In the process of biogas power generation, biogas generator units are particularly sensitive to the composition, temperature, pressure, and amount of biogas. The generator unit requires that the intake system must maintain a constant and moderate air-fuel ratio to avoid excessive air-fuel ratio that may cause flameout; It is also necessary to avoid excessively low air-fuel ratio, which can cause high temperature and detonation. The air-fuel ratio is one of the important factors that affect the combustion performance of biogas generators, and changes in the temperature, pressure, and composition of biogas can affect the air-fuel ratio. Therefore, before choosing to use biogas for power generation, not properly controlling the biogas quality will lead to the inability of the generator unit to operate normally, resulting in significant economic losses.

 

1. Biogas quality

 

To generate electricity using biogas, it is first necessary to meet the requirements for biogas quality of biogas generating units. The requirements for biogas quality in the Medium and Large Power biogas generating units (GBT29488-2003) and the Technical Specifications for Biogas Power Plants (NY/T1704-2009) are as follows.

 

(1) The low calorific value of biogas is ≥ 14MJ/Nm, which is equivalent to the methane volume fraction in biogas not less than 30%. The change rate of methane volume fraction in biogas within 30s shall not exceed 2%, and the change rate of methane volume fraction in biogas during continuous operation of the unit shall not exceed 5%.

 

(2) The temperature of biogas shall not be higher than 50 ℃.

 

(3) The specific requirements for biogas components of biogas generator units are shown in the table below.

 

Note: The standard reference condition for biogas volume is 101.3kPa, 20 ℃; Calculated based on the conversion of all sulfur containing components in biogas into hydrogen sulfide.

 

① Impact of H2S on generator sets

During biogas power generation, H2S in biogas can cause serious corrosion to the intake pipes, superchargers, intercoolers, spark plugs, cylinder liners, exhaust pipes, and silencers of the biogas generator unit, thereby affecting the unit's life. Therefore, biogas pretreatment must be conducted before biogas enters the generator unit to reduce H2S to the allowable range of the unit to ensure reliable operation of the unit.

 

② Effect of moisture on generator sets

 

During biogas power generation, excessive moisture content in biogas can lead to excessive intake pressure loss of the generator set. In severe cases, it can cause engine power fluctuations, cylinder knocking, shutdown, and other issues, seriously affecting its service life. The specific performance is as follows:

 

(a) Difficulty in ignition of the engine;

 

(b) Reduce combustion chamber temperature and reduce internal combustion engine efficiency;

 

(c) The presence of reactive gases such as water vapor can pressurize and increase power consumption;

 

(d) The intermediate products produced by the combination of water vapor and other acidic substances can cause corrosion to the generator set, shorten its service life, and reduce its reliability.

 

③ Impact of dust on generator sets

 

Dust is a clearly restricted impurity in the engine, and its main impact is:

 

(a) Blocking the pipeline, impeded circulation, increased pressure loss, and increased operating costs;

 

(b) Increase mechanical wear and reduce equipment service life.

 

2. Biogas pressure

The biogas pressure of the biogas generator unit at a distance of 1m from the unit gas branch pipe shall not be lower than 3kPa. When the gas supply pressure is lower than 3kPa, gas boosting equipment shall be equipped; When the air supply pressure exceeds 5kPa, it is necessary to add a suitable pressure reducing valve. When the biogas generator unit is running, the allowable biogas pressure fluctuation of the engine is generally less than 0.4 kPa/s.

 

3. Biogas consumption

The amount of biogas used in a biogas generator unit should be determined based on the efficiency and load of the generator unit, and the amount of biogas supplied during power generation should be relatively stable. When calculating the amount of biogas used, it should be less than 1.2 times the rated power consumption of the biogas generator. The gas supply system shall be equipped with facilities for regulating gas consumption and gas metering instruments. During continuous operation of biogas generating units, the capacity of the gas storage device should be designed based on the gas consumption with the total rated power of the operating unit greater than 3h. When the biogas generator unit operates intermittently, the capacity of the gas storage device should be designed based on the total amount of biogas that is greater than the intermittent power generation time.
 

Among them, the gas consumption regulation system for biogas generating units is generally achieved by using an electronic speed regulation system to control the opening of the disc door. The speed regulation system consists of a rotational speed sensor, a speed controller, an actuator, and a driver, forming a closed-loop speed regulation and control system to achieve control of the intake amount of mixed gas. Gas metering instruments generally use biogas flow meters, such as the Ruiyi Automatic Ultrasonic Gas Flowmeter BF-3000/BF-3000B, to measure the instantaneous and cumulative flow of biogas in real time, providing scientific data support for calculating the amount of biogas used.

 

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