Process design of biogas safety system for anaerobic digestion of sludge?

What is the process design of biogas safety system for anaerobic digestion of sludge? The following Zhong Da Consulting will introduce relevant contents for your reference.

Anaerobic digestion of sludge is a very effective treatment method to make sludge reach a stable state. Under anaerobic conditions, the organic matter in sludge is decomposed into digested gas (biogas) with methane and carbon dioxide as the main products through acid digestion stage and alkali digestion stage under the sequential action of acid-producing bacteria and methanogens. Large and medium-sized sewage treatment plants can recycle biogas produced by digestion, which can save energy and reduce operating costs.

Biogas consists of 55% ~ 70% methane, 25% ~ 40% carbon dioxide and 1% ~ 5% nitrogen sulfide and hydrogen sulfide. When biogas is mixed with air in the volume ratio of 1: 5 ~ 14, it will explode when it meets an open flame. When the mixing ratio exceeds this value, it is the fuel with a calorific value of about 4500 ~ 6000 kcal/m3. The methane content in the air will be toxic when it reaches a certain concentration. Therefore, biogas is a kind of energy with high utilization value, and it will be dangerous if the system is not designed and operated properly.

Shown is the flow chart of a typical sewage treatment plant sludge anaerobic digestion biogas utilization system.

1 Process design and safety device setting of biogas system

Biogas flows into the pipeline from the biogas digester, and first passes through the condensate removal tank and desulfurization device, with the purpose of purifying biogas.

1. 1 biogas purification

1. 1. 1 Removing condensed water and impurities

Biogas is a mixture with high humidity. When biogas enters the pipeline from the biogas digester, the temperature gradually decreases, and a large amount of condensed water containing impurities will be produced in the pipeline. If it is not removed from the system, it is easy to block and destroy pipeline equipment.

The biogas pipeline is closest to the biogas digester, and the temperature drop of biogas is the largest, which produces the most condensed water. At this point, a condensate removal tank is provided. In the biogas system, the pipeline is generally designed with a slope of about 1% or more, and a condensate tank is set at the low point. For a long pipeline, considering a certain distance, a cleaning pool is set up. In addition, in important equipment such as biogas compressor, biogas boiler, biogas generator, waste gas burner and desulfurization tower, condensate removal tanks are set at the entrance of dry gas holder and the entrance and exit of wet gas holder. Sometimes the outlet of some equipment needs to be provided with a high-pressure water removal tank, such as a biogas compressor. ?

During normal operation, operators will find that some dehumidifiers (especially those near the digester) discharge a lot of condensed water during daily inspection. When the structure and equipment are overhauled, water can also be used as a water seal tank to be injected into the condensate drainer.

1. 1.2 removal of hydrogen sulfide

The biogas produced by mesophilic digestion of municipal sewage sludge contains 0.005 ~ 0.0 1% H2S, and the biogas produced by fecal digestion accounts for about 0.5 ~ 1.0% of the total volume. General biogas utilization equipment requires that the content of H2S in biogas should be less than 0.009%. Therefore, when biogas generated by sludge generated from municipal sewage is used as raw material, a desulfurization device is not needed. However, the biogas utilization system of biogas digesters with feces as raw materials must be equipped with desulfurization devices.

1.2 Safe utilization of biogas

Biogas enters the gas holder after purification, and the gas holder plays a role in storing gas and stabilizing pressure for the whole system.

1.2. 1 flame suppression and explosion protection

A flame arrester is installed on the inlet pipeline of the gas holder. In addition, it is installed in all parts of the biogas system that communicate with the outside world (such as vacuum pressure safety valve and mechanical exhaust valve). ), the import and export of biogas compressor, biogas boiler, biogas generator and other equipment, and the entrance of biogas pipe of waste gas burner are all equipped with flame arresters. The flame arrester is filled with metal filler. When the flame passes through the gap between the fillers of the flame arrester, the heat is absorbed, and the temperature of the gas drops below the ignition point, thus achieving the purpose of suppressing the flame. A certain mixing ratio of biogas and air and encountering an open flame are two conditions for biogas explosion or combustion. The arrangement of flame arrester effectively prevents the external flame from entering the biogas system and the flame from spreading in the pipeline, thus preventing the system from exploding. Biogas from biogas digesters often contains impurities such as foam and scum, which is easy to block fillers, hinder gas passage and increase pipeline resistance. The operator of the treatment plant can measure and record the pressure change of biogas when it passes through flame arresters at different positions to determine the cycle of cleaning the filler. In actual operation, due to the untimely cleaning of flame arresters, system pressure fluctuation and operation problems often occur. When designing, the valve is generally set in front of and behind the flame arrester, which is convenient for maintenance. ?

1.2.2 biogas utilization

The purified biogas enters the subsequent biogas utilization system from the gas holder, which generally has three branches:

(1) biogas mixing system

Biogas is sent back to the digester by biogas compressor to stir and digest sludge.

(2) Biogas utilization system

Biogas utilization system generally includes: using biogas boiler to directly provide heat energy for biogas digesters or other buildings; Biogas generator is used to generate electricity for internal use of power treatment plant, and waste heat can also provide heat energy for biogas digesters or other buildings; Biogas engine is used to directly drive some equipment in the treatment plant, and the waste heat can also provide heat energy for biogas digesters or other buildings; Directly supplied as combustible gas.

(3) Exhaust gas combustion system

When the biogas utilization equipment cannot completely consume the biogas generated by the biogas digester, in order to prevent the system pressure from exceeding the normal range due to the increase of biogas quantity, the excess biogas will be burned by the waste gas burner. ?

1.2.3 pressure safety protection device The biogas utilization system is a pressure system. If the biogas collection and use are unbalanced, the system pressure may rise above the allowable value; Excessive discharge of sludge or biogas from digester or gas holder may lead to vacuum state inside the structure; In order to prevent the possible destructive influence of overpressure or vacuum on structures and equipment, ensure the operating pressure of the system, and prevent methane from being frequently discharged into the atmosphere, a vacuum pressure safety valve is set at the top of the digester and gas holder.

Because the vacuum pressure safety valve is installed in the part where the biogas system communicates with the outside atmosphere, it is necessary to connect a flame arrester to prevent the outside flame from entering the biogas system. There is a normally open valve at the joint of vacuum pressure safety valve, flame arrester and digester, which is convenient for equipment maintenance. ?

The setting of pressure relief valves in different positions should be based on the basic value of system working pressure, and the corresponding values should be taken according to the pipeline loss between various structures. Operators also need to clean the sealing surfaces of overpressure exhaust port and vacuum air inlet regularly.

Install negative pressure prevention valves at the biogas compressor and desulfurization device to prevent the system in front of the valve from continuing to inhale when the biogas quantity is insufficient.

2 Pressure design of biogas system and interlocking control between structures and equipment

After the hardware realization of biogas system is completed, it is necessary to complete the software part of the safe operation of the system, that is, the setting of the overall pressure of the system and the interlocking control between various structures and equipment.

2. 1 System working pressure setting

Setting the working pressure of the whole biogas system requires comprehensive consideration of the following factors:

(1) The biogas system shall be operated under positive pressure.

② Static pressure of gas holder under its own weight.

③ Calculate the pipeline loss from the biogas digester to the gas holder and finally to the biogas utilization equipment, so as to ensure that the population pressure from the gas to the booster or exhaust gas burner of the biogas utilization equipment meets the equipment requirements under normal working conditions.

④ The gas pressure in the digester should be as low as possible, so that the biogas can be released from the sludge to the maximum extent.

Based on the above definition, the normal working pressure of digester can be obtained, and the pressures at other points in the system can be calculated according to different pipeline losses. During operation, the working pressure of the digester (i.e. system pressure) is usually set and adjusted by changing the counterweight at the top of the gas holder.

2.2 Pressure protection of structure and equipment's own system

Biogas digesters, gas holders, biogas compressors and various biogas utilization equipment operate normally under the working pressure of the system, and the pressure will fluctuate on the basis of this value, but the control and safety devices of each equipment and structure's own system ensure that the pressure change at a certain equipment and structure will not affect the whole system.

① Biogas compressor sends biogas into digester for sludge mixing. The system of biogas compressor itself should have protection function. Alarm when the inlet pressure is low to a certain set value, and stop when it continues to decrease; When the outlet pressure is higher than a set value, it will give an alarm and stop when it continues to rise. These settings of the compressor's own system can avoid negative pressure caused by excessive inhalation of biogas when the system pressure is lower than normal, or make the pressure of the rear system exceed the allowable value.

② The gas inlets of biogas boilers, biogas generators, biogas engines and other equipment also have the same protection settings.

(3) The vacuum pressure relief valve is installed at the top of the digester and gas holder to prevent the air tightness of the structure from being damaged by excessive pressure and the structure from being damaged by negative pressure.

④ An overpressure exhaust valve should also be installed at the top of the spherical tank used in the high pressure system.

2.3 Interlocking control between equipment and structures in the system

In the biogas system, the structure and equipment are connected by pipelines, so they are not isolated from each other, but can be controlled by interlocking.

The total gas volume of the low-pressure system is reflected in the change of the height value of the gas holder. The gas holder has the function of regulating and stabilizing the gas volume of the whole system, and the system can supplement gas with each other to balance the pressure. At the same time, the height value of the gas holder is transmitted to the central control room, which is not only convenient for monitoring, but also can control the start and stop of each equipment in the system according to the height of the gas holder in the following chains:

① The normal operation area is that the height of the gas holder is between 5% and 90% of the total height.

(2) When the height of the gas holder is lower than 5%, stop the biogas compressor and stop the biogas equipment such as boilers, generators and engines. If the system control fails and the pressure drops to the set value of the vacuum safety valve, the vacuum safety valve will act to prevent the negative pressure from damaging the structure.

(3) When the height of the gas holder is higher than 90%, start the waste gas burner. Due to the use of the waste gas burner, the height of the gas holder will begin to decrease. When the height of the gas holder is lower than 90% (which can be set according to the specific conditions of the project), the waste gas burner will stop.

(4) If the waste gas burner fails, the height of the gas holder continues to rise to 93%. If there is a mechanical exhaust valve at the top of the gas holder, open the exhaust valve to exhaust. That is, the set value of the mechanical exhaust valve action can be 93% of the height of the gas holder.

⑤ If the displacement of the mechanical exhaust valve is insufficient and the height of the gas holder is mechanically raised to 95%, the central control room will display a signal alarm.

⑥ If the height of the gas holder continues to rise to the peak, the system pressure will start to rise. When the set opening value of the pressure relief valve is reached, the pressure relief valve at the top of the digester and the gas holder will be opened to protect the structure from damage. It should be noted that the working pressure value of the pressure relief valve must be lower than the maximum pressure value of the air tightness protection of the gas holder.

Through interlocking control, various structures and equipment of biogas system are truly combined into a complete system, which further ensures the normal operation of the system.

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