m3/hr.

In contrast, in Run 3*4 in the control example, the processed biogas included unprocessed air, which lowered the value of the biogas as fuel.

In addition, in Run 3*6, oxygen ran short of the amount of required for the conversion into sulfuric acid and therefore the ratio of the removed hydrogen sulfide is lowered and at the same time sulfur deposited and caused clogging within the tower.

According to the present invention, supplied the amount of the oxygen containing gas was controlled using the hydrogen sulfide load amount so that an appropriate amount of oxygen was supplied to the load, and thus a stable process could be achieved with regard to the performance removing of hydrogen sulfide and with the ratio of conversion into sulfuric acid being 100%.

[Table 2] Test results in present invention (Air amount was controlled on basis of hydrogen sulfide load amount) Run Concentration of hydrogen sulfide in biogas Biogas flow amount Concentration of hydrogen sulfide in mixture gas Supplied amount of air Amount of oxygen in air Hydrogen sulfide load amount Ratio of removed hydrogen sulfide Ratio of conversion into sulfuric acid [ppm] [m3/hr] 3 [ppm] [m3/hr] 3 [kg/hr] [kg/(m -day)] 3 [%] w 3-1 1000 8.3 300 0.14 0.039 0.3 100 100 3-2 3000 8.3 300 0.42 0.116 1.0 100 100 3-3 6000 8.3 300 0.85 0.233 2.0 100 100

[Table 3] Test results in control example (Supplied amount air was 5.1% of biogas flow amount as volume ratio) Run Concentration of hydrogen sulfide in biogas Biogas flow amount Concentration of hydrogen sulfide in mixture gas Supplied amount of air Amount of oxygen in air Hydrogen sulfide load amount Ratio of removed hydrogen sulfide Ratio of conversion into sulfuric acid Run [ppm] [m3/hr] 3 [ppm] [m3/hr] 3 [kg/hr] [kg/(m 3 • day)] [*] M 3-4 1000 8.3 300 0.42 0.116 0.3 100 100 3-5 3000 8.3 300 0.42 0.116 1.0 100 100 3-6 6000 8.3 300 0.42 0.116 2.0 60 60

Example 4

The same test device as in Example 3 was used to examine the process performance in two runs where oxygen containing gases having different oxygen concentrations were prepared.

The mixture gas was made to flow in the downward direction through the biological desulfurization tower.

The oxygen containing gas was mixed through the biogas inflow line.

Active sludge was used as the circulation hquid, and was stored in the circulation liquid storage tank in the lower portion of the biological desulfurization tower.

The circulation liquid was sent to the upper portion of the biological desulfurization tower by means of a pump, and was sprayed at a rate of 1.6 m3/day in parallel to the direction of the gas.

The process temperature was set at 35 °C.

The concentration of methane in the biogas was 80% as volume ratio and the concentration of carbon dioxide was 20% as volume ratio, which remained approximately constant throughout the entire 28 period while biological desulfurization was performed.

The concentration of oxygen was adjusted to 30% of the used oxygen containing gas as a volume ratio, and the concentration of nitrogen was adjusted to 70% as volume ratio in Run 4-1.

The concentration of oxygen was adjusted to 60% as a volume ratio, and the concentration of nitrogen was adjusted to 40% as volume ratio in Run 4-2.

The concentration of hydrogen sulfide in the biogas was adjusted to 6000 ppm.

The biogas flow amount was constant at 8.3 m3/hr, 3 and the hydrogen sulfide load amount was 2.0 kg/( m 3 • day).

The concentration of hydrogen sulfide in the mixture gas remained constant as 300 ppm by adjusting the circulation gas flow amount.

Table 4 shows the test results (Run 4-1 to Run 4-2) when gases having different concentrations of oxygen were supplied.

In Run 4-1, the concentration of oxygen in the gas was 30% as a volume ratio, and the supplied amount of the gas was 0.59 m3/hr.

Run 4*1 resulted in the ratio the of removed hydrogen sulfide that was 100% and the ratio of conversion into sulfuric acid that was also 100%.

In Run 4*2, the concentration of oxygen in the gas was 60% as a volume ratio, and the supplied amount of the gas was 0.30 m3/hr.

Run 4*2 resulted in the ratio the of removed hydrogen sulfide that was 100% and the ratio of conversion into sulfuric acid that was also 100%.

Here, in Run 3-3 in Example 3, the concentration of oxygen in air was 21% as a volume ratio, and the supplied amount of the gas was 0.85 m3/hr.

Run 3*3 resulted in the ratio of the removed hydrogen sulfide that was 100% and the ratio of conversion into sulfuric acid that was also 100%.

In the process system according to the present invention, hydrogen sulfide could be removed by 100% and the ratio of conversion into sulfuric acid was 100% throughout the test period even when a gas having a different concentration of oxygen was used, and thus the process was satisfactory.

Accordingly, the present invention could provide satisfactory process where an appropriate amount of oxygen could be supplied even when the oxygen containing gas had a different concentration of oxygen.

In addition, a smaller amount of the oxygen containing gas could be supplied when the 29 concentration of oxygen was high, which increased the value of the biogas.

[Table 4]

Test results when gases having different oxygen concentrations were supplied Concentration of hydrogen sulfide in biogas:6000 ppm Biogas flow amount:8. 3m3/hr Concentration of hydrogen sulfide in mixture gas:300 ppm Hydrogen sulfide load amount:2. 0kg/(m day) 3, Run Oxygen concentration in oxygen containing gas Supplied amount of oxygen containing gas Ratio of removed hydrogen sulfide Ratio of conversion into sulfuric acid Run [%] [m3/hr] 3 [%3 [*] 4-1 30 0.59 100 100 4-2 60 0.30 100 100 Run3-3 in Example 3 21 0.85 100 100

Example 5

A portion with a height of two meters in the biological desulfurization device in FIG. 2 was filled in with cylindrical fillers of a diameter of 15 mm x a height of 15 mm with a specific surface area of 1000 m /m 2 3 made of polyethylene, and the total volume of the fillers was 1 m3. The mixture gas was made to flow in the downward direction through the biological desulfurization tower.

The oxygen containing gas was mixed through the mixture gas line.

Active sludge was used as the circulation liquid, and was stored in the circulation liquid storage tank in the lower portion of the biological desulfurization tower.

The circulation liquid was sent to the upper portion of the biological desulfurization tower by means of a pump, and was sprayed at a rate of 1.6 m3/day in parallel to the direction of the gas.

The process temperature was set at 35 °C.

Air (with oxygen concentration of 21% as a volume ratio) was used as the oxygen containing gas and a range from 15 L/day to 120 L/day was supplied. The concentration of methane in the biogas was 80% as volume ratio and the concentration of carbon dioxide was 20% as volume ratio, which remained approximately constant throughout 30 the entire period while biological desulfurization was performed.

In this test, the concentration of hydrogen sulfide in the biogas was adjusted at three stages: 1000 ppm; 3000 ppm and 6000 ppm.

The biogas flow amount remained constant at 8.3 m3/hr.

The concentration of hydrogen sulfide in the mixture gas remained constant at 300 ppm by adjusting the circulation gas flow amount.

The evaluation period for the test was five days for each run.

The values of the test results in Table 5 are values on the fifth day of the evaluation.

Table 5 shows the test results (Run 5-1 to Run 5*3).

In Run 5-1, the hydrogen sulfide load amount was 0.3 kg/( m 3 • day), and the supplied amount of air was 0.14 kg/( m 3 • day).

Run 5-1 resulted a in ratio of the removed hydrogen sulfide that was 100% and a ratio for conversion into sulfuric acid that was also 100%.

In Run 5-2, the hydrogen sulfide load amount was 1.0 kg/( m 3 * day), and the supplied amount of air was 0.42 kg/( m 3 • day).

Run 5*2 resulted in a ratio of the removed hydrogen sulfide that was 100% and a ratio for conversion into sulfuric acid that was also 100%.

In Run 5-3, the hydrogen sulfide load amount was 2.0 kg/( m 3 • day), and the supplied amount of air was 0.85 kg/( m 3 • day).

Run 5*3 resulted a in ratio of the removed hydrogen sulfide that was 100% and a ratio for conversion into sulfuric acid that was also 100%.

Accordingly, the process was satisfactory even in the case where an oxygen containing gas was made to flow in the mixture gas line.

[Table 5] Test results in case where air flows into mixture gas line Run Concentration of hydrogen sulfide in biogas Biogas flow amount Concentration of hydrogen sulfide in mixture gas Supplied amount of air Amount of oxygen in air Hydrogen sulfide load amount Ratio of removed hydrogen sulfide Ratio of conversion into sulfuric acid Run [ppm] [m3/hr] 3 [ppm] [m3/hr] 3 [kg/hr] [kg/Cm 3 • day)] [%] M 5-1 1000 8.3 300 0.14 0.039 0.3 100 100 5-2 3000 8.3 300 0.42 0.116 1.0 100 100 5-3 6000 8.3 300 0.85 0.233 2.0 100 100

As described above, according to the present invention, hydrogen sulfide can be efficiently processed under a high load, and the processed 31 hydrogen sulfide can be converted to sulfuric acid so that the device can be prevented from being clogged inside, and thus, it is possible to provide a device and a method for biological desulfurization of a biogas where no cleaning process is required, which makes the process possible at a low cost.

Explanation of Symbols

Oa biogas Ob oxygen containing gas Oc process gas Od circulation liquid Oe blow water Of refilling water 1 biological desulfurization tower la filler layer lb circulation liquid storage tank 2 biogas inflow line 3 gas flow meter 4 hydrogen sulfide concentration gauge 5 mixture gas line 6 oxygen containing gas inflow line 7 oxygen containing gas amount supply adjusting mechanism 8 process gas outflow line 9 circulation gas line 10 circulation gas amount adjusting mechanism 11 spraying line 12 operational unit 13 circulation gas signal transfer mechanism 14 oxygen containing gas signal transfer mechanism 15 gas flow amount signal input line 16 hydrogen sulfide concentration signal input line

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